S. H. Cheung

Role of Mn and H in formation of cubic and hexagonal GaMnN

Electron microscopy studies are carried out to determine the structure of GaMnN films grown on 6H-SiC(0001) and MgO(111) substrates by plasma-assisted molecular beam epitaxy. For the Mn∕GaN multilayer growth, cubic zinc blende GaMnN is observed in the nominal Mn∕GaN multilayer region and surface microprotrusions, indicative of high Mn mobility. In contrast, for the H-assisted growth of Mn-doped GaN, single phase hexagonal wurtzite GaMnN is obtained, but with columnar structure, indicative of reduced Mn mobility during growth.

Incorporation of Ge on GaN(0001)

We investigate Ge incorporation into the Ga-rich GaN(0001) pseudo-1×1 surface using scanning tunneling microscopy and first-principles calculations. We find that the Ge induces a structural change by forming covalent bonds with the Ga. The liberated Ga atoms, together with additional Ge atoms, reside on the T4 sites, forming a 2×2 structure. Different domains of hcp and fcc stacking are formed to relieve the strain caused by the size mismatch between Ga and Ge.

Structural and magnetic properties of MBE-grown GeMnN2 thin films

Epitaxial GeMnN{sub 2} thin films are synthesized by plasma-assisted molecular beam epitaxy. Transmission electron microscopy and x-ray diffraction measurements confirm that it is the orthorhombic variant, consistent with the predictions of first-principles calculations. The magnetic properties of the films are related to defects, with samples grown under Ge-rich conditions exhibiting a net magnetic moment above room temperature. These results are explained by first-principles calculations, indicating that the preferential substitution of one magnetic sublattice of GeMnN{sub 2} by impurities and/or intrinsic defects such as Ge antisites produces a net magnetic moment in an antiferromagnetic background, and also introduces spin-polarized carriers near the Fermi level.