Toru Itakura

Photoluminescence of porous Si, oxidized then deoxidized chemically

We examined the change in photoluminescence spectra of porous Si when it is oxidized then deoxidized chemically. After both steps, photoluminescence shifted to higher frequencies and increased in intensity. These shifts to higher frequencies indicate the photoluminescence is a result of the quantum size effect. Moreover, the increase in photoluminescence intensity after oxidation suggests that termination by hydrogen on the porous Si surface does not always play a key role in the photoluminescence mechanism.

Measurement and analysis of neutron-induced soft errors in sub-half-micron CMOS circuits

Neutron-induced soft error rates (SERs) of subhalf-micron CMOS SRAM and Latch circuits were studied both experimentally and analytically to investigate cosmic ray neutron-induced soft errors (SEs). Because the neutron beam used in the measurement has an energy spectrum similar to that of sea-level atmospheric neutrons, our SER data corresponds to those induced by cosmic ray neutrons. The /spl alpha/-particle induced SERs were also measured for comparison with the neutron-induced SERs. Neutron-induced SEs occurred in both circuits. On the other hand, /spl alpha/-induced SEs occurred in SRAM, but not in the Latch circuits. The measured SERs agreed with simulated results. We discussed the significance of how cosmic ray neutrons affects CMOS circuits at ground level.

Simple method for estimating neutron-induced soft error rates based on modified BGR model

Recently the importance of cosmic ray neutron-induced soft errors has been recognized. We propose a simple model to estimate the neutron-induced soft error rates (SERs), which is a modified version of the burst generation rate (BGR) model. Our model can be used to easily and quickly estimate neutron-induced soft error rates and provides a useful guideline for device and circuit engineers to estimate neutron-induced soft errors (SEs),.

Gettering of iron impurities in p/p+ epitaxial silicon wafers with heavily boron‐doped substrates

We studied gettering effects in p/p+ silicon epitaxial wafers that have no dislocations near the epi/sub interface and oxygen precipitates inside the p+ substrate. The wafers were contaminated with high‐ and low‐level Fe surface concentrations to clarify the relationship between gettering effects and Fe concentration. After annealing at 1000 °C for 60 min followed by quenching, the p/p+ epitaxial wafers showed the same gettering effect independent of the Fe contamination level. The intrinsic gettered reference wafers, however, did not show the gettering effect because Fe impurities do not supersaturate at 1000 °C. We concluded that the gettering effect of Fe in the p/p+ epitaxial wafers is due to the difference in solubility between the p epitaxial layer and the p+ substrate.

Microstructure of porous silicon

We examined the microstructure of luminescent porous silicon by cross‐sectional high‐resolution transmission electron microscopy and found a threadlike structure consisting of Si microcrystals. We also found Si microcrystals with sizes ranging about 3–20 nm randomly distributed throughout the porous silicon.

Cosmic ray neutron-induced soft errors in sub-half micron CMOS circuits

We numerically investigated cosmic ray neutron-induced soft errors in sub-half micron CMOS SRAM and latch circuits at sea level. For our purpose, we developed an original simulator which reproduces well the experimental charge collection data. We investigated soft error rates (SERs) and showed that the neutron-induced SERs in the SRAM are the same order as those due to /spl alpha/-particles and the SERs in the latch are dominated by neutrons.

Alpha-particle-induced collected charge model in SOI-DRAM's

We have developed a model for collected charges induced by an alpha-particle for SOI-DRAMs which assumes that the body capacitance equals the gate capacitance and that holes do not recombine with electrons. The validity of our model was supported by three-dimensional (3-D) device simulations that considered various gate lengths, gate oxide thicknesses, and flat-band voltages. The work function difference between the gate and body materials caused a significant increase in the current gain. The vertical band of the body region should therefore be flat to suppress the collected charge. A thinner gate oxide would also suppress the collected charge during a refresh interval. This finding could not be obtained from the conventional equation.

Transmission-electron-microscopy observation of dislocation networks of oxide vertical-cavity surface-emitting lasers

Oxide vertical-cavity surface-emitting lasers (VCSELs) are widely used in high-speed fiber optical communications. Burn-in tests are run on oxide VCSELs to weed out defective devices, thereby fulfilling the related reliability requirements. Nevertheless, some oxide VCSELs fail in the field. Oxide VCSELs that failed in the field were examined with transmission electron microscopy (TEM). Besides cross-sectional views, plan views of the entire active region were observed. Development of a dislocation network in the active region was found in all samples, and that is the cause of the failures. The Burgers vectors of these dislocations are parallel to [101]. Although the trigger of the dislocations has not been identified, electrostatic discharge (ESD) does not cause the dislocation in our investigated devices, since no ESD-damaged region was found. Strain and/or crystal defects introduced during VCSEL fabrication are considered to be the cause.

Simplified Simulator for Neutron-Induced Soft Errors Based on Modified BGR Model

Recently the importance of cosmic ray neutron-induced soft errors has been recognized. We proposed a simple model for neutron-induced soft error rates, which is a modified version of the BGR model and developed a simulator, MBGR. MBGR can easily and quickly estimate neutron-induced soft error rates with high accuracy.