Schottky Barrier Diode and MESFET based on Ge-Doped α-Ga₂O₃ Thin Film Grown by Mist-CVD

Gallium oxide (Ga₂O₃), which is one of the ultra-wide bandgap semiconductors, has attracted considerable attention because of its potential applications for power electronics. Among polymorphs of Ga₂O₃, β-Ga₂O₃ is the most stable phase. Although β-Ga₂O₃-based devices such as Schottky barrier diode (SBD), MESFET and MOSFET have been reported thus far, the fact that β-Ga₂O₃ substrate is expensive and has a low thermal conductivity is a problem from a point of view of practical applications. On the other hand, α-Ga₂O₃, which is one of the metastable phases, can be grown on a sapphire substrate by heteroepitaxial growth techniques such as mist-CVD, HVPE and MEB. The sapphire substrate is cost-effective and has a high thermal conductivity, which is preferable for device applications. Nonetheless, there are not so many reports on the α-Ga₂O₃-based devices including SBD, MESFET and MOSFET. This is partly because the control of carrier density and achievement of high mobility are not so easy compared with β-Ga₂O₃. Recently, we successfully obtained Ge-doped α-Ga₂O₃ thin films by using mist-CVD method with the wider carrier density ranging from 10¹⁶ to 10¹⁹ cm^-3 and the higher mobility 65.9 cm²V^-1s^-1 than previous reported Sn-doped α-Ga₂O₃.^[1] In this work, we report on the fabrication of SBD and MESFET based on the Ge-doped α-Ga₂O₃ thin films and confirm that the resultant devices exhibit rather high performance.<br/>Ge-doped α-Ga₂O₃ thin films were grown on <i>m</i>-plane sapphire substrates by mist-CVD method. An α-Ga₂O₃n⁺ layer (900-1000 nm thick, the donor density, <i>N</i>_d = 1×10¹⁹ cm³) and an α-Ga₂O₃n⁻ drift layer (500-600 nm thick, UID) were grown on the <i>m</i>-plane sapphire substrate for SBD. To make a mesa structure, the n⁻ layer was etched by ICP-RIE following photolithography. As an ohmic electrode, Ti/Au (20 nm/100 nm thick) or Ti/Al/Ti/Au (30 nm/100 nm/30 nm/ 30 nm thick) was deposited on the n⁺ layer by electron beam (EB) deposition. After that, the samples were annealed at 400, 450, 470 and 500 °C for 1 min in an N₂ atmosphere. Then, Ti/Au (20/100 nm thick) Schottky electrode was formed by EB deposition. The <i>I</i>-<i>V</i> characteristic of the resultant SBDs was measured and analyzed in terms of the thermionic emission model. The <i>n</i> value and the barrier height are 1.06-1.19 and 0.95-1.13 eV, respectively. The on-resistance decreases as the annealing temperature is increased. The lowest on-resistance is 3.7 mΩcm² achieved for the device with Ti/Au/Ti/Au ohmic electrode annealed at 500 °C.<br/>Also, we prepared MESFET as follows. First, Fe-doped buffer layer (800 nm thick), Ge-doped channel layer (300 nm thick) and n⁺ layer (30 nm thick) were grown on an <i>m</i>-plane sapphire substrate. For the mesa device isolation, 500 nm-thick α-Ga₂O₃ was etched by ICP-RIE. A Ti/Au (20/100 nm thick) was deposited as source and drain electrodes. The n⁺ layer on the channel layer was etched by ICP-RIE. A Ni/Au (30/100 nm thick) was deposited as a gate electrode. From the Hall effect measurements, the carrier concentration, the sheet carrier density, and the electron mobility were evaluated to be 2.1×10¹⁷ cm^-3, 4.9×10¹² cm^-2 and 44 cm²V^-1s^-1, respectively. The <i>I</i>−<i>V</i> measurements were conducted for the device with <i>L</i>_GS/<i>L</i>_G/<i>L</i>_GD = 3/3/12 μm, where <i>L</i>_GS, <i>L</i>_G, and <i>L</i>_GD are gate-to-source, gate and gate-to-drain lengths, respectively. From the output curve (<i>V</i>_DS−<i>I</i>_D characteristic), the maximum <i>I</i>_D and the on-resistance were estimated to be 24 mA/mm and 587 Ωmm (at <i>V</i>_GS = 2 V), respectively. The break down voltage is 364 V (at <i>V</i>_GS = -10 V). From the transfer curve (<i>I</i>_D−<i>V</i>_GD characteristic), the threshold voltage, the subthreshold slope and the on-off ratio were evaluated to be -9 V, 164 mV/dec and 10⁹, respectively. These values are superior compared to a-Ga₂O₃-based MESFET reported previously, for which the break down voltage, the on-off ratio and the maximum <i>I</i>_D are 48 V, 2×10⁷ and 35 mA.^[2]<br/>[1]<i>Phys. status solidi A</i> <b>217</b>, 1900632 (2020). [2]<i>IEEE Trans. Electron Devices</i> <b>62</b>, 3640 (2015).