ICNST2020 TEMPLATE

The 17th International Conference on Nano Science and Nano Technology 2020 (ICNST 2020)

Jeung Hun Park1,2,*, Richard S. Kim3, Se-Jeong Park4, Gye-Choon Park5,and Choong-Heui Chung6,*

1Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States

2Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California 90095, United States

3Opto-Diode Corporation, Camarillo, California 93012, United States4Korea ITS Application R&D Center, Korea I.T.S. Co., Ltd., Seoul 06373, Republic of Korea

5Department of Electrical Engineering, Mokpo National University, Muan, Jeonnam 58554, Republic of Korea

6Department of Materials Science and Engineering, Hanbat National University, Daejeon 34158, Republic of Korea

*Corresponding Author: Dr. J. H. Park, E-mail: jeungp@princeton.edu Prof. C.-H. Chung, E-mail: choong@hanbat.ac.kr Submitted/Received: April 23, 2020 Revised: June 24, 2020 Accepted: July 23, 2020

AbstractHere we describe the guidelines for the abstract/summary. The abstract should give a concise summary of

the work. Please use an 11-point Times Roman font. The final paper submission deadline is May 18,

2020. All manuscripts must contain an informative 150 to 300 words abstract explaining the essential

contents of the work, key ideas and results.

Keywords: GaAs Nanowire, Molecular Beam Epitaxy, Raman Spectroscopy, VLS Process.

First Time Use of Abbreviations: USE OF ABBREVIATIONS IN TEXT: No abbreviations are allowed in the title and abstract and should be defined the first time they are used within the title and text. For example, First time use as; Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Visible/near-infrared (Vis/NIR) spectroscopy, X‐ray absorption fine structure (EXAFS) spectroscopy, etc. The "Journal of Nanoscience and Nanotechnology" should be abbreviated as J. Nanosci. Nanotechnol. for the citation purpose in research articles.

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1. Introduction Gallium Arsenide (GaAs) has been widely studied owing to a direct band gap semiconductor with high

carrier mobility and potential application of electro-optic devices [1,2]. Vapor-liquid-solid (VLS) growth

is the most commonly used growth process in which a liquid metal acts as a catalyst for preferential

nucleation and growth of nanowires [3]. Since the performance of GaAs nanowire-based device is

strongly influenced by the crystal structure, size, and shape, detailed studies have been made to realize

controlled VLS grown GaAs nanowires [4,5,6]. Morphological dependences of nanowires on Raman

scatterings such as phonon confinement [7], surface optical phonon mode [8], and aspect ratio of length

over diameter [9] have been largely studied for GaAs nanowires that were mostly focused on optical

properties of individual nanowire. However, very little is known about Raman spectroscopic studies of

GaAs nanowire bundle.

In this paper, we report the relation between the catalyst patterning conditions and the changes of the 1 st

order Raman active modes (transverse- and longitudinal-optical phonons) in Au-catalyzed GaAs nanowire

bundles on GaAs(111)B wafer. we characterized the nanowire bundles within the wafer using

complementary µ-Raman spectroscopy and scanning electron microscopy (SEM) as a function of the e-

beam dose rate, dot-size and inter-dot spacing.

2. Experimental Details We fabricated e-beam lithographically Au-patterned 4inch GaAs(111)B substrates by changing the

patterning parameters (the e-beam dose rate: 145 – 595 µC/cm2, dot-size/-spacing: 100 and 150 nm) and

grew wafer-scale GaAs nanowires using a solid-source molecular beam epitaxy. There were thirty-one

rows of nano-patterns made by E-beam lithography at 50 KeV. Each pattern was made up of 250 × 150

ordered arrays of dots which have nominal sizes of 100 nm and 150 nm with the inter-dot spacings of 100

nm and 150 nm, respectively. As increasing the E-beam dose from 145 μC/cm2 to 595 μC/cm2, with which

the rows are patterned, the Au disk boundaries of the nano-patterns gradually become clearer in which

more amounts of Au element is incorporated into GaAs(111)B matrix as a catalyst layer. From the SEM

image analysis of the dot patterns [10], we measured the dot sizes to be 106 ± 5 nm for 100 nm,

respectively and 144 ± 5 nm and inter-dot spacing 104 ± 5 nm and 156 ± 4 nm. For the convenience, we

use the nominal values for the dot-size and interdot-spacing as the patterning condition of 100 nm and

150 nm. Detailed descriptions of the growth conditions and characterizations of GaAs nanowires were

described in the previous publication [10].

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3. Results and Discussion Figure 1 shows the SEM images and size distribution of as-grown GaAs nanowires for three dose rates

(400, 430, 520 µC/cm2). Upon annealing at 485 oC in the presence of As vapor, an Au pattern forms into

Au-Ga eutectic droplets with a broad size distribution in which either Ga or Au adatoms migrates with a

mean length of 3 µm [10,11]. GaAs nanowires are randomly oriented with the respect to the substrate

rather than vertically aligned due to large variations in Au-Ga catalyst [12], catalyst migration of Au

[13,14] or Ga [15,16] and relatively lower growth temperature [17]. Their shapes were highly tapered

along with most of their length, and they have larger base diameters than original patterning conditions

(100 nm and 150 nm), indicating that the lateral growth should induce the increase of the base diameter

[11]. Multiple nanowires can be grown within single Au dot pattern [10,18].

Figure 2(a) shows the surface coverage changes of the Au catalyzed GaAs nanowires evaluated from

100 nm and 150 nm patterning conditions, respectively. Due to random growth orientation of GaAs

nanowires, the fractional coverage of nanowires rather than the number density was utilized [19]. As the

e-beam dose increases, the surface coverage of the GaAs nanowires for both patterning sizes rapidly

increases and is saturated at a rate of ~80%. The nanowires with 150 nm patterns do not likely to be

grown with an e-beam dose below 340 μC/cm2. It implies that there would be a threshold e-beam dose

rate which creates a minimum thickness of Au-Ga catalyst layer to enable the reliable growth of the GaAs

nanowires. The nanowires with the patterning condition of100 nm exhibit relatively higher threshold dose

rate ~ 400 μC/cm2. Figure 2(b) estimates the average base diameter changes of as-grown nanowires from

multiple SEM investigations. The obtained data was fitted to a log-normal distribution [20], and then their

average diameter was calculated from the fitted slope. With increasing the e-beam dose, the base diameter

decreases for both cases. As a result, the cross-over of the base diameter size between 100 nm and 150 nm

patterns can be found at the dose rate of 480 μC/cm2. Figure 2(c) depicts the average aspect ratio α as a

function of e-beam doses ranging from 250 μC/cm2 to 600 μC/cm2. More than N = 50 nanowires were

investigated, and then α was estimated for each patterning condition by using a formula α= 1N ∑

i=1

N

( Li / Di )

, where N is the number of nanowires, L is the average length, and D is the base diameter. For the pattern

condition of 150 nm, α appears to be relatively independent from the dose rate and is evaluated to be 22.7

~ 25.7 when the dose rate is larger than 340 μC/cm2. For the pattern condition of 100 nm, however, α is

more sensitively changed to the dose rate and is linearly increased from 13.2 to 25.1 when the dose rate

increases from 400 μC/cm2 and 520 μC/cm2.

Figure 3 shows Raman spectroscopic characterization of GaAs nanowire bundles with variations in

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dose rates and pattering sizes. Two Raman spectra (175 μC/cm2 and 250 μC/cm2) exhibit two clear peaks

which the peak positions at 268.7 cm-1 and 292 cm-1 are corresponded to the scattering from bulk GaAs

TO and LO phonon modes, respectively. It implies that these two lower dose rates would not be

optimized patterning conditions for the growth of nanowires. In contrast, the peak positions of TO and LO

for the dose rates of 355, 400, and 520 μC/cm2 are identified around 265 cm-1 and 286 cm-1, respectively.

As the increased dose rate results in decreasing the base diameter (see Fig. 2(b)), both the TO and LO

bands are broadened and shifted considerably toward a lower wave numbers relative to bulk crystalline

GaAs. For both patterning conditions 100 nm and 150 nm, it is consistently shown that the LO peaks are

more sensitive to the dose rate change (and the resulting morphological change) than TO peaks.

Interestingly, as shown in Figs. 2(b-c), the effects of the dose rate and morphology on Raman spectra is

more pronounced in the 100 nm pattern than those in the 150 nm.

4. Conclusions We studied the relation between the catalyst patterning conditions and the spectral changes of the 1 st

order Raman active modes in Au-catalyzed GaAs nanowire bundles. We fabricated e-beam

lithographically Au-patterned 4inch GaAs(111)B substrate by varying the patterning conditions (e-beam

dose rate, dot-size and interdot-spacing), and grew GaAs nanowires via VLS growth process using a

solid-source molecular beam epitaxy. We have experimentally shown that Raman spectra were dependent

on initial pattern sizes (100 nm and 150 nm) and thickness of Au layers by varying e-beam dose. Our

understanding of the Raman spectroscopic characterizations of the nanowire bundles would benefit the

cost-effective nondestructive characterization of semiconducting nanostructures within supporting

substrates and development of nanoscale optoelectronic devices.

Acknowledgments The authors are grateful to Profs. Bruce E. Koel (Princeton University), Suneel Kodambaka (UCLA)

and Dr. Vincent Gambin (Northrop Grumman Space Technology) for providing valuable discussion in the

nanowire growth and characterization, and Prof. Yang Yang (UCLA) for providing access to µ-Raman

spectroscopy system. JHP acknowledges the supports in part of the research projects from the Princeton

Catalysis Initiatives, the UC Discovery — Northrop Grumman Aerospace Systems, and the NSF CMMI

Grant No. 0926412. CHC acknowledges the financial support from Basic Science Research Program

through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT

(Grant No. NRF-2019R1F1A1058917).

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REFERENCES (Use Harvard Referencing): References should be in the proper Harvard style on a separate page, numbered in the sequence in which they occur in the text. Cite references numerically in a bracket [ ] in the text and list in the same numerical order at the end of the manuscript. References should be listed in the HARVARD Referencing format (http://jnn.aspbs.org/). Authors could directly copy references in a Harvard style from the Google Scholar (https://scholar.google.com) and then paste in the Reference List of the manuscript as such.

References and Notes1. Duan, X., Wang, J. and Lieber, C.M., 2000. Synthesis and optical properties of gallium arsenide

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More examples for References and Notes

Journal Articles

1. Duan, X., Wang, J. and Lieber, C.M., 2000. Synthesis and optical properties of gallium arsenide

nanowires. Applied Physics Letters, 76(9), pp. 1116–1118.

2. Shtrikman, H., Popovitz-Biro, R., Kretinin, A. and Heiblum, M., 2009. Stacking-faults-free zinc blende

GaAs nanowires. Nano Letters, 9(1), pp. 215–219.

Book

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Book Chapter

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Table I. Summarized Raman peaks of Au catalyzed GaAs nanowire bundles grown on GaAs(111)B. A

unit is cm-1.

Type E1(TO) E1(LO) E2h SOBulk-GaAs 268.7 292.2 N/A N/ANano-GaAs 268.6 288.7 255.8 281.8 – 287.4

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Figure captions

Figure 1. 30 degree-tilt SEM images (top-row) and the morphological analysis (bottom-row) of as-grown

GaAs nanowires on Au patterned GaAs(111)B substrates via SS-MBE. The dot-size and inter-dot spacing

of (a) 100 nm and (b) 150 nm for the dose rates of 400 (blue), 430 (yellow), and 520 (green) µC/cm2.

Figure 2. (a) Surface coverage, (b) average base diameter changes, and (c) the aspect ratio of GaAs

nanowire bundles as a function of electron beam dose for the pattering conditions 100 nm (red dot) and

150 nm (black square). The solid lines are displayed for the eye-guidance.

Figure 3. Raman spectra of GaAs nanowire bundles as a function of electron beam dose for the pattering

condition of (a) 100 nm and (b) 150 nm. The five peaks marked with dashed lines are assigned to be As

anti-site (~258 cm-1), nano-GaAs TO (~265 cm-1), bulk-GaAs TO (~268.7 cm-1), nano-GaAs LO (~286

cm-1), and bulk-GaAs LO (~292 cm-1).

Use Large Fonts for numbering/ligands in the Figures

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Figures and Captions

Figure 1. 30 degree-tilt SEM images (top-row) and the morphological analysis (bottom-row) of as-grown

GaAs nanowires on Au patterned GaAs(111)B substrates via SS-MBE. The dot-size and inter-dot spacing

of (a) 100 nm and (b) 150 nm for the dose rates of 400 (blue), 430 (yellow), and 520 (green) µC/cm2.

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Figure 2. (a) Surface coverage, (b) average base diameter changes, and (c) the aspect ratio of GaAs

nanowire bundles as a function of electron beam dose for the pattering conditions 100 nm (red dot) and

150 nm (black square). The solid lines are displayed for the eye-guidance.

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Figure 3. Raman spectra of GaAs nanowire bundles as a function of electron beam dose for the pattering

condition of (a) 100 nm and (b) 150 nm. The five peaks marked with dashed lines are assigned to be As

anti-site (~258 cm-1), nano-GaAs TO (~265 cm-1), bulk-GaAs TO (~268.7 cm-1), nano-GaAs LO (~286

cm-1), and bulk-GaAs LO (~292 cm-1).

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Graphical Abstract

Non-destructive characterization of GaAs nanowires grown on a 4inch GaAs(111)B substrate was performed using micro-Raman spectroscopy. The relation between the catalyst patterning conditions (e-beam dose rate, dot-size and interdot-spacings) and the intensity of the 1st order Raman active modes in Au-catalyzed GaAs nanowire bundles was studied. Ensembles of single crystalline GaAs nanowires covered with different Au-thickness exhibit a downshift and asymmetric broadening of the 1st order TO and LO phonon peaks relative to GaAs bulk modes. The sensitivity of a downshift and broadening of Raman spectra are directly related to morphology and surface coverage variations in as-grown nanowires.

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