Abstract: Composite ohmic contacts designed for SiC devices operating in air at 350°C have
been studied. Ohmic contacts to n- and p-4H-SiC were protected against inter-diffusion and
oxidation by Ta-Si-N layers obtained by sputter deposition from a TaSi2 target in a mixture of
Ar and N2. Platinum was sputter-deposited at 250°C to promote adhesion between the Ta-Si-
N barrier layer and a thick Au cap layer. Platinum also acts as a barrier to the diffusion of
Au. The electrical and mechanical characteristics of the composite contacts were stable after
hundreds of hours of annealing in air at 350°C. We report the effects of thermal aging on the
specific contact resistance and the semiconductor sheet resistance, and the results of wire
bond pull and shear tests following aging for Ta-Si-N / Pt / Au stacks deposited on both SiO2
dielectric layers and the ohmic contact layers.
Abstract: Tantalum-ruthenium diffusion barriers for contacts to SiC were investigated in this work.
Stable specific contact resistances of (2 ± 1) x 10-5 Ω cm2 and (4 ± 2) x 10-5 Ω cm2 were measured
on p-type 4H SiC for Al/Ni and Ni ohmic contacts, respectively, when they were beneath Ru-rich
Ta-Ru barriers aged at 350 °C for 3000 h in air. Annealed Ni ohmic contacts on n-SiC aged at
350 °C in air for 1000 h (the longest time tested) are also very stable. Pull tests revealed greatly
improved adhesion between layers in metallization stacks that contained Ta-Ru barriers in place of
previously studied Ta-Ru-N barriers. A 5 nm Ta layer inserted between the Ru-rich Ta-Ru barriers
and Au was found to further improve the adhesion of the metallization stacks.
Abstract: One important challenge in SiC Bipolar Junction Transistor (BJT) fabrication is to form
good ohmic contacts to both n-type and p-type SiC. In this paper, we have examined contact study
in a SiC BJT process with sputter deposition of titanium tungsten contacts to both n-type and p-type
regions followed by annealing at different temperatures between 750 oC and 950 oC. The contacts
were characterized using linear transmission line method (LTLM) structures. To see the formation
of compound phases, X-ray Diffraction (XRD) θ-2θ scans were performed before and after
annealing. The results indicate that 5 minutes annealing at 950 oC of the n+ contact is sufficient
whereas the p+ contacts remain non-ohmic after 30 minutes annealing. The n+ emitter structure
contact resistivity after 5 min annealing with 750 oC and 950 oC was 1.08 × 10-3 5cm2 and
4.08 × 10-4 5cm2, respectively. Small amorphous regions of silicon and carbon as well as titanium
tungsten carbide regions were observed by high-resolution transmission electron microscopy
(HRTEM), whereas less carbide formation and no amorphous regions were found in a sample with
unsuccessful formation of TiW ohmic contacts.
Abstract: We have used depth-resolved cathodoluminescence and Auger electron spectroscopies,
DRCLS and AES, respectively, to probe the electronic structure and the composition of Ti/Al
ohmic contacts to p-type SiC on a nanometer scale. A continuous Ti-Si-C compound layer was
observed using the Auger depth profile. No interfacial Al segregation was found. The secondary
electron threshold technique showed a continuous decrease in work function from the p-type SiC to
the Ti-Si-C compound layer. Our results support an ohmic contact mechanism by an intermediate
semiconductor layer which reduces the otherwise large interfacial Schottky barrier height. DRCLS
revealed a ~2.78 eV sub-band gap transition enhanced by interfacial reaction in the near-interface
SiC, suggesting the formation of additional C or Si vacancies.
Abstract: Nickel ohmic contacts to p-type epitaxial and heavily implanted 4H-SiC are
described. Room and elevated temperature results are presented. Elevated temperature
measurements of specific contact resistance are compared to theoretical calculations. The
calculations require the acceptor doping concentration and the contact’s barrier height.
Epitaxial material has a known acceptor value thereby allowing the barrier height to be
deduced by requiring agreement between the calculated and measured values of the contact
resistance. Calculations of the contact resistance for implanted material use the barrier height
from the epitaxial results along with a variable activated acceptor doping concentration which
is adjusted to give agreement with measured room temperature specific contact resistances.
Specific contact resistances as low as 7x10-6 ohm-cm2 fabricated on the Si face have been
obtained to epitaxial 4H p-type material whereas contacts to implanted material result in
much larger contact resistance values of 4x10-5 ohm-cm2. These results, when compared to
theoretical calculations, indicate that activated acceptor doping concentrations in heavily
implanted material are on the order of 2% of the implant concentration.
Abstract: Al based alloys, such as Ti/Al, are commonly used for ohmic contacts on p-type SiC.
The interfacial structures of a metal alloy film on SiC are very complicated after annealing. Al is
considered as the key element responsible for forming ohmic contacts on p-type SiC, and reacts
with C from SiC and forms Al4C3 and Si during annealing. In this study, we have investigated
ohmic contact formation of a single component Al4C3 film on p-type SiC. Based on the
stoichiometric formation of Al4C3 between Al and C at high temperatures, several samples with
various Al/C mole ratios have been examined for ohmic contact formation after different annealing
temperatures. Carbon rich and stoichiometric Al4C3 films form ohmic contacts on p-type 4H-SiC
(~2.8 x1018 cm-3 ) after annealing at 800 and 900°C. X-ray diffraction (XRD) data have shown that a
single component Al4C3 is formed when an ohmic contact on p-type SiC is activated. Al/SiC, as the
control sample, does not form ohmic contacts under the same conditions. This study reveals that
Al4C3 can be responsible for forming ohmic contacts on p-type SiC. However, its chemical
instability requires that the secondary metal is necessary to form stable ohmic contacts when Albased
films are used.
Abstract: AlNi and Ni2Si based ohmic contacts to p-type 4H-SiC have been produced using low
energy ion implantation, a Ti contact layer, and sequential anneals. Low resistivities were promoted
by degenerately (>1020 cm-3) doping the surface region of 4H-SiC epilayers via Al+ implantation.
High acceptor activation and improved surface morphology was achieved by capping the samples
with pyrolized photoresist and using a two-step anneal sequence in argon. Ti/AlNi/W and
Ti/Ni2Si/W stacks of varying Ti and/or binary layer thickness were compared at varying anneal
temperatures. AlNi based samples reliably and repeatedly achieved specific contact resistivities as
low as 5.5 x10-5 ohm-cm2 after annealing at temperatures of 700-1000°C. For the Ni2Si samples,
resistivities as low 4.5x10-4 ohm-cm2 were reached after annealing between 750 and 1100°C.
Similarly, a set of Ti/AlNi/Au samples, with or without Ge as an additional contact layer, were
prepared via the same procedures. In this case, specific contact resistivities as low as 5.0 x10-4
ohm-cm2 were achieved after annealing the Ti/AlNi/Au samples between 600 and 700°C for 30
minutes in a dynamic argon atmosphere or under high vacuum. The lowest resistivities were
realized using thicker (~ 40 nm) Ti layers. I-V analysis revealed superior linear characteristics for
the AlNi system, which also exhibited a more stable microstructure after anneal. SIMS and RBS
were used to analyze the stability of the stacks subsequent to thermal treatment. AFM analysis
demonstrated the superiority of photoresist capping over alternatives in minimizing surface
roughness. Linear ohmic behavior after significantly reduced anneal temperature is the main
observation of the present study.
Abstract: We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) to correlate
subsurface deep level emissions and double barrier current-voltage (I-V) characteristics across an
array of Ni/4H-SiC diodes on the same epitaxial wafer. These results demonstrate not only a
correspondence between these optical features and measured barrier heights, but they also suggest
that such states may limit the range of SB heights in general. DRCLS of near-ideal diodes show a
broad 2.45 eV emission at common to all diode areas and associated with either impurities or
inclusions. Strongly non-ideal diodes exhibit additional defect emissions at 2.2 and 2.65 eV. On the
other hand, there is no correlation between the appearance of morphological defects observed by
polarized light microscopy or X-ray topography and the presence of double barrier characteristics.
The DRCLS observations of defect level transitions that correlate with non-ideal Schottky barriers
suggest that these sub-surface defect features can be used to predict Schottky barrier behavior.
Abstract: In this study, we performed a statistical analysis of 500 Ni Schottky diodes
distributed across a 2-inch, n-type 4H-SiC wafer with an epilayer grown by chemical
vapor deposition. A majority of the diodes displayed ideal thermionic emission when
under forward bias, whereas some diodes showed ‘double-barrier’ characteristics with
a ‘knee’ in the low-voltage log I vs. V plot. X-ray topography (XRT) and polarized
light microscopy (PLM) revealed no correlations between screw dislocations and
micropipes and the presence of double-barrier diodes. Depth resolved
cathodoluminescence (DRCLS) indicated that certain deep-level states are associated
with the observed electrical variations.
Abstract: The impact of high temperature annealing using graphite encapsulation (formed by
baking photoresist) on the electrical properties of Ni Schottky diodes formed on the annealed
surfaces is studied. The surface morphology is also characterized by atomic force microscopy
(AFM). Annealing for 10 minutes at temperatures up to 1800 °C with graphite encapsulation
actually reduces the high-current ideality factor of the diodes while raising the current-voltage
barrier height (linearly extrapolated to unity ideality factor) from 1.453 V to 1.67-1.73 V. Excess
leakage current occurs only in a subset of diodes, which are believed to be affected by extended
defects. The AFM images show no significant surface roughening, and the graphite can be removed
after processing. This encapsulation method is found to be highly effective in preserving the
electronic properties of the surface during high temperature annealing.