Amorphous hydrogenated

Hydrogenated amorphous silicon has shown promise in producing economical cells for converting solar energy into electricity. 

The tetrahedrally bonded materials, such as Si and Ge, possess only positional disorder however, materials of this type exhibit high density of defect states (DOS). It is only with the addition of elements such as hydrogen and/or a halogen, typically fluorine, that the DOS is reduced to a point such that electronic device appHcations emerge. These materials contain up to - 10 atomic % hydrogen, commonly called hydrogenated amorphous siHcon (i -Si H). 

The optoelectronic properties of the i -Si H films depend on many deposition parameters such as the pressure of the gas, flow rate, substrate temperature, power dissipation in the plasma, excitation frequency, anode—cathode distance, gas composition, and electrode configuration. Deposition conditions that are generally employed to produce device-quahty hydrogenated amorphous Si (i -SiH) are as follows gas composition = 100% SiH flow rate is high, --- dO cm pressure is low, 26—80 Pa (200—600 mtorr) deposition temperature = 250° C radio-frequency power is low, <25 mW/cm and the anode—cathode distance is 1-4 cm. 

Street, R.A. (1991) Hydrogenated Amorphous Silicon (Cambridge University Press, Cambridge). 

Weiler, M., Lang, K., Li, E., and Robertson, J., "Deposition of Tetrahedral Hydrogenated Amorphous Carbon Using a Novel Electron Cyclotron Wave Resonance Reactor, Appl. Phys. Lett., Vol. 72,1998,pp. 1314-1316. 

Methods of Deposition of Hydrogenated Amorphous Silicon for Device Applications... 

Material Aspects of Hydrogenated Amorphous Silicon 1.1.2.1. Atomic Structure... 

Hydrogenated amorphous silicon is not a homogeneous material. Its structure is thought to consist of voids embedded in an amorphous matrix [62, 63]. The size and number density of the voids depend on the deposition conditions. Poor-quality material can have a void fraction around 20%, while device quality a-Si H has been shown to contain fewer voids, 1%. with a diameter of 10 A [64-66]. The surfaces of the voids may be partly covered with hydrogen atoms [62, 67],... 

Over the past decades the term device quality has come to refer to intrinsic PECVD hydrogenated amorphous silicon that has optimum properties for application in a certain device. Of course, depending on the type of device, different optimum values are required nevertheless the properties as listed in Table I are generally accepted, e.g. [6, 11]. Many of these properties are interrelated, which has to borne in mind when attempting to optimize only one of them. 

Selected Properties of Device Quality Hydrogenated Amorphous Silicon Films... 

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