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Silicon-based electronic-photonic integrated circuits, which are compatible with state-of-the-art complementary metal-oxide-semiconductor processes, offer promising opportunities for on-chip mid-infrared photonic systems. However, the lack of efficient mid-infrared optical modulators on silicon hinders the utilization of such systems. Here, we demonstrate the Franz-Keldysh effect in GeSn alloys and achieve mid-infrared electro-absorption optical modulation using GeSn heterostructures on silicon. Our experimental and theoretical results verify that the direct bandgap energy of GeSn can be widely tuned by varying the Sn content, thereby realizing wavelength-tunable optical modulation in the mid-infrared range with a figure-of-merit greater than 1.5 and a broadband operating range greater than 140 nm. In contrast to conventional silicon-photonic modulators based on the plasma dispersion effect, our GeSn heterostructure demonstrates practical and effective Franz-Keldysh mid-infrared optical modulation on silicon, helping to unlock the potential of electronic-photonic integrated circuits in a wide range of applications. Silicon-based electronic-photonic integrated circuits are promising for various applications, but their mid-infrared optical modulation is elusive. Here, tunable mid-infrared electro-absorption modulation, with broadband operation range >140 nm, is achieved in GeSn alloys on Si by controlling the Sn content.

We study the effect of temperature and doping in Si-based GeSn heterojunction phototransistors (HPTs) for low-power-consuming, low-cost, and high-speed mid-infrared (MIR) applications. The incorporation of ${\rm{G}}{{\rm{e}}_{1 - x}}{\rm{S}}{{\rm{n}}_x}$ alloy in the base of our HPTs significantly shortens the emitter-to-collector transit time, leading to high cut-off frequency ( ${f_T}$ ) due to an increase in mobility. Furthermore, the ${\rm{G}}{{\rm{e}}_{1 - x}}{\rm{S}}{{\rm{n}}_x}$ base extends the optical detection over a wide range (up to 2500 nm) due to the shrinkage of the bandgap energy caused by alloying with Sn. Additionally, spectral responsivity increases with Sn alloying due to the increased absorption coefficient. Our results show that ${f_T}$ and responsivity are strongly dependent not only on doping but also on temperature. The impact of temperature on the noise behavior of phototransistors was also analyzed for frequencies up to 100 GHz. As the temperature increased, the signal-to-noise ratio (SNR) decreased; however, spectral responsivity significantly improved. The high SNR, responsivity, and ${f_T}$ values of the GeSn HPT make it a potential candidate for future Si-based uncooled high-speed MIR photodetection.

We report on lateral p-i-n GeSn waveguide photodetector on silicon substrates for complementary metal-oxide-semiconductor (CMOS)-compatible mid-infrared silicon photonics.

We have demonstrated high-performance flexible germanium (Ge) vertical p-i-n photodetectors (PDs) based on a resonant cavity structure by a direct flip transfer of Ge nanomembranes on polyethylene terephthalate (PET) substrates. Finite-difference time-domain simulation proves that the vertical cavity structure composed of the bottom gold and top SU-8 layers as a reflector and an anti-reflection surface, respectively, could enhance the average absorption in the near-infrared (NIR) region (i.e., 1520-1640 nm) from 0.06 to 0.20 by 233%. Strains introduced into Ge NMs by convex and concave fixtures are measured to be 0.37 and-0.32%, respectively. The fabricated PDs exhibit a low dark current density of 9.6 mA/cm2 at-1 V and a high forward-reverse current ratio of 105 under the flat condition. Responsivity at 1550 nm increases from 52.5 to 133.8 mA/W by tensile strain, while it slightly decreases to 32.6 mA/W under comparable compressive strain. Furthermore, the devices show no degradation in their optoelectronic responses after 200 bending cycles at convex fixtures with a radius of 30 mm. Overall, such flexible Ge PDs with the capabilities of both excellent optoelectronic performance and mechanical durability represent significant advances in the field of group IV NIR optoelectronic devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Submitted/Accepted version This work was supported by the A*STAR Advanced Manufacturing and Engineering (AME) Young Individual Research Grant (YIRG) under Project A2084c0066, the Nanyang Technological University (NTU) start-up grant (M4082289.040), and partly by the National Research Foundation, Singapore, under its Competitive Research Program (CRP award NRF-CRP19-2017-01) and the Ministry of Education Tier 2 (MOE2018-T2-1-137). The work at the CCU was supported by the Ministry of Science and Technology, Taiwan, grant number MOST 109-2636-E-194- 002.

Metal-semiconductor-metal photodetectors (MSM PDs) are effective for monolithic integration with other optical components of the photonic circuits because of the planar fabrication technique. In this article, we present design, growth, and characterization of GeSn MSM PDs that are suitable for photonic integrated circuits. The introduction of 4% Sn in the GeSn active region also reduces the direct bandgap and shows a redshift in the optical responsivity spectra, which can extend up to 1800 nm wavelength, which means it can cover the entire telecommunication bands. The spectral responsivity increases with an increase in bias voltage caused by the high electric field, which enhances the carrier generation rate and the carrier collection efficiency. Therefore, the GeSn MSM PDs can be a suitable device for a wide range of short-wave infrared (SWIR) applications. National Research Foundation (NRF) Published version National Research Foundation Singapore Competitive Research Programme under Grant NRF-CRP19-2017-01.

GeSn alloys have emerged as promising materials for silicon-based optoelectronic devices. However, the epitaxy of pseudomorphic GeSn layers on a Ge buffer is susceptible to a significant compressive strain that significantly hinders the performance of GeSn-based photonic devices. Herein, we report on a new strategy to produce strain-free GeSn nanomembranes for advanced optoelectronic applications. The GeSn alloy was grown on a silicon-on-insulator substrate using Ge buffers, and it has a residual compressive strain. By transfer-printing the GeSn/Ge/Si multi-layers, followed by etching the Si template and the Ge buffer layers, respectively, the residual compressive strain was completely removed to achieve strain-free GeSn layers. A bandgap reduction was also observed as a result of strain relaxation. Furthermore, theoretical analysis was performed to evaluate the effect of strain relaxation on the GeSn-based optoelectronic devices. The proposed approach offers a practical and viable method for preparing strain-free GeSn alloys for advanced optoelectronic applications. Ministry of Education (MOE) Nanyang Technological University Accepted version This work at CCU was supported by the Ministry of Science and Technology of Taiwan (MOST) under the grant Nos. MOST 108-2221-E-194-055 and MOST 109-2636-E-194-002, and the Advanced Institute of Manufacturing with High-tech Innovations (AIM-HI) from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The work at NTU was supported by NTU start-up grant (M4082289.040) and Singapore MOE AcRF Tier 1 grant (M4012124.040). The authors would like to thank Dr. Hui Li at National Taiwan University, Taiwan, for providing assistance in the material growth, and Dr. I-Fang Cheng at Taiwan Semiconductor Research Institute, Taiwan, for providing assistance in the Raman experiments.

We have demonstrated flexible GeSn metal–semiconductor–metal (MSM) photodetectors (PDs) by exploring the effect of mechanical strain on their optoelectronic properties. The PDs were fabricated from transfer-printed GeSn nanomembranes on polyethylene terephthalate (PET) substrates. Strain was introduced into the GeSn PDs under bend-down (uniaxial tensile strain) and bend-up (uniaxial compressive strain) conditions and their values were measured by Raman spectroscopy. The applied strain can affect the band-structure of the GeSn alloys, leading to a modulation of the electrical and optical characteristics of the PDs. Accordingly, dark current characteristics show an increase from 8.1 to 10.3 μA under the bend-down conditions and a decrease to 7.2 μA under the bend-up conditions, respectively. The optical responsivity at a wavelength of 2 μm increased by 151% under bend-down conditions, while it decreases by 35% under bend-up conditions. A theoretical study was carried out to support the fact that the responsivity enhancement is attributed to the change in the absorption coefficient of the strained GeSn. The results offer a new pathway to modulate the optical properties of GeSn for flexible applications. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version The work was supported by the Nanyang Technological University (NTU) start-up grant (M4082289.040) and partly by the National Research Foundation, Singapore, under its Competitive Research Program (CRP Award NRF-CRP19-2017-01) and Ministry of Education, Singapore, under its Tier 2 (MOE2018-T2-1-137). The authors would like to thank Prof. Hong Wang at the NTU for providing a laser setup.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well (MQW) photoconductors (PCs) on silicon substrate were investigated. The high quality GeSn/Ge MQW epitaxial structure was grown on a silicon substrate using low temperature molecular beam epitaxy techniques with atomically precise thickness control. Surface-illuminated GeSn/Ge MQW PCs were fabricated using complementary metal-oxide-semiconductor-compatible processing and characterized in a wide temperature range of 55–320 K. The photodetection range was extended to λ = 2235 n m at T = 320 K due to bandgap shrinkage with Sn alloying. Measured spectral responsivity was enhanced at reduced temperatures. These results provide better understanding of GeSn/Ge MQW structures for efficient short-wave infrared photodetection.

We demonstrate silicon-based p - n - p floating-base GeSn heterojunction phototransistors with enhanced optical responsivity for efficient short-wave infrared (SWIR) photodetection. The narrow-bandgap GeSn active layer sandwiched between the p - G e collector and n - G e base effectively extends the photodetection range in the SWIR range, and the internal gain amplifies the optical response by a factor of more than three at a low driving voltage of 0.4 V compared to that of a reference GeSn p - i - n photodetector (PD). We anticipate that our findings will be leveraged to realize complementary metal-oxide-semiconductor-compatible, sensitive, low driving voltage SWIR PDs in a wide range of applications.

We present a design of Ge 1−x Sn x waveguide electroabsorption modulators on silicon and analyze the Franz-Keldysh modulation properties. The introduction of Sn into Ge allows for considerably narrowing the direct bandgap energy, thereby extended the absorption edge toward mid-infrared region to achieve mid-infrared optical modulation. We calculate the change in absorption coefficient of Ge 1−x Sn x induced by the Franz-Keldysh effects. The results show that significant absorption coefficient variation associated with the Franz-Keldysh effect can be achieved for optical modulation in the mid-infrared region.

A n-p-n heterojunction phototransitor with Ge 1-x Sn x base is presented. The work is focussed on effect of Sn concentration on the gain performance of the device based on optimised base and collector doping.

We report mid-infrared optical modulation in GeSn photodiodes on silicon. The incorporation of Sn into Ge shrinks the direct bandgap energy, redshifting the absorption edge to longer wavelengths and enabling electro-absorption optical modulation based on Franz-Keldysh effect in the mid-infrared range.

We report cavity-enhanced direct band electroluminescence from GeSn p-i-n diodes on silicon-on-insulator substrates. A Fabry-Perot cavity was created between the buried oxide layer and the deposited oxide layer to enhance electroluminescence from the GeSn active layer.

Micromilling is a straightforward approach to the manufacture of polymer microfluidic devices for applications in chemistry and biology. This fabrication process reduces costs, provides a relatively simple user interface, and enables the fabrication of complex structures, which makes it ideal for the development of prototypes. In this study, we investigated the influence of micromilling parameters on the surface roughness of a cyclic olefin copolymer (COC) substrate. We then employed factor analysis to determine the optimal cutting conditions. The parameters used in all experiments were the spindle speed, the feed rate, and the depth of cut. Roughness was measured using a stylus profilometer. The lowest roughness was 0.173 μm at a spindle speed of 20,000 rpm, feed rate of 300 mm/min, and cut depth of 20 μm. Factor analysis revealed that the feed rate has the greatest impact on surface quality, whereas the depth of cut has the least impact.

We present a variational study of direct transitions in GeSn/Ge quantum well system for photonic applications. The exciton radiuses, binding energies, and oscillator strengths are calculated for various Sn contents are calculated and discussed.

We report on tensile-strained Ge/Sisub0.11/subGesub0.89/subquantum-well (QW) metal-semiconductor-metal (MSM) photodetectors on Si substrates. A tensile strain of 0.21% is introduced into the Ge wells by growing the QW stack on in-situ annealed Ge-on-Si virtual substrates (VS). The optical characterization of Ge/Sisub0.11/subGesub0.89/subQW MSM photodetectors indicates that the optical response increases to a wavelength of 1.5 μm or higher owing to the strain-induced direct bandgap shrinkage. Analysis of the band structure by using a k · p model suggests that by optimizing the tensile strain and Ge well width, tensile-strained Ge/SiGe QW photodetectors can be designed to cover the telecommunication C-band and beyond for optical telecommunications and on-chip interconnection.

We propose and develop an intensity-detection-based refractive-index (RI) sensor for low-cost, rapid RI sensing. The sensor is composed of a polymer bent ridge waveguide (BRWG) structure on a low-cost glass substrate and is integrated with a microfluidic channel. Different-RI solutions flowing through the BRWG sensing region induce output optical power variations caused by optical bend losses, enabling simple and real-time RI detection. Additionally, the sensors are fabricated using rapid and cost-effective vacuum-less processes, attaining the low cost and high throughput required for mass production. A good RI solution of 5.31 10−4 × RIU−1 is achieved from the RI experiments. This study demonstrates mass-producible and compact RI sensors for rapid and sensitive chemical analysis and biomedical sensing.

We report a theoretical investigation on the electrical properties of a Sn-based group-IV structure for a resonant tunneling diode (RTD). The analysis on the composition-dependent strain, energy profile, and current-voltage characteristic of a double-barrier heterostructure shows that the peak current density and peak-to-valley ratio are enhanced with a moderated tensile strain in the barrier layer, providing an alternative approach for group-IV RTDs.

Ge1-xSnx alloys have recently attracted great research attention for new optoelectronic devices, especially efficient Si-based light emitters [1]. While Ge is an indirect semiconductor, its direct conduction band lies just 136.5 meV above the lowest conduction band at L pint in the Brillouin zone. This energy difference can be greatly reduced by alloying Ge with Sn, another group-IV element, leading to remarkably improved light-emitting efficiency. With a Sn composition of ~7%, direct-bandgap Ge1−xSnx alloys can be created [2] for laser gain media. In this paper, we present a study of photoluminescence (PL) from strained Ge1-xSnx films on Ge virtual substrate (VS) on Si wafer. The transparent carrier densities are also predicted.

We propose and analyze a strain-free GeSn/SiGeSn quantum-well structure for elec-troabsorption modulators based on the quantum-confined Stark effect at 1550 nm wavelength.

We report an investigation on GeSn p-i-n waveguide photodetectors grown on a Ge-buffered Si wafer. In comparison with a reference Ge detector, the GeSn detector shows an enhanced responsivity in the measured energy range, mainly attributed to the smaller bandgap caused by Sn-alloying. Analysis of the quantum efficiency indicates that increasing the Sn content in the active layers can significantly shorten the required device length to achieve the maximum efficiency. The present investigation demonstrates the planar photodetectors desired for monolithic integration with electronic devices.

We report an investigation of GeSn-based p-i-n photodiodes with an active GeSn layer that is almost fully strained. The results show that (a) the response of the Ge/GeSn/Ge heterojunction photodiodes is stronger than that of the reference Ge-based photodiodes at photon energies above the 0.8 eV direct bandgap of bulk Ge (

We report the first observation of room-temperature quantum-confined photoluminescence (PL) from low-dimensional Ge(1-x)Sn(x)/Ge superlattices (SLs) up to a high Sn content of 6.96%. Both direct and indirect emissions associated with the interband transitions between minibands in the conduction bands and valence band were observed at room temperature. As the Sn content is increased, the energy difference between the lowest direct and indirect transitions is reduced, indicating an effective modification of the band structure desired for optoelectronics. The integrated PL intensity ratio of direct to indirect recombinations is significantly enhanced with increasing Sn content due to the reduced Γ-L energy separation and quantum confinement effect. Those results suggest that Sn-based low-dimensional structures are promising material for efficient Si-based lasers.

We investigate the effects of biaxial tensile stress and n-type doping on the direct-transition optical gain in germanium infrared lasers, in which three differently oriented systems of (0 0 1), (1 1 0) and (1 1 1) Ge are analysed. We show our theoretical model for the strain tensor, strained electronic band structure, carrier occupation, optical gain and free-carrier absorption to evaluate the net optical gain under different strain and n-doping levels. Our analysis shows that for a large strain, (0 0 1) Ge can produce a larger optical gain than the others due to its clear direct-bandgap behaviour for mid-infrared lasers. For a moderate strain combined with the use of n-type doping, (1 1 1) Ge can generate a larger net optical gain than the others for near-infrared lasers. Our modelling results provide an insight into the improvement of optical emission in Ge for lasers.