Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

Abstract

Group III-nitride semiconductors have been recognized as very important materials for opto-electronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) for applications in the visible and ultra-violet (UV) regions. To produce highly efficient light emission in LEDs, it is important to realize pure and almost perfect semiconductor materials in epitaxial growth. In most LED materials, it is desirable to have dislocation densities of 10 4 cm -2 or less for highly efficient light emission.In practice, GaN layers are grown on sapphire substrates (Al 2 O 3 ) by metalorganic chemical vapor deposition (MOCVD) at high temperature. Because of the difference in crystal structure and atomic spacing of sapphire substrate and GaN layer, so-called lattice mismatch, the GaN layers are formed and grown with a high density of threading dislocations with a typical density of 10 8 -10 9 cm -2 . This high density of threading dislocations associated with these GaN devices was a major obstacle in commercializing blue and green LEDs. Extended defects such as threading dislocations decrease the efficiency of LEDs because threading dislocations are associated with non-radiative recombination centers. Therefore, the efficiency of LEDs made in hetero-epitaxial growth decreases dramatically as the dislocation density increases.But it was discovered that adding a certain amount of In (indium) to the GaN layers increases the LED efficiency dramatically despite very high dislocation densities. This discovery has been the most important advance in allowing high efficiency blue and green LEDs. To explain the unexpected highly efficient light emission in InGaN based LEDs despite the existence of high dislocation density above 10 8 -10 9 cm -2 , a high density of In clustering or In rich regions was proposed to be essential in InGaN based active layers. Because the bandgap of the In rich regions is smaller than that of the surrounding InGaN materials, radiative carrier recombination is favored in these regions. The existence of In clustering has been often observed by photo-luminescence, cathodoluminescence, and high resolution transmission electron microscopy (HR-TEM) measurements. Especially, cross-sectional TEM and X-ray fluorescence measurements have been believed to be direct tools for detecting In clustering and used to optimize the density of In clusters or In rich quantum dots in the InGaN active layers to increase the internal quantum efficiency of InGaN based LEDs.The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). The average In mole fraction by three-dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis.The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be nonuniform in the InGaN active layer. In clustering or In ...

Composition Fluctuation of In and Well-Width Fluctuation in InGaN/GaN Multiple Quantum Wells in Light-Emitting Diode Devices

“…sapphire substrates by metal-organic vapor-phase epitaxy. To analyze the crosssectional microstructure of InGaN/GaN MQWs, crosssectional TEM specimens and needle-type APT specimens with a radius of curvature ,50 nm were prepared by using a focused ion beam~FIB; Gu et al, 2009!. To minimize any surface damage associated with Ga ions during initial rough milling at 30 kV, the samples were protected by a 100-nmthick sputter-deposited Ni layer and a 2-mm-thick ion-beaminduced Pt layer deposited in the FIB. In addition, the sample was milled in the final stage at a low acceleration voltage of 1-5 kV for a few minutes.…”

An efficiency droop model of the saturated radiative recombination rate and its verification by radiative and nonradiative carrier lifetime measurements in InGaN-based light emitting diodes

“…The lateral sizes of In-rich area are reported in a wide range from a few nm to hundreds of nm and they form a sort of Inrich quantum disks (Q-disks) or quantum dots (Q-dots) [13,14]. The volume of Q-disks (or Q-dots) are much smaller than the physical volume of a QW, namely V eff << V a .…”

“…Lots of experimental evidences have suggested the existence of In-rich areas in the active layer of the InGaN-based blue LED, which can act as localization centers of carriers and behave more like quantum disks (Q-disks) or quantum dots (Q-dots) with reduced density of states [13][14]. The existence of In-rich areas implies that the effective active volume is smaller than the nominal volume of QWs.…”

Analysis of statistical compositional alloy fluctuations in InGaN from aberration corrected transmission electron microscopy image series

“…To calculate a a parameter, suiting to the considered mean alloy composition and relaxation state, we use a linear fit between the c-lattice constant in GaN and the (pseudomorphic) uniaxially strained c-lattice parameter of In x Ga 1Àx N for x ¼ 0.3, calculated from Eqs. (3) and (5). Thus, we calculate a Uniaxial according to a Uniaxial % c 0 ðx ¼ 0:3Þ À cðx ¼ 0Þ 0:3 ;…”

“…1-3 Whether clustering, short-range ordering or random alloying dominates and how the alloy composition is influenced by the growth conditions is still under debate in both theoretical and experimental works. [4][5][6][7] A deeper insight into the relation between the compositional fluctuations and the optical properties requires, in the first instance, experimental access to the structural parameters on the atomic scale. A method that is commonly applied to such problems is high resolution transmission electron microscopy (HRTEM), e.g., in Refs.…”