Wide bandgap based diluted magnetic semiconductors (DMS) are very promising materials for room temperature spintronics [1]. Even though room temperature ferromagnetism was already demonstrated for (Ga,Mn)N and (Zn,Co)O, the mechanisms underlying this behaviour are still under speculations. The characterization of atomically flat GaN and ZnO based DMS using spin polarised scanning tunnelling microscopy (SP-STM) would allow to differentiate long range magnetic coupling between single atoms from magnetic aggregates. However atomically flat surfaces required for STM characterisation are not available from traditional growth methods.
The present work focuses on the preparation of atomically flat (Ga,Mn)N and (Zn,Co)O by thermal diffusion of the transition metal into the semiconductor. In addition to a flat surface, partial substitution of the host metal (resp. Ga or Zn) by the transition element (resp. Mn or Co) must be achieved. The structural properties of the materials were characterised by STM, Time of Flight Secondary Ion Mass Spectroscopy (ToF SIMS), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES).
According to our results, an atomically flat (Zn,Co)O surface can be achieved by thermal diffusion of Co into atomically flat ZnO surfaces. The substitution process occurs but is confined to the first nanometer below the surface leading to a stable Co rich surface. Prolonged annealing (12 hours) does not allow the diffusion of the Co deeper into the semiconductor.
Atomically resolved STM images of GaMnN surfaces were obtained after thermal diffusion of Mn into GaN. ToF-SIMS reveals that Mn diffuses several nanometers into the semiconductors while AES reveals the segregation of the Ga excess to the surface. According to the literature substitution of Ga by Mn occurs [2].
[1] S.J. Pearton, C.R. Abernathy, D.P. Norton, A.F. Hebard, Y.D. Park, L.A. Boatner, J.D. Budai, Materials Science and Engineering R 40 (2003) 137.
[2] I.A. Kowalik, B.J. Kowalski, B.A. Orlowski, E. Lusakowska, R.J. Iwanowski, S. Mickevicius, R.L. Johnson, I. Grzegory, S. Porowski, Surf. Sci. 566–568 (2004) 457. |