High-quality hexagonal ZnO crystals grown by chemical vapor deposition

High-quality zinc oxide (ZnO) crystals were grown on a (0001) sapphire substrate by chemical vapor deposition at 830 °C under atmospheric pressure. The hexagonal crystals had an average diameter of about 150 μm, and a thickness of about 15 μm, as observed under a polarizing microscope. The large (0002) facet was flat, regular, and neat. In the X-ray diffraction pattern, strong (0002) and weak (0004) peaks indicate that the crystals had a wurtzite structure. The crystalline quality was characterized by Raman scattering, and the E₂(high), E₂(low), and A_l(LO) modes confirm the high quality of the ZnO crystals. Photoluminescence (PL) spectra of the crystals had a strong and sharp ultraviolet emission peak at 379 nm. The PL mechanism was also discussed.     

Conformal aluminum oxide coating of high aspect ratio structures using metalorganic chemical vapor deposition

The metalorganic chemical vapor deposition of aluminum oxide has been studied over a wide process parameter range. Electrical properties of as-grown and annealed layers have been investigated using planar aluminum/aluminum oxide/silicon capacitors. The best processing conditions resulted in a leakage current of 10 nA/cm² at an equivalent oxide thickness of 3.6 nm. In addition, the film conformality was evaluated on silicon trench structures with aspect ratios of up to 60. Excellent step coverage of over 90% (thickness at trench bottom to thickness at trench middle) was achieved at temperatures below 400 °C and a pressure of 100 Pa. After annealing the electrical properties of these layers, analyzed on planar test structures, were comparable to the results obtained at higher deposition temperature.     

Annealing and partial pressure ratio effects on ZnO films grown by metal-organic chemical vapor deposition using tert-butanol as oxidant

ZnO films were deposited by metal-organic chemical vapor deposition on (0001) sapphire substrates at various partial pressure ratios of oxygen and zinc precursors (R_VI/II). The annealing and the R_VI/II ratio effects on the vibrational and optical properties of ZnO films have been investigated by Micro-Raman scattering and low temperature photoluminescence (PL) spectroscopy. As confirmed by characterizations used in this study, the quality of the ZnO films was improved by thermal annealing at 900 °C in oxygen ambient. Raman spectra of the as-deposited films show a broad band (BB) centered at about 518 cm⁻¹ whose intensity increases when the R_VI/II ratio decreases. After annealing, the intensity ratio of the BB to the E₂ high (E₂^H) peak decreases rapidly with increasing the annealing time (t_an). The vibrational properties of the annealed films grown at R_VI/II = 1 need only 1 h to be improved in contrast to those of films grown in Zn-rich condition, which need 4 h. From the E₂^H mode frequency, the residual stress in both the as-grown and the annealed films has been estimated. Micro-Raman measurements show that as-grown films are under a compressive stress which vanishes upon annealing and is not strongly dependent on t_an for t_an up to 1 h. PL spectra show that sharp donor bound exciton and A-free exciton emissions are observed for the as-deposited films grown at R_VI/II ≥ 0.5 and are enhanced after annealing for 1 h. However, in ZnO films grown in Zn-rich condition these emissions are absent and a t_an = 4 h is needed to annihilate non-radiative recombination centers and improve their luminescent efficiency.     

Epitaxial growth of lithium niobate film using metalorganic chemical vapor deposition

Lithium niobate films grown epitaxially on sapphire substrate were prepared using a thermal chemical vapor deposition method from the metalorganic compounds Li(C₁₁H₁₉O₂) and Nb(OC₂H₅)₅. The range of operating conditions for obtaining pure epitaxially grown LiNbO₃ without other oxides is within that for obtaining pure polycrystalline LiNbO₃ grown on silicon substrate. On analyzing the composition of the epitaxially grown LiNbO₃ film, the composition of the film was similar to that of the LiNbO₃ solid solution in the phase diagram of the Li-Nb composite oxide obtained for crystal growth from a molten solution.     

Characteristics improvement of metalorganic chemical vapor deposition grown MgZnO films by MgO buffer layers

MgZnO films with a small quantity of Mg were grown on c-sapphire substrates coated with a thin MgO buffer layers by metalorganic chemical vapor deposition. The MgO buffer layer causes improvement in the structural, optical, and electrical properties of subsequently deposited MgZnO thin films, when compared to MgZnO films deposited without a buffer layer. The MgZnO films with a MgO buffer layer grown at 330 °C showed the best performance. Transmission electron microscopy revealed that the cubic phase MgO buffer layer promoted the epitaxial behavior of MgZnO, where the planar relationships of the wurtzite-MgZnO/cubic-MgO/sapphire heterostructures mainly were MgZnO(0001)//MgO(001)//sapphire(0001) and MgZnO(11?00)//MgO(110)//sapphire(112?0). It resulted in lower lattice mismatch between MgO and MgZnO by domain epitaxy of 2/1 and enhancement in preferred growth of the MgZnO films along the c-axis.     

In situ X-ray studies of metal organic chemical vapor deposition of PbZrxTi1 − xO3

In situ synchrotron X-ray scattering and fluorescence techniques were used to simultaneously observe the evolution of the strain and composition of a growing crystal surface in real time. Control of the X-ray incidence angle allows us to obtain high surface sensitivity. We studied metal organic chemical vapor deposition (MOCVD) of epitaxial PbZr_xTi_1 − xO₃ (PZT) onto SrTiO₃ (001) substrates under various growth conditions. We observe a strong increase in Zr incorporation as strain relaxation occurs, consistent with the effect of compositional strain on the thermodynamic driving force for growth.     

Effects of growth variables on the properties of single crystalline ZnO thin film grown by inductively coupled plasma metal organic chemical vapor deposition

Single crystalline undoped and Ga-doped n-type zinc oxide (ZnO) films were grown on sapphire (Al₂O₃) substrates by inductively coupled plasma (ICP) metal organic chemical vapor deposition. Effects of growth variables on the structural, optical, and electrical properties of ZnO films have been studied in detail. Single crystal films with flat and smooth surfaces were reproducibly obtained, with application of sample bias and O₂ ICP. The best film properties were obtained at the growth condition of 650 °C, 400 W ICP power, − 94 V bias voltage, O/Zn (VI/II) ratio of 75. Single crystalline Ga doped n-ZnO films were also obtained, with free carrier concentration of about 1.5 × 10¹⁹/cm³ at 1 at.% Ga concentration.     

Templated growth of tungsten oxide micro/nanostructures using aerosol assisted chemical vapour deposition

Porous anodized alumina (PAA) and macroporous silicon (MS) substrates have been used to template the growth of tungsten oxide via aerosol assisted chemical vapour deposition from the precursor tungsten hexaphenoxide. The results show that thin PAA substrates have potential as templates for growing microstructured tungsten oxide films and MS substrates cause the growth of ‘grids’ of polycrystalline tungsten oxide.     

Texture control of ZnO films by inductively coupled plasma assisted chemical vapor deposition

ZnO films were grown on Si (100) and quartz substrates by inductively coupled plasma-assisted chemical vapor deposition using diethylzinc, O₂, and Ar. ZnO films with the (002) preferred orientation (PO) were formed at substrate temperatures > 250 °C regardless of any other changes made to process variables, since the (002) plane has the lowest formation energy with the highest number of unsaturated Zn-ZnO or O-ZnO bonds. At temperatures < 250 °C, the a-axis plane PO such as (100), (110), and (101) as well as the c-axis (002) plane PO were able to form by varying the temperature, plasma power, and deposition rate. The a-axis PO was formed when the radio frequency power was high enough to produce a crystalline ZnO film but was insufficient to form a (002) PO. The a-axis PO was also formed at higher deposition rates ≥ 20 nm/min when the radio frequency power was high enough to produce crystalline ZnO film. Since the (002) plane grew slowly, the grain exposing (002) plane was overgrown by the grains of the a-axis plane at higher deposition rates.     

BiFeO3 thin films prepared using metalorganic chemical vapor deposition

BiFeO₃ thin films were grown on (001) SrTiO₃ and (001) ZrO₂(Y₂O₃) substrates by single source metalorganic chemical vapor deposition in the temperature range T = 500 ÷ 800 °C using Fe(thd)₃ and Bi(C₆H₅)₃ as volatile precursors. X-ray diffraction analysis shows cube-on-cube epitaxial growth of BiFeO₃ on (001) SrTiO₃. The strongly reduced bismuth transfer into the film due to the high thermal stability of Bi(C₆H₅)₃ was counterbalanced by the increase of the total pressure as well as of the residence time of the precursor flow in the reactor; the Bi/Fe ratio in the film thus becomes close to that in the precursor mixture. Optical second harmonic generation measurements have evidenced the ferroelectric ordering in BiFeO₃ films and the apparent decrease of the Curie temperature of the strained films as compared to BiFeO₃ single crystal.     

Study of plasma enhanced chemical vapor deposition of ZnO films by non-thermal plasma jet at atmospheric pressure

Plasma enhanced chemical vapor deposition using a non-thermal plasma jet was applied to deposition of ZnO films. Using vaporized bis(octane-2,4-dionato)zinc flow crossed by the plasma jet, the deposition rate was as high as several tens of nm/s. From the results of infrared spectra, the films deposited at the substrate temperature T_sub = 100 °C contained a significant amount of carbon residue, while the films prepared at T_sub = 250 °C showed less carbon fraction. The experimental results confirmed that the plasma jet decomposed bis(octane-2,4-dionato)zinc in the gaseous phase and on the substrate, and that there should be the critical T_sub to form high-quality ZnO films in the range from 100 to 250 °C.     

Influence of alcohol on grain growth of tin oxide in chemical vapor deposition

Morphologies of tin oxide micro-grains in the early stage of film growth were analyzed for films deposited by chemical vapor deposition using tin chloride as a source material. Atomic force microscopy observations revealed increased micro-grain density and decreased size by adding methanol into the reaction system, but X-ray photoelectron spectroscopy analyses suggested that the total deposited volume was unchanged. The relative amount of chlorine contamination at the bottom of alcohol-added films increased in the order of isopropyl alcohol < ethanol < methanol. A model of chlorine desorption through reaction with alcohol, which occurred in the early stage of film growth, can explain the results.     

Selective ultrathin gold deposition by organometallic chemical vapor deposition onto organic self-assembled monolayers (SAMs)

We demonstrate a selective deposition of ultrathin gold layers via OMCVD (organometallic chemical vapor deposition) onto self-assembled dithiols. Dithiols have been self-assembled to produce a thiolated surface. The gold layer deposited from a gold precursor, present in the vapor around the sample, is bound to the exposed thiol groups. We demonstrate that it is possible to deposit gold only onto the areas where the binding thiol groups are located, and investigate the growth process with spontaneous desorption time-of-flight mass spectrometry, Rutherford backscattering spectroscopy, atomic absorption spectroscopy and atomic force microscopy.     

The growth of nanoscale ZnO films by pulsed-spray evaporation chemical vapor deposition and their structural, electric and optical properties

Great interest in nanoscale thin films (sub-100 nm) has been stimulated by the developing demands of functional devices. In this paper, nanoscale zinc oxide (ZnO) thin films were deposited on glass substrates at 300 °C by pulsed-spray evaporation chemical vapor deposition. Scanning electron micrographs indicate uniform surface morphologies composed of nanometer-sized spherical particles. The growth kinetics and growth mode are studied and the relationship between the film thickness and the electric properties with respect to the growth mode is interpreted. X-ray diffraction shows that all ZnO films grown by this process were crystallized in a hexagonal structure and highly oriented with their c-axes perpendicular to the plane of the substrate. Optical measurements show transparencies above 85% in the visible spectral range for all films. The absorbance in the UV spectral range respects well the Beer-Lambert law, enabling an accurate optical thickness measurement, and the absorption coefficient was measured for a selected wavelength. The measured band gap energies exhibit an almost constant value of 3.41 eV for all films with different thicknesses, which attributed to the thickness-independent crystallite size.     

Growth and morphological studies of (100) textured diamond thin films by microwave plasma-enhanced chemical vapor deposition

Textured (100) diamond films have been successfully grown using the plasma-enhanced chemical vapor deposition technique and characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. The thickness of such a (100)-oriented diamond film can be as thin as 4 μm, and the just-emerged transitional layer is found to be only 1.5 μm, which is very thin compared with the computer simulation value of 700d₀, where d₀ is the average distance between the nuclei. A systematic study of various parameters in the carburization and bias steps on the growth of textured (100) diamond films and the subsequent change of surface morphology has been investigated. Experimental results show that these two pre-growth steps seem to ease the growth of textured (100) diamond films and they should be optimized for a set of growth conditions. It is suggested that varying these parameters in the pre-growth steps may cause a change of microstructure, alignment of nuclei, and defect states in the diamond-like layer, resulting in the morphological change of textured (100) diamond films.     

One-minute microwave-assisted chemical bath deposition of nanostructured ZnO rod-arrays

Microwave-assisted chemical bath deposition (MACBD) is an emerging route for rapid synthesis of films and nanostructured particles. In this paper we report MACBD of ZnO rod-array films on bare glass substrates from an aqueous bath of tetra ammonium zinc complex. The deposition time is reduced to about 1 min as compared to around 60 min for conventional CBD. X-ray diffraction study shows that as-deposited films are uniaxially out-of-plane textured along the c-axis. Scanning Electron Microscopy reveals that the films consist of elongated elliptical tapered rods of diameters 250 to 350 nm. Atomic Force Microscopy shows that the films consist of about 350 nm grains. The RMS roughness is about 60 nm. The energy band gap is 3.27 eV as estimated from optical data. The films are n-type with electrical conductivity of 1 × 10^− 4 S/cm.     

Characterizations of gallium-doped ZnO films on glass substrate prepared by atmospheric pressure metal-organic chemical vapor deposition

Ga-doped zinc oxide (ZnO:Ga) films were grown on glass substrate by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as an n-type dopant gas. The structural, electrical and optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga-doping plays an important role in forming microstructures in ZnO films. A smooth surface with a predominant orientation of (101) was obtained for the ZnO:Ga film grown at a flow rate of TEG = 7.5 sccm. Moreover, a lowest resistivity of 3.6 × 10^− 4 Ω cm and a highest mobility of 30.4 cm² V^− 1 s^− 1 were presented by the same sample, as evaluated by Hall measurement. Otherwise, as the flow rate of TEG was increased, the average transmittance of ZnO:Ga films increased from 75% to more than 85% in the wavelength range of 400-800 nm, simultaneously with a blue-shift in the absorption edge. The results obtained suggest that low-resistivity and high-transparency ZnO films can be obtained by AP-MOCVD using Ga-doping sufficiently to make the films grow degenerate and effect the Burstein-Moss shift to raise the band-gap energy from 3.26 to 3.71 eV.     

Effect of the substrate on the properties of ZnO-MgO thin films grown by atmospheric pressure metal-organic chemical vapor deposition

The ZnO-MgO alloys possess attractive properties for possible applications in optoelectronic and display devices; however, the optical properties are strongly dependent on the deposition parameters. In this work, the effect of the glassy and metallic substrates on the structural, morphological and optical properties of ZnO-MgO thin films using atmospheric pressure metal-organic chemical vapor deposition was investigated at relatively low deposition temperature, 500 °C. Magnesium and zinc acetylacetonates were used as the metal-organic source. X-ray diffraction experiments provided evidence that the kind of substrates cause a deviation of c-axis lattice constant due to the constitution of a oxide mixture (ZnO and MgO) in combination with different intermetallic compounds(Mg₂Zn₁₁ and Mg₄Zn₇) in the growth films. The substitutional and interstitial sites of Mg²⁺ instead of Zn²⁺ ions in the lattice are the most probable mechanism to form intermetallic compounds. The optical parameters as well as thickness of the films were calculated by Spectroscopic Ellipsometry using the classical dispersion model based on the sum of the single and double Lorentz and Drude oscillators in combination with Kato-Adachi equations, as well as X-ray reflectivity.     

Imposed layer-by-layer growth of ZnO on GaN/sapphire substrates using pulsed laser interval deposition

The oxide semiconductor ZnO is of high interest for electrooptical applications due to its direct and wide band gap in the UV region. We present our results on pulsed laser deposition growth of ZnO on GaN-buffered Al₂O₃ substrates. Using in-situ reflection high energy electron diffraction, intensity oscillations were recorded and used to apply the technique of interval deposition. A significant improvement of structural thin film quality was achieved due to the expansion of the high quality from the first layers to the whole film thickness.