Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Photoconductivity in crystalline phthalocyanines

The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H₂Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H₂Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H₂Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H₂Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H₂Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.     

Space-charge-limited currents and carrier trapping in plasma-polymerized malononitrile films

The dark electrical conductivity of plasma-polymerized malononitrile (PPMa) films in an Ag/PPMa/Ag sandwich structure was investigated over the temperature range 300–525 K at a reduced pressure of 10^-5 Torr. The room temperature current-voltage characteristics indicate space-charge-limited currents. The results for the conductivity as a function of inverse temperature are in accordance with the band model proposed by Barbe and Westgate, and the thermal energy gap between the top of the valence band and the bottom of the conduction band was determined to be 0.86±0.01 eV. At temperatures below about 380±5 K the conduction process is consistent with the presence of an electron trapping level situated 0.34±0.05 eV below the conduction band edge with a density of (5.02±0.05) × 10¹⁶ cm^-3. It is assumed that above about 435±5 K the conduction process is intrinsic. Experiments on the chemisorption of oxygen suggest that electrons are the majority carriers in PPMa films.     

Deep electron trap level in semi-insulating GaAs

The experimental data on the Hall measurements have been used to characterize deep electron trapping levels in Cr doped semi-insulating GaAs crystals. The energy of the level below the conduction band edge has been found to be ∼ 0·8 eV and is thought to be related to Ga vacancies in the host crystal and Cr impurities.     

Hydride vapour phase epitaxy growth and characterisation of GaN layers

A vertical hydride vapour phase epitaxial reactor was applied for the growth of GaN layers on (0001) sapphire. In this paper, the growth parameters and the results of optical investigations are reported. It is seen that the layers grown at high temperature have better crystallographic properties than those grown at 600–800°C, although the latter show a smoother surface. The absorption spectra exhibited a tail from the absorption edge down to about 1 eV that may be fitted by the Lukovsky model considering the presence of a level at about 1.2 eV from the conduction band. The cathodoluminescence spectra show three main emissions: the yellow band, a blue band (BB) centred at 2.8 eV and the near band edge recombination. The BB is mostly originated close to the layer/substrate interface and practically extinguished at the layer surface. It is seen that when the intensity of the BB increases that of the fundamental recombination decreases and vice versa.     

Study of photoluminescence and computation of configuration coordinate diagram of Cu related deep levels in InP

Photoluminescence has been studied in Cu diffusedn andp-InP. Inp-InP a Cu related photoluminescence (PL) band was observed at 1·216eV. The temperature dependence of line-width was studied and line-shape and line-width analysis carried out. The configuration coordinate diagram of the band was calculated which showed a small lattice relaxation of 0·079 Å. Inn-InP two PL bands at 1·20 and 1·01 eV were found at 10 K. The former was similar to the 1·216eV band inp-InP. The PL of the 1·01 eV band was also studied in detail and the corresponding configuration coordinate diagram derived.     

Temperature effects on the optoelectronic properties of AgIn5S8 thin films

Polycrystalline AgIn₅S₈ thin films are obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates under a pressure of ~ 1.3 × 10⁻³ Pa. The temperature dependence of the optical band gap and photoconductivity of these films was studied in the temperature regions of 300-450 K and 40-300 K, respectively. The heat treatment effect at annealing temperatures of 350, 450 and 550 K on the temperature dependent photoconductivity is also investigated. The absorption coefficient, which was studied in the incidence photon energy range of 1.65-2.55 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge which corresponds to a direct allowed transition energy band gap of 1.78 eV exhibited a temperature coefficient of −3.56 × 10⁻⁴ eV/K. The 0 K energy band gap is estimated as 1.89 eV. AgIn₅S₈ films are observed to be photoconductive. The highest and most stable temperature invariant photocurrent was obtained at an annealing temperature of 550 K. The photoconductivity kinetics was attributed to the structural modifications caused by annealing and due to the trapping-recombination centers' exchange.     

Investigation of electrical conduction in polyvinyl formal

Current-voltage (I–V) characteristics of pure polyvinyl formal (PVF) were investigated at different fields, range 5–100 kV/cm, as a function of temperature, range 313–363 K. It was observed that while at low fields (up to 25 kV/cm), the conduction was Ohm’s law-dependent at high fields (beyond 25 kV/cm), the conduction was Poole-Frenkel (P-F) mechanism-dependent. An attempt was made to identify the nature of the current by comparing its observed dependence on temperature, electric field and electrode materials with their respective characteristic features of the existing theories of electrical conduction. The current showed a strong dependence on temperature. To identify the possible mechanism of conduction, current versus square root of field characteristics were drawn with aluminium, silver, copper and gold as upper electrodes and Al as the lower electrodes. The observed characteristic suggested that the charge carriers were generated by the field-assisted lowering of coulombic barriers at the traps, and were subsequently conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari-modified P-F mechanism. The calculated value of the modified P-F barrier was ⋍ 1·94×10⁻¹⁹ J (1·21 eV).     

Effect of sintering temperature on structural and electrical properties of gadolinium doped ceria (Ce<Subscript>0·9</Subscript>Gd<Subscript>0·1</Subscript>O<Subscript>1·95</Subscript>)

Gadolinium doped ceria oxide is one of the promising materials as an electrolyte for IT-SOFCs. Ce_0·9Gd_0·1O_1·95 (GDC10) powder was prepared by solid state reaction and sintered at 1473 K, 1573 K, 1673 K and 1773 K. All samples were studied using X-ray diffraction, scanning electron micrograph and d.c. conductivity measurement. The crystallinity and surface morphology of the samples improved with sintering temperature. Further, the electrical conductivity measurement indicated that the conduction mechanism is mainly ionic. The conductivity of samples sintered at 1673 K and 1773 K at 800°C are of the order of 0·1 S-cm⁻¹. The activation energies decreased from 1·25–0·82 eV with increase in sintering temperature.     

Band gap study on zinc-doped cadmium sulphide

Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.     

1.4 eV photoluminescence in chlorine-doped polycrystalline CdTe with a high density of defects

The 1.4 eV photoluminescence (PL) band in a polycrystalline CdTe : Cl with a high density of defects was studied as a function of the chlorine dopant concentration and temperature. For a material with a high density of defects this band has a smooth, non-symmetrical shape without any apparent phonon structure. The energy of the intensity maximum of the 1.4 eV band depends on the concentration of chlorine and varies from 1.389 to 1.408 eV. The temperature quenching of the PL intensity for all samples was measured and the activation energy (ET) was found. Minimum and maximum values of the ET were 0.10 and 0.20 eV, respectively. A configurational – coordinate model is proposed for the 1.4 eV PL band in which the excited state of the recombination centre lies within the conduction band. In this model the temperature quenching of the PL is pictured as being essentially due to an electronic transition from the excited state directly to the ground state (the internal mechanism). This revised version was published online in November 2006 with corrections to the Cover Date.     

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to ε _xx  = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.     

Temperature dependence of AlInAs band gap energy and AlInAs/InP band offsets

Abstract

The temperature variations of the AlInAs photoluminescence (PL) transition energies and the AlInAs/InP interface staggered lineup luminescence (SLL) energy are reported. The S shape appearing from 4 to 90 K on the energy v. temperature curves of these PL energies are owing to extrinsic recombinations. In particular, the S shape of the SLL energy curve v. temperature is probably a result of acceptor impurities localised in AlInAs at the interface (on edge impurities). The band offsets were determined by solving the Schrödinger and Poisson equations with a self consistent calculation program. At 4.5 K, the conduction and valence band offsets are 0.384 and 0.295 eV respectively. Their temperature variation is shown to be important: 35 and 23 meV at 4.5 and 300 K respectively. The Van Vechten and Malloy model (following a thermodynamic approach) for the temperature variation of the band offsets is compared to the results in the present work.     

Electrical conductivity and optical properties of ZnO films annealed in hydrogen atmosphere after chemical vapor deposition

The d.c. electrical conductivity and optical properties of polycrystalline zinc oxide films (220–450 nm thick) annealed in hydrogen after chemical vapor deposition are investigated. A minimum film resistivity after the annealing gives 0.31 cm for the film as-deposited at a substrate temperature of 823 K. From the temperature dependence of conductivity, band conduction is confirmed for the films at temperatures above 250 K. The effect of grain-boundary scattering is due to thermionic emission of electrons over grain boundary barriers. At temperatures below 250 K, variable-range hopping transport is found to be dominant. The films are transparent in the wavelength range 400 to 1000 nm and have sharp ultraviolet absorption edges at 380 nm. The absorption edge analysis reveals the optical band gap energy for the films to be 3.18–3.23 eV. The Urbach tail analysis gives the width of localized states E_e=0.06-0.14eV.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Photorefractive and Photovoltaic Effects in Doped LiNbO 3

Photo-induced currents and voltages determine the photorefractive process in doped LiNbO 3, which can be utilized for the reversible storage of thick phase holograms. Optical recording and erasure are studied in the spectral region between 1·5 and 4 eV (lambda; = 0·8 ? 0·3 μm). The influence of various transition metal dopants, chemical treatments, temperature and external electric fields on the storage properties is investigated. The results yield consequences for write-read-erase applications and for read-only storage of stacked phase holograms.     

Photoelectrochemical solar cells using electrodeposited GaAs and AlSb semiconductor films

Electrodeposited GaAs and AlSb thin film semiconductors were prepared under various deposition conditions. Reasonable stoichiometry could be attained as revealed by EDAX studies. The best stoichiometry was obtained are Ga_1·04As_0·96 and Al_1·12Sb_0·88. The band gap of the GaAs and AlSb films was ∼1·5 eV and 1·6 eV respectively. The electrochemical and photoelectrochemical studies on these films are reported with different redox-couples in aqueous and non-aqueous medium.     

Synthesis of nanocrystalline CdS thin films in PVA matrix

Nanocrystalline thin films of CdS are deposited on glass substrates by chemical bath deposition technique using polyvinyl alcohol (PVA) matrix solution. Crystallite sizes of the nanocrystalline films are determined from broadening of X-ray diffraction lines and are found to vary from 5·4–10·2 nm. The band gap of the nanocrystalline material is determined from the UV spectrograph. The absorption edge is shifted towards the lower wave length side (i.e. blue shift) and are found to be within the range from 2·48–2·8 eV as grain sizes decrease from 10·2–5·4 nm. This is also supported by the spectral response curves. An increase of molarity decreases the grain size which in turn increases the band gap.     

Electrical transport properties of bulk amorphous Gex Se1−x

d. c. -Conductivity measurements have been made as a function of temperature (80–330 K) on bulk amorphous samples of Ge_xSe_1?x (0.1 ? x ?0.7), in order to identify the conduction mechanism and to observe the effect of selenium -doping on the d. c.-conductivity of germanium. It is found that for samples with a low concentration of selenium (0. 5? x ? 0. 7 ), conduction in the high temperature region (330-180 K) is due to thermally assisted tunneling of electrons in the localized states at the conduction band edge, and in the low temperature region (80–190 K), conduction takes place through variable range hopping in the localized states near the Fermi level. The addition of selenium is found to increase the amorphicity of the samples. In the samples with a relatively high content of selenium (0.1 ? x ? 0.4), conduction in the entire temperature range of investigation is due to the thermally assisted tunneling of holes in the localized states at the valence band edge.