Effect of irradiation-induced disorder on the optical absorption spectra of CdS thin films

PHYSICA ELSEVIER

Physica B 240 (1997) 8 12

Effect of irradiation-induced disorder on the optical absorption spectra of CdS thin films K.L. Narayanan a, K.P. Vijayakumar a, K.G.M. Nair b,*, N.S. Thampi b aDepartment of Physics, Cochin University of Science & Technology, Cochin, 682 022, India bMaterials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, 603 102, India

Received 26 September 1996; received in revised form 15 May 1997

Abstract

Optical absorption studies have been carried out on CdS thin-films irradiated with low energy Ar + ions. The optical band gap was found to reduce with the increase in the irradiation dose. This was accompanied by a progressive reduction in the sharpness of the absorption band edge. These effects have been attributed to the irradiation-induced lattice disorder. Postirradiation annealing at 773 K resulted in the recovery of the optical band gap and absorption edge sharpness to unirradiated values. PACS: 61.80.-x; 78.50.Ge; 78.66.-w Keywords: Optical properties; CdS; Ion implantation

1. Introduction

Cadmium sulphide thin films find extensive applications in the area of solar cells and other photovoltaic devices. The large optical band gap of the material makes it ideally suited for application as a window layer of solar cells. CdS is also a promising material for the fabrication o f light-emitting devices in the green region. However, the preparation of homojunctions is difficult as CdS thin films prepared by various techniques are normally found to be n-type and conventional doping techniques are rather ineffective in converting n to p-type. Ion implantation is a wellestablished non-equilibrium technique for doping of * Corresponding author. 0921-4526/97/$17.00 Published by Elsevier Science B.V. PII S0921-4526(97)00428-6

semiconductors and there are a number of studies on CdS thin films where the investigators have observed n- to p-type conversion during implantation with N +, P+ and Bi + [1-3]. Ion implantation introduces considerable amount of lattice defects, which will modify the optical and electrical properties of the material. Since the presence o f defects and irradiationinduced disorder significantly affect the optical properties, optical absorption spectrometry is an ideal technique for investigating the effect of irradiation in semiconductor thin films. There have been a number o f optical absorption studies o f ion-implanted GaAs [4], GaP [5] and a-Si [6] thin films. However, there is not much published work on optical properties of ion implanted CdS thin films. The present paper discusses the optical absorption characteristics of argon ion

K.L. Narayanan et al./ Physica B 240 (1997) 8-12

implanted CdS thin films. The results of postirradiation annealing studies are also presented.

9

1.5

' ' 2. Experimental

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Thin films of cadmium sulphide were prepared by chemical bath deposition technique [7]. The thickness of the films was measured using Sloan Dek-Tak 3030 surface profilometer. The films were irradiated with 80 keV Ar + ions from a low-energy accelerator at room temperature to various doses in the range of 1015-1017 ions/cm 2. The optical absorption spectra of the as-deposited and irradiated samples were recorded in the wavelength range 400-800 nm by an UV-VIS Chimito 2500 spectrophotometer. Annealing of the irradiated films was carried out in argon atmosphere at various temperatures in the range from 473 to 773 K. The annealing time was kept fixed at 120 rain.

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Fig. 1. Optical absorption spectra of as-deposited and implanted films. (o) As deposited; (m) dose 5 ×1015 ions/cm2; (o) dose 1 × 1016 ions/cm2; ([]) dose 1 × 1017 ions/cm 2.

1.5 3. Results and discussion 1.3

The thickness of CdS thin films prepared by chemical bath deposition was measured using Sloan DekTak 3030 surface profilometer and was found to be 1.8 ~m. The structure of the films was examined using X-ray diffraction technique and it was found to be a mixture of both cubic and hexagonal phases. The thickness of the implanted region is about 1000 (100nm). The range of the 80keV Ar + ions in CdS is 63 nm with a straggling of 31 nm (calculated using TRIM code). The optical absorption spectra of the as-deposited and irradiated films are given in Fig. 1. It is seen that the absorption coefficient increases with irradiation dose. The absorption coefficient of the films at 550 nm is plotted as a function of irradiation dose in Fig. 2. The increased absorption during irradiation can arise due to several factors, such as increased carrier concentration due to implantation, production of metallic cadmium clusters due to the loss of sulphur and production of defect levels in the band gap. Each of this absorption process has their characteristic energy dependence [8]. The clear exponential increase of absorption coefficient with energy in this case strongly suggests that the increased absorption is primarily due to the production of defect levels in the band gap.

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A careful examination of the optical absorption spectra in Fig. 1 reveals that the absorption edge shifts to higher wavelengths with increasing irradiation dose suggesting a decrease in the optical band gap. CdS is a direct band gap material and the optical absorption coefficient, ~, is given by CX ( h v - E g ) 1/2,

(1)

where hv is the photon energy and Eg the optical band gap. The optical band gap can be found by plotting

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K L . Narayanan et al. / Physica B 240 (1997) 8-12

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coefficient, ~, in the band edge region to a function of the form

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Fig. 3. (a) Plot showing ~2 versus energy for the as-deposited and implanted films; (o) As deposited; (11) dose 5 x l015 ions/cm2; (e) dose 1 x l016 ions/cm2; (E3) dose 1 X 1017 ions/cm2. (b) The optical band gap Eg as a function of irradiation dose.

(0~)2 versus hv and extrapolating the linear portion of the curve to intersect the energy axis (Fig. 3(a)). The optical band gap Eg determined by this method is shown as a function of the irradiation dose in Fig. 3 (b), where we find a progressive reduction of the band gap with increasing irradiation dose. Another parameter which gets significantly affected by irradiationinduced disorder is the sharpness of the band edge, which is found to reduce with the increase in the irradiation dose. In order to quantify the band edge sharpness, it is common practice [9] to fit the absorption

where E0 is normally referred to as the inverse logarithmic slope. E0 as a function of the irradiation dose is given in Fig. 4 where it is seen that E0 increases with irradiation dose. Irradiation with Ar + ions produce point defects such as vacancies, interstitials and antisite defects. Defect clusters such as argon bubbles, dislocation loops, etc. can also be expected to form during irradiation. The defects produce band tailing. The observed reduction in the optical band gap as well as the loss of sharpness of the absorption edge both arise due to this band tailing. The relationship between the lattice disorder, the band edge sharpness and the optical band gap has been first established by Cody et al. [10] for explaining the optical absorption characteristics of hydrogenated amorphous silicon. They have considered both the static structural disorder caused by defects and dynamic phonon disorder both of which have the same effect on the electronic energy levels under adiabatic approximation. The relationship between E0 and the irradiationinduced disorder has the form =

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K.L. Narayanan et al. / Physica B 240 (1997) 8-12

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Fig. 6. Optical absorption spectra of CdS thin film in the as-deposited condition, after implantation to a dose of 5 x 1016ions/cm2 and after various stages of isochronal annealing; (a) as deposited; (b) implanted; (c) annealing at 473K; (d) annealing at 573 K; (e) annealing at 673 K; (f) annealing at 773K.

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(4) where Eg(0, 0) and E0(0, 0) are the values o f Eg and E0 in a defect free crystal at 0 K and D is the secondorder deformation potential which decides the effect o f temperature on the band gap. Eq. (4) suggests a linear relationship between Eg and E0. The optical band gap o f the unirradiated CdS film and those of films irradiated to various doses are plotted as a function of the corresponding E0 values in Fig. 5. A good linear dependence between E0 and Eg can be seen from Fig. 5. The observed strong correlation between Eg and E0 suggests that the decrease in band gap as a result of irradiation is predominantly due to lattice disorder produced during irradiation. Post-irradiation annealing studies were carried out on a sample irradiated to a dose of 5 x 1016 ions/cm z with 80 keV Ar + ions. The optical absorption spectra of the as-deposited, irradiated and annealed samples

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Fig. 7. Absorption coefficientof the films at 550nm as a function of annealing temperture. The absorption coefficient values of the as-deposited film is marked by * on the Y-axis. are given in Fig. 6. The isochronal annealing upto a temperature of 573 K did not give rise to any significant change in the optical absorption spectra of the sample as can be seen from Fig. 7 where the absorption coefficient at 5 5 0 n m is shown as a function o f annealing temperature. A sharp reduction in absorption is seen on annealing at 573 K. The band gap is

12

K.L. Narayanan et al./ Physica B 240 (1997) 8 12

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accomodation of such atoms. Since both E0 and Eg which are very sensitive to matrix strain recover to original values it is unlikely that argon atoms are still retained in the matrix. However, if argon atoms cluster to form bubbles during annealing the strain due to the accommodation of argon atoms is relieved. The bubbles may still reduce the optical quality of the film and give rise to increased absorption. The linear relationship between E0 and Eg is also obeyed during annealing, as can be seen from Fig. 5 where E0 and Eg values obtained from various absorption spectra recorded during annealing are also included.

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ANNEALING TEmPERATUrE (K) Fig. 8. Variation of E0 and Eg during post-irradiation annealing shown as a function of annealing temperature. (E0 and Eg values of the as-deposited samples are marked by * on the corresponding axes. The arrow marks on the axes indicate the as-irradiated values.

found to increase on annealing and recovers back to the as-deposited values when annealed at 773 K for 2 h (Fig. 8). The band edge also progressively sharpens on annealing and the inverse logarithmic slope returns to the as-deposited values at 773 K. The increase of the band gap and sharpening of the band edge signify the reduction of lattice disorder, arising out of defect annealing. The near-complete recovery of these parameters to the pre-irradiation values indicate that almost complete annihilation of the defects produced during irradiation has occurred. However, the absorption coefficient does not completely recover to as-deposited values on annealing as seen from Fig. 7. The possible reasons for this are as follows. Though most of the defects produced during implantation will get annealed out, the implanted argon atoms will be still retained in the matrix. Further, the film quality could also reduce substantially during annealing giving rise to cracking. The remnant absorption might be attributed to such effects. It is interesting to note that though absorption coefficient does not completely recover back to as-deposited value during annealing, E0 and Eg values practically go back to as-deposited values. Such an observation is significant. If the residual absorption is due to argon atoms substitutionally incorporated in the CdS matrix we should have also observed the effect of strain created by the

4. Summary and conclusions

The effect of irradiation on the optical absorption characterisation of CdS thin films has been investigated. A progressive decrease in Eg and the band edge sharpness is observed with increasing irradiation dose which has been attributed to irradiation produced damage. The dependance of band gap on the irradiationproduced lattice disorder might be profitably used in tailoring the properties of semiconductor materials. Acknowledgements

This work has been carried out under IUC-DAEF collaboration scheme. References [1] Y. Shiraki, T. Shimada, K.F. Komatsubara, J. Appl. Phys. 43 (1972) 710. [2] W.W. Anderson, J.T. Mitchell, Appl. Phys. Lett. 12 (1968) 334. [3] F. Chernow, G. Rose, L. Wahlin, Appl. Phys. Lett. 12 (1968) 339. [4] E.V.K. Rao, Phys. Stat. Sol. A 33 (1976) 683. [5] T. Pankey, J.E. Davey, J. Appl. Phys. 41 (1970) 697. [6] U. Zammit, K.N. Madhusoodanan, F. Scudieri, F. Mercuri, E. Wendler, W. Wesch, Phys. Rev. B 49 (1994) 2163. [7] K.L. Narayanan, K.P. Vijayakumar, K.G.M. Nair, N.S. Thampi, Bull. Mater. Sci., 20 (1997) 1. [8] J.l. Pankove, Optical processes in semiconductors, PrenticeHall, Englewood Cliffs, NJ, 1971, pp. 34-86. [9] U. Zammit, K.N. Madhusoodanan, M. Marinelli, F. Scudieri, R. Pizzoferrato, F. Mercuri, E. Wendler, W. Wesch, Phys. Rev. B 49 (1994) 14322. [10] G.D. Cody, T. Tiedje, B. Abeles, Y. Goldstein, Phy. Rev. Lett. 47 (1981) 1480.