Key Experimental Findings

Ferroelectric Properties of E-beam Evaporated HZO Thin Films

• P–Ecurves of MFM structures show remnant polarization (P_r) increasing with annealing temperature up to 800 °C; optimized at 600 °C with P_r reaching 60 µC/cm² at 30 °C/s heating rate.
• Powder XRD patterns confirm orthorhombic (o-phase) peak emergence at 30.5° above 600 °C and stronger with higher heating rates.
• Piezoelectric force microscopy exhibits 180° phase reversal, confirming ferroelectric switching.

Fabrication and Electrical Performance of FE-HEMTs

• One-step E-beam deposition of 2 nm Al₂O₃/20 nm HZO/Ni/Au (30/100 nm) gate stack on MOCVD-grown GaN HEMT (L_G= 10 µm, W_G= 110 µm) without breaking vacuum.
• Transfer characteristics yield I_on/I_off = 10⁸ and SS = 64.4 mV/dec.

Programmable Current Switching

• Pre-polarization voltages induce V_TH shift from –2.14 V (negative poling) to –0.93 V (positive poling), total swing of 1.21 V.
• Saturation current I_DS modulates from <100 mA/mm to 275 mA/mm; repeatable cycling between 50 mA/mm and 250 mA/mm over three cycles.

Mechanistic Insights

• Ferroelectric polarization charges couple with the 2DEG at the AlGaN/GaN interface, causing carrier accumulation (negative poling) or depletion (positive poling).
• Sheet carrier density n_schanges from 5.73 × 10¹² cm^-2 (+5 V poling) to 9.68 × 10¹² cm^-2 (–10 V poling) via C–V measurements.
• Carrier mobility extracted from R_SD–V_GS linear fitting decreases from 1 947 cm²/(V·s) (positive poling) to 1 561 cm²/(V·s) (negative poling) due to electron-electron and polar optical phonon scattering.

Technological Implications

The findings directly enable:

• Integration of D-mode and E-mode HEMTs within a single architecture via V_TH modulation.
• Programmable current-switching behavior for GaN-based logic circuits with non-volatile ferroelectric phase storage.
• Low-cost, CMOS-compatible fabrication alternative to ALD using E-beam evaporation.

Challenges and Future Directions

The work highlights the impact of ferroelectric polarization on electron transport at the AlGaN/GaN interface. Future integration should focus on further optimization of annealing conditions and gate stack thickness to enhance coupling efficiency.