Atomic Force Microscope (AFM) and Scanning Tunneling Microscope (STM).
The STM uses the principle of Electron Tunnelling. It passes a very sharp typically tungsten tip right above a sample and measures the induced current across the tip. This gives really high resolution if you get it close as a 10x decrease in distance corresponds with a increase in current.
The AFM rubs a cantilever and tip across the surface and uses the actual cantilever deflection to measure the surface.
Two forms that we can discuss are
- stimulated emission
- spontaneous emission
Spotaneous emission occurs when an electron falls from the conduction band to the valence band, emitting a photon. This photon’s wavelength is directly determined by the band gap between the conduction and the valence band. This is the principle behind LEDs
Stimulated emission occurs when a photon approachs a system with electrons in the conduction band, these electrons then choose to lose energy by emitting a precise clone of the incoming photon, with the same wavelength, and same phase. This is the principle behind Optical Amplifiers, and therefore LASERs
initially seems like a great choice, as it contains 6 very reactive fluorine ions, that can very effectively etch the Silicon
is notable for being Electronegative. This means that unlike most chemicals that dont like being an ion, due to imbalance of charge, SF6 is very likely to scoop up an electron and become and remain stable that way. What this practically means, is that we lose electrons consistently from our RIE Plasma. This is important as electrons are fundamentally what sustain the plasma, so if we remove these electrons, we run the risk of the plasma collapsing and arcing happening.
Specifically in the context of etching Silicon with KOH, Crystal orientation is very important. Silicons <111> orientation is a very dense layer with not a lot of free bonds which affects KOH’s etching ability, in fact, we observe that etching basically stops once it reaches the <111> plane. What this means is that in the case of etching <100> plane Silicon, we get a diagonal channel/inverse pyramid where our etch will stop. This is very important as it means that if we have a high aspect ratio, there is a possibility that we simply wont make it deep enough before hitting the <111> plane. In the case of <110> , the <111> plane is perpendicular and therefore we get a perfectly anisotropic etch
We need to be careful of our temperature as this impacts our wet etch significantly as well as considering macro-loading factors.
Q6- a)
b)
c) iv
7a) Like a disc with 4 cut its so it looks like a 4 leaf clover You can measure resististivty, conductivity, mobility
We need to ensure there are no isolated holes and the sample is basically two dimensional, as well as it being homogenous and the contacts being at the edges of the sample
b)
c) We are now potentially in intrinsic, and if its negative this means we are n-type
Q8 a) Optical confinement essentially means trapping the photons, which then means we can keep them in the optical amplification area, more effectively applying gain to the input photon
Carrier Confinement essentially revolves around trapping charge carriers in an state that they dont have enough energy to escape, if this is narrow enough we actually begin getting quantisable energy levels that these trapped charge carriers can be, what this means is that unlike the smooth hump we see with bulk emitters, with QWs we get a staircase of states that charge carriers can be. This means we get less modes of light coming out, and get closer to very pure wavelengths of laser
c) A is the actual quantum well B is the optical confinement zone and C is the cladding ii) to prevent light from refracting as much and leaving the confinement zone
iii) MOCVD and MBE Metal Organic Chemical Vapour Deposition and Molecular Beam Epitaxy
MOCVD is much higher throughput, and is much cheaper, but MBE is much more precise
d) Optical Fibre Communications use lasers to modulate very fast pulses to transmit information with significantly less loss than regular wires.
We can use lasers to deliver very precise heat to cut materials such as plsatic
9a) The surface composition, if the analyser magnet isnt working well or slit is too wide, youll get unwanted ions being implanted
b) They should investigate how deep the implanted ions are going, as well as a distrubtion of depths to check for channeling
c) This is pointing towards a channeling issue, [diagram of crystal ortientation and of channleing].
We could try fix this by tilting the sample a little, by about 7 degrees, or adding a thin amorphous layer on top of the sample for implantation, before removing later