Structural investigations of ZnO thin films grown by reactive pulsed magnetron sputtering at different substrate temperatures

Zinc oxide (ZnO) is a promising material for electronic and optical applications [1]. As in the case of other wide bandgap compounds (i.e. GaN) grown in heteroepitaxy conditions, the final electrical properties of the film are affected by its crystal quality. Rutherford backscattering spectrometry (RBS) in channeling mode (RBS/C) is a well proved method to obtain this physical information in such structures, with the additional advantage of in-depth information [2]. In this work, we report RBS/C and Raman characterization of ZnO thin films grown onto (0001) oriented sapphire [3] by reactive pulsed magnetron sputtering at substrate temperatures (Ts) spanning from 22ºC to 550ºC .
Random and RBS/C measurements were performed along the and axes of the wurtzite lattice (P63mc) of ZnO [1] with a 3.035 MeV He+ beam. The sample position was controlled by a 3-axis goniometer. The micro-Raman spectra were collected at an excitation wavelength of 532 nm. The influence of substrate temperature onto low- and high-frequency E2 modes of the resulting film was monitored. The low-frequency E2 mode is associated with the vibration of the heavy Zn sublattice, while the high-frequency E2 mode involves only the oxygen atoms.
Stoichiometry of the oxide layer was determined from the random RBS spectra, which showed a constant composition except for an O-rich interface (associated with Zn in-diffusion). RBS/C angular scans along the axis revealed a progressive enhancement of crystal quality of wurtzite ZnO with substrate temperature, with a <0001> textured growth even for low substrate temperatures (100ºC). The film crystallinity is greatly increased for substrate temperatures above 400ºC, a behaviour also confirmed by Raman measurements. The latter shows significant decrease of the line width which in the case of low-frequency E2 mode at Ts>400 °C approaches and even becomes smaller than the values of reference single crystal sample. Also, the Raman peak positions do not change for the films grown at temperatures above 400°C. Further RBS/C analysis along the oblique axis was carried out to determine the possible influence of strain. The angular scan showed no-shift among different ZnO depths, revealing a non-strained film under the technique limits. From these measurements, it can be concluded that epitaxial ZnO films can be grown by pulsed reactive magnetron sputtering without appreciable strain on sapphire substrates at Ts ~550ºC.

References:

1. Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005).
2. L.C. Feldman, J.W. Mayer and S.T. Picraux, Material analysis by ion channeling, NY, Academic Press (1982).
3. M. Vinnichenko, N. Shevchenko, A. Rogozin, R. Grötzschel, A. Mücklich, A. Kolitsch and W. Möller, J. Appl. Phys. 102, 113505 (2007)