Evidence for hydrogen two-level systems in atomic layer deposition oxides

TL;DR

This study investigates TLS defects in atomic layer deposited oxide films at millikelvin temperatures, revealing hydrogen as the primary source of dielectric loss and providing insights into material-specific TLS behavior.

Contribution

The paper provides the first direct evidence linking hydrogen impurities to TLS loss in ALD oxides, combining microwave loss measurements with impurity analysis.

Findings

Hydrogen impurities dominate TLS loss in ALD oxides.

Crystalline BeO exhibits higher bulk loss tangent than amorphous films.

Carbon impurities are negligible in TLS loss contribution.

Abstract

Two-level system (TLS) defects in dielectrics are known to limit the performance of electronic devices. We study TLS using millikelvin microwave loss measurements of three atomic layer deposited (ALD) oxide films--crystalline BeO ($\rm{c-BeO}$), amorphous $\rm{Al_2O_3}$ ($\rm{a-Al_2O_3}$), and amorphous $\rm{LaAlO_3}$ ($\rm{a-LaAlO_3}$)--and interpret them with room temperature characterization measurements. We find that the bulk loss tangent in the crystalline film is 6 times higher than in the amorphous films. In addition, its power saturation agrees with an amorphous distribution of TLS. Through a comparison of loss tangent data to secondary ion mass spectrometry (SIMS) impurity analysis we find that the dominant loss in all film types is consistent with hydrogen-based TLS. In the amorphous films excess hydrogen is found at the ambient-exposed surface, and we extract the associated hydrogen-based surface loss tangent. Data from films with a factor of 40 difference in carbon impurities revealed that carbon is currently a negligible contributor to TLS loss.