Growth and Electronic Properties Of ZnO Epilayers by Plasma-Assisted Molecular Beam Epitaxy

Journal of Electronic Materials, Jun 2005 by Murphy, T E, Chen, D Y, Phillips, J D

ZnO thin films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The crystalline properties of the layers as measured by x-ray diffraction were found to improve with lower growth temperatures, where the full-width at half-maximum (FWHM) of the x-ray rocking curves was shown to be in the range of 100 to 1,100 arcsec. The electronic properties were found to improve for higher growth temperatures, with n-type carrier concentration and electron mobility in the range of 1 × 10^sup 17^-5 × 10^sup 18^ cm^sup -3^ and 80-36 cm^sup 2^/Vs, respectively. Photoluminescence (PL) measurements indicated that growth at higher temperatures provided superior band edge radiative emission, while growth at lower temperatures resulted in significant deep level radiative emission centered at 2.35 eV. Photoconductive decay measurements exhibit a slow decay indicating the presence of hole traps, where Zn vacancies are believed to be the source of both the slow decay and the deep level emission observed in PL spectra.

Key words: ZnO, molecular beam epitaxy (MBE), electrical properties

INTRODUCTION

ZnO is a material of interest for optoelectronics applications operating in the visible and ultraviolet (UV) spectral range. ZnO is also attractive for applications requiring radiation hardness and high-temperature operation due to its high bond strength and large bandgap energy (-3.35 eV). The large excitonic binding energy of ZnO, approximately 60 meV, also makes it an attractive material for realizing efficient electrically injected UV light emitters operating at room temperature. One distinct advantage of this material over other wide-bandgap semiconductors is the availability of native substrates, enabling lattice-matched homoepitaxy. High-quality ZnO substrates are currently available, though the relatively high cost has confined much of the research on ZnO epitaxy to growth on sapphire substrates. Sapphire is a suitable substrate for ZnO epitaxy due to its similar lattice structure (hexagonal) with ZnO, although there is a large lattice mismatch (- 16%, c-plane) between the two materials. Significant effort has been channeled into growth on c-plane sapphire substrates, which results in c-plane polar ZnO.1-4 Somewhat less effort has been directed toward the study of growth on a-plane5,6 and r-plane7 sapphire substrates. It is clear that for advanced optoelectronic devices to be realized with precisely controlled doping profiles or heterostructure interfaces, advanced growth techniques such as molecular beam epitaxy (MBE) or vapor phase epitaxy must be used. In this work, the growth of ZnO on c-plane Al^sub 2^O^sub 3^ by MBE and corresponding electronic and optical properties are presented.

EXPERIMENT

ZnO films were grown on c-plane sapphire substrates by plasma-assisted MBE. The system used for these growths was a Riber 32P system that was modified for oxide growth. Monatomic oxygen was supplied by an Oxford Scientific (Oxfordshire, United Kingdom) electron cyclotron resonance (ECR) plasma source fed by high-purity oxygen gas. The oxygen flow rate used for these experiments was 4 seem with an ECR power of approximately 270 W. The Zn was supplied by a standard dual-filament effusion cell where Zn flux was monitored by the beam equivalent pressure (BEP) measured by a standard ion gauge. For these experiments, the Zn BEP varied between 1 × 10^sup -7^ and 2 × 10^sup -6^ Torr. The sapphire substrates were purchased as "epi-ready" and were not chemically cleaned prior to loading in the chamber. Prior to growth, the substrates were thermally cleaned at 700°C for 15 min and then exposed to oxygen plasma for 15 min at 600°C. Samples were grown at a substrate temperature of either 400°C or 600°C. Reflection high energy electron diffraction (RHEED) was used during the growths to monitor the crystalline growth and the evolving surface morphology. Samples were characterized by x-ray diffraction, optical spectral reflectance, Hall effect measurements, and photoluminescence (PL) to determine structural, electronic, and optical properties.

STRUCTURAL PROPERTIES

In-situ RHEED was used to verify a clean sapphire surface prior to initiating growth. Upon the introduction of Zn flux, the RHEED pattern changed to the characteristic streaky_ZnO (0001) pattern shown in Fig. 1a along the [1120] azimuthal. For most of the growths in these experiments, the RHEED pattern eventually transitioned to the spotty pattern shown in Fig. 1b. This reflects a change from two-dimensional growth (streaky) to a three-dimensional surface morphology (spotty).8 Optical reflectance spectrometry was used to measure the thickness of the samples and to estimate the quality of the surface morphology. The reflectance spectra of a representative layer are shown in Fig. 2, where a sample thickness of 700 nm and root-mean-square (rms) surface roughness of approximately 5 nm were inferred. The samples ranged in thickness from 100 nm to 1400 nm and growth rates ranged from 0.01 µm/h to 0.2 µm/h.