My Undergraduate Thesis

Thesis Title

Modeling Quantum Dot in a Well Infrared Photodetector by NEGF Method

Thesis Supervisor

Supervised by: Dr. Farseem Mannan Mohammedy Also we got valuable suggestion from : Dr. Anisuzzaman Talukder, Md. Zunaid Baten (leaving for univ of Michigan shortly), Ahmad Zubair (leaving for MIT shortly), Redwan Noor Sajjad (doing PhD at univ of virginia), Sishir Bhowmick (doing PhD at univ of Michigan) Special Thanks to: Professor Supriyo Datta (Purdue Univ.)

Thesis Group

Electronics

Group Members

Soumitra Roy Joy (0606001)

Golam Md. Imran Hossain (0606029)

Tonmoy Kumar Bhowmick (0606049)

Motivation behind the thesis

QD photodectors, among many field of applications, will probably be most interesting if we can use it in Night Vision Camera, i.e. thermal imaging.

Electron in QD is like “particle in a 3D box”, its energy bands are discrete, spike-type Density of State. Thats why Quantum Dot is called “Artificial Atom”.

Due to this atom-like behavior , QD detector is fundamentally resistant to thermal noise. Unfortunately, we still have to operate these QD detectors at very low temperature, say, at 80 degree kelvin, and the cooling system increases both cost and size of detectors.

However, if researchers can find a way to remove some problems in QD fabrication technique (i.e. uniform growth of QDs), it will definitely outdo all present IR detectors both in terms of performance and costing. Thats the key for us, the engineers !

Prerequisites

1. Necessary courses:

Optoelectronics

Heterojunction Semiconductor

Fiber Optics

These courses are to give you the basic understanding on photo-detectors, their performance scale (i.e. absorption co-efficient, responsivity, background noise), their limitation etc.

2. Necessary Books:

Atom to Transistor (prof. Supriyo Datta)

Physics of Optoelectronic Devices (Shun Chuang)

These books are to give u the concept on how to model a device for mathematical/ numerical analysis.

3. Other documents:

various introductory articles papers on Quantum Dot Photodetectors

a) “Hot Dot Detectors”, by prof. Sanjay Krishna, prof. Pallab Bhattacharya (university of Michigan)

b) “Evaluation of the fundamental properties of quantum dot infrared detectors”, by prof. Jamie Phillips (univ of Michigan)

These articles will give you the idea why Quantum dot detectors are promising in IR detection mechanism in near future, what are the main limitations of QD that researchers are yet to solve, etc.

Significant Outcome

QD detectors have two very interesting properties:

1. It provides discrete energy band for electron, and the intersubband gap can be modified by changing few parameters, i.e. bias voltage, quantum well width etc.

Thus QD detector will be a very useful tool for multispectral detection, since you can tune its spectral response both before and after its fabrication.

Our 1st paper, “Effect of Asymmetric Well Quantum Dots-in-a-well Infrared Photodetectors on Density of States Using NEGF Formalism”, has numerically verified the fact of the above mentioned tuning capability of QD detectors.

You can read this paper from the ScienceDirect archive:

Effect of Asymmetric Well Quantum Dots-in-a-well Infrared Photodetectors on Density of States Using NEGF Formalism

2. Another promising aspect of QD is, they, unlike other detectors, can detect normal incident light. This property will facilitate us to reduce cost associated with special optical arrangement (i.e.- no need for grating, opto-coupler any more).

Our 2nd Paper, “Influence of Quantum Dot Dimensions in a DWELL Photodetector on Absorption Co-efficient” mathematically substantiates that, if the QD dimensions can somehow be changed, the absorption of normal incidence light will then improve significantly.

Link for the second paper:

Influence of Quantum Dot Dimensions in a DWELL Photodetector on Absorption Co-efficient

Major Problems Encountered

1. The QD heterostructure, due to its asymmetry, is very tough to model in finite domain.

2.The mathematical operation, i.e. the simulation not only is time consuming (will take more than a day to complete), but also requiring immense memory (one computer can hardly manage to provide all memory necessary)

3. The modeling and analysis of QD devices is relatively a new chapter in theoretical research. We had to go with insufficient samples and experimental data.

Areas of Further Improvement

1. Modeling of coupling between two neighbor QD is really a challenge to overcome.

2. It is yet to explore, what impact does “Non-uniform distribution of QD island” exert on device behavior.

3. Our model has overlooked the effect of the doping concentration of the dot material, an important parameter for absorption co-efficient.

4. “Phonon-bottleneck” effect, which causes increase in relaxation time, a much expected phenomenon for detectors, is yet to include in the device model.