JP2000068597A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a reliable semiconductor laser element by preventing degradation of an element caused by the fact that, in a semiconductor laser, Zn which is an impurity element diffuses as chose to as an active layer from a high-concentration p-type InGaAs contact layer or a high-concentration p-type Inp substrate. SOLUTION: A supperlattice intermediate layer 17 is formed at a high- concentration p-type InGaAs contact layer 18 or between a high-concentration p-type InP substrate and a p-type Inp clad layer 16. The super lattice intermediate layer 17 comprises a super lattice of InAs and GaAs or InAs and GaP, while either of two kinds of semiconductor layer forming it is modulatedly doped with p-type impurity element. Thus, diffusion Zn from a p-type contact layer or p-type substrate to the proximity of an active layer is suppressed, allowing manufacture of reliable semiconductor laser element with less degradation of element characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device,
In particular, the present invention relates to a semiconductor laser device made of a III-V compound semiconductor used for an optical transmission device or the like.

[0002]

2. Description of the Related Art A semiconductor laser used as a light source in an optical transmission device, which plays a central role in high-capacity, high-speed communication, is mainly composed of an InP-based III-V compound semiconductor. The semiconductor laser composed of the InP-based semiconductor includes a first conduction type cladding layer formed on a first conduction type semiconductor substrate, an active layer for generating laser light formed thereon, and a first conduction type cladding layer formed thereon. And a cladding layer of the second conductivity type formed on the substrate. Electrodes are formed so as to be in contact with the semiconductor substrate of the first conductivity type and the cladding layer of the second conductivity type, and electrons and holes are injected into the active layer via the electrodes. Therefore, it is necessary to reduce the contact resistance between the first conductive type semiconductor substrate and the second conductive type semiconductor layer and the respective electrodes and the element resistance, and to reduce the operating voltage of the element and increase the high temperature. This is important for achieving operation and extending the life. For this reason, a p-type or n-type substrate having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more is usually used as the first conductivity type semiconductor substrate. Further, a contact layer having a carrier concentration of the second conductivity type of 1 × 10 ¹⁸ cm ⁻³ or more is formed between the electrode and the second conductivity type clad layer.

For example, in an InP-based semiconductor laser formed on an n-type InP substrate and operating at a wavelength in the 1.3 to 1.6 μm band, an n-type InP substrate having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more is used. Is used. In addition, as shown in p. 302 of the 1994 IEICE Autumn Meeting, a high-concentration p-type InGaAs crystal is formed as a p-type contact layer between a p-type InP cladding layer crystal and a p-electrode. Have been reported. This InGaAs crystal is lattice-matched to the InP substrate when the In composition is 0.53 and the Ga composition is 0.47,
Since it is possible to dope a p-type at a high concentration of × 10 ¹⁹ cm ⁻³ or more, it is possible to reduce the contact resistance with the p-electrode.

Further, in a heterojunction of a p-type InGaAs contact layer and a p-type InP cladding layer, the band gap difference is large. Therefore, in order to reduce device resistance, the band gap between them is an intermediate value between the two. May be inserted as an intermediate layer. FIG. 5 shows an example of a laser structure manufactured by these methods.

An n-type (100) InP substrate 51 has an n-type I
nP cladding layer (thickness 0.5 μm) 52, n-type InGa
AsP lower light guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 53, InGaAsP (composition wavelength 1.45)
a five-period strained multiple quantum well (MQW) active layer 54 with InGaAsP (composition wavelength 1.10 μm, 10 nm thick) as a barrier layer and InGaAsP as a well layer.
Upper light guide layer (composition wavelength 1.10 μm, thickness 0.1 μm)
m) 55, p-type InP cladding layer (1.5 μm thickness) 5
6, p-type InGaAsP intermediate layer (thickness 0.2 μm) 5
7. High concentration p-type InGaAs contact layer (thickness 0.2
μm) 58 are sequentially formed, and a high concentration p-type InGa
A p-electrode 61 is provided in contact with the As contact layer. This high-concentration p-type InGaAs contact layer 58
Carrier concentration of 1 × 10 using zinc (Zn) as an impurity element
^Doped at ¹⁹ cm ^-3 . On the other hand, p-type InGaAs
Similarly, the sP intermediate layer 57 and the p-type InP layer cladding layer 56 use Zn as an impurity element, and their carrier concentrations are usually about 5 × 10 ¹⁸ cm ⁻³ and 7 × 10 ¹⁷ cm ⁻³ , respectively. Further, in the active layer 54, an undoped crystal is usually used.

[0006]

However, in the above-described semiconductor laser device having the conventional structure, the high-concentration p-type
During the growth of the nGaAs contact layer, it has been reported that Zn, which is a p-type impurity element in the InGaAs layer, abnormally diffuses into the p-type InGaAsP intermediate layer, the p-type InP cladding layer, and the active layer thereunder. I have. If Zn, which is a p-type impurity element, diffuses to the vicinity of the active layer, an increase in threshold current is confirmed during operation of the semiconductor laser device, which adversely affects the reliability of the device. The degree of diffusion of the Zn element into the vicinity of the active layer depends on the doping concentration of the InGaAs layer. Therefore, as the Zn concentration of the p-type InGaAs layer is increased to reduce the contact resistance with the p-electrode, the Z
The diffusion of the n element into the vicinity of the active layer increases.

For this reason, there has been reported a method of using carbon (C) having a smaller diffusion constant than Zn as an impurity element of a p-type InGaAs layer. However, in this method, since it is difficult to increase the doping concentration of the p-type InGaAs layer to 1 × 10 ¹⁹ cm ⁻³ or more, it is difficult to sufficiently reduce the contact resistance between the electrode and the element. However, there is a problem that the operating voltage becomes high.

A similar problem has been confirmed in an InP-based semiconductor laser formed on a p-type InP substrate. In this case, for example, Zn is transferred from a p-type InP substrate having a carrier concentration of about 5 × 10 ¹⁸ cm ⁻³ to a p-type InP cladding layer having a carrier concentration of about 7 × 10 ¹⁷ cm ⁻³ and further to an active layer thereon. Due to the diffusion of the element, the threshold current increases during the operation of the semiconductor laser device, and the reliability of the device decreases. For this reason, p-type In
A method of reducing the influence of Zn diffusion from a p-type InP substrate by increasing the thickness of a P cladding layer to several μm has been reported. However, in this method, the crystal growth time for fabricating the device increases, and at the same time, the p-type InP
There has been a problem that the resistance of the cladding layer increases the element resistance.

The present inventor has studied the technology described in Japanese Patent Application Laid-Open No. 5-259577 in order to solve this problem, but has not found enough technical content to solve the above-mentioned problem.

The present invention suppresses the diffusion of an impurity element from a high-concentration p-type contact layer or a high-concentration p-type substrate in a semiconductor device such as a semiconductor laser, reduces the element resistance, and provides a highly reliable semiconductor device. The purpose is to provide.

[0011]

In order to achieve the above object, a high-concentration p-type InGaAs contact layer and a p-type InP
A method of forming a superlattice intermediate layer between clad layers or between a high-concentration p-type InP substrate and a p-type InP clad layer has been devised. Further, the superlattice intermediate layer is formed by modulating and doping a p-type impurity element into only one of the two types of semiconductor layers forming the superlattice intermediate layer. It is possible to prevent the diffusion of Zn from the layer or the high-concentration p-type InP substrate. As a semiconductor material most suitable for forming the superlattice layer, there is a combination of InAs and GaAs or a combination of InAs and GaP.

Hereinafter, the In formed on the n-type InP substrate will be described.
A case of a high-concentration p-type contact layer in a semiconductor laser made of a P-based semiconductor will be described as an example.

The p-type InGaAs contact layer adjacent to the p-electrode is lattice-matched to the InP substrate, and is doped to 1 × 10 ¹⁹ cm ⁻³ or more using Zn as an impurity element. As a superlattice intermediate layer formed between the p-type InGaAs contact layer and the p-type InP clad layer, for example, In
Consider a case where a superlattice layer made of As and GaAs is formed. The lattice mismatch between InAs and GaAs and the InP substrate is + 3.2% and -3.7%, respectively.
By stacking an nAs layer and a GaAs layer with a thickness of several nm to form a superlattice structure, it is possible to maintain a good crystallinity and obtain an In layer.
Almost lattice matching with the P cladding layer is possible.
The InAs / GaAs superlattice intermediate layer is made of GaAs.
By modulating and doping only the layer with Zn as an impurity element, a p-type of about 1 × 10 ^{18 to} 5 × 10 ¹⁸ cm ⁻³ can be obtained.

By introducing the modulation-doped InAs / GaAs superlattice intermediate layer, the barrier against the injection of holes from the p-type InGaAs contact layer into the p-type InP cladding layer is reduced, and the device resistance is reduced. Also, InGaAs
Diffusion of Zn from the contact layer is prevented by the plurality of InAs / GaAs hetero interfaces in the superlattice structure, and the
Abnormal diffusion of Zn near the W active layer is reduced.

Similarly, when an intermediate layer made of an InAs / GaP superlattice is formed, it is possible to reduce the element resistance and prevent the abnormal diffusion of Zn.

While the above description has been made with reference to the case of an InP-based semiconductor laser formed on an n-type InP substrate, the case of an InP-based semiconductor laser formed on a high-concentration p-type InP substrate is also described. A similar effect can be obtained by forming a modulation-doped superlattice intermediate layer between the cladding layers. Further, a semiconductor laser comprising another III-group compound semiconductor such as GaAs has a high concentration p.
A similar effect can be obtained by forming a modulation-doped superlattice intermediate layer between the mold layer or the high-concentration p-type substrate and the p-type clad layer. Further, the effects of the present invention are not limited to semiconductor lasers,
It is also effective in a semiconductor device made of an I-group compound semiconductor.

Based on the above discussion, the present inventors provide a semiconductor device having the following structural features.

A semiconductor device (such as an electronic device or an optical device) according to the present invention includes a semiconductor multilayer structure having a pn junction formed on a semiconductor substrate (for example, a laminated structure including a channel of an electronic device and a light emitting region of an optical device). In a semiconductor device, a p-type semiconductor layer or a p-type substrate having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more and a p-type semiconductor layer having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or less adjacent thereto are provided. The structure having a semiconductor superlattice layer is a basic feature. Device structures in which a superlattice layer is provided in the vicinity of the light-emitting region are described in, for example, JP-A-7-99365, JP-A-6-334265, and JP-A-5-7051. Differently, no guidance is given to optimize its structure in light of the above-mentioned objects of the present invention. The effects achieved by such a basic structure of the present invention will be understood from the description with reference to FIG.

In order to enhance the effect of the semiconductor device of the present invention, zinc is used as the p-type impurity element of the p-type semiconductor layer or the p-type substrate having the carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more. Recommend. Further, of the two types of semiconductor layers forming the semiconductor superlattice layer, only one of the semiconductor layers may be modulation-doped with a p-type impurity element. When using InP for the semiconductor substrate, it is recommended that the semiconductor superlattice layer be a superlattice made of InAs and GaAs or a superlattice made of InAs and GaP.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 A ridge waveguide type semiconductor laser was fabricated on an n-type InP substrate. FIG. 1 is a sectional view of the device. First, an n-type InP cladding layer (thickness 0.5 μm) 12 and an n-type InGaAsP lower optical guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 1
3, InGaAsP (composition wavelength 1.45 μm, 6 nm
InGaAsP (having a composition wavelength of 1.10 μm)
m, 10 nm thick) as a barrier layer, a five-period strained multiple quantum well (MQW) active layer 14, an InGaAsP upper optical guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 15, p
Type InP cladding layer (thickness 1.5 μm, carrier concentration 7
× 10 ¹⁷ cm ⁻³ ) 16, InAs (5 nm) and GaAs
An InAs / GaAs superlattice intermediate layer 17 composed of 10 (5 nm) periods and a p-type InGaAs contact layer (thickness 0.2 μm, carrier concentration 2 × 10 ¹⁹ cm ⁻³ ) 18 are sequentially formed. Here, the InAs / GaAs superlattice intermediate layer 1
In No. 7, only the GaAs layer is modulation-doped into a p-type of 5 × 10 ¹⁸ cm ⁻³ . The substrate temperature is 600 ° C., and the p-type impurity element is Zn. Raw materials include trimethylindium, triethylgallium, arsine, phosphine,
And dimethyl zinc. Here, the Zn concentration in the crystal in this structure was determined by secondary ion mass spectrometry (SIMS).
FIG. 2 shows the results of the measurement. For comparison, FIG.
The results of analysis of the laminated structure using the conventional InGaAsP intermediate layer shown in FIG. In each measurement, the contact layers 18, 58 and the intermediate layers 17, 57
Is removed by etching, and then the InP cladding layer 1 is removed.
Analysis was performed from 6, 56. Modulation dope I of Embodiment 1
In the sample in which the nAs / GaAs superlattice intermediate layer was formed, p
Concentration in the InP cladding layer 16 is 7 × 10 ¹⁷ cm
^−3, and the Zn concentration in the InGaAsP light guide layer 15 and the MQW active layer 14 is below the detection limit. On the other hand, when the conventional superlattice intermediate layer is not used,
The Zn concentration in the p-type InP cladding layer 56 increases, and furthermore, the Z concentration in the InGaAsP light guide layer 55 and the active layer 54 is increased.
The diffusion of n is observed. This is probably because Zn, which is an impurity element, diffused toward the active layer during the growth of the InGaAs contact layer 58. As a result, it has been clarified that the use of the modulation-doped superlattice intermediate layer suppresses the diffusion of Zn into the active layer.

Next, after processing into a ridge stripe waveguide structure using a known laser manufacturing method, the SiO ₂ film 19 is formed.
Is formed, and the current constriction portion 20 is embedded with polyimide,
A p-electrode 21 and an n-electrode 22 are formed. The ridge width of the fabricated ridge waveguide type laser is 2 μm, and the total element length is 4
00 μm.

This laser device oscillates at a wavelength of 1.3 μm under a continuous condition at room temperature, and has a threshold current of 8 to 10 mA.
The operating voltage was 1.3 to 1.4 V at the time of output of 10 mW, showing good characteristics. In addition, a threshold current of 18 to 22 mA was obtained even at a high temperature of 85 ° C. 85 ° C, 1
After conducting a power-on experiment at 0 mW for 100 hours, the change in threshold value at room temperature was measured.
It was about 2 mA. Also, the long-term reliability of the device is 85 ° C.
As a result of the evaluation under the high temperature condition, an estimated life of 100,000 hours or more was confirmed.

On the other hand, the conventional InGaAsP shown in FIG.
In the case of a semiconductor laser having an intermediate layer, the threshold current at room temperature is 8 to 10 mA, and the operating voltage is 1.3 at the time of output of 10 mW.
The threshold current at ~ 1.4 V and 85 ° C was 25 to 30 mA. Further, in the case of this element, 85 ° C., 10 m
W, the increase in threshold current after 100 hours of energization experiment is 3-5
mA was observed, and the estimated lifetime of the device was about 50,000 hours.

Embodiment 2 A ridge waveguide type semiconductor laser was manufactured on an n-type InP substrate. FIG. 3 is a sectional view of the device. First, an n-type InP cladding layer (thickness 0.5 μm) 12 and an n-type InGaAsP lower optical guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 1
3, InGaAsP (composition wavelength 1.45 μm, 6 nm
InGaAsP (having a composition wavelength of 1.10 μm)
m, 10 nm thick) as a barrier layer, a five-period strained multiple quantum well (MQW) active layer 14, an InGaAsP upper optical guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 15, p
Type InP cladding layer (thickness 1.5 μm, carrier concentration 7
× 10 ¹⁷ cm ⁻³ ) 16, InAs (5 nm) and GaP (3
nm), a p-type InGaAs contact layer (thickness of 0.2 μm),
m and a carrier concentration of 2 × 10 ¹⁹ cm ⁻³ ) 18 are sequentially formed. Here, the InAs / GaP superlattice intermediate layer 27 is made of G
Only the aP layer is modulation-doped into a p-type of 2 × 10 ¹⁸ cm ⁻³ . The substrate temperature is 600 ° C., and the p-type impurity element is Z
n. As raw materials, trimethyl indium, triethyl gallium, arsine, phosphine, and dimethyl zinc were used. Here, the result of measuring the Zn concentration in the crystal in this structure by secondary ion mass spectrometry (SIMS) showed substantially the same result as in the case of the first embodiment.

Next, after processing into a ridge stripe waveguide structure using a known laser manufacturing method, the SiO ₂ film 19 is formed.
Is formed, and the current constriction portion 20 is embedded with polyimide,
A p-electrode 21 and an n-electrode 22 are formed. The ridge width of the fabricated ridge waveguide type laser is 2 μm, and the total element length is 4
00 μm.

This laser device oscillates at a wavelength of 1.3 μm under a continuous condition at room temperature, and has a threshold current of 8 to 10 mA.
The operating voltage was 1.3 to 1.4 V at the time of output of 10 mW, showing good characteristics. In addition, a threshold current of 18 to 22 mA was obtained even at a high temperature of 85 ° C. 85 ° C, 1
After conducting a power-on experiment at 0 mW for 100 hours, the change in threshold value at room temperature was measured.
It was about 2 mA. Also, the long-term reliability of the device is 85 ° C.
As a result of the evaluation under the high temperature condition, an estimated life of 100,000 hours or more was confirmed.

Embodiment 3 A ridge waveguide type semiconductor laser was fabricated on a p-type InP substrate. FIG. 4 is a sectional view of the device. The carrier concentration of the p-type (100) InP substrate 31 is 5 × 10 ¹⁸ cm ⁻³ . First, InAs (5 nm) and GaAs (5 nm) are formed on the p-type InP substrate 31 by metalorganic chemical vapor deposition.
InAs / GaAs superlattice intermediate layer 3 consisting of 10 periods
Form 2 In this InAs / GaAs superlattice intermediate layer, only the GaAs layer is modulation-doped into a 5 × 10 ¹⁸ cm ⁻³ p-type. Subsequently, a p-type InP cladding layer (thickness 0.5 μm) 33, an InGaAsP lower optical guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 34, InGaP
InGaAsP (composition wavelength 1.10 μm, 10 nm) was formed using AsP (composition wavelength 1.45 μm, 6 nm thick) as a well layer.
-Thickness strained multiple quantum well (MQW)
An active layer 35, an InGaAsP upper light guide layer (composition wavelength 1.10 μm, thickness 0.1 μm) 36, an n-type InP cladding layer (thickness 1.5 μm) 37, and an n-type InGaAsP contact layer 38 are sequentially formed.

Next, after processing into a ridge stripe waveguide structure using a known laser manufacturing method, the SiO 2 film 39 is formed.
Is formed, and the current constriction portion 40 is embedded with polyimide.
A p-electrode 41 and an n-electrode 42 are formed. The ridge width of the fabricated ridge waveguide type laser is 2 μm, and the total element length is 4
00 μm.

This laser device oscillates at a wavelength of 1.3 μm under continuous conditions at room temperature, and has a threshold current of 8 to 10 mA.
The operating voltage was 1.3 to 1.4 V at the time of output of 10 mW, showing good characteristics. In addition, a threshold current of 18 to 22 mA was obtained even at a high temperature of 85 ° C. 85 ° C, 1
After conducting a power-on experiment at 0 mW for 100 hours, the change in threshold value at room temperature was measured.
It was about 2 mA. Also, the long-term reliability of the device is 85 ° C.
As a result of the evaluation under the high temperature condition, an estimated life of 100,000 hours or more was confirmed.

On the other hand, p of the conventional structure without the superlattice intermediate layer
In a semiconductor laser element on a p-type InP substrate, diffusion of Zn from the p-type InP substrate into the active layer occurs. Therefore, the operating voltage at 10 mW output at room temperature is 1.4 to 1.6 V, 85 ° C. An increase in the threshold current after a 10-mW, 100-hour energization experiment was observed at 3 to 5 mA, and the estimated lifetime of the element was about 50,000 hours.

In the above embodiment, the case of the semiconductor laser device formed on the InP substrate has been described. However, also in the case of the semiconductor laser device formed on the GaAs substrate, the modulation doped superlattice intermediate layer is similarly formed. By introducing it, it is possible to improve reliability.

[0033]

According to the semiconductor device of the present invention, a p-type modulation doped superlattice intermediate layer is formed between a high-concentration p-type InGaAs contact layer or a high-concentration p-type InP substrate and a p-type cladding layer. As a result, a semiconductor laser with small device characteristic deterioration due to abnormal diffusion of Zn to the vicinity of the active layer can be realized. Furthermore, by using the semiconductor laser in an optical transmission device, the reliability of the optical transmission device can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an element structure according to a first embodiment of the present invention.

FIG. 2 is a view showing a Zn concentration profile in a device having the structure of FIGS. 1 and 5 measured by secondary ion mass spectrometry.

FIG. 3 is a diagram for explaining an element structure according to a second embodiment of the present invention.

FIG. 4 is a diagram for explaining an element structure according to a third embodiment of the present invention.

FIG. 5 is a diagram showing an element structure using a conventional contact layer and an intermediate layer.

[Explanation of symbols]

11 ... n-type (100) InP substrate, 12 ... n-type InP clad layer, 13 ... n-type InGaAsP lower light guide layer,
14 ... InGaAsP strained multiple quantum well active layer, 15 ... I
nGaAsP upper light guide layer, 16 ... p-type InP clad layer, 17 ... InAs / GaAs superlattice intermediate layer, 18 ...
p-type InGaAs contact layer, 19 ... SiO ₂ film, 2
0 ... polyimide current confinement layer, 21 ... p electrode, 22 ... n electrode, 27 ... InAs / GaP superlattice intermediate layer, 31 ... p-type (100) InP substrate, 32 ... InAs / GaAs superlattice intermediate layer, 33 ... p Type InP cladding layer, 34 ... InG
aAsP lower optical guide layer, 35 ... InGaAsP strained multiple quantum well active layer, 36 ... InGaAsP upper optical guide layer, 37 ... n-type InP cladding layer, 38 ... n-type InGa
AsP contact layer, 39: SiO ₂ film, 40: polyimide current confinement layer, 41: p electrode, 42: n electrode, 51:
n-type (100) InP substrate, 52 ... n-type InP cladding layer, 53 ... n-type InGaAsP lower light guide layer, 54 ...
InGaAsP strained multiple quantum well active layer, 55 ... InGa
AsP upper light guide layer, 56... P-type InP clad layer,
57: p-type InGaAsP intermediate layer; 58: p-type InGa
As contact layer, 59: SiO ₂ film, 60: polyimide current confinement layer, 61: p electrode, 62: n electrode.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Toshinobu Tsuchiya 1-280 Higashi-Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. Central Research Laboratory (72) Inventor Masahiro Aoki 1-280 Higashi Koigakubo, Kokubunji, Tokyo 5F041 AA44 CA04 CA05 CA34 CA35 CA37 CA39 5F073 AA13 AA71 AA74 CA12 CB19 EA28 EA29

Claims (5)

[Claims] In a semiconductor device having a semiconductor multilayer structure having a pn junction formed on a semiconductor substrate, a p-type semiconductor layer or a p-type substrate having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more and a p-type substrate adjacent thereto are provided. A semiconductor device, wherein a semiconductor superlattice layer is formed between a p-type semiconductor layer having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or less. 2. The semiconductor device according to claim 1, wherein the p-type impurity element of the p-type semiconductor layer or the p-type substrate having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more is zinc. 3. The semiconductor device according to claim 1, wherein, of the two types of semiconductor layers forming the semiconductor superlattice layer, only one of the semiconductor layers is modulation-doped with a p-type impurity element. 4. The semiconductor device according to claim 1, wherein said semiconductor substrate is InP, and said semiconductor superlattice layer is made of a superlattice of InAs and GaAs. 5. The semiconductor device according to claim 1, wherein said semiconductor substrate is InP, and said semiconductor superlattice layer is made of a superlattice of InAs and GaP.