V. I. Mashanov

Mid-infrared electroluminescence from a Ge/Ge0.922Sn0.078/Ge double heterostructure p-i-n diode on a Si substrate

We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318 A/cm2–490 A/cm2 show two distinct profiles peaked at 0.545 eV (2.275 μm) and 0.573 eV (2.164 μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities.

GeSn-based p-i-n photodiodes with strained active layer on a Si wafer

We report an investigation of GeSn-based p-i-n photodiodes with an active GeSn layer that is almost fully strained. The results show that (a) the response of the Ge/GeSn/Ge heterojunction photodiodes is stronger than that of the reference Ge-based photodiodes at photon energies above the 0.8 eV direct bandgap of bulk Ge (<1.55 μm), and (b) the optical response extends to lower energy regions (1.55–1.80 μm wavelengths) as characterized by the strained GeSn bandgap. A cusp-like spectral characteristic is observed for samples with high Sn contents, which is attributed to the significant strain-induced energy splitting of heavy and light hole bands. This work represents a step forward in developing GeSn-based infrared photodetectors.

GeSi films with reduced dislocation density grown by molecular-beam epitaxy on compliant substrates based on porous silicon

To grow high-quality heteroepitaxial layers, we propose a compliant silicon substrate consisting of a thin epitaxial silicon film on a high-density porous layer as a membrane and an expansive low-density porous layer as a mechanical damper which shields the overlying layers from the massive wafer. GeSi films over the critical thickness have been grown by molecular-beam epitaxy on these substrates. Transmission electron microscopy analysis shows that Ge0.2Si0.8 films have no dislocations owing to just elastic strain relaxation whereas plastic flow in the pseudomorphic films that are being grown on conventional Si substrates occurs with generation of dislocations in a regular manner. The experimental data on porous silicon structure are presented in some detail and are briefly discussed in connection with substrate compliance.

GeSn p-i-n waveguide photodetectors on silicon substrates

We report an investigation on GeSn p-i-n waveguide photodetectors grown on a Ge-buffered Si wafer. In comparison with a reference Ge detector, the GeSn detector shows an enhanced responsivity in the measured energy range, mainly attributed to the smaller bandgap caused by Sn-alloying. Analysis of the quantum efficiency indicates that increasing the Sn content in the active layers can significantly shorten the required device length to achieve the maximum efficiency. The present investigation demonstrates the planar photodetectors desired for monolithic integration with electronic devices.

Strain-induced wrinkling on SiGe free standing film

This study reports both experimental and theoretical investigation on the strain-induced wrinkling on Si1−xGex∕Si free standing film. Clear periodical pattern is observed and attributed to the strain relaxation of the SiGe film. With increasing lateral length of the film, both wavelength and amplitude increase. Nonlinear Von Karman plate theory is employed to model the structure. From the analysis, it shows that the formation of the wrinkling pattern is a trade-off between bending energy and stretching energy. Based on the modeling, it is found that wavelength decreased with increasing Ge content, while vice versa for the amplitude.

Formation of Ge-Sn nanodots on Si(100) surfaces by molecular beam epitaxy

The surface morphology of Ge0.96Sn0.04/Si(100) heterostructures grown at temperatures from 250 to 450°C by atomic force microscopy (AFM) and scanning tunnel microscopy (STM) ex situ has been studied. The statistical data for the density of Ge0.96Sn0.04 nanodots (ND) depending on their lateral size have been obtained. Maximum density of ND (6 × 1011 cm-2) with the average lateral size of 7 nm can be obtained at 250°C. Relying on the reflection of high energy electron diffraction, AFM, and STM, it is concluded that molecular beam growth of Ge1-xSnx heterostructures with the small concentrations of Sn in the range of substrate temperatures from 250 to 450°C follows the Stranski-Krastanow mechanism. Based on the technique of recording diffractometry of high energy electrons during the process of epitaxy, the wetting layer thickness of Ge0.96Sn0.04 films is found to depend on the temperature of the substrate.

Local intermixing on Ge/Si heterostructures at low temperature growth

We report experimental investigation on a series of strained Ge/Si heterostructure with various Ge thicknesses (the order of nanometers) grown at low temperatures (260 °C). In addition to the conventional uniform intermixing at the Ge/Si interface for structures with thin Ge layer, local intermixing characterized by a pattern structure is observed for structures with thick Ge layer. The pattern is formed beneath the Ge layer with an island shape and exhibits a Ge concentration dependent profile. From the analysis, it shows that the growth temperature and the strain energy stored in the Ge layer play a dominated role for the observation. In comparison with the conventional high temperature growth, this investigation shows that the self-assembly process is suppressed at low growth temperatures and intermixing plays a dominant role for strain relaxation.

Strained multilayer structures with pseudomorphic GeSiSn layers

The temperature and composition dependences of the critical thickness of the 2D–3D transition for a GeSiSn film on Si(100) have been studied. The regularities of the formation of multilayer structures with pseudomorphic GeSiSn layers directly on a Si substrate, without relaxed buffer layers, were investigated for the first time. The possibility of forming multilayer structures based on pseudomorphic GeSiSn layers has been shown and the lattice parameters have been determined using transmission electron microscopy. The grown structures demonstrate photoluminescence for Sn contents from 3.5 to 5% in GeSiSn layers.

Sn-Based Group-IV Structure for Resonant Tunneling Diodes

We report a theoretical investigation on the electrical properties of a Sn-based group-IV structure for a resonant tunneling diode (RTD). The analysis on the composition-dependent strain, energy profile, and current-voltage characteristic of a double-barrier heterostructure shows that the peak current density and peak-to-valley ratio are enhanced with a moderated tensile strain in the barrier layer, providing an alternative approach for group-IV RTDs.

Initial growth stages of Si–Ge–Sn ternary alloys grown on Si (100) by low-temperature molecular-beam epitaxy

Temperature dependence of the critical thickness of the transition from two-dimensional to threedimensional growth of the Ge1–5xSi4xSnx films grown on Si (100) by molecular-beam epitaxy in the temperature range 150–450°C has been experimentally determined. This dependence is nonmonotonic and is similar to that of the critical thickness for the transition from two-dimensional to three-dimensional growth in the case of the deposition of pure Ge on Si (100) and is caused by a change in the mechanism of two-dimensional growth. Data on the average size and the density of islands, and the ratio between the height of the islands and their lateral size are obtained by the methods of atomic force microscopy and scanning tunneling microscopy. As the growth temperature is increased from 200 to 400°C, the average size of the nanoislands increases from 4.7 to 23.6 nm.