Semiconductor laser devices and process for making them

Abstract: Carrier injection layers (5, 7) are formed on an Al x Ga 1-x As clad layer (2) of high resistance by embedding and re-growth with the use of crystal growth processes such as MO-VPE or MO-MBE where material supply sources are all provided by gas sources, whereby it is not required to mesa-etch embedding regions to a depth reaching a substrate (1); the formation of any separate blocking layer is dispensed with; any possible influence of the substrate (1) is eliminated; and the time constant is decreased by decreasing the inter-electrode capacity.

Description: The present invention relates to a process for making an embedded type semiconductor laser device. Figure 2 illustrates a typical structure of a conventional semiconductor laser device in which the transverse mode is controlled by the provision of an embedded layer. In Fig. 2, reference numeral 22 stands for a semi-insulating substrate, 23 a lower clad layer, 24 an active layer, 25 an upper clad layer, 26 a p-type carrier injection layer, 27 a blocking layer, 28 an n-type carrier injection layer, and 29 and 30 electrodes. The laser device shown in Figure 2 is of the structure that carriers are transversely injected in the active layer. The lower clad layer 23 of Al x Ga 1-x As, the active layer 24 of Al y Ga 1-y As and the upper clad layer 25 of Al x Ga 1-x As, provided that x > y, are laminated on the semi-insulating substrate 22 of GaAs in that order. The active layer 24 may be not only of the above structure but also of a singlet or multiplet quantum well structure or a multiplet active layer structure thereof. Figure 3 is a view for illustrating a conventional multiplet quantum well structure. When the thickness of the active layer is 20 nm or below, electrons occupy only limited levels due to the resulting quantum effect. For that reason, transition occurs on the thus limited levels so that the efficiency of radiation is increased, resulting in a decrease in threshold value currents. The multiplet quantum well structure makes use of such phenomena. As illustrated, a very small thickness is given to an active layer 33 formed between upper and lower clad layers 31 and 32, while barrier layers 34 of AlGaAs and quantum wells 35 of GaAs are repeatedly formed. To effectively confine light in the active layer, the mixed crystal ratio of Al of the barrier layers is smaller than that of the clad layers. Usually, certain limitations are imposed upon the number of wells, the thickness of barrier layers and the mixed crystal ratio of barrier layers (generally, the number of wells is 5, the thickness of barrier layers is 6 to 20 nm and the Al to As ratio of barrier layers is 0.2 to 0.3). This is because in the case of the structure where currents flow in the longitudinal direction, there is a change in the injection of carriers in the upper and lower quantum wells. However, the transverse junction type laser device is freed of such limitations. Thus, an energy band structure of a conduction band Ec and a valence band Ev is obtained, as illustrated. Carriers are injected in the quantum wells, so that radiation takes place due to the recombination of electrons with holes, and the coherency of output light is maintained by reason that the quantum wells interact. It is noted that the reason why the space between the clad layer and the first quantum well is increased is to prevent the similarity of other quantum well to the energy band structure from deteriorating because of the energy level of the clad layer being high. By using the quantum wells for the active layer, it is also possible to obtain a very decreased inter-electrode capacity. Usually, regions for embedding p- and n-type carrier injection layers 26 and 28 are formed by the LPE (liquid-phase epitaxial) technique. A blocking layer 27 is provided to prevent any leak from occurring between the p- and n-type carrier injection layers 2 through a substrate. However, difficulty is involved in the formation of the regions for embedding carrier injection layers by the LPE technique, since a melt tends to be oxidized to form a surface oxide film on Al x Ga 1-x As, if the value of x is increased as is the case with the clad layers of semiconductor laser devices. For that reason, it has been required to previously mesa-etch a region to a depth reaching the GaAs substrate 22 and embed a carrier layer in the GaAs substrate 22. When currents flow in the transverse direction in this structure, leak currents occur through the substrate. In order to prevent this, it is required that the blocking layer 27 be provided in either one of the embedding regions (in Figure 2, a p-type AlGaAs layer having a molar Al to As ratio of 0.2 to 0.4 be provided in the lower portion of the n-side carrier injection layer 27). It is difficult to form a blocking layer having a uniformly controlled large area, as experienced with a 2- or 3-inch (1 inch = 2.54 cm) substrate, thus leading to a lowering of device process yields. JP-A-60-235491 discloses an embedded (transverse junction) semiconductor laser composed of a substrate; a mesa form double heterostructure in which an active layer is sandwiched between upper and lower Al x Ga 1-x As clad layers with an Al mole fraction o < x < 1; and n⁺- and n⁻-type contact layers having electrodes on their surfaces. JP-A-61-079284 discloses a transverse junction (quantum-well active layer) semiconductor laser comprising two high specific resistance cladding layers formed of Al x Ga 1-x As, 0.3 ≦ x ≦ 0.8 which enclose an active layer and barrier layers. JP-A-58-225680 discloses a transverse junction (quantum-well active layer) semiconductor laser similar to that made according to the process of this invention, but having no n⁺- and n⁻-type contact layers provided between the carrier injection layers and the corresponding electrodes. An object of the present invention is to provide a process to make an embedded type of semiconductor laser device by embedding and regrowing carrier injection layers on Al x Ga 1-x As wherein 0.3 ≦ x ≦ 0.85 with the use of crystal growth processes capable of growing ternary or more multiple system crystals containing aluminium, by the MO-VPE (organic metal vapor-phase epitaxial) technique or the MO-MBE (organic metal molecular beam epitaxial) technique. According to the present invention we provide a process for making a semiconductor laser device comprising a semi-insulating substrate; a mesa-form double hetero structure in which an active layer of Al y Ga 1-y As is sandwiched between upper and lower clad layers, each having a high specific resistance and being formed of Al x Ga 1-x As, provided that 0.3 ≦ x ≦ 0.85, 0 ≦ y ≦ 0.45 and 0 ≦ y < x; n- and p- type carrier injection layers of Al z Ga 1-z As and between which said double hetero structure is interposed, provided that y < z ≦ x; and n⁺- and p⁺-type contact layers having electrodes formed on their surfaces; in which process mesa etching is applied to halfway through the lower clad layer and afterwards the carrier injection layers are embedded and re-grown on said lower clad layer of Al x Ga 1-x Ga with the use of the crystal growth process MO-VPE or MO-MBE where the material supply sources are all provided by gas sources, and wherein just prior to re-growth, an oxide film is removed from the surface of the Al x Ga 1-x As by vapor-phase etching. In the accompanying drawings:- Figure 1 is a cross-sectional view showing the structure of a semiconductor laser device made according to the process of the present invention; Figure 2 is a view showing the structure of a conventional semiconductor laser device; and Figure 3 is a view illustrating a conventional multiple quantum well structure. Referring now to Figure 1, there is shown a structure of the semiconductor laser device according to the present invention, wherein reference numeral 1 stands for a semi-insulating substrate, 2 a lower clad layer, 3 an active layer, 4 an upper clad layer, 5 a p-type carrier injection layer, 6 a p⁺-contact layer, 7 an n-type carrier injection layer, 8 an n⁺-contact layer and 9 and 10 electrodes. As in the case with the structure of Fig. 2, on the semi-insulating substrate 1 of, e.g., 0.3 mm to 0.5 mm in thickness, there is formed a double hetero (DH) structure including the Al y Ga 1-y As active layer 3 having a thickness of 3 nm to 500 nm, preferably 10 to 300 nm, which is sandwiched between the Al x Ga 1-x As clad layers 2 and 4, provided that 0.3 ≦ x ≦ 0.85, 0 ≦ y ≦ 0.45 and 0 ≦ y ≦ x. The upper and lower clad layers 4 and 2 have a thickness of 0.5 to 2 µm and 1 to 5 µm, respectively. It is noted that the substrate may be any amorphous substrate of glass or any low-resistance substrate, if the vapor-phase growth of Al y Ga 1-y As takes place thereon. It is also understood that the structure of the active layer 3 is not limited to the above type, and may be of a singlet or multiplet quantum well type. Alternatively, the active layer 3 may be formed of a laminate of two or more of such active layers. Afterwards, in order to form an embedding region in either one of the p- and n-sides as shown in Figure 1, mesa etching is applied to halfway through the lower clad layer 2 of Al x Ga 1-x As (0.3 ≦ x ≦ 0.85), leaving behind a portion of about 0.2 to 1 µm, to form a p-side (11) or n-side (12) carrier injection region on Al x Ga 1-x As. In this case, the carrier concentration may be about 5 × 10¹⁷ to 2 × 10¹⁸cm⁻³ for the p-type carrier injection layer 5, about 2 × 10¹⁸ to 5 × 10¹⁹cm⁻³ for the p⁺-contact layer 6, about 5 × 10¹⁷ to 2 × 10¹⁸cm⁻³ for the n-type carrier injection layer 7 and about 1 × 10¹⁸ to 3 × 10¹⁸cm⁻³ for the n⁺-contact layer 8, as measured by the Van der Pauw's method. Subsequently, the other region 12 or 11 is subjected to mesa etching and embedding growth. Regions 11 and 12 comprising the embedded layers 5, 7 and the contact layers 6, 8 are formed in this manner. In the prior art having recourse to the LPE technique, it was required to dig a region to a depth reaching the GaAs substrate because of crystal growth being made difficult due to the presence of the surface oxide film of Al x Ga 1-x As. In contrast, not only does the present invention render re-growth on Al x Ga 1-x As possible, but it is also possible to carry out embedding and re-growth by making use of a crystal growth process which allows selective growth with the use of a protective film such as SiO₂ or SiN x , for instance, the MO-VPE or MO-MBE technique where such material supply sources as a gallium source (trimethyl gallium), an arsenic source (arsine) and an aluminium source (trimethyl aluminium) are all provided by gas sources. It is to be noted, however, that just prior to re-growth, an oxide film is removed in situ from the surface of Al x Ga 1-x As by vapor-phase etching using HCl (hydrogen chloride), etc. to form the Al z Ga 1-z As embedded layers 5 and 7, provided that x ≧ z > y. At this time, the layers 5 and 7 are grown to the surface side beyond the active layer 3. Subsequently, the contact layers of p⁺-GaAs 6 and n⁺-GaAs 8 are formed. Finally, the electrodes 9 and 10 are formed on the contact layers by vapor-deposition. Examples The present invention will now be explained with reference to the following examples. By the vacuum MO-VPE technique, 1.5-µm undoped Al 0.42 Ga 0.50 As, 0.2 µm-undoped GaAs and 1.0 µm-undoped Al 0.42 Ga 0.50 As were grown on a semi-insulating substrate in that order at a growth temperature of 710 to 790°C and a growth pressure of 76 Torr. An SiN x film of 150 nm (1500 Å) is then deposited on the surface of the substrate by PCVD (plasma chemical vapor-deposition). The SiN x film, now applied with a resist, is exposed to ultraviolet rays to pattern an one-side embedding region, and is then windowed with a hydrofluoric acid buffer solution (1HF:24NH₄F) followed by the removal of the resist. Thereafter, about 1.7-µm mesa etching is applied to halfway through the lower clad layer of undoped Al 0.42 Ga 0.50 As with an aqueous etching solution containing phosphates in excess (1H₃PO₄:1H₂O₂:3HOC₂H₅C₂H₅OH). By the vacuum MO-VPE technique, 1-µm n-(or p-)Al 0.32 Ga 0.60 As and 0.7-µm n-(or p-)GaAs were embedded at 710°C in that order for growth. At this time, just prior to re-growth, HCl is admitted into a reactor to remove about 0.05 to 0.2 µm of the re-growth surface layer by vapor-phase etching. With the use of a similar process, the activated layer of GaAs (y = 0) is mesa-etched to a transverse width of about 0.15 µm to form another p-(or n-)type embedding region. Afterwards, (epi-side) AuZn/Ni/Au (surface side) and (epi-side) AuZn/Ni/Au (surface side) are respectively vapor-deposited on the surfaces of the n- and p-side regions. Alternatively, in the process of making the above laser device, vapor-phase mesa etching may be carried out in place of wet etching, while a gas such as HCl is admitted in the reactor using the above protective film, immediately followed by embedding and re-growth. The thus obtained laser device was found to have superior reproducibility and succeeded in laser oscillation. However, samples not subjected to vapor-etching in the reactor just prior to growth all failed to oscillate. The transverse junction type of embedded hetero structure shown in Figure 1 was continuously oscillated in the form of a quintuplet quantum well structure at normal temperature, a resonator wavelength of 160 µm, an active layer's width of 2.0 µm, an active layer's thickness of 230 nm, a barrier layer's thickness of 19 nm and a quantum well's thickness of 11 nm. As a result, output light of 1.8 mW at an oscillation wavelength of 868 to 870 nm was obtained at a threshold value current of 9.5 mA, a forward inter-electrode voltage of 34 Ω and 14 mA. For the inter-electrode capacity, a value as small as 0.042 pF (at zero bias time) was obtained. According to the present invention as detailed above, in the laser device of the structure wherein carriers are injected in the transverse direction, the carrier injection layers are formed on the clad layers of Al x Ga 1-x As of high resistance by embedding and re-growth with the use of crystal growth techniques such as MO-VPE or MO-MBE where material supply sources are all provided by gas sources, whereby: (1) it is not required to perform mesa etching to a depth reaching the substrate; nor is the formation of any separate blocking layer needed; (2) any possible influence of the substrate is eliminated; (3) the inter-electrode capacity can be reduced, resulting in a decrease in the time constant; (4) these devices may be used for the production of superfast response devices, e.g., photocouplers and OEIC; (5) since these devices may be made by planar processes while electrical insulation is provided between the elements by the clad layers, they be monolithically integrated with other electronic circuits and photocircuits; and (6) the process may be simplified because of no need of forming blocking layers, which is one cause of yield reductions.

Claims: A process for making a semiconductor laser device comprising a semi-insulating substrate (1); a mesa-form double hetero structure in which an active layer (3) of Al y Ga 1-y As is sandwiched between upper (4) and lower (2) clad layers, each having a high specific resistance and being formed of Al x Ga 1-x As, provided that 0.3≦ x ≦ 0.85, 0≦ y ≦ 0.45 and 0≦ y< x; n- and p- type carrier injection layers (5,7) of Al z Ga 1-z As and between which said double hetero structure (2,3,4) is interposed, provided that y < z ≦ x; and n⁺- and p⁺-type contact layers (6,8) having electrodes (9,10) formed on their surfaces; in which process mesa etching is applied to halfway through the lower clad layer (2), and afterwards the carrier injection layers (5,7) are embedded and re-grown on said lower clad layer of Al x Ga 1-x Ga with the use of the crystal growth process MO-VPE or MO-MBE where the material supply sources are all provided by gas sources, and wherein just prior to re-growth, an oxide film is removed from the surface of the Al x Ga 1-x As by vapor-phase etching. A process as claimed in Claim 1, which includes a pn-junction in the transverse direction.

Inventors: GOTOH HIDEKI C O MITSUBISHI KA, SHIMOYAMA KENJI C O MITSUBISHI, GOTOH, HIDEKI C/O MITSUBISHI KASEI CORPORATION, SHIMOYAMA, KENJI C/O MITSUBISHI KASEI CORPORATION
Applicants: MITSUBISHI CHEM IND, MITSUBISHI KASEI CORPORATION
Application year: 1989
Main classification: H01S3
IPC codes: H01S5, H01L21