S. B. Qadri

Microwave properties of tetragonally distorted (Ba0.5Sr0.5)TiO3 thin films

A strong correlation is observed between the structure and the microwave dielectric properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films deposited onto (001) MgO by pulsed laser deposition. Films were deposited at 750 °C in an oxygen pressure that was varied from 3 to 1000 mTorr. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D=a/c) of the films depends on the oxygen deposition pressure. D varied from 0.996 at 3 mTorr to 1.003 at 800 mTorr. At microwave frequencies (1–20 GHz), BST films with low distortion have higher dielectric constants (e∼500), and lower dielectric loss (tan δ∼0.02) compared to films with higher distortion. The correlation of the microwave properties with the film structure can be attributed to stresses and polarizability in the film. The BST film grown at the oxygen deposition pressure of 50 mTorr exhibits a large dielectric constant change and a low dielectric loss at the same time, which corresponds to the film in low stress (D=1.0004). For tu...

Epitaxial growth of Al-doped ZnO thin films grown by pulsed laser deposition

Epitaxial thin films of aluminum-doped zinc oxide (AZO) have been deposited on single crystal sapphire (000l) substrates by pulsed laser deposition. 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 at substrate temperatures ranging from 25 to 680 °C in oxygen pressures ranging from 0.13 to 9.31 Pa. For the 200-nm-thick single crystal AZO film, the electrical resistivity was observed to be 2.2×10−4 Ω·cm and the average transmission in the visible range (400–700 nm) was 90%. This low resistivity value for the single crystal AZO film is due to the high carrier mobility of the film, which results from a decrease in the grain boundary scattering, compared to polycrystalline AZO films.

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Molecular beam epitaxy of Zn1−xCdxSe epilayers and ZnSe/Zn1−xCdxSe superlattices

We have investigated the epitaxial growth of Zn1−x CdxSe epilayers and ZnSe/Zn1−x CdxSe superlattices (0≤x≤1) on (100)GaAs. Although thick epilayers of Zn1−x CdxSe are prone to defect formation with increasing Cd content, the structural and optical characteristics improve remarkably when Zn1−x CdxSe is  in the  form of  thin layers  within ZnSe/Zn1−x CdxSe  superlattices. High  quality superlattices  can be grown for x≤0.35. The characterization of these systems using transmission electron microscopy, x‐ray diffraction, reflectivity, and photoluminescence is reported.

Intermetallic phase formation during annealing of Al/Ni multilayers

The phase evolution during annealing of Al/Ni multilayer samples prepared by ion‐beam sputtering with composition modulation wavelengths Λ between 10 and 400 nm was determined using x‐ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of Al0.40Ni0.60 and Al0.75Ni0.25 were investigated. For the Al0.40Ni0.60 samples the following results were obtained. A measure of the degree of periodicity and the sharpness of the interfaces in a sample with Λ=80 nm was the large number (over 20) of peaks observed in small‐angle x‐ray scattering measurements. A sample with Λ=10 nm was transformed by heat treatment directly to the AlNi phase. In the Λ=80 nm sample, the first phase formed after annealing was the metastable η phase. The η phase was identified as Al9Ni2. In the 400 nm wavelength sample, both the metastable η phase and the stable Al3Ni formed after the first exothermic reaction. For the Al0.75Ni0.25 samples two results were obtained. A Λ=11.4 nm sample transfor...

Transparent conducting Zr-doped In2O3 thin films for organic light-emitting diodes

Zirconium-doped indium oxide (ZIO) thin films (∼2000 A thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical and optical properties of these films have been investigated as a function of substrate temperature and oxygen partial pressure during deposition. Films were deposited at substrate temperatures ranging from 25 °C to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. The films (∼2000 A thick) deposited at 200 °C in 25 mTorr of oxygen show electrical resistivities as low as 2.5×10−4 Ω cm, an average visible transmittance of 89%, and an optical band gap of 4.1 eV. The ZIO films were used as a transparent anode contact in organic light emitting diodes and the device performance was studied. The external quantum efficiency measured from these devices was about 0.9% at a current density of 100 A/m2.

Pressure induced structural transitions in nanometer size particles of PbS

At elevated pressure, PbS undergoes a first order phase transition from the NaCl or B1 structure to an orthorhombic structure. The effects of particle sizes in the nanometer range on this transition have been investigated using energy‐dispersive x‐ray diffraction of synchrotron produced wiggler radiation. Relative to the bulk crystals, the onset of transition pressure showed a significant increase with decreasing particle size. The results also show that compressibility increases with decreasing particle size: this increase is continuous through the phase transition.

Evidence of strain and lattice distortion in lead sulfide nanocrystallites

X-ray diffraction studies on nanometer sized lead sulfide particles reveal the presence of a compressive strain. A number of samples with particle sizes ranging from 2 to 16 nm were synthesized using the three dimensional periodic, bicontinuous cubic phase as a matrix. Samples of the larger size particles could be indexed to an fcc lattice. As the particle size decreased below 6 nm, a tetragonal distortion of the cubic lattice was observed, accompanied by a decrease in the unit cell volume.

Anode material based on Zr-doped ZnO thin films for organic light-emitting diodes

Zr-doped ZnO (ZZO) thin films have been investigated as an anode material, a potential alternative to indium tin oxide (ITO), for organic light emitting diode (OLED) devices. ZZO films have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were studied as a function of substrate temperature and oxygen pressure during deposition. For a 200-nm-thick ZZO film grown at 250 °C in 1 mTorr of oxygen, a resistivity of 5.6×10−4 Ω cm was measured and an average optical transmittance of 84% was measured in the visible range (400–700 nm). The ZZO films, grown at different oxygen pressures, were used as an anode contact for OLED devices. External electroluminescence quantum efficiencies (0.8%–0.9%) comparable to those (0.9%–1.0%) measured for control devices fabricated on commercial ITO anodes were obtained at high current densities (1000 A/m2). These results demonstrate that ZZO is a good anode material. In addition, it is an attractive alternative to ...

Low-dimension structural properties and microindentation studies of ion-beam-sputtered multilayers of Ag/Al films

Artificially modulated silver/aluminum multilayer films with modulation wavelengths Λ between 1.35 and 21.3 nm, and total film thickness of 1 μm were prepared by ion-beam sputtering. The mechanical properties of these films were investigated by a low load muhardness indentation technique. The films displayed increased hardness as the modulation wavelength decreased. X-ray diffraction scans showed well-defined small angle peaks and satellite peaks for high angles indicating a coherent multilayer structure. The hardness enhancement as the modulation wavelength decreases may be partially attributed to the Ag2Al formation at the interfaces. The high quality of the well-defined layer structure is confirmed by the presence of interference maxima corresponding to the number of bilayers in the low angle diffraction data for films with total film thicknessesess than 33.0 nm.