Growth of High Quality Poly-SiGe on Glass Substrates
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ABSTRACT The composition variation and strutural properties of poly-SiGe thin films were investigated by Reactive Thermal CVD with Si2H6 and GeF4. Deposition of the films was carried out at a low temperature of 450°C on oxidized silicon substrates using different growth parameters, i.e., the source gas flow ratio (Si2H6/ GeF4) and thegas flow rate. The structural profiles of as-deposited films were characterized by X-ray diffraction (XRD) and Raman scattering spectroscopies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All these films show (220) preferential orientation. The mole fractions of Si in poly-SixGel-x films were estimated to be from 0.95 to 0.05 for x by using Vegard's law for the XRD peaks. TEM observation revealed that high crystallinity was well established even in poly-Sio.95Geo.05 films owing to the direct nucleation on the substrate surface.
INTRODUCTION Polycrystalline Si thin films have many applications in integrated circuit technology. Examples span a wide range of applications, from metal oxide semiconductors (MOS) to bipolar silicon technologies. In recent years, a new area has emerged where the application of poly-Si films to largearea electronic devices such as thin film transistors (TFTs)[] for manufacturing active matrix liquid crystal displays (AM-LCDs) and solar cells 31 has been addressed. For these devices, low temperature processing (lower than 450°C) , which is required so that low cost glass substrates can be used, and establishing the film uniformity over large areas are desirable. High quality poly-Si films have been obtained so far by deposition of amorphous silicon thin films and subsequent crystallization upon a low-temperature anneal (thermal annealing)1 2] or laser anneal131 . The films crystallized are well-suited for TFT application and such TFTs exhibit high field effect mobility (>100 cm 2Ns)l4J. For thermal annealing, the associated processing time for the crystallization, however, was shown to increase significantly as the annealing temperature was 1 21 lowered. Recent efforts have been made in metal-induced crystallization in order to solve this problem. In fact, considerable reduction of both annealing temperature and time, e.g., at 450°C for 151 5 hours, was achieved in nickel-induced crystallization . On the other hand, for laser annealing, there is no serious problem in terms of process time and temperature. Thus, its practical application to TFTs is in progress, although there remains the technical problem ofthe difficulty in establishing the large-area uniformity of the films because of the stepwise laser illuminations of limited area and the beam homogeneity. Alternatively, low temperature CVD is a potential technique for cost-effective and productive deposition of large-area poly-Si thin films because of the one-step process and the feasibility of depositing large-area thin films. Plasma CVD has been most extensively studied for the low temperature growth of poly-Si films and has achieved it at considerably high growth rates
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