Method of diffusing Si into compound semiconductor and compound semiconductor device

Abstract: A method of diffusing Si into compound semiconductor (1b) from a Si film (2) provided on a surface region of the compound semiconductor (1b), wherein the diffusion is conducted with providing a diffusion stopper layer (4) at a position of predetermined depth from the surface of the compound semiconductor (1b), which stopper layer (4) has a lower diffusion speed than that of the compound semiconductor (1b). A compound semiconductor device includes a compound semiconductor substrate (1a), a diffusion stopper layer (4) provided on the semiconductor substrate (1a), a compound semiconductor layer (1b) provided on the diffusion stopper layer (4), a Si film (2) provided on the semiconductor layer (1b), and Si diffusion regions (5) into which Si is diffused from the Si film (2) so as to reach the interface between the diffusion stopper layer (4) and the semiconductor substrate (1a).

Description: FIELD OF THE INVENTION The present invention relates to a method of diffusing Si into compound semiconductor and compound semiconductor device, and more particularly to that capable of controlling the diffusion depth at high reproducibility and high precision. BACKGROUND OF THE INVENTION Figure 6 shows a cross-sectional view of a wafer for explaining a prior art method of diffusing Si. In figure 6, reference numeral 1 designates Al x Ga 1-x As layer (0 ≦ x < 1) as a III - V group compound semiconductor layer. A Si film 2 is provided on a surface of the Al x Ga 1-x As layer 1 by such as a vacuum evaporation method, a CVD method (Chemical Vapor Deposition), or a sputtering method. A protection layer 3 comprising such as SiO₂ or Si₃N₄ is one for protecting the surfaces of Al x Ga 1-x As layer 1 and the Si film 2. Si diffusion may be conducted without providing this protection layer 3. Reference numeral 5 designates a Si diffusion region. The diffusion process will be described. The Al x Ga 1-x As layer 1 having Si film 2 thereon is heated up to a temperature of about 800 to 950 °C in arsenide ambient having arsenic partial pressure of about 3.3775 x 10⁵ N/m² (0.3 atmosphere). Then, Si is diffused into the Al X Ga 1-X AS layer 1 by heating the Si film 2 in arsenide ambient. A method of this kind is known from Applied Physics Letters, vol.50, no.5, 2 February 1987, pp.265-266. In this prior art diffusion method, diffusion time, diffusion temperature, arsenic pressure, Si film thickness, protection layer thickness, and material of protection layer can be selected as desired, as parameters which determine the diffusion speed and diffusion depth. For example, as the diffusion temperature is low, arsenic pressure is low, Si film is thick, and protection layer is thick, the diffusion speed is low. The reason why the diffusion speed is low as the protection layer is thick, is described in "1986 International Conference on Solid State Devices and Materials, pp 41 to 44". Since the annealing is conducted in As ambient, As diffuses through the protection layer to reach the interface between the protection layer and GaAs layer, thereby creating an As rich (that is, Ga vacancies rich) region in GaAs layer, and Si will diffuse in association with these vacancies, thereby promoting the diffusion. When the protection layer is thick, As slowly reaches the interface, thereby lowering the diffusion speed. Furthermore, it is reported in "Appl. Phys. Lett. 44(8), 15 April 1984, pp 750 to 752" and "J. Electrochem. Soc. 129, No. 4, 1982, pp 837 to 840" that the diffusion speed is lower when using a Si₃N₄ protection layer than when using a SiO₂ protection layer. When SiO₂ protection layer is used, Ga in the GaAs layer is likely to diffuse into SiO₂ at the SiO₂ protection and GaAs layer interface during annealing. Thus, Ga vacancies will occur in the GaAs layer and Si will diffuse in association with these vacancies, thereby promoting the diffusion. On the other hand, when Si₃N₄ protection layer is used, Ga is not likely to diffuse into Si₃N₄. In addition, since annealing is conducted in As ambient, As will diffuse through the protection layer to reach the SiO₂ protection and GaAs layer interface, thereby creating an As rich region in GaAs layer, and Si will diffuse in association with these vacancies, thereby promoting the diffusion. In more dense Si₃N₄ film than SiO₂, As slowly reaches the interface, thereby reducing the diffusion speed. When the Si diffusion process is employed in a device, the diffusion depth needs to be precisely controlled. In the prior art diffusion method, however, since the diffusion depth varies dependent on a lot of parameters as described above, it is difficult to control the diffusion depth at high reproducibility and high precision with suppressing variations of these parameters. Patent Abstracts of Japan, vol.8, no.39, (E-228) [1476], 21 February 1984 & JP-A-58 197 786 discloses a method of diffusing Zn into a compound semiconductor wherein the diffusion is conducted with providing a diffusion control layer at a position of predetermined depth from the surface of the compound semiconductor. SUMMARY OF THE INVENTION According to the present invention there is provided a method of diffusing Si into a compound semiconductor comprising: diffusing from a Si film provided on a surface region of said compound semiconductor into said compound semiconductor, said compound semiconductor being Al x Ga 1-x As, (0 ≤ x < 1); providing a diffusion stopper layer at a predetermined depth from the surface of said compound semiconductor; the stopper layer having a lower diffusion speed than that of said compound semiconductor, the stopper layer comprises Al Y Ga 1-Y As (1>y ≧ x+0.3) and has a thickness of about 0.1 µm. According to the present invention there is provided a compound semiconductor device comprising a compound semiconductor substrate; a diffusion stopper layer provided on the semiconductor substrate, a compound semiconductor layer provided on the diffusion stopper layer; a Si film provided on the semiconductor layer; and Si diffusion regions extending from the Si film to the interface between the diffusion stopper layer and the semiconductor substrate, the stopper layer comprising Al Y Ga 1-Y As (1 > y ≥ x+0.3), (0 ≤ x < 1) and having a thickness of about 0.1 µm. According to the present invention there is provided a laser diode comprising: a substrate made of a first conductivity type compound semiconductor; a lower cladding layer having itself upper and lower portions being provided on the semiconductor substrate and being made of the first conductivity type compound semiconductor; a diffusion stopper layer made of the first conductivity type compound semiconductor and being located between the upper and lower portions of the lower cladding layer; an active layer made of the first conductivity type compound semiconductor provided on the upper portion of the lower cladding layer; an upper cladding layer provided on the active layer and being made of a second conductivity type compound semiconductor; a contact layer provided on the upper cladding layer and made of the second conductivity type semiconductor; Si films provided on the contact layer; Si diffusion regions extending from the Si films to the interface between the diffusion stopper layer and the lower portion of the lower cladding layer, wherein the stopper layer comprises Al Y Ga 1-Y As (1>y ≥ x+0.3), (0 ≤ x < 1), and has a thickness of about 0.1 um. According to the present invention there is provided a laser diode comprising: a substrate made of a second conductivity type compound semiconductor; a lower cladding layer provided on the semiconductor substrate and being made of the second conductivity type compound semiconductor; an active layer provided on the lower cladding layer and being made of a first conductivity type compound semiconductor; an upper cladding layer having upper and lower portions being provided on the active layer and being made of the first conductivity type compound semiconductor; a diffusion stopper layer being located between the upper and lower portions of the upper cladding layer and being made of the first conductivity type compound semiconductor; a contact layer being provided on the upper portion of the upper cladding layer and being made of the first conductivity type compound semiconductor; Si films provided on the contact layer; and Si diffusion regions extending from the Si films to to the interface between the diffusion stopper layer and the lower portion of the upper cladding layer, wherein the stopper layer comprises Al Y Ga 1-y As (1>y ≥ x+0.3), (0 ≤ x < 1) and has a thickness of about 0.1 µm. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) and (b) are diagrams showing cross-sectional view of a wafer for explaining a method of diffusing Si according to a first embodiment of the present invention; Figure 2 is a graph diagram showing Al concentration vs diffusion depth characteristics according to the present invention; Figures 3, 4 and 5 are diagrams showing cross-sectional views of laser diodes according to a second, a third and a fourth embodiment of the present invention respectively; and Figure 6 is a diagram showing a cross-sectional view of a wafer for explaining a method of diffusing Si according to the prior art. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figures 1(a) and (b) show cross-sectional views for explaining a method of diffusing Si according to a first arrangement. Figure 1(a) shows a cross-sectional view of a wafer in a state prior to Si diffusion and Figure 1(b) shows the wafer in which Si diffusion is conducted. In Figures 1(a) and (b), reference numerals 1a and 1b designate an Al X Ga 1-X As substrate and an Al X Ga 1 X As layer, respectively, as III-V group compound semiconductor layers. A Si film 2 is provided on the Al X Ga 1-X Ga layer 1b. A protection layer 3 is one for protecting the surfaces of the Al X Ga 1-X As layer 1b and the Si film 2. A diffusion stopper layer 4 comprising Al Y Ga 1-y As (y > x) is provided at a position of predetermined depth from the surface of the Al X Ga 1-X As layer 1b, which stopper layer has a lower diffusion speed than that of the Al X Ga 1-X As layer 1b. Reference numeral 5 designates a Si diffusion region. The diffusion process will be described. Si diffusion is generally carried out in a closed quartz ampul in which arsenic partial pressure is applied by metal arsenide, or in an opened quartz ampul wherein arsenic partial pressure is applied by injecting arsine (AsH₃). Here, the former process is employed. GaAs is employed as a sample, and the protection layer 3 is omitted. Si film 2 is deposited on the GaAs substrate to a thickness of 300 x 10-¹nm (300 Å) by vacuum evaporation method. The wafer and metal arsenide are set in the quartz ampul, and the quartz ampul is evacuated and closed. The weight of metal arsenide is about 40 mg per ampul capacity of 40 cc. The quartz ampul is put in the diffusion furnace and diffusion is conducted at a temperature of 850 °C in arsenide ambient having arsenic partial pressure of about 3.3775 x 10⁵ N/m² (0.3 atmosphere). As previously described, diffusion depth varies dependent on Si film thickness, arsenic pressure, and temperature. When diffusion is carried out for four hours under the above described condition, Si is diffused up to a depth of 2.3 µm. On the other hand, when diffusion is carried out into Al 0.5 Ga 0.5 As substrate under the same condition as above, Si is diffused up to a depth of only 1.5 µm. Figure 2 is a graph diagram showing Al concentration vs. diffusion depth characteristics. As is apparent from figure 2, as the Al concentration is high, the diffusion depth is shallow, meaning lower diffusion speed. Suppose that diffusion depth is set to 2.3 µm, the diffusion stopper layer 4 of 0.3 µm, comprising Al Y Ga 1-Y As (y ≧ 0.3) is provided at a little above the diffusion front, (for example, at a position of 2.0 µm depth), diffusion can be easily stopped in the stopper layer 4. This means that even if a little variation occurs in the Si film thickness, arsenic partial pressure, and diffusion temperature, diffusion up to a predetermined depth is realized by only conducting diffusion a little longer than usual, without unfavorable effects. While in the above-illustrated embodiment the process of Si diffusion into a GaAs layer as discussed, Si diffusion into Al X Ga 1-X As layer will be carried out by a similar process to that discussed above. In this case the diffusion depth can be easily controlled by providing a diffusion stopper layer 4 comprising Al Y Ga 1-Y As (1>y ≧ x + 0.3). Figure 3 shows a cross-sectional view of a laser diode as a first embodiment of the present invention, wherein a Si diffusion method of the present invention is employed. In figure 3, reference numeral 6a designates a p type GaAs substrate. A p type Al 0.42 Ga 0.58 As (or Al 0.33 Ga 0.67 As) lower cladding layer 7a is provided on the substrate 6a. A diffusion stopper layer 4 of about 0.1 µm, comprising p type Al 0.72 Ga 0.28 As (or Al 0.63 Ga 0.37 As) is provided at a position of 0.5 to 1 µm distance from an active layer 8 and between upper and lower portions of the lower cladding layer 7a. A p type Al 0.08 Ga 0.92 As (or GaAs) active layer 8 is provided on the upper portion of the lower cladding layer 7a. An n type Al 0.42 Ga 0.58 As (or Al 0.33 Ga 0.67 As) upper cladding layer 9a is provided on the active layer 8. An n type GaAs contact layer 10a is provided on the upper cladding layer 9a. Si films 2 are provided on the contact layer 10a. Reference numeral 5 designates Si diffusion regions. In this laser diode, since the diffusion stopper layer 4 is provided at a position lower than the active layer 8, the Si diffusion portion in the active layer 8 becomes to have a lower refractive index than that of the other region in the active layer 8, thereby producing a light waveguide mechanism. Figure 4 shows a cross-sectional view of a laser diode as a second embodiment of the present invention, wherein the Si diffusion method of the present invention is employed. In figure 4, reference numeral 6b designates an n type GaAs substrate. An n type Al 0.42 Ga 0.58 As (or Al 0.33 Ga 0.67 As) lower cladding layer 7b is provided on the substrate 6b. A p type Al 0.08 Ga 0.92 As (or GaAs) active layer 8 is provided on the lower cladding layer 7b. A p type Al 0.42 Ga 0.58 As (or Al 0.33 Ga 0.67 As) upper cladding layer 9b is provided on the active layer 8. A diffusion stopper layer 4b of about 0.1 µm, which comprises Al 0.72 Ga 0.28 As (or Al 0.63 Ga 0.37 As) is provided at a position of 0.2 to 0.3 µm distance from the active layer 8 and between the upper and lower portions of the upper cladding layer 9b. A p type GaAs contact layer 10b is provided on the upper portion of the upper cladding layer 9b. In this laser diode, the transverse mode is controlled by the Si diffusion. When Si diffusion is carried out, the Si diffusion portion in the upper cladding layer 9b and the diffusion stopper layer 4b comes to have a lower refractive index than that of the other portions of the cladding layer 9b and the stopper layer 4b. When the portions of high refractive index and low refractive index exist near the active layer, the active layer 8 comes to have aneffective refractive index distribution in the transverse direction, thereby confining the light in the center of the laser chip. Accordingly, the transverse mode can be controlled by controlling the diffusion depth, and the longitudinal direction mode can be further controlled. As discussed above, the distance between the diffusion front and the active layer greatly affects the laser diode characteristics, meaning the importance of the control of diffusion depth. Thus, the present invention can be effectively utilized. Figure 5 shows a cross-sectional view of a laser diode which further comprises a lightguide layer 11 in the device as shown in figure 4. In this laser diode, the lightguide layer 11 comprises an A1GaAs series superlattice and is provided between the diffusion stopper layer 4b and the upper cladding layer 9b. The superlattice is disordered in the Si diffusion region 5, and becomes a mixed crystal of single composition. Since the portion of mixed crystal has a refractive index which is different from that of the portion of superlattice, a light waveguide mechanism is produced. In this structure, the diffusion stopper layer 4 is provided under the light guide layer 11, and the Si diffusion is precisely stopped in the stopper layer 4, thereby obtaining a laser diode with high reproducibility and uniform characteristics. As discussed above, according to the present invention, Si diffusion into compound semiconductor is conducted to provide a diffusion stopper layer at a position of predetermined depth from the surface of the compound semiconductor, which stopper layer has a lower diffusion speed than that of the compound semiconductor. Thus, the diffusion depth can be controlled at high reproducibility and high precision.

Claims: A method of diffusing Si into a compound semiconductor comprising: diffusing from a Si film provided on a surface region of said compound semiconductor into said compound semiconductor, said compound semiconductor being Al x Ga 1-x As, (0 ≤ x < 1); providing a diffusion stopper layer (4) at a predetermined depth from the surface of said compound semiconductor; the stopper layer (4) having a lower diffusion speed than that of said compound semiconductor, the stopper layer (4) comprising Al Y Ga 1-Y As (1 > y ≧ x+0.3) and having a thickness of about 0.1 µm. A compound semiconductor device comprising: a compound semiconductor substrate (1a); a diffusion stopper layer (4) provided on the semiconductor substrate (1a), a compound semiconductor layer (1b) provided on the diffusion stopper layer (4); a Si film (2) provided on the semiconductor layer (1b); and Si diffusion regions (5) extending from the Si film (2) to the interface between the diffusion stopper layer (4) and the semiconductor substrate (1a), the stopper layer (4) comprising Al Y Ga 1-Y AS (1 >y ≧ x+0.3), (0 ≤ x < 1) and having a thickness of about 0.1 µm. A laser diode comprising: a substrate (6a) made of a first conductivity type compound semiconductor; a lower cladding layer (7a) having itself upper and lower portions being provided on the semiconductor substrate (6a) and being made of the first conductivity type compound semiconductor; a diffusion stopper layer (4) made of the first conductivity type compound semiconductor and being located between the upper and lower portions of the lower cladding layer (7a); an active layer (8) made of the first conductivity type compound semiconductor provided on the upper portion of the lower cladding layer (7a); an upper cladding layer (9a) provided on the active layer (8) and being made of a second conductivity type compound semiconductor; a contact layer (10a) provided on the upper cladding layer (9a) and made of the second conductivity type semiconductor; Si films (2) provided on the contact layer (10a); Si diffusion regions (5) extending from the Si films (2) to the interface between the diffusion stopper layer (4) and the lower portion of the lower cladding layer (7a), wherein the stopper layer (4) comprises Al y Ga₁ ₋ y As(1>y ≥ x+0.3), (0 ≤ x < 1), and has a thickness of about 0.1 µm. A laser diode utilising a compound semiconductor as defined in claim 3, wherein said semiconductor substrate (6a) comprises p type GaAs, said lower cladding layer (7a) comprises p type Al Z Ga 1-z As, said diffusion stopper layer (4) comprises p type Al Y Ga 1-Y As(y > z), said active layer (8) comprises p type Al W Ga 1-w As, (w=0 or 0.08) said upper cladding layer (9a) comprises n type Al Z Ga 1-z As, and said contact laye (10a) comprises n type GaAs. A laser diode comprising: a substrate (6b) made of a second conductivity type compound semiconductor; a lower cladding layer (7b) provided on the semiconductor substrate (6b) and being made of the second conductivity type compound semiconductor; an active layer (8) provided on the lower cladding layer (7b) and being made of a first conductivity type compound semiconductor; an upper cladding layer (9b) having upper and lower portions being provided on the active layer (8) and being made of the first conductivity type compound semiconductor; a diffusion stopper layer (4b) being located between the upper and lower portions of the upper cladding layer (9b) and being made of the first conductivity type compound semiconductor; a contact layer (10b) being provided on the upper portion of the upper cladding layer (9b) and being made of the first conductivity type compound semiconductor; Si films (2) provided on the contact layer (10b); and Si diffusion regions (5) extending from the Si films (2) to the interface between the diffusion stopper layer (4b) and the lower portion of the upper cladding layer (9b), wherein the stopper layer (4) comprises Al Y Ga 1-y As (1 > y ≥ x+0.3), (0 ≤ x < 1) and has a thickness of about 0.1 µm. A laser diode utilising a compound semiconductor as defined in claim 5, wherein said semiconductor substrate (6b) comprises n type GaAs, said lower cladding layer (7b) comprises n type Al z Ga 1-z As, said active layer (8) comprises p type Al w Ga 1-w As, (w = 0 or 0.08) said upper cladding layer (9b) comprises p type Al z Ga 1-z As, said diffusion stopper layer (4b) comprises p type Al Y Ga 1-Y As (y > z), and said contact layer (10b) comprises p type GaAs. A laser diode utilising a compound semiconductor as defined in claim 5 or claim 6, which further comprises a light guide layer (11) provided between said diffusion stopper layer (4b) and said upper portion of said upper cladding layer (9b). A laser diode utilising a compound semiconductor as defined in claim 7, wherein said semiconductor substrate (6b) comprises n type GaAs, said lower cladding layer (7b) comprises n type Al z Ga 1-z As, said active layer (8) comprises p type Al w Ga 1-w As, (w = 0 or 0.08) said upper cladding layer (9b) comprises p type Al z Ga 1-z As, said diffusion stopper layer (4b) comprises p type Al y Ga 1-y As (y > z), said contact layer (10b) comprises p type GaAs, and said light guide layer (11) comprises AlGaAs series superlattice.

Inventors: MURAKAMI TAKASHI, MURAKAMI, TAKASHI, MURAKAMI, TAKASHI C/O MITSUBISHI DENKI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H01L21
IPC codes: H01S5, H01L21