Electrical properties of undoped bulk ZnO substrates

Undoped bulk ZnO crystals obtained from Tokyo Denpa show either resistive behavior [(5×10⁴)−(3×10⁵) Ohm cm) or low n-type conductivity (n ⋍10¹⁴ cm⁻³) with mobilities in the latter case of 130–150 cm²/V sec. The variation in resistivity may be related to the thermal instability of Li that is present in the samples. The Fermi level is pinned by 90-meV shallow donors that are deeper than the 70 meV and hydrogen-related 35-meV shallow donors in Eagle Pitcher and Cermet substrates. In all three cases, 0.3-eV electron traps are very prominent, and in the Tokyo Denpa material they dominate the high-temperature capacitance-frequency characteristics. The concentration of these traps, on the order of 2×10¹⁵ cm⁻³, is about 20 times higher in the Tokyo Denpa ZnO compared to the two other materials. The other electron traps at E_c −0.2 eV commonly observed in undoped n-ZnO are not detected in conducting Tokyo Denpa ZnO samples, but they may be traps that pin the Fermi level in the more compensated high-resistivity samples.     

Free carrier absorption and lattice vibrational modes in bulk ZnO

The electronic properties of semiconductors are highly dependent on carrier scattering mechanisms determined by crystalline structure, band structure, and defects in the material. Experimental characteristics of lattice vibrational modes and free carrier absorption in single-crystal ZnO samples obtained from different sources are presented in this work to provide a further understanding of carrier scattering processes pertaining to electronic properties. Infrared absorption measurements indicate strong absorption peaks due to a combination of optical and nonpolar phonon modes in the 9–13 μm spectral region. The Raman spectra obtained for these samples similarly reveal the presence of these phonon modes. Infrared absorption measurements also demonstrate free carrier absorption in the 3–9 μm spectral region for higher conductivity samples, where a λ^m dependence is observed with m=2.7–3, indicating both longitudinal optical phonon scattering and ionized impurity scattering. From these results, we show that infrared absorption can be used as a routine nondestructive technique to determine the material characteristics and quality of bulk ZnO.     

Properties of electrical contacts on bulk and epitaxial n-type ZnO

The electrical properties of several metal contacts to n-type ZnO (0001) were studied. The ZnO samples consisted of bulk single-crystal material, epitaxial layers on sapphire grown by molecular beam epitaxy (MBE), and polycrystalline thin films on sapphire obtained by pulsed laser deposition (PLD). Ohmic and rectifying contacts were observed dependent upon both the metal material and the ZnO surface. Ohmic contacts were characterized using the circular transmission line method (c-TLM), where contact resistivity was found to be in the range of 10⁻⁴−10⁻⁵ Ω-cm². Schottky behavior was observed using Ag contacts exhibiting varying leakage current and breakdown voltage dependent on the polarity of the ZnO surface.     

Correlation between nitrogen and carbon incorporation into MOVPE ZnO at various oxidizing conditions

Nitrogen-doped ZnO films with preferential (0 0 0 1) orientation were synthesized on c-Al₂O₃ and Si substrates by metal organic vapor phase epitaxy (MOVPE) using tertiary butanol (t-BuOH) and/or N₂O as oxidizers for diethylzinc (DEZ). A striking correlation between nitrogen and carbon incorporation into ZnO was revealed by concentration versus depth profiling employing secondary ion mass spectrometry (SIMS), consistently with recently reported simulations of nitrogen-carbon complexing.     

Thermal conductivity of bulk ZnO after different thermal treatments

Thermal conductivities (κ) of melt-grown bulk ZnO samples thermally treated under different conditions were measured using scanning thermal microscopy. Samples annealed in air at 1050°C for 3 h and treated with N-plasma at 750°C for 1 min. exhibited κ=1.35±0.08 W/cm-K and κ=1.47±0.08 W/cm-K, respectively. These are the highest values reported for ZnO. Atomic force microscopy (AFM) and conductive-AFM measurements revealed that surface carrier concentration as well as surface morphology affected the thermal conductivity.     

Solar hydrogen production of ZnO horn electrodeposited on carbon film

Solar hydrogen production by ZnO electrodeposited onto carbon films was systematically investigated by controlling the amount of the deposited ZnO, property of carbon, and surface activation by KOH treatment. Carbon films have been prepared by polymer solution casting method using PAN solution, and subsequently annealing process. The surface of carbon film became made more porous by chemically activating using KOH. And then, by electrochemical deposition, single crystalline ZnO horn was loaded on both carbon film and activated carbon film to ZnO-based hetero photocatalysts. Electrochemical deposition has been carried out to investigate the effect of ZnO-loading amount by varying deposition condition such as deposition time and concentration of zinc acetate solution. The size of ZnO horn deposited was highly dependent on initial concentration of Zn precursor and deposition time. The measurement of hydrogen production showed that photocatalytic ZnO has produced hydrogen 1.65 and 1.68 times more when deposited on non-activated carbon film and activated carbon film, respectively, than ZnO nanoparticle alone, on account of fast electron transfer due to high electrical conductivity of carbon film. Whereas, by chemically activating carbon film to activated carbon film through the base treatment, hydrogen production was increased only 1.02 times as much as that by ZnO deposited on non-activated carbon film. In this work, the results were discussed and reasoned and it was suggested that characteristics of carbon film would play the most important key role on enhancement of hydrogen production using ZnO-based hetero photocatalysts among various factors.     

Effect of hydrogenation on the cathodoluminescence properties of ZnO single crystals

The effect of hydrogenation on the intensity evolution of ZnO single crystal under electron beam irradiation was investigated by cathodoluminescence (CL). Without hydrogenation, the ultraviolet (UV) emission for O-face decreases exponentially and reaches a constant value with e-beam irradiation, while that for Zn-face once increases and then decreases. The hydrogenation significantly increases the UV emission for both faces. On the other hand, the amplitude of the decreasing component of the O-face becomes larger, while the increasing component is not affected. These results suggest that hydrogen only affects the decreasing component of the UV intensity evolution.     

The effect of growth conditions,point defects and hydrogen on the electronic structure and properties of p-type (Al,N) codoped ZnO: A first principles study

The effects of point defects, hydrogen, and growth conditions on the electronic structure and properties of the (Al,N) codoped p-type ZnO have been investigated using the first principles method. The obtained results showed that the Al_Zn–N_O–V_Zn complex is a shallow acceptor that can play an important role in achieving the p-type conductivity in the (Al,N) codoped ZnO films. Our results showed also that the electrical conductivity type in the (Al,N) codoped ZnO films strongly depends on the donor/acceptor concentrations ratio. The codoped ZnO films prepared under both Zn-rich and O-rich growth conditions with a donors/acceptors ratio of 1:2 have a p-type conductivity, while those prepared with a ratio of 1:1 cannot be p-type unless if they are prepared under O-rich conditions. The achieved p-type quality depends also on the used nitrogen doping source. To prepare p-type ZnO film of high quality using the (Al,N) codoping method, the use of NO or NO₂ is recommended. The presence of donor defects such as oxygen vacancies and hydrogen will significantly affect the electronic properties of the (Al,N) codoped ZnO films, and if the concentration of these defects in the sample is high enough, the material can be easily converted to n-type.     

Enhanced photovoltaic performance in inverted polymer solar cells using Li ion doped ZnO cathode buffer layer

We have proposed an approach to improve the photovoltaic performance of inverted polymer solar cells (i-PSC) using lithium ion doped ZnO (LiZnO) as cathode buffer layer (CBL). The LiZnO CBL was prepared using the diffusion technique, performed by inducing the Li ion of 8-hydroxyquinolatolithium (Liq) to diffuse into ZnO film through annealing the bi-layer ZnO/Liq film. Doping concentration of Li ion was controlled by using various thickness of Liq film and annealing temperature. Based on LiZnO CBL, the poly (3-hexylthiophene) [6,6]:-phenyl C61-butyric acid methyl ester (P3HT:PCBM) i-PSC device possessed a optimal power conversion efficiency (PCE) of 4.07%, which was 30% improved than that of the device with neat ZnO as CBL. The enhancement of the device performance could be attributed to the enhanced electron mobility and better band matching of the LiZnO CBL. Our finding indicates that the LiZnO film fabricated with relatively low temperature treatment has great potential for high-performance i-PSCs.     

Structural,optical and photocatalytic properties of ZnO nanoparticles modified with Cu

ZnO and ZnO modified with Cu nanoparticles have been prepared by a simple forced hydrolysis method. The concentration of Cu incorporated in ZnO ranged from 1% to 5% by atomic weight, and the influence of Cu concentration on the physical properties of ZnO and the relation to the photocatalytic performance has been investigated. The prepared ZnO and ZnO:Cu samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and UV–vis transmittance spectroscopy. The results show that the ZnO nanomaterial was crystalline with the hexagonal wurtzite structure, with the preferential orientation of the grains along the (101) plane. The average grain size for samples with 1–5% Cu was in the range of 11–29 nm. The ZnO nanoparticles annealed at 420 °C showed an increased photocatalytic activity for the decomposition of methylene blue.     

High-quality ZnO growth, doping, and polarization effect

The authors have reported their recent progress in the research field of ZnO materials as well as the corresponding global advance. Recent results regarding(1) the development of high-quality epitaxy techniques,(2) the defect physics and the Te/N co-doping mechanism for p-type conduction, and(3) the design, realization, and properties of the ZnMgO/ZnO hetero-structures have been shown and discussed. A complete technology of the growth of high-quality ZnO epi-films and nano-crystals has been developed. The co-doping of N plus an iso-valent element to oxygen has been found to be the most hopeful path to overcome the notorious p-type hurdle. High mobility electrons have been observed in low-dimensional structures utilizing the polarization of ZnMgO and ZnO. Very different properties as well as new physics of the electrons in 2DEG and 3DES have been found as compared to the electrons in the bulk.     

Characterization of ZnO and ZnO:Al films deposited by MOCVD on oriented and amorphous substrates

Zinc oxide (ZnO) and ZnO:Al-doped films were deposited by metal organic chemical vapour deposition (MOCVD) using the Zn(tta)₂·tmeda (H-tta=2-thenoyltrifluoroacetone, tmeda=N,N,N′,N′-tetramethylethylendiamine) and Al(acac)₃ (H-acac=acetylacetone) precursors on different substrates. The deposited layers were characterised by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Film structure is strongly dependent on the substrate nature and deposition conditions. AFM and XRD measurements show a good film texture and a preferential orientation along the c-axis. The Al concentration of ZnO:Al film has been confirmed by energy dispersive X-ray (EDX) analysis. Optical transparency of these ZnO layers has been studied in order to evaluate their applications as a transparent conducting oxide (TCO) material.     

Fabrication of ZnO nanorod-based hydrogen gas nanosensor

We report a first work on nanofabrication of hydrogen nanosensor from single ZnO branched nanorods (tripod) using in-situ lift-out technique and performed in the chamber of focused ion beam (FIB) system. Self-assembled ZnO branched nanorod has been grown by a cost-effective and fast synthesis route using an aqueous solution method and rapid thermal processing. Their properties were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersion X-ray spectroscopy, transmission electron microscopy, and micro-Raman spectroscopy. These analyses indicate high quality ZnO nanorods. Furthermore, our synthesis technique permits branched nanorods to be easily transferred to other substrates. This flexibility of substrate choice opens the possibility of using FIB system for handling.

The main advantage of the proposed in-situ approach is a controllable lift-out procedure which permitted us to obtain a 90% success rate for building nanodevices. The fabricated nanosensor uses only single self-assembled ZnO branched nanorod (tripod) to gauge the 150 ppm H₂ in the air at room temperature. The hydrogen sensitivity is in the range of 0.6–2% depending on which two branches to use. The nanosensor has selectivity against other gases such as O₂, CH₄, CO and LPG, which shows sensitivity of <0.02%. The single ZnO branched nanorod sensor can operate at low power of <5 μW.     

Ga Al In掺杂ZnO电子结构的第一性原理计算

计算了Ga、Al、In掺杂ZnO体系电子结构,分析了掺杂对ZnO晶体的结构、能带、电子态密度、差分电荷分布的影响。所有计算,都是基于密度泛函理论(DFT)框架下的第一性原理平面波超软赝势方法。计算结果表明:在导带底引入了大量由掺杂原子贡献的导电载流子(Ga:2.57×1021cm–3;Al:2.58×1021cm–3;In:2.53×1021cm–3),明显提高了体系的电导率。同时,光学带隙展宽,且向低能方向漂移,可作为优良的透明导电薄膜材料。     

紫外辐射ZnO纳米棒的生长及性能

用退火法在玻璃、硅片衬底上先生长ZnO籽晶,然后在90℃下在醋酸锌和六亚甲基四胺溶液中生长了直径约为17 nm的ZnO纳米棒.采用X射线衍射仪(XRD)分析了不同衬底上生长的ZnO纳米棒的结构和择优生长取向,用扫描电子显微镜(SEM)观察了ZnO的形态,用荧光光谱仪分析了纳米棒的发光特性,讨论了籽晶、衬底类型和衬底放置方式对纳米棒的尺寸、排列趋向性和光学性能的影响.纳米棒的直径和排列依赖于衬底的初始状态,籽晶可以减小纳米棒的尺寸,增强纳米棒的排列有序性;一旦衬底上生长了籽晶,后续生长的纳米棒的尺寸、排列和性能与衬底的类型无关,纳米棒都具有强的紫光发射.但衬底的放置方式会影响其上纳米棒的形态,竖直放置的衬底易生长尺寸分布均匀的准有序排列的纳米棒.     

Effect of H2O2 concentration on electrochemical growth and properties of vertically oriented ZnO nanorods electrodeposited from chloride solutions

In this work, ZnO nanostructures are electrodeposited on a transparent conducting glass from chloride baths. The influence of H₂O₂ concentration on the electrochemical characteristics has been studied using cyclic voltammetry (CV) and chronoamperometry (CA) techniques. From the analysis of the current transients on the basis of the Scharifker–Hills model, it is found that nucleation mechanism is progressive with a typical three-dimensional (3D) nucleation and growth process; independently with the concentration of H₂O₂. However, the nucleation rate of the ZnO changes with the increase of H₂O₂ concentration. The Mott–Schottky measurements demonstrate an n-type semiconductor character for all samples with a carrier density varying between 5.14×10¹⁸ cm⁻³ and 1.47×10¹⁸ cm⁻³. Scanning electron microscopy (SEM) observations show arrays of vertically aligned ZnO nanorods (NRs) with good homogeneity. The X-ray diffraction (XRD) patterns show that the ZnO deposited crystallises according to a hexagonal Würtzite-type structure and with the c-axis perpendicular to the electrode surface. The directional growth along (002) crystallographic plane is very important for deposits obtained at 5 and 7 mM of H₂O₂. The high optical properties of the ZnO NRs with a low density of deep defects was checked by UV–vis transmittance analyses, the band gap energy of films varies between 3.23 and 3.31 eV with transparency around 80–90%.     

铟掺杂ZnO体单晶的生长及其性质

研究了In掺杂n型zno体单晶的化学气相传输法生长和材料性质.利用霍尔效应、x射线光电子能谱、光吸收谱、喇曼散射、阴极荧光谱等手段对晶体的特性和缺陷进行r分析.掺In后容易获得浓度为1018～lO19cm-3的n型ZnO单晶,掺人杂质的激活效率很高.随着掺杂浓度的提高,znO单晶的带边吸收和电学性质等发生明显的变化.分析了掺In-ZnO单晶的缺陷及其对材料性质的影响.     

Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting

Photoelectrochemical cells based on traditional and nanostructured ZnO thin films are investigated for hydrogen generation from water splitting. The ZnO thin films are fabricated using three different deposition geometries: normal pulsed laser deposition, pulsed laser oblique‐angle deposition, and electron‐beam glancing‐angle deposition. The nanostructured films are characterized by scanning electron microscopy, X‐ray diffraction, UV‐vis spectroscopy and photoelectrochemical techniques. Normal pulsed laser deposition produces dense thin films with ca. 200 nm grain sizes, while oblique‐angle deposition produces nanoplatelets with a fishscale morphology and individual features measuring ca. 900 by 450 nm on average. In contrast, glancing‐angle deposition generates a highly porous, interconnected network of spherical nanoparticles of 15–40 nm diameter. Mott‐Schottky plots show the flat band potential of pulsed laser deposition, oblique‐angle deposition, and glancing‐angle deposition samples to be ?0.29, ?0.28 and +0.20 V, respectively. Generation of photocurrent is observed at anodic potentials and no limiting photocurrents were observed with applied potentials up to 1.3 V for all photoelectrochemical cells. The effective photon‐to‐hydrogen efficiency is found to be 0.1%, 0.2% and 0.6% for pulsed laser deposition, oblique‐angle deposition and glancing‐angle deposition samples, respectively. The photoelectrochemical properties of the three types of films are understood to be a function of porosity, crystal defect concentration, charge transport properties and space charge layer characteristics.     

Electrical properties of templateless electrodeposited ZnO nanowires

Electrochemical deposition allows the preparation of ZnO nanostructures with precisely controlled morphology and properties, by finely tuning the process parameters. ZnO nanowires were deposited onto gold substrates by electrodeposition from a low concentration zinc nitrate bath. Photolithography was employed for patterning interdigitated electrode systems onto silicon/silicon dioxide substrates and ZnO electrodeposition lead to wires connected to each other by bridging neighboring interdigits allowing electronic transport characterization. Optical measurements, i.e. reflection and photoluminescence spectroscopy, were performed and the results were correlated to electronic transport data. We found that we deal with a system for which one can apply a model of space charge limited currents with different traps energy distribution as a consequence of electrodeposition rate. Current versus temperature measurements show different behavior for lower and higher range of temperatures. Such nanowires, fabricated and contacted in a straightforward way, allow a wide area of applications ranging from conductometric bio- or chemo-sensors to optoelectronic devices.     

ZnO spintronics and nanowire devices

ZnO is a very promising material for spintronics applications, with many groups reporting room-temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during pulsed laser deposition (PLD), we find an inverse correlation between magnetization and electron density as controlled by Sn-doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for ferromagnetism include the bound magnetic polaron model or exchange that is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. The progress in ZnO nanowires is also reviewed. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for microlasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼−3 V, and a maximum transconductance of 0.3 mS/mm. Under ultraviolet (UV) illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼5 mS/mm. The channel mobility is estimated to be ∼3 cm²/Vss, comparable to that for thin film ZnO enhancement mode metal-oxide semiconductor field effect transistors (MOSFETs), and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. The Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25°C, very low reverse currents, and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25°C to 150°C, the resistivity of nanorods treated in H₂ at 400°C prior to measurement showed an activation energy of 0.089 eV and was insensitive to ambient used. By contrast, the conductivity of nanorods not treated in H₂ was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. Sensitive pH sensors using single ZnO nanowires have also been fabricated.