Nanocrystalline Gallium Nitride from Gallium Azide Precursors
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INTRODUCTION Only very few chemical routes to nanocrystalline group-13 nitrides have been described in the literature to date. Among the group-13 nitrides, the wide band gap (3.45 eV) direct semiconductor GaN has gained considerable interest for applications as blue light-emitting diodes and high-power devices, also because of its chemical inertness, radiation resistance and capability of working at elevated temperatures [1]. Synthetic approaches to GaN nanopowders include the following. The treatment of GaCl3 with Li3N in benzene in a pressure vessel at 280'C yielded a powder with an average particle size of 32nm. The solid state pyrolyses of {Ga(NH) 3/2) (450'C, vacuum) and of ([H2GaNH 2]3) (600'C, closed tube) into GaN gave disperse powders consisting of a mixture of cubic and hexagonal GaN with broad particle size distributions. Decomposition of Ga2[N(CH 3)2]6 yielded agglomerated GaN nanoparticles of 5 nm size. We recently demonstrated the preparation of 5nm GaN particles by careful pyrolysis of the polymeric precursor [Ga(N 3)3], [2a-e]. However, a simple and reliable route to highly crystalline, defined, and free standing GaN nanocrystals is still to be developed.
PRECURSOR CONCEPT AND SYNTHESIS The synthesis of high quality nanocrystals generally requires a fast nucleation, a rapid termination of the growth and effective thermal annealing of the particles. This is very difficult to achieve for GaN. Owing to the strongly ionic character of the GaN lattice, the surface mobilities of the reacting species are poor and the kinetics at the growth sites are slow. Favorable conditions would resemble those of an arc discharge, with local temperatures up to 70000 C and shock waves exerting GPa pressure acting on a microsecond time scale. 15 Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
An alternative and more straight-forward molecular way is to take advantage of the explosivity of suitable gallium azides. Detonations closely resemble the conditions of an arc discharge [3]. We have developed a safe and easy way to conduct controlled detonations of these N-rich precursors. By this method, nanocrystalline GaN can be obtained on a reliable and reasonable preparative scale. The ideal molecular precursors to obtain pure (i.e. C, H and heteroatom free) EN nanocrystals appear to be the triazido species E(N 3). The sensitivity towards heat or shock of these extremely dangerous compounds is substantially reduced in donor-stablized derivatives which can be handled without special precautions. The monomeric and soluble amine adducts are quantitatively obtained as white solids from Na[E(N 3)4] and the respective amine in toluene in solution (scheme 1). For a detailed discussion as well as solid state structures see [2e], [4]. Scheme 1: Precursor chemistry of EN nanoparticles ECI 3
+ 3 NaN 3
thf -
25 0 C
n
Na[E(N 3)4] (E = AI, Ga, In)
toluene, 25 'C -NaN 3
+
3 NaCI
Do (e.g. py, NR 3, NMe 2R)
DoxE(N 3)3 E =Ga, Do =NE, x I E = Al, Ga, In, Do = py, x = 3 The low-melting NEt 3Ga(N 3) 3 (1) (m.p. 4
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