Sandip Das

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detectors operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Layered GaTe Crystals for Radiation Detectors

In this work we investigated a new method of growing detector grade large GaTe layered chalcogenide single crystals. GaTe ingots (2″ diameter) were grown by a novel method using graphite crucible by slow crystallization from a melt of high purity (7N) Ga and Te precursors in an argon atmosphere. GaTe samples from the monocrystalline area of the ingot have been cleaved mechanically and characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by x-rays (EDAX), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), transmission line matrix method (TLM), resistivity measurements using van der Pauw technique, Hall Effect, and Capacitance-Voltage measurements. Our investigations reveal high potential for developing superior quality GaTe crystals using this growth technique for growing large volume inexpensive GaTe single crystals for nuclear radiation detectors.

Defect levels in Cu2ZnSn(SxSe1−x)4 solar cells probed by current-mode deep level transient spectroscopy

Defect levels in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have been investigated by current-mode deep level transient spectroscopy. Experiments were carried out on two CZTSSe cells with photoconversion efficiencies of 4.1% and 7.1% measured under AM 1.5 illumination. The absorber layer of the 4.1% efficiency cell was prepared by annealing evaporated ZnS/Cu/Sn stacked precursor under S/Se vapor, while the absorber of the 7.1% efficiency cell was prepared by co-evaporation of the constituent elements. The 4.1% efficiency CZTSSe cell with a S/(S + Se) ratio of 0.58 exhibited two dominant deep acceptor levels at Ev + 0.12 eV, and Ev + 0.32 eV identified as CuZn(-/0) and CuSn(2-/-) antisite defects, respectively. The 7.1% efficiency cell with purely Se composition S/(S + Se) = 0 showed only one shallow level at Ev + 0.03 eV corresponding to Cu-vacancy (VCu). Our results revealed that VCu is the primary defect center in the high-efficiency kesterite solar cell in contrast to the detrimental CuZn and CuSn a...

Cu 2 ZnSnSe 4 Photovoltaic Absorber Grown by Vertical Gradient Freeze Technique

High quality large grain single phase Cu2ZnSnSe4 (CZTSe) photovoltaic absorber material was grown by vertical gradient freeze (VGF) technique for the first time. Polycrystalline CZTSe ingot was grown in a vacuum sealed quartz ampoule inside a modified three-zone vertical Bridgman furnace employing a directional cooling. Structural and compositional analyses of the grown crystals were performed by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The grown crystals exhibited highly crystalline tetragonal structure corresponding to kesterite Cu2ZnSnSe4 with lattice parameters of a = 5.696 A and c = 11.338 A as evidenced from XRD pattern. Raman spectra showed three characteristic peaks at 171.5, 194.6, and 231.1 cm-1 attributed to kesterite phase CZTSe. No other secondary phases were detected in the grown crystals. Thermoelectric probe measurements showed p-type conductivity of the grown crystals and energy dispersive X-ray spectroscopy (EDS) along the crystal growth direction showed uniform and stoichiometric elemental distribution. Our results show that VGF technique can be used to grow high quality kesterite compounds for photovoltaic application.

Comparison of Cu 2 ZnSnS 4 thin film properties prepared by thermal evaporation of elemental metals and binary sulfide sources

Cu2ZnSnS4 (CZTS) thin films have been prepared by thermal evaporation of elemental copper and binary sulfides of zinc and tin. Stacked precursor layers on corning glass substrates were thermally evaporated followed by sulfurization at 560°C under H2S flow to fabricate CZTS films. Six different sequences of stacked layers were studied. Structural, compositional, and morphological characterizations of the resulting films were carried out by x-ray diffraction, scanning electron microscopy, atomic force microscopy and energy dispersive x-ray spectroscopy. Electrical and optical properties were measured via Hall effect, van-der Pauw technique and UV-Vis spectroscopy. Our study showed that the best quality films were obtained with ZnS/Cu/SnS2 stacking order of the precursor layers. Details of film fabrication and comparative study of the film properties are discussed. Further study of all the film properties and their corresponding photovoltaic response is presently under investigation and will be reported.

Surface Passivation of p-GaTe Layered Crystals for Improved p-GaTe/n-InSe Heterojunction Solar Cells

GaTe and GaTe:In single crystals were grown from high purity Ga (7N) and zone refined Te (>7N) precursor materials. InSe thin films were deposited by thermal evaporation onto the sulfur passivated GaTe:In substrates at various substrate temperatures from 450K-550K to fabricate p-GaTe:In/n-InSe heterojunction solar cells. Scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS) were used to characterize GaTe:In crystals and InSe thin film surfaces. The current-voltage characteristics of p-GaTe:In/n-InSe solar cells were measured under dark and under illumination of 75mW/cm 2 . Dark J-V measurements showed that the reverse saturation current density (J 0 ) decreased from 3.8 x 10 -6 A/cm 2 to 1.5 x 10 -9 A/cm 2 and the ideality factor was reduced from 2.04 to 1.15 as a result of surface passivation. Under illumination of 75 mW/cm 2 , the open-circuit voltage (V oc ) increased from 0.54V to 0.68V and short-circuit current density (J sc ) increased from 7.19 mA/cm 2 to 8.65 mA/cm 2 for solar cells with surface passivated GaTe:In substrates, leading to an increased solar cell efficiency of 5.03%. EPMA measurements revealed that the InSe thin films deposited at 550 K on GaTe:In substrates were near stoichiometric with enhanced grain size contributing also to better solar cell performance.

Low-cost Cu 2 ZnSnS 4 thin films for large-area high-efficiency heterojunction solar cells

Cu2ZnSnS4 (CZTS) thin films have been deposited on soda-lime glass (SLG) and Mo-coated SLG substrates using a low-cost spray pyrolysis technique followed by sulfurization under H2S flow at 540°C. Aqueous solution containing CuCl, ZnCl2, SnCl4 and thiourea was used as precursor. Spray deposition was carried out at three different substrate temperatures of 280°C, 360°C and 440°C. Deposition conditions were optimized to obtain the best film properties. Structural, morphological and compositional analysis of the as-deposited and sulfurized CZTS films were carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw and Hall effect measurements. XRD spectra confirmed the formation of kesterite CZTS films. Grown CZTS films showed an absorption coefficient >;104 cm-1 and the bandgaps were found to lie between 1.42-1.72 eV at room temperature depending on deposition conditions and post deposition sulfurization treatments. All films were found to be of p-type conductivity with an average carrier concentration in the order of 1018-1020 cm-3. Optimum quality films were obtained for films fabricated at 360°C and no secondary phases were observed. n-CdS window layer was deposited on p-CZTS films prepared at 360°C substrate temperature to fabricate p-CZTS/n-CdS heterojunction solar cells. The heterojunction exhibited an open-circuit voltage (VOC) of 290 mV and a short-circuit current density (JSC) of 3.1 mA/cm2 under AM 1.5 illumination. Details of CZTS thin film fabrication, processing, and characterization results are presented.

Radiation detectors based on 4H semi-insulating silicon carbide

In this work, radiation detectors were fabricated using 8 mm × 8 mm substrates, ~ 390 μm in thickness, diced from commercial (0001) 4H-SiC semi-insulating wafer (> 109 Ohm-cm). Our characterization results, including x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, thermally stimulated current (TSC) measurements, chemical etching and Raman spectroscopy, show the high quality of the semiinsulating SiC crystals, which are believed to meet the requirements of fabricating high performance radiation detectors. Current-voltage characteristics showed very low leakage current (~ 1.5 pA at -500 V) and the capability of detectors operation up to 200°C.

Fabrication and characterization of improved p-GaTe/n-InSe heterojunction solar cells

InSe thin films were deposited by thermal evaporation of polycrystalline InSe onto the sulfur passivated p-GaTe:In substrates at 450K-550K to fabricate improved p-GaTe:In/n-InSe heterojunction solar cells. Dark J-V measurements showed that the reverse saturation current density decreased from 4 × 10^-6 A/cm² to 2 × 10^-9 A/cm² and the ideality factor was reduced from 2.04 to 1.12 as a result of surface passivation of p-GaTe substrate materials. Under illumination of 75 mW/cm², the open-circuit voltage increased from 0.53 V to 0.62 V and short-circuit current density increased from 7.93 mA/cm² to 10.34 mA/cm² for solar cells with surface passivated p-GaTe:In substrates, leading to an increased solar cell efficiency of ~6.09%.

Low-cost fabrication of improved n-Si/p-AgGaSe2 heterojunction solar cells

The fabrication of low-cost n-Si/p-AgGaSe2 heterojunction solar cells by controlled thermal evaporation method is reported. It is observed that in the case of p-AgGaSe2 films deposited on H-terminated n-Si (H-Si) substrates at higher temperature of 650K, the photovoltaic properties of the n-Si/p-AgGaSe2 junctions are considerably improved. The improved junction, under solar simulator AM1 illumination, demonstrated an efficiency of ~5.2% on an active area of 0.18 cm2 without any antireflection coating whereas the AgGaSe2 films thermally evaporated at room temperature of 300K on H-Si substrates showed the efficiency ~2.1%.