Surface energies for different GaAs surface orientations have been calculated as a function of the chemical potential. We use an energy density formalism within the first-principles pseudopotential density-functional approach. The equilibrium crystal shape has been derived from the surface energies for the ͑110͒, ͑100͒, ͑111͒, and ͑1 1 1͒ orientations. Under As-rich conditions all four considered surface orientations exist in thermodynamic equilibrium, in agreement with experimental observations. Moreover, our calculations allow us to decide on previous contradictory theoretical values for the surface energies of the ͑111͒ and ͑1 1 1͒ facets.

The package fhi96md is an efficient code to perform density-functional theory totalenergy calculations for materials ranging from insulators to transition metals. The package employs first-principles pseudopotentials, and a plane-wave basis-set. For exchange and correlation both the local density and generalized gradient approximations are implemented. The code has a low storage demand and performs efficiently on low budget personal computers as well as high performance computers. Method of solutionAb initio molecular dynamics on the Born-Oppenheimer surface is implemented by a two step method. In the first step the Kohn-Sham equation [26] is solved self-consistently to obtain the electron ground state and the forces on the nuclei. In a second step these forces are used to integrate the equations of motion for the next time step. The calculation of the total-energy and the Kohn-Sham operator in a plane-wave basis-set is done by the momentum-space method [27].To solve the Kohn-Sham equation the package fhi96md employs the iterative schemes of Williams and Soler [28] and Payne et al. [29]. We use the frozen-core approximation and replace the effect of the core electrons by normconserving pseudopotentials [30][31][32][33] in the fully separable form [34]. The equations of motion of the nuclei are integrated using standard schemes in molecular dynamics such as the Verlet algorithm. Optionally an efficient structure optimization can be performed by a second order algorithm with a damping term. Restrictions on the complexity of the problemOnly pseudopotentials with s-, p-, and d-components are implemented. For highly correlated systems (e.g. f -electrons) the treatment of the exchange-correlation interaction is not appro-2 priate. Relativistic effects are included via scalar relativistic pseudopotentials. The system is assumed to be non-magnetic, but a generalization of the program to magnetic states is straight forward.3 LONG WRITE-UP

Employing first principles total energy calculations we have studied the behavior of Ga and Al adatoms on the GaAs(001)-b2 surface. Beside the adsorption site we identify two diffusion channels that are characterized by different adatom-surface dimer interaction. Both affect the adatom migration: in one channel the adatom jumps across the surface dimers and leaves the dimer bonds intact; in the other one the dimer bonds are broken. The two channels are taken into account to derive effective adatom diffusion barriers. We find a strong diffusion anisotropy for both Al and Ga adatoms and, in agreement with experiments, higher diffusion barriers for Al than for Ga. [S0031-9007 (97)04835-7] PACS numbers: 68.35.Fx, 68.35.Ja Growth techniques, such as molecular beam epitaxy, operate under conditions far away from thermodynamic equilibrium. Particularly for growth at low temperatures or for structures with length scales smaller than the adatom diffusion length, features driven by the growth kinetics have been observed [1]. In the GaAs͞AlAs heteroepitaxy the differences between the growth kinetics of AlAs and GaAs have been utilized to create low dimensional structures [2]. Recently Kapon et al. [3]successfully fabricated a quantum wire heterostructure in which stimulated emission has been observed.Despite these successes the underlying microscopic processes such as adsorption, surface diffusion, desorption, and nucleation are poorly understood. A key mechanism in growth is cation surface diffusion, which is considered to be a rate limiting process [4,5]. Experimentally the surface diffusion is difficult to access. The deduced migration barriers [6-10] for Ga adatoms span a range between 1.1 and 4.0 eV. Even the anisotropy of surface diffusion on the (001) surface is controversially debated: Shitara et al. [11] speculate that the fastest diffusion is along the ͓110͔ direction; in contrast, Kawabe and Sugaya [4] propose the ͓110͔ direction. We have therefore performed first-principles total-energy calculations which constitute a powerful tool to study surface diffusion.Adatoms on solid surfaces occupy well-defined binding sites. The migration of the chemisorbed adatoms can be described as a hopping between these sites. The activation energies for the individual hops are determined by the energy differences between the binding and the transition sites. These positions can be identified as the minima and saddle points of the potential energy surface over the configurational space spanned by the coordinates of the adatom and the substrate atoms. In order to find all minima and transition sites a mapping of the entire configurational space is in principle required. However, this is computationally neither possible nor useful. For the study of surface diffusion the mapping is commonly restricted to a subspace given by the lateral coordinates of the adsorbate [12][13][14][15][16]. This mapping gives the potential energy surface (PES) E͑x, y͒ for a given lateral position ͑x, y͒ of the adatom where all substrate atoms and the z ...

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