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However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors.
Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination.
Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime 0.
Our results demonstrate improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.
Graphene has rapidly become an attractive candidate material for broadband and ultrafast photodetection because of its distinct optical and electronic characteristics 1 , 2. These characteristics stem from the unique band structure of graphene, which enables carrier generation via optical absorption over an extremely broad spectral range from the ultraviolet to microwave regime. Furthermore, the compatibility of graphene photodetectors with silicon-based fabrication platforms enables integration with low-cost and high-performance complementary metal oxide semiconductor read-out and post-processing circuits.
Most graphene photodetectors utilize graphene—metal junctions or graphene p—n junctions to spatially separate and extract photogenerated carriers. Various techniques have been explored to address these challenges and to enhance the responsivity of graphene photodetectors.
Despite the significant advantages of these techniques for offering high photodetection responsivities, the scope and potential use of existing graphene photodetectors remain limited by tradeoffs between high responsivity, an ultrafast temporal response, and broadband operation. For example, responsivity-enhanced photodetection in graphene from the visible to mid-infrared wavelengths has been achieved by increasing the photocarrier lifetime through band-structure engineering and defect engineering.
For this purpose, carrier trapping mechanisms and patterned graphene nanostructures have been utilized to introduce bandgap and midgap defect states in graphene 7 — However, the response times for these graphene photodetectors have been limited by long carrier trapping times due to the introduced defect and edge states.
Hybrid graphene—quantum dot photodetectors offer a powerful alternative for enhancing the photodetection responsivity by increasing light absorption and introducing large carrier multiplication factors 11 — However, the bandwidth and response time for this type of graphene photodetector are restricted by the narrow spectral bandwidth and long carrier trapping times of the quantum dots.
Photodetectors based on two graphene layers separated by a thin tunnel barrier have also led to enhanced broadband responsivity via separation of the photogenerated electrons and holes through quantum tunneling and minimization of their recombination However, the response times for this type of graphene photodetector have been limited by the long carrier trapping times in the tunneling barriers utilized. Waveguide-integrated graphene photodetectors have been considered to be another promising alternative and offer enhanced ultrafast responsivity by increasing the interaction length of light within graphene 16 — These graphene photodetectors have the additional advantage of process compatibility with standard photonic integrated circuits.
However, their spectral bandwidth has been restricted by the bandwidth limitations of the waveguides utilized. Moreover, microcavities, plasmonic structures, and optical antennae have all been integrated with graphene to achieve high responsivities by increasing the interaction length of light within graphene 20 — However, the bandwidth of these types of graphene photodetectors has been limited by the resonant nature of the structures utilized. In this work, we use engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which have unique electrical and optical characteristics that enable simultaneous broadband and ultrafast photodetection with high responsivity.
The key novelty that enables the superior performance of these photoconductive nanostructures is the confinement of most of the photocarrier generation and conduction to the graphene nano-stripes and gold patches, respectively.
Therefore, such nanostructures benefit from the broadband optical absorption and photocarrier multiplication capabilities of graphene and avoid the negative effects of the short photocarrier lifetime of graphene. Commercially available chemical vapor deposition CVD -grown graphene is first transferred to a high-resistivity silicon wafer covered with a nm-thick thermally grown SiO 2 layer.
The graphene nano-stripes are then patterned by another electron beam lithography step and formed by oxygen plasma etching. The responsivity values are calculated from the photocurrent measured using a source measure unit Keithley; SourceMeter , and the optical power measured using a calibrated near-infrared photodetector Thorlabs; SC.
The calorimeter is positioned at a distance of 1 cm from the Globar output, where the infrared intensity is uniform across the calorimeter input aperture. The uniformity of the infrared beam is confirmed by replacing the calorimeter with a graphene photodetector and monitoring its output photocurrent while moving it in the plane normal to the incident beam.
The responsivity values are calculated from the measured photocurrent and measured infrared power using the calorimeter and scaled according to the ratio between the active area of the graphene photodetector and calorimeter.
The operation principle of the photodetector, which is based on utilization of the photoconductive nanostructures, is illustrated in Fig. The carrier concentration and sheet resistance of the graphene nano-stripes used in this study are measured to be 1.
The photoconductive nanostructures are formed by connecting arrays of nanoscale gold patches to either side of the graphene nano-stripes. The geometry of the gold patches is engineered to concentrate a major portion of the incident optical beam onto the graphene nano-stripes over a broad optical spectrum ranging from the visible to infrared regime.
The graphene nano-stripes are designed to be narrower than the effective metal—graphene junction regions, where the photogenerated electrons and holes separate. This design enables a fast photocarrier transit time to the gold patches under an applied bias voltage because this transit time is much faster than the graphene photocarrier lifetime.
Therefore, unlike previously demonstrated graphene photodetectors, which show high photoconductive gains via an increase in the photocarrier lifetime, the gold-patched graphene nano-stripes afford high photoconductive gains by reducing the photocarrier transport time to the gold patches. The photocarriers transported to the gold patches are all collected together illustrated by red arrows in Fig. High-responsivity and broadband photodetection via gold-patched graphene nano-stripes.
The photodetector is fabricated on a high-resistivity Si substrate coated with a nm-thick SiO 2 layer. The gate voltage applied to the Si substrate, V g , controls the Fermi energy level of the graphene nano-stripes. Numerical finite difference time domain simulations Lumerical are carried out to demonstrate the unique capability of the designed gold patches to efficiently concentrate the incident optical beam onto the graphene nano-stripes over a broad optical wavelength range.
Despite the broadband optical coupling to the graphene nano-stripes, the optical absorption in graphene is estimated to be highly wavelength dependent, as shown in Fig.
This strong wavelength dependence stems from the optical absorption in graphene being dominated by interband transitions in the visible and near-infrared spectral ranges and by intraband transitions in the infrared spectral range 33 , leading to much lower optical absorption in the visible and near-infrared regimes.
Gold-patched graphene nano-stripes can exploit the enhanced carrier multiexcitation generation that occurs at higher photon energy levels to compensate for the lower optical absorption at lower wavelengths 3 — 5. Such carrier multiplication factors have not been previously exploited in monolayer graphene photodetectors without the use of quantum dots because of the short photocarrier lifetime in graphene 11 , However, carrier multiplication can be used to boost the photoconductive gain of the gold-patched graphene nano-stripes at lower wavelengths because of the fast photocarrier transport time to the gold patches.
This figure also shows a scanning electron microscopy image of the gold patches for further details, see the Methods.
The responsivity spectrum for the fabricated photodetector at an optical power of 2. The photodetector has an ultrabroad operation bandwidth from the visible to the infrared regime with high-responsivity levels ranging from 0. This graphene photodetector exhibits the widest photodetection bandwidth with the high responsivity reported to date, which are enabled by the use of the gold-patched graphene nano-stripes.
In the following sections, we describe how even higher responsivity levels can be realized when the bias and gate voltages are optimized for the photodetector. In addition, the asymmetric geometry of the gold-patched graphene nano-stripes leads to a highly polarization-sensitive responsivity Supplementary Fig.
This strong polarization sensitivity for the photodetector has many applications in polarimetric imaging and sensing systems. The photoconductive gain of the fabricated photodetector is calculated from the measured responsivity and estimated optical absorption in graphene, as shown in Fig. As expected, higher photoconductive gains are obtained at lower wavelengths for which the photogenerated electrons are excited to higher energy levels in the conduction band.
Excitation to higher energy levels gives rise to the excitation of secondary electron—hole pairs by transferring more energy during relaxation, as illustrated in the inset of Fig. The effect of the gate voltage is best described by the device band diagrams at various gate voltages Fig.
This tuning modifies the band-bending slope at different gate voltages a detailed analysis of the device band diagram is included in Supplementary Fig. When an optical beam is incident on the device, the photogenerated electrons and holes move according to the induced electric field determined by the band-bending slope. Therefore, the photogenerated holes move to the center of the graphene nano-stripes, where they eventually recombine, with the photogenerated electrons moving to the anode and cathode junctions.
The induced photocurrent is proportional to the difference in the number of photogenerated electrons that drift to the anode and cathode junctions Supplementary Fig.
Impact of the gate voltage on the photodetector performance in the visible and near-infrared regimes. The carrier concentration in the graphene nano-stripes used in this study is measured to be 1.
The responsivity values at each gate voltage are divided by the photodetector responsivity at nm to eliminate the influence of variations in the band diagram at different gate voltages. The dashed lines show the predicted responsivity spectra assuming interband transitions are allowed over the entire wavelength range.
As the gate voltage decreases from 65 V, the p-type carrier concentration in the graphene nano-stripes increases and a steeper energy band bending is introduced on the anode side compared to the cathode side 34 , The variation in the band-bending slopes leads to an increase in the induced photocurrent when the gate voltage decreases.
The highest responsivity level is obtained at a gate voltage of 22 V. The photodetector responsivity decreases at higher optical powers Supplementary Fig. This decrease is due to the increase in the carrier recombination rate at high photogenerated carrier densities, which reduces the carrier scattering time and carrier multiplication efficiency 3 — 5.
Larger area gold-patched graphene nano-stripe arrays can be used to maintain high photodetection efficiencies at high optical powers. As expected, the measured photodetector responsivity shows a linear dependence on the applied bias voltage Supplementary Fig. This dependence suggests that higher responsivity levels can be realized by increasing the bias voltage at the expense of an increased dark current.
The impact of the gate voltage is best described by the graphene band diagrams at various gate voltages and wavelengths Fig. These diagrams show that the gate voltage tunes the Fermi energy level, which changes the number of available states because of the cone-shaped band diagram of graphene.
Since a larger number of states are available at higher Fermi energies, the photodetector responsivity values are increased following a decrease in the gate voltage at all wavelengths. At a given Fermi energy level, a larger number of states is available to be filled by lower-energy photons, which results in an increase in photodetector responsivity values at longer wavelengths.
Responsivity values as high as The measurement bandwidth of our experimental setup was limited by the detection bandwidth of the calibrated infrared detector used for the measurements for further details, see the Methods.
Influence of the gate voltage on the photodetector performance in the infrared regime. The inset shows the estimated noise current as a function of the gate voltage. One of the drawbacks of the presented photodetector based on gold-patched graphene nano-stripes is the relatively large dark current that occurs due to the photoconductive nature of the photodetector. Therefore, the noise equivalent power NEP of the fabricated photodetector is calculated to assess the noise performance. Under this operation condition, the photodetector noise current, which is dominated by Johnson Nyquist and shot-noise sources, is extracted from the measured photocurrent and resistance data Supplementary Fig.
A high-frequency electrical model of the graphene photodetector is shown in Fig. Additionally, the gold-patch capacitance, C g , and substrate resistance, R sub , are estimated to be As can be observed from the device electrical model, the use of a high-resistivity silicon substrate leads to a large resistance, R sub , in series with the SiO 2 capacitance, C ox , eliminating the negative impact of this capacitance on the ultrafast photodetection speed.
Therefore, the photodetector frequency response predicted by this electrical model is dominated by the parasitic resistance of the graphene nano-stripes and parasitic capacitance of the gold patches, leading to a predicted photoresponse cutoff frequency of GHz.
Characterization of the operation speed of the fabricated photodetector. The beams from two wavelength-tunable DFB lasers at frequencies of f 1 and f 2 are focused onto the gold-patched graphene nano-stripes to generate a photocurrent, I ph , at the optical beating frequency, f 1 — f 2.
The measured photoresponse values exhibit no roll-off up to 50 GHz, which is the frequency limitation of the utilized experimental setup. The dashed lines show the estimated frequency response for the graphene photodetector calculated from the device electrical model.
RLE Recent Papers
This mix of the exploration of new ideas and their reduction to practice remains the hallmark of the present-day RLE. Abstract: Graphene-based photodetectors have attracted strong interest for their exceptional physical properties, which include an ultrafast response across a broad spectrum4, a strong electron—electron interaction5 and photocarrier multiplication However, the weak optical absorption of graphene limits its photoresponsivity. To address this, graphene has been integrated into nanocavities, microcavities and plasmon resonators, but these approaches restrict photodetection to narrow bands. Hybrid graphene—quantum dot architectures can greatly improve responsivity, but at the cost of response speed.
Chip-integrated ultrafast graphene photodetector with high responsivity
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Heinz and Kenneth L. A chip-integrated graphene photodetector with a high responsivity of over 0. View via Publisher. Save to Library.