We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In 0.25 Ga 0.75 N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s- shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.

Nominal InN monolayers grown by molecular beam epitaxy on GaN(0001) are investigated combining in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and density functional theory (DFT). TEM reveals a chemical intraplane ordering never observed before. Employing DFT, we identify a novel surface stabilization mechanism elastically frustrated rehybridization, which is responsible for the observed chemical ordering. The mechanism also sets an incorporation barrier for indium concentrations above 25% and thus fundamentally limits the indium content in coherently strained layers.

Previously, we found that N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 °C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 °C. This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells exhibit a spatially uniform green luminescence band up to room temperature, but the intensity of this band is observed to strongly quench with temperature. Temperature-dependent photoluminescence transients show that this thermal quenching is related to a high density of nonradiative Shockley-Read-Hall centers with large capture coefficients for electrons and holes.

We explore an alternative way to fabricate (In, Ga) N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In adsorbate structure manifesting itself by a (root 3 x root 3)R30 degrees surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In0.33Ga0.67N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In, Ga) N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25-0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretically predicted values for these structures. Published by AIP Publishing.

We explore an alternative way to fabricate (In,Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In adsorbate structure manifesting itself by a $(sqrt{3}times!sqrt{3})text{R}30^{circ}$ surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In$_{0.33}$Ga$_{0.67}$N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25-0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretically predicted values for these structures.

We investigate the formation of submonolayer InN quantum sheets and their embedment into GaN to form (In, Ga)N/GaN superlattices. While supplying InN on GaN(0001), we have monitored in-situ the formation of an In adlayer manifesting itself by the (1 x 3) reflection high-energy electron diffraction pattern, that is thought to be established by a third of a monolayer of In adatoms consistent with a (root 3 x root 3)R30 degrees structure. We have utilized this In adsorbate structure on GaN(0001) as a template for the synthesis of laterally ordered InGaN quantum sheets with a self-limited thickness of 1 monolayer and an In content expected to be 0.33 as defined by the (root 3 x root 3)R30 degrees structure. Repeatedly inserting these quantum sheets into GaN, we have synthesized (In, Ga) N/GaN short-period superlattices with abrupt interfaces and high periodicity, as concurrently determined ex-situ by X-ray diffractometry. The embedded quantum sheets consist of single and coherent (In, Ga)N monolayers with an In content of 0.22-0.28, even when the InN supply is increased up to 4.7 MLs. Therefore, these results also evidence the self-limitation of the In content to the maximum expected of 0.33 within coherent monolayer-thick (In, Ga) N quantum sheets.

We investigate sub-monolayer InN quantum sheets embedded in GaN(0001) by temperature-dependent photoluminescence spectroscopy under both continuous-wave and pulsed excitation. Both the peak energy and the linewidth of the emission band associated with the quantum sheets exhibit an anomalous dependence on temperature indicative of carrier localization. Photoluminescence transients reveal a power law decay at low temperatures reflecting that the recombining electrons and holes occupy spatially separate, individual potential minima reminiscent of conventional (In,Ga)N(0001) quantum wells exhibiting the characteristic disorder of a random alloy. At elevated temperatures, carrier delocalization sets in and is accompanied by a thermally activated quenching of the emission. We ascribe the strong nonradiative recombination to extended states in the GaN barriers and confirm our assumption by a simple rate-equation model.

We investigate the luminescence of Ga- and N-polar In$_{x}$Ga$_{1-x}$N/In$_{y}$Ga$_{1-y}$N quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6H-SiC substrates. In striking contrast to their Ga-polar counterparts, the N-polar QWs prepared on freestanding GaN do not exhibit any detectable photoluminescence. Theoretical simulations of the band profiles combined with resonant excitation of the QWs allow us to rule out carrier escape and subsequent surface recombination as the reason for the absence of luminescence. To explore the hypothesis of a high concentration of nonradiative defects at the interfaces between wells and barriers, we analyze Ga- and N-polar QWs prepared on 6H-SiC as a function of the well width. Intense luminescence is observed for both Ga- and N polar samples. As expected, the luminescence of the Ga-polar QWs quenches and red-shifts with increasing well width due to the quantum confined Stark effect. In contrast, both the intensity and the energy of the luminescence from the N-polar samples are essentially independent of well width. Transmission electron microscopy reveals that the N-polar QWs exhibit abrupt interfaces and homogeneous composition, excluding emission from In-rich clusters as the reason for this anomalous behavior. The microscopic origin of the luminescence in the N-polar QWs is elucidated using spatially resolved cathodoluminescence spectroscopy. Regardless of well width, the luminescence is found to not originate from the N-polar QWs, but from the semipolar facets of v-pit defects. These results cast serious doubts on the potential of N-polar QWs grown by plasma-assisted molecular beam epitaxy for the development of long-wavelength light emitting diodes. What remains to be seen is whether unconventional growth conditions may enable a significant reduction in the concentration of nonradiative defects.

Corrigendum to “Growth mechanisms in semipolar (2021) and nonpolar m plane (1010) AlGaN/GaN structures grown by PAMBE under N-rich conditions” [Cryst. Growth 377C (2013) 184–191] M. Sawicka , C. Cheze , H. Turski , J. Smalc-Koziorowska , M. Kryśko , S. Kret , T. Remmele , M. Albrecht , G. Cywinski , I. Grzegory , C. Skierbiszewski a,b a Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland b TopGaN Sp. z o.o., Sokolowska 29/37, 01-142 Warsaw, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland d Leibniz Institute of Crystal Growth, Max-Born Strasse 2, Berlin 12489, Germany

Based on the evaluation of lattice parameter maps in aberration corrected high resolution transmission electron microscopy images, we propose a simple method that allows quantifying the composition and disorder of a semiconductor alloy at the unit cell scale with high accuracy. This is realized by considering, next to the out-of-plane, also the in-plane lattice parameter component allowing to separate the chemical composition from the strain field. Considering only the out-of-plane lattice parameter component not only yields large deviations from the true local alloy content but also carries the risk of identifying false ordering phenomena like formations of chains or platelets. Our method is demonstrated on image simulations of relaxed supercells, as well as on experimental images of an In0.20Ga0.80N quantum well. Principally, our approach is applicable to all epitaxially strained compounds in the form of quantum wells, free standing islands, quantum dots, or wires.

The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar (2 0 (2) over bar 1), m-plane (1 0 (1) over bar 0) and c-plane (0 0 0 1) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar (2 0 (2) over bar 1) structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m-plane one, as compared to the c-plane MQW structure. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed. (c) 2013 Elsevier B.V. All rights reserved.