Christopher A. Broderick

Tight-binding analysis of the electronic structure of dilute bismide alloys of GaP and GaAs

We develop an atomistic, nearest-neighbor sp3s* 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 anti-crossing (BAC) interaction. By extending this analysis to GaBiAs we demonstrate that the observed strong variation of the band gap Eg and spin-orbit-splitting (SO) energy 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 Eg also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. Our calculated values of Eg and SO are in good agreement with experiment throughout the investigated composition range x less than 13%. In particular, our calculations reproduce the experimentally observed crossover to an Eg < SO regime at approximately 10.5% Bi composition in bulk GaBiAs. 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 GaBiAs electronic structure.

Band engineering in dilute nitride and bismide semiconductor lasers

Highly mismatched semiconductor alloys such as GaNAs and GaBiAs have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin orbit- 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 e?fficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

Derivation of 12- and 14-band k · p Hamiltonians for dilute bismide and bismide-nitride semiconductors

Using an sp3s* tight-binding (TB) 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 (VBE) and highly localized Bi-related resonant states lying below the GaAs VBE. 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 TB 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 TB 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 TB 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.

Impact of alloy disorder on the band structure of compressively strained GaBixAs1-x

The incorporation of bismuth (Bi) in GaAs results in a large reduction of the band gap energy (E

Optical gain in GaAsBi/GaAs quantum well diode lasers

_g

Determination of type-I band offsets in GaBixAs1–x quantum wells using polarisation-resolved photovoltage spectroscopy and 12-band k.p calculations

) accompanied with a large increase in the spin-orbit splitting energy (

12‐band k · p model for dilute bismide alloys of (In)GaAs derived from supercell calculations

\bigtriangleup_{SO}

Experimental and modelling study of InGaBiAs/InP alloys with up to 5.8% Bi, and with Δso > Eg

), leading to the condition that

GaAs1−xBix/GaNyAs1−y type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics

\bigtriangleup_{SO} > E_g

Theory of the Electronic Structure of Dilute Bismide Alloys: Tight-Binding and k · p Models

which is anticipated to reduce so-called CHSH Auger recombination losses whereby the energy and momentum of a recombining electron-hole pair is given to a second hole which is excited into the spin-orbit band. We theoretically investigate the electronic structure of experimentally grown GaBi