Growth of centimeter-scale atomically thin MoS2 films by pulsed laser deposition

We are reporting the growth of single layer and few-layer MoS2 films on single crystal sapphire substrates using a pulsed-laser deposition technique. A pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns) was used to ablate a polycrystalline MoS2 target. The material thus ablated was deposited on a single crystal sapphire (0001) substrate kept at 700 °C in an ambient vacuum of 10−6 Torr. Detailed characterization of the films was performed using atomic force microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence (PL) measurements. The ablation of the MoS2 target by 50 laser pulses (energy density: 1.5 J/cm2) was found to result in the formation of a monolayer of MoS2 as shown by AFM results. In the Raman spectrum, A1g and E12g peaks were observed at 404.6 cm−1 and 384.5 cm−1 with a spacing of 20.1 cm−1, confirming the monolayer thickness of the film. The UV-Vis absorption spectrum exhibited two exciton absorption bands at 672 nm (1.85 eV) and 615 nm (2.02 eV), with an energy split of 0.17 eV, which is in excellent agreement with the theoretically predicted value of 0.15 eV. The monolayer MoS2 exhibited a PL peak at 1.85 eV confirming the direct nature of the band-gap. By varying the number of laser pulses, bi-layer, tri-layer, and few-layer MoS2 films were prepared. It was found that as the number of monolayers (n) in the MoS2 films increases, the spacing between the A1g and E12g Raman peaks (Δf) increases following an empirical relation, Δ f = 26 . 45 − 15 . 42 1 + 1 . 44 n 0 . 9 cm − 1

Two step growth mechanism of Cu2ZnSnS4 thin films

Cu2ZnSnS4 (CZTS) semiconductor is rapidly emerging as the best absorber layer for next generation solar photovoltaics. Its cost effectiveness, environment-friendly nature, wide presence of chemical constituents in nature and high absorption coefficient with suitable energy band gap for effective utilization of solar spectrum makes it a viable alternative. The present work summarizes the preparation of CZTS films through a two-step process consisting of co-sputtered metallic precursors on glass substrates kept at 230 degrees C Followed by sulfurization for 2 h in the ambience of elemental sulfur vapor at different temperatures ranging from 300 to 550 degrees C. The X-ray diffraction (XRD) and Raman analysis make it explicit that the sulfurization temperature has significant impact on reaction mechanism resulting in various hi-metallic, mono and binary metal sulfides. The diffraction pattern noticed at 500 degrees C corresponding to (112), (220) and (312) planes confirms the single phase CZTS as evidenced by weak and strong Raman modes at 285, 337 and 352 cm(-1), The transmittance and reflectance measurements of optimized CZTS films revealed that the films have an energy band gap of similar to 1.56 eV. The optimized films were characterized by scanning electron microscopy (SEM) attached with EDS to know the morphological features and elemental quantification. The single phase CZTS films have exhibited p-type conductivity with sheet resistance similar to 6.8 x 10(3) Omega/sq., carrier concentration similar to 9.1 x 10(17) cm(-3) and hole mobility similar to 16.6 cm(2)V(-1) s(-1)

Influence of sulfurization temperature on physical properties of Cu2ZnSnS4 thin films

Copper Zinc Tin Sulfide (Cu2ZnSnS4 or CZTS) is gaining much attention recently as a potential light absorber alternative to CuInGaSe2 due to its suitable energy band gap similar to 1.5 eV with p-type conductivity, high optical absorption coefficient of similar to 10(5) cm(-1). Moreover, all its constituents are abundant in the crust of the earth and environmentally harmless. In the present investigation, CZTS thin films were prepared using simple two step process of, sulfurization of sequentially sputtered stack, Glass/Zn/Sn/Cu (hereafter CTZ) metallic precursors on soda lime glass substrate held at temperature 200 degrees C. The sputter power was optimized individually for Zn, Sn, and Cu layers. The sputtered CTZ precursors were annealed at different temperatures in the range, 300-550 degrees C with an increment of 50 degrees C for 2 h in the ambience of vaporized elemental sulfur. The XRD pattern revealed that the films sulfurized in the temperature range 300-400 degrees C showed various spurious (binary and ternary) phases and the films sulfurized at 450 degrees C exhibited a clear phase corresponding to CZTS that becomes predominant at 500 degrees C. The optimized (500 degrees C) CZTS thin films showed kesterite structure with (112) preferred orientation. The sharp Raman shift centered at 336 cm(-1) confirms the single phase CZTS for the precursors sulfurized at 500 degrees C. From the transmittance measurements, the energy gap is found to be 1.62 eV for optimized CZTS films. The optical profilometer studies indicated an increase in the surface roughness with the sulfurization temperature. AFM measurements revealed compact morphology with pyramidal texture

Effect of metal layer stacking order on the growth of Cu(2)ZnSnS(4)thin films

In this paper we have presented an in-depth study of effect of metallic precursor stacking order on the growth of the Cu2ZnSnS4 (CZTS) thin films. The CZTS films were prepared by employing a two-step process comprising of sequential sputtering of the metal precursors followed by sulfurization. An optimized stacking sequence as well as growth mechanism for obtaining the single phase CZTS has been proposed based on the results of XRD, Raman, XPS, UV-Vis and electrical studies. A combination of Raman analysis and XPS has been carried out to confirm the CZTS phase formation and to detect any minor phases, if present. The occurrence of Raman modes at around 286 and 336 cm(-1) for the Zn/Cu/Sn/Cu stack sulfurized at 500 degrees C indicated the existence of prominent Kesterite CZTS phase. The perfect homogeneous mixing of sequential precursors together with the elemental sulfur was observed in the case of sulfurized stack order of Zn/Cu/Sn/Cu, which yielded single phase CZTS films, and further confirmed by high resolution core level XPS measurements. Stack dependent micro structural features and elemental analysis were also carried out using FESEM attached to EDS. The p-type charge carriers as detected using hot-probe measurement technique and the band-gap energy of similar to 1.52 eV as estimated from the absorbance spectrum, suggested that the Zn/Cu/Sn/Cu stack order is most appropriate for realizing single phase CZTS growth using two step method

Effect of metal layer stacking order on the growth of Cu(2)ZnSnS(4)thin films

In this paper we have presented an in-depth study of effect of metallic precursor stacking order on the growth of the Cu2ZnSnS4 (CZTS) thin films. The CZTS films were prepared by employing a two-step process comprising of sequential sputtering of the metal precursors followed by sulfurization. An optimized stacking sequence as well as growth mechanism for obtaining the single phase CZTS has been proposed based on the results of XRD, Raman, XPS, UV-Vis and electrical studies. A combination of Raman analysis and XPS has been carried out to confirm the CZTS phase formation and to detect any minor phases, if present. The occurrence of Raman modes at around 286 and 336 cm(-1) for the Zn/Cu/Sn/Cu stack sulfurized at 500 degrees C indicated the existence of prominent Kesterite CZTS phase. The perfect homogeneous mixing of sequential precursors together with the elemental sulfur was observed in the case of sulfurized stack order of Zn/Cu/Sn/Cu, which yielded single phase CZTS films, and further confirmed by high resolution core level XPS measurements. Stack dependent micro structural features and elemental analysis were also carried out using FESEM attached to EDS. The p-type charge carriers as detected using hot-probe measurement technique and the band-gap energy of similar to 1.52 eV as estimated from the absorbance spectrum, suggested that the Zn/Cu/Sn/Cu stack order is most appropriate for realizing single phase CZTS growth using two step method

Impact of selenization pressure on the micro-structural properties of Cu2ZnSnSe4 thin films

The Cu2ZnSnSe4(CZTSe) thin films were prepared by a two-step process consisting of high vacuum sequential evaporation of precursors stack (Sn/Se/ZnSe/Se/Cu/Se) in 4-folds followed by selenization at 350 degrees C in tubular furnace under varied argon gas pressure from 1 mbar to 600 mbar with an interval of 150 mbar. The Cu/(Zn + Sn) and Zn/Sn ratios found to vary from 1.22 to 0.93 and 1.94 to 1.08 with an increase in pressure and the stacks selenized for 600 mbar showed nearly stoichiometric composition with slight Cu-poor and Zn-rich values required for CZTSe growth. The X-ray diffraction studies revealed similar diffraction pattern with a preferred orientation along (112) plane, indicating the formation of kesterite-type CZTSe for all the selenization pressures. Raman spectra recorded using different excitation wavelength sources (785, 532 and 458 nm), revealed two main peaks at 192 and 172 cm(-1) and two supplementary weak peaks at 82 and 232 cm-1 corresponding to kesterite-ordered CZTSe phase for films selenized at a pressure of 600 mbar. Appreciable changes in morphology have been noticed with increase in selenization pressure from low dense irregular rod like morphology to compact spherical grain morphology. All the samples showed high absorption coefficient (>10(4)cm(-1)). A slight variation in optical band gap from 0.90 to 1.01 eV was found with increase in selenization pressure. The Hall effect measurements reveal that all the films are p-type conductive. The precursor stack films selenized at 600 mbar exhibit high mobility of 7.88 cm(2)(Vs)(-1) with lower carrier concentration of 2.54 x 10(19) cm(-3) and resistivity of an order of 10(-2) Omega cm, respectively. (C) 2017 Elsevier Ltd. All rights reserved

Low temperature crystallization of Cu2ZnSnSe4 thin films using binary selenide precursors

In the present paper, a novel process for synthesis of Cu2ZnSnSe4 thin films via low temperature selenization (350 A degrees C) of multiple stacks of binary selenides has been reported. Further, the influence of selenization temperature (250-450 A degrees C) on the physical properties of Cu2ZnSnSe4 thin films was studied and discussed herein. The Rietveld refinement from X-ray diffraction data of Cu2ZnSnSe4 films grown at a selenization temperature of 350 A degrees C was found to be single phase with kesterite type crystal structure and having lattice parameters a = 5.695 , c = 11.334 . Raman spectra recorded using multi excitation wavelength sources under non-resonant and near resonant conditions confirms the formation of single phase Cu2ZnSnSe4 films. Secondary ion mass spectroscopic (SIMS) analysis demonstrated that composition of elements across the thickness is fairly uniform. Energy dispersive X-ray analysis measurement reveals that the obtained films are Cu-poor and Zn-rich. The scanning electron micrographs of binary selenide stacks selenized at a temperature of 350 A degrees C shows randomly oriented cylindrical grains. The optical absorption studies indicated a direct band gap of 1.01 eV. The films showed p-type conductivity with electrical resistivity of 4.66 a"broken vertical bar cm, Hall mobility of 15.17 cm(2) (Vs)(-1) and carrier concentration of 8.82 x 10(16) cm(-3)