Relationship between photoluminescence and structural properties of the sputtered Zn1−xCdxO films on Si substrates

Zn_1−xCd_xO (x=0.2, 0.4) alloyed crystal thin films have been deposited on Si(1 1 1) substrates at different temperatures by using dc reactive magnetron sputtering technique. The Zn_1−xCd_xO films are of highly (0 0 2)-preferred orientation possessing the hexagonal wurtzite structure of pure ZnO. At 450 °C, the films have better crystal quality and photoluminescent characteristics. For the films with x=0.2 and 0.4, the corresponding near-band-edge (NBE) energies are 3.10 and 3.03 eV, respectively, both have red-shifts compared with that of ZnO (3.30 eV). For the substrate temperatures lower or higher than 450 °C, the other NBE emission peak appears, the X-ray diffraction intensity of (0 0 2) peak decreases and the related FWHM increases. With the Cd addition up to x=0.4 both the XRD and PL intensity of the Zn_1−xCd_xO films decrease sharply in comparison with x=0.2.     

Composition and substrate temperature dependence of structural and optical studies on ZnSexCdS1−x alloy films

Thin (t−0.60 μm) films of ZnSe_xCdS_1−x were formed by vacuum evaporation on glass substrates held at 350 and 470 K. XRD studies showed that all the films were polycrystalline in nature. Films with x0.70 were hexagonal whereas films with x0.80 were cubic in structure. The structural transition was in the range 0.70<x<0.80. The lattice parameter was higher in films formed at a higher temperature. The lattice parameter followed Vegard's law. Grain size increased with substrate temperature. From the optical transmission spectra recorded in the wavelength range 300–2500 nm, the extinction coefficient, refractive index and band gaps were obtained. Band gap values showed a downward bowing with ‘x' with a bowing parameter of 0.40 eV.     

Photoelectrochemical investigations of n-CdSe1−xTex thin film electrodes

An electrode/electrolyte interface has been formed between an n-type CdSe_1−xTe_x (0≤x≤1) alloyed/mixed type semiconductor and a sulphide/polysulphide redox electrolyte. It has been investigated through the current–voltage, capacitance–voltage and spectrally selective properties. The dependence of the dark current through the junction and the junction capacitance on the voltage across the junction have been examined and analysed. It appeared that the current transport mechanism across the junction is strongly influenced by the recombination mechanism at the interface and series resistance effects. Upon illumination of the interface with a light of 20 mW cm⁻², an open circuit voltage of the order of 0.35 V and a short circuit current of 212 μA cm⁻² have been developed (for x=0.2), yielding an efficiency of energy conversion equal to 0.2% and a form factor of 45%. The action spectra in the 450–1000 nm wavelength range showed presence of the interface states at the electrode/electrolyte interface. The magnitudes of the barrier heights at the interfaces were also determined. It has been seen that a significant improvement in the electrochemical performance of a cell is noticed for the electrode composition with x=0.2.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

High quality GaN epitaxial layers were obtained with Al_xGa_1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al_xGa_1−xN (0≤x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al_xGa_1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

Doping and electrical characteristics of in-situ heavily B-doped Si_1−x−yGe_xC_y (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si_1−x−yGe_xC_y with y=0.0054 or below, the carrier concentration was nearly equal to C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ cm⁻³, regardless of the Ge fraction. The B-doped Si_1−x−yGe_xC_y with high Ge and C fractions contained some electrically inactive B even at the lower C_B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Characterization of CuIn1 − xAlxS2 thin films prepared by thermal evaporation

Ingots containing single crystals of the quaternary alloys CuIn_1 − xAl_xS₂ (CIAS) were grown by a horizontal Bridgman method for compositions with x = 0, 0.2 and x = 0.4. (CIAS) thin films were prepared by thermal evaporation technique on to glass substrates. Structural and optical properties of the films were studied in function of the Al content. Band gap, and absorption coefficients were determined from the analysis of the optical spectra (transmittance and reflectance as a function of wavelength) recorded by a spectrophotometer. The samples have direct bandgap energies of 1.95 eV (x = 0), 2.06 eV (x = 0,2) and 2.1 eV (x = 0,4). These optical results were correlated with the structural analysis by X-Ray diffraction.     

Lattice dynamics of Ga1−xAlxAs studied by ab initio calculations

We have calculated the phonon dispersion and phonon density of states of superlattices of Ga_1−xAl_xAs for x=0.0, 0.25, 0.75 and 1.0 using the ab initio method within the supercell approach and calculating the Hellmann–Feynman forces. A noticeable difference of force constants between Ga–As and Al–As atomic pairs has been found. In any case, Al atoms vibrate in well-separated high-frequency optic modes.     

Magnetoresistance effect in thin films of Eu1−xSmxO on silicon

In the present study, the temperature-dependence of magnetic field effect on the resistance of films of oxides of rare earth elements (Gd, Eu, and Eu-Sm solid solution) prepared by vacuum thermal evaporation was considered and analyzed at approximately the Curie temperature. It was demonstrated that the temperature range of the effect, its sign and magnitude are determined by the dependence of the magnetic moment of magnetic clusters on both the external magnetic field strength and film parameters such as the spin of the magnetic ion, exchange energy, defect density, etc.     

Spectroscopic ellipsometry analysis of GaAs1 − xNx layers grown by molecular beam epitaxy

In this work, we present the effect of nitrogen incorporation on the dielectric function of GaAsN samples, grown by molecular beam epitaxy (MBE) followed by a rapid thermal annealing (for 90 s at 680 °C). The GaAs_1 − xN_x samples with N content up to 1.5% (x = 0.0%, 0.1%, 0.5%, 1.5%), are investigated using room temperature spectroscopic ellipsometry (SE). The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. It was found that the features associated with E₁ and E₁ + Δ₁ transitions are blue-shifted and become less sharp with increasing nitrogen incorporation, in contrast to the case of E₀ transition energy in GaAs_1 − xN_x. An increase of the split-off Δ₁ energy with nitrogen content was also obtained, in agreement to results found with MOVPE GaAs_1 − xN_x grown samples.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Thermally activated rotation of Co1−xO particles within zirconia grains

Yttria partially stabilized zirconia (Y-PSZ) and Co_1−xO powders in 4:1 molar ratio were sintered and then annealed at 1300 and 1600°C to investigate the orientation change of Co_1−xO particles within Y-PSZ grains. Transmission electron microscopic observations indicated the Co_1−xO particles remained nonepitaxy in Y-PSZ grains after annealing at 1300°C for 300 h. When fired at 1600°C for 1–100 h, submicro-sized Co_1−xO particles (denoted as C) reached parallel epitaxy relationship, i.e. [100]_C//[100]_Z, [010]_C//[010]_Z and another relationship, i.e. [111]_C//[100]_Z, //[011]_Z with respect to the host zirconia grain (denoted as Z) nearly free of tetragonal precipitates. On the other hand, larger intragranular Co_1−xO particles (>1 μm in diameter) failed to reach epitaxial orientations even subject to prolonged annealing (100 h) at 1600°C. The temperature and size dependence of orientation change of the intragranular particle is in accordance with theoretical consideration of Brownian type rotation of the particle above a critical temperature for anchorage release at interface.     

Epitaxial growth of strained Si1−yCy films by the hot-wire cell method and its application to metal oxide semiconductor devices

We succeeded in obtaining strained Si_1−yC_y films at a substrate temperature of 200 °C by the hot-wire cell method. The substitutional carbon concentration in films annealed at 700 °C was 0.9%, while it was limited to 0.13% for a sample grown by gas-source molecular beam epitaxy (MBE) at a substrate temperature of 700 °C. We investigated the thermal stability of strained Si_1−yC_y films for device application. Annealing at over 900 °C caused the formation of 3C-SiC and relaxation of the strain occurred. From this result, we found that the process temperature should be lower than 800 °C. A low-temperature MOSFET process, in which all process temperatures after deposition of Si_1−yC_y were lower than 800 °C, was developed and a strained Si_1−yC_y MOSFET was fabricated.     

Photoconductors based on ZnxCd1−xS and CdSe1−ySy thin films, fabricated with multilayer structure

Photoconductors based on evaporated Zn_xCd_1−xS and CdSe_1−yS_y thin films, with a novel multilayer structure, were fabricated and characterized through spectral response measurements. The device structure is constituted by several layers of different energy gaps, sequentially deposited side by side and interconnected in series or in parallel. The influence of the preparation conditions, number of layers and electrode configuration, on the performance of the photoconductors was determined. Photoresistors with a detection range between 400 and 750 nm were developed, using four layers with energy gaps varying between 1.8 eV and 2.8 eV and interconnected in parallel.     

Combinatorial synthesis and rapid characterization of Mo1−xSnx (0x1) thin films

Thin films of Mo_1−xSn_x, continuously and linearly mapped for 0<x<1, have been prepared by d.c. magnetron sputter deposition under various growth conditions. X-ray diffraction results indicate that as x in high-pressure deposited Mo_1−xSn_x increases from 0 to approximately 0.45, the bcc lattice expands and no new phases are formed. At low deposition pressures, Mo₃Sn, a β-tungsten structured phase, is formed along with the bcc Mo–Sn solid solution for 0.1<x<0.3. The variation of the lattice parameter for this intermetallic phase also indicates that solid solutions, possibly of the form Mo_3+ySn, are being formed. These materials are of special interest as anode candidates in lithium-ion batteries.     

Photoelectrochemical properties of sol–gel-derived Ti1−xVxO2 solid solution film photoelectrodes

Ti_1−xV_xO₂ solid solution film photoelectrodes were prepared by the dip-coating sol–gel method. X-ray diffraction and X-ray photoelectron spectroscopy were employed to ensure the formation of the solid solution and their composition. Obvious photoresponses were observed in the visible region for the solid solution film electrodes with x0.05 and the red shift of the photoresponse was enhanced with increasing x. Moreover, the solid solution film electrodes were found to be photoelectrochemically stable. However, the onset potential of photocurrent shifted positively with increasing x. Band model of the solid solution was suggested to explain the effects of the vanadium incorporation on the photoelectrochemical properties.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Phase formation process of IrxSi1−x thin films Structure and electrical properties

Experimental data on the phase formation process of amorphous Ir_xSi_1−x thin films with 0.30 ≤ x ≤ 0.41 are presented and discussed in relation to electric transport properties. The structure formation process at temperatures from 300 K up to 1223 K was investigated by means of X-ray diffraction. Distinct phases were observed in the final stage in dependence on the initial composition: Ir₃Si₄, Ir₃Si₅, and IrSi₃. An unknown metastable phase was found in films with a silicon concentration of 61 at.% to 64 at.% after annealing above the crystallization temperature T = 970 K. The crystal structure of this phase was determined by the combined use of X-ray diffraction and electron diffraction. It was found to be monoclinic, basic-face centred with lattice constants a = 1.027 nm, b = 0.796 nm, c = 0.609 nm, and γ = 113.7°. Additionally, microstructure and morphology of the films were investigated by transmission electron microscopy (TEM). The annealing process was studied by means of mechanical stress investigations as well as by electrical resistivity and thermopower measurements. Correlations between the structure, the phase formation and the electrical transport behaviour are discussed on the basis of conduction mechanism.     

Magnetism in Si1 − xMnx diluted magnetic semiconductor thin films

We have studied the electrical and magnetic properties of p-type semiconductor thin films of Si_1 − xMn_x/Si (x = 0.036 and 0.05) grown by molecular beam epitaxy. Experimental results reveal that the resistivity of the samples decreases gradually with increasing measurement temperature, which can be described well by Mott's variable-range-hopping model. All the samples exhibit the ferromagnetic ordering above room temperature. Among these samples, Si_0.95Mn_0.05 has a higher hole density and magnetization. This indicates an enhancement of hole-mediated ferromagnetic exchange interactions when the Mn-doping concentration is increased.     

Hardness, internal stress and fracture toughness of epitaxial AlxGa1−xAs films

Vickers microhardness indentations of 10 μm (001) oriented epilayers of Al_xGa_1−xAs on GaAs substrates have been utilized to evaluate the hardness H_v, the internal stress, and the fracture toughness K_Ic of the layers as a function of their composition parameter x. The hardness H_v varies linearly according to: (6.9-2.2x) GPa and K_Ic increases linearly with x according to: K_1c = (0.44+1.30x) MPa m^1/2. The influence of the substrate on these measurements was found to be negligible for the layer thickness (10 μm) and the indentation load (0.25 N) used, disregarding internal stresses.

Internal film stresses were evaluated by the bimorph buckling method, and were found to depend on the composition parameter according to σ = 0.13x GPa. These stresses did not notably affect the H_v measurements, but for K_Ic corrections as large as 25% had to be made.

The radial cracks observed were of the shallow Palmqvist type. In contradiction to previous reports on this type of cracking, it was found to initiate during unloading, not during loading, and a physical explanation for this deviation is given. No deep radial/median cracks were observed. It was found important to use expressions based on the correct crack geometry in the K_Ic evaluation. Also, a simple theory for the influence of internal stresses on the K_Ic results has been developed.