Monte Carlo Modeling of Phonon-assisted Carrier Transport in Cubic and Hexagonal Gallium Nitride

Monte Carlo method is employed for the calculations of electron and hole transport characteristics of cubic and hexagonal GaN at T = 300 K in the fields of E ≤ 1000 kV/cm⁻¹. It is shown that electron drift velocity and mobility is heavily reduced in hexagonal crystals due to additional phonon modes (~ 26 meV) and by fast electron scattering between the lowest Γ₁ valley and the minimally (~ 400 meV) up-shifted Γ₃ valley. Intervalley scattering is mediated most efficiently by the low-energy (~ 2 meV) acoustic phonons. The randomizing scattering is even more pronounced in p-type crystals where the sub-bands of light and heavy holes merge at the Γ-point of Brillouin zone. Cubic phase crystals are concluded to be advantageous for ultrafast electronic and photonics device performance because electron drift mobility is higher by an order of magnitude, and the hole mobility is several times higher than those in hexagonal phase.     

Photoplasticity of As2Se3 films investigated with combined nanoindentation and AFM methods

Thin As₂Se₃ amorphous chalcogenide films have been studied by nanoindentation and atomic force microscopy in darkness and under illumination by band-gap light. The combination of these two methods has been used to study the peculiarities of the photoplastic effect in amorphous semiconductors. It has been shown by multiple loading indentation experiments that a non-linear mechanism of the formation of the strain response is realized in the As₂Se₃ films subjected to the combined action of light and external mechanical loading. We have observed that light illumination alters the internal friction of the films and their shear modulus. These observations have been considered in the frame of the two-phase model of chalcogenide glasses. Some arguments in favor that the self-organization processes take place in the structure of irradiated film are given.     

Hall effect of FeTe and Fe(Se1–xTex) thin films

The Hall effect is investigated in thin-film samples of iron–chalcogenide superconductors in detail. The Hall coefficient (R_H) of FeTe and Fe(Se_1–xTe_x) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se_0.4Te_0.6) and Fe(Se_0.5Te_0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se_1–xTe_x), it is suggested that the coexistence of ‘itinerant’ carriers both in electron and hole channels is indispensable for the occurrence of superconductivity.     

Transient electrical transport in pure and doped chalcogenide glasses

Transient hole transport measurements in undoped As₂Se₃ and Se and in doped As₂Se₃ are explained without recourse to the presence of deep traps. We suggest that the conduction process involves small polarons as originally proposed by Emin. Correlations between the measured activation energies and the dispersion in the Se and As₂Se₃ systems as well as the dependences of the drift mobilities and the dc mobilities on doping in As₂Se₃ are explained.     

Electrical transport in a-As2Se3 containing metallic impurities

Significant changes of the hole drift mobility are observed in a-As₂Se₃ containing concentrations less than ～ 10²⁰ cm^?3 of metallic impurities (Mn, Fe, Ni, Cu, Zn, Ga, In, Tl and Na). Depending upon metal concentration the results suggest a modification of the hopping transport channel in Ga, In and Tl doped samples and the buildup of new traps that are either isoenergetic with the intrinsic trap population (Tl, Ga, In, Cu) or lie deeper in the gap (Mn, Fe, Ni, Cu). Trap-limited hopping transport provides a consistent explanation of the experimental data.     

Preparation and thermoelectric power of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n?=?1.1?×?10¹⁹?cm^?3 for Bi₂Se₃ and n?=?0.53?×?10¹⁹?cm^?3 for SnBi₄Se₇. The thermoelectric power has been measured over the temperature range 90–420?K. The thermoelectric power of Bi₂Se₃ is higher than that for SnBi₄Se₇, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi₂Se₃ with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS ². Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi₂Se₃ compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, Sn_Bi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi₂Se₄.     

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si?:?H) and amorphous As₂Se₃ (a-As₂Se₃) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As₂Se₃ than for a-Si?:?H has been observed and this is interpreted in terms of the higher structural flexibility of a-As₂Se₃. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.     

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi₂Se₃ thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi₂Se₃ grew on amorphous Al₂O₃ even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi₂Se₃ channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.     

Influence of thermal annealing on optical and structural properties change in Bi-doped Ge30Se70 thin films

The effect of annealing on the structural and optical properties of thermally evaporated Ge₃₀Se₇₀ and Ge₃₀Se₆₀Bi₁₀ thin films is reported in this paper. The prepared films were thermally annealed at 250°C to optimize the optical properties which can be used for the optical device fabrication. X-ray diffraction study revealed no structural transformation whereas the surface morphology changed as observed from scanning electron microscopy. The optical properties of the deposited and annealed films have been investigated by using a UV–VIS–NIR spectrophotometer in the wavelength range of 400–1100?nm. The optical band gap of the Ge₃₀Se₇₀ annealed film is found to be increased while the energy gap of the Bi-doped annealed Ge₃₀Se₆₀Bi₁₀ thin film decreased which is explained by the chemical disorderness, defect states and density of localized states in the mobility gap. The Tauc parameter and Urbach energy which measure the degree of disorder changed with the annealing process. The transmittivity increases and the absorption power decreases in the Ge₃₀Se₇₀ annealed film, whereas the reverse effect is noticed for the annealed Ge₃₀Se₆₀Bi₁₀ thin film. The irreversible nature of this change can be useful for optical recording purposes.     

Optical parameters of In–Se and In–Se–Te thin amorphous films prepared by pulsed laser deposition

The thin films of materials based on In–Se are under study for their applicability in photovoltaic devices, solid-state batteries and phase-change memories.The amorphous thin films of In₂Se_3−xTe_x (x=0–1.5) and InSe were prepared by pulsed laser deposition method (PLD) using a KrF excimer laser beam (λ=248 nm, 0.5 J cm⁻²) from polycrystalline bulk targets. The compositions of films verified by energy-dispersive X-ray analysis (EDX) were close to the compositions of targets. The surfaces of PLD films containing small amount of droplets were viewed by optical and scanning electron microscopy (SEM).The optical properties (transmittance and reflectance spectra, spectral dependence of index of refraction, optical gap, single-oscillator energy, dispersion energy, dielectric constant) of the films were determined.The values of index of refraction increased with increasing substitution of Te for Se in In₂Se₃ films, the values of the optical gap decreased with increasing substitution of Te for Se in In₂Se₃ films.     

Kinetics of photo‐darkening and self‐bleaching in amorphous As2S3 and As2Se3 thin films

The kinetics of photo‐darkening of amorphous As₂S₃ and a‐As₂Se₃ thin films follows a single exponential, but the magnitude and the rate of the process is higher in case of As₂S₃. The kinetics of self‐bleaching (dark relaxation) in advance photo‐darkened state follows a stretched exponential (SRE) with different stretching parameter for a‐As₂S₃ and a‐As₂Se₃. Within the J. C. Phillips approach we suppose that photo‐darkening in amorphous As₂S₃ films is, to some extent, accompanied by changes in short‐range order interactions, while photo‐darkening of amorphous As₂Se₃ is accompanied rather by changes in Coulomb interactions. The self‐bleaching process reduced the magnitude of photo‐darkening up to 45% and 60% for amorphous As₂S₃ and As₂Se₃ films, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of oxygen and chlorine on charge transport in vacuum deposited pure a-Se films

We have investigated the effects of oxygen and chlorine addition in the ppm range to pure amorphous selenium (a-Se) in terms of electron and hole transport in vacuum-deposited films. We examined the electron range (μ_eτ_e) and hole range (μ_hτ_h) by carrying out time-of-flight (TOF) and interrupted field time-of-flight (IFTOF) transient photoconductivity measurements. We have found that even small amounts of oxygen in the ppm range can significantly affect both electron and hole lifetimes. Oxygen addition increases the hole lifetime and decreases the electron lifetime. As the oxygen doped a-Se films age, the hole lifetime has a tendency to decrease while the electron lifetime shows a slight improvement. In contrast, chlorine addition almost totally annihilates electron transport while the hole transport remains relatively unaffected.     

The heterostructure and electrical properties of Sb2Se3/Bi2Se3 grown by molecular beam epitaxy

We report the growth and characterization of Sb₂Se₃/Bi₂Se₃ bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb₂Se₃ grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi₂Se₃ (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb₂Se₃ and Bi₂Se₃, which is important for the protection of surface states Bi₂Se₃. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb₂Se₃. The insulating nature of Sb₂Se₃ is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb₂Se₃ into Bi₂Se₃, and such effect can be reduced in the Sb₂Se₃/(Bi_0.7Sb_0.3)₂Se₃ bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.     

Carrier transport in Bi2Se3 topological insulator slab

Electron transport in Bi₂Se₃ topological insulator slabs is investigated in the thermal activation regime (>50 K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi₂Se₃ slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments.     

Photodarkening of amorphous Se and Se95Te5 films under μs light pulses

Photodarkening phenomena in the amorphous Se and Se₉₅Te₅ films at the μs light pulses irradiation are investigated. The process of photodarkening during each of the individual 1, 10 or 100 μs pulses and also the processes proceeding between successive pulses are recorded and analyzed. The darkened film was shown to restore initial optical properties after some stay in darkness without any additional irradiation or heating. Photodarkening is very strongly dependent on the light pulse fluence.     

Photoinduced phenomena in mAs2S3·nAs2Se3 amorphous films and their application in micro-optics technology

Photoinduced phenomena, such as photodarkening, photorefraction, and photodissolution, were studied in different compositions of three-component mAs₂S₃·nAs₂Se₃ amorphous films. The main emphasis was on the effect of photoinduced change of dissolution rate since this effect is the basis of many applications of amorphous chalcogenide films. The results of the investigation were compared with photoinduced effects in binary As₂S₃ and As₂Se₃ films. Advanced micro-optical devices: micro-lens and micro-mirror arrays, diffractive gratings, and photonic bandgap crystals, based on three-component amorphous films which possessed optimal photodissolution characteristics were developed. The primary parameters of the micro-optical devices developed are discussed.     

Electronic and structural changes induced by irradiation or annealing in pulsed laser deposited As50Se50 films. An XPS and UPS study

The response of amorphous As₅₀Se₅₀ pulsed laser deposited (PLD) films to annealing and irradiation, was investigated by X-ray and UV-photoelectron spectroscopies. It is shown that annealing smoothes out structural as well as electronic defects, while irradiation with near band-gap light enhances the film's “chemical disorder” and causes the creation of electronic defects. The observed behavior of the films is compared to that of thermally evaporated films and the corresponding bulk glasses.     

Model fitted and model free approaches for crystallization kinetics in Se85Te15-xBix glasses

In this research work, we have described the model-fitted and model free approaches for the study of crystallization kinetics in Se₈₅Te_15-xBi_x chalcogenide glasses. Se₈₅Te_15-xBi_x bulk alloys were synthesized by melt quenching technique. High Resolution X- Ray diffraction (HRXRD) was used to confirm the amorphous nature of prepared alloys. Non-isothermal Differential Scanning Calorimetry (DSC) measurements were done at heating rates of 5, 10, 15, 20 and 25 K/min for crystallization kinetics studies in Se₈₅Te_15-xBi_x glasses. The various characteristic temperatures, such as glass transition (T_g), on-set crystallization (T_c) temperature, peak crystallization temperature (T_p) and melting temperatures (T_m) have been obtained from various DSC thermograms. The activation energies of glass transition (ΔE_t) were calculated by using Kissinger and Moynihan approaches and found to be minimum for Se₈₅Te₁₂Bi₃ chalcogenide glass which indicates that this alloy has maximum probability to jump into a less configurational energy state and has larger stability. The model-free approaches; Kissinger–Akahira–Sunose (KAS), Flynn-Wall-Ozawa (FWO), Tang and Straink (TS) reveal that the activation energy of crystallization varies with crystallization degree and temperature both. This variation shows that amorphous to crystalline phase transformation in Se₈₅Te_15-xBi_x chalcogenide glasses is a complex process with various nucleation and growth mechanisms.     

Electrical and Thermal Transport Properties of n-type Bi6Cu2Se4O6 (2BiCuSeO + 2Bi2O2Se)

New thermoelectric materials, n-type Bi₆Cu₂Se₄O₆ oxyselenides, composed of well-known BiCuSeO and Bi₂O₂Se oxyselenides, are synthesized with a simple solid-state reaction. Electrical transport properties, microstructures, and elastic properties are investigated with an emphasis on thermal transport properties. Similar to Bi₂O₂Se, it is found that the halogen-doped Bi₆Cu₂Se₄O₆ possesses n-type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi₂O₂Se, an extremely low thermal conductivity can be observed in Bi₆Cu₂Se₄O₆. To reveal the origin of low thermal conductivity, elastic properties, sound velocity, Grüneisen parameter, and Debye temperature are evaluated. Importantly, the calculated phonon mean free path of Bi₆Cu₂Se₄O₆ is comparable to the interlayer distance for BiO─CuSe and BiO─Se layers, which is ascribed to the strong interlayer phonon scattering. Contributing from the outstanding low thermal conductivity and improved electrical transport properties, the maximum ZT ≈0.15 at 823 K and ≈0.11 at 873K are realized in n-type Bi₆Cu₂Se_3.2Br_0.8O₆ and Bi₆Cu₂Se_3.6Cl_0.4O₆, respectively, indicating the promising thermoelectric performance in n-type Bi₆Cu₂Se₄O₆ oxyselenides.     

Influence of Se atoms on the properties of amorphous Ge

Electron spin resonance (ESR), electrical and optical measurements have been made for Ge_1?xSe_x (0?x?0.35) films in order to elucidate relations between tetrahedrally bonded amorphous semiconductors and chalcogenide amorphous semiconductors. The ESR signal due to dangling bonds in amorphous Ge decreases by increasing Se content. For more than about 20 at. % Se, the electrical conductivity is the activation type and the optical gap increases with the increase of Se content. The model of charged dangling bonds by Street and Mott seems to explain the experimental results.