Query: Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys

Candidate papers:

abstract: Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si) germanium (Ge) and SiGe alloys are all indirect bandgap semiconductors that cannot emit light efficiently. The goal 1 of achieving efficient light emission from group IV materials in silicon technology has been elusive for decades 2 6 Here we demonstrate efficient light emission from direct bandgap hexagonal Ge and SiGe alloys. We measure a sub nanosecond, temperature insensitive radiative recombination lifetime and observe an emission yield similar to that of direct bandgap group III V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information processing technologies. A hexagonal (rather than cubic) alloy of silicon and germanium that has a direct (rather than indirect) bandgap emits light efficiently across a range of wavelengths, enabling electronic and optoelectronic functionalities to be combined on a single chip.
author_names: Elham M T Fadaly

Alain Dijkstra

Jens Rene Suckert

Dorian Ziss

Marvin A J van Tilburg

Chenyang Mao

Yizhen Ren

Victor T van Lange

Ksenia Korzun

Sebastian Kolling

Marcel A Verheijen

David Busse

Claudia Rodl

Jurgen Furthmuller

Friedhelm Bechstedt

J Stangl

Jonathan J Finley

Silvana Botti

Jos E M Haverkort

Erik P A M Bakkers
title: Direct bandgap emission from hexagonal Ge and SiGe alloys
venue: Nature
year: 2020

abstract: Novel group IV nanostructures were fabricated and the optical properties of such nanostructures were investigated for monolithic integration of optically active materials with silicon. The SnxGe1 x alloy system was studied due to the previous demonstration of an indirect to direct energy bandgap transition for strain relieved SnxGe1 x films on Si(001) In addition, quantum confined structures of Sn were fabricated and the optical properties were investigated. Due to the small electron effective mass of a Sn, quantum confinement effects are expected at relatively large radii. Coherently strained, epitaxial SnxGe1 x films on Ge(001) substrates were synthesized with film thickness exceeding 100 nm for the first time. The demonstration of dislocation free SnxGe1 x films is a step toward the fabrication of silicon based integrated infrared optoelectronic devices. The optical properties of coherently strained SnxGe1 x/Ge(001) alloys were investigated both theoretically and experimentally. Deformation potential theory calculations were performed to predict the effect of coherency strain on the extrema points of the conduction band and the valence band. The energy bandgap of SnxGe1 x/Ge(001) alloys was measured via Fourier transform infrared spectroscopy. Coherency strain did not change the SnxGe1 x energy bandgap when the strain axis was along [001] but deformation potential theory predicted the absence of an indirect to direct energy bandgap transition when the strain axis was along [111] In addition to being the only group IV alloy exhibiting a direct energy bandgap, when grown beyond a critical thickness, SnxGe1 x/Ge(001) exhibits an interesting phenomenon during MBE growth. Sn segregates via surface diffusion to the crest of a surface undulation during growth and forms ordered Sn enriched SnxGe1 x rods oriented along [001] The SnxGe1 x alloy system was used as a model system to gain insight to the physical mechanisms governing self assembly and ordering during molecular beam epitaxy. Sn nanowires were fabricated in anodic alumina templates with lengths exceeding 1 mm and diameters on the order of 40 nm. Anodic alumina templates can be fabricated non lithographically with ordered domains of hexagonally packed pores greater than 1 mm and pore densities on the order of 1011 cm 2. The achievement of single crystal Sn nanowires fabricated using pressure injection in porous alumina templates was demonstrated. The fabrication of a Sn quantum dots embedded in Ge was achieved by annealing 1 mm thick SnxGe1 x films at 750degC. The measured diameter of the quantum dots was 32 nm and a 10% size variation was observed. Quantum size effects were observed in a Sn quantum dots. Optical transmittance measurements yield a value of 0.45 eV for the direct energy bandgap as a result of quantum confinement. A high degree of tunability of the bandgap energy with the quantum dot radius is expected for a Sn. Thus quantum confined structures of a Sn are promising for optoelectronic device applications.
author_names: Regina Ragan
title: Direct Energy Bandgap Group IV Alloys and Nanostructures
year: 2002

abstract: 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 G 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.
author_names: Michael Oehme

Martin Gollhofer

Daniel Widmann

Marc Schmid

Mathias Kaschel

Erich Kasper

Jorg Schulze
title: Direct bandgap narrowing in Ge LED's on Si substrates.
venue: Optics express
year: 2013

abstract: Recent advances in the synthetic growth of nanowires have given access to crystal phases that in bulk are only observed under extreme pressure conditions. Here, we use first principles methods based on density functional theory and many body perturbation theory to show that a suitable mixing of hexagonal Si and hexagonal Ge yields a direct bandgap with an optically permitted transition. Comparison of the calculated radiative lifetimes with typical values of nonradiative recombination mechanisms indicates that optical emission will be the dominant recombination mechanism. These findings pave the way to the development of silicon based optoelectronic devices, thus far hindered by the poor light emission efficiency of cubic Si.
author_names: Xavier Cartoixa

Maurizia Palummo

Hakon Ikaros T Hauge

Erik P A M Bakkers

Riccardo Rurali
title: Optical Emission in Hexagonal SiGe Nanowires.
venue: Nano letters
year: 2017

abstract: We present a comprehensive, up to date compilation of band parameters for the technologically important III V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin orbit, and crystal field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.
author_names: Igor Vurgaftman

Jerry R Meyer

L R Ram-Mohan
title: Band parameters for III V compound semiconductors and their alloys
year: 2001

abstract: Contributors. Preface. Gallium Nitride (GaN) (V. Bougrov, et al. Aluminum Nitride (AIN) (Y. Goldberg) Indium Nitride (InN) (A. Zubrilov) Boron Nitride (BN) (S. Rumyantsev, et al. Silicon Carbide (SiC) (Y. Goldberg, et al. Silicon Germanium (Si 1 xGe x) (F. Schaffler) Appendix 1: Basic Physical Constants. Appendix 2: Periodic Table of the Elements. Appendix 3: Rectangular Coordinates for Hexagonal Crystal. Appendix 4: The First Brillouin Zone for Wurtzite Crystal. Appendix 5: Zinc Blende Structure. Appendix 6: The First Brillouin Zone for Zinc Blende Crystal. Additional References.
author_names: M E Levinshtein

Sergey Rumyantsev

Michael S Shur
title: Properties of advanced semiconductor materials GaN, AlN, InN, BN, SiC, SiGe
year: 2001

abstract: Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e. single crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct bandgap material with strongly enhanced light emission efficiency, capable of supporting population inversion as required for providing optical gain.
author_names: Jose R Sanchez-Perez

Cicek Boztug

Feng Chen

Faisal F Sudradjat

Deborah M Paskiewicz

R B Jacobson

Max G Lagally

Roberto Paiella
title: Direct bandgap light emitting germanium in tensilely strained nanomembranes
venue: Proceedings of the National Academy of Sciences
year: 2011

abstract: Semiconductor alloy nanowires with spatially graded compositions (and bandgaps) provide a new material platform for many new multifunctional optoelectronic devices, such as broadly tunable lasers, multispectral photodetectors, broad band light emitting diodes (LEDs) and high efficiency solar cells. In this review, we will summarize the recent progress on composition graded semiconductor alloy nanowires with bandgaps graded in a wide range. Depending on different growth methods and material systems, two typical nanowire composition grading approaches will be presented in detail, including composition graded alloy nanowires along a single substrate and those along single nanowires. Furthermore, selected examples of applications of these composition graded semiconductor nanowires will be presented and discussed, including tunable nanolasers, multi terminal on nanowire photodetectors, full spectrum solar cells, and white light LEDs. Finally, we will make some concluding remarks with future perspectives including opportunities and challenges in this research area.
author_names: Xiujuan Zhuang

Cun-Zheng Ning

Anlian Pan
title: Composition and bandgap graded semiconductor alloy nanowires.
venue: Advanced materials
year: 2012

abstract: Abstract This paper presents simple but accurate closed form equations for Band Gap Narrowing (BGN) for n and p type, Si, Ge, GaAs and GexSi1 x alloys and strained layers. The equations are derived by identifying the four components of BGN: exchange energy shift of the majority band edge, correlation energy shift of the minority band edge and impurity interaction shifts of the two band edges. In the simple parabolic band approximation, the BGN is determined by the effective masses of the carriers and the relative permittivity of the semiconductor. For real semiconductors, known corrections due to anisotropy of the bands, due to multi valleys in a band and due to interactions between sub bands are used. The values of BGN for n Si, n Ge and n and p GaAs calculated using this simple formulation agree closely with the theoretical values calculated by other authors using advanced but complex many body methods and the Random Phase Approximation for screening effects. For p Si and p Ge ours appear to be the first calculations taking all interactions into account. Experimental values of BGN for all semiconductors except for p Ge for which no data could be found, are also in very good agreement with our theory. The Fermi level for n and p Si and p GaAs is determined using the published luminescence data. In heavily doped p type semiconductors, the values are found to be considerably smaller than those calculated using the known values of the effective density of states. The values of apparent BGN for n and p Si and p GaAs calculated using experimentally determined Fermi levels are in remarkably good agreement with the experimental values derived from device measurements. All results are presented in a form which lends itself to numerical computer simulation studies.
author_names: S C Jain

David J Roulston
title: A simple expression for band gap narrowing (BGN) in heavily doped Si, Ge, GaAs and GexSi1 x strained layers
year: 1991

abstract: In recent years, quantum dots have been successfully grown by self assembling processes. For optoelectronic device applications, the quantum dot structures have advantages such as reduced phonon scattering, longer carrier lifetime, and lower detector noise due to low dimensional confinement effect. Comparing to traditional optoelectronic III V and other materials, self assembled Ge quantum dots grown on Si substrates have a potential to be monolithically integrated with advanced Si based technology. In this paper, we describe the growth of self assembled, guided Ge quantum dots, and Ge quantum dot superlattices on Si. For dot growth, issues such as growth conditions and their effects on the dot morphology are reviewed. Then vertical correlation and dot morphology evolution are addressed in relation to the critical thickness of Ge quantum dot superlattices. In addition, we also discuss the quantum dot p i p photodetectors (QDIPs) and n i n photodetectors for mid infrared applications, and the quantum dot p i n photodetectors for 1.3 1.55 mum for communications applications. The wavelength of SiGe p i p QDIP can be tuned by the size as grown by various patterning methods. Photoresponse is demonstrated for an n i n structure in both the mid infrared and far infrared wavelength ranges. The p i n diodes exhibit low dark current and high quantum efficiency. The characteristics of fabricated light emitting diode (LED) devices are also discussed, and room temperature electroluminescence is observed for Ge quantum dot LED. The results indicate that Ge dot materials are potentially applicable for mid infrared (8 12 mum) detectors as well as fiber optic (1.3 1.55 mum) communications.
author_names: Kang L Wang

Dongho Cha

Jianlin Liu

Christopher Chen
title: Ge/Si Self Assembled Quantum Dots and Their Optoelectronic Device Applications
venue: Proceedings of the IEEE
year: 2007