Condensed
Matter & Surface Sciences
COLLOQUIUM
Jacek K. Furdyna
Electronic
Effects Determining the Formation
of
Ferromagnetic III1-xMnxV Alloys During
Epitaxial Growth
Substitutional incorporation of Mn into the III-V lattice in sufficient concentrations to form ferromagnetic III1-xMnxV alloys (e.g., Ga1-xMnxAs) must be carried out by non-equilibrium low-temperature epitaxy, whereby Mn concentrations x approaching 0.10 can be obtained. The ferromagnetism of these alloys occurs because, in addition to providing magnetic moments, the Mn ions also act as acceptors, thus providing large concentrations of holes. Our research has shown, however, that the incorporation of Mn acceptors at the Group-III sites becomes energetically unfavorable above a certain concentration (i.e., above reaching a certain limiting value of the Fermi energy); and it is then more favorable for Mn to occupy interstitial positions in the III-V lattice. This has deleterious effects on the ferromagnetic properties of the III1-xMnxV alloy, since interstitials are double donors, thus reducing the hole concentration by compensating the substitutional Mn acceptors; and, in addition, by forming antiferromagnetic pairs with the substitutional Mn ions, and thus canceling their magnetic moments. Both these effects result in a lowering of the Curie temperature of the III1-xMnxV alloys. In this talk we will show that incorporation of substitutional Mn is determined by the Fermi level during the growth process itself. To demonstrate this, we will describe a variety of growth experiments that involve co-doping III1-xMnxV alloys with Be, including modulation co-doping of Al1-yGayAs/Ga1-xMnxAs/Al1-yGayAs heterostructures. We will then discuss possible strategies to circumvent this obstacle to the incorporation of substitutional Mn at the Group-III sites.