Spectral Response Of Photoconductivity Research Articles

Photoconductivity in germanium selenide single crystals

AbstractThe photoconductivity spectral response is measured for GeSe single crystals at different temperatures from room temperature to near liquid nitrogen temperature. Measurements are carried out with plane polarized light with the plane of polarization parallel to the a‐ and c‐crystallographic axes which lie in the plane of cleavage. The photon energy range of measurements extends from 0.5 to 3.0 eV. The photoconductivity energy gap is determined from the spectral response by the Moss rule. The room temperature photoconductivity energy gap is found to be (1.144 ± 0.011) and (1.167 ± 0.025) eV for the a‐ and c‐axes, respectively. The temperature dependence of the photoconductivity energy gap is also deduced. It is found that it is linear between 93 and 250 K with a negative temperature coefficient equal to −(0.35 ± 0.01) meV/K for the a‐axis. The results for the c‐axis show a large discrepancy which is attributed to crystal lattice bending introduced during the cleavage process. The minority carrier lifetime is determined from the photoconductivity frequency response using square modulated illumination. The photoconductivity frequency response is found to be characterized by two different lifetimes.

Research note Variation of band gap with temperature in cadmium phosphate glass

The spectral response of photoconductivity of 50 mol.% CdO-50 mol. % P2O5 glass has been measured under different applied fields in the photon energy range l·5–6·2eV at temperatures ranging from 192 K to 310 K. A numerical technique has been employed to analyse the data. The band gap obtained has been found to be independent of the applied field and the field average of the band gap shows a slow decrease with the increase in temperature.

The room temperature photoconductivity in Zn 3P 2

Photoconductivity (a.c.) spectral response curves have been measured on high quality Zn 3P 2 single crystals in the 0.65–2.6 eV energy range. Absorption plots in the 0.65–1.5 eV region for the same samples have also been determined. A set of optical transitions has been found. Excitations from the low energy region are interpreted in terms of impurity transitions and from the high energy region as band-to-band ones.

Growth and characterization of Bi<inf>12</inf>SiO<inf>20</inf>films by metalorganic chemical vapor deposition

The metalorganic chemical vapor deposition (MOCVD) technique was applied to the epitaxial growth of Bi <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</inf> films. The growth was carried out in an atmospheric and low pressure reactor using Bi(CH <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and Si (OCH <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> (or Si(OC <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> ) as metal sources, and O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> or N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O gas as an oxidizing gas. The single crystal films have been grown over a wide composition range of 5.5 < Bi <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> /SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> < 8. The lattice constants of the films increase in proportion with Si deficiencies. Films with good compositional uniformity were obtained using the N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O gas as an oxidizing gas. For these epitaxial films, it was found that the photoconductivity was ten to fifteen times as large as that of bulk crystals and there was a strong relationship between the film composition and the photoconductivity spectral response.

Investigation of photoconductivity sensitising centres in CdGaInS4 layered crystals

Localised levels inducing high photosensitivity in CdGaInS4 crystals are investigated. An electrical conductivity increase of about 108 and 105 in magnitude was measured at 110 K and 300 K respectively, under an illumination power of about 1 mW cm-2. A photoconductivity spectral response displaying a maximum at 2.64 eV and 300 K with a thermal shift of about 5*10-4 eV K-1 in the range 77-300 K has been observed. The high photosensitivity and a super-linear region in the lux-ampere characteristics (LAC) have been explained by assuming the presence of sensitising and recombination centres following the Rose-Bube model. By studying the thermal quenching of photoconductivity, a concentration Ns>1015 cm-3, an ionisation energy Es=0.24 eV and a ratio of the electron to hole capture cross section of sensitising centres Sn/Sp=(2-5)*10-4 were calculated. The origin of these centres is hypothesised to be due to anti-structural defects caused by a partial redistribution of the Cd-Ga atoms within the cationic sub-lattice.

Photoelectronic properties of HgI2

Two types of photoelectronic measurements have been used to determine the properties of HgI2 crystals and photodetector cells: thermally stimulated conductivity, and spectral response of photoconductivity in both the lateral and transverse geometries. Characteristic trap depths are identified that compare well with previously reported values, and a decrease in apparent density with time and temperature cycling is described. Analysis of photoconductivity spectral response using the DeVore model for lateral contacts and the Goodman model for transverse contacts allows the determination of photoelectronic parameters for the material.

Surface effects and grain-boundary domination in thin-film Zn3P2 photoconductivity

Photoconductivity spectral response, linearity, and response time measurement indicate that surface effects and intergrain boundaries can play important roles in the characteristics of thin-film Zn3P2. Response time of the order of a few seconds and the dependence of the spectral response on vacuum operation indicate the existence of long-lived states at the surface. Photoconductivity response was found to be a maximum at a wavelength of 400 nm. Vacuum operation and heat treatment were observed to improve responsivity, shift the peak responsivity wavelength towards longer wavelengths, and increase the spectral width of maximum responsivity. The photoconductivity spectral dependence and the response times were found to be light–intensity dependent. It is proposed that effects of the barrier heights at the surface and at the intergrain junctions are light-illumination dependent. The properties of the surface and the grain boundary depend on chemisorption or chemical reactions due to adsorbed gases or impurities.

Electrical properties of Bi2S3 thin films prepared by the dip-dry method

Bi2S3 thin films have been prepared by the dip-dry method and their electrical properties such as conductivity, thermoelectric power and spectral response of photoconductivity were investigated. Some uncommon findings, such as a simultaneous increase in conductivity and thermoelectric power with thickness of film, are explained by Seto's model for polycrystalline silicon film duly modified for in homogeneities.

The photoconductive effect in Pb2CrO5 thin films prepared by an electron-beam evaporation technique

Photoconductivity in Pb2CrO5 thin film prepared by an electron-beam evaporation technique is described. Crystallographically, three kinds of thin films are fabricated which depend on substrate temperature. A sample showing a similar x-ray diffraction profile to the evaporation source material gives the highest photoconductive response. Light illumination from the glass substrate onto the sample improves photoconductivity. A pair of interdigital electrodes is more effective than a pair of planar electrodes on the photoconductive measurement. A band gap energy level of Pb2CrO5 thin film is around 2.2–2.4 eV as a result of the spectral photoconductive response.

On the photoconductivity of copper sulphide polycrystalline thin films

We have studied the spectral response of the photoconductivity of copper sulphide polycrystalline films obtained by thermal evaporation. The phase content of the samples was determined by electron diffraction and the stoichiometry by potentiostatic methods. The electrical properties, resistivity and Hall effect, were determined by the Van der Pauw method. The photoconductivity quantum efficiency spectra show structures clearly characteristic of the phases chalcocite and djurleite. Chalcocite shows peaks at 900, 720 and 500 nm and Djurleite at 620 and 500 nm. Samples with less copper always show the 500 nm peak. This work shows that a peak at 500 nm appears in the photoconductivity spectral response of all copper sulphides studied: Cu x S (1.89 ⩽ x ⩽ 2) .

Spectral response of photoconductivity in polycrystalline semiconductors

Photoconductivity in polycrystalline semiconductors can arise from the modulation, by the optical illumination, of the diffusion potentials at the grain boundaries, which in turn control the majority carrier current in these materials. The photogenerated carriers are captured by interface states at the grain boundaries through Shockley-Read-Hall processes, and under constant illumination this affects the steady-state charge at these boundaries so as to reduce the diffusion potentials from their dark values. The dependence of the diffusion potentials and of the photoconductivity upon position is calculated from the dependence of the optical absorption on wavelength. Finally, the average photoconductivity (expected from measurement) of polycrystalline semiconductor thin films is determined as a function of the intensity of the optical illumination, the grain size, the film thickness and the grain-boundary interface parameters.

Spectral response of ac photoconductivity in Zn3P2

ac (80 Hz) photoconductivity spectral response curves have been measured for seven single crystal samples of Zn3P2 prepared by different preparation techniques and subjected to different heat and surface treatments prior to measurement over the spectral range from 0.6 to 1.2 μm. Surface ac photoconductivity lifetimes range from 0.4 to 40 μsec, and bulk ac photoconductivity lifetimes range from 3 to 100 μsec. Major contributions to the photoconductivity near the band edge of Zn3P2 come from transitions of about 1.41 and 1.52 eV, probably corresponding to direct transitions from the two higher-lying valence bands split by spin-orbit and crystal fields.

Photoelectronic techniques for material diagnostic

In this paper, the most importnat parameters characterizing the physical properties of electronic materials are briefly analysed. Special attention is given to the intrinsic parameters such as energy gap, density of states and effective masses in the bands, to the extrinsic ones such as energy and concentration of trapping or recombination centers in the forbidden gap, to the carrier lifetimes and surface recombination velocities. The knowledge of all these parameters is very important, since they strongly influence the electronic behaviour of the materials. To this purpose, some of the main electronic and photoelectronic techniques have been selected, which, in our opinion, are particularly suitable to give information about the electronic properties of solids, specially in the case of semiconductor materials. Thus, intrinsic parameters are analysed by the behaviour of resistivity, mobility and carrier concentration as a function of temperature; the extrinsic parameters by techniques such as Space-Charge-Limited-Currents (SCLC), Thermally-Stimulated-Currents (TSC); carrier lifetimes and surface recombination velocities by the spectral response of photoconductivity and photomagnetoelectric effect (PME). The basic physical principles and a short theoretical treatment are given for each method of analysis, together with typical examples of application, indicative of the ranges of validity and sensitivity.

Shallow defect levels in neutron-irradiated p-type extrinsic silicon

Two new acceptor levels, at 0.027±0.001 eV and 0.039±0.004 eV from the valence band, have been observed by measurements of Hall effect versus temperature in neutron-irradiated float-zone-grown Si : Ga. The presence of these very shallow electronic energy levels was also indicated by photoconductive spectral response measurements at 5 K. The purpose of neutron irradiation was to counterdope residual boron by neutron transmutation for infrared detector applications. These shallow acceptors are observed after anneals at temperatures as high as 625 °C, but annealing at 700–850 °C reduces their concentration below detectable levels. Shallow levels are also observed in neutron-irradiated Si : Al and Si : In grown by the floating-zone method. Results indicate that formation of these shallow defects depends upon the presence of column-III dopant atoms (Ga, In, Al).

Photoelectronic properties of LPE GaAs : Cu

The photoelectronic properties of liquid-phase epitaxial GaAs : Cu, either grown from a Ga melt with Cu impurity added or with incorporation of Cu by diffusion after growth, have been investigated through measurements of photocapacitance; conductivity in light and dark as a function of photon energy, light intensity, and temperature; thermal and optical quenching of photoconductivity; thermally stimulated conductivity; and photoluminescence. When Cu is present in the growth melt, the electron density in the resulting LPE GaAs layer is increased; when Cu is diffused into a grown layer, it behaves as a conventional acceptor and converts the layers to high-resistivity p-type conductivity. A Cu-related center with energy level about 0.43 eV above the valence band, electron-capture cross section of 10−21 cm2, and hole-capture cross section of 10−16 cm2 is observed in the layers produced with Cu in the melt by spectral response of photocapacitance and photocapacitance decay, and in the layers with diffused Cu by thermal and optical quenching of photoconductivity, spectral response of photoconductivity, and thermally stimulated conductivity. Six other levels, three of them electron traps and three of them hole traps, are also observed and described.

Photoconductive CdS Films by a Chemical Bath Deposition Process

A new technique for the preparation of thin films by chemical bath deposition is described. The films prepared by this method have many of the advantages of those made by the spray deposition method, such as the ease of coating large areas and simplicity of the process. The stoichiometry is easy to maintain in both these methods. The films are formed on rotating substrates from a bath containing cadmium salt in a complex form. Electrical, photoconducting, and optical properties such as I–V characteristics, decay of photoconductivity, spectral response of photoconductivity, and optical absorption were studied. Addition of cations like Ag, Cu, In, and Zn to the deposition bath makes the films amorphous. Such amorphous layers have a density of state of the order of 1019–1020/cm3/eV near the Fermi level, as determined by photodecay and field effect measurements. The films produced are strong, adhering tightly to the substrates, show little or no degradation, and are photosensitive as deposited.

ＣｄＳ沈殿の結晶構造と光電導

The CdS precipitates were prepared from aqueous solutions of cadmium chloride, bromide, nitrate, sulphate or acetate, and the relation between crystal structure and photoconductivity of these CdS precipitates was investigated. All CdS precipitates were found to be a mixture of hexagonal and cubic phases, and their mean particle sizes were about 1.5 to 4μm. All precipitates were found by chemical analysis to be slightly sulphur-rich.Using an interdigital electrode, the spectral response of photoconductivity and the rise and decay of photocurrent were measured. All specimens showed photoconductive response in the visible wavelength region, but the peak wavelength scattered in the wavelength region of 550∼570 nm from one specimen to another. The rise or decay time was very long for the specimens prepared from cadmium chloride and bromide. The specimen prepared from cadmium sulphate had the highest photoconductive sensitivity, while the one from cadmium bromide had the lowest sensitivity.

Photoconductivity in anodic Ta2O5 formed on nitrogen-doped tantalum films

Photocurrent spectroscopy (or the spectral response of photoconductivity) and transient photoconductivity have been used to probe the defect structure (trap states) of anodic Ta2O5 films formed on pure and nitrogen-doped sputter-deposited tantalum. Results of this investigation indicate the presence of two trap-related responses at 1.5 and 2.8 eV, respectively. Nitrogen in the oxide is found to compensate these traps, that is, the amplitude of the photocurrent response is observed to decrease with increasing nitrogen concentration. The recombinationlike trap at 2.1 eV, present in pure Ta2O5, is not observed in samples doped with as little as 2 at.% N2. Transient results indicate a spatial localization of the 1.5-eV trap which may be related to the nonuniform nitrogen distribution in the oxide. Nitrogen in the oxide was found to have little effect on the band-gap energy (4.4 eV).

Photoconduction of glasses in the TeSeSb system

The photoconductivity of bulk glasses of the TeSeSb system is measured as a function of light intensity and photon energy. The relative sensitivity ( ΔI/ I d) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.

Exponential photoconductive edge in amorphous GeTe∗

The photoconductive spectral response of amorphous GeTe has been studied in evaporated films grown under various conditions and subjected to vacuum heat treatments and proton bombardment. The response at ∼ 200°K was found to vary exponentially with photon energy with slope 20eV −1 for all samples, independent of growth conditions or post-growth treatment. The spectral position of the photoconductive edge and its stability to environmental changes, such as demonstrated here, make these films attractive for application in near infrared imaging devices.