Nanoscale characterization of InSb/InAs novel functional semiconductor nanostructures for LEDs / Caracterización a nanoescala de nanoestructuras funcionales novedosas de InSb/InAs para LEDs
Light emitting diodes (LEDs) are becoming increasingly popular day-by-day for lighting applications as they require low maintenance and low fabrication costs, and have long lifetime and low energy consumption. As a result, extensive research and funding have been dedicated in the last years in order to obtain superior LED fabrication techniques and designs which may maximize their optical performances. III-V epitaxial quantum dots (QDs) have been considered to design the active layers of these LEDs as these QDs offer the highest level of optoelectronic efficiency. Among various III-V epitaxial QDs, InSb/InAs QDs emit light at the mid-infrared (MIR) range (3-5 µm) through the tuning of Sb composition and as a result, these QDs can be used to detect various hazardous gases with MIR signatures, among other applications. In this framework, the objective of the present Thesis is to explore the applicability of the migration enhanced epitaxy (MEE) growth technique to fabricate these InSb/InAs QDs, as an alternative to the conventional Stranski-Krastranov (SK) QDs, in order to configure optimum design parameters for highly efficient gas and bio-sensing LEDs at MIR range.
The optoelectronic properties of the MEE grown InSb/InAs QDs highly depend on corresponding Sb distribution. Because of this, atomic column resolved high angle annular dark field (HAADF) – scanning transmission electron microscopy (STEM) characterization technique has been considered, where Sb compositions are realized through HAADF-STEM atomic column intensities. In order to interpret the Sb composition through HAADF-STEM intensities, the quantitative HAADF (qHAADF) method can be used that quantifies Sb induced intensity through the intensity ratio of a Sb-containing region to a Sb-absent region. However, this tool requires both regions to be present in the same micrograph. As a result, the application of this tool becomes limited if InSb/InAs QDs exist in a complex heterostructure where locating a reference area is complicated. Consequently, a modified version of the qHAADF tool has been developed in this Thesis that allows locating the reference region from a second micrograph and hence, the aforementioned limitation could be overcome. The specimen thickness variation between these two areas imposes complications in the Sb compositional analysis by the either qHAADF tool. Therefore, a corresponding thickness variation compensation process has also been discussed in this Thesis to assure atomic column resolved precise Sb compositional analysis.
The MEE grown InSb/InAs QDs associated to sub-monolayer (SML) insertion of InSb may offer increased maximum gain and a larger modulation bandwidth (BW) than its conventional SK counterpart as these QDs are surrounded by a thin/no wetting layer (WL) underneath. However, it has been demonstrated through this PhD Thesis that a high growth temperature facilitates a high Sb segregation into the InAs capping layer. As a result, continuous InSbAs WL form with nm thickness encapsulating a few random InSbAs agglomerates, realized through the corresponding HAADF-STEM and 002 dark field (DF) conventional TEM (CTEM) analyses. The corresponding high Sb segregation seems to induce relatively low average Sb composition in the InSbAs agglomerates.
Typically, Sb segregation from InSb/InAs heterostructures is reduced by decreasing the corresponding growth temperature. However, this PhD Thesis illustrates that Sb segregation can also be reduced from the MEE grown InSb/InAs heterostructures by reducing only InAs cap growth temperature. This results in an increase in Sb composition in the InSbAs agglomerates observed through both HAADF-STEM and 002 DF CTEM analyses. As a result, the random InSbAs agglomerates become bigger or more continuous within the InSbAs WLs. This InAs cap temperature associated compositional variation in Sb composition allows tuning the InSb emission wavelength in the MIR range, realized through the corresponding photoluminescence (PL) emission spectra
