High-Mobility Ga-Polarity GaN achieved by NH 3 -MBE
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High-Mobility Ga-Polarity GaN achieved by NH3-MBE J. X. Wang, X. L. Wang, D. Z. Sun, J. M. Li, Y. P. Zeng, G. X. Hu, H. X. Liu, L. Y. Lin Materials Center, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing 100083, People’s Republic of China ABSTRACT GaN epilayers were grown on (0001) sapphire substrates by NH3-MBE and RF-MBE (radio frequency plasma). The polarities of the epilayers were investigated by in-situ RHEED, chemical solution etching and AFM surface examination. By using a RF-MBE grown GaN layer as template to deposit GaN epilayer by NH3-MBE method, we found that not only Ga-polarity GaN films were repeatedly obtained, but also the electron mobility of these Ga-polarity films was significantly improved with a best value of 290 cm2/V.s at room temperature. Experimental results show it is an easy and stable way for growth of high quality Ga-polarity GaN films. INTRODUCTION
-Nitrides
have been studied intensively in recent years because of their excellent properties for potential applications in optical and electronic devices [1-3]. Many important characters of the materials were explored by researchers to improve the quality of the material and further more the performance of the device. Among these characters, the polarity of GaN epilayer has become a hot topic due to its crucial influence on both materials and devices [4-6]. Since the first paper about the polarity of GaN was reported by Sasaki and Matsuoka [7], subsequent research results clearly show its huge effects on surface morphology[8], RHEED pattern[9-10], selectivity of chemical reagent[11-12], material electrical and optical properties, and two-dimensional electron gas (2DEG) sheet density and mobility of AlGaN/GaN heterostructure. Generally speaking, Ga-polarity or Ga-face epilayer has distinct differences with N-polarity one. For example, Surface of Ga-polarity epilayer is usually much smoother than that of N-polarity epilayer as observed by atomic force microscope (AFM) [8]. With in situ RHEED, a 2 2 pattern was usually appeared indicating a Ga-polarity during growth interruption or at a substrate temperature below 400 after growth. However, a N-polarity only showed 1 1 or 1 3 RHEED patterns during this period[9-10]. Additionally, Seelmann-Eggebert et al[13] found that GaN films having N-polarity were etched in a solution of KOH while films having Ga-polarity were resistant to etching in the same solution. So, it was easy to distinguish the polarity of GaN epilayer by etching. Excepting above methods, some other important techniques were used to clarify polarity such as coaxial impact collision ion scattering spectroscopy[14-15], converged beam electron diffraction (CBED)[16], x-ray photoemission spectroscopy[7], hot (160 ) H3PO4 solution[17], and hemispherically scanned x-ray photoelectron diffraction (HSXPD)[13]. Between two polarities of GaN layer, Ga-polarity is desired for both material growth and device fabrication because it has better properties than N-polarity. It has been proven that
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