Hidekazu Yamamoto

Accurate Thickness Determination of Both Thin SiO2 on Si and Thin Si on SiO2 by Angle-Resolved X-Ray Photoelectron Spectroscopy

Thicknesses of both ultrathin silicon oxide on silicon substrate and ultrathin silicon on silicon oxide are accurately determined by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The effective attenuation lengths of Si 2p photoelectrons in silicon oxide, λO, and silicon substrate, λS, are accurately determined by considering the photoelectron yields in both materials, which were obtained experimentally from the damping of intensities of the plasmon-loss peaks therein. Photoelectron yields for silicon oxide and silicon substrate are YO = 0.91 and YS = 0.74, respectively, and consequently the relationship between λO and λS is λO=1.4 λS. The value of λS is accurately determined from the silicon-on-insulator (SOI) sample with a thickness of 5 nm to be λS = 2.3 nm and the value of λO is subsequently determined to be 3.2 nm. Finally, the value of λO is confirmed by comparing the oxide thicknesses of SiO2 on Si(100) systems determined by AR-XPS with those determined by ellipsometry.

A Study of Metal Impurities Behavior due to Difference in Isolation Structure on ULSI Devices

We have studied the behavior of metal impurities such as Cu and Ni with different isolation techniques in ULSI devices. We have found by using convergent beam electron diffraction (CBED) that the stress in a shallow trench isolation (STI) structure is larger than that of a local-oxidation-of-silicon (LOCOS) structure. With LOCOS, the highly doped substrate efficiently gettered Cu. However, with STI, a small amount of Cu diffused to the device area because of the high stress. On the other hand, high density of bulk micro defect (BMD) generated with the thermal processing of STI effectively getters Ni impurities. With the LOCOS-based wafer, a large amount of Ni diffuses to the device region because of the low BMD density.

Oxygen-Related Defects Introduced by As+-Implantation through Cap Layers in Si Probed by Monoenergetic Positron Beams

The depth distributions and species of defects in 50 keV As+-implanted Si with a cap layer (SiO2 or SiN) were determined from measurements of the Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons. Before annealing, the main species of defects below the amorphous region was determined to be divacancies. Upon annealing at 800°C, oxygen-related defects were introduced into the subsurface region (<20–40 nm). The species of these defects was identified as oxygen microclusters. The positrons were trapped by the open spaces adjacent to the clusters, and the size of these spaces was estimated to be close to that of monovacancies. The oxygen-related defects were annealed out after rapid thermal annealing at 1050°C (10 s). As+-implantation through the SiN film suppressed the introduction of recoiled oxygen atoms; as a result, the concentration of the oxygen-related defects was decreased.

Polycrystalline and Amorphous Si MOS Solar Cells by Anodization

Anodization in various modes for fabrication of polycrystalline and amorphous Si MOS solar cells is investigated. Anodization is found to be very effective for increasing the open-circuit voltage without reducing photocurrent. It is also found that anodized polycrystalline and amorphous Si surfaces give very much improved yields of successfully operating cells as compared with chemically polished surfaces. The mechanism is explained in terms of passivation of material defects in poly-Si and a-Si by selective anodization.

Investigation of the depletion layer by scanning capacitance force microscopy with Kelvin probe force microscopy

We have developed a scanning probe microscope (SPM) that combines atomic force microscopy (AFM) with both Kelvin probe force microscopy (KFM — to measure the surface potential) and scanning capacitance force microscopy (SCFM — to measure the differential capacitance). The surface physical characteristics of a commercial Si Schottky barrier diode (Si-SBD), with and without an applied reverse bias, were measured over the same area by our AFM/KFM/SCFM system. We thus investigated the discrete power device by calculating the depletion-layer width and drawing an energy-band diagram.

Observation of silicon carbide Schottky barrier diode under applied reverse bias using atomic force microscopy/Kelvin probe force microscopy/scanning capacitance force microscopy

We have observed a commercial silicon-carbide Schottky barrier diode (SiC-SBD) using our novel analysis system, in which atomic force microscopy (AFM) is combined with both Kelvin probe force microscopy (KFM; for surface-potential measurement) and scanning capacitance force microscopy (SCFM; for differential-capacitance measurement). The results obtained for the SiC-SBD under an applied reverse bias indicate both the scan area in the sample and a peak value of the SCFM signal in the region where the existence of trapped electrons is deduced from the KFM analysis. Thus, our measurement system can be used to examine commercial power devices; however, novel polishing procedures are required in order to investigate the Schottky contact region.

Evaluation of carrier concentration reduction in GaN-on-GaN wafers by Raman spectroscopy and Kelvin force microscopy

The carrier concentration in a gallium nitride (GaN) substrate of a GaN-on-GaN wafer grown by hydride vapor phase epitaxy (HVPE) was evaluated by Raman spectroscopy and Kelvin probe force microscopy (KFM). On the basis of the longitudinal optical phonon–plasmon coupled (LOPC) mode of Raman spectra and surface potential measurements by KFM, the carrier concentration at the periphery of the HVPE-GaN substrate was found to be about one order of magnitude lower than that at the center. The decrease in carrier concentration is considered to be due to the out-diffusion of dopants during the metal organic chemical vapor deposition (MOCVD) of the epitaxial layer. In silicon (Si) epitaxial wafers, the autodoping of out-diffused dopants introduces nonuniform device characteristics. This undesirable effect needs to be suppressed to successfully move from prototype GaN-on-GaN power devices to commercial products.

High Efficiency MOS Solar Cells by Anodic Oxidation Processes

Preparation of GaAs, InP and Si single-crystal MOS solar cells by anodization in various modes is investigated. Both passive and active modes of anodization are found to increase the open-circuit voltage with respect to that of the bare Schottky cell. Best result is obtained by active anodization of n-GaAs. Passive anodization with a high formation voltage results in an anomalous reduction in photocurrent. A detailed analysis of cell currentvoltage characteristics indicates that cell behavior is dominated by interface-states.