Communications ADVANCED MATERIALSmaterials can be visualized in terms of different connectivity patterns of the dimer building blocks. This topological idea is illustrated in Figure 5 b,c for the R-SnS-1 and R-SnS-3 structure types. Periodic arrays of interconnected Sn,S, broken-cube clusters result from this kind of assembly scheme. The unique three-coordinate capping sulfide sites of adjacent Sn,S, cluster units, which are found to alternate up then down around the 24-atom and 32-atom ring pores of the R-SnS-1 and R-SnS-3 structure types, respectively, are found to be a natural consequence of the interlinking of (Sn,SJ4" building blocks in the patterns shown in Figures 1 and 5.A cautionary note is in order at this juncture in the discussion of the mode of formation of the R-SnS-n materials. Although the crystallization and structure determination of such species as (CHAH),Sn,S,, (TMA),Sn,S,, and (DAB-COH),Sn(S,), are interesting and informative, and spectroscopic evidence has been found for (SII,S,)~@ dimers in the mother liquors of the R-SnS-n preparations, their actual relationship, if any, to the reactions and mechanisms involved in the formation of the R-SnS-n materials could be circumstantial and must currently be regarded as interesting and motivation for further work on this system. Work is continuing that is aimed at probing the nature and chemistry of the gas, solution, and solid phase species present at various stages in the R-SnS-n syntheses in order to clarify the matter further. Esperimen tal I ( VC H I4~rlugsgetellr~haJr nzbH D-69469 Wemherm, 1994 0935-964819411111-0868 $ 5 00+ 2510

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Properties of carbon-doped low-temperature GaAs and InP grown by solid-source molecular-beam epitaxy using CBr 4

We illustrate the use of micro-Raman mapping to study the local effect of defects on device temperature in active AlGaN∕GaN heterostructure field-effect transistors. Significant temperature rises in active devices, 50–100% above average device temperatures, were identified in the vicinity of defects. Measured temperature distributions were compared to finite difference simulations. Reduced thermal conductivity in the defect vicinity was found to be responsible for the local temperature rises in these devices, combined with possible changes in the current flow distribution.

An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1−xN layers (x<0.3) with thickness >20nm can be made from a single measurement.