CSTACrystal Structure Theory and Applications2169-2491Scientific Research Publishing10.4236/csta.2013.23014CSTA-37250ArticlesChemistry&Materials Science ZnO Heteroepitaxy on Sapphire Using a Novel Buffer Layer of Titanium Oxide: Optoelectronic Behavior atoshiYamauchi1*YohImai2Department of Biomolecular Functional Engineering, Ibaraki University, Hitachi, JapanDepartment of Electric and Electronic Engineering, Ibaraki University, Hitachi, Japan* E-mail:ysatoshi@mx.ibaraki.ac.jp(AY);250920130203100105April 16, 2013May 21, 2013 June 17, 2013© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

Optoelectronic property of ZnO epitaxial layer grown by plasma-assisted epitaxy at temperature as low as 340°C using Ti2O3 buffer layer on a-sapphire were studied by low temperature photoluminescence at 10 K comparing to the layers on c-sapphire and a-sapphire without the buffer layer. The near band-edge emission consisting of free-exciton emissions and neutral-donor bound exciton emissions was significantly dependent on the buffer thickness and dominated by the free-exciton emissions in the layer grown on the very thin buffer layer about 0.8 nm, whereas the intense emissions by neutral-donor bound excitons were observed in the ZnO layer on c-sapphire. The structural behavior indicated the donor was originated from the three-dimensional growth of ZnO layer and details of the optoelectronic feature suggested the residual donors were Al and interstitial-Zn.

ZnO; Ti2O3; Plasma-Assisted Epitaxy; Photoluminescence; Free-Exciton; Bound-Exciton1. Introduction

ZnO is candidate for highly efficient blue or ultraviolet light-emitting devices using the wide direct band gap of 3.37 eV and the large exciton binding energy of 60 meV at room temperature. For such device applications, advanced processes are required for the high-quality growth and highly efficient impurity doping. To date, MBE using oxygen-plasma cell [1], MOMBE using H2O vapor [2], PLD [3] etc. have been attractively studied to improve the structural and optoelectronic properties. At the first stage for ZnO growth, reduction of oxygen deficiency which generates strong green-emission due to oxygen vacancies in ZnO had been required to improve the optoelectronic property. For the purpose, plasmaassisted epitaxy (PAE) using oxygen gas plasma was also an useful process as demonstrated for undoped-ZnO growth at the temperature as low as 400˚C on c-sapphire [4,5], in which the green-emission was sufficiently decreased comparing to the band-edge emissions. However, the near band-edge emissions were dominated by neutral donor bound emissions as same as the other processes, which indicated unexpected shallow donors were included with relatively high-density in the layer. It is important to prevent the donors during ZnO growth, especially for p-type ZnO growth. Therefore, the origin and the removal process have been studied at the second stage, in addition to the effective acceptor doping into the layer [6,7]. As well recognized, low-temperature photoluminescence (PL) is so useful for evaluation of the impurities. In the case of ZnO, the bound exciton emissions concerned with shallow donor impurities such as Ga, Al, In and H have been established with the free-exciton emissions [8]. In addition to such impurity donors, interstitial-Zn should also be taken into account for the shallow donor defect in ZnO as suggested by electron paramagnetic resonance [9]. In the growth process, the defect-donor can be easily introduced during three-dimensional growth of ZnO by high-sticking coefficient of Zn-adatom at steps or kinks on the growth surface. Further, the unfavorable feature is probably enhanced in non-equilibrium growth at low temperatures required for highly efficient doping. Therefore, suitable two-dimensional ZnO growth at low temperatures with reducing impurity-donors and the intrinsic donor-defects is required for the optoelectronic device applications. For the purpose, we previously demonstrated drastic improvement of ZnO growth by Ti2O3 buffer layer on a-sapphire [10], where two-dimensional epitaxial growth of ZnO layer was successfully achieved without the rotational domain.

In this paper, optoelectronic property of PAE-ZnO layer grown on the Ti2O3 buffer layer is examined by low-temperature photoluminescence and shows significant reduction of the residual donor. In addition, details of the near band-edge emissions suggest the donor species.

2. Experimental2.1. Layer Growth

Titanium oxide layer was grown by LPCVD using titanium tetra-iso-propoxide (TTIP: Ti-(OC3H7)4) and oxygen gas. Details of the apparatus and the condition was shown elsewhere [10]. In the case of buffer layer growth for ZnO layer, the thickness was controlled by the growth period estimated from the growth rate.

ZnO layer was grown at 340˚C or 400˚C in 3 mtorr by plasma-assisted epitaxy (PAE) using oxygen gas plasma generated by 10 W radio-frequency (rf) power at 13.56 MHz through a capacitively coupled rf-electrode. Details of the PAE-apparatus and the growth process were described elsewhere [4,10]. ZnO growth rate was different by change of the growth mode on c-, aand the buffer layer as shown elsewhere [10], but effective Zn/O supply ratio during the growth was kept at 1.0 by control of Znflux determined by dependence of the growth rate on the Zn-flux.

Single crystalline aand c-sapphire with mirror surface and 300 μm-thick were used as substrates after cleaning by organic-solvents and hot H2SO4 + H2O2. In the case of etching, the substrates were treated in a hot 3H2SO4 + H3PO4 solution at 130˚C for 15 min.

2.2. Evaluation

Thickness of ZnO layers and Ti2O3 layers were checked by a contact-type surface profiler (Veeco, DEKTAK150). Surface morphology of the ZnO films was observed by Nomarski differential interference microscope (OLYMPUS, BX60). Optoelectronic property was examined by photoluminescence (PL) at 10 K using a cryogenic system (Janis Research, CCS-150). UV-light around 313 nm radiated from a deep UV-lamp (USHIO, UXM-501MA) was selected by a band-pass filter and irradiated on the ZnO layer through a sapphire window equipped on the cryostat chamber with the light power density about 2 mW/cm2. The luminescent light modified to 20 Hz-AC by an optical chopper was introduced into a monocrometor (JASCO, SS-50), then detected and amplified by a photomultiplier (Hamamatsu R374) driven by 1 kV. The current signal was amplified and converted to voltage signal by an amplifier (FEMTO, DLPCA-200), and then the noise components was removed in lock-in-amplifier (NF, LI5640) synchronous to the frequency by the chopper. Then, the signal was recorded in a PC after A/D conversion. The relative sensitivity of the system for photon energy was corrected by black-body radiation spectrum from a standard lamp.

3. Results and Discussions3.1. ZnO Layer on C-Sapphire

Growth rate of ZnO layers at 400˚C (closed circles) and 340˚C (open circles) on c-sapphire for various Znflux in oxygen plasma excited by 10 W rf-power are shown in Figure 1. The growth rates same at both temperatures were increased with the Zn-flux then saturated by the flux above 80 μmol/min. The results indicated the effective supply flux rate of Zn/O could be determined at both temperatures since the growth rate was limited by smaller flux of Zn or O. Figure 2 shows typical near band-edge PL spectrum of PAE-ZnO layer with the thickness of 1.0 m grown at 400˚C on csapphire by the Zn/O supply ratio of 1.0 (Zn-flux: 80 μmol/min). The inset shows surface morphology of the layer observed by Nomarski microscope. The layer consisting of three-dimensionally grown columnar grains was preferentially oriented along c-axis but including 30˚-rotational domains. In the PL spectrum, prominent emission peak at 3.3623 eV originated from neutral donor-bound exciton emission (DBE) was observed with a weak DBE emission peak 3.3666 eV and free-exciton emissions of A-Ex and B-Ex at 3.3774 eV and around 3.385 eV respectively [8]. FWHM of the DBE emission of the layer grown by the supply ratio around 1.0 was below 2 meV and the intensity was 4- orders magnitude larger than the green-emission due to deep-level. It is noted that the PL-spectrum was obviously degraded in the ZnO layer grown at temperatures lower than 400˚C and the sharp near band-edge emissions could not be observed.

3.2. ZnO Layer on Ti<sub>2</sub>O<sub>3</sub> Buffer Layer3.2.1. PL Spectrum on A-Sapphire and Buffer Layer

Figure 3 shows near band-edge spectra of PAE-ZnO layers grown on (a) a-sapphire and (b) Ti2O3 buffer layer grown at 320˚C with the thickness of 0.8 nm, where the ZnO layers were grown at 340˚C in oxygen plasma excited by 10 W rf-power. It is noted here the near bandedge emissions could be observed in ZnO layers grown at the temperature around 340˚C but obviously degraded by the high temperature growth around 400˚C. The insets show the surface morphologies observed by Nomarski microscope. The layer was epitaxially grown on the buffer layer with the epitaxial relationship of [1-100] ZnO//[1-100] Ti2O3, whereas the layer was polycrystallized on a-sapphire. The hexagonal pyramid-like grains with the facets according to the epitaxial relationship as shown in the inset of (b) were observed in ZnO

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