Electron\u2013hole superfluidity in strained Si/Ge type II heterojunctions

Sara Conti,Giordano Scappucci,Samira Saberi-Pouya,Michele Virgilio,Alexander R Hamilton,Andrea Perali,François M Peeters,David Neilson

doi:10.1038/s41535-021-00344-3

Abstract

Excitons are promising candidates for generating superfluidity and Bose–Einstein condensation (BEC) in solid-state devices, but an enabling material platform with in-built band structure advantages and scaling compatibility with industrial semiconductor technology is lacking. Here we predict that spatially indirect excitons in a lattice-matched strained Si/Ge bilayer embedded into a germanium-rich SiGe crystal would lead to observable mass-imbalanced electron–hole superfluidity and BEC. Holes would be confined in a compressively strained Ge quantum well and electrons in a lattice-matched tensile strained Si quantum well. We envision a device architecture that does not require an insulating barrier at the Si/Ge interface, since this interface offers a type II band alignment. Thus the electrons and holes can be kept very close but strictly separate, strengthening the electron–hole pairing attraction while preventing fast electron–hole recombination. The band alignment also allows a one-step procedure for making independent contacts to the electron and hole layers, overcoming a significant obstacle to device fabrication. We predict superfluidity at experimentally accessible temperatures of a few Kelvin and carrier densities up to ~6 × 1010 cm−2, while the large imbalance of the electron and hole effective masses can lead to exotic superfluid phases.

Highlights

Indirect excitons in a semiconductor system are a highly sought alternative for achieving quantum condensation and superfluidity in solid-state devices at experimentally accessible temperatures
A supercurrent in the electron–hole superfluid could carry an electric current if the electron and hole layers are independently contacted in a counterflow configuration, directly leading to applications in dissipationless solid-state electronics[3,4]
In this letter we propose as a candidate for electron–hole superfluidity and Bose–Einstein condensation (BEC), an alternative mass-imbalanced solid-state system: a latticematched strained Si/Ge bilayer embedded into a germaniumrich SiGe crystal

Summary

Introduction

Indirect excitons in a semiconductor system are a highly sought alternative for achieving quantum condensation and superfluidity in solid-state devices at experimentally accessible temperatures. When the electron and hole masses in a semiconductor are different, there are predictions of exotic superfluid phases[5], including the Fulde–Ferrell–Larkin–Ovchinnikov phase[6] and the Sarma phase with two Fermi surfaces[7]. These exotic phases are predicted to occur at much higher temperatures than in mass-imbalanced ultracold atomic gas Fermi mixtures[8,9]

Methods

Results

Conclusion