Interplay between interstitial and substitutional hydrogen donors in ZnO

Abstract

A Raman study on hydrogen donors in ZnO reveals the properties of bond-centered hydrogen (H BC ) and hydrogen trapped in the oxygen vacancy (H O ). The donors are identified by their electronic 1s → 2s(2p) transitions and their characteristic local vibrational modes. The H O donor was detected preferentially below the sample surface, where a high oxygen vacancy concentration is generated by thermal treatment of the samples. H BC and H O exhibit different thermal stabilities and their concentrations depend strongly on the sample history. Molecular hydrogen is an essential part of the interplay of the two hydrogen donors.

Towards understanding the hydrogen molecule in ZnO

“…Figure 3(b) shows the results of an identical annealing series conducted at the surface of the same sample. The behavior of H 2 is basically similar to that in the bulk except that the signal disappears already at 600 • C. On the other hand, no spectroscopic features of H BC were detected for the annealing temperatures above 200 • C. Instead, the hydrogen is trapped by the oxygen vacancies generated at the sample surface by high temperature treatment resulting in the formation of the H O shallow donor [27].…”

“…At elevated hydrogenation temperatures the line intensity drops below the detection limit of our setup preventing us from a direct verification of the correlation between the free carriers and the LVM intensity of H BC . Previously, however, we have established that the H BC concentration after hydrogenation at 1000 • C is about 5×10 17 cm −3 [27]. From the comparison of the LVM intensities in Fig.…”

“…Subsequent annealing at 500 • C eliminates the H BC -related signals and recovers the E 1 (LO) phonon line, since the concentration of shallow donors is significantly reduced. The 1s → 2s(2p) transition of another hydrogen-related shallow donor H O at around 265 cm −1 is too weak to be detected in our spectra, because the amount of oxygen vacancies in the bulk of the sample is marginal compared to that at the sample surface [27].…”

“…2(b) and 2(d). We take this as an indication that the high temperature treatment creates additional traps for hydrogen, e.g., zinc and/or oxygen vacancy agglomerates, which decreases the amount of interstitial hydrogen [27]. Annealing at 500 • C releases hydrogen from these traps, thus enhancing the concentration of H 2 .…”

“…The behavior of the hydrogen-related defects, e.g., H BC and H O , for annealing temperatures below 200 • C is discussed elsewhere (see Ref. [27]). No LVMs due to the hydrogen molecule are detected for these annealing temperatures.…”