Trap-related photoconductivity in ZnO epilayers

A strong photoconductive response is observed for ZnO epilayers in the presence of both above bandgap and below bandgap photoexcitation. Photoexcitation for energies larger than the bandgap results in a photoconductive response with fast and slow time constants on the order of nanoseconds and larger than milliseconds, respectively. The fast and slow time constants are attributed to minority carrier recombination and slow escape of holes from traps, respectively. Photoexcitation in the visible spectral region, below the bandgap energy, results in slow rise and fall time constants on the order of minutes and hours. A model for the photoconductive response based on rate equations is presented providing an accurate fit to measured photoconductivity data. The rate equation model suggests the presence of hole trap levels in the energy range of 0.6 eV to 1.0 eV relative to the valence bandedge. The passivation of the ZnO surface with SiO₂ shows significantly reduced photoconductive transient decay time constants, suggesting a significant reduction of deep surface defects on the ZnO material.     

Specific features of the liquid-phase epitaxial growth of SiC epilayers in vacuum

4H-SiC epilayers were grown by a liquid-phase epitaxy in vacuum. It was found that the seed layer with steps characteristic of liquid-phase epitaxy should be preliminary deposited on the substrate. It is demonstrated that growth in a vacuum leads to a decrease in the concentration of uncompensated carriers N _d–N _a to the level of 2×10¹⁶ cm^?3.     

Specific features of intrinsic photoconductivity spectra of copper-compensated indium phosphide

The intrinsic photoconductivity of copper-compensated indium phosphide has been studied. It is found that mechanical polishing of a sample surface gives rise to an additional photoconductivity peak in the region of the fundamental absorption edge. This peak disappears upon storage of the sample. The dependence of the shape of the photoconductivity spectrum on the storage time, electric-field strength, and position of the light spot with respect to the contacts was determined. The results are explained in terms of variation in the lifetime of nonequilibrium carriers across the sample thickness. An expression qualitatively describing the photoconductivity spectra is presented.     

Specific temperature-related features of photoconductivity relaxation in PbSnTe:In films under interband excitation

The time dependences of variations in the photoconductivity of PbSnTe:In films in the range of T ≈ 19—25 K upon interband excitation are studied. It is found that the character of conductivity relaxation after switch-off of illumination depends on the duration and intensity of the preceding illumination. In this case, the characteristic times of relaxation for various modes of illumination can differ by more than an order of magnitude. The obtained results are discussed in the context of a model assuming the presence of a quasicontinuous spectrum of capture levels in the band gap of PbSnTe:In and also a possible effect on the parameters of these levels of the ferroelectric phase transition, the temperature of which is found to be in the temperature range under study     

Specific features of photoconductivity spectra of the CdTe/CdHgTe epitaxial graded-gap heterostructures

Based on studies the structural and photoelectric properties of CdTe/CdHgTe graded-gap heterostructures, it is found that the metallurgical interface between the materials, which is located in the structure bulk, is enriched with structural defects. An increased recombination rate is characteristic of this interface. This circumstance is the cause of the loss of photosensitivity in the mid-IR range of the broadband photoconductivity spectra of these heterostructures. For the epitaxial layers thinner than 10–20 μm, the effect of the metallurgical interface can manifest itself in the reduction or complete loss of photosensitivity.     

Specific features of implementation of the photoconductivity decay method in measurements of recombination parameters of a semiconductor

Concentration of excess charge carriers as a function of time in the course of photoconductivity decay has been calculated. The calculation takes into account the local disturbance of electroneutrality, which necessitates the use of the Poisson equation in the set of equations describing the given process. A rather cumbersome nonlinear differential equation has been derived by solving the new system. Criteria have been established and with the observance of these criteria it is possible to reduce that equation to a well-known equation conventionally used to describe the process of photoconductivity decay upon termination of a pulse of light.     

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe₂-ZnIn₂Se₄ alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn₂Se₄ are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe₂-ZnIn₂Se₄ single crystals containing 15 and 20 mol % of ZnIn₂Se₄, the hopping mechanism of conductivity is dominant at temperatures of T ～ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ～15 and 20 mol % of ZnIn₂Se₄ is a single narrow peak at a wavelength of λ_max = 1190–1160 nm.     

Intrinsic photoconductivity in chromium disilicide epitaxial thin films

Spectral and integrated photoconductivities in chromium-disilicide epitaxial films grown on single-layer silicon substrates have been studied in the photon energy range 0.5–1.6 eV. The region of the photoconductivity maximum observed at 1.23 eV corresponds to the third direct interband transition in chromium disilicide at 0.9–0.95 eV. Possible reasons for the weak photoconductivity signal in the region of the fundamental absorption edge are analyzed. Fiz. Tekh. Poluprovodn. 31, 969–972 (August 1997)     

Specific features of the ion-beam synthesis of Ge nanocrystals in SiO2 thin films

Semiconductors - The systematic features of the formation of Ge nanocrystals in SiO2 thin films implanted with Ge+ ions and then subjected to high-temperature annealing (1130°C) are studied in...     

Submillimeter wave photoconductivity observation of shallow donors in In0.53Ga0.47As

The hydrogen like 1s → 2p (m=?1,0,+1) transitions of two donors have been observed in high intensity magnetic fields up to 8.5T. The m=?1 transitions ocurred between 2 cm^?1 and 25 cm^?1. The signature curves for donors in ternary semiconductor In_0.53Ga_0.47As have now been established.     

Growth and characterization of ZnO thin films on GaN epilayers

Dense ZnO(0001) films formed at 500°C via coalescence of islands grown via metalorganic vapor phase epitaxy (MOVPE) either on GaN/AlN/SiC(0001) substrates or on initial, coherent ZnO layers. Conical crystallites formed due to thermal expansion-induced stresses between the ZnO and the substrate. Interfaces between the ZnO films on GaN epilayers exposed either simultaneously to diethylzinc and oxygen or only to diethylzinc at the initiation of growth were sharp and epitaxial. Interfaces formed after the exposure of the GaN to O₂ were less coherent, though an interfacial oxide was not observed by cross-sectional transmission electron microscopy (TEM). Threading dislocations and stacking faults were observed in all films.     

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg_{1 ? x} Cd _x Te epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.     

Mg and Al co-doping of ZnO thin films: Effect on ultraviolet photoconductivity

Thin films of Zinc Oxide were deposited by the sol-gel technique on glass substrates. The films were doped with Al, Mg or co-doped with both by introduction of appropriate compounds in the solution before dip-coating and annealing in air at 500 °C. Energy Dispersive X-Ray Spectroscopy was employed to measure the dopant incorporation. X-ray diffraction studies indicate that Mg doping increases grain size, while Al doping reduces it. Photoluminescence (PL) measurements indicate that undoped and Al-doped films show, along with a broad near band-edge (NBE) peak, additional peaks at longer wavelengths related to various defect states. However Mg doped films show only a sharp NBE peak, which is blue shifted compared to undoped ZnO, and there are no prominent sub band gap luminescence peaks. This is also the case for Mg and Al co-doped ZnO samples, provided the Mg content is low. Photocurrent measurements were carried out using silver contacts using a De source under atmospheric conditions. Undoped and Mg doped ZnO films showed high resistances and low photocurrent levels. With low Al doping, both the dark current and the photocurrent increase significantly, but the films show very long photocurrent transients. With optimized concentration of Mg/Al co-doping in ZnO, the photocurrent increased by ~98 times compared to ZnO films doped only with Mg. Simultaneously, the photocurrent transients became ~44 times faster than ZnO films doped only with Al.     

Certain features of Ga diffusion in ZnS powders

Spectra of photoluminescence and electron spin resonance were investigated for ZnS powders annealed in the presence of metallic Ga with limited access of atmospheric air. Analysis of these spectra showed that there was no Ga impurity in the annealed ZnS powders. It was established that subsequent free access of air to annealed ZnS:Ga promotes active Ga introduction into the ZnS lattice. A mechanism of Ga diffusion in ZnS is suggested.     

Lattice mismatch induced morphological features and strain in HgCdTe epilayers on CdZnTe substrates

HgCdTe epilayers on CdZnTe substrates can exhibit a cross-hatch pattern of periodically varying strain and surface undulations as revealed by x-ray topography, and in some cases by Nomarski optical microscopy. On { 111 } oriented material, the pattern appears as three sets of parallel lines in the 〈 110 〉 slip directions (60° apart). To investigate this phenomenon and its impact on photovoltaic device performance, we have characterized several liquid phase epitaxy (LPE)-grown HgCdTe epilayer samples by means of Lang x-ray reflection topography, synchrotron white beam x-ray topography (SWBXT), etch pit density, and other techniques. The cross hatching generally shows a correlation with the ZnTe mole fraction of the substrate. In particular, the pattern is likely to appear when the natural lattice parameter of the layer at room temperature is slightly larger or smaller than that of the substrate in the same region. We also find the corresponding pattern in { 211 } oriented layers grown by MBE. Although substantial compositional interdiffusion occurs at the layer/substrate interface during LPE growth at around 500°C, this is not a necessary condition for the cross-hatch pattern, as demonstrated by the occurrence of the pattern in MBE material grown at less than 200°C. In terms of device performance, the pattern is manifested as lines of diodes in an array having greater leakage than their neighbors. In addition to these results, we have investigated other anomalies, by means of SWBXT applied to large-area diodes that have been electrically tested. A novel technique called absorption edge contour mapping, using synchrotron white beam x-rays with a molybdenum filter, was applied to reveal the longer range lattice strain.     

Steady state and transient photoconductivity in amorphous thin films of Se0.8Te0.2

The present paper reports on steady state and transient photoconductivity measurements in amorphous thin films of Se_0.8Te_0.2. The paper also reports the effect of crystallization on the photoconductive behaviour. The photosensitivity(I _ph /I _d ) reduces by a factor of 5 and the decay of photocurrent becomes much slower in the films annealed above the crystallization temperature (state B) as compared to the films annealed below the crystallization temperature (state A). A detailed analysis of the photoconductive decay in the state B shows that the recombination within localized states may be the predominant recombination mechanism in this state.     

Dark conductivity and photoconductivity of amorphous Hg0.78Cd0.22Te thin films

This paper reports the dark conductivity and photoconductivity of amorphous Hg0.78Cd0.22 Te thin films deposited on an Al2O3 substrate by RF magnetron sputtering.It is determined that dark conduction activation energy is 0.417 eV for the as-grown sample.Thermal quenching is absent for the as-grown sample during the testing temperature zone,but the reverse is true for the polycrystalline sample.Photosensitivity shows the maximum at 240 K for amorphous thin films,while it is higher for the as-grown sample than for polycrystalline thin films in the range from 170 to 300 K.The recombination mechanism is the monomolecular recombination process at room temperature,which is different from the low temperature range.Theμτ-product is low in the range of 10-11-10-9 cm2/V,which indicates that some defect states exist in the amorphous thin films.     

Microwave photoconductivity and photodielectric effect in thin PbS films obtained from thiocarbamide coordination compounds

The dependences of photoresponse parameters of the microwave absorption caused by the effect of a short pulse of a nitrogen laser (337 nm, 10 ns) in micrometer-thick PbS films on the frequency of a microwave oscillator, intensity of light, and conditions of the film growth are investigated by a cavity method in the 3-cm frequency band. The films were obtained by pyrolysis of aerosol of thiocarbamide coordination compounds at a substrate temperature of 250–500°C. Using the analysis of frequency dependences of the photoresponse, the microwave photoconductivity and the photodielectric effect, which cause a variation in the cavity quality factor and a variation in its resonance frequency, respectively, are investigated separately.     

Specific features of anisotropic optical thermoelements

The effect of different temperature distributions on the main parameters of anisotropic optical thermoelements are studied in the optical transmission mode. It is shown that such devices implementing the ingenious “transparent wall” method are promising for recording radiation fluxes of different densities in wide spectral and power ranges.