Preparation of CuIn1 − xGaxSe2 films by metalorganic chemical vapor deposition using three precursors

CuInGaSe₂ thin films have been prepared by a low pressure metalorganic chemical vapor deposition technique using three precursors without additional Se. The properties of the resultant films have been examined by scanning electron microscopy, X-ray diffraction, micro-Raman scattering and absorption spectroscopy.     

CuIn1 − xAlxSe2 thin films obtained by selenization of evaporated metallic precursor layers

CuIn_1 − xAl_xSe₂ (CIAS) thin films were grown by a two stage process. Cu, In and Al layers were sequentially evaporated and subsequently heated with elemental selenium in a quasi-closed graphite box. Different x values (0 ≤ x ≤ 0.6) were obtained by varying the Al and In precursor layers thicknesses. Selenization conditions such as Se amount provided during the selenization process were adjusted in order to optimize the film properties. Polycrystalline CuIn_1 − xAl_xSe₂ thin films with chalcopyrite structure were obtained. Referred to CuInSe₂ thin films the lattice parameters, the (112) orientation and the average crystallite size decreased and the band gap energy increased with increasing Al content. To optimize structural properties of the CIAS films a higher Se amount was required as the x value increased. The incorporation of Al changed the thin film morphology towards smaller grain sizes and less compact structures.     

Tuning the optical properties of alloyed CdSexS1 − x nanoparticles by changing the constituent stoichiometry

The spherical particles CdSe_xS_1 − x with 30-80 nm in radius have been successfully prepared by the hydrothermal reaction at 200 °C. The structure characterization which has been carried out using X-ray diffraction (XRD) shows hexagonal crystal structure. Novel properties have been observed via UV-visual absorption spectra and photoluminescence (PL) spectra. The absorption shoulder and the luminescence emission peaks have been tuned by changing the mole ratio of Se in the CdSe_xS_1 − x samples.     

Performance of CuIn1 − xGaxSe2/(PVD)In2S3 solar cells versus gallium content

The present work studies the influence of the Ga content (x = Ga / (Ga + In)) in the absorber on the solar cell performance for devices using (PVD)In₂S₃-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In₂S₃-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E_gCIGSe < 1.1 eV. However, the parameters of the cells buffered with (PVD)In₂S₃ are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In₂S₃ interface.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Structural study of Si1 − xGex nanocrystals embedded in SiO2 films

We have investigated the structural properties of Si_1 − xGe_x nanocrystals formed in an amorphous SiO₂ matrix by magnetron sputtering deposition. The influence of deposition parameters on nanocrystal size, shape, arrangement and internal structure was examined by X-ray diffraction, Raman spectroscopy, grazing incidence small angle X-ray scattering, and high resolution transmission electron microscopy. We found conditions for the formation of spherical Si_1 − xGe_x nanocrystals with average sizes between 3 and 13 nm, uniformly distributed in the matrix. In addition we have shown the influence of deposition parameters on average nanocrystal size and Ge content x.     

Enhanced ferromagnetic properties of multiferroic BiCoxFe1 − xO3 synthesized by hydrothermal method

The BiCo_xFe_1 − xO₃ samples have been successfully synthesized by hydrothermal process. The resulting products were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray (EDS), differential thermal analysis (DTA), and physical property measurement system (PPMS).It was found that the magnetization of the obtained products was greatly enhanced by Co substituting for Fe ions. Furthermore, the value of magnetism of BiCo_xFe_1 − xO₃ samples can be adjusted by Fe doping concentration. DTA curve indicates the ferroelectric properties of the obtained BCFO samples are not affected by Co substitution. Therefore, it would be interesting to realize thin films with similar compositions and study their properties in the interest of device applications.     

Enhancing epitaxial SixC1 − x deposition by adding Ge

In this work, a possible way to enhance the epitaxial growth of metastable, tensile strained Si_xC_1 − x layers by the addition of germanium is demonstrated. During ultra-high vacuum chemical vapor deposition growth, the co-mixing of germane to the Si_xC_1 − x precursors was found to enhance the growth rate by a factor of ~ 3 compared to the growth of pure Si_xC_1 − x. Furthermore, an increase of the amount of substitutional incorporated carbon has been observed. Selective Si_xGe_yC_{1 − x − y} deposition processes utilizing a cyclic deposition were developed to integrate epitaxial tensile strained layers into source and drain areas of n-channel transistors.     

Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoxCo2xBi1 − 3xO3

Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCo^II_xCo^III_2xBi_1 − 3xO₃ (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO₃ thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ₂₃), thermistor constant (B_25/85) and activation energy are in the range of 1.995-2.975 Ω cm, 1140-1234 K and 0.102-0.111 eV, respectively.     

Structural, electrical and optical properties of transparent Zn1 − xMgxO nanocomposite thin films

Zn_1 − xMg_xO thin films of various Mg compositions were deposited on quartz substrates using inexpensive ultrasonic spray pyrolysis technique. The influence of varying Mg composition and substrate temperature on structural, electrical and optical properties of Zn_1 − xMg_xO films were systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70 mol%. AFM images of the Zn_1 − xMg_xO films (x = 0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zn_1 − xMg_xO. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zn_1 − xMg_xO decreases with corresponding increase in Mg content, which result in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16 eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side.     

Parameterization of CuIn1−xGaxSe2 (x = 0, 0.5, and 1) energy bands

Parameterization of the electronic band structure of CuIn_1−xGa_xSe₂ (x = 0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [E_j(k); j = v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the Γ-point valence-band maximum E_v1(0). Also the lowest conduction band E_c1(k) is anisotropic and non-parabolic for energies ~ 0.05 eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn_1−xGa_xSe₂ need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses m_j(k), and also an average energy-dependent approximation of the masses m_j(E).     

Synthesis of ternary ZnxCd1 − xS nanowires by thermal evaporation and the study of their photoluminescence

Ternary Zn_xCd_1 − xS nanowires were synthesized on Au-coated Si (100) substrates by thermal evaporation method. The nanowires obtained under ambient and reduced pressure of ∼ 100 Torr were studied and compared. Both of them were single crystalline wurtzite structured with similar chemical composition. The tip of the nanowires obtained in ambient pressure contained an Au-rich particle, but that obtained under reduced pressure was impurity-free. Hence, both vapor-liquid-solid (VLS) and mixed vapor-solid (VS)-VLS growth mechanisms were proposed for these two types of nanowires. Furthermore, photoluminescence studies revealed that their intrinsic and extrinsic emission bands were within the visible region.     

Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals

The optical and electrical properties of vapour phase grown crystals of diluted magnetic semiconductor Zn_1 − xCr_xTe were investigated for 0 ≤ x ≤ 0.005. The diffuse reflectance spectra exhibited an increase in the fundamental absorption edge (E₀) with composition x and were also dominated by a broad absorption band around 5200 cm^− 1 arising from ⁵T₂ → ⁵E transition. The ⁵T₂ and ⁵E levels originate from the crystal field splitting of the ⁵D free ion in the ground state. The electrical resistivity measurements revealed semiconducting behaviour of the samples with p-type conductivity in the temperature range of 200-450 K.     

Instantaneous preparation of CuIn(S1 − x, Sex)2 films by means of sparks using microwave irradiation

CuIn(S_1 − x,Se_x)₂ (CISSe) films aimed at flexible solar cells were directly prepared on Ti foils from elemental In, Cu, S, and Se particle precursor using microwave irradiation. The formation of the CISSe phase was deduced from X-ray diffraction (XRD) patterns. The (112) peaks of CISSe were well defined and the lattice constants increased in direct proportion to the S/(S + Se) ratio almost satisfying Vegard's law. In particular, CuInSe₂ was formed in the desired chalcopyrite lattice as indicated by the presence of (101), (103) and (211) peaks in the XRD pattern. Porous surfaces and formation of by-products were avoided by employing an evaporated In and Cu films instead of In and Cu particles.     

Quantum-confinement effect on recombination dynamics and carrier localization in cubic InN and InxGa1 − xN quantum boxes

In this study, the quantum confinement effect on recombination dynamics and carrier localization in cubic InN (c-InN) and cubic In_xGa_1 − xN (c-In_xGa_1 − xN) low dimensional structures are theoretically examined. The small InN and In-rich In_xGa_1 − xN low dimensional structures show a strong quantum confinement effect, which results in ground states away from the band edge and discrete eigen-states. Depending on composition, temperature, and size of the InN and In_xGa_1 − xN low dimensional structures, quantum confinement effect can affect the exciton dimensions (D). In InN quantum cubes, the strong quantum confinement effect leads to temperature-dependent radiative lifetimes showing a large size variation. The nearly-temperature-independent and shorter radiative lifetimes in small InN and In-rich In_xGa_1 − xN low dimensional structures suggest that the strong quantum confinement leads to 0 D carrier confinement, stronger carrier localization, and high recombination efficiency. Reported radiative lifetimes were found to match very well with our simulation results of In-rich quantum cubes, which indicates that spontaneous emissions come from the radiative recombination of localized excitons in In-rich In_xGa_1 − xN clusters. The simulation results could provide important information for the designs and interpretations of c-InN and c-In_xGa_1 − xN devices.     

Dye-sensitized solar cells composed of Ti1−xSnxO2 nanocrystals

Ti_1−xSn_xO₂ nanocrystals were successfully synthesized by using a simple solvothermal route, and its band energy gap broaden and flat band potential can be rationally regulated with increasing x value. Furthermore, Ti_1−xSn_xO₂ nanocrystals were first used as the photoelectrode material for dye-sensitized solar cells. A cell made of Ti_1−xSn_xO₂ (x = 0.3) exhibited the best photovoltaic performance. This is due to its most narrow band gap energies, most negative flat band potential and lowest dark current densities. After the surface of Ti_1−xSn_xO₂ (x = 0.3) electrode was treated with TiCl₄ solution, the cell sensitized by a mixed solution of N719 and D131 dye exhibited the best efficiency of 4.64% under the illumination of 1 sun (AM1.5, 100 mW cm⁻²).     

Evolution of the optical transitions in AlxGa1 − xAs/GaAs quantum well structures grown on GaAs buffers with different surface treatments by molecular beam epitaxy

Al_0.3Ga_0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl₂ etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8-300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Cd1−xMnxTe thin films deposited by close-spaced sublimation for application in solar cells

High-quality Cd_1−xMn_xTe polycrystalline films with (1 1 1) preferred orientation were deposited by close-spaced sublimation (CSS) method. The XRD and optical absorption analysis indicated that the band gap of the film was about 1.6 eV. The as-grown Cd_1−xMn_xTe films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. The effect of various post-deposition treatments with vapors of chlorine-containing materials (CdCl₂ and/or MnCl₂), in Ar or H₂/Ar ambient, on the properties of Cd_1−xMn_xTe cells was studied.     

Band alignment of Cd(1 − x)ZnxS produced by spray pyrolysis method

Cd_(1 − x)Zn_xS thin films have been grown on glass substrates by the spray pyrolysis method using CdCl₂ (0.05 M), ZnCl₂ (0.05 M) and H₂NCSNH₂ (0.05 M) solutions and a substrate temperature of 260 °C. The energy band gap, which depends on the mole fraction × in the spray solution used for preparing the Cd_(1 − x)Zn_xS thin films, was determined. The energy band gaps of CdS and ZnS were determined from absorbance measurements in the visible range as 2.445 eV and 3.75 eV, respectively, using Tauc theory. On the other hand, the values calculated using Elliott-Toyozawa theory were 2.486 eV and 3.87 eV, respectively. The exciton binding energies of Cd_0.8Zn_0.2S and ZnS determined using Elliott-Toyozawa theory were 38 meV and 40 meV, respectively. X-ray diffraction results showed that the Cd_(1 − x)Zn_xS thin films formed were polycrystalline with hexagonal grain structure. Atomic force microscopy studies showed that the surface roughness of the Cd_(1 − x)Zn_xS thin films was about 50 nm. Grain sizes of the Cd_(1 − x)Zn_xS thin films varied between 100 and 760 nm.