Band-Gap Engineering in ZnO Thin Films: A Combined Experimental and Theoretical Study

Vani Pawar, Pardeep K. Jha, S. K. Panda, Priyanka A. Jha, and Prabhakar Singh

Phys. Rev. Applied 9, 054001 – Published 2 May 2018

Abstract

Zinc oxide thin films are synthesized and characterized using x-ray diffraction, field-emission scanning electron microscopy, atomic force microscopy, and optical spectroscopy. Our results reveal that the structural, morphological, and optical properties are closely related to the stress of the sample provided that the texture of the film remains the same. The anomalous results are obtained once the texture is altered to a different orientation. We support this experimental observation by carrying out first-principles hybrid functional calculations for two different orientations of the sample and show that the effect of quantum confinement is much stronger for the (100) surface than the (001) surface of ZnO. Furthermore, our calculations provide a route to enhance the band gap of ZnO by more than 50% compared to the bulk band gap, opening up possibilities for wide-range industrial applications.

Received 5 October 2017
Revised 27 February 2018

DOI:https://doi.org/10.1103/PhysRevApplied.9.054001

Physics Subject Headings (PhySH)

Band gap Confinement Optoelectronics Photovoltaic absorbers

II-VI semiconductors Thin films Transparent conducting oxides

Sol-gel process

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Vani Pawar¹, Pardeep K. Jha^1,*, S. K. Panda², Priyanka A. Jha¹, and Prabhakar Singh¹

¹Department of Physics, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, India
²Centre de Physique Théorique, Ecole Polytechnique, CNRS UMR 7644, Université Paris–Saclay, 91128 Palaiseau, France

^*pardeepjha.jiit@gmail.com, pardeepk.pf.phy17@iitbhu.ac.in

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Vol. 9, Iss. 5 — May 2018

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Images

Figure 1
(a) A schematic diagram of the layer deposition of ZnO thin films by a spin-coating unit, abbreviated as $Z_{N}$ ( $N = 1$ , 2, 3, and 4, where $N$ indicates the number of ZnO layers). $Z_{0}$ represents the glass substrate. (b) FESEM and (c) AFM micrographs. (d) The x-ray diffraction pattern of the thin films. (e) The normalized optical absorption spectrum. (Inset) Images of the thin films when the are exposed to daylight, UV light with a shorter wavelength (UV- $C$ ) in the range of 100–280 nm, and UV light with a longer wavelength (UV- $A$ ) in the range of 315–400 nm.
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Figure 2
(a) XRD pattern of thin films ( $Z_{1} - Z_{4}$ ) including a glass substrate ( $Z_{0}$ ). The presented data are smoothed for $10 ° \leq 2 θ \leq 30 °$ to show the broadness and the peak of an amorphous glass substrate, while they are unprocessed for $2 θ > 30 °$ , revealing the crystalline nature of ZnO. (b) The magnified FESEM image and the corresponding surface plots of thin films $Z_{1}$ and $Z_{4}$ , revealing the difference between the very crystalline fiber (case 1) and the partial amorphous bedding in the background (case 2). (c) A comparative XRD pattern of $Z_{1}$ and $Z_{4}$ (with $Z_{0}$ subtracted) showing that the presence of amorphicity in thin films reduces upon an increase in layers. (Inset) The XRD background of the $Z_{4}$ sample for two different glancing angles, $α = 0.1 0 °$ and 0.15°.
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Figure 3
Atomic percentage of Zn, O, and glass (Si) on (a) the nanofiber (case 1; the SEM image is shown in the inset) and on (b) the background (case 2; the SEM image is shown in the inset) as obtained from energy-dispersive x-ray analysis measurements. (c) The average fiber widths, (d) tetragonality ( $c / a$ ), (e) the in-plane stress and texture along the $c$ axis ( $I_{002} / I_{100}$ ) of the four samples. The inset of (c) shows the SEM image of the fiber width for case 1. The respective error bars are shown in red.
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Figure 4
(a) Variation of $(α h ν)^{2}$ as a function of the photon energy. (b) Urbach energy vs the band-gap plot. (c) The fiber width and (d) the band gap as a function of the in-plane stress along the $c$ axis. The error bars are shown in red.
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Figure 5
Total and partial density of states for (a) bulk ZnO and its (001) polylayers of three different thicknesses ( $w$ ): (b) 18.273 Å, (c) 13.052 Å, and (d) 7.831 Å. Similarly, (e) bulk ZnO and (100) polylayers of three different thicknesses, (f) 20.666 Å, (g) 12.245 Å, (h) 6.594 Å, are shown in the right-hand panels. For the partial density of states, only the projections of the top surface atoms are shown. The vertical dashed lines indicate the Fermi level.
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Figure 6
The calculated band gap as a function of the thickness of the sample for the (001) and (100) surfaces.
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