Mobility free carrier

As discussed previously, inherent disorder possessed by tf-SiH alloy limits the mobility of the free carriers (electrons and holes) to about 10 cm2/(s-V) this is compared with crystalline Si, in which the electron mobility is 1500 cm2/(s-V). However, crystalline Si is expensive to manufacture and its size is limited to about 20 cm in diameter. Many applications discussed have either emeiged or been identified which preclude the use of crystalline Si because of cost, size, or both. The basic commonality in these applications is the ability to fabricate devices on areas much larger than can be addressed by crystalline Si Furthermore, these applications are not demanding in terms of speed, which then provides -SiH alloy with a distinct competitive advantage. 

Electrical conductivity, a, is given by the product of free-carrier concentration, n, carrier mobility, p, and carrier charge, e ... 

In Ref. 54, XRD showed the deposit to be hexagonal CuSe. Analysis of the absorption spectrum gave a direct bandgap of 2.02 eV. As commonly seen for these compounds, there was still strong absorption at lower energies (e.g., at 1.9 eV, the absorption coefficient was >7 X 10" cm ), possibly due to an indirect transition but likely due, at least in part, to free-carrier absorption. From Hall measurements, the doping (acceptor) density was ca. 10 cm (heavily degenerate) and the mobility ca. 1 cm V sec The dependence of film thickness and deposition rate on the deposition parameters has been studied in a separate paper [62]. 

In the case of material with a significant concentration of localized states, it is possible to assnme that transport of a carrier over any macroscopic distance will involve motion in states confined to a single energy. Here it is necessary to note that a particn-larly important departnre from this limiting situation is (according to Rose [4]) a trap-limited band motion. In this case, transport of carrier via extended states is repeatedly interrnpted by trapping in localized states. The macroscopic drift mobility for such a carrier is reduced from the value for free carriers, by taking into acconnt the proportion of time spent in traps. Under steady-state conditions, we may write... 

If the generation process is characterized by the primary quantum efficiency rj, the transport properties by the mobility /1, and the recombination by the free-carrier liefetime t, the steady-state photoelectric current can in the case of mobile electrons and immobile holes generally be described by Eq. (13) 14> ... 

This is a very reactive pair which, under free diffusing conditions, would immediately quench each other with the back-reaction. The back-reaction was inhibited by entrapment of both Py and MV+2 while allowing them to communicate chemically with a mobile charge carrier, A,Af,-tetramethylene-2,2 -bipyridinium, which diffused freely in the aqueous solution within the porous network. 

This process describes the scattering of free carriers by the screened Coulomb potential of charged impurities (dopants) or defects theoretically treated already in 1946 by Conwell [74,75], later by Shockley [10] and Brooks and Herring [76,77]. In 1969, Fistul gave an overview on heavily-doped semiconductors [78]. A comprehensive review of the different theories and a comparison to the experimental data of elemental and compound semiconductors was performed by Chattopadhyay and Queisser in 1980 [79]. For nondegenerate semiconductors the ionized impurity mobility is given by [79] ... 

Figures 6.37 and 6.38 show the variation of electrical properties as a function of the dopant content of ZnO films. Figure 6.37 shows the case of AP-CVD ZnO F with fluorine as dopant (here, the fluorine atomic fraction is considered as dopant content). Figure 6.38 shows the case of LP-CVD ZnO B with boron as dopant (here, the B2H6/DEZ ratio is considered as dopant content). The electrical properties taken into consideration are the conductivity a, the resistivity p, the mobility //, and the free carrier density N.

Concerning the free carrier mobility, it has been observed that, except for very low doping ratios (e.g., fluorine content <0.4 at. % for AP-CVD ZnO F or gas doping ratio B2He/DEZ<0.2 for the LP-CVD ZnO B), p continuously decreases when the doping ratio is increased. This is attributed to the following ... 

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