PROCESS FOR FORMING THIN FILM OF COMPOUND SEMICONDUCTOR

Abstract: A process for forming a thin film of a compound semi­ conductor, which comprises forming a group III-V compound semiconductor by feeding a group III element and a group V element, each in an atomic or molecular state, onto a sub­ strate placed in a vacuum chamber and heated, with the feed of the group V element being reduced to such a minute amount in at least a certain period of the growth of the group III-V compound that the group III-V compound semiconductor does not grow during which period the group III element is fed onto the substrate. This process enables a highly pure group III-V compound epitaxial layer having less crystal defect and smooth development surface and heterojunction interface on an atomic level to grow over a wide range of temperatures. This process is effective in preparing various optical devices, super-high speed electronic devices, etc.

Description: TECHNICAL FIELD The present invention relates to a method for forming thin III-V compound semiconductor films and more particularly a method for forming not only thin III-V compound films in which each atomic layer has a flat grown surface but also multilayer thin film structures in which each atomic layer has a flat and a heterojunction interface at which elements of compounds are sharply distributed. BACKGROUND ART Compound semiconductors have been noted as the most suitable semiconductors for obtaining the semiconductor devices which can satisfy the recent demands for super high-speed operation and high performance. In some compound semiconductors, semiconductor mixed crystal can be synthesized while fundamentally maintaining zinc-blende structure, and it is able to obtain heterojunctions at which lattice matching between various dissimilar semiconductor crystal is possible. Therefore, by utilizing such heterojunctions, devices such as ultra-high-speed electronic devices utilizing modulated doping, various optical devices whose typical examples are laser diodes and novel electronic devices utilizing the superlattice structure are developed and some of them have been already used in practice. Furthermore, novel devices utilizing the new physical properties of such heterojunction structures or superlattice structures and especially compound semiconductor devices typically represented by lasers and HEMT (high electron mobility transistor) have been recently extensively studied at a rapid speed. Such elements comprise in general a substrate and a thin film which is formed on the substrate and has a complicated structure consisting of many layers of dissimilar compounds. And it is well known to those skilled in the art that their performance is greatly influenced by the structure of a thin film, especially the sharp distribution of composition or the sharp distribution of doping concentration at the interface between the adjacent layers. It follows, therefore, that the most fundamental technique for fabricating the heterojunction structures or the superlattice structures is to grow the layers each in a desired thickness of different compounds in such a way that the interface between the adjacent layers can be defined as sharply as possible, without causing any crystal defect. One process for growing a thin III-V compound semiconductor film over the surface of a substrate is molecular beam epitaxy (MBE). In the case of growing a III-V compound crystal by the conventional MBE method, at least one element of Group III and at least one element of Group V of the periodic table are simultaneously supplied on the surface of a substrate which is disposed in a vacuum chamber and is heated. (Change, L.L. et al.; J.Vac. Sci, Technol., Vol. 10, P. 11, 1973). In this case, in order to prevent the escape of the Group V element having a high vapor pressure from the grown layer, a large amount of the Group V element is normally supplied on the surface of the growing layer. As a result, the Group III element immediately combines with the Group V element on the growing surface so that III-V molecules are formed. In order to obtain a flat grown surface in each atomic layer, the molecules thus obtained must be sufficiently diffused over the grown surface. However, the diffusion coefficient of such molecules is extremely low as reported by Neave, J.H. et al. (Appl. Phys. Lett. Vol. 47, P. 100, 1985), so that small hills and small valleys which are different in height by a few atomic layers always exit on the grown surface as shown in Figs. 1A-1C. In Fig. 1, reference numeral 1 represents a substrate; 2, Ga atoms; and 3, As atoms. The Ga-As molecules thus formed on the substrate 1 are gradually increased in number in the order of steps as shown Figs. 1A, 1B and 1C, but as shown in Fig. 1B, the second Ga-As molecular layer is formed before the first Ga-As molecular layer has not completely covered the surface of the substrate 1. Since the mobility of Ga-As molecules is low, the second and third Ga-As layers are formed before the second Ga-As molecular layer is displaced to fill the openings of the first Ga-As layer, so that part of the second and third Ga-As molecular layers tends to be directly adsorbed in the openings of the first Ga-As layer and consequently small hills and valleys are formed as shown in Fig. 1C. The results of the recent studies on MBE show that when a RHEED (reflection high energy electron diffraction) pattern is observed in the case of growing a thin film, the intensity of light of the diffraction pattern varies periodically in time and that a period is equal to a length of time required for growing one Ga-As layer. Fig. 2 shows the mode of oscillation and the intensity of light of the diffraction pattern is plotted along the ordinate while the time, along the abscissa. It is readily understood that the oscillations of the intensity of light are damped in time. In the conventional MBE method, prior to the growth of a layer, As molecules are supplied over the surface of the substrate and as a result of the heat treatment for a long period of time, the surface of the substrate becomes relatively flat in terms of an atomic layer. The intensity of RHEED before the growth of a film; that is, t≦0 represents the flatness of the surface of the substrate. When the supply of Ga atoms is started in addition to that of As molecules (that is, t>0), the intensity of RHEED is rapidly reduced and then becomes a minimum level. The intensity of RHEED reflects the interference effect due to the surface roughness and the fact that the the absolute value of the intensity of RHEED drops means that the ratio of the flat areas of the upper layers to those of the adjacent lower layers formed by respective steps becomes smaller. On the other hand, the reduction in amplitude of oscillations means the increase in number of steps so that only white noise signals are obtained. Therefore, the minimum level of RHEED means that one half of a single molecular layer has been grown and that the upper and lower surface areas of a step becomes substantially equal to each other. Thereafter, as the layer is grown further, the intensity of RHEED becomes a maximum level again. This means that just one molecular layer has been grown in a microscopic view. It is very important to note the face that the maximum value of each period is considerably lower than that of the preceding period. The reason is that since the mobility of the Ga-As molecules on the grown surface is not sufficient as described above with reference to Figs. 1A-1C, a flat atomic surface cannot be reproduced after the growth of one molecular layer and steps of heights of one to a few atomic layers remain from point to point. The above-described tendency is further intensified as the growth of the film is increased, so that the oscillations of the intensity of RHEED disappear after the growth of tens molecular layers as shown in Fig. 2. The reason is that many steps are formed on the grown surface as shown in Fig. 1C. In the case of growing the thin film layers by the conventional MBE method, the grown surface is rough, that is, has little hills and valleys. As a result, when a dissimilar semiconductor layers are grown over the grown surface so as to form a heterojunction interface, it cannot be avoided that the roughness of the interface of the heterojunction is intensified. Therefore, there has been proposed a method for accelerating the surface diffusion of molecules by interrupting the growth of film and maintaining the grown layer at a high temperature (Sakaki, H. et al.; Japan. J. Appl. Phys. Vol. 24, pg L. 417, 1985). However, since the diffusion rate of the molecules is low inherently, the above-described method can improve the surface roughness to a small extent. Referring to Figs. 3A-3C, the method for smoothing the surface of the grown layer by maintaining the surface at a high temperature according to the conventional process will be described in conjunction with the formation of the heterojunction by forming an AlAs layer over a GaAs layer. After a GaAs layer 4 has been grown to a predetermined thickness (Fig. 3A), the surface of the GaAs layer 4 is smoothed by thermally displacing "lands" of the thickness of one to three atomic layers on the surface of the GaAs layer 4 (Fig. 3B) and then the AlAs layer 5 is grown over the surface of the GaAs layer 4 (Fig. 3C). However, according to the above-described process, the interface 6 of the heterojunction can be made flat within the range between a few µm to 100µm, but in macroscopic view, the surface smoothness is increased as shown in Fig. 3C. Because of the roughness of the interface of the heterojunction, the devices such as quantum well semiconductor lasers and superlattice optical modulators which are constructed by various combinations of heterojunctions defined between the thin films are increased in breadth of spectrum and degraded in performance to a considerable extent. In order to improve the conventional MBE method, Japanese Patent Application Laid-Open No. 60-112692 discloses a method in which the Ga molecular beam and the As molecular beam are alternately irradiated on a GaAs substrate. However, according to this method between the As molecular beam irradiation period and the Ga molecular beam irradiation period, there is a period of time in which no molecule is irradiated, so that there is a danger that the As atoms escape from the grown crystal. Furthermore, it has a defect that it takes a long period of time to grow a crystal. Furthermore, the adverse effects caused by a low diffusion rate are further pronounced as the crystal growing temperature is lowered. For instance, it is impossible to obtain a high-quality epitaxially grown crystal of GaAs or InP at temperatures lower than 500°C and it is also impossible to obtain a high-quality AlAs epitaxial crystal at a growing temperature lower than 600°C. Moreover, it becomes almost impossible to grow a GaAs or InP crystal at a temperature lower than 400°C and it also becomes almost impossible to grow an AlAs crystal at a temperature lower than 450°C. As a result, the crystal growth is carried out at a high temperature so that the diffusion of impurities results during the crystal growth. As a result, the distribution of impurities cannot be sharply defined accordingly the characteristics of lasers, transistors and the like are limited to levels remarkably lower than their theoretical characteristics. Another process for growing a thin III-V compound semiconductor film over the surface of a substrate is a metallo-organic chemical vapor deposition process (MOCVD). The same Inventors improved MOCVD and proposed a method in which while a very small quantity of a hydride of an element of Group V is normally made to flow over the surface of a heated substrate together with a carrier gas, a metallo-organic compound containing an element of Group III and a hydride of an element of Group V which is diluted to a high concentration are alternately made to follow over the surface of the substrate together with the carrier gas, whereby a III-V compound is grown over the surface of the substrate (Inst. Phys. Conf. Ser. No. 79; Chapter 13, pp 737-738, 1985). However, according to this method, the growth of a compound semiconductor is carried out by the thermal decomposition of the metallo-organic compound containing a Group III element and the hydride of a Group V element, so that it is impossible to lower the crystal growth temperature. In addition, the efficiency of raw materials or compounds to contribute to the growth of the compound semiconductor is low, and expensive and toxic gases must be used. Moreover, there arises the problem that impurities from the raw compounds are diffused into the grown crystal. EP-A-0 215 436 discloses a method of growing a thin film layer for use in a composite semiconductor. The method includes the step of removing impurity molecules on a substrate or lower layer of the composite semiconductor by projecting a Group III molecular beam, or Group V molecular beam onto a substrate, or a lower layer of the composite semiconductor, before growing the epitaxial layer. It also includes the step of growing a thin film layer of Group III and Group V atom composite semiconductor on the substrate, or the lower layer of the composite semiconductor. The "Extended Abstracts of the 17th Conference on Solid State Devices and Materials, 1985, Pages 221-224" disclose a process in which single-crystalline films of ZnSe and ZnTe are grown by ALE. Initial and successive stages of epitaxy employ electron diffraction techniques. In "Applied Physics Letters, Vol. 29, No. 6, 15th September 1976, Pages 323-325" epitaxial structures are disclosed which are grown by alternate monolayer depositions of GaAs and AlAs. US-A-4 058 430 discloses a molecular beam deposition technique in which a gaseous source is used to form a molecular beam comprising M₂ or M₄ molecules, where M is a group V element. As₂ and P₂ molecular beams are used for molecular beam epitaxy of Group III-V semiconductors, such as GaAs and InP. In "Japan. Applied Physics, 23, (1984), Pages L 657-659" there is disclosed a phase-locked epitaxy process similar to that disclosed in the Applied Physics Letters reference mentioned above. DISCLOSURE OF THE INVENTION In view of the above, the primary purpose of the present invention is to provide a method for the epitaxial growth of semiconductors which can substantially overcome the problems encountered in the conventional method for forming thin III-V compound semiconductor films; that is, (1) the grown surface and the interface of the heterojunction have extremely small hills and valleys which are different in height by one or few atomic layers; (2) the epitaxial growth must be carried out at high temperatures; (3) it requires a long period of time to grow a desired crystal and (4) the impurity atoms tend to be trapped in the grown crystal, and which can grow at a higher rate thin III-V compound semiconductor films having high crystal qualities and having flat grown surfaces and flat interfaces of heterojunctions over a substrate maintained at low temperature. In order to achieve the above object, the present invention provides a method of forming a thin compound semiconductor film with a high growth rate on the surface of a substrate according to Claim 1 In one embodiment of the invention, said atoms or molecules of said Group V element are supplied both (a) constantly in said first amount, and (b) periodically in an additional amount so that the sum amount of said Group V element supplied is sufficient to react with said Group III element present on the substrate surface to form said layer of said Group V element on the surface of said substrate. The atoms or molecules of said Group III element are supplied when said atoms or molecules of Group V element are supplied in said first amount, so to form a layer of said Group III element on said layer of said Group V element, at least at one stage of the growth of said thin compound semiconductor film, thereby forming said Group III-V compound on the surface of said substrate. In the latter embodiment, the Group III element is supplied periodically, the Group III element being supplied when the Group V element is supplied in said first amount. According to a second embodiment, the atoms or molecule of the Group III element are supplied constantly to the substrate surface. In the first mentioned embodiment, the supply amount of the atoms or molecules of the Group III element, during at least one period, may be 90-110% of the amount required for forming one atomic plane consisting of the Group III element on the substrate surface. The first amount, of the supply of the atoms or molecules of the Group V element, may be such that the partial pressure of the Group V element is within a range of from the equilibrium dissociation vapor pressure of the Group V element on the growing surface to a pressure equal to 10 times the equilibrium dissociation vapor pressure. The second amount of the atoms or molecules of the Group V element may be greater than the number of atoms required for forming one atomic plane consisting of the Group V element and less than five times the number of atoms. Si may be supplied as a doping atom synchronously with the atoms or molecules of the Group III element. Polyatomic molecules of the Group V element may be thermally cracked into atoms or polyatomic molecules, with a reduced number of atoms, by causing the polyatomic molecules to bombard a heated metal plate and then to be supplied to the surface of the substrate. A hydride of the Group V element may be thermally cracked into atoms or diatomic molecules by passing the hydride through a heated metal pipe, the atoms or molecules of the Group V element then supplied to the substrate surface. Molecular hydrogen may supplied to the surface of the substrate between the periods of supply of the atoms or molecules of the Group III element. The molecular hydrogen may be passed through a heated palladium film to produce atomic hydrogen before it reaches the substrate surface. The amount of the supply of hydrogen may be 50-100 times as much as the number of atoms of one atomic plane of the Group III element. The amount of supply of the atoms or molecules of the Group III element, at one period, may be 90-200% of the amount required for forming one atomic plane consisting of the Group III element. The substrate may be maintained at a temperature between 100 and 700°C. Embodiments of the invention will now be described with reference to the accompanying Drawings, in which:- Figs. 1A-1C are schematic views for explaining the growth of a crystal by conventional MBE; Fig. 2 shows the variation of the intensity of RHEED in the process of growing a GaAs crystal by the conventional method; Figs. 3A-3C are schematic views for explaining the process of flattening the interface of a heterojunction by the conventional method; Figs. 4 is a schematic view of an apparatus according to an embodiment of the invention; Figs. 5A-5C are time charts illustrating a continuous supply of As molecules and periodic alternate supplies of Ga atoms and As molecules according to one embodiment of the invention. Fig. 6 shows the variation in intensity of RHEED when GaAs layers are grown in accordance with the present invention; Figs. 7A-7C are schematic views used to explain the formation of an atomic layer by the migration of Ga atoms in accordance with the present invention; Fig. 8A is a time chart illustrating the timing of the supplies of Ga atoms and As molecules in another embodiment of the present invention, Figs. 8B and 8C are diagrams illustrating the recovery of the intensity of RHEED in the process of crystal growth; Fig. 9 is a diagram illustrating the intensity of RHEED of a GaAs layer grown over the surface of a substrate maintained at 100°C in accordance with the present invention; Fig. 10 is a photoluminescence spectral diagram at 4.2K of a GaAs layer grown on the surface of a substrate maintained at 200°C; Figs. 11A and 11B show a structure of an AlAs/GaAs quantum well grown over the surface of a substrate maintained at a temperature of 300°C and a photoluminescence spectrum thereof at 4.2K, respectively; Figs. 12A and 12B are time charts illustrating the timing of the supplies of As molecules and Ga atoms in a further embodiment of the present invention, respectively; Fig. 13 is a view illustrating a quantum well structure; Fig. 14 shows the photoluminesence spectra of the quantum well structures, respectively, constructed by the present invention and by the prior art method; Figs. 15A and 15B are time charts illustrating the timing of the supplies of Ga atoms and As molecules to a substrate in a yet further embodiment of the present invention, respectively; Fig. 16 shows the variation in intensity of RHEED of a thin film grown in accordance with the time charts shown in Figs. 15A and 15B; Fig. 17 shows the variation in time of the intensity of RHEED when the quantity of Ga atoms supplied is made equal to the coordination number of the surface being grown; Figs. 18A-18E and Figs. 19A-19E are schematic views used to explain the self-flattening phenomenon attained by the present invention; Figs. 20A and 20B are time charts illustrating the timing of the supplies of Ga atoms and As molecules in a further embodiment of the present invention, respectively; Fig. 21 shows the variation in time of the intensity of RHEED of a GaAs layer grown by the method of the present invention; Figs. 22A and 22B show the time charts illustrating the timing of the supplies of Ga atoms and As molecules and the variation in time of the intensity of RHEED in a further embodiment of the present invention; Figs. 23A and 23B show the time charts illustrating the timing of the supplies of Ga atoms and As molecules and the variation in time of the intensity of RHEED in a yet further embodiment of the present invention, respcetively; Fig. 24 shows the structure of the quantum well laser fabricated in accordance with the present invention; Figs. 25A and 25B show the structure of another quantum well constructed in accordance with the present invention and the variation in time of the intensity of RHEED of each grown layer, respectively; Figs. 26A and 26B show a two-dimensional electronic structure device fabricated in accordance with the present invention and the variation in time of the intensity of RHEED of each grown layer, respectively; Fig. 27 is a schematic sectional view of a cracker cell; Fig. 28 is a top view of a shutter; Fig. 29 shows the variation in time of the intensity of RHEED of a grown GaAs crystal; Figs. 30A and 30B are schematic views used to explain the effects obtained by the irradiation of hydrogen; Figs. 31A and 31B show the variations in time of the intensities of RHEED of GaAs crystals grown by increasing the quantity of Ga atom supply and grown by supplying the hydrogen to the surface of Ga atoms supplied excessively, respectively; Figs. 32A, 32B, 32C and 32D are timing charts illustrating the timing of the normally supplies As and the timing of the supplies of Ga, As and H which are intermittently supplied in a predetermined order, respcetively; and Fig. 33 is a schematic view used to explain a method for supplying the hydrogen atoms to a substrate. BEST MODES FOR CARRYING OUT THE INVENTION Now, the present invention will be described in detail with reference to the accompanying drawings. Fig. 4 is a schematic view of an apparatus adapted to carry out the present invention. Reference numeral 11 represents a vacuum chamber which is evacuated to a super high vacuum state of less than 1.33 x 10⁻⁸ Pa (10⁻¹⁰ Torr) by a diffusion pump, a cryogenic pump, an ion pump or the like; 13, a substrate upon which is grown a thin compound semiconductor film; 14, a substrate holder incorporating therein a heater; 15A-15D, molecular beam sources; and 16A-16D, shutters adapted to turn on or off the molecular beams and to control the magnitudes of the molecular beams. For the sake of simplicity, only four molecular beam sources 15A-15D are shown in Fig. 4, but the apparatus can be provided eight molecular beam sources. Depending upon the kinds of thin compound semiconductor films to be grown, from the molecular beam sources the beams of elements of Group III and V are irradiated on the substrate. The quantity of each element supplied to or deposited on the substrate, the timing for opening or closing the shutters and the temperature of the substrate are all controlled by a controller 17 such as a microcomputer. During the growth of a compound semiconductor, the accelerated electron beam emitted from an electron gun 18 is made to irradiate at an angle of incidence of 2-3° on the surface of the grown film and an reflection electron diffraction pattern is focused on a fluorecent screen 19, picked up by a television camera 20 and then displayed on a cathode-ray tube 21, whereby the ongoing growth of the material can be monitored. Furthermore, the intensity of light of the image displayed on the cathode-ray tube is detected by a photoelectric transducer 22 such a solar cell and the output signal is applied to the controller 17 so as to analyze the data. Reference numeral 23 denotes a shutter controller which controls the opening or closing of the shutters in response to the control signal derived from the controller 17; and 24, a thermostat or a temperature controller for controlling the temperature of the heater incorporated in the substrate holder 14 as instructed by controller 17. The apparatus is further equipped with a mass spectrometer (not shown) which measures the mass of a material of a beam irradiated from each molecular beam source. The apparatus described above with reference to Fig. 4 was used to grow a GaAs crystal over the (100) surface of a GaAs single crystal substrate. The super-high-vacuum chamber into which were disposed material elements and a substrate was evacuated at vacuum ranging from 1.33 x 10⁻⁴ to 1.33 x 10⁻⁹ Pa (10⁻⁶ to 10⁻¹¹ Torr). The substrate was heated to 580°C and metallic Ga and metallic As were heated to form the As and Ga beams which in turn were supplied on the substrate. The supply of each element was carried out according to the time charts as shown in Figs. 5A-5C. More particularly, while the As₄ molecular beam was normally made to irradiate the surface of the substrate at a rate of about 1 x 10¹³ atoms/cm²·sec, the Ga atom beam of 6.4 x 10¹⁴/cm²·sec and the As molecular beam of 4 x 10¹⁴ atoms/cm²·sec were alternately irradiated for one second on the substrate. The quantity of the As molecular beam normally irradiated on the substrate is too small to react with Ga to form GaAs, but is sufficient to prevent the escape of As atoms from the growing crystal. Therefore, the quantity of the normally irradiated As₄ molecular beam must be higher than the equilibrium dissociation pressure of As in the growing surface and must be less than 10 times the equilibrium dissociation pressure. For instance, in the case of GaAs (at 600°C), the quantity of the As₄ molecular beam must be from 0.5 x 10¹³ atoms/cm²·sec to 1 x 10¹⁴ atoms/cm²·sec. The quantity of Ga atoms supplied to the substrate shown in Fig. 5B is so selected that one atomic layer is formed by one irradiation of the Ga beam or is equal to the coordination number of the atoms on the growing surface. Furthermore, the quantity of the As atoms supplied to combine with Ga to form GaAs is about 4 times the quantity required for forming one atomic layer. As is irradiated on the substrate in the form of a molecule having four atoms. Therefore, the supply is expressed in terms of the number of As molecules. The quantity of supply of each element is the product of the intensity of the beam and the irradiation time and the beam intensity can be controlled by adjusting the heating temperature of the element source. The quantity of supply of Ga required for forming one atomic layer was determined by the period of oscillations of the intensity of RHEED during the conventional MBE process. While the GaAs crystal was being grown in the manner described above, the electron beam accelerated to about 10 KeV was irradiated on the substrate to measure the intensity of RHEED. Fig. 6 shows the variation in time of the intensity of RHEED thus measured. As is seen from Fig. 6, the intensity of RHEED decreases as soon as the Supply of Ga is started and becomes minimum when a Ga layer is formed, but it increases as the supply of As is started and becomes maximum when an As layer is formed. That is, the intensity of RHEED oscillates in correspondence with the period of supply of the source elements. In this embodiment, unlike the variation obtained from the conventional process as shown in Fig. 2, the oscillations of the intensity of RHEED still continue without being damped even after several thousand layers have been formed. This means that the flatness of each atomic layer of the growing surface is not degraded at all during the growth of layers. The same inventors found the fact that when only atoms of Group III of the periodic table instead of III-V molecules are supplied on the growing surface; that is, in the case of the growth of GaAs, when only Ga atoms are supplied, the mobility of Ga atoms on the surface is 100 times or more as high as the mobility of the GaAs molecules. Because of this phenomenon, the Ga atoms supplied to the growing surface form a flat atomic layer within an extremely short period of time. Atoms of Group V (As in the case of GaAs) are supplied after the supply of atoms of Group III has been completed and are adsorbed by the flat Group III atomic surface, whereby the III-V molecules are formed. The above-described process is completed within a very short period of time when the atoms of Group V are in the form of atoms or a simple molecule. The process for growing a flat atomic surface or plane is shown in Figs. 7A-7C. Since the mobility of Ga atoms 2 is higher, when a predetermined quantity is supplied to the substrate 1 during the steps shown in Figs. 7A and 7B, they rapidly cover the substrate. Furthermore, since the metallic bond in Ga-Ga is weaker than the bond in Ga-As, the Ga atoms on the Ga atoms are attracted by the As surface and completely cover the substrate, thereby forming the Ga atomic surface. Next, when the As atoms 3 are supplied in sufficient quantities to combine with Ga to form GaAs, a flat GaAs layer is grown as shown in Fig. 7C. In practice, these steps are periodically repeated to grow the layers. The surface mobility of the Ga atoms is very higher even at low temperatures, so that it is possible to grow a thin film at remarkably low temperatures. It is considered that the fast migration of Ga, which is an element of Group III of the periodic table, is due to the fact that the Ga atoms supplied to the growing surface have a bond similar to the metallic bond. The migration of In and Al are also fast because their electronic state or configuration is similar to that of Ga. As a result, the growth of one atomic layer can be accomplished at a high rate. In addition, according to the present invention, a very small quantity of an element such as As, which is an element of Group V, is normally supplied to the substrate so that the evaporation of the Group V element from the grown thin film can be prevented during the growth of a crystal. Therefore, a high quality thin film can be obtained. As described above, the method in accordance with the present invention is considerably improved as compared with the prior art methods, therefore a crystal in which the flatness of the surface of each atomic layer can be grown according to the present invention. Then same inventors also made extensive studies and experiments in order to investigate the quantity of supply of Ga required for growing one atomic layer. That is, the amounts of Ga atoms supplied to the substrate were varied over a wide range while irradiating an extremely small quantity of As on the substrate, and after a predetermined time interval the supply of the As molecules for producing a compound was resumed. In the above-described process, the variations in the intensity of RHEED were measured. The results are shown in Figs. 8A-8C. Fig. 8A shows the time charts of the As and Ga beam intensities in order to produce a compound and the supply of Ga was varied by changing the beam intensity or the beam irradiation time interval τ. Fig. 8B shows the variations in the intensity of RHEED in response to the irradiation of the Ga and As beams. Fig. 8C shows the state of recovery of the intensity of RHEED in response to the quantity of the Ga atoms supplied. In Fig. 8C, the quantities of supply of the Ga atoms are normalized in such a way that the quantity of the Ga atoms required for growing one atomic layer is represented by unity (l). This quantity is 6.4 x 10¹⁴/cm² when the Ga layer is grown on the (100) surface of the GaAs substrate. The recovery is expressed in terms of the ratio b/a, where "a" is the difference between the initial value of the intensity of RHEED and the intensity of RHEED when the Ga surface is grown and "b" is a value of rapid recovery immediately after the supply of As, and in terms of (a-c)/a which in turn represents a degree of recovery 10 seconds after the supply of As. In response to the supply of Ga, the reflectivity of the surface drops, so that the intensity of RHEED beam is rapidly degraded or decreased. When the supply of the As molecules is resumed after a predetermined time interval, the intensity of RHEED is recovered and it is seen that the recovery rate is greatly dependent upon the quantity of Ga supplied. When the quantity of Ga supplied is just sufficient to grow one atomic layer, the recovery rate becomes fastest. However, the range of the optimum quantity is not limited to an extremely narrow range and, as shown in Fig. 8C, when the quantity of Ga supplied is between 90 and 110% of the quantity required for growing one atomic layer, the recovery rate is high and there arises no problem in the formation of a compound. The above-described results were obtained when the substrate was maintained at 580°C, but the similar tendency was observed even when the temperature of the substrate was varied. When the quantity of Ga to be supplied is determined within the above-described range and when Ga and As are alternately supplied to the substrate, the amplitudes of oscillation of RHEED become extremely great and it was found that such oscillations continue as long as the growth of a thin film is continued. The quantity of Ga supply will be described in more detail below. The time interval from the interruption of Ga supply to the start of As supply shown in Fig. 8A is completely independent of the formation of a compound and the flatness of the Ga surface. That is, even when this time interval is zero, the characteristics as shown in Fig. 8C remain unchanged. This means that the mobility of the Ga atoms is high. As to the quantity of supply of As, when the quantity is one to 50 times the quantity required for growing one As atomic layer, the flat As surface can be formed. Since the vapor pressure of As is high, excess As atoms which do not contribute to the combination between Ga and As are vaporized out of the surface of the substrate therefore the flat As surface can be formed. Fig. 9 shows the variations in the intensity of RHEED when the GaAs crystal was formed on the GaAs substrate maintained at 100°C by normally irradiating the As₄ beam of 1 x 10¹³ molecules/cm²·sec and by alternately irradiating the Ga atomic beam of 3 x 10¹⁴ atoms/cm²·sec for 2.2 seconds and the As₄ molecular beam of 6 x 10¹⁴/cm²·sec for four seconds on the substrate. In this case, the intensity of RHEED becomes maximum when Ga is supplied, but becomes minimum when As is supplied. Therefore, the oscillations of the intensity of RHEED correspond to the source element supply period. These oscillations continued even after the growth of thousand periods (a few microns in thickness of a film). It is, therefore, considered that the flatness of the atomic surface can be maintained even through the temperature of the substrate is maintained at a surprising low temperature of 100°C because only Ga atoms are supplied. It is possible to grow Al x Ga 1-x As by the process as shown in Figs. 5A-5C. In this case, Al was supplied simultaneously with the supply of Ga. The Ga molecular beam cell and the Al molecular beam cell must be maintained at temperatures so that their beam ratio is maintained at a predetermined value x. When the temperature of the Ga molecular beam cell was 970°C and the temperature of the Al molecular beam cell was 1030°C, the ratio x=0.35 was obtained. The shutter opening time interval for the Ga and Al molecular beam cells was so controlled that the total number of Al and Ga atoms become equal to the quantity just sufficient to grow one atomic layer. With the above-described cells, the opening time interval was 0.5 sec. The step of doping impurities which is required for the fabrication of various PN junction devices was carried out in the manner described below. Be as a p-type impurity and Si as an n-type impurity were supplied simultaneously with the supply of atoms of Group III. According to this doping step, the impurities are efficiently introduced at the lattice positions of the atoms of Group III and the doping was accomplished with a high degree of activation rate as compared with the conventional MBE method. Fig. 10 shows a photoluminescence spectrum at 4.2K of a GaAs film consisting of 4000 atomic layers (1.1µm in thickness) grown on the substrate maintained at 200°C by the process as shown in Figs. 5A to 5C. Because the exciton peaks (FX and DX) are sharply observed and because the intensity of the acceptor type light emission (eAo) is relatively low, it is expected that the quality and purity of the grown crystal are considerably enhanced. Fig. 11A shows a structure of an AlAs-GaAs quantum well grown on a substrate maintained at 300°C by the process described above with reference to Figs. 5A-5C. In Fig. 11A, reference numeral 31 represents a semi-insulating GaAs substrate; 32, a non-doped GaAs layer 500 nm in thickness; 33, 35 and 37, non-doped AlAs layers 50 nm in thickness; 34 and 36, quantum well layers each having a depth of 22 molecular layers; and 38, a GaAs cap layer. The AlAs layer can be grown by supplying Al instead of Ga described above. Fig. 11B shows a photoluminescence spectrum at 4.2K of the quantum well structure. It is seen that even though the quantum well structure exhibits a wide photoluminescence spectrum but it can emit light almost as intense as that emitted by a quantum well structure grown at a high temperature. Therefore, according to the present invention, it becomes possible to grow GaAs and AlAs crystals each having high optical qualities even at low temperatures. In the process described above with reference to Figs. 5A-5C, two As supply sources are used, but as shown in Figs. 12A and 12B, only one As source may be used in such a way that the As beam intensity is switched between a high intensity mode and a low intensity mode. Therefore, while a small quantity of As is normally irradiated on a substrate, the Ga and As beams are alternately irradiated thereon. The AlAs-GaAs single quantum well structures having the same construction were grown on the substrates maintained at 580°C by the conventional MBE method in which Ga (or Al) and As are simultaneously supplied and by the method of the present invention in which while a small quantity of As is normally irradiated on the substrate, Ga (or Al) and As are alternately supplied thereto, and the photoluminescence spectra of these two structures were compared with each other. The structure is shown in Fig. 13. Both the quantum well structures were 6 nm in width (20 atomic layers). In Fig. 13, reference numeral 41 represents a GaAs substrate; 42, a GaAs buffer layer; 43, an AlAs barrier layer (60 nm); 44, a GaAs quantum well (6 nm) and 45, a GaAs cap layer (5 nm). Fig. 14 shows the photoluminescence spectrum 46 of the quantum well structure grown in accordance with the present invention and the photoluminescence spectrum 47 of the quantum well structure grown by the conventional method. It is readily seen that the spectrum of the quantum well structure grown in accordance with the present invention is very narrow and is 2.3 nm and that the hetero interface forming the quantum well is very flat. As in the case of the GaAs crystal, an AlAs crystal having high crystal qualities can be grown on a substrate maintained at a low temperature. The high-quality quantum well was grown on a substrate maintained at 200-300°C by the combination of the AlAs-GaAs heterojunction. The intensive light emission at 720 nm corresponding to the quantum level jump was obtained from the single quantum well 3 nm in width (that is, the width of GaAs), accordingly it is clear that AlAs has a sufficiently high degree of quality. Next, the quantity of supply of an element of Group III such as Ga will be described and the novel phenomena discovered by the same inventors will be explained. The phenomena discovered by the same inventors are as follows: 1) When the quantity of a Group V element on the growing surface is decreased, the diffusion of the material grown on the growing surface when a Group III element is supplied is remarkably accelerated. That is, in the case of the diffusion of metal atoms (the Group III element: Ga) alone on the growing surface, the bond between the metal atoms and the atoms of the surface of the grown layer is not strong chemical bond and the metal atoms having an electron coordination similar to the metal bond are diffuse, consequently the diffusion rate is extremely fast. Therefore, the metal atoms (Ga) can move at high velocities over the growing surface having some little hills and valleys. This phenomenon is called the high speed migration phenomenon. 2) Due to the above-described high speed migration phenomenon, the Group III metal atoms supplied in the succeeding period are so distributed to bury the small valleys on the scale of an atom selectively so that the growing surface becomes flat. That is, when the growing surface has the surface roughness on the scale of an atom, the edge portions of the top surfaces of the steps are instable from the standpoint of energy while the edge portions of the lower surfaces are stable. Therefore, when a statistically large quantity of the Group III atoms migrate across the stepped portions with high speed, these atoms are trapped at the edge portions of the lower steps and the lower step edge portions are gradually displaced so that the lower steps or the whole small valleys are buried. This phenomenon is called the self-flattening phenomenon. Both of the above-described phenomena are extremely ideal factors in obtaining the flat atomic surface or heterojunction interface. By utilizing the above-described phenomena, it becomes possible to grow a compound semiconductor having a flat grown surface or a heterojunction interface on a substrate maintained at relatively low temperatures by controlling the supplies to the substrate of a Group III element and a Group V element. According to the present invention, not only the surface of the crystal grown on the flat substrate can be made flat on the scale of an atom, but also even when the underlying layer has small hills and valleys on the scale of an atom, when metal atoms of Group III are supplied while atoms of Group V are supplied in a very small quantity, these metal atoms are so distributed to bury small valleys, thereby flattening the surface. When the above-described phenomena are utilized, the quantity of supply of atoms of Group III during each period as shown in Figs. 5B and 12B is not needed to be strictly controlled. In addition, while atoms of Group III are normally supplied to the substrate, the quantity of supply of a Group V element is varied to a sufficient low quantity as compared with the quantity required for periodically or continuously growing the crystal. Thus, the surface of the grown layer can be maintained flat. Next referring to Figs. 15A, 15B and 16, the experimental results of the above-described self-flattening phenomenon will be described. In the experiments, the apparatus as shown in Fig. 4 was used and Ga was selected as a Group III element while As was selected as a Group V element. The crystal was grown on the (100) crystal face of the GaAs single crystal substrate maintained at 580°C. Figs. 15A and 15B show the variations in time of the quantities of Ga and As supplied to the substrate. Ingots were heated and evaporated to supply Ga and As, respectively. In the experiments, the quantity of Ga supply at one second interval was 80% of the coordination number (6.4 x 10¹⁴/cm²) of the (100) face of the GaAs substrate. The supply of As is alternately switched in a large quantity R₁ and a small quantity R₂. R₁»R₂ and in this case the quantity R₂ is 5% of the large quantity R₁ so that these quantities are not sufficient to continue the growth of GaAs at the above-described growing temperature. Fig. 16 shows the variations in time of the intensity of the beam of RHEED reflected from a mirror surface (to be referred to the intensity of RHEED) during the growth of GaAs under the above-described conditions. The intensity of RHEED oscillates as it decreases in response to the supply of Ga and increases in response to the supply of As. Fig. 16 shows a feature that a beat occurs at about every five periods. When the peak of the beat is considered to be a time point at which the maximum flatness is recovered, this result shows that the flat surface appears at every five periods. When the quantity of supply of Ga for one period is selected to be equal to the coordination number of the surface of the grown layer; that is, the quantity required for forming one atomic layer (6.4 x 10¹⁴/cm²) the oscillations of the intensity of RHEED continues permanently as long as the growth continues as shown in Fig. 17, but in this case no beat is observed. When the quantity of supply of Ga for one period is selected 0.9 of the coordination number of the surface of the grown layer, a beat appears at every 10 periods and when the quantity of supply of Ga is selected at one thirds (1/3) of the coordination number, a beat is observed at every three periods. The reason of such phenomena can be explained only when it is considered that the Ga atoms are selectively distributed to the small valleys of the surface of the grown layer during the growth of the thin film. Furthermore, a flat grown surface can be formed on a substrate having surface roughness. Referring next Figs. 18A-18E, the self-flattening mechanism in a thin film grown on a flat substrate 1 will be explained. The Ga and As atoms are supplied in the manner described above with reference to Figs. 15a and 15B. When the Ga atoms are supplied in quantity of 80% of the coordination number of the surface (Fig. 18A) of As atoms deposited on the substrate, a step equivalent to one atom results. The surface covered by the Ga atoms is 80% (Fig. 18B). When the As atoms are supplied, they combine only with the Ga atoms and do not combine with the As atoms in the lower layer, so that a step equivalent to one GaAs molecule appears (Fig. 18C). When the Ga atoms are supplied in quantity equal to 80% of the coordination number, some of the newly supplied Ga atoms combine with 20% of the As atoms remaining in the lower layer but do not combine with the As atoms at the step of the upper layer as shown in Fig. 18D. As a result, the area of the upper layer occupied by the Ga atoms becomes 60% of the surface of the grown layer. When the As atoms are further supplied, they combine only with the Ga atoms so that the grown surface in the form of a step whose upper layer occupies 60% and whose lower layer occupies 40% is formed. In like manner, every time when the Ga and As atoms are supplied for one period, the lower layer of the step is increased while the upper layer thereof is decreased. Therefore, a flat surface is repeatedly formed at every five periods. This is the cause for producing beats in the oscillations of the intensity of RHEED shown in Fig. 16. In the respective growth steps shown in Figs. 18A-18E, a very small quantity of the As atoms is supplied during each period when the Ga atoms are supplied, therefore the escape of the As atoms from the surface of the grown layer can be prevented and there is almost no chance that the simultaneously supplied As and Ga atoms combine with each other. The quantity of supply of the As atoms during the period when the As atoms are supplied is greater than the quantity of the Ga atoms exposed at the surface of the grown layer, but an excess of the As atoms is discharged out of the surface of the grown layer. Referring next to Figs. 19A-19E, the self-flattening mechanism in the case of the growth on a substrate 1 with a step will be described. It is assumed that the height of the step is equal to four atomic layers (two GaAs molecular layers) and that Ga and As are supplied in the manner described above with reference to Figs. 15A and 15B. As shown in Fig. 19A, the As surface is formed over the surface of the grown layer. When the Ga atoms are supplied in quantity equal to 80% of the coordination number as shown in Fig. 19B, they readily combine with the As atoms of the layer over the lower surface of the step and do not combine with the As atoms in the layer over the upper surface of the step. As a result, as shown in Fig. 19C, when the Ga and As atoms are supplied for two periods, the initial step of the substrate disappears. As shown in Fig. 19D, the Ga atoms supplied during the third period combine with the As atoms in the lower layer of the As atoms exposed at the surface and when the As atoms are supplied again, the height of the step becomes equal to one molecular layer (Fig. 18E). Thereafter, the surface of the grown layer is flattened in a manner substantially similar to that described above with reference to Figs. 18A-18E. The above-described self-flattening phenomenon is caused mainly due to the behavior of the Ga atoms during the periods when the Ga atoms are supplied. Therefore, it becomes possible to attain the self-flattening of the surface of the grown layer by the combination of the conventional MBE technique in which the Ga atoms and the As atoms sufficient in quantity to combine with the Ga atoms to form GaAs are simultaneously supplied, thereby forming a compound semiconductor layer with the method in accordance with the present invention in which a very small quantity of the As atoms is supplied during the Ga atom supply period. In other words, during the conventional molecular-beam epitaxial growth, the quantity of supply of a Group V element is decreased temporarily while a Group III element is normally supplied so that a flat grown surface can be obtained. Then, a flat grown surface and a flat hetero interface can be formed at low temperatures. In addition, as compared with the Ga supply method described above with reference to Fig. 15A, the switching of a Group III element can be eliminated, so that the growth control can be simplified and a high growth rate can be obtained. The self-flattening phenomenon can be observed not only in the case of the deposition of the Ga atoms but also almost all the cases of metal atoms of Group III. In order to attain the self-flattening phenomenon, it is preferable that the quantity of metal atoms of Group III will not excessively become higher than the coordination number of the surface of the grown surface and it is more preferable that the quantity is equal to or less than the coordination number. Figs. 20A and 20B show the variations in time of the supplies of the Ga atoms and the As molecules, respectively, in one embodiment of the present invention. Fig. 20A shows the supply of Ga which is continued at a predetermined quantity as in the case of the conventional MBE method. Fig. 20B shows the supply of As which is switched between a large quantity R₁ and a small quantity R₂ equal to those described above with reference to Fig. 15B. The quantities R₁ and R₂ are also equal to those described above with reference to Fig. 15B. When the time interval for R₁ is represented by τ₁ while the time interval for R₂ is represented by τ₂, τ₁ is substantially equal to that in the case of the conventional MBE method and the small hills and valleys on a scale of an atom appear over the surface of the grown layer. However, the Ga atoms which are supplied to the succeeding τ₂ period (the period for supplying the As atoms in a small quantity R₂) bury the small valleys, so that a flat surface is reproduced and the continuous oscillations of the intensity of RHEED are observed. Fig. 21 shows the mode of oscillations of the intensity of RHEED when the number of the Ga atoms supplied at the period τ₁ is equal to the number required for forming one atomic layer. Fig. 21 shows that even when the quantity of supply is varied a little, the continuous oscillations can be observed, so that the grown surface is smoothly flattened. Such continuous oscillations can be observed even when the temperature of the substrate is lowered as low as 200°C. The difference in height between the small hills and valleys on growing surface by ordinary MBE method is of the order of five atomic layers at most, accordingly this method is used immediately before the formation of a desired flat surface such as a hetero interface, the object of attaining a flat surface can be attained. That is, the difference in height between the small hills and valleys of the order of five atomic layers can be completely eliminated by repeating the process described above with reference to Figs. 20A and 20B for a few periods and the grown surface is completely flattened. In order to confirm the above-described self-flattening phenomenon, the number of the Ga atoms supplied during one period τ₁ was increased up to the number sufficient to form five atomic layers in the experiments. Figs. 22A and 22B show the variations in time of the supplies of the Ga atoms and the As molecules and the resulting variations in time of the intensity of RHEED, respectively. Even in this case, it is apparent that the surface flattening is accelerated during the period τ₂. The above-described process was used after the growth for a long period of time by the conventional molecular-beam epitaxial technique. Fig. 23A shows the Ga and As supply programs while Fig. 23B shows the variations in time of the intensity of RHEED corresponding to the Ga and As supply program. After the GaAs layer 2µm in thickness has been grown over the substrate maintained at 580°C by conventional MBE method, when the process in accordance with the present invention is repeated for a few periods, it is apparent that the amplitudes of oscillations which once disappeared reappear and the flatness is recovered. Next, some applications of the method in accordance with the present invention to the fabrication of various devices will be described. Fig. 24 shows a single quantum well semiconductor laser having an active layer 10 nm in thickness. In the case of the fabrication of this laser, the supply programs as shown in Figs. 20A and 20B were used in such a way that the quantity of the Ga atoms supplied during the period τ₁ became equal to the quantity required for forming a single atomic layer and τ₂=τ₁ . The substrate was maintained at 580°C. Fig. 24 shows the structure of the laser thus fabricated. Reference numeral 51 represents an n-GaAs substrate; 52, an n-AlGaAs layer 1,000 nm in thickness; 53, a GaAs quantum well 10 nm in thickness; 54, a p-AlGaAs layer 1,000 nm in thickness and 55, a p-PaAs layer 500 nm in thickness. The AlGaAs layer can be grown by simultaneously supplying the Al atoms and Ga atoms in the same quantity to the substrate. The threshold current of the laser diode thus fabricated was as low as about 50% as compared with that of the laser diodes fabricated by the conventional MBE method. Figs. 25A and 25B show one example of the laser diode substantially similar in construction to that described above with reference to Fig. 24, to fabricate this laser diode the process in accordance with the present invention described in detail with reference to Figs. 20A and 20B were applied only immediately before the formation of heterojunction interfaces. Fig. 25A shows the structure of the laser diode and Fig. 25B shows the variations in time of the intensity of RHEED corresponding to the growth of respective layers. The structure of the laser diode in accordance with this embodiment is substantially similar to that of the laser diode shown in Fig. 24. The growing temperature was also same and was 580°C. An n-AlGaAs layer 62 was grown over the surface of an n-GaAs substrate 61 by the conventional MBE method and its last portion 1 nm in thickness (indicated by 62A in Figs. 25A and 25B) was grown by the process described above with reference to Figs. 20A and 20B (in which Ga and Al were supplied in the same quantity). Thereafter, a quantum well layer 63 with a thickness of 10 nm was grown by the conventional MBE method and its last portion 1 nm in thickness (indicated by 63A in Figs. 25A and 25B) was grown by the process described above with reference to Figs. 20A and 20B. Next, a Si-doped p-AlGaAs layer 64 and then an Si-doped p-GaAs cap layer 65 were grown by the conventional method. The variations in time of the intensity of RHEED during the growth of respective layers are indicated by same reference numbers and it is seen that when the present invention is applied, the oscillations are recovered; that is, the flatness is recovered. Fig. 26A shows a two-dimensional electronic device fabricated by the process substantially similar to that described above and Fig. 26B shows the variations in time of the intensity of RHEED corresponding to the growth of respective layers. In Fig. 26A, reference numeral 71 represents a semi-insulating GaAs substrate; 72, a non-doped GaAs layer 1µm in thickness grown by the conventional MBE method; 72A, a non-doped GaAs layer 1 nm in thickness grown by the process in accordance with the present invention described above with reference to Figs. 20A and 20B; 73, a non-doped AlGaAs layer 10 nm in thickness; and 74, a Si-doped p-AlGaAs layer 50 nm in thickness. The two AlGaAs layers 72 and 73 were grown by the conventional MBE method. Si can be supplied during the growth of the AlGaAs layer so as to be doped therein. From the variation of RHEED intensity during growth, it is seen that the flatness is recovered at the heterojunction interfaces. With this structure, a high degree of mobility (2 x 10⁵ cm²/V·sec at 4.5K in the case of the electron density of 1 x 10¹²/cm²) can be obtained at an especially high two-dimensional electron density. These results are considered to be attained due to the improvement of the surface flatness of the heterojunction interface. According to the present invention, it becomes possible to grow high-quality AlAs and GaAs layers at any temperature in excess of 100°C. Furthermore, in the cases of the processes described above with reference to Figs. 20A, 20B and 23A, it is not needed to switch the supply of atoms of Group III and the growth rate at low temperatures can be improved from 0.25µm/h to 1 µm/h as compared with the embodiment described above with reference to Figs. 5A-5C. In the above-described embodiments, the grown surface was flattened by the material supply process described above with reference to Figs. 20A and 20B immediately before the formation of a hetero interface, but it is, of course, possible to attain the similar effects when the process described above with reference to Figs. 5A-5C in which the supply of Ga is switched or the material supply process described above with reference to Figs. 12A and 12B is applied immediately before the formation of a hetero interface. So far the Group V element has been described as the As₄ molecule having four atoms. It is possible to grow a thin III-V compound film at a high growth rate and at lower temperatures by cracking the molecules of Group V which are to be supplied to the substrate, into molecules having less atoms or into atoms. The process for growing a thin GaAs film over a GaAs substrate will be described. Of the molecular beam sources in the apparatus shown in Fig. 4, the As molecular beam source is provided with a cracker cell. Fig. 27 is a sectional view of a cracker cell. In Fig. 27, reference numeral 81 represents a cell main body incorporating therein metal arsenic 82. Reference numeral 83 denotes a heater incorporated in the cell. As₄ molecules each having four atoms are heated and evaporated by the heater 83 and flow into a cracking portion 85 where As₄ molecules are heated to a high temperature. Reference numeral 87 is metal plates having a high melting point and made of, for instance, tantalum and the heated As₄ vapor impinges on them. In addition to the thermal dissociation at a high temperature, the dissociation from As₄ → As₂ is accelerated because of the impingement. Reference numeral 88 is a diffuser such as a mesh which causes As₂ molecules each having two atoms to uniformly irradiate the substrate. Reference numeral 89 is a heat insulating filter for thermally isolating between the cell main body and the cracking portion; 90, a shutter; and 91, a shaft for opening or closing the shutter. The number of atoms which constitutes an As molecule can be measured by the mass spectrometer mounted on the apparatus shown in Fig. 4. Two cracker cells are prepared. The shutter of one of the two cracker cells is kept opened and the temperature of the cell is maintained at a relatively low temperature so that a relatively low intensity beam is made to irradiate on the substrate. The other cell is maintained at a relatively high temperature and its shutter 90 is opened and closed so that the pulse-like high-density beams are directed to the substrate. In this manner, the irradiation of As as shown in Figs. 5A-5C can be carried out. As shown in Fig. 28, a plurality of holes 92 are formed through the shutter 90 so that when the shutter 90 is opened, the beam intensity is increased but when the shutter 90 is closed the beam intensity is decreased. In this case, the intensity of the As beam from a single As source can be varied so that the quantity of As to be supplied to the substrate can be varied. A thin GaAs film was grown on the (100) crystal face of a single GaAs crystal maintained at 200°C. The Ga source was maintained at 970°C and one Ga beam irradiation time was one second. In this case, the beam intensity is 6.4 x 10¹⁴ atoms/cm²·sec. The partial pressure of As was maintained at 1.33 x 10⁻⁵ Pa (1 x 10⁻⁷Torr) (₀) during the Ga beam irradiation period and Ga and As were alternately maintained at the partial pressure of 1.33 x 10⁻⁴ Pa (1 x 10⁻⁶Torr) (₁) for two seconds. As was cracked at 1000°C. Prior to the growth, the substrate was heated to 580-600°C and the As beam was made to irradiate the substrate to remove an oxide film on the substrate and then the temperature of the substrate was lowered to a predetermined temperature to start the growth. The curve A shown in Fig. 29 indicates the variations of the intensity of RHEED during the growth of the thin GaAs film carried out in the manner described above. The shutter of the Ga cell is opened at t=0 and then the intensity of RHEED is decreased. When one atomic layer is grown after t₁, the Ga shutter is closed while the As beam intensity is increased from ₀ to ₁ then the intensity of RHEED is recovered and increased to the initial value after t₂. It is considered that, reflectivity of the electron beam at the Ga surface is low, so that in the case of the supply of Ga the intensity of RHEED is decreased, but when As is supplied, the flat GaAs surface is formed, so that the intensity of RHEED is increased. The curve B shown in Fig. 29 shows the variations of the intensity of RHEED under the same conditions described above except that cracking of As is not carried out. When no cracking is carried out and when the As₄ molecules each having four atoms are used, the recovery of the intensity of RHEED takes a long time, therefore after a predetermined time interval t₂, the intensity of RHEED is not completely recovered yet as indicated by the curve B. The reason is that since the substrate is maintained at a low temperature, the cracking process of As₄ → As₂ → As is not satisfactorily carried out and the absorption of As on the Ga surface takes a long period of time. The higher the cracking temperature, the better, and the cracking of the As₄ molecules can be carried out satisfactorily at a temperature in excess of 900°C. Furthermore, instead of metallic As, a hydride of As such as AsH₃ may be introduced into the apparatus through the supply line which is heated so that the atoms resulting from the cracking of AsH₃ may be supplied to the substrate. In the latter case, the oscillations of the intensity of RHEED will not be damped as shown in curve A in Fig. 29. In the above-described embodiments, the substrates were maintained at 200°C, but according to the method of the present invention, it becomes possible to grow a high-quality crystal on a substrate maintained at a temperature lower than 400°C or at the lowest temperature of 100°C at a high growth rate while according to the conventional method the substrate must be maintained at about 600°C. The cracking process of a Group V element can be applied to the supply processes described above with reference to Figs. 20A, 20B and 23A. So far the present invention has been described in conjunction with GaAs, but a Group V element to be cracked is not limited to As and a Group III element is not limited to Ga. It is apparent that the present invention can be equally applied to other III-V compound semiconductors. According to this method, the molecules of a Group V element are thermally cracked into molecules each having two atoms or into individual atoms which in turn are supplied to the substrate, so that it becomes possible to grow high-quality crystals and furthermore the growth time at a low temperature can be considerably shortened. As described above with reference to Figs. 8A-8C, from the standpoint of the recovery of the intensity of RHEED, it is not needed to make the quantity of a Group III element to be supplied to the substrate completely equal to the quantity required for forming a single atomic layer and no problem arises when the quantity to be supplied to the substrate is between 90 and 100% of the quantity required for forming a single atomic layer. In the case of the self-flattening phenomenon, it has been described that the quantity of the supply of a Group III element is not greater than the quantity required for forming a single atomic layer and is preferably equal to or less than the coordination number. However, the same inventors further found out the fact that when the hydrogen is supplied to the growth surface of a Group III element, the upper limit of the quantity of the supply of a Group III element is considerably relaxed; the control of the supply quantity is simplified; and the more perfect growing surface can be obtained. First the effects attained by supplying the hydrogen to the growing surface will be schematically described. As shown in Fig. 30A, atoms 101 of Group III are supplied to a substrate 100 in a quantity 100-200% greater than the quantity required for forming a single atomic layer and then the hydrogen 102 are made to irradiate the atomic layers in a quantity of 50-100 times the quantity of the Group III element supplied to the substrate. Then, excess atoms of Group III are evaporated from the growing surface in the form of a hydride 103 as schematically shown in Fig. 30B. In this case, the reason why the first atomic layer is not evaporated is that it is maintained in a state more stable than the state of the second atomic layer because of the combination of the first atomic layer with atoms of Group V in the lower layer. When the atoms of Group V are supplied to the flat atomic surface of the atoms of Group III, a III-V molecular layer with a flat atomic surface can be obtained. Atoms of Group V have a high vapor pressure, therefore, the V-V combination is easily broken even when no hydrogen is supplied, and a single layer is grown in the atmosphere at a reduced pressure. In the case of GaAs, As which has a high vapor pressure is evaporated even without the introduction of the hydrogen, but Ga which has a low vapor pressure is hardly vaporized. Therefore, when Ga is deposited on Ga, the former is combined with the hydrogen to form a hydride and then removed. As a result, only one Ga atomic layer remains on the As atomic layer. In like manner, in the cases of AlAs, InAs, InP or GaP, the vapor pressures of Ga, Al and In are considerably lower than those of As and P, it becomes possible to form one atomic layer of Ga, Al or In by the introduction of the hydrogen. Another atomic layer of As or P is deposited on the atomic layer of Ga, Al or In. These steps are repeated so that the epitaxial growth in which atomic layers are sequentially grown on top of each other can be realized. When the apparatus as shown in Fig. 4 was used and when Ga and As were alternately supplied to a substrate maintained at 580°C in accordance with the time charts shown in Figs. 5A and 5B except that the Ga supply quantity is 150% of the quantity required for forming one atomic layer, the oscillations of RHEED disappeared after ten periods as shown in Fig. 31A. However, the hydrogen gas was made to irradiate the substrate crystal at a flow rate of 5cc/min for one second as shown in Figs. 32A-32D, the intensity of RHEED was recovered as shown in Fig. 31B and the oscillations continued for a long period of time. The reason is considered that the introduced hydrogen reacts with excessively adsorbed Ga to form a hydride which in turn is removed away from the surface of the substrate. In this case, the adsorbed Ga atoms forming one atomic layer are combined with the As atoms in the lower layer, so that the Ga atoms in the atomic layer cannot be removed by this process. Thus, it becomes possible to grow one atomic layer in a strict sense. The flow rate of the hydrogen gas may be so selected that the number of the hydrogen atoms supplied to the substrate are 50-100 times as many as the number of the Ga atoms in one atomic layer, and even when the supply of Ga atoms is increased by 200% of a number of atoms required for forming a single atomic layer, the flatness of the Ga surface on a scale of an atom can be maintained by the irradiation of the hydrogen. It suffices to irradiate the hydrogen for about one second per period of the supply of Ga. The hydrogen which is irradiated on the Ga surface may be in the form of a molecule, but more pronounced effects can be attained when the hydrogen atoms are irradiated. Fig. 33 shows a method for irradiating the hydrogen atoms. In Fig. 33, reference numeral 100 represents a substrate disposed in a process chamber; 104, a hydrogen introduction pipe; 105, a heater; and 106, a palladium (Pd) film maintained at about 400°C. A hydrogen molecule H₂ which is introduced into the introduction pipe is adsorbed by the Pd film and decomposed into H atoms. Since the H atoms are small in size, they are diffused through the Pd film and injected into the process chamber evacuated to a high degree of vacuum, irradiating the substrate 100. The power of reaction of the hydrogen atoms is very strong, so that the Ga atoms in the two layer structure can be efficiently removed as shown in Figs. 30A and 30B. As described above, the Ga atom supply quantity is 90-110% of the quantity required for forming one atomic layer when no hydrogen is irradiated, but when the hydrogen is irradiated after the formation of the Ga layer, a flat atomic surface can be formed even when the Ga supply is increased by 200% of the quantity required for forming one atomic layer. In the supply process described above with reference to Figs. 20A and 20B, the hydrogen can be irradiated after the supply of Ga is temporarily interrupted and then the Ga supply can be resumed. As described above, according to the method of the present invention, a flat atomic surface of atoms of Group III is formed by utilizing high mobility of the atoms of Group III; a III-V compound is formed by supplying atoms or molecules of Group V to the flat atomic surface; and the hydrogen is irradiated over the atomic surface after the formation of the atomic surface of atoms of Group III, whereby a more perfect atomic surface can be obtained. INDUSTRIAL APPLICABILITY According to the present invention, in the case of forming or growing a III-V compound semiconductor by supplying an element in the form of atoms or molecules of Group III and an element in the form of atoms or molecules of Group V of the periodic table to a substrate which is disposed in a vacuum process chamber and is heated, while a Group V element is normally supplied to a substrate, the quantity of supply of a Group V element is decreased to such a level that the growth of the III-V compound semiconductor is interrupted at least one time point of the growth of the III-V compound and such small quantity of the Group V element and the Group III element are simultaneously supplied to the substrate. According to the present invention, in order to accomplish the epitaxial growth of high-quality crystals, the fast migration of materials to be supplied to the growing surface of the crystal is an essential factor and the growth of the III-V compound semiconductor is carried out not by the slow migration of III-V molecules but by the fast migration of atoms of Group III, so that the fast migration over the growing surface can be ensured. Therefore, it becomes possible to grow a high-quality epitaxial layer on a substrate maintained at low temperatures and furthermore it also becomes possible to form flat heterojunction interfaces. In addition, a very small quantity of the Group V element is supplied during the period when the Group III element is supplied, so that the evaporation of the Group V element from the regions adjacent to the grown surface can be prevented. As a result, it becomes possible to grow epitaxial crystals having a high degree of purity. Especially, due to controlling the quantity of the supply of the Group III element during the period of the supply of the Group III element or during the period when the quantity of the supply of the Group V element is decreased, to decrease to a quantity required for forming one atomic surface or a quantity not sufficient to form one atomic surface not only the growing surface but also the surface of the substrate are self-flattened, whereby a flat grown surface and flat heterojunction interfaces on a scale of an atom can be obtained. Furthermore, due to switching the supply of a Group V element between a high intensity mode and a low intensity mode after a compound semiconductor has been grown by simultaneously supplying a Group III element and a Group V element, a flat grown surface and flat heterojunction interfaces can be obtained. In addition, the quality of grown crystals can be improved by thermally cracking a Group V element before the element is supplied to the substrate. Moreover, by supplying the hydrogen to the atomic surface of a Group III element a multilayer structure of the Group III element can be removed, whereby the grown surface can be flattened and the qualities of the grown crystal can be improved. According to the present invention, it becomes possible to grow a thin III-V compound semiconductor film and containing only a few impurities and a lattice defect and having high quality, on a substrate maintained at a low temperature. Furthermore, the grown surface and the heterojunction interfaces can be flattened on a scale of an atom and the growth rate can be increased. Therefore, the present invention can be effectively applied to the fabrication of super-high-speed electronic devices, various optical devices such as laser diodes and so on and novel electronic devices utilizing a superlattice.

Claims: A method of forming a thin compound semiconductor film with a high growth rate on the surface of a substrate, said substrate being disposed in a vacuum chamber and heated, the method including supplying atoms or molecules of at least one Group III element and atoms or molecules of at least one Group V element of the periodic table to the surface of said substrate, thereby forming a thin Group III-V compound semiconductor film having a flat surface, the method further including: alternating the supply of the atoms or molecules of said at least one Group V element between: (a) a first amount, said first amount being such that a partial pressure of said Group V element in said vacuum chamber is higher than the equilibrium dissociation vapour pressure of said Group V element on the substrate surface, so that evaporation of said Group V element from a previously grown thin compound semiconductor film is prevented, but said first amount being insufficient for said Group V element to react with said Group III element on said substrate, and (b) a second amount, different from said first amount and sufficient for said Group V element to react with said Group III element, thereby forming a Group III-V compound on the surface of said substrate while supplying the atoms and molecules of said at least one Group III element either in a constant amount or only when the atoms or molecules of said Group V element are supplied in said first amount. A method according to Claim 1, wherein said atoms or molecules of said Group V element are supplied both (a) constantly in said first amount, and (b) periodically in an additional amount so that the sum amount of said Group V element supplied is sufficient to react with said Group III element present on the substrate surface to form said layer of said Group V element on the surface of said substrate; and said atoms or molecules of said Group III element are supplied only when said atoms or molecules of Group V element are supplied in said first amount, so to form a layer of said Group III element on said layer of said Group V element, at least at one stage of the growth of said thin compound semiconductor film, thereby forming said Group III-V compound on the surface of said substrate. A method according to Claim 1 or 2 wherein said Group III element is supplied periodically, said Group III element being supplied only when said Group V element is supplied in said first amount. A method according to Claim 1, wherein said atoms or molecules of said Group III element are supplied in a constant amount to the substrate surface. A method according to Claim 3, wherein the supply amount of said atoms or molecules of said Group III element during at least one period, is 90-110% of the amount required for forming one atomic plane consisting of said Group III element on the substrate surface. A method according to Claim 3, wherein said first amount is such that said partial pressure of said Group V element is within a range of from the equilibrium dissociation vapour pressure of said Group V element on the growing surface to a pressure equal to 10 times said equilibrium dissociation vapour pressure. A method according to Claim 3, wherein said second amount is greater than the number of atoms required for forming one atomic plane consisting of said Group V element and is less than five times said number of atoms. A method according to Claim 3, wherein Si is supplied as a doping atom synchronously with said atoms or molecules of said Group III element. A method according to Claim 3 wherein polyatomic molecules of said Group V element are thermally cracked into atoms or polyatomic molecules, with a reduced number of atoms, by causing said polyatomic molecules to bombard a heated metal plate and then to be supplied to the surface of said substrate. A method according to Claim 3, wherein a hydride of said Group V element is thermally cracked into atoms or diatomic molecules by passing said hydride through a heated metal pipe and said atoms or molecules of said Group V element are then supplied to the substrate surface. A method according to Claim 3, wherein molecular hydrogen is supplied to the surface of said substrate between the periods of supply of said atoms or molecules of said Group III element. A method according to Claim 11, wherein said molecular hydrogen is passed through a heated palladium film to produce atomic hydrogen before it reaches said substrate surface. A method according to Claim 11, wherein the amount of the supply of said hydrogen is 50-100 times as much as the surface cite number of atoms of one atomic plane of said Group III element. A method according to Claim 11, wherein the amount of the supply of said atoms or molecules of said Group III element, at one period, is 90-200% of the amount required for forming one atomic plane consisting of said Group III element. A method according to Claim 3 or 11, wherein said substrate is maintained at a temperature between 100 and 700°C.

Inventors: HORIKOSHI YOSHIJI, KOBAYASHI NAOKI, SUGIURA HIDEO, HORIKOSHI, YOSHIJI, KOBAYASHI, NAOKI, SUGIURA, HIDEO
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, C30B23