Nanolaminate Films Research Articles

Electrical properties of Al 2O 3–HfTiO laminate gate dielectric stacks with an equivalent oxide thickness below 0.8 nm

We report high quality nanolaminate films consisting of five Al 2O 3–HfTiO layers with a dielectric constant of about 29. The dielectric stack was deposited on unheated p-Si substrate from Al 2O 3 and 1HfO 2/1TiO 2 targets using an electron beam gun evaporation system without addition of oxygen. A dielectric constant for a thick HfTiO film of about 83 was also demonstrated. The electrical characteristics of as deposited structures and ones which were annealed for 5–10 min in an O 2 atmosphere at up to 950 °C were investigated. Two types of gate electrodes: Pt and Ti were compared. The dielectric stack which was annealed up to 500 °C exhibits a leakage current density as small as ∼ 1 × 10 − 4 A/cm 2 at an electric of field 1.5 MV/cm for a quantum mechanical corrected equivalent oxide thickness of ∼ 0.76 nm. These values change to ∼ 1 × 10 − 8 A/cm 2 and 1.82 nm respectively, after annealing at 950 °C for 5 min.

Mosaic nanostructure of TiO2 with rutile short-range atomic order

A nanolaminate film of six 36nm TiO2–7nm Al2O3 bilayers is sputter deposited at room temperature and examined by high resolution transmission electron microscopy (HRTEM). Neither the TiO2 nor the Al2O3 layers have long-range crystallographic order. Previous Raman spectroscopy of the nanolaminate showed that short-range atomic order in the TiO2 component is characteristic of bulk rutile. The HRTEM images of the Al2O3 layers consist entirely of random contrast speckle characteristic of a material with no atomic ordering beyond the nearest-neighbor level. However, the predominant feature in the images of the TiO2 layers is a mosaic structure, with fewer regions of random contrast speckle. The mosaic consists of four repetitive elements: (1) domains of {110} planes terminating along ⟨100⟩ directions, (2) planar faults along ⟨100⟩ directions, (3) {110} facets in steps along the [001] direction, and (4) a herringbone structure of short strands of (110) and (−110) planes on either side of a ⟨100⟩ midrib. We show how two combined growth operations can generate this nanostructure: These operations are the preferential three-dimensional growth of a rutile nucleus with a {110} habit and the formation of growth faults with 12⟨10−1⟩{011} and 12⟨10−1⟩{121} displacement vectors. The results explicitly show that TiO2 with rutile short-range atomic order self-assembles into units beyond the nearest-neighbor level. This behavior is different from oxides that are continuous random network formers, such as SiO2 and Al3O3, in which the metal-oxygen bonds are predominantly covalent.

Composition, surface morphology and electrical characteristics of Al 2O 3–TiO 2 nanolaminates and AlTiO films on silicon

We propose and demonstrate Metal-Oxide-Semiconductor structures comprising Al 2O 3–TiO 2 nanolaminate and AlTiO films. Composition, structural and electrical characteristics were studied in detail and compared to TiO 2 thin film-based structures. All dielectric films were evaporated using an electron beam gun (EBG) system on unheated p-Si substrate without adding O 2. MOS structures were investigated in detail before and after annealing at up to 950 °C in O 2 and N 2 + O 2 environments. The nanolaminate films remain in an amorphous state after annealing at 950 °C. The smallest quantum mechanical corrected equivalent oxide thickness measured was ∼1.37 nm. A large reduction of the leakage current density to 1.8 × 10 − 8 A/cm 2 at an electric field of 2 MV/cm was achieved by the annealing process.

Effect of Alumina Addition on Bi-Ti-Al-O Dielectric Thin Films

The alumina addition effect on Bi 1-x-y Ti x Al y O z (BTAO) dielectric films was investigated with nanolaminate films of bismuth titanate and alumina layers. By the addition of alumina layers to the Bi-Ti system, the alumina content gradually increases but the Bi and Ti contents conversely decrease. The bandgap of BTAO increases to a certain level (4.1 eV) with the addition of alumina and then saturates. However, the difference between Fermi level and valence bandedge gradually increases as the alumina layers are added, making them more n-type material. The dielectric constant of BTAO decreases but the insulating property was improved by the addition of alumina layers.

Microstructure and interfacial properties of HfO 2–Al 2O 3 nanolaminate films

High- k HfO 2–Al 2O 3 composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO 2–Al 2O 3 films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO 2–Al 2O 3 composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO 2 thin films. FTIR characterization indicates that the growth of the interfacial SiO 2 layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO 2–Al 2O 3 composite films has been increased compared to pure HfO 2 films. Al 2O 3 as a passivation barrier for HfO 2 high- k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.

Zirconia-metal (Al, Y, Ti) oxide nanolaminate films

Our work on ZrO 2 when this oxide is combined with other metal oxides in multilayer nanolaminate films is reviewed in this invited paper. ZrO 2–Al 2O 3, ZrO 2–Y 2O 3, and ZrO 2–TiO 2 nanolaminates were studied. The films were grown by sequential sputter deposition of individual oxide layers from metal targets using radio-frequency-excited, O 2-bearing discharges. As the thickness of individual layers decreases, interfaces play an increasingly important role in determining the nanolaminates' overall properties. These nanolaminates have different thermodynamic driving forces for interfacial cation mixing and interfacial atomic registry (heteroepitaxy or pseudomorphism). Three examples illustrate how strongly ZrO 2 growth is influenced by its partner metal oxide through different interfacial reactions. These examples are superplastic tetragonal ZrO 2 formation in ZrO 2–Al 2O 3 films, cubic (Zr,Y)-oxide formation in ZrO 2–Y 2O 3 films, and monoclinic (Zr,Ti)O 2 formation (occurring in nature only at high pressure) in ZrO 2–TiO 2 films.

Effects of nitrogen flow rates on reactively sputtered nanocrystal-(Ti,Al)xN1-x/amorphous-SiyN1-y nanolaminate films

Nanocrystal-(Ti,Al)xN1-x/amorphous-SiyN1-y nanolaminate films were deposited periodically under different nitrogen flow rates. The composition, microstructure and mechanical properties of nanolaminate films were investigated by x-ray photoelectron spectroscope, x-ray diffractometer, scanning and transmission electron microscopy, atomic force microscope, and nanoindentation apparatus. Results indicated that the formation of the compound on the target surface was substantially influenced by the deposition rate, composition and crystallite size of the nanolaminate films. Nanolaminate structure with periodic compositional modulation and sharp interfaces were deposited at different nitrogen flow rate. Smaller nanocrystallite size, round-shaped grain features, smoother surface morphology, higher hardness, and reduced elastic modulus were obtained for nanolaminate films with increasing the nitrogen flow rate.

Investigation of nanocrystal-(Ti 1− xAl x)N y/amorphous-Si 3N 4 nanolaminate films

Nanocrystal-(Ti 1− x Al x )N y /amorphous-Si 3N 4 nanolaminate films were deposited periodically via reactive magnetron sputtering technique. The effects of the thickness of multilayer period on the microstructure and mechanical properties were investigated by X-ray diffraction, scanning and transmission electron microscopies, nanoindentation and scratching adhesion testing. Results indicate that the nanolaminate structure is uniform and good flatness interfaces under different multilayer periods. The nanolaminate films exhibited a maximum hardness when the multilayer structure had a period of λ=25 nm and were harder than monolayer (Ti 1− x Al x )N y or Si 3N 4 films of the same thickness. The critical scratching load of the nanolaminates was higher than that of monolayer specimens with the same thickness. The mechanisms of fracture and toughening of nanolaminate films are also discussed.

Optical absorption behavior of ZrO2–TiO2 nanolaminate films

The near-ultraviolet fundamental optical absorption edge of sputter-deposited ZrO2–TiO2 nanolaminate films on SiO2 substrates was studied by transmission-reflection spectrophotometry. Seven different bilayer architectures were investigated, with nominal ZrO2 volume fractions ranging from 0.10 to 0.91 (Zr atom fractions of 0.1–0.9). The absorption coefficient, α(E), was determined as a function of the incident photon energy, E, in the 3.5–5.8 eV range (350–215 nm wavelength). α(E) vs E curves show a systematic blueshift and a change in shape with an increase in the Zr atom fraction in a bilayer. Neither amalgamation nor persistence models can adequately explain the experimental results. The reason why is that an extensive and structurally complex mixed cation interfacial structure formed even during room temperature deposition. A model that takes into account contributions to α(E) from Ti–O–Ti and Zr–O–Zr linkages far from the interfaces between constituents and Ti–O–Zr linkages at these interfaces is successfully applied to the data.

Fundamental optical absorption edge of undoped tetragonal zirconium dioxide

The high-frequency optical absorption edge of pure tetragonal ZrO2, isolated in a ZrO2–Al2O3 nanolaminate film structure, was determined using transmission spectrophotometry. The functional dependence of the absorption coefficient on photon energy shows two interband transitions: an initial indirect transition at 5.22 eV (i.e., the band gap) followed by a direct transition at 5.87 eV. The edge structure is associated with O 2p→Zr 4d electron states and discussed in terms of ab initio calculations reported in the literature.

In situ TEM observations of fracture in nanolaminated metallic thin films

Fracture of single crystal nanolaminated thin films has been investigated through in situ straining of cross-sectional samples of Cu/Ni nanolaminates grown on Cu (001) single crystal substrates. The earlest stages of deformation exhibits a confined layer slip mechanism. With continued straining, unstable fracture occurs creating a mixed-mode crack that propagates across the nanolaminate, roughly perpendicular to the interfaces. Eventually, stable crack growth with intense plastic deformation ahead of the crack tip occurs over many bilayers in the direction of crack growth. Simultaneously, plasticity was seen to spread only 1 or 2 bilayer distances normal to the crack, creating an extremely localized plastic zone. Transmission electron microscopic (TEM) examination after the test did not reveal the presence of dislocations in the crack wake, except where severe crack deflection was observed. By comparison, the plastic zone size in the substrate was greater by several of orders of magnitude.

ZnO/Al 2O 3 nanolaminates fabricated by atomic layer deposition: growth and surface roughness measurements

Nanolaminates are unique nanocomposites that allow various thin film properties to be tuned by changing the composition and interfacial density. ZnO/Al 2O 3 nanolaminates allow the surface roughness to be controlled because ZnO is crystalline and Al 2O 3 is amorphous at low deposition temperatures. ZnO/Al 2O 3 nanolaminates were grown using atomic layer deposition (ALD) methods. The ZnO and Al 2O 3 films were deposited at 450 K using alternating diethyl zinc/H 2O exposures and trimethyl aluminum/H 2O exposures, respectively. The growth rate and surface topography of the pure oxide films were examined using ex situ ellipsometry, stylus profilometry and atomic force microscopy (AFM) techniques. The ZnO ALD films grew at 2.01 Å/cycle and roughened significantly versus film thickness because of the presence of ZnO nanocrystals. In contrast, the Al 2O 3 ALD films grew at 1.29 Å/cycle and remained remarkably smooth versus film thickness because of their amorphous structure. In situ quartz crystal microbalance measurements were employed to study the ZnO/Al 2O 3 nanolaminate growth. The individual ZnO and Al 2O 3 nanolayers nucleated and grew easily on each other with minimal interfacial effects. ZnO/Al 2O 3 nanolaminate films were prepared where the number of ZnO/Al 2O 3 interfaces was varied but the total thickness remained constant at ∼1250 Å. Using AFM techniques, the RMS roughness of the nanolaminates was observed to decrease substantially with increasing ZnO/Al 2O 3 interfacial density. The Al 2O 3 nanolayer interrupts the ZnO crystal growth and forces the ZnO nanolayer to renucleate on the Al 2O 3 surface. AFM measurements revealed that a single trimethyl aluminum/H 2O reaction cycle was sufficient to reduce markedly the surface roughness of the ZnO/Al 2O 3 nanolaminate films. ZnO/Al 2O 3 nanolaminates should be useful to fabricate a surface topography with a controlled surface roughness.

( Zr,Ti ) O 2 interface structure in ZrO2–TiO2 nanolaminates with ultrathin periodicity

A mixed cation interfacial structure in ZrO2–TiO2 nanolaminate films with ultrathin bilayer periodicity grown by sputter deposition at 297 K was identified by x-ray diffraction and nonresonant Raman spectroscopy. This structure consists of an amorphous phase at a ZrO2-on-TiO2 bilayer interface, followed by an extensive crystalline monoclinic (Zr,Ti)O2 solid solution predicted by Vegard’s law. Monoclinic (Zr,Ti)O2 has previously been reported only once, in bulk powder of a single composition (ZrTiO4) at high pressure. Its stabilization in the nanolaminates is explained by the Gibbs–Thomson effect. This complex interfacial structure is shown to be a means of accommodating chemical mixing in the absence of a driving force for heteroepitaxy.

Ultrathin nitrided-nanolaminate (Al2O3/ZrO2/Al2O3) for metal–oxide–semiconductor gate dielectric applications

An ultrathin nanolaminate (Al2O3/ZrO2/Al2O3) film prepared by atomic layer chemical vapor deposition was investigated for use in metal–oxide–semiconductor field effect transistor (MOSFET) gate dielectric applications. Based on transmission electron microscopy and medium-energy ion scattering spectroscopy (MEIS) analysis, an abrupt interface between stoichiometric top-layer Al2O3 and ZrO2 was found. Interfacial layers such as Zr–Al–O and Al–Si–O were also observed. An electrical equivalent oxide thickness as thin as 10.2 Å with a quantum mechanical correction was obtained. Additional plasma nitridation of nanolaminte in N2 led to a significant reduction in the interfacial oxidation of nanolaminate which was confirmed by x-ray photoelectron spectroscopy, MEIS, and capacitance–voltage (C–V) analysis. The nanolaminate film represents a promising alternative for gate dielectric applications of future sub-100 nm MOSFET.

Phase development in annealed zirconia-titania nanolaminates

Phase development was studied in sputter-deposited and annealed ZrO2–TiO2 nanolaminate films on fused silica substrates. The goal was to understand phase evolution as these structures moved toward thermodynamic equilibrium. The results show that diffusive amorphization to form α-ZTO was the first reaction of the as-deposited constituents at low temperature (700 °C). Growth of α-ZTO was self-limited, and a second metastable reaction product with an incommensurate α-PbO2-type lattice, ss-ZTO, formed with time at temperature. Terminal phases formed only when the annealing temperature was raised to 1000 °C, thereby lifting kinetic constraints to diffusion. The terminal phases were ZrTiO4 or ZrTi2O6, depending upon film stoichiometry. This study demonstrates that in the absence of a physical driving force to promote layer registration upon low temperature annealing, constituents react to lower the system’s free energy via a series of metastable phases that involve limited atomic rearrangement. Equilibrium phases are formed only after the kinetic constraints to diffusion are relaxed.

Transmission electron microscopy study of zirconia–alumina nanolaminates grown by reactive sputter deposition. Part I: zirconia nanocrystallite growth morphology

Pure zirconia films and zirconia–alumina nanolaminate films grown by reactive sputter deposition are studied by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The phase composition and morphology associated with zirconia crystallite growth are investigated by examining films containing zirconia layers of varying thickness. These studies, performed at room temperature, suggest that the zirconia crystallites initially grow in the tetragonal phase to a critical size of 6.0±0.2 nm, in agreement with a value of 6.2 nm predicted by end-point thermodynamics. Past the critical size, incorporation of additional zirconia molecules into the zirconia layers is accomplished predominantly by transformation of the growing crystallites to the monoclinic phase, and less frequently by deposition of amorphous zirconia. Transformation to the monoclinic phase is accompanied by a highly faulted intermediary phase. The subsequent growth behavior of monoclinic crystallites is consistent with a three-dimensional interface-controlled, diffusion-limited growth process with a growth exponent between 3 and 4. Nanoindentation measurements of nanolaminates with 5-nm thick zirconia layers give a hardness of ~8 GPa for the upper strata where the morphology of the tetragonal zirconia layers contains an intrinsic roughness. The hardness increases to ~10 GPa closer to the substrate where the laminar morphology is more pronounced. Young's modulus is between 156 and 195 GPa for these same nanolaminates.

Reactive spulter deposition of ceramic oxide nanolaminates: ZrO2–Al2O3 and ZrO2–Y2O3 model systems

The growth of ceramic nanolaminate films by reactive sputter deposition is reviewed. Phase formation in nanolaminates with the same nominal architecture but with different chemical constituents is investigated. Two model systems, zirconia–alumina and zirconiayttria, allow comparison of the effects of chemical reactivity between constituents at the interface. In zirconia–alumina nanolaminates, each component is a separate entity and the interface is incoherent. Phase evolution in zirconia layers of decreasing thickness is predicted by the finite crystal size effect. Tetragonal zirconia is produced in layers whose thickness is less than the critical thickness for stabilisation of monoclinic zirconia (the STP phase). The amorphous structure of alumina is a consequence of its structural flexibility. Overall morphological roughness of the film arises from the polycrystalline nature of the zirconia layers. In zirconia–yttria nanolaminates, an interfacial reaction between components completely obliterates yttria as a separate entity. The reaction product, cubic zirconiayttria, forms needlelike crystallites and accentuates the overall morphological roughness resulting from the polycrystalline nature of the deposit. As zirconia layer thickness increases, monoclinic zirconia is produced along with interfacial cubic zirconia–yttria. This research clearly demonstrates the ability to forminterfacial oxide solid solutions at low temperature in a system in which the bulk equilibrium phase diagram predicts reaction between oxide components. Thus, thin films consisting entirely of interface reaction products can be fabricated if the bilayer spacing is small enough.

Nanoindentation and friction studies on Ti-based nanolaminated films

Multilayered Ti/TiN and Ti/Cu films of 12–15 μm thickness were produced using an r.f. triode-magnetron sputtering technique. The thickness of Ti layers in both types of composites varied between 150 and 1000 nm. TiN and Cu layers were thinner, and their thicknesses varied within a narrower range of 20–120 nm. The volume fractions of the constituents in the composites were kept constant in order to study the effect of reducing Ti layer thickness (λ Ti) on the mechanical and tribological properties of the laminates. Nanoindentation tests were performed to determine hardnesses and elastic moduli of the composites and to analyze the energy expenditure during the indentation process. The elastic modulus and hardness of the Ti/TiN films were both higher when tested in a direction parallel to the plane of the layers compared to a direction normal to their planes. The strength of Ti/TiN also increased with (λ Ti) and showed a good agreement with the strength levels calculated by considering the difference in the elastic moduli of the constituents on the basis of the Koehler strengthening mechanism. Thin Cu layers in Ti/Cu were not effective as barriers to dislocation motion, and Ti/Cu interfaces were susceptible to delamination during indentation tests and sliding wear tests. A linear relationship was observed between the wear rates of Ti/TiN and (λ Ti) −0.5 which suggests that the wear resistance of the laminated composites can be improved by reducing the distance between the TiN layers.

Mechanical properties and tribological behaviour of nanolayered Al/Al 2O 3 and Ti/TiN composites

Mechanical and tribological properties of nanolayered composites of Al/Al 2O 3 and Ti/TiN were investigated. Alternating layers of metals and ceramics were deposited using an r.f. magnetron sputtering technique. Nonoindentation tests were performed to determine force-displacement curves which were used to calculate elastic moduli and nanohardness of composites as a function of distance between layers. It was observed that both elastic modulus and hardness of composites increased with decreasing layer thickness. A good agreement was found between experimentally determined values for elastic modulus and predictions based on the rule of mixtures for isostress conditions. The hardness of Al/Al 2O 3 and Ti/TiN could be described in the formalism of the Hall-Petch-type equation indicating that ceramic layers inhibited slip transfer across metallic layers. A deviation from the Hall-Petch type of strengthening was observed in Al/Al 2O 3 at small interlayer spacings. Friction and wear behaviour of laminates was studied using a pin-on-disc type of tribometer. A systematic increase in wear resistance with decreasing layer thickness was observed. The peak friction coefficient decreased about 70% in Al/Al 2O 3 (with 200 nm Al layer thickness) while a significant 60% improvement in steady state friction coefficient was measured in Ti/TiN (with 150 nm Ti layer thickness) in comparison with as-sputtered monolithic metallic films. These observations indicated that the nanolaminated films are suitable for applications where a combination of low coefficient of friction, high wear resistance and hardness is required.

Characterization of Zinc Oxide Nanolaminate Films

This project sought to increases the understanding of fundamental properties of the transparent conducting oxide materials needed to improve solar energy device efficiency. Specifically, this study focused on the development of Cu:ZnO nanolamintate materials where Cu and ZnO form alternating layers on the nanomerter scale. Initial results show no intermixing of materials, and suggest that these nanolaminates may be employed as infrared mirrors.Â