GeSn Material Research Articles

A Real-Time Chemical Vapor Deposition Optical Monitoring System for Ge1-XSnx Growth Optimization

GeSn has attracted tremendous attention due to its tunable bandgap in recent decades. While the material has been produced through various methods, the Chemical Vapor Deposition (CVD) process has become one of the most essential for fabricating GeSn material since it was first demonstrated. However, the process is generally studied as a black box through input versus output. For example, one could study the change in temperature versus growth rate where changes in film thickness due to temperature change can be plotted only after the growth is finished. Process improvements from the study of these black box studies have yielded understandings in the growth kinematics and the development of growth recipes defining specific growth windows for different films that in many cases are very well understood. The black box treatment of CVD growth studies lacks near real-time feedback for the growth of the film leading to unsuccessful growths that in some cases slow research and in others lead to reduced yields of useful samples for device fabrication.Real-time monitoring of the process both in-situ and ex-situ is essential, as these could enhance understanding of the CVD process and growth dynamics. In this research, we have been developing a sophisticated optical imaging system to monitor the wafer surface morphology with desired magnification and specific areas of a sample during growth. By coupling an optical microscope system and a high-resolution portable imaging system via ports into the CVD reactor, the growth of a sample can be monitored in real time at both the micro and macro scale. As schematically shown in Figure 1, a microscope and an LED light source are mounted to the two 2¾” flanges with an angle of incidence symmetrical along the chamber's vertical center line, and the portable camera is nearly vertically mounted at one of the reactor's optical windows below the horizontal axis. The microscope and the portable camera are designated for distinct functions, a characteristic that is determined by their respective locations and magnification capabilities. By meticulously calculating and applying geometric and optical design principles, such as the imaging area, solid angles, and reflection, we can capture real-time images of the sample surface during its growth.Specifically, using this optical monitoring setup, we could observe the surface morphological change during a GeSn growth on Ge-Vs sample on a Si (100) substrate. This observation led to the early identification of defect formation and post-growth progression of defect formation during the cooling down process. Figure 2 is an example picture taken during the growth for an as-grown GeSn sample showing surface defects indicated by grey spots and edge and surface roughening by the orangish center region noted in the image. This capability would allow for the optimization of growth parameters throughout the entire process that result in larger areas of useful material or provide feedback that allows CVD operators to promptly respond and decide on the subsequent growth early to prevent lost time due to failed growths.Future work is to be accomplished by improvements in the optical system to not only allow for better image quality by using a different camera allowing better focusing, and higher frame rates as well as adjusting light source angle and/or incident camera angle. Acknowledgments The work is supported by the Air Force Office of Scientific Research (AFOSR) (Grant No.: FA9550-19-1-0341, FA9550-20-1-0168). Figure 1

Strained GeSn laser with multiple fins structure based on SiN stress

Among the IV group materials, Germanium (Ge) stands out due to its unique bandgap structure, which can be engineered to achieve direct bandgap emission. This has important applications in the fabrication of efficient integrated light sources using IV group materials. In this paper, a strained GeSn laser with Multiple Fins structure based on SiN stress is proposed, through the Multi-fins structure, the biaxial tensile stress of about 0.7GPa is predicted to be uniformly introduced into the GeSn material in the active region and the laser is predicted to exhibit a threshold current density of 180 kA cm−2 and an emission peak wavelength at 2429 nm. The design presented in this paper provides an effective solution for silicon-based integrated light source.

Role of tin clustering in band structure and thermodynamic stability of GeSn by atomistic modeling

Synthesis of device-quality GeSn materials with higher Sn compositions is hindered by various factors, such as Sn segregation, clustering, and short-range ordering effects. In the present work, the impact of the clustering of Sn atoms in a GeSn semiconductor alloy was studied by density functional theory using SG15 pseudopotentials in a Synopsys QuantumATK tool, where the thermodynamic stability, effective band structure, indirect and direct bandgaps, and density of states (DOS) were computed to highlight the difference between a cluster-free random GeSn alloy and a GeSn alloy with Sn–Sn clusters. A 54-atom bulk Ge1–xSnx (x = 3.71%–27.77%) supercell was constructed with cluster-free and a first nearest neighbor Sn–Sn clustered GeSn alloy at each composition for this work. Computation using the generalized gradient approximation exchange-correlation functional showed that the thermodynamic stability of GeSn was reduced due to the clustering of Sn, which increased the formation energy of the GeSn alloys by increasing the Hartree potential energy and exchange-correlation energy. Moreover, with the effective band structure of the GeSn material at a Sn composition of ∼22%, both direct (Eg,Γ) and indirect (Eg,L) bandgaps decreased by a large margin of 40.76 and 120.17 meV, respectively, due to Sn–Sn clustering. On the other hand, Eg,Γ and Eg,L decrease is limited to 0.5 and 12.8 meV, respectively, for Sn composition of ∼5.6%. Similar impacts were observed on DOS, in an independent computation without deducing from the electronic band structure, where the width of the forbidden band reduces due to the clustering of Sn atoms in GeSn. Moreover, using the energy bandgaps of GeSn computed with the assumption of it being a random alloy having well-dispersed Sn atoms needs revision by incorporating clustering to align with the experimentally determined bandgap. This necessitates incorporating the effect of Sn atoms clustered together at varying distributions based on experimental characterization techniques such as atom probe tomography or extended x-ray absorption fine structure to substantiate the energy bandgap of the GeSn alloy at a particular composition with precision. Hence, considering the effect of Sn clusters during material characterization, beginning with the accurate energy bandgap characterization of GeSn would help in mitigating the effect of process variations on the performance characteristics of GeSn-based group IV electronic and photonic devices such as varying leakage currents in transistors and photodiodes as well as the deviation from the targeted wavelength of operation in lasers and photodetectors.

The growth of Ge and direct bandgap Ge1-xSnx on GaAs (001) by molecular beam epitaxy.

Germanium tin (GeSn) is a tuneable narrow bandgap material, which has shown remarkable promise for the industry of near- and mid-infrared technologies for high efficiency photodetectors and laser devices. Its synthesis is challenged by the lattice mismatch between the GeSn alloy and the substrate on which it is grown, sensitively affecting its crystalline and optical qualities. In this article, we investigate the growth of Ge and GeSn on GaAs (001) substrates using two different buffer layers consisting of Ge/GaAs and Ge/AlAs via molecular beam epitaxy. The quality of the Ge layers was compared using X-ray diffraction, atomic force microscopy, reflection high-energy electron diffraction, and photoluminescence. The characterization techniques demonstrate high-quality Ge layers, including atomic steps, when grown on either GaAs or AlAs at a growth temperature between 500-600 °C. The photoluminescence from the Ge layers was similar in relative intensity and linewidth to that of bulk Ge. The Ge growth was followed by the growth of GeSn using a Sn composition gradient and substrate gradient approach to achieve GeSn films with 9 to 10% Sn composition. Characterization of the GeSn films also indicates high-quality gradients based on X-ray diffraction, photoluminescence, and energy-dispersive X-ray spectroscopy measurements. Finally, we were able to demonstrate temperature-dependent PL results showing that for the growth on Ge/GaAs buffer, the direct transition has shifted past the indirect transition to a longer wavelength/lower energy suggesting a direct bandgap GeSn material.

Dark Current Analysis on GeSn p-i-n Photodetectors.

Group IV alloys of GeSn have been extensively investigated as a competing material alternative in shortwave-to-mid-infrared photodetectors (PDs). The relatively large defect densities present in GeSn alloys are the major challenge in developing practical devices, owing to the low-temperature growth and lattice mismatch with Si or Ge substrates. In this paper, we comprehensively analyze the impact of defects on the performance of GeSn p-i-n homojunction PDs. We first present our theoretical models to calculate various contributing components of the dark current, including minority carrier diffusion in p- and n-regions, carrier generation-recombination in the active intrinsic region, and the tunneling effect. We then analyze the effect of defect density in the GeSn active region on carrier mobilities, scattering times, and the dark current. A higher defect density increases the dark current, resulting in a reduction in the detectivity of GeSn p-i-n PDs. In addition, at low Sn concentrations, defect-related dark current density is dominant, while the generation dark current becomes dominant at a higher Sn content. These results point to the importance of minimizing defect densities in the GeSn material growth and device processing, particularly for higher Sn compositions necessary to expand the cutoff wavelength to mid- and long-wave infrared regime. Moreover, a comparative study indicates that further improvement of the material quality and optimization of device structure reduces the dark current and thereby increases the detectivity. This study provides more realistic expectations and guidelines for evaluating GeSn p-i-n PDs as a competitor to the III-V- and II-VI-based infrared PDs currently on the commercial market.

Electrically injected GeSn laser with stairs-structure based on SiN stressor

After decades of advancement, optoelectronic technology has emerged as a pivotal player in various domains such as optical communication, optical interconnection, and optical computer However, the challenge of integrating optoelectronic devices with complementary metal-oxide-semiconductor (CMOS) technology remains unresolved. Germanium, with its unique band structure, can be transformed into a direct bandgap semiconductor through the introduction of tensile strain, enabling efficient light emission. This provides a solution for the development of integrated light sources based on Group IV materials. A GeSn laser with a stairs-structure based on SiN stressor is proposed in this paper. The combination of the high-stress SiN film and GeSn alloy technology allows for the modification of Ge material, enabling efficient light emission in the laser. The stairs-structure facilitates a higher tensile strain in the GeSn material. The laser utilizes an F–P resonator as the optical cavity. Through simulation experiments, the laser exhibits a threshold current density of 90kA/cm2 at room temperature, a laser wavelength of 2477 nm, and an electrical-to-optical conversion efficiency of 6.5%.

Extended-SWIR GeSn LEDs with Reduced Footprint and Efficient Operation Power

Complementary metal oxide semiconductor-compatible short- and midwave infrared emitters are highly coveted for the monolithic integration of silicon-based photonic and electronic integrated circuits to serve a myriad of applications in sensing and communication. In this regard, the group IV germanium–tin (GeSn) material epitaxially grown on silicon (Si) emerges as a promising platform to implement tunable infrared light emitters. Indeed, upon increasing the Sn content, the bandgap of GeSn narrows and becomes direct, making this material system suitable for developing an efficient silicon-compatible emitter. With this perspective, microbridge PIN GeSn LEDs with a small footprint of 1520 μm2 are demonstrated, and their operation performance is investigated. The spectral analysis of the electroluminescence emission exhibits a peak at 2.31 μm, and it red-shifts slightly as the driving current increases. It is found that the microbridge LED operates at a dissipated power as low as 10.8 W at room temperature and just 3 W at 80 K. This demonstrated low operation power is comparable to that reported for LEDs having a significantly larger footprint reaching 106 μm2. The efficient thermal dissipation of the current design helped reduce the heat-induced optical losses, thus enhancing light emission. Further performance improvements are envisioned through thermal and optical simulations of the microbridge design. These simulations indicate that the use of GeSn-on-insulator substrate for developing a similar microbridge device is expected to improve the optical confinement toward the realization of electrically driven GeSn lasers.

GeSn based heterojunction double-gate tripple metal layer vertical TFET with enhanced DC and Analog/RF performance

In this article, low-bandgap material GeSn and high bandgap Si material heterojunction (GeSn/Si) based double gate triple metal layer Vertical TFET is designed and analyzed by ATLAS Silvaco TCAD simulation tool. Higher ON-state current is obtained by this device owing to (i) the usage of novel low band-gap material i.e., GeSn and (ii) Vertical tunneling as the gate overlaps the source region. The incorporation of GeSn material not only enhances the ON-state current but also improves the SS, albeit trading off the OFF-state current and ambipolar current of the device. To control the ambipolar current and OFF-state current, variation in the gate to drain overlap and auxiliary gate metal work function has been done. The impact of the gate to drain overlap and auxiliary gate workfunction variation has been studied for various electrical parameters. Further through extensive simulation analysis, optimized dimensions have been obtained. An enormously high ION/IOFF ratio of the order of 1.6 × 1012, steepest point, and average subthreshold swing of 7.5 mV/dec and 12.3 mV/dec respectively has been achieved through this proposed device. In addition to this, comparison of the proposed device with other novel TFET devices reported in literature has been done. From the comparison, it has been concluded that better results are obtained by the proposed device in terms of ION, IAMB, ION/IOFF ratio, SSavg, and gm. Therefore, this novel proposed device can be used as a promising device for various small power applications. Further, this device not only provides improved DC and Analog/RF electrical parameters but also provides small footprint due to vertically aligned architecture.

Enhancing the incorporation of Sn in vapor–liquid–solid GeSn nanowires by modulation of the droplet composition

We report on the influence of the liquid droplet composition on the Sn incorporation in GeSn nanowires (NWs) grown by the vapor−liquid−solid (VLS) mechanism with different catalysts. The variation of the NW growth rate and morphology with the growth temperature is investigated and 400 °C is identified as the best temperature to grow the longest untapered NWs with a growth rate of 520 nm min−1. When GeSn NWs are grown with pure Au droplets, we observe a core–shell like structure with a low Sn concentration of less than 2% in the NW core regardless of the growth temperature. We then investigate the impact of adding different fractions of Ag, Al, Ga and Si to Au catalyst on the incorporation of Sn. A significant improvement of Sn incorporation up to 9% is obtained using 75:25 Au–Al catalyst, with a high degree of spatial homogeneity across the NW volume. Thermodynamic model based on the energy minimization at the solid–liquid interface is developed, showing a good correlation with the data. These results can be useful for obtaining technologically important GeSn material with a high Sn content and, more generally, for tuning the composition of VLS NWs in other material systems.

Growth and Strain Modulation of GeSn Alloys for Photonic and Electronic Applications.

GeSn materials have attracted considerable attention for their tunable band structures and high carrier mobilities, which serve well for future photonic and electronic applications. This research presents a novel method to incorporate Sn content as high as 18% into GeSn layers grown at 285–320 °C by using SnCl4 and GeH4 precursors. A series of characterizations were performed to study the material quality, strain, surface roughness, and optical properties of GeSn layers. The Sn content could be calculated using lattice mismatch parameters provided by X-ray analysis. The strain in GeSn layers was modulated from fully strained to partially strained by etching Ge buffer into Ge/GeSn heterostructures . In this study, two categories of samples were prepared when the Ge buffer was either laterally etched onto Si wafers, or vertically etched Ge/GeSnOI wafers which bonded to the oxide. In the latter case, the Ge buffer was initially etched step-by-step for the strain relaxation study. Meanwhile, the Ge/GeSn heterostructure in the first group of samples was patterned into the form of micro-disks. The Ge buffer was selectively etched by using a CF4/O2 gas mixture using a plasma etch tool. Fully or partially relaxed GeSn micro-disks showed photoluminescence (PL) at room temperature. PL results showed that red-shift was clearly observed from the GeSn micro-disk structure, indicating that the compressive strain in the as-grown GeSn material was partially released. Our results pave the path for the growth of high quality GeSn layers with high Sn content, in addition to methods for modulating the strain for lasing and detection of short-wavelength infrared at room temperature.

Systematic study on photoexcited carrier dynamics related to defects in GeSn films with low Sn content at room temperature

Germanium–tin (GeSn) alloys have received much attention thanks to their optical/electrical properties and their operation in the mid-infrared range. However, dislocations/defects in GeSn films serve as trap states, limiting radiative recombination/generation via band-edges. In this work, the impact of the trap states in GeSn with varying Sn contents is investigated. The systematic study reveals that the defects/dislocations in GeSn contribute to the carrier dynamics, mainly originated from the trap states near GeSn/Ge interface. Through photoluminescence (PL) study, the broad PL peak of the trap state for GeSn exists at ∼0.57 eV. The increase in Sn content mitigates the trap-related carrier dynamics. Besides, the increase in GeSn thickness effectively suppresses the interface-related carrier dynamic. By increasing thickness from 180 to ∼900 nm, the external quantum efficiency is enhanced by ∼10×. This study provides a comprehensive understanding of trap-related carrier dynamics in a GeSn material system at room temperature.

Study of strain evolution mechanism in Ge1−xSnx materials grown by low temperature molecular beam epitaxy

Ge1−xSnx materials with constant and step-graded compositions have been successfully grown on Ge/Si(001) substrates by using low temperature molecular beam epitaxy (LT-MBE). It has been observed that both completely strained and partially relaxed GeSn materials with the same composition could be formed within the same sample, without adjusting any growth parameters. The residual in-plane strain in GeSn changes in a specific pattern from the GeSn/Ge interface towards the surface. The lower section of the GeSn material remains fully strained and free of dislocations, while most of the threading dislocations are located in the upper section of the layer, causing considerable strain relaxation within this section while maintaining the composition unchanged. This behavior could be explained by kinetic roughening and dislocation generation mechanisms at low temperatures and is remarkably different from GeSn materials grown by chemical vapor deposition (CVD), which exhibit a gradual strain relaxation process as growth continues. This work contributes to the fundamental understanding of the strain relaxation mechanisms of GeSn materials grown by MBE, which is instructive for improving the material quality in the future.

Extension of spectral sensitivity of GeSn IR photodiode after laser annealing

Photo-detection in the near-infrared is commonly performed by Ge or InGaAs-based photodetectors. Further extension of the detection range to the mid-infrared region can be performed by germanium-tin (GeSn) material which shows promising characteristics and is fully compatible with silicon electronics as can be directly grown on silicon substrates. In this study, we focused on the optoelectronic properties of the photodiodes prepared by using 200 nm thick Ge0.95Sn0.05 epitaxial layers on Ge/n-Si substrate with aluminium contacts. Photodiodes were formed on non-irradiated and Nd:YAG laser irradiated Ge0.95Sn0.05 layers. The samples were irradiated by pulsed Nd:YAG laser with 61.5–259.2 MW/cm2 intensity. The photodiodes were characterized by using short laser pulses with the wavelength in 2.0–2.6 μm range. The laser-irradiated diode was found more sensitive in the long wavelength range due to laser induced Sn atoms redistribution providing formation of graded bandgap structure. Sub-millisecond photocurrent relaxation in the diodes revealed their suitability for image sensors. Our findings open the perspective for improving the photo-sensitivity of GeSn alloys in the mid-infrared by pulsed laser processing.

(G03 Best Paper Award Winner) Analysis of Sn Behavior During Ni/GeSn Solid-State Reaction by Correlated X-ray Diffraction, Atomic Force Microscopy, and Ex-situ/In-situ Transmission Electron Microscopy

GeSn material has been envisioned as an alternative to the complex integration of III-V lasers on Silicon (Si) for photonic applications [1]. GeSn heterostructures with 16 at.% of Sn were demonstrated [2]. Increasing the Sn at.% is of high interest in these devices. Additionally, high quality contacts are needed to achieve electrical pumping, for which Nickel intermetallics were proposed. Indeed, Ni(GeSn) exhibit low temperatures formation and low resistances values. Currently, it is not entirely known how Sn incorporates in Ni-GeSn phases. Sn might: (i) be soluble in the phase, (ii) segregate, and/or (iii) form Ni-Sn alloys [3,4]. The Sn content is otherwise expected to impact different aspects of the phase sequence [5], as well as morphological and electrical properties.We propose a comprehensive study of the Ni-GeSn intermetallics focusing on Sn segregation and its impact on phase stability. 60 nm-thick Ge and Ge1-xSnx layers with 6, 10 and 15 at.% of Sn, were epitaxially grown on Ge-buffered Si (100) substrates. Surface preparation was performed with 1% diluted HF. Then, 10 nm of Ni were deposited by physical vapor deposition and capped with 7 nm of TiN. The Ni-Ge1-xSnx phase sequence and crystalline evolution were monitored by in-situ X-ray diffraction (XRD). Cross-section transmission electron microscopy (XTEM) measurements coupled with energy-dispersive X-ray spectrometry (EDS) and electron energy-loss spectroscopy (EELS) atomic maps were acquired to follow Sn distribution and segregation. Figure 1 shows the phase sequence of the Ni-Ge0.90Sn0.10 system. The growth was sequential. The first phase appearing was the Ni5(GeSn)3 phase. It was followed by the growth of the Ni(GeSn) phase [5 – 7]. EDS profiles confirmed that Sn was incorporated in both the Ni-rich and Ni(GeSn) phases. Similar phase sequences were obtained for all systems, from a straightforward comparison of in-situ XRD patterns. The threshold temperature above which the Ni-rich phase disappeared (and the less resistive Ni(GeSn) phase appeared) increased with Sn content: from 180 °C up to 235 °C and 245 °C for 0, 6 and 10 at.% of Sn, respectively (Figure 2). A dependence of the phase transition on the Sn content in the layers was evidenced. This trend was not observed for the Ni-Ge0.85Sn0.15 system due to Sn segregation at lower temperatures. XTEM analyses coupled with EDS and EELS (Figure 3) enabled us to track Sn segregation. Flat and continuous layers were obtained from the as-deposited state up to 350 °C. At 350 °C, Sn segregation started. It happened first around grain boundaries (GB) and then near the surface. At 400 °C, the Ni(GeSn) layers lost their continuity due to grain boundary grooving and agglomeration (confirmed by atomic force microscopy (AFM)). Grains were mainly composed of Ni and Ge atoms. Sn was predominantly localized at GB and close to the surface. At 550 °C, Sn coming from the intermetallic and from the GeSn thick layers underneath, completely migrated towards the surface. Figure 4 presents a Sn behavior schematic summary. Sn distribution and segregation impacted the Ni-Ge(Sn) phase sequences. Due to the role GB have on atomic transport, Sn segregation in those highly energetic areas might have reduced atoms mobility. Indeed, GB normally offer preferential paths, with enhanced atomic mobility [8]. GBs modified/saturated by Sn, can reduce the diffusion rate for phase formation. We thus believe that Sn acts as a “stuffed barrier”, hampering two-way atomic traffic. The higher the Sn content, the greater the hampering effect is, which slowed down Ni diffusion and delayed phase formation.We showed that Sn segregation in Ni-GeSn layers resulted in higher thermal budgets needed to obtain the less resistive phase (Ni(GeSn)) and have proper electrical contacts. Those thermal budgets could be detrimental for layers’ stability. In-situ TEM-EDS, AFM and sheet resistance results will also be presented, together with alternative solutions to delay Sn segregation. Acknowledgements: We would like to thank Virginie Loup for the wet surface treatment. This work was supported by the French National Research Agency (ANR) under the “Investissements d’avenir” programs: ANR-10-AIRT-0005 (IRT-NANOELEC), ANR-10-EQPX-0030 (EQUIPEX-FDSOI-11) and by the CEA DSM-DRT Phare project “Photonics”.

(Invited) Tensile Strain Engineering and Defects Management in GeSn Laser Cavities

We present GeSn material engineering to address both defect managements and band structure via tensile strain in laser cavities. We show that GeSnOI technologies allow to address both issues, with drastic reduction of power density thresholds in lasing devices at 2-3 µm wavelengths range.First set of analysis were performed on GeSn micro-disks with removed defects from their active region. The GeSn layers, grown on Ge strain-relaxed-buffers (SRBs) on silicon, were etched into micro-disks cavities. A specific chemistry were used to remove the dense misfit dislocation array from the suspended area of the GeSn disk, which constitutes the laser gain media. We obtained reduced lasing thresholds from hundreds of kW/cm2, as commonly obtained in the literature at low temperature, to around 10 kW/cm2 in pulsed regime with Sn content as low as 7 % to 10.5 %.In a second set of analysis we demonstrate a new approach relied on GeSn layers transfer on insulator, GeSnOI, with the aim to introduce SiNx stressor layers in an all-around geometry. [1,2] Tensile strain transfer from the stressor layers to GeSn micro-disks of 1.6% were reached, homogenously distributed in the disks volume. The defective interface were removed from the GeSn layer once it has been transferred on the host Si-substrate. This approach was applied to a GeSn layer with only 5.4% of Sn-content, which is nominally an indirect band gap semiconductor that cannot support lasing. The applied tensile strain transformed the Ge0.95Sn0.05 alloy into a direct-band gap material which support lasing with dramatically reduced thresholds, to 0.8 kW/cm2 at low temperature [3]. Tensile strain indeed yields to reduction of the valence band density of state by lifting the HH-LH degeneracy at the Γ-point, and thus contributes to reduce the lasing thresholds. The tensile strained GeSn lasers operate under continuous-wave-pumping up to 70 K thanks to further thermal management of the GeSnOI devices [3]. These results show the potential of new GeSn material engineering for future integration of laser sources on silicon platform using fully CMOS-compatible technology.[1] M. El Kurdi, M. Prost, A. Ghrib, et.al. ACS Photonics 3, 443 (2016).[2] A. Ghrib, M. El Kurdi, M. Prost, et. al. Adv. Opt. Mater. 3, 353 (2015).[3] A. Elbaz et. al. Nature Photonics, March (2020) doi.org/10.1038/s41566-020-0601-5

Electrically injected GeSn lasers on Si operating up to 100 K

Monolithic lasers on Si have long been anticipated as an enabler of full photonic integration, and significant progress in GeSn material development shows promise for such laser devices. While there are many reports focused on optically pumped lasers, in this work, we demonstrate electrically injected GeSn lasers on Si. We grew a GeSn/SiGeSn heterostructure diode on a Si substrate in a ridge waveguide laser device and tested it under pulsed conditions, giving consideration to the structure design to enhance the carrier and optical confinement. The peak linewidth of 0.13 nm (0.06 meV) and injection current curves indicated lasing, which was observed up to 100 K with emission peaks at 2300 nm. We recorded a threshold of 598 A / c m 2 at 10 K. The peak power and external quantum efficiency were 2.7 mW/facet and 0.3%, respectively. The results indicate advances for group-IV-based lasers, which could serve as a promising route for laser integration on Si.

Ge1\u2212xSnx alloys: Consequences of band mixing effects for the evolution of the band gap \u0393-character with Sn concentration

In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1−xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.

Electrically Injected GeSn Vertical-Cavity Surface Emitters on Silicon-on-Insulator Platforms

An efficient electrically injected group-IV light source compatible with the complementary metal-oxide-semiconductor (CMOS) process is the holy grail for realizing functional, intelligent electronic-photonic integrated circuits for a wide range of applications. The group-IV GeSn material is considered as a promising solution for efficient light sources because its bandgap can be fundamentally transformed from indirect to direct with appropriate Sn compositions. However, an important challenge in realizing efficient electrically injected light emitters is the incorporation of an optical cavity with electrical structures. Here we demonstrate, to the best of our knowledge, the first electrically injected GeSn vertical-cavity surface emitter on the silicon-on-insulator platform. A vertical cavity employing a buried oxide layer and a deposited SiO2 top layer as reflectors is developed for enhancing the electroluminescence in the GeSn active layer. Room-temperature electroluminescence experiments reveal clear c...

Enhanced B doping in CVD-grown GeSn:B using B δ-doping layers

Highly doped GeSn material is interesting for both electronic and optical applications. GeSn:B is a candidate for source-drain material in future Ge pMOS device because Sn adds compressive strain with respect to pure Ge, and therefore can boost the Ge channel performances. A high B concentration is required to obtain low contact resistivity between the source-drain material and the metal contact. To achieve high performance, it is therefore highly desirable to maximize both the Sn content and the B concentration. However, it has been shown than CVD-grown GeSn:B shows a trade-off between the Sn incorporation and the B concentration (increasing B doping reduces Sn incorporation). Furthermore, the highest B concentration of CVD-grown GeSn:B process reported in the literature has been limited to below 1 × 1020 cm−3. Here, we demonstrate a CVD process where B δ-doping layers are inserted in the GeSn layer. We studied the influence of the thickness between each δ-doping layers and the δ-doping layers process conditions on the crystalline quality and the doping density of the GeSn:B layers. For the same Sn content, the δ-doping process results in a 4-times higher B doping than the co-flow process. In addition, a B doping concentration of 2 × 1021 cm−3 with an active concentration of 5 × 1020 cm−3 is achieved.

Fabrication, Characterization, and Analysis of Ge/GeSn Heterojunction p-Type Tunnel Transistors

We present a detailed study on fabrication and characterization of Ge/GeSn heterojunction p-type tunnel-field-effect-transistors (TFETs). Critical process modules as high-k stack and p-i-n diodes are addressed individually. As a result an ultrathin equivalent oxide thickness of 0.84 nm with an accumulation capacitance of $3~\mu \text{F}$ /cm2 was achieved on an extremely scaled tri-layer stack of GeSnOx/Al2O3/HfO2 deposited by atomic-layer deposition monitored in situ by spectroscopic ellipsometry. Combining these process modules, Ge/GeSn heterojunction pTFETs are fabricated and characterized to demonstrate the best in-class pTFET performance in the GeSn material system. The transfer characteristics of the TFETs show signatures of the trap-assisted thermal generation in the subthreshold regime which is explained by a modified Shockley–Read–Hall model. For the ON-state current, we used band-to-band tunneling models calculated using parameters from the density functional theory. We then use the calibrated model to project performance of GeSn pTFETs with increased Sn content (lower bandgap), reduced trap density and ultrathin body geometry. Both experimental and projected results are benchmarked against state-of-the art III–V (e.g., In0.65Ga0.35/GaAs0.4Sb0.6) pTFETs. We demonstrate the ability of GeSn to achieve superior performance with both high ON-current and sub-60mV/decade switching benefiting from the small and direct bandgap for higher Sn contents.