JP2008211142A - Optical semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To execute distortion compensation, improve crystallinity and reduce heat resistance in a distortion quantum well structure on an InGaAs substrate. <P>SOLUTION: A distortion quantum well structure α is formed on an InGaAs substrate 1. Quantum well layers 4, 6, 8 of the quantum well structure α have compression distortion. In order to execute distortion compensation of the compression distortion, InGaAs/GaAs barrier layers 3, 5, 7, 9 including a GaAs layer as a tensile distortion are laminated and formed among the quantum well layers 4, 6, 8. In this way, misfit transition due to distortion and generation of defects can be relaxed. The heat resistance of GaAs having a binary crystal is reduced to suppress temperature rise. Meanwhile. AlAs, AlGaAs or InGaAs may be used instead of the GaAs as tensile distortion. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to an optical semiconductor device, which is devised so as to satisfactorily compensate for strain in a strained quantum well structure and improve crystallinity and thermal conductivity.
An optical semiconductor device is a device including a laser, an optical amplifier, an optical modulator, and the like.

  Conventionally, in an optical fiber communication with a light source wavelength of 1.3 μm to 1.55 μm, an InGaAsP laser on an InP substrate, which is easy to produce due to the band gap and the lattice constant, has been used. Usually, a strained quantum well structure is employed in the active layer in order to improve the oscillation characteristics. Generally, it is known that increasing the amount of distortion improves the laser characteristics because the differential gain is improved. However, too large strain leads to deterioration of crystallinity. Therefore, considering the lattice constant difference from the InP substrate, InGaAsP having a compressive strain of about 1% is used for the quantum well layer as a constituent material. In general, InGaAsP having a composition lattice-matched to the InP substrate is used for the layer.

  In such a conventional laser on an InP substrate, since the band offset between the quantum well layer on the conduction band side and the barrier layer is small, the optical gain is reduced due to the overflow of electrons under high temperature conditions. Increases and decreases efficiency. The characteristic temperature indicating the temperature dependence of the threshold current is as low as about 50K, and the use of a temperature regulator such as a Peltier cooler has been indispensable.

  Also, an AlGaInAs laser that is said to have a larger band offset than the InGaAsP laser on the same InP substrate has been developed, but its temperature characteristics are inferior to those of the short wavelength InGaAs laser on the GaAs substrate. Furthermore, there is a concern about reliability deterioration due to oxidation inherent to materials containing Al.

  On the GaAs substrate, 0.78 μm, 0.85 μm, 0.98 μm, and 1.06 μm band lasers having relatively short wavelengths have been put into practical use, and excellent temperature characteristics exceeding 150K are shown. This is due to a large band offset on the conduction band side. However, in order to obtain light emission at 1.3 μm by the InGaAs strained quantum well structure on the GaAs substrate, it is necessary to increase the In composition to about 50%. As the In composition increases, the lattice mismatch with the GaAs substrate increases, resulting in three-dimensional growth and misfit dislocations. For this reason, it is difficult to form a high-quality quantum well layer in a wavelength band of 1.3 μm or more.

  As a means for improving the problem of lattice mismatch and band offset, a strained quantum well structure on an InGaAs ternary substrate having a lattice constant larger than that of GaAs has been proposed. (K. Otsubo, et al., IEEE Photonics Technology Letter, Vol. 10, No. 8, pp.1073-1075, 1998.)

FIG. 15 shows the relationship between the strain of the InGaAs strained quantum well layer and the emission wavelength. The thickness of the quantum well layer is 10 nm. Here, the InGaAs barrier layer has a composition lattice-matched to the substrate.
The amount of strain in the quantum well layer for obtaining light emission of 1.3 μm is 3.2% on the GaAs substrate, 2.3% on the InGaAs substrate having the In composition of 0.1, and 1 on the InGaAs substrate having the In composition of 0.2. 8%.
In this structure, as the difference in In composition between the quantum well layer and the barrier layer increases, the band discontinuity increases, so that carrier overflow can be suppressed and temperature characteristics are improved. However, since the amount of strain in the quantum well layer increases, the crystallinity deteriorates. Therefore, a technique for improving the crystallinity of a high strain quantum well on a ternary substrate is required.

In addition, there is a problem of the thermal resistance of the element as having a large influence on the high-temperature operating characteristics of the semiconductor laser. When an InGaAs ternary substrate is used in a semiconductor laser structure, the physical property value is not proportional to the composition due to the mixed crystal of the crystal, and there is a nonlinear factor. Therefore, the physical property values of ternary and quaternary mixed crystals show a quadratic function behavior compared to binary. For example, InGaAs has a problem that the thermal resistance of the material is increased as compared with InAs and GaAs as a base material. FIG. 16 shows the In composition dependence of the thermal conductivity of InGaAs. According to this, it can be seen that In _0.3 Ga _0.7 As is about 8 times higher than the thermal conductivity 2.3 of GaAs. As the thermal resistance of the cladding layer and the active layer increases, the efficiency with which the heat generated in the active layer escapes to the heat sink decreases, leading to an increase in the temperature of the active layer.

  In particular, in the vertical cavity surface emitting laser type, the influence is greater than that in the edge emitting type laser due to the use of a mixed crystal in the semiconductor multilayer mirror and the structure in which current injection is concentrated in a small active region.

K. Otsubo, et al., IEEE Photonics Technology Letter, Vol.10, No.8, pp.1073-1075, 1998. Author: Peter S.Zory.Jr, book title "Quantum Well Lasers", publisher: Academic Press, publication date: May 19, 1993, p.379

  As described above, a strained quantum well laser on a ternary substrate is effective for improving the temperature characteristics of a semiconductor laser light source for communication, but in order to increase the oscillation wavelength to 1.3 μm or more, which is a communication wavelength band, It is necessary to increase the strain of the well layer. Therefore, crystallinity deterioration due to misfit dislocations has been a problem. In addition, an increase in thermal resistance when using a ternary crystal has also been a problem for improving high-temperature operating characteristics. The present invention has been made in view of the above points, and its object is to improve the crystallinity and reduce the thermal resistance of a high strain quantum well structure on an InGaAs ternary substrate where excellent temperature characteristics are expected. It is in.

The configuration of the optical semiconductor device of the present invention that solves the above problems is as follows.
In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strain quantum well structure comprises a compression strain quantum well layer and a barrier layer,
The barrier layer is a tensile strain barrier layer, wherein the tensile strain amount is greater than 0 and 1.5% or less.

The configuration of the optical semiconductor device of the present invention is as follows.
In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strained quantum well structure includes a compressive strained quantum well layer and a barrier layer containing GaAs, AlAs, or AlGaAs.

The configuration of the optical semiconductor device of the present invention is as follows.
In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strain quantum well structure includes a compressive strain quantum well layer and a barrier layer containing GaAs or AlAs.

The configuration of the optical semiconductor device of the present invention is as follows.
In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strain quantum well structure includes a compressive strain quantum well layer and a barrier layer containing In (y) Ga (1-y) As.

The configuration of the optical semiconductor device of the present invention is as follows.
The In composition ratio y of In (y) Ga (1-y) As contained in the barrier layer is in the range of 0 <y ≦ 0.05.

The configuration of the optical semiconductor device of the present invention is as follows.
The barrier layer has an In (z) Ga (1-z) As layer on the interface side with the compressive strain quantum well layer.

The configuration of the optical semiconductor device of the present invention is as follows.
The composition ratio x of the substrate made of semiconductor crystal In (x) Ga (1-x) As is in the range of 0 <x ≦ 0.2.

The configuration of the optical semiconductor device of the present invention is as follows.
The strained quantum well structure has an emission wavelength of 1.1 to 1.6 μm.

The configuration of the optical semiconductor device of the present invention is as follows.
The material of the compressive strain quantum well layer is any one of InGaAs, GaInNAs, AlGaInAs, and InGaAsP.

The configuration of the optical semiconductor device of the present invention is as follows.
The strained quantum well structure is processed in a mesa stripe shape, and both sides of the strained quantum well structure are embedded with a semiconductor crystal.

The configuration of the optical semiconductor device of the present invention is as follows.
The semiconductor crystal filling both sides of the strained quantum well structure is a Ru-doped semi-insulating semiconductor crystal.

The configuration of the optical semiconductor module of the present invention is as follows.
In the above optical semiconductor device,
The surface of the laminated structure side opposite to the substrate side is in contact with a heat sink.

  According to the present invention, it is possible to realize an optical semiconductor device that can perform good distortion correction and that has improved crystallinity and improved thermal conductivity.

The best mode for carrying out the present invention will be described below.
The present invention is based on a new idea of strain compensation, crystallinity improvement and thermal conductivity improvement by introducing binary GaAs or AlAs in a compressive strain quantum well formed on a ternary substrate made of InGaAs. It is a thing.

  A strain compensation structure is obtained by introducing a tensile strained GaAs or AlAs layer with a small lattice constant relative to the substrate into all or part of the barrier layer against the quantum well that is compressive strained against the substrate. , The occurrence of dislocation due to strain is alleviated. In addition, low strain growth is necessary for high strain quantum well growth, but even under such conditions, the phase separation that occurs in the binary crystal as seen in the ternary does not occur in principle, so that the crystallinity can be improved. Furthermore, by using a binary GaAs or AlAs layer instead of a ternary mixed crystal, the effect of improving the thermal conductivity in the vertical and in-plane directions of the substrate in the active layer where the temperature rises the most is also produced. Therefore, an increase in the temperature of the active layer of the laser can be suppressed, and a low-cost module that does not require a temperature regulator even in a high temperature environment can be realized.

Here, Embodiment 1 of the present invention will be described with reference to FIG.
Example 1 is a structure for realizing laser oscillation at a wavelength of 1.3 μm in an InGaAs laser structure on a ternary substrate as shown in FIG. The growth was performed using metal organic vapor phase epitaxy (MOVPE). The substrate 1 is an n-In _0.1 Ga _0.9 As substrate having an In composition of 0.1 cut out from a bulk crystal and polished. An n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on this substrate at a growth temperature of 700 ° C. and a growth pressure of 76 Torr, and an n-In _0.58 Ga _0.42 P cladding layer 2 doped with 5 × 10 ^{17 of} Si is further formed. Grown to a thickness of 1.5 μm. On top of that, strains in which InGaAs / GaAs strain compensation barrier layers 3, 5, 7, and 9, which are tensile strains, are arranged as active layers on both sides of the compressive strain quantum well layers 4, 6, and 8, as shown in FIG. A quantum well structure α was grown at a growth temperature of 550 ° C.

More specifically, as shown in FIG. 2, an In _0.1 Ga _0.9 As barrier layer having a thickness of 5 nm is disposed on both sides of the quantum well layers 4, 6, and 8 having a thickness of 10 nm, and a GaAs strain compensation barrier layer is further provided. Arrange. The GaAs strain compensation barrier layer has a thickness of 15 nm and is disposed between the In _0.1 Ga _0.9 As barrier layers. It is a three-layer quantum well active layer with these three periods. In _0.5 Ga _0.5 As, which is a composition capable of emitting 1.3 μm light, was used for the quantum well layer.

FIG. 3 shows the result of calculating the relationship between the In composition of the InGaAs substrate and the critical thickness of GaAs. In a multiple quantum well structure usually used for a semiconductor laser, a modulator, or the like, the thickness of the barrier layer is set to 10 nm or more in order to increase the gain for the mode of light. In this figure, since the In composition at which the critical thickness of GaAs is 15 nm is 0.2, the In composition of the InGaAs substrate must be 0.2.
The composition dependence of the band gap (Eg) of InGaAs is as follows.
In _x Ga _1-x As Eg = 1.424-1.62x + 0.56x ² (eV)
However, in the region with a large In composition, a change in band gap due to strain appears. Incorporating this effect, the band gap of In _0.5 Ga _0.5 As becomes 0.88 eV. (Non-patent document 2: Quantum Well Lasers, p.379)

Further, the band offset to band gap offset ratio: ΔEc / ΔEg of the conduction band was set to 0.65.
With GaAs / In _0.5 Ga _0.5 As, 354 meV, which is a large band offset compared to the InP substrate. This is also a large value compared to the conduction band offset of 252 meV of In _0.1 Ga _0.9 As / In _0.5 Ga _0.5 As quantum wells without strain compensation.

Here, the presence or absence of a binary GaAs structure was compared, and the distortion compensation effect was investigated. As for the structure, an InGaAs barrier layer having the same composition as that of a conventional substrate was compared with a structure in which a GaAs strain compensation barrier layer as shown in FIG. 2 was introduced.
FIG. 4 shows the photoluminescence measurement results of two types of quantum well structures with different barrier layer structures. Comparing two spectra with a peak at a wavelength of 1280 nm, the one with the GaAs strain compensation structure was about three times stronger than the one without. This is presumably because the strain compensation effect reduced the average strain amount of the three-layer quantum well and improved crystallinity.

Returning to FIG. 1 and continuing the explanation, a p-In _0.58 Ga _0.42 P cladding layer 10 doped with zinc 5 × 10 ¹⁷ (cm ⁻³ ) is grown on the strained quantum well structure α to a thickness of 1.5 μm, A 100 nm thick In _0.1 Ga _0.9 As contact layer 11 doped with 2 × 10 ¹⁹ (cm ⁻³ ) in a p-type is grown.

A SiO ₂ layer is deposited thereon by sputtering, and a striped mask having a width of about 2 μm is formed by photolithography. A mesa stripe having a width of 2 μm and a height of 1.6 μm is formed by dry etching and wet etching. Both sides were filled with polyimide, and after polishing the substrate, electrodes 12 and 13 were formed on the upper and lower sides and processed into a ridge laser.

The fabricated laser has an oscillation wavelength of 1.3 μm, a threshold current density of 400 A / cm ² , and an optical output of 20 mW at room temperature and 10 mW at 85 ° C. The characteristic temperature indicating the temperature dependence of the threshold current was 130K, indicating excellent temperature characteristics.
Further, as the barrier layer, all of the barrier layer may be made of only GaAs without using InGaAs. In short, it is sufficient that the barrier layer contains GaAs.
Here, the InGaAs layer thickness in the barrier layer may not be 5 nm, and the GaAs layer thickness may not be 15 nm. Further, the In composition of InGaAs in the barrier layer is preferably larger than 0 and less than the In composition of InGaAs in the quantum well layer. However, when the GaAs layer thickness is too thick, or when the In composition of the InGaAs layer is small and the layer thickness is too thick, etc., if the tensile strain of the barrier layer is too large, the crystal deteriorates.

Next, a second embodiment of the present invention will be described.
In the first embodiment, the edge emission type has been described, but an apparatus that employs a vertical cavity surface emitting laser structure will be described below. FIG. 5 shows vertical resonant surface light emission using a semiconductor multilayer reflector in which an InGaAs layer 14 which is a high refractive index material and an InAlAs layer 15 which is a low refractive index material are laminated on an InGaAs substrate 1 every quarter period of wavelength. The laser structure is shown. By keeping the In composition of the substrate as small as 0.1, the difference in refractive index between InGaAs / InAlAs approaches 0.5, which is the difference in refractive index between GaAs / AlAs, enabling the formation of high-quality reflectors with a reflectivity of 99% or more. It becomes.
In FIG. 5, the same reference numerals are given to the constituent members that perform the same functions as those in the embodiment shown in FIG. 1, and duplicate descriptions are omitted.

  FIG. 6 shows the results of thermal conduction calculation of the InGaAs substrate top emission laser structure. The calculation was performed assuming that the active layer was 4 μm square, the bias current was 8 mA, the voltage was 4 V, and the optical output was 3 mW. Similarly to Example 1, the temperature distribution in the vicinity of the active layer is shown in the case where a binary GaAs layer is introduced into a part of the barrier layer and in the case of a uniform InGaAs barrier layer having the same composition as the substrate. As a result, the introduction of binary GaAs with high thermal conductivity was estimated to reduce the active layer temperature by 10K. This result demonstrates the effectiveness of the binary GaAs strain compensation structure for the barrier layer of the ternary substrate top-emitting laser, and realizes a 1.3μm wavelength surface emitting laser operating up to 85 ° C.

(Example when AlAs is used for the barrier layer)
In Examples 1 and 2, GaAs is used as the material to be introduced into the strain compensation barrier layer, but binary AlAs can also be used. Since the lattice constants of GaAs and AlAs are close to 5.6533 mm and 5.6611 mm, respectively, the difference between the lattice constants is as small as 0.1%. Therefore, AlAs has the same distortion compensation effect as GaAs.

As shown in FIG. 7, an n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on an n-In _0.1 Ga _0.9 As substrate 1, and further, n-In doped with 5 × 10 ¹⁷ (cm ⁻³ ) of Si. _{A 0.1} Al _0.9 As cladding layer 2 ′ is grown to a thickness of 1.5 μm, and an active layer structure is grown thereon. The active layer has a strained quantum well structure α in which InGaAs / AlAs barrier layers 3 ′, 5 ′, 7 ′, and 9 ′ that cause tensile strain are arranged on both sides of the compressive strain quantum well layers 4, 6, and 8.

More specifically, as shown in FIG. 8, an AlAs strain compensation barrier layer is provided. The AlAs strain compensation barrier layer has a thickness of 10 nm. The In _0.18 Ga _0.82 As quantum well layer was 8 nm. The active layer α has four quantum well layers, and the emission wavelength is 0.98 μm.

A p-In _0.1 Al _0.9 As cladding layer 10 ′ doped with zinc 5 × 10 ¹⁷ (cm ⁻³ ) is grown on the strained quantum well structure α to a thickness of 1.5 μm, and a p-type layer 2 × 10 ¹⁹ (cm ^-3 ) A doped In _0.1 Ga _0.9 As contact layer 11 having a thickness of 100 nm is grown. By processing this grown wafer into a broad contact type having a width of 40 μm, a 0.98 μm wavelength laser was produced.

Since the band gap of AlAs is as large as 2.16 eV, the difference in band gap between the InGaAs quantum well and the barrier layer is larger than when GaAs is used for the barrier layer. At high temperatures, the distribution of electrons with a small effective mass to the high energy side increases, so that the characteristics of the laser deteriorate due to carrier overflow that overflows from the quantum well. Increasing the band offset ΔEc of the conduction band is effective for suppressing this and operating the laser even at high temperatures. In such a case, if an AlAs material containing Al is used for the barrier layer, in addition to the strain compensation effect similar to that of GaAs described in Example 1, the band gap is increased, so that carrier overflow is effectively suppressed. can do. In the GaAs / In _0.18 Ga _0.82 As quantum well, the band offset of the conduction band estimated to be 165 meV is 644 meV in the AlAs / In _0.18 Ga _0.82 As quantum well, enabling stable laser operation even at high temperatures. Become. The fabricated laser has an oscillation wavelength of 0.98 μm, a threshold current density of 500 A / cm ² , and an optical output of 50 mW at room temperature and 30 mW at 85 ° C. The characteristic temperature indicating the temperature dependence of the threshold current was 150K, indicating excellent temperature characteristics.

(Example when AlGaAs is used for the barrier layer)
In Examples 1 and 3, the case where GaAs and AlAs are used for the barrier layer is shown, but AlGaAs as a mixed crystal thereof has the same strain compensation effect as GaAs and AlAs.
Further, since the band gap is larger than that of GaAs, carrier overflow can be effectively suppressed.
The composition dependence of the band gap of AlGaAs is as follows.
Al _x Ga _1-x As Eg = 1.424 + 1.247x (eV) (0 <x <0.45)
Therefore, the band offset of the conduction band is estimated to be 403 meV in the Al _0.2 Ga _0.8 As / In _0.3 Ga _0.7 As quantum well. This was confirmed to be a large value compared with 142 meV of the conduction band offset of In _0.1 Ga _0.9 As / In _0.3 Ga _0.7 As quantum wells without strain compensation.

As an example of using AlGaAs as a strain compensation layer, as shown in FIG. 9, an n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on an n-In _0.1 Ga _0.9 As substrate 1, and Si is further added. A 5 × 10 ¹⁷ (cm ⁻³ ) doped n-In _0.58 Ga _0.42 P cladding layer 2 is grown to a thickness of 1 μm, and an active layer structure is grown thereon. In the active layer, as shown in FIG. 9, AlGaAs barrier layers 3 ″, 5 ″, 7 ″, and 9 ″ that cause tensile strain are arranged on both sides of the compression strain quantum well layers 4, 6, and 8. It is a strained quantum well structure α.

More specifically, as shown in FIG. 10, an Al _0.2 Ga _0.8 As strain compensation layer is provided. The Al _0.2 Ga _0.8 As strain compensation layer has a thickness of 20 nm. The In _0.5 Ga _0.5 As quantum well layer was 8 nm. The active layer has three quantum well layers. It is a three-layer quantum well active layer with these three periods. For the quantum well layer, In _0.3 Ga _0.7 As having a composition capable of emitting 1.1 μm was used.

A p-In _0.58 Ga _0.42 P cladding layer 10 doped with zinc 5 × 10 ¹⁷ (cm ⁻³ ) is grown on the strained quantum well structure α to a thickness of 1 μm, and a p-type layer 2 × 10 ¹⁹ (cm ⁻³⁾ is formed thereon. ) A doped In _0.1 Ga _0.9 As contact layer 11 having a thickness of 100 nm is grown. SiO2 was deposited by sputtering, a striped mask was formed by photolithography, and a mesa stripe having a width of 2 μm and a height of 1 μm was formed by using RIE (reactive ion etching) as a mask. Subsequently, a Ru-doped InGaP layer was grown as a current blocking layer on the substrates on both sides of the mesa stripe by a MOVPE method to a thickness of 2 μm. Bisethylcyclopentadienyl ruthenium (II) was used as a raw material for Ru. Further, the mask made of SiO2 was removed by HF, and a p-In _0.58 Ga _0.42 P overclad layer 10 having a layer thickness of 2 μm and a Zn doping concentration of 5 × 10 ¹⁷ (cm ⁻³ ) was grown. A 100 nm thick In _0.1 Ga _0.9 As contact layer 11 doped with 2 × 10 ¹⁹ (cm ⁻³ ) p-type was grown thereon.

The fabricated laser has an oscillation wavelength of 1.1 μm, a threshold current density of 500 A / cm ² , and an optical output of 20 mW at room temperature and 15 mW at 85 ° C. The characteristic temperature indicating the temperature dependence of the threshold current was 135K, indicating excellent temperature characteristics.

(Example when InGaAs is used for the barrier layer)
In the first embodiment, the case where GaAs is used for the barrier layer is shown. However, even in InGaAs into which a small amount of indium is introduced, if the In composition of the barrier layer is less than the In composition of the substrate, it has an effect as a strain compensation structure. .
In addition, there is a problem of phase separation, which is often a problem for ternary crystals when crystal growth is performed at low temperatures. However, if InGaAs is less than 0.1, it is almost unaffected by the miscibility gap, so it is binary. As in GaAs, stable crystal growth is possible. In addition, the addition of a small amount of In to GaAs has the effect of suppressing the incorporation of dislocations. Also, the smaller the In composition of the barrier layer than the In composition of the substrate, the larger the band gap of InGaAs, so that the band offset of the conduction band increases and the effect of confining electrons in the conduction band of the quantum well increases. Similar to the first, third, and fourth embodiments, this effect suppresses carrier overflow and is effective in improving the temperature characteristics of the laser. For example, In _0.02 Ga using InGaAs with an In composition of 0.02 as a barrier layer in a quantum well on an InGaAs substrate with an In composition of 0.1, and using InGaAs with an In composition of 0.5 that realizes light emission with a wavelength of 1.3 μm in the quantum well. _{In a 0.98} As / In _0.5 Ga _0.5 As quantum well, the band offset of the conduction band is 333 meV. This is a large value compared to the conduction band offset of 252 meV in In _0.1 Ga _0.9 As / In _0.5 Ga _0.5 As quantum wells using a lattice-matched barrier layer. Here, an In composition of 0.02 was used. If the In composition is 0.05 or less, the band offset is sufficiently large and effective.

A structural diagram of this embodiment is shown in FIG. An n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on the n-In _0.1 Ga _0.9 As substrate 1 at a growth temperature of 700 ° C. and a growth pressure of 76 Torr, and further, Si is doped with 5 × 10 ¹⁷ n-In _0.58 Ga. _{A 0.42} P clad layer 2 was grown to a thickness of 1.5 μm. A strained quantum well structure α with an InGaAs strain compensation barrier layer as an active layer was grown at a growth temperature of 550 ° C. In _0.02 Ga _0.98 As strain compensation barrier layers 3 ″ ″, 5 ″ ″, 7 ″ ″, 9 ″ ″ are arranged on both sides of the 10 nm thick quantum well layers 4, 6, and 8. In _0.02 Ga _0.98 As strain compensation barrier layers 3 "", 5 "", 7 "", 9 "" are each 15 nm in thickness. It is a three-layer quantum well active layer with these three periods. In _0.5 Ga _0.5 As, which is a composition capable of emitting 1.3 μm light, was used for the quantum well layer.

A 40 nm-thick non-doped InGaAsP optical confinement layer is grown on the strained quantum well structure, a diffraction grating is formed, and p-InGaP doped with 1 × 10 ¹⁸ (cm ⁻³ ) zinc is grown thereon. Here, SiO2 was deposited by sputtering, a stripe-shaped mask was formed by photolithography, and a mesa stripe having a width of 2 μm and a height of about 1.5 μm was formed by RIE (reactive ion etching) using this as a mask. Subsequently, on the substrates on both sides of the mesa stripe, a Ru-doped InGaP layer was grown to a thickness of 3 μm as a current blocking layer by the MOVPE method. Bisethylcyclopentadienyl ruthenium (II) was used as a raw material for Ru. Further, the mask made of SiO2 was removed by HF, and a p-In _0.58 Ga _0.42 P overclad layer 10 having a layer thickness of 2 μm and a Zn doping concentration of 5 × 10 ¹⁷ (cm ⁻³ ) was grown. A 100 nm thick In _0.1 Ga _0.9 As contact layer 11 doped with 2 × 10 ¹⁹ (cm ⁻³ ) p-type was grown thereon.

The laser produced has an oscillation wavelength of 1.3 μm, a threshold current density of 300 A / cm ² , and an optical output of 20 mW at room temperature and 15 mW at 85 ° C. The characteristic temperature indicating the temperature dependence of the threshold current was 140K, indicating excellent temperature characteristics.

(Application example to EA modulator)
This distortion compensation structure can be applied to an electroabsorption modulator. If a quantum well structure is used for the electroabsorption modulator, the quantum confined Stark effect can be used. This effect is that when an electric field is applied perpendicularly to the quantum well, the light absorption peak due to excitons in which electrons and holes are paired changes, and a large change in absorption coefficient occurs even at a small applied voltage.

As shown in FIG. 12, an electroabsorption optical modulator was fabricated on the InGaAs substrate 21. An n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on the n-In _0.1 Ga _0.9 As substrate 21, and further an n-In _0.58 Ga _0.42 P cladding layer 22 doped with Si 5 × 10 ¹⁷ (cm ⁻³ ). Is grown to a thickness of 1.5 μm, and a light absorption layer structure is grown thereon. The active layer has a strained quantum well structure (quantum well layer) 23 in which an AlAs barrier layer that becomes tensile strain is disposed on both sides of the InGaAs compressive strain quantum well layer. The AlAs strain compensation layer has a thickness of 10 nm. This is a 6-layer quantum well light absorption layer with 6 periods. The quantum well layer 23 is made of In _0.4 Ga _0.6 As, which has a gain wavelength with an optimum detuning amount for controlling the absorption coefficient of light having a wavelength of 1.3 μm. The thickness of the quantum well is 10 nm, and a p-In _0.58 Ga _0.42 P cladding layer 24 doped with zinc 5 × 10 ¹⁷ (cm ⁻³ ) is grown on the strained quantum well structure to a thickness of 1.5 μm, A 100 nm thick In _0.1 Ga _0.9 As contact layer 25 doped with 2 × 10 ¹⁹ (cm ⁻³ ) in a p-type is grown. The grown wafer was processed into a ridge type to produce an electroabsorption modulator that controls light having a wavelength of 1.3 μm.
In FIG. 12, 26 is a polyimide buried layer, 27 is an n-electrode, and 28 is a p-electrode.

In the electroabsorption modulator using InGaAsP, since the band offset between the conduction band and the valence band is 4: 6, the band offset of the conduction band is small, and sufficient carrier confinement cannot be ensured. In such an environment, a sufficient extinction ratio cannot be obtained, resulting in a problem of increasing the applied voltage during modulation. In the modulator of this embodiment, the band offset ratio between the conduction band and the valence band is 6.5: 3.5, and the band gap difference between AlAs and InGaAs is large. The band offset of the conduction band is as large as 730 meV. Become. As a result, sufficient carrier confinement can be secured, and a modulator that is less susceptible to the environmental temperature and a drive voltage reduction has been realized. In this example, an electroabsorption modulator for a wavelength of 1.3 μm was shown, but an electroabsorption modulator for a wavelength of 1.55 μm was fabricated by using an In _0.3 Ga _0.7 As substrate with an increased In composition of the substrate. Is also possible.

(Example of application to EA-DFB)
In the sixth embodiment, an example of an electroabsorption optical modulator has been described. However, the present invention can also be applied to an EA-DFB laser in which this modulator and a distributed feedback semiconductor laser (DFB-LD) are monolithically integrated.

  FIG. 13 shows an EA-DFB laser in which an electroabsorption (EA) optical modulator 20 and a distributed feedback semiconductor laser (DFB-LD) are monolithically integrated.

First, the configuration of the distributed feedback semiconductor laser part is that an n-In _0.1 Ga _0.9 As buffer layer doped with Si is grown on an n-In _0.1 Ga _0.9 As substrate 21 and further Si is doped 5 × 10 ¹⁷ (cm ⁻³ ). An n-In _0.58 Ga _0.42 P clad layer 32 is grown to a thickness of 1.5 μm, and a 40 nm thick non-doped InGaAsP guide layer 31 is introduced thereon. An active layer structure is grown thereon. The active layer has a strained quantum well structure (quantum well layer) 33 in which GaAs barrier layers that become tensile strain are arranged on both sides of the InGaAs compressive strain quantum well layer. The GaAs strain compensation layer has a thickness of 15 nm. It is a three-layer quantum well active layer with these three periods. The quantum well layer 33 was made of In _0.5 Ga _0.5 As for oscillation at a wavelength of 1.3 μm. The thickness of the quantum well layer was 10 nm. A 40 nm-thick non-doped InGaAsP guide layer 39 is grown thereon to form a diffraction grating, and an InGaP cladding layer 34 is buried thereon.
Incidentally, 35 is an In _0.1 Ga _0.9 As contact layer, and 38 is a p-electrode.

  A butt joint technique is used to couple the semiconductor laser 30 and a later-described EA optical modulator 20. A mask is attached on the laser structure growth layer, and an island shape having a quantum well active layer only in a region having a width of 15 μm and a length of 400 μm is formed by dry etching and wet etching. By growing the modulator structure in that state, the modulator structure is grown and integrated around the mesa portion of the laser.

The configuration of the electroabsorption optical modulator 20 is a strained quantum well structure (quantum well layer) 23 in which an Al _0.8 Ga _0.2 As barrier layer that becomes tensile strain is arranged on both sides of an InGaAs compressive strain quantum well layer. The Al _0.8 Ga _0.2 As strain compensation layer has a thickness of 10 nm. This is a 6-layer quantum well light absorption layer with 6 periods.

The quantum well layer was made of In _0.4 Ga _0.6 As, which has a gain wavelength with an optimum detuning amount for controlling the absorption coefficient of light having a wavelength of 1.3 μm, and its thickness was 10 nm. After the butt joint growth, the mask was removed, and a p-In _0.58 Ga _0.42 P cladding layer 24 doped with 5 × 10 ¹⁷ (cm ⁻³ ) zinc was grown to a thickness of 1.5 μm, and a p-type 2 × 10 ¹⁹ was formed thereon. An In _0.1 Ga _0.9 As contact layer 25 having a thickness of 100 nm and doped with (cm ⁻³ ) is grown. A mask having a width of about 2 μm is formed thereon, and a mesa stripe having a width of about 2 μm and a height of about 3 μm is formed by dry etching and wet etching. Both sides are filled with polyimide, and electrodes 27 and 28 are formed on the upper and lower sides after the substrate is polished. In addition, the InGaAs contact layer between the laser part and the modulator part is removed for electrical insulation.

  The EA-DFB laser has a threshold current of 20 mA, a characteristic temperature of 100 K, an extinction ratio of 30 dB or more, and the laser threshold current does not fluctuate even at high temperatures, realizing stable operation.

(Face-down mounting semiconductor laser)
In this embodiment, a module on which a semiconductor laser is mounted will be described. In this example, as shown in FIG. 14, the semiconductor laser produced in Example 1 was die-bonded on the heat sink 40 by junction down with the growth surface down. As a result, when a ternary substrate having a lower heat dissipation than a binary GaAs substrate or InP substrate is used, heat can be efficiently radiated from the growth surface side to the heat sink side. In the junction-down type laser mounted in this way, the maximum oscillation temperature increased by 20 ° C compared to the conventional junction-up type laser.

  In all the embodiments described above, the following configurations can be adopted.

  First, in addition to the above InGaAs, GaInNAs, InGaAsP, InGaAlAs, and the like can be used for the electroabsorption optical modulator and the laser quantum well.

Also, in the barrier layer, when GaAs is used as the barrier layer when the In composition of the substrate InGaAs is 0.1, the strain amount of the tensile strain of GaAs is about 0.7%, and the In composition of the substrate InGaAs is 0.2. When GaAs is sometimes used as the barrier layer, the strain amount of tensile strain of GaAs is about 1.5%, and the barrier layer is effective when the strain amount is 1.5% or less.
In addition to the above, the material of the barrier layer may be InGaAlAs, GaNAs, GaInNAs, GaAsP, or the like.

  In addition to InGaP, InAlAs, InGaAlAs, InAlP, InGaAlP, or the like having a large band gap can be used as a material for the cladding layer.

The composition ratio x of the substrate 1 made of the semiconductor crystal In _x Ga _1-x As may be in the range of 0 <x ≦ 0.2.

  Similarly to a normal semiconductor laser structure, by introducing an AlGaInAs layer or InGaAsP layer of about 100 nm as an optical confinement layer between the cladding layer and the quantum well structure, the optical amplification is performed more efficiently and the threshold value is reduced. It becomes possible.

  In Embodiment 6, the insulating polyimide is used for embedding, but it is also possible to embed with BCB or a semi-insulating semiconductor such as InGaP or InAlAs doped with Fe or Ru.

  In addition, when the emission wavelength of the strained quantum well structure is 1.1 to 1.6 μm, each example can be favorably applied.

It is sectional drawing which shows the principal part of Example 1 concerning this invention. It is a figure which shows the In composition change of the quantum well active layer corresponding to FIG. It is a characteristic view which shows the relationship between the wavelength and strain of a strained quantum well on an InGaAs substrate. It is a comparison figure of the presence or absence of distortion compensation of the photoluminescence spectrum of the InGaAs quantum well on the InGaAs substrate of In composition 0.1. 4 is a structural diagram showing a vertical cavity surface emitting laser on an InGaAs substrate of Example 2. FIG. It is comparison explanatory drawing of the difference by the barrier structure of the temperature distribution around an active layer. It is sectional drawing which shows the principal part of Example 3 concerning this invention. It is a figure which shows In composition change of the quantum well active layer corresponding to FIG. It is sectional drawing which shows the principal part of Example 4 concerning this invention. It is a figure which shows In composition change of the quantum well active layer corresponding to FIG. It is sectional drawing which shows the principal part of Example 5 concerning this invention. It is a block diagram which shows the EA modulator which concerns on Example 6 of this invention. It is a block diagram which shows the EA-DFB laser which concerns on Example 7 of this invention. It is a block diagram which shows the face-down mounting semiconductor laser which concerns on Example 8 of this invention. It is explanatory drawing which shows the relationship between the wavelength of a strain quantum well on an InGaAs substrate, and distortion. It is a characteristic view which shows In composition dependence of the thermal resistance of an InGaAs ternary mixed crystal.

Explanation of symbols

1 n-InGaAs substrate 2 n-InGaP cladding layer 3, 5, 7, 9 InGaAs / GaAs barrier layer 4, 6, 8 InGaAs quantum well layer
10 p-InGaP cladding layer 11 contact layer 12 p electrode 13 n electrode 14 InGaAs layer 15 InAlAs layer 20 EA optical modulator 21 InGaAs substrate 22, 32 n-InGaP cladding layer 23, 33 InGaAs / AlAs quantum well layers 24, 34 p -InGaP cladding layer 25, 35 p-InGaAs contact layer 26 polyimide buried layer 27 n electrode 28, 38 p electrode 30 distributed feedback semiconductor laser 31 InGaAsP guide layer 40 heat sink α strained quantum well structure

Claims (12)

In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strain quantum well structure comprises a compression strain quantum well layer and a barrier layer,
The optical semiconductor device, wherein the barrier layer is a tensile strain barrier layer, and the amount of tensile strain is greater than 0 and 1.5% or less. In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strained quantum well structure comprises a compressive strained quantum well layer and a barrier layer containing GaAs, AlAs, or AlGaAs. In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The strained quantum well structure includes a compressive strained quantum well layer and a barrier layer containing GaAs or AlAs. In a strained quantum well structure formed as an active layer on a substrate composed of a ternary mixed crystal semiconductor crystal In (x) Ga (1-x) As,
The optical semiconductor device, wherein the strain quantum well structure comprises a compressive strain quantum well layer and a barrier layer containing In (y) Ga (1-y) As.   5. The optical semiconductor device according to claim 4, wherein the In composition ratio y of In (y) Ga (1-y) As contained in the barrier layer is in the range of 0 <y ≦ 0.05.   6. The optical semiconductor according to claim 1, wherein the barrier layer has an In (z) Ga (1-z) As layer on the interface side with the compressive strain quantum well layer. apparatus.   7. The composition ratio x of the substrate made of semiconductor crystal In (x) Ga (1-x) As is in a range of 0 <x ≦ 0.2. Item optical semiconductor device.   The optical semiconductor device according to claim 1, wherein an emission wavelength of the strained quantum well structure is 1.1 to 1.6 μm.   9. The optical semiconductor device according to claim 1, wherein the material of the compressive strain quantum well layer is any one of InGaAs, GaInNAs, AlGaInAs, and InGaAsP.   10. The optical semiconductor device according to claim 1, wherein the strained quantum well structure is processed in a mesa stripe shape, and both sides of the strained quantum well structure are filled with a semiconductor crystal.   11. The optical semiconductor device according to claim 1, wherein the semiconductor crystal filling both sides of the strain quantum well structure is a Ru-doped semi-insulating semiconductor crystal. The optical semiconductor device according to any one of claims 1 to 11,
An optical semiconductor module, wherein a surface of the laminated structure side opposite to the substrate side is in contact with a heat sink.

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