Single phase a-plane MgZnO epilayers for UV optoelectronics: substitutional behaviour of Mg at large contents

Abstract

High quality 1 \im thick a-plane Mg^Zn^^O layers were produced by molecular beam epitaxy with Mg contents higher than 50%. Resonant Rutherford backscattering spectrometry combined with ion channeling revealed a uniform growth in both composition and atomic order. The lattice-site location of Mg, Zn and O elements was determined independently, proving the substitutional behavior of Mg in Zn-sites of the wurtzite lattice. X-Ray diffraction pole figure analysis also confirms the absence of phase separation. Optical properties at such high Mg contents were studied in Schottky photodiodes.

Deep levels in a-plane, high Mg-content MgxZn1−xO epitaxial layers grown by molecular beam epitaxy

“…The Mg content, its substitutional character, and the crystal structure, were previously determined in these films by Rutherford backscattering (RBS). 15 In that work, uniform growth in terms of both layers composition and atomic order as well as excellent crystal quality of the films was confirmed along with ideal substitutional behavior of Mg on Zn sublattice sites within the wurtzite lattice. Growth was initiated with a ZnO nucleation layer having a nominal thickness of 30 nm at a temperature of 400 C, which was subsequently annealed at 600 C for 10 min in order to obtain a smooth surface.…”

“…While the source of these levels remains unclear, separately reported RBS and high resolution x-ray diffraction on these samples do not show evidence of any phase segregation for any of the alloys studied here, and a single wurtzite structure is observed even at the highest Mg contents. 15 While a complete determination of the physical sources for the detected traps is beyond the scope of this study, this is certainly a topic deserving of attention, especially considering the mid-gap positions of these levels that would imply their importance as generation-recombination centers.…”

ZnMgO-based UV photodiodes: a comparison of films grown by spray pyrolysis and MBE

“…Details of the growth can be found in [4]. The stoichiometry, substitutional character of Mg, and crystallinity was assessed by a combination of Rutherford Backscattering Spectroscopy (RBS), channeling RBS, and XRD [5]. The Mg content shown in Table 1 was extracted from these measurements, and the Mg substitutional character was confirmed for almost all Mg contents, whereas no cubic intrusions where observed.…”

“…The high quality of the Schottky photodiodes is evident from the very large UV/VIS spectral rejection ratios that are in the 10 4 -10 6 range. Indeed, detail analysis of the crystal structure of these films [5] shows that there are no cubic intrusions up to 56% Mg, the Mg sits in a substitutional site even in the high Mg content films, and the RBS channel yield is close to the state of the art commercial single crystal ZnO substrates. Thus, the high crystal quality allows reaching the solar blind region while keeping the wurtzite structure.…”

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

“…ZnO crystallizes in the hexagonal wurtzitetype structure while MgO crystallizes in the cubic rocksalt-type structure. Therefore, once the solubility limit is reached in wurtzite-type Mg x Zn 1−x O solid solution, ∼ 4% in bulk [12] and ∼ 30% or ∼ 50% in thin films depending on the growth method [8,13], phase separation appears [14] and both rock-salt and wurtzite type phases coexist. A lot of effort has been put on trying to reach the highest incorporation limit of Mg 2+ in phase pure wurzite-type Mg x Zn 1−x O.…”