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.