Normally-Off Hydrogen-Terminated Diamond FET with High Temperature ALD Grown HfO₂

Diamond is one of the best candidates for high-power, high-frequency applications due to desirable properties like high breakdown electric field (10 MV/cm), high carrier mobility (3800 cm² V^-1s^-1 for holes and 4500 cm² V^-1s^-1 for electrons) and high saturation velocity (&gt;10⁷cm/s) [1].<br/>Due to the difficulty in doping by conventional methods, the two-dimensional hole gas (2DHG) layer below the hydrogen-terminated diamond surface has been widely explored for fabricating field effect transistors. Prior studies have reported sheet density in the range of 10¹²-10¹⁴cm^-3[2] and mobility of 50-200 cm² V^-1s^-1 [3] for the 2DHG, including integrated CMOS operation at high temperatures[4]. For FET fabrication, it is essential to choose a dielectric that can minimize the gate leakage current as well as preserve the 2DHG in the channel. Various dielectrics such as Al₂O₃, ZrO₂, AlN and MoO₃ have been used as a gate dielectric for diamond HFETs. HfO₂ is a widely used high-k dielectric material due its superior dielectric properties, high breakdown field and large band offsets (ΔE_c=2.29 eV and ΔE_v=1.98 eV) with respect to H-terminated diamond [5]. In this work, Al₂O₃ and HfO₂were compared as gate dielectric for HFETs. Normally off operation with two orders of higher current was achieved with high-temperature atomic layer deposition (ALD) grown HfO₂ in comparison with Al₂O₃ counterpart, as the gate dielectric.<br/><br/>For device fabrication, a (100) CVD-grown diamond substrate was used. The sample was first oxygen terminated by boiling in a 3:1 mixture of H₂SO₄ and HNO₃ at 200^oC for 30 minutes. Then, hydrogen termination was performed in a microwave plasma CVD reactor at 850^oC. This was followed by photolithography and oxygen plasma treatment for device isolation. Then, Ti/Pt/Au adhesion pads were deposited using e-beam evaporation. This was followed by gold deposition on hydrogen terminated regions to form the source and drain ohmic contacts. Subsequently, a 10 nm thick HfO₂layer was deposited at 300^oC using ALD. Then, Al/Au gate metal contact with a gate length (L_G) of 1.5 μm was formed. The gate to drain distance (L_GD) varied from 1.75 μm to 3.25 μm. Finally, source and drain contacts were opened using a 6:1 buffered oxide etchant (BOE) wet etch.<br/><br/>After device fabrication, DC output characteristics were measured using a probe station. A maximum drain output current of 2.05 mA/mm was achieved for a V_GS=-4V. Gate leakage current ≤ 10^-6 mA/mm and a current on/off ratio of 10⁷ were measured. A maximum transconductance (g_m) of 2.06 mS/mm was obtained at V_GS=-2V. The threshold voltage was determined to be -1.2V, indicating a normally-off performance. In contrast, our Al₂O₃-based FETs showed poor performance which requires further investigation.<br/><br/>The normally-off behavior in this device is likely due to the decrease in the upward bend bending at the H-diamond surface. This is possibly a result of the desorption of acceptor species during the high-temperature ALD growth. Dielectrics grown at high temperatures are thermally more stable, making the device suitable for high-temperature applications. Further improvement in the device performance can be possible by exposing the substrate to a NO₂gas environment and by using multiple dielectric layer stacks to improve the channel mobility. The current results show the potential of using high-temperature ALD grown HfO₂ for achieving normally-off behavior in diamond HFETs[6].<br/><br/>References:<br/>1. C. J. H. Wort et al, Mater. Today, vol. 11, Issues 1-2, pp. 22–28, 2008.<br/>2. H. Sato et al, Diamond and Related Materials, Volume 31, 2013.<br/>3. C.E. Nebel et al, Diamond and Related Materials, Volume 13, Issues 11–12, 2004.<br/>4. C. Ren et al, ACS Applied Electronic Materials, Oct. 2021.<br/>5. Z. Ren et al, AIP Advances 11, 035041 (2021).<br/>6. J. W. Liu et al, Appl. Phys. Lett. 103, 092905 (2013).