Chalcogenide electrocatalysts for oxygen-depolarized aqueous
Chalcogenide semiconductor research and applications ... CSRA - Jaramillo... · Chalcogenide...
Transcript of Chalcogenide semiconductor research and applications ... CSRA - Jaramillo... · Chalcogenide...
Chalcogenide semiconductor research and applications
Tutorial 1: Thin film synthesis
Rafael JaramilloMassachusetts Institute of Technology
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Section 1: Thin film deposition techniques
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Categorizing by the 2-phase interface
Solid-vapor interface
• Good control of material fluxes• Flow controllers, shutters,
etc.• High deposition rates can
be low-cost and scalable • Limited kinetics at solid-vapor
interface• Metastable phases and
morphology• Superlattices, doping
gradients• Conformal coatings, ultra-
thin films
Solid-liquid interface
• Enhanced kinetics relative to solid-vapor interface growth• Enables crystallization even
for low-temperature growth• Tendency to approach
thermodynamic equilibrium• Easier to predict process
outcome• Challenging to control
doping, morphology
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Solid-vapor techniques
• Physical vapor deposition (PVD)• Transfer material from source(s) to film using “physical” methods under vacuum• Typically no byproducts• May include background pressure of co-reactant (e.g. O2)• Different types:
• Thermal / ebeam evaporation, closed-space sublimation (CSS)• Pulsed laser deposition (PLD)• Sputtering (magnetron, ion, electron)• Molecular beam epitaxy (MBE)
• Chemical vapor deposition (CVD)• Transfer material from source(s) to film in gas flow systems, often at ambient pressure• Typically all-inorganic precursors• Assume chemical reaction between precursors (e.g. S(g) + MoO3(s) = MoS2(s) + O2(g))
• Metal-organic chemical vapor deposition (MOCVD)• Variant of CVD, using metal-organic precursors (e.g. (CH3)3Ga + AsH3 = GaAs + 3CH4
• Chemical byproducts are removed in the vapor• Atomic layer deposition
• Variant of MOCVD, using kinetic control at relatively low tempereatures to achieve layer-by-layer growth
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Solid-liquid techniques
• Chemical bath deposition (CBD)• Super-saturation of precursors in solution leads to precipitation of desired phase at solid-
liquid interface• Use a combination of salts and organic molecular precursors• Relatively straightforward to setup, but control is limited and throughput is low
• Spin coating, sol-gel, antisolvent, doctor-blading, etc• Deposit film of (semi-)liquid precursors on the substrate, then use thermal and/or chemical
treatment to encourage desired reactions• Vapor-solid-liquid (VLS)
• Transport precursors in the vapor phase to a liquid precursor solution, which remains in contact with the growing solid surface
• Combines flux control of solid-vapor techniques with superior kinetics and crystallization of solid-liquid techniques
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Tradeoffs
High optoelectronic quality
Low optoelectronic quality
High (throughput) / (cost)
Low (throughput) / (cost)
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Section 2: Phase and morphology
• Phase diagrams determine equilibrium product for given conditions and starting materials– Equilibrium may be desirable,
or not
• Understanding and controlling thermodynamic variables can be a great challenge– Conditions at film interface
can differ from indicators
– Precursor mix at film interface can be difficult to control
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Phase control and (avoiding) equilibriumCZTS: Equilibrium phase required for good performance
CIGS: Non-equilibrium phases are used
Angus Rockett
Du et al., J. Appl. Phys. 115, 173502 (2014).
• Formation of non-equilibrium phases may be desirable but hard to understand
• Case of Cu2O:
– Under typical PLD conditions, equilibrium phase is CuO
– Cu2O found for wide range of conditions
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Phase control and (avoiding) equilibrium
Ellingham diagram showing Cu2O – CuO equilibrium
Subramaniyan … Zakutayev, APL Mater. 2, 022105 (2014)
• Origins of MBE: the “three-temperature technique– Two temperatures to
determine Ga and As source fluxes
– Third temperature to determine growth conditions
• Use experimental knobs to achieve thermodynamic equilibrium between desired solid phase and vapor– “Adsorption-limited”
growth window
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Phase control in MBE
Ga-As phase diagram at UHV pressure
• Low vapor pressure of many binary, ionic crystals means that adsorption-limited growth is inaccessible
• Consider quasi-binary SrO-TiO2 phase diagram– Growth window for Sr-rich
conditions defined by SrOsublimation
– Parameter regime is unrealistic
• Growth windows enabled by development of metal-organic sources– e.g. Titanium(IV) isopropoxide
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MBE growth windows for oxides/chalcogenides?
Adsorption-limited growth window for SrTiO3. Red bar indicates achievable parameter regime.
Engel-Herbert, in Molecular Beam Epitaxy: From Research to Mass Production. Elsevier (2012).
• Relatively high vapor pressure of some binary chalcogenides suggests that adsorption-limited growth of ternary phases may be possible
• Promising for some chalcogendide perovskites
– Compounds including main-group metals are most promising, e.g. SrGeS3
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MBE growth windows for oxides/chalcogenides?
Growth window for BaZrSe3 in the corner-sharing structure, determined from DFT-calculated formation energy
• Equilibrium crystal shapes are determined by surface energies
– Analyzed using Wulffplots and constructions
• Equilibrium shape resembles a thin film only for highly anisotropic materials
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Morphology control and (avoiding) equilibrium
Orientation-dependence of surface energy
Equilibrium crystal shape
• Films will approach equilibrium shape locally, e.g. when annealed
• Facets form to create lower-energy surfaces at the expense of higher-energy surfaces
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Faceting and grain boundaries
Li et al., APL 94, 263111 (2009)
• Grain boundaries are high-energy, non-equilibrium structures
• Grains will approach equilibrium locally– Triple-point angles, relative
growth
• Local thermodynamics and kinetics drives “primary” (bulk-like) grain growth
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Grain growth 120°-120°-120° intersections of high-angle (“random”) grain boundaries
Primary grain-growth until reaching stagnant columnar structure
Thompson, Ann. Rev. Mater. Sci. 20, 245 (1990)
• Primary grain growth can yield equiaxed grains in thin films, but not much larger
• For larger grains, need “secondary” grain growth, driven by energetic preference derived from thin film morphology
– Surface energy, interface energy, epitaxy
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Grain growth
Secondary grain growth driven by energetic preference for select surface orientation
Thompson, Ann. Rev. Mater. Sci. 20, 245 (1990)
• Trade-off is inevitable for all but high-end, epitaxial growth methods
• Choose approach based on desired performance
– e.g. minority carrier performance optimize grain size & crystallinity
– e.g. optical coating optimize morphology
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Crystal quality vs. film morphology
Todorov et al., Adv. Mater. 22, E156 (2010)
CZTS solar cell showing evidence of primary grain growth in absorber and conformal coating by ultra-thin CdS/ZnO buffer layer.
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Section 3: Controlling phase and optoelectronic properties
through anion activity
• Phase and defect profiles are strongly controlled by both anion and cation activity– Cation activity is often
close to unity
– Anion activity varies over many orders of magnitude
• e.g. challenge of fully-oxidizing complex copper oxides
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Controlling anion activity is key to happinesscontrolling ionic materials for electronics
Ellingham diagram for binary metal oxides
Ellingham diagram emphasizing CuO formation
• Perovskite nickelates have tunable metal-insulator transition and interesting magnetic properties
• With exception of LaNiO3, all LnNiO3 are thermodynamically unstable
• Use steric trends in thermodynamics of perovskite oxides to estimate stability regime at high p(O2)
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Anion activity and phase stability in the nickelatesElectronic-magnetic phase diagram of LnNiO3 perovskite nickelates [1]
Predicted stability regime for SmNiO3 [2]
1. Catalan, Phase Transitions 81, 729 (2008).2. Jaramillo et al., J. Mater. Chem. C 1, 2455 (2013)
• Use high-pressure oxygen annealing to stabilize SmNiO3 and determine the phase diagram
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Anion activity and phase stability in the nickelates
High pressure annealing of sputtered SmNi films Stability of SmNiO3 bulk-like and epitaxial films
Jaramillo et al., J. Mater. Chem. C 1, 2455 (2013)
• Coupled defect equilibrium in an ionic solid: case of ZnS
– Hypothesize that Zn interstitials will enhance n-type conductivity, as they do in ZnO. How to test this hypothesis?
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Anion activity and point defect profiles
1 SSx = 𝑉S
x + S 𝑔 , 𝑉S 𝑝 S = 𝑒ൗ−∆𝐺10
𝑅𝑇
2 𝑉Sx 𝑉Zn
x = 𝑒ൗ−∆𝐺20
𝑅𝑇
3 𝑉Znx Zni
x = 𝑒ൗ−∆𝐺30
𝑅𝑇
Znix = 𝑝−1 S 𝑒
൘−∆𝐺30+∆𝐺2
0−∆𝐺10
𝑅𝑇
Anion exchange with gas
Schottky defect equilibrium
Frenkel defect equilibrium
Net result – controlling properties with p(S)
• Anion vacancies and persistent photoconductivity in chalcogenides: case of CdS
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Anion activity and point defect profiles
Theory predicts that anion vacancies in chalcogenides result in persistent photoconductivity due to lattice relaxation following photoexcitation [1]
Experiment finds that photoconductivity in CdS can be controlled by varying S-2 activity in the solution during CBD [2]
1. Lany & Zunger, PRB 72, 035215 (2005). 2. Yin & Jaramillo, under review
• Deposition techniques can be sorted along different axes, making different trade-offs evident
• Phase control is usually difficult because of poor understanding and control of thermodynamic variables
• Morphology and crystal quality are often antagonistic
• For oxide and chalcogenide electronic materials, controlling anion activity during processing is key to controlling phase and performance
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Concluding remarks: Thin film synthesis