Surface processing of CdZnTe crystals

Surface processing of CdZnTe crystals V.A. Gnatyuk*a,d, O.I. Vlasenkoa, S.N. Levytskyia, E. Dieguezb, J. Croccob, H. Bensalahb, M. Fiederlec, A. Faulerc, T. Aokid a V.E. Lashkaryov Institute of Semiconductor Physics of National Academy of Sciences of Ukraine, Prospekt Nauky 41, Kyiv 03028, Ukraine; b Departemento de Fisica de Materiales, Universidad Autonoma de Madrid, Carretera de Colmenar km 15, Madrid 28049, Spain; с Freiburger Materialforschungszentrum, Albert-Ludwigs-Universität, Stefan-Meier-Straße 21, Freiburg 79104, Germany; d Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan ABSTRACT The Cd1-xZnxTe (x = 0.1) crystals from two different manufacturers were studied by photoconductivity (PC) measurements. The samples 1 and 2 were subjected to chemical etching and irradiation with nanosecond laser pulses. The IR transmittance spectra of the crystals before and after laser irradiation were also monitored. The PC spectrum of the sample 1 had a typical one-band shape while the spectrum of the sample 2 exhibited two bands roughly corresponding to the bandgaps of CdTe and Cd1-xZnxTe that could be attributed to inhomogeneities in the surface region of the crystal. The positions of the maximum and red edge of the PC spectra did not correspond to the component compositions x in the bulk of Cd0.9Zn0.1Te crystals, however chemical polishing etching of the samples in a brominemethanol solution or/and laser irradiation led to this correspondence. Moreover, depending of laser pulse energy density, irradiation of Cd1-xZnxTe crystals resulted in a short-wavelength shift of the PC spectra, transformation of two bands to one in the case of the sample 2, and an increase in the photosensitivity of the semiconductor. The laser processing provided equalization of parameters in the surface and bulk regions. Keywords: CdZnTe crystals, surface processing, chemical treatment, laser irradiation, photoconductivity spectra 1. INTRODUCTION Recent progress in growth technologies of Cd1-xZnxTe semiconductors has provided the production of detector-grade crystals with a low number of accidental impurities and native defects in the bulk, and with suitable electrical charge transport properties1-3. In fabrication of X/gamma-ray detectors with either ohmic or barrier contacts, it is important to obtain a clean, structurally perfect and smooth surface of CdTe-based crystals before applying various technological procedures, thus the crystal surface state and hence surface pretreatments play a key role in device performance4-8. Chemical polishing with Br-based solutions is usually used to remove damaged surface regions of crystals after cutting, lapping and mechanical polishing3-10. However, chemical etching of CdTe-based semiconductors, particularly in bromine solutions results in the crystal surface enriched with Te which can be oxidized with time. Another etching solutions also led to contamination of the crystal surface with products of chemical reactions. Recently, pulsed laser irradiation has been successfully used for surface processing of CdTebased crystals and modification of the properties of a surface layer of semiconductors8-12. In the present work, surface processing of Cd1-xZnxTe crystals (x = 0.1), obtained from different manufacturers, was carried out by chemical etching and irradiation with nanosecond pulses of the second harmonic of a YAG:Nd laser. The state and properties of the surface region of Cd0.9Zn0.1Te crystals was studied by photoconductivity (PC) and transmittance measurements were carried out. It has been shown that laser irradiation can be used for homogenization of the component composition of the solid solution and modification of a surface layer which has parameters different than those in the bulk of CdTe-based crystals that is usually observed in semiconductor samples after their storage in ambient air conditions8-11. The obtained results can be used for optimization of surface processing of the grown Cd0.9Zn0.1Te crystals which have been developed for instruments and devices of identification of radioactive sources13. *gnatyuk@lycos.com; phone 38 044 5258437; fax 38 044 5258342 Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIV, edited by Ralph B. James, Arnold Burger, Larry A. Franks, Michael Fiederle, Proc. of SPIE Vol. 8507, 85071S · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.937953 Proc. of SPIE Vol. 8507 85071S-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms 2. EXPERIMENT The investigated detector-grade single crystals were grown by the Crystal Growth Laboratory of the Department of Materials Physics of the Autonomous University of Madrid (samples 1) and by the Freiburg Materials Research Center of the Albert-Ludwig University of Freiburg (samples 2)6,7,13. The resistivity of all crystals was ρ ~ 1010 Ω·cm at T = 300 K. Three kinds of treatment (surface processing) of the semiconductor samples were used: 1. Cd0.9Zn0.1Te crystals, polished by manufacturers, were stored in the ambient air for a long time (several months). 2. The Cd0.9Zn0.1Te wafers were subjected to chemical cleaning and etching by using the following procedures: (1) cleaning in acetone and methanol; (2) polishing etching in a 5% bromine-methanol solution; (3) thorough rinsing in methanol. 3. The surface of samples was entirely and uniformly irradiated in air by nanosecond (t = 7 ns at FWHM) single pulses of the second harmonic (λ = 532 nm) of a YAG:Nd laser. The incident laser pulse energy density was varied in a wide range (below and above the melting threshold of the semiconductor surface region). The photoconductivity (PC) spectra were measured at a fixed modulation frequency (400 Hz) using a MDR diffraction spectrometer at T = 300 K. The data processing was performed taking into account the correction for the spectral distribution of a light source (incandescent lamp). In addition, the transmittance spectra of the Cd0.9Zn0.1Te crystals before and after laser irradiation were measured in the near-IR and fundamental absorption regions. 3. RESULTS AND DISCUSSION Figure 1(a, b) shows two kinds of the PC spectra measured for the Cd0.9Zn0.1Te samples 1 and 2, respectively. Curves 1 were obtained for samples after storage in air before surface processing and curves 2 were measured for crystals subjected to chemical polishing etching. Sample 1 (a) (1) before etching (2) after etching (b) Sample 2 (1) before etching (2) after etching 40 10 1 2 750 λ, nm 800 1 20 5 0 700 60 IPC, arb. units IPC, abs. units 15 850 0 700 2 750 λ, nm 800 850 Figure 1. PC spectra of Cd0.9Zn0.1Te samples 1 (a) and 2 (b) before (curves 1) and after (curves 2) chemical polishing etching. The bandgap Eg determined from the PC spectra of the samples before surface treatment (Figure 1(a, b), curves 1) does not correspond to the component composition x = 0.1 of the semiconductor1,14,15. Chemical etching results in a shift of the maximum and red edge of the spectra toward shorter wavelengths (Figure 1(a, b),curves 2), and thus the values of Eg calculated from the spectra become higher, i.e. closer to the bandgap of Cd0.9Zn0.1Te (Eg ≈ 1.53 eV). Two bands in the PC spectrum of the sample 2 (Figure 1(b)) are associated with the fundamental absorption edges of CdTe (1.48-1.49 eV) and Cd0.9Zn0.1Te (1.52-1.53 eV)1-3. These bands can be attributed to inhomogeneities of the surface region of the sample 2 which contains the areas with different x = 0-0.1. Proc. of SPIE Vol. 8507 85071S-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms IPC, arb. units 30 20 3 4 Sample 1 (1) after etching after irradiation with J: 2 (2) - 78 mJ/cm 2 (3) - 128 mJ/cm 2 (4) - 197 mJ/cm 10 0 700 (a) 80 2 1 4 3 750 2 1 λ, nm 800 Sample 2 4 (1) after etching after irradiation with J: 2 (2) - 67 mJ/cm 2 (3) - 83 mJ/cm 2 (4) - 108 mJ/cm 100 IPC, arb.units 40 60 40 3 0 700 850 4 3 2 1 2 20 (b) 4 1 λ, nm 750 800 850 Figure. 2. PC spectra of Cd0.9Zn0.1Te samples 1 (a) and 2 (b) before (curves 1) and after (curves 2-4) irradiation by laser pulses with increasing energy density J. Laser irradiation of the chemically polished Cd0.9Zn0.1Te crystals with energy density in the range J ≈ 45-140 mJ/cm2 increases the PC signal IPC for both the samples 1 and 2 (Figure 2(a, b), curves 2-4). However, when J > 140 mJ/cm2 the intensity of IPC decreases (Figure 2(a), curve 4) in comparison with the maximally photosensitized crystals (curve 3). In the case of the sample 2, irradiation with the certain energy density J ≈ 100 mJ/cm2 leads to the transformation of the PC spectrum profile and only one band with the peak corresponding to the bandgap of Cd0.9Zn0.1Te (Eg ≈ 1.53 eV) is observed (Figure 2(b), curve 4). This is the evidence of an equalization of the component composition (x ≈ 0.1) and improvement in homogeneity of the structural characteristics in the surface area of the sample 2. 0.6 2 Sample 1 3 4 1 0.4 IT, arb. units 5 (1) after etching after irradiation with J: 2 (2) - 47 mJ/cm 2 (3) - 102 mJ/cm 2 (4) - 140 mJ/cm 2 (5) - 194 mJ/cm 0.2 0 800 820 λ, nm 840 860 Figure 3. Transmittance spectra of the chemically etched Cd0.9Zn0.1Te crystals (samples 1) before (curve 1) and after (curves 2-5) irradiation by laser pulses with increasing energy density J. Laser irradiation of the chemically etched Cd0.9Zn0.1Te crystals with energy density J ≈ 45-140 mJ/cm2 results in an increase in the transmittance IT in the near-IR region (Figure 3, curves 2-4). This can be attributed to removing a thin film with a narrower bandgap which is usually formed at the surface of the CdTe-containing semiconductor crystals after chemical surface processing3-10. An additional reason of an increase in IT can be annihilation of point defects in the bulk of the sample and/or gettering them by extended defects. These processes could be stimulated by laser-induced stress and shock waves16. Irradiation of Cd0.9Zn0.1Te crystals with laser pulse energy density J > 140 mJ/cm2 leads to a decrease of the transmittance (Figure 3, curve 5) because of significant alterations of the morphology and structure of the surface region and formation of point defects in the bulk of the semiconductor. Proc. of SPIE Vol. 8507 85071S-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms 1.535 2.6 2.2 (1) - from the literature (2) - from the literature (3) - from the equation below 2.0 1 Sample 1 1.530 2.4 2 1.520 1 (1) - from transmittance (2) - from the PC maximum (3) - from the PC red edge 3 1.515 1.510 1.505 Eg, eV Eg, eV 1.525 3 1.8 2 1.6 2 Eg = 1.49 + 0.4x + 0.51x (eV) 1.4 0 50 100 J, mJ/cm 150 2 Figure 4. Dependencies of the bandgap Eg of the surface region of Cd0.9Zn0.1Te crystals on the energy density of laser pulses J under irradiation of the sample 1. Estimations are based on the transmittance (curve 1) and PC spectra: using the positions of the PC maximum (curve 2) and the red edge of the PC spectrum (curve 3), respectively. 200 CdTe 0.2 0.4 x 0.6 0.8 ZnTe Figure 5. Calculated dependencies of the bandgap of Cd1-xZnxTe semiconductor solid solutions on the component composition x (0 ≤ х ≤ 1). The calculations were carried out with expressions taken from the literature2,14,15 (curves 1 and 2) and using the computed equation shown in the figure(curve 3). Figure 4 demonstrates the bandgap Eg in the surface region of the irradiated crystals in dependence on energy density J of incident laser pulses. The values of Eg were determined from the transmittance measurements (curve 1), and from the positions of the maximum (curve 2) and red edge (curve 3) of the PC spectra. The calculated values of Eg of Cd1-xZnxTe solid solution in dependence on its component composition x are shown in Figure 5. The best estimation of the bandgap of the investigaterd Cd0.9Zn0.1Te crystals has been obtained at the use of the PC maximum positions (Figures 1(a) and 2(a, b)) and computed equation: Eg = 1.49 + 0.4x + 0.51x2. 4. SUMMARY AND CONCLUSION It has been found by investigation of the PC spectra of Cd1-xZnxTe (x = 0.1) that the surface region of the crystals has a bandgap narrower than the corresponding value in the bulk of the semiconductor. A thin surface film with the component composition x < 0.1 has been revealed on the surface of Cd0.9Zn0.1Te crystals after storage of the samples in ambient air conditions. This film is the reason of the narrow bandgap surface region in the samples before surface processing. Chemical polishing etching of Cd0.9Zn0.1Te crystals removes a surface film with a narrow bandgap. The same effect can be achieved by irradiation of the crystal surface with nanosecond laser pulses. Laser irradiation of Cd0.9Zn0.1Te crystals with nanosecond pulses results in an increase in homogeneity in the component composition x of the surface area and equalization of its structural characteristics. The bandgap Eg of Cd0.9Zn0.1Te crystals has been estimated based on the PC and transmittance spectra and calculated using the computed equation. The best fit is when Eg is found based on the position of the PC spectra maximum. The developed techniques of the surface processing of Cd1-xZnxTe crystals, particularly nanosecond laser treatment can be used for homogenization of the surface region and modification of the surface state before formation of electrical contacts and electrode deposition in the fabrication of Cd1-xZnxTe-based high energy radiation detectors. ACKNOWLEDGMENTS This research has been performed in the framework of the Collaborative Project COCAE (grant agreement No 218000) of the European Community’s Seventh Framework Programme (FP7/2007-2013). 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