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).

In epitaxial growth, two main methods have been employed to form quantum nanostructures (QNs) over the past three decades: the Stranski–Krastanov (S–K) mode and the droplet epitaxy (DE) method [1, 2]. Among them, DE offers the unique advantage of enabling the formation of QNs even under lattice-matched conditions, unlike the S–K mode [3]. This technique has been successfully applied not only to lattice-matched systems such as GaAs/AlGaAs but also to lattice-mismatched systems such as InAs/GaAs [2,3]. Recently, it has been reported that QNs can also be formed in the type-II band alignment system of InAs/GaSb, which is lattice-mismatched [4].

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.

Bonding plays an important role in advanced microelectronics integration and packaging by bringing together components and devices produced separately.Surface-pretreatments on bonding materials have been consistently found to be crucial to achieving a high-quality bond, despite incomplete understanding of why certain treatments have greater success than others.Our project aims to improve understanding of some of these bonding pre-treatment effects to provide information to enable more efficient development of bonding protocols.

Indium is a critical material for conductive contacts in the fabrication of cryogenic low-temperature electronics and optical detectors for infrared and microwave applications, because In is a superconducting material which retains its ductility and adhesion properties during thermal cycling.Certain pre-bond plasma treatments increase the success of In-In bond adhesion.We have observed that plasma chemistries such as H₂/He, which do not include N₂ as a plasma gas, promote more successful In-In bonding.To understand why, we investigate the effects of atmospheric plasma exposure on In surfaces for several plasma chemistries including He/H₂/N₂, He/H₂ and He/N₂.X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are used to characterize In surfaces prior to and after ambient pressure plasma treatment.All plasma treatments decreased the amount of carbon and increased the amount of In-oxide present when compared to an untreated film.N­₂-containing plasmas resulted in the appearance of an additional high binding energy peak in the N 1s XPS spectrum.We postulate that this may be due to nitrate species formed on the In native oxide surface. Additional experiments are planned to assess the plausibility of this explanation.In foil was measured prior to and after Ar sputter cleaning for comparison to treated samples.

UPS measurements showed that all plasma chemistry treatments lowered the work function compared to the non-treated control.Greater spatial variation in work function was observed for chemistries with high N₂ versus those with little to no N₂.This finding possibly correlates with poorer bonding for N₂ containing plasmas.