Ion Beam Treatment of Functional Layers in Thin-Film Silicon Solar ...

2.2 Transparent conducting oxide
ZnO:Al films are also characterized by low material density, relatively high resistivity
and high HCl etch rate.
2.2.4.4 As-grown rough ZnO films
As-grown rough ZnO:B films have been prepared by several deposition techniques such as
low pressure chemical vapor deposition (LPCVD) [85, 86] and expanding thermal plasma
CVD [87]. As already discussed, magnetron sputtered ZnO films tend to exhibit c-axis
preferred orientation. Compared with those CVD prepared films, sputter deposited ZnO
films generally exhibit smaller lateral features. Thus, the flat sputtered ZnO:Al films
have to be etched in wet chemicals in order to provide a light scattering effect at the
ZnO/Si interface. The replacement of wet chemical etching by dry etching or as-grown
rough ZnO seems very attractive for industry process, because the vacuum break can
be avoided. There have been many attempts to sputter as-grown rough ZnO films by
magnetron sputtering [11–15]. In the following, the literature related to as-grown rough
ZnO films by magnetron sputtering are briefly reviewed.
Generally, as-grown rough ZnO surfaces were sputtered at very high pressures or with
water vapor. It was reported by Minami et al. and Sato et al. that wedge-like surface
features were sputtered on undoped ZnO and ZnO:Al films with gas pressure between
3 and 10 Pa and substrate temperature of about 300 ◦ C. The sheet resistance of these
films is approximately 1.8 Ω at thickness of 3 µm [11, 13, 14]. Nakada et al. added H 2 O
together with Ar gas in the sputtering chamber. With increasing H 2 O partial pressure,
the columnar structure which can be seen in ZnO films deposited in pure Ar disappeared.
The maximum lateral feature size for ZnO films grown in pure H 2 O was approximately
1 µm. With Al doping, the change of surface morphology was not significant. The
mobility of the ZnO:Al film sputtered in mixed water vapor and Ar, however, is quite
low 3 cm 2 /Vs. After annealing in vacuum, the mobility can be improved to 9 cm 2 /Vs.
As a result, the electrical properties of these ZnO:Al films are not sufficient for the front
contact in solar cells. Similar results were found by Kluth et al. [16]. As-grown rough
ZnO:Al films were obtained as long as the fraction of water vapor in the Ar gas is larger
than 1% and the film thickness is larger than 500 nm. However, these films also suffered
high resistivity. It was also noted by the above authors that gas mixtures such as Ar/O 2
and Ar/H 2 do not lead to as-grown rough ZnO:Al films.
Additionally, it was found by Krantz et al. that adding N 2 into the sputter chamber,
as-grown rough ZnO:Al films were obtained by reactive sputtering [88]. The surface
lateral size was increased from 50 nm to 500 nm as the flow rate of N 2 was increased
from 0 to 10 sccm. The sheet resistance of the film, however, was increased from 4 to
4277 Ωcm due to the large decreasing mobility.
To summarize, as-grown rough ZnO:Al films have been deposited by magnetron sputtering
at very high pressure or by adding water vapor or N 2 to the sputter process.
The as-grown rough ZnO:Al films, however, tend to exhibit low mobility as well as high
resistivity, making them unlikely to be applied in solar cells. One important focus of
this work is the sputtering of as-grown rough ZnO:Al films without strongly deteriorated
electrical properties.
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