PCSI2026 Session PCSI-MoM2: Materials for QIS

Optically active spins in solid-state systems present significant potential for a range of quantum information science applications, including their use as entanglement distribution nodes within quantum networks, single-photon sources for linear optical quantum computing, and as platforms for cluster state quantum computation. Furthermore, the inherent optical connectivity of these systems enables the implementation of low-density parity check (LDPC) error correction codes. Among these platforms, erbium ions implanted in silicon and silicon carbide are particularly promising due to their superior optical and electron spin coherence characteristics, erbium’s emission compatibility with the Telecom C band, and the advanced state of silicon nanofabrication technology. In this work, we report on erbium sites in silicon that simultaneously exhibit extended optical coherence and long electron spin lifetimes. Specifically, we observed spin coherence times of 1 ms in nuclear spin-free silicon crystals. The measured homogeneous linewidths were below 100 kHz, with inhomogeneous broadening approaching 100 MHz [1]. Spectral hole burning and optically detected magnetic resonance techniques were employed to examine both the homogeneous linewidth and spin coherence properties. The demonstration of long spin coherence times and narrow optical linewidths in multiple sites underscores the exceptional suitability of erbium in 28Si for future quantum information and communication technologies, including single-photon frequency multiplexing schemes. Further discussions address the integration of these systems into silicon-on-insulator nanophotonic devices as well as thin-film 4H-SiC-on-insulator (SiCOI) devices. In silicon carbide, we observed an inhomogeneous broadening of 6.22 GHz and homogeneous linewidths as narrow as 440 kHz from a weak ensemble of emitters [2]. Site-selective spectroscopy identified that Er ions primarily occupy two distinct lattice sites in 4H-SiCOI. Additionally, we characterized the optical lifetimes and magneto-optical properties of these narrowband transitions. Collectively, these findings position Er-doped SiCOI as a highly promising solid-state platform for integrated, on-chip quantum information processing applications.

[1] Berkman, I.R., Lyasota, A., de Boo, G.G. et al. Long optical and electron spin coherence times for erbium ions in silicon. npj Quantum Inf11, 66 (2025). https://doi.org/10.1038/s41534-025-01008-x

[2] Alexey Lyasota, Joshua Bader, Shao Qi Lim, Brett C. Johnson, Jeffrey McCallum4, Qing Li, Sven Rogge, Stefania Castelletto, in preparation

+ Author for correspondence: S.Rogge@unsw.edu.au

This work explores two promising approaches for fabricating single-photon emitters operating at telecom wavelengths (1.3–1.55 µm), using epitaxially grown quantum dots (QDs). The first approach involves the growth of InAs nanostructures on InP substrates, producing a mix of quantum dots and dashes. On standard InP (100) substrates, growth proceeds via a modified Stranski–Krastanov (SK) mechanism, where a thick, planar wetting layer forms prior to 3D island nucleation. These elongated dashes, preferentially aligned along the (1-10) direction, exhibit limited three-dimensional confinement, behaving more like quantum wells. In contrast, growth on (311)B-oriented InP yields discrete QDs with strong single-dot emission characteristics.[1]

The second approach leverages the GaSb/GaAs material system to realize both coherently strained SK-mode QDs and strain-relaxed islands, the latter mediated by interfacial misfit dislocation arrays. Despite its promise, this system faces challenges due to likely type-II band alignment and the complexity introduced by GaSb/GaAs interdiffusion during capping.

Growth experiments are conducted using a solid-source VG V80 MBE reactor. InAs QDs are deposited on n-doped InP (001) and (311B) substrates with In₀.₅₃Ga₀.₄₇As buffer layers, while GaSb QDs are grown on semi-insulating GaAs (001) with GaAs buffer layers. Substrate desorption, growth temperatures, and layer thicknesses are carefully optimized.

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

[1] N. A. Jahan et al., Temperature dependent carrier dynamics in telecommunication band InAs quantum dots and dashes grown on InP substrates, J. Appl. Phys. 113, 033506 (2013).

High-performance solution processing is essential for next-generation electronic and optical devices, and Scanning Probe Microscopy (SPM) enables atomic-scale evaluation. This report aims to characterize wet-treated Si(111) surfaces in detail using multiple SPM modes.

Si(111) surfaces were treated by anisotropic etching in NH₄F [1]. These surfaces were first imaged in air by conventional amplitude-modulated Atomic Force Microscopy (AFM). Figure 1(a) shows a familiar step/terrace structure. The same surface was then examined by noncontact AFM (nc-AFM) and Scanning Tunneling Microscopy (STM). The nc-AFM/STM hybrid system was operated at room temperature under ultrahigh-vacuum conditions of 10⁻⁸–10⁻⁷ Pa. For nc-AFM, the oscillation amplitude and frequency shift were set to 1.0 nm and −1.0 Hz, respectively, and for STM, the sample bias and tunneling current were −2.0 V and 0.6 pA. It is evident that the nc-AFM image in Fig. 1(b) resolves smaller particles or adsorbates with much higher resolution than Fig. 1(a). More importantly, these small particles visible in Fig. 1(b) are absent in the STM image in Fig. 1(c). A subsequent nc-AFM scan of the same area after acquiring Fig. 1(c) reproduced the result in Fig. 1(b), confirming that Fig. 1(c) does not arise from probe-induced manipulation of adsorbates but rather from the different imaging mechanisms of nc-AFM and STM.

Superconductivity in thin films can deviate significantly from bulk behavior, especially as dimensionality and disorder come into play. This is particularly true for aluminum, where critical temperature (T_C) and film morphology are highly sensitive to thickness and growth conditions. Here, we present an in-situ scanning tunneling microscopy (STM) study, performed at 78 K, of Al thin films grown on atomically clean Si(111) substrates by molecular beam epitaxy at cryogenic temperatures down to 6 K. The morphology is characterized across a wide range of coverages, from sub-monolayer up to 20 monolayers (ML). Cryogenic growth results in oriented hexagonal islands that begin to coalesce into a continuous film around 5 ML, with a typical roughness of a few monolayers. This roughness is constant up to 20 ML. Upon annealing to room temperature, the surface becomes nearly atomically smooth, though grain boundaries remain visible in STM. In contrast, room temperature growth produces significantly rougher films with large, disconnected islands of varying shape and orientation. We also investigated the superconducting properties of cryogenically grown films after exposure to atmospheric conditions, as required for ex-situ transport measurements. To stabilize the films, we used different post-growth treatments, including low temperature capping, cold oxidation, and room temperature oxidation. The films show critical temperatures approaching 3 K and critical fields above 5 T, which are significantly above the bulk value of 1.2 K.

Control of semiconductor nanostructures becomes more important to achieve the further device miniaturization [1]. Especially, critical significance of charge and spin states becomes evident at atomic-scale precision [2]. Donor-related defect-complex, DX center, in III-V semiconductors is one of lattice defects. Charge states of this defect as well as the bistable switching on the long lifetime were studied for Si-doped n-GaAs [3]. In the case of InAs, static transition of acceptor charge states in Mn-doped p-InAs is only reported [4].

In this presentation, the DX center in the near-surface region of n-type InAs is demonstrated as a quasi-equilibrated metastable state by using the scanning tunneling microscope (STM). After obtaining the cleaved (110) surface of sulfur-doped n-InAs (N_S : 4 × 10¹⁷ cm^-3) in ultra-high vacuum, the sample was transferred to the STM stage kept at 77 K. First, it is found that donor charge states showed striking dependence on the STM tip at the same scan condition. This suggests that the degree of the tip-induced band bending plays the crucial role of the charge state determination. Second, it is found that electrons tunneled from the STM tip cause an impact-ionization by the tip-induced electric field acceleration at the sample bias voltage V > 0 (tip is neutral). When the electric field exceeds the Avalanche breakdown field (〜8 × 10^-5 V/nm) [5], such hot electrons cause the impact-ionization. Generated secondary electrons are spread by radially diverging tip-induced field. At this non-equilibrium situation, the electron quasi-Fermi level becomes locally dominant to charge states of donor-related defects [6] beneath the tip. This quasi-Fermi level effect is detected as the bias voltage dependent topography. No evident bistable switching among charge states is imaged. These suggest that the imaged DX center in the n-InAs is a quasi-equilibrated metastable state available at the non-equilibrium condition and the lifetime of the captured electron by such DX center is expected much shorter than that of n-GaAs.

[1] S. Fölsch, J. Yang, C. Nacci, and K. Kanisawa, Phys. Rev. Lett. 103, 096104 (2009).

[2] P. M. Koenraad and M. E. Flatté, Nat. Mater. 10, 91 (2011).

[3] E. P. Smakman, P. L. J. Helgers, J. Verheyen, P. M. Koenraad and R. Möller, Phys. Rev. 90, 041410(R) (2014).

[4] F. Marczinowski, J. Wiebe, J.-M. Tang, M. E. Flatté, F. Meier, M. Morgenstern and R. Wiesendanger, Phys. Rev. Lett. 99, 157202 (2007).

[5] G. Bauer and F. Kuchar, Phys. Lett. A 30, 399 (1969).

[6] M. Asche and O.G. Sarbey, Physica B 308-310, 788 (2001).

Helical spin structures are expressions of magnetically induced chirality, entangling the dipolar and magnetic orders in materials. The recent discovery of helical van der Waals multiferroics down to the ultrathin limit raises prospects of large chiral magnetoelectric correlations in two dimensions. However, the exact nature and magnitude of these couplings have remained unknown so far. Here we perform a precision measurement of the dynamical magnetoelectric coupling for an enantiopure domain in an exfoliated van der Waals multiferroic. We evaluate this interaction in resonance with a collective electromagnon mode, capturing the impact of its oscillations on the dipolar and magnetic orders of the material with a suite of ultrafast optical probes. Our data show a giant natural optical activity at terahertz frequencies, characterized by quadrature modulations between the electric polarization and magnetization components. First-principles calculations further show that these chiral couplings originate from the synergy between the non-collinear spin texture and relativistic spin–orbit interactions, resulting in substantial enhancements over lattice-mediated effects. Our findings highlight the potential for intertwined orders to enable unique functionalities in the two-dimensional limit and pave the way for the development of van der Waals magnetoelectric devices operating at terahertz speeds.

Reference:

Gao, F.Y., Peng, X., Cheng, X. et al. Giant chiral magnetoelectric oscillations in a van der Waals multiferroic. Nature632, 273–279 (2024). https://doi.org/10.1038/s41586-024-07678-5