Structural disordering studies of Cu6PS5I-based thin films deposited by magnetron sputtering

Cu6PS5I-based thin films were deposited onto silicate glass substrates by magnetron sputtering. With Cu content increase, a red shift of the optical transmission spectra as well as increase of the total electric conductivity are observed. A typical Urbach bundle are revealed, the temperature behaviour of the Urbach absorption edge in thin films are explained by strong electron-phonon interaction. Temperature dependences of the absorption edge energy position and the Urbach energy for thin film are well described in Einstein model. The influence of different type of disordering on the Urbach tail is studied.


Introduction
Cu 6 PS 5 I superionic conductors belong to the compounds with argyrodite-type structure, characterized by high ionic conductivity and intrinsic structural disordering [1].Due to the remarkable physical properties Cu 6 PS 5 I are promising materials for creating renewable energy sources, electrochemical and optical sensors.During the last decades the optical properties of Cu 6 РS 5 I crystals have been studied in detail [2].However, the studies of the optical properties of Cu 6 РS 5 I thin films only begins.For the first time Cu 6 PS 5 I thin films were deposited and studied in Ref. [3].It should be noted that the thin films based on superionic conductors can be applied to the production of supercapacitors of new generation [4].
Therefore, investigation of physical properties of Cu 6 PS 5 I-based thin films is of great interest for argyrodite-type superionic conductors.In the present paper, we study the electrical conductivity and temperature behaviour of optical absorption edge of Cu 6 PS 5 I-based thin films deposited by magnetron sputtering.

Experimental
For Cu 6 PS 5 I-based thin films sputtering we used the codeposition technique from two tilted magnetrons, one equipped with Cu 6 PS 5 I target (pressed powder) and second with pure Cu target.The deposition was carried out at room temperature in Ar atmosphere.For target preparing the microcrystalline powder with the average grain size of 50 Pm was obtained by grinding of the synthesized material in an agate mortar.
Energy-dispersive X-ray spectroscopy was used to ensure the thin films chemical composition.It is shown that thin films in interval from Cu 6.31 P 1.10 S 4.68 I 0.91 to Cu 7.20 P 0.83 S 4.14 I 0.83 are enriched by copper and phophorous (at low concentrations of Cu) atoms, and deficient in phosphorous (at high concentrations of Cu), sulphur and halogen atoms, besides with copper content increase the content of phosphorous, sulphur and halogen decrease.
Electrical conductivity was measured by impedancemeter at frequency 1 MHz.Optical transmission spectra were studied in the interval of temperatures 77-300 K by an MDR-3 grating monochromator, UTREX cryostat was used for lowtemperature studies.Spectral dependences of the absorption coefficient and dispersion dependences of refractive index of thin films were calculated from the transmission spectra [5].

Results and discussion
Electrical studies have shown that the total electric conductivity of the thin films increase with increase of Cu atoms content.Thus, with Cu content increase in interval from Cu 6.31 P 1.10 S 4.68 I 0.91 to Cu 7.20 P 0.83 S 4.14 I 0.83 the electric conductivity increase from 0.053 S/m to 0.060 S/m, respectively.The high value of electrical conductivity in thin films under investigation make them the promising material for creation of solid state batteries and supercapacitors.
Futhermore, with Cu content increase, a red shift of the optical transmission spectra as well as absorption edge spectra is observed.Temperature variations of optical transmission in Cu 6 PS 5 I-based thin films are similar for all investigated samples (8 samples), but we will illustrate it for Cu 6.35 P 1.12 S 4.57 I 0.95 thin film.Thus, interferential transmission spectra of Cu 6.35 P 1.12 S 4.57 I 0.95 thin film at various temperatures within 77-300 K are shown in Fig. 1.With temperature increase, a red shift of both the short-wavelength part of the transmission spectrum and the interferential maxima is observed.Figure 2 shows that the optical absorption edge spectra in the range of their exponential behaviour in Cu 6.35 P 1.12 S 4.57 I 0.95 thin film are described by the Urbach rule [6] where U E is the Urbach energy, V is the absorption edge steepness parameter,  The temperature behaviour of the Urbach absorption edge in Cu 6.35 P 1.12 S 4.57 I 0.95 thin film is explained by electron-phonon interaction (EPI) which is strong in the film under investigation.The EPI parameters are obtained from the temperature dependence of absorption edge steepness parameter (Fig. 2) using the Mahr formula [7] where p Z ! is the effective phonon energy in a oneoscillator model, describing the electron-phonon interaction (EPI), and 0 V is a parameter related to the EPI constant g as V 0 are given in Table 1).For the Cu 6.35 P 1.12 S 4.57 I 0.95 thin film V 0 < 1 that is the evidence for the strong EPI [8].
It should be noted that in the range of exponential behaviour of optical absorption for their.For spectral characterisation of optical absorption edge we used the energy position of an exponential absorption edge D g E values taken at D=5u10 4 cm -1 (Table 1).It should be noted that at T=300K D g E =2.521 eV, while U E =256 meV.
The temperature dependences of D g E and the Urbach energy U E for Cu 6.35 P 1.12 S 4.57 I 0.95 thin film (Fig. 2) can be described in Einstein model by relations [9,10] ( U  E , and 1 ) ( U E parameters for the thin film are given in Table 1. An essential characteristic of the absorption edge spectra of the thin films under investigation is a lengthy Urbach tail which results in the high values of Urbach energy U E (Table 1).In Ref. [11] it was shown that temperature and structural disordering affect Urbach absorption edge shape, i.e. the Urbach energy U E is described by the equation Table 1.Parameters of Urbach absorption edge and EPI for Cu 6.35 P 1.12 S 4.57 I 0.95 thin film where The dispersion dependences of the refractive index for the Cu 6.35 P 1.12 S 4.57 I 0.95 thin film was obtained from the interference transmission spectra (Fig. 3).With temperature increase the nonlinear increase of refractive index in Cu 6.35 P 1.12 S 4.57 I 0.95 thin film is revealed.Optical transmission spectra of Cu 6.35 P 1.12 S 4.57 I 0.95 thin film was investigated in the temperature interval 77-300 K.With Cu content increase as well as with temperature increase, a red shift of the optical transmission spectra was revealed.Temperature variations of optical transmission in Cu 6 PS 5 I-based thin films are similar for all investigated samples, however it illustrated for Cu 6.35 P 1.12 S 4.57 I 0.95 thin film.It is shown that the optical absorption edge spectra in the range of their exponential behaviour in Cu 6.35 P 1.12 S 4.57 I 0.95 thin film are well described by the Urbach rule.Temperature dependences of the energy position of absorption edge, Urbach energy and refractive index of Cu 6.35 P 1.12 S 4.57 I 0.95 thin film were analysed.The origin of lengthy Urbach tail and role of the different type of disordering were discussed.
convergence point coordinates of the Urbach bundle.The coordinates of the Urbach bundle convergence point D and E 0 for the Cu 6.35 P 1.12 S 4.57 I 0.95 thin film are 1.13×10 6 cm -1 and 3.322 eV.

S
are the energy position of absorption edge at 0 K and a dimensionless constant, respectively; E T is the Einstein temperature, corresponding to the average frequency of phonon excitations of a system of non-coupled oscillators, It is shown that the contribution of static structural disordering into the film Urbach energy equals 95%.Static structural disordering in Cu 6.35 P 1.12 S 4.57 I 0.95 thin film may be additionally increase due to (1) the absence of long-range order in the atomic arrangement and chemical bond breakdown; (2) lower density of the atomic structure packing due to the presence of pores; (3) the transition from the three-dimensional bulk structure to the twodimensional planar structure.

4 Conclusions
Cu 6 PS 5 I-based thin films were deposited onto silicate glass substrates by magnetron sputtering.Electrical conductivity of Cu 6 PS 5 I-based thin films was studied depends on chemical composition, with Cu content increase of the total electric conductivity was increased.