Novel heterostructures for UV-LEDs
and biosensors
Martin Stutzmann
Walter Schottky Institut
Technische Universität München
85748 Garching, Germany
www.wsi.tum.de
Financial support: Deutsche Forschungsgemeinschaft
The people who did the work:
III-Nitrides
Diamond
Electronic Properties
Oliver Ambacher
Jose Garrido
Martin Brandt
Roman Dimitrov
Claudio Miskys
Sebbl Gönnenwein
Martin Eickhoff
Christoph Nebel
Tobias Graf
Martin Herrmann
Roland Zeisel
Andrea Lehner
(now at TU Ilmenau)
Uwe Karrer
Georg Steinhoff
Olaf Weidemann
Outline
• AlGaN UV-B and UV-C applications
• AlGaN/diamond heterostructures
• AlGaN biosensors
1) AlGaN devices for UV-B and UV-C
6.5
PIMBE
Alx Ga 1-x N
6.0
T = 300 K
225 nm: bandgap of diamond
Bandgap E g [eV]
5.5
5.0
254 nm: ozone detection
265 nm: maximum DNA damage
UVC
4.5
UVB
308 nm: OH-emission for flame control
4.0
3.5
UVA
3.0
0.0
0.2
0.4
0.6
Al Content
0.8
1.0
Conventional AlGaN UV detectors
cutoff-characteristics
Better wavelength selectivity: integrated filter layer
Wavelength [nm]
Active
Layer
3.0
400
360
320
280
303 nm
Isolator
2.5
Al0.33Ga0.67N
2.0
Al 0.6Ga 0.4 N
1.5
Al 0.4 Ga 0.6N
c-Al2 O 3 Substrate
FWHM
0.35 eV
]
A
[m
n
re
u
tc
o
h
P
1.0
Filter
0.5
Light
0
3.0
Responsivity 200-500 A/W
3.5
4.0
4.5
Photon Energy [eV]
5.0
Passive UV optics: Bragg reflectors
1.0
Al x Ga 1-x N/Al x Ga 1-x N
1
2
1
2
ao (x 1) - ao(x )
2
ao (x )
1.0
30 Periods
0.01
0.9
1
20 Periods
x2
0.8
60
0.6
x1
0.4
selfabsorption
55
0.7
50
Tickness d n [nm]
Reflectivity Rmax
Al-content x n
0.8
0.6
0.5
45
40
d1
35
d2
30
25
0.2
0.4
20
200
T = 300 K
0
200
250
300
350
400
Wavelength [nm]
450
500
0.3
200
250
250
300 350 400 450
Wavelength λ [nm]
300
350
400
Wavelength λ [nm]
450
500
500
1.0
373 nm
3.324 eV
Laser Structure:
Reflectivity [%]
0.8
Bragg Reflectors
20 Periods
Al0.2Ga 0.8 N/Al0.62 Ga0.38 N
(33 nm/38 nm)
0.6
Active Layer
GaN (150 nm)
0.4
368 nm
3.369 eV
0.2
0
1.5
2.0
2.5
3.0
3.5
Photon Energy [eV]
4.0
4.5
2) AlGaN/diamond heterostructures
Motivation:
Combination of different wide-gap semiconductor
materials to overcome some basic problems:
• efficient n- and p-type doping in the same material
system
• direct versus indirect band structure
• largely different thermal, mechanical, electronic,
etc. properties
Doping:
Diamond: p-type doping with boron easy,
n-type doping with phosphorus, sulfur,
etc. still difficult.
AlGaN:
n-type doping with silicon „easy“,
p-type doping with Zn, Mg, Be, C, ...
possible for GaN, but still very
problematic for AlGaN with increasing
Al content...
First pn-diode for epitaxial diamond:
Phosporus: Ec – Ed = 0.6 eV, Boron: Ea – Ev = 0.36 eV
S. Koizumi et al., Science 292 (2001) 1899
0
300
Si:AlxGa1-xN
T = 300 K
-1
250
10
n-Typ
p-Typ
0.01
10
n-Typ
p-Typ
200
0
150
-1
10
-2
100
10
-200
-3
18
[Si] = 6x10 cm
Korakakis et al.
19
[Si] = 5x10
-4
10
0
0.2
50
-3
cm
0.6
0.4
Al-content x
-3
0
0.8
1.0
200
-2
p-Typ AlGaN
[Mg] 5x10
19
cm
-3
-3
10
GaN
Ea = 170 meV
Al0.12Ga0.88N
Ea = 280 meV
-4
10
100
0
-100
∆T = TB - TA [K]
10
σ [1/Ωcm]
Conductivity σ [1/Ω cm]
thermopower
Al0.27Ga0.73 N
∆U
[mV] 0
1
10
0.02
0.01
Activation Energy E A [meV]
10
2
10
10
Al0.27Ga0.73N
Ea = 360 meV
-5
10
0
1
2
3
4
3
5
6
-1
10 /T [K ]
7
8
9
N-Type Doping of AlGaN with Si:
dark conductivity activation energy
Activation energy (meV)
700
AlGaN
600
500
undoped (residual oxygen)
400
300
200
Si doped
100
0
0,0
0,2
0,4
0,6
Al mole fraction
0,8
1,0
The DX-Model for Silicon in AlGaN
Electronic + lattice energy
2. photoionization
N
EB
ED
N
1
Eo
DX
-
γ-BB
Si
Al
N
Si
thermal activation
E A=320 meV
Qd
Si
Al
1. photoionization
d0+e
Al 2
Al
2
Eo
d+ +2e-
-
α-BB
DX +e
0
QDX
Configuration coordinate Q
substitutional site
(shallow donor)
relaxed site
(DX state)
Al1
Diamond/AlN heterostructure
Growth:
• Substrate: type IIb diamond, (100) orientation, 2 x 2 x 0.5 mm3,
[B] = 1017 cm-3, p(300K) = 8.5 x 1013 cm-3, µ = 850 cm2/Vs
• AlN: 200 – 500 nm by plasma-induced MBE, 815 °C, 0.2 µm/h,
[Si] = 1019 cm-3 (non-optimized doping and growth conditions!)
(100) diamond
(0001) AlN
Contacts:
• AlN: Ti/Al or Ti/Pt/Au, 150 µm diameter
• diamond: Ti/Al, 300 µm diameter, on backside
• reasonable rectification
• ideality factor n = 2 ... 3
• built-in voltage approx. 1V
• large reverse currents =>
interface defects
• high series resistance
• room for improvement!
Improved contacts by Excimer Laser annealing:
(ArF, 193 nm, 2J/cm2)
Current (A)
10
10
10
10
10
-8
-9
Current (A)
10
untreated
after Excimer shot
-10
-11
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
10
-10
10
-11
10
-12
10
-13
10
-14
10
-15
-12
-13
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
untreated
after Excimer shot
after HCl bath
-1,0
-0,5
Voltage (V)
0,0
0,5
1,0
Voltage (V)
•Reduction of series resistance by one order of magnitude!
•Strain cracks for AlN thicker than 200nm!
1,5
2,0
Photocurrent of the p/n-junction:
under focused Deuterium lamp illumination
10
-10
2
P = 0,5 µJ/cm (UV-range up to 385 nm)
20
10
-11
-20
-40
-60
10
-12
0,0
0,2
0,4
Voltage (V)
0,6
0,8
1,0
Phase (deg)
Photocurrent (A)
0
Photocurrent cont.
under focused Excimer laser ilumination
10
2
0.85 J/cm - 500 nm
-6
• maximum open circuit
voltage: 1.1 V
2
2.60 J/cm - 500 nm
2
2.00 J/cm - 250 nm
2
Photocurrent (A)
0.20 J/cm - 250 nm
10
-7
10
-8
• laser light strongly absorbed
in AlN (extinction length 70
nm)
-9
• performance limited by
interface defects...
10
10
-10
-0,5
0,0
0,5
Voltage (V)
1,0
1,5
Two-dimensional carrier systems at hetero-interface?
n-AlGaN, Ga(Al)-Face
n.i.d-AlGaN, N-Face
µe ≈ 4500 cm2/Vs, µh ≈ 3800 cm2/Vs
Electroluminescence:
255 nm
• defect luminescence dominates
• weak band-to-band emission
(self-absorption in sample?)
Subgap absorption of AlN and Diamond
Wavelength (nm)
Without bias light
With bias light (325 nm)
Cross section
Photon Energy (eV)
Future plans:
Diamond/AlGaN/AlN quantum well structures...
Efficient and tunable UV light source ?
Summary :
• Heteroepitaxy of AlGaN on diamond possible, but
still needs a lot of improvement ( substrate orientation,
growth conditions, buffer layers, ...)
• Systematic investigation of epitaxy relation and
interface defect states
• Optimization of doping profiles and contacts
• Promising applications for electronic and
optoelectronic devices on diamond ( UV-LEDs, laser
diodes, HBT-transistors, ...)
3) AlGaN biosensors
Motivation:
• Sensitive electronic detection of cell activity
(e. g. action potentials of nerve cells): living
cells as environmental alarm systems
• Combination with established fluorescence
spectroscopy and microscopy
(=> no adsorption in visible and near UV)
• Possible integration of optical and electronic
functionality, SAWs, MEMS, etc...
Basic Electrostatics:
Is equivalent to
Inversion
Domain
Boundary
Ga-face
++++++++++
---------
N-face
--------++++++++++
c, E
c, E
P
++++++++++
P
Compensating
surface charges
Polarization-induced
fixed charges
---------
Substrate
P = σ = ε0χE
σ−
E = σ/ε0 (ε-1)
Example:
P = 0.05 Cm -2
n = σ/e = 3 x 1013 cm-2
E = P/ε0 (ε-1) = 5 x 106 V/cm = 0.5 V/nm
Ga-face
IDB
N Ga
N
N-face
[0001]
σ+
E
[1210]
Polarization-induced 2-dimensional electron gases
Polar surfaces and charged interfaces
+σ
PPE
AlGaN
-σ
PSP
GaN
-σ
PSP
10
-σ
10
19
10
2DEG
10
15
-σ
13
+σ
Al 2 O 3
GaN
10
4
Ga-face
17
PPE
PSP
3
5
10
21
-σ
+σ
10
10
AlN
PSP
2
2DEG
GaN
10
-σ
1
10
10
Al 2 O 3
PSP
1
10
2DEG
+σ
13
10
AlGaN
Ga
+σ
PSP
15
10
GaN
-
PSP
-σ
17
10
AlGaN
Ga
+
+σ
10
GaN
+
-
1/100 e
for every
atom
N-face
19
PSP + PPE
2DEG
Al
N
Al
21
10
GaN
N
Ga(Al)-face
NCV [cm-3 ]
[0001]
N-face
+σ
2
10
3
depth [Å]
10
4
5
10
Biosensor Applications of AlGaN/GaN Heterostructures
• biocompatible?
• deposition of lipid bilayer
membranes possible ?
• surface preparation ?
-σ
AlGaN
+σ
Ti/Al
lipid bilayer / artificial membrane/cell
• performance and ion sensitivity
in electrolyte solutions
AlGaN/GaN HEMT structure:
ion-sensitive, pH-sensitive ?
2DEG
GaN
AlN
Al2 O3
-σ
+σ
Cell proliferation on GaN:
III-nitrides are naturally biocompatible...
1,40
GaN, N-face, after 24 h
re la t iv e d e n s it y o f a d h e re d F ib ro b la s t s
1,20
1,00
0,80
3h
24h
0,60
0,40
0,20
0,00
GaN Ga-face
oxidised
GaN Ga-face
GaN N-face
oxidised
GaN N-face
AlGaN oxidised
AlGaN
AlN oxidised
AlN
• Adhesion of fibroblasts to III-Nitride and
polystyrene surfaces compared after 3h and
24h
AlN, after 24 h
Deposition of Lipid Bilayers on III-Nitrides
Deposition of lipid bilayer membranes
by vesicle fusion after hydrophilization
of the surface
Electrolyte
Membrane
Electrolyte
III-Nitride
E. Sackmann, Science 271,43-48 (1996)
Small unilamellar Vesicles (diameter < 50nm) adhere at
the surface and rupture due to their high tension
Resulting membrane patches form a continuous lipid
bilayer after 24h
Wetting Properties of Differently Treated
AlxGa1-xN Surfaces
Wetting angle of N-face GaN
90
GaN Ga-face
GaN N-face
AlGaN (x=30%)
AlN
80
stored in air
Y oung Angle [°]
70
60
50
40
30
20
10
0
stored in air
HF (10%)
HCl (37%)
wet thermal
Oxide
RCA Clean
Oxidation leads to hydrophilization
of the surface
after wet thermal oxidation at 800°C
Differently charged lipids
DOTAP:
SOPC:
SOPS:
negatively charged
neutral
O
CH 3
H 3C
N
H 3N
positively charged
CH 3
(CH 2 ) 2
CH 2
O
O
P
O
CH 3
O
H 3C
O
H 2C
O
HC
O
O
C
H 2C
CH 2
O
O
CH
O
H 2C
HC
O
O
C
C
N
O
CH 3
O
O
CH 2
O
P
hydrophilic
headgroup
O
H 2C
HC
O
O
C
C
CH 2
O
hydrophobic
hydrocarbon tail
1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (SOPC)
1,2-Dioleoyl-3-Trimethylammonium-Propane (DOTAP)
1-Stearoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (SOPS)
Diffusion Constant Measurement by Photobleaching
Continuous Bleaching
2500
Intensity [a.u.]
2000
1500
1000
0
50
100
x-Position [µm]
150
200
Intensity Profile
Substrate
AlN
SOPC
(0)
DOTAB
(+)
SOPS
(-)
750
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
800
3.9
9.4
1.1
Diffusion constants [µm²/s]
Intensity [a.u.]
Lipid
Intensity Profile
Exponential Fit
700
650
600
550
500
0
50
100
x-Position [µm]
Decay Length
λ=
150
D
B0
(Bleaching Rate B0)
C. Dietrich, R. Merkel, R. Tampe, Biophys. Journal, Vol. 72,1701-1710 (1997)
200
Ion-induced modulation of the channel current
in GaN/AlGaN/GaN HEMTs
14
10
- -flux
+ -flux
-
10 1
as grown
13
10
100
current density [mA/cm]
sheet carrier concentration [cm -2 ]
Non passivated gate area
+
12
10
11
10
-1
10
-2
10
- -flux
-3
10
10
10
-4
10
9
10
0
100
200 300
time [s]
400
500
-30
-20
-10
0
10
voltage [V]
Modulation of ion concentration at the surface
=> modulation of the carrier concentration in the 2DEG
20
30
Pt:GaN Schottky diodes as chemical sensors: hydrogen detection
ohmic contact
(Ti/Al/Ti/Au)
Pt/Pd-Schottky-contact
Depth Distribution of D and O after
storage in 60mbar D2 (from high
energy elastic recoil detection):
GaN (2µm), N e~10 18cm -3
AlN (5nm)
Al2O 3 substrate
15
14
2.0x10
2.0x10
D-content
O-content
15
0
4% O2
1000ppm H
2 in 4% O
2
1% H2
-4
14
1.5x10
2
15
1.0x10
14
14
5.0x10
1.0x10
13
5.0x10
T = 400°C
-8
-1.0
1.5x10
-0.5
0.0
0.5
voltage [V]
1.0
O-content [At/cm ]
2
4
D-content [At/cm ]
current [mA]
8
0.0
0
10
20
depth [nm]
30
0.0
Oxide-free interface:
in-situ deposition of Pd
in-situ Schottky diodes:
• Higher reverse current
• Lower barrier (in-situ:
0.7eV, ex-situ: 0.9eV)
• Approx. same ideality factor
10
2
Stromdichte [A/cm ]
Pd in-situ deposition
• 10-9 mbar base pressure
• 1260 °C, 60 min, ~ 15 nm
Pd on GaN
0
-2
10
-4
10
-6
10
-8
10
in-situ
ex-situ
-10
10
-1,0
-0,5
0,0
Spannung [V]
0,5
1,0
2
Stromdichte [A/cm ]
1,0
0,5
0,0
0,0
0,2
2.
At
m
os
ph
ph
At
mo
s
Spannung [V]
e
är e
är
(+
90
560 mV
0,6
0,8
1. e v a
At ku
mo ie r
sp t
hä
re
6.
0,4
5.
3.
10
mb
ar
4.
ev
H
ak
u ie 2
rt
n)
1,5
mi
Ex-situ Schottky diode: normal hydrogen response
2,0
1,0
In-situ Schottky diode: no hydrogen response!
2,0
1,0
1,71
t
ui
er
e
2
1 . v ak
At uie
m
o s rt
2.
ev p h ä
ak
re
1,74
4.
1,77
m
ba
rH
1,5
15 mV
3.
10
2
Stromdichte [A/cm ]
1,80
0,79 0,80 0,81 0,82
0,5
0,0
0,0
0,2
0,4
0,6
Spannung [V]
0,8
1,0
Surface oxide necessary for hydrogen detection
Sensitivity to ions in electrolytes:
GaN as a pH sensor
PTFE stirer
Ag/AgCl reference
Potentiostat
Pt 100 thermoelement
-
+
pH electrode
Pt counter
electrode
pH-meter
100mM NaCl
10mM Hepes-Buffer
VG
GPIB Interface
PC
-
+
Keithley 2400
Sample Structures
Al/Ti contacts
silicon-glue
GaXOY (thermal oxide)
60 nm GaN:Si
60 nm GaN:Si
1500 nm GaN:Mg
1500 nm GaN:Mg
3nm GaN-cap
35nm AlGaN-barrier
1500 nm GaN
AlN nucleation layer
sapphire substrate
sample A
sapphire substrate
sapphire substrate
sample B
• samples grown by PIMBE
• gate area: 1mm x 0.5 mm
sample C
0.5mm
1mm
Performance of a GaN/AlGaN/GaN ISFET
122.0
120.0
ISD [µA]
118.0
116.0
4.12
4.25
4.36
3.46
3.56
3.66
4.00
6.00
118.3
7.04
GaN-cap
AlGaN-barrier
7.23
510
7.42
8.01
1500 nm GaN
110.0
2.86
< 0.02 pH
118.4
6.11
6.27
AlN nucleation layer
3.13 2.98
118.6
118.5Resolution
114.0
112.0
3.20
3.30
sample C
Time [s]
540
100mM NaCl & 10mM HEPES
VSD=250mV VG = 0V
8.15
8.41
sapphire substrate
400
600
800
1000
time [s]
Ion sensitivity around pH 7: one charge per 100 nm2 !
Corresponding change of surface potential
600
400
∆Φ [mV]
200
0
-200
GaN/AlGaN/GaN: 53.2 mV/pH
(GaN:Si)Ox/GaN:Mg: 56.1 mV/pH
GaN:Si/GaN:Mg: 56.8 mV/pH
-400
1
2
3
4
5
6
7
8
9
10
11
12
13
pH
• High pH-sensitivity of 53 mV/pH to 57 mV/pH
(SiO2: 32-42 mV/pH, Al2O3: 52-57 mV/pH, Ta2O5: 55-57 mV/pH,
Nernstian response limit: 59 mV/pH)
• Non-oxidized and oxidized surfaces show similar sensitivity
=> Natural oxide sufficient
Electrolyte/Oxide Interface - The Gouy-Chapman-Stern Theory
M
M
OH2+
Cl -
OH
• Perfectly polarizable electrode
• No in-diffusion of ions
M
O-
Na +
M
OM
M
OH2+
Cl -
OH
M
M
OH2+
O-
Inner Helmholtz Plane (IHP):
• amphoteric hydroxyl groups
• specifically adsorbed counter ions at ionized
surface sites (e.g. Na+, Cl-)
Na +
M
OH
M
M
OH2+
Ψ 0 σ0
Ψβ
IHP
OHP
Outer Helmholtz Plane (OHP):
• plane of closest approach for hydrated ions
(not specifically adsorbed ions)
σβ
σd
Ψd
Gouy-Chapman
Layer
CStern
CGC
Gouy-Chapman-Layer:
• diffuse charge region from the OHP to the bulk
electrolyte
• described by Poisson-Boltzmann equation
Site-Binding Model for Oxide Surfaces *
M
OH
H+
M – OH + H+
M
M
M
O H 2+
OH
OH
M
M–
-
OH
M
OH
M
O-
+
H+
1/Kb
1/Ka
M – OH2++
M – OH+
H+
NS = Σ[OH] + Σ[OH2+] + Σ[O-]
QS(pH) = Σ[OH2+] – Σ[O-]
solid / liquid
6
D. E. Yates, S. Levine, T. W. Healy, J. Chem. Soc. Faraday Trans. I, 70, 1807 (1974)
O1s
Counts [a.u.]
Surface oxidation:
XPS analysis
as deposited
native Oxide
thermal Oxide @ 600°C
thermal Oxide @ 700°C
thermal Oxide @ 800°C
536
534
532
Binding Energy [eV]
530
Effect of thermal oxidation on carrier density
in HEMTs
dry oxide
wet oxide
1.0
0.8
nS/n0
0.6
0.4
0.2
0.0
no oxide 600
625
650
675
oxidation temperature [°C]
700
Biosensor concepts with AlGaN/GaN ISFETs...
lipid bilayer
ion channel
K+
K+
K+
K+
K+
Unique combination of:
K+
K+
AlGaN
2DEG
GaN
sapphire substrate
Fluorescence detection
• ion sensitivity
• chemical inertness
• biocompatibility
• optical transparency
• possibility to integrate
other functionality
The interface of GaN or AlGaN with carbon is
interesting:
Dead....
Diamond/AlGaN heterostructures for
UV-LEDs
... or alive!
GaN ChemFETs as biosensors
K+
K+
K+
K+
K+
K+
K+