JP7804816B2 - Light-emitting device - Google Patents

Description

This disclosure relates to a light-emitting device.

Patent Document 1 discloses a VCSEL (Vertical Cavity Surface Emitting Laser) that includes a saturable absorbing layer. The VCSEL in Patent Document 1 can generate optical pulses with short optical pulse widths and high peak values.

Patent document 2 discloses a VCSEL having multiple active regions and tunnel junctions arranged between the active regions.

Japanese Patent Application Laid-Open No. 2022-176886 US Patent Application Publication No. 2022/0190559

Further performance improvements are required for light-emitting devices such as those described in Patent Documents 1 and 2.

The present disclosure therefore aims to provide a light-emitting device that can further improve performance.

According to one disclosure of the present specification, there is provided a semiconductor device including a first reflecting mirror disposed on a semiconductor substrate, a first active layer disposed on the first reflecting mirror, a first tunnel junction layer disposed on the first active layer, a second active layer disposed on the first tunnel junction layer, and a second reflecting mirror disposed on the second active layer, wherein the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, a second barrier layer disposed on the plurality of first quantum well layers, and a third barrier layer disposed between the plurality of first quantum well layers. the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, a fifth barrier layer disposed on the plurality of second quantum well layers, and a sixth barrier layer disposed between the plurality of second quantum well layers, wherein when the number of the plurality of second quantum well layers is N, the thickness of each of the plurality of second quantum well layers is w (nm), the number of active layers is M, and the number of quantum well layers required to obtain an optical intensity with a predetermined peak value ratio is Q, the thickness Ta (nm) of the second active layer satisfies the following formula (1):
Ta≧{w×N×M+(Q-N×M)×w/0.2}/M (1)
The distance between the second active layer and the first tunnel junction layer is 40 nm or more, and any one of the plurality of first quantum well layers and the plurality of second quantum well layers is thinner than any one of the first barrier layer to the sixth barrier layer.

Furthermore, according to yet another disclosure of this specification, there is provided a light-emitting device comprising: a first reflecting mirror disposed on a semiconductor substrate; a first active layer disposed on the first reflecting mirror; a first tunnel junction layer disposed on the first active layer; a second active layer disposed on the first tunnel junction layer; and a second reflecting mirror disposed on the second active layer, wherein the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, and a fifth barrier layer disposed on the plurality of second quantum well layers, and the fourth barrier layer is thinner than the fifth barrier layer.

Furthermore, according to yet another disclosure of this specification, there is provided a light-emitting device comprising: a first reflecting mirror disposed on a semiconductor substrate; a first active layer disposed on the first reflecting mirror; a first tunnel junction layer disposed on the first active layer; a second active layer disposed on the first tunnel junction layer; and a second reflecting mirror disposed on the second active layer, wherein the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, and a second barrier layer disposed on the plurality of first quantum well layers, and the first barrier layer is thinner than the second barrier layer.

This disclosure provides a light-emitting device that can further improve performance.

1 is a schematic cross-sectional view showing a light emitting device according to a first embodiment. 1 is an enlarged cross-sectional view showing a light emitting device according to a first embodiment. 4 is a graph showing an example of a light output waveform of the light emitting device according to the first embodiment. FIG. 2 is an energy band gap diagram near the active layer in the light emitting device according to the first embodiment. FIG. 10 is an enlarged cross-sectional view showing a light emitting device according to a second embodiment. FIG. 10 is an energy band gap diagram near the active layer in the light emitting device according to the second embodiment. FIG. 10 is an enlarged cross-sectional view showing a light emitting device according to a third embodiment. FIG. 11 is an energy band gap diagram near the active layer in the light emitting device according to the third embodiment. FIG. 10 is an enlarged cross-sectional view showing a light-emitting device according to a fourth embodiment. FIG. 10 is an enlarged cross-sectional view showing a light-emitting device according to a fifth embodiment. FIG. 10 is an enlarged cross-sectional view showing a light-emitting device according to a sixth embodiment. FIG. 13 is an enlarged cross-sectional view showing a light-emitting device according to a seventh embodiment. FIG. 13 is a block diagram showing a schematic configuration of a distance measuring device according to an eighth embodiment. FIG. 13 is a block diagram showing an example of the configuration of a moving body according to the ninth embodiment.

Embodiments of the present disclosure will be described below with reference to the drawings. Identical or corresponding elements throughout the drawings are designated by common reference numerals, and their description may be omitted or simplified. In the following description, a VCSEL capable of generating an optical pulse with a short optical pulse width and a high peak value may be referred to as a high peak value VCSEL.

[First embodiment]
FIG. 1 is a schematic cross-sectional view showing a light-emitting device 1 according to this embodiment. The light-emitting device 1 of this embodiment is a vertical-cavity surface-emitting laser (VCSEL) having a distributed Bragg reflector (DBR). The light-emitting device 1 includes a semiconductor substrate 11, a lower DBR layer 12, a spacer portion 13, a resonator portion 20, an upper DBR layer 32, electrodes 40 and 41, and a protective film 42. The lower DBR layer 12 (first reflector) is disposed on the semiconductor substrate 11. The spacer portion 13 is disposed on the lower DBR layer 12. The resonator portion 20 is disposed on the spacer portion 13. The upper DBR layer 32 (second reflector) is disposed on the resonator portion 20. Layers (spacer portion 13 and resonator portion 20) located between the lower DBR layer 12 and the upper DBR layer 32 form a resonator spacer portion. A saturable absorbing layer 131 is disposed within the spacer portion 13 .

Figure 2 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. Figure 2 is an enlarged view of region R shown in Figure 1. The configuration of the light-emitting device 1 will be described in more detail with mutual reference to Figures 1 and 2.

The resonator section 20 has an n-type layer 21 disposed on the spacer section 13, and an active layer 22 (first active layer) disposed on the n-type layer 21. The resonator section 20 also has an active layer 25 (second active layer) disposed on the active layer 22 via a spacer section, and a p-type layer 26 disposed on the active layer 25.

The active layer 22 contains a barrier layer 222 (first barrier layer), three quantum well layers 221 (first quantum well layers), two barrier layers 224 (third barrier layers) disposed between the quantum well layers 221, and a barrier layer 223 (second barrier layer). The three quantum well layers 221 and the two barrier layers 224 form a multiple quantum well structure.

The active layer 25 contains a barrier layer 252 (fourth barrier layer), three quantum well layers 251 (second quantum well layers), two barrier layers 254 (sixth barrier layer) and a barrier layer 253 (fifth barrier layer) disposed between the quantum well layers 251. The three quantum well layers 251 and the two barrier layers 254 form a multiple quantum well structure.

The quantum well layers 221 and 251 may be undoped InGaAs layers. The barrier layers 222, 223, 224, 252, 253, and 254 may be undoped GaAs layers.

The spacer portion between the active layer 22 and the active layer 25 includes a p-type layer 27 disposed on the active layer 22, an oxide constriction layer 23 (first oxide constriction layer) disposed on the p-type layer 27, and a p-type layer 28 disposed on the oxide constriction layer 23. The spacer portion between the active layer 22 and the active layer 25 also includes a tunnel junction layer 24 (first tunnel junction layer) disposed on the p-type layer 28, and an n-type layer 29 disposed on the tunnel junction layer 24. An oxide constriction layer 31 (second oxide constriction layer) is disposed within the upper DBR layer 32.

The active layer 22, active layer 25, p-type layer 26, upper DBR layer 32, and the spacer between the active layer 22 and active layer 25 are processed into a mesa shape. An electrode 40 electrically connected to the n-type layer 21 is disposed on the n-type layer 21 exposed by this mesa processing. An electrode 41 electrically connected to the upper DBR layer 32 is disposed on the upper DBR layer 32. A protective film 42 is disposed on the top surface of the n-type layer 21 and the side and top surfaces of the mesa, excluding at least a portion of the surfaces of the electrodes 40 and 41.

The semiconductor substrate 11 may be formed, for example, by a GaAs substrate. The lower DBR layer 12 may be formed, for example, by stacking 35 pairs of stacked layers, each stacked layer consisting of an Al _0.1 GaAs layer and an Al _0.9 GaAs layer, each having an optical film thickness of ¼λc. Here, λc is the center wavelength of the high reflection band of the lower DBR layer 12, which is 940 nm in this embodiment. In this embodiment, the semiconductor substrate 11 and the lower DBR layer 12 are n-type.

The spacer section 13 has a structure not found in typical VCSELs. The saturable absorbing layer 131 may be composed of a multiple quantum well structure including three quantum well layers, each with an 8-nm-thick InGaAs quantum well layer sandwiched between 10-nm-thick AlGaAs barrier layers. The rest of the spacer section 13 other than the multiple quantum well layers may be composed of doped or undoped GaAs, AlGaAs, or other layers.

The resonator section 20 includes an nip junction including an n-type layer 21, an active layer 22, a p-type layer 27, an oxide constriction layer 23, and a p-type layer 28, a tunnel junction layer 24, and an nip junction including an n-type layer 29, an active layer 25, and a p-type layer 26. The tunnel junction layer 24 includes, in order from the semiconductor substrate 11 side, a highly doped p-type layer and a highly doped n-type layer. As described above, the light-emitting device of this embodiment is a multi-junction VCSEL in which multiple nip junctions are connected by the tunnel junction layer 24. The tunnel junction layer 24 may be composed of three or more semiconductor regions. The oxide constriction layer 23 can be formed by oxidizing a portion of an Al _0.98 Ga _0.02 As layer.

The upper DBR layer 32 can be formed by stacking, for example, 20 pairs of stacked layers, each of which is _{an Al0.1Ga0.9As} layer and _{an Al0.9Ga0.1As} layer, each having an optical film thickness of ¼λc. An oxidized constriction layer 31 is disposed within the upper DBR layer 32. The oxidized constriction layer 31 is, for example, an _{Al0.98Ga0.02As} layer having a thickness of ₃₀ nm. In this embodiment, the upper DBR layer 32 is p-type.

The oxidized constriction layers 23, 31 _have a non-oxidized portion in the center of the mesa and an oxidized portion near the sidewall of the mesa, formed by oxidizing the _{Al0.98Ga0.02As} layer from the side of the mesa with water vapor during manufacturing, for example. The diameter of the non-oxidized portion in plan view can be approximately 10 μm. As a result, current injected into the light-emitting device 1 flows only through the non-oxidized portion, and only the portion of the light-emitting device 1 that overlaps with the center portion of the mesa in plan view lases. However, the oxidized constriction layer 23 is not essential and may not be provided.

The laser light generated by the light emitting device 1 may be configured to be emitted from the upper DBR layer 32 side, or the laser light generated by the light emitting device 1 may be configured to be emitted from the semiconductor substrate 11 side. If the laser light is emitted from the upper DBR layer 32 side, the reflectivity of the upper DBR layer 32 is designed to be lower than that of the lower DBR layer 12.

The light-emitting device 1 of this embodiment is based on the configuration of a typical VCSEL, but the following three elements have been added. The first of these three elements is to substantially increase the volume of the active layer. For example, a typical VCSEL is composed of three quantum wells, but the volume of the active layer in the light-emitting device 1 of this embodiment is even larger. The second is to introduce a saturable absorbing layer 131. The third is to increase the effective cavity length of the VCSEL compared to that of a typical VCSEL. The effective cavity length is the cavity length experienced by light within the cavity. More specifically, it is the average distance traveled by light that passes through the active layer in the resonant direction, is reflected by the two reflecting mirrors that make up the cavity, and then passes through the active layer again. If this effective cavity length is comparable to that of a typical VCSEL, the peak pulse width may be too short. The effective cavity length is preferably designed to obtain the required pulse width. For example, the effective cavity length of the light-emitting device 1 of this embodiment can be set longer than the effective cavity length of a normal VCSEL. By adding at least one, and preferably three, of these elements, it is possible to realize a VCSEL that can generate optical pulses with high peak values and short pulse widths.

The light-emitting device 1 has two electrodes for injecting current into the active layer. Specifically, the light-emitting device 1 of this embodiment has electrodes 40 and 41, which inject current into the active layer. However, modified light-emitting device configurations are also possible in which the optical absorption characteristics are controlled by injecting current into the saturable absorbing layer or applying a reverse bias voltage. Such a configuration is referred to as a comparative example. The comparative light-emitting device is, for example, a VCSEL using a saturable absorbing layer, and has a p-i-n configuration in which both the active layer and the saturable absorbing layer are sandwiched between p-type and n-type layers. Furthermore, the comparative example has two electrodes for electrically controlling the p-type and n-type layers sandwiching the active layer, and two electrodes for electrically controlling the p-type and n-type layers of the saturable absorbing layer. It is also possible to configure the device so that one electrode serves as any two of the four electrodes. Therefore, the number of electrodes may be three, four, or more.

In contrast to the comparative example, the light-emitting device 1 of this embodiment does not perform electrical control on the saturable absorption layer, such as current injection or reverse bias voltage application. That is, the number of electrodes in the light-emitting device 1 is smaller than the number of electrodes in the light-emitting device of the comparative example. This configuration enables the light-emitting device 1 to be miniaturized. This configuration also makes it possible to reduce the number or size of the control circuits that control each electrode. Furthermore, since the greater the number of electrodes, the greater the distribution of electrical characteristics between electrodes, reducing the number of electrodes makes it possible to reduce the variation in electrical characteristics.

Note that the design concept of the light-emitting device of the comparative example differs from that of the light-emitting device 1 of this embodiment. In the light-emitting device of the comparative example, a voltage is applied to the active layer in the forward direction of the p-i-n, thereby injecting carriers for light emission into the active layer. Furthermore, a voltage is applied to the saturable absorbing layer in the forward or reverse direction of the p-i-n. The oscillation timing of the light-emitting device is controlled by the conditions or time changes of the voltage applied to the saturable absorbing layer. In this configuration, the oscillation timing is controlled by controlling the voltage or current applied to the saturable absorbing layer to change the characteristics of the saturable absorbing layer, such as the absorption coefficient. Therefore, it is preferable that the absorption coefficient of the saturable absorbing layer in a state where laser oscillation is not occurring is designed to be sufficiently large so as not to cause oscillation. On the other hand, it is preferable that the absorption coefficient of the saturable absorbing layer in a state where oscillation is occurring is designed to be sufficiently small. In the configuration of the comparative example, it is not desirable to have a gain and an absorption coefficient of the saturable absorbing layer close to the oscillation conditions that transition from a non-oscillation state to an oscillation state after a certain period of time; therefore, a design with a margin of error is desirable. Therefore, in the comparative example configuration, the light-emitting device is designed to achieve a large absorption coefficient relative to the gain that is sufficient to achieve a non-oscillation state, and a small absorption coefficient that is sufficient to achieve an oscillation state, and the absorption coefficient is controlled electrically.

In addition, Figure 2 schematically shows the positions of the antinodes and nodes of the standing wave of light propagating within the light-emitting device 1 using dashed lines. As shown in Figure 2, the materials and thicknesses of each layer of the quantum well layers 221 and 251 are designed so that they are located at the antinodes of the standing wave. Furthermore, the materials and thicknesses of each layer of the tunnel junction layer 24 are designed so that they are located at the nodes of the standing wave. This is because the tunnel junction layer 24 is composed of highly doped p-type and n-type layers, and it is desirable to place it near the nodes of the standing wave to avoid light absorption. Furthermore, the materials and thicknesses of each layer of the oxidized constriction layers 23 and 31 are designed so that they are located at the nodes of the standing wave. This reduces the optical effect of the edges of the oxidized portions of the oxidized constriction layers 23 and 31. Note that if the optical effect of the edges of the oxidized portions does not affect the laser characteristics, the oxidized constriction layer 23 does not necessarily have to be located near the nodes, and may be located at a position offset from the nodes.

Figure 3 is a graph showing an example of a calculated optical output waveform of the light-emitting device 1 according to the embodiment. The horizontal axis of Figure 3 represents time in arbitrary units, and the vertical axis of Figure 3 represents the amount of light in arbitrary units (solid line) and the carrier density in arbitrary units (dashed line).

The light-emitting device 1 according to this embodiment emits light having a maximum peak value and a profile that converges to a stable value, which is a predetermined light intensity, after the maximum peak value. That is, in the light-emitting device 1 according to this embodiment, oscillation begins approximately several tens to several hundreds of picoseconds after the start of current injection. This delay in the start of oscillation is due to the large effective volume of the active layers 22 and 25 and the fact that oscillation is inhibited for a certain period of time after the start of current injection due to light absorption in the saturable absorbing layer 131. When light is absorbed in the saturable absorbing layer 131, the absorbed light accumulates as carriers within the saturable absorbing layer 131. The number of carriers increases as light is absorbed, and when the carrier density in the saturable absorbing layer 131 reaches the transparent carrier density, the saturable absorbing layer 131 no longer absorbs light. As a result, the effect of inhibiting laser oscillation disappears, and the semiconductor light-emitting element begins laser oscillation.

The purpose of inhibiting laser oscillation for a certain period of time using the saturable absorbing layer 131 is to accumulate carriers exceeding the threshold carrier density in the active layers 22 and 25. Here, the threshold carrier density is the carrier density that generates the gain necessary for laser oscillation.

Figure 4 is an energy bandgap diagram near the active layer 25 in the light-emitting device 1 according to this embodiment. The vertical axis of Figure 4 represents the energy gap Eg, and the horizontal axis of Figure 4 represents the depth direction of the light-emitting device 1. Note that the scale of the horizontal axis in Figure 4 has been appropriately changed for emphasis, and the width of each layer shown in Figure 4 does not reflect the actual thickness of each semiconductor layer. Furthermore, the numerical values for the thickness of the semiconductor layers shown in Figure 4 represent an example design of the light-emitting device 1.

As shown in FIG. 4, the active layer 25 has three 8-nm-thick quantum well layers 251 arranged therein. A 6-nm-thick barrier layer 254 is arranged between each of the three quantum well layers 251. An 86-nm-thick barrier layer 252 is arranged below the three quantum well layers 251, and an 86-nm-thick barrier layer 253 is arranged above the three quantum well layers 251. The total thickness Ta of the active layer 25 is 208 nm. Although omitted in FIG. 4, the active layer 22 may also have a similar design. In this embodiment, the thickness of the barrier layer 252 and the thickness of the barrier layer 253 are the same.

Also, as shown in FIG. 4, an n-type layer 29 is disposed between the barrier layer 252 and the tunnel junction layer 24 as a spacer layer. The thickness Ts of the n-type layer 29 is 219 nm. The n-type layer 29 may be composed of multiple layers. These multiple layers may include, for example, an oxide constriction layer, a carrier blocking layer, a strain relaxation layer, etc.

The design of the thickness Ta of the active layer 25, the design of the thickness Ts of the n-type layer 29, and the relationship between the thicknesses of the quantum well layers and barrier layers in the multiple quantum well structure will be described in more detail. First, the design of the thickness Ta of the active layer 25 will be described. According to the inventors' investigations, in a light-emitting device 1 having an effective cavity length of 2 μm, when a peak ratio of 5 or more is required, 7.5 effective quantum well layers are required. Furthermore, the minimum carrier density required to generate the stimulated amplification necessary for laser oscillation is approximately 2×10 ¹⁸ cm ⁻³ . At this time, the ratio of the density of carriers accumulated in the barrier layers to the density of carriers accumulated in the quantum well layers is approximately 0.2. In this embodiment, the number of quantum well layers per active layer is three, and since two active layers 22 and 25 are stacked, the total number of quantum well layers is six (3×2). Furthermore, the thickness of each quantum well layer is 8 nm. The thickness Ta of the active layer 25 is the sum of the total thickness of the quantum well layers and the total thickness of the barrier layers. Therefore, the minimum thickness of the active layer 25 to ensure a thickness equivalent to 7.5 quantum well layers, including the quantum well layers and barrier layers, is {8 × 3 × 2 + (7.5 - 3 × 2) × 8/0.2}/2 = 54 nm. Therefore, under the above assumptions, it is desirable that the thickness Ta of the active layer 25 be 54 nm or greater. Here, the peak value ratio is the ratio between the peak value of the optical pulse waveform and the steady-state value after stabilization. For example, a peak value ratio of 5 means that the peak light intensity is five times the steady-state light intensity.

The above discussion will now be generalized. Let N be the number of quantum well layers per active layer, w (nm) be the thickness of each quantum well layer, M be the number of active layers in the light-emitting device 1, and Q be the number of quantum well layers required to obtain a light intensity with a predetermined peak-to-peak ratio. In this case, it is desirable that the thickness Ta of the active layer 25 satisfy the following formula (1):
Ta≧{w×N×M+(Q-N×M)×w/0.2}/M (1)

As described above, by setting the thickness Ta of the active layer 25 so as to satisfy formula (1), a light-emitting device 1 is provided that can obtain a sufficient peak value ratio. For the same reason, it is also desirable to set the thickness Ta of the active layer 22 so as to satisfy formula (1).

Furthermore, depending on the application of the light-emitting device 1, a greater peak-to-peak ratio may be required. For example, if a peak-to-peak ratio of 10 or greater is required, 11 effective quantum well layers are required. In this case, the minimum thickness of the active layer 25 to ensure an active layer thickness equivalent to 11 quantum well layers, including the quantum well layers and barrier layers, is {8 x 3 x 2 + (11 - 3 x 2) x 8/0.2}/2 = 124 nm when there are two active layers. Therefore, if a peak-to-peak ratio of 10 or greater is required, it is desirable that the thickness Ta of the active layer 25 be 124 nm or greater. An example of such an application is a light-emitting device used for distance measurement when the object to be measured is located far away.

As such, the peak value ratio can be improved as the thickness Ta of the active layer 25 increases. However, considering the carrier diffusion length, it is desirable that the thickness Ta be 1 μm or less. Therefore, it is desirable that the thickness Ta range be 54 nm or more and 1 μm or less, and it is even more desirable that the thickness Ta range be 124 nm or more and 1 μm or less.

Next, the relationship between the thickness of the quantum well layers and the barrier layers will be explained in more detail. As described above, in a high-peak VCSEL such as this embodiment, carriers accumulate not only in the quantum well layers but also in the barrier layers. Therefore, a design concept is adopted in which the number of quantum well layers is kept to the minimum necessary for oscillation, and carrier accumulation in the barrier layers compensates for any shortfall in the quantum well layers. The amount of radiative recombination is proportional to the square of the carrier density. Therefore, by accumulating carriers widely across the quantum well layers and barrier layers, as in this embodiment, energy loss due to spontaneous emission is reduced. To achieve the above design, it is desirable in this embodiment to ensure a certain level of barrier layer thickness. This configuration is advantageous over the active layer configuration of a typical VCSEL or a configuration that increases carrier accumulation by increasing the number of quantum well layers. To achieve wide and thin carrier accumulation using the barrier layers as well, it is preferable to increase the thickness of the non-doped region. As described below, the energy difference (band gap difference) between the emission level of the quantum well layer and the band gap of the barrier layer is preferably in the range of 105 meV to 230 meV when it is important to avoid the influence of light absorption by the band edge. If a decrease in light extraction efficiency of about 4% due to light absorption by the band edge is acceptable, the band gap difference is preferably in the range of 60 meV to 230 meV. Here, 60 meV to 230 meV means 60 meV or more and 230 meV or less. In contrast, the design guidelines for the active layer of a normal VCSEL, which is not the high peak VCSEL described above, are presented below to explain the differences from a high peak VCSEL. In a normal VCSEL, the band gap difference between the quantum well layer and the barrier layer is made large to prevent carrier accumulation in the barrier layer. This is to avoid carrier accumulation in the barrier layer, increase response speed, and lower the oscillation threshold. Furthermore, the thickness of the non-doped portion, including the barrier layer, is made as thin as possible. This is to increase response speed and reduce electrical resistance. Furthermore, the non-doped portion is kept to a minimum, and the resonator length is adjusted to a predetermined thickness, such as 1λc, to match the intensity distribution of the standing wave. The non-doped portion of the resonator is made into a doped layer, and the resonator length is adjusted to a predetermined thickness, such as 1λc, by the thickness of the doped layer. Therefore, dopant diffusion from the tunnel junction layer is not a problem, as it is contained within the doped portion.

Furthermore, even if dopants are mixed into the barrier layer for some reason, degradation of the characteristics of ordinary semiconductor lasers (including VCSELs) is unlikely to occur. Conversely, there are cases where dopants are intentionally doped into part of the barrier layer to improve the characteristics of the semiconductor laser. For example, a configuration may be possible in which the modulation speed of a semiconductor laser is improved by doping part of the barrier layer between quantum wells with a p-type dopant.

In a typical VCSEL configuration, it is preferable that the thickness of the non-doped section, which is composed of the quantum well layer and barrier layer of the resonator section, be as thin as possible. Furthermore, since the positions of each layer, such as the active layer and tunnel junction layer, and the positions of the refractive index interfaces between layers are aligned with the intensity distribution of the standing wave, the doped layers end up being thicker, and dopant diffusion from the tunnel junction layer is not a problem. Furthermore, even if dopants are mixed into the barrier layers, the characteristics of the semiconductor laser may still improve.

On the other hand, high-peak VCSELs require a thick barrier layer, which means that the barrier layer is closer to the tunnel junction layer than in normal VCSELs. The inventors have discovered that if dopants are mixed into the barrier layer of a high-peak VCSEL, the barrier layer's ability to store carriers is reduced, resulting in a problem of reduced high-peak pulse energy. This will be discussed in more detail later.

The following describes a case where a peak-to-peak ratio of 5 or greater is required for a high-peak VCSEL. That is, the quantum well layer thickness is 8 nm, there are three quantum well layers per active layer, and the thickness Ta of the active layer 25 is 54 nm. In this case, the total thickness of the quantum well layers is 8 x 3 = 24 nm, and the total thickness of the barrier layers is 54 - 24 = 30 nm. Therefore, the ratio of the total thickness of the barrier layers to the total thickness of the quantum well layers is approximately 1.25. This is an example where the barrier layer thickness is at the lower limit, so it is desirable for the ratio of the total thickness of the barrier layers to the total thickness of the quantum well layers to be 1.25 or greater.

Next, we will consider the case where a peak ratio of 10 or greater is required, that is, where the quantum well layer thickness is 8 nm, there are three quantum well layers per active layer, and the thickness Ta of the active layer 25 is 124 nm. In this case, the total thickness of the quantum well layers is 8 x 3 = 24 nm, and the total thickness of the barrier layers is 124 - 24 = 100 nm. Therefore, the ratio of the total thickness of the barrier layers to the total thickness of the quantum well layers is 4.17. This is an example where the barrier layer thickness is at the lower limit, so it is more desirable for the ratio of the total thickness of the barrier layers to the total thickness of the quantum well layers to be 4.17 or greater.

Furthermore, because the multiple quantum well layers are designed to be located near the antinodes of the standing wave, the barrier layers between the multiple quantum well layers are often designed to be relatively thin. Therefore, as mentioned above, in order to ensure the total thickness of the barrier layers, the barrier layers above and below the multiple quantum well layers are often designed to be relatively thick. Therefore, it is desirable that at least one of the multiple barrier layers in the active layer be designed to be relatively thick, and that the barrier layers between the quantum well layers be designed to be relatively thin. That is, in the example of Figure 2, it is desirable that at least one of the multiple quantum well layers 221, 251 be thinner than at least one of the barrier layers 222, 223, 224, 252, 253, 254. This makes it possible to realize a design in which the quantum well layers are located near the antinodes of the standing wave.

Next, we will explain the design of the thickness Ts of the n-type layer 29. As mentioned above, in the light-emitting device 1 of this embodiment, the barrier layer needs to be thick to a certain extent in order to ensure the peak value ratio. Meanwhile, in this embodiment, a tunnel junction layer 24 is disposed within the light-emitting device 1.

However, if an attempt is made to simply replace the active layer with a thick barrier layer, which is necessary for a high peak power VCSEL, at the position where the active layer is placed in the design concept of a normal VCSEL, the distance between the tunnel junction layer and the barrier layer will be closer than in a normal VCSEL. Therefore, the inventors have discovered that in the configuration of this embodiment, the manufacturing process for the tunnel junction layer 24 can affect the characteristics of the light-emitting device 1 due to the following factors.

The tunnel junction layer 24 may include highly doped p-type and n-type layers with a doping concentration of approximately 10 ¹⁹ cm ⁻³ or more to improve tunneling efficiency and reduce electrical resistance. Unlike typical VCSELs, in which carriers do not accumulate in the barrier layer, high-peak VCSELs also accumulate carriers in the barrier layer. Therefore, when the dopants of the tunnel junction layer 24 are mixed into the barrier layer, the carrier concentration of the barrier layer increases due to the presence of carriers generated by doping in addition to carriers injected by current injection. As a result, the probability of radiative recombination of electrons and holes increases, and the rate at which accumulated carriers disappear through radiative recombination increases. Furthermore, dopants may generate nonradiative recombination centers, which increase the nonradiative recombination of electrons and holes through these centers, resulting in the disappearance of carriers through nonradiative recombination. As a result, the carrier accumulation function is reduced, which may result in reduced luminous efficiency. The reduction in light emission efficiency due to the incorporation of dopants into the barrier layer does not occur in a normal VCSEL, which does not store carriers in the barrier layer, but occurs in a high peak power VCSEL.

The dopants of the tunnel junction layer 24 may be mixed into the barrier layer due to the influence of gas remaining in the deposition apparatus during epitaxial growth of the barrier layer and the influence of dopant diffusion from the tunnel junction layer 24 due to heat during the manufacturing process. Note that gas remaining in the deposition apparatus during epitaxial growth of the barrier layer affects the barrier layer farther from the substrate than the tunnel junction layer 24 (i.e., the barrier layer deposited after the tunnel junction layer 24).

The impact of dopant contamination due to the above factors depends on the distance between the active layer 25 and the tunnel junction layer 24, i.e., the thickness Ts of the n-type layer 29. According to the inventors' study of the range of dopant contamination from the tunnel junction layer, it is desirable for the thickness Ts to be 40 nm or greater, which reduces the impact of dopant contamination. Furthermore, it is even more desirable for the thickness Ts to be 70 nm or greater, which further reduces the impact of dopant contamination.

Next, we will explain the preferred range of the band gap difference between the barrier layer and the quantum well layer. In this disclosure, carriers are accumulated not only in the quantum well layer but also in the barrier layer, thereby increasing the amount of accumulated carriers. The band gap difference between the barrier layer and the quantum well layer required to accumulate carriers in the barrier is 230 meV or less. Assume that the difference between the band gap of the ground state of the quantum well and the band gap of the barrier layer is small, and the light-emitting device is used within the normal temperature range in which semiconductor lasers are used, rather than near absolute zero. In this case, some of the carriers present in the quantum well layer may exist at the same energy position as the conduction band and valence band of the barrier layer in the energy direction distribution. The preferred range for such a distribution is 230 meV or less. Therefore, from the perspective of having carriers exist in the barrier layer, a band gap of 230 meV or less is effective.

On the other hand, from the perspective of avoiding light absorption by the semiconductor that makes up the barrier layer, it is preferable that the band gap difference be equal to or greater than a specific value. In this embodiment, the material for the barrier layer is GaAs, and the light extraction efficiency can be calculated using the wavelength dependence of the absorption coefficient of GaAs as follows. When the band gap difference between the barrier layer and the quantum well layer is 105 meV, the light extraction efficiency decreases by 2% compared to when there is no band gap absorption. Similarly, when the band gap differences between the barrier layer and the quantum well layer are 60 meV, 48 meV, and 44 meV, the light extraction efficiency decreases by 3%, 4%, and 5%, respectively, compared to when there is no band gap absorption. Focusing on these differences, the difference between a 2% and a 3% decrease is relatively large at 45 meV. However, the difference between a 3% and a 4% decrease is 12 meV, and the difference between a 4% and a 5% decrease is 4 meV, so it rapidly decreases.

In ternary or higher compound semiconductors, the target controllability of elemental composition during crystal growth is approximately 1%, and this level of controllability is easily achievable. A 1% change in composition results in a bandgap change of 12 meV for AlGaAs-based materials and 14 meV for InGaAs-based materials. Therefore, a 1% change in composition results in a similar energy difference when the light extraction efficiency mentioned above changes from 3% to 4%. Therefore, considering a 1% change in composition during crystal growth, it is preferable for the bandgap difference to be 60 meV or more, which corresponds to the design value of a 3% decrease in light extraction efficiency mentioned above.

Based on the above, we consider the optimal range of the band gap difference, taking into account both the need to allow carriers to exist in the barrier layer and the need to avoid light absorption in the barrier layer. To minimize the decrease in light extraction efficiency to 2% or less, while prioritizing the avoidance of the effects of light absorption due to the band edge, the band gap difference is preferably in the range of 105 meV to 230 meV. Furthermore, if a decrease in light extraction efficiency of approximately 4% due to light absorption due to the band edge is acceptable, then the band gap difference is preferably in the range of 60 meV to 230 meV, taking into account controllability during crystal growth.

Even if a compound semiconductor material different from the above is used, the wavelength dependence of the absorption coefficient for wavelengths below the band gap does not change significantly as long as it is a direct transition semiconductor material, so the above values can be applied.

An example of a method for manufacturing the light-emitting device 1 will now be described. First, the semiconductor layers that make up the lower DBR layer 12, spacer portion 13, resonator portion 20, and upper DBR layer 32 are grown on the semiconductor substrate 11 by metalorganic vapor phase epitaxy or molecular beam epitaxy.

Next, photolithography and etching techniques are used to pattern the active layer 22, active layer 25, p-type layer 26, upper DBR layer 32, and the spacer between the active layer 22 and active layer 25. This forms a columnar mesa with a diameter of, for example, approximately 30 μm.

Next, thermal oxidation is performed in a water vapor atmosphere at approximately 450°C to oxidize the _{Al0.98Ga0.02As} oxidized constriction layers ₃₁ and 23 in the upper DBR layer 32 and between the active layers 22 and 25, starting from the sidewalls of the mesa. By controlling the oxidation time, an unoxidized portion in the center of the mesa and an oxidized portion near the sidewalls of the mesa are formed in _{the Al0.98Ga0.02As} layer. The diameter of the unoxidized portion of the _{Al0.98Ga0.02As} layer is controlled to be approximately 10 _μm .

Next, photolithography and vacuum deposition are used to form electrode 41, which will serve as the p-side electrode, on the top surface of the mesa, and then etching is performed to form electrode 40, which will serve as the n-side electrode, on the top surface of the n-type layer 21 that is exposed. Electrode 41 has a circular ring-shaped pattern, and the central opening serves as a circular window for light extraction.

Next, photolithography and plasma CVD are used to form a protective film 42 to cover the top and side surfaces of the mesa on which the electrodes 40 and 41 are provided, as well as the top surface of the n-type layer 21. Note that the above process procedure may also be modified so that the entire mesa is covered with a protective film before the electrodes are formed, and then a portion of the protective film on the top surface of the mesa is removed and the electrode is formed in that area.

Next, to obtain good electrical properties, heat treatment is performed in a nitrogen atmosphere to alloy the interface between the electrode material and the semiconductor material, completing the light-emitting device 1 of this embodiment.

As described above, this embodiment provides a light-emitting device 1 with improved performance in a configuration in which a tunnel junction layer 24 is disposed between multiple active layers 22, 25. In the first embodiment, a saturable absorbing layer is used to suppress laser oscillation for a certain period of time, and carriers are accumulated inside the semiconductor laser before generating a short pulse and releasing the accumulated carriers. This embodiment, which suppresses laser oscillation for a certain period of time and accumulates carriers inside the semiconductor laser, has a carrier accumulation layer and quantum well layer suitable for carrier accumulation. Therefore, similar effects are achieved in semiconductor lasers that have a configuration that suppresses laser oscillation for a certain period of time other than the saturable absorbing layer. In the above description, the mesa is formed up to the n-type layer 21, but this is not limited thereto. For example, it may be formed down to the oxide constriction layer. It is preferable that the mesa be formed above the saturable absorbing layer 131. This is because forming a mesa in the saturable absorbing layer 131 increases non-radiative recombination in the saturable absorbing layer 131.

Second Embodiment
In this embodiment, a modification of the first embodiment will be described in which the thicknesses of the barrier layers 252 and 253 are changed. In this embodiment, the description of elements common to the first embodiment may be omitted or simplified.

Figure 5 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. This embodiment differs from the first embodiment in that the barrier layer 252 is thinner than the barrier layer 253 in the active layer 25. In addition, in accordance with this change, the thicknesses of the n-type layer 29 and the p-type layer 26 have been changed so that the position of the quantum well layer 251 is shifted lower than the position of the quantum well layer 251 in the first embodiment.

Figure 6 is an energy bandgap diagram near the active layer 25 in the light-emitting device 1 according to this embodiment. Three 8-nm-thick quantum well layers 251 are arranged in the active layer 25. A 6-nm-thick barrier layer 254 is arranged between each of the three quantum well layers 251. A 68-nm-thick barrier layer 252 is arranged below the three quantum well layers 251. A 104-nm-thick barrier layer 253 is arranged above the three quantum well layers 251. Therefore, the total thickness Ta of the active layer 25 is 208 nm, which is the same as the total thickness of the active layer 25 in the first embodiment. Although omitted in Figure 4, the active layer 22 may have a similar design to that of the first embodiment.

In this embodiment, barrier layer 252 is thinner than barrier layer 253. As such, the thicknesses of the upper and lower barrier layers of the multiple quantum well do not need to be the same. Compared to the first embodiment, the distance from the upper DBR layer 32 to the active layer 25 is longer in this embodiment. Therefore, for example, in a configuration in which part of the upper DBR layer 32 is etched and an electrode is disposed on the resonator section 20, if diffusion of this electrode material adversely affects the active layer, the effect of further reducing the effects of diffusion can be achieved by positioning the active layer away from the electrode.

Therefore, this embodiment also provides a light-emitting device 1 that can improve performance.

[Third embodiment]
In this embodiment, a modification in which the thickness of the active layer is changed from that of the second embodiment will be described. In this embodiment, the description of elements common to the first and second embodiments may be omitted or simplified.

Figure 7 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. The difference between this embodiment and the second embodiment is that the overall thickness of the active layers 22 and 25 has been reduced. Accordingly, the thickness of the p-type layer 26 has also been reduced so that the length of the resonator portion 20 is shortened by half a wavelength.

Figure 8 is an energy bandgap diagram near the active layer 25 in the light-emitting device 1 according to this embodiment. The active layer 25 has three 8-nm-thick quantum well layers 251 arranged therein. A 6-nm-thick barrier layer 254 is arranged between each of the three quantum well layers 251. A 68-nm-thick barrier layer 252 is arranged below the three quantum well layers 251. An 86-nm-thick barrier layer 252 is arranged above the three quantum well layers 251. Therefore, the total thickness Ta of the active layer 25 is 190 nm, which is thinner than the total thickness of the active layer 25 in the second embodiment. Although omitted in Figure 8, the total thickness Ta of the active layer 22 is also 190 nm.

In this embodiment, the thickness of the active layers 22 and 25 is thinner than in the second embodiment, but the thickness of the Ta is 124 nm or more, and a high peak value ratio of 10 or more can be obtained. In this way, the thickness of the active layers 22 and 25 can be changed as appropriate.

Therefore, in this embodiment, as in the first and second embodiments, a light-emitting device 1 is provided that can improve performance.

In this embodiment, the distance between the active layer 25 and the oxide constriction layer 31 is closer than in the second embodiment. This reduces the lateral spread of carriers, potentially improving light-emitting efficiency.

In addition, in this embodiment, the active layers 22, 25 and p-type layer 26 are made thinner, thereby reducing the length of the resonator portion 20 per junction by half a wavelength compared to the first embodiment. This allows the etching depth for mesa formation to be shallower, simplifying the manufacturing process and improving yield. By shortening the resonator length, the mesa etching depth can be made shallower, as described above. Furthermore, shortening the resonator length offers benefits such as cost reduction due to the reduction in material used, a reduced risk of longitudinal mode hopping due to a wider longitudinal mode spacing, and reduced light absorption by the semiconductor, improving light extraction efficiency. These benefits become more pronounced as the number of junctions increases. By shortening the resonator length, the above effects can also be achieved in this embodiment.

If the effective resonator length is short due to a short resonator length, and the required high peak pulse width cannot be obtained, the effective resonator length may be adjusted, for example, by providing a spacer layer or the like closer to the substrate than the oxide constriction layer closer to the substrate. Even in this case, the oxide constriction layer 23 is located higher (opposite the substrate) compared to the first embodiment, which has the advantage of allowing the etching depth to be shallower.

Next, we will explain the preferred range of the band gap difference between the barrier layer and the quantum well layer. The band gap difference between the barrier layer and the quantum well layer required to store carriers in the barrier is 230 meV or less, the same as in the first embodiment. The reason the maximum value of the preferred range of the band gap difference is the same as in the first embodiment is because the principle for storing carriers in the barrier layer is as described above, and this value does not depend on the film thickness of the barrier layer, etc.

The results of calculations performed in the same manner as in the first embodiment to determine the effect of light absorption by the semiconductor constituting the barrier layer on light extraction efficiency are as follows: When the band gap difference between the barrier layer and the quantum well layer is 95 meV, the light extraction efficiency decreases by 2% compared to when there is no band gap absorption. Similarly, when the band gap differences between the barrier layer and the quantum well layer are 55 meV, 46 meV, and 42 meV, the light extraction efficiency decreases by 3%, 4%, and 5%, respectively, compared to when there is no band gap absorption. Focusing on these differences, the difference between a 2% and 3% reduction is 40 meV, which is relatively large. However, the difference between a 3% and 4% reduction is 9 meV, and the difference between a 4% and 5% reduction is 4 meV, which rapidly decreases.

In ternary or higher compound semiconductors, the target controllability of elemental composition during crystal growth is approximately 1%, and this level of controllability is easily achievable. A 1% change in composition results in a bandgap change of 12 meV for AlGaAs-based materials and 14 meV for InGaAs-based materials. Therefore, a 1% change in composition results in a similar energy difference when the light extraction efficiency mentioned above changes from 3% to 4%. Therefore, considering a 1% change in composition during crystal growth, it is preferable that the bandgap difference be 55 meV or more, which corresponds to the design value for a 3% decrease in light extraction efficiency mentioned above.

Based on the above, we consider the optimal range of the band gap difference, taking into account both the need to allow carriers to exist in the barrier layer and the need to avoid light absorption in the barrier layer. To minimize the decrease in light extraction efficiency to 2% or less, while prioritizing the avoidance of the effects of light absorption due to the band edge, the band gap difference is preferably in the range of 95 meV to 230 meV. Furthermore, if a decrease in light extraction efficiency of approximately 4% due to light absorption due to the band edge is acceptable, then the band gap difference is preferably in the range of 55 meV to 230 meV, taking into account controllability during crystal growth.

The preferred range of the band gap difference between the barrier layer and the quantum well layer, which is common to both the first and third embodiments, is as follows: To minimize the reduction in light extraction efficiency to 2% or less while prioritizing the avoidance of the effects of light absorption due to the band edge, the band gap difference is preferably in the range of 105 meV to 230 meV (105 meV or more, 230 meV or less). Furthermore, if a reduction in light extraction efficiency of approximately 4% due to light absorption due to the band edge is acceptable, and taking into account controllability during crystal growth, the band gap difference is preferably in the range of 60 meV to 230 meV (60 meV or more, 230 meV or less).

[Fourth embodiment]
In this embodiment, a modification of the first embodiment will be described in which the thicknesses of the barrier layers 222 and 223 are changed. In this embodiment, the description of elements common to the first embodiment may be omitted or simplified.

Figure 9 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. This embodiment differs from the first embodiment in that the barrier layer 222 is thinner than the barrier layer 223 within the active layer 22. This change also changes the thicknesses of the n-type layer 21 and the p-type layer 27. However, the total thickness Ta of the active layer 22 is the same as the total thickness of the active layer 22 in the first embodiment. Furthermore, the active layer 25 may have the same design as in the first embodiment.

In this embodiment, barrier layer 222 is thinner than barrier layer 223. In this way, the thicknesses of the barrier layers above and below the multiple quantum well do not need to be the same, and the same effect as in the first embodiment can be obtained.

Therefore, in this embodiment, as in the first embodiment, a light-emitting device 1 is provided that can improve performance.

In this embodiment, the distance between the active layer 25 and the oxide constriction layer 31 and the distance between the active layer 22 and the oxide constriction layer 23 can be set to closer values than in the first embodiment. Therefore, the degree of carrier diffusion between the multiple active layers 22, 25 can be made uniform, potentially improving the performance of the light-emitting device 1.

Fifth Embodiment
In this embodiment, a modification of the first embodiment will be described in which the thicknesses of the barrier layers 222 and 223 are changed as in the second embodiment, and the thicknesses of the barrier layers 252 and 253 are also changed as in the fourth embodiment. That is, this embodiment combines the features of the second embodiment and the fourth embodiment. In this embodiment, the description of elements common to the first, second, or fourth embodiment may be omitted or simplified.

Figure 10 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. This embodiment differs from the first embodiment in that the barrier layer 252 is thinner than the barrier layer 253 in the active layer 25, and furthermore, the barrier layer 222 is thinner than the barrier layer 223 in the active layer 22. In addition, due to these changes, the thicknesses of the n-type layer 29, p-type layer 26, n-type layer 21, and p-type layer 27 have also been changed. However, the total thickness Ta of the active layers 22 and 25 is the same as the total thickness of the active layers 22 and 25 in the first embodiment.

In this embodiment, barrier layer 252 is thinner than barrier layer 253, and barrier layer 222 is thinner than barrier layer 223. In this way, the thicknesses of the barrier layers above and below the multiple quantum well do not need to be the same. Furthermore, if differences in the shape of the active layer at each junction cause gain or other characteristic differences between the active layers, the shape of the active layer at each junction may be made the same. This has the effect of reducing characteristic differences compared to when the shape of the active layer is different for each junction.

Therefore, this embodiment also provides a light-emitting device 1 that can improve performance.

Sixth Embodiment
In this embodiment, a modification in which the conductivity type of each semiconductor region is changed from that of the second embodiment will be described. In this embodiment, the description of elements common to the first and second embodiments may be omitted or simplified.

Figure 11 is an enlarged cross-sectional view showing a light-emitting device 1 according to this embodiment. In this embodiment, the conductivity types of each semiconductor region other than the non-doped portions are reversed compared to the second embodiment. That is, the n-type layer 21, p-type layer 27, p-type layer 28, n-type layer 29, and p-type layer 26 of the second embodiment are replaced with p-type layer 21a, n-type layer 27a, n-type layer 28a, p-type layer 29a, and n-type layer 26a, respectively, in this embodiment. Furthermore, the n-type lower DBR layer 12 and p-type upper DBR layer 32 of the second embodiment are replaced with p-type lower DBR layer 12a and n-type upper DBR layer 32a, respectively, in this embodiment. Furthermore, in this embodiment, the tunnel junction layer 24a (first tunnel junction layer) includes, in order from the semiconductor substrate 11 side, a highly doped n-type layer (first n-type layer) and a highly doped p-type layer (first p-type layer). In this embodiment, the conductivity type of the semiconductor substrate 11 is p-type.

In this embodiment, as in the first embodiment, a light-emitting device 1 is provided that can improve performance.

In addition, in this embodiment, the p-type layer 29a is arranged on the side of the barrier layer 252, which is thinner than the barrier layer 253. Generally, the mobility of holes is smaller than the mobility of electrons, so arranging the p-type layer 29a on the side of the thin barrier layer 252 makes the injection of carriers into the quantum well more efficient. Therefore, in this embodiment, the injection of carriers can be made more efficient than in the second embodiment.

Seventh Embodiment
In this embodiment, a modification in which a tunnel junction layer is further added to the sixth embodiment will be described. In this embodiment, the description of elements common to the first, second, or sixth embodiment may be omitted or simplified.

Figure 12 is an enlarged cross-sectional view showing the light-emitting device 1 according to this embodiment. In the light-emitting device 1 of this embodiment, an n-type layer 51 is disposed on the spacer portion 13, a tunnel junction layer 52 (second tunnel junction layer) is disposed on the n-type layer 51, and a p-type layer 21a is disposed on the tunnel junction layer 52. The tunnel junction layer 52 includes, in order from the semiconductor substrate 11 side, a highly doped n-type layer (second n-type layer) and a highly doped p-type layer (second p-type layer). The configuration above the p-type layer 21a is the same as in the sixth embodiment. In this embodiment, the semiconductor substrate 11 and the lower DBR layer 12 have n-type conductivity. The tunnel junction layer 52 serves to invert the conductivity types of the layers above and below the tunnel junction layer 52. The tunnel junction layer 52 is disposed between the lower DBR layer 12 and the active layer 22, allowing the use of an n-type semiconductor substrate 11. In this embodiment, the portions of the spacer portion 13 other than the multiple quantum wells may be composed of an undoped GaAs layer or the like.

In this embodiment, as in the first embodiment, a light-emitting device 1 is provided that can improve performance. Furthermore, in this embodiment, as in the sixth embodiment, carrier injection can be made more efficient.

Furthermore, in this embodiment, an n-type semiconductor substrate 11, which is generally of higher quality than a p-type semiconductor substrate, can be used, which can improve the performance of the light-emitting device 1 compared to the sixth embodiment.

The tunnel junction layer 52 may be arranged below the spacer portion 13 (on the substrate side), but it is preferable to arrange it above the spacer portion 13 (on the opposite side from the substrate) as shown in Figure 12. This is because n-type semiconductors absorb less light and can improve quality.

Eighth Embodiment
A distance measuring device according to the eighth embodiment will be described with reference to Fig. 13. Fig. 13 is a block diagram showing a schematic configuration of the distance measuring device according to this embodiment.

The distance measuring device 700 according to this embodiment is a distance measuring device (LiDAR device) that uses a surface-emitting laser array, in which the light-emitting devices 1 described in any one of the first to seventh embodiments are arranged in an array, as a light source. The distance measuring device 700 can be configured with a control unit 710, a surface-emitting laser array driver 712, a surface-emitting laser array 714, an emitting-side optical system 718, a receiving-side optical system 720, an image sensor 722, and a distance data processing unit 724.

The surface-emitting laser array 714 is a packaged semiconductor device in which light-emitting devices 1 according to any one of the first to seventh embodiments are arranged in an array. The surface-emitting laser array driver 712 is a driver that receives a drive signal from the control unit 710, generates a drive current for oscillating the surface-emitting laser array 714, and outputs this to the surface-emitting laser array 714. Note that the surface-emitting laser array 714 and the surface-emitting laser array driver 712 do not necessarily have to be separate components, and the surface-emitting laser array driver 712 may have the function of the surface-emitting laser array driver 712.

The light-emitting optical system 718 is an optical system that emits laser light generated by the surface-emitting laser array 714 toward the distance measurement target range. The light-receiving optical system 720 is an optical system that guides laser light reflected by the measurement target 1000 to the image sensor 722. Note that while Figure 13 shows the light-emitting optical system 718 and the light-receiving optical system 720 as a single convex lens-shaped element, they are not composed of only a single convex lens-shaped element, but are composed of a lens group combining multiple lenses.

The image sensor 722 is a photoelectric conversion device in which multiple pixels, each including a photoelectric conversion unit, are arranged in a two-dimensional array, and is a light-receiving device that outputs an electrical signal in response to incident light. The image sensor 722 may be an imaging device such as a CMOS image sensor or a SPAD image sensor. The distance data processing unit 724 functions as a distance information acquisition unit that generates and outputs information regarding the distance to the measurement object 1000 present in the distance measurement range based on the signal from the image sensor 722. Note that the distance data processing unit 724 only needs to be electrically connected to the image sensor 722, and may be located in the same package as the image sensor 722 or in a package separate from the image sensor 722.

The control unit 710 is composed of an information processing device including a microcomputer and logic circuits, and functions as a central processing device that controls the operation of each component and performs various calculations within the distance measuring device 700.

Next, the operation of the distance measuring device according to this embodiment will be described with reference to Figure 13. First, the control unit 710 outputs a drive signal to the surface-emitting laser array driver 712. The surface-emitting laser array driver 712 receives the drive signal from the control unit 710 and injects a current of a predetermined value into the surface-emitting laser array 714. This causes the surface-emitting laser array 714 to oscillate, and laser light is output from the surface-emitting laser array 714.

The laser light generated by the surface-emitting laser array 714 is emitted toward the distance measurement range by the light-emitting optical system 718. Of the laser light irradiated onto the measurement object 1000 within the distance measurement range, the laser light reflected by the measurement object 1000 and incident on the light-receiving optical system 720 is guided by the light-receiving optical system 720 to the image sensor 722.

Each pixel of the image sensor 722 generates an electrical signal pulse according to the timing of the laser light incidence. The electrical signal pulse generated by the image sensor 722 is input to the distance data processing unit 724.

The distance data processing unit 724 generates information about the distance to the object 1000 along the light propagation direction based on the timing of receiving the electrical signal pulse output from the image sensor 722. For example, it generates information about the distance to the object 1000 based on the time difference between when light is emitted from the surface-emitting laser array 714 and when it is received by the image sensor 722. By calculating distance information based on the electrical signal pulse output from each pixel of the image sensor 722, three-dimensional information about the object 1000 can be obtained.

The ranging device 700 of this embodiment can be applied, for example, in the automotive field, to control devices that perform control to prevent collisions with other vehicles, and to control automatic driving by following other vehicles. Furthermore, the ranging device 700 of this embodiment can be applied not only to automobiles, but also to other moving bodies (moving devices) such as ships, aircraft, and industrial robots, as well as moving body detection systems. The ranging device 700 of this embodiment can be widely applied to equipment that uses information about objects recognized three-dimensionally, including distance information. These moving bodies can be configured to include the ranging device of this embodiment and control means that controls the moving body based on the distance information acquired by the ranging device.

In addition, the three-dimensional information including depth that can be acquired by the distance measuring device 700 of this embodiment can also be used in image capturing devices, image processing devices, display devices, etc. For example, the three-dimensional information acquired by the distance measuring device 700 of this embodiment can be used to seamlessly display a virtual object on top of an image of the real world. Furthermore, by storing the three-dimensional information along with the image information, it is possible to correct the blurring of the captured image after shooting.

Ninth Embodiment
A moving body according to the ninth embodiment will be described with reference to Figures 14(a) and 14(b), which are block diagrams showing an example of the configuration of a moving body according to this embodiment.

Figure 14(a) shows an example configuration of equipment mounted on a vehicle as an in-vehicle camera. Equipment 80 has a distance measurement unit 803 that measures the distance to an object to be measured, and a collision determination unit 804 that determines whether or not there is a possibility of a collision based on the distance measured by distance measurement unit 803. Distance measurement unit 803 may be configured, for example, by the distance measurement device 700 described in the eighth embodiment. Here, distance measurement unit 803 is an example of a distance information acquisition means that acquires distance information to the object to be measured. In other words, distance information is information related to the distance to the object to be measured, etc.

Device 80 is connected to vehicle information acquisition device 810 and can acquire vehicle information such as vehicle speed, yaw rate, and steering angle. Device 800 is also connected to control ECU 820, a control device that outputs a control signal to generate braking force for the vehicle based on the determination result of collision determination unit 804. Device 80 is also connected to warning device 830, which issues a warning to the driver based on the determination result of collision determination unit 804. For example, if collision determination unit 804 determines that there is a high possibility of a collision, control ECU 820 performs vehicle control to avoid the collision and mitigate damage by applying the brakes, releasing the accelerator, or suppressing engine output. Warning device 830 warns the user by sounding an alarm, displaying warning information on the screen of a car navigation system, or vibrating the seat belt or steering wheel. These devices of device 80 function as a mobile object control unit that controls the operation of controlling the vehicle as described above.

In this embodiment, the device 80 measures the distance around the vehicle, for example, the front or rear. Figure 14(b) shows the device when measuring the distance in front of the vehicle (measurement range 850). The vehicle information acquisition device 810, which acts as a distance measurement control means, sends an instruction to the device 80 or the distance measurement unit 803 to perform a distance measurement operation. This configuration can further improve the accuracy of distance measurement.

The above describes an example of control to prevent collisions with other vehicles, but the invention can also be applied to control of autonomous driving by following other vehicles, and control of autonomous driving to prevent vehicles from veering out of their lanes. Furthermore, the invention is not limited to vehicles such as automobiles, but can be applied to moving objects (mobile devices) such as ships, aircraft, artificial satellites, industrial robots, and consumer robots. In addition, the invention can be applied to a wide range of devices that use object recognition or biometric recognition, such as intelligent transport systems (ITS) and surveillance systems, and is not limited to moving objects.

[Modified embodiment]
The present disclosure is not limited to the above-described embodiments and various modifications are possible. For example, an example in which part of the configuration of one embodiment is added to another embodiment, or an example in which part of the configuration of one embodiment is replaced with part of the configuration of another embodiment, is also an embodiment of the present disclosure.

In the first to seventh embodiments described above, a two-junction configuration including two active layers is illustrated, but the number of junctions is not limited to this. For example, a configuration similar to the above-described embodiments can be applied even if the number of junctions is three or more. For example, in a three-junction configuration, a structure in which two groups including layers from the tunnel junction layer 24 to the n-type layer 21 are stacked can be applied.

In addition, in the above-described first to seventh embodiments, GaAs, AlGaAs, and InGaAs were given as examples of semiconductor materials on which crystal growth is possible when a GaAs substrate is used as the semiconductor substrate 11, but the semiconductor substrate 11 is not limited to a GaAs substrate. For example, an InP substrate can also be used as the semiconductor substrate 11. Examples of semiconductor materials on which crystal growth is possible when an InP substrate is used as the semiconductor substrate 11 include InP, InGaAs, InGaP, and InGaAsP.

Furthermore, the DBR layer in the light-emitting devices according to the first to seventh embodiments does not necessarily have to be made of a semiconductor material, and may be made of a material other than a semiconductor material. In this case, too, by configuring the light-emitting device to perform the same functions as the first to seventh embodiments, it is possible to achieve the same effects as this embodiment.

Furthermore, the oxide constriction layer 23 is not necessarily required in the light-emitting devices according to the first to seventh embodiments, and the oxide constriction layer 23 may not be provided. In such cases, for example, in FIG. 2, a configuration can be applied in which the layers below the active layer 22 are shifted upward in the plane of the drawing by a distance of 1/2λc. This shortens the cavity length by 1/2λc, thereby reducing the amount of material used during manufacturing and lowering costs. Other advantages include a wider longitudinal mode spacing, which reduces the risk of longitudinal mode hopping, and reduced light absorption by the semiconductor, improving light extraction efficiency. These advantages become more pronounced as the number of junctions increases. If a short cavity length results in a short effective cavity length and therefore prevents the required high peak pulse width from being obtained, the effective cavity length may be adjusted, for example, by providing a spacer layer closer to the substrate.

Furthermore, the electrode 40 in the light-emitting device according to the first embodiment does not necessarily have to be formed on the upper surface of the n-type layer 21; it may be formed, for example, on the lower surface of the semiconductor substrate 11. In this case, too, the same effects as in the first embodiment can be achieved as long as the saturable absorption layer is sandwiched between a p-type layer and an n-type layer in a p-i-n configuration and is not electrically controlled from the outside.

The disclosure of this specification includes the complement of the concepts described herein. In other words, if this specification states, for example, that "A is B" (A = B), then even if the statement that "A is not B" (A ≠ B) is omitted, this specification is deemed to disclose or suggest that "A is not B." This is because when it states that "A is B," it is assumed that the case where "A is not B" is taken into consideration.

The disclosure of this specification includes the following configurations.
(Configuration 1)
a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, a second barrier layer disposed on the plurality of first quantum well layers, and a third barrier layer disposed between the plurality of first quantum well layers;
the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, a fifth barrier layer disposed on the plurality of second quantum well layers, and a sixth barrier layer disposed between the plurality of second quantum well layers;
When the number of the plurality of second quantum well layers is N, the thickness of each of the plurality of second quantum well layers is w (nm), the number of active layers is M, and the number of quantum well layers required to obtain a light intensity with a predetermined peak value ratio is Q, the thickness Ta (nm) of the second active layer satisfies the following formula (1):
Ta≧{w×N×M+(Q-N×M)×w/0.2}/M (1)
a distance between the second active layer and the first tunnel junction layer is 40 nm or more;
a light-emitting device, wherein any one of the plurality of first quantum well layers and the plurality of second quantum well layers is thinner than any one of the first barrier layer to the sixth barrier layer;
(Configuration 2)
2. The light emitting device according to claim 1, wherein the second active layer has a thickness Ta of 54 nm or more.
(Configuration 3)
3. The light emitting device according to claim 1, wherein the second active layer has a thickness Ta of 124 nm or more.
(Configuration 4)
4. The light emitting device according to any one of the first to third aspects, wherein the distance between the second active layer and the first tunnel junction layer is 70 nm or more.
(Configuration 5)
5. The light-emitting device according to any one of structures 1 to 4, wherein a ratio of a total thickness of the first barrier layer to the sixth barrier layer to a total thickness of the plurality of first quantum well layers and the plurality of second quantum well layers is 1.25 or more.
(Configuration 6)
6. The light-emitting device according to any one of structures 1 to 5, wherein a ratio of a total thickness of the first barrier layer to the sixth barrier layer to a total thickness of the plurality of first quantum well layers and the plurality of second quantum well layers is 4.17 or more.
(Configuration 7)
7. The light-emitting device according to any one of the preceding claims, wherein the fourth barrier layer is thinner than the fifth barrier layer.
(Configuration 8)
7. The light emitting device according to any one of the preceding claims, wherein the first barrier layer is thinner than the second barrier layer.
(Configuration 9)
the first barrier layer is thinner than the second barrier layer;
7. The light emitting device according to any one of the first to sixth aspects, wherein the fourth barrier layer is thinner than the fifth barrier layer.
(Configuration 10)
a first oxide constriction layer disposed between the first active layer and the second active layer;
a second oxide constriction layer disposed on the second active layer;
10. The light emitting device according to any one of configurations 1 to 9, further comprising:
(Configuration 11)
10. The light emitting device of any one of configurations 1 to 9, further comprising a second oxide constriction layer disposed on the second active layer.
(Configuration 12)
a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, and a fifth barrier layer disposed on the plurality of second quantum well layers;
The light emitting device, wherein the fourth barrier layer is thinner than the fifth barrier layer.
(Configuration 13)
the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, and a second barrier layer disposed on the plurality of first quantum well layers;
13. The light emitting device of claim 12, wherein the first barrier layer is thinner than the second barrier layer.
(Configuration 14)
the semiconductor substrate is p-type;
14. The light emitting device of claim 12 or 13, wherein the first tunnel junction layer includes a first n-type layer and a first p-type layer disposed on the first n-type layer.
(Configuration 15)
a second tunnel junction layer disposed between the semiconductor substrate and the first active layer;
the semiconductor substrate is n-type;
the first tunnel junction layer includes a first n-type layer and a first p-type layer disposed on the first n-type layer;
14. The light emitting device of claim 12 or 13, wherein the second tunnel junction layer includes a second n-type layer and a second p-type layer disposed on the second n-type layer.
(Configuration 16)
a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, and a second barrier layer disposed on the plurality of first quantum well layers;
The light emitting device, wherein the first barrier layer is thinner than the second barrier layer.
(Configuration 17)
17. The light emitting device according to any one of structures 1 to 16, further comprising a saturable absorbing layer between the first reflecting mirror and the second reflecting mirror.
(Configuration 18)
18. The light-emitting device according to configuration 17, wherein the saturable absorbing layer is disposed between the first reflector and the first active layer.
(Configuration 19)
19. The light emitting device of any one of configurations 1 to 18, wherein the light emitting device is configured to emit light having a maximum peak value and a profile that converges to a stable value that is a predetermined light intensity after the maximum peak value.
(Configuration 20)
20. The light-emitting device according to any one of structures 1 to 19, wherein a band gap difference between any one of the plurality of first quantum well layers and the plurality of second quantum well layers and any one of the first barrier layer to the sixth barrier layer is 60 meV or more and 230 meV or less.
(Configuration 21)
21. The light-emitting device according to any one of structures 1 to 20, wherein a band gap difference between any one of the plurality of first quantum well layers and the plurality of second quantum well layers and any one of the first barrier layer to the sixth barrier layer is 105 meV or more and 230 meV or less.
(Configuration 22)
The light-emitting device according to any one of configurations 1 to 21,
a light receiving device that receives light emitted from the light emitting device and reflected by the object to be measured;
a distance information acquisition unit that acquires information about the distance to the measurement object based on the time difference between the timing at which light is emitted from the light emitting device and the timing at which light is received by the light receiving device;
A distance measuring device comprising:
(Configuration 23)
A mobile object,
23. The distance measuring device according to claim 22,
a control means for controlling the moving object based on information about the distance acquired by the distance measuring device;
A moving object characterized by having:

The present disclosure can also be realized by supplying a program that realizes one or more of the functions of the above-described embodiments to a system or device via a network or storage medium, and having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that realizes one or more functions.

The above-described embodiments are merely examples of specific embodiments for implementing the present disclosure, and should not be construed as limiting the technical scope of the present disclosure. In other words, the present disclosure can be implemented in various forms without departing from its technical concept or main features.

REFERENCE SIGNS LIST 1 Light emitting device 11 Semiconductor substrate 12, 12a Lower DBR layer 22, 25 Active layer 24, 52 Tunnel junction layer 32, 32a Upper DBR layer 221, 251 Quantum well layer 222, 223, 224, 252, 253, 254 Barrier layer

Claims (23)

a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, a second barrier layer disposed on the plurality of first quantum well layers, and a third barrier layer disposed between the plurality of first quantum well layers;
the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, a fifth barrier layer disposed on the plurality of second quantum well layers, and a sixth barrier layer disposed between the plurality of second quantum well layers;
When the number of the plurality of second quantum well layers is N, the thickness of each of the plurality of second quantum well layers is w (nm), the number of active layers is M, and the number of quantum well layers required to obtain a light intensity with a predetermined peak value ratio is Q, the thickness Ta (nm) of the second active layer satisfies the following formula (1):
Ta≧{w×N×M+(Q-N×M)×w/0.2}/M (1)
a distance between the second active layer and the first tunnel junction layer is 40 nm or more;
a light-emitting device, wherein any one of the plurality of first quantum well layers and the plurality of second quantum well layers is thinner than any one of the first barrier layer to the sixth barrier layer; The light emitting device according to claim 1 , wherein the second active layer has a thickness Ta of 54 nm or more. The light emitting device according to claim 1 , wherein the second active layer has a thickness Ta of 124 nm or more. The light emitting device according to claim 1 , wherein the distance between the second active layer and the first tunnel junction layer is 70 nm or more. 2. The light-emitting device according to claim 1, wherein a ratio of a total thickness of the first barrier layer to the sixth barrier layer to a total thickness of the first quantum well layers and the second quantum well layers is 1.25 or more. 2. The light emitting device according to claim 1, wherein a ratio of a total thickness of the first barrier layer to the sixth barrier layer to a total thickness of the first quantum well layers and the second quantum well layers is 4.17 or more. The light emitting device according to claim 1 , wherein the fourth barrier layer is thinner than the fifth barrier layer. The light emitting device according to claim 1 , wherein the first barrier layer is thinner than the second barrier layer. the first barrier layer is thinner than the second barrier layer;
The light emitting device according to claim 1 , wherein the fourth barrier layer is thinner than the fifth barrier layer. a first oxide constriction layer disposed between the first active layer and the second active layer;
a second oxide constriction layer disposed on the second active layer;
The light emitting device according to claim 1 , further comprising: 2. The light emitting device according to claim 1, further comprising a second oxide constriction layer disposed on the second active layer. a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the second active layer includes a fourth barrier layer, a plurality of second quantum well layers disposed on the fourth barrier layer, and a fifth barrier layer disposed on the plurality of second quantum well layers;
The light emitting device, wherein the fourth barrier layer is thinner than the fifth barrier layer. the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, and a second barrier layer disposed on the plurality of first quantum well layers;
The light emitting device of claim 12 , wherein the first barrier layer is thinner than the second barrier layer. the semiconductor substrate is p-type;
13. The light emitting device of claim 12, wherein the first tunnel junction layer includes a first n-type layer and a first p-type layer disposed on the first n-type layer. a second tunnel junction layer disposed between the semiconductor substrate and the first active layer;
the semiconductor substrate is n-type;
the first tunnel junction layer includes a first n-type layer and a first p-type layer disposed on the first n-type layer;
13. The light emitting device of claim 12, wherein the second tunnel junction layer includes a second n-type layer and a second p-type layer disposed on the second n-type layer. a first reflector disposed on a semiconductor substrate;
a first active layer disposed on the first reflector;
a first tunnel junction layer disposed on the first active layer;
a second active layer disposed on the first tunnel junction layer;
a second reflector disposed on the second active layer; and
and
the first active layer includes a first barrier layer, a plurality of first quantum well layers disposed on the first barrier layer, and a second barrier layer disposed on the plurality of first quantum well layers;
The light emitting device, wherein the first barrier layer is thinner than the second barrier layer. The light emitting device according to claim 1 , further comprising a saturable absorbing layer between the first reflector and the second reflector. 18. The light emitting device according to claim 17, wherein the saturable absorbing layer is disposed between the first reflector and the first active layer. 2. The light emitting device according to claim 1, wherein the light emitting device is configured to emit light having a profile that has a maximum peak value and that converges to a stable value that is a predetermined light intensity after the maximum peak value. 2. The light-emitting device according to claim 1, wherein a band gap difference between any one of the plurality of first quantum well layers and the plurality of second quantum well layers and any one of the first barrier layer to the sixth barrier layer is 60 meV or more and 230 meV or less. 2. The light-emitting device according to claim 1, wherein a band gap difference between any one of the plurality of first quantum well layers and the plurality of second quantum well layers and any one of the first barrier layer to the sixth barrier layer is 105 meV or more and 230 meV or less. A light emitting device according to any one of claims 1 to 21;
a light receiving device that receives light emitted from the light emitting device and reflected by the object to be measured;
a distance information acquisition unit that acquires information about the distance to the measurement object based on the time difference between the timing at which light is emitted from the light emitting device and the timing at which light is received by the light receiving device;
A distance measuring device comprising: A mobile object,
a distance measuring device according to claim 22;
a control means for controlling the moving object based on information about the distance acquired by the distance measuring device;
A moving object characterized by having: