MICRO RAMAN/PL INVESTIGATIONS OF MATERIALS FOR IR AND PV APPLICATIONS (III-V TYPE II SUPERLATTICES AND II-VI COMPOUNDS)
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Abstract
Raman and photoluminescence (PL) spectroscopy are two of the major spectroscopic techniques that are currently used in studies on solid state materials. Raman spectroscopy, which is based on Raman scattering process, is widely used in characterizing structures, determining compositions, and probing various types of excitations of the material, etc. On the other hand, PL spectroscopy is closely related to the generation and re-combination of photo-generated carriers, thus usually used in determining electronic structure near the fundamental band gap, studying impurities and defects, and so on. Apparently, these two techniques enable material research regarding different aspects, which are both vital for understanding the material properties. Despite the difference in physical principles on which they are based, these two techniques share the same feature of being invasive, which is preferable or mandatory in many of the applications. Furthermore, these two techniques are usually integrated in a single experimental instrument which is commercially available: for instance, a Raman spectroscopy microscope system. These days, the spatial resolution of a confocal optical system can reach the sub-micron range with appropriately choosing excitation wavelength, thus making micro-Raman and micro-PL measurements available. Apparently, this high resolution is making the Raman and PL measurements more effective when studying materials, which also enables related analyses with a sub-micron spatial accuracy.In our work, we conduct micro-Raman or micro-PL spectroscopic investigations on several types of solid state materials, which are all playing significant roles in IR detection and/or PV applications. These materials, in general, fall into three categories: (1) Type II Superlattices (T2SLs) based on InAs/GaSb; (2) T2SLs based on InAs/InAsSb; and (3) CdTe single crystalline epilayers with varying defect density levels. The first two types of materials could be regarded to form one material system, since these two T2SLs are both model systems from the aspect of lattice vibrational properties of semiconductor SLs, where the study of their phonon modes is far from adequate and completed. On the other hand, CdTe, by itself, form another material system. Although being a rather "well-known" semiconductor material when compared to the two T2SLs, the study on defects in CdTe is still developing and far from being complete. In short, for these materials, investigations regarding structural properties and defect behaviors are of great interest, for either theory or application field. These related studies are vital in understanding the physics of the material or assessing the material quality. Correspondingly, micro-Raman and micro-PL technique are extraordinarily suitable for these investigations: the former is a powerful tool in structural characterization, while the latter is extensively used in defect-related studies.In the study of InAs/GaSb and InAs/InAsSb T2SLs, micro-Raman spectroscopy is used as the major tool to study the lattice vibrations and explore the phonon modes. Several new phonon modes have been observed, which for the first time confirms the existence of three distinctly different types of phonon modes (confined, quasi-confined, and extended modes) that have long been predicted theoretically but never experimentally identified. In the study of CdTe, spatially resolved micro-PL spectroscopy is used to investigate the effects of extended defects in the epilayers with high density defects. By comparison with XRD and chemical etching method, the PL method have generally show consistent results, and also exhibiting higher sensitivity in revealing defects that are actually detrimental to radiative recombination of photo-generated carriers. For the first time, we have been able to make direct comparison between etch pits and PL dark spots, and identified three different scenarios when correlating the results of the two characterization tools.