The Impact of Surface Defects on SiC Schottky and Ohmic Contact Formation
Silicon carbide is a leading semiconductor for high power, high temperature electronics, yet SiC devices still face problems of electrical contact control and reproducibility. Recent work worldwide shows that point and extended defects in SiC as well as chemical reactions at metal-SiC interfaces can influence contact transport properties. Yet Schottky barrier and ohmic contact formation at the intimate metal-SiC junction are relatively unexplored on the microscopic level. We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) and Auger electron spectroscopy (AES) to measure the optical emissions of defects near 4H-SiC surfaces and metallized interfaces on a nanometer scale. These techniques reveal that a variety of metals on clean 4H-SiC surfaces in ultrahigh vacuum (UHV) promote the formation of mid-gap defects extending only nanometers away from the junction. These states vary in their ranges of depth and depend sensitively on interface reactivity and subsequent UHV annealing
Electronic defect states at annealed metal∕4H–SiC interfacesa)
We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to study the formation of electronic surface states at metal∕4H–SiC contacts. These junctions were formed using both low and high reactivity metals to study how the nature of interface chemical bonding affects the interface state formation. We observe evidence for the formation and removal of localized states at energies that have been associated with morphological SiC defects. Metals such as Au and Ag with no strong chemical reactivity exhibited the most pronounced changes. Conversely, chemically-reactive metals such as Ti and Ni exhibited only minor changes and only with high temperature annealing. These observations suggest that native defects rather then metal-specific chemical bonding dominate the interface electronic features.
Do Potential People Have Moral Rights?
By a potential person I shall mean an entity which is not now a person but which is capable of developing into a person, given certain biologically and/or technologically possible conditions. This is admittedly a narrower sense than some would attach to the term ‘potential'. After all, people of the twenty-fifth century, if such there will be, are in some sense potential people now, even though the specific biological entities from which they will develop, i.e. the particular gametes or concepti, do not yet exist. For there do exist, in the reproductive capacities of people now living and in the earth's resources, conditions adequate to produce these future people eventually, provided of course that various possible catastrophes are avoided. Indeed, in some sense of ‘potential’ there have been countless billions of potential people from the beginning of time. But I am concerned not with such remote potentialities but with currently existing entities that are capable of developing into people.