A composite material

Abstract: A composite material useful as a diode capable of operating at a high temperature, i.e., 500 to 600°C or a semiconductor optical device capable of emitting ultraviolet rays is provided which comprises single crystal diamond having electric insulation and single crystal cubic boron nitride formed on one surface of the single crystal diamond as a substrate in such a manner that the single crystal cubic boron nitride has the same plane index on the substrate.

Description: This invention relates to a composite material consisting of an insulating substrate of single crystal diamond and cubic boron nitride epitaxially grown thereon. It is generally known that cubic boron nitride (hereinafter referred to as "cBN") has the largest band gap (6 to 7 eV) as a semiconductor material and both P type and N type semiconductor can be prepared therefrom by the addition of impurities. Thus, it has eagerly been desired to develop the cBN semiconductors for the devices operated at high temperature or ultraviolet semiconductor laser device. When a crystal is grown, in general, the growth is carried out on a substrate and as a substrate for the growth of a cBN single crystal, there has hitherto been used only cBN itself. However, cBN having such a size and quality as to be suitable for use as a substrate has not been obtained up to the present time. Therefore, a good single crystal substrate as a substitute for cBN itself is essential considering the above described application and development of cBN as a semiconductor material. As a substrate material for the growth of cBN, diamond having a lattice mismatch with cBN of only about 1.3% and a substantially equal thermal expansion coefficient has been taken into consideration and up to the present time, a hetero junction of diamond and cBN has been proposed, for example, by W.E. Picket, Phys. Rev. B. vol. 38, 1316, 1988. However, no report has been made on the growth of cBN on diamond as a substrate. There has been no such precedent as succeeding in the growth of cBN on the heterogeneous substrate. That is, when considering the application of cBN to semiconductor devices operated at high temperatures, semiconductor optical devices capable of emitting ultraviolet rays, etc., which have hitherto not been prepared, there arises a problem that there is no suitable substrate for the growth of cBN. We have now developed a composite material comprising cBN formed on a diamond substrate, the composite maerial containing a P type or N type semiconductor of cBN single crystal. We have made various studies and experiments for the growth of cBN using diamond considered most suitable as a substrate material for cBN so as to solve the above described problems and consequently, have found surprisingly that diamond can stably exist even under the cBN-forming conditions by reducing the oxygen content from the atmosphere in which cBN is formed on a diamond substrate. Based on this finding, we have reached the composite material comprising cBN formed on diamond. Accordingly, the present invention provides a composite material comprising a single crystal diamond having electric insulation and single crystal cBN formed on one surface of the single crystal diamond as a substrate in such a manner that the single crystal cBN has the same plane index on the substrate wherein the single crystal cBN is a semiconductor of the P type which is doped with Be as an impurity or a semiconductor of the N type which is doped with at least one of Si, S, Ge or Se. In general, the cBN layer has a thickness of 10 to 100 µm and the diamond substrate has a thickness of 1 to 5 mm. In particularly preferred embodiments of the present invention, the above described composite materials can be provided in which the single crystal diamond substrate is of an artificial Ib type diamond single crystal or wherein the single crystal diamond substrate is of an artificial IIa type diamond single crystal. In the composite material of the present invention, the diamond as a substrate for the growth of cBN is electrically insulating and has a resistivity of at least 10⁴ Ωcm (25°C). Such diamond includes, for example, natural or artificial diamonds of the Ia type, Ib type and IIa type, each having an electric resistivity of 10⁴ to 10¹⁶ Ωnm (25°C). Almost all natural diamonds are of the Ia type, and diamonds synthesized artificially at an ultra-high pressure are usually of the Ib type, but diamond of the IIa type can be synthesized at an ultra-high pressure by adding a material which is a nitrogen lover to the catalyst. In the present invention, therefore, artificial diamonds of the Ib type and IIa type are particularly preferred as the substrate. The P type semiconductor is obtained by doping the grown cBN layer with Be as an impurity and the N type semiconductor is obtained by doping the cBN layer with Si, S, Ge or Se. The composite material of the present invention can be prepared by reducing the oxygen content from the atmosphere in which the cBN is formed on a diamond substrate by a high temperature and high pressure method, or by a CVD method. Preferably, hexagonal boron nitride (hereinafter referred to as "hBN") as a raw material and a solvent material are used which are purified to a high purity and which have a very small oxygen content, or hBN and a solvent material are used each or both of which contain a low oxygen formation energy material such as Mg, Al, Ca, V, Ti, Zr and the like. In the conventional high temperature and high pressure method, the raw material usually conatins 100 to 200 ppm of oxygen and in the conventional CVD method, the raw material gas usually contains 1.33 x 10⁻⁵ to 10⁻⁴ Pascal.ℓ/sec (10⁻⁷ to 10 ⁻⁶, torr.ℓ/sec). In the present invention, on the contrary, the oxygen contents are generally reduced to 5 to 10 ppm and 1.33 x 10⁻⁶ to 10⁻⁷ Pascal.ℓ/sec (10⁻⁸ to 10⁻⁹ torr.ℓ/sec), respectively. The accompanying drawings are to illustrate in detail the principle and merits of the present invention. Fig. 1 is a schematic view of the crystal structure of cBN single crystal. Fig. 2 is a cross-sectional view of an apparatus for the preparation of the composite material of the present invention, used in Examples 1 to 3. Fig. 3 is a Raman spectrum of cBN growth on a diamond substrate according to the present invention in Examples 1 to 3. Fig. 1 is a schematic view of the crystal structure of a cBN single crystal, in which N atoms are represented by  and B atoms are represented by o. As can be seen from this figure, cBN has a crystal structure of the zincblende type, similar to diamond. The lattice constants of diamond and cBN are respectively 3.567 Å and 3.615 Å, corresponding to a lattice mismatch of about 1.3%. Since their thermal expansion coefficients are also close, i.e. respectively 4.50x10⁻⁶ K⁻¹ (700°C) and 4.30x10⁻⁶ K⁻¹ (750°C), it is not impossible to grow cBN on a diamond substrate, but actually, this has not been realised. That is, cBN crystal could not be grown on a diamond substrate in the prior art under such a condition that the diamond substrate was stable. We have made various studies to find the reason that the diamond substrate cannot stably be present and consequently, have found that this is due to the reaction of the diamond with oxygen in the atmosphere for the growth of cBN. Based on this finding, it can be confirmed that cBN can readily be grown on a diamond substrate if oxygen is completely removed from the atmosphere or reduced to the oxygen content hereinbefore described for the growth of cBN and the present invention is thus realized. It is found by the reflection high energy electron ray diffraction (RHEED) or Raman spectrum that the cBN grown on a diamond substrate with an electric insulation according to the present invention is single crystal cBN. When this cBN is doped with an additional element as an impurity, a semiconductor cBN with a diamond substrate can be obtained. As illustrated above, according to the present invention, a semiconductor cBN grown on an insulating substrate can be provided and making the best use of the property peculiar to cBN, i.e. its wide band gap, a semiconductor device capable of operating at a high temperature and a semiconductor optical device capable of emitting ultraviolet rays can be realized. The composite material of the present invention consists of cBN and a diamond substrate, which can be applied to a diode capable of operating at a high temperature, i.e. 500 to 600°C,or a semiconductor optical device capable of emitting ultraviolet rays by the best use of the excellent properties peculiar to cBN as a semiconductor, for example, wide band gap and thermal and chemical stability. Furthermore, since in the composite material of the present invention, diamond having electric insulation is used as a substrate, there is no electric interaction between the elements in an integrated circuit and high integration thereof is thus rendered easy, whereby very large scale integration can be realized by the joint use of the thermal property of the cBN semiconductor. The following examples are given in order to illustrate the present invention in detail without limiting the same. Example 1 Using a high temperature and pressure generating apparatus as shown in Fig. 2, the composite material of the present invention was produced by growing cBN on a diamond substrate. In this apparatus, a reaction chamber was surrounded by a molybdenum capsule 3 to prevent it from direct contact with a pressure medium 2 such as hBN and heated at a predetermined temperature by passing AC or DC current through a graphite heater 1. In the reaction chamber, a boron nitride source 4 was arranged so as to be at a higher temperature than a diamond substrate 5 and a solvent 6 such as Li₃SrBN₂ was arranged between the boron nitride source 4 and diamond substrate 5. The boron nitride was dissolved in the solvent 6 from the boron nitride source 4 at a high temperature and high pressure at which cBN was stable and precipitated on the substrate crystal 5 at the relatively lower temperature, based on the solubility difference of boron nitride due to the temperature difference. In Fig. 2, 7 designates an anvil, 8 designates a die, 9 designates a heat insulating pressure medium and 10 designates a power supply part. ΔT means a temperature gradient between the boron nitride source 4 and diamond substrate 5. The specific data of this experiment were as follows: As the boron nitride source 4, there was used a compact of 4 mm in diameter and 2 mm in height, consisting of powdered hBN and cBN which had been subjected to a pretreatment for completely removing oxygen, and as the solvent 6, there was used a compact of 4 mm in diameter and 7 mm in height, consisting of Li₃BrBN₂. The pretreatment for the removal of oxygen was carried out by heating and calcining the boron nitride source for 1 to 2 hours in high purity nitrogen at 2000 to 2100°C to reduce the oxygen content to 10 ppm or less. As the substrate crystal 5, there was used a diamond single crystal of artificial Ib type with a maximum side of 3 mm, which was arranged in such a manner that its (1 1 1) plane was contacted with the solvent 6. When growth of cBN was carried out at a pressure of 65 kbar and a temperature of 1700°C for a retention time of 2 hours using the above described reaction system, a yellow film with a thickness of about 100 µm was formed on the diamond substrate. Identification of this film by a Raman spectrum told that it consisted of cBN crystal having a peak near 1304 cm⁻¹ and 1055 cm⁻¹ as shown in Fig. 3. Example 2 cBN was grown under the similar conditions to Example 1 but using artificial diamond of IIa type as a substrate instead of the artificial diamond of Ib type used in Example 1 and the boron nitride source further containing 4% by weight of Be powder. The thus resulting film was a blue film with a thickness of about 100 µm, having an electric resistance of about 10³ Ωcm and being electrically conductive. Example 3 cBN was grown under the similar conditions to Example 1 but using the boron nitride source 4 of Example 2, to which 5% by weight of silicon powder was further added. The thus resulting film was an orange film with a thickness of about 80 µm, having an electric resistance of about 10⁴ Ωcm and being electrically conductive. Example 4 Using the (1 1 1) plane of an artificial diamond single crystal as a substrate, a cBN film was grown by a vapor phase method wherein B₂H₆, N₂ and H₂ as raw material gases were fed to a reaction chamber respectively at flow rates of 10 cc/min, 10 cc/min and 100 cc/min and while adjusting the pressure inside the reaction chamber to 1 torr, and these gases were reacted for 4 hours at a DC plasma discharge output of 500 W, substrate temperature of 600°C and tungsten filament temperature of 200°C for preheating, and consequently, a film with a thickness of about 10 µm was deposited on the substrate surface. The raw material gas had an oxygen content corresponding to 1.33 x 10⁻⁷ Pascal.ℓ/sec (10⁻⁹ torr.ℓ/sec). Identification of the thus resulting film by a Raman spectrum told that it consisted of cBN having sharp peaks near 1304 cm⁻¹ and 1055 cm⁻¹.

Claims: A composite material comprising a single crystal diamond having electric insulation and single crystal cBN formed on one surface of the single crystal diamond as a substrate in such a manner that the single crystal cBN has the same plane index on the substrate wherein the single crystal cBN is either a semiconductor of the P type which is doped with Be as an impurity or a semiconductor of the N type which is doped with at least one of Si, S, Ge or Se. A composite material as claimed in claim 1, wherein the single crystal diamond is that of the artificial Ib type. A composite material as claimed in claim 1, wherein the single crystal diamond is that of the artificial IIa type. A process for the production of the composite material as claimed in claim 1 by a high temperature and high pressure method, characterized in that an atmosphere in which cBN is formed on a diamond substrate has an oxygen concentration of 10 ppm or less. A process as claimed in claim 4, wherein hBN and a solvent material, used as raw materials, are purified to decrease the oxygen content thereof. A process as claimed in claim 4, wherein hBN and a solvent material, used as raw materials, contain at least one of Mg, Al, Ca, V, Ti and Zr. A process for the production of the composite material as claimed in claim 1 by the CVD method, characterized in that an atmosphere in which cBN is formed on a diamond substrate has an oxygen partial pressure of 1.33 x 10⁻⁶ to 10⁻⁷ Pascal.ℓ/sec (10⁻⁸ to 10⁻⁹ torr.ℓ/sec).

Inventors: TSUJI KAZUSWO C O ITAMI WORKS, YOSHIDA KATSUHITO C O ITAMI WO, TSUJI, KAZUSWO, C/O ITAMI WORKS OF, YOSHIDA, KATSUHITO, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C30B29
IPC codes: H01L33, B01J3, C01B31, H01S5, H01L29, H01L21, C30B29, C01B21, C30B9