在较低温度下实现平整ZnO薄膜的生长有利于ZnO的可控p型掺杂以及获得陡峭异质界面。本文使用分子束外延方法, 采用a面蓝宝石为衬底, 在450 ℃下生长了一系列ZnO薄膜样品。在富氧生长的条件下, 固定氧流量不变, 通过调节锌源温度来改变锌束流, 以此调控生长速率。样品的生长速率为40~100 nm/h。通过扫描电镜 (SEM) 表征发现: 在高锌束流的生长条件下, 样品表面有很多不规则的颗粒; 降低锌的供应量后, 样品表面逐渐平整。原子力显微镜(AFM)测试结果表明: 样品的均方根表面粗糙度(RMS)只有0.238 nm, 接近于原子级平整度。这种平整表面的获得得益于较低的生长速率, 以及ZnO外延薄膜与a面蓝宝石衬底之间小的晶格失配。

ZnO thin film that grows at low temperature benefits the sharp interface in heterostructure and enough doping level of acceptor in ZnO. In general, the smooth growth can be easily obtained at temperature higher than 600 ℃, but difficult in the case of low growth temperature. In this work, by controlling the growth ambient and growth rate, a series of ZnO thin films with smooth surface were grown on a-plane sapphire substrates at 450 ℃ by the plasma-assisted molecular beam epitaxy (P-MBE). The growth was performed in oxygen-rich atmosphere. To tune the growth rate, the zinc flux was changed by varying the K-cell temperature of zinc source (Tk) while keeping oxygen flux constant. The growth rate of the samples is only 40~100 nm/h. Scanning electron microscopy (SEM) images indicate that there are lots of irregular grains on the thin film surface at high zinc flux, and most of the grains disappear gradually from the surface with the zinc flux decreasing. This smooth growth with low growth rate is conducive to control finely the layer thickness and smoothness in the multilayer structure. The root mean square (RMS) surface roughness is only 0.238 nm, measured by atomic force microscopy (AFM). The smooth surface benefits from the low growth rate and small sub-lattice mismatch between the c-plane-ZnO thin film and a-plane-sapphire substrate.