Design and experimental studies of multilayer coatings for applications in gallium nitride light emitting devices
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Date
2006-06
Authors
Ahmed, Naser Mahmoud
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Abstract
In this work mirrors and microcavties for GaN-UV light enhancement were
fabricated and studied. This work discusses methods of designing two types of mirror:
first is the semiconductor distributed Bragg reflector (OBR) (Alo4Gao 6N/GaN) mirrors
MATLAB software and theoretical analysis based on Transfer Matrix Method (TMM)
are used to investigate reflection due to depth errors that occur in films growth, number
of layers, type of substrate and incidence angle. We assume that there is 10% depth
error in high and low index materials. Our simulation results showed that the depth
error caused the reflective band shift about 36nm for semiconductor OBR structure and
20nm for dielectric OBR structure. The optical properties of Alo11Gao89N, Aloo3Gao97N,
and GaN grown on sapphire were investigated. The ellipsometry measurement
operating at wavelength 632.8nm yields If/ the relative amplitude change for J7 and s
polarizations, L\ relative phase shift between the two polarization direction and
refractive index, important parameters to develop the model for GaN on AI20 3
substrate. We demonstrated a GaN/sapphire microcavity exhibiting 1.3-1.6 fold
enhancement in light extraction efficiency by using flat and dome epoxy as external
medium compared with air. as the external medium. Simulations results showed that by
using Hf02 and MgF2 can improve the light extraction by diffracting is the internal light
with a large solid angle into the escape light cone.
Three types of microcavity types were fabricated using metal and dielectric
DBR mirror for the enhancement and inhibition of photoluminescence in GaN. A
GaN/sapphire structure as an active layer was sandwiched between two mirrors, which
were silver metal reflectors. GaN layer on sapphire showed a photoluminescence (PL)
peak around 364 nm. Photoluminescence measurements showed intensity
enhancement of 2 and 16 fold in silver half-cavity back mirror and dielectric DBR mirror
respectively. In the full cavity samples, the amplitude of the photoluminescence is
enhanced 10-times when we used 50 nm silver as a front mirror. A tremendous
enhancement of more than 16-fold is obtained when silver mirror of 25nm thickness
was used as a front mirror. Further, a tremendous enhancement was obtained when a
small hole of 4mm2 was made in the front mirror with 50nm thickness.
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Keywords
In this work mirrors and microcavties for GaN-UV light , enhancement were fabricated