Why are arsenic clusters situated at dislocations in gallium arsenide?

Abstract

The interaction of arsenic with dislocations in gallium arsenide has been studied. By performing molecular dynamics simulations with a Tersoff-type interaction potential and simulating the microscopic diffusion-drift-aggregation process based on the kick-out mechanism, we obtained that arsenic clusters are preferably situated at dislocations from energetic and kinetic reasons. This formation of arsenic clusters explains why the radius of the cylinder with an increased free electron concentration around dislocations in n-type sulfur-doped gallium arsenide extends up to about 10 m, as revealed by Raman scattering.Metal-semiconductor composites, in which metal clusters are dispersed in a crystalline semiconductor, present a fruitful field of investigation for various electrical and optical properties, with some potential applications. Special attention has been paid to arsenic clusters in GaAs. Arsenic clusters are assumed to be semimetallic in nature and to form buried Schottky barriers. 1 Many investigations have shown that arsenic clusters could be formed after annealing above 550°C of low-temperature-grown GaAs containing 2 vol % excess arsenic in the form of As Ga ϩ antisites and I As interstitials. 2 Other works reported that arsenic clusters exist around dislocations, despite their absence in the matrix of semi-insulating or doped bulk GaAs. [3][4][5] The formation of arsenic clusters around dislocations has been related to the increase in the free-electron concentration ͑FEC͒ from the matrix to dislocations in GaAs:Si. 6 It is thus of fundamental interest to clarify why arsenic clusters prefer to stay at dislocations and not in the matrix. The computer simulations presented here will show that it is energetically and kinetically favorable for arsenic clusters to stay at dislocations. Two kinds of simulations have been carried out. The first approach is molecular dynamics ͑MD͒ simulations to explore the difference in the total energy of the As-GaAs system as a function of the distance of the arsenic cluster from the dislocation. In the second approach, the microscopic diffusion-drift-aggregation processes of point defects is simulated to elucidate ͑i͒ whether there are kinetic reasons to form arsenic clusters at dislocations and not in the matrix and ͑ii͒ why the FEC obtained in previous Raman scattering experiments increases from the matrix to the dislocation in a cylinder having a radius as large as 10 m in n-type GaAs, such as GaAs:S. The mechanism of the spatial redistribution of free carriers due to the existence of dislocations is still controversial ͑see Ref. 5 and references therein͒.In the present MD simulations, a Tersoff-type interaction potential has been applied to construct Ga-Ga, As-As, and Ga-As interactions. 7 A perfect GaAs matrix was built as a 7&a 0 ϫ8ͱ6a 0 ϫ6)a 0 block in ͓11 0͔, ͓112 ͔, and ͓111͔ directions ͑lattice parameter a 0 ϭ5.65 Å). It contains 16 128 atoms. Next, all the atoms were displaced according to the isotropic elastic theory to generate a 90°partial glide dislocation in t...

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