Synthesis of hexagonal boron nitride exhibiting intense deep ultraviolet light emission

六方氮化硼（hBN）在室温下具有215 nm的强激子发光，是一种很有前途的紫外发光材料。由于传统上通过高温高压法（HPHT）生长的hBN晶体的尺寸最多限于几mm，因此需要替代的大面积工艺或在大气压或减压下的工艺。因此，在本研究中，我们尝试在减压下通过热化学气相沉积（CVD）来合成这种紫外发光的hBN，并获得了以下结果：1。我们已经成功地合成了hBN，它表现出强烈的紫外光发射在215 nm在室温下的阴极发光（CL）测量多晶镍基板。1170摄氏度的沉积温度比用于常规hBN工艺的沉积温度低几百摄氏度。拉曼测量的结果显示hBN的结晶度与HPHT单晶hBN的结晶度相当。紫外发光hBN的形成伴随着不均匀的横向发光和表面粗糙化，这可能是由于BCL_3和Ni衬底在高温下发生刻蚀反应的结果.我们尝试了一种新的方法，该方法涉及低温沉积以抑制蚀刻反应，并在高温下进行后退火以提高结晶度。通过这种方法成功地形成了具有更光滑表面的UV发光hBN。在10×10mm^2的样品中，可以观察到均匀分布的几μm大小的激子发光。这些结果表明，该方法为提高发光均匀性提供了一条途径。虽然其机制尚不清楚，但hBN在界面处的溶解和沉淀可能与此现象有关。

Hexagonal boron nitride (hBN) is one of promising materials which emit ultraviolet (UV) light due to its intense exciton luminescence at 215 nm at room temperature. Since the size of hBN crystals, which are conventionally grown by the high temperature high pressure method (HPHT), is limited at most to a few mm, an alternative large-area process or a process under atmospheric or reduced pressures is required. Thus, in this present study, we tried to synthesize such UV luminous hBN by a thermal chemical vapor phase deposition (CVD) under reduced pressures, and obtained the results as follow:1. We have successfully synthesized hBN which exhibits intense UV light emission at 215 nm at room temperature in cathodoluminescence (CL) measurements on polycrystalline Ni substrates. The deposition temperature of 1170 degree C is lower by several hundreds of degree C than those used for the conventional hBN processes. The results of Raman measurements show crystallinity of the hBN is comparable to that of a HPHT single crystal hBN.2. The formation of UV luminous hBN was accompanied by non-uniform lateral luminescence and surface roughening, which probably resulted from etching reaction between BCL_3 and Ni substrates at the high temperature.3. We tried a new method which involved low-temperature deposition for suppression of the etching reaction and post-annealing at high temperatures for the improvement in crystallinity. The UV luminous hBN with a smoother surface was successfully formed by this method. The exciton luminescence was observed from spots with the size of a few μm, which were uniformly scattered in the 10×10mm^2 specimen. These results indicate that this method provides a way for the improvement in uniformity of luminescence. Although the mechanism is not clear, dissolution and precipitation of hBN at the interface are probably related to this phenomenon.