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Quantum devices > Semester 1 > Flashcards

Semester 1 Flashcards

1
Q

What is bottom-up fabrication?

A

An additive process where the structures and features can be built as required from atoms and molecules.

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2
Q

What is top-down fabrication?

A

a subtractive process based on a bulk crystal material, which is then engineered/modified to form a particular product.

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3
Q

What are some requirements for nanoscale features?

A
  • Controllable sub nanoscale layer deposition.
  • Flexible growth modes ( in BU fabrication) towards quantum confinement for 2D, 1D and 0D structures.
  • Uniform and conformal deposition.
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4
Q

What is the main technique used for nanoscale features?

A

Epitaxy

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5
Q

What is epitaxy?

A

When a semiconductor layer or set of layers are deposited/grown on a bulk crystal substrate wafer, it is considered an epitaxial layer.

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6
Q

What are the two distinct types of epitaxy?

A

Homoepitaxy and heteroepitaxy

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7
Q

What is homoepitaxy?

A

The composition of the layer grown is essentially the same as that of the substrate (e.g. Si/Si, n-GaAs etc)

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8
Q

What is heteroepitaxy?

A

The composition of the layer grown differs from the substrate.

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9
Q

What are the three most common groups of epitaxial semiconductor layers?

A

Si, III-V and II-IV compound semiconductors

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10
Q

What are the 3 main epitaxial growth techniques?

A

Liquid phase epitaxy (LPE), vapour phase epitaxy (VPE) and molecular beam epitaxy (MBE).

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11
Q

What is liquid phase epitaxy (LPE)?

A

Melting liquid to a solid surface

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12
Q

What is vapour phase epitaxy (VPE)?

A

Transporting vapour to a solid surface

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13
Q

What is molecular beam epitaxy (MBE)?

A

Transporting solid to a solid surface.

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14
Q

What is MOVPE?

A

metal organic vapour phase epitaxy

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15
Q

What is chemical vapour deposition (CVD)?

A

A method to deposit a solid thin film from gaseous phase material.

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16
Q

What is physical vapour deposition(PVD)?

A

Mainly, a vaporization coating technique

17
Q

What is Moore’s law?

A

The observation that the number of transistors in a dense integrated circuit doubles about every two years

18
Q

What are the two main principles behind Moore’s law?

A
  • Density of transistors doubles every two years.

- Complexity of transistors for minimum cost doubles every two years.

19
Q

What are the four benefits of moving to nanoscale devices?

A
  • Cost of manufacturing.
  • Expanding functionality of devices/chips.
  • Energy consumption per devices/chips.
  • Reduce consumption of raw (sometimes scarce) materials.
20
Q

What are 2 drawbacks of nanoscale features?

A
  • Methods of fabrication can end up being too slow or complex, leading to high cost of manufacture.
  • Smaller devices suffer larger current leakage,
21
Q

Name 2 examples of bottom up fabrication.

A
  • MBE

- MOVPE

22
Q

Name a device made by bottom-up fabrication.

A
  • VCSEL laser

- Multijunction solar cells

23
Q

Name an example of top-down fabrication.

A

Erosion of the bulk crystal is carried out by wet or plasma etches.

24
Q

Name a device made by top-down fabrication.

A

Crystalline Silicon solar cell.

25
Q

What are the 3 main methods to create III-V nanowires by either MBE or MOVPE?

A

1) Catalyst assisted growth.
2) Self catalysed nanowire growth.
3) Selective area growth (MOVPE only).

26
Q

How do you derive the expression for growth rate (GR)?

A

Though using the expressions for flux at the grade boundary (j1) and flux at the gas-solid interface (j2) in steady state where j1=j2.

27
Q

What is the limitation of conventional UV photolithography?

A

It relies on the optics and optical path for ensuring even the smallest feature can be achieved, but due to the refractive index being higher for immersion lithography the resolution limit cannot be reached.

28
Q

What are some alternative techniques to conventional UV photolithography?

A
  • Extreme UV (13nm) but this would be complex.
  • E-beam lithography.
  • Nano imprints
29
Q

What type of emission does the semiconductor diode laser use?

A

Stimulated emission.

30
Q

What is stimulated emission?

A

When a photon of energy equal to that of the bandgap of the semiconductor, impinges on an excited atom which can be “stimulated” to relax in the ground state and as such can emit a photon with the same energy and direction.

31
Q

What semiconductor group is usually used to fabricate a diode laser?

A

III-V semiconductor group.

32
Q

What deposition method is usually used to create a semiconductor diode laser?

A

Either MBE or MOVPE to control the interfaces with low defects.

33
Q

What type of structure is usually required for a semiconductor diode laser?

A

A double heterostructure where the heterostructure is composed of a narrow bandgap semiconductor sandwiched between two wider bandgap semiconductors.

34
Q

What two confinement principles are required to achieve emission in semiconductor diode lasers?

A
  • Carrier confinement (population inversion of carriers).

- Optical confinement (total internal reflection)

35
Q

How can energy consumption be reduced in semiconductor diode lasers?

A

Reducing the threshold current by narrowing down the dimension of the confinements using quantum wells and achieve population inversion with lower injected current.

36
Q

Name 3 types of photonic device (other than the laser diode) using a pn junction and provide the bias state for each device during normal operation.

A
  • LED based on spontaneous emission (forward bias).
  • Photodiodes based on light absorption (reverse bias).
  • PV cells based on energy conversion from photon absorption (forward bias).
37
Q

What benefits does a p-I-n structure have over a pn junction?

A
  • Higher quantum efficiency due to extending depletion region.
  • Widening the depletion region also lowers capacitance increasing response time.
38
Q

What is the ITRS?

A

International technology roadmap for semiconductors.

39
Q

What is the primary goal of ITRS?

A

To develop and maintain a document representing a 15 year assessment for the nano-electronic industries future technology.

Quantum devices (3 decks)

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