Robert Laibowitz

Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties

Advanced applications for high k dielectric and ferroelectric materials in the electronics industry continues to demand an understanding of the underlying physics in decreasing dimensions into the nanoscale. We report the synthesis, processing, and electrical characterization of thin (<100nm thick) nanostructured thin films of barium titanate (BaTiO3) built from uniform nanoparticles (<20nm in diameter). We introduce a form of processing as a step toward the ability to prepare textured films based on assembly of nanoparticles. Essential to this approach is an understanding of the nanoparticle as a building block, combined with an ability to integrate them into thin films that have uniform and characteristic electrical properties. Our method offers a versatile means of preparing BaTiO3 nanocrystals, which can be used as a basis for micropatterned or continuous BaTiO3 nanocrystal thin films. We observe the BaTiO3 nanocrystals crystallize with evidence of tetragonality. We investigated the preparation of wel...

Ni–NiO–Ni tunnel junctions for terahertz and infrared detection

We present complete experimental determinations of the tunnel barrier parameters (two barrier heights, junction area, dielectric constant, and extrinsic series resistance) as a function of temperature for submicrometer Ni-NiO-Ni thin-film tunnel junctions, showing that when the temperature-invariant parameters are forced to be consistent, good-quality fits are obtained between I-V curves and the Simmons equation for this very-low-barrier system (measured phi approximately 0.20 eV). A splitting of approximately 10 meV in the barrier heights due to the different processing histories of the upper and lower electrodes is clearly shown, with the upper interface having a lower barrier, consistent with the increased effect of the image potential at a sharper material interface. It is believed that this is the first barrier height measurement with sufficient resolution for this effect to be seen. A fabrication technique that produces high yields and consistent junction behavior is presented as well as the preliminary results of inelastic tunneling spectroscopy at 4 K that show a prominent peak at -59 meV, shifted slightly with respect to the expected transverse optic phonon excitation in bulk NiO but consistent with other surface-sensitive experiments. We discuss the implications of these results for the design of efficient detectors for terahertz and IR radiation.

Photocurrent spectroscopy of low-k dielectric materials: Barrier heights and trap densities

Measurements of photoinduced current have been performed on thin films of porous low-k dielectric materials comprised of carbon-doped oxides. The dielectric films were deposited on silicon surfaces and prepared with a thin gold counterelectrode. From the spectral dependence of the photoinduced current, barrier heights for the dielectric∕silicon and dielectric∕gold interface were deduced. Transient currents were also found to flow after the photoexcitation was abruptly stopped. An estimate of the density of shallow electron traps within the low-k material was obtained from the measurement of the net charge transported from this detrapping current. A density of traps in the range of 6×1016traps∕cm3 was inferred for the low-k films, far exceeding that observed by the same technique for reference dielectric films of pure SiO2. This behavior was also compatible with photocurrent I‐V measurements on the low-k dielectric films and SiO2 reference sample.

Charge trapping at the low-k dielectric-silicon interface probed by the conductance and capacitance techniques

Trap states close to the interfaces in thin films of porous low-k dielectric materials are expected to affect interfacial barriers with contacts and consequently electrical leakage and reliability in these materials. These interfacial traps were investigated using capacitance and conductance measurements in metal/insulator/silicon capacitor structures composed of carbon-doped oxide low-k dielectric films with gold counterelectrodes. The measurements yielded information on the charge state of the low-k dielectric and an estimated density of traps near the Si interface of 2×1011 cm−2 eV−1, considerably greater than in typical SiO2 films. The effects of temperature and annealing were also investigated. An activation energy of 0.36±0.04 eV for trap filling and emptying was inferred.

Fabrication of patterned single-crystal SrTiO3 thin films by ion slicing and anodic bonding

A new technique for directly fabricating patterned thin films (<1μm thick) of fully single-crystal strontium titanate uses deep H+ implantation into the oxide sample, followed by anodic bonding of the sample to a Pyrex or Pyrex-on-Si substrate. The dielectric properties and crystal structure of such thin films are characterized and are found to be essentially those of the bulk single crystal.

Probing the Interface Barriers of Dopant-Segregated Silicide–Si Diodes With Internal Photoemission

An experimental study is presented to probe the interface barriers of dopant-segregated silicide-Si diodes with internal photoemission. The spatial information of the interface dipoles, which is believed to be the cause of the effective Schottky barrier height (SBH) modification, is extracted from the field dependence of the barrier heights. A clear difference between the dopant segregation (DS) junctions and a pure Schottky junction is found: Boron DS modifies the effective SBH by forming a p+- n junction while arsenic DS forms a “Shannon” junction with a fully depleted 1.5-nm doping depth in front of the silicide.

Tunneling in Ultrathin Glass Films

Ultrathin phosphosilicate glass films have been formed on degenerate n-type Si wafers in a high temperature vapor transport process. The glass thickness (determined ellipsometrically) could be controllably varied in the range from 30 to 100 A. A quadratic dependence of the glass thickness vs growth time was observed in most cases. This dependence indicates that the glass film forms via a diffusion-controlled reaction. After depositing metal electrodes on the glass, the tunneling current was studied down to temperatures of about 1.5 K. The thickness dependence of the junction resistance was measured and an estimate of the tunneling barrier height was obtained. It will be shown that the barrier height depends on the history of the samples. Structures in the i-v curve are discussed in terms of properties of the glass film and the type of metal electrode.

Detection of charge carrier confinement into mobile ionic defects in nanoporous dielectric films for advanced interconnects

Reliability and robustness of low-k materials for advanced interconnects has become one of the major challenges for the continuous down-scaling of silicon semiconductor devices. Metal catalyzed time dependent breakdown is a major force preventing integration of sub-32 nm process technology nodes. Here, the authors demonstrate that ions can behave as trapping points for charge carriers. A mechanism for describing trapping of charge carriers into mobile ions under bias and temperature stress is presented. Charge carrier confinement into ionic center was found to be dominated by ionic transport. After extended bias and temperature stress, the magnitude of charge trapping into ionic centers decreased. Simulations suggest that built-in fields could reduce the effect of externally applied fields in directing ionic drift, therefore inhibiting the trapping mechanism. This work depicts the dual role of ionic species when catalyzing dielectric failure (mobile defect and local field distortion).

Effects of photoinduced carrier injection on timedependent dielectric breakdown

Time-dependent dielectric breakdown experiments were performed under broadband UV illumination in order to investigate the effects of increased electron concentration on time to breakdown. Preliminary results show that breakdown can be achieved at shorter time scales and lower fields than in standard reliability tests.

The effect of voltage bias stress on temperature-dependent conduction properties of low-k dielectrics

The change in leakage current in porous low-k interconnect dielectrics (LKDs) arising from time-dependent dielectric breakdown (TDDB) has been investigated. Using periodic measurements of temperature-dependent IV curves and photocurrent decay during voltage bias stress the apparent change in trap density and conduction mechanism in LKDs are investigated. A substantial change in the temperature-dependence of the current is observed, suggesting a move from a thermally excited conduction process to a tunnel-like or percolative process. This change is correlated with a large increase in trap density after long-term bias stress, consistent with a trap to trap tunneling interpretation. However, throughout the high-voltage bias stress, the leakage current continues to obey a √E field dependence.