Takeshi Uruma

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.

Nanoscale observation of organic thin film by atomic force microscopy

Organic photovoltaics (OPVs) fabricated using organic semiconductors and hybrid solar cells (HSCs) based on organic semiconductors/quantum dots (QDs) have been attracting significant attention owing to their potential use in low-cost solar energy-harvesting applications and flexible, light-weight, colorful, large-area devices. In this study, we observed and evaluated the surface of a photoelectric conversion layer (active layer) of the OPVs and HSCs based on phenyl-C61-butyric acid methyl ester (PCBM), poly(3-hexylthiophene) (P3HT), and zinc oxide (ZnO) nanoparticles. The experiment was performed using atomic force microscopy (AFM) combined with a frequency modulation detector (FM detector) and a contact potential difference (CPD) detection circuit. We experimentally confirmed the changes in film thickness and surface potential, as affected by the ZnO nanoparticle concentration. From the experimental results, we confirmed that ZnO nanoparticles possibly affect the structures of PCBM and P3HT. Also, we prepared an energy band diagram on the basis of the observation results, and analyzed the energy distribution inside the active layer.