Lateral GeSn p-i-n photodetectors on insulator prepared by the rapid melting growth method.

In this work, GeSn lateral p-i-n photodetectors (PDs) on insulator were fabricated with an active GeSn layer grown by the rapid melting growth (RMG) method. Taking advantages of the defect-free GeSn strips, GeSn PDs with 5.3% Sn content have low dark current and high responsivities, which are about 0.48, 0.47, and 0.24 A/W for wavelengths of 1550, 1630, and 2000 nm, respectively. The radio frequency of the lateral GeSn PDs was also studied and a 3 dB bandwidth of about 3.8 GHz was achieved. These results indicate that the GeSn grown by the rapid melting growth method is capable of fabricating high-performance Si-based optoelectronic devices.

Sn component gradient GeSn photodetector with 3†dB bandwidth over 50†GHz for extending L band telecommunication.

In this work, high-performance GeSn photodetectors with a Sn content gradient GeSn layer were fabricated on SOI substrate by CMOS-compatible process for C and L band telecommunication. The active GeSn layer has a Sn component increased from 9 to 10.7% with the controlled relaxation degree up to 84%. The responsivities of GeSn detectors at 1550 nm and 1630 nm are 0.47 A/W and 0.32 A/W under -4 V bias, respectively. Over 50 GHz 3 dB bandwidth with the eye pattern about 70 Gb/s was also evidenced at 1630 nm. These results indicate that the GeSn photodetectors have a promising application for extending the silicon photonics from C band to L band.

Harnessing microcomb-based parallel chaos for random number generation and optical decision making.

Optical chaos is vital for various applications such as private communication, encryption, anti-interference sensing, and reinforcement learning. Chaotic microcombs have emerged as promising sources for generating massive optical chaos. However, their inter-channel correlation behavior remains elusive, limiting their potential for on-chip parallel chaotic systems with high throughput. In this study, we present massively parallel chaos based on chaotic microcombs and high-nonlinearity AlGaAsOI platforms. We demonstrate the feasibility of generating parallel chaotic signals with inter-channel correlation <0.04 and a high random number generation rate of 3.84 Tbps. We further show the application of our approach by demonstrating a 15-channel integrated random bit generator with a 20 Gbps channel rate using silicon photonic chips. Additionally, we achieved a scalable decision-making accelerator for up to 256-armed bandit problems. Our work opens new possibilities for chaos-based information processing systems using integrated photonics, and potentially can revolutionize the current architecture of communication, sensing and computations.

Modeling, analysis, and demonstration of a carrier-injection electro-absorption modulator at 2 µm on Ge-on-Si platform.

In this paper, a carrier-injection electro-absorption modulator (EAM) at 2 µm is demonstrated on Ge-on-Si platform. The EAM shows a compact size and high modulation efficiency due to the strong free-carrier electroabsorption (FCEA) effect in Ge. A modulation depth of 40 dB can be obtained under the injection current of only 420 mA. Small-signal frequency response measurement is performed and a small-signal equivalent circuit model is proposed. Based on reflection coefficients and equivalent circuit, the frequency response of carrier-injection EAM is discussed in detail. The 500 Mbps open eye diagram verifies the data-processing capacity of our EAM at 2 µm wavelength for its application in biological, chemical molecular detection, and infrared imaging systems.

GeSn resonance cavity enhanced photodetector with gold bottom reflector for the L band optical communication.

In this work, GeSn resonant cavity enhanced (RCE) p-i-n photodetectors (PDs) with 3.7% Sn content in a GeSn layer were fabricated on a silicon on insulator (SOI) substrate. The gold (Au) layer and the deposited SiO2 layer constitute the bottom reflector and top reflector of the RCE detectors, respectively. The GeSn RCE PD has three resonant peaks and its responsivity is improved about 4.5 times at 1630 nm, compared with GeSn PDs without a gold bottom mirror. The cutoff wavelength of GeSn RCE PDs is up to 1820 nm, while it is only 1730 nm for GeSn PDs without a gold reflector. The responsivity of RCE PDs at 1630 nm reaches 0.126 A/W and 3-dB bandwidth at about 36 GHz is achieved. These results indicate that the RCE structure is an effective approach for enhancing the GeSn PD performance operated at the L band.

High-performance waveguide-coupled lateral Ge/Si avalanche photodetector.

A high-performance waveguide-coupled lateral avalanche photodetector (APD) is experimentally demonstrated without silicon epitaxy and charge layer ion implantation. At the wavelength of 1550 nm, it shows a high responsivity of 48 A/W and a gain-bandwidth product (GBP) of 360 GHz. Wide-open eye diagrams at 25 Gbps can be observed at various avalanche gains. These outstanding performances indicate the proposed APD has great potential in high-speed optical transceivers for optical links.

High-speed and high-power germanium photodetector based on a trapezoidal absorber.

A compact high-power germanium photodetector (Ge PD) is experimentally demonstrated by re-engineering light distribution in the absorber. Compared with a conventional Ge PD, the proposed structure shows a DC saturation photocurrent improved by 28.9% and 3 dB bandwidth as high as 49.5 GHz at 0.1 mA. Under the same photocurrent of 10.5 mA, the proposed Ge PD shows a 3 dB bandwidth of 11.1 GHz, which is almost double the conventional Ge PD (5.6 GHz). The 25 Gb/s eye-diagram measurement verifies the improved power handling capability. The compact size and manufacturing simplicity of this structure will enable new applications for integrated silicon photonics.

Spontaneously Conversion from Film to High Crystalline Quality Stripe during Molecular Beam Epitaxy for High Sn Content GeSn.

Two series of Ge0.8Sn0.2 samples were grown on Ge buffered Si substrate by molecular beam epitaxy (MBE) to investigate the influence of growth temperature and film thickness towards the evolution of surface morphology. A novel phenomena was observed that the Ge0.8Sn0.2 film was segregated and relaxed by the formation of GeSn stripes on the film. Under specific growth condition, the stripes can cover nearly the whole surface. XRD, TEM, AFM, PL and TEM results indicated that the stripes are high quality single crystalline GeSn with Sn content around 5%. The formation of GeSn stripes proposes an effective strategy to fabricate high crystalline quality GeSn stripe on Si, where the Ge0.8Sn0.2 film serves as precursor and the segregated Sn works as catalyst droplets. This technique has great potential for future optoelectronic and microelectronic applications.

High-power back-to-back dual-absorption germanium photodetector.

A high-power germanium photodetector is designed and fabricated using a cold-wall ultrahigh vacuum chemical vapor deposition. A back-to-back dual-absorption structure improves high-power characteristics by reducing the space-charge effect. Compared to a typical p-i-n photodetector, the saturated photocurrent of the back-to-back dual-absorption photodetector is improved from 16.2 to 21.3 mA at $ - {3}\;{\rm V}$-3V. At a bias voltage of $ - {1}\;{\rm V}$-1V, the dark current is 1.31 µA. The optical responsivities are 0.31 and 0.52 A/W at 1550 and 1310 nm, respectively. The 3 dB bandwidth of 4.14 GHz is achieved at $ - {3}\;{\rm V}$-3V. Theoretically, the 3 dB bandwidth can be further optimized in future device fabrication.

Study of GePb photodetectors for shortwave infrared detection.

Ge0.998Pb0.002 photodetectors (PDs) with a GePb layer grown on n-type Ge (100) substrate by magnetron sputtering epitaxy were fabricated by complementary metal-oxide semiconductor (CMOS)-compatible technology. For Ge0.998Pb0.002 PDs, the room-temperature dark current density at -1 V was 3.3 A/cm2. At room temperature, the GePb PDs demonstrated a longwave cutoff of 2.5 µm and the optical responsivities of GePb PDs ranging from 1500 nm to 2000 nm were measured. A temperature dependence optical characterization of these detectors was conducted and temperature-dependent energy bandgaps of Ge0.998Pb0.002 were derived from the photocurrent spectra.

Theoretical Analysis of InGaAs/InAlAs Single-Photon Avalanche Photodiodes.

Theoretical analysis and two-dimensional simulation of InGaAs/InAlAs avalanche photodiodes (APDs) and single-photon APDs (SPADs) are reported. The electric-field distribution and tunneling effect of InGaAs/InAlAs APDs and SPADs are studied. When the InGaAs/InAlAs SPADs are operated under the Geiger mode, the electric field increases linearly in the absorption layer and deviate down from its linear relations in the multiplication layer. Considering the tunneling threshold electric field in multiplication layer, the thickness of the multiplication layer should be larger than 300 nm. Moreover, SPADs can work under a large bias voltage to avoid tunneling in absorption layer with high doping concentrations in the charge layer.

Theoretical study of the bandgap regulation of a two-dimensional GeSn alloy under biaxial strain and uniaxial strain along the armchair direction.

Recently, two-dimensional germanium-tin (2D-GeSn) alloys have attracted considerable attention because they have been predicted to possess a direct bandgap, and this bandgap can be tuned by changing the Sn concentration. However, the tuning efficiency of alloying Sn is still relatively low, and alloying more Sn in 2D-GeSn is difficult to accomplish. To address this issue, the band structures for 2D-GeSn under different strain types (including biaxial and uniaxial strain along the armchair direction, as well as compressive and tensile strain) are investigated using a first-principles method based on density functional theory combined with a GGA+U method and special quasirandom structures. For tensile strain, the results indicate that both biaxially and uniaxially strained 2D-GeSn alloys exhibit direct bandgaps, and their bandgaps decrease as the strain strength increases. The bandgap tuning efficiency for biaxial strain is higher than that for uniaxial strain. For compressive strain, both biaxially and uniaxially strained 2D-GeSn alloys exhibit a large indirect bandgap area, and their bandgaps increase as the strain strength increases; however, their distribution shapes are slightly different. To uncover the physical origin of the difference between them, the projected band, the projected density of the states, the bond length and the bond angle for 2D-GeSn are analyzed. Overall, these results indicate that the combination of alloying Sn and applying an external strain is a good way to reduce the necessary Sn concentration, and this may provide comprehensive theoretical guidance for the strain energy band engineering of 2D-GeSn.

Theoretical Studies on InGaAs/InAlAs SAGCM Avalanche Photodiodes.

In this paper, we provide a detailed insight on InGaAs/InAlAs separate absorption, grading, charge, and multiplication avalanche photodiodes (SAGCM APDs) and a theoretical model of APDs is built. Through theoretical analysis and two-dimensional (2D) simulation, the influence of charge layer and tunneling effect on the APDs is fully understood. The design of charge layer (including doping level and thickness) can be calculated by our predictive model for different multiplication thickness. We find that as the thickness of charge layer increases, the suitable doping level range in charge layer decreases. Compared to thinner charge layer, performance of APD varies significantly via several percent deviations of doping concentrations in thicker charge layer. Moreover, the generation rate (G btt ) of band-to-band tunnel is calculated, and the influence of tunneling effect on avalanche field was analyzed. We confirm that avalanche field and multiplication factor (M n ) in multiplication will decrease by the tunneling effect. The theoretical model and analysis are based on InGaAs/InAlAs APD; however, they are applicable to other APD material systems as well.

Multilayer Graphene-GeSn Quantum Well Heterostructure SWIR Light Source.

The problem of light source always prevents silicon-based photonics from achieving a final integration. Although some optical pump lasers have been reported in recent years, an electrical pumping laser is considered as the ultimate solution. To fabricate a Si-based laser, there are some crucial obstacles that need to be solved such as difficulties in material epitaxy, light absorption by metal electrodes, and compatibility with the existing complementary metal-oxide-semiconductor transistor process. Here, a multilayer graphene and GeSn/Ge quantum well (QW) heterostructure is designed and fabricated as a Si-based light source. Specially designed Ge0.9 Sn0.1 /Ge QWs are used as active layer, which achieves a photoluminescence (PL) peak at 2050 nm. Graphene, which has a high transmittance for all bands of light, lessens the burden of growing thick cladding layer and perfectly breaks the deadlock of light disappearance in metal contacts. The electroluminescence (EL) spectrum of the device is achieved at a peak of 2100 nm under an injection current density of 100 A cm-2 . Both the PL and EL measurements show the heterostructure has good performance as a short-wave infrared (SWIR) light source. Therefore, the results provides a good alternative for the light source in silicon-based photonics.

Theoretical study of the effect of different n-doping elements on band structure and optical gain of GeSn alloys.

N-Doping is an effective approach for improving the lighting efficiency of GeSn alloys. As each doping element has an atomic radius and electronegativity value different from those of the host atoms, the shape of the GeSn band is affected. However, no recent studies considering this phenomenon have been reported. For this reason, first-principles calculations combined with the GGA+U method and supercell models have been employed to precisely investigate the structural properties, band structures, and optical gains of Ge0.9375Sn0.0625 when doped with different V-group elements (including P, As, Sb, and Bi). With regard to the structural properties, the results indicate that they all exhibit a positive deviation from Vegard's law; Ge0.9375-mSn0.0625Pm has the largest bowing coefficient. The bandgap results indicate that doping with P and As does not assist in converting GeSn into a direct bandgap material, while doping with Sb and Bi has positive effects on the transition of GeSn; the corresponding crossover values are 1.89 and 1.58%, respectively. The calculated optical gain indicates that the net gain of Ge0.9375-mSn0.0625 will reach a maximum when the injected carrier density is ∼1 × 1019 cm-3, and it will increase as the doping concentration increases. The effects of the doping elements on the optical gain of GeSn can be ranked as Bi > Sb > As > P.

Ultrathin Broadband Germanium-Graphene Hybrid Photodetector with High Performance.

Germanium-based photodetector is a key component in silicon based photonics because of its unique properties of response at telecommunication band and compatibility with CMOS techniques. However, the limitations of low quantum efficiency and high surface recombination in ultrathin germanium film, especially in the near-infrared range, put huge obstructions on the road toward applications. Nowadays, practical applications require more nanoscale devices with lower power consumption as well as higher responsivity and response speed. In this work, we first demonstrate a germanium-graphene hybrid structure photodetector that consists of an ultrathin 20 nm germanium layer and a monolayer graphene. The photodetector can achieve a broadband detection from ultraviolet to near-infrared range. A conductive gain of 155 and a responsivity of 66.2 A W-1 are achieved, which is about 3 orders of magnitude higher than pure graphene photodetectors and about 4 times larger than pure germanium photodetectors. Such enhancement owes to effective generation, separation and transfer of photogenerated carriers at material interface. The photodetector based on germanium--graphene hybrid structure presents a new paradigm for the realization of small but high performance device in the process of integration in silicon-based optical chips. And it offers new opportunities for imaging, sensing, and other optoelectronic field applications.

Characterization of a Ge1-x-ySiySnx/Ge1-xSnx multiple quantum well structure grown by sputtering epitaxy.

A high-quality Ge0.88Si0.08Sn0.04/Ge0.94Sn0.06 multiple quantum well (MQW) structure was grown on a Ge (001) substrate by sputtering epitaxy. The MQW structure was characterized by high-resolution x-ray diffraction and transmission electron microscopy. Surface-illuminated Ge0.88Si0.08Sn0.04/Ge0.94Sn0.06 MQW pin photodetectors were fabricated with cutoff wavelengths of up to 2140 nm. The analysis of transitions from spectral response was fitted well with the theoretical calculations. Results suggest that sputtering epitaxy is a promising method for preparing high-quality low-dimensional Sn-based group IV materials and that Ge1-x-ySiySnx/Ge1-xSnx MQWs have potential applications in the development of efficient Si-based photonic devices.

All-inorganic perovskite quantum dot/mesoporous TiO2 composite-based photodetectors with enhanced performance.

High-performance all-inorganic perovskite-based metal/semiconductor/metal (MSM) photodetectors with a bilayer composite film of mesoporous TiO2 and CsPbBr3 quantum dots as a photosensitizer were prepared. The photodetectors demonstrated significantly improved on/off ratios of nearly three orders of magnitude compared to those of pure bromine-based perovskite nanocrystal photodetectors with an MSM structure.

Defect-free high Sn-content GeSn on insulator grown by rapid melting growth.

GeSn is an attractive semiconductor material for Si-based photonics. However, large lattice mismatch between GeSn and Si and the low solubility of Sn in Ge limit its development. In order to obtain high Sn-content GeSn on Si, it is normally grown at low temperature, which would lead to inevitable dislocations. Here, we reported a single-crystal defect-free graded GeSn on insulator (GSOI) stripes laterally grown by rapid melting growth (RMG). The Sn-content reaches to 14.2% at the end of the GSOI stripe. Transmission electron microscopy observation shows the GSOI stripe without stacking fault and dislocations. P-channel pseudo metal-oxide-semiconductor field effect transistors (MOSFETs) and metal-semiconductor-metal (MSM) Schottky junction photodetectors were fabricated on these GSOIs. Good transistor performance with a low field peak hole mobility of 402 cm2/Vs is obtained, which indicates a high-quality of this GSOI structure. Strong near-infrared and short-wave infrared optical absorption of the MSM photodetectors at 1550 nm and 2000 nm were observed. Owing to high Sn-content and defect-free, responsivity of 236 mA/W@-1.5 V is achieved at 1550 nm wavelength. In addition, responsivity reaches 154 mA/W@-1.5 V at 2000 nm with the optical absorption layer only 200 nm-thick, which is the highest value reported for GeSn junction photodetectors until now.

High-responsivity vertical-illumination Si/Ge uni-traveling-carrier photodiodes based on silicon-on-insulator substrate.

Si/Ge uni-traveling carrier photodiodes exhibit higher output current when space-charge effect is overcome and the thermal effects is suppressed. High current is beneficial for increasing the dynamic range of various microwave photonic systems and simplifying high-bit-rate digital receivers in many applications. From the point of view of packaging, detectors with vertical-illumination configuration can be easily handled by pick-and-place tools and are a popular choice for making photo-receiver modules. However, vertical-illumination Si/Ge uni-traveling carrier (UTC) devices suffer from inter-constraint between high speed and high responsivity. Here, we report a high responsivity vertical-illumination Si/Ge UTC photodiode based on a silicon-on-insulator substrate. When the transmission of the monolayer anti-reflection coating was maximum, the maximum absorption efficiency of the devices was 1.45 times greater than the silicon substrate owing to constructive interference. The Si/Ge UTC photodiode had a dominant responsivity at 1550 nm of 0.18 A/W, a 50% improvement even with a 25% thinner Ge absorption layer.