Doping of CVD ZnO films

To understand why two ZnO layers having a similar thickness can possess such a large difference in grain size, TEM micrographs have been compared (Fig. 6.34), one for the slice of an undoped LP-CVD ZnO film (i.e., a film with a large grain size) and the other one for the slice of a heavily boron-doped LP-CVD ZnO film (i.e., a film with a small grain size). 

Fay et al. [3] used a pressure lower than 1 Torr, but also BoHf, as dopant gas. They, however, did not observe any substantial modification of crystal orientation by doping. Indeed, as illustrated in Fig. 6.35b, the (1120)/(1010) peak ratio observed here is quite high for undoped ZnO films deposited at 155°C, and remains at a high value even when the dopant concentration is increased. Moreover, in this case, the deposition rate does not vary significantly with the introduction of I 21 h , as is shown in Fig. 6.36. 

In conclusion, it seems that for both the AP-CVD and the LP-CVD processes, deposition parameters have been found, for which the introduction of a dopant component does not significantly perturb the crystallographic orientation of the ZnO films. However, it is, at this stage, not possible to identify the mechanisms and critical parameters governing the influence of dopants on film structure. Nevertheless, some speculative statements can be made in the case of the AP-CVD process, the deposition temperature seems to be the critical factor, i.e., above 400°C the growth orientation of ZnO films is no more perturbed by the introduction of a dopant. For the LP-CVD 

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). 

This means that the fraction of gallium atoms that are electronically active within the ZnO Ga films decreases when the doping level increases. This can explain that N does not continue to increase for higher doping levels. 

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