EPITAXIAL LAYER STRUCTURE GROWN ON GRADED SUBSTRATE AND METHOD OF GROWING THE SAME

Abstract: A crystal structure consisting of a single crystal bulk substrate, a graded substrate and an epitaxial layer grown on the graded substrate, using a lattice-constant mismatch to alleviate the strains caused by the different layers and a method which allows for alleviating strains resulting from the lattice constant variation in the graded substrate.

Description: BACKGROUND OF THE INVENTION 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 quaternary crystal, for example, it is possible to achieve a wide range of energy bandgaps from 0,36 e V to 2,25 e V by rasing 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, lightemitting 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 e V to 1,28 e V . When the GaAs substrate is used, the bandgap is restricted to the range between 1,48 e V and 1,88 e V . To expand the possible bandgap range, ternary or quarternary substrate is required instead of conventional binary substrate. However, it is very difficult to obtain bulk single crystal substrate for the ternary or quarternary system. To this end, so-called graded substrate is commercially available (see DE-A-22 49 858). 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 0,61 P 0,39 wafers consists of a 30 µm-thick graded composition GaAsP layer on a GaAs substrate and a 20µm-thick constant composition GaAs 0,61 P 0,39 layer on top of 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 eleminate the above mentioned defects. Furthermore, from GB-A-2030767 a crystal structure is known comprising a single crystal bulk substrate, epitaxial layers, a constant-composition layer and an epitaxial layer, whereby there is an intentional lattice mismatch between the epitaxial layer and the constant-composition layer of 3.3 x 10⁻³. However, the lattice constant of the epitaxial layer is a greater than that of the constant-composition layer. It is the object of the present invention 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. This object is achieved by a semiconductor structure as defined in claim 1 and by a method of growing an epitaxial layer on a graded substrate as defined in claim 2. 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 Δa/a = 1 x 10⁻³ to 5 x 10⁻³ (Δa : a difference in the lattice constant between the top layer of the graded substrate and the grower 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. DETAILED DESCRIPTION OF THE PREFERRENT EMBODIMENTS Double heterojunctions (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 1-y P 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 intentionally 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. An epitaxial layer may be grown by the hydride vapor- phase- epitaxy (VPE) method. A GaAsP graded substrate consists of a bulk single crystal, a graded composition layer grown thereon, and a constant composition layer thereon. As the bulk crystal, approximately 300 µm-thick GaAs bulk single crystal was used. The graded layer is a grown layer in which 30 µm-thick composition is changed by small degrees from GaAs 1-y P y (y = 0) to GaAs 1-y P y (y = 0,40) in thickness direction. The constant layer is on about 30 µm-thick GaAs 1-y P y (y = 0,40) layer, its composition being constant. Then, the following were consecutively grown on the constant layer : a 2 µm-thick n-InGaP layer as an n-type clad layer 4, a 200 nm(2000 Å)-thick InGaAsP layer as an active layer, a 2µm-thick P-InGaP layer as a p-type clad layer, and a 1 µm-thick InGaAsP layer as a cap layer, which constitutes a DH structure for a semiconductor laser. The n-type InGaP clad layer was Se-doped by approximately 7 x 10¹⁷ cm⁻³ as an n-type dopant, while the p-type InGaP clad layer was Zn-doped by approximately 2 x 10¹⁸ cm⁻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 of GaAs 1-y P y (y = 0,4) with lattice match and the case where growth was effected by introducing a lattice mismatch Δa/a of about + 1 x 10⁻³ to + 5 x 10⁻³ 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 1-x Ga x P (x = 0,71) for both the n-clad layer and the p-clad layer, and In 1-x Ga x As 1-y P y (x = 0,97, y = 0,45) for both the cap layer and the active layer. The lattice constants of these mixed crystals are matched to the lattice constant (a = 5,5 Å) of the constant layer of the GaAs 1-y P y (y = 0,40) graded substrate by about Δa/a < 5 x 10⁻⁴. 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 1-x Ga x P (x = 0,67 ± 0,02) for both the n-clad layer and the p-clad layer, and In 1-x Ga x As 1-y P y (x = 0,93 ± 0,02, y = 0,45) for both the cap layer and the active layer. The lattice mismatch (Δa/a) of these epitaxial layers with respect to the lattice constant (a = 5,57 Å) of the constant layer of the GaAs 1-y P y (y = 0,40) graded substrate was about + 1 x 10⁻³ to 5 x 10⁻³ at roon temperature, and the lattice mismatch between each adjacent layer of both clad layers, active layer, and cap layer was within ± 5 x 10⁻⁴. As a result of preparing DH structure for various LD₉, 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 x 10⁻³ to + 5 x 10⁻³ at room temperature was introduced to alleviate a strain, the results of similar observation were favorable and showed a good specular surface. If Δa/a exceeded + 5 x 10⁻³, the inherent adverse effect of lattice mismatch emerges instead of the effect of alleviating a strain. The lower limit of Δa/a at which the effect of the present invention can be detected is + 1 x 10⁻³, 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 µm were fabricated, and a comparison was made of their oscillating threshold current density by pulse operation at room temperature. The selective Zn-diffusion was performed through the window of SiO₂ layer so as to reach the P-type clad layer. Ohmic contacts to the p-side and the n-side were made by An-Zn layer and An-Ge-Ni layer, respectively. The LDs prepared from wafers of lattice matched DH structures for LDs were pulsed at a threshold current density of I th = 25 KA/cm² to 35 KA/cm² 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 Δa/a = 3,7 x 10⁻³ at room temperature, the CW operation at room temperature was obtained at a threshold current density of I th = 3 KA/cm². 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 Δa/a: + 1 x 10⁻³ to + 5 x 10⁻³ with the constant composition layer. Preferably, Δa/a is selected to be in the range of 3 x 10⁻³ to 4 x 10⁻³. The aforementioned value of lattice mismatch is very large as compared with the critical value of allowable lattice mismatch (generally Δa/a : I 5 x 10⁻⁴ 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 reason 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 alleviated 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 larger, a favorable specular surface can be obtained for the aforementioned reason if an epitaxial layer having a small lattice constant of Δa/a being - 1 x 10⁻³ to - 5 x 10⁻³ 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 1-y P y (y = 0,90) 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 hydrid VPE method was used to achieve growth, and its structure is identical as the one described above. More specifically, as for the InAsP graded substrate, an approximately 300 µm-thick InP bulk single crystal was used as the crystal 1. The graded layer is a grown layer in which a 30 µm-thick mixed crystal composition layer is changed by small degrees from InAs 1-y P y (y = 1,0) to InAs 1-y P y (y = 0,70). The constant layer is an about 30 µm-thick InAs 1-y P y (y = 0,70) layer, its mixed crystal composition being constant. Then, the following were consecutively grown on the constant layer : a 2 µm-thick n-InAsP layer as the n-type clad layer, a 200 nm(2000 Å)-thick InGaAsP layer as the active layer, a 2 µm-thick p-InAsP layer as the p-type clad layer, and a 1 µm-thick InGaAsP layer as the cap layer, which constitutes a DH structure for a LD. The n-type InAsP clad layer was Se-doped by approximately 1 x 10¹⁸ cm⁻³ as a n-type dopant, while -the p-type InAsP clad layer was Zn-doped by approximately 2 x 10¹⁸ cm⁻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 LD s were grown on the constant layer of InAs 1-y P y (y = 0,70) with lattice match and the case of the present invention where growth was effected by introducing a lattice mismatch Δa/a of about - 1 x 10⁻³ to - 5 x 10⁻³ to alleviate the strain. In the case where lattice match was effected, the composition of each grown layer was InAs 1-y P y y = 0,70) for both the n-clad layer and the p-clad layer, and In 1-x Ga x As 1-y P y (x = 0,29, y = 0,10) for both the cap layer and the active layer. The lattice constants of these mixed crystals are matched to the lattice constant (a = 0,5925 nm(5,925 Å) of the InAs 1-y P y (y = 0,70) graded substrate by about Δa/a - 5 x 10⁻⁴. In the case of the present invention in which lattice mismatch was introduced so as to alleviate the strain, the composition was InAs 1-y P y (y = 0,20 ± 0,06) for both the n-clad layer  and the p-clad layer, and In 1-x Ga x As 1-y P y (x = 0,33 ± 0,03, y = 0,10) for both the cap layer and the active layer. The lattice mismatch (Δa/a) of these epitaxial layers with respect to the lattice constant (a = 0,5925 nm(5,925 Å) of the InAs 1-y P y (y = 0, 0) graded substrate was about - 1 x 10⁻³ to - 5 x 10⁻³, and the lattice mismatch between adjacent layers of both clad layers, active layer and cap layer was within ± 5 x 10⁻⁴. As a result of preparing DH structures for various LDs, observation with a Normalsky differential interference microscope revealed a deterioration 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 stipe-type LDs with a stripe width of 7 µm 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 I th = 6 to 8 KA/cm² at room temperature. However, the average oscillating threshold current density was I th = 5 KA/cm² during room-temperature CW operation of LDs of the present invention, prepared from wafers of DH structures to which lattice mismatch was introduced so as to alleviate a strain. The minimum value was I th 2 KA/cm² in the case of those prepared from wafers of DH structures for LDs to which a lattice mismatch of about Δa/a = - 3,1 x 10⁻³ 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.

Claims: A semiconductor structure comprising: A single crystal bulk substrate; a graded-composition epitaxial semiconductor layer formed on said bulk substrate and having a lattice constant that varies across the layer thickness; a constant-composition epitaxial semiconductor layer formed on said graded-composition layer and having a lattice constant a, said graded-composition layer being lattice matched with said substrate at the substrate/graded-composition layer interface and with said constant composition layer at the graded-composition layer/constant-composition layer interface;and an epitaxial semiconductor layer grown on said constant-composition layer with an intentional lattice mismatch Δa/a, where Δa is the difference in the lattice constants between said constant-composition layer and said epitaxial layer, the value of Δa/a being selected to be between 1 x 10⁻³ to 5 x 10⁻³, and where the lattice constant of said epitaxial layer is smaller than that of said constant-composition layer for the case that the lattice constant of said graded-composition layer increases in the direction from said bulk substrate to said constant-composition layer, but said lattice constant of said epitaxial layer is larger than that of said constant-composition layer for the case that the lattice constant of said graded-composition layer decreases in the direction from said bulk substrate to said constant-composition layer. A method of growing an epitaxial layer on a graded substrate comprising growing a graded-composition epitaxial semiconductor layer having a lattice constant that varies across the layer thickness on a single crystal bulk substrate; and then growing a constant-composition epitaxial semiconductor layer having a lattice constant a on said graded-composition layer, said graded-composition layer being lattice matched with said substrate at the substrate/graded-composition layer interface and with said constant composition layer at the graded-composition layer/constant-composition layer interface; and then growing said epitaxial semiconductor layer on said constant-composition layer with an intentional lattice mismatch Δa/a of about 1 x 10⁻³ to 5 x 10⁻³, where Δa is the difference in the lattice constant between said constant-composition layer and said epitaxial layer and where the lattice constant of said epitaxial layer is smaller than that of said constant-composition layer for the case that the lattice constant of said graded-composition layer increases in the direction from said bulk substrate to said constant-composition layer, but said lattice constant of said epitaxial layer is larger than that of said constant-composition layer for the case that the lattice constant of said graded-composition layer decreases in the direction from said bulk substrate to said constant-composition layer.

Inventors: MATSUMOTO, TAKASHI
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L21, H01L31, H01S5, H01L29, H01L33