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Premise of research. Ferns are vital components of temperate and tropical ecosystems, but they have not been examined in the context of a central-peripheral hypothesis. Dryopteris fragrans is an ideal species to examine the genetic variation between central and peripheral populations because of its arctic north to temperate south distribution pattern. In addition to understanding colonization dynamics, our study also addresses the issue regarding the conservation value of peripheral plant populations.Methodology. We examined 82 individuals from 22 populations from northern Canada (N-CA) and the northeastern United States (NE-US), which represent central and peripheral populations of D. fragrans, respectively. Two-hundred two loci were resolved using inter–simple sequence repeat markers, allowing analyses of genetic diversity and population structure, insights into gene flow and mating system, and correlations of genetic diversity with geographical distance, population size, and air temperature.Pivotal res...

American hart's-tongue fern (AHTF) is one of the rarest ferns in the United States and concern over its conservation and management has highlighted the need for genetic analysis. Genetic analysis also provides insights into the species' mating system which contributes to our understanding of its rarity and persistence. We analyzed 88 individuals from 11 populations in NY and MI based on variations in 108 loci as revealed through ISSR markers using Nei's gene diversity index, percent polymorphic loci and other measures. Low genetic diversity predominates in the populations from NY, and even lower for the populations in MI. Our results also indicate that AHTF from NY and MI are genetically differentiated from each other, as well as the populations within them. There is no positive correlation between genetic and geographic distances, as well as between genetic distance and census population size. The significantly high among population genetic variation and low gene flow value are common indicators of a predominant inbreeding mating strategy within populations, limited spore dispersal, and genetic drift. Our results also indicate that each AHTF population is an important contributor to the overall genetic variation of the species and thus, represents a significant unit for conservation efforts.

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project., La lista completa de autores puede consultarse en el documento o en la página web de la revista., Facultad de Ingeniería

We present results of electron spin resonance (ESR) experiments on benzimidazobenzophenanthroline (BBL) samples in which the relative concentration of unpaired electrons was monitored as a function of electrochemical doping potential and the number of electrons per repeat unit (ERU). An increasing spin concentration along with increasing conductivity at low doping levels (less than 1/2 ERU) indicates that the conducting species have spin 1/2. An increasing spin concentration along with decreasing conductivity at intermediate doping levels (between 1/2 and 1 ERU) indicates that conduction in dry BBL involves a redox hopping process. The polymer has 2 ERU when fully doped.

We report deuteron and proton nuclear magnetic resonance (NMR) measurements on high quality plasma enhanced chemical vapor deposition amorphous silicon films containing from 5 to 15 at.% hydrogen (H and/or D). Our results show that from 2% to 40% of the hydrogen in these samples is not atomically bonded by the Si–H chemical bond. We present conclusive evidence that in highest quality films nearly all of this non-bonded hydrogen is present as isolated hydrogen molecules. These molecules are located in centers of atomic dimensions, perhaps in the analogue of T-sites in crystalline Si. In high quality films these molecules are not the small population of densely packed molecules in the occasional microvoids. The photoelectronic quality of the films (as measured by the ημτ photoresponse product) increases as the fraction of non-bonded hydrogen increases. Our results are consistent with IR spectral measures of hydrogen. Our experiments also suggest that the non-bonded hydrogen is in the vicinity of light induced defects. Finally by comparing successive annealings and other studies it appears that the molecular hydrogen can be identified with the less-clustered proton narrow NMR line.

We present the first synthesis of acetylene-terminated polyaniline oligomers (ATPAs) by two chemical methods. One method, a modified Moore—Honzl—Wudl synthesis, involves the condensation of p -phenylenediamine (PDA) with 2,5-dioxo-1,4-cyclohexanedicarboxylic acid, followed by endcapping with 3-ethynylaniline (EA) and subsequent air oxidation. The other, more convenient, method is based on a modified Manassen—Khalif Schiff-base approach involving the condensation of PDA with commercially available 1,4-cyclohexanedione, followed by endcapping with EA and subsequent air oxidation. Both methods give oligomers with molecular weight distributions. The ATPAs have been characterized by FT-IR, UV—Vis, 1 H NMR, MS and ESR methods. Bronsted acid doping gives conducting ATPAs (up to 10 −2 S/cm). The results of comparative studies of the ATPAs prepared by both methods are presented.

Amorphous hydrogenated/deuterated silicon nitride (a-SiN:H,D) thin films were studied by deuteron magnetic resonance (DMR). DMR spectra show two quadrupolar broadened Pake-type doublets with splittings of 65.4 kHz for SiD, and 154 kHz for ND bonds; the ratio of these DMR doublet cusp-splittings ( Δ ND Δ SiD = 2.35 ) is, to within experimental error, equal to the ratio of the squares of the respective bond-stretching frequencies (( ω ND ω SiD ) 2 = 2.36 ), as determined by analysis of Fourier transform infrared spectra. The center frequencies of the SiD and ND DMR doublets are not coincident, but rather are shifted by ∼ 2 kHz, indicating that the SiD and ND groups are interacting, and must therefore be near-neighbors in the amorphous network. This explanation consistent with an interpretation of the shifts in bond-stretching frequencies of IR modes that occur when Si(H)D and N(H)D modes are found in the same film.

Deuteron magnetic resonance measurements on high quality a-Si:H,D films reveal three principal features: a sharp 66 kHz quadrupolar Pake doublet corresponding to silicon-bonded D passivating dangling bonds, a broad central line which has been shown to arise primarily from relatively isolated D2 and HD molecules in nanovoids, and a small narrow central line arising from mobile molecular D2 and HD in greater than 3 A microvoids. The results of measurements on high quality plasma-deposited partially deuterated amorphous silicon cyclically exposed to a 2 h 150°C dark anneal and a 36 h photoillumination at 0.5 W/cm2 with a 400–700 nm xenon are lamp are described. The spectra show that, following an initial spin-lattice relaxation-related transient, the intensities of all three components of the deuterium may vary by less than 1% upon illumination or dark anneal. The broad central line, primarily from isolated molecules, shows a small diamagnetic shift upon illumination. The shift becomes strongly paramagnetic at short magnetization recovery times following radio frequency saturation. The SiD doublet and the small narrow central line do not shift appreciably. This absence strongly suggests that Staebler-Wronski magnetic changes are remote from the environments of most of the silicon bonded hydrogen.

Thin films of the ladder polymer poly(benzimidazobenzophenanthroline) (BBL) were irradiated with 180 keV argon ions under high vacuum conditions. Ion-beam exposures ranging from 5 × 1013 up to 3 × 1018 ions/cm2 result in measured BBL film electrical conductivities approaching 100 S/cm. After ion irradiation and a short annealing treatment the electrical conductivity is found to be stable in air at 250 °C, changing less than 5% after several hundred hours. Optical absorption spectra measured in very thin (about 200 nm) ion-irradiated films show a pronounced absorption coefficient reduction in the BBL π-π* conjugated band transition region. In addition, increased absorption occurs at wavelengths both above and below this band transition. As the ion-beam exposure is increased further these spectral features broaden and exhibit increased absorption, extending into the near-infrared spectral range. Infrared absorption measurements indicate that structural changes begin to occur in BBL for low ion irradiation levels, although an extremely broadened infrared mode structure having characteristics reminiscent of BBL remains up to moderately high irradiation levels. These results combined with electron spin resonance data suggest that BBL films undergo heavy structural damage and atomic relaxation, along with some degree of bond reformation, causing a total loss of long-range conjugation order. The irradiation-induced damage and subsequent healing processes result in a stable structure, where electronic transport proceeds through localized midgap defect states near the Fermi level.

Five different Schiff base monomers were used to form thermoset polymers by hot-melt processing. The study included three monomers with the same backbone structure but terminated with acetylene, vinyl or epoxy end groups, respectively, as well as two additional monomers with acetylene end groups but having different backbone structures. In this way we have compared the effect of the backbone and the crosslink structures on the electrical conductivities following p-type doping by molecular iodine. Two thermosets formed from acetylene-terminated monomers exhibited conductivities which were approximately a factor of ten higher than that measured in the thermosets formed from either vinyl- or epoxy-terminated monomers. The iodine dopant showed a non-Fickian Case II type of diffusion behavior in all thermosets. Electron spin resonance and optical spectroscopy showed evidence for the existence of polarons.

An increase in quantum efficiency in photodetectors could result in a proportional reduction in the area of atmospheric Cherenkov telescopes and an even larger reduction in cost. We report on the development of high quantum efficiency, high gain, UV/blue photon‐counting detectors based on AlGaN/InGaN photocathode heterostructures grown by molecular beam epitaxy. This research could eventually result in nearly ideal light detectors with a number of distinct advantages over existing technologies for numerous applications in high‐energy physics and particle astrophysics. Potential advantages include much lower noise detection, better stability and radiation resistance than other cathode structures, high VUV sensitivity and very low radioactive background levels for deep underground experiments, and high detection efficiency for the detection of individual VUV‐visible photons. We are also developing photocathodes with intrinsic gain, initially improving the detection efficiency of hybrid semiconductor‐vacuum tube devices and eventually leading to a new type of all‐solid‐state photomultiplier device.

Mise en evidence de l'existence de fortes non-linearites attribuees a un effet tunnel d'electrons et de trous a travers la couche barriere. Ces non-linearites proviennent de la difference entre les masses effectives des electrons et des trous lourds. La courbure de bande a une forte influence sur les intensites de photoluminescence pour les pics associes aux transitions entre les sous-bandes de trous et d'electrons de plus basse energie. Influence d'un effet tunnel resonnant des trous lourds

Proton NMR studies on hydrogen in numerous plasma deposited a-Si:H thin film samples over many years have established that there are primarily two dipolar broadened lines, representing different distinct degrees of proton clustering involving most of the contained hydrogen. In addition, both of these configurations were thought to be hydrogen tightly bonded to silicon. However, deuteron magnetic resonance established long ago that a substantial fraction of the hydrogen (deuterium) population was a weakly bonded species, very different than the silicon bonded hydrogen typically seen in infrared measurements. Deuteron magnetic resonance can easily distinguish bond strength because deuterons have spin, an electric quadrupole moment, and are sensitive to electric field gradients. The much weaker deuteron dipolar coupling results in NMR line shapes and widths that are determined by distributions of electric field gradients and angular averages about the applied magnetic field direction. The primary features of the deuteron line shape are a sharp powder-pattern Pake doublet attributed to the tightly silicon bonded species and a broad central Gaussian component originally interpreted as a weakly bound component, but now recognized as molecular deuterium and HD trapped in the amorphous equivalent of T-sites. This latter result is confirmed by spin-echo double resonance experiments using 1H and 29Si nuclides, which reveal a very appreciable population of molecular hydrogen in T-like sites. NMR line shape subtractions also indicate that the majority of this species resides in the less-clustered hydrogen phase. Also a comparison of the T-site molecular hydrogen fraction with measured photoresponse, combined with light-induced changes in the DMR line shape, indicate that this species is in regions of a-Si:H that control the optoelectronic properties of the material.

Proton NMR near 200 MHz has been used to examine the dynamics of hole-burning in HWCVD a-Si:H films deposited from SiH4. Radiofrequency hole-burning is a tool to distinguish inhomogeneous broadening from homogeneous broadening. The 3 kHz FWHM resonance line from T-site-trapped H2 molecules shows a hole-burn behavior similar to that found for PECVD a-Si:H films, as does the 24 kHz FWHM line from clustered silicon-bonded hydrogens. However the ∼80 kHz FWHM superbroad line varies with tip angle uniformly across the line for hole-burns applied at any location within ∼50 kHz from the resonance center. The superbroad line is homogeneously broadened and apparently arises from superclose SiH clusters with inter-proton distances considerably less than 2 Å.

Proton NMR and proton-29Si double resonance NMR have been performed on hot wire a-Si:H films deposited from SiH4. Results are compared with those from conventional plasma enhanced chemical vapor deposition a-Si:H. Proton nutational angle studies and proton-29Si spin-echo double resonance (SEDOR) signals at 80 and 294 K indicate that a significant proton resonance population arises from T-site-trapped molecular H2. The hot wire films also display a ≥80 kHz FWHM uperbroad proton line and a sharp feature diamagnetically shifted by 25 ppm.

Proton-29Si double resonance NMR measurements on high quality plasma-enhanced chemical vapor deposition a-Si:H deposited from SiH4 show that more than one third of the contained hydrogen is present as H2 molecules residing in the amorphous equivalent of T sites. The NMR signal from these trapped H2 appears in the narrow 4 kHz proton line, which arises from the less clustered hydrogen population. Very little of the molecular component is in the broad ~24 kHz line, which arises mostly from clustered hydrogen tightly bonded to silicon.

It is demonstrated that in high quality hydrogenated amorphous silicon films nearly 40% of the contained hydrogen is in the form of hydrogen molecules individually trapped in the amorphous equivalent of tetragonal T sites. The majority of these molecular hydrogens are in the less-clustered component of the contained hydrogen. These results could have major implications for the understanding of light-induced defects and hydrogen mobility.

New NMR measurements show that interstitial T site-trapped molecular hydrogen can amount to more than one third of the contained hydrogen in high quality PECVD amorphous silicon. Microvoid-contained dense molecular hydrogen is negligible in these good films. Experiments on a sequence of hydrogenated and/or deuterated a-Si films have characterized individually-trapped molecular HD and D2 in films deposited from SiD4, and from SiH4+D2. The T site-trapped molecular hydrogen fraction observed here is larger than previously reported because of recent efforts to measure very slowly relaxing molecular components and the employment of radiofrequency pulse sequences to detect ortho-D2 with nuclear spin I=2. The population of interstitially trapped molecular hydrogen increases with increasing photovoltaic quality over a range of an order of magnitude in photoresponse product ημτ. Above 200 K, hopping transport of molecular hydrogen among the amorphous equivalent of interstitial T sites occurs with an activation energy near 50 meV.

Deuteron magnetic resonance (DMR) has been used to further examine hydrogen (deuteron) populations in amorphous-silicon (a-Si) and in n-type crystalline silicon (x-Si). In both a-Si and x-Si DMR shows central components arising at least in part from isolated molecular deuterium and sharp doublet features from Si-bonded hydrogen (D). Our new results include the observation in x-Si of molecule-specific DMR multiple echoes from trapped ortho-D2. A second new result is the observation in x-Si of a substantialn Si-bonded D population with splittings between 46 and 80 kHz and perhaps arising from (deuterated) hydrogen bond-centered and antibonding configurations.

We report deuteron magnetic resonance (DMR) measurements on aged deuterium-implanted single crystal n-type silicon and comparisons with amorphous silicon spectra. The sample film was prepared six years ago by deuteration from a-D2 plasma and evaluated by a variety of experimental methods. Deuterium has been evolving with time and the present DMR signal shows a smaller deuteron population. A doublet from Si-D configurations along 〈111〉 has decreased more than have central molecular DMR components, which include 47 and 12 kHz FWHM gaussiane. Transient DMR magnetization recoveries indicate spin lattice relaxation to para-D2 relaxation centers.

Proton NMR and deuteron NMR (DMR) have been used to measure hydrogen populations in a series of PECVD a-Si:H,D films. The sharp DMR doublet from Si-D is fitted and subtracted out. The residual spectra then show specific signatures for molecular D2 and HD. The fitting procedures yield quantitative measures of Si-bonded and molecular species. A particular comparison is made between a pair of films prepared as the powered and unpowered electrodes in the same plasma deposition. Both silicon-bonded and molecular populations axe significantly different in the two films and correlate with photoresponse products ημT and with IR as well as other materials characterizations.

Proton and deuteron NMR in hydrogenated amorphous silicon yield quantitative measures of species-specific structural configurations and their dynamics. Populations of silicon-bonded and molecular hydrogens correlate with photovoltaic quality, doping, illumination/dark anneal sequences, and with infrared and other characterizations. High quality films contain substantial populations of nanovoid-trapped molecular hydrogen.

Magnetic resonance nondestructive evaluation (NDE) methods are being used successfully in the medical, biological and petroleum industries due to their sensitivity to human tissue and other fluid materials. Application to NDE of solid materials, such as those utilized in the aerospace industry, has been limited due to the lower resolution and increased bandwidth of the measurements. With the recent development of measurement techniques and instrumentation appropriate for the NDE of solids, the sensitivity of NMR applied to solid materials has improved; as a result, increasing attention is now being given to the technique’s ability to evaluate aerospace materials and structures. During this program [1], magnetic resonance techniques were found to be useful for evaluation of corrosion in aluminum and heat damage in composite materials. In a previous study [2], corrosion and heat damage were shown to be detectable using nuclear magnetic resonance (NMR). In this program, these phenomena were evaluated in greater detail using additional NMR parameters and the electron spin resonance (ESR) technique.

Light-induced rearrangements in a-Si:H,D during and after illumination have been studied by deuteron magnetic resonance (DMR). A film sample was deposited on a 230°C Al substrate by plasma decomposition of S1H4 and D2. The high quality film contained 4 at.% H and 4 at. % D. DMR spectra of fast-relaxing components were examined at 700 msec or less after radio frequency saturation. At 30 K these showed a broad central signal (primarily nanovoid-trapped HD and D2), part of which shifted paramagnetically during illumination. No comparable shifts were shown by spectral components from Si-bonded hydrogen or from microvoid-contained dense fluid hydrogen. The observed post-illumination shifts vanished upon 150°C dark anneal and reflected light-induced metastable magnetic changes in the surroundings of hydrogen trapped in nanovoids.

A systematic study of both proton and deuteron NMR in hydrogenated amorphous silicon films has revealed significant resonance shifts among various resolved components. The shifts include both paramagnetic and diamagnetic displacements of resolved spectral features from trapped molecular hydrogens. The shifts depend on film quality and deposition conditions. Some of the shifts vary as 1/T and reflect Curie susceptibilities characteristic of local regions of differing dangling bond densities. Spectra from relatively immobile hydrogen molecules trapped in nanovoids are shifted diamagnetically and broadened as temperature decreases. Hydrogens tightly bound to Si do not show similar changes and thus are more remote from dangling bonds and other magnetic defects. Similar spectrally-resol ved shifts have been observed in a-Ge and a-SiGe films and are correlated with film photovoltaic quality as measured by mobility-lifetime products.

Enormous technological breakthroughs have been made in optoelectronic devices through the use of advanced heteroepitaxial-semiconductor crystal-growth techniques. This technology is being extended toward enhanced ultraviolet/blue single-photon detection through the design and fabrication of atomically tailored heteroepitaxial GaAlN∕GaInN photocathode device structures. The AlGaN∕InGaN system is ideal because the band gap can be tailored over an energy range from 0.8 to 6.2 eV and epitaxial thin-film layers can be grown directly on optically transparent sapphire substrates. Although a single p-type GaN layer activated with cesium can produce reasonably high quantum efficiency in the ultraviolet wave band, a more complex design is necessary to achieve high levels extending into the blue region. In the present work, band-gap engineering concepts have been utilized to design heterostructure photocathodes. The increased level of sophistication offered by this approach has been exploited in an attempt to precis...

KELSALL, N., C. HAZARD, AND D. J. LEOPOLD (Department of Environmental and Forest Biology, SUNY College of Environmental Science and Forestry, 1 Forestry Drive Syracuse, New York 13210). Influence of climate factors on demographic changes in the New York populations of the federally-listed Phyllitis scolopendrium (L.) Newm. var. americana J. Torrey Bot. Soc. 131:161-168. 2004.-Approximately 92% of the U.S. population of American hart's-tongue fern (Phyllitis scolopendrium var. americana) is found in Onondaga and Madison counties, New York, in 17 distinct colonies. All New York colonies have been periodically censused since 1916, though rarely in the same year. This study examines census data of the total New York population during and following the severe drought of 1999 and during the summers of 2000 and 2002. Additionally, the long-term (1922-2002) census data of seven colonies are examined relative to climate data for the same period. The total New York population has declined overall since the last previous census in 1995. No climate variables explained more than 23% of variation in population size of any growth phase. The lack of controlling, largescale climate factors suggests that the distribution of hart's-tongue fern in New York is determined mostly by the occurence of habitat capable of buffering climatic fluctuations. Future work should combine small-scale habitat and large-scale climate factors in any analysis.

Reproducible and controlled hole concentrations were achieved by silver doping during molecular‐beam epitaxial growth of p‐type Hg1−xCdxTe. (100)‐ and (111)‐oriented p‐type layers with 0.18

Deuteron and proton NMR measurements have been made on a plasma-deposited $a$-Si: (D,H) sample. The spin-lattice relaxations include resolvable contributions arising from relaxations which proceed via molecular orthohydrogen and paradeuterium impurities. Several alternate relaxation mechanisms are considered. The deuteron resonance shows two distinct components, one of which exhibits a long spin-lattice relaxation time ${T}_{1}$ and a 66-kHz quadrupole-split resonance doublet line shape, which is shown to be characteristic of D bonded to Si.

Nuclear spin relaxation times and resonance line shapes have been measured at 30 MHz for deuterons in a-Ge:H, D plasma-deposited from a 10% GeH 4 /D 2 gas mixture. The molecular D 2 component of the resonance is relatively much larger, in comparison to the lattice-bound D components, than has been observed for similarly prepared a-Si:H, D samples. The increased D 2 /D ratio is interpreted to reflect a larger fractional void volume in a-Ge. The lattice-bound D in a-Ge shows a resolved quadrupolar doublet splitting in good agreement with infra red stretching mode absorptions. The deuteron spin-lattice relaxation shows a component dominated by relaxation via effectively dilute p-D 2 relaxation center molecules located in microvoids. Another deuteron T 1 component, associated with the resolved doublet, shows a distribution of relaxation times over five orders of magnitude and reflects the distribution of bonded D with respect to microvoid locations.

A plasma-deposited sample of a-Si:D,H was incrementally annealed in a series of 30′ heatings at temperatures between 250 and 600°C. Subsequent deuteron magnetic resonance measurements resolve changes in various 2D components. A weakly bound deuterium fraction vanishes after a 350°C anneal. Reduced, but detectable, strongly bound deuterium and molecular deuterium signals persist after progressive anneals to 600°C. The magnetic resonance line shapes of the weakly bound deuterium and the strongly bound deuterium show that they reside in very different local environments. Several characteristics of these environments can be deduced.

We demonstrate the use of In and Ag as n‐ and p‐type dopants, respectively, to controllably dope (100)‐oriented HgTe–CdTe superlattices during molecular‐beam epitaxial (MBE) growth. When normalized by the superlattice growth rate, the low‐temperature Hall‐carrier concentrations of both In‐ and Ag‐doped superlattices are shown to have an exponential dependence on the respective effusion‐cell temperatures in the electron and hole concentration ranges of ∼1016 to 1018 cm−3 . The upper limit on the diffusion coefficient for In at the low MBE growth temperature of ∼160 °C is determined to be 5×10−15 cm2 /s by use of secondary‐ion mass spectrometry. Hall‐effect and current–voltage measurements verify that the combination of In and Ag doping allows the formation of p–n electrical junctions. These results provide the first evidence of p–n junction formation in a HgTe–CdTe superlattice.

Nuclear resonance line shapes and spin relaxation rates have been measured for deuterons and protons in five a-Si samples plasma-deposited from SiH4/D2 and SiD4/Ar gas mixtures. Some samples show spin-lattice relaxation rates which increase exponentially with temperature above 50 K to sharp maxima near 400 K. In all five samples one component of the nuclear spin-lattice relaxation proceeds via dilute molecular D2 and H2 relaxation centers. The corresponding temperature variations of molecular electronic relaxation rates are given by CE★T★2 where C varies systematically with sample preparation conditions.

We report identification of a reliable p‐type impurity dopant that can be used during molecular‐beam‐epitaxial (MBE) growth of HgTe‐CdTe superlattices. Silver doping during the epitaxial growth of (100)‐oriented HgTe‐CdTe superlattices is shown to yield reproducible hole concentrations in the range of 1016–1018 cm−3. When normalized by the growth rate, the hole concentrations show an exponential dependence on the temperature of the Ag effusion cell. The diffusion of Ag in step‐doped junctions was studied by use of secondary‐ion mass spectrometry. The diffusion coefficient of Ag at the low MBE growth temperatures was found to be in the range of (1–5)×10−13 cm2/s. These values are comparable to recently reported values in MBE‐grown (Hg,Cd)Te alloys and suggest that strain does not produce any new diffusion paths in the superlattices.

Two important quantities in determining the electrical properties of a heterojunction are the dopant and compositional grading profiles. Although molecular‐beam epitaxy (MBE) has been used to grow high‐quality layers of (Hg,Cd)Te, there has been no report of attempts to control these important quantities by use of conventional MBE growth techniques. We report the identification of a reliable p‐type impurity dopant that can be used for MBE‐(Hg,Cd)Te and we demonstrate, by use of secondary‐ion mass spectroscopy, that conventional MBE growth techniques can be used with silver to produce the required dopant and compositional grading profiles to fabricate p‐on‐n heterojunctions.

A systematic study has been made of silicon dopant compensation in GaAs grown by molecular‐beam epitaxy. The compensation ratio is derived from Hall measurements for a number of samples grown under identical conditions except for silicon dopant flux. Results are compared with low‐temperature (4.2 K) photoluminescence measurements. The donor and acceptor concentrations are both related to the silicon effusion cell temperature by Arrhenius relations; however, each is fitted with a different activation energy. This leads to increasing compensation with increasing dopant flux.

HgTe‐CdTe superlattices have been grown on GaAs (100) substrates by molecular beam epitaxy. X‐ray diffraction analysis shows the superlattices to be single crystalline with well defined periods. Temperature‐dependent infrared absorbance measurements indicate that the superlattices have band gaps with positive temperature coefficients. The band‐gap temperature dependence is in semiquantitative agreement with theoretical calculations.

HgTe‐CdTe superlattices of high crystalline quality, and with negligible interdiffusion, have been successfully grown at low substrate temperatures (150–170 °C) by molecular beam epitaxy. Optical spectra exhibiting multiple steplike absorption edge features have been measured on as‐grown and thermally annealed samples. The data provide valuable information on the superlattice subband structure when coupled with a suitable model. These results then can be used to obtain semiquantitative measures of layer interdiffusion along with such quantities as valence‐band offset and strain. To observe subband structure in the optical data, it is necessary to fabricate samples having homogeneous HgTe and CdTe layer thicknesses in the superlattice growth direction. Homogeneity of layers was achieved by carefully controlling elemental source fluxes and shutter times during the epitaxial growth process. Substantial changes in the subband‐related absorption structure could be induced by annealing the superlattice samples ...

We report photoluminescence in HgTe‐CdTe superlattices that originates in the CdTe layers. We see a near‐band‐gap line at 1.493 eV, which demonstrates that strain and Hg‐Cd substitutional diffusion are small in the CdTe layers. In addition, a deep‐level line at 0.775 eV, possibly due to recombination along dislocation lines, is observed.

Optical transmittance spectra of HgTe‐CdTe superlattices grown on GaAs substrates by molecular beam epitaxy were measured as a function of temperature between 40 and 360 K. Low substrate temperatures (150–170 °C) were required during growth to minimize interdiffusion and improve crystal quality. The highest quality superlattices exhibit sharp absorption edges with positive temperature coefficients in reasonable agreement with theory. In addition, many superlattices exhibit higher energy absorption features with temperature dependences similar to the lower energy fundamental absorption edge. The higher energy absorption features are associated with optical transitions involving higher order (n=2,3) superlattice minibands.

HgTe‐CdTe heterostructures have been grown by molecular beam epitaxy on (100) InSb substrates. Separate elemental Hg and Te beams were used for the HgTe growth at a substrate temperature of 160 °C. X‐ray diffraction measurements indicate that thin epitaxial layers are of high crystalline quality. Secondary‐ion mass spectroscopy measurements show substantial In and Sb diffusion into the epitaxial layers with a concentration enhancement at the HgTe‐CdTe interface.

Progressive annealing of a-Si:D and a-Si:DH samples between 250 and 600°C is found to eliminate the broad central deuteron magnetic resonance (DMR) spectral component associated with weakly bonded D. The well-resolved doublet arising from tightly bound D diminishes in intensity. The narrow central line associated with microvoid-contained molecular D2 and HD increases and then decreases for the warmest anneals. DMR relaxation times, line shapes, and hole-burning spectra reflect changes in sample morphology upon annealing.

Deuteron magnetic resonance (DMR) measurements have been made on plasma-deposited samples of a-Ge: D, H; a-Si:D,F; and a-SiGe:D,F. The experiments were performed at 30.7 MHz with a Bruker CXP 200 spectrometer and a probe constructed to permit operation at high rf power down to temperatures near 4 K. The 50 to 100 mg samples comprise less than 1 mm flakes in Kel-F containers of 5 mm diameter.

Deuteron magnetic resonance line shapes and spin lattice relaxation times are presented for a-Si:D, F and a-SiGe:D, F. These parameters differ from those for typical a-Si:D, H samples, but in some respects are similar to those for an annealed a-Si:D, H sample. The a-SiGe:D, F spectra display an unusually large broad central weakly bound D resonance component and a barely-resolved Ge-D quadrupolar doublet. Comparisons indicate substantial differences in void morphology between the a-Si:D, F and a-SiGe:D, F.

We report the observation of a broad photoluminescence line at 0.75 eV in CdTe grown by molecular‐beam epitaxy at both high‐ and low‐substrate temperatures. This line is in excellent agreement with the E2 line observed by deep‐level transient spectroscopy by previous workers. The origin of both of these lines is probably a deep defect or impurity level native to CdTe.

Single‐crystal epitaxial layers of (100) and (111) oriented CdTe were grown on (100) oriented GaAs substrates by molecular beam epitaxy. Low‐temperature (4.2 K) photoluminescence spectra exhibit free‐exciton and bound‐exciton peaks having linewidths on the order of 2 meV for both CdTe crystallographic orientations. However, defect‐related photoluminescence is found to be stronger in (100) oriented CdTe.

HgTe–CdTe superlattices were grown by molecular beam epitaxy on GaAs(100) substrates with CdTe buffer layers. The large (14.6%) lattice mismatch between CdTe and GaAs is relieved by a two‐dimensional array of misfit dislocations with a period of 3.1 nm. X‐ray diffraction, infrared transmittance, and Hall effect measurements are reported for thick (>5 μm) single crystalline superlattices.