Negative FIR-photoconductivity in n-GaAs

Negative photoconductivity in n-GaAs has been observed in the far infrared spectral range between 20 and 29 cm^–1. Negative photoconductivity occurs when a magnetic field is applied to the samples and impact ionization of shallow donors by the electric bias field is the dominant mechanism of electron excitation to the conduction band. A conceivable model qualitatively explaining the experimental results is proposed, which involves optical transitions from the lowest Landau subband to higher bound states of shallow donors.     

Defect-dominated optical emission and enhanced ultraviolet photoconductivity properties of ZnO nanorods synthesized by simple and catalyst-free approach

We report on the defect-dominated light emission and ultraviolet (UV) photoconductivity characteristics of ZnO nanorods (NRs) fabricated using a facile, cost-effective, and catalyst-free thermal decomposition route under varying reaction temperatures. The morphological and structural studies reveal the formation of homogeneous quality nanorods in large scale at the highest reaction temperature of 600 ^°C. The luminescence feature of the nanorods is dominated by the defect related emission over the typical band edge emission. The variation of band-edge and native defect-related emission response of the samples has been correlated to the morphology and microstructure. In photoconductivity studies, the I–V characteristics of the ZnO NRs prepared at different reaction temperatures in dark and under UV illumination (λ=365 nm) follow the power law, i.e., IαV ^r . An enhanced ultraviolet photodetection has been observed in the nanorods fabricated at the highest reaction temperature of 600 ^°C. The sample prepared at highest reaction temperature of 600 ^°C exhibits UV photosensitivity value (photo-to-dark current ratio) of around 1.18×10³, which is much higher in magnitude compared to that of the samples prepared at lower reaction temperatures. The enhanced photoconductivity may be assigned to the development of uniformity and homogeneity of the nanorods. Further development of such ZnO nanostructures can form the basis of promising prototype luminescent and UV photodetecting devices.     

AgGaSe2 : A highly photoconductive material

High resistivity single crystals of AgGaSe₂ were grown by the horizontal Bridgman technique. The near band edge photoconductivity of the grown crystal at room temperature was found to be up to 2×10⁴ times higher than the dark conductivity, under the illumination of 10⁻³ W/cm². The photoconductivity spectrum consists primarily of three peaks, which are attributed to the transitions from Γ₇(A), Γ₆(B) and Γ₇(C) states of valence band to the conduction band Γ₆. The crystal field splitting and the spin-orbit splitting were determined from these peak energy positions of the photoconductivity spectrum.     

Saturation of ionization edge absorption by donors in germanium

A study of saturation of the absorption and photoconductivity of Sb and P donors in Ge for radiation of 90 m wavelength, i.e., of energy very closely above their ionization edges is presented at T=9.3 K. Under these conditions negligible heating by the excess radiation energy is expected, which provides a convenient opportunity to study the kinetics of photoionization and recombination. From these measurements we have determined the donor capture cross section of electrons at 9.3 K to be _c=(1.2±0.7)×10^–12cm^–2, and the relaxation time from the 2s to the ground state as ₂₁=(5.8±1.0)×10^–10s. The saturation intensity of the absorption coefficient is around three orders of magnitude higher than the saturation intensity of the photoconductivity. We explain the nonlinear photoconductivity by the Debye-Conwell dependence of the mobility on the number of photoionized donors and compensating acceptors.     

Magneto-optical study of shallow donors in transmutation-doped GaAs

Lineshapes and peak positions of 1s→2p^?1, donor transitions in epitaxial GaAs samples of relatively low compensation have been studied as functions of magnetic field by use of photoconductivity measurements. Some of these samples were produced by transmutation doping using thermal neutrons—a method which is useful for the controlled introduction of donor impurities in GaAs. Two new effects, tentatively attributed to van der Waals interactions between neutral donor atoms, are observed: (1) although both Se and Ge donors are introduced by thermal neutron transmutation, the Se line is much broader than the Ge line, and (2) deviations from isolated-donor behavior occur in the magnetic field dependence of the chemical shift of the shallowest donor present. The separation of lines from two deeper donors, Ge and Si, verified the simple phenomenological theory of the magnetic field dependence of central cell corrections of isolated donors up to at least 10T.     

Photoconductivity in high purity,semiconducting CdF2

The photoconductivity spectra of high purity, semiconducting CdF₂, containing low concentrations of rare-earth donors, are interpreted in terms of a shallow trap 1260 ± 100 cm⁻¹ below the bottom of the conduction band and an excited bound state 1030 cm⁻¹ above the ground state.     

Compensation effect in undoped polycrystalline CdTe synthesized under nonequilibrium conditions

The compensation effect has been revealed in undoped polycrystalline CdTe synthesized during rapid crystallization. The revealed effect leads to an increase in the electrical resistivity to 10⁸–10¹⁰ Ω cm at a background impurity concentration of ～1015 cm^?3 (Ga_Cd and Cl_Te donors, unidentified acceptors). For some samples, this effect is accompanied by the appearance of persistent photoconductivity, which disappears at a temperature of ～200 K. It has been shown that all the polycrystals studied are characterized by a three-level compensation mechanism in which the fundamental properties of the material are determined by deep donors and/or acceptors with a concentration of 10¹² cm^?3. Depending on the specific growth conditions, the electrical resistivity at room temperature is determined by deep centers with activation energies of 0.59 ± 0.10 and 0.71 ± 0.10 eV, which are supposedly related to intrinsic point defects, and deep centers with activation energies of 0.4 ± 0.1 eV, which belong to the DX center formed by the Ga_Cd donor.     

Photoconductivity on europium oxyde single crystals and thin films

In this paper, results of photoconductivity measurements on four EuO samples are given. Low frequency photoconductivity versus temperature (10°K < T < 300°K) and magnetic field H is investigated for two wavelengths: 6600 Å and 9000 Å. The photoconductivity kinetic is also described, and is characterized by a distribution in decay times. Temperature, magnetic field and carrier concentration have small effects on this kinetic. Quenching effect is obtained by adding a continuous illumination (λ₂) to the weak modulated light (λ₁). The kinetic is strongly affected by quenching and becomes more simple. Quenching effect is maximum for the wavelength associated to the 4?⁷–4?⁶ 5d, transition. In contrast to the Penney-Kasuya[1] model we propose another one in which the conduction of equilibrium carriers as photo-excited carriers takes place in a broad band. The variation of low frequency photoconductivity versus temperature is attributed to the mobility variation. This variation agrees well with the model of mobility controlled by spin-disorder. The photoconductivity kinetic is interpreted by a three levels recombination model: the conduction band, the 4f levels and a distribution of trap levels. The lack of variation of photoconductivity decay in the range of metal-semiconductor transition is discussed.     

Microwave photoconductivity relaxation time in a bifacial silicon solar cell base in the vicinity of a p-n junction

Microwave photoconductivity relaxation time depending on light intensity is studied in n ⁺-p-p ⁺ silicon solar cells. The results from experiments performed under conditions of open-circuit and short-circuit currents are in agreement with the simulated data. The relaxation times of microwave photoconductivity are found for a part of the base region adjacent to the n ⁺-p junction.     

High magnetic field studies of donor excitation spectra in InSb

The two lowest energy spectral lines of the shallow donors in InSb involving ground to excited state transitions are studied in photoconductivity using higher spectral resolution and stronger magnetic fields than achieved previously. The observed line positions are compared with recent calculations of the high field hydrogenic donor levels and difference of the order of the effective Rydberg R¹ at zero field are found at magnetic fields where the zero point cyclotron energy exceeds R¹ by two orders of magnitude. Central-cell components of the 1s–2p transition, corresponding to four donor species are resolved, and the magnetic field dependence of the relative chemical shifts are analysed. The broader 1s–2p₀ line undergoes a coupling at an interaction energy of 37 cm^-1, the origin of which is uncertain at present.     

Direct measurement of the gap states and band tail absorption by constant photocurrent method in amorphous silicon

A direct way to obtain the spectra dependence of the optical-absorption coefficient in the low-absorption region (10^?1?10³cm^?1) on thin amorphous silicon films is presented. The constant photocurrent method (CPM) is described compared with normalized photoconductivity spectra measurement and the results on a-Si:H are presented. Implications of the CPM for the gap states spectroscopy are suggested.     

Impurity photoconductivity in epitaxial layers of gallium arsenide

Impurity photoconductivity spectra in the range 0.5–1.5 eV are studied in epitaxial layers of n-GaAs grown on substrates of semiconductive GaAs withρ > 10⁶ Ω·cm in the system Ga-AsCl₃ -H₂. The effect of uncontrolled acceptor impurities on the impurity photoconductivity spectrum is evaluated.     

Polarization dependence of intersubband absorption and photoconductivity in p-type SiGe quantum wells

We present a detailed study of the polarization dependence of subband absorption and photoconductivity in Si/SiGe quantum wells. For samples with a hole concentration ofp_s2.8×10¹² cm², bothp- ands-polarized absorptions have been observed and transitions to several excited states are clearly identified by comparison with self-consistent Luttinger-Kohn type calculations. The photoconductivity is surprisingly insensitive to the polarization, which indicates the importance of the subsequent transport process on the photocurrent responsitivity.     

Frozen photoconductivity in PbTe films

We have studied PbTe films of thicknessd=200/10000 A made with telluride vapour deposition on glass substrate at room temperature. The estimate of the donor concentration ～10¹⁹ cm^?3 of the fresh-deposited film compared with the impurity content in the bulk raw material ～10¹⁷ cm^?3 shows that the donors were mainly film defects or nonstoichiometric Pb atoms. Electrical conductivity of the freshly deposited film increased with lowering of the temperature. After deposition the donors were compensated with an oxidation in the laboratory air. Transition to the thermally activated conductivity resulted from oxidation. At temperatures belowT≈100 K the resistance of the compensated films followed Mott’s ruleR=R ₀ exp(T ₀/T)^1/3. The square film value 1 Mohm andT ₀≈100 K ford=1000 A. At low temperatures an exposure to light resulted in sharp decrease of the film resistance. At liquid helium temperatures the resistance dropped 10³–10⁶ times and stayed at the low value for an indeterminate time. The heating of the film aboveT=100 K gave rise to an initial high resistive state. The critical temperatureT _c, when the frozen photoconductivity became negligible, varied with samples in the temperature region 90–120 K. Near the critical temperature we could measure the time dependence of the film resistance after the light exposure, which followed the equationR=A+B.lnt fort>1 sec with the empirical constantsA andB. After a time intervalτ the resistance gained the initial “dark” value and remained stationary. The value lnτ≈α.(T _c?T), where the factorα approximately wasα≈0.5 K^?1. Some results of these experiments were published earlier (Krylov and Nadgorny 1982; Krylov and Pojarkov 1984).     

Fir photoconductivity in epitaxial InP

FIR photoconductivity from shallow donors in n-type InP was studied using Fourier transform spectroscopy. The ls → 2p transition frequency was found to be 45.5 ± 0.2 cm⁻¹ in zero magnetic field. Spectral response measurements made in magnetic fields from 0 to 40 kG reveal Zeeman transitions of the ls → 2p (m = 0, ±1) states. Laser magnetospectroscopy was also investigated, employing an optically-pumped FIR laser. There is excellent agreement between the results obtained by the two experiments. From the transition energies, an effective mass of 0.077 ± 0.003 m₀ is obtained. With the experimentally-determined ls → 2p transition energy and the effective mass, the static dielectric constant of InP at 4.2 K was found to be 11.8 ± 0.2.     

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.     

First photoconductivity measurement following photoionization of rare-earth dopant in a dielectric powder

Transient response of the dielectric permittivity of dielectric powder of LSO:Ce³⁺ under pulsed laser excitation were recorded by the 8-mm microwave resonator technique at room temperature. The signal in absorption mode is the signature of Ce³⁺ photoionization and photoconductivity effect in the rare-earth-doped dielectric grains. The signal in dispersion mode results from surface defects around each grain leading to electronic traps. This opens new perspectives for photoconductivity measurements on rare-earth-doped insulating nanopowders.     

Photoconductivity in amorphous thin films of Ge22Se78−xBix

Temperature and intensity dependence of photoconductivity is studied in amorphous thin films of Ge₂₂Se_78−x Bi_x with x = 0, 2 and 10. Transient photoconductivity measurements have also been made on the same samples. Our results show that photosensitivity decreases as Bi concentration is increased from x = 0 to x = 2. However, at high concentration of Bi(x = 10), photosensitivity again increases. Transient photoconductivity also show a different behaviour at low and high concentration of Bi. Results have been explained in terms of defect states produced due to Bi incorporation in GeSe system.     

Photoconductivity decay in Te-Ge-Sb glasses

Photoconductivity kinetics in Te-Ge-Sb glasses in the region of bimolecular recombination is investigated as a function of temperature and time of illumination. An untypical shape of photoconductivity decay is interpreted in terms of Street and Mott model of recombination by tunnelling between randomly distributed D⁰ centers of concentration 10¹⁸–10¹⁹ cm^-3.     

The electrical resistivity of 241Am metal

The electrical resistivity of ²⁴¹Am metal was measured between 3.6 and 300 K using a layer of 1.89 μm and a bulk sample of 0.16 mm thickness. The analysis of the low temperature power law gave a T^2.8 dependence for the layer and a T^4.4 dependence for the bulk sample. At higher temperatures, deviations from a linear temperature dependence were found as in uranium and neptunium. The room temperature resistivity of 68.90 μΩcm, and some other physical and chemical properties of americium, are similar to those of the light lanthanide metals. The residual resistivity ratio of the bulk americium sample is 28. The experimental results can be explained by applying the s-d interband scattering model of Mott. Americium has to be considered the first lanthanide-like metal in the actinide series, with the 5<f; level lying more than about 5 eV below the Fermi energy.