Process for forming a diamond-metal junction product

Abstract: A diamond product comprising a semiconductor layer having an outer surface region of graphite in contact with the metal electrode to form an ohmic junction with the metal electrode, wherein the outer surface region of graphite has a thickness of at least about 12 Å.

Description: The present invention relates to a process for forming a diamond-metal junction product in which a diamond semiconductor and a metal are integrated through an ohmic junction. DESCRIPTION OF THE RELATED ART In general, diamond has a large band gap of about 5.5 eV and a large carrier mobility (1800 cm 2 /V·S for electrons and 1600 cm 2 /V·S for holes), as well as a large thermal conductivity of about 20 W/cm·k. In addition, diamond exhibits a very high level of hardness and, hence, superior wear resistance. Thus, diamond possesses various advantageous properties which can never be offered by other materials. In recent years, remarkable progress has been achieved in the technique for synthesizing diamond from gaseous phases, and it is now possible to form a diamond film by chemical vapor deposition (CVD). Studies also have been made to develop a technique for producing semiconductors by doping diamond crystals with impurities. Shiomi et al. reported that a field effect transistor can be produced by forming, on an artificial diamond substrate, a diamond film containing boron as an impurity by a vapor synthesis technique and then forming Schottky junction metal and ohmic junction metal. Hitherto, however, it has been difficult to form metal electrodes on diamond crystal by diamond-metal junction with a high degree of reproducibility of the characteristics of the product. Namely, characteristics of a diamond-metal junction material largely vary according to factors such as the conditions for forming the diamond, the conditions of processing after formation of the diamond and the methods of forming the electrodes. For instance, whether the type of junction formed is an ohmic junction or a Schottky junction depends on whether a methane-hydrogen type gas or a carbon monoxide-hydrogen type gas is used as the material gas in forming the diamond. It is also known that a metal junction which has exhibited an ohmic nature is changed into a metal junction exhibiting a Schottky nature when diamond crystal is scrubbed with a chromic acid mixed solution. Characteristics also vary largely depending on whether the metal electrode is formed to make a point conduct or evaporation-deposited on the diamond layer. Thus, it has been difficult to obtain a product having metal electrodes by a diamond-metal junction with a high degree of reproducibility. Moreover, JP-A-30-58 481 discloses a diamond product having a graphite layer which is obtained by treating a surface of diamond by means of RF glow-discharge and ion implanting. In addition, JF-A-20-34 979 discloses a process of forming a semiconductor diamond layer on a silicon substrate, subsequently changing a condition by elevating the concentration of gas such as methane or CO and increasing the temperature of the silicon substrate, and forming an amorphous carbon or a graphite layer on the surface of the semiconductor layer. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a process for forming a diamond-metal junction product which exhibits excellent ohmic characteristics. A second object of the present invention is to provide a process for forming ohmic junction electrodes having a high degree of reproducibility. A third object of the present invention is to provide a process for forming a diamond-metal junction product in which an ohmic junction is formed between a semiconductor diamond layer and a metal layer through the intermediary of a graphite layer having a thickness of at least 1,2 nm (12 Å). A fourth object of the present invention is to provide a process for forming a diamond product comprising a stabilized outer surface of graphite formed on a diamond layer, wherein said outer surface of graphite has a thickness of at least 1,2 nm (12 Å). A fifth object of the present invention is to provide a process for forming a diamond product comprising a semiconductor diamond layer having an outer surface region of graphite in contact with a metal electrode to form an ohmic junction with the electrode, wherein the outer surface region has a thickness of at least about 1.2 nm (12Å). According to claim 1 the present invention, the above objects are achieved by a process for forming a diamond product which comprises a diamond layer and a metal layer, wherein a stabilized outer surface layer of graphite having a thickness of at least 1.2 nm is formed on said diamond layer, comprising the steps of forming the diamond layer on a substrate using a carbon containing gas, and forming the surface layer of graphite by increasing the carbon source concentration to 1.2 % or higher in terms of the carbon containing gas and increasing the substrate temperature in a final stage of forming of the diamond layer. Moreover, the above objects are achieved by a process for forming a diamond-metal junction product of a semiconductor diamond layer having an outer surface region of graphite in contact with a metal-electrode to form an ohmic junction with said metal-electrode, comprising the steps of the process for forming a diamond product as defined above and the step of forming the metal-electrode on the surface of graphite. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is an illustration of a diamond layer having an outer surface of graphite; Fig. 1B is an illustration of a diamond layer having an outer surface of graphite in contact with a metal electrode; Fig. 1C is an illustration of a diamond layer having an outer surface of graphite in contact with a metal electrode, wherein the diamond layer is supported by a substrate; Fig. 2 is a schematic illustration of a diamond forming apparatus suitable for use in forming a diamond layer of a diamond-metal junction product in accordance with the present invention; Fig. 3 is a schematic illustration of a method for measuring ohmic junction characteristics; and Fig. 4 is. a graph showing I-V characteristics of a diamond-metal junction product not according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Through an intense study, the present inventors have found that the metal junction characteristics are largely governed by the state of the surface of the diamond layer, e.g., the molecular nucleus adsorbed on the diamond surface. The inventors have found that the production of a diamond-metal junction product is best achieved with the stabilization of the state of the diamond surface. With this knowledge, the inventors have conducted experiments to discover conditions for stabilizing the diamond surface and found that superior ohmic junction characteristics can be obtained with a high degree of reproducibility when a graphite region is formed on the surface of the diamond layer. By forming a graphite layer of about 1,2 nm (12 Å) or thicker, it is possible to reduce the influence of nucleus adsorbed on the diamond surface, thus stabilizing the state of the diamond layer surface. The reason why such stabilization is attained has not yet been theoretically determined. It is, however, considered that this advantageous effect owes to the fact that the graphite layer having metallic characteristics can be directly formed on the diamond layer surface without any intermediary because both graphite and diamond contain carbon atoms. Amorphous carbon which has comparatively high resistivity is not suitable for use as the junction layer in the diamond-metal junction product of the present invention, and graphite layer having low resistivity is preferably used as the junction layer. Discrimination between diamond crystals, graphite crystals and amorphous carbon is possible by various methods such as observation of lattice image through an electron microscope or electron bean diffraction. It is also possible to discriminate by spectral analysis of valence band such as by using XPS (X-ray photoelectron spectroscopy) or ELS (energy loss spectroscopy). The thickness of the graphite layer is about 1,2 nm (12 Å) or greater, preferably about 1,5 to 50 nm (15 to 500 Å), and more preferably about 2 to 10 nm (20 to 100 Å). It is not always necessary that the graphite layer is formed uniformly over the entire area of the diamond layer. When the thickness of the graphite layer varies according to position, it is preferred that the mean thickness falls within the range stated above. Thus, the graphite layer may be formed only on an area where a metal electrode is to be formed. The above-described effect of graphite layer for stabilizing the state of the diamond layer surface is not appreciable when the thickness of the graphite layer is about 1 nm (10 Å) or less, and is saturated when the graphite layer thickness is increased beyond 50 nm (500 Å). According to the present invention, the graphite layer is formed by increasing the concentration carbon source and substrate temperature in the final stage of synthesis of the diamond. When the graphite layer is formed by elevating the carbon source concentration and substrate temperature in the final stage of the diamond synthesis, the elevated carbon source concentration is 1.2 % or higher in terms of carbon-containing gas, e.g., methane, and the elevated substrate temperature is, for example, 1000°C or higher. The graphite layer is a region of graphite forming the outer surface of the diamond layer. The graphite layer can be a region of graphite as formed above. Further, a discrete graphite layer can be formed by conventional methods on the diamond layer such as by chemically vapor depositing a graphite layer onto the diamond layer or by bonding or laminating a graphite layer to the diamond layer to form a graphite surface on the diamond layer. In this specification, the term "metal" is used to generally cover metals and alloys usable as the material of the ohmic electrode, unless otherwise specified. More specifically, such metal includes Pt, Au, Ag, Cu, Ti, Zr, Hf, V, Nb, Cr, Mo, Mn, W, Sn, Zn, Ta, Al, Si, Fe, Co, Ni and other metals, alloys of such metals, as well as carbides, sulfides and borides thereof. The electrode may be formed by a known technique such as vacuum evaporation, sputtering and so forth. Fig. 1A shows a diamond layer 12 having an outer surface region or layer of graphite 16. Fig. 1B shows the diamond layer 12 having an outer surface region or layer of graphite 16 in contact with a metal electrode 14. Fig. 1C shows the diamond layer 12 having an outer region or layer of graphite 16 in contact with the metal electrode 14, where the diamond layer 12 is superposed by a substrate 11. The vapor phase synthesis of diamond is conducted preferably but not exclusively by CVD (chemical vapor deposition). Fig. 2 shows a CVD apparatus suitably employed in the production of diamond. The apparatus has a quartz reaction tube 1, a heater 2 and a tungsten filament 3. Numeral 4 designates a silicon substrate. The quartz reaction tube 1 has a gas inlet port 5 and a discharge port 6 which is connected to an evacuating device and a pressure regulating valve which are not shown. The material gas for forming diamond crystal may be a hydrocarbon gas such as methane or ethane or a gas obtained by heating a liquid organic compound such as alcohol and acetone. The material gas, when introduced into the reaction tube, may be diluted by hydrogen or oxygen as required. In order to form a semiconductor diamond, a compound containing boron, phosphor, nitrogen or lithium is added to the material gas. The material gas containing such compound is decomposed by the heat from the filament 3 which is heated to about 2000°C, so that diamond crystals are formed on the substrate. The described method of forming diamond, however, is not exclusive and various other methods can be employed such as microwave plasma CVD, radio-frequency plasma CVD, D.C. plasma CVD, field microwave plasma CVD and burning flame method. Examples Examples of production of diamond-metal junction product of the present invention are shown below. Preliminary Example 1 A semiconductor diamond was formed by using the apparatus shown in Fig. 2. A single-crystal silicon substrate was placed as the substrate 4 within the quartz reaction tube 1. The single-crystal silicon substrate had a diameter of 25 mm, thickness of 0.5 mm and specific resistivity of 1 Ω·cm and was a p-type semiconductor. The evacuation device (not shown) was activated to reduce the pressure in the reaction tube down to 1,33 · 10 -4 Pa (10 -6 Torr) and then the heater 2 was energized to heat the silicon substrate to 800°C. Subsequently, methane gas, hydrogen gas and diborane gas (concentration 1000 ppm diluted with hydrogen) were introduced into the reaction tube 1 from respective cylinders. The rates of supply of the methane gas, hydrogen gas and diborane gas were controlled to 1 standard cm 3 /min (SCCM), 200 standard cm 3 /min (SCCM) and 1 standard cm 3 /min (SCCM), respectively, by using flowmeters and flow rate controllers. Then, the pressure regulating valve (not shown) was opened to set the pressure in the reaction tube to 6,65 · 10 3 Pa (50 Torr), and electrical power was supplied to the filament 3 from a power supply (not shown) so as to red-heat the filament to about 2000°C. The material was decomposed by the heated filament, whereby diamond was formed on the substrate 4. A diamond crystal layer of about 4 µm thick was formed on the substrate after 5-hour reaction. The diamond crystal layer was then annealed in the reaction tube 1 so that the surface was graphitized. More specifically, the graphitizing annealing was conducted in a nitrogen atmosphere at a pressure of 6,65 · 10 4 Pa (500 Torr) and a temperature of 950°C for 10 minutes. Then, Ti was evaporation-deposited by vacuum evaporation method on the thus graphitized surface of the diamond crystal layer, and I-V characteristics were measured by a method illustrated in Fig. 3. In Fig. 3, numeral 11 designates a silicon substrate, 12 denotes a diamond layer, 13 denotes a silver paste (electrode with ohmic junction with silicon), 14 denotes the evaporation-deposited Ti film and 15 denotes a D.C. source and an ammeter. The line A in Fig. 4 shows the I-V characteristics as measured by the method shown in Fig. 3. It will be seen that good ohmic junction characteristics has been obtained. Diamond crystal after graphitization conducted under the same conditions as above was observed through an electron microscope to confirm that a graphite layer of 1,5 nm (15 Å) thick was formed on the surface of the diamond. Comparative Example 1 Diamond layer was formed under the same conditions as Preliminary Example 1 except that the graphite layer was not formed, and the I-V characteristics of a sample thus obtained were measured. The characteristics as measured were shown by the curve B in Fig. 4. It will be seen that ohmic junction characteristics are inferior in this case. Preliminary Example 2 After forming a diamond crystal layer by the same method as Preliminary Example 1, a graphite layer was formed by treating the diamond crystal layer for 10 seconds in the same apparatus at a pressure of 1,33 · 10 3 Pa (10 Torr) and oven temperature of 900°C under the supply of CH 4 and H 2 at rates of 8 standard cm 3 /min (SCCM) and 100 standard cm 3 /min (SCCM), respectively, while heating the filament to 2000°C. The diamond crystal layer with the graphite layer thus formed was subjected to measurement of I-V characteristic conducted under the same conditions as Preliminary Example 1. Superior ohmic junction characteristics well comparing with that of Preliminary Example 1 was confirmed. Diamond crystal after a graphitization conducted under the same conditions as above was observed through an electron microscope to confirm that a graphite layer of 3 nm (30 Å) thick was formed on the surface of the diamond. Examples 1 to 5 and Comparative Example 2 Diamonds were formed on Si substrates by microwave plasma CVD method, by supplying methane, hydrogen and diborane (concentration 100 ppm, diluted with hydrogen) at rates of 1.5 standard cm 3 /min (SCCM), 200 standard cm 3 /min (SCCM) and 0.8 standard cm 3 /min (SCCM), respectively, while setting the microwave power to 400 W. The substrate temperature and the pressure in the quartz reaction tube were maintained at 830°C and 6,65 · 10 3 Pa (50 Torr), respectively. After 6-hour synthesis operation, semiconductor diamond films of abut 5 µm were formed on the substrates. A graphite layer was formed on each of the thus-formed diamond crystal layer by using the same apparatus as that used in Preliminary Example 1. The graphitization was conducted while setting the microwave output to 500 W, at a pressure of 1,33 · 10 4 Pa (100 Torr) and substrate temperature of 980°C, under supply of CH 4 and H 2 at 10 standard cm 3 /min (SCCM) and 150 standard cm 3 /min (SCCM), respectively. The graphitization time was varied according to the samples. The samples thus obtained were subjected to measurement of ohmic junction characteristic to obtain results as shown in Table 1 below. The graphite layer thickness values were determined through electron microscopic observation. graphitizing time Graphite layer thickness Electrode material Ohmic junction characteristic Ex.1 15 sec 1,5 nm (15Å) Au Good Ex.2 60 sec 5 nm (50Å) Au Excellent Ex.3 300 sec 25 nm (250Å) Pt Good Ex.4 10 sec 1,2 nm (12Å) Ti Acceptable Ex.5 600 sec 50 nm (500Å) Pt Good Comp.Ex. 2 5 sec 0,7 nm (7Å) Ti Not good Comp.Ex. 3 8 sec 1 nm (10Å) Ti Not good While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims: A process for forming a diamond product comprising a diamond layer (12) and a metal layer (14), wherein a stabilized outer surface layer of graphite (16) having a thickness of at least 1.2 nm is formed on said diamond layer (12), comprising the steps of: forming the diamond layer (12) on a substrate (11) using a carbon containing gas; and forming the surface layer of graphite (16) by increasing the carbon source concentration to 1.2 % or higher in terms of the carbon containing gas and increasing the substrate temperature in a final stage of forming the diamond layer; and forming a metal layer (14) on the surface layer of graphite (16). A process for forming a diamond product according to claim 1, wherein the steps of forming the diamond layer (12) and forming the surface layer of graphite (16) are conducted by chemical vapor deposition. A process for forming a diamond product according to any of claims 1 and 2, wherein in the step of forming the surface layer of graphite (16) the substrate temperature is elevated to 1000°C or higher. A process for forming a diamond product according to claim 3, wherein the carbon-containing gas is methane. A process for forming a diamond product according to any of the preceding claims, wherein the surface layer of graphite (16) has a thickness within the range of from 1.5 nm to 50 nm (15 Å to 500 Å). A process for forming a diamond product according to claim 5, wherein the surface layer of graphite (16) has a thickness within the range of from 2 nm to 20 nm (20 Å to 200 Å). A process for forming a diamond-metal junction product according to any of claims 1 to 6; wherein the metal layer is a metal - electrode.

Inventors: HIRABAYASHI KEIJI, MATSUSHIMA MASAAKI, HIRABAYASHI, KEIJI, MATSUSHIMA, MASAAKI, HIRABAYASHI, KEIJI, C/O CANON KABUSHIKI KAISHA, Matsushima, Masaaki, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, C30B29, H01L21, C04B37