Transmission electron microscopy investigation of FeAs precipitates in GaAs/AlGaAs heterostructures

Furnace annealing at 850 degrees C of GaAs/Al0.4Ga0.6As heterostructures grown by liquid phase epitaxy (LPE) causes the formation of regularly shaped precipitates in a subsurface layer. Cross-sectional transmission electron microscopy (XTEM) observation of the heterostructures shows that the main boundaries of these precipitates are parallel to the low-index crystallographic planes. A < 110 > lattice image of the GaAs layer containing the precipitates was digitized and processed on a computer. From a computer diffractogram of the image we have concluded that the precipitates are FeAs (orthorhombic) crystallites, with the orientation [110](111)(GaAs)//[101](010)(FeAs). Another orientation found by TEM analysis was [110](-111)(GeAs)//[001](010)(FeAs). The composition of precipitates was confirmed by energy-dispersive X-ray spectroscopy (EDX). On the EDX spectra the Fe peak is clearly visible. We believe that Fe atoms are deposited on the heterostructure surface during the final stage of the LPE growth or during later processing steps. The annealing causes the migration of Fe atoms from the surface and the formation of precipitates.