G. W. Auner

Slater model applied to polarization graded ferroelectrics

Ferroelectric thin films of BaxSr1−xTiO3 with compositional gradients normal to the growth surface have been formed by the successive deposition and annealing of films having step-variable Ba to Sr ratios. By suitably tailoring the magnitude and sense of the gradient in Ba to Sr ratio, directional potentials can be built into the structures yielding a new, but controllable, hysteresis phenomenon. Slater’s empirical model for ferroelectric materials has been extended to also describe thin films with polarization gradients normal to the growth surface, i.e., graded ferroelectric devices. This model accounts for several aspects of these structures, including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset.

Giant effective pyroelectric coefficients from graded ferroelectric devices

Effective pyroelectric coefficients as large as 5 μC/cm2 °C, with peak responsivities at approximately 50 °C, were obtained from compositionally graded barium strontium titanate ferroelectric thin film devices formed on silicon using unbalanced magnetron sputter deposition. These effective pyroelectric coefficients are nearly two orders of magnitude larger than those observed from conventional pyroelectric thin film ferroelectric detectors.

Microstructure and thermal conductivity of epitaxial AlN thin films

Thin films of AIN were grown by plasma source molecular beam epitaxy system on Si(111), Si(100), Al2O3(0001) and Al2O3(1102) substrates. X-ray studies indicated that the films were highly textured on Al2O3(0001) and Al2O3(1102), and less so on Si(111) and Si(100). The thermal conductivity of these thin films was investigated by the thermal wave-mirage technique, and a high value of 25.2 W-1m-1K-1 was found for the AlN film on Al2O3(1102). The physical properties were correlated to the microstructures obtained from atomic resolution transmission electron microscopy and atomic force microscopy images.

Growth of Cu films on hydrogen terminated Si(100) and Si(111) surfaces

We have employed reflection high energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HREM) to study Cu films grown on hydrogen terminated Si(100) and Si(111) substrates by molecular beam epitaxy. X‐ray diffraction and RHEED studies indicate 〈100〉Cu growth on Si(100) and 〈111〉Cu growth on Si(111). HREM reveals orientation relationships of [001]Cu∥[011]Si, (010)Cu∥(011)Si and [112]Cu∥[011]Si, (220)Cu∥(111)Si for Si(100) and Si(111), respectively. A copper silicide layer forms on Si(100) with deposition and appears to aid in proper lattice matching. No significant interdiffused region was detected in the films deposited on Si(111), however, distinct orientational variants were observed in this case.

Optical and electrical properties of Al1−xInxN films grown by plasma source molecular-beam epitaxy

Epitaxial Al1−xInxN thin films with 0⩽x⩽1 have been grown by plasma source molecular beam epitaxy on sapphire (0001) substrates at a low temperature of 375 °C. Both reflection high-energy electron diffraction and x-ray diffraction measurements confirm the c-plane growth with the following epitaxial relations: nitride [0001] ∥ sapphire [0001] and nitride 〈0110〉 ∥ sapphire 〈2110〉. However, the degree of crystalline mosaicity and the compositional fluctuation increase with increasing x. The observed direct energy band gap, determined using optical transmission and reflection measurements show relatively less bowing compared to some earlier studies. Electrical resistivity and Hall effect measurements show n-type electrical conductivity in these alloys with carrier concentrations n⩾1019 cm−3 for In-rich alloys and n⩽1010 cm−3 for Al-rich alloys.

Silicon-carbide MOS capacitors with laser-ablated Pt gate as combustible gas sensors

Abstract Sensing certain components present in the exhaust gas stream of internal combustion engines has become a very important application for chemical sensor technology. SiC-based metal-oxide-semiconductor (MOSiC) capacitors were prepared with platinum gates deposited by pulsed laser ablation. The response of their complex admittance, between 62.5 kHz and 1 MHz, to individual combustible species is presented. These devices with laser-ablated gates behaved similarly to their counterparts with sputtered gates exhibiting high sensitivity to propane, propylene, as well as to carbon monoxide. However, contrary to the sputtered Pt, the laser-ablated Pt showed no adhesion problems to the SiO 2 even after prolonged operation at high temperature. The transient combustible response of these MOSiC devices revealed a fast and a slow component with the slow time constant being species-dependent. Furthermore, the clear response to CO strongly suggests that oxygen-related charged defects in the SiO 2 may be playing a role in the sensing mechanism.

Controllable excimer-laser fabrication of conical nano-tips on silicon thin films

We have found conditions for the reproducible, direct laser fabrication of sharp conical tips with heights of about 1 μm and apical radii of curvature of several tens of nanometers. An individual cone is formed when single-crystalline silicon on a silica substrate is irradiated with a single pulse from a KrF excimer laser, homogenized and shaped to a circular spot several microns in diameter. Atomic force microscopy and field-emission scanning electron microscopy were used to characterize these structures. A simple mechanism of formation based on movement of melted material is proposed. Our results suggest that this technique could produce even smaller structures by optimizing the laser processing geometry.

Microstructure of low temperature grown AlN thin films on Si(111)

AlN thin films were grown on HF-etched Si(111) substrates at 400–600 °C by plasma source molecular beam epitaxy. Reflection high energy electron diffraction and transmission electron microscopy studies show that AlN films grown at 400 °C form an initial amorphous region at the interface, followed by c-axis oriented columnar grains with slightly different tilts and twists. AlN films grown at 600 °C showed a significantly reduced amorphous region near the interface promoting an epitaxial growth of AlN with AlN[0001]∥Si[111] and AlN[0110]∥Si[112] orientations. However, all the films show numerous defects such as stacking faults, dislocations, and grain boundaries.

Raman spectroscopy detects and distinguishes neuroblastoma and related tissues in fresh and (banked) frozen specimens

BACKGROUND Raman spectroscopy has been shown to accurately distinguish different neural crest-derived pediatric tumors. This study tests the ability of Raman spectroscopy to accurately identify cryopreserved tissue specimens using a classification algorithm designed from fresh tumor data and vice versa. METHODS Fresh specimens of neuroblastoma and other pediatric neural crest tumors were analyzed with Raman spectroscopy. After analysis, the specimens were stored at -80 degrees C. At a later date, the specimens were thawed and reanalyzed by Raman spectroscopy. A computer algorithm was used to classify the spectra from the frozen tissue against a computer model built on the fresh tissue data. This classification process was then reversed, testing fresh spectra against a model built from frozen data. RESULTS We collected 1114 spectra (862 fresh and 252 frozen) from 62 tissue samples, including 8 normal adrenal glands, 29 neuroblastomas, 14 ganglioneuromas, 8 nerve sheath tumors, and 3 pheochromocytomas. At the tissue level, frozen neuroblastoma, ganglioneuroma, nerve sheath tumor, and pheochromocytoma were distinguished from normal adrenal tissue with 100% sensitivity and specificity. Fresh tissue had the same results except for the misclassification of one specimen of nerve sheath tumor. CONCLUSIONS The representative spectra show a high correlation between fresh and frozen tissue, and a clear difference between pathologic conditions. Spectra from frozen tissue can be accurately classified against spectra from fresh tissue and vice versa. This modality makes it possible to determine in a few minutes a result that often takes 12 to 36 hours for tissue processing and consideration by a trained pathologist to achieve.

Temperature dependence of mobility and carrier density in InN films

We investigate the temperature dependence of Hall mobility μ and carrier density Ne for thin InN films grown by molecular-beam epitaxy and plasma source molecular-beam epitaxy over three orders-of-magnitude difference in their carrier density: for the low-density film Ne=5.8×1017∕cm3 and for the high-density film Ne=3.2×1020∕cm3. In both the films, for temperature up to 300 K, a large temperature-independent concentration of carriers is observed. For higher temperatures, however, carrier density increases with temperature. The characteristic behavior of the mobility for the low-density film is different from that of the high-density film, particularly for temperatures less than 300 K. The low-density film shows a peak behavior in the mobility around 250 K in contrast to the temperature-independent mobility observed for the high-density film for T<300K. We investigate theoretically the concentrations of donor, acceptor, and threading dislocations for both the films and also discussed various electron-scatt...