铜箔上生长的六角氮化硼薄膜的扫描隧道显微镜研究

利用扫描隧道显微镜研究了采用化学气相沉积法在铜箔表面生长出的高质量的六角氮化硼薄膜. 大范围的扫描隧道显微镜图像显示出该薄膜具有原子级平整的表面, 而扫描隧道谱则显示, 扫描隧道显微镜图像反映出的是该薄膜样品的隧穿势垒空间分布. 极低偏压的扫描隧道显微镜图像呈现了氮化硼薄膜表面的六角蜂窝周期性原子排列, 而高偏压的扫描隧道显微镜图像则呈现出无序和有序排列区域共存的电子调制图案. 该调制图案并非源于氮化硼薄膜和铜箔衬底的面内晶格失配, 而极有可能来源于两者界面处的氢、硼和/或氮原子在铜箔表面的吸附所导致的隧穿势垒的局域空间分布.

Scanning tunneling microscopy study of h-BN thin films grown on Cu foils

Analogous to graphite, hexagonal boron nitride (h-BN) has a layered structure composed of boron and nitrogen atoms that are alternatively bond to each other in a honeycomb array. As the layers are held together by weak van der Waals forces, h-BN thin films can be grown on surfaces of various metal crystals in a layer-by-layer manner, which is again similar to graphene sheets and thus attracts a lot of research interests. In this work, scanning tunneling microscope and spectroscope (STM and STS) were applied to the study of an h-BN thin film with a thickness of about 10 nm grown on Cu foil by means of chemical vapor deposition. X-ray diffraction from the Cu foil shows only one strong peak of Cu(200) in the angle range of 40°-60°, indicating that the Cu foil is mainly Cu(100). After sufficient annealing in an UHV chamber, the h-BN film sample is transferred to a cooling stage (77 K) for STM/STS measurement. Its high quality is confirmed by a large-scale STM scan that shows an atomically flat topography. A series of dI/dV data taken within varied energy windows all exhibit similar U shapes but with different bottom widths that monotonously decrease with the sweeping energy window. The dI/dV curve taken in the energy window of -1 V, +1 V] even shows no energy gap in spite that h-BN film is insulating with a quite large energy gap of around 6 eV, as observed in a large-energy-window dI/dV curve (from -5 V to +5 V). These results indicate that the STM images reflect the spatial distribution of tunneling barriers between Cu(100) substrate and STM tip, rather than the local density of states of the h-BN surface. At high sample biases (from 4 V to 1 V), STM images exhibit an electronic modulation pattern with short range order. The modulation pattern displays a substructure in low-bias STM images (less than 100 mV), which finally turns to the (1×1) lattice of h-BN surface when the sample bias is extremely lowered to 3 mV. It is found that the electronic modulation pattern cannot be fully reproduced by superimposing hexagonal BN lattice on tetragonal Cu(100) lattice, no matter what their relative in-plane crystal orientation is. This implies that the electronic modulation pattern in the STM images is not a Morié pattern due to lattice mismatch. We speculate that it may originate from spatial distribution of tunneling barrier induced by adsorption of H, B and/or N atoms on the Cu(100) surface in the CVD growth process.