The intrinsic photocurrent generation mechanism of a self-assembled graphene p–n junction operating at 1.55 μm is investigated experimentally.It is concluded that both a photovoltage effect and a photothermoelectric effect contribute to the final photocurrent.The photocurrent signal at the p–n junction was found to be dominated by photothermoelectric current,arising from different self-assembled doping levels.
We have provided optical simulations of the evanescently coupled waveguide photodiodes integrated with a 13-channels AWGs. The photodiode could exhibit high internal efficiency by appropriate choice of layers geometry and refractive index. Aseamless joint structure has been designed and fabricated for integrating the output waveguides of AWGs with the evanescently coupled waveguide photodiode array. The highest simulation quantum efficiency could achieve 92% when the matching layer thickness of the PD is 120 nm and the insertion length is 2 μm. The fabricated PD with 320-nm-thick matching layer and 2-μm-length insertion matching layer present a responsivity of 0.87 A/W.
We present the design of a diffractive grating structure and get the optimal parameters which can achieve more than 75%coupling efficiency(CE) between single-mode fiber and silicon-on-insulator(SOI) waveguide through 2D finite-different time-domain(FDTD) simulation.The proposed architecture has a uniform structure with no bottom reflection element or silicon overlay.The structure,including grating couplers,adiabatic tapers and interconnection waveguides can be fabricated on the SOI waveguide with only a single electron-beam lithography(ICP) step,which is CMOS-compatible.A relatively high coupling efficiency of 47.2%was obtained at a wavelength of 1562 nm.
Rongrui LiuYubing WangDongdong YinHan YeXiaohong YangQin Han
High-speed avalanche photodiodes are widely used in optical communication systems. Nowadays, separate absorption charge and multiplication structure is widely adopted. In this article, a structure with higher speed than separate absorption charge and multiplication structure is reported. Besides the traditional absorption layer,charge layer and multiplication layer, this structure introduces an additional charge layer and transit layer and thus can be referred to as separate absorption, charge, multiplication, charge and transit structure. The introduction of the new charge layer and transit layer brings additional freedom in device structure design. The benefit of this structure is that the carrier transit time and device capacitance can be reduced independently, thus the 3 dB bandwidth could be improved by more than 50% in contrast to the separate absorption charge and multiplication structure with the same size.
Graphene field-effect transistors have been intensively studied.However,in order to fabricate devices with more complicated structures,such as the integration with waveguide and other two-dimensional materials,we need to transfer the exfoliated graphene samples to a target position.Due to the small area of exfoliated graphene and its random distribution,the transfer method requires rather high precision.In this paper,we systematically study a method to selectively transfer mechanically exfoliated graphene samples to a target position with a precision of sub-micrometer.To characterize the doping level of this method,we transfer graphene flakes to pre-patterned metal electrodes,forming graphene field-effect transistors.The hole doping of graphene is calculated to be 2.16×10^(12) cm^(-2).In addition,we fabricate a waveguide-integrated multilayer graphene photodetector to demonstrate the viability and accuracy of this method.A photocurrent as high as 0.4 μA is obtained,corresponding to a photoresponsivity of 0.48 mA/W.The device performs uniformly in nine illumination cycles.
Recently, graphene-based photodetectors have been rapidly developed. However, their photoresponsivities are generally low due to the weak optical absorption strength of graphene. In this paper, we fabricate photoconductive multi-layer graphene(MLG) photodetectors on etched silicon-on-insulator substrates. A photoresponsivity exceeding 200 A·W-1is obtained, which enables most optoelectronic application. In addition, according to the analyses of the high photoresponsivity and long photoresponse time, we conclude that the working mechanism of the device is photoconductive effect. The process of photons conversion into conducting electrons is also described in detail. Finally, according to the distinct difference between the photoresponses at 1550 nm and 808 nm, we estimate that the position of the trapping energy is somewhere between 0.4 e V and 0.76 e V, higher than the Fermi energy of MLG. Our work paves a new way for fabricating the graphene photoconductive photodetectors.
In this paper,we present the design,fabrication,and measurement of an evanescently coupled waveguide photodetector operating at 1.55 μm,which mainly comprises a diluted waveguide,a single-mode rib waveguide and a p–i–n photodiode with an extended optical matching layer.The optical characteristics of this structure are studied by using a three-dimensional finite-difference time-domain(3D FDTD) method.The photodetector exhibits a high 3-dB bandwidth of more than 35 GHz and a responsivity of 0.291 A/W at 1550 nm directly coupled with a cleaved fiber.Moreover,a linear response of more than 72-mW optical power is achieved,where a photocurrent of more than 21 mA is obtained at a reverse bias voltage of 3 V.
Graphene is an alternative material for photodetectors owing to its unique properties.These include its uniform absorption of light from ultraviolet to infrared and its ultrahigh mobility for both electrons and holes.Unfortunately,due to the low absorption of light,the photoresponsivity of graphene-based photodetectors is usually low,only a few milliamps per watt.In this letter,we fabricate a waveguide-integrated graphene photodetector.A photoresponsivity exceeding0.11 A·W ^(-1) is obtained which enables most optoelectronic applications.The dominating mechanism of photoresponse is investigated and is attributed to the photo-induced bolometric effect.Theoretical calculation shows that the bolometric photoresponsivity is 4.6 A·W ^(-1).The absorption coefficient of the device is estimated to be 0.27 dB·μm ^(-1).
