III-Nitride Vertical-Cavity Surface-Emitting Lasers: Growth, Fabrication, and Design of Dual Dielectric DBR Nonpolar VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from ~100 kA/cm2 to ~3 kA/cm2, while simultaneously increasing the output power from ~10 µW to ~550 µW. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs

Growth of Group IV and III-V Semiconductor Materials for Silicon Photonics: Buffer Layer and Light Source Development

High data transmission speeds, high levels of integration, and low manufacturing costs have established Si photonics as a crucial technology for next-generation data interconnects and communications systems. It involves a variety of components including light emitters, photodetectors, amplifiers, waveguides, modulators, and more. Because of its indirect bandgap, silicon is unable to serve as an efficient light source on a chip, hence this has been one of the formidable challenges. Within the framework of the monolithic approach, this thesis presents the study of two essential aspects of this challenge, the optimisation of buffer layers and development of light sources, by incorporating and improving different systems of Group IV thin films and III-V quantum dots (QDs) semiconductor materials. The monolithic approach focuses on the direct epitaxial growth of highly efficient light sources, usually by the epitaxy of III-V semiconductors lasers on a single Si chip. However, because of the material dissimilarities between III-V materials and Si, during the heteroepitaxy, a high density of crystalline defects such as threading dislocations (TDs), thermal cracks and anti-phase domains are introduced, severely impeding the performance and yield of the laser. For instance, TDs act as non-radiative recombination centres, while thermal cracks cause issues with the efficient evanescent coupling of the emitted light with Si waveguide. To address these defects, typically complex buffer growth techniques with micron-scale thickness are employed. The research in this thesis is divided into two parts, namely buffer layer optimisation and light source development. Each part outlines alternative strategies for overcoming the above-mentioned hurdles for monolithic growth. The first part highlights the optimisation of buffer layer growth to reduce threading dislocations for the monolithic integration of high-performance direct-bandgap III-V and group IV light sources on Si. The growth optimisation of low defect-density Ge buffer layers epitaxially grown on Si was first investigated. Defect elimination in Ge buffers with doped and undoped seed layers of increasing total thickness is studied under a variety of growth regimes, doping techniques, and annealing processes. This study demonstrates that a 500 nm thin Ge achieves the same defect level (1.3 × 108 cm -2) as 2.2 μm GaAs grown on Si, which greatly increases the thickness budget for the subsequent dislocation filter layers (DFLs) and laser structure growth before the formation of thermal cracks. Meanwhile, a low threading dislocation density of 3.3 × 107 cm -2 is obtained for 1 μm Ge grown on Si. The second part places emphasis on the development of light sources in the near-infrared wavelength range for Si photonics. 1) The development of GeSn, an emerging direct bandgap light source for Si photonics, is shown, which has wide bandgap tuneability and full compatibility with Si complementary metal-oxide semiconductor (CMOS). Growing the high Sn composition of GeSn required for efficient light generation is challenging and its growth generally severely affected by large surface roughness and Sn segregation. In this work, first, ex-situ rapid thermal annealing for the grown GeSn layer is investigated, showing that by proper annealing the strain can be relaxed by 90% without intriguing Sn segregation. This method shows its potential for both material growth and device fabrication. Besides, strain compensated layer and in-situ annealing techniques have been developed. Significantly improved surface quality has been confirmed by in-situ reflection high-energy electron diffraction (RHEED) observations and atomic force microscopy (AFM) images. Transmission electron microscopy (TEM) results reveal the high crystal quality of the multiple quantum wells (MQWs) grown on such buffer layers. 2) The final section details the development of InAs/InP QDs emitting near the strategic 1.55 μm, the lowest optical fibre loss window. The InAs/InP QDs growth is prone to inhomogeneous quantum dash morphologies which broaden the photoluminescence (PL) spectra and degrade the carrier confinement. Research has been conducted on growth parameters and techniques including deposition thickness, growth temperature and Indium-flush technique is applied to improve the uniformity of the dots, and narrow room temperature PL linewidths of 47.9 meV and 50.9 meV have been achieved for single-layer and five-layer quantum dot samples, respectively. The structures enable the fabrication of small footprint microdisk lasers with lasing thresholds as low as 30 μW

The development of components for ultrafast all-optical communication networks

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (p. 149-158).The need for ultrafast (greater than 100 Gbps) all-optical communication networks is amplified as the amount of data-containing communication traffic continues to grow at an exorbitant rate. Multiplexing schemes are attractive, as they enable increased transmission over silica fiber already buried and in use. One of the key components enabling Wavelength Division Multiplexed (WDM) propagation is the distributed feedback (DFB) laser. A periodic index modulation, typically buried within the device structure (hence requiring an overgrowth step), is used to select the specific operating wavelength of the laser. As the WDM wavelength spacing continues to decrease, the increasing demands placed on the fabrication of the DFB lasers bring the fabrication issues to the forefront. Similar patterned surface overgrowth issues need to be addressed for realization of wavelength-selective filters, e.g. Bragg-resonant filters, for all-optical routing. Issues pertaining to the formation of buried index contrasts, for application to future WDM network components, are addressed. Of importance to Time Division Multiplexed (TDM)-based propagation schemes is the multiplexing and demultiplexing of the ultrafast data streams. All-optical switching as well as optical (de)multiplexing are very attractive alternatives to optoelectronic conversion. Many optical switching schemes require the use of a semiconductor optical amplifier (SOA) as the nonlinear medium, as well as optical clock sources with repetition rates on the order of GHz (for integration with electronic components). For TDM network propagation rates on the order of Tbps, lasers must be capable of producing pulses shorter than a picosecond. Mode-locking of laser cavities is an attractive method of ultrashort pulse generation. A semiconductor saturable absorber mirror is a monolithically integrated device utilized for passive mode-locking of a wide variety of lasers. The development of SOAs as well as semiconductor saturable absorber mirrors is discussed.Elisabeth Marley Koontz.Ph.D

Advanced Photovoltaic Devices Enabled by Lattice-Mismatched Epitaxy

Thin-film III-V semiconductor-based photovoltaic (PV) devices, whose light conversion efficiency is primarily limited by the minority carrier lifetimes, are commonly designed to minimize the formation of crystalline defects (threading dislocations or, in extreme cases, fractures) that can occur, in particular, due to a mismatch in lattice constants of the epitaxial substrate and of the active film. At the same time, heteroepitaxy using Si or metal foils instead of costly III-V substrates is a pathway to enabling low-cost thin-film III-V-based PV and associated devices, yet it requires to either use metamorphic buffers or lateral confinement either by substrate patterning or by growing high aspect ratio structures. Mismatched epitaxy can be used for high-efficiency durable III-V space PV systems by incorporation of properly engineered strained quantum confined structures into the solar cells that can enable bandgap engineering and enhanced radiation tolerance. One of the major topics covered in this work is optical and optoelectronic modeling and physics of the triple-junction solar cell featuring planar Si middle sub-cell and GaAs0.73P0.27 and InAs0.85P0.15 periodic nanowire (NW) top and bottom sub-cells, respectively. In particular, the dimensions of the NW arrays that would enable near-unity broad-band absorption for maximum generated current were identified. For the top cell, the planarized array dimensions corresponding to maximum generated current and current matching with the underlying Si sub-cell were found to be 350 nm for NW diameter and 450 – 500 nm for NW spacing. For the GaAs0.73P0.27, resonant coupling was the main factor driving the absorption, yet addressing the coupling of IR light in the transmission mode in the InAs0.85P0.15 nanoscale arrays was challenging and unique. Given the nature of the Si and bottom NW interface, the designs of high refractive index encapsulation materials and conformal reflectors were proposed to enable the use of thin NWs (300 – 400 nm) for sufficient IR absorption. A novel co-simulation tool combining RSoft DiffractMOD® and Sentaurus Device® was established and utilized to design the p-i-n 3D junction and thin conformal GaP passivation coating for maximum GaAs0.73P0.27 NW sub-cell efficiency (16.5%) mainly impacted by the carrier surface annihilation. Development of a highly efficient GaAs solar cell enhanced with InxGa1-xAs/GaAsyP1-y quantum wells (QWs) is also demonstrated as one of the key parts of the dissertation. The optimizations including design of GaAsP strain balancing that would support efficient thermal (here, 17 nm-thick GaAs0.90P0.10 for 9.2 nm-thick In0.10Ga0.90As QWs) and/or tunneling (4.9 nm-thick GaAs0.68P0.32) carrier escape out of the QW while maintaining a consistent morphology of the QW layers in extended QW superlattices were performed using the principles of strain energy minimization and by tuning the growth parameters. The fundamental open-circuit voltage (V¬oc) restraints in radiative and non-radiative recombination-limited regimes in the QW solar cells were studied for a variety of InxGa1-xAs compositions (x=6%, 8%, 10%, and 14%) and number of QWs using spectroscopic and dark current analysis and modeling. Additionally, the design and use of distributed Bragg reflectors for targeted up to 90% QW absorption enhancement is demonstrated resulting in an absolute QW solar cell efficiency increase by 0.4% due to nearly doubled current from the QWs and 0.1% enhancement relatively to the optically-thick baseline device with no QWs

18th Space Photovoltaic Research and Technology Conference

The 18th Space Photovoltaic Research and Technology (SPRAT XVIII) Conference was held September 16 to 18, 2003, at the Ohio Aerospace Institute (OAI) in Brook Park, Ohio. The SPRAT conference, hosted by the Photovoltaic and Space Environments Branch of the NASA Glenn Research Center, brought together representatives of the space photovoltaic community from around the world to share the latest advances in space solar cell technology. This year s conference continued to build on many of the trends shown in SPRAT XVII-the continued advances of thin-film and multijunction solar cell technologies and the new issues required to qualify those types of cells for space applications

Photonic integrated circuits for optical logic applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.The optical logic unit cell is the photonic analog to transistor-transistor logic in electronic devices. Active devices such as InP-based semiconductor optical amplifiers (SOA) emitting at 1550 nm are vertically integrated with passive waveguides using the asymmetric twin waveguide technique and the SOAs are placed in a Mach-Zehnder interferometer (MZI) configuration. By sending in high-intensity pulses, the gain characteristics, phase-shifting, and refractive indices of the SOA can be altered, creating constructive or deconstructive interference at the MZI output. Boolean logic and wavelength conversion can be achieved using this technique, building blocks for optical switching and signal regeneration. The fabrication of these devices is complex and the fabrication of two generations of devices is described in this thesis, including optimization of the mask design, photolithography, etching, and backside processing techniques. Testing and characterization of the active and passive components is also reported, confirming gain and emission at 1550 nm for the SOAs, as well as verifying evanescent coupling between the active and passive waveguides. In addition to the vertical integration of photonic waveguides, Esaki tunnel junctions are investigated for vertical electronic integration. Quantum dot formation and growth via molecular beam epitaxy is investigated for emission at the technologically important wavelength of 1310 nm. The effect of indium incorporation on tunnel junctions is investigated. The tunnel junctions are used to epitaxially link multiple quantum dot active regions in series and lasers are designed, fabricated, and tested.by Ryan Daniel Williams.Ph.D