K. Y. Zang

Nanoscale lateral epitaxial overgrowth of GaN on Si (111)

We demonstrate that GaN can selectively grow by metalorganic chemical vapor deposition into the pores and laterally over the nanoscale patterned SiO2 mask on a template of GaN∕AlN∕Si. The nanoporous SiO2 on GaN surface with pore diameter of approximately 65 nm and pore spacing of 110 nm was created by inductively coupled plasma etching using anodic aluminum oxide template as a mask. Cross-section transmission electron microscopy shows that the threading-dislocation density was largely reduced in this nanoepitaxial lateral overgrowth region. Dislocations parallel to the interface are the dominant type of dislocations in the overgrown layer of GaN. A large number of the threading dislocations were filtered by the nanoscale mask, which leads to the dramatic reduction of the threading dislocations during the growth within the nano-openings. More importantly, due to the nanoscale size of the mask area, the very fast coalescence and subsequent lateral overgrowth of GaN force the threading dislocations to bend t...

Effects of periodic delta-doping on the properties of GaN:Si films grown on Si (111) substrates

In this study, the effects of periodic Si delta-doping on the morphological and optical properties of GaN films grown on Si (111) substrate have been investigated. It is found that the flow rate of Si dopant during growth significantly affects the surface morphology, structural and optical quality of GaN. Compared to undoped GaN on Si(111), films grown using periodic delta-doping show a significant reduction of the in plane tensile stress, which is confirmed by the blueshift of the E2(TO) phonon and band edge photoluminescence peaks. The crack density in GaN films also reduces due to delta-doping.

Nanoheteroepitaxial lateral overgrowth of GaN on nanoporous Si(111)

Nanoheteroepitaxial (NHE) lateral overgrowth of GaN on nanoporous Si(111) substrates has been demonstrated. Nanopore arrays in Si(111) surfaces were fabricated using anodized aluminum oxide templates as etch masks, resulting in an average pore diameter and depth of about 60 and 160–180nm, respectively. NHE growth of AlN and GaN was found to result in a significant reduction in the threading dislocation density (<108cm−2) compared to that on flat Si(111). Most dislocations that originate at the Si interface bent to lie in the GaN (0001) basal plane during lateral growth over the pore openings. E2 phonon blueshifts in the Raman spectra indicate a significant relaxation of the tensile stress in the coalesced GaN films, due to three-dimensional stress relaxation mechanisms on porous substrates. Our results show that a single step lateral overgrowth of GaN on nanopatterned Si(111) substrates without a dielectric mask is a simple way to improve the crystalline quality of GaN layers for microelectronic applications.

Defect reduction by periodic SiNx interlayers in gallium nitride grown on Si (111)

Periodic SiNx interlayers were employed during the metal-organic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111) substrates. The growth and the evolution of defects were studied in this paper. A reduction of the threading dislocation density to ∼109cm−2 was observed on the surface of GaN by counting the surface pit density from the atomic force microscopy results. Besides the observation of the continuous bending and subsequent recombination of the threading dislocations related to the periodic conduction of the SiNx interlayer characterized using cross-sectional transmission electron microscopy, we observed a different behavior induced by the SiNx interlayers: Si-rich inverted hexagonal pyramids with their base on the (0001) plane and six sidewalls on the (101¯1) plane were found near the top surface of the GaN film at the location of SiNx insertion layer characterized using electron energy loss spectroscopy. The preferential deposition of the SiNx on the sidewalls of th...

Improvement of microstructural and optical properties of GaN layer on sapphire by nanoscale lateral epitaxial overgrowth

Nanoscale lateral epitaxial overgrown (NLEO) GaN layers were investigated for the improvement of microstructural and optical properties of GaN. Nanoporous SiO2 films on the surfaces of GaN/sapphire (0001) were fabricated by inductively coupled plasma etching using anodic alumina templates as etch masks, resulting in an average pore diameter and interpore distance of 60 and 110nm, respectively. GaN was grown over the nanoporous SiO2 layer using metal organic chemical vapor deposition to realize a continuous and smooth film. NLEO GaN was found to result in a significant reduction of threading dislocation density as characterized by transmission electron microscopy and atomic force microscopy. The threading dislocation density was ∼107cm−2 at the surface of NLEO GaN. The narrower band-edge excitonic transition in photoluminescence spectrum shows a better optical quality in the NLEO GaN film.

Catalyst-free growth of uniform ZnO nanowire arrays on prepatterned substrate

Uniform and ordered ZnO nanowire arrays have been fabricated on the nanopatterned SiO2∕GaN substrate without metal catalysts using hydrothermal synthesis. The nanopatterns on SiO2∕GaN substrate with an average diameter of 65nm are produced by inductively coupled plasma etching using anodic alumina template as a mask, which provides nucleation sites for the vertical ZnO nanowires growth. High quality of the aligned uniform ZnO nanowire arrays grown on GaN substrate was confirmed by x-ray diffraction, transmission electron microscopy, and photoluminescence. This growth technique provides a cost-effective approach to fabricate ordered nanowire arrays with controlled size, which may benefit the nanowire device applications.

Structural Properties of ZnO Grown on GaN ∕ Sapphire Templates The Transition from Nanorods to Thin Films

ZnO nanorods were synthesized on GaN/sapphire substrates using a modified thermal-evaporation process. The as-synthesized ZnO nanorods and thin films were characterized using scanning electron microscopy, micro-Raman, and X-ray diffraction techniques. The morphology of the ZnO changes from nanorods to continuous thin films when the growth temperature increases to 800°C. Further increase in the growth temperature leads to a lower growth rate of ZnO along the (0001) direction. Micro-photoluminescence measurements show ultraviolet band-edge emission peaks around 378 nm from both nanorods and thin films. Realization of such ZnO structures may be useful for the fabrication of hybrid ZnO/GaN optoelectronic devices.

Template-nonlithographic nanopatterning for site control growth of InGaN nanodots

A site-control nucleation and growth approach for dense InGaN nanodots has been demonstrated on the surface of GaN using a nonlithographic nanopatterning technique by metal organic chemical vapor deposition. Shallow nanopore arrays with a depth of ∼15nm are created by inductively coupled plasma etching in the GaN surface using anodic aluminum oxide films as etch masks. The nanopores are found to be the preferential sites for the InGaN nanodot formation. Uniform InGaN nanodot arrays with a density as high as 1010∕cm2 as defined by the nanopores in GaN were observed on the surface. A strong photoluminescence (PL) emission peak near 2.8eV is observed from the InGaN nanodots. The temperature dependence of PL shows the enhanced carrier localization with higher activation energy in the InGaN nanodots when compared to the InGaN thin layer grown simultaneously on the nonpatterned GaN surface.

The Effect of Periodic Silane Burst on the Properties of GaN on Si (111) Substrates

The periodic silane burst technique was employed during metalorganic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111). Periodic silicon delta doping during growth of both the AlN and GaN layers led to growth of GaN films with decreased tensile stresses and decreased threading dislocation densities, as well as films with improved quality as indicated by x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The possible mechanism of the reduction of tensile stress and the dislocation density is discussed in the paper.

Nanopatterning and selective area epitaxy of GaN on Si substrate

Due to lack of suitable lattice matched substrates, III-Nitride materials are usually grown on sapphire, SiC, and silicon. The heteroepitaxy of GaN on these substrates often incorporates a high density of dislocation and point defects due to lattice and thermal mismatch. It is desirable to reduce the defect density in III-Nitrides in order to fabricate longer lifetime and high brightness light emitting diodes, lasers, and high-electron mobility transistors. In this context, nano-scale epitaxy on patterned Si substrates allows lateral growth, which eventually leads to a reduction of defect density and strain in the overgrown GaN films. Large area nano-patterning with dielectric masks would also be useful to fabricate highly-ordered and dense nitride nanostructures by selective area homo- and hetero-epitaxy.