The study of below and above band-edge imperfection states in In2S3 solar energy materials

β-In₂S₃ is a nontoxic buffer layer material usually used in a thin-film solar cell due to a lot of vacancies and surface states naturally existing in the crystal to assist in photoelectric conversion. Transition metal (TM)-incorporated β-In₂S₃ has also been proposed to increase conversion efficiency in In₂S₃ since multi-photons absorption by intermediate band (IB) would happen in the sulfide. In this paper, single crystals of undoped and Nb-doped β-In₂S₃ have been grown by the chemical vapor transport (CVT) method using ICl₃ as a transport agent. Optical properties of the imperfection states of the crystals are probed by thermoreflectance (TR), photoconductivity (PC), photoluminescence (PL), surface photoconductive response (SPR), optical absorption and photo–voltage–current (photo V–I) measurements. The TR and optical-absorption measurements confirmed that the undoped and Nb-doped β-In₂S₃ are direct semiconductors with energy gap of 1.935 eV for undoped β-In₂S₃, 1.923 eV for β-In₂S₃:Nb_0.005, and 1.901 eV for β-In₂S₃:Nb_0.01. For undoped β-In₂S₃, PC and PL measurements are used to characterize defect transitions below band gap. The above band-edge transitions of undoped β-In₂S₃ have also been evaluated using PL, PC, and SPR measurements. For the evaluation of Nb-doped β-In₂S₃, an intermediate band with energy of ∼0.4 eV below the conduction band edge has been detected in the TR measurements in both β-In₂S₃:Nb_0.005 and β-In₂S₃:Nb_0.01. The photo V–I measurements also verified that the photoelectric-conversion efficiency would be enhanced in the β-In₂S₃ with higher niobium content. Based on the experimental analyses, the optical behavior of the defects, surface states, and IB (formed by Nb) in the In₂S₃ crystals is thus explored.     

Ternary semiconductor compounds CuInS2 (CIS) thin films synthesized by electrochemical atomic layer deposition (EC-ALD)

In this paper the formation and characterization of the I-III-VI₂ semiconductor compound CuInS₂ (CIS) on gold substrate at room temperature by electrochemical atomic layer deposition (EC-ALD) method are reported. Optimum deposition potentials for each element are determined using cyclic voltammetry (CV) technique and Amperometric I-t method is used to prepare the semiconductor compound. These thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared spectroscopy (FT-IR). XRD results indicate that the CIS thin films have a (1 1 2) preferred orientation. The XPS analyses of the films reveal that Cu, In and S are present in an atomic ratio of approximately 1:1:2. And their semiconductor band gaps are found to be 1.50 eV by FT-IR.     

Optimization of annealing conditions of In2S3 thin films deposited by vacuum thermal evaporation

In₂S₃ thin films were grown on glass substrates by means of the vacuum thermal evaporation technique and subsequently thermally annealed in nitrogen and free air atmosphere from 250 to 350 °C for different durations. Experimental parameters have been adjusted in order to optimize the annealing conditions, and to obtain high band gap energy at low deposition temperature, as required for photovoltaic applications. In order to improve our understanding of the influence of the deposition and annealing parameters on device performance, we have investigated our indium sulfide material by X-ray diffraction, energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM) and spectrophotometry. The optical and structural properties of the films were studied as a function of the annealing temperature and durations. X-ray diffraction analysis shows the initial amorphous nature of deposited In-S thin films and the phase transition into crystalline In₂S₃ upon thermal annealing. Films show a good homogeneity and optical direct band gap energy about 2.2 eV. An annealing temperature of 350 °C during 60 min in air atmosphere were the optimal conditions.     

Synthesis and CO2 capture property of high aspect-ratio Li2ZrO3 nanotubes arrays

High aspect-ratio Li₂ZrO₃ nanotubes were prepared by hydrothermal method using ZrO₂ nanotubes layers as templates. Characterizations of SEM, XRD, TEM and CO₂ adsorption were performed. The results showed that tetragonal Li₂ZrO₃ nanotubes arrays containing a little monoclinic ZrO₂ can be obtained using this simple method. The mean diameter of the nanotubes is approximately 150 nm and the corresponding specific surface area is 57.9 m² g⁻¹. Moreover, the obtained Li₂ZrO₃ nanotubes were thermally analyzed under a CO₂ flow to evaluate their CO₂ capture property. It was found that the as-prepared Li₂ZrO₃ nanotubes arrays would be an effective acceptor for CO₂ at high temperature.     

CuInS2 thin films obtained through the annealing of chemically deposited In2S3-CuS thin films

In this work, we report the formation of CuInS₂ thin films on glass substrates by heating chemically deposited multilayers of copper sulfide (CuS) and indium sulfide (In₂S₃) at 300 and 350 °C in nitrogen atmosphere at 10 Torr. CIS thin films were prepared by varying the CuS layer thickness in the multilayers with indium sulfide. The XRD analysis showed that the crystallographic structure of the CuInS₂ (JCPDS 27-0159) is present on the deposited films. From the optical analysis it was estimated the band gap value for the CIS film (1.49 eV). The electrical conductivity varies from 3 × 10⁻⁸ to 3 Ω⁻¹ cm⁻¹ depending on the thickness of the CuS film. CIS films showed p-type conductivity.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Second-harmonic generation in transparent surface crystallized GeS2-Ga2S3-CdS chalcogenide glasses

Transparent surface crystallized glasses containing CdGa₂S₄ nonlinear optical crystal were prepared by the 70GeS₂ · 15Ga₂S₃ · 15CdS (GGC15) chalcogenide glass. Average diameters of crystallites are about 150 nm and 600 nm for heating at 405 °C for 48 and 108 h (named GGC15-48 and GGC15-108), respectively, and the thickness of the surface crystallized layer was approximately 15 μm. By using the Maker fringe measurement, prominent second-harmonic generation was observed from these crystallized glasses, and the χ⁽²⁾ of the GGC15-48 sample is calculated to be as well as 38.85 pm/V, and the value is 13.7 pm/V for the GGC15-108. They are promising to be applied in photoelectric and all-optical field in the future.     

Investigations on co-evaporated Cu2SnSe3 and Cu2SnSe3-ZnSe thin films

Cu₂SnSe₃ is an important precursor material for the growth of Cu₂ZnSnSe₄, an emerging solar cell absorber layer via solid state reaction of Cu₂SnSe₃ and ZnSe. In this study, we have grown Cu₂SnSe₃ (CTSe) and Cu₂SnSe₃-ZnSe (20%) films onto soda-lime glass substrates held at 573 K by co-evaporation technique. The effect of annealing of these films at 723 K for an hour in selenium atmosphere is also investigated. XRD studies of as-deposited Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films indicated SnSe as secondary phase which disappeared on annealing. The direct optical band gap of annealed Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films were found to be 0.90 eV and 0.94 eV respectively. Raman spectroscopy studies were used to understand the effect of ZnSe on the properties of Cu₂SnSe₃.     

Collision-induced first overtone band of D2 in binary mixtures D2-X where X is Ar, Kr, and Xe

Enhancement spectra of the collision-induced absorption in the first overtone region 5500-6750 cm⁻¹ of D₂ in the D₂-Ar, D₂-Kr, and D₂-Xe binary mixtures were studied at 298 K for base densities of D₂ in the range 55-251 amagat and for partial densities of Ar, Kr, and Xe in the range 46-384 amagat. The observed spectra consist of the following quadrupolar transitions: O₂(3), O₂(2), Q₂ (J), J = 1-5 and S₂ (J), J = 0-5 of D₂. Binary and ternary absorption coefficients were determined from the integrated absorption coefficients of the band. Profile analyses of the spectra were carried out using the Birnbaum-Cohen (BC) lineshape function and characteristic lineshape parameters were determined from the analyses.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

TiO2/polyaniline nanocomposite films prepared by magnetron sputtering combined with plasma polymerization process

Radiofrequency plasma polymerization in combination with direct current reactive magnetron sputtering is utilized for the synthesis of TiO₂/plasma polymerized aniline nanocomposite thin films. In the composite film, X-ray diffraction measurements reveal formation of nanocrystalline rutile TiO₂ of crystallite size 3.6 nm. Due to continuous bombardment of plasma species during simultaneous magnetron sputtering and plasma polymerization, the precursors of polymerization are broken and few functional groups are retained in the composite film. The plasma polymerized aniline has the direct optical band gap of 3.55 eV and the nanocrystalline rutile TiO₂ is wide gap semiconductor with indirect gap of 3.20 eV which suggests the existence of an energy barrier at the interface in the composite form. The ac conductivity of composite film shows significant improvement as compared to plasma polymerized aniline film and sputtered rutile TiO₂ film. The composite film may find potential application as antistatic coatings.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Influence of annealing temperature on properties of Cu(In,Ga)(Se,S)2 thin films prepared by co-sputtering from quaternary alloy and In2S3 targets

Pentanary Cu(In,Ga)(Se,S)₂ (CIGSS) thin films were deposited on soda-lime glass substrate by co-sputtering quaternary alloy, and In₂S₃ targets. In this study, we investigated the influence of post-annealing temperature on structural, compositional, electrical, and optical properties of CIGSS films. Our experimental results show that the CIGS quaternary target had chalcopyrite characteristics. All CIGSS films annealed above 733 K exhibited a polycrystalline tetragonal chalcopyrite structure, with (1 1 2) preferred orientation. The carrier concentration and resistivity of the resultant CIGSS layer annealed above 763 K was 4.86×10¹⁶ cm⁻³ and 32 Ω cm, respectively, and the optical band-gap of the CIGSS absorber layer was 1.18 eV. Raman spectral analysis demonstrated the existence of many different phases, including CuInSe₂, CuGaSe₂, and CuInS₂. This may be because the vibration frequencies of In-Se, In-S bonds are similar to the Ga-Se and Ga-S bonds, causing their absorption bands overlap.     

Characterization of Y2O3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y₂O₃ films on NH₄OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y₂O₃ on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10¹² eV⁻¹ cm⁻²). The results show that the deposition of Y₂O₃ on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high-k/GaAs interface. The Al/Y₂O₃/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10⁻⁶ A cm⁻² at a V_FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole-Frenkel emission takes over at high electric fields. The energy band alignment of Y₂O₃ films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y₂O₃/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.     

Preparation and luminescent characteristic of Li3NbO4 nanophosphor

Synthesis and luminescence properties of Li₃NbO₄ oxides by the sol-gel process were investigated. The products were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and absorption spectra. The PL spectra excited at 247 nm have a broad and strong blue emission band maximum at 376 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectra of the samples sintered at temperatures of 600 and 700 °C exhibited the band-gap energies of 4.0 and 4.08 eV.     

Full potential calculation of electronic properties of rutile RO2 (R=Si, Ge, Sn and Pb) compounds via modified Becke Johnson potential

The electronic properties of RO₂ (R=Si, Ge, Sn and Pb; a group IVA element) compounds in rutile structure have been calculated using WIEN2k implementation of full potential linearized augmented plane wave (FPLAPW) method. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke Johnson (MBJ) potential as coupled with Local Density Approximation (LDA) for all the compounds except for PbO₂ where only Generalized Gradient Approximation (GGA) is considered for the same. We predict a direct band gap in all these compounds with continuous decrease as the atomic size of IVA element increases such that there is an appearance of semimetallic band structure for the last compound, PbO₂. The largest band gap (7.66 eV) has been found for SiO₂, which governs its insulating nature. We observe that MBJLDA results for band gaps of these compounds are far better than those obtained using GGA and Engel-Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gaps with corresponding experimental values as compared to other calculations. The electronic band structures are also analyzed in terms of contributions from various electrons.     

Influence of [S2O3]/[Sm] on Sm2S3 thin films deposited by liquid phase deposition method on self-assembled monolayers

Sm₂S₃ thin films were prepared on Si (1 0 0) substrates using SmCl₃ and Na₂S₂O₃ as precursors by liquid phase deposition method on self-assembled monolayers. The influence of the molar concentration ratio of [S₂O₃²⁻]/[Sm³⁺] on the phase compositions, surface morphologies and optical properties of the as-deposited films were investigated. The as-deposited Sm₂S₃ thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), ultraviolet-visible (UV-vis) and photoluminescence spectrum (PL). Results show that it is important to control the [S₂O₃²⁻]/[Sm³⁺] during the deposition process and monophase Sm₂S₃ thin films with orientation growth along (0 1 1) direction can be achieved when [S₂O₃²⁻]/[Sm³⁺] = 2.0, pH 3.0, with citric acid as a template agent. The as-deposited thin films exhibit a dense and crystalline surface morphology. Good transmittance in the visible spectrum and excellent absorbency of ultraviolet light of the thin films are observed, and the band gap of the thin films first decrease and then increase with the increase of the [S₂O₃²⁻]/[Sm³⁺]. The as-deposited thin films also exhibit red photoluminescence properties under visible light excitation. With the increase of the [S₂O₃²⁻]/[Sm³⁺] in the deposition solution, the PL properties of Sm₂S₃ thin films are obviously improved.     

Structural and optical properties of an InxGa1−xN/GaN nanostructure

The structural and optical properties of an In_xGa_1−xN/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.     

Aerosol influence on polarization and intensity in near-infrared O2 and CO2 absorption bands observed from space

We study the intensity and degree of linear polarization of reflected solar radiation at the top of the atmosphere within two carbon dioxide bands and one oxygen absorption band in the near-infrared. In particular, we are interested in the sensitivity of the degree of linear polarization and intensity to changes of aerosol and cirrus cloud layer heights, microphysical properties, and surface albedo. For the simulations we use spectral response functions representative of the Orbiting Carbon Observatory (OCO). Inside the O₂A band at 760 nm and strong CO₂ band at 2060 nm we find a strong influence of the aerosol and cirrus cloud layer height on the degree of linear polarization. An increase of the aerosol or cirrus cloud layer height can lead either to a decrease or increase of the polarization within the band, depending on the microphysical and optical properties of the scatterers, surface albedo, and absorption strength in the bands. The results for the O₂A band also indicate that even over land OCO enables an estimation of the height of an aerosol or cirrus cloud layer. Inside the weak CO₂ band at 1610 nm the influence of aerosol or cirrus cloud layer heights is lower as compared to the O₂A band and CO₂ band at 2060 nm, due to the relatively stronger surface influence. Here an increase of aerosol or cirrus cloud layer height leads to an increase of the degree of linear polarization even in case of low surface albedo and for weakly polarizing scatterers. For the weak CO₂ band at 1610 nm we also study the influence of the aerosol or cirrus cloud layer height on the column CO₂ estimate and the errors resulting from ignoring polarization in simulations of backscatter measurements by space-based instruments such as OCO. Depending on the surface albedo, misinterpretations of the height of atmospheric scatterers might strongly affect the column CO₂ estimates.     

Effects of the substrate temperature on the properties of CuIn5S8 thin films

Structural, optical and electrical properties of CuIn₅S₈ thin films grown by thermal evaporation have been studied relating the effects of substrate heating conditions of these properties. The CuIn₅S₈ thin films were carried out at substrate temperatures in the temperature range 100-300 °C. The effects of heated substrate on their physico-chemical properties were investigated using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), optical transmission and hot probe method. X-ray diffraction revealed that the films are strong preferred orientation along the (3 1 1) plane upon substrate temperature 200 °C and amorphous for the substrate temperatures below 200 °C. No secondary phases are observed for all the films. The composition is greatly affected by heated substrate. From the optical transmission and reflection, an important absorption coefficient exceeds 10⁵ cm⁻¹ at 800 nm was found. As increasing the substrate temperature, the optical energy band gap decreases from 1.70 eV for the unheated films to 1.25 eV for the deposited films at 300 °C. It was found that CuIn₅S₈ thin film is an n-type semiconductor at 250° C.