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We report the comparison of the atomic layer deposited polycrystalline (p-)/amorphous (a-) HfO2 bilayer gate dielectric with p-HfO2/a-Al2O3 for enhanced $\beta $ -Ga2O3 metal oxide semiconductor capacitors (MOSCAPs). A discrepancy in the temperature-dependent hysteretic behaviors is observed, and thus, pulse capacitance–voltage ( ${C}$ – ${V}$ ) measurements are employed to investigate interface properties at the border between the amorphous (Al2O3 or HfO2) and polycrystalline (HfO2) oxide layers. The proposed p-HfO2 /a-HfO2 exhibits similar interface trap densities ( ${D}_{\text {it}}$ ) at the interface between the bilayer dielectric and $\beta \text {-Ga}_{{2}}\text {O}_{{3}}$ , and shows lower border trap density ( ${N}_{\text {bt}}$ ). An effective barrier height of 1.62 eV for the p-HfO2 /a-HfO2 with a breakdown field of 9.1 MV/cm, which is higher than 1.01 eV of p-HfO2 /a-Al2O3, is obtained from the current density-voltage ( ${J}$ – ${V}$ ) characteristics using the Folwer–Nordheim model. Moreover, as compared with a p-HfO2 single layer, the additional a-HfO2 layer on top of p-HfO2 is beneficial to improve hysteretic and leakage characteristics, while maintaining its high-quality interface. The p-HfO2/a-HfO2 bilayer with improved interface properties as well as a large positive flat-band voltage of 4.7 V is a promising candidate for the metal/high- ${k}$ stack of $\beta $ -Ga2O3 MOSCAPs.

Germanium (Ge) has gained great attention not only for future nanoelectronics but for back-end of line (BEOL) compatible monolithic three-dimensional (M3D) integration recently. For high performance and low power devices, various high-k oxide/Ge gate stacks including ferroelectric oxides have been investigated. Here, we demonstrate atomic layer deposited (ALD) polycrystalline (p-) HfO2/GeOX/Ge stack with an amorphous (a-) HfO2 capping layer. The consecutively deposited a-HfO2 capping layer improves hysteretic behaviors ( ${\Delta } {V}$ ) and interface state density ( $\text{D}_{\mathrm{ it}}$ ) of the p-HfO2/GeOX/Ge stack. Furthermore, leakage current density ( ${J}$ ) is significantly reduced $(\times 100)$ by passivating leakage paths through grain boundaries of p-HfO2. The proposed HfO2 layer with the graded crystallinity suggests possible high-k/Ge stacks for further optimized Ge MOS structures.