Masato Morifuji

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 model of impact ionization due to the primary hole in silicon for a full band Monte Carlo simulation

The rate of impact ionization due to the primary hole in silicon is numerically derived from pseudo‐wave‐functions and realistic energy band structure based on a nonlocal empirical pseudopotential method including the spin‐orbit interaction. The calculated impact‐ionization rate SII [s−1] is well fitted to an analytical formula with a power exponent of 3.4, indicating a soft threshold of the impact ionization rate: SII [s−1]=1.14×1012 [s−1 eV−3.4]×(e [eV]−1. 49 [eV])3.4, where e [eV] is the energy of the primary hole relative to the valence band edge. The soft threshold originates from the complexity of the silicon band structure. The calculated impact‐ionization rate shows strong anisotropy at low hole energies (e<3 eV), while it becomes isotropic at high hole energies, indicating the isotropy of the joint density of states at high energies. Numerical calculation also makes it clear that average energies of secondary generated carriers e depend linearly on primary hole energies at the moment of their ge...

Theoretical calculation of impact ionization rate in SiO2

Impact ionization rate in SiO2 was numerically calculated using both pseudo‐wave functions and energy band structure based on a self‐consistent pseudopotential method. To avoid numerical complexity due to amorphous structure, SiO2 was assumed to be a crystalline α‐quartz. The calculated impact ionization rate shows a strong wave vector anisotropy near a threshold energy regime, primary electrons existing at Γ point yield the strongest impact ionization rate. It was found that calculated results are not expressed by a Keldysh formula since SiO2 has complex band structure (e.g., indirect transition gap and nonparabolic bands). The magnitude of the theoretical impact ionization rate was very close to the experimental results recently reported by E. Cartier and F.R. McFeely [Phys. Rev. B 44, 10689 (1991)]. Detailed theoretical study clearly demonstrates that the average energy of secondary generated carriers depends linearly on the energy of primary electrons.

Novel Design of Current Driven Photonic Crystal Laser Diode

Contrary to the great possibilities of photonic crystal (PC), a practical laser diode with a PC cavity has not yet been developed. This is mainly due to the lack of electrically driven devices; Since a two-dimensional PC slab cladded by air is often employed to realize strong light confinement, it is difficult to inject carriers into a cavity in such an air-bridge structure. An electrically driven laser using a tiny semiconductor pillar for current injection has been reported; however, a large cross-section of the pillar (or low electrical resistance) trades-off a large Q-factor sufficient for lasing. In this letter, we propose a novel design of PC laser diode in which light confinement and carrier injection are highly compatible.

A New Surface Potential Based Poly-Si TFT Model for Circuit Simulation

A new surface potential based poly-Si TFT model for circuit simulation was developed, accounting for the influence of both deep and tail states across the band gap. The model describes the drain current for all regions of operation using the unified equation. Calculations using the drain current model produce results that are in good agreement with the measured I-V characteristics of poly-Si TFTs

Nitrogen δ-doping for band engineering of GaAs-related quantum structures

We study energy-band engineering with nitrogen delta (δ)-doping in GaAs-related quantum structures. A tight-binding calculation indicates that the band structure can be engineered by introducing the one-dimensional doping profile of nitrogen into GaAs. Using molecular beam epitaxy, we prepare δ-doped samples of AlGaAs/GaAs quantum wells and GaAs/δ-doped nitrogen superlattice structures at the growth temperature 560 °C. Photoluminescence obtained from the samples shows a clear redshift of the spectral peak positions dependent on the nitrogen coverage. The transition energies of the superlattice structures agree well with those obtained from photoreflectance, indicating the feasibility of band modification with a single or a multiple nitrogen δ-doped layer.

OBSERVATION OF RESONANT OPTICAL-PHONON ASSISTED TUNNELING IN ASYMMETRIC DOUBLE QUANTUM WELLS

Laser Raman microscope measurements in asymmetric double quantum wells with coupled narrow and wide quantum wells were performed to observe the nonequilibrium longitudinal-optical (LO) phonons that are generated by electrons during the phonon assisted tunneling. Both the Stokes and the anti-Stokes intensities show maxima at a certain applied voltage, where the calculated subband spacing between the wide and the narrow quantum well states is found to be equal to the LO phonon energy. This fact indicates that the population of nonequilibrium LO phonons becomes maximum when resonant LO phonon scattering occurs. A strong reduction in the photoluminescence intensity for the narrow quantum well is also observed at the same bias condition.

Wannier-Stark effect in superlattices

Electroreflectance measurements have been carried out in order to investigate Stark-ladder transitions in a GaAs (40 AA)/AlGaAs (20 AA) superlattice under various uniform electric fields, and compared with the transition energies calculated on the basis of a microscopic tight-binding theory. The observed electroreflectance spectra over a wide range of photon energies (1.5-2.2 eV) shift in proportion to an applied electric field. The signals in a higher photon energy region (1.4-2.2 eV) indicate the existence of a transition from the spin-orbit split-off band in the valence band to the Wannier-Stark localization states in the conduction band. The assignment is supported by the tight-binding calculation. Resonant coupling between the localized states is also observed.

Numerical Design of Photonic Crystal Cavity Structure with AlAs/AlOx Cladding Layers for Current-Driven Laser Diodes

Current injection into a photonic crystal cavity is an important task in material science and technology. A photonic crystal built in a two-dimensional semiconductor slab is a promising candidate for novel optical devices since it has excellent light confinement capability. However, the presence of insulating cladding layers to reduce light loss makes current injection difficult, and hence practical active devices have not yet been developed. In this study, we numerically investigate the effect of selectively oxidized AlAs layers designed so as to make carrier injection possible. Results of finite-difference time-domain (FDTD) calculations show that both carrier injection with sufficiently low resistivity and light confinement are feasible for the structure with the oxidized AlAs/AlOx cladding layers.