Yasuji Kimoto

Direct measurement of band offset at the interface between CdS and Cu2ZnSnS4 using hard X-ray photoelectron spectroscopy

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

Formation of helical dislocations in ammonothermal GaN substrate by heat treatment

GaN substrate produced by the basic ammonothermal method and an epitaxial layer on the substrate was evaluated using synchrotron radiation x-ray topography and transmission electron microscopy. We revealed that the threading dislocations present in the GaN substrate are deformed into helical dislocations and the generation of the voids by heat treatment in the substrate for the first observation in the GaN crystal. These phenomena are formed by the interactions between the dislocations and vacancies. The helical dislocation was formed in the substrate region, and not in the epitaxial layer region. Furthermore, the evaluation of the influence of the dislocations on the leakage current of Schottky barrier diodes fabricated on the epitaxial layer is discussed. The dislocations did not affect the leakage current characteristics of the epitaxial layer. Our results suggest that the deformation of dislocations in the GaN substrate does not adversely affect the epitaxial layer.

Band slope in CdS layer of ZnO:Ga/CdS/Cu2ZnSnS4 photovoltaic cells revealed by hard X-ray photoelectron spectroscopy

For compound semiconductor photovoltaic cells with a common structure of the window-layer (WL)/buffer-layer (BL)/absorbing-layer (AL), the band slope in BLs, affecting the conversion efficiency, was directly and non-destructively measured by hard X-ray photoelectron spectroscopy. We demonstrated that the band slope in CdS-BLs sandwiched between WLs and Cu2ZnSnS4 (CZTS)-ALs reflected the trend of the work functions of WLs ( ϕ WL). This result implies that the larger downward band slope to the WL can be achieved using a smaller ϕ WL. The relatively large downward band slope of ∼0.5 eV to the WL was estimated in our ZnO:Ga/CdS/CZTS sample with a higher conversion efficiency of 9.4%, which indicates that the conversion efficiency of CZTS cells can be improved by a larger downward band slope to the WL.

Local atomic structure analysis of SiC interface with oxide using chemical-state-selective X-ray absorption spectroscopy

A local atomic structure analysis of the interface between chemical vapor-deposited SiO2 and 4H-SiC was achieved via a combination of chemical-state-selective X-ray absorption spectroscopy and the use of a sample with a very thin oxide film. The Si K-edge spectrum, which monitors the SiC-assigned Auger peak, allows the SiC side of the SiO2/SiC interface to be selectively measured through the SiO2 film. We estimate the coordination number of the first nearest neighbor to be reduced by 17% with respect to the SiC bulk. This suggests that C vacancy defects exist at the SiC side of the interface.

X-ray absorption spectroscopy to determine originating depth of electrons that form an inelastic background of Auger electron spectrum

In Auger electron spectroscopy (AES), the spectral background is mainly due to inelastic scattering of Auger electrons that lose their kinetic energy in a sample bulk. To investigate the spectral components within this background for SiO2(19.3 nm)/Si(100) with known layer thickness, X-ray absorption spectroscopy (XAS) was used in the partial-electron-yield (PEY) mode at several electron kinetic energies to probe the background of the Si KLL Auger peak. The Si K-edge PEY-XAS spectra constituted of both Si and SiO2 components at each kinetic energy, and their component fractions were approximately the same as those derived from the simulated AES background for the same sample structure. The contributions of Auger electrons originating from layers at different depths to the inelastic background could thus be identified experimentally.

Chemical-state-selective X-ray absorption spectroscopy by detecting bond-specific Auger electrons for SiO2/SiC interface

Chemical-state-selective Si K-edge extended X-ray absorption fine structure (EXAFS) measurements of SiO2 and SiC are demonstrated by detecting bond-specific Auger electrons in SiC coated with a very thin SiO2 film. Differential-electron-yield (DEY) mode is used for the measurements. Each EXAFS spectrum may be subject to the following two spectrally overlapping influences: (i) the background spectrum formed by energy-losing SiC Auger electrons overlaps the SiO2 Auger peak, and (ii) the resonant SiO2 Auger peak overlaps the SiC Auger peak. The SiO2- and SiC-selective DEY–EXAFS spectra differ from each other and are similar to the spectra of bulk SiO2 and SiC, respectively, in the EXAFS regions, indicating that the two influences are negligible, and that this method can be considered valid for selection of chemical states.

Enhancements of photoluminescence intensity in high-quality floating-zone Si by thermal annealing in vacuum

Inactivation of non-radiative defects by hydrogen and their thermal stabilities in a high-quality floating-zone Si wafer depending on annealing conditions have been studied using in-situ photoluminescence (PL) and thermal desorption under an ultra-high vacuum. The PL intensity increased to ~400% of its initial value after annealing at 450 °C and decreased to ~6% of its initial value after annealing at 600 °C due to inactivation and activation of non-radiative defects, respectively. Based on the annealing temperature- and duration-dependence of the PL intensity, we propose two types of hydrogenated defects with different thermal stabilities.

Purification of Silicon Thin Films Containing Nitrogen and Oxygen Impurities using Aluminum-Induced Crystallization

We have presented a method for eliminating impurities in low-purity Si films using aluminum-induced crystallization (AIC). When AIC is applied to a silicon (Si) film containing oxygen (O) and nitrogen (N) atoms, polycrystalline Si grains are grown and they are covered with an Al layer. By Auger electron spectroscopy (AES) measurement, we found that both concentrations of N and O atoms were lower than the AES detection limit (1 at%) in the polycrystalline Si grains, and N and O atoms were condensed in the surface Al layer. These results indicate that AIC can be one of the methods for obtaining high-purity Si grains from low-purity Si films at relatively low temperature