11,113 research outputs found
Epitaxial growth of 6H silicon carbide in the temperature range 1320 C to 1390 C
High-quality epitaxial layers of 6H SiC have been grown on 6H SiC substrates with the grown direction perpendicular to the crystal c-axis. The growth was by chemical vapor deposition from methyltrichlorosilane (CH3SiCl3) in hydrogen at temperatures in the range of 1320 to 1390 C. Epitaxial layers up to 80 microns thick were grown at rates of 0.4 microns/min. Attempts at growth on the (0001) plane of 6H SiC substrates under similar conditions resulted in polycrystalline cubic SiC layers. Optical and X-ray diffraction techniques were used to characterize the grown layers
Electronic Materials with Wide Band Gap: Recent Developments
The development of semiconductor electronics is shortly reviewed, beginning
with the development of germanium devices (band gap eV) after world
war II. Quickly a tendency to alternative materials with wider band gap became
apparent, starting with silicon ( eV). This improved the signal/noise
ratio for classical electronic applications. Both semiconductors have
tetrahedral coordination, and by isoelectronic alternative replacement of Ge or
Si with carbon or several anions and cations other semiconductors with wider
are obtained, that are transparent for visible light and belong to the
group of wide band gap semiconductors. Nowadays some nitrides, especially GaN
and AlN, are the most important materials for optical emission in the
ultraviolet and blue spectral region. Oxide crystals, such as ZnO and
-GaO, offer similarly good electronic properties but suffer
still from significant difficulties in obtaining stable and technically
sufficient -type conductivity.Comment: 25 pages, 8 figures, 4 table
Exchange Anisotropy in Epitaxial and Polycrystalline NiO/NiFe Bilayers
(001) oriented NiO/NiFe bilayers were grown on single crystal MgO (001)
substrates by ion beam sputtering in order to determine the effect that the
crystalline orientation of the NiO antiferromagnetic layer has on the
magnetization curve of the NiFe ferromagnetic layer. Simple models predict no
exchange anisotropy for the (001)-oriented surface, which in its bulk
termination is magnetically compensated. Nonetheless exchange anisotropy is
present in the epitaxial films, although it is approximately half as large as
in polycrystalline films that were grown simultaneously. Experiments show that
differences in exchange field and coercivity between polycrystalline and
epitaxial NiFe/NiO bilayers couples arise due to variations in induced surface
anisotropy and not from differences in the degree of compensation of the
terminating NiO plane. Implications of these observations for models of induced
exchange anisotropy in NiO/NiFe bilayer couples will be discussed.Comment: 23 pages in RevTex format, submitted to Phys Rev B
Strain-induced ferroelectricity in simple rocksalt binary oxides
The alkaline earth binary oxides adopt a simple rocksalt structure and form
an important family of compounds because of their large presence in the earth's
mantle and their potential use in microelectronic devices. In comparison to the
class of multifunctional ferroelectric perovskite oxides, however, their
practical applications remain limited and the emergence of ferroelectricity and
related functional properties in simple binary oxides seems so unlikely that it
was never previously considered. Here, we show using first-principles density
functional calculations that ferroelectricity can be easily induced in simple
alkaline earth binary oxides such as barium oxide (BaO) using appropriate
epitaxial strains. Going beyond the fundamental discovery, we show that the
functional properties (polarization, dielectric constant and piezoelectric
response) of such strained binary oxides are comparable in magnitude to those
of typical ferroelectric perovskite oxides, so making them of direct interest
for applications. Finally, we show that magnetic binary oxides such as EuO,
with the same rocksalt structure, behave similarly to the alkaline earth
oxides, suggesting a route to new multiferroics combining ferroelectric and
magnetic properties
Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain
Recently a metallic state was discovered at the interface between insulating
oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional
electron gas (2DEG) have attracted significant interest due to its potential
applications in nanoelectronics. Control over this carrier density and mobility
of the 2DEG is essential for applications of these novel systems, and may be
achieved by epitaxial strain. However, despite the rich nature of strain
effects on oxide materials properties, such as ferroelectricity, magnetism, and
superconductivity, the relationship between the strain and electrical
properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely
unexplored. Here, we use different lattice constant single crystal substrates
to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial
strain. We have found that tensile strained SrTiO3 destroys the conducting
2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier
concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface.
We have also found that the critical LaAlO3 overlayer thickness for 2DEG
formation increases with SrTiO3 compressive strain. Our first-principles
calculations suggest that a strain-induced electric polarization in the SrTiO3
layer is responsible for this behavior. It is directed away from the interface
and hence creates a negative polarization charge opposing that of the polar
LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer,
and reduces carrier concentration above the critical thickness, in agreement
with our experimental results. Our findings suggest that epitaxial strain can
be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface
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