Dense Ge nanocrystals embedded in TiO₂ with exponentially increased photoconduction by field effect

Area of Science:

• Materials Science
• Nanotechnology
• Semiconductor Physics

Background:

• Semiconductor nanocrystals, specifically silicon (Si) and germanium (Ge), are crucial for photo-effect applications.
• Quantum confinement in nanocrystals allows for tunable optical bandgaps, essential for advanced electronic and photonic devices.
• Fabricating dense, functional nanocrystal structures is key to unlocking their potential in applications like photoconduction.

Purpose of the Study:

• To fabricate dense germanium (Ge) nanocrystals within titanium dioxide (TiO2) for enhanced photoconduction.
• To investigate the impact of quantum confinement on the optical and electrical properties of Ge nanocrystals.
• To elucidate the charge transport mechanisms within Ge nanocrystal composite films.

Main Methods:

• Fabrication of Ge nanocrystals in amorphous TiO2 layers using low-temperature rapid thermal annealing (550°C).
• Characterization of nanocrystal size and structure using High-Resolution Transmission Electron Microscopy (HRTEM) and X-ray Diffraction (XRD).
• Measurement of photocurrent, optical absorption, reflection-transmission spectra, and temperature-dependent dark conduction.

Main Results:

• Dense Ge nanocrystals (approx. 5 nm) were successfully fabricated, exhibiting quantum confinement effects.
• An exponential increase in photocurrent with applied voltage was observed, attributed to field-effect control.
• A significant blue-shift in the absorption gap to 1.14 eV was confirmed, consistent with quantum confinement.
• Nonmonotonic spectral dependence of the refractive index and T^-1/2-type Coulomb gap hopping conduction were observed.

Conclusions:

• The study successfully demonstrates the fabrication of Ge nanocrystals in TiO2 suitable for enhanced photoconduction.
• Quantum confinement in Ge nanocrystals leads to tunable optical properties and bandgap broadening.
• Field-effect control and Coulomb gap hopping are identified as key mechanisms governing the electrical transport properties.