CdTe thin films grown by pulsed laser deposition ...

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close space sublimation (CSS) [2], sputtering [3], chemical deposi- tion [4], and pulsed laser deposition (PLD) [5] among others. Most of these growth techniques ...
Journal of Crystal Growth 386 (2014) 27–31

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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

CdTe thin films grown by pulsed laser deposition using powder as target: Effect of substrate temperature F. de Moure-Flores a,n, J.G. Quiñones-Galván b, A. Guillén-Cervantes b, J.S. Arias-Cerón b, A. Hernández-Hernández b, J. Santoyo-Salazar b, J. Santos-Cruz a, S.A. Mayén-Hernández a, M. de la L. Olvera c, J.G. Mendoza-Álvarez b, M. Meléndez-Lira b, G. Contreras-Puente d a

Facultad de Química-Materiales, Universidad Autónoma de Querétaro, Querétaro 76010, México Departamento de Física, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07360, México c Departamento de Ingeniería Eléctrica, Sección de Estado Sólido, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07360, México d Escuela Superior de Física y Matemáticas del IPN, México D.F. 07738, México b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 June 2013 Received in revised form 19 September 2013 Accepted 20 September 2013 Communicated by P. Rudolph Available online 27 September 2013

CdTe thin films were deposited by pulsed laser deposition on Corning glass slides using CdTe powder as target. Films were grown at substrate temperatures ranging from room temperature (  25 1C) to 300 1C. The structural, compositional and optical properties were analyzed as a function of substrate temperature. X-ray diffraction shows that CdTe films grown at room temperature have hexagonal phase, while for higher temperatures the films have cubic phase. Raman and EDS indicate that films grew with Te excess, which suggests that CdTe films have p-type conductivity. & 2013 Elsevier B.V. All rights reserved.

Keywords: A3. Pulsed laser deposition B2. Semiconducting II-VI materials B3. Solar cells

1. Introduction Cadmium telluride (CdTe) is a II–VI semiconductor compound which is considered of great importance due to its optoelectronic applications, in particular, it is a very promising material for photovoltaic applications [1]. The most common solar cell is based on CdS/CdTe, in this system the p-CdTe film acts as a photoabsorbing layer to generate carries. CdTe has a direct bandgap of 1.5 eV at room temperature and a high absorption coefficient (4105 cm  1), which means that a layer thickness of few micrometers is enough to absorb 90% of incident photons [1]. CdTe films exhibit n- or p-type conductivity; cadmium excess yields n-type whereas tellurium excess yields p-type conductivity [1]. CdTe polycrystalline films can be prepared by several techniques such as vapor transport deposition (VTD), physical vapor deposition (PVD), spray deposition, close space sublimation (CSS) [2], sputtering [3], chemical deposition [4], and pulsed laser deposition (PLD) [5] among others. Most of these growth techniques require high growth temperature (450– 600 1C). The high deposition temperature and the difference in vapor pressure for these materials make that CdTe films present Te excess. PLD has one great advantage over other techniques; the high n

Corresponding author. Tel.: þ 52 442 192 1200. E-mail address: [email protected] (F. de Moure-Flores).

0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.036

energy atoms and ions in the laser-induced plasma produce a higher surface mobility as consequence it is possible to grow high quality films at low substrate temperature [6]. In this work, we report the influence of substrate temperature on structural, compositional and optical properties of CdTe films grown by PLD using CdTe powder as target. Using powder as target-material have advantages and disadvantages. The advantages are: (a) the technique employs less than a half gram of source material, in fact with 0.5 g of material around ten thin films can be obtained and (b) the growth rate is greater than when using a solid target [7]. One disadvantage is that the amount of ablated material is not always the same due to the volume variation of the glass container [8], which hinders controlling the thickness of the films.

2. Experiment CdTe thin films were grown on Corning glass substrates by pulsed laser deposition using CdTe powder enclosed in a glass ampule as target. The laser beam incides perpendicularly to the glass ampule (at a grazing-incidence on source material surface), and the orifice of the glass ampule faces the substrate plane, a schematic of this configuration is shown in Fig. 1. The films were prepared using a Nd:YAG laser with a pulse width and frequency of 5 ns and 10 Hz,

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3. Results and discussion

Fig. 1. Schematic diagram showing the configuration geometry of the substratetarget-laser of PLD system.

Table 1 Substrate temperature and crystallite size of CdTe films grown by PLD. Sample

Substrate temperature (1C)

Crystallite size (nm)

CdTe-25 CdTe-100 CdTe-200 CdTe-300

25 100 200 300

22 24 25 28

respectively. The wavelength used to irradiate the powder-material was 1064 nm with a laser fluence of 2 J/cm2, previously we found no significant differences between CdTe films deposited at wavelengths 1064 nm and 532 nm [7]. The growth time was 10 min and the distance between target-material and substrates was 2.2 cm. The films were grown in vacuum at substrate temperatures ranging from room temperature (E25 1C) to 300 1C. The films were labeled according to the substrate temperature, see Table 1. Before characterization, CdTe films were rinsed in methanol in ultrasonic bath to remove possible contaminants and immediately heated in a nitrogen atmosphere at 100 1C for 20 min in order to remove residues from ambient. X-ray diffraction patterns were obtained in a Siemens D5000 diffractometer, using the Cu-Kα line (1.5406 Å). The surface morphology and grain size of the films were observed by scanning electron microscopy (SEM) using a FESEM-Carl Zeiss Auriga. Raman spectroscopy measurements were carried out in a Labram Dilor micro Raman system employing a HeNe laser (632.8 nm) as excitation source. Atomic concentration measurements of samples were determined by energy dispersive spectrometry (EDS) with a Bruker XFlash detector 5010 installed in a Jeol JSM-6300 scanning electron microscope, using an acceleration voltage of 20 kV. Film thicknesses were measured in a KLA Tencor P15 profilometer. A Perkin-Elmer Lambda 25 UV–vis spectrophotometer was used to obtain optical transmittance. Photoluminescence (PL) spectra were obtained at room temperature using an Omnichrome-Series 56 He–Cd laser emitting at 325 nm with an optical excitation power of 18 mW.

XRD patterns of CdTe films grown by pulsed laser deposition at different substrate temperature are shown in Fig. 2. The diffractogram 2a presents three diffraction peaks at 22.421, 23.721 and 39.241. The diffraction peak at 22.421 corresponds to the hexagonal CdTe phase [9], and the diffraction plane is (100)H. Diffraction peaks at 23.721 and 39.241 may correspond to the cubic [10] or hexagonal [9] phase and the diffraction planes are (111)C/(002)H and (220)C/(110)H, respectively. Due to the presence of the peak at 22.421, which corresponds to the hexagonal phase, peaks at 23.721 and 39.241 can be assigned to the hexagonal CdTe phase [5,7], although exists the possibility of a mixture of phases. The diffractograms 2b–d present three diffraction peaks located at 23.681, 39.261 and 46.421, due to the absence of hexagonal phase peaks in these diffractograms, these peaks can be assigned to the cubic CdTe phase [11] and the diffraction planes are (111)C, (220)C and (311)C, respectively. It should be mentioned here that the physical and chemical properties of films obtained by PLD depend strongly on growth parameters like the substrate temperature, the geometry of substrate-target-laser configuration, wavelength, etc. [11,12]. It has been reported that CdTe films grown at room temperature by PLD using a Nd:YAG laser have hexagonal phase [7], while CdTe films deposited at higher temperature have the cubic stable phase [11]. On the other hand, the crystallite size was calculated from XRD patterns using the Scherrer formula: d ¼0.9λ/B cosθB, where d is the crystallite size, λ is the wavelength (1.5406 Å), B is the full width at half maximum (FWHM) of the peak and θB is the Bragg angle. The crystallite size of CdTe films is summarized in Table 1. It can be appreciated that the crystallite size increases as the substrate temperature increases, a similar behavior has been observed in other II–VI semiconductors [13]. Because the crystallite size is inversely proportional to the FWHM, this increase in the crystallite size indicates an improvement in the crystalline quality. In order to know the surface morphology, SEM pictures of samples were imaged. Fig. 3 shows SEM images of the CdTe films morphology. It can be appreciated that there is a strong relationship between the substrate temperature and the surface morphology of CdTe films grown by PLD. Fig. 3a shows the surface morphology of the CdTe film grown at room temperature (CdTe25 sample), which has a smooth morphology with particles of different sizes on the surface, these particles have size ranging from 80 nm to 1.0 mm. It is important to mention that SEM measurements were performed on different areas of the film, this sample has a high density of large particles (1.0 mm) on the surface.

Fig. 2. XRD patterns of CdTe films grown by PLD at different substrate temperatures. (a) The film grown at room temperature has hexagonal structure. (b and c) Films grown at temperatures ranging from 100 to 300 1C have cubic phase.

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Fig. 3. SEM images of CdTe thin films grown by PLD at different substrate temperatures; (a) room temperature (E25 1C) (CdTe-25), (b) 100 1C (CdTe-100), (c) 200 1C (CdTe-200) and (d) 300 1C (CdTe-300).

The CdTe-100 sample has a smooth morphology with few particles with size ranging from 100 nm to 580 nm. The film grown at 200 1C (Fig. 3c) has a smooth surface. Finally, Fig. 3d shows the morphology of the CdTe-300 sample, which presents a morphology composed of flakes. Particles on the surface (Fig. 3a and b) can be related to an effect called splashing, this effect occurs commonly in films obtained by PLD. The splashing is an undesirable and difficult to avoid effect in films obtained by PLD technique, from Fig. 3 it can be observed that CdTe films grown at a substrate temperature of 200 1C and 300 1C have no splashing. It should be noted that SEM images show the grain size while XRD gives the crystallite size and can be different. The SEM and XRD analysis indicate that grains are composed of small crystallites. Fig. 4 shows the Raman spectra of CdTe films grown by PLD at different substrate temperatures. All spectra showed the longitudinal optical (LO) mode at a frequency of 166.5 cm  1 and their second order mode (2LO) at 333 cm  1, characteristic of CdTe [3]. It can be appreciated that the 2LO mode intensity increases as the substrate temperature increases, indicating an improvement of crystalline quality. It can be also observed that all samples present two shoulders at 123 cm  1 and 142 cm  1, this shoulders correspond to the A1 and E1 phonon vibration modes, respectively, of the hexagonal Te structure [3,11]. This indicates that the CdTe films grown by PLD grew with Te excess. In order to know the composition of the films, EDS measurements were performed. In Table 2 the Cd and Te atomic concentration as well as the Cd:Te ratio are displayed. From Table 2 it can be seen that all CdTe films grew with Te excess. It can be also observed that the Te atomic concentration decreases with increasing the growth temperature, which indicates that the increase of substrate temperature improves the stoichiometry of CdTe films grown by PLD. It is widely accepted that CdTe films with Te excess have p-conductivity [1]; this suggests that CdTe thin films grown by PLD probably have p-type conductivity.

Fig. 4. Raman spectra of CdTe thin films grown by PLD. Spectra show the A1 and E1 of tellurium modes and LO CdTe modes. Table 2 Cd and Te atomic concentrations, thickness and bandgap of CdTe films grown by PLD. Sample

Cd (%)

Te (%)

Cd:Te ratio

Thickness (μm)

Eg (eV)

CdTe-25 CdTe-100 CdTe-200 CdTe-300

43.77 47.48 48.61 48.96

56.23 52.52 51.39 51.04

0.78 0.90 0.94 0.96

1.50 3.66 4.74 5.03

1.34 1.38 1.46 1.50

The optical transmittance spectra of CdTe films are shown in Fig. 5. It can be appreciated that the transmittance of CdTe films is practically zero in the visible region of electromagnetic spectrum

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Fig. 5. Optical transmittance spectra of CdTe films grown by PLD at substrate temperatures ranging from 25 1C to 300 1C. It can be appreciated that the transmittance of CdTe films is practically zero in the range of 400–700 nm (visible region).

Fig. 7. Photoluminescence spectra at room temperature of CdTe-200 and CdTe-300 samples. Spectra have a shoulder at 1.09 eV which is related to defects in the samples and a shoulder around 1.5 eV associated to a band to band recombination.

two PL peaks, one at 1.09 eV and other at 1.50 eV. The peak at 1.50 eV coincides with the bandgap value and the peak at 1.09 is associated to defects [15]. From Fig. 7 it can be observed that the PL spectrum of the CdTe-200 sample is of lower intensity than the CdTe-300 sample, which indicates that the CdTe films grown by PLD at a substrate temperature of 300 1C have the best crystalline quality.

4. Conclusions

Fig. 6. Bandgap calculations for CdTe films grown at different substrate temperatures by PLD.

(400–700 nm). The absorption coefficient (α) was calculated by the relation: T ¼(1 R)2exp(  αl), where T is the transmittance, R is the reflectance and l is the film thickness [14]. The film thicknesses are shown in Table 2. The absorption coefficient was used to determine the bandgap (Eg) for each film, using the relation αhν ¼ (hν Eg)1/2, where hν is the photon energy. Fig. 6 shows the graphic of (αhν)2 vs. hν, the Eg was determined by fitting the lineal part of the curve (the arrows in the Fig. 6 are for eye guide purposes). The Eg values are compiled in Table 2, it can be observed that the Eg value increases when the substrate temperature rises. The bandgap increases from 1.34 eV (CdTe-25 sample) to 1.50 eV for the CdTe-300 sample. From Table 2 it can be observed that as the substrate temperature is increased the stoichiometry of CdTe films improved, and as the stoichiometry of CdTe films improved the bandgap value approaches to the bulk CdTe. This behavior has been observed in other II–VI semiconductors [13]. The photoluminescence spectra at room temperature of the CdTe-200 and CdTe-300 samples are shown in the Fig. 7. The CdTe-200 sample has a shoulder at 1.09 eV and a PL peak at 1.47 eV, the PL peak at 1.47 is close to the Eg value calculated from transmittance measurements, thus this signal was associated to a band to band recombination [11]. The CdTe-300 sample presents

Cadmium telluride thin films grown at different substrate temperatures using powder as target by PLD were obtained. The structural, compositional and optical properties were analyzed as a function of the substrate temperature. X-ray diffraction showed that CdTe films grown at room temperature have hexagonal phase, while for higher temperatures the films have cubic phase. The structural analysis indicates that the crystalline quality improves with increasing the substrate temperature. The compositional analysis implied that the CdTe films grew with Te excess and this excess is reduced when the substrate temperature increases. The excess of Te in the films suggested that the CdTe films have p-conductivity. The structural, compositional and optical analysis showed that the physical properties are enhanced when the substrate temperature increased, which indicates that CdTe films grown at high temperature by PLD using powder as target can be used in the manufacture of solar cells.

Acknowledgments We acknowledge the technical support of Marcela Guerrero, A. García-Sotelo, Josue Esau Romero Ibarra and Zacarías Rivera from the CINVESTAV-IPN and the partial support by CONACyT-México. References [1] Brian E. McCandless, James R. Sites, Cadmium telluride solar cells, in: Antonio Luque, Steven Hegedus (Eds.), Handbook of Photovoltaic Science and Engineering, John Wiley & Sons Ltd, England, 2003, pp. 617–662. [2] Nazar Abbas Shah, Abid Ali, Asghari Maqsood, Preparation and characterization of CdTe for solar cells, detectors and related thin-film materials, Journal of Electronic Materials 37 (2008) 145–151.

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