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.
We present a method of in-plane modification of the refractive index using ion implantation and electrochemical etching of GaN layers. Proposed method allows for the fabrication of embedded air-GaN channels that can be periodically arranged inside III-nitride heterostructures. Importantly, a flat top surface is preserved for further regrowth. High refractive index contrast between air and GaN makes the proposed technology attractive for the fabrication of embedded photonic structures such as diffraction gratings for distributed feedback laser diodes (DFB LDs). We discuss the impact of the different design of air-GaN channels on the properties of DFB LDs.
III-nitride semiconductor system is widely used in many electronic and optoelectronic applications. The presence of extremely high piezoelectric field in quantum wells (QWs) is known to cause severe separation of electron and hole wavefunctions and limits the thickness of QWs used in devices. We have recently shown that wide QWs are also a viable solution in optoelectronic devices. In this paper we will discuss the physics of recombination in wide InGaN QWs. We will show that the piezoelectric field is fully screened in case of wide InGaN QW and light emission occurs through excited states with high wavefunction overlap.
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.
The physics of the bottom tunnel junction (BTJ) and its improvement over standard p-up geometry in InGaN blue LEDs is quantified through pulsed power measurements. It is found that the peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) for a p-down BTJ LED is about threefold that of its counterpart, the p-up top tunnel junction (TTJ) LED. This is contributed to increased radiative recombination and reduced electron overflow. Further, the peaks occur at lower current densities for the BTJ device, suggesting earlier saturation of Shockley-Read-Hall traps. In the droop regime, where electron overflow, device heating, and 3-particle interactions are significant, the performance of the BTJ is found to be consistently better than that of the TTJ, converging at large current densities where the polarization fields are screened.
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.
In this work we study the growth mechanisms of InGaN in plasma-assisted molecular beam epitaxy (PAMBE). We investigate growth of InGaN layers on vicinal GaN (0001) substrates. Indium incorporation as a function of gallium and nitrogen fluxes was examined. We propose microscopic model of InGaN growth by PAMBE postulating different indium adatom incorporation mechanisms on two nonequivalent atomic step edges of wurtzite crystal. The different roles of gallium and nitrogen fluxes during the growth of InGaN layers is discussed.
We present recent progress in the growth of nitride based laser diodes (LDs) made by Plasma Assisted Molecular Beam Epitaxy (PAMBE). In this work we demonstrate LDs grown by PAMBE operating in the range 450 – 460 nm. The LDs were grown on c-plane bulk GaN substrates with threading dislocation density (TDD) ranging from 103 cm-2 to 104 cm-2. The low TDD allowed us to fabricate cw LDs with the lifetime exceeding 2000 h at 10 mW of optical output power. The maximum output power for 3 LDs array in cw mode was 280 mW and 1W in pulse mode. The low temperature growth mode in PAMBE allow for growth of AlGaN-free LDs with high In content InGaN waveguides. The key element to achieve lasing wavelengths above 450 nm was the substantial increase of the nitrogen flux available during the growth by PAMBE.
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