等离子增强原子层沉积低温生长GaN薄膜

Low temperature depositions of GaN thin films by plasma-enhanced atomic layer deposition

采用等离子增强原子层沉积技术在低温下于单晶硅衬底上成功生长了Ga N多晶薄膜,利用椭圆偏振仪、低角度掠入射X射线衍射仪、X射线光电子能谱仪对薄膜样品的生长速率、晶体结构及薄膜成分进行了表征和分析.结果表明,等离子增强原子层沉积技术生长Ga N的温度窗口为210—270?C,薄膜在较高生长温度下呈多晶态,在较低温度下呈非晶态;薄膜中N元素与大部分Ga元素结合成N—Ga键生成Ga N,有少量的Ga元素以Ga—O键存在,多晶Ga N薄膜含有少量非晶态Ga_2O_3.

Metalorganic chemical vapour deposition and molecular beam epitaxy have already been demonstrated to be suc-cessful techniques for obtaining high-quality epitaxial GaN layers with low impurity concentrations and pretty good electrical properties. However, high growth temperature employed in both of these methods give rise to some intrinsic defects of the thin films, such as high background-carrier concentrations. As a low-temperature thin film deposition method, plasma-enhanced atomic layer deposition （PE-ALD） has more unique advantages compared to both methods for epitaxial growth of GaN. In this paper, the polycrystalline GaN thin films were fabricated on Si （100） substrates at 150-300 ℃ by PE-ALD. Trimethylgallium and N2/H2 plasma gas mixture were used as the Ga and N precursors. The growth rate of the thin films was demonstrated by the spectroscopic ellipsometer. The crystal structrue and composi- tion of the GaN thin films were characterized by X-ray diffractometer and X-ray photoelectron spectrometer （XPS）. It is showed that the growth window for PE-ALD grown GaN thin films is 210-270 ~C, where the growth rate remains constant at 0.70 A/cycle. And it is known that it is the self-limiting nature of PE-ALD that is ascribed to the plateau of the growth rate. Films grown at relatively higher temperature are polycrystalline with a hexagonal wurtzite structure, while films grown under relatively lower temperature are amorphous. The grazing incidence X-ray diffraction （GIXRD） patterns of the polycrystalline thin films reveal three main peaks located at 20 = 32.4°, 34.6° and 36.9°, which are corresponding to the （100）, （002） and （101） reflections. It is showed that the Ga, N atoms would get higher energy for more effective migration to positions with lowest energy to form ordered crystalline arrange at higher growth temperature. The XPS results show that all the N elements of the as-grown thin films are in the form of N--Ga bond, indicating that all the N elements are formed into GaN thin films; and there is a little amount of the Ga elements that exist in Ga--O bond. The fact that there is no Ga203-related peaks in the GIXRD pattern suggests that there is small amount of amorphous Ga203 dispersed in the polycrystalline GaN thin films. In the future work, reducing the concentration of the C and O impurities may be achieved by increasing the time of the reaction and plasma pules in the process formula and replacing the inductively coupled plasma with the hollow cathode plasma, respectively.