Myungkoo Kang

Mechanisms of droplet formation and Bi incorporation during molecular beam epitaxy of GaAsBi

We have examined the mechanisms of droplet formation and Bi incorporation during molecular beam epitaxy of GaAsBi. We consider the role of the transition from group-V-rich to group-III-rich conditions, i.e., the stoichiometry threshold, in the presence of Bi. For As-rich GaAsBi growth, Bi acts as a surfactant, leading to the formation of droplet-free GaAsBi films. For films within 10% of the stoichiometric GaAsBi growth regime, surface Ga droplets are observed. However, for Ga-rich GaAsBi growth, Bi acts as an anti-surfactant, inducing Ga-Bi droplet formation. We propose a growth mechanism based upon the growth-rate-dependence of the stoichiometry threshold for GaAsBi.

Surface plasmon resonances of Ga nanoparticle arrays

We have examined the influence of particle and chain diameter on surface plasmon resonance (SPR) energy of 2D and 1D Ga nanoparticle (NP) arrays fabricated using focused-ion-beam irradiation of GaN surfaces. Maxima in the extinction spectra suggest the presence of SPR at visible and near-infrared wavelengths. The SPR energies increase with decreasing NP or chain diameter, due to particle diameter-dependent dipole interactions within the metallic NPs. The SPR quality factors are comparable to those reported from Ag and Au NPs, suggesting Ga NPs as a promising alternative plasmonic material.

Universal mechanism for ion-induced nanostructure formation on III-V compound semiconductor surfaces

We have examined the formation of nanostructures on ion-irradiated compound semiconductor surfaces. We computed the ion doses needed to fully deplete group V elements from the surfaces. These group V depletion doses are in good agreement with the measured threshold ion doses for nucleation of group III-rich nanostructures on a wide variety of III-V compound semiconductor surfaces. Since the group V depletion doses decrease with increasing sputtering yield, these results suggest a universal nanostructure formation mechanism which depends upon the total sputtering yield of each III-V compound.

Formation and evolution of ripples on ion-irradiated semiconductor surfaces

We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga+ FIB irradiation of InSb, we tuned the local beam incidence angle (θeff) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θeff induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θeff leads to island formation on the ripple crests, followed by nanorod (NR) growth. This ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.

Nanodot formation induced by femtosecond laser irradiation

The femtosecond laser generation of ZnSe nanoscale features on ZnSe surfaces was studied. Irradiation with multiple exposures produces 10–100 nm agglomerations of nanocrystalline ZnSe while retaining the original single crystal structure of the underlying material. The structure of these nanodots was verified using a combination of scanning transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. The nanodots continue to grow hours after irradiation through a combination of bulk and surface diffusion. We suggest that in nanodot formation the result of ultrafast laser induced point defect formation is more than an order of magnitude below the ZnSe ultrafast melt threshold fluence. This unique mechanism of point defect injection will be discussed.

Origins of ion irradiation-induced Ga nanoparticle motion on GaAs surfaces

We have examined the origins of ion irradiation-induced nanoparticle (NP) motion. Focused-ion-beam irradiation of GaAs surfaces induces random walks of Ga NPs, which are biased in the direction opposite to that of ion beam scanning. Although the instantaneous NP velocities are constant, the NP drift velocities are dependent on the off-normal irradiation angle, likely due to a difference in surface non-stoichiometry induced by the irradiation angle dependence of the sputtering yield. It is hypothesized that the random walks are initiated by ion irradiation-induced thermal fluctuations, with biasing driven by anisotropic mass transport.

Ga nanoparticle-enhanced photoluminescence of GaAs

We have examined the influence of surface Ga nanoparticles (NPs) on the enhancement of GaAs photoluminescence (PL) efficiency. We have utilized off-normal focused-ion-beam irradiation of GaAs surfaces to fabricate close-packed Ga NP arrays. The enhancement in PL efficiency is inversely proportional to the Ga NP diameter. The maximum PL enhancement occurs for the Ga NP diameter predicted to maximize the incident electromagnetic (EM) field enhancement. The PL enhancement is driven by the surface plasmon resonance (SPR)-induced enhancement of the incident EM field which overwhelms the SPR-induced suppression of the light emission.

Formation of embedded plasmonic Ga nanoparticle arrays and their influence on GaAs photoluminescence

We introduce a novel approach to the seamless integration of plasmonic nanoparticle (NP) arrays into semiconductor layers and demonstrate their enhanced photoluminescence (PL) efficiency. Our approach utilizes focused ion beam-induced self-assembly of close-packed arrays of Ga NPs with tailorable NP diameters, followed by overgrowth of GaAs layers using molecular beam epitaxy. Using a combination of PL spectroscopy and electromagnetic computations, we identify a regime of Ga NP diameter and overgrown GaAs layer thickness where NP-array-enhanced absorption in GaAs leads to enhanced GaAs near-band-edge (NBE) PL efficiency, surpassing that of high-quality epitaxial GaAs layers. As the NP array depth and size are increased, the reduction in spontaneous emission rate overwhelms the NP-array-enhanced absorption, leading to a reduced NBE PL efficiency. This approach provides an opportunity to enhance the PL efficiency of a wide variety of semiconductor heterostructures.

Evolution of ion-induced nanoparticle arrays on GaAs surfaces

We have examined the evolution of irradiation-induced Ga nanoparticle (NP) arrays on GaAs surfaces. Focused-ion-beam irradiation of pre-patterned GaAs surfaces induces monotonic increases in the NP volume and aspect ratio up to a saturation ion dose, independent of NP location within the array. Beyond the saturation ion dose, the NP volume continues to increase monotonically while the NP aspect ratio decreases monotonically. In addition, the NP volumes (aspect ratios) are highest (lowest) for the corner NPs. We discuss the relative influences of bulk and surface diffusion on the evolution of Ga NP arrays.