The role of stacking faults and their associated 0.13 ev acceptor state in doped and undoped ZnO layers and nanostructures

Cathodoluminescence spectra recorded with high spatial and wavelength resolution on tilted ZnO epitaxial layers allow to identify a very prominent emission peak at 3.314 eV as a free electron to shallow acceptor (E A % 130 meV) transition. By correlation with TEM cross-section images recorded on the same samples we can find these acceptor states to be located on basal plane stacking faults (BSFs). Locally, high concentrations of acceptor states are found. Since this spectral feature is often reported in literature especially after attempts to obtain p-type or transition metal doping, we conclude that stacking faults are a common by-product when group V or other extrinsic atoms are incorporated in ZnO layers or nanostructures. 1 Introduction ZnO layers, bulk crystals, and also nanostructures frequently show an emission band at 3.314 eV in low temperature photoluminescence (PL) measurements, which at room temperature often remains as the dominant spectral PL feature. The band is strong especially after doping with group V elements and is mostly treated as an indicator for successful p-type doping, but also appears in Al-doped ZnO and even in nominally undoped ZnO layers and nanostructures of limited crystal quality. All types of assignments are found for this band: LO-or TO-phonon replicas of free excitons, acceptor-bound excitons (A 0 ,X), donor–acceptor pair (D 0 ,A 0) transitions, two-electron satellites (TES), surface-related excitons, or free-to-bound transitions [(e,A 0) or (h,D 0)]. This band is reported to be unstable when annealing is performed. The correlation of low-temperature cathodolumines-cence measurements with very high spatial resolution and high-resolution (HR) transmission electron microscopy (TEM) on the same epitaxial ZnO sample allows us to unambiguously identify this emission band with high-density acceptor states located at c-plane stacking faults (BSFs).