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Semimagnetic semiconductors for spin electronics > Determination by TEM of the local concentrations of point defects in GaMnAs
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GaMnAsTEM

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Main investigators: F. Glas, G. Patriarche and A. Lemaître
Former PhD student: L. Thevenard

Point defects in GaMnAs

Since ferromagnetism in GaMnAs is mediated by holes released by the Mn substitutional atoms, the introduction of donors in the matrix is deleterious. Unfortunately, GaMnAs grown by low-temperature MBE may contain substantial concentrations of point defects acting as donors, which it is essential to measure. In addition to substitutional defects, we considered specifically AsGa antisites and Mn interstitial atoms, of which there may be two my types, having respectively Ga and As neighbors.
Unit Cell
Fig 1: GaMnAs Unit Cell. In addition to Mn substitutional, two kinds of defects are present: Arsenic antisites on Ga sites, and Mn interstitials.
We considered two types of intersticials, 1 and 2, with either As or Ga neighbors. Our method allows us to identify the first species as the dominant one.


TEM contrast analysis method

We devised an original method for the determination of the local concentrations of these four species (which vary between a fraction of % and several %). The experiments are carried out in a TEM. The method relies on the sensitivity of the structure factors of weak reflections (for fast electron scattering) to the concentrations and crystal-lattice locations of these minority constituents. High spatial resolution is obtained by combining one structure factor measurement with two X-ray analyzes at the same location in the specimen. These three measurements are not enough to determine fully the four concentrations, but assign them to narrow ranges.

[002] TEM cross-sectional Image
Fig 2: Cross-sectional TEM Dark-Field [002] image of a GaMnAs/GaAs/GaMnAs trilayer. The GaMnAs layers appear darker due to the presence of Mn interstitials (type 1).

Moreover, simply examining the image contrast with respect to the GaAs substrate is sufficient to determine which type of Mn interstitial dominates. We found that the Mn interstitials having As nearest neighbors prevail in all the as-grown layers examined. It is highly unusual for a TEM method to be capable of determining the concentrations and the lattice locations of point defects.

References
Determination of the local concentrations of Mn intertitials and antisite defects in GaMnAs, F. Glas, G. Patriarche, L. Largeau, A. Lemaître, Phys. Rev. Lett. 93, 86107 (2004)

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