Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Interface engineering and chemistry of Hf-based high-k dielectrics on III–V substrates

We report on the effect of oxygen plasma treatment of two-dimensional multilayer MoS2 crystals on the subsequent growth of Al2O3 and HfO2 films, which were formed by atomic layer deposition (ALD) using trimethylaluminum and tetrakis-(ethylmethylamino)hafnium metal precursors, respectively, with water oxidant. Due to the formation of an ultrathin Mo-oxide layer on the MoS2 surface, the surface coverage of Al2O3 and HfO2 films was significantly improved compared to those on pristine MoS2, even at a high ALD temperature. These results indicate that the surface modification of MoS2 by oxygen plasma treatment can have a major impact on the subsequent deposition of high-k thin films, with important implications on their integration in thin film transistors.

Improved growth behavior of atomic-layer-deposited high-k dielectrics on multilayer MoS2 by oxygen plasma pretreatment.

Tertbutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) is shown to have strong substrate selectivity during atomic layer deposition of metallic cobalt. The interaction of tBu-AllylCo(CO)3 with SiO2 surfaces, where hydroxyl groups would normally provide more active reaction sites for nucleation during typical ALD processes, is thermodynamically disfavored, resulting in no chemical reaction on the surface at a deposition temperature of 140 °C. On the other hand, the precursor reacts strongly with H-terminated Si surfaces (H/Si), depositing ∼1 ML of cobalt after the first pulse by forming Si–Co metallic bonds. This remarkable substrate selectivity of tBu-AllylCo(CO)3 is due to an ALD nucleation reaction process paralleling a catalytic hydrogenation, which requires a coreactant that acts as a hydrogen donor rather than a source of bare protons. The chemical specificity demonstrated in this work suggests a new paradigm for developing selective ALD precursors. Namely, selectivity can be achieved by tailoring the li...

Substrate Selectivity of (tBu-Allyl)Co(CO)3 during Thermal Atomic Layer Deposition of Cobalt

The prospect of enhanced device performance from III–V materials has been recognized for at least 50 years, and yet, relative to the phenomenal size of the Si-based IC industry, these materials fulfilled only specific niches and were often referred to as “the material of the future” [1] . A key restriction enabling widespread use of III–V materials is the lack of a high quality, natural insulator for III–V substrates like that available for the SiO2/Si materials system [2] . The prospect of impending scaling challenges for technologies based on silicon metal oxide semiconductor field effect transistor (MOSFET) devices has brought renewed focus on the use of alternate surface channel materials from the III–V compound semiconductor family. The performance of the traditional MOSFET device structure is dominated by defects at the semiconductor/oxide interface, which in turn requires a high quality semiconductor surface. In this review, reflecting the authors’ current opinion, the recent progress in the understanding of the dielectric/III–V interface is summarized, particularly in regard to the interfacial chemistry that impacts the resultant electrical behavior observed. The first section summarizes the nature of the oxidation states of surface oxides on InxGa1−xAs. Then the atomic layer deposition of such oxides on the InxGa1−xAs surface is summarized in view of the interfacial chemical reactions employed. Finally the resultant electrical properties observed are examined, including the effects of substrate orientation. Portions of this review have been published previously [3] , [4] .

Interfacial chemistry of oxides on InxGa(1-x)As and implications for MOSFET applications

https://userpages.umbc.edu/~gougousi/papers/HfO2_ALD_TSF_2009.pdf

Properties of atomic layer deposited HfO2 thin films

HfO2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis-ethylmethylamino hafnium and H2O precursors at 250 °C. The as-deposited films are mainly amorphous and nearly stoichiometric HfO2 (O/Hf ratio ∼1.9) with low bonded carbon content (∼3 at. %). A comparison of the nucleation stage of the films on OH- and H-terminated Si(100) surfaces has been performed using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). We find for the initial 5–7 process cycles that the film nucleates more efficiently on the OH-terminated surface. However, after the 7th cycle both surfaces exhibit similar surface coverage, which takes about 40 cycles to reach a steady growth rate per cycle. Angle resolved XPS measurements reveal the formation of a ∼6 A interfacial layer after four ALD cycles on the H-terminated surface and the thickness of the interfacial layer does not change substantially between the 4th...

Nucleation of HfO2 atomic layer deposition films on chemical oxide and H-terminated Si

HfO2 films have been deposited by using a tetrakis(dimethylamino)hafnium/H2O atomic layer deposition (ALD) process on GaAs. X-ray photoelectron spectroscopy measurements show that the HF and NH4OH predeposition surface treatment results in efficient removal of the Ga and As native oxides. No interface oxidation is detected after 15cycles of HfO2 ALD implying effective passivation of the GaAs surface. Spectroscopic ellipsometry confirms linear growth at 1.0A∕cycle on both starting surfaces, while Rutherford backscattering spectrometry indicates steady-state coverage after about 10 ALD cycles. For films grown on native oxide GaAs, complete removal of the As oxide is observed after 20 ALD cycles.

Interface of atomic layer deposited HfO2 films on GaAs (100) surfaces

HfO2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis(dimethyl)amino hafnium and H2O precursors at a substrate temperature of 200–325°C. The initial stage of film growth on OH- and H-terminated Si(100) surfaces is investigated using Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The authors observe an initial growth barrier on the Si–H surface for the first approximately four process cycles, where film growth is more efficient on the OH-terminated surface. Both starting surfaces require about 15cycles to reach a steady growth rate per cycle, with the OH-terminated surface displaying a slightly higher growth rate of 2.7×1014Hf∕cm2 compared to 2.4×1014Hf∕cm2 for Si–H. Combining the RBS and SE data we conclude that the films deposited on the OH-terminated surface are denser than those deposited on the Si–H surface. Angle-resolved XPS measurements reveal the formation of a...

Growth and interface of HfO2 films on H-terminated Si from a TDMAH and H2O atomic layer deposition process

The initial film growth 2–100 cycles and the interface evolution of HfO2 thin films on GaAs surfaces were investigated for an atomic layer deposition chemistry that utilizes tetrakis ethylmethyl amino hafnium and H2O at 250°C. Starting surfaces include native oxide and HF or NH4OH-etched substrates. X-ray photoelectron spectroscopy shows that deposition on native oxide GaAs surfaces results in the gradual consumption of the arsenic and gallium oxides. Arsenic oxides are easier to remove, leaving some metallic arsenic–arsenic suboxide at the interface. The removal of the gallium oxides is slower, and some residual Ga2O3 and Ga2O are detected after 100 process cycles. High resolution transmission electron microscopy confirms the presence of an almost sharp interface for the 100 cycle 12 nm film and indicates that the as-deposited film is polycrystalline. The depositions on either HF or NH4OH-etched substrates result in a sharp interface with very little residual gallium oxide and arsenic suboxide present. Rutherford backscattering spectroscopy shows that steady-state growth comparable to that achieved on SiO2 is reached after 20 ALD cycles for all GaAs surfaces; however, high initial surface activity is detected for the etched surfaces. © 2010 The Electrochemical Society. DOI: 10.1149/1.3353166 All rights reserved.

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Justin C. Hackley

Papers

Properties of atomic layer deposited HfO2 thin films

Nucleation of HfO2 atomic layer deposition films on chemical oxide and H-terminated Si

Interface of atomic layer deposited HfO2 films on GaAs (100) surfaces

Growth and interface of HfO2 films on H-terminated Si from a TDMAH and H2O atomic layer deposition process

Growth and Interface Evolution of HfO2 Films on GaAs(100) Surfaces