Effect of annealing on structural, optical and electrical properties of pulse electrodeposited tin sulfide films

Materials Science in Semiconductor Processing 16 (2013) 29–37

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Effect of annealing on structural, optical and electrical properties of pulse electrodeposited tin sulfide films N.R. Mathews n, C. Colı´n Garcı´a, Ildefonso Z. Torres ´n en Energı´a, Universidad Nacional Auto ´noma de Me´xico, Temixco, Morelos 62580, Me´xico Centro de Investigacio

a r t i c l e i n f o

abstract

Available online 11 August 2012

Polycrystalline tin sulfide (SnS) thin films were grown on conducting glass substrates by pulse electrodeposition. The effect of annealing on the physical properties such as structure, morphology, optical, and opto-electronic properties were evaluated to understand the effect of post-deposition treatment for SnS films. Annealing at temperatures higher than 250 11C resulted in the formation of SnS2 as a second phase, however, no significant grain growth or morphological changes were observed for films after annealing at 350 1C. A small change in band gap of 0.1 eV observed for films annealed at 350 1C was interpreted as due to the formation of SnS2 rather than due to morphological changes. This interpretation was supported by X-ray diffractometry, scanning electron microscopy, and Raman spectral data. The electric conduction in the films is controlled by three shallow trap levels with activation energies 0.1, 0.05, and 0.03 eV. The trap with energy 0.03 eV disappeared after annealing at higher temperature, however, the other two traps were unaffected by annealing. & 2012 Elsevier Ltd. All rights reserved.

Keywords: SnS Pulse electrodeposition Raman spectra

1. Introduction Tin sulfide (SnS) thin films have adequate optical characteristics and p-type conductivity, in addition to its qualities such as non-toxicity and the abundance of constituent materials, which makes it one of the promising absorber layers for solar cells. The optical band gap of SnS thin films varies in the range of 1.1–1.7 eV depending on the deposition conditions, crystal structure [1–3] and the presence of traces of other phases SnS2 or Sn2S3 [4]. The value of optical absorption coefficient of SnS is higher than 104 cm  1 in the visible region. SnS films have been deposited by various deposition techniques such as chemical bath deposition (CBD) [2,5], thermal evaporation [6], spray pyrolysis [7,8], and electrodeposition (ED) [3,9–12]. ED is one of the economical methods for depositing large area thin films and can be

n Corresponding author. Tel.: þ52 777 3620090; fax: þ 52 777 3250018. E-mail address: [email protected] (N.R. Mathews).

1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.07.003

deposited by constant current, constant potential, and pulse electrodeposition (PED). The structural, optical and electrical properties of the electrodeposited SnS films have been reported by various authors [3,9–12]. Yue et al. [13] studied annealing of SnS films in air and reported that the film decomposes and changed completely to SnO2 at 250 1C. Devika et al. [14] observed that in evaporated SnS films resistivity and grain size decreases by increasing the annealing temperature; however, activation energy and band gap showed a tendency to increase when annealed above 100 1C. The effect of annealing on the contact characteristics of metal/SnS interfaces were investigated by Ghosh et al. and found that indium (In) maintains ohmic contact to SnS in all annealing conditions. Al forms a barrier at the interface with SnS, however it turns to ohmic after annealing at 350 1C. One possible reason for this was the change in stiochiometry due to desulfurization, and a change in doping profile [15]. There are recent reports on post-deposition annealing of SnS thin films and its effect on film stoichiometry and formation of secondary phases. In the case of films grown

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N.R. Mathews et al. / Materials Science in Semiconductor Processing 16 (2013) 29–37

by vacuum thermal evaporation at 250 1C, it was observed that the film is relatively tolerant to annealing at temperature of 300 1C; however, higher annealing temperature has resulted in the segregation of Sn phase [16]. We have reported the physical properties of pulse electrodeposited SnS thin films and the charge transport mechanism in an Al/SnS Schottky device [17]. The CdS/SnS hetrojunction has showed conversion efficiencies of 1.3% and 0.4% for SnS deposited by spray pyrolysis and electrochemical methods respectively [18,19]. In chalcogenide thin film solar cell fabrication, post-deposition annealing is one of the important steps which optimize compositional profile, structural, morphological, and optoelectronic properties of devices. In CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) solar cells, a postdeposition selenization at an appropriate temperature is performed to adjust the film stoichiometry and to achieve recrystallization [20,21]. In the case of CdTe, vapor chloride annealing is a routine step to promote grain growth, and activate the junction [22]. Hence, understanding the material properties at various stages of annealing is necessary to develop the SnS into a promising absorber material for solar cells. In this work we are discussing the effect of annealing on the structural, electrical and optical properties of the pulse electrodeposited SnS thin films. SnS thin films were subjected to heat treatments at different temperatures, and the optoelectronic parameters were studied as a function of the annealing temperature. Raman spectroscopic technique was used to explore formation of other phases due to annealing and compare with the XRD data. The activation energies of the as-deposited and annealed films are compared and discussed. The observed differences in the modification of activation energies due to annealing reported in the literature and our studies are interpreted as due to the difference in film preparation techniques [14]. To the best of our knowledge there is no report in the literature discussing the effect of annealing on the activation energy of SnS films prepared by electrodeposition. The results discussed in this paper will update the current knowledge about the influence of post-deposition annealing on the physical and opto-electronic properties of electrodeposited SnS thin films. 2. Experimental 2.1. Material processing A three-electrode cell was used for the PED of SnS thin films. The deposition conditions were same as reported [10]. The working electrode (the cathode) was tin oxide coated glass substrates (TCO) with an approximate area of 4 cm2. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum mesh served as the counter electrode. The bath used for the deposition consisted of 2  10  3 M SnCl2 and 8  10  3 M Na2S2O3. The pH of the solution was adjusted to 2.5 and the temperature was maintained at 45 1C. The solution was constantly stirred during deposition, and was purged with pure N2 gas for 10 min prior to the deposition in order to remove dissolved oxygen. Pulse duration of the potential applied to the cathode was 10 s. The deposition (Von) and

dissolution (Voff) potentials with respect to a SCE were Von ¼  0.9 V, and Voff ¼0.1 V respectively. The samples were annealed at different temperatures ranging from 150 to 350 1C in air for 30 min. 2.2. Characterization techniques XRD patterns of the films were recorded using a Rigaku X-ray diffractometer using monochromatized CuKa radia˚ by scanning 2y in the range 20–701, at tion (l ¼1.54056 A) a grazing incidence angle of 0.51. Morphology of the SnS films was investigated using an atomic force microscopy (Nanoscope IV, Digital Instrument) and scanning electron microscopy (Hitachi, FESEM S-5500). The composition of the films was obtained from energy dispersive X-ray spectroscopy (EDS) analysis. For EDS measurements films were deposited on stainless steel substrate. Raman spectra were recorded using a Yvon Jobin Horiba (T64000) spectrometer equipped with a confocal microscope (Olympus BX419), and a CCD camera detector. The sample was excited with the 514.5 nm line of an argon laser operating at a power level of 10 mW. Optical transmittance spectra were recorded in the wavelength range of 200–1000 nm using a Shimadzu UV–vis spectrophotometer. The conductivity type of the SnS films were determined using photoelectrochemical (PEC) technique. In order to perform the electrical measurements, the electric contact to SnS film was prepared by painting a carbon electrode of known area on top of the SnS films. The experimental setup consists of a Kethley 230 programmable voltage source and a 619 electrometer, a tungsten halogen lamp, and a sample holder assembly. The electrical contacts to the sample were taken from the carbon electrode and the transparent conducting oxide coated glass substrate. Illumination of the sample was through the glass substrate (see inset of Fig. 7). During photoconductivity measurements, data acquisition software controlled the ON and OFF periods of illumination and at the same time acquired the current as a function of time. Activation energy measurements were done with Al/ SnS Schottky diode prepared by vacuum evaporating a thin layer of Al electrode. Al is known to form a Schottky barrier with SnS [11]. The sample (Al/SnS/conducting glass) was mounted on the cold finger of a cryostat (Cryogenic Advanced Research systems DE202AE) and the I–V data was collected under dark in the temperature range 100–330 K at an interval of 5 K using a computer controlled Keithley 236 source measure unit. From the I–V data, the current corresponding to a particular value of the voltage was obtained for each temperature interval and analyzed to estimate the activation energy. The temperature ramp-up rate was sufficiently low to ensure thermal equilibrium during data capture. 3. Results and discussion 3.1. Structural studies XRD patterns of the as-deposited and annealed SnS thin films are shown in Fig. 1. As-deposited films and

(111)

31

350°C

(002)

(112)

PDF 41-1455 (131)

(021)

(001)

(100) (101)

100

(110) (120)

N.R. Mathews et al. / Materials Science in Semiconductor Processing 16 (2013) 29–37

---

PDF 23-0677

Intensity (a.u)

0 200

250°C

0 as deposited

200

PDF 39-0354

---

PDF 23-0677

0 10

20

30

50

40 2θ (°)

60

70

Fig. 1. X-ray diffraction patterns of films annealed at different temperatures, along with standard patterns of orthorhombic SnS (PDF 39-0354, continuous line), SnS2 (PDF 23-0677, dotted line), and SnO2 (mineral cassiterite PDF 41-1455, continuous line).

those annealed at temperatures below 250 1C showed characteristic XRD peaks corresponding to orthorhombic SnS (JCPDS 39-0354). However, the films annealed at 350 1C showed additional XRD patterns corresponding to SnS2 as revealed by the reflection (001) at 151 and (100) at 28.41 (JCPDS-23-0677). No secondary phase was observed in the films when heated in air up to 300 1C for 30 min. Quantitative analysis of XRD pattern was performed using the software PXDL and the method of reference intensity ratio, the estimated percentage of SnS and SnS2 were 97.9% and 2.1% respectively. In the XRD pattern of the films annealed at 350 1C a very weak reflection was observed at 33.6 1C which was not observed in the X-ray diffraction pattern of the films annealed at 250 1C, showing very small traces of SnO2 (mineral cassiterite PDF 411455). This is due to the oxidation of Sn (II) to Sn (IV). When the annealing temperature was raised to 500 1C almost the entire SnS component was converted to SnS2 and SnO2 (XRD pattern not showed). The reaction mechanism can be written as O2

2SnS-SnS2 þ SnO2

ð1Þ

The enthalpy of formation of SnS is  100 kJ/mol, whereas for SnS2 and SnO2 the corresponding values are 167 kJ/mol and  581 kJ/mol respectively. Hence the formation of SnS2 and SnO2 due to annealing in air is possible. A similar observation was reported in the case of SnS films deposited by CBD, annealing at 300 1C resulted in the formation of SnS2 and as the temperature was raised to 550 1C SnO2 was formed [2]. In our case, the SnS to SnO2 transformation is at a higher temperature than that reported by Yue et al. [13]. The average size of the crystal grains was estimated using the Scherer formula: D¼

kl b cos y

ð2Þ

Table 1 Lattice parameters of the as-deposited and annealed SnS thin films.

As deposited 150 1C 250 1C 350 1C

˚ a (A)

˚ b (A)

˚ c (A)

4.342 4.306 4.315 4.309

11.200 11.230 11.373 11.420

3.974 3.985 3.977 3.965

where D is the crystalline size, l is the wavelength of the incident radiation, k¼0.90 is the shape factor, y is the Bragg angle, and b is the full width at half maximum (FWHM) in radians. Average crystallite size of as-deposited and the films annealed at 350 1C were 10 nm and 14 nm respectively. The lattice parameters for the orthorhombic phase were calculated using the equation 1 d

2

2

¼

2

2

h k l þ þ a2 b2 c2

ð3Þ

where (hkl) are the lattice planes, and d is the inter planar distance which is given in Table 1. Lattice parameters of as-deposited and annealed films are close to the values reported for orthorhombic SnS (orthorhombic SnS: ˚ b ¼11.192 A˚ and c ¼3.984 A). ˚ a ¼4.329 A, The preferred orientation of the crystallites along a crystal plane (hkl) in a thin film can be described by the term called texture coefficient [23,24] NðI =I Þ P i ¼ PN i io i ¼ 1 ðIi =Iio Þ

ð4Þ

where Pi is the texture coefficient of the plane i, N is the number of reflections taken for the analysis, Ii is the measured integral intensity of the reflection corresponding to plane i, and Iio is the integral intensity of the powder diffraction pattern corresponding to plane i.

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Table 2 Texture coefficient and preferred orientation grade of the SnS films.

(021)

0.5453 0.8614 0.814 1.006

0.9665 1.48 1.478 1.12

1.2841 1.2546 0.8732 1.105

Texture coefficient (101) (111) 1.458 1.1455 1.005 0.799

In this analysis six peaks of orthorhombic SnS in the region 2y ¼10–701 was considered. The value of Pi gives an indication about preferential orientation of the films, Pi less than unity corresponds to randomly oriented samples whereas Pi greater than unity indicates a preferred orientation for the crystallites in that particular direction. Standard deviation s of all the Pi values gives an indication about the degree of preferred orientation of the sample. A value of zero for s indicates that the sample is completely at random orientation. Values of the texture coefficient and s of as-deposited and annealed films are shown in Table 2. The value of s for as-deposited films was greater than 0.7 which is an indication of certain degree of preferential orientation in those films. For annealed samples the value of s is less than 0.6 indicating a tendency to decrease the preferred orientation due to recrystallization. However, from texture coefficient data we can see that for all samples the more preferred growth is for the plane (111).

2.7615 2.208 2.2 1.993

(131)

(141)

(002)

s

1.4028 1.108 1.201 0.849

0.2524 0.4228 0.7854 0.7687

0.7621 0.5434 0.5852 0.3587

0.7398 0.5417 0.4972 0.4386

SnS

200

SnS

0

0 200

SnS2

SnS SnS

200

SnS2

Sn2S3

(120)

intensity (a.u)

As deposited 150 1C 250 1C 350 1C

(110)

SnS SnS SnS SnS2

0 100

200

300

400

Raman shift cm-1 Fig. 2. Raman spectra in the range 50–400 cm  1: (a) as-deposited, (b) 250 1C and (c) 350 1C annealed films.

3.2. Raman spectra

3.3. Morphological properties

Raman spectra of the as-deposited and annealed SnS thin films recorded in the range 50–400 cm  1 are shown in Fig. 2. Reported Raman bands for SnS are at 288, 220, 189, 163 and 96 cm  1, that of SnS2 are at 315 and 215 cm  1 and for Sn2S3 the bands are at 307, 251, 234, 183, 71 and 60 cm  1 [25]. In the present study, for asdeposited and annealed films the Raman spectra showed vibrational modes of SnS at 95, 160, 181, and 220 cm  1. The Raman band observed at 312 cm  1 can be due to the presence of SnS2. It can be observed that in the case of asdeposited films the band at 312 cm  1 is very weak, but as the annealing temperature is increased this peak becomes more intense confirming the formation of SnS2 with annealing. This is in agreement with the XRD pattern (Fig. 1) which shows the presence of SnS2 for films annealed at 350 1C. One noticeable difference between XRD and Raman spectra is that the XRD spectrum of as-deposited SnS film showed only diffraction bands corresponding to SnS phase; however, Raman spectrum showed clear evidence for the presence of SnS2 with the appearance of a characteristic band at 312 cm  1 [25]. This difference in XRD and Raman spectra can be due to the fact that in the as-deposited films only trace amounts of SnS2 is present as a secondary phase which is not shown in the XRD spectrum; however Raman spectroscopy is very sensitive since it is detecting molecular vibrations.

3.3.1. AFM images Fig. 3 shows the three-dimensional AFM images of the as-deposited and annealed SnS films. AFM image shows good surface coverage and agglomerization of grains for as-deposited and annealed films. No significant morphological changes are observed except for coalescence of smaller grains when annealed at 350 1C, this is in agreement with the SEM pictures discussed in Section 3.3.2. In AFM analysis, rms is the most widely used parameter to evaluate surface roughness. The rms values of the asdeposited, and the films annealed at 250 1C and 350 1C were 80, 90 and 120 nm respectively. 3.3.2. SEM analysis SEM images of the as-deposited and annealed films are shown in Fig. 4, as-deposited films has crystallites with developed facets. The image shows the presence of big and small grains. The films show a porous surface morphology which may be due to the technique (PED) used to deposit the films where each pulse can initiate nucleation and grain growth. The grain size as seen in SEM image of as-deposited films varies from 100 to 250 nm, which is significantly higher than the grain size reported by Yue et al. [26], and much higher than that calculated from the XRD data of the same films. In general the grain size measured in SEM image can be greater than the crystallite size estimated from XRD since the XRD provides

N.R. Mathews et al. / Materials Science in Semiconductor Processing 16 (2013) 29–37

33

Fig. 3. AFM images of the SnS films: (a) as-deposited, (b) annealed at 250 1C, and (c) annealed at 350 1C. The corresponding rms values are 80 nm, 90 nm, and 120 nm respectively.

Fig. 4. SEM images of the SnS thin films: (a) as-deposited, (b) annealed at 150 1C, (c) annealed at 250 1C, (d) annealed at 350 1C in air.

information of single crystals whereas the bigger features observed in SEM can be agglomeration of smaller crystals. The annealing has no significant effect on film morphology, however, grain fragmentation and grain coalescence can be observed for the films annealed at 350 1C. Atomic percentage of Sn and S in the as-deposited and annealed films were analyzed using EDS. The EDS analysis was done on the film surface and at the cross-section (Fig. 5), the estimated atomic percentage in all cases is given in Table 3. EDS analysis of the film surface revealed an atomic percentage of 51.1% and 48.9%, for Sn and S respectively in the case of as-deposited films, indicating that the films are slightly rich in Sn as we have reported earlier [10]. The cross-sectional analysis revealed that the atomic percentage of Sn and S are identical in film surface

Fig. 5. Cross-sectional view of the film–substrate interface; the oval ring with label ‘‘spectrum’’ represents the film–substrate interface region from where the EDS data was collected.

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8

S (K)

Sn (L)

Atomic % (surface) as deposited Annealed at 350 1C

48.9 46.15

51.1 53.55

Atomic % (cross-section) As deposited Annealed at 350 1C

48.6 48.2

51.4 53.2

and at the cross-section indicating uniform film growth. However, in the case of annealed films, the S deficiency is higher at the surface compared to cross-section, which is not unexpected. The Sn/S ratio measured at the surface of as deposited film is 1.05, which was increased to 1.16 after annealing at 350 1C. The S deficiency is due to evaporation of S from the SnS films because of its high vapor pressure. A similar observation has been reported by Gosh et al. [16] for vacuum evaporated SnS films, where the Sn/S ratio reached about 2.9 after annealing for 4 h at 400 1C. 3.4. Optical and electrical studies Optical absorption coefficient (a) of the as-deposited and annealed films was calculated from the transmittance and reflectance spectra using the equation 2 2 312 3 !2 1 6ð1RÞ2 4 ð1RÞ2 25 7 þ a ¼ ln4 þR ð5Þ 5 d 2T 2T where d is thickness of the film. The band gap energy can be calculated using the equation ðahnÞn ¼ AðhnEg Þ

ð6Þ

where n is 2 or 2/3 for direct allowed or forbidden transitions respectively, and is 1/2 or 1/3 for indirect allowed and forbidden transitions respectively [27,28]. Direct allowed [3], direct forbidden [1,2], and indirect allowed [29] band gap have been reported for SnS thin films depending on the preparation methods and conditions during deposition. Fig. 6(a)–(c) shows the plot of ðahnÞ2=3 vs. hn of as-deposited and annealed SnS films. In the present case the plot ðahnÞn vs. hn gave a best straight line fit when n¼2/3, and hence our films are having direct forbidden transitions. Based on these calculations the band gaps of the films are determined as 1.3 eV, 1.28 eV, 1.33 eV and 1.42 eV for as-deposited and annealed at 150 1C, 250 1C, and 350 1C. The observed increase in band gap after annealing at 350 1C can be either due to grain fragmentation of larger grains leading to decrease in grain size or a change in composition due to the formation of SnS2 or Sn2S3 with band gap higher than SnS. In the literature there are reports about the change in band gap of SnS after annealing; however, there are discrepancies in the reports by different groups regarding the change in band gap. Devika et al. [14] reported that in the case of SnS films prepared by vacuum evaporation, band gap and activation energy decreased when annealed

6 x 103

Film

(αhν)2/3 (cm-1eV)2/3

Table 3 Atomic percentage of Sn and S obtained from EDS analysis of the as-deposited and annealed SnS films.

4

2

0

Eg=1.3 eV

1.5

Eg= 1.33 eV

2.0

1.5 hν (eV)

2.0

Eg=1.42 eV

1.5

2.0

Fig. 6. Variation of (ahn)2/3 with hn: (a) as-deposited SnS films, (b) annealed at 250 1C in air, and (c) annealed at 350 1C in air.

at temperatures below 100 1C; however, when annealing temperature was higher than 100 1C both band gap and activation energy increased continuously with annealing temperature. On the other hand the grain size was observed to decrease continuously with annealing temperature. Possible reason for the low values of activation energy and band gap at lower annealing temperature was explained as due to the high degree of preferred orientation maintained by the crystallites at low annealing temperature. Contrary to the observation by Devika et al., Gosh et al. [16] reported that in the case of vacuum evaporated SnS films the band gap decreased continuously with annealing temperature. In both cases mentioned above, the films were annealed up to 400 1C. In the case of SnS films prepared by PED, Yue et al. [13] observed that with annealing band gap increases. Our studies show that annealing at 350 1C do not provoke a significant grain fragmentation (see Fig. 4) which can lead to an increase in band gap due to smaller grain size, and hence we consider that the reason for the increase (0.1 eV) in band gap is due to formation of traces of SnS2, which is supported by both XRD (Fig. 1) and Raman (Fig. 2) results. As discussed in Section 3.1, the crystallite size increased from 10 to 14 nm after annealing indicating that the observed increase in band gap cannot be due to the recrystallization. Raman spectra clearly showed enhancement in intensity of the band corresponding to SnS2 after annealing, which supports the conclusion that the increase in band gap is due to the formation of SnS2 with band gap higher than that of SnS. The conductivity type (n or p) of the films was determined from the PEC measurement; the cell used for this study was identical to that used for the electrodeposition. A 0.1 M NaCl solution was used as the electrolyte and the experiment was carried out under intermittent illumination produced using an optical chopper and a tungsten halogen lamp as discussed in Ref. [11]. The intensity of illumination was 100 mW/cm2, and the cathodic potential scan range was from 0 to  0.6 V. Under illumination there was an increase in the current in the cathodic direction (Fig. 7) indicating p-type conductivity for the films. The anodic bias (0 to þ0.3 V) was also applied and there was no change in the current, confirming the conductivity type.

N.R. Mathews et al. / Materials Science in Semiconductor Processing 16 (2013) 29–37

1

I (x10-4Ampers)

Idark

(a)

0 (b) -1 Ilight

-2 -0.8

-0.6

-0.4 -0.2 V(Volts)

0.0

0.2

Fig. 7. PEC response under intermittent illumination, response is measured by applying both cathodic (negative) and anodic (positive) potentials: (a) as deposited SnS and (b) SnS film annealed at 350 1C. Idark and Ilight represent the current in the dark and light respectively.

Conductivity (x10-6 Ω cm)-1

(d) 350°C (c) 250°C

3.5

Thermally stimulated current generation in the film was studied under dark conditions using Al/SnS Schottky diodes to understand the activation energy of traps which control the conduction in SnS thin films. The details of diode fabrication and the current collection are explained in Section 2.2. The activation energy can be estimated using the equation [30]:

(b) 150°C

3.0 2.5

(a) as deposited

s ¼ KeEa =kT

50

where s is the conductivity, Ea is the activation of energy of the trap, k is the Boltzman constant, T is the temperature, and K is a constant related to the hopping-tunneling nature of impurity conduction. Using Eqs. (7) and (8)

0.5

0

100 150 time(s)

200

250

Fig. 8. The photoconductivity response of SnS films: (a) as-deposited, and annealed at; (b) 150 1C, (c) 250 1C, and (d) 350 1C. Inset is the sketch of the device structure used in this measurement.

Fig. 8 shows the photoconductivity data of as-deposited and annealed SnS films, measured in the sandwich configuration using a carbon paint electrode on the film surface and the transparent conductive oxide substrate as the electrical contacts. The area of carbon electrode was 0.06 cm2 (0.2 cm  0.3 cm). Conductivity was calculated from the measured current using the equation Id VA

the annealing results in loss of S and hence the film is slightly rich in Sn. The presence of excess Sn may lead to higher conductivity for the film [14]. The photosensitivity (sl–sd)/sd, where sl and sd are the electrical conductivities under light and dark, was calculated for these films. The photosensitivity of the as-deposited and the film annealed at 150 1C was about 0.08 and as the annealing temperature is increased the photosensitivity decreased. Hence, low temperature annealing is proposed for SnS films since it maintains the photoconductivity of the films and increases its conductivity by one order. The possible reason for the decrease in photosensitivity when annealed at higher temperature can be due to oxidation of the material leading to the formation of SnS2 or Sn2S3 as discussed above. The as-deposited and annealed films exhibited a transient current in dark after the illumination period, indicating the emission of carriers from trap states. Activation energies of various trap states are discussed in Section 3.5. 3.5. Activation energy

4.0

s¼

35

ð7Þ

where I is the measured current, d is the film thickness, V is applied voltage which is much higher than the open circuit voltage of the device, and A is the device area. This equation was derived using the equation of resistivity and the Ohms law. A constant bias of 0.1 V was applied in all the cases. These results show that the as-prepared and the annealed films are photosensitive. Electrical conductivity of the as-deposited films in dark is about 5  10  7 O cm  1 and that of annealed at 150 1C is about 3  10  6 O cm  1. The increase in dark conductivity with annealing temperature can be due to recrystallization and change in composition of the films. According to the EDS analysis (Section 3.3.2),

I¼

VA Ea =kT Ke d

ð8Þ

ð9Þ

In order to estimate the activation energy, the current values were plotted against 1000/T in a semi-logarithmic scale [31–33] and the results are shown in Fig. 9. The data of as-deposited film exhibited three activation energies: 0.1, 0.05, and 0.03 eV at distinct temperature regions. These activation energies correspond to three shallow traps in the forbidden gap of the SnS. The experimentally determined values of the activation energies of as-deposited and annealed films are presented in Table 4. The shallowest level with energy 0.03 eV is pushed more close to the band edge after annealing at 150 1C, and finally disappeared after annealing at 350 1C. However, from Table 4 it can be seen that there is no noticeable change in activation energies of the two traps with energies 0.1 and 0.05 eV due to annealing, indicating that the annealing temperature of 350 1C is not enough to induce recrystallization leading to changes in these trap states. The above argument is supported by the AFM and SEM images shown in Figs. 3 and 4 where the effect of recrystallization is negligible. This study reveals that in order to achieve recrystallization in SnS films, higher annealing temperature is needed. However, annealing at higher temperatures induce the formation of SnS2 and

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N.R. Mathews et al. / Materials Science in Semiconductor Processing 16 (2013) 29–37

T1

0.002479 T2

T1

0.000912 ln(i) (A)

T1

T3

0.000335 T2

as deposited

T2

T3

0.000123 0.000045

150οC

350οC

0.000017 2

3

4

5 6 1000/T(K-1)

7

8

9

Fig. 9. The plot of Ln (I) vs. 1000/T at an applied bias of 0.3 V, the notations T1, T2 and T3 corresponds to the three traps.

Table 4 Activation energy values (in eV) of the three shallow levels in the band gap of SnS film.

As deposited 150 1C annealed 350 1C annealed

Trap1 (T1)

Trap2 (T2)

Trap3 (T3)

0.11 0.10 0.11

0.052 0.05 0.053

0.03 0.022

Sn2S3 as seen in Figs. 1 and 2. SnS2 is an n-type material with band gap higher than 2 eV, hence it is undesirable in the SnS films. The above discussions reveal that in order to develop SnS as a competitive solar cell material it is imperative to develop adequate post-deposition treatments which can maintain the film stoichiometry and enhance the recrystallization of SnS thin films. 4. Conclusions In this paper, we have presented the results of the postdeposition annealing of SnS thin films in air. Structural studies showed that the film has reasonably good stability up to 250 1C annealing temperature and no phase segregation was observed; however, annealing at higher temperatures resulted in the formation of SnS2 as a secondary phase. Raman spectra showed characteristic modes of the SnS and evidence of trace amounts of SnS2 was observed, which is in agreement with the XRD data. Annealing at 350 1C resulted in an increase in band gap by 0.1 eV, which we interpret as due to the oxidation of SnS at higher temperatures forming traces of SnS2 rather than due to the morphological changes. Dark conductivity of the film increased by one order after annealing at 150 1C; however, annealing at higher temperatures resulted in decrease of photosensitivity due to the formation of secondary phases. The conductivity of the film is controlled by three shallow trap levels with activation energies 0.1, 0.05, and 0.03 eV. Our studies conclude that in order to achieve a noticeable recrystallization for SnS prepared by pulse electrodeposition, annealing temperatures higher than 350 1C are needed; however, instability of the SnS at higher temperatures is an obstacle in high temperature annealing, and

hence it is necessary to develop alternate technologies such as the use of a fluxing agent which can promote the recrystallization at lower temperatures.

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