MOCVD Grown High-Al Content β-(Al_xGa_1-x)₂O₃ for Enabling 2DEGs

The unique properties of <i>β</i>-Ga₂O₃including its ultra-wide bandgap of ~4.8 eV, critical breakdown field of ~8 MV/cm, availability of large-area native substrates, and controllable n-type doping have enabled rapid development in both growth and device processing. However, compared to other wide bandgap semiconductors, the intrinsic channel mobility is relatively low. A potential pathway to overcome this mobility limit is through two-dimensional electron gas (2DEG) formed at the interface of δ-doped <i>β</i>-(Al<i>_x</i>Ga_1-<i>x</i>)₂O₃and undoped <i>β</i>-Ga₂O₃. In this work, abrupt and smooth heterojunctions were growth at high Al concentrations using metal organic chemical vapor deposition (MOCVD) with TEGa and TEAl precursors.<br/>Prior to growth, (010) <i>β</i>-Ga₂O₃ substrates were etched in hydrofluoric acid (HF) for 30 minutes to minimize parasitic interfacial Si doping. In pure <i>β</i>-Ga₂O₃, we have demonstrated mobilities of 156 and 114 cm²/Vs for 1x10¹⁷ and 1x10¹⁸ /cm³ Si doping, respectively, with sub-nm RMS roughness. However, as the Al concentration increases, multiple material challenges must be addressed including potential segregation, dopant activation, and maintaining film smoothness. Using TMAl and TEGa precursors, we demonstrate sub-2 nm RMS roughness for 130 nm <i>β</i>-(Al_0.09Ga_0.91)₂O₃films. However, the growth temperature of 900 °C is too high for δ-doping; to reduce the growth temperature, TMAl was replaced with TEAl as the Al precursor due to the reduced cracking temperature. Additionally, TEAl shows potentially lower carbon incorporation because it pyrolyzes via β-hydrogen elimination whereas the pyrolysis of TMAl can leave behind highly reactive methyl radicals that may incorporate into the film.<br/>Surface roughness, carrier activation, and mobility as a function of Al concentration and doping conditions are reported.<br/>Combining effective substrate etching to remove a parasitic conducting channel with smooth highly doped <i>β</i>-(Al<i>_x</i>Ga_1-<i>x</i>)₂O₃will enable high mobility 2DEGs which will open the opportunity for devices with high switching speeds to operate in extreme environments.