Research Project

Design and implementation of a home-built tunable 3.4um measurement setup

Our setup is quite unique since we set up a tunable 3.4um light source, where, we leveraged nonlinear optical properties of periodically poled crystals. The crystal is a custom fabricated Periodically Poled Congruent LiNbO3. This crystal is used for non-linear frequency conversion. Here, we are particularly interested in difference frequency generation (DFG) and hence the grating size is designed so we can generate 3.4um wavelength.

Design and fabrication of optical components for mid-IR (Silicon Photonics)

I fabricated and simulated passive optical compoents using an SOI wafer with 500nm device layer thickness(Soitech). Both e-beam lithography (Elionix ELS-7500EX) and direct laser writer (Heidelberg DWL 66+) were used for lithography.Si etch was done using Deep RIE SPTS Rapier Si DRIE and Oxford 80 Plus to pattern the components.

Thermoelectric and Thermoionic Phenomenon

Solid-state thermionic energy conversion can be more efficient than conventional thermoelectric energy conversion based on bulk Peltier and Seebeck effects, if the thermionic barriers can be properly engineered. However, there have been relatively few studies on solid-state thermionic energy conversion, mainly because of the difficulty of fabricating interfaces with the appropriate energy barriers, characterizing thermal transport across these interfaces, and separating the bulk thermoelectric properties from the interfacial properties. 2D Layered heterostructures (e.g., graphene-MoS2, grapheme-WSe2) enable us to overcome these difficulties, and can potentially create a paradigm shift in the design of thermoelectric power generators and coolers with high efficiency. We aimed to optimizing and measuring the thermoelectric figure of merit (ZT) of these novel devices.

Estimation of doping concentration of n-type GaAs nanowires (NW)

Photoluminescence properties of n-type doped GaAs nanowires, grown by the MOCVD method are measured at 4K. These nanowires are grown at different temperatures (760˚C and 780˚C). Our measurements indicate that increase in carrier concentration increases the complexity of the optical response, attributed to formation of different recombination centers and mechanism. At high carrier concentrations, I observe a blue shift in the band gap energy by up to 25meV. The spectra indicated doping carrier concentrations for various cases, which vary from 6x1017 cm-3 (lightly doped), 1.5x1018 cm-3 (moderately doped), to 3.5x1018 cm-3 (heavily doped). I find that the growth temperature variation does not affect radiative recombination mechanism, however, it does lead to a slight enhancement in the optical emission intensities.

Observation of Fabry-Perot Resonance along GaAs Nanowires

We report substantial improvements in the photoluminescence (PL) efficiency and Fabry-Perot (FP) resonance of individual GaAs nanowires through surface passivation and local field enhancement, enabling Fabry-Perot peaks to be observed even at room temperature. For bare GaAs nanowires, strong FP resonance peaks can be observed at 4K, but not at room temperature. However, depositing the nanowires on gold substrates leads to substantial enhancement in the PL intensity (5X) and 373% to infinite enhancement of FP peaks. Finite difference time domain (FDTD) simulations show that the gold substrate enhances the PL spectra through enhanced absorption rather than enhanced emission. Alternatively, the surface states in the nanowires can be passivated by either an ionic liquid (EMIM-TFSI) or an AlGaAs surface layer to achieve up to 12X enhancement of the photoluminescence intensity and observation of FP peaks at room temperature by reducing non-radiative surface recombination.

Formation of Fabry-Perot cavity in GaAs nanosheets

GaAs nanosheets with no twin defects, staking faults, or dislocations are excellent candidates for optoelectrical applications. Their outstanding optical behavior and twin free structure make them superior to traditionally studied GaAs nanowires. While many research groups have reported optically resonant cavities (i.e., Fabry-Perot) in 1D nanowires, here, I report an optical cavity resonance in GaAs nanosheets consisting of complex 2D asymmetric modes, which are fundamentally different from one-dimensional cavities. These resonant modes are detected experimentally using photoluminescence (PL) spectroscopy, which exhibits a series of peaks or “fringes” superimposed on the bulk GaAs photoluminescence spectrum. Finite-difference time-domain (FDTD) simulations confirm these experimental findings and provide a detailed picture of these complex resonant modes. Here, the complex modes of this cavity are formed by the three non-parallel edges of the GaAs nanosheets. Due to the asymmetrical nature of the nanosheets, the mode profiles are largely unintuitive. I also find that by changing the substrate from Si/SiO2 to Au, I enhance the resonance fringes as well as the overall optical emission by 5X at room temperature. Our FDTD simulation results confirm that this enhancement is caused by the local field enhancement of the Au substrate and indicate that the thickness of the nanosheets plays an important role in the formation and enhancement of fringes.

Effective AlGaAs passivation of GaAs nanosheets (NS)

Unlike nanowires, GaAs nanosheets exhibit no twin defects, stacking faults, or dislocations even when grown on lattice mismatched substrates. As such, they are excellent candidates for optoelectronic applications, including LEDs and solar cells. I report substantial enhancements in the photoluminescence efficiency and the lifetime of passivated GaAs nanosheets produced using the selected area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). Measurements are performed on individual GaAs nanosheets with and without an AlGaAs passivation layer. Both steady state photoluminescence and time-resolved photoluminescence spectroscopy are performed to study the optoelectronic performance of these nanostructures. Our results show that AlGaAs passivation of GaAs nanosheets leads to a 30- to 40-fold enhancement in the photoluminescence intensity. The photoluminescence lifetime increases from less than 30ps to 300ps with passivation, indicating an order of magnitude improvement in the minority carrier lifetime. I attribute these enhancements to the reduction of non-radiative recombination due to the compensation of surface states after passivation. The surface recombination velocity decreases from an initial value of 2.5x105 cm/s to 2.7 x104 cm/s with passivation.

Hydrothermal synthesis of non-doped, n-type and p-type doped TiO2

I presented a low-temperature, hydrothermal synthesis method for Ta-doped TiO2. Here, alkoxide-based precursors are mixed at low temperatures to suppress differential hydrolysis and phase separation. This method ensures homogeneous, molecular mixing of the Ta-dopant with the native oxide up to a concentration of ca. 2.5 at.%. XRD and EDS analyses confirm a uniformly doped rutile TiO2. SEM and TEM analyses reveal a highly branched structure. Optoelectronic properties of these structures were investigated using UV-VIS spectroscopy and low-temperature photoluminescence.

First Principle Calculation of Stability and Effects of Doping on Sulfur-doped TiO2

I present the band gap reduction effect of sulfur doping on TiO2 in anatase phase. This study is based on Density Functional Theory (DFT). For these calculations, several types of supercells consisting of 48 atoms in anatase phase are used to study the optoelectronic properties and band gap energy of sulfur-doped TiO2. The band gap reduction effect of sulfur doping as a function of concentration is also studied here. The most stable substitution site for sulfur is predicted based on theoretical calculations. Based on the previous experimental results and the recent theoretical calculations in this paper, it is proven that sulfur doping is a promising approach for band gap reduction of TiO2 for a wide variety of energy-based applications.