Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC

Optical properties of ZnO nanorods (NRs) grown by vapour-liquid-solid (VLS) technique on 4H-p-SiC substrates were probed by cathodoluminescence (CL) measurements at room temperature and at 5 K complemented with electroluminescence. At room temperature the CL spectra for defect related emission intensity was enhanced with the electron beam penetration depth. We observed a variation in defect related green emission along the nanorod axis. This indicates a relatively poor structural quality near the interface between ZnO NRs and p-SiC substrate. We associate the green emission with oxygen vacancies. Analysis of the low-temperature (5 K) emission spectra in the UV region suggests that the synthesized nanorods contain shallow donors and acceptors.     

Comparative PL study of individual ZnO nanorods,grown by APMOCVD and CBD techniques

The photoluminescence properties of individual ZnO nanorods, grown by atmospheric pressure metalorganic chemical vapor deposition (APMOCV) and chemical bath deposition (CBD) are investigated by means of temperature dependent micro-PL. It was found that the low temperature PL spectra are driven by neutral donor bound exciton emission D⁰X, peaked at 3.359 and 3.363 eV for APMOCVD and CBD ZnO nanorods, respectively. The temperature increase causes a red energy shift of the peaks and enhancement of the free excitonic emission (FX). The FX was found to dominate after 150 K for both samples. It was observed that while APMOCVD ZnO nanorods possess a constant low signal of visible deep level emission with temperature, the ZnO nanorods grown by CBD revealed the thermal activation of deep level emission (DLE) after 130 K. The resulting room temperature DLE was a wide band located at 420–550 nm. The PL properties of individual ZnO nanorods can be of importance for their forthcoming application in future optoelectronics and photonics.     

PLD方法制备的ZnO纳米柱结构及光学特性

采用无催化脉冲激光沉积(PLD)方法,在InP(100)衬底上生长纳米ZnO柱状结构。采用扫描电子显微镜(SEM)、X射线衍射(XRD)以及光致发光(PL)谱等表征手段对ZnO纳米柱的形貌、晶体结构和光学特性进行了观察。SEM图像观察到ZnO纳米柱状结构具有一定的取向性;XRD测试在2θ=34.10°处观测到强的ZnO(002)衍射峰,证实ZnO纳米柱具有较好的c轴择优取向;室温PL谱在379nm处观察到了强的自由激子发射峰(半峰全宽为19nm),未探测到深能级跃迁发射峰,表明生长的纳米ZnO结构具有很高的光学质量。     

High-quality ZnO nanorods grown on graphite substrates by chemical solution method

ZnO nanorods (NRs) were grown perpendicularly on graphite substrates using low-temperature wet chemical bath deposition (CBD) with sputtered ZnO film as seed layer. The individual ZnO NRs exhibit single-crystalline feature with well defined hexagonal prism shape and smooth side facets. The high optical qualities of ZnO NRs on graphite substrates were demonstrated by the dominant near-band edge emission and nearly undetectable deep level emissions in room-temperature photoluminescence spectra. The extremely low average reflectance of 0.45 % was obtained for the ZnO NRs/graphite structure in the spectra range from 200 to 1100 nm, indicating that the reported ZnO NRs/graphite structures have significant opportunity for potential application in high-performance photovoltaic devices. Considering the excellent material characteristics of ZnO NRs and the versatile and fascinating features of graphite substrates, the achievements make it possible for the development of high-performance ZnO-based nano-devices even in transferable, flexible, or stretchable forms.     

Influence of deep defects on electrical properties of Ni/4H-SiC Schottky diode

In this paper, we investigate the influence of deep level defects on the electrical properties of Ni/4H-SiC Schottky diodes by analyzing device current-voltage(I-V) characteristics and deep-level transient spectra(DLTS). Two Schottky barrier heights(SBHs) with different temperature dependences are found in Ni/4 H-SiC Schottky diode above room temperature. DLTS measurements further reveal that two kinds of defects Z_(1/2) and Ti(c)~a are located near the interface between Ni and SiC with the energy levels of E_C-0.67 eV and E_C-0.16 eV respectively. The latter one as the ionized titanium acceptor residing at cubic Si lattice site is thought to be responsible for the low SBH in the localized region of the diode, and therefore inducing the high reverse leakage current of the diode. The experimental results indicate that the Ti(c)~a defect has a strong influence on the electrical and thermal properties of the 4 H-SiC Schottky diode.     

Effects of electrolyte on ZnO nanostructures synthesized by galvanostatic electrochemical deposition and their UV sensing properties

In this study, ZnO nanorods (NRs) and nanocombs (NCs) are synthesized by simple galvanostatic electrochemical deposition technique, without prepared any ZnO seed-layer or catalyst. The effect of the different morphologies on the UV sensing characteristics has been studied under ambient conditions. The photoluminescence (PL) spectra and time-dependent photoresponse of the ZnO nanostructures exhibited good optical properties. At room temperature, NCs showed superior response with 9% change of its resistance, few seconds response time and fully recovery. Inversely, in high temperature ZnO NRs indicated better response than NCs with the variation of 25% of its resistance. The dependence photoresponse on temperature demonstrated clearly how surface-defects affect on UV response of ZnO nanostructures. Our approach is to provide a simple and cost-effective way to fabricate UV detectors.     

Enhancement of the ultraviolet emission of ZnO nanorods by terphenyl liquid-crystalline ligands modification

We have developed a novel method to modifying the surface of ZnO nanorods (ZnO NRs) using p-hexoxyterphenylol (HTph-OH) as liquid crystal ligands. The structure and morphology of the modified ZnO NRs were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and atomic force microscopy (AFM). AFM measurement showed that the dispersion of ZnO NRs could be dramatically improved by the surface modification of HTph-OH and further annealing treatment at its liquid crystal state temperature (150 °C). The remarkable decrease of the annealed composite film roughness is because the HTph-OH chains self-organize into more ordered structure induced by mesogens after annealing treatment, which may push the ZnO NRs to form oriented nano-dispersing structure. The optical properties of the modified ZnO NRs were investigated by UV-vis absorption spectroscopy and photoluminescence spectroscopy (PL). Markedly enhanced band-edge ultraviolet photoluminescence and significantly reduced defect-related emission were observed. We attribute this observation to the nearly perfect surface passivation of the ZnO NRs by the HTph-OH molecules. Meanwhile, UV emission of modified ZnO NRs could be further enhanced by increasing the concentration of HTph-OH and annealing treatment at its liquid crystal state temperature.     

Controllable growth of well-aligned ZnO nanorod arrays by low-temperature wet chemical bath deposition method

Well-aligned ZnO nanorod arrays were synthesized by low-temperature wet chemical bath deposition (CBD) method on Si substrate under different conditions. Results illustrated that dense ZnO nanorods with hexagonal wurtzite structure were vertically well-aligned and uniformly distributed on the substrate. The effects of precursor concentration, growth temperature and time on nanorods morphology were investigated systematically. The mechanism for the effect of preparation parameters was elucidated based on the chemical process of CBD and basic nucleation theory. It is demonstrated that the controllable growth of well-aligned ZnO nanorods can be realized by readily adjusting the preparation parameters. Strong near-band edge ultraviolet (UV) emission were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO nanorod arrays could be achieved via this easy process chemical approach at low temperature.     

Defect-dominated optical emission and enhanced ultraviolet photoconductivity properties of ZnO nanorods synthesized by simple and catalyst-free approach

We report on the defect-dominated light emission and ultraviolet (UV) photoconductivity characteristics of ZnO nanorods (NRs) fabricated using a facile, cost-effective, and catalyst-free thermal decomposition route under varying reaction temperatures. The morphological and structural studies reveal the formation of homogeneous quality nanorods in large scale at the highest reaction temperature of 600 ^°C. The luminescence feature of the nanorods is dominated by the defect related emission over the typical band edge emission. The variation of band-edge and native defect-related emission response of the samples has been correlated to the morphology and microstructure. In photoconductivity studies, the I–V characteristics of the ZnO NRs prepared at different reaction temperatures in dark and under UV illumination (λ=365 nm) follow the power law, i.e., IαV ^r . An enhanced ultraviolet photodetection has been observed in the nanorods fabricated at the highest reaction temperature of 600 ^°C. The sample prepared at highest reaction temperature of 600 ^°C exhibits UV photosensitivity value (photo-to-dark current ratio) of around 1.18×10³, which is much higher in magnitude compared to that of the samples prepared at lower reaction temperatures. The enhanced photoconductivity may be assigned to the development of uniformity and homogeneity of the nanorods. Further development of such ZnO nanostructures can form the basis of promising prototype luminescent and UV photodetecting devices.     

Investigation of electrical and ammonia sensing characteristics of Schottky barrier diode based on a single ultra-long ZnO nanorod

Novel ultra-long ZnO nanorods, with lengths about 0.5-1.5 mm and diameters ranging from 100 to 1000 nm, in mass production have been synthesized via the vapor-phase transport method with CuO catalyst at 900 °C. Rectifying Schottky barrier diodes have been fabricated by aligning a single ultra-long ZnO nanorod across paired Ag electrodes. The resulting current-voltage (I-V) characteristics of the SBD exhibit a clear rectifying behavior. The ideality factor of the diode is about 4.6, and the threshold voltage is about 0.54 V at room temperature (300 K). At the same time the detailed I-V characteristics have been investigated in the temperature range 423-523 K. In addition, after exposure of the Schottky diodes to NH₃, the forward and reverse currents increase rapidly, indicating a high sensitivity to NH₃ gas.     

In掺杂的ZnO纳米棒的共振Raman光谱研究

本文报道In掺杂ZnO纳米棒的成功制备和对其结构以及光荧光性能的详尽研究。在室温条件下ZnO的共振拉曼谱线容易受到很强的荧光干扰, 甚至导致共振拉曼谱线完全被湮没。微量In掺杂入ZnO纳米棒中, 调控紫外发光峰由378 nm(纯ZnO)红移至397 nm; 另外, 在制备过程中引入过量的氧, 在样品中产生大量缺陷, 降低了ZnO的紫外近带边发光峰强度。这两方面导致在室温下可清楚的观察到In掺杂ZnO纳米棒的6阶LO拉曼峰。     

Studies of 6H–SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H–SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H–SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p⁺n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H–SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H–SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of poly-silicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to γ rays.     

Studies of luminescence properties of ZnO and ZnO:Zn nanorods prepared by solution growth technique

Pyramidal ZnO nanorods with hexagonal structure having c-axis preferred orientation are grown over large area silica substrates by a simple aqueous solution growth technique. The as-grown nanorods were studied using XRD, SEM and UV-vis photoluminescence (PL) spectroscopy for their structural, morphological and optical properties, respectively. Further, the samples have also been annealed under different atmospheric conditions (air, O₂, N₂ and Zn) to study the defect formation in nanorods. The PL spectra of the as-grown nanorods show narrow-band excitonic emission at 3.03 eV and a broad-band deep-level emission (DLE) related to the defect centers at 2.24 eV. After some mild air annealing at 200 °C, fine structures with peaks having energy separation of ∼100 meV were observed in the DLE band and the same have been attributed to the longitudinal optical (LO) phonon-assisted transitions. However, the annealing of the samples under mild reducing atmospheres of N₂ or zinc at 550 °C resulted in significant modifications in the DLE band wherein high intensity green emission with two closely spaced peaks with maxima at 2.5 and 2.7 eV were observed which have been attributed to the V_O and Zn_i defect centers, respectively. The V-I characteristic of the ZnO:Zn nanorods shows enhancement in n-type conductivity compared to other samples. The studies thus suggest that the green emitting ZnO:Zn nanorods can be used as low voltage field emission display (FED) phosphors with nanometer scale resolution.     

磁控溅射制备ZnO薄膜的受激发射特性的研究

用射频磁控反应溅射法在二氧化硅衬底上制备ZnO薄膜。得到了在不同温度下ZnO薄膜的吸收与光致发光。观测到了纵光学波 (LO)声子吸收峰与自由激子吸收峰 ;室温 (30 0K)下 ,PL谱中仅有自由激子发光峰。这些结果证实了ZnO薄膜具有较高的质量。探讨了变温ZnO薄膜的发光特性。研究了ZnO薄膜的受激发射特性。     

Growth and optical study of ZnO nanorods

ZnO nanorods were grown by a simple near room temperature, chemical solution method on ZnO-seeded silicon substrates. Study of the ZnO nanorods over different growth times by electron microscopy methods revealed that the resulting ZnO nanorods were single crystalline with a highly preferential growth perpendicular to the substrate and a very good c-axis alignment. The size of the nanorods increased with increasing growth time. The growth mechanism is briefly discussed. Post-annealing in oxygen slightly improved the surface roughness of the ZnO nanorods. Photoluminescence experiments at 1.6 K revealed a major emission peak of the nanorods at around 3.36 eV which is attributed to the band edge transition of ZnO, while defect-related emission is relatively weak.     

微纳跨尺度ZnO结构的紫外发射机理研究

利用气相输运的方法在Si(100)衬底上生长了ZnO的微纳跨尺度结构.扫描电镜照片可以明显地看到样品表面为椎顶六角微米柱-纳米棒的复合结构.样品在室温下的光致发光谱出现了很强的紫外发射峰,没有观察到与杂质或缺陷相关的深能级发射,表明样品有很好的光学质量.通过详细的研究样品的紫外发射谱与温度(83—307K)的依赖关系,发现在室温下样品的近带边发射包含两个部分,分别与自由激子发射和自由载流子到施主(受主)的跃迁(FB跃迁)相关,这个施主(受主)束缚态的离化能为124.6meV. 关键词： ZnO微纳跨尺度结构 光致发光谱 自由载流子到施主(受主)的跃迁 自由激子发射     

Characterization and optical property of ZnO nano-, submicro- and microrods synthesized by hydrothermal method on a large-scale

In the present paper, well-dispersed ZnO nano-, submicro- and microrods with hexagonal structure were synthesized by a simple low temperature hydrothermal process from zinc nitrate hexahydrate without using any additional surfactant, organic solvent or catalytic agent. The phase and structural analysis were carried out by X-ray diffraction (XRD), the morphological analysis was carried out by field emission scanning electron microscopy (FESEM) and the optical property was characterized by room-temperature photoluminescence (PL) spectroscopy. The results revealed the high crystal quality of ZnO powder with hexagonal (wurtzite-type) crystal structure and the formation of well-dispersed ZnO nano-, submicro- and microrods with diameters of about 50, 200 and 500 nm, and lengths of 300 nm, 1 μm and 2 μm, respectively, on a large-scale just using the different temperatures. Room-temperature PL spectrum from the ZnO nanorods reveals a strong UV emission peak at about 360 nm and no green emission band at ∼530 nm. The strong UV photoluminescence indicates the good crystallization quality of the ZnO nanorods. Room-temperature PL spectra from the ZnO submicro- and microrods reveal a weak UV emission peak at ∼400 nm and a very strong visible green emission at 530 nm, that is ascribed to the transition between V_oZn_i and valence band.     

Observation of quantum confinement in ZnO nanorods fabricated using a two-temperature growth method

We observed a quantum confinement effect in vertically well-aligned ultrafine ZnO nanorods using polarized excitation photoluminescence measurements. Room-temperature and low-temperature photoluminescence spectra revealed that free excitons were confined in the nanorods. The magnitude of the energy shift due to the quantum confinement in the ultrafine ZnO nanorods was 6 meV at room temperature, which corresponded to the luminescence from ZnO nanorods 12.8 nm in diameter. The diameter estimated from the spectra was comparable to the value measured from SEM images.     

Effects of high-dose Ge ion implantation and post-implantation annealing on ZnO thin films

This paper reports that ion implantation to a dose of 1×10¹⁷ ions/cm² was performed on c-axis-orientated ZnO thin films deposited on (0001) sapphire substrates by the sol-gel technique. After ion implantation, the as-implanted ZnO films were annealed in argon ambient at different temperatures from 600-900℃. The effects of ion implantation and post-implantation annealing on the structural and optical properties of the ZnO films were investigated by x-ray diffraction (XRD), photoluminescence (PL). It was found that the intensities of (002) peak and near band edge (NBE) exitonic ultraviolet emission increased with increasing annealing temperature from 600-900℃. The defect related deep level emission (DLE) firstly increased with increasing annealing temperature from 600- 750℃, and then decreased quickly with increasing annealing temperature. The recovery of the intensities of NBE and DLE occurs at \sim 850℃ and \sim 750℃ respectively. The relative PL intensity ratio of NBE to DLE showed that the quality of ZnO films increased continuously with increasing annealing temperature from 600 - 900℃.     

CL study of yellow emission in ZnO nanostructures annealed in Ar and O2 atmospheres

The yellow emission of thermally treated undoped and In doped ZnO nanostructures was studied by the cathodoluminescence (CL) technique. CL spectra acquired at room temperature of the as-grown samples revealed two emissions at about 3.2 eV and 2.13 eV, corresponding to the near band edge and defect related emissions, respectively. On annealing the samples at 600  ^°C in Ar and O₂ atmospheres, the defect emission suffers a red shift, irrespective of the annealing atmosphere. This red shift is explained in terms of variations in the relative intensities of the two component bands centered at about 2.24 eV and 1.77 eV, which were clearly resolved in the CL spectra acquired at low temperature of the annealed samples. A decrease of the relative intensity of the yellow emission (2.24 eV) was observed for all thermally annealed samples. The annealing of zinc interstitial point defects is proposed as a possible mechanism to explain this intensity decrease.