Homeworks Samuelson Paper

 

What is meant by the “bottom–up” and “top-down” approaches to nanoscience and nanotechnology?

 

Bottom-up is building larger structures and devices from smaller structures such as individual atoms or molecules.

 

Top-down is the classical approach of the current semiconductor industry, a miniature device is produced by starting with something larger and removing the parts with one does not need and adding other parts such as interconnects, etc. which one does need.     4 points in total

 

 

What limits the dimension of devices that are achievable by the top-down approach?

 

Achievable line widths of lithography, perhaps 50 nm will become viable, but below this probably there is no viable parallel processing (mass production) technique available anymore. 2 points

 

 

What are Leo Esaki and Zhores Alferov are exactly credited for?

 

They received Nobel prizes in physics for:

 

Esaki, quantum well and tunneling diodes by various means, employing quantum effects for electronic devices.

 

Alferov, epitaxial heterostructures and double-heterostructure devices, employing quantum effect devices for optoelectronic devices.                                                                              2 points

 

 

Why are quantum dots sometimes called artificial atoms?

 

The can be modeled by the particles in a box model and have discrete energy levels, because of this similarity to atoms, and because they are man made, they are called artificial atoms.   2 points

 

 

What is a quantum well, how can one model it?

 

Quantum wells consist of ultra-thin layers of small band gap semiconductors sandwiched between larger band gap semiconductor materials. One can model a quantum well as a square box with finite potential energy barriers of finite thickness.                                                                                   3 points

 

 

Does GaAs have a smaller or larger band gap than AlGaAs? Hint see figure 1

 

Smaller, it is the area where the charge carriers are bound as one can see in Fig. 1.   1 point

What is the potential energy function one needs to input into Schrödinger’s equation in order to model an atom?

 

Columbic spherical:           1 point

 

Are the energy levels in a hydrogen atom evenly spaced? If not, what is the spacing of the energy levels of the first and second excited states?

 

no, 13.6 eV divided by n2  is the general rule, so 13.6 eV / 4 = 3.4 eV is the first state and 13.6 eV / 9 » 1.5 eV is the second exited state, making the difference about 1.9 eV    3 points

 

 

Are the energy levels in an infinite square well evenly spaced? If not, what is the general formulae for their spacing?

 

no, it’s                                                               2 points

 

 

Are the energy levels of a harmonic quantum oscillator evenly spaced? If not what is their spacing?                                                                                     1 point

 

yes,                                                                                      

 

 

At which energy is the lowest energy level of a bound electron in a hydrogen atom?

 

-13.6 eV                                                                                              1 point

 

 

At which energy is the lowest energy level of an electron of frequency 105 Hz in the potential of a harmonic oscillator.

 

0.5 times 6.625 10-29 J   corresponding values in eV are also OK                   1 point

 

What is the typical potential energy depth of a finite square well that models a semiconductor quantum well? Again consider bound states.

 

- 0.3 eV                                                                                               1 point

 

 

 

How does one construct a potential energy barrier in a semiconductor?

 

One inserts a layer with a larger band gap into a structure with a smaller band gap.  2 points

 

 

How does the tunneling probability through a barrier depends on the thickness of the barrier?

 

It’s an exponential decay.                                                                                            2 points

 

 

What is the main reason that top-down approaches to nanostructures had limited success?

 

Process induced damage during patterning and etching prevents the devices from working properly

                                                                                                                                                1 point

 

What is the Stranski-Krastanow growth mode?

 

A thin compressively strained semiconductor film of a smaller bandgap that the substrate spontaneously converts into nanoscale island – which become “proper” quantum dots when they are coved by another layer of a larger band gap material                                                                                                2 points

 

 

By which methods other than Stranski-Krastanow growth are self-assembled semiconductor quantum dots formed ?

 

Colloidal or by means of an aerosol, i.e. chemical methods                                                   2 points

 

 

What are two important differences between Samuleson’s rod-like nanostructures and carbon nanotubes?

 

Samuelson’s nanocrystals are tiny semiconductor cylinders when the whole diameter is made of common semiconductor material such as GaAs, while carbon nanotubes are tubes of carbon, i.e. the resemble cylinders that are hollow inside.                                                                              2 points

 

 

How is a quantum dot inserted into one of Samuleson’s nano wires?

 

A thin InAs or InP sheet is directly grown in a GaAs or GaP nanowire,                                 1 point

 

all three dimensions are then in the nanometer range, so one can model it as a 3D finite potential barrier box