Xinyu Sun

Nanoheteroepitaxy for the integration of highly mismatched semiconductor materials

We describe an ongoing study of nanoheteroepitaxy (NHE), the use of nanoscale growth-initiation areas for the integration of highly mismatched semiconductor materials. The concept and theory of NHE is briefly described and is followed by a discussion of the design and fabrication by interferometric lithography of practical sample structures that satisfy the requirements of NHE. Results of NHE growth of GaAs-on-Si and GaN-on-Si are described, following the NHE process from nucleation through to coalescence. Micro-Raman measurements indicate that the strain in partially coalesced NHE GaN-on-Si films is <0.1 GPa.

Fabrication of GaN nanowire arrays by confined epitaxy

The authors report the fabrication of GaN nanowire arrays inside a thick SiNx, selective growth mask that was patterned by interferometric lithography and dry etching. The GaN nanowires are molded by the apertures in the selective growth mask and the growth is epitaxial with respect to the underlying GaN layer. The precise location and diameter of each nanowire in the array are controlled by the growth mask patterning, and the resulting array has a long-range order that is compatible with photonic crystal applications. This process uses conventional metal organic precursors and does not require any additional metal catalysts.

Defect reduction mechanisms in the nanoheteroepitaxy of GaN on SiC

This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.

Nanoheteroepitaxial growth of GaN on Si nanopillar arrays

Nanoheteroepitaxial growth of GaN by metal-organic chemical-vapor deposition on dense arrays of (111) Si nanopillars has been investigated. Scanning electron microscopy, cross-sectional transmission electron microscopy, and electron-diffraction analysis of 0.15-μm-thick GaN layers indicate single-crystal films. Most of the mismatch defects were in-plane stacking faults and the threading dislocation concentration was <108cm−2 at the interface and decreased away from the interface. High-resolution transmission electron microscopy indicated that grain-boundary defects could heal and were followed by high quality, single-crystal GaN. Facetted voids were also present at the GaN∕Si interface and are believed to be an additional strain-energy reduction mechanism. The unusual defect behavior in these samples appears to be related to the high compliance of the nanopillar silicon substrate.

Spatial phase separation of GaN selectively grown on a nanoscale faceted Si surface

This letter reports the growth of spatially separated hexagonal and cubic phases of GaN on a patterned Si(001) substrate by metalorganic vapor-phase epitaxy. The substrate surface was patterned with grooves having a 355 nm period. Each groove consisted of two opposed Si{111} facets that were separated by Si(001) surfaces. Epitaxial growth of GaN on this substrate began selectively on the Si{111} facets and yielded the GaN hexagonal phase. With further growth, the two hexagonal phase regions separately grown on the opposed Si{111} facets coalesced, with strongly misaligned c axes (∼110°). The GaN grown after coalescence was subsequently confirmed, by transmission electron microscopy and photoluminescence, to be of cubic phase.

Photoelectrochemical etching measurement of defect density in GaN grown by nanoheteroepitaxy

The density of dislocations in n-type GaN was measured by photoelectrochemical etching. A 10× reduction in dislocation density was observed compared to planar GaN grown at the same time. Cross-sectional transmission electron microscopy studies indicate that defect reduction is due to the mutual cancellation of dislocations with equal and opposite Burger’s vectors. The nanoheteroepitaxy sample exhibited significantly higher photoluminescence intensity and higher electron mobility than the planar reference sample.