Tomonori Nishimura

Evidence for strong Fermi-level pinning due to metal-induced gap states at metal/germanium interface

The purpose of this paper is to understand metal/germanium (Ge) junction characteristics. Electrode metals with a wide work function range were deposited on Ge. All metal/p-Ge and metal/n-Ge junctions have shown Ohmic and Schottky characteristics, respectively, with the strong Fermi-level pinning. The charge neutrality level (CNL) at metal/Ge interface is close to the branch point calculated for the bulk Ge. Moreover, the pinning level is hardly modulated by annealing in forming gas, forming metal-germanide/Ge interfaces or changing the substrate orientation. These results suggest that Fermi level at metal/Ge interface is intrinsically pinned at the CNL characterized by the metal-induced gap states model.

A Significant Shift of Schottky Barrier Heights at Strongly Pinned Metal/Germanium Interface by Inserting an Ultra-Thin Insulating Film

At any metal/germanium (Ge) interfaces, Schottky junctions to n-Ge and ohmic ones to p-Ge are formed by the strong Fermi level pinning to the valence band edge of Ge. In this paper, we report that Schottky-ohmic characteristics are reversed by inserting an ultra-thin oxide film into the metal/Ge interface. A gradual change of Schottky barrier heights (SBHs) with increasing insulating film thickness has been found, which supports that the origin of Fermi level pinning at the metal/Ge junction is caused by the metal-induced gap states. Furthermore, the SBH change enables us to operate metal source/drain Ge n-channel metal–oxide–semiconductor field effect transistors (n-MOSFETs) without any impurity doping. We demonstrate the metal source/drain Ge n-MOSFET with a peak mobility of 270 cm2/(Vs).

Direct Evidence of GeO Volatilization from GeO2/Ge and Impact of Its Suppression on GeO2/Ge Metal–Insulator–Semiconductor Characteristics

From the studies on the thermal desorption behaviors of GeO2 film and its impact on the electrical properties of GeO2/Ge metal–insulator–semiconductor (MIS) capacitors, it was clarified that the GeO volatilization is driven by the interface reaction at GeO2/Ge, and that volatilization is the origin of the interface deterioration of the MIS capacitors. We found that a Si cap layer formed on top of the GeO2 film suppresses the GeO desorption very efficiently. Then, a marked improvement of the capacitance–voltage (C–V) characteristics was successfully demonstrated with the GeO2/Ge MIS capacitors fabricated by capped annealing process, where a Ni silicide electrode was used as the cap layer. These results provided us quite an important guide for realizing high-quality Ge/dielectric interfaces.

Ge/GeO2 Interface Control with High-Pressure Oxidation for Improving Electrical Characteristics

High-pressure oxidation (HPO) of germanium (Ge) for improving electrical properties of Ge/GeO2 stacks was investigated. The capacitance–voltage (C–V) characteristics of metal/GeO2/Ge capacitors fabricated with HPO revealed improved electrical properties without any post-deposition annealing, and the interface states density (Dit) was reduced to 2×1011 eV-1 cm-2 near the midgap. Moreover, the refractive index of thermally oxidized GeO2 was increased by HPO. It is also discussed from a thermodynamic viewpoint of the Ge/GeO2 system that the GeO desorption from Ge/GeO2 stacks could be efficiently suppressed by HPO.

High-Electron-Mobility

We propose a two-step oxidation with high-pressure oxidation and low-temperature oxygen annealing to form ideal Ge/GeO2 stacks based on thermodynamic and kinetic control. The capacitance-voltage (C-V) characteristics of Ge/GeO2 MISCAPs with two-step oxidation revealed significant improvements of electrical properties, and the interface states density (Dit) estimated with a low-temperature conductance method that was below 1011 eV-1 cm-2 near the midgap. On the basis of our understanding of Ge oxidation, we demonstrated very high electron mobility in Ge n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) that exceeded the universal mobility in Si-MOSFETs. The peak electron mobility in Ge n-MOSFETs with the two-step oxidation was 1100 cm2/V · s in the Al/GeO2/Ge stack. This was achieved by taking care of the Ge/GeO2 channel interface. Since we clarified that mobility was still limited by the remaining extrinsic scattering sources, the present results promise much higher performance Ge complementary metal-oxide-semiconductor.

\hbox{Ge/GeO}_{2}

High-κ dielectrics on Ge have recently attracted much attention as a potential candidate to replace planar silicon transistors for sub-32-nm generations. However, the instability of the high-κ/Ge interface, especially the desorption of germanium monoxide (GeO), hampers the development of Ge-based devices. Therefore, the typical GeO2/Ge structure was chosen to investigate GeO desorption. In this contribution, we describe the desorption kinetics of GeO, including Ge/GeO2 interface reaction, the diffusion process during GeO desorption, the desorption activation energy of GeO, the different mechanisms of GeO desorption, and the active oxidation of Ge. Through annealing GeO2/Ge in an ultrahigh vacuum (UHV), direct evidence for the consumption of Ge substrate has been shown by atomic force microscopy (AFM) measurements of the consumption depth. By using thermal desorption spectroscopy (TDS) measurements and studying oxygen-18 isotope tracing, we have clarified that the GeO desorption is not caused by the GeO di...

n-MOSFETs With Two-Step Oxidation

The electric properties of mono- and multi-layer graphene films were systematically studied. The current modulation increased monotonically with a decrease in the layer number due to the reduction of the interlayer scattering. The carrier mobility in the monolayer was greater than that in the multilayer due to the linear dispersion relation. On the other hand, in the monolayer, the carrier transport was significantly sensitive to the charged impurity density due to the reduction in the screening effect, which caused larger mobility variation. The reduction of the charged impurity density is thus key for high mobility.

Desorption kinetics of GeO from GeO2/Ge structure

The intrinsic channel properties of monolayer and multilayer graphene were systematically investigated as a function of layer number by the exclusion of contact resistance using four-probe measurements. We show that the continuous change in normalized sheet resistivity from graphite to a bilayer graphene is governed by one unique property, i.e., the band overlap, which markedly increases from 1 meV for a bilayer graphene to 11 meV for eight layers and eventually reaches 40 meV for graphite. The monolayer graphene, however, showed a deviation in temperature dependence due to a peculiar linear dispersion. Additionally, contact resistivity was extracted for the case of typical Cr/Au electrodes. The observed high contact resistivity, which varies by three orders of magnitude (from ~103 to 106 Ω µm), might significantly mask the outstanding performance of the monolayer graphene channel, suggesting its importance in future research.

Mobility Variations in Mono- and Multi-Layer Graphene Films

This letter presents a significant improvement of electron mobility in a germanium (Ge) n-channel metal–oxide–semiconductor field-effect transistor with a yttrium oxide (Y2O3) gate dielectric film annealed in high-pressure O2. Interface state density in the upper half of the band gap is reduced to 1011 cm-2 eV-1 and the peak effective mobility is increased up to 1,500 cm2 V-1 s-1. This mobility enhancement is attributed to the suppression of GeO desorption or to passivation of the imperfect interface GeO2 layer by diffused Y2O3. There is no temperature dependence of the observed mobility, which suggests that intrinsic phonon scattering is still not dominant.

Systematic Investigation of the Intrinsic Channel Properties and Contact Resistance of Monolayer and Multilayer Graphene Field-Effect Transistor

We have demonstrated very high electron mobility in Ge n-MOSFETs which exceeds the universal one in Si-MOSFETs. The peak electron mobility on Ge n-MOSFETs is about 1100 cm2/Vsec in Al/GeO2/Ge stack. This has been achieved by taking care of Ge/GeO2 channel interface based on thermodynamic and kinetic control. Since it is clarified that the mobility is still limited by remaining scattering sources, the present results promise us to expect much higher performance Ge CMOS.