Electron Spin Coherence of Shallow Donors in Natural and Isotopically Enriched Germanium

Abstract

Germanium is a widely used material for electronic and optoelectronic devices and recently it has become an important material for spintronics and quantum computing applications. Donor spins in silicon have been shown to support very long coherence times (T2) when the host material is isotopically enriched to remove any magnetic nuclei. Germanium also has nonmagnetic isotopes so it is expected to support long T2's while offering some new properties. Compared to Si, Ge has a strong spin-orbit coupling, large electron wave function, high mobility, and highly anisotropic conduction band valleys which will all give rise to new physics. In this Letter, the first pulsed electron spin resonance measurements of T2 and the spin-lattice relaxation (T1) times for As75 and P31 donors in natural and isotopically enriched germanium are presented. We compare samples with various levels of isotopic enrichment and find that spectral diffusion due to Ge73 nuclear spins limits the coherence in samples with significant amounts of Ge73. For the most highly enriched samples, we find that T1 limits T2 to T2=2T1. We report an anisotropy in T1 and the ensemble linewidths for magnetic fields oriented along different crystal axes but do not resolve any angular dependence to the spectral-diffusion-limited T2 in samples with Ge73.