- Nano Express
- Open Access
Infrared Properties and Terahertz Wave Modulation of Graphene/MnZn Ferrite/p-Si Heterojunctions
© The Author(s). 2017
- Received: 19 May 2017
- Accepted: 26 July 2017
- Published: 8 August 2017
MnZn ferrite thin films were deposited on p-Si substrate and used as the dielectric layer in the graphene field effect transistor for infrared and terahertz device applications. The conditions for MnZn ferrite thin film deposition were optimized before device fabrication. The infrared properties and terahertz wave modulation were studied at different gate voltage. The resistive and magnetic MnZn ferrite thin films are highly transparent for THz wave, which make it possible to magnetically modulate the transmitted THz wave via the large magnetoresistance of graphene monolayer.
Infrared (IR) and terahertz (THz) devices are highly important for many electronic systems such as radar , wireless communication , and security systems . Thus it is critical to explore the materials [4–7] and structures [8–14] that can be used in the infrared and terahertz range. Recently it is found that the transmission of THz wave can be modulated with graphene field effect transistor (GFET) by tuning the intraband transitions of graphene monolayer . In their original GFET THz modulator, B. Sensale-Rodeiguez and coworkers use 92 nm SiO2 as the gate dielectric material, which achieved modulation depth of 15% and modulation speed of 18 Kb/s of THz wave . D. Zhang and coworkers investigated the optical THz modulation of graphene/SiO2 (150 nm)/p-Si GFET, which can be tuned by gate voltage .
Later, it was found that the THz wave modulation of GFET could be improved by replacing the gate dielectric with high-k and dense Al2O3 thin film, which is grown by atomic layer deposition . Modulation depth of 22% and speed of 170 kHz was achieved in the graphene/Al2O3 (60 nm)/p-Si GFET by varying the gate voltage . The improved modulation is attributed to the reduced Coulomb impurity scattering and cavity effect . Further, by using Bi-doped YIG (k ~12.0) as dielectric materials in the graphene/Bi:YIG (50 nm)/p-Si heterostructure, modulation depth of 15% and speed of 200 kHz were achieved from 0.1 to 1.2 THz by applying gate voltage .
According to previous studies, dielectric layer can largely affect the performance of GFET that was used for THz and infrared wave devices. By carefully screening the dielectric materials, it is possible to tune the performance of GFET. In prior studies, nonmagnetic high-k dielectric layers were used for terahertz and infrared GFET devices, where electrical signal is extracted or applied. However, bifunctional magnetic and dielectric layers have not been studied for GFET for terahertz and infrared applications, which could be tuned by external magnetic field. Here, we introduce 150 nm sputtered MnZn ferrite thin films as the dielectric materials of GFET for THz and infrared applications. As a high-k  and magnetic materials, MnZn ferrite thin films could perform as an excellent dielectric layer and also introduce new functionalities in the GFET THz and infrared devices. Response of the graphene/MnZn ferrite/p-Si GFET to the infrared illumination was observed by comparing the I-V curves with and without infrared illumination at different gate bias. Meanwhile, electrical modulation of THz wave was achieved by the GFET as the gate voltage was varied. Subtle change of transmitted THz wave was also observed as the external magnetic field was varied.
Mn1-xZnxFe2O4 thin films were prepared by RF magnetron sputtering. The target material was produced by co-precipitation of Fe(NO4)3, Mn(NO4)3, and Zn(NO4)2 solution, which is calcined at 950–1000 °C for 2 h, then pressed into a 60-mm disc, and finally sintered at 1250 °C for 3.5 h. The films were deposited on (100) p-Si substrates at 200–300 °C under base pressure of 4 × 10−4 Pa and oxygen concentration of 0–25% (PO2/(PO2 + PAr)). The film (150 nm) was annealed in vacuum between 400 and 700 °C under pressure of 0.08 Pa–5.0 Pa for 1.5 h.
The crystal structures of Mn1-xZnxFe2O4 thin films were characterized using Cu Kα X-ray diffraction (XRD, D/max 2400 X Series X-ray diffractometer, Tokyo, Japan) at 40 kV and 100 mA. The microstructures of the Mn1-xZnxFe2O4 thin films were investigated using a scanning electron microscope (SEM: JOEL JSM6490LV). The surface arithmetic average roughness (Ra) and root mean squared roughness (RMS) have been measured by an atomic force microscope (AFM: Veeco Mutimode Nano4). The saturation induction was tested by an Iwatsu BH analyzer (SY8232). The magnetic properties of the films were measured by a vibrating sample magnetometer (VSM, MODEL: BHV-525).
The roughness and grain size of MnZn ferrite thin film deposited at different RF power
RF sputtering power (W)
The length of maximum grains (nm)
The width of maximum grains (nm)
Figure 4c, d shows the comparison of the I-V curves under dark environment and infrared illumination for GFETs using 100 and 150 W sputtered MnZn ferrite thin films, respectively. The infrared light is at wavelength of 915 nm and power of 1 W in a window of ~1 cm2. The applied voltage between source and drain is 1 V. The I-V curve of the GFET under infrared illumination is analogous to that measured in the dark environment, however, with significantly enhanced current. The enhancement is much stronger for the GFET using 100 W sputtered MnZn ferrite thin films as dielectric layer than that using 150 W sputtered MnZn ferrite thin film. The enhancement is ~7.5 times at gate voltage of 10 V for 100 W sputtered MnZn ferrite thin film, which is ~2.5 times for the 150 W sputtered MnZn ferrite thin film. Namely, the surface roughness of MnZn ferrite thin films could also affect the infrared optoelectronic properties.
Graphene/MnZn ferrite/p-Si heterostructure was fabricated for IR and THz device applications. The MnZn ferrite thin film was deposited on the p-Si by magnetron sputtering, which was annealed before used for GFET fabrication. The MnZn ferrite thin films provide an alternative dielectric material for the GFET IR and THz devices. As a magnetic and high-resistive thin film, it can strengthen the magnetoresistance of graphene and modulation of transmitted THz without introducing additional insertion loss. The surface roughness of the MnZn ferrite thin film can largely affect the performance of the IR and THz devices. Higher performance could be achieved by making MnZn ferrite thin film smoother. Such work is in progress.
This work was financially supported by National Key Research Development Program (No. 2016YFA0300801), National Natural Science Foundation of China (Nos. 51401046, 61131005, 51572042), International Cooperation Projects (No.2015DFR50870), Sichuan Science and Technology Projects (Nos. 2014GZ0091, 2015GZ0069, 2014GZ0003), Fundamental Research Funds for the Central Universities (ZYGX2016J045), and the startup fund from the UESTC.
DNZ conceived the idea, made the devices, and did the characterization. MQW optimized and fabricated the MnZn ferrite thin films. TLW supervised the work and wrote the paper. YLL and LCJ participated in the infrared light characterizations. JL participated in the MnZn thin film fabrication. QYW analyzed the data. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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