Strain evolution in GaN nanowires: From free-surface objects to coalesced templates

Abstract

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence, and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained

Disentangling the effects of nanoscale structural variations on the light emission wavelength of single nano-emitters: InGaN/GaN multiquantum well nano-LEDs for a case study

“…1,24 Strain distribution simulations revealed that the decrease in the GaN nanorod diameter limits the strain relaxation depth to the surface, which can be extended along the entire nanorod length (strain still present at the base) by increasing its height up to $300 nm. 25,26 We adopted these conrmed dimensions to produce nano-LEDs using a combination of nanosphere lithography (NSL) and reactive ion etching (RIE). 27 The NSL/RIE procedure has been proven to result in noncoalesced, tightly size-controlled nanorods in contrast to the self-assembled Ni particle mask and selfor catalyst-induced epitaxial growth methods, 4,11,12,26 and to be faster, far less equipment demanding than the electron beam lithography and the focused ion beam patterning approaches.…”

“…25,26 We adopted these conrmed dimensions to produce nano-LEDs using a combination of nanosphere lithography (NSL) and reactive ion etching (RIE). 27 The NSL/RIE procedure has been proven to result in noncoalesced, tightly size-controlled nanorods in contrast to the self-assembled Ni particle mask and selfor catalyst-induced epitaxial growth methods, 4,11,12,26 and to be faster, far less equipment demanding than the electron beam lithography and the focused ion beam patterning approaches. 1,14,28 Despite the fact that the nano-objects in this work were optically excited and not electrically pumped by means of contacts, they will still be termed nano-LEDs.…”

High-resolution X-ray diffraction analysis of strain distribution in GaN nanowires on Si(111) substrate

“…The deformation profiles of ε || (z) and ε ||| (z) along the NW's growth axis are defined assuming exponential deformation decay [3,4,8,9]:…”

“…In [2], the assessment of macro-and micro-deformation in GaN NWs grown on Si(111) was performed with X-ray diffraction, where the NWs were found to be free of deformations on the macro scale but not on the micro scale. In comparison to self-assembled GaN NWs [2], an exponential relaxation of the macro-strain along the NW height was observed for top-down fabricated GaN NWs grown on silicon and sapphire substrates [3,4]. An exponential decay of the mean-squared micro-strain along the self-assembled GaN NW was considered to describe the X-ray diffraction peaks in [8].…”

“…The main sources of deterioration of NW's properties are their crystalline imperfection and residual strain. Generally, NWs are considered almost strain-free crystalline-objects without extended defects that propagate into their structure [2][3][4][5]. In comparison with thick planar epilayers, where the mechanism of lattice accommodation is preferably plastic and where the formation of misfit dislocation networks takes place, NWs are considered to be predominantly free of dislocations [6,7].…”