Elsevier

Applied Surface Science

Volume 258, Issue 13, 15 April 2012, Pages 5200-5205
Applied Surface Science

Stress control in ZnO films on GaN/Al2O3 via wet oxidation of Zn under various temperatures

https://doi.org/10.1016/j.apsusc.2012.01.114Get rights and content

Abstract

Stress and lattice constants are significant factors considered in evaluating the deformation mode of crystalline materials. Zinc oxide (ZnO) nanostructured thin films were prepared via the process of wet oxidation of Zn at temperatures varying from 420 to 570 °C. Structural, elemental and optical characterizations were carried out using various techniques, to investigate the properties of the samples. Scanning electron microscopy (SEM) images showed improvement in the ZnO structure and the grain size of the ZnO became larger as the oxidation temperature was increased, while maintaining a constant flow rate of wet oxygen. X-ray diffraction (XRD) patterns also showed that ZnO films suffered from compressive stress due to elongation in the lattice constant. With increase in the oxidation temperature, the compressive stress became tensile stress because of the decrease in the lattice constants. Photoluminescence (PL) spectra revealed the influence of stress on the energy band gap, with wet oxidation further giving rise to the transition of stress from compressive to tensile.

Highlights

► Epitaxial ZnO thin films were synthesized, using the wet oxidation process. ► High quality of synthesized ZnO films were obtained by applying two temperatures (420 and 470 °C). ► ·The lattice constant of ZnO films shrinked by increasing oxidation temperature through increase of oxygen diffusion in ZnO lattice. ► ·It is possible to grow controllable stress of ZnO on GaN substrate using simple method for high quality thin films.

Introduction

Zinc oxide (ZnO) has many interesting properties such as a wurtzite-hexagonal structure with a wide direct band gap of 3.37 eV, which is very similar to that of gallium nitride (GaN) at 3.4 eV [1]. The ZnO nanostructure has received increasing attention for its potential applications in optoelectronic devices due to a variety of factors, including its morphologies and the availability of simple and low-cost processing [2]. Several workers have focused on the growth of ZnO on Al2O3 (sapphire) and GaN buffer layer grown on Al2O3, which have effects of reducing the density of the defects caused by the lattice and thermal mismatch between ZnO and sapphire [3], [4], [5]. However, as-grown films are known to contain stress and strain due to factors of mismatch between ZnO and substrates.

Unfortunately, very few workers have examined the growth of ZnO on GaN as buffer layer on Al2O3 substrate, to reduce the dislocation caused by the thermal and lattice mismatch between ZnO and Al2O3. This is because to the best of our knowledge, no studies have reported on the control of thermal and lattice mismatch between layers. Many techniques have been employed to produce high-quality ZnO films including, chemical vapor deposition (CVD) [4], metal organic CVD, pulsed laser deposition and thermal evaporation [3], [6], [7]. Thermal evaporation is relatively simple and low cost and could be applied to low melting point, low decomposition or low sublimation point oxides. However, the synthesis of ZnO by oxidizing Zn nanoparticles at variable wet oxidation temperatures has received little attention [8], [9].

On the other hand, some other studies have focused on the evolution of the stress and the structure as a function of thermal or oxygen partial pressures [10], [11]. These two parameters are known to strongly influence the residual stresses in sputtered ZnO films. However, thermal pressure has the larger influence of the versions of pressure on the stoichiometry of ZnO films because the crystallization of a thin film of ZnO is improved by thermal energy [12]. The stress arises during the growth process and a perfect crystallization of ZnO free from stress, could be obtained by adjusting the temperature and oxygen flow rates at optimal conditions during the interaction between Zn and O. Intrinsic stress is directly related to the size of the ZnO grains at the interaction surfaces during the growth process.

The growth stress, which results into defects in the thin film, is due to the unbalanced stoichiometry between Zn and O in ZnO crystals. In other words, the intrinsic stress in ZnO could be controlled by adjusting the growth temperature while maintaining the rate of flow of oxygen. The present paper describes the process of controlling stress in ZnO thin films via wet oxidation. The growth of ZnO films on GaN/Al2O3 for the present experiment was obtained using thermal oxidation at various temperatures. The objective of the current trial is, first, to show how stress in thermal oxidation-grown ZnO films could be converted from compressive to tensile, by controlling the oxidation temperature at a constant wet oxygen flow rate during the process of growth of ZnO on GaN/sapphire. Second, the present study demonstrates the correlation between the stress state of ZnO films and variation in the lattice constant. The intrinsic stress generated when temperature was increased was extracted from the experimental data. Finally, the present experiment shows the influence of thermal wet oxidation growth conditions on lattice constant and stress.

Section snippets

Experimental procedure

The samples used in this study were prepared by depositing Zn nanoparticles (4 N) on GaN/Al2O3. The substrates were cleaned using the standard cleaning method before they were loaded into the evaporating chamber [11]. Pure Zn powder (4 N) was then evaporated on the GaN surface with a thermal evaporator under a constant chamber pressure of 6 × 10−4 mbar, using a direct current of 3.5 A for 2 min. The samples were oxidized twice; first, the samples were placed with wet oxygen in the middle of a

Results and discussion

Fig. 1 shows the top-view SEM images of ZnO samples grown on GaN/Al2O3. A high density of nano crystals grew uniformly over the entire film surface. The thin films of ZnO grown on the substrate had small grains-sized structures, with the morphology changing according to the variation in the oxidation temperatures. In the sample oxidized at 420 °C, as shown in Fig. 1(a), a homogenous layer of ZnO nanostructure grew on the c-plane of the substrate. However, the sample oxidized at 470 °C (Fig. 1

Conclusion

Epitaxial ZnO thin films were synthesized on GaN/sapphire substrates, using the wet oxidation method. SEM images of the samples showed homogenous nanostructures, suggesting that wet oxidation temperatures have a strong effect on the morphology of ZnO films. The XRD pattern revealed that the tip is a single-face crystalline hexagonal ZnO with the (0 0 2) crystal phase and the sharp peak, indicating the quality of the ZnO nanocrystal. The XRD pattern at 2θ showed that the ZnO film suffers from a

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