Chalcogenide semiconductor research and applications ... CSRA - Jaramillo... · Chalcogenide...

Chalcogenide semiconductor research and applications

Tutorial 1: Thin film synthesis

Rafael JaramilloMassachusetts Institute of Technology

August 20, 2017 Jaramillo - CSRA 2017 - Cancun, Mexico 2

Section 1: Thin film deposition techniques


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


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


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


Tradeoffs

High optoelectronic quality

Low optoelectronic quality

High (throughput) / (cost)

Low (throughput) / (cost)


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


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


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


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


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


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


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


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


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


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


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.


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


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)


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


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?


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


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


Concluding remarks: Thin film synthesis

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