Unintentional F doping of the surface of SrTiO3(001) etched in HF acid -- structure and electronic properties

TL;DR

This study reveals that HF acid etching introduces significant fluorine doping on SrTiO3(001) surfaces, affecting electronic properties and electron mobility, but can be mitigated with alternative cleaning methods.

Contribution

It demonstrates that HF etching causes unintentional fluorine doping on SrTiO3 surfaces and proposes a water boil and annealing process to reduce this doping.

Findings

~13% of surface O replaced by F

F does not penetrate deeper layers

F doping does not pin the Fermi level

Abstract

We show that the HF acid etch commonly used to prepare SrTiO3(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO2. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~13 % of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO2 terminated.