MnAs dots with two qualitatively different electronic structures have been grown on the GaN(0001) surface by modification of the first stages of the MBE processes. We present the results of photoemission studies of electronic structures of the systems, morphology investigations by atomic force microscopy (AFM) and magnetization measurements of such MnAs/GaN systems.
MnAs is a ferromagnetic metal with NiAs-type crystal structure at room temperature. It forms metal/semiconductor junctions on various substrates, like GaAs or Si. We grew MnAs layers on GaN crystals because both materials have hexagonal structure and, in the plane perpendicular to the c axis, aMnAs > aGaN. Due to that, spontaneous formation of MnAs dots in this system was expected. Our results confirmed this supposition. In situ electron diffraction (RHEED) observations and ex situ AFM results showed that dots of MnAs (the diameter of 20-60 nm, the average height of 4 nm) were formed for the layers thicker than 7 ML. Two methods of growth initiation were applied - a regular MBE procedure (manganese and arsenic were delivered simultaneously) and the MBE process initiated by subsequent deposition of manganese and arsenic. Both of them led to dots formation. The growth process initiated by subsequent deposition of manganese and arsenic led to the system with electronic structure characteristic of metallic MnAs. However, the other one resulted in the density of states distribution similar to that of half-metallic MnAs dots (as reported by Okabyashi et al. [1]).
The electronic structure studies were carried out by means of resonant photoemission spectroscopy in the synchrotron radiation laboratory MAX-lab, Lund, Sweden. This technique enabled us to determine the electronic states distribution of MnAs layers and the Mn 3d - related contribution to it.
Magnetization measurements showed that the systems exhibited superparamagnetic properties. In particular, the half-metal-like MnAs had the blocking temperature of about 200 K while the intra-dot TC > 330 K. Such a system can be suitable for spintronic applications.
[1] J. Okabayashi, M. Mizuguchi, K. Ono, M. Oshima, A. Fujimori, H. Kuramochi, H. Akinaga, Phys. Rev. B 70, 233305 (2004) |