We demonstrate a stack of two III-nitride laser diodes (LDs) interconnected by a tunnel junction grown by plasma-assisted molecular beam epitaxy. Hydrogen-free growth is used to obtain as-grown p-type conductivity essential for buried tunnel junctions (TJ). We show the impact of the design of tunnel junction. In particular, we show that, apart from the beneficial piezoelectric polarization inside the TJ, heavy doping reduces the differential resistivity even further. The device starts to lase at a wavelength of 459 nm with a slope efficiency (SE) of 0.7 W/A followed by lasing at 456 nm from the second active region doubling the total SE to 1.4 W/A. This demonstration opens new possibilities for the fabrication of stacks of ultraviolet and visible high power pulsed III-nitride LD.

We demonstrate the applications of tunnel junctions (TJs) for new concepts of monolithic nitride-based multicolor light emitting diode (LED) and laser diode (LD) stacks. The presented structures were grown by plasma-assisted molecular beam epitaxy (PAMBE) on GaN bulk crystals. We demonstrate a stack of four LDs operated at pulse mode with emission wavelength of 453 nm. The output power of 1.1 W and high slope efficiency of 2.3 W/A is achieved for devices without dielectric mirrors. Atomically flat surface after the epitaxy of four LD stack and low dislocation density is measured as a result of proper TJ design with optimized doping level. The strain compensation design with InGaN waveguides and AlGaN claddings is shown to be crucial to avoid cracking and lattice relaxation of the 5 µm thick structure. Vertical connection of n-LDs allows for cascade emission of photons and increases the quantum efficiency n-times. The two-color (blue and green) LEDs are demonstrated. Application of TJs simplifies device processing, reducing the need for applications of p-type contact. The key factor enabling demonstration of such devices is hydrogen-free PAMBE technology, in which activation of buried p-type layers is not necessary.

Traditionally, Nitride semiconductors have suffered from poor p-type conductivity, requiring Mg activation by removing hydrogen from grown layers either through thermal annealing or electron irradiation. This requirement restricts the growth of buried p-type layers. Here, we report structures obtained using a Hydrogen-free growth technique – plasma assisted molecular beam epitaxy. Using this method, top and bottom tunnel junctions are realized for top and bottom contacts to traditional Ga-polar devices. Advantages of using both constructions are discussed. The efficiency of the bottom-tunnel junction design is presented through realization of a stable laser diode operating at room temperature. Further work needed to improve tunnel junction performance as well as optical mode confinement to fully benefit from this design is outlined.

Properties of tunnel junctions (TJs) grown by plasma-assisted molecular beam epitaxy are investigated. Examined TJs consist of a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum well (QW) sandwiched between ${\mathrm{In}}_{0.02}{\mathrm{Ga}}_{0.98}\mathrm{N}$ barriers. The influence of piezoelectric polarization, doping, and width of a TJ QW on resistivity of TJs is studied. A decrease of TJ resistivity is observed for high doping and high ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ composition. Additionally, the crystal quality of TJs as a function of magnesium and silicon doping in a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ QW is discussed. Generation of defects in highly doped layers is observed in transmission electron microscopy and by defect selective etching. High doping leads to deterioration of surface morphology, which is corroborated by atomic force microscopy imaging. TJs with low resistance and high crystalline quality, without formation of structural defects, are demonstrated. A theoretical model for calculation of tunneling currents is proposed based on Kane tunneling theory, which considers a realistic electric field distribution in a TJ. A good agreement with the experimental data is obtained. The model is used to predict electrical parameters in a wide range of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ compositions (x = 0–0.40), doping ($n_{\mathrm{Mg},\mathrm{Si}}={10}^{17}-5\times {10}^{21}{\mathrm{cm}}^{-3}$), and TJ QW width (d = 2 – 12 nm).

Gallium nitride-based light-emitting diodes have revolutionized the lighting market by becoming the most energy-efficient light sources. However, the power grid, in example electricity delivery system, is built based on alternating current, which raises problems for directly driving light emitting diodes that require direct current to operate effectively. In this paper, we demonstrate a proof-of-concept device that addresses this fundamental issue – a gallium nitride-based bidirectional light-emitting diode. Its structure is symmetrical with respect to the active region, which, depending on the positive or negative bias, allows for the injection of either electrons or holes from each side. It is composed of two tunnel junctions that surround the active region. In this work, the optical and electrical properties of bidirectional light emitting diodes are investigated under direct and alternating current conditions. We find that the light is emitted in both directions of the supplied current, contrary to conventional light emitting diodes; hence, bidirectional light-emitting diodes can be considered a semiconductor light source powered directly with alternating current. In addition, we show that bidirectional light-emitting diodes can be stacked vertically to multiply the optical power achieved from a single device.

Improved process for the growth of NbN by plasma-assisted molecular beam epitaxy (PAMBE) is needed for the fabrication of Josephson junctions, that are building blocks of qubits in quantum computers. Here we propose an approach to grow cubic δ-NbN on (0001) GaN substrates by PAMBE. We use metal-rich conditions with indium as a surfactant to facilitate the growth of NbN layers, InAlN/NbN, and InGaN/NbN superlattices. For In-rich conditions: (i) structural and electrical quality of NbN is improved as confirmed by larger grain size and higher transition temperature to superconducting phase, (ii) the growth of thin InAlN and InGaN layers on NbN is possible with flat interfaces. High critical current density up to 1 kA cm⁻² was obtained for NbN/InAlN/NbN Josephson junctions with 3 nm tunneling barrier. Indium-rich conditions allow the integration of δ-NbN with other III-N compounds by PAMBE preserving the high structural quality of the grown heterostructures.