Optical Characterization of Semiconductor Nanostructures

Abstract

Self assembled semiconductor nanostructures, such as Quantum wells, nanowires and Quantum Dots, offer a variety of novel properties different from the bulk material. The new properties of low dimensional structures make them a potential candidate in optoelectronic industry. Efforts are now being made to reveal the underlying physics and phenomena of quasi one-dimensional and zero dimensional structures. The work presented herein deals with optical characterization of III-V semiconductor nanowires and III-N-V based emitters i.e., quantum wells and quantum dots, in long wavelength range. Spintronic is an emerging field where dilute magnetic semiconductors are used to achieve magnetic properties. Nanowires with magnetic impurity is considered to be a step towards one-dimensional spintronic devices. Au is the most commonly used catalyst for the VLS growth of NWs. But it also introduces the deep acceptor levels. One way to avoid deep acceptor levels and induce a magnetic impurity is the use of Mn as catalyst. In this thesis, gold (Au) and manganese (Mn) catalyzed self-assembled GaAs and InAs nanowires (NWs) were characterized. The samples were fabricated by molecular beam epitaxial (MBE) technique, on various substrates, at various temperature (540 to 620) oC. Scanning electron microscope (SEM) images revealed high density one-dimensional nanostructures with diameters in the range of 20 to 200 nm and lengths of few microns. Mn was found to diffuse into the stem of wires. HRTEM images show the presence of defects (stacking faults) in nanowires. Raman spectroscopy was used for optical characterization of nanowires and thus to determine the quality of these wires. Defects (stacking faults) were analyzed as the violation of Raman selection rules, which resulted in the asymmetrical broadening and the downshift of the LO and TO modes. We also observed some peaks at the low energy side of the TO peak of the GaAs and InAs NWs, irrespective of the catalyst used for the growth of NWs due to the oxide layer that surrounds the NWs. Surface optical phonons (SO) were found to be activated in both GaAs and InAs NWs. Phonon confinement model (PCM) was used to fit the LO phonon peaks, which also takes into account the contribution for asymmetry in the line shape caused by the presence of SO phonons and structural defects. This allowed to determine the correlation lengths in these wires, the average distance between defects and the defect density in these nanowires. Influence of these defects on SO phonon was also investigated. A good agreement between the experimental results and calculated for SO phonon mode by using the model presented by Ruppin and Englman was obtained. Statistical analysis ofthe data showed a distribution pattern of correlation length related to the growth conditions. Both Au and Mn catalyzed nanowires were found to exhibit similar quality, which indicates that Mn can replace Au catalyst resulting in magnetic impurity in the nanowires and giving us the opportunity to avoid the Au activated deep acceptor levels. To obtain the optical communication wavelength of 1.31 and 1.55 μm on GaAs substrates, InGaAs(N)/GaAs quantum wells and InAs(N)/GaAs(N) quantum dot structures were studied using photoluminescence spectroscopy. The samples were grown by MBE with a proper design of the samples by using stepped barriers to improve carrier trapping efficiency. Comparison of the luminescence from InAsN/GaAs and InAsN/GaAsN quantum dots was made with InGaNAs/GaAs quantum wells, grown under the same experimental conditions. Quantum dot emitters were found to exhibit higher thermal stability. The use of GaAsN barriers as opposed to GaAs barriers provided for narrower and more intense quantum-dot luminescence. Efficient room temperature emission of 1.41 μm (0.88 eV) has been obtained.