Structure and properties of hydrogen-impurity pairs in elemental semiconductors

A variety of experiments have revealed several puzzling properties of hydrogen-impurity pairs. For example, H atoms passivate the electrical activity of some impurities, whereas they induce electrical activity in others; they appear to tunnel around some impurities but not around others. We report first-principles pseudopotential-density-functional calculations for several hydrogen-impurity complexes and unravel the origins and intricacies of the rich behavior of H bound to different substitutional impurities in Si and Ge.FWN – Publicaties zonder aanstelling Universiteit Leide

Microscopic structure of the hydrogen-boron complex in crystalline silicon

The microscopic structure of hydrogen-boron complexes in silicon, which result from the passivation of boron-doped silicon by hydrogen, has been extensively debated in the literature. Most of the debate has focussed on the equilibrium site for the H atom. Here we study the microscopic structure of the complexes using parameter-free total-energy calculations and an exploration of the entire energy surface for H in Si:B. We conclusively show that the global energy minimum occurs for H at a site close to the center of a Si-B bond (BM site), but that there is a barrier of only 0.2 eV for movement of the H atom between four equivalent BM sites. This low energy barrier implies that at room temperature H is able to move around the B atom. Other sites for H proposed by others as the equilibrium sites are shown to be saddle points considerably higher in energy. The vibrational frequency of the H stretching mode at the BM site is calculated and found to be in agreement with experiment. Calculations of the dissociation energy of the complex are discussed.FWN – Publicaties zonder aanstelling Universiteit Leide

Microscopic structure of the hydrogen-phosphorus complex in crystalline silicon

The existing discrepancy between theoretical models and experimental results for hydrogen-donor complexes in crystalline silicon is resolved using first-principles pseudopotential-density-functional calculations for the hydrogen-phosphorus pair. In the configuration which is the global energy minimum, H is located on the extension of a P-Si bond on the Si side, with the Si-H pair relaxing away from P by 0.6 Å, leaving the P atom threefold coordinated. The calculated stretching and wagging vibrational frequencies associated with this configuration are in accord with experiment.FWN – Publicaties zonder aanstelling Universiteit Leide

Theory of hydrogen diffusion and reactions in crystalline silicon

FWN – Publicaties zonder aanstelling Universiteit Leide

Ab initio study of hydrogenic effective mass impurities in Si nanowires

The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations. � 2017 IOP Publishing Ltd

Infrared activity of hydrogen molecules trapped in Si

The rovibrational-translational states of a hydrogen molecule moving in a cage site in Si, when subjected to an electrical field arising from its surroundings, are investigated. The wave functions are expressed in terms of basis functions consisting of the eigenfunctions of the molecule confined to move in the cavity and rovibrational states of the free molecule. The energy levels, intensities of infrared and Raman transitions, effects of uniaxial stress, and a neighboring oxygen defect are found and compared with existing experimental data

Interwell coupling effect in Si/SiGe quantum wells grown by ultra high vacuum chemical vapor deposition

Si/Si0.66Ge0.34coupled quantum well (CQW) structures with different barrier thickness of 40, 4 and 2 nm were grown on Si substrates using an ultra high vacuum chemical vapor deposition (UHV-CVD) system. The samples were characterized using high resolution x-ray diffraction (HRXRD), cross-sectional transmission electron microscopy (XTEM) and photoluminescence (PL) spectroscopy. Blue shift in PL peak energy due to interwell coupling was observed in the CQWs following increase in the Si barrier thickness. The Si/SiGe heterostructure growth process and theoretical band structure model was validated by comparing the energy of the no-phonon peak calculated by the 6 + 2-bandk·pmethod with experimental PL data. Close agreement between theoretical calculations and experimental data was obtained

Macroscopic polarization and band offsets at nitride heterojunctions

Ab initio electronic structure studies of prototypical polar interfaces of wurtzite III-V nitrides show that large uniform electric fields exist in epitaxial nitride overlayers, due to the discontinuity across the interface of the macroscopic polarization of the constituent materials. Polarization fields forbid a standard evaluation of band offsets and formation energies: using new techniques, we find a large forward-backward asymmetry of the offset (0.2 eV for AlN/GaN (0001), 0.85 eV for GaN/AlN (0001)), and tiny interface formation energies.Comment: RevTeX 4 pages, 2 figure

Stability of Ge-related point defects and complexes in Ge-doped SiO_2

We analyze Ge-related defects in Ge-doped SiO_2 using first-principles density functional techniques. Ge is incorporated at the level of ~ 1 mol % and above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency, with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and O vacancies binding strongly to Ge impurities. All the centers considered exhibit potentially EPR-active states, candidates for the identification of the Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure

Linear and Second-order Optical Response of the III-V Mono-layer Superlattices

We report the first fully self-consistent calculations of the nonlinear optical properties of superlattices. The materials investigated are mono-layer superlattices with GaP grown on the the top of InP, AlP and GaAs (110) substrates. We use the full-potential linearized augmented plane wave method within the generalized gradient approximation to obtain the frequency dependent dielectric tensor and the second-harmonic-generation susceptibility. The effect of lattice relaxations on the linear optical properties are studied. Our calculations show that the major anisotropy in the optical properties is the result of strain in GaP. This anisotropy is maximum for the superlattice with maximum lattice mismatch between the constituent materials. In order to differentiate the superlattice features from the bulk-like transitions an improvement over the existing effective medium model is proposed. The superlattice features are found to be more pronounced for the second-order than the linear optical response indicating the need for full supercell calculations in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.