Interface Phonons in Semiconductor Heterostructures and Phonons in Molybdenum Di-sulphide

The systematic collective vibrations of the constituent atoms of a crystal poessess energy which is quantized – the quantum of energy is referred to as phonons. The presence of phonons in the semiconductor lattice introduces anomaly in the otherwise periodic arrangement of atoms which significantly modifies the electrical, thermal and optical properties of the semiconductors. Phonons in semiconductor give rise to many significant effects such as: (a) It is one of the most dominant scattering mechanism for electron transport (b) Non-vertical electron transitions in Indirect band gap semiconductors (c) Creation and annihilation of excitons. Recent advancements in fabrication of low dimensional nanostructures have found various applications in optoelectronics and novel devices to be used in switches. The dimensional confinement of nanostructutres in one or more spatial dimensions modifies the bulk phonons, for example in quantum well new modes such as : Interface modes, confined modes, propagating modes and half space modes arise. In polar semiconductors the most significant mechanism of electron-phonon interaction is through the frohlich interaction. My work concentrates on Interface modes in wurtzite heterostructure and out-of plane modes in 2D materials (for which I have considered MoS2) In the present thesis I have worked on the following problems: 1. Derivation of analytical expressions for dispersion relation and electron Interaction (frohlich potential) potential for the Interface modes in various two interface wurtzite heterostructure. 2. Derivation of analytical expressions and conditions for existence of Interface modes in two interface ternary alloy heterostructures such as Al(1-x)Ga(x)N and quantum well heterostructure GaN/In(x)Ga(1-x)N/GaN as a function of composition “which is labelled by x”. This work extends the work of komirenko et al (1999) applied to wurtzite material heterostructures. 3. Derivation of analytical expressions for dispersion relation and electron Interaction (frohlich potential) potential for the Interface modes in metal terminated two interface wurtzite heterostructure. This problem investigates the role of metal terminations in nanostructures to reduce scattering of electrons by Interface modes. 4. Inspired by the work of Yu et al (1997), I have Developed a Transfer Matrix Theory for determination of dispersion relation and frohlich potential amplitudes in any heterostructure with any arbitrary number of layers. This theory is then applied to a 4-period superlattice of AlN/GaN layer. 5. Determination of Electron interaction potential due to out-of-plane phonon mode in MoS2.