K.-S. Chang

Monolithic multichannel ultraviolet detector arrays and continuous phase evolution in MgxZn1−xO composition spreads

We have fabricated MgxZn1−xO epitaxial composition spreads where the composition across the chip is linearly varied from ZnO to MgO. By using a scanning x-ray microdiffractometer and transmission electron microscopy, we have mapped the phase evolution across the spread. We have discovered a unique growth relationship between cubic and hexagonal MgxZn1−xO where their basal planes are coincident in the phase-separated region of the phase diagram where 0.37⩽x⩽0.6. The continuously changing band gap across the spread is used as a basis for compact broadband photodetector arrays with a range of detection wavelengths separately active at different locations on the spread film. The composition-spread photodetector is demonstrated in the wavelength range of 290–380 nm using the ZnO to Mg0.4Zn0.6O region of the spread.

Exploration of artificial multiferroic thin-film heterostructures using composition spreads

We have fabricated a series of composition spreads consisting of ferroelectric BaTiO3 and piezomagnetic CoFe2O4 layers of varying thicknesses modulated at nanometer level in order to explore artificial magnetoelectric thin-film heterostructures. Scanning microwave microscopy and scanning superconducting quantum interference device microscopy were used to map the dielectric and magnetic properties as a function of continuously changing average composition across the spreads, respectively. Compositions in the middle of the spreads were found to exhibit ferromagnetism while displaying a dielectric constant as high as ≈120.

Multimode quantitative scanning microwave microscopy of in situ grown epitaxial Ba1−xSrxTiO3 composition spreads

We have performed variable-temperature multimode quantitative microwave microscopy of in situ epitaxial Ba1−xSrxTiO3 thin-film composition spreads fabricated on (100) LaA1O3 substrates. Dielectric properties were mapped as a function of continuously varying composition from BaTiO3 to SrTiO3. We have demonstrated nondestructive temperature-dependent dielectric characterization of local thin-film regions. Measurements are simultaneously taken at multiple resonant frequencies of the microscope cavity. The multimode measurements allow frequency dispersion studies. We observe strong composition-dependent dielectric relaxation in Ba1−xSrxTiO3 at microwave frequencies.

Combinatorial libraries of semiconductor gas sensors as inorganic electronic noses

We have fabricated thin-film combinatorial gas sensor libraries based on doped semiconducting SnO2 thin films. Combinatorial pulsed-laser ablation was used to deposit compositionally varying arrays of sensor elements onto a prepatterned device electrode configuration. Using multiplexing electronics, we have demonstrated the detection of chloroform, formaldehyde, and benzene gases at concentrations down to 12.5 ppm through pattern recognition of signals from the arrays of sensors. This technique of fabricating gas sensor arrays as combinatorial libraries can be applied to the manufacturing of portable electronic noses.We have fabricated thin-film combinatorial gas sensor libraries based on doped semiconducting SnO2 thin films. Combinatorial pulsed-laser ablation was used to deposit compositionally varying arrays of sensor elements onto a prepatterned device electrode configuration. Using multiplexing electronics, we have demonstrated the detection of chloroform, formaldehyde, and benzene gases at concentrations down to 12.5 ppm through pattern recognition of signals from the arrays of sensors. This technique of fabricating gas sensor arrays as combinatorial libraries can be applied to the manufacturing of portable electronic noses.

Dissociation and evolution of threading dislocations in epitaxial Ba0.3Sr0.7TiO3 thin films grown on (001) LaAlO3

The defect structure of a 350-nm-thick epitaxial Ba0.3Sr0.7TiO3 (BSTO) film grown on (001) LaAlO3 has been investigated using conventional and high-resolution transmission electron microscopy. The dominant defects in the film are edge-type threading dislocations (TDs) with Burgers vectors b=〈100〉 and 〈110〉. Pure-screw TDs and partial TDs of mixed character were also observed. A rapid reduction of defect density occurred after the growth of the first 100 nm BSTO adjacent to the interface. In the top layer of the film, all TDs with b=〈100〉 are perfect while those with b=〈110〉 are usually dissociated into two partials with a small separation (a few nanometers). However, in the near-interface layer of the film, many TDs with b=〈100〉 are split into two or three partials. A high density of extended stacking faults with displacement vectors of 12〈110〉 type were observed. The stacking faults are associated with dissociated dislocations and partial half loops. The mechanisms for the generation, dissociation and ev...

Microwave frequency tuning and harmonic generation in ferroelectric thin film transmission lines

We evaluate dielectric tuning on nanosecond time scales in ferroelectric Ba0.3Sr0.7TiO3 thin films by measuring nonlinear harmonic generation at a fundamental frequency of 3 GHz. We compare the form of the distributed nonlinear capacitance per unit length C(Vrf) extracted from a simple model of harmonic generation in coplanar waveguide transmission line structures with the nonlinear capacitance C(Vdc) measured using a dc bias voltage, and obtain excellent agreement for temperatures in the range 235–295 K. This demonstrated agreement implies that full dielectric tuning can be expected in these ferroelectric thin films on nanosecond time scales, and also demonstrates that detrimental high-frequency nonlinear effects in device structures can be accurately predicted based on dc biased measurements.

Ferromagnetic resonance in Ni–Mn–Ga films

Textured thin films of nominal composition Ni0.50(MnGa)0.50 were sputter deposited on Si substrates and studied by x-ray diffraction, micromechanical displacement, dc magnetization, and ferromagnetic resonance (FMR). We report the observation of spin wave resonances in this alloy, yielding a spin wave stiffness of D=200 meV A2 at 300 K. A marked thermal hysteresis is observed in the temperature-dependent FMR data arising from the reversible martensitic transition.

Microstructural study of epitaxial Zn1−xMgxO composition spreads

We have fabricated Zn1−xMgxO epitaxial thin-film composition spreads by layer-by-layer pulsed laser deposition (PLD) method where the composition across the chip is linearly varied from ZnO to MgO. In this paper we discuss the cross-sectional transmission electron microscopy study of the combinatorial spreads. The full orientation relationships between substrate and the ZnO and MgO phases were established. Formation of twin-related domain structure was found for the [111]-oriented MgO. Formation of the [100]-oriented cubic MgO in [0001]-oriented ZnO was observed in the mixed region in the middle of the spread. This relationship can potentially be used to grow [100]-oriented cubic structures epitaxially on c-axis-oriented hexagonal substrates. For the extended solid solution of ZnO, a high density of defects was found. Analysis of the defects established that they are translational domains formed by three-dimensional island growth of PLD and specific to the crystallography of ZnO. The ZnO phase appears wit...

Microstructural properties of (Ba, Sr)TiO3 films fabricated from BaF2/SrF2/TiO2 amorphous multilayers using the combinatorial precursor method

We have investigated the microstructure of (Ba, Sr)TiO3 films fabricated from BaF2/SrF2/TiO2 amorphous multilayers. Rutherford backscattering spectroscopy and x-ray diffraction studies show that a controlled thermal treatment can interdiffuse the multilayers so as to create predominantly single-phase epitaxial (Ba, Sr)TiO3 films. A high resolution cross-sectional transmission electron microscopy investigation of the processed films shows that they consist of large epitaxial grains of (Ba, Sr)TiO3 with atomically sharp interfaces with the LaAlO3 substrates. In addition, we have identified regions where polycrystalline and transient phases exist in small pockets in the film matrix. The results here indicate that the combinatorial thin-film synthesis using precursors can produce (Ba, Sr)TiO3 films in combinatorial libraries which exhibit properties similar to those films made by conventional techniques.

Combinatorial pulsed laser deposition using a compact high-throughout thin-film deposition flange

Abstract We describe the design and operation of our high-throughput thin-film deposition flange which is used to perform combinatorial pulsed laser deposition (PLD). The portable flange allows us to “convert” different PLD systems into combinatorial PLD systems. The use of the flange is illustrated in fabrication of a discrete gas sensor library and an epitaxial composition spread.