Quantum Confined Gallium Nitride in a Mesoporous Matrix of MCM-41
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ABSTRACT We report the synthesis of a nanocomposite materials consisting of gallium nitride (GaN) confined in the well ordered mesoporous transparent matrix MCM-41 with the help of the metalorganic single source precursor triazido(trimethylamine)gallium using solution impregnation technique. The loading of the pores of MCM-41 with GaN was confirmed by chemical analysis, XRD, TEM, BET, NMR and optical spectroscopy (PL and PL-excitation spectroscopy). The composites exhibit a blue shift of the bandgap of GaN due to quantum confinement.
INTRODUCTION In the beginning of the nineties, gallium nitride has been changed from a laboratory obscurity into a rapidly increasing field of research. This has been due to some interesting properties: For example the direct bandgap of about 3.4 eV which makes this semiconducting III-V compound suitable for optoelectronic devices operating in the blue region of light with high quantum efficiencies. Blue light emitting diodes and laser diodes based on thin films of GaN are commercial available products. The technology for producing these devices is well established using chemical vapour deposition methods [1]. In the case of nanoparticles of GaN, no such well established chemical practicability is available. Reliable chemical approaches for nanoparticles of II-VI semiconductors cannot be transferred to GaN because of different properties. Most of the chemical pathways leading to nano confined GaN are transformations of solid precursors, like the thermal decomposition of polymeric gallium imide, gallium amide or the detonation of derivates of triazido gallium [2 a, b]. With all these conversions, one is fighting against problems like particle-agglomeration leading to a broad size distribution, and it is difficult to get access to particle size control. In addition, a uniform arrangement of the particles is almost impossible. To overcome these problems, we decided to grow GaN nanoparticles in a well-ordered porous matrix. We chose the silica mesoporous molecular sieve MCM-41 which can be characterisized by its porous system consisting of hexagonally arranged hollow cylinders [3]. The molecular sieve will act as a template according to the GaN particles which will be grown in the pores: the pore diameter of MCM-41 will limit the particle size leading to a small size distribution. The hexagonal pore arrangement will be transferred to the arrangement of the GaN particles. The application of triazido(trimethylamine)gallium 1 as precursor, whose successful transformation into GaN was recently described by our group, combines several advantages [2 417 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
b]: the donator stabilization with trimethylamine leads to a nonexplosive decomposition of triazido gallium into GaN and volatile compounds under low heating rates in ammonia atmosphere in a temperature region below 500 0 C where the host matrix MCM-41 is still intact. The precursor 1 is transferred into gallium amide 2 and polymeric gallium imide 3 (fig. 1). The conversion of
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