Ellipsometric characterization of Cd1−x Mn x Te thin films in the presence of perturbative fields

Ellipsometric studies in the 1·5 to 5 eV range have been carried out on thin films of Cd_1−x Mn _x Te and exhibitE ₀,E ₁ andE ₁+Δ₁ transitions of the cubic semiconductors. The effect on these transitions upon irradiation by HeNe laser light, application of small alternating magnetic fields and the influence of both the fields, applied simultaneously, has been investigated. The observed critical point shifts have been interpreted on the basis of changes in the band structure of these thin films. This has been corroborated by theoretically calculating the effective number of electrons contributing to the transition per atom —N _eff. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995.     

掺锡氧化锌纳米阵列的制备及其结构和光学特性的研究

在旋涂法制成的种子层基底上,用水热法生长氧化锌纳米阵列(ZNAs)和掺锡氧化锌纳米阵列(ZNAs∶Sn)。为了研究掺锡对氧化锌纳米阵列透光性的影响,用X射线衍射(XRD)、扫描电镜(SEM)、透射谱和椭圆偏振仪(SE)对样品进行了表征,并结合等效介质模型(EMA)理论模拟样品光学常数,根据太阳能光谱AM1.5计算透过光的总功率。XRD分析表明纳米棒垂直于基底沿(002)晶向生长,SEM和椭圆偏振仪表明样品表面形貌良好,制得的纳米棒直径约300 nm,长度约1.6μm。透射谱表明ZNAs掺锡之后,在300~800nm波段透过率增大。根据AM1.5计算获得在样品厚度相同的情况下,掺杂5%的ZNAs∶Sn透过光总功率是ZNAs的1.6倍。     

Spin drift and spin diffusion currents in semiconductors

Abstract

On the basis of a spin drift-diffusion model, we show how the spin current is composed and find that spin drift and spin diffusion contribute additively to the spin current, where the spin diffusion current decreases with electric field while the spin drift current increases, demonstrating that the extension of the spin diffusion length by a strong field does not result in a significant increase in spin current in semiconductors owing to the competing effect of the electric field on diffusion. We also find that there is a spin drift-diffusion crossover field for a process in which the drift and diffusion contribute equally to the spin current, which suggests a possible method of identifying whether the process for a given electric field is in the spin drift or spin diffusion regime. Spin drift-diffusion crossover fields for GaAs are calculated and are found to be quite small. We derive the relations between intrinsic spin diffusion length and the spin drift-diffusion crossover field of a semiconductor for different electron statistical regimes. The findings resulting from this investigation might be important for semiconductor spintronics.     

Spin drift and spin diffusion currents in semiconductors

On the basis of a spin drift-diffusion model, we show how the spin current is composed and find that spin drift and spin diffusion contribute additively to the spin current, where the spin diffusion current decreases with electric field while the spin drift current increases, demonstrating that the extension of the spin diffusion length by a strong field does not result in a significant increase in spin current in semiconductors owing to the competing effect of the electric field on diffusion. We also find that there is a spin drift-diffusion crossover field for a process in which the drift and diffusion contribute equally to the spin current, which suggests a possible method of identifying whether the process for a given electric field is in the spin drift or spin diffusion regime. Spin drift-diffusion crossover fields for GaAs are calculated and are found to be quite small. We derive the relations between intrinsic spin diffusion length and the spin drift-diffusion crossover field of a semiconductor for different electron statistical regimes. The findings resulting from this investigation might be important for semiconductor spintronics.     

Optical properties and DC electrical conductivity of Ge28−xSe72Sbx thin films

Thin films of Ge_28−xSe₇₂Sb_x (x=0, 8, 16, 24 at%) with thickness of 200 nm are prepared by thermal evaporation onto glass substrates under vacuum of 5.3×10⁻⁵ mbar. Optical reflectance and transmittance of these films are measured at room temperature in the light wavelength region from 200 to 1100 nm. The estimated optical energy gap, E_g, is found to decrease from 2 eV (0 at% Sb) to 1.5 eV (24 at% Sb), whereas the band tail width, E_e, increases from 0.062 to 0.077 eV, respectively. The refractive index, n, and extinction coefficient, κ, are determined as functions of wavelength. The DC electrical conductivity, σ, of films is measured as a function of temperature in the range from 300 to 360 K. The extracted value of activation energy, ΔE, is found to decrease from 0.95 eV (0 at% Sb) to 0.74 eV (24 at% Sb). Optical and electrical behavior of films can be explained in terms of cohesive energy (CE) and Se-Se defect bonds.     

a-Si:H/a-Si_(1-x)C_x:H 多层膜的电学和光学性质

采用等离子体辉光放电单室系统制备的 a-Si:H/a-Si_(1-x)C_x:H 多层膜结构具有低的暗电导和高的光电导特性,且不同于单层膜,有明显的 S-W 效应。随着子层厚度 L 减小,多层膜曝光态(B)的暗电导率σdB 较退火态(A)的σdA 减小快,即光诱导衰退程度增大,而光电导无明显变化。本文还测定了此多层膜结构的光能隙 E_g,得到随子层厚度减小“蓝移”的结果。用一维单量子阱模型作了讨论,实验值与理论计算符合较好。     

Dc conductivity of consolidated nanoparticles of zinc sulfide

Abstract

Zinc sulfide nanoparticles with average grain sizes ranging from 3 to 12 nm were prepared by arrested chemical preparation, followed by suitable thermal processing. The size of the grains was determined by x-ray line broadening. Dc measurement was performed on compacts of nanoparticles using a Keithley electrometer in the temperature range of 313–423 K. The dc conductivity, σ_dc, of our samples is much higher than that of ZnS single crystals; σ_dc increases with a decrease in the grain size.     

Photodegradation of organic light-emitting devices observed in nitrogen-filled environment

Degradation of organic light-emitting devices (OLEDs) upon ultraviolet (UV) irradiation has been studied by measuring luminance-voltage (L-V) and current-voltage (I-V) characteristics of the devices in a nitrogen-filled glove-box. Photo-oxidation or reaction is no longer the main origin of the degradation for the devices protected by nitrogen. Conventional double-layer OLEDs with tris(8-hydroxyquinoline) aluminum (Alq₃) as the electron transport material and single-layer devices containing Alq₃ as the only organic material exhibit different degradation behaviors: both L-V and I-V characteristics degrade severely for the irradiated double-layer devices, whereas whether I-V degrades or not in a single-layer device is closely related to the species of the charge carriers flowing in the device. By comparing electroluminescent and photoluminescent degradation behaviors of the single-Alq₃-layer devices, we conclude that lowered fluorescent quantum efficiency and hole current after UV irradiation are two origins of the degraded characteristics of the devices isolated from the moist environment.     

Some key properties of low-dimensional electron gas in semiconductors

Conditions for the formation of low-dimensional electron gas in semiconductors and different structures supporting it are discussed. Some of the new devices in which carriers have low-dimensional motion are introduced. The properties of electrons needed to be studied for the optimisation of the device performance are mentioned. In particular, the charge control and mobility of electrons in high electron mobility transistors, gain and loss processes in quantum well lasers, and excitonic line width in multiple quantum wells are discussed.     

Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi₅Ge_xSe_95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi₅Ge_xSe_65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi₅Ge_xSe_65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (E_g) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.     

Fullerene thin-film transistors fabricated on polymeric gate dielectric

Thin-film transistors with fullerene as n-type organic semiconductor have been fabricated. A polymeric gate dielectric, polymethyl methacrylate, has been used as an alternative to usual inorganic dielectrics. No significant differences in the microstructure of fullerene thin-films grown on polymethyl methacrylate were observed. Devices with either gold or aluminium top electrodes have been fabricated. Although the lower work-function of aluminium compared to gold should favour electron injection, similar field-effect mobilities in the range of 10^− 2 cm² V^− 1 s^− 1 were achieved in both cases. Actually, the output characteristics indicate that organic thin-film transistors behave more linearly with gold than with aluminium electrodes. These results confirm that not only energy barriers determine carrier injection at metal/organic interfaces, but also chemical interactions.     

Size quantization effects in optical and electrical properties of II–VI semiconductor films in nanocrystalline form

The electrical properties of CdTe and optical properties of ZnS in nanocrystalline thin film form are studied with a view to have a clearer understanding of the optical processes and the carrier transport mechanisms in nanocrystalline II–VI semiconductors, in general. Nanocrystalline ZnS and CdTe films were deposited by magnetron sputtering of respective targets in argon plasma. The optical absorption data of nanocrystalline ZnS films (thickness 10–40 nm) could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. The conductivity of CdTe (grain size within 4–4·7 nm) showed (T ₀/T) ^p dependence withp ∼ 0·5 indicating the presence of a Coulomb gap near the Fermi level. The width of the Coulomb gap varied within 0·02–0·04 eV depending on the deposition condition. The existing theoretical models were used for estimating hopping energy (0·02–0·04 eV) and hopping distance (2·8–5·1 nm) in nano CdTe films.     

透明导电膜ZnO:Al(ZAO)的组织结构与特性

ZnO:Al(ZAO)是一种简单并半导体氧化物薄膜材料,具有高的载流子浓度和光学禁带宽度,因而具有优异的电学和光学性能,极具应用价值,对于其能级高度简并的ZAO半导体薄膜材料,在较低的温度下,离化杂质散射占主导地位,在较高的温度下,晶格振动散射将成为主要的散射机制;晶界散射仅当晶粒尺寸较小（与电子的平均自由程相当）时才起作用,本文分析了ZAO薄膜的制备方法,晶体结构特性,电子和光学性能以及载流子的散射机制。     

过渡金属(Mn2+、Ni2+、Fe3+、Cu2+)掺杂ZnO基稀磁半导体的制备及性质

利用溶液腐蚀法制备了Mn2+、Ni2+、Fe3+、Cu2+离子掺杂的ZnO基稀磁半导体。XRD表明掺杂后的ZnO仍然保持单一的纤锌矿结构,没有任何杂质相产生。由紫外-可见光反射谱可知掺杂后吸收边发生了红移。掺杂前ZnO的带隙为3.20eV,对样品分别掺入Mn、Ni、Fe和Cu后的带隙分别为3.19eV、3.15eV、3.08eV和3.17eV。掺杂后样品的室温PL谱除了紫外发射峰外,对于Mn掺杂的样品还在蓝光区域出现了2个分别位于424nm和443nm的发射峰,Fe掺杂的样品出现了一个位于468nm的微弱发射峰,Cu掺杂的样品出现了位于469nm及535nm的很宽的发射峰。室温磁滞回线显示掺杂后样品有明显的铁磁性,掺入Mn、Ni、Fe和Cu样品的剩余磁化强度(Ms)分别为0.3902×10-3emu/cm3、0.454emu/cm3、0.372emu/cm3和0.962×10-3emu/cm3,矫顽力分别为47Oe、115.92Oe、99.33Oe和23Oe。经分析室温铁磁性来源于缺陷调制的Mn2+-Mn2+长程铁磁交换相互作用。     

Thin films containing Mn-doped ZnS nanocrystals synthesised by chemical method and study of some of their optical properties

In this article, we present the optical properties of thin films containing Mn-doped ZnS nanocrystals synthesised by the chemical method. The ZnS nanoparticles within the polymer matrix (polyvinyl alcohol) were investigated by SEM and TEM images and analysed by X-ray diffraction. The effect of polymer concentration on the direct band gap of Mn-doped ZnS thin films was calculated from the data for absorption measurements. The values of the band gap are in the range of 3.73–3.90?eV. In addition, we discuss the photoluminescence of these films.