Electronic and optical properties of III-Nitride nanostructures

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Patra, Saroj K.
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University College Cork
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Quantum dots (QDs) based on gallium nitride (GaN), indium nitride (InN), aluminium nitride (AlN) and their respective alloys (e.g. InGaN, AlGaN) have attracted significant interest for “non-classical” light emitters such as single-photon or entangled-photon sources. This originates from the fact that these emitters form the cornerstone for quantum cryptography and quantum computing applications. Thanks to their large band offsets and exciton binding energies, when compared to “standard” III-V based systems (indium gallium arsenide), nitride QDs are attractive to realize non-classical light emission near room temperature. By utilizing InGaN QDs, in principle the emission wavelength regime of these emitters can be tailored; an important feature given that commercial single-photon detectors operate in the visible spectral range. However, a major drawback of conventional nitride-based QD systems originates from the "standard" growth along the polar c-axis of the underlying wurtzite crystal lattice, which results in very strong electrostatic built-in fields. These fields significantly affect the radiative recombination rate of these systems, consequently limiting their efficiency. In this thesis, in a first step, we have targeted the electronic and optical properties of InGaN QD structures grown along a so-called non-polar crystallographic direction. Such an approach allows to keep the benefits of the nitride system, e.g. large band offsets, but at the same time offering distinct new features such as significantly reduced electrostatic built-in fields. We have shown, in conjunction with experiment, that these non-polar InGaN QDs indeed exhibit much faster radiative carrier recombination when compared to a c-plane counterpart. Furthermore, we found here that fundamental changes in the underlying electronic structure, when compared to c-plane systems, lead to strongly linearly polarized light emission with a deterministic axis, from nonpolar InGaN QDs. This feature is of great interest for quantum cryptography applications. Additionally, we were able to show that this high degree of optical linear polarization survived up to temperatures of up to 200K and even beyond. Therefore, on-chip operating conditions are within reach. Our theoretical predictions are in excellent agreement with measurements carried out by our colleagues at the University of Oxford (UK) on structures grown at the University of Cambridge (UK). In addition to single-photon emission properties, we also studied the potential of InGaN based QDs for entangled-photon emission. Here, we demonstrated that InGaN QDs are in principle ideal candidates for such applications. Our theoretical studies, combining fully atomistic electronic structure calculations with many-body theory, show that while intrinsic properties of InGaN alloys forming the QD on the one hand side give significant advantages over conventional indium gallium arsenide emitters, these features, on the other hand, could present a major roadblock for polarization entanglement. Overall, in this work, we have shown that these nanostructures are promising systems for achieving next-generation non-classical light emitters required for quantum cryptography applications.
Quantum dots , III-Nitrides , Single photon , Entangled photon , Nonpolar InGaN , k.p theory
Patra, S. K. 2019. Electronic and optical properties of III-Nitride nanostructures. PhD Thesis, University College Cork.