Indium phosphide films prepared by flash evaporation technique: Synthesis and characterization

Indium phosphide films were deposited by flash evaporating InP powder (99.995%) on glass substrates. Microstructural information was obtained from transmission electron microscope and atomic force microscope (AFM) studies. The average value (~ 0.33 nm) of surface roughness of the films was determined by AFM. X-ray diffraction traces indicated reflections from (111), (220) and (311) planes only. The band gap was found to vary between ~ 1.94 eV and 1.96 eV. ε_∞ varied between 11.58 and 11.89 while the plasma frequency (ω_p,) were seen to vary between 8.52 and 8.59 × 10¹⁴ s^− 1. The bonding environment in the films was determined from Raman and Fourier transformed infrared measurements. The experimental absorption spectra could be faithfully described by considering the effect of scattering by the ultra small crystallites in the film alone. Photoluminescence peak located at ~ 1.5 eV may be ascribed due to transitions from states arising out of phosphorous vacancy to the valance band. The shoulders of the peak ~ 1.5 eV may originate from the DA transitions between V_P and In_P.     

Temperature-dependent high-resolution X-ray photoelectron spectroscopic study on Ge1Sb2Te4

Oxygen-free and amorphous Ge₁Sb₂Te₄ thin film was obtained in an ultra-high vacuum and then annealed in situ to the stable-phase temperature. High-resolution X-ray photoelectron spectroscopy using synchrotron radiation was performed on the film at the different annealing temperatures of 100, 130, 150, 180, and 250 °C. The Te 4d, Sb 4d, and Ge 3d shallow core levels as well as the valence-band spectra were acquired. In the shallow core-level spectra, we observed distinguishable changes in the Sb 4d and Ge 3d levels as the film phase changed. As the temperature increased, a higher binding-energy (BE) component appeared at the Sb 4d level, the intensity of the component increased, and the spin-orbit split feature was enhanced at the Ge 3d level. In the valence-band spectra, a slight increase was observed at 0-1, ~ 3, ~ 9, and ~ 12 eV BE, and a decrease, at ~ 1.5 and ~ 4.5 eV BE. The energy resolution employed in this study was about 150 meV.     

Effects of oxygen flow ratios and annealing temperatures on Raman and photoluminescence of titanium oxide thin films deposited by reactive magnetron sputtering

Titanium oxide (TiO_x) thin films were deposited on the Si(100) substrates by direct-current reactive magnetron sputtering at 3-15 % oxygen flow ratios (FO₂% = FO₂/(FO₂ + FAr) × 100%), and then annealed by rapid thermal annealing (RTA) at 350-750 °C for 2 min in air. The phase, bonding and luminescence behaviors of the as-deposited and annealed TiO_x thin films were analyzed by X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy, respectively. The as-deposited TiO_x films were amorphous from XRD and showed weak Raman intensity. In contrast, the distinct crystalline peaks of anatase and rutile phases were detected after RTA at 550-750 °C from both XRD and Raman spectra. A mixture of anatase and rutile phases was obtained by RTA at 3 FO₂% and its amount increased with annealing temperature. Only the anatase phase was detected in the 6-15 FO₂% specimens after RTA. The PL spectra of all post-annealed TiO_x films showed a broad peak in visible light region. The PL peak of TiO_x film at 3 FO₂% at 750 °C annealing can be fitted into two Gaussian peaks at ~ 486 nm (2.55 eV) and ~ 588 nm (2.11 eV) which were attributed to deep-level emissions of oxygen vacancies in the rutile and anatase phases, respectively. The peak around 550 nm was observed at 6-15 FO₂% which is attributed to electron-hole pair recombination from oxygen vacancy state in anatase phase to valence band. The variation of intensity of PL peaks is concerned with the formation of the rutile and anatase phases at different FO₂% and annealing temperatures.     

Electronic properties of n-ZnO(Al)/p-Si heterojunction prepared by dc magnetron sputtering

The electronic properties of the interface between p-type Si and Al-doped ZnO have been investigated. Films of ZnO(Al) with a thickness of 300 nm were deposited at room temperature by dc magnetron sputtering and subsequently subjected to heat treatment in air in the temperature range 100-400 °C. Current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements were used to characterize the electrical properties of the heterostructure. The I-V measurements show a diode-like behavior with a rectification of ~ 3-4 orders of magnitude. However, annealing above 200 °C gives rise to a pronounced recombination/generation current in the depletion region, which correlates with an increase of the carrier concentration close to the interface and indicates defect formation. Indeed, DLTS reveals the presence of two prominent defect states, one at 0.38 eV above the valence band edge (E_v), and the other, formed during the heat treatment above 250 °C, around E_v + 0.43 eV, which is consistent with the I-V and C-V data.     

Influence of Cu off-stoichiometry on wide band gap CIGSe solar cells

The electric properties of solar cells based on co-evaporated Cu(In,Ga)Se₂ (CIGSe) thin film show a good tolerance regarding the absorber Cu content (y = [Cu]/([In] + [Ga])) for standard Ga concentration, i.e. x = [Ga] / ([In] + [Ga]) ~ 0.3. In the present contribution, we show that this tolerance is lost when the gallium content is increased. Wide bandgap CIGSe samples (x ~ 0.55) with a variation in y from 0.97 to 0.84 have been grown. The efficiency of the cells decreases from 12.6% to 6.5% for y = 0.97 and 0.84 respectively. For the lowest y, the efficiency is harmed because of a low short-circuit current density (J_sc), an increased voltage dependency in the current collection, which affects the fill factor (FF), and a decrease of the open-circuit voltage (V_oc). For y = 0.97 and 0.84 respectively, the decrease of the activation energy (E_a) from 1.36 to 1.24 eV indicates a shift of the area of the dominant recombination from the space charge region towards the interface. There seems to be evidence that reducing the Cu-content in the CIGSe thin film will cause a decrease in the width of the space charge region. Solar cells based on Cu-rich CIGSe (1.03 < y < 1.09) have also been fabricated and characterized. A strong deterioration of their electrical properties is observed despite the KCN etch of the segregated Cu_2 − xSe binary phases at the surface, suggesting the presence of residual Cu_2 − xSe precipitates within the layer.     

Electrical and optical properties of nanocrystalline yttrium-doped hafnium oxide thin films

Yttrium-doped hafnium oxide (YDH) films have been produced by sputter-deposition by varying the growth temperature (T_s) from room-temperature (RT) to 400 °C. The electrical and optical properties of YDH films have been investigated. Structural studies indicate that YDH films grown at T_s = RT − 200 °C were amorphous and those grown at 300-400 °C are nanocrystalline. The crystalline YDH films exhibit the high temperature cubic phase of HfO₂. Spectrophotometry analysis indicates that all the YDH films are transparent. The band gap of YDH films was found to be in the range of 6.20-6.28 eV. Frequency variation of frequency dependent resistivity indicates the hopping conduction mechanism operative in YDH films. While the electrical resistivity (ρ_ac) is ~ 1 Ω-m at low frequencies (100 Hz), ρ_ac decreases to ~ 10^− 4 Ω-cm at higher frequencies (1 MHz).     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Textured surface boron-doped ZnO transparent conductive oxides on polyethylene terephthalate substrates for Si-based thin film solar cells

Textured surface boron-doped zinc oxide (ZnO:B) thin films were directly grown via low pressure metal organic chemical vapor deposition (LP-MOCVD) on polyethylene terephthalate (PET) flexible substrates at low temperatures and high-efficiency flexible polymer silicon (Si) based thin film solar cells were obtained. High purity diethylzinc and water vapors were used as source materials, and diborane was used as an n-type dopant gas. P-i-n silicon layers were fabricated at ~ 398 K by plasma enhanced chemical vapor deposition. These textured surface ZnO:B thin films on PET substrates (PET/ZnO:B) exhibit rough pyramid-like morphology with high transparencies (T ~ 80%) and excellent electrical properties (Rs ~ 10 Ω at d ~ 1500 nm). Finally, the PET/ZnO:B thin films were applied in flexible p-i-n type silicon thin film solar cells (device structure: PET/ZnO:B/p-i-n a-Si:H/Al) with a high conversion efficiency of 6.32% (short-circuit current density J_SC = 10.62 mA/cm², open-circuit voltage V_OC = 0.93 V and fill factor = 64%).     

Synthesis of AgInSnS4 thin films by adding tin (Sn) into the chalcopyrite structure of AgInS2 using spray pyrolysis

AgInSnS₄ thin films were prepared by adding a tin salt to the starting solution used for preparing chalcopyrite AgInS₂ thin films by spray pyrolysis The AgInSnS₄ films were grown at substrate temperatures in the 300-400 °C range, using an alcoholic solution comprised of silver acetate, indium chloride, tin chloride and thiourea. The tin chloride content in the starting solution was gradually varied in terms of the molar ratio x = [Sn]/([S] + [Ag]) from 0 to 0.5 to obtain Sn-doped chalcopyrite AgInS₂ (x < 0.2) and spinel-like AgInSnS₄ (x = 0.2-0.4). X-ray diffraction studies indicated that AgInSnS₄ has a cubic spinel-like structure with lattice parameter of 10.77 A. All AgInSnS₄ thin films exhibited p-type conduction, and their room temperature conductivity ranged from 10^− 1 to 10^− 2 S/cm. The conductivity versus 1/T plots for this material showed an Arrhenius-like behavior, from which two activation energies of E_a1 = 0.23-0.40 eV and E_a2 = 0.07-0.20 eV were determined. These results suggest that the grain boundary scattering and the ionization of shallow acceptors dominate the charge carrier transport in the sprayed AgInSnS₄ thin films. The AgInSnS₄ absorption spectrum revealed an energy gap around E_g = 1.89 eV, which was associated to direct-allowed transitions. To our knowledge, the quaternary compound has been prepared for the first time using spray pyrolysis.     

An analysis of temperature dependent current-voltage characteristics of Cu2O-ZnO heterojunction solar cells

Carrier transport and recombination mechanisms in Cu₂O-ZnO heterojunction thin film solar cells were investigated through an analysis of their current-voltage characteristics in the dark and under various illumination intensities, as a function of temperature between 100 K and 295 K. The Cu₂O-ZnO heterojunction solar cells were prepared by metal organic chemical vapor deposition of Cu₂O on ZnO films sputtered on transparent conducting oxide coated glass substrates. Activation energies extracted from the temperature dependence of the J-V characteristics reveals that interface recombination is the dominant carrier transport mechanism. Tunneling across an interfacial barrier also plays an important role in current flow and a thin TiO₂ buffer layer reduces tunneling. A high open circuit voltage at low temperature (~ 0.9 V at around 100 K) indicates that Cu₂O-ZnO heterojunction solar cells have high potential as solar cells if the recombination and tunneling at the interface can be suppressed at room temperature.     

Metalorganic chemical vapor deposition of β-FeSi2 on β-FeSi2 seed crystals formed on Si substrates

We have fabricated a β-FeSi₂ film by metalorganic chemical vapor deposition on a Si(001) substrate with β-FeSi₂ seed crystals grown by molecular beam epitaxy, and investigated the crystallinity, surface morphology and temperature dependence of photoresponse properties of the β-FeSi₂ film. The surface of the grown β-FeSi₂ film was atomically flat, and step-and-terrace structure was clearly observed. Multi-domain structure of β-FeSi₂ whose average size was approximately 200 nm however was revealed. The photoresponse was obtained in an infrared light region (~ 0.95 eV) at temperatures below 200 K. The external quantum efficiency reached a maximum, being as large as 25% at 100 K when a bias voltage was 2.0 V.     

Role of substrate temperature on the structural, morphological and optical properties of CuGaSe2 thin films grown by Pulsed Electron Deposition technique

CuGaSe₂ (CGS) thin films were grown on uncoated and Mo-coated soda lime glass by Pulsed Electron Deposition (PED) technique at substrate temperatures comprised between 25 °C and 475 °C. X-ray diffraction analysis reveals that CGS samples exhibit a noteworthy crystal quality even at low growth temperature, T_g = 100 °C, whereas the out-of-plane preferential orientation of CGS chalcopyrite phase switches from < 220 > to < 112 > by increasing the substrate temperature. Annealing treatments seem to enhance the crystallinity of the film and to release the residual strain energy. Visible/near-infrared absorbance spectra show a monotonic decrease of CGS optical bandgap (from 1.75 to 1.65 eV) by enhancing the substrate temperature. Yet the morphology of CGS films strongly depends on T_g, which promotes the formation of larger columnar grains perpendicular to the growth plane. Grain dimensions of ~ 2 μm are achieved when CGS films are grown at high temperature (> 400 °C) on Mo-coated glass. The results indicate that PED is a promising growth technique for achieving good-quality CGS that can be useful as absorber layers in thin film solar cells.     

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Large tensile stresses (up to 3 GPa) were previously observed in low-mobility metallic Mo_1 − xSi_x films grown on amorphous Si and they were ascribed to the densification strain at the amorphous-crystalline transition occurring at a critical film thickness. Here, we focus on the influence of the nucleation conditions on the subsequent stress build-up in sputter-deposited Mo_0.84Si_0.16 alloy films. For this purpose, growth was initiated on various underlayers, including amorphous layers and crystalline templates with different lattice mismatch, and the stress evolution was measured in situ during growth using the wafer curvature technique. Tensile stress evolutions were observed on amorphous SiO₂ and (111) Ni underlayers, similarly to the stress behaviour found on amorphous Si. For these series, the films were characterized by large in-plane grain size (~ 500 nm). However, on a (110) Mo buffer layer, a different stress behaviour occurred: after an initial tensile rise ascribed to coherence stress, a reversal towards a compressive steady state stress was observed. A change in film microstructure was also noticed, the typical grain size being ~ 30 nm. The origin of the compressive stress source in the metastable Mo_0.84Si_0.16 alloy grown on (110) Mo is discussed based on the stress evolutions measured at varying deposition rates and Ar working pressures, as well as in comparison with stress evolutions in pure Mo films.     

Fabrication of light absorbing TiO2 μ-donuts

TiO₂ µ-donuts were fabricated on glass and silicon by sol-gel technique using a mask of PMMA nanopillars created by removing PS from a spin-coated composite polymer of PS and PMMA. X-ray diffraction confirmed the anatase TiO₂ phase. X-ray photoelectron spectra showed signals from Ti 2p_3/2 at 458.8 eV and O 1s at 530.4 eV confirming presence of TiO₂. The heights and diameters of the TiO₂ µ-donuts are ~ 353 nm and ~ 2.8 µm, respectively as revealed by atomic force microscopy. UV-vis absorption spectra of TiO₂ µ-donuts showed an unusual light absorption at ~ 524 nm making its potential use in solar-cell applications.     

Na2Cd2I6L2(H2O)6 [L = Urotropine]: An interesting precursor for synthesizing CdO particles

A coordination polymer, Na₂Cd₂I₆L₂(H₂O)₆ [L = Urotropine] has been employed as sole precursor to synthesize CdO particles. Two different preparation methods viz (i) pyrolysis of the title compound at 700 °C for 2 h and (ii) forming cadmium hydroxide from the title compound followed by pyrolysis at 700 °C for 2 h have been used for the synthesis of nano sized CdO-I and CdO-II, respectively. From powder XRD data the lattice parameters (0.4701 and 0.4696 nm respectively for the two samples) and particle size (~ 77 and 30 nm for CdO-I and CdO-II) have been evaluated. Morphology of the two varieties of CdO is widely different as is evident from their SEM images. The estimated values of the band gap of 2.53 eV and 2.59 eV for CdO-I and CdO-II respectively are obtained from the optical spectral analysis.     

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 effect of bilayer period and degree of unbalancing on magnetron sputtered Cr/CrN nano-multilayer wear and corrosion

Cr/CrN nano-multilayers were grown on H13 steel and silicon (100), having different periods (Λ), at room temperature using the unbalanced magnetron sputtering technique by varying the degree of unbalancing (K_G) to investigate the effect on multilayer properties. The multilayers' total thickness was ~ 1 μm and the total number of layers varied from 10 (Λ = 200 nm), 20 (Λ = 100 nm) to 100 (Λ = 20 nm) layers. Film microstructure, hardness, wear and corrosion resistance were measured regarding bilayer period and degree of unbalancing. The results showed that wear resistance was lower for low K_G values and that corrosion resistance was higher and hardness was improved. Nano-hardness was found to be higher for multilayers grown with Λ = 20 nm for all K_G values, reaching a maximum 25 GPa value for K_G = 0.87.     

Improved dielectric properties of lead lanthanum zirconate titanate thin films on copper substrates

Thin films of lead lanthanum zirconate titanate (PLZT) were directly deposited on copper substrates by chemical solution deposition and crystallized at temperatures of ~ 650 °C under low oxygen partial pressure (pO₂) to create film-on-foil capacitor sheets. The dielectric properties of the capacitors formed have much improved dielectric properties compared to those reported previously. The key to the enhanced properties is a reduction in the time that the film is exposed to lower pO₂ by employing a direct insertion strategy to crystallize the films together with the solution chemistry employed. Films exhibited well-saturated hysteresis loops with remanent polarization of ~ 20 μC/cm², dielectric constant of > 1100, and dielectric loss of < 0.07. Energy densities of ~ 32 J/cm³ were obtained at a field of ~ 1.9 MV/cm on a ~ 1 μm thick film with 250 μm Pt electrodes.     

Epitaxial growth and rectification characteristics of double perovskite oxide La2NiMnO6 films on Nb-SrTiO3 single crystal substrates

High-quality thin films of double perovskite La₂NiMnO₆ (LNMO) were epitaxially grown on Nb-doped SrTiO₃ (NSTO) substrates by pulsed laser deposition. The films were found to undergo a ferromagnetic-to-paramagnetic transition at ~ 280 K, which is consistent with the literature report. In the electrical measurements, typical rectifying behavior was observed in the LNMO/NSTO heterojunction. The diffusion voltage (V_D) increases linearly with temperature (T) during cooling until T = 170 K. At T < 170 K, V_D increases at a higher rate and the V_D-T relationship becomes non-linear. A disordered phase related spin polarization was used to understand such behaviors in the heterojunctions.     

Wet chemical route to transparent BiFeO3 films on SiO2 substrates

BiFeO₃ nanoparticles were prepared by a wet chemical synthesis method. Transparent films were deposited on glass and quartz substrates by dip and spin coating processes from the synthesized sol. We obtained thicker films (~ 2 µm) by dip coating process and thinner films (~ 200 nm) by spin coating process. Transmission electron microscopy images confirmed that the particles are nanocrystalline in size. From the optical transmittance spectra the band gap of the BiFeO₃ nanoparticles was determined in the range of ~ 3.03-2.88 eV (~ 410-430 nm). Electrical resistivity, polarization, zero-field-cooled and field-cooled magnetizations versus temperature characteristics were also studied for these films.