Degradation and breakdown characteristics of thin MgO dielectric layers

MgO has been suggested as a possible high-k dielectric for future complementary metal-oxide semiconductor processes. In this work, the time dependent dielectric breakdown (TDDB) characteristics of 20 nm MgO films are discussed. Stress induced leakage current measurements indicate that the low measured Weibull slopes of the TDDB distributions for both n-type and p-type devices cannot be attributed to a lower trap generation rate than for SiO2. This suggests that much fewer defects are required to trigger breakdown in MgO under voltage stress than is the case for SiO2 or other metal-oxide dielectrics. This in turn explains the progressive nature of the breakdown in these films which is observed both in this work and elsewhere. The reason fewer defects are required is attributed to the morphology of the films

Formation of nanoporous InP by electrochemical anodization

Porous InP layers can be formed electrochemically on (100) oriented n- InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains which appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually, the domains meet forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Numerical models of this process have been developed. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value

Pitting and porous layer formation on n-InP anodes

Surface pitting occurs when InP electrodes are anodized in KOH electrolytes at concentrations in the range 2 - 5 mol dm-3. The process has been investigated using atomic force microscopy (AFM) and the results correlated with cross-sectional transmission electron microscopy (TEM) and electroanalytical measurements. AFM measurements show that pitting of the surface occurs and the density of pits is observed to increase with time under both potentiodynamic and potentiostatic conditions. This indicates a progressive pit nucleation process and implies that the development of porous domains beneath the surface is also progressive in nature. Evidence for this is seen in plan view TEM images in which individual domains are seen to be at different stages of development. Analysis of the cyclic voltammograms of InP electrodes in 5 mol dm-3 KOH indicates that, above a critical potential for pit formation, the anodic current is predominantly time dependent and there is little differential dependence of the current on potential. Thus, pores continue to grow with time when the potential is high enough to maintain depletion layer breakdown conditions

A mechanistic study of anodic formation of porous InP

When porous InP is anodically formed in KOH electrolytes, a thin layer ~40 nm in thickness, close to the surface, appears to be unmodified. We have investigated the earlier stages of the anodic formation of porous InP in 5 mol dm-3 KOH. TEM clearly shows individual porous domains which appear triangular in cross-section and square in plan view. The crosssections also show that the domains are separated from the surface by a ~40 nm thick, dense InP layer. It is concluded that the porous domains have a square-based pyramidal shape and that each one develops from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain, and these domains eventually form a continuous porous layer. This implies that the development of porous domains beneath the surface is also progressive in nature. Evidence for this was seen in plan view TEM images. Merging of domains continues to occur at potentials more anodic than the peak potential, where the current is observed to decrease. When the domains grow, the current density increases correspondingly. Eventually, domains meet, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Quantitative models of this process are being developed

Effect of electrolyte concentration on anodic nanoporous layer growth for n-InP in aqueous KOH

The surface morphology and sub-surface porous structure of (100) n-InP following anodization in 1 - 10 mol dm-3 aqueous KOH were studied using linear sweep voltammetry (LSV) in combination with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). LSV of n-InP in 10 mol dm-3 KOH showed a single anodic current peak at 0.41 V. As the concentration of electrolyte was decreased, the peak increased in current density and charge and shifted to more positive potentials; eventually individual peaks were no longer discernable. Porous layers were observed in SEM cross-sections following linear potential sweeps and the porous layer thickness increased significantly with decreasing KOH concentration, reaching a maximum value at ~2.2 mol dm-3. At concentrations less than 1.8 mol dm-3 the layer thickness decreased sharply, pore diameters became wider and pore walls became narrower until eventually, at 1.1 mol dm-2 or lower, no porous layers were observed. It was also observed that the pore width increased and the inter-pore spacing decreased with decreasing concentration. It is proposed that preferential pore propagation occurs along directions, contrary to previous suggestions, and that the resulting anoporous domains, initially formed, have triangular cross-sections when viewed in one of the {110} cleavage planes, ‘dove-tail’ crosssections viewed in the orthogonal {110} cleavage plane and square profiles when viewed in the (100) plane of the electrode surface

Anodic formation and characterization of nanoporous InP in aqueous KOH electrolytes

The anodic behavior of highly doped (> 1018 cm-3) n-InP in aqueous KOH was investigated. Electrodes anodized in the absence of light in 2- 5 mol dm-3 KOH at a constant potential of 0.5- 0.75 V (SCE), or subjected to linear potential sweeps to potentials in this range, were shown to exhibit the formation of a nanoporous subsurface region. Both linear sweep voltammograms and current-time curves at constant potential showed a characteristic anodic peak, corresponding to formation of the nanoporous region. No porous region was formed during anodization in 1 mol dm-3 KOH. The nanoporous region was examined using transmission electron microscopy and found to have a thickness of some 1- 3 μm depending on the anodization conditions and to be located beneath a thin (typically ∼40 nm), dense, near-surface layer. The pores varied in width from 25 to 75 nm and both the pore width and porous region thickness were found to decrease with increasing KOH concentration. The porosity was approximately 35%. The porous layer structure is shown to form by the localized penetration of surface pits into the InP, and the dense, near-surface layer is consistent with the effect of electron depletion at the surface of the semiconductor

The calpain system is associated with survival of breast cancer patients with large but operable inflammatory and non-inflammatory tumours treated with neoadjuvant chemotherapy

The calpains are a family of intracellular cysteine proteases that function in a variety of important cellular functions, including cell signalling, motility, apoptosis and survival. In early invasive breast cancer expression of calpain-1, calpain-2 and their inhibitor, calpastatin, have been associated with clinical outcome and clinicopathological factors. The expression of calpain-1, calpain-2 and calpastatin was determined using immunohistochemistry on core biopsy samples, in a cohort of large but operable inflammatory and non-inflammatory primary breast cancer patients treated with neoadjuvant chemotherapy. Information on treatment and prognostic variables together with long-term clinical follow-up was available for these patients. Diagnostic pre-chemotherapy core biopsy samples and surgically excised specimens were available for analysis. Expression of calpastatin, calpain-1 or calpain-2 in the core biopsies was not associated with breast cancer specific survival in the total patient cohort; however, in patients with non-inflammatory breast cancer, high calpastatin expression was significantly associated with adverse breast cancer-specific survival (P=0.035), as was low calpain-2 expression (P=0.031). Low calpastatin expression was significantly associated with adverse breast cancer-specific survival of the inflammatory breast cancer patients (P=0.020), as was low calpain-1 expression (P=0.003). In conclusion, high calpain-2 and low calpastatin expression is associated with improved breast cancer-specific survival in non-inflammatory large but operable primary breast cancer treated with neoadjuvant chemotherapy. In inflammatory cases, high calpain-1 and high calpastatin expression is associated with improved breast cancer-specific survival. Determining the expression of these proteins may be of clinical relevance. Further validation, in multi-centre cohorts of breast cancer patients treated with neoadjuvant chemotherapy, is warranted

Formation and characterization of porous InP layers in KOH Solutions

Porous InP layers were formed electrochemically on (100) oriented n-InP substrates in various concentrations of aqueous KOH under dark conditions. In KOH concentrations from 2 mol dm-3 to 5 mol dm-3, a porous layer is obtained underneath a dense near-surface layer. The pores within the porous layer appear to propagate from holes through the near-surface layer. Transmission electron microscopy studies of the porous layers formed under both potentiodynamic and potentiostatic conditions show that both the thickness of the porous layer and the mean pore diameter decrease with increasing KOH concentration. The degree of porosity, estimated to be 65%, was found to remain relatively constant for all the porous layers studied

Frequency of occurrence of numbers in the World Wide Web

The distribution of numbers in human documents is determined by a variety of diverse natural and human factors, whose relative significance can be evaluated by studying the numbers' frequency of occurrence. Although it has been studied since the 1880's, this subject remains poorly understood. Here, we obtain the detailed statistics of numbers in the World Wide Web, finding that their distribution is a heavy-tailed dependence which splits in a set of power-law ones. In particular, we find that the frequency of numbers associated to western calendar years shows an uneven behavior: 2004 represents a `singular critical' point, appearing with a strikingly high frequency; as we move away from it, the decreasing frequency allows us to compare the amounts of existing information on the past and on the future. Moreover, while powers of ten occur extremely often, allowing us to obtain statistics up to the huge 10^127, `non-round' numbers occur in a much more limited range, the variations of their frequencies being dramatically different from standard statistical fluctuations. These findings provide a view of the array of numbers used by humans as a highly non-equilibrium and inhomogeneous system, and shed a new light on an issue that, once fully investigated, could lead to a better understanding of many sociological and psychological phenomena.Comment: 5 pages, 4 figure

Nanoporous domains in n-InP anodized in KOH

A model of porous structure growth in semiconductors based on propagation of pores along the A directions has been developed. The model predicts that pores originating at a surface pit lead to porous domains with a truncated tetrahedral shape. SEM and TEM were used to examine cross- sections of n-InP electrodes in the early stages of anodization in aqueous KOH and showed that pores propagate along the A directions. Domain outlines observed in both TEM and SEM images are in excellent agreement with the model. The model is further supported by plan-view TEM and surface SEM images. Quantitative measurements of aspect ratios of the observed domains are in excellent agreement with the predicted values