JP6540434B2 - Semiconductor optical element - Google Patents

Description

  The present invention relates to a semiconductor optical device, and more particularly to a semiconductor light emitting device in which both ap electrode and an n electrode are formed on one side of a semiconductor.

The semiconductor light emitting devices that cover the emission wavelength range from infrared to ultraviolet are almost all of Group 3-5 compound semiconductors although the band gap energy differs depending on the material, such as GaAs based material, InP based material, and GaN based material. It is manufactured using the material.
As a background art of the present technical field, there is Japanese Patent No. 3233139 (Patent Document 1) when taking a GaN-based semiconductor as an example. In this publication, regarding the n-type contact layer of a light emitting diode (LED) fabricated on a sapphire substrate, "the n-type contact layer is formed of a nitride semiconductor, and in particular, is formed of GaN or AlGaN Particularly when using as an n-type contact layer for providing an n-side electrode, an n-type layer having a high electron concentration is obtained by using GaN doped with an n-type impurity such as Si or Ge. And the contact resistance with the n-side electrode can be reduced "(see [0019]).

There is also a patent 3434162 (patent document 2). In this publication, a layer consisting of Al _0.1 Ga _0.9 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Si is grown to a thickness of 20 angstrom, followed by the same amount of Si, regarding the layer in contact with the n electrode of the semiconductor laser diode (LD). And grow a layer consisting of n-type GaN doped in 20 angstroms, and repeat this operation 200 times each to form a superlattice layer having a carrier concentration of 1 × 10 ¹⁹ / cm ^{3 and} a total film thickness of 0.8 μm. See Example 2).

Patent No. 3233139 Patent 3434162

In the semiconductor light emitting device in which both the p electrode and the n electrode are formed on one side of the semiconductor, as in the prior art described in Patent Document 1, an n-type GaN layer or n When the carrier density is increased by increasing the concentration of the impurity added in the same type AlGaN layer, the absorptivity of the light emitted from the active layer is increased by the impurity, and the light extraction efficiency to the outside is lowered. There is a problem that the light emission characteristics of the light emitting diode and the laser diode are degraded.
Further, in the prior art described in Patent Document 2 as well, the impurity concentration is increased to 1 × 10 ¹⁹ / cm ³ , so there is a problem of the characteristic deterioration due to the light absorption.

On the other hand, when the impurity concentration is lowered to lower the carrier density of the n-type contact layer in order to solve the above problems, the ohmic connection with the n electrode is lowered and the resistance of the n-type contact layer itself is also increased. The series resistance from the n electrode to the active layer is increased to cause an increase in the operating voltage of the light emitting device and a decrease in the reliability.
These are not only the nitride semiconductor light emitting elements that emit light in the ultraviolet to blue to green wavelength bands, but also other red / near infrared light emitting elements that are GaAs / AlGaAs based light emitting elements, In the case of an InP / InGaAsP light emitting element or the like used for a communication laser, etc., the n electrode is formed on the same surface side as the p electrode, not on the back side with respect to the p electrode formed on the surface side Have similar problems.
Therefore, an object of the present invention is to provide a semiconductor optical device having a low operating voltage and excellent optical characteristics.

  In order to solve the above problems, one aspect of a semiconductor optical device according to the present invention comprises a substrate, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, a p electrode, and an n electrode. The n-type semiconductor layer is a first n-type semiconductor layer having a film thickness of 10 nm or more, a film thickness is twice or more that of the first n-type semiconductor layer, and a band gap energy is the first n A second n-type semiconductor layer smaller than the n-type semiconductor layer is alternately stacked in plurality; and the n-electrode is in contact with the n-type multi-heterojunction layer.

According to such a semiconductor optical device, as will be described in detail later, a two-dimensional electron gas sufficient between the first n-type semiconductor layer and the second n-type semiconductor layer in the n-type multi hetero junction layer (2DEG) occurs to improve the carrier density and to improve the electron mobility. As a result, even if the impurity concentration in the n-type multiheterojunction layer is suppressed low, a sheet resistance sufficiently low can be realized, and a semiconductor optical device having a low operating voltage and excellent optical characteristics can be obtained.
The layer in contact with the electrode is generally referred to as a contact layer, and the n-type multi-hetero junction layer is a contact layer for the n-electrode.

  In the semiconductor optical device, a film thickness is formed at an interface between the first n-type semiconductor layer and the second n-type semiconductor layer located closer to the substrate than the first n-type semiconductor layer. An undoped semiconductor layer made of the same semiconductor material as the second n-type semiconductor layer at 1 nm to 5 nm, or the first n-type semiconductor layer and the first n-type semiconductor layer The semiconductor device further includes an undoped semiconductor layer having a thickness of 1 nm or more and 5 nm or less and made of the same semiconductor material as the second n-type semiconductor layer at an interface with the second n-type semiconductor layer located far from the substrate. Is preferred.

According to the semiconductor optical device having these preferable structures, the carrier density is further improved by the presence of the undoped semiconductor layer.
In the semiconductor optical device, the first n-type semiconductor layer preferably has an impurity concentration of 5.0 × 10 ¹⁶ cm ⁻³ or more and 5.0 × 10 ¹⁸ cm ⁻³ or less, and the second n The impurity concentration of the semiconductor layer is preferably 5.0 × 10 ¹⁶ cm ⁻³ or more and 5.0 × 10 ¹⁸ cm ⁻³ or less.
When the impurity concentration is 5.0 × 10 ¹⁸ cm ⁻³ or less, light absorption can be suppressed. Moreover, the influence of the compensation by other impurities can be avoided by impurity concentration being 5.0 * 10 < ¹⁶ > cm < ^-3 > or more.

In the semiconductor optical device, the first n-type semiconductor layer preferably has a thickness of 10 nm or more and 20 nm or less. In addition to the fact that sufficient 2DEG occurs when the film thickness is 10 nm or more, and conversely when the film thickness exceeds 20 nm, the electrical characteristics of the 2DEG do not improve so much, this time the influence of alloy scattering accompanying film thickening By this, as the film thickness increases, the crystallinity in the depth direction of the heterojunction interface is impaired, and the electron mobility of the 2DEG tends to decrease.
In the semiconductor optical device, the second n-type semiconductor layer may have a thickness of 20 nm or more and 200 nm or less.
In the semiconductor optical device, both the p electrode and the n electrode may be provided on one side of the front and back sides of the substrate.

In the semiconductor optical device, the substrate may be a substrate of a material selected from the group consisting of GaAs, InP, GaN, sapphire, Si, SiC, ZnO, and AlN.
In the semiconductor optical device, the n-type multi hetero junction layer may be a cladding layer.
The semiconductor optical device may be a light emitting diode, a laser diode, or a light receiving device.

  According to the semiconductor optical device of the present invention, the operating voltage is low and the optical characteristics are excellent.

It is the simplified cross-section figure which showed the comparative example of the semiconductor optical element which formed both p electrode and n electrode in the one side of the board | substrate. FIG. 1 is a cross-sectional view showing a semiconductor laser which is a first embodiment of a semiconductor optical device. It is the elements on larger scale which showed the structure of the n-type multi hetero junction layer in 1st Embodiment. It is the figure which showed the another structure of the n-type multi hetero junction layer in 1st Embodiment. It is a cross-sectional block diagram which shows the light emitting diode (LED) which is 2nd Embodiment of a semiconductor optical element. It is the elements on larger scale which showed the structure of the n-type multi hetero junction layer in 2nd Embodiment. It is a cross-sectional block diagram which shows the semiconductor laser which is 3rd Embodiment of a semiconductor optical element. It is the elements on larger scale which showed the structure of the n-type multi hetero junction layer in 3rd Embodiment. It is a cross-sectional block diagram which shows the light emitting diode which is 4th Embodiment of a semiconductor optical element. It is the elements on larger scale which showed the structure of the n-type multi hetero junction layer in 4th Embodiment. It is the figure which showed the another structure of the n-type multi heterojunction layer in 4th Embodiment. It is a cross-sectional block diagram which shows the semiconductor laser which is 5th Embodiment of a semiconductor optical element. It is the elements on larger scale which showed the structure of the n-type multi hetero junction layer in 5th Embodiment.

Hereinafter, although an embodiment of the present invention is described based on a drawing, a comparative example is explained prior to it.
FIG. 1 is a simplified cross-sectional view showing a comparative example of a semiconductor optical device in which both p and n electrodes are formed on one side of a substrate, and a semiconductor laser using a nitride semiconductor is shown as a comparative example. There is.
The semiconductor laser 200 of the comparative example shown in FIG. 1 is a semiconductor laser 200 having a conventional structure. In the semiconductor laser 200, an undoped GaN layer 21 as a buffer layer, an n-type GaN layer 22 as an n-type contact layer, an n-type AlGaN layer 23 as an n-type cladding layer, and light emission activity on a substrate 20 A layer 24, a p-type AlGaN layer 25 as a p-type cladding layer, and a p-type GaN layer 26 as a p-type contact layer are stacked in this order. As the substrate 20, in addition to sapphire substrates, SiC substrates, Si substrates, GaN substrates, AlN substrates and the like can be applied.

Although the active layer 24 is shown as a single layer in FIG. 1 to simplify the illustration, in practice, for example, a well-known multiple quantum well (MQW: Multi using InGaN, AlGaN, GaN, etc.) is shown. The semiconductor material, the composition ratio, and the like differ depending on the light emission wavelength, which comprises a -Quantum-Well) structure or a Single-Quantum-Well (SQW) structure.
Hereinafter, for convenience of description, a direction perpendicular to the substrate 20 may be referred to as “longitudinal”, and a direction along the substrate 20 may be referred to as “horizontal”. Further, the direction from the substrate 20 to the active layer 24 may be referred to as “upper”, and the direction from the active layer 24 to the substrate 20 may be referred to as “lower”. Further, the “upper” side of the substrate 20 may be referred to as “front side”, and the “lower” side of the substrate 20 may be referred to as “back side”.

In the p-type GaN layer 26 and the p-type AlGaN layer 25, a stripe-shaped ridge portion having a desired shape for current constriction is formed by known nitride semiconductor dry etching. A protective film 27 made of SiO ₂ is formed on the side surfaces and the surface of the p-type AlGaN layer 25 except for the p-type GaN layer 26 exposed at the top.
In addition to being in contact with the p-type GaN layer 26 present at the top of the ridge portion, the p-electrode 28 in ohmic contact with the p-type GaN layer 26 extends laterally to a desired region on the protective film 27 And is formed so as to cover a part of the protective film 27.

The n-electrode 29 is in contact with the n-type GaN layer 22 exposed by etching the p-type AlGaN layer 25, the InGaN quantum well layer 24 and the n-type AlGaN layer 23 in the desired region apart from the ridge portion in the stripe shape. And is ohmically connected to the n-type GaN layer 22.
The above shows the configuration of a simplified semiconductor optical element, taking a nitride semiconductor laser as an example, and p-type layer / active layer / even when semiconductor materials are different such as GaAs and InP. There is no change in the basic semiconductor laminate configuration of the combination of n-type layers.

In addition, although the laser diode (LD) and the light emitting diode (LED) differ in the shape around the p electrode, the presence or absence of cladding layers located above and below the active layer, the film thickness, etc. There is no significant difference between the two.
According to the above configuration, the p electrode 28 is located directly on the active layer 24. Therefore, the current from the p electrode 28 to the active layer 24 is substantially perpendicular to the p electrode 28 from the p electrode 28 (see FIG. In the example of 1, the flow path is traced through the p-type AlGaN layer 25). In the case of a nitride semiconductor laser, the thickness of the p-type cladding layer is generally in the range of 500 nm to 700 nm.
Therefore, the resistance component between the p electrode 28 and the active layer 24 mainly changes according to the contact resistance and the contact area between the p electrode 28 and the p-type contact layer 26 and the film thickness of the p-type cladding layer 25 Do.

On the other hand, the flow of electrons from the n-electrode 29 to the active layer 24 flows from the n-electrode 29 first in the lateral direction to the n-type contact layer (in the example of FIG. Follow the path through the n-type cladding layer.
In the semiconductor light emitting device having the above configuration, the film thickness of the n-type contact layer 22 is generally in the range from 1 μm to at most 3 μm, and the film thickness varies depending on the light emitting device and material system to be manufactured.
On the other hand, the distance from the n-electrode 29 to the side surface of the n-type cladding 23 after patterning by etching is often 5 μm to several tens of μm, which is longer than the thickness of the n-type contact layer.

  Therefore, in the case of a semiconductor optical device having a structure in which both the p electrode 28 and the n electrode 29 are formed on one side of the front and back surfaces of the substrate, the n-type contact layer 22 is lateral to the film thickness of the n-type contact layer 22 Since the distance of the current path flowing to the above tends to be long, the ratio of the lateral resistance component of the n-type contact layer 22 among the resistance components in the entire element of the above electrode arrangement configuration is relatively large.

Therefore, in the low voltage operation of the light emitting element configured as described above, it is necessary to reduce the lateral resistance component of the n-type contact layer 22 involved in the lateral energization to the n electrode 29, in other words, the sheet resistance of the n-type contact layer 22. It is necessary to reduce as much as possible.
One solution to this is to lower the sheet resistance by increasing the impurity concentration of the n-type contact layer 22 to increase the carrier density, but as described above, absorption of light emitted from the active layer 24 Since the amount also increases, the reduction in sheet resistance due to the increase in the impurity concentration of the n-type contact layer 22 is a factor that causes the deterioration of the light emission characteristics of the device.

Another solution is to reduce the sheet resistance by increasing the thickness of the n-type contact layer 22. Usually, the semiconductor layer of the semiconductor light emitting device is formed using an epitaxial growth method such as MOCVD, and thus the process cost From the viewpoint of etc., it is general to limit the film thickness to at most about 3 μm.
From the above, in the semiconductor light emitting device having a configuration in which both the p electrode 28 and the n electrode 29 are on one side of the substrate, the requirement for the n type contact layer in contact with the n electrode is to increase the impurity concentration too much. Without making the sheet resistance of the contact layer itself as low as possible.
The present inventors diligently studied to search for the configuration of an n-type semiconductor layer satisfying the above requirements.
As a result, it has been found that, by using an n-type multi-heterojunction structure described below, sheet resistance can be reduced as compared with the conventional configuration such as an n-type GaN single layer or an n-type AlGaN single layer.

Below, the experimental result which the present inventors performed is demonstrated. Here, epitaxial growth may be referred to as "epi growth", layers obtained by epi growth may be referred to as "epi layers", and substrates on which epi layers are formed may be referred to as "epi substrates". .
As an epi substrate used in the experiment, a total of three types of epi substrates were prepared in which any one of the following "structure 1" to "structure 3" was formed on a sapphire substrate on which a high resistance GaN buffer layer was formed. did.
<Structure 1>
An n-type GaN single layer (Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) with a film thickness of 480 nm.
<Structure 2>
A pair consisting of n-type Al _0.08 GaN (film thickness: 3 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) and n-type GaN (film thickness: 3 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) An n-type superlattice layer in which 80 pairs of layers (total film thickness: 480 nm) are stacked.

<Structure 3>
From an n-type Al _0.1 GaN layer (film thickness: 10 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) and an n-type GaN layer (film thickness: 30 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) N-type multi-heterojunction layer in which 12 pairs of heterojunction layers (total film thickness: 480 nm) are stacked.
"Structure 1" corresponds to the layer structure described in Patent Document 1 described above, and "Structure 2" corresponds to the layer structure described in Patent Document 2 described above. "Structure 3" is a layer structure newly proposed by the present inventors.

The electrical characteristics of each n-type epi layer were evaluated by Hall effect measurement using these three types of epi substrates.
As a result, the sheet resistance of “Structure 1” is 345 Ω / sq, the sheet resistance of “Structure 2” is 376 Ω / sq, and the sheet resistance of “Structure 3” is 273 Ω / sq. The result is obtained that the sheet resistance of the structure 3 "is the lowest.
The main cause of lowering the resistance is that the effect and effect of improving the electron mobility is the largest, while the electron mobility of “structure 1” and “structure 2” is 213 and 264 cm ² / Vs, respectively. Even though the structure 3 ′ ′ is 358 cm ² / Vs and it is a multi-hetero structure composed entirely of Si doping layers, the effect of improving the electron mobility by using the film structure of “structure 3” was gotten. This is presumed to be an improvement in mobility in the two-dimensional electron gas (2DEG) region generated at the heterointerface.

  On the other hand, the superlattice structure of “Structure 2” resulted in the highest sheet resistance. This is because using a laminated structure in which both the n-type AlGaN layer and the n-type GaN layer are 3 nm thick, sufficient 2DEG does not occur at the hetero interface, and the distance between hetero junctions is 3 nm. It is considered that due to the narrowness, the sheet resistance is not reduced due to the deterioration of the overall crystallinity under the influence of alloy scattering.

Furthermore, in the film thickness configuration of “Structure 3”, the GaN layer side is changed to a two-layer structure of a 3 nm thick undoped GaN layer (upper layer side) and a 27 nm thick n-type GaN layer (lower layer side) As a result of producing “Structure 4” in which the heterojunction layer is formed, the electron mobility is further improved than that of “Structure 3” to 397 cm ² / Vs, and the sheet resistance is reduced to 239 Ω / sq.
Here, when the carrier densities of “structure 3” and “structure 4” are compared, “structure 3” is 1.27 × 10 ¹⁸ cm ⁻³ while “structure 4” is 1.40 × 10 ¹⁸ It was confirmed that the carrier density increased to cm ⁻³ .

From the above, the factor that the sheet resistance of the n-type multi hetero junction layer is reduced is that 2DEG sufficiently occurs on the GaN side of the hetero junction interface between each AlGaN (upper layer) and GaN (lower layer) by the above film thickness configuration. Thus, it is estimated that this is a synergetic effect of pseudo increase in carrier density and improvement in electric mobility.
On the other hand, the superlattice structure disclosed in Patent Document 2 described a structure in which the impurity concentration is increased to 1 × 10 ¹⁹ cm ⁻³ , but according to the above experiment, the impurity concentration is 1 × 10 ^When it was lowered to ¹⁸ cm ⁻³ , the effectiveness for reducing the resistance in the superlattice structure was not confirmed.

  In the “structure 3” and the “structure 4”, although the Al composition of the n-type AlGaN layer was experimented and evaluated as 0.1, the band gap energy of the same layer is increased by increasing the Al composition. Since the band offset amount with the GaN layer applied as a layer with a small gap energy is large, and the sheet carrier density of 2DEG tends to further increase, the sheet resistance reduction effect becomes larger.

In addition, with respect to the film thickness of the n-type AlGaN layer, the effect of polarization is increased by thickening the film more than 10 nm, and the sheet carrier density of 2DEG tends to be higher. Also, the effect of reducing the sheet resistance can be obtained.
At this time, by simultaneously thickening the film thickness on the layer side with small band gap energy, further increase in sheet carrier density can be expected.

From the above, the second n-type semiconductor layer and the second n-type semiconductor layer have a band gap energy smaller than that of the first n-type semiconductor, and in particular, a band offset occurs with respect to the first n-type semiconductor layer on the conduction band side It has been found that the n-type multi-heterojunction layer in which a plurality of n-type semiconductor layers are alternately stacked is effective to reduce the sheet resistance.
In the n-type multi hetero junction layer, the film thickness of the first n-type semiconductor layer is preferably 10 nm or more, and specifically, 10 nm or more and 20 nm or less. If the film thickness is less than 10 nm, the sheet carrier density of 2DEG will be insufficient, and if it is more than 20 nm, in addition to the fact that the electrical characteristics of 2DEG do not improve so much, alloy scattering this time As a result, it is experimentally found that the crystallinity in the depth direction of the heterojunction interface is impaired as the film thickness becomes thicker due to the influence of H. Conversely, the electron mobility of 2DEG tends to decrease.

Furthermore, there is also a negative effect that a crack is likely to be generated on the surface of the epi layer due to the influence of stress due to the thick film formation.
Further, the film thickness of the second n-type semiconductor layer is preferably twice or more the film thickness of the first n-type semiconductor layer, and specifically, may be 20 nm or more and 200 nm or less preferable.
By forming an n-type multi-hetero junction layer with the above configuration, a plurality of 2DEGs generated at each hetero junction interface due to band offset of each semiconductor material sufficiently increases the carrier density enhancement effect different from that by n-type impurity doping. Since it can be obtained, the sheet resistance of the same layer can be significantly reduced even if the n-type impurity concentration is low.

Further, the film thickness of the first n-type semiconductor having a band gap energy larger than that of the second n-type semiconductor may be dispersed in a plurality of layers alternately with the second n-type semiconductor layer as the film thickness range. Even when the film thickness is exceeded, the strain applied to the semiconductor epilayer can be alleviated, and the occurrence of cracks on the surface of the epilayer can be prevented to a certain extent.
On the contrary, when the film thickness is made thinner than the lower limit of the above film thickness range, as is clear from the examination result regarding the super lattice structure, when the n-type impurity concentration is low, 2DEG occurs at each heterojunction interface It is obvious that the effect by the above can not be obtained sufficiently, and the target sheet resistance reducing effect of the n-type layer can not be obtained.

In the n-type multi hetero junction layer, the same semiconductor as the second n-type semiconductor layer is formed at the interface between the first n-type semiconductor layer on the upper side and the second n-type semiconductor layer on the lower side. It is preferable to have a structure in which an undoped semiconductor layer made of a material is provided.
With this configuration, the electron mobility of 2DEG generated at each heterojunction interface in the n-type multiheterojunction layer is improved, and the sheet resistance of the entire n-type multiheterojunction layer is further reduced.
At this time, the film thickness of the undoped semiconductor layer is preferably 1 nm or more and 5 nm or less.
When the film thickness of the undoped semiconductor layer is made thicker than 5 nm, the resistance component in the vertical direction inside the n-type multi-heterojunction layer tends to increase, so making it thicker than 5 nm is contrary to the electric It becomes a factor causing deterioration of the characteristics.

In the n-type multi hetero junction layer, the impurity concentration to be added to the first n-type semiconductor layer and the second n-type semiconductor layer is preferably 5 × 10 ¹⁸ cm ⁻³ or less.
The high impurity concentration does not adversely affect the sheet resistance reduction of the n-type multi hetero junction layer, but the active layer is, for example, a light emitting layer by setting the impurity concentration to 2 × 10 ¹⁸ cm ⁻³ or less. In this case, the sheet resistance of the n-type contact layer is reduced while suppressing absorption of the emitted light, which is effective in reducing the operating voltage of the light emitting element. Also in the case where the active layer is a light receiving layer, the effects of light absorption suppression and operating voltage reduction can be similarly obtained.

The impurity concentration does not need to have a lower limit in particular, but if the impurity concentration is lower than 5 × 10 ¹⁶ cm ^−3, it is affected by compensation by other impurities contained in the semiconductor such as C (carbon). Depending on the semiconductor material, the conductivity type may not be distinguishable. Therefore, it is preferable to set 5 × 10 ¹⁶ cm ⁻³ as the lower limit value of the impurity concentration.
As another effect, the n-type multi-heterojunction layer described above has a large refractive index difference with respect to the active layer composed of an InGaN multiple quantum well layer generally used in, for example, a blue-violet laser diode. In addition to the function as a contact layer, it also functions as a cladding layer that confines light emitted from the active layer.

In addition to the above n-type Al _0.1 GaN / n-type GaN multi-hetero junction layer, the semiconductor layer constituting the above n-type multi hetero junction layer may be, for example, an AlN molar ratio if it is a nitride semiconductor. Different Al _x Ga ₁ -x N / Al _y Ga _1-y N (X> Y) multi-heterojunction layer, GaN / InGaN multi-heterojunction layer, InAlN (InAlGaN) / GaN (AlGaN) multi-heterojunction layer, etc. Is considered.

By adding an n-type impurity such as Si to each semiconductor material in these multi-hetero junction layers, n-type contact layers of semiconductor light emitting devices of all emission wavelength bands that can be manufactured using a GaN-based nitride semiconductor Multiple heterojunction layers can be applied.
In addition, even in these multi-hetero junction layers, if they have a high refractive index difference with respect to the active layer material by the combination of the nitride semiconductor materials constituting the layer, they can be applied as a cladding layer as described above. It is clear in

The above-described n-type multi-heterojunction layer is also applicable to compound semiconductor materials different from nitride semiconductors. For example, in the case of a GaAs-based compound semiconductor material, for example, n-type AlGaAs / n-type GaAs multiheterojunction layer Besides, and InAlP / GaAs multi-heterojunction layer, InAlP / InGaP multi heterojunction _{layer, Al M Ga 1-M As} / Al N Ga 1-N As (M> N) multi-heterojunction layer, InAlGaP / InGaP multi Heterojunction layers and the like are conceivable.

By adding an n-type impurity such as Si to each semiconductor material in these multi-heterojunction layers, n-type contacts of semiconductor light emitting devices of all emission wavelength bands that can be manufactured using a compound semiconductor that can be epitaxially grown on a GaAs substrate Multiple heterojunction layers can be applied to the layers. Specifically, the present invention can be applied to, for example, a red light emitting element fabricated on a GaAs substrate.
Further, it is needless to say that the present invention can also be applied as a cladding layer when there is a high refractive index difference with respect to the active layer material due to the combination of GaAs-based compound semiconductor materials constituting these multiheterojunction layers.

The above-described n-type multi-heterojunction layer is also applicable as a structure fabricated on an InP substrate which is another compound semiconductor substrate. For example, in the case of an InP-based compound semiconductor material, for example, an InP / InGaAs multiheterojunction layer In addition, InGaAsP / InP multiheterojunction layers, InGaAs / InGaAsP multiheterojunction layers, and the like are conceivable.
By adding an n-type impurity such as Si to each semiconductor material in these multi-heterojunction layers, n-type contacts of semiconductor light emitting devices of all emission wavelength bands that can be manufactured using a compound semiconductor that can be epitaxially grown on an InP substrate Multiple heterojunction layers can be applied to the layers. Specifically, the present invention can be applied to, for example, a semiconductor light emitting device having an emission wavelength band of 1 μm or more.

Further, it is needless to say that the combination of the InP-based compound semiconductor materials constituting these multi-heterojunction layers can be applied as a cladding layer as described above if they have a high refractive index difference with respect to the active layer material.
Furthermore, in the n-type multi-heterojunction layer of various semiconductor materials described above, as in the n-type multi-heterojunction layer of nitride semiconductor material, the band gap energy of each heterojunction interface is narrow on the material side By providing the undoped layer having the film thickness of the same semiconductor material as the material in the range of 1 nm to 5 nm, the electron mobility of 2DEG generated at each interface can be increased, so the sheet resistance of the multi-hetero junction layer Has the effect of being able to further reduce

Specific embodiments to which the n-type multi-heterojunction layer described above is applied will be described below with reference to the drawings.
First Embodiment
FIG. 2 is a cross-sectional view showing a semiconductor laser which is the first embodiment of the semiconductor optical device, and FIG. 3 is a partially enlarged view showing the structure of the n-type multiheterojunction layer in the first embodiment.
The semiconductor laser 100 of the first embodiment is a semiconductor laser using a nitride semiconductor. A light emitting element using a nitride semiconductor has an emission wavelength range of about 300 nm to 520 nm, and can be applied as a light emitting element that emits ultraviolet to green light.
The semiconductor laser 100 is formed in a desired region on the n-type contact layer 3 and the n-type contact layer 3 having the GaN substrate 1, the n-type GaN buffer layer 2, the above-mentioned n-type multi hetero junction layer, Layered semiconductor structure in which an n-type AlGaN cladding layer 4, an active layer (light emitting layer) 5 composed of an undoped InGaN-based multiple quantum well layer, a p-type AlGaN cladding layer 6, and a p-type GaN contact layer 7 are stacked in this order Have.

A stripe-shaped ridge portion for current confinement is formed on part of the p-type GaN contact layer 7 and the p-type AlGaN cladding layer 6 by well-known nitride semiconductor dry etching, and exposed at the top of the ridge portion A protective film 8 made of, for example, SiO ₂ is formed on the side surfaces and the surface of the p-type AlGaN cladding layer 6 except for the p-type GaN contact layer 7.
The p electrode 9 is in ohmic contact with the p-type GaN contact layer 7 exposed at the top of the ridge, and the electrode metal constituting the p electrode extends laterally to a desired region on the protective film 8 And is formed to cover a portion of the protective film 8.
The n-electrode 10 is formed in contact with the n-type contact layer 3 exposed in a region separated from the stripe-shaped ridge portion, and is in ohmic contact with the n-type contact layer 3.

Here, the configuration of the n-type multi hetero junction layer in the region surrounded by the broken line in FIG. 2 will be described in detail with reference to FIG.
n-type contact layer 3, an n-type _Al 0.15 GaN layer 3a is n-type impurity (Si) concentration of for example 1 × ¹⁰ 18 ^{cm -3} with a thickness located on the upper side, for example 10 nm, the lower side And 20 pairs of n-type GaN layers 3b having a thickness of, for example, 70 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ are alternately stacked. It has an n-type multi-heterojunction layer of 6 μm.

  Compared to an n-type GaN layer having the same impurity concentration and the same film thickness as the n-type contact layer 3, the n-type contact layer 3 having an n-type multi hetero junction layer as shown in FIG. Since the resistance is reduced, the ohmic characteristics of the n-electrode 10 maintain a good state, and the lateral resistance affecting the series resistance component on the n-electrode side is reduced. That is, since the lateral resistance from the vicinity of the n electrode 10 to the n-type AlGaN cladding layer 4 via the n-type contact layer 3 is reduced, the operation is possible even with a nitride semiconductor laser fabricated on a GaN substrate The voltage is reduced.

FIG. 4 is a view showing another structure of the n-type multi hetero junction layer in the first embodiment.
In the n-type multi-heterojunction layer shown in FIG. 4, the upper n-type Al _0.15 GaN layer 3a has the same film thickness and the same impurity concentration as the n-type Al _0.15 GaN layer 3a shown in FIG.
On the other hand, the GaN layer located on the lower layer side has a two-layer structure divided into the lower layer side and the upper layer side, and the lower layer side has a film thickness of eg 67 nm and an n-type impurity (Si) concentration of eg 1 × The n-type GaN layer 3bb is 10 ¹⁸ cm ⁻³ , and the upper layer side is the undoped GaN layer 3c with a thickness of, for example, 3 nm.

That is, the configuration of one heterojunction layer has a three-layer structure in the order of n-type Al _0.15 GaN layer 3 a / undoped GaN layer 3 c / n-type GaN layer 3 bb from the top. In the n-type multiheterojunction layer shown in FIG. 4, for example, 20 heterojunction layers of this configuration are stacked to form an n-type multiheterojunction layer having a total film thickness of, for example, 1.6 μm.
By using the n-type multi-heterojunction layer having the configuration shown in FIG. 4, the influence of scattering by impurities in the 2DEG generation region is reduced, so the electron mobility of the entire n-type contact layer 3 is smaller than that shown in FIG. There is an effect that the sheet resistance of the n-type contact layer 3 can be further reduced.
Therefore, the operating voltage of the laser can be further reduced by applying the n-type contact layer 3 having the configuration of the n-type multiheterojunction layer shown in FIG. 4 to the nitride semiconductor laser shown in FIG. There is.

Here, the first embodiment will be described by taking a more specific example.
In the semiconductor laser 100 having the configuration shown in FIG. 2, the stripe width of the ridge portion is 2.5 μm, the cavity length is 800 μm, and any of the following “structure 1” to “structure 3” is applied to the n-type contact layer 3 A total of three applied semiconductor lasers were prepared.
<Structure 1>
n-type GaN layer (film thickness: 1.6 μm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ). Since this “structure 1” is not a multi-hetero structure, a semiconductor laser to which “structure 1” is applied is a comparative example.
<Structure 2>
From an n-type Al _0.15 GaN layer (film thickness: 10 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) and an n-type GaN layer (film thickness: 70 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ) N-type multi-heterojunction layer in which 20 layers (total film thickness 1.6 μm) of heterojunction layers are stacked. This "structure 2" is a specific example of the configuration shown in FIG.

<Structure 3>
n-type Al _0.15 GaN layer (film thickness: 10 nm, Si impurity concentration: 1 × 10 ¹⁸ cm ⁻³ ), undoped GaN layer (film thickness: 3 nm) and n-type GaN layer (film thickness: 67 nm, Si impurity concentration: An n-type multi-heterojunction layer in which 20 heterojunction layers consisting of 1 × 10 ¹⁸ cm ⁻³ ) (total film thickness 1.6 μm) are stacked. This "structure 3" is a specific example of the configuration shown in FIG.
With respect to the semiconductor lasers of these three types of structures, the operating voltages were compared when the laser output was 100 mW. As a result, “Structure 1” is 4.41 to 4.68 V, whereas “Structure 2” is 4.03 to 4.35 V, and “Structure 3” is 3.94 to 4.21 V, By applying the configurations shown in FIG. 3 and FIG. 4 to the n-type contact layer, an effect of reducing the operating voltage of the laser as compared with the “structure 1” of the comparative example was obtained.

Second Embodiment
FIG. 5 is a sectional view showing a light emitting diode (LED) which is a second embodiment of the semiconductor optical device, and FIG. 6 is a partially enlarged view showing the structure of the n-type multi hetero junction layer in the second embodiment. It is.
The light emitting diode 300 shown in FIG. 5 is a light emitting diode of a nitride semiconductor material, and includes a sapphire substrate 30, an undoped GaN buffer layer 31, an n-type contact layer 32 having an n-type multi-heterojunction layer, and an undoped A semiconductor multilayer structure in which an active layer (light emitting layer) 33 composed of an InGaN-based multiple quantum well layer, a low Mg concentration p-type GaN layer 34, and a high Mg concentration p-type GaN contact layer 35 are stacked in this order Have.

The p-type GaN contact layer 35 to the active layer (light emitting layer) 33 are etched in the desired region for forming the n-electrode to expose the n-type contact layer 32, and the n-electrode on the n-type contact layer 32 38 are provided and are in ohmic contact with the n-type contact layer 32.
A transparent electrode 36 is provided in a desired region on the p-type GaN contact layer 35, and a p electrode 37 is provided on a part of the transparent electrode 36. A well-known ITO (indium tin oxide) film can be applied as the transparent electrode 36.

Here, the configuration of the n-type multi heterojunction layer in the region surrounded by the broken line in FIG. 5 will be described in detail with reference to FIG.
The n-type contact layer 32 is located on the upper layer side, an n-type Al _0.1 GaN layer 32 a having a film thickness of, for example, 20 nm and an n-type impurity (Si) concentration of, for example, 5 × 10 ¹⁷ cm ^−3. And a pair of n-type GaN layers 32b having a thickness of, for example, 200 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁷ cm ⁻³ are alternately stacked, for example, 15 pairs. It has an n-type multiheterojunction structure of 3 μm.

The light emitting diode 300 shown in FIG. 5 has a structure in which multiple layers of AlGaN are provided only in the n-type contact layer 32 located below the active layer 33 in order to mainly emit the light emitted from the active layer 33 to the surface side. There is. Thus, the n-type contact layer 32 also functions as a reflection layer of the emitted light.
Furthermore, in the n-type AlGaN layer 32a and the n-type GaN layer 32b constituting the n-type contact layer 32, the Si impurity concentration on the side of the n-type GaN layer 32b which is a thick film and has a small difference in refractive index from the active layer 33 is lowered. Therefore, light absorption in the n-type contact layer 32 is suppressed.

Even when the Si impurity concentration is different between the n-type AlGaN layer and the n-type GaN layer constituting the n-type multi-heterojunction layer as described above, the ohmic connection with the n electrode is not affected. Showed good ohmic characteristics.
Therefore, the impurity concentration of each semiconductor material constituting the n-type multi-hetero junction layer can be made different for each layer without departing from the scope of the present invention, and the impurity concentration is 5.0 × 10 ¹⁶ cm ^-3 to 5.0 × ¹⁰ 18 ^{cm -3} it is preferably less.

Third Embodiment
FIG. 7 is a cross-sectional view showing a semiconductor laser which is a third embodiment of the semiconductor optical device, and FIG. 8 is a partially enlarged view showing the structure of the n-type multiheterojunction layer in the third embodiment.
The semiconductor laser 600 of the third embodiment is also a semiconductor laser using a nitride semiconductor.
The third embodiment and the first embodiment described above are different in the adhesion surface on which the n electrode is provided, and the point in which the back surface electrode 69 is provided.
The semiconductor laser 600 includes a GaN substrate 60, an n-type GaN buffer layer 61, an n-type contact layer 62 having an n-type multiheterojunction layer, and an active layer (light emitting layer) 63 composed of an undoped InGaN-based multiple quantum well layer. , A p-type AlGaN cladding layer 64, and a p-type GaN contact layer 65 are laminated in this order.

A stripe-shaped ridge portion for current confinement is formed on a part of the p-type GaN contact layer 65 and the p-type AlGaN cladding layer 64 by well-known nitride semiconductor dry etching, and exposed at the top of the ridge portion A protective film 66 made of, for example, SiO ₂ is formed on the side surfaces and the surface of the p-type AlGaN cladding layer 64 except for the p-type GaN contact layer 65.
The p-electrode 67 is in ohmic contact with the p-type GaN contact layer 65 exposed at the top of the ridge portion, and the electrode metal constituting the p-electrode 67 extends laterally to the desired region on the protective film 66 It is extended and formed to cover a part of the protective film 66.
The n-electrode 68 is formed in contact with the n-type contact layer 62 exposed in a region separated from the stripe-shaped ridge portion, and is in ohmic contact with the n-type contact layer 62. However, the n-electrode 68 is provided not in the outermost surface of the n-type contact layer 62 but in contact with the surface where a part of the n-type contact layer 62 is dug by etching.

In addition, as shown in FIG. 8, the n-type contact layer 62 is located on the upper layer side and has a film thickness of, for example, 10 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁸ cm ^−3. _-Type Al _0.15 GaN layer 62a, and an n-type GaN layer 62b having a film thickness of, for example, 70 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ located alternately. It has an n-type multi-heterojunction structure in which 20 pairs of layers are stacked and the total film thickness is, for example, 1.6 μm.

The n-electrode 68 is deposited on the exposed surface of the n-type GaN layer 62b exposed by etching halfway through the n-type AlGaN layer 62a on the outermost surface and the n-type GaN layer 62b below it.
The situation as described above is a case that can occur in a normal manufacturing process from the viewpoint of etching control of the semiconductor layer, but the n-type multi-heterojunction layer shown in FIG. Because of the conductivity type, defects such as deterioration of ohmic contact with the n electrode do not occur.
Therefore, even when the n electrode is formed on the surface where a part of the n-type multiheterojunction layer is recessed by etching, the advantage obtained by applying the n-type multiheterojunction layer is not impaired.
Further, in FIG. 7, the back surface electrode 69 is provided on the back surface side of the GaN substrate 60. The back surface electrode 69 is an electrode necessary for mounting the back surface side of the semiconductor laser 600 by solder. When mounting using AuSn solder or the like, the outermost surface of the back surface electrode 69 is preferably Au.
Further, the electrical connection between the back electrode 69 and the GaN substrate 60 is not particularly limited, and either ohmic connection or Schottky connection may be used.

In the third embodiment, an example of using a GaN substrate as the material of the substrate provided with the back surface electrode has been described. However, even if any substrate material is used, the back surface may be solder-mounted. It goes without saying that the back electrode may be provided regardless of the type of material.
In the first to third embodiments described above, the semiconductor material of the n-type multi-heterojunction layer uses a multi-layered n-type AlGaN / n-type GaN heterojunction layer. Al _X Ga _1-X N / Al _Y Ga _1-Y N (X> Y) multi-heterojunction layer or GaN / InGaN multi-heterojunction, each of which has a different AlN molar ratio depending on the material system, emission wavelength, and device structure used It goes without saying that other semiconductor materials and device structures that can be fabricated on a GaAs substrate, such as layers, InAlN (InAlGaN) / GaN (AlGaN) multiheterojunction layers, etc. may be used.

In the first to third embodiments using a nitride semiconductor, the example using a GaN substrate or sapphire substrate as a substrate has been described, but in addition to this, a SiC substrate, a Si substrate, an AlN substrate, a ZnO substrate, etc. It goes without saying that other substrate materials capable of epitaxially growing a nitride semiconductor may be used.
In particular, by applying an n-type multi-heterojunction layer to a semiconductor optical device fabricated on a substrate material having no conductivity, the effects of the present invention are sufficiently exhibited over semiconductor optical devices fabricated on a conductive substrate. can do.
Fourth Embodiment
FIG. 9 is a cross-sectional view showing a light emitting diode which is a fourth embodiment of the semiconductor optical device, and FIG. 10 is a partially enlarged view showing the structure of the n-type multiheterojunction layer in the fourth embodiment.

The light emitting diode 900 shown in FIG. 9 is manufactured on an n-type GaAs substrate. Thus, the semiconductor light emitting device manufactured on a GaAs substrate mainly has a light emission wavelength range of 650 nm or more, and can be applied as a red to near infrared light emitting device.
The light emitting diode 900 comprises an n type GaAs substrate 90, an n type contact layer 91 having an n type multi hetero junction structure, an active layer (light emitting layer) 92 made of undoped AlGaAs, and Zn (zinc) as a p type impurity. A low Zn concentration p-type GaAs layer 93 and a high Zn concentration p-type GaAs contact layer 94 are stacked in this order. The n-type multi-heterojunction structure in the n-type contact layer 91 shown in FIG. 9 is formed by alternately laminating a plurality of n-type AlGaAs and n-type GaAs layers, as will be described in detail later.

In the desired region for forming the n-electrode 96, the p-type GaAs contact layer 94 to the active layer (light emitting layer) 92 are etched by the known selective wet etching method to expose the n-type contact layer 91. A well-known AuGe-based n electrode 96 is provided on the mold contact layer 91 and is in ohmic contact with the n-type contact layer 91.
A plurality of thin wire-like p electrodes 95 are provided in desired regions on the p-type GaAs contact layer 94, and all p electrodes 95 are mutually connected by pad portions (not shown) provided for wire wiring. It is connected.

A back surface electrode 97 for solder mounting is provided on the back surface side of the n-type GaAs substrate 90.
Here, the configuration of the n-type multi hetero junction layer in the region surrounded by the broken line in FIG. 9 will be described in detail with reference to FIG.
The n-type contact layer 91 is located on the upper layer side, an n-type Al _0.2 GaAs layer 91 a having a film thickness of 15 nm and an n-type impurity (Si) concentration of 2 × 10 ¹⁸ cm ⁻³ , 40 layers of n-type GaAs layers 91b alternately having a thickness of 55 nm and an n-type impurity (Si) concentration of 5 × 10 ¹⁷ cm ^-3 to a total film thickness of 2.8 μm It has a multi-heterojunction structure.

According to the GaAs-based light emitting diode 900 to which such an n-type multi hetero junction layer is applied, the n-type GaAs layer having the same film thickness reflects the reduction of the resistance component from the n electrode 96 to the active layer 92 There is an effect that the operating voltage can be reduced as compared with the case of using n in the n-type contact layer.
FIG. 11 is a view showing another structure of the n-type multi heterojunction layer in the fourth embodiment.
In the n-type multi hetero junction layer of the n-type contact layer 91 shown in FIG. 11, the upper layer side n-type Al _0.2 GaAs layer 91a has the same thickness as the n-type Al _0.2 GaAs layer 91a shown in FIG. It has the same impurity concentration.

On the other hand, the GaAs layer located on the lower layer side has a three-layer structure divided into the lower layer side, the upper layer side, and the intermediate layer, and the intermediate layer has a film thickness of, for example, 66 nm and n-type impurity (Si) concentration For example, the n-type GaAs layer 91bb is 5 × 10 ¹⁷ cm ^-3 , and the undoped GaAs layer 91c having a thickness of 2 nm is disposed in the upper layer and the lower layer of the intermediate layer. That is, the n-type GaAs layer is sandwiched between undoped GaAs layers.
The n-type contact layer 91 shown in FIG. 11 has a total film thickness of 1.6 μm by laminating 20 hetero junction layers consisting of such a three-layer structure GaAs layer and an n-type Al _0.2 GaAs layer 91a. It has an n-type multi-heterojunction layer.

In the case of a GaAs-based material, unlike a nitride semiconductor, it is a semiconductor material that is difficult to polarize, so if, for example, a heterojunction structure is formed above and below a GaAs layer such as n-type AlGaAs layer / undoped GaAs layer / n-type AlGaAs layer, 2DEG occurs at the heterojunction interface between the n-type AlGaAs layers above and below the undoped GaAs layer.
Therefore, in the n-type multi-heterojunction layer, the lower surface side of the n-type GaAs layer heterojunction with the lower n-type AlGaAs layer and the upper surface side of the n-type GaAs layer heterojunction with the upper n-type AlGaAs layer are By making both layers of undoped GaAs layers, it is possible to increase the electron mobility of 2DEG generated at both the upper and lower junction interface, and therefore, the sheet resistance of the n-type contact 91 can be further reduced.

The thickness of the undoped GaAs layer is preferably 1 nm or more and 5 nm or less.
When the film thickness of the undoped semiconductor layer is made thicker than 5 nm, the resistance component in the vertical direction inside the n-type multi-heterojunction layer tends to increase, so making it thicker than 5 nm is contrary to the electric It becomes a factor causing deterioration of the characteristics.
In the fourth embodiment described above, an example in which the n-type multi hetero junction layer is applied to the n-type contact layer of the light emitting diode fabricated on the GaAs substrate has been described. However, the n-type contact layer of the laser fabricated on the GaAs substrate It is also apparent from the application to the laser using the nitride semiconductor and the light emitting diode described in the first to third embodiments described above that they may be applied to

In the fourth embodiment, the semiconductor material of the n-type multiheterojunction layer is a multi-layered n-type AlGaAs / n-type GaAs heterojunction layer. However, the material system used for the active layer, the emission wavelength , and the configuration of the device, InAlP / GaAs multi-heterojunction layer and, InAlP / InGaP multi heterojunction _{layer, Al M Ga 1-M As} / Al N Ga 1-N As (M> N) multi-heterojunction layer, InAlGaP It goes without saying that other semiconductor materials and device structures that can be fabricated on a GaAs substrate, such as an InGaP multi-heterojunction layer or the like may be used.

Fifth Embodiment
FIG. 12 is a cross-sectional view showing a semiconductor laser which is a fifth embodiment of the semiconductor optical device, and FIG. 13 is a partial enlarged view showing the structure of the n-type multiheterojunction layer in the fifth embodiment.
The semiconductor laser 1100 is fabricated on a semi-insulating InP substrate. A semiconductor light emitting element fabricated on an InP substrate has an emission wavelength range of 1.2 μm to 1.7 μm, and can be applied as a light emitting element of a long wavelength band used in a communication system.
The semiconductor laser 1100 includes a semi-insulating InP substrate 110, an n-type InP buffer layer 111, an n-type contact layer 112 having an n-type multi hetero junction structure, an n-type cladding layer 113 formed of an n-type InP layer, -Type guide layer 114 made of n-type InGaAsP layer, active layer 115 made of undoped InGaAs layer, p-type guide layer 116 made of p-type InGaAsP layer, p-type cladding layer 117 made of p-type InP layer, p-type A p-type contact layer 118 formed of an InGaAs layer has a semiconductor laminated structure laminated in this order. The n-type multi-heterojunction structure in the n-type contact layer 112 shown in FIG. 12 is formed by alternately laminating a plurality of layers of n-type InP and n-type InGaAs, as described in detail later.

A desired region where the n electrode 121 is to be formed is etched by the known wet etching from the p-type contact layer 118 to the cladding layer 113, and in order to carry out a process of selectively etching only InP material as a final etching. On the surface of the n-type contact layer 112, the n-type InGaAs layer 112b (see FIG. 13) is exposed.
Further, in the desired region, a stripe-shaped ridge portion for current constriction is also formed by etching. A protective film 119 made of, for example, SiO ₂ is provided on the side surface of the ridge portion and on the p-type cladding layer 117 exposed by etching.

A well-known AuGe-based n electrode 121 is provided on the n-type contact layer 112 and is in ohmic contact with the n-type contact layer 112.
The p-electrode 120 is in ohmic contact with the p-type contact layer 118 exposed at the top of the ridge portion, and the electrode metal constituting the p-electrode 120 extends laterally to a desired region on the protective film 119 And is formed so as to cover a part of the protective film 119.
A back surface electrode 122 for solder mounting is provided on the back surface side of the semi-insulating InP substrate 110.

Here, the configuration of the n-type multiheterojunction layer in the region surrounded by the broken line in FIG. 12 will be described with reference to FIG.
The n-type contact layer 112 is located on the upper layer side, an n-type InP layer 112 a having a film thickness of, for example, 10 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁸ cm ^−3; The total film thickness is, for example, 1 μm by alternately laminating, for example, 20 pairs of n-type InGaAs layers 112b having a film thickness of, for example, 40 nm and an n-type impurity (Si) concentration of, for example, 1 × 10 ¹⁸ cm ^−3. It has a multi-heterojunction structure.

As described above, since InP selective etching is performed as the final process of semiconductor etching for opening the n-electrode formation region, the n-electrode 121 is formed in contact with the n-type InGaAs layer, but ohmic contact is There is no problem such as lowering, and although the thickness of the contact layer is as thin as 1 μm to reduce the capacity of the device, the lateral resistance in the n-type contact layer is achieved by applying the n-type multi-heterojunction structure. It is possible to provide a high quality semiconductor laser excellent in high frequency characteristics by the effect of reduced components.
In the fifth embodiment, the n-type multi-heterojunction structure is applied to the n-type contact layer of the semiconductor laser fabricated on the InP substrate, but the n-type contact layer of the light emitting diode fabricated on the InP substrate is described. It goes without saying that an n-type multi-heterojunction structure may be applied.

In the fifth embodiment, the semiconductor material of the n-type multiheterojunction layer uses a multi-layered n-type InP / n-type InGaAs heterojunction layer, but the InGaAsP / InP multiheterojunction layer, It goes without saying that other semiconductor material structures and device structures that can be fabricated on InP substrates, such as InGaAs / InGaAsP multi-heterojunction layers, etc., may be used.
In the first to fifth embodiments described above, the light emitting element is shown as an example of the semiconductor optical element of the present invention, but the semiconductor optical element of the present invention is a light receiving element using a compound semiconductor. May be Even when an n-type multi-hetero junction layer is used as the n-type contact layer of such a light receiving element, the effect of reducing the element resistance is effective in improving the characteristics of the light receiving element. In the case of a light receiving element, it has a light receiving layer as an active layer in the present invention.

In the above-described first to fifth embodiments, the example in which SiO ₂ is applied to the protective film has been described, but in addition to this, SiN, PSG, Al ₂ O _{3 and the} like are generally used in the manufacturing process of semiconductor elements. There is no problem in applying the insulating film used in
In addition, as described in the first to fifth embodiments, the n-type multi-heterojunction layer according to the present invention is formed of heterojunctions of semiconductor materials different in band gap energy depending on the band offset at the interface. Can be made of any compound semiconductor material capable of generating a two-dimensional electron gas, and various n-type multi-heterojunction layers in which the semiconductor material is changed according to the emission wavelength, device structure, and semiconductor material By providing the semiconductor optical device as an n-type contact layer of the semiconductor optical device, a semiconductor optical device having a low operating voltage and excellent optical characteristics can be provided.

100, 200, 600 ...... Semiconductor laser, 1, 60 ...... GaN substrate, 20 ...... substrate,
2, 61 ...... n-type GaN buffer layer, 21 ...... undoped GaN buffer layer,
3, 62 ... n-type contact layer, 22 ... n-type GaN contact layer,
3a, 62a ... n-type Al _0.15 GaN layer, 3b, 3bb, 62b ... n-type GaN layer, 3c ... undoped GaN layer, 4, 23, 64 ... n-type AlGaN cladding layer,
5, 24 63: Active layer (light emitting layer), 6, 25 p-type AlGaN cladding layer,
7,26,65 ...... p-type GaN contact layer, 8,27,66 ...... protective film,
9,28,67 ...... p electrode, 10,29,68 ...... n electrode, 69 ...... back electrode,
300: light emitting diode, 30: sapphire substrate, 32: n-type contact layer,
31 ...... Undoped GaN buffer layer, 32a ...... n-type Al _0.1 GaN layer,
32b: n-type GaN layer, 33: active layer, 34: low Mg concentration p-type GaN layer,
35: High Mg concentration p-type GaN contact layer 36: Transparent electrode 37: p electrode
38: n electrode, 900: light emitting diode, 90: n type GaAs substrate,
91: n-type contact layer, 91a: n-type Al _0.2 GaAs layer,
91b, 91bb ... n-type GaAs layer, 91c ... undoped GaAs layer,
92: active layer, 93: low Zn concentration p-type GaAs layer, 95: p electrode,
94: p-type GaAs contact layer with high Zn concentration, 96: n electrode, 97: back electrode, 1100: semiconductor laser, 110: semi-insulating InP substrate,
111: n-type InP buffer layer, 112: n-type contact layer,
113: n-type cladding layer, 114: n-type guide layer, 115: active layer,
116 ...... p-type guide layer, 117 ...... p-type cladding layer 117,
118: p-type contact layer, 119: protective film, 120: p electrode,
121 ... n electrode, 122 ... back electrode

Claims (10)

A first n-type semiconductor layer comprising a substrate, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, a p-electrode and an n-electrode, wherein the n-type semiconductor layer has a thickness of 10 nm or more. And a plurality of second n-type semiconductor layers alternately stacked each having a film thickness twice or more than that of the first n-type semiconductor layer and having a band gap energy smaller than that of the first n-type semiconductor layer. With an n-type multi-heterojunction layer,
The n electrode is in contact with the n-type multi hetero junction layer ,
The film thickness is 1 nm or more and 5 nm or less at an interface between the first n-type semiconductor layer and the second n-type semiconductor layer located closer to the substrate side than the first n-type semiconductor layer. 2. A semiconductor optical device comprising an undoped semiconductor layer made of the same semiconductor material as the n-type semiconductor layer of No. 2 . The semiconductor optical device according to claim 1, wherein the first n-type semiconductor layer has an impurity concentration of 5.0 × 10 ¹⁶ cm ⁻³ or more and 5.0 × 10 ¹⁸ cm ⁻³ or less. 3. The semiconductor optical device according to claim 1, wherein the second n-type semiconductor layer has an impurity concentration of 5.0 × 10 ¹⁶ cm ⁻³ or more and 5.0 × 10 ¹⁸ cm ⁻³ or less. element.   The semiconductor optical device according to any one of claims 1 to 3, wherein the first n-type semiconductor layer has a thickness of 10 nm to 20 nm.   The semiconductor optical device according to any one of claims 1 to 4, wherein the second n-type semiconductor layer has a thickness of 20 nm or more and 200 nm or less.   The semiconductor optical device according to any one of claims 1 to 5, wherein both the p electrode and the n electrode are provided on one side of the front and back sides of the substrate.   The semiconductor according to any one of claims 1 to 6, wherein the substrate is a substrate of a material selected from the group consisting of GaAs, InP, GaN, sapphire, Si, SiC, ZnO and AlN. Optical element.   The semiconductor optical device according to any one of claims 1 to 7, wherein the n-type multi hetero junction layer is a cladding layer.   The semiconductor optical device according to any one of claims 1 to 8, wherein the semiconductor optical device is a light emitting diode.   The semiconductor optical device according to any one of claims 1 to 8, wherein the semiconductor optical device is a laser diode.

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