Strained quantum well semiconducteur laser device and method for fabricating the same

Abstract: The semiconductor laser device of the invention includes: a strained quantum well structure including a well layer and a barrier layer, and a semiconductor substrate for supporting the strained quantum well structure. In the semiconductor laser device, at least one of the well layer and the barrier layer is composed of a mixed crystal where an atomic ordering is generated.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a semiconductor laser device and a method for fabricating the same, and a strained quantum well crystal and a method for fabricating the same. 2. Description of the Related Art: Referring to Figures 15A and 15B, a conventional strained quantum well type semiconductor laser device will be described. Such a strained quantum well type semiconductor laser device is disclosed, for example, in T. Tanbun-Ek et al., Appl. Phys. Lett., 57(3), pp. 224-226, (1990), or also in T. Tsuchiya et al., Electr. Lett., vol. 30, no. 10,p. 788-789 (May 12, 1994), and Osinski et al. IEEE Phot. Technol. Lett., vol. 4, no. 1, p. 10-13 (Jan. 1992). As shown in Figure 15B , the semiconductor laser device includes: an InP single crystalline substrate 1 ; a stripe-shaped multi-layered structure 10 provided on the substrate 1 ; and Fe-InP current blocking layers b for covering both sides of the multi-layered structure. An n-type electrode 9 is provided on the bottom surface of the semiconductor laser device and a p-type electrode 8 is provided on the upper surface of the semiconductor laser device. A dielectric film containing SiO 2 and/or Si is provided on an end surface of the semiconductor laser device. The stripe-shaped multi-layered structure 10 includes: a first optical waveguide layer 2 ; a multiquantum well portion 5 consisting of a strained quantum well layer 3 and a barrier layer 4 ; and a second optical waveguide layer 6. The energy band gaps in this portion are shown in Figure 15A . When a current is supplied from the p-type electrode 8 to the n-type electrode 9, holes and electrons are confined in the multi-quantum well portion 5, so that the holes and the electrons are recombined so as to generate an optical emission. As a result, a laser oscillation is generated in the semiconductor laser device. In this conventional example, the strained quantum well layer 3 is composed of mixed crystals having a composition of In 0.7 Ga 0.3 As, and is formed so as to have a compression strain of 1%. By introducing the compression strain into the well layer 3, the band structure of the holes can be similar to the band structure of the electrons. As a result, even if a small amount of carriers are injected, the laser oscillation is generated in the semiconductor laser device. However, in such a semiconductor laser device, as the temperature of the semiconductor laser device increases, the wavelength of the laser oscillation is shifted to the longer wavelength side. Therefore, such a semiconductor laser device has a problem in that the oscillation wavelength thereof is variable in accordance with the variation of the temperature of the semiconductor laser device. In addition, if the amount of the current to be injected into the semiconductor laser device is increased, the temperature of the semiconductor laser device is also increased; consequently the wavelength of the laser oscillation is considerably varied depending upon the amount of the injected current. On the other hand, in the case where such a strained quantum well structure is applied to a distributed feedback type laser device (hereinafter, simply referred to as a "DFB type laser device"), the variation in the temperature of the semiconductor laser device makes it difficult to realize a single node oscillation. In other words, a side mode is generated in the semiconductor laser device, and the difference between the peak at the oscillation wavelength and the peak of the side node cannot be set to be large. Such a phenomenon is remarkable in a strained quantum well laser device, in particular, because the introduction of the strain to the laser device reduces the threshold carrier density and causes an abrupt gain for the laser device. Figure 12A shows the profiles of the gain spectra varying depending upon the temperature in a semiconductor laser device having a non-strained multiple-quantum well (MQW) structure, while Figure 12B shows the profiles of the gain spectra varying depending upon the temperature in a semiconductor laser device having a strained MQW structure. As shown in Figure 12A , since the gain is not so large in the laser device having the non-strained MQW structure, substantially flat gain spectrum can be obtained in a wide wavelength region. The wavelength at the peak of this gain spectrum is shifted to a longer wavelength side as the temperature (T) of the laser device is set to be higher (high T). However, since the gain spectrum is flat in the laser device having the non-strained MQW structure, the difference between the peak at the oscillation wavelength and the peak of the side mode can be set to be large. In other words, it is easier to realize the single mode oscillation for the laser device. The reason is as follows. Even when the variation of the temperature makes longer the distance between the wavelength at the peak of the gain spectrum and the DFB oscillation wavelength, the gain can still be obtained at the DFB oscillation wavelength, so that the laser device can preferentially oscillate at the DFB oscillation wavelength. On the other hand, in the case where the laser device has a strained MQW structure, the introduction of the strain locally increases the gain in a particular wavelength region, as shown in Figure 12B . If the ambient temperature is low (low T), the DFB oscillation can still be generated even when the wavelength at the peak of the gain spectrum deviates from the DFB oscillation wavelength to a certain degree. As a result, the difference between the peak at the oscillation wavelength and the peak of the side mode can be set to be large. However, if the temperature becomes high (high T), the wavelength at the peak of the gain spectrum is shifted to the longer wavelength side in the same way as in the laser device having the non-strained MQW structure, so that substantially no gain can be obtained at the DFB oscillation wavelength. As a result, not only the decrease in the gain at the DFB oscillation wavelength but also the laser oscillation at the wavelength at the peak of the gain spectrum are caused. Consequently, the difference between the peak at the oscillation wavelength and the peak of the side mode cannot be set to be large and it becomes difficult to realize a single mode oscillation for the laser device. That is to say, since the strained quantum well laser device, in particular, has an abrupt gain profile, even when the distance between the DFB oscillation wavelength and the wavelength at the peak of the gain spectrum is short, i.e., the amount of the detuning to be performed is small, the difference between the peak at the oscillation wavelength and the peak of the side mode becomes disadvantageously small and it becomes difficult to realize the single mode oscillation. Therefore, even if a smaller amount of the detuning is required for a strained quantum well laser device as compared with a non-strained quantum well laser device, the single mode oscillation cannot be realized easily. Accordingly, a semiconductor laser device with a small degree of the temperature dependence of a gain wavelength is desired. SUMMARY OF THE INVENTION The present invention thus concerns a semiconductor laser device as defined in claim 1. In one embodiment, the semiconductor laser device further includes a first optical waveguide layer and a second optical waveguide layer so as to sandwich the strained quantum well structure therebetween. In another embodiment, the semiconductor substrate is composed of InP. According to another aspect of the present invention, a method for fabricating a semiconductor laser device is provided as defined in claim 4. In one embodiment, the process step for forming the well layer includes the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.0 - 0.10 × (a number of the well layers), or more; and growing the well layer at a temperature of 580°C or lower. In another embodiment, the process step for forming the well layer includes the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.2 - 0.02 × (a number of the well layers), or more; and growing the well layer at a temperature of 600°C or lower. In still another embodiment, the process step for forming the well layer includes the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.6 - 0.02 × (a number of the well layers), or more; and growing the well layer at a temperature of 700°C or lower. According to still another aspect of the present invention, a strained quantun well crystal is provided according to claim 8. Thus, the invention described herein makes possible the advantages of providing: (1) a semiconductor laser device where the temperature dependence and the injection current dependence of the oscillation wavelength is small, and a single mode oscillation is realized in a wide temperature range and a wide current injection range and a method for fabricating the same; and (2) providing a semiconductor crystal used for such a semiconductor laser device and a method for fabricating the same. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a graph showing the temperature dependence of a PL emission spectrum to be shifted from a peak wavelength at 77K to a peak wavelength at room temperature in the case where an atomic ordering is not generated. Figure 1B is a graph showing the temperature dependence of a PL emission spectrum to be shifted from a peak wavelength at 77K to a peak wavelength at room temperature in the case where an atomic ordering is generated. Figure 2A is a cross-sectional view showing a strained quantum well crystal structure according to a first example of the present invention, while Figure 2B is an energy band diagram of the strained quantum well crystal structure of the first example. Figures 3A and 3B are graphs illustrating a difference in the PL emission spectra depending upon the generation of the atomic ordering. Figure 4 is a graph illustrating a difference in a shift amount of the wavelength at a peak of the gain spectrum depending upon the generation of the atomic ordering. Figure 5 is a graph showing the relationship between the number of the well layers and the amount of the lattice strain to be varied depending upon a crystal growth temperature as a parameter and the generation of the atomic ordering. Figure 6 is a graph showing the relationship between the strain amount (%) where the atomic ordering is generated and the crystal growth temperature (°C) when the number of the InGaAsP strained quantum well layers is ten. Figure 7A is a cross-sectional view showing a strained quantum well semiconductor laser device according to a second example of the present invention, while Figure 7B is an energy band diagram of the strained quantum well structure of the second example. Figures 8A to 8D are cross-sectional views showing the respective process steps for fabricating the strained quantum well laser according to the second example of the present invention. Figure 9 is a graph illustrating a difference in a shift amount of the wavelength at a peak of the gain spectrum depending upon the generation of the atomic ordering. Figure 10 is a graph showing the difference in the injection current dependence of the oscillation wavelength between the present invention and a conventional example. Figure 11A is a cross-sectional view showing a strained quantum well semiconductor laser device according to a third example of the present invention, while Figure 11B is an energy band diagram of the strained quantum well structure of the third example. Figures 12A and 12B are graphs illustrating the difference in the emission spectra of the multiple-quantum well structure depending upon the introduction of the strain. Figure 13 is a graph illustrating the difference in the temperature dependence of the realizability of the single mode oscillation in accordance with the generation of the atomic ordering. Figure 14 is a graph illustrating the difference in the injection currant dependence of the realizability of the single mode oscillation in accordance with the generation of the atomic ordering. Figure 15A is an energy band diagram of the strained quantum well structure of the conventional semiconductor laser device, while Figure 15B is a perspective view showing a conventional strained quantum well type semiconductor laser device. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, by introducing an atomic ordering into at least one of a well layer and a barrier layer, it is possible to provide a strained quantum well crystal suitable for a light-emitting element such as a semiconductor laser device where the wavelength at the peak of the gain spectrum has a small temperature dependence. In general, in the case of producing a mixed crystal composed of a diatomic compound represented by AC and a diatomic compound represented by BC , a mixed crystal represented by (AB)C 2 where the atoms A and B are uniformly mixed can be obtained. However, in the case where such a mixed crystal is formed at a low temperature, a mixed crystal where the atoms A and B are not mixed uniformly and the compounds AC and BC are arranged in a multi-layer manner is sometimes obtained. As a result, observed is a superlattice structure where the lattice planes of the compounds AC and the lattice planes of the compounds BC are alternately arranged is observed. In addition, when a crystal is grown at a low temperature, a superlattice is automatically formed. For the purpose of distinction, this type of superlattice is called a "natural superlattice", as opposed to an "artificial superlattice" such as a quantum well structure. In this specification, if such a natural superlattice is formed, then an expression "a structure has an atomic ordering" or "an atomic ordering is generated in a structure" will be used even when such a superlattice is only partially formed. In the InGaAsP crystal grown at a temperature of 620°C or higher, the atomic ordering is not generated when the well is not strained. According to the experimental results obtained by the present inventors with respect to this crystal while varying the amount of the strain, the number of the well layers, and the crystal growth temperature, the atomic ordering is generated under certain conditions. More specifically, the present inventors have found that the atomic ordering is likely to be generated under the following conditions: (1) the amount of the strain in the InGaAsP crystal is large; (2) the number of the well layers is large; and (3) the crystal growth temperature is low. Although it was found that the atomic ordering is likely to be generated under any of the conditions (1) to (3), the present inventors thought at first that some dislocations were introduced into the crystal. However, the observation of the crystal using a transmission electron microscope revealed that no dislocation existed in the crystal, but that the atomic ordering was generated in the InGaAsP crystal. The temperature dependence of the PL emission spectrum of this crystal is shown in Figure 1B . As shown in Figure 1B , the difference between the wavelength at the peak of the PL emission spectrum at room temperature (RT) and the wavelength at the peak of 77K has been found to be small. In the case where the atomic ordering is not generated, the difference between the wavelength at the peak of the PL emission spectrum at room temperature and the wavelength at the peak of 77K is large as shown in Figure 1A . Conventionally, based on the results shown in Figure 1B , it has been considered that the degradation of the PL characteristics of a strained quantum well structure is caused by the generation of the atomic ordering. However, the present inventors have found that a semiconductor laser device where the oscillation wavelength is varied a little depending upon the temperature and the injection current can be provided by positively utilizing the atomic ordering phenomenon. Hereinafter, the present invention will be described by way of illustrative examples using the InGaAsP crystal with reference to accompanying drawings. Example 1 Referring to Figures 2A and 2B , an exemplary strained quantum well crystal according to the present invention will be described. Figure 2A is a cross-sectional view schematically showing a strained quantum well crystal including an Sn-doped InP semiconductor single crystalline substrate 1 , and a multi-layered structure provided on the Sn-doped InP semiconductor single crystalline substrate 1 . This multi-layered structure is obtained by epitaxially growing an InGaAsP (λg = 1.05 µm) first optical waveguide layer 2 ; a multiple-quantum well structure 5 ; and an InGaAsP (λg = 1.05 µm) second optical waveguide layer 6 , in this order on the InP semiconductor single crystalline substrate 1 . The multiple-quantum well structure 5 includes InGaAsP strained well layers 3 (compression strain of 0.8%) and InGaAsP (λg = 1.15 µm) barrier layers 4 which are alternately deposited so as to form seven quantum wells. The atomic ordering is generated in the multiple-quantum well structure 5 . The photoluminescence (PL) emission wavelength of the multiple-quantum well structure 5 is 1.3 µm. In this example, a compression strain of 0.8% is introduced into the strained well layers 3 and the crystal growth temperature is set to be 580 degrees, so that the atomic ordering is generated in the barrier layers 4 . The energy band gap of the multi-layered structure is shown in Figure 2B . The present inventors have confirmed that the atomic ordering exists in the barrier layers 4 by a transmission electron microscopy. However, it is possible that the atomic ordering is also generated in quantum well layers 3 . In this example, since the compression strain is introduced into the quantum well layers 3 , a tensile strain is generated in the barrier layers 4 . Therefore, the atomic ordering is more likely to be generated by the tensile strain. In order to generate the atomic ordering preferentially within the quantun well layers 3, it seems to be preferable that the tensile strain is generated in the well layers by adjusting the composition of the well layers. The PL emission spectra of the strained quantum well crystal are measured. Figure 3A shows the PL emission peaks of the strained quantum well crystal having no atomic ordering, while Figure 3B shows the PL emission peaks of the strained quantum well crystal having an atomic ordering. In the case where no atomic ordering is generated, by lowering the temperature from room temperature (T = 300K) to 77K, a short wavelength shift of 60 meV is generated. On the other hand, in the case where an atomic ordering is generated, the short wavelength shift is a negligible one, e.g., 10 meV. In addition, if the temperature is maintained at room temperature, then the emission spectrum is not reduced irrespective of the generation of the atomic ordering. Consequently, the strained quantum well crystal having an atomic ordering exhibits an emission characteristic having a low temperature dependence. Figure 4 shows the temperature dependence of the PL emission peak wavelength. In the case where the atomic ordering is generated, the temperature dependence of the PL emission peak wavelength is reduced to one fifth or less of the dependence in the case where no atomic ordering is generated, as shown in Figure 4 . The present inventors analyze the reasons why the atomic ordering is more likely to occur by giving a strain to the crystal and lowering the crystal growth temperature T in the following nanner. For example, in the case where a compression strain is introduced into the InGaAsP strained quantum well layer, the internal energy of the crystal is increased by an amount corresponding to the energy of the strain. However, if In-As bonds having a large lattice constant are naturally oriented in the crystal growth direction so as to generate the atomic ordering, then the strain energy of the strained quantum well layer is reduced. An internal energy U of a crystal is generally expressed by the following equation: U = H - TS where H is an enthalpy, T is a temperature and S is an entropy (S < O). The smaller the internal energy U becomes, the stabler the crystal becomes. Since the introduction of the strain into the crystal increases the enthalpy H , the crystal tends to reduce the internal energy U thereof by reducing the entropy S . In other words, it is necessary that the enthalpy H is increased and the entropy S is reduced by introducing a large strain into the quantum well. On the other hand, the relationship between the crystal growth temperature and the atomic ordering is presumed to be as follows. Since U and H vary in proportion to a square of T , the decrease in the crystal growth temperature T causes the decrease in S in proportion to T . As a result, the decrease in S, i.e., the atomic ordering, is likely to be generated. In addition, the present inventors analyze the reasons why the generation of the atomic ordering suppresses the temperature dependence in the following manner. If the generation of the atomic ordering reduces the entropy S , then the temperature dependence of the internal energy U is suppressed. Since the energy band gap Eg determining the oscillation wavelength is variable depending upon the internal energy U , it is possible to suppress the variation of the energy band gap Eg depending upon the temperature by suppressing the temperature dependence of the internal energy U . Consequently, the generation of the atomic ordering can suppress the variation of the wavelength depending upon the temperature. Figure 5 is a graph showing the relationship between the amount of the strain and the number of the quantum well layers for generating the atomic ordering at the respective crystal growth temperatures Tg of 580, 600 and 700°C. As shown in Figure 5 , if the amount of the strain and the number of the well layers are set to be large, then the atomic ordering is more likely to be generated. Based on the results obtained in this experiment, when the crystal growth temperature Tg is 580°C, the strain amount 580 where the atomic ordering is generated can be expressed by following Equation 1 using the number of the well layers: strain amount 580 (%) = 1.0 - 0.10 × the number of the well layers That is to say, if a strain in an amount larger than the strain amount 580 obtained by Equation 1 is introduced, then the atomic ordering is generated. On the other hand, if a strain in an amount smaller than the strain amount 580 obtained by Equation 1 is introduced, then the atomic ordering is not generated. As shown in Figure 5 , the higher the crystal growth temperature Tg is set to be, the lower the possibility of the generation of the atomic ordering becomes. Therefore, even when the crystal growth temperature Tg is lower than 580°C, the atomic ordering is possibly generated in the region expressed by the inequality "strain amount (%) ≥ 1.0 - 0.10 × the number of the well layers". In the same way, when the crystal growth temperatures Tg are 600°C and 700°C, then the atomic ordering is generated in the regions expressed by the following Equations 2 and 3, respectively: strain amount (%) ≥ 1.2 - 0.02 × the number of the well layers (when Tg is 600°C) strain amount (%) ≥ 1.6 - 0.02 × the number of the well layers (when Tg is 700°C) During an actual fabrication of a semiconductor laser device, the crystal growth is frequently performed at a temperature of 600°C or higher. In addition, an optimal number of well layers for preventing the decrease in the optical output from a semiconductor laser device is required to be 10 or less. Accordingly, in order to generate an atomic ordering in the case where the crystal growth is performed at a temperature of about 600°C, it is necessary to form a compression strain of about 1%. On the other hand, in the case where the crystal growth temperature is lowered to 580°C and the number of the well layers is set to be seven, the atomic ordering can be generated by giving a compression strain of about 0.7%. It is noted that the strain amount can be controlled by adjusting the composition of the crystal to be grown so as to grow a crystal with a desired lattice constant. A strained quantum well crystal structure according to the present invention is suitably applied to an emission region of a semiconductor laser device usable for a pump light source of a fiber amplifier, in particular. In order to obtain a sufficient amount of gain by a fiber amplifier, a laser beam at a particular wavelength is required to be used. However, in the case where a large driving current of 500 mA or more is applied to a semiconductor laser device, the modulation of the driving current considerably varies the radiation amount of the heat from the semiconductor laser device, so that the wavelength of the laser oscillation is adversely varied. However, in the semiconductor laser device of the present invention, the oscillation wavelength is varied little depending upon the temperature. Therefore, even if the driving current is increased, the oscillation wavelength can be easily maintained within a predetermined range required for a fiber amplifier. Furthermore, even when the width of the active layer in a semiconductor laser device is varied in respective elements and the radiation amount of the heat with respect to the same amount of the driving current (or the injection current) differs in the respective laser devices, the oscillation wavelength is maintained to be constant. Therefore, according to the present invention, it is possible to considerably increase the production yield of a semiconductor laser device satisfying a desired design rule. Figure 6 is a graph showing the relationship between the strain amount (%) required for generating the atomic ordering and the crystal growth temperature (°C) when the number of the InGaAsP strained quantum well layers is set to be ten. The atomic ordering is generated in the region having a strain larger than the strain indicated by the curve shown in Figure 6 . The InGaAsP strained quantum well layer is generally grown at a temperature in an approximate range of 600 to 620°C. Accordingly, in order to prevent the generation of the atomic ordering, the crystal growth is required to be performed in the region A . On the other hand, in the case where the InGaAsP strained quantum well layer is grown at a temperature of 620°C or higher, the generation of the atomic ordering can be prevented by employing the conditions indicated by the region B shown in Figure 6. An objective of the present invention is to provide a strained quantum well crystal having an emission characteristic which is less likely to be varied depending upon the temperature by intentionally causing the atomic ordering. However, in the course of defining the conditions for generating the atomic ordering, the contrary conditions, i.e., the conditions for preventing the generation of the atomic ordering are also clarified. In order to realize the PL characteristic shown in Figure 1A while supplying a large strain to the quantum well layer, the crystal growth is required to be performed under the conditions indicated by the region B shown in Figure 6 . Example 2 Referring to Figures 7A and 7B, another exemplary strained quantum well semiconductor laser device according to the present invention will be described. Figure 7A schematically shows a structure of a semiconductor laser device including an Sn-doped InP semiconductor single crystalline substrate 1 having a stripe-shaped ridge (width: 1.2 to 1.5 µm; height: 1.5 to 2.5 µm), and a stripe-shaped multi-layered structure provided on the ridge of the Sn-doped InP semiconductor single crystalline substrate 1 . This stripe-shaped multi-layered structure is obtained by epitaxially growing an InGaAsP (λg = 1.05 µm) first optical waveguide layer 2 ; a multiple-quantum well structure 5 ; and an InGaAsP (λg = 1.05 µm) second optical waveguide layer 6, in this order on the InP semiconductor single crystalline substrate 1 . The thickness of the first optical waveguide layer 2 and that of the second optical waveguide layer 6 are set to be 30 nm and 150 nm, respectively. The multiple-quantum well structure 5 includes InGaAsP strained well layers 3 (compression strain of 0.8%) and InGaAsP (λg = 1.15 µm) barrier layers 4 which are alternately deposited so as to form seven quantum wells. In the multiple-quantum well structure 5 , the atomic ordering is generated. The thickness of the InGaAsP strained well layer 3 is sat to be in a range of 3 to 6 nm, while the thickness of the InGaAsP barrier layer 4 is set to be in a range of 5 to 20 nm. The photoluminescence (PL) emission wavelength of the multiple-quantum well structure 5 is 1.3 µm. In this example, a compression strain of 0.8% is introduced into the strained well layers 3 and the crystal growth temperature thereof is set to be 580 degrees, thereby generating the atomic ordering in the barrier layers 4 . The energy band gaps of the multi-layered structure are shown in Figure 7B. As shown in Figure 7B , a p-InGaAsP/p-InP/n-InP/p-InP current confining structure 7 with a thickness in a range of 3 to 6 µm is provided on both sides of the stripe-shaped multi-layered structure. A groove is provided in a part of the current confining structure 7 . The surface of the groove is covered with an SiO 2 layer. A p-type electrode 8 is provided on the upper surface of the semiconductor laser device, while an n-type electrode 9 is provided on the bottom surface of the semiconductor laser device. Next, referring to Figures 8A to 8D, a method for fabricating the strained quantum well laser device shown in Figure 7A will be described. First, as shown in Figure 8A , a first crystal growth process step for epitaxially growing an InGaAsP first optical waveguide layer 2 ; a multiple-quantum well structure 5 comprised of alternately stacked layers of an InGaAsP quantum well layer 3 and an InGaAsP current barrier layer 4 ; and an InGaAsP second optical waveguide layer 6 in this order on the InP substrate 1 is performed. Then, as shown in Figure 8B , an etching process for etching these layers in a stripe shape is performed. Thereafter, as shown in Figure 8C , a second crystal growth process step for epitaxially growing the current confining structure 7 is performed. Finally, as shown in Figure 8D , an electrode formation process step for forming the p-type electrode 8 and the n-type electrode 9 is performed. In this example, in order to generate the atomic ordering, a strain of 0.8% is introduced into the well layers, and the crystal growth temperature is set to be 580°C, and the supply ratio of a Group V gas to a Group III gas is set to be 250. These crystal growth conditions are indicated at a point A in Figure 5 . As described above, the semiconductor laser device of this example includes in the stripe-shaped multi-layered structure a similar crystal to the strained quantum well crystal of the first example. In this example, such a strained quantum well crystal is also formed under the same conditions as those described in the first example. The oscillation wavelength of the semiconductor laser device including a strained quantum well crystal having an atomic ordering is 1.3 µm, and the maximum optical output therefrom is 300 mW, which is at the same level as that of the semiconductor laser device having no atomic ordering. The laser oscillation is generated at a threshold current at a similar level. Figure 9 is a graph showing a temperature dependence of the laser oscillation wavelength. In the case where the temperature of the semiconductor laser device is varied from -40°C to 100°C, the oscillation wavelength of the strained quantum well type semiconductor laser device having no atomic ordering is varied from 1.26 µm to 1.44 µm, while the oscillation wavelength of the strained quantum well type semiconductor laser device having an atomic ordering is varied from 1.29 µm to 1.31 µm. That is to say, the generation of the atomic ordering reduces the variation amount of the wavelength to one fifth or less. Since the variation amount of the oscillation wavelength is reduced with respect to the temperature, it is possible to provide a semiconductor laser device stably oscillating in a temperature range of -40°C to 100°C without performing any special control of the temperature of the semiconductor laser device. Figure 10 shows the injection current dependence of the oscillation wavelength. In general, as the amount of the current injected into the semiconductor laser device increases, a joule heat is generated because of the internal resistance of the semiconductor laser device, so that the temperature of the semiconductor laser device is increased. As a result, the oscillation wavelength is varied. However, in the semiconductor laser device of this example, since the temperature dependence of the oscillation wavelength is small, the variation in the amount of the injection current causes almost no variation in the oscillation wavelength. For a high-output semiconductor laser device to which a large amount of current is injected, in particular, a small variation in the oscillation wavelength is preferable. In addition, the oscillation wavelength at the time of a high output can be expected advantageously easily from the PL emission wavelength. As is apparent from the foregoing description, it is possible to provide a semiconductor laser device where the oscillation wavelength is varied a little depending upon the temperature and the amount of the injection current in this example. Example 3 Referring to Figures 11A and 11B , a still another exemplary strained quantum well semiconductor laser device according to the present invention will be described. Figure 11A schematically shows a structure of a semiconductor laser device including an Sn-doped InP semiconductor single crystalline substrate 1 having a stripe-shaped ridge, and a stripe-shaped multi-layered structure provided on the ridge of the Sn-doped InP semiconductor single crystalline substrate 1 . A diffraction grating 10 with a pitch of 180 nm is formed on the upper surface of the stripe-shaped ridge. The stripe-shaped multi-layered structure is obtained by epitaxially growing an InGaAsP (λg = 1.05 µm) first optical waveguide layer 2 ; a multiple-quantum well structure 5; and an InGaAsP (λg = 1.05 µm) second optical waveguide layer 6, in this order on the InP semiconductor single crystalline substrate 1 . The multiple-quantum well structure 5 includes InGaAsP strained well layers 3 (compression strain of 0.8%) and InGaAsP barrier layers 4 (λg = 1.15 µm) which are alternately deposited so as to form seven quantum wells. In the multiple-quantum well structure 5 , the atomic ordering is generated. The thickness of the respective semiconductor layers can be set to be in respective ranges described in the third example. The photoluminescence (PL) emission wavelength of the multiple-quantum well structure 5 is 1.3 µm. In this example, a compression strain of 0.8% is introduced into the strained well layers 3 and the crystal growth temperature is set to be 580 degrees, thereby generating the atonic ordering in the barrier layers 4 . The energy band gaps of the multi-layered structure are shown in Figure 11B . As shown in Figure 11B , a p-InGaAsP/p-InP/n-InP/p-InP current confining structure 7 is provided on both sides of the stripe-shaped multi-layered structure. A groove is provided in a part of the current confining structure 7 . The surface of the groove is covered with an SiO 2 layer. A p-type electrode 8 is provided on the upper surface of the semiconductor laser device, while an n-type electrode 9 is provided on the bottom surface of the semiconductor laser device. The semiconductor laser device of this example is characterized by oscillating at an oscillation wavelength determined by the pitch of the diffraction grating 10 , and is called a "distributed feedback (DFB) type laser device". The wavelength at the peak of the gain spectrum in the semiconductor laser device of this example is varied depending upon the temperature or the amount of the injection current in the same way as the semiconductor laser device of the second example. It is ideal for a DFB type laser device to oscillate in a single mode. However, as shown in Figure 12B , in the case where the difference between the DFB oscillation wavelength determined by the pitch of the diffraction grating 10 and the wavelength at the peak of the gain spectrum is larger than 20 nm, the possibility of the single mode oscillation of the laser device is drastically reduced. Figure 13 shows the temperature dependence of the realizability of the single mode characteristics. In the case where a semiconductor laser device having no atomic ordering is driven without performing any temperature control, the temperature at which the single mode oscillation is realized ranges from 0°C to 40°C. On the other hand, in the case where a semiconductor laser device is modeled by using a strained quantum well crystal having an atomic ordering, the single mode oscillation can be maintained even if the temperature of the semiconductor laser device is varied in a range of -80°C to 120°C. As a result, it is no longer necessary to use a Peltier device for controlling the temperature, so that the cost of a laser module can be considerably reduced. In addition, a high-output driving of a DFB type laser device is also realized. As shown in Figure 14, in the case where the atomic ordering is not generated, the realizability of the single mode oscillation decreases if the amount of the injection current is 300 mA or more. On the other hand, in the case where the atomic ordering is generated, the single mode oscillation characteristics are maintained even when the amount of the injection current is 600 mA or more. As a result, the production yield of a high-output DFB type laser device oscillating in a single mode is high even when the optical output therefrom widely ranges from a low output to a high output. As described above, in this example, it is possible to provide a DFB type laser device for realizing a single mode oscillation characteristic which is unlikely to be affected by the temperature or the amount of the injection current, and provide an inexpensive module for which a Peltier device is not required. In the foregoing examples, the barrier layer 4 is composed of InGaAsP (λg = 1.15 µm). However, if the barrier layer 4 is composed of InGaAsP (λg = 1.05 µm), then an atomic ordering is more likely to be generated in the barrier layer. The present invention has been described as being applied to a semiconductor laser device having a spectrum confinement heterostructure. Alternatively, the present invention is applicable to other kinds of semiconductor laser devices. The semiconductor laser device to which the present invention is applicable is not limited to a Fabry-Perot type laser device and a DFB type laser device, but the present invention is applicable to a semiconductor laser device having a high additional value such as a DBR type laser device. In addition, a strained quantum well crystal of the present invention is applicable not only to a light-emitting element such as a semiconductor laser device, but also to a photodetector and a high-speed electron device such as an HFET. In such cases, the temperature dependence of these devices can also be improved. As is apparent from the foregoing description, according to the present invention it is possible to provide a semiconductor laser device where the oscillation wavelength is not affected by the ambient temperature and the injection current by generating an atomic ordering in a strained quantum well layer, and provide an inexpensive DFB type laser device for which a temperature control device such as a Peltier device is not required. Therefore, much practical effect can be attained by the present invention. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims: A semiconductor laser device comprising: a strained quantum wall structure (5) including a well layer (3) and a barrier layer (4); and a semiconductor substrate (1) for supporting the strained quantum well structure (5), wherein the well layer (3) and the barrier layer (4) are composed of an InGaAsP mixed crystal having respectively different energy band gaps from each other, CHARACTERISED IN THAT in at least one of the well layer (3) and the barrier layer (4), an atomic ordering leading to a natural superlattice is generated without dislocation so that a lattice plane of a first compound and a lattice plane of a second compound are alternately stacked one by one to constitute the layer. A semiconductor laser device according to claim 1, further comprising a first optical waveguide layer (2) and a second optical waveguide layer (6) so as to sandwich the strained quantum well structure (5) therebetween. A semiconductor laser device according to claim 1, wherein the semiconductor substrate (1) is composed of InP. A method for fabricating a semiconductor laser device comprising a strained quantum well structure (5) including a well layer (3) and a barrier layer (4), the method comprising the step of forming the well layer (3) and the barrier layer (4) using an InGaAsP mixed crystal having respectively different energy band gaps from each other, and the step of forming an optical waveguide layer (2), the strained quantum well structure (5) being formed on the optical waveguide layer (2), the optical waveguide layer (2) having a bandgap energy larger than that of the barrier layer (4) included in the strained quantum well structure (5), CHARACTERISED IN THAT the method further comprises the step of forming at least one of the well layer (3) and the barrier layer (4) in such a way that in at least one of the well layer (3) and the barrier layer (4), an atomic ordering leading to a natural superlattice is generated without dislocation so that a lattice plane of a first compound and a lattice plane of a second compound are alternately stacked one-by-one to constitute the layer. A method for fabricating a semiconductor laser device according to claim 4, wherein the process step for forming the well layer (3) comprises the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.0 - 0.10 x, wherein x represents the number of the well layers, or more; and growing the well layer (3) at a temperature of 580°C or lower. A method for fabricating a semiconductor laser device according to claim 4, wherein the process step for forming the well layer (3) comprises the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.2 - 0.02 x, wherein x represents the number of the well layers, or more; and growing the well layer (3) at a temperature of 600°C or lower. A method for fabricating a semiconductor laser device according to claim 4, wherein the process step for forming the well layer (3) comprises the steps of: introducing a lattice strain in an amount expressed by an equation: strain amount (%) = 1.6 - 0.02 x, wherein x represents the number of the well layers, or more; and growing the well layer (3) at a temperature of 700°C or lower. A strained quantum well crystal comprising: a semiconductor crystal; and an InGaAsP crystal constituting a strained quantum well having a lattice constant different from a lattice constant of the semiconductor crystal, CHARACTERISED IN THAT an atomic ordering leading to a natural superlattice is generated without dislocation in the InGaAsP crystal so that a lattice plane of a first compound and a lattice plane of a second compound are alternately stacked one by one to constitute the InGaAsP crystal.

Inventors: ISHINO MASATO, KITO MASAHIRO, MATSUI YASUSHI, OTSUKA NOBUYUKI, ISHINO, MASATO, KITO, MASAHIRO, MATSUI, YASUSHI, OTSUKA, NOBUYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H01S5
IPC codes: H01S5, H01L33