Study On Growth And N-type Doping Of Cubic Boron Nitride Thin Films By RF Sputter

Cubic boron nitride (cBN) thin film has been received a considerable attention because of its excellent physical and chemical properties such as high hardness, high chemical stability and large thermal conductivity. The preparation and properties study of cubic boron nitride thin films are attractive and also very difficult to international scientific wold for recent years. In this paper we mainly study preparation, properties and n-type doping of cubic boron nitride thin films.1 Successively depositing cBN thin films on Si substrates which reaches international advanced level, the impact of negative substrate bias voltage and RF powers on the formation of cBN thin films were studied.Boron nitride (BN) films were deposited on (100)-oriented p-type silicon substrate (8I SQcm) with RF sputtering system. The target was hexagonal boron nitride (hBN) of 4N purity, and the working gas was the mixture of nitrogen and argon. The substrates were biased by dc voltage negatively with respect to ground. The films were characterized by infrared spectra and X-ray photoelectron spectroscopy (XPS). Furthermore, the surface morphology of the deposited film was observed by atomic force microscopy (AFM). The experiments show that the condition window for cubic boron nitride (cBN) formation is very narrow. So the formation of cubic boron nitride film is very difficult. For obtaining cBN thin films, it is necessary that substrate negative bias voltage is not lower than 90V and R.F power is not lower than 200W. At 200V of substrate negative bias voltage and 300W of RF power, the content of cubic phase in BN films reaches 92.8%.2 Studying of the properties of cBN thin filmsAFM showed that cBN thin film delaminated from substrate obviously. Basing XPS, we calculate the NIB ratio to be 0.90 that is closing to unity, and the thickness of hBN layer on cBN layer that is about 0.80 nm. The refractive index (at 632.8nm) of cBN thin film with 92.8% cubic phase content is measured to be2. 19 by ellipsometer. The compressive stress of cBN films varied from 1.lGpato 9.3 Gpa.3 For the first time the Gibbs free energy of cBN thin films as a function of compressive stress and temperature was calculated.The elastic strain energy and Gibbs free energy of cubic BN (cBN) thin film in biaxial stress field are calculated. The results show that at a given substrate temperature, there is a compressive stress threshold, below which cBN phase is thermodynamically stable andpabove which hexagonal BN (hBN) phase is thermodynamically stable. At room temperature the compressive stress threshold is calculated to be 9.5 Gpa.4 The influence of substrates on the formation of c-BN thin filmsThe cBN thin films were deposited on different substrates with hot filament assisted plasma CVD and R.F sputter. The substrates are Si .~ Ni Co~ stainless steel and other materials. It is concluded that for CVD method the cubic phase content and adhesion are highly effected by the crystal lattice mismatch between c-BN and substrate materials, however, for sputter method the crystal lattice mismatch between c-BN and substrate materials affects the quality of c-BN thin films very little.5 N-type doping of BN thin films and preparing of BN(n-type)/Si(p-type) heterojunctionsAdding S into the mixture of argon and nitrogen used as working gas, we sputtered 1iBN target to deposit BN thin films so as to study the n-type doping of BN thin films, and BN(n-type)/Si(p-type) heterojunctions were prepared. It is showed that after doping S the BN thin films of n-type conductivity are obtained. The electrical resistivity is estimated to be l.8xIO?~cm and 2.5xIO9~1cm for undoped and S-doped BN films, and the impurity concentration for S-doped BN films is estimated to be 6.52x10?1cm3. The I-V and C-V characteristics of BN(n-type)/Si(p-type) heterojunctions have been studied to close to that of ideal heterojunct ion.6 In this paper the mechanism of cBN formation and BN films n-type doping as well as...