Compound semiconductor vapor phase epitaxial device

Abstract: A compound semiconductor vapor phase epitaxial device comprises a cylindrical reactor vessel (2), a plurality of flow channels (17, 27) disposed in the reactor vessel, a crystal substrate (7) disposed in one of the flow channels, a plurality of gas supply pipes for respectively supplying gas containing element of compound to be grown on the crystal substrate and at least one slit (10) or linearly arranged fine holes (P) communicating adjacent two flow channels so as to extend in a direction normal to a direction of the gas flow to form a laminate layer flow consisting of two or more than two gases at an upstream portion of location of the crystal substrate.

Description: This invention relates to a compound semiconductor vapor phase epitaxial device conceived for improving uniformity in composition and reproducibility of a compound semiconductor crystal formed on a crystal substrate. There is well known a metalorganic chemical vapor deposition (MOCVD) which is one typical example of compound semiconductor vapor phase film formation method capable of controlling thickness and composition of the film and being excellent in productivity, thus being widely utilized for the crystal growth of GaAs which is one typical compound semiconductor of the groups III-V elements. In the MOCVD of the groups III-V elements compound semi-conductor, the crystal growth is generally performed under an atmosphere in which the group V elements material is excessively supplied, and in such case, crystal growth rate is in proportion to supply amount of the group III elements material. Further, nitride semiconductors such as GaN or AlGaN in the group III-V semiconductors are the material series being studied or searched as short wavelength light emitters or emitting elements, and experimental examples of the MOCVD utilizing, as materials, NH 3 , TMG (trimethylgallium) or TMA (trimethylaluminium) have been announced. In a system, however, in which such materials are utilized, the NH 3 and the TMG, and the NH 3 and the TMA react respectively at room temperature (particularly, the NH 3 and the TMA are remarkably reacted), and accordingly, when the materials are supplied, these material are preliminarily mixed and the mixture is then guided into a reactor or reaction device. In such method, there are problems such that the composition of the film formed on the crystal substrate is not uniform and good reproducibility is not provided, and these problems are remarkably observed in the case of AlGaN mixed crystal. In order to eliminate such problems, there is provided the Japanese Patent Laid-open Publication No. 61-202422, in which material gases are independently supplied to reaction areas, i.e. growth areas on the crystal substrate. However, it is difficult to uniformly grow the crystal on the crystal substrate only by independently supplying the material gases and mixing them in the reaction areas because of generation of the material gas vortex and the turbulent flow in the gas flow. Furthermore, there is provided the Japanese Patent Laid-open Publication No. 61-186288 showing an angular reaction tube in consideration of the flow of the material gases. However, the above-mentioned problems cannot be solved by this angular reaction tube. FR-A-2661554 discloses an epitaxial reactor in which first and second parallel gas flow means open at a common position to a crystal growth region. An object of this invention is to substantially eliminate defects or drawback encountered in the prior art and to provide a compound semiconductor vapour phase epitaxial device capable of forming a crystal, on a crystal substrate, having a uniform composition and improved reproducibility. Another object of this invention is to provide a compound semiconductor vapour phase epitaxial device provided with a plurality of gas flow channels through which material gases composed of NH 3 and TMG and TMA are respectively supplied to achieve the above object. According to the present invention, there is provided a compound semiconductor vapour phase epitaxial device, the device comprising: a cylindrical reactor vessel; a plurality of flow channel means disposed in the reactor vessel and each having a substantially rectangular cross section; and, a plurality of gas supply means for respectively supplying gas containing an element or elements of a compound to be grown; characterised by: a susceptor mounted on one of partition walls of one of the flow channel means for supporting a crystal substrate; and by: an aperture in a partition wall opposite the wall on which the susceptor is mounted for providing gas communication between adjacent flow channel means so as to form a laminar flow consisting of two or more gases at an upstream portion in the flow channel means provided with the susceptor. Preferably, said flow channel means is composed of a first flow channel and a second flow channel, said first and second flow channels being provided with a common wall structure through which both flow channels are connected, the aperture for communicating the flow channel means with each other being formed in said common well structure. The present invention also includes a method of using an epitaxial device as described above, characterised by the steps of supplying an excessive amount of a first gas containing a first material from a first gas supply means to a first flow channel in which the susceptor is disposed and supplying a second gas, for controlling a growing speed of the crystal grown on a crystal substrate supported by the susceptor, from a second gas supply means to a second flow channel. According to the structures and characters of the epitaxial device described above, a plurality of gas flow channel means are disposed in a reactor vessel to flow different kinds of material gases, which are mixed together in one of the flow channel means through communicating means at a portion just before the location of the crystal substrate. The communicating means is preferably formed as a slit extending in a direction normal to the gas flow direction. Accordingly, the gases passing the slit constitute no-turbulence laminated gas flows, thus forming a crystal on the crystal substrate having uniform composition and improved reproducibility. For a better understanding of this invention and to show how the same is carried out, reference is made to, by way of preferred embodiments, the accompanying drawings, in which: FIG. 1 is a front view, partially in section, of a compound semiconductor vapour phase epitaxial device according to this invention; FIG. 2 is a schematic perspective view, partially broken away, of the epitaxial device of FIG. 1; FIG. 3 is a sectional view taken along the line III-III in FIG. 1; FIG. 4 is a view showing a dispersion of a gas of III group; FIG. 5 is a schematic view showing positional relationship of various element constituting the epitaxial device of FIG. 1; FIG. 6 is a schematic front view of a modification having a plurality of slits utilized for the device of this invention; FIGS. 7 and 8 are schematic front views of further modifications in relation to FIG. 6; FIG. 9A is an enlarged perspective view showing slit in one preferred embodiment; FIG. 9B is a modification of FIG. 9A, in which the slit is replaced with a plurality of fine holes; FIG. 10 is a modification in which a taper is provided for a first flow channel; and FIG. 11 is a graph showing a positional dependence of the AlGaN alloy composition along the gas flow direction. A first embodiment of a compound semiconductor vapor phase epitaxial device according to this invention will be first described hereunder with reference to FIGS. 1 to 5. Referring to FIG. 1, a compound semiconductor vapor phase epitaxial device 1, called merely epitaxial device 1 hereinafter, comprises a cylindrical reactor vessel 2, first and second flow channel means 17 and 27 both accommodated in the reactor vessel 2, and first and second material gas supply pipes 15 and 25 respectively communicated with the first and second flow channel means 17 and 27. The both ends of the cylindrical reactor vessel 2 are closed by flanged portions 6 and 16 as end plates. These supply pipes 15 and 25 have one ends inserted into the reactor vessel 2 and communicated with the first and second flow channel means 17 and 27, respectively, and have other ends extending outward through the flanged portion 6, and the extended ends of the gas supply pipes 15 and 25 are connected to material gas supply sources 15a and 25a. A hydrogen supply pipe 35 is further disposed so as to extend into the reactor vessel 2 through the flanged end 6 so as to supply hydrogen gas to outer peripheral areas of the flow channel means 17 and 27. The hydrogen supply pipe 35 is connected to the hydrogen gas supply source 35a. To the other flanged end 16 is connected a discharge line 30. In a preferred and illustrated embodiment, two channel means are tightly secured at both longitudinal ends to the end plates 6 and 16 and supported thereby. The reactor vessel 2 is formed, for example, of quartz material, and an RF coil 3 for heating a susceptor 8 is wound around the outer peripheral portion of the reactor vessel 2. As shown in FIGS. 2 and 3, the first flow channel means 17 extends longitudinally in the reactor vessel 2 and has a rectangular cross section defined by four wall structures 17a, 17b, 17c and 17d. Referring to FIG. 1, the upper wall structure 17a forms a lower wall structure of the second flow channel means 27, and the upper wall structure 17a is provided with a slit 10 communicating the first and second flow channel means 17 and 27 so that first and second material gases respectively flowing in the first and second channel means 17 and 27 are mixed with each other. The slit 10 has a linearly elongate hole shape, as shown in FIG. 2 or 5, extending in a direction normal to the gas flow in the channel means 17 and 27 and has a length W1 larger than a width W2 of a crystal substrate 7, which is disposed on the lower wall structure 17b of the first channel means 17 at substantially the inner central portion thereof. The crystal substrate 7 is supported by a graphite susceptor 8 which is disposed on the wall structure 17b. As shown in FIG. 2 or 3, the wall structure 17b has a hollow structure so as to provide some thickness necessary for locating the susceptor 8 on which the crystal substrate 7 is supported. The hollow structure is made, in a sense, for the purpose of reducing weight and cost, but it may be composed of a solid structure having some thickness. In both cases, it will be desired that the surface of the crystal substrate 7 lies in substantially the same level as that of the lower wall structure 17b. The lower wall structure 17b has a ramp portion having an inclination for smoothly guiding the first material gas to the susceptor 8. The second channel means 27 is also defined by four wall structures 27a, 27b, 27c and 27d so as to provide a rectangular cross section and the second channel means 27 is disposed above the first flow channel means 17, in an installed or operating state, so that the wall structure 27b is commonly formed to the wall structure 17a of the first channel means 17. The front end, i.e. right-hand end as viewed in FIG. 1, of the second flow channel means 27 is closed so as to facilitate the gas flow into the first flow channel means 17 through the slit 10. The first and second flow channel means 17 and 27 have the same width W3 larger than the width W4 of the susceptor 8 as shown in FIG. 5. By setting the respective widths W1 to W4 as described above, the crystal can be grown stably with uniform composition in the width direction. An excessive amount of the first gas is supplied into the first flow channel means 17 and the second gas, which controls the growth rate of the crystal which is formed on the crystal substrate 7, is supplied into the second flow channel means 27 positioned adjacent, i.e. above in the installed state, to the first flow channel means 17. The gas supply amounts and the cross sectional areas of the first and second flow channel means 17 and 27 are determined so that the ratio of flow velocities, that is, flow velocity of the first gas with respect to flow velocity of the second gas, is ranged within 5 to 200, preferably 10 to 100. When this ratio exceeds a value of 200, the switching speed of the second gas is delayed, which results in loss of sharpness of a hetero boundary when a hetero structure is formed and in increased consumption of the first gas. On the contrary, when the ratio is below a value of 5, a pressure in the reactor vessel may adversely increase or a turbulent flow may be caused at a portion near the communication portions, thus being also disadvantageous. The velocity of the first gas flow is generally ranged within 50 to 200 cm/sec. In the case of the groups III-V element compound semiconductor, particularly in the case of GaN or AlGaN, NH 3 gas as the first material gas and H 2 gas as carrier gas are utilized as the first gas excessively supplied into the first flow channel means 17, and TMG (trimethylgallium) or a mixed gas of TMG and TMA (trimethylaluminium) as the second material gas and H 2 gas as carrier gas are utilized as the second gas supplied into the second flow channel means 27. On the other hand, in the case of the groups II-VI elements compound semiconductor, particularly in the case of ZnS or ZnSe, H 2 S or H 2 Se is preferably utilized as the first material gas supplied into the first flow channel means 17 and diethylzinc or dimethylzinc is preferably utilized as the second material gas supplied into the second flow channel means 27. The H 2 gas is supplied from the hydrogen supply source 35a into the reactor vessel 2 through the hydrogen supply pipe 35, and this H 2 gas performs a sealing function for preventing the material gases from leaking from the first and second flow channel means 17 and 27 or removes the leaking gases if they leak. The function and operation of the epitaxial device 1 according to this invention described above will be described hereunder with reference to an example of AlGaN compound semiconductor, in which the NH 3 being the material of nitrogen N of the group V elements is supplied into the first flow channel means 17 together with the hydrogen H 2 as carrier gas. The mixed gas composed of TMG and TMA being materials of the group III elements is also supplied into the second flow channel means 27 together with the hydrogen H 2 gas, and in such case, since it is desired that the NH 3 is added in large excess, the ratio of concentration will be naturally determined as (TMG + TMA concentration) << (NH 3 concentration), and the flow rate which is determined in consideration of the carrier flow amount and the cross sectional areas of the flow channel means is set to the range to make the gas flow rate V1 of the first gas sufficiently large. According to the conditions thus set or determined, the second gas flowing in the second channel means 27 can pass through the slit 10 formed to the wall section between the first and second channel means 17 and 27 as if it is sucked by the first material gas flowing at high velocity in the first channel means 17, thus two-phase flow can be obtained without causing a turbulent flow. Namely, as shown in FIG. 4, by the provision of the slit 10, a laminated gas flow of the material gas, i.e. a laminar flow rich in the group III elements material on the side of the slit 10 and a laminar flow rich in the group V elements material on the side of the crystal substrate 7 supported by the susceptor 8 is caused on the down stream, right-hand as viewed. The group III elements gas flow gradually diffuses into the group V elements gas flow and then reaches the reaction area of a temperature of more than about 1000 °C above the susceptor 8. The material is then decomposed and a crystal is hence grown on the crystal substrate 7. As described, according to the epitaxial device 1 of this invention, laminated gas flow of two or more than two material gases can be surely formed by suitably utilizing in combination various conditions such as concentrations of the material gases, gas flow rate, etc.. Alternations or modifications of the epitaxial device according to this invention will be described hereunder with reference to FIGS. 6 to 10. FIG. 6 shows a modification in which a plurality of slits 10a are formed, in place of the single slit 10 of FIG. 4, to the wall structure defining the first and second flow channel means 17 and 27, and in the illustrated example, four slits 10a are formed so as to extend in a direction normal to the flow direction of the gases. The number of the slits 10a may be determined in consideration of the size of the crystal substrate 7, the working accuracy of the slits 10a and the like. FIG. 7 shows an alternative construction in which the downstream end side of the second flow channel means 27 is opened, and in this case, a portion of the material gas flowing in the second flow channel means 27 flows downward and into the first flow channel means 17 through the slit 10, thus forming the two-phase flow without causing any turbulent flow. FIG. 8 shows another modification of this invention, in which the second flow channel means is divided further into two flow channel means 37 and 47, to which slits 30 and 40, each of the character like the slit 10, are formed, respectively so as to communicate with the first flow channel means 17. In this case, when it is desired to prepare a III-V group AlGaN compound semiconductor, the III group TMG and TMA can be separately fed to the reaction areas through the channel means 37 and 47, respectively. FIG. 9B is further modification of the first embodiment, in which the slit 10 of the first embodiment (FIG. 9A) is replaced with a plurality of fine holes P extending linearly in a direction normal to the flow direction of the gas. FIG. 10 shows a modification of the first embodiment, in which the first flow channel means 57 has a lower, in an installed state or illustrated state, wall structure which is tapered so as to incline upwardly towards the downstream side of the gas flow to thereby narrow the flow passage of the first channel means. According to this alternate embodiment, the composition of the compound semiconductor can be preferably adjusted uniformly. An actual experiment utilizing the compound semiconductor vapor phase epitaxial device according to this invention shown in FIG. 1 was performed in the following manner. The crystal substrate was first set to a predetermined position in the reactor vessel of the epitaxial device, and thereafter, air in the reactor vessel was vacuum exhausted by means of a rotary vacuum pump, for example, through the discharge line 30 connected to the downstream side of the reactor vessel. In the next step, the NH 3 material gas and the H 2 carrier gas were supplied into the first flow channel means 17 and the TMG and TMA material gases and the H 2 carrier gas were supplied into the second flow channel means 27. In such occasion, the average flow rate of the gas (NH 3 + H 2 ) passing the first channel means 17 was set to 110 cm/sec. and the average flow rate of the gas (TMG + TMA + H 2 ) was set to 5.0 cm/sec.. The temperature of the crystal substrate of 1050 °C was maintained by heating the susceptor 8 by means of the RF coil 3 for one hour to thereby epitaxially grow an AlGaN series compound semiconductor. The positional dependence of the AlGaN alloy composition along the gas flow direction was measured. The measured result is shown in FIG. 11, in which the solid line represents the distribution of the experiment according to this invention. For the sake of comparison, the distribution of a mixed crystal ratio according to a prior art device in which no flow channel means is utilized, so called a normal pressure type MOCVD device, is also shown in FIG. 11 with a dotted line. As can be easily seen from the result of FIG. 11, according to the epitaxial device of this invention, the composition of the film-formed crystal formed on the crystal substrate can be made substantially uniform and the reproducibility can be also improved.

Claims: A compound semiconductor vapour phase epitaxial device, the device comprising: a cylindrical reactor vessel (2); a plurality of flow channel means (17,27; 17, 37, 47) disposed in the reactor vessel and each having a substantially rectangular cross section; and, a plurality of gas supply means for respectively supplying gas containing an element or elements of a compound to be grown; characterised by: a susceptor (8) mounted on one of partition walls (17a,17b,17c,17d) of one of the flow channel means for supporting a crystal substrate (7); and by: an aperture (10; 10a,10a,10a,10a; P) in a partition wall opposite the wall on which the susceptor is mounted for providing gas communication between adjacent flow channel means so as to form a laminar flow consisting of two or more gases at an upstream portion in the flow channel means provided with the susceptor. An epitaxial device according to claim 1, wherein said susceptor (8) is disposed on an lower wall (17b) of the flow channel and said aperture (10; 10a,10a,10a,10a; P) is in an upper wall (17a) of the flow channel. An epitaxial device according to claim 2, wherein the lower wall (17b) of the flow channel means on which said susceptor is disposed is tapered so as to narrow the cross-sectional area of the flow channel towards a downstream side thereof. An epitaxial device according to any of claims 1 to 3, wherein the aperture is composed of at least one slit (10; 10a,10a,10a,10a) extending in a direction normal to a gas flow direction in the flow channel means. An epitaxial device according to claim 4, wherein said aperture is composed of a plurality of slits (10a,10a,10a,10a). An epitaxial device according to any of claims 1 to 3, wherein said aperture is composed of a plurality of fine holes (P) which are arranged in a direction normal to a gas flow direction in the flow channel means. An epitaxial device according to claim 1, wherein said flow channel means is composed of a first flow channel (17) and a second flow channel (27), said first and second flow channels being provided with a common wall structure through which both flow channels are connected, the aperture (10; 10a,10a,10a,10a:P) for communicating the flow channel means with each other being formed in said common well structure. An epitaxial device according to claim 7, wherein said second flow channel (27) comprises a plurality of flow channel sections (37,47), said flow channel sections having end portions on the downstream side which are closed, and comprising means (30,40) formed in said common wall structure for communicating the second flow channel sections and the first flow channel section with each other. An epitaxial device according to claim 7 or 8, wherein said communication means is composed of at least one slit (30,40) extending in a direction normal to a gas flow direction in the flow channel means. An epitaxial device according to claim 7 or 8, wherein said communication means is composed of a plurality of fine holes (P) which are arranged in a direction normal to a gas flow direction in the flow channel means. An epitaxial device according to any of claims 1 to 10, including a coil means (3) disposed around an outer peripheral surface of the reactor vessel (2) for heating the susceptor (8). A method of using an epitaxial device according to any of claims 1 to 11, characterised by the steps of supplying an excessive amount of a first gas containing a first material from a first gas supply means to a first flow channel in which the susceptor is disposed and supplying a second gas, for controlling a growing speed of the crystal grown on a crystal substrate supported by the susceptor, from a second gas supply means to a second flow channel. A method according to claim 12, wherein a flow velocity ratio (V1/V2) of a first gas flow velocity (V1) in the first flow channel (17) with respect to a second gas flow velocity (V2) in the second flow channel (27) is in a range of 5 to 200. A method according to claim 13, wherein said flow velocity ratio (V1/V2) is in a range of 10 to 100. A method according to any of claims 12 to 14, wherein the first gas contains ammonium gas as the first material and the second gas for controlling the crystal growing speed contains trimethylgallium gas or a mixture of trimethylgallium gas and trimethylaluminium gas.

Inventors: AKASAKI ISAMU, AMANO HIROSHI NIJIGAOKA-HIGASH, HIRAMATSU KAZUMASA, WATANABE ATSUSHI, AKASAKI, ISAMU, AMANO, HIROSHI, HIRAMATSU, KAZUMASA, WATANABE, ATSUSHI, AMANO, HIROSHI, NIJIGAOKA-HIGASHIDANCHI, Watanabe, Atsushi, c/o Pioneer Electronic Corp.
Applicants: AKASAKI ISAMU, AMANO HIROSHI, PIONEER ELECTRONIC CORP, AKASAKI, ISAMU, AMANO, HIROSHI, PIONEER ELECTRONIC CORPORATION
Application year: 1993
Main classification: C30B25
IPC codes: C30B25, C23C16, H01L21