Electron Spin Resonance Experiments on Shallow Donors in Germanium

At liquid helium temperatures, spin resonance of localized donor electrons has been observed in phorphorus-, arsenic-, and bismuth-doped germanium. The presence of hyperfine splitting confirms the singlet as the ground state for all three. The separation of the excited triplet states has been measured by uniaxially stressing the samples. The triplet states are all found to lie close to the effective-mass value of 0.009 eV. The anisotropy of the $g$ tensor has also been measured by uniaxial stress measurements giving a value for the $g$ anisotropy $g_{\mathrm{II}}-g_{\perp }=-1.05\mathrm{}$ for arsenic-doped germanium. The large $g$ anisotropy gives rise to an anisotropic linewidth which is caused by built-in strains in the crystal. Measurements show a strong correlation of this line broadening with the number of dislocations. The broadening is larger than predicted as a result of condensation of the impurities in the neighborhood of dislocations. The linewidth for magnetic fields in the [100] direction, where strain broadening of the line vanishes, has been shown to arise from unresolved hyperfine interactions with ${\mathrm{Ge}}^{73}$ nuclei. The linewidths are in good agreement with values calculated using an isotropic approximation to the effective-mass wave function. The spin-lattice relaxation times have been measured and compared with the theory of Roth and Hasegawa for the one-phonon process. The temperature dependence, the dependence on amplitude and orientation of the magnetic field, and effects of strain predicted by their theory were observed.