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Terahertz emission from GaN-based nanophononic structures: the nexus between
scale and frequency
H. Jeong, and Y. D. Jho
Citation: AIP Conference Proceedings 1399, 439 (2011);
View online: https://doi.org/10.1063/1.3666442
View Table of Contents: http://aip.scitation.org/toc/apc/1399/1
Published by the American Institute of Physics
Terahertz emission from GaN-based nanophononic
structures: the nexus between scale and frequency
H. Jeong and Y. D. Jho*
Department of Info. and Comm., Gwangju Institute of Science and Technology(GIST), 1 Oryong-dong, Buk-gu,
Gwangju 500-712, Korea
Abstract. We report a newly-found terahertz generation mechanism related with acoustic standing waves confined
within GaN-based piezoelectric layers and its frequency control by adapting relevant active layer thicknesses.
Keywords: Terahertz, Acoustic phonon, Piezoelectricity.
PACS: 63.20.kd, 43.35.+d, 78.20.hb.
INTRODUCTION
In piezoelectric heterostructures, coherent acoustic
phonons are significantly amplified by transient
piezoelectric field screening by photo-generated
carriers [1]. The relationship between piezoelectricity
and the acoustic phonon waves have been widely
studied; e.g., the zone-folded [2], propagating modes
[3], or terahertz wave radiation from propagating
acoustic waves at the hetero-interfaces [4]. Here in this
work, we have investigated THz emission
characteristics in terms of confined acoustic phonon
dynamics from reversely biased p—i—n piezoelectric
structures.
EXPERIMENT
As depicted in Fig. 1(a), we have measured THz
radiations from reversely biased p—i—n diode
structures by ultrafast laser-pumping in UV range. A
conventional time-domain THz spectroscopy scheme
was employed for the generation and detection of the
THz wave signals. A frequency-doubled Ti:sapphire
laser was used as “pump” for the carrier excitation
whereas another Ti:sapphire laser was used for the
“signaling” beam in IR range based on
synchronization scheme among lasers. To detect the
THz signals, we have employed GaAs-based photoconductive antenna (PCA) which is activated by the
signaling beam. The excitation laser power density
FIGURE 1.(a) Energy band diagram of the sample used
in this work. (b) Relation between THz frequency and
AlGaN layer thickness. (c) Acoustic standing modes in
AlGaN layer. (d) A temporal comparison between a
measured THz signal and a simulated acoustic phonon
oscillation.
was intentionally maintained to be as high as possible
(400 MW/cm2) to abruptly screen out the piezoelectric
field at room temperature. The samples used in this
work were comprised of InxGa1-xN/GaN multiple
quantum wells (MQWs) and an AlxGa1-xN electron
blocking layers (EBLs) with various thickness L
Physics of Semiconductors
AIP Conf. Proc. 1399, 439-440 (2011); doi: 10.1063/1.3666442
© 2011 American Institute of Physics 978-0-7354-1002-2/$30.00
439
ACKNOWLEDGMENTS
embedded between p-GaN and MQWs. We note that,
as illustrated in Fig. 1(a), the depletion region can
reach through EBL up to p-GaN side where the
electrons are accumulated in the case of UV laser
excitations (denoted by shaded regions)
This work was supported by the Bioimaging
Research Center and “Fusion-Tech. Developments for
THz Info. and Comm.” program of GIST in 2010..
RESULTS
REFERENCES
1.
2.
3.
4.
5.
6.
As summarized in Fig. 1(b), the prominent THz
frequency component were those of fundamental
acoustic standing modes in AlGaN layers (with
thickness L) evaluated to be f=vs/2L, where vs is the
sound velocity. The signal amplitude linearly
increased with the external reverse bias (not shown
here), implying that the accumulated carriers in pGaN/AlGaN interface play an important role for the
THz radiations as the depletion region expands beyond
the AlGaN layer (CF: Fig. 1a). The THz radiation
from confined acoustic wave modes has not been
reported in the previous studies with samples without
AlGaN layer [1-3], whereas significant acoustic
impedance mismatch with surrounding layers were
recently known to induce the standing wave modes in
the case of Au film on a prism [5] and in free-standing
Si-membranes[6].
Without considering the dynamic influence from
photocarriers accumulated at the interfaces under high
reverse bias, the acoustic impedance mismatch
between GaN and AlGaN was estimated to be about
3% in our compositions of samples. As manifested in
the THz signal amplitude increment with reverse bias,
the carrier accumulation at GaN/AlGaN interface is
essential condition for both the acoustic confinement
and THz radiations. Fig. 1(c) shows the simulation
results on the superposition of three multiple harmonic
modes of acoustic standing waves right after the laser
excitation. Each component would decays as it evolves
in time, where higher modes tend to decay faster. In
Fig. 1(d), the different time evolution of the standing
wave components at the p-GaN/AlGAN interface was
integrated in the simulation result (solid line) which
agreed very well with the experimentally measured
THz radiation patterns in time domain (dotted line).
C.-K. Sun et al, Phys. Rev. Lett. 84, 179 (2000).
See, e.g., K.H. Lin et al., Nature Nanotech. 2, 704 (2007).
J.S. Yhang et al, Appl. Phys. Lett. 80,. 4723 (2002).
M.R. Armstrong et al., Nature Phys. 5, 285 (2009).
S. Yamaguchi et al., J. Raman Spectrosc. 39, 1703 (2008).
F. Hudert et al., Phys. Rev. B 79, 201307 (R) (2009).
* e-mail: jho@gist.ac.kr
CONCLUSION
In this work, THz emission measurements were
performed in GaN-based biased heterostructures as a
function of the piezoelectric layer thickness in
nanoscale. We have confirmed that the THz
electromagnetic radiations could be closely associated
with the confined acoustic phonon modes of standing
waves, which possibly finds new was in tuning the
single THz emission frequencies.
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