ABSTRACT
AlSb is a material that exists in 3D as well as 2D regimes. also have high electron mobility is useful for application in high-speed electronic devices.
AlSb has potential applications in radiation detection. Defects, including intrinsic and extrinsic have been shown to influence the performance of AlSb for applications in electronic and .
In this thesis, density functional with the aid of the generalised gradient approximation was used to model the stability as well as the formation of Al and Sb vacancies, Li and Be substitutions in 2D hexagonal AlSb.
The structural and electronic properties of the aforementioned defects in AlSb were reported. Under conditions, the aluminium vacancy (VAl) is energetically more favourable than the antimony vacancy (VSb).
While the Be is more stable at the Al atomic site, the Li substitution is more stable at the Sb atomic site. The defects investigated modulated the bandgap of the AlSb.
Whereas the p orbital of the Sb atom contributed to the dominant states in the bandgap of the host for all the defects, the p orbital of Al contributed immensely to the defects states.
The results further show that using the generalised gradient approximation predicts defective AlSb as well as the pristine AlSb to be metallic. This has paved the way for further investigation using more accurate exchange-correlation approximations.
TABLE OF CONTENTS
Abstract i
Acknowledgements ii
Dedication iii
Table of Contents vi
List of Abbreviation vii
List of Figures viii
List of Tables ix
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation and problem statement . . . . . . . . . . . . . . . . . . . . . 2
1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.4 Overview and synopsis . . . . . . . . 3
2 Electronic structure methods and calculations 4
2.1 Variational principle . . . . . . . . . . 4
2.2 Many-body Hamiltonian . . . . . . . . . 4
2.3 Born-Oppenheimer approximation . . . . . 6
2.4 Hartree approximation . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1 Hartree-Fock approximation . . . . . . . . . . . . . . . . . . . . 9
2.5 Density functional theory . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5.1 Hohenberg-Kohn formalism . . . . . . . . . . . . . . . . . . . . 13
2.5.2 The Kohn-Sham equations . . . . . . . 14
2.6 Exchange-correlation functionals . . . . .. . . . 16
2.6.1 The local density approximation . . . . . . . . . . . . . . . . . . 17
2.6.2 The generalized gradient approximation . . . . . . . . . . . . . . 17
2.6.3 Hybrid functionals . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.7 Pseudopotential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.7.1 Norm-conserving pseudopotentials . . . . . . . . . . . . . . . . 20
2.7.2 Ultra-soft pseudopotentials . . . . . . . . . . . . . . . . . . . . . 20
2.7.3 The projector-augmented wave method . . . . . 21
2.8 Basis set . . . . . . . . . . . . . . . . . . 21
2.9 Brillouin zone . . . . . . . . 21
3 Literature review 23
3.1 Defects in material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.1 Surface defects . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.2 Line defects (dislocations) . . . . . . . . . . . . . . . . . . . . . 24
3.1.3 Point defect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Defects in AlSb and other alloys . . . . . . . 26
4 Methodology 30
4.1 Computational details . . . . . . . . . . 30
4.2 Test of supercell size . . . . . . . . . 30
4.3 Test of cut-off energy . . . . .. . . . . 31
4.4 Test of k-points . . . . . . . . . . 32
4.5 Calculation details . . . . . . . . . . . 33
5 Results 34
5.1 Structural properties . . . . . 34
5.2 Stability and energetics of defects . . . . . . . . . . . . . 36
5.3 Electronic properties . . . . . . . .. . . . 37
6 Conclusions 39
6.1 Summary . . . . . . . . . . . . 39
6.2 Recommendation . . . . . . . 39
Bibliography 41
INTRODUCTION
This chapter presents a brief introduction of the topic of interest – substitutional and vacancy defects in two-dimensional hexagonal AlSb. The motivation of the study, the objectives and the synopsis of the thesis are presented in this chapter.
1.1 Background
Properties of high-pressure materials including AlSb, AlSi, CdTe, C-BN, C-BC2N, TiB2, SiC, TiN, amongst others ranges from superconducting behaviour to super hard properties,
high density, highly incompressible, high thermal conductivity, non-linear optical properties (frequency-doubling) and unusual electronic and magnetic properties.
These unique properties can be utilized for applications in semiconductor diodes and lasers [1], energy storage materials [2], high-temperature electronics and optoelectronics applications [3].
In recent years, two-dimension (2D) materials such as graphene, phosphorene, transition metal dichalcogenides, boron nitride (BN) in their monolayer regime,
have the potentials to enhance existing technologies and also create a range of new applications for future optoelectronic nano-devices, because of the extraordinary electrical, mechanical and optical properties they possess [4].
2D AlSb is of considerable research and technical interest because of its many features like monolayer materials. It has a direct and indirect energy gap of 1.62 eV and 1.31 eV respectively coupled with a unique band alignment [5].
In the past, AlSb has been utilized in optoelectronic devices [6], radiation detection [7], application for lithium-ion and sodium-ion storage [8], and as a buffer layer for the epitaxial growth of GaSb [9].
BIBLIOGRAPHY
Drain, Laser ultrasonics techniques and applications. Routledge, 2019.
Klein, A. Altman, R. Saballos, J. Walsh, A. Tamerius, Y. Meng, D. Puggioni, Jacobsen, J. Rondinelli, and D. Freedman, “High-pressure synthesis of the BiVO3 perovskite,” Physical Review Materials, vol. 3, no. 6, p. 064411, 2019.
Cheng, C. Wang, X. Zou, and L. Liao, “Recent advances in optoelectronic de- vices based on 2D materials and their heterostructures,” Advanced Optical Materi- als, vol. 7, no. 1, p. 1800441, 2019.
Long, P. Wang, H. Fang, and W. Hu, “Progress, challenges, and opportunities for 2D material based photodetectors,” Advanced Functional Materials, vol. 29, no. 19, p. 1803807, 2019.
Singh, S. K. Gupta, and Y. Sonvane, “Structural and opto-electronic properties of 2D AlSb monolayer,” in AIP Conference Proceedings, vol. 1731, p. 120018, AIP Publishing, 2016.
Cheewajaroen, P. Saengkaew, S. Sanorpim, V. Yordsri, C. Thanachayanont, Nuntawong, and W. Rathanasakulthong, “Characterization of N-type and P-type aluminum antimonides on Si substrates for room-temperature optoelectronic de- vices,” Materials Science in Semiconductor Processing, vol. 88, pp. 224–233, 2018.
StudentsandScholarship Team.
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