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 to the basal plane within the first 50 nm of the film thickness. The structure of overgrown GaN is a truncated hexagonal pyramid which is covered with six {11¯01} side facets and (0001) top surface depending on the growth conditions.

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.

The morphological evolution of AlN buffer layers grown on (111) silicon at high-temperature has been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The structure and morphology of subsequently grown GaN films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. It was found that even though thicker AlN buffer layers are the poor single crystal with some defects like mis-oriented domains, stacking faults, they provide better templates for subsequent growth of GaN films with better crystalline and optical properties, compared to the GaN grown on a thin AlN buffer layer. To prevent the formation of SiN x layers at the interface, TMA was flowed for several seconds prior to the introduction of ammonia into the chamber. The AFM result showed that quasi 2-dimensional growth was quickly achieved for the AlN buffer layer with TMA pre-treatment.

In this study, micro-Raman spectroscopy has been used to investigate the vibrational properties of laterally epitaxial overgrown ͑LEO͒ GaN. The LEO GaN films were grown by metal organic chemical vapor deposition on a 2 in. sapphire substrate with SiN mask. Photoluminescence and polarized Raman scattering measurements have been performed in the two regions of GaN growth ͑wing and window regions͒. Raman scattering results are consistent with the lateral growth of GaN in the overgrown region. We have observed second-order Raman scattering in the wing and window regions of GaN. The observations of longitudinal optical phonon plasmon modes in the overgrown region demonstrate that LEO GaN is doped. We have carried out micro-Raman mapping of the local strain and free carrier concentration in the LEO GaN. Anharmonicity due to temperature in LEO GaN has also been investigated. The anharmonicity was found to increase with increasing temperature, and such temperature-induced anharmonicity introduces changes in the linewidth and line center position of the Raman active phonons. The phonon lifetimes in GaN are estimated in the LEO region as well as in the coherently grown region ͑window region͒.

We report growth of InGaN∕GaN multiple quantum wells (MQWs) on (111)-oriented bonded silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition (MOCVD). Prior to MOCVD growth of MQWs, about a 1.2μm thick GaN layer was deposited on SOI substrate with a high-temperature transitional buffer layer. The growth conditions were tuned to realize blue-green emission peaks centered around 420–495nm from such MQWs on SOI. X-ray diffraction, atomic force microscopy, scanning electronic microscopy, and photoluminescence techniques were used to characterize these MQWs. Such an approach to realize multicolor light-emitting layers on SOI substrates is suitable for the integration of InGaN∕GaN-based optoelectronic structures on SOI-based micro-optoelectromechanical systems and sensors.

Nanoporous GaN films were prepared by UV-assisted electrochemical etching. It was observed that by employing rapid thermal annealing ͑RTA͒, optical properties of porous GaN films could be considerably improved. The nanoporous GaN films were subjected to RTA within the temperature range of 500-1100°C under nitrogen and oxygen ambient. The band-edge photoluminescence ͑PL͒ intensity from the porous GaN films showed a monotonic increase with the annealing temperature up to 1000°C in N 2 . However, the band-edge PL intensity showed a drastic reduction for the porous samples annealed under oxygen ambient. Besides this, investigation of the effects of annealing conditions on Eu-implanted nanoporous GaN in N 2 and O 2 ambient was also carried out. We have observed that annealing in N 2 significantly enhances the Eu-related red luminescence around 621 nm, which implies that annealing conditions could play an influential role for light extraction from nanostructured GaN surfaces.Because of its thermal, chemical, and mechanical stability, GaN and its alloys have attracted considerable attention for their applications toward high-temperature, high-power, and optoelectronic devices. 1-4 The efficiency of optical devices using GaN is still restricted due to lack of suitable substrates. GaN and its related nitrides are generally grown on sapphire ͑13.6% lattice mismatch͒ or SiC substrates ͑4% lattice mismatch͒, where larger lattice mismatch degrades the layer quality. To improve the layer quality, a reduction in the defect and dislocation density in the material is desirable. In this regard, porous GaN can be used as a buffer or intermediate layer to reduce the strain and defect density in the overgrown film as reported recently. 5-12 These reports suggest that the pore size, etching depth, or the transverse dimension of the pores can be easily controlled by varying the anodization conditions. It has also been reported that optical and electrical properties of GaN not only depend on the growth conditions but also depend on the postgrowth annealing. Thermal annealing is an important step for the fabrication of light emitters 13 and high-temperature electronic devices. 14 More importantly, it is seen that the rapid thermal annealing ͑RTA͒ of GaN under nitrogen ambient leads to a substantial improvement of the photoluminescence ͑PL͒ intensity, conductivity, and surface morphology. 15-19 Therefore, it is important to understand the PL and morphological properties of porous GaN subjected to RTA where further advancement of the postgrowth processes of overgrown layers or postgrowth implantation can be explored. Ion-implantation or regrowth on such a porous template may lead to high optical quality epi layers by using an intermediate annealing step. Because of wide bandgap and efficient light extraction through scattering from the crystallite facets, nanoporous GaN film would act as a suitable host for rare earth implantation. Among the various rare earth species, Eu seems to be the most studied material to produce strong luminescence...

Articles you may be interested inPorosity-induced relaxation of strains in GaN layers studied by means of micro-indentation and optical spectroscopy Effect of carrier density on the surface morphology and optical properties of nanoporous GaN prepared by UV assisted electrochemical etching Appl. Phys. Lett. 91, 083110 (2007); 10.1063/1.2772753Effect of growth temperature on morphology, structure and luminescence of Eu-doped GaN thin films A systematic optical activation study of Eu-implanted nanoporous GaN films has been carried out as a function of ion dose and annealing temperature. The nanoporous GaN films are prepared by photoelectrochemical etching of n-type GaN films in HF-based electrolyte. Eu ions are implanted in both n-type GaN and n-type porous GaN films at 200 keV with doses ranging from 5 ϫ 10 14 to 5 ϫ 10 15 cm −2 . For the implantation damage recovery and optical activation of Eu 3+ ions, rapid thermal annealing is performed in the temperature range of 900-1200°C under nitrogen ambient. The surface morphology of implanted porous GaN after different processing steps is characterized by scanning electron microscopy and the results show that porous morphology remains uniform even after ion implantation and high temperature processing. Microphotoluminescence and micro-Raman techniques have been used to investigate the optical properties of these Eu-implanted nanoporous films. Postimplantation annealing of both as-grown GaN and porous GaN films leads to the observation of strong photoluminescence ͑PL͒ peak around 622 nm, which is associated with the 5 D 0 -7 F 2 intraionic transition of Eu 3+ ions. We have observed that PL intensity of Eu-related luminescence peaks increases with annealing temperature up to 1100°C. In addition, due to efficient light extraction by surface nanostructuring, Eu-implanted porous GaN films show much stronger luminescence when compared to Eu-implanted as-grown GaN. Raman spectral analyses also indicate the optimum annealing condition for the implantation damage recovery and the compressive stress state in the Eu-implanted films.

InGaN self-organized quantum dots (QDs) have been grown on sapphire (0001) substrates by lowpressure metalorganic chemical vapor deposition (MOCVD). Atomic force microscopy (AFM) measurements revealed the formation of InGaN QDs grown at 760 o C on a very thin AlN template. No InGaN QDs were formed when the amount of InGaN deposited was less than 5 monolayers (MLs), whereas selforganized QDs appear on the surface with further increase in the number of InGaN MLs. For the typical 15 MLs InGaN QDs grown by this technique, the average diameter and height of the QDs are 19.5 and 7.5 nm, respectively, with a dot density of about 1.3 × 10 11 cm -2 . Room temperature microphotoluminescence measurements show blue (2.86 eV) to green (2.31 eV) emission when the InGaN QDs deposited on the thin AlN layer increased from 15 to 35 MLs.