Graphene Quantum Dot Bolometer Camera: Practical Approaches and Preliminary Results

Graphene is an exciting candidate for the detection of high-frequency electromagnetic radiation. Here, we have studied the bolometric performance of epitaxial graphene quantum dots (Q.D.s) on the silicon carbide (SiC) substrate in the terahertz (THz) range. The graphene Q.D. having a diameter in the 200 nm range, exhibited an extremely high resistance variation with temperature up to 4.7 M$\Omega$ K-1, a crucial parameter for the hot electron bolometers. The graphene Q.D.s bolometers have been fabricated in different geometrical configurations, such as variations in electrode spacing (2.5 and 5.0 $\mu$ m) and parallelly connected arrays of 4 and 8 Q.D.s. It is demonstrated that the absorbed power can be improved by tuning the bolometer geometrical configuration and the active graphene area, and the electrical responsivity is still very high for an extensive range of absorbed power. Additionally, we report that the photo response of graphene Q.D. bolometer devices is meagerly affected by the presence of a magnetic field as high as 15 T. The results presented here open ways to continue to optimize and realize the chip-scale matrix of the graphene Q.D.s bolometers for THz imaging and magneto-optical spectroscopy applications.


I. INTRODUCTION
he efficient detection of the mid-infrared (m-IR) to terahertz (THz) part of the electromagnetic spectrum has become essential for several applications, including imaging, free-space communication, meteorology, particle physics, chemistry, and astronomy [1][2][3][4].Considering the current technological demand for imaging, communication, security, and THz application [5][6][7][8], an efficient light detector is an essential instrument for detecting electromagnetic radiation [8,9].Graphene has been proven an excellent candidate due to its wide-band spectral response [9,10].Out of several remarkable properties, weak electron-phonon coupling, and low electronic heat capacity make graphene a suitable candidate for bolometric detection.The bolometric response of graphene has been described well in the existing literature.However, graphene bolometer devices suffer from critical technological challenges such as complex fabrication processes, weak temperature dependence of the resistance, difficulty for scaling up and integration of the device design, the requirement of external electric or magnetic fields to open a bandgap, etc.For example, the higher temperature dependence of resistance has been obtained either by the bandgap opening in graphene using dualgated bilayer structure [11] or applying robust localization of charge carriers by embodying defects in graphene [12].These techniques resulted in bolometric detection up to 10 6 V W -1 and the temperature coefficient of resistance up to 22 kΩ.K -1 at 1.5 K.However, these devices require a multilayer structure, which enhances the complexity of the fabrication process and complicates the scale-up.For bilayer graphene, top and bottom gates are required to electrically induce a bandgap, whereas the disordered graphene requires an additional dielectric layer (typically boron nitride) as the tunneling barrier between the graphene and the electrodes to reduce the thermal conductance.
Furthermore, the bandgap opening in graphene by applying a magnetic field to split Landau levels has also been reported [13].Bolometers based on superconducting tunnel junction [14], and nanobolometers based on superconducting transition-edge sensor [15,16] have already been proposed.The nano-bolometers based on superconducting transition edge have already met the single-photon requirement regarding noise equivalent power (NEP).However, their operation is only possible at a superconducting transition temperature of around 0.1 K.These challenges of graphene bolometers can be overcome by using other methods, as the hot electron graphene quantum dot (Q.D.) based bolometer devices reported in our previous studies [17,18,19] addresses a new direction, i.e., designing epitaxial graphene Q.D.s on SiC substrate where the bandgap is induced via quantum confinement.The graphene Q.D. bolometer devices exhibit an extraordinarily high variation of resistance with temperature (higher than 430 MΩ K-1), leading to responsivity of 10 10 V W -1 .This letter presents preliminary results of the technical feasibility of fabricating multiple Q.D. bolometers using epitaxial graphene grown on SiC substrate.In addition, we show that tuning the design can improve the bolometer's absorbed radiation power with an electrical responsivity that is still very high for an extensive range of absorbed power.
The bolometer devices have been fabricated in multiple geometrical configurations using electron beam lithography following the procedure developed by Yang and colleagues [20].The geometrical configuration of graphene Q.D. bolometer devices fabricated and characterized in this report is as follows: Chip number 1 contains two single Q.D. separated from each other and attached to the sharp edges of isosceles graphene triangular strips on both sides of the dot.The metal electrodes are deposited so that the separation between electrodes is 2.5 and 5 μm.Chip number 2 possess two devices made on an array of 4 and 8 Q.D.s and in which each dot is separately attached to the sharp edges of isosceles graphene triangular strips.The metal electrode spacing in 4 and 8 Q.D.s array devices are fixed at 6.0 μm, and electrodes are patterned so that each dot is electrically connected in parallel.The average diameter of Q.D.s is about 200 nm in each case else otherwise specified.In each case, the electrodes are patterned using Cr/Au on the base side of isosceles graphene triangular strips.Using this facile and straightforward scalable approach, we can reduce the total device impedance and improve the absorbed power values.The bolometers reported here do not require an additional buffer layer or intense magnetic field.The figure 1 shows the THz response of all the bolometer devices at base temperature 3 K under 0.15 THz irradiation.The absorbed THz power for the bolometer device having a single 200 nm graphene Q.D. with 2.5 μm electrode spacing is found to be 3.8 pW (Fig. 1 (a)).The voltage change (ΔVDC) corresponding to the constant bias current ~ 165 pA due to THz radiation is 102.5 mV providing the electrical responsivity as high as 2.7 × 10 10 V W -1 .The bolometer device (single graphene Q.D.) having 5.0 μm electrode separation demonstrates an increase in absorbed THz power compared to the former due to increased graphene area.The absorbed THz power for the bolometer device with the 5.0 μm channel length is 10.9 pW, as depicted in Fig. 1 (b).Under THz irradiation at 3 K, the 5.0 μm channel length device demonstrates the voltage change of 73.5 mV at a fixed bias current of 371 pA, which results in the electrical responsivity of 6.8 × 10 9 V W -1 .This electrical responsivity value is referred to as absorbed THz power and is consistent with the expected responsivity estimated based on Joule heating only.The validity of this concept has been elaborately discussed in our previous reports [18,19,21].Moreover, the bolometer device having 4 Q.D.s array as the active layer absorbs THz power around 0.9 pW and results in the voltage change ΔVDC = 53.4mV, which leads to the electrical responsivity of 5.9 × 10 10 V W -1 at a fixed current of 155 pA.Likewise, the absorbed THz power in the bolometer device having 8 Q.D.s array as the active area is found to be ~ 16.6 pW.Therefore, the voltage change at 3 K due to absorbed THz radiation is 70.8 mV at a fixed current of 1.3 nA, which results in electrical responsivity as high as 4.3 × 10 9 V W -1 .The electrical responsivity of the 8 Q.D.s array bolometer device decreases by one order of magnitude compared to the single Q.D. bolometer device having electrode separation of 2.5 μm and 4 Q.D.s array bolometer device.As the photo response is sublinear as a function of absorbed power, the responsivity decreases with increasing radiation power, but it is still very high for an extensive range of absorbed power [19].Furthermore, the electrical responsivity can be improved by adding a gate electrode to enhance the performance of bolometer devices in terms of the temperature dependence of the resistance.However, a complete and careful study of bolometric response versus the gate voltage and the multiple Q.D. bolometers is in progress.

II. SUMMARY
In conclusion, we demonstrate an easy way to scale up the bolometer array while preserving its intrinsic properties.

Fig 1 .
Fig 1.The response of 0.15 THz radiation on the I-V characteristics of the bolometer devices.Single graphene Q.D. bolometer devices (a) having 2.5μm length between electrodes and (b) having 5.0 μm length between electrodes.Multiple Q.D.s array (c) 4 Q.D.s array bolometer device and (d) 8 Q.D.s array bolometer device.The diameter of Q.D.s is 200 nm in each case.The data is recorded at 3 K.Each plot's red/black lines correspond to THz ON/OFF states.