| GroupⅢ-nitride semiconductor, aluminum nitride (AlN) has attracted considerable attention recently. Because of its large band gap and unique properties, it is suitable for many device applications, such as high power high speed transistors, ultraviolet (UV) light emitting diodes and laser diodes. In addition, AlN can be used to form alloy with other nitrides (GaN and InN), the alloys (AlGaN, GaInN and InAlN) cover a bandgap range from 1.9eV to 6.2eV. Furthermore, AlN has some outstanding physical properties such as super hardness, high thermal conductivity, resistance to high temperature and caustic chemicals, which make AlN an attractive material for electronic and optical devices. Therefore, it is very important to prepare high quality of AlN crystal.Various techniques have been used to grow AlN such as Molecular Beam Epitaxy (MBE), Metal Organic Chemistry Vapour Deposition (MOCVD), Pulsed Laser Deposition (PLD) and reaction sputtering etc. MBE is a very effective method to prepare high quality crystal, one of significant advantages of MBE is its ability to precisely control doping and composition of crystal by changing growth conditions.In this work, RF-MBE system was used to grow AlN epilayer. It is possible to monitor the growth condition by using in-situ surface characterization equipments such as reflection high energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM).By repeated debugging, the RHEED patterns along different azimuths ofα-Al2O3 substrate were observed. Surface texture was studied via these patterns. Our results indicate that the surface morphology of sapphire substrate is improved in the process of thermal cleaning, chemical etching and nitridation, which reveals the treatment process for the substrate is effective. In order to study nitridation conditions, XPS was used to detect the chemical states at the surface. By changing the generating power of nitrogen, optimized parameters has been preliminarily established. After the 15 minutes nitridation, the survey spectrum of the sample indicates the presence of element N. The binding energy at 397.33eV and 398.76eV for N1S are attributed to Al-N and AlOXN1-X bonds respectively. When the output power of N plasma source is 200 W and the nitridation time is 60 minutes, Al-N bond disappeared and the other peak for N1S shifts to lower binding energy. It is possibility that the N ions obtained higher energy when the output power of N-plasma source is larger, and phenomenon mentioned above is due to the etching effect of plasma. But further investigation will be needed to prove it.In addition, nitridation carried out on silicon was studied, too. First a thin Al layer was deposited at low temperature (500℃) on silicon (100), then high temperature nitridation was performed. Finally AlN layer was grown at a little lower temperature. The growth mechanics was preliminary analyzed based on XPS, RHEED and AFM measurements. The XPS results indicate that a higher composition of N was obtained after nitridation. Core level results got from XPS confirm the formation of AlN on the substrate, but the sample has higher surface roughness, which was characterized by AFM. Maybe it is due to that Al atoms couldn't get sufficient energy to move across the surface when the growth temperature is lower. And the growth pattern tends to be three-dimension (3D) mode. Maybe high quality AlN film could be obtained by adjusting growth conditions in the future. |
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