ZnO Epitaxy Research Articles

The effects of buffer growth parameters on heteroepitaxial ZnO films grown by pulsed laser deposition

The effects of buffer layer deposition conditions on subsequent ZnO epitaxy on sapphire (0001) were examined. An initial ZnO buffer layer improves surface roughness for a wide range of buffer layer growth temperatures and pressures. Changes in buffer layer growth pressure and temperature have a moderate effect on the roughness of subsequent film growth. However, the conditions for buffer layer deposition have a large impact on crystallinity of subsequent films. In particular, the out-of-plane X-ray diffraction rocking curve full-width half-maximum decreased as buffer deposition temperature or O 2/O 3 pressure increases. Carrier mobility in the subsequent thick ZnO film was enhanced with increase in buffer layer deposition temperature. Carrier concentration decreased with increasing buffer layer deposition pressure.

Crystal chemistry of interfaces formed between two different non-metallic, inorganic structures

Abstract Epitaxial relations are often described by the mismatch of lattice parameters between the two materials. When the structures forming the epitaxial interface are dissimilar, the lattice mismatch is often less meaningful because of the different geometrical arrangement of atoms in the two structures. In a previous paper, it was shown that two rules of crystal chemistry, widely known as Pauling's rules, namely cation/anion coordination and charge balance, could be applied to understand the epitaxy of (0001) ZnO (wurtzite) on (111) MgAl2O4 (spinel) substrates. It was then shown that the same rules could be applied to predict and confirm the epitaxy of (101) SiO2 (cristobalite) on (0001) Al2O3 (sapphire). Here, the same rules are applied to explain the epitaxy of four other dissimilar structures that have been previously published by others, including SrTiO3 on Ni c(2 × 2) S, TiO2 (anatase) on LaAlO3, TiO2 (rutile) on r-plane Al2O3, and Zr1−x (Y x )O2 on (0001) Al2O3.

Low-temperature interface engineering for high-quality ZnO epitaxy on Si(111) substrate

Zn O ( 0001 ) ∕ Si ( 111 ) interface is engineered by using a three-step technique, involving low-temperature Mg deposition, oxidation, and MgO homoepitaxy. The double heterostructure of MgO(111)∕Mg(0001)∕Si(111) formed at −10°C prevents the Si surface from oxidation and serves as an excellent template for single-domain ZnO epitaxy, which is confirmed with in situ reflection high-energy electron diffraction observation and ex situ characterization by transmission electron microscopy, x-ray diffraction, and photoluminescence. The low-temperature interface engineering method can also be applied to control other reactive metal/Si interfaces and obtain high-quality oxide templates accordingly.

Thermal Analysis of Liquid Phase Epitaxy of ZnO and Characteristics of Some Li-Doped Mg0.06Zn0.94O ∕ ZnO Heterostructure

Thermogravimetry and differential thermal analysis (TG/DTA) were used to examine a process capable of fabricating homoepitaxial, single-crystalline (, Ga, etc.) films under thermodynamically stable conditions by liquid phase epitaxy. A liquid solution of alkaline metal (Li, Na, Cs) chlorides is employed at temperatures of . A reaction of with polycrystalline is applied to continuously form over a process cycle under ambient air atmosphere and pressure as shown by TG/DTA. Mirrorlike films with a thickness have been grown. The high structural quality of the film with sharp interfacial transition from substrate to film has made it possible to observe a surface conductive channel. Temperature-dependent Hall measurement on Li-doped heterostructures under vacuum conditions discloses a high-resistivity state with at and a low-resistivity state with at .

Strain effects in ZnO layers deposited on 6H-SiC

Correlation in crystallite sizes and defects of epitaxial ZnO layers deposited on 6H-SiC substrates has been addressed. The biaxial strain governs the ZnO crystallites for the layer thickness of ∼400nm. The misfit dislocations were observed in nucleation and theater is the columnar growth mode diffracted in transmission electron microscopy. The columnar growth mode is a symbol of stacking faults that appear due to imbalanced interface chemistry in the II-VI/IV materials system, together with the complex impurity matrix. These defects are the main source of nonradiative recombination centers in ZnO epitaxy resulting in shorter exciton lifetimes examined in time-resolved photoluminescence measurements.

Correlation of self-organized surface nanostructures and anisotropic electron transport in nonpolar ZnO (10−10) homoepitaxy

Self-organized surface nanostructures were formed during laser molecular beam epitaxy of nonpolar ZnO (10−10). The growth mechanism of the nanostructures was related to a Schwoebel barrier effect generated during ZnO homoepitaxy. Small islands elongated along the [001] direction were generated on two-dimensional (2D) growing surfaces above the critical thickness. With increasing film thickness, the anisotropic islands developed homogeneous nanostripe arrays with stripe lengths above 5μm and connected by some branches. Highly anisotropic surface morphology markedly influenced electron transport of ZnO films and Mg0.12Zn0.88O∕ZnO multiquantum wells (MQWs) with conductivity parallel to the nanostripe arrays by more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays. We propose that the origin of anisotropic conductivity of ZnO films resulted mainly from anisotropically distributed scattering centers of electrons related to grain boundaries between nanostripe arrays. Furthermore, the surface nanostructures consisting of nanostripe arrays gave rise to the anisotropic surface roughness at the heterointerface between Mg0.12Zn0.88O and ZnO layers and contributed to the anisotropy of 2D electron transport of the MQWs at low temperatures.

Growth of ZnO thin films on c-plane Al 2O 3 by molecular beam epitaxy using ozone as an oxygen source

The growth of c-axis oriented ZnO thin films on c-plane Al 2O 3 via molecular beam epitaxy (MBE) using dilute ozone (O 3) as an oxygen source was investigated. Four-circle X-ray diffraction (XRD) indicates that films grown at 350 °C are epitaxial with respect to the substrate, but with a broad in-plane and out-of-plane mosaic. The films were highly conductive and n-type. Epitaxial film growth required relatively high Zn flux and O 3/O 2 pressure. The growth rate decreased rapidly as growth temperature was increased above 350 °C. The drop in growth rate with temperature reflects the low sticking coefficient of Zn at moderately high temperatures and limited ozone flux for the oxidation of the Zn metal. Characterization of the films included atomic force microscopy (AFM), X-ray diffraction, photoluminescence, and Hall measurements. These results show that molecular beam epitaxy of ZnO using ozone is rate limited by the ozone flux for growth temperatures above 350 °C.

Interface engineering for lattice-matched epitaxy of ZnO on (La,Sr)(Al,Ta)O3(111) substrate

Zn O ∕ ( La , Sr ) ( Al , Ta ) O 3 ( LSAT ) heterointerface is engineered to control the crystallographic orientation of ZnO films grown by plasmas-assisted molecular beam epitaxy. Lattice-matched in-plane alignment of [112¯0]ZnO‖[112¯]LSAT has been realized using Mg modification of the substrate surface, which is confirmed with in situ reflection high-energy electron diffraction observation, and ex situ characterization of x-ray diffraction and transmission electron microscopy. The low-temperature deposition and high-temperature treatment of the Mg layer on the oxygen-terminated LSAT(111) surface results in selective nucleation of a MgO interface layer which serves as a template for single-domain epitaxy of ZnO. Oxygen-polar ZnO film with an atomically smooth surface has been obtained, which is favorable for metal-ZnO Schottky contact with high barrier height.

MOCVD of pure and Ga-doped epitaxial ZnO

Here we report the expansion of the Ga 2O 3 solubility range in ZnO films grown by MOCVD on single-crystal substrates due to the epitaxial stabilization contribution. The epitaxial relations were characterized for ZnO and a broad spectrum of the substrates including c- and r-Al 2O 3, (111) ZrO 2(Y 2O 3), (111) SrTiO 3, (111) Gd 3Ga 5O 12, (111) MgO, (111) MgAl 2O 4. First, the deposition kinetics was measured in the range 300–800 ∘C using Zn(acac) 2 and Ga(acac) 3, where acac = the acetylacetonate-anion. The maximum deposition rate corresponds to the range 500–550 ∘C. The particular feature of the deposition process is switching to the growth of whiskers with the increase of the deposition rate. The films on (111) ZrO 2(Y 2O 3), (111) SrTiO 3, are characterized by a higher epitaxial quality than films on sapphire traditionally used for ZnO epitaxy. This results in the higher solubility of Ga 2O 3 in ZnO films on (111) ZrO 2(Y 2O 3) (at least up to 7.5 at.% Ga in the sum of Ga and Zn) as compared to the films on r-Al 2O 3 (below 3 at.% Ga) due to the epitaxial stabilization. The intensity of the UV photoluminescence (PL) correlates with the epitaxial quality of ZnO films. Ga-doping resulted in the quenching of green PL band because of Ga 3+ occupancy of the octahedral interstitials in the ZnO structure (in agreement with the variation of the ZnO(Ga 2O 3) lattice parameters).

Enhancement of ZnO nucleation in ZnO epitaxy by atomic layer epitaxy

Dry cleaning techniques employing argon, oxygen and hydrogen ECR plasma were used, respectively, to remove organic contaminants and native oxides on sapphire and silicon (Si) substrates prior to the growth of the ZnO film by atomic layer epitaxy. The cleaning efficiency was assessed by investigating the nucleation density and the incubation period for ZnO nucleation using scanning electron micrography (SEM) and Auger electron emission spectroscopic analysis (AES). ECR plasma pretreatment increased the ZnO nucleation density and reduced the incubation period for ZnO nucleation. Oxygen ECR plasma pretreatment was found to be more effective in enhancing ZnO nucleation than any other plasma pretreatment, and the effects are more prominent on the sapphire substrate than the silicon substrate. ZnO nucleation on the sapphire substrate is substantially enhanced by treating the substrate surface with oxygen ECR plasma prior to ZnO atomic layer epitaxy (ALE) because the hydroxyl packing density at the substrate surface is increased by oxygen plasma.

In situ monitoring of Zn* and Mg* species during helicon-wave-excited-plasma sputtering epitaxy of ZnO and Mg0.06Zn0.94O films

Atomic species in the sputtered plumes from pure ZnO and ZnO–MgO (94:6 mol %) targets were identified by means of plume emission spectroscopy (PES) during helicon-wave-excited-plasma sputtering epitaxy. Dominant Zn and Mg species detected by PES were excited neutral zinc (Zn*) and magnesium (Mg*), respectively. Their densities were uniquely controlled by the target bias (Vt) that accelerates the velocity of Ar cations, keeping the gross plasma density constant. The growth rate was principally limited by the densities of Zn* and Mg*, as well as nonradiative species such as Zn–O and Mg–O, and the MgO molar fraction x in the MgxZn1−xO epilayers nearly agreed with that of the target between 500 and 650 °C. Structural qualities of the epilayers were significantly improved by the use of nearly lattice-matched substrates having proper surface arrangements. The a-axis-locked single-domain (0001) MgxZn1−xO epitaxy was accomplished on an uniaxially quasi-lattice-matched (112̄0) Al2O3 substrate. The epilayers had atomically smooth surfaces, and 0.26-nm-high monolayer steps were observed in ZnO epilayers. The ZnO and Mg0.06Zn0.94O epilayers exhibited a predominant near-band-edge photoluminescence peak at room-temperature (3.28 and 3.36 eV, respectively).

Molecular beam epitaxy of high-quality ZnO using hydrogen peroxide as an oxidant

ZnO layers were grown on (0 0 0 1) GaN/sapphire substrates by molecular beam epitaxy (MBE) with the use of hydrogen peroxide as an oxidant. Single-crystal ZnO films were found to grow at substrate temperatures from 400°C to 650°C, whereas textured ZnO layers were prepared at the temperatures below 400°C. Structural and optical properties as well as surface morphology of the single-crystal ZnO films were investigated as a function of growth parameters. The single-crystal material showed strong bound-exciton (BE) emission with a full-width at half-maximum down to 4.7 meV. The increase in both substrate temperature and Zn flux was found to result in higher crystal perfection and stronger BE emission. The data obtained prove the feasibility of the peroxide-assisted MBE for the growth of high-quality ZnO layers.

Importance of lattice matching and surface arrangement for the helicon-wave-excited-plasma sputtering epitaxy of ZnO

Importance of lattice matching and proper surface arrangement of the substrate was shown to obtain better epilayer qualities of ZnO grown by an epitaxy method, the helicon-wave-excited-plasma sputtering epitaxy. The a-axis-locked single-domain (0001) ZnO epitaxy was accomplished on the uniaxially nearly lattice-matched (112̄0) Al2O3 substrate. The epilayer had approximately 50-100-nm-diam atomically smooth two-dimensional terraces with 0.26-nm-high monolayer steps, and exhibited a predominant near-band-edge photoluminescence peak, whose full width at half maximum value was 107 meV at room temperature. The a-axis-locked epitaxy was also realized on a lattice-mismatched (0001) Al2O3 substrate, indicating the compatibility of oxide substrates. However, the ZnO epilayer grown on the (0001) AlN epitaxial template prepared on (0001) Al2O3 had an in-plane 12-fold multidomain structure, though the effective lattice-mismatch between [112̄0] ZnO and [112̄0] AlN is smaller than that between [112̄0] ZnO and [101̄0] Al2O3. Proper purification and polarity-control techniques for the AlN surface, and/or insertion of an appropriate buffer layer are required to form single-domain heterovalent ZnO/AlN interfaces.

Role of hydrogen in molecular beam epitaxy of ZnO

The role of hydrogen in the growth and material properties of ZnO films grown at temperature as low as 300 °C by plasma-assisted molecular beam epitaxy with and without hydrogen irradiation was investigated. Results showed that during growth, the surface morphology changed from small hemispherical grains (10 nm) to large hexagonal islands (100 nm) by molecular hydrogen irradiation. The observed changes in the surface morphology correlated with the surface migration length of Zn adatoms on the surface with or without H termination. X-ray diffraction and photoluminescence measurements showed that the structural and optical properties of ZnO films were significantly improved by H2 irradiation during growth and that the ZnO films grown here at low temperature (300 °C) by H2 irradiation had crystalline quality as high as that of ZnO grown at higher temperature (600 °C) without H2 irradiation. Secondary ion mass spectrometry and Hall effect measurements indicated that most of hydrogen incorporated in the ZnO films grown here did not act as shallow donors.

Monocrystalline ZnO Films on GaN/Al2O3 by Atomic Layer Epitaxy in Gas Flow

Atomic layer epitaxy (ALE) of ZnO on a GaN template was performed in a low-pressure gas flow system using ZnCl2 and O2 precursors, and the evidence of monocrystallinity of an ALE-grown ZnO film is presented. The growth is achieved under the pressure of about 5 mbar with 480 °C substrate temperature and the ZnCl2-source temperature of 390 °C. According to the AFM analyses, the film surface exhibits a low roughness value of 0.45 nm RMS, which fulfills the expectations with respect to high morphological quality of the ALE products. The structural properties of the produced ZnO are characterized by means of X-ray diffraction methods based on wide range 2θ/ω scans and high-resolution studies of 00.2 and 11.4 reciprocal lattice points. Lattice parameters for ALE-grown ZnO layer are a = 0.3253(1) nm and c = 0.52060(5) nm, as derived from the high-resolution data. The mosaicity level of the GaN template layer is reproduced in the grown ZnO film.

Rotation‐domains suppression and polarity control of ZnO epilayers grown on skillfully treated c‐Al2O3 surfaces

AbstractRotation domains are easily and often introduced into ZnO epilayers grown on c‐Al2O3 substrate, resulting in degraded epilayer quality. In this paper, effects of the sapphire surface properties on the epitaxy of ZnO were studied; the surface was carefully treated by high temperature thermal cleaning (TC), atomic hydrogen (H*), oxygen radical (O*) treatments, decoration by a few monolayers gallium and nitridation, and then the ZnO epilayers grown on these surfaces were characterized. Three kinds of rotation domains were observed in total and the sub‐domains were completely suppressed by Ga decoration and sapphire nitridation. The effect of the above treatments on polarities of ZnO epilayer was also studied. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ spectral control of Zn species during helicon-wave-excited-plasma sputtering epitaxy of ZnO

Atomic species in the sputtered plumes from undoped ZnO target were identified by means of emission spectroscopy during helicon-wave-excited-plasma sputtering epitaxy. Luminescent Zn species were found to be excited neutral zinc (Zn*) and zinc cations (Zn+*), and Zn* density was independently controlled by the target bias (Vt) that accelerates the velocity of Ar cations while keeping the plasma density constant. The ZnO film formation seemed to have certain threshold Vt (around −200 V at 600 °C), and the growth rate increased with the magnitude of Vt but decreased with the substrate temperature, indicating that the growth rate is limited by the sticking coefficient of Zn. As a result of the supply mode of Zn that is similar to the case for laser molecular-beam epitaxy, the epitaxial (0001) ZnO on (112̄0) sapphire substrate had ultra-smooth surfaces having atomically flat terraces, and exhibited excitonic reflectance anomalies and luminescence peaks at low temperature.

Crystal chemistry of epitaxial ZnO on (1 1 1) MgAl 2O 4 produced by hydrothermal synthesis

ZnO powders and epitaxial thin films were synthesized hydrothermally in aqueous solutions of zinc nitrate and ammonia at 150°C. Powder yield and particle size varied with changes in pH from 7 to 10.5. The films grew with a c-axis orientation on (1 1 1) MgAl 2O 4 substrates, with an out-of-plane orientation given by (1 1 1) s ||(0 0 0 1) Zn , and an in-plane orientation given by ( 112) s ||(11 2 ̄ 0) Zn . Based on etching studies, the polarity of the films was determined to be zinc faced (0 0 0 1). The in-plane orientation relation corresponds to a 30° rotation of the close packed oxygen directions of the two respective structures. This rotation produces periodic, tetrahedrally coordinated Zn sites at the spinel/ZnO interface and facilitates the epitaxy of ZnO with only a small lattice strain. The crystal chemistry of the interfacial atomic layer that lies between the spinel and wurtzite is discussed in relation to cation coordination and charge balance.

Recent progress in ZnO epitaxy

Recent progress in ZnO epitaxy

Interface Engineering in Heteroepitaxy

We report the importance of interface engineering in heteroepitaxy with examples of plasma-assisted molecular beam epitaxial ZnO growths on (0001) sapphire substrates and on (0001) GaN/sapphire templates, whose interfacesare engineered to improve and to control properties of ZnO films. The growth of rocksalt structure MgO buffer on Al 2 O 3 (0001) is developed for ZnO epitaxy. By employing the MgO buffer layer, the formation of 30° rotated mixed domains is prohibited and two-dimensional layer-by-layer growth of ZnO on sapphire substrate is achieved. High-resolution X-ray diffraction reveals the superior improvement in a crystal quality of ZnO films with an MgO buffer. Polarity of wurtzite structure ZnO films on Ga-polar GaN/sapphire templates is controlled by changing interface structures. By forming a single crystalline, monoclinic Ga 2 O 3 interfacial layer between GaN and ZnO through O-plasma pre-exposure on the Ga-polar GaN surface, O-polar ZnO films are grown. By forming the ZnO/GaN heterointerface without an interfacial layer through the Zn pre-exposure on the Ga-polar GaN surface, Zn-polar ZnO films are grown.