High-temperature gallium phosphide dynistors

Elements of the technology of gallium phosphide (GaP) epitaxial multilayer heterostructures grown by chemical vapor deposition in a chloride system are described and the main characteristics of GaP dynistors obtained by this method and intended for operation in a high-temperature (up to 400°C) medium are presented. A characteristic feature of the obtained initial epitaxial n-p-n-p structure is the binary zinc-magnesium doping of the p-emitter layer, which improves the device switching parameters (in particular, reduces the residual voltage at an acceptable level of reverse current) in comparison to those of the traditional Mg-doped structures. The observed optimization of the current-voltage characteristic is related to differences in the mechanisms of Zn and Mg diffusion in the GaP lattice, which can lead under certain conditions to suppression of the effect of structural defects present in the epilayers.     

Generating broadband random Gaussian signals

General approaches to the problem of generating random Gaussian signals are considered, a variant of practical implementation of such a generator is proposed, some technological aspects of the formation of active device structures are discussed, and spectral characteristics of the output noise signal are presented. The technical solution is based upon the principle of additive formation of a Gaussian process as a result of the joint operation of a group of parallel connected pulsed signal sources of the microplasma type. The proposed devices were implemented as GaAs microchips with p-n structures fabricated by epitaxial-diffusion technology, on which a matrix relief of elementary noise sources was formed using the microlithography techniques.     

Growth and X-ray Diffraction Study of Tl4HgI6 Crystals

Single crystals of red and black Tl₄HgI₆ phases are grown, and their structures are determined by x-ray diffraction. All of the crystals are found to have the noncentrosymmetric space groupP4nc. The crystals contain iodine vacancies in different positions, depending on the iodine content of the growth charge, and mercury vacancies. Black Tl₄HgI₆ crystals, prepared in the presence of excess iodine, are shown to consist of a nearly racemic mixture of two structures differing in polarity.     

Optical properties of lithium indium selenide

Conditions of growth of LiInS₂ and LiInSe₂ crystals with high optical quality are studied. Data on the dispersion of refractive indices in red-phase LiInSe₂ crystals are presented. Possible nonlinear three-frequency processes in LiInS₂ and LiInSe₂ crystals used for SHG and OPO are analyzed. Potentials of these crystals for application in OPO devices are shown.     

Formation of a single base mismatch impedes spontaneous DNA branch migration

DNA branch migration, a process whereby two homologous DNA duplexes exchange strands, is an essential component of genetic recombination. Models for homologous recombination have invoked spontaneous branch migration as one mechanism for the generation of large regions of heteroduplex DNA. During recombination, two homologous parental duplexes that contain similar, but not identical, sequences are paired and undergo strand exchange. An important issue is whether spontaneous branch migration is capable of traversing sequence heterology such as mismatches, insertions and deletions. We use a model four-strand system to examine the effect of mispaired or unpaired bases on branch migration. The assay consists of annealing two short duplexes having defined sequence heterologies. Following annealing, a Holliday junction is formed that is free to branch migrate. Our results demonstrate that a single base mismatch, insertion or deletion is sufficient to pose a substantial barrier to spontaneous branch migration. In the presence of magnesium, branch migration through such sequence heterologies is almost completely blocked. Others have shown that non-mobile four-way junctions undergo a dramatic shift in conformation in the presence of magnesium. Our data suggest that a similar transition occurs for the mobile Holliday junction. We also discuss how proteins may facilitate branch migration through sequence heterologies in vivo.     

Studying the Pyroelectric Effect in AlN Epilayers

We have studied the pyroelectric effect in 6- to 12-μm-thick AlN epilayers grown by hydride–chloride vapor phase epitaxy (HVPE) on 4H-SiC substrates. The pyroelectric current was generated by means of dynamic laser action. Evolution of the temperature fields was monitored by direct measurements of the surface temperature with allowance for the specific thermal heterogeneity of the AlN/SiC system. In combination with the results of pyroelectric current measurements, these data allowed the pyrocoefficient of the AlN/SiC structure to be estimated at p ≈ 3.0 μC/(m² K). Pyroelectric figures of merit of the given structure are calculated that can be used for the comparison of pyroelectric materials intended for the creation of IR radiation detectors.     

Growth and X-ray Diffraction Study of YCa4O(BO3)3:Ce,Er,Yb Crystals

YCa₄O(BO₃)₃crystals (sp. gr. Cm), nominally pure and activated with Yb, Er, and/or Ce, were grown by the Czochralski technique. The crystal structure of YCa₄O(BO₃)₃was refined by single-crystal x-ray diffraction, and the site compositions were determined: (Y_0.792Ca_0.208(6))(Ca_0.938Y_0.062(4))₂Ca₂O_0.96(2)[ ]_0.04(BO₃)₃([ ] = vacancy). The data set for YCa₄O(BO₃)₃was found to contain additional reflections which were inconsistent with space group Cmbut could be indexed in a cell with doubled a. No superlattice reflections were detected in the case of the activated crystals. Structural data for LnCa₄O(BO₃)₃were used to derive relations for the lattice parameters and average cation–anion distances as functions of r _Lnand weighted-average cation radii on the mixed-occupancy sites, which made it possible to assess the distributions of Y, Yb, Er, Ce, and Ca over the mixed-occupancy sites. Absolute structure determination showed that the pure and Yb³⁺-activated YCa₄O(BO₃)₃crystals differed in polarity.