Self-Assembled Ge QDs Formed by High Temperature Annealing on GaAs and AlxGa1-xAs (001)

TL;DR

This study investigates the self-assembly of germanium quantum dots on various semiconductor surfaces via high-temperature annealing, and demonstrates their potential for enhancing photovoltaic devices through upconversion.

Contribution

It presents a novel method for forming Ge quantum dots on GaAs and AlGaAs surfaces via high-temperature annealing in MBE, and explores their integration into solar cells for improved efficiency.

Findings

Ge QDs are nearly spherical with diameters up to 10 nm and no wetting layer.

QDs formation decreases with lower Al-content in the buffer.

Embedding Ge QDs in PIN diodes significantly increases photocurrent below GaAs bandgap.

Abstract

This work studies the spontaneous self-assembly of Ge QDs on AlAs, GaAs, and AlGaAs by high temperature in-situ annealing in molecular beam epitaxy (MBE). The morphology of Ge dots formed on AlAs are observed by atom probe tomography, which revealed nearly spherical QDs with diameters approaching 10 nm and confirmed the complete absence of a wetting layer. Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) of Ge annealed under similar conditions on GaAs and Al0.3Ga0.7As surfaces reveal the gradual suppression of QD formation with decreasing Al-content of the buffer. To investigate the prospects of using encapsulated Ge dots for upconverting photovoltaics, in which photocurrent can still be generated from photons with energy less than the host bandgap, Ge QDs are embedded into the active region of III-V PIN diodes by MBE. It is observed that orders of magnitude higher short-circuit current is obtained at photon energies below the GaAs bandgap compared with a reference PIN diode without Ge QDs. These results demonstrate the promise of Ge QDs for upconverting solar cells and the realization of device-quality integration of group IV and III-V semiconductors.