Resonant infrared laser deposition of polymer-nanocomposite materials for optoelectronic applications

Polymers find a number of potentially useful applications in optoelectronic devices. These include both active layers, such as light-emitting polymers and hole-transport layers, and passive layers, such as polymer barrier coatings and light-management films. This paper reports the experimental results for polymer films deposited by resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) and resonant infrared pulsed laser deposition (RIR-PLD) for commercial optoelectronic device applications. In particular, light-management films, such as anti-reflection coatings, require refractive-index engineering of a material. However, refractive indices of polymers fall within a relatively narrow range, leading to major efforts to develop both low- and high-refractive-index polymers. Polymer nanocomposites can expand the range of refractive indices by incorporating low- or high-refractive-index nanoscale materials. RIR-MAPLE is an excellent technique for depositing polymer-nanocomposite films in multilayer structures, which are essential to light-management coatings. In this paper, we report our efforts to engineer the refractive index of a barrier polymer by combining RIR-MAPLE of nanomaterials (for example, high refractive-index TiO₂ nanoparticles) and RIR-PLD of host polymer. In addition, we report on the properties of organic and polymer films deposited by RIR-MAPLE and/or RIR-PLD, such as Alq₃ [tris(8-hydroxyquinoline) aluminum] and PEDOT:PSS [poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)]. Finally, the challenges and potential for commercializing RIR-MAPLE/PLD, such as industrial scale-up issues, are discussed.     

An experimental investigation of inhomogeneities in resonant infrared matrix-assisted pulsed-laser deposited thin polymer films

Thin polymer films are deposited using matrix-assisted pulsed-laser evaporation and subsequently are characterized by scanning electron and atomic force microscopies. An Er : YAG laser (2937 nm, 350 μs) is used as a light source and the effect of the energy density supplied by the laser on the morphology of the deposited films is investigated. It is found that the appearance of undesirable non-uniform morphological features arises from either poor solubility of the guest molecules or insufficient energy density provided by the laser to vaporize the entire ejected volume. In addition, the surface roughness of two guest–host systems is found to depend linearly on the polymer concentration. These results allow us to better understand earlier work in the field and to establish a framework by which MAPLE films may be improved.     

Pulsed laser deposition of lead-zirconate-titanate thin films and multilayered heterostructures

There is an increasing interest in lead-zirconate-titanate (PZT) based ferroelectric thin film and devices in recent years. Pulsed laser deposition (PLD) technique has been demonstrated to be a versatile and successful tool for the deposition of epitaxial multi-component metal oxide films and heterostructures. This review presents a reasonable understanding of the relationship between PLD processing and composition, crystal structure and orientation of PZT ferroelectric thin films, and heterostructures. Processing-related issues from PLD of PZT thin films and material-integration strategies developed to fabrication of highly oriented or epitaxial PZT thin film based capacitors with excellent ferroelectric properties are discussed in detail. PACS 81.15.Fg; 68.55.Jk; 77.22.Ej     

Aryltriazene photopolymer thin films as sacrificial release layers for laser-assisted forward transfer systems: study of photoablative decomposition and transfer behavior

Thin films of a tailor-made photodecomposible aryltriazene polymer were applied in a modified laser-induced forward transfer (LIFT) process as sacrificial release layers. The photopolymer film acts as an intermediate energy-absorbing dynamic release layer (DRL) that decomposes efficiently into small volatile fragments upon UV laser irradiation. A fast-expanding pressure jet is generated which is used to propel an overlying transfer material from the source target onto a receiver. This DRL-assisted laser direct-write process allows the precise deposition of intact material pixels with micrometer resolution and by single laser pulses. Triazene-based photopolymer DRL donor systems were studied to derive optimum conditions for film thickness and laser fluences necessary for a defined transfer process at the emission wavelength of a XeCl excimer laser (308 nm). Photoablation, surface detachment, delamination and transfer behavior of aryltriazene polymer films with a thickness from 25 nm to ∼400 nm were investigated in order to improve the process control parameters for the fabrication of functional thin-film devices of microdeposited heat- and UV-sensitive materials.     

Resonant infrared pulsed laser deposition of thin biodegradable polymer films

Thin films of the biodegradable polymer poly(DL-lactide-co-glycolide) (PLGA) were deposited using resonant infrared pulsed laser deposition (RIR-PLD). The output of a free-electron laser was focused onto a solid target of the polymer, and the films were deposited using 2.90 (resonant with O-H stretch) and 3.40 (C-H) μm light at macropulse fluences of 7.8 and 6.7 J/cm², respectively. Under these conditions, a 0.5-μm thick film can be grown in less than 5 min. Film structure was determined from infrared absorbance measurements and gel permeation chromatography (GPC). While the infrared absorbance spectrum of the films is nearly identical with that of the native polymer, the average molecular weight of the films is a little less than half that of the starting material. Potential strategies for defeating this mass change are discussed. Received: 22 August 2001 / Accepted: 23 August 2001 / Published online: 17 October 2001     

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic,biological and nanoparticle thin films: a review

The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, …) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.     

Resonant infrared matrix-assisted pulsed laser evaporation of TiO2 nanoparticle films

The successful development of flexible, high performance thin films that are competitive with silicon-based technology will likely require fabricating films of hybrid materials that incorporate nanomaterials, glasses, ceramics, polymers, and thin films. Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is an ideal method for depositing organic materials and nanoparticles with minimal photochemical or photothermal damage to the deposited material. Furthermore, there are many nonhazardous solvents containing chemical functional groups with infrared absorption bands that are accessible using IR lasers. We report here results of recent work in which RIR-MAPLE has been employed successfully to deposit thin films of TiO₂ nanoparticles on Si substrates. Using an Er:YAG laser (λ=2.94 μm), we investigated a variety of MAPLE matrices containing –OH moieties, including water and all four isomers of butyl alcohol. The alcohol isomers are shown to provide effective and relatively nontoxic solvents for use in the RIR-MAPLE process. In addition, we examine the effects of varying concentration and laser fluence on film roughness and surface coverage.     

Characterization of polymer thin films obtained by pulsed laser deposition

The development of laser techniques for the deposition of polymer and biomaterial thin films on solid surfaces in a controlled manner has attracted great attention during the last few years. Here we report the deposition of thin polymer films, namely Polyepichlorhydrin by pulsed laser deposition. Polyepichlorhydrin polymer was deposited on flat substrate (i.e. silicon) using an NdYAG laser (266 nm, 5 ns pulse duration and 10 Hz repetition rate).The obtained thin films have been characterized by atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and spectroscopic ellipsometry.It was found that for laser fluences up to 1.5 J/cm² the chemical structure of the deposited polyepichlorhydrin polymer thin layers resembles to the native polymer, whilst by increasing the laser fluence above 1.5 J/cm² the polyepichlorohydrin films present deviations from the bulk polymer.Morphological investigations (atomic force microscopy and scanning electron microscopy) reveal continuous polyepichlorhydrin thin films for a relatively narrow range of fluences (1-1.5 J/cm²).The wavelength dependence of the refractive index and extinction coefficient was determined by ellipsometry studies which lead to new insights about the material.The obtained results indicate that pulsed laser deposition method is potentially useful for the fabrication of polymer thin films to be used in applications including electronics, microsensor or bioengineering industries.     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Effect of gamma radiation on the electrical properties of Polyaniline/silicon carbide heterojunctions

Polyaniline thin films have been deposited by a very simple technique on p-type Silicon Carbide (SiC) substrates to fabricate heterojunctions devices with good electrical properties. In this work two heterojunctions devices of Polyaniline (PANI) on p-type 4H–SiC and 6H–SiC substrates were electrically characterized using current- voltage (I-V) in the temperature range 20–430 K Capacitance–frequency (C-f) measurements. Furthermore, impedance and capacitance measurements are carried out to study the effect of gamma irradiation on these devices. Additionally, we demonstrate not only the ease of fabrication of PANI/p-SiC heterostructures, but also we show strong indication that these heterostructures have potential applications as sensors of gamma irradiation.     

Impurity effects on the laser-induced crystallization of thin amorphous silicon film on glass substrate

Crystallization of 100 nm thick amorphous silicon (a-Si) films deposited on glass substrates was carried out using a dual-green-laser method. Depending on a-Si deposition method, either low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD), the density of impurities such as Al, K, and Na within the a-Si thin films significantly varied. For the high impurity case of LPCVD, grains of 200–300 nm in size were obtained, whereas for the PECVD case a maximum grain size of about 4 μm was achieved, satisfying the requirements for applications in commercial TFT devices. These results confirm that for the use of glass substrates in polycrystallization of a-Si, controlling the impurity density during substrate preparation is critical.     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Properties of ZnO thin films grown on Si substrates in vacuum and oxygen ambient by pulsed laser deposition

Epitaxial ZnO thin films have been synthesized directly on Si(1 1 1) substrates by pulsed laser deposition (PLD) in vacuum. The reflection high-energy electron diffraction (RHEED) indicates that streaky patterns can be clearly observed from the ZnO epilayers prepared at 600 and 650 °C, revealing a two-dimensional (2D) growth mode. While the ZnO thin film deposited in oxygen ambient shows ring RHEED pattern. There is a compressive in-plane stress existing in the ZnO epitaxial film, but a tensile one in the polycrystalline film. Compared with the ZnO epilayer, the ZnO polycrystalline film shows more intense ultraviolet emission (UVE) with a small full width at half maximum (FWHM) of 89 meV. It is suggested that the atomically flat epilayers may be powerfully used as transitive stratums to grow high-quality ZnO films suitable for the fabrication of optoelectronic devices.     

Structural and surface analysis of thin-film ZnTe formed with pulsed-laser deposition

Nanosecond pulses of a Nd:YAG laser have been employed to deposit thin-film zinc telluride (ZnTe) on silicon (Si) and glass without heating these substrates. We present and discuss the structural and surface properties of films deposited at 1064 nm and 532 nm. X-ray diffraction and analysis of the surface roughness with atomic force microscopy reveal that the material texture and surface morphology depend on the ablation laser line used rather than on the substrate. The observations contribute to improved understanding of pulsed-laser deposition and provide tools to optimize the optoelectronic and photonic properties of ZnTe thin-films as well as their incorporation into Si-based technologies in order to fabricate cost-effective and functional optoelectronic devices and all-optical laser digitizer.     

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

Nanosecond laser ablation and deposition of silicon

Nanosecond-pulsed KrF (248 nm, 25 ns) and Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) lasers were used to ablate a polycrystalline Si target in a background pressure of <10⁻⁴ Pa. Si films were deposited on Si and GaAs substrates at room temperature. The surface morphology of the films was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Round droplets from 20 nm to 5 μm were detected on the deposited films. Raman Spectroscopy indicated that the micron-sized droplets were crystalline and the films were amorphous. The dependence of the properties of the films on laser wavelengths and fluence is discussed.     

Star-shaped oligofluorene nanostructured blend materials: controlled micro-patterning and physical characteristics

Star-shaped oligofluorene consists of highly-fluorescent macromolecules of considerable interest for organic electronics. Here, we demonstrate controlled micro-patterning of these organic nanostructured molecules by blending them with custom-synthesized photo-curable aliphatic polymer matrices to facilitate solventless inkjet printing. The printed microstructures are spherical with minimum dimensions of 12 μm diameter and 1 μm height when using a cartridge delivering ∼1 pL droplets. We evaluate the physical characteristics of the printed structures. Photoluminescence studies indicate that the blend materials possess similar fluorescence properties to neat materials in solid films or toluene solution. The fluorescence lifetime consists of two components, respectively 0.68±0.01 ns (τ ₁) and 1.23±0.12 ns (τ ₂). This work demonstrates that inkjet printing of such blends provides an attractive method of handling fluorescent nano-scaled molecules for photonic and optoelectronic applications.     

Solvent effects on the electrical and optical properties of composite carbon nanotube/MEH-PPV films

Optical signals were used to modulate the resistance of single-walled carbon nanotube–polymer composite films for development of optical switching devices. Films were fabricated from nanotubes dispersed in poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) solutions in either tetrahydrofuran (THF) or toluene. MEH-PPV was affected by the solvent choice, with THF causing an absorbance shift to shorter wavelengths. Composite films formed from THF dispersions showed 1–4% resistance decreases when illuminated with a green diode laser. Illumination of toluene-based films showed up to 5.4% resistance decreases, smaller than expected based on the polymer solution absorbance. Fluorescent emission from the polymer in toluene appears to inhibit charge transfer from the polymer to the nanotubes. THF films were unaffected by illumination with a red diode laser, while toluene dispersion films showed resistance decreases up to 1%. Nanotube dispersion and film formation reproducibility, evaluated for both the solvents, showed that the solvent affected film resistance and dispersion stability.     

Structural characterization and optoelectronic properties of GaN thin films on Si(111) substrates using pulsed laser deposition assisted by gas discharge

GaN films have been grown on Si(111) substrates with a thin AlN buffer layer using pulsed laser deposition (PLD) assisted by gas discharge. The crystalline quality, surface morphology and optoelectronic properties of the deposited films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and room-temperature Van der Pauw–Hall measurements. The influence of the deposition temperature in the range 637–1037 K on the crystallinity of GaN films, the laser incident energy in the range 150–250 mJ/pulse on the surface morphology and the optoelectronic properties were systematically studied. The XRD analysis shows that the crystalline quality of the GaN films improves with increasing deposition temperature to 937 K, but further increase of the deposition temperature to 1037 K leads to the degradation of the crystalline quality. AFM results show that the surface roughness of the GaN films can be decreased with increasing laser incident energy to 220 mJ/pulse. Further increase of the laser incident energy to 250 mJ/pulse leads to an increase in the surface roughness. The optoelectronic properties of GaN films were also improved by increasing the laser incident energy to 220 mJ/pulse. GaN films which have a n-type carrier concentration of 1.26×10¹⁷ cm^-3 and a mobility of 158.1 cm²/Vs can be deposited at a substrate temperature of 937 K, a deposition pressure of 20 Pa and a laser incident energy of 220 mJ/pulse. Their room-temperature PL spectra exhibit a strong band-edge emission at 365 nm. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 73.61.Ey; 78.66.Fd     

Mid infrared interband cascade lasers for sensing applications

The growth of Interband Cascade Laser material to cover the wavelength range from 3–4 μm is presented along with the fabrication and characterization of Broad Area (BA) and Ridge Waveguide (RWG) devices based on this material. Pulsed operation slightly below room temperature is observed for both device types, and a strong reduction of threshold currents can be observed in the RWG lasers. Variation of the active Quantum Well width in the epitaxial structures enables laser emission in the 3–4 μm wavelength region.