1. The pore diameter in macroporous silicon can be periodically modulated.
a) True
b) False
Explanation: Following the approach based on the Lehmann’s model, the pore diameter in macroporous silicon can be periodically modulated. This approach takes advantage from the fact that the porosity of the resulting MpSi structure is established by the ratio between the total current density and the critical current density (i.e. JPS—current density limit between the formation of pSi and electro-polishing of silicon)
2. Increase in the total current density produced by the generation of electronic holes increases the pore diameter.
a) True
b) False
Explanation: The critical current density is constant at the pore bottom tips and thus the increment of the total current density produced by the generation of electronic holes widens the pore diameter. Following this procedure, the pore structure of MpSi can be modulated to produce periodic ratchet-type (i.e. asymmetric) or circular (i.e. symmetric) pore modulations.
3. If the etching conditions are not controlled, then _____ decreases with pore depth.
a) the period length
b) diameter of the pores
c) orderliness of the pores
d) reflectivity of the material
Explanation: If the etching conditions are not controlled, the period length, which is defined as the distance between two consecutive pore modulations, decreases with the pore depth due to diffusion limitations (i.e. lower concentration of HF at the pore bottom tips). As a result, the critical current density decreases and the pore growth rate becomes slower.
4. Post-treatment starts with a thermal treatment of the MpSi structure at _____
a) 900°C
b) 1100 °C
c) 1570°C
d) 1650°C
Explanation: A post-treatment is required after the fabrication of MpSi. This post-treatment starts with a thermal treatment of the MpSi structure at 1100 °C for 100 min under oxygen atmosphere
5. During post treatment of MpSi, a layer of silicon dioxide is formed.
a) True
b) False
Explanation: A thermal treatment of the MpSi structure at 1100 °C for 100 min under oxygen atmosphere generates a silicon dioxide layer of 200 nm along the inner surface of the pores of MpSi
6. Pore formation in μpSi is independent of quantum confinement.
a) True
b) False
Explanation: Pore formation in μpSi is a complex phenomenon involving different mechanisms, i.e. quantum confinement, crystallographic face selectivity, tunnelling and enhanced electric field
7. μpSi structures have _____________
a) high porosity
b) longer life
c) organised pores
d) low density
Explanation: 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.
8. In the quantum confinement model for μpSi structures, energy band gap in the wall region varies.
a) True
b) False
Explanation: 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
9. In μpSi structures, depletion of holes is affected by the energy barrier generated in the wall region.
a) True
b) False
Explanation: 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.
10. 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
Explanation: 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.