Effect of Al excess on the band structure of ZnO using Density Functional Theory

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Effect of Al excess on the band structure of ZnO using Density Functional Theory

Lim Joon Hoong

^a

, Yeoh Choew Keat

^b,*

, Abdullah Chik

^c

and Teh Pei Leng

^d

School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis.

ajhlim07@gmail.com, ^b,*ckyeoh@unimap.edu.my, ^cabdullahchik@unimap.edu.my,

dplteh@unimap.edu.my

Keywords: Density Functional Theory, Modelling, Al-doped ZnO, Band gap.

Abstract. The effect of Al doping to the band structure of ZnO was studied in this paper. The electronic band structure of Al doped ZnO was determined by using first-principles based on density functional theory. ABINIT was used to perform the band structure calculation. The calculated band structure of ZnO and Al doped ZnO shows that ZnO is a direct band gap semiconductor. The band structure become narrow with Al doping compared pure ZnO. With Al doping, the band gap of ZnO (0.749 eV) become smaller as the concentration Al doping increased to 4wt% (0.551 eV). The electrical conductivity of Al doped ZnO was studied as a references value for the band gap. The electrical conductivity of ZnO (8.21 S/cm) was enhanced with Al doping increased to 4wt% (71.87 S/cm).

Introduction

Zinc oxide (ZnO) is a degenerate semiconductor [1] with a direct band gap of 3.22eV at room temperature, large exciton binding energy (60 meV) and optical transparency [2,3]. Zinc oxide has widely uses in semiconductors, solar cells, varistors and paints [4,5]. Recently it was shown that transition metal oxides can exhibit outstanding thermoelectric properties [6]. The main drawback of ZnO as thermoelectric material is low electrical conductivity [7]. The advantage of using ZnO is the opportunities for control of carriers by doping. The addition of dopants is an alternative route to enhance the thermoelectric performance of existing materials by improving electrical conductivity [8]. The energy band gap of a material is related to the electrical properties.

In recent years, there are some reports about the ZnO by Al doping [9]. However, most of the reports are belong to experimental study. This present work will study the effects of Al doping on the electrical conductivity of ZnO and band structure of ZnO using density functional theory (DFT).

Based on the Hohenberg-Kohn theorem, the DFT theory consist two theorems [10]. The first theorem states that the potential Vext(r) is determined uniquely for any system of interacting in an external potential Vext(r). The second theorem states that the energy E[n] for a universal functional can be defined in terms of density n(r). The minimum value of this functional is the exact ground state density n0(r). The electron density is the central quantity in DFT [11]. The electron density with a system of n electron is defined from wave functions as show in Equation 1.

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The energy is rewritten to a functional that depends on the electron density. This reduces the complex wave functions that finding all the ground state physical properties of a system. The energy functional as shown in Equation 2 is finding the minimum energy regarding to the electron density [12]. Where, EHK[n] is the total energy functional, is the kinetic energy and is the electronic interaction.

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Materials Science Forum Submitted: 2016-01-14

ISSN: 1662-9752, Vol. 857, pp 106-110 Accepted: 2016-01-25

doi:10.4028/www.scientific.net/MSF.857.106 Online: 2016-05-20

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