This set of Manufacturing Processes Multiple Choice Questions & Answers (MCQs) focuses on “Electrochemical Etching – 11”.
1. mpSi and μpSi structures are used to develop distributed Bragg reflectors.
Explanation: Multilayered mpSi and μpSi structures can be used to develop distributed Bragg reflectors (DBR). Basically, a pSi-based DBR structure consists of multiple layers of pSi with different porosity levels and thicknesses.
2. The structure of the mpSi- or μpSi-based DBR affects their refractive indices.
Explanation: The interfaces between two consecutive layers of dielectric material reflect incident light due to the effective refractive index contrast. Therefore, constructive light interference can be engineered by designing the structure of the mpSi- or μpSi-based DBR (i.e. by designing the porosity and thickness of each layer).
3. DBR structures based on_____ can be implemented into Fabry–Pérot filters.
Explanation: DBR structures based on mpSi and μpSi can be implemented into Fabry–Pérot filters, also called optical microcavities, which are composed of two parallel DBRs with a space layer of different effective refractive index in between.
4. The effective optical thickness of the space layer is taken equal to _____
d) product of refractive index and density of the material
Explanation: The effective optical thickness of the space layer (i.e. product between the physical thickness and the effective refractive index of the layer) is typically engineered as the wavelength of the incident light or half of it.
5. Microcavities based on mpSi and μpSi can be used to develop optical pass-band filters.
Explanation: The reflectivity spectrum of microcavities based on mpSi and μpSi shows a narrow pass-band centred at the wavelength where the light is reflected with the highest intensity. Therefore, the structural design of microcavities based on mpSi and μpSi makes it possible to develop optical pass-band filters and sensors.
6. The effective refractive index of mpSi and μpSi are modulated to produce rugate filters.
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.
7. Which of the following is one of the important optical property of mpSi and μpSi structure?
b) Stimulated emission
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.
8. The discovery of bright red-orange photoluminescence was the origin for the use of mpSi materials for optoelectronic devices.
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.
9. PL properties of mpSi and μpSi structures depend on _____
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.
10. An increment of the current density can lead to blue shifts in the PL peak of mpSi.
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.
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