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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.

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.

Strain states in wurtzite III-nitride semiconductor quantum wells (QWs) are investigated. X-ray diffraction (XRD) reciprocal space mapping using semipolar (11math2) and (1math01) 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.

Strain states in wurtzite III-nitride semiconductor quantum wells (QWs) are investigated. X-ray diffraction (XRD) reciprocal space mapping using semipolar (11math2) and (1math01) 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.

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.

The optical polarization in [0001]-oriented AlxGa1−xN/AlN multiple quantum wells (QWs) in the deep-ultraviolet region (x>0.69) was studied. Photoluminescence spectroscopy performed at 8.5 K revealed that the predominant polarization direction in QWs with a well width of ∼1.5 nm switched from GaN-like E⊥[0001] to AlN-like E∥[0001] at an Al composition x of ∼0.83, where E is the electric field vector of emitted light. This Al composition is much higher than the previously reported critical compositions for polarization switching phenomena. Furthermore, decreasing the well width from more than 10 to 1.5 nm promoted E⊥[0001] polarization. These results can be explained by the effect of strain and quantum confinement on the valence-band structures.

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.

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 {11[overline 2]2} and {1[overline 1]01} semipolar QWs and {11[overline 2]0} and {1[overline 1]00} 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.

We investigated correlations between nanoscopic optical and structural properties in violet-emitting, blue-emitting, and green-emitting InxGa1−xN/GaN quantum wells (QWs) by means of scanning near-field optical microscopy (SNOM) and atomic force microscopy. Only in the blue-emitting QW, threading dislocations were not major nonradiative recombination centers (NRCs). SNOM data indicated that NRCs in the blue-emitting QW are surrounded by energy levels higher than those for radiative recombination. Such potential distributions realize “antilocalization” of carriers to NRCs, which is the cause of high emission quantum efficiencies in blue emitters.

We report the observation of optical polarization switching in InxGa1−xN/GaN quantum well active layers, using semipolar {11[overline 2]2} planes. When the In composition is less than ~30%, the emissions related to the top and second valence bands are polarized along the [1[overline 1]00] and perpendicular [[overline 1][overline 1]23] directions, respectively, similar to earlier studies. On the contrary, as the In composition increases above 30%, the polarizations switch, indicating a crossover between the two valence bands. Because the polarization degree is less sensitive to the well width, the observed polarization switch is ascribed to the InN deformation potentials.

We investigated correlations between nanoscopic optical and structural properties in violet-emitting, blue-emitting, and green-emitting InxGa1−xN/GaN quantum wells (QWs) by means of scanning near-field optical microscopy (SNOM) and atomic force microscopy. Only in the blue-emitting QW, threading dislocations were not major nonradiative recombination centers (NRCs). SNOM data indicated that NRCs in the blue-emitting QW are surrounded by energy levels higher than those for radiative recombination. Such potential distributions realize “antilocalization” of carriers to NRCs, which is the cause of high emission quantum efficiencies in blue emitters.

We demonstrate that the apparent emission colors of InGaN-based light-emitting diodes using microstructured multifacet quantum wells as active layers can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000 K to blue along the Planckian locus. The controllability relies on facet-dependent polychromatic emissions. The pulsed current operation with the appropriate duties varied their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation.

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 {11[overline 2]2} and {1[overline 1]01} semipolar QWs and {11[overline 2]0} and {1[overline 1]00} 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.

The stimulated emissions from semipolar InGaN laser diode (LD) structures grown on (11[overline 2]2) GaN substrates are observed at room temperature under photopumped conditions. The measured emission peaks are in the photon energy range from 2.62 eV (474 nm) to 3.05 eV (405 nm), and the emission intensity has a threshold behavior with respect to the pumping power. A strong in-plane optical anisotropy is observed between the two perpendicular directions, [[overline 1] [overline 1]23] and [1[overline 1]00], due to anisotropic matrix elements, which depend on the crystal orientation; the stimulated emission measured along the [[overline 1] [overline 1]23] direction occurs with a lower threshold pumping power at a lower energy compared to that obtained along the [1[overline 1]00] direction. The experimental results and the valence band calculations indicate that the transverse-electric mode with an electric vector along the [1[overline 1]00] direction is dominant for gain formation in semipolar and nonpolar InGaN LDs. Compared to c-plane InGaN LDs, semipolar InGaN LDs have comparable or less threshold pumping powers.

The stimulated emissions from semipolar InGaN laser diode (LD) structures grown on (11[overline 2]2) GaN substrates are observed at room temperature under photopumped conditions. The measured emission peaks are in the photon energy range from 2.62 eV (474 nm) to 3.05 eV (405 nm), and the emission intensity has a threshold behavior with respect to the pumping power. A strong in-plane optical anisotropy is observed between the two perpendicular directions, [[overline 1] [overline 1]23] and [1[overline 1]00], due to anisotropic matrix elements, which depend on the crystal orientation; the stimulated emission measured along the [[overline 1] [overline 1]23] direction occurs with a lower threshold pumping power at a lower energy compared to that obtained along the [1[overline 1]00] direction. The experimental results and the valence band calculations indicate that the transverse-electric mode with an electric vector along the [1[overline 1]00] direction is dominant for gain formation in semipolar and nonpolar InGaN LDs. Compared to c-plane InGaN LDs, semipolar InGaN LDs have comparable or less threshold pumping powers.