Manufacturing Processes Puzzles on “Electrochemical Etching – 10”.
1. Pore formation in μpSi is independent of quantum confinement.
a) True
b) False
Answer: b
Clarification: Pore formation in μpSi is a complex phenomenon involving different mechanisms, i.e. quantum confinement, crystallographic face selectivity, tunnelling and enhanced electric field.
2. μpSi structures have _____________
a) high porosity
b) longer life
c) organised pores
d) low density
Answer: a
Clarification: Microporous silicon structures can be obtained by electrochemical etching of p-type silicon wafers. μpSi structures have high porosity and also feature a porous structure with silicon walls of a few nanometres thick separating adjacent micropores.
3. In the quantum confinement model for μpSi structures, energy band gap in the wall region varies.
a) True
b) False
Answer: a
Clarification: The formation of μpSi structures can be explained by the quantum confinement model, in which the energy band gap in the wall region increases as a result of quantum confinement effect, generating an energy barrier for electronic holes.
4. In μpSi structures, depletion of holes is affected by the energy barrier generated in the wall region.
a) True
b) False
Answer: a
Clarification: If the energy barrier in the wall region( as stated in the previous question) is bigger than that of the bias-dependent energy of electronic holes, the porous structure is depleted of holes and thus passivated from dissolution during the etching process.
5. The effective medium concept, where the optical properties of the material are established by its ________________
a) pore distribution in the structure
b) band gap
c) pore thickness
d) wavelength of the incident light
Answer: b
Clarification: The interaction of light with mpSi and μpSi structures is explained by the effective medium concept, where the optical properties of the material are established by its band gap. Therefore, the interaction between light and matter in these porous structures can be designed by engineering their effective refractive index.
6. The refractive index of mpSi and μpSi structures has some anisotropy.
a) True
b) False
Answer: a
Clarification: The refractive index of mpSi μpSi structures and presents certain anisotropy along the specific crystallographic axes. This anisotropy, which can range from 1 to 10 %, is associated with the elongated pore shape along the growth direction.
7. The porosity of mpSi and μpSi relies mainly on the ______________
a) thickness of the material
b) current density
c) etching agent
d) temperature
Answer: b
Clarification: The porosity of mpSi and μpSi relies mainly on the current density applied during the etching process. For this reason, multi-layered mpSi and μpSi structures, which consist of stacks of porous silicon layers featuring different levels of porosity, can be produced from top to bottom by modifying the current density in the course of the etching process.
8. μpSi and mpSi structures are widely used to produce optical structure.
a) True
b) False
Answer: a
Clarification: As a result of their geometric and optoelectronic properties, these structures have been extensively used to produce a variety of optical structures such as micro-cavities, distributed Bragg reflectors, waveguides, omnidirectional mirrors and rugate filters.
9. The effective refractive index of pSi can be varied with the current density.
a) True
b) False
Answer: a
Clarification: The effective refractive index of each layer is directly related with the porosity level. So, the effective refractive index of pSi can be engineered in depth by alternating the current density during the etching process.
10. Thickness of layers in the mpSi and μpSi structures can be controlled by_____
a) current density
b) etching agent
c) etching time
d) porosity
Answer: c
Clarification: The thickness of each layer can be precisely controlled by the etching time as the former is directly proportional to the latter. Therefore, the interaction between incident light and matter in mpSi and μpSi structures can be engineered to produce optical nanostructures for a broad range of applications.
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