Doping efficiency

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],... 

Process techniques such as filtered arc [124-126] deposition, or pulsed laser deposition [79,127-130] generate outstanding film properties for a variety of TCO materials. The main feature is that the film formation is achieved with ions instead of using mostly neutral atoms. The state of the art is the pulsed laser deposition of ZnO Al films, where resistivity of 85 pQ cm has been reported at a substrate temperature of 230°C [79]. This result can be attributed to the high doping efficiency because of the suppression of AI2O3 formation when ionized species are used for film formation. 

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). 

Fig. 6.39. Variations of electron density TVe and doping efficiency 77de (calculated from the gallium content and the electron density) as a function of the gallium content in AP-CVD ZnO Ga films deposited at 370°C. Reprinted with permission from [10]...

Mg is still the most commonly employed acceptor in LEDs and LDs, though p-type conductivity has also been observed in GaN using C, Be and Ca. The common drawback to all these acceptors is their inconveniently large activation energy of order 200 meV, which results in doping efficiencies of about 1%, though, as we discuss below, there is evidence for the existence of shallower acceptors in a few limited cases. 

There are two mechanisms that limit the built- in potential in a-Si H solar cells. One is the existence of band-tail states as mentioned earlier, and the other is the low doping efficiency of a-Si H. Spear and LeComber (1976)... 

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. 

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. 

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. 

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