Growth and characterization of PbI2 thin films by ...

Journal of Nano Research Vol. 24 (2013) pp 1-6 Online available since 2013/Sep/18 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/JNanoR.24.1

Growth and characterization of PbI2 thin films by vacuum thermal evaporation Harshita Agrawal1, a, A. G. Vedeshwar2 and Vibhav K. Saraswat1, b 1

Deptt. of Physics, School of Physical Sciences, Banasthali University, Rajasthan, India 2

Deptt. of Physics and Astrophysics, University of Delhi, Delhi, India a

[email protected], [email protected]

[Submitted: May 31, 2012; revised: October 10, 2012; accepted: March 18, 2013] Keywords: PbI2 Thin films, Vacuum thermal evaporation, direct band gap, XRD, UV-Vis, and SEM.

Abstract:The aim of this study was to investigate the morphology and optical absorption of PbI2 thin films. PbI2 films of different thickness were deposited on glass substrate at ambient temperature by thermal evaporation technique. The deposition rate was optimized at 1-2 nm/s to grow uniform good quality films. The structural analysis of the films was carried out by X-ray diffraction (XRD). The morphology of so prepared films was studied by Scanning Electron Microscope (SEM). The absorption spectra were recorded using a UV-Vis fiber optics based spectrophotometer. All the measurements in the present work were carried out at room temperature (~300 K). PbI2 seems to have a very strong affinity for the growth of preferred (001) orientation. The d spacing of (00l) peaks match quite well with ASTM data for 2H polytype of PbI2 which confirms the stoichiometry. A uniform spherical grain size growth is clearly evident from SEM micrographs. Optical absorption analysis indicates that the Lead Iodide is having a direct optical band gap of about 2.45 eV. The variation of band gap with film thickness could be qualitatively understood as due to the quantum confinement effect. Introduction: Lead Iodide (PbI2) is a toxic, yellowish solid, having melting point at 675 K and boiling point at 1145 K. It displays a range of colors with varying temperature from bright yellow at room temperature to brick red. On cooling, its color returns to yellow. Lead Iodide (PbI2) is a direct band-gap semiconductor compound and one of the most promising materials for γ-ray and X-ray detectors. Due to its wide band gap energy (2.45 eV), the detector can be operated at room temperature without cooling. Its high density (D=6.2 g/cm3) makes it possible to fabricate a thinner detector. The most important feature of PbI2 is its large atomic number (ZPb=82, ZI=53), from which the detective efficiency is expected to be higher than that of the other detectors being used. Recently, many researchers have published the work on the development of the method for PbI2 thin film preparation from solutions, vapor, melts and gels. Also, the recently published results reported on the influence of rare earth (RE) elements on the quality of materials for radiation detectors [1]. The aim of this work was to deposit PbI2 films of different thickness on glass substrate at ambient temperature by thermal evaporation technique to investigate the morphology and optical absorption of PbI2 thin films. Due to the optical properties of this material, the preliminary results obtained in this approach, could be a way to develop thinner detector with higher efficiency. Experimental details: PbI2 films were grown on glass substrate at ambient temperature by thermal evaporation technique. The high purity (99.999 %) stoichiometric powder was palletized for evaporation as starting material. All the films were grown at a base vacuum of 10-6 Torr [2]. The deposition rate was optimized at 1-2 nm/s to grow uniform good quality films. Thickness of these films was All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 117.199.192.244-01/05/14,11:37:03)

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monitored using the quartz crystal thickness monitor (HINDHIVAC-Quartz Crystal Digital Thickness Monitor Model -101). Density and impedance were fed for PbI2 in the thickness monitor. The high vacuum has been used for preparation of films to remove the friction of air & other gases and to avoid contamination possibilities. Using the similar procedure, films of different thickness: 25nm, 50nm, 100nm, 150nm were deposited. As prepared film were characterized for their structural nature using XRD machine (PHILLIPS X-Pert model- 1830). The morphological studies were carried out using SEM. The absorption spectra, for the calculation of band gap using Tauc relation, was recorded using optical fiber based UV-Vis Spectrophotometer (Ocean Optics HR-4000) at room temperature. Results and discussion: The structural analysis of the films was carried out by X-ray diffraction (XRD). Figure 1 shows XRD patterns for films of different thickness (e.g. 25, 50, 100, 150 nm).The morphology (and also the structure by diffraction mode) of the films were studied by Scanning Electron Microscope (SEM). Figure 2 shows the SEM micrographs of the films. The optical absorption measurements were carried out using a UV-Vis. Further, these absorption spectra were used to calculate band gap of PbI2 films. Figure 3 shows the absorption spectra of films. Figure 4 shows the band gap calculations for films of thickness 25nm and 150nm, as representative case. Small pieces (1X1 cm2) of the same film were used for various analyses. All these measurements and analysis can be considered as very accurate because of the modern computer interfacing and analysis software. All the measurements in the present work were carried out at room temperature (~300 K).

Fig 1: X-ray diffractograms of PbI2 films for different thicknesses (indicated in each pattern).


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25nm

50nm

100nm

150nm

Abs orption Vs Waveleng th

Fig 2: SEM micrographs for PbI2 films of different thickness. 30

25nm

A bs orption

20

50nm 100nm 10

150nm 0 340

440

540 waveleng th

Fig 3: Absorption spectra of films vs wavelength.

640

4


Thickness-25 nm Bandgap-2.5eV 1E+15 9E+14

(αhν)2 (eV/cm)2

(ahn)2(eV/cm)2

8E+14 7E+14 6E+14 5E+14 4E+14 3E+14 2E+14 1E+14 0 0

0.5

1

1.5

2

2.5

3

3.5

4

hn(eV)

hν (eV) Thickness-150 nm Bandgap-2.4eV 2.5E+14

(ahn )2(eV/cm)2

2

(αhν) (eV/cm)

2

2E+14

1.5E+14

1E+14

5E+13

0 0

0.5

1

1.5

2

2.5

3

(αhν)2 (eV/cm)2

hn(eV) hν (eV)

hν (eV)

Fig 4: Band gap calculations for films.

3.5

4


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The XRD patterns (figure 1) clearly show the growth of preferred (00l) orientation (parallel to substrate plane or c-axis perpendicular to substrate plane) indicated by the presence of only (00l) peaks. PbI2 seems to have a very strong affinity for the growth of this preferred orientation. This is further evidenced by the fact that no other peaks are observed in any sample without exception irrespective of the growth/process parameters [7, 8]. The d-spacing of (00l) peaks match quite well with ASTM card No. 07-0235 for 2H polytype of PbI2 which confirms the stoichiometry. Uniform spherical grain size growth is clearly evident from SEM micrographs (figure 2). The optical band gap was determined by the measured absorbance of the film as the function of incident wavelength [3]. The absorbance (A) is related to absorption coefficient (α) for an absorbing medium as; .

(1)

where i0 and i are incident and transmitted intensities respectively and t is film thickness [4]. In a crystalline or polycrystalline material the nature of optical transitions (direct or indirect) near the absorption edge can be determined by the relation between and the optical energy gap Eg [5]. Assuming the bands to be parabolic in nature the absorption coefficient in an allowed direct transition is related to the band gap by; α

.

(2)

Table 1: Values of thickness for different values of PbI2 Thickness of film [nm] 25 50 100 150

Bandgap [eV] 2.5 2.48 2.45 2.4

There are plenty of evidences that PbI2 is a direct band gap material [7, 9]. Keeping this in mind, (αhν) 2 is plotted against hν, as shown in figure 4. Since the film under test are direct band gap type, so, single linear portion is observed, which was extrapolated to x-axis in under to determine the band gap [6]. The calculated band gap is listed in table 1. The observed variation (2.5eV for 25nm thin film to 2.4eV for 150nm film) in band gap of different thickness film can be accounted for the quantum confinement effect. As the confining dimension decreases and reaches a certain limit, typically in nanoscale, the energy spectrum turns to discrete. As a result, the band gap becomes size dependent. Quantum confinement is responsible for the increase of energy difference between energy states and band gap. Conclusions: Lead Iodide is found to have a strong tendency for an oriented (i.e. (00l) planes parallel to substrate plane or c-axis perpendicular to substrate plane) film growth even on glass substrate as indicated by X-ray diffraction analysis. Matching of XRD data of films and the starting powder indicate that the grown films are stoichiometric. The morphological analysis (SEM micrograph figure 2) indicates a progressive growth in grain size with film thickness. Optical absorption analysis indicates that the Lead Iodide is having a direct optical band gap of about 2.45 eV. The variation of band gap with film thickness could be qualitatively understood as due to the quantum confinement effect. The extreme variation of band gap observed in two film thicknesses could be due to other causes like residual stress in the film and needs a better and extended analysis.

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References: [1] S.S. Jamil, A. M. Mousa, M. A. Mohammad, K.M. Thajeel, Structural and Optical Properties of Lead Iodide Thin Films Prepared By Vacuum Evaporation Method, Eng. & Tech. Journal, 29 (2011). [2] D.M. Hata, Introduction to Vacuum technology, Prentice Hall, (2008). [3] S. Bandopadhyay, K.K. Roy, S. Kar, A.K. Lahiri, A.K. Maiti, K. Goswami, Optical and structural properties of lead iodide thin films Prepared by vacuum evaporation method, Crystal Research and Technology, 43 (2008), 959–963. [4] T.L. Alford, l. C. Feldman, J.W. Mayer, Fundamentals of Nanoscale Film Analysis, Springer (2007). [5] J.F. Condeles, R.C. Z. Lofrano, J.M. Rosolen, M. Mulato, Stoichiometry, Surface and Structural Characterization of Lead Iodide thin Films, Braz. J. Phys., 36, 2a São Paulo (2006). [6] S. Brunauer, The Absorption of Gases and Vapors: Physical absorption, Volume 1, Princeton University Press, (1943). [7] D.S. Bhavsar, Structural studies of Vacuum evaporated Lead Iodide Thin films, Pelagia Research Library, Advances in Applied Science Research, 2407-413, (2011). [8] J.F. Condeles, R.A. Ando, M. Mulato: Optical and Structural properties of PbI2 thin films, J Mater Sci, 43 (2008)) 525–529. [9] N. Preda, L. Mihut, I. Baltog, T. Velula, V. Teodorescu, Optical properties of low-dimensional PbI2 particles embedded in polyacrylamide matrix, Journal of Optoelectronics and Advanced Materials, 8 (2006) 909 – 913.

Journal of Nano Research Vol. 24 10.4028/www.scientific.net/JNanoR.24

Growth and Characterization of PbI2 Thin Films by Vacuum Thermal Evaporation 10.4028/www.scientific.net/JNanoR.24.1 DOI References [3] S. Bandopadhyay, K.K. Roy, S. Kar, A.K. Lahiri, A.K. Maiti, K. Goswami, Optical and structural properties of lead iodide thin films Prepared by vacuum evaporation method, Crystal Research and Technology, 43 (2008), 959–963. http://dx.doi.org/10.1002/crat.200811160