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We have used atomic force microscopy (AFM) to study the (0 0 0 1) growth surface of a 6H-SiC single crystal at the points where micropipes emerge on the growth surface. All of the micropipes examined are origins of spiral steps, indicating that dislocations intersect the surface at these points. The dislocations observed at the surface/micropipe intersections have Burgers vectors of at least 4b0, where b0 is the Burgers vector of a unit screw dislocation aligned along the c-axis (b0 = 15.19A). Single and double unit dislocations were also observed, but they are not associated with micropipes. Micron-scale deposits of a heterogeneous phase were observed in the vicinity of the micropipes. The curvature of growth steps around these heterogeneities indicates that they impeded step motion while the crystal was growing. Based on our observations, we propose a model for the formation of super-dislocation/micropipe complexes that involves the coalescence of unit screw dislocations that are forced towards one another as large steps grow around heterogeneous material on the surface.

The physical vapor transport technique can be employed to fabricate large diameter silicon carbide crystals (up to 50 mm diameter) exhibiting uniform 4H-polytype over the full crystal volume. Crystal growth rate is controlled to first order by temperature conditions and ambient pressure. 4H-polytype uniformity is controlled by polarity of the seed crystal and the growth temperature. 4H-SiC crystals exhibit crystalline defects mainly in the form of dislocations with densities in the 10 4 cm -2 range and micropipe defects, the latter having densities as low as 10 cm -2 in best crystals. Electrical conductivity in 4H-SiC bulk crystals ranges from 10 15 Ω cm) at room temperature.

Micropipes in high resistivity (p≥5 kΩcm) SiC are highly activated in parallel electric fields (vertical devices) at room temperature starting at very low fields of 5-10 kV/cm, especially in the doped material. No activation of micropipes is observed in high fields (>100 kV/cm) perpendicular to their orientation (lateral devices). In the last case, the high field limitation is due to surface flashover phenomena taking place at 100-175 kV/cm in vacuum ambient and depending strongly on the material growth technology and the gap length. Non-ohmic behavior was not observed in lateral devices up to high applied fields. The high field characterization method is proposed as a powerful tool for the evaluation of the quality of SiC material for next-generation high voltage/high power devices.

Undoped SiC crystals grown by physical vapor transport have been characterized by temperature dependent Hall effect and near infrared optical absorption measurements. Crystals with reduced nitrogen content were found to exhibit p‐type conductivity with carrier concentrations in the 5×1014–1×1016 cm−3 range at room temperature. The Fermi level position determined from Hall effect measurements at elevated temperatures was 0.35 eV above valence band. The primary acceptor‐type impurity was identified as substitutional boron with total concentration of uncompensated acceptors in the 1×1017–5×1018 range. This interpretation was confirmed by near infrared absorption spectra, which were dominated by a broad photoionization band with a threshold at 0.7 eV and a maximum at 1.75 eV. The shape of the band was fitted, and the thermal ionization energy of the defect was found to be in the 0.3–0.4 eV range. A correlation between the photoionization band intensity, and the uncompensated boron content was used to determin...

Because of its wide bandgap and physical stability silicon carbide (SiC) is currently being viewed as a potentially important semiconductor material for high power and high temperature solid-state devices. The nature and extent of surface damage in 6H-SiC substrates prepared by mechanical polishing have been studied using backscattering of ultraviolet light and cross-sectional transmission electron microscopy. When the basal plane surface is prepared by lapping or polishing with large size diamond abrasives, the surface roughness is about one-fifth the particle size, while the subsurface damage extends to a depth of about half the abrasive size. Under optimum conditions of particle size, vertical load, and relative rotation speed, the extent of subsurface damage can be minimized resulting in a nominally defect-free specular surface exhibiting a uniform strained layer of less than 8 nm.

Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi‐insulating silicon carbide.

Results of the high field performance of single‐crystal bulk 6H–SiC of relatively high resistivity (∼500 Ω cm) are reported. Prebreakdown and breakdown phenomena of SiC at high fields are studied using lateral device geometries, particularly suitable for photoconductive power switches and other high voltage power devices. The influence of the electrode configuration, gap length, sample geometry, and contact technology on the high field responses of SiC is discussed. Ohmic response and relatively large hold‐off fields (≳80 kV/cm) are reported for this material in vacuum and SF6 gas.

Semi‐insulating 6H–SiC crystals have been achieved by using controlled doping with deep‐level vanadium impurities. High resistivity undoped and semi‐insulating vanadium‐doped single‐crystals with diameters up to 50 mm were grown by physical vapor transport using an induction‐heated, cold‐wall system in which high purity graphite materials constituted the hot zone of the furnace. Undoped crystals were p‐type due to the presence of residual acceptor impurities, mainly boron, and exhibited resistivities ranging up to 3000 Ω cm. The semi‐insulating behavior of the vanadium‐doped crystals is attributed to compensation of residual acceptors by the deep‐level vanadium V4+(3d1) donor located near the middle of the band gap.

For the past six years, the Arkansas Foundation for Medical Care, Inc. (AFMC), Health Care Quality Improvement Program (HCQIP), has focused primarily on inpatient projects. In 1996, we began expanding project information to include outpatient issues. Earlier ambulatory topics included management of thyroid disease, diabetes and flu immunization. This AFMC project focuses on the prevalence of facility resources to manage hypertension and asthma as part of quality improvement efforts for Medicare and Medicaid patients in Arkansas. AFMC understands that outpatient facilities frequently lack an infrastructure to conduct outpatient chart audits in an efficient and effective fashion. This difficulty in data acquisition reflects a significant barrier. Nevertheless, certain processes and structural elements can be assessed to improve management of common outpatient conditions.

SiC is an important wide bandgap semiconductor material for high temperature and high power electronic device applications. Purity improvements in the growth environment has resulted in a two-fold benefit during growth: (a) minimized inconsistencies in the background doping resulting in high resistivity (>5000 ohm-cm) wafer yield increase from 10–15% to 70-85%, and (b) decrease in micropipe formation. Growth parameters play an important role in determining the perfection and properties of the SiC crystals, and are extremely critical in the growth of large diameter crystals. Several aspects of growth are vital in obtaining highly perfect, large diameter crystals, such as: (i) optimized furnace design, (ii) high purity growth environment, and (iii) carefully controlled growth conditions. Although significant reduction in micropipe density has been achieved by improvements in the growth process, more stringent device requirements mandate further reduction in the defect density. In-depth understanding of the mechanisms of micropipe formation is essential in order to devise approaches to eliminate them. Experiments have been performed to understand the role of growth conditions and ambient purity on crystal perfection by intentionally introducing arrays of impurity sites on one half of the growth surface. Results clearly suggest that presence of impurities or second phase inclusions during start or during growth can result in the nucleation of micropipes. Insights obtained from these studies were instrumental in the growth of ultra-low micropipe density (less than 2 micropipes cm−2 ) in 1.5 inch diameter boules.

4H and 6H-SiC epitaxial layers exhibit characteristic morphological defects caused by process and substrate interferences with the a-axis directed step-flow growth. 4H-SiC is shown to typically exhibit worse morphology than 6H-SiC for a given off-axis orientation. SiC epitaxial layer defects are significantly reduced by the optimization of growth conditions and substrate surface preparation. The remaining highly variable defects are shown to emanate from the substrate surface with densities of ≥1000 cm−2

The growth surface of a 6H-SiC boule, grown by physical vapor transport, was examined using scanning force microscopy. The dimensions of surface/micropipe intersections and screw dislocation Burgers vectors have been determined from topographic data. All micropipes are positioned along the lines of super screw dislocations with a Burgers vectors of at least 4 times the c-axis repeat distance (15.2 Å). Perfect c-axis screw dislocations with Burgers vectors of only 15.2 Å are stable and do not have open cores. Measurements show that micropipe core radii, determined indirectly from the width of the craters formed at the surface/micropipe intersections, increase with the square of the dislocation Burgers vector.

Hall effect, deep level transient spectroscopy, optical absorption, and optical admittance spectroscopy were employed to determine the position of the vanadium acceptor and vanadiumnitrogen complex in vanadium- and nitrogen-doped 4H and 6H SiC. Hall effect results indicate that the acceptor position in 4H(6H) SiC is 0.80(0.66) eV beneath the conduction band edge. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 806 meV and a capture cross section of 1.8×10−16 cmr−2 The optical absorption measurements proved that the acceptor level investigated is related to isolated vanadium, and therefore the vanadium acceptor level. Based upon DLTS and SIMS measurements, the maximum solubility of vanadium in SiC was determined to the 3×10−17 crn3. An examination of polarized light experiments indicates that vanadium also complexes with another element to form electronic(at 5000 cm−1) and vibrational absorption(at 683 cm−1) bands. While the other constituent cannot be identified, evidence suggests that nitrogen is a likely candidate. This complex introduces a deep level at Ec−0.78 eV as determined using optical admittance spectroscopy.

Because of its wide band gap and physical stability, silicon carbide is an important semiconductor material for high power, high temperature solid state devices. In such applications, any surface damage introduced during wafer fabrication is detrimental to optimum device fabrication and operation. A typical wafer fabrication procedure consists of mechanical lapping and polishing, followed by chemical etching for removal of any residual mechanical damage. However, SiC is a refractory material with strong atomic bonding and high mechanical hardness, chemical etching of its surface is often difficult. Mechanical polishing is thus the crucial step during wafer preparation. Though a number of techniques are currently in use for assessment of polishing induced surface damage, such as grazing incidence xray diffraction and Rutherford backscattering, none of them is effective for studying the hidden damage beneath the polished surface. In the present work cross-sectional transmission electron microscopy (XTEM) was used to observe directly the nature and extent of subsurface damage in 6HSiC substrates after different stages of mechanical polishing, and to elucidate the possible mechanism for material removal by the abrasive treatments.

The effects of various metallic impurities, both singly and in combinations, on the performance of silicon solar cells have been studied. Czochralski crystals were grown with controlled additions of secondary impurities. The primary dopants were boron and phosphorus while the secondaires were: A1, B, C, Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, P, Pd, Ta, Ti, V, W, Zn, and Zr. Impurity concentrations ranged from 1010to 1017/cm3. Solar cells were made using a conventional diffusion process and were characterized by computer reduction of I-V data. The collected data indicated that impurity-induced performance loss was primarily due to reduction of the base diffusion length. Based on this observation, an analytic model was developed which predicts cell performance as a function of the secondary impurity concentrations. The calculated performance parameters are in good agreement with measured values except for Cu, Ni, and Fe, which at higher concentrations, degrade the cell substantially by means of junction mechanisms. This behavior can be distinguished from base diffusion length effects by careful analysis of the I-V data. The effects of impurities in n-base and p-base devices differ in degree but submit to the same modeling analysis. A comparison of calculated and measured performance for multiple impurities indicates a limited interaction between impurities, e.g., copper appears to improve titanium-doped cells.

The deformation behavior of Tl3AsSe3 (TAS) single crystals has been studied by hardness tests in four different crystal surfaces, the (10$\overline 1$0), (1$\overline 2$10), (0001), and the plane where the normal is tilted 19° off the hexagonal c axis in the b–c plane. The dominant slip planes are of the (10$\overline 1$1) type; the possibility of (10$\overline 1$0) and (0001) slip was also observed. The observed dependence of force on the length of diamond pyramid indentations implies that TAS behaves to some degree like a ductile metal. Measurements of hardness anisotropy are very useful for defining crystal growth and handling conditions that minimize the effect of stresses. X-ray topographs of Knoop hardness indentations indicate that the measured x-ray diffraction contrast is closely related to the rotational component of each individual slip system activated by the hardness indentation.

The effect of Ti, Cu and Fe on silicon solar cells has been investigated. Ti severely degrades cell performance above a concentration of 10 11 cm −3 . The presence of 2 × 10 14 cm −3 Ti results in a 63% loss in cell performance and more than an order of magnitude reduction in carrier lifetime. Ti gives rise to two deep levels in Si at Ev + 0.30 eV and Ec − 0.27 eV. Copper, at concentrations below 10 16 cm −3 , has negligible effect on cell performance and carrier lifetime. Above 10 16 cm −3 copper occasionally produces a 10–15% loss in cell performance with a noticeable increase in junction excess current. No recombination centers were found due to Cu, instead considerable precipitation in the starting material was observed. Fe begins to hurt the cell performance above a concentration of 2 × 10 14 cm −3 . Iron at 1.7 × 10 15 cm −3 results in 46% loss in cell efficiency and about an order of magnitude reduction in lifetime. Fe induces a deep level in silicon at Ev + 0.4 eV. The active center density, for both Ti and Fe, is only a very small fraction of the total impurity content in the starting silicon wafer.

Powders of NaLaS2 were synthesized and hot pressed into translucent discs about 95% dense whose hardness was 274 kg/mm2 (Knoop). The transmission range of the compound is from approximately 0.45-20 μm. Though optically clear discs could not be produced with our equipment, the trend of our data indicates that increasing pressure promotes increased density and transparency. The cubic symmetry of NaLaS2 was confirmed; a LiFeO2-type structure is proposed.

Deep level transient spectroscopy coupled with dark and lighted I–V measurements were used to study the electrical properties of silicon crystals and solar cells purposely contaminated with controlled amounts of molybdenum. Mo severely degrades minority carrier lifetime, and hence solar cell performance, by inducing a recombination center at EV + 0.30 eV. Neither HCl nor POCl3 gettering at temperatures as high as 1100°C and times up to five hours mitigate the effects of Mo. Because the Mo segregation coefficient is small, 4.5 × 10−8, impurity contamination of silicon during crystal growth can be kept below the levels for which electrical properties are affected.

By coupling the results of electrical measurements, such as spectral response, lighted and dark I-V determinations, and deep-level-transient spectroscopy with optical and laser scan photomicroscopy, we have evaluated the effects of grain boundaries and impurities on silicon solar cells. Titanium which produces two deep levels, E_{v} + 0.30 and E_{c} - 0.26 eV, in silicon, degrades cell performance by reducing bulk lifetime and thus cell short-circuit current. Electrically active grain boundaries induce carrier recombination in the bulk as well as in the depletion region of the solar cell. Experimental data imply a small but measurable segregation of titanium into some grain boundaries of the polycrystalline silicon containing high Ti concentration (2 × 1014cm-3). However for the titanium-contaminated polycrystalline material used in this study, solar cell performance is dominated by the electrically active titanium concentration in the grains. Microstructural impacts on the devices are of secondary importance.

Model analyses indicate that sophisticated solar cell designs including, e.g., back surface fields, optical reflectors, surface passivation, and double layer antireflective coatings can produce devices with conversion efficiencies above 20 percent (AM1). To realize this potential, the quality of the silicon from which the cells are made must be improved; and these excellent electrical properties must be maintained during device processing. As the cell efficiency rises, the sensitivity to trace contaminants also increases. For example, the threshold Ti impurity concentration at which cell performance degrades is more than an order of magnitude lower for an 18-percent cell. Similar behavior occurs for numerous other metal species which introduce deep level traps that stimulate the recombination of photogenerated carriers in silicon. Purification via crystal growth in conjunction with gettering steps to preserve the large diffusion length of the as-grown material can lead to the production of devices with efficiencies aboved 18 percent, as has been verified experimentally.

The proportion of a-amylase extractable by water (21° C.) varies from one-fifth (green malt) to four-fifths (finished brewery malt) of that extractable by 0·2% calcium acetate solution; papain solution alone is inferior to calcium acetate as an extracting agent, and causes little or no enhancement of extraction when used simultaneously with calcium acetate. Moreover, calcium acetate extraction (21° C.) yields results comparable with those obtained on mashing (65° C.). For investigating α-amylase changes during malting and mashing, aqueous extraction may accordingly give misleading results, though ease of extraction increases as kiln treatment proceeds. Using the calcium acetate method, it has been shown that resting barley contains no more than a trace of α-amylase, which first appears in an appreciable amount after two days of germination, increasing thereafter to a maximum at the sixth day. There is little subsequent change until after the early stages of kilning, but there is substantial decline by the time a temperature of 60° C. (4–5% moisture) has been reached. During mashing (65° C.), α-amylase dissolution is substantially complete in 5–10 min., relatively slow inactivation thereafter taking place.

Mn2+-doped single crystals of strontium fluorophosphate were grown from the melt. ESR and optical measurements were performed on the crystals to identify the optical properties of Mn2+ ions located on various sites in the structure. We have observed and identified the optical excitation and fluorescence spectra of Mn2+ ions located on the Sr(I) site and a “modified” Sr(II) site. Mn2+ ions located on Sr(II) and many varieties of a “modified” Sr(I) site were observed in ESR but could not be detected by optical means. Mn2+ ions show a strong preference for locating on the Sr(I) site over the Sr(II) site and ratios of Mn(I) to Mn(II) of as high as 125:1 were observed. Defect centers involving fluorine vacancies and substitutional oxygen were found to form in crystals pulled from stoichiometric melts, and their optical properties are described. A comparison is made between the types of Mn2+ and defect centers that form in calcium fluorophosphate and reasons for the similarities and differences will be discussed.

The character of the products of the breakdown of starch by diastase is influenced, among other things, by the proportions present in the diastase of its constituent α-and β-amylases. The extents to which the latter respectively hydrolyse starch and amylose depend on the pre-treatment to which the amylose may have been submitted in its preparation. Potato starch, well ground to rupture the granules, or alternatively made directly into paste but dispersed at 120°C., can be separated by electrophoresis into amylopectin and amyloamylose. Such amylose is much more completely degraded by barley diastase than starch albeit at a slower rate. By alternate redispersion and electrophoresis, the amylopectin can be made to yield high proportions of the amylose. Thus, by such mild means starch can be largely converted into material which is much more completely degraded by barley diastase than the original starch.

In Part 1 of the paper, the author gives an outline of the physical pro perties of proteins, especially in relation to the reaction of the medium. This subject has developed considerably in recent years, and has an important bearing on brewing problems. The iso-electric point is dis cussed along with its relationship to coagulation, turbidity, viscosity, surface tension and other physical properties. Indications are given of the applications of these matters to practical problems. In Part II., the nature of swelling is described, with some experiments performed on isinglass. The latter is found to give the most rapid swelling when the reaction of the interior of the swelling material is at about pn 3.4. Protection by proteins is described, and shown to be minimum at the isoelectric point of the protecting protein. In the case of finings, this is at about pn 5.0.

The protein isolated from sweet wort by Hopkins, Amphlett and Berridge and described by them (this Journ., 1941, 106) has been investigated further. Its most out standing feature is its susceptibility to calcium salts, in presence of which it is coagulated at much greater rates than in salt-free solution. It also binds calcium. Coagulation is greatly accelerated by the first small quantities of coagulum to be formed, and initial precipitation is facilitated by surface activity and by calcium, which reduce the lag period. The lower the value of pH the more rapidly does coagulation take place. Maximum turbidity is given at pH 3.8, but turbidity is reduced by the presence of salts, including those of calcium, over the range of pH 3–5. Precipitates and coagula contain nucleic acid, the proportions of which tend to increase with fall in pH. This nucleic acid is responsible for the formation of turbidity and may be associated with the sensitivity of the protein complex to calcium.

The pressure deri vative of the band gap, (∂E/∂P)T, has been determined to be + 3 × 10−6 e Vbar−1 for Mg2Sn by measurements of the pressure derivative of the electrical conductivity. Similar measurements on Mg2Si and Mg2Ge were inconclusive, but estimates of this parameter were made from the shift of the optical obsorption edge with pressure. (∂E/∂P)T = (0 ± 0.5) × 10−6 e Vbar−1 for both compounds. The similarity of the coefficients for Mg2Si and Mg2Ge suggests that the symmetries of the band edges of both compounds are similar, while either the conduction band edge, the valence band edge, or both edges in Mg2Sn may have a different symmetry.

Vitamin B1 has been determined in a large number of beers by the fermentation method of Schultz, Atkin and Frey which has not previously been applied to worts and beers. In general, the highest amounts were found in beers of high original gravity and in those made almost entirely from malt. Well-attenuated and unprimed beers give the highest ratio, units of B1: calories. During the brew the malt, almost the sole source of vitamin B1, yields up the vitamin to the wort, which in its turn loses most of it to the yeast. Most of the vitamin B1 results have been confirmed by the thiochrome method involving an entirely different principle, whilst the general validity of both methods was further confirmed by animal feedings tests. Determination of Riboflavin (vitamin B2) by the microbiological method of Snell and Strong has led to the following conclusions:—Riboflavin is formed during germination of barley and in growing yeast. Most of the riboflavin of the malt passes into the wort during mashing and sparging and remains in the beer, the yeast not taking out any appreciable quantity. The number of microgrms. of riboflavin per 100 ml. of beer, in general, exceeds somewhat the number of degrees of original gravity.

The starch-splitting enzymes of malt, α- and β-amylases, are capable of converting the starch entirely to fermentable sugars save for a small fraction of limit dextrins which contain the α-1.6-linkage. The enzyme, limit dextrinase, which is also present in malt, has been shown to split glucose from limit dextrin on which, in its absence, α- and β-amylase have no action. In its presence, following upon the release of glucose, maltose is also concurrently produced. The limit dextrinase splits the α-1.6-linkage, thus liberating unbranched dextrins on which α- and β-amylases can act further. Some of the conditions of its action are reported. Experiment indicated that a small amount of limit dextrinase action takes place in brewery worts prior to inactivation by boiling. Activity of limit dextrinase in a range of brewery malts is reported.

Some observations on the microstructure of Czochralski‐grown fluorapatite, Ca5F(PO4)3, single crystals are presented. From etching and x‐ray topographic studies we have found that slip in this material is on (1010) planes, with one Burgers vector in the c direction, and another in the basal plane and probably along the [1010] direction.

The pantothenic acid content of brewing materials and products has been determined by microbiological assay using Lactobacillus arabinosus as test organism. Pantothenic acid is synthesized by the germinating barley corn during the malting process. Most of the vitamin in the malt passes into the worts. The content in finished beers depends largeiyon the yeast used, some yeasts removing a portion of the pantothenic acid during fermentation, whilst others leave the wort unchanged and still others enrich the wort with respect to the vitamin. Two single-cell cultures of brewers' yeast were found to need pantothenic acid or β-alanine for growth; a third required neither.