Optical semiconductor device

Abstract: A semiconductor laser of multiple quantum well structure includes a multiple quantum well active layer (32) having a well layer of In x Ga 1-x As (0 < x ≦ 1), a first p-type clad layer (33) which is formed on the active layer (32) and lattice-matches with InP, and a second p-type clad layer (37) having a higher acceptor concentration than the first p-type clad layer (33). The acceptor concentration of the first p-type clad layer (33) is set to be not more than 2 x 10¹⁷ cm⁻³ and that of the second p-type region (37) is set to be not less than 1 x 10¹⁸ cm⁻³ in the range of 0.25 µm from the active layer. The well layer of the active layer (32) is formed of In x Ga 1-x As (0.53 < x ≦ 1). A laser in which the efficiency of injection into the active layer is increased and which has a small threshold value and excellent high-speed characteristic can be provided.

Description: This invention relates to an optical semiconductor device having a buried quantum well structure, and more particularly to an optical semiconductor device designed for improving the characteristic of a laser of multiple quantum well structure or the like. In recent years, a so-called quantum well semiconductor laser having at least one quantum well structure obtained by forming a well layer of a film thickness less than the de Broglie wavelength of electrons in an active layer and barrier layers whose forbidden band width is larger than that of the well layer and which are disposed on both sides of the well layer has been developed. The quantum well semiconductor laser has various advantages over a semiconductor laser of double hetero structure having no quantum well formed in the active layer in that the oscillation threshold value can be lowered, the modulation bandwidth can be widened, the oscillation spectrum width can be narrowed, the temperature characteristic can be improved, and a high output power can be obtained, for example. Further, recently, the degree of freedom of the oscillated wavelength may be enhanced and the characteristic may be further improved by using material which does not lattice-match with the substrate to form the quantum well layer. In general, if a material which does not lattice-match with the substrate exists near the active layer, a large number of lattice defects occur and the reliability and characteristic of the laser may be significantly deteriorated. However, even when a material which does not lattice-match with the substrate is used in the quantum well layer, the layer may be elastically strained so as not to generate lattice defects which may deteriorate the reliability and characteristic of the laser if the thickness of the layer is smaller than a critical thickness. The result of calculation obtained by use of a model of Matthews and Blakeslee may be used as one measure of the critical thickness. A high power laser and a laser with an extremely small threshold value are realized by use of an InGaAs/AℓGaAs semiconductor laser formed on a GaAs substrate of 0.98 µm bandwidth utilizing the above-described strained quantum well structure. Various possibilities of changing the TE/TM mode characteristic and changing the switching characteristic and absorption characteristic can be obtained by introducing a strained optical waveguide layer in an optical semiconductor device other than the semiconductor laser, for example, a semiconductor laser amplifier, optical switch and optical modulator. In the case of an InGaAs/InGaAsP series semiconductor laser formed on the InP substrate mainly used in the optical fiber communication, a so-called buried heterostructure in which the right and left portions of the active layer are filled with material having a larger forbidden bandwidth than the active layer is frequently used in order to obtain the single lateral mode oscillation of small threshold value. In this case, the strained quantum well layer comes to have a strained hetero interface on the side surface thereof and a large strain and stress are generated. Now, a laser of In 0.7 Ga 0.3 As strained quantum well structure formed on a (001) InP substrate 1 shown in Fig. 15 is explained in detail as an example. The semiconductor laser is constructed by forming a multi-layered structure of an active layer (optical waveguide layer) 4 of strained quantum well structure, p-type InP clad layer 5 of 1.5 µm thickness and p-type InGaAsP (composition corresponding to a photoluminescence wavelength of 1.2 µm (1.2 µm-wavelength composition)) contact layer 6 of 0.8 µm thickness on the n-type InP substrate 1 (which is also used as a clad layer) and burying the laminated structure in an Fe-doped semi-insulative InP layer 7. The active layer 4 has undoped In 0,7 Ga 0,3 As well layers 2 of 4.2 nm thickness and undoped InGaAsP (1.2 pm-wavelength composition) barrier layers 3 which lattice-match with InP and are respectively disposed on both sides of a corresponding one of the well layers. The barrier layers 3 are formed with a thickness of 12 nm between the well layers 2 and a thickness of 20 nm on each of the uppermost and lowermost well layers and thus have a total thickness of 76 nm. Therefore, the total thickness of the active layer 4 is 92.8 nm. Further, the width of the buried active layer 4 is 2 µm. Electrodes 11 and 12 for current supply are formed on opposite sides of the substrate. Further, a laser resonator (optical cavity) with a length of 1 mm is formed by cleavage. The lattice constant of the InP substrate is 0.58688 nm and the lattice constant of In 0.7 Ga 0.3 As is 0.59381 nm. Assuming now that there is an infinite plane having no side surface, the well layer 2 is elastically strained since it is as thin as 4.2 nm. The degree of strain is ε xx = ε yy = -0.01167, ε zz = -2(C 12 /C 11 )xε xx = 0.011974, ε yz = ε zx = ε xy = 0. C 12 /C 11 = 0.504 is the Poisson's ratio of In 0.7 Ga 0.3 As. As a result, the lattice constant of the strained In 0.7 Ga 0.3 As well layer 2 becomes equal to that of InP in the xy plane and the lattice constant in the z direction is 0.60092 nm. However, in practice, the active layer 4 is buried in the Fe-doped InP blocking layer 7 in a stripe form with a width of 2 µm. As a result, lattice-mismatching occurs in the boundary between the well layer 2 and the blocking layer 7. Since the thickness of the well layer 2 is 4.2 nm, it may be formed of approx. 6.99 unit layers on average. In the face-centered cubic zinc blende structure, four atomic layers are present in the lattice interval a with a positional deviation in the <001> direction, and in this case, four layers (lattice constant a) which are aligned with each other are counted as one unit lattice layer. When the thickness is divided by the lattice constant of InP, 7.16 unit lattice layers can be obtained. The well layer 2 has four layers and therefore lattice-mismatching corresponding to the 0.68 unit lattice layer will occur in the side surface of the active layer 4. Since the width of the active layer 4 is as large as 2 µm, the entire portion of the active layer 4 will not be elastically compressed. As a result, lattice defects such as dislocations may tend to occur near the side surface of the active layer 4. Since the degree of lattice-mismatching of the upper side surface of the strained quantum well layer is larger than the 0.5 unit lattice layer, the lattice plane of the strained quantum well layer becomes nearer to a lattice plane of the blocking layer lying directly above the strained quantum well layer and will be more easily connected to the lattice plane on the side surface thereof, and the possibility of occurrence of dislocations becomes high. With such lattice defects, segregation of impurity tends to occur. Further, with such lattice defects, significant reduction in the reliability and deterioration in the laser characteristic such as reduction in the light emission efficiency rise in the oscillation threshold value and reduction in the differential gain may occur. In an optical semiconductor device other than the semiconductor laser, defects due to the lattice-mismatching occur in the side surface of the buried strained quantum well optical waveguide layer to cause various demerits. For example, in the buried strained quantum well optical waveguide, an increase in the absorption coefficient and an increase in the light scattering coefficient may be caused by the defects. Further, in a light detection device having a buried strained quantum well light absorption layer, various problems such as an increase in a generation recombination dark current caused by the defects on the side surface, irregularity of the internal electric field caused by impurity segregated by the defects, reduction in the quantum efficiency due to recombination in the defects and reduction in the reliability may occur. Further, in a semiconductor device other than the optical semiconductor device, a strained semiconductor layer such as a pseudomorphic HEMT strained channel layer or a strained In(Ga)As ohmic contact layer is used to improve the characteristic. When the strained semiconductor layer is buried in part of the substrate instead of the entire portion of the substrate, various problems such as reduction in the carrier lifetime, reduction in the electron mobility due to an increase in the scattering centers, an increase in noises, and reduction in the reliability which are caused by segregation of impurity and lattice defects due to the lattice-mismatching on the side surface may occur. In a quantum well laser having InGaAsP or InGaAs at the well layer widely used in the optical communication field, the well barrier of the conduction band is low and the well barrier of the valence band is high so that overflow of electrons to the barrier layer and optical waveguide layer may easily occur and the injection efficiency of holes into the well layer will become low. For this reason, the utilization efficiency of carriers becomes low and the effect of reduction in the oscillation threshold value and extension of the modulation bandwidth is not significant in comparison with a case of a bulk type double hetero semiconductor laser having the same oscillation wavelength. Since the injection efficiency of holes into the well layer becomes low when stimulated emission becomes large and the number of carriers becomes insufficient, a difference in the hole density occurs between the wells and the gain saturation ε for light becomes large. This means that damping becomes large and the frequency bandwidth will not be extended irrespective of an increase in the differential gain caused by the effect of the quantum well. A high power laser may have a preferable characteristic when it is of long resonator and a large number of wells are used, but since the above problems are present, the number of wells cannot be fully increased in practice. In general, since saturation of the gain G with respect to the carriers is significant in the quantum well laser, the differential gain becomes significantly deteriorated when the oscillation threshold carrier density becomes high. Therefore, reduction in the oscillation threshold value is important in view of the high-speed response characteristic. Since the oscillat ing condition can be expressed by ΓG=α, the optical total loss α must be set to a small value to decrease the gain necessary for the oscillation in order to reduce the oscillation threshold value. In a quantum well laser, particularly, in a strained quantum well laser having strains in the well layer, the waveguide loss of the active layer can be suppressed to an extremely small value, but the loss may become large when the acceptor density of a p-clad layer is high. Since the optical confinement coefficient Γ of the quantum well laser is small, influence by absorption in the clad layer is extremely large. Therefore, in order to obtain a quantum well laser of low threshold value, it is necessary to reduce the acceptor density of the p-clad layer. In this case, however, since a large band barrier is created in the hetero junction portion between the p-clad layer and the active layer, there occurs a problem that the injection efficiency of holes into the quantum well active layer is lowered. This means that it is difficult to form a quantum well laser which has a small threshold value and a high-speed response. In order to solve the problem of the barrier of the hetero interface, use of GRIN (graded index) is proposed, but in this case, precise control for the crystal growth becomes necessary and the process becomes complicated. In addition, a problem that defects of dislocations tend to occur in the side surface of the active layer by the lattice-mismatching has occurred in the strained quantum well laser of buried structure. In particular, when a layer doped with a transition metal such as Fe is formed on the side surface, there occurs a possibility that composite defects of dislocations and the transition metal serving as impurity deteriorate the laser characteristic. Thus, in the conventional semiconductor device having the strained semiconductor layer of buried structure, defects such as lattice defects and impurity segregation caused by the lattice-mismatching of the side surface of the strained semiconductor layer may occur, thereby significantly deteriorating the characteristic and reliability. Further, in the multiple quantum well laser of InGaAs or InGaAsP, the injection efficiency of holes into the well layer is low and a difference in the hole density tends to occur between the wells, thereby making it difficult to increase the number of wells. Further, it is difficult to realize a laser which has a small absorption loss and high injection efficiency of holes and reduction in the oscillation threshold value and the improvement of the high-speed response characteristic cannot be easily attained. This invention has been made in view of the above problems and an object of this invention is to provide a semiconductor device of buried strained quantum well structure having an excellent characteristic and reliability by alleviating the lattice-mismatching in the side surface of a strained semiconductor layer. Another object of this invention is to provide a semiconductor device having a small threshold value and an excellent high-speed response characteristic and capable of enhancing the injection efficiency of carriers into a quantum well active layer. This invention is roughly divided into the following two features. That is, the first feature is the alleviation of the lattice-mismatching in the side surface of the strained quantum well layer and the second feature is the improvement of the injection efficiency. According to a first aspect of this invention, there is provided an optical semiconductor device comprising a stripe-form optical waveguide layer formed on a main surface of a semiconductor substrate and having a strained quantum well constituted by first semiconductor layers which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on both lateral sides of said optical waveguide layer; and cladding layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on upper and lower sides of said optical waveguide layer; wherein the side surfaces of the optical waveguide layer are tapered with respect to the normal of said substrate at an angle of not less than 45 degrees. According to a second aspect of this invention, there is provided an optical semiconductor device comprising: a stripe-form optical waveguide layer formed on a main surface of a semiconductor substrate and having a strained quantum well constituted by first semiconductor layers which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on both lateral sides of said optical waveguide layer; cladding layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on upper and lower sides of said optical waveguide layer; wherein the average lattice constant of each blocking layer has a value between the lattice constants of said substrate and said first semiconductor layer. The optical semiconductor device may be constructed by use of a semiconductor layer of tetragonal system having the c axis set in the direction of the normal to the substrate to form part of the semiconductor layer of the clad layer which extends in the right and left directions of the strained quantum well layer and lies in a plane parallel to the substrate and the lattice constant of the a axis thereof is set to be less than that of the semiconductor substrate. In each of the above mentioned arrangements, it is assumed that the stripe-form optical waveguide layer (corresponding to the active layer in the semiconductor laser and the light absorption layer in the optical detection device) is surrounded at the right, left, upper and lower portions thereof by the clad layer having a forbidden bandwidth larger than that of the barrier layer of the strained quantum well. Of course, the first arrangement can be used together with the other two arrangements. The important factor of a third aspect of this invention is to alleviate the lattice-mismatching caused in the side surface of the strained semiconductor layer of the first region buried in the second region. This is achieved by an optical semiconductor device comprising: a stripe-form optical waveguide layer formed on a main surface of a semiconductor substrate and having a strained quantum well constituted by first semiconductor layers which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on both lateral sides of said optical waveguide layer; cladding layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on upper and lower sides of said optical waveguide layer; wherein a composition changing region whose lattice constant is changed continuously or stepwise between that of said optical waveguide layer and that of each blocking layer is formed between the blocking layers and the optical waveguide layer. In this case, the strained semiconductor layer indicates a semiconductor layer having strains elastically and intentionally introduced therein by laminating semiconductor layers of compositions which have different lattice constants and each of which has a film thickness less than the critical thickness and it is not assumed that the strains are inadvertently introduced by the incomplete composition control of crystal growth. In general, in the case of a semiconductor layer grown on the GaAs and InP substrate, the maximum value of strains inadvertently introduced by the incomplete composition control of crystal growth is 0.2% or less, and if a larger degree of lattice-mismatching has occurred, it can be considered that the strained semiconductor layer is intentionally formed. In an optical semiconductor device using a buried optical waveguide having the strained semiconductor layer as a portion thereof, it can be considered that a portion in which the optical waveguide is formed is a first region and a buried portion adjacent to the side surface of the optical waveguide is a second region. The composition changing region can be formed by diffusing impurities to activate the mutual diffusion of atoms in the boundary portion, for example. when one of the first and second regions is formed by selective growth, it can be formed by utilizing variation in the crystal composition according to the distance from a mask formed on the other region. The important factor of a fourth aspect of this invention is to reduce the possibility of generation of dislocations by reducing the accumulated lattice-mismatching in the side surface of the buried strained semiconductor layer to less than half the unit lattice interval. In this case, as described before, the strained semiconductor layer indicates a semiconductor layer having strains elastically introduced therein by laminating semiconductor layers of composition having different lattice constants with the film thickness of each strained semiconductor layer set less than the critical thickness. Therefore, this is achieved by an optical semiconductor device comprising: a stripe-form optical waveguide layer formed on a main surface of a semiconductor substrate and having a strained quantum well constituted by first semiconductor layers which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on both lateral sides of said optical waveguide layer; cladding layers formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer and formed on upper and lower sides of said optical waveguide layer; wherein the expression that absolute (Z2(n) - Z1(n)) < δ min (n)/2 is satisfied when said first and second semiconductor layers are alternately laminated, the lower main surface of the lowermost one of said first semiconductor layers of said optical waveguide layer is used as a reference plane, the coordinate system Z is set in the upward direction along the normal of a low index plane which is the nearest to said reference plane, the coordinate of an n-th unit lattice plane of said optical waveguide layer from said reference plane is Z1(n), the coordinate of an n-th unit lattice plane of said blocking layers from said reference plane is Z2(n), and the smallest one of four values of "Z2(n+1)-Z2(n)", "Z2(n)-Z2(n-1)", "Z1(n+1)-Z1(n)" and "Z1(n)-Z1(n-1)" is δ min (n) . In the semiconductor device having a strained semiconductor layer buried in the first region, this invention is made to satisfy the following-expression (1) for a given value n; absolute {Z2(n) - Z1(n)} < δ min (n)/2 where the lower main surface of the lowermost strained semiconductor layer is used as a reference lattice plane (z = 0), the coordinate system Z is set in the upward direction along the normal thereof, the coordinate of an n-th unit lattice plane from the reference lattice plane of the first region is Z1(n) when the first region is assumed to infinitely extend, the coordinate of an n-th unit lattice plane from the reference lattice plane of the second region is Z2(n) when the second region adjacent to the first region is assumed to infinitely extend, and the smallest one of four values of "Z2(n+1)-22(n)", "Z2(n)-Z2(n-1)", "Z1(n+1)-Z1(n)" and "Z1(n)-Z1(n-1)" is δ min (n). In this case, absolute{x} indicates the absolute value of x. In other words, the maximum degree of accumulated lattice-mismatching in the boundary between the first and second regions is suppressed to a value less than half that of the unit lattice layer. In this case, the term "unit lattice layer" is defined as a period of the crystal structure in the Z direction. For example, in the case of zinc blend structure, the lattice constant a comes to correspond to one unit lattice layer when the Z direction is set to the <001> direction and 3 1/2 a comes to correspond to one unit lattice layer when the Z direction is set to the <111> direction. When the reference plane is not a low index plane, for example, when it is an off substrate, the theory as described above can be similarly applied by setting the normal of the nearest low index plane to the Z direction. As one means for satisfying the expression (1), a method of laminating at least one first strained semiconductor layer having a larger lattice constant than the semiconductor substrate and at least one second strained semiconductor layer having a smaller lattice constant than the semiconductor substrate and canceling the accumlated lattice-mismatching of the side surface caused by one of the first and second strained semiconductor layer groups with the lattice-mismatching of the side surface caused by the other strained semiconductor layer group. This method is effective when the total thickness of one of the strained semiconductor layer groups becomes too large and the expression (1) cannot be satisfied only by use of the strained semiconductor layer group. In some cases, m atomic planes of the same group of elements exist in one unit lattice in the Z direction. For example, if the reference lattice plane of Z=0 is a Group-III atomic plane in the (001) plane of III-V compound semiconductor of zinc blende structure, m becomes 2 since a Group-III atomic plane exists in a /2. In the case of (110) plane, since Group-III atoms and Group-V atoms are present on the same plane, the same group atomic planes of m=4 exist in the unit lattice layer 2 1/2 a . In such a case, lateral deviation may occur and dislocations will be caused in different atomic planes of the same group in the unit lattice layer. At this time, it is preferable to satisfy the following expression (2) for a given value n. absolute {Z2(n) - Z1(n)} < δ min (n)/(2m) In an optical semiconductor device using a buried optical waveguide which is partly formed of a strained semiconductor layer, it can be considered that a portion in which the optical waveguide is formed is a first region and a buried portion adjacent to the optical waveguide is a second region. In the case of a strained quantum wire or strained quantum box, it can be considered that a semiconductor region forming the strained quantum wire or strained quantum box is a first region and a semiconductor region forming the barrier of the side surface thereof is a second region. Of course, the above embodiments can be used in combination. In the optical semiconductor device of the first aspect, since the lattice-mismatching of the side surface of the optical waveguide is distributed in the tapered plane, distortion of the lattice gradually occurs over the taper width so as to suppress occurrence of large strains and stresses. As a result, an optical semiconductor device in which lattice defects such as dislocations and segregation of impurity can be prevented and which has an excellent characteristic and high reliability can be attained. In the optical semiconductor device of the second aspect, since the average lattice constant in a plane parallel to the substrate including the stripe-form optical waveguide layer takes a value between the lattice constant of the substrate and the lattice constant of the strained quantum well layer, the lattice-mismatching of the side surface of the optical waveguide layer can be alleviated. As a result, an optical semiconductor device in which lattice defects such as dislocations and segregation of impurity can be prevented and which has an excellent characteristic and high reliability can be attained. Further, since the tetragonal system material (semiconductor of chalcopyrite structure) is used in a plane parallel to the substrate including the stripe-form optical waveguide layer and the lattice constant a of the tetragonal system material is set to be smaller than the lattice constant of the substrate. A value c /2 obtained by dividing the lattice constant of the semiconductor mixed crystal of chalcopyrite structure in the direction along the axis c by 2 is generally smaller than the lattice constant a in the direction of axis a . Therefore, if the thickness thereof is properly determined, the average lattice constants of the blocking layer and active layer in the direction of the normal to the substrate can be approximately matched to each other in the optical semiconductor device having a buried strained quantum well optical waveguide layer whose strained quantum well layer has a lattice constant smaller than that of the substrate. Thus, it becomes possible to alleviate the lattice-mismatching in the side surface of the optical waveguide layer without causing the lattice-mismatching with respect to the semiconductor substrate. As a result, an optical semiconductor device in which lattice defects such as dislocations and segregation of impurity can be prevented and which has an excellent characteristic and high reliability can be attained. In the optical semiconductor device of the third aspect, since the stress caused by the lattice-mismatching between the first and second regions is distributed in the entire portion of the composition changing region formed therebetween. Therefore, an optical semiconductor device in which generation of dislocations caused by concentration of stress on the boundary surface can be suppressed and whose characteristic and reliability will not be degraded by the dislocations can be attained. In a case where impurity is doped into the second region with high impurity concentration, mutual diffusion of host atoms in the diffusion regions can be activated if impurity diffusion from the boundary portion into the first region has occurred. As a result, in the boundary portion between the first and second regions, the mutual diffusion of host atoms between the regions proceeds while the strained quantum well is being disordered, thus forming a composition changing region. When the semiconductor mixed crystal is subjected to the selective growth, the composition, film thickness and distortion amount of the mixed crystal depend on the distance from the mask and the opening ratio thereof. Based on this fact, a region having varying lattice constants can be formed in the boundary portion. The above composition changing region can be formed by setting the varying direction of the lattice constant from one region to the other region such that the lattice constant at a portion closer to the other region will become nearer to the average lattice constant of the other region. In the fourth aspect, if the maximum degree of accumlated lattice-mismatching of the boundary portion between the first and second regions is suppressed to a value less than 1/2 of that of the unit lattice layer to satisfy the expression (1), generation of dislocations in the boundary between the first and second regions can be efficiently suppressed since the n'th unit lattice plane in the second region is located at a shorter distance from the n'th unit lattice plane in the first region than the (n±1)'th unit lattice plane in the second region. It is possible to increase the film thickness and the number of layers of the strained semiconductor layer which satisfies the expression (1) by laminating at least one first strained semiconductor layer having a larger lattice constant than the semiconductor substrate and at least one second strained semiconductor layer having a smaller lattice constant than the semiconductor substrate and canceling the lattice-mismatching of the side surface caused by one of the first and second strained semiconductor layer groups with the lattice-mismatching of the side surface caused by the other strained semiconductor layer group. If m atomic planes of the same group of elements exist in one unit lattice in the Z direction, an atomic plane of the same group of the adjacent region which is not deviated can be set at a shorter distance than an atomic plane of the same group of the adjacent region which is deviated by one to satisfy the expression (2). Generation of dislocations in the atomic planes of the same group accompanied by lateral deviation can be efficiently prevented since the maximum degree of accumlated lattice-mismatching in the Z direction of the boundary between the first and second regions is suppressed to a value less than 1/2 of the interval of the same group atomic planes. In either case, an optical semiconductor device in which generation of lattice defects such as dislocations in a portion near the boundary between the first and second regions can be prevented and which has an excellent characteristic and high reliability can be attained. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view showing the schematic structure of a semiconductor laser according to a first embodiment of this invention; Figs. 2A to 2C are cross sectional views showing a process of manufacturing the laser according to the first embodiment; Fig. 3 is a cross sectional view showing the schematic structure of a semiconductor laser according to a second embodiment of this invention; Fig. 4 is a cross sectional view showing the schematic structure of a semiconductor laser according to a third embodiment of this invention; Figs. 5A to SD are cross sectional views showing a process of manufacturing the laser according to the third embodiment; Figs. 6A and 6B are cross sectional views showing a process of manufacturing a semiconductor laser according to a fourth embodiment; Fig. 7 is a characteristic diagram showing the dependency of the laser differential gain on the number of well layers in the fourth embodiment; Fig. 8 is a cross sectional view showing the schematic structure of a semiconductor laser according to a fifth embodiment of this invention; Fig. 9 is a cross sectional view showing the main structure of the laser of the fifth embodiment; Fig. 10 is a model diagram showing deviation of a lattice plane in the Z direction of the side surface of an active layer in the fifth embodiment; Fig. 11 is a cross sectional view showing the schematic structure of a semiconductor laser according to a sixth embodiment of this invention; Fig. 12 is a model diagram showing the band structure of an active layer of the laser in the sixth embodiment; Fig. 13 is a model diagram showing another example of the band structure of an active layer in the sixth embodiment; Fig. 14 is a model diagram showing still another example of the band structure of an active layer in the sixth embodiment; and Fig. 15 is a cross sectional view showing the schematic structure of a conventional multiple quantum well semiconductor laser. There will now be described an embodiment of this invention with reference to the accompanying drawings. First, the first and second embodiments are explained. Fig. 1 is a cross sectional view showing the schematic structure of a semiconductor laser according to the first embodiment of this invention. An n-type InP buffer layer (clad layer) 1b is formed on an n-type InP substrate la. A multiple quantum well active layer 4 having InGaAs strained quantum well layers 2 and InGaAsP barrier layers 3 alternately laminated is formed in a stripe form on the buffer layer 1b. A blocking layer 8 having p-type InAℓAs layers 18 and p-type InP layers 17 alternately laminated is formed on the side surface of the active layer 4. A p-type InP clad layer 5 is formed on the active layer 4 and the blocking layer 8 and a p-type InGaAsP contact layer 6 is formed on the clad layer 5. A p-side electrode 11 is formed on the contact layer 6 and an n-side electrode 12 is formed on the rear surface of the substrate 1a. The semiconductor laser is manufactured as follows. First, as shown in Fig. 2A, an n-type InP buffer layer 1b of a thickness of 2 µm is formed on an n-type InP semiconductor substrate 1a having a (001) main surface by a metal organic chemical vapor deposition (MOCVD) method, then an undoped InGaAs/InGaAsP multiple quantum well active layer 4 is formed on the buffer layer 1b and a thin undoped InP dummy layer 14 of 10 nm thickness is formed on the active layer 4. The active layer 4 is formed by alternately laminating four undoped In 0.7 Ga 0.3 As strained quantum well layers 2 of 4.2 nm thickness and five undoped InGaAsP (1.2 µm-wavelength composition) barrier layers 3 of 12 nm thickness (20 nm on both ends), which lattice-match with InP. An SiO 2 film 15 is deposited on the laminated structure and patterned by a conventional PEP technique so as to etch out both end portions of each stripe of 2 µm width serving as an active layer by 1 µm width. At this time, hydrochloric-series etchant is used to etch the thin InP dummy layer 14 and SH series (mixture of sulfuric acid, hydrogen peroxide liquid and water) is used as etchant for etching the active layer 4. A photoresist is removed after the process of etching the SiO 2 film 15. The cross section of an active layer having a recess at the center thereof as shown in Fig. 2A can be obtained by the SH series etching using the SiO 2 film 15 and InP dummy layer 14 as a mask. The width of the tapered portion on each side is 0.4 µm and the width of the active layer at the central portion is 1 µm. Next, as shown in Fig. 2B, strained blocking layers 8 are grown on both side portions 16 of the active layer 4 by the MOCVD method without removing the SiO 2 film 15. The strained blocking layer 8 is formed by alternately laminating four p-type In 0.7 Aℓ 0.3 As layers 18 of 4.2 nm thickness and five p-type InP layers 17 of 12 nm thickness (but the lowermost and upermost layers are 20 nm thick) and the total thickness thereof is substantially the same as that of the active layer (92.8 nm). After this, as shown in Fig. 2C, the SiO 2 film 15 is removed, and a p-type InP clad layer 5 of 2 µm thickness and a p-type InGaAsP contact layer 6 of 0.8 µm thickness are sequentially grown on the resultant structure by the MOCVD method. Then, a p-type ohmic electrode (Ti/Pt/Au) 11 is formed on the contact layer 6 and an n-type ohmic electrode (Au/Ge) 12 is formed on the rear surface of the substrate 1. Next, the contact layer 6 and p-type clad layer 5 are selectively etched to form a mesa structure of 10 µm width including the active layer 4. Further, the outside portion of the active layer 4b is removed by use of SH series etchant to form a semiconductor laser wafer of self aligned constricted mesa (SACM) structure having a cross section of mushroom form as shown in Fig. 1. The wafer is cleaved into a bar form of 1 mm length, then cut into chips of 300 µm width and mounted on a module (not shown). The oscillation wavelength is approx. 1.55 µm. In the above structure, the In 0.7 Aℓ 0.3 As layer 18 and the InP layer 17 have a forbidden bandwidth larger than the active layer 4. The In 0.7 Aℓ 0.3 As layer 18 has substantially the same lattice constant as the In 0.7 Ga 0.3 As strained quantum well layer 2. Therefore, the blocking layer 8 has a structure having substantially the same lattice constant as the active layer 4 and the average lattice constant (0.58813 nm) thereof takes a value between those of the InP substrate 1 and the In 0.7 Ga 0.3 As strained quantum well layer 2. It is ideal to completely align the positions of the strained quantum well layer 2 and the In 0.7 Aℓ 0.3 As layer 18, but the positional difference caused by the crystal growth may occur. However, since the active layer 4 is formed with a tapered side surface, influence by the lattice-mismatching may be distributed on the tapered portion, thus preventing generation of extremely large strain and stress even if the position difference occurs between the strained quantum well layer 2 and the In 0.7 Aℓ 0.3 As layer 18. Therefore, a strained quantum well laser of buried structure in which segregation of impurity and lattice defects such as dislocations can be prevented and whose characteristic and reliability will not be degraded can be realized. With the above structure, an excellent characteristic of the strained quantum well of the linewidth enhancement factor (α parameter) as small as 2 can be stably obtained, and as a result, a semiconductor laser having an excellent characteristic that the chirping (-20 dB average spectrum width) at the time of 10 Gb/s modulation (bias of the threshold value, modulation pulse current 40 mA pp ) is less than 0.3 nm can be realized. Further, with the semiconductor laser of this invention having the high reliability and small chirping, a several hundred km optical fiber transmission system of 10 Gb/s direct intensity modulation-direct detection system having no external optical modulator can be put into practice. In a modification of the above embodiment, the blocking layer 8 may be formed of a single layer of In 0.667 Aℓ 0.333 As instead of using the laminated structure. In this case, the average lattice constants (0.58813 nm) of the active layer 4 and the blocking layer 8 are substantially equal to each other and the value thereof lies between the lattice constants of the substrate and the In 0.7 Ga 0.3 As strained quantum well layer 2. The microscopic lattice-mismatching occurs in the side surface of the active layer 4, but since the side surface of the active layer is formed in a tapered form, the strain and stress are dispersed in the tapered portion and the segregation of impurity and lattice defects can be prevented in the same manner as in the former example. Further, even though the average lattice constants of the active layer 4 and the blocking layer 8 do not completely coincide with each other, the lattice-mismatching occurring in the side surface of the active layer can be effectively alleviated by setting the average lattice constant of the blocking layer to an intermediate value between those of the InP substrate 1 and the strained quantum well layer 2. Fig. 3 is a cross sectional view showing the schematic structure of a semiconductor laser according to a second embodiment of this invention. The construction and the manufacturing process of the semiconductor laser are substantially the same as those of the semiconductor laser of the first embodiment, but this is different from that of the first embodiment in that the strained quantum well layer 2 is formed of In 0.48 Ga 0.52 As and a layer 9 of Ag 0.65 Cu 0.35 GaSe 2 of chalcopyrite structure (tetragonal system) of 1.56 nm thickness is formed in the InP blocking layer 17 instead of the InAℓAs layer. The oscillation wavelength is 1.48 µm which is suitable for optical fiber amplifier excitation. The lattice constant of the In 0.48 Ga 0.52 As strained quantum well layer 2 is 0.58485 nm and is smaller than the lattice constant 0.58688 nm of the InP substrate unlike the former embodiment. The magnitude of the strain of the strained quantum well layer 2 obtained when it is assumed that the strained quantum well layer 2 infinitely extends is ε xx = ε yy = 0.003471, ε zz = -2(C 12 /C 11 )ε xx = -0.003422, ε yx = ε zx = ε xy = 0. In this case, it is assumed that the Poison's ratio of In 0.48 Ga 0.52 As is C 12 /C 11 = 0.493. The lattice interval of the strained quantum well layer 2 in the normal direction of the substrate after it is strained is 0.58284 nm. The lattice constant of the Ag 0.65 Cu 0.35 GaSe 2 layer 9 is a = 0.5845 nm and c = 1.092 nm, and if the thickness thereof is 1.56 nm and the strain thereof in the direction of axis a is neglected, the microscopic lattice matching with respect to the strained quantum well active layer in the strained state can be attained in the direction of the normal to the substrate. In practice, since the strain of the Ag 0.65 Cu 0.35 GaSe 2 layer 9 occurs, complete matching cannot be attained unless the composition and thickness thereof are adjusted. Further, even when the perfect lattice matching is attained for the average of the film thicknesses, microscopic lattice-mismatching may occur in the side surface of the active layer 4. However, even if the lattice matching is imperfect, influence of the lattice-mismatching is dispersed on the entire portion of the tapered portion and the segregation of impurity and lattice defects can be prevented since the side surface of the active layer 4 is formed in a tapered form like the former embodiment. Further, the forbidden bandwidth of the Ag 0.65 Cu 0.35 GaSe 2 layer 9 is approx. 1.77 eV which is larger than that of the InP substrate and this is preferable for the current constriction and light confinement. Since various other chalcopyrite materials are present, the degree of freedom of selection can be increased in view of the lattice constant and forbidden bandwidth. Therefore, a high power and highly reliable buried type strained quantum well semiconductor laser can be realized by use of the above method. This invention is not limited to the above embodiments and various modifications can be made. The barrier layer is not necessarily formed of constant composition but may be formed with the GRIN structure in which the refractive index is decreased (the forbidden bandwidth is increased) continuously or stepwise from the active layer towards the clad layer. The quantum well layer may be a double quantum well structure or double barrier structure. The material of the active layer may be of various combinations including InGaAs, InGaAsP, InGaAℓAs, InGaAsSb, GaAℓAsSb, GaAℓPSb, InAℓSb, and InGaAℓSb. The lateral light (and current) confinement structure of the optical waveguide layer may be formed with various structures such as buried hetero structure and semi-insulative InP buried structure in addition to the above SACM structure. In the case of a semiconductor laser, a distributed feedback type (DFB) laser, distributed Bragg reflector type (DBR) laser, wavelength tunable laser having a plurality of electrodes, complex resonator laser, monitor integrated laser, optical waveguide integrated laser, or bistable laser can be used. Of course, this invention can be applied to various optical semiconductor elements such as a semiconductor optical waveguide, semiconductor light detector (photodiode), semiconductor optical switch, semiconductor directional coupler, semiconductor light modulator, and photonic IC having the above units integrated therein in addition to the above semiconductor laser. In the case of a semiconductor optical waveguide, the stripe is not limited to a straight line, and this invention can be applied to various optical waveguides having side surfaces, such as a bent optical waveguide, a total reflection type optical waveguide, a Y-branch, a cross and a taper. The cross sectional taper form is not limited to the tapered form in which the central portion is formed in a concave configuration. Next, a third embodiment is explained. Fig. 4 is a cross sectional view showing the schematic structure of a semiconductor laser according to the third embodiment of this invention. Figs. 5A to 5D are cross sectional views showing the semiconductor laser. The semiconductor laser is manufactured as follows. First, as shown in Fig. 5A, an undoped InGaAs/InGaAsP multiple quantum well active layer 4 is formed over an n-type InP semiconductor substrate 1a having a (001) main surface with an n-type InP buffer layer (clad) 1b of 2 µm thickness disposed therebetween by the metal organic vapor deposition (MOCVD) method. As shown in Fig. 5B, the active layer is formed with a structure constituted by seven undoped In 0.7 Ga 0.3 As strained quantum well layers 2 of 3 nm thickness, and undoped InGaAsP layers 3 (1.2 µm-wavelength composition) which each have 10 nm thickness (but the lowermost layer 3x is 30 nm thick and the uppermost layer 3y is 100 nm thick), lattice-match with InP and are respectively disposed on both sides of a corresponding one of the well layers 2. After a diffraction grating is formed on the uppermost InGaAsP layer 3y, an SiO 2 film 15 is formed on the diffraction grating and patterned by use of a normal PEP technique to remove the both end portions of the stripe of 2 µm width serving as an active layer by the width of 1 µm as shown in Fig. 5C. At this time, if SH series etchant (mixture of sulfuric acid, hydrogen peroxide liquid and water) is used as etchant for etching the active layer 4, the cross section of an active layer having a recess at the center thereof as shown in Fig. 5C can be obtained. The width of the tapered portion on each side is 0.4 µm and the width of the active layer at the central portion is 1 µm. Next, as shown in Fig. SD, after the SiO 2 film 15 is removed, a p-type InP clad layer 5 of 2 pm thickness having an acceptor concentration of 2 × 10 18 cm -3 and a p-type InGaAsP contact layer 6 of 0.8 pm thickness are sequentially grown on the entire portion of the resultant structure by the MOCVD method. During the growing process, zinc which is a p-type dopant is diffused from the re-growth interface into the active layer 4. As is well known in the art, if impurity such as zinc is diffused into semiconductor superlattices, mutual diffusion of Group-III atoms is activated and disordering occurs. As a result, the zinc diffused region in the side surface of the active layer 4 becomes a region of intermediate composition between the compositions of the strained quantum well layer 2 and the no-strain barrier layer 3. The crystal mixing also occurs in a portion between the p-type InP clad layer 5 and the active layer 4, but since the side surface thereof is tapered, the intermediate composition region of the side surface of the active layer serves as a composition changing region 27 having a composition nearer to the composition of InP in a portion closer to the outermost end portion thereof. Since the lattice-mismatching of the side surface is distributed in the entire portion of the composition changing region 27, excessive concentration of the stress can be alleviated and generation of defects such as dislocations can be prevented. Further, diffusion of zinc from the upper portion into the active layer 4 can be prevented by the presence of the InGaAsP layer 3b. After this, a p-type ohmic electrode (Ti/Pt/Au) 11 is formed on the contact layer 6, an n-type ohmic electrode (Au/Ge) 12 is formed on the rear surface of the substrate 1, and the contact layer 6 and p-type clad layer 5 are sequentially and selectively etched to form a mesa structure of 10 µm width including the active layer 4. Further, the outside active layer is removed by use of SH series etchant to form a semiconductor laser wafer of self aligned constricted mesa (SACM) structure having a cross section of mushroom form as shown in Fig. 4. The wafer is cleaved into a bar form of 1 mm length, then cut into chips of 300 µm width and mounted on a module (not shown). The oscillation wavelength is approx. 1.55 µm. As described above, since generation of dislocations can be suppressed irrespective of the lattice-mismatching in the side surface of the active layer, a strained quantum well distributed feedback type semiconductor laser having a small threshold value, high power, high speed, low chirping, narrow spectrum line width, and high reliability can be realized. In this embodiment, diffusion of zinc is effected in the overgrowth step but can be effected in a step independent from the overgrowth step. Further, the diffused impurity is not limited to zinc but may be another impurity such as cadmium, magnesium, beryllium, carbon, or silicon. Next, a method of manufacturing a semiconductor laser according to a fourth embodiment is explained with reference to Fig. 6. The process from the initial crystal growth step to the active layer patterning step is the same as that shown in Figs. 5A to 5C. After this, as shown in Fig. 6A, a blocking layer 26 of multi-layered structure having an In 0.52 Aℓ 0.48 As layer 26a and an InP layer 26b is selectively grown on a portion in which the active layer has been removed without removing the SiO 2 film 15 used for patterning the active layer. The forbidden bandwidth of the blocking layer 26 is larger than that of the active layer 4 so that the leak of current which does not pass the active layer can be prevented. At this time, since the diffusion length of In is larger than that of Aℓ in a portion on the SiO 2 film 15, In of a larger amount than that of Aℓ is excessively supplied to a portion near the SiO 2 film 15 at the time of growth of the In 0.52 Aℓ 0.48 As layer 26a. As a result, the In 0.52 Aℓ 0.48 As layer 26a comes to have a ratio of In/(In+Aℓ) larger than 0.52 in a portion 28 near the SiO 2 film 15. Since the ratio In/(In+Aℓ) becomes larger than 0.52 and the lattice constant becomes larger in a portion nearer to the active layer 4, the average lattice constant of the blocking layer 26 becomes nearer to that of the active layer 4. That is, the composition changing region 28 acts to disperse the influence of the lattice-mismatching in the side surface of the active layer in combination with the effect obtained by the tapered side surface. As a result, concentration of the stress can be alleviated and generation of defects such as dislocations can be prevented. The average lattice constant of a portion of the blocking layer 26 near the active layer 4 can be adjusted by changing the crystal growth condition, the thickness and the number of layers of the In 0.52 Aℓ 0.48 As layer 26a. After this, a semiconductor laser as shown in Fig. 6B is completed by removing the SiO 2 film 15, forming a p-InP clad layer 5 and p-InGaAsP contact layer 6 and forming electrodes and mesa structure like the former embodiment. Also, in this embodiment, generation of defects due to the lattice-mismatching in the side surface of the active layer can be suppressed and a semiconductor laser having a high performance and high reliability can be realized. It is also possible to form the active layer by selective growth after formation of the blocking layer. The characteristic of the quantum well laser depends on the number of quantum well layers. Fig. 7 shows examples of calculations for the dependency of the differential gain of the In 0.7 Ga 0.3 As/InGaAsP-seriesstrained quantum well laser on the number of well layers. In order to attain a good characteristic, it is necessary to provide a certain number of layers. However, unless means for alleviating influence of the lattice-mismatching in the side surface of the active layer is used as in this embodiment, serious dislocations will be generated near the side surface when the number of layers is large (the total film thickness is large). According to the method of this embodiment, the total film thickness can be made larger without generation of dislocations near the side surface of the active layer. A highly reliable semiconductor laser which has a small threshold value, small line width increasing coefficient (α parameter) and large differential gain can be stably attained by use of the structure of this embodiment. The strained quantum well semiconductor laser having the excellent characteristic may be used as a light source in the very high speed optical communication or coherent optical communication, or high power light source for optical amplifier excitation, for example. This invention is not limited to the above embodiments and various modifications can be made. For example, each semiconductor layer is not necessarily formed of constant composition but may be formed with the GRIN structure in which the refractive index is decreased (the forbidden bandwidth is increased) continuously or stepwise from the active layer towards the clad layer. The lateral light (and current) confinement structure of the optical waveguide layer may be formed with various structures such as buried hetero structure and semi-insulative InP buried structure in addition to the above SACM structure. The shape of the side surface is not limited to the tapered shape having a recess in the central portion as in the above embodiment, but may be a vertical plane, and the cross section of the active layer may be a trapezoid, inverted trapezoid or any desired form. In the case of a semiconductor laser, a Fabry-Perot (FP) laser, distributed Bragg reflector type (DBR) laser, wavelength tunable laser having a plurality of electrodes, complex resonator laser, monitor integrated laser, optical waveguide integrated laser, or bistable laser can be used in addition to the distributed feedback type (DFB) laser. Of course, this invention can be applied to various optical semiconductor elements such as a semiconductor laser amplifier, semiconductor optical waveguide, semiconductor light detector (photodiode), semiconductor optical switch, semiconductor directional coupler, semiconductor light modulator and photonic IC having the above units integrated therein and electronic devices such as a pseudomorphic HEMT and hetero junction bipolar transistor in addition to the above semiconductor laser. The shape of the first region is not limited to the linear stripe, but may be a desired form such as a rectangular, polygon, circle, comb-shape, Y-branch, cross (X), or curve, for example. It will be easily understood that even if the first region is not completely surrounded by another region, if a strained semiconductor layer is also formed in the second region, or if a third region is formed, the effect of this invention can be attained by forming a composition changing region in the side surface of each region including a strained semiconductor layer. The manufacturing method is not limited to the MOCVD method. For example, another method such as the ALE (atomic layer epitaxial growth) method can be used and the order of steps for forming the respective regions can be changed. The material may be of various combinations including SiGe, SiC, ZnSSe, CdSSe, HgCdTe and chalcopyrite in addition to Group-III-V mixed crystal such as InGaAs, GaAℓAs, InGaAsP, InGaAℓAs, InGaAℓP, InGaAsSb, GaAℓAsP, GaAℓAsSb, GaAℓPSb, InAℓPSb, InGaAℓSb and AℓGaN. Next, a fifth embodiment is explained. Fig. 8 is a cross sectional view showing the schematic structure of a semiconductor laser according to a fifth embodiment of this invention. The semiconductor laser is formed as follows. An n-type InP buffer layer (clad) 1b of 2 µm thickness is formed on an n-type InP semiconductor substrate 1a having a (001) main surface by the metal organic chemical vapor deposition (MOCVD) method, then an undoped InGaAs/InGaAsP multiple quantum well active layer 4 is formed on the buffer layer 1b. As shown in Fig. 9, the active layer 4 is constructed by using seven laminated structures each formed of an undoped In 0.7 Ga 0.3 As strained quantum well layer 2 of 4.206 nm thickness and undoped In 0.48 Ga 0.52 As 0.78 P 0.22 strained barrier layers 3a of 2.005 nm thickness disposed on both sides of the strained quantum well layer 2 and disposing each of the seven laminated structures between corresponding two of undoped InGaAsP layers 3b of 1.2 µm-wavelength composition which lattice-match with InP and each have a thickness of 10 nm (but the lowermost layer has a thickness of 30 nm and the uppermost layer has a thickness of 70 nm). After a diffraction grating is formed on the uppermost InGaAsP layer 3b, an SiO 2 is formed thereon and then a patterning process is effected by use of a conventional PEP technique to etch out portions of 1 µm width on both sides of the stripe of 2 µm width serving as an active layer. At this time, if SH series etchant (mixture of sulfuric acid, hydrogen peroxide liquid and water) is used as etchant for etching the active layer 4, the tapered cross section of an active layer having a recess at the center thereof as shown in Fig. 8 can be obtained. The width of the tapered portion on each side is 0.4 µm and the width of the active layer at the central portion is 1 µm. Next, after the SiO 2 film is removed, a p-type InP clad layer 5 of 2 µm thickness and a p-type InGaAsP contact layer 6 of 0.8 µm thickness are sequentially grown on the resultant structure by the MOCVD method. Then, a p-type ohmic electrode (Ti/Pt/Au) 11 is formed on the contact layer 6 and an n-type ohmic electrode (Au/Ge) 12 is formed on the rear surface of the substrate 1. Next, the contact layer 6 and p-type clad layer 5 are selectively etched to form a mesa structure of 10 µm width including the active layer 4. Further, the outside active layer 4b is removed by use of SH series etchant to form a semiconductor laser wafer of self aligned constricted mesa (SACM) structure having a cross section of mushroom form as shown in Fig. 8. The wafer is cleaved into a bar form of 1 mm length, then cut into chips of 300 µm width and mounted on a module (not shown). The oscillation wavelength is approx. 1.55 µm. The lattice constant of the InP substrate 1 is 0.58688 nm, and the lattice constants of the In 0.7 Ga 0.3 As strained quantum well layer 2 and the In 0.48 Ga 0.52 As 0.78 P 0.22 strained barrier layer 3a are respectively 0.59381 nm and 0.57986 nm. Assuming now that there is an infinite plane having no side surface, the strained quantum well layer 2 and the strained barrier layer 3 are as thin as 4.206 nm and 2.005 nm and elastically strained. The degree of strain of the strained quantum well layer 2 is ε xx = ε yy = -0.01167, ε zz = -2(C 12 /C 11 )xε xx = 0.011974, ε yz = ε zx = ε xy = 0. C 12 /C 11 = 0.504 is the Poison's ratio of In 0.7 Ga 0.3 As. The degree of strain of the strained barrier layer 3a is ε xx = ε yy = 0.012106, ε zz = -2(C 12 /C 11 )xε xx = -0.012106, and ε yz = ε zx = ε xy = 0. Since, the Poison's ratio of In 0.48 Ga 0.52 As 0.78 P 0.22 is not exactly derived, it is assumed that C 12 /C 11 = 0.5. As a result, the lattice constant of the strained In 0.7 Ga 0.3 As quantum well layer 2 becomes equal to that of InP in the xy plane and the lattice constant in the z direction is 0.60092 nm. The thickness of 4.206 nm corresponds to seven unit lattice layers. The lattice constant of the strained In 0.48 Ga 0.52 As 0.78 P 0.22 barrier layer 3a becomes equal to that of InP in the xy plane and the lattice constant in the z direction is 0.57284 nm. The thickness of 2.005 nm corresponds to 3.5 unit lattice layers. The degrees of deviation of the lattice plane in the <001> direction and (Z direction) of the side surface of the active layer 4 are shown in Fig. 10. The maximum deviation of the lattice plane is 0.04914 nm and is sufficiently smaller than 0.14321 nm which is 1/4 of δ min = 0.57284 nm and satisfies the expression (2). Therefore, generation of dislocations caused by deviation in the (001) same group atomic plane accompanied by lateral deviation can be prevented. Of course, since it is difficult to completely control the film thickness and composition by use of the normal MOCVD method which is different from the ALE (atomic layer epitaxial growth), it is considered that a slight variation occurs in the thickness of each strained semiconductor layer. Further, there is a possibility that the values of the above lattice constant and C 12 /C 11 are not precise. However, in either case, if the deviation of the lattice plane in the Z direction of the seven-layered layer can be controlled so as not to exceed 0.28642 nm, that is, so as to satisfy the expression (1), generation of dislocations corresponding to deviation of the unit lattice plane can be prevented. The microscopically small lattice-mismatching occurs in the surface of the active layer 4, but since the side surface of the active layer is formed in a tapered form, the strain and stress are dispersed in the tapered portion and alleviated, and each semiconductor layer can be elastically strained also in the boundary portion of the side surface. Therefore, a buried type strained quantum well laser in which the segregation of impurity and lattice defects such as dislocations in the side surfaces of the strained semiconductor layers 2 and 3a can be prevented and the characteristic and reliability can be kept high can be provided. The characteristic of the quantum well laser depends on the number of quantum well layers. Fig. 7 shows examples of calculations for the dependency of the differential gain of the In 0.7 Ga 0.3 As/InGaAsP-series strained quantum well laser on the number of well layers. In order to attain a desired characteristic, it is necessary to provide a certain number of layers. However, unless means for alleviating influence of the lattice-mismatching in the side surface of the active layer is used as in this embodiment, serious disloca tions will be generated near the side surface when the number of layers is large (the total film thickness is large). For example, if the In 0.48 Ga 0.52 As 0.78 P 0.22 barrier layer 3a is omitted in the example of Fig. 8, the accumulated degree of mismatching of the side surface obtained by calculation becomes 0.68796 nm and exceeds the magnitude of the unit lattice layer so that dislocations will occur in the side surface. According to this invention, the total film thickness can be increased without generation of dislocations in the side surface of the active layer. With the structure of this embodiment, the threshold value can be kept small and an excellent characteristic of the strained quantum well of the linewidth enhancement factor (α parameter) as small as 2 can be stably obtained, and as a result, a semiconductor laser having an excellent characteristic that the chirping (-20 dB average spectrum width) at the time of 10 Gb/s modulation (bias of the threshold value, modulation current 40 mA pp ) is less than 0.3 nm can be realized. Further, with the semiconductor laser of this invention having the high reliability and small chirping, a several hundred km optical fiber transmission system of 10 Gb/s direct intensity modulation-direct detection system having no external optical modulator can be put into practice. In a modification of the above embodiment, the barrier layer except the uppermost and lowermost layers may be formed of a single strained InGaAsP layer instead of the multi-layered structure of the strained layers 3x and the non-strained layers 3y. Also, in this case, the same effect can be obtained by setting the composition of the strained InGaAsP barrier layer and the thickness thereof in the lattice matching state with respect to the substrate on the xy plane so as to satisfy the expressions (1) and (2). This invention is not limited to the above embodiment, and can be variously modified as explained in the fourth modification of this invention. It will be easily understood that even if the first region is not completely surrounded by another region, if a strained semiconductor layer is also formed in the second region, or if a third region is formed, the effect of this invention can be attained by setting the amount of deviation, thickness and position of each layer such that the deviation of the lattice plane in the side surface of each region including the strained semiconductor layer may satisfy the expression (1). Further, this invention can be used to alleviate the influence by the lattice-mismatching of the side surface of quantum wires and quantum boxes containing strains. Next, the sixth embodiment is explained. Fig. 11 is a cross sectional view showing the schematic structure of a semiconductor laser according to the sixth embodiment of this invention. The semiconductor laser is manufactured as follows. An n-type InP buffer layer 1b of 2 µm thickness is formed on an n-type InP semiconductor substrate la having a (001) main surface by the metal organic chemical vapour deposition (MOCVD) method, then an undoped InGaAsP optical waveguide layer 3a, undoped InAs/InGaAsP multiple quantum well active layer 4 and undoped InGaAsP optical waveguide layer 3b are sequentially laminated on the buffer layer 1b. As shown in Fig. 12, the active layer 4 is constituted by twelve undoped InAs strained quantum well layers 2 of 2 nm thickness and undoped InGaAsP barrier layers 3 of 5 nm thickness each disposed between corresponding two of the well layers. After a diffraction grating is formed on the InGaAsP optical waveguide layer 3b, an SiO 2 film is deposited on the resultant structure and patterned by use of the conventional PEP technique to etch out end both portions of the stripe of 2 µm width serving as the active layer by 1 µm width. After the SiO 2 film is removed, a p-type InP clad layer 5 of 2 µm thickness is formed on the entire portion of the resultant structure and then a p-type InGaAsP contact layer 6 of 0.8 µm is formed on the clad layer 5. A p-type ohmic electrode (Ti/Pt/Au) 11 is formed on the contact layer 6 and an n-type ohmic electrode (Au/Ge) 12 is formed on the rear surface of the substrate 1. The contact layer 6 and the p-type clad layer 5 are selectively etched into a mesa shape of 10 µm width including the active layer 4. Further, the outside portion of the active layer is removed by use of SH series etchant to form a semiconductor laser wafer of self-alignment constricted mesa (SACM) structure having a cross section of mushroom form as shown in Fig.11. The wafer is cleaved into a bar form of 1 mm length, then cut into chips of 300 µm width and mounted on a module (not shown). Assume that the composition of the InGaAsP barrier layer 3 is so selected that the forbidden band end thereof may coincide with the band edge E LH for light holes in the strained InAs layer 2. At present, the band alignment of the In 1-u Ga u As v P 1-v (0 ≦ u ≦ 1, 0 ≦ v ≦ 1) on the InP substrate is not completely known. However, since it is known in the art that the strained InAs layer with InP disposed on both sides thereof forms a shallower quantum well for light holes than that which the In 0.53 Ga 0.47 As layer with InP disposed on both sides thereof forms, it is sure that non-strained In 1-x Ga x As y P 1-y composition (0 < x < 0.47, 0 < y < 1) which satisfies the above condition is present. When the above condition is set, no barrier is created for light holes so that light holes can freely move in the respective well layers 2 and barrier layers 3. Further, light holes are partly coupled with heavy holes in the internal portion of the well layer because the overlapping integral of the wave function for the light holes and heavy holes is not 0. Therefore, holes can be efficiently injected into a well which is far apart from the p-clad layer and the wells can be effectively coupled. As a result, a strained multiple quantum well laser having a small threshold value and high-speed response can be realized. It is possible to confirm that no barrier is created for light holes by measurement of photoluminescence, for example. That is, light emission by recombination of heavy holes and electrons is permitted only for polarized components parallel to the well layer, but light emission by recombination of light holes and electrons is permitted for polarized components parallel to and perpendicular to the well layer. Therefore, the light emissions can be discriminated from each other and whether the valence band ends E LH for light holes are substantially aligned or not can be determined by checking whether or not light emission by light holes is caused by the quantized level. This invention is not limited to the above embodiment and can be variously modified. For example, the InGaAsP barrier layer 3 can be formed of material having a lattice constant smaller than that of InP. The conceptional diagram of the band structure obtained at this time is shown in Fig. 13. In this case, since influence of the lattice-mismatching in the side surface can be cancelled by the positive and negative strains in the strained multiple quantum well laser having the active layer buried therein, generation of the lattice defects can be suppressed when the number of laminated layers of the strained well layer is increased. When a semiconductor layer having a lattice constant smaller than that of the substrate is used as a well layer, the hole injection efficiency can be enhanced and the coupling efficiency between the wells can be improved also with respect to the quantum well for light holes by setting the valence band edges E HH for heavy holes in the strained layer and the barrier layer to substantially the constant level. The band structure obtained at this time is shown in Fig. 14. As the lateral light (and current) confinement structure of the optical waveguide, various structures other than the above SACM structure, for example, buried hetero structure and semi-insulative InP buried structure can be considered. A Fabry-Perot (FP) laser, distributed Bragg reflector type (DBR) laser, wavelength tunable laser having a plurality of electrodes, complex resonator laser, monitor integrated laser, optical waveguide integrated laser, or bistable laser can be used in addition to the distributed feedback type (DFB) laser. Of course, this invention can be applied to various optical semiconductor elements such as a semiconductor laser amplifier, carrier injection type semiconductor optical switch, semiconductor light modulator and photonic IC having the above units integrated therein. Of course, material is not limited to InAs or InGaAsP.

Claims: An optical semiconductor device comprising: a stripe-form optical waveguide layer (4) formed on a main surface of a semiconductor substrate (1a) and having a strained quantum well constituted by first semiconductor layers (2) which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers (3) which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers (8) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on both lateral sides of said optical waveguide layer (4); and cladding layers (1b, 5) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on upper and lower sides of said optical waveguide layer (4); wherein the side surfaces of the optical waveguide layer (4) are tapered with respect to the normal of said substrate at an angle of not less that 45 degrees. A device according to claim 1, characterised in that the average lattice constant of each blocking layer (8) which is in contact with said side surfaces of the optical waveguide layer (4) has a value between the lattice constants of said substrate (1a) and said first semiconductor layer (2). A device according to claim 1, characterised in that the expression that absolute (Z2(n) - Z1(n)) < δ min (n)/2 is satisfied when said first and second semiconductor layers (2,3) are alternately laminated, the lower main surface of the lowermost one of said first semiconductor layers of said optical waveguide layer (4) is used as a reference plane, the coordinate system Z is set in the upward direction along the normal of a low index plane which is the nearest to said reference plane, the coordinate of an n-th unit lattice plane of said optical waveguide layer from said reference plane is Z1(n), the coordinate of an n-th unit lattice plane of said blocking layers from said reference plane is Z2(n), and the smallest one of four values of "Z2(n+1)-Z2(n)", "Z2(n)-Z2(n-1)", "Z1(n+1)-Z1(n)" and Z1(n)-Z1(n-1)" is δ min (n). A device according to claim 3, characterised by further comprising a compensation semiconductor layer (3a) having a forbidden bandwidth larger than that of said first semiconductor layer and in which the sign of a difference between the lattice constant of said compensation semiconductor layer and the average lattice constant of each blocking layer (8) is opposite to the sign of a difference between the lattice constant of said first semiconductor layer (2) and said average lattice constant, and influence of the lattice-mismatching between said first semiconductor layer (2) and each blocking layer (8) is removed by said compensation semiconductor layer (3a). A device according to claim 1, characterised in that said first and second semiconductor layers (2,3) respectively serve as quantum well layers and barrier layers and are alternately laminated, and the forbidden band edge of a light hole band in said first semiconductor layer substantially coincides with that of a light hole band in said second semiconductor layer. A device according to claim 1, characterised in that said first and second semiconductor layers (2,3) respectively serve as quantum well layers and barrier layers and are alternately laminated, and the forbidden band edge of a heavy hole band in said first semiconductor layer substantially coincides with that of a heavy hole band in said second semiconductor layer. A device according to claim 3, characterised in that each blocking layer (8) is formed of third and fourth semiconductor layers (18,17) each having a forbidden bandwidth larger than that of said first semiconductor layer (2) and said third semiconductor layer (18) has substantially the same lattice constant as said first semiconductor layer (2) and is formed in substantially the same plane as said first semiconductor layer. A device according to claim 2, characterised by further comprising a composition changing region (27,28) which is formed between the blocking layers (8) and the optical waveguide layer (4) and whose lattice constant is changed continuously or stepwise between that of each blocking layer (8) and that of said optical waveguide layer (4). A device according to claim 1, wherein the cladding layer (5) formed on the upper side of the optical waveguide layer forms the blocking layers. An optical semiconductor device comprising: a stripe-form optical waveguide layer (4) formed on a main surface of a semiconductor substrate (1a) and having a strained quantum well constituted by first semiconductor layers (2) which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers (3) which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers (8) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on both lateral sides of said optical waveguide layer (4); cladding layers (1b, 5) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on upper and lower sides of said optical waveguide layer (4); wherein the average lattice constant of each blocking layer (8) has a value between the lattice constants of said substrate (1a) and said first semiconductor layer (2). A device according to claim 10, characterised in that each blocking layer (8) includes a semiconductor layer of tetragonal structure having a c axis set in the direction of the normal to said substrate and the lattice constant of the a axis of said semiconductor layer of the tetragonal structure is smaller than that of said substrate. A device according to claim 10, characterised in that the expression that absolute (Z2(n) - Z1(n)) < δ min (n)/2 is satisfied when said first and second semiconductor layers (2,3) are alternately laminated, the lower main surface of the lowermost one of said first semiconductor layers of said optical waveguide layer (4) is used as a reference plane, the coordinate system Z is set in the upward direction along the normal of a low index plane which is the nearest to said reference plane, the coordinate of an n-th unit lattice plane of said optical waveguide layer from said reference plane is Z1(n), the coordinate of an n-th unit lattice plane of said blocking layers from said reference plane is Z2(n), and the smallest one of four values of "Z2(n+1)-Z2(n)", "Z2(n)-Z2(n-1)", "Z1(n+1)-Z1(n)" and "Z1(n)-Z1(n-1)" is δ min (n). A device according to claim 12, characterised by further comprising a compensation semiconductor layer (3a) having a forbidden bandwidth larger than that of said first semiconductor layer and in which the sign of a difference between the lattice constant of said compensation semiconductor layer and the average lattice constant of each blocking layer (8) is opposite to the sign of a difference between the lattice constant of said first semiconductor layer (2) and said average lattice constant, and influence of the lattice-mismatching between said first semiconductor layer (2) and each blocking layer (8) is removed by said compensation semiconductor layer (3a). A device according to claim 10, characterised in that said first and second semiconductor layers (2,3) respectively service as quantum well layers and barrier layers and are alternately laminated, and the forbidden band edge of a light hole band in said first semiconductor layer substantially coincides with that of a light hole band in said second semiconductor layer. A device according to claim 10, characterised in that said first and second semiconductor layers (2,3) respectively serve as quantum well layers and barrier layers and are alternately laminated, and the forbidden band edge of a heavy hole band in said first semiconductor layer substantially coincides with that of a heavy hole band in said second semiconductor layer. A device according to claim 12, characterised in that each blocking layer (8) is formed of third and fourth semiconductor layers (18,17) each having a forbidden bandwidth larger than that of said first semiconductor layer (2) and said third semiconductor layer (18) has substantially the same lattice constant as said first semiconductor layer (2) and is formed in substantially the same plane as said first semiconductor layer. A device according to claim 10, characterised by further comprising a composition changing region (27,28) which is formed between the blocking layers and the optical waveguide layer and whose lattice constant is changed continuously or stepwise between that of each blocking layer (8) and that of said optical waveguide layer (4). An optical semiconductor device comprising: . a stripe-form optical waveguide layer (4) formed on a main surface of a semiconductor substrate (la) and having a strained quantum well constituted by first semiconductor layers (2) which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers (3) which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers (5, 26) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on both lateral sides of said optical waveguide layer (4); cladding layers (1b, 5) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on upper and lower sides of said optical waveguide layer (4); wherein a composition changing region (27,28) whose lattice constant is changed continuously or stepwise between that of said optical waveguide layer (4) and that of each blocking layer (5, 26) is formed between the blocking layers and the optical waveguide layer. A device according to claim 18, wherein the cladding layer (5) formed on the upper side of the optical waveguide layer forms the blocking layers. An optical semiconductor device comprising: a stripe-form optical waveguide layer (4) formed on a main surface of a semiconductor substrate (1a) and having a strained quantum well constituted by first semiconductor layers (2) which each have a thickness of less than the de Broglie wavelength of electrons and which do not lattice-match with said substrate, and second semiconductor layers (3) which each have a forbidden bandwidth larger than that of said first semiconductor layers and which are respectively disposed on upper and lower sides of a corresponding one of said first semiconductor layers; blocking layers (8) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on both lateral sides of said optical waveguide layer (4); cladding layers (1b, 5) formed of semiconductor having a forbidden bandwidth larger than that of said second semiconductor layer (3) and formed on upper and lower sides of said optical waveguide layer (4); wherein the expression that absolute (Z2(n) - Z1(n)) < δ min (n)/2 is satisfied when said first and second semiconductor layers (2,3) are alternately laminated, the lower main surface of the lowermost one of said first semiconductor layers of said optical waveguide layer (4) is used as a reference plane, the coordinate system Z is set in the upward direction along the normal of a low index plane which is the nearest to said reference plane, the coordinate of an n-th unit lattice plane of said optical waveguide layer from said reference plane is Z1(n), the coordinate of an n-th unit lattice plane of said blocking layers from said reference plane is Z2(n), and the smallest one of four values of "Z2(n+1)-Z2(n)", "Z2(n)-Z2(n-1)", "Z1(n+1)-Z1(n)" and "Z1(n)-Z1(n-1)" is δ min (n). A device according to claim 20, characterised by further comprising a compensation semiconductor layer (3a) having a forbidden bandwidth larger than that of said first semiconductor layer and in which the sign of a difference between the lattice constant of said compensation semiconductor layer and the average lattice constant of each blocking layer (8) is opposite to the sign of a difference between the lattice constant of said first semiconductor layer (2) and said average lattice constant, and influence of the lattice-mismatching between said first semiconductor layer (2) and each blocking layer (8) is removed by said compensation semiconductor layer (3a). A device according to claim 20, characterised in that each blocking layer (8) is formed of third and fourth semiconductor layers (18,17) each having a forbidden bandwidth larger than that of said first semiconductor layer (2) and said third semiconductor layer (18) has substantially the same lattice constant as said first semiconductor layer (2) and is formed in substantially the same plane as said first semiconductor layer.

Inventors: HIRAYAMA YUZO, ONOMURA MASAAKI, SUZUKI NOBUO, HIRAYAMA, YUZO, ONOMURA, MASAAKI, SUZUKI, NOBUO, Hirayama, Yuzo, c/o Intell. Property Div., Onomura, Masaaki, c/o Intell. Property Div., Suzuki, Nobuo, c/o Intell. Property Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01S3
IPC codes: H01S5