Hyperfine interactions at dangling bonds in amorphous germanium

Isotope-engineered amorphous germanium (a-Ge) films with⁷³Ge concentrations in the range of 0.1 to 95.6 % have been investigated by electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR) at microwave frequencies between 0.434 and 9.35 GHz. The hyperfine interactions of dangling bond (DB) defects with many⁷³Ge nuclei and their spin localization radius have been extracted from the broadening of the EDMR signals in isotope enriched samples at different⁷³Ge concentrations. Linewidths as low as (Formula presented) have been observed at 0.434 GHz in a sample without⁷³Ge nuclear spins. At low⁷³Ge concentrations, the frequency-dependent linewidth (Formula presented) is determined by g-factor anisotropy and disorder. A frequency-independent linewidth contribution of about 1 G is attributed to dipolar broadening between the DB electronic spins. Over a large range of intermediate concentrations, the statistically distributed nuclear spins of⁷³Ge atoms on sites close to the DB defect atom are responsible for the overall linewidth. The large linewidth (Formula presented) of samples with⁷³Ge concentrations of 95.6% requires a model wave function with a Fermi contact interaction of A_iso = 29 G × gμ_B at the defect atom, indicating that a fraction of 3.4% of the DB wave function originates from s-like orbitals there. The decay of the rest of the DB wave function can be described with a spin localization radius of 3.5 Å by a numerical model for the statistical hyperfine broadening. The delocalization of the DB spin is much smaller than that of the DB charge density determined in transport measurements.