Daniel Widmann

GeSn p-i-n detectors integrated on Si with up to 4% Sn

GeSn heterojunction photodetectors on Si substrates were grown with Sn concentration up to 4%, fabricated for vertical light incidence, and characterized. The complete layer structure was grown by means of ultra low temperature (100 °C) molecular beam epitaxy. The Sn content shifts the responsivity into the infrared, about 310 nm for the 4% Sn sample. An increase of the optical responsivity for wavelengths higher than 1550 nm can be observed with increasing Sn content. At 1600 nm, the optical responsivity is increased by more than a factor of 10 for the GeSn diode with 4% Sn in comparison to the Ge reference diode.

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.

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 LEDs 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 10²⁰cm⁻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 10²⁰cm⁻³ doping density.

GeSn/Ge multiquantum well photodetectors on Si substrates

Vertical incidence GeSn/Ge multiquantum well (MQW) pin photodetectors on Si substrates were fabricated with a Sn concentration of 7%. The epitaxial structure was grown with a special low temperature molecular beam epitaxy process. The Ge barrier in the GeSn/Ge MQW was kept constant at 10 nm. The well width was varied between 6 and 12 nm. The GeSn/Ge MQW structures were grown pseudomorphically with the in-plane lattice constant of the Ge virtual substrate. The absorption edge shifts to longer wavelengths with thicker QWs in agreement with expectations from smaller quantization energies for the thicker QWs.

Franz-Keldysh effect in GeSn pin photodetectors

The optical properties and the Franz-Keldysh effect at the direct band gap of GeSn alloys with Sn concentrations up to 4.2% at room temperature were investigated. The GeSn material was embedded in the intrinsic region of a Ge heterojunction photodetector on Si substrates. The layer structure was grown by means of ultra-low temperature molecular beam epitaxy. The absorption coefficient as function of photon energy and the direct bandgap energies were determined. In all investigated samples, the Franz-Keldysh effect can be observed. A maximum absorption ratio of 1.5 was determined for 2% Sn for a voltage swing of 3 V.

Sealed and compact fiber links to integrated photonics using grating couplers

We present a sealed, permanent, compact and efficient optical fiber-to-chip interface utilizing the wide-spread grating coupler. The easily produced fiber link is based on the reflection in an angle-polished fiber with a reflective metal coating. Efficiencies for different coupling methods to grating couplers are compared.

A SiGe-HBT 2:1 analog multiplexer with more than 67 GHz bandwidth

This paper presents a 2:1 Analog Multiplexer (AMUX) in a SiGe-HBT technology. The AMUX is used for time interleaving operation of two digital-to-analog converters (DACs) and therefore extends both the sampling rate and the bandwidth compared to a single DAC. The linear AMUX signal path allows for generation of broadband signals with higher order modulation schemes which is essential for raising data rates in optical communication networks. The AMUX provides a differential peak-to-peak output voltage of up to 1 V with linear gain. A signal path 3-dB bandwidth exceeding 67 GHz has been measured. The clock path exhibits a 3-dB bandwidth of 60 GHz. S-parameter measurements are presented. Measured PAM4 eye diagrams at 56 GS/s from the time interleaving operation of two DACs are reported.

Ge and GeSn Light Emitters on Si

The heteroepitaxial growth of GeSn and Ge crystals on Si substrates are investigated for Si-based photonic applications. Light Emitting Diodes with emission wavelengths from 2,100 to 1,550 nm could be demonstrated with active intrinsic GeSn light emitting layers between Ge barriers. A clear shift of the direct band gap toward the infrared beyond 2 μm is measured. Emission intensity is increased compared to Ge Light Emitting Diodes. Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices are demonstrated. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Pérot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1,660 nm and 1,700 nm.

Extraction of GeSn absorption coefficients from photodetector response

Germanium tin (GeSn) shifts the photo-response of infrared (IR) light detectors from a typical cutoff wavelength of Ge detectors of 1550nm toward the mid infrared (MIR) [1]. The determination of the optical absorption coefficient of GeSn is difficult because it requires both high grade GeSn material and appropriate measurement structure. A series of vertical pin photodetector with the absorber material GeSn of excellent quality were fabricated achieving quantitative extraction of the absorption coefficient from photo-response. In this paper the fabrication of vertical GeSn photodetectors, the measurement of the background doping level in the intrinsic region (i-region) and the determination of the absorption coefficient are presented and the influence of high doped contact layers and electro-absorption effects from the built-in electric field are discussed.