Structural order and clustering in annealed a -SiC and a -SiC:H Cesar R. S. da Silva, 1 J. F. Justo, 2 and A. Fazzio 1 1 Instituto de Fı ´sica da Universidade de Sa ˜ o Paulo, CP 66318, CEP 05508-970, Sa ˜ o Paulo, SP, Brazil 2 Dep. Eng. Sistemas Eletroˆnicos, Escola Polite´cnica da Universidade de Sa ˜ o Paulo, CP 61548, CEP 05424-970, Sa ˜ o Paulo, SP, Brazil ~Received 14 August 2001; published 28 February 2002! We carried out a theoretical investigation of the structural properties of annealed a -SiC and a -SiC:H. The calculations were performed using the free volume Monte Carlo method combined with interatomic potentials. Our prototype nanoparticle contained as much as 85 000 atoms which allows for a superior statistical sampling of the material. The results show that C atoms segregate, forming small clusters embedded in an extensive Si network. Si atoms are mostly contained in a single extensive network and a few small clusters. Unprecedented detailed ring statistics analysis shows that C clusters do not form strictly C rings and that the introduction of H increases the occurrence of microvoids in the structure. Moreover, the incorporation of H slightly reduces the chemical order in the material and, to a larger extent, reduces the midrange structural order. Hydrogen also relaxes bonding stress around atoms in the network. The results are consistent with the available experimental data. DOI: 10.1103/PhysRevB.65.104108 PACS number~s!: 61.43.Bn, 61.43.Dq I. INTRODUCTION Due to its electronic properties, amorphous silicon carbide ( a -SiC) has potential applications to optoelectronic devices. 1 As a result, considerable effort has been devoted over the last decade in order to refine the experimental tech- niques and obtain SiC with the most desirable optical prop- erties. However, to achieve that, several fundamental ques- tions concerning the structural properties of SiC still need to be addressed. For example, experimental results provide con- troversial interpretations of the degree of chemical order 2,3 and of the role of hydrogen in the ordering of the alloy. Amorphous silicon carbide films are usually grown by plasma-enhanced chemical vapor deposition, from a mixture of silane ~SiH 4 ) and methane ~CH 4 ) or other hydrocarbons. Subsequent thermal annealing of these films leads to a large increase in their photoluminescence, suggesting that relevant structural changes occur under annealing. 4 Most of these changes are the result of processes at the atomistic level and, consequently, of chemical rebinding. It is therefore important to understand the microscopic properties of a -SiC in order to eventually improve its photoluminescence properties. In that context, atomistic simulations can be a useful tool in inves- tigating the structural properties of the material. Atomistic simulation of the growth of a massive sample is a formidable task in terms of computational resources. For- tunately, a substantial amount of information can still be ob- tained by studying the amorphous material generated by a fast quench from the melt. Using this approach, amorphiza- tion of semiconductors by laser ablation has already been investigated. 5–9 Here, we report atomistic simulations of the structural properties of unhydrogenated and hydrogenated amorphous silicon carbide. Our investigations were carried out by combining Monte Carlo ~MC! methods with inter- atomic potentials in a simulated annealing procedure. Our simulation system comprised as much as 85 000 atoms, which allowed for a proper statistical sampling of the prop- erties of SiC. By analyzing the ring and cluster statistics, to investigate the medium-range order, we observe that carbon atoms form small clusters, while silicon form large networks. We also find that hydrogen changes the chemical order and midrange structural order in the material. II. METHODOLOGY Atomistic simulations on the properties of amorphous sili- con carbide have been performed using several levels of ap- proximation, from ab initio 10 to empirical methods. 11,12 However, ab initio or even empirical methods still cannot afford simulation cells comprising more than a hundred or a few thousand atoms, respectively. Therefore, they do not provide a relevant statistical sampling of the material, and important questions could not be properly addressed. Re- cently, an efficient algorithm, based on a free volume Monte Carlo approach, has been developed 9 to study structural properties of a -Si. In order to describe the interatomic inter- actions, we used the Tersoff empirical potential. 13–15 This potential has already been shown to provide a good descrip- tion of several structural properties of silicon carbide. 12 Us- ing that algorithm, we generated amorphous silicon carbide from a molten material, which is slowly cooled down in a simulated annealing procedure. The prototype system is a cube with 32 000 Si and 32 000 C atoms in a zinc-blende structure organized as an array of (20320320) unit cells, oriented in ^ 100& directions. The simulation started with the system at a very high tempera- ture, above the melting point. Consequently, the system could lose any structural memory from the crystalline phase after annealing. In simulating hydrogenated SiC, we added 21 000 hydrogen atoms in random positions after complete melting of the system, followed by a subsequent annealing. We emphasize that during annealing a small amount of H was released on the surface of the system, but the total amount of hydrogen inside the system was kept at around 20% atomic concentration. Both systems ( a -SiC and a -SiC:H) were slowly quenched down to 10 K, when the statistical analysis were performed. Our simulations were carried out using open volume, so PHYSICAL REVIEW B, VOLUME 65, 104108 0163-1829/2002/65~10!/104108~5!/$20.00 ©2002 The American Physical Society 65 104108-1