As laser micromachining is applied to ever smaller structures and more complex materials, the demand for greater control of the laser energy budget, in space and time, grows commensurately. Here we describe materials modification using picosecond resonant laser excitation in the mid-infrared spectral region to create spatially and temporally dense vibrational, rather than electronic, excitation. Examples include ablation of fused silica and machining of crystalline quartz; deposition of functionalized polymers on microstructures, and laser-directed transfer of proteins and nucleotides from a matrix of water ice. The experiments demonstrate that high spatial and temporal density of vibrational excitation can be achieved by ultrafast resonant infrared excitation of selected vibrational modes of these materials. In some cases, resonant infrared materials modification is far more successful than techniques based on ultraviolet excimer lasers. The laser used for most of the experiments was a tunable, high pulse-repetition frequency free-electron laser. However, a comparison of polymer deposition using a conventional nanosecond laser at a wavelength of 2.94 μm shows that the possibility exists for transferring the concept to conventional table-top devices. Mechanistic considerations nevertheless suggest that utlrashort pulses are likely to be more useful than longer pulses for many applications. A figure of merit is proposed for self-consistent comparisons of processing efficiency among different lasers.
It has been known for a long time that many systems, including disordered metals and metallic oxides, undergo a metal/insulator transition (MIT). We have found that the superconducting transition temperature, Tc, of such materials is enhanced in the vicinity of the MIT. We have constructed superconductivity phase diagrams (Tc vs σ, the conductivity) for many materials whose only common feature is their proximity to the MIT and found that they are remarkably similar. These results suggest that there is a common mechanism for the enhancement of superconductivity near the MIT. Following this clue, we developed a simple, heuristic model based on scaling theory near the MIT that accounts for the observed features.
Pulsed laser deposition of complex, chemically sensitive polymers using tunable, picosecond infrared laser excitation has shown great promise for producing films of these materials appropriate for a wide variety of sensor and coating applications. Fourier-transform infrared spectra of the bulk starting polymers and those of the deposited thin films are nearly identical, verifying that the short-range order and chemical functionality of the polymers are preserved during the process. Gel permeation chromatography and mass spectrometry have been used to characterize the polydispersity of the mass distribution; here the results are mixed, with the mass distributions of some poly-mers being preserved while others show significant bond scission. Most recently, we have demonstrated that it is possible to coat cantilever structures with the polymer SFXA; the deposited polymer responds as desired when 'challenged' by appropriate chemicals.
Transparent conducting indium tin oxide (ITO) tin films were grown by pulsed-laser deposition (PLD) on glass and single crystal yttria-stabilized zirconia (YSZ) substrates. The structural, electrical and optical properties of these films were investigated as a function of substrate deposition temperature and background gas pressure. Films were deposited using a KrF excimer laser (248nm, 30 ns FWHM) at a fluence of 1.2 J/cm2. Films were deposited at substrate temperature of 300 degree(s)C in mixed gases (12 mTorr of argon and 1-50 mTorr of oxygen). X-ray diffraction, scanning electron microscopy and atomic force microscopy were used to characterize the structure and morphology of the deposited films. UV/VIS/NIR spectroscopy and Hall effect measurements were used to characterize the optical and electrical properties of the films. ITO films (300 nm thick), deposited by PLD on YSZ at 300 degree(s)C in a gas mixture of 12 mTorr of argon and 5 mTorr of oxygen, exhibit a low electrical resistivity (1.6 x 10-4(Omega) -cm) with a high transparency (~74%) at 550 nm. ITO films deposited by PLD on both glass and YSZ substrates have been used as an anode contact in organic light-emitting diodes. A comparison of the device performance for the two substrates shows that the device fabricated on the ITO/YSZ has a higher external quantum efficiency than that of the device fabricated on the ITO/glass.
Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on glass and on flexible polyethylene teraphthalate (PET) substrates. The structural, electrical and optical properties of these films were investigated as a function of substrate deposition temperature and background gas pressure. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 1.2 J/cm2. Films were deposited at substrate temperatures ranging from 25 degree(s)C to 150 degree(s)C in oxygen pressures ranging from 10 to 60 mTorr. ITO films (280 nm thick), deposited by PLD on PET at 25 degree(s)C and 45 mTorr of oxygen, exhibit a low electrical sheet resistance (20 - 25 (Omega) /sq.) and high transparency (approximately 85%) in the visible range (400-700 nm). We have also used the ITO thin films deposited on both glass and PET substrates by PLD as the anode contact in organic light emitting devices (OLEDs) and measured the device performance. The external quantum efficiency measured at a current density of 250 A/m2 for the device on PET was approximately 0.9%, which is higher than that (approximately 0.5%) for the device on glass. The reduction in the driving voltage and high external quantum efficiency made an ITO coated PET substrate very promising for future large scale OLED application.
Transparent and highly conducting zirconium-doped indium oxide (ZIO) thin films have been grown by pulsed laser deposition (PLD) on glass substrates without a post- deposition anneal. The structural, electrical and optical properties of these films were investigated as a function of film composition and substrate deposition temperature. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 1 J/cm2 at growth temperatures ranging from 20 degrees Celsius to 400 degrees Celsius in oxygen pressure ranging from 1 mTorr to 25 mTorr. The films (approximately 2000 angstrom thick) deposited at 200 degrees Celsius in 25 mTorr of oxygen show electrical resistivities as low as 2.7 X 10-4 (Omega) -cm, the average visible transmittance of 89%, the refractive index of 1.99 and optical band gap of 4.1 eV. These ZIO films were used as a transparent anode contact in organic light emitting diodes (OLEDs) and the device performance was studied. The external quantum efficiency measured at 100 A/m2 for the [ZIO/TPD/Alq3/MgAg] diodes was about 0.9%. Low driving voltage and high light emission were observed for the OLEDs with the ZIO anode.
A feasibility study of megapixel microcalorimeter arrays, based on thermoelectric energy to voltage conversion and digital superconducting readout, is presented. The design concept originated from the philosophy of employing the simplest principles at the single-pixel level to enable large arrays without sacrificing energy resolution, fast operation speed, and quantum efficiency. Initial experimental tests confirm the basic predictions of theory, and show no major obstacle in achieving the desired characteristics.
Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on glass substrates. The structural, electrical and optical properties of these films were investigated as a function of film thickness. Films were deposited using a KrF excimer laser at a fluence of 2 J/cm2, at substrate temperature of 300 degrees C and 10 mTorr of oxygen pressure. For ITO films deposited at 300 degrees C in 10 mTorr of oxygen pressure, the resistivity of 2-4 X 10-4 (Omega) - cm was observed and the average transmission in the visible range was about 85-90 percent. The Hall mobility and carrier density for ITO films were observed to be in the range of 24-27 cm2/V-s and 5-9 X 1020 cm-3, respectively. We have used the ITO thin films, deposited by PLD on silica substrates, as the anode contact in organic light emitting devices and studied the effect of ITO film thickness on the device performance. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 500-1000 angstrom. The device shoed an external quantum efficiency of about 0.8 percent at 100 A/m2.
Two transparent conducting oxides (TCO) thin films including tin-doped indium oxide (ITO) and aluminum-doped zinc oxide (AZO) were grown on glass substrates by pulsed laser deposition (PLD). The structural, electrical and optical properties of these films were investigated as a function of target composition and film growth temperature. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2 at growth temperatures ranging from 25 degrees Celsius to 400 degrees Celsius in oxygen pressures ranging from 1 to 100 mTorr. For a 300 nm thick ITO film deposited at 300 degrees Celsius in oxygen pressure of 10 mTorr, the resistivity was 2 X 10-4 (Omega) -cm and the average transmission in visible range (400 - 700 nm) was 85%. The Hall mobility and carrier density for a 150 nm thick ITO film deposited at 300 degrees Celsius were 27 cm2/V-s and 1.4 X 1021 cm-3, respectively. For a 100 nm thick AZO film deposited at 200 degrees Celsius in an oxygen pressure of 5 mTorr, the resistivity was 3.8 X 10-4 (Omega) -cm and the average transmission in visible range (400 - 700 nm) was 90%. The Hall mobility and carrier density for the same AZO film were 18 cm2/V-s and 9.1 X 1020 cm-3, respectively. AFM measurements indicated that the RMS surface roughness of the ITO films (approximately 5 angstrom) was slightly lower than that of the AZO films (approximately 7 angstrom). XPS measurements showed that the work function of ITO films grown at 250 degrees Celsius was 4.51 plus or minus 0.05 eV, which is higher than that (4.05 plus or minus 0.05 eV) of the AZO films grown at 200 degrees Celsius. The PLD ITO films were used to fabricate organic light-emitting diodes (OLEDs). The electroluminescent (EL) performance was measured and the luminous power efficiency was calculated to be 0.6 lm/W, which is comparable to that measured with commercially available sputter-deposited ITO anodes.
The effect of spin-polarized injection on the superconductivity order parameter is investigated in a device consisting of YBa2Cu3O7-(delta )/Au/Ni0.8Fe0.2 layers. A non-equilibrium theory which qualitatively agrees with the results of measurements made on superconductor/insulator/ferromagnet structures is presented. A quantitative analysis shows that this theory predicts injection currents that are several orders of magnitude too large. Recent results suggest that superconductivity in thin films can be strongly influenced by the injection of a spin- polarized current from a ferromagnetic material. The effect has been found to occur in both low Tc (Sn) and high Tc (YBa2Cu3O7-(delta )) superconductors when either a conventional ferromagnetic metal, permalloy (Ni0.8Fe0.2), or a colossal magnetoresistive material were used as the source of spin polarization. Control experiments showed that unpolarized current from a nonmagnetic metal had comparatively little effect on the same superconductors. A phenomenological model, in which the energy gap of the superconductor is perturbed by the presence of excess spin polarized electrons, has been shown to qualitatively mimic the experimental results. However, an estimate of the current needed to significantly suppress the gap is shown to be several orders of magnitude larger than is observed.
Pulsed laser deposition has been used for the growth of high quality YBa2Cu3O7 and La0.67Sr0.33MnO3 thin films and multilayers for electronic device applications. In particular, YBa2Cu3O7 - (SrTiO3, CeO2) - La0.67Sr0.33MnO3 trilayer devices were fabricated to study the supercurrent suppression by the injection of a spin-polarized quasiparticle current. Our results show that the critical current for a YBa2Cu3O7 - 50 angstroms SrTiO3 - La0.67Sr0.33MnO3 device was found to decrease from 120 mA to 15 mA, for an injection current of 60 mA of spin polarized current yielding a negative current gain of approximately 1.8. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties. Once optimized, spin injection represents a new approach to fabricating superconducting transistors which could impact electronic systems for many important next generation.
Low loss ferroelectric thin films deposited by pulsed laser deposition (PLD) are currently being used to develop a new class of tunable microwave circuits based on the electric field dependence of the dielectric constant. Single phase, (100) oriented Ba0.5Sr0.5TiO3 (BST) films have been deposited onto (100) LaAlO3, SrTiO3, and MgO substrates. Interdigitated capacitors have been used to measure the dielectric constant and dissipation factor of these films as a function of DC bias and temperature at 1 MHz and as a function of DC bias at 1 to 20 GHz at room temperature. A low phase noise voltage controlled oscillator is currently being developed for use at frequencies from 1 - 20 GHz. To achieve low phase noise in the oscillator will require the loss tangent in the ferroelectric to be <EQ 5 X 10-3. Origins of the dielectric loss are being investigated using optical techniques. Optical imaging of the ferroelectric films using confocal scanning optical microscopy shows reproducible polarization fluctuations over sub-micrometer length scales for BST films deposited onto SrTiO3 which are not observed for films deposited onto MgO. Dielectric loss in the ferroelectric film is reduced through a combination of post deposition processing and donor/acceptor doping of the films. The lowest dielectric loss measured at microwave frequencies (tan(delta) equals 0.01 at 1 - 10 GHz) has been in a post-deposition annealed Ba0.5Sr0.5TiO3 film doped with approximately 1 - 2 atomic % Mn.
High quality thin films of SrxBa(1-x)TiO3 are currently being grown using pulsed laser deposition (PLD). These films are being used for the construction of frequency tunable microwave electronic devices. In particular, a low phase noise, voltage controlled oscillator (1.5 - 2.5 GHz) is currently being developed. Single phase and oriented SrxBa(1-x)TiO3 films have been deposited by PLD onto (100) LaAlO3 and MgO and single crystal Ag films. The dielectric properties of these films has been measured at 1 MHz and between 1 and 20 GHz. A 75% change in the capacitance can be achieved using a 40 V bias across a 5 micrometer interdigital capacitor gap (80 kV/cm). The dissipation factor (measured at 1 MHz) depends on film composition and temperature. Dielectric loss measurement at 1 - 20 GHz have shown a dielectric loss tangent as small as 1.25 multiplied by 10-2.
Pulsed laser deposition (PLD) has been used to deposit high quality thin films of Ni81Fe19/Au/YBa2Cu3O7- (delta ) onto (100) oriented substrates of MgO and SrTiO3 for the purpose of fabricating a novel high temperature superconducting three terminal device. The ferromagnet-normal metal-superconductor (F-N-S) structure is currently being investigated to determine the effect of the injection of a spin-polarized current on the order parameter of a high temperature superconducting thin film. High quality films with sharp interfaces, free of defects, are required in order to maximize the spin-injection effect. The surface morphology and transport properties of the YBa2Cu3O7-(delta ) films have been investigated using scanning electron microscopy and ac susceptibility measurements, respectively, as a function of increasing laser fluence. Deposition at 2.0 - 2.4 J/cm2, 790 degrees Celsius and 320 m Torr O2 produces films with a sharp superconducting transition and a smooth surface. The growth of Au on YBCO under different PLD conditions has been observed by atomic force microscopy. Surface clustering of Au occurs at elevated temperatures and is attributed to increased surface mobility. The presence or absence of a background gas influences the cluster size. These results are discussed within the framework of the role of excess energy of PLD adatoms with changing laser fluence and background gas.
James Horwitz, Paul Dorsey, N. Koon, M. Rubinstein, J. Byers, D. Gillespie, Michael Osofsky, V. Harris, K. Grabowski, D. Knies, Edward Donovan, Randolph Treece, Douglas Chrisey
The effect of substrate temperature and oxygen deposition pressure on the structure and properties of thin films of LaxCa1-xMnO(delta ) has been investigated. Thin films (approximately 1000 angstroms) of La0.67Ca0.33MnO(delta ) were deposited onto LaAlO3 (100) substrates by pulsed laser deposition at a substrate temperature of 600 and 700 degree(s)C. A series of films were grown on different oxygen pressures, between 15 and 400 mTorr, which systematically changed the oxygen concentrations in the films. As-deposited films exhibited an oriented orthorhombic structure. At low oxygen deposition pressures films were preferentially (202) oriented. At high pressures deposited films had a (040) preferred orientation. A 900 degree(s)C anneal in flowing oxygen of a film deposited at low oxygen pressure resulted in a decrease in the a lattice parameter and a change in the preferred orientation from (202) to (040). Vacuum annealing at 550 degree(s)C resulted in an increase in the a lattice parameter. The resistivity as a function of temperature showed a significant variation as a function of growth conditions. The peak in the resistivity curve (Tm) varied between 73 and 150 K depending upon the growth conditions. The activation energy associated with the semiconducting phase was approximately the same for all films (approximately 100 meV).
Pulsed laser deposition is a superior technique for the growth of high quality thin films (<EQ 1 micrometers ) of electronic ceramics and has satisfied many applications. To meet developing applications, there is a need for thick films (>= 1 micrometers ) of electronic ceramics. Two film qualities principally control the growth of thick films: the film surface morphology and film stress. The deposition parameters which affect these qualities include: film deposition rate, film-substrate lattice mismatch, film-substrate thermal coefficient of expansion mismatch, and film growth kinetics. Our results suggest that it will be difficult to fabricate thick ceramic films of suitable electronic quality by conventional physical vapor deposition techniques.
We describe the deposition of SrxBa1-xTiO3 (0.5 <EQ x <EQ 0.8) thin films by pulsed laser deposition and the issues related the their application as active microwave device components. The SrxBa1-xTiO3 thin films (approximately equals 5000 angstrom) deposited at 775-850 degree(s)C in 350 mTorr of oxygen onto (100) MgO and LaAlO3 were smooth, single phase, and epitaxial with the underlying substrate. Highly oriented Sr0.5Ba0.5TiO3 films on LaAlO3 with x- ray rocking curves of 72 arc seconds were observed. The dielectric constant of Sr0.5Ba0.5TiO3 thin films, determined from the signal in patterned transmission lines between 100 kHz and 0.1 GHz, was approximately equals 20% of that observed for the bulk and the zero field temperature dependence was broad in comparison to the sharply peaked behavior seen in bulk. The dielectric loss tangent was measured as a function of stoichiometry for SrxBa1-xTiO3 (0.2 <EQ x <EQ 0.8) thin films (3-5 micrometers ) at room temperature and at 9.2 GHz. Loss tangent values were found to be highly sensitive to the Curie temperature of the film. Loss tangent values as low as 0.1% were obtained for Sr0.8Ba0.2TiO3. The results for SrxBa1-xTiO3 thin films presented in this paper are encouraging for future applications in active microwave devices.
The ejection of sub-micron size particulates from a metal target as a result of the interaction of an excimer laser with a Pt target was investigated. To study the effect of laser fluence, particulates were collected on Pt films prepared on MgO(100) substrates over a fluence range of 0.83 - 3.3 J/cm2. Films were prepared at several substrate temperatures: 25 degree(s)C, 350 degree(s)C, 450 degree(s)C, and 550 degree(s)C, and under 0.05 Torr of an inert gas (Ar). It was found that, for a substrate temperature of 450 degree(s)C, the Pt particulates would stick easily to the growing Pt film with little evidence for deformation. The size distributions and areal densities of the metal particulates were measured from low magnification scanning electron micrographs of the thin film surfaces prepared at 450 degree(s)C and compared. In general, spherical particulates were produced with diameters ranging from about 0.05 to 1.0 micrometers . The shape of the size distribution of particulates on the film surface was roughly constant radially outward from the plume center, although the magnitude of the particulates decreased from the plume center. As the fluence increased, the mean diameter of the particulates increased slightly (from 0.25 to 0.35 micrometers ) while the number density of particulates decreased by over two orders of magnitude (from 1.04 X 105 to about 1.0 X 103 particulates/cm2 per angstrom of film deposited) near the plume center.
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