Al Capping Layer Research Articles

Improvement of thermal stability of NiAl via employing Al capping layer for advanced interconnect applications

This letter reports on the study of employing an Al capping layer to improve the thermal stability of NiAl films for advanced interconnect applications. We prepare NiAl films with an Al capping layer of various thicknesses. Transmission electron microscopy analysis of NiAl with a 1 nm thick capping layer annealed at 450 °C discloses an effective suppression of Al out-diffusion from the NiAl layer and hence enhanced thermal stability. Measurement of thickness-dependent resistivity unravels a much slower resistivity increase for the capping layer sample than that of the sample without capping layer and low resistivity below 10 nm (49.7 μΩ·cm for 3.2 nm).

Improvement of Thermal Stability of NiAl Via Employing Al Capping Layer for Advanced Interconnect Applications

Improvement of Thermal Stability of NiAl Via Employing Al Capping Layer for Advanced Interconnect Applications

Three-dimensional bulk Fermi surfaces and Weyl crossings of Co2MnGa thin films underneath a protection layer

Angle-resolved photoelectron spectroscopy utilizing soft x-ray synchrotron radiation was applied to Heusler-type ${\mathrm{Co}}_{2}\mathrm{MnGa}$ thin films that have a 1-nm Al capping layer. The bulk Fermi surfaces and band structures varied along the out-of-plane momentum, stemming from the three-dimensional crystal structure, in the absence of any in situ surface treatment. In addition, there were characteristic intersecting bands (Weyl cones), with crossing points near the Fermi level, which were consistent with computed results. The Weyl cones are of bulk origin and are responsible for the high anomalous Nernst and the anomalous Hall coefficients. A close comparison of the experimental band structures in ${\mathrm{Co}}_{2}\mathrm{MnGe}$ and ${\mathrm{Co}}_{2}\mathrm{MnGa}$ indicated that the rigid band picture is valid in both alloys and that fine carrier tuning is possible by replacing Ga with Ge to improve the anomalous conductivity.

Electrical Performance and Stability Improvements of High-Mobility Indium–Gallium–Tin Oxide Thin-Film Transistors Using an Oxidized Aluminum Capping Layer of Optimal Thickness

We examined the effects of aluminum (Al) capping layer thickness on the electrical performance and stability of high-mobility indium–gallium–tin oxide (IGTO) thin-film transistors (TFTs). The Al capping layers with thicknesses (tAls) of 3, 5, and 8 nm were deposited, respectively, on top of the IGTO thin film by electron beam evaporation, and the IGTO TFTs without and with Al capping layers were subjected to thermal annealing at 200 °C for 1 h in ambient air. Among the IGTO TFTs without and with Al capping layers, the TFT with a 3 nm thick Al capping layer exhibited excellent electrical performance (field-effect mobility: 26.4 cm2/V s, subthreshold swing: 0.20 V/dec, and threshold voltage: −1.7 V) and higher electrical stability under positive and negative bias illumination stresses than other TFTs. To elucidate the physical mechanism responsible for the observed phenomenon, we compared the O1s spectra of the IGTO thin films without and with Al capping layers using X-ray photoelectron spectroscopy analyses. From the characterization results, it was observed that the weakly bonded oxygen-related components decreased from 25.0 to 10.0%, whereas the oxygen-deficient portion was maintained at 24.4% after the formation of the 3 nm thick Al capping layer. In contrast, a significant increase in the oxygen-deficient portion was observed after the formation of the Al capping layers having tAl values greater than 3 nm. These results imply that the thicker Al capping layer has a stronger gathering power for the oxygen species, and that 3 nm is the optimum thickness of the Al capping layer, which can selectively remove the weakly bonded oxygen species acting as subgap tail states within the IGTO. The results of this study thus demonstrate that the formation of an Al capping layer with the optimal thickness is a practical and useful method to enhance the electrical performance and stability of high-mobility IGTO TFTs.

Metal-induced n+/n homojunction for ultrahigh electron mobility transistors

A self-organized n+/n homojunction is proposed to achieve ultrahigh performance of thin film transistors (TFTs) based on an amorphous (Zn,Ba)SnO3 (ZBTO) semiconductor with sufficiently limited scattering centers. A deposited Al layer can induce a highly O-deficient (n+) interface layer in the back channel of a-ZBTO without damaging the front channel layer via the formation of a metal-oxide interlayer between the metal and back channel. The n+ layer can significantly improve the field-effect mobility by providing a relatively high concentration of free electrons in the front n-channel ZBTO, where the scattering of carriers is already controlled. In comparison with a Ti layer, the Al metal layer is superior, as confirmed by first-principles density functional theory (DFT) calculations, due to the stronger metal-O bonds, which make it easier to form a metal oxide AlOx interlayer through the removal of oxygen from ZBTO. The field-effect mobility of a-ZBTO with an Al capping layer can reach 153.4 cm2/Vs, which is higher than that of the pristine device, i.e., 20.8 cm2/Vs. This result paves the way for the realization of a cost-effective method for implementing indium-free ZBTO devices in various applications, such as flat panel displays and large-area electronic circuits.

Ultra-High Mobility Transistors Via Metal Induced Conductive Region

The amorphous oxide semiconductors (AOSs) such as In-Ga-Zn-O (IGZO) and Zn-Sn-O (ZTO) have been extensively studied as the active layers for thin film transistors (TFTs) in display backplanes due to its high mobility (>10cm2/Vs), transparency, low process temperature and large area uniformity. However, many studies on AOSs have reported that there is trade-off relationship between electrical performance and device stability. Generally, the mobility increases with increasing indium or tin content, while the stability is degraded simultaneously. Furthermore, because the indium is a rare metal and expensive, incorporating indium in active materials is one of the critical reasons for increasing overall process costs. For this reason, the development of an indium-free oxide semiconductor materials with high mobility and stability is continuously required.Recently, (Zn1-xBax)SnO3 (ZBTO) has been reported to be a very stable oxide semiconductor with high electron mobility results from its small effective mass and less defective structure. Based on these desirable material properties, it has been adopted as an active material for cost-effective and high-performance TFTs. Also, in order to improve electrical performance dramatically, an additional metal layers are introduced.The metal capping layers have been reported to improve the performance of TFTs by various mechanisms, including the elimination of the scattering/trapping centers, low-temperature crystallization, and electron injection. However, although many research have been performed to demonstrate the origin of the mobility enhancement by the metal capping layer, the exact mechanism has not been confirmed. Also, former reports of this method showed several drawback such as the induced crystallization and need for stabilization time, which may limit its application on large-area displays.To investigate the effect of metal capping layer, the device was fabricated using In-free active material. The ZBTO active layer were deposited via RF magnetron sputtering on the p++ Si/SiOx substrate. All sample was annealed at 350℃ for 1 hour in air ambient, and patterned using shadow masks. The channel width (W) and length (L) were 800 and 200 μm, respectively. In case of metal capped device, 20 nm-thick Al or Ti metal layer was sputtered selectively between the source/drain electrodes with dimensions W/L = 1300/100 μm.In this work, the formation of conductive region in active layer induced by metal capping layer was suggested as new mechanism of the mobility improvement. Due to the strong oxidation power of Al metal, oxygen deficiency is easily induced in active layer near the interface between Al and ZBTO during Al oxidation. This conductive region in active layer generates free electrons, which can be transported without serious scattering owing to the low defect density of ZBTO. These phenomena were confirmed by TEM and XPS results. As a result, the ZBTO TFTs with an Al metal layer exhibited significantly improved field-effect mobility from 20.8 cm2/Vs to 153.4 cm2/Vs without degradation of other transfer parameters. Furthermore, the ZBTO TFTs showed far superior mobility improvement compared to other conventional oxide TFTs. The carrier transport mechanisms was investigated by 1/f noise measurements, which indicated that carrier transport of ZBTO TFTs with the Al capping layer is enhanced by the reduced bulk trap sites. Moreover, reliability tests including NBS, PBS indicated that the front channel of ZBTO active layer was relatively less affected. Our results indicate that adopt of stable active material with small defect density, in this case ZBTO, is a key factor in the fabrication of metal-assisted TFTs capable of ultra-high performance.

Improved Field-Effect Mobility of In–Ga–Zn–O TFTs by Oxidized Metal Layer

The application of an aluminum (Al) capping layer on top of an In–Ga–Zn–O (IGZO) active layer is proposed to enhance the mobility of IGZO thin-film transistors (TFTs). The Al metal layer forms a very thin and dense oxide film on the surface that prevents further internal oxidation, and an additional AlO x interlayer between Al and IGZO is formed using oxygen in the IGZO back-channel region due to its strong oxidation power. The formation of an aluminum oxide interlayer induces an oxygen-deficient region in the active layer, inducing free carriers that enhance the field-effect mobility from 11.3 to 72.6 cm2/Vs. The device reliability under positive and negative bias stress in the dark is relatively unaffected by the presence of the Al capping layer; however, the stability under negative bias illumination stress is accelerated, likely originating from the ionization of oxygen vacant sites.

Improving electrical conductivity and its thermal stability of Al-doped ZnO polycrystalline films using ultrathin Al film as a passivation layer

Aluminum-doped ZnO (AZO) has been considered as a prospective material for application as transparent electrodes in solar cells. In this application, an improvement of the Hall mobility is desired to achieve the high conductivity, because an increase in the carrier concentration results in a decrease in transmission at near-infrared wavelengths. However, the achievement of high Hall mobility is still a challenge. Another challenge associated with AZO is the low thermal stability of the electrical properties, which limits the application of AZO. This limitation originates from Zn desorption and migration of oxygen from the environment into the AZO film. It has been shown that using an ultrathin Al capping layer on an AZO film can greatly improve its thermal stability. An improvement in the Hall mobility was obtained for an Al capped AZO film after annealing in N2 gas at 400 °C. The Hall mobility reached 49.1 cm2/V, which is close to the theoretical limit of the mobility for an AZO polycrystalline film, with a moderate carrier concentration of 2.8 × 1020 cm−3, ensuring high transparency in the visible and near-infrared ranges. The obtained results are due to the passivation effect of the AlOx film formed by natural oxidation of Al film. Zn desorption was suppressed and the migration of O from the environment was reduced, leading to an improvement in the thermal stability. Once Zn desorption is prevented, improvement of the crystalline perfection due to thermal annealing can contribute to an improvement in the Hall mobility.

Separation of the two-magnon scattering contribution to damping for the determination of the spin mixing conductance

We present angle dependent measurements of the damping properties of epitaxial Fe layers with MgO, Al and Pt capping layers. Based on the preferential distribution of lattice defects following the crystal symmetry, we make use of a model of the defect density to separate the contribution of two-magnon scattering to the damping from the isotropic contribution originating in the spin pumping effect, the viscous Gilbert damping and the magnetic proximity effect. The separation of the two-magnon contribution, which depends strongly on the defect density, allows for the measurement of a value of the effective spin mixing conductance which is closer to the value exclusively due to spin pumping. The influence of the defect density for bilayers systems due to the different capping layers and to the unavoidable spread in defect density from sample to sample is thus removed. This shows the potential of studying spin pumping phenomena in fully ordered systems in which this separation is possible, contrary to polycrystalline or amorphous metallic thin films.

AZO Thin Film Transistor Performance Enhancement by Capping an Aluminum Layer

Coplanar bottom-gate aluminum-zinc-oxide (AZO) thin film transistors (TFTs) with aluminum (Al) capping layers were fabricated in this work. The influences of the Al capping layer thickness and the post-annealing condition on the performance of the AZO-TFTswere investigated. Results show that the performance of the AZO-TFTs are enhanced significantly by introducing the Al capping layer on back channel, with saturation mobility increasing dramatically from 0.128 to 12.6 cm2/ $\text {V}\cdot \text {s}$ . The enhancement is ascribed to the diffusion of Al atoms into the AZO thin film and thus induced crystallization improvement.

Patterning the two dimensional electron gas at the LaAlO3/SrTiO3 interface by structured Al capping

We demonstrate an approach for patterning a quasi-two dimensional electron gas (q-2DEG) at the interface of LaAlO3 (LAO) and SrTiO3 (STO) utilizing a structured Al capping layer. The capping of Al enables the formation of q-2DEG at the interface of 1–3 unit cells (uc) of LAO on STO, which was originally insulating before capping. The properties of the q-2DEG induced by the Al capping layer are essentially the same as those of q-2DEG without Al. Therefore, we can pattern q-2DEG by simply patterning the Al film on LAO (2 or 3 uc)/STO using a one-step liftoff process. Our approach circumvents the difficulty of direct patterning of oxide materials and provides a simple and robust patterning method for future device applications based on complex oxide interfaces.

Synchrotron radiation X-ray photoelectron spectroscopy of Ti/Al ohmic contacts to n-type GaN: Key role of Al capping layers in interface scavenging reactions

Interface reactions between Ti-based electrodes and n-type GaN epilayers were investigated by synchrotron radiation X-ray photoelectron spectroscopy. Metallic Ga and thin TiN alloys were formed at the interface by subsequently depositing Al capping layers on ultrathin Ti layers even at room temperature. By comparing results from stacked Ti/Al and single Ti electrodes, the essential role of Al capping layers serving as an oxygen-scavenging element to produce reactive Ti underlayers was demonstrated. Further growth of the metallic interlayer during annealing was observed. A strategy for achieving low-resistance ohmic contacts to n-GaN with low-thermal-budget processing is discussed.

Al capping layers for nondestructive x-ray photoelectron spectroscopy analyses of transition-metal nitride thin films

X-ray photoelectron spectroscopy (XPS) compositional analyses of materials that have been air exposed typically require ion etching in order to remove contaminated surface layers. However, the etching step can lead to changes in sample surface and near-surface compositions due to preferential elemental sputter ejection and forward recoil implantation; this is a particular problem for metal/gas compounds and alloys such as nitrides and oxides. Here, the authors use TiN as a model system and compare XPS analysis results from three sets of polycrystalline TiN/Si(001) films deposited by reactive magnetron sputtering in a separate vacuum chamber. The films are either (1) air-exposed for ≤10 min prior to insertion into the ultrahigh-vacuum (UHV) XPS system; (2) air-exposed and subject to ion etching, using different ion energies and beam incidence angles, in the XPS chamber prior to analysis; or (3) Al-capped in-situ in the deposition system prior to air-exposure and loading into the XPS instrument. The authors show that thin, 1.5–6.0 nm, Al capping layers provide effective barriers to oxidation and contamination of TiN surfaces, thus allowing nondestructive acquisition of high-resolution core-level spectra representative of clean samples, and, hence, correct bonding assignments. The Ti 2p and N 1s satellite features, which are sensitive to ion bombardment, exhibit high intensities comparable to those obtained from single-crystal TiN/MgO(001) films grown and analyzed in-situ in a UHV XPS system and there is no indication of Al/TiN interfacial reactions. XPS-determined N/Ti concentrations acquired from Al/TiN samples agree very well with Rutherford backscattering and elastic recoil analysis results while ion-etched air-exposed samples exhibit strong N loss due to preferential resputtering. The intensities and shapes of the Ti 2p and N 1s core level signals from Al/TiN/Si(001) samples do not change following long-term (up to 70 days) exposure to ambient conditions, indicating that the thin Al capping layers provide stable surface passivation without spallation.

High thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealing

We demonstrate that the magnetic anisotropy of the CoFeB/MgO bilayer can be manipulated by adding an aluminum capping layer. After rapid thermal annealing, we can achieve large perpendicular magnetic anisotropy of CoFeB with a high thermal stability factor (Δ = 72) while the Gilbert damping constant can be reduced down to only 0.011 simultaneously. The boron and residual oxygen in the bulk CoFeB layer are properly absorbed by the Al capping layer during annealing, leading to the enhanced exchange stiffness and reduced damping. The interfacial Fe-O bonding can be optimized by tuning annealing temperature and thickness of Al, resulting in enhanced perpendicular anisotropy.

Microtextured Surfaces with Gradient Wetting Properties

Patterned surfaces with microwrinkled surface structures were prepared by thermally evaporating thin aluminum (10-300 nm thick) (Al) layers onto thick prestrained layers of a silicone elastomer and subsequently releasing the strain. This resulted in the formation of sinusoidal periodic surface wrinkles with characteristic wavelengths in the 3-42 μm range and amplitudes as large as 3.6 ± 0.4 μm. The Al thickness dependence of the wrinkle wavelengths and amplitudes was determined for different values of the applied prestrain and compared to a recent large-amplitude deflection theory of wrinkle formation. The results were found to be in good agreement with theory. Samples with spatial gradients in wrinkle wavelength and amplitude were also produced by applying mechanical strain gradients to the silicone elastomer layers prior to deposition of the Al capping layers. Sessile water droplets that were placed on these surfaces were found to have contact angles that were dependent upon their position. Moreover, these samples were shown to direct the motion of small water droplets when the substrates were vibrated.

Tuning the coercive field of Ni and CuNi thin films with the embedding of Co nanoparticles: An element-specific study

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.

Nanophononic thin-film filters and mirrors studied by picosecond ultrasonics

Optimized acoustic phonon thin-film filters are studied by picosecond ultrasonics. A broadband mirror and a color filter based on aperiodic multilayers were optimized to work in the subterahertz range, and grown by molecular beam epitaxy. Time resolved differential optical reflectivity experiments were performed with pump and probe pulses incident on opposite sides of the substrate. We provide broadband transmission curves for the phonon devices. The results are in good agreement with standard transfer matrix method simulations. In addition, we analyze the effects of the free surface and the influence of an Al capping layer on the response of the aperiodic devices.

Modification of optical and electrical properties of ITO using a thin Al capping layer

In this study, the work function, transmittance, and resistivity of indium tin oxide (ITO) thin films were successfully modified by depositing an Al capping layer on top of ITO with subsequent thermal annealing. The 5 nm thick Al layer was deposited by a conventional dc magnetron sputtering method and the layer was converted into an aluminum oxinitride by subjecting the sample to rapid thermal annealing (RTA) under a nitrogen atmosphere. The films exhibited a high transmittance of 86% on average within the visible wavelength region with an average resistivity value of 7.9 × 10 − 4 Ω cm. Heat-treating the Al/ITO films via RTA resulted in the decrease of the optical band gap from that of bare ITO. In addition, the films showed red-shift phenomena due to their decreased band gaps when the heat-treatment temperature was increased. The resultant electrical and optical characteristics can be explained by the formation of aluminum oxinitride on the surface of the ITO films. The work function of the heat-treated films increased by up to 0.26 eV from that of a bare ITO film. The increase of the work function predicts the reduction of the hole-injection barrier in organic light-emitting diode (OLED) devices and the eventual use of these films could provide much improved efficiency of devices.

Atomistic molecular dynamics study of interface formation: Al on poly(p-phenylene vinylene)

We model interface formation by metal deposition on the conjugated polymer poly-para-phenylene vinylene, studying direct aluminum and layered aluminum-calcium structures Al/PPV and Al/Ca/PPV. To do that we use classical molecular dynamics simulations, checked by ab initio density-functional theory calculations, for selected relevant configurations. We find that Al not only migrates easily into the film, with a strong charge transfer to the neighboring chains, but also promotes rearrangement of the polymer in the interfacial region to the hexagonal structure. On the other hand, our results indicate that a thin Ca layer is sufficient to protect the film and maintain a well-defined metal/polymer interface, and that also a thin Al capping layer may protect the whole from environmental degradation.

Improvement of Interface Properties of W/La2O3/Si MOS Structure Using Al Capping Layer

Electrical characteristics of W/La2O3/Si MOS capacitors have been investigated under various annealing condition. Relatively high interface state densities (Dit) of 1012 cm-2eV-1 observed after annealing at 200 to 500 oC was effectively reduced down to 1011 cm-2eV-1 by using an Al capping layer in the annealing process. However, a trade off between high capacitance density and low Dit has been observed in this method.