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Improved AlN/SiC Device Structures and Ohmic Contact Process

IP.com Disclosure Number: IPCOM000116485D
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 6 page(s) / 155K

Publishing Venue

IBM

Related People

Strite, S: AUTHOR

Abstract

SiC electronics will find very wide application in high temperature, high power, and high field electronics [1]. To perform at a level comparable to more prevalent semiconductors, SiC devices will require a high quality insulator and better ohmic contacts [2].

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Improved AlN/SiC Device Structures and Ohmic Contact Process

      SiC electronics will find very wide application in high
temperature, high power, and high field electronics [1].  To perform
at a level comparable to more prevalent semiconductors, SiC devices
will require a high quality insulator and better ohmic contacts [2].

Here, the use of lattice matched epitaxial AlN as a high quality
insulator for SiC devices and a C-depleting plasma processing step
for the formation of low resistance n-type ohmic contacts to SiC is
proposed.

Figure 1: Comparison of the AlN and 6H-SiC lattice constants and
thermal expansion coefficients.

      SiC, like Si, suffers from a high surface state density, but
cannot be completely passivated by SiO sub 2 because C does not form
a stable oxide (3).  AlN (4) is a wide bandgap semiconductor which is
nearly perfectly lattice and thermally matched to SiC as illustrated
in Fig. 1.  AlN can be grown epitaxially onto SiC substrates (5), is
very resistive, has a reasonable dielectric constant (8.5) and a high
breakdown field (1.5 times 10 sup 7 'V/cm').  These properties make
AlN an excellent candidate for the formation of a nearly ideal
heterointerface with SiC which can be exploited in MIS device
structures.  Thermally stable to temperatures up to 800 sup 0C with a
bandgap energy of 6.2 eV, AlN is also suitable for high temperature
applications.

      The high density of surface states and large bandgap of SiC
make low resistance ohmic contact formation difficult.  The surface
Fermi energy is pinned near midgap, and both electrons and holes must
surmount large Schottky barriers to injection.  Both higher doping
and a narrow bandgap are known to improve ohmic contacts by
respectively reducing the width and height of the barrier.

      The proposed process for n-type ohmic contact formation to SiC
relies on two effects likely to occur in C-depleted SiC.  The bandgap
should decrease as the material becomes more Si-like, and the free
electron concentration should rise as a result of C sub Si antisite
defect donor formation (6).

Figure 2: Band diagram of a metal/SiC interface in which the electron
injection barrier is lowered by surface C depletion and thinned by an
increased free electron concentration.

      H plasmas etch C atoms via CH sub 4 formation and desorption.
However, studies of the (0001) sub Si SiC face indicate that only
disordered C bonds are etched by H plasma exposure (5).  In order to
etch C atoms in a H plasma, it will be necessary to promote
disordering of the surface material.  This can be accomplished by
including a relatively massive, but inert gas like Ar in the plasma,
tuning the plasma for higher ion energies, and possibly heating the
SiC sample.  High kinetic energy atoms bombarding the surface will
create disorder allowing H atoms to carry off surface C.  The C
depletion and antisite doping results in an ohmic contact band
structure, as illustrated in Fig....