What do we study and do? Light comes from electrons transitioning from higher energy to lower energy levels. Wave-particle nature of light Wave nature: refraction, diffraction, interference (labs) Particle nature: photons Generating light LED, laser Detecting light Solar cell, photodetector Transmitting light Fiber Manipulating light Lens, structural coloration Using light (projects) Spectrometer, holography, plastic lens, (berry-juice) dye-sensitized solar cell 1
Lecture 4 Semiconductor Fabrication, Laser 2
I am still confused about energy levels A. Yes B. No 3
I am still confused about energy band gap A. Yes B. No 4
After getting a wafer, we do microfabricationà Photolithography Steps (1) Photo Resist http://conocimientosintegratedcircuit.blogspot.com/2010/05/integrated-circuitfabrication-process.html 5
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Photolithography Steps (2) Remove photoresist http://conocimientosintegratedcircuit.blogspot.com/2010/05/integrated-circuitfabrication-process.html 7
The Fabrication of Integrated Circuits (or LEDs, lasers) 8
Properties of Laser Light It s coherent no natural source is. Two types of coherence: Temporal highly monochromatic Spatial highly directional 9
Occupation of Energy Levels in Thermal Equilibrium Boltzmann Distribution n 2 = exp n1 E2 E k T B 1 At 300 K, for E 2 E 1 = 1 ev, n 2 /n 1 = 1.7 x 10-17 0.1 ev 2% 10
How is the Boltzmann Distribution different from the probability distribution for electrons? Electrons are fermions, and atoms are bosons. 11
spontaneous emission Optical Processes absorption stimulated emission Before After Light Amplification by Stimulated Emission of Radiation 12
Can population inversion achieved with a 2-level system when you pump energy into the system continuously? A Yes B No C Don t know 13
Principle of LASER hυ 32 E 3 E 3 E 3 E 3 hυ 13 E 2 Metastable state E 2 E 2 IN E 2 OUT hυ 21 hυ 2 E 1 E 1 E 1 E 1 Coherent photons (a) (b) (c) (d) The principle of the LASER. (a) Atoms in the ground state are pumped up to the energy level E 3 by incoming photons of energy hυ 13 = E 3 E 1. (b) Atoms at E 3 rapidly decay to the metastable state at energy level E 2 by emitting photons or emitting lattice vibrations; hυ 32 = E 3 E 2. (c) As the states at E 2 are long-lived, they quickly become populated and there is a population inversion between E 2 and E 1. (d) A random photon (from a spontaneous decay) of energy hυ 21 = E 2 E 1 can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emitted. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 14
Population Inversion Mirror Mirror Optical resonant cavity 15
Purpose of the Cavity Standing wave Cavity Modes Natural Emission Monochromatic Directional 16 λ
Beam divergence Directional D φ φ = (4/π)(λ/D) Nd:YAG, λ = 1.06 µm, D = 3 mm, φ = 0.020 o He-Ne, λ = 0.6328 µm, D = 1 mm, φ = 0.036 o 17
As we increase the pumping power, so does the population inversion (N 2 N 1 ). What happens to population inversion after threshold? A increases B becomes constant C decreases D don t know 18
Output Power vs. Pump Rate http://www.ni.com/white-paper/14878/en/ 19
What does it take to make a laser? Find an appropriate material Find an appropriate pumping method Find an appropriate optical cavity 20
Type of Lasers Doped insulator lasers Ruby laser Nd:YAG laser Gas laser HeNe laser Argon-ion laser Carbon dioxide laser Excimer laser Semiconductor laser 21
Ruby Laser http://universe-review.ca/f13-atom07.htm 22
Ruby Laser Heat Xenonfilled flash tube ~4 millisec http://universe-review.ca/f13-atom07.htm 23
Nd:YAG Laser How many level (broadly speaking) system is this? Is it easier to achieve population inversion than a ruby laser? Why? http://perg.phys.ksu.edu/vqm/laserweb/ch-6/f6s2t2p2.htm 24
Nd:YAG Laser neodymium ions in yittrium aluminum garnet Ophthalmology Skin cancer removal Prostate surgery Hair removal Engraving Drilling Rangefinder Spectroscopy http://directedlight.com/laser-components/catalog/nd-yag-rods/ 25
He-Ne Laser http://www.thorlabs.com/images/tabimages/hene_energylevels.jpg 26
He-Ne Laser http://en.wikipedia.org/wiki/helium%e2%80%93neon_laser 27
Carbon Dioxide Laser http://www.intechopen.com/books/laser-pulses-theory-technology-andapplications/longitudinally-excited-co2-laser 28
Applications of CO 2 Lasers Medical, laser scalpel 80% of soft biological tissue (skin) is water, which absorbs light at ~10 µm wavelength. à Penetration length ~ 50 µm. Vaporizes tissue Industrial Laser cutting (~100-200 W) 29
Semiconductor Laser Population Inversion p + Junction n + E c E v E F p E g Holes in V B Electro ns ev o Electrons in C B E F n E c E c E g p + Inv ers ion reg io n n + E c ev E F n E F p (a) E v (b) The energy band diagram of a degenerately doped p-n with no bias. (b) Band diagram with a sufficiently large forward bias to cause population inversion and hence stimulated emission. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) V 30
Cleaved surface mirror Semiconductor Laser Diode Current (001) p + n + L L GaAs GaAs Electrode Electrode (110) Cleavage plane Active region (stimulated emission region) A schematic illustration of a GaAs homojunction laser diode. The cleaved surfaces act as reflecting mirrors. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 31
Light Output vs. Current (L-I Curve) threshold http://www.newport.com/tutorial-laser-diode-technology/852182/1033/content.aspx 32
Heterostructure Laser Refractive index Photon density (a) (b) (c) (d) Electrons in CB E c E v 2 ev n AlGaAs p GaAs (~0.1 µm) Active region 1.4 ev Holes in VB p AlGaAs ΔE c Δn ~ 5% 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 2 ev E c E v (a) A double heterostructure diode has two junctions which are between two different bandgap semiconductors (GaAs and AlGaAs). (b) Simplified energy band diagram under a large forward bias. Lasing recombination takes place in the p- GaAs layer, the active layer (c) Higher bandgap materials have a lower refractive index (d) AlGaAs layers provide lateral optical confinement. 33 Nobel Prize in Physics 2000
Common Lasers # photons/#electrons Type" Wavelength" Power" Efficiency" Helium-Neon" 632.8nm" 0.1-50mW" <0.1%" Ruby" 694.3nm" 0.03-100J" <0.5%" Carbon-Dioxide" 10.6µm" 3-100W" 5-15%" Nd:YAG" 1.064µm" 0.04-600W" 0.1-2%" Argon" 488/514nm" 5mW-20W" <0.1%" Dye" 400-900nm" 20-800mW" 10-20%" Hydrogen- Fluoride" 2.6-3µm" 0.01-150W" 0.1-1%" Gallium- Arsenide" 780-900nm" 1-40mW" 1-20%" 34
Principles of laser What We Learned Population inversion in a 3-level system Optical cavity Types of laser Ruby laser YAG laser HeNe laser Semiconductor laser 35
How a Laser Works 36
In a scale of 1 (easy) to 5 (difficult), rate today s lecture on semiconductor fabrication. A. 1 (easy) B. 2 C. 3 D. 4 E. 5 (difficult) 37
In a scale of 1 (easy) to 5 (difficult), rate today s lecture on laser. A. 1 (easy) B. 2 C. 3 D. 4 E. 5 (difficult) 38