Molecular beam epitaxial growth and magnetotransport properties of GaMnAs

Abstract

GaMnAs is a canonical ferromagnetic semiconductor with ferromagnetic phase induced by exchange interaction between Mn 2 + ions with 5/2 spins, located at Ga sites of GaAs matrix and valence band holes. Due to the limited solubility of Mn in galium arsenide, GaMnAs ternary alloy can only be grown by a method implying growth conditions which are far from thermodynamic equilibrium, such as molecular beam epitaxy (MBE). Moreover, in case of GaMnAs, very low growth temperatures are necessary to employ, in order to incorporate Mn in concentrations high enough (higher than about 0.5 at.%) to provide a ferromagnetic phase transition. Hence the GaMnAs growth temperature must be lower than about 300 °C, where as for GaAs MBE growth the temperatures in the range 580–640 °C are typically applied. In the low growth temperature conditions, Mn occupies Ga sites ( Mn Ga ) in the GaAs host. To avoid segregation of secondary MnAs phase, increasing the Mn content in GaMnAs necessary for high ferromagnetic phase transition temperature, must be correlated with the decreasing growth temperature. On the other hand, the low growth temperatures imply high concentrations of point defects in the GaMnAs lattice. Arsenic atoms located at Ga sites (arsenic antisites) antisites and Mn ions located at interstitial positions (Mn interstitials) are the two main defects identified so far in GaMnAs. Being double donors both defects partially compensate Mn Ga acceptors lowering the concentration of holes and ferromagnetic phase transition temperature in GaMnAs. Thus their concentrations should be the lowest possible. Minimization of the content of As antisites requires optimized growth conditions, the concentration of Mn interstitials can be lowered by suitable post-growth annealing procedures. Optimized MBE growth and post-growth annealing procedures are discussed for GaMnAs under compressive strain in case of layers grown directly on GaAs(1 0 0) substrates and for GaMnAs under tensile strain in case of layers grown on thick, relaxed InGaAs buffers. Basic magnetic and transport properties of GaMnAs in these two different strain states are reviewed.