ALD2019 Session EM1-WeA: Ternary and Quaternary Oxide Materials

Oxides of rhenium, such as ReO₂ and ReO₃, exhibit high conductivity in the order σ = 10³ – 10⁴ (Ωcm)^‑1, close to that of metals. This property is of high importance regarding applications with high demands on tailored electrical properties. An example of such an application are microchannel plates (MCPs), which are used as signal amplifiers in advanced detector units for UV-light (space aviation) and photoelectrons (XPS). Also, MCPs play a crucial role in the development of large area photodetectors.^[1] MCPs are comprised of a glass capillary array. By applying a material with high secondary electron emission (SEE) yield, an incident electron/photon can create an electron avalanche that eventually increases the signal-to-noise ratio and thereby allows the detection of event starters. Below the SEE layer, a resistive coating must be applied, exhibiting the right range of resistivity to avoid electrons from being drained and to act as electron supplier. This material must be operable at elevated temperatures, i.e. it must have a low temperature-coefficient of resistance (TCR) to avoid decreased device performance at higher temperatures. Metal nanoparticles embedded in an Al₂O₃-matrix have been shown to provide the right resistivity for MCP application.^[2] However, their TCR values demand for the development of materials with improved properties.

In this context, the development of ternary oxides, comprising ReO_x units with electrical conductivity and other components with dielectric character, provide a good opportunity to overcome the limitations of existing coatings.

Herein, we present two new materials, namely ReAl_yO_x and ReSi_yO_x. These materials, where ReSi_yO_x is reported for the first time, have been deposited using ALD and detailed mechanistic studies revealed that during the growth, reductive elimination reactions cause the formation of uncommon but stable Re(III) oxide species. Furthermore, fascinating mechanisms allowing the formation of unsaturated hydrocarbons and hydrogen, were identified during the growth. The processes were explored in great detail employing in-situ tools such as QCM, QMS and FTIR. Furthermore, the materials were fully characterized in terms of composition, structure, and electrical properties using XPS, RBS/HFS, XRD, XRR and XAS.

[1 ] J. Wang, K. Byrum, M. Demarteau, J. Elam, A. Mane, E. May, R. Wagner, D. Walters, L. Xia, J. Xie, and H. Zhao, Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 2015, 804, 84

Process, structural, chemical, and electrical characterization is presented for PbZr_xTi_1-xO₃ (PZT) deposited by ALD using different combinations of precursors with thicknesses suitable for piezoelectric microelectromechanical systems (piezo-MEMS). PZT grown by ALD is desired for integration into 3-D MEMS [1] whereby actuators are grown on high aspect-ratio sidewalls, which greatly increases the areal work-density. To determine the viability of ALD PZT for MEMS, the films were integrated into micromachined, released cantilever structures. ALD PbTiO₃ (PTO) has been previously grown using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)₂] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively [2]. Incorporation of tetrakis dimethylamino zirconium [TDMAZ] yielded PZT, but the growth rate of PbZrO₃ (PZO) and PZO-rich PZT was suppressed compared to PTO-rich PZT, resulting in a Zr/Ti composition less than the desired morphotropic phase boundary (MPB) composition of 52/48. The PTO-rich ALD PZT (Zr/Ti = 22/78) films were annealed at 700 °C in O₂ to crystallize into the perovskite phase. Micromachined trench structures 45 µm deep were coated with conformal ALD PbZr₂₂Ti₇₈O₃ to demonstrate that the process conditions fell within the ALD-window. Transmission electron microscopy images revealed that the ALD PZT films crystallized with grains tens of nanometers in diameter. Rutherford backscattering was used to characterize the chemical composition of the films. In-situ ellispometry indicated that the as-grown PZT thickness was a linear function of the number of supercycles. For 200 nm-thick ALD PbZr₂₂Ti₇₈O₃ films, planar metal-insulator-metal capacitors were created to evaluate the ferroelectric and dielectric properties. The films exhibited an average remnant polarization of 20 µm/cm², a dielectric constant at zero volts of 475, a tuning range of 212-548 from 0 to 450 kV/cm, and a corresponding Tan delta of 0.025 (see supplemental). The ferroelectric properties of the ALD PZT presented here rivals PZT grown at similar compositions and thicknesses by other well-accepted processing methods. Several other zirconium precursors were investigated for inclusion into ALD PZT including zirconium tert-butoxide [Zr(OtBu)₄] and tetrakis ethylmethylamino zirconium [TEMAZ]. The incorporation of Zr(OtBu)₄ led to improved tunability of the zirconium concentration but also resulted in a lower overall growth rate of PZT.

References

[1] Three dimensional piezoelectric MEMS, US8966993 B2, J.S. Pulskamp and R. G. Polcawich (19 Dec 2012)

[2] N. A. Strnad et al. J. Vac. Sci. Technol. A 37(2) Mar/Apr 2019

ALD of ternary films has grown in interest as applications requiring the advantages of ALD (including sub-nanometer thickness control, uniformity, compositional control and conformality) have demanded more complex materials. However, depositing ternary materials by ALD is typically more complicated than the binary ALD systems from which the ternary materials are derived. As a result, the growth of a ternary material by ALD often deviates from ideal growth behavior described by the “rule of mixtures.” In this work, ALD of NiAl_yO_x was studied using the nickelocene (NiCp₂)/O₃ and trimethyl aluminum (TMA)/H₂O precursor systems for nickel oxide and aluminum oxide respectively. Depositions of NiAl_yO_x were performed using the supercycle method, allowing a wide range of compositions to be achieved by altering the cycle ratio of nickel oxide to aluminum oxide ALD cycles. However, composition as a function of cycle ratio was found to be nickel deficient compared to the ideal rule of mixtures. The cause of the observed non-ideality was explored to help elucidate the ALD reaction mechanisms. Characterization of the films revealed that the growth rate of aluminum oxide was significantly enhanced following a NiCp₂/O₃ cycle. Modification of the rule of mixtures to account for the enhanced growth rate of Al₂O₃ resulted in a model which fits observed properties well, accounting for both the observed compositions and growth rates. In addition, NiAl_yO_x has shown to be an interesting case study for understanding the deposition of ternary materials of first-row transition metal oxides grown with O₃. Similar to ALD of Fe₂O₃ and MnO_x, which utilize metallocene-derivatives and O₃, NiO ALD requires long precursor and O₃ exposures to achieve fully self-limiting growth. The root cause for this behavior is hypothesized to stem from the use of the strong oxidizer O₃, which may oxidize the metal centers to higher oxidation states or otherwise introduce extra oxygen into the film. For the deposition of NiAl_yO_x, low O₃ exposures were used to develop the initial NiAl_yO_x system. To understand the effects of O₃ exposure on the deposition of the ternary ALD film and binary NiO system, growth of NiAl_yO_x as a function of O₃ exposure was explored with an aim to understand the ALD reaction mechanisms at play when using O₃ as an oxidizer. The effect of high O₃ exposures will be discussed.

Preparation of B_xMg_1-xO films through an ABC-type ALD program employing bis(cyclopentadienyl) magnesium (II), trimethyl borate (B(OCH₃)₃), and water as precursors is presented herein. The thin film properties are extensively characterized by X-ray photoelectron (XPS), diffraction (XRD), and reflectivity (XRR). Moreover, optical properties of the as-grown and post-deposition annealed films are assessed through spectroscopic ellipsometry measurements with a particular focus on the effective optical properties of the B₂O₃ film component, which is an attractive shallow-boron dopant source material. Instead of using the highly toxic diborane (B₂H₆) as a boron doping source, the present work employs ALD to generate MgO-stablized B­₂O_3. Interestingly, as observed from in-situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) analysis, stabilization of B₂O₃ occurs by dosing Mg(Cp)₂ after the B(OCH₃)₃ pulse which forms Mg-B-O* surface species that undergoes further reaction to B₂O₃ in the succeeding water pulse. Without the Mg(Cp)₂ pulse (or MgO “AB” sub-cycle) preferential formation of boric acid (B(OH)₃) occurs instead of B­₂O₃. Post-deposition annealing treatments along with follow-up spectroscopic ellipsometry and high-resolution XPS/depth profiling measurements were performed to characterize the composite films’ optical and electronic structural properties, respectively. The thermal ALD synthesis of B_xMg_1-xO composite films on silicon in concert with post-deposition annealing is a potentially viable approach towards shallow-boron dopant layer or junction formation through the stable B₂O₃ component and could find use in solid-state electronics applications.

The introduction of controlled amounts of dopants into logic and memory devices has been extensively used in nanoelectronics in order to alter the properties of a material and make it suitable for a specific application. Doping thin films can be complicated because of the difficulty to control thickness and dopant gradient distribution.

Atomic Layer Deposition allows for excellent thickness control and doping is usually achieved by varying the cycle ratio of two metal oxides during deposition, M1O and M2O, in what are called super-cycles. We refer to this method as the standard nanolaminate process [1]. Here, we propose a modified ALD process that allows for finetuning the dopant concentration down to a few atomic % in the metal oxide (MO) thin film. This doping control can happen at thicknesses much thinner than standard nanolaminates thus allowing for a better trade-off between stack functionality and leakage. Doping levels of less than 10cation% are desired, specifically for higher k phases of HfO₂ for ferroelectric devices [2].

In figure 1, a comparison of the minimum deposition thickness required for each method, is given for varying dopant concentrations. This illustrates that the modified ALD process allows for films to be ~6x thinner than the standard nanolaminate process from the studied precursors for Zr-doped HfO₂, while exhibiting less than 5% 1-sigma thickness variation across a 200mm wafer. The process does not involve BEOL thermal budget or partial reactions associated with plasma oxidation and can therefore be very conformal. This makes it compatible with FEOL integration for 3D thin film depositions.

It has been demonstrated that doping can be beneficial for non-volatile memories as it can enhance the endurance and the switching stability of the devices [3]. Here, HfZrO thin films with varying dopant levels of Zr are deposited through the modified ALD method on 200mm substrates. Dopant concentrations are determined by RBS analysis. Film properties are analyzed with spectroscopic ellipsometry and XPS. Thin-film HfO₂-based ferroelectric devices with different dopants will be evaluated.

[2] Fischer D., Kersch A., J. of Appl. Phys., 104(8), 084104 (2008).

[3] Ryu S. W., Cho S., Park J., Kwac J., Kim H., Nishi Y., Appl. Phys. Lett., 105, 072102 (2014).

Depositing complex oxides by ALD is a rapidly emerging field, with new materials with excellent properties published every month. LaNiO₃ is a perovskite type oxide with metallic behaviour, thought to play a major role in functional multilayer stacks and integrated electronics. Although routes for depositing LaNiO₃ has been reported in literature, an annealing step has always been necessary to obtain eptaxial films. In many applications this hinders monolithic device integration, and can cause unwanted strain effects and cracking.

In this presentation we show a facile route for direct epitaxial growth of LaNiO₃ by thermal ALD at temperatures as low as 250 °C. The films show excellent conductive properties as deposited, with specific resistivity as low as 10^-4 Ohm cm. The functional properties of the films are slightly improved upon annealing, reaching specific resitivites in the 10^-5 Ohm cm order of magnitude. Furthermore, we show that the order of precursor pulses plays a fundamental role in obtaining high quality as deposited epitaxial films.

LaNiO₃ is a remarkable member of the RENiO₃ series of materials (RE = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Y, Lu),as it is the only member that remains metallic to 0 K. All other members observe a metal-insulator-transition (MIT) at temperature varying from ~100 K (Pr) to ~600 K (Lu). Controllably tuning the MIT is highly awaited by the community.

In this presentation we also show how substution by Cu on the Ni-site alters the electric properties and MIT temperature of LaNiO₃. We show that a full range of compositions is attainable, from LaCuO_x to LaNiO₃, and that the electronic properties vary smoothly over this composition range. ALD can be used to obtain a remarkable cation composition control. Furthermore, we hypothesize on the reason behind the varying properties of this material system, and try to explain it by showing how oxygen vacancies are introduced by copper substitution. This may help shed light on the surprising properties of LaNiO₃ itself, but more importantly the films can be directly used in applications where a tunable MIT is necessary.

This presentations highlights the use of complex oxide ALD as a real alternative to MBE, with several key advantages such as conformality and low temperature deposition.

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HfO₂ based ferroelectrics are a promising family of ferroelectrics stable as thin films. The ferroelectric response in HfO₂ thin films is associated with a metastable orthorhombic phase, typically seen in films < 30 nm thick, stabilized by doping, electrode material, deposition conditions and annealing. Thin HfO₂ based ferroelectrics are attractive for use in memory, energy harvesting, and sensing applications, however the polarization response of the thinnest of these films, <~5 nm, has been reported to be unstable over time. In this work we show that the pyroelectric response of 5 nm Hf_0.58Zr_0.42O₂ with TaN electrodes decreases logarithmically after biasing, decreasing from an initial value of -58 µCm^-2K^-1 to ~-40 µCm^-2K^-1 in the first 24 hours. While films 10 nm and thicker have a pyroelectric response that is constant on the same timescale.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.