Photoluminescence spectra of MnIn2S4

MnIn₂S₄ single crystals grown by the directional crystallization method were investigated by using the temperature and excitation power dependencies of photoluminescence (PL) spectra. PL spectra consist of one broad band resulting from donor-acceptor pair recombination. The analysis of the temperature quenching of the PL intensity yields one defect donor level with a thermal ionization energy of about 0.17 eV. The broad band of PL spectra indicates that radiative recombination is related to multiphonon optical processes. The energy of the involved phonon was found to be around 0.025 eV and the energy of the acceptor level is about 0.86 eV.     

The copper influence on the PL spectra of CdTe thin film as a component of the CdS/CdTe heterojunction

The influence of annealing in the presence of CdCl₂ and a thin copper layer deposited onto CdTe on the photoluminescence spectra of CdTe, as a component of CdS/CdTe heterojunction, has been studied for two excitation wavelengths: 0.337 μm and 0.6328 μm. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. At 0.6328 μm excitation, the interface PL consists of a known 1.43X band, and the chloride annealing enhances radiative transitions at 1.536 eV. The intensity of the 1.536 eV transitions increases when Cu is present. The PL of as-deposited CdTe films prepared in the presence of oxygen has the 1.45X band attenuated when excited with 0.337 μm excitation wavelength.     

Oxygen vacancy in Al2O3: Photoluminescence study and first-principle simulation

Broad photoluminescence (PL) band at 2.97 eV excited in the band near 6.0 eV in amorphous chemical vapor deposition films is related to the neutral oxygen vacancy by analogy with crystalline Al₂O₃. The identification of this PL band was supported by the results of first-principle quantum chemical simulation, which showed 6.3 and 6.4 eV bands in the extinction spectra for α- and γ-Al₂O₃, respectively. Other PL bands are attributed to ionized single vacancies (F⁺-centers), divacancies (F₂) and, probably, interstitial Al.     

Photoluminescence studies of CdTe films and junctions

Device quality CdTe films and junctions have been studied using low-temperature photoluminescence (PL) measurements. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. The CdTe films and junctions were prepared under various deposition conditions to determine the effect of film deposition and solar cell fabrication parameters, such as the effect of oxygen, and chloride treatment. A PL band located at 1.232 eV has been attributed to the presence of oxygen. This band is present only in as-deposited samples excited at the CdTe surface. Samples annealed in the presence of CdCl₂ exhibit a single PL band located at 1.42 eV. A model explaining the behavior of these bands is presented.     

Investigation of potential and compositional fluctuations in CuGa3Se5 crystals using photoluminescence spectroscopy

We studied the photoluminescence (PL) properties of the ordered defect compound CuGa₃Se₅. Different single crystals were grown by the vertical Bridgman method and by the solid phase crystallization method. Their crystal structure and cell parameters were determined by X-ray diffraction. The PL spectra were recorded at T = 10-300 K. Also, laser power dependences were studied. We found an asymmetric PL band at 1.76 eV. PL band shifts towards higher energies with increasing laser power. The shape and properties of this band assure the presence of potential and compositional fluctuations. The influence of both fluctuations on the PL properties of CuGa₃Se₅ is studied and the radiative recombination processes are explained.     

Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell

The phase formation, morphology and luminescent properties of ZnNb₂O₆ nanocrystals by the sol-gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb₂O₆ grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20-40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV.     

Effect of interdiffusion and impurities on thin film CdTe/CdS photovoltaic junctions

We have used low temperature photoluminescence (PL) to study thin film CdTe/CdS solar cell structures. The devices were produced by close space sublimation (CSS) and have undergone a post-growth treatment, a vital step in increasing device efficiency. The treatment consisted of evaporating a thin layer of CdCl₂ onto the back CdTe surface and heat treating in air at 400 °C for between 10 and 120 min. This produced a range of device efficiencies from 2% to 9%. The efficiency improvements are the result of a complex interaction between the CdCl₂, impurities and sulfur interdiffusion. The structures were prepared for PL by a chemical bevel etching technique which allows the luminescence emission to be studied as a function of depth throughout the sample. The main features in the PL spectra have been identified as being due to the Cl-A center and the Te-dislocation-related Y luminescence band. Using PL we have quantified the S diffusion into the CdTe which has a maximum of 20% at the interface in the most efficient samples. We have also obtained the profiles of recombination and non-radiative recombination centers in the device. We observe correlations between impurity centers and device efficiency which can help explain the effects of the CdCl₂ treatment on the optoelectronic properties of the CdTe/CdS junction.     

Radiative recombination in Cu2ZnSnSe4 monograins studied by photoluminescence spectroscopy

In this study we investigated the optical properties of Cu₂ZnSnSe₄ monograin powders that were synthesized from binary compounds in the liquid phase of flux material (KI) in evacuated quartz ampoules. The monograin powder had p-type conductivity. Radiative recombination processes in Cu₂ZnSnSe₄ monograins were studied using photoluminescence spectroscopy. The detected low-temperature (T = 10 K) photoluminescence band at 0.946 eV results from band-to-impurity recombination in Cu₂ZnSnSe₄. The ionization energy of the corresponding acceptor defect was found to be 69 ± 4 meV. Additional photoluminescence bands detected at 0.765 eV, 0.810 eV and 0.860 eV are proposed to result from Cu₂SnSe₃ phase whose presence in the as-grown monograins was detected by Raman spectroscopy and SEM analysis. Considering photoluminescence results, it is proposed that the optical bandgap energy of Cu₂ZnSnSe₄ is around 1.02 eV at 10 K.     

Optical characterization of Cu2ZnSnSe4 grown by thermal co-evaporation

Cu₂ZnSnSe₄ thin films with different substrate temperature and Cu flux were grown by thermal co-evaporation. Raman scattering, photoluminescence, and contactless electroreflectance (ER) measurements were performed. The Raman spectra of Cu₂ZnSnSe₄ show two main peaks at 170 and 192 cm^− 1. The photoluminescence spectrum shows a peak below 1.0 eV. Franz-Keldysh oscillations (FKOs) were observed in the ER spectra. From the analysis of the FKOs, the bandgap energy of Cu₂ZnSnSe₄ thin films is estimated to be 1.07 eV at 90 K and 0.99 eV at room temperature. We conclude that the bandgap energy of Cu₂ZnSnSe₄ thin films is around 1.0 eV.     

Effect of Sb doping on the opto-electronic properties of SnO2 nanowires

Antimony (Sb) doping of SnO₂ nanowires (NWs) was investigated for its optical and electrical effects. The low-temperature photoluminescence spectra of SnO₂ NWs varied significantly with increasing Sb content, where the temperature-dependence of the visible emission at ca. 400 nm was distinctive with Sb-doping, indicating different defect states, such as neutral and positively charged oxygen vacancies. Field effect transistors (FETs) with low-level Sb-doped SnO₂ NW channels exhibited higher mobility, charge concentration, and faster response and recovery to UV light than intrinsic SnO₂ NW FETs.     

Synthesis and characterization of Mg2B2O5

Magnesium borate of the form Mg₂B₂O₅ has been prepared and its structural and thermal properties were studied using X-ray diffraction and differential thermal analysis. An investigation of the electrical and optical properties of Mg₂B₂O₅ system has been carried out. The electrical resistivity of the sample was measured in the temperature range of 170-400 K. The data analysis revealed an extrinsic nature of the conductivity with two impurity levels located at 0.13 and 0.71 eV in the temperature ranges of 170-230 K and 240-400 K, respectively. The optical transmission and reflection was recorded at 300 K in the incident photon energy range of 3.0-6.0 eV. The absorption coefficient data analysis revealed an indirect optical energy band gap of 4.73 eV. In addition, two impurity levels located at 3.43, and 4.49 eV were observed in the absorption spectra.     

Structural and optical properties of Al-doped SnO2 nanowires

We report a facile thermal evaporation method for the syntheses of Al-doped SnO₂ nanowires using Al-doped SnO₂ nanoparticles as precursors. High-density, single-crystalline Al-doped SnO₂ nanowires were directly grown on the 6H-SiC substrates without any catalyst. X-ray diffraction patterns show that the Al dopants are incorporated into the rutile SnO₂ nanowires. The X-ray photoelectron spectra confirm the SnO₂ nanowires doped with 5 at.% Al. The photoluminescence spectra of the Al-doped SnO₂ nanowires exhibit that the large blue shift of the emission band can be observed in the Al-doped SnO₂ nanowires compared with undoped nanowires. The distortion of the crystal lattices caused by incorporation of Al atoms at the interstitials should be responsible for the large blue shift of the emission band.     

Vapor phase growth and optical properties of single-crystalline SnO2 nanobelts

Mass production of single-crystalline SnO₂ nanobelts was successfully achieved through a thermal evaporation of metallic Sn powders at 900 °C. The as-prepared SnO₂ nanobelts were typically 30-200 nm in width, 10-50 nm in thickness, and about tens of micrometers in length. In addition to the classical Raman models, two new Raman bands at 498 and 698 cm⁻¹ are observed for rutile-phased SnO₂ nanobelts, which can be attributed to the IR-active A_2u TO and A_2u LO modes, respectively. Photoluminescence (PL) spectrum of SnO₂ nanobelts featured an emission band at 615 nm (with a small shoulder at 585 nm), which might correspond to the existence of oxygen deficiencies in the produced belts. The formation of SnO₂ nanobelts followed a vapor-solid (VS) growth mechanism.     

Characteristics of n-Cd0.9 Zn0.1S/p-CdTe heterojunctions

CdTe thin films were prepared by thermal evaporation under a vacuum of 10⁻⁶ Torr and with a deposition rate of about 60 nm/min. X-ray diffraction studies of the as-deposited films revealed polycrystalline films with cubic structure. The effect of heat treatment with or without CdCl₂ enhances the grain size and improves the crystallinity of the films. Moreover, the activation energy decreases upon heat treatment with or without CdCl₂ for CdTe thin films. The optical spectra of CdTe films show interference oscillations indicating the good optical quality of these films. The calculated energy gap decreases with or without CdCl₂ treatments. The current-voltage and capacitance-voltage characteristics for dark and illuminated three junction cells are measured. By analysing these measurements the different junction parameters are obtained and the effect of CdCl₂ treatment on the performance of the heterojunctions is investigated.     

Luminescent and photocatalytic properties of hollow SnO2 nanospheres

Size tunable solid SnO₂ (STO) and hollow SnO₂ (HTO) nanospheres were prepared by a sacrificing template method. The peaks around 390 nm were observed in photoluminescence (PL) spectra. Based on the results, the PL intensity exhibits morphology-dependence and size-dependence, and HTO structure displays better optical properties than STO structure. The degradation of Methyl Orange (MO) in aqueous solution is selected as a probe reaction to evaluate the catalytic activity of nano-SnO₂. The result shows that HTO structure presents stronger photocatalytic (PC) activity. According to the result of specific surface area testing, the improved PL and PC properties of HTO structure can be mainly explained by the surface effect induced by large specific surface area. This work is meaningful for developing nanomaterials with enhanced optical and photochemical properties.     

Non-equilibrium charge carriers generation — recombination mechanisms at the interface of the SnO2/GaSe heterojunction

The interface of the SnO₂/GaSe heterojunctions, having SnO₂ layer obtained by different deposition methods have been studied using the photoluminescence and photocurrent spectral distribution. The SnO₂/GaSe structures are photosensitive in the 1.7 ÷ 3.6 eV spectral range and reveal its maximum value for the heterojunctions with magnetron sputtered SnO₂ layer.     

Structural, optical and electrical properties of reactively sputtered Ag2Cu2O3 films

Ag₂Cu₂O₃ thin films were deposited on glass substrates by RF magnetron sputtering of an equiatomic silver-copper target (Ag_0.5Cu_0.5) in reactive Ar-O₂ mixtures. The reactive sputtering was done at varying power, oxygen flow rate and deposition temperature to study the influence of these parameters on the deposition of Ag₂Cu₂O₃ films. The film structure was determined by X-ray diffraction, while the optical properties were examined by spectrophotometry (UV-vis-NIR) and photoluminescence. Furthermore, the film thickness and resistivity were measured by tactile profilometry and 4-point probe, respectively. Additional mobility, resistivity and charge carrier density Hall effect measurements were done on a few selected samples. The best films in terms of stoichiometry and crystallography were achieved with a sputtering power of 100 W, oxygen and argon flow rates of 20 sccm (giving a deposition pressure of 1.21 Pa) and a deposition temperature of 250 °C. The optical transmittance and photoluminescence spectra of films deposited with these parameters indicate several band gaps, most prominently, a direct one of around 2.2 eV. Electrical characterization reveals charge carrier concentrations and mobilities in the range of 10²¹-10²² cm^− 3 and 0.01-0.1 cm²/Vs, respectively.     

Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni

SnO₂ hollow spheres with interstitial Sn²⁺ defect were fabricated by the hydrothermal method without any surfactant or polymer, whose shell is constructed by two layers of tetragonal prism nanorod arrays. The growth mechanism of the hollow spheres was investigated and attributed to the nucleation and arrangement of SnO₂ tetragonal prism nanorods on the surface of the hydrothermal reaction formed NO bubbles in the aqueous solution. After illumination by 275 nm wavelength light, narrow peak emissions centered at about 587-626 nm have been found in the photoluminescence spectrum, which have been ascribed to the interstitial Sn²⁺ defect in the SnO₂ hollow spheres.     

Solvo-solid preparation of Zn3N2 hollow structures; their PL yellow emission and hydrogen absorption characteristics

Zinc nitride (Zn₃N₂) hollow structures with 10-25 μm size have been prepared by solvo-solid approach using aqueous ammonia treated Zn precursor at reaction temperature of 600 °C for reaction time of 240 min under ammonia gas flow. The structural, compositional and morphological characterizations of the as-obtained product were performed by XRD, EDS and SEM. Room temperature photoluminescence (PL) spectrum of zinc nitride hollow structures (ZNHSs) exhibited UV emission band at 384 nm and a defect related yellow emission band at 605 nm. The first ever studies on hydrogen absorption characteristics of ZNHSs performed at 373 K showed an absorption capacity of 1.29 wt.%. Growth mechanism proposed for the formation of ZNHSs is also discussed briefly.     

Ag2СdSnS4 single crystals as promising materials for optoelectronic

Single crystals of the quaternary single crystals Ag₂CdSnS₄ were grown for the first time using the horizontal gradient freeze technique. Optical spectral and photoelectric properties of obtained crystals were investigated. The band gap energy at 77 K according to the photoconductivity spectra is 1.94 eV. The energy levels of the major donor centers in the band gap were determined. The role of intrinsic defects in the observed dependences is analyzed. The energy levels of the major donor centers in the band gap were determined. A small photoconductivity maximum at low temperature is observed at wavelength λ_m = 640 nm (hν ∼ 1.94 eV); situated in the fundamental absorption band, which unambiguously corresponds to the intrinsic origin of photoconductivity. The increase of the extrinsic photoconductivity with the maximum at λ_m ∼ 800 nm with temperature leads to its domination above 240 K. The observed peculiarity can be explained by the photoexcitation of electrons from the valence band to the donor centers which are empty at high temperatures and with further thermal excitation to the conduction band.