Interface states atSiO2/6H−SiC(0001)interfaces observed by x-ray photoelectron spectroscopy measurements under

Abstract

Interface states in almost the entire SiC band gap are observable by means of x-ray photoelectron spectroscopy ͑XPS͒ measurements under bias, although SiC is a wide-gap semiconductor having 2.9 eV band-gap energy. When a SiO 2 layer is formed by wet oxidation at 1000°C on 6H-SiC(0001) Si-faced surfaces, only a broad interface state peak is observed at ϳ2 eV above the SiC valence-band maximum ͑VBM͒, while for dry oxidation at the same temperature, an additional sharp interface state peak is caused at 1.8 eV above the VBM. When the wet-oxidation temperature is increased to 1150°C, this 1.8-eV interface-state peak also appears. The concentration of graphitic carbon at the SiO 2 /SiC interface is found to increase with the heat treatment temperature. The 1.8-eV interface-state peak is tentatively attributed to graphitic carbon with a special structure near the interface. On the other hand, the broad 2-eV interface-state peak is attributed to Si dangling bonds at the interface. Without the 1.8-eV interface-state peak, current-voltage (I-V) curves measured under x-ray irradiation deviate only slightly from the ideal I-V curve ͑ϳ0.4 V͒, while with this peak, the deviation becomes much larger ͑ϳ0.8 V͒. XPS measurements under bias show that the I-V curves under x-ray irradiation are determined by the magnitude of band bending in SiC. Therefore, the deviation from the ideal I-V curve is attributed to the accumulation of holes ͑i.e., minority carriers͒, generated by x-ray irradiation, at interface states with energies between the SiC and metal Fermi levels, causing a downward SiC band-edge shift and thus resulting in a decrease in the magnitude of band bending in SiC. This result demonstrates that the interface states affect I-V characteristics by a static effect ͑i.e., interface state charges͒, not by a dynamical effect ͑i.e., electron-hole recombination at the interface states͒.

Characterization of oxide films on 4H-SiC epitaxial (0001¯) faces by high-energy-resolution photoemission spectroscopy: Comparison between wet and dry oxidation

“…22 There are many previous reports that have suggested that the high interface state density for oxide-4H-SiC systems originates from carbon clusters at the interfaces. 14,[23][24][25] However, as shown in Fig. 2, the intensity of the peak dramatically increases with a decrease in e , which means that the origin of this peak exists at the sample surface rather than the oxidesemiconductor interface.…”

“…However, the difference in the electrical properties between the C and Si faces was not discussed. For the comparison between wet and dry oxidations, Kobayashi et al 14 have investigated 6H-SiC Si-face-oxide interfaces fabricated by wet oxidation at low temperatures ͑Ͻ1000°C͒ and at high temperatures ͑Ͼ1000°C͒, and dry oxidation at high temperatures. They found that the interface layer in the case of high-temperature oxidation contains more interface states, attributed to graphitic-carbon-related states, than in the case of low-temperature oxidation.…”

Theoretical study of the mechanism of dry oxidation of4H-SiC

“…X-ray photoelectron spectroscopy ͑XPS͒ studies on MOS structures under bias revealed a peak of states coinciding with D 3 in 6H-SiC. 3 The high density of interface states near the conduction band was found to be connected to traps with slower response times. 4 These states were also investigated 5 by photon stimulated tunneling ͑PST͒, locating a peak at E C − 0.1 eV.…”

“…[11][12][13] The nature of this transition layer, i.e., its actual atomistic structure, is not yet well understood, but an enhanced carbon concentration could clearly be established. 3,14 Many authors suggest the presence of graphitic or sp 2 -bonded carbon islands at the interface. 1,15,16 The observation of "platelet-shaped inhomogeneities" with lateral dimensions of 30-80 nm and thicknesses of 3-4 nm by atomic force microscopy ͑AFM͒ on the SiC side of the interface after etching off the oxide layer, 16 and the detection of increased carbon concentrations at the interface in areas of similar size by electron energy loss spectroscopy ͑EELS͒ 17 have been presented as the main evidence for the presence of such islands.…”

On the topographic and optical properties of SiC/SiO2 surfaces

“…The most important are 3C SiC with a cubic unit cell and 4H SiC with a hexagonal unit cell structure. SiC is much more chemically stable compared to Si and therefore a higher temperature (above 1100°C) is necessary for conventional thermal oxidation [1][2][3]. Carbon clusters are formed at the SiC/SiO 2 interfaces during this high temperature oxidation and they strongly influence the interface state density, which is greater than one order of magnitude higher than in Si [3][4][5].…”