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An analysis of the main recombination modes in nitrides, based on new method of data treatment is proposed for the determination of the carrier recombination processes in optically excited matter measured by time-resolved photoluminescence (PL). The analysis includes basic recombination modes: nonradiative Shockley-Read-Hall (SRH), radiative and Auger recombination in relation to monomolecular, bi-molecular, and tri-molecular processes of optical relaxation. The method is based on the introduction of instantaneous PL decay rate r P L plotted as a function of the PL intensity or of the time. Such an approach provides deep insight into the time evolution of the recombination of the optically excited semiconductor systems and can be applied to the time evolution of a variety of optically excited systems. The demonstration of its strength is given by the application to III-nitride based systems, including nitride highly doped and semi-insulating thick layers, polar and non-polar multi-quantum wells (MQWs). At low temperatures (5 K), the mono- and bi-molecular processes determine the carrier relaxation, and the tri-molecular Auger recombination contribution is negligible. At room temperature the data indicate an important contribution of Auger processes. It is also shown that asymptotic (low excitation), one-exponential recombination rate has different character depending on the presence of the electric fields across the structure.

The potential fluctuations in III-nitride quantum wells lead to many effects like emission broadening and S-shape energy vs. temperature dependence. The best description of the energy dependence comes from calculations based on Gaussian density of states. However, in most of the published reports, changes of carrier lifetime with energy and temperature are not taken into account. Since experimental evidence shows that lifetime significantly depends on energy and temperature, here we propose a model that describes two basic parameters of luminescence: lifetime of carries and emission energy as a function of temperature in the case of quantum wells and layers that are characterized by potential fluctuations. Comparison of the measured energy and lifetime dependences on temperature in specially grown InGaN/GaN quantum wells and InAlGaN layer shows very good agreement with the proposed theoretical approach.

Optical phenomena in an ensemble of high-quality GaN nanowires (NWs) grown on a Si substrate have been studied by reflectance and time-resolved luminescence. Such NWs form a structure that acts as a virtual layer that specifically reflects and polarizes light and can be characterized by an effective refractive index. In fact we have found that the NW ensembles of high NW density (high filling fraction) behave rather like a layer of effective medium described by the Maxwell Garnett approximation. Moreover, light extinction and strong depolarization are observed that we assign to scattering and interference of light inside the NW ensemble. The wavelength range of high extinction and depolarization correlates well with transverse localization wavelength estimated for such an ensemble of NWs, so we suppose that these effects are due to Anderson localization of light. We also report results of time-resolved measurements of polarization of individual emission centers including free and bound excitons (D0XA, 3.47 eV), inversion domain boundaries (IDB, 3.45 eV) and stacking faults (SF, 3.42 eV). The emission of the D0XA and SF lines is polarized perpendicular to GaN c-axis while the 3.45 eV line is polarized along the c-axis which supports a hypothesis that this line is emitted from IDBs. Time-dependent depolarization of luminescence is observed during the first 0.1 ns after excitation and is interpreted as the result of interaction of the emission centers with hot particles existing for a short time after excitation.

Growth of high-quality nitride-based structures requires use of miscut substrates, which means that the growth plane is tilted with respect to the c lattice plane (0001). In order to investigate the miscut influence on grown layers, patterned GaN substrates with separate regions tilted between 0.34° and 0.97° to the c-plane were used to grow multiquantum well InGaN/GaN structures by metalorganic vapor phase epitaxy. The quantum wells had a nominal 6% indium content. Precise values of miscut in each region were determined by atomic-force microscopy. Recombination dynamics in each region was studied using time-resolved microphotoluminescence (PL) from 4 K to room temperature. It was found that the PL energy increases with miscut angle from 3.17 to 3.29 eV (at 4 K). The PL lifetime also correlated with the miscut angle, it decreased from 1.2 down to 0.8 ns (at 4 K). These results suggest that an increase of the angle causes a decrease of the indium content. Less indium naturally leads to a higher bandgap, and to reduction of electric field inside the quantum wells (QWs). Both effects lead to a PL energy increase. Moreover, the decrease of the electric field reduces the electron–hole separation that explains the shorter radiative recombination at 4 K at regions of high-miscut angles. The shape of temperature dependence of PL decay times suggests that at low temperatures PL decay is determined by the localization of carriers, whereas in room temperature LO phonon-assisted delocalization processes play a dominant role.

We describe surface patterning as a method to obtain discrete regions of different local miscut angles, δ, in semipolar ( 20 2 ¯ 1 ) GaN substrates. During patterning the region angle δ was varied between 0° and ±2° with respect to the growth plane. We chose two nonequivalent miscut directions: a 〈 1 ¯ 2 1 ¯ 0 〉 and c ׳ 〈 1 ¯ 01 4 ¯ 〉. On such patterned substrates In x Ga 1− x N/GaN quantum wells were grown by a Metalorganic Vapor Phase Epitaxy method. Angled regions were not planarized during growth and their initial miscut angles remained. We studied structural, morphological and optical properties of In x Ga 1− x N/GaN quantum wells as a function of δ . The intended In concentration, x , was about 13% on exact oriented ( 20 2 ¯ 1 ) planes. For miscut towards the a -direction, indium content decreases and luminescence energy increases with δ . For miscut towards c ׳, we observe quite different behaviors, indium content and quantum well width are constant and luminescence energy increases with δ . We propose the explanation of these effects on the basis of sample morphology, In-content and built-in electric field. The obtained results are important for understanding the role of miscut (intentional and unintentional) in basic properties of semipolar InGaN/GaN quantum well structures. It is also worth to note that, the spectral width of luminescence was the smallest for regions miscut 2° towards c ׳ direction, which can be important for semipolar optoelectronic devices.

International audience; The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

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.

In the case of InGaN alloys grown by metalorganic vapour phase epitaxy on a c-plane GaN, indium content decreases as the substrate miscut is increased. This phenomenon has been previously used to fabricate laser diodes with variable wavelength on one chip [Appl. Phys. Express 5, 021001 (2012)]. In that work, however, wavelength variation was only 5 nm. In the present work we show independent, electrically driven array of light emitting diodes (LED), covering 40 nm emission wavelength range on one chip. This is achieved by a particular patterning technique, which enables the change in the local miscut of the substrate by introducing large enough slopes for practical devices. This technological approach offers a new degree of freedom for InGaN/GaN bandgap modification and device engineering. It can be applied to freestanding GaN as well as to GaN/sapphire templates used for mass production of LEDs. Once optimized, this approach could eventually lead to truly monolithic RGB LEDs.

Molecular beam epitaxy growth and basic physical properties of quaternary AlInGaN layers, sufficiently thick for construction of electron blocking layers (EBL), embedded in ternary InGaN layers are presented. Transmission electron microscopy (TEM) measurement revealed good crystallographic structure and compositional uniformity of the quaternary layers contained in other nitride layers, which are typical for construction of nitride based devices. The AlInGaN layer was epitaxially compatible to InGaN matrix, strained, and no strain related dislocation creation was observed. The strain penetrated for limited depth, below 3 nm, even for relatively high content of indium (7%). For lower indium content (0.6%), the strain was below the detection limit by TEM strain analysis. The structures containing quaternary AlInGaN layers were studied by time dependent photoluminescence (PL) at different temperatures and excitation powers. It was shown that PL spectra contain three peaks: high energy donor bound exciton pea...