Anion substitution with bismuth (Bi) in III-V semiconductors is an effective method for experimental engineering of the band gap Eg at low Bi concentrations (≤ 2%), in particular in gallium arsenide (GaAs). The inverse Bi-concentration dependence of Eg has been found to be linear at low concentrations x and dominated by a valence band-defect level anticrossing between As and Bi occupied p levels. Predictive models for the valence band hybridization require a first-principle understanding which can be obtained by density functional theory with the main challenges being the proper description of Eg and the spin-orbit coupling. By using an efficient method to include these effects, it is shown here that at high concentrations Eg is modified mainly by a Bi-Bi p orbital interaction and by the large Bi atom-induced strain. In particular, we find that at high concentrations the Bi-Bi interactions depend strongly on model periodic cluster configurations, which is not captured by tight-binding models. Averaging over various configurations supports the defect level broadening picture. This points to the role of different atomic configurations obtained by varying the experimental growth conditions in engineering arsenide band gaps, in particular for telecommunication laser technology.

While being of persistent interest for the integration of lattice-matched laser devices with silicon circuits, the electronic structure of dilute nitride III/V-semiconductors has presented a challenge to ab initio computational approaches. The root of this lies in the strong distortion N atoms exert on most host materials. Here, we resolve these issues by combining density functional theory calculations based on the meta-GGA functional presented by Tran and Blaha (TB09) with a supercell approach for the dilute nitride Ga(NAs). Exploring the requirements posed to supercells, we show that the distortion field of a single N atom must be allowed to decrease so far, that it does not overlap with its periodic images. This also prevents spurious electronic interactions between translational symmetric atoms, allowing to compute band gaps in very good agreement with experimentally derived reference values. These results open up the field of dilute nitride compound semiconductors to predictive ab initio calculations. 71.15.Mb, 71.20.Nr, 71.55.Eq 1 arXiv:1705.10763v1 [cond-mat.mtrl-sci]

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Combining ab initio density functional theory with the Dirac-Bloch and gap equations, excitonic properties of transition-metal dichalcogenide hetero-bilayers with type-II band alignment are computed. The existence of interlayer excitons is predicted, whose binding energies are as large as 350 meV, only roughly 100 meV less than those of the coexisting intralayer excitons. The oscillator strength of the interlayer excitons reaches a few percent of the intralayer exciton resonances and their radiative lifetime is two orders of magnitude larger than that of the intralayer excitons.

A procedure is presented that combines density functional theory computations of bulk semiconductor alloys with the semiconductor Bloch equations, in order to achieve an ab initio based prediction of the optical properties of semiconductor alloy heterostructures. The parameters of an eight-band k · p-Hamiltonian are fitted to the effective band structure of an appropriate alloy. The envelope function approach is applied to model the quantum well using the k · p-wave functions and eigenvalues as starting point for calculating the optical properties of the heterostructure. It is shown that Luttinger parameters derived from band structures computed with the TB09 density functional reproduce extrapolated values. The procedure is illustrated by computing the absorption spectra for a (AlGa)As/Ga(AsP)/(AlGa)As quantum well system with varying phosphide content in the active layer.An ab initio based approach to optical properties of semiconductor heterostructures 2 this purpose. Today, up to five different semiconductors are mixed resulting in quinary alloys, e.g. (GaIn)(NAsSb) [1,2,3,4,5] or (AlInGa)(AsSb) [6,7,8]. In addition, modern growth techniques allow the production of very pure compounds which is essential for optical properties. With this ever increasing number of available materials, it becomes increasingly important to predict the optical properties of new compounds to determine their usability for application in opto-electronic devices such as semiconductor lasers or solar cells.In this work, we present a method to calculate the optical properties of III-V heterostructures based on a first principles approach. For this purpose, the band structure of the semiconductor heterostructure is obtained within k · p-theory [9] and the envelope function approach [10] as described in section 2.2. However, this approach heavily depends on material parameters such as effective masses which ultimately need to be extracted from experiments. To cope with this issue, we use density functional theory (DFT) [11] to calculate the bulk band structures of each quantum well (QW) within the sample. To overcome the common shortcoming of DFT to underestimate the band gap of semiconductors [12], an advanced functional is used. A bulk k · p-band structure [9] is then fitted to its DFT counterpart. This approach results in a set of effective material parameters which are used to obtain the band structure of the QW sample within the envelope function approach.The resulting energy bands and wave functions from the k · p-calculation serve as starting point for the calculation of the optical properties. We calculate the absorption using the semiconductor Bloch approach [13] as outlined in section 2.4. The computation of other quantities such as the refractive index or photo luminescence is also possible within our microscopic approach. However, in this work we only aim at demonstrating the possibility of combining DFT and the semiconductor Bloch approach. Therefore, we chose a Ga(AsP) QW with (AlGa)As barriers, a well known III-V material s...