METHOD FOR GROWING SINGLE CRYSTAL THIN FILMS OF ELEMENT SEMICONDUCTOR

Abstract: A method for growing a single crystal thin film of an element semiconductor which comprises repeating the successive operations of feeding a single kind of gas containing the element semiconductor as a component element onto a substrate heated in a growth chamber and then exhausting the gas in the growth chamber under controlled conditions and thereby growing the single crystal thin film of said element semiconductor in a desired thickness with a precision of monomolecular layer. In an alternate method for growing a single crystal thin film of an element semiconductor, the sole gas containing the element semiconductor is continuously fed onto the substrate for a given period of time, thereby forming a single crystal thin film of element semiconductor having a desired thickness. According to the present invention, there can be obtained single crystal thin films with high quality in a high reproducibility, by simplified operating parameters and growing apparatus.

Description: The present invention relates to a method for growing a single crystal thin film of an element semiconductor, comprising the steps of feeding a single kind of halogen compound gas containing said element semiconductor onto a substrate heated in a growth chamber up to a temperature laying within a temperature range of 750°C to 900°C. Such a method for growing a single crystal thin film of an element semiconductor is known from GB-A-2 162 206. According to this known method a growth vessel is evacuated to about 10⁻⁷ to 10⁻⁸ Pa, then, for example, a Si substrate is heated up to 300 to 1000°C and a gaseous SiH₂Cl₂ is introduced into the vessel by opening a valve for 0,5 to 10 seconds, while maintaining the internal pressure at 1 to 10⁻⁶ Pa. Then the said valve is closed and H₂ gas is introduced into the vessel, wherein this H₂ gas reacts with the SiH₂Cl₂, the internal pressure being maintained at 1 to 10⁻⁶ Pa. The introduction of the H₂ gas is realized by opening another valve for a predetermined time period as, for example, 2 to 200 seconds. As a result, at least one molecular layer of Si is grown on the substrate. However, with this known method for growing a single crystal thin film of an element semiconductor there are still remaining several problems as for example the relation between the crystalline quality and the direction of the electric field and the surface. JP-A-56 124 228 (Kokusai Denki K.K.) 29/09/1981 discloses an epitaxial growth furnace for growing Si epitaxial films on wafers, wherein the wafer is heated up to a temperature laying in the range of 900°C to 1100°C. According to one embodiment of this known method a plurality of substrate wafers are arranged in an upwrite position on a quartz boat and placed in a quartz reaction tube. The interior of the quartz reaction tube is evacuated by a vacuum pump. The wafers are heated up to about 900°C to 1100°C. Furthermore SiH₂Cl₂ is introduced as a source gas together with hydrogen (H₂) as a carrier gas into the reaction tube and the interior of the reaction tube is maintained at a reduced pressure of 0,1 to 100 Torr to grow an epitaxial layer. From Japanese Journal of Applied Physics, vol.25, no. 4, part 1, April 1986; pages 524-527, Tokyo, Japan; T. Meguro et al.: "Low-temperature Silicon Epitaxial Growth by CO₂ Laser CVD using SiH₄ gas" and from the Journal of the Electrochemical Society, No. 131, no. 10, October 1984, pages 2430-2435, Manchester, New Hampshire, USA; R. Reif: "Plasma-enhanced Chemical Vapor Deposition of Silicon Epitaxial Layers" processes of silicon epitaxial growth using a halogen-free 100% SiH₄ as a source gas without requireing the use of a carrier gas are known. With the known process of silicon epitaxial growth as it is known from the above-mentioned Journal of the Electrochemical Society, No. 131, No. 10, October 1984, pages 2430-2435, Manchester, New Hampshire, USA much more work is still needed to evaluate the quality of the low temperature films. Generally, it is acknowledged in the art that SiH₄, which is decomposed on a surface of Si, is apt to bring about problems in crystalline quality in comparision with chlorides such as SiH₂Cl₂, SiHCl₃, SiCl₄, etc. Furthermore in connection with the plasma-enhanced CVD there are also existing problems caused by high energy particles produced by plasma discharge. As epitaxy techniques for the formation of an epitaxially grown single crystal layer of an element semiconductor consisting solely of one element such as silicon, there have been heretofore known, for example the already mentioned chemical vapor deposition (hereinafter referred to as "CVD") or the molecular beam epitaxy (hereinafter referred to as "MBE"), etc. Further recently, one of the inventors (Junichi Nishizawa) proposed a molecular layer epitaxy method (hereinafter abbreviated as "MLE") which made possible the formation of a grown single crystal layer with a precision of monomolecular layer by alternately introducing different kinds of gases. (Japanese Patent Application No. 59-153978 which has been laid open to public inspection as Laid-Open No. 61-34928.) However, the above-mentioned known methods have not been completely satisfactory because of the disadvantages set forth below. For example, in CVD, a source gas (e.g.,SiH₄) is introduced simultaneously with a carrier gas (e.g., hydrogen) into a reaction chamber and then single crystals are grown by pyrolysis. In a such manner, contamination by impurities is apt to cause due to the introduction of the carrier gas, thereby causing not only deterioration of the quality of a grown layer, but also other problems, for example, difficulties in controlling with a precision of monomolecular layer. On the other hand, in the case of the MBE in which crystal growth is carried out under ultrahigh vacuum, it is difficult to maintain the rate of crystal growth constant over a long time. Further, the MBE is still inferior in the quality of the resulting crystals as compared to the CVD set forth above. The MLE has been developed with the object of overcoming the disadvantages of these former methods, but this method is disadvantageous in that a gas feeding system is complicated, since different gases are repeatedly alternately fed. Therefore, improvement has been awaited. The present invention has been developed with a view of eliminating the disadvantages associated with the above prior art. More specifically, the present invention is directed to a further improvement in the above MLE. It is accordingly an object of the present invention to provide a novel method for growing a single crystal thin film of an element semiconductor with a precision of monomolecular layer on a substrate by introducing only one kind of gas. According to the present invention this object is solved by the features of the characterizing part of the claim 1. Further improved embodiments of the inventive method can be learned from the subclaims 2 to 4. As described above, in the MLE, different kinds of gases are alternately introduced onto a substrate so that single crystals of an element semiconductor are grown with a precision of the monomolecular layer. In contrast to such a prior art method, the present invention covers a method for growing a single crystal thin film of an element semiconductor which is characterized in that a) said single kind of halogen compound gas containing said element semiconductor as a component element is fed onto said substrate for a period of time in the range of 1 to 60 seconds, b) said gas in the growth chamber is then exhausted for a given period of time in the range of 1 to 120 seconds, and c) the steps a) and b) are repeated successively, thereby growing said single crystal thin film of said element semiconductor in a desired thickness with a precision of monomolecular layer. Brief Description of the Drawings Fig. 1 is a schematic diagram illustrating the construction of the crystal growing apparatus used for carrying out the present invention. Fig. 2 is a graph showing the substrate temperature dependence of the film thickness grown per cycle of gas feeding and exhausting. Fig. 3 is a graph showing the dependence of the film thickness grown per cycle of gas feeding and exhausting upon the gas feeding pressure. FIGS. 4(a) to 4(d) are sequence charts showing the pulses of gas feeding. FIG. 5 is a graph showing the dependence of the growth rate upon the gas feeding pressure when gas is continuously fed for a given period of time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As the result of the inventors' detailed studies, the operating conditions set forth above are practically controlled in such a manner that the pressure inside the growth chamber, the gas feeding time and exhausting time are in the ranges of 10⁻⁴ to 10² Pa (pascal), 1 to 60 seconds and 1 to 120 seconds, respectively, and other conditions such as the heating temperature of the substrate and the flow rate of the gas fed are appropriately adjusted. In practicing the first feature of the present invention wherein gas feeding and exhausting operations are alternately repeated, the operating cycle of feeding a single gas containing an element semiconductor as a component element onto a substrate for a predetermined period of time and then exhausting the gas for a predetermined period of time is repeated until a single crystal thin film having a desired thickness is formed. The operating conditions are appropriately adjusted depending on the intended single crystal thin film and they are not necessarily required to be the same throughout all of the cycles. As the gas containing an element semiconductor as a component element, SiH₂Cl₂, SiHCl₃, SiCl₄, SiH₄ or Si₂H₆ is preferably used in the present invention. The present invention will hereinafter be described more specifically with reference to the following examples. EXAMPLES FIG. 1 is a schematic diagram illustrating the construction of a crystal growing apparatus used for growing a single crystal thin film of an element semiconductor in accordance to the present invention. In FIG. 1, reference numerals 1 and 2 designate a growth chamber made of a metallic material, such as stainless steel, and a gate valve, respectively. Numeral 3 indicates an exhaust for evacuating the growth chamber 1 to an ultrahigh vacuum and numeral 4 indicates a nozzle for introducing gas 13 containing a semiconductor component element. Numeral 5 indicates a heater for heating a substrate 8 of (100) oriented silicon semiconductor crystal. The heater 5 is made of a tungsten wire sealed within a quartz and a infrared lamp 6 is disposed on the upper portion of a lamp house 11 in order to heat the substrate 8. A quartz plate 10 divides the lamp house 11 from the growth chamber 1. Leads, etc., are not shown in FIG. 1. There are also shown a pyrometer 7 for measuring temperatures and a B-A (Bayard-Alpert) gauge 9 for measuring the degree of vacuum inside the growth chamber 1. As a practical example of growing single crystals of IV-Group element semiconductor using such an apparatus, particulary silicon will hereinafter be explained. Silicon single crystal thin films were formed using SiH₂Cl₂ (dichlorosilane) as a silicon-containing gas at various substrate temperatures while keeping the gas feeding pressure constant. FIG. 2 shows the substrate temperature dependence of the thickness of silicon single crystal thin films grown per cycle at various substrate temperatures. Any of the thin films was grown by repeating a cycle of feeding the dichlorosiliane gas into the growth chamber 1 and evacuating the growth chamber 1. In each cycle, gas feeding time and exhausting time were 40 seconds and 20 seconds, respectively. That is, the total time required for one cycle was 60 seconds. The gas feeding pressure was 1.3 x 10⁻² pascal (Pa). The thickness of the thin film grown per cycle varies on the order of one monomolecular layer to several monolayers depending on the temperature of the substrate, as shown in FIG. 2. Particularly, the operating conditions for growing thin films on the order of monomolecular layer, dimolecular layer and half-molecular layer per cycle are respectively described hereinafter. Firstly, in order to grow a monomolecular layer of Si single crystal per cycle, the gate valve 2 was opened and the interior of the growth chamber 1 was evacuated to a pressure of the order of 10⁻⁷ to 10⁻⁸Pa by using the ultrahigh vacuum exhaust 3. The substrate 8 was heated to a temperature of 800°C by the heater 5 or the infrared lamp 6 and its surface was subjected to a heat cleaning for five minutes. Then, the substrate 8 was heated to 855°C by the heater 5 or the infrared lamp 6 and valve 12 was opened to introduce SiH₂Cl₂ gas into the growth chamber 1 over 40 seconds while the interior pressure of the growth chamber 1 was maintained at 1.3 x 10⁻² Pa. Thereafter, the valve 12 was closed and the gas inside the growth chamber 1 was exhausted for 20 seconds. Such an operating cycle grew a monomolecular layer of Si single crystal on the substrate 8. By repeating such a cycle under the same conditions, monomolecular layers were successively grown one after another and thereby a thin film having the desired thickness could be grown with the precision of monomolecular layer. For example, when the above-mentioned cycle was repeated 500 times, a grown thin film approximately 68,0 nm (680 Å) in thickness was formed on the (100) substrate 8. Similarly, in order to grow a dimolecular layer of silicon single crystal per cycle, the growth chamber 1 was evacuated to ultrahigh vacuum and the surface of the substrate 8 was subjected to a heat cleaning. After heating the substrate 8 to 890°C, the valve 12 was opened to introduce SiH₂Cl₂ gas into the growth chamber 1 for 40 seconds while maintaining the interior pressure of the growth chamber 1 at 1.3 x 10⁻² Pa. Thereafter, the valve 12 was closed and the gas inside the growth chamber 1 was exhausted for 20 seconds. By one cycle of such successive operations, a dimolecular layer of Si single crystal was grown on the substrate 8. As one practical example, when the same cycle was repeated 500 times under the same conditions, there was obtained a grown thin film of Si single crystal with a thickness of approximately 136,0 nm (1360 Å) on the (100) substrate. Further, in order to grow a half-molecular layer of Si single crystal per cycle, the growth chamber 1 was evacuated to an ultrahigh vacuum and the surface of the substrate 8 was subjected to a heat cleaning. After heating the substrate 8 to 820°C, the valve 12 was opened to introduce SiH₂Cl₂ gas into the growth chamber 1 for 40 seconds while maintaining the pressure inside the growth chamber 1 at 1.3 x 10⁻² Pa. Thereafter, the valve 12 was closed and the gas inside the growth chamber 1 was exhausted for 20 seconds. By one cycle of such successive operations, a half-molecular layer of Si single crystal was grown on the substrate 8. In an exemplary crystal growing process, when the same operations were repeated 500 times under the same conditions, there was obtained a grown Si single crystal thin film with a thickness of approximately 34,0 nm (340 Å) on the (100) substrate. As set forth above, according to the present invention, it is possible to grow single crystals of one monolayer to several monolayers in thickness with a precision of monomolecular layer by appropriately varying the temperature of the substrate under the same gas feeding pressure. FIG. 3 is a graph showing the dependence of the film thickness grown per cycle upon the gas feeding pressure in which SiH₂Cl₂ gas was used and the gas feeding pressure was varied while keeping the temperature of the substrate constant. In each cycle, the gas feeding time and the exhausting time were 40 seconds and 20 seconds. Thus, 60 seconds was required for each cycle. As practical examples of growing element semiconductor single crystals with a precision of monomolecular layer at a substrate temperature of 850 °C, we will specifically described about the operating conditions which make it possible to grow thin films of the order of one monolayer, dimolecular layer and half-molecular layer in thickness per cycle. Firstly, in order to grow a monomolecular layer of Si single crystal in each cycle, the gate valve 2 was opened and the growth chamber was evacuated to the degree of vacuum where the interior pressure of the growth chamber 1 was about 10⁻⁷ to 10⁻⁸ Pa, using the ultrahigh vacuum exhaust 3. The substrate 8 was heated to 800°C by the heater 5 or the infrared lamp 6 and the surface of the substrate 8 was subjected to heat cleaning for about 5 minutes. Then, the substrate 8 was heated to 850°C by the heater 5 or the infrared lamp 6 and the valve 12 was opened to introduce SiH₂Cl₂ gas into the growth chamber 1 for 40 seconds. Throughout the introduction of the gas, the pressure inside the growth chamber 1 was of 1.5 x 10⁻² Pa. Thereafter, the valve 12 was closed and the gas in the growth chamber 1 was exhausted for 20 seconds. In one cycle of such successive operations, a monomolecular layer of Si single crystal was grown on the substrate 8. These successive operations were repeated under the same conditions to grow successively monomolecular layers. Similarly, in order to grow a dimolecular layer of Si single crystal per cycle, SiH₂Cl₂ gas was fed into the growth chamber 1 for 40 seconds so that the pressure inside the growth chamber 1 was brought to 7.0 x10⁻² Pa and then the growth chamber 1 was evacuated for 20 seconds. A dimolecular layer of Si single crystal was grown on the substrate 8 per cycle. Further, in order to grow a half-molecular layer of Si single crystal for each cycle, SiH₂Cl₂ gas was introduced into the growth chamber 1 for 40 seconds so that the pressure within the growth chamber 1 was brought to 6.0 x 10⁻³ Pa and then the growth chamber 1 was evacuated for 20 seconds. In one cycle of these successive operations, there was obtained a grown single crystal thin film with a film thickness equivalent to the thickness of a half-molecular layer on the substrate 8. The above examples were carried out at a substrate temperature of 850°C, but, also within the substrate temperature range of 750 to 900°C, it is possible to grow single crystals with a precision of the order of monomolecular layer, as in the same manner as described in the above examples, by appropriately selecting optimum conditions for gas feeding pressure and time and the evacuating time. In the foregoing examples, the single crystal thin films having the desired thickness were grown by repeating the successive steps of gas feeding and evacuation under the same conditions throughout the growing process, as shown in a sequence chart in FIG. 4(a). However, it is not always required to conduct every cycle under the same conditions. If necessary, as shown in the sequence charts of FIGS. 4(b) to 4(d), gas feeding pressure and time may be changed. In FIGS. 4(a) to 4(d), feeding pressure and feeding time are indicated by P₀ and P₁ and t₁ and t₃, respectively, and exhaust time is indicated by t₂. In such cases, single crystals are grown to a film thickness of monolayers corresponding to the operating conditions of each cycle. Further examples are illustrated in which element semiconductor single cyrstal thin films having a desired thickness are grown by continuously introducing a single kind of gas on the substrate 8 heated in the growth chamber 1 over a given period of time. FIG. 5 shows the dependence of growth rate of thin films upon the gas feeding pressure. The thin films were formed using SiH₂Cl₂ (dichlorosilane) as an Si-containing gas. The temperature of the substrate 8 was maintained constant and the gas feeding pressure was varied. As noted from FIG. 5, the growth rate (µm per minute) is increased on the order of 0.1 µm. For example, in order to form a grown thin film of 1 µm in thickness, the gate valve 2 was opened and the growth chamber 1 was evacuated to the degree where the pressure within the growth chamber 1 reached 10⁻⁶ Pa or less. The surface of the substrate 8 was subjected to heat cleaning and then the substrate 8 was heated to 1050°C by the heater 5 or by the infrared lamp 6. The valve 12 was opened to introduce SiH₂Cl₂ gas into the growth chamber 1 for 10 minutes, while keeping the pressure inside the growth chamber 1 at 50 Pa. This procedure formed a 1 µm thick single crystal thin film onto the substrate 8. As the ultrahigh vacuum exhaust in the foregoing examples, turbo-molecular pump, cryopump, ion pump or other known vacuum pumps can be employed. Further, in the above described examples, only Si single crystals are described. However, the present invention can also be applied to other IV-Group semiconductors, such as Ge. Further, the material of the substrate is not limited to Si. Other materials, such as sapphire or spinel, can also be employed. As described above, according to the present invention, it is possible to grow element semiconductor single crystal thin films on a substrate with the precision of monomolecular layer using only a single kind of gas containing a component element of an element semiconductor. Since only such a single gas is used as a source gas, there can be readily obtained single crystal thin films of high quality in a high reproducibility, by simplified operating parameters, without causing contamination problems by impurities (e.g., oxygen, carbon, etc.), which are associated with the use of carrier gas such as hydrogen. Further, the single crystal growth of the present invention is advantageous in that the operating temperature for single crystal growth is approximately 250 °C lower than usual epitaxial temperature (approximately 1100 °C). In addition, the present invention has an industrial merit that the growing apparatus can be also simplified, since a gas feeding line is simplified.

Claims: A method for growing a single crystal thin film of an element semiconductor, said method comprising the steps of feeding a single kind of halogen compound gas containing said element semiconductor onto a substrate heated in a growth chamber up to a temperature lying within a temperature range of 750°C to 900°C, characterized in that a) said single kind of halogen compound gas containing said element semiconductor as a component element is fed onto said substrate for a period of time in the range of 1 to 60 seconds, b) said gas in the growth chamber is then exhausted for a given period of time in the range of 1 to 120 seconds, and c) the steps a) and b) are repeated successively, thereby growing said single crystal thin film of said element semiconductor in a desired thickness with a precision of monomolecular layer. A method as claimed in claim 1, wherein said halogen compound gas is any one of SiH₂Cl₂, SiHCl₃, SiCl₄ and SiH₄. A method for growing a single crystal thin film of an element semiconductor, said method comprising continuously feeding a single kind of halogen compound gas containing said element semiconductor onto a substrate heated in a growth chamber at a pressure of 30 to 300 Pa, and thereby growing said single crystal thin film of said element semiconductor in a desired thickness. A method as claimed in claim 3, wherein said halogen compound gas is any one of SiH₂Cl₂, SiHCl₃ and SiCl₄.

Inventors: AOKI, KENJI, NISHIZAWA, JUNICHI
Applicants: NISHIZAWA, JUNICHI, RESEARCH DEVELOPMENT CORPORATION OF JAPAN, SEIKO INSTRUMENTS INC.
Application year: 1987
Main classification: C30B25
IPC codes: C30B25, C30B29, H01L21