1. 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

2. 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)

3. 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

4. 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

5. Semiconductor materials

6. Semiconductor materials
Examples:
IV: C, Si, Ge
III-V: GaAs, GaN, InP, AlSb, GaAlAs, GaInN
II-VI: ZnSe, CdTe

7. Physical Structure About 1022 atoms in each cm3. Basic lattice
Face-centered cubic
(fcc)
Diamond structure
Si, Ge
Zincblende
GaAs, InP, ZnS,...
Zincblende: ABCABC…
Wurtzite: ABABAB…
About 1022 atoms in each cm3.

8. Electronic Structure
Bands analogous to electronic energy levels of single atoms
Band gap between 0 and 5 eV (1 eV = 3.83 x 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

9. Band structure of Si
Chelikowski and Cohen, Phys. Rev. B 14, 556 (1976)

10. 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

11. Growth (layers) MOCVD (Metal-Organic Chemical Vapor Deposition)
Also known as MOVPE, etc.
Growth temperatures near melting point
Growth rate ~1 µm/min.
From

12. 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.)

13. Growth (layers) MBE (Molecular-Beam Epitaxy) Low growth temperature
Growth rate ~few µm/hr.
Can grow atomically flat surfaces and monolayers
From

14. Doping Adding impurities to alter the electrical properties
n-type (donors) or p-type (acceptors)
Deep or shallow
Single/double/triple
n-type
p-type
Si Doped with Group V
Si Doped with Group III

15. 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.

16. Doping Grown in Diffusion
Neutron transmutation (30Si(n,g)31Si --> 31P + b-, T1/2=2.6 hr.)
Ion implantation

17. Characterization (electrical)
Hall effect enables determination of:
charge of carriers
density of carriers
binding energy of carriers (temperature dependent)

18. Characterization (optical)
Infrared (IR) spectroscopy allows determination of:
impurity species
electronic and vibrational energies of impurities
Agarwal et al., Phys. Rev. 138, A882 (1965).

19. Applications
The pn-junction is the basis of many semiconductor devices.
Three semiconductor devices
Field effect transistor
Light-emitting diode
Laser diode

20. 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

21. 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. + + + + + + + +

22. 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. + + + + + + + +

23. Another view of the pn-junction
No bias
Reverse bias
(no current)
Forward bias
(current) – + – +

24. Field Effect Transistor (FET)

25. 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).

27. 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.

28. 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 electron-hole pair which is forced out of the material as electric current by the electric field in the depletion region.