Properties of ZnO

Jimenez-Gonzalez, A. Suarez-Parra, R. 1996. Effect of heat treatment on the properties of ZnO thin films prepared by successive ion layer adsorption and reaction (SILAR)./. Cryst. Growth 167 649-655. 

Zahir, MH Katayama, S Awano, M. Synthesis and de-NOx properties of ZnO-Ga20s-AI2O3 spinel, Afofer. Chem. Phys., 2004, Volume 86, Issue 1, 99-104. 

The highest purity material is calcined with additives such as Bi203 and used in the manufacture of varistors [2.80]. The photoconducting properties of ZnO are used in photoreproduction processes. Doping with alumina causes a reduction in electrical resistance hence, it can be used in the coatings on the master papers for offset reproduction [2.81]. 

The semiconducting properties of ZnO have nevertheless proved to be advantageous because they permit the extensive application of modern surface spectroscopies (such as XPS, UPS, EELS, and LEED) for the investigation of the surface structures since the charging problems that are usually encountered with oxides are avoided (393). For this reason, ZnO is one of the most thoroughly investigated oxides. 

Because of the high purity, the excellent morphological definition, and the good optical properties of the microcrystals, the surface and catalytic properties of ZnO powders prepared by Zn combustion have been investigated extensively by spectroscopic techniques. The results of these investigations are well suited to comparison with results obtained with single crystals and less well-defined samples. 

Acidic properties of ZnO have been established by various methods (20). However, only very little information on the spectroscopic behavior of pyridine adsorbed on ZnO is available. Tanabe et al. (20,220) report on the observation of the PyL species, whereas PyH+ could not be detected. 

Acknowledgement. The authors are grateful to Paul Erhart and Karsten Albe for extensive discussions of the defect properties of ZnO and for the supply of original versions of their figures. 

The electrical parameters, especially the conductivity, were investigated early in history of ZnO research. An overwiew over electronic properties of ZnO up to the end of the 1950s was given by Heiland et al. in 1959 [7]. Most of the early investigations up to about 1955 were performed on sintered polycrystalline ZnO samples [4,8], which suffered from the general problem of conduction in porous, inhomogeneous materials with a lower density compared with... 

So far, all observed free-charge-carrier contributions to the optical properties of ZnO and related materials are only due to n-type conductivity, because reproducible and sufficiently high p-type conductivity is still a challenge. Therefore, all data presented here address n-type conductive samples. 

ZnO films can provide substantial information on chemical and electronic properties of ZnO surfaces and interfaces, which occur in real thin film solar cell structures. In addition, general information on the interface formation of oxide materials can be extracted. In the following we describe ... 

Motivated by the application of ZnO in gas sensors and catalysis and by the more general desire to understand surface properties of ionically bonded solids, electronic properties of ZnO surfaces have been investigated for many years [20,76-80]. An overview of the early work on ZnO surface properties is included in the book of Henrich and Cox [81]. 

First sputtering processes for ZnO deposition were developed in the late 1960s for manufacturing surface acoustic wave devices [2]. The piezoelectric properties of ZnO films are crucial for that application and major efforts were made to develop ZnO sputtering processes which enabled c-axis oriented growth, high resistivity and unique termination of the ZnO crystallites [3,4]. 

The properties of ZnO films depend strongly on stoichiometry and phase composition. This holds for TCO films as well as for dielectric films ... 

The properties of ZnO Al films for the application as the front electrode in a-Si H p-i-n solar cells have been compared by Muller et al. [106],... 

In the following, each step will be described, both for the AP-CVD and for the LP-CVD cases. The results presented are focused on those properties of ZnO films that are useful for thin film solar cell applications, i.e., transparency, conductivity, and light scattering capability. The last part of paragraph 6.2 comments briefly on alternative methods of CVD processes that have been investigated for ZnO deposition (PE-CVD, photo-CVD,. ..). 

For LP-CVD ZnO, similar variations in the structural properties of ZnO films occur with an increase of substrate temperature, but this takes place at a lower temperature, i.e., around 160°C, and with different crystallographic orientations. Indeed, the preferential orientation is (1120) for LP-CVD ZnO, instead of (0002) for the AP-CVD ZnO [3,16,23], Figure 6.21 shows XRD patterns of undoped LP-CVD ZnO films deposited with increasing substrate temperature, along with the corresponding SEM micrographs of their surface. 

As has been discussed above, there is a strong influence of grain size on the electrical and optical properties of ZnO films. Take as a first example, ZnO films grown by the LP-CVD process in the substrate temperature range between 155 and 180°C they have a microstructure as described in Sect. 6.2.2.1 with conical crystallites that form pyramids at the surface (see Fig. 6.6). This microstructure has, via the pyramidal structure of the surface, a pronounced influence on the optoelectronic properties of the films, specifically on their light scattering capability. 

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