tube furnacesubstrate holder

Synthesis from the Vapor Phase: Gas-Solid Reactions inCatalysis and Electronic Materials

Hock GroupDepartment of Biological and Chemical Sciences, Illinois Institute of Technology

The typical picture of chemical synthesis is one of bubbling beakers, colored solutions andwhirring stir bars. However, much of modern chemistry is conducted in the gas phase or bysolid-gas interactions. This poster presents an introduction to vapor phase synthesis, thetools of the trade, and how we use solution synthesis to enable some of the cutting-edgeapplications under development in the Hock Group.

Chemical vapor deposition (CVD) is a process for forming thin films from vapors of one ormore precursors. Atomic layer deposition (ALD) is a technique related to CVD in which aheated substrate is exposed alternately to two complementary precursors. The two ALDprecursors undergo self-limiting chemisorption reactions producing highly conformal filmsof varying thickness and composition. ALD can provide fine control of film thickness, on thescale of a single atom, pin-hole free surfaces and conformity of products, making it aninvaluable tool for applications such as electronic materials, photovoltaics, heterogeneouscatalysts, piezoelectric materials and superconductors.

Energy: Earth-abundant and inexpensive photovoltaics (PV)

Chemical Vapor Deposition

precursor dosed into reaction chamber

film growth growth continuesuntil gas phase depleted

Atomic Layer Deposition of AB

1) excess precursor pumped away

ALx precursorHyB

precursor A dosed into reaction chamber

film growth by self-limited surface reaction

reactant B also undergoesself-limited surface reaction

2) Second reactantintroduced

repeat cycles to grow film

M

O

O

H

O

HH

- 2 HL

OH

repeat cycles to grow film

Specific example: ALD of M2O3 (e.g. Al2O3)

valves are computer controlled

vacuum pump

oven w/ precursors

ALx precursorbubbler

N2

N2 + ALx

HyB

Schematic of an ALD System

High material purity and quality interface contacts are key attributes ofefficient solar cells. ALD provides exquisite control over the compositionof our materials. Thus, we invent, test, and use vapor precursors ofinexpensive and earth-abundant elements like tin, sulfur, iron, and otherelements in photovoltaic devices. The rational study of ALD-prepared PVdevices provides invaluable knowledge for the future adaptation of ALD-based processes to faster, industrially relevant fabrication procedures.

For example, SnS (herzenbergite) is very promising nontoxic and earth-abundant absorbing material for solar cells. Researchon SnS had reached a standstill due to poor properties and stoichiometry. We conceived a novel ALD precursor, tested itsreactivity with H2S surrogates, and optimized ALD conditions.

200 cycles 1000 cycles 3000 cycles 5000 cycles

# of cycles

Areal density on thermal oxide (at/cm2)

Sn SSn:S

ratio

200 2.37E+16 2.39E+16 0.99

1000 1.20E+17 1.20E+17 1.00

1500 1.70E+17 1.71E+17 0.99

3000 3.44E+17 3.42E+17 1.01

5000 5.99E+17 6.03E+17 0.99

y = 0.0752x - 2.9461R² = 0.9973

0

50

100

150

200

250

300

350

400

0 1000 2000 3000 4000 5000 6000

Fil

m t

hic

kn

ess

(n

m)

# of ALD cycles

Number of cycles vs Film Thickness

Growth Rate = 0.75 Å/cycle

Stoichiometric and high quality SnS wasprepared by atomic layer depositionemploying well-defined surface – gasreactivity. We use this approach todevelop novel materials for all componentsof solar cells.

Collaborators: Jeffrey T. Miller,2 Aditya Unni,1 Marc J. A. Johnson,2 Peter C. Stair,2, 3 Christopher L. Marshall,2 Jeffrey Elam,2 Alex B. Martinson,2 Thomas

Prolier,2 Jeff Terry,1 Carlo Segre,1 Larry A. Curtiss2

1Illinois Institute of Technology , 2Argonne National Laboratory, 3Northwestern University

Acknowledgements: We thank the Illinois Institute of Technology, Argonne NationalLaboratory, the Fieldhouse Research Fellowship Fund, the Department of Energy, and theDreyfus Foundation for generous support and our colleagues at IIT and ANL for stimulatingscientific discussions.

a. Volatile Metal Precursor attached to nucleation siteb. Second precursor covers initial support nucleation sitesc. Third precursor activates new surface sites for nucleationd. Cycle is repeated until metal is unprotectede. Produces metal nanoparticles embedded in surfacef. Removal of protecting ligands

1. Stair, et al. Angew. Chem. 49 (2010) 2547

Pd/Alumina 80 ⁰C

Catalysis: Single metal sites,* advanced ALD

techniques and higher-throughput screening

Electronic Materials: Superconductors, resistive random

access memory (RRAM), and other applications

“ABC” ALD for high catalyst nucleation density1*For our work with targeting surface bound single-atom sites, please see

the “X-Ray Methods in Surface Chemistry and Catalysis “ poster by Hu, et al.

Initial ALD nucleation sites are one to hundreds of atoms in size. We aredeveloping novel routes to small nanoscale clusters and studying their catalyticability for hydrogenation/dehydrogenation reactions, non-oxidative coupling ofalkanes, photocatalytic water splitting, artificial biodiesel synthesis, and otherreactions. In addition to heterogeneous catalyst synthesis, we are developingnovel catalyst support treatments and ALD deposition chemistries.

Integrated ALD-Catalysis Reactor for high-throughput ALD catalyst synthesis and testing

ALD is currently used to fabricate HfO2 in modern processors and is under development formany other components of electronic devices. The self-limiting nature of ALD provides thelevel of synthetic control necessary to fabricate billions of transistors per chip without asingle mistake (faulty device component). Currently only a few materials are easilysynthesized by ALD but the demand for more precursors and processes increases every year.At its heart, this materials science problem is one of chemical synthesis for precursordevelopment and chemical reactivity of the surface species during the deposition.

Main Group Precursors

Metal oxides are often only a dose of water away. However, manyfascinating applications of metal selenides and tellurides await thedevelopment of alternatives to the highly toxic and unstable H2Se andH2Te.

Precursor Development

Se-based superconductor

Although the number of elements amenable to ALD grows every year,many frontiers await, such as ALD at mild temperatures compatiblewith plastics and fabrics. These inexpensive, “soft” substrates areideal for medical diagnostic, plastic-based solar cells, LEDs, and otherapplications. We are also interested in integrating ALD with otherelectronic device fabrication methods.

Hock Group

Inert-atmosphere filtration ALD deposition apparatus(and Mike!)

controlled growthand

conformal coating

Pro: FAST growthCons: “pinch off”,

impurities

Left to right: Adam, Han Li, Bo Hu, Matt Weimer, Wen Li

Dr. Michael Lanci

Nick Shattuck