Growth and structure of amorphous silicon

Despite being a relatively simple reactor, there are many variables in the deposition process which must be controlled to give good material. The gas pressure determines the mean free path for collisions of the gas molecules and influences whether the reactions are at the growing surface or in the gas. The gas flow rate determines the residence time of the gas species in the reactor. The rf power controls the rate of dissociation of the gas and therefore also the film growth rate, and the temperature of the substrate controls the chemical reactions on the growing surface. 

There are numerous modifications which can be made to the basic reactor design. The initial dissociation of the silane gas can be by ultraviolet (UV) light illumination (photo-CVD), which either excites the silane directly, or by energy transfer from mercury vapour introduced into the chamber. Photo-CVD reactors eliminate the electric discharge and prevent the bombardment of the growing film by ions from the plasma, which may be a source of defects. A different way of reducing bombardment is to separate the plasma from the growing 

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The term polysilicon arises from the structure of these silicon layers, which are essentially polycrystalline in contrast to single-crystalline silicon substrates, due to the growth of these films on amorphous starting layers (in silicon surface micromachining, a silicon oxide layer is usually used as both a seed layer and a sacrificial layer). Polysilicon is widely used in sensor technology it can be used as part of a membrane layer, as an electrical connector, or as a part of a thermopile structure. In this contribution, we focus on its most important function - as the functional layer in surface-micromachined structures. In surface micromachining basically two approaches for producing polysilicon films are used ... 

Figure 16 (Street et al., 1986) shows the typical sample structure, consisting of three layers of a-Si H. Results using this technique have been reported for samples grown by the rf glow discharge of silane and by rf sputtering (Shinar et al., 1989). The first layer is hydrogenated amorphous silicon, deposited under conditions that yield high quality films (i.e., deposition temperature of 230°C, low growth rate) and is typically two microns thick. Next a layer of approximately 1000 A is deposited, whereby...

This book describes the properties and device applications of hydrogenated amorphous silicon. It covers the growth, the atomic and electronic structure, the properties of dopants and defects, the optical and electronic properties which result from the disordered structure, and finally the applications of this technologically very important material. There is also a notable chapter on contacts, interfaces, and multilayers. The main emphasis of the book is on the new physical phenomena which result from the disorder of the atomic structure. 

This book describes the material properties and physical phenomena of hydrogenated amorphous silicon (a-Si H). It covers the growth of material, the atomic structure, the electronic and optical properties, as well as devices and device applications. Since it focusses on the specific properties of one amorphous material, there is a considerable emphasis on describing and interpreting the experimental information. Familiarity with semiconductor physics is assumed, and the reader is also referred to the excellent books by Mott and Davis, Elliott, and Zallen for further information about the general properties of amorphous semiconductors and glasses.f... 

In dry oxidation the first reaction dominates whereas the second reaction dominates in wet oxidation. The growth rate is a function of temperature, oxide thickness, and substrate orientation. As an example, the average growth rate on the (100) surface in wet oxygen at 1000 °C after Ih of oxidation is about 1 A/s. The structure of thermally grown oxide is amorphous and typically has exact stoichiometric composition. The oxide layer formed on a silicon substrate is about 2.27 times the thickness of the consumed silicon and contains about 2.2 x 10 molecules/cm of Si02. 

When the surface is completely covered with an oxide film, dissolution becomes independent of the geometric factors that are responsible for the formation and directional growth of pores, such as surface curvature and orientation. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus show intrinsically identical structure in all orientations. Also, on the oxide-covered surface the rate-determining step is no longer electrochemical but rather the chemical dissolution of the oxide. 

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