GaAs quantum well structures investigation by local cathodoluminescence

The article shows the cathodoluminescence technique application to a quality analysis of a semiconductor multilayer heterostructures. Two structures with a GaAs quantum well embedded between the AlGaAs and GaInP barriers were investigated. The AlGaAs/GaAs/GaInP and GaInP/GaAs/AlGaAs structures were grown by MOCVD on a GaAs substrate. In this work we study the interface quality of quantum-dimensional GaAs layer by means of the local cathodoluminescence. Degradation and broadening of GaAs/GaInP interface occurring during the growth process of GaAs on GaInP layer was assumed to result in the formation of a layer with mixed composition at the interface. In addition, the presence of the layer prevented the formation of a quantum well in the GaAs layer. The transition layer was clearly observed by the cathodoluminescence. In the other case it was found that the growth of a structure with GaAs layer on top of AlGaAs produced a quantum well with a 10 nm thickness. The interface quality and layer thicknesses were also confirmed by the X-ray diffraction investigation of these structures.     

Utility of the tandem Pauson-Khand reaction in the construction of tetracycles

The scope of the tandem Pauson-Khand reaction has been explored for the regiospecific construction of [5.5.5.5]- and [5.6.6.5]tetracyclic systems via the photolytic method of Livinghouse. The rapid regiospecific entry into the two dicyclopentapentanoid systems 17 and 29 was accomplished from the key diene-diynes 11 and 19b. A photochemically mediated catalytic tandem Pauson-Khand cyclization was employed to prepare the parent ring systems of dicyclopenta[a,e]pentalene (from 19b) and dicyclopenta[a,f]pentalene (from 11) in regiospecific fashion in a one-pot process. Under these conditions, conversion of acyclic diene-diyne 16 into tetracyclic system 17 was achieved in 74% yield, while a similar process was employed to convert 28 into tetracycle 29 in 90% yield. This is much improved over the previous conditions that employed NMO. Six carbon-carbon bonds were generated in this process constituting up to 98% yield for each carbon-carbon bond so formed. Furthermore, tetracyclic [5.6.6.5] systems such as dicyclopenta[b,g]decalins 37, 38, and 40 were prepared from similar diene-diyne precursors via the tandem Pauson-Khand cyclization. Importantly, acetal 36 provided the desired cis-fused [5.6.6.5] system 38a (via 40a/b) in stereospecific fashion. This reaction is unique in that it provides a cis-decalin ring system; moreover, the yield of each of the six carbon-carbon bonds formed in this process was at least 89%. The structure of cis diol 38a was confirmed by X-ray crystallography.     

Calculation of the Green function for the Laplace equation

A modification of the traditional method of calculating the Green function in a layered medium is suggested that allows one to substantially increase its accuracy. In addition, a technique for calculating the Green function that makes it possible to control the accuracy of the calculated potential is offered. This technique is based on the properties of the Bessel and Struve functions. An example of calculation using the suggested technique is illustrated. The results may be extended for a wide class of problems the solution to which requires calculation of the Green function for the Laplace equation in a layered medium.     

Investigation of multiple layer heterojunction structures by means of an electron-beam microprobe-analyzer

Multiple layer heterojunction structures in the AlAs GaAs system are investigated by means of an X-ray microanalyzer. The quantitative determination of Ga and Al is extensively discussed. The correction for atomic number, absorption and secondary fluorescence are considered. For Al_xGa_1–xAs layers it is favourable for quantitative analysis the AlK_α-emission for x < 0,3 and the GaK_α-emission for x > 0.3. Some multilayer heterojunction structures of laser and luminescence diodes are shown in form of their AlK_α microprobe diagrams.     

Liquid phase epitaxy of AlxGa1–xAs-GaAs heterostructures

This paper is concerned with the investigation of the peculiarities of the application of liquid phase epitaxy for obtaining multilayer structures with heterojunctions in the AlAs–GaAs system. Segregational depletion of Al in liquid and in solid phases is characteristic of this system. It is proved that supercooling of the substrate and numerous inclusions of Al allow to decrease this segregational depletion of Al and to obtain layers of nearly constant composition up to 600 μm in thickness. A new technique of crystallization of structures from a limited volume of melt with the change of solutions by way of compulsory squeezing out of the foregoing solution by the following one is put forward with the view to perfect the planarity of multilayer structures with abrupt heterojunctions and to improve the complete change of solutions on a substrate. The results of the study of various methods of controlling the value and direction of the Al concentration gradient are listed. It has been proved that structures with fluent increase of Al concentration during the crystallization process and also structures with alteration in the gradient direction of Al concentration may be obtained by way of artificial slowing down or acceleration of the process of mixing up solutions of various compositions.