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An optical absorption properties investigation of CuInSe2 by a simultaneous photoacoustic-photoconductivity measuring technique
Infrared Physics and Technology 105 (2020) 103194
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
An optical absorption properties investigation of CuInSe2 by a simultaneous photoacoustic-photoconductivity measuring technique Fatima Zohra Satour, Ameur Zegadi
T
⁎
Laboratoire: Croissance et Caractérisation de Nouveaux Semiconducteurs (LCCNS), Département d’Electronique, Faculté de Technologie, Université Ferhat Abbas Sétif 1, 19000 Sétif, Algeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoconductivity Photoacoustic CuInSe2 Near-infrared absorption Chemical defects
In this paper we describe a new photoacoustic cell design that allows simultaneous photoacoustic and photoconductive measurements of solid semiconductor samples. The two techniques are complementary. Photoacoustic spectroscopy is known for being a direct monitor of nonradiative de-excitation processes, while photoconductivity spectroscopy is a direct monitor on the photocurrent generated following the absorption of the radiation source. If used simultaneously, they should unveil valuable information on the properties of materials. As an application to the refined spectrometer we present a comparative study on the optical absorption properties of CuInSe2 single. The measurements were performed at room temperature close to the absorption edge. Appropriate theoretical models have been used to evaluate the absorption coefficient spectral dependence from measured data. The combined techniques showed varying sensitivities in detecting defects levels in different regions of the absorption spectrum of the compound, and thus offering more information on the defects structure of semiconductors in a quicker way.
1. Introduction Photoacoustic spectroscopy (PAS) continues to attract increasing attentions for its analytical capabilities in various research fields [1]. PAS is a non-destructive technique that can be employed in the study of optical, thermal and acoustic properties of materials of any form (solid, liquid, gel, powder or thin films). The method does not require electrical contacts, and therefore offers distinct advantages over many conventional techniques. PAS gives direct information on non-radiative absorption interactions. These are known to be directly linked to the band structure and to defects that are responsible of the mechanisms of energy losses [2]. Photoconductivity (PC) is a well established and documented technique, which is used in the analysis of the photoelectrical properties of semiconductors [3,4]. The alloys Cu(In,Ga)(Se,S)2, denoted as CIGS, when used as the absorber layer of solar cells have yielded high efficient devices. In fact, solar cells based on these materials have reached an efficiency of 22.9% [5]. Further improvements are possible by a better understanding of the material and device optoelectronic properties [6,7]. Several experimental photoacoustic and photoconductive studies of CuInSe2 (CIS) have been reported in the literature [8–13]. Yet, there is no reported study in which both techniques were employed in analyzing the same sample at the same time, this work redresses this issue. ⁎
In this paper, PAS and PC are combined together in order to analyse samples of p and n-conducting CuInSe2 single crystals. The spectra are measured close the fundamental absorption edge of the compound at room temperature in the photon energy range 0.7 eV ≤ hν ≤ 1.4 eV. Based on appropriate existing theoretical models of both techniques, the spectral dependence of the absorption coefficients is determined from measured spectra. A comparison between the results obtained from the two techniques is provided in the light of published data. 2. Materials and methods In photoacoustic spectroscopy when a modulated monochromatic light source illuminates a sample enclosed in an air-filled PA cell; the sample after absorbing the incident radiation generates a small amount of heat within the sample thermal diffusion length, lth. lth = (2βs/2πf)½ where βs is the thermal diffusivity of CuInSe2, βs ≈ 5⋅10−6 m2/s, and f being the modulating radiation frequency. The heat diffuses then across the sample and gives rise to temperature changes close to the sample surface. As a result, acoustic waves arise in the sample surrounding that is detected by an appropriate sensor. The spectrometer used in this study is an improved version of the previously described one [14], and is shown in Fig. 1(a). Its constituents are a xenon short-arc lamp as the exciting light source, a chopper, a
Corresponding author. E-mail address: [email protected] (A. Zegadi).
https://doi.org/10.1016/j.infrared.2020.103194 Received 11 November 2019; Received in revised form 9 January 2020; Accepted 10 January 2020 Available online 13 January 2020 1350-4495/ © 2020 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
Fig. 1. Schematic diagrams of: (a) the spectrometer; (b) the cell in which: (1) Backing material, (2) Sample, (3) Metal contacts, (4) Conducting wires, (5) Gas (air), (6) Optical window and (7) Microphone.
monochromator, and the cell. A near-infrared filter (λ = 900–2000 nm), is inserted after the monochromator for the infrared range study case. The measuring cell, which is the heart of the spectrophotometer, is a new design that allows both photoacoustic and photoconductivity measurements to be performed simultaneously of solid samples. This has been optimized in its geometric parameters to enhance the spectrometer responses (PAS and PCS). It is rectangular in shape and is made of aluminum. Its schematic diagram is shown in Fig. 1(b). The microphone gives the photoacoustic response. Simultaneously, the photoconductive response is obtained through the contacts deposited on the sample under test. The two signals (PA and PC) are, at first, amplified separately by highly sensitive ac amplifiers, and then phase sensitively detected using a multifunction digital lock-in amplifier. A computer is used for the acquisition and for the control. Good quality single crystal samples of CuInSe2 are used in this work. The crystal structure was analysed using a Bruker X-ray diffractometer. The main chalcopyrite structure peaks were present with a preferred orientation of (1 1 2). The samples were cut from ingots grown from stoichiometric compositions using the vertical Bridgman technique [15]. Both n and p-type samples, for comparison, are used. Prior to any measurement, the samples were polished on both sides, chemically etched in a 1% bromine in methanol solution, and finally washed with deionized water. The final thickness of the samples was around 1 mm. Ohmic Ni contacts have been deposited from a solution of NiSO4 on pconducting samples, while Cu contacts have been deposited from a solution of CuSO4 on n-conducting samples using the electrodeposition method [16]. The photoacoustic response is corrected for the spectral distribution of the optical system by normalising the response of the specimen to that carbon black powder. Similarly, PC spectra are also normalized to constant photon flux and to the dark conductivity of the samples. All spectra were measured at room temperature at f = 112 Hz as the modulating frequency. In photoacoustic spectroscopy, the modulating frequency serves to choose the part of the sample close to the surface to be exited thermally in relation to the constant of the thermal diffusion length (lth). In our case we have chosen a moderate frequency where both responses of the techniques were stable in the photon energy range of interest.
Table 1 Electrical conductivity type, thickness (ls) and elemental composition of the samples together with their corresponding Δm and Δs. Sample
S-1 S-2
Type
n p
ls (µm)
1050 1020
Composition Cu (at. %)
In (at. %)
Se (at. %)
23.23 22.75
26.59 27.02
50.18 50.23
Δm
Δs
−0.126 −0.158
−0.025 −0.032
microscope (JEOL JSM-7001F). The measurement was taken from several points of the samples surface and an average value was determined and the result is listed in Table 1. Also included are the two parameters Δm and Δs which determine the deviations from molecularity and valence stoichiometry, respectively. These are defined as [17]:
Δm =
Δs =
|u| −1 |In|
2 |Se| −1 |Cu| + 3 |In|
(1) (2)
According to theoretical models published on the defect formation in CuInSe2 [17–18], the signs of Δm and Δs and since all samples show deficiency in Cu and an excess in In, it is expected that VCu and InCu will play important roles in establishing the chemical defects present in the samples. 3.2. Photoacoustic and photoconductivity measurements Fig. 2 depicts the photoacoustic and photoconductivity spectra measured simultaneously at room temperature as a function of the photon energy from n and p-type CuInSe2 single crystal specimens in the photon energy range 0.7 eV ≤ hν ≤ 1.4 eV. The minimum detectable signal is not the same in both techniques. In PC, the baseline is 0.0, while in PA it is 0.2. It is worth mentioning that in PA such an aspect has a dependency on the modulating frequency, and thus, on the thermal diffusion length (lth). The spectra exhibit three distinct regions. The first region, labelled as A, where hν ≤ 0.92 eV, the peaks that are observed in here are associated with transitions between non-radiative defect states and the conduction/valence bands. The first part of the spectra (PA and PC), hν ≤ 0.84 eV, looks alike. In between 0.84 and 0.92 eV some differences can be noticed. The second region, labelled as B, where 0.92 ≤ hν ≤ 1.06 eV, is that of the fundamental edge of CuInSe2, known to be around hν = 1.02 eV [12]. The sharp rise in the curves of all spectra clearly indicates the direct nature of the band to band transition edge [3]. Clear differences in the curves up rise of the amplitude of both PA and PC signals are observed characteristics of shallow defect transition states. The third region, labelled as C, where
3. Results and discussion The electrical conductivity type and the thickness of the samples used in this work are given in Table 1. 3.1. Elemental composition The determination of the composition of the samples was performed using Energy Dispersive X-ray employing a scanning electron 2
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
The dependence of the absorption coefficient, α, on the normalized amplitude of the PA signal, q, for a sample of thickness ls is given by the relation [20]:
q = q0
a (1 − R) 1 − RRb e−2xy
1 + 2ARb + BRb2
(3)
where
x
q0 =
x 2 + 2x + 2
a = 1 + e−2y (x + 1) − 2 cos(y ) e−y (x + 1)
A e−y (x + 1) {(x 2 − 2) (1 + e−2xy ) cos(y ) + 2x (1 − e−2xy ) sin(y ) a (x 2 − 2x + 2)
=
− 2 (x 2 − 2) e−xy cosh(y )} B=
(x 2 + 2x + 2) −2xy −2y e {e + e−2xy − 2 e−y (x + 1) cos(y )} a (x 2 − 2x + 2)
with x = αlth and y = ls/lth. R and Rb are the optical and thermal reflection coefficients at the interfaces sample-gas and backing materialsample, respectively. The relation that gives the dependence of the absorption coefficient on the photoconductive signal is [9]:
Fig. 2. Comparative plots showing the spectral dependence of the normalized amplitude of the photoacoustic and photoconductive signals measured simultaneously of n and p-conducting CuInSe2 specimens.
hν ≥ 1.06 eV, the two techniques (PAS and PCS) gave a different picture in spectral behavior. In this particular region the incident light beam is absorbed within a thin layer of thickness in the order of the optical diffusion length, a few microns. While the recorded PA signal is almost constant and no feature is detected for these particular samples, the recorded PC signal looks different showing some characteristic peaks. Tokumoto et al. [19] showed that in this region the spectra are determined by the reflection effect but not by the change of the absorption coefficient. This effect is more pronounced in PC spectra [9].
Δσ =
⎞ ητμI0 ⎛ αls + Z − (αLD − Z ) exp(−αls ) ⎟ ⎜1 − exp(−αls ) − αLD ls 1 − (αLD2 )2 ⎜ ⎟ 1 + Z cot 2L D ⎝ ⎠ (4)
( )
where I0 is the incident monochromatic light flux, η is the quantum efficiency and Z = soτ/LD. so is the surface recombination velocity, τ is the volume lifetime, LD is the diffusion length, and µ is the mobility. Fig. 3 (a and b) show the absorption coefficient spectral distributions which were determined from PC and PA data using the relations (3) and (4) close to the samples fundamental edge, respectively. The limit of the reported bandgap of CuInSe2, Eg = 1.02 eV, is indicated in both figures. A first look at the curves reveals that the upper limit of the absorption coefficient edges in both modes of operation (the maximum for PCS is α ≈ 104 cm−1 and for PAS is α ≈ 105 cm−1) is shifted in comparison to the reported Eg, it is around 1.08 eV. On the uprising part
3.3. Absorption coefficient analysis In fact, there are more differences between the spectral behavior of the measured PA and PC techniques. They come out clearer when the absorption coefficient spectral distribution is extracted from both measurements.
Fig. 3. Comparative plots of the spectral dependence of the absorption coefficient close to the fundamental edge of n and p-CuInSe2. (a) Photoconductivity; (b) Photoacoustic. 3
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
Fig. 4. Comparative plots of the extracted absorption spectral dependence of the shoulders. (a) Feature a; (b) feature b; (c) feature c.
Fig. 5. Determination of the bandgap (Eg). (a) Photoacoustic; (b) Photoconductivity. Table 2 Results summary of the chemical defects detected using PCS and PAS close to CuInSe2 band edge. Feature
Defect detected at hν (eV)
Operation mode
Nature
Activation energy (Eg = 1.02 eV)
a
1.000 and 1.005 0.961 0.970 0.973 0.977 0.936
PCS, PAS
Acceptor
15–20 meV
PCS PCS, PAS
Donor Acceptor or Donor
59 meV 43–50 meV
PCS, PAS
Donor
84 meV
b b
c
of the absorption spectra three features (labelled as a, b and c) are observed in forms of shoulders or in changes in the curve trend. Beneath these shoulders, the absorption coefficient follows an exponential dependence on the photon energy. It is worth noting that the feature a is only observed in p-conducting samples, while the feature c is only detected in n-conducting ones. The peaks observed at photon energies above Eg are due to interband transitions [9]. In order to determine the activation energies of these defect levels we have used the method which we have previously described [14].
Fig. 6. Spectral distributions of the absorption coefficient from measured photoacoustic and photoconductivity spectra in the transparency region.
4
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
two different channels of absorption, if used together, they should bring a wealth of information on the band and defect structures of semiconducting materials in order to optimize devices.
Fig. 4(a, b and c) depicts the extraction of the defect levels of the features a, b and c, respectively. Starting with the feature a, this is only observed in p-type conducting samples in both modes (PC and PA) and therefore it is due to shallow acceptor. Its central position is in the range 1.000 ≤ hν ≤ 1.005 eV. It is determined from the absorption curves using the method described by Lange et al. [21]. The latter is based on subtracting the slope data from that of the concerned spectrum in the photon energy region of interest. As for the feature b, shown in Fig. 4(b), four peaks are resolved, two in operation modes (PCS and PAS), and one in each sample type (n and p). The peak centered at hν = 0.961 eV, which is only observed in the n-conducting sample and detected only by PCS is due to a shallow donor. The three other peaks observed at hν = 0.97 eV (p-type, PCS mode), at hν = 0.973 eV (p-type, PAS mode) and at hν = 0.977 eV (n-type, PAS mode) are believed to be due to the same defect level if we take in consideration the system error estimated by ± 5 meV. As a result, we assume the same defect level that is centered at hν = 0.973 eV. It could be due to either a donor or an acceptor state. Finally, the feature c, resolved and shown in Fig. 4(c), is only observed in n-conducting samples and has been detected by both operational modes. It is probably due to a donor state that is centered at hν = 0.936 eV. To be able to determine the activation energies of all these defect states, we need the value of the bandgap (Eg). This has been carried out by plotting the spectral dependence of (αhν)2 shown in Fig. 5 (a and b) from measured PA and PC data in the region close to the absorption edge. The extrapolation of the linear parts of curves reveals the direct nature of the fundamental transition with Eg = 1.02 eV. Table 2 resumes these results obtained in the spectral region of the fundamental edge of CuInSe2. The nature of the defect (acceptor/donor) was determined by considering the samples’ characteristics such as the electrical conductivity type, the signs of Δm and Δs from Table 1 and taking into account reported data in the literature [18,22–25]. The activation energy is determined from the difference in photon energies between Eg and the level peak position [3]. The features observed at photon energies above the bandgap (hν > Eg), i.e. the region C, have been interpreted as due to a very photosensitive surface layer, and the additional structures observed in the form of peaks have attributed to indirect optical transitions [9,26]. Fig. 6 shows comparative curves of the absorption coefficient spectral distribution deduced from measured photoacoustic and photoconductivity spectra in the transparency region, that is in the range 0.7 ≤ hν ≤ 0.92 eV. First, it is important to emphasize that this is the first time that we have seen the spectral distribution of CuInSe2 in this spectral region using the photoconductivity technique and this is due to the high resolution of the present spectrophotometer and its high sensitivity of detection. Second, as detected by photoacoustic technique, we note the presence of five major peaks, which are well documented in the literature [12,14]. In photoconductivity spectra, on the other hand, we note the presence of only four peaks (E1, E2, E3 and E4), but there is no sign E5. These results agree with those recently published using deep levels transient spectroscopy (DLTS) [27,28].
Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support from the Directorate-General for Scientific Research and Technological Development (DGRSDTAlgeria). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.infrared.2020.103194. References [1] 20th Int. Conf. Photoacoustic and Photothermal Phenomena (ICPPP20), Moscow, Russia, 7 – 12 July, 2019. [2] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, Wiley, New York, 1981. [3] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971. [4] R.H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, 1992. [5] M.A. Green, Y. Hishikawa, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, M. Yoshita, A.W.Y. Ho-Baillie, Solar cell efficiency tables (Version 53), Prog. Photovolt. Res. Appl. 27 (2019) 3–12. [6] S. Siebentritt, What limits the efficiency of chalcopyrite solar cells? Sol. Energy Mater. Sol. Cells 95 (2011) 1471. [7] M. Powalla, W. Witte, P. Jackson, S. Paetel, E. Lotter, R. Wuerz, F. Kessler, C. Tschamber, W. Hempel, D. Hariskos, R. Menner, A. Bauer, S. Spiering, E. Ahlswede, T.M. Friedlmeier, D. Blàzquez-Sànchez, I. Klugius, W. Wischmann, CIGS cells and modules with high efficiency on glass and flexible substrates, IEEE J. Photovolt. 4 (2014) 440. [8] V. Ramanathan, T. Datta, R. Noufi, Photoconductivity in CuInSe2 thin films, Appl. Phys. Lett. 51 (1987) 746. [9] M.A. Slifkin, A. Al-Rahmani, M. Imanieh, R.D. Tomlinson, H. Neumann, Photoconductivity spectra of n-type CuInSe2 single crystals, Cryst. Res. Technol. 26 (1991) 109. [10] A.G. Valyomana, T.P. Sajeev, Photoconductivity studies of CuInSe2 thin films prepared by the chemical bath deposition technique, Phys. Stat. Sol. (a) 127 (1991) K113. [11] M. Igalson, Photoconductivity of p-type CuInSe2, Phys. Stat. Sol. (a) 139 (1993) 481. [12] A. Zegadi, M.A. Slifkin, M. Djamin, R.D. Tomlinson, H. Neumann, Photoacoustic spectroscopy of defect states in CuInSe2 single crystals, Sol. Stat. Commun. 83 (1992) 587. [13] K.T.R. Reddy, M.A. Slifkin, A.M. Weiss, Characterization of inorganic materials with photoacoustic spectrometry, Opt. Mater. 16 (2001) 87. [14] A. Zegadi, M. Rouha, F.Z. Satour, A study on the effect of oxygen implants in CuInSe2 by photoacoustic spectroscopy, Cryst. Res. Technol. 50 (2015) 49. [15] R.D. Tomlinson, Fabrication of CuInSe2 single crystals using melt-growth techniques, Solar Cells 16 (1986) 17. [16] S.M. Wasim, Transport properties of CuInSe2, Sol. Cells 16 (1986) 289. [17] A. Groenink, P.H. Janse, A generalized approach to the defect chemistry of ternary compounds, Z. Phys. Chem. 110 (1978) 17. [18] S.B. Zhang, Su-Huai Wei, A. Zunger and H. Katayama-Yoshida, Defect physics of the CuInSe2 chalcopyrite semiconductor, Phys. Rev. B 57 (1998) 9642. [19] H. Tokumoto, M. Tokumoto, T. Ishiguro, Photoacoustic spectra of semiconductors in the strong absorption region, J. Phys. Soc. Jpn. 50 (1981) 602. [20] H. Neumann, On measuring impurity absorption spectra of semiconductors by photoacoustic spectroscopy, Cryst. Res. Technol. 28 (1993) 73. [21] P. Lange, H. Neff, M. Fearheiley, K.J. Bachmann, Photoluminescence and photoconductivity of CuInSe2, Phys. Rev. B 31 (1985) 4074. [22] M. Djabar, F.Z. Satour, A. Zegadi, M.V. Yakushev, Near-infrared optical analysis of CuInSe2(1 ± x) crystals using transmission, photoacoustic and transmission-photoacoustic spectroscopies, Infrared Phys. Technol. 100 (2019) 37. [23] C. Spiendler, F. Babbe, M.H. Wolter, F. Ehré, K. Santhosh, P. Hilgert, F. Werner, S. Siebentritt, Electronic defects in Cu(In, Ga)Se2: towards a comprehensive model,
4. Conclusion A new photoacoustic cell has been designed and was used to analyze the optical properties of CuInSe2. we have presented simultaneous measurements in which the spectroscopic techniques of photoacoustic and photoconductivity have been employed to analyze the spectral behavior of CuInSe2. Chemical defects have been observed and analyzed. Some of these defects activation energies were similar in values and others were different. A pronounced difference in the spectral behavior in the sample saturation region between the results obtained from the two techniques has been noticed. It is the first time that the spectral distribution of CuInSe2 in the region 0.7 eV ≤ hν ≤ 0.92 eV is revealed using the photoconductivity technique. In summary, these are 5
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
26 (1991) 1011. [27] A. Benhenni, F.Z. Satour, A. Zouaoui, A. Zegadi, Deep defect levels in CuInSe2 single crystals using DLTS, MCTS and photoacoustic spectroscopy, Infrared Phys. Technol. 99 (2019) 172. [28] V. Nadazdy, M.V. Yakushev, E.H. Djebbar, A.E. Hill, R.D. Tomlinson, Switching of deep levels in CuInSe2 due to electric field-induced Cu ion migration, J. Appl. Phys. 84 (1998) 4322.
Phys. Rev. Mater. 3 (2019) 090302. [24] R. Marquez, C. Rincon, Defect physics of ternary chalcopyrite semiconductors, Mater. Lett. 40 (1999) 66. [25] C. Rincon, S.M. Wasim, Defect chemistry of AIBIIIC2VI chalcopyrite semiconducting compounds, in: S.K. Deb, A. Zunger (Eds.), Ternary and Multinary Compounds, Materials Research Society, Pittsburgh, 1987, pp. 443–453. [26] M.A. Slifkin, A. Al-Rahmani, M. Imanieh, R.D. Tomlinson, H. Neumann, Annealing effects on photoconductivity spectra of CuInSe2 single crystals, Cryst. Res. Technol.
6
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
An optical absorption properties investigation of CuInSe2 by a simultaneous photoacoustic-photoconductivity measuring technique Fatima Zohra Satour, Ameur Zegadi
T
⁎
Laboratoire: Croissance et Caractérisation de Nouveaux Semiconducteurs (LCCNS), Département d’Electronique, Faculté de Technologie, Université Ferhat Abbas Sétif 1, 19000 Sétif, Algeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoconductivity Photoacoustic CuInSe2 Near-infrared absorption Chemical defects
In this paper we describe a new photoacoustic cell design that allows simultaneous photoacoustic and photoconductive measurements of solid semiconductor samples. The two techniques are complementary. Photoacoustic spectroscopy is known for being a direct monitor of nonradiative de-excitation processes, while photoconductivity spectroscopy is a direct monitor on the photocurrent generated following the absorption of the radiation source. If used simultaneously, they should unveil valuable information on the properties of materials. As an application to the refined spectrometer we present a comparative study on the optical absorption properties of CuInSe2 single. The measurements were performed at room temperature close to the absorption edge. Appropriate theoretical models have been used to evaluate the absorption coefficient spectral dependence from measured data. The combined techniques showed varying sensitivities in detecting defects levels in different regions of the absorption spectrum of the compound, and thus offering more information on the defects structure of semiconductors in a quicker way.
1. Introduction Photoacoustic spectroscopy (PAS) continues to attract increasing attentions for its analytical capabilities in various research fields [1]. PAS is a non-destructive technique that can be employed in the study of optical, thermal and acoustic properties of materials of any form (solid, liquid, gel, powder or thin films). The method does not require electrical contacts, and therefore offers distinct advantages over many conventional techniques. PAS gives direct information on non-radiative absorption interactions. These are known to be directly linked to the band structure and to defects that are responsible of the mechanisms of energy losses [2]. Photoconductivity (PC) is a well established and documented technique, which is used in the analysis of the photoelectrical properties of semiconductors [3,4]. The alloys Cu(In,Ga)(Se,S)2, denoted as CIGS, when used as the absorber layer of solar cells have yielded high efficient devices. In fact, solar cells based on these materials have reached an efficiency of 22.9% [5]. Further improvements are possible by a better understanding of the material and device optoelectronic properties [6,7]. Several experimental photoacoustic and photoconductive studies of CuInSe2 (CIS) have been reported in the literature [8–13]. Yet, there is no reported study in which both techniques were employed in analyzing the same sample at the same time, this work redresses this issue. ⁎
In this paper, PAS and PC are combined together in order to analyse samples of p and n-conducting CuInSe2 single crystals. The spectra are measured close the fundamental absorption edge of the compound at room temperature in the photon energy range 0.7 eV ≤ hν ≤ 1.4 eV. Based on appropriate existing theoretical models of both techniques, the spectral dependence of the absorption coefficients is determined from measured spectra. A comparison between the results obtained from the two techniques is provided in the light of published data. 2. Materials and methods In photoacoustic spectroscopy when a modulated monochromatic light source illuminates a sample enclosed in an air-filled PA cell; the sample after absorbing the incident radiation generates a small amount of heat within the sample thermal diffusion length, lth. lth = (2βs/2πf)½ where βs is the thermal diffusivity of CuInSe2, βs ≈ 5⋅10−6 m2/s, and f being the modulating radiation frequency. The heat diffuses then across the sample and gives rise to temperature changes close to the sample surface. As a result, acoustic waves arise in the sample surrounding that is detected by an appropriate sensor. The spectrometer used in this study is an improved version of the previously described one [14], and is shown in Fig. 1(a). Its constituents are a xenon short-arc lamp as the exciting light source, a chopper, a
Corresponding author. E-mail address: [email protected] (A. Zegadi).
https://doi.org/10.1016/j.infrared.2020.103194 Received 11 November 2019; Received in revised form 9 January 2020; Accepted 10 January 2020 Available online 13 January 2020 1350-4495/ © 2020 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
Fig. 1. Schematic diagrams of: (a) the spectrometer; (b) the cell in which: (1) Backing material, (2) Sample, (3) Metal contacts, (4) Conducting wires, (5) Gas (air), (6) Optical window and (7) Microphone.
monochromator, and the cell. A near-infrared filter (λ = 900–2000 nm), is inserted after the monochromator for the infrared range study case. The measuring cell, which is the heart of the spectrophotometer, is a new design that allows both photoacoustic and photoconductivity measurements to be performed simultaneously of solid samples. This has been optimized in its geometric parameters to enhance the spectrometer responses (PAS and PCS). It is rectangular in shape and is made of aluminum. Its schematic diagram is shown in Fig. 1(b). The microphone gives the photoacoustic response. Simultaneously, the photoconductive response is obtained through the contacts deposited on the sample under test. The two signals (PA and PC) are, at first, amplified separately by highly sensitive ac amplifiers, and then phase sensitively detected using a multifunction digital lock-in amplifier. A computer is used for the acquisition and for the control. Good quality single crystal samples of CuInSe2 are used in this work. The crystal structure was analysed using a Bruker X-ray diffractometer. The main chalcopyrite structure peaks were present with a preferred orientation of (1 1 2). The samples were cut from ingots grown from stoichiometric compositions using the vertical Bridgman technique [15]. Both n and p-type samples, for comparison, are used. Prior to any measurement, the samples were polished on both sides, chemically etched in a 1% bromine in methanol solution, and finally washed with deionized water. The final thickness of the samples was around 1 mm. Ohmic Ni contacts have been deposited from a solution of NiSO4 on pconducting samples, while Cu contacts have been deposited from a solution of CuSO4 on n-conducting samples using the electrodeposition method [16]. The photoacoustic response is corrected for the spectral distribution of the optical system by normalising the response of the specimen to that carbon black powder. Similarly, PC spectra are also normalized to constant photon flux and to the dark conductivity of the samples. All spectra were measured at room temperature at f = 112 Hz as the modulating frequency. In photoacoustic spectroscopy, the modulating frequency serves to choose the part of the sample close to the surface to be exited thermally in relation to the constant of the thermal diffusion length (lth). In our case we have chosen a moderate frequency where both responses of the techniques were stable in the photon energy range of interest.
Table 1 Electrical conductivity type, thickness (ls) and elemental composition of the samples together with their corresponding Δm and Δs. Sample
S-1 S-2
Type
n p
ls (µm)
1050 1020
Composition Cu (at. %)
In (at. %)
Se (at. %)
23.23 22.75
26.59 27.02
50.18 50.23
Δm
Δs
−0.126 −0.158
−0.025 −0.032
microscope (JEOL JSM-7001F). The measurement was taken from several points of the samples surface and an average value was determined and the result is listed in Table 1. Also included are the two parameters Δm and Δs which determine the deviations from molecularity and valence stoichiometry, respectively. These are defined as [17]:
Δm =
Δs =
|u| −1 |In|
2 |Se| −1 |Cu| + 3 |In|
(1) (2)
According to theoretical models published on the defect formation in CuInSe2 [17–18], the signs of Δm and Δs and since all samples show deficiency in Cu and an excess in In, it is expected that VCu and InCu will play important roles in establishing the chemical defects present in the samples. 3.2. Photoacoustic and photoconductivity measurements Fig. 2 depicts the photoacoustic and photoconductivity spectra measured simultaneously at room temperature as a function of the photon energy from n and p-type CuInSe2 single crystal specimens in the photon energy range 0.7 eV ≤ hν ≤ 1.4 eV. The minimum detectable signal is not the same in both techniques. In PC, the baseline is 0.0, while in PA it is 0.2. It is worth mentioning that in PA such an aspect has a dependency on the modulating frequency, and thus, on the thermal diffusion length (lth). The spectra exhibit three distinct regions. The first region, labelled as A, where hν ≤ 0.92 eV, the peaks that are observed in here are associated with transitions between non-radiative defect states and the conduction/valence bands. The first part of the spectra (PA and PC), hν ≤ 0.84 eV, looks alike. In between 0.84 and 0.92 eV some differences can be noticed. The second region, labelled as B, where 0.92 ≤ hν ≤ 1.06 eV, is that of the fundamental edge of CuInSe2, known to be around hν = 1.02 eV [12]. The sharp rise in the curves of all spectra clearly indicates the direct nature of the band to band transition edge [3]. Clear differences in the curves up rise of the amplitude of both PA and PC signals are observed characteristics of shallow defect transition states. The third region, labelled as C, where
3. Results and discussion The electrical conductivity type and the thickness of the samples used in this work are given in Table 1. 3.1. Elemental composition The determination of the composition of the samples was performed using Energy Dispersive X-ray employing a scanning electron 2
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
The dependence of the absorption coefficient, α, on the normalized amplitude of the PA signal, q, for a sample of thickness ls is given by the relation [20]:
q = q0
a (1 − R) 1 − RRb e−2xy
1 + 2ARb + BRb2
(3)
where
x
q0 =
x 2 + 2x + 2
a = 1 + e−2y (x + 1) − 2 cos(y ) e−y (x + 1)
A e−y (x + 1) {(x 2 − 2) (1 + e−2xy ) cos(y ) + 2x (1 − e−2xy ) sin(y ) a (x 2 − 2x + 2)
=
− 2 (x 2 − 2) e−xy cosh(y )} B=
(x 2 + 2x + 2) −2xy −2y e {e + e−2xy − 2 e−y (x + 1) cos(y )} a (x 2 − 2x + 2)
with x = αlth and y = ls/lth. R and Rb are the optical and thermal reflection coefficients at the interfaces sample-gas and backing materialsample, respectively. The relation that gives the dependence of the absorption coefficient on the photoconductive signal is [9]:
Fig. 2. Comparative plots showing the spectral dependence of the normalized amplitude of the photoacoustic and photoconductive signals measured simultaneously of n and p-conducting CuInSe2 specimens.
hν ≥ 1.06 eV, the two techniques (PAS and PCS) gave a different picture in spectral behavior. In this particular region the incident light beam is absorbed within a thin layer of thickness in the order of the optical diffusion length, a few microns. While the recorded PA signal is almost constant and no feature is detected for these particular samples, the recorded PC signal looks different showing some characteristic peaks. Tokumoto et al. [19] showed that in this region the spectra are determined by the reflection effect but not by the change of the absorption coefficient. This effect is more pronounced in PC spectra [9].
Δσ =
⎞ ητμI0 ⎛ αls + Z − (αLD − Z ) exp(−αls ) ⎟ ⎜1 − exp(−αls ) − αLD ls 1 − (αLD2 )2 ⎜ ⎟ 1 + Z cot 2L D ⎝ ⎠ (4)
( )
where I0 is the incident monochromatic light flux, η is the quantum efficiency and Z = soτ/LD. so is the surface recombination velocity, τ is the volume lifetime, LD is the diffusion length, and µ is the mobility. Fig. 3 (a and b) show the absorption coefficient spectral distributions which were determined from PC and PA data using the relations (3) and (4) close to the samples fundamental edge, respectively. The limit of the reported bandgap of CuInSe2, Eg = 1.02 eV, is indicated in both figures. A first look at the curves reveals that the upper limit of the absorption coefficient edges in both modes of operation (the maximum for PCS is α ≈ 104 cm−1 and for PAS is α ≈ 105 cm−1) is shifted in comparison to the reported Eg, it is around 1.08 eV. On the uprising part
3.3. Absorption coefficient analysis In fact, there are more differences between the spectral behavior of the measured PA and PC techniques. They come out clearer when the absorption coefficient spectral distribution is extracted from both measurements.
Fig. 3. Comparative plots of the spectral dependence of the absorption coefficient close to the fundamental edge of n and p-CuInSe2. (a) Photoconductivity; (b) Photoacoustic. 3
Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
Fig. 4. Comparative plots of the extracted absorption spectral dependence of the shoulders. (a) Feature a; (b) feature b; (c) feature c.
Fig. 5. Determination of the bandgap (Eg). (a) Photoacoustic; (b) Photoconductivity. Table 2 Results summary of the chemical defects detected using PCS and PAS close to CuInSe2 band edge. Feature
Defect detected at hν (eV)
Operation mode
Nature
Activation energy (Eg = 1.02 eV)
a
1.000 and 1.005 0.961 0.970 0.973 0.977 0.936
PCS, PAS
Acceptor
15–20 meV
PCS PCS, PAS
Donor Acceptor or Donor
59 meV 43–50 meV
PCS, PAS
Donor
84 meV
b b
c
of the absorption spectra three features (labelled as a, b and c) are observed in forms of shoulders or in changes in the curve trend. Beneath these shoulders, the absorption coefficient follows an exponential dependence on the photon energy. It is worth noting that the feature a is only observed in p-conducting samples, while the feature c is only detected in n-conducting ones. The peaks observed at photon energies above Eg are due to interband transitions [9]. In order to determine the activation energies of these defect levels we have used the method which we have previously described [14].
Fig. 6. Spectral distributions of the absorption coefficient from measured photoacoustic and photoconductivity spectra in the transparency region.
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Infrared Physics and Technology 105 (2020) 103194
F.Z. Satour and A. Zegadi
two different channels of absorption, if used together, they should bring a wealth of information on the band and defect structures of semiconducting materials in order to optimize devices.
Fig. 4(a, b and c) depicts the extraction of the defect levels of the features a, b and c, respectively. Starting with the feature a, this is only observed in p-type conducting samples in both modes (PC and PA) and therefore it is due to shallow acceptor. Its central position is in the range 1.000 ≤ hν ≤ 1.005 eV. It is determined from the absorption curves using the method described by Lange et al. [21]. The latter is based on subtracting the slope data from that of the concerned spectrum in the photon energy region of interest. As for the feature b, shown in Fig. 4(b), four peaks are resolved, two in operation modes (PCS and PAS), and one in each sample type (n and p). The peak centered at hν = 0.961 eV, which is only observed in the n-conducting sample and detected only by PCS is due to a shallow donor. The three other peaks observed at hν = 0.97 eV (p-type, PCS mode), at hν = 0.973 eV (p-type, PAS mode) and at hν = 0.977 eV (n-type, PAS mode) are believed to be due to the same defect level if we take in consideration the system error estimated by ± 5 meV. As a result, we assume the same defect level that is centered at hν = 0.973 eV. It could be due to either a donor or an acceptor state. Finally, the feature c, resolved and shown in Fig. 4(c), is only observed in n-conducting samples and has been detected by both operational modes. It is probably due to a donor state that is centered at hν = 0.936 eV. To be able to determine the activation energies of all these defect states, we need the value of the bandgap (Eg). This has been carried out by plotting the spectral dependence of (αhν)2 shown in Fig. 5 (a and b) from measured PA and PC data in the region close to the absorption edge. The extrapolation of the linear parts of curves reveals the direct nature of the fundamental transition with Eg = 1.02 eV. Table 2 resumes these results obtained in the spectral region of the fundamental edge of CuInSe2. The nature of the defect (acceptor/donor) was determined by considering the samples’ characteristics such as the electrical conductivity type, the signs of Δm and Δs from Table 1 and taking into account reported data in the literature [18,22–25]. The activation energy is determined from the difference in photon energies between Eg and the level peak position [3]. The features observed at photon energies above the bandgap (hν > Eg), i.e. the region C, have been interpreted as due to a very photosensitive surface layer, and the additional structures observed in the form of peaks have attributed to indirect optical transitions [9,26]. Fig. 6 shows comparative curves of the absorption coefficient spectral distribution deduced from measured photoacoustic and photoconductivity spectra in the transparency region, that is in the range 0.7 ≤ hν ≤ 0.92 eV. First, it is important to emphasize that this is the first time that we have seen the spectral distribution of CuInSe2 in this spectral region using the photoconductivity technique and this is due to the high resolution of the present spectrophotometer and its high sensitivity of detection. Second, as detected by photoacoustic technique, we note the presence of five major peaks, which are well documented in the literature [12,14]. In photoconductivity spectra, on the other hand, we note the presence of only four peaks (E1, E2, E3 and E4), but there is no sign E5. These results agree with those recently published using deep levels transient spectroscopy (DLTS) [27,28].
Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support from the Directorate-General for Scientific Research and Technological Development (DGRSDTAlgeria). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.infrared.2020.103194. References [1] 20th Int. Conf. Photoacoustic and Photothermal Phenomena (ICPPP20), Moscow, Russia, 7 – 12 July, 2019. [2] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, Wiley, New York, 1981. [3] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971. [4] R.H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, 1992. [5] M.A. Green, Y. Hishikawa, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, M. Yoshita, A.W.Y. Ho-Baillie, Solar cell efficiency tables (Version 53), Prog. Photovolt. Res. Appl. 27 (2019) 3–12. [6] S. Siebentritt, What limits the efficiency of chalcopyrite solar cells? Sol. Energy Mater. Sol. Cells 95 (2011) 1471. [7] M. Powalla, W. Witte, P. Jackson, S. Paetel, E. Lotter, R. Wuerz, F. Kessler, C. Tschamber, W. Hempel, D. Hariskos, R. Menner, A. Bauer, S. Spiering, E. Ahlswede, T.M. Friedlmeier, D. Blàzquez-Sànchez, I. Klugius, W. Wischmann, CIGS cells and modules with high efficiency on glass and flexible substrates, IEEE J. Photovolt. 4 (2014) 440. [8] V. Ramanathan, T. Datta, R. Noufi, Photoconductivity in CuInSe2 thin films, Appl. Phys. Lett. 51 (1987) 746. [9] M.A. Slifkin, A. Al-Rahmani, M. Imanieh, R.D. Tomlinson, H. Neumann, Photoconductivity spectra of n-type CuInSe2 single crystals, Cryst. Res. Technol. 26 (1991) 109. [10] A.G. Valyomana, T.P. Sajeev, Photoconductivity studies of CuInSe2 thin films prepared by the chemical bath deposition technique, Phys. Stat. Sol. (a) 127 (1991) K113. [11] M. Igalson, Photoconductivity of p-type CuInSe2, Phys. Stat. Sol. (a) 139 (1993) 481. [12] A. Zegadi, M.A. Slifkin, M. Djamin, R.D. Tomlinson, H. Neumann, Photoacoustic spectroscopy of defect states in CuInSe2 single crystals, Sol. Stat. Commun. 83 (1992) 587. [13] K.T.R. Reddy, M.A. Slifkin, A.M. Weiss, Characterization of inorganic materials with photoacoustic spectrometry, Opt. Mater. 16 (2001) 87. [14] A. Zegadi, M. Rouha, F.Z. Satour, A study on the effect of oxygen implants in CuInSe2 by photoacoustic spectroscopy, Cryst. Res. Technol. 50 (2015) 49. [15] R.D. Tomlinson, Fabrication of CuInSe2 single crystals using melt-growth techniques, Solar Cells 16 (1986) 17. [16] S.M. Wasim, Transport properties of CuInSe2, Sol. Cells 16 (1986) 289. [17] A. Groenink, P.H. Janse, A generalized approach to the defect chemistry of ternary compounds, Z. Phys. Chem. 110 (1978) 17. [18] S.B. Zhang, Su-Huai Wei, A. Zunger and H. Katayama-Yoshida, Defect physics of the CuInSe2 chalcopyrite semiconductor, Phys. Rev. B 57 (1998) 9642. [19] H. Tokumoto, M. Tokumoto, T. Ishiguro, Photoacoustic spectra of semiconductors in the strong absorption region, J. Phys. Soc. Jpn. 50 (1981) 602. [20] H. Neumann, On measuring impurity absorption spectra of semiconductors by photoacoustic spectroscopy, Cryst. Res. Technol. 28 (1993) 73. [21] P. Lange, H. Neff, M. Fearheiley, K.J. Bachmann, Photoluminescence and photoconductivity of CuInSe2, Phys. Rev. B 31 (1985) 4074. [22] M. Djabar, F.Z. Satour, A. Zegadi, M.V. Yakushev, Near-infrared optical analysis of CuInSe2(1 ± x) crystals using transmission, photoacoustic and transmission-photoacoustic spectroscopies, Infrared Phys. Technol. 100 (2019) 37. [23] C. Spiendler, F. Babbe, M.H. Wolter, F. Ehré, K. Santhosh, P. Hilgert, F. Werner, S. Siebentritt, Electronic defects in Cu(In, Ga)Se2: towards a comprehensive model,
4. Conclusion A new photoacoustic cell has been designed and was used to analyze the optical properties of CuInSe2. we have presented simultaneous measurements in which the spectroscopic techniques of photoacoustic and photoconductivity have been employed to analyze the spectral behavior of CuInSe2. Chemical defects have been observed and analyzed. Some of these defects activation energies were similar in values and others were different. A pronounced difference in the spectral behavior in the sample saturation region between the results obtained from the two techniques has been noticed. It is the first time that the spectral distribution of CuInSe2 in the region 0.7 eV ≤ hν ≤ 0.92 eV is revealed using the photoconductivity technique. In summary, these are 5
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26 (1991) 1011. [27] A. Benhenni, F.Z. Satour, A. Zouaoui, A. Zegadi, Deep defect levels in CuInSe2 single crystals using DLTS, MCTS and photoacoustic spectroscopy, Infrared Phys. Technol. 99 (2019) 172. [28] V. Nadazdy, M.V. Yakushev, E.H. Djebbar, A.E. Hill, R.D. Tomlinson, Switching of deep levels in CuInSe2 due to electric field-induced Cu ion migration, J. Appl. Phys. 84 (1998) 4322.
Phys. Rev. Mater. 3 (2019) 090302. [24] R. Marquez, C. Rincon, Defect physics of ternary chalcopyrite semiconductors, Mater. Lett. 40 (1999) 66. [25] C. Rincon, S.M. Wasim, Defect chemistry of AIBIIIC2VI chalcopyrite semiconducting compounds, in: S.K. Deb, A. Zunger (Eds.), Ternary and Multinary Compounds, Materials Research Society, Pittsburgh, 1987, pp. 443–453. [26] M.A. Slifkin, A. Al-Rahmani, M. Imanieh, R.D. Tomlinson, H. Neumann, Annealing effects on photoconductivity spectra of CuInSe2 single crystals, Cryst. Res. Technol.
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