Epitaxial Layer Structure Grown on Graded Substrate and Method of Growing the Same

The present invention relates to an epitaxial layer structure grown on a graded substrate and a method of epitaxial growth on the graded substrate.

At present, mixed crystals of various semiconductor material such as GaAlAs, InGaAsP, InGaAlAsP, and ZnCdSSe are used as epitaxial layer for a semiconductor device, which is grown on a substrate of a similar mixed crystal. In the case of InGaAsP quarternary crystal, for example, it is possible to achieve a wide range of energy bandgaps from 0.36 eV to 2.25 eV by varying the composition of the mixed crystal from InAs to GaP. For this reason, the InGaAsP epitaxial layer is available for various optoelectric semiconductor devices such as laser diodes, light-emitting diodes, and photo detectors wherein the InGaAsP layer can have a desired wavelength ranging from the infrared region to the visible region.

Generally speaking, if a crystal having a lattice constant different from that of the substrate is grown on the substrate, a lot of misfit dislocations are introduced from its interface into a grown layer, which undermines the crystal quality of the grown layer. Consequently, the surface condition of the grown crystal is also adversely affected, making it impossible to obtain usable crystals.

Since the epitaxial layer should be lattice-matched to the substrate, possible bandgap range of the epitaxial layer depends on the composition of the substrate. For instance, when the InGaAsP layer is grown on an InP substrate with a lattice match condition, the bandgap range is restricted to 0.75 eV to 1.28 eV. When the GaAs substrate is used, the bandgap is restricted to the range between 1.48 eV and 1.88 eV. To expand the possible bandgap range, ternary or quarternary substrate is required instead of conventional binary substrate. However, it is very difficult to obtain a bulk single crystal substrate for the ternary or quarternary system.

To this end, so-called graded substrate is commercially available. The graded substrate comprises a binary bulk single crystal substrate, a graded composition layer formed on the binary substrate, and a constant composition layer formed on the graded composition layer. A typical example of such a commercially available GaAs.sub.0.61 P.sub.0.39 wafers consists of a 30 .mu.m-thick graded composition GaAsP layer on a GaAs substrate and a 20 .mu.m-thick constant composition GaAs.sub.0.61 P.sub.0.39 layer on top of the wafer. Using this wafer as the graded substrate for growing a desired ternary or quarternary epitaxial layer such as InGaP or InGaAsP thereon, however, the grown surface has a so called cross-hatched pattern and a number of small surface defects appear when InGaP and InGaAsP layers are grown under the lattice matched condition on the GaAsP graded substrate. To achieve a good semiconductor device, therefore, it is necessary to eliminate the above mentioned deffects.

Accordingly, an object of the present invention is to provide a crystal structure having a graded substrate and an epitaxial layer grown on the graded substrate and a method making the same with a good surface condition and crystal quality by eliminating the aforementioned conventional drawbacks.

The present invention is featured in that an intentional lattice mismatch is introduced between the epitaxial layer crystal and the graded substrate surface by the value of .DELTA.a/a=1.times.10.sup.-3 to 5.times.10.sup.-3 (.DELTA.a: a difference in the lattice constant between the top layer of the graded substrate and the grown layer thereon, a: the lattice constant of the top layer of the graded substrate) so as to alleviate a strain resulting from a change in the lattice constant within the graded substrate. When the lattice constant in the graded layer is increased from a lower layer (bulk substrate) toward the top layer (the constant layer), the lattice constant of the epitaxial layer should be selected to be smaller than that of the constant layer. On the other hand, when the lattice constant of the graded layer is decreased from bulk substrate toward the constant layer, the lattice constant of the grown layer should be selected to be larger than that of the constant layer.

FIG. 1 shows a schematic cross-sectional view of a double heterojunction structure for a planer stripe semiconductor laser.

Double heterojunction (DH) structures for semiconductor laser diodes are prepared for the following respective cases: a conventional case where lattice match is carried out on a graded substrate in which a mixed crystal of GaAs.sub.1-y P.sub.y (y=0.4) is subjected to graded epitaxial growth on a GaAs bulk crystal and a case of the present invention where lattice mismatch is intensionally introduced so as to alleviate a strain. Then, a comparison was made in surface conditions and lasing characteristics which are substantially affected by crystal quality.

FIG. 1 shows the structure of an epitaxial layer grown by the hydride vapor-phase-epitaxy (VPE) method. A GaAsP graded substrate 10 consists of a bulk single crystal 1, a graded composition layer 2 grown thereon, and a constant composition layer 3 thereon. As the bulk crystal 1, approximately 300 .mu.m-thick GaAs bulk single crystal was used. The graded layer 2 is a grown layer in which 30 .mu.m-thick composition is changed by small degrees from GaAs.sub.1-y Py (y=0) to GaAs.sub.1-y P.sub.y (y=0.40) in thickness direction. The constant layer 3 is an about 30 .mu.m-thick GaAs.sub.1-y P.sub.y (y=0.40) layer, its composition being constant. Then, the following were consecutively grown on the constant layer 3: a 2 .mu.m-thick n-InGaP layer as an n-type clad layer 4, a 2000 .ANG.-thick InGaAsP layer as an active layer 5, a 2 .mu.m thick P-InGaP layer as a P-type clad layer 6, and a 1 .mu.m-thick InGaAsP layer as a cap layer 7, which constitute a DH structure for a semiconductor laser.

The n-type InGaP clad layer 4 was Se-doped by approximately 7.times.10.sup.17 cm.sup.-3 as an n-type dopant, while the p-type InGaP clad layer 6 was Zn-doped by approximately 2.times.10.sup.18 cm.sup.-3 as a p-type dopant, respectively. The InGaAsP active and cap layers were undoped.

A comparison was made between the case where the above-described DH structure for laser diodes (LD) were grown on the constant layer 3 of GaAs.sub.1-y P.sub.y (y=0.4) with lattice match and the case where growth was effected by introducing a lattice mismatch .DELTA.a/a of about +1.times.10.sup.-3 to +5.times.10.sup.-3 so as to alleviate the strain which is inherently present in the graded substrate.

In the case where lattice match was effected, the composition of each grown layer was In.sub.1-x Ga.sub.x P (x=0.71) for both the n-clad layer 4 and the p-clad layer 6, and In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y (x=0.97, y=0.45) for both the cap layer 7 and the active layer J. The lattice constants of these mixed crystals are matched to the lattice constant (a=5.5 .ANG.) of the constant layer 3 of the GaAs.sub.1-y P.sub.y (y=0.40) graded substrate by about 66 a/a<5.times.10.sup.-4.

Meanwhile, in the case of the present invention in which intentional lattice mismatch was introduced in the direction of alleviating a strain which is internally present in the graded epitaxial substrate, the composition was In.sub.1-x Ga.sub.x P (x=0.67.+-.0.02) for both the n-clad layer 4 and the p-clad layer 6, and In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y (x=0.93.+-.0.02, y=0.45) for both the cap layer 7 and the active layer 5. The lattice mismatch (.DELTA.a/a) of these epitaxial layers with respect to the lattice constant (a=5.57 .ANG.) of the constant layer 3 of the GaAs.sub.1-y P.sub.y (y=0.40) graded substrate was about +1.times.10.sup.-3 to 5.times.10.sup.-3 at room temperature, and the lattice mismatch between each adjacent layer of both clad layers, active layer, and cap layer was within .+-.5.times.10.sup.-4.

As a result of preparing DH structure for various LDs, observation by the Normalsky differential interference microscope revealed that many fine irregularities occurred on the surface of the lattice-matched epitaxial layers, and a deteriorated specular surface was detected. However, as for the surface condition of the epitaxial layers of the present invention to which a lattice mismatch of +1.times.10.sup.-3 to +5.times.10.sup.-3 at room temperature was introduced to alleviate a strain, the results of similar observation were favorable and showed a good specular surface. If .DELTA.a/a exceeded +5.times.10.sup.-3, the inherent adverse effect of lattice mismatch emerges instead of the effect of alleviating a strain. The lower limit of .DELTA.a/a at which the effect of the present invention can be detected is +1.times.10.sup.-3, and a deteriorated specular surface can be noted at a level lower than that. Furthermore, using wafers of these DH structures for LDs, Zn-diffused planner stripe-type LDs with a stripe width of 18 .mu.m were fabricated, and a comparison was made of their oscillating threshold current density by pulse operation at room temperature. The reflective Zn-diffusion was performed through the window of SiO.sub.2 layer 8 so as to reach the P-type clad layer 7. Ohmic contacts to the p-side and the n-side were made by Au-Zn layer 11 and Au-Ge-Ni layer 12, respectively. The LDs prepared from wafers of lattice matched DH structures for LDs were pulsed at a threshold current density of Jth=25 kA/cm.sup.2 to 35 kA/cm.sup.2 at room temperature, but no continuous wave (CW) operation was obtained and yield was low (less than 50%). However, in the case of LDs of the present invention prepared from wafers of DH structures to which lattice mismatch was introduced by about .DELTA.a/a=3.7.times.10.sup.-3 at room temperature, the CW operation at room temperature was obtained at a threshold current density of Jth=3 kA/cm.sup.2.

As described above, it has been found that the aforementioned problems can be resolved if growth is effected by intentionally carrying out a lattice mismatch of approximately .DELTA.a/a: +1.times.10.sup.-3 to +5.times.10.sup.-3 with the constant composition layer. Preferably, .DELTA.a/a is selected to be in the range of 3.times.10.sup.-3 to 4.times.10.sup.-3.

The aforementioned value of lattice-mismatch is very large as compared with the critical value of allowable lattice mismatch (generally .DELTA.a/a: I5.times.10.sup.-4 or thereabout) at which it is conventionally considered that an epitaxial layer without any misfit dislocation can be obtained in cases where a layer of InGaP or InGaAsP is epitaxially grown on a GaAs bulk crystal substrate. The fact that the epitaxial layer with such a great lattice mismatch yields a better specular surface than a lattice-matched epitaxial layer is a remarkable phenomenon.

The reunion for this phenomenon can be surmised as follows: It can be considered that a portion of a stress caused as a result of a difference in the lattice constant between a GaAs substrate and a GaAsP epitaxial growth layer is aleviated by misfit displacements or the like, while some other portion is stored in the GaAsP graded layer and GaAsP constant layer as a strain. It is considered that when epitaxial growth is effected on a substrate where such a strain is inherently present, a better epitaxial layer can be obtained if a lattice mismatch to the extent of alleviating the strain is incorporated into the epitaxial growth.

With respect to the growth on a GaAsP graded substrate subjected to graded epitaxial growth using a GaP bulk single crystal in such a manner that the lattice constant will become large, a favorable specular surface can be obtained for the aforementioned reason if an epitaxial layer having a small lattice constant of .DELTA.a/a being -1.times.10.sup.-3 to -5.times.10.sup.-3 is grown.

More specifically, DH structures for LDs were prepared for the following respective cases: a case where lattice match is carried out on a substrate in which an InAs.sub.1-y P.sub.y (y=0.70) mixed crystal is subjected to graded epitaxial growth on an InP bulk single crystal and case of the present invention where lattice mismatch is introduced intentionally so as to alleviate a strain. Then, a comparison was made in surface conditions and laser characteristics. The hybrid VPE method was used to achieve growth, and its structure is identical as the one shown in FIG. 1.

More specifically, as for the InAsP graded substrate, an approximately 300 .mu.m-thick InP bulk single crystal was used as the crystal 1. The graded layer 2 is a grown layer in which a 30 .mu.m-thick mixed crystal composition layer is charged by small degrees from InAs.sub.1-y P.sub.y (y=1.0) to InAs.sub.1-y P.sub.y (y=0.70). The constant layer 3 is an about 30 .mu.m-thick InAs.sub.1-y P.sub.y (y=0.70) layer, its mixed crystal composition being constant. Then, the following were consecutively grown on the constant layer 3: a 2 .mu.m-thick n-InAsP layer as the n-type clad layer 4, a 2000 .ANG.-thick InGaAsP layer as the active layer 5, a 2 .mu.m-thick P-InAsP layer as the p-type clad layer 6, and a 1 .mu.m-thick InGaAsP layer as the cap layer 7, which constitute a DH structure for a LD. The n-type InAsP clad layer 4 was Se-doped by approximately 1.times.10.sup.18 cm.sup.-3 as an n-type dopant, while the p-type InAsP clad layer 6 was Zn-doped by approximately 2.times.10.sup.18 cm.sup.-3 as a p-type dopant, respectively. The InGaAsP active and cap layers were undoped.

A comparison was made between the case where the above-described DH structures for LDs were grown on the constant layer 3 of InAs.sub.1-y P.sub.y (y=0.70) with lattice match and the case of the present invention where growth was effected by introducing a lattice mismatch .DELTA.a/a of about -1.times.10.sup.-3 to -5.times.10.sup.-3 to alleviate the strain.

In the case where lattice match was effected, the composition of each grown layer was InAs.sub.1-y P.sub.y (y=0.70) for both the n-clad layer 4 and the p-clad layer 6, and In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y (x=0.29, y=0.10) for both the cap layer 7 and the active layer 8. The lattice constants of these mixed crystals are matched to the lattice constant (a=5.925 .ANG.) of the InAs.sub.1-y P.sub.y (y=0.70) graded substrate by about .DELTA.a/a.ltoreq.5.times.10.sup.-4.

In the case of the present invention in which lattice mismatch was introduced so as to alleviate a strain, the composition was InAs.sub.1-y P.sub.y (y=0.20.+-.0.06) for both the n-clad layer .phi. and to p-clad layer 6, and In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y (x=0.33.+-.0.03, y=0.10) for both the cap layer 7 and the active layer 5. The lattice mismatch (.DELTA.a/a) of these epitaxial layers with respect to the lattice constant (a=5.925 .ANG.) of the InAs.sub.1-y P.sub.y (y=0.70) graded substrate was about -1.times.10.sup.-3 to -5.times.10.sup.-3, and the lattice mismatch between adjacent layers of both clad layers, active layer and cap layer was within .+-.5.times.10.sup.-4. As a result of preparing DH structures for various LDs, observation with a Normalsky differential interference microscope revealed a determination in the surface morphology in terms of the surface condition of the lattice-matched epitaxial layers. However, the same observation was made of the surface condition of the epitaxial layers of the present invention to which lattice mismatch was introduced to alleviate a strain, and the surface morphology was favorable. Furthermore, using wafers of these DH structures for LDs, Zn-diffused planar stripe-type LDs with a stripe width of 7 .mu.m were fabricated, and a comparison was made of their oscillating threshold current density. The LDs prepared from wafers of lattice-matched DH structures for LDs continuously operated at a threshold current density of Jth=6 to 8 kA/cm.sup.2 at room temperature.

However, the average oscillating threshold current density was Jth=5 kA/cm.sup.2 during room-temperature CW operation of LDs of the present invention, prepared from wafers of DH structures to which lattice mismatch was introduced is as to alleviate a strain. The minimum value was Jth=2 kA/cm.sup.2 in the case of those prepared from wafers of DH structures for LDs to which a lattice mismatch of about .DELTA.a/a=-3.1.times.10.sup.-3 was introduced.

As described above, if the epitaxial technique of the present invention is used, it becomes possible to form epitaxial layers with good surface conditions and crystal qualities on graded substrates as compared with the conventional method. Needless to say, the present invention, can be applied to any other known semiconductor device which are grown on a graded substrate.