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Development of SiC heterojunction power devices
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Gammon, P. M. (2011) Development of SiC heterojunction power devices. PhD thesis, University of Warwick.
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Abstract
Silicon carbide (SiC), with its wide bandgap, high thermal conductivity and natural
oxide is a substrate that has given rise to a new generation of power devices than can
operate at high temperature, high power and high frequency, though the material is not
without its problems. SiC "heterojunction devices" are layers of germanium (Ge) or silicon
(Si) that are deposited via molecular beam epitaxy (MBE) or wafer bonded onto the SiC
surface. These narrow bandgap thin films can provide a high mobility channel region
overcoming SiC's crippling channel mobility, which is most often made worse by a high
density of interface states. Concentrating predominantly on Ge/SiC heterojunctions, this
thesis characterises the physical and electrical nature of these structures, investigating
the rectifying properties of the heterojunction interface and the ability of these layers to
support a depletion region.
A physical analysis of the layers revealed that the Ge formed in an unexpectedly
uniform fashion, given the large lattice mismatch involved. At a deposition temperature of
500oC the Ge initially clumped into wide, shallow islands before merging, forming at best
a 300 nm polycrystalline layer with a surface roughness of only 6 nm. This was in contrast
to MBE deposited Si/SiC layers that formed tall islands that at 1 μm thick, still had not
merged. After being formed into Ge/SiC heterojunction diodes they were electrically
characterised. The layers displayed near ideal (η = 1:05) turn-on characteristics, low
turn-on voltage (approximately 0.3 V less than Ni/SiC SBDs), reasonable on-resistance
(12 mΩcm2) and minimal leakage current. The devices were shown to suffer severe Fermi
level pinning that defined the way the materials' bands aligned. This occurred as a result
of an inhomogeneous interface that also caused fluctuations in the size of the Schottky
barrier height across the interface. New characterisation techniques relating to these
phenomena were applied to a heterojunction for the first time.
MBE formed Ge/SiC layers and wafer bonded Si/SiC layers were formed into MOS
capacitors through the deposition of the high-K dielectric hafnium oxide (HfO2). The
increased conduction band offset between oxide and narrow bandgap semiconductor suppressed leakage problems often seen in HfO2/SiC structures. Capacitance-voltage results
showed that they could both support a depletion region, though the best results came
from the MBE Ge/SiC diodes. Current-voltage results showed that the more uniform
Si/SiC devices could block 3.5 MV/cm.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TP Chemical technology | ||||
Library of Congress Subject Headings (LCSH): | Silicon-carbide thin films, Heterojunctions, Germanium, Wide gap semiconductors | ||||
Official Date: | May 2011 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Mawby, Phil ; Covington, James A., 1973- | ||||
Extent: | xxii, 287 leaves : ill., charts | ||||
Language: | eng |
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