JP4068535B2 - Semiconductor laser device, optical semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a semiconductor laser element, an optical semiconductor element, and a manufacturing method thereof, and more particularly, to a semiconductor laser element for optical communication, an optical semiconductor element, and a manufacturing method thereof.

  A long-wavelength semiconductor laser element having an emission wavelength of 1.2 μm to 1.6 μm has been attracting attention as an optical semiconductor element for optical communication. As this long-wavelength semiconductor laser element, a structure in which a light emitting layer made of a GaInAsP-based material is formed on a substrate made of InP is mainly used. In this structure, the light emitting layer and the substrate are close to each other in lattice constant, and practical light emission efficiency is obtained.

  However, a semiconductor laser device using an InP substrate has a problem that it is expensive and expensive. Therefore, in recent years, attention has been paid to an element that uses a GaAs substrate that is inexpensive and easy to process as a semiconductor laser element for long-wavelength optical communication, and forms a GaInAsN light emitting layer (well layer) on the GaAs substrate.

  In the long wavelength semiconductor laser device using the GaAs substrate, the In composition of the light emitting layer is increased to 30 to 40% in order to realize light emission in the 1.2 μm to 1.6 μm band. However, when the In composition of the light emitting layer is increased, the lattice constant of the active layer becomes larger than that of the substrate. Therefore, in this element, by mixing N with a small atomic radius in the light emitting layer, the difference in lattice constant between the light emitting layer and the substrate is prevented from becoming too large. In addition, the active layer is made a thin film of about 3 to 8 nm so that the strain amount to the GaInAsN light emitting layer is not increased too much. This GaInAsN is generally produced by MOCVD using dimethylhydrazine (DMH) as a nitrogen source. And in order to fully take in N and In in a GaInAsN active layer, it grows at the low growth temperature of 420 to 550 degreeC. By using such a manufacturing method and structure, a laser device using the GaAs substrate described above is intended to obtain an inexpensive device with a long wavelength.

In addition, a method of adding antimony (Sb) to a GaInAsN active layer after using a GaAs substrate has been attempted. This method is described in, for example, Japanese Patent No. 3209266.
Japanese Patent No. 3209266

However, the semiconductor laser device in which the GaInAsN light emitting layer is formed on the GaAs substrate has a problem that the light emission efficiency is low and the threshold value is high. This is because the GaInAsN light-emitting layer and the GaAs underlayer are grown at a low temperature of 420 ° C. to 550 ° C. In such a low temperature growth, a high-quality crystal is obtained from the viewpoint of crystal growth compared to the growth at a high temperature. Because it is difficult. In addition, GaInAsN-based materials are special material systems with extremely large strains, and it has been sufficiently elucidated experimentally and theoretically how to improve the luminous efficiency and threshold characteristics. Because there was not. A brief description is given below.

  In general, in the III-V compound semiconductor light emitting device, when the strain of the light emitting layer increases, the dislocation of the light emitting layer increases and the optical characteristics are likely to deteriorate. For example, it has been found that in many III-V compound semiconductors, it is difficult to make the film thickness larger than a predetermined value in a strained heterojunction system. This is because in a strained heterojunction system, the strain increases as the film thickness increases. The critical value of the amount of strain is explained by Mathews Blakeslee's theory that it is an amount of energy that is more advantageous when strain relaxation proceeds by plastic deformation than by elastic deformation. However, with GaInAsN-based materials, it is known that device characteristics such as light emission efficiency are not significantly deteriorated even if crystal growth is performed beyond the critical strain amount according to the above theory. As described above, the GaInAsN-based material is one of III-V group compound semiconductors, but the mechanism relating to the relationship between strain and device characteristics is not fully understood. Further, the structure of several atomic layers at the interface portion between the GaAs underlayer and the GaInAsN light-emitting layer has not been clarified, and it has been found that it is advantageous for the optical semiconductor element of this material system to relax the strain. Absent.

  Further, paying attention to the distortion of the light emitting layer, a method for reducing the distortion of the GaInAsN light emitting layer to solve the problem of deterioration of optical characteristics has been performed, but a sufficient effect has not been obtained.

  That is, in the GaInAsN-based material, since the base layer and the light emitting layer are grown at a low temperature as described above, step bunching (atomic size irregularities) is likely to occur in the base layer. And thermodynamically, it is easier to reduce the strain of the active layer if the surface of the GaAs layer serving as the underlying layer is uneven rather than flat. In addition, if the underlying layer is uneven, the GaInAsN crystal is easily phase-separated into an InAs rich region and a GaN rich region, which is advantageous in terms of energy. A method for reducing the distortion of the light emitting layer has been performed using these. Specifically, a GaInAsN layer having an In composition higher than the average of the GaInAs layer is formed in the recess of the base layer. As a result, GaInAs having a high In composition in the recess is easily elastically deformed. This is because the ductility is increased by increasing the In composition of GaInAsN in the recess, and the volume is small. And the distortion of the whole GaInAsN active layer is relieved by the elastic deformation of GaInAs in this recessed part. In addition, as a limit, a method of forming quantum dots in the recess is also performed. However, even if such a method for reducing the strain of the active layer is used, the luminous efficiency and threshold characteristics cannot be sufficiently improved.

  Further, a method of adding antimony to the light emitting layer as described in the above-mentioned Japanese Patent No. 3209266 has been proposed with a focus on planarization of the light emitting layer and improvement of crystallinity of the light emitting layer. This method is intended to improve the characteristics by flattening the light emitting layer and improving the crystallinity of the light emitting layer. However, even with this method, the luminous efficiency and threshold characteristics cannot be sufficiently improved.

  The present invention has been made on the basis of recognition of such a problem, and an object of the present invention is to achieve high luminous efficiency by using a GaAs substrate in an optical semiconductor element having a wavelength of 1.2 μm to 1.6 μm for optical communication. Another object of the present invention is to provide a semiconductor laser device and an optical semiconductor device having a low oscillation threshold.

According to one aspect of the invention, a GaAs substrate;
An n-type semiconductor layer formed on the GaAs substrate;
An active layer formed on the n-type semiconductor layer;
A p-type semiconductor layer formed on the active layer and containing Al, Ga, and As and doped with a p-type impurity;
With
The active layer is
An n-side guide layer formed on the n-type semiconductor layer, including Ga and As, and having a smaller band gap energy than the n-type semiconductor layer;
A GaAs layer formed on the n-side guide layer and doped with Se at a concentration of 2 × 10 ¹⁸ cm ^{−3 to} 2 × 10 ¹⁹ cm ⁻³ ;
Ga _1-x In _x As _1-y N _y (0 ≦ x ≦ 1, 0 <y) is formed on the GaAs layer to which Se is added in contact with the Se-added GaAs layer or at a predetermined interval. A well layer having a smaller band gap energy than the n-side guide layer and the Se-added GaAs layer,
A p-side guide layer formed on the well layer, including Ga and As, and having a larger band gap energy than the well layer;
A semiconductor laser device is provided.

According to one aspect of the present invention, a GaAs substrate;
An n-type semiconductor layer formed on the GaAs substrate, including Al, Ga, and As and doped with Si;
An active layer formed on the n-type semiconductor layer;
A p-type semiconductor layer formed on the active layer and containing Al, Ga, and As and doped with a p-type impurity;
With
The active layer is
An n-side guide layer formed on the n-type semiconductor layer, including Ga and As, and having a smaller band gap energy than the n-type semiconductor layer;
An Se layer formed on the n-side guide layer and made of GaAs to which Se having a concentration of 2 × 10 ¹⁸ cm ⁻³ or more and 2 × 10 ¹⁹ cm ⁻³ or less is added and has a thickness of 0.8 nm or more and 20 nm or less. A GaAs layer doped with
Ga _1-x In _x As _1-y N is formed on the GaAs layer to which Se is added in contact with the GaAs layer to which Se is added or at a distance of 3 nm or less from the GaAs layer to which Se is added. _y (0 ≦ x ≦ 1, 0 <y <1), the band gap energy is 1.01 eV or less, and the band gap energy is smaller than the n-side guide layer and the GaAs layer to which Se is added, A well layer having a thickness of 3 nm to 8 nm;
A p-side guide layer formed on the well layer, including Ga and As, and having a larger band gap energy than the well layer;
A semiconductor laser device is provided.

Further, according to one embodiment of the present invention, by MOCVD,
Forming an n-type semiconductor layer containing Al, Ga, and As and containing Si on a GaAs substrate;
On the n-type semiconductor layer, an n-side guide layer containing Ga and As and having a smaller band gap energy than the n-type semiconductor layer is formed.
The n-side guide layer is made of GaAs to which Se having a concentration of 2 × 10 ¹⁸ cm ⁻³ or more and 2 × 10 ¹⁹ cm ⁻³ or less is added, and Se is added with a thickness of 0.8 nm or more and 20 nm or less. Forming a GaAs layer,
Ga _1-x In _x As _1-y N is formed on the GaAs layer to which Se is added in contact with the GaAs layer to which Se is added or at a distance of 3 nm or less from the GaAs layer to which Se is added. _y (0 ≦ x ≦ 1, 0 <y <1), the band gap energy is 1.01 eV or less, and the band gap energy is smaller than the n-side guide layer and the GaAs layer to which Se is added, Forming a well layer having a thickness of 3 nm to 8 nm;
On the well layer, a p-side guide layer including Ga and As and having a larger band gap energy than the well layer is formed.
Provided is a method for manufacturing an optical semiconductor element, wherein a p-type semiconductor layer containing Al, Ga, and As and doped with a p-type impurity is formed on the p-side guide layer.

  According to the present invention, in a long-wavelength optical semiconductor device using a GaAs substrate and a GaInAsN well layer, since the GaAs layer to which Se is added is used as the underlying layer of the GaInAsN well layer, the light emission efficiency is high. An element having a low oscillation threshold can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. One of the features of the embodiment of the present invention is that, as shown in FIG. 1, for example, a GaAs layer to which Se is added is used as the base layer 5 of the GaInAsN well layer 6. Thereby, the surface of the underlayer 5 can be flattened, the crystallinity of the light emitting layer 6 can be improved, and the characteristics of light emission efficiency and oscillation threshold can be enhanced. In the following, first, the structure and manufacturing method of the main part will be described in the embodiments, and then specific element structures and manufacturing methods will be described in the first to third examples.

(Embodiment)
FIG. 1 is a sectional view of a semiconductor laser device according to an embodiment of the present invention. This semiconductor laser element is an element having a long wavelength (1.2 μm to 1.6 μm) for optical communication. On an n-type GaAs substrate 1 having a thickness of several hundred μm, a buffer layer 2 made of n-type GaAs, an n-type clad layer 3 made of AlGaAs having a thickness of 1.5 μm, and an n-side made of n-type GaAs having a thickness of 90 nm optical guide layer 4, the base layer having a thickness of 10nm which Se is made of the added n-type GaAs (GaAs layer Se is _{added) 5, Ga 1-x in} x As 1-y n y (0 ≦ x ≦ 1 , 0 <y <1) 3 to 8 nm thick light-emitting layer (well layer) 6, p-type GaAs 100 nm thick p-side light guide layer 7, AlGaAs 1.5 μm thick p-type A clad layer 8 and a p-type contact layer 9 made of GaAs and having a thickness of 0.1 μm are sequentially formed. Among these, the n-side light guide layer 4 to the p-side GaAs light guide layer 7 function as the active layer 10. In practice, the thickness of the substrate 1 is several hundred μm, and the thicknesses of the stacked bodies 2 to 9 are several μm. However, in FIG. 1, the scale is changed for easy understanding.

  In the element of FIG. 1, an n-side electrode (not shown) is formed on the lower side of the n-type GaAs substrate 1 in the drawing. A p-side electrode (not shown) is formed on the upper side of the p-type contact layer 9 in the figure. Then, light is emitted from the light emitting layer 6 by current injection from the p-side electrode and the n-side electrode. This light is amplified in the active layer 10. The amplified light is emitted as a laser beam in a direction perpendicular to the paper surface.

  The semiconductor layers 2 to 9 are grown by MOCVD. This MOCVD method is suitable for mass production because the crystal growth time is short, the price of the crystal growth apparatus is low, and the cost of the raw material is low compared with the MBE method or the like. In this crystal growth, first, the n-type cladding layer 3 is grown at a high temperature of 600 ° C. or higher in order to increase the crystallinity and lower the operating voltage. Next, the layers 4 to 7 of the active layer 10 are continuously grown at substantially the same temperature. This is because if the layers 4 to 7 are grown at intervals, the surfaces of the layers 4 to 7 are exposed to heat for a long period of time, and the crystals are likely to deteriorate. Here, the GaInAsN light-emitting layer 6 of the active layer 10 needs to be grown at a low temperature of 550 ° C. or lower so that N is taken in. For this reason, the growth temperature of the active layer 10 continuously grown is set to 550 ° C. or lower. Next, the p-type cladding layer 8 is grown at a high temperature of 600 ° C. or higher in order to increase the crystallinity and lower the operating voltage.

One of the features of the semiconductor laser device of FIG. 1 is that a Se-doped GaAs layer is formed as the underlying layer 5 of the GaInAsN light-emitting layer 6. Thereby, the surface of the foundation layer 5 becomes flat.
That is, as described above, the active layer 10 is formed at a low temperature of 550 ° C. or lower. However, such low temperature growth tends to cause surface roughness due to step bunching (atomic size irregularities). In particular, when Si, which is a general n-type impurity, is added to a GaAs-based material like the n-side light guide layer 4, surface roughness is likely to occur. Therefore, in the present embodiment, the base layer 5 is doped with an appropriate amount of Se, and the surface of the base layer 5 is planarized with an atomic size. Hereinafter, a description will be given with reference to FIG.

FIG. 2 shows the conditions of the growth temperature and the Se content necessary for the surface of the GaAs layer (underlayer) 5 to which Se is added to be flattened with an atomic size. The vertical axis represents Se concentration [cm ⁻³ ], and the horizontal axis represents growth temperature [° C.]. As can be easily seen from the figure, the vertical axis is a logarithmic scale, and the horizontal axis is a linear scale. The conditions in FIG. 2 are obtained by the inventor through simulations and experiments. In this simulation and experiment, the base layer 5 is formed on the n-side guide layer 4 by changing the temperature and the doping concentration of Se in the structure of FIG. The flatness of the surface of the underlayer 5 is observed with an atomic force microscope (AFM).

As shown in FIG. 2, the Se concentration that provides a planarization effect is 2 × 10 ¹⁸ cm ⁻³ or more and 2 × 10 ¹⁹ cm ⁻³ or less. However, when the growth temperature is 400 ° C. or lower, planarization becomes difficult even if Se is doped. Also, the higher the growth temperature, the wider the Se concentration margin. Thus, by setting the growth temperature and Se concentration of the underlayer 5 within the hatched range in FIG. 2, the flatness of the underlayer 5 can be enhanced. The reason why the flatness is high in the shaded area is mainly analyzed as follows.

(1) When Se is added at the time of crystal growth of GaAs constituting the underlayer 5, the step forward speed (lateral growth speed) is increased. Thereby, the ratio of the step forward speed to the growth speed (longitudinal growth speed) increases, and the flattening of the crystal surface proceeds. This flattening effect is prominent when an average of one or more Se is taken into each Ga site on each step during the growth of one atomic layer. For this reason, flatness becomes high by adding Se density | concentration more than fixed level.
(2) In order for crystal growth in the step flow mode (layer growth mode) to occur, the moving speed of surface adsorbed atoms needs to be faster than the growth speed. Here, the growth rate of the underlayer 5 must be higher than 0.15 μm / h in order to prevent In phase separation in the light emitting layer 6. On the other hand, the moving speed of the surface adsorbed atoms increases as the temperature is increased. For this reason, if the growth temperature is set to 400 ° C. or higher, a flattening effect can be obtained.
(3) When the temperature is increased, the moving speed of the surface adsorbed atoms is increased as described above, and flattening is facilitated. For this reason, the higher the temperature, the lower the Se concentration.
(4) On the other hand, if the concentration of Se is too high, the impurity concentration becomes too high and it is difficult to grow a good quality crystal. Specifically, when the Se concentration exceeds about 2 × 10 ¹⁹ cm ⁻³ , Se metal is deposited on the crystal surface, and the crystallinity of the GaAs crystal is lowered.

  In this way, by providing a GaAs layer to which Se of the concentration shown in FIG. 2 is added as the underlayer 5, the surface of the underlayer 5 is flattened with an atomic size.

  In the element of FIG. 1, the base layer 5 is planarized as described above. The present inventor has found that, in the element of FIG. 1, the planarization increases the luminous efficiency of the light emitting layer 6 and decreases the oscillation threshold current value. In order to elucidate the reason, the present inventor conducted detailed experiments. As a result, it was found that this reason is related to the growth process. This will be described below.

As a result of diligent research, the present inventor has found that DMH (dimethylhydrazine) as a growth raw material for the GaInAsN light-emitting layer 6 has a special etching effect. Specifically, the crystal growth rate decreased when DMH was allowed to flow through the GaAs crystal. Further, when DMZ was allowed to flow during the growth of GaInAs at a growth temperature of 520 ° C., the growth rate of GaInAs decreased by about 30%. From DMH, NH ₂ having an etching effect is supplied. Therefore, this NH _2, is believed etching effect occurs. Due to this etching effect, when the GaInAsN light emitting layer 6 is formed on the base layer 5 made of GaAs, the base layer 5 is etched. If surface roughness (step bunching) is present in the underlayer 5, the etching effect is particularly strong at the step portion, so that the surface roughness increases. That is, if the crystal surface of the underlayer 5 has irregularities, the irregularities become larger due to the etching of NH ₂ when the GaInAsN light emitting layer 6 is formed. Thus, when the large unevenness | corrugation arises in the base layer 5, the crystallinity of the light emitting layer 6 on it will deteriorate. On the contrary, if the underlayer 5 is flat, the influence of the etching effect is reduced. For this reason, the crystallinity of the GaInAsN light emitting layer 6 becomes high by planarizing the underlayer 5.

  In addition, the effect of preventing the surface roughness of the underlayer 5 from becoming too large is particularly great in this GaInAsN-based material. This is because in the GaInAsN-based material, the thickness of the light emitting layer 6 is as thin as 3 to 8 nm, so that when the surface roughness of the underlayer 5 increases, a large fluctuation in the band gap occurs. This will be described with reference to FIG.

FIG. 3 is a diagram showing the relationship (theoretical value) between the thickness and the band gap energy in the Ga _0.63 In _0.37 As _0.995 N _0.005 well layer. The horizontal axis indicates the thickness [nm] of the well layer, and the vertical axis indicates the band gap energy [eV] of the well layer. Here, there is a relationship of λ = 1.24 / E between the band gap energy E [eV] and the wavelength λ [μm], and when E = 0.9 [eV], λ = 1.38 [μm]. It is. In FIG. 3, the dotted line A indicates the band gap energy determined from the composition (calculation ignoring the quantum effect), and the solid line indicates the band gap energy to which the quantum effect is added. A dotted line B indicates an approximate straight line of the rate of change of the band gap energy. As can be seen from the solid line in FIG. 3, in the Ga _0.63 In _0.37 As _0.995 N _0.005 well layer, when the thickness is 15 nm or more, there is almost no quantum effect, and the band gap energy is about 0. 9 eV. However, as the thickness decreases, the band gap energy increases due to the quantum effect. For example, when the thickness is about 4 to 6 nm, as can be seen from the dotted line B, the band gap energy is reduced by about 12.5 eV (the band gap wavelength is about 100 nm) as the film thickness is reduced by 1 nm.

  As can be seen from FIG. 3 described above, the light emitting layer 6 of FIG. 1 has a thin film thickness of 3 to 8 nm, so that the change in band gap energy due to the change in thickness is large. For this reason, if the unevenness | corrugation of the surface of the base layer 5 becomes large, the unevenness | corrugation of the light emitting layer 6 will also become large and the fluctuation | variation of a band gap will become large. For example, if there is a roughness of 0.5 nm or more on the surface of the underlayer 5, as shown in FIG. 3, there may be a fluctuation of 10 meV or more in a band gap. From this, it can be seen that the effect that the surface roughness of the underlayer 5 does not become too large is particularly great in this GaInAsN-based material.

  Thus, in the element of FIG. 1, it is analyzed that the crystallinity of the light emitting layer 6 is increased and the light emission efficiency is increased by flattening the base layer 5. Also. It is analyzed that the oscillation threshold current value is reduced by reducing the band gap fluctuation of the light emitting layer 6.

  On the other hand, it has been conventionally considered that the presence of step bunching in the underlayer 5 is not necessarily bad. This is because it is considered that the crystallinity of the light emitting layer 6 is hardly deteriorated even if the base layer 5 has step bunching. Moreover, in order to relieve the distortion of the light emitting layer 6, it is preferable that the underlayer 5 has step bunching. However, as a result of the experiment, the inventor of the present invention has an etching action on DMH as a growth raw material of the light emitting layer 6 as described above, and the crystallinity of the base layer 5 and the light emitting layer 6 is greatly deteriorated by step bunching of the base layer 5. I knew what I was doing. Furthermore, according to the experiments by the present inventors, it was found that the underlayer 5 can be flattened by making the underlayer 5 GaAs doped with an appropriate amount of Se. As a result, the inventor has found that the crystallinity of the light emitting layer 6 can be greatly improved by providing the base layer 5 made of GaAs doped with Se.

  However, in the present embodiment, the distortion of the light emitting layer 6 increases due to the planarization of the base layer 5. Thus, when the distortion of the light emitting layer 6 increases, it is difficult to obtain high characteristics in other material systems. However, in the device of FIG. 1, high characteristics can be obtained despite the large strain of the light emitting layer 6. For this reason, the present invention believes that the GaInAsN-based material is less deteriorated in characteristics due to an increase in strain than other material systems.

  As described above, since the base layer 5 made of GaAs doped with Se is provided in the element of FIG. 1, the crystallinity of the InGaAsN light emitting layer 6 can be improved and the light emission efficiency can be increased. In addition, the fluctuation of the quantum effect of the InGaAsN light-emitting layer 6 can be reduced to reduce the oscillation threshold current value.

  In the element shown in FIG. 1, the dopant of the underlayer 5 is Se. This Se is an n-type impurity. For this reason, even if Se is doped into the underlayer 5, the electrical characteristics are not deteriorated.

  Next, the thickness of the underlayer 5 made of GaAs to which Se is added will be examined. That is, in the element of FIG. 1, although the thickness of the underlayer 5 is 10 nm, it can be manufactured with other thicknesses, so the range of the thickness will be examined.

  According to the experiments by the present inventors, when the thickness of the underlayer 5 is 0.8 μm or more and 20 nm or less, the effect of improving the light emission efficiency and the oscillation threshold characteristics as described above was obtained. The inventor considers the reason as follows.

  The underlayer 5 made of GaAs to which Se is added is provided for the purpose of flattening the surface by preventing surface roughness due to step bunching. For example, in the structure of FIG. 1, surface roughness due to step bunching exists on the surface of the n-side light guide layer 4. For this reason, the base layer 5 needs to have a thickness that can cover the surface roughness of the n-side light guide layer 4. This step bunching is usually about two atomic layers. For this reason, if the thickness of the underlayer 5 is 3 atomic layers or more, and the thickness is 0.8 nm or more, most of the step bunching is covered and the surface can be flattened. Thereby, when the thickness of the underlayer 5 is 0.8 nm or more, the effect of improving the characteristics of the light emission efficiency and the oscillation threshold value can be obtained.

  On the other hand, the GaAs layer 5 to which Se having the concentration shown in FIG. 2 is added has a larger light absorption coefficient than the undoped GaAs layer. For this reason, if the GaAs layer 5 to which Se is added is too thick, the internal loss coefficient increases. This will be described with reference to FIG.

  FIG. 4 is a graph showing a difference in internal absorption loss between a GaAs layer to which Se is not added and a GaAs layer to which Se is added. In the figure, the solid line shows the relationship between the thickness and the internal loss factor of the GaAs layer containing Se at the concentration shown in FIG. A dotted line A in the figure indicates the relationship between the thickness of the GaAs layer to which Se is not added and the internal absorption loss. A dotted line B indicates an approximate straight line of the rate of change in internal absorption loss. According to FIG. 4, it can be seen that when the thickness of the GaAs layer containing Se is set to 20 nm or more, the internal loss increases rapidly. From the viewpoint of the internal absorption loss, it is considered that the thickness of the GaAs layer 5 to which Se is added is preferably 20 nm or less in order to obtain the effect of improving the light emission efficiency and the oscillation threshold characteristics.

  Thus, the thickness of the GaAs layer 5 to which Se is added is preferably 0.8 μm or more and 20 nm or less in order to improve the light emission efficiency and the oscillation threshold characteristics.

In the device of FIG. 1 described above, according to the experiment of the present inventor, when hydrogen selenide (H ₂ Se) is used as a Se raw material during crystal growth of the underlayer 5 made of GaAs doped with Se, The effect of conversion can be further enhanced. This mechanism is not well understood. However, in the MBE (Molecular Beam Epitaxy) method, it is known that the presence of hydrogen radicals in the vicinity of the growing surface has a flattening effect on the crystal surface. As described above, GaInAsN-based crystal growth is characterized by low-temperature growth. From these, the present inventor believes that H ₂ Se, which has a high thermal decomposition efficiency at a low temperature, produces the same planarization effect by generating hydrogen radicals.

  In the element shown in FIG. 1, the GaInAsN light-emitting layer 6 is provided immediately above the base layer 5, but a thin film layer may be interposed between the base layer 5 and the light-emitting layer 6 as necessary. However, according to the experiments of the present inventors, in order to sufficiently obtain the above-described flattening effect, it is preferable that the thickness of the intervening thin film is 3 nm or less.

  In the element of FIG. 1, the case where the GaAs layer 5 to which Se is added is used for the semiconductor laser element has been described. However, this can also be used for other optical semiconductor elements such as an LED and a light receiving element. Here, in a light emitting element such as a semiconductor laser element or an LED, a well layer is a light emitting layer. In the light receiving element, the well layer serves as a light receiving layer.

(Example)
Next, a specific structure of the optical semiconductor element using the basic structure described in the above embodiment will be described. Hereinafter, a case where a semiconductor laser element is used as the optical semiconductor element will be described.

Example 1
FIG. 5 is a cross-sectional view showing the semiconductor laser device of Example 1 of the present invention. This semiconductor laser element is a ridge stripe type semiconductor laser element for optical communication. In FIG. 5, the ridge shape and electrodes are omitted, and the layer structure is shown. The semiconductor laser device of FIG. 5 has a structure in which semiconductor layers 12 to 23 are stacked on a GaAs substrate 11. The material, layer thickness, and impurity content of each layer are listed in order from the GaAs substrate 11 side as follows.
n-type GaAs substrate 11
n-type GaAs buffer layer 12 (thickness 0.5 μm)
n-type Al _0.55 Ga _0.45 As cladding layer 13
(Thickness 1.5 μm, Si addition concentration 1 × 10 ¹⁸ cm ⁻³ )
n-type GaAs optical guide layer 14
(Thickness 0.09 μm, Si addition concentration 5 × 10 ¹⁷ cm ⁻³ )
Underlayer (GaAs layer to which Se is added) 15
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Ga _0.63 In _0.37 As _0.995 N _0.005 well layer 16 (thickness 6 nm)
Barrier layer (GaAs layer added with Se) 17
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Ga _0.63 In _0.37 As _0.995 N _0.005 well layer 18 (thickness 6 nm)
Barrier layer (GaAs layer to which Se is added) 19
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Ga _0.63 In _0.37 As _0.995 N _0.005 well layer 20 (thickness 6 nm)
p-type GaAs light guide layer 21
(Thickness 0.1 μm, Zn addition concentration 1 × 10 ¹⁷ cm ⁻³ )
p-type Al _0.55 Ga _0.45 As cladding layer 22
(Thickness 1.5 μm, Zn addition concentration 1 × 10 ¹⁸ cm ⁻³ )
p-type GaAs contact layer 23
(Thickness 0.1 μm, Zn addition concentration 7 × 10 ¹⁸ cm ⁻³ )

  In the semiconductor laser device of FIG. 5, the n-type GaAs light guide layer 14 to the p-type GaAs light guide layer 21 act as the active layer 24. The active layer 24 has a multiple quantum well structure (MQW structure) in which well layers (well layers) 16, 18, and 20 and barrier layers (barrier layers) 17 and 19 are alternately stacked a plurality of times. .

  In the semiconductor laser device of FIG. 5, a GaAs layer to which Se is added is formed as the base layer 15 of the first well layer 16. The underlayer 15 is the same layer as the underlayer 5 described with reference to FIGS. By forming the base layer 15 made of GaAs to which Se is added in this way, the crystallinity of the first well layer 16 can be increased and the characteristics of the element can be improved.

  Further, in the semiconductor laser device of FIG. 5, a GaAs layer to which Se is added is formed as the first and second barrier layers 17 and 19 of the multiple quantum well structures 16 to 20. These barrier layers 17 and 19 are also layers similar to the underlayer 5 described with reference to FIGS. By using such a multiple quantum well structure, the crystallinity of the second and third well layers 18 and 20 can be increased and the characteristics can be further improved.

  That is, conventionally, in other material systems, a method of obtaining a laser with high characteristics by a multiple quantum well structure has been known. However, in the crystal growth of a GaInAsN-based material, as described above, it is common to grow at a low temperature of 480 to 550 ° C. under conditions of a high In composition and a large amount of strain. For this reason, in GaInAsN crystal growth, it is very difficult to control the composition and increase the crystal quality. As a result, even if multiple quantum wells are formed, the quality of the crystal is poor, so the more quantum well layers are stacked, the more difficult it is to control the composition and film thickness, and a semiconductor laser device with excellent characteristics cannot be obtained. There was a point. Therefore, when the GaAs layer added with Se of the concentration shown in FIG. 2 is applied to the barrier layers 17 and 19 as in this embodiment, the GaInAsN well layers 18 and 20 can be grown flat due to the above-described planarization effect. The crystal quality is improved. For this reason, a semiconductor laser device having excellent characteristics can be obtained.

  Next, a specific manufacturing method of this embodiment will be described.

(1) First, an n-type GaAs buffer layer 12 and an n-type AlGaAs cladding layer 13 are grown on an n-type GaAs substrate 11 at 670 ° C.
(2) Next, the growth temperature is lowered to 520 ° C., and the n-side GaAs optical guide layer 14 is grown. Thereafter, an underlayer 15 made of GaAs doped with Se at the same temperature is grown, and well layers 16, 18, 20 made of GaInAsN at the same temperature, a barrier layer 17 made of GaAs added with Se, 19, a p-type GaAs optical guide layer 21 is grown sequentially.
(3) Next, the temperature is raised to 640 ° C., and the p-side AlGaAs cladding layer 22 is grown.
(4) Next, the temperature is lowered to 550 ° C., and a GaAs contact layer 23 is grown.

  In the manufacturing method described above, there is no increase or decrease in the growth temperature around the GaInAsN well layers 16, 18, 20, so that the layers 17, 19, 21 thereon are grown immediately after the growth of the GaInAsN well layers 16, 18, 20. Can do. Thereby, deterioration of the well layers 16, 18, and 20 can be prevented, and crystallinity can be improved. On the other hand, when the time period from the growth of the GaInAsN well layers 16, 18, 20 to the growth of the upper layers 17, 19, 21 exceeds 1 minute, the GaInAsN well layers 16, 18, 20 are directly heated to high temperatures. The surface of the GaInAsN crystal is very rough.

  The growth rate of the active layer 24 is preferably 0.3 to 1.5 μm / h. If the growth rate is slower than this range, the luminous efficiency is lowered due to the phase separation of the GaInAsN crystal. Further, when the growth rate is high, the steepness at the heterojunction interface is impaired, and the quantization order broadening occurs. In this embodiment, the growth rate is 0.6 μm / h.

As raw materials for MOCVD, TMG (trimethyl gallium) as Ga raw material, TMA (trimethyl aluminum) as Al raw material, TMI (trimethyl indium) as In raw material, SiH ₄ (silane) as Si raw material, AsH ₃ as As raw material (Arsine) and DMH (dimethylhydrazine) can be used as the N raw material.

Here, as the group III (Ga, Al, In) organic metal, an ethyl group, a chloride group, or the like may be used. TBA (tertiary butyl arsine) may be used as the As raw material. As a raw material of N, it is preferable to use DMH. However, ammonia, hydrazine or monomethylhydrazine can be used in combination. It is preferable to use H ₂ Se (hydrogen selenide) as a Se raw material. However, an organometallic raw material may be used.

After the crystal growth shown in FIG. 5 is completed, a mesa structure or the like is formed, an n-side electrode is formed on the lower side of the n-type GaAs substrate 11 in the drawing, and a p-side electrode is formed on the upper side of the p-type contact layer 23 in the drawing. Further, an insulating film made of SiO ₂ is formed.

  The semiconductor laser device of this example obtained by the above manufacturing method is a ridge stripe type having a stripe width of 3 μm, an oscillation wavelength of about 1.3 μm, a threshold current value of about 5 mA, and a light emission efficiency of about 0.3 W / A at room temperature. Excellent characteristics can be obtained. On the other hand, in the comparative example in which the GaAs layers 15, 17 and 19 to which Se is added are not provided, the threshold current value is about 8 mA or more.

(Example 2)
One of the features of the semiconductor laser of Example 2 is that, as shown in FIG. 6, an undoped GaAs layer 30 is grown by 0.5 nm between a GaAs layer 29 to which Se is added and a GaInAsN light emitting layer 31. This is the point. Thereby, the diffusion of impurities (Si, Se) from the n-type light guide layer 28 or the GaAs layer 29 to which Se is added to the light emitting layer 31 can be prevented, and the oscillation threshold can be further lowered.

FIG. 6 is a cross-sectional view of a ridge stripe type semiconductor laser device for optical communication according to a second embodiment of the present invention. This semiconductor laser device has a structure in which semiconductor layers 26 to 34 are stacked on a GaAs substrate 25. The material, layer thickness, and impurity content of each layer are listed in order from the GaAs substrate 25 side as follows.
n-type GaAs substrate 25
n-type GaAs buffer layer 26 (thickness 0.5 μm)
n-type Al _0.55 Ga _0.45 As cladding layer 27
(Thickness 1.5 μm, Si addition concentration 1 × 10 ¹⁸ cm ⁻³ )
n-type GaAs optical guide layer 28
(Thickness 0.09 μm, Si addition concentration 5 × 10 ¹⁷ cm ⁻³ )
GaAs layer 29 doped with Se
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Undoped GaAs layer 30 (thickness 0.5 nm, undoped)
Ga _0.63 In _0.37 As _0.995 N _0.005 light emitting layer 31 (thickness 6 nm)
p-type GaAs light guide layer 32
(Thickness 0.1 μm, Zn addition concentration 1 × 10 ¹⁷ cm ⁻³ )
p-type Al _0.55 Ga _0.45 As cladding layer 33
(Thickness 1.5 μm, Zn addition concentration 1 × 10 ¹⁸ cm ⁻³ )
p-type GaAs contact layer 34
(Thickness 0.1 μm, Zn addition concentration 7 × 10 ¹⁸ cm ⁻³ )

  In the semiconductor laser device of FIG. 6, an undoped GaAs layer 30 is grown to a thickness of 0.5 nm between a GaAs layer 29 to which Se is added and a GaInAsN light emitting layer 31. Thereby, the diffusion of impurities into the light emitting layer 31 can be suppressed.

That is, as described above, GaInAsN is generally grown at a low temperature. For this reason, when the p-side AlGaAs cladding layer 33 is grown at a temperature higher than the growth temperature of the GaInAsN light emitting layer 31, impurities (Si) contained in the n-type light guide layer 28 are diffused into the GaInAsN light emitting layer 31. There is. In the element of FIG. 6, the Se concentration of the GaAs layer 29 to which Se is added is as high as 2 × 10 ¹⁸ cm ⁻³ or more and 5 × 10 ¹⁹ cm ⁻³ or less. For this reason, the impurity (Se) contained in the GaAs layer 29 to which Se is added may diffuse into the GaInAsN light emitting layer 31. In particular, when the Se concentration is set high or the growth temperature of the p-side AlGaAs cladding layer 33 is set high, Se easily diffuses into the GaInAsN well layer 31. When impurities (Se, Si) are diffused in the light emitting layer 31 as described above, the carrier concentration of the GaInAsN light emitting layer 31 is increased, and the light output characteristics of the semiconductor laser element may be deteriorated. Therefore, in this embodiment, an undoped GaAs layer 30 is grown to a thickness of 0.5 nm between the GaAs layer 29 to which Se is added and the GaInAsN light emitting layer 31. This suppresses the diffusion of impurities into the GaInAsN light emitting layer 31 in the process of growing at a high temperature.

  Thus, in the element of FIG. 6, diffusion of impurities to the light emitting layer 31 can be suppressed. Thereby, the controllability of the carrier concentration of the entire active layer 35 can be improved, and the deterioration of the light output characteristics can be suppressed. Specifically, the oscillation threshold value of the element of the comparative example in which the undoped GaAs layer 30 is not grown is 6 mA, whereas the oscillation threshold value of the element of this example can be 3 mA.

  In the element of FIG. 6 described above, the thickness of the undoped GaAs layer 30 is 0.5 nm, but this thickness can be changed. However, if the undoped GaAs layer 30 is too thick, the effect of planarization by the GaAs layer 29 to which Se is added is lost. In order to enhance the effect of planarization by the GaAs layer 29 to which Se is added, the thickness of the undoped GaAs layer 30 is set to 3 nm or less, preferably 1 nm or less.

(Example 3)
The semiconductor laser device of Example 3 is a surface emitting laser. FIG. 7 is a sectional view of a surface emitting optical communication semiconductor laser element according to Example 3 of the present invention. FIG. 8 is an enlarged view of the resonator 50 of this element. This semiconductor laser device has a structure in which semiconductor layers 37 to 49 are stacked on a GaAs substrate 36. The layers are listed in order from the GaAs substrate 36 side as follows.
n-type GaAs substrate 36
n-type GaAs buffer layer 37 (thickness 0.5 μm)
n-side DBR layer _{38 (GaAs / Al 0.7 Ga 0.3} As 35pair)
Selective oxidation current confinement layer 39
(Al _0.98 Ga _0.02 As / Al _x O _y thickness 6 nm)
n-type GaAs cladding layer 40
Underlayer (GaAs layer to which Se is added) 41
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Ga _0.63 In _0.37 As _0.995 N _0.005 well layer 42 (thickness 6 nm)
First barrier layer (undoped GaAs layer) 43
Second barrier layer (GaAs layer to which Se is added) 44
(Thickness 0.01 μm, Se addition concentration 6 × 10 ¹⁸ cm ⁻³ )
Ga _0.63 In _0.37 As _0.995 N _0.005 well layer 45 (thickness 6 nm)
p-type GaAs cladding layer 46
Selective oxidation current confinement layer 47
(Al _0.98 Ga _0.02 As / Al _x O _y thickness 6 nm)
p-side DBR layer 48 (Al _0.7 Ga _0.3 As / GaAs 24 pair)
GaAs contact layer 49
(Thickness 0.1 μm, Zn addition concentration 7 × 10 ¹⁸ cm ⁻³ )

In the element of FIG. 7, the design oscillation wavelength λ = 1.3 μm. The thickness d ₁ of the Al _0.7 Ga _0.3 As layer in the n-side DBR 38 is determined to be d ₁ n ₁ = λ / 4 = 0.325 μm, where n ₁ is the refractive index. Similarly, the thickness d ₂ of the GaAs layer in the n-side DBR 38 is determined to be d ₂ n ₂ = λ / 4 = 0.325 μm, where n ₂ is the refractive index. The same applies to the p-side DBR 48. By determining the film thickness in this way, light of wavelength λ is effectively reflected by the DBRs 38 and 48 on both sides.

  A resonator 50 is sandwiched between the DBRs 38 and 48 described above. The total thickness of the resonator 50 is designed so that the sum of the product of the refractive index and the thickness of each layer becomes a design oscillation wavelength of 1.3 μm. The resonator 50 has a multiple quantum well structure including first and second well layers 42 and 45 and barrier layers 43 and 44 therebetween. In this embodiment, the barrier layers 43 and 44 have a two-layer structure of a first barrier layer 43 made of undoped GaAs and a second barrier layer 44 made of GaAs doped with Se. ing. When the thickness of the barrier layer is 1 nm or more, the barrier layer can have a two-layer structure in this way. However, in order to obtain the above-described planarization effect of the GaAs layer to which Se is added, the thickness of the second barrier layer 44 is set to 0.8 nm or more.

Further, the selective oxidation current confinement layer 39 and 48 _after laminating the Al _{0.98 Ga 0.02} As layer, obtained by selective oxidation of the _Al 0.98 _{Ga 0.02} As layer with a water vapor atmosphere. The selective oxidation current confinement layers 39 and 48 squeeze and inject current into the resonator 50.

  7 and 8, the GaAs layer to which Se is added is used as the base layer 41 and the barrier layer 44. Therefore, the layers 41 and 44 are flattened, and the crystals of the well layers 42 and 45 are crystallized. Sexuality can be increased. Thereby, the characteristic of an element can be made high. As described above, even in the surface emitting semiconductor laser element, the characteristics can be improved by forming the GaAs layers 41 and 44 to which Se is added.

  Specifically, in the element shown in FIG. 7, characteristics with an oscillation wavelength of 1.3 μm and an oscillation threshold current value of 1.5 mA can be obtained in a current confinement structure in which the current confinement system is 5 μmφ. On the other hand, in the comparative example in which the GaAs layers 41 and 44 to which Se is added are not formed, the threshold current value is about 3.0 mA.

  In the first to third embodiments described above, the case where the GaAs layer to which Se is added is used for the semiconductor laser element has been described. However, the GaAs layer to which Se is added can also be used for other optical semiconductor elements such as LEDs and light receiving elements.

Sectional drawing of the semiconductor laser element of embodiment of this invention. The figure which shows Se content and the growth temperature in which the planarization effect of the GaAs layer to which Se was added occurs. In Ga _0.63 In _0.37 As _0.995 N _0.005 well layer is a diagram showing the thickness, and the band gap energy, the relationship (theoretical value). It is a graph which shows the difference in the internal absorption loss of the GaAs layer which does not add Se, and the GaAs layer which added Se. Sectional drawing of the semiconductor laser element of Example 1 of this invention. Sectional drawing of the semiconductor laser element of Example 2 of this invention. Sectional drawing of the semiconductor laser element of Example 3 of this invention. The enlarged view of the resonator 50 of the semiconductor laser element of Example 3 of this invention.

Explanation of symbols

1 n-type GaAs substrate 3 n-type cladding layer 5 underlayer (GaAs layer to which Se is added)
6 GaInAsN light emitting layer (well layer)
8 p-type cladding layer 11 n-type GaAs substrate 13 n-type cladding layer 15 underlayer (GaAs layer added with Se)
16 Ga _0.63 In _0.37 As _0.995 N _0.005 well layer (first well layer)
17 First barrier layer (GaAs layer added with Se)
18 Ga _0.63 In _0.37 As _0.995 N _0.005 well layer (second well layer)
19 Second barrier layer (GaAs layer added with Se)
20 Ga _0.63 In _0.37 As _0.995 N _0.005 well layer (third well layer)
22 p-type cladding layer 25 n-type GaAs substrate 27 n-type cladding layer 29 GaAs layer 30 doped with Se 30 undoped n-type GaAs layer 31 Ga _0.63 In _0.37 As _0.995 N _0.005 light emitting layer (well layer)
33 p-type cladding layer 36 n-type GaAs substrate 40 n-type cladding layer 41 Underlayer (GaAs layer to which Se is added)
42 Ga _0.63 In _0.37 As _0.995 N _0.005 well layer (first well layer)
43 First barrier layer (undoped GaAs layer)
44 Second barrier layer (GaAs layer doped with Se)
45 Ga _0.63 In _0.37 As _0.995 N _0.005 well layer (second well layer)
46 p-type cladding layer

Claims (6)

A GaAs substrate;
An n-type semiconductor layer formed on the GaAs substrate;
An active layer formed on the n-type semiconductor layer;
A p-type semiconductor layer formed on the active layer and containing Al, Ga, and As and doped with a p-type impurity;
Equipped with a,
The active layer is
An n-side guide layer formed on the n-type semiconductor layer, including Ga and As, and having a smaller band gap energy than the n-type semiconductor layer;
A GaAs layer formed on the n-side guide layer and doped with Se at a concentration of 2 × 10 ¹⁸ cm ^{−3 to} 2 × 10 ¹⁹ cm ⁻³ ;
Ga _1-x In _x As _1-y N _y (0 ≦ x ≦ 1, 0 <y) is formed on the GaAs layer to which Se is added in contact with the Se-added GaAs layer or at a predetermined interval. A well layer having a smaller band gap energy than the n-side guide layer and the Se-added GaAs layer,
A p-side guide layer formed on the well layer, including Ga and As, and having a larger band gap energy than the well layer;
The semiconductor laser element characterized by having a. The thickness of the GaAs layer to which Se is added is 0.8 nm or more and 20 nm or less,
2. The optical semiconductor device according to claim 1, wherein the thickness of the well layer is 3 nm or more and 8 nm or less, and the band gap energy of the well layer is 1.01 eV or less. The optical semiconductor device according to claim 1, wherein the distance between the GaAs layer in which the said well layer Se is added is equal to or is 3nm or less. A GaAs substrate;
An n-type semiconductor layer formed on the GaAs substrate and including Al, Ga, and As and doped with Si;
An active layer formed on the n-type semiconductor layer;
A p-type semiconductor layer formed on the active layer and containing Al, Ga, and As and doped with a p-type impurity;
With
The active layer is
An n-side guide layer formed on the n-type semiconductor layer, including Ga and As, and having a smaller band gap energy than the n-type semiconductor layer;
An Se layer formed on the n-side guide layer and made of GaAs to which Se having a concentration of 2 × 10 ¹⁸ cm ⁻³ or more and 2 × 10 ¹⁹ cm ⁻³ or less is added and has a thickness of 0.8 nm or more and 20 nm or less. A GaAs layer doped with
Ga _1-x In _x As _1-y N is formed on the GaAs layer to which Se is added in contact with the GaAs layer to which Se is added or at a distance of 3 nm or less from the GaAs layer to which Se is added. _y (0 ≦ x ≦ 1, 0 <y <1), the band gap energy is 1.01 eV or less, and the band gap energy is smaller than the n-side guide layer and the GaAs layer to which Se is added, A well layer having a thickness of 3 nm to 8 nm,
A p-side guide layer formed on the well layer , including Ga and As, and having a larger band gap energy than the well layer;
A semiconductor laser device comprising: By MOCVD method
Forming an n-type semiconductor layer containing Al, Ga, and As and containing Si on a GaAs substrate;
On the n-type semiconductor layer, an n-side guide layer containing Ga and As and having a smaller band gap energy than the n-type semiconductor layer is formed.
The n-side guide layer is made of GaAs to which Se having a concentration of 2 × 10 ¹⁸ cm ⁻³ or more and 2 × 10 ¹⁹ cm ⁻³ or less is added, and Se is added with a thickness of 0.8 nm or more and 20 nm or less. Forming a GaAs layer,
Ga _1-x In _x As _1-y N is formed on the GaAs layer to which Se is added in contact with the GaAs layer to which Se is added or at a distance of 3 nm or less from the GaAs layer to which Se is added. _y (0 ≦ x ≦ 1, 0 <y <1), the band gap energy is 1.01 eV or less, and the band gap energy is smaller than the n-side guide layer and the GaAs layer to which Se is added, Forming a well layer having a thickness of 3 nm to 8 nm;
On the well layer, a p-side guide layer including Ga and As and having a larger band gap energy than the well layer is formed.
A method of manufacturing an optical semiconductor element, comprising: forming a p-type semiconductor layer containing Al, Ga, and As and doped with a p-type impurity on the p-side guide layer . The growth temperature of the GaAs layer to which Se is added is 420 ° C. or higher and 550 ° C. or lower,
Method for manufacturing an optical semiconductor device according to claim 5, wherein the growth temperature of the well layer is 550 ° C. or less 420 ° C. or higher.

Images