Transcript Nitride-based Semiconductors and their Applications

Nitride semiconductors and
their applications
Part II: Nitride semiconductors
Nitride papers published
# papers published
1000
100
10
1980 1985 1990 1995 2000 2005
Year
Nitride-based semiconductors
• A III-V semiconductor in which N is one of
the elements. Examples include AlN, GaN,
InN, and alloys such as AlxGa1-xN.
• These materials have high melting points
(strong bond with N) and a wide span of
bandgaps (from 0.7 eV for InN to 3.4 eV for
GaN to 6.2 eV for AlN).
Applications
• High-temperature, high-power electronics
• Ultraviolet (UV) radiation detectors
– feedback systems in furnaces and engines
– solar-blind missile early warning systems
– astronomical applications
• LEDs and LDs
Blue LEDs and LDs
• Full color day-visible displays
• Energy efficient lighting
– Traffic lights, replace every 6 months, LEDs,
every 5-10 years (60,000 hours)
– Consumer lighting applications.
• Higher storage density on optical media
(>50 GB on a single DVD)
• Faster on-time
• Fluorescent photosensitizers accumulate
preferentially in cancerous cells.
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Short history
1971
GaN LED demonstrated (Pankove)
1986
“High” quality GaN grown (Akasaki)
1988 P-type GaN grown (Akasaki); Nakamura starts work
on GaN
1990 Two-flow MOCVD system developed (Nakamura)
1991 High quality p-type GaN grown; first pn-junction
GaN LED created
1992
ZnSe-CdZnSe blue laser developed (3M)
1993
Commercial blue GaN LEDs introduced (Nichia)
1996
Room temperature nitride LDs developed (Nichia)
1999
Commercial nitride LDs introduced (Nichia)
2004
SONY markets blue laser DVD writers (23 GB/layer)
Nitride problems
1. Inability to grow good quality crystals
2. Inability to grow p-type crystals
Crystal quality
• Problem: No lattice matched substrates, high
growth temperature results in convection currents
– Sapphire is closest but is 15% off.
– SiC is too expensive
– MOCVD growth too fast for good control (few mm/min)
• Solution: Buffer layers, new growth system
– First grow GaN or AlN buffer layer
– Two-flow MOCVD system
– Still many many dislocations in material (1010 cm-2) but
dislocations don’t matter?
Two-flow MOCVD
Nakamura, Harada, and Seno, J. Appl. Phys 58, 2021 (1991)
Buffer layer
Dislocation
High quality GaN
Low quality GaN (0.2 mm)
AlN or GaN buffer layer
(0.05 mm)
Substrate
p-type GaN
• Problem: No one could dope GaN p-type
• Solution:
– At first, LEEBI (Low Energy Electron Beam
Irradiation)
– Later, annealing at 700°C in non H-containing gas
– Hydrogen was passivating the acceptors!
Factors leading to success
• Small bureaucracy (N. Ogawa and S.
Nakamura)
• A 3.3 M$ USD gamble (1.5% of annual
sales)
• Large companies tend to be conservative,
both in funds and in research outlook
Current research on nitrides
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•
•
Fundamental physics
Improving crystal quality (still very poor)
Ultraviolet lasers
Lattice matching with quaternary alloys
(AlGaInN)
• Nitride heterostructures and accompanying
applications
Nitride heterostructures
AlGaN
GaN
Energy
Conduction band
AlGaN
GaN
Valence band
Heterostructure usefulness
Charge carriers are
spatially separated from
the (now) ionized impurity
atoms, leading to higher
carrier mobilities.
Electrons form a 2Dimensional Electron Gas
(2-DEG).
Research questions
• Even undoped, carrier densities in AlGaN/GaN
heterostructures is 10 to 100 times larger than
those in similar (AlGaAs/GaAs) systems.
What is the source of these carriers?
• Carrier mobilities in AlGaN/GaN heterostructures
are 10 to 100 times lower than in the
AlGaAs/GaAs system.
What are the principle mechanisms limiting the
mobility?
Origin of carriers
Current theory: surface donor defects on AlGaN
Concentration of surface defects and transfer of electrons to
GaN well is enhanced by strain-induced electric field.
Pseudomorphic growth
AlGaN
GaN
lattice matched
non-lattice matched
AlGaN/GaN interface
• GaN and AlN have ~2.5% lattice mismatch
• Grown on polar c-axis
• Spontaneous and induced piezoelectric fields are present
Band structure
CB
• When AlGaN barrier is thick
enough to pull defect level above
bottom of GaN well, electrons
begin to transfer to well.
defect
level
• Formation energy of donor defects
is reduced because electrons can
drop to lower energy level by
transferring to GaN well
GaN
AlGaN
Electron transfer
-2
cm )
10
2-DEG density (10
12
12
14
T=4K
Al fraction
25%
25% Al
15%
8
5%
15% Al
6
4
2
0
GaN
5% Al
0
100
200 300 400
Barrier width (Ѓ)
500
AlGaN
cm -2 )
20
12
15
10
5
0
5
31 nm barrier
T = 13K
2 DEG density (10
2 DEG density (10
12
cm -2 )
Comparison with experiment
14
12
10
8
6
4
27% Al
T = 13 K
2
0
10 15 20 25 30 35
Barrier Al composition (%)
0
100 200 300 400 500
Barrier thickness (Ѓ)
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)
Transport properties
Semiconductor with no applied field:
• Electrons move randomly with an average velocity
of zero
• Mean time between electron-electron collisions is t.
With an applied field:
• Electrons have an acceleration of a = eE/m*
• Average velocity of electrons is at, parallel to field.
The mobility is a measure of how easily charge carriers
respond to an applied electric field.
vavg
m=
E
Limiting factors
Scattering mechanisms
• Coulomb fields
• Phonons
• Alloy disorder
Coulomb scattering
Electrons are affected by the long-range Coulomb
fields of randomly distributed ionized donor atoms.
• Thicker barriers move ionized surface donors
further away from carriers.
• Large 2-DEG densities screen the effect of these
Coulomb fields.
Phonon scattering
Phonons are lattice vibrations in a crystal.
Acoustic phonons
• Both types of atoms move “in-phase”
• Low energy vibrations
Optical phonons
• Atoms of different types move “out-of-phase”
• High energy vibration
Phonon scattering
• Phonons scatter carriers by creating small
fluctuating dipoles between atoms
(piezoelectric mode).
• Phonons scatter carriers by disturbing the
periodicity of the crystal lattice
(deformation potential mode).
Alloy disorder
• Electron wavefunction
penetrates into AlGaN
barrier.
• Al and Ga atoms are
distributed randomly in
AlGaN
• Randomly varying potential
scatters electrons.
2-DEG mobilities
106
2-DEG mobility (cm2/Vs)
Phonon
Coulomb
105
Alloy
104
total
0
4K
Al 0.15Ga0.85N/GaN
100 200 300 400
Barrier thickness (Å)
500
Improving the mobility
Strategies:
• Reduce 2-DEG density
– Smaller Al alloy fractions
– Thinner barriers
(Highest mobility heterostructures have Al fractions
of ~10% and barrier thicknesses of ~130 Å)
• Reduce alloy disorder scattering
– AlN spacer
AlN spacer
GaN
AlN
AlGaN
Conclusions
• Nitride-based semiconductors are a promising field
for a wide variety of new technological applications.
• 2-DEG mobilities are limited by two factors:
– Coulomb scattering (N < 2 x 1012 cm-2)
– Alloy disorder scattering (N > 4 x 1012 cm-2)
– We predict maximum low temp mobilities of 105 cm2/V s
without a AlN spacer.
• Using a AlN spacer seems a promising way to
improve the conductivity of nitride heterostructures.
Subband structure
Confining potential results in
quantized energy levels.
2  

2
2m *
4


4

2
2   V z  E
Trial wavefunction
 0   x,y
b 2 3 / 2 bz / 2
z
e
6
Energy (meV)
Subband structure
100
E
10 11
10
1
EF
E0
1012
1013
-2
2-DEG density (cm )
Comparison with experiment
2-DEG mobility (cm2/Vs)
3 104
104
8 103
6 103
13 K
Al 0.05Ga0.95N/GaN
4 103
0
100 200 300 400
Barrier thickness (Å)
500
Comparison with experiment
31 nm barrier
T = 13 K
2 104
1 104
0 100
5
2
2-DEG mobility (cm /Vs)
2
2-DEG mobility (cm /Vs)
3 104
4
2 10
T = 13 K
27% Al
4
1 10
0
10 15 20 25 30 35
Barrier Al composition (%)
0 10
0
100 200 300 400 500
Barrier thickness (Ѓ)
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)
Comparison with experiment
31 nm barrier
T = 13 K
2
2-DEG mobility (cm /Vs)
2
2-DEG mobility (cm /Vs)
3 104
2 104
1 104
0 100
5
4
2 10
T = 13 K
27% Al
4
1 10
0
10 15 20 25 30 35
Barrier Al composition (%)
0 10
0
100 200 300 400 500
Barrier thickness (Ѓ)
Smorchkova et al., J. Appl. Phys 86, 4520 (1999)