Crystallographic texture study of pulsed laser deposited Cr2O3 thin films
Abstracts:Chromium oxide thin films of thickness ~140 nm were grown on c-plane (0001) and a-plane (110) sapphire substrates by Pulsed Laser Deposition (PLD) technique. The films of Cr2O3 on Si (100) substrates have been synthesized at an optimized oxygen partial pressure of 2 Pa with varying substrate temperature in the range of 473-973 K. It was observed that the films deposited at temperatures ≥673 K are crystalline as confirmed by X-Ray Diffraction (XRD) studies. The X-Ray Photoelectron Spectroscopy (XPS) profile showed that the films were grown with Cr3+ oxidation state. Atomic Force Microscopy (AFM) revealed smooth, uniform surface morphology with small amount of porosity. The epitaxial nature of the film on sapphire substrate and crystallographic orientation of the Cr2O3 over Si substrate were studied using rocking curve, ϕ (Phi) scan and pole figure measurement. Pole figure analysis suggested that the films deposited on Si substrate were fiber textured. Rocking curve Full Width at Half Maximum (FWHM) for Cr2O3 (0006) and Cr2O3 (110) was about 0.1° and 0.06° respectively, which indicate the good quality of crystalline film. High Resolution X-ray Diffraction (HRXRD) with reciprocal space mappings (RSM) measurements indicate that the films grown on sapphire substrates are in relaxed state.
Fabrication of SnS solar cells via facile nanoparticle synthesis based on non-toxic solvents
Abstracts:Tin monosulfide (SnS) thin film is one of the most promising absorbers layer for high efficiency solar cells using low cost earth-abundant materials. To fabricate high quality SnS thin films, a simple nanoparticle based approach was employed in this work for phase pure material preparation using non-toxic solvents. Uniform SnS thin films were fabricated via a spin-coating process using phase pure SnS-ORT nanoparticle precursor solutions based on a methanol solvent, followed by post-annealing for grain growth. SnS thin film solar cells were then fabricated and their photovoltaic performance was characterized. The best SnS solar cell fabricated with the as-prepared SnS films with a band gap energy of ~1.25 eV showed a conversion efficiency of 0.12%.
Highly stable silver nanowires based bilayered flexible transparent conductive electrode
Abstracts:A flexible transparent conductive electrode (TCE) with silver nanowires (AgNWs) is an essential component in flexible optoelectronic devices. Considerable efforts have been made to improve the transmittance and sheet resistance of AgNWs TCE for large area applications. However, long-term usages of AgNW TCEs are limited by their poor thermal and chemical stabilities under different atmospheric conditions. In this work, we propose to improve the opto-electrical properties by fabricating AgNWs layer on a metal mesh (MM) polyethyleneterephthalate (PET) substrate (AgNW/MM/PET). A bilayer structure of the AgNW/MM/PET was fabricated by bar coating, and it exhibited excellent opto-electrical properties with a transmittance of 90.3% and a sheet resistance of 0.745 Ω/sq. In addition, the fabricated AgNW/MM/PET showed more stable electrical performances than mono layer TCEs with AgNWs or metal mesh on PET under various environmental conditions, such as light, temperature, and humidity. The AgNW/MM/PET electrodes with excellent opto-electrical properties could be a possible replacement for indium tin oxide for large area flexible optoelectronic applications.
Fabrication of high band gap kesterite solar cell absorber materials for tandem applications
Abstracts:Using the thermal annealing of evaporated metallic precursors in successive H2Se and H2S atmospheres, it was possible to reproducibly manufacture kesterite absorber material for solar cell applications with a sulfur content varying from 30% to 100%. Respective band gaps for these sulfur inclusions were measured at approximately 1.45 eV and 2.0 eV. A recipe was devised for which results could be reproduced within an error margin of ±5% and the influence of the H2S pressure during the post sulfurization was negligible on all measurable and observable parameters. The evolution of the S/Se ratio in the sample was observed to be linearly dependent on the annealing time. It was also observed that at very early stages of the post-sulfurization, both the original Cu2(Zn,Ge)Se4 (CZGSe) and a primary Cu2(Zn,Ge)(S,Se)4 (CZGSSe) phase with a sulfur inclusion of ~30% coexist in the sample. The (112) x-ray diffraction (XRD) reflection of the CZGSe phase progressively disappears in favor for the first mixed CZGSSe phase. Using grazing incidence-XRD, the S/Se ratio was shown to be inhomogeneous. Indeed, the XRD measurement of the top layers led to the calculation of higher sulfur inclusions than was the case when measuring the bulk material. Top-scanning electron microscopy (SEM) as well as cross-SEM measurements were taken in order to determine the impact of the sulfur inclusion on the crystal growth and the overall quality of the produced absorber layers. The obtained images revealed a reduction in crystal size and the appearance of numerous holes in the layer as the S/Se ratio is increased.
Physical properties of ZnSe1-xTex,thin films prepared with a single three-chamber evaporation coaxial source
Abstracts:ZnSe1-xTex thin films were prepared using a single evaporation source specially designed to produce ternary material from the simultaneous sublimation of ZnSe and ZnTe precursor materials. A detailed description of the evaporation source and the procedure for depositing the thin films are given. The substrate temperature allows adequately controlling the Te concentration in the samples. The samples were studied using X-ray Diffraction, Energy Dispersive X-ray, transmittance, room temperature photoluminescence, and photoconductivity in the visible spectral region. The percentage of Te as a function of the substrate temperature was determined using Vegard's Law. Samples grew with the stable cubic structure and were oriented along plane . The crystallite size and thickness of the samples were dependent on the substrate temperature. Absorption values on the order of 108 cm−1 and a gap change from 3,0 (x = 0,98) to 3,3 (x = 0,13) eV as the substrate temperature increased from 333 to 623 K were found. The photoluminescence and photoconductivity spectra have contributions of charge recombination through the band gap edge, with localized states below the band gap associated with Zn and Te vacancies in the material. The photoconductivity and photoluminescence signals changed according to the composition of the films.
Transparent conductors for Mid-infrared liquid crystal spatial light modulators
Abstracts:Transparent conductors (TCs) are required for liquid crystal spatial light modulators (LC-SLMs) in order to set up an electric field across the LC layer. In the middle infrared (Mid-IR) range (λ = 2 to 5 μm), LC-SLMs can offer a low-cost, non-mechanical, random-access and compact alternative to the gimbaled mirrors used currently for Mid-IR laser beam-steering. Indium tin oxide (ITO) is the industry standard for applications in the visible spectrum but it performs poorly in the IR, with a transmittance <20% for Mid-IR wavelengths. Little work has been done to develop a comparable material which fulfils the required properties in the Mid-IR: A sheet resistance allowing operation at typical frequencies (≈1 kHz) and, if patterned, with minimal voltage drop along the electrode, a transmittance >50% in the target range, chemical, thermal and mechanical robustness which can endure subsequent processing, and ability to be patterned at low-cost to a resolution comparable to the wavelengths investigated. Ni and Cu ultra thin metallic films (UTMFs), Cu x O thin films (TFs), and chemical vapour deposition (CVD) grown mono-layer graphene were investigated. Ni UTMFs and graphene were found to have the best performance with sheet resistance values of and respectively for samples having a transmittance of 65% and 97% at λ = 2.3 μm. Both Ni UTMFs and CVD mono-layer graphene were found to be suitably stable with age. An increase of sheet resistance after baking was recorded due to oxidation and desorption of contaminating dopants respectively. Ni UTMFs were found to be patternable down to a 3 μm resolution, limited by the mask, using a standard photo-lithographic lift-off process. Transmissive LC cells with a maximum phase shift of 3π at λ = 2.3 μm were assembled with both Ni UTMFs and mono-layer graphene as TCs on sapphire, with the former having transmittance of 18.7% and contrast ratio of 25.0, and the latter having transmittance of 81.1% and contrast ratio of 50.7.
Synthesis of nanocrystalline δ-MoN by thermal annealing of amorphous thin films grown on (100) Si by reactive sputtering at room temperature
Abstracts:We report on the synthesis and characterization of nanocrystalline δ-MoN by crystallization of amorphous thin films grown on (100) Si by reactive sputtering at room temperature. Films with chemical composition MoN were grown using a deposition pressure of 5mTorr with a reactive mixture of Ar/(Ar + N2) = 0.5. The as-grown films display mostly amorphous structure. Nanocrystalline δ-MoN phase is obtained after annealing at temperatures above 600 °C. The superconducting critical temperature T c depends on film thickness. Thick films (170 nm) annealed at 700 °C for 30 min display a T c = 11.2 K (close to the one reported for bulk specimens: 13 K), which is gradually suppressed to 7.2 K for 40 nm thick δ-MoN films. Our results provide a simple method to synthesize superconducting nitride thin films on silicon wafers with T c above the ones observed for conventional superconductors such as Nb.
Two-stage processed Cu4SnS4 thin films for photovoltaics - Effect of (N2 + S2) pressure during annealing
Abstracts:In this paper, we report the fabrication of Cu4SnS4 thin films by annealing chemically deposited SnS–CuS precursors at 823 K for 90 min, and we studied the effect of the (N2 + S2) pressure during annealing on the growth and properties of the Cu4SnS4 films. Films prepared at a (N2 + S2) pressure of 1.3 kPa exhibited an orthorhombic crystal structure with lattice parameters of a = 1.371 nm, b = 0.766 nm, and c = 0.643 nm, a grain size of 3–6 μm, a direct optical band gap of 1.0 eV, p-type electrical conductivity, and a hole mobility of 69.5 cm2 V−1 s−1. Increasing the (N2 + S2) pressure from 1.3 kPa to 66.7 kPa increased the grain size to more than 6 μm and the hole mobility to 150 cm2 V−1 s−1 at 26.7 kPa and then decreased to 86 cm2 V−1 s−1 at 66.7 kPa. Further increasing the (N2 + S2) pressure to 101.3 kPa resulted in the formation of a monoclinic Cu2SnS3 secondary phase. This study reveals that an annealing temperature of 823 K and a (N2 + S2) pressure of 1.3–66.7 kPa are the optimized conditions to obtain large-grained Cu4SnS4 films free of secondary phases with good optical and electrical properties.
Study of polycrystalline CdTe films by contact and contactless pulsed photo-ionization spectroscopy
Abstracts:The methodology of sample preparing and combining of measurement techniques in comprehensive characterization of thin cadmium telluride (CdTe) heterostructures has been demonstrated by recording spectra of deep (bulk and surface) traps, acting coherently within photo-ionization and thermal emission processes. The CdTe heterostructures on silicon (Si) substrates have been examined by combining measurements of the microwave probed and contact photoconductivity transients varying pulsed laser excitation wavelengths. The pulsed photo-ionization spectra have been analysed using the Lucovsky model. It has been shown that microwave probed photoconductivity transients in polycrystalline CdTe films with bare surfaces are governed by diffusion limited surface recombination. The surface recombination is there mainly influenced by deep traps of 1.23 eV photo-activation energy. The bulk traps in CdTe - Si structure containing metal electrodes exhibited a spectrum with five photo-ionization steps, having the threshold photo-ionization energy of the E b1 = 0.57 eV, E b2 = 0.94 eV and E b5 = 1.42 eV, E b4 = 1.13 eV, and E b3 = 1.05 eV which are attributed using literature data to the Te interstitials (E b1 ), to Cd (E b2 and E b5 ) and Te (E b4 ) vacancies, and to anti-site (Te at Cd) defect (E b3 ), respectively. Shallow emission centres with thermal activation energy of 100 meV have been estimated from carrier lifetime variations with temperature.
Investigation of porous anodic alumina templates formed by anodization of single-crystal aluminum substrates
Abstracts:Ordered porous anodic alumina (PAA) templates are of great interest as they facilitate the future development of nanodevices. The present study focuses on the impact of substrates with different crystallographic orientations on the template's pore structure. Characteristics such as pore diameter, interpore distance, pore regularity, porosity, and circularity are calculated as a function of the anodization potential for three different Al crystal orientations. The presented experiments reveal that the different crystallographic orientations mainly impact the pore ordering, while other structural parameters, such as the pore diameter and interpore distance, are not significantly affected.