1. The effective refractive index of mpSi and μpSi are modulated to produce rugate filters.
a) True
b) False
Explanation: If the current density profile is modified in a sinusoidal manner in the course of the electrochemical etching, the effective refractive index of mpSi and μpSi can be modulated to produce rugate filters, which are another type of optical structure used in optoelectronic and sensing applications.
2. Which of the following is one of the important optical property of mpSi and μpSi structure?
a) Photo-conductivity
b) Stimulated emission
c) Photo-luminescence
d) Photo-emissivity
Explanation: Another important optical property of mpSi and μpSi structure is their photo-luminescence (PL), which has been extensively studied during the last decades. Bulk crystalline silicon presents a very weak PL peak at 1100 nm.
3. The discovery of bright red-orange photoluminescence was the origin for the use of mpSi materials for optoelectronic devices.
a) True
b) False
Explanation: Photo-luminescence limits the use of silicon to develop optoelectronic devices, which are aimed at converting light into electricity (e.g. photodetectors, solar cells, etc.). However, the discovery of bright red-orange photoluminescence and the identification of confinement effects in the absorption spectrum of pSi were the starting point of a flood of studies focused on the development of optoelectronic devices based on mpSi and μpSi structures such as switches, displays and lasers
4. PL properties of mpSi and μpSi structures depend on _____
a) temperature
b) etching conditions
c) light intensity
d) refractive index
Explanation: PL properties of mpSi and μpSi structures depend on the etching conditions. The different PL bands in pSi can be tuned from blue-green, red-orange to infrared by adjusting the etching conditions.
5. An increment of the current density can lead to blue shifts in the PL peak of mpSi.
a) True
b) False
Explanation: An increment of the current density, a decrease in the concentration of HF or an increase of the illumination intensity lead to blue shifts in the PL peak of mpSi and μpSi. In addition, it is worth stressing that PL in pSi is dependent on the porosity level as well as the doping density
6. In metal-assisted etching of porous silicon, the oxidant is reduced at the noble metal surface.
a) True
b) False
Explanation: The oxidant is reduced at the noble metal surface due to its catalytic activity. The reduction of the oxidant generates electronic holes, which are subsequently injected into the silicon substrate through the noble metal interface
7. Electronic holes in metal-assisted etching of porous silicon oxidise silicon atoms.
a) True
b) False
Explanation: Electronic holes oxidise silicon atoms at the interface noble metal-silicon, which are then etched away by HF molecules. The concentration of electronic holes is maximum at the noble metal-silicon interface and thus its dissolution rate is much faster there.
8. Which of the following is used as an oxidant in metal-assisted etching?
a) Sulphuric acid
b) Nitric acid
c) Oxygen bubbles
d) Peroxy-di-sulfuric acid
Explanation: Regarding the type of oxidant, hydrogen peroxide is the most commonly used although other oxidants such as oxygen bubbles, oxygen dissolved in water, silver nitrate (AgNO3), chloroauric acid (HAuCl4), potassium gold (III) chloride (KAuCl4), chloroplatinic acid (H2PtCl6) have been explored as well.
9. The holes which are left out after oxidation of silicon atoms enter the bulk silicon region.
a) True
b) False
Explanation: The remaining electronic holes (i.e. those not consumed during the dissolution of silicon) diffuse from the interface noble metal-silicon to the bulk silicon
10. The structural features of pSi produced by metal-assisted method independent on illumination.
a) True
b) False
Explanation: The resulting structural features of pSi produced by metal-assisted approach rely on different parameters such as the etchant composition and its temperature, the nature of the noble metal, the illumination and the intrinsic properties of the silicon substrate (e.g. doping type and level, crystallographic orientation, etc.).