Heteroepitaxy on nanoscale substrates: Application to solid -state lighting

Abstract

Thin film heterostructures are limited by a maximum critical thickness before introduction of extended defects. One-dimensional form factors like nanowires/nanorods, due to the possibility of lateral elastic relaxation, can tolerate much larger lattice mismatch than their thin film counterparts. The present document begins with the description of some modeling work employing solution thermodynamics and finite element analysis as a motivation to the nanoheteroepitaxy approach to achieve a monolithic phosphor-free white light emitting diode (LED). A nanorod with a pointed tip morphology has been shown to be required for pushing the emission wavelengths from the InN-GaN system to longer values (red). The process developed to synthesize diameter controlled GaN pyramidal tipped nanorods without the use of any catalysts has also been developed in the course of the present research work. The same template-based process along with optical lithography techniques has been demonstrated to yield controlled nanorod diameter variation on the same substrate. Due to the large surface-area-to-volume ratio of the synthesized nanorods, it is required to ascertain that the nanorods are not devoid of charge carriers due to the surface depletion effect. Electrical characterization of the nanorods in the form of single and multiple GaN nanorod Schottky and p-n junctions diodes employing conductive atomic force microscopy has also been performed and described in the document. Finally, cathodoluminescence spectra from (In,Ga)N nanorods have been used to show the potential of the nanorod form to incorporate higher InN mole fractions as compared to thin film counterparts. Along with applications in solid-state lighting, the pointed tip morphology of the nanorods, resulting in a very high field enhancement factor, are contenders as field emitters. Coupled with such a high field enhancement factor, the incorporation of (Al,Ga)N on the nanorod tip helps to reduce the effective surface work function resulting in a significant reduction in the turn-on field from (Al,Ga)N/GaN nanorod heterostructures as compared to GaN nanorods. Such an approach circumvents the doping problem of (Al,Ga)N, still utilizing its low electron affinity. Results from vacuum field emission experiments from (Al,Ga)N/GaN nanorod heterostructures and their analysis have also been presented in the document