GaN Based Chemical Sensors
B. Baur, O. Weidemann, G.Steinhoff, J. Schalwig1, M. Hermann, G. Müller1, M. Stutzmann and M. Eickhoff
Walter Schottky Institut, Technische Universität München, Am Coulombwall 3
D-85748 Garching, Germany
1EADS-Deutschland GmbH, D-81663 Munich, Germany
Chemical Sensors
Device Improvement by Thermal Oxidation
• Pt, Pd:GaN Schottky diodes
• Pt:AlGaN/GaN GasFETs
• temperature dependence, detection mechanism, improvement of stability
GaN und AlGaN/GaN Ion Sensitive Field Effect Transistors (ISFET)
Pt
Oxide
1
10
• Wet and dry thermal
oxidation for 2 h at
700 °C
0
10
-1
10
-2
10
-3
1x10
-4
1x10
-5
10
-6
10
-7
10
-8
10
-9
2
• pH-sensitivity
• detection mechanism: site-binding model
Outlook: Biosensors
• Reduction of reverse
current by more than
3 orders of magnitude
•
•
• Enhancement of the
barrier height from
0.5 eV to 0.8 eV (dry
oxidation) and 0.9 eV
(wet oxidation)
diffusion of phospholipids on AlGaN-surfaces
biocompatibility
current density [A/cm ]
10
GaN
without oxidation
wet, 700°C, 2h
dry, 700°C, 2h
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
TEM micrograph
showing enhanced
oxidation at dislocations.
voltage [V]
Pt:GaN Schottky diodes on oxidized and non
oxidized samples.
Gas Sensitivity of Pt:GaN and Pd:GaN Schottky Diodes
-3
10
0
10
-5
10
-9
1% H2 in N2 on
-6
10
-11
0.5
1.0
1.5
2.0
0
2.5
120
240
360
350
H2 2000ppm
C
D
F
G
NO2 1000ppm
300
200
300
400
Temperature [°C]
-100
62.5 ppm
120
GaN-cap
AlGaN-barrier
1500 nm GaN:Mg
500
AlN nucleation layer
sapphire substrate
sapphire substrate
A
B
150
100
600
50
0
200 °C
0 100 200 300 400 500
[H2]/ppm
100
200
300
400
500
reference electrode (Ag/AgCl)
600
Sensitivity of a Pt:GaN Schottky diode upon
exposure to different test gases as a function
of temperature.
VDS
118.0
100 mM NaCl
10 mM HepesBuffer
drain
A+
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.0
0.0
1.0
0.1
0.2
0.3
2.
ev
ac
ua
1.
t
in
atm ed
os
ph
ere
er
e
os
ph
atm
0.4
-3
4,6x10
-3
4,5x10
-3
4,4x10
-3
4,3x10
-3
4,2x10
-3
4,1x10
-3
2
4,7x10
current density @ +0.2 V [A/cm ]
In-situ diodes show 50
times lower H2 sensitivity
compared to ex-situ diodes
in-situ
-3
0
20
40
60
0.5
0.6
0.7
0.8
0.9
High pH-response of 53-57 mV/pH (A,B) and 53 mV/pH (C)
•
For chemically equivalent surfaces the sensitivity of the detection
mechanism (∆Φ) is independent from the device structure
1.0
80
100
-1
2,0x10
-1
1,5x10
-1
1,0x10
-1
5,0x10
-2
120
•
7.23
510
Time [s]
540
7.42
8.15
8.41
600
800
1000
400
200
Thermally oxidized surface and native surface oxide result in the same
sensitivity
•
ex-situ
0
-200
sample C: 53.19 mV/pH
sample B: 56.05 mV/pH
sample A: 55.10 mV/pH
pH-sensitivity can be explained by site-binding model
•
Device sensitivity (∆I) strongly depends on device structure
•
pH-resolution: < 0.02 pH
•
Device drift 8µ10-3 pH/h at pH 5
-400
1
2
3
4
5
6
7
8
9
10 11 12 13
pH
pH-induced change of the gate potential in
a 100 mM NaCl solution, 10 mM HEPES.
G. Steinhoff, M. Hermann, W. J. Schaff, L. F. Eastman, M. Stutzmann, M. Eickhoff, Appl. Phys. Lett. 83, (2003) 177
Outlook: Biosensors
10
I
5
0
0
15
30
time [min]
45
60
75
•
time [min]
Transient response of a Pd:GaN Schottky diode
to H2 pulses of 1 and 10 mbar.
118.3
7.04
600
pH-sensitivity is a pure surface effect
0,0
5
0
-1
2,5x10
p(H2) [mbar]
p(H2) [mbar]
4,0x10
10
3,0x10
118.4
6.11
6.27
time [s]
(SiO2: 32 mV/pH, Al2O3: 53-57 mV/pH, Ta2O5: 55-59 mV/pH)
Shift of the room temperature I-V curves of
an ex-situ deposited Pd:GaN Schottky
diode upon exposure to molecular
hydrogen at a partial pressure of 10 mbar
H2 in vacuum.
Shift of the room temperature I-V curves of an
in-situ deposited Pd:GaN Schottky diode upon
exposure to molecular hydrogen at a partial
pressure of 10 mbar H2 in vacuum.
118.5
6.00
400
•
bias [V]
bias [V]
Comparison of in-situ and
ex-situ deposited contacts
on MBE-grown GaN
Intermediate oxide
provides adsorption sites
for atomic hydrogen
in
0.5
5.
2
3.
10
m
ba
4
rH
1 . . re 2
in ev
at ac
2. mos uat
e
ev
p
ac her d
ua
e
te
d
0.82
560 mV
in-situ
2
•
0.81
0.5
0.0
0.0
current density @ +0.2 V [A/cm ]
•
0.80
2.98 2.86
pH-dependence of the channel current in a
GaN/AlGaN/GaN HEMT-structure (sample
C), USD = 250 mV .
0m
ba
rH
4.
re2
ev
ac
ua
ted
Influence of interfacial
oxide layer?
0.79
Transient response of a Pd:GaN Schottky
diode to H2 pulses of 1 mbar.
High chemical stability of AlGaN surfaces in aqueous
solutions (native metal oxide)
•
t
A+
•
Transparent electronic devices for simultaneous
electronical and optical analysis
•
Control of surface wetting behaviour
A+
B+
High ion sensitivity
O. Weidemann, M. Hermann, G. Steinhoff, H. Wingbrant, A. Lloyd Spetz, M. Stutzmann and M. Eickhoff, Appl. Phys. Lett. 83, (2003), in press
B+
lipid bilayer
B+
ion channel
A+
B+
AlGaN
Gas Sensitivity of AlGaN/GaN FETs
GaN
2DEG
Device cross section
400
UGate= 0V
T = 300°C
4
UGate= -2V
3
2
UGate= -4V
1
sensor signal [µA]
channel current [mA]
2DEG
200
0
-100
4
6
8
10
12
14
16
0,00
Voltage USD [V]
Current-voltage characteristics of a Pt:Ga-faceGaN/AlGaN HEMT in a gas ambient of 4% O2
and of 1% H2 diluted in 4% O2.
0,05
0,10
0,15
0,20
0,25
concentration [%]
Relative change in the source-drain current upon
exposure to reducing and oxidizing gases.
• High sensitivity towards hydrogen
0,30
SOPC
DOTAB
DOTAP
SOPS
sapphire substrate
0.8
0.6
0.4
optical analysis
0.2
0.0
(0)
(+)
(-)
AlN
4.1
9.5
not
measurable
AlGaN
2.9
15.0
0.2
GaN Ga-face
3.1
7.5
not
measurable
GaN N-face
3.9
9.4
1.1
Substrate
100
0
2
Lipid
400°C in 4% O2
H2
Acetylene
F
H
NO2
300
-1
0
Diffusion Constant [µm²/s]:
500
4% O2
1% H2 in 4% O2
5
optical density
1.0
6
300 400 500 600 700 800 900 1000
λ [nm]
Cell adhesion and survival on AlGaN-surfaces:
Diffusion constants of lipides with differently charged headgroup
•
•
High diffusion constants for lipids with positively
charged headgroups
Low influence of the different surfaces
after 3h
after 24h
Fibroblasts (3T3-cells) on thermally oxidized AlN
•
No difference in adhesion of fibroblasts on
untreated and thermally oxidized surfaces
•
No difference in adhesion on GaN, AlGaN, AlN
• Cross sensitivity determined by porosity of gate metal
• Limited lifetime above 400 °C
III-Nitrides are naturally biocompatible
J. Schalwig, G. Müller, M. Eickhoff, O. Ambacher and M. Stutzmann, Sensors and Actuators B 87, (2002) 425
0.6
118.6
114.0
source
1.5
1.0
4.00
8.01
A+
3.
1
•
1.71
0.4
3.203.13
3.30
110.0
ex-situ
current density [A/cm ]
Chemical nature of
adsorption sites?
current density [A/cm ]
•
1.74
1.0
116.0
4.12
4.25
4.36
112.0
A+
2.0
15 mV
0.2
3.46
3.56
3.66
A+
Ion sensitive gate
1.77
122.0
120.0
Detection Mechanism – Hydrogen Adsorption at the Interfacial Oxide of Pd:GaN Schottky Diodes
1.80
0.0
potentiostat
Pt counter electrode
A+
1.5
-0.2
VG
J. Schalwig, G. Müller, U. Karrer, O. Ambacher, M. Eickhoff, M. Stutzmann, L. Görgens, G. Dollinger, Appl. Phys. Lett. 80, (2002) 1222
Adsorption of dissociated
hydrogen at Pd/GaN
interface results in
decrease of Schottky
barrier height.
-0.4
Dependence of the channel current on the
electrolyte gate.
VGS
Transient response of Pd:GaN Schottky
diodes to hydrogen pulses of 62.5 ppm,
125 ppm, 187.5 ppm, 250 ppm, 375 ppm and
500 ppm in 4 % oxygen at atmospheric
pressure, measured at a constant forward
current of 75 µA.
2.0
B
-0.6
0
0
A
15
Schematics of the investigated transistor structures
50
20
sapphire substrate
C
time [s]
•
100
1500 nm GaN
200 °C
-200
• Immediate desorption
above 200 °C
(fast response)
C
2nm GaxOy (thermal oxidation)
60 nm GaN:Si
200
100
0
140
Al/Ti contacts
500 ppm
∆V [mV]
∆V [mV]
100
(1 µm x 1 µm).
Ion-Sensitive GaN and AlGaN/GaN Field Effect Transistors (ISFETs) in Electrolyte Solutions
840
250
200
• Hydrogen storage at
electronically active sites at
low temperatures
(long recovery time)
100 °C
300
voltage [mV]
400
720
silicon-glue
600
• Decrease of Schottky
barrier height
600
Transient behaviour of the response to H2 at
different temperatures.
Response of a Ga-face Pt:GaN Schottky diodes
to exposure to 1% H2 in 4% O2
500
480
time [min]
voltage [V]
• Dipole adsorption of atomic
hydrogen at metal/GaN
interface
800 °C
RT
150°C
300°C
T = RT
0.0
700 °C
as grown
-4
10
ISD [µA]
• Dissociation of molecular
hydrogen at catalytic
Schottky contact
4% O2
∆Φ [mV]
10
4% O2
1% H2
AFM micrograph of as
grown and dry oxidized
GaN surfaces
-2
10
ISD [µA]
-7
10
Device cross section
Voltage shift of I-V
characteristics
current [mA]
current [A]
10
G. Steinhoff, O. Purrucker, M. Tanaka, M. Stutzmann, M. Eickhoff, Advanced Functional Materials (2003), in press