THESIS
2020
xix, 113 pages : illustrations ; 30 cm
Abstract
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
1×10
10 cm
-2 and range from 60–75 nm, respectively. Time-resolved photoluminescence studies reveal a mono-expon...[
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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
1×10
10 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 1.1×10
19 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/cm
2. 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/cm
2 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/cm
2 to below100 W/cm
2 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.
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