Theses and Dissertations from UMD
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Item III-V Optoelectronic Devices: Room temperature CW operation of interband cascade laser & High efficiency p-side down InGaN/GaN solar cell(2011) Ryu, Geunmin; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During the past two decades, the field of III-V optoelectronic devices has gained widespread interest as a result of advances in the performance and reliability of epitaxial structures. In principle, III-V materials can provide sources, detectors and optoelectronic components over wavelengths from UV to IR. During my Ph.D study, I have focused on two III-V optoelectronic devices: Mid-IR interband cascade lasers and group III-Nitride solar cells. In the first part of this dissertation, we will discuss development of a room temperature CW operation interband cascade laser and in the second part, we will discuss the concept of high efficiency III-N solar cells. Part I Lasers that emit in the mid-IR (3~5um) spectral region can be used in many civilian and military applications such as chemical sensing, free space optical communication and IR countermeasures. There are three types of lasers that can cover the Mid-IR region. First, conventional type-I quantum well (QW) lasers on GaSb substrates, second, inter-subband quantum cascade lasers (QCLs) on InP substrates and finally interband cascade laser with type-II alignment of the conduction and valence bands on GaSb substrates. Gallium Antimonide based type II interband cascade lasers (ICLs) cover the 3~4 um wavelength range, and it is the most natural match to the mid-IR. For most applications, it is required that the laser operates in continuous wave (CW) mode either at room temperature or at temperatures accessible to thermoelectric coolers. Recently, we have been able to operate interband cascade lasers in CW mode at room temperature with 62mW of output power, internal loss of 4.8cm-1, 170mW/A slope efficiency, and a threshold current density as low as 300 A/cm2 which are a significant milestone toward many applications. In the first part of this thesis, we are going to talk about the fundamental principles of operation of the ICLs and their applications. Secondly, we will present the development of a fabrication process. Third, we will discuss the performance characteristics of ICLs. Lasers were characterized by doing series of length dependent pulsed/CW measurements to obtain critical parameters at low temperature and at room temperature; such as wall plug efficiency, threshold current density, internal loss, and thermal impedance. For low temperature CW measurement, a specially designed vacuum chamber was used to prevent water condensation. Finally, we will present ICL optimization processes. For laser optimization, we re-designed the device structure, in particular the lower cladding region, the injection region, and the active region thickness, to achieve a higher confinement factor and lower loss, thus increasing the operating temperature and the output power. Part II Since the 1950s, silicon solar cells have been intensively studied and developed. Solar cell technology has greatly benefited from the maturity of silicon technology developed originally for the IC industry. This has led to the development of high quality single crystal silicon wafers with low dislocation densities. However, because of the poor spectral overlap between the absorption of silicon cells and the spectrum of solar light, silicon solar cells cannot fundamentally produce high efficiency solar cells. In order to achieve high efficiency solar cells, researchers have investigated many alternatives including tandem cells, GaAs, and III-Nitride materials. In the second part of this thesis, we will talk about the development of high efficiency III-Nitride solar cells using novel p-side InGaN/GaN materials, including device background, new solar cell design, fabrication process development, and preliminary device characterizations.Item Design and fabrication of high-performance interband cascade tunable external cavity lasers(2010) Chryssis, Athanasios Nicholas; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The 3-4 &mum spectral region, which contains a number of important lines for chemical detection and other civilian and military applications, has proven to be one of the most challenging for semiconductor laser development. Recently alternative interband semiconductor diode laser technologies that have the potential to meet today's high power performance requirements have made groundbreaking progress. The interband cascade laser is one of the leading candidates in that particular spectral region and has recently been shown to operate continuously at room temperature, which is a significant milestone towards the widespread commercialization of the technology. In this work we are going to talk about the fundamental principles of operation of the interband cascade laser, its performance characteristics, and about the design and fabrication of an interband cascade external cavity laser that will impact the chemical sensing and free space communications industries. Our first goal for this work was to successfully develop a fabrication process for this new laser, based on a Gallium Antimonide (GaSb) material system. Based on our established Indium Phosphide (InP) process, we fine tuned our etching, dielectric deposition, metalization, annealing and mounting techniques in order to successfully produce ridge interband cascade lasers. Devices were then characterized by doing a series of length dependence experiments to determine critical physical parameters. Parameters like slope efficiency, threshold current density, internal loss, characteristic temperature, wall plug efficiency were extracted for different laser designs, as part of an optimization process of the quantum well structure of our interband cascade lasers. Lower cladding thickness, number of cascades, separate confinement heterostructure doping, heat dissipation and the confinement factor &Gamma, were found to be five of the most dominant effects affecting the device performance and high temperature operation. Based on our extensive dielectric coating experience for the near infra red, we were able to create new, broadband, anti reflection coatings specifically designed for the mid-IR. We created a simulation platform that designs double layer dielectric coatings. Based on the desired wavelength, spectral response, reflectivity and the modal nature of the laser's output beam, our program was able to predict coating designs, optimized for ultra low reflectivity. Our designs were grown in our e-beam evaporation facility, with very high control on the thicknesses and indices of refraction of the coating layers, using an in situ monitoring technique. Finally we were able to demonstrate wavelength tunability using our anti-reflection (AR) coated interband cascade lasers inside an external cavity configuration. This is the first demonstration of an interband cascade external cavity laser.Our tunable laser also demonstrated the widest to date tuning range of 300 nm (208 cm-1).