Response of thin NE102A scintillator films to fission fragments

The pulse height response of NE102A plastic scintillator films to fragment ions from ²⁵²Cf spontaneous fission has been investigated as a function of fragment time-of-flight and film thickness t in the range 1–15 μm. For velocities close to the two peaks in the fragment velocity spectrum and for films of either t ? 9 μm or t ? 3 μm, an approximately linear relationship between response and fragment velocity is found. For films of thickness 3–6 μm or for fragments from symmetric or highly asymmetric fission. deviations from the linear dependence are observed. The response increases rapidly with film thickness in the range t = 3–6 μm and saturates at t ? 9 μm for both light and heavy fragments. The possible role of surface effects in these phenomena is discussed.     

Ultrafast thin film GaAs photoconductive detectors

Recent progress in the development of GaAs thin film photoconductive detectors for future optical communication systems at near-IR wavelengths is reported. The devices consist of a thin high purity epitaxial layer of GaAs grown on top of a chromium-doped semi-insulating substrate, two metal contacts and a contact layer on top of the “active” layer. Intrinsic detector response times of the order of 28 ps are found after excitation with fast laser pulses. The initial fast photocurrent decay is followed, if and only if the contacts are ohmic, by a second decay which is orders of magnitude slower. The amplitude of the slow tail decreases and finally disappears at higher excitation intensities or after the superposition of extremely faint continuous wave light. It is suggested that a small number of deep centres which are present in the active layer or close to the interface with the substrate (probably chromium) and which trap and re-emit charge carriers are responsible for the long tail. These centres are saturable. The results of studies of current gain and noise equivalent power of GaAs and a comparison with In_0.53Ga_0.47As/InP detectors are given and possible means of future improvements are indicated.     

Ultrafast thin film GaAs photoconductive detectors

The technique of reactive d.c. plasmatron sputtering with elemental targets is characterized by strong interactions of the reactive gas with the target surface and with the condensing target material. These interactions have a marked influence on the current-voltage (I-U) behaviour of the gas discharge which can be used to control the film deposition process in both the single-target mode and in cosputtering. Model calculations of I-U characteristics are carried out for single and multiple targets of silicon and titanium. On the basis of the formula derived it is possible to predict the influence of the process parameters (pressure, deposition rate, target area etc.) on the I-U curves as well as the interactions of various targets. The calculations agree qualitatively with the measured I-U characteristics. Experiments were carried out with silicon and titanium targets in Ar-O₂ mixtures.     

Zn掺杂TiO2薄膜紫外探测器及其光电性能研究

采用射频磁控溅射的方法制备Zn掺杂TiO2薄膜,用XRD、SEM和UV-Vis分别表征TiO2薄膜的晶体结构、表面形貌及其紫外-可见光吸收谱.并用此材料制备Au/TiO2/An结构MSM光电导型薄膜紫外光探测器,研究其光电特性.实验结果表明,Zn掺杂TiO2紫外探测器在250 nm、5 V偏压紫外光照下光电流约为500μA,其响应度为100 A/W,平均暗电流约为0.5μA;由于ZnO/TiO2复合薄膜之间的费米能级不同而形成的内建电场作用,减少了产生的光生电子与空穴的复合,得到较强的光电流.且其光响应的上升迟豫时间约为22 s,下降响应时间约为80 s;响应时间较长是由于广泛分布于薄膜中的缺陷而造成的.结果表明Zn掺杂TiO2可作为一种良好的紫外探测材料.     

Deformation characteristics of an organic thin film transistor

An organic thin film transistor (OTFT) on a flexible substrate with electroplated electrodes has many advantages in the fabrication of low cost sensors, e-paper, smart cards, and flexible displays. In this study, we simulated the mechanical characteristics of an OTFT with various compressive stress conditions using COMSOL. An analysis model, which was limited to channel, source, and drain, was used to investigate deformation and internal stress concentrations. The channel length is 40 microm and the OTFT structure is a top-contact structure. The OTFT was fabricated using pentacene as a semiconducting layer and electroplated Ni as a gate electrode. The deformation characteristics of the fabricated OTFT were predicted in terms of strain and internal stress.     

The operation characteristics of a thin film CdSe transducer

CdSe thin films and CdSe thin film active transducers (TFAT) have been studied under similar deposition conditions in an attempt to understand closely the dynamic anomalous behavior reported earlier. The observance of a positive and negative change in drain current on the application of a positive strain, according as the applied gate voltage is kept below or above a characteristic level, is believed to be a critical factor in the development of the TFAT as a fast sensing, miniature pressure sensor or a switching device. The work contained here reveals that the over-all response is the combined effect of three main factors: (a) the piezoelectricity of CdSe; (b) the strain dependence of the conductivity of the CdSe film alone; and (c) the presence of traps and donor states at the semiconductor-insulator interface and inside the insulator. In the inverted structure of the device discussed here, (a) and (b) are shown to act in opposition and therefore the device performance is modified according to their relative magnitudes. Piezoelectricity is shown to be the principal mechanism when the device is operated at a gate voltage V_g above the flat band voltage V_FB. The strain dependence of V_FB and its value are obtained by following a simple curve-fittinh procedure of the data relating the transconductance g_m to V_g. Three possible mechanisms for (b), namely piezoresistivity, band gap change and carrier mobility modulation based on Petritz's polycrystalline model, are discussed.     

氧化镍薄膜气体传感器性能提升

本文的目的在于优化甲醛气体传感器.微型甲醛气体传感器设计是以石英玻璃当作基材,白金(Pt)被当作微型加热器电阻来加热感测层,并以氧化镍(NiO)薄膜作为感测层.当环境内有甲醛气体存在时,NiO薄膜层上导电度会增加,因而导致感测层电阻值降低.此微传感器,膜厚为0.34μm,在300℃反应时间只需6秒,灵敏度可达13.5 kΩ/ppb,最低侦测限度可以量测到40 ppb. 而本研究中针对不同的甲醛气体浓度,分别添加金当其催化剂、玻璃基材上共溅镀氧化镍与氧化铝、并比较有无指叉电极、改变基材温度…等,以提升其氧化镍薄膜感测性能.     

Phase transformation and optical characteristics of porous germanium thin film

In this study, we proposed a method to prepare GeO₂ by treating porous Ge thin film with thermal annealing in O₂ ambient. After annealing, the morphological transformation from porous thin film to an island structure was observed. The crystallization and composition of the porous Ge thin film prepared using different annealing time in O₂ ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO₂ with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO₂, it was found that the visible photoluminescence originated from the germanium oxide. Photoluminescence measurements obtained at different temperatures exhibited a maximum integrated PL intensity at around 200 K. A possible explanation for this behavior might be the competition between radiative recombination and nonradiative hopping process.     

Influence of finite semiconductor thickness on thin film transistor characteristics

Through an exact numerical analysis of the electron charge in the conduction band as a function of the gate voltage, the influence of the finite thickness of the semiconductor channel on thin film transistor characteristics is investigated. It is shown that, in various experimentally interesting situations, sufficiently accurate approximations can be found which are much easier to calculate. However, if bulk trapping occurs in a heavily doped semiconductor, no such approximation can be found.     

Response of neutron detectors to high-energy mixed radiation fields

Radiation protection around CERN's high-energy accelerators represents a major challenge due to the presence of complex, mixed radiation fields. Behind thick shielding neutrons dominate and their energy ranges from fractions of eV to about 1 GeV. In this work the response of various portable detectors sensitive to neutrons was studied at CERN's High-Energy Reference Field Facility (CERF). The measurements were carried out with conventional rem counters, which usually cover neutron energies up to 20 MeV, the Thermo WENDI-2, which is specified to measure neutrons up to several GeV, and a tissue-equivalent proportional counter. The experimentally determined neutron dose equivalent results were compared with Monte Carlo (MC) simulations. Based on these studies field calibration factors can be determined, which result in a more reliable estimate of H(*)(10) in an unknown, but presumably similar high-energy field around an accelerator than a calibration factor determined in a radiation field of a reference neutron source.     

掺杂比和氧分压对脉冲激光沉积ZnO:Al膜性能影响的研究

利用脉冲激光法制备了ＺｎＯ：Ａｌ透明导电膜，通过对膜进行霍尔系数测量及ＳＥＭ、ＸＲＤ测试分析，详细研究了靶材中的化学配比（掺杂比）对膜的透射比和电阻率的影响。结果表明：掺杂比、氧分压强曩响着膜的电学、光学性能和膜的结晶状况。从电学分析看出。掺杂比从０．７５％增至１．５Ｒ过程中膜的载汉子浓度、透射比（在波长大于５００ｎｍ的范围）和光隙能相应增大，在氧分压强为０Ｐａ、掺杂比为１．５％左右时沉积的膜，其     

铁电薄膜极化反转电流的电场和温度特性研究

应用铁电薄膜极化反转的KA模型,讨论了极化反转电流随电场和温度的变化关系.结果表明,极化反转电流随电场的增加而增加,极化反转时间在低电场时成指数关系下降,高电场时幂律关系下降;在居里温度附近,极化反转电流随温度增加而减小,温度远低于居里温度时,随温度增加而增加.     

Tribological characteristics of thin films and applications of thin film technology for friction and wear reduction

Thin film deposition technologies based on chemical and physical vapor deposition are now well established to improve the wear resistance of cutting tools. Although severe tribological conditions occur here, coated tools perform exceptionally well. Despite this, the growth in use of thin film deposition technology is slow in the general mechanical industry. In part, this is due to lack of data on the tribological characteristics of bodies coated with thin films.In this paper, tribological test data gathered in a systematic program of sliding and rolling contact tests are presented to demonstrate the usefulness of thin film coatings for wear protection and friction reduction. Samples coated by reactive magnetron sputtering were tested in sliding and rolling contact tests at low and high speeds and at varying contact stresses. These results are presented and used to identify the primary consideration guiding the choice of coating materials, coating properties, film thickness needed etc. A newly developed film-to-substrate adhesion strength test is also briefly described and the results obtained are cited. It is shown that film material selection and coating process selection have a reasonable physical basis.     

Resistive switching characteristics of ytterbium oxide thin film for nonvolatile memory application

This paper studies the effects of both the positive and negative forming processes on the resistive switching characteristics of a Pt/Yb₂O₃/TiN RRAM device. The polarity of the forming process can determine the transition mechanism, either bipolar or unipolar. Bipolar behavior exists after the positive forming process, while unipolar behavior exists after the negative forming process. Furthermore, the bipolar switching characteristics of the Pt/Yb₂O₃/TiN device can be affected by using a reverse polarity forming treatment, which not only reduces the set and reset voltage, but also improves the on/off ratio.     

Effects of the integrity of silicon thin films on the electrical characteristics of thin dielectric ONO film

Capacitors with two kinds of lower electrodes were fabricated and their effects evaluated on the electrical characteristics of oxide–nitride–oxide (ONO) film. One of the electrodes was made of amorphous silicon film chemically deposited using a gas mixture of Si2H6–PH3; the other was made of poly-Si film deposited by SiH4 decomposition and doped by As+ ion implantation. The ONO thin dielectric layer was composed of natural oxide, CVD silicon nitride and thermal oxide formed on the silicon nitride. The capacitance, the leakage current, the dielectric breakdown field and the time-dependent dielectric breakdown (TDDB) were tested to evaluate the electrical properties of the capacitors. The leakage current and the dielectric breakdown voltage showed similar values between the two capacitors, whereas the TDDB under negative bias showed a great difference. This indicates that, with respect to electrical properties, the integrity of the oxide grown on the in situ P-doped amorphous silicon is better than the oxide grown on the As+ ion-implanted poly-Si. What is more, phosphorus in the amorphous silicon did not lead to any problems with junction depth, even after post heat treatment at 950°C. This revised version was published online in July 2006 with corrections to the Cover Date.