Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ～ 389 nm and a broad green emission peak at ～ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.     

脉冲激光沉积法在蓝宝石衬底上制备a轴取向YSZ薄膜

采用脉冲激光沉积方法,通过金红石相TiO2(200)纳米诱导层在蓝宝石衬底上生长了(200)取向的萤石相钇稳定氧化锆(YSZ)薄膜。通过反射式高能电子衍射对薄膜生长过程进行原位监测;用XRD分析进行后位的薄膜结构表征;使用原子力显微镜来观测薄膜的形貌及晶粒大小。结果表明,我们成功地在蓝宝石衬底上外延了具有三重旋转织构的a轴取向萤石相YSZ薄膜,其外延关系为YSZ(200)∥TiO2(200)∥Al2O3(0001);YSZ[010]∥TiO2[001]∥Al2O3[11-20]。     

Epitaxial growth of ZnO layers using nanorods with high crystalline quality

Epitaxial ZnO films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition using a ZnO multi-dimensional structure having the sequence of ZnO film/ZnO nanorods/sapphire. The vertically well-aligned one-dimensional ZnO nanorods were grown epitaxially on the sapphire substrate with in-plane alignment under suitable growth conditions and then used as seeds for the subsequent epitaxial ZnO layer. For the transition of the ZnO structures from the nanorods to the film, the growth temperature and working pressure were controlled, while keeping the other conditions fixed. The growth of the ZnO films on the well-aligned ZnO nanorods results in homoepitaxial growth with the identical orientation relationship along the in-plane direction as well as the same c-axis orientation. The microstructural analysis of the multi-dimensional structure and analysis of the microstructural evolution from the one-dimensional nanorods to the two-dimensional film were conducted using transmission electron microscopy.     

AlN epilayers and nanostructures growth in a homebuilt alumina hot-wall high temperature chemical vapor deposition system

AlN epilayers and nanostructures were grown in the range from 500 to 1500 °C in a homebuilt alumina hot-wall high temperature chemical vapor deposition system. The results revealed that high quality AlN epilayers can be grown at high temperature beyond 1100 °C and versatile AlN nanostrctures can be grown at low temperature below 900 °C, enabling the system to tailor AlN structures just by changing the growth temperature. High growth temperature as well as low N/Al ratio was preferable to surface mobility of the adatoms and lateral growth, resulting in a series of morphology changes. Meanwhile, the crystal quality improved with the increasing growth temperature, as proved by the decreasing FWHM of (0002) plane rocking curve of the epilayer and narrowing peaks in θ-2θ XRD pattern of the nanostructures. The epitaixal relationship was proven to be AlN (0001) ‖ sapphire (0001) and AlN [1-210] ‖ sapphire [1-100]. The layer was in tensile stress state in several tens of nanometers range near the interface and turned into compressive stress state out of the range. Tens of atoms layers of sapphire interface were substituted for AlN lattice due to nitridation. Low growth temperature produced versatile AlN nanostructures, whose crystal structures varied from amorphous in 500 °C case to defective crystal in 700 °C case and improved crystallinity in 900 °C case.     

Low temperature epitaxial growth of ZnO layer by plasma-assisted epitaxy

Plasma-assisted epitaxial growth of ZnO layers were achieved on C- and R-plane sapphire substrates in oxygen plasma excited by radio frequency power at 13.56 MHz with evaporation of pure elemental Zn. The ZnO layers were grown at 300–400°C with high growth rate around 1.7 μm/h. Surface cleaning of sapphire substrates using Ar-plasma was crucial for good quality ZnO growth. Photoluminescence spectra at 10 K were dominated by band-edge emission due to bound excitons without deep level emission in green-light region. The intensity of band-edge emission was strongly dependent on applied radio frequency power to excite Ar- and O₂-plasma for sapphire surface cleaning and ZnO growth, respectively, and was about 50 times larger on the layer grown in oxygen plasma than that grown in non-excited oxygen gas. The ZnO layer grown on R-plane sapphire was epitaxially grown above 300°C in oxygen plasma, however, on C-plane sapphire the ZnO layer was easily polycrystallized for thick films even at 400°C. Growth mode and surface morphology of ZnO layers were drastically changed with the substrate orientation.     

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.     

Epitaxial growth of InN on c-plane sapphire by pulsed laser deposition with r.f. nitrogen radical source

We have grown InN films on c-plane sapphire substrates by pulsed laser deposition (PLD) with a radio frequency nitrogen radical source for the first time and investigated the effect of the substrate surface nitridation on the structural and electrical properties of InN films with reflection high energy electron diffraction (RHEED), atomic force microscope, the Hall effect measurements and high-resolution X-ray diffraction (HRXRD). RHEED and HRXRD characterizations revealed that high-quality InN grows epitaxially on sapphire by PLD and its epitaxial relationship is InN (0 0 0 1)∣∣sapphire (0 0 0 1) and InN [2 -1 -1 0]∣∣sapphire [1 0 -1 0]. The InN crystalline quality and the electron mobility are improved by the substrate nitridation process. The area of the pits at the InN surface is reduced by the substrate nitridation process probably due to the reduction in the interface energy between InN and the substrate. The full width at half maximum of the -1 -1 2 4 X-ray rocking curve for InN grown by the present technique without using any buffer layers was as small as 34.8 arcmin. These results indicate that the present technique is promising for the growth of the high-quality InN films.     

Electrical and luminescent properties and deep traps spectra of N-polar GaN films

Electrical and luminescent properties of N-polar undoped GaN films grown using low temperature GaN buffers on on-axis and miscut sapphire and on-axis AlN buffers are compared to the properties of Ga-polar films grown on low temperature GaN buffers. It is shown that the concentration of residual donors increases by about an order of magnitude for on-axis N-polar growth and by two orders of magnitude for off-axis growth compared to Ga-polar films. On-axis films for both Ga-polar and N-polar polarities show the presence of n⁺ interfacial layers greatly influencing the apparent electron concentration and mobility deduced from capacitance–voltage C–V measurements. These interfacial layers are much less prominent in the miscut N-polar films. Growth on N-polar greatly increases the concentration of electron traps with activation energy of 0.9 eV possibly related to Ga-interstitials.     

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.     

制备条件对电致变色氧化镍薄膜特性的影响

在充氧气的真空室内 ,用电子束蒸发NiO粉末颗粒的方法分别以 0 1和 0 8nm/s的淀积速率制备了氧化镍薄膜 ,并在不同的环境中对薄膜进行热处理。研究了薄膜结构和电致变色特性与淀积速率的关系 ,发现以较慢和较快速率淀积的薄膜分别具有NiO晶粒的 (2 0 0 )和 (111)不同择优取向 ,前者致色范围较小 ,后者致色范围较大。还研究了热处理对薄膜的结构、动态致色范围、致色效率 ,以及红外光谱特性的影响 ,发现热处理对薄膜的致色效率影响较小 ,然而对动态致色范围的影响很大。     

Substrate effects on the structure and optical properties of GaN epitaxial films

The phase composition and luminescent properties of GaN films grown by molecular beam epitaxy on (0001) sapphire and 6H-SiC substrates were studied. The films grown on SiC were found to consist only of the hexagonal phase and contain a lower concentration of impurities. Grains of cubic GaN, as well as donor and acceptor impurities, were found in the GaN film grown on sapphire. The formation of impurity centers is caused by the diffusion of oxygen and aluminum from the sapphire substrate during crystal growth.     

Effect of interface on mid-infrared photothermal response of MoS2 thin film grown by pulsed laser deposition

This study reports on the mid-infrared (mid-IR) photothermal response of multilayer MoS2 thin films grown on crystalline (p-type silicon and c-axisoriented single crystal sapphire) and amorphous (Si/SiO2 and Si/SiN) substrates by pulsed laser deposition (PLD).The photothermal response of the MoS2 films is measured as the changes in the resistance of the MoS2 films when irradiated with a mid-IR (7 to 8.2 μm) source.We show that enhancing the temperature coefficient of resistance (TCR) of the MoS2 thin films is possible by controlling the film-substrate interface through a proper choice of substrate and growth conditions.The thin films grown by PLD are characterized using X-ray diffraction,Raman,atomic force microscopy,X-ray photoelectron microscopy,and transmission electron microscopy.The high-resolution transmission electron microscopy (HRTEM) images show that the MoS2 films grow on sapphire substrates in a layer-by-layer manner with misfit dislocations.The layer growth morphology is disrupted when the films are grown on substrates with a diamond cubic structure (e.g.,silicon) because of twin growth formation.The growth morphology on amorphous substrates,such as Si/SiO2 or Si/SiN,is very different.The PLD-grown MoS2 films on silicon show higher TCR (-2.9％ K-1 at 296 K),higher mid-IR sensitivity (△R/R =5.2％),and higher responsivity (8.7 V·W-1) compared to both the PLD-grown films on other substrates and the mechanically exfoliated MoS2 flakes transferred to different substrates.     

Ion-etch characteristics of epitaxial copper on sapphire

For the first time a face-centred-cubic metal film has been epitaxially grown on sapphire. Film crystallinity and ion-etch characteristics were studied using back reflection X-ray diffraction and scanning electron microscope techniques. Single-crystal copper films have been deposited on basal plane sapphire substrates in the temperature range 240 to 375° C. The films exhibited bulk metal resistivity. Ion-etching studies of the films, using argon ions, have demonstrated superior quality, resolution and vertical etch profile of interconnection transmission lines over those of polycrystalline or less crystalline films.     

A facile room temperature synthesis of ZnO nanoflower thin films grown at a solid–liquid interface

Hierarchical ZnO films consisting of nanoflower particulates are successfully grown by a solid–liquid interface reaction technique at room temperature without additives like surfactants, capping agent, or complexing agent. The structural, morphological, and photocatalytic properties of these films are studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV–Vis spectroscopy. The nucleation, growth processes and hence the resulting morphology of the end product can be regulated by changing the concentration of LiOH and the time of reaction. SEM throws light on the chronology of the flower formation by studying the intermediate morphology. Electron microscopy results indicated that these ZnO nanostructures self-assembled to produce flower-like nanostructures. The highest photocatalytic efficiency was observed for the films prepared at the concentration of LiOH 0.5 mg/mL in ethanol at 24 h. On the basis of the results, a plausible growth mechanism for the formation of flower-like ZnO nanostructures is discussed.     

Reduction of microwave dielectric losses in KTa1 − xNbxO3 thin films by MgO-doping

KTa_1 − xNb_xO₃ (KTN) thin films were grown by pulsed laser deposition on sapphire and MgO substrates. Their structural and high frequency dielectric characteristics evidenced the strong influence of the substrate and suggested possible KTN/MgO interdiffusion that could be responsible for the lower dielectric losses obtained on this substrate. Both undoped and 6% MgO-doped KTN thin films were then grown on sapphire. Dielectric measurements performed at 12.5 GHz by a resonant cavity perturbation method evidenced reduction of losses by MgO-doping. Loss tangent (tan δ) was reduced by a factor of 3 in comparison with undoped films grown on sapphire.     

Achieve 2-inch-diameter homogeneous GaN films on sapphire substrates by pulsed laser deposition

The 2-inch-diameter homogeneous GaN films have been epitaxially grown on sapphire substrates by pulsed laser deposition (PLD) technique with optimized laser rastering and PLD growth conditions. The as-grown GaN films are characterized by in situ reflection high-energy electron diffraction, white-light interferometry, scanning electron microscopy, atomic force microscopy (AFM), grazing incidence angle X-ray reflectivity, reciprocal space mappings, and micro-Raman spectroscopy for surface morphologies and structural properties. The as-grown 2-inch-diameter single-crystalline GaN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity less than 3.4 % and very smooth surface with a RMS roughness less than 1.3 nm measured by AFM. There is a maximum of 1.2 nm thick interfacial layer existing between the as-grown GaN films and sapphire substrates, and the as-grown 310 nm thick GaN films are almost fully relaxed only with an in-plane compressive strain of 0.044 %. This work demonstrates a possibility for achieving high-quality large-scale GaN films with uniform thickness and atomically abrupt interface by PLD, and is of great interest for the commercial development of GaN-based optoelectronic devices.     

Effects of substrate parameters on structure and optical properties of ZnO thin films fabricated by pulsed laser deposition

Highly oriented zinc oxide thin films have been grown on quartz, Si (1 1 1) and sapphire substrates by pulsed laser deposition (PLD). The effect of temperature and substrate parameter on structural and optical properties of ZnO thin films has been characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmission spectra and PL spectra. The experimental results show that the best crystalline thin films grown on different substrate with hexagonal wurtzite structure were achieved at growth temperature 400–500 °C. The growth temperature of ZnO thin film deposited on Si (1 1 1) substrate is lower than that of sapphire and quartz. The band gaps are increasing from 3.2 to 3.31 eV for ZnO thin film fabricated on quartz substrate at growth temperature from 100 to 600 °C. The crystalline quality and UV emission of ZnO thin film grown on sapphire substrate are significantly higher than those of other ZnO thin films grown on different substrates.     

Realization of nonpolar a-plane ZnO films on r-plane sapphire substrates using a simple single-source chemical vapor deposition

Nonpolar (112?0) ZnO thin films (a-plane ZnO) have been grown on (11?02) sapphire substrates (r-plane sapphire) by a simple atmospheric pressure single-source chemical vapor deposition (SSCVD) approach. The crystallinity, surface morphology and optical property of the films were investigated using high-resolution X-ray diffraction (HRXRD), scanning electron microscope (SEM) and transmission spectrum, respectively. XRD results revealed that the ZnO films were grown on the substrates epitaxially along (112?0) orientation, and the epitaxial relationship between the ZnO films and the substrates was determined to be (112?0)ZnO∥(11?02) Al2O3, and [1?101]ZnO∥[022?1]Al₂O₃. The SEM image exhibited that the a-plane ZnO films showed a high density of well-aligned ZnO sheets with rectangular structure. The transmission spectrum showed that the ZnO films were highly transparent in the visible region.     

Effects of crystallinity and chemical variation on apparent band-gap shift in polycrystalline indium nitride

The nature of the apparent band-gap shift in polycrystalline indium nitride thin-films, grown by remote-plasma-enhanced chemical vapour deposition at 535 ± 10 °C, has been investigated separately in relation to growth temperature dependent crystallinity and chemical variation. Substrates of sapphire and gallium nitride on sapphire were used to study the effect of a stress-reduced template on indium nitride crystallite quality and apparent band-gap. To mimic surface growth temperature variations two glass substrates of differing thickness and thermal conductivity were intentionally used for the same growth conditions. The samples were characterised using optical transmission, scanning electron microscope, X-ray diffraction, and high-resolution X-ray photoelectron spectroscopy. The results indicate that the apparent band-gap shift in polycrystalline narrow band-gap indium nitride thin-films is not primarily determined by the quality of indium nitride crystallites but rather it is associated with growth temperature dependent chemical variations in the films.     

Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL).