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Strain Engineered Electrically Pumped SiGeSn Microring Lasers on Si
SiGeSn holds great promise for enabling fully group-IV integrated photonics operating at wavelengths extending in the mid-infrared range. Here, we demonstrate an electrically pumped GeSn microring laser based on SiGeSn/GeSn heterostructures. The ring shape allows for enhanced strain relaxation, leading to enhanced optical properties, and better guiding of the carriers into the optically active region. We have engineered a partial undercut of the ring to further promote strain relaxation while maintaining adequate heat sinking. Lasing is measured up to 90 K, with a 75 K T0. Scaling of the threshold current density as the inverse of the outer circumference is linked to optical losses at the etched surface, limiting device performance. Modeling is consistent with experiments across the range of explored inner and outer radii. These results will guide additional device optimization, aiming at improving electrical injection and using stressors to increase the bandgap directness of the active material.
Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells
n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
Subnanometer Control of the Heteroepitaxial Growth of Multimicrometer-Thick Ge /(Si, Ge) Quantum Cascade Structures
The fabrication of complex low-dimensional quantum devices requires the control of the heteroepitaxial growth at the subnanometer scale. This is particularly challenging when the total thickness of stacked layers of device-active material becomes extremely large and exceeds the multi-μm limit, as in the case of quantum cascade structures. Here, we use the ultrahigh-vacuum chemical vapor deposition technique for the growth of multi-μm-thick stacks of high Ge content strain-balanced Ge/SiGe tunneling heterostructures on Si substrates, designed to serve as the active material in a THz quantum cascade laser. By combining thorough structural investigation with THz spectroscopy absorption experiments and numerical simulations we show that the optimized deposition process can produce state-of-the-art threading dislocation density, ultrasharp interfaces, control of dopant atom position at the nanoscale, and reproducibility within 1% of the layer thickness and composition within the whole multilayer. We show that by using ultrahigh-vacuum chemical vapor deposition one achieves simultaneously a control of the epitaxy down to the sub-nm scale typical of the molecular beam epitaxy, and the high growth rate and technological relevance of chemical vapor deposition. Thus, this technique is a key enabler for the deposition of integrated THz devices and other complex quantum structures based on the Ge/SiGe material system.
CMOS-Compatible Bias-Tunable Dual-Band Detector Based on GeSn/Ge/Si Coupled Photodiodes
Infrared (IR) multispectral detection is attracting increasing interest with the rising demand for high spectral sensitivity, room temperature operation, CMOS-compatible devices. Here, we present a two-terminal dual-band detector, which provides a bias-switchable spectral response in two distinct IR bands. The device is obtained from a vertical GeSn/Ge/Si stack, forming a double junction n-i-p-i-n structure, epitaxially grown on a Si wafer. The photoresponse can be switched by inverting the bias polarity between the near and the short-wave IR bands, with specific detectivities of 1.9 × 1010 and 4.0 × 109 cm·(Hz)1/2/W, respectively. The possibility of detecting two spectral bands with the same pixel opens up interesting applications in the field of IR imaging and material recognition, as shown in a solvent detection test. The continuous voltage tuning, combined with the nonlinear photoresponse of the detector, enables a novel approach to spectral analysis, demonstrated by identifying the wavelength of a monochromatic beam. © 2021 The Authors. Published by American Chemical Society.
Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device
A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at <100 nm and >1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology.
Raman shifts in MBE-grown SixGe1 − x − ySny alloys with large Si content
We examine the Raman shift in silicon–germanium–tin alloys with high silicon content grown on a germanium virtual substrate by molecular beam epitaxy. The Raman shifts of the three most prominent modes, Si–Si, Si–Ge, and Ge–Ge, are measured and compared with results in previous literature. We analyze and fit the dependence of the three modes on the composition and strain of the semiconductor alloys. We also demonstrate the calculation of the composition and strain of SixGe1 − x − ySny from the Raman shifts alone, based on the fitted relationships. Our analysis extends previous results to samples lattice matched on Ge and with higher Si content than in prior comprehensive Raman analyses, thus making Raman measurements as a local, fast, and nondestructive characterization technique accessible for a wider compositional range of these ternary alloys for silicon-based photonic and microelectronic devices.
Ge(Sn) nano-island/Si heterostructure photodetectors with plasmonic antennas
We report on photodetection in deep subwavelength Ge(Sn) nano-islands on Si nano-pillar substrates, in which self-aligned nano-antennas in the Al contact metal are used to enhance light absorption by means of local surface plasmon resonances. The impact of parameters such as substrate doping and device geometry on the measured responsivities are investigated and our experimental results are supported by simulations of the three-dimensional distribution of the electromagnetic fields. Comparatively high optical responsivities of about 0.1 A W-1 are observed as a consequence of the excitation of localized surface plasmons, making our nano-island photodetectors interesting for applications in which size reduction is essential. © 2020 The Author(s). Published by IOP Publishing Ltd.
Advanced GeSn/SiGeSn Group IV Heterostructure Lasers
Growth and characterization of advanced group IV semiconductor materials with CMOS-compatible applications are demonstrated, both in photonics. The investigated GeSn/SiGeSn heterostructures combine direct bandgap GeSn active layers with indirect gap ternary SiGeSn claddings, a design proven its worth already decades ago in the III–V material system. Different types of double heterostructures and multi-quantum wells (MQWs) are epitaxially grown with varying well thicknesses and barriers. The retaining high material quality of those complex structures is probed by advanced characterization methods, such as atom probe tomography and dark-field electron holography to extract composition parameters and strain, used further for band structure calculations. Special emphasis is put on the impact of carrier confinement and quantization effects, evaluated by photoluminescence and validated by theoretical calculations. As shown, particularly MQW heterostructures promise the highest potential for efficient next generation complementary metal-oxide-semiconductor (CMOS)-compatible group IV lasers.
Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models
n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron-phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter-and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1!3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3. © 2019 by the authors.
Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology
Buried-channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 × 10 5 cm 2 V −1 s −1 ) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top-gate of a dopant-less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths ≈ 6 µm, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 × 10 11 cm −2 , light effective mass (0.09m e ), and high effective g-factor (up to 9.2) highlight the potential of undoped Ge/SiGe as a low-disorder material platform for hybrid quantum technologies. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim