Tangali S. Sudarshan

Flashover in wide‐band‐gap high‐purity insulators: Methodology and mechanisms

The electron beam of a scanning electron microscope (SEM) is used to charge an unmetallized insulator (Al2O3, Y2O3, SiO2) in vacuum. The charging is found to be stable in time after the e‐beam is switched off. The SEM is also used to measure the implanted charge by measuring the resulting electrostatic potential. The distribution of potential around the trapped charges is determined by the classical laws of electrostatics. The electrostatic energy stored in the polarized dielectric can thus be determined. The implanted charge can be removed by the introduction of carriers into the polarized sample by flooding the surface with electron beams of varying energies. Slow or rapid relaxation occurs depending on the operating conditions of the flood gun (energy, intensity, etc.) which introduces the electrons. When the relaxation kinetics are slow, the electrostatic charge decreases slowly as a function of time. On the other hand, a rapid relaxation of the dielectric leads to the appearance of a high‐density pla...

Mechanisms of Surface Flashover Along Solid Dielectrics in Compressed Gases: a Review

Surface flashover in compressed-gas insulated systems is a much studied, but poorly explained phenomenon. In this paper we review the literature of surface flashover with primary emphasis on the understanding of physical processes leading to discharge initiation and insulator flashover under high voltage excitation. The flashover models presently in vogue will first be discussed, followed by the results of some recent experiments which are likely to have an impact on further modeling. Included in this context are phenomena such as ionization, surface charging, partial discharges, optical activity, and gas/dielectric interactions. Finally, the influence of system parameters such as insulator size, shape, surface condition, triple junction geometry, voltage waveform, gas formulation and particle contamination are discussed with regard to their effect on the flashover characteristics. Mechanisms are suggested in an effort to provide a physical explanation for the observed phenomena. Although the physics of the discharge initiation and propagation processes are presently not well understood, and the present models only account for a few of the mechanisms known to be important in the discharge development, all the work points to an interaction between the spacer and the various electron/photon processes in the surrounding gas volume. This interaction has not been accounted for in the discharge models proposed to date. Further modeling work should incorporate these interactions and the intrinsic properties of the dielectric ric which are related to these interactions. More basic research is suggested to provide a better understanding of the physics of the discharge initiation and breakdown phenomena.

In-situ insulator surface charge measurements in dielectric bridged vacuum gaps using an electrostatic probe

Surface charge measurements on alumina and polymer insulators were carried out after stressing them with DC voltages in a high vacuum. The order of magnitude of surface charge density was found to be the same for materials with supposedly widely varying secondary-emission yields. Surface coatings on alumina insulators reduced charge accumulation because of increased surface conductivity and/or reduced secondary-emission yield, which led to significant improvement in voltage hold-off for alumina ceramics. Removing the cathode triple junction from the main body of the cylindrical insulator, reducing the X-ray activity in the gap, or relieving the stress at the critical junction did not significantly alter the surface charge characteristics of cylindrical insulators. Wet hydrogen firing of plain alumina reduced the voltage hold-off by 25% without altering the surface charge density. It is postulated that the charging of insulators in bridged vacuum gaps with DC stresses is due to internal secondary emission produced by ionization of the lattice in the surface layer of the insulating material, by primary electrons injected at the cathode triple junction. This mechanism of charge production differs from the current models where charging is believed to occur due to electrons hopping along the surface/vacuum interface. >

Design rules for field plate edge termination in SiC Schottky diodes

Practical design of silicon carbide (SiC) Schottky diodes incorporating a field plate necessitates an understanding of how the addition of the field plate affects the performance parameters and the relationship between the diode structure and diode performance. In this paper, design rules are presented for SiC Schottky diodes that incorporate field plate edge termination. The use of an appropriate field plate edge termination can improve the reverse breakdown voltage of a SiC Schottky diode by a factor of two. Reverse breakdown voltage values can be obtained that are up to 88% of the theoretical maximums.

Prebreakdown Processes Associated with Surface Flashover of Solid Insulators in Vacuum

A computer simulation of the insulator surface charging in vacuum under the application of a high voltage, is presented for alumina (A1203) insulator and for alumina insulators coated with cuprous oxide (Cu2O) and chromium oxide (Cr2O3). It is shown that the mode of surface charge distribution is dependent on the secondary emission yield of the insulator surface. Mleasurements of insulator- electrode junction fields, using an electronic microforce balance, show the effect of surface coatings on the cathode and anode junction fields. High speed streak photography showing the time development of an arc in the case of coated insulators is presented. The experimental results support the insulator surface charging model which is simulated on a digital computer.

dc surface flashover mechanism along solids in vacuum based on a collision‐ionization model

Selected experimental results are presented from an extensive experimental investigation of dc prebreakdown and breakdown processes in vacuum along solid insulators made of a variety of alumina ceramics and polymers. The investigations included measurements of predischarge current, x‐ray intensity, insulator surface charge density, breakdown behavior as influenced by insulator surface characteristics and insulator profiles, and the analysis of the spectral characteristics associated with insulator surface luminescence. The prebreakdown and breakdown phenomena were found to be strongly influenced by surface microstructure and the chemical state of the insulator surface. The experimentally observed phenomena in our studies strongly point to a new breakdown model based on collision‐ionization by electrons of defect sites and/or traps within the dielectric subsurface at the vacuum interface.

Tunable Reverse‐Biased Graphene/Silicon Heterojunction Schottky Diode Sensor

A new chemical sensor based on reverse-biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias-dependent molecular detection sensitivity and low operating power. The device takes advantage of graphenes atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO₂ and 3 times higher for NH₃ in ambient conditions, while consuming ∼500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confirmed using capacitance-voltage measurements.

Basal plane dislocation-free epitaxy of silicon carbide

Molten KOH etching was implemented on SiC substrates before growing epilayers on them. It was found that the creation of basal plane dislocation (BPD) etch pits on the substrates can greatly enhance the conversion of BPDs to threading edge dislocations during epitaxy, and thus low BPD density and BPD-free SiC epilayers are obtained by this method. The reason why BPD etch pits can promote the earlier conversion is discussed. The SiC epilayer growth by this method is very promising in overcoming forward voltage drop degradation of SiC PiN diodes.

Dielectric surface preflashover processes in vacuum

Surface preflashover phenomena across alumina ceramics in vacuum are presented. The localized plasmas are identified by the spatially resolved images that appear in a pulse surface preflashover phase using an intensified charge‐coupled‐device camera, along with coordinated, time‐resolved preflashover current and luminosity. It was observed that significant preflashover activity associated with the localized plasmas on the surface appears after a flashover event and disappears after a few pulse voltage applications. Large bursts of current pulses (several hundred milliamperes) with no counterpart in the luminosity signal were also observed during preflashover. These phenomena cannot be explained satisfactorily by electron stimulated gas desorption. The effect of adsorbed gases and surface polishing on preflashover are presented and discussed. The experimental results indicate that preflashover essentially depends on the surface state (physical and chemical) of the solid insulator. The preflashover conducti...

Cathodoluminescence study of the properties of stacking faults in 4H-SiC homoepitaxial layers

In-grown stacking faults in n-type 4H-SiC epitaxial layers have been investigated by real-color cathodoluminescence imaging and spectroscopy carried out at room and liquid helium temperatures. Stacking faults with 8H stacking order were observed, as well as double layer and multilayer 3C-SiC structures and a defect with an excitonic band gap at 2.635 eV. It was found that 8H stacking faults and triangular surface defects can be generated from similar nucleation sources. Time-resolved measurements reveal that compared to defect-free regions, the carrier lifetimes are severely reduced by the presence of stacking faults corresponding to triangular surface defects and three-dimensional 3C-SiC inclusions.