July 11, 2003
Material Defects in 4H-Silicon Carbide Diodes
U. Zimmermann1, J. Österman1, A. Hallén2, D. Kuylenstierna2, A.O. Konstantinov3, W.M. Vetter4 and M. Dudley4
1Department of Microelectronics and Information Technology, Royal Institute of Technology, Sweden;
2Department of Microelectronics, Chalmers University of Technology, Sweden;
3ACREO AB, Sweden;
4Department of Materials Science and Engineering, State University of New York at Stony Brook, NY
The role of crystallographic defects on the performance of semiconductor devices has been studied using the example of pn junction diodes in the modern semiconductor compound silicon carbide. The correlation of the locations of crystallographic defects as observed by the Synchrotron White-beam X ray Diffraction Topography technique and the electrical characteristics of the electronic devices allows us to draw conclusions about the importance of high-quality semiconductor material for reliable device operation.
The operation of all semiconductor components is based on the
regular arrangement of atoms in the crystal lattice of the
underlying material. Deviations from this regular arrangement can be
introduced intentionally in the form of strained layers,
compositional variations and impurity atoms in order to control and
modify the electrical properties of the semiconductor material.
However, this requires a base material of high purity and high
crystalline quality.
Silicon carbide was discovered by the Swedish scientist Jöns Jakob Berzelius in 1824. Silicon carbide has long been used as an abrasive material only, even though its semiconducting properties had already been discovered in the beginning of the twentieth century. During the last decade, a new interest in silicon carbide was driven by its unique properties, which make this material a promising candidate for high-voltage, high power and high temperature electronics.
The growth of large crystals of silicon carbide for device
fabrication has long been hindered by the tendency of the material
to form different crystal lattices with different properties. These
so-called polytypes are all built up by the stacking of bi-atomic
silicon-carbon layers. Of technological interest are mainly the
hexagonal crystals, 4H and 6H, and the cubic 3C variant. Although
the technique of large crystal growth is under control nowadays,
there still exists a diversity of crystallographic defects in
state-of-the-art silicon carbide wafers. Of these defects, edge and
screw dislocations are the most abundant ones. Figure 1 shows a
model of a screw dislocation in the 4H polytype of silicon carbide.
In our experiments, the location of screw dislocations on samples containing pn junction diodes was determined by the technique of Synchrotron White-beam X-ray Diffraction Topography, performed at the Synchrotron Topography Facility, Beamline X19C, at the NSLS. This technique delivered a two-dimensional map representing the crystalline structure of the sample, as shown in Figure 2. The data were then compared to electrical and electro-optical measurements on the individual diodes. The results showed a clear correlation between the presence of crystallographic defects and a reduced performance of the affected diodes.
The direct comparison between the light emission of the silicon
carbide junction under reverse bias, the X-ray topograph, and the
pictorial data of an electron-beam induced current (EBIC)
measurement can be seen in Figure 3. While the electroluminescence
under reverse bias shows inhomogeneities in the leakage current
distribution (Figure 3a), the focused electron beam of a scanning
electron microscope was used to generate electron-hole pairs
adjacent to the junction in the EBIC measurement, which can be
measured as an external current through the diode (Figure 3c). It is
obvious that the electrical characteristics of the diode are
affected by the presence of screw dislocations, as revealed in
Figure 3b.
The presence of screw dislocations, where silicon carbide can have a hollow or a closed core, was found to have a dramatic influence on the critical electric field strength of the silicon carbide material. The critical electric field, which is ten times higher in silicon carbide as compared to silicon, allows a semiconductor diode to block high reverse voltages. A local reduction of this parameter inside the active area can lead to a catastrophic failure of the whole device. This study furthermore demonstrates that Synchrotron White-beam X-ray topography is a non-destructive technique suitable to locate these defects without the need for extensive sample preparation.
BEAMLINE
X19C
FUNDING
Swedish Foundation for Strategic Research (SSF)
PUBLICATION
U. Zimmermann, J. Österman, D. Kuylenstierna, A. Hallén, A. O.
Konstantinov, W. M. Vetter, and M. Dudley, "Material defects in
4H-silicon carbide diodes", Journal of Applied Physics, 93,
611-8 (2003)
FOR MORE INFORMATION
Uwe Zimmermann
Dept. of Microelectronics and Information Technology
Royal Institute of Technology SWEDEN
Email: uwe@imit.kth.se