[1,2] and photoconducting [3,4] properties of silver sulphide thin films. In the visible and near IR region, Ag2S barrier layers are used as detectors. Recently, the ...
Materials Chemistry and Physics 63 (2000) 226±229
A new chemical method for the preparation of Ag2S thin ®lms B.R. Sankapal, R.S. Mane, C.D. Lokhande* Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Received 17 March 1999; received in revised form 19 July 1999; accepted 30 September 1999
Abstract Semiconducting Ag2S thin ®lms were deposited onto amorphous glass, and single crystalline wafer of Si(1 1 1) by using a new, simple and less expensive successive ionic layer adsorption and reaction (SILAR) chemical method. The XRD studies show that the crystallinity of Ag2S ®lm depends on the type of substrate. The microstructural, optical and electrical properties of Ag2S ®lms deposited onto glass substrate and subsequently annealed at various (373±573 K) temperatures in air were studied and results are reported. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Ag2S thin ®lms; Chemical method; Structural; Optical and electrical properties
1. Introduction Considerable work has been done on the photovoltaic [1,2] and photoconducting [3,4] properties of silver sulphide thin ®lms. In the visible and near IR region, Ag2S barrier layers are used as detectors. Recently, the use of Ag2S in the electrochemical photovoltaic (ECPV) storage cells as a storage electrode has created much interest as the current storage ef®ciency of Ag/Ag2S couple is about 90% [5,6]. Different chemical methods of thin ®lm preparations such as spray pyrolysis, controlled precipitation, electrodeposition, electroless deposition, electroconversion etc. have been used to prepare Ag2S ®lms. Dhumure and Lokhande have reported the chemical deposition of Ag2S ®lms from acidic solutions containing thioacetamide and thiosulphate [7±10]. Successive ionic layer adsorption and reaction (SILAR) method was introduced by Nicolau [11±13] to deposit CdS, ZnS and CdS/ZnS multilayer ®lms. The SILAR method is based on immersion of the substrate into separately placed cation and anion precursor solutions and rinsing between every immersion with ion-exchanged water to avoid homogeneous precipitation [11,12]. Due to low deposition temperature, diffusion of ions in the thin ®lms is low and hence SILAR method is suitable for growing thin multilayered structures. In this communication, we report deposition of Ag2S ®lm from aqueous medium by using SILAR method. The dependence of crystallinity on the nature of the substrate and the *
Corresponding author.
effect of annealing on structural, optical and electrical properties of the ®lms are presented. 2. Experimental details 2.1. Substrate cleaning Microscope glass slides were cleaned in a commercial detergent solution, rinsed well in de-ionized water and dried. While single crystal wafer of Si(1 1 1) was etched by using the following procedure: Si(1 1 1) wafers were dipped in 40% NH4F at 108C for 15 min and then rinsed in double distilled water. 2.2. Preparation of Ag2S films Loba analytical reagent grade silver nitrate and thiourea were used for the deposition of silver sul®de thin ®lms. The cationic precursor was 0.05 M silver nitrate with pH 8. The source for sulphide ions was 0.4 M thiourea with pH 6. For the deposition of Ag2S thin ®lm, well cleaned substrate was immersed in cationic precursor solution (silver nitrate) for 12 s where the silver ions were adsorbed on the surface of the substrate. Such substrate was immersed in double distilled water for 10 s to remove loosely bound or excess silver ions. Further the substrate was immersed in anionic precursor solution (thiourea) for 12 s where sulphide ions were reacted with adsorbed silver ions on the substrate to form Ag2S on the substrate. Subsequently, substrate was
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Table 1 The optimized deposition conditions for the Ag2S films Precursor solutions Deposition conditions Silver nitrate (cation) Thiourea (anion) Concentration (M) Complexing agent pH Immersion time (s) No. of immersions Deposition temperature (8C)
0.05 EDTA 8 12 15 27
0.4 ÿ 6 12 15 27
immersed in double distilled water for 10 s. to remove loosely bound or powdery Ag2S material on the substrate. This completes one cycle of Ag2S ®lm formation. By repeating such SILAR cycles for 15 times, we have obtained Ag2S ®lm of thickness about 0.22 mm. The optimized deposition conditions are shown in Table 1. Amorphous microslide glass and single crystal wafer of Si(1 1 1) were used as the substrates. The Ag2S ®lms deposited on glass substrates were annealed at temperatures between 373 to 573 K for the duration of 1 h in air. 2.3. Characterization of Ag2S films X-ray diffraction (XRD) patterns of Ag2S the ®lms were recorded on a Philips model PW - 1710 in the range of scanning angles 10±1008 (2) with CuKa radiation using Ê . The scanning electron microthe wavelength of 1.5406 A scope (Cambridge Stereoscan, MK -III, after coating with gold palladium via a polaron SEM coating unit E-2500) was used to study the surface morphology of Ag2S ®lm deposited on glass substrate. The optical absorption spectra of the ®lms were recorded on Hitachi - 330 (Japan) UV±VIS±NIR spectrophotometer in the wavelength range 350±2000 nm. Electrical resistivity measurement was carried out using dc two point probe method. The area of the ®lm (0.5 cm2) was de®ned and silver paste was applied to ensure good ohmic contacts to Ag2S ®lm. A chromel-alumel thermocouple was used to measure the temperature.
Fig. 1. (XRD) patterns of Ag2S thin film deposited on (a) glass and single Crystal wafer of (b) Si(1 1 1) substrates.
comitant growth of many grain families having different possible epitaxial orientations. A comparison of observed `d' values with standard ASTM data `d' values [14] is made in Table 2 for various substrates. Ag2S ®lms deposited onto amorphous glass substrates were annealed at 373±573 K temperature in air for 1 h and the effect of annealing on crystallinity of Ag2S ®lm was studied. It is seen that by annealing from 373±473 K, (0 3 1) plane was disappeared and (1 0 3) plane was formed due to recrystallization during annealing, the similar result has been observed for Bi2Se3 ®lms [15]. The increase in intensities of existing planes is also observed as a consequence of annealing which may be attributed to increase in particle size. The scanning electron microscopy studies for Ag2S ®lms on glass substrate reveal that the ®lms formed are homogeneous and uniform on the substrate with the presence of
3. Results and discussion
Table 2 Comparison of observed and standard d' values of Ag2S film deposited on various substrates and annealed films
3.1. Structural studies
Substrates
Std. `d' Ê) values (A
Observed `d' Ê) values (A
Planes (hkl)
Amorphous glass Annealed at (K) 373
2.213
2.208
031
2.836 2.213 2.836 2.606 2.383 2.383 3.437 2.836 2.606 1.554
2.818 2.206 2.809 2.606 2.372 2.337 3.415 2.842 2.604 1.550
112 031 112 121 103 103 111 112 121 204
Fig. 1 shows X-ray diffraction (XRD) patterns of Ag2S ®lms deposited onto substrates as (a) amorphous glass, and (b) single crystal wafer of Si(1 1 1) substrates. The XRD studies show the formation of Ag2S thin ®lm by SILAR method. The XRD patterns of Ag2S ®lm deposited onto glass substrate show a broad hump due to nature of Ag2S ®lm. The crystallinity was improved signi®cantly with Si(1 1 1) substrate. The improvement in crystallinity using single crystalline substrate in case of CdS and ZnS over InP(1 1 1), GaAs(0 1 1 1) and Ge(1 1 0) has been discussed by Nicolau et al. [12]. Generally one can observe a con-
473 573 Si(1 1 1)
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Fig. 3. Plots of ( h)2against (h) for (a) as-deposited, annealed at (b) 373 K (c) 473 K and (d) 573 K Ag2S thin films in air for 1 h.
Fig. 2. Scanning electron micrographs of (a) as-deposited and (b) etched Ag2S films at 30,000 magnification.
some loosely bound Ag2S particles (Fig. 2 a). The Ag2S ®lm etched in 3% HCI for 5 s. shows removal of some of the such loosely bound Ag2S particles (Fig. 2b). 3.2. Optical adsorption studies The optical absorption spectra of Ag2S ®lms on glass substrates were studied at room temperature in the wavelength range 350±2000 nm. The nature of the transition involved (direct or indirect) during absorption process can be determined by studying the dependence of absorption coef®cient ` ' on photo energy h as /
K
hÿEgn=2 h
(1)
where, K is constant, `Eg' is the separation between the valence band and conduction bands, `n' is a constant equal to 1 for direct gap materials and 4 for indirect gap material. The variation of ( h)1/2 versus h and ( h)1/2 versus h for as-deposited and annealed Ag2S films was studied and it was found that only the plots of ( h)1/2 versus h are linear as shown in Fig. 3a±d. Thus band gap energy was determined by extrapolating the straight portion of plot to the energy axis at 0 which is found to be 1.1 eV for asdeposited Ag2S film, this value of `Eg' is in good agreement with the bandgap `Eg' reported earlier [16]. However no significant bandgap change was observed for annealed Ag2S films (shown in Table 3). 3.3. Electrical resistivity studies The dark electrical resistivity of the as-deposited and annealed Ag2S ®lm was measured at room temperature. The room temperature electrical resistivity of Ag2S ®lm decreases after annealing the ®lm at higher temperatures. This may be attributed to the increase in particle size of Ag2S ®lm. The dc electrical resistivity of the as-deposited and annealed Ag2S ®lms was measured in the temperature range of 348±423 K. The plots of variation of log resistivity with reciprocal temperature are shown in Fig. 4a±d. The resistivity plots show two activation energies. In case of
Table 3 Electrical and optical properties of Ag2S films deposited on glass substrate Ag2S film
As-deposited Annealed at (K) 373 473 573
Optical bandgap Eg (eV)
Dark resistivity 104 cm
Activation energy, Ea (eV) Low temperature region
High temperature region
1.10
3.16
0.03
0.18
1.07 1.04 1.0
2.58 2.05 1.99
0.09 0.13 0.1
0.24 0.25 0.18
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studies showed that ®lms of Ag2S with monoclinic crystal structure. The crystallinity of Ag2S ®lms deposited onto various substrates depends crystallinity of the substrate itself. The SEM study shows that ®lms are uniform and well covered to the substrate. The annealing of Ag2S (upto 573 K) shows no signi®cant changes in electrical and optical properties. Acknowledgements Authors are thankful to University Grants Commission, New Delhi, India for the ®nancial support under the project: F. 10 - 7 / 97 (SR-I). References Fig. 4. Plots of log p against 1000/T for (a) as-deposited, annealed at (b) 373 K (c) 473 K and (d) 573 K Ag2S thin films in air for one hour.
Ag2S, the change in activation energies has been attributed to transition from to phase. For Ag2S single crystal such transition temperature is around 450 K has been reported by Paul and Pillai [17]. In the present case, such transition temperature is around 416 K. The difference in transition temperature of to phase in case of chemical deposited Ag2S thin ®lms and single crystal Ag2S has been reported earlier [7±9]. 4. Conclusions A simple method, successive ionic layer adsorption and reaction (SILAR) is employed to deposit Ag2S thin ®lms. Quality of ®lms depends upon preparative conditions. XRD
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