Fabrication of Surface Nitridation Mask on GaAs Substrate for Nano-Lithography
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Fabrication of Surface Nitridation Mask on GaAs Substrate for Nano-Lithography Yo Yamamoto1, Kohji Saito2, Toshiyuki Kondo2, Takahiro Maruyama1,2 and Shigeya Naritsuka1,2 1 Meijo Univ. 21st Century COE Program, Nagoya, Japan 2 Dept. of Materials Science & Engineering, Meijo University, Nagoya, Japan. ABSTRACT The surface nitridation of GaAs substrate under various conditions were performed and its surface morphology was studied by using atomic force microscope. The RMS roughness of the surfaces was closely related to formation of the particles which was formed by As evaporation and subsequent Ga migration, which was enhanced by higher substrate temperature. INTRODUCTION Recently, the hetero epitaxial growth of III-V semiconductors on Si, such as GaAs/Si, has been extensively studied for making opto-electronic integrated circuit (OEIC). In this field, the formation of dislocations caused by the lattice mismatch and the difference in thermal expansion coefficient between GaAs and Si are recognized as unavoidable problem. As one of the effective techniques to solve these problems, epitaxial lateral overgrowth (ELO) has been proposed. It is composed by photo-lithography and selective-area growth, provides a flat epitaxial layer with very few dislocations for Si, GaAs and GaP alloy [1-6]. Also, a remarkable reduction of the dislocation density even in the hetero epitaxial system with large lattice mismatching, such as GaAs on Si, also has been reported [1-6]. By using hetero epitaxial lateral overgrowth, there is a possibility to design a novel nano-structure device with a high reliability enough for commercial applications. However, in order to fabricate nano structure devices, finer machining technique such as electron lithography and/or scanning tunnel microscopy (STM) lithography is necessary instead of a conventional photolithography. Combination of STM lithography and selective-area growth allows us to design a nano structured optical and electronic devices. We have chosen an ultra-high-vacuumed process containing STM lithography and molecular beam epitaxy (MBE). To take full advantages of STM lithography, it is important to develop the applicable mask with good machinability and stability. One of the candidates is GaN thin layer. We studied about the formation of GaN thin layer on GaAs substrates by the nitridation. This paper especially focused on the surface morphology of the nitrided film on the GaAs substrate prepared under various conditions of nitridation and growth.
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EXPERIMENTAL PROCEDURE After chemical etching with a solution of NH4OH:H2O2:H2O=4:1:20, GaAs (001) substrate was heated up to 580oC in MBE chamber to remove the surface oxide layer. Then, the surface of the substrate was nitrized by RF nitrogen source, changing the RF power between 200-400W, N2 flow rate between 1.2-0.6 SCCM, substrate temperature between 300-500oC which was set, and the nitridation time for 15-60 min. Microstructure of the specimens were studied by using atomic force microscope (AFM) and scanning electron micr
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