Effect of laser fluence on structural and optical properties of CuxS films grown by pulsed laser deposition at different wavelengths

CuxS thin films were grown onto soda-lime glass substrates by pulsed laser deposition at two different wavelengths: 1064 and 532 nm. X-ray diffraction, Raman and UV–vis spectroscopies were used to characterize the CuxS films. Results are presented as a function of laser fluence. XRD patterns indicate that covellite and chalcocite phases were obtained. Raman spectra showed that chalcocite is the predominant phase in the crystalline samples. Optical band gap values are between 2.29 and 2.74 eV for ablation with 1064 nm wavelength; meanwhile, for 532 nm band gap values varied from 2.24 to 2.66 eV; which are in the range of expected values for CuxS films. At 1064 nm and 4.4 J cm−2 sample presented the highest optical absorbance in the visible range, which corresponds to the highest thickness. These are the best growth parameters for CuxS films in order to be used as absorber films for solar cells applications.


Introduction
Semiconductors have been considered promising materials with a wide application range in science and technology. Within them, copper sulfide is an interesting p-type semiconductor material which has near ideal solar control characteristics: transmittance in the infrared range and less than 10% reflectance in the visible range [1]. Variations in x (1x2) can provide different crystalline phases depending on the temperature, producing significant variation in the film's properties [2]. At room temperature, Cu x S has five stable phases: (CuS) covellite, anilite (Cu 1.7 S), digenite (Cu 1.8 S), djurteite (Cu 1.95 S) and chalcocite (Cu 2 S) [1,2]. It is a material which has applications in fields such as solar cells, electroconductive coatings, electrodes and it is also used in catalysis [3]. The optical band gap of Cu x S varies in the range of 1.2-2.5 eV. On the other hand, electrical conductivity can vary from 0.07 to 2400 Ω cm −1 as x changes from 2 to 1.8 [1,2].
Several methods have been used for growing Cu x S films, such as chemical bath deposition (CBD), spray pyrolysis, atomic layer deposition (ALD), chemical vapor deposition (CVD), vacuum evaporation, among others [3,4]. In order to improve and control some stoichiometric phases, several physical techniques have been used. However, stoichiometry transfer from the target to the substrate is difficult to obtain with evaporation or magnetron sputtering using a single target. Pulsed laser deposition (PLD) is an alternative deposition technique to keep this property required for the growth of complex systems [5].
Some of the advantages that make PLD an important technique are: (a) it has been used to fabricate high quality thin films of materials with a complex stoichiometry, or multilayered structures from a target with similar composition [6,7]; (b) some process parameters such as background pressure, type of background gas, type of substrate and substrate temperature can be separated from the laser-target interaction; (c) the number of particles arriving to the substrate can be controlled by the number of pulses and the fluence [7]. In addition, a lot of variables can be manipulated in order to improve the properties of the films in the PLD technique, such as Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
background gas pressure, substrate temperature and laser incident energy. It is well-known that laser incident energy could change morphology, microstructure and promote formation of specific crystalline phases [8].
To the authors knowledge, copper sulfide films grown by pulsed laser deposition have not been reported. For that reason, the main objective in this work is to evaluate the effect of laser fluence on structural and optical properties of Cu x S films grown onto soda-lime glass substrates by PLD, changing the wavelength and the distance between the target and lens. Results are discussed as a function of the laser fluence.

Materials and methods
Growth of Cu x S films Copper sulfide films were grown onto soda-lime glass substrates with a size of 1.5×2.0 cm 2 by PLD in a vacuum chamber evacuated to a base pressure of 2.6×10 -4 Torr, using a CuS (covellite) solid target. The ablation was induced by a Nd:YAG laser beam with two different wavelengths 1064 and 532 nm, a pulse width and frequency of 5 ns and 10 Hz, respectively. The growth time was 300 s and the distance between substrate and target-material was 2 cm. The fluence was varied from 0.7 to 13.6 J cm −2 at 1064 nm. At 532 nm the laser fluence varied from 0.6 to 13.1 J cm −2 by changing the spot size. To investigate the influence of the laser fluence on the properties of the films, the position of the target surface relative to the laser focus was adjusted by moving the lens. Spot size was changed because of the variation in the distance between lens and target, as shown in table 1 Characterization X-ray Diffraction patterns were obtained in a Siemens D5000 Diffractometer, using the Cu-K µ line (1.5406 Å). Raman spectroscopy measurements were carried out in a Thermo Scientific Raman microscopy DXR2, employing a 532 nm laser line as excitation source. An UV-vis Perkin Elmer Lambda 25 spectrophotometer was used to obtain the optical transmittance spectra. Finally, film thicknesses were measured with a KLA Tencor D-100 profilometer. Energy laser fluence was measured with the Handheld Laser Power Meter-Nova II.
Results and discussion XRD analysis XRD patterns for Cu x S films grown by PLD with wavelengths of 1064 and 532 nm are shown in figures 1 and 2, respectively. The diffractogram of the sample grown at 0.7 J cm −2 showed in figure 1, presents two peaks centered at 26.49°and 21.87°The highest intensity peak at 2q=26.49°corresponds to Cu 2 S hexagonal phase (PDF 26-1116) and the diffraction plane is (002); the peak at 21.87°is related to S (PDF No. 08-0247) with (220) diffraction plane [9]. The diffractogram of the sample grown at 4.4 J cm −2 , shows eight peaks. The dominant peak at 2q=26.49°corresponds to the Chalcocite (Cu 2 S) and the (002) diffraction plane, as well as the peak at 23.08°, is associated to S and the diffraction plane is (222); the peaks at 29.27, 31.78, 47.94, 52.71 and 59.34°, are related to covellite (CuS) hexagonal phase (PDF No. 06-0464) [10,11]. The crystallinity for samples at 8.7 and 13.6 J cm −2 is missed out, as diffraction patterns show. At 0.7 J cm −2 fluence, sample is mostly Cu 2 S phase with a small quantity of elemental sulfur. On the other hand, when laser fluence is increased, some peaks of covellite phase are present; however, the highest peak is still corresponding to chalcocite hexagonal phase. The increase in the laser fluence causes an increase in plasma density and kinetic energy. Mobility of deposited atoms are supposed to be improved with highest laser fluences; however, when this fluence is too high, the crystallinity is degraded due to the bombardment of the surface with high kinetic energy species, this is a particular behavior that has been reported for other materials [8]. X-ray diffraction patterns of Cu x S films grown with a wavelength of 532 nm show that they present lower crystallinity than the samples grown at 1064 nm as it is shown in figure 2. Moreover, when fluence is increased, a sulfur-impurity related signal appeared at 2 q=21.87°(PDF No. 08-0247). At 0.6 J cm −2 the peak at 23.08°c orresponds to S and the diffraction plane is (222); there are some small diffraction peaks at 32.85 and 47.94 degrees, which correspond to covellite phase. A mixture of amorphous and crystalline phases is favored by 532 nm wavelength ablation, as XRD patterns indicate [12]. A proper laser energy incident is acquired to vaporize the target and the kinetic energy in the plume can be enhanced, in this way the crystallinity could be improved, as well [8,13].   authors have reported a smaller band around 267 cm −1 attributed to lattice vibrations of covellite [14]. The intensity of the 470 cm −1 Raman mode in the sample grown at 8.7 J cm −2 , is lower than the signal corresponding to the films grown at low fluence and the peak is not well defined, which could indicate a poor crystallinity. Finally, the 13.6 J cm −2 sample does not present any signal, this means that an amorphous material was grown, which is in agreement with the data obtained from XRD measurements. According to XRD patterns and Raman spectra for samples grown using 1064 nm, lower laser fluence values favor the growth of crystalline materials, being chalcocite the predominant phase.

Raman spectroscopy analysis
Raman spectra of Cu x S films grown by PLD at different laser fluences for 532 nm are shown in figure 4. A Raman signal centered at 450 cm −1 can be observed and attributed to a n S-S vibration, which is different from octasulfur (471 cm −1 ) or covellite at 474 cm −1 signals [14]. As it was mentioned before the peak around 470 cm −1 has been identified to chalcocite phase, more specifically to S-S stretching mode of S 2 ions at the 4e sites [10]. The vibrational mode observed at 482 cm −1 in the Raman spectra is assigned to the S−S stretching vibrations in Cu x S copper-deficient sulfides with x=1.12 [15]. Raman spectra confirms XRD results, the ablation with 532 nm wavelength induces multiple phases of CuS.   Figures 5 and 6 show the optical transmittance of Cu x S films deposited using 1064 and 532 nm wavelengths, respectively, in the electromagnetic spectral region of 300-1000 nm. In the samples grown at 1064 nm, it can be noticed that optical transmittance increases for increasing laser fluence, however, sample grown at 4.4 J cm −2 presents the highest absorbance in the visible range (400-700 nm). for Cu x S films at 532 nm, sample at 0.6 J cm −2 presents the highest light absorbance in the visible range. When laser fluence is increased, a shoulder in the blue region appear due to quantum confinement effects in CuS nanomaterial [16]; this effect is more evident at 4.3 and 13.1 J cm −2 with a wavelength of 532 nm, in which there is a shoulder in the blue region and a slightly blue-shift due to quantum confinement effects of covellite or the free-carrier intra-band absorption [17][18][19].

Optical properties
The absorption coefficient (α) was calculated from equation  [16,18]. In fact, these values are similar to the reported for amorphous CuS thin films; most of the Cu x S films grown in the present work, show the same behavior as XRD  patterns indicate [20]. It is observed that the thickness does not follow a particular trend, the highest thickness is obtained at 1064 nm and 4.4 J cm −2 in which the lowest transmittance was observed. It is known that the optical properties of a semiconductor depends on the crystallinity [16] and this sample has a highest crystallinity as XRD   and Raman results show. Moreover, the increase in the band gap could be related to a change in the composition [18]. However, the smallest band gap value (2.24 eV) corresponds to the sample grown at 532 nm and 8.4 J cm −2 . It is known that the optical properties of semiconductors depend on their crystallite size, moreover, a variation on the band gap may be related to a change in stoichiometry [18], as it can be seen in

Conclusions
Copper sulfide films were grown onto soda-lime glass substrates by pulsed laser deposition with incident wavelengths of 1064 and 532 nm and varying the laser fluence. According to XRD patterns, at 1064 nm and low fluence values, Cu 2 S preferential phase films with high crystallinity and Cu 2 S preferential phase are obtained. At 532 nm films, lower crystallinity was observed as compared for samples grown at 1064 nm. Furthermore, a mixture of amorphous and crystalline phases was obtained. Raman spectra showed the highest Raman mode corresponding to chalcocite phase at 0.7 and 4.4 J cm −2 for 1064 nm. The 532 nm wavelength promotes multiple phases of CuS. As XRD and Raman analysis demonstrated, the crystallinity is decreased when laser fluence is increased for both wavelengths. A low laser energy beam causes a crystallinity improvement. Energy band gap values were between 2.22 to 2.49 eV. Sample grown at 1064 nm and 4.4 J cm −2 presented the highest optical absorbance in the visible range, which corresponds to the highest thickness. At these growth parameters, Cu x S films present the best conditions to be used as absorber films for solar cells applications.