Michael A. Meeker

Enhanced piezoluminescence in non-stoichiometric ZnS:Cu microparticle based light emitting elastomers

Piezoluminescence (PZL), also referred to as mechanoluminescence (ML), is a promising energy conversion mechanism for realizing mechanically driven photon sources including handheld displays, lighting, bioimaging and sensing applications. However, the realization of a visible PZL intensity at room temperature from low mechanical stresses has been fundamentally challenging. Herein, we describe a PZL elastomer exhibiting significantly enhanced brightness under ambient conditions. The elastomer consisted of defect-engineered non-stoichiometric Cu-doped ZnS (ZnS:Cu) microparticles in a polydimethylsiloxane (PDMS) matrix. The role of the defect structure was found to be the controlling parameter in the nature of PZL emission. Hydrogenation treatment was designed to induce a controlled concentration of sulfur vacancies that provided the trapped electrons, which had a strong correlation with the PZL performance of ZnS:Cu. An optimum electron concentration was necessary in order to maximize the PZL intensity due to an adequate electron energy transfer ratio between non-radiative recombination (NRR) and thermal radiative recombination (TRR). The light-emitting elastomer with an optimum content of PZL particles maximized the stress-mediated electroluminescence–piezoelectric coupling, enabling visible PZL brightness under indoor light conditions.

Photoluminescence lineshape and dynamics of localized excitonic transitions in InAsP epitaxial layers

The excitonic radiative transitions of InAsxP1−x (x = 0.13 and x = 0.40) alloy epitaxial layers were studied through magnetic field and temperature dependent photoluminescence and time-resolved photoluminescence spectroscopy. While the linewidth and lineshape of the exciton transition for x = 0.40 indicate the presence of alloy broadening due to random anion distribution and the existence of localized exciton states, those of x = 0.13 suggest that this type of compositional disorder is absent in x = 0.13. This localization is further supported by the behavior of the exciton transitions at low temperature and high magnetic fields. InAs0.4P0.6 exhibits anomalous “S-shaped” temperature dependence of the excition emission peak below 100 K as well as linewidth broadening at high magnetic fields due to the compression of the excitonic volume amid compositional fluctuations. Finally, photoluminescence decay patterns suggest that the excitons radiatively relax through two channels, a fast and a slow decay. While ...

Growth and characterization of metamorphic InAs/GaSb tunnel heterojunction on GaAs by molecular beam epitaxy

The structural, morphological, optical, and electrical transport characteristics of a metamorphic, broken-gap InAs/GaSb p-i-n tunnel diode structure, grown by molecular beam epitaxy on GaAs, were demonstrated. Precise shutter sequences were implemented for the strain-balanced InAs/GaSb active layer growth on GaAs, as corroborated by high-resolution X-ray analysis. Cross-sectional transmission electron microscopy and detailed micrograph analysis demonstrated strain relaxation primarily via the formation of 90° Lomer misfit dislocations (MDs) exhibiting a 5.6 nm spacing and intermittent 60° MDs at the GaSb/GaAs heterointerface, which was further supported by a minimal lattice tilt of 180 arc sec observed during X-ray analysis. Selective area diffraction and Fast Fourier Transform patterns confirmed the full relaxation of the GaSb buffer layer and quasi-ideal, strain-balanced InAs/GaSb heteroepitaxy. Temperature-dependent photoluminescence measurements demonstrated the optical band gap of the GaSb layer. Str...

Growth, structural, and electrical properties of germanium-on-silicon heterostructure by molecular beam epitaxy

The growth, morphological, and electrical properties of thin-film Ge grown by molecular beam epitaxy on Si using a two-step growth process were investigated. High-resolution x-ray diffraction analysis demonstrated ∼0.10% tensile-strained Ge epilayer, owing to the thermal expansion coefficient mismatch between Ge and Si, and negligible epilayer lattice tilt. Micro-Raman spectroscopic analysis corroborated the strain-state of the Ge thin-film. Cross-sectional transmission electron microscopy revealed the formation of 90  ° Lomer dislocation network at Ge/Si heterointerface, suggesting the rapid and complete relaxation of Ge epilayer during growth. Atomic force micrographs exhibited smooth surface morphology with surface roughness < 2 nm. Temperature dependent Hall mobility measurements and the modelling thereof indicated that ionized impurity scattering limited carrier mobility in Ge layer. Capacitance- and conductance-voltage measurements were performed to determine the effect of epilayer dislocation densi...

Nanoscale Texturing and Interfaces in Compositionally Modified Ca3Co4O9 with Enhanced Thermoelectric Performance

Oxide thermoelectric materials are nontoxic, chemically and thermally stable in oxidizing environments, cost-effective, and comparatively simpler to synthesize. However, thermoelectric oxides exhibit comparatively lower figure of merit (ZT) than that of metallic alloy counterparts. In this study, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. Results demonstrated that nanoscale platelets (having layered structure with nanoscale spacing) and metallic inclusions provide effective scattering of phonons, resulting in lower thermal conductivity and higher ZT. Thermoelectric measurement direction was found to have a significant effect on the magnitude of ZT because of the strong anisotropy in the transport properties induced by the layered nanostructure. The peak ZT value for the Ca2.85Lu0.15Co3.95Ga0.05O9 specimen measured along both perpendicular and parallel directions with respect to the SPS pressure axis is found be 0.16 at 630 °C and 0.04 at 580 °C, respectively. The peak ZT of 0.25 at 670 °C was observed for the spark plasma-sintered Ca2.95Ag0.05Co4O9 sample. The estimated output power of 2.15 W was obtained for the full size model, showing high-temperature thermoelectric applicability of this nanostructured material without significant oxidation.

Time resolved magneto-optical studies of InAsP ternary alloys

The recent rapid progress in the field of spintronics requires extensive studies of carrier and spin relaxation dynamics in semiconductors. In this work, we employed time and spin resolved differential transmission measurements in order to probe carrier and spin relaxation times in several InAsP ternary alloys. In addition, the dynamics of the excitonic radiative transitions of InAs0.13P0.87 epitaxial layer were studied through the time-resolved photoluminescence spectroscopy.