Mikołaj Żak

The origin of hillocks in AlGaN layers grown on high-temperature annealed (HTA) AlN templates has been reinvestigated. During the HTA process, AlON precipitates form within the AlN matrix due to oxygen contamination. The thermal mismatch between the precipitates and the compressively strained AlN generates shear stress on pyramidal glide planes, which provide the glide system for (a + c) dislocations. Importantly, the proximity of a precipitate to the free surface amplifies this localized shear field and lowers the barrier for half-loop formation, making subsurface precipitates the most effective dislocation sources. Once the shear stress reaches a critical value, a single (a + c) dislocation half-loop nucleates. The threading segments of the loop terminate at the HTA-AlN surface and trigger spiral growth during AlGaN epitaxy, ultimately leading to hillock formation.

This study investigates the impact of growth polarity on the optical and electrical performance of GaN-based light-emitting diodes (LEDs) by comparing structures grown on N-polar and Ga-polar GaN bulk substrates. For a direct comparison, the Ga-polar LED utilized a bottom tunnel junction to align the built-in polarization field with the current flow, replicating the configuration of the N-polar LED. It exhibited an emission intensity over three orders of magnitude higher, despite having a similar layer composition to its N-polar counterpart. However, the N-polar LED featured a smoother surface and a lower turn-on voltage. The Ga-polar LED demonstrated lower differential resistance and superior quantum well optical quality, as indicated by the current density value at maximum external quantum efficiency. Ga-polar LEDs with a bottom tunnel junction outperformed N-polar devices in terms of emission intensity while maintaining the advantageous built-in field alignment typically associated with N-polar LEDs. This suggests that the inferior performance of N-polar LEDs stems from challenges in the epitaxy process on the N-side of the GaN crystal.

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.

We analyze theoretically an InGaN/GaN n-i-p diode with a single quantum well supporting only one bound state. The bottom parts of the diode, namely the first barrier and the quantum well, are heavily n-doped with silicon at 5 × 10¹⁹ cm⁻³ to ensure a high electron concentration in the well. The voltage drop in the diode occurs in the second AlGaN barrier, which is undoped, and structure ends with a p-doped GaN. The band structure of the diode is calculated by a Schrodinger–Poisson drift-diffusion solver. Next, we calculate the absorption from the bound state in the well to the “continuum” above the well. We show the oscillatory behavior of the spectrum, with the amplitude decreasing with more negative voltage applied to the diode. Oscillations are due to interferences of the wavefunctions between the edges of the well and the slope of the potential barrier.

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.

The GaN-based light emitting diodes (LEDs) brought a revolution in the lighting market by becoming the most energy-efficient light sources. However, the power grid, i.e. electricity delivery system, is built based on alternating current (AC), which raises problems for directly driving LEDs that require direct current (DC) to operate effectively. In this paper, we demonstrate a proof of concept device that addresses this fundamental issue – a GaN-based bidirectional light emitting diode (BD LED). The BD LED structure is symmetrical with respect to the active region, which, depending on the negative or positive 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. The optical and electrical properties of such BD LEDs under DC and AC conditions are investigated. We find that the light is emitted in both directions of supplied current, contrary to the standard LEDs, hence BD LEDs can be considered as a semiconductor light source powered directly with AC. In addition, we show that BD LEDs can be stacked vertically to multiply the optical power achieved from a single device.

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.

In this work, we present a GaN-based blue LED construction utilizing bottom tunnel junction (TJ) grown by plasma-assisted molecular beam epitaxy. The setup allows for N-polar-like built-in field alignment while being grown on a Ga-polar substrate. In this study, we present an efficient bottom TJ LED in which the distance between the quantum well and device surface is only 25 nm. This is achieved by the utilization of an n-type current spreading layer consisting of 20 nm thick In_0.02Ga_0.98N with a Ge doping concentration of 7 × 10¹⁹ cm⁻³. Heavily Ge and Mg doped bottom TJs allowed achieving a low LED turn-on voltage of 2.75 V at 20 A/cm² and a differential resistivity of 4.7 × 10⁻⁴ Ω cm² at 1 kA/cm². The device maintained high crystal quality and smooth morphology, which allows for its use as a light emitting platform for further integration. Although the p-up reference LED exhibits lower resistivity at high current, its luminous efficiency is lower than for bottom TJ LEDs.

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.