Photoconductivity and electrical properties of epitaxial Pb1 − xCdxSe films

Photosensitive monocrystalline films Pb_{1 − x}Cd_xSe (x⩽ 0.03) of low carrier concentration (n = 0.3 ÷ 4 × 10¹⁷ cm⁻³ have been grown by hot wall epitaxy method on cleaved (111)BaF₂ or (100)KCl substrates.Photoconductive thresholds determined by spectral response measurements (at temperatures 4.2, 77 and 300 K) were in good agreement with the fundamental absorption edges and showed a large increase in the energy gap E_g with Cd content. Photoconductivity in the impurity range (hv < E_g) was observed.The temperature dependence of the Hall coefficient and the mobility between 4.2 and 300 K have been studied. Temperature freeze out of the free carriers was observed.Discussion of the carrier scattering mechanisms and impurity photoconductivity phenomena is presented.     

Anomalies of the static dielectric constant of Pb1−x Ge x Te

The static dielectric constant of Pb_1–x Ge _x Te (0x0.05) has been determined from differential capacitance measurements on Schottky-barriers in the temperature range of 4.2–300 K. A comparison with data deduced from the phonon frequencies via the Lyddane-Sachs-Teller relation shows substantial discrepancies which are attributed to lattice defects.Work supported by Jubiläumsfonds der Österreichischen Nationalbank     

Composition dependence of optical and electrical characteristics in flash evaporated Pb1−xHgxTe films

Structural, optical and electrical analyses of flash evaporated lead-mercury telluride films indicate that under appropriate conditions of growth ternary Pb_1−xHg_xTe compounds are formed in the single phase crystalline films. Optical band gaps of the crystalline films, grown on substrates at temperatures (Ts) of 25 and 100°C, are observed to decrease with increase in mercury concentration in the films. The optical gap decreases between 0.27 and 0.14 eV in the 0.07 ⩽x⩽ 0.50 Ts of 25°C films and between 0.23 and 0.09 eV in the Ts of 100°C 0.07 ⩽x⩽ 0.93 films. Activation energy values, calculated from electrical conductivity measurements, indicate that between 350 and 500 K, the conduction is mainly due to thermally generated charge carriers, whereas impurity conduction dominates at temperatures <250K. All crystalline films are observed to be photoconducting.     

Anomalies of the refractive index and the optical energy gap of ferroelectric Pb1−x Ge x Te

The refractive index and the optical energy gap of Pb_1–x Ge _x Te (0x0.11) have been determined from transmission- and reflectivity measurements in the temperature range from 4.2 K to 300 K. At the ferroelectric phase transition a change of the temperature coefficient of both quantities is observed. A two bandk·p model calculation demonstrates a correlation of the optical energy gap with the high frequency dielectric constant. For higher values ofx (x=0.09) a splitting of absorption edge and birefringence have been observed.     

Optical and electrical properties of flash-evaporated Pb1−xHgxSe films

Results of optical and electrical properties of flash-evaporated Pb_1−xHg_xSe films, in the composition range 0.07 ⩽ x ⩽ 0.93, show that the alloy films are formed by alloying of PbSe and HgSe. The optical band gaps of films grown on substrates at 25 and 100°C decrease linearly from 0.20 to 0.09 eV and from 0.15 to 0.065 eV, respectively, as the Hg concentration in the films is increased from 0.07 to 0.93. Activation energy values, as obtained from conductivity measurements, suggest that between 225 and 400 K the conduction results mainly from thermally-generated charge carriers, whereas at the lower temperatures (<225 K) impurity conduction dominates. Irreversible changes in the properties of the films are observed at > 400 K.     

Structural,electrical and optical properties of Cdx Zn1−xSe thin films

X-ray diffraction and electron diffraction techniques indicate that Cd_x Zn_1–xSe thin films on glass substrates have a polycrystalline nature, with sphalerite structure for x0.5 and wurtzite structure for x0.6. The crystalline size in each composition increases with increasing the film thickness. The room temperature dark resistivity varies from one composition to another showing a transition at x=0.55The temperature dependence of of the deposited films revealed two conduction mechanisms, one below 352 K due to shallow levels, surface states, and defects introduced during the film growth, and over 352 K due to deep-level ionization following the ordinary semiconducting behaviour.The thermal activation energy of the free charge carriers decreases linearly with increasing the molar fraction x of the CdSe content up to x=0.55, above which it increases with increasing x.The optical constants of Cd_x Zn_1–xSe thin films of different compositions were determined in the spectral range 400–2000 nm. The analysis of the absorption coefticient at and near the absorption edge indicates the existence of allowed direct transition energy gaps decreasing with increasing x.     

Effect of NaF on optical and structural properties of CdxZn1−xS nano crystalline films

This work investigates the effect of NaF on optical and structural properties of nano crystalline Cd_xZn_1?xS films. The Cd_xZn_1?xS films are prepared through chemical bath deposition (CBD) technique in aqueous alkaline bath and their subsequent condensation on substrates. The as-obtained samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–VIS absorption spectroscopy. Micro structural features, obtained from XRD analysis confirm the formation of cubic phase of undoped as well as NaF doped Cd_xZn_1?xS nano particles while SEM observations depict non-uniform distribution of grains. These results show the average grain size of pure as well as NaF doped samples to range from 50 to 90 nm. Tauc's plots, extracted from absorption spectra exhibit absorption to be dominating mainly in blue-green region of visible spectrum. The room-temperature photoluminescence (PL) spectra of Cd_xZn_1?xS samples show a peak around 425 nm, which gets blue shifted for doped sample indicating improvement in PL properties on its addition.     

Composition and lattice mismatch dependent dielectric constants and optical phonon modes of InAs1−x−ySbxPy quaternary alloys

Based on the pseudopotential formalism under the virtual crystal approximation, the dielectric and lattice vibration properties of zinc-blende InAs_1−x−ySb_xP_y quaternary system under conditions of lattice matching and lattice mismatching to InAs substrates have been investigated. Generally, a good agreement is noticed between our results and the available experimental and theoretical data reported in the literature. The variation of all features of interest versus either the composition parameter x or the lattice mismatch percentage is found to be monotonic and almost linear. The present study provides more opportunities to get diverse high-frequency and static dielectric constants, longitudinal and transversal optical phonon modes and phonon frequency splitting by a proper choice of the composition parameters x and y (0 ⩽ x ⩽ 0.30, 0 ⩽ y ⩽ 0.69) and/or the lattice mismatch percentage.     

Energy gaps,charge distribution and optical properties of AlxIn1−xSb ternary alloys

The electronic and optical properties of Al_xIn_1−xSb ternary alloys have been investigated using a pseudopotential approach within the virtual crystal approximation. The effect of alloy disorder on the studied properties has been examined and found to be weak. The extent of the direct-to-indirect band gap transition is found to occur at x = 0.73. Our results agree well with those reported in the literature. Trends in bonding and ionicity are discussed by means of the electron charge distribution. The present study may be a useful information for mid-infrared inter band cascade lasers applications and other antimonide device structures.     

Structure,optical, and magnetic properties of rutile Sn1−xMnxO2 thin films

Sn_1?xMn_xO₂ (x ≤ 0.11) thin films were fabricated by sol–gel and spin-coated method on Si (1 1 1) substrate. X-ray diffraction revealed that single-phase rutile polycrystalline structure was obtained for x up to about 0.078. Evolution of the lattice parameters and X-ray photoelectron spectroscopy studies confirmed the incorporation of Mn³⁺ cations into rutile SnO₂ lattice. Optical transmission studies show that the band gap energy (E_g) broadens with the increasing of Mn content. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. Interestingly, the magnetic moment per Mn atom decreases with the increasing Mn content. The origin of RTFM can be interpreted in terms of the bound magnetic polaron model.     

The superconducting transition temperature of Pb1−x Mn x films

Results of the superconducting transition temperatureT _c of amorphous and microcrystalline films of lead doped with manganese as magnetic impurity are reported in this work. The amorphous films show an Abrikosov-Gor'kov behaviour, whereas for the crystalline films there is a much smaller depression and a peak for higher Mn concentrations, which indicates a region of coexistence of superconductivity and magnetic ordering as a spinglass.Supported by the Deutsche Forschungsgemeinschaft in the Sonderforschungsbereich 125, Fehlordnung in Metallen — Aachen, Jülich, Köln     

Optical and photoelectrical properties of Hg1−xCdxTe/CdTe epitaxial films with graded band gap

The infrared transmission and photoconductivity spectra of HgCdTe epitaxial films with graded band gap were investigated both theoretically and experimentally. Theoretical calculations were performed in the framework of the WKB approximation. The composition profile has been obtained from a fitting procedure. In order to reduce the total number of fitting parameters as well as to improve accuracy of this procedure the differential of the transmission versus photon energy curves was used. The best fit was obtained for an exponential composition gradient.     

A study of Ba x Mg1 − x F2 optical films

Ba _x Me_{1 ? x} F₂ binary fluoride films (“Me” denotes calcium or magnesium fluoride) are studied. A method of processing the reflection and transmission spectra is proposed to determine the optical constants. The dispersion dependences of the refractive indices and absorption coefficients of films in the range of 1.3–12 μm are found. Dispersion in films in the regions of additional absorption bands, which are absent in single crystals, is observed for the first time. It is shown that the films of binary fluorides have a higher packing density, a lower absorption, and better operating characteristics than do films of pure fluorides. The films are promising for application as optical interference coatings in the mid-IR spectral region.     

Surface nanostructure effects on optical properties of Pb(Zr x Ti1−x )O3 thin films

Optical scattering properties of nanostructured matter have crucial impact on performance efficiency of various photonic components, such as waveguides, display elements, and solar cells. In this paper, diffuse transmission properties of nanocrystalline Pb(Zr _x Ti_1?x )O₃ thin films with a high refractive index of ~2.5 and optical transmittance are presented. Thin films with a thicknesses ranging from 50 to 500 nm were studied using integrating sphere technique and results were compared to simulations performed by a scalar scattering theory. Thin films were deposited by pulsed laser deposition at room temperature on MgO(100) substrates and post-annealed at a temperature of 800 °C. Structural phase evolution-induced surface effects, which introduced periodicity on the film surface, cause the definite diffuse elements in transmission spectra of the films. Low and evenly distributed scattering amplitudes in k-space were seen for highly tetragonal- or trigonal-oriented films with non-textured surfaces, which led to low diffuse transmission values (T _D ≈ 5 %), while confined and increased scattering amplitudes in k-space were seen for tetragonal–trigonal-oriented films, with phase co-existence, which led to microstructure-induced textured surfaces and increased diffuse transmission values (T _D ≈ 50 %). For highly textured surfaces, scattering amplitudes distributed in tilted ellipsoid shape in k-space was observed. Difference between modeled and measured values was 3.8 % in maximum.     

Photoluminescence and transmittance of CdS1−xTex thin films

CdS_1?xTe_x thin films were prepared by first producing CdS:In thin films by the spray pyrolysis (SP) technique and then annealing the films in the presence of Te vapor in nitrogen atmosphere. X-ray diffraction (XRD) measurements showed that the films are polycrystalline with mixed (cubic and hexagonal) structure. Transmittance measurements were recorded at room temperature in the wavelength range 400–1100 nm. The first derivative of the absorbance – which was deduced from the transmittance – was calculated and used to find the values of the bandgap energy. The presence of two minima in the first derivative curves was interpreted by the presence of a mixed (hexagonal and cubic) phase in accordance with the XRD measurements. The photoluminescence was recorded at T=60 K and a deconvolution peak fit was performed for each spectrum. The dependence of the PL spectra on the tellurium content of the films is obvious through the number, position and amplitude of the peaks in the different bands.     

Temperature and composition dependence of the energy band gap of Pb1−x Mn x S solid solution

Photovoltaicp-n junctions inn-type Pb_1–xMn _x Sx0.04, have been made by sulphur diffusion. Current-voltage and resistance-voltage characteristics have been examined at various temperatures. The spectral responses of the diodes have been measured within the temperature range from 5 to 300 K at a zero bias. From these measurements the energy band gap of Pb_1–x Mn _x S solid solution has been determined as a function of temperature and manganese content. A phenomenological expression describing the variation of the energy gap of Pb_1–x Mn _x S with temperature and alloy composition has been proposed.     

Features of the plasma sputtering of polycrystalline Pb1 − x Sn x S films

The processes of the sputtering and modification of surfaces of polycrystalline films of the ternary solid solution Pb_{1 ? x} Sn _x S (x = 0.9–1.0) in a high-density Ar plasma of high-frequency low-pressure inductive discharge are studied. Films with thicknesses of 1–4 μm are grown on glass substrates using the “hot-wall” method and consist of plate-like crystallites. It is established that the sputtering rate for lead-tin sulfide films does not exceed 2.0 nm/s, which is determined by the presence of oxygen-containing compounds on the surfaces. In the case of plate-like crystallites with nanodimensional thicknesses, the effect of smoothing of the developed surfaces of the polycrystalline Pb_{1 ? x} Sn _x S layers during plasma treatment is observed; this is important for fabricating multilayer device structures.     

Room-temperature ferromagnetism and optical properties of Zn1−xMnxO nanoparticles

The room-temperature ferromagnetism is observed in Zn_0.98Mn_0.02O nanoparticles, which is related to the host-lattice defects induced by doping Mn. The ferromagnetism in Zn_0.95Mn_0.05O nanoparticles can be suppressed by Mn clusters. The additional peak at 519 cm^?1 is observed in Raman scattering spectra of the Zn_1?xMn_xO nanoparticles associated with intrinsic host-lattice defects, which become activated due to the Mn doping. The decrease in band gap and the weak intensity of absorption peak in the nanoparticles may be due to the sp–d interaction between transition metal and Zn anions.