Enhancement-Mode Buried Strained Silicon Channel Double Quantum Dot with Integrated Electrometer

Abstract

Silicon and silicon-germanium heterostructure devices are interesting test beds for exploring solid-state quantum computing. Electron spin lifetimes and coherence times in fully relaxed SiGe / strained Si (sSi) quantum well devices are much longer than competing systems (e.g. gallium- arsenide heterostructures), with further improvements expected. Additionally, highly crystalline and high mobility interfaces allow fabrication of few electron dots and sensitive electrometers more easily. We propose and demonstrate a relaxed-SiGe/sSi enhancement-mode gate stack for quantum dots. The wafers are grown to our specification using CVD process by Lawrence SQI. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. Polysilicon depletion gates are used to form few electron dots in the sSi quantum well. High density plasma silicon dioxide was used as a secondary dielectric, followed by a tungsten/titanium nitride enhancement gate to draw electrons into the system. A modified implant, polycrystalline silicon formation and annealing conditions were utilized to minimize the thermal budget that potentially leads to Ge/Si interdiffusion. A mobility of 1.6E5 cm^2/Vs at 5.8E11 /cm^2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade conductance oscillations are measured in a single quantum dot nanostructure, evidence of discrete electron number. The Coulomb blockade diamonds increase to at least ±10 mV of dc voltage across the device after the last transition, a strong indication of single electron dot occupation. Charge transitions in a double quantum dot system are observed using the integrated electrometer, with tunable coupling between the dots. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi- program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.