Oxide muonics: I. Modelling the electrical activity of hydrogen in semiconducting oxides

Abstract

A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxide semiconductors is revealed by a study of the electronic structure and electrical activity of their muonium counterparts, a methodology that we term ‘muonics’. In CdO, Ag2O and Cu2O, paramagnetic muonium centres show varying degrees of delocalization of the singly occupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO and RuO2, on the other hand, show only electronically diamagnetic muon states, mimicking those of interstitial protons. Muonium in CdO shows shallow-donor behaviour, dissociating between 50 and 150 K; the effective ionization energy of 0.1 eV is at some variance with the effective-mass model but illustrates the possibility of hydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. For Ag2O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete by room temperature. Striking examples of level-crossing and RF resonance spectroscopy reveal a more complex interplay of several metastable states in this case. In Cu2O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbital is substantially more delocalized than in the trapped-atom states known in certain wide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effective ionization energy of 1 eV, defines an electrically active level near mid-gap in this material.

Conductivity in transparent oxide semiconductors

“…Several subsequent µSR studies have revealed the shallow donor-nature of muonium in the TCO materials In 2 O 3 and SnO 2 [102], CdO [103], and Ga 2 O 3 [104] (see, for example, Fig. 6).…”

Hydrogen impurity in yttria:Ab initioandμSR perspectives

“…18,19 The short lifetime (2.2 μs) of the implanted positive muonium is reflected on the short time-scale measurements performed under nonequilibrium conditions and allows the observation of the isolated defect centers that are responsible for the electrical activity of hydrogen. 17,20,21 Although the muon is only one ninth the mass of the proton, the reduced mass of muonium is 99.6% of that of hydrogen, so that the respective electronic properties are basically the same in both atoms. 20 μSR spectroscopy has also the advantage of being sensitive to both neutral (paramagnetic) and charged (electronically diamagnetic) states of muonium and of being able to distinguish deep 22 and shallow centers.…”

“…17,20,21 Although the muon is only one ninth the mass of the proton, the reduced mass of muonium is 99.6% of that of hydrogen, so that the respective electronic properties are basically the same in both atoms. 20 μSR spectroscopy has also the advantage of being sensitive to both neutral (paramagnetic) and charged (electronically diamagnetic) states of muonium and of being able to distinguish deep 22 and shallow centers. 16,23 Thus, extensive studies have already been carried out to verify whether deep muonium (resembling the atom-like localized state of hydrogen) or shallow-donor muonium are formed in different semiconductors and oxides, hence providing a reasonably good comparison to whether hydrogen behaves as a deep or shallow impurity, respectively.…”

Hydrogen impurities and shallow donors in SnO2studied by infrared spectroscopy

“…[11][12][13][14] Similar to the situation in ZnO, hydrogen recently has been proposed to be an important n-type dopant in several transparent conducting oxides. [4][5][6][15][16][17][18] SnO 2 is a prototypical transparent conducting oxide. 11,12,19 Recent theory finds that native defects are unlikely to be the source of conductivity.…”