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强的激子发光，是一种很有前途的紫外光发光材料。由于传统的高温高压法(HPHT)生长的hBN晶体的尺寸最大限制为几毫米，因此需要一种替代的大面积工艺或常压或减压工艺。因此，在本研究中，我们尝试在减压下用热化学气相沉积法(CVD)合成了这种紫外发光的hBN，并获得了如下结果：1.我们成功地在多晶Ni衬底上合成了室温下在215 nm处有强烈紫外光发射的hBN。1170℃的沉积温度比传统的hBN工艺低几百℃。拉曼测量结果表明，hBN的结晶度与HPHT单晶hBN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，这可能是由于高温下BCl3和Ni衬底之间的刻蚀反应造成的。我们尝试了一种新的方法，包括低温沉积来抑制刻蚀反应，并在高温下进行后退火来提高结晶度。用这种方法成功地制备了表面更加光滑的紫外光致发光的hBN。在10×10 mm^2样品中观察到了激子发光，这些斑点的大小为几个μm，并均匀地散布在样品中。这些结果表明，这种方法为提高发光的均匀性提供了一条途径。虽然机理尚不清楚，但hBN在界面的溶解和析出可能与这一现象有关。

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.