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Influence of deposition temperature on the properties of sputtered films grown from a Cu2OCdOTeO2 composite target: Electronic properties of CdTe2O5
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Influence of deposition temperature on the properties of sputtered films grown from a Cu2OeCdOeTeO2 composite target: Electronic properties of CdTe2O5 A. Beristain Bautistaa, Emilia Olivosb, R. Arroyaveb, F. Rodríguez Melgarejoa, S. Jiménez Sandovala,∗ a
Centro de Investigación y de Estudios Avanzados del I. P. N., Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla, C. P. 76230, Querétaro, Qro, Mexico b Texas A&M University, Department of Materials Science & Engineering, College Station, Tx, 77840, USA
ABS TRA CT
Thin films were grown by sputtering targets made by compressing a mixture of the binary oxides Cu2O, CdO and TeO2. Their properties were studied as a function of the substrate temperature, which was varied from room temperature up to 450 °C. All the analyses were carried out on as-grown films. The elemental composition was analyzed by energy dispersive spectroscopy. A structural analysis was carried from grazing-angle X-ray diffraction patterns, which showed the formation of Cu2O (at 350 °C and above) and CdTe2O5 (at 400 °C and above) clusters immersed in an amorphous background. Raman scattering (room temperature) and photoluminescence (room temperature and 80 K) spectra were dominated by the signals originating from the Cu2O clusters. The optical transmittance spectra presented characteristics in agreement with the phases present in the films, as determined by X-ray diffraction. Due to the lack of information on CdTe2O5 in the literature, density functional theory calculations were carried out to analyze the optical properties of the films containing this compound in terms of the calculated electronic band structure. It was determined that CdTe2O5 is an indirect gap material. The theoretical band gap was 3.06 eV, which compares well with the experimental one ∼3.16 eV.
1. Introduction Prevailing technological developments have produced the need for novel materials that could meet the increasingly demanding functionality in new technologies, and for the development of fast and robust sensors, electronic and opto-electronic devices. Along this line, binary and a large number of ternary compounds have been extensively studied to date. Quaternary compounds and composite systems are materials that have not been fully exploited due to their complexity and to the lack of control over deposition methods. These materials, however, possess an enormous potential that has led to a wide variety of research topics, aimed not only to take advantage of the unique properties that these materials can present, but also to get a better understanding of their physical properties. Among these efforts, it may be mentioned the studies on films of solid solutions and composites based on cadmium telluride with copper and oxygen. The growth of such films has been carried out following two approaches: i) by reactive cosputtering using two targets, one of Cu and the other of CdTe, with a flow of O2 and Ar in the chamber [1–3]; and ii) by means of sputtering a single target prepared by pressing mixtures of CdTe and CuO powders [4,5]. As a result, in both cases it was possible to improve and control the electronic properties of the films. In recent years, the synthesis of transparent conductive oxides has been directed, among other routes, towards the investigation of quaternary materials by means of the combination of binary oxides. Examples of this approach are the systems Ga2O3 − ZnO e In2O3
∗
Corresponding author. E-mail address: [email protected] (S. Jiménez Sandoval).
https://doi.org/10.1016/j.spmi.2018.09.025 Received 21 July 2018; Received in revised form 11 September 2018; Accepted 20 September 2018 0749-6036/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Bautista, A.B., Superlattices and Microstructures, https://doi.org/10.1016/j.spmi.2018.09.025
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[6], SnO2 − ZnO e In2O3 [7], SnO2 − CdO e In2O3 [8] and TeO2 − CdO e In2O3 [9]. Following this approach and our previous work, we have investigated the CdeTeeCueO system by sputtering targets made of a mixture of the binary oxides Cu2O, CdO and TeO2, in the 1:1:1 M ratio. We report here the results of the characterization carried out on the chemical composition, structure, microstructure and photoluminescence of the as-grown films, as a function of substrate temperature during growth. This experimental characterization is followed by density functional theory calculations of the structural and electronic properties of CdTe2O5, one of the resulting materials when high substrate temperatures were employed. 2. Experimental details Films were grown by rf sputtering on glass and silicon substrates using a single cold-pressed composite target prepared from a mixture of the binary compounds: copper(I) oxide, cadmium(II) oxide and tellurium dioxide. For that end, a target was made in our laboratory by cold-pressing Cu2O (99.99%), CdO (99.5%) and TeO2 (99.995%) powders from Sigma-Aldrich in a 1:1:1 mol ratio. For different runs, the substrate temperature was fixed to values starting from room temperature (i.e. unheated on purpose: at the end of the deposit the temperature was ∼70 °C), 100 °C, and up to 450 °C in 50°C-steps. The substrates were cleansed prior to deposition following standard procedures. For a single run, glass and silicon substrates were placed simultaneously in the substrate holder to assure the same chemical composition. During film growth, the Ar flow in the chamber was 11 sccm, resulting in a working pressure of ∼1 × 10−3 Torr. The substrate-to-target distance was 8 cm. A radio frequency power of 40 W was applied to the target, which was previously pre-sputtered for 5 min. The deposition times were 150 min for all growths. The growth rate was of ∼10.7 nm/min for substrate temperatures (Ts) not higher than 250 °C; when Ts was in the range from 300 to 450 °C, the growth rate decreased to ∼7 nm/min as a consequence of atomic re-evaporation at the film surface during growth on a hotter surface. The chemical composition of the samples was determined from the films deposited on Si substrates through wavelength dispersive spectroscopy (WDS) in an electron probe micro analyzer (EPMA) JXA – 8530F. The photoluminescence (PL) studies were performed at 80 K and room temperature, in a micro-Raman system (Labram from Dilor) using the 488 nm excitation line of an Ar+-laser. For low-temperature PL measurements, a liquid-nitrogen Oxford cryostat was employed. The X-ray diffraction experiments were obtained in a Rigaku, D/Max-2100, using the Cu Kα radiation (1.5406 Å), with 30 kV and 20 mA as working parameters. 3. Results and discussion 3.1. Elemental analysis The chemical composition in the films presented some deviations from the nominal concentrations used in the targets. Fig. 1 presents the results of the chemical composition obtained by WDS as a function of substrate temperature (Ts). According to the 1:1:1 ratio of Cu2O, CdO and TeO2, the target was composed of 50 at. % of oxygen, 25% at. of copper, 12.5% at. of cadmium and 12.5% at. of tellurium. These nominal concentrations in the target were the same for all growths. As a precaution, the target in use was ground and re-manufactured after each growth to avoid variations in the chemical composition on its surface, as a consequence of preferential sputtering during the previous deposition. In Fig. 1 the nominal concentrations are indicated by the dotted lines. For the growth at room temperature (RT), the amount of oxygen in the films was around 10 at. % below the nominal concentration, while the amount of tellurium is ca. 8 at. % above the nominal concentration. However, the concentrations of cadmium and copper in this film were close to their corresponding nominal concentrations. In general, we can see that as Ts increases in the substrate, the amount of oxygen increases until reaching values close to its nominal value. The tellurium concentration remained higher, around 20 at.%, than
Fig. 1. Chemical composition of the films obtained from wavelength dispersive spectroscopy as a function of the substrate temperature. 2
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Fig. 2. XRD patterns for the series of films that were obtained with a substrate temperature of RT, and from 250 to 450 °C. The Miller indices identify c-cubic Cu2O and m-monoclinic CdTe2O5 structures.
the nominal (12.5 at. %) as Ts increased achieving values close to 25 at.% above 300 °C. In contrast, copper and cadmium concentrations decreased to values around 19 and 6 at. %, respectively, for Ts = 450 °C. For all depositions, the lowest elemental concentration obtained was for Cd, due to its characteristic high vapor pressure. 3.2. Crystalline structure The structural characteristics of the films, determined through X-ray diffraction (XRD), showed that the films are a mixture of amorphous and polycrystalline regions. Fig. 2 shows the diffraction patterns of the films for the employed substrate temperatures. A clear evolution is observed starting from the amorphous film obtained at room temperature up to the formation of diffraction peaks corresponding to polycrystalline regions of Cu2O and CdTe2O5. The identification of the peaks has been carried out using the powder diffraction files of cubic Cu2O (PDF # 01–1142) and monoclinic CdTe2O5 (PDF # 24–0169). The (111) reflection of Cu2O is formed with high intensity from 350 °C up to 450 °C. Other Cu2O reflections correspond to the planes (200) and (220). That is, the first well defined crystalline phase formed in the films as a result of using higher-than-room-temperature Ts was Cu2O. The formation of Cu2O before CdTe2O5 must be driven by energy considerations involving all the interactions among the four chemical elements present during deposition. That is, upon arrival of the sputtered species to the substrate surface at temperature Ts, energetically the formation of Cu2O was favored over the ternary compound CdTe2O5. The formation of any other compound containing copper, besides Cu2O, was energetically more costly at the temperature and pressure in the growth chamber. As a result, polycrystalline grains of Cu2O initiated the nucleation process at the substrate surface. Then, when higher substrate temperatures were used (i.e. larger thermal energy available) Cd, Te along with abiding oxygen atoms, gave rise to crystalline CdTe2O5, in addition to the Cu2O already present. The reflections of planes (111), (200) and (220) of Cu2O were located at 36.5°, 42.3° and 61.6°, respectively. The average full width at half maximum (FWHM), lattice parameter a, interplanar distance d and crystallite size (XS) were obtained from the high intensity peak (111) of the cubic Cu2O phase for films grown at substrate temperatures of 350, 400 and 450 °C. These structural parameters are reported in Table 1. The interplanar distance d was obtained from the following relation
(h2 + k 2 + l 2) 1 = , d2 a2
(1)
where h, k and l are the Miller indices of the corresponding family of planes. From the structural parameters in Table 1, it may be 3
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Table 1 Structural parameters of the crystalline aggregates of Cu2O in the films. The full width at half maximum and the interplanar distance were derived from the (111) peaks. Substrate temperature (°C)
FWHM
d (Å)
Lattice parameter a (Å)
Crystallite size (Å)
350 400 450
0.664 ( ± 0.049) 0.373 ( ± 0.012) 0.374 ( ± 0.019)
2.4703 ( ± 0.0077) 2.4650 ( ± 0.0020) 2.4584 ( ± 0.0019)
4.2786 4.2695 4.2580
127 ( ± 10) 233 ( ± 08) 236 ( ± 11)
observed that, as the temperature of the substrate increases, the lattice parameter a decreases from 4.2786 to 4.2580 Å. The value for a reported in the literature is 4.2696 Å [13], which is in the middle of the values in Table 1. The crystallite size increased from 127 to 236 Å, as Ts was raised from 350 to 450 °C. From Fig. 2 it may be observed that films grown at ambient temperature and up to 300 °C have amorphous characteristics. That is, a broad band characteristic of amorphous material is noticeable with its center around ∼29°. Even when Ts = 450 °C, a non-planar background baseline is observed, indicative of the existence of an amorphous fraction in the film. In the samples grown with substrate temperature of 400 and 450 °C, reflections were obtained from the (100) and (200) planes located at 9.9° and 19.8°, respectively, corresponding to the monoclinic phase of CdTe2O5. The peak observed around 29°, however, has contributions from both Cu2O and CdTe2O5. The former presents a diffraction peak at 29.756°, (110) reflection, the latter at 30.043°, the (300) reflection. Fig. 3 presents a deconvolution with Gaussian functions of this peak, which shows the contribution of both phases. Energetically, Cu2O was favored over the formation of any other copper-containing ternary or quaternary compound. The remaining elements found their lowestenergy condition through the formation of CdTe2O5, although it must be noticed that even at Ts = 450 °C, some amorphous material was still present. The thicknesses of the films are given in Table 2. Two regimes can be observed: i) from RT to 250 °C the thickness of the films were around 1.6 μm; ii) for Ts between 300 and 450 °C, the thickness fluctuated around 1 μm. The higher the Ts, the larger the escaping probability for the atoms that arrive to the surface of the growing film. As a result, the thicknesses of the films are smaller for those deposited at higher Ts.
3.3. Lattice dynamics: Raman spectroscopy From RT to 300 °C, the spectra of the films did not show any distinguishable peaks, given their amorphous structure. The vibrational spectra of the polycrystalline films, i.e. for Ts = 350–450 °C, are presented in Fig. 4. It is possible to identify some vibrational frequencies at 216, 410, 466, 635 and 730 cm−1. Some of them correspond to Cu2O aggregates (216, 410 and 635 cm−1) [10], as it can be noticed by comparing with the Raman spectrum of Cu2O powder shown in the inset of Fig. 4. Two other bands correspond to vibrational modes of tellurium-oxygen structural units present in the crystalline structure of CdTe2O5. The mode at 466 cm−1 can be identified with vibrations of TeeOeTe chains between [TeO4] units, while the 730 cm−1 mode with vibrations of Te=O involving three-coordinate Te atoms or more, known as [TeO3+1] units [11]. It is noticed that the band at 730 cm−1 matches as well with a reported Raman band of CdTe2O5 [12].
Fig. 3. Deconvolution with Gaussian functions of the diffraction peak around 29° (Figs. 2 and 450 °C pattern) showing the contributions of both materials: Cu2O and CdTe2O5. The dotted lines correspond to the powder-diffraction-file data for each compound: cubic Cu2O (PDF # 01–1142) and monoclinic CdTe2O5 (PDF # 24–0169). 4
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Table 2 Thicknesses of the as-grown films for the various substrate temperatures employed. Ts (°C)
RT
100
150
200
250
300
350
400
450
Thickness (μm)
1.6
1.7
1.6
1.6
1.6
0.9
1.1
1.0
1.1
Fig. 4. Raman spectra of the films grown at room temperature and at Ts = 350–450 °C. The spectra of the films grown at temperatures lower than 350 °C were similar to the one at RT. The inset shows the Raman spectrum of Cu2O powder.
3.4. Photoluminescence spectra The luminescence of the films was obtained using an Ar+ laser emitting at 488 nm. Through the analysis of the photoluminescence spectra, information can be obtained about defect-related electronic levels, as well as the energetics of excitonic recombination processes. Fig. 5 shows the PL spectra of the films grown at substrate temperatures from 300 to 450 °C. The spectra are correlated with the X-ray diffraction data in the sense that the amorphous films show only extremely weak and broad emissions. Two measurements were made for each sample: room temperature (black continuous line) and 80 K (red dotted line). In general, it is possible to observe five emissions: 611, 618, 627, 633 and 728 nm. All these signals are related to excitonic relaxation processes in the crystalline structure of Cu2O. Indeed, the broad emission at 728 nm (1.7 eV) is due to the recombination of excitons bound to oxygen vacancies with double positive charge (VO2+) [13,14]. This signal is no longer observable in the film grown at 450 °C, which is indicative that this type of vacancies was significantly reduced for this substrate temperature. Some of this emission is noticeable in the spectrum of the film grown at 350 °C. The PL signal was the weakest for the film deposited at 300 °C, as a result of the structural disorder in this sample as revealed by the X-ray diffraction pattern, Fig. 2. In the case of the well-defined PL signal of the film grown at 450 °C, top spectrum Fig. 5, its analysis was carried out through a deconvolution with Gaussian functions to determine the components that make up the luminescent emission. Fig. 6a shows the deconvolution for the RT spectrum. Three emissions were determined at 1.95 (635 nm), 1.98 (626 nm) and 2.02 eV (613 nm). These emissions originate from free-exciton recombinations. That is, the emission at 1.95 eV corresponds to excitonic relaxation with phonon emission of 515 cm−1 (F2g mode [10]). The emission at 1.98 eV is due to the same type of relaxation with the emission of a 350 cm−1 phonon (Bu mode [10]). Finally, the emission at 2.02 eV corresponds to pure excitonic relaxation with no phonon emission [14]. Fig. 6b presents the deconvolution of the PL spectrum measured at 80 K. It is observed that the intensity of the 2.02 eV emission increased; while the 1.95 eV one decreased, with respect to the RT spectrum. There is a shift of the RT 1.98 eV emission to 2.0 eV as the temperature was lowered to 80 K due to the lattice contraction on cooling. Another consequence of decreasing the temperature was a reduction of the width of the emissions due to a reduction in the thermal fluctuations of the atoms around their equilibrium positions. Fig. 7 shows a schematic representation (Jablonski diagram) to illustrate the radiative transitions present in the films, which originate from the Cu2O clusters.
3.5. Optical transmittance In Fig. 8 the transmittance spectra of the films are presented as a function of the substrate temperature from RT to 450 °C. Initially, we can see that the RT films up to 200 °C show a slightly higher percentage of transmission with respect to those from 300 to 400 °C. Because of their amorphous nature, it is possible to observe a non-abrupt slope of the absorption edge for the RT-200 °C films, with respect to those of 300 and 350 °C. The slightly sharper slope of the absorption edge of the film grown at 350 °C indicates that 5
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Fig. 5. Photoluminescence spectra of the films grown at different substrate temperatures. The measurements were performed at room temperature (black continuous line) and 80 K (red dotted line). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Deconvolution of the emission spectra of the measurements made at (a) RT and (b) 80 K of the film deposited at 450 °C.
the amount of defect-related electronic levels decreased, in accordance with the X-ray diffraction data. The transmittance spectra of the 400–450 °C films present different characteristics. It must be remember that these films consist of three different phases: amorphous material and aggregates of Cu2O and CdTe2O5. Thus, the optical spectra contain signatures of the absorption characteristics of the three components. These contributions are clearly noticeable in the absorption spectrum of the film grown at 450 °C, Fig. 9. For the Cu2O-like and CdTe2O5-like absorptions, estimates of the effective band gap for each case were obtained by using the Tauc relation: αhν = B(hν-Eg)n,
(2) 6
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Fig. 7. Jablonski diagram of the radiative transitions observed in the photoluminescence measurements. All the emissions originate from the Cu2O clusters present in the films.
Fig. 8. Transmittance spectra of the films as a function of the substrate temperature from RT to 450 °C.
Where B is a constant, hν is the energy of the photon, Eg is the band gap energy. The exponent n has a value of 2 for amorphous and indirect gap materials, and ½ for direct bandgap crystalline materials [16]. The optical absorption coefficient α(λ) was determined from:
7
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Fig. 9. Absorbance spectrum of the film deposited at 450 °C. The insets show Tauc's model fittings to determine the effective band gap of the (a) Cu2O-like and (b) CdTe2O5-like regions.
α (λ ) = 2.303
A , d
(3)
where A is the absorbance and d is the thickness of the film. The insets in Fig. 9 show that the effective band gap obtained from the Tauc's model fittings for the Cu2O-like and CdTe2O5-like regions were 2.67 and 3.16 eV, respectively. The former is close to the Cu2O band gap, which is ∼2.17 eV [15], the latter is discussed below in regard to our theoretical calculations for CdTe2O5. It must be taken into consideration that the curve shown in Fig. 9 represents an effective absorption made of the absorption of the three phases: amorphous, Cu2O and CdTe2O5. In that sense, the values of the band gap obtained from the Cu2O-like and CdTe2O5-like absorption regions represent approximations to the real values. A value for the pseudo band gap for the amorphous films was obtained by applying Tauc's model with n = 2 in equation (2). These values ranged from 1.7 to 1.8 eV, as shown in Fig. 10 which may be understood as corresponding to the photon energies for which the absorption of the materials start being significant. 4. Electronic properties of CdTe2O5: theoretical calculations As mentioned above, the information in the literature on CdTe2O5 is remarkably scarce. For instance, there is no experimental or theoretical report on its electronic band gap, a fundamental property. Here we report density functional theory (DFT) calculations on the electronic band structure and the corresponding density of states for CdTe2O5. These calculations are used to discuss the value of the band gap determined from the CdTe2O5-like region in the absorption spectrum shown in Fig. 9. 4.1. Computational details The band structure and total energy calculations were performed using the frozen-core projector-augmented wave (PAW) method [17] within the local density approximation (LDA) as implemented in the VASP software package [18–21]. The plane wave cutoff and k point mesh (Monkhorst-Pack [22]) were set to 528 eV and a k point mesh of 4 × 8 × 4 were used. The optimal plane wave cut-off energy and k-point mesh have been chosen based on performed total energy convergence test. The monoclinic crystal structure of CdTe2O5 is described by the space group P21/c, number 14, with 32 atoms in the unit cell (4 atoms of Cd, 20 atoms of O and 8 atoms of Te) Fig. 11a. 8
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Fig. 10. Estimates of the pseudo gap for the amorphous films grown at the indicated temperatures using Tauc's model.
Fig. 11. (a) Monoclinic unit cell of CdTe2O5 structure. Cd atoms (larger spheres) are represented by pink color, green and red spheres denote tellurium and oxygen atoms, respectively. (b) The corresponding first Brillouin Zone of the monoclinic structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
To determine at ground state, the values of a, b and c, the lattice constants of monoclinic CdTe2O5 we have fitted the total energy versus volume data to the nonlinear Murnaghan equation of state [23]. The set of explicitly DFT-computed lattice parameters and the calculated band gap energy are given in Table 3. Our results agree remarkably well with the available experimental data of structural properties for the closely related compound CaTe2O5 [24], Table 3. 4.2. Band structure The calculated band structure for CdTe2O5 along the high symmetry directions of its Brillouin zone is shown in Fig. 12 (left). Table 3 Calculated values of the lattice constants, volume and band gap energy of the unit cell of CdTe2O5 within LDA approximation. For comparison purposes, the available measured structural parameters for the similar compound CaTe2O5 are shown.
Cd(Te2O5) Ca(Te2O5)
a (Å)
b (Å)
c (Å)
α
β
γ
V (Å3)
Band gap (eV)
9.176 9.464
5.484 5.757
10.811 11.229
90 90
114.295 115.14
90 90
495.790 553.746
3.06 This work - Exp. Ref [25]
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Fig. 12. The LDA electron band structure (left) and total density of states of CdTe2O5 (right). The Fermi level is set to zero.
According to these calculations, this compound possesses an indirect energy band gap of about 3.06 eV with valence band maximum (VBM) and the conduction band minimum (CBM) located at the Y and Γ points of the BZ (Fig. 11b), respectively. The calculated value is in very good correspondence with the experimental optical band gap of ∼3.16 obtained from the fitting in Fig. 9. It is pointed out that this experimental value was not determined from data of a pure CdTe2O5 film, but from the CdTe2O5-like region in the absorption curve in Fig. 9, which could be somewhat affected by the absorption of Cu2O, and vice versa. Moreover, it has been reported an optical band gap for CdTe2O5 crystals with a different crystalline structure (triclinic) than the one reported here (monoclinic) with a value of 3.63 eV [25].
5. Conclusions The physical properties of thin films obtained by sputtering from a composite target made of a mixture of Cu2O, CdO and TeO2 (1:1:1 mol ratio) at different substrate temperatures have been studied. In general, an increment in substrate temperature produced that the concentration in the films of Cd decreased, while that of O increased. At 350 °C the first crystalline aggregates of Cu2O, with an average size of 127 ± 10 Å, were formed. Then, the growth of CdTe2O5 crystals followed requiring more energy, since this type of aggregates formed at 400 °C. At 450 °C, the films consisted of Cu2O and CdTe2O5 clusters immersed in an amorphous background. Film thickness profiling was obtained by interferometric methods obtaining values from ∼1.0 to 1.7 μm. The presence of polycrystalline Cu2O clusters was confirmed by the appearance of Raman bands at frequencies corresponding to the longitudinal and transverse optical modes of Cu2O. In addition to these bands, low intensity vibrational modes belonging to structural units of the crystalline structure of CdTe2O5 were observed. The room temperature and 80 K photoluminescence spectra consisted of emissions from the Cu2O crystallites corresponding to free excitons relaxations with and without phonon emissions. Density functional theory calculations were carried out to determine the electronic band structure of monoclinic CdTe2O5. The optimized lattice parameters were very close to those of the related compound CaTe2O5. The calculations showed that CdTe2O5 in an indirect band gap material with a magnitude of 3.06 eV. This value compares well with the experimentally determined from the CdTe2O5 clusters (3.16 eV) in the films grown at 450 °C.
Acknowledgements A. Beristain-Bautista acknowledges the scholarship from Conacyt-Mexico. The authors acknowledge the technical assistance of M. A. Hernández Landaverde and C.I. Zúñiga Romero. S. Jiménez Sandoval acknowledges the partial financial support of ConacytMexico through grant No. 257166. The financial support provided by the Sectorial Fund CONACYT-SENER Energy Sustainability under grant No. 202784; 2015–07 “Mexican postdoctoral Projects in Energy Sustainability” is also acknowledged. 10
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2018.09.025. References [1] A. Mendoza-Galván, S. Jiménez-Sandoval, J. Carmona-Rodríguez, Spectroscopic ellipsometry study of CuCdTeO thin films grown by reactive co-sputtering, Thin Solid Films 519 (2011) 2899–2902. [2] S. Jiménez-Sandoval, J. Carmona-Rodríguez, R. Lozada-Morales, O. Jiménez Sandoval, M. Meléndez-Lira, C.I. Zúñiga-Romero, D. Dahlberg, Effect of high copper and oxygen concentrations on the optical and electrical properties of (CdTe)xCuyOz films, Sol. Energy Mater. Sol. Cells 90 (2006) 2248–2254. [3] J. Carmona-Rodríguez, R. Lozada-Morales, P. del Ángel-Vicente, O. Jiménez-Sandoval, G. López-Calzada, D. Dahlberg, S. Jiménez-Sandoval, Properties of Cux(CdTe)yOz thin films: composition-dependent control of band gap and charge transport, J. Mater. Chem. 21 (2011) 13001–13008. [4] G. Arreola-Jardón, S. Jiménez-Sandoval, A. Mendoza-Galván, Growth and characterization of CuCdTeO thin films sputtered from CdTe-CuO composite targets, Vacuum 101 (2014) 130–135. [5] A. Mendoza-Galván, G. Arreola-Jardón, L.H. Karlsson, P.O.Å. Persson, S. Jiménez-Sandoval, Optical properties of CuCdTeO thin films sputtered from CdTe-CuO composite targets, Thin Solid Films 571 (2014) 706–712. [6] T. Kamiya, K. Nomura, H. Hosono, Present status of amorphous In-Ga-Zn-O thin-film transistors, Sci. Technol. Adv. Mater. 11 (2010) 044305. [7] T. Minami, Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications, Thin Solid Films 516 (2008) 1314–1321. [8] T.O. Mason, D.R. Kammler, B.J. Ingram, G.B. Gonzalez, D.L. Young, T.J. Coutts, Key structural and defect chemical aspects of Cd–In–Sn–O transparent conducting oxides, Thin Solid Films 445 (2003) 186–192. [9] A. Martel, F. Caballero-Briones, R. Castro-Rodríguez, J. Méndez–Gamboa, N. Romeo, A. Bosio, J.L. Peña, Physical properties of transparent conducting Cd-Te-InO thin films: outlining a thermodynamic system for transparent conducting oxides, Thin Solid Films 518 (2009) 413–418. [10] A. Sanson, A first-principles study of vibrational modes in Cu2O and Ag2O crystals, Solid State Commun. 151 (2011) 1452–1454. [11] D. Tatar, G. Özen, F.B. Erim, M.L. Öveçoğlu, Raman characterizations and structural properties of the binary TeO2-WO3, TeO2-CdF2 and ternary TeO2-CdF2-WO3 glasses, J. Raman Spectrosc. 41 (2010) 797–807. [12] T. Komatsu, H. Mohri, Raman scattering spectra and optical properties of tellurite glasses and crystalline phases containing PbO and CdO, Phys. Chem. Glasses 40 (1999) 257–263. [13] H. Solache-Carranco, G. Juárez-Díaz, A. Esparza-García, M. Briseño-García, M. Galván-Arellano, J. Martínez-Juárez, G. Romero-Paredes, R. Peña-Sierra, Photoluminescence and X-ray diffraction studies on Cu2O, J. Lumin. 129 (2009) 1483–1487. [14] S.V. Gastev, A.A. Kaplyanskii, N.S. Sokolov, Relaxed excitons in Cu2O, Solid State Commun. 42 (1982) 389–391. [15] Liangmin Zhang, Lyndsey McMillon, Jeremiah McNatt, Gas-dependent bandgap and electrical conductivity of Cu2O thin films, Sol. Energy Mater. Sol. Cells 108 (2013) 230–234. [16] J. Tauc, Amorphous and Liquid Semiconductors, first ed., Plenum press, London and New York, 1974. [17] P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953–17979. [18] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1993) 558–561. [19] G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B 49 (1994) 14251–14269. [20] G. Kresse, J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6 (1996) 15–50. [21] G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169–11186. [22] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [23] F.D. Murnaghan, The compressibility of media under extreme pressures, Proc. Natl. Acad. Sci. U.S.A. 30 (1944) 244–247. [24] N. Barrier, J.M. Rueff, M.B. Lepetit, J. Contreras-Garcia, S. Malo, B. Raveau, A new polymorph with a layered structure ε-CaTe2O5, Solid State Sci. 11 (2009) 289–293. [25] Jalal Nawash, Crystal growth of CdTe2O5 by a modified bridgman technique, Mater. Res. Soc. Symp. Proc. 1799 (2015) 13–18.
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of deposition temperature on the properties of sputtered films grown from a Cu2OeCdOeTeO2 composite target: Electronic properties of CdTe2O5 A. Beristain Bautistaa, Emilia Olivosb, R. Arroyaveb, F. Rodríguez Melgarejoa, S. Jiménez Sandovala,∗ a
Centro de Investigación y de Estudios Avanzados del I. P. N., Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla, C. P. 76230, Querétaro, Qro, Mexico b Texas A&M University, Department of Materials Science & Engineering, College Station, Tx, 77840, USA
ABS TRA CT
Thin films were grown by sputtering targets made by compressing a mixture of the binary oxides Cu2O, CdO and TeO2. Their properties were studied as a function of the substrate temperature, which was varied from room temperature up to 450 °C. All the analyses were carried out on as-grown films. The elemental composition was analyzed by energy dispersive spectroscopy. A structural analysis was carried from grazing-angle X-ray diffraction patterns, which showed the formation of Cu2O (at 350 °C and above) and CdTe2O5 (at 400 °C and above) clusters immersed in an amorphous background. Raman scattering (room temperature) and photoluminescence (room temperature and 80 K) spectra were dominated by the signals originating from the Cu2O clusters. The optical transmittance spectra presented characteristics in agreement with the phases present in the films, as determined by X-ray diffraction. Due to the lack of information on CdTe2O5 in the literature, density functional theory calculations were carried out to analyze the optical properties of the films containing this compound in terms of the calculated electronic band structure. It was determined that CdTe2O5 is an indirect gap material. The theoretical band gap was 3.06 eV, which compares well with the experimental one ∼3.16 eV.
1. Introduction Prevailing technological developments have produced the need for novel materials that could meet the increasingly demanding functionality in new technologies, and for the development of fast and robust sensors, electronic and opto-electronic devices. Along this line, binary and a large number of ternary compounds have been extensively studied to date. Quaternary compounds and composite systems are materials that have not been fully exploited due to their complexity and to the lack of control over deposition methods. These materials, however, possess an enormous potential that has led to a wide variety of research topics, aimed not only to take advantage of the unique properties that these materials can present, but also to get a better understanding of their physical properties. Among these efforts, it may be mentioned the studies on films of solid solutions and composites based on cadmium telluride with copper and oxygen. The growth of such films has been carried out following two approaches: i) by reactive cosputtering using two targets, one of Cu and the other of CdTe, with a flow of O2 and Ar in the chamber [1–3]; and ii) by means of sputtering a single target prepared by pressing mixtures of CdTe and CuO powders [4,5]. As a result, in both cases it was possible to improve and control the electronic properties of the films. In recent years, the synthesis of transparent conductive oxides has been directed, among other routes, towards the investigation of quaternary materials by means of the combination of binary oxides. Examples of this approach are the systems Ga2O3 − ZnO e In2O3
∗
Corresponding author. E-mail address: [email protected] (S. Jiménez Sandoval).
https://doi.org/10.1016/j.spmi.2018.09.025 Received 21 July 2018; Received in revised form 11 September 2018; Accepted 20 September 2018 0749-6036/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Bautista, A.B., Superlattices and Microstructures, https://doi.org/10.1016/j.spmi.2018.09.025
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[6], SnO2 − ZnO e In2O3 [7], SnO2 − CdO e In2O3 [8] and TeO2 − CdO e In2O3 [9]. Following this approach and our previous work, we have investigated the CdeTeeCueO system by sputtering targets made of a mixture of the binary oxides Cu2O, CdO and TeO2, in the 1:1:1 M ratio. We report here the results of the characterization carried out on the chemical composition, structure, microstructure and photoluminescence of the as-grown films, as a function of substrate temperature during growth. This experimental characterization is followed by density functional theory calculations of the structural and electronic properties of CdTe2O5, one of the resulting materials when high substrate temperatures were employed. 2. Experimental details Films were grown by rf sputtering on glass and silicon substrates using a single cold-pressed composite target prepared from a mixture of the binary compounds: copper(I) oxide, cadmium(II) oxide and tellurium dioxide. For that end, a target was made in our laboratory by cold-pressing Cu2O (99.99%), CdO (99.5%) and TeO2 (99.995%) powders from Sigma-Aldrich in a 1:1:1 mol ratio. For different runs, the substrate temperature was fixed to values starting from room temperature (i.e. unheated on purpose: at the end of the deposit the temperature was ∼70 °C), 100 °C, and up to 450 °C in 50°C-steps. The substrates were cleansed prior to deposition following standard procedures. For a single run, glass and silicon substrates were placed simultaneously in the substrate holder to assure the same chemical composition. During film growth, the Ar flow in the chamber was 11 sccm, resulting in a working pressure of ∼1 × 10−3 Torr. The substrate-to-target distance was 8 cm. A radio frequency power of 40 W was applied to the target, which was previously pre-sputtered for 5 min. The deposition times were 150 min for all growths. The growth rate was of ∼10.7 nm/min for substrate temperatures (Ts) not higher than 250 °C; when Ts was in the range from 300 to 450 °C, the growth rate decreased to ∼7 nm/min as a consequence of atomic re-evaporation at the film surface during growth on a hotter surface. The chemical composition of the samples was determined from the films deposited on Si substrates through wavelength dispersive spectroscopy (WDS) in an electron probe micro analyzer (EPMA) JXA – 8530F. The photoluminescence (PL) studies were performed at 80 K and room temperature, in a micro-Raman system (Labram from Dilor) using the 488 nm excitation line of an Ar+-laser. For low-temperature PL measurements, a liquid-nitrogen Oxford cryostat was employed. The X-ray diffraction experiments were obtained in a Rigaku, D/Max-2100, using the Cu Kα radiation (1.5406 Å), with 30 kV and 20 mA as working parameters. 3. Results and discussion 3.1. Elemental analysis The chemical composition in the films presented some deviations from the nominal concentrations used in the targets. Fig. 1 presents the results of the chemical composition obtained by WDS as a function of substrate temperature (Ts). According to the 1:1:1 ratio of Cu2O, CdO and TeO2, the target was composed of 50 at. % of oxygen, 25% at. of copper, 12.5% at. of cadmium and 12.5% at. of tellurium. These nominal concentrations in the target were the same for all growths. As a precaution, the target in use was ground and re-manufactured after each growth to avoid variations in the chemical composition on its surface, as a consequence of preferential sputtering during the previous deposition. In Fig. 1 the nominal concentrations are indicated by the dotted lines. For the growth at room temperature (RT), the amount of oxygen in the films was around 10 at. % below the nominal concentration, while the amount of tellurium is ca. 8 at. % above the nominal concentration. However, the concentrations of cadmium and copper in this film were close to their corresponding nominal concentrations. In general, we can see that as Ts increases in the substrate, the amount of oxygen increases until reaching values close to its nominal value. The tellurium concentration remained higher, around 20 at.%, than
Fig. 1. Chemical composition of the films obtained from wavelength dispersive spectroscopy as a function of the substrate temperature. 2
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Fig. 2. XRD patterns for the series of films that were obtained with a substrate temperature of RT, and from 250 to 450 °C. The Miller indices identify c-cubic Cu2O and m-monoclinic CdTe2O5 structures.
the nominal (12.5 at. %) as Ts increased achieving values close to 25 at.% above 300 °C. In contrast, copper and cadmium concentrations decreased to values around 19 and 6 at. %, respectively, for Ts = 450 °C. For all depositions, the lowest elemental concentration obtained was for Cd, due to its characteristic high vapor pressure. 3.2. Crystalline structure The structural characteristics of the films, determined through X-ray diffraction (XRD), showed that the films are a mixture of amorphous and polycrystalline regions. Fig. 2 shows the diffraction patterns of the films for the employed substrate temperatures. A clear evolution is observed starting from the amorphous film obtained at room temperature up to the formation of diffraction peaks corresponding to polycrystalline regions of Cu2O and CdTe2O5. The identification of the peaks has been carried out using the powder diffraction files of cubic Cu2O (PDF # 01–1142) and monoclinic CdTe2O5 (PDF # 24–0169). The (111) reflection of Cu2O is formed with high intensity from 350 °C up to 450 °C. Other Cu2O reflections correspond to the planes (200) and (220). That is, the first well defined crystalline phase formed in the films as a result of using higher-than-room-temperature Ts was Cu2O. The formation of Cu2O before CdTe2O5 must be driven by energy considerations involving all the interactions among the four chemical elements present during deposition. That is, upon arrival of the sputtered species to the substrate surface at temperature Ts, energetically the formation of Cu2O was favored over the ternary compound CdTe2O5. The formation of any other compound containing copper, besides Cu2O, was energetically more costly at the temperature and pressure in the growth chamber. As a result, polycrystalline grains of Cu2O initiated the nucleation process at the substrate surface. Then, when higher substrate temperatures were used (i.e. larger thermal energy available) Cd, Te along with abiding oxygen atoms, gave rise to crystalline CdTe2O5, in addition to the Cu2O already present. The reflections of planes (111), (200) and (220) of Cu2O were located at 36.5°, 42.3° and 61.6°, respectively. The average full width at half maximum (FWHM), lattice parameter a, interplanar distance d and crystallite size (XS) were obtained from the high intensity peak (111) of the cubic Cu2O phase for films grown at substrate temperatures of 350, 400 and 450 °C. These structural parameters are reported in Table 1. The interplanar distance d was obtained from the following relation
(h2 + k 2 + l 2) 1 = , d2 a2
(1)
where h, k and l are the Miller indices of the corresponding family of planes. From the structural parameters in Table 1, it may be 3
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Table 1 Structural parameters of the crystalline aggregates of Cu2O in the films. The full width at half maximum and the interplanar distance were derived from the (111) peaks. Substrate temperature (°C)
FWHM
d (Å)
Lattice parameter a (Å)
Crystallite size (Å)
350 400 450
0.664 ( ± 0.049) 0.373 ( ± 0.012) 0.374 ( ± 0.019)
2.4703 ( ± 0.0077) 2.4650 ( ± 0.0020) 2.4584 ( ± 0.0019)
4.2786 4.2695 4.2580
127 ( ± 10) 233 ( ± 08) 236 ( ± 11)
observed that, as the temperature of the substrate increases, the lattice parameter a decreases from 4.2786 to 4.2580 Å. The value for a reported in the literature is 4.2696 Å [13], which is in the middle of the values in Table 1. The crystallite size increased from 127 to 236 Å, as Ts was raised from 350 to 450 °C. From Fig. 2 it may be observed that films grown at ambient temperature and up to 300 °C have amorphous characteristics. That is, a broad band characteristic of amorphous material is noticeable with its center around ∼29°. Even when Ts = 450 °C, a non-planar background baseline is observed, indicative of the existence of an amorphous fraction in the film. In the samples grown with substrate temperature of 400 and 450 °C, reflections were obtained from the (100) and (200) planes located at 9.9° and 19.8°, respectively, corresponding to the monoclinic phase of CdTe2O5. The peak observed around 29°, however, has contributions from both Cu2O and CdTe2O5. The former presents a diffraction peak at 29.756°, (110) reflection, the latter at 30.043°, the (300) reflection. Fig. 3 presents a deconvolution with Gaussian functions of this peak, which shows the contribution of both phases. Energetically, Cu2O was favored over the formation of any other copper-containing ternary or quaternary compound. The remaining elements found their lowestenergy condition through the formation of CdTe2O5, although it must be noticed that even at Ts = 450 °C, some amorphous material was still present. The thicknesses of the films are given in Table 2. Two regimes can be observed: i) from RT to 250 °C the thickness of the films were around 1.6 μm; ii) for Ts between 300 and 450 °C, the thickness fluctuated around 1 μm. The higher the Ts, the larger the escaping probability for the atoms that arrive to the surface of the growing film. As a result, the thicknesses of the films are smaller for those deposited at higher Ts.
3.3. Lattice dynamics: Raman spectroscopy From RT to 300 °C, the spectra of the films did not show any distinguishable peaks, given their amorphous structure. The vibrational spectra of the polycrystalline films, i.e. for Ts = 350–450 °C, are presented in Fig. 4. It is possible to identify some vibrational frequencies at 216, 410, 466, 635 and 730 cm−1. Some of them correspond to Cu2O aggregates (216, 410 and 635 cm−1) [10], as it can be noticed by comparing with the Raman spectrum of Cu2O powder shown in the inset of Fig. 4. Two other bands correspond to vibrational modes of tellurium-oxygen structural units present in the crystalline structure of CdTe2O5. The mode at 466 cm−1 can be identified with vibrations of TeeOeTe chains between [TeO4] units, while the 730 cm−1 mode with vibrations of Te=O involving three-coordinate Te atoms or more, known as [TeO3+1] units [11]. It is noticed that the band at 730 cm−1 matches as well with a reported Raman band of CdTe2O5 [12].
Fig. 3. Deconvolution with Gaussian functions of the diffraction peak around 29° (Figs. 2 and 450 °C pattern) showing the contributions of both materials: Cu2O and CdTe2O5. The dotted lines correspond to the powder-diffraction-file data for each compound: cubic Cu2O (PDF # 01–1142) and monoclinic CdTe2O5 (PDF # 24–0169). 4
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Table 2 Thicknesses of the as-grown films for the various substrate temperatures employed. Ts (°C)
RT
100
150
200
250
300
350
400
450
Thickness (μm)
1.6
1.7
1.6
1.6
1.6
0.9
1.1
1.0
1.1
Fig. 4. Raman spectra of the films grown at room temperature and at Ts = 350–450 °C. The spectra of the films grown at temperatures lower than 350 °C were similar to the one at RT. The inset shows the Raman spectrum of Cu2O powder.
3.4. Photoluminescence spectra The luminescence of the films was obtained using an Ar+ laser emitting at 488 nm. Through the analysis of the photoluminescence spectra, information can be obtained about defect-related electronic levels, as well as the energetics of excitonic recombination processes. Fig. 5 shows the PL spectra of the films grown at substrate temperatures from 300 to 450 °C. The spectra are correlated with the X-ray diffraction data in the sense that the amorphous films show only extremely weak and broad emissions. Two measurements were made for each sample: room temperature (black continuous line) and 80 K (red dotted line). In general, it is possible to observe five emissions: 611, 618, 627, 633 and 728 nm. All these signals are related to excitonic relaxation processes in the crystalline structure of Cu2O. Indeed, the broad emission at 728 nm (1.7 eV) is due to the recombination of excitons bound to oxygen vacancies with double positive charge (VO2+) [13,14]. This signal is no longer observable in the film grown at 450 °C, which is indicative that this type of vacancies was significantly reduced for this substrate temperature. Some of this emission is noticeable in the spectrum of the film grown at 350 °C. The PL signal was the weakest for the film deposited at 300 °C, as a result of the structural disorder in this sample as revealed by the X-ray diffraction pattern, Fig. 2. In the case of the well-defined PL signal of the film grown at 450 °C, top spectrum Fig. 5, its analysis was carried out through a deconvolution with Gaussian functions to determine the components that make up the luminescent emission. Fig. 6a shows the deconvolution for the RT spectrum. Three emissions were determined at 1.95 (635 nm), 1.98 (626 nm) and 2.02 eV (613 nm). These emissions originate from free-exciton recombinations. That is, the emission at 1.95 eV corresponds to excitonic relaxation with phonon emission of 515 cm−1 (F2g mode [10]). The emission at 1.98 eV is due to the same type of relaxation with the emission of a 350 cm−1 phonon (Bu mode [10]). Finally, the emission at 2.02 eV corresponds to pure excitonic relaxation with no phonon emission [14]. Fig. 6b presents the deconvolution of the PL spectrum measured at 80 K. It is observed that the intensity of the 2.02 eV emission increased; while the 1.95 eV one decreased, with respect to the RT spectrum. There is a shift of the RT 1.98 eV emission to 2.0 eV as the temperature was lowered to 80 K due to the lattice contraction on cooling. Another consequence of decreasing the temperature was a reduction of the width of the emissions due to a reduction in the thermal fluctuations of the atoms around their equilibrium positions. Fig. 7 shows a schematic representation (Jablonski diagram) to illustrate the radiative transitions present in the films, which originate from the Cu2O clusters.
3.5. Optical transmittance In Fig. 8 the transmittance spectra of the films are presented as a function of the substrate temperature from RT to 450 °C. Initially, we can see that the RT films up to 200 °C show a slightly higher percentage of transmission with respect to those from 300 to 400 °C. Because of their amorphous nature, it is possible to observe a non-abrupt slope of the absorption edge for the RT-200 °C films, with respect to those of 300 and 350 °C. The slightly sharper slope of the absorption edge of the film grown at 350 °C indicates that 5
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Fig. 5. Photoluminescence spectra of the films grown at different substrate temperatures. The measurements were performed at room temperature (black continuous line) and 80 K (red dotted line). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Deconvolution of the emission spectra of the measurements made at (a) RT and (b) 80 K of the film deposited at 450 °C.
the amount of defect-related electronic levels decreased, in accordance with the X-ray diffraction data. The transmittance spectra of the 400–450 °C films present different characteristics. It must be remember that these films consist of three different phases: amorphous material and aggregates of Cu2O and CdTe2O5. Thus, the optical spectra contain signatures of the absorption characteristics of the three components. These contributions are clearly noticeable in the absorption spectrum of the film grown at 450 °C, Fig. 9. For the Cu2O-like and CdTe2O5-like absorptions, estimates of the effective band gap for each case were obtained by using the Tauc relation: αhν = B(hν-Eg)n,
(2) 6
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Fig. 7. Jablonski diagram of the radiative transitions observed in the photoluminescence measurements. All the emissions originate from the Cu2O clusters present in the films.
Fig. 8. Transmittance spectra of the films as a function of the substrate temperature from RT to 450 °C.
Where B is a constant, hν is the energy of the photon, Eg is the band gap energy. The exponent n has a value of 2 for amorphous and indirect gap materials, and ½ for direct bandgap crystalline materials [16]. The optical absorption coefficient α(λ) was determined from:
7
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Fig. 9. Absorbance spectrum of the film deposited at 450 °C. The insets show Tauc's model fittings to determine the effective band gap of the (a) Cu2O-like and (b) CdTe2O5-like regions.
α (λ ) = 2.303
A , d
(3)
where A is the absorbance and d is the thickness of the film. The insets in Fig. 9 show that the effective band gap obtained from the Tauc's model fittings for the Cu2O-like and CdTe2O5-like regions were 2.67 and 3.16 eV, respectively. The former is close to the Cu2O band gap, which is ∼2.17 eV [15], the latter is discussed below in regard to our theoretical calculations for CdTe2O5. It must be taken into consideration that the curve shown in Fig. 9 represents an effective absorption made of the absorption of the three phases: amorphous, Cu2O and CdTe2O5. In that sense, the values of the band gap obtained from the Cu2O-like and CdTe2O5-like absorption regions represent approximations to the real values. A value for the pseudo band gap for the amorphous films was obtained by applying Tauc's model with n = 2 in equation (2). These values ranged from 1.7 to 1.8 eV, as shown in Fig. 10 which may be understood as corresponding to the photon energies for which the absorption of the materials start being significant. 4. Electronic properties of CdTe2O5: theoretical calculations As mentioned above, the information in the literature on CdTe2O5 is remarkably scarce. For instance, there is no experimental or theoretical report on its electronic band gap, a fundamental property. Here we report density functional theory (DFT) calculations on the electronic band structure and the corresponding density of states for CdTe2O5. These calculations are used to discuss the value of the band gap determined from the CdTe2O5-like region in the absorption spectrum shown in Fig. 9. 4.1. Computational details The band structure and total energy calculations were performed using the frozen-core projector-augmented wave (PAW) method [17] within the local density approximation (LDA) as implemented in the VASP software package [18–21]. The plane wave cutoff and k point mesh (Monkhorst-Pack [22]) were set to 528 eV and a k point mesh of 4 × 8 × 4 were used. The optimal plane wave cut-off energy and k-point mesh have been chosen based on performed total energy convergence test. The monoclinic crystal structure of CdTe2O5 is described by the space group P21/c, number 14, with 32 atoms in the unit cell (4 atoms of Cd, 20 atoms of O and 8 atoms of Te) Fig. 11a. 8
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Fig. 10. Estimates of the pseudo gap for the amorphous films grown at the indicated temperatures using Tauc's model.
Fig. 11. (a) Monoclinic unit cell of CdTe2O5 structure. Cd atoms (larger spheres) are represented by pink color, green and red spheres denote tellurium and oxygen atoms, respectively. (b) The corresponding first Brillouin Zone of the monoclinic structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
To determine at ground state, the values of a, b and c, the lattice constants of monoclinic CdTe2O5 we have fitted the total energy versus volume data to the nonlinear Murnaghan equation of state [23]. The set of explicitly DFT-computed lattice parameters and the calculated band gap energy are given in Table 3. Our results agree remarkably well with the available experimental data of structural properties for the closely related compound CaTe2O5 [24], Table 3. 4.2. Band structure The calculated band structure for CdTe2O5 along the high symmetry directions of its Brillouin zone is shown in Fig. 12 (left). Table 3 Calculated values of the lattice constants, volume and band gap energy of the unit cell of CdTe2O5 within LDA approximation. For comparison purposes, the available measured structural parameters for the similar compound CaTe2O5 are shown.
Cd(Te2O5) Ca(Te2O5)
a (Å)
b (Å)
c (Å)
α
β
γ
V (Å3)
Band gap (eV)
9.176 9.464
5.484 5.757
10.811 11.229
90 90
114.295 115.14
90 90
495.790 553.746
3.06 This work - Exp. Ref [25]
9
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Fig. 12. The LDA electron band structure (left) and total density of states of CdTe2O5 (right). The Fermi level is set to zero.
According to these calculations, this compound possesses an indirect energy band gap of about 3.06 eV with valence band maximum (VBM) and the conduction band minimum (CBM) located at the Y and Γ points of the BZ (Fig. 11b), respectively. The calculated value is in very good correspondence with the experimental optical band gap of ∼3.16 obtained from the fitting in Fig. 9. It is pointed out that this experimental value was not determined from data of a pure CdTe2O5 film, but from the CdTe2O5-like region in the absorption curve in Fig. 9, which could be somewhat affected by the absorption of Cu2O, and vice versa. Moreover, it has been reported an optical band gap for CdTe2O5 crystals with a different crystalline structure (triclinic) than the one reported here (monoclinic) with a value of 3.63 eV [25].
5. Conclusions The physical properties of thin films obtained by sputtering from a composite target made of a mixture of Cu2O, CdO and TeO2 (1:1:1 mol ratio) at different substrate temperatures have been studied. In general, an increment in substrate temperature produced that the concentration in the films of Cd decreased, while that of O increased. At 350 °C the first crystalline aggregates of Cu2O, with an average size of 127 ± 10 Å, were formed. Then, the growth of CdTe2O5 crystals followed requiring more energy, since this type of aggregates formed at 400 °C. At 450 °C, the films consisted of Cu2O and CdTe2O5 clusters immersed in an amorphous background. Film thickness profiling was obtained by interferometric methods obtaining values from ∼1.0 to 1.7 μm. The presence of polycrystalline Cu2O clusters was confirmed by the appearance of Raman bands at frequencies corresponding to the longitudinal and transverse optical modes of Cu2O. In addition to these bands, low intensity vibrational modes belonging to structural units of the crystalline structure of CdTe2O5 were observed. The room temperature and 80 K photoluminescence spectra consisted of emissions from the Cu2O crystallites corresponding to free excitons relaxations with and without phonon emissions. Density functional theory calculations were carried out to determine the electronic band structure of monoclinic CdTe2O5. The optimized lattice parameters were very close to those of the related compound CaTe2O5. The calculations showed that CdTe2O5 in an indirect band gap material with a magnitude of 3.06 eV. This value compares well with the experimentally determined from the CdTe2O5 clusters (3.16 eV) in the films grown at 450 °C.
Acknowledgements A. Beristain-Bautista acknowledges the scholarship from Conacyt-Mexico. The authors acknowledge the technical assistance of M. A. Hernández Landaverde and C.I. Zúñiga Romero. S. Jiménez Sandoval acknowledges the partial financial support of ConacytMexico through grant No. 257166. The financial support provided by the Sectorial Fund CONACYT-SENER Energy Sustainability under grant No. 202784; 2015–07 “Mexican postdoctoral Projects in Energy Sustainability” is also acknowledged. 10
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