Solid State Communications, Vol. 108, No. 7, pp. 413–417, 1998 Pergamon c  1998 Elsevier Science Ltd. All rights reserved 0038–1098/98 $ - see front matter PII: S0038–1098(98)00401-3 DENSITY DEPENDENCE OF THE STRUCTURAL AND ELECTRONIC PROPERTIES OF AMORPHOUS GaN M. Yu ∗ and D.A. Drabold Department of physics and Astronomy, Condensed Matter and Surface Sciences Program, Ohio University, Athens, OH 45701-2979, U.S.A. (Received 27 April 1998; accepted 21 August 1998 by F. Yndurain) We have performed quantum molecular-dynamics to simulate amor- phous GaN. The network models at different densities are generated in such a way that the charge transfer between ions is included self- consistently throughout the simulation. The pair distribution function indicates that the network models are topologically disordered, and the detailed structural analysis implies the existence of a certain chemical short-range order. An important property is that the network models (64 atoms in a supercell) have no wrong pairs (homopolar bonds) or odd-membered rings in all the densities studied, indicating the strong ionicity in amorphous GaN. The valence band tail states are mostly lo- calized on the threefold coordinated N sites and the conduction band tail states are mostly localized on the threefold coordinated Ga sites. There are no midgap states at any density, and the band gap is 2.6– 3.6 eV depending on the density. The possible existence of amorphous GaN is suggested in a small range of density near 90 % of experimen- tal value of wurtzite GaN. c  1998 Elsevier Science Ltd. All rights reserved Keywords: A. disordered systems, semiconductors, D. electron–electron interactions, electronic states (localized) On both experimental and theoretical grounds there is reason to believe that amorphous GaN (a-GaN) may have interest as a novel electronic material. Nonomura et al. [1,2] produced a-GaN films by reactive sputter- ing, and observed Tauc optical gap up to 3.95 eV as well as a low midgap defects density of states. They suggested that amorphous GaN is a kind of large opti- cal gap semiconducting material. Stumm and Drabold [3] performed an ab initio molecular dynamics study by constructing atomistic models at two different den- sities (one is at the wurtzite crystalline experimental density and the other is at 82% of the experimental density). They found a large state-free optical gap and weakly delocalized band tails in the less dense model, indicating the novel electronic properties in a-GaN. The results provide new insight into the character of bonding, defects, and their experimental signatures, ∗ E-mail; myu@helios.phy.ohiou.edu and also require us to answer the following questions: (1) at what density might a-GaN exist? and (2) do the novel electronic properties still exist in that den- sity? In the absence of the detailed information about the density and structure of a-GaN from experimen- tal measurement, a further study of molecular dynam- ics simulation is indispensable. In the present work, we focus on the density dependence of the structural and electronic properties and optimize the density to find the most probable condition for the existence of amorphous GaN. In constructing our network models at different den- sities, we utilized the program package of “Fireball96” proposed by Demkov, Ortega, Sankey, and Grum- bach [4]. The program generalizes the local basis den- sity functional method of Sankey and Niklewski [5] to an approximate self-consistent form for charge trans- fer. The exchange-correlation functional used is the Ceperley–Alder form [6, 7]. The basis functions are 413