III-NITRIDE BASED ULTRAVIOLET SURFACE ACOUSTIC WAVE SENSORS

Introduction

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.

For sensing purposes, it is very convenient to use the SAW delay-line 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.

*J. D. Maines, E. G. S. Paige, A. F. Saunders, A. S. Young, Electron. Lett. 5, 678 (1969).

Making use of the unique combination of wide energy gap and piezoelectric properties, we were the first to implement the GaN-based surface acoustic wave III-nitride-based SAW oscillator and to apply it for UV sensing.

D. Ciplys, R. Rimeika, M. S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J. Yang, M. Asif Khan, Appl. Phys. Lett. 80, 2020 (2002)

Sapphire substrate

ULTRAVIOLET RADIATION

GaN layer

AMPLIFIER

SAW DELAY LINE

SAW

Input IDTOutput IDTRF SPECTRUM

ANALYZER

Schematics of the SAW-based UV sensor

Remote signal pickup is possible

Transmission characteristic of the SAW delay line

f0 = 200 MHz

K2 = 0.1 %

196 197 198 199 200 201 202 203 204-60

-50

-40

-30

-20

-10

0

Frequency (MHz)

Tra

nsm

issi

on

(dB

)

K2 = 0.1 %

GaN on sapphire

K2 = 0.5 %

Bulk a-AlN

Basic principles

Amplitude condition for oscillations: amplifier gain must exceed the total insertion loss of the SAW delay line.

,22 mVLf

Phase condition: the phase shift around the loop must be

Calculation parameters:

transducer aperture W = 1 mm,

number of transducer electrode pairs N = 100,

dielectric constant = 10,

source and load resistances

RL = Rs = 50 Ohm.

K2 is the electromechanical coupling constant.

where L is the distance between IDTs, V is the SAW velocity, is the phase shift introduced by the amplifier, cables and transducer circuitry.

Any change in V or leads to the change in oscillator frequency f.

SAW oscillator frequency up-shift due to UV illumination of SAW transducers

UV light: from mercury lamp through 330 nm filter

Illuminated: entire surface of the sample, including transducer area

Frequency shift is due to the change of SAW transducer parameters by UV 0 200 400 600 800

220.84

220.88

220.92

220.96

off

on

off

UV on

Fre

quen

cy (

MH

z)

Time (s)

D. Ciplys, R. Rimeika, A. Sereika, R. Gaska, M. S. Shur, J. W. Yang, and M. A. Khan, Electron. Lett. 37, 545 (2001).

Possibilities of solar-blind operation

-10 -5 0 5 10

Xenon lamp

Sun

Dark

Sig

na

l po

we

r (a

rb. u

nits

)

Frequency deviation (kHz)

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.

These differences might serve for the development of solar-blind UV sensors.

Predicted optical wavelength cut-off as function of AlGaN composition

Calculated using  bowing parameter b:

–0.39 eV for MOCVD-grown layers M. J. Bergmann et al , Appl. Phys. Lett. 75, 67 (1999)

1.08 eV for MBE-grown layers U. Ozgur et al, Appl. Phys. Lett. 79, 4103 (2001)

0.0 0.2 0.4 0.6 0.8 1.0

200

250

300

350

x=0.238

x=0.36Solar radiation cut-off

Layers grown by MOCVD

Grown by MBE

Sen

sor

cut-

off

wav

elen

gth

(n

m)

Molar fraction of Al, x

xxbxxEg 1)1(42.313.6 (eV)

Separation by wavelength

Separation by line width

D. Ciplys, R. Rimeika, M.S. Shur, R. Gaska, A. Sereika, J.Yang, M. Asif Khan, Electron. Lett. 38, 134 (2002)

0 5 10 15 20 25 30-24

-20

-16

-12

-8

-4

0

4

8

12

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

Fre

que

ncy

Jum

p (

kHz

)

Input UV Power ( W )

SAW line 1

Amplifier 1

UV light

Spectrum analyzer

Mixer

SAW line 2

Amplifier 2

Differential SAW oscillator with improved thermal stability

UV-induced frequency down-shift vs. optical power

20 21 22 23 24-100

-80

-60

-40

-20

0

Device #2

Differential SAW oscillator output

Feb. 27, 2002

Out

put s

igns

l (dB

m)

Frequency (kHz)

Output signal

Schematics

365 nm

The SAW oscillator frequency is temperature dependent.

The temperature drift can be minimized by using the differential scheme

The temperature coefficients of frequency (TCF):

GaN on sapphire: -50 to -60 ppm/K H. H. Jeong et al , Physica Stat. Sol. (a) 188, 247 (2001)

Bulk AlN: -19 ppm/K G. Bu, D. Ciplys, M. Shur, L. J. Schowalter, S. Schujman, R. Gaska , Electron. Lett. (accepted for publication in 2003)

320 340 360 380 4000.01

0.1

1

Fre

que

ncy

shift

(a

.u.)

Optical wavelength (nm)

1 mm10 mm

1.3 mm0 5 10 150

1

2

3

4

365 nm

0.37 kHz / W

Freq

uenc

y sh

ift (k

Hz)

Optical power (W)

sR

Band gap width of GaN 3.4 eV

Visible blind operation

No frequency shift (with accuracy of 1 %) was observed at optical wavelengths above 400 nm

SAW oscillator frequency down-shift due to UV illumination of SAW propagation path

221.28 221.30 221.32 221.34 221.36-100

-80

-60

-40

-20

0

DarkIlluminated by UV <= 365 nm

Sig

nal

po

wer

(d

Bm

)

Frequency (MHz)

UV light: from Xenon lamp

Illuminated: area between SAW transducers

UV through 330 nm filter

Wavelength tuning:

1 nm bandwidth

monochromator

Optical wavelength dependence of the frequency down-shift

Optical power dependence of the frequency down-shift

UV light spot spot between transducers

,

1

1

21

2

2

L

L

VR

K

f

ff

sdark

darkUV

Frequency shift is due to the change of SAW velocity by UV via screening the piezoelectric fields by photoconductivity

where L1 is the length of illuminated region, Rs is the sheet resistivity.

NATO Advanced Research Workshop UV Solid-State Light Emitters and Detectors June 17-21, 2003 Vilnius, Lithuania

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.

D. Ciplys1,3, A. Sereika1, R.Rimeika1, R. Gaska2, M. Shur3, J. Yang4, M. Asif Khan4

daumantas.ciplys@ff.vu.lt; gaska@s-et.com ; shurm@rpi.edu 1Vilnius University, Physics Faculty, Dept. of Radiophysics, Vilnius, Lithuania

2Sensor Electronic Technology, Inc., Columbia, SC, USA3Rensselaer Polytechnic Institute, Dept. of Electrical, Computer, and Systems Engineering, Troy, NY, USA

4University of South Carolina, Dept. of Electrical Engineering, Columbia, SC, USA