Advanced Devices & Sustainable Energy Laboratory (ADSEL), Bradley Department of Electrical and Computer Engineering and ‡Fluids Research Laboratory, Department of Geosciences, Virginia Tech , Blacksburg, Virginia 24061, United States.
ACS Appl Mater Interfaces. 2017 Dec 13;9(49):43315-43324. doi: 10.1021/acsami.7b06601. Epub 2017 Nov 30.
Because of the high carrier mobility of germanium (Ge) and high dielectric permittivity of amorphous niobium pentoxide (a-NbO), Ge/a-NbO heterostructures offer several advantages for the rapidly developing field of oxide-semiconductor-based multifunctional devices. To this end, we investigate the growth, structural, band alignment, and metal-insulator-semiconductor (MIS) electrical properties of physical vapor-deposited NbO on crystallographically oriented (100), (110), and (111)Ge epilayers. The as-deposited NbO dielectrics were found to be in the amorphous state, demonstrating an abrupt oxide/semiconductor heterointerface with respect to Ge, when examined via low- and high-magnification cross-sectional transmission electron microscopy. Additionally, variable-angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to independently determine the a-NbO band gap, yielding a direct gap value of 4.30 eV. Moreover, analysis of the heterointerfacial energy band alignment between a-NbO and epitaxial Ge revealed valance band offsets (ΔE) greater than 2.5 eV, following the relation ΔE > ΔE > ΔE. Similarly, utilizing the empirically determined a-NbO band gap, conduction band offsets (ΔE) greater than 0.75 eV were found, likewise following the relation ΔE > ΔE > ΔE. Leveraging the reduced ΔE observed at the a-NbO/Ge heterointerface, we also perform the first experimental investigation into Schottky barrier height reduction on n-Ge using a 2 nm a-NbO interlayer, resulting in a 20× increase in reverse-bias current density and improved Ohmic behavior.
由于锗（Ge）的高载流子迁移率和非晶五氧化铌（a-NbO）的高介电常数，Ge/a-NbO 异质结构为基于氧化物半导体的多功能器件这一日益发展的领域提供了多个优势。为此，我们研究了物理气相沉积在结晶取向的（100）、（110）和（111）Ge 外延层上的 NbO 的生长、结构、能带排列和金属-绝缘体-半导体（MIS）电特性。通过低倍和高倍横截面透射电子显微镜检查，发现沉积的 NbO 电介质处于非晶态，相对于 Ge 显示出氧化物/半导体异质界面的突然转变。此外，变角光谱椭圆偏振术和 X 射线光电子能谱（XPS）被用来独立地确定 a-NbO 的带隙，得到直接带隙值为 4.30 eV。此外，对 a-NbO 和外延 Ge 之间的异质界面能带排列的分析表明，价带偏移（ΔE）大于 2.5 eV，符合关系 ΔE > ΔE > ΔE。同样，利用经验确定的 a-NbO 带隙，发现导带偏移（ΔE）大于 0.75 eV，同样符合关系 ΔE > ΔE > ΔE。利用在 a-NbO/Ge 异质界面观察到的减小的 ΔE，我们还首次使用 2nm 的 a-NbO 中间层对 n-Ge 进行肖特基势垒降低的实验研究，导致反向偏压电流密度增加 20 倍，并改善了欧姆行为。
