Hideaki Murotani

Temperature dependence of localized exciton transitions in AlGaN ternary alloy epitaxial layers

The optical properties of Ga-rich AlxGa1−xN (x=0.019, 0.038, 0.057, 0.077, and 0.092) ternary alloy epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence intensity of excitons in five epitaxial layers indicated a thermal quenching process with two activation energies. The two quenching activation energies were attributed to the delocalization of excitons and thermal dissociation of excitons. Anomalous temperature dependence of the PL peak energy was also observed in the epitaxial layers, which enabled the evaluation of the localization energy of the excitons. The localization energy increased as the 1.7th power of the PL linewidth, which reflected a broadening of the density of localized exciton states. In addition, the luminescence decay of the localized excitons for the five epitaxial layers became longer with decreasing emission energy. These observations suggest that the decay of excitons is caused not only by ra...

Photoluminescence from highly excited AlN epitaxial layers

Excitonic optical properties of AlN epitaxial layers have been studied by means of photoluminescence and optical reflectance spectroscopies. The binding energy of free excitons was estimated to be 57meV on the basis of the energy separation between the n=1 ground and n=2 excited states. In addition, the luminescence line due to radiative recombination of biexcitons was observed under high density excitation. The energy separation between free-exciton luminescence and biexciton luminescence was 19meV, which corresponded to the binding energy of biexcitons. Therefore, the ratio of the biexciton binding energy to the exciton binding energy was approximately 0.33.

Effects of exciton localization on internal quantum efficiency of InGaN nanowires

The optical properties of InGaN nanowires with different emission wavelengths of 485, 515, 555, and 580 nm have been studied by means of photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The PL peak energy of the nanowires exhibited an anomalous shift to higher energy and then to lower energy with increasing temperature. Analysis of the temperature-dependent variations in the PL peak energy let us evaluate the localization energies of excitons, which increased with increasing indium composition. TRPL measurements also revealed that the PL decay time of the nanowires increased and then became constant with decreasing emission energy, which was typical of localized excitons and enabled us to evaluate the characteristic energies of localized states. The characteristic energy increased with increasing indium composition, indicating that the density of localized states broadened with increasing indium composition. In addition, a correlation was clearly observed between the internal quantum effic...

Dependence of internal quantum efficiency on doping region and Si concentration in Al-rich AlGaN quantum wells

The internal quantum efficiency (IQE) of Si-doped AlGaN quantum wells has been studied by means of photoluminescence spectroscopy. Analysis of the IQE as a function of doping region revealed that the IQE increased from 19% to 40% with doping of the well layers. This increase was attributed to an improvement in the interface quality between well and barrier layers as well as a reduction in point defect density. Moreover, the IQE increased to a maximum of 50% and then decreased with increasing Si concentration of the well layers. This indicated the existence of an optimum Si concentration for IQE improvement.

Silicon concentration dependence of optical polarization in AlGaN epitaxial layers

The optical polarization of Si-doped AlxGa1−xN epitaxial layers (x=0.37–0.95) has been studied by means of photoluminescence (PL) spectroscopy. The predominant polarization component of the band-edge PL switched from E⊥c to E∥c at an Al composition between 0.68 and 0.81. This critical Al composition was much higher than in previous reports for AlGaN epitaxial layers. In addition, the predominant polarization in Al0.55Ga0.45N epitaxial layers switched from E⊥c to E∥c with increasing Si concentration. Therefore, the topmost valence band changed from the heavy-hole band to the crystal-field split-off-hole band with decreasing in-plane compressive strain induced by Si doping.

Localization-induced inhomogeneous screening of internal electric fields in AlGaN-based quantum wells

The influence of both localization and internal electric field on the microscopic photoluminescence (PL) properties of AlGaN-based quantum wells (QWs) has been studied by means of scanning near-field optical microscopy (SNOM). SNOM-PL images of three QWs with different well-layer thicknesses were measured under an illumination-collection mode. A correlation was observed between the PL intensity and the PL peak wavelength: a shorter-PL wavelength indicates a stronger intensity for a wider QW. The correlation is caused by an inhomogeneous screening of the internal electric field.

Temperature dependence of excitonic transitions in a-plane AlN epitaxial layers

The excitonic optical properties of a-plane AlN epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and optical reflectance (OR) spectroscopy. An exciton resonance and free-exciton luminescence were clearly observed up to room temperature in the OR and PL measurements, respectively. Analysis of the low temperature OR spectrum enabled us to obtain a splitting energy of 7.3 meV between longitudinal and transverse-exciton resonances for AlN, which was approximately one order of magnitude larger than that for GaN. An emission from the upper branch of the excitonic polariton was also observed at temperatures above 100 K, reflecting thermal repopulation of excitonic polaritons from the lower to the upper branch. In addition, the temperature dependence of the transverse-exciton resonance could be well described using an empirical equation based on Bose–Einstein statistics, in which the Einstein characteristic temperature was estimated to be 455 K.

Correlation between in-plane strain and optical polarization of Si-doped AlGaN epitaxial layers as a function of Al content and Si concentration

We have investigated the relationship between in-plane strain and optical polarization in AlGaN epitaxial layers as a function of Al content and of Si concentration. Al content and in-plane strain were measured by reciprocal space mapping using a simple relationship from the tensor of elastic moduli. The change of valence band energy separation based on the measured in-plane strain was in good agreement with the change of polarization degree as a function of Al content. The dependence of polarization degree on Si concentration was also explained by the change of in-plane strain, and polarization switching occurred around the Al content and strain as calculated by the k · p approach with the cubic approximation.

Huge binding energy of localized biexcitons in Al-rich AlxGa1−xN ternary alloys

Excitonic optical properties of Al-rich AlxGa1−xN ternary alloy epitaxial layers have been studied by means of photoluminescence excitation spectroscopy. On the basis of the energy separation between exciton resonance and two-photon biexciton resonance, the binding energy of biexcitons was estimated to be 56±5 and 48±5 meV for the sample with x=0.81 and 0.89, respectively. The biexciton binding energy of 56 meV was approximately three times as large as the biexciton binding energy of 19 meV in AlN. The large enhancement of the biexciton binding energy resulted from the strong localization of biexcitons due to alloy disorder.

Emission Wavelength Dependence of Internal Quantum Efficiency in InGaN Nanowires

The internal quantum efficiency (IQE) of InGaN nanowires with different emission wavelength of 485, 515, 555, and 580 nm has been studied by means of photoluminescence (PL) spectroscopy. It was found from the analysis of IQE as a function of excitation power density that the IQE was unchanged at about 100% under weak excitation conditions at low temperature. This indicated that the effects of nonradiative recombination processes were negligibly small at low temperature. Moreover, the IQE increased from 5 to 12% with increasing emission wavelength from 485 to 580 nm. Since the clear correlation between the IQE and the PL blue shift due to band filling effects of localized states was observed, the increase in the IQE reflected the increase in the effect of exciton localization with increasing indium composition.