Method of making tunable semiconductor laser

Abstract: In a method of making a tunable twin guide (TTG) type tunable semiconductor laser, over the surface of a semiconductor substrate (5) of one conductivity type, a tuning layer (22), a central layer (23) of the opposite conductivity type, and an active layer (24) are sequentially deposited over one another by a selective epitaxy method such as metal organic vapor phase epitaxy (MOVPE). Each layer is stripe-shaped. This method differs from conventional methods in which the processing of semiconductor elements for defining the current path/optical waveguide inside the laser is carried out by etching.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of making a tunable semiconductor laser capable of controlling the wavelength of the coherent light to be output. Description of the Prior Art The tunable semiconductor lasers is used as a local oscillation light source for use in coherent light communications systems, and also as a carrier-wave light source of frequency-division multiplex light transmitter. Not only for increasing channel capacity in optical communications systems but also for enabling longer distance between communication relay stations by longer transmittable distance, it is necessary to improve the performance of the semiconductor laser which is a major component. Effort is being devoted to the development for this. Tunable lasers proposed so far are of different types such as distributed feedback (DFB) type, distributed Bragg Reflector (DBR) type, and tunable twin guide (TTG) type. DFB type and DBR type of these conventional tunable semiconductor lasers are described, for example, in U.S. patent No. 4,949,350 issued on August 14, 1990, and hence the detail of these is omitted herein. A typical example of TTG tunable semiconductor laser is described in U.S. patent No. 5,048,049 issued on September 10, 1991 which is made by the following process: On the surface of a p-type indium phosphide (InP) substrate is a layer sequence consisting of a 2 µm-thick buffer layer of the same material, a 0.15 µm-thick diffraction grating layer of p-type indium gallium arsenide phosphide (InGaAsP), an n-type InGaAsP active layer of 0.1 µm thick, a 0.05 µm-thick antimelt-back layer of InGaAsP, a n-type InP central layer of 0.1 µm thick, a n-type InGaAsP tuning layer of 3 µm thick, a p-type InP cover layer of 1.5 µm thick, and a InGaAsP contact layer of 0.2 µm thick. These are grown by the liquid phase epitaxy technique each on the top of the preceding one. Then the layer sequence is etched to leave a stripe-shaped mesa having a predetermined width. The remaining where etched away is filled as lateral n-type InP regions. The periphery of the lateral regions is inactivated by proton implantation or p-type impurity diffusion. Then each of conducting films insulated from each other is applied by the usual patterning onto the surfaces of the p-type InP substrate, the contact layer, and the lateral regions, respectively, and connected to a laser oscillation drive power input electrode, a tuning power input electrode, and a common lateral layer electrode, respectively. Electric current injection of charge carriers (holes and electrons) bringing about laser oscillation reaches the common electrode from the laser oscillation drive power input electrode via the conductor layer on the substrate undersurface, substrate, barrier layer, diffraction grating layer, active layer, and lateral regions. On the other hand, the current injection for tuning reaches the common electrode from the tunable power input electrode via the cover, tuning, and lateral regions. A TTG tunable semiconductor laser achieving independent definition of the current path and optical waveguide, respectively, by different means is described in U.S. patent No. 5,008,893 issued on April 16, 1991, and its manufacturing process is summarized as follows: On the surface of a p-type InP substrate of 80 µm thick, a multi-layer structure consisting of a 5 µm thick buffer layer of the same material, a 0.15 µm-thick barrier layer composed of three layers of n-type InGaAsP, n-type InP and p-type InP are formed sequentially. Then a part of the multilayer structure is etched away in stripe form corresponding to the current path for supply of carriers for laser oscillation. A 1 µm-thick compensating layer of p-type InP is formed over the overall surface in addition to filling the etched part with the same material, and thereon an active layer of InGaAsP is formed. After applying a 0.03 µm-thick protective film of InGaAsP onto this active layer, an n-type InP central layer of 0.15 µm thick, and an n-type InGaAsP tuning layer of 0.2 µm thick are formed. After forming the first contact layer of n-type InGaAsP of 0.1 µm thick on the entire surface of the tuning layer, a p-type InP cladding layer of 1.5 µm thick and the second contact layer of p-type InGaAsP of 0.2 µm thick are formed. Then the first etching is carried out to remove a part of the periphery of the multilayer structure including from the second contact layer to the active layer to form an edge parallel to the stripes. Then the second etching is carried out to leave a part of the multilayer structure from the second contact layer to the cladding layer to be a ridge waveguide having the same form as the stripe of the current path and to remove the remaining part. Subsequently to the substrate undersurface, and the first and second contact layers, respectively, are connected each of a laser oscillation drive power input electrode, common electrode, and tunable power input electrode. In the TTG tunable semiconductor laser with this ridge waveguide, current for carrier injection bringing about laser oscillation reaches the common electrode from the laser oscillation drive power input electrode through the compensating layer filling the stripe-shaped window in the barrier layer, the active layer and the central layer. On the other hand, the current for carrier injection for tuning reaches the common electrode from the tuning power input electrode through the cladding, and tuning layers. As apparent from the above-described structure, the TTG tunable semiconductor laser is tunable through variation of the refractive index which can be caused by carrier injection from the wavelength control electrode under the condition of single mode oscillation kept by carrier injection from the laser oscillation drive electrode and in turn. The change in refractive index of the tuning layer which varies laser oscillation wavelength is due to production of plasma in the layer resulting from the carrier injection. The tuning efficiency of the TTG laser is dependent on how much carriers are enclosed into the tuning layer and on what extent the laser oscillation light and the tuning layer interact. The above-described process of manufacturing TTG laser includes a processing step of building a striped-shaped mesa or a waveguide ridge by selective etching of the multilayer structure as described above, and therefore these geometries vary considerably with the temperature, concentration, and agitation of the etchant used in this step. In other words, progress of etching varies finely with site of the multilayer structure and variable processing time, and this makes it difficult to ensure the geometrical uniformity of the stripe-shaped mesa or waveguide ridge. Further the variation in progress in etching causes geometrical disorder of stripe-shaped mesa and waveguide ridge, which in turn results in scattering of laser light, loss of laser light output, and lowered efficiency of the current to laser light conversion. Besides the scattering of laser light reduces the coupling efficiency between the semiconductor laser light and an optical fiber constituting optical light transmission path. Two attempts to address these problems are described in Japanese patent application JP-A-1055887 (Patent Abstracts of Japan vol. 13, No. 264 (E-774) 19 June 1989) and European patent application EP-A-0433051. JP-A-1055887 describes a method for making a stripe-shaped, laminated heterostructure on a substrate by vapour phase epitaxy, in which a crushing layer is first formed on regions of the substrate surface on which growth is not required. After formation of the desired heterostructure, the crushing layer must be completely removed by vapour phase etching and replaced by a current blocking structure. EP-A-0433051 describes a method of making a semiconductor laser in which a non-crystalline, dielectric film is formed on a substrate so as to define a channel. A mesa stripe structure is then formed on the substrate in the channel by a selective growth technique such as MOCVD (metal organic chemical vapour deposition) and a current blocking layer grown on the dielectric film. SUMMARY OF THE INVENTION The invention provides a method for making a tunable semiconductor laser of the above mentioned type as defined in the appended independent claims. Preferred features of the invention are defined in dependent subclaims. In the method of this invention the stripe-shaped, multilayered ridge is formed by the deposition of a tuning layer, a central layer and an active layer by a selective growth method such as a selective epitaxy method. By eliminating the etching process conventionally used to form the shape of the multilayered ridge, the method according to the invention may advantageously provide improved precision and uniformity in the fabrication of the multilayered ridge, and thus lead to improved laser performance. Advantageously, the selective growth method may be metal organic vapour phase epitaxy (MOVPE), which may lead to further improved realization of the stripe structure with high precision and good uniformity. In addition it may enable formation of a thinner layer between the active layer and the tuning layer, contributing to a wide tunable bandwidth of laser light. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a) through 1(c) and Figs. 2(a) and through 2(c) are sectional views illustrating in sequence the processing steps of the first embodiment of the present invention; Fig. 3 is a sectional view of the TTG semiconductor laser manufactured by the first embodiment of the present invention; Figs. 4(a) through 4(c) are sectional views correspondent to Figs. 2 (a) through 2(c), which illustrate in sequence the processing steps of a modified form of the first embodiment of the present invention; Figs. 5(a) through 5(d) are sectional views correspondent to Figs. 2(a) through 2(c), which illustrate in sequence the processing steps of the second embodiment of the present invention; Fig. 6 is a sectional view of the TTG semiconductor laser manufactured by the second embodiment of the present invention; and Figs. 7a and 7b are sectional views correspondent to Figs. 5(c) and 5(d), which illustrate in sequence the processing steps of a modified form of the second embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the present invention as process used in fabricating a 1.5 µm band TTG semiconductor laser will be set forth with reference Figs. 1(a) through 1(c), Fig. 2(a) through 2(c) and Fig. 3 illustrating in sequence the processing steps of it. Over the entire surface of a p-type InP substrate 1,a 1 µm-thick n-type InP layer 2 is grown to a carrier concentration of 2 x 10 17 cm -3 by MOVPE, and thereon a stripe mask 3 of SiO 2 provided with a stripe-shaped window of 3 µm wide is formed (Fig. 1(a)). Then selective diffusion of Zn is carried out through mask 3, thereby the exposed region being inverted with respect to type of conductivity to form a p-type InP region 4 (Fig. 1(b)). After removing mask 3, a 1 µm-thick p-type InP layer 5 is grown over the whole surface to a carrier concentration of 7 x 10 17 cm -3 (Fig. 1(c)). After providing a diffraction grating (not shown) on the surface of p-type InP layer 5, a mask 21 of SiO 2 provided with a 2.0 µm wide window is formed. On the exposed surface of p-type InP layer 5 through this window, multilayer structure is formed by using selective MOVPE. That is, each on the top of the preceding one, a 0.25 pm-thick InGaAsP tuning layer 22, a 0.1 µm-thick central layer 23 of n-type InP (4 x 10 18 cm -3 in carrier concentration), an active layer 24, and a 0.2 µm-thick p-type InP layer 25 are sequentially grown (Fig. 2(a)). Active layer 24 is constructed of a 10 nm-thick (100 A-thick) InGaAsP layer, a seven-period multi-quantum well structure (which consists of a 7 nm-thick (70 Å-thick) InGaAsP layer and a 10 nm-thick (100 A-thick) InGaAsP layer), and a 50 nm-thick (500 Å-thick) InGaAsP layer. After removing mask 21, and covering p-type InP layer 25 with a SiO 2 film 26 formed on the surface of it, n-type InP is grown over the whole surface of InP layer 5 on the sides of the multilayer ridge 22 to 25, thereby a compensating contact layer of 0.7 µm thick being formed (Fig. 2(b)). Besides after removing mask 26, using a stripe mask 28 of SiO 2 having a window of 6 µm wide, a 1.5 µm-thick p-type InP cladding layer 29 and a 0.1 µm-thick p + -type InGaAs cap layer 30 are grown directly on the multilayer ridge by using selective MOVPE (Fig. 2(c)). After forming SiO 2 film 31 covering the whole resulting exposed surface including the sides of the cladding layer 29, and the surfaces of a cap layer 30 and a compensating contact layer 27, metal film is formed by patterning on layers 27, 30 as anode 33 and cathode 32, respectively. Similarly on the undersurface of substrate 1, a conductor film is formed as another anode 33 (Fig. 3). In the tunable semiconductor laser (Fig. 3) manufactured in this embodiment, the drive current bringing about the carrier injection for laser oscillation excitation flows from the anode 33 through the cap layer 30, cladding layer 29, p-type InP layer 25, an active layer 24 including a quantum well structure, a central layer 23, and a compensating contact layer 27 to a common cathode electrode 32. On the other hand, tuning current causing the carrier injection for controlling the refractive index of the tuning layer 22 reaches a common electrode 32 through the aforesaid layers 1, 4, 5, 22, 23 and 27. These current paths are indicated by dashed lines in Fig. 3. In stead of the tuning layer 22 and the central layer 23 of the multilayer ridge 22 to 25 in the first embodiment described above, as formed by selective growth, layers 22A, 23A covering the whole surface of the p-type InP layer 5 can be grown by using MOVPE (Fig. 4(a)). In this case SiO 2 film 21A provided with a stripe-shaped window having a width smaller than 2 µm is formed on the surface of the central layer 23A, and then an active layer 24 is formed by using the selective MOVPE on the surface 23A exposed through the window. After enlarging the width of the stripe window in a mask 21A to 6 µm, a cladding layer 29A and a cap layer 30A are selectively grown (Fig. 4(b)). After covering the surface of the cladding layer 29A, cap layer 30A and a part of the central layer 23A with a mask 31A, a contact layer 32A and a cap layer 32B are formed by using selective MOVPE (Fig. 4(c)). The cap layer 30A, the cap layer 32B, and the conductor film (not shown) on the undersurface of the substrate 1 (Fig. 4) correspond to the cap layer 30, and the electrodes 32 and 33, respectively, of the above-described first embodiment. The tunable semiconductor laser according to this embodiment recorded 20 mA as excitation current threshold for generating laser oscillation and 15 mW or more as laser light output power. Besides it has been demonstrated the oscillation wavelength to be continuously variable within the range of up to 8.5 nm (85 Å) under the condition of laser light output kept at more than 10 mW, and the spectral linewidth was within up to 10 MHz within the same wavelength range. The second embodiment of the present invention will be described with reference to Figs. 5(a) through 5(d) illustrating in sequence the processing steps like Figs. 2(a) through 2(c). On the surface of a p-type InP substrate 70, a diffraction grating (not shown) is formed, and then a mask 71 of SiO 2 provided with a stripe-shaped window of 2 µm wide is formed. On the surface of a p-type InP substrate 70 exposed through this window, the multilayer structure is formed by using selective MOVPE, i.e., a 0.25 µm-thick InGaAsP tuning layer 72, a 0.1 µm-thick central layer 73 of n-type InP (4 x 10 18 cm -3 in carrier concentration), an active layer 74, and a 0.82 µm-thick p-type InP layer 75. The active layer 74 is constructed of a 10 nm-thick (100 A-thick) InGaAsP layer, a seven-period multi-quantum well structure (which consists of a 7 nm-thick (70 Å-thick) InGaAsP layer and a 10 nm-thick (100 Å-thick) InGaAsP layer), and a 50 nm-thick (500 Å-thick) InGaAsP layer. The above-mentioned thicknesses of those layers cause disappearance of (111) B plane of layer, and in turn result in a pyramid configuration of the multilayer ridge 72 to 75 (Fig. 5(a)). After removing mask 71, a 0.7 µm-thick n-type InP layer 76 is formed over the whole upper surface of substrate 70 and the side of the pyramid (Fig. 5(b)). Subsequently the entire surface of the layer 76 is etched with a sulfuric acid-containing etchant, resulting in removing the top portion of p-type InP layer 75 (Fig. 5(c)). After bringing a mask 77 provide with,a 6 µm-wide, stripe-shaped SiO 2 window correspondent in area to the multilayer ridge, on the surface of n-type InP layer 76 exposed through this window, a 1.5 µm-thick p-type InP cladding layer 78 and a 0.1 µm-thick p-type InGaAs cap layer 79 are formed by using selective MOVPE (Fig. 5(d)). Additionally referring to Fig. 6, after forming SiO 2 film 84 over, and covering the sides of the cladding layer 78, cap layer 79, and compensating contact layer 76, a metal film is formed by patterning on layers 79 and 76 as an anode 86 and a cathode 85, respectively. On the undersurface of the substrate 70, another conductor film is formed as another anode 86. Besides in the second embodiment described above, the process of forming the compensating contact layer 76 which covers the summed surface of the surface of the substrate 70 and the sides of the active layer (Fig. 5(b)) is followed by the process of removing the top portion of the multilayer ridge (Fig. 5(c)). An alternative process can be used which comprises forming a SiO 2 mask 80 to cover the whole surface of layer 76, opening in this mask a stripe window of 2 µm wide just above the multilayer ridge, and then selectively diffusing Zn through the window to form an inverted to-p-type region 81 reaching the layer 75 (Fig. 7(a)). In this case after enlarging the width of window of mask 80 to 6 µm following Zn diffusion step, the cladding layer 78 and the cap layer 79 are formed on the surface of the compensating contact layer 76 exposed through mask 80 (Fig. 7(b)). The subsequent steps of forming anodes 86 and a common cathode 85 are as described above with reference to Fig. 6. The tunable semiconductor lasers according to the second embodiment and the modified form thereof (Figs. 7(a) and 7(b)) described above showed almost the same performance as that of the first embodiment and the modified form thereof. Under the condition of the laser light output power kept to be 10 mW or more, they showed the continuously adjustable range of oscillation wavelength to be about 7 nm (70 Å) while the first embodiment showed to be 8.5 nm (85 Å) as stated above. As described above, the process of fabricating TTG tunable semiconductor lasers according to the present invention comprising a step of processing the semiconductor elements for defining the internal current path and/or optical waveguide, which is accomplished not by etching but by using a selective growth method, preferably selective MOVPE. This enables scattering of laser light in the active layer/quantum well structure and the tuning layer to be prevented, with the consequent increase of the efficiency of the drive current to laser light conversion, and in turn a reduced loss of coupling efficiency between an optical fiber and laser output light. Further effect of enabling smaller thickness from the active layer to the tuning layer produces the possibility to increase the laser oscillation tuning efficiency.

Claims: A method of making a tunable twin guide semiconductor laser comprising: a step of preparing a semiconductor substrate provided at the surface thereof with a first semiconductor layer (5) having a first conductivity type; a step of forming a diffraction grating on a surface of said first semiconductor layer; a step of forming a first mask (21) on the surface of said first semiconductor layer, said first mask having a stripe-shaped window; a step of forming through said window a stripe-shaped multilayer ridge comprising a tuning layer (22), a central layer (23) having a second conductivity type, and an active layer (24) by using selective epitaxy; a step of removing said first mask; a step of forming a second mask (26) for preventing epitaxial growth on a surface of said multilayer ridge; a step of forming by epitaxy a compensating semiconductor layer (27) having said second conductivity on the surface of said multilayer ridge exposed through said second mask; and a step of forming a cladding layer (29) on a surface of said multilayer ridge and a neighboring surface of said compensating semiconductor layer. A method of making a tunable twin guide semiconductor laser comprising: a step of preparing a semiconductor substrate provided at a surface thereof with a first semiconductor layer (1) having a first conductivity type; a step of growing epitaxially a second semiconductor layer (2) having a second conductivity type on a surface of said first semiconductor layer; a step of forming a first mask (3) for diffusion on a surface of said second semiconductor layer, said first mask for diffusion being provided with a stripe-shaped window; a step of forming a diffusion region (4) having said first conductivity type by performing selective diffusion of impurity from the surface of said second semiconductor layer exposed through said window of said first mask; a step of removing said first mask; a step of forming by epitaxy a third semiconductor layer (5) having said first conductivity type on the surface of said second semiconductor layer containing said diffusion region; a step of forming on a surface of said third semiconductor layer a diffraction grating; a step of forming by epitaxy in sequence a tuning layer (22A) and a central layer (23A) on the surface of said third semiconductor layer having said diffraction grating formed thereon; a step for forming a second mask (21A) on a surface of said central layer, said second mask being provided with a stripe-shaped window; a step of forming by epitaxy an active layer (24) on the surface of said central layer exposed through said stripe-shaped window of said second mask; a step of enlarging said stripe-shaped window of said second mask near said active layer to produce an exposed area of said central layer; and a step of forming by epitaxy a cladding layer (29A) of said first conductivity type so as to cover an upper surface and sides of said active layer, and the surface of said central layer (23A) exposed through said enlarged window of said second mask (21A). A method of making a tunable twin guide semiconductor laser comprising: a step of preparing a semiconductor substrate provided at the surface thereof with a first semiconductor layer (70) of one conductivity type; a step of providing a diffraction grating on a surface of said first semiconductor layer; a step of forming a first mask (71) on the surface of said first semiconductor layer, said first mask being provided with a stripe-shaped window; a step of forming through said window a stripe-shaped multilayer ridge comprising a tuning layer (72), a central layer (73) and an active layer (74) by using selective epitaxy; a step of removing said first mask; a step of forming by epitaxy a second semiconductor layer (76) of the opposite conductivity on a surface of said multilayer ridge and said first semiconductor layer; a step of removing a part of said second semiconductor layer so as to expose a surface of said multilayer ridge; and a step of forming a cladding layer (78) so as to cover an exposed surface of said multilayer ridge and a neighboring region of said second semiconductor layer (76). A method of making a tunable semiconductor laser comprising: a step of preparing a semiconductor substrate provided at a surface thereof with a first semiconductor layer (70) of one conductivity type; a step of providing a diffraction grating on a surface of said first semiconductor layer; a step of forming a first mask (71) on the surface of said first semiconductor layer, said first mask being provided with a stripe-shaped window; a step of forming through said window a stripe-shaped multilayer ridge comprising a tuning layer (72), a central layer (73) and an active layer (74) by using a selective epitaxy growth method; a step of removing said first mask; a step of forming by epitaxy a second semiconductor layer (76) of the opposite conductivity on a surface of said multilayer ridge and said first semiconductor layer; a step of diffusing selectively impurity to form a diffused region (81) including a part of said multilayer ridge, thereby the opposite conductivity of said second semiconductor layer at the neighboring surface being inverted to said one conductivity type; and a step of forming by epitaxy a cladding layer (78) so as to cover the surface of said diffused region (81) and a neighboring part of said semiconductor layer (76) of said opposite conductivity type. A method of making the tunable twin guide semiconductor laser according to any preceding claim, further comprising a step of growing epitaxially a cap layer (30, 30A, 79) on a surface of said cladding layer (29, 29A, 78). A method of making a tunable twin guide semiconductor laser according to any preceding claim, further comprising a step of forming selectively conductor layers on an undersurface of said semiconductor substrate and on a surface of said cap or cladding layer, respectively, and a step of forming conductor layers located away from said multilayer ridge or active layer in a plane of said semiconductor substrate and connected to said compensating semiconductor layer (27) or said central layer (23A) or said second semiconductor layer (76). A method of fabricating a tunable twin guide semiconductor laser according to any preceding claim, wherein said epitaxy is carried out by using a metal organic vapour phase epitaxy (MOVPE).

Inventors: SAKATA YASUTAKA, SASAKI TATSUYA, YAMAGUCHI MASAYUKI, SAKATA, YASUTAKA, SASAKI, TATSUYA, YAMAGUCHI, MASAYUKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01S3
IPC codes: H01S5, H01L33