In this work, the results are presented of a nanorod LED array. If the lateral size of the nanorods is small enough, it is
possible to achieve a degree of lateral confinement. If the nanorods are ordered into a suitable photonic lattice, then this
will reduce the lateral spontaneous emission and enhance emission along the vertical axis via the Purcell effect.
Additionally there is a degree of dislocation filtering that can occur [1]. However, one potential drawback of this device
is the large free surface that borders the multi-quantum well active region. Nevertheless, it has been shown that the
surface recombination in the nitride materials is the lowest of all III-V semiconductors.
Results of SEM, PL, EL, and far field pattern are presented to compare the progressive effect of using photo-assisted
electroless and wet etching [2]. It can be seen that over time the photo-assisted electroless method clearly delineates the
active MQW region, possibly as a result of the different etch rate of InGaN. Alternatively, a purely chemical etching
method was used. With a narrowing of the nanorods, there is a progressive blue shift of the photoluminescence peak. The
optical image of the emission shows that there are well-defined lines of enhanced light propagation that match the
symmetry of the nanorod array, thus showing there is a photonic crystal effect.
In this paper we investigate the utilization of a patterned photonic crystal layer positioned below the Multiple
Quantum Well region of a p-side up LED to improve light extraction and investigate the benefit of adding additional top
surface photonic crystal patterning to the same device. Performance for each design is evaluated in terms of angular
extraction efficiency and far field angular beam profile for fixed lattice geometry and a range of etch depths. We show
that a buried photonic crystal lattice improves extraction for emission angles below the critical angle for total internal
reflection, and does not have a detrimental effect over the normal extraction cone of the LED. An improvement of (33%
to 57% in extraction efficiency is obtained using a buried photonic crystal lattice alone. The introduction of additional
top surface patterning has potential to further improve the extraction efficiency up to 73%, and to tailor the shape of the
emission profile.
A resonance tunnelling LED structure having a high efficiency, low droop and negligible wavelength shift with current
is reported in this study. The LED structure contains a thick InGaN bottom spacer between an n-GaN contact layer and a
multiple quantum well (MQW) active region, and a thin InGaN top spacer between the MQW and an AlGaN electron
blocking layer (EBL). The observed high efficiency and negligible wavelength shift with applied current are attributed
to the thick InGaN bottom spacer that nucleates V-pits and acts as a strain control layer for the MQW. The thick InGaN
layer also provides an electron reservoir for efficient electron tunnelling injection into the MQW and reduces the electropotential
difference between the n-emitter and the p-emitter, to suppress current leakage at high driving current and
reduce droop. The top InGaN spacer was designed to act as a magnesium back-diffusion barrier and strain relief layer
from EBL so as to obtain high efficiency.
We investigate improvement in performance attainable by etching Photonic Crystals and Photonic Quasicrystals
(PQC) into the top emitting surface of LEDs (PQC-LEDs). We describe the physical mechanisms of extraction
enhancement through ordered surface patterning and investigate benefits in terms of total extraction enhancement,
beam directionality, and far field beam quality. We show that a great variety of far field beam profiles tailored for
specific applications such as projection TV light engines and direct flat panel display illumination can be obtained
simply by adjusting geometric design parameters. Our results show that PQC-LEDs can provide around 50%
improvement in extraction enhancement and coupling efficiency for applications requiring non Lambertian beam
shapes when etched into standard epitaxy wafers in comparison to "state of the art" surface roughened GaN LED
devices. We also show that by tailoring the epi-structure to complement the properties of the PQC, extraction as
well as directional light coupling enhancements can be improved by a factor of 9 amounting to a total coupling
enhancement of around 78%.
Effect of degradation processes on transient currents in LEDs has been studied. It has been found that transient currents are several orders of magnitude higher than steady-state currents. The transient current time dependencies are non-exponential and show a distribution of relaxation times in the range of 1-100 microseconds. The charge associated with the transient currents is Q ~3x10-10 C which corresponds to high number of carrier traps Nt ~ 2x109 in the investigated chips. For one-year old chips an increase of charge and trap number by ~ 25% has been found compared to the fresh chips. Two probable reasons have been suggested to explain the observed increase of number of carrier traps: first one is related to increase of the number of trap sites at dislocations, and second one is a gradual phase separation process in quantum wells resulting in degradation of their quality.
A III-nitride blue LED structure based on the system of two wells with charge asymmetric resonance tunneling (CART), which allows enhancing the number of the electrons captured into the active region with the quantum well, was systematically studied. The barrier design uses the charge asymmetric resonance-tunneling phenomenon, which allows making the barrier transparent for electrons and blocking for holes. The growth and post-growth processes were optimized to achieve an efficient CART LED. The output power of 4 mW at the operating current of 20 mA has been achieved, corresponding to the external efficiency of 6%. Results presented in this report include the optimization of the quantum well growth parameters, bowing parameter for InGaN alloys grown on GaN, dry etching of III-nitride materials, Ohmic contacts to p- and n- type GaN, electrostatic discharge (ESD) problems related with the reliability of LEDs. The results presented include also modulation-technique LED characterization to tune the maximum radiative-recombination efficiency in accordance with the common operating current density.
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