Department of Physics

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GaSb Quantum Dots

Associate Professor Ewa M. Goldys

April 2001

Quantum Dots as New Materials

Quantum dots are semiconductor nanostructures with all three dimensions of less than tens of nanometres, either deposited on or embedded in another semiconductor. The interest in quantum dots was initially driven by a desire to create a material with electronic density of states strongly modified by quantum confinement effects (a reduction in size to less than tens of nanometers) and approaching a delta-like density of states for a truly zero-dimensional system. Such a medium was perceived to offer significant advantages for example in ultra-low threshold semiconductor diode lasers, and also presented interesting opportunities for fundamental research in the area of light-matter interaction.

An important breakthrough has been realized with the development of the self-assembled growth procedures, which can be carried out both using MOCVD as well as MBE techniques (see for example D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett., 64, 1943, (1990)). In the self-assembled growth the quantum dots are created from ultrathin layers (typically about 2 monolayers thick) which spontaneously break up due to strain between the substrate and the grown film, and minimize their energy by forming small scale islands. Size quantization in such islands has been demonstrated.

Self-assembled growth has proven to be an extremely fruitful technique which is now widely used. At Macquarie University we have made significant advances in material growth and understanding of the self-assembly growth process and its control. (Appl. Phys Lett. 73 , 1233, (1998)). We deposit GaSb quantum dots on GaAs using atmospheric pressure metalorganic chemical vapour deposition. The GaSb dots (islands) self-organise due to lattice mismatch of several percent between GaAs and GaSb. The dots can be visualised using a technique called Atomic Force Microscopy producing photographic imges.

Studies of quantum dots attract significant interest worldwide, because of their fascinating new physics and unique potential for innovative electronic and optoelectronic devices. Actually, these innovative applications are just beginning to emerge. One of them involves using quantum dots for the detection of infrared light in devices similar to the previously explored quantum well intersubband detectors. Other interesting applications include use in quantum gates at the centre of a quantum computer.

The aim of our research on GaSb quantum dots was to establish a technology to fabricate a three dimensional quantum dot composite material, a building block for future electronic and optoelectronic devices. This is achieved by depositing multiple layers of quantum dots interspersed with quantum barriers of a different material. Interestingly, the dots show some degree of vertical correlation.

Our recent results include

· Demonstration of feasibility of QD growth using atmospheric pressure MOCVD. This is significant, because of an extremely rapid turnover time possible in such systems. We are able to complete the growth process (from loading the chamber to taking the sample out) within 1 hour, while the actual QD growth takes several seconds. Such short times indicate a process which may be industrially relevant.

· Establishment of growth protocols for growth of QDs with varying sizes and densities. Our aim was to grow a high density of small dots, and we have achieved ???? give numbers here.

· identification and understanding of growth evolution. Our systematic studies of growth evolution with variation of growth parameterts indicate a variety of different scenarios, where the dilution of precursors and the growth time both play a role, in addition to the commonly recognised influences of growth temperaature and the lattice mismatch.

· optical characterisation and analysis of optical emission. In embedded films we have observed optical (photoluminescence and cathodoluminescence)emission at energies about 1.0 eV, with peak energies following the trend in dot sizes. We interpret this by a combined effect of quantum confinement and interface intermixing.

· We have also compared GaSb dots embedded in GaAs with an opposite system of GaAs embedded in GaSb, and with a II-VI system of ZnTe dots in CdSe.

Follow this link to learn about the new physics in quantum dots.

Joint Funding Sought

Our GaSb QD growth technology has reached the stage of maturity so that device applications can now be envisaged. We are interested in securing joint funding for quantum dot device research, for example on quantum dot light detectors . For more information contact Associate Professor Ewa Goldys

Our selected publications concerning quantum dots include:

1) "Microstructural evolution of GaSb self-assembled islands grown by metalorganic chemical vapour deposition", B.M. Kinder and E.M. Goldys, Applied Physics Letters, vol. 73 no 9, p. 1233-5 (1998).

2) "Cathodoluminescence study of multilayer GaSb/GaAs self-assembled quantum dots grown by MOCVD", Motlan, E.M. Goldys submitted to Applied Physics Letters

3) "Cathodoluminescence studies of self-organised CdTe/ZnTe quantum dots", M. Godlewski, S. Mackowski, G. Karczewski, E.M. Goldys, M.R. Phillips, accepted in Semiconductor Science and Technology.

4) ''GaAs in GaSb - a new type of heterostructure emitting at 2 um wavelength'', A.A. Toropov, V.A Solov'ev, B.Ya. Mel'tser, Ya.V. Terent'ev, S.V. Ivanov, P.S. Kop'ev,Motlan, and E. M. Goldys, submitted to Applied Physics Letters.

5) "Growth Optimisation of GaSb/GaAs Self-assembled Quantum Dots Grown by MOCVD", Motlan, E.M. Goldys, T.L. Tansley, submitted to Journal of Crystal Growth.

6) "Size and density control of MOCVD grown self-organized GaSb islands on GaAs" Motlan, E.M. Goldys, K. Drozdowicz-Tomsia, T.L. Tansley, COMMAD 1998, 14-16 December 1998, Perth, page 460-463.

7) " Stages of formation and self-assembly of GaSb grown on GaAs by metalorganic chemical vapour deposition" B. M. Davies, E.M. Goldys, T.L. Tansley.,18 IUVSTA Workshop "Diffusion and Growth in Ultrathin Layers", Newcastle 17-21 November 1997 November (invited talk)

8) "Metalorganic Chemical Vapour Deposition of GaSb Quantum Dots on Germanium", A. Subekti, M.J. Paterson, E. Goldys, T.L. Tansley, Thin Solid Films, vol 320, p. 166-8, (1998).

9) "GaSb films and self-assembled islands grown by MOCVD", K. Drozdowicz-Tomsia, Agus Subekti, E.M. Goldys and Melissa J. Paterson, 16 General Conference of the Condensed Matter Division of the European Physical Society, 28 August - 28 August 1997, Leuven, Belgium.

10) "The Influence of a Substrate on Self-Organised Island Nucleation Morphology of Metalorganic Chemical Vapour Deposited GaSb", A. Subekti, M.J. Paterson, {\bf E.M. Goldys} and T.L. Tansley, Applied Surface Science, vol 140, 190-6, (1999).

11) " The effects of growth temperature on the structure of GaSb/GaAs quantum dots by MOCVD" Motlan, T.L. Tansley and E.M. Goldys, 11-th International Semiconducting and Insulating Materials Conference Canberra Australia 3-7 July 2000.

12) "Scanning cathodoluminescence and electron microscopy of self-organised CdTe quantum dots" M. Godlewski, S. Mackowski, G. Karczewski, E.M. Goldys, M.R. Phillips, XXIX International School of Physics of Semiconducting Compounds. Jaszowiec-Ustron, Poland June 2-9 2000, abstract booklet p.593rd Polish-French Symposium on Vacuum Science, Technology and Applications, May 18-19, 2000, Warsaw, Poland, Elektronika (in press)

13) "Spectroscopy of CdMnS nanocrystals embedded in glass", .M.Godlewski, V.Yu.Ivanov, A.Khachapuridze, Motlan, E.M.Goldys and M.R.Phillips, XXIX International School on the Physics of Semiconducting Compounds, Jaszowiec Jaszowiec-Ustron, Poland June 2-9 2000, abstract booklet p.139

14) "MOCVD GaSb/GaAs Quantum Dots" Motlan, E.M. Goldys, T.L. Tansley, Fall Meeting, Materials Research Society, Boston, MA, USA, 27 November - 1 December 2000.

15) "Optical spectroscopy of GaSb/GaAs self-assembled quantum dots grown by MOCVD"Motlan, K. S. A. Butcher, E. M. Goldys, and T. L. Tansley, 2000 Conference on Optoelectronic and Microelectronic Materials and Devices, Melbourne, Australia, 6-8 December 2000

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