AlN layers were grown directly on sapphire ͑0001͒ substrates using three different growth sequences based on metal-organic vapor phase epitaxy with an emphasis on initial nucleation processes. These three methods were simultaneous, alternating supply of aluminum and nitrogen sources, and a combination of the two. In all the methods, nucleation was initiated by three-dimensional ͑3D͒ islands with a typical diameter of ϳ20 nm. Enhanced migration by the alternating source supply caused highly 3D AlN ridge structures at the sapphire molecular steps. These ridge structures prevented a flattened AlN surface and, in addition, moderated lattice relaxation, suggesting the importance of controlling the initial nucleation in determining the film's properties. In fact, the hybridized method, derived from the simultaneous and alternating supply methods, was able to control the initial nucleation, and provided the best film quality; the 600-nm-thick AlN grown by this method had an atomically flat surface free of pits and particles, and the x-ray diffraction line widths were ϳ45 and ϳ250 arcsec for the ͑0002͒ and ͑1012͒ planes, respectively.
The optical polarization in ͓0001͔-oriented Al x Ga 1−x N / AlN multiple quantum wells ͑QWs͒ in the deepultraviolet region ͑x Ͼ 0.69͒ was studied. Photoluminescence spectroscopy performed at 8.5 K revealed that the predominant polarization direction in QWs with a well width of ϳ1.5 nm switched from GaN-like E Ќ ͓0001͔ to AlN-like E ʈ ͓0001͔ at an Al composition x of ϳ0.83, 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 Ќ ͓0001͔ polarization. These results can be explained by the effect of strain and quantum confinement on the valenceband structures.Realizing deep-ultraviolet ͑DUV͒ semiconductor-based emitters will provide compact, nontoxic, and high-efficiency light sources for various applications, ranging from biological agent detection to next-generation data storage. 1 Although such devices primarily require Al x Ga 1−x N-based quantum wells ͑QWs͒ with a high Al content, 1-5 their fundamental optical properties remain controversial. For example, it has experimentally been demonstrated that surface emissions from ͓0001͔-oriented Al 0.66 Ga 0.34 N / Al 0.76 Ga 0.24 N QWs ͑Refs. 3 and 4͒ and Al 0.11 In 0.03 Ga 0.86 N / Al 0.2 In 0.03 Ga 0.77 N QWs ͑Ref. 6͒ are quite weak because of the predominant optical polarization along the ͓0001͔ c direction. In contrast, a theoretical study has predicted that valence-band engineering through the well width and/or in-plane compressive strain in c-oriented Al x Ga 1−x N QWs may remarkably enhance the emission polarized perpendicular to the c axis, 7 which suggests that surface emitters can be fabricated by better-established growth on the c plane.Those discussions originate from the valence-band structure in AlN strikingly different from that in conventional GaN. In wurtzite AlN or GaN, the degeneracy of the p-like states at the ⌫ point is lifted by both crystal-field splitting and spin-orbit splitting, resulting in three valence bands at the Brillouin zone center. Interestingly, AlN has a negative crystal-field splitting energy ͑⌬ cr = −217 meV͒, 8-12 whereas GaN has a positive ⌬ cr of 11 meV. 13 These splittings lead to valence-band arrangements in the order of ⌫ 7 , ⌫ 9 , and ⌫ 7 from the top for AlN, and in the order of ⌫ 9 , ⌫ 7 , and ⌫ 7 for GaN. The topmost ⌫ 7 in AlN is the crystal-field split off hole ͑CH͒ band governed by p z -like state, and ⌫ 9
The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.
AlN layers are homoepitaxially grown on (0001) AlN substrates. The surfaces are atomically smooth, and the X-ray diffraction rocking curves for the symmetric and asymmetric planes indicate narrow line widths in the range of 10-30 arcsec. The oxygen, silicon, and carbon concentrations are below the detection limits of secondary ion mass spectroscopy. Due to these superior structural properties and low impurity concentrations, sharp free and donor-bound excitons dominate the photoluminescence spectra at low temperatures, while free excitons dominate at elevated temperatures. #
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