Features of the electrical and photoelectrical properties of nanocrystalline indium and zinc oxide films

Electrical and photoelectrical properties of nanocrystalline zinc oxide and indium oxide films are studied. For these oxides the temperature dependences of conductance are observed to be consisting of two parts with different activation energy. Also photoconductivity relaxation of the oxides can be described by a sum of two exponential functions. The spectral dependencies of nanocrystalline zinc oxide and indium oxide photoconductivity are presented. The photoconductivity arises as samples are illuminated with energy less than band gap. The data are discussed on the basis of model by which the localized states in the band gap play major role.     

Hopping and drift mechanisms of photoconductivity in ZnO:Li films

The mechanisms of dark- and photoconductivity in ZnO:Li films are investigated. The obtained results are interpreted on the basis of hopping mechanism of charge carriers transport for the dark conductivity and the hopping or drift mechanism for the photoconductivity depending on the energy of an exciting photon. For photons with the energy more than the forbidden band gap the drift mechanism of carriers transport takes place, while for photons with the energy less than the forbidden band gap the hopping mechanism takes place.     

Photogeneration,optical absorption and transport in hydrogenated sputtered amorphous silicon

By combining direct optical transmission with steady state photocurrent and photoconductivity gain measurements we evaluated the optical absorption constant of sputtered hydrogenated a-Si between 10⁵ to 10^?1cm^?1. The photogeneration process involves excitation from the valence band or from defect states in the middle of the gap to the conduction band, with the electrons making the major contribution to the photocurrent. The electron drift mobility, determined from the photoconductivity studies, is considerably smaller than that determined from the time of flight technique, due to trapping at deep centers.     

Photoquenching effect and its consequence in p type gallium arsenide

In order to examine the origin of the occasionally observed shift in the photoconductivity peak with respect to the band gap value; optical and electro-optical investigation of a p type GaAs crystal was carried out. The absorption and photochonductivity spectra were recorded and by comparing both spectra, a model based on the transition of electrons from the oxygen “adsorbate surface states” is proposed to explain the strong photoquenching observed on the high energy side of the band gap value. This also explains the observed shift.     

第一性原理研究B掺杂Mn_4Si_7的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及B掺杂Mn_4Si_7的电子结构和光学性质进行理论计算.研究结果表明,未掺杂Mn_4Si_7是间接带隙半导体,其禁带宽度为0.786 eV,B掺杂后其禁带宽度下降为0.723 eV. B掺杂Mn_4Si_7是p型半导体材料.未掺杂Mn_4Si_7在近红外区的吸收系数达到10~5 cm~(-1),B掺杂引起Mn_4Si_7的折射率、吸收系数、反射系数及光电导率增加.     

Mn掺杂6H-SiC的第一性原理研究

本文通过密度泛函理论第一性原理平面波超软赝势计算方法计算了Mn掺杂6H-SiC的电子结构与光学性质。计算结果显示掺杂Mn后的6H-SiC为间接带隙p型半导体,且带隙较本征体有所降低,带隙由2.022 eV降为0.602 eV,电子从价带跃迁所需能量减少。掺杂后的Mn的3d能级在能带结构中以杂质能级出现,提高了载流子浓度,导电性增强。光学性质研究中,掺杂Mn后的介电函数虚部在低能处增加,电子激发态数量增多,跃迁概率增大。掺杂后的光吸收谱能量初值也较未掺杂的3.1 eV扩展到0 eV,反射谱发生红移。由于禁带宽度的降低使得光电导率起始范围得到扩展。     

Oxygen-related band gap state in single crystal rubrene

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.     

Ge，Al掺杂锰硅化合物Mn4Si7的第一性原理计算

基于密度泛函理论（DFT）的第一性原理计算方法,对本征Mn4Si7以及Ge,Al单掺杂和共掺杂Mn4Si7的晶体结构,能带结构,态密度以及光学性质进行了计算和分析.计算结果表明：本征态Mn4Si7的禁带宽度为0.810 eV,为直接带隙半导体材料,掺杂后晶体结构稍微变化,禁带宽度减小,且共掺杂时禁带宽度最小,电导率最好.Al以及Ge,Al共同掺杂时会产生杂质能级.掺杂后光子能量向低能级方向移动,光电导率,光吸收,反射系数都有所增大,说明掺杂改善了Mn4Si7的光学性质,从而可以提高光伏发电效率.     

Detection of photoquenching in CuGaTe2

The photoconductivity spectrum of a single crystal of p type of CuGaTe₂, has been examined at 300K and 77K. Structural details, which appeared on the high energy side of The band gap transition, are attributed to the valance band structure. Pronounced photoquenching is also observed on the high energy side and it is due to the recombination of holes with the ejected electrons from the inner 3d level of copper atom. Such a photoquenching effect has not been reported so for.     

Localized states in amorphous arsenic

Optical absorption and photoconductivity spectra of bulk amorphous arsenic are presented and analyzed using a general density of states model for amorphous semiconductors. It is concluded that localized states do exist in the forbidden gap and at the band edges and that transitions between localized states are significant.     

Investigation of the Occupation Function of Localized States in Amorphous Semiconductors Under Infrared-Light Excitation

The occupation function of localized states in a-Si:H under i.r. light excitation is directly calculated from the observed infrared-stimulated photocurrent spectra. The calculated occupation function in dual-beam and one-beam excitation cases are compared. The overshot phenomenon in the dual-beam experiment is explained as the result of the difference of the initial condition between two cases when i.r. light is turned on. In the dual-beam excitation case, at the beginning, in the upper part of the band gap, there are many trapped electrons to be excited into the conduction band to cause the photoconductivity overshot.     

Optical and photoelectrical properties of Hg1−xCdxTe/CdTe epitaxial films with graded band gap

The infrared transmission and photoconductivity spectra of HgCdTe epitaxial films with graded band gap were investigated both theoretically and experimentally. Theoretical calculations were performed in the framework of the WKB approximation. The composition profile has been obtained from a fitting procedure. In order to reduce the total number of fitting parameters as well as to improve accuracy of this procedure the differential of the transmission versus photon energy curves was used. The best fit was obtained for an exponential composition gradient.     

Photocurrent spectroscopy of indirect transitions in Ge/Si multilayer quantum dots at room temperature

Lateral photoconductivity spectra of multilayer Ge/Si heterostructures with Ge quantum dots were studied in the work proposed at room temperature. The photocurrent with minimal energy 0.48-0.56 eV that is smaller than Ge band gap was observed from such structures at the geometry of waveguide excitation. Generation of the photocurrent with the limit energy 0.48-0.56 eV was explained by spatially indirect electron transitions from heavy hole states of SiGe valence band into Δ₂-valley of the conduction band of Si surrounding. It was found out that the limit energy of such transitions decreased, as the number of SiGe quantum dot layers increased.     

Intriguing photoconductivity behaviors of SrTiO3−δ thin films with Pt contacts

The photoconductivity spectra of oxygen-deficient SrTiO_3−δ thin films with Pt contacts show two intriguing peaks at and above the band gap (∼3.2 eV). The first (second) peak, located at (above) the band gap, shows hysteretic (linear) photocurrent-voltage characteristics with a long (short) relaxation time. In addition, the first (second) peak shows a weak (strong) dependence on the surface treatment of the substrate as well as the surrounding atmosphere. These results have been discussed in terms of the Schottky contact, built-in voltage, and trapping/recombination process.     

First observation of photoconductivity in the semiconducting phase of VO2

We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of VO₂ single crystals. At low temperatures the frequency dependence of the photosensitivity (ps) exhibits an edge which gradually flattens at higher temperatures. The resulting mobility gap at 4.2 K amounts to 0.95 eV and thus differs from the optically determined band gap (0.65–0.75 eV).¹ These measurements, together with other known data suggest a “quasi-amorphous” electronic behavior of crystalline and semiconducting VO₂.     

Negative photoconductivity of selectively doped SiGe/Si: B heterostructures with a two-dimensional hole gas in the middle-infrared range

The spectra of lateral photoconductivity in selectively doped SiGe/Si: B heterostructures with a two-dimensional hole gas are analyzed. It is revealed that the lateral photoconductivity spectra of these heterostructures exhibit two signals opposite in sign. The positive signal of the photoconductivity is associated with the impurity photoconductivity in silicon layers of the heterostructures. The negative signal of the photoconductivity is assigned to the transitions of holes from the SiGe quantum well to long-lived states in silicon barriers. The position of the negative photoconductivity signal depends on the composition of the quantum well, and the energy of the low-frequency edge of this signal is in close agreement with the calculated band offset between the quantum-confinement level of holes in the quantum well and the valence band edge in the barrier.     

Free and bound exciton decay in indium phosphide

Excitonic recombination and photoconductivity near the energy gap of vapour grown InP epitaxial layers are investigated. Besides the free exciton several bound exciton complexes are observed and studied as a function of temperature. A new value of the band gap is derived (E_g = 1.424 ± 0.001 meV).     

Photoconductivity in the ordered vacancy compound CuIn5Se8

Thin films of the ordered vacancy compound CuIn₅Se₈ are deposited on glass substrates by multisource co-evaporation method and photoconductivity measurements are done on the films at various temperatures from 10 up to 300 K. The two photoactive bands that are identified in the spectral response spectra of CuIn₅Se₈ thin films at all measured temperatures are attributed to photoactive transitions between acceptor V_Cu to donor In_Cu and valence band to conduction band transitions respectively. From the spectra, a shift in band-to-band gap from 1.36 to 1.3 eV is observed with a temperature variation from 10 to 300 K. The non-exponential long-term decay observed in the transient photoconductivity spectra suggests a deep level trap-emptying process to be associated with the decay process and the decay constants are calculated by the differential life-time concept. From the steady state photocurrent analysis, a reduction in intergrain potential barrier on illumination has been noted as one reason for increase in conductivity on illumination. X-ray photoelectron spectroscopy analysis has been done to determine the binding energies of Cu, In and Se in the compound.     

Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.