Study of the III-nitride based nanostructures

The III-nitrides has wide application in opto-electronic devices, including green/blue
light-emitting diodes (LEDs), laser diodes and solar cells. This thesis addresses the
fundamental issues of these devices from a concern of efficiencies.
InGaN-based LEDs present efficiency losses (droop) at higher injection intensities.
By relieving the quantum-confined-stark effect (QCSE), the nanorod structure
reduces the electron-longitudinal-optical phonon coupling strength by 40% on
average, as measured by the Huang-Rhys Factor. Consequently, a weaker indirect
Auger recombination could potentially curb the droop. Besides, surface plasmon
polariton–exciton coupling leads to 5.5 times enhancement of the internal quantum
efficiency (IQE) and is found to be motivated by the carrier delocalization effect.
Both SP and nanorods contribute to high LED efficiencies.
The whispering gallery mode ring cavities are studied in the Finite Difference Time
Domain (FDTD) approach in order to design high efficiency nitride laser diodes. For
InGaN based rings with a radius of 1000-1500nm, the ideal Q-factor reaches 106 at
the resonance wavelength around 500nm. The mode splitting effect with separation
of bonding and anti-bonding modes is observed as a result of the interference
between rings. A 2-3 times enhancement of the Q-factor can be realized when two
rings is 25-50nm apart. Q-factors reduce when two rings have a deviation in size.
InGaN based quantum dot (QD) solar cell taking advantages of both the
intermediate band solar cell (IBSC) structure and prominent piezoelectric fields in
III-nitrides is theoretically researched. The IB provided by QDs increases the short
circuit current density, and the piezoelectric field enhances the open circuit voltage.
The optimized structure reaches a highest conversion efficiency of 55.4%. The
InGaN QD structures are viable for high performance nitride solar cells.