(Invited) Two-Dimensional Characterization of Wide-Bandgap Materials and Contact Interfaces by Using Scanning Internal Photoemission Microscopy

Abstract

Scanning internal photoemission spectroscopy (SIPM) has been developed to map the electrical characteristics of metal/semiconductor interfaces nondestructively. Our experimental demonstrations of the mapping characterization are reviewed from the several aspects. (1) Thermal degradation: For the development of high-power devices, the thermal degradation is more important. In the case of Ni/n-SiC Schottky contacts, the local interfacial reaction between Ni and SiC was observed in the photoyield (Y)image after annealing at 500° C or higher. The lower Schottky barrier height was determined in such regions. In the case of Au/Ni/n-GaN Schottky contacts, we found that surface scratches on the metal dot enhanced interfacial degradation.(2) Device degradation by applying high-voltage: Applying reverse bias voltage (Vbias) down to −45 V during theYmeasurement is possible in our SIPM. For most of the Ni/n-GaN Schottky contacts formed on a thick n-GaN layer grown on a freestanding GaN substrate, uniform distribution of Y was observed over the electrode. On the other hand, for the contacts with a slightly larger reverse current, the Y distribution was also uniform at Vbias = 0 V, but over Vbias = −36 V, Y was intensively increased at small spots. After the SIPM measurements, the I–V characteristics became leaky, and the same spots were observed in the microscope image. These results indicate that SIPM is useful for in situ monitoring of the initial stage of the degradation under applying reverse bias voltage.(3) Process-induced surface damages: The electrical characteristics of the Schottky contacts have a sensitive nature to the damages located on the semiconductor surface. We applied SIPM to the damage characterization. Firstly, the surface damages were intentionally induced by focused ion beam, or selective ion-implantation: Ga ions to n-GaAs, N ions to n-GaN, and N ions to n-SiC. The implanted images were clearly observed as the same Y images, and no extension of the induced damages was found. For the GaAs and SiC contacts, Y increased in the implanted regions, while for the GaN, Y decreased. Different roles of the damages as generation or recombination centers were clarified.(4) Grain boundaries of semiconductors and printed metal particles: SIPM is also available for the contacts formed on a semiconductor surface with poor crystal quality. Among the SiC poly-types, 3C-SiC has the advantages of an isotropic crystal structure and high electron and hole mobilities for electron device applications. However, owing to a lack of 3C-SiC bulk crystals, heteroepitaxial growth on Si, 4H-, or 6H-SiC substrates is unavoidable. Thus, the crystal quality of 3C-SiC is not as good as those of 4H- and 6H-SiC. We measured Ni Schottky contacts on p-3C-SiC layers grown on 4H or 6H-SiC substrates. The sample surface consists of 3C-SiC domains with a flat top. The domain pattern was clearly visualized in a Y map. By combining Ymaps measured with red and green lasers, we found that Schottky barrier height is smaller and larger recombination occurs in the boundary regions than in the flat regions. (5) Expansion to semiconductor/ semiconductor and metal-insulator-semiconductor interfaces: Finally, the structural expansion of the samples is described. As long as a hetero junction and an electrical field at the interface exist, a photocurrent can be generated upon monochromatic light irradiation. We characterized p+-Si/n−-SiC heterojunctions formed by surface-activated bonding. In the internal photoemission spectra, a linear relationship was found between Y 1/2 and hν, and threshold energy was reasonably obtained to be 1.34 eV. In the SIPM results, Y maps were successfully obtained, and nanometer-deep scratches, which were formed on the SiC surface in wafer polishing, were sensitively visualized as a pattern.This technique was confirmed to be useful for the development of the wide-bandgap-semiconductor high-power devices.