The aim of this project is to make quantum dots and wires using materials and process steps which naturally self-organise into low-dimensional systems. This could provide a viable alternative to the use of nanolithography in producing quantum size effects in zero and one dimensions. The work will be targeted at optical and transport properties.
Electronic material systems are being investigated with a view to making quantum dots and wires using materials and process steps that naturally self organize into low dimensional systems.
Growth techniques have been developed to produce a range of zeolites suitable for semiconductor containment. Adequately sized single crystal zeolite Y, silicalite II, ALPO5, SAPO4, rho, silicate I and MCM41 have been grown. Sufficient stocks of these are now held for semiconductor incorporation. These zeolites should enable us to produce dots and wires in the size range 5.3 to 38 angstroms. Indium phosphorus and gallium phosphorus have been inserted into the channels of zeolite Y and silicon into silicates. Nuclear magnetic resonance (NMR) indicates that some measure of success has been achieved, but photoluminescence (PL) is very weak.
Work on the chemical structure of the porous silicon surface has demonstrated that the role of hydrogen is as a passivating agent not as a luminescence centre. Total replacement of the hydrogen by oxygen can result in highly luminescent material. Transport measurements in conjunction with transmission electron microscopy (TEM) indicate that the porous silicon examined does not contain quantum wires but is made up of isolated dots of silicon in a silica matrix. Calculations using a new ab initio real space technique are under way to examine the properties of these structures.
It is now possible for us to controllably grow dots of the semimetal erbium arsenide in a gallium arsenide matrix with diameters in the range 10 to 25 angstroms. A number of wires with a diameter of about 20 angstroms have also been grown and made into samples for transport studies. Universal conductance fluctuations have been observed in these structures.
APPROACH AND METHODS
Three very different systems are being investigated with a view to achieving self-organising low-dimensional structures. The first is to use a class of solids known as zeolites to provide template for the growth of semiconductors. Zeolite crystals grow with naturally occurring cages and columns. The dimensions of these are uniquely defined by the crystal structure, and to some extent, this can be tailored to meet specific requirements. We plan to grow a range of custom designed zeolites and to incorporate semiconductors into them, primarily by MOCVD. The growth work will be guided by theoretical calculations of the optical and transport properties, and supported by detailed luminescence and transport measurements.
The second system that is being investigated is based on a phase separation process which has been observed to occur during MBE. Using this technique it is possible to produce a three dimensional array of quantum dots with well defined sizes chosen within the range 12 - 50 angstroms. So far, this work has produced erbium arsenide dots in a gallium arsenide matrix, but other materials will be produced using analogous techniques.
The third system is produced by an electrochemical etching technique. This is a naturally limiting process which is believed to produce narrow wires of silicon which exhibit quantum confinement. This work will be conducted in collaboration with the EOLIS (7228) project.
The main attraction of using these techniques for producing low-dimensional systems is their potential low cost and high yield. Applications in optical devices, communications and fast signal processing are foreseen.