Robert Scott Mclean

High-performance ZnO thin-film transistors on gate dielectrics grown by atomic layer deposition

We fabricated high-performance ZnO thin-film transistors on gate dielectrics of HfO2, HfSiOx, and Al2O3, grown by atomic layer deposition (ALD). Devices on HfO2 had a mobility of 12.2cm2∕Vs with a threshold voltage of 2.6V and subthreshold slope of 0.5V∕decade. Device performance on Al2O3 depended on synthesis temperature. For 100nm thick Al2O3, synthesized at 200°C, ZnO devices had a mobility of 17.6cm2∕Vs with a threshold voltage of 6V and less than ∼0.1nA gate leakage at 20V. The overall trends were that devices on Hf oxides had a lower threshold voltage, while the gate leakage current density was lower on Al2O3. Device characteristics for all ALD dielectrics exhibited negligibly small hysteresis, suggesting a low defect density at the interface of ZnO with the gate dielectric.

Influence of energetic bombardment on stress, resistivity, and microstructure of indium tin oxide films grown by radio frequency magnetron sputtering on flexible polyester substrates

Using rf magnetron sputtering, we have identified conditions for growing indium tin oxide (ITO) thin films at room temperature that simultaneously exhibit low resistivity (∼3×10−4 Ω cm), high optical transparency (>80%), and near-zero stress on polyester substrates. From transport measurements, we deduced that Sn donor atoms had little effect on electrical conduction in ITO films. We further concluded from an analysis of sputtered ions and atoms that bombardment by energetic (>35 eV) negative oxygen ions caused high stress (∼1 GPa) in films grown at lower (6 mTorr) pressure. We further concluded that bombardment by lower-energy (1–2 eV) sputtered oxygen species at the growing film surface was likely responsible for the dependence of ITO crystallization and microstructure on oxygen partial pressure during deposition.

Critical tensile strain and water vapor transmission rate for nanolaminate films grown using Al2O3 atomic layer deposition and alucone molecular layer deposition

Critical tensile strains (CTSs) and water vapor transmission rates (WVTRs) were measured for nanolaminate films grown on polyimide substrates using Al2O3 atomic layer deposition (ALD) and alucone molecular layer deposition (MLD). Nanolaminate composition was controlled by varying the ratio of ALD:MLD cycles during film growth. For ∼100 nm film thicknesses, the CTS obtained its highest value of ∼1.0% for the 3:1 nanolaminate. The WVTR decreased dramatically versus nanolaminate composition and reached the measurement sensitivity limit at WVTR ∼1 × 10−4 g/(m2day) for the 7:2, 5:1, and 6:1 nanolaminates. The ALD:MLD nanolaminates may be useful as flexible gas/vapor diffusion barriers on polymers.

Permeability and corrosion in ZrO2/Al2O3 nanolaminate and Al2O3 thin films grown by atomic layer deposition on polymers

The authors studied moisture permeation and corrosion in Al2O3 and Al2O3/ZrO2 nanolaminate (NL) thin films grown by atomic layer deposition (ALD) at 100 °C on polyester substrates. A percolation model accurately described the dependence of permeation on the volume fraction of ZrO2 in the nanolaminates. As the fraction of ZrO2 was reduced to ∼0.5, moisture permeation in the NLs approached the measurement limit ∼1 × 10−4 g-H2O/m2-day, equivalent to Al2O3 with the same total thickness. However, resistance to corrosion by water was modestly better for Al2O3 than for the NL, and we proposed that corrosion in ALD Al2O3 films was associated with hydrogen incorporation and a consequent film chemical composition that is an oxy-hydroxide, AlOx(OH)3−2x. The authors present x-ray diffraction evidence for conversion of ALD Al2O3 to hydroxide corrosion products, AlO(OH) and Al(OH)3, after aging films in damp heat (85 °C/85% relative humidity) for two weeks.

Alucone Interlayers to Minimize Stress Caused by Thermal Expansion Mismatch between Al2O3 Films and Teflon Substrates

Alucone films were employed as interlayers to minimize stress caused by thermal expansion mismatch between Al(2)O(3) films grown by atomic layer deposition (ALD) and Teflon fluorinated ethylene propylene (FEP) substrates. The alucone films were grown by molecular layer deposition (MLD) using trimethylaluminum (TMA), ethylene glycol (EG), and H(2)O. Without the alucone interlayer, the Al(2)O(3) films were susceptible to cracking resulting from the high coefficient of thermal expansion (CTE) mismatch between the Al(2)O(3) film and the Teflon FEP substrate. Cracking was observed by field emission scanning electron microscopy (FE-SEM) images of Al(2)O(3) films grown directly on Teflon FEP substrates at temperatures from 100 to 160 °C and then cooled to room temperature. With an alucone interlayer, the Al(2)O(3) film had a crack density that was reduced progressively versus alucone interlayer thickness. For Al(2)O(3) film thicknesses of 48 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >60 nm on 50 μm thick Teflon FEP substrates. For thinner Al(2)O(3) film thicknesses of 21 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >40 nm on 50 μm thick Teflon FEP substrates. Slightly higher alucone interlayer thicknesses were required to prevent cracking on thicker Teflon FEP substrates with a thickness of 125 μm. The alucone interlayer linearly reduced the compressive stress on the Al(2)O(3) film caused by the thermal expansion mismatch between the Al(2)O(3) coating and the Teflon FEP substrate. The average compressive stress reduction per thickness of the alucone interlayer was determined to be 8.5 ± 2.3 MPa/nm. Comparison of critical tensile strains for alucone films on Teflon FEP and HSPEN substrates revealed that residual compressive stress in the alucone film on Teflon FEP could help offset applied tensile stress and lead to the attainment of much higher critical tensile strains.

Morphological Changes During Crystallization and Melting of Polyoxymethylene Studied by Synchrotron X-Ray Scattering and Modulated Differential Scanning Calorimetry

Temperature scanning techniques, including synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction(WAXD), and temperature-modulated differential scanning calorimetry (TMDSC), were used to study melting recrystallization in semicrystalline polyoxymethylene (POM). The isothermal crystallization of POM was also studied by time-resolved SAXS. From SAXS profiles, several morphological variables were calculated, including the long period L, lamellar thickness lc , interlamellar amorphous thickness la , and scattering invariant Q. The lamellar parameters were also obtained using tapping atomic force microscopy (AFM) for two thermal histories, and some lamellar-scale and larger-scale morphological changes were characterized before and after partial melting. These real-space images also provided support to interpretation of SAXS analysis in the interpretation of first- and second-order intensity maxima. During melting at constant heating rates, la from SAXS increased slowly starting at about 100°C, suggesting melting of thin inserted lamellae, and at about 150°C, lc began to increase combined with a more rapid increase in la due to further melting of inserted lamellae and some recrystallization into separate stacks of lamellae. The end of melting was about 182°C. TMDSC data also characterized the level of melting and recrystallization starting at low temperatures for a quenched sample. The DSC data provided the total extent of melting, and this was contrasted with the drop in SAXS and WAXD intensities. SAXS and TMDSC temperature scans on a quenched, but still highly crystalline, POM sample were compared with the data from a high-temperature (145°C) isothermally crystallized POM sample with a higher degree of crystal perfection.

Characterization of CZTSSe photovoltaic device with an atomic layer-deposited passivation layer

We describe a CZTSSe (Cu2ZnSn(S1−x,Sex)4) photovoltaic (PV) device with an ALD (atomic layer deposition) coated buffer dielectric layer for CZTSSe surface passivation. An ALD buffer layer, such as TiO2, can be applied in order to reduce the interface recombination and improve the devices open-circuit voltage. Detailed characterization data including current-voltage, admittance spectroscopy, and capacitance profiling are presented in order to compare the performance of PV devices with and without the ALD layer.

Effect of early stage growth on moisture permeation of thin-film Al2O3 grown by atomic layer deposition on polymers

The authors investigated moisture permeation for Al2O3 barrier films grown by atomic layer deposition (ALD) on the two individual chemically distinct surfaces of a commercial (DuPont Teijin Films, Chester, VA) polyethylene terephthalate (PET) film. One surface is “bare” PET polymer, whereas the opposite surface is coated with a low friction “slip” layer to facilitate winding and unwinding of the polymer film on a roll. For early stage Al2O3 film growth (<8 nm), the authors found that moisture permeation was less on the bare polymer side than the slip side but converged to same instrument limited value (∼1 × 10−4 g H2O/m2 day) for thicker films. However, thicker barrier films grown on the slip side experienced abrupt permeation breakthrough in longer-term Ca-testing at 60 °C/85% RH, whereas equivalently grown barrier films on the bare PET-side were stable. Mildly etching the slip side in Ar, before depositing Al2O3, resulted in permeation performance equivalent to barriers grown on the bare polymer side. T...

Photostimulable luminescence and imaging in vapor-deposited BaFCl:Eu thin film phosphors

Thin films of BaFCl:Eu with photostimulable luminescence (PSL) were grown by vapor deposition. For substrate temperatures at or below 350 °C, films were optically transparent and exhibited higher resolution in x-ray imaging than a commercial, particulate PSL phosphor screen, when luminescence was stimulated by scanning with a focused HeNe laser. Although post annealing films in a hydrogen atmosphere at 600–900 °C increased their PSL signal, which was relatively weak in as-deposited films, annealing reduced imaging resolution, apparently because of an increase in optical scattering of the focused laser. The implications of hydrogen annealing for the PSL mechanism are also discussed.

Application of Atomic Layer Deposition for Gas Permeation Barrier Thin Films

In many electronics applications, especially portable displays--cell phones, iPods, PDAs etc., replacing glass with a flexible polymer would reduce weight, improve durability, and enable new design shapes. However, glass is impermeable to atmospheric gases, whereas polymers are not. Water vapor permeation can corrode traditional electronic materials, such as Al conductors, while it can seriously degrade emerging organic electronic materials, targeted for emissive displays (OLEDs) and flexible electronics. Packaging materials comprised of thin (<50 nm) coatings of Al, SiOx, or AlOx, do not provide adequate protection for these emerging electronic applications. OLEDs, for example, need a gas diffusion barrier that reduces water vapor permeation by more than 10x, whereas packaging materials reduce atmospheric gas permeation by only 1050x. Coating defects are what limit barrier performance. Most barrier coatings for packaging films are made by a physical vapor deposition (PVD) process, such as sputtering or thermal evaporation. Pinhole defects are common in PVD films, because surface asperities and particles can shadow the arriving coating flux. Coating by chemical vapor deposition (CVD) mitigates shadowing effects, but CVD often needs higher temperature for film growth, which may be incompatible with temperature-sensitive polymers. By either process, film growth proceeds by nucleation and coalescence of islands of the coating material. This creates granular or columnar boundaries, which facilitate gas permeation. On the other hand, atomic layer deposition (ALD) is uniquely suited to producing high performance gas diffusion barrier coatings. The sequential introduction of reactants and their monolayer, selflimiting surface adsorption force a layer-by-layer film growth, which is highly conformal. Pinholes and film boundaries are essentially eliminated. Table 1 summarizes early permeation test results for thin ALD Al2O3 barriers on 5-mil thick polyester (PEN) and 2-mil thick (Kapton®) polyimide. These permeation rates for oxygen at 50% RH, measured with commercial MOCON instrument, were at or below its lower measurement limit of ~5x10 cc-O2/m/day.