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U.S. ARMY COMBAT CAPABILITIES
DEVELOPMENT COMMAND –
ARMY RESEARCH LABORATORY
Lesly Henrriquez-Saravia
Electrical Engineering Undergraduate, University of Maryland, College Park
Optical and Power Devices Branch – Advanced Electronics Division – SEDD
22 007 2020
Optic Spectroscopy Characterization ofIII-Nitride Semiconductor Materials for Lasers
Mentor: Dr. Gregory Garrett
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Advantages
• Lasers made with cubic-GaN and m-plane
hexagonal-InGaN have a smaller polarization
field compared to hexagonal-GaN.
• Semiconductor lasers can be smaller, more
compact and more efficient.
Applications
• Efficient semiconductor lasers will help enable
compact atomic clocks that will improve PNT
technology.
• A cubic-GaN laser at 369.5 nm would be used for
Yb+ ion cooling.
• An m-plane InGaN laser at 460.9 nm is needed
for Sr atom lattice cooling.
APPLICATION OF SEMICONDUCTOR LASERS
AND ADVANTAGES
Figure 1: Chip based atomic
clock being developed by
the DARPA A-PhI program.
Efficient semiconductor
lasers are needed to cool
atoms.
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• Samples of GaN were grown on
3C-SiC using plasma assisted MBE
at ARL– Research determined optimal growth
occurred at 800 °C at a growth rate of
210 nm/h.
– Optimal predeposit time of Ga was
determined at 3 seconds.
– samples have 2.5 mm square areas of
GaN layers grown on 3C-SiC
surrounded by a hatch of polycrystalline
SiC as shown in figure 2
• Optical Spectroscopy was used to
determine if the GaN was
hexagonal or cubic.
– Raman spectroscopy: different crystal
structures will have different vibrational
modes.
– Photoluminescence (PL) spectroscopy:
tells us the band gap, which we expect
to be different.
GROWTH OF CUBIC-GaN
cubic-GaN
3C-SiC
Si(100)
Figure 3: GaN layers were grown on 3C-SiC (cubic)
templates that were provided on Si(100) oriented
substrates.
Figure 2: surface image of GaN layer grown at 800 °C at
a rate of 210 nm/h
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RAMAN SPECTROSCOPY OF GaN SAMPLES
Figure 4: Raman Spectra for h-GaN excited by three different
laser wavelengths. (a) 5145 Å, (b) 4880 Å (c) 4579 Å (Image
from A.Tabata and R. Enderlein, J. Appl. Phys. 79, 4137
(1996).)
Figure 5: Raman Spectra for c-GaN excited by three
different laser wavelengths. (a) 5145 Å, (b) 4880 Å,
(c) 4579 Å (Image from A.Tabata and R. Enderlein, J.
Appl. Phys. 79, 4137 (1996)
• The Raman spectroscopy can be taken for the samples to determine if it is cubic or
hexagonal, or a mixture of both.
• Based on literature, cubic-GaN has an LO mode peak at about 740 cm-1 and a TO
mode peak at about 553 cm-1
• h-GaN exhibits an E2 peak at about 570 cm-1.
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PHOTOLUMINESCENCE (PL) SPECTROSCOPY
OF GaN SAMPLES
Figure 7: PL spectra of cubic-GaN measured using a
325 nm laser. Each plot represents a different focus
point of the laser.
• PL can also characterize samples.
• Cubic-GaN has a PL peak at ~380 nm
while h-GaN has a PL peak at 364 nm.
• PL was taken at 12K with the 325 nm
line of a HeCd laser. Sample was excited
with 8 mW.
• The PL spectrum shows a cubic peak
(380 nm) when the laser is focused in
the central area, which consists of the
GaN layers grown on 3C-SiC.
• The red line shows the PL when the
laser line is crossing the hatches
between the square, which consist of
polycrystalline SiC. The graph exhibits
the PL peaks for both h-GaN (359 nm)
and cubic-GaN (380 nm).
Figure 6: Sample of cubic-GaN. PL was taken with
laser focused in different areas of square.
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• Samples studied were grown by Prof.
Dan Feezell’s group at the University
of New Mexico using MOCVD.
• M-plane h-InGaN does not have a
strong polarized field perpendicular
to the wells as seen in c-plane
InGaN.
• Growing on the m-plane also makes
it easier to incorporate more Indium,
which leads to a longer wavelength.
• The goal is to have the samples that
emit at 461 nm.
• PL spectroscopy was used to
characterize the multi-quantum well
(MQW).
NONPOLAR M-PLANE h-InGaN
Barrier GaN
Well InxGa1-xN
Barrier GaN
GaN
Bulk m-Plane GaN Substrate
Figure 8: Multi-quantum well structure. Samples
had 7 barriers and 6 wells. Typical thickness of
wells is 4.4 nm. Thickness of barriers is 1.8 nm.
The x in the well represents the percentage of
Indium.
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INTERNAL QUANTUM EFFICIENCY OF M-PLANE
InGaN MULTI-QUANTUM WELLS
• PL spectra were taken for multiple m-
plane InGaN samples at various
temperature. Temperatures ranged
from 12K – 300K
• PL was taken using a 405 nm laser.
Sample was excited with 10 mW.
• The Internal Quantum Efficiency (IQE)
tells of a sample tells us of the quality
of the MQWs and reflects the amount
of non-radiative defects in the
sample.
• IQE can be estimated by taking the
ratio of the area under the PL curve at
room-temperature to the area under
the PL curve at low-temperature.
• The samples were grown to have the
same thickness of barriers and wells.
The percentage in of Indium in the
wells varied.
• The IQE measured in the samples
were 34%,35% and 37%.
Figure 9: PL spectra of the longest wavelength sample
at various temperature. IQE was calculated as 37%.
Sample consisted of 19% Indium.
PL Spectra of InxGa1-xN with x = 0.19
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PL SPECTROSCOPY OF InGaN SAMPLES
Figure 10: Temperature dependent PL of a sample
where the percentage of Indium was estimated to be
12.5%. IQE was calculated as 35%.
Figure 11: Temperature dependent PL of a sample
where the percentage of Indium was estimated to be
13.5%. IQE was calculated as 34%.
PL Spectra of InxGa1-xN with x = 0.125 PL Spectra of InxGa1-xN with x = 0.135
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CENTRAL PEAK AND FWHM
• Central peak of PL at room temperature reflects the percentage of indium and the
width of the quantum wells. ‒ Central peak for sample with 12.5% Indium located at 444.0 nm.
‒ Central peak for sample with 13.5% 452.5 nm.
‒ Central Peak for sample with 19.0% Indium located at 459.5 nm
• FWHM of PL at room temperature is used to analyze the local uniformity of the
percentage of Indium and the thickness of the quantum wells . A smaller width
indicates a more uniform sample. ‒ FWHM of sample with 12.5% Indium was 29.6 nm
‒ FWHM of sample with 13.5% Indium was 29.5 nm
‒ FWHM of sample with 19.0% Indium was 36.2 nm
Figure 12: PL spectra with marked peaks of the three samples at room temperature.
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• Cubic GaN Epilayers
– GaN samples grown at ARL were characterized
by PL spectroscopy.
– Comparing the PL peak that was measured to
the PL peaks in literature, we have confirmed
that the samples were cubic with little indication
of hexagonal inclusions.
– We will continue to further characterize the
sample using Raman spectroscopy. We also
have temperature dependent data on three
cubic-GaN which will be used to examine IQE.
• M-plane InGaN MQW
– Similarly, we used PL spectroscopy to estimate
the Internal Quantum Efficiency of InGaN multi-
quantum wells. The highest IQE measured was
at 37%.
– To get an efficient laser, we will try to achieve an
IQE of 70%.
– We will continue to analyze new samples sent
from UNM.
SUMMARY
Indium IQE Wavelength Width
12.5% 35% 444 nm 29.6 nm
13.5% 34% 452.5 nm 29.5 nm
16% 58% 459.5 nm 36.2 nm
Table 1: Data collected from PL spectra showing
percent of Indium to IQE and wavelength emited.
Figure 13: PL showing of h-GaN (359 nm) and
cubic-GaN (380 nm) peaks in poly-crystalline area