Electroluminescence of GeSn/Ge MQW LEDs on Si substrate.

Multi-quantum well light-emitting diodes, consisting of ten alternating GeSn/Ge-layers, were grown by molecular beam epitaxy on Si. The Ge barriers were 10 nm thick, and the GeSn wells were grown with 7% Sn and thicknesses between 6 and 12 nm. The electroluminescence spectra measured at 300 and 80 K yield a broad and intensive luminescence band. Deconvolution revealed three major lines produced by the GeSn wells that can be interpreted in terms of quantum confinement. We interpret that the three lines represent two direct lines, formed by transitions with the light and heavy hole band, respectively, and an indirect line. Biaxial compressive strain causes a splitting of light and heavy holes in the GeSn wells. This interpretation is supported by an effective mass band structure calculation.

Electrically pumped lasing from Ge Fabry-Perot resonators on Si.

Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices has been observed. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Perot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1660 nm and 1700 nm. This fits the theoretically predicted behavior for the n-type Ge material system. With further pulsed electrical injection of 500 kA/cm2 it was possible to reach the lasing threshold for such edge emitters. Different lengths and widths of devices have been investigated in order to maintain best gain-absorption ratios.

GeSn-on-Si normal incidence photodetectors with bandwidths more than 40 GHz.

GeSn (Sn content up to 4.2%) photodiodes with vertical pin structures were grown on thin Ge virtual substrates on Si by a low temperature (160 °C) molecular beam epitaxy. Vertical detectors were fabricated by a double mesa process with mesa radii between 5 µm and 80 µm. The nominal intrinsic absorber contains carrier densities from below 1 · 10(16) cm(-3) to 1 · 10(17) cm(-3) for Ge reference detectors and GeSn detectors with 4.2% Sn, respectively. The photodetectors were investigated with electrical and optoelectrical methods from direct current up to high frequencies (40 GHz). For a laser wavelength of 1550 nm an increasing of the optical responsivities (84 mA/W -218 mA/W) for vertical incidence detectors with thin (300 nm) absorbers as function of the Sn content were found. Most important from an application perspective all detectors had bandwidth above 40 GHz at enough reverse voltage which increased from zero to -5 V within the given Sn range. Increasing carrier densities (up to 1 · 10(17) cm(-3)) with Sn contents caused the depletion of the nominal intrinsic absorber at increasing reverse voltages.

Direct bandgap narrowing in Ge LED's on Si substrates.

In this paper we investigate the influence of n-type doping in Ge light emitting diodes on Si substrates on the room temperature emission spectrum. The layer structures are grown with a special low temperature molecular beam epitaxy process resulting in a slight tensile strain of 0.13%. The Ge LED's show a dominant direct bandgap emission with shrinking bandgap at the Γ point in dependence of n-type doping level. The emission shift (38 meV at 10²°cm⁻³) is mainly assigned to bandgap narrowing at high doping. The electroluminescence intensity increases with doping concentrations up to 3x10¹9cm⁻³ and decreases sharply at higher doping levels. The integrated direct gap emission intensity increases superlinear with electrical current density. Power exponents vary from about 2 at low doping densities up to 3.6 at 10²°cm⁻³ doping density.

Energy-band engineering for improved charge retention in fully self-aligned double floating-gate single-electron memories.

We present a new fully self-aligned single-electron memory with a single pair of nano floating gates, made of different materials (Si and Ge). The energy barrier that prevents stored charge leakage is induced not only by quantum effects but also by the conduction-band offset that arises between Ge and Si. The dimensions and position of each floating gate are well-defined and controlled. The devices exhibit a long retention time and single-electron injection at room temperature.