Bicrystalline (GaN)1−x(ZnO)x(GaN)_{1-x}(ZnO)_x solid-solution nanowires containing only about 6.4 at.% ZnO exhibit ultraviolet-to-red broadband photodetection, with highest responsivities of 60, 86 and 43 A/W under 365, 532 and 650 nm irradiation (response times 170, 320 and 160 ms), the two longer wavelengths lying well below the absorption edge and so not explicable by the roughly 3.06 eV band edge alone. The performance was attributed to Zn- and O-related point-defect levels (substitutional Zn on Ga or N sites, O on N sites) plus a zigzag axial bicrystal boundary acting as two parallel carrier-transfer channels, but these assignments were inferred from literature on related materials rather than directly measured on the nanowires. The open problem is to identify microscopically and energetically the in-gap electronic states of this quaternary alloy nanostructure and to separate the individual roles of point defects, basal stacking faults, and the bicrystal boundary in the sub-bandgap optical response and carrier transport.

Background

GaN and ZnO are wide-band-gap (about 3.4 eV) wurtzite semiconductors. When they form a substitutional solid solution written (GaN)1−x(ZnO)x(GaN)_{1-x}(ZnO)_x (equivalently (Ga1−xZnx)(N1−xOx)(Ga_{1-x}Zn_x)(N_{1-x}O_x), often abbreviated GaN-ZnO or GZNO), Zn replaces Ga on cation sites and O replaces N on anion sites in the same wurtzite lattice. This alloy became famous through the discovery by Maeda and Domen that it splits water photocatalytically under visible light, and its optical absorption edge can be tuned from the ultraviolet deep into the visible. Fifteen years of work have still not settled even the basic question of why the band gap narrows on alloying: first-principles calculations by Huda et al., Jensen et al., and Wei et al. attribute it to band-structure effects (repulsion between occupied Zn 3d and N 2p states raising the valence-band maximum), while photoluminescence (PL) studies such as Lee et al. interpret the visible emission through impurity levels, and Olsen et al. presented evidence from ZnO-rich films that a defect band, not a shifted fundamental gap, controls the absorption-edge evolution. Here 'PL' denotes emission spectra after optical excitation, and an 'impurity' or 'defect level' is an electronic state inside the band gap introduced by a point defect (vacancy, antisite, interstitial, or substituent), an extended defect such as a stacking fault, or an impurity.

In wurtzite crystals a stacking fault is a local change in the stacking sequence of the atomic planes; in both GaN and ZnO, basal-plane stacking faults act as thin zincblende-like quantum wells that localize carriers and introduce near-band-edge states, as shown for ZnO nanowires by Khranovskyy et al. A 'bicrystal boundary' is a grain boundary running the length of a nanowire that separates two single-crystal domains with a fixed crystallographic relationship. Point-defect levels in the dilute-alloy context are written with Kröger-Vink-like notation: ZnGaZn_{Ga} is Zn substituting for Ga (an acceptor, i.e. it can capture electrons and introduces a level above the valence band), ZnNZn_N is Zn on a N site (a deep acceptor), and ONO_N is O on a N site (a shallow donor, i.e. easily ionized). For GaN specifically, Demchenko and Reshchikov analyzed the Zn-acceptor blue luminescence, so Zn-related acceptor energies in GaN are known in outline, but not in the alloy.

Building on earlier synthesis of pseudobinary solid-solution nanowires (Liu et al.) and composition/band-gap tailoring of (GaN)1−x(ZnO)x(GaN)_{1-x}(ZnO)_x nanowire arrays with 10.3-47.6% ZnO (Li et al.), Ma et al. (2021) grew dilute (~6.4 at.% ZnO) bicrystalline (GaN)1−x(ZnO)x(GaN)_{1-x}(ZnO)_x nanowires by Au-catalyzed chemical vapor deposition. X-ray diffraction without peak splitting and spatially homogeneous elemental maps from scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) established a single-phase dilute solid solution. Each wire contains two crystal domains separated by a long zigzag boundary with the relationship (1ˉ101)//(01ˉ11)(\bar{1}101)//(0\bar{1}11), plus a high density of stacking faults near that boundary. The wires show an absorption edge near 405 nm with a pronounced visible tail and broad PL from ~380 to ~800 nm; a single-nanowire photodetector reached responsivities of 60, 86, and 43 A/W at 365, 532, and 650 nm with specific detectivities (the standard sensitivity figure of merit for photodetectors) on the order of 10910^9 Jones. The authors explained the broadband response by Zn- and O-related in-gap levels (ZnGaZn_{Ga} acceptors ~300-400 meV above the valence band, ZnNZn_N deep acceptors, ONO_N shallow donors with ~20-30 meV ionization energy), while the axial bicrystal boundary was proposed to form two parallel carrier-transfer channels that suppress recombination. All of these assignments were imported from literature on related systems; none was measured directly on these nanowires. Most recently, time-resolved X-ray excited optical luminescence (TRXEOL, in which synchrotron X-ray pulses excite the sample and emission decay is recorded with a streak camera) by Ward et al. managed, for the first time, to separate near-band-gap from defect-emission components in Ga1−xZnxN1−xOxGa_{1-x}Zn_xN_{1-x}O_x — and even there the assignment of individual visible bands remained partly conjectural.

Problem Statement

Identify, microscopically and energetically, the in-gap electronic states that produce the visible absorption tail, the broad 380-800 nm photoluminescence, and the sub-bandgap photoresponse measured under 532 nm and 650 nm excitation of bicrystalline (GaN)1−x(ZnO)x(GaN)_{1-x}(ZnO)_x solid-solution nanowires, and separate the individual roles of point defects, basal stacking faults, and the bicrystal boundary in that response.

Concretely, an answer must establish, for the dilute solid-solution nanowires (around 6.4 at.% ZnO as synthesized):

  1. Which point-defect and impurity centers are present and optically/electrically active — among the candidates ZnGaZn_{Ga}, ZnNZn_N, ONO_N, Ga and Zn vacancies, and O- or Zn-related complexes — with their charge states, densities, and charge-transition level energies inside the band gap, established by direct measurement (for example deep-level transient or optical spectroscopy, electron paramagnetic resonance, or composition-, temperature-, and excitation-power-dependent photoluminescence including time-resolved X-ray excited optical luminescence) and/or quantitative electronic-structure calculation (for example hybrid-density-functional defect thermodynamics on the alloy).
  2. The separate contributions of the basal stacking faults that cluster near the boundary and of the axial zigzag bicrystal boundary itself — each of which introduces localized states or transport channels — to the sub-bandgap optical absorption, the broad luminescence, and the photocarrier separation and transport, as distinct from the point-defect contribution; this includes testing the proposal that the bicrystal boundary acts as two parallel carrier-transfer channels that reduce recombination.

A complete answer provides a quantitative assignment of the measured spectral and photoresponse features to identified defects or structural elements with stated energies and uncertainties, consistent across the spectroscopic, structural, and device-level observations.

References

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