Analysis of the spectra of trivalent erbium in multiple sites of hexagonal aluminum nitride

The 12 K cathodoluminescence spectra of Er3+ doped into single crystals of aluminum nitride (2H-AlN) in the hexagonal phase are reported between 320 nm and 775 nm. The emission spectra represent transitions from the lower Stark level of 2P(3/2) to the Stark levels of the 4I(15/2), 4I(13/2), 4I(11/2), 4I(9/2), 4F(9/2), and 4S(3/2) multiplet manifolds of Er3+(4f(11)). Emission spectra from 4S(3/2) to 4I(15/2) are also reported. All observed strong line emission are accounted for in terms of two principle sites, denoted site a and site b , with a few line spectra attributed to additional sites. A parameterized Hamiltonian that includes the atomic and crystal-field terms for Er3+(4f(11)) (2S+1)L_J was used to determine the symmetry and the crystal field splitting of the a and b sites. A descent in symmetry calculation was carried out to determine if distortion due to the size difference between Er, Al and the vacancies can be discerned. Modeling results assuming C_3v and C_1h are discussed. It appears that the sensitivity to a C_1h model is not sufficient to invalidate the choice of C_3v as an approximate symmetry for both sites. The g-factors reported from an EPR study of Er3+ in single-crystal AlN are in reasonable agreement with calculated g-factors for Er3+ in the a site assuming C_3v symmetry

Electronic properties of graphite-like ion tracks in insulating tetrahedral amorphous carbon

We investigated the formation of quasi one-dimensional conducting filaments in tetrahedral amorphous carbon (ta-C) films created by swift heavy ion irradiation. Various ta-C films with thicknesses of about 100 nm were grown using mass-separated ion beam deposition on highly conducting Si and Ni substrates. After deposition, the films were irradiated with 1 GeV U-238 ions at fluences between 109 and 10(11) ions/cm(2). Due to their high electronic energy loss of about 40 kev/ nm, the swift heavy ions graphitize the predominantly (70%) sp(3)-bound tertahedral amorphous carbon film (ta-C) along their trajectories, yielding conducting nanowires embedded in an insulating matrix. Using atomic force microscopy (AFM) with conducting cantilevers and an applied bias voltage the presence of conducting tracks was confirmed and their conductivities were determined to be several orders of magnitude higher than that of the host matrix. Temperature-dependent electrical measurements were performed on the irradiated samples at 300K - 15K with fields of up to 5V/mu m

Crystal field and Zeeman splittings for energy levels of Nd3+ in hexagonal AlN

The crystal-field and Zeeman splittings of the energy levels of Nd3+(4f3) 2S+1LJ in hexagonal phase AlN have been investigated. The multiplet manifolds of Nd3+(4f3) analyzed include the ground state, 4I9/2, and excited states 4I11/2, 4I13/2, 4F3/2, 4F5/2, 2H(2)9/2, 4F7/2, 4S3/2, 4G5/2, and 2G7/2. Experimental energy levels were obtained from analyses of the 12 K cathodoluminescence spectra from Nd3+-implanted films of AlN, and from the 15 K photoluminescence excitation spectra and the site-selective combined excitation-emission spectra (CEES) recently reported for in situ Nd-doped hexagonal AlN grown by plasma-assisted molecular beam epitaxy (PA-MBE). CEES results identify a main site and two minority sites for Nd3+ in both samples. Transition line strengths attributed to the ion in minority sites are relatively stronger in Nd:AlN than in Nd:GaN. The 15 K experimental Zeeman splitting of Nd3+ are analyzed in the PA-MBE grown AlN samples and compared with the Zeeman splitting observed in Nd:GaN. The crystal-field and Zeeman splittings were modeled using a parametrized Hamiltonian consisting of atomic and crystal-field terms. We considered possible site distortion due to the size of the implanted Nd ion that would reduce the site symmetry from C3v to C3 or C1h. However, no significant improvement was obtained using these lower symmetry models, leading us to conclude that C3v symmetry is a reasonable approximation for the main site Nd3+ ions in AlN

Spectroscopic analysis of Eu3+ in single-crystal hexagonal phase AlN

A detailed spectroscopic analysis of the crystal-field splitting of the energy levels of Eu3+(4f6) in single crystals of hexagonal phase aluminum nitride is reported based on assignments made to the high-resolution cathodoluminescence spectra observed between 500 nm and 750 nm obtained at 11 K and room temperature. Single crystals doped with trivalent europium were grown by high pressure, high temperature technology, and the crystal structure was confirmed by x ray diffraction methods to be the hexagonal phase. The Eu3+ ions substitute for Al3+ ions in sites of C3v symmetry during crystal growth. More than 97% of the observed spectra are attributed to Eu3+ in the majority site. The spectra are identified as transitions from the excited 5D0 and 5D1multiplets of Eu3+ to the ground-state multiplets 7F0, 7F1, 7F2, 7F3,7F4, 7F5, and 7F6 split by the crystal field into energy (Stark) levels. A parameterized Hamiltonian defined to operate within the 4f6 electronic configuration of Eu3+ was used to model the experimental Stark levels and their symmetry assignments or irreducible representations (irreps). The crystal-field parameters were determined through use of a Monte Carlo method in which the six Bqk were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting, which involved 20 Stark levels and their irreps from 5D1, 5D0, and 7F0–4, resulted in a rms deviation of 6.7 cm−1. The predicted splitting of the 7F5 and 7F6 multiplets was used to assign the experimental splitting for these manifolds since the spectra involved are weak and broad, precluding detailed Stark-level assignments