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In this work, the optical properties of thin-film α-Ga2O3 are examined and contrasted with those of β-Ga2O3 thin films grown on similar substrates. Thin films of α-Ga2O3 were synthesized employing mist chemical vapor deposition (mist CVD) on double-side polished c-plane sapphire. Optical properties were studied through absorbance, photoluminescence (PL), and photoluminescence excitation (PLE). Absorbance measurements showed spectra consistent with that previously reported in the literature, with Eg ~ 5.19 eV for α-Ga2O3. Despite an absorption edge laying at higher energies, the PL spectra in α-Ga2O3 films yield similar features to those of metal-organic chemical vapor deposition (MOCVD) grown β-Ga2O3 thin-films. In both cases, when excited above the band gap, the PL spectra consist of UV, UV′, blue and green broad bands. As the excitation photon energy was varied, the relative intensity of these PL bands changed. This is attributed to a non-uniform defect concentration distribution across the sample depth. With smaller concentration of defects associated with blue and green emission in the bulk compared to the surface in the α-phase films.

Ultra-fast thermal annealing of semiconductor materials using a laser can be revolutionary for short processing times and low manufacturing costs. Here we investigate Cu–In–Se thin films as precursors for CuInSe2 semiconductor absorber layers via laser annealing. The reaction mechanism of laser annealed metal stacks is revealed by measuring ex situ X-ray diffractograms, Raman spectra and composition. It is shown that the formation of CuInSe2 occurs via the formation of CuxSe/InxSey binary phases as in conventional annealing routes, despite the entirely different annealing time scale. Pre-alloying the Cu and In metals prior to laser annealing significantly enhances the selenisation reaction rate. Laser annealing for six seconds approaches a near phase-pure material, which exhibits similar crystalline quality to the reference material annealed for ninety minutes in a tube furnace. The estimated quasi Fermi level splitting deficit for the laser annealed material is only 60 meV lower than the reference sample, which implies a high optoelectronic quality.

Exfoliated gallium oxide (Ga2O3) has been reported as an ultrathin channel material in field-effect transistors. Unlike in 2D materials with van der Waals forces between stacked layers, weak bonding in β-Ga2O3 along the (100) direction enables mechanical exfoliation. The thickness of these exfoliated films has been limited to tens to hundreds of nanometers. Here we summarize our latest work, which followed the process developed by Carey et al., enabling us to attain ~2 nm thick Ga2O3 films over a large surface area (< 1 mm2). The films are characterized using optical microscopy, AFM, XPS, Raman spectroscopy, photoluminescence (PL), and TEM. Optical microscope images showed color changes to the film upon annealing. AFM revealed the film thickness to be as thin as 2 nm over areas << 1 mm2. XPS and Raman spectra revealed characteristic signatures of β-phase Ga2O3. Characteristic PL of Ga2O3 was seen in all samples with overall intensity of luminescence increasing after annealing, attributed to increased crystallinity and grain size. Changes in PL after annealing are associated with an increase in oxygen interstitials and a decrease in oxygen vacancies. Lastly, TEM analysis revealed the film as β-phase Ga2O3 polycrystalline. Overall, our results demonstrate that annealing of thin films obtained from oxide printing of liquid metal Ga is a non-expensive and straightforward process that can lead to β-Ga2O3 films that are nanometer-thin over wafer-scale areas.

The degradation of methyl-ammonium lead iodide (MAPbI3) upon exposure to air is a potentially limiting effect for large scale MAPbI3 photovoltaic production. Here, we report a systematic study on effects of air-exposure on the structural and optical properties of MAPbI3 thin films. The X-ray diffraction studies indicate a shrinking volume of MAPbI3 upon air-exposure as the material decomposes back to its precursors, methyl amine (CH3NH2) and lead iodide (PbI2). However, the photoluminescence (PL) yield and carrier lifetime measured with time-resolved photoluminescence, show an increasing trend upon air-exposure. These phenomena can be explained by self-passivation of MAPbI3 grains by PbI2 layer that reduces the number of non-radiative recombination centres at the grain boundaries. However, this process is not self-limiting and it eventually leads to a film that has completely reverted back to its precursor state. It is shown that this conversion of MAPbI3 film back to its precursors is also accelerated by exposure to laser illumination. Furthermore, we report unusual variation of PL intensity on a shorter time scale of a few seconds in all the films used for the experiment. The variations are found to follow different trends in the encapsulated samples as compared to the un-encapsulated samples. We propose that the decomposition followed by the ionic diffusion through film is responsible for such unusual behaviours.

We report on silicon delta doping of metalorganic vapor-phase epitaxy-grown \b{eta}-Ga2O3 thin films using silane precursor. Delta-doped \b{eta}-Ga2O3 epitaxial films are characterized using capacitance-voltage profiling and secondary-ion mass spectroscopy. Electron sheet charge density in the range of 2.9e12 cm-2 to 8e12 cm-2 with a half width at half maximum ranging from 6.2 nm to 3.5 nm is measured. We also demonstrate a high density (6.4e12 cm-2) degenerate two-dimensional electron gas using a delta-doped \b{eta}-(Al0.26Ga0.74)2O3/\b{eta}-Ga2O3 heterostructure.The total charge could also include a contribution from a parallel channel in the \b{eta}-(Al0.26Ga0.74)2O3 alloy barrier.

We correct three labeling errors, one in Fig. 4(a), and the other two in Section 3.2 of the manuscript text [Opt. Mater. Express 9, 826 (2019)10.1364/OME.9.000826].

We report on the growth and characterization of Ge-doped β-Ga2O3 thin films using a solid germanium source. β-Ga2O3 thin films were grown using a low-pressure chemical vapor deposition reactor with either an oxygen or a gallium delivery tube. Films were grown on 6° offcut sapphire and (010) β-Ga2O3 substrates with growth rates between 0.5 and 22 μm/h. By controlling the germanium vapor pressure, a wide range of Hall carrier concentrations between 1017 and 1019 cm−3 were achieved. Low-temperature Hall data revealed a difference in donor incorporation depending on the reactor configuration. At low growth rates, germanium occupied a single donor energy level between 8 and 10 meV. At higher growth rates, germanium doping predominantly results in a deeper donor energy level at 85 meV. This work shows the effect of reactor design and growth regime on the kinetics of impurity incorporation. Studying donor incorporation in β-Ga2O3 is important for the design of high-power electronic devices.

Laser annealing can reduce the annealing time required to form Cu(In,Ga)(S,Se)2 (CIGSe) thin films for use in thin film photovoltaics to a single second timescale, if not faster. In this work, we use microstructural characterization coupled with modeling of the optical and thermal properties to understand the laser annealing of three types of electrodeposited precursor stacks for the CIGSe parent compound CuInSe2. The precursor films are: stacked elemental layers Cu/In/Se, stacked binary selenides In2Se3/Cu2−xSe, and a single layer of coelectrodeposited Cu–In–Se. Conceptually, these stacks are ordered in terms of decreasing stored chemical and interfacial potential free energy, consideration of which predicts that the formation of large grained CuInSe2 from the stacked elemental layers would be the most exothermic and thus most rapid process. However we find that microstructural details of the electrodeposited films such as void fraction present in the stacked binary selenides dramatically alter the heat and mass flow. Additionally, modeling of the optical absorption within the elemental stacked precursor suggests extremely localized heating at the In/Se interface resulting in significant Se loss. Despite its lower chemical potential energy, the coelectrodeposited CuInSe2 precursor's more uniform optical absorption of near-bandgap light coupled with its compact, low void fraction microstructure of nano-sized grains results in the most optimal recrystallization and compositional homogenization via interdiffusion. Furthermore this annealed layer formed a working device with a short circuit current density of 23 mA cm−2. This combined modeling and experimental investigation underscores the need to consider practical micro- and nanostructure-dependent properties as well as the optical absorption and not simply thermodynamics when designing accelerated two step deposition and annealing processes for compound semiconductors.

Homoepitaxial growth of β-Ga 2O 3 using metalorganic vapor-phase epitaxy (MOVPE) on several different crystal orientations has previously been studied, but growth on the ( 2 ¯ 01 ) plane has remained comparatively unexplored. To investigate this, we grew Si-doped and unintentionally doped (UID) homoepitaxial layers simultaneously on Sn-doped ( 2 ¯ 01 ) and ( 010 ) substrates under conditions optimized for ( 010 ) growth. We report herein on results from current–voltage and capacitance–voltage (IV and CV) and deep level transient spectroscopy (DLTS) characterization of Schottky diodes fabricated on these samples. Devices on ( 010 ) display nearly ideal Schottky diode JV and CV behaviors while for ( 2 ¯ 01 ) evidence of complete depletion of >2 μm thickness, UID epilayers are observed. The ( 2 ¯ 01 ) UID diodes exhibited Mott–Gurney space charge-limited transport and were completely depleted even at zero bias. Doping ( 2 ¯ 01 ) samples heavily with Si was sufficient to overcome background compensation and reproduce near-ideal diode behavior. DLTS data from these doped devices show, in addition to typical negative majority-carrier transients, positive transients with a broad energy distribution, possibly indicating traps on structural defects or surface states. An etch pit analysis under SEM revealed intricate structures of the grown layers. Cross-sectional TEM characterization of a ( 2 ¯ 01 ) sample revealed a high density of structural defects within the epitaxial layer, likely stacking faults. Hypotheses for the origins of the compensation in ( 2 ¯ 01 ) β-Ga 2O 3 homoepitaxial MOVPE growth under ( 010 ) growth conditions are discussed; the most likely explanation is the presence of defect states introduced by the stacking faults visible under TEM.

In this work, we report the different effects of CdCl2 treatment on CdTe films deposited by thermal evaporation onto CdS and MgZnO (MZO) buffer layers. The main finding, which is relevant for understanding recent advances in CdTe device efficiency, is that few- ${\mu}$m thick CdTe films deposited on MZO can be induced to completely recrystallize forming a film consisting of grains that span the film thickness and are up to 30 $\mu$m laterally. On CdS buffer layers, the changes in microstructure with Cl treatment are much less pronounced and the final microstructure is less ideal for thin film photovoltaics. We propose a thermodynamic framework for understanding the microstructural changes during CdCl 2 treatment which can assist in understanding the wide range of behaviors observed across the many CdTe thin film solar cell fabrication procedures.

The effect of Indium (In) doping on CdTe thin film solar cells was investigated. CdTe thin films were deposited using the elemental vapor transport (EVT) technique under various Cd/Te gas phase ratios and In vapor concentrations. TCO/Buffer/n-CdTe/p-ZnTe solar cells have been fabricated and characterized. Indium doping has been found to affect the device performance. Capacitance-voltage (C-V) measurements revealed that net n-type doping increased with In concentration during the EVT deposition. Spectral response (SR) measurements indicated an enhanced carrier collection at higher In concentrations. The open-circuit voltage ($\text{V}_{\mathbf {OC}}$) and fill factor (FF) of the devices was found to increase with increased n-type doping and therefore builtin voltage. To-date n-type CdTe solar cells yielded devices with $\text{V}_{\mathbf {OC}} \quad =$ 730 mV, $\text{J}_{\mathbf {SC}} \quad =$ 20 mA/cm$^{\mathbf {2}} {, \mathbf {FF}} =$textbf 62 % and efficiency near 9 %.

We discuss the optoelectronic property variation between grains and grain boundaries of CdTe polycrystalline thin films using a confocal microscopy system. Single-photon photoluminescence (PL) and time-resolved photoluminescence spectroscopy (PL) is used to map $10 \times 10 \mu \mathrm {m}^{2}$ area at the back surface of CdTe with an optical resolution of 104 nm. TRPL maps show that different grain boundaries have different near-surface lifetimes. Surprisingly, grain-boundaries with high near-surface lifetime are associated with regions of the sample that have low PL yield. This study demonstrates the potential of confocal PL and TRPL mapping to understand carrier lifetime variations in thin films.

One industrially relevant fabrication method for CuInSe2 absorber layers begins with electrodeposition, which is highly resource efficient and easily up-scalable, followed by a high temperature annealing step. Commonly a furnace is used for annealing, although it is possible to reduce the duration of this step by 2–3 orders of magnitude using laser heating. Past work demonstrated that rastering a 1064 nm, continuous wave, neodymium-doped yttrium aluminium garnet laser beam over an electrodeposited precursor film with 1 s dwell time, promoted CuInSe2 formation, grain growth, and gave a photovoltaic device with 1.6% power conversion efficiency. However, this device showed inhomogeneous current collection correlating approximately to the Gaussian flux profile of the laser beam. This work demonstrates how deviations in the incident laser flux on the precursor lead to local temperature variation in the film. The temperature affects the rates of atomic diffusion, grain growth and chemical reactions, with the hottest region having the largest crystallites and a stoichiometric and constant composition through the film depth. Spatially resolved photoluminescence yield shows a positive correlation to the temperature profile caused by the Gaussian flux. It is expected that with a homogeneous laser beam flux, spatial variations in absorber layer properties would be eliminated, leading to uniform device properties.

Despite the many similarities between Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)(S,Se)2 materials and device architecture, open questions remain about the optimal surface preparation steps for CZTSSe absorbers, including whether differences exist for absorber layers deposited by different methods. In this work, we investigate KCN etching and NH4OH treatment as surface preparation methods for the absorber/CdS interface for two-stage processed CZTSSe and co-evaporated CZTSe absorber layers. Ambient-exposed, thus oxidized and contaminated, thin film absorbers are utilized to examine the effectiveness of these surface preparation methods and to elucidate their effects on device performance. Topography and surface potential images simultaneously obtained by Kelvin probe force microscopy (KPFM) show the existence of an overlayer on the ambient-exposed absorbers. Moreover, KPFM results also demonstrate that although NH4OH treatment removes much of the overlayer from the CZTSSe surface, KCN etching removes the overlayer completely. In addition, differences in the deposited CdS layer and depletion region width result depending on the surface preparation method, with the NH4OH-treated solar cells having narrower depletion region. This is reflected in device results in which KCN-etched solar cells outperform their NH4OH-treated counterparts due to increases in external quantum efficiency at long wavelengths and in open circuit voltage. KPFM measurements also demonstrate that grain boundaries (GBs) in the KCN-etched two-stage processed CZTSSe thin films are either negatively charged or neutral. On the other hand, KCN etching makes the surface almost equipotential in the co-evaporated CZTSe thin film by causing disappearance of positively charged GBs that existed before etching.

In this paper we study the structural properties of Cu 2 ZnSnS 4 (CZTS) synthesized by cosputtering binary sulfide followed rapid thermal annealing (RTA) as functions of temperature and composition. By annealing identical samples at different temperatures we were able to find the optimum temperature for annealing samples in our RTA furnace. With this information, we annealed samples with a variety of discrete compositions at the optimum temperature to study the compositional differences on the optical, electrical and the structural differences on CZTS thin film crystallization. Samples were evaluated by x-ray diffraction, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Sample rich in Cu tended to form areas of CU 2-x S. It was confirmed that Sn loss during annealing increases with increased annealing temperature. Samples rich in tin before annealing grew larger grains indicating that Sn can also playa role in grain growth.

Photon absorption is a critical process for photovoltaic operation, yet the effect of illumination on semiconductor growth and processing is rarely investigated. In this work we investigate the effect of illumination during the cadmium chloride (CdCl2) annealing treatment of cadmium telluride (CdTe) thin films for photovoltaic applications while keeping the sample temperature constant. Illumination at 808 nm with varying intensity (0, 0.5, 1 and 1.5 W) when incident on CdTe during thermal CdCl2 treatment of CdTe lead to enhancement of grain size and optoelectronic properties of CdTe. However photoassisted etching of the CdTe surface is also observed at illuminations sufficiently low that the sample temperature was barely affected during annealing. Additionally, annealing under high illumination sufficient to drive CdCl2 annealing in the absence of a furnace has no such effect on the CdTe surface. We attempt to reconcile these observations and provide rationale for further experiments.

Larger grain sizes and crystallographic texture in the absorber layer are correlated with performance in polycrystalline inorganic thin film solar cells because they lead to reduced densities of recombination centers and traps at grain boundaries. In this work, we compare the effects of rapid thermal annealing (RTA) and furnace annealing on film morphology, grain growth, and crystallographic texture in Cu2ZnSnS4 (CZTS) thin films deposited on Mo-coated glass by RF cosputtering. Using x-ray diffraction (XRD),scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD), we show that RTA induces significant grain growth and texture without the degradation of the substrate caused in furnace annealing by the high temperatures required for CZTS phase formation. CZTS grains grown in RTA annealed samples were 2–3 times larger than those in furnace-annealed samples. Significantly, this is demonstrated in RTA on sodium-free glass which implies that RTA and similar processing methods may be capable of producing desirable CZTS grain distributions on alternative substrates such as metal foil, plastic, or thin glass.

CuInSe2 (CISe) is a prototype material for the I–III–VI chalcopyrites such as Cu(In,Ga)(S,Se)2 used as absorber layers in thin film photovoltaic cells. Carefully-controlled pulsed-laser annealing (PLA) is a unique annealing process that has been demonstrated to improve the device performance of chalcopyrite solar cells. Here, we investigate the changes in defect populations after PLA of electrochemically-deposited CISe thin films previously furnace annealed in selenium vapor. The films were irradiated in the sub-melting regime at fluences inducing temperatures up to 840 ± 100 K. Deep-level transient spectroscopy on Schottky diodes reveals that the activation energy of the dominant majority carrier trap changes non-monotonically from 215 ± 10 meV for the reference sample, to 330 ± 10 meV for samples irradiated at 20 and 30 mJ/cm2, and then back to 215 ± 10 meV for samples irradiated at 40 mJ/cm2. A hypothesis involving competing processes of diffusion of Cu and laser-induced generation of In vacancies may explain this behavior.

We present a novel method for depositing crystalline cadmium telluride (CdTe) thin films from aqueous solutions. The films were deposited via a spin-coating technique by direct conversion alternating layers of Cd and Te precursor solutions. Powder x-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images of the films show crystallization and preferential orientation of the grains to the (111) following a post-deposition thermal annealing in the presence of cadmium chloride (CdCl 2 ). This result is a promising first step towards the fabrication of a CdTe-based photovoltaic device using a solution-deposited absorber layer.

We demonstrate a low cost solution based process for polycrystalline CdTe deposition followed by a fast laser annealing step. A chemical bath deposition (CBD) process using a Cd(OH) 2 and a Te precursor solution to form polycrystalline CdTe thin films via ion-exchange reaction. This is followed by a fast laser annealing process by a 248nm excimer laser to improve the grain size of the deposited CdTe layers. X-ray diffraction analysis shows that the CBD process favors the growth of (111) CdTe. Additionally the laser treatment improves the crystalline quality of the films as is evidenced by a decreased FWHM of the (111) XRD peaks of the laser treated samples.

A large amount of current research in thin film photovoltaics based on inorganic semiconductors aims to deposit and process these active layers using inexpensive and low temperature processing methods resulting in high densities of electronic states in the bandgap associated with structural and point defects. We define semiconductors in which the Fermi level is pinned throughout the bulk as being at the defective limit and explore the consequences for heterojunctions designed to separate photocarriers. Using this novel concept of the defective limit, heterojunction band offsets can be predicted for less-common materials such as the earth-abundant materials also under intense investigation. Because of the Fermi level pinning in the bulk, only Type II heterojunctions will separate charges but the high defect concentrations will also result in small minority carrier diffusion lengths. These limitations require the use of nanostructured device architectures for cells of such materials with appreciable power conversion efficiency.

article i nfo Tin monosulfide (SnS) is of interest as a potential solar cell absorber material. We present a preliminary investigation of the effects of sputtering conditions on SnS thin-film structural, optical, and electronic properties. Films were RF sputtered from an SnS target using an argon plasma. Resistivity, stoichiometry, phase, grain size and shape, bandgap, and optical absorption coefficient can be varied by modifying argon pressure for a fixed deposition time. Most films have an indirect bandgap in the range of 1.08-1.18 eV. XRD patterns confirmed the films as mostly crystalline, and grain morphology was examined using profile and surface SEM images.

We report on the heteroepitaxial growth of thin films of rocksalt GdN on c-plane (0 0 0 1) wurtzite GaN by molecular beam epitaxy (MBE) using either an N2 plasma or NH3 as the nitrogen source. In both cases, epitaxial films with fully oriented GdN (1 1 1)∥GaN (0 0 0 1) were deposited as demonstrated by θ–2θ X-ray diffraction. φ scans of GdN peaks demonstrate 6-fold symmetry along the growth axis implying the presence of two 3-fold-symmetric GdN (1 1 1) crystal variants in-plane. Electrical transport and magnetometry measurements on films grown using N2 plasma show that these GdN films are ferromagnetic below TC=70 K and degenerately doped or metallic from 10 to 300 K with magnetotransport signatures associated with TC.

The performance of polycrystalline CdTe photovoltaic thin films is expected to depend on the grain boundary density and corresponding grain size of the film microstructure. However, the electrical performance of grain boundaries within these films is not well understood, and can be beneficial, harmful, or neutral in terms of film performance. Electron backscatter diffraction has been used to characterize the grain size, grain boundary structure, and crystallographic texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with microstructure. Weak fiber textures were observed in the as-deposited films, with (111) textures present at lower deposition pressures and (110) textures observed at higher deposition pressures. The CdCl2-treated samples exhibited significant grain recrystallization with a high fraction of twin boundaries. Good correlation of solar cell efficiency was observed with twin-corrected grain size while poor correlation was found if the twin boundaries were considered as grain boundaries in the grain size determination. This implies that the twin boundaries are neutral with respect to recombination and carrier transport.

Post-deposition laser processing of CdTe has potential for reducing manufacturing costs of CdTe PV modules. In this work we demonstrate the applicability of lasers to thermally processing CdTe modules by using a continuous wave Nd:YAG 1064 nm laser to drive CdCl2 treatment of CdTe. The data show that, similar to traditional annealing, laser annealing for only 15 seconds at higher than typical temperatures, the annealed CdTe displays an increase in A-center peak (1.435 eV) in photoluminescence yield. These indicate that chlorine activation of the absorber layer occurs in a mere 15 s and demonstrates the potential of accelerating the post-deposition CdCl2 treatment of CdTe by utilizing laser annealing at higher temperatures.

Ternary and quaternary dilute II–VI oxides were synthesised using a highly non-equilibrium method: the combination of O ion implantation and pulsed-laser melting. CdOxTe1−x thin films have been produced with x up to 0.015 and with the energy gap reduced by 0.15 eV. Optical transitions corresponding to both the lower (E−) and upper (E+) conduction sub-bands, resulting from the anticrossing interaction between the localised O states and the extended conduction states of the matrix, are clearly observed in quaternary Cd0.6Mn0.4OxTe1−x and Zn0.88Mn0.12OxTe1−x layers. These results have important implications for the existing theoretical models of the electronic structure of the highly mismatched alloys. In Zn1−xMnxTe, where the O level lies below the conduction band edge, it was demonstrated that incorporation of a small amount of oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the ZnMnTe matrix. The three absorption edges of this material (∼0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for multiband, single-junction, high-efficiency photovoltaic devices.

We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdO{sub x}Te{sub 1-x} thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O{sup +}-implantation in CdTe followed by pulsed laser melting. Quaternary Cd{sub 0.6}Mn{sub 0.4}O{sub x}Te{sub 1-x} and Zn{sub 0.88}Mn{sub 0.12}O{sub x}Te{sub 1-x} with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E{sub -}) and upper (E{sub +}) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, single-junction solar cells.

Using oxygen ion implantation and pulsed laser melting, we have synthesized thin films of highly mismatched ternary (y = 0) and quaternary (y = 0.12) Zn 1-y Mn y O x Te 1-x alloys with oxygen content in excess of x ∼ 0.01. We show that incorporation of a small amount of isoelectronic oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the Zn 1-y Mn y Te matrix. The structure of the conduction band is well described by the anticrossing interaction between O localized states and the extended states of the host semiconductor matrix. As a result the conduction band splits into two subbands with distinctly non-parabolic dispersion relations. The three absorption edges of this material (-0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for the multi-band, single junction, high efficiency photovoltaic devices.

We present a systematic investigation on the formation of the highly mismatched alloy GaN{sub x}As{sub 1-x} using N{sup +}-implantation followed by a combination of pulsed laser melting and rapid thermal annealing. Thin films of GaN{sub x}As{sub 1-x} with x as high as 0.016 and an activation efficiency of the implanted N up to 50% have been synthesized with structural and optical properties comparable to films grown by epitaxial deposition techniques with similar substitutional N content. The effects of N{sup +} implantation dose, laser energy fluence and rapid thermal annealing temperature on the N incorporation as well as optical and structural properties of the GaN{sub x}As{sub 1-x} films are discussed.

One method for producing CuInSe 2 (CISe) absorber layers is electrodeposition followed by annealing. Replacing the commonly used furnace annealing step with a laser can reduce annealing times by 2–3 orders of magnitude: from 30 minutes to 1 s. However, laser processing has, to date, not resulted in absorber layers which can form functioning final devices. One reason is due to Se loss during annealing even on these short timescales. We show how this Se loss is reduced by using a background partial pressure of Se (P Se ) during annealing. Higher P Se results in increased grain size and drastically increased photoluminescence yield. The introduction of an elevated P Se in the laser annealing chamber enabled the fabrication of the first known CuInSe 2 photovoltaic device using electrodeposition followed by laser annealing which gave 1.6% efficiency.

Pulsed UV laser treatments have recently been applied to polycrystalline CdTe solar cells to create an ohmic back contact. In this work, we investigate the surface stoichiometry variations produced by pulsed laser excitation using X-ray photoelectron spectroscopy (XPS). These results reveal surfaces that are becoming more Cd-rich, counter to what is expected from calculations and what has been measured by others. To understand this apparent disagreement, we have constructed a geometric model of the sample surface considering surface topography and a segregation of phases at the surface. From this, we infer that the laser treatments do produce a Te enrichment as expected, but that this enrichment is localized near the grain boundaries. Furthermore, the apparent Cd-enrichment occurs because the Te-rich region's contribution to the XPS intensity is lessened by topographic and surface sensitivity effects as well as a sub-monolayer of Cd found elsewhere on the film surface.

As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10 000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 A of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 A thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.

We describe how the techniques of X-ray reflectivity (XRR), electron spectroscopy for chemical analysis (ESCA), and atomic force microscopy (AFM) can be used to obtain the structural parameters-thickness, coverage, and topography-of thin films used on magnetic recording disks. We focus on ultra-thin amorphous nitrogenated carbon (CNx) overcoats on disks. Each technique has its own strengths: XRR measures film thickness absolutely, ESCA determines the chemical composition of the films, and AFM maps topography accurately. For the CNx overcoats investigated, we find incomplete coverage for thicknesses less than 20 /spl Aring/, and we find a small surface roughness with rms roughness /spl les/11 /spl Aring/.

Presented here are the results of a three dimensional, finite element simulation that models pulsed, ultraviolet (UV) laser annealing of polycrystalline CdTe. The model considers heat generated by the absorption of a 25 ns, 248 nm laser pulse normally incident to a 5 μm thick CdTe thin film deposited on a polycrystalline alumina substrate. In particular, focus is on the spatial and temporal distribution of temperature from laser fluences that achieve a sub-melting condition. The model shows that there are very large temperature gradients both in depth and in-plane directions. These predictions, as well as the onset of melting, are confirmed with cross sectional scanning electron microscopy. Additionally, the model predicts that the heat generated dissipates rapidly after the pulse has ended. This has implications if pulse trains are to be used experimentally.

For the production of high efficiency thin film, Cu(In,Ga)Se2 solar cells, absorber layers with grain sizes of a few hundred nanometers and without detrimental secondary phases are favored. Co-electrodeposition offers a low-cost and material efficient synthesis route, where, in a single step, films containing CuInSe2 are formed. However, the material is nanocrystalline, constitutes multiple phases and has poor photovoltaic properties 1. Therefore a subsequent annealing step is required to produce absorber layers suitable for use in photovoltaic devices. Laser annealing has been demonstrated to improve crystallinity, stimulate atomic diffusion and develop opto-electronic properties when compared to the precursor 2. In this work, high resolution X-ray diffraction was used in order to assess the presence of secondary phases in the absorber layer. All diffractograms of laser annealed films exhibited an additional, unknown peak, measurable through the full depth of the material which is independent of precursor composition, annealing time or laser flux. Evaluation of literature on codeposited CuInSe2, combined with Rietveld refinement suggests a number of possible identities for this peak. The candidates in order of most likely to least likely are structural defects, In2Se3, and CuIn3Se5. We consider the impact that each of these would have on a device formed via this process and thus its success as a new manufacturing route for CuInSe2 solar cells.

Cu 2 ZnSnS 4 (CZTS) may exhibit both kesterite and stannite polymorphs and shows promise as an absorber layer in thin film photovoltaic solar cells to be produced at terawatt scales. This study examines the effects of CZTS polymorphism and inhomogeneous distributions of CZTS polymorphs on device characteristics under scenarios of single phase films, a sinusoidal variation between kesterite and stannite with depth, and single phase films with thin layers of the other polymorph at both interfaces. In general, stannite-only devices are predicted to have higher efficiency than kesterite-only devices and sinusoidal grading results in efficiency between those of the pure phases. However, the device performance is relatively insensitive to the wavelength of the sinusoidal grading and rather is very sensitive to the phase present at the CdS interface. Predicted AM1.5 current-voltage (J-V) curves and descriptive metrics as well as wavelength-resolved quantum efficiencies are reported for all models. Based on these results, we propose control of cation ordering in CZTSSe as a mechanism for device design using bandgap grading and interface engineering without variation of stoichiometry.

The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while ...

The high performance of polycrystalline CdTethin film solar cells is enabled by annealing in the presence of Cl. This process is typically carried out for tens of minutes resulting in reduction of defect states within the bandgap among other beneficial effects. In this work, we investigate laser annealing as a means of rapidly annealing CdTe using a continuous wave sub-bandgap 1064 nm laser. The partial transmission of the beam allows us to monitor the annealing process in-situ and in real time. We find that optoelectronic and structural changes occur through two distinct kinetic processes resulting in the removal of deep defects and twinned regions, respectively. A multilayer optical model including surface roughness is used to interpret both the in-situ transmission as well as ex-situreflectivitymeasurements. These experiments demonstrate beneficial material changes resulting from sub-bandgap laser-driven CdCl2 treatment of CdTe in minutes, which is an important step towards accelerating the processing of the CdTe absorber layer.

Cu(In,Ga)Se2 (CIGS) thin film photovoltaic absorber layers are primarily synthesized by vacuum based techniques at industrial scale. In this work, we investigate non-vacuum film synthesis by electrochemical deposition coupled with pulsed laser annealing (PLA) and or continuous wave laser annealing (CWLA) using 1064 nm laser. PLA results indicate that at high fluence (≥100 mJ/cm2) CuInSe2 films melt and dewet on both Mo and Cu substrates. In the submelt PLA regime (≤70 mJ/cm2) no change in XRD results is recorded. However CWLA at 50 W/cm2 for up to 45 s does not result in melting or dewetting of the film. XRD and Raman data indicate more than 80% reduction in full width at half maximum (FWHM) in their respective main peaks for annealing time of 15 s or more. No other secondary phases are observed in XRD or Raman spectrum. These results might help us in setting up the foundation for processing CIGS through an entirely non-vacuum process.

We investigate the light-trapping effects of dielectric nanoparticles embedded within the active semiconductor layer of a thin-film solar cell. The baseline model consists of a 1.0 μm slab of crystalline silicon on an aluminum back contact topped with a 75 nm Si 3 N 4 anti-reflective coating. Using finite-difference time-domain (FDTD) simulations, we calculate the absorption gain due to a periodic array of SiO 2 nanospheres with characteristic depth, diameter, and pitch. Under optimal conditions, spectrally integrated absorption gain due to embedded spheres can reach as high as 23.4 % relative to the baseline geometry. Using a geometry with an Au-core and SiO 2 shell, it is even possible to reach 30 % after accounting for Ohmic losses. We also discuss the trade-offs between broadband scattering efficiency, poor absorption at long-wavelengths, and semiconductor displacement due to the embedded nanospheres.

Electron backscatter diffraction (EBSD) has been used to characterize the grain size, grain boundary structure, and texture of sputtered CdTe at varying deposition pressures before and after CdCl 2 treatment in order to correlate performance with film microstructure. It is known that twin boundaries may have different electrical properties than high-angle grain boundaries and in this work we have included the effects of twin boundaries. We found better correlation of solar cell device performance to the twin-corrected grain size than to the standard grain size. In addition, we have correlated the photoluminescence (PL) spectra with device performance and with the EBSD results. We find that sputtering at 18 mTorr yields the highest efficiency, largest twin-corrected grain size and the strongest PL.

In this report we investigate the effects of pulsed laser annealing (PLA) on both as-electrodeposited (ED) and electrodeposited-furnace annealed (EDA) CuInSe 2 (CIS) samples by varying the laser fluence (J/cm2) and number of pulses. Results for as-ED samples indicate that liquid CIS-phase formation during PLA with 248 nm laser is to be avoided as liquid CIS dewets on Mo [1] as well as MoSe 2 . In the case of EDA-PLA samples, scanning electron microscopy (SEM) images suggest no apparent change in surface morphology but photoluminescence (PL) indicates change in PL yield and FWHM after PLA processing, a possible indication of annealing of defect states. The effects of PLA on defects are further explored using deep level transient spectroscopy (DLTS).

We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) polycrystalline thin films using cosputtering from binary sulfide targets followed by annealing in sulfur vapor at 500 {\deg}C to 650 {\deg}C. The films are the kesterite CZTS phase as indicated by x-ray diffraction, Raman scattering, and optical absorption measurements. The films exhibit (112) fiber texture and preferred low-angle and Sigma3 grain boundary populations which have been demonstrated to reduce recombination in Cu(In,Ga)Se2 and CdTe films. The grain growth kinetics are investigated as functions of temperature and the addition of Na. Significantly, lateral grain sizes above 1 um are demonstrated for samples grown on Na-free glass,demonstrating the feasibility for CZTS growth on substrates other than soda lime glass.

Copper zinc tin sulfide (CZTS) is a promising Earth-abundant thin-film solar cell material; it has an appropriate band gap of ∼1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu 2 SnS 3 , which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.