We describe here new areas of research in low dimensional structures in the Central Facility at Sheffield. In both the quantum wire and quantum dot cases, progress so far has been very good with high quality structures now available for physics research purposes, with device structures expected to be available in the near future for CF users both at Sheffield and elsewhere. An important feature of both the wire and dot work is that structures of quantum dimensions are produced with high quantum efficiency, by the use of growth related techniques without the involvement of small scale lithographic etching.
MOVPE growth of GaAs quantum wires was initiated at Sheffield about two years ago, under grant GR/J08652. The growth technique relies on the widely differing growth rates of GaAs and AlGaAs along <100> and <111> directions. The growth (performed by J S Roberts) is carried out in etched v-grooves and leads to the production of crescent-shaped quantum wires of lateral dimensions 30-500 Å and height ~50 Å. Progress has been very good with highly reproducible quantum wires being obtained on 2, 4 and 8 micron pitch gratings. Good PL linewidths of <10 meV are obtained reproducibly, with high sub-band spacing >30 meV, as required for 300K device operation. Vertical stacking of up to five quantum wires has also been achieved, with no degradation of PL linewidths. Recent successes include the production of modulation-doped quantum wires, with strongly enhanced PL emission intensity relative to that of the sidewall quantum wells, and the growth of p-i-n structures. Strong electroluminescence is observed from the p-i-n structures with quantum wire EL intensity, relative to that from the sidewall quantum wells, strongly enhanced compared to the quantum wire/quantum well intensity ratio observed in PL. Future work will focus on the study of the physics of the carrier capture processes in the modulation-doped and p-i-n structures, the investigation of many body effects in the optical spectra and the fabrication of quantum wire laser structures.
Exploratory growth of InAs-GaAs quantum dots has been carried out by M Hopkinson in the last six to twelve months. The growth technique relies on the Stranski-Krastanov growth mode which arises for strained InAs layers grown beyond the critical thickness of 1.7 monolayers. At this point the growth mode changes from 2D to 3D, mass transport occurring from the InAs "wetting" layer to the 3D islands in order to achieve strain relief. Pyramid shaped islands are formed (base size ~150 Å and height ~50 Å) of dimensions required to observe strong quantum phenomena. The samples produced in Sheffield have high quantum efficiency comparable to that found in good quality quantum wells, and show very good PL linewidths of ~35 meV indicative of size fluctuations amongst the quantum dots of only ±5%. We have obtained clear evidence for the zero dimensional nature of the electronic states involved from power dependent PL measurements and resonantly excited PL studies. The resonantly excited PL studies show no evidence for ground state PL for excitation energies less than 60 meV above the detection energy, a clear signature of the 0D character of the states, since in 0D no absorption can occur at energies between the fully quantised states of the system.
Future work will focus on further control of the growth process, magneto-optical studies to identify the symmetries of the states involved, study of carrier relaxation mechanisms in 0D systems and the fabrication of quantum dot laser structures.
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This page was created by Peter Parbrook on the 7th May 1996.