Charles J. Reyner

GaSb/GaAs type-II quantum dots grown by droplet epitaxy

We demonstrate the formation of GaSb quantum dots (QDs) on a GaAs(001) substrate by droplet epitaxy using molecular beam epitaxy. The high crystal quality and bimodal size distribution of the QDs are confirmed using atomic force and transmission electron microscope images. A staggered type-II QD band structure is suggested by a photoluminescence peak that is blue shifted with increasing excitation intensity, a large emission polarization of 60%, and a long carrier decay time of 11.5 ns. Our research provides a different approach to fabricating high quality GaSb type-II QDs.

Strong interband transitions in InAs quantum dots solar cell

Solar cells fabricated from a stack of ten periods of InAs quantum dots sandwiched in a GaAs p-n junction were fabricated and tested. The 300 K photoresponse spectrum exhibits two strong peaks and several weak peaks related to band-to-band transitions within the quantum dots. A few of these peaks were also observed in the photoluminescence and external quantum efficiency spectra. The power conversion efficiency was obtained from the current-voltage characteristics. Surface plasmon effect on the solar cell was investigated by coupling gold nanoparticles to the surface of the device using dithiol ligands with an enhancement on the order of 10%.

800 meV localization energy in GaSb/GaAs/Al0.3Ga0.7As quantum dots

The localization energies, capture cross sections, and storage times of holes in GaSb quantum dots (QDs) are measured for three GaSb/GaAs QD ensembles with different QD sizes. The structural properties, such as height and diameter, are determined by atomic force microscopy, while the electronic properties are measured using deep-level transient spectroscopy. The various QDs exhibit varying hole localization energies corresponding to their size. The maximum localization energy of 800 (±50) meV is achieved by using additional Al0.3Ga0.7As barriers. Based on an extrapolation, alternative material systems are proposed to further increase the localization energy and carrier storage time of QDs.

Characterization of GaSb/GaAs interfacial misfit arrays using x-ray diffraction

We report a nondestructive, large-area method to characterize dislocation formation at a highly lattice-mismatched interface. The analysis is based on x-ray diffraction and reciprocal space mapping using a standard, lab-based diffractometer. We use this technique to identify and analyze a two-dimensional array of 90° misfit dislocations at a GaSb/GaAs interface. The full width at half maximum of the GaSb 004 reciprocal lattice point is shown to decrease with increasing GaSb epilayer thickness, as expected from theoretical models. Based on these measurements, the variation in the spatial dislocation frequency is calculated to be 1%.

Short-Wave Infrared GaInAsSb Photodiodes Grown on GaAs Substrate by Interfacial Misfit Array Technique

We report GaInAsSb-based <i>p</i>-<i>i</i>-<i>n</i> photodiodes operating in the 2-2.4-μm wavelength range grown on GaAs (100) substrates using the interfacial misfit (IMF) array technique. A zero-bias dynamic-resistance-area product of 260 Ωcm² and a room temperature peak responsivity of 0.8 A/W (at 2 μm) with an estimated maximum detectivity (D*) of ~3.8×10¹⁰ cm Hz^1/2 W^-1 is obtained in the photodiodes at -0.2 V. These preliminary results of the IMF-based GaInAsSb detectors are comparable to similar detectors grown on native GaSb substrates demonstrating the potential of the IMF array growth mode to realize high-quality Sb-based infrared detectors on GaAs substrates.

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases ∼7 times while the QD height decreases ∼50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

Structural and optical properties of InAs/AlAsSb quantum dots with GaAs(Sb) cladding layers

We investigate the effect of GaAs1−xSbx cladding layer composition on the growth and properties of InAs self-assembled quantum dots surrounded by AlAs0.56Sb0.44 barriers. Lowering Sb-content in the GaAs1−xSbx improves the morphology of the InAs quantum dots and reduces cladding layer alloy fluctuations. The result is a dramatic increase in photoluminescence intensity from the InAs quantum dots, with a peak at 0.87 eV. The emission energy exhibits a cube root dependence on excitation power, consistent with the type-II band alignment of the quantum dots. The characteristics of this quantum dot system show promise for applications such as intermediate band solar cells.

Band Alignment Tailoring of InAs1-xSbx/GaAs Quantum Dots: Control of Type I to Type II Transition

We report the growth of InAs(1-x)Sb(x) self-assembled quantum dots (QDs) on GaAs (100) by molecular beam epitaxy. The optical properties of the QDs are investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL). A type I to type II band alignment transition is demonstrated by both power-dependent PL and TRPL in InAs(1-x)Sb(x) QD samples with increased Sb beam flux. Results are compared to an eight-band strain-dependent k x p model incorporating detailed QD structure and alloy composition. The calculations show that the conduction band offset of InAs(1-x)Sb(x)/GaAs can be continuously tuned from 0 to 500 meV and a flat conduction band alignment exists when 60% Sb is incorporated into the QDs. Our study offers the possibility of tailoring the band structure of GaAs based InAsSb QDs and opens up new means for device applications.

InGaAs/InAsSb strained layer superlattices for mid-wave infrared detectors

Investigation of growth and properties of InGaAs/InAsSb strained layer superlattices, identified as ternary strained layer superlattices (ternary SLSs), is reported. The material space for the antimony-based SLS detector development is expanded beyond InAs/InAsSb and InAs/(In)GaSb by incorporating Ga into InAs. It was found that this not only provides support for strain compensation but also enhances the infrared (IR) absorption properties. A unique InGaAs/InAsSb SLS exists when the conduction band of InGaAs aligns with that of InAsSb. The bandgap of this specific InGaAs/InAsSb SLS can then be tuned by adjusting the thickness of both constituents. Due to the enhanced electron-hole wavefunction overlap, a significant increase in the absorption coefficient was theoretically predicted for ternary SLS as compared to current state-of-the-art InAs/InAsSb SLS structures, and an approximately 30%–35% increase in the absorption coefficient was experimentally observed. All the samples examined in this work were des...

Unipolar infrared detectors based on InGaAs/InAsSb ternary superlattices

Growth and characteristics of mid-wave infrared (MWIR) InGaAs/InAsSb strained layer superlattice (SLS) detectors are reported. InGaAs/InAsSb SLSs, identified as ternary SLSs, not only provide an extra degree of freedom for superlattice strain compensation but also show enhanced absorption properties compared to InAs/InAsSb SLSs. Utilizing In1-yGayAs/InAs0.65Sb0.35 ternary SLSs (y = 0, 5, 10, and 20%) designed to have the same bandgap, a set of four unipolar detectors are investigated. These demonstrate an enhancement in the detector quantum efficiency due to the increased absorption coefficient. The detectors exhibit dark current performance within a factor of 10 of Rule 07 at temperatures above 120 K, and external quantum efficiencies in the 15%–25% range. This work demonstrates ternary SLSs are a potential absorber material for future high performance MWIR detectors.