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.     

Photoconductivity of chemically deposited lanthanum/neodymium doped (Cd - Pb)S films

Abstract

Results of photoconductivity rise and decay, optical absorption and photoconductivity excitation spectra are presented for different chemically deposited (Cd–-Pb)S films. The ratio of saturated photocurrent of rise curves (I_pc) to dark current (I_DC) is found to be of the order of 10⁵ for undoped (Cd_0.95–-Pb_0.05)S films which improves to 10⁶ for the systems prepared with NaF end Lanthanum/Neodymium nitrate. When annealing the films at 400°C for 2 minutes, the dark current decreases to almost zero, along with a decrease in the photocurrent, but the I_pc/I_DC ratio goes to a very high value. From the analysis of the decay curves lifetime of carriers are found to be 31.11 sec, 48.01 sec, 47.77 sec and 94.66 sec for (Cd_0.95–-Pb_0.05)S; (Cd_0.95–-Pb_0.05)S: NaF; (Cd_0.95–-Pb_0.05) S: NaF, La and (Cd_0.95 –- Pb_0.05)S: NaF, Nd respectively. Further the mobility of carriers for these systems are found to be 51.44 cm²/V-s, 93.33 cm²/V-s, 94.83 cm²/V-s and 164.78 cm²/V-s respectively. Band gaps determined from optical absorption and photoconductivity excitation spectra gave similar results. The direct band gap nature is found for mixed films. Results of different irradiation conditions during preparation of the films are also investigated.     

Space-charge-limited currents and photoconductive properties of Tl2InGaSe4 layered crystals

The extrinsic electronic parameters of Tl₂InGaSe₄ layered crystals were investigated through measurement of the temperature-dependent dark conductivity, space-charge-limited currents and photoconductivity. Analysis of the dark conductivity reveals the existence of two extrinsic energy levels at 0.40 and 0.51 eV below the conduction band edge, which are dominant above and below 260 K, respectively. Current–voltage characteristics show that the one at 0.51 eV is a trapping energy level with a concentration of (4.8–7.7) × 10¹⁰ cm^?3. Photoconductivity measurements reveal the existence of another energy level located at 0.16 eV. In the studied temperature range, the photocurrent increases with increasing temperature. The dependence of the photoconductivity on the incident light intensity exhibits a linear recombination character near room temperature and a supralinear character as the temperature decreases. The change in recombination mechanism is attributed to an exchange in the behavior of sensitizing and recombination centres.     

Optical and electrical properties of amorphous arsenic

Amorphous arsenic prepared by plasma decomposition of arsine has been characterized using field-effect conductance, thermopower, optical absorption, and photoconductivity measurements. It is found that the Fermi level is located in a density of states ～ 10¹⁷ cm^?3 eV^?1 approximately in the center of the forbidden gap, that conduction occurs via electrons in extended states in the conduction band, and that the optical and photoelectrical properties are very similar to those of bulk a-As. It is concluded that a model involving a negative correlation energy for localized states is inappropriate for this material.     

Tetrazen als Defektelektronenhaftstelle im Anthrazenkristall

Tetracene doped anthracene crystals with molar impurity concentrations ranging from 10^?8 to 10^?4 were grown from the melt by the Bridgman technique. To describe the impurity distribution in the crystals the equation of normal freezing had to be modified concerning the change in density when the crystals were lowered down into the cooler part of the furnace. As a result the equilibrium distribution coefficient was determined to be 1.3 · 10^?2. Photocurrent measurements were performed by a transient photoconductivity technique (Kepler andLe Blanc). The behaviour of the hole photocurrent in doped crystals showed that measuring the drift mobility one gets a very sensitive tool for determining quantitatively tetracene contents down to molar concentrations of 10^?8. It was found that tetracene is a shallow hole trap having an activation energy of 0.42 eV and a capture cross section of 3.5 · 10^?14 cm².     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

Photoconductive properties of some chemically deposited (Cd–Pb)s films

Films of (Cd–Pb)S have been prepared using chemical deposition in aqueous alkaline bath and their subsequent condensation on substrates. Important achievements in terms of electrical response, optical absorption and photoconductivity (PC) excitation spectra, SEM, XRD and photoluminescence (PL) studies are presented and discussed. From the photocurrent curves, the ratio I_PC (saturated photocurrent)/I_DC (dark current) was observed to be of the order of 10⁶ for the systems prepared with CdCl₂, and to be 10⁷ when doped with samarium nitrate. Values of trap depth E, lifetime and mobility are evaluated from the PC decay. Band-gaps are determined from the two spectra. Diffraction lines in XRD studies are associated to CdS and PbS, and according to SEM studies layered growth of the films takes place. PL of samarium doped (Cd–Pb)S films shows an emission peak in the green-yellow region under 365?nm excitation. The PL brightness decreases with temperature.     

The temperature dependence of optical gap and photoconductivity threshold in undoped hydrogenated amorphous silicon films

We report on the temperature dependences of the optical gap E_o and the photoconductivity threshold (?ω)_o for undoped hydrogenated amorphous silicon films. When increasing the temperature, both E_o and (?ω)_o are seen to linearly decrease at respective rates β= 3.5 10^?4 eV K^?1 (temperature range 290 K–460 K) and γ= 5.2 10^?4 eV K^?1 (temperature range 220 K – 360 K). At higher temperatures E_o decreases at the rate β = 14.3 10^?4 ev K^?1. Our results are discussed in terms of conduction in extended states. We show there is no physical reason in relating the temperature dependence of the activation energy and that of the gap as generally assumed. From optical absorption we deduce a minimum metallic conductivity σ_min the value of which agrees with Mott's predictions. On the contrary, σ_min measured from dark conductivity is nearly two orders of magnitude lower. A discussion is proposed infering band bending at the film substrate interface.     

Photoelectronic investigations of semi-insulating p-type GaAs:Cr containing neutral chromium acceptors

A photoelectronic analysis of p-type GaAs:Cr, i.e. measurements of thermally stimulated currents and the dependences of photoconductivity and photo-Hall effect on photon energy, temperature and light intensity, have enabled trap locations and densities as well as properties of neutral chromium acceptors to be determined. Hole traps proved to be located at 0.15 and 0.23 eV above the valence band, and their densities have been estimated to be 10¹⁵ cm^?3 and 5 × 10¹⁶ cm^?3 respectively. Their occurrence is related to the presence of copper in the samples investigated. Neutral chromium acceptors are located at 0.77 eV above the valence band and are at a constant distance from the conduction band. Their photoionization cross-section is 3 × 10^?17 cm² while the photoexcited electron escape cross-section is about 10^?20 cm². The potential of a neutral Cr acceptor is of the delta function type with weak coulombic tails. The maximum radius of the Bohr orbit of an electron in the ground state is 4 atomic units.     

Design of multiple layered a-Si:H(F)/a-SiGex:H(F) films for enhancement in photoresponse in the near-infrared spectrum

Photo-electric properties of a-Si:H(F)/a-SiGe_x:H(F) multilayer films were investigated by measurements of optical absorption, and photoconductivity in both steady and transient modes with the repetition length and the difference in the optical gap between a-Si:H(F) and a-SiGe_x:H(F) as the variables. Measurements of primary photocurrent clarified that photosensitivity for the multilayer films extended to longer wavelengths of around 725 nm, while high resistivity was maintained despite of lowering the band gap.The drift mobility of electrons was measured by the time-of-flight technique, showing 10^–2-10^–3 cm²/Vs, while the drift mobility-lifetime products of electron was maintained to be 10^–7 cm²/V. On the other hand, the drift mobility of holes was 10^–3 cm²/Vs, which was the similar magnitude to that of a-Si:H(F).     

Photoconduction of natural cinnabar (α-HgS)

The photoconductivity measurements presented in this paper permit us to reveal a sensitizing process in mercury sulphide (α-HgS) which is common to a number of II–VI compounds. The excitation spectra of the photocurrent as well as the results obtained in terms of excitation density and temperature allow us to locate the slow recombination center at 165 meV from the valence band. The ratio of capture cross sections for holes and electrons for this center is around 10⁵. Experiments on photoresponse to a cut off of the excitation show that, in the temperature range corresponding to the quenching of the photocurrent, the recombination process of free carriers is close to the bimolecular. However, the influence on our results of an electron trap located at 50 meV from the conduction band is pointed out.     

Photoconductive properties of some chemically deposited (Cd-Pb)S films

Films of (Cd-Pb)S have been prepared using chemical deposition in aqueous alkaline bath and their subsequent condensation on substrates. Important achievements in terms of electrical response, optical absorption and photoconductivity (PC) excitation spectra, SEM, XRD and photoluminescence (PL) studies are presented and discussed. From the photocurrent curves, the ratio I _PC (saturated photocurrent)/I _DC (dark current) was observed to be of the order of 10⁶ for the systems prepared with CdCl₂, and to be 10⁷ when doped with samarium nitrate. Values of trap depth E, lifetime and mobility are evaluated from the PC decay. Band gaps are determined from the two spectra. Diffraction lines in XRD studies are associated with CdS and PbS, and according to SEM studies, layered growth of the films takes place. PL of samarium-doped (Cd-Pb)S films shows an emission peak in the green-yellow region under 365-nm excitation. The PL brightness decreases with temperature.     

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.     

Comment on the optical absorption edge in a-Si:H

The optical absorption edge of undoped amorphous silicon hydride has been measured using optical transmission, photoconductivity, and photothermal deflection spectroscopy. The results obtained by these techniques agree in the exponential edge region. An apparent inconsistency pointed out by Redfield between the optical absorption edge and the valence band tail density of states as measured by drift mobility is attributed to the non-exponential behavior of the absorption edge above α～10³ cm^-1.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

Electrical conductivity and photoconductivity of single crystals of lithium hydride

Nonactivated LiH crystals and crystals of LiH with Mg, In, Tl, Sn, Sb, and Bi impurities are investigated. Photoconductivity is found in luminescent crystals. The temperature dependence of the electrical conductivity and photoconductivity in the temperature range of 100–570 ° K is measured. Volt-ampere characteristics of the dark current and photocurrent in fields up to 10⁴ V/cm at various temperatures are obtained. Data is presented on the inertia of the photocurrent and its dependence on the intensity of the exciting light. Conclusions are drawn concerning some connection of LiH photoconductivity with the type of activating impurity, the absence of a connection with the brightness of the luminescence, and the specific role of photoconductivity in color centers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 94–98, October, 1971.     

Optical and electrical properties of GaN: Si-based microstructures with a wide range of doping levels

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (～2.5 × 10¹⁸ cm^?3) and to the overlap of the impurity band with the conduction band (～2 × 10¹⁹ cm^?3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.     

Study of trapping and recombination centres in Tl2InGaTe4 chain crystals by dark electrical conductivity and photoconductivity measurements

Dark electrical conductivity and photoconductivity of Tl₂InGaTe₄ single crystals have been measured and analyzed in the temperature region 100–300 K. The dark electrical conductivity measurements revealed an intrinsic- or extrinsic-type of conductivity above or below 210 K, respectively. From intrinsic conductivity data analysis, the energy band gap of Tl₂InGaTe₄ crystals was determined as 0.85 eV. In the extrinsic region, the dark conductivity arises from a donor energy level located at 0.30 eV below the conduction band. The photocurrent increases with increasing illumination intensity. The recombination mechanism in the crystal changes as temperature decreases due to the effect of exponential trapping centres. Two trapping and/or recombination centres located at 89 and 27 meV were determined from the temperature dependence of the photocurrent, which decreased or increased with increasing temperature in the regions above or below 180 K, respectively.     

Wechsellichtmethode zur Messung von Haftstellen-Einfangquerschnitten beim Cadmiumsulfid

An extension of a method previously described is used to determine capture crosssections for shallow traps of CdS-crystals in the vicinity of the conduction band. The data are obtained by measurement of photoconductivity induced by sinusoidally modulated light ofλ ≈ 600 mμ. The results are as follows:σ ≈ 1,5 to 6 · 10^?15 cm² for traps at 0.12 ev (below the conduction band);σ ≈ 5 to 9 · 10^?15 cm² for traps between 0.2 and 0.6 ev;σ ≈ 10^?14 to 10^?13 cm² for traps at 0.7 to 0.8 ev.     

Radiative and photoelectric properties of CuInS2 single crystals

The photoluminescence and photocurrent spectra of CuInS₂ single crystals grown by the Bridgman method are studied at temperatures of 80 and 300 K. The photosensitivity spectrum is observed in the shortwave photoluminescence band. From the watt-ampere characteristics of photoconductivity, a linear mechanism of recombination of minority charge carriers is established for an illumination level of up to 100 mW/cm² in the temperature range of 80–300K.