Optoelectronical properties of polycrystalline β ‐GaSe thin films

Polycrystalline β ‐GaSe thin films were obtained by the thermal evaporation of GaSe crystals onto glass substrates kept at 300 °C under a pressure of 10^–5 Torr. The transmittance and reflectance of these films was measured in the incident photon energy range of 1.1–3.70 eV. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct allowed transitions band gap of 1.83 eV. The data analysis allowed the identification of the dispersive optical parameters by calculating the refractive index in the wavelength region of 620–1100 nm. In addition, the photocurrent of the samples was studied as function of incident illumination‐intensity and temperature. The photocurrent is found to exhibit sublinear and supralinear character above and below 270 K, respectively. The temperature dependent photocurrent data analysis allowed the calculation of photocurrent activation energies as 603, 119 and 45 meV being dominant in the temperature regions of 250–300 K, 180–240 K and 80‐160 K, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,electrical and optical properties of Ge implanted GaSe single crystals grown by Bridgman technique

Structural, optical and electrical properties of Ge implanted GaSe single crystal have been studied by means of X‐Ray Diffraction (XRD), temperature dependent conductivity and photoconductivity (PC) measurements for different annealing temperatures. It was observed that upon implanting GaSe with Ge and applying annealing process, the resistivity is reduced from 2.1 × 10⁹ to 6.5 × 10⁵ Ω‐cm. From the temperature dependent conductivities, the activation energies have been found to be 4, 34, and 314 meV for as‐grown, 36 and 472 meV for as‐implanted and 39 and 647 meV for implanted and annealed GaSe single crystals at 500°C. Calculated activation energies from the conductivity measurements indicated that the transport mechanisms are dominated by thermal excitation at different temperature intervals in the implanted and unimplanted samples. By measuring photoconductivity (PC) measurement as a function of temperature and illumination intensity, the relation between photocurrent (I_PC) and illumination intensity (Φ) was studied and it was observed that the relation obeys the power law, I_PC αΦⁿ with n between 1 and 2, which is indication of behaving as a supralinear character and existing continuous distribution of localized states in the band gap. As a result of transmission measurements, it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as‐grown GaSe; however, a slight shift of optical band gap toward higher energies for Ge‐implanted sample was observed with increasing annealing temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical properties of nitrogen implanted GaSe single crystal

N‐implantation to GaSe single crystals was carried out perpendicular to c‐axis with ion beam of 6 × 10¹⁵ ions/cm² dose having energy values 30 keV and 60 keV. Temperature dependent electrical conductivities and Hall mobilities of implanted samples were measured along the layer in the temperature range of 100‐320 K. It was observed that N‐implantation decreases the resistivity values down to 10³ Ω‐cm depending on the annealing temperature, from the room temperature resistivity values of as‐grown samples lying in the range 10⁶‐10⁷ Ω‐cm. The temperature dependent conductivities exhibits two regions (100‐190 and 200‐320 K) with the activation energies of 234‐267 meV and 26‐74 meV, for the annealing temperatures of 500 and 700 °C, respectively. The temperature dependence of Hall mobility for the sample annealed at 500 °C shows abrupt increase and decrease as the ambient temperature increases. The analysis of the mobility‐temperature dependence in the studied temperature range showed that impurity scattering and lattice scattering mechanisms are effective at different temperature regions with high temperature exponent. Annealing of the samples at 700 °C shifted impurity scattering mechanism toward higher temperature regions. In order to obtain the information about the defect produced by N‐implantation, the carrier density was analyzed by using single donor‐single acceptor model. We found acceptor ionization energy as E_a = 450 meV, and acceptor and donor concentration as 1.3 × 10¹³ and N_d = 3.5 × 10¹⁰ cm⁻³, respectively. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermally Stimulated Current Observation of Trapping Centers in Undoped GaSe Layered Single Crystals

Undoped p‐GaSe layered single crystals were grown using Bridgman technique. Thermally stimulated current measurements in the temperature range of 10‐300 K were performed at a heating rate of 0.18 K/s. The analysis of the data revealed three trap levels at 0.02 , 0.10 and 0.26 eV. The calculation for these traps yielded 8.8 × 10^‐27, 1.9 × 10^‐25, and 3.2 × 10^‐21 cm² for capture cross sections and 3.2 × 10¹⁴, 1.1 × 10¹⁶, and 1.2 × 10¹⁶ cm^‐3 for the concentrations, respectively.     

Influence of ion implantation on structural and photoconductive properties of Bridgman grown GaSe single crystals

The structure and temperature dependent spectral photoconductivity of as‐grown and N‐and Si‐implanted GaSe single crystals have been studied. It was observed that post‐annealing results in a complete recovery of the crystalline nature that was moderately reduced upon implantation. The band edge is shifted in the implanted sample which is attributed to the structural modifications and continuous shallow levels introduced upon implantation and annealing. Our calculations showed that the trap density is increased upon implantation and annealing which confirms a possibility of explanation the phenomenon within a framework of continuous trap levels. Photocurrent measurements as a function of photo‐excitation intensity also support continuous distribution of localized states in the band gap. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Annealing effect on electrical and photoconductive properties of Si implanted GaSe single crystal

GaSe single crystals grown by Bridgman method have been doped by ion implantation technique. The samples were bombarded in the direction parallel to c‐axis by Si ion beam of about 100 keV to doses of 1 × 10¹⁶ ions/cm² at room temperature. The effects of Si implantation with annealing at 500 and 600 °C on the electrical properties have been studied by measuring the temperature dependent conductivity and photoconductivity under different illumination intensities in the temperature range of 100–320 K. It is observed that Si implantation increases the room temperature conductivity 10⁻⁷ to 10⁻³ (Ω‐cm)⁻¹ depending on the post annealing temperature. The analysis of temperature dependent conductivity shows that at high temperature region above 200 K, the transport mechanism is dominated by thermal excitation in the doped and undoped GaSe samples. At lower temperatures, the conduction of carriers is dominated by variable range hopping mechanism in the implanted samples. Annealing of the samples at and above 600 °C weakens the temperature dependence of the conductivity and photoconductivity. This indicates that annealing of the implanted samples activates Si‐atoms and increases structural deformations and stacking faults. The same behavior was observed from photoconductivity measurements. Hence, photocurrent‐illumination intensity dependence in the implanted samples obeys the power low I_pc ∝ Φⁿ with n between 1 and 2 which is an indication of continuous distribution of localized states in the band gap. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resonant Raman Scattering near the Free‐to‐Bound Transition in Undoped p‐GaSe

Raman spectra of GaSe layered crystal have been measured using a He‐Ne laser and temperature tuning the free‐ to‐bound gap in the range 10‐290 K. Resonance enhancement of E″⁽²⁾ mode has been observed for both incident and scattered photon energies equal to the free‐to‐bound transition energy.     

Numerical simulation of heat conduction for the growth of anisotropic layered GaSe crystals

In this report, we present the usage of a second rank cylindrical conductivity tensor which we derived to simulate the crystal growth processes of a layered compound GaSe in a cylindrical enclosure by directional solidification. Use of such a tensor is inevitable in the simulations of the growth of highly anisotropic crystals having layered structure, since the crystallographic orientation of the grown material is not necessarily aligned with the ampoule symmetry. Using the finite difference control volume approach in 3D, we solved transient heat conduction equation for a highly anisotropic solid in a cylindrical enclosure. We obtained sloped thermal fields and isothermal surfaces and the magnitudes of the slopes are strong functions of both azimuthal angle and growth orientation. The results showed that the orientation of the crystallographic axes of GaSe in the enclosure is quite effective in the steady and the transient fields, isotherms, and axial and radial temperature gradient within the material. Increase of Bi number decreases the magnitude of the slope of isothermal surface. Anisotropy of the conductivity seems to be effective in the orientation of the growth direction of the resulting crystal within the cylindrical ampoule. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

一维硅纳米材料的光学特性

硅纳米线与硅纳米管是两种重要的一维硅纳米材料,由于具有量子限制效应等性能在光电子器件方面具有潜在的应用前景.总结了近年来硅纳米线在光学特性方面的研究进展,重点介绍了本征硅纳米线、掺杂硅纳米线及硅纳米线阵列的光致发光光谱(PL)的最新进展情况,同时涉及了硅纳米管在PL发射光谱方面的研究结果.并对其发展作了展望     

Refractive index,band gap and oscillator parameters of amorphous GaSe thin films

GaSe thin films are obtained by evaporating GaSe crystals onto ultrasonically cleaned glass substrates kept at room temperature under a pressure of ∼10^–5 Torr. The X‐ray analysis revealed that these films are of amorphous nature. The reflectance and transmittance of the films are measured in the incident photon energy range of 1.1–3.0 eV. The absorption coefficient spectral analysis revealed the existence of long and wide band tails of the localized states in the low absorption region. The band tails width is calculated to be 0.42 eV. The analysis of the absorption coefficient in the high absorption region revealed an indirect forbidden band gap of 1.93 eV. The transmittance analysis in the incidence photon wavelength range of 500–1100 nm allowed the determination of refractive index as function of wave length. The refractive index–wavelength variation leads to the determination of dispersion and oscillator energies as 31.23 and 3.90 eV, respectively. The static refractive index and static dielectric constant were also calculated as a result of the later data and found to be 9.0 and 3.0, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical Studies on Smectic-Isotropic Phase Transition Behavior of Two Thermotropic Liquid Crystals

The binary mixture of two compounds, viz., Cholesteryl laurate (CL) and diethyl 4, 4′-azoxy benzoate shows the cholesteric and smectic phases sequentially when the specimen is cooled from its isotropic phase. Refractive indices, birefringence, and optical transmittance have been measured by the optical technique. With the help of measured data, the macroscopic ordered parameter has been discussed. The temperature dependence of these parameters has also been discussed.     

Optical Characterization of Lyotropic Chromonic Liquid Crystals

We report the results of our studies on the optical and thermal properties of binary mixture of compounds, viz., Berberine (BBE) and Poly-ethylene glycol (PEG). The mixture shows very interesting coexistent biphasic regions N + I and N + C phases respectively at different concentrations of BBE sequentially when the specimen is cooled from its isotropic liquid phase. The temperature variations of optical anisotropy and optical textures have also been discussed. Formation of above molecular orientations has been confirmed by X-ray studies.     

Fe掺杂GaN晶体非极性面的光学各向异性研究

本文利用低温光致发光谱(PL)研究了Fe掺杂GaN晶体非极性a面{1120}、m面{1100} 的带边峰和Fe³⁺相关峰(⁴T₁(G)- ⁶A₁(S))的偏振发光特性。结果表明:a面与m面光学各向异性差别较小,线偏振光的电矢量E平行于c轴[0001]时(E∥c),GaN带边峰强度最小,而Fe³⁺零声子峰(1.299 eV)强度最强。带边峰线偏振度小,而Fe³⁺零声子峰线偏振度大,a面带边峰的线偏振度为26％,Fe³⁺零声子峰的偏振度在a面和m面分别达到55%和58%。在5 K低温下,进一步测量了Fe³⁺精细峰和声子伴线的偏振特性,结果表明,除了一个微弱的峰外,其他精细峰和声子伴线与Fe³⁺零声子峰偏振特性一致。本研究有助于拓展Fe掺杂GaN晶体材料在新型偏振光电器件领域的应用。     

Use of reflectance anisotropy spectroscopy for mapping the anisotropy of lyotropic liquid crystal dispersions in formulations

Lyotropic liquid crystal dispersions have commercial importance in the formulation of cleaning products and pharmaceuticals. The anisotropy of such dispersions is an important aspect of their properties and a simple method for measurement would be valuable to formulators. The relatively new optical technique of reflectance anisotropy spectroscopy (RAS) was considered to have potential in this respect. To test the idea, the anisotropy of a binary mixture of an anionic surfactant Aerosol OT (AOT) and glycerol (used as a model for water) was investigated using RAS. The variation in the measured anisotropy parallels the expected behaviour of a dispersion of lyotropic liquid crystals as a function of concentration and temperature. A response surface of anisotropy generated as a function of temperature and surfactant concentration demonstrates the use of RAS as a tool for mapping liquid anisotropy to facilitate the formulation of structured liquids.     

Effects of surface tension anisotropy on interfacial instability in directional solidification

Effects of surface tension anisotropy on the planar interfacial stability are studied with asymptotic analysis method in both the solidification from undercooled pure melts and the unidirectional solidification of binary alloys. The asymptotic approach developed by Xu is adopted to study the interfacial stability here, which is different from that used by other investigators previously in their works. A simple linear analysis result is obtained, i.e., the surface tension anisotropy may compete to determine interfacial stability near some critical conditions in unidirectional solidification of binary alloys. The exsitstence of the surface tension anisotropy enlarges the instability region of disturbed wave number. And the threshold of instability is strongly affected by surface tension anisotropy, especially at high pulling velocity or high temperature gradient. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase Transition and Optical Characterization of Mesophase Stability of Nematic and Columnar Biphasic Regions

We report the results of our studies on the optical and thermal properties of binary mixture of two compounds viz., abietic acid and alizarin dye. The mixture shows a very interesting co-existent biphasic regions of nematic (N + I) and columnar smectic (C + I) phases, sequentially when the specimen is cooled from its isotropic phase respectively at different concentrations of given molecule. The temperature variations of optical anisotropy, optical textures and electrical conductivity have also been discussed. Aggregated molecular size has been confirmed by X-ray studies.     

Polarization anisotropy of photoluminescence from triphenylamine‐based molecular single crystals

We present an experimental study of the polarization of the photoluminescence (PL) emission from a (Z)‐3‐(4‐(diphenylamino)phenyl)‐2‐(pyridin‐2‐yl)‐acrylonitrile ( DPPyACN) single organic crystal. Our measurements show that the photoluminescence (PL) emission spectrum of the crystal consists of three highly polarized PL bands with different polarization ratios. The spectral position of one of these bands is the same as that of the PL emission of the molecule in dilute solutions in different “poor” solvents. Thus, we relate the different polarization ratios of the PL bands to different physical origin of the PL emission.     

Optical properties of ZnO nanotubes

Tubular ZnO nanostructures have been obtained via a hydrothermal method at low temperature (90 °C) without any catalysts or templates. The XRD measurement reveals that the tubes are single crystals with hexagonal wurtzite structure. SEM shows that the diameters of ZnO nanotubes ranged from 400 to 550 nm. The Raman and PL spectra indicate that oxygen vacancies or Zn interstitials are responsible for the green emission in the ZnO nanotubes. A possible growth mechanism on the formation of crystalline ZnO nanotubes has been presented. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

空气退火对ZnS薄膜结构和光学特性的影响

采用X射线衍射、扫描电子显微镜和光致发光等技术研究了空气退火对ZnS薄膜的结构和光学特性的影响.薄膜在500℃以下退火后结晶质量得到改善,仍呈ZnS立方相结构.退火温度达到550℃时,薄膜中出现ZnO六方相结构.薄膜退火后,大气中的氧掺入薄膜中,出现ZnS-ZnO复合层.随退火温度升高,薄膜晶粒尺寸增大,透过率增加,带隙逐渐接近ZnO带隙.薄膜光致发光结果表明,复合层内ZnS和ZnO绿色发光的叠加替代了来自ZnS缺陷能级间的绿色发光.     

Influence of density of states on optical properties of GaSe thin film

A systematic investigation on the effect of substrate temperature on the structure, optical absorption and density of states of vacuum evaporated gallium monoselenide (GaSe) thin films is reported. The X‐ray diffraction analysis shows an occurrence of amorphous to polycrystalline transformation in the films deposited at higher‐temperature substrates (573K). The compositional analysis is made with Auger Electron Spectroscopy (AES). The thickness of the film (175nm) is measured by a multiple beam interferometery. Optical characteristics of the GaSe sample have been analyzed using spectrophotometer in the photon energy range of 1.0 ‐ 4 eV. The absorption mechanism has been recognized and the allowed indirect as well as forbidden direct transitions have been found. As‐deposited films show two indirect and allowed transitions due to spin‐orbit splitting of the valence band, as reported here for the first time. Low field conduction have enabled us to determine the density of states in amorphous and poly‐GaSe films. The amorphous and polycrystalline GaSe thin films have localized states density values of N (E_F) = 1.686 × 10¹⁷ cm^‐3 eV^‐1 and 1.257 × 10¹⁵ cm^‐3 eV^‐1 respectively. The experimental results are interpreted in terms of variations in the density of localized states due to progressive decrease of the unsaturated bonds during deposition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)