Method of forming semiconductor thin film

Abstract: A method of forming a semiconductor thin layer on a silicon substrate comprising the steps of depositing a first amorphous layer of a compound semiconductor (e.g., GaAs) on the silicon substrate, and growing a first epitaxial layer of the compound semiconductor on the amorphous layer, characterized in that the method comprises the steps of: after the epitaxial growth step, depositing a second amorphous layer of the compound semiconductor on the first epitaxial layer, and growing a second epitaxial layer of the compound semiconductor on the second amorphous layer. The obtained GaAs/Si substrate has a reduced dislocation density.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the formation of a semiconductor thin film, and more particularly, to a method of forming a compound semiconductor thin film on a silicon substrate. The present invention is applied to a GaAs/Si substrate for GaAs devices, which substrate is produced by growing an epitaxial GaAs thin film on a silicon (Si) substrate. 2. Description of the Related Art A compound semiconductor substrate, for example, gallium arsenide (GaAs) substrate is produced by growing a single-crystal (GaAs) lump (ingot) by the use of a pulling method (Czochralski method) or the like, and then cutting the lump. Such a GaAs substrate is expensive, easily broken, and has a relatively small size. Recently, proposals have been made to produce GaAs/Si substrates corresponding to the GaAs substrate, to enlarge the wafer size, increase the mechanical strength and thermal conductivity, and reduce costs. In this case, a large lattice mismatch of about 4% exists between GaAs and Si, and this hinders the epitaxial growth of GaAs, and accordingly, a buffer layer having a strained layer superlattice (SLS) structure is inserted between the Si substrate and the GaAs layer to relax the lattice mismatch. The SLS structure buffer layer is composed of InGaAs-GaAs, GaAsP-GaAs or InGaAs-GaAsP system as disclosed, for example, in N. El-Masry, et al. "Defect Reduction in GaAs Epilayers on Si Substrates Using Strained Layer Superlattices", Mat. Res. Symp. Proc. Vol. 91, ′1987, Materials Research Society, pp. 99 - 103. Nevertheless, the formation of the SLS structure buffer layer is complicated. Attempts have been made to grow the GaAs layer directly on the Si substrate by adopting a two-step growth sequence (process), as disclosed in, for example, M. AKIYAMA et al., "Growth of GaAs on Si and its Application to FETs and LEDs", Mat. Res. Soc. Symp. Proc. Vol. 67, 1986, pp. 53-64, or K. Kaminishi : "GaAs on Si Technology", Solid State Technology, Vol.30, No.11,pp.91-97. In the two-step growth sequence, the Si substrate (wafer) is heated to about 900°C in a gas flow of H₂ and AsH₃ to clean its surface, whereupon it is cooled to 450°C or less and a first GaAs thin layer is deposited on the Si substrate by a metalorganic chemical vapor deposition (MOCVD) method. Subsequently, the Si substrate is reheated to a temperature of from 700 to 750°C, which is a conventional GaAs growth temperature for the MOCVD method, and then a second GaAs layer is epitaxially grown by MOCVD. The first GaAs thin layer has an amorphous or unstable crystal structure upon deposition and is changed into an epitaxial GaAs thin layer by the annealing (heating) of the second GaAs layer growth. In the GaAs epitaxial thin film formed directly on the Si substrate, however, crystal defects, such as a high density of dislocations and many etch pits, may occur, owing to differences in the lattice constants and thermal expansion coefficients of Si and GaAs. These crystal defects cause problems in devices formed using the GaAs epitaxial film; for example, they reduce electron mobility which brings a lower switching speed of a GaAs transistor, or cause a large dispersion of threshold voltages of GaAs transistors in a wafer (i.e. the GaAs/Si substrate), and lead to a shorter life of light-emitting devices such as lasers and LED's. An object of the present invention is therefore to reduce crystal defects generated in the GaAs epitaxial thin film of a GaAs/Si substrate, and to provide a method of forming a semiconductor (especially a compound semiconductor) thin film having fewer crystal defects on a Si substrate. This in turn should lead to improvements in the properties of devices formed using the substrate. The present invention provides a method of forming a semiconductor thin layer on a silicon substrate, comprising the steps of depositing a first amorphous layer of a compound semiconductor on the silicon substrate, and growing a first epitaxial layer of the compound semiconductor on the amorphous layer; according to the present invention, after the epitaxial growth step a second amorphous layer of the compound semiconductor is deposited on the first epitaxial layer, and a second epitaxial layer of the compound semiconductor is grown on the second amorphous layer. Preferably the deposition of an amorphous layer and the growth of an epitaxial layer, which layers are made of the same compound semiconductor material, e.g. a III-V compound semiconductor such as GaAs, AlGaAs, InP, GaP, InGaAs and InGaAsP, are repeated. Dislocations generated at the interface between the Si substrate and the compound semiconductor layer appear in the first epitaxial compound semiconductor layer and then propagate upwards into the subsequently grown compound semiconductor layer. The amorphous semiconductor layer deposited on the first epitaxial compound semiconductor layer has the effect of reducing the number of threading dislocations (i.e. the dislocation density), and therefore the repetition of the amorphous layer deposition and the epitaxial layer growth further decreases the dislocation density. Preferably, the amorphous semiconductor layer has a thickness of from 5 to 20 nm. The deposition and the growth of the compound semiconductor layers are performed by an MOCVD method or by a molecular beam epitaxy (MBE) method. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more apparent from the description of the preferred embodiments set forth below with reference to the accompanying drawings, in which: Figs. 1a to 1c are schematic sectional views of a compound semiconductor/Si substrate in various stages of the formation of the compound semiconductor thin film in accordance with the present invention; Fig. 2 is a graph showing a temperature variation in the compound semiconductor thin film formation; Fig. 3 is a microphotograph of the surface of the top GaAs epitaxial layer of the GaAs/Si substrate formed according to the present invention; Fig. 4 is a microphotograph of the surface of the top (first) GaAs epitaxial layer formed according to the prior art; and Fig. 5 is a schematic sectional view of a field effect transistor produced by using the GaAs/Si substrate. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1a to 1c and Fig. 2, a compound semiconductor (for example; GaAs) thin film is formed on a Si substrate, namely, a GaAs/Si substrate is produced in accordance with the present invention. First, as shown in Fig. 1a, a Si substrate (wafer) 1 oriented 4° off (001) toward [110] is cleaned with a hydrofluoric acid (HF) solution, and the cleaned Si substrate 1 is then placed in an MOCVD apparatus. The Si substrate 1 is heated to 900°C or more (e.g., about 950°C) and a hydrogen (H₂) gas (at 10 ℓ/min) and an arsine (AsH₃) gas (at 200 cm³/min) are passed through the apparatus to clean the surface of the Si substrate at step A of Fig. 2. As shown in Fig. 1b, by adopting an MOCVD method and the above-mentioned two-step growth sequence, a first amorphous GaAs layer 2 and a first epitaxial GaAs layer 3 are formed on the Si substrate 1, at steps B and C of Fig. 2, respectively. In the MOCVD process for GaAs, trimethyl gallium (TMG; (CH₃)₃Ga) or triethyl gallium (TEG; (C₂H₅)₃Ga) are used as the Ga source, which is carried into a reactor of the MOCVD apparatus by a carrier gas of hydrogen (H₂), and an arsine (AsH₃) gas as an As source. The cleaned Si substrate 1 is cooled to 450°C or less (e.g., 430°C), and at this temperature, the H₂ carrier gas (at 10 ℓ/min) containing TMG (0.5 to 5 cm³/min) and the AsH₃ gas (at 200 cm³/min) are passed through the reactor to deposit the first amorphous GaAs layer 2 having a thickness of from 5 to 20 nm (e.g., 10 nm) on the Si substrate 1, at step B. The deposition is stopped, and subsequently, the Si substrate 1 is reheated to 650 to 750°C (e.g., 650°C). At this temperature the H₂ carrier gas (at 10 ℓ/min) containing TMG (1 to 5 cm³/min) and the AsH₃ gas (at 200 cm³/min) are passed through the reactor to epitaxially grow the first epitaxial GaAs layer 3 having a suitable thickness (e.g., 1 µm), at step C. The total thickness of the formed layers 2 and 3 is preferably from 0.1 to 1.0 µm. Next, in accordance with the present invention, the Si substrate 1 is cooled to a temperature of 450°C or less (e.g., 430°C), and then the H₂ carrier gas containing TMG and the AsH₃ gas under the same conditions as those at step B are passed through the reactor for 1 to 5 minutes, as shown in Fig. 2, to deposit a second amorphous GaAs layer 4 having a thickness of from 5 to 20 nm (e.g., 10 nm) on the first epitaxial GaAs layer, at step D of Fig. 2. Then the Si substrate 1 is reheated to 650 to 750°C (e.g, 650°C). At this temperature the H₂ carrier gas containing TMG and the AsH₃ under the same conditions as those at step C are passed therethrough for 20 to 40 min, to epitaxially grow a second epitaxial GaAs layer 5 having a thickness of 2 µm. Accordingly, a GaAs/Si substrate having a GaAs thin film consisting of the layers 2 to 5 is produced. To detect crystal defects of the GaAs/Si substrate, the surface of the second epitaxial GaAs layer 5 is etched for 10 minutes with molten potassium hydroxide (KOH). The etched surface is shown in Fig. 3 as a differential interference microphotograph (x 870). It can be seen from Fig. 3 that the dislocation density is 1.8 x 10⁷ cm⁻² and no etching pits exist. As a comparative example, in step C the first epitaxial GaAs layer 3 is grown, to obtain a thickness thereof of 3 µm. The thus obtained GaAs/Si substrate is produced in accordance with the two-step growth sequence. The surface of the first epitaxial GaAs layer is etched with molten KOH at 350°C, and is shown in Fig. 4 as a differential interference microphotograph (x 870). It can be seen from Fig. 4 that the dislocation density is 9.2 x 10⁷ cm⁻² and two etching pits exit. In Figs. 3 and 4, one of the small recesses corresponds to one of the dislocation appearing on the surfaces. The size of the recess in Fig. 3 is larger than that of the recess in Fig. 4, and the number of recesses in Fig. 3 is smaller than that of the recesses in Fig. 4. Therefore the dislocation density of the top epitaxial layer (Fig. 3) of the GaAs thin film formed in accordance with the present invention is as low as one fifth that of the comparative example (Fig. 4). In the above embodiment, a pair of the second amorphous GaAs layer and the second epitaxial GaAs layer are formed. Where the deposition of the amorphous GaAs layer and the growth of the epitaxial GaAs layer are repeated to form additional pairs of the amorphous and epitaxial GaAs layers on the formed epitaxial GaAs layer, the additional amorphous GaAs layers contribute to a further reduction of the dislocation density. By using the produced GaAs/Si substrate (Fig. 1c), a field effect transistor (FET) is produced, as shown in Fig. 5. In this case, the second epitaxial GaAs layer 5 is slightly doped with Si or Se during the epitaxial growth. The GaAs layer 5 is selectively and slightly etched to form a recess and a gate electrode 8 of Au/Au-Ge is formed on the GaAs layer 5 within the recess, and a source electrode 6 and a drain electrode 7 of Pt/Au-Ge are formed on the GaAs layer 5 in a conventional manner. The characteristics of the produced FET are improved due to a reduction of the dislocation density. Instead of GaAs, another III-V compound semiconductor, such as AlGaAs, InP, GaP, InGaAs and InGaAsP can be used.

Claims: A method of forming a semiconductor thin layer on a silicon substrate, comprising the steps of depositing a first amorphous layer (2) of a compound semiconductor on the silicon substrate (1), and growing a first epitaxial layer (3) of the compound semiconductor on the amorphous layer (2), characterised in that the method further comprises the steps of: depositing a second amorphous layer (4) of the compound semiconductor on the first epitaxial layer (3), and growing a second epitaxial layer (5) of the compound semiconductor on the second amorphous layer. A method according to claim 1, in which, after the second epitaxial growth step, a further stage of deposition of an amorphous layer and growth of an epitaxial layer is performed. A method according to claim 1 or 2, in which the compound semiconductor is a III-V compound semiconductor. A method according to claim 3, in which the III-V compound semiconductor is GaAs. A method according to any preceding claim, in which the deposition of the amorphous layers and the growth of the epitaxial layers are performed by a metalorganic chemical vapor deposition process. A method according to claim 5, in which the deposition of the amorphous layers is performed at a temperature of from 350 to 450°C. A method according to claim 5 or 6, in which the growth of the epitaxial layers is performed at a temperature of from 650 to 750°C. A method according to any of claims 1 to 4, in which the deposition of the amorphous layers and the growth of the epitaxial layers are performed by a molecular beam epitaxy process.

Inventors: ESHITA TAKASHI, FURUMURA YUJI, MIENO FUMITAKE SAGINUMADAI SKY, WATANABE TAKUYA, ESHITA, TAKASHI, FURUMURA, YUJI, MIENO, FUMITAKE SAGINUMADAI SKY MANSION 105, WATANABE, TAKUYA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H01L21
IPC codes: H01L21