  Evidence of the absence of the clear electron spin-resonance signal from Ge
dangling bonds (DBs) at Ge/GeO$_2$ interfaces is explored by means of
first-principles electronic-structure calculations. Comparing the electronic
structures of the DBs at Si/SiO$_2$ and Ge/GeO$_2$ interfaces, we found that
the electronic structure of the Ge-DB is markedly different from that of the
Si-DB; the Ge-DB states does not position in the energy band gap of the
Ge/GeO$_2$ interface while the Si-DB states clearly appears. In addition, the
charge density distribution of the Ge-DB state spreads more widely than that of
the Si-DB state. These features are explained by considering the metallic
properties of the bonding network of the Ge/GeO$_2$ interface and the
structural deformation of the Ge bulk at the Ge/GeO$_2$ interface due to the
lattice-constant mismatch.
