Abstract: The high density of interface states of thermally grown oxides on silicon carbide has
prompted research into alternative oxidation methods and post oxidation anneals. One such
alternative is oxidation of a deposited sacrificial silicon layer. A recent variation of this technique is
a partial oxidation of the deposited Si layer, so that a thin Si layer remains between the SiO2 and
SiC layers. If the SiO2/Si interface has lower interface state densities than the SiO2/SiC interface,
the SiO2/Si/SiC hetero-structure could yield improved channel mobilities in MOS devices.
Moreover, by correct optimization of the MOSFET device structure, breakdown can be designed to
occur in the bulk SiC layer, thus maintaining a high blocking voltage. Post oxidation annealing in
N2O is another technique often used to reduce interface state densities. However, little is known
about the chemical and physical nature of these N2O oxidized dielectrics. Ellipsometric and
Medium Energy Ion Scattering (MEIS) investigations of conventional SiO2/SiC interfaces
compared with SiO2/Si/SiC hetero-junction structures and N2O oxidized samples are reported.
Abstract: Hydrogen etching of 4H-SiC has been performed in a hot-wall chemical vapor deposition
reactor to reduce surface damage and to create a bilayer-stepped surface morphology, optimal for
initiation of growth on 4H-SiC substrates offcut 4° and 8° towards the <11-20> direction. To
understand how step bunching evolves during the ramp to growth temperature, samples were etched
ending at temperatures from 1400 to 1580°C under 0, 2 or 10 sccm of propane (C3H8) addition to
hydrogen. Initial exploratory growth of 5 μm thick epilayers on the 4° etched surfaces are also
discussed. Atomic force microscopy (AFM) and Nomarski microscopy were employed to
investigate changes in the surface morphology. The 8° substrates subjected to H2-C3H8 etching up
to growth temperature routinely exhibited bilayer steps. However, when the 4° substrates were
etched with a 10 sccm C3H8 flow, considerable step bunching was observed. At 1450°C, with a
10 sccm of C3H8 flow (partial pressure is 1.25x10-5 bar), step bunching started with the formation of
ribbon-like steps. Progression to higher temperature etches have shown the coalescence of the
ribbons into larger macro-steps up to 30 nm in height. Etching 4° substrates under 2 sccm of C3H8
(partial pressure is 2.5x10-6 bar) or in pure H2 up to 1500°C results in minimal step bunching.
Abstract: The performance of SiC MOSFET devices to date is below theoretically expected
performance levels. This is widely considered to be attributed to defect at the SiO2/SiC interface that
degrade the electrical performance of the device. To analyze the relationship between defect
structures near the interface and electrical performances, advanced computer simulations were
performed. A slab model using 444 atoms for an amorphous oxide layer on a 4H-SiC (0001) substrate
was made by using first-principles molecular dynamic simulation code optimized for the
Earth-Simulator. Simulated heating and rapid quenching was performed for the slab model in order to
obtain a more realistic structure and electronic geometry of a-SiO2/4H-SiC interface. The heating
temperature, the heating time and the speed of rapid quenching were 4000 K, 3.0 ps and -1000 K/ps,
respectively. The interatomic distance and the bond angles of SiO2 layers after the calculation are
agree well with the most probable values of bulk a-SiO2 layers, and no coordination defects were
observed in the neighborhood of SiC substrate.
Abstract: Graphitization of the 6H-SiC(0001) surface as a function of annealing temperature has
been studied by ARPES, high resolution XPS, and LEED. For the initial stage of graphitization –
the 6√3 reconstructed surface – we observe σ-bands characteristic of graphitic sp2-bonded carbon.
The π-bands are modified by the interaction with the substrate. C1s core level spectra indicate that
this layer consists of two inequivalent types of carbon atoms. The next layer of graphite (graphene)
formed on top of the 6√3 surface at TA=1250°C-1300°C has an unperturbed electronic structure.
Annealing at higher temperatures results in the formation of a multilayer graphite film. It is shown
that the atomic arrangement of the interface between graphite and the SiC(0001) surface is
practically identical to that of the 6√3 reconstructed layer.
Abstract: Ordered reconstruction phases on the 4H-SiC(1102) surface have been investigated using
low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling
microscopy (STM). After initial hydrogen etching, the samples were prepared by Si deposition and
annealing in ultra-high vacuum (UHV). Two distinct reconstruction phases develop upon annealing,
first with a (2×1), and at higher temperatures with a c(2×2) LEED pattern. After further annealing
the fractional order LEED spots vanish and a (1x1) pattern develops. For the (2×1) phase, STM
micrographs show that adatom chains develop on large flat terraces, which in view of AES consist
of additional Si. These highly linear and equidistant chains represent a self-assembled well-ordered
pattern of nanowires developing due to the intrinsic structure of the 4H-SiC(1102) surface. For the
c(2×2) phase AES indicates a surface composition close to the bulk stoichiometry. For the (1×1)
phase a further Si depletion is observed.
Abstract: In order to give experimental insights on the atomic structure of the Si atomic wires
developing on the β-SiC(100) surface, we use synchrotron radiation-based x-ray diffraction at
grazing incidence to study a network of such atomic lines in a 5x2 surface array. Our results lead to
an accurate surface and sub-surface structure determination evidencing a structure in agreement
with a two adlayer symmetric dimer reconstruction. This atomic structure is significantly different
from the 3x2 surface structure, giving new insights on the Si atomic lines stability.
Abstract: Al2O3 has been grown by Atomic Layer Chemical Vapour Deposition (ALCVD) on ntype
4H-SiC using O3 as an oxidant and tri-methyl-aluminum (TMA) as a precursor. After
deposition, annealing at 1000°C during 3h in different atmospheres (Ar, N2 and O2) was performed.
Interface properties were studied by Capacitance-Voltage (CV) and Thermal Dielectric Relaxation
Current (TDRC) measurements. The highest near-interface trap density (Nit) was deduced to be
4x1012 eV-1cm-2 between 0.36 eV and 0.5 eV below the conduction band, Ec, for O2 annealed
samples, 2.8x1012 eV-1cm-2 between 0.42 eV and 0.56 eV below Ec for Ar annealed samples and
2.2x1012 eV-1cm-2 between 0.4 eV and 0.6 eV below Ec for N2 annealed samples. Only samples
annealed in Ar exhibit a nearly trap free region close to Ec. Annealing in N2 is found to decrease Nit
between 0.3 and 0.7 eV but shows a slightly higher Nit close the conduction band compared to the
Abstract: Preliminary results of a systematic theoretical study on the reactions of NO with a model
4H-SiC/SiO2 interface are presented. We show, that nitridation is a complex process, in which the
balance between various mechanisms depends on doping and temperature. For weakly doped
(1015-16 cm-3) n-type SiC, the crucial effect is an additional oxidation without creation of excess carbon
at the interface.
Abstract: We present a novel approach based on conductive atomic force microscopy (c-AFM) for
nano-scale mapping of the Schottky barrier height (SBH) between a semiconductor and an ultrathin
(1-5 nm) metal film. The method was applied to characterize the uniformity of the Au/4H-SiC
Schottky contact, which is attractive for applications due to the high reported (∼1.8 eV) SBH value.
Since this system is very sensitive to the SiC surface preparation, we investigated the effect on the
nano-scale SBH distribution of a ∼2 nm thick not uniform SiO2 layer. The macroscopic I-V
characteristics on Au/SiC and Au/not uniform SiO2/SiC diodes showed that the interfacial oxide
lowers the average SBH. The c-AFM investigation is carried out collecting arrays of I-V curves for
different tip positions on 1μm×1μm area. From these curves, 2D SBH maps are obtained with 10-
20 nm spatial resolution and energy resolution <0.1 eV. The laterally inhomogeneous character of
the Au/SiC contact is demonstrated. In fact, a SBH distribution peaked at 1.8 eV and with tails from
1.6 eV to 2.1 eV is obtained. Moreover, in the presence of the not uniform oxide at the interface, the
SBH distribution exhibits a 0.3 eV peak lowering and a broadening (tails from 1.1 eV to 2.1 eV).