Growth of ZnO Films

Zinc oxide films can be prepared by a modification of the procedure used for the preparation of Zr02 films. A mixed solution of zinc acetate ( Baker Analyzed Reagent, J. C. Baker Chemical Co.) and acetic acid (Reagent A.C.S., Fisher Scientific Co.) was used as source material. A stream of oxygen gas carried the nebulized solution into the reaction chamber. Both the efficiency of the deposition and the uniformity of the films were affected by the deposition parameters used. Table 1 shows a set of typical reaction 

Substrate temperature Substrate to nozzle distance I.D. of nozzle I.D. of reactor Oxygen flow rate 

Films of zinc oxide were characterized by the same procedures used for Zr02. The three strongest X-ray diffraction peaks were found to be at 36.3 , 31.8°, and 34.5°, and these corresponded to the (101), (100), and (002) reflections of zinc oxide. The measured grain sizes for a 0.3 pm thick zinc oxide film was 0.1 pm and approximately 0.3 pm for a film of 1 pm thickness. The resistivity of zinc oxide, in the dark, was 1 x 10 2 cm. 

Minami, H. Nanto, and S. Takata, Thin Solid Films, 124, 43 (1985). 

Bahadur, M. Hamdani, J. F. Koenig, and P. Chartier, Solar Energy Mater, 14, 107 (1986). 

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Oxygen-Related Surface Species and Initial Growth of ZnO Films... 

A1 + N codoping has been undertaken during the growth of ZnO films in an N2O + O2 atmosphere on glass substrates by DC reactive magnetron sputtering. At a substrate temperature of 500 °C, p-ZnO was realized with hole densities as high as 1.1 X 10 cm ... 

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]. 

Figure 5.14 shows the X-ray diffraction patterns of ZnO films deposited at different substrate temperatures at p(O2) = 33mPa, where the maximum deposition rate is achieved. Only the (002) reflection of the wurtzite phase can be identified, which indicates textured growth of the polycrystalline films with the c-axis perpendicular to the substrate even at room temperature. 

The thickness d of LP-CVD ZnO B films is represented in Fig. 6.9 as a function of deposition time t. For d > 500 nm, d is linearly dependent on t. For d < 500 nm, the growth of ZnO is slower than that for higher thickness, indicating an incubation phase during which nuclei are formed on the glass... 

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... 

Fig. 7.9. RHEED images of optimized ZnO thin film surfaces on r-plane, a-plane, and c-plane sapphire, in the two azimuthal orientations (top and bottom) separated by 45° (left) or 30° (middle and right), respectively. The RHEED patterns of the a-axis textured film on r-plane sapphire (left) indicate an epitaxial and three-dimensional, island-like growth. The ZnO films on a-plane (middle) and c-plane sapphire (right) exhibit a smoother surface structure, as indicated by the streaky RHEED patterns and the observation of additional weak reflections in the top images due to 3 x 3 surface reconstruction [51]...

Fig. 7.16. Growth temperature-dependent reduction of carrier concentration (300 K) of ZnO films on sapphire due to introduction of Ce02 buffer layers. The growth temperature is about 50° C lower as the given heater temperature. Hall measurements by H. von Wenckstern...

ZnO films were deposited by RTF, MS, and PEMOCVD. In the case of RTF the ZnO films have a large-grained structure with no preferred crystallite orientation. On the contrary, ZnO films deposited by PFMOCVD and MS have fine-grained structure with an average grain-size of 40-60 nm. Under certain growth conditions ZnO films show a preferred orientation with the c-axis normal to the substrate. However, ZnO films prepared by... 

Figure 1. X-ray diffraction patterns of ZnO films at different growth temperature.

Lau et al. [198] have carefully investigated the growth kinetics of ZnO films with DEZ and different reactant gases (H2O, CO2, N2O) (Table 3-11). In contrast to the results with DMZ, their observations with respect to the mass flow of DEZ show a LH-mechanism. The dependence of the deposition on the substrate temperature was... 

Spray pyrolysis of ZnO films was carried out with Zn(ac)2 solutions in pure water or mixtures of water and alcohols, such as isopropanol and ethanol. Typical growth rates were in the range of 0 to 60 nm/min. Goyal et al. [229] observed an increase of the deposition rate with the molarity of the spray solution and Islam et al. [225] observed a maximum value at a deposition temperature of about 300°C. For higher temperatures, the growth rate drops slowly. To our knowledge, no further studies on the kinetics in spray pyrolysis have been published. 

Electrochemical growth of ZnO/eosin Y hybrid thin film. Trans. Mater. Res. Soc. Jpn., 32, 417 20. 

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