In this paper we will study the influence of InGaN underlayer on efficiency of InGaN-based LEDs grown by plasma-assisted molecular beam epitaxy (PAMBE). We observed that LEDs with the thinnest underlayer have the highest efficiency. This finding agrees with the theory that the defects, which are buried in standard LEDs are in fact generated during the growth of GaN in MOVPE at high temperature. In case of PAMBE, the growth temperature of GaN is 300°C lower, and these defects are not generated in the first place and there is no need for an InGaN underlayer.
The innovative method of μLEDs fabrication is presented. The light emission area was defined by a size of the tunnel junction (TJ) embedded inside diode. The epitaxial structures were grown entirely by plasma assisted molecular beam epitaxy (PAMBE) on (0001) bulk GaN crystals. The PAMBE grown LED structure emitting light at 450 nm was capped with TJ region and 100 nm n-type GaN. The emission size of μLEDs was defined by ion implantation of n-type GaN and TJ region. The entire surface of the wafer is atomically flat, ready for the next epitaxial process, which is important e.g. for TJ µLEDs red-green-blue displays with a stack of 3 µLEDs.
Incorporation of tunnel junctions (TJs) to device structure enabled vertical integration of multicolor light emitting diodes (LEDs) and laser diodes (LDs). The TJs allows to control the current path in distributed-feedback LDs and micro-LEDs. It opens possibility to design new architecture devices like “inverted” LEDs or LDs with TJs located below active region. These devices have the sequence of p and n type layers similar to structures grown on hypothetical p-type (0001) GaN substrate, which is beneficial for high carrier injection efficiency, and enables operation at cryogenic temperatures. Finally, we also discuss the properties of bi-directional LEDs and wavelength-tunable LEDs.
We present LED profiting from the bottom-tunnel junction (BTJ) construction. The BTJ design aligns the polarization fields in a desired direction in the vicinity of active region and inverts the ordering of the layer stack in the structure. This leads the situation were conductive, n-type layer is on the very top of the structure. Since current spreading in n-type material is much better than in p-type, BTJ-based light emitters open new possibilities in heterostructure design. In this talk we present new light emitting structures grown by plasma-assisted MBE based on BTJ platform and compare prospects for bottom and top tunnel junction devices.
New approach towards efficient light emission with bottom-tunnel junctions is developed. The bottom-tunnel junction design aligns the polarization fields in a desired direction in the vicinity of quantum well, while simultaneously eliminating the need for p-type contacts, and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the heterostructures, increasing injection efficiency and opening new possibilities in heterostructure design. InGaN-based buried-tunnel junction is used to construct first monolithically grown p-type-down laser diode on n-type, Ga-polar bulk GaN substrate. Unique advantages of such construction that enables to separate design of carrier injection and optical mode confinement for such laser diode structures is discussed.
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