Strain engineering of Ge/GeSn photonic structures

Silicon compatible light sources have been referred to as the \holy grail" for Si photonics. Such devices would give the potential for a range of applications; from optical interconnects on integrated circuits, to cheap optical gas sensing and spectroscopic devices on a Si platform. Whilst numerous heterogeneous integration schemes for integrating III-V lasers with Si wafers are being pursued, it would be far easier and cheaper to use the epitaxial tools already in complementary-metal-oxide-semiconductor (CMOS) lines, where Ge and SiGe chemical vapour deposition is used in a number of advanced technology nodes. Germanium is an efficient absorber, but a poor emitter due to a band-structure which is narrowly indirect, but by only 140 meV. Through the application of strain, or by alloying with Sn, the Ge bandstructure can be engineered to become direct bandgap, making it an effcient light emitter. In this work, silicon nitride stressor technologies, and CMOS compatible processes are used to produce levels of tensile strain in Ge optical micro-cavities where a transition to direct bandgap is predicted. The strain distribution, and the optical emission of a range of Ge optical cavities are analyzed, with an emphasis on the effect of strain distribution on the material band-structure. Peak levels of strain are reported which are higher than that reported in the literature using comparable techniques. Furthermore, these techniques are applied to GeSn epi-layers and demonstrate that highly compressive GeSn alloys grown pseudomorphically on Ge virtual substrates, can be transformed to direct bandgap materials, with emission >3 m wavelength { the longest wavelength emission demonstrated from GeSn alloys. Such emission is modeled to have a good overlap with methane absorption lines, indicating that there is huge potential for the such technologies to be used for low cost, Si compatible gas sensing in the mid-infrared

Electronic Transport Properties of Silicon-Germanium Single Photon Avalanche Detectors

Single photon avalanche detectors (SPADs) have uses in a number of applications, including time-of-flight ranging, quantum key distribution and low-light sensing. Germanium has an absorption edge at the key communications wavelengths of 1.3-1.55um, and can be grown epitaxially on silicon, however, SiGe SPADs exhibit a number of performance limitations, including low detection efficiencies, high dark counts and afterpulsing. Unintentional doping may affect electronic performance, and band-to-band tunnelling at high operational voltages SPADs may lead to noise currents. Additionally, defects in the Si/Ge interface lead to trap states within the bandgap and contribute to afterpulsing. This work investigates a range of critical performance parameters in SiGe SPADs. The effect of intentional and unintentional doping in SPADs on electric fields, potential profiles and carrier transport in the device is investigated, and optimal dopant profiles for a SiGe SPAD discussed. The dependence of band-to-band tunnelling currents in Ge on bias voltage, Ge thickness and temperature is investigated, and these currents are compared to other sources of noise currents in SPADs. DFT calculations of misfit dislocation structures in Ge are undertaken, to establish electronic bandstructures and optimised geometries for these defects, and identify trap states in the bandgap, which may contribute to afterpulsing and dark counts in SPADs. A number of directions for continuing work are identified, to progress understanding of noise currents and afterpulsing in SPADs

Optical Detection Properties of Silicon-Germanium Quantum Well Structures

A study has been carried out on Si/SiGe multi quantum well structures to determine their applicability as normal incidence infrared detectors in the spectral range of 2-12 micrometers. The research effort was primarily experimental; however, extensive calculations were performed to initially explain the experimental data and then used to design subsequent structures. Multiple quantum well structures grown on both Si[001] and Si[110] substrates via molecular beam epitaxy were studied by photoluminescence, absorption, and photoresponse measurements over a wide parameter space. Variables included quantum well depth and width, well doping, number of wells and growth temperature. Well widths were varied from 20Å to 50Å, Ge composition from 10% to 60%, boron doping from 1 x 1018 cm-3 to 8 x 1019 cm-3, number of wells from 5 to 30 and growth temperature from 550 to 710 °C. Calculations using k.p theory and the envelope function approximation were performed to determine the position of the bound states in the wells, the amount of band mixing and the transition strengths for bound-to-bound transitions for Si[001]/Si1-xGe sub x, Si[110]/Si1-xGex and GaAs/AlGaAs quantum well structures. The Si[110] structures have more allowed energy bands which are significantly mixed. A comparison was made between Si[001]/Si1-xGex, Si[110]/Si1-xGex and GaAs/AlGaAs quantum well structures designed to operate in the 8-12 µm region, and all three showed comparable momentum matrix elements

FWP executive summaries, Basic Energy Sciences Materials Sciences Programs (SNL/NM)

The BES Materials Sciences Program has the central theme of Scientifically Tailored Materials. The major objective of this program is to combine Sandia`s expertise and capabilities in the areas of solid state sciences, advanced atomic-level diagnostics and materials synthesis and processing science to produce new classes of tailored materials as well as to enhance the properties of existing materials for US energy applications and for critical defense needs. Current core research in this program includes the physics and chemistry of ceramics synthesis and processing, the use of energetic particles for the synthesis and study of materials, tailored surfaces and interfaces for materials applications, chemical vapor deposition sciences, artificially-structured semiconductor materials science, advanced growth techniques for improved semiconductor structures, transport in unconventional solids, atomic-level science of interfacial adhesion, high-temperature superconductors, and the synthesis and processing of nano-size clusters for energy applications. In addition, the program includes the following three smaller efforts initiated in the past two years: (1) Wetting and Flow of Liquid Metals and Amorphous Ceramics at Solid Interfaces, (2) Field-Structured Anisotropic Composites, and (3) Composition-Modulated Semiconductor Structures for Photovoltaic and Optical Technologies. The latter is a joint effort with the National Renewable Energy Laboratory. Separate summaries are given of individual research areas

Growth and Characterization of β-Iron Disilicide, β-Iron Silicon Germanide, and Osmium Silicides

The semiconducting silicides offer significant potential for use in optoelectronic devices. Full implementation of the materials, however, requires the ability to tailor the energy gap and band structure to permit the synthesis of heterojunctions. One promising approach is to alloy the silicides with Ge. As part of an investigation into the synthesis of semiconducting silicide heterostructures, a series of β-Fe(Si1−xGex)2 epilayer samples, with nominal alloy content in the range 0 0.04. Osmium silicide films have been grown by molecular beam epitaxy on Si(100). The silicides have been grown using e-beam evaporation sources for both Os and Si onto Si(100) substrates at varying growth rates and temperatures ranging from 600-700ºC. The resulting films have been analyzed using reflection high-energy electron diffraction, Raman spectroscopy, reflectivity measurements, in-plane and out of plane X-ray diffraction and temperature dependent magnetotransport. A change in crystalline quality is observed with an increase in Si overpressure. For a lower silicon to osmium flux ration (JSi/JOs=1.5) both OsSi2 and Os2Si3 occur, whereas with a much larger Si overpressure (JSi/JOs>4), crystalline quality is greatly increased and only a single phase, Os2Si3, is present. The out-of-plane X-ray diffraction data show that the film grows along its [4 0 2] direction, with a good crystal quality as evidenced by the small FWHM in the rocking curve. The in-plane X-ray diffraction data show growth twins with perpendicular orientation to each other

Germanium-tin-silicon epitaxial structures grown on silicon by reduced pressure chemical vapour deposition

Crystalline germanium-tin (GeSn) binary alloys have been subject to a significant research effort in recent years. This research effort is motivated by the myriad of potential applications that GeSn alloys offer. Crystalline epitaxial layers of GeSn and silicon-germanium-tin (SiGeSn) have been grown onto Si(001) substrates on a relaxed Ge buffer using reduced pressure CVD and commercially available precursors. X-ray diffraction, transmission electron microscopy, atomic force microscopy, secondary ion mass spectrometry and Raman spectroscopy were used to determine layer composition, layer thickness, crystallinity, degree of strain relaxation, surface features and roughness of the samples investigated in this work. The epilayers produced have been both fully strained to their growth platform and partially relaxed. The Sn fraction of the alloy layers varied from 1 to 12 at. % Sn. Using N2 as the carrier gas during growth is observed to inhibit Ge1-xSnx growth. Off-axis substrates are determined to hinder the production of crystalline layers of GeSn. In-situ material characterization of GeSn layers during thermal treatment has identified the existence of a critical temperature for higher Sn fraction layers, beyond which the material quality degrades rapidly. This critical temperature is dependent on the layer composition, layer thickness, layer strain state and annealing environment. Layers of germanium-tin-oxide are produced by thermal oxidation and shown to have similar oxide formation rates to pure Ge. The low thermal budget limit for the high Sn fraction alloys has driven research into forming Ohmic metal contacts on GeSn layers with processes limited to low temperatures. Gold is determined to be the optimum electrical contact material