AIJCSR-425
ISSN 2349-4425
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Simple optical system design to produce Continuous wave visible emission of direct-bandgap semiconductor (n-type GaN/sapphire -yellow emission) and indirectbandgap semiconductor (monocrystalline p-type Si- red emission) at room temperature for nanodevices Kifah Q. Salih Ministry of Higher Education and Scientific Research, Iraq-Baghdad h/p:+964-07711973084
[email protected]*
[email protected] ABSTRACT There has been considerable interest in nanoemitters for different applications with low cost. A key motivation of this work in using a metallic thin film on glass or semiconductor substrate as optical system is to keep reducing the device size & its cost. Simple optical system has been used to amplify Continuous wave visible emission of n-type GaN/sapphire (as a type of direct-bandgap semiconductor) and monocrystalline p-type Si (as a type of indirect-bandgap semiconductor) to produce better emitters with low cost. We have designed and constructed simple PL setup using He-Ne laser (543.5nm) and Ar+ laser (514.5nm) to get visible emissions from Si & GaN at room temperature respectively. We used optical system (Metal thin AL film (120 nm in thickness) / Glass (0.5 mm in thickness) substrate/air)) as a reflector for incident laser beam and (Metal thin AL film (120 nm in thickness)/Si wafer substrate/air) as a second reflector for emission light from samples. At different excitation intensity, red emission monocrystalline Si wafer is dominated at wavelengths (~700-750) nm and strong yellow emission ntype GaN/sapphire is dominated at wavelengths (~560-565) nm. Optical design coupled with convenient surface texture of samples, wavelength excitation source and incident angle of laser beam. AFM revealed the surface texture of samples. We calculated absorption coefficient (α) of GaN and Si using a technical computing language Matlab in order to understanding the effect of absorption coefficient on visible emission of samples according to energy of excitation source. All measurements were carried out at room temperature.
Keywords: Continuous wave PL techniques, Silicon visible emission, GaN yellow emission, absorption coefficient, surface texture, Microptical cavity, & optical properties of Lasing mirror.
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AIJCSR-425
ISSN 2349-4425
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INTRODUCTION Photoluminescence spectroscopy has become a standard
Importantly, we calculated absorption coefficient (α) of GaN
method for the characterization of semiconductors properties
and Si using a technical computing language Matlab.
by analyzing the emitted light from a semiconductor material after interaction with laser source of PL system [1-3]. The investigation of silicon & GaN emissions are a very active field of research, due to the fundamental physical properties and promising applications in advanced optoelectronic and electronic devices. In this work, we have designed simple optical system to produce
Continuous
wave
visible
emitters of
n-type
GaN/sapphire (as type of direct-bandgap semiconductoryellow emission) and monocrystalline p-type Si (as type of indirect-bandgap semiconductor-red emission) using Ar+ laser(514.5nm)
and
He-Ne
laser
(543.5nm)
at
room
temperature. The main aim of this study is to demonstrate the results of two very important materials in optoelectronics applications, Si (indirect-bandgap semiconductor) and GaN (direct-bandgap semiconductor) at room temperature with simple design & low
EXPERIMENTAL RESULTS In this work, two material systems, n-type GaN/sapphire (as a type of direct-bandgap semiconductor), and monocrystalline p-type Si (as a type of indirect-bandgap semiconductor) have been
studied
mainly
using
a
size, where the semiconductor gain core is only a few tens of nanometers in size [4].
CW
Photoluminescence set up " He-Ne laser (543.5nm) and Ar+ laser (514.5nm) to get visible emissions from Si & GaN at room temperature respectively, shown schematically in Fig.(1) . Simple optical design /optical amplification system ((Metal thin AL film (120 nm in thickness) / Glass (0.5 mm in thickness) substrate/air)) as a reflector for incident laser beam)) and (( Metal thin AL film (120 nm in thickness)/Si wafer substrate/air)) as a second reflector for emission light)) has been used, shown schematically in Fig.(2). PL of sample was collimated to a monochromater slit (150μm) and a Photomultiplier
(200-900nm)
in
cost. A key motivation of this work is to keep reducing the device
home-made
connection
with
a
monochromator. The measurements of AFM and CW PL of two material systems were performed at room temperature, Results illustrate in Figures (3-6)
Fig.(1): A schematic diagram of a simple optical cavity arrangement for visible emission
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AIJCSR-425
ISSN 2349-4425
He-Ne laser (543.5nm)
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Optial cavity of sample
Fig. (2) A schematic diagram of a home-made CW Photoluminescence Setup using excitation sources: He-Ne laser (543.5nm) and Ar+ laser (514.5nm) for Si & GaN respectively.
Fig. (3) AFM of monocrystalline p-type Si sample
Fig(4) AFM of n-type GaN/sapphire sample.
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Excitation source He-Ne laser (543.5nm) ±3nm
Fig. (5): Visible photoluminescence emission of p-type Si at room temperature.
n-type GaN/sapphire Excitation source: Ar+ laser (514.5nm) ±5nm
Fig.(6) Visible photoluminescence emission of n-type GaN/Sapphire at Room temperature.
We have calculated absorption coefficient (α) of GaN and Si energy in dependence on the energy of excitation source using a technical computing language Matlab, The result is illustrated in Figure 7.
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Fig.(7)Absorption coefficient of GaN and c-Si using a technical computing language Matlab. At different excitation intensity, red emission monocrystalline
donor and a deep acceptor .The deep acceptor could be a Ga or
Si wafer is dominated at wavelengths (~700-750) nm and
a nitrogen vacancy [5].
strong yellow emission n-type GaN/sapphire is dominated at
Visible emission intensity and its profile of GaN/sapphire &
wavelengths (~560-565) nm. In this study, Optimum results
monocrystalline Si are created mainly by four factors:
requirements are that Optical system , convenient surface
1- Surface texture
texture of samples are coupled with wavelength excitation
2- Penetration depth 1/α (α is absorption coefficient).
source/intensity laser beam and incident angle of laser beam(
3- Ratio of the rate of radiative recombination to the total
θ=400-650 )
recombination rate 4- Absorbed photon flux
DISCUSSION PL spectra, AFM and the technical computing language Matlab results are used to characterize the room temperature
From Figure7, the intensity profile of the emitted light (Ip) can explain by equation (1) as given below [6]:
visible emission of samples as shown in figures (3-7). AFM
Ip=
images of monocrystalline p-type Si sample (Figure 3) and ntype GaN/Sapphire (Figure 4) displays pit-like defects and pits types (dislocations , some of micro pipes defects) respectively . All types of defects play main effect in penetration depth of laser radiation energy .This effect causes main change in out put intensity profile and spot size/profile distribution of light
(1+αL)
exp
(-αW)
................................ (1) Where Ф is the absorbed photon flux and k is the ratio of the rate of radiative recombination to the total recombination rate. c-Si α is larger than GaN for 2.28eV(He-Ne laser ) .So ,c-Si penetration depth is less than GaN. It suggests to the
emission( Figures:5-6). PL spectra of GaN/sapphire
(kФ)/
with a maximum energy
difference in minority carrier diffusion length (L ) and the
"~2.2eV" is usually referred to as the “yellow luminescence at
thickness of the space charge (W )will recombine which
( suitable excitation power "0.6-0.8 watt"- room temperature”.
creating the PL features.
Yellow luminescence is a common feature of n-type III nitrides, it arises from the recombination involving a shallow
If the radiative quantum efficiency and reflectance remain constant, the luminescence intensity should saturate in the larger absorption region. The saturation of the luminescence
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AIJCSR-425
ISSN 2349-4425
intensity, namely no decrease of the luminescence intensity at
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REFERENCES
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wavelength excitation source/intensity laser beam and incident angle of laser beam. From our experimentally observed, we have shown that Si red emission is dominated at different experimental conditions. Absorption coefficient of our samples, we have seen the surface texture
plays a role in
visible emission even if the excitation source is not equal to their energy bandgap. The results of this study stimulate the search for new way to modify the visible emission of direct and
indirect
bandgap
semiconductor
materials
for
optoelectronics applications using external suitable conditions.
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