Schottky barriers and Schottky barrier based devices on Si and SiC
Abstract: This work is devoted to a study of the formation andcharacterisation of Schottky Barriers on differentsemiconductors with an extension to the development of devicesbased on Schottky barriers. The major part of the work has beendone on 6H-SiC, but Si and SiGe alloys have also been used.Different metals have been deposited on p-type and n-type6H-SiC and the properties of the created barriers have beeninvestigated by CV, IV and photoelectric techniques. Data fromother research groups have been collected in order toinvestigate the correlation between the barrier height and themetal work function. The conclusion is that for a number ofmetals there is a strong correlation between the barrier heightand the metal work function, while other metals showsignificant deviation from the Schottky-Mott theory. Largescatter in the data exists between different investigationsindicating that the method of sample preparation is veryimportant for the results. This is not surprising since theSchottky Barrier is mainly a surface device, which makes itvery sensitive to variations in the surface conditions.Extensive work has been done in order to explain thebehaviour of Schottky diodes fabricated on SiC and to qualifythe measurement techniques in presence of different anomaliesin the devices. A highly resistive interfacial layer presentbetween the bulk wafer and the epitaxial top layer on certainp-type 6H-SiC wafers has been found.A process for fabrication of buried Schottky and ohmiccontacts in Silicon by wafer bonding has been developed usingCo as a buried metal layer. During the heat treatment the metalreacts with the Silicon and forms CoSi2. The processed contacts exhibit the sameproperties as similar contacts produced by other techniques.Schottky contacts formed by deposition of W and Ti on differentSiGe alloys have been characterised. The pinning of the Fermilevel in these alloys is stronger than in pure Si.The device work included fabrication of Schottky diodes foruse as photodetectors and Permeable Base Transistors (PBT).Grid shaped Ti based Schottky diodes were fabricated in 6H-SiC.Optical characterisation shows that the diodes are sensitive inthe UV-range with a peak sensitivity around 300 nm. The diodesare insensitive to visible light. Diodes on p-type materialshow higher sensitivity than diodes on n-type material. This isexpected due to the higher barriers on p-type and the highermobility of electrons, increasing the contribution fromdiffusion.A process for the fabrication of PBT:s on Silicon using selfaligned CoSi2contacts was developed. Devices were fabricatedand DC-characterised. The first PBT on 6H-SiC was fabricatedusing a process based on etching of epitaxial layers. Ni wasused as etch mask and ohmic contact. Ti was used as gatemetal.
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