INVESTIGATION OF Al2O3 AND TiO2 AS GATE INSULATORS FOR
4H-SiC PULSED POWER DEVICES
Q. Shuia, M. S. Mazzolab, X. Gua, C. W. Mylesc, and M. A. Gundersena, ζ
a
Department of Electrical Engineering-Electrophysics, University of Southern California
Los Angeles, CA 90089-0271
b
Department of Electrical and Computer Engineering, Mississippi State University
Mississippi State, MS 39762
c
Department of Physics, Texas Tech University
Lubbock, TX 79409-1051
Abstract
Achieving high quality and high electric breakdown of
oxide on 4H-SiC substrates is still a major research
challenge. 500Å Al2O3 and TiO2 were deposited on 4HSiC by high vacuum and low-temperature e-beam
evaporation. AFM (Atomic Force Microscopy) results
show that a surface mean height of 4-7 Å of Al2O3 was
found on Si substrate and 74 Å of Al2O3 was found on
SiC substrate. MIS capacitors were fabricated using one
mask to evaluate the quality of these gate materials. C-V
measurements at 10kHz show that Al2O3 and TiO2 have
dielectric constants of more than 9 and 20, respectively.
The leakage current of an Al2O3 MIS capacitor is about
10-9A on a capacitor area of 150m 150m at a gate
voltage of 10V. An Al2O3 MIS capacitor has a lower
leakage current than a TiO2 MIS capacitor due to its large
band gap offsets. Its breakdown electric field strength is
more than 8MV/cm. This research indicates that e-beam
deposited Al2O3 may be a promising dielectric material
for the pulsed power SiC devices.
I. INTRODUCTION
It is important to develop solid-state-based pulsed
power switches with hold off voltage considerably in
excess of 10 kV, peak currents of kA to many kA, and
turn-on in 10's of nanoseconds, for varying pulse lengths.
4H-SiC is an attractive candidate at this time because it
has a large bandgap energy (3.26eV) relative to GaAs
(1.43eV) and Si (1.12eV), breakdown electric field
strength of 3-4 MV/cm, electron saturation velocity of 2.2
107cm/s and thermal conductivity of 4.9 W/cm.K [1-2].
Up to now, 5kV 4H-SiC MOSFETs with 100-m [3] Ndrift region and several kilovolts of VJFETs have been
proposed [4-8]. 4H-SiC-based switches are thus of
interest as potentially efficient, low-cost, and compact
alternatives to gas phase switches [9]. MOSFETs,
although the most widely used power devices, have poor


oxide quality and oxide breakdown which has thus far
limited the performance of 4H-SiC MOSFETs [10], and
the high breakdown field strength of SiC has not been
fully exploited. Utilizing a material with higher dielectric
constant will reduce the electric field across the dielectrics
[11], and provides a possible solution to the abovementioned problems.
The relatively large lattice mismatch between Al2O3 or
TiO2 and SiC substrates restricts the use of these layers in
SiC technology as SiO2 and AlN. So far, very limited
research has been done in this field [12-13]. Nevertheless,
Al2O3 (  =8-15, Eg=6-9eV) and TiO2 (  =20-110, Eg=35eV) have higher dielectric constants and high bandgap
energies as well as the acceptance of Ti and Al in most
modern CMOS fabrication facilities. A band alignment of
Al2O3 on 4H-SiC with offsets of 2.2 and 1.5eV for the
valence and conduction bands further makes it a
potentially serious competitor to SiO2 [12].
In this paper, we study Al2O3 and TiO2 grown by ebeam evaporation. This method is a high vacuum and
low-temperature process (T<200ºC), thus it is effective in
limiting the formation of the interfacial SiO2 layer
between the SiC and Al2O3 as well as the formation of
high-temperature induced defects. Surface morphology of
Al2O3 or TiO2 were investigated by AFM, and both Al2O3
and TiO2 MIS capacitors were fabricated to evaluate their
potentials as gate dielectrics in pulsed power SiC devices.
II. EXPERIMENTAL PROCEDURE
4-8m-thick n- layer (~11016cm-3) and 0.3 m-thick
buffer layer, grown on the (0001) Si-faced 4H-SiC
substrate (~11019cm-3), were provided by Mississippi
State University. Fig.1 illustrates the schematic structure
of the MIS capacitors. Only one mask was used to
fabricate these capacitors. Before the deposition of nickel
and gate dielectric materials, the samples were cleaned
and dipped in buffered oxide etch (7:1) for 1min. 2000 Å
nickel was e- beam evaporated and annealed at 900 ºC for
1min to form backside ohmic contacts. 500 Å Al2O3 and
This work supported by the AFOSR MURI Program and the ARO.
email: mag@usc.edu
TiO2 were also e-beam deposited at temperatures less than
200ºC, respectively. Ti (200 Å) /Au (1500Å) were chosen
as gate electrodes. They were patterned by standard
photolithography techniques, and annealed at 450ºC for
30s by rapid thermal annealing (RTA).
frequency of 10kHz at room temperature. Since an nepilayer is below the gate insulators, at positive voltages,
Figure 2(a). An AFM image (10m10m) of the
starting material (4H-SiC n- epilayer).
Figure 1. The schematic structure of investigated 4H-SiC
MIS capacitors.
The capacitance-voltage (C-V) measurements of these
MIS capacitors were done by a HP 4263 LCR meter at a
frequency of 10kHz. The small ac voltage is 1V on an
applied dc gate voltage. Dielectric constants of Al2O3 and
TiO2 grown by e-beam evaporation were calculated.
Current-voltage (I-V) measurements were performed by
using a HP 4145B semiconductor parameter analyzer on
150m150m gate patterns.
III. RESULTS AND DISCUSSION
A. Film morphology
Fig. 2 (a) shows the surface morphology of the n- layer
(~11016cm-3) grown on heavily doped 4H-SiC substrate.
The surface shows a regular terrace structure with width
of about 1m. The surface mean height is 84Å. An AFM
image was also taken after the deposition of Al2O3 or
TiO2 (Fig. 2(b)). We found that the surface morphology
improves and the surface mean height is about 74 Å. We
also measured Al2O3 (500 Å) deposited by the same ebeam evaporator on Si substrate, and the surface mean
height is 4-7 Å (Fig.2(c)). It is obvious that the surface
morphology not only depends on the growth conditions,
but also depends on the substrate. We conclude that from
the point of surface morphology, high vacuum and lowtemperature e-beam evaporation proves to be a simple and
reliable way for the growth of good surface morphology
of Al2O3 and TiO2 gate dielectrics.
B. Electrical Characterization
Figures 3(a) and 3(b) show the C-V characteristics of
metal-Al2O3-SiC and metal-TiO2-SiC capacitors measured
on different contact areas, 120m 120m and 100m
100m, respectively. Measurements were performed at a
Figure 2(b). An AFM image (10m10m) of 500 Å
Al2O3 deposited on 4H-SiC n- epilayer.
Figure 2(c). An AFM image (10m10m) of 500Å
Al2O3 deposited on Si substrate.
majority carrier electrons would experience a force
toward the oxide-semiconductor interface. At large
positive voltages, the capacitance saturates, indicating that
an accumulation layer is formed below the gate materials.
The capacitance C acc for the MIS capacitor in the
accumulation mode is just oxide effective capacitance.
The oxide effective capacitance may also include the
contribution of a very thin SiO2 formed in the subsequent
dielectric constant of the insulating systems. Similarly, the
theoretical value of CFB for TiO2 (A=120m 120m) is
determined to be 20 pF. This value is also below our
expected value deduced from the experimental data in Fig
3 (b). Since the dielectric constant TiO2 is several times
larger than that of Al2O3, the thin SiO2 layer doesn’t
reduce the dielectric constant of TiO2/SiO2 system as
much as it does for Al2O3/SiO2 system. There are two
approaches to determine the thicknesses of SiO2 and
Al2O3 or TiO2. The first is to fabricate another set of MIS
capacitors with different thicknesses of Al2O3 or TiO2
under the same fabrication conditions. The second is to
employ material characterization equipments, such as
AES (Auger Electron Spectroscopy), SIMS (Secondary
Ion Mass Spectrometry) and TEM (Transmission Electron
Microscope). That is part of our future work.
Figure 3(a) Capacitance-voltage characteristics of MetalAl2O3 (500 Å)-SiC capacitors measured at 10kHz.
Figure 3(b). Capacitance-voltage characteristics of
Metal-TiO2 (500 Å)-SiC capacitors measured at 10kHz.
450C gate electrode annealing process. A minimum
capacitance Cmin at large negative voltages in Fig. 3(a)
and 3(b) mean that a maximum depletion width
xdT is
reached. We first assume that there is no thin interfacial
SiO2 layer formed, the dielectric constants for Al2O3 and
TiO2 on two different areas are consistent and they are 9
and 20, respectively, based on the accumulation
capacitance C acc . The theoretical flat-band capacitance
then can be determined from the following equation [14]:
CFB 
 0 ox A
 
tox   ox 
  SiC 
(1)
 kT   0 SiC 



 q  qN a 
We note that the calculated flat-band capacitance for
Al2O3 (A=120m 120m) is 13.5pF. This value is even
below Cmin , indicating that a thin interfacial layer SiO2 is
in series with the Al2O3 and that it reduces the effective
Figure 4. Current-voltage characteristics of MIS
capacitors. The capacitor area is 150m 150m and the
thicknesses of Al2O3 and TiO2 are all 500 Å.
Leakage currents were measured on MIS capacitors
with area of 150m 150m. From Fig. 4 it is found that
the leakage current is dependent on the polarity of the
gate bias. The dielectric of Metal-Al2O3-SiC capacitors
even doesn’t break down at a gate voltage of 40V, which
corresponds to an electric field strength of 8MV/cm.
Al2O3 has band offsets of 2.2eV for the valence band, and
1.5eV for the conduction band [12]. These band offsets
promise higher barriers for both electrons and holes. Fig.4
also shows that Al2O3 is highly resistive, and that its
resistivity is more than 11011cm. The experiments
demonstrate that Al2O3 is a low-leakage current gate
material. TiO2 has a higher leakage current at negative
gate voltages than Al2O3. This can be explained by the
observations [15-17] that materials with higher dielectric
constants tend to suffer from higher leakage currents due
to smaller bandgaps. It is known that the bandgap of TiO2
is 3-5eV, the smaller bandgaps lead to lower band offsets
with respect to 4H-SiC.
Nitridation is widely used in silicon technology and it is
usually achieved by annealing the sample in nitric oxide
(NO), nitrous oxide (N2O) or NH3 atmosphere. Nitridation
is not only very effective in limiting the formation of an
unnecessary interfacial SiO2, but also reduce the Si
dangling bond and Si-O-Si strained bonds at the interface,
hence decreasing the leakage current. To gain insight into
the role that NO plays at the interface between the
insulator Al2O3 and 4H-SiC, after depositing Ni and
alloying of the backside Ni ohmic contacts at 900ºC, the
samples were subjected to annealing in NO at 1000ºC for
10min. then dipped in buffered oxide etch (7:1) for 1min.
The subsequent fabrication conditions are the same as
described above. We found that C acc decreases to 16.8pF
for a 120m 120m capacitor. We think that a thicker
SiO2 is formed and nitric oxide doesn’t seem to limit the
formation of SiO2 in this case. I-V tests show that the
leakage currents remain almost the same.
IV. CONCLUSIONS AND FUTURE WORK
We have performed e-beam deposition of Al2O3 and
TiO2 on 4H-SiC for the purpose of pulsed power
application. High-vacuum and low-temperature e-beam
evaporation shows a mean height of about 4-7 Å on Si
substrate. The MIS C-V characteristics show that the
dielectric constant of Al2O3 is more than 9, and TiO2 is
more than 20. I-V curves show that Al2O3 is highly
resistive and has much lower leakage current than TiO2
due to its large band gap offsets. Therefore, Al2O3
deposited by e-beam evaporation can be considered as a
promising material among the gate insulators for high
power SiC devices which will be used in pulsed power in
the future.
Further research is needed to identify detailed
information of gate materials by X-Ray Diffraction
(XRD), TEM, etc. Dielectric constants, flat-band voltage,
fixed oxide charges, and interface trapped charge
densities need to be determined.
V. REFERENCES
[1] B. Jayant Baliga, “Power semiconductor device figure
of merit for high-frequency applications,” IEEE Electron
Device Lett., 10, pp. 455-457, 1989.
[2] J. A. Cooper, JR., and A. Agarwal, “SiC powerswitching devices-the second electronic revolution?”
Proceeding of the IEEE, 90, pp. 956-968, 2002.
[3] I. A. Khan, J. A. Cooper, Jr., M. A. Capano, T. IsaacsSmith, and J. R. Williams, “High-voltage UMOSFETs in
4H SiC,” Power Semiconductor Devices and IC's, 2002.
Proceedings of the 14th International Symposium on,
June 2002, pp. 157-160.
[4] H. Mitlehner, W. Bartsch, K. O. Dohnke, P. Friedrichs
and R. Kaltschmidt, “Dynamic characteristics of high
voltage 4H-SiC vertical JFETs,” Power Semiconductor
Devices and IC's, 1999. ISPSD '99, 1999 IEEE
International Symposium on, pp. 339-342, 1999.
[5] A. Mihaila, R. Udrea, R. Azar, J. Liang, G.
Amaratunga, A. Rusu, and G. Brezeanu, “ Thearetical and
numerical investigation of SiC JFET and MOSFET at
6.5kV,” Semiconductor Conference, 1999. CAS '99
Proceedings. 1999 International, vol. 1, pp. 191-194, Oct.
1999.
[6] K. Asano, Y. Sugawara, S. Ryu, R. Singh, J. Palmour,
T. Hayashi, and D. Takayama, “5.5 kV normally-off low
Rons 4H-SiC SEJFET,” Power Semiconductor Devices
and IC's, 2001. ISPSD '01, 2001 IEEE International
Symposium on, pp. 25-32, 2001.
[7] K. Tone, J. H. Zhao, L. Fursin, P. Alexandrov, and M.
Weiner, “4H-SiC normally-off vertical junction fieldeffect transistor with high current density,” IEEE Electron
Device Lett., 24, pp. 463-465, 2003.
[8] Q. Shui, X. Gu, C. W. Myles, M. S. Mazzola, and M.
A. Gundersen, “Simulations of a high power 4H-SiC
VJFET and its GaAs counterpart,” Proc. of 14th
International Pulsed Power Conference, pp. 123-126,
Dallas, Texas USA, June 15-18, 2003.
[9] M. A. Gundersen, “Gas-phase pulsed power
switches,” IEEE Transaction on Plasma Science, 19, pp.
1123-1131, 1991.
[10] J. A. Cooper, JR., A. Agarwal, “SiC power-switching
devices-the second electronics revolution?,” Proceedings
of the IEEE, 6, pp. 956-968, 2002.
[11] L. A. Lipkin and J. W. Palmour, “Insulator
investigation on SiC for improved reliability,” IEEE
Trans. Electron Devices, 46, pp. 525-532, 1999.
[12] K. Y. Gao, Th. Seyller, L. Ley, F. Ciobanu, G. Pensl,
A. Tadich, J. D. Riley, and R. G. C. Leckey, “Al2O3
prepared by atomic layer deposition as gate dielectric on
6H-SiC (0001),” Appl. Phys. Lett., 83, pp.1830-1832,
2003.
[13] A. Werbowy, K. Zdunek, E. Dusinski, J. Szmidt, and
M. Elert, “Impulse plasma deposition of aluminum oxide
layers for Al2O3/Si, SiC, GaN systems,” Surface and
Coatings Technology, 174, pp.170-175, 2003.
[14] D. A. Neamen, “Semiconductor Physics and
Devices,” McGraw-Hill Higher Education, Third Edition.
[15] R. Jammy, U. Scjroeder, K. Wong, J. Bruley, J.
Shepard, and P. DeHaven, “Synthesis and characterization
of TiO2 films for deep trench capacitor applications,” in
Proc. of the 2000 12th IEEE International Symposium on
Applications of Ferroelectrics, 1, pp.147-150, 2000.
[16] S. A. Campbell, D. C. Gilmer, X. –C. Wang, M. –T
Hsieh, H. –S Kim, W. L. Gladfelter, and Jinhua Yan,
“MOSFET transistors fabricated with high permittivity
TiO2 dielectrics,” IEEE Trans. Electron Devices, 44,
pp.104-109, 1997.
[17] G. D. Wilk, R. M. Wallace, and J. M. Anthony,
“High-κ gate dielectrics: current status and materials
properties considerations,” J. Appl. Phys, 89, pp.52435275, 2001.