(Invited) Thermally Oxidized Yttrium Oxide on Germanium for n-Mos Capacitor and Field-Effect Transistor
Authors: Yamamoto Keisuke , Wen Wei-Chen , Wang Dong , Nakashima Hiroshi 
Published: February 2024

Germanium (Ge) has many exciting material characteristics, such as high carrier mobility and narrow bandgap in the near-infrared range. Thus it is suitable for various applications. A high-quality insulating film on Ge is required to apply Ge to novel electronic devices successfully. It is well known that GeO2 on Ge has good electrical characteristics as a gate insulator or interlayer (IL), similar to SiO2 on Si [1]. Unlike SiO2, however, GeO2 is thermodynamically unstable under atmospheric pressure at typical oxidation temperatures (~400 °C) and volatilizes as GeO [2,3]. Several methods have been developed to suppress GeO volatilization from the surface. For example, transition metals such as yttrium (Y) have been introduced into GeO2. Yttrium (Y)-doped GeO2 has excellent thermal, chemical, and electrical characteristics. In the capacitance–voltage measurement, it shows a low interface state density (D
it) and a narrow hysteresis corresponding to the border trap (BT) density (N
bt) of gate insulators [4]. Based on the above studies, we focused on metal Y deposition with subsequent thermal oxidation as an efficient alternative method to deposit Y-oxide as a gate insulator on Ge. This method has been studied to deposit Y2O3 as a gate insulator on Si [5]. Moreover, metal oxidation has been used to form various gate insulators on Ge, such as Al [6] and Hf [7]. In this study, we expect a capping layer of Y and oxidized Y to suppress GeO volatilization and stabilize the interface. We fabricated and evaluated metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (FETs) with either a thermally oxidized Y or a thermally oxidized Ge oxide layer. The structural analysis found that the insulator was divided into three layers: Y2O3, YGeO3, and GeOx from the top. The oxidation temperature affected only the thickness of the bottom GeOx layer. We found that the Y-oxide gate stack had better electrical characteristics and a lower D
it and N
bt than the thermally oxidized GeOx insulator. In contrast, the D
it–energy distribution and N
bt temperature dependence of the Y-oxide gate insulator were similar to those of the GeOx gate insulator. We examined these observations, including the structural analysis results. We found that thermally oxidized Y had a distinct advantage over thermally oxidized Ge oxide: the possibility of controlling the structure and electrical characteristics of the Ge gate stack, such as the GeOx thickness and the BT signal origin.

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