# Christopher A. BroderickTyndall National Institute · Theory, Modelling and Design Research Centre

Christopher A. Broderick

B.Sc. (Hons.) Physics, Ph.D. (Physics)

## About

93

Publications

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Introduction

Researcher working in theoretical and computational condensed matter physics.
Current research interests include:
1. Electronic, optical and spin properties of condensed matter, with a focus on emerging semiconductor alloys
2. Development of scientific software for the modelling of semiconductor materials and nanostructures
3. Theory/design of semiconductor materials/nanostructures for applications in photonic and photovoltaic technologies
http://github.com/christopherbroderick

Additional affiliations

December 2016 - January 2017

January 2016 - March 2016

April 2015 - July 2016

Education

October 2010 - February 2015

September 2006 - May 2010

## Publications

Publications (93)

The electronic structure of highly mismatched semiconductor alloys is characterized by carrier localization and strongly influenced by the local alloy microstructure. First-principles calculations can deliver valuable quantitative insight, but their associated computational expense limits alloy supercell size and imposes artificial long-range order...

We present a theoretical analysis of mid-infrared radiative recombination in InAs/GaSb superlattices (SLs). We employ a semi-analytical plane wave expansion method in conjunction with an 8-band k·p Hamiltonian to compute the SL electronic structure, paying careful attention to the identification and mitigation of spurious solutions. The calculated...

We present a theoretical analysis of highly-strained InAs quantum well lasers grown on InP for use in next-generation hollow-core fibre optical communications close to 2 µm, and validate our calculations against recent experimental data.

We theoretically analyse strain-compensated GaAs 1−x Bi x /GaN y As 1−y "W-type" quantum wells, demonstrating a viable approach to achieve efficient GaAs-based 1.3 and 1.55 µm lasers in which non-radiative Auger recombination is expected to be mitigated by type-II band offsets.

We theoretically analyze band-to-band tunneling (BTBT) in highly mismatched, narrow-gap dilute nitride and bismide alloys, and quantify the impact of the N- or Bi-induced perturbation of the band structure—due to band anticrossing (BAC) with localized impurity states—on the electric-field-dependent BTBT generation rate. For this class of semiconduc...

We present a combined experimental and theoretical analysis of the evolution of the near-band-gap electronic and optical properties of SixGe1−x−ySny alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on SixGe1−x−ySny epitaxial layers grown via chemical vapor deposition for Si (...

We theoretically analyse strain-compensated GaAs 1 −x Bi x /GaN y As 1 −y “W-type” quantum wells, demonstrating a viable approach to achieve efficient GaAs-based 1.3 and 1.55 µ m lasers in which non-radiative Auger recombination is expected to be mitigated by type-II band offsets.

Ge$_{1-x}$Sn$_{x}$ alloys are a promising candidate material to realise direct-gap group-IV semiconductors for applications in Si-compatible electronic and photonic devices. Here, we present a combined theoretical and experimental analysis of Raman spectroscopy in Ge$_{1-x}$Sn$_{x}$ alloys. We describe liquid-vapour-solid growth and structural char...

Si$_{y}$Ge$_{1-x-y}$(C,Sn,Pb)$_{x}$ alloys have attracted significant attention as a route to achieve a direct-gap group-IV semiconductor. Using density functional theory (DFT) - employing local density approximation and hybrid Heyd-Scuzeria-Ernzerhof exchange-correlation functionals - we compute the lattice parameters, relaxed and inner elastic co...

We discuss a new class of type-II quantum wells (QWs) that exploit the impact of Bi and N on the GaAs band-structure. Via growth, experiment, and theoretical calculations we highlight the properties of GaAsBi/GaNAs "W" QWs, demonstrating a potential pathway to uncooled telecom-wavelength laser operation.

Highly-mismatched alloys constitute a promising approach to extend the operational range of GaAs-based quantum well (QW) lasers to telecom wavelengths. This is challenging using type-I QWs due to the difficulty to incorporate sufficient N or Bi via epitaxial growth. To overcome this, we investigate a novel class of strain-compensated type-II QWs co...

Solving the multi-band k·p Schrödinger equation for a quantum-confined heterostructure using a reciprocal space plane wave approach presents several advantages compared to conventional real space approaches such as the finite difference or element methods. In addition to allowing analytical derivation of the heterostructure Hamiltonian, a desired l...

We present a combined experimental and theoretical analysis of the evolution of the near-band gap electronic and optical properties of Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ epitaxial layers grown via chem...

We theoretically analyse band-to-band tunneling (BTBT) in highly-mismatched, narrow-gap dilute nitride and bismide alloys, and quantify the impact of the N- or Bi-induced perturbation of the band structure -- due to band-anticrossing (BAC) with localised impurity states -- on the electric field-dependent BTBT generation rate. For this class of semi...

The continued exponential growth of the internet dictates the need to develop energy-efficient “green” photonics technologies for telecoms.Conventional temperature sensitive 1.55μm InP-based telecom lasers require external cooling, greatly increasing the energy consumption of optical communication networks. This is due to strong intrinsic losses as...

We calculate the electronic structure of germanium-tin (Ge1-x Sn x ) binary alloys for 0 ≤ x ≤ 1 using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition x are identified, and the impact of epitaxial strain is investigated by constraining supercell lattice constants perpendi...

We present a magneto-optical study of the carrier dynamics in compressively strained Ge1−xSnx films with Sn content up to 10% epitaxially grown on Ge on Si(001) virtual substrates. We leverage the Hanle effect under steady-state excitation to study the spin-dependent optical transitions in the presence of an external magnetic field. This allows us...

We calculate the electronic structure of germanium-tin (Ge$_{1-x}$Sn$_{x}$) binary alloys for $0 \leq x \leq 1$ using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition $x$ are identified, and the impact of epitaxial strain is included by constraining supercell lattice const...

We present a theoretical analysis of the electronic properties of type-II GaAs 1−x Sb x /GaAs quantum rings (QRs), from the perspective of applications in intermediate band solar cells (IBSCs). We outline the analytical solution of Schrödinger's equation for a cylindrical QR of infinite potential depth, and describe the evolution of the QR ground s...

We present a magneto-optical study of the carrier dynamics in compressively strained Ge(1-x)Sn(x) films having Sn compositions up to 10% epitaxially grown on blanket Ge on Si (001) virtual substrates. We leverage the Hanle effect under steady-state excitation to study the spin-dependent optical transitions in presence of an external magnetic field....

Conventional diamond-structured silicon (Si) and germanium (Ge) possess indirect fundamental band gaps, limiting their potential for applications in light-emitting devices. However, SiGe alloys grown in the lonsdaleite ("hexagonal diamond") phase have recently emerged as a promising direct-gap, Si-compatible material system, with experimental measu...

In recent years there has been an increase in interest around the applications of dilute bismide and dilute nitride alloys. This is owing to their potential application in optoelectronic devices that operate in the near- and mid- infra-red regime. As well as the ability to produce longer operational wavelengths, these materials also offer decreased...

Alloying of Ge with other group-IV elements-C, Sn or Pb-represents a promising route to realise direct-gap group-IV semiconductors for applications in Si-compatible devices, including light-emitting diodes and lasers, as well as tunnelling field-effect transistors and multi-junction solar cells. To develop a quantitative understanding of the proper...

We present a theoretical analysis of the electronic properties of type-II GaAs$_{1-x}$Sb$_{x}$/GaAs quantum rings (QRs), from the perspective of applications in intermediate band solar cells (IBSCs). We outline the analytical solution of Schr\"{o}dinger's equation for a cylindrical QR of infinite potential depth, and describe the evolution of the Q...

Group IV alloys have attracted interest in the drive to create Si compatible, direct bandgap materials for implementation in complementary metal oxide semiconductor (CMOS) and beyond CMOS devices. The lack of a direct bandgap in Si and Ge hinders their incorporation into optoelectronic and photonic devices, without the induction of undesirable stra...

We present a theoretical analysis of electronic structure evolution in the highly-mismatched dilute carbide group-IV alloy Ge1−xCx. For ordered alloy supercells, we demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving the hybridization of A1-symmetric linear combinations of Ge states lying close in energy...

We present a theoretical analysis of electronic structure evolution in the highly-mismatched dilute carbide group-IV alloy Ge$_{1-x}$C$_{x}$. For ordered alloy supercells, we demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving hybridisation of $A_{1}$-symmetric linear combinations of Ge states lying clos...

We present a theoretical analysis of electronic structure evolution in the group-IV alloy Ge$_{1-x}$Pb$_{x}$ based on density functional theory. For ordered alloy supercells we demonstrate the emergence of a singlet conduction band (CB) edge state, suggesting the emergence of a direct band gap for Pb compositions as low as $x \approx 1$%. However,...

We present and compare three distinct atomistic models—based on first principles and semi-empirical approaches—of the structural and electronic properties of \(\hbox {Ge}_{1-x}\hbox {Sn}_{x}\) alloys. Density functional theory calculations incorporating Heyd–Scuseria–Ernzerhof (HSE), local density approximation (LDA) and modified Becke–Johnson (mBJ...

We analyse the optical properties of InAs1−xSbx/AlyIn1−yAs quantum wells (QWs) grown by molecular beam epitaxy on relaxed AlyIn1−yAs metamorphic buffer layers (MBLs) using GaAs substrates. The use of AlyIn1−yAs MBLs allows for the growth of QWs having large type-I band offsets, and emission wavelengths >3 μm. Photoluminescence (PL) measurements for...

We present and compare three distinct atomistic models -- based on first principles and semi-empirical approaches -- of the structural and electronic properties of Ge$_{1-x}$Sn$_{x}$ alloys. Density functional theory calculations incorporating Heyd-Scuseria-Ernzerhof (HSE) and modified Becke-Johnson (mBJ) exchange-correlation functionals are used t...

The requirement for improved mid-infrared sensing technologies motivates the development of photodiodes displaying high signal-to-noise ratio. Key to achieving this goal is minimi-sation of the dark current, to which band-to-band tunneling (BTBT) contributes significantly in narrow-gap materials. We present a theoretical analysis of BTBT in narrow-...

Type-II quantum-confined heterostructures constitute a promising approach to realise highly efficient intermediate band solar cells (IBSCs), due to their long radiative lifetimes and the flexibility with which their electronic and optical properties can be engineered. To quantify the potential of type-II GaSb/GaAs quantum rings (QRs) for IBSC appli...

We present a comparative analysis of electronic structure evolution and the indirect-to direct-gap transition in Ge1−x(Sn,Pb)x group-IV semiconductor alloys. We present first principles disordered alloy electronic structure calculations for Ge1−x(Sn,Pb)x special quasi-random structures, and find a significantly larger band gap reduction in Ge1−xPbx...

Using spectroscopic ellipsometry measurements on GaP1−xBix/GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap (\({E}_{g}^{{\rm{\Gamma }}}\)) and valence band spin-orbit splitting energy (ΔSO). \({E}_{g}^{{\rm{\Gamma }}}\) (ΔSO) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorpora...

We present a theoretical investigation of the optical properties of metamorphic InNy(As1−xSbx)1−y/AlzIn1−zAs type-I quantum wells (QWs) designed to emit at mid-infrared wavelengths. The use of AlzIn1−zAs metamorphic buffer layers has recently been demonstrated to enable growth of lattice-mismatched InAs1−xSbx QWs having emission wavelengths 3 μm on...

The use of AlInAs metamorphic buffer layers to facilitate the growth of lattice-mismatched InNAsSb quantum wells on GaAs or InAs substrates has recently been demonstrated to constitute an attractive approach to developing light-emitting devices at application-rich mid-infrared wavelengths. However, little information is available regarding the fund...

We present a detailed analysis of the electronic structure of GeSn alloys using density functional theory. Special attention is paid to Sn-induced conduction band mixing effects. Our calculations indicate a continuous evolution from an indirect to a direct band gap material with increasing Sn content. This finding is in stark contrast to the litera...

We analyse the optical properties of InAsSb/AlInAs quantum wells (QWs) grown by molecular beam epitaxy on relaxed AlInAs metamorphic buffer layers (MBLs) using GaAs substrates. The use of AlInAs MBLs allows for the growth of QWs having large type-I band offsets, and emission wavelengths > 3 μm. Photoluminescence (PL) measurements for QWs having Sb...

We perform a systematic theoretical analysis of the nature and importance of alloy disorder effects on the electronic and optical properties of GaNAsBi alloys and quantum wells (QWs), using large-scale atomistic supercell electronic structure calculations based on the tight-binding method. Using ordered alloy supercell calculations, we also derive...

We present a theoretical analysis and optimisation of the properties and performance of mid-
infrared semiconductor lasers based on the dilute bismide alloy InGaAsBi, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band...

We present a theoretical analysis and optimisation of the properties and performance of mid-infrared semiconductor lasers based on the dilute bismide alloy InGaAsBi, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band...

We perform a systematic theoretical analysis of the nature and importance of alloy disorder effects on the electronic and optical properties of GaN$_{y}$As$_{1-x-y}$Bi$_{x}$ alloys and quantum wells (QWs), using large-scale atomistic tight-binding electronic structure calculations. Based on ordered alloy supercell calculations we also derive and pa...

Using spectroscopic ellipsometry measurements on GaP$_{1-x}$Bi$_{x}$/GaP epitaxial layers up to $x = 3.7$% we observe a giant bowing of the direct band gap ($E_{g}^{\Gamma}$) and valence band spin-orbit splitting energy ($\Delta_{\textrm{SO}}$). $E_{g}^{\Gamma}$ ($\Delta_{\textrm{SO}}$) is measured to decrease (increase) by approximately 200 meV (2...

Adding dilute concentrations of nitrogen (N) or bismuth (Bi) into conventional III-V semiconductor alloys causes a large bowing of the bandgap energy due to the modification of the electronic band structure. This behaviour has attracted significant interest due to the resulting optical and electronic properties. Firstly, the authors present theoret...

The potential to extend the emission wavelength of photonic devices further into the near-and mid-infrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N...

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performan...

We present an analysis of dilute bismide quantum well (QW) lasers grown on GaAs and InP substrates. Our theoretical analysis is based upon a 12-band k·p Hamiltonian which directly incorporates the strong impact of Bi incorporation on the band structure using a band-anticrossing approach. For GaBiAs QWs grown on GaAs we analyse the device performanc...

There is a need to improve the performance of materials underlying optical communications devices. Performance parameters such as modulation speed, temperature insensitively and band width can be tailored at the material level. In this talk I will show modelling and comparison with experiment of some highly-mismatched alloys: dilute nitride and dil...

The use of InGaAs metamorphic buffer layers (MBLs) to facilitate the growth of lattice-mismatched heterostructures constitutes an attractive approach to developing long-wavelength semiconductor lasers on GaAs substrates, since they offer the improved carrier and optical confinement associated with GaAs-based materials. We present a theoretical stud...

We present a theoretical analysis of the properties and performance of mid-infrared dilute bismide quantum well (QW) lasers grown on InP substrates. We analyse the band structure of strained InGaBiAs alloys and quantify their potential for the development of mid-infrared semiconductor lasers. In addition to identifying the permissible growth combin...

We present a theoretical analysis of the electronic and optical properties of near-infrared dilute bismide quantum well (QW) lasers grown on GaAs substrates. Our theoretical model is based upon a 12-band k·p Hamiltonian which explicitly incorporates the strong Bi-induced modifications of the band structure in pseudomorphically strained GaBi x As 1−...

The use of InGaAs metamorphic buffer layers (MBLs) to facilitate the growth of lattice-mismatched heterostructures constitutes a novel approach to developing GaAs-based long-wavelength semiconductor lasers. Such devices are attractive since they approach the improved electronic and optical confinement associated with GaAs-based materials. As a resu...

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...

Using photovoltage (PV) spectroscopy we analyse the electronic structure of a series of GaBiAs/(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 fr...

Temperature dependent photo-modulated reflectance is used to measure the band gap E g and spin–orbit splitting energy Δ so in dilute-Bi In0.53Ga0.47As1-x Bix /InP for 1.2% ≤ x ≤ 5.8%. At room temperature, E g decreases with increasing Bi from 0.65 to 0.47 eV (∼2.6 μm), while Δ so increases from 0.42 to 0.62 eV, leading to a crossover between E g an...

GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.

The electron Land\'e $g$ factor $({g}^{*})$ is investigated both experimentally and theoretically in a series of ${\mathrm{GaBi}}_{x}{\mathrm{As}}_{1$-${}x}/\mathrm{GaAs}$ strained epitaxial layers, for bismuth compositions up to $x=3.8%$. We measure ${g}^{*}$ via time-resolved photoluminescence spectroscopy, which we use to analyze the spin quantu...

Systematic photoluminescence measurements on a series of GaBixAs1-x samples are analyzed theoretically using a fully microscopic approach. Based on sp(3)s* tight-binding calculations, an effective k. p model is set up and used to compute the band structure and dipole matrix elements for the experimentally investigated samples. With this input, the...

The electron Land\'e g factor ($g^{*}$) is investigated both experimentally
and theoretically in a series of GaBi$_{x}$As$_{1-x}$/GaAs strained epitaxial
layers, for bismuth compositions up to $x = 3.8$%. We measure $g^{*}$ via
time-resolved photoluminescence spectroscopy, which we use to analyze the spin
quantum beats in the polarization of the ph...