Identification of the I$_{10}$ Donor in ZnO as a Sn--Li Complex with Large Hyperfine Interaction

Abstract

Donor impurities in wide direct band gap semiconductors provide a promising platform for spin--photon quantum technologies by combining a donor spin qubit with optically addressable transitions. In ZnO, the shallow donor with the largest reported binding energy has long been associated with the I$_{10}$ bound exciton line, but its microscopic origin has remained unresolved. Here we demonstrate the controlled formation and identification of this donor as a Sn--Li complex through a combination of ion implantation, annealing, optical spectroscopy, and first-principles calculations. Resonant two-laser coherent population trapping measurements reveal an electron--$^{119}$Sn hyperfine interaction of $392 \pm 15$\,MHz, establishing a coupled electron--spin--1/2, nuclear--spin--1/2 system with one of the largest hyperfine couplings reported for shallow donors in semiconductors. Density functional theory calculations show that a nearest-neighbor Sn$_{\mathrm{Zn}}$--Li$_{\mathrm{Zn}}$ complex has favorable formation energetics, donor character with the electron localized on Sn, and an extrapolated hyperfine interaction consistent with experiment. The large donor binding energy and excited-state structure indicate enhanced thermal robustness of the optical transition relative to conventional group--III donors, while the strong hyperfine interaction enables fast electron--nuclear spin control and prospects for direct nuclear--spin--photon interfaces. We further observe efficient optically induced nuclear spin polarization, highlighting a path toward nuclear spin initialization. More broadly, our results reveal how a donor--acceptor complex can access previously unexplored regimes of shallow donor physics, extending the design space of quantum defects beyond isolated substitutional dopants.

Figures (7)

• Figure 1: Dependence of bound exciton localization energy (blue) and hyperfine interaction strength (red) as a function of donor binding energy. Circles and squares are Ref. Meyer_2004. Star represents this work.
• Figure 2: (a) Zero-field photoluminescence spectrum of Sn-implanted sample A and a non-implanted piece of the same substrate. $T= 7$ K. (b) Magneto-PL spectrum of sample A. T=8.9 K. The color bar shows PL intensity in counts/seconds. (c) SIMS measurement of the Sn and Li density in sample A and an unimplanted piece of the same substrate. In the unimplanted sample, Sn is at the detection limit and Li deeper than 100 nm is at the detection limit in the non-implanted sample. (d) Formation of I$_{10}$ (Sn-Li) with and without Li implantation. Li implantation and annealing showing enhanced formation with Li implantation. $T=9.5$ K.
• Figure 3: Experimental energy configurations in the (a) low (LE) energy fixed-frequency pump case and (b) high-energy (HE) fixed-frequency pump case. (c) Photoluminescence excitation probe scan over the high-energy transition with and without the pump excitation. (d) and (e) Power series of the pump laser for (a) LE pump and (b) HE pump, respectively. (f) Nuclear spin polarization corresponding to (d) and (e). All scans are taken at B=6 T and T=8 K.
• Figure 4: (a) Structural illustration of three symmetry-inequivalent Sn$_\textnormal{Zn}$+Li$_\textnormal{Zn}$ defect complexes (denoted as a, b and c). (b) Formation energy $E^f$ of Li$_\textnormal{Zn}$, Sn$_\textnormal{Zn}$ and Sn$_\textnormal{Zn}$+Li$_\textnormal{Zn}$ defect complexes in ZnO as a function of Fermi level $E_{\mathrm{F}}$ under Zn-rich (left panel) and O-rich conditions (right panel). Three Sn$_\textnormal{Zn}$ + Li$_\textnormal{Zn}$ defect complexes are considered, and they exhibit very similar, nearly overlapping formation energies.
• Figure 5: Isotropic hyperfine parameters for Sn-Li donors as a function of supercell size. GGA data points are in black squares and HSE data points are in red circles.The black line is a linear fit to GGA data for $N$$\geq$ 432: $A$(GGA) = 117.99 + 258.26 $\times$ 1000/$N$. The red line is a rigid shift of the black line, passing through the largest HSE data ($N$$=$ 1024). The intercept of the red line yields $A= 466.7$ MHz.