About

Steven M. George is a Professor of Chemistry at the University of Colorado Boulder with a Ph.D. from the University of California, Berkeley, and a B.S. from Yale University. His research focuses on surface chemistry, nanotechnology/materials, physical chemistry, and renewable energy. He specializes in the fabrication, design, and properties of ultrathin films and nanostructures, developing new surface chemistries for thin film growth, measuring thin film growth using in situ techniques, and characterizing thin film properties. His work is relevant to technological areas such as semiconductor processing, flexible displays, MEMS/NEMS, Li-ion batteries, and fuel cells. Professor George's research bridges multiple disciplines and involves collaborations across various departments and institutions, including industry and national laboratories. He extensively utilizes atomic layer deposition (ALD) techniques for thin film growth, which are based on sequential, self-limiting surface reactions, allowing atomic layer control and conformal coatings on high aspect ratio structures. His contributions include advancing ALD methods for binary materials like Al2O3, MgO, and TaN, as well as for single-element metal films such as tungsten, and organic polymer films through molecular layer deposition (MLD). His research also explores hybrid organic-inorganic polymers, with applications in gas diffusion barriers, battery electrode stabilization, catalysis, and nano-photovoltaic devices. Professor George has developed various reactor systems for film growth and employs multiple in situ and ex situ techniques for film analysis. His professional activities include leadership roles in the American Vacuum Society and editorial positions, reflecting his active engagement in the scientific community.

Research topics

• Materials science
• Nanotechnology
• Chemistry
• Metallurgy
• Thermodynamics
• Polymer chemistry
• Optoelectronics
• Physics
• Engineering physics
• Chemical engineering

Selected publications

• Thermal Atomic Layer Etching of Indium Gallium Zinc Oxide (IGZO), In ₂ O ₃ , Ga ₂ O ₃ , and ZnO Using Sequential Hydrogen Fluoride and Acetylacetone Exposures

  The Journal of Physical Chemistry C · 2025-10-30 · 3 citations

  articleSenior authorCorresponding

  Thermal atomic layer etching (ALE) of indium gallium zinc oxide (IGZO), In2O3, Ga2O3, and ZnO was achieved using sequential hydrogen fluoride (HF) and acetylacetone (Hacac) exposures. The HF exposure fluorinates the metal oxide surface to a metal fluoride layer. The Hacac exposure then undergoes ligand substitution and hydrogen transfer to volatilize the metal fluoride layer. The etching of IGZO films was studied using in situ spectroscopic ellipsometry (SE). Repeated exposures of Hacac or sequential exposures of Hacac and O3 on IGZO films at temperatures up to 250 °C observed no etching. However, fluorination prior to Hacac exposures achieved IGZO etching. Sequential exposures of HF and Hacac produced etch rates of 0.3 Å/cycle at 200 °C. The etch rates increased with temperature up to an etch rate of 0.6 Å/cycle at 250 °C. Similar experiments were performed using hexafluoroacetylacetone (Hhfac) instead of Hacac. Sequential exposures of HF and Hhfac revealed slightly lower etch rates of 0.2 Å/cycle at 230 °C. The etch rates using HF and Hhfac exposures increased with temperature up to an etch rate of 0.5 Å/cycle at 270 °C. In situ quadrupole mass spectrometry (QMS) studies were performed to study the volatile products during etching of In2O3, Ga2O3, and ZnO powders by sequential HF and Hacac exposures at 200 °C. These QMS investigations observed H2O during HF fluorination of In2O3 and Ga2O3. M(acac)x species where M = In or Ga and HF were also monitored as etch products during Hacac exposures on fluorinated In2O3 and Ga2O3. These etch products were consistent with a ligand substitution and hydrogen transfer mechanism. The time dependence of the In(acac)3+ and Ga(acac)2+ ion signals was monitored during Hacac exposures after fluorination of In2O3 and Ga2O3. The decay of the M(acac)x+ ion signal intensity versus Hacac exposure was consistent with a self-limiting surface reaction. In contrast, ZnO was spontaneously etched by Hacac. There was a constant Zn(acac)2+ ion signal during Hacac exposure on ZnO with no previous fluorination of ZnO. However, this spontaneous etching of ZnO by Hacac did not affect the self-limiting behavior of IGZO thermal ALE films using sequential HF and Hacac exposures.

  PublisherDOI

• Film and surface stress during Al2O3 and AlF3 atomic layer deposition using in situ wafer curvature measurements

  Applied Surface Science · 2025-12-20 · 1 citations

  articleSenior authorCorresponding
  PublisherDOI

• Electron-Enhanced Deposition of Titanium-, Silicon- and Tungsten-Containing Films at Low Temperatures Using Volatile Precursors with Various Reactive Background Gases

  Chemistry of Materials · 2025-06-16 · 2 citations

  articleSenior authorCorresponding

  Electron-enhanced atomic layer deposition (EE-ALD) and electron-enhanced chemical vapor deposition (EE-CVD) can be employed for the low temperature deposition of thin films using volatile precursors with various reactive background gases (RBGs). EE-CVD expands on the previous demonstration of TiN EE-ALD using alternating Ti(N(CH3)2)4 (tetrakisdimethylamino titanium (TDMAT)) and electron beam exposures with NH3 RBG. During EE-CVD, the electron beam and the RBG are present continuously. Together with the RBG and electron beam incident on the surface, the volatile precursor is pulsed into the vacuum chamber to control the film growth. In this survey, the metal or metalloid precursors were TDMAT, Si2H6, and W(CO)6. The RBGs were O2, NH3, CH4, and H2. The study focused on TiO2 EE-ALD and SiN, SiO2, SiCx, SiHx, W2N, WOx, and WCx EE-CVD. Thin film growth was monitored using in situ 4-wavelength ellipsometry. To first illustrate EE-ALD, TiO2 EE-ALD was performed at T < 80 °C using alternating TDMAT and electron beam exposures together with O2 RBG. The growth rate for the TiO2 EE-ALD was ∼0.7 Å/cycle. The TiO2 EE-ALD films were nearly stoichiometric, displayed crystallinity, and were smooth as measured by atomic force microscopy (AFM). Other Ti-containing EE-ALD films were deposited using CH4 and H2 RBGs. Subsequently, to demonstrate EE-CVD, SiCx EE-CVD was performed at T < 100 °C using repeating Si2H6 pulses with continuous electron beam and CH4 RBG exposures. XPS revealed a 1:1 Si/C stoichiometry for a CH4 RBG pressure of 0.45 mTorr and C-rich films for higher CH4 RBG pressures. The SiC EE-CVD growth rate was ∼0.4 Å per Si2H6 pulse. The stoichiometric SiC EE-CVD films were smooth as measured by AFM. Other Si-containing EE-CVD films that were deposited included SiO2, SiN and SiHx. In addition, W2N was deposited with EE-CVD at T < 120 °C using repeating W(CO)6 pulses with continuous electron beam and NH3 RBG exposures. The W2N EE-CVD growth rate was ∼0.17 Å per W(CO)6 pulse. The W2N films had a resistivity of ∼450 μΩ cm. The W2N EE-CVD films also displayed crystallinity and high purity. Other W-containing EE-CVD films that were deposited included WOx and WCx. This survey shows that the EE-ALD technique can be extended to EE-CVD with various RBGs to deposit a broad range of materials at low temperatures including oxides, nitrides and carbides.

  PublisherDOI

• Strongly and Weakly Adsorbed H₂O Layer Thicknesses on Hydroxylated SiO₂ Surfaces versus H₂O Pressure at Various Substrate Temperatures

  The Journal of Physical Chemistry C · 2025-01-08 · 5 citations

  articleSenior authorCorresponding

  The H2O layer thickness on flat hydroxylated SiO2 surfaces was measured at various H2O pressures and substrate temperatures using in situ real-time spectroscopic ellipsometry (SE). The in situ SE measurements were conducted at 18.1, 27.2, and 30.4 °C (291.25, 300.35, and 303.55 K) in a warm-wall vacuum chamber designed with a cooled sample stage. The H2O pressures were varied up to the saturation H2O vapor pressures of 15.6, 27.0, and 32.5 Torr at 18.1, 27.2, and 30.4 °C, respectively. The SE measurements showed that there were two distinct types of H2O layers on the hydroxylated SiO2 surface: a thin strongly adsorbed layer and a weakly adsorbed layer that was much thicker at high H2O pressures. The strongly adsorbed layer had thicknesses ≤1.2 Å and was not lost by removing the H2O pressure. The strongly adsorbed layer could be desorbed by heating the sample stage to 124 °C (397.15 K). The stability of the strongly adsorbed layer was consistent with an adsorption energy of >20 kcal/mol. In contrast, the weakly adsorbed layer could be added or removed by increasing or decreasing the H2O pressure. The weakly adsorbed layer obtained much higher multilayer H2O thicknesses at larger H2O pressures. For example, the weakly adsorbed multilayer thickness was 7.5 Å at 92% relative humidity at 30.4 °C (32.5 Torr). Complementary in situ Fourier transform infrared (FTIR) studies were also performed on hydroxylated SiO2 nanoparticles that were in qualitative agreement with the SE results.

  PublisherDOI

• Mechanism of Thermal Atomic Layer Etching of Hafnium Zirconium Oxide, HfO₂ and ZrO₂ Using Sequential HF and Acetylacetone Exposures

  Chemistry of Materials · 2025-07-29 · 1 citations

  articleSenior authorCorresponding

  The mechanism of hafnium zirconium oxide (HZO), HfO2 and ZrO2 thermal atomic layer etching (ALE) was explored using sequential hydrogen fluoride (HF) and acetylacetone (Hacac) exposures. HF modifies the metal oxide surface by forming a metal fluoride layer. Hacac then removes the metal fluoride layer by forming volatile metal acac products. Initial in situ spectroscopic ellipsometry investigations revealed that amorphous and crystalline HZO, HfO2 and ZrO2 films were etched by sequential HF and Hacac exposures. Amorphous HZO was etched at a rate of 0.80 Å/cycle at 230 °C. Crystalline HZO was etched at rates of 0.50 and 0.74 Å/cycle at 250 and 270 °C, respectively. Amorphous HfO2 and ZrO2 were etched at rates of 0.20 and 0.36 Å/cycle at 250 °C, respectively. The crystalline HfO2 and ZrO2 films were etched at 250 °C at rates of 0.02 and 0.18 Å/cycle, respectively. To understand the etching mechanism, the volatile etch products during HfO2 and ZrO2 thermal ALE at 250 °C were identified with high sensitivity quadrupole mass spectrometry (QMS) techniques using HfO2 and ZrO2 powder samples. The QMS studies determined that HF exposures yielded H2O during fluorination of the metal oxide at 250 °C. After fluorination of HfO2 and ZrO2, QMS analysis identified Hf(acac)2F2 and Zr(acac)2F2 etch products, respectively. These mixed ligand fluoro(acetylacetonate) compounds of Hf and Zr have not been reported earlier in the literature. The volatile fluoro(acetylacetonate) compounds appeared immediately with the onset of Hacac exposures. The main Hf(acac)F2+ and Zr(acac)F2+ ion intensities from Hf(acac)2F2 and Zr(acac)2F2, respectively, also decreased with time during the Hacac exposures. This decrease was consistent with a self-limiting surface reaction. In addition, Hacac exposures on the fluorinated metal oxide surfaces were observed to produce volatile HF at the onset of Hacac exposures at 250 °C. Some of the HF reaction product from the Hacac reaction can potentially return to refluorinate the metal oxide surface and minimize the amount of HF required for additional etching. Experiments with multiple Hacac exposures after the HF fluorination reaction revealed that the Hf(acac)F2+ and Zr(acac)F2+ ion intensities decreased progressively with each Hacac exposure after the initial HF exposure. These results confirm that the Hacac reaction is self-limiting even though some HF etch product can remain and refluorinate the HfO2 and ZrO2 surfaces.

  PublisherDOI

• 497 IV Fluid Optimization (IV FLO): A Comparison of Three Pressurization Methods

  Annals of Emergency Medicine · 2025-08-22

  articleOpen accessSenior author
  PublisherOA PDFDOI

• Thermal Atomic Layer Etching of Zinc Oxide from 30–300 °C Using Sequential Exposures of Hydrogen Fluoride and Trimethylgallium

  Chemistry of Materials · 2025-03-31 · 2 citations

  articleSenior authorCorresponding

  Thermal atomic layer etching (ALE) of zinc oxide (ZnO) was demonstrated over a large temperature range from 30–300 °C using sequential exposures of HF (hydrogen fluoride) and Ga(CH3)3 (trimethylgallium (TMG)). In contrast to earlier studies of thermal ZnO ALE using sequential exposures of HF and trimethylaluminum (TMA), ZnO ALE with sequential HF and TMG exposures occurred without competing GaF3 atomic layer deposition (ALD) or ZnO conversion. Quartz crystal microbalance (QCM) studies during ZnO ALE revealed a stepwise mass increase during fluorination by HF exposures and a larger mass decrease during ligand-exchange by TMG exposures. The mass changes per cycle (MCPC) were self-limiting versus HF and TMG exposures at 100 °C. Spectroscopic ellipsometry measured etch rates over a wide temperature range. The etch rates varied from 0.24 Å/cycle at 30 °C to 3.82 Å/cycle at 300 °C. The temperature-dependent etch rates were consistent with an activation barrier of Ea = 3.3 kcal/mol. TMG exposures were also compared with TMA exposures at 100 °C on fresh ZnO surfaces grown by ZnO ALD. TMG exposures led to a mass gain consistent with TMG adsorption. In contrast, TMA exposures produced a mass loss consistent with the conversion of ZnO to Al2O3. Previous studies showed that conversion of ZnO to Al2O3 prevented ZnO ALE using HF and TMA exposures at temperatures less than 205 °C. Etching at <205 °C was restricted because HF adsorption on fluorinated Al2O3 led to competing AlF3 ALD. In contrast, ZnO ALE at temperatures as low as 30 °C is possible because no competing GaF3 ALD occurs using HF and TMG exposures. Quadrupole mass spectrometry (QMS) experiments were also performed to identify the etch products during ZnO ALE. The QMS experiments support fluorination and ligand-exchange reactions without conversion during ZnO ALE using HF and TMG exposures. The HF and TMG exposures were selective for ZnO ALE compared with HfO2, ZrO2 or Al2O3 ALE. ZnO ALE could also smooth ZnO surfaces progressively versus number of ZnO ALE cycles.

  PublisherDOI

• Low-temperature etching of silicon oxide and silicon nitride with hydrogen fluoride

  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films · 2024-11-18 · 32 citations

  articleOpen access

  Etching of high aspect ratio features into alternating SiO2 and SiN layers is an enabling technology for the manufacturing of 3D NAND flash memories. In this paper, we study a low-temperature or cryo plasma etch process, which utilizes HF gas together with other gas additives. Compared with a low-temperature process that uses separate fluorine and hydrogen gases, the etching rate of the SiO2/SiN stack doubles. Both materials etch faster with this so-called second generation cryo etch process. Pure HF plasma enhances the SiN etching rate, while SiO2 requires an additional fluorine source such as PF3 to etch meaningfully. The insertion of H2O plasma steps into the second generation cryo etch process boosts the SiN etching rate by a factor of 2.4, while SiO2 etches only 1.3 times faster. We observe a rate enhancing effect of H2O coadsorption in thermal etching experiments of SiN with HF. Ammonium fluorosilicate (AFS) plays a salient role in etching of SiN with HF with and without plasma. AFS appears weakened in the presence of H2O. Density functional theory calculations confirm the reduction of the bonding energy when NH4F in AFS is replaced by H2O.

  PublisherDOI

• Building Semipermeable Films One Monomer at a Time: Structural Advantages via Molecular Layer Deposition vs Interfacial Polymerization

  Chemistry of Materials · 2024-01-18 · 10 citations

  articleOpen accessSenior author

  Molecular layer deposition (MLD) provides the opportunity to perform condensation polymerization one vaporized monomer at a time for the creation of precise, selective nanofilms for desalination membranes. Here, we compare the structure, chemistry, and morphology of two types of commercial interfacial polymerzation (IP) membranes with lab-made MLD films. M-phenylenediamine (MPD) and trimesoyl chloride (TMC) produced a cross-linked, aromatic polyamide often used in reverse osmosis membranes at MLD growth rates of 2.9 Å/cycle at 115 °C. Likewise, piperazine (PIP) and TMC formed polypiperazine amide, a common selective layer in nanofiltration membranes, with MLD growth rates of 1.5 Å/cycle at 115 °C. Ellipsometry and X-ray reflectivity results suggest that the surface of the MLD films is comprised of polymer segments roughly two monomers in length, which are connected at one end to the cross-linked bulk layer. As a result of this structure as well as the triple-functionality of TMC, MPD-TMC had a temperature window of stable growth rate from 115 to 150 °C, which is unlike any non-cross-linked MLD chemistries reported in the literature. Compared to IP films, corresponding MLD films were denser and morphologically conformal, which suggests a reduction in void volumes; this explains the high degree of salt rejection and reduced flux previously observed for exceptionally thin MPD-TMC MLD membranes. Using X-ray photoelectron spectroscopy and infrared spectroscopy, MLD PIP-TMC films evidenced a completely cross-linked internal structure, which lacked amine and carboxyl groups, pointing to a hydrophobic bulk structure, ideal for optimized water flux. Grazing-incidence wide-angle X-ray scattering showed broad features in each polyamide with d-spacings of 5.0 Å in PIP-TMC compared to that of 3.8 Å in MPD-TMC. While MLD and IP films were structurally identical to PIP-TMC, MPD-TMC IP films had a structure that may have been altered by post-treatment compared to MLD films. These results provide foundational insights into the MLD process, structure–performance relationships, and membrane fabrication.

  PublisherDOI

• Selectivity between SiO₂ and SiN_<i>x</i> during Thermal Atomic Layer Etching Using Al(CH₃)₃/HF and Spontaneous Etching Using HF and Effect of HF + NH₃ Codosing

  Chemistry of Materials · 2024-07-01 · 13 citations

  articleSenior author

  Selectivity was examined between SiO2 and SiNx during thermal atomic layer etching (ALE) and spontaneous etching. Thermal ALE of SiO2 and SiNx was explored using sequential trimethylaluminum (TMA) and hydrogen fluoride (HF) with reactant exposures of 3 Torr for 45 s at 275 °C. SiO2 thermal ALE achieved an etch per cycle (EPC) of 0.20 Å/cycle and near-ideal synergy up to 95%. SiNx thermal ALE exhibited a higher EPC of 1.06 Å/cycle. The selectivity factor was ∼5:1 for SiNx etching compared to SiO2 etching (preferential SiNx removal) during thermal ALE using TMA and HF. Spontaneous etching was then quantified using repeated exposures of HF vapor alone at 3 Torr and 275 °C. SiO2 spontaneous etching was minor at an etch rate of 0.03 Å/min, enabling near-ideal synergy for SiO2 thermal ALE. In contrast, major SiNx spontaneous etching displayed an etch rate of 1.72 Å/min and predominated over SiNx thermal ALE. The selectivity factor was ∼50:1 for SiNx spontaneous etching compared to SiO2 spontaneous etching using an HF pressure of 3 Torr. This selective SiNx spontaneous etching was attributed to F– surface species during HF exposures. NH3 codosing with HF was then examined during thermal ALE and spontaneous etching. Thermal ALE of SiO2 and SiNx was examined using sequential TMA and HF + NH3 codosing with reactant exposures of 3 Torr for 45 s at 275 °C. SiO2 thermal ALE with HF + NH3 codosing had a high EPC of 8.83 Å/cycle. In contrast, SiNx thermal ALE with HF + NH3 codosing was negligible. The selectivity factor was reversed and much higher at >1000:1 for SiO2 etching compared to SiNx etching (preferential SiO2 removal) during thermal ALE with HF + NH3 codosing. Rapid SiO2 spontaneous etching with HF + NH3 codosing at 3 Torr had an etch rate of 27.50 Å/min. In contrast, SiNx spontaneous etching with HF + NH3 codosing produced a very low etch rate of 0.02 Å/min. The selectivity factor was >1000:1 for SiO2 spontaneous etching compared to SiNx spontaneous etching with HF + NH3 codosing. This selective SiO2 spontaneous etching was attributed to HF2– surface species during HF + NH3 exposures. These studies revealed that the NH3 coadsorbate during HF exposures modified the active etch species and dramatically influenced the etch selectivity between SiO2 and SiNx. Reciprocal etch selectivity should be important for the selective removal of SiO2 or SiNx in composite structures.

  PublisherDOI

Recent grants

• Fundamental Issues for Thermal Atomic Layer Etching

  NSF · $475k · 2016–2019

• In Situ Probing of Atomic Layer Deposition: Surface Chemistry, Film Growth and Electrical Properties

  NSF · $471k · 2004–2007

• Molecular Layer Deposition of Polymers: Nucleation, Surface Chemistry and Nanocomposite Films

  NSF · $480k · 2007–2010

• Hybrid Organic-Inorganic Films Grown by Molecular Layer Deposition as Precursors to Conformal Metal Oxide-Carbon Composite and Porous Metal Oxide Films

  NSF · $375k · 2014–2016

• Molecular Layer Deposition of Hybrid Organic-Inorganic Polymer Films with Tunable Mechanical Properties

  NSF · $480k · 2010–2013

Frequent coauthors

• Andrew S. Cavanagh

  University of Colorado Boulder

  91 shared

• Anne C. Dillon

  Queen's University Belfast

  58 shared

• Victor M. Bright

  University of Colorado Boulder

  46 shared

• Ronggui Yang

  Peking University

  45 shared

• А. И. Абдулагатов

  Dagestan State University

  44 shared

• Alan W. Weimer

  University of Colorado Boulder

  43 shared

• Young‐Hee Lee

  37 shared

• Markus D. Groner

  Forge Nano (United States)

  37 shared

Education

• Ph.D., Chemistry

  University of California, Berkeley

  1983

• B.S., Chemistry

  Yale University

  1977

Awards & honors

• DPS Nishizawa Award from the International Symposium on Dry…
• Faculty Research Award from College of Engineering and Appli…
• University of Colorado at Boulder Faculty Assembly Excellenc…
• American Chemical Society Colorado Section Award, 2004
• R&D 100 Award for Particle-ALD, 2004