Graphene-silicon Hybrid Research Articles

High Photocurrent in Gated Graphene-Silicon Hybrid Photodiodes.

Graphene/silicon (G/Si) heterojunction based devices have been demonstrated as high responsivity photodetectors that are potentially compatible with semiconductor technology. Such G/Si Schottky junction diodes are typically in parallel with gated G/silicon dioxide (SiO2)/Si areas, where the graphene is contacted. Here, we utilize scanning photocurrent measurements to investigate the spatial distribution and explain the physical origin of photocurrent generation in these devices. We observe distinctly higher photocurrents underneath the isolating region of graphene on SiO2 adjacent to the Schottky junction of G/Si. A certain threshold voltage (VT) is required before this can be observed, and its origins are similar to that of the threshold voltage in metal oxide semiconductor field effect transistors. A physical model serves to explain the large photocurrents underneath SiO2 by the formation of an inversion layer in Si. Our findings contribute to a basic understanding of graphene/semiconductor hybrid devices which, in turn, can help in designing efficient optoelectronic devices and systems based on such 2D/3D heterojunctions.

Graphene-silicon hybrid photonic integrated circuits

Silicon photonics is considered as a promising technology to realize high-performance photonic integrated circuits, owing to its complementary metal oxide semiconductor-compatibility which is applicable for large-scale integration at low cost. However, due to the limitation of optoelectronic properties of silicon, the challenge to the realization of high-performance active device on the silicon integrated platform still exists. The recent development of graphene-silicon hybrid photonic integrated circuit provides a practical solution to this problem, because graphene, as a superior two-dimensional material, possesses many advantageous optoelectronic properties, such as high mobility, high electro-optical coefficient, and broadband absorption, which can be fully exploited to break through the material limitation of silicon. Moreover, compared with other active integrated materials such as germanium and compound semiconductors, graphene is cost-effective and can be conveniently integrated with silicon photonic device. Here, we review some important research progress of graphene-silicon hybrid photonic integrated circuits that include optical sources, optical waveguides, optical modulators, and photodetectors. The challenges and prospects of these devices are also analyzed, which are expected to be beneficial to the relevant research communities.

Polarization-independent terahertz wave modulator based on graphene-silicon hybrid structure**Project supported by the National Natural Science Foundation of China (Grant No. 61565004), the Natural Science Foundation of Guangxi Zhuang Autonomous Region, China (Grant Nos. 2013GXNSFDA019002 and 2014GXNSFGA118003), the Guangxi Scientific Research and Technology Development Program, China (Grant No. 1598017-1), the Guilin Scientific Research and Technology Development Program, China (Grant Nos. 20140127-1

In this study, we propose and demonstrate a broadband polarization-independent terahertz modulator based on graphene/silicon hybrid structure through a combination of continuous wave optical illumination and electrical gating. Under a pump power of 400 mW and the voltages ranging from –1.8 V to 1.4 V, modulation depths in a range of –23%–62% are achieved in a frequency range from 0.25 THz to 0.65 THz. The modulator is also found to have a transition from unidirectional modulation to bidirectional modulation with the increase of pump power. Combining the Raman spectra and Schottky current–voltage characteristics of the device, it is found that the large amplitude modulation is ascribed to the electric-field controlled carrier concentration in silicon with assistance of the graphene electrode and Schottky junction.

Enhancing cycling durability of Li-ion batteries with hierarchical structured silicon-graphene hybrid anodes.

Hybrid anode materials consisting of micro-sized silicon (Si) particles interconnected with few-layer graphene (FLG) nanoplatelets and sodium-neutralized poly(acrylic acid) as a binder were evaluated for Li-ion batteries. The hybrid film has demonstrated a reversible discharge capacity of ∼1800 mA h g-1 with a capacity retention of 97% after 200 cycles. The superior electrochemical properties of the hybrid anodes are attributed to a durable, hierarchical conductive network formed between Si particles and the multi-scale carbon additives, with enhanced cohesion by the functional polymer binder. Furthermore, improved solid electrolyte interphase (SEI) stability is achieved from the electrolyte additives, due to the formation of a kinetically stable film on the surface of the Si.

Proposed high-speed micron-scale spatial light valve based on a silicon-graphene hybrid structure.

We propose a new ultracompact CMOS-compatible variable-transmission spatial light valve based on a silicon-graphene hybrid structure. Normally incident ∼1560 nm light can be coupled to a silicon-graphene-based 1D photonic crystal cavity through a perturbation-based diffractive coupling scheme. The lightwave modulation is achieved by tuning the Fermi level of the graphene, which can change both the loss and the resonant wavelength of the cavity. Based on finite-difference time-domain simulation, the modulation depth is larger than 10 dB with driving voltage of about 4.8 V(pp) while the modulation speed is estimated to be higher than 45 GHz.

Analysis of an electro-optic modulator based on a graphene-silicon hybrid 1D photonic crystal nanobeam cavity.

We propose and numerically study an on-chip graphene-silicon hybrid electro-optic (EO) modulator operating at the telecommunication band, which is implemented by a compact 1D photonic crystal nanobeam (PCN) cavity coupled to a bus waveguide with a graphene sheet on top. Through electrically tuning the Fermi level of the graphene, both the quality factor and the resonance wavelength can be significantly changed, thus the in-plane lightwave can be efficiently modulated. Based on finite-difference time-domain (FDTD) simulation results, the proposed modulator can provide a large free spectral range (FSR) of 125.6 nm, a high modulation speed of 133 GHz, and a large modulation depth of ~12.5 dB in a small modal volume, promising a high performance EO modulator for wavelength-division multiplexed (WDM) optical communication systems.

Enhanced parametric frequency conversion in a compact silicon-graphene microring resonator.

We demonstrate four-wave mixing (FWM) in a 10-μm-radius silicon microring resonator with the assistance of giant nonlinearity of the monolayer graphene. A maximum enhancement of 6.8 dB of conversion efficiency in the silicon-graphene microring (SGM) resonator is observed. A nonlinear propagation model is established and the optical Kerr coefficient of the silicon-graphene hybrid waveguide is three times larger than that of the silicon waveguide.

Active graphene-silicon hybrid diode for terahertz waves.

Controlling the propagation properties of the terahertz waves in graphene holds great promise in enabling novel technologies for the convergence of electronics and photonics. A diode is a fundamental electronic device that allows the passage of current in just one direction based on the polarity of the applied voltage. With simultaneous optical and electrical excitations, we experimentally demonstrate an active diode for the terahertz waves consisting of a graphene–silicon hybrid film. The diode transmits terahertz waves when biased with a positive voltage while attenuates the wave under a low negative voltage, which can be seen as an analogue of an electronic semiconductor diode. Here, we obtain a large transmission modulation of 83% in the graphene–silicon hybrid film, which exhibits tremendous potential for applications in designing broadband terahertz modulators and switchable terahertz plasmonic and metamaterial devices.

Theoretical study of graphene-silicon nitride-silicon hybrid photonic crystal waveguides for four-wave mixing enhancement

In this paper, we propose and theoretically investigate a silicon-based hybrid nonlinear photonic device with a silicon nitride (Si3N4) film sandwiched between a graphene sheet and a silicon photonic crystal waveguide (PCW). With properly designed structural parameters of the silicon PCW and the presence of Si3N4 buffer layer, enhanced interaction between the optical field and graphene can be achieved to improve four-wave mixing (FWM) process in the device. The influences of the silicon PCW and the Si3N4 buffer layer thicknesses on FWM conversion efficiency are also analyzed. Numerical simulation results show that up to 19.66 dB improvement of the FWM conversion efficiency can be obtained in the proposed device.

Dual control of active graphene–silicon hybrid metamaterial devices

Metamaterials offer enormous opportunities and unprecedented functionalities to manipulate electromagnetic waves enabling promising applications such as invisibility cloaking, superfocusing and subwavelength confinement. The exotic electromagnetic behavior of metamaterials was dramatically empowered by dynamic control through incorporation of active media. A prominent example is a graphene metamaterial – an integration of a two-dimensional monolayer graphene with a planar metasurface, where the unique optical and electronic properties of graphene is inherited by the metamaterial, thus opening up fascinating possibilities in electromagnetic wave control. Here, through a combination of continuous wave (CW) optical illumination and electrical gating, we demonstrate a giant active modulation of terahertz waves in a graphene–silicon hybrid metamaterial at extremely low bias voltages. The highly tunable characteristics of the graphene metamaterial device under electrical bias and optical illumination open up new avenues for graphene-based high performance integrated active photonic devices compatible with the silicon technology.

Local and nonlocal optically induced transparency effects in graphene-silicon hybrid nanophotonic integrated circuits.

Graphene is well-known as a two-dimensional sheet of carbon atoms arrayed in a honeycomb structure. It has some unique and fascinating properties, which are useful for realizing many optoelectronic devices and applications, including transistors, photodetectors, solar cells, and modulators. To enhance light-graphene interactions and take advantage of its properties, a promising approach is to combine a graphene sheet with optical waveguides, such as silicon nanophotonic wires considered in this paper. Here we report local and nonlocal optically induced transparency (OIT) effects in graphene-silicon hybrid nanophotonic integrated circuits. A low-power, continuous-wave laser is used as the pump light, and the power required for producing the OIT effect is as low as ∼0.1 mW. The corresponding power density is several orders lower than that needed for the previously reported saturated absorption effect in graphene, which implies a mechanism involving light absorption by the silicon and photocarrier transport through the silicon-graphene junction. The present OIT effect enables low power, all-optical, broadband control and sensing, modulation and switching locally and nonlocally.

Dynamic microstructural evolution of graphite under displacing irradiation

Graphitic materials and graphite composites experience dimensional change when exposed to radiation-induced atomic displacements. This has major implications for current and future technological ranging from nuclear fission reactors to the processing of graphene–silicon hybrid devices. Dimensional change in nuclear graphites is a complex problem involving the filler, binder, porosity, cracks and atomic-level effects all interacting within the polygranular structure. An improved understanding of the atomistic mechanisms which drive dimensional change within individual graphitic crystals is required to feed into the multiscale modelling of this system.In this study, micromechanically exfoliated samples of highly oriented pyrolytic graphite have been ion irradiated and studied in situ using transmission electron microscopy (TEM) in order to gain insights into the response of single graphitic crystals to displacing radiation. Under continuous ion bombardment, a complex dynamic sequence of deformation evolves featuring several distinct stages from the inducement of strain, the creation of dislocations leading to dislocation arrays, the formation of kink band networks and localised doming of the sample. Observing these ion irradiation-induced processes using in situ TEM reveals previously unknown details of the sequence of microstructural developments and physics driving these phenomena. A mechanistic model consistent with the microstructural changes observed is presented.

Optical bistability and free carrier dynamics in graphene–silicon photonic crystal cavities

We introduce graphene–silicon hybrid nonlinear devices operating at a few femtojoule cavity circulating energies, including: (1) dual- and single-cavity optical bistability; (2) detailed and broadband switching dynamics, and (3) free-carrier and thermal effects in regenerative oscillations. Sub-wavelength nanostructures confine light in a single mode silicon resonator with high Q/V ratio, enabling strong light interaction with the graphene cladding layer.

Regenerative oscillation and four-wave mixing in graphene optoelectronics

The unique linear and massless band structure of graphene, in a purely two-dimensional Dirac fermionic structure, have led to intense research spanning from condensed matter physics to nanoscale device applications covering the electrical, thermal, mechanical and optical domains. Here we report three consecutive first-observations in graphene-silicon hybrid optoelectronic devices: (1) ultralow power resonant optical bistability; (2) self-induced regenerative oscillations; and (3) coherent four-wave mixing, all at a few femtojoule cavity recirculating energies. These observations, in comparison with control measurements with solely monolithic silicon cavities, are enabled only by the dramatically-large and chi(3) nonlinearities in graphene and the large Q/V ratios in wavelength-localized photonic crystal cavities. These results demonstrate the feasibility and versatility of hybrid two-dimensional graphene-silicon nanophotonic devices for next-generation chip-scale ultrafast optical communications, radio-frequency optoelectronics, and all-optical signal processing.