Surface Acoustic Wave Detection of Large Lattice Relaxation of Metastable EL2 in LT-GaAs
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SURFACE ACOUSTIC WAVE DETECTION OF LARGE LATTICE RELAXATION OF METASTABLE EL2 IN LT-GaAs KEN KHACHATURYAN*, EICKE R.WEBER*, RICHARD M.WHITE* *Dept. of Materials Science and Mineral Engineering, University of California and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA **Berkeley Sensor & Actuator Center, Electronics Research Laboratory, Dept. of Electrical Engineering & Computer Sciences, University of California, Berkeley, CA
ABSTRACT For the first time, surface acoustic waves (SAWs) were used to study the lattice relaxation of metastable defects. A persistent increase of as much as 0.4% of the SAW velocity at low temperatures was observed after illumination of LT-GaAs; this increase could be quenched by annealing at 120-130OK. This behaviour is caused by the metastable transition of EL2-like AsGa defects and constitutes the direct experimental proof of the illumination induced large lattice relaxation of this defect. INTRODUCTION GaAs, grown by molecular beam epitaxy (MBE) at very low temperatures (LT-GaAs) between 2000 and 3000C has a number of unusual properties, which make it a technologically important material, particularly when used to eliminate sidegating and backgating in GaAs integrated circuits [1]. Its most important property is the presence of as much as 1% superstoichiometric As [2,3], which in as-grown layers is incorporated as AsGa antisite defects. AsGa in LT GaAs has properties very similar to those of EL2 in bulk GaAs [4,5] giving rise to the similar spectral shape of the optical absorption band, which is quenchable by illumination at low temperatures [6]. For the first time, the metastability of EL2 in LT-GaAs is studied with the surface acoustic waves. SAWs, similar to ocean waves, propagate parallel to the surface, but decay with depth so that all displacements are essentially zero at a depth of about one wavelength [7,8]. The displacements in the SAW are both longitudinal and transverse, each with its own variation of magnitude vs. depth. The SAW can be of particular importance as a tool to study illumination induced persistent lattice rearrangement of metastable defects. The lattice rearrangement causes a change in elastic constants, which can be directly measured by the SAW techniques. The SAW measurements can therefore provide unique information on the mechanism of defect metastability, an issue of great current interest. A surface acoustic wave delay line [9] was used to measure the SAW velocity and propagation loss. It consists of two periodic gratings of electrodes of alternating polarity (called interdigital transducers, IDTs ) as source and as detector of the SAW (Fig.1). Excitation and detection of the SAW is due to the piezoelectric character of GaAs. From the condition of constructive interference of the SAWs launched from the individual fingers of the IDT, it follows that the excitation of the SAW is the most efficient if
fo- v/X Mat. Res. Soc. Symp. Proc. Vol. 241. @1992 Materials Research Society
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where fo is the SAW frequency, v is the SAW
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