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Thesis advisor: Pradip Bakshi / This thesis is a theoretical study of the electron transport and response properties of epitaxially grown, low-dimensional semiconductor quantum well heterostructures, under steady-state, current driven (nonequilibrium) conditions. These structures operate in the Terahertz (THz) frequency and submillimeter wavelength range, and are the leading candidates for compact, coherent sources of THz radiation. This work is divided into two parts: Part I consists of an analytical study of the individual quantum well units, and the tunneling transmission characteristics, for which reasonably accurate algebraic expressions are obtained. An underlying philosophy of this work is the desire to describe each of the key components involved, independently, through these simple analytical expressions. In Part II the numerical study of the transport and radiation response of the quantum well structures specially designed to generate THz radiation based on the plasma instability concept is presented. Several models are proposed which describe the overall electron transport and which determine the underlying nonequilibrium steady state. In particular, the key features of the experimental current-voltage (IV) curves for such structures are explained, and the corresponding response properties are determined. The modeling and simulation of these potential optoelectronic devices is a crucial tool for elucidating the precise mechanisms and interplay of the many microscopic processes which give rise to the observed behavior. Key features of the radiation response arise from the intersubband plasma instability which occurs due to the resonant interaction of an emission and an absorption mode, and these features are compared with the experimental observations. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

I describe mid/far-infrared photodetectors based on frequency upconversion in a near-resonant cascade of interband and intersubband transitions in high optical non-linearity asymmetric quantum well structures. Such structures can yield high detectivity and responsivity in the mid/far-infrared range. Resonant uponversion detectors can be designed for both collinear and perpendicular pump and signal beams. They can be integrated with semiconductor pump lasers to yield compact devices. Single photon counting is also achieved by these detectors. I present specific device designs based on GaAs/AlGaAs and InGaAs/AlInAs heterostructures and calculations of their expected figures of merit. This includes a study of the intersubband nonlinear absorption of asymmetric double quantum wells designed for mid/far-IR range. The dependance of second order nonlinear susceptibility on various parameters of the structure is studied. In particular, different values for barrier and well widths are considered. The nonlinear absorption can be obtained by using perturbative calculation of the linear susceptibility up to second order with density matrix approach. The intersubband linear and nonlinear asymmetric double quantum well can be tuned using two design parameters. One is the width of the barrier between the wells that controls the coupling and the second is the width of the narrow well that controls the asymmetry of the structure. As the barrier width narrows the energy gap at the anticrossing increases. The asymmetry of the two well potentials is essential for sum frequency and difference frequency generation since in a symmetric well hZ31i = 0 due to the same parity of the ground and second excited states so that Â(2) = 0. In our detection scheme using frequency up-conversion we demonstrate that these devices can achieve high detectivity, very low noise and high value for Â(2) hence good efficiency. This can be an important advantage for low signal detection and single photon counting.

There are two sections in this thesis, the first section we measured the photoluminescence (PL) spectra¡Bphotocurrent spectra and electro absorption spectra of InGaAsN single quantum well structures grown by MBE. From temperature-dependent PL spectra of InGaAsN, we observed a localized level at low temperature, and the carrier localization effect increases when the mole fraction of nitrogen increases (2.1%~3.25%). This peculiarity influences the PL peak position and the PL linewidth, and it can be improved by adequate annealing. We also obtained the activation energies about 52~59meV by Arrhenius plot and thermal quenching model. For the photocurrent spectra we observe the sub-band transition and quantum confined stark effect. From the electro-absorption spectra, we obtain the maximum absorption changes (

In this thesis, we have set up a SiO2 sputter system. The system includes sputter gun, RF power supply, RF power controller, turbo pump, quick access door, cooling water tubes, gas lines, electric circuits etc. We applied sputter techniques for quantum well intermixing (QWI) process. We can adjust the pressure, gas, RF power etc. of the sputter system to fit the best QWI conditions and then sputter a SiO2 film on the samples. The samples with multiple quantum wells were grown by our team members using molecular beam epitaxy system. After SiO2 film deposition, the samples were annealed by Rapid Thermal Process. The annealing temperatures are about 650¢J-750¢J. Following the thermal annealing, room-temperature PL measurements were used to study the blue shift and intensity change after QWI process. After our hard working, we had fixed many problems of sputter system. We have obtained useful data through many QWI experiments. Our results are listed as follows : PL intensity : We use RF power = 100W, sputter time = 5 min., annealing temperature = 675¢J, annealing time = 30 sec. PL intensity has been enhanced by 25 times. Blue shift : there is no clear blue shift.

In this thesis, we have set up a SiO2 sputter system. We applied sputter techniques for quantum well intermixing (QWI) process to increase the bandgap of the quantum well structure. The samples with multiple quantum wells were grown by our team members using molecular beam epitaxy system, and the MQW structure were grown by MOCVD. Before sputtering, some samples will use ICP to enhance the vacancy on the surface. First, we will sputter SiO2 on the surface of sample. After SiO2 film deposition, the samples were annealed by Rapid Thermal Process. And ordinary annealing were about 700¢J~800¢J. A later period, the annealing temperatures will be above 850¢J. Room-temperature PL (Photoluminescence) measurements were used to study the blue shift and intensity change after QWI process. And we will do the mesa process to measure the characteristic of optoelectronics. If the conditions are RF power = 100W, sputter time = 30 min, ICP enhance 250W for 2 min, annealing temperature = 825¢J, annealing time = 60 sec. The PL signal have a blue shift of 64nm(wavelength from 1506nm to 1444nm).When annealing temperature =700¢J, and annealing time = 60 sec, we have a blue shift of 12nm(wavelength from 1572nm to 1560nm) on the C116 sample. We do the mesa process on the MQW which contain P and it have a large blue shift. After the process, we success to compare the different between EL and Photocurrent. But the structure of samples which certain Al do not have a apparent blue shift. And the annealing temperature is too large, samples will be damaged. We think that the reason have relation to materials of the sample.

We measure the energy gap of two-dimensional electron gas in AlAs0.56Sb0.44/Ga0.47In0.53As at low temperature by photoluminescence measurement. We intend to observe the intersubband transition in these samples, especially the first and second subband. We have discovered that the sample ( AlAs0.56Sb0.44/Ga0.47In0.53As ) has three subband by SdH and electron density will increase with illumination time. Although we did not observe the intersubband transition in the experiment, we did know the performance and controls about TRIAX320 monochrometer. It means we could measure the better result in further.

Abstract The purpose of this thesis is to fabricate l = 1.55mm ridge waveguide spot size converter lasers for high laser-to-fiber coupling. The laser structure design includes the lateral and vertical directions. In the vertical aspect, we use six asymmetric InGaAsP/InP quantum wells of 50, 50, 100, 100, 150, 150Å. The asymmetric multi-quantum-well structure is designed to gather the optical field in the 150Å-wide quantum wells. Besides, we add a passive waveguide near the 150Å-wide quantum wells to expand optical field profile. In the lateral aspect, we use tapered ridge waveguides which have two different fashions: one is tapered down from 2.5mm to 1mm, and the other is tapered up from 2.5mm to 4mm. The results show that the tapered-up ridge waveguide laser has a CW operation with Ith = 34mA and diverge angles of 14¢X¡Ñ36¢X (q//¡Ñq^) at room temperature. The tapered-down ridge waveguide laser exhibits only a pulsed operation at room temperature. At lower temperature T = 13¢XC, the tapered-down laser has a CW operation of Ith = 36mA and diverge angles of 14¢X¡Ñ36¢X(q//¡Ñq^). From I-V and C-V measurements, we attribute the heating problem to the large series resistance caused by the incorrect n-type doping profile in the epitaxy process.