A precursor to the breakdown of homogeneous growth in low temperature MBE GaAs detected by ellipsometry

Abstract

GaAs grown by MBE at low temperature (LT) in the range of 150-250°C forms a highly resistive film after it has been annealed. The high resistance of annealed LT-GaAs has been use to provide a semi-insulating layer lattice-matched to GaAs. For this reason, LT layers must have reproducible properties and must be of high crystalline quality for subsequent epitaxial layers. The as-grown LT-GaAs typically has 1-2% excess As which is responsible for an increase in the lattice constant of about 0.1 %. The thickness of the LT-GaAs layer is limited by the strain energy in the film which leads to the formation of pyramidal defects which is known as the critical thickness, hc. In-situ single wavelength ellipsometry has been used to precisely set a reference temperature for a given As beam equivalent pressure (BEP) which can be used to grow reproducible LT-GaAs layers. In the past, XTEM has been used to determine hc, however, in-situ ellipsometry can also be used to monitor the film’s optical properties during growth and can detect the different regions from breakdown of homogeneous growth through the film’s strain relaxation by the generation of the pyramidal defects to the formation of polycrystalline GaAs, if the growth is allowed to continue that far. An XTEM micrograph of such a film with the growth regions defined is shown in Fig. 1.