F. Schiettekatte

Ultrasensitive, fast, thin-film differential scanning calorimeter

The equipment for an ultrasensitive, fast, thin-film differential scanning calorimetry [(TDSC) or nanocalorimetry] technique is described. The calorimetric cell (∼0.30 cm2) operates by applying a short (∼10 ms) dc current pulse (∼10 mA) to a thin (∼50 nm) patterned metal strip, which is supported by a thin (∼50 nm) SiNx membrane. The calorimeter operates at high heating rates (15–200 K/ms) and is very sensitive (30 pJ/K). The design of the calorimeter, the timing/synchronization methods, as well as the choice of key components of the instrument are discussed. Comparisons are made between two dc pulsing circuits that generate the current, a battery powered system and a system based on discharge of an assembly of charged capacitors (recommended). Design concepts for the differential as well as a simplified nondifferential technique are discussed and evaluated via experiments on thin films of indium. The differential design shows an increase in sensitivity, making it suitable for small samples. The custom made electronic circuits are also described, including the design of a preamplifier with low (28×) and high (700×) gain options, which are also compared using experimental data. Noise considerations are critical for the method. Simple models which describe noise levels in the calorimetric data are given and methods for reducing noise are discussed in detail. The sources of noise in the instrument are discussed in terms of both fundamental factors such as Johnson noise of the metal strip, as well as the limiting attributes of the sensing and pulsing circuits and instrumentation. These limiting attributes include spurious signals generated by desorption of ambient gases from the sensor, ground loops, switching regulators, and missing codes in analog-to-digital converter instruments. Examples of the experimental data of heat capacity Cp(T) of various thin films of indium, tin, and polystyrene are presented. A complete data set of raw experimental values is included for a 20 nm sample of Sn which shows the values of current and voltage of both the sample and reference sensors, as well as the differential voltage and the final values of the heat capacity.

High photoluminescence in erbium-doped chalcogenide thin films

Abstract The spectral properties of the chalcogenide glasses As2S3 and As24S38Se38-doped with Er3+ are presented and discussed. Thin films were formed by thermal evaporation and the erbium doping was obtained by subsequent ion implantation. Strong Er3+ emission at 1.54 μm has been observed. The high refractive index of these chalcogenide glasses lead to Er3+ emission cross-sections (15×10 −21 cm 2 ) which are two times higher than for doped silica glass. The lifetime of the Er3+ metastable 4I13/2 energy level was measured to be 2.3 ms. This short lifetime is consistent with the high emission cross-section. Furthermore, the very low phonon energies of chalcogenide glasses lead to relatively long lifetimes of the Er3+ 4I11/2 pump level, which have been measured to be of the order of 0.25 ms. These spectral properties make this glass a good candidate for applications in the field of integrated optics.

Scanning calorimeter for nanoliter-scale liquid samples

We introduce a scanning calorimeter for use with a single solid or liquid sample with a volume down to a few nanoliters. Its use is demonstrated with the melting of 52 nL of indium, using heating rates from 100 to 1000 K/s. The heat of fusion was measured to within 5% of the bulk value, and the sensitivity of the measurement was ±7 μW. The heat of vaporization of water was measured in the scanning mode to be within ±23% of the bulk value by actively vaporizing water droplets from 2 to 100 nL in volume. Results within 25% were obtained for the heat of vaporization by using the calorimeter in a heat-conductive mode and measuring the passive evaporation of water. Temperature measurements over a period of 10 h had a standard deviation of 3 mK.

Band gaps of the dilute quaternary alloys GaNxAs1−x−yBiy and Ga1−yInyNxAs1−x

We report strong band gap photoluminescence at room temperature in dilute quaternary GaNxAs1−x−yBiy alloys (x<1.6%,y<2.6%) grown by molecular beam epitaxy. The band gap of the alloy can be approximated by the band gap of GaAs minus the reduction in gap associated with the effects of N and Bi alloying individually. A one-parameter method for fitting the composition dependence of the band gaps of dilute quaternary semiconductor alloys is proposed which is in excellent agreement with data for Ga1−yInyNxAs1−x.

Synthesis and Characterization of Single-Layer Silver—Decanethiolate Lamellar Crystals

We report the synthesis of silver-decanethiolate (AgSC10) lamellar crystals. Nanometer-sized Ag clusters grown on inert substrates react with decanethiol vapor to form multilayer AgSC10 lamellar crystals with both layer-by-layer and in-plane ordering. The crystals have strong (010) texture with the layers parallel to the substrates. The synthesis method allows for a precise control of the number of layers. The thickness of the lamellae can be manipulated and systematically reduced to a single layer by decreasing the amount of Ag and lowering the annealing temperature. The single-layer AgSC10 lamellae are two-dimensional crystals and have uniform thickness and in-plane ordering. These samples were characterized with nanocalorimetry, atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), and Rutherford backscattering spectroscopy (RBS).

Microstructural evolution in H ion induced splitting of freestanding GaN

We investigated the microstructural transformations during hydrogen ion-induced splitting of GaN thin layers. Cross-sectional transmission electron microscopy and positron annihilation spectroscopy data show that the implanted region is decorated with a high density of 1 – 2 nm bubbles resulting from vacancy clustering during implantation. These nanobubbles persist up to 450 ° C. Ion channeling data show a strong dechanneling enhancement in this temperature range tentatively attributed to strain-induced lattice distortion. The dechanneling level decreases following the formation of plateletlike structures at 475 ° C. Extended internal surfaces develop around 550 ° C leading to the exfoliation of GaN thin layer.

Fabrication of high resistivity cold-implanted InGaAsP photoconductors for efficient pulsed terahertz devices

A multiple-energy, high fluence, MeV Fe ion implantation process was applied at 83 K to heavily damage a low band gap (0.79 eV) epitaxial InGaAsP layer. Optimal rapid thermal annealing conditions were found and produced a fast photoconductor with high resistivity (up to 2500 Ωcm) and Hall mobility around 400 cm2V−1s−1. Short photocarrier trapping times (0.3 ps – 3 ps) were observed via transient differential reflectivity measurements. Furthermore, photoconductive terahertz devices with coplanar electrodes were fabricated and validated. Under pulsed excitation with a 1550 nm femtosecond fiber laser source, antennas based on Fe-implanted InGaAsP are able to emit broadband radiation exceeding 2 THz. Given such specifications, this new material qualifies as a worthy candidate for an integration into optical terahertz spectrometer designs.

Influence of curvature on impurity gettering by nanocavities in Si

Competition for Au gettering in Si between two cavity layers of different diameter (34 and 12 nm) is examined. Au is initially contained in the large cavity layer made by He implantation. Transport of Au towards the second, small diameter cavity layer is measured by ion scattering. The true surface in both layers is determined by electron microscopy. Small cavities are found to be four times more efficient gettering sites than large cavities for the same amount of internal surface. This difference is explained by a simple model based on curvature thermodynamics, faceting, and surface reconstruction.

Ion-beam modification of Co/Ag multilayers II: Variation of structural and magnetic properties with Co layer thickness

The structural, magnetic and transport properties of rf sputtered Co/Ag multilayers with Co-layer thicknesses ranging from 1 to 14 A have been studied by a combination of x-ray diffraction, magnetic and transport measurements. The magnetoresistance at room temperature has a maximum value of more than 12% for a Co-layer thickness around 5 A. Magnetic measurements demonstrate that samples near this Co-layer thickness are in the transition region from superparamagnetic to ferromagnetic behavior. X-ray analysis indicates that, during deposition, a significant quantity of Co is dispersed throughout a highly textured Ag matrix. Upon irradiation with 1 MeV Si+ ions up to a dose of 5×1016 Si+/cm2, an initial demixing of the Co is followed by segregation into grains with the same texture as the Ag. The resulting changes in the magnetization and magnetoresistance are characterized on the basis of a log-normal distribution of the volume of the magnetic particles. As the particle sizes increase, a systematic evolutio...

Evolution of the Potential-Energy Surface of Amorphous Silicon

The link between the energy surface of bulk systems and their dynamical properties is generally difficult to establish. Using the activation-relaxation technique, we follow the change in the barrier distribution of a model of amorphous silicon as a function of the degree of global relaxation. We find that while the barrier-height distribution, calculated from the initial minimum, is a unique function that depends only on the level of relaxation, the reverse-barrier height distribution, calculated from the final state, is independent of global relaxation, following a different function. Moreover, the resulting gained or released energy distribution is a simple convolution of these two distributions indicating that the activation and relaxation parts of the elementary relaxation mechanism are completely independent. This characterized energy landscape can be used to explain nanocalorimetry measurements.