ZnO薄膜的晶体性能的分析

在硅基上制备出了c轴取向高度一致的ZnO薄膜 ,这将有可能成为新型GaN单晶薄膜的过渡层。对ZnO薄膜的晶体性能进行了分析 ,研究不同衬底和不同衬底温度对ZnO薄膜的结晶状况的影响 ,并着重用TEM研究了硅基ZnO薄膜的晶体性能。     

An attempt on triple doping in ZnO by Pulsed Laser Deposition

An attempt has been made on triple doping of B, Ga and N into ZnO films using B₂O₃ and GaN doped ZnO targets by KrF excimer laser. From XRD, it has been found that, the films doped with equal percentage of B₂O₃ and GaN, show better structural properties. The suppression of the deep levels observed from PL studies reveals their good optical properties. The Hall measurement shows that films have n-type conductivities due to lack of nitrogen to compensate the donors. The low resistivity and high mobility have been observed for the film having better structural properties.     

Specific Features of the Luminescence of ZnO:Te/GaN/Al2O3 Heterostructures

High-quality heteroepitaxial (0001)ZnO:Te/(0001)GaN/(0001)Al₂O₃ structures have been grown by hydrogen vapor-phase epitaxy in a low-pressure continuous flow reactor. X-ray diffraction analysis of these heterostructures has been carried out, which revealed the high structural quality of thin zinc oxide layers. The surface morphology and specific features of UV photoluminescence of the ZnO/GaN/Al₂O₃ heterostructure have been analyzed.

     

ZnO/GaN heterostructure for LED applications

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of Zn:In onto GaN:Mg/GaN structures MOCVD-grown on Al₂O₃ substrates has been observed. The white light is produced by superposition of two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al₂O₃ structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN is constructed and a qualitative explanation of the EL is given. Conditions of ZnO deposition strongly affects the properties of the recombination emission and predetermines the EL spectrum of the LED structure if it does not have high quantum efficiency (more than 1%) such as in commercial LEDs.     

Nematic liquid crystal alignment on the ion beam-exposed ZnO film

This paper investigates the nematic liquid crystal (NLC) alignment on ion beam-exposed zinc oxide (ZnO) films. The ZnO films are deposited by a radio frequency magnetron sputtering. During the deposition of ZnO film, we supplied sufficient oxygen gas for high resistivity and transmittance. The deposited films show a high transmittance of over 90% and high resistivity of over 10¹⁰ Ω cm. The ZnO films show a high deposition rate of 26.7 Å/min. Images obtained via scanning electron microscopy of the ZnO film surfaces, before and after the ion beam exposure, show that groove patterns are formed being to be parallel to the ion beam exposure direction. LC cells are fabricated with the ion beam-exposed ZnO films. The NLC molecules align parallel to the ion beam exposure direction. The electro-optic and response characteristics of fabricated cells show the possibility of application to liquid crystal displays.     

Improved zinc oxide film for gas sensor applications

Zinc oxide (ZnO) is a versatile material for different commercial applications such as transparent electrodes, piezoelectric devices, varistors, SAW devices etc because of its high piezoelectric coupling, greater stability of its hexagonal phase and its pyroelectric property. In fact, ZnO is a potential material for gas sensor applications. Good quality ZnO films were deposited on glass and quartz substrates by a novel CVD technique using zinc acetate as the starting solution. X-ray diffraction confirmed the crystallinity of the zinc oxide film and SEM study revealed uniform deposition of fine grains. Undoped ZnO films were used for detection of dimethylamine (DMA) and H₂ at different temperatures by recording the change in resistivity of the film in presence of the test gases. The response was faster and the sensitivity was higher compared to the earlier reported ZnO based sensors developed in our laboratory. The main objective of this work was to study the selectivity of the ZnO film for a particular gas in presence of the others. The operating temperature was found to play a key role in the selectivity of such sensors.     

Preparation and optimization of freestanding GaN using low-temperature GaN layer

In this work, a method to acquire freestanding GaN by using low temperature (LT)-GaN layer was put forward. To obtain porous structure and increase the crystallinity, LT-GaN layers were annealed at high temperature. The morphology of LT-GaN layers with different thickness and annealing temperature before and after annealing was analyzed. Comparison of GaN films using different LT-GaN layers was made to acquire optimal LT-GaN process. According to HRXRD and Raman results, GaN grown on 800 nm LT-GaN layer which was annealed at 1090 °C has good crystal quality and small stress. The GaN film was successfully separated from the substrate after cooling down. The self-separation mechanism of this method was discussed. Cross-sectional EBSD mapping measurements were carried out to investigate the effect of LT-buffer layer on improvement of crystal quality and stress relief. The optical property of the obtained freestanding GaN film was also determined by PL measurement.     

Structural properties of non-stoichiometric zinc oxide films

We report a study of the crystal structure and microstructure of sputtered non-stoichiometric ZnO _x thin films for which 0.7 < x < 1. A substrate r.f. discharge was used to control film stoichiometry during deposition. All films had a columnar microstructure, and the film surface progressed from rough to smooth with increased oxidation. X-ray diffraction analysis detected no presence of crystalline zinc in any film. The crystallite size, strain and orientation of ZnO, detected in all films, was dependent on film composition and substrate r.f. discharge power. A model of film structure incorporating the competing effects of ion bombardment (causing amorphization) and increased oxygen content (creating improved crystallinity and orientation) is used to explain the observed variation of ZnO _x crystal structure.     

Vertical ZnO Nanotube Transistor on a Graphene Film for Flexible Inorganic Electronics

The bottom‐up integration of a 1D–2D hybrid semiconductor nanostructure into a vertical field‐effect transistor (VFET) for use in flexible inorganic electronics is reported. Zinc oxide (ZnO) nanotubes on graphene film is used as an example. The VFET is fabricated by growing position‐ and dimension‐controlled single crystal ZnO nanotubes vertically on a large graphene film. The graphene film, which acts as the substrate, provides a bottom electrical contact to the nanotubes. Due to the high quality of the single crystal ZnO nanotubes and the unique 1D device structure, the fabricated VFET exhibits excellent electrical characteristics. For example, it has a small subthreshold swing of 110 mV dec^?1, a high I_max/I_min ratio of 10⁶, and a transconductance of 170 nS µm^?1. The electrical characteristics of the nanotube VFETs are validated using 3D transport simulations. Furthermore, the nanotube VFETs fabricated on graphene films can be easily transferred onto flexible plastic substrates. The resulting components are reliable, exhibit high performance, and do not degrade significantly during testing.     

Sputter deposition of ZnO thin films at high substrate temperatures

ZnO thin films have been deposited on GaN and ZnO substrates at substrate temperatures up to 750 °C by radio-frequency sputtering using ZnO ceramic targets in pure argon or in a mixture of argon and oxygen. By optimizing the sputter parameters, such as sputtering power, Ar/O₂ sputtering gas ratio and temperature of the substrates high quality films were obtained as judged from the X-ray rocking curve half width and luminescence line width. The crystallinity of the ZnO films increases with increasing substrate temperature. Yet there are distinct differences between films grown on GaN templates and on O- and Zn-polar ZnO substrates.     

Effect of GaN doping on ZnO films by pulsed laser deposition

The present study reveals the codoping of Ga and N into ZnO films on sapphire substrates by pulsed laser deposition (PLD) with GaN doped ZnO targets. The glow discharge mass spectroscopy (GDMS) spectra confirm the presence of Ga and N in the doped films. The XRD measurements show that for GaN concentration up to 0.8 mol%, the full width at half maximum (FWHM) is almost unchanged thereby maintaining the crystallinity of the films, while for 1 mol% the FWHM increases. The PL spectra only show the strong near band edge (NBE) emission, whereas the deep level emissions are almost undetectable, indicating that they have been considerably suppressed. Our Hall measurements indicate that all the GaN doped ZnO films are of n-type. However, as the GaN concentration is greater than 0.6 mol%, the film shows a decrease in the carrier concentration, suggesting that N acceptors are not sufficient to compensate the native donor defects.     

Influence of the PZT film thickness on the structure and electrical properties of the ZnO/PZT heterostructure

In this study, ZnO/PZT films have been continuously deposited on SiO₂/Si substrate using radio frequency reactive magnetron sputtering method. The influence of PZT film thickness on the crystallization, surface microstructure and electrical properties of ZnO films were investigated. The X-ray diffraction results showed that the ZnO/PZT-330-nm-thick films had a perfect c-axis preferred orientation and better crystal structure compared to other samples. Simultaneously, the stress in the films has been shown to change from compressive to tensile with the increase of PZT film thickness. Atomic force microscopy displayed that the microstructures of the ZnO/PZT films with little thickness were loosened, and the grains presented a lamellar structure. However, the ZnO films showed a dense, uniform and crack-free uniform microstructure as increasing the PZT film thickness. The relative dielectric constant and dielectric loss of the ZnO/PZT-330-nm-thick films were approximately 21.7 and 0.52 %, respectively. The leakage current density of the ZnO/PZT films first decreased and then increased with increasing PZT film thickness. The ZnO/PZT-330-nm-thick films had a lowest leakage current density of approximately 10^?6 Acm^?2 at ?5 V, which showed excellent insulating characteristic.     

Epitaxial silicon carbide on a 6″ silicon wafer

The results of the growth of silicon-carbide films on silicon wafers with a large diameter of 150 mm (6″) by using a new method of solid-phase epitaxy are presented. A SiC film growing on Si wafers was studied by means of spectral ellipsometry, SEM, X-ray diffraction, and Raman scattering. As follows from the studies, SiC layers are epitaxial over the entire surface of a 150-mm wafer. The wafers have no mechanical stresses, are smooth, and do not have bends. The half-width of the X-ray rocking curve (FWHM_ω?θ) of the wafers varies in the range from 0.7° to 0.8° across the thickness layer of 80–100 nm. The wafers are suitable as templates for the growth of SiC, AlN, GaN, ZnO, and other wide-gap semiconductors on its surface using standard CVD, HVPE, and MBE methods.     

Growth of highly oriented ZnO films by the two-step electrodeposition technique

Compact and transparent ZnO films were deposited on the ITO/glass substrates from zinc nitrate aqueous solution by the two-step electrodeposition technique. While the first potentiostatic step was used to produce ZnO seed layer, the ZnO film growth has been done galvanostatically. Effects of the potentiostatic parameters on the crystal structure, morphology and optical properties of ZnO films were investigated. Results show that ZnO films with highly c-axis preferred orientation can been obtained when the potentiostatic deposition at −1.2 V for 15 s has been applied. Such an observation might be attributed to the etching process of ITO substrate in the diluted HCl solution. The film exhibits smooth and compact morphology, high transmittance in the visible band (>80%) and sharp absorption edge (at ∼370 nm). The analysis on the growth mechanism indicates that the short potentiostatic process prior to the film growth can produce ZnO seed layer and substitute the initial nucleation process in the conventional one-step galvanostatic deposition, thus increasing the nucleation density and preventing the formation of loose structures.     

Catalyst-free ZnO nanowires grown on a-plane GaN

ZnO nanowires were grown on a-plane GaN templates by chemical vapor deposition (CVD) without employing a catalyst. The a-plane GaN templates were pre-deposited on an r-plane sapphire substrate by metal-organic CVD. The resulting ZnO nanowires grow in angles off- related to the GaN basal plane. X-ray diffraction (XRD) spectra showed that the ZnO layer was grown with a heteroepitaxial relationship of (110)_ZnO||(110)_GaN. Photoluminescence spectra measured at 17 K exhibited near-band-edge emission at 372 nm with a full width at half maximum of 10 nm. The growth mechanism on a-GaN was the Volmer-Weber (VW) mode and differed from the Stranski-Krastanow (SK) mode observed for growth on c-GaN. This difference results from the higher interfacial free-energy on the a-plane between ZnO and GaN than that on the c-plane orientation.     

Growth of nonpolar m-plane GaN epitaxial film on a lattice-matched (100) β-LiGaO2 substrate by chemical vapor deposition

We report the growth of nonpolar GaN epitaxial films on nearly lattice-matched LiGaO₂ substrate by a chemical vapor deposition (CVD) method. The structural, morphological and optical properties of GaN films were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), atomic force microscopy, and photoluminescence (PL) measurements. We found that growth temperature plays an important role in the preparation of pure m-plane films by CVD method. Pure m-plane GaN was achieved by optimized growth condition. Epitaxial relationship was revealed by TEM study. The PL spectrum at room temperature has a strong near-band-edge emission at 3.41 eV and a weak yellow luminescence band.     

Enhanced magnetic properties of chemical solution deposited BiFeO3 thin film with ZnO buffer layer

Magnetic properties of BiFeO₃ films deposited on Si substrates with and without ZnO buffer layer have been studied in this work. We adopted the chemical solution deposition method for the deposition of BiFeO₃ as well as ZnO films. The x-ray diffraction measurements on the deposited films confirm the formation of crystalline phase of BiFeO₃ and ZnO films, while our electron microscopy measurements help to understand the morphology of few micrometers thick films. It is found that the deposited ZnO film exhibit a hexagonal particulate surface morphology, whereas BiFeO₃ film fully covers the ZnO surface. Our magnetic measurements reveal that the magnetization of BiFeO₃ has increased by more than ten times in BiFeO₃/ZnO/Si film compared to BiFeO₃/Si film, indicating the major role played by ZnO buffer layer in enhancing the magnetic properties of BiFeO₃, a technologically important multiferroic material.     

High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

The effect of parallel electric fields on the orientation of polycrystalline zinc oxide thin films produced by reactive evaporation and oxidation of zinc

The effect of an electric field (detached electrodes) on the crystal orientation of zinc oxide thin film was investigated. Two methods, direct and indirect, were utilized to produce zinc oxide thin films. In the direct method (reactive evaporation) oxygen was introduced into the zinc vapour stream and ZnO films were deposited on cold substrates in a vacuum system. Parallel electric fields were applied during the reaction process. It was observed that the electric field had no effect on the preferred c-axis orientation of the ZnO crystals. There was no significant difference in the crystal size or surface texture of ZnO samples subjected to electric fields with respect to the sample that experienced no electric field. In the indirect method (oxidation) electric fields were applied during the evaporation of zinc then the samples were oxidized in air in a furnace at 600 C. Here the application of electric field improved the c-axis orientations. The crystal size remained unchanged, but the surface morphology was affected by the application of the electric field. Whenever the c-axis orientation improved, crystals on the surface became rounded and a more ordered microstructure was observed.     

Fabrication of GaN epitaxial thin film on InGaZnO4 single-crystalline buffer layer

Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO₄ (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH₃ source. The epitaxial relationships are (0001)_GaN//(0001)_IGZO//(111)_YSZ in out-of-plane and [112¯0]_GaN//[112¯0]_IGZO//[11¯0]_YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30° with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.