Papers by Keyword: Wannier-Stark Localization

Abstract: Bloch oscillations in semiconductors have been studied for several decades. Oscillations were observed in artificial superlattices based on AlGaAs-GaAs proposed Esaki and Tsu, and the natural superlattice based on superstructures SiC. In this paper we considers some base properties of the SiC natural superlattices that are the ground for the Bloch oscillations existence in these materials and attract attention to the some main disadvantages of the artificial superlattices. The experimental detection of the Bloch oscillation-dependent physical effects in semiconductors has been an actual problem over decades.

Abstract: Terahertz electroluminescence of unipolar n⁺⁺-n⁻-n⁺ SiC structures at helium temperatures and I-V characteristics of bipolar n⁺⁺-π-n⁺ SiC structures with natural superlattices at 300~K at strong electrical fields are studied. The properties of the THz electroluminescence and I-V characteristics testify that these phenomena due to Bloch oscillations.

Abstract: Recently the intense terahertz electroluminescence from monopolar n⁺⁺–n^– –n⁺ structures of 6H- and 8H-SiC of natural superlattices at helium temperatures due to Bloch oscillations was discovered. In the present work we present the THz emission spectra of bipolar n⁺⁺–π–n⁺ structures (π is a high-resistance layer of p-type conductivity) of natural superlattices 4H-, 8H- and 15R-SiC at 7 K. The bipolar n⁺⁺–π–n⁺ structures of 4H- and 8H-SiC were analogous to those of structures for which the negative differential conductivity effect was observed earlier for three polytypes (4H, 6H and 8H) at T=300 K. We demonstrate resemblance and differences of the spontaneous THz emission spectra for the monopolar and bipolar 4H-, 6H- 8H- and 15R-SiC natural superlattices caused by Bloch oscillations of electrons in the SiC natural superlattice.

Abstract: The comprehensive study of the terahertz electroluminescence caused by the Bloch oscillations of the electrons in the natural superlattices of 8H-, 6H-SiC with strong electrical field applied along the natural superlattices axis is represented. The electroluminescence spectra become much broader when the bias field exceeds substantially the threshold field of the Bloch oscillations. This spectral broadening can be explained by an appearance of a new spectral line that is much wider and its maximum is localized at higher energy than lines induced by Bloch oscillations. This line has no link with the Bloch oscillations mechanism and it is a result of the presumed changes in the SiC conduction band with complex electron spectrum structure by applied electrical field.

Abstract: We report on efficient terahertz emission in the region of 1.5-2 THz from high electric field biased 6H-SiC structures with a natural superlattice at liquid helium temperatures. The shape of the emission spectrum, the linear dependence of its maximum on the bias and the characteristic field strength required to achieve the emission allow the emission to be attributed to steady-state Bloch oscillations of electrons in the SiC natural superlattice.

Abstract: A natural superlattice (NSL) in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to depressing of the electron impact zone-zone ionization at 300K with the electric field directed along the crystal axis or the NSL axis. The NSL influence can be also observed in the nitrogen impact ionization at 4.2K. In this case for the parallel field the impurity breakdown has not been fixed up to the fields 1.6 MV/cm in 6H-SiC. These results are explained by the insufficient gain in miniband for the ionization electron energy. Therewith the impurity breakdown at the electric field perpendicular to this axis correlates with a classical picture.

Abstract: In materials with a small degree of ionicity ranging 10-15%, such as in SiC, carrier scattering on polar optical potential is possible. Unlike scattering on deformation potential, the drift mobility in this case increases continuously. As this phenomenon may be realized in SiC hot hole transport, I-F characteristics in 6H-SiC with Na-Nd ~ 5x1017 cm-3 have been studied at electrical field 1-150 kV/cm for temperature from 300 to 600K. Furthermore, we studied the breakdown of Al impurity.

Abstract: A natural superlattice (NSL) in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to a number of effects in phenomena of quantum-mechanical transport and impact ionization when the electric field directed along an axis of NSL (axis C in crystal). These processes are absolutely traditional when the electric field is perpendicularly to this axis. The parallel field phenomena are explained by the effects of the Wannier–Stark localization (WSL) among them the Bloch oscillations effect is most prominent today.

Abstract: In the 6H-SiC p+-n--n+ junction the effect of the premature breakdown has been revealed. This effect stimulated by the small temperature increase and illumination by light with energy greater than the bandgap energy of 6H-SiC. The breakdown field appears to be 20% less than the intrinsic breakdown field in these structures.

Abstract: The work deals with the highly important problem of the qualitative temperature dependence of avalanche breakdown voltage in p-n junctions based on 4H-SiC. As it has been shown before, the temperature coefficient of avalanche breakdown voltage (TCABV) is negative in seven SiC polytypes, including 4Н-SiC. This effect has been explained by the Wannier-Stark localization (WSL). It is worth noting that the plane of the investigated p-n junctions coincided with the basal plane (0001). However the current SiC device technology prefers 4H-SiC p-n junction formation on a plane that has 8о disorientation from (0001). This may result in a weakening of the WSL and, correspondingly, in a positive TCABV. This problem has been elucidated in the present paper. The photocurrent of 4H-SiC p-n junctions in a strong electric field has been scrutinized, that has allowed to discover a negative differential conductivity region and it has testified to the WSL process and negative TCABV.