Download Solutions - T. Serkan Kasirga

MSN 517 - Autumn’14-Rev.TSK.1
Homework 3- Solutions
Due date: 27/12/2014 in-class
1. Calculate the wavelength of a photon emitted from a 1.1 nm radius CdSe
quantum dot. There are several parameters that you need to look up.
We have done almost the same exercise in class: Here radius is given,
diameter is twice the radius so
𝐸 ≈ 1.74 𝑒𝑉 +
ℏ2 𝜋2
2𝑙 2 𝜇
−
1.8𝑒
𝜀𝑙
plug
in
the
numbers:
𝑙 = 2.2 𝑛𝑚 µ =
0.1 𝑚, 𝜀 = 10.1 and you’ll find this: 𝐸 ≈ 1.74 𝑒𝑉 + 3.1 − 1.36 = 3.48 𝑒𝑉
This corresponds to a wavelength of 359 nm roughly.
2. Picture on the right shows a two input circuit (inputs
A&B) with a single output. I would like you to work out
what happens on the output side depending on the 4
different voltage combinations you can have on the
inputs. What are the active elements on this circuit
called and what is this circuit useful for?
A
1
1
0
0
This circuit is a NAND (not-and) gate and used for logic
operations. Active elements here are MOSFETS.
Here is the truth table for different voltage
configurations:
B
Out
1
0
0
1
1
1
0
1
3. Explain why there is a photonic band gap in photonic crystals (a material that
contains periodically alternating stacks of layers with different refractive
indices). For the sake of simplicity you may use a 1D photonic crystal for your
explanations.
MSN 517 - Autumn’14-Rev.TSK.1
The reason for the photonic bandgap is the periodicity of the structures that
are in the same length range as the wavelength of the light. If you recall, the
Bloch theorem states that electrons can only scatter from impurities in a
crystal but not the ion cores. Same applies for light, so when we introduce
periodically arranged impurities into the system, interference of the light
scattered from these impurities eventually leads to forbidden and allowed
bands.
4. If I ask you to get me an electronics grade Si wafer, what method is probably
used to grow it and why do you think so? You may list more than one
technique if you like.
Most probably what you’ll get would be fabricated by floating zone or
Czochralski method since these techniques can produce high purity, high
crystallinity Si.
5. During the lectures I’ve used the term “epitaxy” or “epitaxial layer” many
time without explaining what it is. If you already haven’t checked it now is a
good time to do so! Explain what epitaxial growth means.
As the mighty Wikipedia says it is deposition of a crystalline overlayer on a
crystalline substrate. Overlayer has to follow the crystal structure of the
substrate. There are various types of epitaxial growth.
6. Explain the differences between MOCVD and ALD.
In both deposition methods some compounds of the materials needed to be
deposited are used. However, in ALD two different precursors are used in a
self-limiting manner and layer by layer deposition is performed whereas in
MOCVD metal-organic precursor is reacted with certain gases to deposit the
material.
7. What are the major disadvantages of the PLD technique?
It is difficult to get uniform film thickness in large areas and it is difficult
apply this technique at an industrial scale.