III-NITRIDE BASED ULTRAVIOLET
SURFACE ACOUSTIC WAVE SENSORS
1,3
Ciplys , A.
D.
1
Sereika ,
1
R.Rimeika ,
2
Gaska ,
3
Shur ,
R.
M.
gaska@s-et.com ;
daumantas.ciplys@ff.vu.lt;
4
Yang ,
J.
M. Asif
shurm@rpi.edu
4
Khan
1Vilnius
University, Physics Faculty, Dept. of Radiophysics, Vilnius, Lithuania
2Sensor Electronic Technology, Inc., Columbia, SC, USA
3Rensselaer Polytechnic Institute, Dept. of Electrical, Computer, and Systems Engineering, Troy, NY, USA
4University of South Carolina, Dept. of Electrical Engineering, Columbia, SC, USA
Introduction
Basic principles
Schematics of the SAW-based UV sensor
Due to a wide energy band gap, AlN, GaN, and their alloys are well
suited for the fabrication of ultraviolet (UV) sensors, particularly, of
visible-blind and solar-blind photodetectors. These materials
possess strong piezoelectric properties making them attractive for
surface acoustic wave (SAW) device applications.
Amplitude condition for oscillations: amplifier gain must exceed
the total insertion loss of the SAW delay line.
ULTRAVIOLET RADIATION
Transmission characteristic of the SAW delay line
SAW DELAY LINE
Input IDT
Output IDT GaN layer
Sapphire substrate
Remote signal
pickup is possible
*J. D. Maines, E. G. S. Paige, A. F. Saunders, A. S. Young, Electron. Lett. 5, 678
(1969).
AMPLIFIER
Making use of the unique combination of wide energy gap and
piezoelectric properties, we were the first to implement the GaNbased surface acoustic wave III-nitride-based SAW oscillator and to
apply it for UV sensing.
SAW oscillator frequency down-shift due to
UV illumination of SAW propagation path
Signal power (dBm)
Frequency (MHz)
Frequency shift is
due to the change of
SAW transducer
parameters by UV
off
Bulk a-AlN
transducer aperture W = 1
mm,
number of transducer
electrode pairs N = 100,
-30
dielectric constant e = 10,
K2 = 0.1 %
-40
source and load resistances
GaN on
sapphire
RL = Rs = 50 Ohm.
off
K2 is the electromechanical
coupling constant.
196 197 198 199
200 201 202 203 204
2 f L V  2 m ,
where L is the distance between IDTs, V is the SAW velocity,  is the phase
shift introduced by the amplifier, cables and transducer circuitry.
Illuminated: area between SAW transducers
on
220.88
K2 = 0.1 %
Phase condition: the phase shift around the loop must be
UV light: from Xenon lamp
220.96
UV
on
Calculation parameters:
Frequency (MHz)
Any change in V or  leads to the change in oscillator frequency f.
0
220.92
K2 = 0.5 %
-20
-60
SAW oscillator frequency up-shift due
to UV illumination of SAW transducers
Illuminated: entire
surface of the
sample, including
transducer area
-10
f0 = 200 MHz
-50
D. Ciplys, R. Rimeika, M. S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J.
Yang, M. Asif Khan, Appl. Phys. Lett. 80, 2020 (2002)
UV light: from
mercury lamp
through 330 nm filter
Transmission (dB)
RF SPECTRUM
ANALYZER
SAW
For sensing purposes, it is very convenient to use the SAW delayline oscillator, which has been first demonstrated in 1969 [*] as
temperature-sensitive device. Since then, various SAW sensors has
been developed but not those for UV.
0
Illuminated
by UV
UV through
nm nm
filter
L330
<= 365
-20
Dark
Frequency shift is due to the change of SAW velocity by UV via screening the
piezoelectric fields by photoconductivity
-40
fUV  f dark
K2
1
L1

,
2
f dark
2 1  Rs Ve  L
-60
where L1 is the length of illuminated region, Rs is the sheet resistivity.
-80
220.84
0
200
400
600
-100
800
221.28
Time (s)
221.30
221.32
221.34
221.36
Possibilities of solar-blind operation
Frequency (MHz)
D. Ciplys, R. Rimeika, A. Sereika, R. Gaska, M. S. Shur, J. W. Yang, and M.
A. Khan, Electron. Lett. 37, 545 (2001).
Separation by wavelength
Rs
Optical wavelength dependence of the frequency down-shift
Predicted optical wavelength cut-off as function of AlGaN composition
Differential SAW oscillator with
improved thermal stability
Schematics
The SAW oscillator
frequency is temperature
dependent.
The temperature drift
can be minimized by
using the differential
scheme
Amplifier 1
Mixer
Spectrum
analyzer
SAW line 2
Amplifier 2
Output signsl (dBm)
GaN on sapphire: -50 to -60 ppm/K
H. H. Jeong et al , Physica Stat. Sol. (a)
188, 247 (2001)
-20
-40
-60
-100
G. Bu, D. Ciplys, M. Shur, L. J. Schowalter, S.
Schujman, R. Gaska , Electron. Lett. (accepted for
publication in 2003)
21
22
23
Frequency (kHz)
380
24
4
Grown by MBE
250
1.08 eV for MBE-grown
layers
Layers grown by MOCVD
U. Ozgur et al, Appl. Phys.
Lett. 79, 4103 (2001)
0.0
0.2
0.4
0.6
0.8
1.0
400
E g  6.13 x  3.42(1  x)  b x1  x 
(eV)
SAW oscillator line widths measured under different
illumination conditions
We attribute the
differences in the line
widths to the different
noise spectra of the
artificial and natural
UV sources.
2
365 nm
1
5
10
15
Optical power (W)
10
mm
1 mm
1.3 mm
Xenon lamp
These differences
might serve for the
development of solarblind UV sensors.
Sun
8
Frequency Jump ( kHz )
x=0.238
M. J. Bergmann et al , Appl.
Phys. Lett. 75, 67 (1999)
200
UV light spot spot
between transducers
0.37 kHz / W
3
0
12
Solar radiation cut-off
300
Molar fraction of Al, x
0
UV-induced frequency down-shift vs. optical power
–0.39 eV for MOCVD-grown
layers
x=0.36
Separation by line width
Feb. 27, 2002
20
360
350
Optical power dependence of the frequency down-shift
-80
Bulk AlN: -19 ppm/K
340
No frequency shift
(with accuracy of 1 %)
was observed at
optical wavelengths
above 400 nm
Calculated using
bowing parameter b:
Optical wavelength (nm)
Device #2
The temperature coefficients of
frequency (TCF):
Band gap width
of GaN 3.4 eV
0.01
320
Output
signal
Differential SAW oscillator output
0
Visible blind
operation
Signal power (arb. units)
SAW line 1
1 nm bandwidth
monochromator
0.1
Frequency shift (kHz)
UV light
Frequency shift (a.u.)
1
Sensor cut-off wavelength (nm)
Wavelength tuning:
Dark
4
UV on L2, Spot Diameter 0.5mm
UV on L1, Spot Diameter 0.5mm
UV on L2, Spot Diameter 7.3mm
UV on L1, Spot Diameter 7.3mm
0
-4
-10
-5
0
5
10
Frequency deviation (kHz)
-8
-12
D. Ciplys, R. Rimeika, M.S. Shur, R. Gaska, A. Sereika, J.Yang, M. Asif Khan,
Electron. Lett. 38, 134 (2002)
365 nm
-16
-20
-24
0
5
10
15
20
Input UV Power ( W )
25
30
Acknowledgments The work at RPI was supported by the National Science Foundation (program monitors Dr. U. Varshney and Dr. James Mink); under a subcontract
from DARPA (Project Manager Dr. Edgar Martinez and monitored by John Blevins at AFRL, contract F33615-02-C-5417). The work at SET, Inc. was partially supported
by the Office of Naval Research and monitored by Dr. Y.-S. Park. The work at SET, Inc. and USC was partially supported by NASA under contract NAG5-10322. Authors
also acknowledge the support by NATO Expert Visit grant .PST.EV.977426.
NATO Advanced Research Workshop UV Solid-State Light Emitters and Detectors June 17-21, 2003 Vilnius, Lithuania