Semiconductor laser device and method of manufacturing same

Abstract: A high-power low-noise semiconductor laser device required for light sources for an erasable optical disc and a method of manufacturing the device are disclosed. Especially, a self-sustained pulsating semiconductor laser device is disclosed in which an MQW structure is used for an active layer (4, 44 or 44a), and thus a kink output power and a light output power by which the self-pulsation is enabled are increased to 10 mW or more and noise is reduced. Another semiconductor laser device of a self-­ pulsation type is disclosed in which impurities are doped in the active layer to reduce the self-pulsating frequency to obtain a low-noise characteristic even if higher optical feeback occurs.

Description: BACKGROUND OF THE INVENTION The present invention relates to semiconductor laser devices and methods of manufacturing same, and more particularly to such devices which have high-power low-noise characteristics effective for use as optical sources for erasable optical discs and methods of manufacturing the devices. Self-sustained pulsating semiconductor laser devices are used as low-noise semiconductor laser devices. However, in conventional self-sustained pulsating semiconductor laser devices, the kink power and the maximum optical output which enables the self-sustained pulsation is low, for example, about 7 - 9 mW, as disclosed in Japanese papers, IECE TECHNICAL REPORT ON OPTICAL AND QUANTUM ELECTRONICS , OQE 84-57 (1984), pp. 39-46 and OQE 84-30, pp. 65-72. Self-sustained pulsating semiconductor laser devices having an active layer of a double heterostructure and an active layer of a multiquantum well structure and the characteristics thereof are disclosed, for example, in the Inventors' article entitled "Self-sustained Pulsating Semiconductor Lasers with a Ridge Stripe Structure", Japanese paper, IECE TECHNICAL REPORT ON OPTICAL AND QUANTUM ELECTRONICS , OQE 88-5 (1988), pp. 33-38. The technique disclosed in this report is incorporated in the present invention. Conventionally, the single quantum well structure of the semiconductor lasers is mainly used for reducing their threshold current. In addition, a pair of superlattice waveguides are provided one on an upper surface of the single quantum well active layer and the other on a lower surface of the layer in order to improve the flatness and crystallinity of the quantum well interface. Semiconductor laser devices of this type are disclosed, for example, in Japanese paper, EXTENDED ABSTRACTS (THE 46TH AUTUMN MEETING, 1985), THE JAPAN SOCIETY OF APPLIED PHYSICS , 1P - N-8, P. 196. Self-sustained pulsating semiconductor lasers are conventionally suitable for low-noise semiconductor lasers required as optical sources for light video discs, etc. The characteristics of the self-sustained pulsating semiconductor lasers are addressed, for example, in Japanese paper, IECE TECHNICAL REPORT ON OPTICAL AND QUANTUM ELECTRONICS , OEQ 84-30, pp. 65-72. In these semiconductor lasers, the kink power and the optical output power which enables self-sustained pulsation are low, and 7 - 9 mW. The control of the self-pulsating frequency is not studied and the frequency is high, for example, about 1 GHz. Conventionally, an optical waveguide is etched with an etchant to form a ridge stripe structure, and the control of the thickness of the waveguide has been provided by setting an etching time in consideration of the etching rate of an etchant. References concerned are, for example, Japanese papers, EXTENDED ABSTRACTS (THE 35TH SPRING MEETING , 1988 ), JAPAN SOCIETY OF APPLIED PHYSICS AND RELATED SOCIETIES , 31p - ZP - 17 (1988), p. 912 and IECE TECHNICAL REPORT ON OPTICAL AND QUANTUM ELECTRONICS , OQE 88 - 5 (1988) pp. 33-38. A self-sustained pulsating semiconductor laser device having the features included in the first part of claim 1 is known form IEE PROCEEDINGS , Sections A to I, vol. 132, No. 1, Part I, February 1985, pp. 29-32. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor laser device which has a high-power low-noise characteristic suitable for a light source for optical discs to thereby suppress optical feedback noise generated when optical feedback to the laser device occurs and hence to provide a low-noise characteristic in which the relative intensity noise is maintained at 10 -13 Hz -1 or less from a low to a high laser output power even if a quantity of optical feedback is large. This object is accomplished by the laser device characterised in claim 1. The self-pulsating frequency of the self-sustained pulsating semiconductor laser device is reduced by doping n-type impurities in the multiquantum well active layer in the device. The reason for this will now be described. Considering that the self-sustained pulsation of the semiconductor laser device is likely to resonate at a frequency similar to that of its relaxation oscillation, the self-pulsating frequency can be described using an equation similar to that representing the relaxation oscillation frequency. The self-pulsating frequency f p is given by where K is the constant, dg/dn is the differential gain, τ p is the photon lifetime, I is the injected current, I th is the threshold current, C is the photon velocity, n r is the effective refractive index, α i is the internal loss, L is the cavity length, and R 1 and R 2 are the facet reflectivities. For example, as disclosed in Japanese paper, IECE TECHNICAL REPORT ON OPTICAL AND QUANTUM ELECTRONICS OQE86 - 63, pp. 17-24, if the active layer has a multi-quantum well structure, the differential gain increases, and hence the self-pulsating frequency increases compared to the regular bulk active layer such that the relaxation oscillation frequency increases. In order to reduce the self-pulsating frequency, it would be effective to increase the photon lifetime by increasing the cavity length and the facet reflectivities in the above equations (1) and (2) as well as to reduce the differential gain. The reduction of the differential gain is achieved by doping n-type impurities in the multiquantum well active layer. Thus, electrons are supplied contributing to recombination emission even if carriers are not injected in the active layer, and thus it is possible to obtain a gain in a wider energy area than a regular one. Therefore, the gain spectrum expands, a rise in the gain due to carrier injection, namely, the differential gain, is reduced compared to the undoped active layer. If the multiquantum well layer is uniformly doped with impurities, adverse effect may occur such as occurrence of an unstable pulsating wavelength due to deformation of the quantized energy level formed in the quantum well by the space charges of impurity ions or disturbance of distribution of the gain spectra. Therefore, it is effective to perform modulation doping in the quantum barrier alone If the overall thickness of the multiquantum well layer decreases, a multi-longitudinal mode self-pulsation is not obtained, and a single-longitudinal mode oscillation tends to occur. An appropriate overall thickness of the multiquantum well layer is 0.05 - 0.09 µm. In addition, if the quantum well is thinned, a single-longitudinal mode oscillation tends to occur, and, even if a self-pulsating is obtained, the frequency would increase, so that an appropriate quantum well is 5 - 20 nm broad and preferably 8 - 12 nm broad. According to the present invention, the self-pulsating frequency was reduced, so that a low-noise characteristic in which a relative noise intensity was 10 -14 - 10 -13 Hz -1 was realized even in an optical output power higher than the conventional one even if an optical feedback quantity of 3 - 6% was generated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross section view of an MQW self-sustained pulsating laser device as examples 1 and 2, which are not embodiments of the present invention. Fig. 2 is the light output power-injected current characteristics of the MQW self-sustained pulsating laser device as the example 1 and a conventional regular DH-type self-sustained pulsating laser device. Fig. 3 illustrates the relationship between the self-pulsating frequency and drive current of the MQW self-sustained pulsating laser device as the example 1 and the conventional regular DH-type self-sustained pulsating laser device. Fig. 4 schematically illustrates the conduction band energy structure of the multiquantum well structure of the MQW self-sustained pulsating laser device as the example 2, which is not an embodiment of the present invention. Fig. 5 is a cross section view of a SQW self-sustained pulsating semiconductor laser device as an example 3, which is not an embodiment of the present invention. Fig. 6 schematically illustrates the conduction band energy structure of a single quantum well structure. Fig. 7 is a cross section view of a self-sustained pulsating semiconductor laser device as each of examples 4 - 7, which are not embodiments of the present invention. Figs. 8 - 11 each schematically illustrates the energy band structure of the active layer in each of the examples 4 - 7. Fig. 12 is a cross section view of an MQW self-sustained pulsating laser device as examples 8, which is not an embodiment of the invention and 9, which is an embodiment of the present invention. Fig. 13 is a cross section view of an MQW self-sustained pulsating laser device as an example 10, which is an embodiment of the present invention, when the active layer is a modulated n-doped MQW as in the example 9. Fig. 14 illustrates the light output power-injected current characteristics of the device. Fig. 15 ilustrates the relationship between the self-pulsating frequency and the drive current of the device. Figs. 16 - 19 are cross section views of self-sustained pulsating laser devices as examples 11 - 14, respectively, which are not embodiments of the present invention. Example 1 An MQW laser as an example 1, which is not an embodiment of the present invention will now be described with reference to Fig. 1. Sequentially formed by crystalline growth on an n-GaAs substrate by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) are an n-GaAs buffer layer 2 (0.5 µm thick), an n-Al x Ga 1-x AS cladding layer 3 (1.5 µm thick, x = 0.45), an undoped AlGaAs MQW active layer 4 (an alternate laminate of five undoped Al y Ga 1-y As quantum barriers (each being 3 - 5 nm thick, y = 0.25 - 0.27) and four undoped Al z Ga 1-z AS quantum wells (each being 9 - 11 nm thick, z = 0.04 - 0.06), a p-Al x Ga 1-x AS cladding layer 5 (1.5 µm thick, x = 0.45), and a p-GaAs layer 6 (0.2 µm thick). Thereafter an insulator film is formed on top of the resulting half-finished product, and an insulator film mask of a stripe-like pattern is formed by photolithography and etching. By using this mask, a stripe-like ridge waveguide 11 is formed by wet etching using a hydrophosphoric acid solution. The thickness d 1 of the layer 5 except in the ridge after etching was selected to be 0.3 - 0.5 µm. Thereafter, the n-GaAs current blocking layer 7 (0.9 µm thick) is grown by selective recrystallization so as to be substantially flush with the ridge with the mask being left. Thus the current-blocking and light-absorbing layer is formed by crystal growth in a self-aligned manner. The insulator film mask is then etched away with a hydrofluoric acid solution, a p-GaAs contact layer (1.5 µm thick) is formed by crystalline growth. Thereafter, a p-side electrode 9 and an n-side electrode 10 are deposited and a device is cut away by cleaving and scribing. The appropriate stripe width of the ridge 11 for providing a laser which oscillates in the fundamental lateral mode in a stabilized manner was 4 - 7 µm in bottom width S 2 and 3-5 µm in top width S 1 . For comparison, a regular double hetero (DH) laser was prepared using undoped A1 0.14 Ga 0.86 As as an active layer. The laser had the same structural parameters as the example except for the active layer. Fig. 2 illustrates the light output power-injected current characteristics of the MQW laser of the particular example 1 and the regular DH laser. The oscillating threshold current was 35 - 40 mA in the MQW laser of the example 1, which was small by 30 - 40% compared to 65 - 70 mA of the regular DH laser. In addition, the MQW laser of the example 1 pulsated at the kink-free fundamental lateral mode up to 20 - 25 mW of a light output power, which was more than twice that of the regular DH laser. Since the laser changes to the single longitudinal mode beyond the kink level, the relative intensity noise cannot be reduced below 10 -13 Hz -1 . Since, however, the multi-longitudinal mode self-sustained pulsation is obtained up to the kink level, the relative intensity noise was on the order of 5 x 10 -15 - 5 x 10 -14 Hz -1 lower than 10 -13 Hz -1 up to 20 - 25 mW of the light power output in the absence of optical feedback. Like this, in the absence of optical feedback, the relative intensity noise of the example 1 was suppressed satisfactorily below the intensity requirement of 10 -13 Hz -1 required as a light source for optical video discs, etc., so that a satisfactory low-noise characteristic from a low output power to a high output power was obtained. Even if optical feedback may be generated, low noise below the relative intensity noise of 10 -13 Hz -1 level is obtained if the self-pulsating frequency is controlled. If the active layer has an MQW structure, the differential gain increases, so that the self-pulsating frequency tends to be higher than that of the regular double heterostructure. Namely, as shown in the relationship between self-pulsating frequency and drive current of Fig. 3, the self-pulsating frequency (shown by small squares) of the MQW active layer is higher than that (shown by circlets) of the regular double heteroactive layer. However, self-sustained pulsating frequency can be suppressed by increasing the facet reflectivities. In the device, the self-pulsating frequency was reduced by coating the opposite face of the cavity from the laser beam emission facet with a high-reflective film of a reflectivity of 90%. In this device, the relative intensity noise was suppressed lower than 10 -13 Hz -1 even if 3 - 6% of optical feedback was generated in 50 - 80 mm of the optical path between an optical disc and the laser device in the optical pickup system. The self-pulsating frequency was successfully lowered than 1.3 GHz in an optical power output of 20 mW. Although optical feedback is produced at an optical output of 5 - 20 mW, the relative intensity noise was suppressed within a range of 8 x 10 -15 - 8 x 10 -14 Hz -1 , and thus a satisfactory low-noise characteristic for a light source for an optical video disc was realized. By applying a facet refractivity lowering antirefractive film to the front facet of the cavity and a facet refractivity increasing refractive film to the rear facet of the cavity, the self-pulsting area was expanded successfully and the above low-noise characteristic was obtained in an optical output power range of 5 - 50 mW. When the device of the particular example had antireflective and reflective coatings, the kink power level was 70 - 80 mW and the catastrophic optical damage level was 90 - 100 mW. At durability test, the device was not deteriorated although it was operated at a surrounding temperature of 50°C and at automatic controlled power of 40 mW for 2000 hours or more. According to the particular example, by constituting the active layer so as to have a multiquantum well (MQW) structure, the maximum optical output power which enables self-pulsation and the kink power level were successfully increased twice-three times and the threshold current was reduced by 30 - 40% compared to the regular double hetero (DH) structure. When the cavity facets had antirefractive and refractive coatings, 70 - 80 mW of the kink level and 50 - 60 mW of the self-pulsating optical output power were obtained. In addition, although the differential gain increases and the self-pulsating frequency increases if an MQW active layer is employed, the differential gain was reduced by coating the rear facet of the cavity with a high-refractive film. Therefore, the self-pulsating frequency of the device was successfully suppressed to about 0.2 - 1.0 GHz up to an optical output power of 25 - 30 mW. Thus, even if 3 - 6% of optical feedback was generated in an optical pickup system optical path of 50 - 80 mm in an optical video disc system, the relative intensity noise was successfully suppressed lower than 10 13 Hz -1 . Like this, the high-power low-noise characteristic of the Semiconductor laser device according to the particular example was improved compared to the conventional device. The use of the MQW active layer improved the temperature characteristic of the device threshold current. A device having antirefractive and refractive coatings was not deteriorated at durability test even if 2000 hours passed at a surrounding temperature of 50°C at an automatic controlled power of 40 mW. While in the particular example the crystal material used was of AlGaAs system, other materials such as AlGaInP/GaAs system, InGaAsP/InP system materials, etc., may be used, of course. Example 2 Fig. 1 is a cross section view of a semiconductor laser device as an example 2, which is not an embodiment of the present invention. Fig. 4 schematically illustrates the conduction band energy structure of a multiquantum well structure constituting the active layer of the laser device. Reference numeral 1 denotes an n-GaAs substrate; 2, an n-GaAs buffer layer; 3, an n-Al x Ga 1-x As cladding layer (lower cladding layer); 4, the AlGaAs active layer of a multiquantum well structure; 5, a p-Al x Ga 1-x As cladding layer (upper cladding layer); 11, a ridge stripe formed integrally at the central portion of the p-Al x Ga 1-x AS cladding layer 5; 6, an oxidation-preventive p-GaAs layer for the ridge stripe 11 formed on to of the stripe 11; 7, an n-GaAs block layer (light-absorbing and current-blocking layer); 8, a p-GaAs buried layer; 9, a p-side electrode and 10, an n-side electrode. This device is fabricated as follows. First, sequentially grown epitaxially on the 100 µm-thick n-GaAs (001) substrate 1 by the crystal growth technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) are the 0.3 - 0.5 µm-thick buffer layer 2; the 1.0 - 1.5 µm-thick Al x Ga 1-x As cladding layer 3 (x = 0.45 - 0.55), and the active layer 4, of a multiquantum well structure (The multiquantum well structure active layer 4 is formed of an alternate laminate of five Al y As 1-y quantum barriers (y = 0.20 - 0.30), each being 3 - 6 nm thick, and four Al z Ga 1-z As quantum wells (z = 0 - 0.10), each being 10 - 15 nm thick. The overall thickness of the active layer is 0.05 - 0.08 µm); the 1.0 - 2.5 µm-thick p-Al x Ga 1-x As cladding layer 5 (x = 0.45 - 0.55); and the 0.2 - 0.4 pm-thick p-GaAs layer 6. Subsequently, a SiO 2 insulator film (not shown) is formed and a stripe mask pattern is formed in the insulator film by photolithography. Thereafter, the layers 6 and 5 are etched 0.5 - 1.5 µm deep with a hydrophosphoric acid solution to form a ridge stripe structure. The n-GaAs blocking layer 7 is selectively grown epitaxially by chemical vapor deposition such that the blocking layer 7 is flush with the ridge with the SiO 2 insulator film mask being left. Thereafter, this mask is etched away with a hydrofluoric solution and the p-GaAs contact layer 8 (1.0 - 2.0 µm thick) is grown. The p-side electrode 9 and n-side electrode 19 are deposited, and the resulting wafer is cleaved and scribed to cut away a device from the wafer. Among the respective device structural parameters, preferably, the ridge top horizontal width S 1 is 2 - 4 µm and the horizontal ridge bottom width S 2 is 4 - 7 µm. Preferably, the overall thickness of d a of the active layer 4 is 0.05 - 0.08 µm, and especially 0.055 - 0.065 µm. One or both of n- and p-type impurities may be doped in a concentration of 1 x 10 18 - 1 x 10 19 cm -2 in the active layer. The thickness d c of the cladding layer 5 including the thickness (height) of the ridge stripe (the thickness of the layer 5 before etching) is preferably 1.0 - 3.0 µm and more preferably 1.3 - 1.8 µm. The thickness d 1 of the cladding layer 5 on each side of the ridge on which no stripes are formed (the thickness of the etched portion of the layer 5) is preferably 0.2 - 0.6 µm, and more preferably 0.3 - 0.5 µm. In the particular example, in a device with no facet coating, a self-sustained pulsation was obtained at a threshold current of 20 - 30 mA and at a light output power of 2 - 7 mW, and the kink power was 40 - 50 mW. In a device with refractive and antirefractive facet coatings, a self-pulsation was obtained at a light output of 2 - 12 mW and the kink power was 80 - 100 mW. Like this, the device of the particular example by itself successfully provided a high power characteristic in which a light output power of 40 - 50 mW or more was obtained which was required for writing and erasing data in a memory in an optical disc, and a low noise characteristic in which the relative intensity noise was 10 13 - 10 14 Hz -1 , which was lower than a limit below which memory reading was possible, at a light output power of 2 - 10 mW even if optical feedback occurred. The device operated with high reliability or without noticeable deterioration being recognized even after a lapse of about 2000 hours under APC (automatic power control) conditions in which the temperature was maintained at 50°C and the light output power was maintained at 50 mW. The semiconductor laser device of the particular example satisfied the high-power low-noise characteristic required for an optical source for erasable optical disc. Example 3 Fig. 5 is a cross section view of a semiconductor laser device, which not an embodiment of the present invention. Fig. 6 schematically illustrates the conduction band energy structure of a single-quantum well structure constituting the active layer of the laser device. The example 3 is the same as the example 2 except in the active laser structure. In more detail, the active layer of the example 2 employed the multiquantum well structure while the active layer of the example 3 employed a single-quantum well structure. The example 3 is the same in device fabricating process as the example 2 except for in the active layer. The active layer of the particular example 3 includes an Al α Ga 1-α As single quantum well layer 112 (10 - 30 nm thick, 0 ≤ α ≤ 0.15), an n-Al β Ga 1-β As layer 111 and a p-Al β Ga 1-β As layer 113 (the layers 111 and 113 each being 15 - 40 nm thick, 0.15 ≤ β ≤ 0.35) which function as a quantum barrier and cladding layer, respectively, provided on an upper and a lower surfaces of the well layer 112. The layers 111 and 113 may have a multiquantum well structure (for example, 4 - 10 nm thick), a superlattice layer-formed well structure (for example, 1 - 4 nm thick) or a graded-index layer, or may be doped separately or together with at least one of n- and p-type impurities in a range of from 1 x 10 18 - 1 x 10 19 cm -3 . The device of the particular example 3 produced effects similar to those produced by the example 2. In addition, since the particular example 3 employed the single quantum well structure, laser oscillation was possible at low carrier density, and the threshold current was reduced to 10 - 20 mA. While in the examples 2 and 3 the material is of the AlGaAs system, other materials, for example, of AlGaInP/GaAs, InGaAsP/InP, and InGaAs/GaAs systems may be used to produce similar effects. As described above, in the semiconductor laser devices of the examples 2 and 3, self-pulsation can be obtained by controlling the effective transverse refractive index difference of the active layer, the light output power which enables self-pulsation can be controlled, and the kink power is improved compared to the conventional device, so that the devices can be used as a light source for an erasable optical discs. In more detail, the device by itself can satisfactorily provide both a high power characteristic in which high power such as 40 - 50 mW or more required for writing and erasing data in an optical disc is obtained, and a low-noise characteristic in which the relative intensity noise is reduced to 10 13 - 10 14 Hz -1 for enabling the memory reading at a light output power of 2 - 10 mW even if optical feedback occurs, and thus the device can directly ovewrite in an optical disc memory. In a device with antirefractive and refractive facet coatings, no noticeable deteriorations were recognized after a lapse of 2000 hours in an accelerated lifetime test under an APC opration of an light output power of 50 mW at a temprature of 50°C. Example 4 An example 4, which is not an embodiment of the present invention will now be described with reference to Fig. 7. First, sequentially grown epitaxially on an n-GaAs (001) substrate 21 (100 µm thick) by MBE or MOCVD are an n-GaAs buffer layer 22 (0.5 µm thick), an n-Al x Ga 1-x As cladding layer 23 (1.0 - 1.5 µm thick, x = 0.45 - 0.55), an undoped or n- or p-multiquantum well layer 24 (of three-six alternate quantum barriers and wells, each quantum barrier including an Al y Ga 1-y As layer 2 - 5 nm broad, y = 0.20 - 0.45, and each quantum well including an Al z Ga 1-z As layer 3 - 10 nm broad, z = 0 - 0.20, or of ten-fifteen alternate Al y Ga 1-y As superlattice barriers (each being 0.5 - 1 nm broad) and Al z Ga 1-z As superlattice wells (each being 0.5 - 2 nm broad) in order to constitute an energy band structure of a conduction band and a valence band in the active layer as shown in Fig. 8.), undoped single-quantum well layer 25 (of Al z′ Ga 1-z′ As layer 10 - 30 nm broad, z′ = 0 - 0.15), an undoped or n- or p-type multiquantum well layer 26 (of three-six Al y Ga 1-y As quantum barriers (each being 2 - 5 nm broad, y = 0.20 - 0.45) and Al z Ga 1-z As quantum wells (each being 3 - 10 nm broad, z = 0 - 0.20), or of ten-fifteen alternate Al y Ga 1-y As superlattice barriers (each being 0.5 - 1 nm broad) and Al z Ga 1-z As superlattice wells (each being 0.5 - 2 nm broad)), a p-Al x Ga 1-x As cladding layer 27 (1.3 - 1.6 µm thick, x = 0.45 - 0.55), and a p-GaAs layer 28 (0.2 - 0.3 µm thick). A SiO 2 film is then formed and a stripe mask pattern is prepared by photolithography. This stripe SiO 2 film is used as a mask and the layers 28 and 27 are etched with a hydrophosphoric acid solution to form a ridge-like optical waveguide. Thereafter, an n-GaAs current block layer 29 (0.7 - 1.0 µm thick) is selectively deposited with the SiO 2 film mask being left. Subsequently, the SiO 2 film mask is etched away with a hydrofluoric solution, and a p-GaAs cap layer 210 (1.0 - 2.0 µm thick) is grown on the layers 28 and 29. Thereafter, a p-type layer side electrode 211 and an n-type layer side electrode 212 are formed, and a device is cut away from the resulting wafer in a cleaved and scribed process. In the particular example, in order to cause a laser oscillation at the fundamental lateral mode and at a low threshold current, an appropriate ridge bottom stripe width S 2 was 4 - 6 µm. When the cladding layer thickness d 1 was 0.3 - 0.6 µm after the ridge waveguide was formed, a laser device which caused a self-pulsation was obtained. The particular device caused a laser oscillation at a threshold current of 10 - 20 mA and performed a self-pulsation at an optical output power of 2 - 30 mW. The kink output power was 50 - 60 mW. In a laser device in which n-type or p-type impurities are doped at a density of 1 x 10 18 - 1 x 10 19 cm -3 in the multiquantum well layer of the active layer, laser oscillation occurred at a threshold current of 10 - 20 mA and the self-pulsating frequency was controlled successfully depending on the polarity and density of the dopants. In a laser device with reflective and anti-reflective facet coatings, self-pulsation occurred at a light output power of 2 - 50 mW, and the kink output power was 80 - 90 mW. A laser device was realized in which the overall thicknesses of the active and cladding layers were controlled to have respective predetermined values (the active layer was 0.04 - 0.08 µm thick and the cladding layer was 0.3 - 0.6 µm thick) such that a self-pulsation occurred at a light output power of 2 - 10 mW and the kink output power was 110 - 120 mW. Thus, the device by itself satisfied a high-power low-noise characteristic required for a light source for writing and erasing data in an optical disc and a light source for reading data from an optical disc. Example 5 An example 5, which is not an embodiment of the present invention will now be described with reference to Fig. 9 which schematically illustrates the energy band structure of a conduction band in the active layer. In the laser device fabricated, the number of Al elements z in the Al z Ga 1-z As quantum well of each of the multiquantum well layers 24 and 26 provided on the upper and lower surfaces of the single quantum well layer in the active layer was selected to be larger than the number of Al elements z′ in the Al z′ Ga 1-z′ As single quantum well layer 5. The structure of the multiquantum well layers 24 and 26 was designed such that the quantized energy level formed in the multiquantum well layer was the same as, or higher than, that formed in the single quantum well layer. According to the particular example, the threshold current was further reduced and laser oscillation was successfully performed at 5 - 10 mA. Other characteristics similar to those in the example 1 were obtained. Example 6 An example 6, which is not an embodiment of the present invention will now be described with reference to Fig.10 which schematically illustrates the energy band structure of a conduction band in the active layer. In the laser device fabricated, the Al z Ga 1-z As quantum wells 24′, 24″ ...; 26′, 26″ ... of the multiquantum well layers 24 and 26 provided on the upper and lower surfaces of the single quantum well layer in the active layer were selected so as to increase gradually in width and in the number of Al elements from the single quantum well 25 toward the cladding layers 23 and 27. The structures of the multiquantum well layers 24 and 26 were designed such that the quantized energy level formed in the multiquantum well layer was the same as, or higher than, that formed in the single quantum well layer. The example 6 produced effects similar to those produced by the example 5. Example 7 An example 7, which is not an embodiment of the present invention will now be described with reference to Fig. 11 which schematically illustrates the energy band structure of a conduction band in the active layer. In the laser device fabricated, the multiquantum well layers 24 and 26, respectively, provided on the upper and lower surfaces of the single quantum well layer in the active layer were a graded-index layer in which the number of Al elements y of the Al y Ga 1-y As quantum barrier was selected so as to increase from the single quantum well layer toward the cladding layers. In the device, the quantized energy levels formed in the quantum wells of the multiquantum well layer were selected so as to increase gradually from the single quantum well layer to the cladding layers. The particular example also produced effects similar those by the example 5. According to the examples 4 - 7, it is possible to control the effective transverse refractive index difference of the active layer of the single quantum well structure, so that it is easy to control the lateral and longitudinal modes to thereby provide a desired laser characteristic. In the laser devices of a ridge stripe structure shown in the examples 4 - 7, self-pulsation occurred and the kink output power was 50 - 60 mW at a light output power of 2 - 30 mW when the cladding layer was 0.3 - 0.6 µm thick and the overall thickness of the active layer was 0.05 - 0.08 µm after the ridge was formed. The threshold current was 5 - 10 mA. In a laser device with refractive and antirefractive facet coatings, self-pulsation occurred in a light output power range of 2 - 50 mW, and the kink output power was 80 - 90 mW. A device was obtained in which the effective transverse refractive index difference of the active layer was controlled such that self-pulsation occurred at a light output power range of 2 - 10 mW and the kink output power was 110 - 120 mW. Thus, these devices satisfied a high power characteristic required for light sources for recording and erasing data in optical discs and a low-noise characteristic required for light sources for reading data in the discs. While the examples 4 - 7 were described using AlGaAs materials, other materials such as AlGaInP/GaAs and InGaAsP/InP system materials may be used for the same purposes, of course. Example 8 An MQW self-sustained pulsation laser as an example 8, which is not an embodiment of the present invention will now be described with reference to Fig. 12. Sequentially deposited by crystalline growth on an n-GaAs substrate 31 by MOCVD or MPE are n-GaAs buffer layer 32 (0.5 µm thick), an n-Al x Ga 1-x As cladding layer 33 (1.5 µm thick, x = 0.45), an AlGaAs multiquantum well structure active layer 34′ (of an alternate laminate of five Al y Ga 1-y As quantum barriers, y = 0.2 - 0.27, each barrier being 3 - 5 nm thick and four Al z Ga 1-z As quantum wells, z = 0.04 - 0.07; with n-type impurities Se being doped uniformly in a concentration of 9 x 10 17 - 2 x 10 18 cm -3 in the quantum barriers and wells), a p-Al x Ga 1-x As cladding layer 35 (1.3 - 1.6 µm thick, x = 0.45), and a p-GaAs layer 36 (0.2 µm thick). Thereafter, an insulator film is formed on top of the resulting crystal, and a stripe-like insultor film mask is prepared by photolithography and etching. This mask is used to form a stripe-like ridge waveguide 313 in an etching process with a hydroiphosphoric acid solution as shown in Fig. 12. The layer 35 on each side of the ridge formed by etching had a thickness d 1 of from 0.3 - 0.6 µm, An n-GaAs current blocking layer 37 (0.7 - 1.1 µm thick) was selectively deposited with the insulator film mask left thereon. Thereafter, the mask was etched away with a hydrofluoridic acid solution, and a p-GaAs layer 38 (1.0 - 1.5 µm thick) was again deposited by crystal growth. Thereafter, a p-side electrode 39 and an n-side electrode 310 were deposited, and a device was cut away from the resulting wafer in a cleaving and scribing process. An appropriate ridge stripe bottom width S 2 was 4 - 7 µm in order to provide a laser device which oscillates at the fundamental lateral mode in a stabilized manner. For comparison, a reference device was fabricated in which the active layer included an undoped AlGaAs multiquantum well layer 34 with other parameters being the same as the corresponding ones in the particular example. Both in the device in which n-type impurities are doped in the active layer and in the device in which no n-type impurities are doped, self-pulsation was maintained up to the kink level. Fig. 14 shows the light output-power injected current characteristics of the device having the uniformly n-doped active layer and the device having the undoped active layer for comparing purposes. Since the device having the uniformly n-doped MQW active layer provides a high gain with a low injection carrier concentration compared to the device having the undoped active layer, the threshold carrier and current densities are reduced to therby provide a reduced threshold current. As shown in Fig.14, in the uniformly n-doped device, the threshold current is 25 - 30 mA which means a 20 - 30% reduction of the threshold current compared to 30 - 40 mA of the threshold current in the undoped device. In addition, the kink level of the particular device increased by 25 - 35 mW compared to the undoped device. According to the particular device, the relative intensity noise was below 10 -13 Hz -1 up to the kink level. Since the differential gain of the MQW active layer is reduced by n-type doping, the self-pulsating frequency is reduced. As shown in Fig. 15, the self-pulsating frequency was reduced to 1/2 - 1/3 in the uniformly n-doped device (shown by A) compared to the undoped device (shown by □). In the particular example, the self-pulsating frequency was further reduced by applying a high reflective coating to the facet opposite to the emission face of the example. Thus, the self-pulsating frequency was maintained successfully at 1.0 GHz or less up to a kink level of 25 - 30 mW and the relative intensity noise was suppressed below 10 -13 Hz -1 although 3 - 6% of optical feedback occurred. In the device with the reflective and anti-reflective facet coatings, the kink level was increased to 60 - 70 mW. In the particular device with the refractive and anti-refractive facet coatings, the catastrophic optical damage level was 80 - 90 mW. The device operated satisfactorily without noticeable deterioration for more 2000 hours at a surrounding temperature of 50°C at an automatic controlled power of 40 mW at lifetime test. Example 9 An example 9, which is an embodiment of the invention will now be described with reference to Fig. 12. The process for fabricating the device was similar to that for the example 8 except that impurities of Se were doped only in the quantum barrier of the MQW active layer 34" in crystal growth to thereby fabricate a modulated n-doped device. The doping level of the impurities of Se was set to 9 x 10 17 - 2 x 10 18 cm -3 . Also in this device, a stabilized self-pulsation was obtained and continued up to the kink level. In the device of the particular example, the threshold current was 20 - 25 mA which was substantially the same as, or lower than, that in the example 8. The self-pulsating frequency was also suppressed to substantially the same as, or less than, that in the example 8. Thus, the self-pulsating frequency was maintained at 1.0 GHz or less up to the kink level. Even if optical feedback of 3 - 6% occurred, the relative intensity noise was suppressed to 10 -13 Hz -1 or less. Example 10 An MQW self-sustained pulsating laser device as an example 10 will now be described with reference to Fig. 13. The process for fabricating the device was similar to that for the example 8 except the use of a so-called GRIN-SCH (Graded-Index Waveguide Separate Confinement Heterostructure) in which n- and p-type AlGaAs graded-index layers 311 and 312 were provided on an upper and a lower surfaces, respectively, of the MQW active layer in the crystal growth process. The MQW active layer included a uniformly n-doped MQW active layer 34' or a modulated n-doped MQW active layer 34", which is an embodiment of the invention as in the example 9. Thus the threshold current obtained was 10 - 20 mA. The self-pulsation continued up to a kink level of 30 mW and the self-pulsating frequency was 0.2 - 1.0 GHz up to a light output power of 2 - 30 mW. Thus, the relative intensity noise was suppressed to 10 -13 Hz -1 or less even if optical feedback of 3 - 6% occurred. According to the examples 8 - 10, the frequency of the self-sustained pulsating semiconductor laser device was reduced advantageously to 1/2 - 1/3 of that of the conventional one. If the active layer is of a multiquantum well layer, the differential gain will increase and the self-pulsating frequency will increase compared to a regular bulk active layer. However, by the examples 8 - 10, the differential gain was reduced to substantially the same as, or less than, that provided by the bulk active layer. The self-pulsating frequency of the device was reduced to about 0.2 - 1.0 GHz up to a light output power of 25 - 30 mW, and the relative intensity noise was reduced to 10 -14 - 10 -13 Hz -1 even if optical feedback quantiy of 3 - 6% occurred. In a device with reflective and anti-reflective facet coatings, self-pulsation was maintained up to 50 - 60 mW. A low noise characteristic was obtained in which the relative intensity noise was 10 -14 - 10 -13 Hz -1 even if optical feedback occurred at a light output power of 5 - 50 mW. By the examples 8 - 10, the semiconductor laser provided high-output, low-noise compared to the conventional one. In a device with refractive and anti-refractive facet coatings, no deterioration was recognized even after a lapse of 2000 hours at a surrounding temperature of 50°C and at an automatic controlled output power of 40 mW. While the examples 8 - 10 used AlGaAs system crystal materials, other materials such as AlGaInP/GaInP and InGaAsP/InP system materials may be used, of course. Example 11 A laser device of an example 11, which is not an embodiment of the present invention is shown in Fig. 16. In fabrication, first, sequentially deposited on an n-GaAs substrate 41 (100 µm thick) by MOCVD are an n-GaAs buffer 42 (0.5 µm thick, 1 x 10 18 cm -3 in impurity density), an n-Al x Ga 1-x As cladding layer 43 (x = 0.45, 1.0 - 1.5 µm thick, 9 x 10 17 cm -3 in impurity density), an undoped Al y Ga 1-y As active layer 44 (0.05 - 0.08 µm thick in total), a p-Al x Ga 1-x As cladding layer 45 (x = 0.45, 0.2 - 0.3 µm thick, 6 x 10 17 cm -3 in impurity density), a highly p-type impurity doped Al x′ Ga 1-x′ As cladding layer 46 (x′ = 0.5, 0.1 - 0.2 µm thick, 5 x 10 18 - 5 x 10 19 cm -3 in impurity density), a p-Al z Ga 1-z As etch-block layer 47 (z = 0, 3 - 10 µm thick, 1 x 10 18 cm -3 in impurity density), a highly p-type impurity doped Al x′ Ga 1-x′ As cladding layer 46′ (x′ = 0.5, 0.1 - 0.2 µm thick, 5 x 10 18 - 5 x 10 19 cm -3 in impurity density), a p-Al x Ga 1-x As cladding layer 48 (x = 0.45, 0.4 - 0.6 µm thick, 6 x 10 17 cm -3 in impurity density), and a p-GaAs cap layer 49 (0.2 µm thick, 5 x 10 18 - 5 x 10 19 cm -3 in impurity density). Thereafter, a stripe-like insulator film mask (of SiO 2 0.2 - 0.3 µm thick) is prepared (not shown). Initially, a p-GaAs layer 9 is chemically etched with a non-selective etchant, for example, of a mixed solution of hydrophosphoric acid, hydroperoxide and ethylene glycol, and then up to the etch-stop layer 47 with a selective etchant, for example, of a mixed aqueous solution of hydrochloric acid and water. The half-finished product is then heated to 800 - 850°C higher than a regular growth temperature of 650 - 700°C in the MOCVD growth furnace and annealed while being supplied with AsH 3 for 3-30 minutes. The half-finished product was then placed again in the furnace and n-GaAs current-blocking and absorbing layer 410 was grown again in the growth furnace with the insulator film mask being left. At this time, the compositions of the etch-block layer 47 and the layers below and above the block layer 47 were disordered (shown hatched) due to the interaction of these layers and the composition of the etch-block layer 47 area became high. Subsequently, the temperature was lowered continuously to the growth temperature 650 - 700°C to selectively grow the n-GaAs blocking layer 410 (0.6 - 0.9 µm thick, 3 - 5 x 10 18 cm -3 in density). The half-finished product was taken temporarily out of the furnace and the mask was removed. Furthermore, the half-finished product was returned into the growth furnace to deposit a p-GaAs cap layer 411 (1.0 - 1.5 µm thick, 2 x 10 18 - 1 x 10 19 cm -3 in impurity density) by crystal growth. A p-electrode 412 and an n-electrode 413 are deposited, and the resulting half-finished product is then cleaved and scribed, and a device is cut away. In order to cause a stabilized laser oscillation at the fundamental lateral mode in the particular example, the stripe width S was preferably 3 - 8 µm and more preferably 4 - 6 µm. The optical waveguide layer other than the ridge portion of the ridge waveguide structure was prepared to have the total d of the thicknesses of the layers 45, 46 and 47 with high controllability and with the accuracy of crystal growth conditions. When the total thickness d was 0.2 - 0.3 µm, laser longitudinal mode was a single mode, and, when the total thickness d was 0.4 - 0.6 µm, a self-pulsating oscillation occurred at a multi-longitudinal mode. The threshold current was low when the total d was small; namely, when the total d was 0.2 - 0.3 µm, the threshold current was 30 - 40 mA, and when the total d was 0.4 - 0.6 µm, the threshold current was 50 - 60 mA. The kink output power was 40 - 50 mW when the total thickness d was 0.2 - 0.3 µm, and 10 - 20 mW when the total d was 0.4 - 0.6 µm. By control of the total thickness d , a laser device was fabricated with high reproducibility which satisfied a high output characteristic for an optical disc or a low-noise characteristic for a video disc. Example 12 An example 12, which is not an embodiment of the present invention will be shown in Fig. 17. The laser device of the example 12 was fabricated in a process similar to that used for fabricating the example 11. An upper cladding layer 48 and a cap layer 49 were etched until the etch-block layer 47 was reached, a highly p-type impurity doped gas layer 414 was selectively deposited, and then an n-GaAs block layer 410 was deposited selectively. Subsequently, the resulting half-finished product was heated continuously up to 800 - 850°C higher than the regular crystalline growth temperature of 650 - 700°C and then was annealed for 10 - 30 minutes to thereby disorder the compositions of the etch-block layer 47 and the layers on the upper and lower surfaces of the blocking layer 47 by diffusing the impurities out of the p-GaAs layer 414 (as shown hatched in Fig. 17). The temperature was then lowered to the crystal growth temperature of 650 - 700°C to grow the n-GaAs blocking layer 410 selectively. This latter process is similar to that in the example 11. Also by this particular example, a device was provided having performances similar to those of the example 11. Example 13 An example 13, which is not an embodiment of the present invention will be described with reference to Fig. 18. The compositions of the etch-block layer 47 and the lower and upper cladding layers 45 and 46 were disordered sufficiently due to the interaction between these layers by providing the same highly impurity-doped layers 46 and 414 as those employed in the examples 11 and 12. Also in the particular example, the device characteristics similar to those of the examples 11 and 12 were obtained. Example 14 An example 14, which is not an embodiment of the present invention will now be described with reference to Fig. 19. The process for fabrication of a device was similar to those for the examples 12 and 13 but was different in that enough heat treatment process is performed to cause impurities to diffuse from a highly impurity-doped layer 49 to the active layer 44 except for in the ridge portion (as shown hatched). By such process, the active layer 44a corresponding to the area except the ridge portion was changed to a mixed crystal and the active layer 44b area corresponding to the ridge portion was a so-called buried structure. Thus, according to the device, the threshold current was 20 - 30 mA, the kink power was 60 - 70 mW and the catastrophic damage level was 100 - 110 mW. According to the examples 11 - 14, the cladding layers of the semiconductor laser having a ridge waveguide structure was prepared so as to have a desired thickness with high controllability and high reproducibility compared to the conventional device and, therefore, the intended stabilized characteristics of the semiconductor laser was obtained. When the thickness of the cladding layer except for the ridge portion was controlled to 0.2 - 0.3 µm, a single longitudinal mode pulsating laser device was obtained in which the threshold current was 30 - 40 mA, the slope efficiency was 0.5 - 0.6 mW/mA and the kink output power was 40 - 50 mW. These devices had charactristics satisfying a high output characteristic required for an optical source for recording and erasing data in optical discs. When the thickness of the cladding layers was controlled to 0.4 - 0.6 µm, a multi-longitudinal mode self-sustained pulsating layer device was obtained in which the threshold current was 50 - 60 mA, the slope efficiency was 0.4 - 0.5 mW/mA, and the kink output power was 10 - 20 mW. The device had a characteristic satisfying a low-noise characteristic (having a relative intensity noise of 10 -14 - 10 -13 Hz -1 ) required for a video disc optical source. According to the methods of the examples 11 - 14, the etch-block layer can produce the above effects easily without adversely affecting the laser characteritics by performing an annealing process alone in addition to the conventional techniques. While the examples 11 - 14 have been described using the AlGaAs materials, AlGaInP or InGaAsP materials may be used to produce similar effects.

Claims: A self-sustained pulsating semiconductor laser device having a double-hetero structure comprising an active layer (34") which includes a multi-quantum well structure doped with at least n-type impurities and is sandwiched between lower and upper cladding layers (33, 35), a ridge step (313) formed in the upper cladding layer, a pair of current blocking and light-absorbing layers (37) one on each of the sides of the ridge to control the effective transverse refractive index difference of the active layer in order to provide self-sustained pulsation, characterised in that the impurities are modulation-doped in either only quantum barriers or only quantum wells of the multi-quantum well structure (34"). The laser device of claim 1, wherein the impurities doped in the active layer (34") are n-type or both p-type and n-type. The laser device of claim 1 or 2, wherein the portion in which the impurities are modulation-doped has a smaller width than any of the quantum wells. The laser device of any preceding claim, wherein the density of the impurities doped is between 7 x 10 18 and 1 x 10 19 cm -3 for the n-type and between 5 x 10 17 and 5 x 10 18 cm -3 for the p-type. The laser device of any preceding claim, wherein the n-type impurities are Se or Si and the p-type impurities are Be or Mg. The laser device of any preceding claim, wherein each of the quantum barriers of the multi-quantum well structure (34") has a thickness of 3 to 7 nm and each of the quantum wells of the multi-quantum well structure has a thickness of 5 to 20 nm. The laser device of any preceding claim, wherein the multi-quantum well structure (34") has a thickness of 0.05 to 0.09 µm. The laser device of any preceding claim, wherein the multi-quantum well structure (34") has a graded-index separate confinement heterostructure.

Inventors: KAJIMURA TAKASHI, KAWANO TOSHIHIRO, ONO YUICHI, TANAKA TOSHIAKI, KAJIMURA, TAKASHI, KAWANO, TOSHIHIRO, ONO, YUICHI, TANAKA, TOSHIAKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: H01S3
IPC codes: H01S5