SWCNT‑FeFETs with HfO₂ Defect Control Achieve Record On/Off Ratios, Endurance, and Retention

Abstract

We have engineered ferroelectric field‑effect transistors (FeFETs) that use single‑walled carbon nanotube (SWCNT) stripes as the channel, a (Bi,Nd)₄Ti₃O₁₂ (BNT) ferroelectric film as the gate dielectric, and a thin HfO₂ layer as a defect‑control buffer. The resulting devices exhibit a high channel conductance, an on/off current ratio exceeding 2×10⁵, a carrier mobility of 395 cm²/V·s, and robust fatigue endurance and data‑retention performance. Despite the very thin effective gate capacitance, the HfO₂ layer limits the gate‑leakage current to 3.1×10⁻⁹ A/cm² at −3 V.

Background

FeFETs are promising for non‑volatile memory and low‑power logic because they combine high speed, a simple single‑device structure, and non‑destructive readout [1–3]. BNT is a lead‑free ferroelectric with excellent chemical stability and fatigue endurance, making it an attractive gate dielectric that can reduce threshold voltage and enhance channel conductance. Carbon nanotubes (CNTs) are widely used in FeFETs for their superior conductivity and mobility [4–7]. Ideal CNTs have no dangling bonds, which minimizes interface reactions with the ferroelectric film [8,9]. However, fabricating a single CNT between source and drain is experimentally challenging, and CNT networks often contain metallic tubes that lower the on/off ratio [7,10]. Prior work has used multi‑walled CNT (MWCNT) stripes to overcome these issues, but fatigue and retention of such devices remain unclear [9]. SWCNTs, being seamless single‑layer graphene tubes, offer higher quality and fewer defects than MWCNTs [11]. Defects such as ion impurities, oxygen vacancies, and dislocations in the ferroelectric film can diffuse and degrade device performance [12–16]. A thin HfO₂ layer can block defect diffusion, act as a lattice‑matching buffer between BNT and Si, and thus reduce dislocation density in the BNT film, improving the on/off ratio, fatigue endurance, and retention.

Methods

FeFETs were fabricated on heavily doped n‑type Si (ρ = 0.0015 Ω·cm) used as the back gate. A 20 nm HfO₂ layer was deposited by pulsed laser deposition (PLD) with a KrF excimer laser (248 nm). A 300 nm BNT film was then grown by PLD as described in earlier work [17]. SWCNTs (length 10–30 µm, diameter 0.8–1.1 nm, 85 % purity) were assembled by evaporation‑induced self‑assembly: a 100 mg/L SWCNT/water dispersion was evaporated at 80 °C with a rate of 9–21 µL/min, forming micron‑wide stripes on the BNT/HfO₂/Si substrate. Pt source/drain electrodes (4.5 mm² each) were sputtered by ion‑beam deposition through a 1 cm² mask. The channel length and width were 200 µm and 1500 µm, respectively. A post‑anneal at 500 °C for 2 h improved electrode–CNT contact. Metallic SWCNTs were selectively removed by a high‑gate‑voltage pulse, leaving a semiconducting network. For comparison, SWCNT/SiO₂‑FETs and MWCNT/BNT‑FeFETs were fabricated under the same conditions. Electrical characterization used a Keithley 4200 parameter analyzer; ferroelectric hysteresis was measured with a RT Precision Workstation analyzer.

Results and Discussion

Figure 1a shows the stripe‑patterned SWCNT network; the stripes appear as bright, densely packed ridges, while the underlying BNT/HfO₂/Si substrate is visible as darker, recessed areas. SEM images (Fig. 1b–d) confirm the crystalline, porous BNT/HfO₂ surface compared to smoother BNT alone. Hysteresis loops (Fig. 1e) reveal that BNT/HfO₂ films exhibit higher polarization than BNT alone, indicating reduced defect diffusion when grown on HfO₂.

Output characteristics (Fig. 2) demonstrate typical p‑channel behavior. Both SWCNT/BNT/HfO₂‑FeFET and SWCNT/BNT‑FeFET reach a saturated source‑drain current of 3.8×10⁻² A and conductance of 9.5×10⁻³ S at VGS = −4 V, VDS = 4 V. However, the HfO₂‑based device shows markedly lower off‑state current and negligible leakage at VGS = 0 V, confirming effective defect blocking.

Transfer curves (Fig. 3) give threshold voltages of 0.2 V (HfO₂) and 0.8 V (no HfO₂) and mobilities of 395 cm²/V·s and 300 cm²/V·s, respectively. Logarithmic transfer plots (Fig. 4) yield on/off ratios of 2×10⁵, 2×10⁴, and 2.3×10² for SWCNT/BNT/HfO₂‑FeFET, SWCNT/BNT‑FeFET, and SWCNT/SiO₂/HfO₂‑FET, respectively. The HfO₂ layer thus significantly improves the on/off ratio by suppressing defect‑driven leakage.

Memory window widths measured from the IDS–VGS hysteresis are 4.2 V (HfO₂) and 4.1 V (no HfO₂), far exceeding the 1.1 V observed in CNT/PZT‑FeFETs [20], indicating strong ferroelectric coupling. The retention study (Fig. 5) shows the SWCNT/BNT/HfO₂‑FeFET maintains an on/off ratio of 3×10⁴ after 10⁶ s, with only 3.2 % degradation, and extrapolation predicts a ratio of 1.9×10⁴ after 10⁸ s. In contrast, devices without HfO₂ retain only 3×10³ (SWCNT/BNT) and 2×10² (MWCNT/BNT). Leakage current measurements (Fig. 6) confirm that BNT/HfO₂ films have an an order‑of‑magnitude lower leakage (1.2×10⁻⁹ A) than BNT alone (1.5×10⁻⁸ A) at −3 V, consistent with reduced space‑charge density (2.1×10¹⁷ cm⁻³ vs 1.4×10¹⁹ cm⁻³).

Fatigue endurance (Fig. 7) shows a 3 % loss after 10¹¹ cycles for the HfO₂ device, compared with 10 % and 25 % for SWCNT/BNT‑FeFET and MWCNT/BNT‑FeFET, respectively. The thin HfO₂ buffer effectively blocks Si diffusion and ion impurities, preserving ferroelectric polarization.

Simulations (Fig. 8) of P–E hysteresis and IDS–VGS curves using asymmetric charge models confirm that the HfO₂ layer reduces defect‑induced asymmetry, lowering off‑state current and enhancing device stability.

Conclusions

Integrating a thin HfO₂ defect‑control layer into SWCNT‑BNT FeFETs yields a low‑leakage (1.2×10⁻⁹ A at −3 V), high‑on/off ratio (2×10⁵), and high mobility (395 cm²/V·s). The device exhibits excellent fatigue endurance (3 % loss after 10¹¹ cycles) and robust data retention (on/off ratio > 1.9×10⁴ after 10⁸ s). These improvements stem from defect suppression and lattice matching provided by HfO₂, as confirmed by both measurements and simulations.

Abbreviations

BNT

(Bi,Nd)₄Ti₃O₁₂

FeFETs

Ferroelectric field‑effect transistors

MWCNT

Multi‑walled CNT

PLD

Pulsed laser deposition

SWCNT

Single‑walled carbon nanotube