published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

<div>Achievements in the growth of ultra-pure III-V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two-dimensional electron gas (2DEG) embedded in GaAs/Al_xGa_1−xAs heterostructures provides an unparalleled platform for many-body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor-semiconductor heterostructures with ultra clean superconductor-semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non-abelian exchange statistics.</div><div><br></div><div><div>Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 × 10⁶ cm⁻²/V s. Utilizing an ultra-high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/Al_xGa_1−xAs heterostructures.</div></div><div><br></div><div>Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures grown by Molecular Beam Epitaxy (MBE). The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two-dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. <br></div><div><br></div><div>Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al_0.15In_0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al_0.15In_0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al_0.15In_0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.<br></div><div><br></div><div><br></div>

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Typical power outputs of commercially available diode lasers are on the order of 5 milliwatts. This thesis discusses the growth, processing and fabrication of high power (lOO's of milliwatts) diode lasers. Devices were grown by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD). The MOCVD diode lasers demonstrated room temperature laser operation with peak output powers of 450 mW/facet pulsed mode. The MBE diode lasers demonstrated room temperature pulsed laser operation of 110 mW/facet. The dynamics of the quantum well structure were studied. The carrier concentration, threshold current density and coatings were modeled. It was demonstrated through transmission line analogies that, depending on the thickness of the high reflective coating, the result would be a high output power diode laser or a superluminescent device. The MBE device was coated with a high power coating resulting in a peak power of 450 mW. The MOCVD device was used to study the superluminescence resulting from specific coatings. / Graduation date: 1992

The thesis focuses on the study of asymmetric multiple quantum wells(AMQWs) grown by molecular beam epitaxy (MBE) in the Riber Compact 21T MBE system. We investigate AMQW structures in which the well width is varied but the material compositions of the wells and the barriers are kept constant. Also, we have investigated AMQWs with p-type modulation doping at the barrier region and the AMQWs of different well widths without changing the well compositions. The AMQW samples are obtained the emission spectra by using photoluminescence (PL), electroluminescence (EL), photo-current and photoreflectance (PR) in the experiments. Also, The AMQW samples (AMQ100-70-40 and AMQ-100-MD70-40) are fabricated into laser diodes to obtain the characteristics of device in this study. The threshold current density Jth of laser diode is measured about 2 kA/cm2.The internal quantum efficiency £bi and the absorption £\ of AMQ-100-70-40 are 34.7% and 9.47 cm-1 respectively. The internal quantum efficiency £bi and the absorption £\ of AMQ-100-MD70-40 are 22.2% and 10.56 cm-1 respectively. Moreover, we present the InGaAsP AMQW samples grown by MOCVD to compare with the InGaAs/InAlGaAs AMQW samples. The broad-band property is valuable for application of optical communication. It is highly desirable to have broadly tunable lasers and broad-band semiconductor optical amplifiers (SOAs) in the 1.3 or 1.55£gm to handle more number of channels increasing the volume of information traffic for future optical communication networks. The band-band light source is also desirable in medical science for the optical coherence tomography (OCT).

Both non-polar (10-10) (m-plane) and polar (0001) (c-plane) zinc oxide (ZnO) have a good lattice match with lithium aluminum (LiAlO2, LAO) (200) substrate, so it is difficult to control the epitaxial orientation. Therefore, this research is to explore how the growth parameters influence on the crystal orientation of ZnO film grown by plasma assisted molecular beam epitaxy. The experimental results show that m-plane ZnO can be grown with low zinc flux and low oxygen pressure. Increasing zinc flux and oxygen pressure will lead to increase in growth rate, and consequently, c-plane ZnO will nucleate on the substrate besides m-plane zinc oxide. The substrate temperature is one of the main factors that influence the choice of zinc oxide epitaxial orientation. High temperature will promote the m-plane zinc oxide nucleation, while low temperature will conduct to the c-plane zinc oxide nucleation. Under low zinc flux and low oxygen pressure, epitaxy of ZnO with different crystalline orientations can be achieved through changing the substrate temperature. The surface morphology and roughness of the substrate will affect the particle size and surface morphology of ZnO epilayers. When the substrate is smooth, the crystal size of the epitaxial film is large and the surface is flat with many rectangular stripes, taking on the platform-like morphology. If the substrate is rough with many scratches, the particle size becomes small and the surface is granular-like and rather rough. Furthermore, when the substrate is rough, it is difficult to control the different orientations of ZnO epitaxial films through changing the substrate temperature.

The main work of this thesis is to design the TE-polarized SOA structures for booster amplifier, and the polarization-independent SOA structures for preamplifier at receiver end. In the SOA structure, we add a lattice-matched ternary compound InGaAs as an extra quantum well in separate-confinement heterostructure (SCH) layer. The purpose is to result in the band-filling/shrinkage and lead to change the absorption coefficient. Therefore, the refractive index change will be increased, and the structure can work as a Mach-Zehnder interferometer under reverse bias. We also added an electron barrier InAlAs layer to reduce the carriers accumulation in the extra InGaAs QW. After the epitaxy of MOCVD, this designed structure was processed to be a ridge laser. From the measurements of ridge laser, the barrier InAlAs could not efficiently stop the carrier injection into the extra InGaAs QW. The other part of this thesis is to set up a digital signal apparatus to analyze the RHEED pattern on the screen of the chamber. We make a connection between CCD camera and PC utilizing the framegrabber in RHEED system, and develop the programs from LabVIEW and IMAQ to obtain the functions we need. Further, from the tests of grabing and analysis for RHEED pattern, the digital signal system on RHEED pattern has been successfully demonstrated.

The thesis consists of two aspects: (1) Research on regrowth by molecular beam epitaxy, and (2) Silicon monoxide coating. In part one, we used (NH4)2Sx to passivate the InAlGaAs/InP surface. From the X-ray photoelectron spectroscopy (XPS), the passivated surface shows a dramatic reduce of oxidation. A preparation chamber for the regrowth has been setup to proceed the sulfur passivation method. We can obtain a clean surface for regrowth after heating and putting samples in the high vacuum chamber. In the design of regrowth layers, we have found the best waveguide structure by regrowth. When the ridge width is 2.5 mm with etching depth 1.4 mm, a circular mode profile can be obtained by Fimmwave simulation. In the integration between devices, we have designed the best waveguide structure after regrowth by BeamProp 3D. The best design will make the propagation loss smaller than 0.21%. The second part is anti-reflection (AR) coating by silicon monoxide (SiO) deposition. The SiO refractive index of 1.8837 was obtained by transmission, and ellipsometer measurements. The corresponding AR coating thickness for InP substrate is 2057 &#x00C5;. In order to make AR coating on lasers of different effective index, we design the double-layer coating. For Beam Expander Semiconductor Optical Amplifier (BESOA), SiO2 / SiO and Si3NX / SiO double-layer coatings were compared with SiO single layer. The reflectance (R) was reduced 16.86 % and 25.12 %, respectively, and the R < 1% bandwidth extends 200 &#x00C5;.

Gamma-phase lithium aluminate (LiAlO2) single crystal is grown by Czochralski pulling method and a-plane LiAlO2(LAO) is chosen as the substrate for subsequent gallium nitride (GaN) epitaxial growth by plasma-assisted molecular beam epitaxy (PAMBE). The lattice mismatch between the nitride and the substrate is greatly reduced due to small lattice mismatch of~0.3% between [0001]GaN and [010]LAO and of~1.7% between [11-20]GaN and [001]LAO in the plane of the substrate LAO(100). Pure hexagonal [10-10]GaN is successfully grown directly on the LAO substrate without buffer layer. Crystal quality and properties are analyzed through a series of measurements, including reflection high-energy electron diffraction (RHEED), field-emission electron microscopy (FESEM), electron backscatter diffraction (EBSD), x-ray diffraction and cathodo luminescence (CL).