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Graphene Schottky Varactor Diodes for High-Performance Photodetection Adi Levi, Moshe Kirshner, Ofer Sinai, Eldad Peretz, Ohad Meshulam, Arnab Ghosh, Noam Gotlib, Chen Stern, Shaofan Yuan, Fengnian Xia, and Doron Naveh ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00811 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019
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Graphene Schottky Varactor Diodes for HighPerformance Photodetection Adi Levi*1,2, Moshe Kirshner*1,2, Ofer Sinai1,2, Eldad Peretz1,2, Ohad Meshulam1,2, Arnab Ghosh1,2, Noam Gotlib1,2, Chen Stern1,2, Shaofan Yuan3, Fengnian Xia3, and Doron Naveh1,2† 1 2
Faculty of Engineering, Bar-Ilan University, Ramat-Gan, Israel, 52900
Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan, Israel, 52900 3 Department of Electrical Engineering, Yale University, New Haven CT 06511, USA * These
authors contributed equally to this work
† Corresponding
author:
[email protected] ABSTRACT Over the past decade graphene devices have inspired the progress of future electronic and optoelectronic technologies. The unique combination of fast carrier dynamics and intrinsic quantum capacitance of graphene is a fertile ground for implementing novel device architectures. Here, we report on a novel device architecture comprising graphene Schottky diode varactors, and assess the potential applications of this type of new devices in optoelectronics. We show that graphene varactor diodes exhibit significant advantages compared with existing graphene photodetectors including elimination of high dark currents and enhancement of the external quantum efficiency (EQE). Our devices demonstrate a large photoconductive gain and EQE of up to 37%, fast photoresponse and low leakage currents at room temperature. Keywords: Graphene, High-Frequency, Photodetector, Varactor, Schottky Diode, DarkCurrent. Photodetection in gapless graphene is being a vigorously active area over last decade, where photovoltaic,1 bolometric2 and photothermoelectric3 physics were studied in depth.4 Photoresponse of pristine graphene devices is fundamentally limited by the low absorption cross-section4-5 and can be enhanced by plasmonic effects,6-7 optical cavities,8-9 semiconductor interfaces,10-11 quantum dot12 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 a CMOS capacitive integrators for digital imaging. Device architectures comprising graphene as a Schottky contact to semiconductor substrates13-19 improved this twin issue of dark currents and low responsivity,4, 20 but graphene
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in this case works as a transparent electrode rather than the active photoconductive channel. Ergo, 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. Conversely to graphene contacts on semiconductors, here we applied thin film polycrystalline semiconductor contacts to a graphene channel to construct lateral Schottky varactor diodes, as shown in Figure 1a-1b. Varactors are voltage-tunable capacitors and therefore can be utilized for impedance matching,21oscillator and modulators in the radiofrequency22 to terahertz23 range. Modern varactor devices are typically fabricated from
Figure 1. Graphene Schottky Varactor Diode. (a) Optical micrograph of transfer-length modulated graphene Schottky varactor diodes, scale bar is 5μm. (b) Graphical illustration of the device structure with Pd/Al/Ge contacts, (c) I-V transfer curves at temperature range of 257-342 K, (d) Temperature dependence of the diode saturation current (blue dots) and fitted Schottky barrier height (slope of red line). (e) Voltage-resolved capacitance and current.
Schottky diodes24 and semiconductor heterostructures.25 More recently the quantum capacitance of graphene was utilized to construct tunable metal-oxide-semiconductor (MOS) capacitors.26 In this work we further utilize the quantum capacitance of graphene to demonstrate Schottky varactor diodes as photodetectors. The importance of this device archetype is associated with the unique combination noise and dark current reduction together with enhanced responsivity. In addition, the photoconductive gain in graphene varactors is frequency dependent and is peaking 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 holds promise for several future applications such as pulse time of flight measurements for 3D imaging - that requires fast and sensitive photodetectors that would not be saturated with ambient light. Moreover, the varactor is
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defined by the Schottky contacts of the device and thus the graphene channel is remained as an additional knob for controlling the spectral sensitivity – possibly with an extension to the infrared regime - by integrating plasmonic structures ,6-7 quantum dots that enhance absorption12 or cavity enhancement of the light-graphene interaction.9, 27
RESULTS & DISCUSSION The varactor diodes were prepared from CVD graphene on a 285nm P+-Si/SiO2 substrate and were metallized with layers of 20/40/40 nm of Ge/Al/Pd in sequence, as shown in Figure 1a and illustrated in schematic device layout of Figure 1b. Owing to the symmetric band structure of graphene, the diodes display a symmetric rectification (Figure 1c) with a voltage threshold of ~1V. The high defect density in evaporated polycrystalline Ge on graphene results in significant Fermi level pinning, rendering the diodes almost irresponsive to a gate bias and therefore in all measurements the gate terminal remained floating. The Schottky barrier height (SBH) was evaluated to be 0.16 eV according to the two-dimensional (2D) thermionic emission model,28 J J s (1 exp qV / k BT ) , where J s is the saturation current density that was fitted (Figure 1d) to J s A2*DT 3 2 exp[ B / k BT ] , A2*D is the twodimensional Richardson constant, B is the SBH, k B is Boltzmann constant and T is the temperature (see supplementary information for details). Length modulated devices (Figure 1a) confirm the dominance of contact resistivity on the overall resistivity of the diodes. The voltage-resolved capacitance of the devices (Figure 1e) shows a typical capacitance signature of a varactor diode peaking at zero voltage: C
C0 , where 0.58 corresponds to (1 V Vb )
an abrupt junctions,29-30 and Vb 0.45V is the fitted barrier potential and also matches the diode turn-on voltage. The control over the impedance of a varactor device is generally characterized by its tuning ratio, defined as maximum to minimum capacitance ratio of the device, Cmax / Cmin .31
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Figure 2. Graphene Varactor Characteristics. Voltage (a) and frequency (b) resolved cutoff frequency. (c) Frequency resolved relative photocurrent (left axis, blue dots) and Q-factor of graphene varactor diodes (right axis; black: deduced from measured impedance; red: simulation). (d) Time-resolved photocurrent at 120 kHz modulation of the laser light.
It is noteworthy to emphasize that our varactor devices exhibit relatively high tuning ratios, greater than 5. The characteristic cutoff frequency g
G is an additional figure of C
merit of the device performance, where G is the conductivity and C is the capacitance (Figure 2a). At frequencies higher than g the device behaves more like a capacitor and below this frequency it behaves like a variable resistor and therefore, a minimal value of g is desired in an efficient varactor. Figure 2a presents the measured cutoff frequency as a function of applied voltage at frequency of 10 kHz and Figure 2b displays the frequency resolved cutoff frequency g . The device quality factor (Q), is the ratio of the imaginary to the real parts of the measured impedance Q Im Z / Re Z g / 2 f .32-33
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The Q-factor is displayed in Figure 2c features a resonance at 10 kHz (black points) is
Figure 3. Characteristics of graphene photodetectors: Photoresponsivity (a), external quantum efficiency (EQE) defined as the ratio between the photocurrent and the incoming photon flux (b) and (c) photoconductive gain (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. (d) Noise equivalent power as a function of frequency recorded with DC bias of 50 mV and corresponding to a photoresponsivity of 0.1 A/W.
shown together with the photocurrent relative amplitude, driven by the modulated laser light (blue points) on an unbiased varactor diode. The model representing the device frequency response (red line) corresponds to an equivalent RC network (see Figure S5, supporting information) comprising the measured capacitance of Schottky contacts, graphene sheet resistivity, quantum capacitance and kinetic inductance. Figure 2d displays a time-domain measurement of the photocurrent measured on an unbiased device with light modulation at 120 kHz. Here each data point represented by blue dot in Figure 2c represent the root mean square of the photocurrent measured in time-domain such as in Figure 2d – providing the relative amplitude of the photocurrent as function of modulation frequency. Considering approximate values for the graphene quantum capacitance and kinetic inductance, we predict (red line) that an additional resonance should exist and our qualitative assessment of the resonance frequency and the Q value correspond to the model provided in the supplementary information.
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Figure 4. Voltage and frequency dependence of graphene varactor photoresponse. (a) Photocurrent as function of applied DC bias, (b) photocurrent (blue) and phase (red) as function of laser modulation frequency, and (c) frequency-dependence of the phase lag measured by electrically excited currents (blue) and optically excited photocurrents (black).
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, we show here that by applying a periodic perturbation on unbiased varactors (or low-voltage DC biased) by illumination, the device resonates with photocurrent at some amplitude in accordance with the cutoff frequency g . Interestingly, as shown in Figure 3, close to the device resonance frequency (at 10 kHz) the measured responsivity reaches a maximal value of 160 mA/W, corresponding to an EQE of 37% and a photoconductive gain of 16, exceeding by around two orders of magnitude compared to the typical responsivity exhibited by Ohmic contact graphene devices.27,
34-36
Overall performance of the varactor photodetectors is summarized in Error! Reference source not found., displaying the photoresponsivity, EQE and photoconductive gain as function of the illumination power (Figure 3a-3c, respectively). The upper most 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 were carried out with a 532nm laser at very low power density (2.4-121 mW/cm2) and with a beam diameter of ~1cm in order to avoid photoresponse induced by photo-thermoelectric effects. Admirably, this strong photoresponse 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 10 pA/µm. The specific detectivity is related to the NEP by 𝐷 ∗ =
𝐴∆𝑓 𝑁𝐸𝑃
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. In other devices photoconductive gain12 may often correspond to an enhanced absorption of long-lived photocarriers by a gain
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medium, putting a limit on the response time of the device. Here, the gain mechanism of varactor devices 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 such photocurrent oscillators can still be further enhanced if combined with a gain medium.12, 37 Interestingly, the capacitive impedance of the devices (Figure 2) and its resulting resonance decouples the DC bias from the photoresponse (Figure 4a). This capacitive nature of the detector also corresponds to the frequency dependence of photoresponse of Figure 4b and to the Q-factor (shown in Figure 2c): at 75 Hz the responsivity 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 (Figure 4c), showing very good agreement between the two measurements.
Reference
Description
Responsivity
Detection Mode
Bandwidth
Wavelength
IQE (%)
EQE (%)
Dark Current
This work
Graphene Varactor Diode
160 mA/W
Photocurrent Oscillator (POSC)
> 1 MHz
Visible
1600
37
< 10 pA/µm
1, 36
Graphenemetal junction
6.1 mA/W
Photocurrent (PV/PTE)
> 40 GHz
Visible, NIR
10
0.5
≥1 µA/µm
3, 38-40
Graphene P-N Junction
10 mA/W
Photocurrent (PTE)
> 20 GHz
Visible
35
2.5
≥1 µA/µm
7
Nanostructured Graphene
16 mA/W
Graphene Plasmonic Bolometer
> 1 GHz
12.2 µm
9
Graphene Bilayer Coupled to Microcavity
21 mA/W
Photocurrent (PV/PTE)
--
850 nm
14
≥1 µA/µm
19
GrapheneSilicon Heterojunction
435 mA/W
Schottky Diode
1 kHz
0.2-1 µm
65
