Free-standing tetracene single crystal field effect transistor

We have fabricated and studied field effect transistors (FETs) on the optically transparent free-standing organic single crystals of tetracene. These FETs exhibit effective hole channel mobility μ_eff up to 0.15 cm²/Vs and on-off ratios up to 2×10⁷. Using measured values of μ_eff, thermal activation energy, and a simple model, we deduce an intrinsic free carrier mobility in the range of tens of cm²/Vs, similar to that found in pentacene crystals. These values should be considered only as a rough indication of achievable mobilities in samples much purer than those presently studied. The obtained results show the possibility of FET behavior in transparent crystals with low intrinsic carrier density.     

Characterization of deep level defects in sublimation grown p-type 6H-SiC epilayers by deep level transient spectroscopy

In this study deep level transient spectroscopy has been performed on boron–nitrogen co-doped 6H-SiC epilayers exhibiting p-type conductivity with free carrier concentration (N_A–N_D)∼3×10¹⁷ cm⁻³. We observed a hole H₁ majority carrier and an electron E₁ minority carrier traps in the device having activation energies E_v+0.24 eV, E_c −0.41 eV, respectively. The capture cross-section and trap concentration of H₁ and E₁ levels were found to be (5×10⁻¹⁹ cm², 2×10¹⁵ cm⁻³) and (1.6×10⁻¹⁶ cm², 3×10¹⁵ cm⁻³), respectively. Owing to the background involvement of aluminum in growth reactor and comparison of the obtained data with the literature, the H₁ defect was identified as aluminum acceptor. A reasonable justification has been given to correlate the E₁ defect to a nitrogen donor.     

Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

Spontaneous spin-flip Raman linewidth and nonlinear processes in InSb

A small-signal gain technique has been used to measure the lineshape of spontaneous spin-flip Raman scattering as a function of magnetic field (H = 0.5–10 kG) for an electron concentration n = 10¹⁵ cm^-3 at T = 2°K with both photons propagating normal to H. Four-wave mixing processes have been observed for varying carrier concentrations together with an interference between the resonant spin-flip nonlinearity and the nonresonant nonlinearity resulting from conduction electron nonparabolicity.     

Magnetic circular dichroism of band-edge exciton absorptions in PbI2

The linear Zeeman behaviour of the n = 1, 2 and 3 exciton absorptions near 20000 cm^-1 in PbI₂ single crystals was determined from measurements of the magnetic circular dichroism. The similarity of the g-value (+ 1.5 ± 0.5) for each of the n = 1, 2 and 3 lines supports the hydrogenic model. Any quadratic shifts of the n ? 2 levels were < 10 cm^-1 in an applied magnetic field of 21 T.     

Hall effect in the Fe<Subscript>x</Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S magnetic semiconductors

The electrical properties and the Hall effect in the Fe_xMn_1?xS magnetic semiconductors (0<x≤0.5) have been experimentally studied in the range 77–300 K in magnetic fields of up to 15 kOe. The cation-substituted sulfides with 0.25≤x≤0.3 possessing colossal magnetoresistance (CMR) were established to be narrow-gap semiconductors with carrier concentrations n ～ 10¹¹–10¹⁵ cm^?3 and high carrier mobilities μ ～ 10²–10⁴ cm² V^?1 s^?1. It is believed that the CMR effect in these sulfides can be explained in terms of the model of magnetic and electron phase separation, which is analogous to the percolation theory in the case of heavily doped semiconductors.     

Reversal of sign of the Hall coefficient in n-InSb in the magnetic freeze out regime

Change in sign of the Hall coefficient has been observed in n-InSb with carrier concentrations ranging from 2 × 10¹⁴ cm^?3 to 6 × 10¹⁴ cm^?3, which occurs at liquid helium temperatures and magnetic fields above 30 kG. The influence of the compensation ratio, temperature, and geometry has been investigated.     

Determination of relaxation times of ground electronic state vibrational levels of mixed molecular crystals

The stimulated emission spectra of mixed molecular crystals were investigated with the Nd glass lasser third harmonic excitation at 4.2 K. Napthalene crystals doped with β-naphthyl-n-biphenylyl ethylene and dibenzyl doped with naphthacene in various concentrations were studied. Relaxation times of the ground state vibrational levels in mixed molecules were obtained by use of the dependence of stimulated emission intensity upon excitation energy for naphthacene in dibenzyl τ₁ of the vibrational level at 317 cm^-1 was ～ 3 × 10^-9 s; for β-naphthyl-n-biphenylyl ethylene in naphthalene τ₁ of the 1629 cm^-1 vibrational level was ～ 10^-10 s.     

Growth and electrical properties of ZnO nanorod arrays prepared by chemical spray pyrolysis

Chemical spray pyrolysis was applied to grow ZnO nanorod arrays from zinc chloride solutions with pH=2 and 5 on glass/ITO substrate at 480 and 550 °C. The obtained structures were characterized by their morphological, electrical and PL properties. According to SEM, deposition of acidic solutions retards coalescence of the growing crystals. The charge carrier density in ZnO nanorods was determined from the C-V characteristics of ZnO/Hg Schottky barrier. Carrier densities ∼10¹⁵ cm⁻³ and slightly above 10¹⁶ cm⁻³ were recorded for ZnO deposited at 550 and 480 °C, respectively. According to PL studies, intense UV-emission is characteristic of ZnO independent of growth temperature, the concentration of oxygen vacancy related defects is lower in ZnO nanorods deposited at 550 °C. Solution pH has no influence on carrier density and PL properties.     

Trapping centers in undoped GaS layered single crystals

Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10–300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2×10^-21, 2.9×10^-23, 2.4×10^-21, 8.0×10^-9, 1.9×10^-9 and 4.3×10^-10 cm² for the capture cross sections and 1.6×10¹³, 5.0×10¹², 7.3×10¹², 1.2×10¹⁴, 8.9×10¹³ and 2.6×10¹³ cm^-3 for the concentrations, respectively. PACS 71.55.-i; 72.20.Jv; 72.80.Jc     

Electron scattering mechanisms in indium-tin-oxide thin films prepared at the various process conditions

The carrier concentrations and mobilities of indium-tin-oxide (ITO) thin films by DC magnetron sputtering at the various process conditions were measured by means of the Hall technique. The relationship between the carrier concentration and mobility showed two distinct features: (i) roughly up to the carrier concentration of 9.0 × 10²⁰/cm³, both the carrier concentration and mobility increased together; (ii) above the carrier concentration of 9.0 × 10²⁰/cm³, the carrier mobility decreased as the carrier concentration further increased. The distinct behavior of the carrier concentration and mobility was due to the transition of the dominant electron scattering mechanism. ITO thin film with a low degree of crystallinity was governed by the grain boundary scattering. However, the ionized impurity scattering was dominant in ITO thin film with a high carrier concentration over 9.0 × 10²⁰/cm³. The overall characterizations related to the carrier concentration and mobility were also performed using X-ray diffractometer, UV-vis-NIR spectrometer, scanning electron microscope, atomic force microscope.     

The mobility of free carriers in PbSe crystals

The free carrier mobility in PbSe varies strongly with temperature and carrier concentration. The electrical properties of single crystals of PbSe with carrier concentrations ranging from 10¹⁶ to 10²⁰ cm^?3 have been measured between 0·3 and 800K. The extrinsic mobility is governed by acoustical phonon scattering. An anomalous behaviour of the mobility at low temperatures and low concentrations was observed although no ionized impurities were present.     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

Electrical and optical properties of ZnO films prepared by sputtering of ZnO targets containing AlN

ZnO films prepared from the ZnO target containing 2% AlN are transparent irrespective of radio frequency (RF) power. The obtained ZnO films have the carrier density of 3.8 × 10²⁰ cm⁻³ or less and the low mobility of 5.3-7.8 cm²/(V s). In the case of 5% AlN target, ZnO films prepared at 40, 60 and 80 W are transparent, whereas ZnO films prepared at 100 and 120 W are colored. As RF power increases from 40 to 120 W, the carrier density increases straightforwardly up to 5.5 × 10²⁰ cm⁻³ at 100 W and is oppositely reduced to 3.2 × 10²⁰ cm⁻³ at 120 W. In the case of 10% AlN target, ZnO films prepared at 60 W or more are colored, and have the carrier density of 4 × 10²⁰ cm⁻³ or less. The N-concentration in these colored films is estimated to be 1% or less. The Al-concentration in the ZnO films prepared from the 5 and 10% AlN targets is higher than 2%. The carrier density of the ZnO films containing Al and N atoms is nearly equal to that of ZnO films doped with Al atoms alone. There is no evidence in supporting the enhancement of the carrier density via the formation of N-Al_xZn_4−x clusters (4 ≥ x ≥ 2).     

ESR spectra of Eu centers in defect Ga2S3 single crystals

In this paper, the author presents the results of measurements of the low-temperature and angular dependences of the ESR spectra of Eu²⁺ centers in defect Ga₂S₃ single crystals in the temperature range 8–29 K and for 0–180° orientations of the static magnetic field. The electron structure of impurity ¹⁵¹Eu atoms in Ga₂S₃:Eu single crystals has been studied by using the ESR method at different doping proportions of Eu atoms. Ga₂S₃ single crystals were grown from the melt using the Bridgman method. The Eu concentration was determined by atomic absorption analysis and X–ray fluorescence analysis (XRFA). By investigation on the ESR spectra, the author has first determined the values of charge states for Eu, which have turned out to be a Eu²⁺(4f⁷) ion with spin S=7/2, g=4.18±0.02 and concentration of the states of Eu N=6.3×10¹⁴ cm⁻³.     

Degradation of superconducting properties in MgB2 by Cu addition

The (MgB₂)_2−xCu_x (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E_F, and the corresponding velocity, V_F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB₂. Two different impurity phases, MgCu₂ and CuB₂₄, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T_c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×10²¹ cm⁻³, but for the x=0.5 sample it decreased to 1.3×10²¹ cm⁻³. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, E_F, calculations the decrease of the carrier concentration by the additions of Cu into MgB₂ gives a decrease in E_F and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band.     

Room temperature minority carrier lifetime and efficiency of p-type GaAs1−xPx

Using sine wave modulated laser excitation, the optical phase shift of the luminescence of vapor grown Zn doped GaAs_1?xP_x layers was measured. From such measurements the minority carrier lifetime can be derived. This lifetime was measured as a function of doping level for crystals with a composition sufficiently remote from the cross-over composition (0.30 < x < 0.35 to ascertain that only a small fraction of the minority carriers is in the indirect minima. In addition, luminescence efficiency measurements were made using cathode ray excitation. From the lifetime and efficiency results it is concluded that for p < 2 × 10¹⁹cm^-3 the lifetime, being between 1 and 3 ns, is dominated by nonradiative transitions. For the radiative transitions a rate constant of 4.5 × 10^-11cm³s^-1 was determined. This value compares reasonably well with values known for GaAs, about 7 × 10^-11cm³s^-1. The efficiency of a few strongly doped samples (p > 10¹⁹cm^-3) may be depressed by self-absorption of the luminescence. In addition, a few samples with a higher phosphorus content, up to x = 0.45, were measured. Their lifetimes being mainly determined by indirect transitions are also in the low nanosecond range. This stands in contrast with the longer lifetimes (up to 150 ns at p < 10¹⁷ cm^-3) of indirect bandgap material (x ≈ 0.8).     

Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique

Tin sulfide (SnS) is a material of interest for use as an absorber in low cost solar cells. Single crystals of SnS were grown by the physical vapor deposition technique. The grown crystals were characterized to evaluate the composition, structure, morphology, electrical and optical properties using appropriate techniques. The composition analysis indicated that the crystals were nearly stoichiometric with Sn-to-S atomic percent ratio of 1.02. Study of their morphology revealed the layered type growth mechanism with low surface roughness. The grown crystals had orthorhombic structure with (0 4 0) orientation. They exhibited an indirect optical band gap of 1.06 eV and direct band gap of 1.21 eV with high absorption coefficient (up to 10³ cm⁻¹) above the fundamental absorption edge. The grown crystals were of p-type with an electrical resistivity of 120 Ω cm and carrier concentration 1.52×10¹⁵ cm⁻³. Analysis of optical absorption and diffuse reflectance spectra showed the presence of a wide absorption band in the wavelength range 300-1200 nm, which closely matches with a significant part of solar radiation spectrum. The obtained results were discussed to assess the suitability of the SnS crystal for the fabrication of optoelectronic devices.     

The optical conductivity of free carriers in GaAs

The optical conductivity of free electrons in polar semiconducting compounds has recently been calculated by use of a generalized Boltzmann equation derived from the equation of motion of the quantum density matrix. This reduces to the quasi-classical Boltzmann transport equation in the low frequency limit: the optical conductivity thus obtained spans a spectral range from around 30cm^?1 to 1.2 × 10⁴cm^?1 in GaAs. In this paper, the optical conductivity is calculated for GaAs as a function of carrier concentration in terms of a frequency dependent relaxation time which reduces to the usual relaxation time in the limit of low frequencies and an elastic scattering mechanism. The low frequency limit of the relaxation time is used to estimate the mobility as a function of carrier concentration. The frequency dependent relaxation time is given for GaAs at 298 K over the spectral region from 45 cm^?1 to 2.3 × 10³cm^?1 for carrier concentrations from 3.4 × 10¹⁵cm^?3 to 8.7 × 10¹⁸cm^?3.     

Minority carrier lifetime in highly doped Ge

The minority carrier lifetime is measured in p-type Ge up to a majority carrier concentration of 10¹⁹ cm⁻³ using the photoconductive effect and the photo-magneto-electric effect. A stronger than linear dependence of the reciprocal lifetime on the carrier density is observed. Therefrom, from the weak temperature-dependence, from previous work and from the comparison with Si-data, we conclude that the dominant recombination mechanism at high carrier concentrations is the phonon-assisted band to band Auger-recombination. A transition coefficient of about 10⁻³¹ cm⁶ sec⁻¹ is estimated.