Electrical investigation of p-type 4H-SiC heavily doped by ion implantation

In the last decades, an increasing interest in the research field of wide bandgap semiconductors was observed, mostly due to the progressive approaching of silicon-based devices to their theoretical limits. 4H-SiC is an example among these, and is a mature compound for applications. The main advantages offered 4H-SiC in comparison with silicon are an higher breakdown field, an higher thermal conductivity, a higher operating temperature, very high hardness and melting point, biocompatibility, but also low switching losses in high frequencies applications and lower on-resistances in unipolar devices. Then, 4H-SiC power devices offer great performance improvement; moreover, they can work in hostile environments where silicon power devices cannot function. Ion implantation technology is a key process in the fabrication of almost all kinds of SiC devices, owing to the advantage of a spatially selective doping. This work is dedicated to the electrical investigation of several differently-processed 4H-SiC ion- implanted samples, mainly through Hall effect and space charge spectroscopy experiments. It was also developed the automatic control (Labview) of several experiments. In the work, the effectiveness of high temperature post-implant thermal treatments (up to 2000°C) were studied and compared considering: (i) different methods, (ii) different temperatures and (iii) different duration of the annealing process. Preliminary p + /n and Schottky junctions were also investigated as simple test devices. 1) Heavy doping by ion implantation of single off-axis 4H-SiC layers The electrical investigation is one of the most important characterization of ion-implanted samples, which must be submitted to mandatory post-implant thermal treatment in order to both (i) recover the lattice after ion bombardment, and (ii) address the implanted impurities into lattice sites so that they can effectively act as dopants. Electrical investigation can give fundamental information on the efficiency of the electrical impurity activation. To understand the results of the research it should be noted that: (a) To realize good ohmic contacts it is necessary to obtain spatially defined highly doped regions, which must have conductivity as low as possible. (b) It has been shown that the electrical activation efficiency and the electrical conductivity increase with the annealing temperature increasing. (c) To maximize the layer conductivity, temperatures around 1700°C are generally used and implantation density high till to 10 21 cm -3 . In this work, an original approach, different from (c), is explored by the using very high annealing temperature, around 2000°C, on samples of Al + -implant concentration of the order of 10 20 cm -3 . Several Al + -implanted 4H-SiC samples, resulting of p-type conductivity, were investigated, with a nominal density varying in the range of about 1-5∙10 20 cm -3 and subjected to two different high temperature thermal treatments. One annealing method uses a radiofrequency heated furnace till to 1950°C (Conventional Annealing, CA), the other exploits a microwave field, providing a fast heating rate up to 2000°C (Micro-Wave Annealing, MWA). In this contest, mainly ion implanted p-type samples were investigated, both off-axis and on-axis <0001> semi-insulating 4H-SiC. Concerning p-type off-axis samples, a high electrical activation of implanted Al (50-70%) and a compensation ratio below 10% were estimated. In the work, the main sample processing parameters have been varied, as the implant temperature, CA annealing duration, and heating/cooling rates, and the best values assessed. MWA method leads to higher hole density and lower mobility than CA in equivalent ion implanted layers, resulting in lower resistivity, probably related to the 50°C higher annealing temperature. An optimal duration of the CA treatment was estimated in about 12-13 minutes. A RT resistivity on the lowest reported in literature for this kind of samples, has been obtained. 2) Low resistivity data: variable range hopping Notwithstanding the heavy p-type doping levels, the carrier density remained less than the critical one required for a semiconductor to metal transition. However, the high carrier densities obtained was enough to trigger a low temperature impurity band (IB) conduction. In the heaviest doped samples, such a conduction mechanism persists till to RT, without significantly prejudice the mobility values. This feature can have an interesting technological fall, because it guarantee a nearly temperature- independent carrier density, it being not affected by freeze-out effects. The usual transport mechanism occurring in the IB conduction is the nearest neighbor hopping: such a regime is effectively consistent with the resistivity temperature behavior of the lowest doped samples. In the heavier doped samples, however, a trend of the resistivity data compatible with a variable range hopping (VRH) conduction has been pointed out, here highlighted for the first time in p-type 4H-SiC. Even more: in the heaviest doped samples, and in particular, in those annealed by MWA, the temperature dependence of the resistivity data is consistent with a reduced dimensionality (2D) of the VRH conduction. In these samples, TEM investigation pointed out faulted dislocation loops in the basal plane, whose average spacing along the c-axis is comparable with the optimal length of the hops in the VRH transport. This result suggested the assignment of such a peculiar behavior to a kind of spatial confinement into a plane of the carrier hops. 3) Test device the p + -n junction In the last part of the work, the electrical properties of 4H-SiC diodes were also studied. In this case, a heavy Al + ion implantation was realized on n-type epilayers, according to the technological process applied for final devices. Good rectification properties was shown from these preliminary devices in their current-voltage characteristics. Admittance spectroscopy and deep level transient spectroscopy measurements showed the presence of electrically active defects other than the dopants ones, induced in the active region of the diodes by ion implantation. A critical comparison with the literature of these defects was performed. Preliminary to such an investigation, it was assessed the experimental set up for the admittance spectroscopy and current-voltage investigation and the automatic control of these measurements.

Eficiência de fontes, dinâmica do magnésio no solo sob a cultura do café no Brasil e comparação da análise de K, Ca e Mg em solos do Arkansas-EUA seco em estufa e úmido de campo

A cultura do café no Brasil tem apresentado frequente deficiência de magnésio (Mg) limitando sua produtividade, portanto faz-se necessário o estudo de fontes que contenham Mg para essa cultura. Por outro lado, o estudo das metodologias de análise de K, Ca e Mg no solo é um outro ponto que precisa ser estudado para melhor manejo da fertilidade do solo e recomendação de adubações. Objetivou-se com o primeiro experimento avaliar a eficiência de fontes de magnésio para a cultura do café e a dinâmica deste nutriente no perfil do solo. E com o experimento desenvolvido em Arkansas-EUA, avaliar as correlações entre as concentrações de nutrientes do solo seco em estufa e úmido de campo extraídos com Mehlich-3 e 1 mol L-1 NH4OAc. Observou-se que o óxido e oxissulfato de Mg elevaram os valores de pH e CTC e diminuíram a concentração de H + Al do solo. As fontes diminuíram a disponibilidade de K e Ca, e aumentaram o Mg no solo. Na planta, óxido e sulfato de Mg proporcionaram maior concentração de Mg foliar. Apenas no segundo ano de avaliação houve aumento de produtividade do café. Os fertilizantes óxido e oxissulfato de Mg obtiveram o maior índice de eficiência agronômica em relação ao carbonato de Mg. No segundo experimento, K, Ca e Mg extraíveis com Mehlich-3 e NH4OAc foram altamente correlacionados (r2> 0,95) tanto para solo úmido de campo quanto para o seco em estufa. A relação entre as concentrações de K no solo seco em estufa e úmido de campo para Mehlich-3 e NH4OAc foram muito semelhantes e altamente correlacionados (r2 = 0,92). A secagem do solo em estufa teve efeito mínimo sobre as concentrações de Ca e reduziu a concentração de Mg tanto para Mehlich-3 quanto para NH4OAc. Entre os nutrientes estudados, a concentração de K foi a mais afetada pela secagem em estufa, necessitando de pesquisas de campo para correlacionar e calibrar novas recomendações agronômicas.

Simultaneous optical and X-ray observations of flares and rotational modulation on the RS CVn binary HR 1099 (V711 Tau) from the MUSICOS 1998 campaign

We present simultaneous and continuous observations of the Hα, Hβ, He I D_3, Na I D_1,D_2 doublet and the Ca II H&K lines for the RS CVn system HR 1099. The spectroscopic observations were obtained during the MUSICOS 1998 campaign involving several observatories and instruments, both echelle and long-slit spectrographs. During this campaign, HR 1099 was observed almost continuously for more than 8 orbits of 2^d.8. Two large optical flares were observed, both showing an increase in the emission of Hα, Ca II H K, Hβ and He I D_3 and a strong filling-in of the Na I D_1, D_2 doublet. Contemporary photometric observations were carried out with the robotic telescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories. Maps of the distribution of the spotted regions on the photosphere of the binary components were derived using the Maximum Entropy and Tikhonov photometric regularization criteria. Rotational modulation was observed in Hα and He I D_3 in anti-correlation with the photometric light curves. Both flares occurred at the same binary phase (0.85), suggesting that these events took place in the same active region. Simultaneous X-ray observations, performed by ASM on board RXTE, show several flare-like events, some of which correlate well with the observed optical flares. Rotational modulation in the X-ray light curve has been detected with minimum flux when the less active G5 V star was in front. A possible periodicity in the X-ray flare-like events was also found.

Letter of Natick Indians about Magunkaqoug; their unwillingness to sell it, 1715 September 5

One-page handwritten petition to the Trustees signed by Natick Indians expressing unwillingness to sell their lands.

[Copy of the Maguntaquoq deed], 1715 October 11

Four-page 18th century handwritten copy of the deed between the "Committee or Agents for the Indian Proprietors of the Plantation of Natick" and the Trustees for 800 acres of land in Middlesex County known as Maguntaquog.

Committee of Natick yr letter of 29th May 1722 to Mr. Leverett, 1722 May 29

One-page handwritten letter signed by Thomas Waban, Solomon Thomas, and Benjamin Tray regarding a "settlement in ye Gospel ministry, among us in our town of Natick."

[Indenture of Anthony Blount & John Jones to the Trustees of Hopkinton], 1724 March

Handwritten indenture between grantors Anthony Blount and John Jones, and the Trustees as grantees for a parcel of land originally included in Mr. Savill Simpson's farm.

[Plan of a part of Hopkinton, 1824 February 26]

Plat of a part of Hopkinton with houses labeled.

Mining Districts, Victoria, Australia, ca. 1868 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: Victoria mining districts, mining divisions & the gold fields, engraved by William Slight under the direction of R. Brough Smyth ; colored by Arthur Everett, August 1st, 1868. It was published by Dept of Mines ca. 1868. Scale [ca. 1:1,000,000].The image inside the map neatline is georeferenced to the surface of the earth and fit to the coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, administrative boundaries, railroads, gold reefs, mining districts, telegraph lines, shoreline features, and more. Relief shown by hachures. Includes notes.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

The Turkish Empire in Europe, Asia, and Africa: dividid into all its governments, together with the other territories that are tributary to it, as also the dominions of ye Emperor of Marocco [sic] according to the newest and most exact observations

by H. Moll.

(Table A1) CaCO3 contents and Sr/Ca ratios in ODP Leg 130, 138 and 154

Strontium/calcium (Sr/Ca) ratios in bulk and foraminiferal calcite have been used to constrain the history of Sr/Ca in the oceans and to evaluate calcite diagenetic alteration. However bulk Sr/Ca records also may be influenced by differences in Sr uptake and/or in the diagenetic susceptibility of different calcium carbonate sedimentary components. We present data on the sediment size fraction and calcium carbonate distribution in bulk samples, Sr/Ca in a range of sedimentary size components, and Sr/Ca in bulk sediments. Ocean Drilling Program samples from sites on Ontong Java Plateau and Ceara Rise (in the western equatorial Pacific and Atlantic, respectively) and from sites in the eastern equatorial Pacific were selected to represent progressive stages in the diagenetic pathway from the sea floor through a range of burial depths equivalent to sediment ages of ~5.6, ~9.4, and ~37.1 Ma. Samples were subdivided by size to produce a unique data set of size-specific Sr/Ca ratios. Fine fraction (<45 ?m) Sr/Ca ratios are higher than those of all corresponding coarse fractions, indicating that fine nannofossil-dominated calcite has a Sr partition coefficient 1.3-1.5 times greater than that of coarse foraminifera-dominated calcite. Thus, absolute values of bulk Sr/Ca in contemporaneous samples reflect, in part, the ratio of fine to coarse calcite sedimentary components. Sr/Ca values in fine and coarse components also behave differently in their response to pre-burial dissolution and to recrystallization at depth. Coarse size components are sensitive to bottom water carbonate ion undersaturation, and they lose original Sr/Ca differences among contemporary samples over not, vert, similar10 my. In contrast, fine components recrystallize faster in more deeply buried samples. Interpretation of the historical Sr/Ca record is complicated by post-depositional diagenetic artifacts, and thus our data do not provide clear evidence of specific temporal changes in oceanic Sr/Ca ratios over the past 10 million years. This paper represents the first systematic attempt to examine trends in calcite Sr/Ca as a function of sediment size fraction and age.

La ratificación constitucional : discurso pronunciado en la Ca��mara de Representantes /

"El Club 'Cnel. Gerónimo de Amilivia' a los nacionalistas de la 19a. Sección."

Family of James Burrill Angell

Standing (Adults) L-R: Marion Waterhouse Angell (Mrs. J.R.); James Rowland Angell; Andrew C. McLaughlin; Fanny C. Cooley. Angell (Mrs. A.C.); Alexis Caswell Angell; Sarah Caswell Angell (daughter of A.C), Lois Angell McLaughlin

Middle Row: James Waterhouse Angell (son of J.R.), Marion Angell (McAlpin) (daughter of James R.) James Burrill Angell, Isabel McLaughlin

Front Row: Constance McLaughlin (Green); Robert Cooley Angell (son of A.C.); Esther Lois McLaughlin (Donahue) David Blair McLaughlin; Rowland Hazard McLaughlin; James Burrill Angell II (son of A.C.): James Angell McLaughlin

Lectura super primum Sententiarum : prologi, quaestiones I & II /

Includes index.

Die Kanzlei Kaiser Konrads II : mit neu bearbeiteten Regesten und drei ungedruckten Urkunden /

Includes documents and quotations in "Anhang": p. 93-163.