Epitaxial lateral overgrowth of AlN on self-assembled patterned nanorods

Abstract

We report an inexpensive nanoscale patterning process for epitaxial lateral overgrowth (ELOG) in AlN layers grown by metal organic vapour phase epitaxy (MOVPE) on sapphire. The pattern was produced by an inductively coupled plasma etch using a self-assembled monolayer of silica spheres on AlN as the lithographic mask. The resulting uniform 1 [small mu ]m length rod structure across a wafer showed a massive reduction in threading dislocations (TDs) when annealed at 1100 [degree]C. Overgrowing homoepitaxial AlN on top of the nanorods, at a temperature of 1100 [degree]C, produced a crack free coalesced film with approximately 4 [small mu ]m of growth, which is formed at a much lower temperature compared to that typically required for microscale ELOG. The improved crystal quality, in terms of TD reduction, of the AlN above the rods was determined by detailed weak beam (WB) electron microscopy studies and showed that the threading dislocation density (TDD) was greatly reduced, by approximately two orders of magnitude in the case for edge-type dislocations. In situ reflectance measurements during the overgrowth allowed for thickness coalescence to be estimated along with wafer curvature changes. The in situ measurements also confirmed that tensile strain built up at a much slower rate in the ELOG AlN layer compared to that of AlN prepared directly on sapphire

Ultra-High-Density Arrays of Defect-Free AlN Nanorods: A “Space-Filling” Approach

“…We first deposited a monolayer of larger 700 nm silica spheres and spaced these features out using a CF4 dry ICP etch to shrink the nanospheres as in Figure 3 The growth mechanism for the evolution in shape of the grown AlN nanorods will be broken into two parts; (i) the formation of the initial apex tipped surface morphology and (ii) vertical growth without coalescence as in Figure 5a. We put forward a mechanism to describe the two possible outcomes of growth at stage (i); case 1/rod formation as reported here and case 2/coalesced bulk layer formation 26 . Our observation is if the c-plane is too small(relative to the rod diameter), as seen in Figure 4 when the top c-plane radius (r) approaches the rod radius (R) the limit of the ratio of the c-plane growth rate (Gc) over the semi-polar growth rate (Gs) equals zero and hence the semi-polar facets disappear completely allowing coalescence of the rods.…”

Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods

“…Using the same self-assembly method as in our previous report for AlN nanorods [48][49] a close packed nano dimensional silica sphere hard mask (SSHM) was coated over the entire 2" wafer, as seen in the photograph of Fig. 1e.…”

Epitaxial Growth of AlN on (0001) Sapphire: Assessment of HVPE Process by a Design of Experiments Approach

“…The Al atoms stick to the masking materials and nucleate polycrystalline growth. The approach consists in growing films on substrates possessing elevated mesas separated by lower trench regions to promote dislocation bending [39]. Despite lateral growth and coalescence over the mesas or pillars [21,34], only one order of magnitude reduction in the threading dislocation density were observed (10 9 to 3 × 10 8 /cm 2 ).…”

“…It is clear that the optimal temperature range is 1200-1400 • C to increase the Al mobility on the surface while avoiding sapphire degradation and/or etching [38]. Very recently, attempts to reduce strain by lateral epitaxial growth (LEO) have been proposed [21,26,34,39]. The common LEO developed for GaN has been difficult to adapt to AlN films.…”