Shoichi Kabuyanagi

Dramatic effects of hydrogen-induced out-diffusion of oxygen from Ge surface on junction leakage as well as electron mobility in n-channel Ge MOSFETs

This paper discusses about effects of oxygen in Ge substrate on MOSFET performance from both viewpoints of advantages and disadvantages. For improvement of electron mobility in Ge n-MOSFETs, oxygen in the channel region should be extracted to suppress additional scattering. On the other hand, oxygen in S/D region is helpful for dramatically reducing junction leakage currents. By understanding these oxygen effects on Ge, high electron mobility Ge n-MOSFETs with the highest Ion/Ioff ratio are demonstrated.

Atomically flat planarization of Ge(100), (110), and (111) surfaces in H2 annealing

We demonstrate that Ge(100), (110), and (111) Ge surfaces are planarized with atomic level step and terrace structures in H2 annealing. The temperature required for such planarization is different among the three orientations. The step edge structure on the Ge(100) surface is composed of alternate smooth and rough steps (Sa + Sb steps) owing to the (2 ? 1) reconstruction on that surface. It is also shown that the terrace widths on the Ge(110) and (111) surfaces are on average controlled by adjusting the off-angle from the respective surface orientation.

Experimental evidence of zone-center optical phonon softening by accumulating holes in thin Ge

We discuss the impact of free carriers on the zone-center optical phonon frequency in germanium (Ge). By taking advantage of the Ge-on-insulator structure, we measured the Raman spectroscopy by applying back-gate bias. Phonon softening by accumulating holes in Ge film was clearly observed. This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart. Furthermore, we propose that the free carrier effect on phonon softening is simply understandable from the viewpoint of covalent bonding modification by free carriers.

Nondestructive characterization of oxide/germanium interface by direct-gap photoluminescence analysis

We demonstrate that direct-gap photoluminescence (PL) analysis is useful for the nondestructive, fast, and in-line characterization of the oxide/germanium (Ge) interface without making actual devices. The fact that Ge has a quasi-direct gap enables us to utilize direct-gap PL intensity as a good indicator for interface quality estimation. We experimentally confirm the validity of the present analysis, by comparing PL spectra with capacitance–voltage characteristics. The impact of the band bending at the interface on PL intensity is also discussed. Furthermore, the effect of forming gas annealing at oxide/Ge interfaces is discussed by taking advantage of the present method.

Energy band gap and phonon frequency in semiconductors are material constants

We report both Raman shift and photoluminescence (PL) in Ge as a function of carrier density instead of dopant concentration. The results show both the energy band gap (Eg) and zone-centered phonon frequency are dependent on free carrier density. In the low voltage device operation, it should be taken into consideration that material constants so far accepted are not really constant anymore.

Effects of free-carriers on rigid band and bond descriptions in Germanium — Key to designing and modeling in Ge nano-devices

This paper discusses free carrier effects on phonon and electronic structures in Germanium (Ge) films. The gate bias-dependent Raman and photoluminescence (PL) spectroscopies have been investigated by using back-biased GeOI FETs. The experimental results indicate that both phonon and electronic structures of Ge are dependent on gate bias in FETs with a fixed doping concentration. The results provide fundamental knowledge not only on semiconductor physics but also on nano-device modeling.

n + Si/pGe Heterojunctions Fabricated by Low Temperature Ribbon Bonding With Passivating Interlayer

A bonding technique via passivating interlayer formation is proposed for bulk material heterojunction fabrication. n+Si/pGe heterojunctions were fabricated by a ribbon bonding with interfaces passivated by an amorphous interlayer. With a highest process temperature as low as 150 °C, the bonded junctions exhibited rectifying characteristics with a turn-on voltage of 0.3 V as an ideal Si/Ge heterojunction and an ideality factor of 2.15. This technique shows a great potential for bulk material heterojunction formation, especially when ultimately abrupt junctions and low temperature processes are needed.

n+Si/pGe Cross Heterojunctions Fabricated by Narrow Membrane Bonding

nSi/pGe cross heterojunctions were fabricated by narrow membrane bonding with interfaces passivated by amorphous interlayer insertion for the first time. The turn-on voltage was around 0.3V indicating no significant charges induced by the bonded interfaces. The ideality factor of the diode was 1.2 indicating a well passivated interface. This narrow membrane bonding technology shows great potential for bulk material heterojunction formation, especially in cases that ultimately abrupt doping profiles are highly needed.

Demonstration of High Electron Mobility in Germanium n-channel Junctionless FETs

We have demonstrated the high electron mobility in Ge n-channel junction-less FET for the first time, and discussed its electrical properties. The on/off ratio of ~10 was achieved by thinning the Ge thickness down to 15 nm. The field effect mobility in the channel was estimated to be ~1000 cm/Vs. Then, a comparison of the value with the bulk mobilities of n-Si and n-Ge is made.