Sadhvikas Addamane

Transmission Electron Microscopy-Based Analysis of Electrically Conductive Surface Defects in Large Area GaSb Homoepitaxial Diodes Grown Using Molecular Beam Epitaxy

We investigate a mechanism causing shorting of large area GaSb diodes grown on GaSb substrates using molecular beam epitaxy (MBE). The source of these shorts is determined to be large crystallographic defects on the surface of the diodes that are formed around droplets of gallium ejected from the gallium Knudsen cells during MBE. The gallium droplets cause defects in the crystal structure, and, as the epitaxy continues, the gallium is incorporated into the surrounding material. The shape of the defects is pyramidal with a central void extending from the epi-surface to the gallium core. Processing a GaSb diode with these surface defects results in the top-side contact metal migrating into the defect and shorting the diode. This prevents realization of large area diodes that are critical to applications such as photovoltaics and detectors. The diodes in this study are electrically characterized and the defect formation mechanism is investigated using cross-section transmission electron microscopy and electron dispersive spectroscopy.

Multi-angle VECSEL cavities for dispersion control and peak-power scaling

We present a novel cavity design for vertical external cavity surface emitting lasers (VECSELs) enabling multiple interactions with the gain structure under different angles in a single round trip. This allows for a low round-trip group delay dispersion (GDD) despite using high-gain resonant VECSEL structures possessing pronounced resonances in their reflective GDD profile. Femtosecond-regime pulses with an average output power of 1.14 W and record peak intensities for mode-locked VECSELs of 6.3 kW are presented with simulations demonstrating the GDD compensating mechanism employed in this scheme.

AlGaSb-Based Solar Cells Grown on GaAs: Structural Investigation and Device Performance

GaSb and alloys based on the 6.1 Å family can be grown metamorphically on substrates such as GaAs allowing for the realization of several multijunction solar cell designs. This paper investigates the molecular beam epitaxy growth, crystal quality, and device performance of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb-based single-junction solar cells grown on GaAs substrates. The focus is on the optimization of the growth of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb on GaAs (001) substrates in order to minimize the threading dislocation density resulting from the large lattice mismatch between GaSb and GaAs. Utilizing optimum growth conditions, solar cells with absorbing layers of different Al<italic>_x </italic>Ga₁<italic>_x</italic>Sb compositions are studied and compared to control cells grown on lattice-matched GaSb substrates. GaSb, Al_0.15Ga_0.85Sb, and Al_0.5Ga_0.5Sb solar cells grown on GaAs substrates show open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. Furthermore, the lattice-mismatched cells demonstrate promising carrier collection with comparable spectral response to lattice-matched control cells grown on GaSb.

Active Mediation of Plasmon Enhanced Localized Exciton Generation, Carrier Diffusion and Enhanced Photon Emission

Understanding the enhancement of charge carrier generation and their diffusion is imperative for improving the efficiency of optoelectronic devices particularly infrared photodetectors that are less developed than their visible counterpart. Here, using gold nanorods as model plasmonic systems, InAs quantum dots (QDs) embedded in an InGaAs quantum well as an emitter, and GaAs as an active mediator of surface plasmons for enhancing carrier generation and photon emission, the distance dependence of energy transfer and carrier diffusion have been investigated both experimentally and theoretically. Analysis of the QD emission enhancement as a function of distance reveals a Förster radius of 3.85 ± 0.15 nm, a near-field decay length of 4.8 ± 0.1 nm and an effective carrier diffusion length of 64.0 ± 3.0 nm. Theoretical study of the temporal-evolution of the electron-hole occupation number of the excited states of the QDs indicates that the emission enhancement trend is determined by the carrier diffusion and capture rates.

Growth and Optimization of 2-μm InGaSb/AlGaSb Quantum-Well-Based VECSELs on GaAs/AlGaAs DBRs

We report the growth of optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) based on InGaSb/AlGaSb quantum wells grown on GaAs/AlGaAs distributed Bragg reflectors (DBRs). The 7.78% lattice mismatch between GaSb and GaAs is accommodated by an array of 90° misfit dislocations at the interface. This results in spontaneous relaxation of the GaSb epilayer and also significantly reduces the threading dislocation density. The VECSELs are operated in both pulsed (with 340-W peak output power) and continuous wave mode (with 0.12-W peak output power). We investigate the effects of the GaSb/GaAs interface by comparing the lattice mismatched III-Sb VECSEL grown on GaAs/AlGaAs DBRs to a lattice matched III-Sb VECSEL grown on GaSb/AlAsSb DBRs. The lattice matched VECSEL outperforms the lattice mismatched VECSEL in terms of threshold pump density, efficiency, and maximum continuous-wave output power. This can be attributed to the presence of threading dislocations throughout the active region of the mismatched VECSEL, which is confirmed by cross-sectional transmission electron microscopy. The optical properties of the III-Sb active regions are characterized by time-resolved photoluminescence, which can be used to optimize the IMF interface.

Degradation mechanism of SESAMs under intense ultrashort pulses in modelocked VECSELs

Mode-locked VECSELs using SESAMs are a relatively less complex and cost-effective alternative to state-of-the-art ultrafast lasers based on solid-state or fiber lasers. VECSELs have seen considerable progress in device performance in terms of pulse width and peak power in the recent years. However, it appears that the combination of high power and short pulses can cause some irreversible damage to the SESAM. The degradation mechanism, which can lead to a reduction of the VECSEL output power over time, is not fully understood and deserves to be investigated and alleviated in order to achieve stable mode-locking over long periods of time. It is particularly important for VECSEL systems meant to be commercialized, needing long term operation with a long product lifetime. Here, we investigate the performance and robustness of a SESAM-modelocked VECSEL system under intense pulse intensity excitation. The effect of the degradation on the VECSEL performance is investigated using the SESAM in a VECSEL cavity supporting ultrashort pulses, while the degradation mechanism was investigated by exciting the SESAMs with an external femtosecond laser source. The decay of the photoluminescence (PL) and reflectivity under high excitation was monitored and the damaged samples were further analyzed using a thorough Transmission Electron Microscopy (TEM) analysis. It is found that the major contribution to the degradation is the field intensity and that the compositional damage is confined to the DBR region of the SESAM.

Structure and cavity geometry optimization for ultrashort and high power pulse generation from a VECSEL

Here we present the gain and SESAM structure design strategy employed for the demonstration of ultrashort pulses and we present a comprehensive study outlining the influence of the cavity geometry on the pulse duration and peak power achievable with a state of the art VECSEL and SESAM structure. We will discuss the physical mechanisms limiting the output power with near 100fs pulses and we will compare experimental results obtained with different cavity geometries, including a V-shaped cavity, a multi-fold cavity, and a ring cavity in a colliding pulse modelocking scheme. The experimental results are supported by numerical simulations.

Pixelated GaSb solar cells on silicon by membrane bonding

We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates.We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates.

Modeling and experimental realization of modelocked VECSEL producing high power sub-100 fs pulses

A microscopic many-body theory driven design and optimization supports the experimental demonstration of sub-100 fs pulse duration directly from a semiconductor laser. A passively modelocked vertical external cavity surface emitting laser producing a pulse duration of 95 fs at a central wavelength of 1025 nm is demonstrated. The semiconductor gain and absorber structures used in the experiment are numerically optimized by modelling the pulse formation dynamic of the system. The resulting structure design is described in detail and the physical limitations in terms of pulse duration and power are discussed. Using a ring cavity geometry, a stable colliding pulse modelocking regime with an output power of 90 mW per beam at a repetition rate of 2.2 GHz is demonstrated. The output pulses are thoroughly characterized and are in good agreement with our predictive model.

Temperature and distance dependence of plasmon enhanced InAs/InGaAs/GaAs dot-in-a-well near IR emission

Using gold nanorods (AuNRs) as plasmonic structures to couple the InAs QD emitter for enhancing carrier generation and photon emission, temperature dependent PL emission over a range of 10K to room temperature has been studied systematically by varying the GaAs thickness.