Yuki Kumagai

Photoacoustic and Photoelectrochemical Characterization of Inverse Opal TiO2 Sensitized with CdSe Quantum Dots

Inverse opal TiO2 may offer a novel and promising solution for enhancing the light harvesting efficiency of dye-sensitized solar cells (DSSCs). Its large interconnected pores enable a better penetration of the dye sensitizers via the matrix pores, making this material surpasses the efficiency of conventional TiO2 electrodes. Moreover, it also exhibits a photonic band gap that may enable a significant change in its dye absorbance by the adjustment of the photon localization near the red edge of the photonic band gap to the position of dye absorption. In this study, we report a simple method of fabrication of inverse opal TiO2, wherein the voids in artificial opal latex are filled with nanosized TiO2 particles by adding a drop of TiCl4 into the latex matrix, hydrolyzing, and heating. In this process, we investigate the effect of different heat treatment times on the properties of inverse opal TiO2. Photoacoustic (PA) characterization shows that longer heat treatment times could produce more defect sites. The presence of defects causes the inhibition of electron transfer and results in a decrease in incident photon-to-current conversion efficiency (IPCE). CdSe quantum dots were adsorbed onto inverse opal TiO2 by chemical deposition. The blue shift of PA spectra relative to the bulk CdSe and the gain in IPCE were clearly observed. This result indicates the quantum confinement effect and photosensitization of CdSe quantum dots.

Very Low Bit Error Rate in Flash Memory Using Tunnel Dielectrics Formed by Kr/O2/NO Plasma Oxynitridation

The gate leakage current which influences the charge hold time of flash memories is characterized as very localized in tunnel oxide area. And the leakage current value is relatively low as the quantity of the current. In conventional test-element-group (TEG) for evaluation of this leakage current, the gate leakage current is measured in relatively large area capacitors or transistors, as a result, the localized and low gate leakage current cannot be measured. In this paper, we propose the new concept TEG in which the localized gate leakage current corresponding to the bit error in the flash memory can be measured in short time. We statistically evaluated the stress induced leakage current (SILC) of all cells in very low gate leakage current region (about 10-16 A) in the very short time (a few seconds) and easily confirmed and categorized the localized cells with the large SILC after the stress. In addition, we show that plasma oxynitridation using Kr/O2/NO gases is effective in suppressing the anomalous SILC and carrier traps by statistically evaluation of 60,000 cells in proposed array TEG.

Large-Scale Test Circuits for High-Speed and Highly Accurate Evaluation of Variability and Noise in Metal--Oxide--Semiconductor Field-Effect Transistor Electrical Characteristics

To develop a new process technology for suppressing the variability and noise in metal–oxide–semiconductor field-effect transistors (MOSFETs) for large-scale integrated circuits, accurate and rapid measurement test circuits for the evaluation of a large number of MOSFET electrical characteristics were developed. These test circuits contain current-to-voltage conversion circuits and simple scanning circuits in order to achieve rapid and accurate evaluation for a wide range of measurement currents. The test circuits were fabricated and the variabilities and noises in drain–source current, gate leakage current, and p–n junction leakage current were evaluated using a large-scale test circuit.

Chemical Structure of Interfacial Transition Layer Formed on Si(100) and Its Dependence on Oxidation Temperature, Annealing in Forming Gas, and Difference in Oxidizing Species

The angle-resolved Si 2p photoelectron spectra arising from a interfacial transition layer formed on a Si(100) were measured with a probing depth of nearly 2 nm. The novel analytical procedure of these spectra was developed by considering that one SiO2 monolayer, two compositional transition layers (CTLs), and one Si monolayer constituting the Si substrate surface are continuously connected with each other to maintain the areal density of Si atoms. It was found for thermally grown transition layers that two CTLs are formed on the oxide side of the CTL/Si interface and the chemical structures correlated with the residual stress appear on the Si substrate side of the interface. The effects of oxidation temperature in the range from 900 to 1050 °C, annealing in the forming gas, and oxidation using oxygen radicals on the chemical structures of transition layers formed on both sides of the interface were also clarified.

Asymmetry of RTS characteristics along source-drain direction and statistical analysis of process-induced RTS

In this work, we investigated random telegraph signal (RTS) amplitude and the probability of trap empty along two different drain current directions for various gate lengths using novel test structures which enable to measure RTS in large numbers. Asymmetry of RTS amplitude along source-drain current direction increases as gate length shortens because a trap near the gate edge dominates RTS phenomenon as gate length shortens. The probability of trap empty shows weak positive correlation between both directions but asymmetric difference of that partially remains. We also investigated RTS characteristics dependence on kinds of gate insulator films and plasma damages of back-end-of-line (BEOL). Silicon oxynitride gate insulator film has bad effect on RTS and plasma damage does not appear as the increase of RTS amplitude up to 51,385 of antenna ratio.

Crystallographic orientation dependence of compositional transition and valence band offset at SiO2/Si interface formed using oxygen radicals

The chemical and electronic-band structures of SiO2/Si interfaces formed utilizing oxygen radicals were investigated by measuring angle-resolved photoelectron spectra arising from Si 2p and O 1s core levels and a valence band with the same probing depth. We clarified that (1) the SiO2/Si interfaces formed exhibited an almost abrupt compositional transition, (2) the valence band offsets at the Si(111)/Si, Si(110)/Si, and Si(551)/Si interfaces are almost the same and are 0.07 eV smaller than that at the SiO2/Si(100) interface.

Atomically flattening of Si surface of silicon on insulator and isolation-patterned wafers

By introducing high-purity and low-temperature Ar annealing at 850 °C, atomically flat Si surfaces of silicon-on-insulator (SOI) and shallow-trench-isolation (STI)-patterned wafers were obtained. In the case of the STI-patterned wafer, this low-temperature annealing and subsequent radical oxidation to form a gate oxide film were introduced into the complementary metal oxide semiconductor (CMOS) process with 0.22 µm technology. As a result, a test array circuit for evaluating the electrical characteristics of a very large number (>260,000) of metal oxide semiconductor field effect transistors (MOSFETs) having an atomically flat gate insulator/Si interface was successfully fabricated on a 200-mm-diameter wafer. By evaluating 262,144 nMOSFETs, it was found that not only the gate oxide reliability was improved, but also the noise amplitude of the gate–source voltage related to the random telegraph noise (RTN) was reduced owing to the introduction of the atomically flat gate insulator/Si interface.

Stress-induced leakage current and random telegraph signal

Stress-induced leakage current (SILC) and random telegraph signal (RTS) in n-type metal-oxide-semiconductor field-effect-transistor (n-MOSFETs) caused by the Fowler-Nordheim tunneling stress are studied by using the author’s newly developed test pattern. MOSFETs having large RTS increase can be induced by electrical stress in parallel with the inducing of SILC. Generation and recovery characteristics of SILC and RTS against the stress time and measurement temperature are very similar. However, the MOSFETs having large RTS are not related to ones having large anomalous SILC in this study. We consider that the traps that cause the RTS and anomalous SILC are the same, but their locations in SiO2 are different.

Statistical evaluation for anomalous SILC of tunnel oxide using integrated array TEG

We propose a new test-element-group (TEG) in order to study anomalous stress-induced leakage current (SILC) of tunnel oxide, which is a problem for flash memory reliability. Using this TEG, gate current through the tunnel oxide in the order of 10-16 A can be measured for about 1,000,000 transistors within 4 minutes. The behavior of anomalous SILC when Fowler-Nordheim (FN) electron tunneling stress is applied or measurement temperature is changed was investigated. It was detected that the anomalous SILC appears or disappears by applying stress and it was annealed out at high temperature measurement, as observed in flash memory cells. This TEG can be fabricated by simple processes, and its peripheral circuits are simple structures, so the tunnel dielectric can be changed drastically. We consider that using this TEG for the development of tunnel dielectric can lead to clarifying the origin of anomalous SILC, and promote the downscaling of tunnel dielectric thickness.

Demonstrating distribution of SILC values at individual leakage spots

Stress induced leakage current (SILC) in the order of 10-17 to 10-13 A were statistically evaluated by using an advanced test circuit. In this paper, the distribution of SILC was evaluated by changing measurement electric fields, electric stress intensities, device area, and oxide thickness. The distribution of SILC is determined by the current values at individual leakage spots when the device area is sufficiently small. When the electric stress intensity and the measurement field are small, the distribution of logarithm of SILC follows the Gumbel distribution because the maximum current values of the leakage spots determine the gate leakage current in small area MOSFETs. We also evaluated the time-dependent characteristics of SILC in small area MOSFETs. The random telegraph signals of gate leakage current were observed which also indicates the current values of individual leakage spots.