反应源温度对ZnO纳米线阵列定向性及发光性能的影响

以Au薄膜为催化剂、ZnO与碳混合粉末为反应源,采用碳热还原法在单晶Si衬底上制备了ZnO纳米线阵列.通过扫描电子显微镜( SEM)、X射线衍射仪(XRD)、荧光分光光度计对样品的表征,研究了反应源温度对ZnO纳米线阵列的定向性和光致发光性能的影响.样品在源温度920℃条件下沿(002)方向择优生长,定向性最好,温度过低不利于ZnO纳米线阵列密集生长,而温度过高导致Zn原子二次蒸发,因而也不利于纳米线阵列的定向和择优生长;样品在源温度880℃有最强的近紫外带边发射,表明温度过高和过低都不利于ZnO晶体结构的优化;由于ZnO纳米线在缺氧氛围下生长,氧空位是缺陷存在的主要形式,因此所有样品都有较强的绿光发射.温度升高导致纳米线生长速度提高而增加了氧空位缺陷数量,从而使样品绿峰强度增强并在源温度920℃时达最大值,但温度的进一步升高可导致ZnO纳米线表面Zn元素的蒸发而降低氧空位缺陷的数量,从而抑制绿峰强度.     

Effects of growth atmosphere and homo-buffer layer on properties of ZnO films prepared on Si(1 1 1) by PLD

ZnO films were synthesized on Si(1 1 1) substrates by pulsed laser deposition (PLD) under four different growth conditions. The structural and optical properties of the samples were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and photoluminescence (PL) measurement. It is found that when ZnO film is directly prepared on Si, oxygen atmosphere can significantly enhance the near-band-edge (NBE) emission and decrease the deep-level (DL) emission, but cause a polycrystalline film. By introducing a homo-buffer layer fabricated at 500 °C in vacuum, epitaxial ZnO film with three-dimensional (3D) growth mode is achieved instead of the polycrystalline film. In particular, the epitaxial film with the buffer layer shows more intensive NBE emission and narrower full-width at half maximum (FWHM) of 98 meV than the film without the buffer layer. The experimental results suggest that both oxygen atmosphere and buffer layer are quite efficient during PLD to grow high-quality ZnO/Si heteroepitaxial films suitable for applications in optoelectronic devices.     

Vertical nanowire array-based light emitting diodes

Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.     

Optical characterization of ZnO thin films deposited by Sol-gel method

In this paper, ZnO thin film is deposited on Pt/TiO₂/SiO₂/Si substrate using the sol-gel method and the effect of annealing temperature on the structural morphology and optical properties of ZnO thin films is investigated. The ZnO thin films are crystallized by the heat treatment at over 400°C. The ZnO thin film annealed at 600°C exhibits the greatest c-axis orientation and the Full-Width-Half-Maximum (FWHM) of X-ray peak is 0.4360°. A dense ZnO thin film is deposited by the growth of uniform grains with the increase of annealing temperature but when the annealing temperature increases to 700°C, the surface morphology of ZnO thin film becomes worse by the aggregation of ZnO particles. In the results of surface morphology of ZnO thin film using atomic force microscope (AFM), the surface roughness of ZnO thin film annealed at 600°C is smallest, that is, approximately 1.048 nm. For the PL characteristics of ZnO thin film, it is observed that ZnO thin film annealed at 600°C exhibits the greatest UV (ultraviolet) exciton emission at approximately 378 nm, and the smallest visible emission at approximately 510 nm among ZnO thin films annealed at various temperatures. It is deduced that ZnO thin film annealed at 600°C is formed most stoichiometrically, since the visible emission at approximately 510 nm comes from either oxygen vacancies or impurities.     

ZnO:Cd薄膜光学性能的研究

本文以载玻片、石英为基底,采用控制掺杂方法,制备了ZnO:Cd薄膜,通过热处理方法获得了优良性能的ZnO:Cd薄膜。研究分析了镉浓度为2%-10%时ZnO薄膜的透射光谱。结果表明,有较强的紫外发射峰,但没有深能级发射,未出现其它的发射峰,说明ZnO:Cd薄膜在紫外线区的发射占主导地位,表明带间跃迁占主导地位。     

Ordered ZnO/AZO/PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition

An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400 °C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO₄ and H₂O₂ solutions at a potential of − 1 V (versus saturated calomel electrode) and temperatures of 65 and 80 °C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140 nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism.     

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.     

Annealing effects on the cathodoluminescence properties of individual ZnO nanowire

Annealing effects on cathodoluminescence of single zinc oxide nanowire are investigated. It is found that ultraviolet emission is gradually decreased and further disappears with temperatures in all the atmospheres. Deep-level emission is blue-shift with temperature in both argon and vacuum, compared with fresh nanowire. Oxygen vacancies are the main defects for the blue-shift in the oxygen-deficient condition such as argon and vacuum. However, deep-level emission is red-shift in oxygen atmosphere with temperature. It is thought that complex defects zinc vacancies and oxygen interstitials should be responsible for the interesting red-shift of deep-level emission.     

Effects of thermal annealing in oxygen plasma for buffer layers on properties of ZnO thin films

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.     

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.     

Catalyst-free synthesis of ZnO nanowire arrays on zinc substrate by low temperature thermal oxidation

High density ZnO nanowire arrays were successfully synthesized without catalyst by direct oxidation of zinc substrate in air below 400 °C, lower than the melting point of zinc metal. The as-grown ZnO nanowires are single crystalline with a Wurzite structure extending in <110> direction. The diameters of the ZnO nanowires range from 20 to 150 nm and the lengths from several to over ten micrometers. Room temperature photoluminescence measurements reveal an intense ultraviolet emission at 397 nm. The present work highlights the promise of the low temperature, direct oxidation process in the high-yield synthesis of high quality semiconductor nanowire arrays for nano-devices.     

Oxygen plasma power dependence on ZnO grown on porous silicon substrates by plasma-assisted molecular beam epitaxy

ZnO thin films were deposited on porous silicon by plasma-assisted molecular beam epitaxy using different radio frequency power settings. Optical emission spectrometry was applied to study the characteristics of the oxygen plasma, and the effects of the radio frequency power on the properties of the ZnO thin films were evaluated by X-ray diffraction, scanning electron microscopy, and photoluminescence. The grain sizes for radio frequency powers of 100, 200, and 300 W were 46, 48, and 62 nm, respectively. In addition, the photoluminescence intensities of the ultraviolet and the visible range increased at 300 W, because the density of the atomic oxygen transitions increased. The quality of the ZnO thin films was enhanced, but the deep-level emission peaks increased with increasing radio frequency power. The structural and optical properties of the ZnO thin films were improved at the radio frequency power of 300 W. Moreover, the optical properties of the ZnO thin films were improved with porous silicon, instead of Si.     

Luminescence and electrical properties of solution-processed ZnO thin films by adding fluorides and annealing atmosphere

To develop an efficient channel layer for thin film transistors (TFTs), understanding the defect-related luminescence and electrical property is crucial for solution-processed ZnO thin films. Film growth with the fluorides addition, especially using LiF, followed by the oxygen ambient post-annealing leads to decreased defect-related emission as well as enhanced switching property. The saturation mobility and current on/off ratio are 0.31 cm² V⁻¹ s⁻¹ and 1.04 × 10³. Consequently, we can visualize an optimized process condition and characterization method for solution-processed TFT based on the fluorine-doped ZnO film channel layer by considering the overall emission behavior.     

Preferential growth of ZnO thin films by the atomic layer deposition technique

Preferred orientation of ZnO thin films deposited by the atomic layer deposition (ALD) technique could be manipulated by deposition temperature. In this work, diethyl zinc (DEZn) and deionized water (H(2)O) were used as a zinc source and oxygen source, respectively. The results demonstrated that (10.0) dominant ZnO thin films were grown in the temperature range of 155-220?°C. The c-axis crystal growth of these films was greatly suppressed. Adhesion of anions (such as fragments of an ethyl group) on the (00.2) polar surface of the ZnO thin film was believed to be responsible for this suppression. In contrast, (00.2) dominant ZnO thin films were obtained between 220 and 300?°C. The preferred orientations of (10.0) and (00.2) of the ZnO thin films were examined by XRD texture analysis. The texture analysis results agreed well with the alignments of ZnO nanowires (NWs) which were grown from these ZnO thin films. In this case, the nanosized crystals of ZnO thin films acted as seeds for the growth of ZnO nanowires (NWs) by chemical vapor deposition (CVD) process. The highly (00.2) textured ZnO thin films deposited at high temperatures, such as 280?°C, contained polycrystals with the c?axis perpendicular to the substrate surface and provided a good template for the growth of vertically aligned ZnO NWs.     

Stable p-type conductivity and enhanced photoconductivity from nitrogen-doped annealed ZnO thin film

We report on the growth of p-type ZnO thin films with improved stability on various substrates and study the photoconductive property of the p-type ZnO films. The nitrogen doped ZnO (N:ZnO) thin films were grown on Si, quartz and alumina substrates by radio frequency magnetron sputtering followed by thermal annealing. Structural studies show that the N:ZnO films possess high crystallinity with c-axis orientation. The as-grown films possess higher lattice constants compared to the undoped films. Besides the high crystallinity, the Raman spectra show clear evidence of nitrogen incorporation in the doped ZnO lattice. A strong UV photoluminescence emission at ~ 380 nm is observed from all the N:ZnO thin films. Prior to post-deposition annealing, p-type conductivity was found to be unstable at room temperature. Post-growth annealing of N:ZnO film on Si substrate shows a relatively stable p-type ZnO with room temperature resistivity of 0.2 Ω cm, Hall mobility of 58 cm²/V s and hole concentration of 1.95 × 10¹⁷ cm^− 3. A homo-junction p-n diode fabricated on the annealed p-type ZnO layer showed rectification behavior in the current-voltage characteristics demonstrating the p-type conduction of the doped layer. Doped ZnO films (annealed) show more than two orders of magnitude enhancement in the photoconductivity as compared to that of the undoped film. The transient photoconductivity measurement with UV light illumination on the doped ZnO film shows a slow photoresponse with bi-exponential growth and bi-exponential decay behaviors. Mechanism of improved photoconductivity and slow photoresponse is discussed based on high mobility of carriers and photodesorption of oxygen molecules in the N:ZnO film, respectively.     

Relationship between ultraviolet emission and electron concentration of ZnO thin films

ZnO thin films were deposited on (0001) Al₂O₃ substrates depending on oxygen partial pressure by pulsed laser deposition. Optical properties of ZnO were investigated by photoluminescence (PL). The relationship between PL and electron concentration has been investigated. Origin of the dominant ultraviolet (UV) emission in ZnO thin film measured at room temperature was identified as a free electron-neutral-acceptor transition (eA⁰) through temperature dependence of PL measurement. The UV emission intensity at room temperature is related to variation of electron concentration because a free-electron-neutral-acceptor transition (eA⁰) as origin of UV emission at room temperature is related to impurity concentration of ZnO.     

Enhanced visible light response of ZnO porous thin film by post-annealing treatment

ZnO thin films were synthesized by a facile electrodeposition method in the aqueous solution. Porous ZnO thin films with wurtzite structure could be achieved by mean of annealing treatment. The growth mechanism of the porous ZnO thin film was discussed. The intensity of the E ₂ mode in the ZnO thin film, which represents crystalline quality of the thin film increases with the increasing of annealing temperature. Optical properties indicate that annealing temperature has strong effect on the optical band gap value and defect concentrations. Both the green and yellow emissions corresponding to respective oxygen vacancies and oxygen interstitials can be identified. The results show that ZnO annealed at 400 °C exhibits a significant photocurrent density enhancement which is about 18 times larger than that of the as-deposited ZnO thin films. The mechanism of the enhanced photoresponse for the ZnO thin film has been discussed in detail.     

Structural and optical properties of hydrothermally grown zinc oxide nanorods on polyethersulfone substrates as a function of the growth temperature and duration

Zinc oxide (ZnO) nanorods were grown on polyethersulfone substrates with a seed layer by hydrothermal synthesis. The effects of the growth temperature and duration on the structural and optical properties of the ZnO nanorods were investigated by X-ray diffraction, field emission scanning electron microscope and photoluminescence measurements. Improvement of the structural properties was confirmed when the ZnO nanorods were grown at a moderate thermal energy. Thermal energies that were too high or too low resulted in structural degradation: low thermal energies did not provide enough energy for the ZnO growth, and high thermal energies contributed to improper growth by creating an uncommon flake-like structure. Photoluminescence measurements showed that the near-band-edge emission to deep-level emission peak ratio increases with increasing growth temperature at growth duration of 5 h.     

Epitaxial growth of ZnO layers on (111) GaAs substrates by laser molecular beam epitaxy

ZnO layers were grown on (111) GaAs substrates by laser molecular epitaxy at substrate temperatures between 200 and 550 °C. X-ray diffraction analysis revealed that c-axis of ZnO epilayer with a wurtzite structure is perpendicular to the substrate surface. X-ray rocking curves and Raman spectroscopy showed that the crystal quality of ZnO epilayers depends on the substrate temperature during the growth. Strong near-band-edge emission in the UV region without any deep-level emissions was observed from the ZnO epilayers at room temperature. The results indicate that laser molecular beam epitaxy is a promising growth method for obtaining high-quality ZnO layers on (111) GaAs substrates.     

Effect of the oxygen pressure on the photoluminescence properties of ZnO thin films by PLD

ZnO thin films on Si(111) substrate were deposited by laser ablation of Zn target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. The experiments were performed at laser energy density of 31 J/cm², substrate temperature of 400 °C and various oxygen pressures (5–65 Pa). X-ray diffraction was applied to characterize the structure of the deposited ZnO films and the optical properties of the ZnO thin films were characterized by photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm. The influence of the oxygen pressure on the structural and optical properties of ZnO thin films was investigated. It was found that ZnO film with random growth grains can be obtained under the condition of oxygen pressure 5–65 Pa. It will be clearly shown that the grain size and the formation of intrinsic defects depend on the oxygen partial pressure and that high optical quality of the ZnO films is obtained under low oxygen pressure (5 Pa, 11 Pa) conditions.