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.     

Micro-Raman spectroscopy of disordered and ordered sulfur phases on a passivated GaAs surface

The depletion layer width and band bending of passivated n-type Sn doped GaAs(1 0 0) between subsequent steps of chemical treatment as well as after a single run treatment were investigated by micro-Raman light scattering by longitudinal optical phonons and coupled phonon-plasmon modes. Experiments were carried out ex situ at room temperature. We conclude that all observed lineshape changes are due to band bending and to an amorphous surface phase represented by a broad spectral component. We applied two passivation methods. One was based on (NH₄)₂S_x solution and lasted 30 min. The second was based on the S₂Cl₂ solution and lasted 10 s. These enabled identification of surface regions of different amorphousness and for faster passivation places of enlarged and completely reduced band bending.     

Theoretical investigation of Cu-containing materials with different valence structure types: BaCu2S2, Li2CuSb,and LiCuS

Optoelectronics research requires cheap materials with a broad spectrum of optical, electronic, and structural properties. The class of Heusler compounds and ternary structures provide many possibilities for finding alternative group IV and III–V semiconductor compounds. This study introduces wider band gap materials for use in solar cells as an alternative to cadmium sulfide buffer layers. The buffer layer is inserted between the absorber layer (p-type) and the transparent window layer (n-type) to enhance the maximum amount of light transmission. Reasonable calculations are reported for the band gaps of copper-containing materials: LiCuS, BaCu₂S₂, and Li₂CuSb. Previous optical analysis measurements of these films determined that the band gaps were 1.8 and 1.9 eV for BaCu₂S₂ and LiCuS, respectively. In general, semiconductor compounds have been studied theoretically, but there are major differences between the experimental and theoretically calculated band gaps. A suitable calculation method for semiconductor compounds is described in this study. For the first time, calculations based on the Engel and Vosko method are introduced for these semiconductor compounds. This method yields band gaps that are comparable to the experimental values, which facilitate the development of microscopic analyses of these compounds. Direct band gaps of 1.15 and 1.7 eV were obtained for BaCu₂S₂ and LiCuS, respectively, whereas the indirect band gap was 0.7 eV for Li₂CuSb.     

Rare-earth gate oxides for GaAs MOSFET application

Rare-earth oxide films for gate dielectric on n-GaAs have been investigated. The oxide films were e-beam evaporated on S-passivated GaAs, considering interfacial chemical bonding state and energy band structure. Rare-earth oxides such as Gd₂O₃, (Gd_xLa_1−x)₂O₃, and Gd-silicate were employed due to high resistivity and no chemical reaction with GaAs. Structural and bonding properties were characterized by X-ray photoemission, absorption, and diffraction. The electrical characteristics of metal-oxide-semiconductor (MOS) diodes were correlated with material properties and energy band structures to guarantee the feasibility for MOS field effect transistor (FET) application.Gd₂O₃ films were grown epitaxially on S-passivated GaAs (0 0 1) at 400 °C. The passivation induced a lowering of crystallization temperature with an epitaxial relationship of Gd₂O₃ (4 4 0) and GaAs (0 0 1). A better lattice matching relation between Gd₂O₃ and GaAs substrate was accomplished by the substitution of Gd with La, which has larger ionic radius. The in-plane relationship of (Gd_xLa_1−x)₂O₃ (4 4 0) with GaAs (0 0 1) was found and the epitaxial films showed an improved crystalline quality. Amorphous Gd-silicate film was synthesized by the incorporation of SiO₂ into Gd₂O₃. These amorphous Gd-silicate films excluded defect traps or current flow path due to grain boundaries and showed a relatively larger energy band gap dependent on the contents of SiO₂. Energy band parameters such as ΔE_C, ΔE_V, and E_g were effectively controlled by the film composition.     

Dry etching of CuCrO2 thin films

Highly conducting films of p-type CuCrO₂ are attractive as hole-injectors in oxide-based light emitters. In this paper, we report on the development of dry etch patterning of CuCrO₂ thin films. The only plasma chemistry that provided some chemical enhancement was Cl₂/Ar under inductively coupled plasma conditions. Etch rates of ∼500 Å min⁻¹ were obtained at chuck voltages around −300 V and moderate source powers. In all cases, the etched surface morphologies were improved relative to un-etched control samples due to the smoothing effect of the physical component of the etching. The threshold ion energy for the onset of etching was determined to be 34 eV. Very low concentrations (≤1 at.%) of residual chlorine were detected on the etched surfaces but could be removed by simple water rinsing.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

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.     

The surface state of URu2Si2

The ‘hidden-order’ (HO) transition of URu₂Si₂ remains a puzzle after 25 years of research. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we found that a hole-like band around Γ having its band maximum at E = −35 meV, and previously thought to be a bulk band of the system, is indeed a surface state not related to the HO phase transition. Here we present our detailed investigations to assign that state to a surface feature, and discuss on the possible origins of this surface band.     

Characterization of nanocrystalline SnO2 thin film fabricated by electrodeposition method for dye-sensitized solar cell application

Nanocrystalline SnO₂ thin film was prepared by cathodic electrodeposition-anodic oxidation and its structure was characterized by X-ray diffraction, SEM, UV-visible absorption and nitrogen adsorption-desorption by BET method. The obtained film has a surface area of 137.9 m²/g with grain sized of 24 nm. Thus the prepared SnO₂ thin film can be applied as an electrode in dye-sensitized solar cell. The SnO₂ electrode was successfully sensitized by Erythrosin dye and photoelectrochemical measurements indicate that the cell present short-circuit photocurrent (J_sc) of 760 μA/cm², fill factor (FF = 0.4), photovoltage (V_oc = 0.21 V) and overall conversion efficiency (η) of 0.06% under direct sun light illumination. The relatively low fill factor and photovoltage are attributed to the reduction of triodiode by conduction band electrons and intrinsic properties of SnO₂.     

Optical properties of dangling-bond states at cleaved silicon surfaces

The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Electronic Emission from Hg-Atoms and HgCl-Radicals Due to Collisions of Nitrogen Ions and HgCl2/Hg2Cl2 Molecules

Emission spectra of the (B-X), (C-X) and (D-X) band systems of HgCl-radical and mercury atomic lines from highly excited levels to various lower levels have been observed during collisions of N⁺ and N⁺ ₂ ions and HgCl₂/Hg₂Cl₂ molecules at different laboratory kinetic energies of the projectile ions. Emission cross-sections of the most intense mercury atomic lines and the (C-X) band system of the HgCl-radicals, have been measured in the laboratory kinetic energy range of 100–900 eV.     

Structural-optical study of high-dielectric-constant oxide films

High-k polycrystalline Pr₂O₃ and amorphous LaAlO₃ oxide thin films deposited on Si(0 0 1) are studied. The microstructure is investigated using X-ray diffraction and scanning electron microscopy. Optical properties are determined in the 0.75-6.5 eV photon energy range using spectroscopic ellipsometry. The polycrystalline Pr₂O₃ films have an optical gap of 3.86 eV and a dielectric constant of 16-26, which increases with film thickness. Similarly, very thin amorphous LaAlO₃ films have the optical gap of 5.8 eV, and a dielectric constant below 14 which also increases with film thickness. The lower dielectric constant compared to crystalline material is an intrinsic characteristic of amorphous films.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Characterization of electrodeposited Bi2S3 thin films by holographic interferometry

Optical non destructive evaluation methods, using lasers as the object illumination source, include holographic interferometry. It is widely used to measure stress, strain, and vibration in engineering structures. Double exposure holographic interferometry (DEHI) technique is used to determine thickness and stress of electrodeposited bismuth trisulphide (Bi₂S₃) thin films for various deposition times. The same is tested for other concentration of the precursors. It is observed that, increase in deposition time, increases thickness of thin film but decreases stress to the substrate. The structural, optical and surface wettability properties of the as deposited films have been studied using X-ray diffraction (XRD), optical absorption and contact angle measurement, respectively. The X-ray diffraction study reveals that the films are polycrystalline with orthorhombic crystal structure. Optical absorption study shows the presence of direct transition with band bap 1.78 eV. The water contact angle measurement shows hydrophobic nature of Bi₂S₃ thin film surface.     

Characterization and properties of ZnO1−xSx alloy films fabricated by radio-frequency magnetron sputtering

A series of ZnO_1−xS_x alloy films (0 ≤ x ≤ 1) were grown on quartz substrates by radio-frequency (rf) magnetron sputtering of ZnS ceramic target, using oxygen and argon as working gas. X-ray diffraction measurement shows that the ZnO_1−xS_x films have wurtzite structure with (0 0 2) preferential orientation in O-rich side (0 ≤ x ≤ 0.23) and zinc blende structure with (1 1 1) preferential orientation in S-rich side (0.77 ≤ x ≤ 1). However, when the S content is in the range of 0.23 < x < 0.77, the ZnO_1−xS_x film consists of two phases of wurtzite and zinc blende or amorphous ZnO_1−xS_x phase. The band gap energy of the films shows non-linear dependence on the S content, with an optical bowing parameter of about 2.9 eV. The photoluminescence (PL) measurement reveals that the PL spectrum of the wurtzite ZnO_1−xS_x is dominated by visible band and its PL intensity and intensity ratio of UV to visible band decrease greatly compared with undoped ZnO. All as-grown ZnO_1−xS_x films behave insulating, but show n-type conductivity for w-ZnO_1−xS_x and maintain insulating properties for β-ZnO_1−xS_x after annealed. Mechanisms of effects of S on optical and electrical properties of the ZnO_1−xS_x alloy are discussed in the present work.     

FT-IR study of the photocatalytic degradation of gaseous toluene over UV-irradiated TiO2 microballs: enhanced performance by hydrothermal treatment in alkaline solution

In this study, photocatalysts of TiO₂ microballls were obtained via a hydrothermal treating of commercial P25 in alkaline solution, and then characterized with SEM, XRD, BET, DRS and surface photovoltage spectroscopy (SPS) techniques. The photovoltage response of the prepared TiO₂ microballs on spectrum features a quantum size effect brought about by the reduced grain size with respect to the precursor. The UV-assisted photodegradation of gaseous toluene over P25 and the prepared TiO₂ microballs was monitored by an in situ infrared technique. The results demonstrated that the prepared TiO₂ microballs in anatase form were more active than commercial P25 in photocatalytic oxidation of gaseous toluene. The promoted activity of the hydrothermal-treated TiO₂ is attributed to the increasing specific surface area and larger band gap induced by the reduced crystallite size.     

Spontaneous emission of photons and electrons during chemisorption of chlorine on sodium

Photons and electrons are emitted when Cl₂ molecules react on a Na surface prepared by UHV evaporation. The emission yield per reacting molecule is 10^?7-10^?6 for photons and approximately 10^?5 for electrons. The dominating light emission band has a maximum at hv = 2.15 eV (width 0.6 eV). A less intense u.v. band has a maximum at about 4.7 eV. A drastic decrease in the photon and electron emission at a Cl₂ exposure of about 5.10^?3 torr·sec, is attributed to the formation of a continuous NaCl film on the Na surface.     

“In situ” XPS study of band structures at Cu2O/TiO2 heterojunctions interface

In an attempt to investigate influence of the defects on electronic structure of Cu₂O/TiO₂ heterojunctions, thin Cu₂O layers were successively deposited on TiO₂ that has different levels of defect concentrations, and the resultant band bending and offset characteristics were studied by in situ X-ray photoelectron spectroscopy (XPS). The TiO₂ substrates with defects were prepared by Ar⁺ sputtering, followed by annealing at different temperatures in oxygen atmosphere. Presence of the defects in TiO₂ surface dramatically influences on the band bending and band offset at the interface: more defects are on TiO₂ surface, less band bending are at the interface, inducing smaller conduction band offsets. On the reduced TiO₂ surface, Cu₂O was disproportionately decomposed to form CuO and Cu.