Investigation of the Structural , Optical and Electrical Properties of AgInSe 2 Thin Films

The Silver1Indium1Selenide (AgInSe2) (AIS) thin1films of (3001±20) nm thickness have been1prepared2from the compound alloys2using thermal evaporation2 technique onto the glass2substrate at room temperature, with a deposition rate2(3±0.1) nm2sec-1. The2structural, optical and electrical3properties have been studied3at different annealing3temperatures (Ta=450, 550 and 650) K. The amount3or (concentration) of the elements3(Ag, In, Se) in the prepared alloy3was verified using an energy dispersive3x-ray spectrometer (EDS)3technology. X-ray diffraction3analysis shows that AIS alloy prepared as (powder) and the thin films3are polycrystalline of tetragonal3structure with preferential orientation3(112). The crystalline3size increases as a function3of annealing temperature. The atomic force3microscope (AFM) technique was used to examine3the topography and estimate3the surface roughness, also the average grain3size of the films. The results show3that the grain size increases3with annealing3temperature. The optical4band gap of the films lies4in the range 1.6-1.9 eV. The films4appear to be4n-type indicating that the electrons4as a dominant charge4carrier. The electrical conductivity4increases with a corresponding4increase in annealing4temperature.


Introduction
In recent 4 years, considerable 4 efforts have been 4 made to grow 4 device-quality thin 4 films by optimization of growth 4 parameters for applications 4 in optoelectronic devices.The ternary semiconductor 4 compounds with the formula 4 A I B III C VI 2 such 4 as CuAlS2, AgAlSe2, etc. have been widely 4 investigated because 4 of their potential applications 4 in electro-optic, the chalcopyrite 4 type crystals the most intensive 4 studies have been 4 carried out in the series 4 of I− III−VI2 family compounds 4 where Cu is involved 4 as a group I element, very few 4 works have appeared in literature where the group I element is Ag 4 [1].AIS semiconductors 4 have been produced by several 4 techniques 5 such as: co-evaporation 5 [2,3], ultra-high vacuum 5 pulsed laser deposition [4], horizontal 5 Bridgman method [5], molecular beam 5 epitaxial [6], vertical gradient temperature 5 freezing method [7] and solid 5 state microwave irradiation 5 [8].In this work, we have 5 prepared Ag-In-Se 5 thin films by thermal evaporation 5 technique, followed by an adequate 5 annealing treatment.

Experimental
The alloy 5 of AgAlSe2 was obtained by fusing 5 the mixture of the appropriate amount 5 of the elements Ag, In and Se of high purity (99.999%) in an evacuating fused quartz 5 ampoules with 5 (2×10 -35 Torr), heated at (1200 K) for five 5 hours.The films of 300±20 nm thickness were deposited 5 by the thermal evaporation technique 5 at room temperature 5 using the Edward 5 coating unit model (E 306) with molybdenum 5 boat.Energy 5 dispersive x-ray spectrometer (EDS) was used to investigate 5 the amount or (concentration 5 ) of the elements 5 (Ag, In, Se) of AgInSe2 5 alloy.The crystal 5 structure of the alloy and films was characterized 5 by using X-ray diffraction 5 (XRD) by (Shimadzu6000 5 X-ray Diffraction) with the copper 5 target of the wavelength 5 (λ=1.5406) Ǻ. Lattice constants, crystalline size 5 were specified, the inter planar 5 spacing dhkl (Å) between consecutive 5 parallel planes was measured 5 by Bragg's law 5 [9]: n λ= 2θ dhkl Sin(θ) ………………………………………….(1) Where, n 5 is the order of diffraction and θ 5 is the angle of incidence.The average crystalline 5 size can be estimated


Where, β is the full width 5 at half maximum 5 intensity in radians.(AFM) was employed 5 to investigate the surface morphology 5 of the AgInSe2 films 5 as a device 5 type of (SPM 5 -AAA3000 contact 5 mode spectrometer, Angstrom).Optical measurement 5 has been constructed using 5 UV-Visible 1800 5 spectrophotometer.The thickness (t) of all prepared films was measured 5 by using the weighing 5 method according to the following 5  Where; m, ρ, A were the mass, density and area of the films.Using a sensitive balance 5 whose sensitivity of the order 5 (10 -4 ) gm.The optical 5 absorption spectrum 5 was utilized to define the optical energy 5 gap (Eg opt ) eV using Tauc 5 formula [12]: αhν=B (hν -Eg opt ) 1/r ………………………………………….(4) Where, B is a constant, hν is the photon 5 energy (eV) and r is constant, that it may take values 5 2, 3, 1/2, 3/2 depending on the material 5 and the type of the optical transition 5 .The absorption 5 coefficient (α) value can be computed 5 from the formula [13]: where k represents 5 the extinction coefficient, which is calculated 5

   c d
Hall Effect 5 measurements have been managed 5 by Van der Pauw 5 (Ecopia HMS -3000) to determining majority 5 carrier concentrations, type of carrier 5 and their mobility 5 in thin 5 films.

Result and discussion
Energy 6 dispersive x-ray spectrometer 6 (EDS) used to examine the amount 6 or (concentration) of the elements 6 (Ag, In, Se) in the alloy.The results are shown in figure (1) and table (1). Figure (2) shows the XRD 6 pattern of (AIS) bulk powder, which illustrates 6 that the AgInSe2 was a chalcopyrite material in polycrystalline (tetragonal phase) structure as it is compared 6 with the standard 6 values in ICDD 6 cards.The spectrum 6 is considered to exhibit sharp 6 peaks at (112),( 200), ( 220), ( 204), ( 312), ( 116),( 400), (316), and (424) corresponding to 2θ values equal to 25.72, 29.45, 41.95, 42.92, 49.84, 51.51, 60.72, 68.42 and 77.12 respectively.The X-ray diffraction 6 parameters inter planar spacing (d), Miller 6 indices and crystalline 6 size for AgInSe2 alloy 6 are listed in table (2), they prefer orientation 6 at (112) planes.Whereas from the X-ray diffraction 6 patterns of AgInSe2 thin 6 films with different annealing 6 temperatures, one can observe that the thin films 6 have the polycrystalline 6 tetragonal structure 7 as shown in figure (3).The figure indicates that the patterns 7 include three sharp peaks 7 referred to (112), ( 220) and (312) direction.As well, this figure confirms 7 that the preferential orientation is in the 7 (112) direction.The structural parameters 7 of annealed AgInSe2 thin 7 films with different annealing 7 temperatures were illustrated in table (3).The crystallite 7 size has been estimated 7 of the FWHM value of the (112) peak 7 by using Scherrers equation and is observed, 7 it increased with Ta as shown in table (3).These results were matched with [3,17].By increasing the Ta the locations of the measured diffraction 7 peaks do not change significantly 7 , but the intensities 7 of the peaks increase.This is due to the improvement 7 of crystalline of the films.Figure (4) shows the three 7 -dimensional (3D) of AgInSe2 thin films with different Ta.From this figure, it can be deduced 7 that these films have spherical 7 grains granting 7 the smooth surface 7 morphology.The values of surface roughness 7 and the grain sizes 7 are calculated.It has been 7 observed that a surface 7 roughness were equal to (0.608, 0.936,1.65 and 0.809) for different 7 Ta (RT, 450,550 and 650) K respectively.The grain 8 size has been observed (86.68, 90.49, 106.54 and 68.06) for (RT, 450, 550 and 650) K respectively.Therefore, the average 8 diameter of AIS thin 8 film with annealing temperature 8 550K (see Fig. 4, c) is larger than the AIS thin film (Figure 4, a, b, d).
The absorbance 8 and transmittance 8 spectrum of AIS thin 8 film were evaluated as a function 8 of wavelength at different annealing 8 temperatures as in Figure ( 5).This figure believed that the absorbance 8 increases in the visible wavelength range as a function of annealing 8 temperature.
The behavior of the transmittance spectra 8 is opposite completely 8 to that of the 8 absorbance spectra.From figure (6), we can observe 8 that the α values, which were calculated using equation 8 (4), indeed, own high amount reached 8 above (10 4 ) cm -1 .It was pointed that the α values in general increases 8 as a function of annealing 8 temperature, which is attributed to an increase 8 in absorbance of used films.From table (4) we found 8 that the value of α increases from (5.8-6.6)×10 4 cm -18 with the increase of Ta.The value of Eg opt decreases 8 (from 1.8 to 1.6) eV and then increases to (1.9) eV with the increase of Ta as shown in table ( 4) and figure ( 6).The decrease 8 in the band gap 8 (Eg opt ) values may be describable of the increase 8 in defect states near the bands, this result 8 is in agreement 8 with ref.  4) indicates 8 that n valuable decreases with the rise of Ta.This behavior maybe because of the decreasing 8 in the reflection, which the refractive index 8 in turn depends on it.Extinction coefficient (k) in general increment 8 with Ta this is attributed to the same reason mentioned previously in the absorption 8 coefficient, because the behavior of k is similar 8 to α.The variables of εr and εi versus 8 photon energy at different 8 Ta are shown in figure 8(c & d).The behavior of εr is similar to that of the refractive index 8 because of the small value of k 2 compared 8 with n 2 , while εi behavior is similar 8 to that of extinction coefficient 8 because it mainly depends on the k value, which is related to the variation 9 of absorption coefficient.The variables 9 of εr and εi with different 8 annealing temperatures film are non 10 -systematic.This means that this material 10 possesses a specialized property 10 with Ta.We can deduce from the variation 10 of the resistivity verse annealing 10 temperature for all samples of AIS films, that the resistivity values decrease 10 as the Ta rises due to the improvement 10 in the film construction.We believe that there was a reduce in dangling 10 bonds, defects like vacancy sites, and point 10 defect in the film structure, therefore 10 the resistivity of the films decreases from (1.32 Ω.cm to 0.30 Ω.cm) as the Ta increases 10 from ( RT to 550 K) as shown in Figure (8), which presented 10 a plot of lnσ versus 10 3 1 /T for different Ta.From this figure 10 the activation energy can be determined.Electrical 10 conductivity increases as the Ta enlarges because of the rise in the number 10 of available transport charge carriers.We can notice 10 from figure ( 8) that all AIS films have two mechanisms 10 for electrical conductivity which means that there are two mechanisms of transport of free carriers with two 10 values of the activation energy (Ea1, Ea2) each one predominating in a different temperature 10 range.The electrical conductivity 10 of these films is affected by the transport 10 of free carriers in extended 10 states beyond the mobility edge at higher temperature ranges 10 (403-473) K, as well as carriers excited into the localized 10 states at the edge of the band 10 and hopping 10 at other range of temperature (300-393) K [13] as shown in Table (5).
The type of charge 10 carriers, concentration (n) and Hall mobility (μH) has been estimated 10 from Hall measurements.These values are listed in Table ( 6).The negative sign 10 of the Hall coefficient 10 indicates that the conductive nature of the film is n-type, i.e. electrons 10 are the majority charge carriers, this result is in agreement with ref.
[18], i.e Hall 10 voltage decreases with the increase of the current.The carrier concentration 10 of the order 10 17 cm -3 is in a good agreement with refs [3,19].We can notice 10 from Table ( 6) that the carrier concentration 10 and mobility increase with increas e10 of Ta.