Physics of 2D transport in IIINitrides

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Physics of 2D transport in III-Nitrides
Michael J. Manfra Bell Labs, Lucent Technologies,
Murray Hill, NJ, USA
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Organization of lecture

• INTRODUCTORY MATERIAL
• 2DEG formation in AlGaN/GaN heterojunctions
• Suitable substrates and MBE growth conditions
• Heterostructure design for low density 2DEGs
• SCATTERING MECHANISMS
• Analysis of scattering mechanisms from mobility
  vs. density data (low temperature)
• Phonon scattering at higher temperatures
• Present limits to low-temperature mobility
• QUANTUM TRANSPORT
• Quantum Hall Effect in GaN
• Quantum vs. mobility lifetimes

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Organization (continued)

• SPIN PHYSICS in GaN
• Spin-orbit coupling in wurtzite crystals
• Anti-localization phenomena in magnetotransport
• BEYOND 2D 1D and 0D NANOSTRUCTURES in GaN
• Quantum point contacts
• Quantum dots

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Wurtzite GaN large polarization fields
a3.189A c5.185A 0001 and 0001 Are not
equivalent

• low symmetry
• ionic bonding
• large polarization fields

David Greve, CMU
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Polarization Doping in AlGaN/GaN
-spol
sPAlGaN - PGaN
-s
2DEG
AlGaN
s
PE
SP
GaN
EF
SP
substrate
s5.5x1013 electron/cm2 (x) AlxGa1-xN/GaN
structure
spol
Surface States
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Motivation for low density 2DEGs

• Extend the study of 2DEGs in GaN to very low
  density lt1012cm-2
• To identify the scattering mechanisms limiting
  low temperature
• mobility and quantum transport
• To improve material quality!
• New parameter space for interesting physics?
• m 0.2me (3 times heavier than GaAs )
• g 2 (4.5 times larger than GaAs)
• Spin physics in GaN
• Nanostructures in GaN

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Sample design low density 2DEGs

• 3nm GaN capping layer
• 16nm Al0.06Ga0.94N barrier layer
• 1.5mm MBE GaN layer
• 100mm semi-insulating HVPE GaN on Al2O3

Conduction band edge

• keep Al reasonably high 6 125meV CB offset
• use thickness of GaN cap layer to reduce density
• Reproducibly yields density of 9x1011cm-2

energy
position
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GaN Substrates Prepared by HVPE Collaboration
with R. Molnar MIT Lincoln Laboratory

• Hydride Vapor Phase Epitaxy
• GaCl NH3 GaN HCl H2
• Very high growth rates 50mm/hr
• Reduced dislocation density
• lt 5x107cm-2 for 100mm films
• Produces smooth stepped surfaces
• Ideal for MBE overgrowth

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Plasma-assisted molecular beam epitaxy
Modified Riber 32
materials control on the atomic scale
nitrogen plasma source
pumped by 2 cryopumps base pressure 5x10-12
torr running pressure 1x10-6 torr
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Surface Morphology
HVPE Substrate
MBE overgrown GaN
Height scale 5nm
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CV Profile of Net Donor Density
ND-NA 5x1014cm-3
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Sample Structure and Devices
GaN (3nm)
AlxGa1-xN (16nm)
Insulated gate Hall bar
2DEG
MBE GaN (1mm)
HVPE GaN (100mm)
0001

• Grown by Plasma assisted MBE
• Tgrowth745C
• pgrowth2x10-6Torr

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Insulated gate Hall bar (FET)
SiO2 insulated gate lt10-12A of leakage T0.3K
0.1mm x 2mm 2 current probes 14 voltage probes
sweep density in a one sample map
transport properties
Density range 2x1011cm-2 - 2x1012cm-2
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Tune conductivity over wide range
rapidly increasing resistance below
2x1011cm-2 look at r vs. T to examine MIT
linear dependence of density on VG tune density
from 1x1011cm-2 to gt2x1012cm-2 Factor of 20 in
single sample!
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Mobility vs. Density (T0.3K)
Mobility increases with increasing density from
1 to 10 x1011cm-2 Peak mobility
80,000cm2/Vs at density 1.7x1012cm-2 Mobility
rolls over 2x1012cm-2 Transition to region
dominated by alloy and/or interface roughness
scattering
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Increase in density reduces scattering angle
T0K, elastic scattering on the Fermi
circle Transport lifetime met/m 1/tt?P(q)(1-co
s(q))dq
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Scattering mechanisms 1-10x1011cm-2

• 1) Surface states mn0.42
• 2) Background impurities m106cm2/Vs
• 3) Charged dislocations mn1.1
• 3x108cm-2 dislocations
• m 80,000cm2/Vs

Charged dislocations limit mobility in the low
density regime
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Use low defect density template
Use template with 5x107cm-2 dislocations 100mm
thick Large jump in mobility peak mobility
167,000cm2/Vs ne9.1x1011cm-2
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Tune magnetotransport
We can now tune to low filling factors
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High field transport with high mobility
QHE at n12 fully developed at 3.1T
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Early onset of SdH oscillations
Onset of oscillations drops from 2T to 0.6T
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Low densityhigh mobility key
hwc
figure of merit ratio m/ne
62,000/0.86x1012 72
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Maintain good mobility at 4x1011cm-2 m/n 89
only the lowest Landau level is filled at 15
Tesla kinetic energy quenched correlations
should develop if mobility remains high
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FQHE in AlGaN/GaN
Al0.03Ga0.97N/GaN single interface
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quantum lifetime tq
Transport lifetime 1/tt?P(q)(1-cos(q))dq
Quantum lifetime 1/tq?P(q)dq
DR4R0X(T)exp(-p/wctq) X(T)(2pkT/?wc)/sinh(2pkT
/ ?wc) R0 is resistance at zero field
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quantum and transport times
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Compare with dislocation and impurity scattering
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Spin Physics in GaN

• Extremely long spin lifetime predicted in GaN
  due to expected weak spin-orbit coupling,
• 3 orders of magnitude longer than GaAs.
• (S. Krishnamurthy et al., APL 83, 1761 (2003))

Experimental exploration of the physics of
spin-orbit coupling in GaN is essential for
device applications Magnetoconductivity
measurements are a classic tool for probing
spin-orbit effects
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Conduction Band Splitting
without spin-orbit coupling
with spin-orbit coupling
E
EF
EC
k
Energy spectrum described by Rashba Hamiltonian
DSO aSO pF aSO (ne) ½
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Quantum interference corrections to Drude
conductivity
Constructive interference of two electron waves
in opposite directions leads to decreased
conductivity at B0 weak localization

Constructive interference can be destroyed by an
additional phase originating from a magnetic
field
A
A
Spin relaxation due to spin-orbit coupling
impurity scattering leads to a positive
contribution to conductivity - antilocalization
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conductivity in weak magnetic field
BSO
B0
B0
antilocalization
general form with SO
Conductivity with weak localization
BSO a2
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High Field Magnetoresistance
Ordinary Sample
In the classical Drude model No resistance
change at low magnetic field
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low field magnetoresistance
Zoom into mT range
weak localization visible
anti-localization caused by spin-orbit coupling,
effect is a few parts in 104
Unambiguous observation of anti-localization
implies existence of spin-orbit coupling in GaN
anti-localization
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Two samples different conductivity
lower conductivity (diffusive regime) theory
works best high conductivity (ballistic
regime) new regime for study
Lower conductivity sample
Plot data as Ds ssquare (B) - ssquare (B0)
Goal Extraction of SO parameters
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Tuning the Quantum Interference

• Two points
• Conductivity minimum does not change its field
  position
• Amplitude does change

If Bso does not change a does not change!
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Extracting Parameters for GaN
Higher conductivity sample ballistic transport
regime
Fit parameters BSO (2 0.1)mT 0.25mT gt Bj gt
0.04mT aSO 6meVÅ DSO0.2meV 0.4meV
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Conclusions (spin-orbit coupling)

• Spin-orbit coupling in GaN 2DEGs is NOT weak, the
  magnitude is comparable to GaAs!
• The spin-orbit coupling is described by Rashba
  Hamiltonian.
• Places constraints on the use of GaN 2DEGs in
  spintronic applications.

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Nanostructures in GaN

• Can we create electrically active 1D and 0D
  structures in the Nitrides?
• Most of what we know about mesoscopic physics
  derives from studies of GaAs
• Exploit unique material parameters of GaN to
  probe new regime of mesoscopia

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1D- quantum point contact
Quantum Point Contact

• Short constriction between two electron
  reservoirs defined by electron beam lithography
• Conductance of constriction is tuned by gate
  voltage - one-dimensional channels are formed for
  electrons with a quantized conductance of 2e2/h
• Analogue to waveguides

Chou et al., APL 86 073108 (2005)
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Spin in the QPC
Zeeman-spin splitting in high magnetic fields
- g-factor of 2.5 is extracted - Zero-field spin
splitting energy of 0.4meV
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0D quantum dots
Lithographically defined Quantum Dot
a GaN/AlGaN single electron transistor
Each Coulomb oscillation is attributed to
charging the dot with one single electron
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Coulomb blockade and conductance diamond
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Conclusions

• High mobility 2DEGs in the density range 1011cm-2
  to 1012cm-2, mobility presently limited by
  charged dislocation scattering
• Spin-orbit coupling in GaN/AlGaN 2DEGs
  investigated utilizing anti-localization effects
  strength comparable to GaAs
• Quantum transport in electrically addressable
  one- and zero-dimensional Nitride systems
  demonstrated