| Cubic boron nitride （cBN） crystal is a synthetic III-V semiconductor material,and natural cubic boron nitride （cBN） crystals have not been found in nature so far.cBN single crystals, with the widest bandgap（6.4eV） in III-V compounds and IVelemental semiconductor, have many interesting properties, such as high hardness andthermal conductivity only second to diamond. As electrical or optoelectronic material,cBN has extensive applications. So, many researchers are interested in the synthesis,physical properties and application of cBN. But, due to rigorous growing conditions,it is difficult to get high-quality and large-size cBN single crystals or film materials.These factors prevent the related researches from making progress.Color zoning and plate-like cBN single crystals （labeled as：#210） under highpressure and high temperature （HPHT） were studied in this dissertation. The cBNsample has eight surfaces, which are all {111} planes. The upper and lower faces,which are parallel and bigger than other faces, are very smooth and flat, so that suchcBN samples are very suitable for substrates of electronic and optoelectronic devices.Therefore, it is very necessary to study on physical and chemical properties of cBNcrystals.The paper mainly studied surface polarity, impurities, defects of cBN singlecrystals by means of chemical corrosion, morphology, XPS and Raman. Somesignificant results were obtained. The dissertation includes six chapters.In chapter one, synthesis and semiconductor characteristics of cBN crystals aresummarized. And my main research results of the whole thesis are outlined too.In chapter two, physical and chemical properties, preparation technologies and some main application researches of cBN crystals are introduced.In chapter three, the relationships between chemical corrosion, surfacemorphology, I-V characteristics and surface polarity are considered. we found aconvenient, rapid and nondestructive method to distinguish the surface polarity ofcBN—microscopic observation.Actual cBN single crystal is color zoning and plate-like sample, whose surfacesare {111} planes. Top and bottom planes are parallel and bigger than side planes. Thesix side planes are parallel and have approximately equal areas. Meanwhile, the cBNcrystals exhibited color zoning: three triangular amber regions and three triangulartransparent regions, and they are interval and symmetric distribution. The six triangleshave common vertex, which located in the central of sample.The results show that cBN crystals can be corroded by molten NaOH. Thecorrosion rate of {111}N plane is fast. Etching pits on {111}N faces are triangular orhexagonal, and the sizes are larger than those of {111}B faces. However, etching pitson {111}B faces are triangular and small. Further observation results indicate that: theside faces adjacent the transparent region are {111}B planes, the side faces adjacentamber regions are {111}N planes; if the angle between one of big surfaces and the{111}N side faces is obtuse, then this big surface is {111}B plane, and the other bigsurface which makes an acute angle with the {111}N side faces is {111}N face. Thus,a convenient method was found to distinguish surface polarity of the color zoning andplate-like cBN crystals by using microscopic observation.We cleaved cBN crystal along {110}planes. There are two corrosionmorphologies on {110}planes. One is that the cleavage surface of cBN crystal issmooth and flat, and no etching pits are observed because of the isotropic corrosionspeed. The other is that strip-shaped etching pits are found on the {110} cleavagesurface.The coplanar I-V characteristics of cBN crystals show that the leakage current of{111}N faces is bigger than that of {111}B faces.In chapter four, impurities, defects and chemical states are analyzed by XPS spectra. It turned out that cBN crystals include C, O, Si besides B and N. A trait of Sipresumably comes from hBN raw materials. Most O elements exist as absorbates onthe surface, because they decrease rapidly after Ar ion sputtering. Around6at%C isstill unexpectedly observed after sputtering in our experiments. So there should be agreat deal of C impurities in cBN samples. Carbon probably comes from the graphitewalls of the growth chamber. According to the peak fit and the analysis of thechemical states, carbon can occupy the nitrogen lattice position （CN） as acceptorimpurity. The atomic ratio of B to N （B: N） before and after sputtering is greater than1, and the deviations from stoichiometry indicate that the existence of N vacancies（VN） as donors. CN-VNas donor-acceptor complex will not only affect the electricalproperties of cBN because of the impurity compensation, but also complicate thestructure of luminescence spectra of cBN single crystals based on the mechanism ofdonor-acceptor pair luminescence.Further studies show that XPS spectra were influenced by surface polarity. Theatomic ratio of B: N for {111}B face is bigger than that of N face before sputtering.After sputtering, the atomic ratio of B: N for {111}B face drops, the atomic ratio of B:N for {111}N face rises. According to fitting results of C1s, C-N-B bonding or moredefects may exist on the surface of {111}N plane, C-B-N bonding may be on thesurface of {111}B plane.In chapter five, Raman spectra were studied for#210cBN crystals. The resultsshow that Raman spectrum is also related with surface polarity. For the {111}B plane,only TO and LO modes were observed. However, the broad bands at925,955and1253cm-1, were discovered for {111}N face besides TO and LO modes. Ramanspectra were analyzed based on LO phonon-plasmon interaction, local mode, richboron, wBN and disorder-activated Raman scattering （DARS） aspects respectively.{111}N face has more defects. So, the broad bands appear as a result of the scatteringof phonons all over the Brilliouin Zone, The bands at925and955cm-1are ascribed tothe TO mode of K and Q points in the Brillouin Zone, respectively. Raman peak at1253cm-1does not correspond to special point of the Brillouin Zone, but, it may also be caused by DARS. The broad bands were not observed on {111}B face because ofless defects.At last, the sixth chapter makes a summary of the above research contents. |
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