Influence of oxygen pressure on the structural, electrical and optical properties of VO2 thin films deposited on ZnO/glass substrates by pulsed laser deposition

The influence of oxygen pressure on the structural and electrical properties of vanadium oxide thin films deposited on glass substrates by pulsed laser deposition, via a 5-nm thick ZnO buffer, was investigated. For the purposes of comparison, VO₂ thin films were also deposited on c-cut sapphire and glass substrates. During laser ablation of the V metal target, the oxygen pressure was varied between 1.33 and 6.67 Pa at 500 °C, and the interaction and reaction of the VO₂ and the ZnO buffer were studied. X-ray diffraction studies showed that the VO₂ thin film deposited on a c-axis oriented ZnO buffer layer under 1.33 Pa oxygen had (020) preferential orientation. However, VO₂ thin films deposited under 5.33 and 6.67 Pa were randomly oriented and showed (011) peaks. Crystalline orientation controlled VO₂ thin films were prepared without such expensive single crystal substrates as c-cut sapphire. The metal-insulator transition properties of the VO₂/ZnO/glass samples were investigated in terms of electrical conductivity and infrared reflectance with varying temperatures, and the surface composition was investigated by X-ray photoelectron spectroscopy.     

Growth of ZnO/sapphire heteroepitaxial thin films by radio-frequency sputtering with a raw powder target

Epitaxial ZnO thin films were deposited by radio-frequency sputtering. In contrast to typical sputter growth, in which a ZnO sintered target was used, raw ZnO powder with a particle size smaller than 1 μm is used as the source material. In order to verify this approach, ZnO thin films were deposited on sapphire(0001) substrates and characterized by X-ray diffraction, atomic force microscopy, ultraviolet-visible-near-infrared (near-IR) transmission spectroscopy, and photoluminescence spectroscopy. The as-deposited ZnO thin films grew epitaxially on the sapphire(0001) substrate. A crossover in the growth mode from an initial 2-dimensional planar layer to later 3-dimensional islands was observed, which is consistent with the results obtained using a ZnO sintered target. The ZnO films showed band-edge emission with a bandgap energy of 3.27 eV and a high optical transmittance > 80% from visible to near-IR region. This shows that ZnO powder targets can be an alternative to relatively expensive sintered ones in the fabrication of ZnO nano-structures and doped ZnO.     

Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering

The Pb(Zr_0.80Ti_0.20)O₃ (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (P_r = 25.6 μC/cm²), and lower coercive field (E_c = 60.5 kV/cm) than that of the films without a PbO buffer layer (P_r = 9.4 μC/cm², E_c = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure.     

Influence of VI/II ratios on the growth of ZnO thin films on sapphire substrates by low temperature MOCVD

We investigated the structural, electrical, and optical properties of ZnO thin films grown at different VI/II ratios on sapphire substrates by metalorganic chemical vapor deposition. Transmission electron microscopy and X-ray diffraction revealed the epitaxial nature with a reduced dislocation density of the ZnO films grown at increased VI/II ratios. The carrier concentration of the films increased to 4.9 × 10¹⁸ cm^− 3 and their resistivity decreased to 1.4 × 10^− 1 Ω cm at a VI/II ratio of 513.4 μmol/min. The ZnO films also showed good optical transmittance (> 80%) in the visible and near-infrared wavelength regions. The room temperature PL revealed a strong band-edge emission with a weak deep level emission, suggesting the good crystalline quality of the ZnO films on the sapphire substrates. Furthermore, the intensity ratio of the band-edge emission to the deep-level emission (I_UV/I_Vis) increased with increasing VI/II ratio.     

Effects of low temperature ZnO and MgO buffer thicknesses on properties of ZnO films grown on (0001) Al2O3 substrates by plasma-assisted molecular beam epitaxy

We investigated the structural properties of Zn-polar ZnO films with low temperature (LT) ZnO and MgO buffer layers grown by plasma-assisted molecular beam epitaxy on (0001) c-Al₂O₃ substrates using X-ray diffraction and transmission electron microscopy (TEM). The effects of MgO buffer layer thickness and LT ZnO buffer layer thickness were also examined. The optimum thicknesses for better crystal quality were 8 and 40 nm. One-pair and two-pair LT ZnO/MgO buffer layers were employed, and the changes in the structural properties of the high-temperature (HT) ZnO films using such buffer layers were studied. Contrary to the general tendency of c-ZnO films, the HT ZnO films on the LT ZnO/MgO buffer layers showed higher full width at half maximum (FWHM) values for X-ray rocking curves (XRCs) with (0002) reflection than those with (101?1) reflection. Compared with the one-pair LT ZnO/MgO buffer layers, the FWHM values of (0002) XRCs markedly decreased, whereas those of (101?1) XRCs slightly increased due to the insertion of one more pair of LT ZnO/MgO buffer layers into the previous film with one-pair LT ZnO/MgO buffer layers. The cross-sectional TEM observations with the two-beam condition confirmed that the screw dislocation was the dominant threading dislocation type—a finding that agreed well with the XRC results. On the basis of the plan-view TEM observations, the densities of the total threading dislocations for the HT ZnO films with the one- and two-pair LT ZnO/MgO buffer layers were determined as 2.3 × 10⁹ cm^− 2 and 1.6 × 10⁹ cm^− 2, respectively. The results imply that the crystal quality of Zn-polar ZnO films can be improved by two-pair LT ZnO/MgO buffer layers, and types of threading dislocations can be modified by adjusting the buffer system.     

Optical properties of the c-axis oriented LiNbO3 thin film

C-axis oriented Lithium Niobate (LiNbO₃) thin films have been deposited onto epitaxially matched (001) sapphire substrate using pulsed laser deposition technique. Structural and optical properties of the thin films have been studied using the X-ray diffraction (XRD) and UV-Visible spectroscopy respectively. Raman spectroscopy has been used to study the optical phonon modes and defects in the c-axis oriented LiNbO₃ thin films. XRD analysis indicates the presence of stress in the as-grown LiNbO₃ thin films and is attributed to the small lattice mismatch between LiNbO₃ and sapphire. Refractive index (n = 2.13 at 640 nm) of the (006) LiNbO₃ thin films was found to be slightly lower from the corresponding bulk value (n = 2.28). Various factors responsible for the deviation in the refractive index of (006) LiNbO₃ thin films from the corresponding bulk value are discussed and the deviation is mainly attributed to the lattice contraction due to the presence of stress in deposited film.     

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112?0) and ZnO (0001?) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance-voltage (C-V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C-V measurements the sheet carrier density in a Zn_0.8 Mg_0.2O/ZnO/Zn_0.8 Mg_0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10¹⁰ cm^− 2. For the films deposited on sapphire 2D growth is observed in the Burton-Cabrera-Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.     

Ferroelectric and dielectric multilayer heterostructures based on KTa0.65Nb0.35O3 and Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y grown by pulsed laser deposition and chemical solution deposition for high frequency tunable devices

Epitaxial growth of Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (BZN) thin films was achieved on (100)_pc LaAlO₃ substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of ε_r = 203 and quite low dielectric losses of tanδ = 5 × 10^− 2. Epitaxial ferroelectric − dielectric KTa_0.65Nb_0.35O₃ (KTN) − Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (KTN on BZN and BZN on KTN) bilayers were obtained by PLD on (100)_pc LaAlO₃ with the insertion of a suitable buffer layer of KNbO₃ in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices.     

Low temperature photoluminescence and infrared dielectric functions of pulsed laser deposited ZnO thin films on silicon

C-axis oriented ZnO thin films were grown on silicon (100) and (111) substrates by pulsed laser deposition. Low temperature photoluminescence spectra show besides the peaks of free excitons, of defect bound excitons, and of a donor-acceptor pair transition a new doublet at 3.328/3.332 eV. The doublet seems to originate from the columnar textured ZnO film structure. A corresponding structural dependence of the broadening parameter of the infrared dielectric functions was derived from spectroscopic ellipsometry in the spectral range from 380 to 1200 cm^− 1. The wave numbers of the E₁ transverse optical and A₁ longitudinal optical phonon modes of the ZnO films on silicon are determined to be 406 and 573 cm^− 1, respectively. These values are slightly smaller than those of single-crystalline ZnO thin films on sapphire.     

The fabrication and application of ZnO:Al thin films in low-frequency inductively coupled plasma fabricated silicon solar cell

A home-made radio frequency magnetron sputtering is used to systematically study the structural, electrical, and optical properties of aluminum doped zinc oxide (ZnO:Al) thin films. The intensity of the (002) peak exhibits a remarkable enhancement with increasing film thickness. Upon optimization, we achieved low resistivity of 4.2 × 10^− 4 Ω cm and high transmittance of ~ 88% for ZnO:Al films. Based on the present experimental data, the carrier transport mechanism is discussed. It is found that the grain boundary scattering needs to be considered because the mean free path of free carrier is comparable to the grain size. The 80 nm-ZnO:Al thin films are then deposited onto low-frequency inductively coupled plasma fabricated silicon solar cells to assess the effect of ZnO:Al thin films on the performance of the solar cells. Optimized ZnO:Al thin films are identified as transparent and conductive oxide thin film layers.     

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Fabrication of KTa0.65Nb0.35O3 film by pulsed laser deposition on glass substrate with various buffer layers

KTa_0.65Nb_0.35O₃ (KTN) thin films were deposited on amorphous glass substrates using a range of single buffer layers such as indium tin oxide (ITO), zinc oxide (ZnO), 3 at% Al-doped ZnO (AZO), and 3 at% Ga-doped ZnO (GZO), as well as a variety of multi-buffer layers such as SrTiO₃ (STO)/ITO, STO/ZnO, STO/AZO, and STO/GZO using a pulsed laser deposition system. All films showed a polycrystalline perovskite phase with the exception of all single buffer layers and STO/ITO multi-buffer layers. The STO buffer layer is important for crystallizing KTN films due to the similar lattice constant and same crystal structure. The optical transmittance of all films exhibited a transmittance ?90% in the wavelength range.     

Synthesis and characterization of 4H-SiC on C-plane sapphire by C60 and Si molecular beam epitaxy

4H-SiC (silicon carbide) films were grown on (0001) sapphire substrate at rather low temperatures(1000-1100 °C) with relative high deposition rate by using fullerene (C₆₀) and silicon solid sources molecular beam epitaxy with substrate nitridation and aluminum nitride (AlN) buffer layer deposition prior to the SiC deposition. The effects of substrate nitridation and AlN buffer layer to the adhesion of the SiC thin films on sapphire have been studied. X-Ray diffraction, pole figure, atomic force microscope, Fourier transform infrared spectroscopy and photoluminescence were employed for the analysis of composition, orientation of the film and surface morphology. Relative high deposition rate at ∼ 165 nm/h was achieved.     

Structural properties of low-temperature grown ZnO thin films determined by X-ray diffraction and X-ray absorption spectroscopy

The epitaxial growth of ZnO thin films on Al₂O₃ (0001) substrates have been achieved at a low-substrate temperature of 150 °C using a dc reactive sputtering technique. The structures and crystallographic orientations of ZnO films varying thicknesses on sapphire (0001) were investigated using X-ray diffraction (XRD). We used angle-dependent X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to examine the variation of local structure. The XRD data showed that the crystallinity of the film is improved as the film thickness increases and the strain is fully released as the film thickness reached about 800 Å. The Zn K-edge XANES spectra of the ZnO films have a strong angle-dependent spectral feature resulting from the preferred c-axis orientation. The wurtzite structure of the ZnO films was explicitly shown by the XRD and EXAFS analysis. The carrier concentration, Hall mobility and resistivity of the 800 Å-thick ZnO film were 1.84 × 10¹⁹ cm^− 3, 24.62 cm²V^− 1s^− 1, and 1.38 × 10^− 2 Ω cm, respectively.     

Influence of the self-buffer layer on ZnO film grown by atmospheric metal organic chemical vapor deposition

ZnO films with and without a self-buffer layer were grown on c-plane sapphire substrates by atmospheric metal organic chemical vapor deposition. The influence of the buffer layer thickness, annealing temperature and annealing time on ZnO films has been investigated. The full width at half maximum of the ω-rocking curve of the optimized self-buffer layer sample is only 395 arc sec. Its surface is composed of regular columnar hexagons. After the buffer layer was introduced, the A₁ longitudinal mode peak at 576 cm^− 1, related to the defects, disappears in Raman spectra. For the photoluminescence, besides the strong donor binding exciton peak at 3.3564 eV, an ionized donor binding exciton and a free exciton peak is respectively observed at 3.3673 and 3.3756 eV at the high-energy side in the spectrum of the sample with the buffer layer.     

Optical and photoconductivity properties of ZnO thin films grown by pulsed filtered cathodic vacuum arc deposition

High quality transparent conductive ZnO thin films with various thicknesses were prepared by pulsed filtered cathodic vacuum arc deposition (PFCVAD) system on glass substrates at room temperature.The high quality of the ZnO thin films was verified by X-ray diffraction and optical measurements. XRD analysis revealed that all films had a strong ZnO (200) peak, indicating c-axis orientation. The ZnO thin films are very transparent (92%) in the near vis regions. For the ZnO thin films deposited at a pressure of 0.086 Pa (6.5 × 10⁻⁴ Torr) optical energy band gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. Urbach tail energy also decreased as the film thickness increased.Spectral dependence of the photoconductivity was obtained from measurements of the samples deposited at various thicknesses. Photoconductivities were observed at energies lower than energy gap which indicates the existence of energy states in the forbidden gap. Photoconductivities of ZnO thin films increase with energy of the light and reach its maximum value at around 2.32 eV. Above this value surface recombination becomes dominant process and reduces the photocurrent. The photoconductivity increases with decreasing the film thickness.     

Electrical transport parameters of heavily-doped zinc oxide and zinc magnesium oxide single and multilayer films heteroepitaxially grown on oxide single crystals

Heavily doped epitaxial ZnO:Al and Zn_1−xMg_xO:Al films were grown by radio frequency magnetron sputtering onto single crystalline substrates (sapphire, MgO, silicon) and characterized by structural and electrical measurements. It is the aim of this investigation to better understand the carrier transport and the doping mechanisms in heavily doped transparent conducting oxide (TCO) films. It was found that the crystallographic film quality determines only partly the mobilities and the carrier concentrations: ZnO:Al films on a-plane (110) sapphire and on MgO (100) exhibit the highest mobilities. The oxygen partial pressure during the deposition from ceramic targets is more important influencing especially the carrier concentration N of the films. Though the films grew epitaxially grain boundaries are still existent, which reduce the mobility due to electrical grain boundary barriers for N < 3 · 10²⁰ cm^− 3. From annealing experiments the role of point defects and dislocations for the carrier transport could be estimated. For carrier concentrations above 3 · 10²⁰ cm^− 3 ionized impurity scattering limits the mobility, which is in agreement with our earlier review [K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097].