GE-Base: Germanium Experimental Platforms

• GE-Base is a series of germanium-based experimental platforms characterized by superior charge transport, high purity, and optimized detector architectures for ultra-sensitive, low-background measurements.
• Key innovations include BEGe detectors with enhanced pulse shape discrimination and precise material processing that achieve excellent energy resolution and minimized cosmogenic activation.
• Advanced implementations extend to quantum sensing and spin qubit integration using Ge/GeSi alloys, phononic crystal cavities, and RF reflectometry for sub-keV dark matter and neutrino detection.

GE-Base refers to a series of germanium-based (“Ge-based”) experimental platforms and device architectures that leverage the unique physical, chemical, and electronic properties of germanium in state-of-the-art applications spanning rare-event physics, quantum technologies, and high-frequency electronics. This term encompasses a foundational layer of materials, detectors, and device designs where Ge’s superior charge transport, large atomic mass, and favorable band structure are exploited for ultra-sensitive, scalable, and background-free measurements. Representative implementations include both macroscopic devices—such as Broad Energy Germanium (BEGe) detectors for neutrinoless double beta decay searches in GERDA—and nanoscale quantum architectures, such as Ge/GeSi single crystals for spin qubit integration and phononic crystal-enhanced quantum sensors.

1. Germanium-based Detector Technologies for Rare-Event Searches

High-purity germanium (HPGe) detectors, pioneered in large-scale experiments such as GERDA, form the backbone of GE-Base for rare-event searches involving neutrinoless double beta decay ($0\nu\beta\beta$). The GERDA experiment’s adoption of BEGe detectors illustrates the paradigm:

• Detector Architecture: BEGe diodes feature a small p$^+$ readout electrode and a wrap-around lithium-diffused n$^+$ electrode. This geometry produces highly localized weighting potentials, critical for distinguishing single-site events (signal) from multi-site backgrounds through the A/E pulse shape parameter.
• Material Purity and Processing: Germanium is isotopically enriched to $\sim$88% $^{76}$Ge, purified to 11N quality via zone refining, and fabricated into detectors with low impurity gradients and optimized depletion voltages (<4 kV).
• Performance Metrics: The detectors routinely achieve energy resolutions of 1.7–1.8 keV FWHM at 1333 keV in vacuum, active volume fractions near 90%, and sub-pA leakage currents. In liquid argon operation, resolution remains 2.8–3.0 keV at 2615 keV, enabling background indices (BI) around $10^{-3}$ c/(keV·kg·yr) (Agostini et al., 2014, Collaboration et al., 2019).
• Signal Discrimination: Pulse shape discrimination (PSD), quantified via the A/E parameter, suppresses gamma-induced backgrounds by a factor of 10–15 for single-site event acceptance of 90%. This level of discrimination is essential for rendering the experiment nearly background-free in the search region of interest.

The optimization of detector mass yield and the reduction of cosmogenic activation during the production chain (by extensive underground storage and shielded transport) are further key facets, ensuring long-lived radioisotope backgrounds (like $^{68}$Ge, $^{60}$Co) remain subdominant.

2. Ge-based Platforms for Quantum Information Science

The GE-Base concept extends to the domain of quantum computing via the use of Ge$_{1-x}$Si$_{x}$ alloys as highly engineered, low-strain substrates for spin qubit integration (Fuhrberg et al., 22 Apr 2025).

• Alloy Synthesis and Atomic Ordering: Bulk Ge$_{0.85}$Si$_{0.15}$ single crystals grown by the Czochralski method exhibit a random alloy structure with Si substituting Ge at lattice sites, as verified by X-ray photoelectron diffraction (XPD) and Bloch wave simulations. This suppresses phase segregation and ensures uniform lattice properties.
• Electronic Band Structure: Momentum-resolved photoemission (HarMoMic) reveals that the heavy/light hole and split-off (SO) bands closely mimic those of pure Ge; measured carrier effective masses remain low ($m^*_\text{LH/HH}\approx0.07\,m_0$, $m^*_\text{SO}\approx0.05\,m_0$). The reduced strain and high crystalline order are essential for minimizing decoherence and maximizing spin lifespan in hole-spin qubits.
• Implication: These materials provide the substrates for planar Ge/GeSi heterostructures supporting scalable, high-fidelity spin qubits, benefiting from efficient electric-field control and absence of valley degeneracy.

3. Advanced Quantum Sensing Architectures

A novel incarnation of GE-Base is realized in the GeQuLEP platform—Germanium-based Quantum Sensors for Low-Energy Physics—which integrates HPGe crystals, engineered phononic crystal (PnC) cavities, and quantum-classical hybrid readout via radio-frequency quantum point contacts (RF-QPCs) (Mei et al., 2 Jul 2025).

• Key Mechanisms:
  • Dipole-bound quantum dots, naturally formed by impurity freeze-out at cryogenic temperatures, act as the sensing elements.
  • Engineered PnC cavities, patterned into the Ge, confine and channel ballistic phonons by creating phononic bandgaps and enhancing the local phonon density-of-states near resonant quantum dots.

  • The detection chain couples phonon-induced charge displacements in quantum dots to RF-QPCs, exploiting:

    $$V(x) \approx -V_0 \exp\left\{ -\frac{x^2}{2\sigma^2} \right\}$$

    for the quantum dot potential, and the induced charge:

    $Q_{\text{ind}} = Q \cdot \frac{\delta x}{d}$

    via the Ramo–Shockley theorem.

  • The absorption cross-section enhancement due to phonon slow-down is parameterized as:

    $$\sigma_{\text{abs}} \propto \frac{D^2}{\rho v_{\text{ph}}^3 \hbar}$$

• Sensitivity: The theoretical minimum detection energy is $0.00745$ eV (single primary phonon level), several orders of magnitude below conventional HPGe detection limits. This regime allows unprecedented access to sub-keV dark matter and low-energy neutrino measurements, including real-time solar $pp$ neutrino detection via CE$\nu$NS.
• Architecture: The approach is contact-free, planar, and designed to permit large-scale integration and low-noise operation, exploiting the synergy between quantum transduction and RF reflectometry.

4. Energy Band Engineering and Materials Integration

Control over energy band alignment—spanning vertical graphene base transistor (GBT) structures, GeSi quantum substrates, and Ge-photonic devices—is a recurring tenet in GE-Base systems (Mehr et al., 2011, Fuhrberg et al., 22 Apr 2025).

• Engineering in GBTs: GBTs utilize a vertical stack of emitter, graphene base, and collector, separated by specifically engineered insulators (e.g., SiO$_2$ and graded TixSi$_{1-x}$O$_2$) to optimize barrier heights for electron tunneling. Quantum capacitance of graphene ($$C_Q \approx | \partial Q_B/\partial V_Q | \approx K|V_o|$$, $K \sim 25~\mu$F/cm$^2$/V) critically impacts transconductance and cutoff frequency.
• SiGe/GeSi Alloys: In quantum substrates, a low Si fraction (15%) in Ge$_{1-x}$Si$_x$ buffers tunes the strain while retaining essential Ge electronic properties. Synchrotron-based methods confirm random alloying with no secondary phases.

The exploitation of tailored band offsets and quantum capacitance effects is essential for achieving the required charge selectivity, low-noise properties, and ballistic carrier dynamics in next-generation devices.

5. Digital Signal Processing and Simulation in HPGe Systems

Advances in digital trace analysis and detailed simulation underpin GE-Base detector calibration, signal extraction, and background rejection (Collaboration et al., 2019, Agostini et al., 20 Jun 2025).

• Pile-up Decomposition: New digital signal processing algorithms (built within the MGDO framework) resolve pile-up signals in HPGe detectors by employing trapezoidal filtering, pole-zero correction, and moving window differentiation. These enable identification of rare delayed coincidence signatures (e.g., $^{77}$Ge–$^{77\mathrm{m}}$As transitions in GERDA) even when the signals overlap with preamplifier decay.
• Pulse Shape Simulation: Codes such as ADL3 (augmented with field/weighting potential routines from siggen/mjd_fieldgen) simulate charge drift and signal formation. These models account for impurity gradients, local field variations, and dead layer evolution, validating and guiding analysis cuts for signal/background separation and optimizing operating voltages.

This integration of large-volume detector modeling, experimental calibration, and cutting-edge DSP is crucial for maximizing sensitivity to rare physics signals while ensuring robustness against background and operational perturbations.

6. Applications and Scientific Impact

The GE-Base paradigm directly enables and advances several frontiers:

• Neutrinoless Double Beta Decay: By supporting background indices at $10^{-3}$–$10^{-5}$ c/(keV·kg·yr) and leveraging high-mass, active-fraction detectors, GERDA and successor experiments like LEGEND achieve half-life sensitivities beyond $10^{26}$ yr, probing the Majorana nature of neutrinos.
• Quantum Information: Ge/GeSi substrates with low lattice strain and low disorder support scalable, long-lived hole-spin qubits, promoting progress in silicon-compatible quantum computing.
• Dark Matter and Neutrino Detection: Single-phonon threshold capabilities in Ge-based quantum sensors theoretically open the detection window for very low-mass (sub-keV) dark matter and CE$\nu$NS interactions, with applications extending to solar and supernova neutrino detection.
• High-Frequency Electronics: GBTs and SiGe/Ge-based heterostructures integrate into Si-compatible high-frequency (THz) electronics, offering enhanced current gain and ballistic transport properties.

The common foundation—meticulous material synthesis and detector engineering, quantum-level measurement, and stringent background controls—underlies the scientific reach of the GE-Base in both particle physics and quantum engineering.