Nitride semiconductors and their applications

statementdizzyeyedSemiconductor

Nov 1, 2013 (4 years and 2 months ago)

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Nitride semiconductors and
their applications

Part II: Nitride semiconductors

Nitride papers published

Nitride
-
based semiconductors


A III
-
V semiconductor in which N is one of
the elements. Examples include AlN, GaN,
InN, and alloys such as Al
x
Ga
1
-
x
N.



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.

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
m
m/min)


Solution: Buffer layers, new growth system


First grow GaN or AlN buffer layer


Two
-
flow MOCVD system


Still many many dislocations in material (10
10

cm
-
2
) but
dislocations don’t matter?

Two
-
flow MOCVD

Nakamura, Harada, and Seno, J. Appl. Phys 58, 2021 (1991)

Buffer layer

High quality GaN


Low quality GaN (0.2
m
m)


AlN or GaN buffer layer

(0.05
m
m)


Substrate



Dislocation

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


Fundamental physics


Improving crystal quality (still very poor)


Ultraviolet lasers


Lattice matching with quaternary alloys
(AlGaInN)


Nitride heterostructures and accompanying
applications

Nitride heterostructures

Heterostructure usefulness

Charge carriers are
spatially separated from
the (now) ionized impurity
atoms, leading to higher
carrier mobilities.


Electrons form a 2
-
Dimensional 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 interface


GaN and AlN have ~2.5% lattice mismatch


Grown on polar c
-
axis


Spontaneous and induced piezoelectric fields are present

lattice matched

non
-
lattice matched

AlGaN

GaN

Band structure


When AlGaN barrier is thick
enough to pull defect level above
bottom of GaN well, electrons
begin to transfer to well.





Formation energy of donor defects
is reduced because electrons can
drop to lower energy level by
transferring to GaN well

Electron transfer

Comparison with experiment

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 a
t
, parallel to field.


The mobility is a measure of how easily charge
carriers respond to an applied electric field.

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

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

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 10
12

cm
-
2
)


Alloy disorder scattering (N > 4 x 10
12

cm
-
2
)


We predict maximum low temp mobilities of 10
5

cm
2
/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.

Trial wavefunction

Subband structure

Comparison with experiment

Comparison with experiment

Smorchkova et al., J. Appl. Phys
86
, 4520 (1999)

Comparison with experiment

Smorchkova et al., J. Appl. Phys
86
, 4520 (1999)