Showing papers by "Stephan Wirths published in 2016"

Optically Pumped GeSn Microdisk Lasers on Si

Abstract: The strong correlation between advancing the performance of Si microelectronics and their demand of low power consumption requires new ways of data communication. Photonic circuits on Si are already highly developed except for an eligible on-chip laser source integrated monolithically. The recent demonstration of an optically pumped waveguide laser made from the Si-congruent GeSn alloy, monolithical laser integration has taken a big step forward on the way to an all-inclusive nanophotonic platform in CMOS. We present group IV microdisk lasers with significant improvements in lasing temperature and lasing threshold compared to the previously reported nonundercut Fabry–Perot type lasers. Lasing is observed up to 130 K with optical excitation density threshold of 220 kW/cm2 at 50 K. Additionally the influence of strain relaxation on the band structure of undercut resonators is discussed and allows the proof of laser emission for a just direct Ge0.915Sn0.085 alloy where Γ and L valleys have the same energies....

Study of GeSn based heterostructures: towards optimized group IV MQW LEDs.

Abstract: We present results on CVD growth and electro-optical characterization of Ge0.92Sn0.08/Ge p-i-n heterostructure diodes. The suitability of Ge as barriers for direct bandgap GeSn active layers in different LED geometries, such as double heterostructures and multi quantum wells is discussed based on electroluminescence data. Theoretical calculations by effective mass and 6 band k∙p method reveal low barrier heights for this specific structure. Best configurations offer only a maximum barrier height for electrons of about 40 meV at the Γ point at room temperature (e.g. 300 K), evidently insufficient for proper light emitting devices. An alternative solution using SiGeSn as barrier material is introduced, which provides appropriate band alignment for both electrons and holes resulting in efficient confinement in direct bandgap GeSn wells. Finally, epitaxial growth of such a complete SiGeSn/GeSn/SiGeSn double heterostructure including doping is shown.

Monolithic integration of multiple III-V semiconductors on Si for MOSFETs and TFETs

Abstract: In this paper we report on our work on the monolithic integration of various III-V compounds on Si using template-assisted selective epitaxy (TASE) and its application for electronic devices. Nanowires, crossbar nanostructures, and micron-sized sheets are epitaxially grown on Si via metal-organic chemical vapor deposition and form the basis for III-V MOSFETs and Tunnel FETs. Epitaxy conditions specific to TASE are discussed and material quality assessed. Here, we focus on InAs and GaSb as a potential all-III-V alternative to complementary group IV technology. Scaled n-FETs as well as both n- and p-channel TFETs are fabricated on Si and illustrate the potential of TASE.

Line Tunneling Dominating Charge Transport in SiGe/Si Heterostructure TFETs

Abstract: This paper provides an experimental proof that both the ON-current $I_{\mathrm{\scriptscriptstyle ON}}$ and the subthreshold swing SS of Si(Ge)-based tunneling FETs (TFETs) drastically benefit from device architectures promoting line tunneling aligned with the gate electrical field. A novel SiGe/Si heterostructure TFET is fabricated, making use of a selective and self-adjusted silicidation, thus enlarging the area for band-to-band-tunneling (BTBT) in a region directly underneath the gate. In addition, a counter-doped pocket within the SiGe layer at the source tunnel junction is introduced in order to sharpen the corresponding doping profile and, consequently, to shorten the resulting tunneling length. Experimental analysis of activation energies $E_{a}$ identifies BTBT, dominating the drain current $I_{d}$ in the SiGe/Si heterostructure TFET over a wide region of the gate voltage $V_{g}$ , thus reducing parasitic influence of Shockley–Read–Hall recombination and trap-assisted tunneling. Both a relatively high $I_{\mathrm{\scriptscriptstyle ON}} = 6.7\mu \text{A}/\mu \text{m}$ at a supply voltage $V_{\mathrm{ DD}} = 0.5$ V and an average SS of about 80 mV/decade over four orders of magnitude of $I_{d}$ were achieved.

Direct bandgap GeSn light emitting diodes for short-wave infrared applications grown on Si

Abstract: The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 µm). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.

GeSn lasers for CMOS integration

Abstract: In search of a suitable CMOS compatible light source many routes and materials are under investigation. Si-based group IV (Si)GeSn alloys offer a tunable bandgap from indirect to direct, making them ideal candidates for on-chip photonics and nano-electronics. An overview of recent achievements in material growth and device developments will be given. Optically pumped waveguide and microdisk structures with different strain and various Sn concentrations provide direct evidence of gain in these alloys and the width of the emission wavelength range that can be covered. Towards the aim of electrically pumped lasers, a set of different homojunction light emitting diodes and more complex heterostructure SiGeSn/GeSn LEDs is presented. Detailed investigation of electroluminescence spectra indicate that GeSn/SiGeSn heterostructures will be advantageous for future laser fabrication.

On the track towards an electrically pumped group IV laser

Abstract: GeSn and SiGeSn alloys have been grown by reactive gas source deposition using a commercial 8″ LPCVD tool. Both alloys exhibit a direct bandgap. Optically pumped GeSn Fabry-Perot and microdisc laser were fabricated etching strong undercuts to efficiently relax the strain. Lasing operation is demonstrated up to 135K and 140K, respectively. The wavelength is adjusted between 2.0 and 2.6 µm at 20K in dependence on the Sn concentration. Electroluminescence has been observed up to 300K in p-i-n GeSn homojunctions. Moreover first p-i-n SiGeSn/GeSn double heterostructures have been successfully deposited.

High Sn‐Content GeSn Light Emitters for Silicon Photonics

Abstract: The present chip technology is based on silicon with increasing number of other materials integrated into electrical circuits. This chapter presents a systematic photoluminescence (PL) study of compressively strained, direct-bandgap GeSn alloys, followed by the analysis of two different optical source designs. First, a direct bandgap GeSn light emitting diode (LED) will be characterized via power-and temperature-dependent electroluminescence (EL) measurements. Then, lasing will be demonstrated in a microdisk (MD) resonator under optical pumping. The integration of direct-bandgap GeSn-based devices as a light source for on-chip communications offers the possibility to monolithically integrate the complete photonic circuit within mainstream silicon technology. The chapter describes material properties using Ge0.875Sn0.125 epilayers of various thicknesses. Temperature-dependent integrated PL intensity is a suitable method to determine whether a semiconductor has a direct or indirect fundamental bandgap. In conclusion, the chapter presents growth and optical characterization of high-quality GeSn alloys with very high Sn content.