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Light-emitting devices that take advantage of the wide bandgap characteristics of InGaN/GaN are widely used in the industry. However, inhomogeneities have been reported in their photoluminescence (PL) mapping at the nanometer and submicrometer scale, even in samples of high crystal quality. In addition, a blinking phenomenon (time variation of PL intensity) under photoexcitation has been reported in relation to these inhomogeneities. The reason why this blinking phenomenon occurs is still unclear; it has been observed in quantum dots and other single and multilayer quantum well structures. Nevertheless, there are very few publications on nanopillar InGaN quantum well samples, which are the focus of this research. Here, we report and analyze the behavior of the blinking phenomena on a nanopillar sample. We noticed that the blinking of the pillars is somehow synchronized on a long timescale among several spatially separated nanopillars. We demonstrated that the synchronization is not due to random intensity fluctuations. We suggest instead that the synchronization is caused by a nonlinear response of the quantum wells to the UV source. In other words, when the stimulation intensity surpasses a certain value, it triggers an ON/OFF state switch in the PL of some of the pillars. Even if preliminary, our study helps to provide clues to understanding the mechanism of the occurrence of the blink phenomenon. [ABSTRACT FROM AUTHOR]

The radiation properties of nonpolar AlGaN quantum wells (QWs) were theoretically investigated by comparing them to those of c-plane AlGaN QWs with heavy holes as the top valence band (VB). First, the conditions to minimize the threshold carrier density of c-plane QW laser diodes were explored. A thin well width (∼1 nm) and reduction of the Al content in the well layer were important to reduce threshold carrier density because narrow wells suppressed the quantum confined Stark effect and AlGaN with a lower Al content had a lower density of states. Moreover, the emission wavelength was widely controlled by tuning the Al contents of both the well and barrier layers under the proposed conditions. Then the properties of nonpolar AlGaN QWs were investigated. Nonpolar AlGaN had several superior characteristics compared to c-plane QWs, including large overlap integrals, optical polarization suitable for both edge and surface emissions, an almost linearly polarized optical dipole between the conduction band and top VB due to the isolated VBs, and a reduced VB density of state. Finally, the threshold carrier densities of both nonpolar and optimized c-plane QWs were compared as functions of the transition wavelength. At a given wavelength, the threshold of nonpolar QWs was lower than that of c-plane ones. Particularly below 260 nm, nonpolar QWs had a low threshold, whereas that of c-plane QWs drastically increased due to the large VB mass of AlN and carrier population in the crystal-field splitting band. [ABSTRACT FROM AUTHOR]

Valence band (VB) effective masses of nonpolar and semipolar nitride heterostructures were studied using k·p perturbation theory, with a particular focus on band mixing. When the absolute value of the energy separation of the topmost two VBs (|ΔE|) is sufficiently larger than the spin-orbit interaction (Δso), the mixing between these two VBs was negligible. This enabled us to calculate an analytical expression for VB mass expression. Under this condition, the effective mass of the top VB was reduced less than 10% along a certain direction, and less than 25% for an in-plane (x′y′-plane) average, compared to the c-plane case. When ΔE was comparable with Δso, however, the VB structure became isotropic due to band mixing. Consequently, the VB band structure approached that of the c-plane and optical polarization was suppressed. On the other hand, VB mass along the growth direction (z′) was always heavy, similar to that of the c-plane. However, the topmost two VBs had slightly different mass values along z′, which affected radiative properties of non-c-plane nitride quantum wells (less than 2 nm wide) through the difference in quantum confinement strengths. [ABSTRACT FROM AUTHOR]

Strain states in wurtzite III-nitride semiconductor quantum wells (QWs) are investigated. X-ray diffraction (XRD) reciprocal space mapping using semipolar [formula] and [formula] InGaN/GaN QWs as test samples demonstrates that the projections of reciprocal lattice vectors of unstrained GaN and pseudomorphically strained InGaN on the interface agrees, indicating continuity of layers across the interface. High resolution transmission electron microscopy supports the XRD analysis. Based on the experimental results, strain tensor elements are extracted for arbitrary crystalline orientation. Furthermore, expansion of the model to arbitrary crystal structures is suggested. [ABSTRACT FROM AUTHOR]

The relaxation of lattice-mismatched strain by deep postetching was systematically investigated for InGaN/GaN multiple quantum wells (MQWs). A planar heterojunction wafer, which included an In0.21Ga0.79N (3.2 nm)/GaN (14.8 nm) MQW, was etched by inductively coupled plasma dry etching, to fabricate high-density nanopillar, nanostripe, and nanohole arrays. The etching depth was 570 nm for all nanostructures. The diameter of the nanopillars was varied from 50 to 300 nm, then the mesa stripe width of the nanostripes and the diameter of the nanoholes were varied from 100 nm to 440 nm and 50 nm to 310 nm, respectively. The effect of strain relaxation on various optical properties was investigated. For example, in an array of nanopillars with diameter 130 nm and interval 250 nm, a large blueshift in the photoluminescence (PL) emission peak from 510 nm (as-grown) to 459 nm occurred at room temperature (RT). PL internal quantum efficiency (defined by the ratio of PL integral intensity at 300 K to that at 4.2 K) was enhanced from 34% (as-grown) to 60%, and the PL decay time at 4.2 K was reduced from 22 ns (as-grown) to 4.2 ns. These results clearly indicate the reduction of lattice-mismatched strain by postetching, which enhanced strain reduction with decreasing nanopillar diameter down to a diameter of 130 nm, where the strain reduction became saturated. The dependence of RT-PL decay time on nanopillar diameter was measured, and the surface nonradiative recombination velocity was estimated to be 5.8×102 cm/s. This relatively slow rate indicates a little etching damage. [ABSTRACT FROM AUTHOR]

The optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 μm, 1 μm, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence (PL) and time-resolved PL measurements suggest inhomogeneous relaxation of the lattice-mismatch induced strain in the InGaN layers. By comparing to a strain distribution simulation, we found that partial stain relaxation occurs at the free side wall, but strain remains in the middle of the pillar structures. The strain relaxation leads to an enhanced radiative recombination rate by a factor of 4–8. On the other hand, nonradiative recombination processes are not strongly affected, even by postgrowth etching. Those characteristics are clearly reflected in the doughnut-shape emission patterns observed by optical microscopy. [ABSTRACT FROM AUTHOR]

Excitonic properties of polar, semipolar, and nonpolar InGaN/GaN strained quantum wells (QWs) were investigated in terms of exciton localization and polarization-induced electric fields. The spontaneous emission lifetimes measured at ∼10 K for the (0001) polar QWs were 1.4 ns at an emission wavelength of 400 nm, but increased monotonically to 85 ns at 520 nm. On the other hand, those for {1122} and {1101} semipolar QWs and {1120} and {1100} nonpolar QWs were on the order of a few hundred picoseconds and independent of the emission wavelength. To quantitatively discuss these results, the crystalline orientation dependence of the spontaneous emission lifetime of 1s heavy hole excitons in InGaN/GaN QWs at 0 K was calculated, when lateral confinements were considered to express well-reported potential fluctuations. It is revealed that both the crystalline orientation and lateral confinement vary the spontaneous emission lifetime by orders of magnitude. Analyses of the experimental results suggest that excitons in the (0001) polar QWs are more strongly localized as the In composition increases, but the semipolar and nonpolar QWs exhibit the opposite tendency. These tendencies are attributed to differences in the growth characteristics. [ABSTRACT FROM AUTHOR]