(Invited) Thermally Oxidized Yttrium Oxide on Germanium for n-Mos Capacitor and Field-Effect Transistor

Abstract

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.[1] H. Matsubara et al., Appl. Phys. Lett. 93 (2008) 032104. https://doi.org/10.1063/1.2959731[2] S. K. Wang et al., J. Appl. Phys. 108 (2010) 054104. https://doi.org/10.1063/1.3475990[3] S. K. Sahari et al., J. Phys. Conf. Ser. 417 (2013) 012014. https://doi.org/10.1088/1742-6596/417/1/012014[4] C. Lu et al., J. Appl. Phys. 116 (2014) 174103. https://doi.org/10.1063/1.4901205[5] M. Gurvitch et al., Appl. Phys. Lett. 51 (1987) 919. https://doi.org/10.1063/1.98801[6] T. Ohno, et al., Appl. Phys. Lett. 107 (2015) 133107. https://doi.org/10.1063/1.4932385[7] T. Hosoi et al., Microelectron. Eng. 109 (2013) 137. https://doi.org/10.1016/j.mee.2013.03.115[8] W.-C. Wen et al., Mat. Sci. Semicond. Processing, accepted (2023).