Selective Area Growth (SAG) by Molecular Beam Epitaxy (MBE) is one of the best approaches to develop a variety of nanostructures on different substrates. Ordered axial InGaN/GaN nanoLED structures were grown on GaN/sapphire templates as well as on GaN buffered Si(111) substrates. Core-shell InGaN/GaN microstructures can also be grown following two approaches: i) from top–down (etched) GaN cores and ii) from bottom-up GaN cores. In both cases a subsequent conformal growth of InGaN layers was achieved. Based on this approach, core-shell nanoLED arrays were successfully fabricated.
A basic aspect of SAG refers to the initial stages of nanocrystals nucleation within the nanoholes that lead to their stable hexagonal structure and the efficient filtering of dislocations coming from the substrate, strongly dependent on the nano/microrod geometry.
A common observed feature is that In incorporation depends strongly on the crystal plane considered, either m- or r-plane, giving rise to two InGaN related emissions. Exploiting this effect, dot-in-a-wire InGaN structures were grown embedded in ordered GaN nanorods acting as Single Photon Emitters.
Nano/microrods can also be used as nanoFET transistors with a semi-cylindrical gate direct contact allowing for a very tight electrostatic control of the channel.
SAG is also used to grow ordered nanostructures on semi-polar and non-polar orientations GaN/sapphire templates with the aim to fabricate ternary pseudo-substrates with tailored lattice constant and very high crystal quality.
Snežana Lazić, Ekaterina Chernysheva, Žarko Gačević, Noemi García-Lepetit, Herko van der Meulen, Marcus Müller, Frank Bertram, Peter Veit, Jürgen Christen, Almudena Torres-Pardo, José González Calbet, Enrique Calleja, José Calleja
The realization of reliable single photon emitters operating at high temperature and located at predetermined positions still presents a major challenge for the development of solid-state systems for quantum light applications. We demonstrate single-photon emission from two-dimensional ordered arrays of GaN nanowires containing InGaN nanodisks. The structures were fabricated by molecular beam epitaxy on (0001) GaN-on-sapphire templates patterned with nanohole masks prepared by colloidal lithography. Low-temperature cathodoluminescence measurements reveal the spatial distribution of light emitted from a single nanowire heterostructure. The emission originating from the topmost part of the InGaN regions covers the blue-to-green spectral range and shows intense and narrow quantum dot-like photoluminescence lines. These lines exhibit an average linear polarization ratio of 92%. Photon correlation measurements show photon antibunching with a g(2)(0) values well below the 0.5 threshold for single photon emission. The antibunching rate increases linearly with the optical excitation power, extrapolating to the exciton decay rate of ~1 ns-1 at vanishing pump power. This value is comparable with the exciton lifetime measured by time-resolved photoluminescence. Fast and efficient single photon emitters with controlled spatial position and strong linear polarization are an important step towards high-speed on-chip quantum information management.
New advances on Selective Area Growth (SAG) of InGaN/GaN nanostructures by plasma-assisted MBE on GaN/sapphire templates and Si (111) substrates are presented. Both, axial and core-shell structures are considered. Very intense green electroluminescence is achieved on axial nanoLEDs grown on Si(111) with very small emission drift with current injection. First results on core-shell InGaN/GaN structures grown by MBE on GaN templates are also presented. Two approaches are followed: i) top down, where cylindrical micro-rods are etched down by ICP from a 3 micron thick GaN/sapphire template, and bottom up, in which very high aspect ratio GaN cores are used. In both cases, GaN and InGaN shell layers are then grown both in axial and radial directions. Potential advantages of this core-shell structure as compared to the axial one are twofold: the increase of emission surface (lateral area) and the absence of internal electric fields (m-plane). The crystal perfection is much better than that of 2D InGaN films of similar In% composition. Ordered arrays of GaN and InGaN axial nanostructures are also grown on non-polar and semi-polar directions and subsequently merged into a continuous film to produce high quality pseudo substrates. The resulting films exhibit a very strong luminescence, orders of magnitude higher that from the substrate used. Semi-polar GaN templates have a huge density of stacking faults (SFs) most of them are filtered upon coalescence of the nanostructures grown on top. In all cases there is a preferential growth direction along the c-plane (0001). PL and spatially resolved CL measurements on individual nanostructures, either polar, non-polar, or semi-polar show that the In% incorporation depends strongly on the crystal plane considered.
In this work, efficient solar-blind metal-semiconductor-metal (MSM) photodiodes grown on Si (111) by molecular beam epitaxy are reported. Growth details are described, including the use of different kinds of buffer layers. AlGaN samples using an AlGaN/GaN superlattice (SL) as a buffer showed the presence of cracks, while AlGaN samples on an AlN buffer were crack-free. The additional strain introduced by the SL and the increase of the lattice mismatch between Si and AlGaN when the Al content increases, are responsible for the cracking. MSM photodiodes were fabricated and characterized using such layers. UV detectors obtained on the sample with cracks presented a dark current above 100 pA at 5 V, while in the crack-free photodiodes the dark current was below 10 pA at 30 V. The ultraviolet/visible contrast was also reduced in order of magnitude due to the presence of cracks. Peak responsivity values of 14 mA/W at 5 V and of 16 mA/W at 10 V were obtained for the photodetectors with cracks and for the crack-free photodetectors, respectively. The spectral noise density was 1 x 10-24 A2/Hz at 5 V for the detectors with cracks, showing at low frequencies a 1/f-type behavior. For the crack-free photodetectors, the spectral noise density value was below the system detection limits (1 x 10-26 A2/Hz) at 10 V. A detectivity value of 5.2 x 1010 cmxHz1/2xW-1 at 10 V was estimated for the crack-free photodiodes.
In the recent years, the depletion of the stratospheric ozone layer has alerted the scientific community about the risks of a solar ultraviolet (UV) radiation overexposure. Biological research has confirmed the very important role of the UV-B (320 - 280 nm) and UV-A (400 - 320 nm) bands on the Earth biosystem. AlxGa1-xN semiconductor alloys, with a bandgap tunable between 3.4 eV and 6.2 eV, are the most suitable materials for the fabrication of solar UV detectors. In this paper we describe the fabrication and characteristics of AlGaN photoconductive and Schottky barrier photodetectors, with Al mole fractions up to 35%. Photoconductive detectors show very high gains, that decrease with increasing incident optical power. They present persistent photoconductivity effects, and a significant below-the-gap response. The physics of this behavior is discussed. On the other hand, AlGaN Schottky barrier photodetectors show a very fast response that is independent of the optical power, and their UV/visible rejection ratio is rather high. As the Al content increases, the evolution of the responsivity and cut-off wavelength is presented. Al0.22Ga0.78N Schottky barriers are very good candidates to monitor the UV-B band. The prospective applications of AlGaN photodiodes to determine the biological action of the solar UV radiation are also discussed.
Conference Committee Involvement (2)
Gallium Nitride Materials and Devices XI
15 February 2016 | San Francisco, California, United States
Gallium Nitride Materials and Devices X
9 February 2015 | San Francisco, California, United States
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