Yoshinari Kamakura

A MONTE CARLO SIMULATION OF ANISOTROPIC ELECTRON TRANSPORT IN SILICON INCLUDING FULL BAND STRUCTURE AND ANISOTROPIC IMPACT-IONIZATION MODEL

The physics of electron transport in bulk silicon is investigated by using a newly developed Monte Carlo simulator which improves the state‐of‐the‐art treatment of hot carrier transport. (1) The full band structure of the semiconductor was computed by using an empirical‐pseudopotential method. (2) A phonon dispersion curve was obtained from an adiabatic bond‐charge model. (3) Electron‐phonon scattering was computed by using a rigid pseudo‐ion model. The calculated scattering rate is consistent with the full band structure and the phonon dispersion curve of silicon, thus leaving no adjustable parameters such as deformation potential coefficients. (4) The impact‐ionization rate was calculated by using Fermi’s golden rule directly from the full band structure. We took into account the dielectric function depending on both wave vector and transition energy in the numerical calculation of the rate. The impact‐ionization rate obtained in the present study strongly depends on both wave vector and band index of t...

Impact ionization model for full band Monte Carlo simulation

The impact ionization rate in silicon is numerically derived from wave functions and energy band structure based on an empirical pseudopotential method. The calculated impact ionization rate is well fitted to an analytical formula with a power exponent of 4.6, indicating soft threshold of impact ionization rate, which originates from the complexity of the Si band structure. The calculated impact ionization rate shows strong anisotropy at low electron energy (e<3 eV), while it becomes isotropic at higher energy. Numerical calculation also reveals that the average energy of secondary generated carriers depends linearly on the primary electron energy at the moment of their generation. A full band Monte Carlo simulation using the newly derived impact ionization rate demonstrates that calculated quantum yield and ionization coefficient agree well with reported experimental data.

A comparison of numerical solutions of the Boltzmann transport equation for high-energy electron transport silicon

In this work we have undertaken a comparison of several previously reported computer codes which solve the semiclassical Boltzmann equation for electron transport in silicon. Most of the codes are based on the Monte Carlo particle technique, and have been used here to calculate a relatively simple set of transport characteristics, such as the average electron energy. The results have been contributed by researchers from Japan, Europe, and the United States, and the results were subsequently collected by an independent observer. Although the computed data vary widely, depending on the models and input parameters which are used, they provide for the first time a quantitative (though not comprehensive) comparison of Boltzmann Equation solutions. >

A new soft breakdown model for thin thermal SiO/sub 2/ films under constant current stress

Soft breakdown properties of thin gate oxide films are investigated using a constant current stress measurement. The soft breakdown can be classified into two different modes from the current conduction characteristics of post breakdown oxides: one of the modes shows a telegraph switching pattern and the other random noise. The generation probabilities of two soft breakdown modes and hard breakdown strongly depend on the stress current. Time-to-breakdown is well characterized by a universal function of stress conditions regardless of the breakdown modes. These experimental findings imply that all types of breakdown originate from the same precursor and the magnitude of the following local heating due to the transient current in a conductive micro spot determines the charge conduction properties after a breakdown event.

A new model of time evolution of gate leakage current after soft breakdown in ultra-thin gate oxides

The post-SBD degradation of ultra-thin gate oxides is investigated by means of experiments, theoretical modeling, and computer simulations. The gate leakage current after SBD increases gradually, and is finally limited by the parasitic resistance. A newly developed model shows that the gate leakage increase of post-SBD MOSFETs even under operating conditions causes a significant impact on LSIs in terms of the power consumption.

A detailed study of soft- and pre-soft-breakdowns in small geometry MOS structures

A new soft breakdown (SBD) model in weakly stressed thin gate oxides is proposed. Anomalous leakage current prior to soft breakdown is observed under weak current stress, which strongly indicates the cause of erratic retention error in flash memories. Further current stress induces two types of soft breakdown originating from the same precursor, i.e. multi-step tunneling. One of the SBD features current fluctuation accompanying non-switching 1/f noise and the other random telegraph noise. The difference between the two SBD modes reflect the consequence of different amount of Joule heat damage.

Electron mobility calculation for graphene on substrates

By a semiclassical Monte Carlo method, the electron mobility in graphene is calculated for three different substrates: SiO2, HfO2, and hexagonal boron nitride (h-BN). The calculations account for polar and non-polar surface optical phonon (OP) scatterings induced by the substrates and charged impurity (CI) scattering, in addition to intrinsic phonon scattering in pristine graphene. It is found that HfO2 is unsuitable as a substrate, because the surface OP scattering of the substrate significantly degrades the electron mobility. The mobility on the SiO2 and h-BN substrates decreases due to CI scattering. However, the mobility on the h-BN substrate exhibits a high electron mobility of 170 000 cm2/(V·s) for electron densities less than 1012 cm−2. Therefore, h-BN should be an appealing substrate for graphene devices, as confirmed experimentally.

Monte Carlo simulations of electron transport properties of diamond in high electric fields using full band structure

Electron transport properties in diamond under high electric fields (⩽5×106 V/cm) have been investigated by means of Monte Carlo simulations which include a full band structure, a wave-vector- and frequency-dependent dielectric function, phonon scattering rates with phonon dispersion relations, and impact ionization rates. The full band structure of diamond was calculated using an empirical pseudopotential method with an expansion of 113 plane waves, and was utilized to evaluate the dielectric function using the Lindhard method while suitable deformation potential coefficients were chosen in an adiabatic bond-charge model. Calculated results such as transition energies at the main points of symmetry and lines in the Brillouin zone as well as phonon dispersions were in good agreement with corresponding experimental data previously reported. The impact ionization rates of electrons in diamond were then evaluated from Fermi’s golden rule using the full band structure and dielectric function. The electric fie...

Boron segregation to extended defects induced by self-ion implantation into silicon

The evolution of boron segregation to extended defects during thermal annealing was studied with secondary ion mass spectrometry and cross-sectional transmission electron microscopy. Czochralski Si wafers with a boron concentration of 3×1017 cm−3 were implanted with 50 keV Si ion for doses from 5×1013 to 2×1015 cm−2 and then annealed at 720, 820, or 870 °C in nitrogen ambient for various annealing times. The evolution of boron segregation peaks to three types of dislocation loops, end-of-range (EOR) dislocation loops, clamshell defects, and Rp (the projected range) defects, is closely related to the evolution of dislocation loops. As annealing temperature and time increase, the boron segregation peaks grow, remain stable, and then disappear together with the dislocation loops. For lower temperature annealing, the boron segregation peaks grow more slowly and reach higher peak concentrations. In addition to the boron segregation to dislocation loops, boron segregation to {311} defects was also found. The bo...

Theoretical performance estimation of silicene, germanene, and graphene nanoribbon field-effect transistors under ballistic transport

Silicene or germanene is a monolayer honeycomb lattice made of Si or Ge, similar to graphene made of C. In this work, we have assessed the performance potentials of silicene nanoribbon (SiNR), germanene nanoribbon (GeNR), and graphene nanoribbon (GNR), which all have a sufficient band gap to switch off, as field-effect transistor (FET) channel materials. We have demonstrated that, by comparing at the same band gap of ∼0.5 eV, the GNR FET maintains an advantage over SiNR or GeNR FETs under an ideal transport situation, but SiNR and GeNR are attractive channel materials for high-performance FETs as well.