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.

Plasma Treatment for Nitrogen-Doped 3D Graphene Framework by a Conductive Matrix with Sulfur for High-Performance Li-S Batteries.

Carbon materials have received considerable attention as host cathode materials for sulfur in lithium-sulfur batteries; N-doped carbon materials show particularly high electrocatalytic activity. Efforts are made to synthesize N-doped carbon materials by introducing nitrogen-rich sources followed by sintering or hydrothermal processes. In the present work, an in situ hollow cathode discharge plasma treatment method is used to prepare 3D porous frameworks based on N-doped graphene as a potential conductive matrix material. The resulting N-doped graphene is used to prepare a 3D porous framework with a S content of 90 wt% as a cathode in lithium-sulfur cells, which delivers a specific discharge capacity of 1186 mAh g-1 at 0.1 C, a coulombic efficiency of 96% after 200 cycles, and a capacity retention of 578 mAh g-1 at 1.0 C after 1000 cycles. The performance is attributed to the flexible 3D structure and clustering of pyridinic N-dopants in graphene. The N-doped graphene shows high electrochemical performance and the flexible 3D porous stable structure accommodates the considerable volume change of the active material during lithium insertion and extraction processes, improving the long-term electrochemical performance.

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.

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.

Infrared tubular microcavity based on rolled-up GeSn/Ge nanomembranes.

Germanium-Tin (GeSn) alloys have attracted great amounts of attention as these group IV semiconductors present direct band-gap behavior with high Sn content and are compatible with current complementary metal oxide semiconductor technology. In this work, three dimensional tubular GeSn/Ge micro-resonators with a diameter of around 7.3 µm were demonstrated by rolling up GeSn nanomembranes (NM) grown on a Ge-on-insulator wafer via molecular beam epitaxy. The microstructural properties of the resonators were carefully investigated and the strain distributions of the rolled-up GeSn/Ge microcavities along the radial direction were studied by utilizing micro-Raman spectroscopy with different excitation laser wavelengths. The values of the strains calculated from Raman shifts agree well with the theoretical prediction. Coupled with fiber tapers, as-fabricated devices present a high quality factor of up to 800 in the transmission spectral measurements. The micro-resonators fabricated via rolled-up nanotechnology and GeSn/Ge NMs in this work may have great potential in photonic micro- and nanodevices.

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.

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.

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.

Compact two-mode (de)multiplexer based on symmetric Y-junction and multimode interference waveguides.

A compact two-mode (de)multiplexer [(DE)MUX] based on symmetric Y-junction and multimode interference (MMI) waveguides was designed by 3D beam propagation method (BPM). The phase evolution in the structure was discussed in detail. Simulations show that the optical bandwidth is as large as 100 nm (1500 nm ~1600 nm). The two-mode (DE)MUX is very compact compared with the other kind of mode (DE)MUX. The length of the structure is only 48.8 µm. Simulation also shows the fabrication tolerance is as large as ± 75 nm.

Machine-learning-assisted and real-time-feedback-controlled growth of InAs/GaAs quantum dots.

The applications of self-assembled InAs/GaAs quantum dots (QDs) for lasers and single photon sources strongly rely on their density and quality. Establishing the process parameters in molecular beam epitaxy (MBE) for a specific density of QDs is a multidimensional optimization challenge, usually addressed through time-consuming and iterative trial-and-error. Here, we report a real-time feedback control method to realize the growth of QDs with arbitrary density, which is fully automated and intelligent. We develop a machine learning (ML) model named 3D ResNet 50 trained using reflection high-energy electron diffraction (RHEED) videos as input instead of static images and providing real-time feedback on surface morphologies for process control. As a result, we demonstrate that ML from previous growth could predict the post-growth density of QDs, by successfully tuning the QD densities in near-real time from 1.5 × 1010 cm-2 down to 3.8 × 108 cm-2 or up to 1.4 × 1011 cm-2. Compared to traditional methods, our approach can dramatically expedite the optimization process and improve the reproducibility of MBE. The concepts and methodologies proved feasible in this work are promising to be applied to a variety of material growth processes, which will revolutionize semiconductor manufacturing for optoelectronic and microelectronic industries.

Enhanced photoluminescence and electroluminescence of multilayer GeSi islands on Si001 substrates by phosphorus-doping.

Ge/Si heterojunction light emitting diodes with 20-bilayers undoped or phosphorus in situ doped GeSi islands were fabricated on n(+)(-)Si(001) substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD). Enhanced room temperature photoluminescence (PL) and electroluminescence (EL) around 1.5 µm were observed from the devices with phosphorus-doped GeSi islands. Theoretical calculations indicated that the emission is from the radiative recombination in GeSi islands. The intensity enhancement of PL and EL is attributed to the sufficient supply of electrons in active layer for radiative recombination.

GeSn p-i-n photodetector for all telecommunication bands detection.

Using a 820 nm-thick high-quality Ge0.97Sn0.03 alloy film grown on Si(001) by molecular beam epitaxy, GeSn p-i-n photodectectors have been fabricated. The detectors have relatively high responsivities, such as 0.52 A/W, 0.23 A/W, and 0.12 A/W at 1310 nm, 1540 nm, and 1640 nm, respectively, under a 1 V reverse bias. With a broad detection spectrum (800-1800 nm) covering the whole telecommunication windows and compatibility with conventional complementary metal-oxide-semiconductors (CMOS) technology, the GeSn devices are attractive for applications in both optical communications and optical interconnects.

Negative Magnetoresistance in the GeSn Strip.

Discovery of topological materials associated with an exotic phenomenon has attracted increasing attention in modern condensed matter physics. A typical example is the chiral anomaly proposed in the Dirac or Weyl semimetals. In addition to the well-known topological semimetals, such as TaAs and Na3Bi, recently, group IV GeSn alloys were also proposed to be Dirac semimetals in theory, demonstrating potential applications compatible with current Si-based technology. Here, we report the observation of large negative magnetoresistance (MR) that is sensitive to the orientation of the magnetic and electric field in the GeSn strip. This negative MR emerges only when the applied magnetic field is parallel to the electric field, which is consistent with the chiral anomaly in topological semimetals. This work paves a new way toward exploring the negative MR behavior and underlying mechanism in a new class of Dirac semimetals.

Broadband Photodetector Based on Inorganic Perovskite CsPbBr3 /GeSn Heterojunction.

Photodetectors with broadband response spectrum have attracted great interest in many application areas such as imaging, gas sensing, and night vision. Here, a high performance broadband photodetector is demonstrated with inorganic perovskite CsPbBr3 /GeSn heterojunction, detection range can be covered from 450 to 2200 nm. The responsivity of heterojunction device can achieve as high as 129 mA W-1 under illuminated light of 532 nm, which is 4.92 times larger than that of a GeSn based device. As the CsPbBr3 can also act as anti-reflective coating for infrared wavelength, the infrared band responsivity at wavelength of 2200 nm can also be raised by 1.42 times. In addition, the device with all inorganic components is showed good stability, while keeping in the dry environment, the device can sustain its 90% original after 550 h storage. These results show the inorganic perovskite/GeSn heterojunction device is of great potential in broadband photodetection with high responsivity.

Fabrication of silicon-based template-assisted nanoelectrode arrays and ohmic contact properties investigation.

A convenient fabrication technology for large-area, highly-ordered nanoelectrode arrays on silicon substrate has been described here, using porous anodic alumina (PAA) as a template. The ultrathin PAA membranes were anodic oxidized utilizing a two-step anodization method, from Al film evaporated on substrate. The purposes for the use of two-step anodization were, first, improving the regularity of the porous structures, and second reducing the thickness of the membranes to 100-200 nm we desired. Then the nanoelectrode arrays were obtained by electroless depositing Ni-W alloy into the through pores of PAA membranes, making the alloy isolated by the insulating pore walls and contacting with the silicon substrates at the bottoms of pores. The Ni-W alloy was also electroless deposited at the back surface of silicon to form back electrode. Then ohmic contact properties between silicon and Ni-W alloy were investigated after rapid thermal annealing. Scanning electron microscopy (SEM) observations showed the structure characteristics, and the influence factors of fabrication effect were discussed. The current-voltage (I-V) curves revealed the contact properties. After annealing in N2 at 700 degrees C, good linear property was shown with contact resistance of 33 omega, which confirmed ohmic contacts between silicon and electrodes. These results presented significant application potential of this technology in nanosize current-injection devices in optoelectronics, microelectronics and bio-medical fields.

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.

Development of microfluidic platform capable of high-throughput absolute quantification of single-cell multiple intracellular proteins from tumor cell lines and patient tumor samples.

Quantification of single-cell proteins plays key roles in cell heterogeneity while due to technical limitations absolute numbers of multiple intracellular proteins from large populations of single cells were still missing, leading to compromised results in cell-type classifications. This paper presents a microfluidic platform capable of high-throughput absolute quantification of single-cell multiple types of intracellular proteins where cells stained with fluorescent labelled antibodies are aspirated into the constriction microchannels with excited fluorescent signals detected and translated into numbers of binding sites of targeted proteins based on calibration curves formed by flushing gradient solutions of fluorescent labelled antibodies directly into constriction microchannels. Based on this approach, single-cell numbers of binding sites of ß-actin, α-tubulin and ß-tubulin from tens of thousands of five representative tumor cell lines were first quantified, reporting cell-type classification rates of 83.0 ± 7.1%. Then single-cell numbers of binding sites of ß-actin, biotin and RhoA from thousands of five tumor cell lines with varieties in malignant levels were quantified, reporting cell-type classification rates of 93.7 ± 2.8%. Furthermore, single-cell numbers of binding sites of Ras, c-Myc and p53 from thousands of cells derived from two oral tumor lines of CAL 27, WSU-HN6 and two oral tumor patient samples were quantified, contributing to high classifications of both tumor cell lines (98.6%) and tumor patient samples (83.4%). In conclusion, the developed microfluidic platform was capable of quantifying multiple intracellular proteins from large populations of single cells, and the collected data of protein expressions enabled effective cell-type classifications.

Germanium/perovskite heterostructure for high-performance and broadband photodetector from visible to infrared telecommunication band.

A high-performance and broadband heterojunction photodetector has been successfully fabricated. The heterostructure device is based on a uniform and pinhole-free perovskite film constructed on top of a single-crystal germanium layer. The perovskite/germanium photodetector shows enhanced performance and a broad spectrum compared with the single-material-based device. The photon response properties are characterized in detail from the visible to near-infrared spectrum. At an optical fibre communication wavelength of 1550 nm, the heterojunction device exhibits the highest responsivity of 1.4 A/W. The performance is promoted because of an antireflection perovskite coating, the thickness of which is optimized to 150 nm at the telecommunication band. At a visible light wavelength of 680 nm, the device shows outstanding responsivity and detectivity of 228 A/W and 1.6 × 1010 Jones, respectively. These excellent properties arise from the photoconductive gain boost in the heterostructure device. The presented heterojunction photodetector provides a competitive approach for wide-spectrum photodetection from visible to optical communication areas. Based on the distinguished capacity of light detection and harvesting from the visible to near-infrared spectrum, the designed germanium/perovskite heterostructure configuration is believed to provide new building blocks for novel optoelectronic devices.

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.

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.