Effects of sapphire substrates surface treatment on epitaxial ZnO thin films grown by MOCVD

The surface treatment effects of sapphire substrate on the quality of epitaxial ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) were studied. The sapphire substrates have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best-quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements. Results show that the intensity of (002) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching is strongest. FWHM of (002) diffraction peak is narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

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.     

Metal organic chemical vapor deposition and investigation of ZnO thin films grown on sapphire

A new type of large area metal organic chemical vapor deposition (MOCVD) system for the growth of high quality and large size ZnO materials is introduced. Materials properties of the un-doped, n- and p-doped ZnO epi-films grown on sapphire substrates by this MOCVD system are studied by various techniques, including high resolution X-ray diffraction (XRD), UV-Visible optical transmission (OT), photoluminescence (PL) and photoluminescence excitation (PLE), synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). The wurtzite (w) ZnO crystal structures grown with primary (0002) orientation were identified. Results have shown the high crystalline quality of MOCVD-grown ZnO films, indicated by the narrow XRD, PL and Raman line widths, strong PL signals, sharp OT edge and smooth surface. In particular, high p-type carrier concentration of > 10¹⁷ cm^− 3 have been achieved besides the good n-type doping in ZnO.     

Influence of lithium doping on the structural and electrical characteristics of ZnO thin films

Thin films of undoped and lithium-doped Zinc oxide, (Zn_1 − xLi_x)O; x = 0, 0.05, 0.10 and 0.20 were prepared by sol-gel method using spin-coating technique on silicon substrates [(111)Pt/Ti/SiO₂/Si)]. The influence of lithium doping on the structural, electrical and microstructural characteristics have been investigated by means of X-ray diffraction, leakage current, piezoelectric measurements and scanning electron microscopy. The resistivity of the ZnO film is found to increase markedly with low levels (x ≤ 0.05) of lithium doping thereby enhancing their piezoelectric applications. The transverse piezoelectric coefficient, e₃₁^? has been determined for the thin films having the composition (Zn_0.95Li_0.05)O, to study their suitability for piezoelectric applications.     

Microstructural evolution of sol-gel derived ZnO thin films

Zinc oxide thin films, with thicknesses between ∼ 20 and 450 nm, were prepared by spin-coating a sol-gel precursor solution (zinc acetate dihydrate and monoethanolamine in an isopropanol solvent) onto glass substrates, followed by heat treatment at temperatures through 773 K. At 298 and 373 K, the films exhibited the structure of a lamellar ZnO precursor, Layered Basic Zinc Acetate (LBZA). At higher temperatures, LBZA released intercalated water and acetate groups and dehydroxylated to form zinc oxide nanograins with wurtzite structure, which were preferentially oriented in the c-axis direction. Both the degree of the films' c-axis orientation and the topography of their surfaces varied with heat treatment and precursor concentration. For films calcined at 773 K, a minimum of micron-scale surface wrinkles coincided with a maximum in c-axis preference at intermediate concentrations, suggesting that release of mechanical stress during densification of thicker films may have disrupted the ordering process that occurs during heat treatment.     

Optical and electrical properties of ZnO nanoparticle thin films deposited on quartz by sparking process

ZnO nanoparticle thin films were deposited on quartz substrates by a novel sparking deposition which is a simple and cost-effective technique. The sparking off two zinc tips above the substrate was done repeatedly 50-200 times through a high voltage of 10 kV in air at atmospheric pressure. The film deposition rate by sparking process was approximately 1.0 nm/spark. The ZnO thin films were characterized by X-ray diffraction, Raman spectroscopy, UV-vis spectrophotometry, and ionoluminescence at room temperature. The two broad emission peaks centered at 483 nm (green emission) and 650 nm (orange-red emission) were varied after two-step annealing treatments at 400-800 °C. Moreover, the electrical resistivity of the films was likely to be proportional to the peak intensity of the orange-red emission.     

Stable p-type ZnO films grown by atomic layer deposition on GaAs substrates and treated by post-deposition rapid thermal annealing

Long-term stable p-type ZnO films were grown by atomic layer deposition on semi-insulating GaAs substrates and followed by rapid thermal annealing (RTA) in oxygen ambient. Significant decrease in the electron concentration and increase in the hole concentration, together with the intensity enhancement of acceptor-related A^oX spectral peak and the shift of bound exciton peak from D^oX to A^oX in the low-temperature photoluminescence spectra, were observed as the RTA temperature increased. Conversion of conductivity from intrinsic n-type to extrinsic p-type ZnO occurred at the RTA temperature of 600 °C. The p-type ZnO film with a hole concentration as high as 3.44 × 10²⁰ cm^− 3 and long-term stability up to 180 days was obtained as the RTA treatment was carried out at 700 °C. The results were attributed to the diffusion of arsenic atoms from GaAs into ZnO as well as the activation of As-related acceptors by the post-RTA treatment.     

Chemical stability of CaCu3Ti4O12 thin films grown by MOCVD on different substrates

Calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), thin films have been fabricated by Metal Organic Chemical Vapor Deposition on silicon substrates buffered with two different low-k interlayers, namely SiO₂ and Si₃N₄. Depositions have been carried out from a molten mixture consisting of the Ca(hfa)₂ • tetraglyme, Ti(tmhd)₂(O-iPr)₂, and Cu(tmhd)₂ [Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; tetraglyme = 2,5,8,11,14-pentaoxapentadecane; Htmhd = 2,2,6,6-tetramethyl-3,5-heptandione; O-iPr = iso-propoxide] precursors. The chemical stability of CCTO films on both the SiO₂ and Si₃N₄ low-k layers has been investigated by transmission electron microscopy in the perspective of their implementation in capacitor devices.     

Growth and characteristics of ZnO films on growth side of freestanding diamond substrate dependent on buffer layer thickness

We report the growth and properties of highly c-axis oriented ZnO films, by radio-frequency magnetron sputtering, on the growth side of freestanding chemical vapor deposited diamond film-substrate. Low-temperature ZnO buffer layer is required for the formation of continuous ZnO films. The morphology, structure, and optical properties of the ZnO films deposited are strongly dependent on the thickness of the buffer layer. The optimized thickness of ZnO buffer layer is about 10 nm to realize high-quality ZnO films having small compressive stress and high intensity ultraviolet emission. The ZnO/diamond (growth side) system is available for the applications in numerous fields, especially for high performance surface acoustic wave devices.     

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.     

On the effect of acetic acid on physical properties of chemically sprayed fluorine-doped ZnO thin films

Fluorine-doped zinc oxide (ZnO:F) thin films onto sodocalcic glass substrates, starting from a highly concentrated starting solution (0.4 M) containing zinc acetate and hydrofluoric acid diluted in a mixture of deionized water, acetic acid , and methanol, using the chemical spray deposition technique, were deposited and characterized. The effect of the acetic acid content in the starting solution, and the substrate temperature on the electrical resistivity, structure, morphology and optical characteristics was studied. The samples were polycrystalline in nature, but as the acetic acid content in the starting solution increases, the preferential orientation shows a switching from (002) to (101). For a predetermined deposition temperature, as the acetic acid content increases, the film resistivity values show an increase. The minimum resistivity in the order of 6 × 10^− 3 Ω cm was found for the films deposited with the lowest acetic acid content used. The surface morphology varies from agglomerated grains to rod-like shaped grains as a function of the acetic acid content.     

Effect of the [Al/Zn] ratio in the starting solution and deposition temperature on the physical properties of sprayed ZnO:Al thin films

Aluminum-doped zinc oxide thin films (ZnO:Al) were deposited on sodocalcic glass substrates by the chemical spray technique, using zinc acetate and aluminum pentanedionate as precursors. The effect of the [Al/Zn] ratio in the starting solution, as well as the substrate temperature, on the physical characteristic of ZnO:Al thin films was analyzed. We have found that the addition of Al to the starting solution decreases the electrical resistivity of the films until a minimum value, located between 2 and 3 at.%; a further increase in the [Al/Zn] ratio leads to an increase in the resistivity. A similar resistivity tendency with the substrate temperature was encountered, namely, as the substrate temperature is increased, a minimum value of around 475 °C in almost all the cases, was obtained. At higher deposition temperatures the film resistivity suffers an increase. After a vacuum-thermal treatment, performed at 400 °C for 1 h, the films showed a resistivity decrease about one order of magnitude, reaching a minimum value, for the films deposited at 475 °C, of 4.3 × 10^− 3 Ω cm.The film morphology is strongly affected by the [Al/Zn] ratio in the starting solution. X-ray analysis shows a (002) preferential growth in all the films. As the substrate temperature increases it is observed a slight increase in the transmittance as well as a shift in the band gap of the ZnO:Al thin films.     

Elaboration and characterization of Co doped, conductive ZnO thin films deposited by radio-frequency magnetron sputtering at room temperature

Nanocrystalline, highly (at.%) Co doped ZnO powder, obtained by a modified sol-gel method, was used as a target material for the growth of µm thin films by radio frequency magnetron sputtering. The films were deposited at room temperature on quartz substrates. The as-deposited films were polycrystalline but highly textured with the c-axis aligned normal to the substrate plane. They present high optical transmittance in the visible range of approximately 90%, a carrier concentration of about 10²⁰ cm^− 3 and electrical resistivity of 10^− 3 Ω cm at room temperature. The analysis of the Co²⁺ spectrum by electron paramagnetic resonance (EPR) showed the Co to be incorporated substitutionally and the angular variation EPR spectrum demonstrates a monocrystal like texturing of the films with the c-axis normal to the film plane.     

Indium-doped ZnO thin films deposited by the sol–gel technique

Conducting and transparent indium-doped ZnO thin films were deposited on sodocalcic glass substrates by the sol–gel technique. Zinc acetate and indium chloride were used as precursor materials. The electrical resistivity, structure, morphology and optical transmittance of the films were analyzed as a function of the film thickness and the post-deposition annealing treatments in vacuum, oxygen or argon. The obtained films exhibited a (002) preferential growth in all the cases. Surface morphology studies showed that an increase in the films' thickness causes an increase in the grain size. Films with 0.18 μm thickness, prepared under optimal deposition conditions followed by an annealing treatment in vacuum showed electrical resistivity of 1.3 × 10^− 2 Ωcm and optical transmittance higher than 85%. These results make ZnO:In thin films an attractive material for transparent electrodes in thin film solar cells.     

Control of epitaxial growth orientation in ZnO nanorods on c-plane sapphire substrates

This paper presents a study on low temperature hydrothermal growth of ZnO nanorods (NRs) on pre-seeded (0001) sapphire substrates. Prior to hydrothermal growth of ZnO NRs, epitaxial ZnO seeds were grown by metal-organic chemical vapour deposition under various process conditions. Findings show that the majority of ZnO NRs inclined at a specific angle of about 38° to the direction perpendicular to the substrate surface and exhibited a preferential in-plane alignment, besides other NRs growing vertically from the sapphire surface. X-ray diffraction φ-scan measurements reveal that the ZnO nanorods displayed two distinct epitaxial relationships with sapphire which were (0001)_ZnO//(0001)_sapphire and (0001)_ZnO//(101?4)_sapphire, respectively. Reduced lattice mismatch between ZnO and sapphire is responsible for the inclined ZnO NRs growth. The growth direction of ZnO NRs is remarkably dependent on the growth conditions of ZnO seeds and sapphire substrate pre-treatment. The epitaxial orientations of ZnO seeds grown on the sapphire substrate dominate the subsequent ZnO NRs growth and can be controlled through adjusting growth conditions.     

Comparative study of hydrothermal treatment and thermal annealing effects on the properties of electrodeposited micro-columnar ZnO thin films

We report a comparison of the role played by different sample treatments, namely, a low-temperature hydrothermal treatment by hot H₂O vapor in an autoclave versus thermal annealing in air on the properties of ZnO films grown by electrochemical deposition (ECD). Scanning electron microscopy studies reveal a homogeneous micro-columnar morphology and changes in the film surface for the two different treatments. It is found that post-growth hydrothermal treatments of ECD ZnO films at 150 °C under an aqueous environment enhance their structural and optical properties (photoluminescence, transmission, Raman spectra, etc.) similar to thermal annealing in air at higher temperatures (> 200 °C). The modifications of the structural and optical properties of ZnO samples after thermal annealing in air in the temperature range of 150-600 °C are discussed. The removal of chlorine from the films by the hydrothermal treatment was evidenced which could be the main reason for the improvement of the film quality. The observation of the enhanced photoluminescence peak at 380 nm demonstrates the superior properties of the hydrothermally treated ZnO films as compared to the films annealed in air ambient at the same or higher temperature. This post-growth hydrothermal treatment would be useful for the realization of high performance optoelectronic devices on flexible supports which might not withstand at high temperature annealing treatments.     

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.     

O2气流量对MOCVD法生长ZnO薄膜性质的影响

采用低压MOCVD方法,在(0001)Al2O3衬底上沉积了ZnO薄膜.研究了Ⅵ族源O2气流量的变化对薄膜结构、表面形貌及光致发光特性的影响.增加O2气流量,ZnO薄膜结晶质量有所降低,半高宽从0.20°展宽至0.30°,由单一c轴取向变成无取向薄膜.同时,生成的柱状晶粒平均尺寸减少,晶粒更加均匀,均方根粗糙度减小.PL谱分析表明随O2气流量加大,带边峰明显增强,深能级峰明显减弱,ZnO薄膜光学质量提高.这些事实说明在本实验条件下,采用低压MOCVD方法生长的ZnO薄膜在光致发光特性主要依赖于Zn、O组份配比,而不是薄膜的微观结构质量.     

Effects of UV-ozone treatment on radio-frequency magnetron sputtered ZnO thin films

The effects of UV-ozone treatment on ZnO thin films prepared by using radio-frequency magnetron sputtering are investigated. Decrease in the density of oxygen vacancy as well as increase in the density of oxygen interstitial were inferred from the UV-ozone treated samples. It was also found that a considerable difference in the work function (0.25 eV) is induced by UV-ozone treatment implying a shift in Fermi level. This shift was confirmed by capacitance-voltage measurements, which demonstrated that the boundary between the inversion region and the depletion region of a ZnO-based metal-oxide-semiconductor (MOS) capacitor positively shifts when UV ozone treated. Our results clearly indicate that the threshold voltage of a thin film transistor can be adjusted by modifying the ZnO surface via UV ozone treatment. MOS capacitors fabricated with UV-ozone treated HfO₂ and/or ZnO also yielded a smaller leakage current (~ 73%-90% smaller) and a larger breakdown voltage (~ 8%-11% larger). The physical mechanism behind the effect of the UV ozone treatment is addressed in this study with the help of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy.     

Nitrogen-doped p-type ZnO thin films and ZnO/ZnSe p-n heterojunctions grown on ZnSe substrate by radical beam gettering epitaxy

We investigate the p-type doping in ZnO prepared by the method of radical beam gettering epitaxy using NO gas as the oxygen source and nitrogen dopant. Secondary ion mass spectroscopy measurements demonstrate that N is incorporated into ZnO film in concentration of about 8 × 10¹⁸ cm^− 3. The hole concentration of the N-doped p-type ZnO films was between 1.4 × 10¹⁷ and 7.2 × 10¹⁷ cm^− 3, and the hole mobility was 0.9-1.2 cm²/Vs as demonstrated by Hall effect measurements. The emission peak of 3.312 eV is observed in the photoluminescence spectra at 4.2  of N-doped p-type ZnO films, probably neutral acceptor bound. The activation energy of the nitrogen acceptor was obtained by temperature-dependent Hall-effect measurement and equals about 145 meV. The p-n heterojunctions ZnO/ZnSe were grown on n-type ZnSe substrate and have a turn-on voltage of about 3.5 V.