Method of making P-type compound semiconductor

Abstract: A P-type compound semiconductor layer doped with carbon is formed on a semi-insulating substrate by placing the substrate in a reactor, and carrying out vapor-phase epitaxy by feeding vapors of an organic metal having a methyl radical, arsine, and an alkyl compound of arsenic substantially simultaneously into the reactor so that a C-doped P-type compound semiconductor is deposited on the substrate.

Description: The present invention relates to a method of forming a P-type compound semiconductor layer which can be used, for example, as a part of a semiconductor light-emitting device, such as a semiconductor laser. There are various types of semiconductor lasers. One example is a TJS ( T ransverse J unction S tripe) laser shown in Figure 1. This TJS laser may be fabricated in the following manner. First, a magnesium-doped (Mg-doped) P-type GaAs layer 4 is grown on a semi-insulating gallium arsenide (GaAs) substrate 2 by liquid-phase epitaxy. Then, a lower tellurium-doped (Te-doped) N-type AlGaAs cladding layer 6 is grown on the Mg-doped P-type GaAs layer 4 by liquid-phase epitaxy. On the lower cladding layer 6, a Te-doped N-type AlGaAs active layer 8 is grown by liquid-phase epitaxy. Again liquid-phase epitaxy is used to grow an upper Te-doped AlGaAs cladding layer 10 on the active layer 8. Then, a Te-doped N-type GaAs layer 12 is grown on the upper cladding layer 10 by liquid-phase epitaxy. Thereafter, Zn is diffused from the top surface of the Te-doped N-type GaAs layer 12 into the structure including the GaAs layer 2, the Mg-doped P-type GaAs layer 4, the lower cladding layer 6, the active layer 8, the upper cladding layer 10, and the Te-doped N-type GaAs layer 12, so as to convert portions respectively of the layers 2, 4, 6, 8, and 10 together into a Zn-diffused P⁺-type region 14. By this Zn diffusion, a portion of the Te-doped N-type GaAs layer 12 is converted into a Zn-doped P⁺-type GaAs layer 16. Then, Zn in the Zn-diffused P⁺-type region 14 and the Zn-doped P⁺-type GaAs layer 16 is driven to diffuse outward of the region 14 and the layer 16 to thereby form a Zn-drive-in-diffused P-type region 18, as shown. Thereafter, that portion of the P-type region 18 which is located between the layer 12 and the layer 16, and portions of the layers 12 and 16 on opposite sides thereof are etched away, so that a Te-doped N-type GaAs layer 12a and a Zn-doped P⁺-type GaAs contact layer 12b result. A TJS laser, similar in structure to that shown in fig. 1, is described in U.S. Patent No. 4334311. In manufacturing the TJS semiconductor laser of Figure 1, liquid-phase epitaxy is used to form the P-type GaAs layer 4, the lower cladding layer 6, the active layer 8, the upper cladding layer 10, and the N-type GaAs layer 12. However, when variations in performance and reproducibility of the semiconductor lasers are taken into consideration, it is desirable to use vapour-phase epitaxy involving thermal decomposition, such as MOMBE (metalorganic molecular beam epitaxy) and MOCVD (metalorganic chemical vapour deposition). However, when vapour-phase epitaxy involving thermal decomposition is used, a problem occurs when the P-type GaAs layer 4 is formed. Assuming that Zn which is usable in vapour-phase epitaxy is used in place of Mg, which cannot be used in vapour-phase epitaxy, to form a Zn-doped P-type GaAs layer in place of the Mg-doped P-type GaAs layer 4 by thermal-decomposition-involving vapour-phase epitaxy, Zn would be diffused from the Zn-doped P-type GaAs layer into the GaAs layer 2 and into the Te-doped N-type AlGaAs layer 6 during the diffusion step for forming the Zn-diffused P⁺-type region 14 and during the drive-in diffusion step for forming the Zn-drive-in-diffused P-type region 8, because the diffusion rate of Zn is high. This causes reduction of the carrier concentration of the Zn-doped P-type GaAs layer. In order to avoid this problem, the Mg-doped P-type GaAs layer 4 is formed, using Mg as a dopant which has a low diffusion rate, but is unusable in thermal-decomposition-involving vapour-phase epitaxy. Carbon (C) is known as a dopant which has a very low diffusion rate and is still usable in vapour-phase epitaxy in which thermal decomposition takes place. It is, therefore, desired to use C to form a P-type GaAs semiconductor layer by vapour-phase epitaxy involving thermal decomposition. Processes for forming a P-type GaAs semiconductor, using C as a dopant, are disclosed in, for example, Japanese Published Patent Application No. SHO 62-104118 and Japanese Published Patent Application No. SHO 63-88820. According to the method of the former Japanese application, a P-type GaAs semiconductor layer is formed by MBE (molecular beam epitaxy), using trimethylgallium and arsenic as vapour sources. In the manufacturing method disclosed in Japanese Published Patent Application No. SHO 63-88820, trimethylgallium and arsine are alternately fed while a P-type GaAs semiconductor layer is being formed by MOCVD. However, the method disclosed in Japanese published Patent Application No. SHO 62-104118 has disadvantages that other process than MBE cannot be used and that adjustment of concentration of C is difficult. A problem encountered in the process of Japanese Published Patent Application No. SHO 63-88820 is that although adjustment of the concentration of C is possible, the manufacturing process is complicated since it is necessary to alternately supply trimethylgallium and arsine. The present invention is provided as a solution to the problems aforesaid. In accordance with the present invention there is provided a method of making a C-doped P-type arsenic compound semiconductor, in which vapour-phase epitaxy involving thermal decomposition is used and in which an organic metal having a methyl radical as source of a constituent of said arsenic compound semiconductor other than arsenic, arsine, and an alkyl compound of arsenic are used as vapour sources. In the method defined above change in growth temperature is not necessary as means of controlling carbon dopant content. Instead the carbon dopant content may be controlled by changing the amount of arsine. This is particularly advantageous. In this case the ratio of arsine to the organic metal having a methyl radical may be varied in a range of 0.6 to 0.9. A preferred organic metal having a methyl radical is trimethylgallium. A preferred alkyl compound of arsenic is trimethyl arsine. These may both be used together with arsine fed in a ratio of between 0.6 and 0.9 arsine to trimethylgallium. The vapour phase epitaxy may be performed by chemical vapour deposition (MOCVD) or by molecular beam epitaxy (MBE). An application of the above defined method to the making of a semiconductor laser device is set out in claim 9 of the claims appended. In the accompanying drawings: Figure 1 is a cross-sectional view of a conventional TJS laser; Figure 2 is a cross-sectional view of a TJS laser according to one embodiment of the present invention; and Figures 3 through 6 are cross-sectional views in various steps of making the TJS laser of Figure 2. Now, the present invention is described in terms of a semiconductor light-emitting device, for example, a TJS semiconductor laser device. As shown in Figure 2, the TJS semiconductor laser device according to one embodiment of the present invention includes a semi-insulating GaAs substrate 22. On the top surface of the substrate 22, a C-doped P-type GaAs layer 24 is disposed, which functions as a layer that is not active optically. The C-doped P-type GaAs layer 24 has a thickness of, for example, 0.1-0.3 µm and has a C concentration of, for example, from 1 × 10¹⁸ cm⁻³ to 10 × 10¹⁸ cm⁻³. A selenium-doped (Se-doped) N-type Al 0.36 Ga 0.64 As lower cladding layer 26 is disposed on the top surface of the C-doped P-type GaAs layer 24. The lower cladding layer 26 has a thickness of, for example, 0.5-3 µm and has a Se concentration of, for example, from 1 × 10¹⁷ cm⁻³ to 10 × 10¹⁷ cm⁻³. A Se-doped N-type Al 0.06 Ga 0.94 As active layer 28 is disposed on the top surface of the lower cladding layer 26. The active layer 28 has a thickness of, for example, 50-150 nm (500-1500Å) and has a Se concentration of, for example, from 1 × 10¹⁷ cm⁻³ to 10 × 10¹⁷ cm⁻³. Because the active layer 28 has a composition of Al 0.06 Ga 0.94 As, its forbidden band gap is narrower than that of the lower cladding layer 26. On the top surface of the active layer 28, a Se-doped N-type Al 0.36 Ga 0.64 As upper cladding layer 30 is formed, which has a thickness of, for example, 0.5-3 µm and a Se concentration of, for example, from 1 × 10¹⁷ cm⁻³ to 10 × 10¹⁷ cm⁻³. The upper cladding layer 30 has the same composition as the lower cladding layer and has a wider forbidden band gap than the active layer 28. On the top surface of the upper cladding layer 30, a Se-doped N-type GaAs contact layer 32 is disposed. The contact layer 32 has a thickness of, for example, 1-3 µm and has a Se concentration of, for example, from 3 × 10¹⁸ cm⁻³ to 10 × 10¹⁸ cm⁻³. A Zn-diffused P⁺-type region 34 is formed to extend through portions respectively of the GaAs layer 22, the P-type GaAs layer 24, the lower cladding layer 26, the active layer 28, and the upper cladding layer 30, as shown in Figure 2. Between the Zn-diffused P⁺-type region 34 and those portions of the layers 22, 24, 26, 28 and 30 into which Zn has not been diffused, a Zn-drive-in-diffused P-type region 36 extends. The width of the Zn-drive-in-diffused P-type region 36 is, for example, 2-4 µm. On the Zn-diffused P⁺-type region 34, a Zn-doped P⁺-type GaAs contact layer 38 is formed, which, similar to the contact layer 32, has a thickness of, for example, 1-3 µm. The thickness of the Zn-diffused P⁺-type region 34 is, for example, 2-4 µm. The Zn concentration of the contact layer 38 and the Zn-diffused P⁺-type region 34 is, for example, 0.8 × 10¹⁹ cm⁻³ to 2 × 10¹⁹ cm⁻³, and the Zn concentration of the Zn-drive-in-diffused P-type region 36 is, for example, 0.5 × 10¹⁸ cm⁻³ to 5 × 10¹⁸ cm⁻³. As is known, when positive and negative voltages are applied to the contact layers 38 and 32, respectively, laser oscillations occur in a portion (black-colored portion in Figure 2) of the Zn-drive-in-diffused P-type region 36 which are located between the active layer 28 and the Zn-diffused P⁺-type region 34. A P-type region has a higher index of refraction than a P⁺-type region. The structure of Figure 2 takes advantage of this fact to improve the confinement of the generated laser light by incorporating the Zn-drive-in-diffused P-type region 36. The P-type GaAs layer 24, the lower cladding layer 26, the active layer 28, the upper cladding layer 30, and the contact layer 32 of the TJS laser device of Figure 2 can be formed by MOCVD, for example. The growth temperature may be, for example, 800 °C. First, in order to fabricate the P-type GaAs layer 24 as shown in Figure 3, an organic metal having a methyl radical, such as, for example, trimethylgallium (CH₃)₃Ga, arsine, AsH₃, and an alkyl compound of arsenic, such as, for example, trimethylarsine, (CH₃)₃As, are used. Trimethylgallium is fed into a reactor, in which the GaAs substrate 22 is placed, at a pressure of, for example, 5.2x10²Pa (39 mmHg) and at a flow rate of, for example, 3 cm³/min. Trimethylarsine is fed to the reactor at a pressure of, for example, 3.10x10⁴Pa (233 mmHg) and at a flow rate of, for example, 50 cm³/min. Arsine is fed in a ratio of, for example, 0.6-0.9 with respect to trimethylarsine. In other words, if the flow rate of arsine is chosen to be the same as the above-stated flow rate of trimethylarsine, namely, 50 cm³/min., for example, a pressure of from 3.10 x 10⁴ x 0.6 Pa to 3.10 x 10⁴ x 0.9 Pa (233 x 0.6 mmHg to 233 x 0.9 mmHg) is employed for arsine to be fed. On the other hand, if the same pressure as trimethylarsine, namely, 3.1 x 10⁴ Pa, (233 mmHg), is used, the flow rate of arsine is from 50 x 0.6 cm³/min to 50 x 0.9 cm³/min. Trimethylgallium, arsine, and trimethylarsine are fed to the reactor substantially simultaneously. It is considered that the following reactions take place in the reactor. (CH₃)₃Ga + AsH₃ → GaAs + 3CH₄ ↑ (CH₃)₃Ga + (CH₃)₃As → GaAs + 4CH₄ ↑ + 2C 2C is solid, and GaAs is doped with this 2C. From the above-described reactions, it is seen that GaAs could be doped with carbon through the reaction of trimethylgallium and trimethylarsine only. In such a case, the control of the C concentration must be done by changing the growth temperature. However, since the growth temperature is an essential factor for MOCVD and, therefore, it is desirable to maintain the growth temperature constant both when the P-type GaAs layer 24 is formed and when the lower cladding layer 26, the active layer 28 and the upper cladding layer 30 are formed, using MOCVD, it is not desirable to change the growth temperature. It is not necessary to change the growth temperature to control the concentration of carbon. Instead, arsine is additionally used and the amount of arsine is changed within the range stated above to control the concentration of carbon. After the P-type GaAs layer 24 is formed in the manner stated above, the lower cladding layers 26, the active layer 28, and the upper cladding layer 30 are successively formed from trimethylaluminum, trimethylgallium, arsine, and hydrogen selenide, as shown in Figure 4. Thereafter, from trimethylgallium, arsine, and hydrogen selenide, the Se-doped N-type GaAs layer 40 from which the contact layers 32 and 38 are to be made is formed on the upper cladding layer 30, as shown in Figure 5. Then, as shown in Figure 6, Zn is diffused from a portion of the upper surface of the GaAs layer 40 to dope portions respectively of the GaAs layer 40, the upper cladding layer 30, the active layer 28, the lower cladding layer 26, the P-type GaAs layer 24, and the GaAs layer 22, with Zn to change these portions together into a region 42. Then, Zn in the Zn-doped P-type diffusion region 42 is drive-in diffused to thereby form a Zn-drive-in-diffused P-type region 44. Finally, a portion of the GaAs layer 40 and that portion of the Zn-doped P-type diffusion region 42 which forms a PN junction with the GaAs layer 40 are removed, which results in the Zn-doped P⁺-type GaAs contact layer 38, the Se-doped N-type GaAs contact layer 32, the Zn-diffused P⁺-type region 34, and the Zn-drive-in-diffused P-type region 36, as shown in Figure 2. In the above-described embodiment, the C-doped P-type GaAs layer 24 is part of a TJS laser, but such a C-doped P-type GaAs layer can be a part of semiconductor lasers of other types, or other semiconductor light-emitting devices, such as a photodiode. It can also be formed as part of an HEMT (high electron mobility transistor) or the like. Further, in the above-described embodiment, it is a GaAs layer that is doped with carbon, but the present invention can be used to dope other compound semiconductors, such as AlGaAs, with carbon. Although, in the above-described embodiment, the Zn-drive-in-diffused P-type region is used, it is not necessarily needed. Furthermore, the Zn-diffused P⁺-type region 34 is described to extend into a portion of the GaAs layer 22, but it may extend only as far as, for example, the P-type GaAs layer 24. The present invention has been described in terms of an embodiment which uses MOCVD, but it can be implemented by MOMBE.

Claims: A method of making a C-doped P-type arsenic compound semiconductor, in which vapour-phase epitaxy involving thermal decomposition is used and in which an organic metal having a methyl radical as source of a constituent of said arsenic compound semiconductor other than arsenic, arsine, and an alkyl compound of arsenic are used as vapour sources. A method of making a C-doped P-type arsenic compound semiconductor according to claim 1, wherein said organic metal having a methyl radical is trimethylgallium. A method of making a C-doped P-type arsenic compound semiconductor according to claim 1 wherein said alkyl compound of arsenic is trimethylarsine. A method of making a C-doped P-type arsenic compound semiconductor according to claim 1 wherein arsine is fed in the ratio of from 0.6 to 0.9 to said organic metal having a methyl radical. A method of making a C-doped P-type arsenic compound semiconductor according to claim 1 wherein said organic metal having a methyl radical is trimethylgallium, said alkyl compound of arsenic is trimethylarsine, and arsine is fed in a ratio of from 0.6 to 0.9 with respect to trimethylgallium. A method of making a C-doped P-type arsenic compound semiconductor according to either of claims 1 or 5 wherein MOCVD is used as said vapour-phase epitaxy. A method of making a C-doped P-type arsenic compound semiconductor according to either of claims 1 or 5 wherein MOMBE is used as said vapour-phase epitaxy. A method of making a C-doped P-type arsenic compound semiconductor according to any one of claims 1 to 7, wherein said method comprises the steps of: placing a semi-insulating substrate (22) in a reactor; and supplying the organic metal having a methyl radical, arsine, and the alkyl compound of arsenic substantially simultaneously to said reactor and causing vapour-phase epitaxy involving thermal decomposition to take place, whereby a P-type compound semiconductor layer (24) doped with carbon is formed on said semi-insulating substrate (22). A method according to any one of claims 1 to 8 for making a semiconductor laser device, said semiconductor laser device comprising a semi-insulating substrate (22), a P-type semiconductor first layer (24) of arsenic compound semiconductor material doped with carbon and disposed on said substrate, an N-type semiconductor second layer (26) disposed on said first layer, an N-type semiconductor third layer (28) disposed on said second layer (26) having a band gap narrower than that of said second layer (26), an N-type semiconductor fourth layer (30) disposed on said third layer (28) having a band gap broader than that of said third layer (28), and a P-type region (34,36) defined in said second, third and fourth semiconductor layers (26,28,30); said method comprising the steps of: forming said first layer (24) by vapour-phase epitaxy involving thermal decomposition with the organic metal having a methyl radical, arsine, and the alkyl compound of arsenic being used as vapour sources; forming said second, third and fourth layers (26,28,30) by vapour-phase epitaxy involving thermal decomposition; and forming said P-type region (34,36) by diffusion.

Inventors: ANDO KOJI, YAGI TETSUYA, ANDO, KOJI, YAGI, TETSUYA, Ando, Koji, c/o Mitsubishi Denki K. K., Yagi, Tetsuya, c/o Mitsubishi Denki K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L21
IPC codes: C30B25, H01L29, H01L21, H01L33, H01S5