ИСТИНА

Boron-doped diamond, a p-type semiconductor, is viewed as a promising material for microelectronics due to its unique properties. At a high dopant concentration, diamond exhibits a number of interesting properties from both scientific and practical viewpoints. For example, at a boron concentration higher than 1020cm-3, diamond has metallic conduction [1], while at a boron concentration of >2*1021 cm-3, it becomes a superconductor at T= 4 K [2]. If diamond film has been created by using of RFPECVD method, the impurity doping process can be realized by adding boron-contained substance to the working mixture. At the same time morphology and doping level in many respects depend on processes occurring in rf-plasma during film growth. These processes may be controlled by insitu diagnostics of optical emission spectra of plasma and thus diamond films of required quality can be grown. In this paper we insitu investigated optical emission spectra of rf-plasma in the region from 200 nm to 800 nm during boron doped polycrystalline diamond films growth. Special attention was given to analysis of intensity of line on 249.7 nm corresponding with transition from 3s to 2p in boron atom. Relative intensity of this line allowed to measure amount of boron in plasma. Hydrogen combined with ethanol was used as working mixture. Trimethylborate (TMB) was added to working mixture in order to get boron doped polycrystalline diamond films. All samples were grown on Si substrate with orientation (100). Thickness of all films was, approximately, 4 µm. The boron content in the films was determined by secondary-ion mass spectroscopy. Polycrystalline diamond films were grown at a TMB concentration of 1, 2, 3, and 4% in the gas mixture. The boron concentration in the films was 0.34, 0.6, 0.9, and 1.2%, respectively. Scanning electron microscope analysis of the diamond films showed that they consist of crystals with, approximately, 1 µm size. We used Raman spectroscopy method for morphology investigation of polycrystalline diamond films. Also, absorption spectroscopy method was used for optical properties investigation of all grown films. In order to measure optical absorption spectra of the grown films (in the transmission mode), we etched part of the silicon substrate in each sample so that the diamond film represented a membrane about 0.5 cm in diameter. In order to record absorption spectra, we used two continuous spectrum sources, namely, a hollow-cathode deuterium lamp operating in the UV range and a tungsten incandescent lamp operating in the visible and near-IR regions. Raman spectra (FIG.1) were measured with using of laser radiation with wavelength 532 nm and power of 40 mW. Correlation dependences between features of optical emission spectra of plasma and spectra of Raman scattering were defined. The effect of the degree of boron doping of polycrystalline diamond films on their absorption spectra has been studied in the region from 200 to 1000 nm (FIG.2). It has been shown that optical adsorption spectra revealed presence of the peak with maximum intensity which corresponds to photon energy with, approximately, 2 eV. Adsorption intensity in this region of energies was proportional to boron concentration in the films. Such dependence of adsorption coefficient was explained by boron segregation on crystallite faces, which indicates that boron atoms are incorporated in both the diamond lattice and amorphous graphite located between diamond crystallites. FIG.1. Raman spectra of the polycrystalline diamond films: (A) undoped polycrystalline diamond film, (B) 0.34%, (C) 0.6%, (D) 0.9%, and (E) 1.2% of boron in the films, respectively. The right-hand inset shows the Raman spectrum of the undoped diamond single crystal; the left-hand inset shows the Raman spectrum of polycrystalline graphite. FIG.2. Absorption coefficient Ka vs. incident photon energy E: (A) undoped diamond film, (B) 0.34%, (C) 0.6%, (D) 0.9%, and (E) 1.2% of boron in the films, respectively. 1. Zhang R.J., Lee S.T., Lam Y.W. // Diamond Relat. Mater. 1996. vol. 5. P. 1288-1294. 2. Ekimov E.A., Sidorov V.A., Bauer E.D. et al. // Letters To Narute 2004. vol. 428, P. 542-545 (2004).