The stability and transport of native point defects at the interface between ammonothermally grown GaN substrates and MOVPE-grown GaN epilayers remain insufficiently understood, despite their relevance for highperformance GaN electronics. In this work, we investigate the long-standing question of whether native vacancies can diffuse from low-dislocation-density ammonothermal GaN into subsequently grown epitaxial layers. A combination of complementary techniques-including Secondary Ion Mass Spectroscopy (SIMS), C-V profiling, photoluminescence, Raman spectroscopy, and detailed modelling of variable-energy positron annihilation spectroscopy (VEPAS)-is employed to resolve vacancy distributions with high depth sensitivity. Our results consistently show that vacancy-related defects do not propagate from the substrate into the MOVPE epilayer. Instead, we identify a highly compensated interfacial region extending several hundred nanometres into the ammonothermal GaN. This region electrically decouples the epilayer from the substrate and generates builtin electric fields that may influence carrier transport and device behavior. These findings provide new insight into defect behaviour at GaN heterointerfaces and have direct implications for the optimization of ammonothermal GaN substrates for power and optoelectronic devices.

Optical absorption and reabsorption of light emitted from the active regions in nitride laser diodes (LDs) reduce the light extraction efficiency of these devices. The presence of Si and Mg can considerably increase optical absorption, an effect that is much stronger in the high-energy (short-wavelength) spectral range. This absorption increase is governed by the ionization of Si donor and Mg acceptor states, which are controlled by the electron and hole quasi-Fermi levels. We demonstrate that absorption increases due to enhanced Mg ionization caused by compensation in the p-type region, alongside high Si ionization in the n-type region. A theoretical explanation is provided for the observed increase in optical efficiency achieved by removing dopants from the waveguides. Furthermore, superior material quality is shown to yield low absorption levels, especially within the Mg-doped p-type region of the device.

The paper presents a systematic investigation of point defects in a series of high-quality GaN crystals grown through halide vapor phase epitaxy (HVPE) and ammonothermal synthesis (AM). The GaN crystals were characterized using positron annihilation spectroscopy (PAS), along with measurements of photoluminescence (PL) and ab-initio modelling of various point defect configurations and calculations of positron lifetimes. The concentration of impurities in each GaN crystal was determined using secondary ion mass spectrometry. Our study revealed that the concentration of gallium vacancies (VGa) in GaN crystals is influenced by the levels of hydrogen and oxygen impurities. Most HVPE GaN crystals exhibit low concentrations of hydrogen and oxygen, with no defects detectable by PAS. Only one HVPE crystal showed an enhanced concentration of oxygen, which is associated with VGa, forming negatively charged VGa+ON complexes, where one nitrogen nearest neighbour of the vacancy is substituted by oxygen. In contrast, AM GaN crystals contain at least two orders of magnitude higher concentrations of hydrogen and oxygen impurities. This results in a high concentration (>= 1018 cm-3) of VGa complexes with H and/or O. The configuration of these complexes is determined by the ratio of hydrogen to oxygen concentration within the crystal. The undoped n-type AM GaN crystal contains VGa+2H+ON complexes, while the manganese-doped semi-insulating AM crystal contains VGa+3H complexes, both complexes being in neutral charge state. The PL results provide evidence that the VGa+2H+ON and VGa+3H complexes are the source of the green emission bands at 2.34 eV and 2.37 eV, respectively, in AM crystals.

The objective of the present work is to explore the potential of germanium-doped gallium nitride (GaN:Ge), focusing specifically on the challenges and opportunities associated with Ge ion implantation (I/I) into GaN. Thin GaN layers were grown on semi-insulating native GaN substrates using the metal-organic vapor-phase epitaxy (MOVPE) technique and subsequently implanted with germanium ions. The implantation parameters were chosen to obtain 200-nm-thick layers with target doping levels ranging from 5 x 10(18) to 10(20) cm(-3). Following implantation, the samples were subjected to Ultra-High-Pressure Annealing (UHPA). High-resolution X-ray diffraction (HRXRD) measurements were performed to assess the structural damage recovery during UHPA. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to confirm the absence of thermal diffusion during this process. Density functional theory (DFT) calculations were carried out to explain the observed lack of diffusion in the implanted samples. Hall-effect measurements determined the free carrier concentration, mobility, and conductivity of the Ge-doped layers. A strong dependence of the electrical parameters on the annealing time was revealed. Prolonging the annealing time from 1 h to 5 h led to an increase in the free carrier concentration in all analyzed cases. While a further increase in annealing time (to 10 h) did not significantly augment the electron concentration, the carrier mobility and conductivity were enhanced. Finally, the ion-implanted samples were systematically compared with bulk HVPE-GaN:Ge crystals doped in situ and MOVPEgrown GaN:Ge layers in terms of their electrical properties.

In situ X-ray reciprocal space mapping was performed during the interval heating and cooling of InGaN/GaN quantum wells (QWs) grown via metal–organic vapor phase epitaxy (MOVPE). Our detailed in situ X-ray analysis enabled us to track changes in the peak intensities and radial and angular broadenings of the reflection. By simulating the radial diffraction profiles recorded during the thermal cycle treatment, we demonstrate the presence of indium concentration distributions (ICDs) in the different QWs of the heterostructure (1. QW, bottom, 2. QW, middle, and 3. QW, upper). During the heating process, we found that the homogenization of the QWs occurred in the temperature range of 850 °C to 920 °C, manifesting in a reduction in ICDs in the QWs. Furthermore, there is a critical temperature (T = 940 °C) at which the mean value of the indium concentration starts to decrease below 15% in 1. QW, indicating the initiation of decomposition in 1. QW. Moreover, further heating up to 1000 °C results in extended diffuse scattering along the angular direction of the diffraction spot, confirming the propagation of the decomposition and the formation of trapezoidal objects, which contain voids and amorphous materials (In-Ga). Heating InGaN QWs up to T = 1000 °C led to a simultaneous decrease in the indium content and ICDs. During the cooling phase, there was no significant variation in the indium concentrations in the different QWs but rather an increase in the defect area, which contributes to the amplification of diffuse scattering. A comparison of ex situ complementary high-resolution transmission microscopy (Ex-HRTEM) measurements performed at room temperature before and after the thermal cycle treatment provides proof of the formation of four different types of defects in the QWs, which result from the decomposition of 1. QW during the heating phase. This, in turn, has strongly influenced the intensity of the photoluminescence emission spectra without any detectable shift in the emission wavelength λMQWs.

We investigate the formation and characteristics of vacancy clusters (VCs) in gallium nitride (GaN) grown by metal–organic chemical vapor deposition. The research identifies vacancy-type defects, particularly V _Ga –H _i complexes, using positron annihilation spectroscopy, in as-grown sample which are transformed into VCs 2V _Ga –2V _N during low-energy electron beam irradiation. The study employs photoluminescence and cathodoluminescence spectroscopy to analyze the luminescence properties, demonstrating that yellow band and excitonic near-band-edge emission are greatly enhanced during the VC process. The results indicate that the luminescence spectra of the samples change significantly during electron beam irradiation, highlighting the dynamic behavior of defects in GaN under different conditions. The dominant change affecting the luminescence properties seems to be the elimination of a deep donor acting like a non-radiative centrum. The experimental evidence suggests that this deep donor is V _N . The findings contribute to a deeper understanding of the role of nitrogen vacancy, vacancy complexes and clusters in GaN, which is crucial for optimizing its applications in optoelectronic devices and their processing.

The focus of this study was the investigation of how the total pressure of reactants and ammonia flow rate influence the growth morphology of aluminum–gallium nitride layers crystallized by Halide Vapor Phase Epitaxy. It was established how these two critical parameters change the supersaturation levels of gallium and aluminum in the growth zone, and subsequently the morphology of the produced layers. A halide vapor phase epitaxy reactor built in-house was used, allowing for precise control over the growth conditions. Results demonstrate that both total pressure and ammonia flow rate significantly affect the nucleation and crystal growth processes which have an impact on the alloy composition, surface morphology and structural quality of aluminum–gallium nitride layers. Reducing the total pressure and adjusting the ammonia flow rate led to a notable enhancement in the homogeneity and crystallographic quality of the grown layers, along with increased aluminum incorporation. This research contributes to a deeper understanding of the growth mechanisms involved in the halide vapor phase epitaxy of aluminum–gallium nitride, and furthermore it suggests a trajectory for the optimization of growth parameters so as to obtain high-quality materials for advanced optoelectronic and electronic applications.

GaN/InGaN quantum wells (QWs), widely used as the active region in blue and green light emitters, are sus-ceptible to structural degradation at temperatures above 900 degrees C. The degradation process is initiated by the diffusion and clustering of gallium vacancies (VGa). The aim of this work is to determine how the vacancy population in different layers surrounding the QWs affects their thermal stability. Silicon and magnesium doping was used to manipulate the vacancy concentrations. Experimental results showed that the availability of VGa in the high-temperature (HT) GaN layer below the QWs has a clear effect on the degradation process. No effect of the vacancy concentration in the layer above the active region indicates that the VGa diffusion associated with QW degradation mainly occurs in the [0001] direction. Magnesium doping (2 x 1019 cm -3) in the HT underlayer reduces the VGa concentration, which improves the thermal stability of the closest QW, showing that vacancies in the barriers also contribute to the degradation process. By using heavy magnesium doping (1019 cm-3) in the barriers alone, the thermal stability of the QWs is significantly improved even when a typical HT n-type GaN underlayer is used. This shows that Mg atoms not only increase the formation energy of VGa, but also limit its diffusivity, enabling the use of GaN:Mg layers as diffusion barriers for VGa.

Refractive index engineering is a key element in the design of optoelectronic device structures. In this work we present a method for the fabrication of buried hollow channels, periodically arranged inside GaN structures. The method is based on selective area doping and electrochemical etching. Channels width is defined by the selective area ion implantation mask geometry while the channel vertical dimension depends on the implantation energy and annealing conditions. In this work we used implantation with Si 100 keV and dose 2.6.1015 cm-2 into GaN. Next, a 100 nm GaN layer was grown by plasma-assisted molecular beam epitaxy (PAMBE). After annealing at 1070 degrees C for 5 min, selectively doped regions were removed in the process of electrochemical etching (ECE), forming periodically arranged empty volumes. Annealing resulted in partial strain removal as observed by X-Ray diffraction. Surface roughness was evaluated at every stage of the process: after implantation, after regrowth and after annealing, and it is promising for the integration of the proposed technology with device fabrication and processing. The flexibility in the mask design for selective ion implantation and a high refractive index contrast between GaN and air, make the proposed technology attractive for the fabrication of buried photonic structures such as diffraction gratings.

Thermal instability of InxGa1-x N quantum wells (QWs) is an obstacle to construct efficient blue and green LEDs and laser diodes. Structural degradation of QWs with indium content above 15% becomes severe at temperatures above 930 degrees C leading to formation of extended non-radiative areas within the active region. Our previous studies (Smalc-Koziorowska, 2021) indicated a relationship between the degradation process and metal vacancies present in the layers adjacent to the QWs. In this work, we show a method to overcome this problem by using heavy Si doping of the GaN barrier layers. In particular, such barrier layer grown on the top of n-type GaN layer below the InGaN QWs can act as a diffusion barrier for vacancies. The presence of silicon atoms increases the energy barrier for gallium vacancies migration. This effectively reduces possibility of diffusion of gallium vacancies from the n-type layer to the active region. As a result, improved thermal stability of QWs was achieved and significant degradation was not observed up to temperatures of 980 degrees C in comparison to 930 degrees C for the undoped structure. (C) 2021 Elsevier B.V. All rights reserved.

The thermal emission rate of electrons from a 0.25 eV trap, widely observed in epitaxial GaN by conventional deep-level transient spectroscopy (DLTS), is investigated by the Laplace DLTS (LDLTS) technique. It is demonstrated that the capacitance relaxation waveforms, from which the properties of this trap have been determined until now by the correlation procedure, contain two exponential components induced by the thermal emission of electrons from two traps with the activation energies of 173 meV and 232 meV. In lightly silicon-doped and carbon-doped epitaxial GaN grown on sapphire substrates, the concentrations of these traps were found to be (5.5–7.2) × 1013 and (1.5–3.2) × 1013 cm−3, respectively. Based on formerly reported experimental results and recently published results of theoretical calculations performed with the implementation of the density functional theory with hybrid functionals, the point defects being a potential source of these traps is proposed. It is shown, that a high resolution of LDLTS gives a new insight into the properties of energy levels related to point defects in epitaxial GaN.

The aim of this paper is to give an experimental evidence that point defects (most probably gallium vacancies) induce decomposition of InGaN quantum wells (QWs) at high temperatures. In the experiment performed, we implanted GaN:Si/sapphire substrates with helium ions in order to introduce a high density of point defects. Then, we grew InGaN QWs on such substrates at temperature of 730 °C, what caused elimination of most (but not all) of the implantation-induced point defects expanding the crystal lattice. The InGaN QWs were almost identical to those grown on unimplanted GaN substrates. In the next step of the experiment, we annealed samples grown on unimplanted and implanted GaN at temperatures of 900 °C, 920 °C and 940 °C for half an hour. The samples were examined using Photoluminescence, X-ray Diffraction and Transmission Electron Microscopy. We found out that the decomposition of InGaN QWs started at lower temperatures for the samples grown on the implanted GaN substrates what provides a strong experimental support that point defects play important role in InGaN decomposition at high temperatures.

The present paper focuses on the high-pressure metal-organic vapor phase epitaxy (MOVPE) upside-down vertical reactor (where the inlet of cold gases is below a hot susceptor). This study aims to investigate thermo-kinetic phenomena taking place during the GaN (gallium nitride) growth process using trimethylgallium and ammonia at a pressure of above 2 bar. High pressure accelerates the growth process, but it results in poor thickness and quality in the obtained layers; hence, understanding the factors influencing non-uniformity is crucial. The present investigations have been conducted with the aid of ANSYS Fluent finite volume method commercial software. The obtained results confirm the possibility of increasing the growth rate by more than six times through increasing the pressure from 0.5 bar to 2.5 bar. The analysis shows which zones vortexes form in. Special attention should be paid to the transitional flow within the growth zone as well as the viewport. Furthermore, the normal reactor design cannot be used under the considered conditions, even for the lower pressure value of 0.5 bar, due to high turbulences.

In this work, we study the thermal degradation of In-rich InxGa1-xN quantum wells (QWs) and propose explanation of its origin based on the diffusion of metal vacancies. The structural transformation of the InxGa1-xN QWs is initiated by the formation of small initial voids created due to agglomeration of metal vacancies diffusing from the layers beneath the QW. The presence of voids in the QW relaxes the mismatch stress in the vicinity of the void and drives In atoms to diffuse to the relaxed void surroundings. The void walls enriched in In atoms are prone for thermal decomposition, what leads to a subsequent disintegration of the surrounding lattice. The phases observed in the degraded areas of QWs contain voids partly filled with crystalline In and amorphous material, surrounded by the rim of high In-content InxGa1-xN or pure InN; the remaining QW between the voids contains residual amount of In. In the case of the InxGa1-xN QWs deposited on the GaN layer doped to n-type or on unintentionally doped GaN, we observe a preferential degradation of the first grown QW, while doping of the underlying GaN layer with Mg prevents the degradation of the closest InxGa1-xN QW. The reduction in the metal vacancy concentration in the InxGa1-xN QWs and their surroundings is crucial for making them more resistant to thermal degradation.

In the first part of the presentation, main applications of the GaN -based laser diodes are shown. These include lighting, RGB projectors, optical communication, Quantum Technologies, welding of copper and gold. In the second part, the main material issues will be discussed, in particular, presence of point- and extended-defects and how they influence the properties of laser diodes. © 2019 The Author(s)

The horizontal excitation energy transport in the range of tens of micrometers was measured in high quality homoepitaxial InGaN quantum wells (QWs) with the use of time and space resolved micro-photoluminescence as a function of (i) applied vertical electric field, (ii) temperature, and (iii) linear density of atomic steps. The investigated structure consisted of InGaN QWs inside a p-n junction. The indium content in QWs was designed to be different in defined areas of the sample (due to mastering of different off-cuts and atomic steps density) so that the wells could emit at energies from 2.6 to 2.86 eV. The horizontal transport range was sensitive to the vertical electric field, which means that it could not be just a radiation transfer, but charge carriers must have been involved as well. We found that the transport range decreased for higher slope angles, possibly due to stronger scattering on atomic steps when their linear density became higher. The diffusion coefficients reached 6 cm2/s and due to long lifetime of even 2 μs, the diffusion length was even LD = 30 μm in areas of low off-cut angles. We discuss possible mechanisms of transport and conclude that for such high diffusion constant, the most probable is the excitonic transport. The LD was maximum at 40 K and then decreased significantly with temperature, which was probably caused by thermal dissociation of excitons.

The distance between the showerhead and the sample surface (GAP) is one of the main growth parameters of the commonly used research reactor, Close Coupled Showerhead. We examine its influence on the growth rate of GaN layers deposited under various conditions (growth temperature, carrier gas, V/III ratio and growth pressure). Regardless of other growth parameters, increasing the GAP value leads to a reduction in the growth rate.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 µm and their height was 4 and 1 µm. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 µm high stripes and less pronounced for the shallower 1 µm high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

The linewidth of the luminescence should be the reflection of the system disorder, and the natural way of thinking is to associate this disorder with local variation of In composition and/or quantum wells thickness. Problem of the emission linewidth of InGaN quantum structures has been discussed over last two decades. In the early work of O’Donnel et al.1, the remarkable, universal character of the emission broadening in InGaN quantum wells and epilayers was reported. Authors argued that since the line broadening is apparently independent of the growth method, it must reflect an inherent feature of these structures. Indeed, up to our knowledge there is presently no data showing a direct relation between the emission linewidth and microscopic landscape of InGaN structures. To look more closely on the origin of emission line broadening we performed cathodoluminescence (CL) mapping, using Hitachi SU-70 scanning electron microscope equipped with Horiba Jobin Yvon optical detection system. We analyzed InGaN/GaN quantum wells in wide range of In content. Our measuments show that over large span of image diameter, the cathodoluminescence linewidth remains almost constant, meaning that what we perceive as the visible In fluctuation landscape is practically irrelevant. Most likely the observed source of broadening is related to submicron size effects related to strong carriers localization. Reference: 1. K. P. O’Donnell, T. Breitkopf, H. Kalt, W. Van der Stricht, I. Moerman, P. Demeester, and P. G. Middleton, “Optical linewidths of InGaN light emitting diodes and epilayers”, Appl. Phys. Lett. 70, 1843 (1997)