Ge/Si(001) heterostructures with dense arrays of Ge quantum dots: morphology, defects, photo-emf spectra and terahertz conductivity

TL;DR

This study investigates the morphology, defects, photo-emf spectra, and terahertz conductivity of dense Ge quantum dot arrays in Ge/Si(001) heterostructures, revealing enhanced conductivity and photo-emf effects influenced by quantum dot charge states.

Contribution

It provides new insights into the formation, optical, and terahertz properties of Ge/Si quantum dot heterostructures, including first terahertz conductivity measurements and effects of wide-band IR irradiation.

Findings

Photo-emf spectra are affected by wide-band IR irradiation.

Terahertz conductivity of quantum dot structures exceeds that of bulk Ge.

Conductivity decreases with temperature in the studied heterostructures.

Abstract

Issues of Ge hut array formation and growth at low temperatures on the Ge/Si(001) wetting layer are discussed on the basis of explorations performed by high resolution STM and in-situ RHEED. Data of HRTEM studies of multilayer Ge/Si heterostructures are presented with the focus on low-temperature formation of perfect films. Heteroepitaxial Si p-i-n-diodes with multilayer stacks of Ge/Si(001) quantum dot dense arrays built in intrinsic domains have been investigated and found to exhibit the photo-emf in a wide spectral range from 0.8 to 5 mcm. An effect of wide-band irradiation by infrared light on the photo-emf spectra has been observed. Photo-emf in different spectral ranges has been found to be differently affected by the wide-band irradiation. A significant increase in photo-emf is observed in the fundamental absorption range under the wide-band irradiation. The observed phenomena are explained in terms of positive and neutral charge states of the quantum dot layers and the Coulomb potential of the quantum dot ensemble. By using a coherent source spectrometer, first measurements of terahertz dynamical conductivity (absorptivity) spectra of Ge/Si(001) heterostructures were performed at frequencies 0.3-1.2 THz in the temperature interval from 300 to 5 K. The effective dynamical conductivity of the heterostructures with Ge quantum dots has been discovered to be significantly higher than that of the structure with the same amount of bulk germanium (not organized in an array of quantum dots). The excess conductivity is not observed in the structures with the Ge coverage less than 8 \AA. When a Ge/Si(001) sample is cooled down the conductivity decreases. We discuss possible mechanisms that can be responsible for the observed effects.