Oxide muonics: A new compendium

Abstract

A new survey of muonium states brings the total of binary non-magnetic oxides studied to 30, with normal muonium—the interstitially trapped atomic state—found in 15 of these. The number of shallow-donor states of the type known in ZnO now also totals 15, but there are hints of several others. Tantalizingly, the shallow-donor and deep-atomic states are found to coexist in several of the candidate high-permittivity dielectrics. Highly anisotropic states, resembling anomalous muonium in semiconductors and including examples of muonium trapped at oxygen vacancies, complete a spectrum of hyperfine parameters covering five powers of ten. Effective ionization temperatures range from 10 K for shallow to over 1000 K for deep states, with corresponding activation energies between several meV and several eV. The oxide band gap emerges as a parameter controlling the systematics of the deep-to-shallow transition for muonium and, by inference, monatomic hydrogen.

Introduction

Since the earliest spectroscopic studies of muonium in quartz, and in a relatively small number of other wide-gap non-magnetic oxides, the use of muonium as a model for hydrogen has taken on a new importance. This follows $\mathrm{\mu }\mathrm{SR}$ confirmation that hydrogen forms a shallow-donor state in ZnO; that is, its electron wave function does not retain atomic character but delocalizes into conduction-band states. The question arises as to whether hydrogen can similarly act as an n-type dopant and induce electronic conductivity in other oxides. A particular concern is that it might do so in those high-permittivity materials such as ${\mathrm{HfO}}_2$ and ${\mathrm{ZrO}}_2$ that are destined to replace ${\mathrm{SiO}}_2$ as nano-scale gate dielectrics. We have therefore undertaken a new survey of muonium states in a wide selection of oxides, both semiconducting and dielectric. We use the term muonics to describe the use of muonium data as a model for the electronic structure and electrical activity of hydrogen in the dilute monatomic limit. Comparable ESR data for H in oxides is sparse, but fortunately include the trapped atoms in ${\mathrm{SiO}}_2$ and ${\mathrm{Li}}_2\mathrm{O}$, as well as the donor in ZnO, thereby validating the muonics principle both for deep and shallow states.