B. L. Liang

InGaAs heterostructure formation in catalyst-free GaAs nanopillars by selective-area metal-organic vapor phase epitaxy

We investigate axial GaAs/InGaAs/GaAs heterostructures embedded in GaAsnanopillars via catalyst-free selective-area metal-organic chemical vapor deposition. Structural characterization by transmission electron microscopy with energy dispersive x-ray spectroscopy(EDS) indicates formation of axial In x Ga 1 − x As ( x ∼ 0.20 ) inserts with thicknesses from 36 to 220 nm with ±10% variation and graded Ga:In transitions controlled by In segregation. Using the heterointerfaces as markers, the vertical growth rate is determined to increase linearly during growth.Photoluminescence from 77 to 290 K and EDS suggest the presence of strain in the shortest inserts. This capability to control the formation of axial nanopillarheterostructures is crucial for optimized device integration.

Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations

We report structural analysis of completely relaxed GaSb epitaxial layers deposited monolithically on GaAs substrates using interfacial misfit (IMF) array growth mode. Unlike the traditional tetragonal distortion approach, strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth. The complete and instantaneous strain relief at the GaSb/GaAs interface is achieved by the formation of a two-dimensional Lomer dislocation network comprising of pure-edge (90°) dislocations along both [110] and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré fringe patterns along with X-ray diffraction measure the long-range uniformity and strain relaxation of the IMF samples. The proof for the existence of the IMF array and low threading dislocation density is provided with the help of transmission electron micrographs for the GaSb epitaxial layer. Our results indicate that the IMF-grown GaSb is completely (98.5%) relaxed with very low density of threading dislocations (105 cm−2), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>109 cm−2).

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%.

Bottom-up Photonic Crystal Cavities Formed by Patterned III–V Nanopillars

We report the demonstration of photonic crystal lasers formed bottom-up by patterned III-V nanopillar (NP) arrays. In this work, we present a method whereby the photonic band gap region and active gain regions are formed simultaneously by selective-area metal-organic chemical vapor deposition. This approach allows us the ability to design device parameters lithographically. By accurate control of position and diameter of the NPs, high-Q cavities can be formed entirely with NPs. This particular model cavity supports a non-degenerate hexapole mode1 with a high overlap between the E-field and the center pillars. Design optimization by finite-difference time-domain simulations yields a cavity Q of ~5000.

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.

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.

Lateral interdot carrier transfer in an InAs quantum dot cluster grown on a pyramidal GaAs surface

InAs quantum dot clusters (QDCs), which consist of three closely spaced QDs, are formed on nano-facets of GaAs pyramidal structures by selective-area growth using metal-organic chemical vapor deposition. Photoluminescence (PL) and time-resolved PL (TRPL) experiments, measured in the PL linewidth, peak energy and QD emission dynamics indicate lateral carrier transfer within QDCs with an interdot carrier tunneling time of 910 ps under low excitation conditions. This study demonstrates the controlled formation of laterally coupled QDCs, providing a new approach to fabricate patterned QD molecules for optical computing applications.

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...

Composite axial/core-shell nanopillar light-emitting diodes at 1.3 μm

Selective-area growth of III-V nanopillars (NPs) is used to demonstrate near-infrared emitters that employ a composite axial/core-shell heterostructure. The axial p-i-n heterostructure allows growth of strain relaxed InGaAs inserts emitting at 1.3 μm. Radial growth of an InGaP shell provides in-situ surface passivation to reduce non-radiative recombination and space-charge limited transport due to mid-gap surface states. The resulting light-emitting diode is comparable to bulk devices with an ideality factor of η = 1.67 and reverse bias leakage of 12 nA at −5 V. This device performance makes the combination of axial current injection with in-situ passivation a promising approach to NP based emitters.