Gregory Triplett

Charge storage characteristics of ultra-small Pt nanoparticle embedded GaAs based non-volatile memory

Charge storage characteristics of ultra-small Pt nanoparticle embedded devices were characterized by capacitance-voltage measurements. A unique tilt target sputtering configuration was employed to produce highly homogenous nanoparticle arrays. Pt nanoparticle devices with sizes ranging from ∼0.7 to 1.34 nm and particle densities of ∼3.3–5.9 × 1012 cm−2 were embedded between atomic layer deposited and e-beam evaporated tunneling and blocking Al2O3 layers. These GaAs-based non-volatile memory devices demonstrate maximum memory windows equivalent to 6.5 V. Retention characteristics show that over 80% charged electrons were retained after 105 s, which is promising for device applications.

Reduced auger recombination in mid-infrared semiconductor lasers

A quantum-design approach to reduce the Auger losses in λ = 2 μm InGaSb type-I quantum well edge-emitting lasers is reported. Experimentally realized structures show a ∼3 × reduction in the threshold, which results in 4.6 × lower Auger current loss at room temperature. This is equivalent to a carrier lifetime improvement of 5.7 × and represents about a 19-fold reduction in the equivalent “Auger coefficient.”

Strained Active Regions in GaAs-based Quantum Cascade Lasers

This paper explores the effects of low indium concentration in deep-well AlGaAs/GaAs quantum cascade laser structures. Photon emission from these strained active regions involves nonlinear optical processes, which reduce the emission wavelength. Designs that incorporate strain in one and two wells of the active region, respectively, on (111) GaAs are compared. Results demonstrate wavelength extendibility in compressively strained deep-well GaAs-based devices.

Pseudomorphic growth of InAs on misoriented GaAs for extending quantum cascade laser wavelength

The authors have studied the impact of epilayer strain on the deposition of InAs/GaAs on (100) and (111)B with 2° offset toward⟨2-1-1⟩ surfaces. Consequences of a 7% lattice mismatch between these orientations in the form of three-dimensional growth are less apparent for (111)B with 2° offset toward⟨2-1-1⟩ surfaces compared to (100). By exploring a range of molecular beam epitaxy process parameters for InAs/GaAs growth and utilizing scanning electron microscopy, atomic force microscopy, and Raman spectroscopy to evaluate the quality of these strained layers, the authors develop empirical models that describe the influence of the process conditions in regards to surface roughness with >92% accuracy. The smoothest InAs/GaAs samples demonstrated average surface roughness of 0.08 nm for 10 μm2 areas, albeit at very low deposition rates. The authors have found the most important process conditions to be substrate temperature and deposition rate, leading us to believe that controlling diffusion length may be th...

Orientation-dependent pseudomorphic growth of InAs for use in lattice-mismatched mid-infrared photonic structures

In this study, InAs was deposited on GaAs (100) and GaAs (111)B 2° → ⟨2-1-1⟩ substrates for the purpose of differentiating the InAs growth mode stemming from strain and then analyzed using in-situ reflection high energy electron diffraction, scanning electron microscopy, Raman spectroscopy, reflectance spectroscopy, and atomic force microscopy. The procession of InAs deposition throughout a range of deposition conditions results in assorted forms of strain relief revealing that, despite lattice mismatch for InAs on GaAs (approximately 7%), InAs does not necessarily result in typical quantum dot/wire formation on (111) surfaces, but instead proceeds two-dimensionally due primarily to the surface orientation.

Infrared Transmission Characteristics and Laser Tissue Interaction of Albino Pigskin Using Pulsed NIR Laser Light

This work explores near infrared transmission through albino pigskin and determines controllable factors that influence transmission efficiency. Pigskin samples of varying thicknesses were irradiated using a 1440 nm near-infrared laser diode, where a photodetector was used to measure the transmitted power, and a two-dimensional real time surface temperature distribution was recorded using infrared thermography. Results demonstrate that this technique could potentially lead to a noninvasive approach for enhancing wound healing.

Strain monitoring in InAs–AlxGa1−xAsySb1−y structures grown by molecular beam epitaxy

A study of strained InAs–AlxGa1−xAsySb1−y quantum well structures produced by molecular beam epitaxy is presented. The ability to manipulate quantum well strain by way of the AlxGa1−xAsySb1−y buffer is examined using statistical experimental design. Results show that anion composition in the buffer (with a target lattice constant, a=6.12A) varies by as much as 3% in the 450–500°C growth temperature range. The data reveal interrelationships between strain, structural characteristics, and conductivity. Results demonstrate that these relationships exist and can be modeled empirically and exploited for the design of near-infrared optoelectronic devices.

Composition and surface deformation variance in highly strained InxGa1−xSb structures on (100) GaSb

InxGa1−xSb is a ternary semiconductor material that offers excellent electronic properties as well as a widely tunable bandgap range (1.7–7.3 μm). However, because of the potentially large lattice mismatch between InxGa1−xSb and GaSb (up to ∼6%), it is inherently difficult to produce large area, high-quality, defect-free InxGa1−xSb epilayers. Studying crystal deformation processes that ultimately enable gliding dislocations in InxGa1−xSb epilayers, as well as the morphologies that result from these processes, is critical for controlling quantum properties in InxGa1−xSb devices. In this study, InxGa1−xSb nanostructures were produced by a solid-source molecular beam epitaxy on undoped GaSb (100) substrates and were examined using various techniques including scanning electron microscopy, energy dispersive spectroscopy, and (micro) Raman spectroscopy. Characterization data demonstrates that with increasing lattice mismatch (compressive strain), there are two distinct regions across the sample, specifically a...

Infrared irradiation of skin for the development of non-invasive health monitoring technologies

Infrared radiation was employed to study the optical transmission properties of pigskin and the factors that influence transmission at room temperature. The skin samples from the forehead of piglets were irradiated using an infrared-pulsed source by varying the beam properties such as optical power, power density, duty cycle, as well as sample thickness. Because infrared radiation in select instances can penetrate through thick-fleshy skin more easily than visible radiation, temperature fluctuations observed within the skin samples stemming from exposure-dependent absorption revealed interesting transmission properties and the limits of optical exposure. Pigskin was selected for this study since its structure most closely resembles that of human skin. Furthermore, the pulsed beam technique compared to continuous operation offers more precise control of heat generation within the skin. Through this effort, the correlated pulsed-beam parameters that influence infrared transmission were identified and varied to minimize the internal absorption losses through the dermis layers. The two most significant parameters that reduce absorption losses were frequency and duty cycle of the pulsed beam. Using the Bouger-Beer-Lambert Law, the absorption coefficient from empirical data is approximated, while accepting that the absorption coefficient is neither uniform nor linear. Given that the optical source used in this study was single mode, the infrared spectra obtained from irradiated samples also reveal characteristics of the skin structure. Realization of appropriate sample conditions and exposure parameters that reduce light attenuation within the skin and sample degradation could give way to novel non-invasive measuring techniques for health monitoring purposes.

Improved optical resonance in mid-infrared GaAs-based modulating retro-reflectors

In this work, we studied a mid-infrared modulating retro-reflector (MRR) design that is GaAs-based because of the flexibility to monolithically incorporate reflective optics along with quantum well modulator region. Using solid-source molecular beam epitaxy, we produced MRR devices, where the GaAs quantum well(s) in the modulator region contained AlxGa1-xAs barriers to tune the wavelength selectivity beyond three microns. The width of the quantum well was also adjusted in order to achieve free electron absorption within the confined energy subbands and modified by way of the quantum confined Stark effect. When the applied electric field varies in polarity, intensity, or frequency, the fabricated MRRs behave as an optional shutter--absorbing or transmitting the incident mid-infrared energy depending on the applied field. Our work shows that the ability for the modulating region to effectively act as a shutter for mid-infrared radiation depends on the number of cascading quantum wells, though increasing the number of wells directly increases the overall thickness of the modulating region and adversely affects the reflected power of the mid-infrared modulated beam. The shutter operation was achieved by applying an alternating square bias across the QWM region, and the speed at which the quantum wells switch from absorbing to non-absorbing was dependent on the physical size of the device. Increasing the physical size increases the associated device capacitance. The maximum achievable contrast ratio for these devices is calculated to be 1.6:1 for applied voltages between 12V and 25V.