1. Quantum Dot Infrared Photo-detector
Quantum Electronics for Engineer
Present by: Chintana Keo
Date: May 3, 2006

2. Agenda What is a Photo-detector?
What is the different between Quantum Dot Infrared Photo-detector (QDIP) and Quantum Well Infrared Photo-detector (QWIP)?
Sample sketch or diagram of QDIP
How does the device work?
Advantage of QDIP
Dark current calculation & Why?
Detection energy calculation
Some possible applications
Conclusion

3. What is a photo-detector?
A photo-detector is a semi-conductor photodiode device that generate electrical current or electrons excitation when light source is shine onto its’ surface or when light source is entering a diode semiconductor device made from such material as GaAs & InGaAs.
A photo-detector is an opto-electronics device that allow us to produce an image of an object as a result of the electrical current produced by shining a light source within a given wavelength range depending on what materials is used.

4. What is a photo-detector? (Continues)
A photo-detector is basically a photodiode in principle. When struck by light source, the electrons within become stimulated and create current across a diode resulting in an exact duplicated image as the source.

5. The different between quantum well & quantum dot
There similarities and different characteristics of photo detectors:
Quantum Well Infrared Photo-Detector (QWIP)
Quantum Dot Photo-Detector (QDIP).
Figure shows the different between quantum dot and quantum well:
Left is quantum well infrared photo-detector
Well between barriers
Right is quantum dot infrared photo-detector
Dots between barriers

6. Schematic Sample of Quantum Dot
Boron doped Ge quantum dots growth sample
Producing using molecular-beam epitaxy (MBE) method in a thin layer of semi-conductor materials.

7. Basic Device Both device has an emitter and a collector
The detection mechanism in both devices is by intraband photo excitation of electrons between energy levels

8. The Advantage of QDIP
QDIP allow direct incident normal to wafer surfaces.
Avoid fabricating grate coupler as in QWIP.
In producing QWIP, a grating coupler required which yield in extra fabrication steps.
It has lower dark current & high detection sensitivity than QWIP.
Better Radiant sensitivity and Efficiency resulting in better detection.
Dominant in normal direction response to growth direction.

9. Dark Current Calculation
Dark current is the current produce internal to the photodetector resulting as noise
Simplest way to calculate dark current density is to count mobile carrier barrier and carrier velocity
Jdark is a dark current
υ is a drift velocity
n3d is current density
Can be calculate using the second formula at left.
mb is a barrier effective mass
Ea is thermal activation energy

10. Radiant Sensitivity and Quantum Efficiency
Current produce when light hitting a semi-conductor radiating electrons excitation.
This can be calculate using the following formula
QE = ((S x 1240) / λ ) x 100%
Where S is the radiant sensitivity
Long exited electron lifetime lead to high responsivity, higher temp and higher dark current which will limited detectivity

11. Responsivity
Responsivity can be calculated using the formula at left, where:
υ - a phonon frequency
η - the absorption efficiency
g - photoconductive gain
Higher absorption efficiency have better detection.

12. Possible Applications
High speed infrared detection
Infrared image application—possible use in security systems to produce image of various objects.
Possible use in IR Spectrophotometer
Possible use in Cell Sorter
Could be use in Infrared Camera

13. Conclusion
There are still many challenges to overcome such fabrication or manufacturing process that will produce quantum dot to meet design requirement
Current manufacturing process limit to size and dot density that it is impractical for commercial used
Due to complex fabrication process and limited size it is expensive to manufacture
Still in its infancy—needs better doping control

14. Question?
Thank You

15. Credit & Reference Prof: Joel Therrien—UMass Lowell.
American Science & Engineering—Billerica, Ma
Prof: Sam Milshtein—UMass Lowell
Photodiodes—Hamamatsu Photonics K.K. Solid State Division
The Photonics Dictionary, 42nd Ed 1996—The Tropel Spectrum
Growth Study of Surfactant-Mediate SiGe graded layers—Thin Solid Film 380 (2000) 54-56
Photoluminescence of multi-layer of SiGe dot growth on Si—J. Wa, H Lou—Device research laboratory, Electrical Engineering Department---University of California at Los Angeles
Reshifting and broadening of quantum well infrared photo-detector—IEEE Journal of selected topic in quantum electronics, vol 4 No 4 July/August 1998
Intersuband absorption in boron dope multiple Ge Quantum Dot—Applied Physic Letter Vol. 74, Number 2, January 11, 1998
Normal Incidence Mid-Range Ge Quantum dot photo-detector—Fei Lou, Song Tong, Jianlin Liu & Kang L. Wang--Journal of Electronics materials, vol. 33, Number 8, 2004
Zhen Yang, Yi Shi, Jianlin Liu, Bo Yan, Rang Zhang, Youdou Zhen & Kanglong Wang—Optical Properties of Ge/Si quantum dot superlatices—Department of Physic and National Laboratory of Solid State Microstructure, Nanjing University & University of California. Science Direct—Material Letters 58 (2004)