21 research outputs found
Bulk and film synthesis pathways to ternary magnesium tungsten nitrides
Bulk solid state synthesis of nitride materials usually leads to
thermodynamically stable, cation-ordered crystal structures, whereas thin film
synthesis tends to favor disordered, metastable phases. This dichotomy is
inconvenient both for basic materials discovery, where non-equilibrium thin
film synthesis methods can be useful to overcome reaction kinetic barriers, and
for practical technology applications where stable ground state structures are
sometimes required. Here, we explore the uncharted Mg-W-N chemical phase space,
using rapid thermal annealing to reconcile the differences between thin film
and bulk powder syntheses. Combinatorial co-sputtering synthesis from Mg and W
targets in a N environment yielded cation-disordered Mg-W-N phases in the
rocksalt (0.1< Mg/(Mg+W) <0.9), and hexagonal boron nitride (0.7< Mg/(Mg+W)
<0.9) structure types. In contrast, bulk synthesis produced a cation-ordered
polymorph of MgWN that consists of alternating layers of rocksalt-like
[MgN] octahedra and nickeline-like [WN] trigonal prisms (denoted
"rocksaline"). Thermodynamic calculations corroborate these observations,
showing rocksaline MgWN is stable while other polymorphs are metastable. We
also show that rapid thermal annealing can convert disordered rocksalt films to
this cation-ordered polymorph near the MgWN stoichiometry. Electronic
structure calculations suggest that this rocksalt-to-rocksaline structural
transformation should also drive a metallic-to-semiconductor transformation. In
addition to revealing three new phases (rocksalt MgWN and MgWN,
hexagonal boron nitride MgWN, and rocksaline MgWN), these findings
highlight how rapid thermal annealing can control polymorphic transformations,
adding a new strategy for exploration of thermodynamic stability in uncharted
phase spaces
Ternary Nitride Semiconductors in the Rocksalt Crystal Structure
Inorganic nitrides with wurtzite crystal structures are well-known
semiconductors used in optoelectronic devices. In contrast, rocksalt-based
nitrides are known for their metallic and refractory properties. Breaking this
dichotomy, here we report on ternary nitride semiconductors with rocksalt
crystal structures, remarkable optoelectronic properties, and the general
chemical formula MgTMN (TM=Ti, Zr, Hf, Nb). These compounds form
over a broad metal composition range and our experiments show that Mg-rich
compositions are nondegenerate semiconductors with visible-range optical
absorption onsets (1.8-2.1 eV). Lattice parameters are compatible with growth
on a variety of substrates, and epitaxially grown MgZrN exhibits
remarkable electron mobilities approaching 100 cmVs. Ab
initio calculations reveal that these compounds have disorder-tunable optical
properties, large dielectric constants and low carrier effective masses that
are insensitive to disorder. Overall, these experimental and theoretical
results highlight MgTMN rocksalts as a new class of
semiconductor materials with promising properties for optoelectronic
applications
Bi-containing n-FeWO_4 Thin Films Provide the Largest Photovoltage and Highest Stability for a sub-2 eV Band Gap Photoanode
Photoelectrocatalysis of the oxygen evolution reaction remains a primary challenge for development of tandem-absorber solar fuel generators due to the lack of a photoanode with broad solar spectrum utilization, a large photovoltage, and stable operation. Bismuth vanadate with a 2.4–2.5 eV band gap has shown the most promise becauses its photoactivity down to 0.4 V vs RHE is sufficiently low to couple to a lower-gap photocathode for fuel synthesis. Through development of photoanodes based on the FeWO_4 structure, in particular, Fe-rich variants with addition of about 6% Bi, we demonstrate the same 0.4 V vs RHE turn-on voltage with a 2 eV band gap metal oxide, enabling a 2-fold increase in the device efficiency limit. Combinatorial exploration of materials composition and processing facilitated synthesis of n-type variants of this typical p-type semiconductor that exhibit much higher photoactivity than previous implementations of FeWO_4 in solar photochemistry. The photoanodes are particularly promising for solar fuel applications given their stable operation in acid and base
Misfit Layer Compounds and Ferecrystals: Model Systems for Thermoelectric Nanocomposites
A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the [(PbSe)1+δ]m(TiSe2)n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor
Autonomous sputter synthesis of thin film nitrides with composition controlled by Bayesian optimization of optical plasma emission
Autonomous experimentation has emerged as an efficient approach to accelerate
the pace of materials discovery. Although instruments for autonomous synthesis
have become popular in molecular and polymer science, solution processing of
hybrid materials and nanoparticles, examples of autonomous tools for physical
vapour deposition are scarce yet important for the semiconductor industry.
Here, we report the design and implementation of an autonomous instrument for
sputter deposition of thin films with controlled composition, leveraging a
highly automated sputtering reactor custom-controlled by Python, optical
emission spectroscopy (OES), and Bayesian optimization algorithm. We modeled
film composition, measured by x-ray fluorescence, as a linear function of
emission lines monitored during the co-sputtering from elemental Zn and Ti
targets in N atmosphere. A Bayesian control algorithm, informed by OES,
navigates the space of sputtering power to fabricate films with user-defined
composition, by minimizing the absolute error between desired and measured
emission signals. We validated our approach by autonomously fabricating
ZnTiN films with deviations from the targeted cation
composition within relative 3.5 %, even for 15 nm thin films, demonstrating
that the proposed approach can reliably synthesize thin films with specific
composition and minimal human interference. Moreover, the proposed method can
be extended to more difficult synthesis experiments where plasma intensity
depends non-linearly on pressure, or the elemental sticking coefficients
strongly depend on the substrate temperature
Synthesis and Calculations of Wurtzite Al1−xGdxN Heterostructural Alloys
Al1−xGdxN is one of a series of novel heterostructural alloys involving rare earth cations with potentially interesting properties for (opto)electronic, magnetic and neutron detector applications. Using alloy models in conjunction with density functional theory, we explored the full composition range for Al1−xGdxN and found that wurtzite is the ground state structure up to a critical composition of x = 0.82. The calculated temperature-composition phase diagram reveals a large miscibility gap inducing spinodal decomposition at equilibrium conditions, with higher Gd substitution (meta)stabilized at higher temperatures. By depositing combinatorial thin films at
high effective temperatures using radio frequency co-sputtering, we have achieved the highest Gd3+ incorporation into the wurtzite phase reported to date, with single-phase compositions at least up to x ≈ 0.25 confirmed by high resolution synchrotron grazing incidence wide angle X-ray scattering. High resolution transmission electron microscopy on material with x ≈ 0.13 confirmed a uniform composition polycrystalline film with uniform columnar grains having the wurtzite structure. Expanding our calculations to other rare earth cations (Pr and Tb) reveals similar thermodynamic stability and solubility behavior to Gd. From this and previous studies on Al1−xScxN, we elucidate that both smaller ionic radius and higher bond ionicity promote increased incorporation of group IIIB cations into wurtzite AlN. This work furthers the development of design rules for new alloys in this materials family
Combinatorial Nitrogen Gradients in Sputtered Thin Films
High-throughput
synthesis and characterization methods can significantly
accelerate the rate of experimental research. For physical vapor deposition
(PVD), these methods include combinatorial sputtering with intentional
gradients of metal/metalloid composition, temperature, and thickness
across the substrate. However, many other synthesis parameters still
remain out of reach for combinatorial methods. Here, we extend combinatorial
sputtering parameters to include gradients of gaseous elements in
thin films. Specifically, a nitrogen gradient was generated in a thin
film sample library by placing two MnTe sputtering sources with different
gas flows (Ar and Ar/N<sub>2</sub>) opposite of one another during
the synthesis. The nitrogen content gradient was measured along the
sample surface, correlating with the distance from the nitrogen source.
The phase, composition, and optoelectronic properties of the resulting
thin films change as a function of the nitrogen content. This work
shows that gradients of gaseous elements can be generated in thin
films synthesized by sputtering, expanding the boundaries of combinatorial
science