Large-scale atomistic simulations demonstrate dominant alloy disorder effects in ${\mathrm{GaBi}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}/\mathrm{GaAs}$ multiple quantum wells

Large-scale atomistic simulations demonstrate dominant alloy disorder effects in ${GaBi}_{x} {As}_{1 - x} / GaAs$ multiple quantum wells

Muhammad Usman

Phys. Rev. Materials 2, 044602 – Published 10 April 2018

Abstract

Bismide semiconductor materials and heterostructures are considered a promising candidate for the design and implementation of photonic, thermoelectric, photovoltaic, and spintronic devices. This work presents a detailed theoretical study of the electronic and optical properties of strongly coupled ${GaBi}_{x} {As}_{1 - x}$ /GaAs multiple quantum well (MQW) structures. Based on a systematic set of large-scale atomistic tight-binding calculations, our results reveal that the impact of atomic-scale fluctuations in alloy composition is stronger than the interwell coupling effect, and plays an important role in the electronic and optical properties of the investigated MQW structures. Independent of QW geometry parameters, alloy disorder leads to a strong confinement of charge carriers, a large broadening of the hole energies, and a red-shift in the ground-state transition wavelength. Polarization-resolved optical transition strengths exhibit a striking effect of disorder, where the inhomogeneous broadening could exceed an order of magnitude for MQWs, in comparison to a factor of about 3 for single QWs. The strong influence of alloy disorder effects persists when small variations in the size and composition of MQWs typically expected in a realistic experimental environment are considered. The presented results highlight the limited scope of continuum methods and emphasize on the need for large-scale atomistic approaches to design devices with tailored functionalities based on the novel properties of bismide materials.

Received 22 November 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.2.044602

Physics Subject Headings (PhySH)

Electronic structure Optoelectronics Photonics Spintronics

Alloys Energy materials Nanostructures Quantum wells Solid-state detectors Thermoelectrics

Tight-binding model

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Muhammad Usman^*

School of Physics, The University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia

^*usman@alumni.purdue.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 2, Iss. 4 — April 2018

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Ordered vs disordered multiple quantum wells. (a), (b) Schematic diagram illustrates the position and type of atoms in a (010)-oriented anion plane of a disordered and an ordered quantum well structure. Both MQW structures consist of three ${GaBi}_{x} {As}_{1 - x}$ quantum wells, separated by GaAs spacer layers. The geometry parameters and the Bi compositions of the two MQW structures are identical, except the spatial distribution of Bi atoms in the QW regions. The MQW structure in (a) has Bi atoms randomly distributed in the QW regions (disordered alloy), whereas the MQW in (b) exhibits uniform distribution of Bi atoms (ordered alloy). The impact of fluctuations in Bi atom positions is clearly evident in the electron and hole charge density plots shown in (c) and (d) for the disordered and ordered cases, respectively. The ordered MQW structure shows strongly coupled charge carrier states, whereas the disorder overcomes the interwell coupling effect leading to much stronger confinement of charge carriers (in particular the hole state) due to the formation of Bi pairs and clusters.
Reuse & Permissions
Figure 2
Schematic diagram of a ${GaBi}_{x} {As}_{1 - x} / GaAs$ multi-quantum-well structure is illustrated. The device consists of $n$ number of QWs with $d$ nm separation between the QWs and the width of each QW is $w$ nm. The thickness of both GaAs substrate and capping layer is 16 nm, and the lateral dimensions are 4 nm in both (100) and (010) directions. The boundary conditions are periodic in all three spatial dimensions.
Reuse & Permissions
Figure 3
Interwell coupling effects in ordered MQW devices. (a)–(d) The line plots of hydrostatic $(ε_{H})$ and biaxial $(ε_{B})$ strain components as a function of distance along the (001) axis are shown through the center of QWs for $n = 1$ , 2, 3, and 4. The separation $d$ between the QWs is 4 nm and the widths of QWs are 8 nm. (e), (f) Plots of the lowest electron and the highest hole energy levels are shown as a function of the separation $(d)$ between the QWs for $n = 2$ , 3, and 4. The horizontal dashed lines indicate energy levels for a single QW device. (g) Plots of the lowest electron and the highest hole state charge densities as a function of distance along the (001) axis are shown for a few selected separations $d$ and for $n = 3$ and 4.
Reuse & Permissions
Figure 4
Dominant role of alloy disorder in MQW structures. (a)–(d) The line plots of hydrostatic $(ε_{H})$ and biaxial $(ε_{B})$ strain components as a function of distance along the (001) axis are shown through the center of QWs for $n = 1$ , 2, 3, and 4. The separation between the QWs is 4 nm and the widths of QWs are 8 nm. (e), (f) Plots of the lowest electron and the highest hole energy levels are shown as a function of the separation $(d)$ between the QWs for $n = 2$ , 3, and 4. The error bars show the broadening of energies computed over five different random distributions of Bi atoms in the QW regions. The dashed lines in (e) are plotted as a guide to eye to indicate the overall trend of variation in electron energies. The shaded areas indicate energy level broadening for a single QW $(n = 1)$ . (g) Plots of the lowest electron and the highest hole state charge densities as a function of distance along the (001) axis plotted are shown for a few selected separations $d$ and for $n = 3$ and 4.
Reuse & Permissions
Figure 5
Polarization-resolved interband optical transition strengths. Plots of the polarization-dependent interband optical transition strengths are shown as a function of the distance along the (001) axis for $n = 2$ . The error bars are for disordered ${GaBi}_{x} {As}_{1 - x}$ QW structures and indicate the broadening of transition strengths due to fluctuations in Bi atom positions.
Reuse & Permissions
Figure 6
Interplay between alloy disorder and geometry parameters. (a)–(e) The Bi fraction of a triple QW structure $(n = 3)$ is varied as indicated by labels { $x_{1}, x_{2}, x_{3}$ }, where $x_{1}, x_{2}$ , and $x_{3}$ are the Bi fractions of the lowest, middle, and top QWs, respectively. The widths $w$ of all three QW regions are 8 nm and the separation between the QWs is 4 nm. (f)–(j) The widths of QWs in a triple QW structure $(n = 3)$ are varied as indicated by labels { $w_{1}, w_{2}, w_{3}$ }, where $w_{1}, w_{2}$ , and $w_{3}$ are the widths of the lowest, middle, and top QWs, respectively. The Bi fraction $x$ of all three QW regions is 3.125% and the separation between the QWs is 4 nm.
Reuse & Permissions

Physical Review Materials

Large-scale atomistic simulations demonstrate dominant alloy disorder effects in GaBixAs1−x/GaAs multiple quantum wells

Muhammad Usman

Phys. Rev. Materials 2, 044602 – Published 10 April 2018

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Large-scale atomistic simulations demonstrate dominant alloy disorder effects in ${GaBi}_{x} {As}_{1 - x} / GaAs$ multiple quantum wells