Carrier relaxation dynamics in defect states of epitaxial GaN/AlN/Si using ultrafast transient absorption spectroscopy

The relaxation dynamics of the carriers through the defect levels in an epitaxial GaN film grown with an AlN buffer layer on Si has been performed on the femto-picosecond timescale, using ultrafast transient absorption spectroscopy (UFTS). The sample was pumped above and below the band gap and probed with a white light continuum (480-800 nm). A combination of bi and triple exponential decay functions at different probe wavelengths were used to fit the kinetic profile of the carriers in the defect continuum. Based on the UFTS measurements, a model is proposed which explains the dynamics in the shallow traps and deep level defects. Furthermore, to determine the role of the lattice in the relaxation dynamics, the experiment was conducted at a low lattice temperature of 4.2 K. The relaxation constants from the UFTS measurements confirm not only the presence of shallow and deep level defects but also the involvement of phonons in one of the relaxation processes

Geometry based local transport phenomena in classical and quantum regimes

In this thesis, we study geometry-based non-equilibrium steady-state transport phenomena theoretically with the overarching goal to understand how the multi-path geometry can affect transport in classical and quantum systems. We begin with an overview of the physics associated with classical and quantum transport and the formalisms used to obtain results. In the non-equilibrium steady-state, one would expect that the local gradient imposed by the reservoirs would define a unique direction of flow from high-to-low. However, through this thesis we show that may not always be the case as one can devise a local steady-state atypical flow which goes from (low-to-high) by using a system with multi-path geometry. We address the universality of these steady-state local atypical flows in systems with multiple paths, through the following undertakings:We show a classical harmonic system of Hookean springs and point masses coupled in a multi-path geometry driven by two Langevin reservoirs at different temperatures can give rise to a steady-state local atypical thermal flow. Through molecular dynamics simulations of Langevin equations for this system, we show that the atypical current depends on both internal and external parameters such as ratio of spring constants, ratio of masses and system-reservoir coupling respectively. We also show the robust nature of this atypical current against substrate induced non-linearity and asymmetric system-reservoir coupling. Two different approaches, namely the Redfield and Lindblad master equation, are used to extract the non-equilibrium steady-state thermal transport of a quantum system of oscillators coupled in a triangular geometry described in the coordinate-momentum space and as a Bose-Hubbard Hamiltonian respectively. Through the third quantization formalism and numerical simulation of the quantum master equations we show that atypical flows are universal to multi-path geometry and arise in both descriptions. We show that these atypical flows give rise to two patterns of internal steady-state circulations, clockwise and counterclockwise. We map out phase diagrams for these flow patterns as a function of system parameters thereby showing its robust nature. Finally, we show that these atypical flows and internal steady-state circulations are not limited to thermal transport but can be achieved for particle transport as well. We phenomenologically describe a hybrid system comprising of photonic structures and electronic quantum dots and show that the triangular geometry of this system can give rise to steady-state photonic circulations. We show the robust nature of these circulations against photon blockade and interactions through numerically calculated phase maps with the ratio of tunneling coefficients and system-reservoir coupling as the parameters. At the end, we elaborate on the applications of these geometry-based steady-state atypical flows and outline possible experimental realizations to observe these atypical flows and circulations

Geometry based local transport phenomena in classical and quantum regimes

In this thesis, we study geometry-based non-equilibrium steady-state transport phenomena theoretically with the overarching goal to understand how the multi-path geometry can affect transport in classical and quantum systems. We begin with an overview of the physics associated with classical and quantum transport and the formalisms used to obtain results. In the non-equilibrium steady-state, one would expect that the local gradient imposed by the reservoirs would define a unique direction of flow from high-to-low. However, through this thesis we show that may not always be the case as one can devise a local steady-state atypical flow which goes from (low-to-high) by using a system with multi-path geometry. We address the universality of these steady-state local atypical flows in systems with multiple paths, through the following undertakings:We show a classical harmonic system of Hookean springs and point masses coupled in a multi-path geometry driven by two Langevin reservoirs at different temperatures can give rise to a steady-state local atypical thermal flow. Through molecular dynamics simulations of Langevin equations for this system, we show that the atypical current depends on both internal and external parameters such as ratio of spring constants, ratio of masses and system-reservoir coupling respectively. We also show the robust nature of this atypical current against substrate induced non-linearity and asymmetric system-reservoir coupling. Two different approaches, namely the Redfield and Lindblad master equation, are used to extract the non-equilibrium steady-state thermal transport of a quantum system of oscillators coupled in a triangular geometry described in the coordinate-momentum space and as a Bose-Hubbard Hamiltonian respectively. Through the third quantization formalism and numerical simulation of the quantum master equations we show that atypical flows are universal to multi-path geometry and arise in both descriptions. We show that these atypical flows give rise to two patterns of internal steady-state circulations, clockwise and counterclockwise. We map out phase diagrams for these flow patterns as a function of system parameters thereby showing its robust nature. Finally, we show that these atypical flows and internal steady-state circulations are not limited to thermal transport but can be achieved for particle transport as well. We phenomenologically describe a hybrid system comprising of photonic structures and electronic quantum dots and show that the triangular geometry of this system can give rise to steady-state photonic circulations. We show the robust nature of these circulations against photon blockade and interactions through numerically calculated phase maps with the ratio of tunneling coefficients and system-reservoir coupling as the parameters. At the end, we elaborate on the applications of these geometry-based steady-state atypical flows and outline possible experimental realizations to observe these atypical flows and circulations

Probing the correlation between structure, carrier dynamics and defect states of epitaxial GaN film on (11(2)over-bar0) sapphire grown by rf-molecular beam epitaxy

A systematic study has been performed to correlate structural, optical and electrical properties with defect states in the GaN films grown on a-plane (11 (2) over bar0) sapphire substrate via rf-plasma molecular beam epitaxy. Morphological analysis reveals the presence of small lateral size (30-70 nm) hexagonally shaped V-pits on the GaN films. These V-defects possibly contribute as the main source of non-radiative decay. High resolution X-ray diffraction reveals highly single crystalline GaN film grown on a-plane sapphire substrate where the threading dislocations are the cause of V-defects in the film. Photoluminescence measurement shows a highly luminescence band to band emission of GaN film at 3.41 eV along with a broad defect band emission centered at 2.2 eV. A detailed optical and electrical analysis has been carried out to study the defect states and related carrier dynamics for determining the efficacy of the film for device fabrication. The variation in the low temperature current voltage measurements confirms the presence of deep level defects in the mid-band gap region while transient spectroscopy shows that non radiative decay is the dominant relaxation mechanism for the photo excited-carriers from these defect states