Materials Science Forum Vol. 725

Abstract: A comparison is made between two kinds of Nitrogen implantations for the formation of thin n+p junctions in p-type Silicon Carbide (SiC) epitaxial layers. The standard beam ion implantations and PULSIONTM processes were performed at two distinct energies (700 eV and 7 keV) and the subsequent annealing was held at 1600°C in a resistive furnace specifically adapted to SiC material. Positron Annihilation Spectroscopy (PAS) and unpolarized infrared reflectivity (IR) measurements were carried out before and after the annealing, respecively. Despite the presence of deep vacancy clusters near the as-implanted sample surfaces, no extended defects were detected after the annealing. Plasma implanted samples prove to contain a lower point defect concentration than beam implanted samples. The concentration of these defects (resulting from plasma process) is higher in the plane parallel to the optical axis, which denotes an energy spreading alongside the dopant distribution.

Abstract: Electron beam induced current (EBIC) and etch pit method have been used to study the dissociation behavior of basal plane dislocations (BPDs) in 4H-SiC under electron beam irradiation. When 20 kV scanning electron beam was applied for 1 h, it has been found that BPDs whose dislocation lines were along [11-20] off-cut direction dissociated into partial dislocations (PDs) forming a stacking fault (SF) between them; while no dissociation was found for BPDs extending along other directions. These results are discussed in terms of different formation energy of SFs expanding from a pure screw type and a mixed type BPD. In addition, the angle between dislocation line of a BPD and the [11-20] off-cut direction might also play a role in determining the minimum energy for SF formation.

Abstract: The etch pits on the C-face 4H-SiC substrate was produced by chlorine trifluoride gas at various temperatures. The etch pit density showed the comparable value to the current dislocation density level of the Si-face 4H-SiC, when the etching was performed around 713 K. Thus, the etch pit density obtained by this technique is expected to reveal the crystal quality.

Abstract: SiC Schottky barrier diodes were fabricated and measured properties were characterized by device simulation. Most of devices show low leakage current, however, a few devices show leakage current larger than the values estimated from deviation of drift layer parameters. The leakage current component remarkable in lower voltage and saturating at higher voltage is related to Schottky barrier tunneling at macroscopic defects. The component remarkable in higher voltage is considered to be due to microscopic defect related generation current, concerning with non-stoichiometry.

Abstract: The reflectance around the longitudinal optical (LO) phonon frequency in the terahertz region changes with the carrier density of silicon carbide (SiC), while the reflectance around the transverse optical (TO) phonon frequency remains constantly high. The relative reflectance obtained from the reflectance at the two frequencies related to the TO and LO phonon was evaluated as a function of the carrier density of SiC. Two waves around these phonon frequencies can be generated easily using a tunable terahertz source. Nondestructive imaging of the carrier density of cubic SiC (3C-SiC) at the rate of 1 s per point was carried out successfully using this tunable terahertz source.

Abstract: GaN crystals grown in supercritical ammonia by the ammonothermal method were studied by cathodoluminescence (CL), both in image and spectrally resolved modes. The main extended defects and the incorporation of point defects and impurities in different growth sectors were revealed. The influence of the seeds, the role of the growth planes and the changes in the crystal quality during the growth run are discussed.

Abstract: (0001) GaN bulk crystals with a thickness of 3.4 mm and a density of in-grown dislocations of 3.5·10⁶ cm^-2 have been deformed at room temperature using a Vickers indenter at two different sample orientations in relation to the indenter. Dislocations and cracks at the indentations were investigated by means of optical microscopy and scanning electron microscopy in secondary electron contrast and cathodoluminescence imaging. The arrangement of indentation-induced dislocations and cracks is described and the orientation effect is discussed.

Abstract: Role of 3C-SiC intermediate layer (3C-SiC IL) and a propagation of crystal defects from 3C-SiC IL to III-nitride epilayers has been investigated by observing the interface between the 3C-SiC IL and III-nitride epilayers. We found that better quality epilayers were obtained by using such intermediate layers than by direct growth on Si substrates. In the case of III-nitride epilayers grown directly on Si, the layers grown at the initial stage are not flat. The 3C-SiC IL grown on Si (111) substrates has many stacking faults (SFs) that form along the 3C-SiC {111} planes. The initial III-nitride films have V-shaped trenches due to the SFs of the 3C-SiC IL. However, some SFs do not lead to the V-shaped defects.

Abstract: The dislocations in GaN thin film with GaN/AlN multilayer (ML) as the buffer layer were evaluated using transmission electron microscopy. A high density of dislocations parallel to the GaN/ML interface and a sudden decrease in the dislocation density at the GaN/ML interface were found. Dislocation propagation in the direction parallel to the GaN/ML interface by turning horizontally on the GaN/ML interface is considered to be effective in decreasing the dislocation density at the top layer of GaN.

Abstract: An AlGaN/GaN high electron mobility transistor (HEMT) stressed at 10 GHz and increased channel temperatures of T ≈ 260 °C has been analyzed by electroluminescence microscopy (ELM) and infrared thermography (IRT). After stress a negative threshold shift is seen in the electrical characteristics. Based on the current dependence of the electroluminescence (EL) intensity image and a local increase of T this shift can be assigned to the degradation of one of its gate fingers. Transmission electron microscopy (TEM) images of this gate finger revealed structural changes along the drain-side edge of the gate.