Ultrafast Carrier Relaxation Study in Indium Nitride
Date Issued
2010
Date
2010
Author(s)
Su, Yi-En
Abstract
Indium nitride (InN) has been considered important recently because of its small direct band gap (~0.6 electron volt (eV)), comparing to Gallium Nitride (GaN) with a direct band gap of ~3.4eV. With different fraction of indium and gallium, GaxIn1-xN was expected to have emitting spectral wavelength ranged from 370nm to 1700nm, covering the region of visible light (about 400nm ~ 700nm) and telecommunication wavelength. InN was regarded as a great potential material applied in light-emitting diode (LED) and solar cells. Therefore, the carrier thermalization dynamics and energy relaxation mechanism in InN is highly important.
Recent investigations of InN carrier dynamics found that InN had many other special properties, very light electron effective mass (0.042m0), relatively light hole effective mass (0.42m0), nonparabolic conduction band, high electron accumulation on surface, high electron mobility, and high electron saturation drift velocity. These special properties make InN not only prospective in the optoelectrical field, but also a potential material for high reactive rate electrical devices. The ultrahigh longitudinal optical phonon (LO-phonon) energy of InN (~73meV) makes it expected to have an ultrafast electron energy relaxation time below 100 femtoseconds (fs). However, experimental results were not consistent with theoretical expectation. All the paper published before 2010 reported an electron energy relaxation time longer than 400fs. Papers published earlier even reported an energy relaxation time near 10 picoseconds (ps).
There are two explanations for the postponed electron energy relaxation time, one is the hot phonon effect, and one is the screening effect between each electron and LO-phonons induced by other electrons. Earlier papers attributed the reason to the hot phonon effect. Until 2006, Wen et al reported some experimental results against the story, and attributed the postponed relaxation time to the screening effect. Since then, the argument between the reasons of relatively slow energy relaxation time has never ended. This thesis will prove that the main reason of the postponed electron energy relaxation time shown in the published papers is the screening effect. This thesis also show that as the screening effect and the hot phonon effect being removed by lowering the electron density, a sub-100 fs electron energy relaxation time can be directly observed, consistent with the theoretical expectation.
This thesis also report an investigation on hole thermalization dynamics at the edge of the valence band, including holes heating time with both theoretical expectation and experimental result. I hope that this thesis would make significant help in understanding InN carrier dynamics. While InN is applied in the electrical or optoelectrical field in the future, the investigation and understanding of InN carrier mechanism in this thesis could help to predict or to solve some carrier property problems, and further more to make breakthrough in the fields of optoelectronics and solid state electronics.
Subjects
InN
carrier dynamics
electron cooling
hole heating
Mg doped InN
Type
thesis
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