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 defects induced outside the Au electrode by infiltrated molecules may enhance the minority carrier generation, and thus acting as a minority carrier source just like MOS field-effect transistors.

FIG. 3. ͑Color online͒ Evaluation of interface quality of GeON/Ge structures. ͑a͒ C-V curves of Au/GeON/Ge capacitor measured at 112 K. Measured frequency ranged from 50 kHz to 1 MHz. ͑b͒ Energy distribution of D it for GeON/Ge structures obtained using low temperature conductance method. Thickness of initial BOs is also indicated.

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.

Adsorbed species and its diffusion behaviors in GeO2/Ge stacks, which are future alternative metal-oxide-semiconductor (MOS) materials, have been investigated using various physical analyses. We clarified that GeO2 rapidly absorbs moisture in air just after its exposure. After the absorbed moisture in GeO2 reaches a certain limit, the GeO2 starts to absorb some organic molecules, which is accompanied by a structural change in GeO2 to form a partial carbonate or hydroxide. We also found that the hydrogen distribution in GeO2 shows intrinsic characteristics, indicative of different diffusion behaviors at the surface and at the GeO2/Ge interface. Because the impurity absorbability of GeO2 has a great influence on the electrical properties in Ge-MOS devices, these results provide valuable information in realizing high quality GeO2/Ge stacks for the actual use of Ge-MOS technologies.

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.