Complete Report - University of New South Wales

4.6.5 Improved SOI LEDs Using Surface Plasmons.
A very effective way to enhance the external luminescence quantum effi ciency from silicon light
emitting diodes is the application of light trapping schemes. Light trapping is conventionally
used in photovoltaic devices to increase the average optical path for incident light, resulting
in better absorption of weakly absorbed light. According to Kirchhoff’s law, an enhanced
absorption corresponds to an enhanced emission. Light that is trapped within a planar device
due to total internal refl ection can be coupled out after multiple paths through a textured
device. The benefi cial effect of light trapping schemes on LED performance has been
demonstrated in high effi ciency bulk silicon light emitting diodes [M.A. Green et al., Nature
412, 805 (2001)].
However, for thin silicon on insulator devices, traditional light trapping methods such as
texturing with pyramids, cannot be used. The excitation of surface plasmons is a promising
alternative way of increasing light absorption and emission from thin-Si based devices. An
eighteen fold absorption enhancement at 800nm wavelength of silicon on insulator (SOI)
devices was reported by Stuart and Hall [H.R. Stuart and D.G. Hall, Appl.Phys.Lett. 73, 3815
(1998)]. Our work aims at increasing the absorption / emission form thin fi lm Si devices at
longer wavelengths.
Surface Plasmon effects are observed on thin silicon layers covered with an array of
nanometer sized metal islands deposited on the surface and separated from the active silicon
layer by a very thin ( ~ 30nm) spacer layer (SiO2). A simple method of silver deposition followed
by annealing is used to deposit Ag nanoparticles on silicon-on-insulator (SOI) light emitting
devices. The fact that the entire island deposition is a low temperature process allows metal
islands to be deposited onto fi nished fully processed devices. Fig.4.6.5.1 shows a typical SEM
image of the nanometer sized silver islands.
Figure 4.6.5.1: SEM photo of the deposited
nanometre silver islands for the surface plasmon
effect.
Changes in the optical device performance were
monitored experimentally by measuring the
spectral electroluminescence (EL) emission and
the spectral response (SR) of SOI LEDs before and
after island deposition. The optical enhancement
is calculated as the ratio of the SR and EL
signals, respectively after and before deposition
of the islands. This combination of EL and SR is
ideal because it provides information about the
optical path lengths enhancement in different
spectral ranges above (SR) and near and below
(EL) the band-gap of silicon. As reported earlier,
our initial proof of concept studies showed only
a 30% enhancement in the electroluminescence
measurements from our Silicon-on-insulator
devices.
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