Iori Hideshima

Comprehensive study and design of scaled metal/high-k/Ge gate stacks with ultrathin aluminum oxide interlayers

Advanced metal/high-k/Ge gate stacks with a sub-nm equivalent oxide thickness (EOT) and improved interface properties were demonstrated by controlling interface reactions using ultrathin aluminum oxide (AlOx) interlayers. A step-by-step in situ procedure by deposition of AlOx and hafnium oxide (HfOx) layers on Ge and subsequent plasma oxidation was conducted to fabricate Pt/HfO2/AlOx/GeOx/Ge stacked structures. Comprehensive study by means of physical and electrical characterizations revealed distinct impacts of AlOx interlayers, plasma oxidation, and metal electrodes serving as capping layers on EOT scaling, improved interface quality, and thermal stability of the stacks. Aggressive EOT scaling down to 0.56 nm and very low interface state density of 2.4 × 1011 cm−2eV−1 with a sub-nm EOT and sufficient thermal stability were achieved by systematic process optimization.

Thermal Robustness and Improved Electrical Properties of Ultrathin Germanium Oxynitride Gate Dielectric

Robustness of ultrathin germanium oxynitrides (GeON) formed by plasma nitridation of thermal oxides (GeO2) on Ge(100) substrates [K. Kutsuki et al.: Appl. Phys. Lett. 95 (2009) 022102] was investigated by means of physical and electrical measurements. The decomposition temperature of a 3.7-nm-thick GeON layer was found to increase up to 550 °C by plasma nitridation, which was about 100 °C higher than that of pure GeO2. While the insulating property of GeON dielectrics begins to degrade just below the decomposition temperature, i.e., at around 540 °C, thermal treatment up to 520 °C effectively improves the electrical properties of the ultrathin GeON dielectrics, such as recovery of bulk defects and quite low interface state density (Dit) even for the ultrathin gate dielectrics. The advantage of GeON dielectrics in designing a fabrication process for Ge-based devices and the physical origins of the improved properties will be discussed.

Insight into unusual impurity absorbability of GeO2 in GeO2∕Ge stacks

Adsorbed species and its diffusion behaviors in GeO(2)∕Ge stacks, which are future alternative metal-oxide-semiconductor (MOS) materials, have been investigated using various physical analyses. We clarified that GeO(2) rapidly absorbs moisture in air just after its exposure. After the absorbed moisture in GeO(2) reaches a certain limit, the GeO(2) starts to absorb some organic molecules, which is accompanied by a structural change in GeO(2) to form a partial carbonate or hydroxide. We also found that the hydrogen distribution in GeO(2) shows intrinsic characteristics, indicative of different diffusion behaviors at the surface and at the GeO(2)∕Ge interface. Because the impurity absorbability of GeO(2) has a great influence on the electrical properties in Ge-MOS devices, these results provide valuable information in realizing high quality GeO(2)∕Ge stacks for the actual use of Ge-MOS technologies.

Interface engineering between metal electrode and GeO2 dielectric for future Ge-based metal-oxide-semiconductor technologies

Interfacial reactions between a metal-gate electrode and GeO2 dielectric in Ge-based metal-oxide-semiconductor (MOS) devices have been investigated by several analytical techniques, and we have demonstrated a method to suppress the interfacial reactions. Although no reaction occurs at the Au/GeO2 interface, a significant reaction was observed at the Al/GeO2 interface, which leads to increases in the leakage current and defect states in an MOS capacitor. While Al is oxidized at the Al/GeO2 interface, GeO2 is reduced to form Ge-Ge and Ge-Al bonding units during the early stage of the Al deposition. Moreover, the Ge-Al alloy segregates to the Al-electrode surface during the sequent Al deposition. These interfacial reactions are dramatically suppressed by insertion of ultrathin Al2O3 into the Al/GeO2 interface.

Improved Electrical Properties and Thermal Stability of GeON Gate Dielectrics Formed by Plasma Nitridation of Ultrathin Oxides on Ge(100)

We demonstrated the impact of plasma nitridation on thermally grown GeO2 for the purposes of obtaining high-quality germanium oxynitride (GeON) gate dielectrics. Physical characterizations revealed the formation of a nitrogen-rich surface layer on the ultrathin oxide, while keeping an abrupt GeO2/Ge interface without a transition layer. The thermal stability of the GeON layer was significantly improved over that of the pure oxide. We also found that although the GeO2 layer is vulnerable to air exposure, a nitrogen-rich layer suppresses electrical degradation and provides excellent insulating properties. Consequently, we were able to obtain Ge-MOS capacitors with GeON dielectrics of an equivalent oxide thickness (EOT) as small as 1.7 nm. Minimum interface state density (Dit) values of GeON/Ge structures, i.e., as low as 3 x 1011 cm-2eV-1, were successfully obtained for both the lower and upper halves of the bandgap.

High-quality GeON gate dielectrics formed by plasma nitridation of ultrathin thermal oxides on Ge(100)

High-quality germanium oxynitride (GeON) gate dielectrics for Ge-based metal-oxide-semiconductor (MOS) devices were fabricated by plasma nitridation of ultrathin thermal oxides on Ge(100) substrates. Although ultrathin oxides with abrupt GeO2/Ge interfaces can be formed by conventional dry oxidation, air exposure results in serious electrical degradation. It was found that plasma nitridation forms a nitrogen-rich capping layer on the ultrathin oxide and significantly improves thermal stability of the GeON layer. The nitrogen-rich layer effectively suppresses electrical degradation during air exposure and provides excellent insulating properties. Consequently, we were able to achieve Ge-MOS capacitors with GeON dielectrics of an equivalent oxide thickness (EOT) as small as 1.7 nm. Minimum interface state density (Dit) values of GeON/Ge structures, i.e., as low as 3 × 1011 cm−2eV−1, were successfully obtained for both the lower and upper halves of the bandgap.

Advantage of high-density plasma nitridation for improving thermal stability of ultrathin GeO 2 on Ge(100)

We have investigated the thermal stability of ultrathin germanium oxynitride (GeON) gate dielectrics fabricated by plasma nitridation of ultrathin thermal oxide (GeO2). Thermal treatment up to 520°C effectively improved the electrical properties of the ultrathin GeON dielectrics, such as reduced bulk and interface defects.