Synthesis of hexagonal boron nitride exhibiting intense deep ultraviolet light emission

六方氮化硼（hBN）在室温下具有215nm的强激子发光，是一种很有前途的紫外发射材料。由于传统的高温高压法（HPHT）生长的hBN晶体的尺寸最多被限制在几毫米，因此需要替代大面积工艺或常压或减压工艺。因此，在本研究中，我们尝试在减压下采用热化学气相沉积（CVD）的方法合成这种紫外发光的hBN，得到了以下结果：我们成功地合成了hBN，在室温下，在多晶Ni衬底上的阴极发光（CL）测量显示出215 nm的强紫外光发射。1170℃的沉积温度比传统hBN工艺的沉积温度低几百℃。拉曼测量结果表明，hBN的结晶度与HPHT单晶hBN.2相当。紫外发光hBN的形成伴随着不均匀的侧向发光和表面粗糙化，这可能是BCL_3与Ni衬底在高温下蚀刻反应的结果。我们尝试了一种新的方法，即低温沉积来抑制蚀刻反应，高温后退火来提高结晶度。该方法成功制备了表面光滑的紫外发光hBN。在10×10mm^2样品中均匀分布了几μm大小的激子发光点。这些结果表明，该方法为提高发光均匀性提供了一条途径。虽然机理尚不清楚，但界面处hBN的溶解和析出可能与此现象有关。

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.