A scanning probe microscopy approach for identifying defects in aluminum oxide

TL;DR

This paper demonstrates the use of cryogenic electrostatic force microscopy to identify and characterize individual defects in aluminum oxide dielectrics, which impact quantum dot qubit coherence, by linking surface potential maps to defect electronic structures.

Contribution

It introduces a cryogenic EFM method combined with electrostatic modeling and DFT comparison for atomic-scale defect identification in aluminum oxide dielectrics.

Findings

EFM can detect charge state transitions of surface defects.

Defects such as vacancies and impurities are identifiable via EFM.

The approach links defect energies to specific atomic structures.

Abstract

The coherence of quantum dot qubits fabricated in semiconductors is often limited by charge noise from defects in gate dielectrics, which are material- and process-dependent. Characterizing these defects is an important step towards reducing their impact and improving qubit coherence. The identification of individual defects requires atomic-scale spatial resolution, however, and sufficient spectral sensitivity to determine their electronic structure. Electrostatic force microscopy (EFM) provides highly resolved maps of the surface potential of dielectrics, and importantly, is also sensitive to single-electron charging processes that reflect the spectral structure of underlying defects. In this work, we use cryogenic EFM to characterize aluminum oxide grown by atomic layer deposition (ALD) on bulk silicon. These measurements reveal defects close to the surface that exchange electrons with the EFM tip as they transition through different charge states. Detailed electrostatic modeling opens the door to powerful techniques for mapping tip-backgate charging voltages onto defect transition energies, allowing defects such as aluminum vacancies, and carbon, oxygen, or hydrogen impurities to be identified, by comparing to density functional theory (DFT). These results point towards EFM as a powerful tool for exploring defect structures in solid-state qubits.