Work Area: Multilayered Materials for Silicon-Compatible Opto-Electronics
Keywords buffer layers, III-V growth, strain relaxation, CBE, ALMBE, MBE, dislocation density
Start Date: 24 July 92 / Duration: 36 months / Status: running
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Abstract BLES aims to establish design rules for the controlled growth of buffer layers on GaAs, InP and Si substrates. Buffer layers should provide the selected lattice parameter and a defect density no greater than normally available in conventional substrates. Growth in this programme will largely be by MBE, atomic layer MBE (ALMBE) and chemical beam epitaxy (CBE).
The major objectives are to:
It is clear that in the field of III-V devices buffer layers are necessary and are of crucial importance to the controlled manufacture of devices. However, there is little understanding of what makes a good buffer layer. BLES is proposing to make a systematic study in order to gain a complete understanding of the way in which defects are incorporated into growing buffer layers, and of the effect which specific defects then have on electronic or optical performance. This three-way correspondence between growth, characterisation and performance requires more detailed and systematic study than most industrial laboratories are able to mount.
The novelty of the approach is that, firstly, BLES intends to avoid generating threading dislocations rather than trying to filter them out, and secondly, that the designs will actively prevent damage from propagating downwards in a structure. A variety of relaxed and partially-relaxed buffer layers will be grown by MBE, ALMBE and CBE in order to test the universality of an empirical law of strain relaxation over a wide range of compositions and strains. It is hoped that the results will enable the specification of design rules for buffer layers that are applicable to many growth methods and materials. The systems to be studied in most detail are InGaAs/GaAs, InGaAsP/GaAs, InGaAs/InP and GaAs/Si.
Buffer layer relaxation has been studied for InGaAs on GaAs for a range of In content up to 50%. All layers follow the same strain relaxation behaviour, following an empirical curve with a threshold then a steady reduction in residual strain. This appears to hold for material grown by MBE, ALMBE and CBE, as long as 2D growth conditions can be maintained.
Layers under tensile strain (eg GaAsP on GaAs) may crack and do not follow the empirical curve even after annealing. Considerable progress is being made by the consortium in the rational design of multilayers involving strain steps with and without tensile components such that relaxation occurs at predictable interfaces. The fundamental basis of the empirical curve is being investigated.
It is clear that there is considerable potential for the design of layers with controlled concentrations of threading dislocations suitable for subsequent device growth.
A European workshop is planned for 1994 at which buffer layer relaxation data will be discussed and initial design rules will be presented.
University of Liverpool - UK
Dept. of Materials Science and Engineering
P.O. Box 147
UK - LIVERPOOL L69 3BX
Centro Nacional de Microelectronica, Madrid - E
Universidad de Cadiz - E
Universidad Politecnica de Madrid - E
Optronics Ireland - IRL
University of Surrey - UK
Prof. P. J. Goodhew
BLES - 6854, August 1994
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html version of synopsis by Nick Cook