Pseudopotential-based full zone k.p technique for indirect bandgap semiconductors: Si, Ge, diamond and SiC

The $k \cdot p$ is a versatile technique that describes the semiconductor band structure in the vicinity of the bandgap. The technique can be extended to full Brillouin zone by including more coupled bands into consideration. For completeness, a detailed formulation is provided where the associated $k \cdot p$ parameters are extracted from the local empirical pseudopotential method in the form of band edge energies and generalized momentum matrix elements. We demonstrate the systematic improvement of the technique with the proper choice of the band edge states for the group-IV indirect bandgap semiconductors: Si, Ge, diamond and SiC of the 3C cubic phase. The full zone agreement is observed to span an energy window of more than 20 eV for Si, and 40 eV for the diamond with the 15-band pseudopotential-based $k \cdot p$ approach.Comment: 8 pages, 6 figures, requires fizik.cls (included

High-dielectric constant and wide band gap inverse silver oxide phases of the ordered ternary alloys of SiO2_{2}, GeO2_{2} and SnO2_{2}

High-dielectric constant and wide band gap oxides have important technological applications. The crystalline oxide polymorphs having lattice constant compatibility to silicon are particularly desirable. One recently reported candidate is the inverse silver oxide phase of SiO$_2$. First-principles study of this system together with its isovalent equivalents GeO$_{2}$, SnO$_{2}$ as well as their ternary alloys are performed. Within the framework of density functional theory both generalized gradient approximation and local density approximation (LDA) are employed to obtain their structural properties, elastic constants and the electronic band structures. To check the stability of these materials, phonon dispersion curves are computed which indicate that GeO$_{2}$ and SnO$_{2}$ have negative phonon branches whereas their ternary alloys Si$_{0.5}$Ge$_{0.5}$O$_{2}$, Si$_{0.5}$Sn$_{0.5}$O$_{2}$, and Ge$_{0.5}$Sn$_{0.5}$O$_{2}$ are all stable within LDA possessing dielectric constants ranging between 10 to 20. Furthermore, the lattice constant of Si$_{0.5}$Ge$_{0.5}$O$_{2}$ is virtually identical to the Si(100) surface. The $GW$ band gaps of the stable materials are computed which restore the wide band gap values in addition to their high dielectric constants.Comment: Published version; two figures merged into on

Interband, intraband and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice

Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studied theoretically using an atomistic pseudopotential approach. From small clusters to large NCs containing on the order of several thousand atoms are considered. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. It is observed that the highest occupied molecular orbital for both Si and Ge NCs and the lowest unoccupied molecular orbital for Si NCs display oscillations with respect to size among the different irreducible representations of the $C_{3v}$ point group to which these spherical NCs belong. Based on this electronic structure, first the interband absorption is thoroughly studied which shows the importance of surface polarization effects that significantly reduce the absorption when included. This reduction is found to increase with decreasing NC size or with increasing permittivity mismatch between the NC core and the host matrix. Reasonable agreement is observed with the experimental absorption spectra where available. The deformation of spherical NCs into prolate or oblate ellipsoids are seen to introduce no pronounced effects for the absorption spectra. Next, intraconduction and intravalence band absorption coefficients are obtained in the wavelength range from far-infrared to visible region. These results can be valuable for the infrared photodetection prospects of these NC arrays. Finally, excited-state absorption at three different optical pump wavelengths, 532 nm, 355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong absorption windows in the case of holes and a broad spectrum in the case of electrons which can especially be relevant for the discussions on achieving gain in these structures.Comment: Published version, 13 pages, 15 figures, local field effects include

Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals

For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger recombination (AR) and carrier multiplication (CM) lifetimes are computed exactly in a three-dimensional real space grid using empirical pseudopotential wave functions. Our results in support of recent experimental data offer new predictions. We extract simple Auger constants valid for NCs. We show that both Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion's share from the total excess energy and can cause a CM. We predict that CM becomes especially efficient for hot electrons with an excess energy of about 1 eV above the CM threshold.Comment: 4 pages, 6 figures (Published version

Nuclear spin squeezing via electric quadrupole interaction

Control over nuclear spin fluctuations is essential for processes that rely on preserving the quantum state of an embedded system. For this purpose, squeezing is a viable alternative, so far that has not been properly exploited for the nuclear spins. Of particular relevance in solids is the electric quadrupole interaction (QI), which operates on nuclei having spin higher than 1/2. In its general form, QI involves an electric field gradient (EFG) biaxiality term. Here, we show that as this EFG biaxiality increases, it enables continuous tuning of single-particle squeezing from the one-axis twisting to the two-axis countertwisting limits. A detailed analysis of QI squeezing is provided, exhibiting the intricate consequences of EFG biaxiality. The initial states over the Bloch sphere are mapped out to identify those favorable for fast initial squeezing, or for prolonged squeezings. Furthermore, the evolution of squeezing in the presence of a phase-damping channel and an external magnetic field are investigated. We observe that dephasing drives toward an anti-squeezed terminal state, the degree of which increases with the spin angular momentum. Finally, QI squeezing in the limiting case of a two-dimensional EFG with a perpendicular magnetic field is discussed, which is of importance for two-dimensional materials, and the associated beat patterns in squeezing are revealed.Comment: Published version in contents, 10 pages, 9 figure

Disorder-free localization around the conduction band edge of crossing and kinked silicon nanowires

We explore ballistic regime quantum transport characteristics of oxide-embedded crossing and kinked silicon nanowires (NWs) within a large-scale empirical pseudopotential electronic structure framework, coupled to the Kubo-Greenwood transport analysis. A real-space wave function study is undertaken and the outcomes are interpreted together with the findings of ballistic transport calculations. This reveals that ballistic transport edge lies tens to hundreds of millielectron volts above the lowest unoccupied molecular orbital, with a substantial number of localized states appearing in between, as well as above the former. We show that these localized states are not due to the oxide interface, but rather core silicon-derived. They manifest the wave nature of electrons brought to foreground by the reflections originating from NW junctions and bends. Hence, we show that the crossings and kinks of even ultraclean Si NWs possess a conduction band tail without a recourse to atomistic disorder.Comment: Published version, 7 pages, 9 figure