1063-7826/01/3511- $21.00 © 2001 MAIK “Nauka/Interperiodica” 1320 Semiconductors, Vol. 35, No. 11, 2001, pp. 1320–1323. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 11, 2001, pp. 1376–1379. Original Russian Text Copyright © 2001 by Golubev, Kurdyukov, Medvedev, Pevtsov, Sorokin, Hutchison. Recently, it has been proposed [1] to use synthetic opals, widely employed as starting materials for creat- ing photonic crystals [2, 3], as matrices for obtaining three-dimensional (3D) arrays of nanosize electronic devices. Recent results obtained for opal–Pt–Si nano- composites indicate that 3D-ordered Schottky barriers can be formed on the inner surface of opal voids [4, 5]. The density of active nanoelements in such structures may be as high as 10 14 cm –3 , which exceeds by approx- imately six orders of magnitude the values achieved by means of modern planar technology. In this study, 3D-ordered synthetic opals were impregnated with GaN. The structural properties of GaN synthesized within interconnected opal voids were investigated by transmission electron microscopy (TEM). A study of optical reflection spectra confirmed that the obtained opal–GaN composites retain 3D- ordering. Use of opal matrices can, first, ensure a large specific surface area (on the order of 10 m 2 /cm 3 ) of GaN-based light-emitting devices at a high density of elements. Second, the photonic-crystal properties of perfect opal–GaN composites, characterized by the presence of a photonic gap in the visible range, allow a substantial improvement and modification of the emit- ting properties of GaN-based nanodevices in this spec- tral region. Synthetic opals were composed of monodisperse amorphous SiO 2 spheres forming a regular fcc lattice. Opal samples with spheres 235 or 250 nm in diameter were used in this study. The opals also contained an ordered sublattice of octahedral and tetrahedral voids accessible to filling with substances, with the volume of the sublattice constituting 26% of the total sample vol- ume. Opal samples impregnated with precursor sub- stances (Ga, Ga 2 O 3 ) were annealed in an atmosphere of nitrogen hydrides at T = 1000–1100°C for 30–50 h [6, 7]. Electron-microscopic studies of the microstructure of opal–GaN nanocomposites were done using an FEG 3000 transmission electron microscope. Figure 1a shows a section of a composite, in which its ordered “crystal” structure, close-packed in the (111) plane, is well seen. Nearly all voids and void-con- necting channels (Figs. 1a–1d) are filled with a guest substance forming a cluster lattice. This conclusion is based on an analysis of the contrast between the SiO 2 sphere and the place that was an octahedral (tetrahe- dral) void prior to filling. The cluster is darker than the sphere. This means that the guest substance is enriched in an element heavier than Si. Figure 1b presents two regions [image plane coincides with the (100) plane] shifted by one half of the SiO 2 sphere diameter with respect to each other. As a result of such a shift, projec- tions of filled octahedral and tetrahedral voids can be seen in the image. The figure shows truncated spheres with section planes not passing through the sphere diameter. Electron microdiffraction patterns obtained from parts of clusters are point electron diffraction patterns characteristic of single-crystal objects. High-resolution images of these regions show a perfect single-crystal structure (Figs. 1e and 1f), with electron diffraction pat- terns obtained from neighboring clusters revealing one and the same crystallite orientation. This means that a single-crystal structure with one and the same azi- muthal orientation is preserved within several micrometers. The substance in the channels connecting the clusters is also in the single-crystal state. Thus, the single-crystal structure extends continuously through a channel from one cluster into another, with the same orientation preserved. An analysis of point electron dif- fraction patterns identified the substance in clusters as hexagonal GaN with lattice constants a = 0.3095 and b = 0.500 nm, which is in rather good agreement with Structural and Photonic Properties of Opal–GaN Nanocomposites V. G. Golubev 1 *, D. A. Kurdyukov 1 , A. V. Medvedev 1 , A. B. Pevtsov 1 , L. M. Sorokin 1 , and J. L. Hutchison 2 1 Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia * e-mail: golubev@gvg.ioffe.rssi.ru 2 Department of Materials, Oxford University, Oxford OXI 3PH, UK Submitted April 19, 2001; accepted for publication April 26, 2001 Abstract—Electron microscopic methods have been applied to demonstrate that gallium nitride synthesized in opal voids has perfect crystal structure. Studies of optical reflection spectra revealed that the obtained opal–〈gal- lium nitride〉 nanocomposites retain the photonic-crystal properties of the ordered host matrix at any (0-100%) degree of opal void filling with gallium nitride. © 2001 MAIK “Nauka/Interperiodica”. LOW-DIMENSIONAL SYSTEMS