Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Physics
Specialization
Applied Physics
Department
Physics
First Advisor
Feruz Ganikhanov
Abstract
Wide-bandgap semiconductors, such as Barium Tin Oxide (BaSnO3) and Strontium Titanate (SrTiO3) are important for advanced electronic device applications. These materials offer high electron mobility and a high breakdown voltage, addressing limitations commonly found in traditional silicon-based transistors, such as low forward voltage and instability in harsh conditions. The high breakdown voltage resulting from wide bandgaps allows for operation at higher voltages, which is essential for high-power electronic devices. Additionally, wide-bandgap semiconductors exhibit significantly high electron mobility, a key factor in switching speed. For instance, BaSnO3 demonstrates electron mobility in the range of 100-300 cm²/Vs at room temperature.
In semiconductors, electron mobility is affected by scattering from lattice vibrations (phonons), impurities, and defects. At the thermal equilibrium, electrons undergo random scattering with no overall movement. However, when an electric field is applied, it causes a net electron drift in the opposite direction of the field. This drift velocity is directly proportional to the applied field and the mobility of the electrons. The mobility of electrons is influenced by their charge, effective mass, and mean free path time, predominantly constrained by electron-phonon scattering in pure materials.
Understanding phonon dynamics is critical for high-frequency applications, as electron-phonon interactions impact carrier mobility. High-energy phonons decay into lower-energy phonons and can interact with electrons and impurities. The phonon damping rate (Γ), related to the relaxation time (T2), affects the dielectric function and can be measured precisely using time-domain coherent anti-Stokes Raman scattering (td-CARS), which provides precise time resolution and sensitivity.
In doped semiconductors, additional plasmon-phonon interactions occur due to collective charge oscillations, forming hybrid modes. These interactions depend on charge carrier concentration, impacting both phonon and plasmon properties. The frequency of these coupled modes is determined by the electron density, electronic charge, dielectric constant, and effective mass.
The total phonon dephasing time (T2) is determined by amplitude decay time (T1) and phase decay time (Tφ). In low-doped semiconductors, T2 equals T1, while in highly-doped semiconductors, T2 equals Tφ, highlighting the dephasing contributions. Using td-CARS, we can accurately measure phonon and hybrid mode decay dynamics in wide-bandgap semiconductors, providing direct insights into phonon relaxation times, Raman linewidths, and phonon interactions. These measurements are crucial for both fundamental physics and practical microelectronic device applications.
In the first manuscript, Time-domain coherent Raman techniques have been utilized to selectively measure ultrafast decay rates of optical phonons in cubic BaSnO3 perovskite. Measurements were made within a 350-1300 cm-1 frequency range with time and equivalent spectral resolution of 120 fs and less than 0.1 cm-1, respectively. The phonon mode damping rates are found to be within 1.27-1.59 ps-1 at room temperature, indicating that the homogeneously broadened Raman linewidths are within 6.7-8.4 cm-1. Phonon decay mechanisms are being discussed within the framework of parametric phonon interactions due to lattice anharmonicity. Characteristics of the Raman active modes are essential in understanding the limiting factors for achieving high carrier mobility in device applications of the material.
In the second manuscript, the Decay of multiple Raman active vibrations has been directly traced in time in technologically important wide bandgap semiconduction oxides such as BaSnO3 (BSO), STiO (STO), and in KTi-OPO4 (KTP) crystal that have important applications in laser frequency conversion. Time-domain coherent Raman technique with excellent time (~120 fs) and spectral resolutions have been applied to measure ultrafast decay rates of optical phonons with 350-1500 cm-1 frequencies. Phonon decay mechanisms via phonon energy loss due to second- and third-order parametric processes have been discussed. The correspondingly high equivalent spectral resolution allowed to determine phonon line bandwidths to be within 7.2-8.3 cm-1 (BSO), 8.5-9.7 cm-1 (STO), and 6.2-18.6 cm-1 (KTP).
In the third manuscript, ultrafast decay of optical phonons has been studied in wide-bandgap BaSnO3 and SrTiO3 perovskites using nonlinear spectroscopy with 120 femtosecond time resolution. The coherent Raman mode excitations have been selected and traced with tunable optical pulses. Decay of symmetry forbidden modes of vibrations have been detected directly in time. Phonon decay rates for the main LO- and TO- phonon modes have been found to be within 1.36-1.78 ps-1 and are explained in terms of parametric phonon interactions and pure dephasing mechanism in the materials that are of interest in microelectronic applications.
Recommended Citation
Neupane, Chandra Prasad, "ULTRAFAST DYNAMICS OF INTRINSIC AND HYBRID PHONON MODES
IN WIDE-BANDGAP SEMICONDUCTORS" (2024). Open Access Dissertations. Paper 1674.
https://digitalcommons.uri.edu/oa_diss/1674