Report—Photonics & Nanotechnology Presentation #3
| March 11, 2014 | Posted by Frank Gomez under CN, COMSOC, CS, EDCAS |
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Report on Lecture #3 on Photonics and Nanotechnology, held on Wednesday, February 12, 2014
Professor H. Modajeri, Devry University Pomona was the presentor.
Where we are going in this Presentation #3: How can we produce a solid-state LASER, with high intensity monofrequency light? What are the fabrication techniques, at the nano-size linear dimension scale, that will use the known, and measured crystallographic orientation of the subject crystal, to produce such a structure?
Order of topics being covered today:
Crystal Lattice Structure
Bulk Crystal Growth in Semiconductors, by both the Bridgeman Method and the Czochralski Method:
Bridgeman Method can produce very large bulk crystals ( 10 inch in diameter; 20 to 30 inches in height; start with a quartz cell, bottom is a melting zone; top is a solidification zone; control the temperature gradient between the zones, say a 1 degree per mm change in heath gradient; pull the crystal from the bottom of the melt zone at a very slow rate, say 0.5 mm per hour. The result, after continuing this process for a week or so, is a single type of crystal. So, break the crucible to obtain this crystal, and cut the crystal with a diamond saw to get the final crystal with the desired orientation, say [111} for a cubic Brillouin zone)
Czochralski Method is similar in concept to the Bridgeman method, except that here have a slow controlled twist of the crystal as it is pulled from the seed at the bottom of the melt zone.
Epitaxial: Metal Organic Chemical Vapor Epitaxial (Here a chemical reaction is generated to get the final desired material. An example is the reaction:
(CH3) + Ph3-àCH4 + InP
An epitaxial growth will occur dependent upon the organization of the substrate. It is necessary to exhaust the volatile gas from the chamber where the reaction occurs.
Epitaxy: Liquid Phase Epitaxy: Here, we do not have a chemical reaction, but build up thin layers of the desired semiconductor material by the process of nucleation; specifically, control the number of layers and the thickness of the sample bycontrolling the time during which the nucleation occurs. So, the process can be simply described a obtaining a slide of graphite or similar substrate material; placing that slide in a cryogenic chamber with very low pressure at near vacuum conditions (say 10^-10 bar); starting the nucleation, and then halting it after a calculated time. Thin crystals of the semiconductors InAlAs and GaAs can be obtained by this method.
Epitaxy: Molecular Beam Epitaxy: This is by far the most expensive method, but it delivers the thinnest semiconductor structures and the most accurate semiconductor regions. No wonder it is preferred for preparing quantum barriers and quantum wells. MBE requires high vacuums and uses several electron beam guns. Multilayers can be grown for numerous II-V semiconductors (say InGaP, GaAs, GaInAs (Sb)) with well defined energy gaps from the valence bands into the conduction bands in the k-space zone structure. The crystal structure of the substrate is maintained.
To illustrate these concepts, Professor Mohajeri finished up his lecture by showing several short YouTube videos illustrating the main lecture points. The first was provided by Sharp Electronics, and featured a multiview of the cryogenic equipment and electron beams developed by Thames Cryogenics. The second was generated by Vladimir Kaganer of the Paul Drude Institute. This was a video modeled-simulation of the Monte Carlo Process that show what is occurring in an ideal MBE process. These are (1) island nucleations where flat two dimensional islands of a single material are formed on a flat substrate;(2) layer by layer growth of the separate materials desired for the compound semiconductor;(3) step flow growth when the crystal surface is inclined with respect to the lattice planes; and (4) roughening, which occurs at low temperature when some atoms do not have enough energy to find the favorable sites near the step edges. These would have to be removed later by polishing. A most intriguing Monte Carlo simulation of the laser formation problem was captured in this video.
No, Prof Mohajeri did not give it as a homework assignment to the attending engineers and engineering students. At the very least, Prof Mohajeri would expect the attendees to appreciate the materials development issues that are involved in producing the photonic / nanotechnology devices that the attendees would use in their electro-optical engineering systems.
The IEEE Foothill Section is looking forward to the next presentation in this Photonics / Nanotechnology Series in April, 2014. Thank you, Professor H Mohajeri.