A 2009 molecular-beam-epitaxy study (Du et al., Adv. Mater. 21, 4625 (2009)) established controlled growth of high-quality ZnO and single-phase wurtzite MgZnO with band gaps up to 4.55 eV, and demonstrated visible-blind (n-ZnO/insulator-MgO/p-Si) and solar-blind (Mg0.55Zn0.45O\mathrm{Mg}_{0.55}\mathrm{Zn}_{0.45}\mathrm{O}) ultraviolet detectors, but left three materials bottlenecks open: (i) no reproducible, stable p-type ZnO, forcing the use of p-Si as the complementary electrode in heterojunction devices; (ii) a purely empirical Mg-incorporation window, in which single-phase wurtzite growth up to x≈0.55x\approx0.55 required the Mg flux to stay below an unquantified composition-dependent critical value, with no microscopic account of Mg incorporation versus rock-salt MgO nucleation; and (iii) no atomic-scale, quantitative description of the nucleation dynamics of the ultrathin AlN and MgO interfacial layers that enable epitaxy on disparate substrates. Device-level engineering has advanced substantially in the years since, but these three materials problems—whose joint solution is what all-oxide (Si-free) ZnO/MgZnO ultraviolet optoelectronics requires—remain open.

Background

Zinc oxide (ZnO) is a transparent oxide semiconductor with a wide direct band gap of about 3.37 eV at room temperature and a large exciton binding energy (about 60 meV; an exciton is the bound electron-hole pair formed when light is absorbed, and its binding energy measures how stable that pair is at device operating temperatures), making it a long-standing candidate for ultraviolet (UV) light emitters and detectors. High-quality device material is grown by molecular beam epitaxy (MBE), in which thermal atomic beams of Zn, Mg, and other elements impinge on a heated substrate; reactive oxygen is supplied by a radio-frequency plasma source that produces oxygen radicals (RF-plasma-assisted MBE). ZnO crystallizes in the wurtzite structure, a hexagonal lattice of stacked Zn–O bilayers whose stacking order fixes the film polarity (Zn-polar versus O-polar); wrong stacking at the substrate interface produces inversion domains (regions of opposite polarity) and 30-degree in-plane rotation domains that degrade crystal quality. Heteroepitaxy, i.e. growth on a dissimilar crystalline substrate, of ZnO on sapphire, spinel (MgAl2O4\mathrm{MgAl_2O_4}), LSAT ((La,Sr)(Al,Ta)O3(\mathrm{La},\mathrm{Sr})(\mathrm{Al},\mathrm{Ta})\mathrm{O}_3), and Si is therefore difficult, and the 2009 source paper introduced ultrathin interfacial layers—AlN formed by controlled nitridation of the sapphire surface (exposing it to activated nitrogen so that its outer atomic layers convert to aluminum nitride), and MgO formed by Mg wetting of oxide surfaces (a deposited Mg layer that oxidizes into a continuous, atomically thin MgO template)—to template single-domain, polarity-controlled ZnO growth, followed by a low-temperature ZnO buffer layer to relax misfit strain. Alloying with Mg raises the band gap of wurtzite MgxZn1−xO\mathrm{Mg}_x\mathrm{Zn}_{1-x}\mathrm{O} toward the solar-blind range: solar-blind UV detection means detecting wavelengths shorter than about 280 nm, the terrestrial cutoff of the solar spectrum, which requires a band gap of roughly 4.4 eV or larger so that sunlight itself generates no carriers. The obstacle is that Mg strongly prefers octahedral (rock-salt, as in MgO) coordination while Zn prefers tetrahedral wurtzite coordination, so high Mg contents drive phase segregation into rock-salt MgO; the source paper showed that a low-Mg quasi-homoepitaxial buffer layer (a Mg0.2Zn0.8O\mathrm{Mg}_{0.2}\mathrm{Zn}_{0.8}\mathrm{O} layer close in composition to the film grown on top, used to relieve misfit strain) plus Mg fluxes kept below a composition-dependent critical value allowed reproducible single-phase wurtzite growth up to x≈0.55x\approx0.55 (Eg=4.55E_g=4.55 eV). On the doping side, ZnO is easily n-type (native donors such as Zn interstitials and oxygen vacancies, plus hydrogen, are abundant), but p-type doping has resisted three decades of effort: candidate acceptors (nitrogen, phosphorus, arsenic, antimony, lithium, sodium) suffer from low solubility, deep acceptor levels, and compensation by native donor defects, and reported p-type films frequently revert to n-type or fail to reproduce (Janotti and Van de Walle, Rep. Prog. Phys. 72, 126501 (2009)). Because stable p-type ZnO was unavailable, the source paper built its visible-blind detector (one that responds to UV light while rejecting visible light) as a double heterojunction n-ZnO/insulator-MgO/p-Si, in which a roughly 50 nm insulating MgO interlayer blocks injection of carriers photo-generated in the Si substrate by visible light while passing UV photocurrent from the ZnO. The demonstrated devices—a solar-blind photoconductive detector (a detector whose electrical conductivity rises under illumination) with a 277 nm cutoff operating at 150 V bias with 25 pA dark current (the current that flows with no illumination), and the visible-blind Si-based heterojunction—worked, but their limitations are the open problems addressed below.

Problem Statement

The common objective uniting the issues left open by Du et al. (Adv. Mater. 21, 4625 (2009)) is to remove the materials bottlenecks that still force ZnO/MgZnO ultraviolet optoelectronics to rely on silicon and on empirical growth recipes, so that all-oxide (Si-free) visible-blind and solar-blind UV detectors—and ultimately ZnO-based UV emitters—can be built reproducibly. Three linked questions constitute the problem. (1) p-type ZnO: establish a growth and doping route that yields p-type ZnO films reproducibly and stably (with controlled hole concentration, shallow-enough acceptor levels, and conductivity that does not revert to n-type), removing the need for p-Si as the complementary electrode in ZnO heterojunction UV devices; determine which acceptor species and defect-compensation mechanisms govern the dopability limit. (2) Mg incorporation window: determine quantitatively the composition-dependent critical Mg flux (and, more generally, the growth-parameter-dependent incorporation window) beyond which single-phase wurtzite MgxZn1−xO\mathrm{Mg}_x\mathrm{Zn}_{1-x}\mathrm{O} gives way to rock-salt MgO segregation, including its dependence on substrate temperature, flux ratios, buffer layers, and substrate choice, and establish the microscopic mechanism by which Mg atoms incorporate into wurtzite sites versus nucleating Mg-rich rock-salt clusters during growth. (3) Interfacial nucleation: provide a microscopic, quantitative description of the atomic-scale nucleation dynamics of the ultrathin AlN (formed by sapphire nitridation) and MgO (formed by Mg wetting) interfacial layers—early-stage stacking-sequence evolution, adatom diffusion, and substitution kinetics—that determine film polarity and domain structure on the different substrates used. Each of (1)-(3) may be answered independently, and it is their joint resolution that converts the demonstrated but empirical 2009 results (single-phase wurtzite Mg0.55Zn0.45O\mathrm{Mg}_{0.55}\mathrm{Zn}_{0.45}\mathrm{O} at 4.55 eV grown below an unquantified critical Mg flux; visible-blind n-ZnO/insulator-MgO/p-Si and solar-blind MgZnO detectors) into a predictive, all-oxide materials technology; the device-level engineering that the same paper left as future work (low bias and dark current with preserved spectral selectivity) has since been substantially advanced by later literature and is treated as context rather than as part of the open core.

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