Elsevier

Journal of Crystal Growth

Volume 309, Issue 2, 1 December 2007, Pages 121-127
Journal of Crystal Growth

Structural and optical properties of non-polar A-plane ZnO films grown on R-plane sapphire substrates by plasma-assisted molecular-beam epitaxy

https://doi.org/10.1016/j.jcrysgro.2007.09.025Get rights and content

Abstract

Non-polar, A-plane (112¯0) ZnO films are epitaxially grown on R-plane (11¯02) Al2O3 substrates by plasma-assisted molecular-beam epitaxy and structural and optical properties are investigated. Epitaxial relationships between ZnO films and Al2O3 substrates are determined to be ZnO(112¯0)//Al2O3(11¯20), ZnO[1¯100]//Al2O3[112¯0]. The misfit is relaxed by the regularly spaced misfit dislocations. Structural properties are anisotropic and surfaces of films show stripes running along the ZnO [0001] direction. Full width at half maximums of X-ray rocking curves for the on-axis (112¯0) with φ=0° and 90°, and the off-axis (101¯1) reflections are 0.41°, 0.36°, and 0.39°, respectively, for the 300 nm-thick ZnO film. Near-band edge emissions are dominant in low-temperature (12 K) photoluminescence while emissions from deep levels are negligible indicating a high optical quality of the layers. The near-band edge emissions are strongly polarized perpendicular to the [0001] axis. Transitions of neutral-donor bound excitons (D0X) and allowed free excitons are observed in photoluminescence and photoreflectance spectra, which are significantly blue-shifted compared to the transitions in C-plane ZnO.

Introduction

As a direct wide-band-gap semiconductor, ZnO has received increasing attention due to its potential applicability to optoelectronic devices such as ultraviolet (UV)-light emitting diodes (LEDs) and laser diodes (LDs) [1], [2]. ZnO films have been usually grown on C-plane (0 0 0 1) Al2O3, where the ZnO films have 〈0 0 0 1〉 orientation in the growth direction. In this case, macroscopic electrostatic field is generated along the growth direction and it results in spontaneous and piezoelectric polarization. The polarization induced electric field make negative effects on device properties such as a decrease in the overlapping of the electron and hole wave function in the quantum well and consequently a decrease of internal quantum efficiency of the emitting devices [3], [4], [5]. In order to eliminate the polarization effects, growing ZnO films without polarity along the growth direction, i.e., growth of non-polar films are needed. There are several reports on growing non-polar A-plane ZnO on R-plane sapphire substrates. Most of the studies report the A-plane ZnO films grown by MOCVD [6], [7], [8], [9] and only a few studies report molecular-beam epitaxy (MBE) of A-plane ZnO [10], [11]. We note that there have been no detail studies on growth and properties of A-plane ZnO grown by MBE on R-plane sapphire so far.

We have grown A-plane ZnO films on R-plane sapphire substrates by plasma-assisted molecular-beam epitaxy (PAMBE) and address following topics in this study; (1) in situ reflection high energy electron diffraction (RHEED) observation and RHEED pattern analysis, (2) epitaxial relationship determined by RHEED, X-ray diffraction (XRD), and transmission electron microscopy (TEM), (3) surface morphology addressed by using an atomic force microscope (AFM), (4) crystal quality evaluated by on-axis and off-axis XRD rocking curves. (5) Interface and misfit dislocations investigated by TEM, and (6) optical properties including photoreflectance (PR) and polarized photoluminescence (PL).

Section snippets

Experimental procedures

ZnO films on R-plane Al2O3 substrates were grown by PAMBE with a radio frequency (RF) plasma radical cell and an effusion cell for solid source. Commercial R-plane Al2O3 substrates were cleaned by conventional chemical and thermal methods used for the C-plane Al2O3 substrates. Elemental zinc (Zn) with 6 N purity and RF plasma-enhanced oxygen (O2) gas were used as group II and IV sources, respectively. All the ZnO films were grown at 700 °C. RF plasma condition was set to 300 W of the power and two

Epitaxial relationship

Fig. 1 shows RHEED patterns for R-plane Al2O3 substrates (a,b) after the thermal cleaning and ZnO films grown for 1 h (c,d). Observed RHEED patterns give us several features. First, the substrate and the ZnO film grown on it are both single crystalline. Therefore, a specific epitaxial relationship between the substrate and the film should be existing. The epitaxial relationship was determined by determining azimuth directions for each pattern based on the camera constant and interplanar spacing

Conclusions

Non-polar, A-plane (112¯0) ZnO films were grown on R-plane Al2O3 substrates by PAMBE. The grown ZnO films were single crystalline. The epitaxial relationship between the ZnO film and the substrate was investigated by three different methods using the RHEED, XRD, and TEM. It was consistently determined to be [1 1 0 1]Al2O3//[0 0 0 1]ZnO and [112¯0]Al2O3//[1¯100]ZnO. The lattice misfits were calculated considering the in-plane translational periods of ZnO and Al2O3 along the ZnO [11¯00] and [0 0 0 1]

Acknowledgements

This work was supported by the Basic Research Program of the Korea Science & Engineering Foundation through Grant no. R01-2004-000-10104-0 and by the Korea Research Foundation through Grant No. KRF-2005-205-D00078, and the Brain Korea 21 Project in 2006.

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