Synthesis and characterization of Mg2B2O5

Magnesium borate of the form Mg₂B₂O₅ has been prepared and its structural and thermal properties were studied using X-ray diffraction and differential thermal analysis. An investigation of the electrical and optical properties of Mg₂B₂O₅ system has been carried out. The electrical resistivity of the sample was measured in the temperature range of 170-400 K. The data analysis revealed an extrinsic nature of the conductivity with two impurity levels located at 0.13 and 0.71 eV in the temperature ranges of 170-230 K and 240-400 K, respectively. The optical transmission and reflection was recorded at 300 K in the incident photon energy range of 3.0-6.0 eV. The absorption coefficient data analysis revealed an indirect optical energy band gap of 4.73 eV. In addition, two impurity levels located at 3.43, and 4.49 eV were observed in the absorption spectra.     

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.     

Ag2СdSnS4 single crystals as promising materials for optoelectronic

Single crystals of the quaternary single crystals Ag₂CdSnS₄ were grown for the first time using the horizontal gradient freeze technique. Optical spectral and photoelectric properties of obtained crystals were investigated. The band gap energy at 77 K according to the photoconductivity spectra is 1.94 eV. The energy levels of the major donor centers in the band gap were determined. The role of intrinsic defects in the observed dependences is analyzed. The energy levels of the major donor centers in the band gap were determined. A small photoconductivity maximum at low temperature is observed at wavelength λ_m = 640 nm (hν ∼ 1.94 eV); situated in the fundamental absorption band, which unambiguously corresponds to the intrinsic origin of photoconductivity. The increase of the extrinsic photoconductivity with the maximum at λ_m ∼ 800 nm with temperature leads to its domination above 240 K. The observed peculiarity can be explained by the photoexcitation of electrons from the valence band to the donor centers which are empty at high temperatures and with further thermal excitation to the conduction band.     

Photoluminescence spectra of MnIn2S4

MnIn₂S₄ single crystals grown by the directional crystallization method were investigated by using the temperature and excitation power dependencies of photoluminescence (PL) spectra. PL spectra consist of one broad band resulting from donor-acceptor pair recombination. The analysis of the temperature quenching of the PL intensity yields one defect donor level with a thermal ionization energy of about 0.17 eV. The broad band of PL spectra indicates that radiative recombination is related to multiphonon optical processes. The energy of the involved phonon was found to be around 0.025 eV and the energy of the acceptor level is about 0.86 eV.     

Electrical properties of Se1−xTex crystals

Single crystals of Se_1?xTe_x (with x varying from 0.1 to 0.9) were successfully grown by the Bridgman method. D.C. Hall effect and temperature dependence of π of these crystals were measured. π at 300°K is found to decrease and the carrier concentration to increase approximately exponentially with x. The temperature dependence of π shows that the conduction is thermally activated. At low temperature (< 300 °K) extrinsic conduction is dominant with the value of the activation energy decreasing from 0.065 to 0.006 eV as x increases from 0.1 to 0.9. At higher temperature, intrinsic conduction is observed for alloys with x = 0.4, 0.5, 0.6 and 0.8 yielding the values of intrinsic band gaps (E_g). The values of E_g are found to decrease with x.     

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.     

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T _C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T _C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.     

Magnetic properties of β-FeSi2 single crystals

We investigated temperature and magnetic field dependences of the magnetization of β-FeSi₂ single crystals in the temperature range of 5-300 K in magnetic fields up to 15 kOe. The temperature dependence of the magnetic susceptibility of the Cr- and Ni-doped sample can be explained by temperature-dependent contributions due to paramagnetic centres and due to the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature as well as activation energy of the donor and acceptor levels are estimated.     

The influence of copper doping on the electrical properties of magnetic semiconducting Cd1−xFexCr2S4 single crystals

The Curie temperature and the dependence of the resistivity on the magnetic flux density and on the temperature of Cu-doped p-type Cd_1?xFe_xCr₂S₄ single crystals have been measured.The Curie temperature is not affected by the doping in spite of the considerably higher electrical conductivity of the doped samples. The dependence of the resistivity on the magnetic flux density (B ≤ 1 Vs/m²) has a behaviour similar to that of n-type CdCr₂Se₄: In, reported earlier /1/. At low iron content, the negative magnetoresistance is enhanced by the copper doping; at high iron content, the magnetoresistance is diminished by the doping. The results are discussed on base of the model of “magnetic impurity states” due to Fe²⁺ states.     

Laser Excitation Effect on the Optical and Dielectric Properties of Ferroelectric KNbO3 Single Crystals

The effects of laser irradiation on the optical absorption (200- to 800-nm range) and dielectric properties (10²- to 10⁷-Hz range) of KNbO₃ single crystals in ferroelectric phase have been studied. All these properties are found to be reduced significantly at lower frequencies with an increase in laser irradiation time. However, the phase transition temperature remains unchanged. It is observed that the dielectric constant (K) and loss (tan ) values of KNbO₃ single crystals are decreased significantly at room temperature or at Curie temperature by laser excitation. Appreciable changes in the band gap (eV) and activation energy are also observed after laser excitation.     

Temperature dependence of dielectric function and optical transitions in TlInSe2 and TlGaTe2

The principal components of the dielectric function of TlInSe₂ and TlGaTe₂ crystals with quasi-one-dimensional chain structure have been studied over the photon energies 1.5-5.0eV in the temperature range 140-400 K, with due regard to the reported phase transitions. For TlGaTe₂, the absolute values of the dielectric function experience a sudden temperature-induced change at 290 K in nearly all the accessed photon energy range. The energy and other parameters of the critical points for inter-band optical transitions related to the obtained dielectric function have then been retrieved for both TlInSe₂ and TlGaTe₂. An abrupt change in the energy of the critical point, positioned at 2.93 eV at room temperature and formed by optical transitions induced by the light polarized along the chains, has then been disclosed for TlGaTe₂ at 290 K.     

Dielectric anomaly in LiCa2V5O15 ceramics

A polycrystalline sample of LiCa₂V₅O₁₅ (LCV) was prepared using a mixed oxide method at low temperature (i.e., at 630 °C). X-ray structural analysis shows the single-phase formation of the compound in the orthorhombic crystal system at room temperature. A study on the surface morphology of the compound showed uniform grain distribution on the surface and in the bulk of the sample with less porosity. A dielectric anomaly suggests that the compound has a transition temperature at 274 °C. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), of the compound was found to be 0.67 eV at 10 kHz. The nature of the variation of conductivity and value of activation energy in different regions, suggest that the conduction process is of mixed type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies).     

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.     

Optical properties and DC electrical conductivity of Ge28−xSe72Sbx thin films

Thin films of Ge_28−xSe₇₂Sb_x (x=0, 8, 16, 24 at%) with thickness of 200 nm are prepared by thermal evaporation onto glass substrates under vacuum of 5.3×10⁻⁵ mbar. Optical reflectance and transmittance of these films are measured at room temperature in the light wavelength region from 200 to 1100 nm. The estimated optical energy gap, E_g, is found to decrease from 2 eV (0 at% Sb) to 1.5 eV (24 at% Sb), whereas the band tail width, E_e, increases from 0.062 to 0.077 eV, respectively. The refractive index, n, and extinction coefficient, κ, are determined as functions of wavelength. The DC electrical conductivity, σ, of films is measured as a function of temperature in the range from 300 to 360 K. The extracted value of activation energy, ΔE, is found to decrease from 0.95 eV (0 at% Sb) to 0.74 eV (24 at% Sb). Optical and electrical behavior of films can be explained in terms of cohesive energy (CE) and Se-Se defect bonds.     

Transmission photoacoustic spectroscopy of flash-evaporated CuIn0.75Ga0.25Se2 thin films

Photoacoustic spectroscopy (PAS) has proved to be an effective technique for the evaluation of inherent defect population in a wide range of materials for various applications. This paper demonstrates the use of this technique in transmission mode and hence evaluates the optical properties of flash evaporated CuIn_0.75Ga_0.25Se₂ (CIGS) thin films. Both the photoacoustic and transmission spectra were recorded at room temperature using high resolution near-infrared of the gas-microphone type PAS which revealed a very broad transmission region (about 300 meV) near the fundamental band edge in the as-grown CIGS thin films due to the presence of several shallow defect levels. The post-deposition heat treatment of the samples under selenium ambient followed by annealing under inert and forming gas ambient showed significant changes in the behavior of the PAS spectra particularly near the fundamental band edge. The absorption coefficient has been derived from these spectra to determine the band gap energy values and the activation energies for several defect related energy levels. Using the PAS, the energy band gap values were in the range of 1.197 to 1.202 eV. The optical transmission spectra were also recorded from the spectrophotometer. The transmission data was used to determine the energy band gap values which were calculated to be in the range of 1.159 to 1.194 eV. These values were found to be in good agreement to each other as well as to those reported in the literature.     

Impurity conduction in ion beam synthesized β-FeSi2/Si

Carrier transport in ion-beam synthesized (IBS) β-FeSi₂ was investigated by using Hall effect measurement at low temperatures (15-300 K). The measurement showed p-type conduction in the temperature range of 15-300 K. The Hall coefficient increased with increasing temperature up to 25 K, and then it decreased, which suggested the two carrier conduction, i.e., the impurity conduction, as well as the conduction in the valence band, play an important role in the carrier transport. Based on the two carrier model, the hole concentration and mobility for the impurity conduction at 25 K were evaluated to be 9.9×10¹⁷ cm⁻³ and 0.85 cm² V⁻¹ s⁻¹, respectively, which suggested that the acceptors were isolated and did not form the impurity band at the impurity concentration of 9.9×10¹⁷ cm⁻³. Thus, the threshold concentration for the impurity band formation was more than three orders of magnitude higher than that for GaAs (2×10¹⁶ cm⁻³), which could be explained on the basis of the Mott criterion.     

Band gap and intergrain barrier activation energies in Bi90Sb10 thin films

The electrical resistivity and Hall effect measurements have been made on vacuum evaporated Bi₉₀ Sb₁₀ alloy films of various thickness (350 A to 4500 A), in the temperature range of 77 to 510 K. As observed earlier, the alloy system behaves like a semiconductor, but with a band gap quite higher than previously reported for bulk single crystals. Also a kind of intergrain barrier is found in these films. The activation energy of these barriers is found to decrease with increasing film thickness and substrate temperature. This trend agrees with the earlier observations in other materials and also in the same alloy system. The higher band gaps in these films are attributed to quantum size effect and high dislocation density in these films. The decrease in the inter-grain barrier activation energy with increasing thickness and substrate temperature has been attributed to increased grain size of the films.     

Magnetic and Electrical Properties of Single-Crystal La1.99Sr0.01Cu1 ? yNiyO4

We report the out-of-plane and in-plane magnetic susceptibility and resistivity of Ni-doped La_1.99Sr_0.01CuO₄ single crystals grown by Traveling-Solvent Floating-Zone technique. Our data shows that in lightly Sr-doped compounds the Néel temperature T _N increased with increasing Ni concentration. At high temperatures, the resistivity is described within the impurity band conduction model. As the temperature is lowered, the resistivity behavior change to the variable range hopping model associated with the evolving of antiferromagnetic ordering.     

Electronic conduction in SiO2:N thin films grown by thermal oxidation of silicon in N2O

Silicon oxynitride [SiO₂:N] thin films have been grown by oxidizing silicon in N₂O at 900, 1000 and 1100 °C and at 760 and 1520 torr. It is shown that the dominant electrical conduction mechanism, for high electric fields, is the field assisted thermionic emission from the traps (Poole-Frenkel effect), and is not direct or Fowler-Nordheim tunneling, as typically occurs in thermal silicon oxide with similar thickness. Electrical conduction in these films occurs by field assisted electron emission from donor traps with energy levels varying in the range from 0.5 to 1 eV from the conduction band. The results shown here indicate that the best quality films are those grown at low temperature and pressure, since they give films with a higher critical electric field, a higher energy barrier depth at the traps and less donors compensated by acceptors than those grown at high temperatures and pressures.     

Melt growth and characterization of Mg2Si bulk crystals

We have grown Mg₂Si bulk crystals by the vertical Bridgman method using a high-purity Mg (6N-up) source. The grown crystals were single-phase Mg₂Si and had well-developed grains (1-5 mm³). Laue observations and SEM-EDX observations confirmed that crystalline quality in the grains was single crystal with stoichiometric composition. Electron concentration of the single crystalline specimens grown from 6N-up-Mg was 4.0 × 10¹⁵ cm^− 3 at room temperature (RT). This value is more than one order of magnitude lower than that of specimens grown from 4N-Mg [(5-7) × 10¹⁶ cm^− 3]. The Hall mobility of 14,500 cm²/Vs was observed at 45 K in the crystals grown from 6N-up-Mg. We also found that Al impurity plays an important role in the crystals grown from a low-purity Mg source. From the optical absorption measurement, we estimated that the indirect energy gap was about 0.66 eV at 300 K and about 0.74 eV at 4 K.