Properties of Ag doped ZnTe thin films by an ion exchange process

Applied Surface Science 191 (2002) 280±285

Properties of Ag doped ZnTe thin ®lms by an ion exchange process Akram K.S. Aqili*, Asghari Maqsood, Zul®qar Ali Thermal Physics Laboratory, Department of Physics, Quaid-I-Azam University, Islamabad, 45320, Pakistan Received 12 October 2001; accepted 17 March 2002

Abstract ZnTe thin ®lms prepared by two sourced thermal evaporation were immersed in AgNO3 solution for different time periods, then heated in vacuum. The resistivity of the doped ®lm reduced to 0.01% of the resistivity of the undoped ®lm. The effect of Ag doping on the structure of the ®lms was studied by X-ray diffraction (XRD), while optical properties such as ®lm thickness, refractive index, absorption coef®cient and optical band gap of the ®lms were calculated by ®tting the transmittance in the range 400±2000 nm. # 2002 Elsevier Science B.V. All rights reserved. PACS: 78.20.Ci; 78.20.-e; 73.61.Ga Keywords: ZnTe ®lm; Ag doped; Optical properties; Electrical properties

1. Introduction Recently the interest in ZnTe thin ®lms increased due to possible application as back contact for CdS/ CdTe thin ®lms solar cells [1±6] and other optoelectronic applications in the visible region [7±10]. The electrical conductivity as well as the optical properties are important parameters for such applications. Ag as group I element acts as acceptor dopant in II±VI semiconductors [11], which lead to an increase of the electrical conductivity of ZnTe. On the other hand, ion exchange process was recently utilized for doping II±VI semiconductors by Cu and Ag [12±14] due to the simplicity of the

*

Corresponding author. Tel.: ‡92-51-828187; fax: ‡92-51-9210256. E-mail addresses: [email protected], [email protected] (A.K.S. Aqili).

method. In this work we present our result of ZnTe ®lms doped with Ag by an ion exchange process. 2. Experimental Pure Zn and Te (>99.99%) were used as source materials for evaporation. The materials were placed into two graphite crucibles with a hole of 2 mm diameter on the top to act as point sources. The crucibles were heated independently by 500 W quartz lamps connected to the main through temperature controllers with K-type thermocouples inserted into the graphite crucibles. An IR heater was used for substrate heating, while a quartz crystal was used for monitoring the evaporation rate. The evaporation was carried out into an E306 coating system. The chamber was evacuated to 10 6 mbar before starting the evaporation process. The substrate temperature

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 1 8 - 0

A.K.S. Aqili et al. / Applied Surface Science 191 (2002) 280±285

was 400 8C and the source temperatures of Zn and Te were 540 and 480 8C, respectively (i.e. the ¯ux ratio Zn/Te was 2). After deposition the ®lm was kept at the same substrate temperature for 30 min, then cooled down below 100 8C before breaking the vacuum. Many ®lms have been prepared with similar deposition parameters in order to study the effect of post treatment with AgNO3 solution. The solution contained 0.4 g (AgNO3) in 1 l of H2O and was kept at a temperature of 60  2 8C. The immersion time was varied from 30 to 240 s for different ®lms. The ®lms after immersion were cleaned in distilled water bath and dried by dried nitrogen. Subsequently, the ®lms were heated in vacuum (10 6 mbar) at 400 8C for 1 h. The composition of the ®lms was measured by SEM attached to the electron microprobe analyzer (EMPA), the structure of the heated ®lms was studied by X-ray diffraction (XRD) using Co Ka1 radiation. The transmission spectra in the range 400± 2000 nm have been recorded by a Perkin-Elmer Lambda19 UV±VIS±NIR spectrophotometer with UV-WinLab software, while the electrical resistivity, determined of cut samples with evaporated Au contacts according to Van der Pauw geometry, was measured as a function of temperature (30±200 8C). 3. Results and discussion The EMPA spectra of undoped and doped sample are shown in Fig. 1 and the Ag concentration for

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Fig. 1. EMPA spectra of undoped (top) and Ag doped (bottom) ®lms.

different immersion times are given in Table 1. The results show that the Ag concentration does not increase linearly with immersion time. It could be due to the ion exchange of Ag ions with the ZnTe ®lm

Table 1 The optical (with comparison) and electrical results of the ®lms Immersion time (s)

Ag concentration d (nm) (at.%)

0 30 60 120 240 ZnTe crystal ZnTe film Cu-doped ZnTe film by rf sputtering

0 2.2 2.8 3.4 3.8 ± ± ±

a

[18]. [19]. c [14]. b

573 512 556 522 531 ± ± ±

n n0

b

2.64 2.65 2.69 2.72 2.76 2.70a 2.65±2.74b 2.55±2.65c

1.22E5 1.22E5 1.22E5 1.25E5 1.23E5 ± ± ±

E1 ˆ n20

Eg (eV)

r (O cm) Ea (eV) s0 (O cm)

6.97 7.02 7.24 7.40 7.62 ± ± ±

2.244 2.238 2.235 2.220 2.218 2.26 2.24±2.23 2.26±2.27

5.5E6 5.1E4 4.7E3 2.5E3 5.5E2 ± ± ±

0.74 0.62 0.53 0.45 0.38 ± ± ±

4.7E5 1.8E5 7.9E4 1.3E4 4.7E3 ± ± ±

1

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s are the refractive indices of the ®lm and the substrate, respectively, d is the thickness of the ®lm, a the absorption coef®cient of the ®lm and l the wavelength. The refractive index of the substrate could be calculated from the following formula [15]:  1=2 1 1 ; sˆ ‡ 2 1 Ts Ts where Ts is the transmission of the substrate. In the wavelength range 400±2000 nm, s was found to be 1:527  008. Using an empirical formula for the n dependence of l [14±16] as: n ˆ n0 ‡

Fig. 2. XRD pattern of the heated ®lms (400 8C) for (I) undoped, immersed for (II) 60 s and (III) 240 s.

that decreased when the ®lm was covered with layers of Ag. The XRD pattern of the heated ®lms (Fig. 2) shows the cubic structure of ZnTe and no peaks related to Ag or Ag2Te were observed. There was a little effect on the ®lms orientation as well as the intensity of the peaks. The thickness and refractive index of the ®lms were calculated by ®tting the transmission data to the following equation [14,15]: Tˆ

B

Ax ; Cx cos…f† ‡ Dx2

(1)

where T is the normal transmittance for the system consisting of a thin ®lm on a transparent substrate surrounded by air (refractive index ˆ 1), and taking into account all multiple re¯ection at the interface for the case of k2 ! n2 , which is true for this kind of semiconductor thin ®lms [15,16]. The other variables are de®ned as: A ˆ 16n2 s, B ˆ …n ‡ 1†3 …n ‡ s2 †, C ˆ 2…n2 1†…n2 s2 †, D ˆ …n 1†3 …n s2 †, f ˆ …4pnd†=l, x ˆ exp… ad†, k ˆ …al†=4p. Here n and

b ; l2

where n0 and b are constants. The absorption in the transparent region of the ®lm could be due to Urbach tail, defect absorption, multi-phonon absorption and light scattering [17]. The wavelength dependence of the absorption process is complicated, therefore, if the total absorption coef®cient is small, it may be expanded in a Taylor series around the photon energy far from any absorption line. If only terms up to second degree are included (a varies slowly with l), the relation for a can be written as follows: f g aˆc‡ ‡ 2 ; l l where c, f and g are constants. Fig. 3. shows the resulting ®t of Eq. (1) to the experimental data. It is clear that it gives a good ®tting in transparent as well as in the medium absorption region. For calculation of a in the high absorption region, the values of n and d from ®tted curve are used. The exact solution of Eq. (1) for x is as follows: xˆ

…C1 ‡ A=T†

‰…C1 ‡ A=T†2 2D

4BDŠ1=2

;

(2)

where C1 ˆ C cos…f† The long wavelength dielectric constant (E1 ˆ n20 ) can be calculated where (l ! 1), i.e. 1 ! 0: l2 The band gap was determined using the well-known dependence for a direct band gap, which is ahn  …hn Eg †1=2 , where hn is the photon energy and Eg

A.K.S. Aqili et al. / Applied Surface Science 191 (2002) 280±285

283

Fig. 3. Transmission curve of a ZnTe ®lm immersed for 240 s after heating (400 8C, 1 h) along with ®tting (‡).

the optical band gap. By extrapolating (ahn)2 against the incident photon energy (hn) plot, the band gap can be obtained. Heating the ®lms in vacuum (for 1 h at 400 8C) leads to an improvement of the transmittance of the ®lms as shown in Fig. 4. The decrease of the transmittance with immersion time of the heated ®lms is shown in Fig. 5. While a slight increase of the refractive index was observed as shown in Fig. 6,

Fig. 7 shows the shift of the optical band gap with increasing immersion time (i.e. Ag concentration in the ®lms). The measured room temperature (25 8C) resistivity of the ®lms after immersion in Ag solution was quite low (3±0.7 O cm), while after heating the resistivity increased (Table 1.). The reason is that the ion exchange takes place at the ®lm surface, which means that high concentrating Ag layer was formed on the

Fig. 4. Transmittance of the ®lm immersed in Ag solution for 240 s before and after heating (400 8C, 1 h).

Fig. 5. Transmittance of heated ®lms (400 8C, 1 h) immersed for different times.

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Fig. 6. Refractive index of the heated (400 8C, 1 h) ®lms immersed for different times.

®lm surface (where contacts were made). Diffusion of Ag into the ®lms took place after heating in vacuum at temperature of 400 8C. The resistivity values of the heated ®lms for different immersion times in Ag solution are listed in Table 1. The dark conductivity s of the ®lms as a function of temperature was studied in the range of 30±200 8C,. No irreversible behavior of the conductivity was observed for heated ®lms. Fig. 8 shows the dark conductivity of the heated ®lms as a function of temperature. The conductivity, in this region, can be represented by the following relation [20]:   …Ef Ev † s ˆ s…0† exp ; kT

Fig. 8. Dark conductivity of the heated ®lms for different immersion times plotted against temperature.

s(0) is the conductivity at 1=T ˆ 0 and Ef is the Fermi energy level. The dark conductivity activation energy (Ea BEf Ev ) was deduced by ®tting ln(s) versus 1/kT where the slope gives the activation energy (Fig. 9). The variation of the activation energy with immersion time is given in Table 1. The dark conductivity activation energy of an undoped ®lm was determined to 0.74 eV, which is little higher than that mentioned (0.65 eV) in the available literature [17]. It indicated that the ®lm prepared at this substrate was more stoichiometric.

where Ev represents the critical energy at which delocalization of states in the valance band occurs,

Fig. 7. Plot of a as a function photon energy of the heated (400 8C, 1 h) ®lms for different immersion times.

Fig. 9. Fitting of ln(s) against 1/kT of the heated (400 8C, 1 h) ®lm immersed for 240 s.

A.K.S. Aqili et al. / Applied Surface Science 191 (2002) 280±285

The decrease in the activation energy with immersion time is a result of the increased doping concentration and a stronger interaction among impurities. However, this activation energy is much greater than the reported ionization energy of Ag acceptors in a ZnTe crystal (0.11 eV) [21], which could be due to a lower doping level. Due to an ion exchange in the AgNO3 solution different ways of Ag incorporation into ZnTe might be possible. For example, by substitution of Zn2‡ by Ag1‡ the following interaction is possible [22]: Zn ‡ 2…Ag‡ ‡ e† ! Zn2‡ ‡ 2e ‡ 2…Ag†; Alternatively, Ag might be incorporated interstitially in ZnTe or it might form Ag2Te. However, the XRD results have not shown any peaks corresponding to Ag or Ag2Te due to the small quantity of Ag in the ®lms. But from the electrical and optical results we can expect that Ag was deposited at the surface of the ®lms and then diffused into the ®lm by heating in vacuum. 4. Conclusions The results indicate that due to immersion of ZnTe ®lms in AgNO3 solution, Ag was deposited at the surface of the ®lms by an ion exchange with AgNO3. The diffusion of Ag into the ®lms occurred by heating in vacuum at a temperature of 400 8C for 1 h. The results show a decrease of the electrical resistivity and of the dark conductivity activation energy with immersion time. No critical effect on the ®lm structure was observed. The optical results show a decrease of the transmittance, a shift of the optical band gap and a slight increase of the refractive indices of the ®lms. Acknowledgements This work was partially supported by The Abdus Salam International Centre for Theoretical Physics (ICTP) Italy, Pakistan Atomic Energy Commission

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(PAEC) and Quaid-i-Azam University Research Fund (URF). One of us (Akram K.S. Aqili) would like to acknowledge ICTP for the award of scholarship for a period of 1 year. References [1] L. Feng, D. Mao, J. Tang, R.T. Culins, J.U. Trenfny, J. Electron. Mater. 25 (1996) 1442. [2] T.A. Gessert, P. Sheidon, X. Li, D. Dunlavy, D. Niles, in: Proceeding of the 26th IEEE PVSC, 29 October±3 November 1997. [3] N.B. Chaure, J.P. Nair, R. Jayakrishan, V. Ganesan, R.K. Pandey, Thin Solid Films 324 (1998) 78. [4] J. Tang, D. Mao, L. Feng, W. Song, J.U. Trenfny, in: Proceedings of the 25th PVSC, 13±17 May 1996, Washington, DC, p. 925. [5] T.A. Gessert, A.R. Mason, R.C. Reedy, R. Maston, T.J. Cutts, P. Sheldon, J. Electron. Mater. 24 (1995) 1443. [6] D. Rioux, D.W. Niles, H. HoÈchst, J. Appl. Phys. 73 (1993) 8381. [7] S. Tatarenko, T. Baron, A. Arnoult, J. Cibert, M. Grum, A. Haury, J. Cryst. Growth 175/176 (1997) 682. [8] J.T. Trexler, J.J. Fijol, L.C. Calhoum, R.M. Park, P.H. Holloway, J. Cryst. Growth 159 (1996) 723. [9] M. Nishio, Q. Guo, H. Ogawa, Thin Solid Films 343/344 (1999) 508. [10] B. Reinhold, M. Wienenecke, Phys. B 273/274 (1999) 856. [11] H. Wolf, T. Filz, V. Ostheimer, J. Hamann, S. Lany, J. Cryst. Growth 214/215 (2000) 967. [12] M. Ristova, M. Ristov, P. Tosev, M. Mitreski, Thin Solid Films 315 (1998) 301. [13] M. Ristova, M. Ristov, Appl. Surf. Sci. 181 (2001) 68. [14] A.K.S. Aqili, A. Maqsood, Z. Ali, Appl. Surf. Sci. 180 (2001) 73. [15] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [16] A.K.S. Aqili, Z. Ali, A. Maqsood, Appl. Surf. Sci. 167 (2000) 1. [17] W. Chengyum, S. Xueguang, M. Junjun, S. Qingde, Z. Guiwen, Measur. Sci. Technol. 8 (1997) 911. [18] R.R. Reddy, Y.N. Ahammed, K.R. Gopal, D.V. Raghuram, Opt. Mater. 10 (1998) 95. [19] H. Bellakhder, A. Qutzourhit, E.L. Ameziane, Thin Solid Films 382 (2001) 30. [20] J.B. Webb, D.E. Brodie, Can. J. Phys. 52 (1974) 2240. [21] M. Aven, B. Segall, Phys. Rev. 130 (1963) 81. [22] Chemical predictor software, version 3.0, Ivan Kassal, Copyright 1997±1998.