Study of the ZnO crystal growth by vapour transport methods

The crystal growth of ZnO by vapour transport is classically made with the assistance of additional species that produce a gaseous mixture, the role of which remains often uncertain in the transport and growth process. Initially, in order to study the mass transport process, a numerical simulation is made to analyse which are the requirements to have an effective transport. As the pressure of each gaseous species is generally unknown, the numerical study has been performed for different total pressures. It is found that, if congruent and equilibrium conditions are assumed at the sublimation and crystallisation interfaces, effective growth conditions can only be attained for a narrow range of total pressures. Nevertheless, it is well known that ZnO growth by vapour transport is possible for a wide range of pressures of gaseous species. As a consequence, partial pressures higher than the equilibrium ones must be present in order to justify the experimental results. We suggest that the thermal decomposition of ZnO is given by an activated process. The analysis of different mechanisms that could justify the activated decomposition, in accord with a systematic set of growth experiments, suggests that some additional species in the growth of ZnO by vapour transport promote the generation of an additional Zn pressure. This zinc pressure would act autocatalytically inducing O₂ and Zn partial pressures higher than the equilibrium ones and promoting thermal decomposition. The above-cited set of experimental growth experiences, that include the presence of C, Zn, Fe, Cu and H₂, will be analysed and interpreted according to this approach.     

Growth and optical absorption spectra of ZnO films grown by pulsed laser deposition

ZnO films on Al₂O₃ substrate were grown by using a pulsed laser deposition method. Through photoluminescence (PL) and X-ray diffraction (XRD) measurements, the optimum growth conditions for the ZnO growth were calculated. The results of the XRD measurement indicate that ZnO film was strongly oriented to the c-axis of hexagonal structure and epitaxially crystallized under constraints created by the substrate. The full-width half-maximum for a theta curve of the (0 0 0 2) peak was 0.201°. Also, from the PL measurement, the grown ZnO film was observed to be a free exciton, which indicates a high quality of epilayer. The Hall mobility and carrier density of the ZnO film at 293 K were estimated to be 299 cm²/V sec and , respectively. The absorption spectra revealed that the temperature dependence of the optical band gap on the ZnO films was − .     

Properties of Si-rich SiO2 films by RF magnetron sputtering

Si-rich silicon oxide (SiO_x, 1<x<2) films were prepared by RF magnetron reactive sputtering or co-sputtering on the Si(1 1 1) substrates. X-ray diffraction patterns showed that the peak of silicon nanocrystals (NCs), separated from SiO_x films, had (1 1 1) preferred orientation. The results of scanning electron microscopy indicated the Si NCs uniting into clusters. We demonstrated that the photoluminescence (PL) peaks at 650 nm was caused by defect center. In particular, we discussed the correlation between the PL and the structure of SiO_x films. The mean size of the Si NCs was estimated to be about 3 nm by the PL peak position.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Microstructure and ferroelectric properties of BaTiO3 films on LaNiO3 buffer layers by rf sputtering

We have fabricated LaNiO₃ and BaTiO₃ films using the rf sputtering method. The LaNiO₃ were deposited on Si substrates, demonstrating a (1 0 0) highly oriented structure and nanocrystalline characteristic with a grain size of 30 nm. The BaTiO₃ thin films were deposited on the LaNiO₃ buffer layers, and have exhibited a (1 0 0) texture with a thickness of 400 nm. A smooth interface is obtained between the LaNiO₃ bottom electrode and the BaTiO₃ film from cross-section observations by scanning electron microscopy. The bi-layer films show a dense and column microstructure with a grain size of 60 nm. Ferroelectric characterizations have been obtained for the BaTiO₃ films. The remnant polarization and coercive field are 2.1 μC/cm² and 45 kV/cm, respectively. The leak current measurements have shown a good insulating property.     

High-quality ZnO thin films prepared by two-step thermal oxidation of the metallic Zn

In this paper, we report the preparation of nanocrystalline ZnO thin films on Si (1 0 0) substrates using a simple method, in which a resistive thermal evaporation of Zn and a two-step annealing process were employed. The aim of the first annealing step in an oxygen ambient at 300°C for 2 h is to form ZnO layers on the surface of the Zn films to prevent the diffusion of the metallic Zn from the films during the high-temperature annealing process. To obtain high-quality ZnO films, a high-temperature annealing step was performed at temperature in the range of 600–900°C. The effects of the annealing temperature on the photoluminescence (PL) and orientation of ZnO nanocrystalline thin films were studied. A very strong near-band-edge emission around 375 nm with a full-width at half-maximum of 105 meV and a relatively weak emission around 510 nm related to deep-level defects were observed, which indicated that high-quality ZnO films have been obtained.     

Epitaxial growth of ZnO thin films on Si substrates by PLD technique

Epitaxial ZnO thin films have been grown on Si(1 1 1) substrates at temperatures between 550 and 700 °C with an oxygen pressure of 60 Pa by pulsed laser deposition (PLD). A ZnO thin film deposited at 500 °C in no-oxygen ambient was used as a buffer layer for the ZnO growth. In situ reflection high-energy electron diffraction (RHEED) observations show that ZnO thin films directly deposited on Si are of a polycrystalline structure, and the crystallinity is deteriorated with an increase of substrate temperature as reflected by the evolution of RHEED patterns from the mixture of spots and rings to single rings. In contrast, the ZnO films grown on a homo-buffer layer exhibit aligned spotty patterns indicating an epitaxial growth. Among the ZnO thin films with a buffer layer, the film grown at 650 °C shows the best structural quality and the strongest ultraviolet (UV) emission with a full-width at half-maximum (FWHM) of 86 meV. It is found that the ZnO film with a buffer layer has better crystallinity than the film without the buffer layer at the same substrate temperature, while the film without the buffer layer shows a more intense UV emission. Possible reasons and preventive methods are suggested to obtain highly optical quality films.     

Heteroepitaxial growth of Cu2O thin film on ZnO by metal organic chemical vapor deposition

Cuprous oxide (Cu₂O) thin films were grown epitaxially on c-axis-oriented polycrystalline zinc oxide (ZnO) thin films by low-pressure metal organic chemical vapor deposition (MOCVD) from Copper(II) hexafluoroacetylacetonate [Cu(C₅HF₆O₂)₂] at various substrate temperatures, between 250 and 400 °C, and pressures, between 0.6 and 2.1 Torr. Polycrystalline thin films of Cu₂O grow as single phase with [1 1 0] axis aligned perpendicular to the ZnO surface and with in-plane rotational alignment due to (2 2 0)_Cu2O(0 0 0 2)_ZnO; [0 0 1]_Cu2O[1 2¯ 1 0]_ZnO epitaxy. The resulting interface is rectifying and may be suitable for oxide-based p–n junction solar cells or diodes.     

Growth and energy band gap of CaSeS thin films prepared by hot-wall epitaxy

This paper investigates preparation of CaSeS thin films using hot-wall epitaxy. These films can be grown epitaxially on cleaved BaF₂(1 1 1) at a substrate temperature of 873 K by tailoring the VI/II flux ratio vaporized from Ca and SeS resources. The optical absorption edge of these films thus tailored can be observed clearly, shifting toward higher photon energy with increasing S content. In particular, the energy band gap of CaSe_0.66S_0.34, capable of lattice-matching to InP was found to be 4.69 eV, producing considerably large band gap difference of 3.34 eV between the CaSe_0.66S_0.34 and InP.     

High intense UV-luminescence of nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

High quality zinc oxide (ZnO) films were obtained by thermal oxidation of high quality ZnS films. The ZnS films were deposited on a Si substrate by a low-pressure metalorganic chemical vapor deposition technique. X-ray diffraction spectra indicate that high quality ZnO films possessing a polycrystalline hexagonal wurtzite structure with preferred orientation of (0 0 2) were obtained. A fourth order LO Raman scattering was observed in the films. In photoluminescence (PL) measurements, a strong PL with a full-width at half-maximum of 10 nm around 380 nm was obtained for the samples annealed at 900°C at room temperature. The maximum PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.     

Morphological,structural and electrical investigations on non-polar a-plane ZnO epilayers

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements (C–V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.     

Epitaxial growth of ZnO films

After summing up the main physical properties of ZnO and its subsequent applications the aim of this article is to review the growth of ZnO epitaxial films by PLD, MBE, MOCVD and sputtering under their various aspects, substrates, precursors, reaction chemistry, assessment of the layers etc. …, keeping constantly in mind some key issues for the device applications of ZnO in optoelectronics, surface acoustic filters and spintronics, amongst which the growth of high quality epitaxial layers of both n- or p-type conductivity, the possibility of dissolving transition elements in the layers, the growth of ZnO related alloys and heterostructures are of major significance.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

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 ∼5×10⁹ 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.     

Influence of oxygen/argon ratio on structural,electrical and optical properties of Ag-doped ZnO thin films

Ag-doped ZnO (ZnO:Ag) thin films were deposited on quartz substrates by radio frequency magnetron sputtering technique. The influence of oxygen/argon ratio on structural, electrical and optical properties of ZnO:Ag films has been investigated. ZnO:Ag films gradually transform from n-type into p-type conductivity with increasing oxygen/argon ratio. X-ray photoelectron spectroscopy measurement indicates that Ag substitutes Zn site (Ag_Zn) in the ZnO:Ag films, acting as acceptor, and being responsible for the formation of p-type conductivity. The presence of p-type ZnO:Ag under O-rich condition is attributed to the depression of the donor defects and low formation energy of Ag_Zn acceptor. The I–V curve of the p-ZnO:Ag/n-ZnO homojunction shows a rectification characteristic with a turn-on voltage of ∼7 V.     

Sn3O4 single crystal nanobelts grown by carbothermal reduction process

This article reports on the growth of single crystal Sn₃O₄ nanobelts and SnO by a carbothermal reduction process in two different regions of a furnace tube. Even though intermediate tin oxide compounds (Sn₃O₄) have been observed experimentally, the study of structures based on them is a challenging task. Characterization data allowed us to propose that Sn₃O₄ nanobelts grew by vapor–solid mechanism while SnO grew by self-catalyst vapor–liquid–solid mechanism. Electrical measurements of a single Sn₃O₄ nanobelt were performed at different temperatures, revealing undoped semiconductor characteristics.     

Preparation and characterization of fiber-textured SrAl2O4:Eu films grown using a homo-buffer layer

This paper describes the preparation and characterization of fiber-textured SrAl₂O₄:Eu films on a quartz glass substrate using a homo-buffer layer. The effect of the buffer layer on the crystallinity and adhesion was investigated by cross-section transmission electron microscopy and X-ray diffraction (XRD). The results show that the prepared film was not only well crystallized but also highly textured. The preferred orientation of this textured film was confirmed to be (0 3 1) by pole figure measurement. In addition, this film exhibits excellent optical transparency, with an average transmittance of more than 80% in the visible range.     

Thermal-induced phase transition and assembly of hexagonal metastable In2O3 nanocrystals: A new approach to In2O3 functional materials

This paper reports on the thermal-induced performance of hexagonal metastable In₂O₃ nanocrystals involving in phase transition and assembly, with particular emphasis on the assembly for the preparation of functional materials. For In₂O₃ nanocrystals, the metastable phase was found to be thermally unstable and transform to cubic phase when temperature was higher than 600 °C, accompanied by assembly as well as evolution of optical properties, but the two polymorphs coexisted at the temperature ranging from 600 to 900 °C, during which the content of product phase and crystal size gradually increased upon increasing temperature. The assembly of In₂O₃ nanocrystals can be developed to fabricate In₂O₃ functional materials, such as various ceramic materials, or even desired nano- or micro-structures, by using metastable In₂O₃ nanocrystals as precursors or building blocks. The electrical resistivity of In₂O₃ conductive film fabricated by a hot-pressing route was as low as 3.72×10⁻³ Ω cm, close to that of In₂O₃ single crystal, which is important for In₂O₃ that is always used as conductive materials. The findings should be of importance for both the wide applications of In₂O₃ in optical and electronic devices and theoretical investigations on crystal structures.