Zero-bias Operation Research Articles

Mid-infrared waveguide-integrated and photo-thermoelectric graphene photodetector based on germanium-on-silicon platform

Mid-infrared (MIR) photonic integration is desirable in the development of MIR spectroscopy and “lab-on-a-chip” sensing. The germanium-on-silicon (GOS) platform offers a promising solution for MIR photonic integration, extending the operational wavelength to a longer band by eliminating the light-absorbing buried oxide layer. However, MIR photodetectors on the GOS platform remain undeveloped due to the challenging heterogeneous integration of active materials on silicon and inadequate light absorption in the photodetection region. Here, we demonstrate a photo-thermoelectric graphene photodetector on the GOS platform, taking advantage of zero-bias operation and easy heterogeneous integration of graphene. By employing split-gate architecture and plasmonic enhancement to strengthen the light-graphene interaction, we achieve a responsivity of 1.97 V W−1 and noise equivalent power of 2.8 nW Hz−1/2 at the wavelength of 3.7 µm. This work enables waveguide-integrated MIR photodetection on the GOS platform for the first time, and it holds great potential for on-chip MIR sensing and imaging applications.

Type-printable photodetector arrays for multichannel meta-infrared imaging

Multichannel meta-imaging, inspired by the parallel-processing capability of neuromorphic computing, offers considerable advancements in resolution enhancement and edge discrimination in imaging systems, extending even into the mid- to far-infrared spectrum. Currently typical multichannel infrared imaging systems consist of separating optical gratings or merging multi-cameras, which require complex circuit design and heavy power consumption, hindering the implementation of advanced human-eye-like imagers. Here, we present printable graphene plasmonic photodetector arrays driven by a ferroelectric superdomain for multichannel meta-infrared imaging with enhanced edge discrimination. The fabricated photodetectors exhibited multiple spectral responses with zero-bias operation by directly rescaling the ferroelectric superdomain instead of reconstructing the separated gratings. We also demonstrated enhanced and faster shape classification (98.1%) and edge detection (98.2%) using our multichannel infrared images compared with single-channel detectors. Our proof-of-concept photodetector arrays simplify multichannel infrared imaging systems and offer potential solutions in efficient edge detection in human-brain-type machine vision.

Group-IV based pin photodetectors for C-band application

GeSn photodetectors with Sn contents between 0% and 6.5% were grown pseudomorphically on Ge virtual substrates on Si by molecular beam epitaxy. Based on the similar functional dependence of the current-voltage characteristics, we conclude that a similar crystal defect is the cause of the recombination/generation centers in both the GeSn and the Ge reference sample. We found that dislocations are not the predominant recombination type. The signal-to-background ratio is a proper quantitative measure for the comparison of heterostructure photodetectors and with near zero bias operation in the C-band, the values of around 104 were obtained with 1 mW light input. For C-band operation, the optimal Sn contents are found to be between 2% and 4%.

Self-powered perovskite photon-counting detectors.

Metal-halide perovskites (MHPs) have been successfully exploited for converting photons to charges or vice versa in applications of solar cells, light-emitting diodes and solar fuels1-3, for which all these applications involve strong light. Here we show that self-powered polycrystalline perovskite photodetectors can rival the commercial silicon photomultipliers (SiPMs) for photon counting. The photon-counting capability of perovskite photon-counting detectors (PCDs) is mainly determined by shallow traps, despite that deep traps also limit charge-collection efficiency. Two shallow traps with energy depth of 5.8 ± 0.8 millielectronvolts (meV) and 57.2 ± 0.1 meV are identified in polycrystalline methylammonium lead triiodide, which mainly stay at grain boundaries and the surface, respectively. We show that these shallow traps can be reduced by grain-size enhancement and surface passivation using diphenyl sulfide, respectively. It greatly suppresses dark count rate (DCR) from >20,000 counts per second per square millimetre (cps mm-2) to 2 cps mm-2 at room temperature, enabling much better response to weak light than SiPMs. The perovskite PCDs can collect γ-ray spectra with better energy resolution than SiPMs and maintain performance at high temperatures up to 85 °C. The zero-bias operation of perovskite detectors enables no drift of noise and detection property. This study opens a new application of photon counting for perovskites that uses their unique defect properties.

Zero Bias Operation: Photodetection Behaviors Obtained by Emerging Materials and Device Structures.

Zero-biased photodetectors have desirable characteristics for potentially next-generation devices, including high efficiency, rapid response, and low power operation. In particular, the detector efficiency can be improved simply by changing the electrode contact geometry or morphological structure of materials, which give unique properties such as energy band bending, photo absorbance and electric field distribution. In addition, several combinations of materials enable or disable the operation of selective wavelengths of light detection. Herein, such recent progresses in photodetector operating at zero-bias voltage are reviewed. Considering the advantages and promises of these low-power photodetectors, this review introduces various zero-bias implementations and reviews the key points.

Terahertz photodetection in scalable single-layer-graphene and hexagonal boron nitride heterostructures

The unique optoelectronic properties of single layer graphene (SLG) are ideal for the development of photonic devices across a broad range of frequencies from x rays to microwaves. In the terahertz (THz) frequency range (0.1–10 THz), this has led to the development of optical modulators, nonlinear sources, and photodetectors with state-of-the-art performances. A key challenge is the integration of SLG-based active elements with pre-existing technological platforms in a scalable way, while maintaining performance level unperturbed. Here, we report room temperature THz detectors made of large-area SLG, grown by chemical vapor deposition (CVD) and integrated in antenna-coupled field effect transistors. We selectively activate the photo-thermoelectric detection dynamics, and we employ different dielectric configurations of SLG on Al2O3 with and without large-area CVD hexagonal boron nitride capping to investigate their effect on SLG thermoelectric properties underpinning photodetection. With these scalable architectures, response times ∼5 ns and noise equivalent powers (NEPs) ∼1 nW Hz−1/2 are achieved under zero-bias operation. This shows the feasibility of scalable, large-area, layered material heterostructures for THz detection.

Highly efficient UV detection in a metal–semiconductor–metal detector with epigraphene

We show that epitaxial graphene on silicon carbide (epigraphene) grown at high temperatures (T >1850 °C) readily acts as material for implementing solar-blind ultraviolet (UV) detectors with outstanding performance. We present centimeter-sized epigraphene metal–semiconductor–metal (MSM) detectors with a peak external quantum efficiency of η ∼ 85% for wavelengths λ = 250–280 nm, corresponding to nearly 100% internal quantum efficiency when accounting for reflection losses. Zero bias operation is possible in asymmetric devices, with the responsivity to UV remaining as high as R = 134 mA/W, making this a self-powered detector. The low dark currents Io ∼ 50 fA translate into an estimated record high specific detectivity D = 3.5 × 1015 Jones. The performance that we demonstrate, together with material reproducibility, renders epigraphene technologically attractive to implement high-performance planar MSM devices with a low processing effort, including multi-pixel UV sensor arrays, suitable for a number of practical applications.

High-Speed 850 nm Photodetector for Zero-Bias Operation

High-speed photodetector operating at 850 nm wavelength with a large diameter and high quantum efficiency is desirable to meet the growing demands of short-reach optical links for high-performance computing systems. Zero-bias operation of the high-speed photodetectors can reduce power consumption, minimize system complexity of the optical transceivers and reduce the radiation damage in a harsh environment. Traditional p-i-n photodetectors for 850 nm applications often require a high reverse bias to accelerate the carrier transport for high-speed data transmission. In this work, we demonstrate a high-speed and low dark current modified uni-traveling-carrier photodiode based on GaAs/AlGaAs at 850 nm wavelength operating under zero bias with a quantum efficiency of 73%. The 3-dB bandwidth of the 20 μm and 40 μm diameter devices is 22.5 GHz and 13.3 GHz, respectively. A clear eye pattern is demonstrated at a 25.8 Gbit/s data rate for the device under zero-bias operation. To the best of our knowledge, this photodetector demonstrates the highest 3-dB bandwidth among all the zero-bias 850 nm photodetectors reported to date.

InAsSb-based detectors on GaSb for near-room-temperature operation in the mid-wave infrared

III-Sb barrier detectors suitable for the mid-wave infrared were grown on GaSb by molecular beam epitaxy. Using both bulk-InAsSb and an InAsSb–InAs strained layer superlattice, operation close to room temperature was demonstrated with cutoff wavelengths of 4.82 and 5.79 μm, respectively, with zero-bias operation possible for the bulk-InAsSb detector. X-ray diffraction, temperature dependent dark current, and spectral quantum efficiency were measured, and an analysis based on calculated specific detectivity was carried out. 1/f noise effects are considered. Results indicate that these optimized devices may be suitable as alternatives to InSb, or even HgCdTe, for many applications, especially where available power is limited.

Towards an optimal MIIM diode for rectennas at 10.6 μm

We investigate, for the first time, optimization of the responsivity of metal-insulator-insulator-metal (MIIM) diodes by carefully considering materials' properties. The diode's resistance is fixed at 100 Ω in order to match the nano antenna's impedance and to increase the total efficiency of the rectifying antenna (rectenna). The optimization is performed at zero-bias responsivity and resistance to ensure zero-bias operation. The diode's resistance and responsivity are calculated from the simulated current-voltage characteristics using Airy functions-based transfer matrix method (TMM) that was verified using experimental results. The design parameters of barrier heights for each insulator and metals, their dielectric constants, and the difference in the metals' work functions, are optimized to enhance the overall diode's performance. Optimal results are presented showing the beneficial effect of simultaneously varying these design parameters. Suggestions for implementing these optimal values with real materials are proposed and presented. Finally, the MIIM capacitance effect on the rectenna cut-off frequency is investigated to ensure that proposed MIIM diodes allow the rectenna to operate efficiently in the IR regime.

Heterogeneously Integrated Graphene/Silicon/Halide Waveguide Photodetectors toward Chip-Scale Zero-Bias Long-Wave Infrared Spectroscopic Sensing.

Mid-infrared absorption spectroscopy plays an important role in molecule identification and quantification for widespread applications. Integrated photonics provides opportunities to perform spectroscopic sensing on-chip for the minimization of device size, cost, and power consumption. The integration of waveguides and photodetectors is an indispensable step toward the realization of these on-chip sensing systems. It is desired to extend the operating wavelengths of these on-chip sensing systems to the long-wave infrared (LWIR) range to utilize more molecular absorption fingerprints. However, the development of LWIR waveguide-integrated photodetectors faces challenges from both waveguide platforms due to the bottom cladding material absorption and photodetection technologies due to the low LWIR photon energy. Here, we demonstrate LWIR waveguide-integrated photodetectors through heterogeneous integration of graphene photodetectors and Si waveguides on CaF2 substrates. A high-yield transfer printing method is developed for flexibly integrating the waveguide and substrate materials to solve the bottom cladding material absorption issue. The fabricated Si-on-CaF2 waveguides show low losses in the broad LWIR wavelength range of 6.3-7.1 μm. The graphene photodetector achieves a broadband responsivity of ∼8 mA/W in these low-photon-energy LWIR wavelengths under zero-bias operation with the help of waveguide integration and plasmonic enhancement. We further integrate the graphene photodetector with a Si-on-CaF2 folded waveguide and demonstrate on-chip absorption sensing using toluene as an example. These results reveal the potential of our technology for the realization of chip-scale, low-cost, and low-power-consumption LWIR spectroscopic sensing systems.

High-Speed Evanescently-Coupled Waveguide Type-II MUTC Photodiodes for Zero-Bias Operation

We demonstrate evanescently-coupled GaAs0.5Sb0.5/In0.53AlyGa0.47-yAs modified uni-traveling carrier (MUTC) waveguide photodiodes (PDs) with high bandwidth (BW) of 66 GHz at zero bias and over 100 GHz bandwidth under low bias condition. An InxAlyGa1-x-yAs dual-integrated waveguide-drift layer which can facilitate efficient light coupling from the waveguide into the absorber over a short length is utilized. A 7-μm long waveguide PD has an internal responsivity of 0.48 A/W at 1550 nm and clearly open eye diagrams at 40 Gb/s are demonstrated at zero bias.

Grain Boundary Josephson Junction Harmonic Mixer Coupled With a Bowtie Loaded Meander Antenna With Zero-Bias Operation

The high- T c superconducting (HTS) bicrystal YBa2Cu3O7–δ (YBCO) zero-bias-operation Josephson junction (JJ) harmonic mixer coupled with a bowtie loaded meander antenna (BLMA) is proposed and studied at 0.21 THz. When the JJ mixer coupled with the BLMA, the maximal harmonic number increases up to 146 with the conversion efficiency of −63.6 dB at the bath temperature around 4.6 K compared to a mixer coupled with a log-periodic antenna (LPA) of only a maximal harmonic number of 46 with the conversion efficiency of −71.6 dB. The mixer coupled with the BLMA has a higher order harmonic mixing with a lower local oscillator (LO) power of 1.26 dBm, which further illustrates its superiority to that coupled with a LPA with the LO power of 5.59 dBm. The experiment verifies the merits of the JJ harmonic mixer operating at zero bias as a promising device in terahertz regions.

High-Speed Characteristics of Uni-Traveling-Carrier Photodiode Under Bias-Free Operation

In this letter, we have conducted experiments on a traditional uni-traveling carrier photodiode (UTC-PD) and a previously modified UTC-PD under the zero-bias operational condition and make a comparison between them. The experimental results demonstrate that the modified UTC-PD is benefit to high speed, and the 3-dB bandwidth can achieve 40 GHz even when the active diameter of ${14}~{\mu }\text{m}$ under the zero-bias operation. Besides, based on the modified UTC-PD structure, we make some improvements in the collector to restrain the trend in which the bandwidth attenuated by the increase of photocurrent under the zero bias operation.

Optimized uni-traveling carrier photodiode and mushroom-mesa structure for high-power and sub-terahertz bandwidth under zero- and low-bias operation

In this paper, physically-based simulations are carried out to investigate and design uni-traveling carrier photodiode (UTC-PD) for high-power sub-terahertz wave generation at zero- and low-bias operation. The reliability of the physically-based simulation is demonstrated by comparing with our experimental result. Both the bandwidth and RF output power of the proposed UTC-PD is significantly improved by careful design the built-in electric field distribution under high-power input. For the optimized UTC-PD with the mesa diameter of 5 μm, its 3dB bandwidth large than 100 GHz even if the photocurrent reaches 6 mA under zero-bias operation. The device can reach a high bandwidth of 92.4 GHz, 105 GHz, and 119.5 GHz under the reverse bias of 0.5 V, 1 V, and 2 V, respectively, even the input photocurrent as high as 18.2 mA. The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero- or low-bias operation. Besides, a novel design of mushroom-mesa UTC-PD (MM-UTC-PD) is proposed which with 4.3% improved high-speed performance. The MM-UTC-PD can trade-off between the external quantum-efficiency and bandwidth when miniaturized junction size is required.

Silver-Nanowire-Embedded Transparent Metal-Oxide Heterojunction Schottky Photodetector.

We report a self-biased and transparent Cu4O3/TiO2 heterojunction for ultraviolet photodetection. The dynamic photoresponse improved 8.5 × 104% by adding silver nanowires (AgNWs) Schottky contact and maintaining 39% transparency. The current density-voltage characteristics revealed a strong interfacial electric field, responsible for zero-bias operation. In addition, the dynamic photoresponse measurement endorsed the effective holes collection by embedded-AgNWs network, leading to fast rise and fall time of 0.439 and 0.423 ms, respectively. Similarly, a drastic improvement in responsivity and detectivity of 187.5 mAW-1 and of 5.13 × 109 Jones, is observed, respectively. The AgNWs employed as contact electrode can ensure high-performance for transparent and flexible optoelectronic applications.

High frequency response of LWIR HgCdTe photodiodes operated under zero-bias mode

High frequency response is an important capability of infrared detectors in many applications. High-temperature long wavelength infrared HgCdTe heterostructure photodiodes exhibit sub-nanosecond time constants while operating under reverse bias. However, the noise, as well as the high current requirements are severe obstacles to their widespread applications. Thus, the present efforts are focused on a zero-bias operation of infrared detectors. A numerical modelling was used for investigation of the device design on the response time and current responsivity of HgCdTe photodiodes operating at 200 K and zero-bias mode. A formulation of the equations for carrier transport in semiconductors is presented in the Fourier space method in order to analyze spectrum characteristic of currents generated by harmonic optical signals. The method is valid in describing the high frequency response of IR devices based on HgCdTe multilayer heterostructures.

Zero-Bias Operation of CVD Graphene Photodetector with Asymmetric Metal Contacts

The responsivity of a graphene photodetector was substantially enhanced by modulating the potential gradient in a chemical vapor deposition-grown graphene channel using asymmetric metal contacts such as Ti, Pd, and Au for the source and drain. The photoresponsivity of a graphene photodetector with an asymmetric Au–Ti contact combination increased to 52 mA/W at an illumination power density of 0.25 μW/cm2; this photoresponsivity is ∼63 times higher than that of a graphene photodetector having a symmetric Au–Au contact combination (0.82 mA/W).

Optimization of a HOT LWIR HgCdTe Photodiode for Fast Response and High Detectivity in Zero-Bias Operation Mode

Fast response is an important property of infrared detectors for many applications. Currently, high-temperature long-wavelength infrared HgCdTe heterostructure photodiodes exhibit subnanosecond time constants while operating under reverse bias. However, nonequilibrium devices exhibit excessive low-frequency 1/f noise that extends up to MHz range, representing a severe obstacle to their widespread application. Present efforts are focused on zero-bias operation of photodiodes. Unfortunately, the time constant of unbiased photodiodes is still at the level of several nanoseconds. We present herein a theoretical investigation of device design for improved response time and detectivity of long-wavelength infrared HgCdTe photodiodes operating at 230 K in zero-bias mode. The calculation results show that highly doped \tp-type HgCdTe is the absorber material of choice for fast photodiodes due to its high electron diffusion coefficient. The detectivity of such a device can also be optimized by using absorber doping of NA = 1 × 1017 cm−3. Reduction of the thickness is yet another approach to improve the device response. Time constant below 1 ns is achieved for an unbiased photodiode with absorber thickness below 4 μm. A tradeoff between the contradictory requirements of achieving high detectivity and fast response time is expected in an optically immersed photodiode with very small active area.

E-beam evaporated ZnO thin films: Fabrication and characterization as UV detector

In the present paper, fabrication and structural, optical and electrical characterization of ZnO thin films grown by electron gun technique are reported and the performances of a prototype of UV photodetector based on them are illustrated. ZnO thin films, fabricated on sapphire by e-beam evaporation followed by a two-step ex situ treatment (annealing and oxidation), are polycrystalline, with a smooth surface and show very good visible transparency and an energy gap of 3.2eV. Preliminary results on fabrication and characterization of an UV detector are reported. The Al interdigitated contacts show a Schottky behavior, which is strongly desired in view of applications since it has many advantages in the aspects of high quantum efficiency, response time, low dark current, high UV/visible contrast and possible zero-bias operation.