SEMICONDUCTOR DEVICE

Abstract: A novel method of easily forming a quaternary group III-V crystal on the group III-V crystal maintaining the lattice superposition relative to the group III-V crystal. It is easy to form a superlattice that consists of a first binary group III-V (written as III¹-V¹) crystal layer and a ternary group III-V (III¹-III²-V²) crystal layer on a substrate of group III-V crystal both maintaining lattice superposition with the substrate. A stabler superlattice layer is obtained if the ratio of film thickness of the III¹-V¹ crystal to that of the III¹-III²-V² crystal is so selected that the compositions thereof maintain the lattice superposition with the original crystal when the superlattice forms a mixed crystal spontaneously or being doped with impurities.

Description: Technical Field The present invention relates to application of a superlattice semiconductor layer to a semiconductor device such as a semiconductor laser, a light emitting diode, a photodiode, an FET and a bipolar transistor, the semiconductor layer being made of a multi-element compound semiconductor which consists of three or less elements and which has an energy band width equivalent in size to a quaternary compound semiconductor. Background Art By virtue of the development of MOCVD (Metal Organo Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy), a compound semiconductor device having its thickness controlled so as to be substantially equal to the thickness of a single atom layer has recently been proposed. One type of semiconductor device which utilizes an energy band width of a multi-element compound consisting of four or more elements has also attracted special interest recently. However, the technique of forming a quaternary compound semiconductor having a film thickness of 100 Å or less, particularly one which consists of two different elements in each of the two different groups e.g., InGaAsp on a III-V group substrate in a lattice matched state still involves many problems in terms of controllability (see H. Kressel et al, "Semiconductor Lasers and Heterojunction LEDs", pp.304 to 318, Academic Press, (1977). To avoid the problems of manufacturing true quaternary semiconductor devices, appl. phys. let. 45 (11), 1 December 1984, pp. 1193-1195 describes a pseudo-quaternary device having a GaInAs/InP superlattice structure. The thicknesses of the layers forming the superlattice are selected to obtain a desired band gap width. However, if each of the individual layers are lattice-matched with the substrate and with each other, the device described in this document is still not free of internal lattice strain and crystal defects and dislocations which either occur during manufacturing or after exposure to extreme operating conditions (heat or high current). Disclosure of the Invention It is therefore the object of the invention to provide a semiconductor device which is easy to manufacture and stable even under extreme operating conditions. The inventors have found that the lattice strain results from the occurrance of mixed crystallization by which true quaternary semiconductor regions are formed which are generally not lattice-matched with the adjacent semiconductor layers. Incidentally, the fact that mixed crystallization may be intentionally produced by ion implantation or impurity diffusion is disclosed in WO 82/03946. The preamble of claim 1 corresponds to Applied Physics Letters, vol. 45, no. 11, December 1984, pages 1193-1195. The symbols III¹, III², V¹ and v² hereinafter denote a first V-group element, a second III-group element, a first V-group element and a second V-group element, respectively. The invention is defined in claim 1. Embodiments are set out in the dependent claims. The present inventors have found that a superlattice structure, which consists of a first III-V group (hereinafter referred to as "III¹-V¹") binary crystal layer formed on a III-V group crystal substrate and lattice-matching with the substrate and a III-V group (III¹-III²-V²) ternary crystal layer composed of two different III-group elements and a V-group element and also lattice-matching with the substrate, is easy to produce and has an average energy band gap (width) equivalent in size to a III-V group (III¹-III²-V¹-V²) quaternary crystal layer. The present inventors have, after further exhaustive study, found that a superlattice structure formed so that the ratio of the film thickness (L₂) of the III¹-V¹ binary crystal layer and the film thickness (L₃) of the III¹-III²-V² ternary crystal layer, i.e., L₂/L₃, is conincident with a value determined by the calculation described later maintains its lattice-matching state with the substrate even after mixed crystallization spontaneously occurring during growth or mixed crystallization caused by Zn diffusion or Si implantation, and the superlattice structure is thus extremely stable. More specifically, the feature of the present invention resides in the employment of a superlattice structure composed of (III¹-V¹) binary and (III¹-III²-V²) ternary mixed crystals (which lattice-match with a substrate concerned) in place of a (III¹-III²-V¹-V²) quaternary crystal layer which is formed on a III-V group binary mixed crystal (III¹-V¹) such as GaAs and which lattice-matches with the substrate. In other words, the present invention is accomplished on the basis of the following finding. Assuming that either one of the binary and ternary layers which has a smaller energy band gap Eg is referred to as "well" and the other layer, which has a larger energy band gap Eg is referred to as "barrier" and their respective thicknesses are denoted by L W and L B , respectively, it is important to adjust the L W /L B in advance for a mixed crystal having a metallurgically average composition, which is formed on the assumption that the above-described superlattice structure induces disordering, so that the mixed crystal is also lattice-matched with the previous crystal. It has also been found that, when the L W /L B ratio is within ± 15% from a calculated value, similar advantageous effects are obtained. This growth method has the following advantages. Namely, it is possible to considerably ease the strictness required for control of the V¹/V² element ratio and the substrate temperature when two different V-group elements are simultaneously supplied. In general, when a binary layer is sandwiched between superlattice crystal layers, microscopic irregularities on the surfaces of the crystal layers are flattened to provide a crystal having an extremely flat surface, i.e., excellent surface morphology, so that it is possible to prevent generation of a surface energy level and growth of a multi-element mixed crystal portion which does not lattice-match with the substrate at the interface. Further, when an optimal L W /L B ratio is selected, even if the superlattice structure is mixed during an element producing process to induce disordering, it becomes a lattice-matched quaternary mixed crystal. Therefore, there is no induction of any large internal lattice strain or misfit dislocation. The thicknesses L W and L B of films to be formed can be controlled to the Å order by MBE (Molecular Beam Epitaxy) and OMVPE (Organometal Vapro Phase Epitaxy). In a superlattice structure wherein L B ≦25Å (1nm=10Å), free tunneling of electrons may take place, and in such case, this superlattice structure may electrically be regarded as substantially equal to a quaternary mixed crystal having a metallurgically average composition as is known from the Kronig- Penny model. Practical examples will be described below. In one example, a superlattice structure consisting of InP and In 0.53 Ga 0.47 As is grown on an InP substrate; in the other example, a superlattice structure consisting of GaAs and In 0.49 Ga 0.51 P is grown on a GaAs substrate. In the former example, InGaAs serves as a well, while InP serves as a barrier, and a superlattice structure which is equivalent to a metallurgically average quaternary mixed crystal In x Ga 1-x As y P 1-y . To allow this superlattice structure to be mixed-crystallized so as to lattice-match with InP, it is necessary to satisfy the condition of y≃2.16 (1-x) (see "Semiconductor Lasers and Heterojunction LEDs" H. Kressel, J.K. Butler Academic Press NewYork (1977) p.392). In the case of a superlattice structure consisting of In z Ga 1-z As (Z=0.537) having a thickness L W and InP having a thickness L B , R≡L W /L B is required to satisfy the quadratic expression: 1.16(1-Z)R²+1.16R-Z=0 . For this reason, it is necessary to meet the condition of L W /L B ≃0.392 . However, it has been found that it suffices to set the L W /L B ratio within a range from 0.33 to 0.45 in practice. Under the above-described condition, the average composition of the quaternary mixed crystal is In 0.87 Ga 0.13 As 0.28 P 0.72 . The energy band gaps of InP, In 0.87 Ga 0.13 As 0.28 P 0.72 and In 0.53 Ga 0.47 As are 1.35eV, 1.14eV and 0.75eV, respectively (see "Heterostructure Lasers, Part B" H. C. Casey, Jr. M. B. Panish Academic Press, NewYork, (1978) p.40). It is therefore possible to design a quantum well type long wavelength laser by following an optimum structure for GaAℓAs multiple quantum well lasers (see W. T. Tsang, Appl Phys Lett 39 (1981) p.780). In the latter example, GaAs serves as a well, while InGaP serves as a barrier, and the condition of y≃1-2.04x , under which the metallurgically average quaternary mixed crystal In x Ga 1-x As y P 1-y lattice-matches with GaAs, has been found to be substantially satisfied irrespective of the L W /L B ratio. This means that it is possible to produce a quaternary layer In x Ga 1-x As y P 1-y having any desired average composition between GaAs and In 0.49 Ga 0.51 P by appropriately setting L W and L B : In addition, when a superlattice structure is grown by MBE or OMVPE, even if the film thickness ratio between GaAs and InGaP is deviated from a predetermined value, since the lattice matching condition is satisfied, it is possible to readily produce a superlattice structure without any fear of misfit dislocation being induced, provided that the device structure concerned permits slight deviation of composition to be ignored. Further, gradual change of the film thickness ratio enables production of a graded layer in which the energy level is inclined with respect to the thickness. Furthermore, employment of a (binary/ternary) superlattice structure in place of a quaternary growth layer enables avoidance of the problem of mixing instability during growth due to the existence of so-called miscibility-gap, so that it becomes possible to obtain a mirror-surface epitaxial film even at a relatively low growth temperature. As has been described above, it has been found that the crystal structure according to the present invention enables semiconductor devices employing a quantum well structure to be readily produced, such as FETs, bipolar transistors, photodiodes, long and short wavelength lasers and light-emitting diodes, and the present invention is considerably effective in improving characteristics of such semiconductor devices. Brief Description of Drawings Fig. 1a schematically shows the conduction band of a quantum well laser in which a cladding layer is defined by InP grown on an InP substrate, a well layer in a quantum well active layer is defined by In 0.53 Ga 0.47 As and a barrier layer therein is defined by an (InP/In 0.53 Ga 0.47 As) superlattice structure (the average composition is the same as that of an In 0.87 Ga 0.13 As 0.28 P 0.72 quaternary mixed crystal). Fig. 1b shows an equivalent model of the conduction band shown in Fig. 1a in the case where electron tunneling effect or disordering of the superlattice structure is induced in the same laser as that shown in Fig. 1a. Figs. 2a and 2b show in combination a quantum well laser in which a cladding layer is defined by InGaAℓP grown on a GaAs substrate, a well layer in a quantum well active layer is defined by GaAs and a barrier layer therein is defined by a (GaAs/In 0.49 Ga 0.51 P) superlattice structure (the average composition can be set so as to be intermediate between the respective compositions of the two different compounds by setting the film thickness ratio therebetween as desired, Fig. 2a schematically showing the conduction band, and Fig. 2b showing an equivalent model of the conduction band. Figs. 3a and 3b are views employed to describe a quantum well layer in which the well layer in the arrangement shown in Fig. 2a is also defined by a (GaAs/In 0.49 Ga 0.51 P) superlattice structure, thereby shortening the oscillation wavelength. Fig. 4 is a sectional view of a device shown in Example 4. Fig. 5 is a sectional view of a device shown in Example 5. Best Mode for Carrying Out the Invention Example 1: An example in which a superlattice layer according to the present invention is employed as a barrier layer in a quantum well active layer will be described below. Figs. 1a and 1b show the conduction band of an active layer portion of a quantum well type long wavelength laser formed by successively growing an n-InP cladding layer (n∼1x10¹⁸, 3µm), an undoped quantum well active layer and a p-InP cladding layer (p∼1x10¹⁸, 2µm) on an n⁺-InP substrate by OMVPE. In this arrangement, the quantum well active layer consists of a quantum well layer which is defined by a single layer of In 0.53 Ga 0.47 As with a thickness of d w =70Å∼120Å and a barrier layer which is defined by an (InP/In 0.53 Ga 0.47 As) superlattice structure according to the present invention. The thickness L B of InP serving as a barrier layer in the superlattice layer was set so as to satisfy the condition of L B ≦25Å so that tunneling of electrons would take place effectively, and two different superlattice layers were prepared in accordance with the condition of L W /L B ≅0.392 , i.e., (L W / L B )≅(8Å/20Å) and (L W /L B )≅(4Å/10Å). The thickness d B of one group of superlattices constituting the barrier layer was set to d B ≅84∼112Å so that electrons within each of the wells in the multiple quantum well structure were prevented from tunneling. The above-described semiconductor layers constituting a quantum well structure were formed by the use of a known MBE method. This crystal was etched into a mesa shape having a width of 2µm by conventional photolithography and then subjected to liquid phase epitaxy to produce a buried type (BH) laser. Thus, it was possible to obtain a device which had a threshold current value that was about a half (10mA or less) of that of conventional InGaAsP/InP lasers produced by liquid phase epitaxy at a wavelength of 1.3 to 1.55µm, and which showed excellent temperature characteristics. A laser produced by MBE also had excellent characteristics similar to the above. It should be noted that similar results were obtained under the condition of L W /L B =0.33 to 0.45. Example 2: Figs. 2a and 2b show the conduction band of an active layer portion of a quantum well type visible light laser formed by successively growing an n-In u Ga v Aℓ 1-u-v P cladding layer (n∼1x10¹⁸, 3µm), an undoped quantum well active layer, a p-(In u Ga v Aℓ 1-u-v ) P cladding layer (p∼1x 10¹⁸, 2µm) and a P-GaAs gap layer (1µm) on an n⁺-GaAs substrate by MBE. In this arrangement, the quantum well active layer consists of a quantum well layer which is defined by a single layer of GaAs with a thickness of d W =40Å∼100Å and a barrier layer which is defined by a (GaAs/In 0.49 Ga 0.51 P) superlattice structure according to the present invention. The thickness L B of InGaP was set so as to satisfy the condition of L B ≦25Å so that tunneling of electrons would take place effectively. The thickness d B of one group of superlattices constituting the barrier layer was set to d B ≃60Å∼100Å so that electrons within each well were prevented from tunneling. In this case, the L W /L B ratio was appropriately adjusted within a range of L W , L B <30Å in accordance with the oscillation wavelength. The composition (In u Ga v Aℓ 1-u-v )P of the cladding layer was appropriately selected from a range from In 0.49 Ga 0.51 P (E g ∼1.88eV) to In 0.59 Ga 0.41 P (Eg∼2.25eV) so that lattice matching with GaAs was obtained in accordance with the oscillation wavelength. From this crystal, a stripe laser employing an SiO₂ film as a current confinement layer was produced. The obtained device had a low threshold current density, i.e., 1KA/cm², at an oscillation wavelength of 0.75µm to 0.65µm and showed excellent temperature characteristics. As to the optical output, current-optical output characteristics which had excellent linearity up to 20mW were obtained. Example 3: Figs. 3a and 3b show in combination a laser in which the well layer in the active layer is also defined by a superlattice structure according to the present invention. It was possible, according to this example, to obtain a device having an oscillation wavelength of 0.70 to 0.65µm. Example 4: An example of an FET having an arrangement shown in Fig. 4 will be explained below. An undoped (an impurity concentration of 10¹⁴cm⁻³ or less) InP buffer layer 102 having a film thickness of 5000Å was formed on a semiinsulator InP substrate 101 by a known epitaxy method, and a channel layer 103 defined by a superlattice structure according to the present invention was formed on the layer 102. The channel layer 103 was formed by a known MBE method by alternately stacking 100 InP layers (film thickness: 15Å) 113 having an impurity concentration of 1x10¹⁷cm⁻³ and 100 InGaAs layers (film thickness: 6Å) 123. An undoped InAℓAs hetero gap layer 124 (film thickness: 5000Å) was formed in a gate forming region on the channel layer 103, and Aℓ having a film thickness of 2000Å was formed on the layer 104 by evaporation to provide a gate electrode 105. AuGe/Ni/Au (film thickness: 2000Å) was formed in each of the source and drain regions on the channel layer 103 by sputtering and alloyed by a heat treatment at 400°C for three minutes, thereby forming source and drain electrodes 106 and 107. The obtained FET had the following characteristics: a gate length of 0.3µm; a source-drain spacing of 1µm; a transconductance of about 300ms/mm; and a cut-off frequency of 60GHz. Example 5: An example in which the present invention is applied to a photodiode is explained below. An n⁺-InP buffer layer 202 was formed on an n⁺-Inp substrate 201 by epitaxy, and a superlattice layer 203 according to the present invention was formed as a light absorbing layer on the buffer layer 202 by molecular beam epitaxy and/or organometal vapor phase epitaxy. The superlattice layer was formed by stacking InGaAs layers 213 and InP layers 223 by a method similar to that in Example 4. An n-InP layer 204 having a film thickness of 1µm was formed on the superlattice layer, and Zn was diffused in a region 205 so as to be a p-region, thereby forming a p-n junction within the n-InP layer 204. Further, a CrAuP electrode 206 and an AuGeNiAu electrode 207 were formed by conventional methods to complete a device. This device had practical sensitivity at a wavelength of 1.33 to 1.55µm. It was also possible to apply the present invention to a light-emitting diode, a bipolar transistor, etc. in addition to the above-described examples. It should be noted that it is possible, in the above-described embodiments, to utilize a partial change in optical or electrical properties caused by spontaneous mixed crystallization of a part of the superlattice structure or mixed crystallization thereof effected by impurity diffusion or ion implantation. Industrial Applicability It is an object of the present invention to provide a high-performance semiconductor device, e.g., an FET (Field Effect Transistor), a bipolar transistor and a phototransistor which operate at high speed, and a semiconductor laser device which stably oscillates in either a long or short wavelength range. Since the present invention enables a quaternary III-V group crystal of excellent quality to be readily produced on a III-V group substrate, it is possible to obtain various types of semiconductor device having remarkably improved characteristics.

Claims: A semiconductor device comprising a III¹-V¹ binary crystal substrate (101; 201) and a superlattice structure (103; 203) composed of a plurality of alternating III¹-V¹ binary crystal layers (113; 223) and III¹-III²-V² ternary crystal layers (123; 213), each of said crystal layers (113, 123; 213, 223) being substantially lattice-matched with the substrate (101; 201), characterized in that the layer thickness ratio L W /L B between said two types of crystal layers (113, 123; 213, 223) is selected so that a substance produced as a result of mixed-crystallization of said superlattice structure (103; 203) is substantially lattice-matched with the substrate (101; 201). The semiconductor device of claim 1, wherein said substrate (101; 201) is defined by InP, wherein said superlattice structure (103; 203) is defined by a combination of InP crystal layers (113; 223) each having a thickness L B and In z Ga 1-z As crystal layers (123; 213) each having a thickness L W with z ≅ 0.537, and wherein the thickness ratio L W /L B is 0.33 to 0.45. The semiconductor device of claim 1 or 2, wherein the superlattice structure defines an active layer and/or an optical guide layer and/or a cladding layer of a semiconductor laser device. The semiconductor laser device of claim 3, wherein said substrate is defined by GaAs, said active layer and/or optical layer guide layer is defined by a superlattice layer made of a combination of GaAs crystal layers and In z Ga 1-z P crystal layers, and wherein the cladding layer is defined by an InGaAℓP layer. The semiconductor device of any one of claims 1 to 4, wherein said superlattice layer is formed by molecular beam epitaxy (MBE) and/or organo metal vapor phase epitaxy (OMVPE). The semiconductor device of any one of claims 1 to 5, wherein the superlattice structure has a mixed crystal region. The semiconductor device of claim 6, wherein the mixed crystal region is formed spontaneously, or by impurity diffusion in the superlattice structure, or by ion implantation in the superlattice structure.

Inventors: KURODA, TAKAO HITACHI KOGANEI-RYO, 5-20-22, MATSUMURA, HIROYOSHI 264-61, KUBO, HIDAKA-MACHI, OHISHI, AKIO HITACHI DAI 4 KYOSHIN-RYO, 4-14-6, TSUJI, SHINJI HITACHI KOYASUDAI-APART, 2-32-A104, WATANABE, AKIYOSHI HITACHI SHOBU-RYO, 5-18-3
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H01S3
IPC codes: H01L29, H01L33, H01S5