1. Brad Gussin John Romankiewicz 12/1/04 Quantum Dots: Photon Interaction Applications

2. 2 Semiconductor Structures Bulk Crystal (3D)  3 Degrees of Freedom (x-, y-, and z-axis) Quantum Well (2D)  2 Degrees of Freedom (x-, and y-axis) Quantum Wire (1D)  1 Degree of Freedom (x-axis) Quantum Dot (0D)  0 Degrees of Freedom (electron is confined in all directions)

3. 3 Structure vs. Energy Quantum Dots are sometimes called “artificial atoms”

4. 4 Infrared Photodetection QWIP Bulk Crystal Wavelength Intensity Quantum Well IP +- Photons CB VB

5. 5 QWIP Drawbacks High Intensity / Low Temp Polarization scatter grating Detector only works when photon hits semiconductor perpendicularly to the two unconfined axis Grating

6. 6 Quantum Dots (Structure and Formation) Self-Assembly (a.k.a Stranski- Krastanow Method) : Mismatched lattice constants cause surface tension which results in Qdot formation with surprisingly uniform characteristics. GaAs  5.6533 Å InAs  6.0584 Å

7. 7 QDIPs 3D e - Confinement: Sharper wavelength discrimination E =  n / R 2  E controlled by dimensional parameter R No need for Polarization “Photon Bottleneck” : e - stays excited for a longer time (i.e. less recombination), resulting in a more efficient detector and resistance to temperature. Higher temperatures and lower intensity.

8. 8 The Future of QDIPs Self-assembly techniques still unstable: tune photodetection properties by manipulating the shape and size of Qdots Possible Solutions: Different Material Combinations More precise control of parameters (T, pressure, physical setup) Combine self-assembly with lithography and etching techniques For example: create crevices or pre-etched holes to encourage qdot growth in specified positions. * Dr. Bijan Movaghar estimates 5-10 years before commercially practical QDIPs are in use.

9. 9 QDIP Applications Increase detectivity  Increase number of applications Medical (Thermal Imaging) Weather Military Astronomy: Infrared Image of the Milky Way

10. 10 Currently, quantum wells and quantum dots are being researched for use in solar cells. Currently, quantum wells and quantum dots are being researched for use in solar cells. Factors that affect solar cell efficiency: Factors that affect solar cell efficiency: –Wavelength of light –Recombination –Temperature –Reflection –Electrical Resistance Solar Cell Applications

11. 11 On an I-V curve characterizing the output of a solar cell, the ratio of maximum power to the product of the open- circuit voltage and the short circuit current is the fill factor. The higher the fill factor, the “squarer” the shape of the I-V curve. On an I-V curve characterizing the output of a solar cell, the ratio of maximum power to the product of the open- circuit voltage and the short circuit current is the fill factor. The higher the fill factor, the “squarer” the shape of the I-V curve.

12. 12 Quantum Well Application Photocurrent and output voltage can be individually optimized – –Absorption edge and spectral characteristics can be tailored by the width and depth of QWs. In GaAs/Al x Ga 1-x As p – i – n structure with inserted QWs, researchers have observed enhancement in the short-circuit current and thus efficiency in comparison with control samples that are identical except without the QWs.

13. 13 Quantum Dot Application Have potential to improve efficiency Have potential to improve efficiency –Reducing recombination   Channeling the electrons and holes through the coupling between aligned QD’s  Photon bottleneck –Increasing the amount of usable incident light –Can be used at higher temperatures Drawbacks Drawbacks –Size is harder to control  Thus harder to control the light absorption spectrum Solutions for quantum dot solar cells are similar to QDIP solutions Solutions for quantum dot solar cells are similar to QDIP solutions

14. 14 Structure and Energy Diagram P-type, intrinsic, n- type structure P-type, intrinsic, n- type structure Based on a self- organized InAs/GaAs system Based on a self- organized InAs/GaAs system Quantum dots used in active regions Quantum dots used in active regions

15. 15 Conclusion Qdot technology will help optimize the photon detection and photovoltaic industries by making devices more efficient and functionally effective. Qdot technology will help optimize the photon detection and photovoltaic industries by making devices more efficient and functionally effective. Possible other areas for commercial development include use in the automobile or robotics industries Possible other areas for commercial development include use in the automobile or robotics industries  Detect object or humans in the vicinity of the device.  Power generation for device.

16. 16 Sources Interview with Interview with Dr. Bijan Movaghar, visiting professor (November 19, 2004). Aroutounian, V. et al. Journal of Applied Physics. Vol. 89, No. 4. February 15, 2001. Razeghi, Manijeh. Fundamentals of Solid State Engineering. Kluwer: Boston. 2002. Center For Quantum Devices. Quantum Dots Introduction