CHARACTERIZATION OF THERMAL EVAPORATION ALUMINUM DOPED TIN SULFIDE THIN FILM
Muhamad Faiz Hashim, Wan Nurulhuda Wan Shamsuri,Bakhtiar Ridhwan Bachiran, Rashid Ahmed, Karim Deraman and Rosli Hussin
Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
Corresponding author: [email protected] ABSTRACT
Tin sulfide (SnS) has caught attention as alternative material for solar cell absorber layer. This is due to the abundant amount of Tin sulfide in nature and lower cost compared to other absorber layer materials such as copper indium gallium selenide (CIGS) and cadmium telluride (CdTe). The high absorption coefficient (α >104 cm-1) and ideal bandgap (in the range of 1.2 – 1.5 eV) made Tin sulfide a promising candidate for the absorber layer. The aims of this research is to introduce aluminum as dopant to optimize the structural and optical properties of the tin sulfide thin film. The variation of the weight percentage and annealing process is analyzed. The thin films are characterized by using X-ray diffraction (XRD) and UV- Visible Spectroscopy (UV-Vis Spectroscopy). All samples were orthorhombic SnS with preferred (111) and (101) crystallites orientation. With the increasing of aluminum doping concentration and annealing process, the average crystallite size increased from 16nm – 57nm. The evaluated energy band gap Eg of the Al:SnS films decreases from 1.49eV to 1.32eV. It is found that doping percentage and annealing process play vital roles in producing high quality and suitable absorber layer.
Keywords: Tin sulfide; Aluminum; Thin Film; Solar Cell; Absorber Layer; Thermal Evaporation; Semiconductor; Annealing, Structural Properties; Optical Properties;
INTRODUCTION
Great attention have been made to find alternative materials to replace copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) based absorbed layer for high effcient solar cell. One of the challenging problem for this materials is to overcome the toxicity and the rarest of the materials. Tin sulfide (SnS) is choose as alternative materials for the absorber layer because it’s posess near optimum bandgap of 1.4 eV [1], high absorption coeffcient (3.06 x 104 cm-1) [2, 3] and considered as non-toxic and abdundant in nature [4].
High absorption and low electrical resistivity of SnS thin film can be achieved by doping SnS with dopant material such as antimony (Sb) [5], silver (Ag) [6], bismuth
(Bi) [7] and many other material. Doping process may alter the structural, electrical and optical properties of the SnS materials. According to Yuying et al. doping process may have affected the electrical resistivity of SnS thin film due to the change amount of carrier concentration [8].
There are several deposition techniques that can be utilized for the preparation of SnS thin films, such as RF sputtering [9], spray pyrolysis [9], electron beam evaporation [10], vacuum evaporation [11] and others. For this research thermal evaporation technique has been used to fabricate aluminum doped SnS. The main objective for this study is to find out the effect of aluminum doping and annealing process toward the properties of SnS thin film.
METHODOLOGY
In this work, Edwards E306 Vacuum evaporator machine was used to deposit all Al:SnS thin film samples. The Al:SnS thin film was deposited on soda lime glass substrate using mixture of 99.99% purity SnS and Aluminum powder as source material. Each substrate was first cleaned using Branson 3210 Ultrasonic cleaner by immersing the substrate into acetone solution and rinsed with distilled water. The deposition process was carried out in a chamber vacuumed by rotary – diffusion pump system. The evaporating system was operated in a pressure ranging from 10-5 to 10-6 torr. The distance between the source and substrate was kept constant at 15cm during the deposition process. The mixture than was placed in molybdenum boat crucible and heated using electrical current. All thin film samples deposition rate and thickness were monitored using quartz film thickness monitor FTM5. The deposition rate and thickness was kept constant at 0.3 nm/s and 450nm thick respectively. The concentration of Al:SnS mixture was prepared with a different weight percentage range from 0 wt%, 5 wt% and 10 wt%. Finally, the thin film was annealed in a vacuum environment at 200
oC for 2 hours using a tube furnace. The annealing process was conducted to alter the properties of the thin film samples.
The effect of aluminium doping toward SnS thin film properties was investigated by using several characterization methods. The X-ray diffraction (XRD) measurement was carried out using Siemens Diffractrometer D5000 (Cu Kα radiation operating at 40 kV, 30 mA) to indentify the crystal structure and phase present on each samples. The optical properties of all samples were measured using Shidmadzu 3101PC UV-VIS NIR spectrophotometer in the wavelength range from 200nm – 1800nm.
RESULTS AND DISCUSSION
Structural Properties
XRD pattern of all Al:SnS samples are as shown in figure 1. Based on figure 1 (a) and (b), it is found that all samples are in crystalline nature. All XRD peaks can be assigned to orthorhombic structure of Herzenbergite SnS with lattice constants: a=4.29 Å,
(JCPDSNo.39-0354). The dominant peaks of all samples are (111) and (101), with weaker peaks of (002) [3, 11, 12].
The average crystallite size of all thin film samples are estimated using Scherer’s equation [12, 13].
(1)
where K=0.89 is the shape factor, is the X-ray wavelength of Cu Kα radiation (0.154056nm),θ is the Bragg angle, and β is the experimental full-width half-maximum (FWHM) of the respective diffraction peak (in units of radians). From the Scherer’s formula the crystallite size of each sample was calculated and listed in table 1 (a) and (b).
Figure 1: (a) The XRD pattern of as-deposited Aluminium doped SnS thin film with different doping concentration. (b) The XRD pattern of annealed Aluminium doped SnS thin film with different doping concentration
By comparing table 1 and table 2 it is found that the average crystalite size increase as it is undergoes annealing process, this means the crystalite size are expended and the atomic arrangement are recrystallized [3, 13]. Table 1 and table 2 also shows that the crystallite size increase with the increase of aluminum doping concentration and it is due to lattice dillation. This is in a well agreement with the study made by Manoharan et al., [7]. They reported that Bi doping increases the crystallite size of the SnS thin film.
Table 1: The crystallite size of as-deposited all Al:SnS thin film with different doping concentration.
Sample ( h k l ) θ(0) FWHM (rad) d (nm)
0% ( 1 1 1 ) 31.90 0.0061 26
5% ( 1 1 1 ) 31.65 0.0028 57
10% ( 1 0 1 ) 30.75 0.0033 48
Table 2: The crystallite size of annealed Al:SnS thin film with different doping concentration.
Sample ( h k l ) θ(0) FWHM (rad) d (nm)
0% ( 1 1 1 ) 31.80 0.56 16
5% ( 1 1 1 ) 31.75 0.51 18
10% ( 1 0 1 ) 30.70 0.29 31
Optical Properties
SnS with different aluminium doping percentage exhibit changes in optical properties in the visible to near infrared region. Figure 2 (a) and (b) show the transmittance spectra of all samples. Based on figure 2 (a) and (b), the optical absorption edge of the sample shifts as the aluminium doping percentage differ. The shift towards the longer wavelength region indicates the decrease of the optical band gap [14].
The energy bandgap, Eg are calculated by plotting (αhν)2vs. hν graph. The energy band gap, Eg of the sample could be obtained by using this following equation [15]
αhν = A (hν –Eg)n (2)
where h is Plank constant, A is a constant and n is ½ for a direct energy gap and 2 for indirect energy gap semiconductor materials. The band gap is calculated by extrapolating the straight vertical section of the (αhν)2 vs. hν curve to the energy axis (x-axis) [16]. From the (αhν)2 Vs. hν graph, energy band gap of each samples are estimated and plot in figure 3. Figure 3 shows The graph of optical energy bandgap (Eg) vs doping percentage (wt%) for all samples.
Based on figure 3, the value of the energy band gap are slightly decreased as the aluminium dopant is introduced to the SnS sample. It may occur because of structural deformation in SnS lattice arrangement from the replacement of tin ion with aluminum ion [12] and band shrinkage effect due to the increase of carrier concentration [14]. By increasing the doping percentage above 5 wt%, the energy bandgap value are increased and it is due to an effective incoporation of dopant into SnS lattice, since more aluminum atom are placed at the substitutional sites, therefore the occupied states and the band gap were increased. This is an agreement with the study made by A. Manohar et al.,[7]. The value of energy band gap also decreases as the sample undergoes the annealing process. This is due to absorption involving defect states of the sample [17].
Figure 2: (a) The transmittance graph of as-deposited Al:SnS thin film with different doping concentration. (b) The transmittance graph of annealed Al:SnS thin film with different doping concentration
Table 3: The energy bandgap of as-deposited and anneal thin film sample
Figure 3.The graph of optical energy bandgap (Eg) vs doping percentage (wt%) Doping Percentage
(%)
Optical Bandgap (Eg) As-Deposited Anneal
0 1.46 1.49
5 1.53 1.32
10 1.44 1.34
CONCLUSIONS
Al:SnS thin film samples are successfully deposited onto glass substrate using thermal deposition technique. Next, all samples underwent annealing process in a vacuum chamber at 2000C for 2 hours. XRD patterns show that all samples are in orthorhombic crystal nature with dominant peak of (111) and (101). The crystallite size of all thin film samples are obtained using Scherer equation. The crystallite size of the sample increases with the increase of doping concentration and annealing process. The energy band gap of each samples are estimated and the values are decreased from 1.49eV to 1.32eV and it’s due to the improved crystalline structure and grain size expantion. From our study, it shows that aluminium doping and annealing process, improved the structural and optical properties of the SnS thin film. The annealed 5% Al:SnS are the most promising SnS condition to applied to solar cell devices, because of it’s lowest energy band gap (1.32 eV).
ACKNOWLEDGEMENTS
The authors also would like to thanks all the individuals involve making this project a sucess. This project was financally support by (Q.J130000.2526.10H77) Ministry of Higher Education (MoHE) and Universiti Teknologi Malaysia (UTM).
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