Optimizing Infrared and Terahertz Modulation in Graphene/MnZn Ferrite/p‑Si Heterojunctions

Background

Infrared (IR) and terahertz (THz) technologies are integral to radar, wireless communication, and security systems [1–3]. Consequently, the search for materials and heterostructures that operate efficiently in these spectral windows remains a priority [4–14]. Recent work has shown that a graphene field‑effect transistor (GFET) can modulate THz transmission by tuning intraband transitions in the graphene monolayer [8]. For example, a GFET with 92 nm SiO₂ as gate dielectric achieved a 15 % modulation depth at 18 kHz [8], while a graphene/SiO₂ (150 nm)/p‑Si device reached 22 % depth at 170 kHz when Al₂O₃ (60 nm) was used as a high‑k gate insulator [16]. A Bi‑doped YIG (k ≈ 12) dielectric similarly enabled 15 % depth at 200 kHz across 0.1–1.2 THz [17].

These studies demonstrate that the dielectric layer largely dictates GFET performance in the IR and THz regimes. While nonmagnetic high‑k dielectrics have been extensively explored, integrating a bifunctional magnetic dielectric—capable of external magnetic tuning—remains largely uncharted. MnZn ferrite, a high‑k (k ≈ 16) and ferromagnetic oxide, offers a promising platform to combine dielectric robustness with magnetic control. Here, we report the deposition of 150‑nm MnZn ferrite films on p‑Si, their integration into GFETs, and the resulting IR phototransport and THz modulation under electrical and magnetic stimuli.

Methods

Mn_1‑xZn_xFe₂O₄ thin films were fabricated by RF magnetron sputtering. The target, produced via co‑precipitation of Fe(NO₃)₃, Mn(NO₃)₃, and Zn(NO₃)₂, was calcined at 950–1000 °C for 2 h, pressed into a 60‑mm disc, and sintered at 1250 °C for 3.5 h. Films were deposited on (100) p‑Si substrates at 200–300 °C under a base pressure of 4 × 10⁻⁴ Pa with 0–25 % O₂ (P_O2/(P_O2 + P_Ar)). A 150‑nm film was then annealed in vacuum (0.08–5.0 Pa) at 400–700 °C for 1.5 h.

Structural analysis employed Cu Kα X‑ray diffraction (XRD, D/max 2400, Tokyo, Japan) at 40 kV/100 mA. Morphology was examined via SEM (JOEL JSM6490LV) and AFM (Veeco Mutimode Nano4) to extract arithmetic average roughness (Ra) and root‑mean‑square roughness (RMS). Magnetic properties were measured with a vibrating sample magnetometer (VSM, BH–V525) and saturation induction with an Iwatsu BH analyzer (SY8232).

After optimizing the MnZn ferrite growth conditions, monolayer graphene was transferred from copper foil onto the ferrite films using a standard PMMA‑assisted method [19,20] to form graphene/MnZn ferrite/p‑Si heterostructures. Gold source, drain, and gate electrodes were evaporated to complete the GFETs (see Scheme 1). Electrical characteristics were recorded with an Agilent 4155B parameter analyzer. IR response was evaluated by measuring I–V curves under 915 nm illumination (1 W, ~1 cm²) versus dark. THz transmission was measured with a time‑domain spectroscopy (TDS) system while varying gate voltage and applying an external magnetic field generated by a custom copper coil.

The GFET architecture incorporating 150‑nm MnZn ferrite as the gate dielectric.

Results and Discussion

Figure 1 shows XRD patterns for Mn_1‑xZn_xFe₂O₄ films deposited at RF powers of 100–180 W. All samples exhibited a spinel lattice, with the (311) reflection dominating at 160 W, indicating optimal crystallinity. Table 1 (not shown) reports that Ra and RMS increase monotonically with sputtering power, suggesting that higher energy deposition roughens the surface—an effect that can degrade GFET performance.

XRD spectra of MnZn ferrite on p‑Si (100) for RF powers 100, 120, 140, 160, and 180 W.

SEM and AFM imaging (Figure 2) confirmed columnar grains that coarsen upon annealing. XRD after annealing (Figure 3a) revealed that 550 °C maximized the (311) intensity. Magnetic hysteresis loops (Figure 3b) measured at room temperature showed a saturation magnetization (M_s) of 330 kA m⁻¹ and coercivity (H_c) of 1600 A m⁻¹ (≈20 Oe) at 550 °C under 0.5 Pa nitrogen, the optimum annealing condition. Elevated temperatures or gas pressures introduced nitrogen‑related defects, reducing M_s and increasing H_c.

SEM (a, b) and AFM (c, d) images of as‑deposited and annealed MnZn ferrite films.

Annealing effects on (a) XRD, (b) hysteresis, (c) M_s versus pressure, and (d) M_s versus temperature.

GFETs were fabricated on films sputtered at 100 and 150 W. The drain‑source current (I_sd) versus gate‑source voltage (V_sg) curves (Figure 4a,b) show a pronounced electron‑channel activation under negative gate bias, while positive bias yields a weaker response, likely due to asymmetric thermionic emission and interband tunneling at the gated/access interface [21]. The device with the 100‑W film exhibits lower resistance than its 150‑W counterpart, reflecting the smoother dielectric surface and reduced charge‑carrier scattering.

IR characterization: (a) I_sd–V_sg for 100‑W film, (b) 150‑W film, (c) and (d) the same curves under 915 nm illumination (1 W).

Under 915 nm illumination, both devices display a marked current increase, but the 100‑W device benefits from a ~7.5× amplification at 10 V, compared to ~2.5× for the 150‑W device. This disparity underscores the influence of dielectric roughness on photo‑induced carrier transport.

THz transmission was evaluated by sweeping gate voltage from +25 V to –25 V. As the channel resistance drops with negative bias, the transmitted THz intensity decreases (Figure 5a), demonstrating gate‑controlled modulation. When an external magnetic field is applied, the transmitted THz intensity falls further, saturating above 50 Oe (Figure 5b). This subtle 5 % modulation is attributed to the large magnetoresistance of graphene under fringe fields from the MnZn ferrite, though surface non‑uniformity limits the effect. A smoother dielectric would likely enhance magneto‑THz modulation.

THz characterization: (a) Transmittance spectrum (0.2–1.0 THz) at gate voltages –25 to +25 V, (b) Spectrum under 0.63–0.70 THz for varying magnetic fields.

Conclusions

We have demonstrated that MnZn ferrite thin films, when deposited and annealed under optimized conditions, serve as a high‑k, magnetically responsive dielectric for graphene‑based heterojunctions. The resulting GFETs exhibit significant IR photoconductive enhancement and gate‑controlled THz transmission, with a modest magnetic field tunability that stems from the large graphene magnetoresistance. Device performance is strongly correlated with dielectric surface roughness; further smoothing of the MnZn ferrite layer is expected to boost both IR and THz modulation depths. Future work will focus on refining film morphology to unlock the full potential of magnetically tunable IR/THz devices.