The Study On Doping And Properties Of Cubic Boron Nitride Crystals

Cubic boron nitride (cBN) possesses many excellent properties, such as veryhigh thermal conductivity and dielectric strength, outstanding thermal and chemicalstabilities, and both N-type doping and P-type doping, therefore, cBN has potentialapplications in high-temperature and high-power electronic devices and deepultraviolet optoelectronic devices. The energy gap of cBN is about6.4eV, which isthe largest in III-V compounds. The resistivity of cBN is usually more than109cmand should be improved by doping. At present, the main doping method is in situdoping. Others are seldom adopted. But in situ doping technology is mainly applied inlaboratory for scientific researches, and often induces the congregation of impuritiesand impure crystal phases.In this dissertation, the conventional semiconductor doping technologies wereused to research the doping of cBN. Firstly, the high temperature thermal diffusionmethod was used for doping Si into cubic BN crystal, and the effects of diffusiontemperature and time on the doped cBN were investgated. The conductivity ofSi–doped cBN increased a little. Because the radius of Si atom is comparable to thelength of B-N bond, thus, Si impurities mainly concentrate in the surface layer of cBNsamples and made little influence on the bulk resistance of cBN crystal. Theactivation energy of Si impurities was calculated according to the current-voltagecurves measured in different temperature. The higher diffusion temperature is, or thelonger diffusion time is, the lower the activation energy could be. The doped cBNwere analyzed by XPS spectra. The result showed that Si impurities mainly occupiedthe lattice B atoms and combined with N atoms, and became donors. However, due toB atoms in cBN are in excess, the small quantity of B-Si bonds cannot be excluded.In order to obtain P-type cBN, ion implantian was used to dope Be into cBNcrystals. The size of Be atom is small, and Be is effective p-type impurity for cBN.The software SRIM2008was used for simulating the ion implantation process. Theresults showed that the ion project range was2646，and lots of defects weregenerated that may lead to the crystals amorphization. Annealing can eliminate thesedefects. In this work, ion implantion did not affect the surface topography of cBN.XPS results indicated cBN had no phase transition, but no Be peaks was found. TheSEM showed the surface of cBN was smooth and had no defects. The EDS verified Be impurities existed in the surface layer of cBN, but the content of Be was hard todetermine since the resolution of EDS was low for light elements. According to thecoplanar I-V charateictics, the surface conductance of cBN decreased after Be ionimplantation, which was probably because of the compensation of Be impurities andthe intrinsic donor defetcs or imprities in cBN.