The doping efficiency

All the information needed to calculate the doping efficiency, T, of a-Si H from Eq. (5.2) is provided by the experiments discussed above, q is obtained by equating the excess electron concentration with the density of donors and is defined in terms of either the gas-phase or solid-phase impurity concentration. 

The two values differ by the impurity distribution coefficient, /,. A crucial assumption is that dangling bonds are the only deep defects which take up donor or acceptor electrons and holes. No other charged gap states of significant density have been found. 

The electrons in the band edge states contribute only about 10% to the total (see Fig. 5.16), so that the simple approximate relation is 

Almost all the donors are compensated by deep defects, a result which is completely different from crystalline silicon in which there is virtually no defect compensation. The increased conductivity of doped a-Si H results from the small excess band tail electron density which is present because slightly larger than iVj, so that 

The defect density varies as the square root of the gas-phase phosphorus concentration, so that from Eq. (5.19), 

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TCO films such as ZnO Al, ZnO In, and ZnO Ga are meta-stable materials since the phase segregation (e.g., formation of the oxides of the dopants instead of substitutional incorporation) is favored by thermodynamics. The doping efficiency for magnetron sputtered ZnO Al films is usually below 50%. Doping efficiency up to 100% has been reported for films by PLD [79],... 

Hu and Gordon calculated the doping efficiency t/de from the gallium content in the ZnO films (determined by electron microprobe analysis) and the electron density, for the case of AP-CVD ZnO Ga. They observed that 7de steadily decreases with an increase of the gallium content (see Fig. 6.39). 

More recently, Faughnan and Hanak (1983) have used spectral response data to determine that the concentration of acceptors is —1019 cm-3 for p-type a-Si H layers containing — 1021 boron atoms cm -3 (as determined by SIMS) for a doping efficiency of — 1 %. Dresner (1983) has estimated that the doping efficiency of boron in a-Si H is — 0.1 % for films containing between 1019 and 1021 boron atoms cm-3. Thus, more recent estimates of the doping efficiency are in the range 0.1-1.0%. Apparently, many of the dopant atoms do not go into electronically active substitutional sites. 

The built-in potential of a-Si H p-i-n cells has been increased by alloying the p layer with carbon (Tawada et al., 1982). However, as shown in Fig. 5, the resistivity of the p layer increases as the optical gap (or carbon content) increases. Thus, the carbon alloying is decreasing the doping efficiency in this case. The increase in with increasing carbon content of the p layer is apparently associated with a suppression of the dark current by the wide-band-gap p layer. 

Given the possibility of both three-fold inactive impurity states and substitutional four-fold dopants, it is important to know the doping efficiency, q, which is defined as the fraction of impurities which are active dopants. 

Here A eep is the density of deep states which can take an extra electron from the donor and Kbt is the density of electrons occupying shallow states near the band edge. Fig. 5.3 shows a schematic diagram of the occupancy of states by donor electrons. Thus the doping efficiency may be obtained from either structural or electronic information. 

The structural measurements are not very successful in giving detailed information about the doping efficiency, except that it is not high and most of the impurities are three-fold coordinated, as would be expected from the % — N rule. The doping is therefore only a partial deviation from the rule. The ability of phosphorus and boron to have a coordination of either three or four is the origin of the distinctive doping properties described below. 

Fig. 5.17 evaluates the doping efficiency of the different dopants in a-Si H and a-Ge H, based on measurements of the defect and band edge carrier densities. 

Te is completely substituted for an As-sublattice site, whereas about 90% of the dissolved Si is substituted for a Ga site forming a donor, the remaining 10% for an As site. Since SIas is an acceptor, self-compensation takes place, reducing the doping efficiency of silicon. In addition, the substituted constituents interact with native... 

It has been found that the doping efficiency depends on the C/Si ratio in the reaction gas (106). For nitrogen doping, a smaller ratio brings about a higher doping efficiency, and for Al... 

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