Hydrogen in ZnO

doi:10.1016/j.physb.2009.08.250

Physica B: Condensed Matter

Volume 404, Issues 23–24, 15 December 2009, Pages 5075-5079

https://doi.org/10.1016/j.physb.2009.08.250 Get rights and content

Abstract

The results of a combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity on ZnO are presented. Two shallow donors—hydrogen at the bond-centered lattice site, $H_{BC}$ , and hydrogen bound in an oxygen vacancy, $H_{O}$ —were identified. Donor $H_{BC}$ has an ionization energy of 53 meV. The recombination of an exciton bound to $H_{BC}$ gives rise to the $3360.1 \pm 0.2 meV$ photoluminescence line. The $1 s \to 2 p$ donor transition at $330 {cm}^{- 1}$ is detected in the Raman scattering and photoconductivity spectra. The stretch mode of the associated O–H bond is detected in IR absorption at $3611 {cm}^{- 1}$ . The $H_{O}$ donor in ZnO has an ionization energy of 47 meV. The excitonic recombination at $H_{O}$ leads to the previously labeled $I_{4}$ line at 3362.8 meV. Photoconductivity and Raman spectra reveal the $1 s \to 2 p$ donor transition at $265 {cm}^{- 1}$ .

It is shown that $H_{BC}$ migrating through the ZnO lattice forms electrically inactive interstitial $H_{2}$ . Vibrational modes of $H_{2}$ , HD, and $D_{2}$ were identified at 4145, 3628, and $2985 {cm}^{- 1}$ , respectively. These results suggest that interstitial $H_{2}$ is responsible for the “hidden” hydrogen in ZnO.

Introduction

Hydrogen is a common impurity in ZnO that strongly influences its electrical and optical properties [1], [2]. First principles investigations suggest that hydrogen acts as a shallow donor in ZnO [3].

The results of a combined Raman scattering, IR absorption, photoluminescence, and photoconductivity study of hydrogen in ZnO are presented [4], [5]. Two hydrogen donors are identified. These are bond-centered hydrogen ( $H_{BC}$ ) and hydrogen bound in an oxygen vacancy ( $H_{O}$ ) with the ionization energies of 53 and 47 meV, respectively. Spectroscopic evidences are presented that the so-called “hidden” hydrogen in ZnO [6] occurs in the form of electrically inactive $H_{2}$ .

Section snippets

Experimental

The ZnO crystals used in this work were hexagonal prisms with a diameter of about 2 mm and a length of $\sim 20 mm$ . The nominally undoped n-type single crystals with resistivity of $10 - 100 Ω cm$ were grown from the vapor phase at the Institute for Applied Physics, University of Erlangen (Germany).

Hydrogen and/or deuterium was introduced into the samples via thermal treatments in a sealed quartz ampoules filled with hydrogen and/or deuterium gas (pressure of 0.5 bar at room temperature). The thermal

Bond-centered hydrogen

Fig. 1 shows the Raman spectra measured at $T \leq 20 K$ . Before hydrogen treatment (bottom spectra), the well-known lattice phonon lines are observed. The $E_{1}$ (LO) phonon at $591 {cm}^{- 1}$ is of particular interest for the current study.

After treatment of the ZnO sample in $H_{2}$ gas at $725^{\circ} C$ , two additional features with frequencies at 330 and $3611 {cm}^{- 1}$ appear in the Raman spectrum (mid spectrum). The $3611 {cm}^{- 1}$ line has been previously observed in IR absorption spectra directly after hydrogen incorporation [7].

Hydrogen molecule

Fig. 7 shows Raman spectra recorded at $T \leq 20 K$ from a ZnO sample treated in $H_{2}$ gas. A line at $3611 {cm}^{- 1}$ due to $H_{BC}$ dominates the spectrum obtained directly after the treatment. For the samples of this study the donor concentration was around $10^{18} {cm}^{- 3}$ .

Bond-centered hydrogen is known to be mobile already at room temperature [4], [8]. After annealing the ZnO sample at $550^{\circ} C$ for 30 min, the $3611 {cm}^{- 1}$ line disappears (see bottom spectrum in Fig. 7). A new line at $4145 {cm}^{- 1}$ shows up in the spectrum at the

Summary

A combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity on ZnO is reported. Two shallow donors—hydrogen at the bond-centered lattice site, $H_{BC}$ , and hydrogen bound in an oxygen vacancy, $H_{O}$ —were identified.

It is shown that $H_{BC}$ migrating through the ZnO lattice forms electrically inactive interstitial $H_{2}$ . These results suggest that interstitial $H_{2}$ is responsible for the hidden hydrogen in ZnO.

Acknowledgments

The author is indebted to R. Helbig for providing him with the ZnO samples. J. Weber and M. Stavola are greatly acknowledged for numerous and helpful discussions. F. Herklotz and F. Börrnert are acknowledged for the help with PL and IR absorption measurements. Part of this work was supported the by the Deutsche Forschungsgemeinschaft (Grant nos. WE 1319/16-1 and LA 1397/3).

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Hydrogen in ZnO

Abstract

Introduction

Section snippets

Experimental

Bond-centered hydrogen

Hydrogen molecule

Summary

Acknowledgments

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