{'fraction_non_alphanumeric': 0.0295021072933781, 'fraction_numerical': 0.018644188870633616, 'mean_word_length': 4.8236272878535775, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Structural and morphological characterization of ternary nanocrystalline Cu-In-S thin films prepared by laser ablation

Thin nanocrystalline Cu-In-S films are prepared by pulsed laser ablation in vacuum with Nd:YAG laser operating at 1064 nm on Si (100) substrates. As a target, a CuInS2 quantum dots powder, synthesized by sonochemical template-free colloidal approach and pressed to form a pellet is used. The structure and morphology of the deposited films were studied employing X-ray diffraction, scanning electron microscopy, and atomic force microscopy techniques. The analysis of the deposited thin films revealed predominant cubic Cu1.7In0.05S phase. It has been found that deposited films were nanocrystalline. The effect of laser pulse energy and deposition time on the structure and morphology of as-prepared films is studied. Slight mean diameter increase from 8 nm to 13 nm with deposition time and applied power was observed, as the time dependence is more pronounced. On the contrary, six fold increase from 7.4 to 44.3 nm of the surface root mean square roughness was estimated with augmentation of the applied laser pulse energy.


Introduction
Members of the ternary Cu-In-S semiconductor system with various compositions and structures play a major role in thin film photovoltaic technology [1]. The ternary compounds from the mentioned system are very similar to the II-IV semiconductors from structural viewpoint, which reflects in the similarity of optoelectronic properties [2], with the advantage of being environmentally friendly. These properties can be controllably tuned if the semiconductors are prepared in low-dimensional form [2][3][4]. Recently, CuInS2 nanocrystals attracted an ever-increasing attention due to their application in quantum dots sensitized solar cells [5,6].
So far, many physical and chemical techniques have been employed for preparing Cu-In-S thin films. Among them, the pulsed laser deposition (PLD) is a simple, but versatile, experimental method that finds use and in extensive areas of thin film deposition and multi-layer research [7,8].
This technique offers the possibility to transfer material to a random substrate with no preliminary treatment (absence of seed layer or molecular linker). Literature data concerning laser ablation as a technique for deposition of thin films composed by QDs are, however, scarce [5,9].
In the context of our efforts towards the development of controllable methods for deposition of lowdimensional semiconductor films, in the present study, we employ this technique as a direct method to deposit thin films from the ternary Cu-In-S semiconductor system starting from a highly nanostructured target material, initially deposited by a template-free, sonochemically-assisted colloidal chemical route. We pay a special emphasis on the structure and surface morphology properties of the synthesized material in thin film form as a function of the laser pulse density and the deposition time.

Sample preparation
As a target for subsequent thin film deposition by the laser ablation technique, powder sample composed of CuInS 2 quantum dots (QDs) pressed in a form of pellet has been used. The QD solid powder was synthesized by our recently developed sonochemically-assisted template-free colloidal chemical technique [10]. Synthesized target material was identified by the X-ray diffraction technique and it was concluded that it was composed by highly nanostructured CuInS2 with cubic crystal structure (zincblende structural type) [10].
Q-Switched Nd:YAG laser source (model -Quanta Ray GCR:3 from Spectra-Physics, Inc.) operating at fundamental wavelength λ = 1064 nm, pulse duration τ = 7 ns was used for target ablation. The depositions were conducted in a stainless steel vacuum chamber which was evacuated down to residual pressure of 10 -4 Torr. The laser beam was incident at 45º on the target surface and the irradiation spot was of 1 mm 2 area. The expulsed material was deposited on the Si (100) substrates placed parallel to the target at a separation distance of 7 cm. Laser power is altered starting from the ablation threshold at fixed distance between the target and substrate, chosen to maximize the surface quality. The laser pulse energies used were 60 mJ, 100 mJ, 300 mJ, and 500 mJ at repetition rate 10 Hz. The deposition time was varied between 10 and 40 min.

Material characterization
The X-ray diffraction method for polycrystalline samples is employed to identify the chemical composition and structural types of the deposited films and also to get much more in-depth insights into their structural characteristics. Grazing Incidence XRD (GIXRD) experiments were performed on a Rigaku SmartLab X-ray thin film diffraction system (Rigaku Corporation, Japan) with CuKα1,2 irradiation (λ=1.5418 Å). The GIXRD patterns realized at an angle of incidence α = 0.35° were registered via continuous method applying 0.01° steps (2θ, θ is the Bragg angle) and a counting speed of 12.000 deg/min. The scattered X-ray radiation was detected with SC-70 detector. All XRD patterns were taken within an angular interval 2θ from 20° to 70° in which the main diffraction peaks of CIS are located. SEM measurements were utilized to complement the XRD studies, e.g. in the course of confirmation of the nanostructured character of the deposited films, as well as to estimate the size distribution of the nanoparticles constituting the films. SEM observations have been made using e-Line EBL equipment in SEM mode (Raith GmbH, Germany), working with accelerating voltage of 10 kV. The Cu-In-S films were observed in plane and on cross-section with magnification from 20 to 150 thousand times. Cross-section SEM was used to determine the film thickness. The grain size was estimated with SPIP tm image processing software version 6.0.9 (Image metrology A/S).
AFM measurements were performed on scanning probe microscope Multimode V (Bruker, ex. Veeco, Santa Barbara, CA). The images were taken in tapping mode, as each sample has been investigated on multiple points of the surface. Measurements in scale of 5 µm have been performed with scanning rate in the interval 0.7-1.0 Hz and images resolution of 512 lines per scan direction. Aluminium coated silicon cantilevers TAP150-Al-G (Budget Sensors Innovative Solutions Bulgaria Ltd., Bulgaria) have been used. All images were just flattened and no further processing was carried out. Roughness measurements were performed using the Nanoscope 7.30 programme.

Structural investigations by X-ray diffraction
XRD patterns of Cu-In-S films obtained at each of the applied laser pulse energies (500, 300, 100, and 60 mJ) are shown in figure 1. The spectra shown are with low intensity; their constituting diffraction bands are obviously broadened, indicating a nanocrystalline character. It can be seen that the spectra intensity differs, as it is quite low for the film, obtained in the vicinity of the ablation threshold even for 40 min deposition time, probably due to ultra-small film thickness. The sharp peak around 50° which appears in some of the patterns is reckoned to belong to c-Si (100)  Although for Cu-In-S crystals the typical stable structure is chalcopyrite CuInS2, a loss of sulphur during the ablation is possible. Deviation from the nominal stoichiometry in terms of S deficiency is reported [8,11] as a consequence of evaporation loss during deposition or annealing in vacuum. The positions of all the peaks in the spectra are slightly shifted to higher 2θ in comparison to referred data, which is likely caused by compressive lattice strain (smaller dhkl values in the case of nanostructured material as compared to the bulk one, as follows from Bragg's law). Some less prominent peaks as well as shoulders of the main peaks are noticed in the spectra. Their reliable indication is embarrassed by the low intensity and quite high noise of the patterns and for trustful results, additional measurements of thicker films are needed. Although the presence of small quantities of CuxS and In2S3 cannot be fully excluded at this stage, more probably there is an additional phase of chalcopyrite stoichiometric CuInS2 (JPDS file 27-0159). The diffraction bands, typical for In are not found-out and, hence, In phase, reported by the others [9] is not revealed.
The most prominent diffraction peak in the patterns, located at 2θ~47 was least-squares fitted with two bands using Voight function. A typical fit is shown in figure 2. The values for θ and full width at half-maximum intensity of the main peak β obtained from the fits were then applied to calculate the average crystal size, using the Scherrer equation and the results are presented in table 1. An increase of quantum dots size with the laser energy was expected, since more CuInS2 plasma should be generated from the target when employed higher laser energy and the CuInS2 plasma continues to grow on the substrate to form quantum dots with larger diameter [5,11,12]. The results listed in table 1 confirmed to some extent our expectation. When compared calculated diameters for the film obtained with    figure 3. With the deposition time, a strong increase in the diffraction peaksintensity and reduction of the FWHM, resulting from the increase of the nanocrystallites size can be seen. In fact, looking at table 1, it is evident that the biggest calculated crystallites mean diameter is for the films deposited with the weakest power -at the ablation threshold, but for the longest time. The bigger grain size in the as-deposited films, despite the assumed from the XRD intensity ultra-small film thickness, implies a low density of nuclei on the substrate surface which may be due to the applied low laser power [13] and hence, the very low deposition rate. Perhaps, the embryo density is power dependent, while the time is crucial for the subsequent growth of the grains. The combination of very low average deposition rate and long deposition time might result in high mobility of ad-atoms on the substrate surface, and hence, in bigger grain size.
Taking into account the fact that nanometer-sized crystals could exhibit an intrinsic lattice strain, caused by a high fraction of surface atoms, the experimental XRD data were also exploited for refinement of unit cell parameters and for estimation of lattice strain. The interplanar distance d hkl was determined using the Bragg equation d{hkl} = λ/2sinθ. The unit cell parameter a in the case of cubic crystal system is calculated using the relation Eq. (1): where (h, k, l are the lattice planes; d{hkl} is the corresponding interplanar distance; for the {220} peak a = √8d{220}. Further on, the average uniform strain is computed, following the equation Eq. (2):  (2) where abulk is the lattice constant value of 5.587 (JCPDS file No 45-1304). From the results listed in the table 1, it can be seen that the calculated lattice constants are quite smaller than the referred one, with the value of ~ 5.53 closer to the parameter a -5.52 of chalcopyrite structure. The high internal stress implies a lattice distortion, which, apart from the expected strain, caused by the size reduction, could also originate from the CuInS2 → Cu1.7In0.05S phase transformation during deposition, combined with the low substrate temperature. Moreover, since CuInS2 compound is slightly non-stoichiometric [8,14] as well as Cu1.7In0.05S is considered [14] as a solid solution based on Cu2S and In2S5, some deviations from the reported bulk units parameters values are not surprising. As can be seen from the table 1, however, the observed trends in lattice strain can be actually related [15] to the crystal size-the smaller size of nanocrystals constituting the films induces larger lattice strain regardless of the film thickness.

Morphology
Information about the grain size has also been obtained from the SEM investigation. SEM surface images of samples deposited with 300 mJ pulse energy are depicted in figure 4(a). It is seen that SEM measurements confirm the nanocrystalline character of the films and that most of the grains have close to spherical shape. The SPIP tm image processing was used to get the statistical information and the results confirm the conclusion based on the XRD data. SEM images in cross section were used to estimate the film thicknesses. A typical image is shown in figure 4 (b). The results have shown film thicknesses up to 100 nm, as in the case with the sample, processed close to the ablation threshold, at 60 mJ, formation of discontinuous layer is not observed. roughness. The root mean square roughness, estimated from AFM images, strongly increases with applied pulse energy from 7.4 nm for 60 mJ, and 22 nm for 100 mJ to 44.3 nm for 300 mJ. The obtained results indicate the suitability of the applied method for deposition of nanostructured CIS films on a random substrate with controllable surface roughness.

Conclusions
Thin nanocrystalline Cu-In-S films are prepared by pulsed laser ablation of CuInS2 quantum dots powder, synthesized by the sonochemical template-free colloidal approach. X-ray diffraction and SEM measurements revealed that deposited films are nanocrystalline with predominant cubic phase and a small grain size (~ 10 nm). Deviation from the nominal stoichiometry in terms of the S deficiency has been noticed. These observations have been related to loss of sulphur during the ablation.
On the basis of analysis of data obtained by XRD and SEM measurements, the evolution of crystal size upon laser pulse energy and time of deposition was followed. Slight mean diameter increase from 8 nm to 13 nm with deposition time and applied power was observed, as the time dependence is more pronounced. It was found, that the embryo density is power dependent, while the time is crucial for the subsequent growth of the grains. AFM investigations revealed a strong anticipated increase of the surface roughness from 7.4 to 44.3 nm with increasing applied laser pulse energy.