Takefumi Kamioka

Source-induced RDF overwhelms RTN in nanowire transistor: Statistical analysis with full device EMC/MD simulation accelerated by GPU computing

We numerically demonstrate that a random dopant fluctuation (RDF) in a source region causes a noticeable variability in the on-current of Si nanowire (NW) transistors, and its effect is much larger than that of a random telegraph noise (RTN). This work assesses the static and dynamic variability of NW device characteristics using the ensemble Monte Carlo/molecular dynamics (EMC/MD) simulation, which employs parallel computing technique using a graphic processing unit (GPU). The current flow in a one-dimensional NW device is determined by the number of dopants at the source edge, indicating the importance of forming an abrupt source-channel boundary to suppress the variability.

Si Island Formation on Domain Boundaries Induced by Ar Ion Irradiation on High-Temperature Si(111)-7 ×7 Dimer-Adatom-Stacking Fault Surfaces

Using a low-energy ion gun/high-temperature scanning tunneling microscope combined system (IG/STM), we observed the atomic scale behavior of a Si surface kept at 500°C before, during and after Ar ion irradiation. We found that Si islands grew up within ion irradiation selectively along the boundaries of domains of 7 ×7 dimer-adatom-stacking (DAS) faults. The Si islands were 1 double atomic layer high and had the 7 ×7 DAS reconstruction. The area of the islands increased linearly with ion doses up to 2.5 ×1014 cm-2.

Fabrication and performance analysis of a mechanical stack InGaP/GaAs//Si solar cell

We present the measurement results and analyses of a InGaP/GaAs//Si mechanical stack four-terminal solar cell. From these results, we performed detailed discussions of the loss mechanisms and efficiency limits of InGaP/GaAs//Si mechanical stack solar cells.

Simulation of interdigitated back-contact silicon heterojunction solar cells with quantum transport model

A simulation of interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells was performed using a quantum transport model to consider the quantum effect at the crystalline/amorphous (c/a) heterojunction interface. It was found that the impact of the quantum effect on the open-circuit voltage is comparable to that of the interface defect density at the c/a interface, indicating the importance of implementation of the quantum model. The optimal back-contact design was also discussed from the viewpoint of mass production, in which the design rule is relaxed. The degradation of the conversion efficiency by widening the gap between the p- and n-aSi:H layers can be compensated by improving passivation quality at the c/a interface.

Current fluctuation in sub-nano second regime in gate-all-around nanowire channels studied with ensemble Monte Carlo/molecular dynamics simulation

The impact of current fluctuation due to discreteness in carrier numbers on high-frequency noise amplitudes is numerically investigated, focusing on the comparison to the impact of a single trapped charge in the oxide layer for gate-all-around nanowire structures. The variation in the amount of the charge transporting through the channel within a single clock cycle is estimated. The transported charge variation due to the current fluctuation clearly shows the universality with respect to the total amount of the transported charge. It concludes that the current fluctuation becomes a dominant noise source over 100 GHz range.

Development of liquid-metal-ion source low-energy ion gun/high-temperature ultrahigh vacuum scanning tunneling microscope combined system

A liquid-metal-ion source low-energy ion gun/high-temperature ultrahigh vacuum scanning tunneling microscope combined system (LMIS-IG/STM) has been developed in order to investigate the ion beam modification process in situ based on our previous ion gun/STM combined system (IG/STM). Various kinds of metal ions can be irradiated with low acceleration energy of 0.01–5keV during STM observation at 400–600°C. As an example, real-time STM observation of Si(111)7×7 surface irradiated with Si2+ ions is demonstrated. The STM results have shown that the surface defects generated by Si2+ ion irradiation exhibit similar behavior of surface defects induced by Ar+ irradiation with IG/STM.

Full-scale whole device EMC/MD simulation of Si nanowire transistor including source and drain regions by utilizing graphic processing units

We have realized the full-scale whole device EMC/MD simulation including source and drain regions by utilizing graphic processing unit. The transfer characteristic of a gate-all-around nanowire Si MOSFET is simulated by reproducing the field effect of the surrounding gate electrode with spreading charged particles on the gate insulator layer. We have found an appreciable impact of the random dopant distribution (RDF) in source and drain regions on the drain current variability. Furthermore, the dynamic fluctuation of the drain current is found to be increase as the channel length decreases. The EMC/MD simulation powered by GPU is a useful method to investigate the dynamic fluctuation as well as the statistical device-to-device variability of nano-scale FETs.

Electrical conduction of Si/ITO/Si junctions fabricated by surface activated bonding

The electrical properties of n-Si/ITO/n-Si, n-Si/ITO/p-Si, and p-Si/ITO/n-Si junctions fabricated by surface activated bonding (SAB) were investigated. The current-voltage (I-V) characteristics of n-Si/ITO/n-Si, n-Si/ITO/p-Si, and p-Si/ITO/n-Si junctions showed excellent linearity. The interface resistance of n-Si/ITO/p-Si junctions was found to be 0.0249 O·cm2, which is the smallest value observed in all the samples.

Real-Time Scanning Tunneling Microscopy Observation of Si(111) Surface Modified by Au+ Ion Irradiation

The real-time scanning tunneling microscopy (STM) observation of Au+ ion irradiation effects on a high-temperature Si surface has been performed using our original ion gun and STM combined system. Sequential STM images of a Si(111)-7×7 surface kept at 500 °C have been obtained before, during, and after Au+ ion irradiation with 3 keV. Vacancy islands, which are two-dimensional clusters of surface vacancies, and 5×2-Au structures were formed on the sample surface, and their size were changed during the heat treatment after the ion irradiation. This method enables us to count exact numbers of vacancies and Au atoms on the surface by measuring the sizes of vacancy islands and 5×2-Au reconstructions. The timescale of the growth of the 5×2-Au domain suggests that the implanted Au atoms diffuse to the surface almost without interacting with point defects induced by the ion irradiation.

Analysis of interface workfunction and process-induced damage of reactive-plasma-deposited ITO/SiO2/Si stack

Workfunction of reactive-plasma deposited indium-tin-oxide (RPD-ITO) at the ITO/SiO2 interface, which is referred as interface workfunction, and the process-induced damage are experimentally extracted for the first time based on capacitance-voltage (C-V) analysis. The estimated interface workfunction value of 4.74 eV for as-deposition condition is about 0.4 eV higher than that in the bulk determined by ultraviolet photoelectron spectroscopy (UPS). The RPD process induces the damage at the Si/SiO2 interface, and the degree of the damage is evaluated as the interface defect density (Dit) to be around 1012 cm-2eV-1. The effects of forming-gas annealing on the interface workfunction and recovery of the damage are also studied. The interface workfunction value once decreases to 4.53 eV by the annealing up to 250 oC and then turns to increase to 4.77 eV after 400 oC annealing. The damage is annihilated by the low-temperature forming-gas annealing at 200 oC.