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Sameer Gupta

Theoretical characterization of point defects in semiconductors: From photovoltaics to quantum engineering applications

Published on 31 March 2023
Thesis presented March 31, 2023

Point defects play an essential role in the technological applications of semiconductors, providing suitable doping conditions. At the same time, defects with carrier trap characteristics can prove to be detrimental to the performance and efficiencies of solid-state devices. Due to their implications on semiconductors, especially electronics and optical properties, defects have been the subject of extensive research for the last several decades.
In this thesis, intentionally added impurities in Cadmium Telluride (CdTe) and their interaction with native defects have been studied using first principle based methods. CdTe has several technological applications, such as solar cells, nuclear radiation detectors, and astronomical spectroscopy. The properties of CdTe have been studied for several decades. However, the growth process related to defects composition, for photovoltaics and other applications is still evolving with a better understanding of native defect properties and the knowledge of doping and alloying strategies.
The two main topics addressed in this thesis are related to CdTe photovoltaics and possible quantum application. The first one deals with the design strategy involving the alloying of polycrystalline CdTe solar cells with Se that has pushed the efficiency of thin film CdTe devices above 22 % In terms of defect physics, experimental studies have elucidated that the Se atom diffuses into CdTe bulk and passivates the intrinsic carrier traps present therein. Finding the Se dopant diffusion mechanism holds significant importance for understanding the depth profile of the dopant and native defect passivation mechanism. We have used the first principle based DFT calculations to identify a unique two-step mechanism to define the Se diffusion in the CdTe bulk. The diffusion process involves the interaction of Se with the Te self interstitial and the promotion of Te interstitial diffusion in the CdTe bulk. The barrier associated with the Se diffusion was calculated to be lower than 0.42 eV, reflecting the fast diffusion of the Se dopant in CdTe. In the next stage, we used the Se diffusion trajectory to understand the interaction of the dopant with the carrier trap native defects, Cd vacancy, and Te antisite, and their passivation on interaction forming bound defect complexes with Se.
Point defects, along with their strong implication for opto-electronic properties of semiconductors, have also proven their suitability for quantum applications such as computing and sensing in recent years. Transition metal doped into semiconductor nanostructures with characteristic localized spin have emerged to be a suitable candidate for these applications, giving rise to an emerging field of solotronics.
In the second part of the thesis, we studied the electronic structure of transition metal Cr and Mn solitary dopant defects in CdTe. We identified the effect of electron-lattice coupling on the local symmetry of the dopants in the lattice and established its correspondence with localized defect electronic wavefunction. Using the DFT calculations of the thermodynamic binding energy associated with dopant and native defects interaction, we then establish that achieving Cr isolated dopant configuration in CdTe nanostructures is challenging as it binds strongly with native defects. In contrast, Mn in suitable growth conditions can be readily obtained in an isolated configuration. We also proposed a phenomenological model to define the interaction of transition metal dopants with native defects. Finally, we explained the experimentally observed change in the spin state of Cr from that of the isolated dopant ground state configuration using the interaction of Cr with a native defect in CdTe.
Together these two studies demonstrate that understanding the underlying mechanism at the atomic scale can be used to define experimentally characterized phenomena at the macroscopic scale and devise growth strategies.

Doping, photonics, defect, spin

On-line thesis.