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Trioctahedral ferromagnesium micas from the Archaean amphibolites of the Kola super-deep borehole (KSDB-3) complex have been compared with those from analogous Archaean surface rocks in an integrated study of their chemical compositions and structural states (using wet chemistry, microprobe, X-ray, Mössbauer and infrared methods). Reports on both the experimental procedure and the crystal chemistry of the trioctahedral micas are given. This has enabled the revision of the assignment of individual infrared (IR) absorption bands for the hydroxyl ion (vOH−), and demonstrated the possibility of determining the content of Mg, Fe2+, R3+; cations and vacancies in the octahedral sheet to within a few hundredths of atomic units on the basis of the infrared and Mössbauer data. Proof was found of the presence of molecular water replacing K+ in the interlayer, which results in anomalous variations in the cell parameters, as the mica structure expands in response to the ingress of H2O. Variations in the degree of non-stoichiometry of the micas is related to the presence of structural H2O and of octahedral M1 vacancies. We found that samples recovered from depth within the borehole display lower degrees of octahedral order than those found in the analogous surface rocks.
A hybrid double heterostructure with large asymmetric band offsets, combining AlAsSb/InAs (as a III–V part) and CdMgSe/CdSe (as a II–VI part), has been proposed as a basic element of a mid-infrared laser structure design. The p-i-n diode structure has been successfully grown by molecular beam epitaxy (MBE) and exhibited an intense long-wavelength electroluminescence at 3.12 μm (300K). A II–VI MBE growth initiation with a thin ZnTe buffer layer prior to the CdMgSe deposition results in a dramatic reduction of defect density originating at the II–VI/III–V interface, as demonstrated by transmission electron microscopy. A less than 10 times reduction of electroluminescence intensity from 77 to 300K indicates an efficient carrier confinement in the InAs active layer due to high potential barriers in conduction and valence bands, estimated as ΔEC = 1.28 eV and ΔEV ∼ 1.6 eV. An increase in the pumping current results in a super-linear raising the EL intensity. The type of band line up at the coherent InAs/Cd1−xMgxSe interface is discussed for 0≤x≤0.2, using experimental data and theoretical estimations within a model-solid theory.
A novel quasi-thermodynamic approach is suggested to simulate surface chemistry in III-V compound MOVPE. Blocking of free adsorption sites by methyl radicals is considered as the mechanism limiting the growth rate at low temperatures. This assumption has provided a good reproduction of experimental data on GaAs MOVPE in various types of reactor. The commercial computational fluid dynamics software CFD-ACE™ has been used to perform a detailed threedimensional modeling of AlGaAs and InGaP deposition in an AIX-200 horizontal reactor. The surface model has been incorporated into the code to obtain the growth rate and layer composition distributions over the substrate. Modeling results demonstrate a reasonable agreement with experimental data.
Surface segregation and phase separation are investigated as processes limiting the indium incorporation in InGaN grown by ammonia Molecular Beam Epitaxy (MBE) and Metal- Organic Vapor Phase Epitaxy (MOVPE). It is shown that a significant concentration of indium on the growing surface may prevent the adsorption of ammonia via site blocking mechanism and result in appearance of In droplets instead of InGaN growth. Another conclusion is that the composition fluctuations in InGaN are related to coexistence of strained and relaxed InGaN islands rather than to the phase separation as commonly assumed.
Multiwafer Planetary Reactor is a promising system for large-scale production of heterostructures for LED's based on III-group nitrides. Analysis of chemical processes occurring in the reactor allows one to get insight into specific mechanisms governing growth of nitride based heterostructures. In the present paper results of modeling analysis of MOVPE of InxGa1−xN layers in AIX-200 Reactor and AIX 2000 HT Planetary Reactor are reported. The model used for MOVPE process analysis accounts for gas flow, heat transfer, and multicomponent mass transport along with gas phase and surface chemical reactions. Results of the modeling analysis of In transport and incorporation into the solid phase are compared with experimental data. It is shown that the model predicts reasonably well the In incorporation during MOVPE of InGaN under In/(In+Ga) ratio in the gas phase less than 20%.
Recent studies revealed specific features of chemical processes occurring on the surface of growing group-III nitrides – extremely low sticking probability of molecular nitrogen, low sticking coefficient and incomplete decomposition of ammonia frequently used as the nitrogen precursor. These features (kinetic by nature) result in the growth process going on under conditions remarkably deviated from the gas-solid heterogeneous equilibrium. In this paper we propose a novel approach to modeling of group-III nitride growth by MOVPE taking into account these features. In the model the sticking/evaporation coefficients of N2 and NH3 extracted from independent experiments are used allowing adequate description of the kinetic effects. The model is applied to analysis of growth of binary (GaN) and ternary (InGaN) compounds in a horizontal tube reactor. The growth rate and the solid phase composition are predicted theoretically and compared with available experimental data. The modeling results reveal lower ammonia decomposition ratio on the surface of the crystal as compared to thermodynamic expectations. The developed model can be used for optimization of growth process conditions.
This paper discusses the distribution of tumor size at detection derived within the framework of a new stochastic model of carcinogenesis. This distribution assumes a simple limiting form, with age at detection tending to infinity which is found to be a generalization of the distribution that arises in the length-biased sampling. Two versions of the model are considered with reference to spontaneous and induced carcinogenesis; both of them show similar asymptotic behavior. When the limiting distribution is applied to real data analysis its adequacy can be tested through testing the conditional independence of the size, V, and the age, A, at detection given A > t*, where the value of t* is to be estimated from the given sample. This is illustrated with an application to data on premenopausal breast cancer. The proposed distribution offers the prospect of the estimation of some biologically meaningful parameters descriptive of the temporal organization of tumor latency. An estimate of the model stability to the prior distribution of tumor size and some other stability results for the Bayes formula are given.
We report the first observations of electroluminescence (EL) and lasing in laser structures with high Al-content (x=0.64, Eg=1.474 eV) cladding layers and a narrow-gap InGaAsSb active layer (Eg=0.326 eV at T=77K). The structures are LPE-grown lattice-matched to GaSb substrate. Band energy diagrams of the laser structures had strongly asymmetric band offsets. The heterojunction between high Al-content layer and InGaAsSb narrow-gap active layer has a type II broken-gap alignment at 300K. In this laser structure spontaneous emission was obtained at λ=3.8μm at T=77K and λ=4.25 μm at T=300K. Full width at half maximum (FWHM) of emission band was 34 meV. Emission intensity decreased by a factor of 30 from T=77K to 300K. Lasing with single dominant mode was achieved at λ=3.774 μm (T=80K) in pulsed mode. Threshold current as low as 60 mA and characteristic temperature T0=26K were obtained at T=80–120K.
The multihit–one target model induces a stochastic ordering of cell survival with respect to the cell sensitivity characteristics. This property can be used for a description of cell killing effects in heterogeneous populations of cells on the basis of randomized versions of the model. In such versions, either the critical number of lesions or the mean number of hits per unit dose (sensitivity), or both, are assumed to be random. We give some new results specifying conditions under which the randomized multihit models are identifiable, with a focus on the following cases: (1) the critical number of radiation-induced lesions, m, is random; (2) the sensitivity parameter, x, is random given m is known or otherwise; (3) x and m form a pair of independent random variables.
Optimization problems in cancer radiation therapy are considered, with the efficiency functional defined as the difference between expected survival probabilities for normal and neoplastic tissues. Precise upper bounds of the efficiency functional over natural classes of cellular response functions are found. The ‘Lipschitz' upper bound gives rise to a new family of probability metrics. In the framework of the ‘m hit-one target' model of irradiated cell survival the problem of optimal fractionation of the given total dose into n fractions is treated. For m = 1, n arbitrary, and n = 1, 2, m arbitrary, complete solution is obtained. In other cases an approximation procedure is constructed. Stability of extremal values and upper bounds of the efficiency functional with respect to perturbation of radiosensitivity distributions for normal and tumor tissues is demonstrated.
Photol uminescence of p-P InAs/InAsPSb and InAs/InAl AsSb heterojunctions grown by LPE method was studied at liquid helium temperature. The recombination spectra contained a new broad band lying between the substrate and the layer lines which was identified as an emission from the interface. This line is characterized by a strong blue shift when the excitation intensity increases. The intensities of bulk and interface lines show an unusual dependence on the pumping power. On the basis of experimental findings the interface line is attributed to emission from electrons confined at the interface due to reflection of elecctrons moving above the barrier.
Stark broadening of Lyman lines in high density plasmas is treated theoretically to study the effects of the fields inside the plasmas, the microfield, and the oscillating fields from Langmuir waves. Resonance features are expected to be observed from the line profiles, especially about the microfield.
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