12.3 Surface Plasmon Enhanced GaN-Based LED
275
to SP energy,c
ext (ω) is the SP’s scattering efficiency. It can be seen from the equation
that the k rad (ω)/k non (ω) and c
ext (ω) are the two key factors determining whether the
SP can improve the quantum efficiency in the quantum well. If the extreme case is that
c
ext is equal to 0 or k non is far less than k rad , then the quantum efficiency of the quantum
well cannot be improved. Only when C
ext (ω) > k rad (ω)/(k non (ω) + k rad (ω)) can the
internal quantum efficiency of quantum well be likely to increase.
Methods to improve the SP scattering efficiency include increasing surface roughness of the metal film, fabricating metal grating structure, using LSP coupling and
selecting metal with low absorption such as Ag, Au or Al.
12.3.3 Coupling Methods for SP Coupling Enhanced
GaN-Based LED
In the early stage, people explored how to improve emission efficiency of InGaN
quantum wells mainly using light injection. In 2004, Okamoto et al. deposited Ag,
Al and Au on three different InGaN quantum well respectively to systematically
study the SP coupling enhanced luminescence [28]. A 50 nm thick metal is deposited
on the cladding layer of the GaN-based blue quantum well. The distance between
the metal film and the quantum well is 10 nm. Then, the PL methods with back
incidence and back light emission are used to compare the luminous efficiency of
different sample with or without metal. The PL intensity of the sample with Ag at
the peak position increase by 14 times compared to the sample with no metal, while
the sample with Al shows a 8-times increase compared to the sample with no metal.
At the same time, Okamoto also studied how the thickness of the cladding layer
between different metal films and quantum wells affects the sample’s PL intensity.
As the distance between the metal film and quantum well increases, the PL intensity’s
multiplication factor decreases and eventually drops to 1 at the thickness of 150 nm.
This verifies that the SP coupling is a near-field effect and exponentially decays with
distance. Temperature-dependent PL can be used to directly calculate the internal
quantum efficiency of quantum wells, to distinguish the effect of SP on internal and
external quantum efficiency. The internal quantum efficiency of quantum wells with
Ag surface plasmon increased by 6.8 times from 6 to 41%. At the same time, results
from time-resolved PL show that the spontaneous emission rate of quantum wells
is 32 times higher than that of conventional quantum well structures due to the SP
coupling [29].
Compared to metal films, metal particles are more advantageous in terms of SP
enhancement because LSP has high scattering rate and adjustable SP resonance
frequency. In 2006, Pompa increased the PL efficiency of CdSe/ZnS by 30 times
using Au particle array with triangular prism shape, and nanostructures were prepared
by electron beam exposure [30].
Undoubtedly, the SP coupling can greatly increase the luminous efficiency of
quantum wells or other radiators. However, the application of SP to LEDs, especially
275
to SP energy,c
ext (ω) is the SP’s scattering efficiency. It can be seen from the equation
that the k rad (ω)/k non (ω) and c
ext (ω) are the two key factors determining whether the
SP can improve the quantum efficiency in the quantum well. If the extreme case is that
c
ext is equal to 0 or k non is far less than k rad , then the quantum efficiency of the quantum
well cannot be improved. Only when C
ext (ω) > k rad (ω)/(k non (ω) + k rad (ω)) can the
internal quantum efficiency of quantum well be likely to increase.
Methods to improve the SP scattering efficiency include increasing surface roughness of the metal film, fabricating metal grating structure, using LSP coupling and
selecting metal with low absorption such as Ag, Au or Al.
12.3.3 Coupling Methods for SP Coupling Enhanced
GaN-Based LED
In the early stage, people explored how to improve emission efficiency of InGaN
quantum wells mainly using light injection. In 2004, Okamoto et al. deposited Ag,
Al and Au on three different InGaN quantum well respectively to systematically
study the SP coupling enhanced luminescence [28]. A 50 nm thick metal is deposited
on the cladding layer of the GaN-based blue quantum well. The distance between
the metal film and the quantum well is 10 nm. Then, the PL methods with back
incidence and back light emission are used to compare the luminous efficiency of
different sample with or without metal. The PL intensity of the sample with Ag at
the peak position increase by 14 times compared to the sample with no metal, while
the sample with Al shows a 8-times increase compared to the sample with no metal.
At the same time, Okamoto also studied how the thickness of the cladding layer
between different metal films and quantum wells affects the sample’s PL intensity.
As the distance between the metal film and quantum well increases, the PL intensity’s
multiplication factor decreases and eventually drops to 1 at the thickness of 150 nm.
This verifies that the SP coupling is a near-field effect and exponentially decays with
distance. Temperature-dependent PL can be used to directly calculate the internal
quantum efficiency of quantum wells, to distinguish the effect of SP on internal and
external quantum efficiency. The internal quantum efficiency of quantum wells with
Ag surface plasmon increased by 6.8 times from 6 to 41%. At the same time, results
from time-resolved PL show that the spontaneous emission rate of quantum wells
is 32 times higher than that of conventional quantum well structures due to the SP
coupling [29].
Compared to metal films, metal particles are more advantageous in terms of SP
enhancement because LSP has high scattering rate and adjustable SP resonance
frequency. In 2006, Pompa increased the PL efficiency of CdSe/ZnS by 30 times
using Au particle array with triangular prism shape, and nanostructures were prepared
by electron beam exposure [30].
Undoubtedly, the SP coupling can greatly increase the luminous efficiency of
quantum wells or other radiators. However, the application of SP to LEDs, especially