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SDM-2, ©Michael Shur 1999-2009
Pyroelectric and Piezoelectric Properties of GaN-based Materials and Devices
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SDM-2, ©Michael Shur 1999-2009
Outline
Pyroelectric effect and pyroelectric sensorsPiezoelectric properties and piezoelectric
sensorsPolarization doping in GaN-based
heterostructuresStrain energy band engineeringConclusions
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SDM-2, ©Michael Shur 1999-2009
It is complex!
“The Universe is full of magical things patiently waiting for our wits to grow sharper”
Eden Phillpotts
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Wide Gap SemiconductorsEnabling Technology for Piezo-electronics
Zinc blende structureNo PE or SP effect in <100 > directionPE coefficient <111> ~0.15 C/m2
No PE/SP ‘doping’ reported
• Wurtzite structure• c-axis growth direction • PE coeficient in c-direction ~1
C/m2
• PE/SP ‘doping’ demonstrated
GaAs GaN/AlN
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Ga and N faces
G a ll i u m f a ce
Ni tr o g e n f a c e
[000 1][0001]
After Hellman, E. S., The Polarity of GaN: a Critical Review. MRS Internet J. Nitride Semicond. Res. 3, 11(1998)
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Polarization sign
GaN
GaN
GaN
AlGaN
AlGaN
AlGaN
Relaxed
Relaxed
TensileStrain
CompressiveStrain
Relaxed
Relaxed
GaN
GaN
GaN
AlGaN
AlGaN
AlGaN
Psp
Psp
Psp
Psp
Ppe
Ppe
Psp
Psp
Ppe
Ppe
Psp
Psp
PspPsp
+σ
+σ
-σ +σ
-σ
-σ
[0001] [0001]
Ga face N face
After O. Ambacher et al. J.Appl. Phys. 85, 3222 (1999)
2D electrons
2D holes
2D holes
2D electrons
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Spontaneous polarization
For comparison, in BaTiO3, Ps = 0.25 C/m2
-0.045-0.057-0.032-0.029-0.081Spontaneous polarization(C/m2)
BeOZnOInNGaNAlNAfter F. Bernardini et al. Phys Rev. 56, 10024(1997)
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Equivalent circuit of pyroelectric sensor
IpPyroelectr icm aterial
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Primary and secondary pyroelectric effects
b
-100-50
050
-4 0 4 8 12 16 20
Time, s
-1
0
1
2
3
-4 0 4 8 12 16 20
0
-40
-80
a
c
-0.2
0.1
0.4
0.7
1
0 2 4 6 8 10 12Time, s
0
50
1000 2 4 6 8 10 120 2 4 6 8 10 12
50
0
Slow heat transferFast heat transfer
(after A. D. Bykhovski, V. V. Kaminski, M. S. Shur, Q. C. Chen, and M. A. Khan "Pyroelectricity in gallium nitride thin films", Appl. Phys. Lett., 69, 3254 (1996)).
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Pyroelectric voltage for primarypyroelectric effect (changing flux magnitude)
From A. D. Bykhovski, V. V. Kaminski, M. S. Shur, Q. C. Chen, and M. A. Khan "Pyroelectricity in gallium nitride thin films", Appl. Phys. Lett., 69, 3254 (1996)
-1
0
1
2
3
4
5
6
7
8
9
-0.5 0 0.5 1 1.5 2 2.5 3Time (s)
Heat Flux Counter!
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Pyroelectric voltage for primarypyroelectric effect (changing flux magnitude
and direction)
0 20
0
0.4 (c )
0
20
(a)
-20
0
(b)
Bat
h te
mp
erat
ure
(oC
)H
eat
flux
(a
.u)
Tim e (s )10 30 40
-0.4Vo
ltage
(m
V)
20
40
(after A. D. Bykhovski, V. V. Kaminski, M. S. Shur, Q. C. Chen, and M. A. Khan "Pyroelectricity in gallium nitride thin films", Appl. Phys. Lett., 69, 3254 (1996)).
Heat Flux Counter!
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Two time constants for primarypyroelectric effect
b
-100-50
050
-4 0 4 8 12 16 20
Time, s
-1
0
1
2
3
-4 0 4 8 12 16 20
0
-40
-80
a
c
τT reflects the rate of the sample cooling due to free convection
τs reflects the pyroelectric charge relaxation.
(after A. D. Bykhovski, V. V. Kaminski, M. S. Shur, Q. C. Chen, and M. A. Khan "Pyroelectricity in gallium nitride thin films", Appl. Phys. Lett., 69, 3254 (1996)).
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Primary pyroelectric effect at 300 oC(High Temperature Operation!)
0
0
4
8
b .
0
a .Ga N
T(o
C)
300
200
100
12
Vol
tag
e (m
V)
10 20 30 40
Tim e (s )
Constant temperature of “cold”contact (94 oC) is shown by dotted line.
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Pyroelectric coefficients
From M. S. Shur, A. D. Bykhovski , R. Gaska, and M. A. Khan, GaN-based Pyroelectronics and Piezoelectronics,to be published
LiTaO3
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
PbTiO3PZTPVDFBaTiO2TGSGaN
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Curie temperature
0
500
1000
1500
2000
0
500
1000
1500
2000
LiTaO3 PbTiO3PZTPVDFBaTiO2TGSGaN
From M. S. Shur, A. D. Bykhovski, R. Gaska, and A. Khan, GaN-based Pyroelectronics and Piezoelectronics,in Handbook of Thin Film Devices, Volume 1: Hetero-structures for High Performance Devices, Edited by Colin E.C. Wood, Handbook edited by Maurice H. Francombe, pp. 299-339, Academic Press, San Diego, 2000
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Piezoelectric Properties of GaN
.
1.00.80.60.40.2-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Ionicity
ZnSe
ZnTe CdTeInPInAs
AlSb InSb
GaAsAlAs
GaSb
GaN
0
e 14
( C/ m
2 )
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Piezoelectric constantselm (C/m2) e33 e31 e15 e14GaN(electromechanicalcoefficients)
1 -0.36 -0.3
GaN(mobility)
0.44 -0.22 -0.22 0.375
GaN (fromoptical phonons)
0.65 -0.33 -0.33 0.56
GaN (ab initio) 0.73 -0.49InN (from opticalphonons)
0.43 -0.22 -0.22 0.37
InN (ab initio) 0.97 -0.57AlN (surfaceacoustic waves)
1.55 -0.58 -0.48
AlN (ab initio) 1.46 -0.60SiC 0.2 0.08ZnO 1.32 -0.57 -0.48GaAs -0.185 0.093 0.093 -0.16
From M. S. Shur, A. D. Bykhovski, R. Gaska, and A. Khan, GaN-based Pyroelectronics and Piezoelectronics,in Handbook of Thin Film Devices, Volume 1: Hetero-structures for High Performance Devices, Edited by Colin E.C. Wood, Handbook edited by Maurice H. Francombe, pp. 299-339, Academic Press, San Diego, 2000
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How Piezoelectric constants are determined?
Electromechanical coefficients (difficult)Optical experiments (indirect)Estimate from transport measurements (very
indirect)
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Strain from the lattice mismatch
uxx = (aGaN -aAlGaN)/aGaNStrain with metal - to be analyzed?Thermal expansion - important for sapphire
substrate, relatively unimportant for AlGaN/GaN interfaces
Ppe = 2 uxx (e31-e33 C13/C33)
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Effect of Strain on Dislocation-Free Growth
Critical thickness as a function of Al mole fraction in AlxGa1-xN/GaN: superlattice (solid line), SIS structure (dashed line).
From A. D. Bykhovski, B. L. Gelmont, and M. S. Shur, J. Appl. Phys. 81 (9), 6332-6338 (1997)
0
10
20
30
40
50
60
0 0.2 0.4 0.6 0.8 1
Al Mole Fraction
Thickness, nm
Can it be exceeded?
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• High Al- Heterostructures• Origin of Doping?
i-GaN
Al0.25Ga0.75N
AlN
SiC100 200 300 400 500 600
1,000
2,000
3,000
300K
77K
Hal
l Mob
ility
(cm
2 /Vs)
AlGaN Thickness (A)
Critical Thickness dcr ~ 30 nm
Effect of Critical Thickness on Electron Mobility
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Model Assumptions
Isotropic approximationAveraged (over the layers) elastic constantsIdeal crystal (except, of course, for misfit
dislocations)The critical thickness is found by minimizing
the deformation energy with respect to the misfit dislocation density
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SIS sensor
Piezoelectric sensorsPyroelectric sensors
Sapphire
n- GaNAlNn- GaN
Contacts
After R. Gaska, J. Yang, A. D. Bykhovski, M. S. Shur, V. V. Kaminski, S. M. Soloviev,
Appl. Phys. Lett. 71(26), 3817 (1997)
c
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SIS structure and Band Diagram
n - GaN 0.4 μm
AlN 30 Å - 100 Å
n - GaN 1.0 μm
i - GaN 1.0 μm
AlN Buffer
Sapphire
Top ContactBottomContact
GaN n
F
(1) (2) (3)
GaN n
Ef
Ev
Ec
L
AlNu
B A B A B A
From A. Bykhovski, B. Gelmont, M. S. Shur, and A. Khan. Strain and Charge Distribution in GaN-AlN-GaN SIS Structure,in: Proceedings of International Conference on Silicon Carbide and Related Materials-1993, Washington DC,
Institute of Physics Publishing, Institute of Physics Conference Series Number 137, 1994, p. 691- 694.
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SIS capacitance
From A. Bykhovski, B. Gelmont, and M. S. Shur, J. Appl. Phys. 74(11), 6734 (1993).
0
1
2
3
4
5
6
-2 -1 0 1 2 3Voltage (V)
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Resistance of SIS structure with 50 nm top GaN layer versus compressive strain.
44.4
44.5
44.6
44.7
44.8
44.9
45
45.1
45.2
45.3
0 0.5 1 1.5 2 2.5 3
After R. Gaska, J. Yang, A. D. Bykhovski, M. S. Shur, V. V. Kaminski, S. M. Soloviev,
Appl. Phys. Lett. 71(26), 3817 (1997)
Twice as largeas in SiC
Short range superlatticeis four timeslarger than inSiC
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Possible domain structure
After A. D. Bykhovski, V. V. Kaminskii, M. S. Shur, Q. C. Chen, and M. Asif Khan, Piezoresistive Effect in Wurtzite n-type GaN, Appl. Phys. Lett., 68 (6), pp. 818-819 (1996)
N
GaGa
GaSurface
N
GaGa
N N
GaGa
NSurface
NGaGaN
Surface
NGaGaN
N
N N
Ga
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F i g . 1 E p i l a y e r d e s i g n o f [ ( A l N ) m - ( G a N ) n ] p S R S L s t r u c t u r e .T h e c o n s t a n t c u r r e n t w a s m a i n t a i n e d b y o h m i c c o n t a c t s 1 , 4a n d s a m p l e r e s i s t a n c e w a s e s t i m a t e d b y m e a s u r i n g v o l t a g ed r o p b e t w e e n c o n t a c t s 2 a n d 3 . A r r o w s s h o w t h e d i r e c t i o n so f t h e s t r a i n c o m p o n e n t , u x x
S a p p h i r e
2 μ m G a N
3 0 0 n m
2 5 n m G a N
[ ( A l N ) m - ( G a N n ) ] p
1 2 3 4
( 0 0 0 1 )
xu x x u x x
Short Range Superlattices
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Superlattice Band Diagram
y
A A A A AB B B BBeffec tiveband gap
(0001)En
erg
y (e
V)
After A. D. Bykhovski, B. L. Gelmont, and M. S. Shur, Elastic Strain Relaxation in GaN-AlN, GaN-AlGaN, and GaN-InGaN Superlattices, J. Appl. Phys. Vol. 81, No. 9, pp. 6332, May (1997
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0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
5
10
15
20
25
3
21
δR/R
(10-3
)
Strain (10-4 )
Fig. 2 Relative change in resistance under applied strain. (o) - correspond to [(AlN)6-(GaN)3]150 SRSL ; (Δ) - [(AlN)3-(GaN)8]150 SRSL; solid line 1 shows dependence measured for GaN-AlN-GaN SIS 1; dashed line 2 - SiC p-n junction 1 ; 3 - GaAs 2. The GF for the measured samples was in the 30 to 90 range.The larger GF (higher sensitivity to strain) was measured in SRSLs with higher Al content.
1 J. S. Shor, L. Bemis, and A. D. Kurtz, IEEE Trans. Electron Dev. , 41 (5), 661 (1994).2 A. Sagar, Phys. Rev., 112, 1533 (1958); 117, 101 (1960).
Resistance change in SRSL
SRSL
SiCGaN
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0.0 0.5 1.0 1.5 2.0 2.5 3.00
20
40
60
80
100
2
1
Gau
ge F
acto
r
Voltage (mV)
Gauge factors for [(AlN)m-(GaN)n]p SRSL structures and different biases. (o) and line 1 correspond to [(AlN)6-(GaN)3]150 SRSL; (Δ) and line 2 - [(AlN)3-(GaN)8]150 SRSL. At each voltage, a number of measurements were taken for different strains in 0.5x10-4 – 2.4x10-4 range.
Gauge Factor in SRSL
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Static Gauge factors (GF)(measured under longitudinal mechanical deformation)
GaNn-type GF=10
p-type GF > 200
GaN/AlN/GaN SapphireGF = 50
AlN/GaN Short Range Superlattices
GF = 30-80
AlGaN/GaN HeterostructuresGF = 10-15
0 1 2 3 4 50 .0 0
0 .0 2
0 .0 4
0 .0 6
0 .0 8
0 .1 0
0 .1 2
p -G a N
S iS iC
G a N /A lN /G a NδR/R
S tra in , 1 0 -4
High Temperature p-GaN Pressure Sensor
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Piezoelectric effect in doped and undoped channel
GaN/AlGaN HFETs.
2
-20 20 40
0
Ener
gy (e
V)
Distance (nm)
Undoped
Nd = 5x1017 cm-3 1
After M. S. Shur, GaN and related materials for high power applications, Mat. Res. Soc. Proc. Vol. 483, pp. 15-26 (1998) M. S. Shur, GaN and related materials for high power applications, Mat. Res. Soc. Proc. Vol. 483, pp. 15-26 (1998)
Up to 4x1013 cm-2 inGaNversus 2x1012 cm-2
in GaAs
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Piezoelectric effect (opposite polarity)
-4
0En
ergy
(eV)
Distance (nm)
-20
-2
20 40
After M. S. Shur, GaN and related materials for high power applications, Mat. Res. Soc. Proc. Vol. 483, pp. 15-26 (1998) M. S. Shur, GaN and related materials for high power applications, Mat. Res. Soc. Proc. Vol. 483, pp. 15-26 (1998)
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C-V of AlGaN/GaN heterostructuresshow the presence of residual strain
••••••••
•••••••••••••
••••••••••••••••••••••••••••••
••••••••••••••••••••••••••
0
0.002
0.004
0.006
0.008
0.01
-16 -12 -8 -4 0Voltage, V
10 nm (100% strained)
20 nm (100% strained)
40 nm (60%)60 nm (30%)100 nm (0%)
From A. D. Bykhovski, R. Gaska, and M. S. Shur, Appl. Phys. Lett. 73 (24) (1998).
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Piezoelectric and Pyroelectric Doping in AlGaN/GaN
A l M o la r F r a c t io n
0
1
2
3
4
5
6
0 0 .2 0 .4 0 .6 0 .8 1
State-of-the-art
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Maximum Electron Sheet Density
0
5
10
15
20
25
30
35
GaA
s/A
l Ga A
s
InG
a As/
A lG
a As
Ga N
/Al G
aN
Ga N
/AlIn
GaN
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Electronic island at the surface of semiconductor grain in pyroelectric matrix
(MQDs -Moveable Quantum Dots)
Control by external field – Zero dimensional Field Effect (ZFE)
Po
Semiconductor
Electronic island
PyroelectricPo
Semiconductor
Electronic inversion island
Pyroelectric
Hole inversion island
Inversion electron and hole islands at the surface of pyroelectric grain in semiconductor matrix
After V. Kachorovskiy and M. S. Shur, APL, March 29 (2004)
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Terahertz oscillations
MOVABLE QUANTUM DOTS (MQD)
CAN SWITCH OR SHIFT FREQUENCY BY EXTERNAL FIELD OR BY LIGHT
1 THz to 30 THz
2D island might oscillate as a whole over grain surface. The oscillations can be exited by AC field perpendicular to Po
00
4( 2 )p
ePmR
=+
πωε ε
Oscillation frequency is of the order of a terahertz
0 2~ω π/
Oscillation frequency
After V. Kachorovskiy and M. S. Shur, APL, March 29 (2004)
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Key references
M. S. Shur, A. D. Bykhovski, R. Gaska, and A. Khan, GaN-based Pyroelectronics and Piezoelectronics, in Handbook of Thin Film Devices, Volume 1: Hetero-structures for High Performance Devices, Edited by Colin E.C. Wood, Handbook edited by Maurice H. Francombe, pp. 299-339, Academic Press, San Diego, 2000
Piezoelectric effect in AlGaN/GaN HFETs-P. M. Asbeck, E. T. Yu, S. S. Lau, G. J. Sullivan, J. Van Hove, and J. M. Redwing, Electron. Lett. 33, 1230 (1997)
Piezoelectric constants and spontaneous polarization in III-N ab-initio-F. Bernardini and V. Fiorentini, Phys. Rev. B 56(16), 10024 (1997);
Predicted red shift in GaN-based QWs- J. Wang, J. B. Jeon, Yu. M. Sirenko, and K. W. Kim, Photonics Technol. Lett. 9 (6), 728 (1997).
Observed red and blue shift in InGaN/GaN QWs- A. Hangleiter, Jin Seo Im, H. Kollmer, S. Heppel, J. Off, F. Scholz, MRS Internet J. Nitride Semicond. Res. 3, 15 (1998).
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Conclusions
•Pyroelectric properties of GaN are comparable to those of the pyroelectric ceramics such as PZT or BaTiO3•Both primary and secondary pyroelectric effects have been observed in GaN based structures. •piezoresistive effect in GaN/AlN multilayer structures up to 4 times larger than in SiC. •Role of spontaneous polarization, the role of surface and interface states, and of the inversion domains must be ascertained•Polarization doping plays dominant role in AlGaInN/GaN HEMTs