WO2021019533A1 - Graphene schottky varactor diodes - Google Patents

Graphene schottky varactor diodes Download PDF

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Publication number
WO2021019533A1
WO2021019533A1 PCT/IL2020/050831 IL2020050831W WO2021019533A1 WO 2021019533 A1 WO2021019533 A1 WO 2021019533A1 IL 2020050831 W IL2020050831 W IL 2020050831W WO 2021019533 A1 WO2021019533 A1 WO 2021019533A1
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Prior art keywords
graphene
schottky
varactor diode
diode
photoresponse
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PCT/IL2020/050831
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French (fr)
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Doron NAVEH
Moshe KIRSHNER
Adi LEVI
Avraham TWITTO
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Bar-Ilan University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/1606—Graphene
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
    • H01L29/92—Capacitors having potential barriers
    • H01L29/93—Variable capacitance diodes, e.g. varactors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier
    • H01L31/108—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier the potential barrier being of the Schottky type

Definitions

  • the present invention relates to graphene Schottky varactor diodes and to the use thereof for high-performance photodetection.
  • Photodetection in gapless graphene has been an active area of research over the last decade, where photovoltaic, bolometric, and photothermoelectric physics were studied in depth.
  • Photoresponse of pristine graphene devices is fundamentally limited by the low absorption cross-section and can be enhanced by plasmonic effects, optical cavities, semiconductor interfaces, quantum dot and other absorption enhancing agents.
  • the progress of graphene photoconductive devices is yet limited by the sizable leakage currents that introduce noise and complicate the coupling with complementary metal-oxide-semiconductor (CMOS) capacitive integrators for digital imaging.
  • CMOS complementary metal-oxide-semiconductor
  • Varactors are voltage-tunable capacitors and can therefore be utilized for impedance matching, as oscillators and modulators in the radiofrequency to terahertz range.
  • Modern varactor devices are typically fabricated from Schottky diodes and semiconductor heterostructures. Recently, the quantum capacitance of graphene was utilized to construct tunable metaloxide-semiconductor (MOS) capacitors.
  • MOS metaloxide-semiconductor
  • the invention relates to a graphene Schottky varactor diode comprising: a. a graphene channel on a substrate;
  • the graphene Schottky varactor diode of the invention is adapted to: a. rectify l-V transfer curves with a voltage threshold of at least 0.1 V b. provide a photoresponse increase with modulation frequency
  • c. provide a photoresponse of pure graphene of at least 100 mA/W; and d. provide a photoresponse independent from DC bias.
  • a typical graphene Schottky varactor diode of the invention has one or more of the following characteristics: a. NEP of 10 14 WHz 1/2 at a bias of 50 mV;
  • the graphene Schottky varactor diode of the invention has a pure capacitive character at the resonant frequency of 10 kHz.
  • the graphene channel is deposited by chemical vapor deposition (CVD).
  • the graphene Schottky varactor diode of the invention has a photoresponse that is enabled by the illumination of light from the visible spectrum, and in another embodiment it has a photoresponse that is enabled by the illumination of light from the infrared (IR) spectrum.
  • the diode is an IR-enhanced diode, it comprises an additional periodic metallic plasmonic structure on top of the graphene mesh.
  • the graphene Schottky varactor diode of the invention may comprise a low-gap semiconductor located under the graphene channel, thereby to enhance the photoresponse of said diode.
  • the low-gap semiconductor is black phosphorus.
  • the graphene Schottky varactor diode of the invention is useful in a variety of applications, for instance for use in LIDAR or 3D laser scanning applications.
  • the invention also encompasses a process for preparing a graphene Schottky varactor diode, comprising performing CVD of graphene on a substrate, and thereafter depositing Schottky contacts, to obtain the graphene Schottky varactor diode.
  • the substrate is P+-Si/Si02
  • the Schottky contacts comprise metallized layers of Ge/Al/Pd.
  • Fig. 1(a) shows an optical micrograph of transfer-length modulated graphene Schottky varactor diodes, scale bar is 5pm;
  • Fig. 1(b) is a graphic illustration of the device structure with Pd/AI/Ge contacts
  • Fig. 1(c) shows l-V transfer curves at temperature range of 257-342 K
  • Fig. 1(d) shows the temperature dependence of the diode saturation current (dots) and fitted Schottky barrier height (slope of solid line);
  • Fig. 1(e) shows voltage-resolved capacitance and current
  • Fig. 2(a) shows the Graphene Varactor voltage
  • Fig. 2(b) shows the Graphene Varactor frequency
  • Fig. 2(c) shows the Graphene Varactor resolved cutoff frequency resolved relative photocurrent (left axis, dots) and Q-factor of graphene varactor diodes (right axis; thick solid line: deduced from measured impedance; thin solid line: simulation).
  • Fig. 2(d) is the time-resolved photocurrent at 120 kHz modulation of the laser light;
  • Fig. 3(a) shows the photoresponsivity of graphene photodetectors;
  • Fig. 3(b) shows the external quantum efficiency (EQE) of graphene photodetectors, defined as the ratio between the photocurrent and the incoming photon flux;
  • Fig. 3(c) shows the photoconductive gain of graphene photodetectors (excess of internal quantum efficiency (IQE) from 100% and IQE is the ratio of photocurrent to the absorbed photon flux as function of illumination power;
  • Fig. 3(d) shows the noise equivalent power of graphene photodetectors as a function of frequency recorded with DC bias of 50 mV and corresponding to a photoresponsivity of 0.1 A/W;
  • Fig. 4(b) Voltage and frequency dependence of graphene varactor: photocurrent (the rising graph) and phase (the descending graph) as function of laser modulation frequency;
  • Fig. 4(c) Voltage and frequency dependence of graphene varactor: frequency-dependence of the phase lag measured by electrically excited currents (bottom graph) and optically excited photocurrents (top graph);
  • Fig. 5(a) Absorption spectrum of graphene hole mesh (bottom) and gold filling in graphene holes (top);
  • Figs. 6(a-d) are optical images of black phosphorus crystal below the graphene channel in several device structure embodiments
  • Fig. 6(e) shows photocurrent versus voltage
  • Fig. 6(f) shows the current-voltage characteristics for symmetric varactor diode.
  • Graphene varactors are devices designed to detect the differential of illumination in time.
  • the enhanced infrared sensation of such devices is therefore very appealing for many applications including night vision and distance measurements.
  • the enhancement of the IR response of graphene is enabled by an additional plasmonic element in the design structure of the varactor diode, the IR response being in the range ⁇ 700nm to ⁇ lmm.
  • the IR enhancement is enabled by the addition of low- gap semiconductors.
  • the graphene Schottky varactor is responsive in the visible light range i.e., wavelength of ⁇ 380 to ⁇ 700 nm.
  • the graphene Schottky varactor diodes are further characterized by characteristics selected from: a. NEP (Noise Equivalent Power) of 10 14 WHz 1/2 at a bias of 50 mV; b. a Schottky barrier height (SBH) of 0.16 eV;
  • the graphene Schottky varactor diodes of the present invention have pure capacitive character, at the resonant frequency of 10 kHz. Therefore, illumination of the device close to a frequency of about 10 kHz drives a higher oscillation amplitude of photocarriers, even without an applied DC voltage.
  • the graphene Schottky varactor diodes of the present invention are intrinsically immune to low-frequency noise owing to the characteristic capacitive frequency cutoff, and are therefore sensitive to rapid changes in illumination rather than to the incident optical power.
  • the invention also encompasses a process for preparing a graphene Schottky varactor diode, comprising performing CVD of graphene on a suitable substrate, for instance P+-Si/Si02, and further depositing Schottky contacts to obtain the graphene Schottky varactor diode.
  • Suitable Schottky contacts may be, for instance, metallized layers of Ge/AI/Pd.
  • the graphene Schottky varactor diodes of the present invention are useful in a variety of applications, including but not limited to LIDAR (3D laser scanning) applications including, such as 3D photography, high sensitivity photodetection and depth imaging, distance measurements, and high-speed communications.
  • LIDAR 3D laser scanning
  • 3D photography high sensitivity photodetection and depth imaging
  • distance measurements distance measurements
  • high-speed communications high-speed communications
  • the novel device architecture of the present invention provides graphene Schottky diode varactors comprising a graphene channel in between two Schottky contacts that is illuminated.
  • the graphene varactor diodes resolve some of the fundamental limitations in graphene optoelectronics, including the elimination of dark currents and the enhancement of the external quantum efficiency (EQE).
  • the devices of the present invention possess a large photoconductive gain and EQE of up to 37%, fast photoresponse, and low leakage currents at room temperature.
  • the graphene Schottky varactor diodes of the invention have a record high photoresponse for a pure graphene device of at least 100 mA/W, for example of 160 mA/W.
  • thin film polycrystalline semiconductor contacts are applied to a graphene channel to construct lateral Schottky varactor diodes, as shown in Figs. l(a)-l(b).
  • the device consists of a sensing material (e.g. the graphene rectangle in Fig. 1(a) and the middle part of Fig. 1(b)) enclosed by 2 Schottky contacts that due to the graphene's quantum capacitance create a Varactor. Photons hit the sensing material, some of which are absorbed and converted to charge carriers. The charge carriers are coupled by the Varactor capacitance to the Schottky contacts and can be read by a read out circuitry or read out equipment. The capacitance coupling inherently supports efficient read out of the changes in the illumination levels intensity that is translated to changes of the charge carriers intensity.
  • a sensing material e.g. the graphene rectangle in Fig. 1(a) and the middle part of Fig. 1(b)
  • 2 Schottky contacts that due to the graphene's quantum capacitance create a Varactor. Photons hit the sensing material, some of which are absorbed and converted to charge carriers.
  • the charge carriers are coupled by the Vara
  • the graphene layer in Figs. 1(a) and 1(b) has 3 main roles in the device: first to absorb and convert photons to charge carriers, second to make the Schottky contacts by its interface with the semiconductor part of the contacts (e.g. Ge in Fig. 1(b)), third to create the Varactor due to the graphene's quantum capacitance.
  • the device sensing material In order to tune the device to the IR spectrum, the device sensing material has to be tuned accordingly. Two different methods are defined for the device sensing material IR tuning:
  • Figs. 5(a) and 5(b) Tune the graphene layer for IR absorbance enhancement by creating graphene hole mesh as can be seen from Figs. 5(a) and 5(b).
  • the graphene rectangle in Fig. 1 (a) is replaced by a graphene hole mesh rectangle as appear in the SEM micrograph in Fig. 5(b).
  • the graphene absorbance spectrum is controlled by the holes pitch, the holes diameter and the holes fill (empty holes vs. gold or other metal filled holes that support plasmonic gain in the IR spectrum).
  • Fig. 5(a) demonstrates the IR absorbance significant enhancement by gold filled graphene holes vs. empty graphene holes as was measured by an FTIR.
  • the graphene layer keeps its 3 roles as described above.
  • Fig. 6(a) through 6(d) display several configurations of black phosphorus layers as an IR sensing material below the graphene layer.
  • Fig. 6(e) shows the device photocurrent at 1550nm
  • Fig. 6(f) shows the symmetric current-voltage characteristics of the Varactor.
  • the graphene Schottky varactor diodes of the present disclosure are prepared from CVD graphene on a substrate followed by deposition of Schottky contacts.
  • the substrate is made of insulating or semi-insulating materials such as polymers, glass and semiconductors, for example GaAs, GaN, InP, GaP, Ge, InSb, InAs, and their alloys.
  • One particularly suitable illustrative substrate is P+-Si/Si0 2 .
  • the substrate is flexible.
  • the substrate is transparent.
  • the size of the substrate is from about 100 nm to several microns, for example, about 285nm.
  • the Schottky contacts may comprise layers of Ge/AI/Pd, T1O2 or Si.
  • the Schottky contacts comprise metallized layers of Ge/AI/Pd.
  • the metallized Ge/AI/Pd layers thickness is set to maintain the Schottky contact, for example, the metallized Ge/AI/Pd layers may have thickness of about 20/40/40 nm, respectively.
  • Si/SiOx as a substrate is therefore provided for the purpose of illustration only, and many other substrates can achieve a similar device structure and performance without detracting from the purpose of the invention.
  • the graphene Schottky varactor diodes of the present disclosure may display a symmetric rectification (Fig. 1(C)) with a voltage threshold of at least 0.1, for example, of IV.
  • the overall resistivity of the Graphene diodes is dominated by the contact resistivity as can be confirmed by length modulated devices (Fig. 1(a)).
  • the control over the impedance of a graphene varactor diode is generally characterized by its tuning ratio, defined as maximum to minimum capacitance ratio of the device, C ma x/ C mi n .
  • the varactor devices of the present disclosure exhibit high tuning ratios, for example, greater than 5.
  • the graphene Schottky varactor diodes of the present invention exhibit a pure capacitive character, at the resonant frequency of 10 kHz as is demonstrated by Figs. 2(a) and 2(b).
  • Fig. 2(a) shows the measured cutoff frequency as a function of applied voltage at frequency of 10 kHz
  • Fig. 2(b) displays the frequency resolved cutoff frequency w 9 .
  • the Q-factor of the diode of the present disclosure is displayed in Fig. 2(c), having a first resonance at 10 kHz (black points).
  • Fig. 2(d) displays a time-domain measurement of the photocurrent measured on an unbiased device with light modulation at 120 kHz. Each data point is represented by a dot, left axis in Fig. 2(c) represents the root mean square of the photocurrent measured in the time-domain such as in Fig. 2(d), providing the relative amplitude of the photocurrent as a function of the modulation frequency.
  • the non-linear capacitance of the diodes can result in harmonic generation when pumped with a sinusoidal signal, and the circuit connected to them can extract a desired harmonic.
  • the device resonates with photocurrent at some amplitude in accordance with the cutoff frequency w 9 .
  • the responsivity of the device of an embodiment of the present invention reaches a maximal value of 160 mA/W, which corresponds to an EQE (External Quantum Efficiency, defined as the ratio of the number of charge carriers collected by the detector to the number of photons of a given energy shining on the detector from outside (incident photons) of 37% and a photoconductive gain of 16 (shown in Fig. 3). This is higher by about two orders of magnitude compared to the typical responsivity exhibited by Ohmic-contact-graphene-devices.
  • EQE External Quantum Efficiency
  • Photocurrent measurements on the devices of the invention were carried out with a 532nm laser at very low power density (2.4- 121 mW/cm 2 ) and with a beam diameter of ⁇ lcm in order to avoid photo-response induced by photo-thermoelectric effects. Accordingly, the device of the present invention has strong photo-response, which is achieved with extremely low dark currents and noises, owing to the intrinsically low response at low frequencies - putting the upper bound of noise equivalent power (NEP) at ⁇ 10 14 (WHz 1/2 ) and dark currents at 100 pA.
  • NEP noise equivalent power
  • the specific detectivity is related to the NEP by where A is the area of the device.
  • A is the area of the device.
  • D* specific detectivity
  • Photoconductive gain may often correspond to an enhanced absorption of long-lived photocarriers by a gain medium, putting a limit on the response time of the device.
  • the gain mechanism of the varactor device is free from dynamical effects associated with the excitation lifetime and is rooted in the oscillatory nature and frequency of the photocurrent. Nonetheless, the performance of this new class of photodetectors can be further enhanced if combined with a gain medium.
  • the capacitive impedance of the device (Fig. 2) and its resulting resonance decouples the DC bias from the photoresponse (Fig. 4(a)).
  • the capacitive nature of the detector also corresponds to the frequency dependence of the photoresponse according to Fig.
  • the responsivity at 75 Hz is ⁇ 1 mA/W and as frequency increases, the photocurrent takes a capacitive character with a phase difference of up to 70° and responsivity of ⁇ 100 mA/W at 10 kHz.
  • the phase difference of the photocurrent is comparable to the phase measured electrically without illumination on the device (Fig. 4(c)), showing very good agreement between the two measurements.
  • the present invention provides the use of the varactor diode for LIDAR (3D laser scanning) applications including, but not limited to 3D photography, high sensitivity photodetection and depth imaging, distance measurements and high-speed communications.
  • LIDAR 3D laser scanning
  • Varactor Diodes The importance of the Graphene Schottky Varactor Diodes of the present disclosure is associated with the unique combination of noise and dark current reduction, together with enhanced responsivity.
  • the photoconductive gain in the graphene. Varactors are frequency dependent and peak at high frequencies - and thus naturally filter the CW with its unique temporal response. This property prevents the saturation of detector elements and pixels from CW illumination and therefore may be used for several applications as described above.
  • the varactor is defined by the Schottky contacts of the device and thus the graphene channel remains an additional knob for controlling the spectral sensitivity by integrating plasmonic structures, quantum dots that enhance absorption or cavity enhancement of the light-graphene interaction.
  • Graphene-based varactor devices are designed to detect the differential illumination over time. In one embodiment of the invention this enhancement is facilitated by the implementation of IR illumination. This embodiment is referred to herein as the "IR-enhanced diode". As mentioned earlier, the atomic thickness of graphene in combination with the long wavelength of IR, reduces the interaction cross-section to negligibly small signals. When used in conjunction with IR wavelengths, the present invention shows an enhancement of the IR response which can be achieved in two ways:
  • Graphene can be patterned and produce conductive/insulating regions that resonate with IR light at room temperature. Such photodetectors have been demonstrated and proved efficient. Such patterns of graphene can also be combined with gold, silver and/or other metals to enhance the plasmonic excitation (Fig. 5).
  • the patterning of hole meshes for example, can resonate at wavelength of ⁇ 15 pm (see lower curve in Fig. 5(a)), and the addition of a gold layer of at least 10 nm, preferably 25nm filling the hole adds a spectral absorbance of ⁇ 10% in the range of 1-10 pm (upper curve in Fig. 5(a)).
  • Plasmonic meshes can be fabricated, for instance, by definition of spot array at electron-beam exposure of a lithographic resist, and the development of the resist to define the mesh at certain spot size and spacing, according to lithographic pattern. After resist development the graphene is etched from the area of the spots (e.g., by an exposure to oxygen plasma).
  • the etched mesh holes are then coated with gold, silver or any other plasmonic metal by physical vapor deposition and then the device is put in solvent for lift-off, where the metal is removed from areas covered by lithographic resist - and remain permanent coating on areas that were not covered with resist after lithography development stage and plasma etching.
  • the varactor diodes are implemented with black phosphorus crystal below the graphene channel, in several architectures of the detector, including bare contacts and (interdigitated) finger-contacts on the channel (Figs. 6(a)-6(d)).
  • the current-voltage curves demonstrate a symmetric varactor diode (Fig. 6(f)).
  • the photocurrent is voltage sensitive and is highly responsive to short-wave infrared at 1550 nm wavelength (Fig. 6(e)).
  • Graphene was synthesized by Chemical Vapor deposition (CVD) primarily as described by Yu, Q. Jauregui et. al. in Nature materials 2011, 10 (6), 443, and was transferred onto a highly p- doped (0.001-0.005 W-cm) Si wafer with 285 nm thermal oxide.
  • Graphene circular voids were patterned in rectangular and triangular meshes, with diameters of 40-100nm in the graphene voids, spaced apart by 100-400 nm, respectively.
  • Contact electrodes were defined by EBL and metallized with electron beam deposition Ge ⁇ AI ⁇ Pd (20 ⁇ 40 ⁇ 40nm) lift off at background pressure of 10-7 Torr, deposition rate of about 0.5 A/sec and with materials.
  • the Si ⁇ SiC>2 chip was attached on a lead less chip carrier (LCC) and the device contact pads were connected to the LCC by wire bonding.
  • Transport characterization Chemical Vapor deposition
  • the photoresponse was measured with a 532nm laser diode with a beam diameter of 10 mm after it was spread with a CaF2 uncoated Plano-Convex lens to achieve optimized power uniformity on the device.
  • Laser beam was modulated using a mechanical chopper (10 kHz) and photocurrents were amplified by a low noise current amplifier (Femto, DLPCA-200) and SR830 lock-in amplifier (Fig. 4(b), 4(c)).
  • Capacitance measurements were performed using Agilent N1301A CMU mounted on an Agilent B1500A. Current spectral density was measured with a Keysight MSO - X 4045A oscilloscope. Time domain photocurrent measurements (Fig. 2(c)) were performed on unbiased varactor devices connected to a Keysight CX3324A Device Current Waveform Analyzer, recording the photocurrent (Fig. 2(c) - blue dots left axis, Fig. 2(d)).
  • the absorption spectrum of graphene meshes shows the enhancement of gold filled graphene voids in the mesh (Fig. 5b).
  • the absorption of graphene on the spectrum is about 2.3% [reference: Tony Heintz Phys. Rev. Lett. 101, 196405, 2008], the absorption of graphene mesh (as defined above) is reduced to ⁇ 1% over the IR spectral range and with plasmonic resonance at wavelength of about 15 microns. If the graphene mesh voids are filled with thin layer of plasmonic metal (gold, silver, etc.), the absorption of IR light is enhanced in the wavelength range of 1.5-11 microns.

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Abstract

A graphene Schottky varactor diode comprises: a) a graphene channel on a substrate; and b) Schottky contacts, wherein said graphene Schottky varactor diode is essentially free from dark currents.

Description

GRAPHENE SCHOTTKY VARACTOR DIODES
Field of the Invention
The present invention relates to graphene Schottky varactor diodes and to the use thereof for high-performance photodetection.
Background of the Invention
Photodetection in gapless graphene has been an active area of research over the last decade, where photovoltaic, bolometric, and photothermoelectric physics were studied in depth. Photoresponse of pristine graphene devices is fundamentally limited by the low absorption cross-section and can be enhanced by plasmonic effects, optical cavities, semiconductor interfaces, quantum dot and other absorption enhancing agents. However, the progress of graphene photoconductive devices is yet limited by the sizable leakage currents that introduce noise and complicate the coupling with complementary metal-oxide-semiconductor (CMOS) capacitive integrators for digital imaging. Device architectures comprising graphene as a Schottky contact to semiconductor substrates improved this twin issue of dark currents and low responsivity, but graphene in this case works as a transparent electrode rather than as an active photoconductive channel. Therefore, in graphene-semiconductor hybrid Schottky diodes, the generation of photo-carriers takes place at the semiconductor side of the junction, and thus carrier dynamics and spectral response depend on the semiconductor properties rather than those of graphene.
Varactors are voltage-tunable capacitors and can therefore be utilized for impedance matching, as oscillators and modulators in the radiofrequency to terahertz range. Modern varactor devices are typically fabricated from Schottky diodes and semiconductor heterostructures. Recently, the quantum capacitance of graphene was utilized to construct tunable metaloxide-semiconductor (MOS) capacitors.
However, to date the art has not resolved fundamental limitations in graphene optoelectronics including enhancing the external quantum efficiency (EQE), and the elimination of dark currents that flow through the diode even when no photons are entering the device, the charges being generated in the detector when no outside radiation is entering the detector. Accordingly it is an object of the invention to provide new graphene varactor diodes that address prior art shortcomings. Other objects and advantages of the invention will become apparent as the description proceeds.
Summary of the Invention
In one aspect the invention relates to a graphene Schottky varactor diode comprising: a. a graphene channel on a substrate;
b. Schottky contacts; wherein said graphene Schottky varactor diode is essentially free from dark currents. In thos context the term "essentially free" should be taken to indicate very low dark currents, which do not adversely affect the operation of the diode, such as when the diode exhibits no dark currents at zero DC bias, or a dark current of not more than 100 pA at a bias of 50 mV, but typically no more than 5 nA.
In another aspect, the graphene Schottky varactor diode of the invention is adapted to: a. rectify l-V transfer curves with a voltage threshold of at least 0.1 V b. provide a photoresponse increase with modulation frequency
c. provide a photoresponse of pure graphene of at least 100 mA/W; and d. provide a photoresponse independent from DC bias.
A typical graphene Schottky varactor diode of the invention has one or more of the following characteristics: a. NEP of 10 14 WHz1/2 at a bias of 50 mV;
b. a Schottky barrier height of 0.16 eV;
c. a tuning ratio greater than 5;
d. a responsivity of 160 mA/W, EQE of 37%, and a gain of above 16, at the device resonance frequency (at 10 kHz); or
e. combinations of the above a-d characteristics. In some embodiments, the graphene Schottky varactor diode of the invention has a pure capacitive character at the resonant frequency of 10 kHz. In one embodiment the graphene channel is deposited by chemical vapor deposition (CVD).
In one embodiment the graphene Schottky varactor diode of the invention has a photoresponse that is enabled by the illumination of light from the visible spectrum, and in another embodiment it has a photoresponse that is enabled by the illumination of light from the infrared (IR) spectrum. In some embodiments in which the diode is an IR-enhanced diode, it comprises an additional periodic metallic plasmonic structure on top of the graphene mesh.
The graphene Schottky varactor diode of the invention may comprise a low-gap semiconductor located under the graphene channel, thereby to enhance the photoresponse of said diode. In some embodiments the low-gap semiconductor is black phosphorus.
The graphene Schottky varactor diode of the invention is useful in a variety of applications, for instance for use in LIDAR or 3D laser scanning applications.
The invention also encompasses a process for preparing a graphene Schottky varactor diode, comprising performing CVD of graphene on a substrate, and thereafter depositing Schottky contacts, to obtain the graphene Schottky varactor diode. In one embodiment the substrate is P+-Si/Si02, and in another embodiment the Schottky contacts comprise metallized layers of Ge/Al/Pd.
Brief Description of the Drawings
Fig. 1(a) shows an optical micrograph of transfer-length modulated graphene Schottky varactor diodes, scale bar is 5pm;
Fig. 1(b) is a graphic illustration of the device structure with Pd/AI/Ge contacts;
Fig. 1(c) shows l-V transfer curves at temperature range of 257-342 K; Fig. 1(d) shows the temperature dependence of the diode saturation current (dots) and fitted Schottky barrier height (slope of solid line);
Fig. 1(e) shows voltage-resolved capacitance and current; Fig. 2(a) shows the Graphene Varactor voltage; Fig. 2(b) shows the Graphene Varactor frequency; Fig. 2(c) shows the Graphene Varactor resolved cutoff frequency resolved relative photocurrent (left axis, dots) and Q-factor of graphene varactor diodes (right axis; thick solid line: deduced from measured impedance; thin solid line: simulation).
Fig. 2(d) is the time-resolved photocurrent at 120 kHz modulation of the laser light; Fig. 3(a) shows the photoresponsivity of graphene photodetectors; Fig. 3(b) shows the external quantum efficiency (EQE) of graphene photodetectors, defined as the ratio between the photocurrent and the incoming photon flux;
Fig. 3(c) shows the photoconductive gain of graphene photodetectors (excess of internal quantum efficiency (IQE) from 100% and IQE is the ratio of photocurrent to the absorbed photon flux as function of illumination power;
Fig. 3(d) shows the noise equivalent power of graphene photodetectors as a function of frequency recorded with DC bias of 50 mV and corresponding to a photoresponsivity of 0.1 A/W;
Fig. 4(a) Voltage and frequency dependence of graphene varactor photoresponse:
Photocurrent as function of applied DC bias
Fig. 4(b) Voltage and frequency dependence of graphene varactor: photocurrent (the rising graph) and phase (the descending graph) as function of laser modulation frequency;
Fig. 4(c) Voltage and frequency dependence of graphene varactor: frequency-dependence of the phase lag measured by electrically excited currents (bottom graph) and optically excited photocurrents (top graph); Fig. 5(a) Absorption spectrum of graphene hole mesh (bottom) and gold filling in graphene holes (top);
Fig. 5(b) Scanning electron micrograph of gold-filled graphene hole mesh;
Figs. 6(a-d) are optical images of black phosphorus crystal below the graphene channel in several device structure embodiments;
Fig. 6(e) shows photocurrent versus voltage;
Fig. 6(f) shows the current-voltage characteristics for symmetric varactor diode.
Detailed Description of the Invention
Graphene varactors are devices designed to detect the differential of illumination in time. The atomic thickness of graphene, combined with the long wavelength of infrared (IR) light, reduces the cross section of interaction to vanishingly small signals. The enhanced infrared sensation of such devices is therefore very appealing for many applications including night vision and distance measurements. In one embodiment of the invention the enhancement of the IR response of graphene is enabled by an additional plasmonic element in the design structure of the varactor diode, the IR response being in the range ~700nm to ~lmm. In another embodiment of the invention the IR enhancement is enabled by the addition of low- gap semiconductors. In a further embodiment of the invention the graphene Schottky varactor is responsive in the visible light range i.e., wavelength of ~ 380 to ~ 700 nm.
The graphene Schottky varactor diodes of the invention are characterized by:
a. rectifying l-V transfer curves with a voltage threshold of at least 0.1 V;
b. a photoresponse increase with modulation frequency;
c. a photoresponse of pure graphene device of at least 100 mA/W;
d. a photoresponse independent from DC bias; and
e. no dark currents at zero DC bias, or a dark current of 100 pA at a bias of 50 mV;
In some embodiments of the invention the graphene Schottky varactor diodes are further characterized by characteristics selected from: a. NEP (Noise Equivalent Power) of 10 14 WHz 1/2 at a bias of 50 mV; b. a Schottky barrier height (SBH) of 0.16 eV;
c. a tuning ratio greater than 5;
d. a responsivity of 160 mA/W, EQE of 37%, and a gain of above 16, at the device resonance frequency (at 10 kHz); and e. combinations thereof.
In another embodiment, the graphene Schottky varactor diodes of the present invention have pure capacitive character, at the resonant frequency of 10 kHz. Therefore, illumination of the device close to a frequency of about 10 kHz drives a higher oscillation amplitude of photocarriers, even without an applied DC voltage.
The graphene Schottky varactor diodes of the present invention are intrinsically immune to low-frequency noise owing to the characteristic capacitive frequency cutoff, and are therefore sensitive to rapid changes in illumination rather than to the incident optical power.
The invention also encompasses a process for preparing a graphene Schottky varactor diode, comprising performing CVD of graphene on a suitable substrate, for instance P+-Si/Si02, and further depositing Schottky contacts to obtain the graphene Schottky varactor diode. Suitable Schottky contacts may be, for instance, metallized layers of Ge/AI/Pd.
The graphene Schottky varactor diodes of the present invention are useful in a variety of applications, including but not limited to LIDAR (3D laser scanning) applications including, such as 3D photography, high sensitivity photodetection and depth imaging, distance measurements, and high-speed communications.
The novel device architecture of the present invention provides graphene Schottky diode varactors comprising a graphene channel in between two Schottky contacts that is illuminated. The graphene varactor diodes resolve some of the fundamental limitations in graphene optoelectronics, including the elimination of dark currents and the enhancement of the external quantum efficiency (EQE). The devices of the present invention possess a large photoconductive gain and EQE of up to 37%, fast photoresponse, and low leakage currents at room temperature. The graphene Schottky varactor diodes of the invention have a record high photoresponse for a pure graphene device of at least 100 mA/W, for example of 160 mA/W. Conversely to graphene contacts on semiconductors, in the present invention thin film polycrystalline semiconductor contacts are applied to a graphene channel to construct lateral Schottky varactor diodes, as shown in Figs. l(a)-l(b).
As can be seen from Fig. 1(a) and Fig. 1(b), the device consists of a sensing material (e.g. the graphene rectangle in Fig. 1(a) and the middle part of Fig. 1(b)) enclosed by 2 Schottky contacts that due to the graphene's quantum capacitance create a Varactor. Photons hit the sensing material, some of which are absorbed and converted to charge carriers. The charge carriers are coupled by the Varactor capacitance to the Schottky contacts and can be read by a read out circuitry or read out equipment. The capacitance coupling inherently supports efficient read out of the changes in the illumination levels intensity that is translated to changes of the charge carriers intensity.
Thus, the graphene layer in Figs. 1(a) and 1(b) has 3 main roles in the device: first to absorb and convert photons to charge carriers, second to make the Schottky contacts by its interface with the semiconductor part of the contacts (e.g. Ge in Fig. 1(b)), third to create the Varactor due to the graphene's quantum capacitance.
In order to tune the device to the IR spectrum, the device sensing material has to be tuned accordingly. Two different methods are defined for the device sensing material IR tuning:
1. Tune the graphene layer for IR absorbance enhancement by creating graphene hole mesh as can be seen from Figs. 5(a) and 5(b). In this method, the graphene rectangle in Fig. 1 (a) is replaced by a graphene hole mesh rectangle as appear in the SEM micrograph in Fig. 5(b). In this method, the graphene absorbance spectrum is controlled by the holes pitch, the holes diameter and the holes fill (empty holes vs. gold or other metal filled holes that support plasmonic gain in the IR spectrum). Fig. 5(a) demonstrates the IR absorbance significant enhancement by gold filled graphene holes vs. empty graphene holes as was measured by an FTIR. In this method, the graphene layer keeps its 3 roles as described above. 2. Add an IR sensitive material thin layer below the graphene layer as appears in Fig. 6. In this method, the bottom IR sensitive layer absorb some of the IR photons that pass through the almost transparent graphene layer, and convert them to charge carriers. The graphene layer acts as high mobility contact that couples the charge carriers to the Schottky Varactor. In this method the graphene layer keeps 2 out of its 3 original roles: it makes Schottky contacts with the semiconductor parts of the device contacts, and create the Varactor due to the graphene's quantum capacitance. Figs. 6(a) through 6(d) display several configurations of black phosphorus layers as an IR sensing material below the graphene layer. Fig. 6(e) shows the device photocurrent at 1550nm, and Fig. 6(f) shows the symmetric current-voltage characteristics of the Varactor.
As said, the graphene Schottky varactor diodes of the present disclosure are prepared from CVD graphene on a substrate followed by deposition of Schottky contacts. Typically, the substrate is made of insulating or semi-insulating materials such as polymers, glass and semiconductors, for example GaAs, GaN, InP, GaP, Ge, InSb, InAs, and their alloys. One particularly suitable illustrative substrate is P+-Si/Si02. In one embodiment of the invention the substrate is flexible. In another embodiment of the invention the substrate is transparent.
Typically, the size of the substrate is from about 100 nm to several microns, for example, about 285nm. The Schottky contacts may comprise layers of Ge/AI/Pd, T1O2 or Si. In some embodiments the Schottky contacts comprise metallized layers of Ge/AI/Pd. Typically, the metallized Ge/AI/Pd layers thickness is set to maintain the Schottky contact, for example, the metallized Ge/AI/Pd layers may have thickness of about 20/40/40 nm, respectively. Flowever, it should be noted that the abovementioned list of materials is not exhaustive and persons skilled in the art of micro- and nano-fabrication/lithography will recognize that the materials disclosed herein for the Schottky contacts are not limited to the few mentioned above, but that they can be exchanged with other standard materials, used in combination, in tandem and in any number of layers, whilst still achieving the purpose of the invention.
Similarly, a person skilled in the art of lithographic techniques will be aware of many options that are available for substrates onto which to fabricate the device. The example of Si/SiOx as a substrate is therefore provided for the purpose of illustration only, and many other substrates can achieve a similar device structure and performance without detracting from the purpose of the invention.
Owing to the symmetric band structure of graphene, the graphene Schottky varactor diodes of the present disclosure may display a symmetric rectification (Fig. 1(C)) with a voltage threshold of at least 0.1, for example, of IV.
The high defect density in evaporated polycrystalline Ge on graphene results in significant Fermi level pinning, rendering the diodes almost irresponsive to a back gate bias. The Schottky barrier height (SBH) is about 0.16 eV as can be determined by the two-dimensional (2D) thermionic emission model, [27], J=Js(l-exp[qV/kBT]), where L is the saturation current density that was fitted (Fig. 1(d)) to JS =A2DT3/2 exp[-<Ds /keT], A* 2D is the two dimensional Richardson constant, CDs is the SBH, ke is Boltzmann constant and T is the temperature.
The overall resistivity of the Graphene diodes is dominated by the contact resistivity as can be confirmed by length modulated devices (Fig. 1(a)). The voltage-resolved capacitance of the devices (Fig. 1(e)) shows a typical capacitance signature of a varactor diode peaking at zero voltage: C = Cc/(l+V/Vb)Y , where y * 0.58 corresponds to an abrupt junction, and 14 = 0.45 V is the fitted barrier potential and also matches the diode turn-on voltage.
The control over the impedance of a graphene varactor diode is generally characterized by its tuning ratio, defined as maximum to minimum capacitance ratio of the device, Cmax/ Cmin . [30] The varactor devices of the present disclosure exhibit high tuning ratios, for example, greater than 5.
The characteristic cutoff frequency, u)g=G/C is an additional figure of merit of the device performance, where G is the conductivity and C is the capacitance (Fig. 2(a)). At frequencies higher than wb the device behaves more like a capacitor and below this frequency it behaves like a variable resistor and therefore, a minimal value of w3 is desired in an efficient varactor.
The graphene Schottky varactor diodes of the present invention exhibit a pure capacitive character, at the resonant frequency of 10 kHz as is demonstrated by Figs. 2(a) and 2(b). Fig. 2(a) shows the measured cutoff frequency as a function of applied voltage at frequency of 10 kHz, and Fig. 2(b) displays the frequency resolved cutoff frequency w9.
The device quality factor (Q), is the ratio of the imaginary to the real parts of the measured impedance Q = lmZ/ReZ = w9/2p/. [31, 32] The Q-factor of the diode of the present disclosure is displayed in Fig. 2(c), having a first resonance at 10 kHz (black points). Fig. 2(d) displays a time-domain measurement of the photocurrent measured on an unbiased device with light modulation at 120 kHz. Each data point is represented by a dot, left axis in Fig. 2(c) represents the root mean square of the photocurrent measured in the time-domain such as in Fig. 2(d), providing the relative amplitude of the photocurrent as a function of the modulation frequency. Considering approximate values for the graphene quantum capacitance and kinetic inductance, it is predicted (solid line) that an additional resonance exists.
In Schottky varactor diodes, the non-linear capacitance of the diodes can result in harmonic generation when pumped with a sinusoidal signal, and the circuit connected to them can extract a desired harmonic. Reciprocally, it is shown that by applying a periodic perturbation on the unbiased varactor (or low voltage DC biased) by illumination, the device resonates with photocurrent at some amplitude in accordance with the cutoff frequency w9.
The responsivity of the device of an embodiment of the present invention, measured close to the device resonance frequency (at 10 kHz) reaches a maximal value of 160 mA/W, which corresponds to an EQE (External Quantum Efficiency, defined as the ratio of the number of charge carriers collected by the detector to the number of photons of a given energy shining on the detector from outside (incident photons) of 37% and a photoconductive gain of 16 (shown in Fig. 3). This is higher by about two orders of magnitude compared to the typical responsivity exhibited by Ohmic-contact-graphene-devices. The overall performance of the varactor photodetectors is summarized in Fig. 3, displaying the photoresponsivity, EQE and photoconductive gain as function of the illumination power (Fig. 3(a)-3(c), respectively). The uppermost value EQE reaches 37% corresponding to a gain of above 16, corresponding to an internal quantum efficiency, where a 2.3% absorbance is assumed.
Photocurrent measurements on the devices of the invention were carried out with a 532nm laser at very low power density (2.4- 121 mW/cm2) and with a beam diameter of ~lcm in order to avoid photo-response induced by photo-thermoelectric effects. Accordingly, the device of the present invention has strong photo-response, which is achieved with extremely low dark currents and noises, owing to the intrinsically low response at low frequencies - putting the upper bound of noise equivalent power (NEP) at ~10 14 (WHz 1/2) and dark currents at 100 pA.
The specific detectivity is related to the NEP by
Figure imgf000012_0001
where A is the area of the device. Here, the unique combination of low NEP and relatively high responsivity corresponds to a specific detectivity (D*) of ~1013 Jones at a bandwidth of 10 kHz.
Photoconductive gain may often correspond to an enhanced absorption of long-lived photocarriers by a gain medium, putting a limit on the response time of the device. In the present invention, the gain mechanism of the varactor device is free from dynamical effects associated with the excitation lifetime and is rooted in the oscillatory nature and frequency of the photocurrent. Nonetheless, the performance of this new class of photodetectors can be further enhanced if combined with a gain medium. The capacitive impedance of the device (Fig. 2) and its resulting resonance decouples the DC bias from the photoresponse (Fig. 4(a)). The capacitive nature of the detector also corresponds to the frequency dependence of the photoresponse according to Fig. 4(b) and to the Q-factor (shown in Fig. 2(c)). The responsivity at 75 Hz is ~ 1 mA/W and as frequency increases, the photocurrent takes a capacitive character with a phase difference of up to 70° and responsivity of ~100 mA/W at 10 kHz. The phase difference of the photocurrent is comparable to the phase measured electrically without illumination on the device (Fig. 4(c)), showing very good agreement between the two measurements.
In some embodiments, the present invention provides the use of the varactor diode for LIDAR (3D laser scanning) applications including, but not limited to 3D photography, high sensitivity photodetection and depth imaging, distance measurements and high-speed communications.
The importance of the Graphene Schottky Varactor Diodes of the present disclosure is associated with the unique combination of noise and dark current reduction, together with enhanced responsivity. In addition, the photoconductive gain in the graphene. Varactors are frequency dependent and peak at high frequencies - and thus naturally filter the CW with its unique temporal response. This property prevents the saturation of detector elements and pixels from CW illumination and therefore may be used for several applications as described above. Moreover, the varactor is defined by the Schottky contacts of the device and thus the graphene channel remains an additional knob for controlling the spectral sensitivity by integrating plasmonic structures, quantum dots that enhance absorption or cavity enhancement of the light-graphene interaction.
Graphene-based varactor devices are designed to detect the differential illumination over time. In one embodiment of the invention this enhancement is facilitated by the implementation of IR illumination. This embodiment is referred to herein as the "IR-enhanced diode". As mentioned earlier, the atomic thickness of graphene in combination with the long wavelength of IR, reduces the interaction cross-section to negligibly small signals. When used in conjunction with IR wavelengths, the present invention shows an enhancement of the IR response which can be achieved in two ways:
1) Enhanced IR absorbance of graphene by plasmons. Graphene can be patterned and produce conductive/insulating regions that resonate with IR light at room temperature. Such photodetectors have been demonstrated and proved efficient. Such patterns of graphene can also be combined with gold, silver and/or other metals to enhance the plasmonic excitation (Fig. 5). The patterning of hole meshes, for example, can resonate at wavelength of ~15 pm (see lower curve in Fig. 5(a)), and the addition of a gold layer of at least 10 nm, preferably 25nm filling the hole adds a spectral absorbance of ~10% in the range of 1-10 pm (upper curve in Fig. 5(a)). The integration of such plasmonic enhancers with graphene varactors is achieved by patterning the varactor channel with standard lithography methods. Plasmonic meshes can be fabricated, for instance, by definition of spot array at electron-beam exposure of a lithographic resist, and the development of the resist to define the mesh at certain spot size and spacing, according to lithographic pattern. After resist development the graphene is etched from the area of the spots (e.g., by an exposure to oxygen plasma). In some cases, the etched mesh holes are then coated with gold, silver or any other plasmonic metal by physical vapor deposition and then the device is put in solvent for lift-off, where the metal is removed from areas covered by lithographic resist - and remain permanent coating on areas that were not covered with resist after lithography development stage and plasma etching.
2) Enhanced IR absorbance by addition of low-gap semiconductors. The varactor diodes are implemented with black phosphorus crystal below the graphene channel, in several architectures of the detector, including bare contacts and (interdigitated) finger-contacts on the channel (Figs. 6(a)-6(d)). The current-voltage curves demonstrate a symmetric varactor diode (Fig. 6(f)). However, the photocurrent is voltage sensitive and is highly responsive to short-wave infrared at 1550 nm wavelength (Fig. 6(e)).
Example 1
Graphene was synthesized by Chemical Vapor deposition (CVD) primarily as described by Yu, Q. Jauregui et. al. in Nature materials 2011, 10 (6), 443, and was transferred onto a highly p- doped (0.001-0.005 W-cm) Si wafer with 285 nm thermal oxide. Graphene circular voids were patterned in rectangular and triangular meshes, with diameters of 40-100nm in the graphene voids, spaced apart by 100-400 nm, respectively. Contact electrodes were defined by EBL and metallized with electron beam deposition Ge\AI\Pd (20\40\40nm) lift off at background pressure of 10-7 Torr, deposition rate of about 0.5 A/sec and with materials. Finally, the Si\SiC>2 chip was attached on a lead less chip carrier (LCC) and the device contact pads were connected to the LCC by wire bonding. Transport characterization
All the measurements were performed with Montana Instruments cryostat under vacuum. I- V curves were measured with a Keysight B2912 source meter unit.
Device characterization
The photoresponse was measured with a 532nm laser diode with a beam diameter of 10 mm after it was spread with a CaF2 uncoated Plano-Convex lens to achieve optimized power uniformity on the device. Laser beam was modulated using a mechanical chopper (10 kHz) and photocurrents were amplified by a low noise current amplifier (Femto, DLPCA-200) and SR830 lock-in amplifier (Fig. 4(b), 4(c)).
Capacitance measurements were performed using Agilent N1301A CMU mounted on an Agilent B1500A. Current spectral density was measured with a Keysight MSO - X 4045A oscilloscope. Time domain photocurrent measurements (Fig. 2(c)) were performed on unbiased varactor devices connected to a Keysight CX3324A Device Current Waveform Analyzer, recording the photocurrent (Fig. 2(c) - blue dots left axis, Fig. 2(d)).
The absorption spectrum of graphene meshes (Fig. 5a) shows the enhancement of gold filled graphene voids in the mesh (Fig. 5b). The absorption of graphene on the spectrum is about 2.3% [reference: Tony Heintz Phys. Rev. Lett. 101, 196405, 2008], the absorption of graphene mesh (as defined above) is reduced to ~1% over the IR spectral range and with plasmonic resonance at wavelength of about 15 microns. If the graphene mesh voids are filled with thin layer of plasmonic metal (gold, silver, etc.), the absorption of IR light is enhanced in the wavelength range of 1.5-11 microns.
While various embodiments of the invention have been shown and described for the purpose of illustration, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

Claims
1) A graphene Schottky varactor diode comprising:
a. a graphene channel on a substrate;
b. Schottky contacts; said graphene Schottky varactor diode being essentially free from dark currents.
2) The graphene Schottky varactor diode of claim 1, which is adapted to: a. rectify l-V transfer curves with a voltage threshold of at least 0.1 V
b. provide a photoresponse increase with modulation frequency
c. provide a photoresponse of pure graphene of at least 100 mA/W; and d. provide a photoresponse independent from DC bias.
3) The graphene Schottky varactor diode of claim 1, which exhibits no dark currents at zero DC bias, or a dark current of not more than 100 pA at a bias of 50 mV.
4) The graphene Schottky varactor diode of claim 3, wherein the dark current is not more than 5 nA.
5) The graphene Schottky varactor diode of claim 1 having one or more of the following characteristics: a. NEP of 10'14 WHz1/2 at a bias of 50 mV;
b. a Schottky barrier height of 0.16 eV;
c. a tuning ratio greater than 5;
d. a responsivity of 160 mA/W, EQE of 37%, and a gain of above 16, at the device resonance frequency (at 10 kHz); or
e. combinations of the above a-d characteristics.
6) The graphene Schottky varactor diode of claim 1, having a pure capacitive character at the resonant frequency of 10 kHz.
7) The graphene Schottky varactor diode of claim 1, wherein said graphene channel is deposited by chemical vapor deposition (CVD).
8) The graphene Schottky varactor diode of claim 1, wherein the photoresponse of said diode is enabled by the illumination of light from the visible spectrum. 9) The graphene Schottky varactor diode of claim 1, wherein the photoresponse of said diode is enabled by the illumination of light from the infrared (IR) spectrum.
10) The IR-enhanced diode of claim 1 comprising an additional periodic metallic plasmonic structure on top of the said graphene mesh.
11) The graphene Schottky varactor diode of claim 1, comprising a low-gap semiconductor located under the graphene channel, thereby to enhance the photoresponse of said diode.
12) The graphene Schottky varactor diode of claim 11, wherein the low-gap semiconductor is black phosphorus.
13) The diode of claim 1, for use in for use in LIDAR or 3D laser scanning applications.
14) A process for preparing a graphene Schottky varactor diode, comprising performing CVD of graphene on a substrate, and thereafter depositing Schottky contacts, to obtain the graphene Schottky varactor diode.
15) The process of claim 14, wherein the substrate is P+-Si/Si02.
16) The process of claim 14, wherein the Schottky contacts comprise metallized layers of Ge/AI/Pd.
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