Abstract
A novel catalyst-free and maskless growth approach is presented to form an ordered geometrical array of three-dimensional (3D) AlGaN/AlN microrods. The growth method is composed of a single growth step using metalorganic vapor phase epitaxy, achieving microstructures with homogeneous diameters, shapes and sizes over relatively large scale (on 2-in. wafer). The 3D AlGaN/AlN heterostructures are grown in a form of micro-sized columns elongated in one direction perpendicular to the substrate surface and with a hexagonal cross section. A careful examination of growth steps revealed that this technology allows to suppress coalescence and lateral overgrowth, promoting vertical 3D growth. Interestingly, two distinct morphologies can be obtained: honeycomb-like hexagonal arrangement perfectly packed and with twisted microrods layout, by controlling strain state in AlN buffer layers. Consequently, 3D AlGaN microrods on tensile-strained AlN templates show a 0° twisted morphology, while on compressive-strained templated a 30° twisted arrangement. Moreover, the optical and crystalline quality studies revealed that the top AlGaN layers of the examined 3D semiconductor structures are characterized by a low native point-defect concentration. These 3D AlGaN platforms can be applied for light emitting devices or sensing applications.Graphic abstract
Highlights
The development of three-dimensional (3D) semiconductor structures with distinct architectures opens new routes for device design with novel features, becoming valuable technology for certain specialized purposes and alternative for conventional planar structures
An in situ monitoring system attached to the metalorganic vapor phase epitaxy (MOVPE) reactor allows to control the growth parameters through measuring the reflectance and temperature of the wafer
During the AlN nucleation layer growth, the different AlN crystal faces grown on the cone-shape array patterns of a patterned sapphire substrates (PSSs) are observed (Fig. 1, scanning electron microscopy (SEM) micrograph I), to GaN growth reported by Wu et al [30]
Summary
The development of three-dimensional (3D) semiconductor structures with distinct architectures opens new routes for device design with novel features, becoming valuable technology for certain specialized purposes and alternative for conventional planar structures. The catalyst-assisted synthesis uses metallic seeds, namely Au and Ni, that act as nucleation sites for III-nitride growth [12, 13] This type of process is rather easy, providing access to versatile shapes and crystal orientations, as well with high surface-to-volume aspect ratio. SAG ( considered as catalyst-free technique) is a more common approach that uses a mask material, such as dielectric SiNx or SiO2 patterned layer (GaN does not nucleate on these materials), to control the distribution and size of nitride structures [11, 14, 15] This technique, with two distinct modes: pulsed and continuous, results in a dense and homogeneous array of rods with hexagonal cross section; the mask borders have been seen to be an additional source of structural defects [16]. SAG is a rather complex technique, composed of multiple growth procedures, including mask pattern deposition and actual growth of desired structures
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