GR-FET application for high-frequency detection device
© Mahjoub and Ochiai; licensee Springer. 2013
Received: 20 July 2012
Accepted: 18 November 2012
Published: 10 January 2013
A small forbidden gap matched to low-energy photons (meV) and a quasi-Dirac electron system are both definitive characteristics of bilayer graphene (GR) that has gained it considerable interest in realizing a broadly tunable sensor for application in the microwave region around gigahertz (GHz) and terahertz (THz) regimes. In this work, a systematic study is presented which explores the GHz/THz detection limit of both bilayer and single-layer graphene field-effect transistor (GR-FET) devices. Several major improvements to the wiring setup, insulation architecture, graphite source, and bolometric heating of the GR-FET sensor were made in order to extend microwave photoresponse past previous reports of 40 GHz and to further improve THz detection.
KeywordsGraphene Microwave application Terahertz detection Frequency response Bolometric effect Nonlinear effect Ambient condition
Graphene (GR) has become one of the most well-known carbon nanomaterials due to its unique optical, electrical, and thermal properties which arise from its unique 2D hexagonal honeycomb crystal structure. This unique structure directly influences the band structure of graphene, which is governed by a quasi-Dirac electron system that varies from the single-layer case with gapless spectrum characteristics to the bilayer case with a small forbidden gap[1, 2]. In both instances, the band gap can be ideally tuned in order to match the low-energy photons in the gigahertz (GHz)/terahertz (THz) regime. This is in marked contrast to conventional semiconductors whose band gaps appear several orders of magnitude larger. For these reasons, graphene field-effect transistors (GR-FETs) have the potential to exceed the detection limit of most existing semiconductor quantum point contacts[3, 4]. This is due to the unique phase-coherent length of open quantum dot structures that can be formed in bilayer graphene when exposed to GHz/THz radiation. An additional benefit of the GR-FET platform in relation to structures based on carbon nanotubes includes the high level of similarity with conventional integrated semiconductor FET fabrication techniques. Considering the mentioned benefits, GR-FETs are emerging as excellent candidates for developing a broadly tunable GHz/THz sensor. In particular, the realization of THz detection will be important for future developments in medical imaging, spectroscopy, and communication, which all exploit the unique linear nonionizing benefits of THz radiation.
Existing GR-FETs have been fabricated by micromechanical exfoliation of highly oriented pyrolytic graphite (HOPG-SG2) contacted with two-terminal submicron-scale metal electrodes (Ti/Au or Pd/Au). The microwave transconductance characteristics show excellent photoresponse around the X band (approximately 10 GHz) but quickly cut off thereafter. The observed cutoff frequency was determined to be a result of the measurement wiring rather than the intrinsic response of the graphene. The positive results of this study indicate that THz detection is possible and that many of the same experimental components could remain constant for THz irradiation experiments. Hence, this study presents the results of such THz irradiation experiment, where the same sample box design used in the previous GHz response measurement was used to test the THz detection capabilities of several GR-FETs. The results of this study and of the former GHz response study revealed numerous complementary areas for improvement. Therefore, this work also investigates experimental improvements to the wiring setup, insulation architecture, graphite source, and bolometric heating detection of the GR-FET sensor in order to extend microwave photoresponse past previous reports of 40 GHz and to further improve THz detection.
Results and discussion
Based on our previous discussion of the microwave transport properties in GR-FET devices, the possibility to utilize GR for THz detection has become a more practical goal. Following the previously discussed approach, a clear response to THz radiation has been observed using the setup shown in Figure2. The fluctuations in the response of the device can be explained by considering the influence of bolometric and nonlinearity effects within the GR material. Exposure to THz radiation will inevitably induce these effects depending on the nature of the sample, whether it is monolayer with semimetallic behavior or bilayer with semiconductor behavior, resulting in a change in the resistance. Referring back to the original resistance's room temperature dependence in Figure3, the outcome of Figure2 can be understood to be the result of a strong bolometric response that increases the resistance in the metallic-type devices and decreases the resistance in the semiconductor-type devices. In addition, nonlinearity effects play an important role in influencing the response of semiconductor-type devices to THz radiation. Nonlinear response occurs because the band gap excitation energy matches the incident wave frequency.
The main aspects of characterization were indicated by the small arrows in the previous response curves of Figure5; the arrows simply indicate two sets of information. The first aspect is the change in the average resistance value for the transition from the THz-OFF state to the THz-ON state. The second aspect is the instantaneous value of the resistance at the two moments where THz radiation starts and the moment where THz radiation is terminated.
Overall, this experiment reveals the interplay of different photoresponse mechanisms primarily involving rectification due to THz radiation in the presence of nonlinearity and bolometric heating effects on the transport properties of GR-FET devices. The observation of such bolometric responses, especially at ultrahigh frequencies, is a highly prized characteristic for a variety of device applications. Similarly, such a response has been observed for GaAs, which confirms the bolometric behavior observed in the GR-FET device, even at ambient conditions.
The observation of a high-frequency response in GR-FETs beyond 40 GHz has clarified the importance of power and intensity in microwave transmission. Following a previous study in semiconductor QD THz sensing, a basic frequency characteristic has already been defined using a conventional microwave transconductance measurement. Building on these findings, this experiment presents a systematic study which explored the GHz/THz detection limit of both bilayer and single-layer GR-FETs. THz irradiation experiments revealed the interplay of different photoresponse mechanisms, primarily involving nonlinearity and bolometric heating effects on the transport properties of the GR-FET device. The bilayer GR samples show a clear visible - faster and larger - photoresponse change in comparison to the monolayer sample. This is a direct result of the small apparent band gap that exists in the bilayer GR materials. The observation of such bolometric responses, especially at ultrahigh frequencies, is a highly prized characteristic for a variety of device applications. Additionally, the microwave response of both the single- and bilayer GR-FET was significantly extended from previous reports by improving the wiring setup, insulation architecture, and heat dissipation of the GR-FET nanosensor. Even in the case of the GR two-terminal system, an excellent response was observed under room-temperature conditions. Therefore, it is possible to conclude that the GR strip line detector system serves as a valuable means to analyze high-frequency response measurements and that GR-FETs will work effectively as room-temperature GHz-THz sensors.
YO is a regent professor; NA is an associate professor; AMM, TA, YI, and TO are graduate students; MK is a postdoctoral candidate; TO is a professor; and KM is an assistant professor from the Graduate School of Advanced Integrated Science at Chiba University. AN is an undergraduate student from the Chemistry Department at the University of Minnesota-Twin Cities. JPB is a professor in the Electrical Engineering Department, SUNY at Buffalo. DKF is a regent professor in the Department of Electrical Engineering, Arizona State University. KI is a professor in the Advanced Device Laboratory at the Institute of Physical and Chemical Research (RIKEN).
Graphene field-effect transistor
This work is supported in part by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (19054016, 19204030, and 16656007) and by the JSPS Core-to-Core Program. This work was also in part supported by the Global COE Program at Chiba University (G-03, MEXT) and promoted by the international research and educational collaboration between Chiba University and SUNY Buffalo. Acknowledgement is extended to the National Science Foundation's Partnerships for the International Research & Education (NSF-PIRE) grant (OISE-0968405).
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