Search

Near‐surface regions of Cd1−xMnxTe and Hg1−xCdxTe epilayers (down to tens of angstroms) on (001) GaAs substrates have been characterized by extremely asymmetric Bragg reflection topography (EABRT) with the laboratory x‐ray source condition and a Lang camera, using x‐ray grazing incidence angles less than the critical angle for total external reflection. The experimental topographs obtained in the present work illustrate the potential of the EABRT technique for nondestructive characterization of near‐surface regions of crystals. The resolution of an image in EABRT is discussed in detail.

Tetraethyldistibine, Et 4 Sb 2 , a new antimony source for MOCVD is evaluated. It is shown to be a useful and safe dopant feedstock for low temperature growth of p-type Hg 1- x Cd x Te. Attempted purification of Et 4 Sb 2 by adduct formation with CdI 2 resulted in isolation of a polymeric chain adduct [Et 4 Sb 2 · 2CdI 2 ] n ( 1 ). The structure of 1 has been determined by X-ray crystallography and consists of chains of iodide-bridged Et 4 Sb 2 · 2CdI 2 units which have CdI 2 bonded to each antimony atom. Antimony and two iodine atoms form a trigonal planar arrangement around cadmium, and two weak axial bonds to the iodine atoms of adjacent units give distorted trigonal bipyramidal coordination to cadmium. The distibine molecules display a skew configuration of the ethyl substituents. The Sb II -Sb II and Sb II -Cd II distances are 2.784(2) and 2.822(2) A respectively. Dropwise addition of Me 2 Cd to Et 4 Sb 2 in the absence of solvent gave an insoluble 1:1 adduct. Et 4 Sb 2 reacts with Te but not with Cd or Hg.

X-ray diffraction has been used to study a GaAs/Si wafer containing microcracks. Two arrays of microcracks running in the [011] and [01¯1] directions have been identified. The FWHM of the rocking curves and the curvature of the wafer show a marked dependence on the azimuthal orientation of the sample, due apparently to the anisotropic distribution of microcracks. A discussion is given of the causes of the observed reversal of diffraction contrast in the images of microcracks in the GaAs epilayer as compared with the Si substrate. The “white-black-white” contrast in images of microcracks and its variation due to the effective misorientation of the lattice was also observed and is discussed in detail. Mosaic blocks existing in the epilayer were observed by Lang topography.

Understanding the genesis of the block haplotype structure of the genome is a major challenge. With the completion of the sequencing of the Human Genome and the initiation of the HapMap project the concept that the chromosomes of the mammalian genome are a mosaic, or patchwork, of conserved extended block haplotype sequences is now accepted by the mainstream genomics research community. Ancestral Haplotypes (AHs) can be viewed as a recombined string of smaller Polymorphic Frozen Blocks (PFBs). How have such variant extended DNA sequence tracts emerged in evolution? Here the relevant literature on the problem is reviewed from various fields of molecular and cell biology particularly molecular immunology and comparative and functional genomics. Based on our synthesis we then advance a testable molecular and cellular model. A critical part of the analysis concerns the origin of the strand biased mutation signatures in the transcribed regions of the human and higher primate genome, A-to-G versus T-to-C (ratio ∼ 1.5 fold) and C-to-T versus G-to-A (≥ 1.5 fold). A comparison and evaluation of the current state of the fields of immunoglobulin Somatic Hypermutation (SHM) and Transcription-Coupled DNA Repair focused on how mutations in newly synthesized RNA might be copied back to DNA thus accounting for some of the genome-wide strand biases (e.g., the A-to-G vs T-to-C component of the strand biased spectrum). We hypothesize that the genesis of PFBs and extended AHs occurs during mutagenic episodes in evolution (e.g., retroviral infections) and that many of the critical DNA sequence diversifying events occur first at the RNA level, e.g., recombination between RNA strings resulting in tandem and dispersed RNA duplications (retroduplications), RNA mutations via adenosine-to-inosine pre-mRNA editing events as well as error prone RNA synthesis. These are then copied back into DNA by a cellular reverse transcription process (also likely to be error-prone) that we have called "reverse transcription-mediated long DNA conversion." Finally we suggest that all these activities and others can be envisaged as being brought physically under the umbrella of special sites in the nucleus involved in transcription known as "transcription factories."

High-resolution X-ray diffraction studies of a twinned (111) CdTe epilayer on (0001) sapphire substrate were carried out. The structural properties and uniformity of the CdTe epilayer were established from analyses of Lang topographs, double-crystal rocking-curve maps and twin-content maps. Maps of the full width at half-maximum (FWHM) of the rocking curve were taken for two twin species (1 and 2), which relate to each other by a rotation of 180° about the [111] axis. The value of the FWHM varied over the sample from 108 to over 1000 arcs. The twin-content maps were determined over the whole layer, and showed significant variations. Lang topographs were taken using the asymmetric 422 Bragg reflection for both twins and the results were consistent with the rocking curves. The topographs showed that clusters of dislocations exist in the layer and many of them lie along the [112] direction parallel to the (111) CdTe layer surface with the Burgers vector in the [11 0] direction. Evidence of double-positioning twins was found in the CdTe epilayer. It is shown that combinations of rocking-curve maps and topographs give a much more informative characterization than a single-point measurement of the rocking curve.

High-resolution X-ray diffraction studies of MOCVD-grown Cd1−xMnxTe/Cd1−yMnyTe superlattices on a (001) GaAs substrate have been carried out. Topographs and rocking-curve maps were taken from two directions at right angles to each other ([110] and [110], the latter being the gas flow direction). The structural properties and uniformity of the Cd1−xMnxTe/ Cd1−yMnyTe superlattice epilayers have been established from analyses of both rocking-curve maps and topographs. An anisotropic mosaic structure, where the mosaic blocks are well ordered along the [110] direction and disordered along the [110] direction, is proposed in order to explain these observations. It is shown that combinations of rocking-curve maps and topographs give a much more informative characterisation than a single-point measurement. The satellite peaks of the double-crystal rocking curve of Cd1−xMnxTe/Cd1−yMnyTe superlattices have been observed using an asymmetric 335 Bragg reflection. The angular positions of the observed satellites have been investigated and compared with the predictions of a standard theoretical formula, extended to the case of asymmetric reflections. The rocking curves of Cd1−xMnxTe/Cd1−yMnyTe superlattices have also been simulated using the Takagi-Taupin equations. The agreement obtained is quite satisfactory.

Three major applications have been found for rare-earth compounds in Metal Organic Chemical Vapour Depostion (MOCVD) or Chemical Vapour Deposition (CVD). Yttrium 2,2,6,6-tetramethyl-3,5-heptanedionates and 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionates have been used in conjunction with barium and copper(II) β-diketonates to deposit YBa2Cu3O7−δ as superconducting thin films. Rare-earth fluorides and chlorides have been used for CFD doping of rare earths into MOCVD-deposited ZnS, whilst yttrium chloride has been used, with barium iodide and copper(I) chloride, to produce YBa2Cu3O7−δ superconducting material by CVD. Lanthanoid (Ln) tris(cyclopentadienyl) compounds, Ln(C5H5)3 or Ln(C5H4Me)3, have been used for doping of rare earths into 13–15 (III–V) semiconductors. Their volatility, structure/volatility relationships, and preparations are discussed. Possible alternative reagents and problems to be faced in doping 12–16 (II–VI) semiconductors are also considered.

The role of normal absorption in x‐ray diffraction measurements of integrated Bragg intensities from epitaxial (or other) layers and their associated substrates is discussed in terms of the transmission factor. It is shown that the technique for the determination of the polarity of an epitaxial layer by means of anomalous scattering of x rays need not be limited to the case of an effectively infinite layer thickness, in Bragg geometry. Some discussion of layer‐thickness determination is also given.

Specular HgTe–CdTe superlattice epilayers have been obtained on two 2‐in. diam GaAs or sapphire wafers per growth run using a horizontal metalorganic chemical vapor deposition (MOCVD) reactor in which the pyrolysis of the organometallics is induced by a cracking susceptor suspended above the substrates. This enables growth of superlattices below 300 °C using the standard precursors dimethyl cadmium, diethyl telluride, and elemental mercury. Annealing the superlattices at 350 °C converts them to homogeneous Hg1−xCdxTe. Compositional profiles obtained by Rutherford backscattering, sputter Auger depth profiling, and e‐beam analysis of ultramicrotomed thin cross sections, were used to study interdiffusion of the CdTe and HgTe layers during growth and annealing. Arrays of metal–semiconductor field effect transistors (MESFETS) and photoconductive detectors with room temperature peak responsivity between 1.3 and 1.6 μm have been prepared from incompletely interdiffused material grown on semi‐insulating GaAs with...

Organometallic feedstocks of cadmium, mercury and tellurium, as well as dopants to create n-type, p-type or diluted magnetic semiconductors, can impose limits on the range of programmable parameters in the MOCVD reactor. The influence of the chosen organometallics on the crystal growth process is critically reviewed.

The new organometallic feedstock MeMn ( CO ) 5 has been used for the MOCVD growth of Cd 1− x Mn x Te (111B) epilayers on GaAs (100) substrates. The epitaxial relationship was established by HRTEM and X-ray diffraction (XRD). The quaternary alloy Hg 1−x−y Cd x Mn y Te was grown by in-situ annealing og HgTe overlayers on Cd 1−x Mn x Te epilayers. Adherent films of Hg 1−x Mn x Te on glass were grown by MOCVD. The electrical properties show semi-metal behaviour, and XRD and other properties indicate a zinc-blende structure.

A general method is described for production of uniform thin films from the vapour phase. A brief review of the prior art reveals that various approaches to this problem, and in particular substrate rotation, fail to achieve uniformity of thickness and composition in such technologies as molecular beam epitaxy (MBE) or chemical vapour deposition (CVD). The new technique, for which patent protection has been obtained, involves establishment of uniform growth conditions in one direction, and translation of the substrate at a constant rate in a direction perpendicular to the first. The effect is to integrate and average any property in the direction of translation, so that every point on the substrate experiences an identical history, ensuring complete uniformity of the deposited film.

The anomalous scattering of X-rays has been used to determine the polarity of CdTe epitaxial layers on sapphire. The results for two samples are presented, one of (111) orientation ('A face'), the other of (III) orientation (,B face'). The (III) layer is twinned, the two twin species being related by a 180� rotation about the [1111 axis. The twin fraction shows considerable variation for different positions on this sample, and must be taken into account when analysing the integrated X-ray intensities, in order to get meaningful Bijvoet ratios. The polarities of the two twin species are found to be the same.

The complexes (η-C5H5)2(η-C5R5)Rh2M(CO)3(CF3C2CF3) are obtained by addition of (η-C5R5)M(CO)2 (R = H or Me; M = Co or Rh) to the binuclear complex (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3). The reactions occur slowly at room temperature in hydrocarbon solvents. Spectroscopic data reveal that the trinuclear complexes contain three different types of carbonyl group: terminal, edge-bridging, and acyl. The molecular geometry of (η-C5H5)2(η-C5Me5)Rh3(CO)3(CF3C2CF3) has been established by X-ray diffraction. There is a V-shaped arrangement of the three rhodium atoms. Within the (η-C5H5)2Rh2(CO)(CF3C2CF3) unit the CO has become terminal and the CF3C2CF3 has twisted to sit askew the RhRh bond. The two RhCO bonds of (η-C5Me5)Rh(CO)2 have added to this binuclear unit in different ways. One gives the new RhRh bond which is bridged unsymmetrically by the carbonyl. The other has undergone a 1,2-addition to the coordinated CF3C2CF3 moiety; this gives a four-membered metallacyclic ring incorporating an acyl carbonyl. Two isomers of (η-C5H5)3Rh3(CO)3(CF3C2CF3) can be isolated. Spectroscopic data indicate that the minor isomer is structurally analogous to (η-C5H5)2(η-C5Me5)-Rh3(CO)3(CF3C2CF3) but that the major isomer has a different but, as yet, undetermined structure. The cobalt complexes (η-C5H5)2(η-C5R5)Rh2Co(CO)3-(CF3C2CF3) are structurally analogous to (η-C5H5)2(η-C5Me5)Rh3(CO)3-(CF3C2CF3). The following products are obtained when the tertiary phosphine complexes (η-C5R5)Rh(CO)(PR′3) are added to (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3); (η-C5H5)2(η-C5R5)Rh3(CO)(CF3C2CF3), (η-C5H5)2Rh2(CO)2-(CF3C2CF3) and (η-C5H5)2Rh2(CO)(PR′3)(CF3C2CF3). Thus, this metal nucleophile adds as the separate components [(η-C5R5Rh], [CO], and [PR′3). The major product obtained when (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3 is treated with (η-C5Me5)2Rh2(μ-CO)2 is (η-C5H5)2(gh-C5Me5)Rh3(CO)3(CF2C2CF3).

Treatment of (η-C5H5)2Rh2(μ-CO) {μ(η2-CF3C2CF3)} with diazo compounds N2CR1R2 gives the μ-alkylidene complexes (η-C5H5)2Rh2(μ-CO)(μ-CR1R2) {μ(η1-CF3C2CF3)} (I, R1  R2  H; II, R1  H, R2  CF3; III, R1  H, R2  CO2Et; IV, R1  R2  CO2Et). In solution, I and III are converted to (η-C5H5)2Rh2{μ(η1,η3-C(CF3)C(CF3)CHR2)} by insertion of the alkylidene unit into an alkyne—rhodium bond. This reaction is not observed for II. With IV, a related insertion reaction produces (η-C5H5)2Rh2{C(CF3)C(CF3)OC(OEt)C(CO2Et)}. The crystal and molecular structure of this complex has been determined from single crystal X-ray diffraction data.

Organoytterbium(III) complexes [(η-C 5 H 5 ) 2 )YbX] (X = O 2 CMe, O 2 CC 6 F 5 , O 2 CC 5 H 4 N, Cl, Br, I, CCPh, C 6 F 5 , (MeCO) 2 CH, (PhCO) 2 CH) have been synthesized in high yield from [η-C 5 H 5 ) 2 ]Yb(MeOCH 2 ) 2 ] and the corresponding HgX 2 , TlX, AgX 2 or CuX species. An X-ray diffraction study of the pentafluorobenzoate has shown it to be dimeric, with a somewhat unsymmetrical bridging carboxylate.

Ultramicrotome techniques are used to prepare thin cross sections of a {111} epilayer of CdTe deposited by metalorganic chemical vapor deposition onto a {100} GaAs substrate. The structure of these samples is investigated by transmission electron microscopy using high‐resolution (HRTEM) and diffraction contrast, and the polarity of the {111} layer by convergent beam electron diffraction (CBED) and characteristic x‐ray emission under various electron channelling conditions, or ALCHEMI. Rutherford backscattering and channelling experiments on the bulk film confirm the presence of a multiply twinned lamellar structure as observed by electron beam techniques. Strong channeling confirms that the crystallinity is good, and that no significant concentration of defects occurs. HRTEM images of the {111} epilayer from the interface across the lamellar twins show few dislocations or crystal defects. Diffraction contrast indicates the presence of a periodic strain in the GaAs and parallel to the interface. CBED and A...

The known complex, trans-(η-C5H5)2Rh2(CO)2(CF3C2CF3) is formed in high yield from (η-C5H5)Rh(CO)2 and CF3FCCF3 at 100°. The less stable cis-isomer of the complex is obtained in low yield from the same reaction. The infrared, 1H, 19F and 13C NMR spectra of the two isomers are compared. The trans-isomer undergoes CO scrambling in solution at room temperature, and the variable temperature 13C NMR spectra are consistent with a pairwise bridge opening and closing mechanism. The mechanism is extended to account for the isomerization of cis to trans isomer, whihc has a half-life of 12 h at room temperature. The 13C spectrum indicates that the cis-isomer is static in solution at room temperature. The trans-isomer is reversibly protonated by protonic acids, and BF4− and PF6− salts of the protonated species can be isolated. The spectroscopic properties of these salts are consistent with protonation at one of the alkynyl-carbons, but it is not possible to distinguish between two alternative structures for the complex cation. Treatment of (η-C5H5)2Rh2(CO)2(CF3C2CF3) with (η-C5H5)Rh(CO)2 gives the trinuclear complex (η-C5H5)3Rh3(CO)(CF3C2CF3) in 80% yield. The analogoug but-2-yne complex is formed from (η-C5H5)3Rh3(CO)3 and MeCCMe. The infrared, 1H, 19F and 13C NMR spectra indicate that the hexafluorobut-2-yne complex exists in two different structural arrangements in solution. One has an edge bridging, and the other a face bridging carbonyl. The proportion of the isomers is affected by the solvent polarity. The spectra of the but-2-yne complex indicate it is fluxional at room temperature, and has a face bridging structure in solution regardless of the polarity of the solvent. Reversible protonation of the hexafluorobut-2-yne complex occurs in protonic acids, and the salt [(η-C5H5)3Rh3(CO)(CF3C2CF3)H]+[BF4]−,H2O can be isolated. The spectroscopic properties of this complex are consistent with a structure incorporating an edge-bridging carbonyl, and probably, an edge-bridging hydride ligand.

Treatment of the μ(η 1 )-alkyne complex (η-C 5 H 5 ) 2 Rh 2 (CO) 2 (CF 3 C 2 CF 3 ) with trimethylamine- N -oxide results in mono-decarbonylation to give the μ(η 2 )- alkyne complex (η-C 5 H 5 ) 2 Rh 2 (μ-CO)(CF 3 C 2 CF 3 ). Coordinative addition of a variety of ligands L to the monocarbonyl complex has been achieved at room temperature, and stable adducts (η-C 5 H 5 ) 2 Rh 2 (CO)L(CF 3 C 2 CF 3 ) (L  CO, CNBu t , PPh 3 , PMePh 2 , P(OMe) 3 , AsPh 3 , PF 3 and PF 2 (NEt 2 )) have been fully characterized by infrared and NMR spectroscopy. In each complex, there is a μ(η 1 )-attachment of the hexafluorobut-2-yne and a trans -arrangement of CO and L. The spectroscopic data establish that there is rapid scrambling of CO and L when L  CNBu t . An unstable adduct is formed when (η-C 5 H 5 ) 2 Rh 2 (μ-CO)(CF 3 C 2 CF 3 ) is dissolved in pyridine.

The complex (η-C5H5)2Rh2(CO)3(CF3C2CF3) is obtained, together with the known cis- and trans-isomers of (η-C5H5)2Rh2(CO)2(CF3C2CF3), when the intractable products formed from [Rh(CO)2Cl]2 and CF3CCCF3 at 25°C are treated with (C5H5)Tl. An X-ray diffraction study on crystals of the tricarbonyl complex has established that it contains two terminal carbonyls in transarrangement, and a 1-ene-3-one unit, C(CF3C(CF3)C(O), which bridges the metalmetal bond. NMR data for solutions of the complex have been interpreted in terms of an equilibrium between the solid state species (II) and an isomer (III) which has two bridging carbonyls. The proportion of III in the equilibrium mixture increases as the temperature is lowered. Coupling constant data indicate that there is rapid carbonyl scrambling in II, even at −100°C. Decarbonylation of (η-C5H5)2Rh2(CO)3(CF3C2CF3) is achieved at 25°C by treatment with Me3NO; this reaction produces cis- and trans-(η-C5H5)2Rh2(CO)2(CF3C2CF3) plus (η-C5H5)2Rh2(μ)-CO)(CF3C2CF3).

Le traitement de (η-C 5 H 5 ) 2 Rh 2 (μ-CO)(μ-CF 3 C 2 CF 3 ) avec les diazoalcanes N 2 CR'R 2 dans l'ether diethylique a 0°C donne (η-C 5 H 5 ) 2 Rh 2 (μ-CO)(μ-CR'R 2 )(μ-CF 3 C 2 CF 3 ). La migration du groupe alkylidene a l'alcyne coordine a lieu lors du stockage en solution a temperature ordinaire. Structure de (η-C 5 H 5 ) 2 Rh 2 [μ-C(CF 3 )=C(CF 3 ) OC(OEt)=C(CO 2 Et)]

The anomalous scattering of x rays has been used to determine the polarity of single‐crystal epitaxial layers, as grown on the substrate material. The method relies on the breakdown of Friedel’s law in noncentrosymmetric structures and is particularly effective when there exists a large difference between the values of the imaginary anomalous dispersion correction, f‘, for the respective atomic species. The method is nondestructive and measurements are taken from only one side of the sample (the side with the epitaxial layer).

Proton n.m.r. spectra of hydrido-osmium species show low intensity satellites as a result of coupling to natural abundance 187Os; a study of the coupling constants, and satellite intensities, observed in the 1H n.m.r spectra of hydrido-osmium clusters provides valuable information about molecular structures and fluxional processes and evidence is presented for the presence of a tetrahedral interstitial hydride in [HOs10C(CO)24]–.

Treatment of [(η5-C5H5)2Rh2(CO)2{µ(η1-CF3C2-CF3)}](1) with Me3NO produces [(η5-C5H5)2Rh2(µ-CO){µ(η2-CF3C2CF3)}](2) which forms adducts with CO, AsPh3, P(OMe)3, PMePh2, Ph2PCH2CH2PPh2, SO2, Me2SO, and ButNC; reaction of (2) with alkynes gives binuclear metallodiene and metallodienone complexes and with H2 under mild conditions yields CF3CH2CH2CF3.

Reaction of the anions [Ru6C(CO)16]2– and [Ru5WC(CO)17]2– with ClAuPR3(R = alkyl, aryl) yields [Ru6C(CO)16(AuPR3)2] and [Ru5WC(CO)17(AuPR3)2], respectively; X-ray crystallography has shown that two gold atoms adopt a different geometry in each case and subsequent low-temperature 31P{1H} n.m.r. studies have shown that these compounds exist as isomeric species in solution.