The role of the interface in germanium quantum dots: when not only size matters for quantum confinement effects

Abstract

Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects.

Optical bandgap of semiconductor nanostructures: Methods for experimental data analysis

“…In a previous work, we observed that PECVD QDs exhibit a sharper interface with lower amount of Ge sub-oxide states in comparison with sputter samples, ensuring a stronger electron-hole confinement into Ge QDs of PECVD samples. 16 Since Tauc plot focuses the threshold energy range of absorption coefficient, which is affected by any midgap levels or bending induced by interface defects, the different behavior of our Tauc plots from those of Liu et al can be regarded as a consequence of the different growth technique used. In our case the Tauc plot is characterized by a unique linear region which extends over a wider spectral range (0.8 eV) with respect to the case of Ref.…”

“…16 Here, we briefly review the three models used for the extraction of optical bandgap in our samples [ Fig. 1(a)].…”

“…3 These characteristics allow tailoring of the light absorption and emission spectra, making possible the application of NS in many fields such as photovoltaics, 4 photodetection, [5][6][7] and optoelectronic 8,9 applications. Actually, the absorption modulation and enhancement in NS are still under investigation, since other factors [10][11][12][13][14][15][16] can interfere with the QCE. In fact, several studies have recently demonstrated how the optical properties of Si and Ge NS can be changed by varying several structural characteristics such as shape, 10 crystalline structure, 11,12 or the potential barriers surrounding the NS.…”

“…The Ge QWs were produced by plasma enhanced chemical vapor deposition (PECVD), as they exhibit a much cleaner and sharper interface compared to Ge NS produced by sputtering techniques, allowing a stronger quantum confinement effect. 16 Moreover, the use of a high refractive index material, such as Ge, in films a few nanometers thin, is a situation that enhances the impact of the approximations, thus representing the most stringent test bed for the optical models. Both the extractions of a from R&T spectra and the determination of E g from a are discussed.…”

“…In fact, several studies have recently demonstrated how the optical properties of Si and Ge NS can be changed by varying several structural characteristics such as shape, 10 crystalline structure, 11,12 or the potential barriers surrounding the NS. [13][14][15][16] For all these reasons, an accurate extraction of optical parameters from experimental data becomes crucial, in order to have a deeper and quantitative understanding of QCE. To date, a long debate has been underway on how to extract the optical bandgap from experimental data.…”