Transition Rate Enhancement in Reduced Dimensional Optoelectronic Devices

Junctions of dissimilar semiconductors, heterojunctions, have been used in a variety of electronic, photonic, and optoelectronic devices. A prime example is the invention of Double Heterojunction (DH) lasers by H. Kroemer and Z. Alferov which was awarded the Nobel Prize in Physics in 2000 for "laying the foundation of modern information technology." When the distance between the two heterojunctions is reduced to a few nanometers, electron motion is confined to two dimensions resulting in a Quantum Well (QW). QW lasers which followed DH lasers show much better optoelectronic properties such as higher efficiency, narrower linewidth, and smaller threshold current. In this thesis, we analyze the effect of dimensionality on the absorption and emission properties of optoelectronic devices. We compare GaAs/AlGaAs DH, Multiple Quantum Well (MQW) and Quantum Wire Lasers where the active region is made of GaAs in each device. Specifically, we calculate the spontaneous emission rate and optical gain of these devices. This analysis helps to determine the underlying physics for the significant increase in the transition rates and optical gain of these devices due to electron confinement. We examine band-to-band optical transition rates for (a) 3D case of DH lasers, (b) 2D case of QW lasers, and (c) 1D case of Quantum wires, and derive and compare, spontaneous emission spectra and the gain coefficient for the three cases. We identify how dimensionality enhances emission spectra through modification of the Joint Optical Density of States (JODS) and oscillator strength. Finally, this study helps explain why heterojunction-based core-shell nanowires show enhanced optoelectronic properties compared to core-only, or homojunction core-shell nanowires.