High Aspect Ratio β-Ga₂O₃ FinFETs with Near-Zero Hystersis and Low On-Resistance by Matel-Assisted Chemical Etching

Beta-Gallium Oxide (β-Ga₂O₃) is a promising material for power electronics due to its ultra-wide band gap of ~ 4.8 eV, a reasonable electron mobility, and the availability of bulk substrates. Nonetheless, β-Ga₂O₃ transistors reported so far still suffer from low current density compared to GaN devices. Thus, forming high aspect ratio channel structures to achieve lower on-resistance is still crucial for β-Ga₂O₃ transistors. Although through RIE, β-Ga₂O₃ vertical transistors with 9.27 aspect ratio has been fabricated,[1] the high energy ion induced damage from RIE degrades the device performance, leading to a limited effective channel mobility. In addition, the ion-induced damages result in a hysteresis in all β-Ga₂O₃ transistor utilizing RIE process (ref?). On the other hand, β-Ga₂O₃ fins with high aspect ratio and low interface trap density have been demonstrated by the metal-assisted chemical etching (MacEtch) process (ref). Moreover, an almost hysteresis-free CV loop was achieved on the MacEtch-formed β-Ga₂O₃ structures, making it suitable for β-Ga₂O₃ transistor fabrication. In this work, we demonstrate hysteresis-free β-Ga₂O₃ FinFETs with 1.5 mΩ-cm² specific on-resistance (R_on,sp) and over 10:1 aspect ratio through the MacEtch process.<br/>A 2 μm-thick lightly doped β-Ga₂O₃ layer was grown on a (010) Fe-doped semi-insulating substrate by MOCVD, with Si doping concentration of ~ 4×10¹⁷ cm^-3. Then, channel and source/drain region were defined through lithography followed by 20 nm Pt MacEtch catalyst deposition with an ebeam evaporator. After a standard lift-off process, the samples were immersed in a mixture of hydrofluoric acid (HF, 49%) and potassium persulfate (K₂S₂O₈) under deep UV illumination to etch and form fin-shaped channels and source/drain mesa. Then, Al₂O₃ was deposited through an ALD process followed by 1 min RTA with N₂ ambient. A Ti/Au gold gate electrodes were sequentially deposited on the high-k layer to form the gate stack. Finally, 1 min RTA with N₂ ambient was applied for source/drain ohmic contact formation. Through the MacEtch process, β-Ga₂O₃ FinFETs with high aspect ratio channels and smooth sidewalls were produced. The top width and fin height are ~ 170nm and ~2.1 μm, respectively, leading to an overall 10:1 aspect ratio. In addition, slight lateral etching on the β-Ga₂O₃ fins also occurred during the MacEtch process, resulting in the trapezoid-like channel shapes.<br/>DC transfer characteristics of the β-Ga₂O₃ FinFETs were measured and the drive current obtained is ~1.9×10^-5A (4.34A/m when normalized to 2×Fin height + top width) under V_ds=5V, V_ov=5V and L_g=1μm. The on/off ratio is around 10⁶ and gate leakage current is at only ~ 10 pA level, suggesting a gate stack with good electrostatic control and low leakage. A dependence of threshold voltages on the fin dimension is also observed, causing the FinFETs to shift from depletion mode (normally on) to enhancement mode (normally off) as the fin width decreases. This tunability of the threshold voltage provides the freedom to choose transistor operation mode depends on the application. The subthreshold swings (SS) are extracted to be 101 mV/dec., indicating the interface trap density is low on the MacEtch-formed sidewalls. Moreover, all the transfer characteristics demonstrate almost zero hysteresis. The largest hysteresis is only 55 mV clockwise, which is small compared to the typical 120 – 800 mV hysteresis of reported β-Ga₂O₃ FETs.[1-3] This hysteresis-free characteristic could be attributed to the absence of RIE-induced ion damages and traps by the virtual of the MacEtch. The R_on,sp extracted from the slope of low V_ds region is 1.5mΩ-cm² with V_ov = 6 V, which is low compared to other published β-Ga₂O₃ FETs (ref). Detailed output characteristics and breakdown measurement of β-Ga₂O₃ FinFET will be presented.<br/>Acknowledgement: NSF ECCS 18-09946.<br/>[1] W.Li, et al., 2019 IEDM, 2019. [2] K.Chabak et al., APL, 2016 [3] Z.Hu et al., EDL, 2018