III-V Optoelectronic Devices: Room temperature CW operation of interband cascade laser & High efficiency p-side down InGaN/GaN solar cell
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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.
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.
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.