K、Ti掺杂Mg2Si电子结构和光学性质的第一性原理研究

采用第一性原理方法,对本征Mg₂Si以及K和Ti掺杂Mg₂Si的几何结构、电子结构和光学性质进行计算分析。计算结果表明本征Mg₂Si是带隙值为0.290 eV的间接带隙半导体材料,K掺杂Mg₂Si后,Mg₂Si为p型半导体,电子跃迁方式由间接跃迁变为直接跃迁,Ti掺杂Mg₂Si后,Mg₂Si为n型半导体,仍然是间接带隙。K、Ti掺杂后的静介电常数ε₁(0)从20.52分别增大到53.55、69.25,使得掺杂体系对电荷的束缚能力增强。掺杂后,吸收谱和光电导率均发生红移现象,这有效扩大了对可见光的吸收范围,此外可见光区的吸收系数、反射系数以及光电导率都减小,导致透射能力增强,明显改善了Mg₂Si的光学性质。     

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

The technique of thermally-stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density-of-states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localized states.     

Photoconductivity of amorphous CdS films

The photoconductivity of amorphous CdS films, vacuum evaporated onto cooled substrates, has been investigated. The conditions and possible reasons for the appearance of a low temperature maximum in the photoconductivity temperature dependence of chalcogen rich samples have been found. The presence of a slow recombination centre related to the lone pair level of the one-fold coordinated negative chalcogen has been assumed. The energy necessary for both the electron (2 eV) and hole (0.4 eV) escape from this centre has been determined. Photoinduced changes in photoconductivity have been examined as well.     

Substitutional doping of chemically vapor-deposited amorphous silicon

Dc conductivity, absorption coefficient, photoconductivity, and magnetoresistance of phosphorus-doped amorphous Si films prepared by chemical vapor deposition (CVD) have been measured as a function of doping ratio. These results indicate that phosphorus doping reduces localized states in the mobility gap, narrows the tailing width below the extended states, and that phosphorus donors form the impurity band at 0.15 eV below E_c at a doping ratio of about 1×10^-2. It is also found that electronic properties of CVD amorphous Si can be controlled in a wide range by substitutional doping of phosphorus atoms.     

Peculiarity of constant photocurrent method for silicon films with mixed amorphous–nanocrystalline structure

The results of conductivity, photoconductivity and constant photocurrent method absorption measurements by DC and AC methods in hydrogenated silicon films with mixed amorphous–nanocrystalline structure are presented. A series of diphasic silicon films was deposited by very high frequency plasma enhanced chemical vapor deposition technique, using different hydrogen dilution ratios of silane. The increase of hydrogen dilution ratio results in five orders of magnitude increase of conductivity and a sharp increase of grain volume fraction. The comparison of the absorption spectra obtained by DC and AC methods showed that they are similar for silicon films with the predominantly amorphous structure and films with high grain volume fraction. However we found a dramatic discrepancy between the absorption spectra obtained by DC and AC constant photocurrent methods in silicon films deposited in the regime of the structure transition from amorphous to nanocrystalline state. AC constant photocurrent method gives higher absorption coefficient than DC constant photocurrent method in the photon energy range of 1.2–1.7 eV. This result indicates the possibility of crystalline grains contribution to absorption spectra measured by AC constant photocurrent method in silicon films with intermediate crystalline grain volume fraction.     

Sc、Ce掺杂CrSi2的电子结构与光学性质的第一性原理

采用基于密度泛函理论的第一性原理赝势平面波方法对Sc、Ce单掺和共掺后CrSi₂的几何结构、电子结构、复介电函数、吸收系数和光电导率进行了计算。结果表明:Sc、Ce掺杂CrSi₂的晶格常数增大,带隙变小。本征CrSi₂的带隙为0.386 eV,Sc、Ce单掺及共掺CrSi₂的禁带宽度分别减小至0.245 eV、0.232 eV、0.198 eV,费米能级均向低能区移动进入价带。由于Sc的3d态电子和Ce的4f态电子的影响,Sc、Ce掺杂的CrSi₂在导带下方出现了杂质能级。掺杂后的CrSi₂介电函数虚部第一介电峰峰值增加且向低能方向移动,说明Sc、Ce掺杂使得CrSi₂在低能区的光跃迁强度增强,Sc-Ce共掺时更明显。Sc、Ce掺杂的CrSi₂吸收边在低能方向发生红移,在能量大于21.6 eV特别是在位于31.3 eV的较高能量附近,本征CrSi₂几乎不吸收光子,Sc单掺和Sc-Ce共掺CrSi₂吸收光子的能力有所增强,并在E=31.3 eV附近形成了第二吸收峰。说明掺杂Sc、Ce改善了CrSi₂对红外和较高能区光子的吸收。在小于3.91 eV的低能区掺杂后的CrSi₂光电导率增加。在20.01 eV<E<34.21 eV时,本征CrSi₂光电导率为零,但Sc、Ce掺杂后的体系不为零,掺杂拓宽了CrSi₂的光响应范围。研究结果为CrSi₂基光电器件的应用与设计提供了理论依据。     

Extrinsic electroabsorption of amorphous As2SeTe2

We report here the first known electroabsorption measurements on an amorphous material for photon energies much less than the absorption edge. Detailed structure is found, which can be interpreted in a consistent way with independent photoconductivity measurements.     

Optical Properties of Poly(Isothianaphthene)

Abstract

The optical properties in poly(isothianaphthene), PITN, have been investigated by photoconductivity measurements and in situ optical absorption spectroscopy measurements during electrochemical n-type doping. Irradiation of the anode induces photoconductive response for a photon energy neighboring the band-gap and upon irradiation of the cathode photoconduction was observed for light of photon energies higher than the band-gap. The origin of this anomalous polarity effect of photoconduction in PITN has been discussed.     

Co元素掺杂CrSi2的第一性原理计算

采用基于密度泛函理论的第一性原理赝势平面波方法,对Co掺杂CrSi₂的几何结构、电子结构和光学性质进行了计算与分析.结果表明,掺杂后的CrSi₂晶格常数无明显变化,禁带宽度增大.由于Co元素3d电子的影响,在费米能级附近出现了杂质能级.掺杂后的CrSi₂复介电函数虚部在低能方向发生红移,在小于1.20 eV,大于2.41 eV的能量范围内光跃迁强度增强.吸收系数的主峰向高能方向移动,峰值增大,在小于1.38 eV,大于3.30 eV的能量范围改善了CrSi₂对红外光子的吸收.光电导率的主峰向高能方向移动,在小于1.16 eV,大于2.36 eV的能量范围内光电导率增强,说明掺杂Co元素后改善了CrSi₂特别是红外光区的光电性质,计算结果为CrSi₂光电器件的研究制造提供了理论依据.     

Electrical and optical properties of natural and synthetic melanin biopolymer

Natural and synthetic melanin have been investigated by means of optical, electrical and photoelectronic measurements. Optical measurements evidence absorption curves which allow to estimate optical gap for synthetic melanin, by using Tauc’s method. Dark conductivity and photoconductivity measurements were performed as a function of temperature and for different duration of thermal treatments. It has been evidenced that both quantities are thermally activated and thermal treatments play a very important role as far as gap states are concerned.     

P和As掺杂Mn4Si7第一性原理计算

采用第一性原理计算方法,对本征Mn₄Si₇以及P和As掺杂的Mn₄Si₇的电子结构和光学性质进行计算解析。计算结果表明本征Mn₄Si₇是带隙值为0.810 eV的间接带隙半导体材料,P掺杂Mn₄Si₇的带隙值增大为0.839 eV,As掺杂Mn₄Si₇的带隙值减小为0.752 eV。掺杂使得Mn₄Si₇的能带结构和态密度向低能方向移动,同时使得介电函数的实数部分在低能区明显增大,虚数部分几乎全部区域增加且8 eV以后趋向于零。此外掺杂还增加了高能区的消光系数、吸收系数、反射系数以及光电导率,明显改善了Mn₄Si₇的光学性质。     

Improvement of photoconductivity in Silicon Tin (SiSn) thin films

Hydrogenated-amorphous silicon tin alloy (a-SiSn (C:H)) films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. We present investigations of the absorption and photoconductivity characteristics of a-SiSn(C:H) thin films as a function of the tin concentration, which could be a pivotal feature for the bottom solar cell development. We demonstrate that the photoconductivity is significantly improved by the film growth under enhanced hydrogen dilution. The results suggest that the hydrogen dilution plays an important role in development of high quality a-SiSn(C:H) photosensitive thin films.     

Drift mobility in amorphous selenium-sulfur alloys

Drift mobility measurements have been made on thin amorphous films of selenium-sulfur alloys in the composition range from 14 at % to 31 at % sulfur. Carrier lifetimes were found to be 10–20 μsec for holes and > 10^?3 sec for electrons. These lifetimes are considerably longer than those reported by Schottmiller et al. in lower sulfur concentration films. The drift mobilities decrease with increasing sulfur concentrations and were characterized by activation energies of 0.33 eV for holes and 0.43 eV for electrons. Only a trap-controlled drift mobility model is consistent with the constant activation energies and decreasing mobility observed with increasing sulfur content. The optical absorption and photoconductivity edges were found to shift to higher photon energies with increasing sulfur content.     

Ionic conductivity of BaFBr

The ionic conductivity of flux grown BaFBr crystals is reported. Measurements were made parallel to the crystallographic C-axis in the temperature range 300 to 800 K. The ionic conductivity of the BaFBr pellet is found to be more than that of a single crystal. A tentative identification of low and high temperature conduction has been made as extrinsic association and extrinsic dissociation state of defect in the crystal while in pellet extrinsic dissociation and intrinsic regions. The probable mechanism for the conduction process has been discussed on the basis of structural considerations of the unit cell.     

Growth and Optical Absorption of Photorefractive Bi12SiO20 Crystals Doped with Zn and Cd and Co-doped with P

Single crystals of Bi₁₂SiO₂₀ (BSO) Zn- and Cd-doped and co-doped with P have been grown by Czochralski technique and optimum conditions for producing optically homogeneous crystals were established. The transmission and reflection spectra have been measured in the range 0.38–0.85 μm and absorption coefficients have been determined. The Zn dopants have been found to bleach the crystals and decrease the transmission shoulder, whereas Cd dopant increases the absorption and makes the shoulder well pronounced. P-doping strongly decreases the transmission shoulder and shifts absorption to shorter wavelengths. The absorption Coefficient of BSO doped with P has been found to decrease with dopant concentration increase up to 3 mol% P and above it increases with P concentration in the whole studied range. Zn + P- and Cd + P-co-doping (P concentration in the crystal less than 3 mol%) decreases strongly the absorption coefficient and shifts the spectra to short wavelengths.     

Photoconductivity of Crystalline Iodoform II: Photocurrents Induced by a Two-Photon Absorption

Photogeneration of charge carriers in iodoform has been studied using a high power pulsed dye laser as an excitation source. In the region of 480-640 nm, i.e. that of weak absorption, the magnitude of the photogenerated charge follows a square light intensity dependence in the intensity range employed (up to 10²³ photons/cm²s). The spectral dependence of the photogeneration efficiency is similar to that observed for the dc photoconductivity at twice shorter wavelengths; the efficiency drops down, however, rapidly when the tail of the first absorption band of iodoform is approached. Temperature anomalies below 270 K render the exact determination of activation energies impossible.     

Temperature dependence of optical absorption in In1‐xGaxAs solid solutions

Absorption spectra near the fundamental absoption edge of n‐type of In_1‐xGa_xAs are studied. The temperature coefficient of the In_1‐xGa_xAs energy gap, dE_g/dT, has been obtained and compared with calculated data. An exponential dependence of the absorption coefficient on photon energy has been found. The slope of the exponential absorption curve is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Die Phasenübergänge und die Störstellenphotoleitung von Sb2S3

The influence of the Sb₂S₃ single crystals growing conditions on the mikrowave dielectric permittivity and the photoconductivity has been investigated. An improper photoconductivity arises on the ferroelectric plane (001) in the crystals with a high dielectric permittivity.     

Photoelectronic and electrical properties of CuIn5S8 single crystals

To identify the impurity levels in CuIn₅S₈ single crystals, the dark electrical conductivity and photoconductivity measurements were carried out in the temperature range of 50–460 K. The data reflect the intrinsic and extrinsic nature of the crystals above and below 300 K, respectively. Energy band gaps of 1.35 and 1.31 eV at 0 K and 300 K, were defined from the dark conductivity measurements and the photocurrent spectra, respectively. The dark and photoconductivity data in the extrinsic temperature region reflect the existence of two independent donor energy levels located at 130 and 16 meV. The photocurrent‐illumination intensity dependence (F) follows the law I_phαF^γ, with γ being 1.0, 0.5 and 1.0 at low, moderate and high intensities indicating the domination of monomolecular, bimolecular and strong recombination at the surface, respectively. In the intrinsic region and in the temperature region where the shallow donor energy level 16 meV is dominant, the free electron life time, τ_n, is found to be constant with increasing F. In the temperature region 140 K < T < 210 K, the free electron life time increases with increasing illumination intensity showing the supralinear character. Below 140 K, τ_n decrease with decreasing illumination intensity. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and optical properties of poly[4-methacryloxy-(4′-carboxy)-azobenzene]

Abstract

In the present work, the homopolymer built from the free radical polymerization of methacrylic monomer incorporating an azobenzene side-group has been synthesized and structurally characterized. The optical properties such as refractive index, extinction coefficient, absorption coefficient and optical energy band gap of poly[4-methacryloxy-(4′-carboxy)-azobenzene] thin film prepared by high vacuum sublimation method were determined using spectroscopic ellipsometry combined with transmittance measurements.

We found that in the spectrum of extinction coefficient there are absorption bands, which are assigned as the n-π* and π-π* electronic transitions of the azo compound.