Group III-nitride based quantum dots (QDs) have attracted considerable attention for optoelectronic device applications due to the strong quantum confinement. However, the uniformity of QDs growth in self-assembled method is difficult to control. It depends critically on the growth conditions. The inhomogeneous size distribution and varying indium composition in each QD can deteriorate the performance of QD devices.

In this thesis, an investigation of self-assembled polar InGaN QDs on c-plane sapphire and Si substrates by metal-organic chemical vapor deposition (MOCVD) is reported. The surface density and diameter of the typical truncated pyramidal QDs are approximately 1×10¹⁰ cm^-2 and range from 60–75 nm, respectively. Time-resolved photoluminescence studies reveal a mono-expon...[ Read more ]

Group III-nitride based quantum dots (QDs) have attracted considerable attention for optoelectronic device applications due to the strong quantum confinement. However, the uniformity of QDs growth in self-assembled method is difficult to control. It depends critically on the growth conditions. The inhomogeneous size distribution and varying indium composition in each QD can deteriorate the performance of QD devices.

In this thesis, an investigation of self-assembled polar InGaN QDs on c-plane sapphire and Si substrates by metal-organic chemical vapor deposition (MOCVD) is reported. The surface density and diameter of the typical truncated pyramidal QDs are approximately 1×10¹⁰ cm^-2 and range from 60–75 nm, respectively. Time-resolved photoluminescence studies reveal a mono-exponential exciton decay with a radiative exciton lifetime of 480 ps for uncapped QDs. With an optimized GaN capping layer grown by a two-step method, a radiative exciton lifetime of 707 ps for the capped QDs is obtained. This short radiative exciton lifetime is much shorter than that for previously studied polar QDs and is even comparable with those grown along non-polar QDs, which is strong evidence of the reduction of built-in fields in these polar InGaN QDs.

To improve the optical properties of InGaN QDs, novel vertically aligned and uniformly separated InGaN nanorods (NRs) underneath an InGaN QD active layer was achieved by selective lateral photoelectrochemical (PEC) etching. The mechanism of this PEC etching is examined in detail by characterizing it under different InGaN doping concentrations, potassium hydroxide (KOH) concentrations and etching times. A lateral etch rate of 80 nm/min was achieved for an n-type doping concentration of 1.1×10¹⁹ cm^-3 and a KOH concentration of 2.2 M. Raman measurements exhibit that around 210 MPa compressive residual strain is released for a GaN-based microdisk after 30 min PEC etching. In addition, the PL intensity of the InGaN QDs is enhanced by as much as 6 times after 30 min PEC etching. The magnitude of this increase is related to the increased light extraction efficiency due to additional light scattering from the InGaN NRs.

GaN-based light-emitting diodes (LEDs) are important to lighting and display applications. In this thesis, we demonstrate green-emission (512 nm) InGaN QD LEDs grown on a c-plane sapphire substrate by MOCVD. The screening of the built-in fields in the QDs effectively improves the performance of QD LEDs. These high quantum efficiency and high temperature stability green QD LEDs are able to operate with negligible efficiency droop and with current density increased up to 106 A/cm². Our results show that InGaN QDs may be a viable option to replace conventional quantum well devices.

Furthermore, green InGaN QD microdisk lasers pivoted on a Si substrate with the lowest measured threshold at a record low of 76 W/cm² at room temperature under continuous-wave optical pumping were studied. Vertically correlated stacking of well-separated InGaN QDs in a GaN matrix as the micro-cavity gain medium is achieved by MOCVD. A comparison of ~1μm microdisk lasers with different sidewall roughnesses is demonstrated. The lasing threshold is improved from above 1 kW/cm² to below100 W/cm² by an improvement of the sidewall smoothness. Moreover, the cavity quality factor (Q-factor) is increased by three times. Our results provide new insights into QDs as the laser gain medium to obtain an ultra-low threshold.