The ability to characterize recombination and carrier trapping processes in group-III nitride-based nanowires is vital to further improvements in their overall efficiencies. While advances in scanning transmission electron microscope (STEM)-based cathodoluminescence (CL) have offered some insight into nanowire behavior, inconsistencies in nanowire emission along with CL detector limitations have resulted in the incomplete understanding in nanowire emission processes. Here, two nanowire heterostructures were explored with STEM-CL: a polarization-graded AlGaN nanowire light-emitting diode (LED) with a GaN quantum disk and a polarization-graded AlGaN nanowire with three different InGaN quantum disks. Most nanowires explored in this study did not emit. For the wires that did emit in both structures, they exhibited asymmetrical emission consistent with the polarization-induced electric fields in the barrier regions of the nano-LEDs. In the AlGaN/InGaN sample, two of the quantum disks exhibited no emission potentially due to the three-dimensional landscape of the sample or due to limitations in the CL detection.

We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

Aberration-corrected scanning transmission electron microscopy images of the In0.15Ga0.85N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer’s upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.

The growth of high-quality indium (In)-rich InXGa1−XN alloys is technologically important for applications to attain highly efficient green light-emitting diodes and solar cells. However, phase separation and composition modulation in In-rich InXGa1−XN alloys are inevitable phenomena that degrade the crystal quality of In-rich InXGa1−XN layers. Composition modulations were observed in the In-rich InXGa1−XN layers with various In compositions. The In composition modulation in the InXGa1−XN alloys formed in samples with In compositions exceeding 47%. The misfit strain between the InGaN layer and the GaN buffer retarded the composition modulation, which resulted in the formation of modulated regions 100 nm above the In0.67Ga0.33N/GaN interface. The composition modulations were formed on the specific crystallographic planes of both the {0001} and {0114} planes of InGaN.

In this paper, we have observed an atomic-scale structure and compositional variation at the interface of the InGaN/GaN multi-quantum wells (MQW) by both scanning transmission electron microscopy (STEM) using high-angle annular dark-field mode and atom probe tomography (APT). The iso-concentration analysis of APT results revealed that the roughness of InGaN/GaN interface increased as the MQW layers were filled up, and that the upper interface of MQW (GaN/InGaN to the p-GaN side) was much rougher than that of the lower interface (InGaN/GaN tot he n-GaN side). On the basis of experimental results, it is suggested that the formation of interface roughness can affect the quantum efficiency of InGaN-based light-emitting diodes.

InGaN alloys with (0001) or (000) polarities are grown by plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy images, interpreted using first-principles theoretical calculations, show that there is strong indium surface segregation on InGaN for both (0001) and (000) polarities. Evidence for the existence and stability of a structure containing two adlayers of indium on the In-rich InGaN(0001) surface is presented. The dependence on growth temperature and group III/V ratio of indium incorporation in InGaN is reported, and a model based on indium surface segregation is proposed to explain the observations.

Multiwafer Planetary Reactor is a promising system for large-scale production of heterostructures for LED's based on III-group nitrides. Analysis of chemical processes occurring in the reactor allows one to get insight into specific mechanisms governing growth of nitride based heterostructures. In the present paper results of modeling analysis of MOVPE of InxGa1−xN layers in AIX-200 Reactor and AIX 2000 HT Planetary Reactor are reported. The model used for MOVPE process analysis accounts for gas flow, heat transfer, and multicomponent mass transport along with gas phase and surface chemical reactions. Results of the modeling analysis of In transport and incorporation into the solid phase are compared with experimental data. It is shown that the model predicts reasonably well the In incorporation during MOVPE of InGaN under In/(In+Ga) ratio in the gas phase less than 20%.

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 °C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 × 1018 cm−3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Å) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed

Recent studies revealed specific features of chemical processes occurring on the surface of growing group-III nitrides – extremely low sticking probability of molecular nitrogen, low sticking coefficient and incomplete decomposition of ammonia frequently used as the nitrogen precursor. These features (kinetic by nature) result in the growth process going on under conditions remarkably deviated from the gas-solid heterogeneous equilibrium. In this paper we propose a novel approach to modeling of group-III nitride growth by MOVPE taking into account these features. In the model the sticking/evaporation coefficients of N2 and NH3 extracted from independent experiments are used allowing adequate description of the kinetic effects. The model is applied to analysis of growth of binary (GaN) and ternary (InGaN) compounds in a horizontal tube reactor. The growth rate and the solid phase composition are predicted theoretically and compared with available experimental data. The modeling results reveal lower ammonia decomposition ratio on the surface of the crystal as compared to thermodynamic expectations. The developed model can be used for optimization of growth process conditions.

We demonstrate that our secondary mass ion spectroscopy (SIMS) method for the determination of the mole fraction in solid InxGa1-xN solutions is accurate and reproduceable without need of reference samples. The method is based on measuring relative current values of CsM+ (M=Ga, In) secondary ions. The claim of reliable SIMS determination without reference samples was confirmed by four independent analytical methods on the same samples with a relative error in the InN mole fraction determination below 15%.

The effect of elastic strain in epitaxial InGaN layers coherently grown on GaN wafers on spinodal decomposition of the ternary compound is examined. The effect results in considerable suppression of phase separation in the strained InGaN layers. To predict correctly the position of the miscibility gap in the T-x diagram it is important to take into account the compositional dependence of the elastic constants of the ternary compound. The contribution of the elastic strain to the Gibbs free energy of InGaN is calculated assuming uniform compression of the epitaxial layer with respect to the underlying GaN wafer. The interaction of binary constituents in the solid phase is accounted for on the base of regular solution model. The enthalpy of mixing is estimated using the Valence Force Field approximation. The strain effect becomes stronger with increasing In content in the InGaN. As a result the miscibility gap shifts remarkably into the area of higher InN concentration and becomes of asymmetrical shape. Various growth surface orientations and the type of crystalline structure (wurtzite or sphalerite) provide different effects of the elastic strain on phase separation in ternary compounds.

We analyse by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) the strain relaxation mechanisms in InGaN layers on GaN as dependent on the In content. At the experimentally given thickness of 100 nm, the layers remain coherently strained, up to an In concentration of 14 %. We show that part of the strain is reduced elastically by formation of hexagonally facetted pinholes. First misfit dislocations are observed to form at pinholes that reach the InGaN/GaN interface. We discuss these results in the framework of the Matthews-Blakeslee model for the critical thickness considering the Peierls force for glide of threading dislocations in the different slip systems of the wurtzite lattice.

In this paper growth of high quality InGaN films on (0001) sapphire substrates by atmospheric pressure organometallic vapour phase epitaxy in a vertical rotating disk reactor is investigated. The InGaN layers grown above 800 °C are transparent and show no In-droplets on the surface. The In-content varies between 56 and 9 % for growth temperatures between 700 and 850 °C. The DC X-ray rocking curve of InGaN typically shows a FWHM between 8 and 15 arcmin. Room temperature PL shows an intense band edge emission with a FWHM between 100 and 200 meV for an In-content of 9 and 56 %. The initial efforts on QW growth are discussed.

InGaN films have been grown on GaN and AlGaN epitaxial layers by metalorganic vapor phase epitaxy. The “composition pulling effect” during the initial InGaN growth stages has been studied as a function of the lattice mismatch between the InGaN and the underlying epitaxial layer. The crystalline quality of the InGaN is good near the InGaN/GaN interface and the composition is close to that of GaN. However, with increasing InGaN film thickness, the crystal quality deteriorates and the indium mole fraction increases. The composition pulling effect becomes stronger with increasing lattice mismatch. It is suggested that indium atoms are excluded from the InGaN lattice during the early growth stages to reduce the deformation energy from the lattice mismatch. TEM observations of the InGaN/GaN structure reveal that the degradation of the crystalline quality of InGaN films grown on GaN is caused by pit formation which arises from edge dislocations propagating through the InGaN film from the underlying GaN.

In the past year, several organizations have fabricated reliable, high-brightness LEDs from III-Nitride materials that emit in the blue and green. Recently, Nichia in Japan have announced lasing action in GaN-based diodes. Quantum well structures are key to all these results, offering higher brightness, narrower EL linewidths, and a wider spectral range. In order for the III-Nitride technology to develop, the material growth technique must offer high volume at low cost in addition to the requisite device performance. To date, only MOVPE has demonstrated this capability. We have previously reported the growth of GaN, InGaN, and AlGaN layers by MOVPE in a multi-wafer, high-speed rotating disk reactor. Both n- and p-doping and high quality optical properties have been achieved. In this paper we extend this earlier work and present results of the performance of InGaN / (Al)GaN quantum well structures. Intense PL spectra were observed in the violet and blue regions. The thinnest wells show evidence from PL and DCXRD measurments of either discontinuous layers (islands) or a diffuse upper interface, with preliminary TEM results showing the latter to be the most likely. We also report excellent uniformity of these quantum well structures, and show electroluminescence from a SQW diode emitting at 473 nm.