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 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

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.

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 n-type p-type Si Doped with Group V 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 (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 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.