Session M03: Defects and Doping in Si, Ge, SiC, and Diamond

Probing NV and SiV charge state dynamics using high-voltage nanosecond pulse and photoluminescence spectral analysis

Nitrogen-vacancy (NV) and silicon-vacancy (SiV) color defects in diamond are promising systems for quantum technology applications. The NV and SiV centers have multiple charge states, and their charge states have different electronic, optical and spin properties. For the NV centers, most investigations for quantum sensing applications are targeted on the negatively charged NV (NV⁻), and it is important for the NV centers to be in the NV⁻ state. However, it is known that the NV centers are converted to the neutrally charged state (NV⁰) under laser excitation. An energetically favorable charge state for the NV and SiV centers depends on their local environments. It is essential to understand and control the charge state dynamics for their quantum applications.

 

In this presentation, we discuss the charge state dynamics of NV and SiV centers under high-voltage (HV) nanosecond pulse discharges. These voltage-induced changes in charge states are probed by their photoluminescence spectral analysis. The analysis result from the present experiment shows that the HV nanosecond pulses cause shifts of the chemical potential and can convert the charge states of NV and SiV centers with the transition rates of ~MHz. This result also indicates that the major population of the SiV centers in the sample is the doubly negatively charged state (SiV²⁻), which is often overlooked because of its non-fluorescent and non-magnetic nature. This demonstration paves a path for a method of rapid manipulation of the NV and SiV charge states.   

Development and Applications of High-field NV-detected NMR-ESR Techniques

The nitrogen-vacancy (NV) center in diamond has enabled studies of nanoscale Nuclear magnetic resonance (NMR) and electron spin resonance (ESR) with high sensitivity in small sample volumes at low magnetic field [1]. While low field NV-detected NMR techniques are useful in many experiments, NV-NMR at high magnetic field (>1T) with finer resolution on chemical shifts and higher thermal polarization can be advantageous for applications including surface NMR study of heterogeneous catalysis, ensemble NMR of microfluidics, and condensed matter studies aimed at probing micro- and nano-scale features [2]. We have recently demonstrated NV-detected ¹³C NMR and ESR at 4.2T and 8.3T [2][3]. In addition, we have shown that dynamic nuclear polarization (DNP) profile at high field reveals spin clustering in diamond [4]. The result shows the importance of DNP analysis for the detection of spin clusters and a pathway for efficient DNP at high magnetic fields. In this presentation, we will discuss about the recent development of our high-field NV-detected NMR and EPR techniques and their applications. The high-field NV-NMR and -ESR is a powerful tool for the investigation of nanoscale spin environments such as spin concentration, spatial distribution, polarization, and dynamics, which are crucial in solid-state quantum sensing and quantum information applications.   

Fast optoelectronic charge state conversion of silicon vacancies in diamond

Group IV vacancy color centers in diamond are promising spin-photon interfaces with strong potential for applications for photonic quantum technologies. Reliable methods for controlling and stabilizing their charge state are urgently needed for scaling to multi-qubit devices. Here, we manipulate the charge state of silicon vacancy (SiV) ensembles by combining luminescence and photo-current spectroscopy. We controllably convert the charge state between the optically active SiV^- and dark SiV^2- with MHz rates and >90% contrast by judiciously choosing the local potential applied to in-plane surface electrodes and the laser excitation wavelength. We observe intense SiV^- photoluminescence under hole-capture, measure the intrinsic conversion time from the dark SiV^2- to the bright SiV^- to be 36.4(6.7)ms and demonstrate how it can be enhanced by a factor of 10⁵ via optical pumping. Moreover, we obtain new information on the defects that contribute to photo-conductivity, indicating the presence of substitutional nitrogen and divacancies.   

Excited States in the Si G-center from First-Principles GW/BSE Calculations

Defects in solid state materials have been identified as candidates for scalable qubits that can be grown on a chip. In diamond structure Si, the G-center—a pair of substitutional C atoms with an interstitial Si atom—has seen particular interest because of its optical activity. A zero-phonon line (ZPL) emission at 968 meV is observed in the O-band of optical telecommunication wavelengths, making it ideal for quantum information infrastructure. Emissions from the Si G-center are therefore compatible with optical fiber technology, which could make Si a material used for long distance coherent transmission in photonics and quantum optics. We investigate the excited states of the Si G-center with the GW approximation and the Bethe-Salpeter Equation (BSE) approach. We calculate the excitation energy, wavefunction character, and transition dipole moments associated with the first few excited states, using the open-source code BerkeleyGW. We consider the implications of the results for single photon emission and quantum technologies.   

Solid state defect emitters with no electrical activity

Point defects may introduce defect levels into the fundamental band gap of the host semiconductors that alter the electrical properties of the material. As a consequence, the in-gap defect levels and states automatically lower the threshold energy of optical excitation associated with the optical gap of the host semiconductor. It is, therefore, a common assumption that solid state defect emitters in semiconductors ultimately alter the conductivity of the host. Here we demonstrate on a particular defect in 4H silicon carbide by means of advanced ab intitio calculations that a yet unrecognized class of point defects exists which are optically active but electrically inactive in the ground state. These findings propose an unexplored avenue for engineering semiconductor devices, suggesting the feasibility of creating defect species that offer independent control over optical and electrical functionalities within the same platform. The implications of this work extend to the design and optimization of highly integrated and miniaturized semiconductor devices that leverage both optical and electrical attributes for enhanced functionality.   

First-principles investigation of near surface donor-acceptor pairs in silicon carbide

We recently proposed a quantum science platform¹ utilizing the dipole-dipole coupling between donor-acceptor pairs (DAPs) in bulk materials to realize optically controllable, long-range interactions between defects in solids. Building on this proposal and using first principles calculations, we investigate the physical properties of several DAPs in proximity of different surfaces in 3C-SiC. Our results indicate that the (2x1) hydrogen terminated surface is a promising surface termination, as it doesn’t introduce any surface states in the bandgap and causes minor changes to the DAP properties, compared to those in the bulk; specifically, we only observe a slight increase in both the zero-phonon line energies and the Huang-Rhys factors. Additionally, we explore the possibility of using transitions between surface defects to near-surface donors and acceptors, to orient the dipole moments of the DAPs perpendicular to the surface. Doing so leads to a beneficial increase of the DAPs dipole coupling compared to their bulk counterpart.   

Quantifying Phosphorous Levels in Ultrathin Si Films using Second Harmonic Generation

Non-destructive assessment of dopant concentration plays a pivotal role in contemporary semiconductor manufacturing. In this investigation, we introduce an inventive method for appraising the dopant concentration in doped Si thin films (DSTF) utilizing second harmonic generation (SHG). This technique involves analyzing the charge trapping triggered by internal photoemission (three photon absorption) and the concomitant electric field-induced SHG. Our proposed method includes the development of a strategy for estimating DSTF's dopant concentration by considering tunneling probability and the Fermi-Dirac distribution, irrespective of its crystalline structure. Our method facilitates the precise measurement of dopant concentrations spanning from 10¹⁷ to 10²⁰ atoms/cm³, allowing for real-time monitoring. 

The non-monotonic variation of the second-order susceptibility χ⁽²⁾ with dopant concentration may be attributed to the presence of interstitial defects and dopant agglomeration in highly doped silicon. The trapping rate at t = 0, denoted as R(0), exhibits a nonlinear dependency on dopant concentration due to the combined effects of ionization energy reduction, bandgap narrowing, and tunneling effect. As the quantity of electron density generated and trapped at the interface or surface directly impacts R(0), a substantial correlation between R(0) and surface electron density (n_sur) is evident.

 

First-principles study of persistent spin helix on OH-terminated diamond, Si, and Ge surfaces

The stability and reduced chemical reactivity of semiconductors largely depend on their surface termination methods. Surface termination studies have been conducted on semiconductors like Si [1], Ge [2], and diamond [3] surfaces. Theoretically, terminating the diamond surface using hydroxyl (OH) exhibited the persistent spin helix with its spin-orbit coupling value, α_PSH, of 14.2 meV・Å [4]. On the other side, terminating the diamond surface by using hydrogen produced the Rashba spin splitting with α_R of 3.6 meV・Å [4]. These findings steer our study to explore the effects of OH termination on the electronic properties of other group IV semiconductors, specifically Si and Ge surfaces, emphasizing their spin-orbit coupling properties to identify alternative materials suitable for spintronic device applications.

 

Reference

[1] D. B. Fenner, D. K. Biegelsen, and R. D. Bringans, J. Appl. Phys. 66, 419 (1989).

[2] S. Rivillon, Y. J. Chabal, F. Amy, and A. Kahn, Appl. Phys. Lett. 87, 253101 (2005).

[3] B. B. Pate, M. H. Hecht, C. Binns, I. Lindau, and W. E. Spicer, J. Vac. Sci. Technol. 21, 364 (1982).

[4] H. P. Kadarisman, N. Yamaguchi, and F. Ishii, Appl. Phys. Express 16, 023001 (2023).   

Dispersive Sensing of an STM-Tip Induced Quantum Dot in P-doped Si(100)

Recently, radio frequency (RF) reflectometry techniques have been employed in dispersive gate-based sensing across multiple quantum dot platforms to perform sensitive charge-based state read-out. Here, we extend such techniques to a mK-STM by integrating an LC tank circuit directly into the tip plate to enable simultaneous measurement of tunneling current and RF-reflectometry. For semiconductor samples of interest, STM tip-induced band bending gives rise to the formation a quantum dot which can then be scanned across a sample surface to sense defects via changes in tip-sample capacitance. This measurement geometry provides a unique method of probing new materials as only a single lead is needed to both create and measure the properties of the tip induced dot. This reduced overhead presents a new non-destructive method of characterization of materials that can be employed in QIS applications. As a demonstration of this capability, we present spectroscopic measurements on a series of Si(100) samples doped with varying concentrations of phosphorus (N_D ~10¹⁷ to 10¹⁹ cm^-3). Our results reveal phase contrast in the vicinity of both surface defects and sub-surface dopants. At lower dopant concentrations, RF phase-resolved spectral features appear to broaden while the apparent STM tip-sample bias voltage hysteresis drastically increases. This hysteresis, which manifests as changes to spectral features pending the sweep direction (i.e. negative bias to positive, and vice versa), suggests conditional charging effects based on the initial state of the induced dot. Here, we specifically focus on extensive ‘line-style’ phase spectroscopy measurements on a Si(100) sample possessing a concentration of N_D = 4.3∙10¹⁸ cm^-3 with a specific focus on the hysteresis dependent features. These measurements yield fine parabolic-like patterns indicative of surface defects, while larger underlying charge structures that emerge in the background can be attributed to sub-surface dopants. From these results, we aim to understand how the local environment of novel materials lead to deformations in the confining potentials of dot-based qubits.     

Photo-induced charge state dynamics of the neutral and negatively charged silicon vacancy centers in room-temperature diamond

The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. However, the dynamics governing SiV charge state interconversion has been less explored mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. In this work we use multi-color confocal microscopy and density functional theory to examine photo-induced SiV recombination from neutral (SiV0), to single (SiV-), to double-negatively charged (SiV2-) over a broad spectral window in chemical-vapor-deposition diamond under ambient conditions. For the SiV0 to SiV‒ transition, we found a linear growth of the photo-recombination rate with laserpower at all observed wavelengths, a hallmark of single photon dynamics. Laser excitation of SiV‒, on the other hand, yields only fractional recombination into SiV2‒, a finding we interpret in terms of a photo-activated electron tunneling process from proximal nitrogen atoms.   

Charged Defects in Germanium: A First-Principles Study

High-purity Germanium (Ge) detectors have been proven to be the best among the current technologies for the detection of particle signatures in double-beta decay due to their excellent energy resolution and lowest background signal. However, large Ge detectors are prone to charge trapping (CT) phenomena due to the presence of defects and impurities in crystalline Ge, resulting in significant energy resolution degradation. It is therefore imperative to identify such defects and eliminate them during the Ge detector production process. In this work, we employ Density Functional Theory and beyond to study the structural and electronic properties of suspicious charged defects (vacancy, substitutional, and interstitial) in Ge. Our preliminary results show that the n-type defects (P, As, Sb) have lower formation energies compared to the vacancy and interstitial, and their charge transtition level lie closer to the conduction band of Ge. Our results are important for identifying defects, thus may make a significant contribution in developing mitigation strategies to improve the performance of Ge detectors.   

Characterising arsenic dopant incorporation in germanium

Germanium is receiving renewed interest for its utility in fabricating quantum technological devices, showcasing advantages over silicon such as higher electron mobility, stronger spin-orbit coupling, and enlarged Bohr radius. Its potential for nuclear spin-free isotopic enrichment and long donor coherence times, coupled with its already-established use in high-performance electronics, underscores its evolving relevance for the fabrication of devices such as a solid-state quantum computer. Recently, we have demonstrated that arsenic incorporates into the Ge(001) surface at room temperature [1]. This remarkable result means that there is no incorporation anneal required after the deposition of the precursor molecule, the incorporation probability of arsenic is unity, and lateral diffusion during fabrication should be minimal. Here, we present new work investigating the conductivity and confinement of arsenic in germanium with parallels to our similar recent results in silicon [2,3].

[1] Hofmann et. al., Angewandte Chemie, 62(7), (2023).

[2] Constantinou, et al., Advanced Science, 10, 2302101, (2023).

[3] D’Anna, Advanced Electronic Materials, 9, 2201212, (2023).