Recently, ternary perovskite oxides have attracted great attention as alternative transparent conducting oxides (TCOs) because their structures are compatible with many other perovskite oxides that allow devices to be fabricated comprised entirely of perovskite oxides. Among these perovskite oxides, BaSnO3 has attracted considerable attention as a promising TCO because of its high mobility at room temperature (~320 cm2V-1s-1 in bulk single crystals and ~100 cm2V-1s-1 in epitaxial thin films) and high temperature stability in oxygen atmospheres compared to other TCOs, such as In2O3, ZnO, and SnO2. The electrical and optical properties of the BaSnO3 can be improved by either inducing oxygen vacancies or cationic doping. We have grown epitaxial La-doped BaSnO3 (LBSO) thin films on MgO (001) substrates by pulsed laser deposition using a La0.04Ba0.96SnO3 target, and investigated their structural, electrical, and optical properties as a function of the oxygen pressure during deposition. The permittivity of the LBSO films can be modified as a function of the oxygen pressure during deposition allowing tuning of their epsilon-near-zero (ENZ) wavelength from 2.2 μm to 7 μm. We will present details of the deposition conditions on the properties of LBSO films and the ability to tune the permittivity in this infrared range.
The formation of laser-induced oxide layers on titanium surfaces has been widely investigated for coloring and marking applications. Complex titanium-based oxides exhibiting multiple phases can be achieved through laser patterning. Laser processing offers several advantages in that discrete areas can be modified leading to patterns with differing optical and electronic properties. To date, most research has focused on the formation and thickness control of TiO2, a wide bandgap semiconductor (~ 3.2 eV), as a means to control coloration. However, for many applications, including photodetectors and photocatalysts, a semiconductor oxide with a narrow bandgap (< 1 eV) is preferred to allow for strong absorption into the mid-IR. Other oxides and sub-oxides such as Ti2O3 have been identified as a byproduct of laser surface processing. In addition to its narrow bandgap, bulk Ti2O3 offers the unique property of having a semiconductor-metal transition at around 150 – 200°C where resistivity switches over an order of magnitude. Because of these properties, we investigate the optimization of laser processing conditions using picosecond and femtosecond laser irradiation to form Ti2O3. The effect of laser fluence, scan speed, pulse frequency, and sample chamber pressure will be discussed. Additionally, Ti2O3 thin films were grown via pulsed laser deposition to study structural phase purity, where the effect of growth temperature on optical and electrical properties is explored.
Plasmonic materials have attracted great attention due to their ability to enhance light-matter interactions. Noble metals such as Au and Ag have been well studied as materials for plasmonic devices. However, these metals are not suitable for mid infrared (IR) plasmonic applications due to their relatively large optical losses, which are detrimental to device efficiency. Metal oxides, on the other hand, have been proposed for low loss metallic components in the mid IR because they can provide a tunable carrier density by varying the concentration of dopants or defects (oxygen vacancies). The epsilon-near zero wavelength of the real part of the dielectric permittivity of these metal oxides, for example, can easily be tuned from 1.5 μm to 4 μm by adjusting doping or defect levels. Optical losses in devices made from these metal oxide materials generally exhibit lower losses than those obtained with conventional metals. We have investigated laser processing techniques for synthesizing several types of metal oxides such as indium tin oxide and phase change materials such as VO2. First, pulsed laser deposition was used to grow these oxide thin films. Second, an ultrafast laser was used to spatially pattern the thin films via a direct laser interference patterning (DLIP) configuration while simultaneously producing laser induced periodic surface structures (LIPSS) resulting in a uniaxial surface morphology. We will present details of the laser processing conditions on surface morphology, electrical, and optical properties of these laser processed metal oxide films.
The use of laser induced forward transfer (LIFT) techniques for printing materials for sensor and electronics applications is growing as additive manufacturing expands into the fabrication of functional structures. LIFT is capable of achieving high speed/throughput, high-resolution patterns of a wide range of materials over many types of substrates for applications in flexible-hybrid electronics. In many LIFT applications, the use of a sacrificial or laser-absorbing donor layer is required despite the fact that it can only be used once. This is because the various types of release layers commonly in use with LIFT are completely vaporized when illuminated with a laser pulse. A better solution would be to employ a reusable laserabsorbing layer to which the transferable ink or material is attached and then released by a laser pulse without damage to the absorbing layer, therefore allowing its repeated use in subsequent transfers. In this work, we describe the use of two types of reusable laser-absorbing layers for LIFT. One is based on an elastomeric donor layer made from poly(dimethylsiloxane) or PDMS, while the other is based on a ceramic thin film comprised of indium tin oxide (ITO). These release layers have been used at NRL to transfer a wide range of materials including fluids, nanoinks, nanowires and metal foils of varying size and thickness. We will present examples of both PDMS and ITO as donor layers for LIFT and their reusability for laser printing of distinct materials ranging from fluids to solids.
The development of rapid prototyping techniques for the fabrication of microelectronic structures has seen rapid growth over the past decade. In particular, laser-induced forward transfer (LIFT) is a non-lithographic direct-write technique that offers the advantages of high speed / throughput, high resolution, materials versatility, and substrate compatibility. Because of the high degree of control over size and shape of printed material, the development of a wide range of microelectronic components, including interconnects, antennas, and sensors, has become possible using LIFT. In this paper, we explore the use of LIFT to print various 3D microstructures including high aspect ratio micro pillars using high viscosity Ag nanopastes. In addition, we demonstrate the fabrication of interconnects via LIFT on RF switches that, after printing and subsequent curing, perform similarly to an analogous wire-bonded device.
The control of light-matter interaction through the use of subwavelength structures known as metamaterials has facilitated
the ability to control electromagnetic radiation in ways not previously achievable. A plethora of passive metamaterials as
well as examples of active or tunable metamaterials have been realized in recent years. However, the development of
tunable metamaterials is still met with challenges due to lack of materials choices. To this end, materials that exhibit a
metal-insulator transition are being explored as the active element for future metamaterials because of their characteristic
abrupt change in electrical conductivity across their phase transition. The fast switching times (▵t < 100 fs) and a change
in resistivity of four orders or more make vanadium dioxide (VO2) an ideal candidate for active metamaterials. It is known
that the properties associated with thin film metal-insulator transition materials are strongly dependent on the growth
conditions. For this work, we have studied how growth conditions (such as gas partial pressure) influence the metalinsulator
transition in VO2 thin films made by pulsed laser deposition. In addition, strain engineering during the growth
process has been investigated as a method to tune the metal-insulator transition temperature. Examples of both the optical
and electrical transient dynamics facilitating the metal-insulator transition will be presented together with specific
examples of thin film metamaterial devices.
Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 68°C, at which point its electrical conductivity changes by several orders of magnitude. This extremely fast transition (Δt < 100 fs) can be induced thermally, mechanically, electrically, or optically. The combination of fast switching times and response to a broad range of external stimuli make VO2 an ideal material for a variety of novel devices and sensors. While the MIT in VO2 has been exploited for a variety of microwave/terahertz applications (i.e. tunable filters and modulators), very few devices exploiting the fast switching time of VO2 have been reported. The electrical properties of thin film VO2 (conductivity, carrier concentration, switching speed, etc.) are highly dependent on growth and post-processing conditions. The optimization of these conditions is therefore critical to the design and fabrication of VO2 devices. This paper will report the effects of various pulsed laser deposition (PLD) growth conditions on the metal-insulator transition in thin film VO2. In particular, we report the effect of PLD growth conditions on the stress/strain state of the VO2 layer, and the subsequent change in electrical properties. Finally, results from fabricated VO2 devices (THz emitters and THz modulators) will be presented.
Additive manufacturing techniques such as 3D printing are able to generate reproductions of a part in free space without the use of molds; however, the objects produced lack electrical functionality from an applications perspective. At the same time, techniques such as inkjet and laser direct-write (LDW) can be used to print electronic components and connections onto already existing objects, but are not capable of generating a full object on their own. The approach missing to date is the combination of 3D printing processes with direct-write of electronic circuits. Among the numerous direct write techniques available, LDW offers unique advantages and capabilities given its compatibility with a wide range of materials, surface chemistries and surface morphologies. The Naval Research Laboratory (NRL) has developed various LDW processes ranging from the non-phase transformative direct printing of complex suspensions or inks to lase-and-place for embedding entire semiconductor devices. These processes have been demonstrated in digital manufacturing of a wide variety of microelectronic elements ranging from circuit components such as electrical interconnects and passives to antennas, sensors, actuators and power sources. At NRL we are investigating the combination of LDW with 3D printing to demonstrate the digital fabrication of functional parts, such as 3D circuits. Merging these techniques will make possible the development of a new generation of structures capable of detecting, processing, communicating and interacting with their surroundings in ways never imagined before. This paper shows the latest results achieved at NRL in this area, describing the various approaches developed for generating 3D printed electronics with LDW.
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