Epitaxial Growth Of ZnO Films Research Articles

The epitaxial growth of ZnO films on Cu(111) surface: Thickness dependence

ZnO ultrathin films supported on Cu, Ag or Au are attracting increasing interest because they can form under special conditions on commercial Cu/ZnO/Al2O3 catalysts for methanol synthesis, or can be used as alternative materials for catalysis, or as prototypes of the so-called inverse catalysts. In the present study, we have investigated the thickness dependence of the structure and electronic properties of free standing and Cu(111) supported ZnO ultrathin films by means of DFT + U calculations. On the free-standing films, a flat graphitic-like morphology is observed up to 10 layers, where a transition to wurtzite-like structures occurs. On Cu(111), a slightly corrugated graphitic structure is obtained for ZnO films up to 4 layers. From 5 layers on, the rumpling becomes substantial and a phase transition to wurtzite takes place. The driving force behind the destabilization of flat graphitic films on Cu(111) is the charge transfer from the underlying Cu support to the film. This finding rationalizes the experimental observations that the transformation from graphitic to wurtzite polymorph of ZnO films on Cu takes place above 5-layer thickness.

Facilitating epitaxial growth of ZnO films on patterned GaN layers: A solution-concentration-induced successive lateral growth mechanism

The hydrothermal epitaxy of ZnO films on a patterned GaN layer with a honeycomb etching hole array is demonstrated. Through m-planes of the GaN layer exposed on the vertical walls of the etching holes, highly crystalline ZnO films via multiple lateral growth stages can be realized. It is found that higher concentrations of zinc nitrate hexahydrate (ZNH) and hexamethylenetetramine (HMT) in hydrothermal solution yield a larger number of ZnO molecules to speed up ZnO growth during the initial stage of hydrothermal growth, also create secondary crystals and initialize further lateral growth stages to bridge neighboring ZnO prisms after smooth surfaces formed on the m-plane of a ZnO prism. A successive lateral growth mechanism that strongly depends on ZNH and HMT concentrations in the hydrothermal solution is proposed and discussed.

Effect of high temperature thermal treatment of (100) γ-LiAlO2 substrate on epitaxial growth of ZnO films by plasma-assisted molecular beam epitaxy

Non-polar ZnO thin films were fabricated on (100) γ-LiAlO2 substrates by plasma-assisted molecular beam epitaxy. The effect of high temperature thermal treatment of substrate on the crystalline orientation and quality of ZnO thin film was investigated. The film grown on (100) γ-LiAlO2 substrate without high temperature thermal treatment consists of domains of both polar and non-polar orientations as identified by the X-ray diffraction pattern. Using high temperature thermal treatment of substrate, the growth of polar ZnO has been suppressed effectively. Besides, high temperature thermal treatment of substrate improves the crystalline quality of epitaxial ZnO thin film, which exhibits a smaller full width at half maximum value of ZnO (101−0) diffraction peak and a weaker deep level emission of photoluminescence. The suppression of polar ZnO growth and the quality improvement of the epitaxial ZnO films are due to the improvement of surface morphology and roughness of the substrate upon high temperature treatment.

Hydrothermal epitaxial growth of ZnO films on sapphire substrates presenting epitaxial ZnAl2O4 buffer layers

This article describes our investigation of the hydrothermal epitaxial growth of c-plane ZnO films on Al2O3 substrates presenting ZnAl2O4 buffer layers. We obtained (111) ZnAl2O4 epitaxial layers on a-plane Al2O3 substrates readily through solid phase epitaxy. Although the ZnAl2O4 buffer layers grew epitaxially with a (111) out-of-plane orientation and comprised two coexisting equivalent azimuthal variants with relative 180° in-plane rotation, the ZnO epitaxial films grown upon them exhibited a c-plane orientation with unitary in-plane epitaxial orientation of <11¯00>ZnO∥<11¯0>ZnAl2O4 on the two different ZnAl2O4 variants. Taking the coincidence of the site lattices between the (0001) plane of ZnO and the (111) plane of ZnAl2O4 into account, a reduction in lattice misfit was achieved through a 30° rotation between the lattices of the ZnO and the ZnAl2O4. We used X-ray diffraction and transmission electron microscopy to obtain detailed microstructural views of the hydrothermally grown ZnO epitaxial films on the ZnAl2O4 buffer layers.

Effect of Co-doping content on hydrothermal derived ZnO array films

Cobalt doped ZnO films are synthesised using a hydrothermal process. The effect of Co2+ concentration on morphology, phase composition, crystallisation and spectroscopic characteristics of ZnO films is investigated. The results indicate that both the structure and morphology of the ZnO films evolve with the concentration of cobalt ions incorporated into the lattice. In the presence of a small amount of Co2+ ions, films are formed that comprise hexagonal ZnO nanorods, oriented with the c-axis perpendicular to the substrate. With increasing amount of Co2+, cracks in the ZnO nanorods can be observed and growth in the [001] direction is significantly inhibited. When the Co2+ concentration exceeds 0.010M, ZnO rods with the typical hexagonal structure are no longer observed and instead, ZnO films comprising close-packed grains with an irregular polygonal structure are formed. The epitaxial growth of ZnO films is nearly completely inhibited when the concentration of Co2+ is increased above 0.050M. This behaviour can be explained by the selective adsorption of the organic substances in the solution onto the (001) ZnO crystal face, thus inhibiting growth in the [001] direction and disrupting the crystallisation of ZnO films. Increasing the Co content deteriorates the crystallisation of ZnO rods and increases tensile stresses present in the ZnO films.

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1) ZnO‖(1 1 1) Si and [1 1 2¯ 0] ZnO‖[0 1 1] Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10 9 cm −2. Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.

Structural characterization of two-step growth of epitaxial ZnO films on sapphire substrates at low temperatures

We have investigated two-step growth of high-quality epitaxial ZnO films, where the first layer—the buffer layer (nucleation layer template)—is grown at a low temperature (230–290 °C) to induce a smooth (two-dimensional) growth. This is followed by growth at a moderate temperature ∼430 °C to form high-quality smooth ZnO layers for device structures. It was possible to reduce the growth temperature to 250–290 °C and obtain a smooth epitaxial template layer on sapphire (0 0 0 1) substrates with surface roughness less than 1 nm. After the high-temperature growth, the film surface undulations (roughness) increased to about 2 nm, but it is still quite smooth. The calculation of c and a lattice parameters by high-resolution x-ray diffraction shows that the a lattice parameter is fully relaxed at the growth temperatures but the c lattice parameter is dependent on the defect concentration in the growing film. A decoupling between a and c lattice parameters of the films is observed, which leads to abnormal Poisson's ratios ranging from 0.08 to 0.54. The decoupling of the lattice parameters is analysed based on growth characteristics and the presence of strain and defects in the grown films. We present our detailed studies on the nature of epitaxy, defects and interfaces by using comprehensive x-ray diffraction and high-resolution TEM studies.

Initial Oxidation and Interfacial Diffusion of Zn on Faceted MgO(111) Films

The interaction of zinc and faceted MgO(111) thin films prepared on a Mo(110) substrate was investigated in situ by using various surface analysis techniques, including X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction. The results revealed that three-dimensional Zn islands exist on the faceted MgO(111) films and that no chemical interaction takes place at the interface at room temperature. Initially, deposited Zn is stable at temperatures below 400 K and diffuses into MgO at temperatures above 425 K. A portion of Zn is oxidized at approximately 10 (-6) mbar O 2 at room temperature. An interfacial phase of Zn x Mg 1- x O was formed after Zn was exposed to approximately 10 (-6) mbar O 2 at temperatures >or=500 K. The faceted structure on the MgO(111) surface is of a disadvantage for the epitaxial growth of ZnO films.

Epitaxial Growth of ZnO Films Prepared by Using MO-CVD with Zn(C9H15O3)2

Epitaxial Growth of ZnO Films Prepared by Using MO-CVD with Zn(C9H15O3)2

Epitaxial growth of ZnO films on (100) and (001) γ-LiAlO2 substrates by pulsed laser deposition

Structural and optical properties were investigated for ZnO films grown on (100) and (001) γ-LiAlO2 (LAO) substrates by pulsed laser deposition method. According XRD results, it is intuitionistic that (100) LAO is suitable for fabricating high quality ZnO film, while (001) LAO is unsuitable. The FWHM of XRD, stress in film and FWHM of UV PL spectra for ZnO films on (100) LAO show a decreasing with increasing substrate temperature from 300 to 600 °C. ZnO film fabricated at 600 °C has the greatest grain size, the smallest stress (0.47 Gpa) and PL FWHM value (∼85 meV). This means that the substrate temperature of 600 °C is optimum for ZnO film deposited on (100) LAO. Moreover, it was found that the UV PL spectra intensity of ZnO film is not only related to the grain size and stoichiometric, but also depends on the stress in the film.

Epitaxial growth of ZnO films by helicon-wave-plasma-assisted sputtering

The epitaxial growth of ZnO films on Al 2O 3 (0 0 0 1) substrates have been achieved at a low-substrate temperature of 400 °C by using a helicon wave-plasma-assisted sputtering technique. X-ray diffraction θ–2θ scan, ω scan and φ scan indicate that the ZnO films have a good c-axis orientation and in-plane epitaxy. The photoluminescence measurement of the films shows that there is only a strong emission peak at 384 nm, and no visible emission related to the structural defects can be detected. A strong absorption in the ultraviolet region and a high transparence of over 85% in visible region are obtained from the optical absorption measurements. Hall-effect measurement reveals that the mobility of the deposited films is 5.0 cm 2/V.s. All above results demonstrate that this newly developed sputtering technique provides a flexible and powerful technique for the growth of high-quality epitaxial ZnO films.

Ａ面サファイア基板を用いた単結晶ＺｎＯ薄膜の分子線エピタキシャル成長

This paper describes radical-source molecular beam epitaxial growth of ZnO films on a-plane sapphire substrates. Reflection high-energy electron diffraction observation and x-ray diffraction measurement show that single-crystalline ZnO (0001) films without any rotational domains are obtained on the sapphire substrates. Photoluminescence spectra measured at 300K exhibit an intense emission peak from free excitons. Hall measurement shows that phonon scattering becomes dominant with increasing temperature and a relatively high electron mobility of -100cm2/Vs with a low residual electron density of -4×1017cm-3 at 300K is achieved.

Epitaxial growth of ZnO films on Si(111)

In this paper, we report the growth of ZnO films on silicon substrates using a pulsed laser deposition technique. These films were deposited on Si(111) directly as well as by using thin buffer layers of AlN and GaN. All the films were found to have c-axis-preferred orientation aligned with normal to the substrate. Films with AlN and GaN buffer layers were epitaxial with preferred in-plane orientation, while those directly grown on Si(111) were found to have random in-plane orientation. A decrease in the frequency of the Raman mode and a red shift of the band-edge photoluminescence peak due to the presence of tensile strain in the film, was observed. Various possible sources for the observed biaxial strain are discussed.

Epitaxial growth of ZnO thin films on ScAlMgO 4 (0 0 0 1) by chemical solution deposition

Epitaxial zinc oxide films were produced on the low misfit (0.3%) substrate ScAlMgO 4 (0 0 0 1) by chemical solution deposition (CSD). Epitaxial growth of ZnO films was achieved by spin coating a precursor solution of zinc acetate dihydrate and ethanolamine in 2-methoxyethanol, heating at 300°C, then at 500°C, and finally at 850°C. X-ray diffraction ( θ/2 θ-scans/off-axis ϕ-scans) as well as electron diffraction show that the axes a and c of ZnO and ScAlMgO 4 are parallel. The absolute orientation of the ZnO film was determined by electron microdiffraction patterns to be [0 0 0 1 ̄ ] . Electron microscopy did not reveal any reaction between film and substrate. The structure of the interface between ZnO and ScAlMgO 4 was characterized in detail by high-resolution TEM methods. Exit wave reconstruction from focus series was carried out to localize the positions of atoms at the interface. It was found that the ZnO film coherently continues the terminating tetrahedral (Mg,Al)O 4 layer of the ScAlMgO 4 substrate to result in an ABAB stacking of the oxygen layers across the interface as in the wurtzite structure. The structural model is in agreement with the absolute orientation of the ZnO film.

Growth of epitaxial ZnO films on Si(111)

AbstractEpitaxial ZnO films have been grown on Si(111) substrates by employing a AlN buffer layer during a pulsed laser-deposition process. The epitaxial structure of AlN on Si(111) substrate provides a template for ZnO growth. The resultant films are evaluated by transmission electron microscopy, x-ray diffraction, and electrical measurements. The results of x-ray diffraction and electron microscopy on these films clearly show the epitaxial growth of ZnO films with an orientational relationship of ZnO[0001]||Aln[0001]||Si[111] along the growth direction and ZnO[2 11 0]||AlN[2 11 0]||Si[0 11] along the in-plane direction. High electrical conductivity (103 S/m at 300 K) and a linear I-V characteristics make these epitaxial films ideal for microelectronic, optoelectronic, and transparent conducting oxide applications.

Atmospheric Pressure Atomic Layer Epitaxy of ZnO Using a Chloride Source

We examined the epitaxial growth of ZnO films on a sapphire (0001) substrate by an atmospheric pressure atomic layer epitaxy technique using ZnCl2 and O2 sources. The films were deposited epitaxially in a substrate temperature range of 450−550 °C with a constant growth rate of 0.26 nm/cycle. It is noteworthy that the rate corresponds to just a half-length of the c axis of hexagonal ZnO, indicating that the alternate deposition of ZnCl2 and O2 on the substrate is governed by a self-limiting mechanism. This was also confirmed by the facts that the film thickness was dependent only on the growth cycles and that the surface was quite smooth. A strong photoluminescence band edge emission of 3.36 eV was observed at 20 K.

Epitaxial growth of ZnO films on Si substrates using an epitaxial GaN buffer

We report on epitaxial growth of ZnO films on Si(111) substrates using an epitaxial GaN buffer layer. A rf magnetron sputtering process has been developed and utilized in growing epitaxial GaN buffers on Si, and then ZnO films on the GaN-buffered Si substrates. X-ray diffraction analysis shows that both the ZnO and GaN films are of a monocrystalline wurtzite structure with an epitaxial relationship of ZnO[0001]//GaN[0001]//Si[111] along the growth direction and ZnO[112_01]//GaN[112_0]//Si[11_0] along the in-plane direction. The successful growth of epitaxial ZnO/GaN films on Si demonstrates the feasibility and promise of integrating various functional devices on the same substrate.

The epitaxial growth of wurtzite ZnO films on LiNbO 3 (0 0 0 1) substrates

ZnO epitaxial films were deposited on LiNbO 3 (0 0 0 1) substrates by pulsed laser deposition. The smaller lattice misfit (−8.5%) between ZnO along 〈1 0 1 ̄ 0〉− direction and LiNbO 3 (0 0 0 1) along 〈1 1 2 ̄ 0〉− direction, as compared with that in the case of normally used sapphire (0 0 0 1) substrates, favored the epitaxial growth of ZnO films. The transmittance spectra of ZnO films deposited in vacuum after annealed in pure oxygen show a sharp absorption edge at 375.6 nm (E g =3.31 eV) .

Structural and Optical Characterizations of ZnO Films Grown by Reactive Electron Beam Evaporation

Low temperature epitaxial growth of ZnO films is achieved on Si(001) substrates by reactive electron beam evaporation. Growth temperature is varied from 125° C to 420° C and the optimum temperature is found between 200° C and 300° C. X-ray diffraction shows that the ZnO films are highly c-axis oriented and the line width of (002) diffraction peak is significantly smaller than that measured from the ZnO films deposited by magnetron sputtering. The combined photoluminescence and photoluminescence excitation (PLE) spectroscopic measurements demonstrate the sharp band-absorption edge and exciton absorption in the ZnO films. PLE has also revealed that the absorption characteristic near the band edge is remarkably improved with the increase of oxygen content in the ZnO films, although x-ray diffraction analysis shows that the crystalline structure of ZnO films grown under different oxygen pressures remains unchanged.

Epitaxial growth of ZnO films on lattice-matched ScAlMgO4(0001) substrates

Abstract The use of lattice-matched (0.09%) hexagonal oxide substrate, ScAlMgO 4 , has considerably improved the quality of ZnO films grown by laser molecular-beam epitaxy. Surface morphology was extremely smooth as represented by atomically flat terraces and 0.26 nm high steps corresponding to the charge neutral unit of ZnO. Crystallinity was comparable to that of bulk single crystals: full-width at half-maximum value of ZnO(0 0 0 2) rocking curve was 39 arcsec. Optical absorption spectra showed clear splitting of A- and B-exciton lines, indicating low damping. Residual carrier concentration in pristine films was as low as 10 15 cm −3 with keeping high electron mobility of ∼100 cm 2 /V s. Therefore, the ZnO films grown on ScAlMgO 4 (0 0 0 1) substrates can be a starting point for realizing p-type ZnO. In fact, high concentration (∼10 21 cm −3 ) nitrogen can be doped with keeping high crystallinity.