Monolayer ScN Coupled with Ferroelectric ScAlN for E-Mode GaN HEMT

As the needs for cleaner energy and electric vehicles keeps growing, there is an inevitable demand for high efficiency power electronics, and group III-nitride materials such as aluminum nitride (AlN), gallium nitride (GaN) and their alloys have been demonstrated to have superior properties in power device applications due to their wide bandgaps. In the meantime, the polarization-induced 2-dimensional electron gas (2DEG) formed between the nitride heterostructure makes them great materials in high-electron-mobility-transistors (HEMTs). However, most (Al)GaN HEMTs operate in the depletion-mode (D-mode) which reduces the power efficiency while increasing the complexity in circuit design. Extensive research on GaN HEMTs have been focused on achieving enhancement-mode (E-mode) operation by recessed gate structure, fluorine plasma treatment and p-GaN gates yet these structures will suffer from poor stability or limited threshold voltage (<i>V</i>_th) tunability. Recent works on incorporating oxide-based ferroelectric materials coupled with charge trap layers in GaN HEMTs have shown to be another route to achieve E-mode operation yet it mostly requires an ex-situ deposition of the ferroelectric layer and a relatively thick charge trap layer. Over the past few years, a novel nitride ferroelectric nitride material, scandium aluminum nitride (ScAlN) has emerged in various applications due to its high breakdown field, high Curie temperature, etc. and can be grown in-situ with conventional HEMT structure in molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) system. Here, we present a fully-epitaxial E-mode HEMT with ferroelectric ScAlN gate coupled with monolayer scandium nitride (ScN) serves as the charge trap layer and AlGaN/GaN channel. The threshold voltage can be tuned to 1.3 V and shows great stability with a large ON/OFF ratio more than 10⁷. Complete structure from the AlGaN/GaN interface to the ScAlN/ScN gate are grown in MBE system under ultra-high vacuum to provide clean heterostructure interfaces, enabling a low subthreshold voltage (<i>SS</i>) of 61 mV/dec, which is close to the Boltzmann limit. This provides a pathway to implement wide-bandgap materials in efficient power devices with fail-safe operation.