We develop an atomistic, nearest-neighbor sp 3 s * tight-binding Hamiltonian to investigate the electronic structure of dilute bismide alloys of GaP and GaAs. Using this model, we calculate that the incorporation of dilute concentrations of Bi in GaP introduces Bi-related defect states in the band gap, which interact with the host matrix valence band edge via a Bi composition dependent band anticrossing (BAC) interaction. By extending this analysis to GaBi x As 1−x , we demonstrate that the observed strong variation of the band gap (E g ) and spin-orbit-splitting energy ( SO ) with Bi composition can be well explained in terms of a BAC interaction between the extended states of the GaAs valence band edge and highly localized Bi-related defect states lying in the valence band, with the change in E g also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. Our calculated values of E g and SO are in good agreement with experiment throughout the investigated composition range (x 13%). In particular, our calculations reproduce the experimentally observed crossover to an E g < SO regime at approximately 10.5% Bi composition in bulk GaBi x As 1−x . Recent x-ray spectroscopy measurements have indicated the presence of Bi pairs and clusters even for Bi compositions as low as 2%. We include a systematic study of different Bi nearest-neighbor environments in the alloy to achieve a quantitative understanding of the effect of Bi pairing and clustering on the GaBi x As 1−x electronic structure.

Using an sp 3 s * tight-binding model we demonstrate how the observed strong bowing of the band gap and spin-orbit-splitting with increasing Bi composition in the dilute bismide alloy GaBixAs1−x can be described in terms of a band-anticrossing interaction between the extended states of the GaAs valence band edge and highly localised Bi-related resonant states lying below the GaAs valence band edge. We derive a 12-band k · p Hamiltonian to describe the band structure of GaBixAs1−x and show that this model is in excellent agreement with full tight-binding calculations of the band structure in the vicinity of the band edges, as well as with experimental measurements of the band gap and spin-orbit-splitting across a large composition range. Based on a tight-binding model of GaBixNyAs1−x−y we show that to a good approximation N and Bi act independently of one another in disordered GaBixNyAs1−x−y alloys, indicating that a simple description of the band structure is possible. We present a 14-band k · p Hamiltonian for ordered GaBixNyAs1−x−y crystals which reproduces accurately the essential features of full tight-binding calculations of the band structure in the vicinity of the band edges. The k · p models we present here are therefore ideally suited to the simulation of the optoelectronic properties of these novel III-V semiconductor alloys.

The incorporation of bismuth (Bi) in GaAs results in a large reduction of the band gap energy (E g ) accompanied with a large increase in the spin-orbit splitting energy ( SO ), leading to the condition that SO > E g , which is anticipated to reduce hot-hole producing Auger recombination losses whereby the energy and momentum of a recombining electron-hole pair are given to a second hole which is excited into the spin-orbit band. We theoretically investigate the electronic structure of experimentally grown GaBi x As 1−x samples on (100) GaAs substrates by directly comparing our data with room temperature photomodulated reflectance (PR) measurements. Our atomistic theoretical calculations, in agreement with the PR measurements, confirm that E g is equal to SO for x ≈ 9%. We then theoretically probe the inhomogeneous broadening of the interband transition energies as a function of the alloy disorder. The broadening associated with spin-split-off transitions arises from conventional alloy effects, while the behavior of the heavy-hole transitions can be well described using a valence bandanticrossing model. We show that for the samples containing 8.5% and 10.4% Bi the difficulty in identifying a clear light-hole-related transition energy from the measured PR data is due to the significant broadening of the host matrix light-hole states as a result of the presence of a large number of Bi resonant states in the same energy range and disorder in the alloy. We further provide quantitative estimates of the impact of supercell size and the assumed random distribution of Bi atoms on the interband transition energies in GaBi x As 1−x . Our calculations support a type-I band alignment at the GaBi x As 1−x /GaAs interface, consistent with recent experimental findings.

Highly mismatched semiconductor alloys such as GaN x As 1−x and GaBi x As 1−x have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spinorbit-splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecomm lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

Using photovoltage (PV) spectroscopy we analyse the electronic structure of a series of GaBi x As x 1− /(Al)GaAs dilute bismide quantum well (QW) laser structures. The use of polarisation-resolved PV measurements allows us to separately identify transitions involving bound light-and heavy-hole states in the QWs, as well as bound-to-continuum transitions from the QWs to the barriers. Analysis of these transitions enables us to probe the GaBi x As x 1− /(Al) GaAs conduction and valence band offsets, thereby quantifying the band offsets. Using a 12band k p• Hamiltonian, we extract the band offsets in the QWs explicitly by constraining the Birelated parameters of the model against the experimentally measured transition energies. The PV measurements and k p• calculations we present provide the first explicit confirmation of a type-I band offset at the GaBi x As x 1− /GaAs heterointerface near x = 2%. This result, combined with the theory we present for calculating the band offsets at GaBi x As x 1− /(Al)GaAs heterointerfaces, can be used to determine the band offsets at arbitrary Bi composition x.

We present and compare three distinct atomistic models -based on first principles and semiempirical approaches -of the structural and electronic properties of Ge1−xSnx alloys. Density functional theory calculations incorporating Heyd-Scuseria-Ernzerhof (HSE) and modified Becke-Johnson (mBJ) exchange-correlation functionals are used to perform structural relaxation and electronic structure calculations for a series of Ge1−xSnx alloy supercells. Based on HSE calculations, a semi-empirical valence force field (VFF) potential and sp 3 s * tight-binding (TB) Hamiltonian are parametrised. Comparing the HSE, mBJ and TB models, and using the HSE results as a benchmark, we demonstrate that: (i) mBJ calculations provide an accurate first principles description of the electronic structure at reduced computational cost, (ii) the VFF potential is sufficiently accurate to circumvent the requirement to perform first principles structural relaxation, and (iii) TB calculations provide a good quantitative description of the alloy electronic structure in the vicinity of the band edges. Our results also emphasise the importance of Sn-induced band mixing in determining the nature of the conduction band structure of Ge1−xSnx alloys. The theoretical models and benchmark calculations we present inform and enable predictive, computationally efficient and scalable atomistic calculations for disordered alloys and nanostructures. This provides a suitable platform to underpin further theoretical investigations of the properties of this emerging semiconductor alloy.

Si)GeSn semiconductors are finally coming of age after a long gestation period. The demonstration of devicequality epi-layers and quantum-engineered heterostructures has meant that tunable all-group IV Si-integrated infrared photonics is now a real possibility. Notwithstanding the recent exciting developments in (Si)GeSn materials and devices, this family of semiconductors is still facing serious limitations that need to be addressed to enable reliable and scalable applications. The main outstanding challenges include the difficulty to grow highcrystalline quality layers and heterostructures at the desired Sn content and lattice strain, preserve the material integrity during growth and throughout device processing steps, and control doping and defect density. Other challenges are related to the lack of optimized device designs and predictive theoretical models to evaluate and simulate the fundamental properties and performance of (Si)GeSn layers and heterostructures. This Perspective highlights key strategies to circumvent these hurdles and bring this material system to maturity to create farreaching new opportunities for Si-compatible infrared photodetectors, sensors, and emitters for applications in free-space communication, infrared harvesting, biological and chemical sensing, and thermal imaging.

We present a theoretical study of the gain characteristics of GaBixAs1−x/(Al)GaAs dilute bismide quantum well (QW) lasers. After providing a brief overview of the current state of development of dilute bismide alloys for semiconductor laser applications, we introduce the theoretical model we have developed for the description of the electronic and optical properties of dilute bismide QWs. Using a theoretical approach based on a 12-band k·p Hamiltonian we then undertake a detailed analysis of the electronic and optical properties of a series of ideal and real GaBixAs1−x/(Al)GaAs QW laser structures as a function of Bi composition x. We theoretically optimize the gain characteristics of an existing low x device by varying the Al composition in the barrier layers, which governs a trade-off between the electronic and optical confinement. The theoretical results are compared to temperature-dependent spontaneous emission measurements at low x, which reveals the presence of significant Bi-induced inhomogeneous broadening of the optical spectra. We also investigate the gain characteristics of GaBixAs1−x/(Al)GaAs QW lasers at higher values of x, including a QW designed to emit at 1.55 µm. Our theoretical results elucidate the impact of Bi incorporation on the electronic and optical properties of GaAsbased QW lasers, and reveal several general trends in the gain characteristics as a function of x. Overall, our analysis confirms that dilute bismide alloys are a promising candidate material system for the development of highly efficient, uncooled GaAsbased QW lasers operating at telecommunication wavelengths.