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Ge on insulators is desired for achieving extremely high performance metal oxide silicon field-effect transistors with sufficiently low leakage current. However, for thin film transistor applications, Ge thin film deposition on SiO2 substrates using conventional CVD techniques has not been demonstrated so far due to the extremely long incubation time needed. Previously, we demonstrated the deposition of polycrystalline Ge films directly on top of fully SiO2 covered Si substrates with nearly no incubation time by using high-density plasma chemical vapor deposition (HDPCVD) technique at 400oC. However, such high temperature will cause overheating of the plasma windows and then frequently lead to the particle formation as well as the cracking of deposited Ge films. The re-crystallization of Ge films deposited directly on top of fully SiO2 covered substrates at a lower temperature of 300oC by HDPCVD have been then performed. Re-crystallizations of Ge films by furnace were used to enhance the crystalline properties of low-temperature grown Ge films. According to the x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) results, we believe that very pure Ge films still can be successfully deposited on SiO2 substrates at lower substrate temperature. After 400oC, 4hr re-crystallization in N2 ambient, the amorphous structure of Ge films will transfer to polycrystalline structure and the cubic structure with primarily (111), (220), and (311) orientations of the re-growth Ge films is mainly composed of fine grains and can be unambiguously identified from x-ray diffraction (XRD) patterns. Lowest surface roughness can be found at the sample after 4hr annealing, but the RMS will increase very rapidly after 6hr annealing due to the reactions between Ge and residual oxygen. The results of sheet resistance show that the implanted poly-Ge films can be activated at lower temperature. In summary, the used HDPCVD technique is able to provide a simple, powerful and reliable approach in the fabrication of polycrystalline Ge thin film transistors. |