M. Haugk

Effect of oxygen on the growth of (101̄0) GaN surfaces: The formation of nanopipes

Local density–functional methods are used to examine the behavior of O and O-related defect complexes on the walls of nanopipes in GaN. We find that O has a tendency to segregate to the (1010) surface and identify the gallium vacancy surrounded by three oxygen impurities [VGa–(ON)3] to be a particularly stable and electrically inert complex. We suggest that during Stranski–Krastanow growth, when interisland spaces shrink, these defects reach a critical concentration beyond which further growth is prevented and nanopipes are formed.

Theory of Ga, N and H terminated GaN (0 0 0 1)(0 0 0 1) surfaces

Abstract We present a theoretical study of atomic structures, electrical properties and formation energies for a variety of possible reconstructions with 1 × 1 and 2 × 2 periodicity of the GaN (0 0 0 1) and (0 0 0 1 ) surfaces. We find that during MBE growth in the (0 0 0 1) direction 2 × 2 structures become stable under N rich growth conditions while Ga rich environment should yield structures with 1 × 1 periodicity. Considering MBE growth on (0 0 0 1 ) surfaces reconstructions with 1 × 1 periodicity have low energies. During MOCVD growth where H terminated surfaces may occur 1 × 1 periodicities are found to be stable for both growth directions.

Structures, Energetics and Electronic Properties of Complex III‐V Semiconductor Systems

A parallel implementation of the selfconsistent-charge density-functional based tight-binding (SCC-DFTB) method is used to examine large scale structures in III-V semiconductors. We firstly describe the parallel implementation of the method and its efficiency. We then turn to applications of the parallel code to complex GaAs systems. The geometries and energetics of different models for the √19 × √19 reconstruction at the (111) surface are investigated. A structure containing hexagonal rings of As at the surface consistent with STM experiments is found to be stable under Ga-rich growth conditions. We then examine voids in the bulk material which are mainly caused by the movement of dislocations. Void clusters of 12 missing atoms are found to be energetically favorable. This is in very good agreement with recent positron annihilation measurements. Additionally, we investigate the diffusion of C in p-type material and suggest a diffusion path with an activation energy of less than 1 eV which is consistent with experimental studies. Finally, focusing on GaN we provide atomistic insight into line defects in wurtzite GaN threading along the growing c-axis. We highlight the stability and electronic properties of screw and edge dislocations, discuss reasons for the formation of nanopipes and relate the yellow luminescence observed in highly defected materials to deep acceptors. V Ga and V Ga -(O N ) n , trapped at threading edge dislocations.

Interaction of Oxygen with Threading Dislocations in GaN

A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {1010} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.

A density-functional based tight-binding approach to GaAs surface reconstructions

A density-functional-based non-orthogonal tight-binding (DF-TB) scheme is used to investigate models for the reconstructions of GaAs(110), (100) and (111) surfaces. The relative stabilities of the competing reconstructions are then determined as a function of the chemical potential, thus simulating a wide range of possible MBE growth conditions. We find a good agreement with recent experiments and ab initio calculations, and establish the validity of the scheme for large-scale applications.

Vacancy clusters in plastically deformed semiconductors

Experimental investigations of plastically deformed elemental and III-V semiconductors prove that a high number of vacancies and vacancy clusters are formed. The formation of point defects by the motion of jogged dislocations is analysed. Vacancies formed behind the jog are not stable as a simple chain of vacancies. Instead, they are transformed immediately to stable three-dimensional agglomerates. The stability of various vacancy clusters has been investigated by means of density-functional calculations. The positron lifetime in such clusters was calculated and compared to experimental results. The association of open-volume defects with the dislocation can be derived from positron lifetime measurements. The analysis in a positron-trapping model characterizes the dislocation as a combined defect. The undisturbed dislocation line is a precursor trap for the positron capture in a deep trap related to the vacancies bound to the dislocation.

Magic number vacancy aggregates in Si and GaAs – structure and positron lifetime studies

Abstract We investigate structural properties of large vacancy agglomerates in Si and GaAs using a self-consistent-charge density-functional based tight-binding method. We also calculated the defect-related positron lifetimes. Strong evidence is found for the existence of vacancy aggregates with unusual magic numbers in GaAs. In contrast to Si – the first stable agglomerate consists 12 vacancies instead of 6. This findings fit into experimental observations on deformed and irradiated samples.

The formation of nanopipes caused by donor impurities in GaN: A theoretical study for the case of oxygen

Local density-functional methods are used to examine the behaviour of O and O-related defect complexes at {1010}-type surfaces in GaN. We find that O has a tendency to segregate to the (1010) surface and we identify the gallium vacancy surrounded by three oxygen impurities (VGa-(ON)3) to be a particularly stable and electrically inert complex. We suggest that these complexes impede growth at the walls of the nanopipes preventing them from growing in. Also, other donor-related defect complexes, in particular gallium vacancies surrounded by three silicon atoms as second nearest neighbours, are expected to have the same effect.

Structural models for the reconstruction of the surface and their relative stabilities

We present a theoretical study of the relative formation energies for possible models of the reconstruction found under Ga-rich growth conditions at the surface. The energetically most favourable model has fourfold-coordinated Ga atoms on the surface, exhibiting metallic bonding character. This structure differs as regards the electron-counting rule (ECR) from the well accepted models for the (100), (110) and (111) surfaces of GaAs. Our results suggest that it is still possible to explain the stability of the metallic surface with a rule similar to the ECR.

A parallel code for a self-consistent charge density functional based tight binding method: Total energy calculations for extended systems

Abstract We present a parallel version of a selfconsistent-charge density-functional based tight-binding (SCC-DFTB) method for total energy calculations and geometry optimizations of clusters and periodic structures. On single processor machines the SCC-DFTB method has been successfully applied to systems up to several hundred atoms with an accuracy comparable to sophisticated selfconsistent field density-functional theory (SCF-DFT) methods. The new parallel code allows to treat systems which are larger by an order of magnitude in reasonable time. The freely available ScaLAPACK and PBLAS libraries are used for linear algebra operations. We tested the scaling of our program for a realistic system (III–V semiconductor surface) with different sizes and give a short outlook on current applications.