Impact of germanium on vacancy clustering in germanium-doped silicon

Recent density functional theory calculations by Chen et al. [J. Appl. Phys. 103, 123519 (2008)] revealed that vacancies (V) tend to accumulate around germanium (Ge) atoms in Ge-doped silicon (Si) to form GeVn clusters. In the present study, we employ similar electronic structure calculations to predict the binding energies of GeVn and Vn clusters containing up to four V. It is verified that V are strongly attracted to pre-existing GeVn clusters. Nevertheless, by comparing with the stability of Vn clusters, we predict that the Ge contribution to the binding energy of the GeVn clusters is limited. We use mass action analysis to quantify the relative concentrations of GeVn and Vn clusters over a wide temperature range: Vn clusters dominate in Ge-doped Si under realistic conditions

Defect interactions in Sn_1-<i>x</i>Ge_<i>x</i> random alloys

Sn1-xGex alloys are candidates for buffer layers to match the lattices of III-V or II-VI compounds with Si or Ge for microelectronic or optoelectronic applications. In the present work electronic structure calculations are used to study relative energies of clusters formed between Sn atoms and lattice vacancies in Ge that relate to alloys of low Sn content. We also establish that the special quasirandom structure approach correctly describes the random alloy nature of Sn1-xGex with higher Sn content. In particular, the calculated deviations of the lattice parameters from Vegard's Law are consistent with experimental results

<i>E</i> centers in ternary Si_1-<i>x-y</i>Ge_<i>x</i>Sn_<i>y</i> random alloys

Density functional theory calculations are used to study the association of arsenic (As) atoms to lattice vacancies and the formation of As-vacancy pairs, known as E centers, in the random Si0.375Ge0.5Sn0.125 alloy. The local environments are described by 32-atom special quasirandom structures that represent random Si1-x-yGexSny alloys. It is predicted that the nearest-neighbor environment will exert a strong influence on the stability of E centers in ternary Si0.375Ge0.5Sn0.125

Fluorine codoping in germanium to suppress donor diffusion and deactivation

Electronic structure calculations are used to investigate the stability of fluorine-vacancy (Fn)Vm) clusters in germanium (Ge). Using mass action analysis, it is predicted that the FnVm clusters can remediate the concentration of free V considerably. Importantly, we find that F and P codoping leads to a reduction in the concentration of donor-vacancy (DV) pairs. These pairs are responsible for the atomic transport and the formation of DnV clusters that lead to a deactivation of donor atoms. The predictions are technologically significant as they point toward an approach by which V-mediated donor diffusion and the formation of inactive D(n)V clusters can be suppressed. This would result in shallow and fully electrically active n-type doped regions in Ge-based electronic devices

Engineering the free vacancy and active donor concentrations in phosphorus and arsenic double donor-doped germanium

In germanium, donor atoms migrate or form larger immobile clusters via their interaction with lattice vacancies. By engineering the concentration of free vacancies, it is possible to control the diffusion of the donor atoms and the formation of those larger clusters that lead to the deactivation of a significant proportion of the donor atoms. Electronic structure calculations in conjunction with mass action analysis are used to predict the concentrations of free vacancies and deactivated donor atoms in germanium doped with different proportions of arsenic and phosphorous. We find, for example, that at low temperatures, the concentration of free vacancies is partially suppressed by increasing the proportion of arsenic doping, whereas at high temperatures (above 1000 K), the concentration of free vacancies is relatively constant irrespective of the donor species. It is predicted that the free vacancy and active donor concentrations vary linearly with the arsenic to phosphorous ratio across a wide range of temperatures

Diffusion of E centers in germanium predicted using GGA+U approach

Density functional theory calculations (based on GGA+U approach) are used to investigate the formation and diffusion of donor-vacancy pairs (E centers) in germanium. We conclude that depending upon the Fermi energy, E centers that incorporate for phosphorous and arsenic can form in their neutral, singly negatively or doubly negatively charged states whereas with antimony only the neutral or doubly negatively charged states are predicted. The activation energies of diffusion are compared with recent experimental work and support the idea that smaller donor atoms exhibit higher diffusion activation energies

Vacancy-mediated dopant diffusion activation enthalpies for germanium

Electronic structure calculations are used to predict the activation enthalpies of diffusion for a range of impurity atoms (aluminium, gallium, indium, silicon, tin, phosphorus, arsenic, and antimony) in germanium. Consistent with experimental studies, all the impurity atoms considered diffuse via their interaction with vacancies. Overall, the calculated diffusion activation enthalpies are in good agreement with the experimental results, with the exception of indium, where the most recent experimental study suggests a significantly higher activation enthalpy. Here, we predict that indium diffuses with an activation enthalpy of 2.79 eV, essentially the same as the value determined by early radiotracer studies

Diffusion and defect reactions between donors, C, and vacancies in Ge. II. Atomistic calculations of related complexes

Electronic structure calculations are used to study the stability, concentration, and migration of vacancy-donor (phosphorus, arsenic, and antimony) complexes in germanium, in the presence of carbon. The association of carbon with mobile vacancy-donor pairs can lead to energetically favorable and relatively immobile complexes. It is predicted that the complexes formed between lattice vacancies, carbon, and antimony substitutional atoms are more stable and less mobile compared to complexes composed of vacancies, carbon, and phosphorus or arsenic atoms. Then, with the use of mass action analysis, the relative concentrations of the most important complexes are calculated, which depend also on their relative stability not just their absolute stability. Overall, the theoretical predictions are consistent with experimental results, which determined that the diffusion of vacancy-donor defects is retarded in the presence of carbon, especially in samples with a high concentration of carbon. In addition, the calculations provide information on the structure and the equilibrium concentration of the most important complexes and details of their association energies

Native defects and self-diffusion in GaSb

The native defects in GaSb have been studied with first-principles total-energy calculations. We report the structures and the formation energies of the stable defects and estimate the defect concentrations under different growth conditions. The most important native defect is the GaSb antisite, which acts as an acceptor. The other important defects are the acceptor-type Ga vacancy and the donor-type Ga interstitial. The Sb vacancies and interstitials are found to have much higher formation energies. A metastable state is observed for the SbGa antisite. The significantly larger concentrations of the Ga vacancies and interstitials compared to the corresponding Sb defects is in accordance with the asymmetric self-diffusion behavior in GaSb. The data supports the next-nearest-neighbor model for the self-diffusion, in which the migration occurs independently in the different sublattices. Self-diffusion is dominated by moving Ga atoms.Peer reviewe

Curie temperature and carrier concentration gradients in MBE grown GaMnAs layers

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic GaMnAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the GaMnAs/GaAs interface. While high resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the GaMnAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance-voltage profiling. The gradient in Curie temperature seems to be a general feature of GaMnAs layers grown at low-temperature. Possible explanations are discussed.Comment: 3 pages, 4 figures, submitted to AP