用等离子体辅助分子束外延生长氧化锌单晶薄膜

Growth of Zinc Oxide Single Crystal Thin Films by Plasma-assisted Molecular Beam Epitaxy

利用等离子体辅助分子束外延(P-MBE)方法,通过优化生长条件,在c平面蓝宝石(Al_2O_3)上生长出氧化锌(ZnO)单晶薄膜。使用反射式高能衍射仪(RHEED)原位监测到样品表面十分平整,X射线摇摆曲线(XRC)测得ZnO薄膜的<002>取向半峰全宽为0.20°,证实为ZnO单晶薄膜。室温下吸收谱(ABS)和光致发光(PL)谱显示了较强的激子吸收和发射,且无深能级(DL)发光。电学性能测量表明,生长的ZnO为n型半导体,室温下载流于浓度为7×10^(16) cm^(-3),与体单晶ZnO中的载流子浓度相当。

Zinc oxide (ZnO) is a wide gap semiconductor with band gap of 3.37 eV and binding energy of 60 meV at RT. This material was expected for ultraviolet emissive device related to exciton effect at RT. For fabrication of high-quality ZnO thin films, most effort was performed. Previous work, including selected lattice match substrate (GaN, bulk ZnO or ScAlMgO_4), employed buffer layer (MgO), annealing at high temperature, was successfully made to obtain high-quality ZnO thin films. In this paper, we chose c-plane sapphire (Al_2O_3) as a substrate of ZnO thin film. At the optimum temperature, the high-quality ZnO thin films were fabricated. During growth process, the high pure Zn source (6N) was evaporated at 245℃ by Knudsen cell and oxygen was cracked by an rf plasma atomic source (13.56 MHz). Both basal pressures of Zn and oxygen were fixed at 5×10^(-5) and 3×10^(-3) Pa, respectively. The chemical cleaned substrate was transferred into growth chamber and then was exposed in plasma oxygen atmosphere at 650℃. Reflection high-energy electronic diffraction pattern indi- cates that flat substrate surfaces appear after plasma oxygen treatment. The ZnO thin films were grown at selected temperature of 650℃. The as-grown sample was further investigated by X-ray rocking curve (XRC), photolumines- cence, and Hall effect measure. ZnO single crystal thin films were prepared on c-plane Al_2O_3 substrate by plasma-assisted molecular beam epi- taxy (P-MBE). A flat surface of the sample was obtained by the monitoring of reflection high-energy electron diffraction. The full width at half maximum of ZnO(002) rocking curve is 0.2°, this indicates that the sample is single crysal thin films. From photoluminescence spectrum at room temperature, an intense free exciton emission located at 3.30 eV was observed and no deep level emission appeared in visible region. Absorption spectrum shows the evident absorption peak of free exciton located at 3.36 eV. Carrier concentration of the sample is 7×10^(16) cm^(-3) measured by Van De pauw method. This result corresponds with the reported carrier concentration in ZnO bulk crystal. The above results indicate that the high quality ZnO single crystal thin film was obtained.