Raman Characterization of a New Te-Rich Binary Compound: CdTe2

J. Phys. Chem. B 2009, 113, 4333–4337

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Raman Characterization of a New Te-Rich Binary Compound: CdTe2 Jean Rousset,*,†,‡ Edouard Rzepka,*,§ and Daniel Lincot*,†,‡ IRDEP Institut de Recherche et De´Veloppement sur l’Energie PhotoVoltaı¨que, UMR 7174, EDF-CNRS-ENSCP6 quai Watier 78401 Chatou Cedex, France, LECAsLaboratoire d’Electrochimie et de Chimie Analytique, UMR 7575, ENSCP 11 rue Pierre et Marie Curie 75231 Paris Cedex 05, France, and GeMacsGroupe d’Etude de la Matie`re Condense´e, Campus du CNRS de Meudon BelleVue 1 place Aristide Briand 92195 Meudon Cedex, France ReceiVed: October 6, 2008; ReVised Manuscript ReceiVed: January 22, 2009

Structural characterization by Raman spectroscopy of CdTe thin films electrodeposited in acidic conditions is considered in this work. This study focuses on the evolution of material properties as a function of the applied potential and the film thickness, demonstrating the possibility to obtain a new Te-rich compound with a II/VI ratio of 1/2 under specific bath conditions. Raman measurements carried out on etched samples first allow the elimination of the assumption of a mixture of phases CdTe + Te and tend to confirm the formation of the CdTe2 binary compound. The signature of this phase on the Raman spectrum is the increase of the LO band intensity compared to that obtained for the CdTe. The influence of the laser power is also considered. While no effect is observed on CdTe films, the increase of the incident irradiation power leads to the decomposition of the CdTe2 compound into two more stable phases namely CdTe and Te. Introduction Recently studies carried out in our laboratory give rise to the formation by electrodeposition of cadmium ditelluride (CdTe2) thin films (1 µm thick). This phase appears under specific bath and potential conditions as described elsewhere.1,2 The Te-rich compound is observed when the applied potential is shifted toward more cathodic values with respect to the classical CdTe deposition potential in the case of one micrometer thick films, but also as a function of the thickness during the deposition of thicker films (>10 µm). The optical and structural (quasi-amorphous) properties of CdTe2 have been explored in a previous study3 and a crystallographic structure (pyrite) has been proposed. The aim of the present work is to determine the Raman response of this new compound in order to understand the phase transition observed depending on both the applied potential and the thickness of the layer. This characterization method allows the clear detection of the presence of a pure tellurium phase and then differentiates the formation of a binary compound (CdTe2) from that of a phase mixture (CdTe + Te). Some authors4-6 have identified the response of the pure CdTe material, and that of pure tellurium formed during the growth process, which causes a loss of crystal quality. The evaluation of the impact of the Te inclusions on the material quality, in terms of crystalline and optical properties of the CdTe crystal,7 can also be evaluated by this technique. Raman spectroscopy has been used to estimate the efficiency of annealing treatments8,9 or chemical etching6,10 to eliminate pure Te from the bulk or from the sample surface. The detection of phase formation (such as CdIn2Te411), or the study of the impurities presence (such as zinc12-14 or bismuth15 in CdTe alloys) is also possible by this technique. * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; [email protected]. † UMR 7174. ‡ UMR 7575. § Ge Mac.

Moreover Raman spectroscopy studies can be carried out in situ in order to follow the growing of a phase in real time16 or to study an electrolyte/solid interface.17 In this work Raman marks of the CdTe2 phase are identified and differentiated from that of the CdTe. In addition, it is well-known that the incident laser beam power can induce changes on the studied sample properties.18 Its influence is particularly important in the case of the characterization of a “metastable” compound such as CdTe2. Thus, the evolution of this material has been studied under different irradiation powers. Experimental Details Electrodepositions were performed at 70 °C in a conventional three electrodes cell using a VMP2 from Princeton applied research and recorded on a PC equipped with the Ec-lab software. The reference electrode was a saturated K2SO4 mercurous sulfate electrode (MSE, +0.65 V vs standard hydrogen electrode), separated from the solution by a fritted AL2O3 junction and maintained at room temperature by a glass bridge. A Pt wire served as the counter-electrode and was placed inside an open glass tube to prevent backdiffusion in the solution of O2 formed during the experiment. For the synthesis of thin films (1 µm thick), the working electrode was made of cadmium sulfide thin films deposited on transparent tin-oxide-coated glass plates by chemical bath deposition. For thicker films graphite substrates polished with 3 and 1 µm alumina successively, were used. A heat treatment at 450 °C during 30 min was carried out to improve the reproducibility of these substrates. In this case a 100 nm pure Cd underlayer is deposited by imposing a more cathodic potential than that of the cadmium ions reduction during a few minutes. This guarantees an ohmic contact between the carbon substrate and the deposit. For all the experiments presented here the electrolytic solution was prepared by dissolving 0.2 mol/L of Cd ions (the precursor salt is 3 CdSO4 · 8H2O from Alfa Aesar) in a saturated solution of HTeO2+/TeO2 (TeO2, 99.995% from Aldrich). For the growth

10.1021/jp8088098 CCC: $40.75  2009 American Chemical Society Published on Web 03/02/2009

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Figure 1. Morphology of films deposited at -1 V (left, Cd/Te ≈ 1) and -0.95 V (right, Cd/Te ≈ 0.5).

of thick films, the saturation of the electrolyte avoids the decrease of the tellurium ions concentration during the deposition. The pH was adjusted with dilute hydrochloric acid to 1.5 or 1 before dissolving TeO2. The water used was deionized and had a resistivity of 18 MΩ · cm (Millipore). The solution was deaerated by the bubbling of an inert gas (argon), and the argon pressure was maintained during all the experiments. Raman measurements were carried out at room temperature; they were acquired using unpolarized line at 5145 Å of the Ar+ laser. The incident power and the accumulated power were set by attenuating the laser beam by the use of neutral density filters. The 2 mW power for a laser spot diameter of 1 µm2 was divided by a hundred-fold factor for the most of the spectra presented here. For the thick films the samples are split to allow the analyze of the cross section of the film. The analyses are spaced every two micrometers which is acceptable regarding the spot diameter to avoid overlap effects. At 5145 Å the investigated depth was very shallow (of the order of 100 nm) and only permitted a surface analysis. Indeed an electrodeposited thin film shows an absorption coefficient determined by transmission spectroscopy of about 105 cm-1 at this wavelength. The data were fitted with labspec3 using Gaussian and Lorentzian profiles. Finally, morphology and composition of the films were studied by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively. Results and Discussion Characterization of the CdTe-CdTe2 Transition as a Function of the Applied Potential. In this study two series (pH ) 1.5 and pH ) 1) of 1 µm thick electrodeposited films synthesized at potentials ranging between -0.7 and -1.1 V are characterized. These potential values correspond to the two theoretical limits of the CdTe electrodeposition potential plateau. The compact morphology of these films and the corresponding Raman spectra are presented respectively in Figure 1 and Figure 2. The evolution of the spectra is connected with the variation of the composition and the optical properties of the material [1; 2] depending on the applied potential. pH ) 1.5, without Basic Treatment. On each spectrum presented (acquired under laser powers of 0.2 mW) in Figure 2a four peaks at 96, 123, 141, and 164 cm-1 are observed. The two first can be assigned respectively to the A1 symmetry phonon and to the E symmetry phonon of the pure tellurium phase.4 The band centered at 141 cm-1 is due to both the pure tellurium phase (E) and the transverse optical phonon (TO) of the CdTe lattice (peak labeled E + TO). The last band (164 cm-1) is also characteristic of the CdTe phase and is due to lattice vibrations along the (110) and (111) crystallographic

directions (the longitudinal-optical phonon; LO). Thus these Raman spectra demonstrate the presence of both CdTe and Te phases. The detection of pure Te phase on a CdTe sample is a classical result. The material is superficially oxidized, and a thin film of pure Te is created in contact with air. Observing Figure 2a, the strong response of this compound is thus superimposed with that of the weak binary signal. As a function of the applied potential, the position of the peaks stays almost unchanged but strong evolutions in terms of band intensities are visible. The samples can be classified into three different groups. The first one concerns the films electrodeposited at potentials close to that of the Cd(II) reduction (-1.1 V e E e -1 V). In this case the LO band is weakly intense and appears as a shoulder of the E + TO band. The composition analyses of these films shows a weak Te excess (52%, 50%, and 50.5% of Te atoms for the samples deposited at -1.1, -1.05, and -1 V, respectively) and the band gap values determined from the transmission spectra (not shown) are close to that of a crystalline CdTe phase, (respectively 1.54, 1.52, and 1.53 eV for the samples deposited at -1.1, -1.05, and -1 V). The second group includes the samples grown at potentials ranging between -0.95 and -0.8 V. A very high tellurium excess is detected in these films and the Cd/Te atomic ratio remains quite constant close to 0.5, (0.54, 0.51, 0.53 for the films deposited at -0.8, -0.9, and -0.95 V, respectively). The measured bandgap (1.3 eV) is lower than that of the precedent group. Concerning the Raman spectra the four peaks are present but we noticed changes on the profiles of the bands. Thus, the increase of the relative LO band intensity is observed when the material is grown in this potential interval; at contrary no shift of the peak position is noticed. Finally the film deposited at the less cathodic potential (-0.7 V) presents the highest excess of tellurium atoms and the ratio II/VI reaches its minimum value. The corresponding Raman spectrum is comparable to those obtained for the films of the first group. Basic Treatment Influence. To eliminate the Te phase present on their surface, the samples were treated in a 1.5 M KOH/methanol5 solution under air at room temperature and the corresponding spectra are presented in Figure 2b. The pure tellurium phase is spontaneously transformed by chemical dissociation into two soluble species (TeO32- and Te22-) and then its signature vanishes from the most of the presented Raman spectra allowing the easier detection of the two CdTe related peaks: the transversal-optical phonon (141 cm-1) and the longitudinal-optical phonon (164 cm-1). The characterization of the basically treated samples confirms the increase of the LO band intensity when the applied potential becomes less and

Raman Characterization of CdTe2

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Figure 2. Raman spectra of as-grown thin films electrodeposited at different potentials (a) (normalized intensities) and after a treatment in 1.5 M KOH/methanol (b).

less cathodic. For the samples showing an high excess of tellurium the vanishing of the tellurium (-0.95 V < E(V) < -0.8 V) bands with the basic treatment allows to eliminate the assumption of the presence of pure tellurium phase in the sample and tends to confirm the formation of a binary CdTe2 compound. The increase of the LO band intensity with the decrease of the ratio II/VI to 0.5 could be regarded as the signature of the formation of the cadmium ditelluride in the rest of this paper. The Raman spectra of the sample synthesized at a potential of -0.7 V contains the A1 band even with the basic treatment showing the presence of pure tellurium in the material bulk. pH ) 1. Similar variations of the Raman spectra, atomic composition and band gap value depending on the potential are observed for samples deposited in a more acidic electrochemical bath (pH ) 1). But the potential interval which allows to deposition of the CdTe binary compound is reduced compared to that obtained for less acidic bath. Thus, stoichiometric CdTe was only synthesized applying a potential close to - 1.1V. Impact of the Laser Power. Raman spectra of the material grown at -0.8 V (assuming that corresponds to CdTe2 phase) were acquired using different laser powers in order to estimate the impact of this parameter (Figure 3). Different neutral filters allows the incident power to be divided by a factor of 1.2, 10, 100, and 1000. The relative intensity of the LO band which clearly appears in the spectrum acquired under the lowest laser powers, decreases with increasing laser power. This band which clearly appears at 0.002 and 0.02 mW is only present as a shoulder of the TO peak in the two spectra acquired with the two highest laser powers. To complete this study two samples deposited at -0.9 and -1 V corresponding respectively to CdTe2 and CdTe phases were analyzed after a basic treatment under two different laser powers (0.2 and 0.02 mW; Figure 4). The response of the CdTe2 (Figure 5a,b) film under the most attenuated beam does not contain a strong band characteristic

Figure 3. Raman spectra of a thin film electrodeposited at -0.8 V acquired under different laser powers: (a) 0.002, (b) 0.02, (c) 0.2, (d) 1.7 mW. Intensities are normalized.

of a pure tellurium phase. On the contrary the use of the most powerful beam clearly leads to the emergence of the strong tellurium A1 band. The metastable CdTe2 compound is transformed under irradiation into two more stable phases such as CdTe and pure tellurium phase. This study has to be connected with some annealing experiments which were carried out in the laboratory3 showing the decomposition of the CdTe2 compound for temperatures higher than 100 °C.

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Figure 4. (Left) Raman spectra of a thin film electrodeposited at -0.9 V acquired under two different laser powers: (a) 0.2 and (b) 0.02 mW. (Right) Raman spectra of a thin film electrodeposited at -1 V acquired under two different laser powers: (c) 0.2 and (d) 0.02 mW.

Figure 5. SEM images of the cross section of 20 µm thick films, growth stopped after (a) and during the CdTe f CdTe2 phase transition (b). Composition of layer in panel a as a function of its thickness (c).

In the case of the quasi-stoichiometric CdTe deposited at -1 V the increasing of the laser power does not change the spectrum profile (Figure 5c,d). No significant evolution of the material is detected. Characterization of CdTe-CdTe2 Transition as the Function of the Thickness. Thick films of 20 µm were synthesized at pH ) 1.5 and E(V) ) -1.05 V to show the composition and morphological changes of the material depending on the thickness of the film. We demonstrated in a previous study2

that CdTe-CdTe2 transition can be experimentally detected during the growth by the appearance of a current drop on the potentiostatic curve, then the electrodeposition can be stopped before of after the change of the material properties. Growth Stopped after the CdTe-CdTe2 Transition. The composition and the morphology of the film are presented respectively in the Figure 5a-c. Two different parts are clearly visible, the border between these two zones is localized approximately 10 µm above the substrate. The layer composition

Raman Characterization of CdTe2

J. Phys. Chem. B, Vol. 113, No. 13, 2009 4337 top of the CdTe columns in the last micrometer of the film. The Raman spectra profile corresponds to that of a CdTe phase and does not change depending on the thickness. No significant increase of the LO band intensity is detected on the cross section of the film (Figure 6). Conclusion

Figure 6. Evolution of the intensity and of the area of LO peak (normalized by those obtained for 2 µm) as the function of the thickness of the layers. ((() intensity and (0) area of the layer presented in Figure 5a; (•) intensity and (∆) area of the layer presented in Figure 5b).

ranges, between 0 and 10 µm, from a Cd-rich phase to a phase containing a great excess of tellurium atoms, with a II/VI ratio which varies from 1.1 to 0.5. Corresponding spectra contain the LO band as a shoulder of the E + TO band and the A1 band of the tellurium is clearly apparent (not shown). From 10 µm to the top of the film the composition remains quite constant with a ratio II/VI close to 0.5 which could correspond to the CdTe2 phase formation. The clear cleaving of this part of the film is characteristic of an amorphous phase. For this thickness range the LO band is the strongest feature of the Raman spectra (not shown). After a basic etching of this film, no response of pure tellurium phase is observed in the most of the spectra, except in some Te-rich areas situated close to the border between CdTe and CdTe2 where tellurium precipitates or inclusions can be detected. This phase disappears with beginning of the CdTe2 deposit. The increase of the LO band intensity as a function of the film thickness (Figure 6) shows that the same transition is obtained as a function of the thickness and as a function of the potential. The evolution of the material nature is due to the ohmic drop which appears during the growth because of the resistivity of the material. This drop provokes a potential shift at the liquid/solid interface and leads to the CdTe-CdTe2 transition. Sample Stopped during the Morphological Transition. To focus on the phase transition, the growth of the second film presented in Figure 5b was stopped during the characteristic current drop. The obtained material clearly contains the two previously described phases but the CdTe2 only appears on the

Raman spectra has been employed to characterize the phase transition which was observed in terms of composition, energy band gap, and crystallinity depending on the applied potential (1 µm thick film) and depending on the thickness of the layer (20 µm thick film). So the hypothesis of the formation of a new binary compound tends to be confirmed by this study. Indeed the vanishing of the strong tellurium signal with the basic treatment of the films eliminates the assumption of a mix of CdTe and Te phases. The increase of the relative intensity of LO band has been considered as the Raman signature of the CdTe2 compound. Finally the metastable character of this compound has been demonstrated by increasing the laser power. References and Notes (1) Lepiller, C.; Lincot, D. J. Electrochem. Soc. 2004, 151, C348. (2) Rousset, J.; Lincot, D. J. Electrochem. Soc. 2007, 154, D310. (3) Rousset, J.; Olsson, P.; McCandless, B.; Lincot, D. Chem. Mater. 2008, 20, 6550. (4) Pine, A. S.; Dresselhaus, G. Phys ReV. B 1971, 4, 356. (5) Shin, S. H.; Bajaj, J.; Moudy, L. A.; Cheung, D. T. Appl. Phys. Lett. 1983, 43, 68. (6) Amitharaj, P. M.; Pollack, F. H. Appl. Phys. Lett. 1984, 45, 789. (7) Rodriguez, M. E.; Zelaya-Angel, O.; Pe´rez Bueno, J. J.; JimenezSandoval, S.; Tirado, L. J. Cryst. Growth 2000, 213, 259. (8) Sochinskii, N. V.; Serrano, M. D.; Die`guez, E.; Agullo`-Rueda, F.; Pal, U.; Piqueras, J.; Fernandez, P. J. Appl. Phys. 1995, 77, 2806. (9) Mora-Sero`, I.; Tena-Zaera, R.; Gonzales, J.; Mun˜oz-Sanjose´, V. J. Cryst. Growth 2004, 262, 19. (10) Levy, M.; Amir, N.; Khanin, E.; Nemiirovsky, Y.; Beserman, R. J. Cryst. Growth 1999, 197, 626. (11) Zapata-Torres, M.; Castro-Rodriguez, R.; Melendez-Lira, M.; Jimenez-Sandoval, S.; Zapta-Navarro, A.; Pen˜a, J. L. Thin Solid Films 2000, 358, 12. (12) Sridharan, M.; Narayandass, S. K.; Mangalaraj, D.; Lee, H. C. J. Alloys Cmpd. 2002, 3460, 100. (13) Prabakar, K.; Narayandass, S. K; Mangalaraj, D. Phys. B 2003, 328, 355. (14) Lee, S. H.; Gupta, A.; Wang, S.; Compaan, A. D.; McCandless, B. E. Sol. Energy Mater. Sol. Cells 2005, 86, 551. (15) Saucedo, E.; Ruiz, C. M.; Martinez, O.; Fornaro, L.; Sochinskii, N. V.; Sanz, L. F.; Die´guez, E. J. Cryst. Growth 2005, 275, e471. (16) Drews, D.; Sahm, J.; Richter, W. J. Appl. Phys. 1995, 78, 4060. (17) De Tacconi, N. R.; Lezna, R. O.; Rajeshwar, K. J. Phys. Chem. 1994, 98, 4104. (18) Rai, B. K.; Bist, H. D.; Katiyar, R. S. J. Appl. Phys. 1996, 80, 477.

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