Polycrystalline Si

Figure 15-31. l/V characteristics of a large area plastic solar cell ( illuminated with 488 nm, 10 rnW/ cm2). FF for the 1TO plastic cell is 0.35. As reference a pholocurrcnl of a polycrystalline Si cell is plotted ( ), 10 limes reduced. 

Silane decomposes to its elements at above 400°C. Process (1) is known as direct thermal decomposition, and produces either amorphous or polycrystalline Si (function of reaction temperature and other processing parameters), and is commonly used, for instance, in the solar cell industry to reduce silane to silicon. 

Fig. 6. I(V) characteristic at AMI illumination of solar cell consisting of p-n junction in polycrystalline Si. [Courtesy of B.W Faughnan]... 

Low-Pressure CVD Processes. Low-pressure CVD (LPCVD) (—101 Pa) is the main tool for the production of polycrystalline Si dielectric and passivation films used in Si IC (integrated-circuit) manufacture (1, 20, 21). The main advantage of LPCVD is the large number of wafers that can be coated simultaneously without detrimental effects to film uniformity. This capability is a result of the large diffusion coefficient at low pressures, which... 

Such efficient minority-carrier injectors have been proposed as emitters for high-power transistors (Kroemer, 1957) and demonstrated using an n-type oxygen-rich polycrystalline Si emitter on a p-type c-Si base (Oh-uchi et al., 1979). The main problem expected to occur at heterojunctions between dissimilar materials is that associated with interfacial states that may either pin the Fermi level or act as generation-recombination centers. However, in the case of a-Si H the abundance of atomic hydrogen should help eliminate the interfacial states. 

The liquid SiHCls is fractionally distilled and is then reacted with H2 to produce high-purity polycrystalline Si ... 

Low pressure CVD (LPCVD) has become a dominant process in the growth of thin films of microelectronic materials. It is widely used to deposit thin films of polycrystalline Si, Si02, and Si3N4 In addition it has been demonstrated for deposition of metals, specifically Al and W. The process is carried out in tubular, hot wall reactors where the wafers are placed perpendicular to the flow direction as illustrated in Figure 2. The very large packing densities that can be realized in LPCVD reactors without adverse... 

The modelling approach behind the LPCVD reactor model is not restricted to any specific deposition kinetics. However, to limit the algebraic complexity and to be able to compare model predictions with experiments we consider the simplest major deposition process, the deposition of polycrystalline Si from SiH4. The model is based on the following kinetic mechanism ... 

Wagner compared structural and electrical properties of polycrystalline Si layers grown by CVD or LPE (In melt, 947°C, 0.12 pm min-1) with similar grain boundary structures [20]. The measured minority carrier lifetime was always higher and the recombination strength of the defects was smaller in the LPE layers than in the CVD layers. They attributed this to the higher purity of the LPE layer and its lower density of defects (rod-like defects). 

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