Our
research is focused on a wide range of interdisciplinary projects dealing with
synthesis of novel molecular and solid-state inorganic systems, including
epitaxial growth of device-quality thin-film materials. These materials will
have applications in microelectronics, optoelectronics, and refractory ceramics.
The common theme running through our effort in materials design and synthesis is
to exploit our ability to build-in precise atomic arrangements (primarily via
novel molecular chemistry) in order to overcome thermodynamic impediments
arising from differences in atomic sizes, electronic level-filling and
electronegativities of constituent atoms and thereby form new alloy and compound
materials of the main group. The range of target materials is very wide and some
of them are described below.
1.
Synthesis of C-Si-Ge-Sn alloys and compounds. New materials for integration of Si-based microelectronics
and optoelectronics:
The
vast majority of research into semiconducting materials for device applications
has been restricted, for practical reasons, to materials whose overall
composition forms a thermodynamically stable bulk phase such as Si, SiC, etc.
Structural metastability is encountered on much higher spatial scales through
e.g. layering and patterning. A major part of our work, since 1994, has been
focused on extending metastability to the atomic level by designing and
producing novel CVD precursors that incorporate desired atomic arrangements.
Using this technique, we have successfully produced new families of C-Si-Ge-Sn
compound and alloy semiconductors as epitaxial
heterostructures or novel nanostructures. Our work in this area bridges
the gap between inorganic materials chemistry, applied physics, and
heterostructure engineering and it is beginning to have a significant impact on
a the rapidly emerging research area of Si-based semiconductors that are
considered important for future generations of high-speed devices.
By
combining powerful synthetic techniques of molecular chemistry and UHV-CVD with
innovative computational materials design (in collaboration with theoretical
physics colleagues) and state of the art characterization tools at ASU, we
continue design and fabrication of new classes of Si-based semiconducting (and
possibly even metallic) systems with interesting and useful properties.
We are particularly interested in direct bandgap materials, new
insulators with simultaneous tuning of dielectric and structural properties,
ordered semiconductor alloys with tunable electronic and structural properties,
and highly doped systems that may exceed standard doping density limits to
produce novel covalent metals.
2.
Development of group III nitrides via chemically designed (C-H) and (N-H)
free precursors and UHV-CVD:
The
main objective of this work is to explore the growth of defect free epitaxial
nitrides and to develop unusual nanostructured morphologies, such as quantum
dots and wires, via novel synthesis methods. The crucial step in this goal is
the successful development of a unique family of inorganic compounds of
composition H2GaN3, D2GaN3, H(Cl)GaN3,
and H(Br)GaN3. The use of H2GaN3, D2GaN3
has led to MBE/CVD growth of semiconductor nanostructures. Specifically,
selective growth of GaN quantum dots directly on (111) Si has been observed.
The same compounds appear to be suitable for use in vapor-solid-liquid (VLS)
experiments to grow nanorods and nanodots directly on substrates such as SiC,
and Al2O3. Current
work is focused on systematic growth and characterization of rationally designed
and assembled periodic arrays of such nanostructures.
The ultimate goal is to obtain material with novel optoelectronic
properties. The development of
standard GaN thin-film heterostructures on Si via AlN buffer layers, at low
temperatures has also been accomplished via this method and represents one of
the simplest possible and versatile synthetic routes to GaN.
3.
Synthesis of Quaternary wide bandgap semiconductors based on covalent carbides and
nitrides, and new methods of
integration of optical materials with Si
The primary objective is to develop tunable epitaxial materials that contain isostructural binary carbides and nitrides of the main group. Phases with the exact stoichiometries such as SiCAlN GeCAlN etc. have been synthesized via reactions of molecular H3MCN (M= Ge, Si, Sn) and Al atoms using MBE methods: [H3MCN (gas) + Al (atoms) ==> MCAlN + 3/2 H2]. The key to the successful synthesis of these new materials is the unique combination of novel inorganic sources and traditional MBE utilizing highly reactive metallic beams. We are exploiting this new synthetic method to fabricate a range of wide gap semiconductors between the hypothetical GeC or SnC with AlN, GaN, and InN etc. The prospect of producing a direct wide bandgap material in these systems may have important consequences in optoelectronic applications.
Development
of thin crystalline Si-Al-O-N templates and buffer layers on Si substrates via self assembly methods is in progress.
The Si-Al-O-N materials are ideal
candidates as semiconducting nucleation layers for the integration of nitride
semiconductors such as AlN, GaN, SiCAlN with silicon.
4. Synthesis
of Diamond-Like Compounds in the Li-Be(Mg)-B-(Al)-C-N-O System:
The
objectives outlined in this section are the development of radically new
approaches to synthesize novel light-element precursors that will yield
superhard and ultrastrong materials via novel solution routes. The exploratory
syntheses have resulted in the development of a new class of materials with
ternary and quaternary compositions that are isoelectronic to diamond or are
related to Si3N4. Examples
of these materials are: LiBC4N4, BC3N3,
BeC2N2, MgC2N2, BeCN2,
LiBC2N4, AlC3N3, and LiAlC2N4.
Application of high pressure or laser ablation should give extremely
dense, superhard materials with structures and properties related to those of
diamond. A goal is to design and synthesize possible alternatives to diamond for
high performance applications under extreme conditions.
These materials are also important because of their high thermal,
chemical and mechanical stability as well as their novel dielectric and
optoelectronic properties. Wide bandgaps, that in some cases may be direct, are
anticipated in these materials.
Another
theme of materials design is bandgap tuning using these light elements.
By incorporating a controllable number of compensated group II, III, and
V constituents in the molecular precursor(s), control of the ionicity of the
crystal potential and thereby the bandgap of the resulting bulk material can be
tuned. Bandgap tuning through MBE is well-known and well-developed, but bandgap
tuning of materials containing the more refractory first-row compounds is a
relatively unexplored field. These materials are of great interest since they
can cover a very wide range of bandgaps and can be very chemically stable and
mechanically hard.
5. Synthesis of Si based superhard dielectric materials:
The
primary objective of this research is the
development of methodologies to synthesize unimolecular precursors composed of
light elements (B, C, N, O, and Si) for preparation of superhard thin films and
coatings as well as superior dielectric materials by CVD.
The synthesis thus far has focused on ternary and quaternary compositions
in the Si-B-O-N and B-C-Si-N systems. New high-hardness systems are synthesized
as thin films on Si(100) substrates. Mechanical, electrical and dielectric properties such as
nanohardness, capacitance-voltage (C-V) as a function of frequency and leakage
current density-voltage (JL-V) characteristics are determined.
6. Synthesis
of Si based nanostructures.
New and practical
methods are developed for deposition of Si quantum dots that are embedded in an
amorphous SiO2 and Si3N4 matrices and emit in
the visible spectral region. Growth of coherent and
defect free Si1-xGex nanostructures
is carried out by CVD of unimolecular precursor molecules incorporating the
target stoichiometries. In situ,
real time observation and characterization of the growth processes are conducted
using low energy electron microscopy (LEEM).
7.
Synthesis of main-group element nanoporous systems and interpenetrating
lattices.
Recent
work has led to the discovery of new crystalline frameworks based upon Al(CN)3,
Ga(CN)3, In(CN)3, which have
microporous open structures. A
continuous series of solid solutions were also obtained in the Al1-xGax(CN)3
and Ga1-xInx(CN)3 systems so that the lattice parameter and hence the size of
the cavity is tunable over a wide range. Corresponding inclusion compounds, and
analogous interpenetrating lattices have also been synthesized. Examples include
InC3N3Kr,
LiGaC4N4, CuGaC4N4, Tl2(C4N4)
etc. We are extending this work to
synthesize structurally related materials that have the C=N
group in the structure replaced with a longer C=C-C=N
moiety. In these the metal atom is
bonded to six such groups with a linear M-C=C-C=N-M
configuration thus allowing for a substantially larger cavities to form inside
the simple cubic cell. These materials will possess interesting zeolitic and
adsorption properties.