Donors in Ge as Qubits: Establishing Physical Attributes

TL;DR

This paper develops a multiscale theoretical framework to accurately predict the properties of donors in germanium, essential for quantum bit applications, revealing unique features that differ from other semiconductors.

Contribution

It introduces a combined DFT and multivalley effective mass approach with central cell corrections to characterize donors in Ge, including binding energies and STM signatures.

Findings

Predicted binding energies of negatively ionized donors in Ge.

Identified unique electronic signatures of buried donors in Ge.

Showed that extrapolations from other semiconductors to Ge are invalid.

Abstract

Quantum electronic devices at the single impurity level demand an understanding of the physical attributes of dopants at an unprecedented accuracy. Germanium-based technologies have been developed recently, creating a necessity to adapt the latest theoretical tools to the unique electronic structure of this material. We investigate basic properties of donors in Ge which are not known experimentally, but are indispensable for qubit implementations. Our approach provides a description of the wavefunction at multiscale, associating microscopic information from Density Functional Theory and envelope functions from state of the art multivalley effective mass calculations, including a central cell correction designed to reproduce the energetics of all group V donor species (P, As, Sb and Bi). With this formalism, we predict the binding energies of negatively ionized donors (D- state). Furthermore, we investigate the signatures of buried donors to be expected from Scanning Tunneling Microscopy (STM). The naive assumption that attributes of donor electrons in other semiconductors may be extrapolated to Ge is shown to fail, similar to earlier attempts to recreate in Si qubits designed for GaAs. Our results suggest that the mature techniques available for qubit realizations may be adapted to germanium to some extent, but the peculiarities of the Ge band structure will demand new ideas for fabrication and control.