Search

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence

Ion Beam Induced Charge (IBIC) and Deep Level Transient Spectroscopy (DLTS) were used to investigate displacement damage caused by MeV energy ion beams in Si diodes. The devices were irradiated with 3 MeV Si ions to create displacement damage and a 2 MeV He ion beam was used for IBIC. The IBIC signal deterioration was measured as the function of the ion fluence and DLTS was used to identify the defects and their quantities. We used a new calculation method based on previous work by Fizzotti et al. [9] and more recently by Vittone [10] to determine the fraction of the active traps that affect the lifetime. The first application of this method is presented in this paper.

We present a series of metrics for comparison between displacement damage due to heavy ion and neutron irradiation in silicon bipolar junction transistors. We have compared ion and fast neutron irradiations to determine an ion-to-neutron damage equivalence. We find that a combination of metrics (damage factor, deep level transient spectroscopy (DLTS) and annealing factor) are needed to ensure a comprehensive understanding of the physics involved in the ion-to-neutron conversion. The linearity of the damage factor (primarily probing the base-emitter junction) is not enough to ensure a valid comparison; rather, we must also use additional techniques (DLTS and capacitance measurements) to ensure that collector compensation is not occurring. As a result, care must be taken in choosing the irradiation beam for ion exposures. The displacement damage should peak in the sensitive region of the device to both ensure maximum gain degradation and to minimize collector compensation.

Following irradiation, the gain of silicon bipolar transistors can be improved by annealing at 350 K. We show that both the number of defects measured by deep level transient spectroscopy (DLTS) and the gain can be restored to the post irradiation state by injection of minority carriers. One can cycle between the post irradiation state and the 350 K annealed state by alternating minority carrier injection at 300 K with zero- or reverse-bias anneals at 350 K. The structure of the bistable defects is not known, but we observe that they affect capture kinetics into the shallow charge state of the silicon divacancy defect, V 2 (=/−). This suggests that the bistable defects are located within the neutron or ion damage cluster.

Although it has been speculated that estrogen therapy may promote changes in breast tissue that could lead to cancer, no information exists as to differences in breast tissue for women who do and do not take hormone replacement (HRT) therapy. This study seeks to determine if there are differences in the tissue of women taking HRT in contrast to those who do not and if these differences are apparent in cases of breast cancer, cellular atypia, fibrocystic (FCD) disease and normal breasts. A total of 327 non-pregnant, nonlactating, pre-menopausal women were enrolled in the study, including 139 women who were actively taking HRT and 188 women who never had taken HRT. Using breast enhanced scintigraphy test (BEST) imaging, differentiation of breast tissue was determined. The groups were then analyzed to determine the effect of hormone therapy within each category of breast tissue. Differentiation between normal, FCD, cellular atypia, and breast cancer represent statistically significant differences (p.001) in metabolic activity and vascularity as demonstrated by differences in both average count activity (ACA) and maximal count activity (MCA). The distinction between cellular atypia and infiltrating breast cancer was statistically (p.05) different when looking at the maximal activity. Normal breast tissue and breasts with FCD appear more homogenous with no statistical differences in variability in breast tissue. Tissue variability is statistically greater when localized processes, such as cellular atypia and breast cancer, are present. Differentiation of cellular metabolic activity in breast tissue can be statistically determined when looking at the average and maximal metabolic activity. The final distinction between cellular atypia and cancer occurs when a focal region of breast tissue becomes metabolically more active than the surrounding breast tissue as shown by statistical increases in MCA. These findings are confirmed by the increased metabolic variability seen in regions of cellular atypia and cancer compared with the homogenous metabolic activity present in normal and fibrocystic breasts.

Ta2O5 is a candidate for use in metal–oxide–metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta2O5 MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films

Pb(Zr0.53Ti0.47)O3 (PZT) on diamond is a potentially robust structure for surface acoustic wave (SAW) device applications. We have studied the growth and physical characteristics of PZT on diamond and other substrates by pulsed laser deposition. Under a broad range of processing conditions we explored, PZT deposited directly on diamond is almost exclusively pyrochlore-type, which is nonferroelectric. Growth of ferroelectric perovskite PZT is promoted via the use of a PbTiO3 buffer layer within a narrow window of processing parameters [i.e., P(O2)=100–200 mTorr, T=550–650 °C, 1–2 J/cm2]. Similar results were also obtained for deposition of PZT on Si, Pt-coated Si, and Pt-coated diamond substrates. The dielectric constants of the perovskite PZT films are 500–650 at 1 V and 100 kHz. The piezoelectric coefficients of these films are in the range of 50×10−12–350×10−12 m/V. The SAW velocity of perovskite PZT films is similar to that of highly oriented sputter deposited ZnO films. The acoustic attenuation in per...

Parallel synthesis together with high-throughput screening was used to identify candidate materials for integrated circuit applications that demand a superior high permittivity dielectric thin film. Specifically, we developed a continuous-composition spread (CCS) technique to synthesize much of a pseudoternary oxide system in a single deposition and used this in conjunction with a high-throughput measurement protocol, thereby allowing each chemical system to be deposited and evaluated in about 24 h. This approach led to the identification of compositions in the Zr-Sn-Ti-O system with promising properties. The same technique was used to determine the optimum compositions as a function of processing parameters. Films with the composition Zr .2 Sn .2 Ti .6 O 2 were then prepared using a conventional synthetic technique (on-axis sputtering) and were verified to have excellent properties. Thus, the CCS technique has demonstrated utility in rapidly identifying and developing a useful new material.

The high brilliance of third generation undulator sources offers an unprecedented opportunity to perform coherent X-ray diffraction studies of the structure and dynamics of materials on length scales down to interatomic spacings. We have measured the coherent diffraction or “speckle” pattern from a GaAsAlAs superlattice. The speckle diffraction is consistent with height fluctuations of the superlattice of ∼10 A over lengths of order 60 μm. Unlike other methods of characterizing the roughness of semiconductor material, such as scanning probe microscopies (STM, AFM) or scanning electron microscopy (SEM), this novel coherent diffraction method is sensitive to lateral variations of the interface height buried within the small illuminated volume of the material, and thus can offer information unavailable from other non-destructive techniques.

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106 cm−2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014 cm−3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is ...

We report x-ray photon correlation spectroscopy studies of the static structure factor and dynamic correlation function of a gold colloid dispersed in the viscous liquid glycerol. We find a diffusion coefficient for Brownian motion of the gold colloid which agrees well with that extrapolated from measurements made with visible light, but which was determined on an optically opaque sample and in a wave-vector range inaccessible to visible light.

Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for x‐ray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries.

An estimated one third of all American and United Kingdom women take hormone therapy. In sharp contrast to these numbers, as many as one half of women diagnosed with breast cancer have taken hormones. Little additional information is available regarding the risk of breast cancer and even less is known about the association between hormone therapy and fibrocystic (FCD) disease or atypia of the breast. Three hundred women between 30 and 50 years of age were enrolled in this study, including 120 taking hormone replacement (HRT) therapy and 180 women who had never taken hormone therapy. These women were divided into four categories including those with normal breast tissue, those with FCD disease, those with cellular atypia, and those with breast cancer. Another group of women were also identified who had breast implants. Using breast enhanced scintigraphy (BEST) imaging, changes in breast tissue were determined and compared according to the use of HRT. Forty percent (122 of 300) had “normal” breasts, of whom 68.8% (84 of 122) did not take HRT. This accounted for 46.7% (84 of 180) of the women not taking hormone therapy, while only 31.7% (38 of 120) of the women taking HRT had normal breasts. This difference was statistically (p.001) significant. There was a greater incidence of breast abnormality in women taking HRT and a lower incidence in pathology among women not taking HRT when cumulatively analyzed for FCD, cellular atypia, and breast cancer. This difference was statistically significant (p.001) for women with breast cancer where 62.5% (10 of 16) were women taking HRT. Although the study was relatively small, it is the first such study to compare a continuum of changes in breast tissue according to the use of HRT. The study suggests that the initial empirical observations regarding higher incidence of HRT among women with breast cancer, may have a relationship to underlying changes in breast tissue that are associated with differences in mitochondrial content and activity. Further investigation is needed.

Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (${\mathrm{C}}_{60}$) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of ${\mathrm{C}}_{60}$ reduces the critical thickness at which Al becomes conducting from \ensuremath{\sim}35 to \ensuremath{\sim}20 \AA{}. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of ${\mathrm{C}}_{60}$. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active ${\mathit{A}}_{\mathit{g}}$(2) pentagonal-pinch mode, imply that up to six electrons per ${\mathrm{C}}_{60}$ are transferred from the Al to the ${\mathrm{C}}_{60}$ layer. This demonstration of charge transfer across planar metal-${\mathrm{C}}_{60}$ interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.

Crystals of C60 or C70 cocrystallized with n-pentane grow as elongated, ten-sided columns. X-ray diffraction shows ordering of C60 or C70 molecules along the column and a remarkable tenfold symmetry normal to the column. The ratio of the x-ray-diffraction vectors of the two lowest-order diffraction spots is nearly equal to the “golden ratio,” τ. Despite these similarities with decagonal, quasicrystalline order, the diffraction indicates crystalline order with a twinned unit cell.

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.

The continuing drive towards miniaturization of electronic devices1 is motivating the search for new materials. Consider, for example, the case of the much-used dynamic random-access memory. The minimum capacitance per cell that can be tolerated is expected2 to remain at 30–40 fF, but as the cell area decreases, the corresponding reduction in geometric capacitance has to be compensated for. So far, this has been achieved by resorting to complex non-planar structures and/or using much thinner films of the dielectric insulator, amorphous silicon dioxide (a-SiOx), although the latter approach is limited by the electric fields that can be supported by a-SiOx before its insulating properties break down. An alternative strategy is to develop thin-film insulators that have a dielectric constant significantly greater than that of a-SiOx, reducing the size of the fields required for device operation. Here we show that a composition-spread technique allows for the efficient evaluating of materials with both a high dielectric constant and a high breakdown field. We apply this approach to the Zr–Sn–Ti–O system, and we find that compositions close to Zr0.15Sn0.3Ti0.55O2−δ are better thin-film dielectrics than high-quality deposited a-SiOx. Although detailed tests of the performance of these materials have not yet been carried out, our initial results suggest that they are likely to be comparable to the best alternatives (such as (Ba, Sr)TiO3) currently being considered for integrated-circuit capacitors.

Differences in breast tissue can be determined using breast-enhanced scintigraphy test (BEST) imaging. Minimal work in vivo has been done previously to determine the effects of soy protein on breast tissue. The authors' earlier work demonstrated reduction in inflammatory changes in breast tissue. This work was conducted to examine the effect of daily soy protein consumption on a larger group of women over the course of 1 year. Sixty-four premenopausal women were studied after initial BEST imaging evaluation revealed fibrocystic changes of the breast. Women were asked to consume a medical-grade soy protein on a daily basis, making no other dietary or lifestyle changes during that time. Each woman underwent BEST imaging 1 year later with the results compared to the initial findings. Women and their physicians reported a subjective reduction in both breast tenderness and fibrocystic disease (FCD). There was a nonstatistical reduction in both the average and maximal count breast activity following 1 year of daily soy consumption. There was a statistically significant reduction ( P< .01) in variability of tissue activity following 1 year of soy protein treatment. This is the first in vivo study looking at the effect of soy protein on breast tissue health. The findings are promising and showed both objective and subjective findings consistent with a reduction in fibrocystic disease of the breast. Further research is needed to confirm these findings in a greater number of women and to determine if soy protein has the same beneficial effect in atypia and breast cancer.

N‐channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on‐off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.

New classes of high dielectric constant materials based on binary and ternary amorphous metal oxides will be discussed. These films are compatible with low temperature processing within the metal layers of a circuit and can therefore be used with conventional metal electrodes like TiN. Different classes of mixed metal oxides will be discussed that are based on alloys of tantalum oxide and titanium oxide.

Epitaxial ferroelectric SrRuO3/Pb(Zr0.52Ti0.48)O3/SrRuO3 heterostructures have been fabricated employing isotropic metallic oxide electrodes on (100) SrTiO3 and (100) Si with an yttria stabilized zirconia buffer layer. The structures have been grown in situ by 90° off‐axis sputtering, which allows the growth of uniform stoichiometric films over large areas with excellent step coverage. X‐ray diffraction, Rutherford backscattering spectroscopy, and cross‐sectional transmission electron microscopy reveal high crystalline quality and coherent interfaces. They exhibit superior fatigue characteristics over those made with metal electrodes, showing little degradation over 1010 cycles, with a large remnant polarization.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

The positions of the electronic levels of an intrinsic bistable defect have been measured using deep level transient spectroscopy (DLTS) in n- and p-type damaged silicon bipolar transistor diodes after minority carrier injection and thermal annealing. The kinetic rates observed during conversion of this defect have been determined using both DLTS and transistor gain measurements on devices irradiated with electrons and neutrons. First order conversion kinetics are observed during both injection and thermal annealing of this defect in electron damaged transistors, but more complicated, stretched-out kinetics are seen in neutron irradiated devices. The latter behavior can be successfully modeled as a small spread in the energy barriers for atomic displacements of this defect probably due to strain or electrostatic variations expected in damage clusters. The measured injection bias dependence of the recombination-driven transition to the bistable state of this defect is unlike that seen for generation-recomb...

Isochronal anneal sequences have been carried out on pnp and npn transistors irradiated with fast neutrons at a variety of fluences. The evolution of base and collector currents was utilized to characterize the annealing behavior of defects in both the emitter-base depletion region and the neutral base. Various annealing biases, theoretical modeling, as well as previous deep level transient spectroscopy (DLTS) data, were used to assign the relative magnitude of each of the important defects to the total recombination current. We find that donor-vacancy pairs in the neutral n-type base of our pnp transistors are responsible for about 1/3 of the postdamage lifetime degradation, while the remaining recombination currents can be largely attributed to a cluster-related divacancylike defect which has no shallow state DLTS emission peak. This latter defect anneals gradually from 350 to 590 K. Generation/recombination currents in the base-emitter junctions in both types of devices were found to anneal in a simila...

Using deep level transient spectroscopy, the authors have measured the defect spectrum in the collector of a n-p-n bipolar transistor following fast neutron irradiation as well as the gain on the same device. They show that a slow change observed in both the gain and deep level traps in the n-type collector at 300K are bistable. The transistor gain and the defects can be returned to the postirradiation condition by forward bias at room temperature, i.e., by operating the transistor (gain) or injection through the base-collector diode (defect spectrum).

We report the synthesis by molecular beam epitaxy of a new insulating phase in the barium bismuth oxide system, BaBi 2 O y , and its use as a building block for bismuth oxide heterostructures.

The compounds Ta2NiS5 and Ta2NiSe5 have a novel layered structure: within the layers are rows of tantalum atoms separated by rows of nickel atoms. The lower dimensional structure of these compounds as well as the expected d1 non-bonding electron configuration of the tantalum atom leads to the expectation that classical charge density wave behavior would be observed in their properties. An examination of the electrical, magnetic and structural properties shows that this is not the case; rather, the properties are more consistent with the materials being small band gap semiconductors. Further, both compounds show structural phase transitions: in Ta2NiS5 a first-order transition occurs at 28 K and in Ta2NiSe5 a second-order transition occurs at 328 K. The latter transition is shown by electron microscopy to be martensitic-like.

Temperature-induced phase transitions in TlReO 4 have been investigated by Raman spectroscopy and by x-ray powder diffraction. The data reveal the occurrence of a hitherto unreported phase transition from orthorhombic to the tetragonal scheelite-type structure near 200 K and this is a first-order transition. At temperatures above 400 K the orthorhombic phase is known to transform to the scheelite-type structure. These phase changes present a unique situation. The lattice parameters of the low-temperature (LT) scheelite phase are a 0 =5.798 A and c 0 =12.920 A at 70 K and for the tetragonal high-temperature (HT) scheelite phase are a 0 =5.761 A, and c 0 =13.33 A at 400 K. The corresponding unit-cell volumes are 434 A 3 and 442 A 3 . For the orthorhombic phase a 0 =5.623, b 0 =5.791, c 0 =13.295, and V=432 A 3 at 293 K. The orthorhombic-LT-scheelite and the orthorhombic-HT-scheelite phase boundaries drawn on the basis of the ΔV's at the transition point intersect at negative pressures in a P-T diagram. This intersection appears to satisfy the criteria required for the occurrence of an "isolated point" in the P-T plane; namely a point at which two first-order phase boundaries intersect to become a continuous transition. This is probably the first experimental indication for such a possibility.

Strandlike domains are observed in the commensurate charge-density wave states of orthorhombic TaS3 using dark field electron microscopy. The images of the strandlike domains are found to be time-dependent and sensitive to the radiation damage induced by the incident high energy electrons. Our observation suggests that the appearance of the strandlike structure may be common to this class of materials regardless of the commensurability of the charge-density waves.

We have used metal molecular beam epitaxy, sputter deposition with single composite target, and spin‐on/pyrolysis with metallo‐organic precursors to prepare thin films of high Tc oxide superconductors mostly in the Y‐Ba‐Cu‐O system. We report the superconducting properties and the structural characteristics of the films. The processing parameters of these thin‐film techniques are also discussed.