Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are ...limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron–hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm.
The paper provides a thermodynamic insight into the metastable nature of InN and In-rich InGaN alloys, based on experimental studies of their plasma-assisted MBE growth and high-temperature ...decomposition, as well as on theoretical modeling of nitrogen vacancy behavior. This instability may easily result in occurrence of metallic In nanoparticles in the bulk of In(Ga)N films and in the vicinity of extended defects at high enough In content, which makes us consider this material as a metal–semiconductor composite. An overview of a wide set of experimental studies performed by us on the epitaxial films grown in many laboratories all around the world is given which proves an existence of such In nanoparticles in the films and shows how they affect optical and electrical properties of the epilayers. Possible applications of epitaxial InN layers for THz emitters and magnetic field sensors are discussed.
•InN and In-rich InGaN need to be regarded as metal-semiconductor composites.•Thermodynamic insight into the metastable nature of InN is provided.•Overview of experimental proofs of complex structures of InN epilayers is presented.•In metal nanoparticles act via particle plasmons and highly conductive inhomogeneities.
The paper reports on elaboration of plasma‐assisted molecular beam epitaxy (MBE) of AlxGa1 − xN‐based quantum‐well (QW) structures with high Al content (up to 50% in the QW) grown directly on ...c‐sapphire. Different elements of the structure design are considered consecutively in detail along with the advanced growth approaches developed for each element. Special attention is paid to the growth conditions of (i) AlN nucleation layers with suppressed generation of threading dislocations (TDs), (ii) 2‐µm thick AlN buffer layers with atomically smooth droplet‐free morphology (rms = 0.46 nm) grown under the strongly metal‐rich conditions, (iii) cladding and waveguide AlGaN layers also possessing the atomically smooth droplet‐free morphology that is ensured by the accurately established phase diagram of metal(Ga)‐rich growth conditions within the temperature range 660–780 °C. Employing several 3‐nm thick strained GaN insertions in the AlN buffer layer and a AlGaN/AlN superlattice (SL) on top of it is shown to result in a significant decrease of TD's density down to 108–109 cm−2 in the top QW region fabricated by a submonolayer digital alloying (SDA) technique. Finally, advanced AlGaN‐based QW structures are presented, which demonstrate optically pumped lasing within the deep‐ultraviolet (UV) wavelength range with the threshold power density below 600 kW cm−2 (at 289 nm).
Plasma‐assisted molecular beam epitaxy of AlxGa1 − xN‐based quantum well (QW) structures with high Al content (up to 50% in the QW) grown directly on c‐sapphire is described in this Feature Article. Special attention is paid to the growth conditions of (i) AlN nucleation layers with suppressed generation of threading dislocations (TDs), (ii) atomically‐smooth 2‐μm‐thick AlN buffer layers with several 3‐nm‐thick strained GaN insertions and AlGaN/AlN superlattice to decrease TD's density down to 108–109cm−2, (iii) cladding and waveguide AlGaN layers with atomically‐smooth droplet‐free morphology and QW region fabricated by a sub‐monolayer digital alloying technique. These QW structures demonstrate optically‐pumped lasing within the deep‐UV (sub‐300 nm) wavelength range with threshold power density below 600 kW/cm2 (at 289 nm).
Crystalline properties of (1–2)-μm-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The ...best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780°C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10–15) x-ray rocking curve peaks of 469 and 1025arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7×108cm−2 and 5.9×109cm−2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures.
► High temperature MEE AlN nucleation layer is the best for PA MBE of AlN buffer layers. ► The initial lateral grain size influences on the TD densities in the AlN buffer layers. ► Incomplete stress relaxation was revealed in MEE AlN layers by XRD measurements.
Low-temperature (<750°C) growth of thick AlN epilayers on c-sapphire by plasma-assisted molecular-beam epitaxy under the Al-rich conditions (FAl/FN⁎<1.4) is reported here. Short periodic Al-flux ...interruptions controlled precisely by laser reflectometry ensure continuous growth of droplet-free and atomically smooth AlN films (rms<2ML over 4μm2) with a growth rate governed by the activated nitrogen flux. Lateral spreading of small accumulated Al clusters with their subsequent incorporation into the AlN layer during the Al-flux interruptions is supposed to be facilitated by activated nitrogen radicals. Strong influence of the remaining Al droplets on the subsequent growth of AlGaN/AlN superlattices is also demonstrated.
► Plasma-assisted molecular beam epitaxy of AlN epilayers at temperatures below 800°C. ► Droplet-free and atomically smooth surface of III-nitrides thin films. ► Segregation effects in superlattices AlGaN/AlN. ► Laser reflectometry and RHEED monitoring.
A 60 mW output power has been achieved in mid‐UV (λ = 270 nm) spontaneous sources with electron‐beam pulse‐scanning pumping, fabricated from AlGaN MQW heterostructures grown by PA MBE on c‐Al2O3 ...substrates. Under the CW pumping at much lower excitation power density the mid‐UV sources demonstrate a 4.7 mW output power. In that regime the power efficiency of the structures is about 0.24%, while their internal quantum efficiency is estimated to be as high as 50%.
•Site-controlled 100nm-thick GaN(000I¯) NRs are grown by PA MBE on apexes of µ-CPSS.•NRs selectively grow due to anisotropy of GaN surface energy and adatom kinetics.•Disk-like InGaN single QWs ...embedded in the top parts of the NRs emit at 510nm.
The site-controlled selective area growth of N-polar GaN nanorods (NR) was developed by plasma-assisted MBE (PA MBE) on micro-cone-patterned sapphire substrates (µ-CPSS) by using a two-stage growth process. A GaN nucleation layer grown by migration enhanced epitaxy provides the best selectivity for nucleation of NRs on the apexes of 3.5-µm-diameter cones, whereas the subsequent growth of 1-μm-high NRs with a constant diameter of about 100nm proceeds by standard high-temperature PA MBE at nitrogen-rich conditions. These results are explained by anisotropy of the surface energy for GaN of different polarity and crystal orientation. The InGaN single quantum wells inserted in the GaN NRs grown on the µ-CPSS demonstrate photoluminescence at 510nm with a spatially periodic variation of its intensity with a period of ∼6µm equal to that of the substrate patterning profile.
The paper describes experimental study of stress relaxation and generation in (1-2)-μm-thick GaN and AlGaN layers grown on AlN/c-Al2O3 buffer layers by low temperature (<720 °C) metal-rich ...plasma-assisted molecular beam epitaxy (PA MBE). The atomically smooth undoped GaN layers demonstrate only gradual relaxation of the compressive stress, which is probably related to thermodynamically driven inclination of threading dislocations (TDs).The slower stress relaxation at the lower growth temperature is explained by kinetic limitation of this process. The switch of compressive to tensile stress in the less-strained undoped Al0.7Ga0.3N layers, attributed mostly to the same effect of TD inclination, occurs in the low-temperature PA MBE conditions at much larger thickness (∼0.6 with) as compared to MOVPE ones. Introduction of high Si doping (n∼1019cm-3) reduces noticeably the initial compressive stress in the AlGaN film due to substitution of small Si atoms in the group-III sublattice. At larger thickness, Si atoms seem to effect the TD propagation and suppress generation of tensile stress related to TD inclination, which makes possible to grow ∼1μm-thick Al0.7Ga0.3N: Si films without cracking.
PDF
