| Summary: | In last two decades, InGaN/GaN light-emitting diodes have been one of the main focus of
research thanks to their low power consumption, high efficiency, long lifetime, high color
purity and color quality, narrow luminescence, possibility to tune the emission
wavelength from near ultraviolet to green by increasing the In content, and several other
promising properties. In early stages of the development of InGaN-based LEDs, growing
high quality epitaxial films on a suitable substrate was the main issue. This issue was
effectively bypassed by growing the main device layers on the lattice-matched buffer
layer. Another big issue was the difficulty to achieve high p-type conductivity in GaN; the
problem was solved by high temperature annealing and low energy electron beam
irradiation methods.
In order to achieve a well operating InGaN/GaN light-emitting diode, both optical and
electrical properties should be in the desired level. The main performance measure of
these devices is the external quantum efficiency. To increase the external quantum
efficiency, the issues with the carrier injection, radiative recombination, light-extraction,
ohmic contacts, and several other factors that limit the overall performance of the device
need to be properly addressed. Although InGaN-based light emitting diodes have been
strongly developed, the performance of the devices still needs to be increased by novel
methods. Moreover, the devices need to be properly characterized in both wafer-level and
chip-level states to deeply investigate the drawbacks and advantages of each element
during the growth and fabrication. Furthermore, the applications of light emitting diodes
needs to be extensively investigated which can be realized through making hybrid systems and combining the advantages of these light emitting devises and novel materials.
Although, there are some studies on this kind of hybrid applications, there is still a long
way to go to make use of InGaN/GaN light emitting diode structures in pumping novel
materials and devices.
In this thesis, we systematically investigate the design, growth, wafer-level
characterization, device fabrication, and device-level characterization of InGaN/GaN
light-emitting diode structures grown on polar sapphire substrates. We demonstrate
optimized growth process and wafer-level characterization of high crystal quality
InGaN/GaN device. The fabrication and device-level characterization of improved
conventional, flip-chip and vertical light-emitting diodes are demonstrated following the
growth of the epitaxial layers. Next, we investigate the advantages and drawbacks of
Ni/Ag/Ni/Au and Ni/Ag/Ti/Au ohmic reflectors and compare the devices fabricated with
both types of the contact-mirrors. The proposed device outperforms the reference device
in terms of electrical (lower forward voltage) and optical properties (higher reflectance).
Moreover, the critical role of the incorporation of sputtered TiW was studied and it was
concluded that the TiW-incorporated light-emitting diodes exhibit higher light extraction,
higher optical power, and higher external quantum efficiency compared with those
without TiW. The enhancement in the device performance was mainly attributed to the
robustness of the device against high temperature annealing. Electroluminescence
measurements further confirmed that TiW-incorporated light emitting diodes possess
better heat management. We also investigated the critical role of InGaN epitaxial thin
layer on the electrical performance of the device. By testing the layers with several
thicknesses and In compositions, we came to the conclusion that a 2 nm thick InGaN layer with high In composition can enhance the current-voltage characteristics of the
device by creating a 2-dimensional hole gas in the interface. The generation of holes in
the interface is attributed to band bending induced by the piezoelectric polarization owing
to the lattice mismatch.
We investigated the effect of grading the InGaN quantum wells along the growth
directions and compared the carrier distribution, radiative recombination rate, optical
power, and external quantum efficiency with those of the conventional structure device.
Moreover, we performed extensive study on the thickness-dependent performance
enhancement of the quantum wells and concluded that the device with 6.5 nm thick
graded quantum wells outperforms the conventional device with 2.5 nm thick quantum
wells. Furthermore, studies also showed that having only three graded quantum wells
with 6.5 nm thickness in the active region is more effective than having eight non-graded
2.5 nm thick quantum wells.
Quantum confined stark effect is a well-known phenomenon which strongly reduces
the performance of the devices owing to the separation of charge carriers due to the
piezoelectric polarization induced-band bending. This bending in both conduction and
valence bands strongly affects the carrier transport. The bending reduces effective barrier
height of AlGaN electron blocking layer for electrons and increases barrier height for
holes. Bearing in mind the low mobility and concentration of holes and the leakage of
electrons, we introduced two additional InGaN quantum wells in the electron blocking
layer to recycle the leaked electrons. The radiative recombination was significantly
increased which was consistent with optical power and external quantum efficiency
results. Moreover, we showed that, by having only six quantum wells in the active region and two quantum wells within the electron blocking layer can still significantly increase
the performance of the conventional device with eight quantum wells.
The synthesis of novel nanomaterials and their hybridization with optoelectronic
materials and devices have recently become one the main research areas which combines
the electrical injection properties of the one and the optical, structural, and geometrical
properties of another material. In that manner, we chemically synthesized novel 2D CdSe
nanoplatelets, and performed optical and morphological characterization. Moreover, we
increased the photoluminescence of CdSe solid films with the incorporation of localized
surface plasmons. The photoluminescence enhancement was attributed to the electric
field enhancement and increased number of radiative channels in the presence of metallic
nanoparticles, which was also confirmed with theoretical studies and time-resolved
photoluminescence spectroscopy experiments. We believe this method will be useful in
devices incorporating CdSe nanoplatelets as main active materials. Moreover, we
investigated the critical role of CdSe nanoplatelets in the performance of color-converted
InGaN/GaN light-emitting diodes as an exciton donor for the color converter CdSe/ZnS
nanocrystal quantum dots. The hybrid device fabricated with the CdSe nanoplatelets
outperformed the one without the nanoplatelets in terms of power conversion efficiency.
The enhancement was ascribed to the exciton migration from donor CdSe nanoplatelets to
acceptor CdSe/ZnS quantum dots both of which were pumped with InGaN/GaN
light-emitting diode. Nonradiative Forster-type excitonic energy transfer between these
donor-acceptor pairs was further confirmed with time-resolved photoluminescence
spectroscopy and photoluminescence excitation measurements.
Furthermore, we investigated Forster resonance energy transfer to CdSe nanoplatelets from InGaN quantum wells of bulk and nanopillar structures. Optical characterization
revealed that the internal quantum efficiency and light extraction efficiency of nanopillar
device structure were higher than those of the as-grown structure owing to the increased
surface to volume ratio and the strain-relaxation. Next, the excitonic energy transfer
between InGaN/GaN nanopillars and chemically synthesized CdSe nanoplatelets were
monitored with time-resolved photoluminescence decay measurements. Resonance
energy transfer from bulk quantum well capped with 3 nm GaN cap layer was also
investigated in a similar manner following short characterization of bulk quantum well
epitaxial structure. The energy transfer efficiency of the bulk quantum well system was
higher than that of the nanopillar structure. The enhanced exciton migration was
attributed to the reduced separation between the quantum wells and the nanoplatelets in
the bulk quantum well structure compared with the nanopillar structure. Stacking of
nanoplatelets is believed to strongly reduce the chance of nanoplatelets to be in the close
proximity of the quantum wells in the InGaN/GaN nanopillar arrays.
In summary, the thesis includes the epitaxial growth, device fabrication, wafer-level
and device-level characterization, studies of novel device designs, and hybrid applications
of InGaN/GaN light-emitting diodes structures with novel CdSe nanoplatelets. Effective
methods to increase the electrical and optical performance of the device were discussed in
detail and the performance of the devices was compared with the conventional structures.
The thesis work has provided important insights for design, growth, fabrication,
characterization, and applications of high performance InGaN/GaN light-emitting diodes
and heterostructures.
|
|---|