Study of S ion-assisted sulfurization of n-GaAs (1 0 0) surface

The chemical structure and site location of sulfur atoms on n-GaAs (1 0 0) surface treated by bombardment of S⁺ ions over their energy range from 10 to 100 eV have been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. The formation of Ga-S and As-S species on the S⁺ ion bombarded n-GaAs surface is observed. An apparent donor doping effect is observed for the n-GaAs by the 100 eV S⁺ ion bombardment. It is found that the S⁺ ions with higher energy are more effective in the formation of Ga-S species, which assists the n-GaAs (1 0 0) surface in reconstruction into an ordered (1 × 1) structure upon subsequent annealing. The treatment is further extended to repair Ar⁺ ion damaged n-GaAs (1 0 0) surface. It is found that after a n-GaAs (1 0 0) sample is damaged by 150 eV Ar⁺ ion bombardment, and followed by 50 eV S⁺ ion treatment and subsequent annealing process, finally an (1 × 1) ordering GaAs (1 0 0) surface with low surface states is obtained.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Electrical properties of KTiOPO4 single crystal in the temperature range from −100 °C to 100 °C

The electrical property of a KTiOPO₄ single crystal was studied by means of a dielectric spectroscopy method in the temperature range from −100 to 100 °C. Dielectric dispersion began at a temperature, T_S=−80 °C. It is believed that this dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of K⁺ ions. The activation energy concerned with hopping conduction is E_a∼0.20 eV above T_S. T_S=−80 °C can be the minimum temperature for the hopping K⁺ ion.     

Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

On viscous mechanism for surface diffusion at high temperatures (T/Tm > 0.75) due to formation of a 2D dense fluid on metallic surfaces

Experimental determinations of temperature dependence of surface self-diffusion coefficient of several metals exhibit a strong increase in D_s values and in activation energy for temperatures near the melting point T_m. This variation is illustrated by a bending of the Arrhenius plot of surface self-diffusion coefficients of tungsten, which are obtained experimentally by tip profile variation technique. For T/T_m < 0.75 the apparent activation energy for W is 2.85 eV and the pre-exponential term is equal to 0.24 cm²/s, while for T/T_m > 0.75 we have respectively 5.57 eV and 1.08 × 10⁴ cm²/s. To account for these unexpected variations in the activation energy and diffusivities, the hypothesis that the surface mass transport mechanism changes from individual atomic jumps at low temperatures towards a cooperative motion at temperatures near the bulk melting point, namely a viscous mechanism, is proposed. This model is based on the postulation of the formation of a 2D dense fluid on the metallic surfaces about 75% of the bulk melting temperature. Discussions of existing models on surface diffusion proposed by Rhead, by Bonzel, or by Tsong are given, and a technique to characterize surface viscosity of a 2D dense fluid is suggested.     

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Frequency dependence of spin-lattice relaxation time T₁ of ²⁷Al in one-dimensional K⁺ ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T₁ is proportional to ω^1.49±0.05 and agrres well with the $\text{ω}{}^{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}$ dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E_NMR=0.029 eV of the d(ln T₁)/dT curve in the low temperature region. The frequency dependence of T₁ in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the $\text{ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence as temperature is raised above 450 K.     

Role of vitreous matrix on the optical activity of Ge-doped silica

We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B_2β) and on the two related emissions at ∼4.2 eV (α_E) and at ∼3.1 eV (β). From the temperature dependence of B_2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the first singlet excited state S₁: the radiative one, giving rise the α_E band, and the thermally activated intersystem-crossing process between S₁ and the triplet state T₁, originating the β band. The intersystem-crossing rate increases on increasing the temperature, determining an opposite thermal behavior of the intensity of the two emissions. However, this temperature dependence cannot be rationalized by a simple Arrhenius law and the α_E decay kinetics at high temperatures do not follow a single exponential law, suggesting a complex landscape of configurational energies of the process.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

Electronic states of the c is - and trans - H3ONO molecules: a CASPT2 study

The CASPT2 calculations for the S₀, T₁, S₁, T₂, and S₂ states of the cis- and trans-CH₃ONO molecules predict the energy levels and geometries of the cis- and trans-isomers in the different states. The CASPT2 adiabatic (T ₀) and vertical (T _v) excitation energies are in good agreement with available experimental data (for the S₁ cis- and trans-isomers). The CH₃O-NO dissociation potential energy curves were calculated at the CASPT2//CASSCF level, and the CASPT2 calculations were performed for the transition states along the T₁, S₁, and T₂ dissociation paths. For the repulsive S₂ state the calculations predict the T _v values larger than 5.4 eV and dissociation products of CH₃O (1²A″) + NO (X²Π).     

Deep levels in gallium arsenide by capacitance methods

The three capacitance methods, i.e., TSCAP, PHCAP, and transient capacitance measurements, are applied to determine electronic properties of deep levels inn-GaAs. In the boat-grown wafer detected are the 0.30 eV electron trap withN _T =3.6×10¹⁶ cm^?3 andS _n =2.4×10^?15 cm², and the 0.75 eV electron trap withN _T =2.0×10¹⁶ cm^?3 andS _n =1.2×10^?14 cm². In the epitaxial wafer, the 0.45 eV hole trap is detected withN _T >1.5×10¹³ cm^?3 andS _p =1.4×10^?14 cm² as well as the 0.75 eV electron trap withN _T =2.4×10¹³ cm^?3.     

Effects of Cr doping in bilayered manganite LaSr2Mn2O7: Resistivity, thermoelectric power, and thermal conductivity

Systematic studies of resistivity, thermoelectric power, and thermal conductivity have been performed on polycrystalline bilayered manganites LaSr₂Mn_2−xCr_xO₇ (0≤x≤0.2). It is found that the temperature dependence of both Seebeck coefficient S(T) and resistivity ρ(T) in the high temperature region follows the small polaron transport mechanism for all the samples. But in the low temperature region, variable-range-hopping (VRH) model matches the experimental data better. In addition, the maximum of absolute S(T) at low temperatures is gradually suppressed for the sample with Cr-doping level of x>0.04, implying that a new FM order probably arises. With decreasing the temperatures further, S(T) has a sign change and becomes positive for the sample with Cr-doping level of x>0.04, indicating that there may occur a variation of the type of charge carrier. As to thermal conduction κ(T), the low-temperature peak is suppressed due to Cr-doping. The variation of κ(T) is analyzed based on the combined effect due to the suppression of local Mn³⁺O₆ Jahn-Teller (JT) lattice distortion because of the substitution of Cr³⁺ ions for Mn³⁺ ions, which results in the increase in thermal conduction, and the introduction of the disorder due to Cr-doping, which contributes to the decrease in thermal conduction.     

Synthesis, structure and magnetic properties of non-crystalline ferrihydrite nanoflakes

Synthesis and characterization of the structural and magnetic properties of a 2-line (2L) ferrihydrite (FHYD) sample based on the composition Fe:Al:Cu=100:25:5 are reported. Typical of 2L-FHYDs, this sample also yields the two broad lines in X-ray diffraction and triplet in the 1400-1700 cm⁻¹ range in IR spectroscopy. However, in transmission electron microscopy, nanoflakes of about 5-20 nm size but without any hint of diffraction fringes characteristic of crystalline order were observed. Temperature dependence (2-380 K) of the magnetization M vs. applied field H (up to ±65 kOe) of this non-crystalline ferrihydrite is used to establish a blocking temperature T_B?20 K, Néel temperature T_N?365 K and a spin-glass ordering of the surface Fe³⁺ spins at T_S?6 K. These magnitudes of T_B and T_S are considerably smaller than those of a 5 nm undoped 2L crystalline ferrihydrite with T_B=70 K and T_S=30 K. The fit of the M vs. H data for several T>T_B to a modified Langevin function is shown to collapse onto a universal curve yielding a temperature-independent average magnetic moment μ_P=70(5)μ_B per nanoflake. Analysis of these parameters obtained from the fits of M vs. H data above T_B is used to show that the effective average volume of the nanoflakes is about 1/3 that of spherical 5 nm crystalline 2L-FHYD. It is argued that these lower magnitudes of μ_P, T_B, and T_S for the nanoflakes result from their smaller effective volume determined here.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Temperature dependence of the radiation-stimulated conductivity of CsI crystals upon picosecond excitation by electron beams

The temperature dependence of the pulse conductivity for CsI crystals upon excitation with an electron beam (0.2 MeV, 50 ps, 400 A/cm²) at a time resolution of 150 ps is investigated. Under experimental conditions, the time of bimolecular recombination of electrons and holes (V _k centers) is directly measured in the temperature range 100–300 K. This made it possible to calculate the temperature dependence of the effective recombination cross section S(T)=7.9×10^?8 T² cm². The temperature dependence of the conductivity σ(T) is interpreted within the model of the separation of genetically bound electron-hole pairs. The activation energy of this process is found to be E _G =0.07 eV.     

Lattice thermal conductivity of compounds with inhomogeneous intermediate rare-earth ion valence

The thermal conductivity κ (within the range 4–300 K) and electrical conductivity σ (from 80 to 300 K) of polycrystalline Sm₃S₄ with the lattice parameter a=8.505 Å (with a slight off-stoichiometry toward Sm₂S₃) are measured. For T>95 K, charge transfer is shown to occur, as in stoichiometric Sm₃S₄ samples, by the hopping mechanism (σ ～ exp(?ΔE/kT) with ΔE ～ 0.13 eV). At low temperatures [up to the maximum in the lattice thermal conductivity κ_ph(T)], κ_ph ～ T ^2.6; in the range 20–50 K, κ_ph ～ T ^?1.2; and for T>95 K, where the hopping charge-transfer mechanism sets in, κ_ph ～ T ^?0.3 and a noticeable residual thermal resistivity is observed. It is concluded that in compounds with inhomogeneous intermediate rare-earthion valence, to which Sm₃S₄ belongs, electron hopping from Sm²⁺ (ion with a larger radius) to Sm³⁺ (ion with a smaller radius) and back generates local stresses in the crystal lattice which bring about a change in the thermal conductivity scaling of κ_ph from T ^?1.2 to T ^?0.3 and the formation of an appreciable residual thermal resistivity.     

Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178 K

We have investigated the molecular motions of TRIS⁺ ([(CH₂OH)₃CNH₃]⁺) and ions in the [(CH₂OH)₃CNH₃]₂SiF₆ crystal below room temperature from the measurements of the spin-lattice relaxation time T₁ and the NMR absorption line of ¹H and ¹⁹F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of T_c=178 K. The narrowing of the ¹⁹F-NMR line was observed around T_c=178 K and the reorientation of the anion appears above T_c. Moreover, from the analysis of the temperature dependence of T₁, we have observed that the activation energy of the reorientational motion of ions changes from 0.168 eV (T>T_c) to 0.185 eV (T<T_c). Based on these results, we found that the reorientational motion of ions is closely related to the origin of the phase transition at T_c. In addition, from the measurement of the ¹H-NMR line, we also found that the reorientational motion of H₂ in the -CH₂OH group becomes active accompanied by the phase transition.     

High-frequency response and wavelength dispersion of the linear electro-optic effect in PZN-PT (88/12) single crystal

We have characterized the wavelength dispersion of the high- and low-frequency values of the linear electro-optic coefficient r _c = r ₃₃–(n _o/n _e)³ r ₁₃ of poled 0.88 Pb(Zn_1/3Nb_2/3)O₃–0.12 PbTiO₃ single crystal. The measurements have been investigated as a function of the laser wavelength from 0.465 to 1.32 m. Clamped r ^S and unclamped r ^T values of the electro-optic coefficient as well as the acoustic contribution r ^a have been established. We obtained a ratio of r ^S/r ^T = 30%, which is independent of the wavelength of the light beam. The value of r ^S _c at the wavelength of 633 nm was found to be equal to 50±5 pm/V, which is a rather high value.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Optical characterization of CuIn5S8 crystals by ellipsometry measurements

Optical properties of CuIn₅S₈ crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2–6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple–DiDomenico and Spitzer–Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn₅S₈ crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis.     

Electrical and optical properties of single crystal In2Te3 and Ga2Te3

Electrical conductivity and thermopower measurements are reported for the defect semiconductors p-In₂Te₃ and n-Ga₂Te₃. The hole mobility μ_p in the former varied as Tⁿwheren=+5.98 forT<350 K and n=-4.13 forT>350 K showing a maximum of 210 cm²V^-1 sec^-1 at 350 K. Electron mobility in n-Ga₂Te₃ also went through a maximum at 320 K. The optical band-gaps for both were found to be direct, with values of 1.01 and 1.08 eV for In₂Te₃ and Ga₂Te₃ respectively at 300 K. Pronounced effects of annealing on TEP and optical absorption gave evidence of defect ordering at low temperatures followed by defect creation at T>350 K.     

Electron cooling in three‐dimensional Dirac fermion systems at low temperature: Effect of screening

Hot electron cooling rate P, due to acoustic phonons, is investigated in three‐dimensional Dirac fermion systems at low temperature taking account of the screening of electron–acoustic phonon interaction. P is studied as a function of electron temperature T_e and electron concentration n_e. Screening is found to suppress P very significantly for about T_e < 0.5 K and its effect reduces considerably for about T_e > 1 K in Cd₃As₂. In Bloch–Grüneisen (BG) regime, for screened (unscreened) case the T_e dependence is P ～ T_e⁹(T_e⁵) and the n_e dependence gives P ～ n_e^–5/3 (n_e^–1/3). The T_e dependence is characteristic of 3D phonons and n_e dependence is characteristics of 3D Dirac fermions. The plot of P /T_e⁴ vs. T_e shows a maximum at temperature T_em which shifts to higher values for larger n_e. Interestingly, the maximum is nearly same for different n_e and T_em/n_e^1/3 being nearly constant. More importantly, we propose, the n_e dependent measurements of P would provide a clearer signature to identify 3D Dirac semimetal phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)