Crystalline Quality Of ZnO Films Research Articles

X-Ray- and Synchrotron-Radiation Study of the Defect Structure of Epitaxial ZnO Films Grown by Magnetron Deposition on Al2O3 and LaMgAl11O19 Substrates with (0001) Orientation

The results of studying the structural features of samples of zinc-oxide films obtained by magnetron deposition on chips of lanthanum-magnesium hexaaluminate and the surface of sapphire substrates with a gold buffer layer are presented. Analysis of the structure and morphology of the films is carried out using a set of methods, including high-resolution X-ray diffractometry, the method of constructing pole figures, and transmission electron microscopy. It is shown that when using cleavages of lanthanum-magnesium hexaaluminate, an epitaxial ZnO film is formed without signs of growth rotating domains. The use of a gold buffer layer during growth on sapphire substrates improves the crystalline quality of ZnO films, but does not completely suppress domain growth.

Stable p‐type ZnO thin films on sapphire and n‐type 4H‐SiC achieved by controlling oxygen pressure using radical‐source laser molecular beam epitaxy

Stable p‐type ZnO thin films on sapphire (0001) and n‐type 4H‐SiC (0001) substrates were achieved in low‐ionized oxygen pressure using radical‐source laser molecular beam epitaxy system. The p conduction type of ZnO films originates from not only zinc vacancy, but also interstitial oxygen acceptor defects, which was investigated by X‐ray diffraction, atomic force microscopy, X‐ray photoelectron spectroscopy, and photoluminescence measurements. The electrical properties were tested 180 days after deposition by Hall measurement. For the ZnO thin film grown on sapphire, a stable mobility of 17.0 cm2 V−1s−1, a resistivity of 1.08 Ωcm, and a hole concentration of 3.4 × 1017 cm−3 were achieved. The selection of 4H‐SiC substrates improved the crystalline quality of ZnO films confirmed by the X‐ray diffraction patterns. An intrinsic p‐type ZnO film on n‐type 4H‐SiC, with a stable mobility of 44.6 cm2 V−1s−1, a resistivity of 1.02 Ωcm, and a hole concentration of 1.4 × 1017 cm−3 were achieved. The current–voltage curve of the p–n ZnO homojunction shows typical diode characteristics, which also confirmed the achievement of the p‐type ZnO. The current–voltage curve of the p‐ZnO/n‐4H‐SiC heterojunction also shows p–n junction rectifier features. These results suggest the possibility of ultraviolet photodetectors and light‐emitting devices.

ZnO deposited on Si (111) with Al2O3 buffer layer by atomic layer deposition

ZnO films were grown on Si (111) substrates by atomic layer deposition (ALD) at a low-temperature of 200 °C, and the structural and optical properties of the films were investigated. By inserting an ultrathin Al2O3 buffer layer between ZnO and Si, the crystalline quality of ZnO films was improved, resulting in the increase of the intensity of the photoluminescence.

Sol–gel derived nanostructure undoped and cobalt doped ZnO: Structural, optical and electrical studies

Nanostructure ZnO and ZnO:Co films were deposited onto borofloat glass substrates by the sol–gel method using spin coating technique. X-ray diffraction (XRD) study was used to investigate the effects of cobalt dopant on the crystalline structure and orientation of the films. XRD observations showed that the crystalline quality of ZnO film decreased with increasing cobalt content. The effect of Co incorporation on the surface morphology was clearly observed from scanning electron microscopy (SEM) images. The chemical composition of the 3% Co doped nanostructure ZnO film was confirmed by X-ray photoelectron spectroscopy (XPS). The doping of ZnO with Co causes a decrease in particle size and crystallinity while it leads to an increase in the band gap, which changes in accordance with the Burstein–Moss effect. The optical constants of the films such as refractive index and extinction coefficient were determined using transmittance and reflectance spectra. The sheet resistance of the films, which was calculated using Van der Pauw method, first increased by about two orders of magnitude and then tended to decrease.

Heteroepitaxial growth and characterization of ZnO films on Gd3Ga5O12 garnet substrates

ZnO (0001) films have been grown epitaxially on (111) and (001) planes of Gd3Ga5O12 (GGG) substrates, which have different orientations and symmetries. The X-ray diffraction revealed that the out-of-plane and in-plane epitaxial relations of ZnO films on the GGG (111) substrates [ZnO/GGG (111)] were the [0001]ZnO || [001]GGG and [10 − 10]ZnO || [1 − 12]GGG ± 21°, while ZnO films on the GGG (001) substrates [ZnO/GGG (001)] showed the out-of-plane and in-plane epitaxial relations with the [0001]ZnO || [001]GGG and [10 − 10]ZnO || [100]GGG, [010]GGG. The mosaic spread of ZnO/GGG (001) along the in-plane and out-of-plane directions was larger than that of ZnO/GGG (111), which was closely related to a number of dislocations generated in the ZnO lattices. These mosaic structures mainly determined crystal quality of ZnO films, as also confirmed by transmittance electron microscopy (TEM). Moreover, high-resolution TEM showed that the lattice of ZnO along the [0001] direction was connected with lattices of GGG along the [111] and [001] directions without interface buffer layers. Finally, crystalline quality of ZnO films was also clarified by temperature-dependent photoluminescent spectroscopy. This study contributes to provide a new device concept on oxide semiconductor/garnet interfaces.

P-type conductivity control of heteroepitaxially grown ZnO films by N and Te codoping and thermal annealing

The authors report on the p-type conductivity in the ZnO films, grown on c-Al2O3 substrates by molecular-beam epitaxy, using N and Te codoping and thermal annealing. In the electrical properties, the N and Te codoping effectively suppresses the background electron concentration of ZnO films, and the thermal annealing causes the conductivity conversion from n-type to p-type. In the structural properties, the N and Te codoping deteriorates the crystalline quality of ZnO films, even if Te doping contributes to improve the crystallinity, and the thermal annealing recovers the degraded crystalline quality again. In the optical properties, the N and Te codoping simultaneously increases the donor-related emission, the acceptor-related emission, and the nonradiative recombination in ZnO films, while the thermal annealing relatively enhances the acceptor-related emission. It is proposed that (i) the N and Te codoping simultaneously induces N-related defects and donor-type defects along with free acceptors in the ZnO films, but the amount the N-related defects is larger than the donor-type defects and the free acceptors; and (ii) the thermal annealing relatively activates the N-related defects and suppresses the donor-type defects.

Growth, optical, and electrical properties of nonpolar m-plane ZnO on p-Si substrates with Al2O3 buffer layers

Nonploar m-plane ZnO films were grown on p-Si (111) substrates by using atomic layer deposition. X-ray diffraction and high resolution tunneling electron microscopy measurements showed that the ZnO films were 〈101¯0〉 oriented, and the crystalline quality of ZnO films was improved with an Al2O3 buffer layer, which significantly enhances the photoluminescence and reduces the reverse leakage current of ZnO/Si heterojuction.

Nitrogen Doped ZnO Thin Films Prepared by Photo-Assisted Metal-Organic Chemical Vapor Deposition

Nitrogen-doped ZnO (ZnO:N) films were prepared by photo-assisted metal-organic chemical vapor deposition technique using NH3 as N doping source. The effects of in-situ light irradiation on the properties of ZnO:N films were studied by Hall measurements, X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy. The results show that stable p-type ZnO:N films with a hole concentration of 3.61 x 10(17) cm(-3) was successfully achieved. Moreover, introducing proper in-situ light irradiation during the growth process can not only effectively improve the crystalline quality of ZnO films, but also enhance the activity of (N)o (N occupies O site) acceptors by removing the undesirable hydrogen atoms from ZnO:N films. Both effects are benefit for the p-type conductivity formation. Our results indicate that photo-assisted MOCVD maybe an effective technology to realize device-quality p-type ZnO:N films.

Improvement in crystalline quality and surface smoothness of ZnO film by multi-step deposition process

The effect of multi-step deposition process on the crystalline quality and surface smoothness of ZnO film was investigated. ZnO films were composed of multi-layers, in which each layer was deposited at different temperatures. The maximum intensity and the smallest FWHM of (002) diffraction peak in XRD spectrum were observed for the multi-layered ZnO film of which each layer was deposited at progressively higher temperatures. In addition, the smoothest film surface was also observed for the ZnO film deposited through multi-step process in which deposition temperatures gradually increase. On the other hand, the large difference between the deposition temperatures in multi-step process did not result in the significant improvement of the crystalline quality of ZnO film. The ZnO film prepared by using multi-step process had high transmittance over 70% in visible region and the optical band gap of 3.22eV.

Epitaxial Growth of m-Plane ZnO Thin Films on (101̅0) Sapphire Substrate by Atomic Layer Deposition with Interrupted Flow

Nonpolar ZnO films were grown epitaxially on (1010) sapphire substrates at 200 °C by atomic layer deposition with interrupted flow. The latter method improved the crystalline quality of ZnO films, transformed the structure from polycrystalline to epitaxial, and enhanced the optical properties of near-band-edge emission. The interfacial structure shows multiple domain phases along sapphire (020) and the disappearance of a minor phase near the surface. As determined by X-ray diffraction, the epitaxial relation between ZnO and sapphire follows [002]ZnO∥[020]sapphire and [020]ZnO∥[006]sapphire. The photoluminescence intensity increased with increasing crystalline quality and thickness of ZnO films.

Effect of a buffer layer on the properties of UV photodetectors based on a ZnO/diamond film structure

In this paper, ZnO films were grown on the nucleation sides of freestanding diamond substrates with and without a ZnO homobuffer layer by the radio-frequency (RF) magnetron sputtering method. The effects of buffer layers on the properties of the ZnO film and UV photodetectors based on a ZnO/freestanding diamond film structure were studied. The experimental results suggested that the buffer layer was helpful in improving the crystalline quality of ZnO/diamond heteroepitaxial films and the electrical property of the ZnO photodetectors was relative to the crystalline quality of ZnO films. For the photodetector based on the ZnO film with a buffer layer, a higher value of photo-responsivity under a 10 V bias voltage and a better time-dependent photocurrent characteristic were obtained.

The structural and optical properties of ZnO/Si thin films by RTA treatments

ZnO/Si thin films were prepared by rf magnetron sputtering method and some of the samples were treated by rapid thermal annealing (RTA) process at different temperatures ranging from 400 to 800 °C. The effects of RTA treatment on the structural properties were studied by using X-ray diffraction and atomic force microscopy while optical properties were studied by the photoluminescence measurements. It is observed that the ZnO film annealed at 600 °C reveals the strongest UV emission intensity and narrowest full width at half maximum among the temperature ranges studied. The enhanced UV emission from the film annealed at 600 °C is attributed to the improved crystalline quality of ZnO film due to the effective relaxation of residual compressive stress and achieving maximum grain size.

Structural and optical properties of ZnO thin films on glass substrate grown by laser-ablating Zn target in oxygen atmosphere

C-axis-orientated ZnO thin films were prepared on glass substrates by pulsed-laser deposition (PLD) technique in an oxygen-reactive atmosphere, using a metallic Zn target. The effects of growth condition such as laser energy and substrate temperature on the structural and optical properties of ZnO films had been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission spectra and room-temperature (RT) photoluminescence (PL) measurements. The results showed that the thickness, crystallite size, and compactness of ZnO films increased with the laser energy and substrate temperature. Both the absorption edges and the UV emission peaks of the films exhibited redshift, and UV emission intensity gradually increased as the laser energy and substrate temperature increased. From these results, it was concluded that crystalline quality of ZnO films was improved with increasing laser energy and substrate temperature.

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm −1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.

Effect of O/Zn Flux Ratio on Crystalline Quality of ZnO Films Grown by Plasma-Assisted Molecular Beam Epitaxy

The effect of O/Zn flux ratio on the crystalline quality of ZnO films grown at 700°C by plasma-assisted molecular beam epitaxy was investigated. Zinc beam flux (FZn) was varied from 2.2≤FZn≤8.3 Å/s with an O2 flow rate of 3 sccm and RF power of 300 W. The surface morphology of the ZnO layers strongly depended on FZn. ZnO epilayers grown under stoichiometric flux conditions (i.e., FZn=5.1 Å/s) had high crystalline quality, as was confirmed by using X-ray diffraction, photoluminescence (PL), and Hall-effect measurements: the full width at half maximum (FWHM) of a skew symmetric (1010) X-ray rocking curve was 720 arcsec; the dominant neutral donor bound exciton emission intensity in the PL spectra became maximum with the narrowest FWHM; the electron mobility was a maximum of 130 cm2V-1s-1; and a residual carrier concentration of 1.2×1017 cm-3 was achieved. We demonstrated that stoichiometric ZnO films have the lowest dislocation density and the highest electron mobility compared with ZnO films grown under nonstoichiometric flux conditions.

Effect of deposition conditions on optical and electrical properties of ZnO films prepared by pulsed laser deposition

We report the optical and electrical properties for highly c-axis oriented ZnO films deposited on Si (0 0 1) and quartz substrates by pulsed laser deposition (PLD) under various deposition conditions such as substrate temperature, laser fluence and oxygen ambient pressure. Dependence of structural, optical and electrical measurements on the substrate temperature reveals improvement of crystalline quality of ZnO films with raising substrate temperature. Optical and electrical properties show different behaviors in vacuum and oxygen ambient with increasing laser fluence. Combined with high laser fluence, oxygen ambient plays an important role in removing defects due to excess zinc or oxygen vacancy.

Role of ZnS buffer layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy

ZnS buffer layers were used to grow ZnO films on GaAs(0 0 1) substrates. The role of ZnS buffer layers in the growth of zincblende ZnO films on GaAs(0 0 1) substrates was investigated by atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. The optimization of the ZnS buffer layer thickness resulted in improvements of the surface morphology and crystalline quality of ZnO films by homogeneous nucleation. With the optimized ZnS buffer layer thickness of ∼72 nm the surface root-mean-square roughness of the grown ZnO film was minimized to ∼14 nm and the deep-level PL intensity was reduced to 1/76 of the near-band edge PL intensity at room temperature.