Abstract
4H-SiC Schottky rectifiers with 
dielectric overlap edge termination were rapid thermal annealed at temperatures from 700 to 1100°C for periods of 60-240 s under 
ambient. The forward turn-on voltage 
ideality factor 
and on-state resistance 
were all improved by anneals <1000°C. At higher temperatures and longer duration, the Ni Schottky contact showed ohmic behavior and degraded surface morphology. Since the reverse characteristics were less affected by the lower temperature anneals than the forward characteristics, the diodes exhibited an increase in on/off ratio of up to ∼20%. © 2003 The Electrochemical Society. All rights reserved.
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There is currently a lot of interest in the application of SiC power rectifiers to high-current switching in the >30 A range because of the high thermal conductivity (∼4.9 W/cm K) and large bandgap relative to Si.1
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15 Of the various polytypes, 4H-SiC seems the most attractive because of its wider bandgap (3.26 eV) compared to 6H-SiC (2.9 eV) and its higher electron mobility.3 These advantages in materials properties mean that on-state resistance, 
for a 4H-SiC rectifier can be over 100 times smaller than that of a Si rectifier at the same breakdown voltage, 
since 
is given by 
(where μ is the electron mobility, ɛ the permittivity, and 
the critical field for breakdown).13
There are numerous situations in which high-temperature annealing might be employed in the fabrication of SiC rectifiers, including activation of implanted dopants or reduction of reverse leakage current in self-aligned 
-implanted edge terminations.16
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19 In the latter case, 
implantation is used to create a high-resistance region around the contact periphery in order to spread the electric field distribution and avoid breakdown due to field crowding. While this process does increase reverse breakdown voltage, it also increases leakage current. Several studies have examined the effect of anneals up to 700°C for reducing this leakage current.17
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In this paper we report the stability of 4H-SiC Schottky power rectifiers to rapid thermal annealing treatments in the temperature range 700-1100°C with the contacts already in place. This is of interest for defining the stability of devices in which the Ni rectifying contact is used as a self-aligned mask for implant edge termination. The performance of the rectifiers is found to show a general improvement for 1000°C anneals.
Experimental
The starting substrates were 
4H-SiC. Approximately 10 μm of lightly n-type 
4H-SiC was grown on these substrate by vapor phase epitaxy, followed by ∼500 Å of thermal 
A full-area back contact of E-beam deposited Ni annealed at 970°C was used for contact to the substrate. The contact resistivity was 
layers (0.7 μm thick) deposited by plasma-enhanced chemical vapor deposition (PECVD) were employed as a part of the dielectric overlap edge termination. Holes were opened in the 
stack by a combination of dry and wet etching and a Schottky contact of E-beam evaporated (2000 Å thick) Ni patterned by lift-off. The contact diameter was fixed at 154 μm. A schematic of the completed structure is shown in Fig. 1. The annealing was performed in a Heatpulse 610T System under flowing 
ambient at temperatures of 700-1100°C for 60-240 s. The current-voltage (I-V) characteristics were measured before and after this procedure. Prior to annealing, the typical on-state resistance was 0.8 mΩ cm2, the reverse breakdown voltage ∼450 V, and the forward turn-on voltage was 2.5 V at 100 A cm−2.
Figure 1. Schematic of 4H-SiC Schottky rectifier.
Results and Discussion
The reverse bias characteristics showed a general increase in current for 1000°C anneals, while the forward I-V characteristics also showed significant changes under these conditions, as shown in Fig. 2. Note the decrease in turn-on voltage as the anneal temperature increased. The forward current density, 
can be expressed as
where 
is the Richardson's constant for SiC, k is Boltzmann's constant, e is the electric charge, 
is the barrier height for Ni on 4H-SiC, 
is the applied voltage, n is the ideality factor, and T is the measurement temperature. The data in Fig. 2 are consistent with a reduction in 
On 6H-SiC, Ni was observed to form 
at temperature beginning at ∼600°C,20 eventually leading to ohmic behavior. In lower temperature anneals, Ni/6H-SiC diodes showed a reduction of leakage current through removal of low-barrier secondary diodes in parallel to the primary diode.16 Our measured 
was ∼1.4 eV prior to annealing (Fig. 3a), which is consistent with past reports.20
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23 It is likely that the reduced turn-on voltage observed in Fig. 2 is the result of silicide formation, as suggested previously.17 The on/off current ratio was 
at 3 V/−450 V in control samples and increased by a maximum of ∼20% after optimum annealing at 700°C.
Figure 2. Forward I-V characteristics as a function of anneal temperature for 60 s anneals.
Figure 3. (a) Forward I-V characteristic from large-area (0.64 mm2) diode from which a barrier height of 0.64 eV was obtained, and (b) change in 
as a function of anneal temperature for 60 s anneals.
The forward current characteristics of a SiC Schottky rectifier are dominated by the Ni barrier height and by series resistance contributions and is related to 
and 
through the relationship
With annealing the value of 
decreased as described, but 
showed improvement (Fig. 3b). Once again the annealing durations had to be kept <120 s at the highest temperature in order to retain some acceptable rectifying behavior of the top Ni contact.
Figure 4 shows the effect of annealing time at 900°C on the forward I-V characteristics (4a), 
(4b), and 
(4c). Note that the largest anneal (240 s) produced essentially ohmic behavior. Examples of the effect of the annealing duration on the top Ni contact morphology are shown in the optical micrograph of Fig. 5. It is clear that there is already a strong metallurgical reaction between the Ni and the SiC at 1000°C for 60 s. To obtain the lowest ohmic contact resistivity for annealed Ni contacts requires ∼3 min at this temperature, and after 60 s the contact still retains some rectifying behavior. Note also the dark appearance of the contact after annealing. Auger electron spectroscopy showed high levels of carbon present in these reacted contacts, suggesting the reaction can be written as
Figure 4. (a) Forward I-V characteristics, and (b) change in 
and (c) 
as a function of anneal time at 900°C.
Figure 5. Optical micrographs of Ni contacts (a) before and after annealing at 1000°C for (b) 60 or (c) 120 s.
Figure 6 shows the dependence of the change in 
(6a) and 
(6b) on measurement temperature for samples annealed at 900°C. There is little significant change in 
while the improvement in 
decreases with increasing temperature as current transports over the barrier. Arrhenius plots of the forward current indicated a barrier height of ∼1.14 eV after the 900°C anneal, consistent with the observed increase in current after annealing. From the intersection of this data with the temperature axis, we inferred an electrically active contact area equivalent to only ∼9% of the physical contact area. This is consistent with the expected behavior for thermionic emission still being the dominant transport mechanism after 900°C annealing.
Figure 6. Change in 
(a) and 
(b) as a function of measurement temperature after 900 C, 60 anneals.
Conclusions
The effects of rapid thermal annealing in the performance of 4H-SiC Schottky rectifiers were investigated. For high-temperature anneals, the Ni rectifying contact becomes ohmic, with severe degradation of the contact morphology. The forward current characteristics are still dominated by thermionic emission until the onset of ohmic behavior. The benefits of annealing at moderate temperature include a decrease in forward turn-on voltage and an increase in the on/off current ratio. The reasons for those improvements are not clear but may include annealing of defects or reduction of interfacial oxides between the Ni rectifying contact and the SiC. We should note that the reverse breakdown voltage in our devices is still only approximately one-half that predicted from the epilayer thickness and doping; thus, the performance is still limited by material defect density rather than purity. The use of more thermally stable rectifying contact metallization would allow higher annealing temperatures and possibly more improvement in device performance.
Acknowledgments
The work at the University of Florida is partially supported by NSF CTS 1109973, while the work at Sterling Semiconductor is partially supported by U.S. Air Force contact no. 33615-01-M-2136 (Dr. James Schofield).
The University of Florida assisted in meeting the publication costs of this article.