Novel Diffusion Mechanism on the GaAs(001) Surface: The Role of Adatom-Dimer Interaction

Abstract

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 ...

Ab initio pseudopotentials for electronic structure calculations of poly-atomic systems using density-functional theory

“…Questions addressed with pseudopotentials provided by this code, or its earlier version, range from phase transitions [10,11], defects in semiconductors [12][13][14], the structure of and diffusion on surfaces of semiconductors [15][16][17], simple metals [18], and transition metals [19][20][21], up to surface reactions [22,23], including molecules [24,25] of first-row species.…”

“…Part 1 (program psgen) serves to generate the pseudopotentials using the schemes by Hamann [33] or by Troullier and Martins [34]. This combination provides efficient pseudopotentials for "canonical" applications like group IV and III-V semiconductors [9,10,[15][16][17]22,23], as well as for systems where first-row, transition or noble metal elements are present [11,[19][20][21]24,25], or where "semicore" d-states must be treated as valence states, like in GaN [41] or InN [14]. In such cases the strongly localized 2p and 3, 4, 5d valence states are readily handled with the Troullier-Martins scheme.…”

First-principles studies of kinetics in epitaxial growth of III-V semiconductors

“…In the context of DFT calculations, the potential energy surface (PES) of an adatom is the adequate concept to address the issue of diffusion. DFT calculations of potential energy surfaces for various GaAs(001) surfaces are available from the literature, both for Ga diffusion [39][40][41] as well as for As diffusion [42]. Most importantly, it was found (see Ref.…”

“…Most importantly, it was found (see Ref. [40]) that Ga has its strongest adsorption site inside As surface dimers, thereby breaking the dimer bond. The picture evolving from these computational results suggests that a possible role of the Ga atoms in growth could be the breaking-up of existing As surface dimers, and the formation of locally Ga-rich environments that act as selective sites for adsorption of As 2 molecules from the gas phase.…”

Atomic processes in molecular beam epitaxy on strained InAs(137): A density-functional theory study

“…There, DFT calculations have shown that Ga adatoms have the ability to break the As-As bond of As surface dimers. 18 Later it was demonstrated that this splitting of surface As dimers by Ga adatoms is one of the key processes in homoepitaxy. 19 Therefore we studied the analogous process of an In adatom on InAs͑137͒A splitting the As surface dimer.…”