Search

We present a theoretical analysis of the electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs quantum rings (QRs), from the perspective of applications in intermediate band solar cells (IBSCs). We outline the analytical solution of Schrodinger’s equation for a cylindrical QR of infinite potential depth, and describe the evolution of the QR ground state with QR morphology. Having used this analytical model to elucidate general aspects of the electronic properties of QRs, we undertake multi-band k·p calculations—including strain and piezoelectric effects—for realistic $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs. Our k·p calculations confirm that the large type-II band offsets in $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs provide strong confinement of holes, and further indicate the presence of resonant (quasi-bound) electron states which localise in the centre of the QR. From the perspective of IBSC design the calculated electronic properties demonstrate several benefits, including (i) large hole ionisation energies, mitigating thermionic emission from the intermediate band, and (ii) electron-hole spatial overlaps exceeding those in conventional $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QDs, with the potential to engineer these overlaps via the QR morphology so as to manage the trade-off between optical absorption and radiative recombination. Overall, our analysis highlights the flexibility offered by the QR geometry from the perspective of band structure engineering, and identifies specific combinations of QR alloy composition and morphology which offer optimised sub-band gap energies for QR-based IBSCs.

There are many applications for light-emitting diodes (LEDs) that can operate in the mid-infrared spectral range. However, the efficiency of these devices at room temperature is limited by competing nonradiative recombination mechanisms, inadequate carrier confinement, and insufficient optical extraction. Earlier devices based on bulk materials and heterojunctions have been quite successful to date, leading to some commercialization, but several new designs containing quantum structures for the active region have since been proposed and are being studied. Similarly, there is growing interest in using more cost-effective substrates requiring the development of metamorphic buffer layers as well as resonant cavity structures to increase optical extraction. An overview of the current status of mid-infrared LED technology is given here together with a brief summary of some recent developments.

The use of Al z In 1−z As metamorphic buffer layers to facilitate the growth of lattice-mismatched InN y (As 1−x Sb x ) 1−y 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 fundamental properties of this newly established platform. We present a theoretical investigation and optimisation of the properties and performance of InN y (As 1−x Sb x ) 1−y /Al z In 1−z As structures designed to emit at 3.3 and 4.2 μm. We quantify the design space available to these structures in terms of the ability to engineer and optimise the optoelectronic properties, and quantify the potential of metamorphic InN y (As 1−x Sb x ) 1−y structures for the development of mid-infrared light emitters, providing guidelines for the design of optimised light-emitting diodes.

We analyse the optical properties of InAs1−x Sb x /Al y In1−y As quantum wells (QWs) grown by molecular beam epitaxy on relaxed Al y In1−y As metamorphic buffer layers (MBLs) using GaAs substrates. The use of Al y In1−y As MBLs allows for the growth of QWs having large type-I band offsets, and emission wavelengths >3 m. Photoluminescence (PL) measurements for QWs having Sb compositions up to x = 10% demonstrate strong room temperature PL up to 3.4 m, as well as enhancement of the PL intensity with increasing wavelength. To quantify the trends in the measured PL we calculate the QW spontaneous emission (SE), using a theoretical model based on an eight-band Hamiltonian. The theoretical calculations, which are in good agreement with experiment, identify that the observed enhancement in PL intensity with increasing wavelength is associated with the impact of compressive strain on the QW valence band structure, which reduces the band edge density of states making more carriers available to undergo radiative recombination at fixed carrier density. Our results highlight the potential of type-I InAs1−x Sb x /Al y In1−y As metamorphic QWs to address several limitations associated with existing heterostructures operating in the mid-infrared, establishing these novel heterostructures as a suitable platform for the development of light-emitting diodes and diode lasers.

We present a theoretical investigation of the optical properties of metamorphic $\mathbf{InN}_{\pmb{y}}(\mathbf{As}_{1-\pmb{x}}\mathbf{Sb}_{x})_{1-\pmb{y}}/\mathbf{Al}_{z}\mathbf{In}_{1-\pmb{z}}$ As type-I quantum wells (QWs) designed to emit at mid-infrared wavelengths. The use of $\mathbf{Al}_{z}\mathbf{In}_{1-z}$ As metamorphic buffer layers has recently been demonstrated to enable growth of lattice-mismatched In. $\mathbf{As}_{1-\pmb{x}}\mathbf{Sb}_{\pmb{x}}$ QWs having emission wavelengths $\underset{\sim}{>}$ 3 $\mu \mathbf{m}$ on GaAs substrates. However, little information is available regarding the properties of this newly established platform. We undertake a theoretical analysis and optimisation of the properties and performance of strain-balanced structures designed to emit at 3.3 and $4.2\ \mu \mathbf{m}$ , where we recommend the incorporation of dilute concentrations of nitrogen (N) to achieve emission beyond $4\ \mu \mathbf{m}$ . We quantify the calculated trends in the optical properties, as well as the ability to engineer and optimise the overall QW performance. Our results highlight the potential of metamorphic $\mathbf{InN}_{y}(\mathbf{As}_{1-x}\mathbf{Sb}_{x})_{1-y}/\mathbf{Al}_{z}\mathbf{In}_{1-z}$ As QWs for the development of mid-infrared light-emitting diodes, and provide guidelines for the growth of optimised structures.