Outline of lectures:
Day 1-2: Research on the physics of nitride semiconductors
Fundamentals of semiconductor physics
Research on nitrides
Day 3-4: Research on the teaching and learning of physics
Research in cognitive science
Research in physics education
Nitride semiconductors and
their applications
Part I: Basic Semiconductor Physics
“One should not work on semiconductors, that is a
filthy mess; who knows whether they really exist.”
Attributed to Wolfgang Pauli (1931)
What are semiconductors?
• Metals, semimetals, semiconductors, insulators
• Characteristics
– Conductivity increases dramatically with temperature
(conductivity at T = 0 K is zero)
– Conductivity changes dramatically with addition of
small amounts of impurities
• Applications
– Anything in which you want to control the flow of
current (transistors, amplifiers, microprocessors, etc.)
– Devices for producing light
– Radiation detectors
History of semiconductors
• 1833 Michael Faraday discovers temperaturedependent conductivity of silver sulfide
• 1873 Willoughby Smith discovers
photoconductivity of selenium
• 1874 Ferdinand Braun discovers that point
contacts on some metal sulfides are rectifying
• 1947 John Bardeen, Walter Brattain, and William
Shockley invent the transistor
Semiconductor materials
Semiconductor materials
Group
Examples:
IV: C, Si, Ge
III-V: GaAs, GaN,
InP, AlSb, GaAlAs,
GaInN
II
III
IV
V
B
C
N
VI
Al Si
P
S
Zn Ga Ge As Se
Cd In
II-VI: ZnSe, CdTe
Hg
Sn Sb Te
Physical Structure
Basic lattice
Face-centered cubic
(fcc)
Diamond structure
Zincblende
Si, Ge
GaAs, InP, ZnS,...
A
B
C
Zincblende: ABCABC…
Wurtzite: ABABAB…
About 1022 atoms in each cm3.
Electronic Structure
• Bands analogous to electronic
energy levels of single atoms
• Band gap between 0 and 5 eV
(1 eV = 3.83 x 10-23 Cal)
• Electrons in valence band are
involved in atomic bonding
• Electrons in conduction band
are free to wander the crystal
• Temperature dependence of
resistance is due to thermal
excitation of electrons across
bandgap
Band structure of Si
Chelikowski and Cohen, Phys. Rev. B 14, 556 (1976)
Growth (bulk)
• Czochralski growth (1918)
• Crystals grown near
melting point of material
(> 1410 °C for silicon)
• Boules up to 12” diameter
and 6 feet long
• Growth rate: ~few mm/min
• Used for Si, Ge, GaAs, InP
From http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter5.htm
Growth (layers)
• MOCVD (MetalOrganic Chemical
Vapor Deposition)
• Also known as
MOVPE, etc.
• Growth temperatures
near melting point
• Growth rate ~1
µm/min.
From http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter5.htm
Fun facts about AsH3
• OSHA Permissible Exposure Limit = 0.05 ppm
(averaged over 8 hour work shift)
• Detection: Garlic-like or fishy odor at 0.5 ppm
• IDLH (Immediately Dangerous to Life or
Health) at 6 ppm. (IDLH for other toxic gases
such as Chlorine or Phosphine are >1000 ppm.)
Growth (layers)
• MBE (MolecularBeam Epitaxy)
• Low growth
temperature
• Growth rate ~few
µm/hr.
• Can grow
atomically flat
surfaces and
monolayers
From http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter5.htm
Doping
• Adding impurities to alter the
electrical properties
• n-type (donors) or p-type
(acceptors)
• Deep or shallow
• Single/double/triple
n-type
Si Doped with Group V
p-type
Si Doped with Group III
Doping
• Shallow donors can be modeled as hydrogen atoms
in a dielectric medium.
• The donor electron level is only a few (6-50) meV
below conduction band.
• Hydrogen-like and helium-like levels are observed.
Doping
• Grown in
• Diffusion
• Neutron transmutation
(30Si(n,g)31Si --> 31P + b-, T1/2=2.6 hr.)
• Ion implantation
Characterization (electrical)
Hall effect enables
determination of:
• charge of carriers
• density of carriers
• binding energy of
carriers (temperature
dependent)
Characterization (optical)
Infrared (IR) spectroscopy
allows determination of:
• impurity species
• electronic and vibrational
energies of impurities
Agarwal et al., Phys. Rev. 138, A882 (1965).
Applications
• The pn-junction is the basis of many
semiconductor devices.
• Three semiconductor devices
– Field effect transistor
– Light-emitting diode
– Laser diode
pn-junction
• Consists of p-type material
next to n-type material.
• Electrons from the n-type
material fill in the acceptors
on the p-type side near the
junction and vice versa.
• Process stops when the
layer of negatively charged
acceptors becomes too
think for the remaining
electrons to get through.
+++
++
++
++
Negatively
charged
acceptors
Positively
charged
donors
pn-junction
• Current will flow if a battery
is hooked up as shown. The
positive terminal of the
battery attracts electrons,
pulling them through the
depletion region.
• A certain minimum voltage is
required to overcome the
repulsion of the depletion
region.
+++
++
++
++
pn-junction
• If the battery is hooked up
in the opposite direction,
then no current flows. (The
depletion region actually
gets bigger.)
• If too much voltage is
applied in this direction,
current flows, but your
junction is unhappy.
+++
++
++
++
Another view of the pn-junction
No bias
Reverse bias
(no current)
–
+
Forward bias
(current)
–
+
Field Effect Transistor (FET)
Light Emitting Diode (LED)
• Is basically a pn-junction
• When an electron and a hole
collide, a photon (light) is emitted.
The energy of the light is “equal”
to the bandgap energy.
Si bandgap ≈ 1.2 eV (infrared)
GaAs bandgap ≈ 1.5 eV (red)
• Defects in crystal can cause
electron-hole collisions to occur
without emission of light (nonradiative recombination).
Laser Diode (LD)
• Is basically a pn-junction
• Same principle as LEDs, however,
waveguides are added to the structure to
enable the light to reach lasing intensities.
Some surfaces are polished mirror-flat to
allow light to reflect back and forth inside
the active region.
• Much better material quality (smaller
density of defects) is required for LDs than
LEDs.
Other applications
• Radiation detectors
Radiation hitting the material knocks an electron
from the valence to the conduction band, creating
a free carrier. An applied voltage sweeps the
carrier out of the material where it is detected as
current.
• Solar cells
Again, a pn-junction. Light creates an electronhole pair which is forced out of the material as
electric current by the electric field in the
depletion region.