Photoluminescence spectra of high-quality bulk AlN crystals are reported. In addition to the expected linear luminescence features like free excitons and donor-bound excitons, nonlinear processes like biexcitons and the exciton-exciton scattering band are seen for higher excitation densities. For temperatures above Ϸ150 K the electron-hole plasma becomes clearly visible in the spectra. A detailed analysis of the data yields an exciton binding energy of 55 meV, a biexciton binding energy of 28.5 meV, a band gap of 6.089 eV at low temperature, and a band gap of 6.015 eV at room temperature.
AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.
The intrinsic anisotropic optical properties of wurtzite AlN are investigated in absorption and emission. Full access to the anisotropy of the optical response of the hexagonal material is obtained by investigating the (1100) plane of a high-quality bulk crystal allowing electric field E polarization perpendicular (E ⊥ c) and parallel (E c) to the optical axis c. Spectroscopic ellipsometry yields the ordinary (ε ⊥ ) and extraordinary (ε ) dielectric functions (DFs) from 0.58 up to 20 eV. The comparison of the experimental data with recently calculated DFs demonstrates that Coulomb interaction has a strong impact not only on the spectral dependence around the fundamental absorption edge but also on the high-energy features usually discussed in terms of van Hove singularities. The fits of the second-order derivatives of ε and ε ⊥ provide the transition energies for the main features in this range. The DFs close to the fundamental absorption edge, dominated by free excitons, exciton-phonon complexes, and the exciton continuum, are independently confirmed by reflectivity and synchrotron-based photoluminescence excitation studies. Values for the band gaps, the crystal field ( cf = −221 ± 2 meV), and spin-orbit splittings ( so = 13 ± 2 meV) are obtained. Furthermore, we obtain accurate values for the static dielectric constants of ε S⊥ = 7.65 and ε S = 9.21, entering, e.g., the calculations of exciton binding energies. Photoluminescence is used to investigate the emission properties of the same sample.
We report on a defect related luminescence band at 2.4 eV in aluminum nitride bulk crystals, for which we find strong indications to be related to silicon DX centers. Time resolved photoluminescence spectroscopy using a sub-bandgap excitation reveals two different recombination processes with very long decay times of 13 ms and 153 ms at low temperature. Based on the results of temperature and excitation dependent photoluminescence experiments, the process with the shorter lifetime is assigned to a donor-acceptor pair transition involving the shallow silicon donor state, which can be emptied with a thermal dissociation energy of 65 meV. The slower process with a thermal quenching energy of 15 meV is assigned to the slightly deeper Si DX state known from electron paramagnetic resonance experiments, which is transferred back to the shallow donor state.
We report on the identification of a two-electron transition for the shallow donor silicon in homoepitaxial aluminum nitride (AlN). One c-oriented sample was analyzed by low temperature photoluminescence spectroscopy on multiple excitation spots. We find a unique correlation of one single emission band, 76.6 meV below the free excitonic emission, with the luminescence of excitons bound to neutral silicon proving the identity as a two-electron transition. The assignment is confirmed by temperature dependent photoluminescence investigations. We find a donor ionization energy of ð63:5 6 1:5Þ meV for silicon in AlN. V C 2013 AIP Publishing LLC.Recent interest in light emitting devices operating in the deep UV spectral region 1 is due to promising applications such as the deactivation of microorganisms by exposure to light with short wavelengths. Several research groups are working on light emitting diodes (LEDs) based on aluminum gallium nitride (AlGaN) fabricated on foreign substrates such as sapphire 2-4 or silicon carbide. 5 As a consequence, these structures suffer from high defect densities due to the lattice and thermal expansion coefficient mismatch between the substrate material and the epitaxial film, directly degrading the efficiency of the device. 6 One approach is to deposit the diode layer, with high Al content, directly on an AlN single crystal. This kind of substrate is promising for an improvement of the crystal quality, 7,8 and recently, well operating LEDs 9 and lasers 10,11 have been presented. Nevertheless, many fundamental properties such as the exact electronic structure for typical dopants are not known. Only recently, homoepitaxial AlN layers of good crystal quality became available allowing the proper identification of single bound excitonic emission bands. 12,13 However, for the mostly present donor silicon (Si), there is no agreement on the ionization energy. There is an ongoing discussion, whether Si Al undergoes a strong lattice relaxation and forms a deep DX center, where the dopant atom captures a second electron. Several theoretical studies predict DX center formation for Si Al , 14,15 whereas others find silicon to be a shallow donor in wurtzite AlN. 16,17 Experimentally, the majority of studies shows n-type conductivity after silicon doping, 18-24 although they find very different values for the ionization energy ranging from 86 to 570 meV. On the other hand, there are two detailed reports about DX center formation found by spectrally resolved photoconductivity and electron paramagnetic resonance. 25,26 In this letter, we present a detailed investigation of a homoepitaxially grown wurtzite AlN layer by means of temperature dependent photoluminescence (PL) spectroscopy. Exposure at multiple excitation spots and the temperature dependent behavior of the emission bands observed allow for an identification of a two-electron transition with shallow silicon donors involved. We further discuss the presented results in the framework of possible DX center formation.The sample under investigat...
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