AZO种子层朝向对ZnO纳米棒形貌和发光特性的影响

在水平管式炉中,采用热蒸发锌粉的方法,在镀有掺铝氧化锌(AZO)薄膜的石英基片上制备了大量高密度的ZnO纳米棒,AZO膜面分别正对和背对锌源。利用扫描电子显微镜、X射线衍射仪以及荧光光谱仪分析AZO膜面朝向对ZnO纳米棒的形貌、微结构及光学性能的影响。结果表明,不同朝向的AZO膜面上所生长的纳米棒具有相似的形貌和微结构。保温时间10min样品的氧空位的缺陷态发光为绿光,强度较强;保温时间15min样品的纳米棒长度较长、相对垂直衬底,其近带边发光较强,氧间隙的缺陷态发光较弱。正对锌源衬底上且保温时间15min样品的近带边发光最强,且缺陷态发光最弱。     

A Comparison of ZnO Nanowires and Nanorods Grown Using MOCVD and Hydrothermal Processes

A comparison of ZnO nanowires (NWs) and nanorods (NRs) grown using metalorganic chemical vapor deposition (MOCVD) and hydrothermal synthesis, respectively, on p-Si (100), GaN/sapphire, and SiO₂ substrates is reported. Scanning electron microscopy (SEM) images reveal that ZnO NWs grown using MOCVD had diameters varying from 20 nm to 150 nm and approximate lengths ranging from 0.7 μm to 2 μm. The NWs exhibited clean termination/tips in the absence of any secondary nucleation. The NRs grown using the hydrothermal method had diameters varying between 200 nm and 350 nm with approximate lengths between 0.7 μm and 1 μm. However, the NRs grown on p-Si overlapped with each other and showed secondary nucleation. x-Ray diffraction (XRD) of (0002)-oriented ZnO NWs grown on GaN using MOCVD demonstrated a full-width at half-maximum (FWHM) of 0.0498 (θ) compared with 0.052 (θ) for ZnO NRs grown on similar substrates using hydrothermal synthesis, showing better crystal quality. Similar crystal quality was observed for NWs grown on p-Si and SiO₂ substrates. Photoluminescence (PL) of the NWs grown on p-Si and SiO₂ showed a single absorption peak attributed to exciton–exciton recombination. ZnO NWs grown on GaN/sapphire had defects associated with oxygen interstitials and oxygen vacancies.     

Defect‐Related Emissions and Magnetization Properties of ZnO Nanorods

A clear correlation between defect‐related emissions and the magnetization of ZnO nanorods synthesized by a one‐step aqueous chemical method is demonstrated. The relative contribution of the emission bands arising from various types of defects is determined and found to be linked with the size of the nanorods and annealing conditions. When the size of the nanorods and the annealing temperature are increased, the magnetization of pure ZnO nanorods decreases with the reduction of a defect‐related band originating from singly charged oxygen vacancies ($V_{\rm o}^ +$ ). With a sufficient increase of annealing temperature (at 900 °C), the nanorods show diamagnetic behavior. Combining with the electron paramagnetic resonance results, a direct link between the magnetization and the relative occupancy of the singly charged oxygen vacancies present on the surface of ZnO nanorods is established.     

Influence of ammonia concentration on the microstructure,electrical and raman properties of low temperature chemical bath deposited ZnO nanorods

Nanostructured zinc oxide synthesized using an easy and low temperature chemical bath deposition method are among the most promising low cost semiconducting nanostructures investigated for a variety of applications. We successfully report the effects of ammonia solution in the growth of ZnO nanorods at a temperature of 60 °C. Successive addition of ammonia altered the degree of supersaturation of the growth solution, causing a significant deviation in the morphology and crystal orientation of ZnO nanorods. Field emission scanning Electron Microscopy images revealed changes in surface morphology of ZnO nanorods with respect to addition of specific amounts of ammonia. X-ray diffraction analysis revealed wurtzite crystal structure of ZnO which was further supported by X-ray photoelectron studies, optical absorbance and Raman spectra that also revealed the existence of wurtzite ZnO. The current-voltage measurement showed the electrical properties of the synthesized ZnO nanorods. The vertically grown nanorods with flat tops, effect more rectifying Schottky contacts to be realized on comparison to needle like structures.     

Self‐Compensated Insulating ZnO‐Based Piezoelectric Nanogenerators

Remarkable enhancement of piezoelectric power output from a nanogenerator (NG) based on a zinc oxide (ZnO) thin film is achieved via native defect control. A large number of unintentionally induced point defects that act as n‐type carriers in ZnO have a strong influence on screening the piezoelectric potential into a piezoelectric NG. Here, additional oxygen molecules bombarded into ZnO lead to oxygen‐rich conditions, and the n‐type conductivity of ZnO is decreased dramatically. The acceptor‐type point defects such as zinc vacancies created during the deposition process trap n‐type carriers occurring from donor‐type point defects through a self‐compensation mechanism. This unique insulating‐type ZnO thin film‐based NGs (IZ‐NGs) generates output voltage around 1.5 V that is over ten times higher than that of an n‐type ZnO thin film‐based NG (around 0.1 V). In addition, it is found that the power output performance of the IZ‐NG can be further increased by hybridizing with a p‐type polymer (poly(3‐hexylthiophene‐2,5‐diyl):phenyl‐C₆₁‐butyric acid methyl ester) via surface free carrier neutralization.     

Processing temperature dependent morphological and optical properties of ZnO nanorods

Zinc Oxide (ZnO) nanorods were synthesized by thermal decomposition of zinc acetate dihydrate at various processing temperatures. The morphology of samples, examined using transmission electron microscopy and field emission scanning electron microscopy, revealed large variations in length and diameter of nanorods. As temperature was increased from 300 °C to 450 °C, ZnO nanocrystal morphology changed from wire-like to rod-like. This morphology change is a result of competition between nucleation and growth rate in the vapor–solid growth mechanism of nanorods. Photoluminescence spectrum of the nanorods showed both band edge emission as well as native defect related visible emission. Relative intensity of UV and visible emission indicated crystal quality of the nanorods, the ratio increased upto 400 °C and deteriorated at higher processing temperature. It is argued that process induced defects dominate at processing temperatures ≤400 °C, whereas equilibrium concentration of native defects is high at temperatures >400 °C.     

缺陷相互作用对HfO2基阻变存储器件均匀性的影响：第一性原理研究

The physical mechanism of doping effects on switching uniformity and operation voltage in Al-doped HfO₂ resistive random access memory（RRAM） devices is proposed from another perspective:defects interactions, based on first principle calculations.In doped HfO₂,dopant is proved to have a localized effect on the formation of defects and the interactions between them.In addition,both effects cause oxygen vacancies（V_O） to have a tendency to form clusters and these clusters are easy to form around the dopant.It is proved that this process can improve the performance of material through projected density of states（PDOS） analysis.For V_O filament-type RRAM devices, these clusters are concluded to be helpful for the controllability of the switching process in which oxygen vacancy filaments form and break.Therefore,improved uniformity and operation voltage of Al-doped HfjO₂ RRAM devices is achieved.     

UV‐Induced Multilevel Current Amplification Memory Effect in Zinc Oxide Rods Resistive Switching Devices

Zinc oxide (ZnO) devices represent an alternative in the semiconductor technology for their application in resistive switching memory devices and ultraviolet (UV) photodetectors due to their chemical and electrical properties. The multilevel current amplification of ZnO rods RRAM devices induced by UV light illumination is reported here for the first time. The resistive switching mechanism underlying in this type of devices is attributed to the formation of conductive filaments composed of oxygen vacancies. The analysis of the photodecay processes carried out on the devices fabricated with different electrodes shows that the type of interface (Ag/ZnO and Au/ZnO) affects the surface barrier height, which influences the photodecay rate. It is shown that by applying UV light, higher relaxation constants (slower photodecay rates) are obtained and lead to multilevel current amplification behavior.     

Bipolar Ni/ZnO/HfO2/Ni RRAM with multilevel characteristic by different reset bias

The authors report the fabrication and characterization of resistive random access memory (RRAM) with Ni/ZnO/HfO₂/Ni structure at room temperature. It was found that the proposed device exhibited bipolar switching behavior with multilevel characteristics in a reset process. It was found that the device exhibited two-step reset stage under high reset bias. By applying a 2nd reset stage after the transformation of the 1st reset stage, it was found that the RRAM could return to the initial state. From I–V curves measured in these two reset stages, it was found that the current conduction was dominated by Schottky emission due to the migration of oxygen ions and recombination with oxygen vacancies. This reaction could break the conducting filament so as to transform carrier transport mechanism to Schottky emission. This also results in the simultaneous transformation from low resistance state (LRS) to high resistance state (HRS).     

Photoluminescence Properties of ZnO Nanorods Synthesized by Different Methods

The photoluminescence properties of ZnO nanorods synthesized by the low-temperature hydrothermal and high-temperature vapor-phase methods are studied. At room temperature, the photoluminescence of ZnO nanorods synthesized by the high-temperature vapor-phase method exhibits only one highintensity ultraviolet peak at a wavelength of 382 nm. At the same time, the luminescence spectra of ZnO nanorod samples grown by the low-temperature hydrothermal method, but with the use of different chemical reagents exhibit, apart from the ultraviolet peak, a violet band or a yellow-orange band at ~401 and ~574 nm, respectively. The violet luminescence band is attributed to defects or zinc vacancy complexes, and the yellow-orange band to defects associated with interstitial oxygen.     

Structural and optical properties of (Al,K)-co-doped ZnO thin films deposited by a sol–gel technique

Thin films of Al-doped ZnO (AZO) and (Al, K)-co-doped ZnO (AKZO) were synthesized by sol–gel spin coating and their structural and optical properties were investigated. All the films had a preferential orientation in which the c-axis was perpendicular to the substrate. The optical bandgap increased after Al doping, but decreased after K doping at a given Al doping concentration. UV emission and a broad visible emission band were observed in photoluminescence (PL) spectra. The intensity of both emission bands decreased after Al and K co-doping. PL excitation (PLE) spectra of the blue emission band indicate that the initial state is possibly the same for all the samples and a similar case occurs for the orange–red emission band. The green emission can be attributed to electronic transitions involving oxygen vacancies. A possible process for the orange–red emission of the thin films is radiative recombination of an electron trapped in a zinc interstitial defect with a hole deeply trapped in interstitial oxygen.     

Stable resistive switching characteristics of Ce:HfOx film induced by annealing process

In this work, Ce:HfO_x films were fabricated and the resistive switching characteristics were investigated. The chemical bonding states of the films were explored by X-ray photoelectron spectroscopy. The annealing process was carried out to modulate the concentration of oxygen vacancies in the film to confirm the dominant role of oxygen vacancies on resistive switching behaviors, which resulted in the elimination of unstable oxygen vacancies and the introduction of oxygen vacancy near Ce dopants due to the reduction of Ce⁴⁺. Benefiting from the oxygen vacancies near Ce dopants, stable resistive switching performance can be achieved for the annealed Ce:HfO_x sample. A schematic diagram based on the formation and rupture of oxygen vacancy filaments was proposed to illustrate the switching behaviors of annealed Ce:HfO_x sample.     

生长时间对氧化锌纳米杆形貌的影响

采用旋涂法在洗净的玻璃衬底上制备了醋酸锌薄膜,并进一步在空气中退火获得了氧化锌(ZnO)薄膜,X射线衍射分析显示退火后获得的ZnO薄膜具有c轴(002)择优取向生长特性.通过水热法以ZnO薄膜为种子层,生长了ZnO纳米杆阵列.研究了在相同的ZnO种子层、前驱液浓度和生长温度条件下,不同生长时间对ZnO纳米杆形貌的影响.扫描电子显微镜照片显示,随着生长时间的增加,ZnO纳米杆阵列的生长具有阶段性规律,并且在经过52h生长后得到了顶端中心被溶解的ZnO纳米管.分析认为该现象和前驱液中Zn2+离子和OH-离子的浓度变化有关,同时也和ZnO的非极性结构有关.     

Formation of Cu<Subscript>2</Subscript>O and ZnO Crystal Layers by Magnetron Assisted Sputtering and Their Optical Characterization

Copper (I) oxide and zinc oxide films are formed on silicon and glassy quartz substrates by magnetron assisted sputtering. The thickness of the films is tens and hundreds of nanometers. The films are grown at different substrate temperatures and different oxygen pressures in the working chamber. The film samples are studied by the X-ray diffraction technique, scanning electron microscopy, and optical methods. It is established that an increase in the substrate temperature yields a change in the surface morphology of copper (I) oxide films towards the formation of well-pronounced crystallites. The reflectance and Raman spectra suggest that the quality of such films is close to that of bulk Cu₂O crystals produced by the oxidation of copper. As concerns ZnO films, an increase in the substrate temperature and an increase in the partial oxygen pressure make it possible to produce films, for which a sharp exciton structure is observed in the reflectance spectra and the emission of excitons bound at donors is observed in the luminescence spectra.     

Optical and thermal properties of precursor-controlled graphene–zinc nanocomposites

Graphene, a monolayer two dimensional carbon sheet can be utilized as a support to anchor functional nanomaterials to form novel nanocomposites for a variety of potential applications. We present an approach for the in situ preparation of graphene–zinc oxide nanocomposites through a reflux process in which either zinc acetate or zinc chloride can serve as precursors. The synthesized samples were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray analysis, ultraviolet–visible spectroscopy and thermogravimetry analysis (TGA–DSC) for structural, optical and thermal properties. It has been found that nanocomposites comprise of zinc oxide (ZnO) nanostructures deposited on graphene sheets, and the choice of zinc precursor has a deterministic influence on the morphology, structure and properties of the graphene–ZnO nanocomposites. In addition, the novel structure of zinc acetate based nanocomposite has induced improved absorption and thermal stability of the graphene/ZnO nanocomposite as compared to zinc chloride based nanocomposite and would be promising for future applications in nanotechnology.     

Synthesis and optical property of ZnO nano-/micro-rods

A series of zinc oxide (ZnO) nano-/micro-rods had been synthesized via solution-based routes. In the hydrothermal route, the obtained ZnO nano-/micro-rods had two topographies. In refluxing procedures, spindly ZnO nanorods were obtained in the presence of poly(vinyl-pyrrolidone) (PVP) and ellipsoid-like nanorods were obtained in the absence of PVP. The products were characterized using X-ray powder diffraction (XRD),scanning electron microscope (SEM), transmission electron microscope (TEM), and electron diffraction (ED)analysis. Room temperature photoluminescence (PL)spectra of the ZnO products demonstrated a weak UV emission and a broad visible emission for each of the sample. The growth mechanism of 1-D ZnO crystals was discussed briefly.     

Temperature dependent study of basal plane stacking faults in Ag:ZnO nanorods by Raman and photoluminescence spectroscopy

We report the specific features of basal plane stacking faults (BSFs) in ZnO nanorods (NRs), studied by temperature dependent photoluminescence and Raman spectroscopy. At low temperature (4 K) the intense band of emission at 3.321 eV is attributed to the presence of BSFs defects and Ag as an acceptor dopant in ZnO. This specific peak red-shifts with the temperature increase, occupying the position 3.210 eV at RT. The nature of the emission is explained as exciton recombination of the electrons, confined in the homo-heterojunction QW, with the holes, localized near the Ag atoms close to SFs. Raman spectroscopy revealed that Ag:ZnO nanorods have slightly downshifted positions of the modes 330 cm⁻¹ and 440 cm⁻¹ by 4 cm⁻¹, which we explain as due to the presence of BSFs. It was also observed, that the longitudinal optical phonon mode A^LO, which is common polar mode for ZnO, was not detected by Raman spectroscopy in the samples with high BSFs density. This feature can be explained as due to existence of the bound charge induced by the BSFs in the NRs.     

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM?) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO₂ and Ta/HfO₂ systems, thereby providing a complete explanation of the RRAM? switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.     

Resistive switching characteristics of MIM structures based on oxygen-variable ultra-thin HfO2 and fabricated at low temperature

We report the resistive switching characteristics of Metal-Insulator-Metal (MIM) structures fabricated at low temperature and having different concentrations of oxygen vacancies in the insulator. The oxygen modulation in HfO₂ is promoted by a very simple variation of standard thermal Atomic-Layer Deposition (ALD), so that different exposure times to H₂O during each half-cycle of the hafnium oxide deposition are used (being Tetrakis Dimethylamino Hafnium–TDMAH the other precursor). We show the correlation of the stoichiometry with the forming voltage, conduction mechanisms and resistance windows of memory devices. All structures present a bipolar operation mode in which the resistive switching mechanism is related to the migration of oxygen vacancies inside the dielectric. These MIM devices have a simple structure, low power consumption and they are fabricated using a very low thermal budget of only 250 °C, thus enabling their integration at the Back-End of Line (BEOL) stage of an integrated circuit in order to increase the density of memory arrays in at least one order of magnitude.     

The preparation of oxygen-deficient ZnO nanorod arrays and their enhanced field emission

Vertical and uniform zinc oxide (ZnO) nanorod arrays (NRAs) with sharp tips were fabricated on Zn substrate by a straightforward hydrothermal method without the assistance of seed layer, template or surfactant. Whereafter, the as-synthesized ZnO NRAs were successfully doped with oxygen vacancies by sodium borohydride (NaBH₄) solution reduction, aiming to generate donor energy levels below the conduction band. More importantly, the doped concentration of oxygen vacancies could be effectively controlled by adjusting the reduction temperature, and we have ultimately achieved the purpose of controllable tailoring the energy band structure of ZnO NRAs. As with design, the oxygen-deficient ZnO NRAs present a lower turn-on field of 0.67 V/μm, higher field enhancement factor of 64601 and better field emission stability. Such excellent FE performance of the as-prepared emitter should originate from the optimization of geometry, the efficient electron transport, as well as the decreased work function.