At cryogenic temperatures, silicon quantum dots (SiQDs) exhibit distinct electronic, optical, and spin properties that diverge significantly from bulk silicon. These characteristics arise from quantum confinement effects, surface states, and reduced phonon interactions, making SiQDs promising candidates for quantum technologies. This analysis focuses on low-temperature photoluminescence (PL), carrier freeze-out, and spin dynamics, alongside their applications in quantum sensing and single-photon emission. Experimental techniques for cryogenic characterization are also discussed, with comparisons to bulk silicon behavior under similar conditions.

Low-temperature photoluminescence in SiQDs reveals sharp emission lines attributed to excitonic transitions confined within the nanoscale dimensions. Unlike bulk silicon, which exhibits weak indirect bandgap emission at cryogenic temperatures due to phonon-assisted processes, SiQDs display enhanced PL efficiency with narrower linewidths. The quantum confinement effect increases the oscillator strength of radiative recombination, leading to observable zero-phonon lines at temperatures below 50 K. Surface passivation plays a critical role in mitigating non-radiative recombination channels, with hydrogen-terminated SiQDs showing superior PL stability compared to oxide-coated counterparts. Spectral diffusion, caused by charge fluctuations in the surrounding matrix, is a key challenge but can be suppressed by stabilizing the dielectric environment.

Carrier freeze-out in SiQDs occurs at lower temperatures compared to bulk silicon due to the increased ionization energy of dopants confined within the dot. In bulk silicon, carrier freeze-out typically begins below 30 K, leading to a sharp decline in conductivity. In contrast, SiQDs exhibit a more gradual freeze-out process, as the discrete energy levels and Coulomb blockade effects alter the ionization dynamics. For phosphorus-doped SiQDs, the binding energy of donors can exceed 50 meV, compared to 45 meV in bulk silicon, resulting in persistent carrier localization even at millikelvin temperatures. This property is exploited in single-electron transistors, where controlled carrier occupation is essential for charge sensing applications.

Spin dynamics in SiQDs are influenced by reduced spin-orbit coupling and hyperfine interactions, leading to longer spin coherence times than those observed in bulk silicon. The nuclear spin-free isotope silicon-28 further enhances coherence, with spin relaxation times (T1) exceeding milliseconds below 1 K. Electron spin resonance (ESR) measurements reveal Landé g-factor anisotropy due to quantum confinement, differing from the isotropic g-factor of bulk silicon. Spin-photon interfaces benefit from these properties, enabling SiQDs to serve as spin qubits with optical addressability. The absence of piezoelectric fields, common in III-V quantum dots, reduces spin decoherence mechanisms, making SiQDs advantageous for scalable quantum networks.

Applications in quantum sensing leverage the spin-dependent PL of SiQDs, where magnetic field variations induce detectable shifts in emission energy. Nitrogen-vacancy centers in diamond are a benchmark for such sensors, but SiQDs offer complementary advantages, including compatibility with silicon photonics and tunable emission wavelengths. For single-photon sources, SiQDs exhibit antibunching in Hanbury Brown-Twiss experiments, confirming non-classical light emission. The narrow linewidth and high quantum yield enable integration with photonic cavities for enhanced emission rates, though spectral stability remains a challenge at sub-Kelvin temperatures.

Experimental techniques for cryogenic characterization include confocal PL microscopy, which resolves individual SiQDs with sub-micron precision. Dilution refrigerators equipped with optical access facilitate measurements below 100 mK, while superconducting magnets enable high-field spin manipulation. Time-resolved PL spectroscopy captures exciton lifetimes, typically ranging from nanoseconds to microseconds depending on surface quality. Electrical transport measurements in this regime require careful shielding to minimize electromagnetic interference, with lock-in amplification detecting faint signals. Cross-correlation techniques between PL and ESR provide insights into spin-photon coupling efficiency.

Compared to bulk silicon, SiQDs demonstrate superior optical activity and spin coherence at cryogenic temperatures, albeit with challenges in spectral stability and charge noise. Bulk silicon remains the platform of choice for ensemble-based quantum devices like donor spin qubits, while SiQDs excel in scenarios requiring nanoscale light-matter interaction. The absence of intervalley scattering in SiQDs simplifies the energy landscape, but surface-related defects introduce variability that must be addressed through advanced passivation schemes.

In summary, the cryogenic properties of SiQDs underscore their potential in quantum technologies, where their unique combination of optical and spin characteristics enables functionalities unattainable with bulk silicon. Advances in cryogenic characterization techniques will further elucidate their behavior, paving the way for robust quantum sensors and on-demand single-photon emitters. The comparison with bulk silicon highlights the trade-offs between scalability and performance, guiding material selection for specific applications in the quantum domain.