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Aluminum nitride (AlN)-based acoustic filters are key devices of radio-frequency communications. However, the performance of electroacoustic resonators remains limited by the crystalline quality of the piezoelectric AlN material. An innovative material strategy that combines different types of materials (2D and 3D materials) and growth techniques is presented here to enhance the crystalline quality of thick AlN films grown on silicon-based substrates. Building upon previous works showing the efficacy of 2D MoS2 in sputtered-AlN texturing, this paper demonstrates how an in situ thermochemical treatment of the sputtered-AlN/MoS2 seed layers succeeded by a metal organic vapor phase epitaxy (MOVPE) AlN regrowth significantly enhances the crystalline quality of the AlN layer and avoids the detrimental stack delamination at the weak AlN/MoS2 interface. This paper shows that a delicate balance in the thickness of the sputtered-AlN film should be found to facilitate species diffusion toward the underlying MoS2 without compromising its texturizing properties for the subsequent MOVPE AlN regrowth. The sputtered-AlN/MoS2 seed layers involve a nanometric AlN film of less than 5 nm deposited onto three monolayers of MoS2. The heat treatment induces an unexpected chemical and structural nanometric reorganization at the interface, converting the initial lamellar MoS2 film into a discontinuous covalently bonded MoNxOy film, followed by the appearance of metallic Mo nanoparticles at an elevated temperature. Subsequent MOVPE AlN regrowth on the annealed seed layer stacks enables the growth of thick AlN films of up to 600 nm with a mosaicity of less than 0.3° without delamination or cracks. This approach not only facilitates the integration of thick AlN films on Si-based wafers but also opens avenues for growing III–N or other 3D materials on Si through the intercalation of 2D materials. [ABSTRACT FROM AUTHOR]

The origin of the nanoscale emission inhomogeneities of red emitting InGaN/InGaN quantum wells (QWs) grown directly on a GaN template and on an InGaN on sapphire (InGaNOS) substrate is investigated. InGaNOS is a partly relaxed InGaN pseudo-substrate fabricated by Soitec. As the latter approach provides an interesting optical internal quantum efficiency of 6.5% at 624 nm at 290 K, a deeper study, at the microstructure level, was conducted. The emission inhomogeneities on InGaNOS were highlighted by cathodoluminescence wavelength mappings where three areas were chosen: one emitting at a shorter wavelength, i.e., 588 nm, and two at a longer wavelength, i.e., 607 and 611 nm. Specimens from these zones were extracted by focused ion beam milling to perform cross-sectional characterization techniques. High-angle annular dark field scanning transmission electron microscopy images demonstrated that, while red emitting areas present homogeneous QWs, shorter wavelength areas exhibit non-uniform QWs, in terms of thickness and In composition. Complementary deformation mappings in the growth direction obtained by geometrical phase analysis show that longer emission wavelengths are originating from homogeneous QWs with an InN mole fraction evaluated at 39.0 ± 1.5%. This result demonstrates the possibility of achieving red emission with a coherent (In,Ga)N alloy when using an adapted substrate. A comparison of identical QWs grown on a GaN template is also given. [ABSTRACT FROM AUTHOR]

The full InGaN structure is used to achieve red light emitting diodes (LEDs). This LED structure is composed of a partly relaxed InGaN pseudo-substrate fabricated by Soitec, namely, InGaNOS, a n-doped buffer layer formed by a set of InxGa1−xN/GaN superlattices, thin InyGa1−yN/InxGa1−xN multiple quantum wells, and a p doped InxGa1−xN area. p-doped InGaN layers are first studied to determine the optimal Mg concentration. In the case of an In content of 2%, an acceptor concentration of 1 × 1019/cm3 was measured for a Mg concentration of 2 × 1019/cm3. Red electroluminescence was then demonstrated for two generations of LEDs, including chip sizes of 300 × 300 μm2 and 50 × 50 μm2. The differences between these two LED generations are detailed. For both devices, red emission with a peak wavelength at 620 nm was observed for a pumping current density of 12 A/cm2. Red light-emission is maintained over the entire tested current range. From the first to the second LED generation, the maximum external quantum efficiency, obtained in the range of 17–40 A/cm2, was increased by almost one order of magnitude (a factor of 9), thanks to the different optimizations. [ABSTRACT FROM AUTHOR]

The distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 × 1019 cm−3. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 × 1020 cm−3, despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential. [ABSTRACT FROM AUTHOR]

The full InGaN structure was grown on two different InGaNOS substrates from Soitec. An electron blocking layer was inserted in the full InGaN light emitting diode (LED). Enhanced internal quantum efficiency of red emitting InGaN/InGaN quantum wells was measured with a value above 10% at 640 nm. 10 μm diameter circular micro-LEDs are emitted at 625 nm with an external quantum efficiency of 0.14% at 8 A cm−2 with an estimated light extraction efficiency below 4%. With a a lattice parameter of 3.210 Ĺ, InGaN based red LED can also emit up to 650 nm. [ABSTRACT FROM AUTHOR]

Planar UV-C light emitting diodes still suffer from low efficiency, mainly due to substrate crystalline quality, p doped conductivity and extraction efficiency. One possible way to overcome partly these issues is to realize the whole UV structure on AlGaN pyramids by selective area growth in order to benefit from the advantages of such structures, i.e. the dislocation filtering and the semi polar planes. We present here a detailed study about the epitaxy of AlGaN nano-sized pyramids by metal organic vapor phase epitaxy on patterned templates presenting different holes apertures and pitches as 1.5 μm and 4 μm or 100 nm and 250 nm respectively. While increasing the Al content, their height decreases while the thickness of the deposition on the mask increases whatever the design of the mask. Those changes of the pyramid shapes and deposition are directly linked to the properties of Al adatoms, i.e. low Al diffusion length. Using the conventional growth mode for the epitaxy of those pyramids did not permit the incorporation of Al from the base of the pyramids to their truncated apex. Its presence was concentrated on the edges and top of the pyramids. On the contrary, a pulsed growth mode, coupled with a strongly reduced pitch, allowed an incorporation of Al since the base of the nanopyramid, and a decrease of the deposition height on the mask. These results can be explained by the desorption of Ga species, due to the presence of H2 in the reactor chamber during the step without the metal precursors, leading to a higher Al/Ga ratio. It is even enhanced inside the holes by the reduced pitch. [ABSTRACT FROM AUTHOR]

Micro light emitting diodes have been grown by metal organic vapor phase epitaxy on standard GaN and partly relaxed InGaNOS substrates with the purpose of incorporating higher concentrations of indium for identical growth conditions. Green emission has been demonstrated at wavelengths of 500 nm for the GaN template and 525 and 549 nm for the InGaNOS substrates, respectively. The structure, deformation, indium concentration and piezoelectric potentials have been measured with nm-scale spatial resolution in the same specimens by transmission electron microscopy. We show by off-axis electron holography that the piezoelectric potential and information about the indium concentration from the mean inner potential are obtained simultaneously. By separating the components using a model, we show that for higher concentrations of indium in the quantum wells (QWs) grown on InGaNOS substrates, the piezoelectric potentials are reduced. The measurements of the indium concentrations by electron holography have been verified by combining energy dispersive x-ray spectrometry, x-ray diffraction and from the tensile deformation made by precession electron diffraction. A discussion of the limitations of these advanced aberration-corrected transmission electron microscopy techniques when applied to nm-scale QW structures is given. [ABSTRACT FROM AUTHOR]

In this work, the growth of InGaN on epitaxial graphene by molecular beam epitaxy is studied. The nucleation of the alloy follows a three-dimensional (3D) growth mode in the observed temperature range of 515 °C–765 °C, leading to the formation of dendrite-like islands. Careful Raman scattering experiments show that the graphene underneath is not degraded by the InGaN growth. Moreover, lateral displacement of the nuclei during an atomic force microscopy (AFM) scan demonstrates weak bonding interactions between the InGaN and the graphene. Finally, a longer growth time of the alloy gives rise to a compact thin film in a partial epitaxial relationship with the SiC underneath the graphene. [ABSTRACT FROM AUTHOR]

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons τ increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing τ depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs. [ABSTRACT FROM AUTHOR]

We present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs. [ABSTRACT FROM AUTHOR]

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and [formula] nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL/dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV(cm GPa)-1. [ABSTRACT FROM AUTHOR]

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I1 stacking faults in the overall recombination mechanism of excitons. [ABSTRACT FROM AUTHOR]

We have studied the properties of Mg-doped GaN epilayers grown by molecular beam epitaxy (MBE) with ammonia as nitrogen source. GaN p-n homojunctions has been developed to determine the optoelectronic characteristics of the junctions as a function of the p-type GaN growth conditions. It is shown that the electrical characteristics strongly depend on the Mg flux and the growth temperature. As a result, the junction characteristics have been drastically improved and state of the art MBE-grown p-type layers have been obtained: the hole concentration, the mobility, and the resistivity are 1×1018 cm-3, 9 cm2/V s, and 0.75 Ω cm, respectively. These characteristics lead to an increase of the homojunction light emitting diode (LED) optical output power by two orders of magnitude. To further assess the quality of these MBE-grown p-type layers, we have prepared a hybrid LED which consists of an InGaN/GaN quantum well active structure grown by metal organic vapor phase epitaxy followed by a p-type region grown by MBE. An optical power in the milliwatt range at 20 mA is demonstrated confirming thereby the quality of low temperature MBE-grown p-type layers. [ABSTRACT FROM AUTHOR]

We analyze the room temperature photoluminescence properties of several multilayer stackings of GaN/AlN quantum dots. We report drastic differences of emission energies and linewidths between continuous wave and time-resolved photoluminescence experiments. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift as well as an unexpected narrowing of the emission line, when the laser intensity is increased. Under intense, pulsed excitation, in time-resolved photoluminescence, a substantial blue-shift is induced, due to the partial cancelation of the quantum confined Stark effect. When the system is again free to relax, we observe a time-dependent red-shift of the line, which maintains a fairly constant width. We attribute the observed behavior of energies and linewidths to the intricate contributions of the in-plane distribution of dot sizes and of the depth-dependent decrease of the degree of excitation of the different planes. We support our interpretations by the use of a model based on a self-consistent solution of the Schrödinger and Poisson equations within the envelope function approximation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) has been applied to study optical transitions between the ground and excited states in polar and a-plane nonpolar 2 nm wide GaN/Al0.12Ga0.88N quantum well (QW) structures. In addition to the fundamental transition, CER features related to optical transitions between excited states were clearly observed for the polar QW structure whereas such features were not observed for the nonpolar QW structure. This experimental result clearly shows that the polarization-related electric field leads to a quantum confinement of some extra states in the polar QW system. Such states are not confined in the nonpolar QW and, therefore, optical transitions between them are not detected, i.e., only the fundamental transition is observed in CER spectrum of the nonpolar QW. [ABSTRACT FROM AUTHOR]

The impact of a relaxed InGaN pseudosubstrate on indium incorporation in a full InGaN heterostructure was investigated. Three types of InGaN pseudosubstrates were tested with different a lattice parameters ranging from 3.190 to 3.205 Å, that is to say, greater than that of a GaN template on sapphire. Samples were loaded together in the growth chamber in order to apply exactly the same growth conditions. The effect of the photoluminescence (PL) emission redshift was observed on InyGa1-yN buffer layers and also on InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs). It was found that these pseudosubstrates have the ability to improve the indium incorporation rate, with an increasing effect as the a lattice parameter increases. A strong PL emission redshift was observed in InxGa1-xN/InyGa1-yN MQWs as a function of the increasing a lattice parameter of the InGaN pseudosubstrate, compared to a reference grown on a GaN template. It has been shown that green and amber emissions can be easily reached. A redshift of up to 42 nm was detected between various InGaN pseudosubstrate samples and up to 62nm compared to a conventional structure emerged from a GaN buffer on the sapphire substrate. The average QW width less than 3 nm indicates a higher In content. The reduced compressive strain originating from the relaxed InGaN substrate allows the reduction in the compositional pulling effect and consequently enables an enhanced In incorporation rate. [ABSTRACT FROM AUTHOR]

Combined thermoelectric-resistivity measurements and micro-Raman experiments have been performed on single heavily Si-doped GaN wires. In both approaches, similar carrier concentration and mobility were determined taking into account the non-parabolicity of the conduction band. The unique high conductivity of Si-doped GaN wires is explained by a mobility μ = 56 cm2·V-1·s-1 at a carrier concentration n = 2.6 × 1020 cm-3. This is attributed to a more efficient dopant incorporation in Si-doped GaN microwires as compared to Si-doped GaN planar layers. [ABSTRACT FROM AUTHOR]

The effect of thermal annealing on In0.25Ga0.75N/GaN quantum wells grown by molecular beam epitaxy at 550 °C is investigated. A strong increase in the 300 K photoluminescence (PL) intensity is observed for samples annealed at 880 °C, while degradation occurs for higher temperatures. The improvement of the optical properties is ascribed to higher internal quantum efficiency (IQE), as indicated by temperature-dependent and time-resolved PL experiments. The effect of carrier localization due to possible quantum dot formation via indium clustering is ruled out based on high-resolution transmission electron microscopy imaging. IQE improvement is thus attributed to a reduction of point defects upon annealing. [ABSTRACT FROM AUTHOR]

The realization of a linear array of 300 pixels, with a 26-mum pitch, based on AlxGa1-xN metal-semiconductor-metal photodetectors, is described. The composition of the active layer is chosen in order to optimize the solar-blind operation: A sharp cutoff at 280 nm is observed, with discrimination between far and medium ultraviolet (UV) of three orders of magnitude. The detector shows a peak responsivity of 12 mA/W and a dark current smaller than 1 fA at the typical polarization of 4 V. The maximum resolution is analyzed in terms of modulation transfer function (MTF): The best result is obtained for a front-side illumination, i.e., when an MTF of 0.45 is measured at the half Nyquist frequency (19.2 lp/mm). Some UV images, which are obtained in a pushbroom model, are reported. The visible rejection is proven by directly imaging the arc of a Xenon lamp: It is shown that only the relatively weak far-UV component contributes to the signal [ABSTRACT FROM PUBLISHER]

A two-color GaN-based quantum cascade detector is demonstrated. This photodetector operates simultaneously at a peak wavelength of 1.7 and 1 μm at room temperature without any external voltage. These peaks correspond, respectively, to the e1e2 and e1e3 intersubband absorption of the active GaN quantum well. The extractor has been designed to allow for efficient transfer of electrons from both the e2 and e3 states to the next period. The 1 μm detected wavelength is the shortest value reported for an intersubband semiconductor based detector. [ABSTRACT FROM AUTHOR]

A report is presented on the fabrication of light emitting diodes (LEDs) based on GaN core/shell wires on conductive substrates by metal organic vapour phase epitaxy. Catalyst-free GaN-based wires are grown spontaneously on 2-inch n-doped silicon substrates without any thick buffer layer. The LED wire heterostructure consists of an n-type GaN:Si core covered radially by five InGaN/GaN quantum wells and a p-type GaN:Mg shell. Macroscopic devices that integrate around 106 wire-LEDs have been obtained thanks to a simple, direct and full-wafer scale contacting process. For the first time, continuous-wave electrical injection at room temperature through the Si substrate into a cm2-chip of GaN-based core/shell wire-LEDs is successfully demonstrated, producing blue electroluminescent emission at 450 nm. [ABSTRACT FROM AUTHOR]

AlGaN/GaN HEMTs have been fabricated directly on (111) oriented single crystal diamond with 1.3 × 1013 cm-2 channel sheet charge density and 731 cm2/Vs mobility. 0.2 µm gate length devices showed 0.73 A/mm maximum drain current density and fT and fmax cutoff frequencies of 21 and 42 GHz. [ABSTRACT FROM AUTHOR]

Time-resolved cathodoluminescence at 27 K has been performed on a-plane GaN grown by epitaxial lateral overgrowth. We detail the relaxation and recombination mechanisms of excitons [free or bound to neutral donors, or bound to I1-type basal stacking faults (BSFs)] in relation to the local density in BSFs. We describe the slow exciton capture rate on isolated BSFs by a diffusion model involving donors via a hopping process. Where BSFs are organized into bundles, we relate the shorter rise time to intra-BSF localization processes and the multiexponential decay to the type-II band alignment of BSFs in wurtzite GaN. [ABSTRACT FROM AUTHOR]

A monolithic white light emitting diode using a (Ga,In)N/GaN multiple quantum well (MQW) light converter is demonstrated. Blue photons emitted under electrical injection by (Ga,In)N/GaN QWs located inside a GaN p-n junction are partly absorbed by another (Ga,In)N/GaN MQW situated outside the junction which emits yellow-green light. The combination of the blue and yellow-green components results in white light emission. [ABSTRACT FROM AUTHOR]

Nonpolar GaN based microcavity (MC) made of a bottom AlN/GaN distributed Bragg reflector (DBR) and a top dielectric SiO2/SiNx DBR has been fabricated on a-plane GaN template. The 13 pair AlN/GaN DBR, centered around 372 nm, exhibits a peak reflectivity of ∼95% together with a flat stopband of 30 nm width. The cavity mode centered around 390 nm is characterized by a full width at half maximum of 4 nm. The optical properties of both the DBR and MC are well reproduced when accounting for linear birefringence effects. [ABSTRACT FROM AUTHOR]

The photoconductive response of AlGaN based UV detectors to 193 nm excimer laser radiation is presented. Two devices have been tested: a metal-semiconductor-metal (MSM) planar structure and a Schottky diode. The transient response of the MSM device closely follows the laser pulses, with a photoconductive decay time constant shorter than 3 ns. Conversely, the Schottky diode shows a slower photoconductive rise and decay kinetics due to the material series resistance coupled with the junction capacitance. Moreover, a longer time constant tail is also evident in this case with a characteristic time of about 40 ns, due to the presence of trap states localized at 0.2–0.3 eV from the band edge. The detection dynamics has been evaluated by changing the beam energy density between 2×10-5 and 0.2 mJ/mm2. The signal increases linearly in the case of the MSM device up to 0.001 mJ/mm2, whereas, for a further intensity rise, the response shows a sublinear behavior. On the contrary, the Schottky diode showed a linear trend inside the reduced 2×10-3–1.5×10-2 mJ/mm2 range. [ABSTRACT FROM AUTHOR]

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free Al[sub x]Ga[sub 1-x]N/GaN distributed Bragg reflectors (DBRs) with high-Al composition (x=0.5). This is achieved by introducing a thick AlN interlayer and strain mediating Al[sub y]Ga[sub 1-y]N layer between the substrate and DBR. The relatively larger refractive index ratio between Al[sub 0.5]Ga[sub 0.5]N and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011. [ABSTRACT FROM PUBLISHER]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

A correction to the article "Dependence of the Mg-related acceptor ionization energy with the acceptor concentration in p-type GaN layers grown by molecular beam epitaxy" that was published in the previous issue is presented.

This article presents the dynamics of excitons in a-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature. [ABSTRACT FROM AUTHOR]

Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA-ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm-3. [ABSTRACT FROM AUTHOR]