Preparation of ZnO films on sapphire (01 $$\bar 1$$ 2) substrates by low-pressure organometallic chemical vapour deposition

Abstract

We have proposed a new photochromic device in which two kinds of transition metal ions are alternatively doped into a host material with a period of a few tens of nanometres [1]. Such a device with superlattice structure has been fabricated by means of low-pressure organometallic chemical vapour deposition (LPOMCVD) [2, 3]. Using LPOMCVD, we succeeded in the preparation of (11 20) orientated ZnO films [4, 5]. Nevertheless, the resultant ZnO fihn exhibited a fairly low resistivity, because of formation of oxygen vacancies. Insulating ZnO films are required as photochromic host materials. Thus, in the work reported in this letter, oxygen gas was introduced into a reaction chamber during the preparation of a ZnO film. The properties of the resultant ZnO films are discussed on the basis of data of X-ray diffraction (XRD) patterns, electric resistivity, absorption spectra and photoluminescence (PL) spectra. Zinc acetate dihydrate (ZnAc) was used as a precursor of Zn source. The LPOMCVD apparatus was the same system used in our previous letter [4]. A reaction chamber was evacuated to 1.0 x 10 -4 Pa then H20 vapour was introduced into the reaction chamber up to 2.7 × 10 -2 or 6.7 × 10 -3 Pa. Furthermore, oxygen gas was introduced into the chamber using a mass flow meter. The total pressure of the reaction chamber was almost unchanged after oxygen gas was introduced into it. ZnO films were deposited on sapphire (01 ]-2) substrates heated at 350°C. The preparation of the ZnO films was carried out at a low sublimation rate, because both (11 20) and ZnO (0002) reflections were observed in an XRD pattern of a ZnO film prepared at a sublimation rate higher than 3 m g h -1 [5]. The preferential orientation of the ZnO film was analysed from rocking curves for (11 20) ZnO reflection; in XRD measurements (CuK~), the angle (2 0) of the Xray detector was fixed to 56.5 °, then the angle (0) of the sample was independently scanned. The thickhess of the film was evaluated by a surface roughness meter (Kosaka SE-30D). Electric resistivity was measured by the Van der Pauw technique. PL spectra were measured using a 50 cm focal length grating monochromator, a lock-in-amplifier and a cw He-Cd laser (325 rim, 10 roW) as an excitor. The conditions for the preparation of ZnO films are listed in Table I. As shown in the results of samples 1 and 4, the deposition rate is not proportional to the sublimation rate alone. The partial pressure of H20 is another factor controlling the deposition rate. As a general tendency, addition of oxygen gas leads to a decrease in the deposition rate, irrespective of the H20 partial pressure. Fig. 1 shows XRD patterns of the ZnO films. Only (11 20) reflections were seen in the XRD patterns for all samples, so that the preferential orientation is not remarkably disturbed by the addition o f oxygen gas. Nevertheless, the intensity of the (11 20) reflection decreases with the flow rate of oxygen gas. As for the samples 1, 2 and 3, the intensity of the (11 20) reflection normalized by the film thickness is decreased by the addition of oxygen gas. In general, orientation of the ZnO film has an influence on the intensity of the XRD patterns. To clarify the relation between orientation and intensity of the ZnO (11 20) reflection, the orientations of these ZnO films were evaluated from rocking curves for the ZnO (11 2_-0) reflections. The rocking curves of the ZnO (11 20) reflections, which are not shown here, have full width at half maximum (FWHM) of 2.8, 3.2, 3.0, 4.2 and 6.1 ° for samples 1, 2, 3, 4 and 5, respectively. The inclination angles of ZnO (1120) plane with respect to A1203 (0224) plane are 2.3, 1.9, 2.4, 2.6 and 1.7 ° for samples 1, 2, 3, 4 and 5, respectively. Except for sample 5, the preferential orientation is not significantly affected by the addition of oxygen gas. Accordingly, the decrease in the intensities of the (1 t 20) reflections caused by introducing oxygen gas is not attributable to degradation of the