Outline of lectures: Day 1-2: Research on the physics of nitride semiconductors Fundamentals of semiconductor physics Research on nitrides Day 3-4: Research.
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Outline of lectures:
Day 1-2: Research on the physics of nitride semiconductorsFundamentals of semiconductor physicsResearch on nitrides
Day 3-4: Research on the teaching and learning of physicsResearch in cognitive scienceResearch 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 temperature-dependent 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
Examples:
IV: C, Si, Ge
III-V: GaAs, GaN, InP, AlSb, GaAlAs, GaInN
II-VI: ZnSe, CdTe
BCNAlSiPGaGeAsInSnSbIIIIVVGroupZnCdHgIISSeTeVI
Physical StructureBasic lattice
Face-centered cubic(fcc)
Diamond structure
Si, Ge
Zincblende
GaAs, InP, ZnS,...
ABCZincblende: 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 (Metal-Organic 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 (Molecular-Beam 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
BCNAlSiPGaGeAsInSnSbIIIIVVGroupZnCdIISSeTeVI
n-type p-typeSi Doped with Group VSi Doped with Group V Si Doped with
Group III
Si Doped with Group III
Doping
Conduction bandValence bandDonor level
• 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,)31Si --> 31P + -, 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)
SourceDrainGateGaten-type-p-typep-typedepl. regiondepl. region--------
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 (non-radiative 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 detectorsRadiation 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 cellsAgain, a pn-junction. Light creates an electron-hole pair which is forced out of the material as electric current by the electric field in the depletion region.
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