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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