Gaku Okamoto

Origin of flatband voltage shift and unusual minority carrier generation in thermally grown GeO2/Ge metal-oxide-semiconductor devices

Improvement in electrical properties of thermally grown GeO2/Ge metal-oxide-semiconductor (MOS) capacitors, such as significantly reduced flatband voltage (VFB) shift, small hysteresis, and minimized minority carrier response in capacitance-voltage (C-V) characteristics, has been demonstrated by in situ low temperature vacuum annealing prior to gate electrode deposition. Thermal desorption analysis has revealed that not only water but also hydrocarbons are easily infiltrated into GeO2 layers during air exposure and desorbed at around 300 °C, indicating that organic molecules within GeO2/Ge MOS structures are possible origins of electrical defects. The inversion capacitance, indicative of minority carrier generation, increases with air exposure time for Au/GeO2/Ge MOS capacitors, while maintaining an interface state density (Dit) of about a few 1011 cm−2 eV−1. Unusual increase in inversion capacitance was found to be suppressed by Al2O3 capping (Au/Al2O3/GeO2/Ge structures). This suggests that electrical d...

Germanium oxynitride gate dielectrics formed by plasma nitridation of ultrathin thermal oxides on Ge(100)

Germanium oxynitride (GeON) gate dielectrics with surface nitrogen-rich layers were fabricated by plasma nitridation of thermally grown oxides (GeO2) on Ge(100). Insulating features of ultrathin GeO2 layers of around 2-nm-thick were found to improve with plasma treatment, in which leakage current was drastically reduced to over four orders of magnitude. Consequently, Au/GeON/Ge capacitors of an equivalent oxide thickness down to 1.7 nm were achieved while keeping sufficient leakage reduction merit. The minimum interface state density values of GeON/Ge structures as low as 3×1011 cm−2 eV−1 were obtained for both the lower and upper halves of the bandgap without any postnitridation treatments. These results were discussed based on the effects of plasma nitridation on a degraded GeO2 surface for recovering its electrical properties by creating stable nitride layers.

Characteristics of Pure Ge3N4 Dielectric Layers Formed by High-Density Plasma Nitridation

We have demonstrated the direct nitridation of Ge substrates to obtain pure germanium nitrides (Ge3N4). Physical characterization revealed that 3.5-nm-thick amorphous Ge3N4 layers with smooth surfaces and abrupt nitride/Ge interfaces were formed by the high-density plasma nitridation of Ge(100) substrates. We have investigated the thermal stability of the Ge3N4 layers, and found that the nitride was stable up to 550 °C and started to decompose around 580 °C under an N2 ambient, while maintaining smooth nitride surfaces during thermal decomposition. We also found that vacuum annealing did not affect the decomposition temperature and that nitrogen was the only desorption species during Ge3N4 decomposition, which led to the regrowth of smooth and crystalline Ge surfaces after the nitrides had been completely removed at 700 °C. These results demonstrate both the superior thermal stability of pure Ge3N4 as a gate insulator and feasibility of using nitride as a surface passivation layer in the fabrication of Ge-based devices.

Humidity-dependent stability of amorphous germanium nitrides fabricated by plasma nitridation

We have investigated the stability of amorphous germanium nitride (Ge3N4) layers formed by plasma nitridation of Ge(100) surfaces using x-ray photoelectron spectroscopy and atomic force microscopy. We have found that humidity in the air accelerates the degradation of Ge3N4 layers and that under 80% humidity condition, most of the Ge–N bonds convert to Ge–O bonds, producing a uniform GeO2 layer, within 12h even at room temperature. After this conversion of nitrides to oxides, the surface roughness drastically increased by forming GeO2 islands on the surfaces. These findings indicate that although Ge3N4 layers have superior thermal stability compared to the GeO2 layers, Ge3N4 reacts readily with hydroxyl groups and it is therefore essential to take the best care of the moisture in the fabrication of Ge-based devices with Ge3N4 insulator or passivation layers.

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.

Synchrotron Radiation Photoemission Study of Ge3N4/Ge Structures Formed by Plasma Nitridation

Chemical bonding states and energy band alignment of pure germanium nitride (Ge3N4) layers formed on Ge(100) surfaces by high-density plasma nitridation were characterized by synchrotron radiation photoemission spectroscopy (SR-PES). The core-level shift of 2.31 eV originating from Ge–N bonds (Ge4+) with respect to the bulk Ge 3d5/2 peak position (Ge0+) was determined by peak deconvolution of Ge 3d core-level spectra. In situ SR-PES study on changes in Ge 3d, N 1s, and O 1s core-level spectra during thermal annealing under ultrahigh vacuum (UHV) conditions revealed that oxidized surface layer on Ge3N4 film could be selectively removed at around 500 °C, which was 50 °C lower than the decomposition temperature of Ge3N4. Ge3+ component was found to increase with decreasing Ge4+ component during thermal decomposition of Ge3N4 while no significant change in Ge1+ and Ge2+ components. The Ge3N4 energy bandgap of 3.68 eV was experimentally determined from energy loss spectra of N 1s photoelectrons. The valence band offset at Ge3N4/Ge(100) interfaces were also estimated to be 1.65 eV from valence band spectra, and thus, the energy band alignment between Ge3N4 dielectrics and Ge substrate was determined.

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.

Thermal and humidity stability of Ge 3 N 4 thin layers fabricated by high-density plasma nitridation

Germanium has recently drawn tremendous attention due to its higher hole and electron mobility than those of Si. Nitride-based dielectrics are important and promising not only as a buffer layer to grow high-k gate dielectrics on Ge, but also as a gate insulator itself for Ge-based FETs, in spite of GeOx being unstable. While it has been reported that fabricating oxygen-free, pure Ge3N4 layers is hard to deposit, the fact that pure Ge nitride is available by using atomic nitrogen radicals has been reported by Maeda et al. Accordingly, first of all, we focused our attention on the direct nitridation of Ge substrates by using our custom-built high-density plasma source. Furthermore, in order to apply this Ge nitride to integrate high-k gate dielectrics with Ge and its alloy for FET-based devices, some issues related to the stabilities should be well understood first to optimize the device fabrication process. For these reasons, in this paper, we investigated thermal stability of Ge3N4 layers. In addition, since it is reported that Ge surface is very sensitive to moisture, we have also characterized stability of the Ge nitride layers against moisture in the air.