Photoconduction and polarization effects in a heat-treated Au/Pb2CrO5/SnO2 film device

2 CrO₅/SnO₂ sandwich-structured film device (1.39 μm thick) was measured at room temperature as a function of frequency in the range 5 Hz-5 kHz, both in the dark and under illumination with visible light of different intensities. The Pb₂CrO₅ film was prepared at a substrate temperature of 200 °C and annealed at a temperature of 460 °C, and has been characterized to be polycrystalline-rich in sizable micro-grains. The experimental frequency-dependence of the total ac-impedance was found to be adequately described over the entire frequency range used by a proposed equivalent RC-circuit model that took into consideration the contributions of the bulk, grain-boundaries, and electrode–Pb₂CrO₅ interface to the device’s ac behavior. Bulk conduction within crystalline grains due to the semiconductivity of Pb₂CrO₅ and space-charge effects in the highly resistive grain-boundary regions have been found to dominate the device’s ac-behavior at frequencies higher than 100 Hz and were strongly dependent on light intensity. The light-intensity behavior of the circuit-parameters associated with grain-boundary effects can be understood by the use of grain-boundary trapping models. The effect of interfacial space-charge polarization in the region near the electrode–Pb₂CrO₅ junction has been noted to be most significant at the low-frequency side and was not highly affected by illumination of the device. Received: 13 February 1997/Accepted: 30 July 1997     

Pb2CrO5 photovoltaic device for the detecting light-beam position

A light position detector operating through a photovoltaic effect of a Pb₂CrO₅ ceramic disk with a pair of Au planar electrodes is investigated. A fabrication technique and the basic characteristics of the photovoltaic device for position detection are described. A peak photovoltage is obtained around the edge of the electrode for an incident light beam. The incident light beam shape and the electrode pattern are important factors for obtaining a linear relation between the light-beam position and the output signal from the device. A device fabricated for detecting one-dimensional light position has a high position resolution of 0.5 m and a good linearity of ±2 m or less. A two-dimensional device can be fabricated in the same way as the one-dimensional device, except for the electrode pattern. A method for two-dimensional light position detection using a Pb₂CrO₅ photovoltaic cell is demonstrated for a green LED as a light source.     

The photoconductive effect in Pb2CrO5 thin films prepared by an electron-beam evaporation technique

Photoconductivity in Pb₂CrO₅ thin film prepared by an electron-beam evaporation technique is described. Crystallographically, three kinds of thin films are fabricated which depend on substrate temperature. A sample showing a similar x-ray diffraction profile to the evaporation source material gives the highest photoconductive response. Light illumination from the glass substrate onto the sample improves photoconductivity. A pair of interdigital electrodes is more effective than a pair of planar electrodes on the photoconductive measurement. A band gap energy level of Pb₂CrO₅ thin film is around 2.2–2.4 eV as a result of the spectral photoconductive response.     

Rectifying resistance switching behaviors of SnO2 microsphere films modulated by top electrodes

Based on the bipolar resistive switching (RS) characteristics of SnO₂ films, we have fabricated a new prototypical device with sandwiched structure of Metal/SnO₂/fluorine-doped tin oxide (FTO). The SnO₂ microspheres film was grown on FTO glass by template-free hydrothermal synthesis, which was evaporated with various commonly used electrodes such as aluminium (Al), silver (Ag), and gold (Au), respectively. Typical self-rectifying resistance switching behaviors were observed for the RS devices with Al and Au electrodes. However, no obvious rectifying resistance switching behavior was observed for the RS device with Ag electrode. Above results were interpreted by considering the different interface barriers between SnO₂ and top metal electrodes. Our current studies pave the ways for modulating the self-rectifying resistance switching properties of resistive memory devices by choosing suitable metal electrodes.     

Direct conversion of a metal organic compound to epitaxial Sb-doped SnO2 film on a (0 0 1) TiO2 substrate using a KrF laser, and its resulting electrical properties

Epitaxial Sb-doped SnO₂ (0 0 1) thin film on a TiO₂ (0 0 1) substrate was successfully prepared by laser-assisted metal organic deposition at room temperature. The effects of the precursor thin film and laser fluence on the resistivity, carrier concentration, and mobility of the Sb-doped SnO₂ film were investigated. The resistivity of the Sb-doped SnO₂ film prepared by direct irradiation to metal organic film is one order of magnitude lower than that of film prepared by irradiation to amorphous Sb-doped SnO₂ film. From an analysis of Hall measurements, the difference between the resistivity of the Sb-doped SnO₂ film prepared using the metal organic precursor film and that of amorphous precursor film appears to be caused by the mobility. Direct conversion of the metal organic compound by excimer laser irradiation was found to be effective for preparing epitaxial Sb-doped SnO₂ film with low resistivity.     

Characterization of nanocrystalline SnO2 thin film fabricated by electrodeposition method for dye-sensitized solar cell application

Nanocrystalline SnO₂ thin film was prepared by cathodic electrodeposition-anodic oxidation and its structure was characterized by X-ray diffraction, SEM, UV-visible absorption and nitrogen adsorption-desorption by BET method. The obtained film has a surface area of 137.9 m²/g with grain sized of 24 nm. Thus the prepared SnO₂ thin film can be applied as an electrode in dye-sensitized solar cell. The SnO₂ electrode was successfully sensitized by Erythrosin dye and photoelectrochemical measurements indicate that the cell present short-circuit photocurrent (J_sc) of 760 μA/cm², fill factor (FF = 0.4), photovoltage (V_oc = 0.21 V) and overall conversion efficiency (η) of 0.06% under direct sun light illumination. The relatively low fill factor and photovoltage are attributed to the reduction of triodiode by conduction band electrons and intrinsic properties of SnO₂.     

Photovoltaic effect in thin film of Pb(Zr,Ti)O3 evaporated on stainless steel

A dc photovoltaic effect has been observed in thin films of Pb(Zr, Ti)O₃ evaporated on stainless steel, in the absence of applied field. The spectral photovoltaic response of the film has a peak sensitivity around 4000 Å. The photovoltage exists only below the Curie temperature. The film device becomes semiconductive under illumination.     

Spin domain mapping of a CrO2 thin film using spin-polarized current microscopy

We investigate spin domain mapping of a CrO₂ thin film using spin-polarized current microscopy at room temperature, where conductive atomic force microscopy (CAFM) with a CrO₂-coated tip is used. The nanoscale spin domains of the CrO₂ thin film were crosschecked by magnetic force microscopy (MFM). Notably, the CAFM exhibits the spin domains of the CrO₂ thin film with higher resolution than the MFM, which may result from a local point contact between the nanoscale CrO₂-coated tip and surface of the CrO₂ thin film.     

Photoconductivity in a Pb2CrO5 ceramic disk with surface electrodes

The photoconductive behavior of a Pb₂CrO₅ ceramic disk with one pair of electrodes on one surface has been studied at room temperature. When illuminated with light in the visible region, the device exhibits photocurrent which is much larger than the dark current. The current-voltage characteristics and the variation of photocurrent with (i) applied field, (ii) light intensity, and (iii) wavelength are presented with transient behavior.     

Interactions at tetraphenyl-porphyrin/InP interfaces observed by surface photovoltage spectroscopy

Illumination induced charge separation processes at tetraphenyl-porphyrin (H₂TPP)/InP interfaces are characterized. The results indicate that upon illumination an electronic transition takes place between the organic highest occupied molecular orbital (HOMO) and the InP conduction band. A 70 meV blue shift in the characteristic modulated surface photovoltage spectrum of a 50 nm H₂TPP film was observed when n-InP substrate was used. Such blue shift was not observed when Au, SnO₂:F or p-InP substrates were used. The shift may suggest a presence of an ordered interfacial sub-layer in an almost lying orientation, due to chemical or physical interactions at the interface. The results indicate that the Fermi level is unpinned at the interface. An exciton diffusion barrier in close proximity to the H₂TPP/n-InP interface has been demonstrated.     

Crystal-orientation dependence of photoconductivity in Pb2CrO5 thin film

Crystal-orientation dependence in photoconductive Pb₂CrO₅ thin film devices, prepared by an electron-beam evaporation deposition technique, with interdigital electrodes is described as functions of film thicknesses, temperature, and electrode widths. There are three types of orientations of (020), (310), and (200). The photoconductive sensitivity of (200) is smaller than the other orientations because of the lower carrier mobility. The observed activation energy for each oriented sample is 0.69 or 0.26 eV for the dark or the photoillumination state. The film thickness influences strongly the photoconductive sensitivity and spectrum. At the shorter wavelengths the difference of the photoconductive current due to the illumination direction is obvious. The ratio of the finger width to the gap of interdigital electrodes on the thin film affects the photoconductive current level without changing the photoconductive spectra.     

Preparation of Pb2CrO5 thin films by an electron-beam evaporation technique

Pb₂CrO₅ thin films have been prepared by an electron-beam evaporation deposition technique on glass substrates using ceramic disks. The thin film fabrication conditions are studied by x-ray diffraction, replica electron micrography and scanning electron microscopy as parameters of substrate temperature, annealing temperature and annealing time. As-deposited Pb₂CrO₅ thin films in the amorphous state are crystallized by heat treatment. Annealed thin films are structurally classified into three types according to the preferred orientations which depend on substrate temperatures (T _s:

1. (i)

   (020) atT _s=room temperature

2. (ii)

   mainly (310) atT _s=100 ‡C, and

3. (iii)

   (200) atT _s=350 ‡C.

The substrate temperature contributes to film orientations; annealing temperature and time-enhance film crystallinity. The annealing temperature is fixed between 400 and 500 ‡C in making appropriate Pb₂CrO₅ thin films. These thin films, ranging between 0.3 and 2.0 Μm in thickness, are prepared at a deposition rate of 1500 å/m.

     

Optical parameters in SnO2 nanocrystalline textured films grown on p-InSb (111) substrates

Spectroscopic ellipsometry and photoluminescence (PL) measurements on SnO₂ nanocrystalline textured films grown on p-InSb (111) substrates by using radio-frequency magnetron sputtering at low temperature were carried out to investigate the dependence of the optical parameters on the SnO₂ thin film thickness. As the SnO₂ film thickness increases, while the energy gap of the SnO₂ film decreases, its refractive index increases. The PL spectra show that the broad peaks corresponding to the donor-acceptor pair transitions are dominant and that the peak positions change with the SnO₂ film thickness. These results can help improve understanding for the application of SnO₂ nanocrystalline thin films grown on p-InSb (111) substrates in potential optoelectronic devices based on InSb substrates.     

ZnO/Si异质结的光电转换特性研究

利用直流反应溅射方法在p型Si衬底上生长掺Al的n型ZnO薄膜,测量了由n型ZnO薄膜和p型Si衬底组成的异质结在黑暗和光照条件下的I-V特性,结果表明该异质结具有优良的整流特性,而且在光照条件下的反向电流迅速增大并很快趋于饱和.通过测量ZnO薄膜的光电流和异质结的光电压的光谱响应,初步分析了异质结的光电转换机理.测量结果显示,在入射光波长为380nm时光电流强度明显下降,反映出光电流与ZnO薄膜禁带宽度的密切关系;同时还发现,在与ZnO禁带宽度相对应的波长前后所产生的光生电压方向相反.推测这一现象与异质结的能带结构密切相关. 关键词： ZnO薄膜 异质结 光电转换 光谱响应     

Low-temperature CO gas sensors based on Au/SnO2 thick film

A study on the low-temperature CO gas sensors based on Au/SnO₂ thick film was reported. Au/SnO₂ powders were prepared by a deposition-precipitation method. Thick films were fabricated from Au/SnO₂ powders. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses were carried out for investigation of morphology and crystalline structure. Au/SnO₂ thick film sensors exhibited high sensitivity to CO gas at relatively low operating temperature (83-210 °C). We also reported the effect of the calcination temperature of Au/SnO₂ on the CO gas sensing behavior. The optimal calcination temperature of Au/SnO₂ was 300 °C.     

Photoinduced charge injection in the metal/organic interface studied by transient photovoltage measurements with bias

Transient photovoltage of ITO/organic/Al cells is studied under different bias polarities and voltages.It is found that for an ITO/NPB/Al cell,light incidence on the Al side induces more bias-dependent transient photovoltage variation when the photovoltage is positive than when it is negative.However,for an ITO/C60 /Al cell,the variation characteristics of transient photovoltage is reversed.These results support the previously proposed mechanism that Al could inject charges into the organic layer upon photon excitation,indicating that the absorption of electrode can also contribute to photovoltaic effect.     

Field emission from patterned SnO2 nanostructures

A simple and reliable method has been developed for synthesizing finely patterned tin dioxide (SnO₂) nanostructure arrays on silicon substrates. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO₂ nanostructures arrays, a unit area is of ∼500 μm × 200 μm, were synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized SnO₂ nanostructures were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanism of formation of SnO₂ nanostructures was also discussed. The measurement of field emission (FE) revealed that the as-synthesized SnO₂ nanorods, nanowires and nanoparticles arrays have a lower turn-on field of 2.6, 3.2 and 3.9 V/μm, respectively, at the current density of 0.1 μA/cm². This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays, sensor arrays and so on.     

Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction

In this article, we report the growth of zinc‐tin nitride (ZnSnN₂) thin films as a potential absorber for photovoltaic applications by fabricating a heterojunction of n‐ZnSnN₂/p‐SnO. The performance of the heterojunction has been monitored through selective deposition of top electrode with different materials (Ni/Au or Al). The electron‐transfer process from the ZnSnN₂ layer to the cathode is facilitated by selecting metal electrode with relatively low work function, which also boosts up the electron injection or/and extraction. The diode exhibits a good J–V response in the dark with a rectification ratio of 3 × 10³ at 1.0 V and an ideality factor of 4.2 in particular with Al as the top electrode. Under illumination, the heterostructure solar cell demonstrates a power conversion efficiency of ≈0.37% with an open circuit voltage of 0.25 V and a short circuit current density of 4.16 mA cm^?2. The prime strategies, on how to improve solar cell efficiency concerning band offsets and band alignment engineering are also discussed.     

Electronic structure of CeAl2 thin films studied by X-ray absorption spectroscopy

We report X-ray absorption near edge structures (XANES) study of CeAl₂ thin films of various thicknesses, 40-120 nm, at Al K- and Ce L₃-edges. The threshold of the absorption features at the Al K-edge shifts to the higher photon energy side as film thickness decreases, implying a decreased in Al p-orbital charges. On the other hand, from Ce L₃-edge spectra, we observed a decrease in the 5d4f occupancy as the surface-to-bulk ratio increases. The valence of Ce in these thin films, as revealed by the Ce L₃-edge spectral results, is mainly trivalent. From a more detailed analysis we found a small amount of Ce⁴⁺ contribution, which increases with decreasing film thickness. Our results indicate that the surface-to-bulk ratio is the key factor which affects the electronic structure of CeAl₂ thin films. The above observations also suggest that charge transfer from Al to Ce is associated with the decrease of the film thickness.     

Structures and photovoltaic properties of copper oxides/fullerene solar cells

Copper oxide (CuO_x) thin films were produced by spin-coating and electrodeposition methods, and their microstructures and photovoltaic properties were investigated. Thin film solar cells based on the Cu₂O/C₆₀ and CuO/C₆₀ heterojunction or bulk heterojunction structures were fabricated on F-doped or In-doped SnO₂, which showed photovoltaic activity under air mass 1.5 simulated sunlight conditions. Microstructures of the CuO_x thin films were examined by X-ray diffraction and transmission electron microscopy, which indicated the presence of Cu₂O and CuO nanoparticles. The energy levels of the present solar cells were also discussed.