Investigation of deep level defects in CdTe thin films

Investigation of deep level defects in CdTe thin films H. Shankar, A. Castaldini, E. Dieguez, E. Dauksta, A. Medvid, S. Rubio, and A. Cavallini Citation: AIP Conference Proceedings 1583, 145 (2014); doi: 10.1063/1.4865623 View online: http://dx.doi.org/10.1063/1.4865623 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1583?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of the origin of deep levels in CdTe doped with Bi J. Appl. Phys. 103, 094901 (2008); 10.1063/1.2903512 Simulated admittance spectroscopy measurements of high concentration deep level defects in CdTe thin-film solar cells J. Appl. Phys. 100, 033710 (2006); 10.1063/1.2220491 Deep energy levels in CdTe and CdZnTe J. Appl. Phys. 83, 2121 (1998); 10.1063/1.366946 Deep level photoluminescence spectroscopy of CdTe epitaxial layer surfaces Appl. Phys. Lett. 56, 1266 (1990); 10.1063/1.102532 Defect levels in electrodeposited ntype CdTe thin films J. Appl. Phys. 61, 2234 (1987); 10.1063/1.337985

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Investigation of Deep Level Defects in CdTe Thin Films H. Shankara, A. Castaldinia, E. Dieguezb, E. Daukstac, A. Medvidc, S. Rubiob and A. Cavallinia a

Department of Physics and Astronomy,University of Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy

b

Crystal Growth Lab, Department of Materials Physics, Faculty of Science, University Autonoma of Madrid, Ciudad Universitaria de Cantoblanco, 28049, Madrid, Spain. c

Institute of Technical Physics, Riga Technical University, 14 Azenes Str, Riga, Latvia, Department of Materials Abstract. In the past few years, a large body of work has been dedicated to CdTe thin film semiconductors, as the electronic and optical properties of CdTe nanostructures make them desirable for photovoltaic applications. The performance of semiconductor devices is greatly influenced by the deep levels. Knowledge of parameters of deep levels present in as-grown materials and the identification of their origin is the key factor in the development of photovoltaic device performance. Photo Induced Current Transient Spectroscopy technique (PICTS) has proven to be a very powerful method for the study of deep levels enabling us to identify the type of traps, their activation energy and apparent capture cross section. In the present work, we report the effect of growth parameters and LASER irradiation intensity on the photo-electric and transport properties of CdTe thin films prepared by Close-Space Sublimation method using SiC electrical heating element. CdTe thin films were grown at three different source temperatures (630, 650 and 700 °C). The grown films were irradiated with Nd:YAG LASER and characterized by Photo-Induced Current Transient Spectroscopy, Photocurrent measurementand Current Voltage measurements.The defect levels are found to be significantly influenced by the growth temperature. Keywords: : Defect levels in II-VI semiconductor, Thin films, Electrical properties, Solar cells. PACS: 71.55.Gs, 68.55.Ln, 73.61.Ga, 84.60.Jt

INTRODUCTION The request of low cost thin-film CdTe/CdS solar cells is growing more and more as their efficiency and reliability continue to improve. Although, the calculated maximum theoretical efficiency of CdTe solar cell is about 27% [1], large commercial module has reachedonly up to 18.7 % [2]. One of the major reasons for this relatively low performance is the presence of defects associated with deep energy levels in the forbidden gap of the CdTe absorber layer. These defects capture the excited charge carriers, which decreases the output current and open circuit voltage, thus lowering of the device efficiency [3]. Hence, the knowledge of the deep levels present in asgrown films and identification of their origin is the key factor in enhancing the efficiency of solar cells. PhotoInduced Current Transient Spectroscopy (PICTS) is the well-known techniques for the characterization of deep and shallow levels in the band gap of CdTe [4]. The nature of the defects is mainly determined by the sample grown technique. Close Space Sublimation (CSS) is one of the most successful techniques for CdTe thin films deposition [5]. However, most of the CSS experiments reported have been carried out using halogen lamps as heating systems [6,7]. Therefore, it is interesting to explore the application of other heating systems which can be easily implemented in CSS chambers, such as electric heaters, in order to simplify and increase the durability of the equipment used in the CSS fabrication of solar cells. The aim of this work is to identify and characterize the deep levels present in CdTe (untreated) and LASER treated CdTe thin films prepared by CSS method,at various source temperatures, using SiC electrical heating element.

EXPERIMENTAL CdTe thin films were deposited on the glass substrate by using a homemade CSS chamber system. The details of the design and construction of the CSS chamber are reported in Ref.8. The typical experimental parameters for CdTe thin film depositionare as follows: substrate temperature was kept close to 590 °C, with source temperatures equal to 630 °C, 650 °C and 700 °C, pressure of 1mbar, distance between source and substrate equal to 1 mm. The International Conference on Defects in Semiconductors 2013 AIP Conf. Proc. 1583, 145-149 (2014); doi: 10.1063/1.4865623 © 2014 AIP Publishing LLC 978-0-7354-1215-6/$30.00

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deposition time was 30 minutes in all cases. Under these conditions, uniform films of about 10 μm thickness were obtained. In order to explore the possibility of reducing the defects in the as grown films, the surface of CdTe was irradiated by pulses of Nd:YAG laser with wavelength λ=532 nm, pulse duration of τ =4 ns and power P=1 MW. The LASER beam spot of 3 mm diameter was moved with 20 μm steps over the surface of the sample. Before irradiation by LASER, the CdTe samples (630 °C and 700 °C) was covered by SiO2 layer, for preventing evaporation of Cd atoms. The thickness of SiO2 layer was ~100nm. All the films have p-type conductivity. On each of the films, two Ohmiccontacts were made by using GaAl alloy on same surface and their volt-ampere (I-V) characteristics were studied. Photocurrent analyses were carried out at room temperaturein planar ohmic contacts configuration. A monochromaticphoton beam of is obtained from a QTH lamp and focused into a monochromator. This photon is focused onto the sample and scanned across the crosssectionalsurface between the two ohmic contacts. The photogenerated carriers are swept out by the electric field due to the external applied bias and the photocurrent signal is collectedby a lock-in amplifier.PICTS analyses were carried out by a SULA Technologiessystem with a sample heating rate of 0.2 K/s. Theemission rate varied from 5 to 2x104 s−1. The excitation wavelength used was880 nm.The theoretical and technological approach of PICTS measurement and the way of extracting defect energy and cross section from the PICTS spectra are reported in Ref.9,10. Surface morphology of the films was studied by SEM (manufactured by Tescan, Czech Republic model:Mira/LMU).

RESULT AND DISUSSION SEM images of non-irradiated and irradiated by Nd:YAG laser CdTe films are shown in Fig.1. The motivation for the LASER treatment is to increase the life time of the excited charge carriers by decreasing the trap levels in the as grown films. From SEM images, it can be seen that the surface of CdTe becomes smoother and small nanoparticles are formed on the surface of the sample after irradiation by the LASER (Fig.1b). This must be due to the conglomeration of Te atoms, which can be used for plasmon resonance in solar cells for enhancement of light absorption. However, EDX (Energy Dispersive X-Ray Analyser) measurements (not reported here) did not show any changes in composition. This can be explained by the fact that electron beam in EDX method gives signal from relatively large volume of the sample, hence nanoparticles on the sample surface have an impact not detectable by EDX on the resulting signal. (a)

(b)

FIGURE 1.Typical SEM images of (a) non-irradiated and (b) irradiated by Nd:YAG laser CdTe films prepared at source temperature equal to 700 °C

The transport properties analyzed by the I-V characteristics were performed on CdTe thin films in dark at 300 K. As shown in Fig.2, the sample resistivity is alteredby the irradiation.It decreases in the CdTe films prepared at 630°C while it increases in the CdTe films grown at 700 °C. This different behavior in resistivity can be attributed to the redistribution of intrinsic defects in volume of the crystal due to presence of gradient of temperature, that is the


thermo-gradient effect[11,12],during the irradiation of the crystal by the laser. It is worth noting that, nevertheless, the Ohmic behavior of the thin films is not altered. From the photocurrent measurement (Fig.3 shows the film responsivity),the band gap of the CdTe films were found to be 1.47 eV for both LASER treated and untreated samples, which is a very promising result for the growth method here adopted as well as for the LASER treatment. In fact, this means that the film stoichiometry is the same of the crystalline CdTe, whose the photocurrent responsivity, also reported in Fig. 3, shows that the bandgap value is 1.44 eV. The shift in the band gap can be attributed to the quantum confinement related to the film nanostructures. There is no significant change in the responsivity of the sample before and after LASER irradiation.

FIGURE 2. IV characteristics of CdTe films before and after LASER irradiation

FIGURE 3. Photocurrent spectra ofCdTe films (a) before and (b) after LASER irradiation. In Fig. 3 (a) the responsivity of the single crystal is also reported (right side scale).

Figure 4 showstypical Arrhenius plotsachieved by means of several PICTS spectra to extract the apparent trap energy and capture cross section of the energy levels in the forbidden gap. PICTS measurements on CdTe thin film showed the presence of two classes of traps: midgap traps with anaverage energy value of 0.84-0.89 eV, and deep traps, the value of which spans between 0.9 and 1.2 eV. The obtained trap parameters are tabulated in table1. Following the conventions reported in literature [13], these traps were named as H1 and I, which are already identified and reported for CdTe single crystal. The origin for the level H1 can be attributed to the Te antisite defect [14] or to VCd associated with an impurity [15]. The trap I, found in all the thin films, is attributed to the Te vacancy


(VTe). Both H1 and I defects levels are electron traps.After LASER treatment only the defect level I is observed while H1 defect level vanishes. This may be due to the improvement in the crystallinity of CdTe thin films by the rapid annealing with LASER treatment. There are very few reports available for the investigation of defect deep levels by PICTS on CdTe thin films grown by CSS. Xavier Mathew et al [16] reported defect levels in CdTe thin films grown by Electro Deposition and CSS method which, however, are different from the here reported defect levels. This likely occurs because of the method of filmdeposition which determines the difference in the morphology and defect energy levels of the samples. TABLE (1). Summary of the deep levels identified by PICTS analyses in CdTe thin films.

Sample CdTe630 °C CdTe650 °C CdTe 700 °C CdTe630 °C LASER CdTe700 °CLASER

Trap Label I H1 I H1 I I I

Activation energy E (ev) 1.08 0.89 1.04 0.85 0.94 1.10 1.2

Capture crossection σ (cm-2) 2e-8 1e-10 3e-10 9e-12 9e-12 7e-10 7e-9

Attribution Te Vacancy (VTe) Teantisite (TeCd) Te Vacancy (VTe) Teantisite ((TeCd) Te Vacancy (VTe) Te Vacancy (VTe) Te Vacancy (VTe)

FIGURE 4. Typical Arrhenius plotsof (a)untreated CdTefilms and (b) LASER treated CdTe films

CONCLUSION The quality of the CdTe thin films grown by CSS method for solar cell device fabrication is tested with respect the defects present in the films as a function of the growth conditions and laser treatment. The usefulness of PICTS technique in identifying the defect levels in the CdTe thin film is demonstrated. Two electron traps was identified and the apparent activation energy and capture cross-section were estimated. The defects in the as grown samples are successfully minimized by LASER irradiation, as demonstrated by the PICTS and SEM results.

ACKNOWLEDGMENTS This work was supported by project “Investigation of the transport properties of nanostructured CdTe solar Cells” Project No. MATERA/MFM-1840 .


REFERENCES 1. K. Zanio, Cadmium Telluride (Semiconductors and Semimetals, vol. 13), Academic Press, New York, San Francisco, London, 1978 2. First Solar Press Release, February 26, 2013 3. J. Versluys, P. Clauws, P. Nollet, S. Degrave, M. BurgelmanThin Solid Films 451 –452 (2004) 434–438 4. A.Cavallini et al. Nuclear Instuments and Methods in Physics Research A 448 (2000) 558 5. Reference need J. Britt and C. Ferekides Appl. Phys. Lett. 62, 2851 (1993) 6. G. P. Herna´ndez, X. Mathew, J. P Enrı´quez, B. E. Morales, M. M. Lira, J. A. Toledo, A. S. Jua´rez and J. Campos, J. Mater. Sci., 2004, 39, 1515. 7. N. Romeo, A. Bosio, V. Canevari and A. Podesta´, Sol. Energy, 2004, 77, 795 8. J. L. Plaza, O. Martınez, S. Rubio, V. Hortelano and E. Dieguez Cryst. Eng. Comm, 2013, 15, 2314–2318 9. M. Tapiero, N. Benjelloun, J. P. Zielinger, S. El Hamd, and C. Noguet, J. Appl. Phys. 64, 4006 (1988). 10. X. Mathew, Sol. Energy Mater. Sol. Cells 76, 225–242 (2003 11. A. Medvid, Defects and on Diffusion Forum, Vols.210 - 212, pp.89-1 01, 2002 12. D. Gilles and E. R.Weber. PhysRev.Lett. vo1.64, pp.196-199, 1990. 13. A. Castaldini, A. Cavallini, and B. Fraboni, P. Fernandez, and J. Piqueras J. Appl. Phys. 83 (1998) 2121. 14. M. Fiederle,C.Eiche,M.Salk,R.Schwarz,K.W.Benz,W.Stadler, D.M. Hofmann,B.K.Meyer J. Appl.Phys 84(1998).6689 15. M. Berding Physical Review B.60(1999)8943–8950. 16. X. Mathew et al, Solar Energy Materials & Solar Cells 70 (2001) 379–393
