Al Film Thickness Research Articles

Nanometer-thick molecular beam epitaxy Al films capped with in situ deposited Al2O3—High-crystallinity, morphology, and superconductivity

Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al(111)⟨21¯1¯⟩∥sapphire(0001)[21¯1¯0]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness.

Analysis of propagation and resonance properties of longitudinal leaky surface acoustic wave on LiNbO3/SiC bonded structure

Abstract The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAW) on a bonded structure comprising an X-cut LiNbO3 (LN) thin plate and a 4H-SiC support substrate are theoretically investigated. The strong LLSAW responses with high Q factors were obtained at the LN thin-plate thickness h where the LLSAW phase velocity was slower than the bulk shear wave of 4H-SiC of 7126 m/s, and a fractional bandwidth (FBW) of 9–10% was obtained for the normalized Al film thickness by wavelength h Al/λ = 0.06–0.07 and h/λ = 0.30–0.40. Moreover, even at h/λ with a faster phase velocity than the bulk shear wave of 4H-SiC, the strong LLSAW responses without spurious response owing to the LLSAW higher-order mode were obtained. Finally, h Al/λ = 0.031 and h/λ = 0.19 were extracted to obtain a phase velocity of 7800 m/s, high Q factors, and FBW of 7.6%.

MoS2-Based MOS Capacitor for In-Memory Light Sensing

Recently, two-dimensional transition metal dichalcogenide (2D-TMD) materials have shown great attention due to their special physical characteristics and their applications in nanodevices [1]. The 2D-TMD materials are possible to develop via chemical vapor deposition (CVD) and physical and chemical exfoliation methods. The mechanical exfoliation method is very promising to the growth of high quality TMDs like MoS2, WS2, ZrS2, MoSe2, NbSe2, and WSe2 [2]. Along with other 2D materials, MoS2 has been used as a channel material in field effect transistors due to its transition from indirect bandgap to direct bandgap. This bandgap is sufficient to balance the limitations of the zero-bandgap graphene for electronic devices [3]. Moreover, researchers have reported that the threshold voltage of the memory or transistor could be shifted when optical light is illuminated. The memory window of the transistor is modulated using the different wavelengths of light. The phenomenon is primarily based on the application of photo-induced charge carriers. The light induced threshold voltage shifting with bias voltage confirms the possibility to use photo-generated carriers in in-memory light sensors [4].MOS capacitor was fabricated on Si substrate. A 3 nm thick Al2O3 tunneling layer was deposited by plasma enhanced atomic layer deposition (PE-ALD) at 2500 C. Next, a 70-nm thick MoS2 was deposited using the drop casting. A 7 nm thick Al2O3 was again deposited by PE-ALD as a blocking layer at 2500 C. Finally, 50 nm thick Al film was deposited as a top electrode by sputter using a metal shadow mask. The complete fabrication process of the Al/Al2O3/MoS2/Al2O3/P+-Si MOS devices is shown in Fig. 1.The charging properties of the device were measured, as shown in Fig. 2. The device was operated with sweep voltage of +6/-6 V in forward and reverse conditions, as depicted in Fig. 2 (a). The memory window of the device was examined of about 2.5 V. To confirm the long-term stability, the endurance of the device was measured with the programming and erasing voltages of +6/-6 and -6/+6, respectively (Fig 2 (b)). The device demonstrates good endurance for more than 1000 cycles. We also measured the memory window of the device by increasing the sweeping voltages from +4/-4 to +8/-8 during the programming and erasing conditions, which is depicted in Fig. 2(c). The memory window was increased with increasing the sweeping voltages and the noted maximum was about 5 V at +8/-8 V, suggesting superb charge trapping effect. The optical characteristics of the device were examined by illuminating light onto the device. The optical features of the device were determined using the different wavelength of light from 600 nm to 450 nm. When the device was in dark, the memory window was observed to be about 2.5 V. When the optical light (Intensity: 2 mW/cm2) was turned on for 1 s onto the device with 600 nm wavelength during the programming of +6/-6 V, the threshold voltage was increased from 2.5 V to 3.2 V. We further checked the effect of different wavelengths of light from 550 nm to 450 nm with the interval of 50 nm using the same programming voltage, same intensity, and same illumination time. We got an enormous shift in threshold voltage from 3.2 V to about 5 V. The large threshold voltage shift confirms the MoS2 layer traps more electrons for 450 nm wavelength.The MOS device displays a decent memory window (2.5 V) and good endurance (1000cycles) without any humiliation. The MOS device reveals a great memory window when the operating voltage was varied from 4/-4 to 8/-8. The memory window of the MOS device was increased from 2.5 V to about 5 V when optical light with different wavelengths was induced onto the device. These decent features of the device make it highly applicable for storage and in memory light sensing in the near future.References Konstantatos et al, “Hybrid graphene–quantum dot phototransistors with ultrahigh gain. Nature Nanotech”, vol 7, pp. 363–368, 2012Shanmugam et al, “A Review of the Synthesis, Properties, and Applications of 2D Materials. Particle, vol 39, 2200031, 2022F. Mak et al, “Atomically Thin MoS2: A New Direct-Gap Semiconductor”, Phys. Rev. Lett, vol 105, 136805, 2010.Hu et al, “Dependency of organic phototransistor properties on the dielectric layers”, Appl. Phys. Lett. vol 89, 072108, 2006 Fig. 1. Schematic of complete fabrication process of the MOS deviceFig. 2. (a) C-V curve of the device. (b) Endurance of the device. (c) C-V curve with the sweeping voltages of +4/-4 to +8/-8 in the programming and erasing. Figure 1

Growth and vacuum ultraviolet characteristics of magnesium fluoride protected aluminum reflective coatings prepared by electron-beam evaporation

Magnesium fluoride protected aluminum (MgF2/Al) vacuum ultraviolet (VUV) coatings were prepared by electron-beam evaporation process with excellent controllability. The effects of preparation conditions on the microstructure and VUV reflectivity of MgF2/Al coatings were studied. The experimental results showed that crystalline MgF2 layers were grown at 250 °C and were beneficial to improving the VUV reflectivity of mirrors. However, the temperature rise led to the growth of Al nanowires on the mirror surfaces with relatively thick Al films, and the Al nanowires continued to grow with the extension of baking time. It was found that reducing the thickness of Al film could not only suppress the generation of Al nanowires, but also decrease the surface roughness of mirrors. The negative impacts of Al nanowires on the performance of VUV mirror were analyzed. The physical mechanism of suppressing the generation of Al nanowires was discussed. This work would provide a reference for the growth of VUV grade Al film by electron-beam evaporation, and also valuable experience for the study of thermal stability and surface roughness control of Al-based VUV coatings.

Diagnosing X-pinch and cylindrical wire array Z-pinch with an all-optical framing camera on the “QiangGuang-I” facility

An all-optical framing camera has been established on the 1-MA “QiangGuang-I” facility for investigating the imploding dynamics of the X-pinch and the cylindrical wire array Z-pinch. This camera measures the photons flux distribution by detecting the photon-induced refractive index changes in the InP semiconductor with a probe laser beam. The InP-based imaging sensor with a 100 nm thick Al film and a 10 μm thick gold transmission grating of 20 μm spatial period was fabricated based on the lithography method. Two sequential frames with 1.5 ns time resolution and 13 ns inter frame separation time can be recorded in one single shot. Framing images indicating the plasma generation and dissipation of the X-pinch and the cylindrical wire array Z-pinch have been obtained, showing that this framing camera is a promising technique for investigating the mechanisms of the Z-pinch and other high energy density physics.

Low loss single crystalline aluminum films obtained on Si (1 1 1) through interfacial modulation

Single-crystalline aluminum (Al) films were grown on Si (111) substrates by molecular beam epitaxy to study the growth mechanism and demonstrate the role of interface modulation. The effect of the growth temperature is demonstrated on a series of 10 nm thick Al films, showing improved surface wetting by lowering growth temperatures. Post-annealing is proved to be effective on the elimination of twins within a thin Al film. The best twin-free Al film in this study was achieved by a two-step method, with a roughness less than 0.2 nm. The transmission electron microscopy results reveal the atomically sharp Al/Si interface. The ellipsometry measurements show an overall reduction on ε2 compared to the Palik’s values, and especially a more than 40% reduction is achieved in the UV regions.

Film thickness and architecture effects in biaxially strained polymer supported Al/Mo bilayers

Multilayers on polymer substrates are of interest for several technical applications. However, the true understanding of how one layer affects the other during mechanical loading is still unknown. In order to address this lack of knowledge, single and bilayer thin films of Al (250, 125, and 75 nm) and Mo (50 nm) were sputter deposited onto polyimide (PI, 50 µm) and biaxially strained in-situ with X-ray diffraction. The technique allows for the simultaneous measurement of lattice strains of Al and Mo to correlate the observed mechanical behavior of both materials. Using the evaluated film stress and full width at half maximum evolutions, four domains of mechanical behavior are identified. The presence of the domains in either Al or Mo depends on the thickness of the Al films and, more importantly, the bilayer architecture (Al/Mo/PI vs. Mo/Al/PI). Domain boundaries in Al were found to correspond well with domain boundaries in the Mo, such as necking in Al with fracture in Mo or simultaneous fracture in both materials. The maximum stress achieved in the Mo layers during biaxial straining was also found to highly depend on the architecture and depend less on the Al film thickness. Results will demonstrate that the film architecture (layer order) is more important than the film thickness to enhance fracture resistance.

Coexistence of superconductivity and spin-splitting fields in superconductor/ferromagnetic insulator bilayers of arbitrary thickness

Ferromagnetic insulators (FI) can induce a strong exchange field in an adjacent superconductor (S) via the magnetic proximity effect. This manifests as spin splitting of the BCS density of states of the superconductor, an important ingredient for numerous superconducting spintronics applications and the realization of Majorana fermions. A crucial parameter that determines the magnitude of the induced spin splitting in FI/S bilayers is the thickness of the S layer d: In very thin samples, the superconductivity is suppressed by the strong magnetism. By contrast, in very thick samples, the spin splitting is absent at distances away from the interface. In this work, we calculate the density of states and critical exchange field of FI/S bilayers of arbitrary thickness. From here, we determine the range of parameters of interest for applications, where the exchange field and superconductivity coexist. We show that for d>3.0ξs, the paramagnetic phase transition is always of the second order, in contrast to the first-order transition in thinner samples at low temperatures. Here ξs is the superconducting coherence length. Finally, we compare our theory with the tunneling spectroscopy measurements in several EuS/Al/AlOx/Al samples. If the Al film in contact with the EuS is thinner than a certain critical value, we do not observe superconductivity, whereas, in thicker samples, we find evidence of a first-order phase transition induced by an external field. The complete transition is preceded by a regime in which normal and superconducting regions coexist. We attribute this mixed phase to inhomogeneities of the Al film thickness and the presence of superparamagnetic grains at the EuS/Al interface with different switching fields. The steplike evolution of the tunnel-barrier magnetoresistance supports this assumption. Our results demonstrate on the one hand, the important role of the S layer thickness, which is particularly relevant for the fabrication of high-quality samples suitable for applications. On the other hand, the agreement between theory and experiment demonstrates the accuracy of our theory, which, originally developed for homogeneous situations, is generalized to highly inhomogeneous systems.Received 6 January 2021Revised 16 March 2021Accepted 28 April 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023131Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasProximity effectSpintronicsSuperconductivityCondensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Picosecond ultrasonics with miniaturized semiconductor lasers

There is a great desire to extend ultrasonic techniques to the imaging and characterization of nanoobjects. This can be achieved by picosecond ultrasonics, where by using ultrafast lasers it is possible to generate and detect acoustic waves with frequencies up to terahertz and wavelengths down to nanometers. In our work we present a picosecond ultrasonics setup based on miniaturized mode-locked semiconductor lasers, whose performance allows us to obtain the necessary power, pulse duration and repetition rate. Using such a laser, we measure the ultrasonic echo signal with picosecond resolution in a 112 nm thick Al film deposited on a semiconductor substrate. We show that the obtained signal is as good as the signal obtained with a standard bulky mode-locked Ti-Sa laser. The experiments pave the way for designing integrated portable picosecond ultrasonic setups on the basis of miniaturized semiconductor lasers.

Topological Transition in a 3 nm Thick Al Film Grown by Molecular Beam Epitaxy

We have performed detailed transport measurements on a 3 nm thick (as-grown) Al film on GaAs prepared by molecular beam epitaxy (MBE). Such an epitaxial film grown on a GaAs substrate shows the Berezinskii-Kosterlitz-Thouless (BKT) transition, a topological transition in two dimensions. Our experimental data shows that the MBE-grown Al nanofilm is an ideal system for probing interesting physical phenomena such as the BKT transition and superconductivity. The increased superconductor transition temperature (~2.4 K) compared to that of bulk Al (1.2 K), together with the ultrathin film quality, may be advantageous for future superconductor-based quantum devices and quantum information technology.

Sputter film deposition to fabricate thick oxide films with extremely smooth surface suitable for room-temperature bonding

Energy-treatment sputtering (ETS), sputter film deposition performed during film growth with slight etching of the advancing surface with an ion-beam, can be applied to deposit SiO2 films on glass wafers. The surface roughness Sa of 700 nm thick ETS-SiO2 film is 0.17 nm, which is remarkably lower than that of films deposited using conventional magnetron sputtering (Sa = 0.78 nm). Moreover, the Sa of 700 nm thick ETS-SiO2 film deposited on thick Al film having a rough surface of Sa = 2.95 nm is only 0.24 nm. Bonding performance using ETS-SiO2 films is identical to that using SiO2 films with smooth surfaces obtained after chemical mechanical polishing.

Optimization of Super-Hydrophobic Property by Two-Step Surface Modification

The super-hydrophobic surface can be used in anti-pollution, self-cleaning, and anti-corrosive properties. Two-step surface treatment process on Al-coated glass was conducted by surface etching using potassium hydroxide (KOH) and surface coating using lauric acid for super-hydrophobic surface. The KOH-etched Al surface (1st etching) was changed to a hydrophilic property with a water contact angle (WCA) of 68° to 48°. On the other hand, the WCA of the etched Al surface was changed to about 153° with super-hydrophobic property when the lauric acid coating (2nd coating) was applied on the KOH-etched Al surface for 30 minutes. We found that the hydrophobicity of Al surface was related to the roughness by surface modification as well as the Al film thickness by sputtering method.

Low-Loss Characteristics of Metal-Foil-Based Passive Components by Surface-Activated Bonding Technologies

Low-loss passive components for RF signals compatible with the on-wafer process are essential for realizing integrated circuits with high-frequency and high-power operations. We successfully fabricate thick-metal-film-based coplanar waveguides (CPWs) and inductors (INDs) by directly bonding a 17- $\mu \text{m}$ -thick Al foil to a sapphire (0001) substrate and wet etching. The surface-activated bonding (SAB) technologies at room temperature are used. RF characteristics of the foil-based passive components are compared with those of components made of 1- $\mu \text{m}$ -thick evaporated Al layers. We obtain a better insertion loss and a higher ${Q}$ -factor for foil-based CPWs and INDs, respectively. The measured characteristics are compared with those obtained by an analysis based on the equivalent circuit scheme. Impacts of side etching of foils and surface oxidation on their characteristics are observed. Characteristics of virtual components made of 1- $\mu \text{m}$ -thick Al foils, i.e., 1- $\mu \text{m}$ -thick Al films with the same resistivity as that of foils, are analytically investigated.

Far‐ and Deep‐Ultraviolet Surface Plasmon Resonance Sensor

Plasmonics in the UV region has been widely focused because of the higher energy and the abundant electronic resonances compared to the conventional visible plasmonics. Recently, we have investigated the surface plasmon resonance (SPR) properties of the Al film, aiming for the application as refractive index sensors. Utilizing the UV lights, we expect three advantages: high sensitivity, material selectivity, and surface selectivity. By using an original attenuated total reflectance spectroscopic instrument, Al-SPR angle and wavelength were investigated with changing environments on the Al film. Al film thickness and materials of prisms on which Al was evaporated were also important factors for the SPR properties. By optimizing the conditions, the Al film worked as a sensor both in air and in liquids. In addition, our established system expands the plasmonics into an even higher energy region than 200 nm, while the UV-plasmonics have been studied in the wavelength region longer than 200 nm.

Effects of content and thickness on the microstructure, optical and electrical properties of oxidized Al-doped ZnO films

It is crucial to control conductivity and optical transmittance of Al doped ZnO (AZO) thin films in application of optoelectronic materials. In this paper, AZO thin films are prepared by oxidizing thermal evaporated Zn-Al thin films in open air. Then, the effects of Al contents and film thicknesses on microstructure, optical and electrical properties of the AZO films are studied. The results show that the optical and electrical properties of the AZO films are affected by the Al content and thickness changing. The Haacke figure of merit reaches 2.91×10-4 Ω-1. The film surface morphology is changed by the Al content. Nanowire is formed when the Al content is 9.58%. The Al2O3 phase appears with an excessive Al content. The transmittance of the AZO films is less than 25% when the Al content is more than 9.58%. The grain size first increases and then decreases with the increase of film thickness when the Al contents remain at 2%. Within the limits of available transmittance, the sheet resistance and transmittance of the AZO thin film decrease exponentially with the film thickness increasing.

Ni-Al-Ti Ohmic Contacts with Preserved Form Factor and Few 10<sup>-</sup> <sup>4</sup> Ωcm<sup>2</sup> Specific Resistance on 0.1-1 Ωcm p-Type 4H-SiC

This study shows that a thin Ni film on Al/Ti/4H-SiC metal pads allows to preserve the pad form factor during a 1000 °C/2 min treatment, provided that the Al and Ti film thicknesses are sufficiently thin. Moreover, by reducing the Al to Ti thickness ratio, droplet formation in the contact area is avoided and a mirror-like appearance is obtained. This optimal contact morphology corresponds to a specific contact resistance of few 10-4 Ωcm2 at room temperature on p-type 4H-SiC with resistivity in the range 0.1 – 1 Ωcm.

Improved C-V Hysteresis and Two-States Characteristics in MIS (p) Structure with Elongated Thin Metal Gate

Enhancement of transient current has been investigated in MIS (p) structure by thinning part of gate metal thickness [1]. To acquire further understanding of the transient phenomenon in C-V characteristic, we use forward and backward C-V hysteresis profiling to observe the delayed carriers’ response due to thin metal gate induced voltage drop. By forward and backward voltage sweeping, two capacitance states can be easily induced and the hysteresis of the C-V characteristic can become noticeable. The schematic cross section and top view of our device is shown in Fig. 1. The MIS structure with elongated thin metal gate (ETGMIS) is formed by thin Al film circles in concentric with the thick Al gate, in which the inner radius R1 is 122.5um and the outer radius R2 is 142.5um, and the thickness of thick and thin Al films are around 250nm and 10nm, respectively. For comparison, the conventional MIS is fabricated by thick Al gate with a radius R2 of 142.5um. From the ratio of accumulation capacitance shown in Fig. 2, it can be inferred the elongated part of metal gate does not contribute to the accumulation capacitance, which indicates the existing resistivity of the thin metal, and therefore induces transient two-states characteristics as mentioned in previous study [1]. Fig. 3 shows the C-V profiling with various frequencies for the ETGMIS and the MIS. It can be seen though C-V curve of the MIS also splits during forward and backward voltage sweeping, two-states characteristic of capacitance is noticeably enlarged by the ETGMIS, especially during low frequency measurement. It’s worth mentioning that due to the nature of carriers’ generation and recombination, the capacitance of backward voltage sweeping will generally be higher than forward voltage sweeping at the same voltage. In addition, the capacitance during forward and backward voltage sweeping starts to split after the device turn into inversion, which implies the two-states phenomenon is attributed to the minority carriers. To further illustrate this enhanced two-states phenomenon, several pulse cycles are imposed to measure the difference of capacitance at a defined read voltage, called ”capacitance window”. The applied pulse voltages are +2V and -2V to initiate the transient two–states characteristic, and the measurement is practiced at a frequency of 1kHz. Moreover, in order to obtain significant capacitance window, we define a parameter ”capacitance window ratio”, which is defined as (Cbackward-Cforward)/Cbackward in the C-V curve. Follow the above definition, we choose the voltage that owns the biggest capacitance window ratio as the defined read voltage after the pulse, which is about -0.2V. As depicted in Fig. 4, the capacitance window in ETGMIS can be about an order larger than in MIS. From another point of view, it can be seen in Fig. 5 that there are obvious voltage shift between forward and backward C-V sweeps in ETGMIS than in MIS with the same EOT. To well compare the hysteresis in the C-V curve for both structures, we define the corresponding voltage as the voltage when the capacitance is 10% larger than the minimum capacitance Cmin. Therefore, the hysteresis can be defined as the difference between two corresponding voltages. Apparently, as shown in Table 1, the ETGMIS owns larger hysteresis than the MIS. It is supposed that the elongated part of gate may trigger more minority carriers to late response due to its resistive nature. Take further consideration into the delayed property, we carry out different pace of the C-V sweeping rate to observe the following hysteresis variation. As shown in Fig. 6, the hysteresis of the ETGMIS decreases as the C-V sweeping rate decreases, while the MIS displays almost insignificant variation of the C-V voltage shift. Consequently, we can reasonably suspect the C-V hysteresis enhancement is mainly attributed to the delayed characteristic of the elongated thin metal. At last, we analyze the variation of the oxide thickness to the hysteresis in ETGMIS. In previous work [1], it is noticed the fringing field is restricted by the surrounded thin metal due to its resistive characteristic. In Fig. 7, when the oxide is thicker, the vertical electric field will become weaker. Hence, the fringing field is therefore restricted more slightly. As a result, this ETGMIS becomes more similar to the MIS; that is, tends to hold a smaller C-V hysteresis, and vice versa. This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-180-MY3 and MOST 106-2221-E-002-196-MY3. [1] Kuan-Hao Tseng, Chien-Shun Liao, and Jenn-Gwo Hwu, IEEE Trans. On Nanotechnology, Vol. 16, No. 6, pp. 1011-1015, Nov. 2017. Figure 1

The Accuracy of Al and Cu Film Thickness Determinations and the Implications for Electron Probe Microanalysis.

The accuracy to which Cu and Al coatings can be determined, and the effect this has on the quantification of the substrate, is investigated. Cu and Al coatings of nominally 5, 10, 15, and 20 nm were sputter coated onto polished Bi using two configurations of coater: One with the film thickness monitor (FTM) sensor colocated with the samples, and one where the sensor is located to one side. The FTM thicknesses are compared against those calculated from measured Cu Lα and Al Kα k-ratios using PENEPMA, GMRFilm, and DTSA-II. Selected samples were also cross-sectioned using focused ion beam. Both systems produced repeatable coatings, the thickest coating being approximately four times the thinnest coating. The side-located FTM sensor indicated thicknesses less than half those of the software modeled results, propagating on to 70% errors in substrate quantification at 5 kV. The colocated FTM sensor produced errors in film thickness and substrate quantification of 10-20%. Over the range of film thicknesses and accelerating voltages modeled both the substrate and coating k-ratios can be approximated by linear trends as functions of film thickness. The Al films were found to have a reduced density of ~2 g/cm2.

Gold nanoparticle plasmon resonance in near-field coupled Au NPs layer/Al film nanostructure: Dependence on metal film thickness

We study the effects of coupling between plasmonic metal nanoparticles and a thin metal film by using light extinction spectroscopy. A planar monolayer of gold nanoparticles located near an aluminum thin film (thicknesses within the range of 0–62 nm) was used to analyze the coupling between the monolayer and the thin metal film. SPR peak area increase for polymer coated Au NPs, non-monotonical behavior of the peak area for bare Au NPs, as well as red shift and broadening of SPR at the increase of the Al film thickness have been observed. These effects are rationalized as a result of coupling of the layer of Au NPs with Al film through the field of localized surface plasmons in Au NPs that causes the excitation of collective plasmonic gap mode in the nanostructure. An additional mechanism for bare Au NPs is the non-radiative damping of SPR that is caused by the electrical contact between metal NPs and film.

Surface Plasmon Resonance Sensing Characteristics of Thin Aluminum Films in Aqueous Solution

Surface plasmon resonance (SPR) sensors, utilizing thin aluminum (Al) films, are reported. The sensor designs are based in the novel optical trapezoidal prism chip, made from either polymer or BK7 glass. Optimum Al-film thickness was determined to 20 ± 5 nm, and deposited by thermal e-beam evaporation and rf-magnetron sputtering. Upon contact to air, and in de-ionized aqueous solution, a self-limited surface oxide layer forms. Determined by ellipsometric recordings, its thickness grows from 4.6 ± 0.6 nm in air to 12.3 ± 2 nm in the wet interface. A single SP-resonance applies to the dry interface. Unusual spectral broadening, due to the presence of multiple resonances, is predicted and experimentally verified at the wet interface. The Al-oxide adlayer is beneficial in the angular interrogation, where the instrumental response approaches the one exhibited by the noble metals. Under wavelength interrogation conditions, the presence of the Al-oxide adlayer causes severe degradation of the sensor response. Both, angular and wavelength interrogation at the metal-aqueous solution interface were exploited experimentally and compared with theoretical predictions regarding the sensing features of gold (Au), copper (Cu), and silver (Ag) metal films as well. Limitations, cost aspects and alternative routes to overcome degradation and the loss of SPR-activity in the presence of ionic phosphate buffered saline aqueous solutions are outlined for Al-films.