The 40th edition of the


an IEEE event (since 1995)

Sinaia, Romania (11-14 October 2017)

Quantum confinement effects and light absorption in Ge nanostructures
Antonio Terrasi, Dipartimento di Fisica e Astronomia, Università di Catania and CNR-IMM - Catania, Italy

In the past years, group-IV nanostructures received great attention as new material for efficient optoelectronics devices, photodetectors and solar cells. In particular, Ge quantum dots (QDs) gained a renewed scientific interest over Si QDs due to the lower synthesis temperature, higher absorption coefficient and larger exciton Bohr radius. The optical behavior and the band-gap tuning of Ge QDs do not simply depend on the size, as quantum confinement effects (QCE) predicts, but also on QD-QD distance and ordering. The determination of the optical bandgap (Eg) in semiconductor nanostructures is a key issue in understanding the extent of QCE on electronic properties and it usually involves some analytical approximation in experimental data reduction and modeling of the light absorption processes. In this paper we first compare some of the analytical procedures frequently used to evaluate the optical bandgap from reflectance (R) and transmittance (T) spectra. Ge quantum wells (QW) and quantum dots (QD) embedded in SiO2 produced by plasma enhanced chemical vapor deposition were used, the former one being the most simple type of nanostructure. Elaboration of R&T data was conducted by two approximated methods (single or double pass approximation, SPA and DPA, respectively) to extract the absorption spectra, followed by Eg evaluation through linear fit of Tauc or Cody plots. Direct fitting of R&T spectra through a Tauc-Lorentz oscillator model is used as comparison. One of the most remarkable results of our investigation is an unprecedented high light absorption efficiency, 15 times larger than in the bulk, for small Ge QDs (2-3 nm in diameter) grown in a multilayer configuration (3-6 nm thick film with Ge QDs, separated by 20 nm thick SiO2 barrier). Structural and optical characterizations have been employed to describe the QCE-induced enhancement of optical bandgap (from 0.8 in bulk up to 2.5 eV in multilayer structure) and of oscillator strength (one order of magnitude). Through a detailed electron energy loss spectroscopy (EELS) analysis, we characterized the structural and chemical properties of Ge QD. A comparison with Ge QDs in single thick layer is also performed. These results add new insights into the role of QD packaging on confined systems, and open the route for reliable exploitation of QC effects.