Structure, optical and electrical characterization of nanostructured Gallium Sulfide Ga2S3 thin films for electronic and
solar cell applications
Ga2S3 Thin films with different thickness (32, 60, 100 and 140 nm) were prepared using inert gas condensation technique using Argon gas. The structure of the ingot powder and films were studied using X-ray diffraction (XRD). The surface topography of these films was carried out using Transmission Electron Microscope (TEM), which showed that, the grain size increase with film thickness for these studied samples, and the structure was explained by diffraction patterns which illustrated that the deposited films have nanocrystalline structure.
The optical transmittance, reflectance and absoprbance of these films were measured, it was noticed that, these samples had a direct opical energy gap, this optical energy gap were calculated for these films, and was noticed that, Egdir increase with film thicknesses, on the other hand the urbch tail (Em) values for these samples decrease with film thicknesses.The electrical resistivty (?) for these films were dtermined and was noticed that, theese values decrease with film thickness.
Keywords:- Ga2S3 , nanostructure thin films, structure, transmission electron microscopy, optical properties, and electrical resistivity.
Gallium (III) sulfide is an important compound of III-VI layered structure semiconductor group. It consider as one of the most important wide band gap chalcogenide semiconductor [1, 2], and has a special interest due to their owing layered structure. This layered structure and physical properties such as mechanical, optical and electrical properties makes it a promising candidate for electrical sensors, nonlinear optical and photoelectric optical applications .
Semiconductor in the range of nanostructures is promising building blocks for photonic and electronic devices in the future applications [4, 5]. In particular, nanosheets of III-VI semiconducting group have been regarded as promising candidates for energy storage devices, ultrathin and flexible optoelectronic devices . Also; nanostructure layered type of chalcogenide semiconductor like InSe, GaTe, GaS, Ga2S3 .etc. are of a diverse interest in solar, X-ray detection, optical, nonlinear elements in laser technology applications [4, 5, 7, 8]. Ga2S3nanostructure used as hydrogen evolution catalysts , Structural, and optical properties of Ga2S3thin films had been investigated , used for fabrication of field effect transistor [11, 12] and Photodetector, energy conversion and energy storage devices . As well, it was used in the fabrication of photo-detectors on flexible and rigid substrates and gas sensors [15, 16]. Meng et al  reported that the single wall carbon nanotube-gallium sulfide composites used as anode materials instead of the conventional anode of graphite in the Lithium-Ion batteries, Ga2S3 has (space group D4 6h for ? -type GaS) [18,19]. GaS crystallizes in the stacking type of layers in the stacking sequence of S Ga Ga S along the c axis . The optical properties of GaS were studied by many authors [22-26], it was found that, these films had energy gap as 2.76 eV and 2.10 eV . While the direct band gap energy of gallium sulfide thin film deposited by using thermal evaporation as 2.55 eV ,and also 3.1 eV at room temperature, while this band gap was (3.05 eV at 300 K) and 3.2 eV . The diectric properties of Ga2S3 crystals and thin films had investigated by many authors [31-33], on the other hand the electronic and magnetic properties of Ga2S3 single crystals was studied [34-35], it was found that, Three electron traps located at 0.17, 0.45, and 0.56 eV from the conduction band have been detected, while The electronic properties of the neutral Ga2S3 and polymorphs of gallium sulphide (Ga2S3) have been investigated [36-40], it was found that, Ga2S3 behave electron approaches from 3.51 to 3.64 eV . The influence of film particle size on optical properties of Ga2S3 thin films, it was found that the energy gap decreased with grain size.
In the existing study, the synthesis of Ga2S3 nanostructure was performed via inert gas condensation (IGC) technique which categorize as one of the physical methods of preparation. We Study the effect of particle size on structure, optical and electrical resistance of Ga2S3 nanostructured thin films.
3. Results and Discussion
X-ray diffraction (XRD) pattern of gallium (III) sulfide powder source sample is illustrated in Fig. 1. It could be observed that Ga2S3 powder has single phase of monoclinic polycrystalline structure which matched with ICCD Card No. (48-1432) as shown in Table 1 . It is noted in XRD pattern that there is no diffraction line appears for any impurities or other phases but all lines for pure Ga2S3 of monoclinic phase. The powder sample was used as an ingot source material for evaporating thin films by inert gas condensation (IGC) method.
The crystallite size (Cs) of the ingot powder has been calculated from the full width at half maximum (FWHM) of the highest intense peak (-321) of the pattern using Sherrer’s formula 
where both ? is the wavelength of the used X-ray, ? is the Braggs angle and ? (the FWHM of the peak) is expressed in radians and corrected for the instrumental broadening by measuring the width of a standard reference.
The dislocation density (?), which refers to the number of defects in the films, is as :
The lattice strain (Ls), which affects the broaden of the X-ray peaks and the optical and dielectric results, which was determined using 
Another important factor for these studied samples was determined, which is the number of crystallites per unit area (N), which has been determined using the following equation 
Where t is the crystal thickness. The calculate values for the above parameters for Ga2S3 powder are shown in table 2.
Fig. 2 revealed the grazing incident in-plane X-ray diffraction (GIIXD) pattern for film of thickness 140 nm deposited on glass substrate. It is illustrated that there is no diffraction lines appear in the pattern; which could be referred that the deposited film has an amorphous structure.
The surface topography and structure analysis of these the films were studied using both of TEM and DEM respectively, as shown in Figs. 3, 4. From Fig. 3 it was seen that, all these films had a nanostructure features with particle size range (0.5-1.6 nm for film thickness 32 nm Fig 3a), (0.6-1.5 nm for film thickness 60nm, Fig 3b), (0.6-1.2 nm for film thickness 100 nm, Fig 3c) and (0.4-0.7 nm for film thickness 140 nm, Fig 3d). Figure 4 emphases that all films under investigation have crystalline features as spots or ringes in diffraction patterns. The diffraction patterns of deposited films revealed that it has nanocrystalline monoclinc (make sure from d values) structure. It could be noted that the electron microscope diffraction pattern has more presize data in this case of low thickness films, the relation between film thickness and grain size is shown in Fig. 5, from this Fig, it was noticed that, the grain size increase with film thickness.
3.2. Optical results
The optical transmission, reflection and absorption spectra for the Ga2S3 thin films of different thicknesses (32, 60, 100 and 140 nm) are shown in Figs 6a, b and c respectively.
From Fig.6a it is clear that, the Ga2S3 thin films of low thicknesses (32 and 60 nm) had no optical compared with another higher two thicknesses (100 and 140 nm), this is logic and could be attributed to, for higher thicknesses (100 and 140 nm) there are sub energy levels in the forbidden energy gap,
so also the same behavior for these samples are shown in absorbance and reflectance spectra (Figs 6b and c).
The absorption coefficient (?) of the investigated films was calculated from both the transmission and reflection spectra due to the following relation :
where d is the film thickness, R is the reflection and T is the transmission of of the studied films. The absorption coefficient (?) as function of photon energy (h?) is seen in Fig. 7.
The optical energy gap (Eg) is determined from the absorption spectra curves using the empirical equation :
where A is a constant, Eg is the energy band gap, ? is the frequency of the incident radiation and h is Planck’s constant. The constant p takes different values depending on the kind of optical transition of these films. p=0.5 for direct allowed transition (direct energy gap) and for indirect allowed transition the value of p will equal 2.
The influence of photon energy (h?) on (?.h?)2 for the Ga2S3 thin films of different thicknesses (32, 60, 100 and 140 nm) respectively is shown in Fig. 7. The direct optical energy gap Eg was estimated from the extrapolation of the linear part of the curves in Fig. 7. for all samples, it was noticed that the optical energy gap (Eg) decreased with film thickness as shown in Fig. 8, this could be attributed to, that the electron mobility increase with film thickness, which caused a decrease of the determined optical energy gap.
The Urbach tail and calculate Em from the following equation 
Where h? is the photon energy, ? is the absorption coefficient, ?o , Em are material dependent constants, h? photon energy, ? is the frequency. From the relation between ln (?) and (h?) we can determine Em as shown in Fig. 9. The Urbach tail on film thicknesses is shown in Fig 10, from this Fig, it was noticed that, the values of Urbach tail decrease with film thicknesses, this is due to electron mobility decrease with film thicknesses.
The extinction coefficient (k) for all films were calculated from the relation:
The dependence of (k) on wave length is shown in Fig.11.from this fig it was seen that, the values of (k( increase with wavelength due to the increase of (?), and also the mobility of electrons µe to the increase of film thicknesses.
3.4. Electrical results
The d.c. electrical resistance (R elect) was measured while the electrical resistivity (?) for these films were calculated as follow:-
Where A is the film area = (w*d), where w is the film width, d is the film thickness , l is the film length. The relation between (?) and film thicknesses (d) is shown in Fig. 12. From this Fig 12, it could be seen that, the (R elect) , resistivity (?) decreased with film thickness, this is due to increase of free carrier concentration with film thickness, which leads to the decrease of film resistance and resistivity (?) .
The structure for Ga3S2 ingot powder source and deposited thin films was determined by XRD and GIIXD techniques and shows single phase of monoclinic polycrystalline structure for powder source.The surface topography and structure of Ga3S2 thin films were detected using (SEM), (TEM), and DEM was found these films had nanostructure monoclinic phase with particle size ranging from to nanos depending on film thicknesses, which meaning that, grain size increase with film thicknesses. Both of optical transmittance, reflectance and absorbance spectra were studied for Ga3S2 films. The studied films have direct energy gap Egdir ranging from 3.7 to 3.2 eV which decreases with rising film thickness. The extinction coefficient (k) increase with film thickness, as a result of increase the direct energy gap, on the other hand the Urbach tail values decrease with film thickness. Finally the electrical resistivity decreased with film thicknesses. The above results for nanocrystalline Ga3S2 thin films give an indication that, the films could be promising for applying at solar cell (Eg 3.20 to 3.7 eV) and UV detector and some other optoelectronic devices.
Fig.1. XRD pattern for Ga2S3 powder sample.
Fig.2. GIIXD pattern for Ga2S3 film of thickness 140 nm.
Fig.3. TEM Images of for Ga2S3 thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig.4. Diffraction patterns for Ga2S3 thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig. 5: Grain size dependence on film thickness for Ga2S3 thin films.
Fig.6: a) Transmittance and b) reflectance and c) absorbance spectra dependance on wave length for Ga2S3 thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig. 7: Relation between (h?.?)2 and photon energy (h?) for Ga2S3 thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig.8: Direct energy gap dependance on film thickness for Ga2S3 thin films.
Fig. 9: Relation between ln (?) and photon energy (h?) for Ga2S3 thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig. 10: Urbach tail (Em) dependence on film thickness for Ga2S3 thin films
Fig. 11: Relation between (k) and wavelength (nm) for Ga2S3thin films with different thicknesses, a) 32, b) 60, c) 100 and d) 140 nm.
Fig. 12: Electrical resistivity dependence on film thickness for Ga2S3 thin films.
Table 1: X-ray diffraction data of Ga2S3 powder
GaS (powder) Monoclinic (Ga2S3)
Card ( 48-1432 )
2-theta d(hkl) I/Io d(hkl) I/Io (hkl)
16.660 5.31699 29.44 5.3040 68 ?111
18.646 4.75490 19.42 4.7460 22 ?220
18.8019 4.71586 14.47 4.7110 8 101
25.3548 3.50995 7.72 3.5063
27.784 3.20832 85.33 3.2018 100 ?321
29.6812 3.00744 100 3.0093 98 020
31.452 2.84203 25.67 2.8358 12 ?331
31.7047 2.81996 31.35 2.8167 67 ?311
32.649 2.74051 14.69 2.7361 12 ?212
33.71 2.65702 2.94 2.6435 15 ?420
35.21 2.54671 3.59
38.27 2.34995 2.25 2.3421 13 ?131
40.883 2.20558 14.48 2.2014 29 3?41
41.1359 2.19260 9.12 2.1912 15 0?22
41.279 2.18533 7.97 2.1640 6 301
43.342 2.08595 6.03 2.0831 17 ?432
44.335 2.04154 9.45 2.0455 22 5?41
49.207 1.85018 43.32 1.8467 95 ?323
53.615 1.70800 17.04 1.7059 25 ?351
53.889 1.69995 20.75 1.6989 30 032
54.113 1.69345 15.23 1.6937 35 311
57.4 1.60405 16.80 1.6003 12 ?642
58.354 1.58006 10.11 1.5818 18 ?660
59.722 1.54711 6.64
64.76 1.43831 2.35
65.621 1.42157 3.54
66.03 1.41372 2.82
75.7 1.25472 1.15
86.44 1.12488 3.28
Table 2. The structural results analysis for Ga2S3 powder.
Ls (N) / cm2 (?) line/cm2 Cs (nm) ? (FWHM)
10.94 8.3E+15 9.1E+12 33.08 0.08
2.73 1.3E+14 5.7E+11 132.33 0.02
3.84 3.6E+14 1.1E+12 94.24 0.07
3.95 3.9E+14 1.2E+12 91.69 0.05
22.01 6.7E+16 3.7E+13 16.44 0.09
14.98 2.1E+16 1.7E+13 24.16 0.06
24.11 8.9E+16 4.4E+13 15.01 0.10
15.62 2.4E+16 1.9E+13 23.16 0.07
2.70 1.3E+14 5.6E+11 133.84 0.07
31.20 1.9E+17 7.4E+13 11.60 0.13
4.81 7.1E+14 1.8E+12 75.17 0.06
4.61 6.2E+14 1.6E+12 78.46 0.07
8.18 3.5E+15 5.1E+12 44.25 0.05
4.00 4.1E+14 1.2E+12 90.41 0.02
15.29 2.3E+16 1.8E+13 23.68 0.08
9.99 6.3E+15 7.6E+12 36.22 0.04
12.34 1.2E+16 1.2E+13 29.32 0.05
19.40 4.6E+16 2.9E+13 18.65 0.10
11.87 1.1E+16 1.1E+13 30.48 0.05
12.10 1.1E+16 1.1E+13 29.90 0.10
17.18 3.2E+16 2.3E+13 21.07 0.07
16.47 2.8E+16 2.1E+13 21.97 0.10
4.94 7.6E+14 1.9E+12 73.29 0.11
25.00 9.9E+16 4.8E+13 14.48 0.10
17.39 3.3E+16 2.3E+13 20.82 0.10
5.12 8.5E+14 2.0E+12 70.70 0.10