Preparation And Doping Of Wide Band Gap Cubic Boron Nitride Films

Cubic boron nitride (cBN) is a superhard and wide band gap semiconductor material with outstanding physical and chemical properties. The cBN crystal synthesized by high-pressure high-temperature (HPHT) method has been the important material for cutting and milling tools because of its extreme hardness and thermal conductivity, which are only second to diamond, excellent oxidation resistance at high temperature and the chemical inertness to ferrous materials. Cubic BN has more attractive potential properties as a wide band gap semiconductor material for it has, except for its great mechanical properties, the widest band gap (Eg= 6.3±0.2 eV) and can be doped for both n- and p-type conductivity. So, cBN is a very promising material for fabrication of high power and high-temperature devices operating in harsh environment and short wavelength emitter or detector.The size of HPHT synthesized cBN crystals is too small to meet the demands of the situations which require a large area, such as electrical devices and coating on cutting tools as well. To fully exhibit the excellent properties of cBN, there are more and more research interesting in depositing cBN films at low pressure and low temperature since the great developments of thin film science and technology had been done. In this work, based on the background above, two parts on preparation and doping of cBN films are involved. The advanced progress in the field which is about epitaxial growth and doping of cBN films will be presented.The cBN films are deposited by three kinds of systems: radio frequency (RF) sputtering, RF magnetron sputtering (RF-MS) and electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-CVD). RF sputtering is the first employed to study the deposition of cBN. The thresholds of substrate bias and temperature are found to be -150 V and 300℃respectively. The film with cubic phase fraction of 76% is prepared with the optimized bias of -210 V and temperature of 500℃. It is confirmed that the two-step process with RF sputtering can produce the film with the better adhesion which due to the lower stress in the film. There are several facts of the RF sputtering system should be improved for deposition of cBN. The heater in the system can only provide the substrate temperature of 500℃and the ultimate vacuum is about 10-4 Pa/10-6 Torr which are not good enough. The matching network and vacuum system are not stable enough for enhancing the quality of cBN and they damage the reproducibility as well.The study by employing RF-MS and ECR-CVD system indicate this two can both produce much better cBN films, especially for ECR-CVD system. The quality can be improved greatly by introducing fluorine chemistry into the microwave plasma and the selective etching the non-cubic phase by fluorine allow cBN film deposition at low substrate bias. So, the film with more cubic phase, lower residual stress, better crystallinity and larger thickness can be obtained. The films can exhibit Raman and X-ray diffraction (XRD) signal well on which the grain size can be estimated.Diamond films on silicon wafer which are deposited by microwave plasma CVD are employed as substrate to improve the cBN film further since their lattice constant and surface energy are similar. The epitaxial cBN films with the cBN fraction close to 100% are achieved by RF-MS and ECR-CVD. High resolution transmittance electronic microscope (HRTEM) images demonstrate the direct growth of cBN on diamond without any aBN/tBN incubation layer. The films have great adhesion and do not delaminate. The polycrystalline cBN films prepared by ECR-CVD have better crystallinity and lower stress than nano-crystalline ones produced by RF-MS and their thickness can achieve to several micrometers, which make the electrical and mechanical applications of cBN more promising.The mechanisms for depositing cBN films by sputtering and ECR-CVD are discussed based on the characterizations of high quality films. It is considered that the subsurface nucleation and growth which accord with subplantation model is responsible for sputtering procedure, while it's the surface growth of cBN in fluorine-based ECR-CVD. Fluorine-containing species play a very important role in this surface CVD growth. Flurione atoms etch hBN surface while BFx and NHx is responsible for crystal growth. The ECR-CVD process is obviously more promising for practical applications of cBN.The research for doping of cBN films are carried by ion implantation. Beryllium (Be) and sulfur (S) are employed as p-type and n-type dopant respectively and the corresponding conductive types are achieved. This is internationally innovative work at a certain extent.Beryllium ions are implanted in the cBN films prepared by RF sputtering. The resistivity decreases by 6 orders after post-implantation high temperature annealing. The p-type conductivity is revealed by Hall-effect measurement. The corresponding mobility are of 14~28 cm~2/V·s and carrier concentration are 1019~1020 cm-3. P-BN/n-Si heterojunction which has obvious rectifying characteristic is achieved between Be-implanted BN film and silicon substrate. Space-charged-limited current is demonstrated by I-V characteristic measured from the surface which indicates the existence of shallow traps in the film. The fitting equation of the forward I-V data of the heterojunction is comparable with the ideal diode equation and Anderson model for heterojunction. The junction is supposed to have other transport mechanism than diffusion.The nano-crystalline and polycrystalline cBN films prepared by RF-MS and ECR-CVD respectively are doped to be p- and n-type semiconductor with Be and S as the dopants respectively. The conductive types are demonstrated by Hall-effect measurements and the corresponding mobility (Be:3~31.2 cm~2/V?s;S:384 cm~2/V?s), carrier concentration (1017~ 1018 cm-3) and resistivity are also obtained. The activation energies of implanted ions are calculated from temperature dependence of the sheet resistance. It is interesting to note the hole mobility (3 cm~2/V·s) and activation energy (0.2 eV) obtained in Be-implanted polycrystalline cBN film are comparable to that measured on Be-doped HPHT cBN single crystals (2 cm~2/V·s and 0.23 eV). The activation efficiency is estimated to be around 4% here which is close to that of the boron ions implanted in polycrystalline diamond films (~1%). The activation energies of Be ions implanted in nano-crystalline cBN films (0.27 eV) is slightly larger than that in polycrystalline film (0.2 eV) which is supposed to be related to the more grain boundaries and defects in the nano-crystalline cBN films. Beryllium is a shallow impurity for cBN.The work shows ion implantation is the effective doping process for cBN film and the electrical properties are affected greatly by dose and ion energy. The proper dose and ion energy are necessary for desired doping effect. The electrical properties will be changed more strongly by larger dose and more obvious doping effect can be observed. The ions implanted with lower ion energy have higher activation efficiency due to the comparative even ion distribution.