GB2162862A - Process for forming monocrystalline thin film of compound semiconductor - Google Patents

Abstract

A cycle of alternately or cyclically introducing external gases containing molecules of component elements of a compound semiconductor to be formed on a substrate is repeated while appropriately controlling the pressure, substrate temperature and gas introduction rate in a crystal growth vessel, so that a monocrystal which is dimensionally as precise as a single monolayer can grow on the substrate by making use of chemical reactions on the heated substrate surface.

Description

SPECIFICATION Process for forming monocrystalline thin film of compound semiconductor This invention relates to a process for forming a monocrystalline thin film of a compound semiconductor.

A Metal Organic Vapour Phase epitaxy process (hereinafter referred to as an MO-CVD process), a molecular beam epitaxial process (hereinafter referred to as an MBE process) and an atomic layer epitaxial process (hereinafter referred to as an ALE process) are well known in the art as vapor phase epitaxial techniques for obtaining crystalline thin film of semiconductors.

In the MO-CVD process, Ill and V group elements as sources, and hydrogen gas or the like as a carrier are simultaneously introduced into a reaction chamber to cause crystal growth by means of thermal decomposition.

The thermal decomposition results in a poor quality of the grown crystal layer. In addition, the thickness control which is dimensionally as precise as a single monolayer is difficult.

The MBE process is well known as a crystal growth process making use of a ultrahigh vacuum. This process, however, includes a first stage of physical adsorption. Therefore, the quality of the crystal obtained is inferior to that obtained by the CVD process which makes use of a chemical reaction. Besides, for the growth of a compound semiconductor such as GaAs of Ill and V group elements, Ill and V group elements are used as sources and are disposed in a growth chamber. There- fore, it is difficult to control the amount and rate of vaporization of gases evaporated as a result of the heating of the sources. In addition, replenishment of the sources is difficult.

Further, it is difficult to maintain a constant growth rate for a long period of time. Furthermore, the evacuating device is complicated in construction. Still further, precise control of the stoichiometric composition of a compound semiconductor is difficult. Consequently, the MBE process is defective in that high quality crystals cannot be obtained.

The ALE process is an improvement over the MBE process. In this process, component elements of a compound semiconductor are alternately supplied in the form of pulses so that monoatomic layers are alternately deposited on a substrate to cause growth of a thin film composed of atomic layers, as disclosed in U.S. Patent No. 4,058,430 (1977) to T.

Suntola et al. Although this process is advantageous in that the film thickness can be controlled with the precision of the atomic layer, it is actually an extension of the MBE process, and the crystal quality is not satisfactory as in the case of the MBE process.

Besides, its application is limited to growth of thin films of compound semiconductors, e.g., those of II and IV group elements, such as CdTe and ZnTe, and th & rocess is not successfully applicable to Si or GaAs, which is the most important semiconductor material presently used for the production of semiconductor devices including ultra LSl's. There are attempts for improving the ALE process so as to absorb molecules to the surface of a crystal thereby to make use of chemical reactions on the surface of the crystal. This approach, however, concerns only with the growth of polycrystals of ZnS or amorphous thin films of Ta2O5, and has not concern with a single crystal growth technique.

Meanwhile, with the recent ever-incr#asing speed of communications and controls, there are strong demands for production of various three-terminal elements and diodes which can exhibit high performance in the range of microwaves and milliwaves and also for semiconductor devices operating in the light wave range (e.g., lasers, light-emitting and lightreceiving elements, etc.). Accordingly, there has been a strong demand for a selective epitaxial process for growth of a crystal of three-dimensional structure which is dimensionally as precise as a single monolayer in the thicknesswise direction.

With the aforesaid MO-CVD process, MBE process and ALE process, however, it is difficult to obtain a selective epitaxial growth layer corresponding to a mask pattern on a substrate with the dimensional precision described above because the crystal grows on the mask material as well.

Since the desired selective epitaxial growth cannot be attained with all of the prior art processes described above, it is difficult to obtain high quality crystals having satisfactory stoichiometric compositions by the MO-CVD process and MBE process, while single crystals cannot also be obtained by the ALE process.

A primary object of the invention is to improve the quality of the crystal growth layer by controlling the stoichiometric composition and to ensure growth of a film which is dimensionally as precise as a single monolayer.

In accordance with one aspect of the present invention which attains this object, there is provided a process which comprises evacuating the interior of a crystal growth vessel to a predetermined pressure, heating a substrate disposed in the vessel to a predetermined relatively low temperature, introducing gaseous molecules containing those of one of component elements of a compound semiconductor into the vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of the vessel to the predetermined pressure again, introducing gaseous molecules containing those of another component element of the compound semiconductor into the vessel under a predet ermined pressure for a predetermined period of time, and repeating a sequence of the above steps to cause growth of a monocrystalline thin film of the compound semiconductor having a desired thickness and dimensionally as precise as a single monolayer.

The vessel is preferably evacuated to a pressure of 10-' to 10-7 Pascal, the substrate is preferably heated up to 300 to 800"C, and the gas introduction period is preferably from 0.5 to 200 seconds.

Under these conditions, growth of a monocrystalline thin film of a compound semiconductor of a desired thickness can grow with dimensional precision as precise as a single monolayer.

Further, by introducing gaseous molecules containing those of an impurity element of the compound semiconductor simultaneously or alternately with the gaseous molecules containing those of at least one of the component elements of the compound semiconductor, any desired -impurity concentration distribution can be provided in the thicknesswise direction, or a molecular layer containing such an impurity element and a molecular layer not containing such an impurity element can be sequentially and continuously formed on the substrate.

Furthermore, since the impurity can be doped in each of the individual layers, a very sharp distribution of the impurity concentration can be obtained, which is very useful for the fabrication of high speed transistors, integrated circuits, diodes, light-emitting elements, etc.

A second object of the invention is to facilitate production of a compound semiconductor having a hetero structure.

In accordance with another aspect of the present invention which attains this object, there is provided a process which comprises evacuating the interior of a crystal growth vessel to a predetermined pressure, heating a substrate disposed in the vessel to a predetermined relatively low temperature and repeating, according to respective predetermined sequences, introduction of gaseous molecules containing those of individual component elements of at least two different compound semiconductors and evacuation of the vessel after completion of introduction of the gaseous molecules.

By this process, a hetero structure consisting of at least two different compound semiconductors can grow continuously on the substrate In this case, gaseous molecules containing those of an impurity element of the compound semiconductors may be introduced simultaneously or alternately with the gaseous molecules containing those of at least one of the component elements of the compound semiconductors, whereby any desired distribution of the impurity concentration in the thicknesswise direction can be provided, or a molecular layer containing such an impurity element and a molecular layer not containing any impurity element can be formed sequentially continuously on the substrate.

A third object of the invention is to attain selective growth of a monocrystal of threedimensional structure whose thickness can be controlled with dimensional precision as precise as a single monolayer.

In accordance with another aspect of the present invention which attains this object, there is provided a process which comprises forming on a crystalline substrate a mask pattern of a material different from that of the substrate, disposing the substrate formed with the mask pattern in a crystal growth vessel after rinsing and drying, then evacuating the interior of the vessel to a predetermined va- cuum, heating the substrate formed with the mask pattern, introducing gaseous molecules containing those of one component element of a compound semiconductor into the vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of the vessel again, introducing gaseous molecules containing those of another component element of the compound semiconductor or containing a gas reacting with the first component element of the semiconductor into the vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of the vessel again, and repeating a sequence of the above procedure to cause growth of single monolayers on the substrate.

By this process, a monocrystalline thin film of the semiconductor having a desired thickness can selectively grow on the substrate surface only with precision as precise as a single monolayer.

The above and other objects and features of the invention will become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view showing the construction of a crystal growth device preferably used for carrying out an embodiment of the invention.

Figure 2A. is a graph showing the relation between the thickness of the film grown and the amount of TMG introduced per cycle of the process carried out by the device illustrated in Fig. 1.

Figure 2B is a graph showing the relation between the thickness of the film grown and the number of valve on-off operations in the process carried out by the device illustrated in Fig. 1.

Figures 3A to 3E are schematic sectional views illustrating the selective growth process carried out by the device shown in Fig. 1.

Figure 4 is a graph showing the relation between the thickness of the film grown and the number of gas introduction cycles in the process carried out by the device shown in Fig. 1.

Figures 5 and 6 are schematic views show ing the construction of crystal growth devices preferably used for carrying out different em bodiments of the invention, respectively.

Figure 7 illustrates how a superstructure is formed by the device shown in Fig. 6, wherein Fig. 7A illustrates the superstructure and Fig. 7B illustrates a gas introduction sequence.

Figures 8 and 9 are schematic views show ing the construction of crystal growth devices preferably used for carrying out further em bodiments of the invention, respectively.

Referring now to Fig. 1, a crystal growth vessel 1 is made of stainless steel or like metal. The vessel 1 is coupled to an evacuat ing unit 3 via a gate valve 2 for evacuating its interior to a ultrahigh vacuum. The vessel 1 includes nozzles 4 and 5 introducing gaseous compounds containing Ill and V group ele ments respectively as components of a Ill-V group compound semiconductor which grows on a substrate 12. The nozzles 4 and 5 are provided with on-off valves 6 and 7 for con trolling the introduced amounts of the gase ous compounds 8 and 9 containing the Ill and V group elements, respectively. A heater 10 for heating the substrate 12 is disposed in the vessel 1, and a thermocouple 11 is coup led to the heater 10 for-measuring the tem perature thereof.The heater 10 includes a tungsten filament sealed in a quartz glass casing, on which the substrate 12 is mounted. The vessel 1 is further provided with a pressure gauge 13 for measuring the value of its internal vacuum.

A monocrystalline thin film of a compound semiconductor is formed in a manner as de scribed below by the crystal growth device of the construction shown in Fig. 1. Suppose, for example, the case of epitaxial growth of a single crystal of GaAs on the substrate 12 of GaAs. First, the vessel- 1 is evacuated to about 10-7 to 108 Pascal (hereinafter abbreviated as Pa) by opening the gate valve 2 and operating the ultrahigh-vacuum evacuating unit 3. Then, the GaAs substrate 12 is heated to 300 to 800"cm by the heater 10. There after, gaseous trimethyl gallium (TMG) 8 is introduced as a gas containing Ga by keeping the valve 6 open for 0.5 to 10 sec. and maintaining the internal pressure of the vessel 1 at 10-' to 10-7 Pa. Then, the valve 6 is closed, and the vessel 1 is evacuated again.

'Thereafter, gaseous arsine (AsH3) 9 is intro duced as a gas containing As by keeping the valve 7 open for 2 to 200 sec. and maintain ing the internal pressure of the vessel 1 at 10- ' to 10-7-Pa. As a result, at least one molecular layer of GaAs grows on the sub strate 12.

Thus, growth of a single crystal growth layer of GaAs having a desired thickness can be attained with precision as precise as a single monolayer by repeating the process of growth of the monomolecular layer in the manner described above.

Fig. 2A shows the thickness of the GaAs film grown per cycle using TMG and AsH3 as component-containing gases at a growth temperature of 500 C, the thickness being plotted relative to the amount of TMG introduced per cycle. It will be seen from this graph that, as the amount of TMG introduced per cycle is increased, the thickness of the film grown is ultimately saturated. This means that a single monolayer or a dimolecular layer can reliably grow in one cycle even with slight fluctuations of the amounts of gases introduced so long as the amounts are in excess of those corresponding to the saturation thickness. Fig. 2B shows the thickness of the epitaxially grown GaAs layer plotted relative to the number of cycles of alternate introduction of TMG and AsH3 to illustrate the condition of saturation.It will be apparent from the graph that a very satisfactory linearity is obtained, so that it is possible to accurately control the film thickness to any desired value. Examination of the GaAs epitaxial growth layer thus obtained by electron beam diffraction and X-ray diffraction reveals that the layer has a monocrystalline thin-film structure of very high perfectness.

The material gas containing Ga is not limited to TMG, and a thin film of GaAs having a satisfactory crystalline structure could also be obtained by introducing a gas such as TEG, ZEGaCI, ZMGaCI, or, GaCI3, GaBr3 or Gal.

A process for selective growth of a single crystal of GaAs on a substrate of GaAs by the device shown in Fig. 1 will now be described with reference to Fig. 3.

First, about 1 to 2 ym of a substrate of GaAs 12 polished to be as smooth as a mirror surface is removed by etching according to an ordinary etching process. Then, an 5i3N4 film 121 having a uniform thickness of approximately 2,000 A (i.e., 200 nm) is formed by a plasma CVD process on the substrate 12, and, then, a photoresist 122 is coated on the film 121, as shown in Fig. 3A. Then, a photoresist pattern as shown in Fig. 3B is formed by an ordinary photoetching process.

Subsequently, the Si3N4 film 121 is selectively etched away by applying a buffer etchant with HF: H20 = 1:5, and, then, the photoresist 122 is removed, thereby leaving a mask pattern of the 5i3N4 film 121 on the substrate 12, as shown in Fig. 3C. The surface of the wafer is then rinsed, and about 100 A of the surface of the GaAs substrate 12 is etched away by applying an organic alkaline etchant containing trialkyl 2,1-hydroxyalkyl ammonium hydroxide (THAH), followed by rinsing and drying.

The substrate 12 formed with the mask pattern is mounted on the heater 10 in the crystal growth vessel 1 shown in Fig. 1, and the vessel 1 is evacuated to 10-7 to 10-8 Pa by opening the gate valve 2 and operating the ultrahigh-vacuum evacuating unit 3. Then, the GaAs substrate 12 is heated to, for example, about 300 to 800 C by the heater 10, and TMG 8 is introduced as Ga-containing gas by holding the valve 6 open for 0.5 to 10 sec.

and maintaining the internal pressure of the vessel 1 at 10-' to 10-7 Pa. Subsequently, the valve 6 is closed, and the vessel 1 is evacuated again. Thereafter, AsH3 9 is introduced as As-containing gas by holding the valve 7 open for 2 to 200 sec. and maintaining the internal pressure of the vessel 1 at 10-' to 10-7 Pa. In this way, selective growth of a single crystal of GaAs corresponding to at least a single monolayer on the exposed mask-free substrate surface can be attained.It is apparent that, by repeating the sequence of steps described above, successive single monolayers grow on the mask-free surface of the substrate 12 to provide a monocrystalline GaAs layer of a desired thickness with dimensional precision as precise as a single monolayer, without causing growth of GaAs on the mask of the 5i3N4 film 121, as shown in Fig. 3D.

Afterwards, the Si3N4 film 121 is removed by etching with a buffer etchant, whereby a monocrystalline selective growth layer of GaAs 123 having a desired thickness is formed on a desired surface area of the substrate 12 according to the mask pattern, as shown in Fig.

3E.

The above selective growth process has not yet been fully theoretically elucidated, but is based on the inventor's experiments.

Fig. 4 shows the results of the experiments.

The graph shows the relation between the thickness of the epitaxial growth layer of GaAs and the number of cycles of alternate introduction of TMG and AsH3 as componentcontaining gases at a growth temperature of 500 C. The thickness of the epitaxial growth layer obtained was 1,100 A when the cycle of alternate introduction of the gases 8 and 9 was repeated 400 times, 0.57 y when the cycle was repeated 2,000 times, and 1.13 y when the cycle was repeated 4,000 times. it should be noted that the thickness of the growth film and the number of gas introduction cycles are very linearly related to each other. It was thus confirmed that the thickness of the selective growth layer can be controlled by controlling the number of gas introduction cycles.

Fig. 5 shows another form of the crystal growth device, which is designed for doping with impurities. In Fig. 5, the same reference numerals are used to designate the same or equivalent parts appearing in Fig. 1. The construction is different from that of Fig. 1 in that nozzles 14 and 15 for introducing gaseous compounds for impurity doping are additionally provided in the crystal growth vessel 1, the nozzles 14 and 15 being provided with respective on-off valves 16 and 17 for control- ling the amounts of gaseous compounds 18 and 19 containing II and Vl group elements as respective impurity components introduced into the vessel 1.

To form a p-type growth layer with this device, TMG 8 and AsH3 9 as main component gases and dimethyl zinc (ZMZn) 18 as an impurity gas are cyclically introduced. As another method, TMG 8 and ZMZn 18 are introduced simultaneously but alternately with AsH3 9, or AsH3 9 and ZMZn 18 are introduced simultaneously but alternately with TMG 8. As a further method, a first cycle consisting of simultaneous introduction of TMG and ZMZn, evacuation, introduction of AsH3 and evacuation, and a second cycle consisting of introduction of TMG only, evacuation, introduction of AsH3 only and evacuation, may be alternately repeated to alternately form a layer doped with Zn and a layer not doped with Zn or to form a plurality of first layers alternated by second layers.

The impurity gas may be dimethyl cadmium (MZCd), dimethyl magnesium (ZMMg), monosilane {SiH4), germane (GeH4), etc. Further, ZMCd and ZMZn may be simultaneously introduced.

To form an n-type growth-layer, dimethyl selenium (ZMSe) 19 as impurity gas is introduced cyclically with TMG 8 and AsH3 9.

Alternately, TMG 8 and ZMSe 19 are introduced simultaneously but alternately -with.

AsH3 9.

The impurity gas in this case may be dimethyl sulfur (ZMS), hydrogen sulfide (H2S), hy-- drogen selenide (H3Se), etc.

In this case, a molecular epitaxial growth layer having a desired distribution of the impurity concentration in the thicknesswise direction can be provided by setting the rate of supply of impurity gas to be lower than that of AsH3 9 and TMG 8, e.g., 10-3 to 10-6 and setting the gas introduction time to 0.5 to 10 sec. Further, it is possible to produce pn junctions, non-uniform impurity concentration distributions, bipolar transistor structures such as npn, npin, pnp and pnip, field-effect transistor structures such as n + in + and n+n#n +, electrostatic induction transistor structures, pnpn thyristor structures, etc. by appropriately controlling the rate and time of supply of the impurity gases; Since the doping with impurities can be done for desired ones of layers, it is possible to provide a very sharp impurity concentration distribution, which is very effective for the fabrication of very high speed transistors, integrated circuits, light-emitting diodes, etc.

Fig. 6 shows a further form of the crystal growth device, which is designed for the growth of a mixed-crystal compound semiconductor. This device will now be described in connection with a case of growth of Ga(,~x) AIxAs as the mixed crystal. In Fig. 6, the same reference numerals are used to designate the same or equivalent parts appearing in Fig. 1.

The construction is different from that of Fig.

1 in that nozzles 20 and 21 for introducing a gaseous compound 24 containing Al as a Ill group element and silane (SiH4) 25 through on-off valves 22 and 23 respectively are additionally provided in the crystal growth vessel 1.

With this-structure, TMG 8, AsH3 9 and trimethyl aluminum (TMA1) 24, for instance, are introduced as component gases cyclically into the vessel 1. A mixed-crystal molecular epitaxial growth layer can be formed, which has a desired component ratio in the thicknesswise direction, by controlling the rate and time of supply of TMA1 24 with respect to TMG 8.

TMG 8 and TMA1 24 may be introduced simultaneously. Further, a mixture gas conta#ining TMG and TMA1 may~ be introduced from the nozzle 20.

Although Ga(1 X)AIxAs has been taken as an example of the mixed crystal, other mixed crystals, e.g., those containing Ill and V group elements such as GaAs# #, P,, In, Ga#1 P and Ifl,Ga1#,AS and those containing II and V -group elements such as Hg(l H91#,Cd,Te, may also be formed.

To form a superstructure of Ga(,,,AI,As as shown in Fig. 7A, a sequence as shown in Fig. 7B may be used. More specifically, in the first two cycles, TMG 8 and AsH3 9 are introduced alternately, in the succeeding five cycles TMA1 24 and AsH3 9 are introduced alternately, and in the succeeding two cycles TMG 8 and AsH3 9 are introduced alternately.

The following four cycles are impurity introduction cycles, in which silane (six4) is introduced simultaneously with AsH3 but alternately with TMG 8 through the nozzle 21 by on-off operating the valve 23. In other words, the on-off phases of the valves 23 and 26 are maintained the same in four successive cycles, followed by two cycles of undoped GaAs growth, in which TMG 8 and AsH3 9 are introduced alternately, and then by two cycles of undoped AlAs growth, in which TMA1 24 and AsH3 9 are introduced alternately.

Fig. 8 shows a further form of the crystal growth device, which is designed for the growth of a lattice-strain compensation, mixed-crystal compound semiconductor of quaternary or higher mixed crystal structure permitting independent control of the forbidden band width and lattice constant. It is well known in the art that lattice strain compensation can be attained when Gas 7AIo 3Aso sgPe o1 for instance, is formed by growth on a GaAs substrate. Accordingly, this device will now be described in connection with a case of growth of Ga(, X)AlxAs(1-y)py as the mixed crystal.

In Fig. 8, the same reference numerals are used to designate the same or equivalent parts appearing in Fig. 1. The construction is diffeernt from that of Fig. 1 in that nozzles 26 and 27 for introducing gaseous compounds 30 and 31 containing Al in the Ill group and P in the V group through on-off valves 26 and 27 respectively are additionally provided in the crystal growth vessel 1.

With this structure, TMG 8, AsH3 9, TMA1 30 and phosphine (PH3) 31, for example, are introduced as component gases cyclically into the vessel 1. In this case, the elements in the Ill and V groups may be introduced simultaneously. Further, a mixture gas prepared in advance may be introduced. The growth temperature and growth pressure are controlled substantially in the same way as in the case of the device shown in Fig. 1. A lattice-strain compensation, mixed-crystal compound semiconductor epitaxial growth layer can be formed by appropriately controlling the mixture ratio and the rate and time of introduction of the gases.

In the various forms of the device described above, the heater for heating the substrate 12 is provided in the crystal growth vessel 1. Fig.

9 shows a further form of the device, in which an infrared lamp 32 housed in a lamp case 33 disposed outside the crystal growth vessel 1 is used as a substrate heater. Infrared rays emitted from the lamp 32 in the lamp case 33 passes through a quartz glass plate 34 to irradiate the substrate 12 which is supported on a susceptor 35, thereby heating the substrate 12. With this structure, parts that are unnecessary for the crystal growth can be removed from the interior of the crystal growth vessel 1, thus eliminating generation of undesired gases of heavy metals or the like by the heat generated from the heater.

Furthermore, the crystal growth vessel 1 is associated with an optical system 36 co-operating with an external light source 37, e.g., a mercury lamp, a heavy hydrogen lamp, a xenon lamp, an excimer laser, an argon laser, etc., to irradiate the substrate 12 with light having a wavelength of 180 to 600 nm. In this case, the substrate temperature can be reduced to ensure growth of a single crystal having a still higher quality.

The ultrahigh-vacuum evacuating unit employed in the above forms of the device may be of well-known type, e.g., an ion pump.

Further, an auxiliary vacuum vessel, a crystal transport device, etc. for the insertion and transport of the monocrystalline substrate into and out of the crystal growth vessel may be readily provided to improve the mass productivity of the device. Further, while GaAs has been referred to by way of example, the invention is of course also;applicable to compounds of Ill and V group elements or compounds of II and IV group elements, e.g., InP, AIP and GaP. Further, the material of the substrate is in no way limited to GaAs, and it is possible to cause heteroepitaxial growth of a compound semiconductor on another compound substrate. Furthermore, various other changes and modifications may be made without departing from the scope and spirit of the invention,

Claims (20)

1. A process for forming a monocrystalline thin film of a compound semiconductor comprising the steps of evacuating the interior of a crystal growth vessel to a predetermined pressure, heating a substrate disposed in said crystal growth vessel to a predetermined relatively low temperature, introducing gaseous molecules containing those of one of component elements of a compound semiconductor into said crystal growth vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of said crystal growth vessel to the predetermined pressure again, introducing gaseous molecules containing those of another component element of said compound semiconductor into said crystal growth vessel under a predetermined pressure for a predetermined period of time, and repeating a sequence of the above steps to cause growth of a monocrystalline thin film of said compound semiconductor having a desired thickness and dimensionally as precise as a single monolayer.

2. A process according to claim 1, wherein the interior of said crystal growth vessel is evacuated to 1 0-- to 10 7 Pascal, and said gaseous molecules containing those of said component elements are introduced for 0.5 to 200 seconds onto said substrate heated to 300 to 800 C.

3. A process according to claim 1, wherein said substrate is irradiated with light having a wavelength of 180 to 600 nm.

4. A process according to claim 1, wherein gaseous molecules containing those of an impurity element of said compound semiconductor are introduced either simultaneously or alternately with the gaseous molecules containing those of at least one of the component elements of said compound semiconductor, whereby any desired impurity concentration distribution is provided in the thicknesswise direction, or a molecular layer containing such an impurity element and a molecular layer not containing such an impurity element are sequentially and continuously formed on said substrate.

5. A process according to claim 4, wherein gaseous molecules containing those of impurity elements of a compound semiconductor consisting of two or more components are introduced.

6. A process according to claim 1, wherein gaseous molecules of a mixture containing those of an impurity element used for doping said compound semiconductor consisting of one or more component elements and gaseous molecules of another mixture containing those of another impurity element used for doping said compound semiconductor are introduced individually in different cycles or introduced for different periods in the same cycle, whereby different impurity elements are doped in different single monolayers.

7. A process according to claim 1, wherein said gaseous molecules containing those of one of component elements of the compound semiconductor are selected from a group consisting of TMG, TEG, ZEGaCI, GaBr3 and Gal3, said gaseous molecules containing those of another component element of the compound semiconductor are selected from a group consisting of TMAs, AsCI3, AsBr3 and AsH3, and said compound semiconductor to be formed is GaAs.

8. A process according to claim- 7, wherein gaseous molecules selected from a group consisting of ZMZn, ZEZn, ZECd, ZMHg, ZEHg and# B3H6 provide gaseous molecules containing a p-type impurity element of said GaAs compound semiconductor, while, those selected from a group consisting of SiH4, GeH4, SnH4, PbH4, ZMSe, ZMTe, H2S, H2Se, H2Te and H2Po provide gaseous molecules containing an n-type impurity element of said compound semiconductor, and said gaseous molecules are introduced simultaneously or alternately with at least one of gaseous molecules containing those of a component element of said GaAs compound semiconductor, whereby a desired impurity concentration distribution in the thicknesswise direction is provided or a molecular layer containing the impurity element and a molecular layer not containing any impurity element are cyclically formed.

9. A process for forming a monocrystalline thin film of a compound semiconductor comprising the steps of evacuating the interior of a crystal growth vessel to a predetermined pressure, heating a substrate disposed in said crystal growth vessel to a predetermined relatively low temperature, and repeating, according to respective predetermined sequences, introduction of gaseous molecules containing those of individual component elements.of at least two different compound semiconductors and evacuation of said crystal growth vessel after completion of introduction of said gaseous molecules, whereby a hetero structure consisting of at least two different compound semiconductors is formed on said substrate by growth.

10. A process according to claim 9, wherein gaseous molecules containing those of an impurity element of said compound semiconductors are introduced simultaneously or alternately with gaseous molecules containing those of at least one of the component elements of said compound semiconductors, whereby a desired impurity concentration dis tribution in the thicknesswise direction is pro vided, or a molecular layer containing the impurity element and a molecular layer not containing any impurity element are cyclically formed.

11. A process according-to claim 9, wherein said substrate is irradiated with light having a wavelength of 180 to 600 nm.

12. A process according to claim 9, wherein one of said at least two different compound semiconductors is GaAs, and another compound semiconductor is Ga(1-x) AIxAs.

13. A process according to claim 9, wherein gaseous molecules of a mixture con taining those of an impurity element used for doping one of said compound semiconductors consisting of two or more component ele ments and gaseous molecules of another mix ture containing those of another impurity ele ment used for doping the other said com pound semiconductor are introduced individu ally in different cycles or introduced for differ ent periods in the same cycle, whereby differ ent impurity elements are doped in different single monolayers.

14. A process according to claim 12.

wherein a cycle of causing growth of at least one molecular layer by introducing gaseous molecules selected from a group consisting of TMA1, TEA1, ZMAIC1, Alp13, AlBr3 and All, as gaseous molecules containing Al simultane ously or alternately with a gas containing components other than Ai of said Ga(, xXAIxAs compound semiconductor into said crystal growth vessel under a predetermined pressure for a predetermined period of time and evacu ating the interior of said crystal growth vessel, is repeated to form a crystalline thin film of said Ga,,~,,AI,As compound semiconductor.

15. A process according to claim 14, wherein gaseous molecules selected from a group consisting of ZMZn, ZEZn, ZECd, ZMHg, ZEHg and B2H6 provide gaseous mole cules containing a p-type impurity element of said compound semiconductor, while, those selected from a group consisting of Six4, Get4, SnH4, PbH4, ZMSe, ZMTe, H2S, H2Se, H3Te and H2Po provide gaseous molecules containing an n-type impurity element of said compound semiconductors, and said gaseous molecules are introduced simultaneously or alternately with at least one of said gaseous molecules containing those of component ele ments of said compound semiconductors, whereby a desired impurity concentration distribution in the thicknesswise direction is provided, or a molecular layer containing the impurity element and a molecular layer not containing any impurity element are cyclically formed.

16. A process for forming a monocrystal line thin film of a compound semiconductor comprising the steps of forming on a crystalline substrate a mask pattern of a material different from that of said substrate, disposing said substrate formed with said mask pattern in a crystal growth vessel after rinsing and -- drying, then evacuating the interior of said crystal growth vessel to a predetermined vacuum, heating said substrate, introducing gaseous molecules containing those of one component element of a compound semiconductor into said crystal growth vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of said crystal growth vessel again, introducing gaseous molecules containing those of another component element of said compound semiconductor or containing a gas reacting with the first component element of said semiconductor into said crystal growth vessel under a predetermined pressure for a predetermined period of time, evacuating the interior of said crystal growth vessel again, and repeating a sequence of the above steps to cause growth of single monolayers, whereby a monocrystalline thin film of said compound semiconductor having a desired thickness is selectively formed on said substrate with precision as precise as a single monolayer.

17. A process according to claim 16, wherein said crystalline substrate is made of a compound semiconductor of Ill and V group elements and said gaseous molecules as the sources of the growth include III and V group elements.

18. A process according to claim 16, wherein said substrate is irradiated with light having a wavelength of 180 to 600 nm.

19. A process according to claim 17, further comprising the steps of using a film of SI,Ny and/or a film of SixOy as the material of said mask pattern and using a liquid containing trialkyl 2,1-hydroxyalkyl ammonium hydroxide (THAH) as a rinsing liquid for rinsing said substrate prior to the growth and also as an etchant.

20. A process according to any of claims 1 to 19 for forming a monocrystalline thin film, substantially as hereinbefore described and exemplified and with reference to the accompanying drawings.