Ions In GaN Research Articles

Droop-free amplified red emission from Eu ions in GaN

Eu-doped GaN (GaN:Eu) are novel candidates for red light-emitting diodes (LEDs). To further improve the luminescent efficiency of the GaN:Eu-based LED, the efficiency-droop under strong excitation conditions should be suppressed. In this paper, we demonstrate droop-free luminescence of GaN:Eu emitted from a sample-edge using a stripe excitation configuration. The Eu emission intensity clearly increases compared to the conventional surface-emission, and the enhancement is more pronounced for stronger excitation conditions. We clarify that the wavelength dependence of the enhancement agrees well with the optical gain spectrum of the GaN:Eu and is attributed to amplified spontaneous emission.

Crystal field model simulations of magnetic response of pairs, triplets and quartets of Mn3+ ions in GaN

A ferromagnetic coupling between localized Mn spins was predicted in a series of ab initio and tight binding calculations and experimentally verified for the dilute magnetic semiconductor Ga1−x Mn x N. In the limit of small Mn concentrations, x ≲ 0.01, the paramagnetic properties of this material were successfully described using a single ion crystal field model approach. In order to obtain the description of magnetization in (Ga,Mn)N in the presence of interacting magnetic centers, we extend the previous model of a single substitutional Mn3+ ion in GaN by considering pairs, triplets and quartets of Mn3+ ions coupled by a ferromagnetic superexchange interaction. Using this approach we investigate how the magnetic properties, particularly the magnitude of the uniaxial anisotropy field, change as the number of magnetic Mn3+ ions in a given cluster increases from 1 to 4. Our simulations are then exploited in explaining experimental magnetic properties of Ga1−x Mn x N with x ≅ 0.03, where the presence of small magnetic clusters gains in significance. As a result the approximate lower and upper limits for the values of exchange couplings between Mn3+ ions in GaN, being in nearest neighbors (nns) J nn and next nns J nnn positions, respectively, are established.

Spin echo studies on Fe3+ ions in GaN: Spin-phonon relaxation and ligand hyperfine interactions

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes (∼T) below 25 K and by Raman two-phonon processes (∼T9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells.

Carrier dynamics and excitation of Eu3+ ions in GaN

The carrier mobility and dynamics of nonequilibrium carriers in Eu-doped GaN have been determined by above-band-gap, optically excited, time-resolved terahertz spectroscopy and compared with that of undoped GaN. The carrier mobility of GaN:Eu was an order of magnitude smaller than that of a similarly grown undoped GaN layer. Two time constants were observed in the carrier relaxation dynamics for GaN:Eu. A fast one (2.2 ps), which we related to an efficient nonradiative carrier trap, and a slower one (\ensuremath{\sim}85 ps) related to Eu-related traps. Ultrafast photoluminescence spectroscopy shows that Eu-related emission is observed within 100 ps after a pulsed excitation. These values set an upper value to the energy-transfer time from the Eu-related trap to $\mathrm{E}{\mathrm{u}}^{3+}$ ion and underlie the high efficiency of the excitation of $\mathrm{E}{\mathrm{u}}^{3+}$ ions in GaN.

Photoluminescence quantum efficiency of Er optical centers in GaN epilayers

We report the quantum efficiency of photoluminescence processes of Er optical centers as well as the thermal quenching mechanism in GaN epilayers prepared by metal-organic chemical vapor deposition. High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN under resonant excitation excitations were performed. Data provide a picture of the thermal quenching processes and activation energy levels. By comparing the photoluminescence from Er ions in the epilayer with a reference sample of Er-doped SiO2, we find that the fraction of Er ions that emits photon at 1.54 μm upon a resonant optical excitation is approximately 68%. This result presents a significant step in the realization of GaN:Er epilayers as an optical gain medium at 1.54 μm.

Temperature dependence studies of Er optical centers in GaN epilayers grown by MOCVD

ABSTRACTWe report the temperature dependence of Er optical centers in GaN epilayers prepared by metal-organic chemical vapor deposition under the resonant excitation (4I15/2 → 4I9/2) excitation using a Ti:Sapphire laser (λexc = 809 nm). High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN have been performed to identify the crystal filed splitting of the first excited state, 4I13/2. Here, we have employed a simple approach to determine activation energies which are related to the thermal population of electrons from the lowest level to the higher level of the crystal field splitting of the first excited state.

Stretching magnetism with an electric field in a nitride semiconductor.

The significant inversion symmetry breaking specific to wurtzite semiconductors, and the associated spontaneous electrical polarization, lead to outstanding features such as high density of carriers at the GaN/(Al,Ga)N interface—exploited in high-power/high-frequency electronics—and piezoelectric capabilities serving for nanodrives, sensors and energy harvesting devices. Here we show that the multifunctionality of nitride semiconductors encompasses also a magnetoelectric effect allowing to control the magnetization by an electric field. We first demonstrate that doping of GaN by Mn results in a semi-insulating material apt to sustain electric fields as high as 5 MV cm−1. Having such a material we find experimentally that the inverse piezoelectric effect controls the magnitude of the single-ion magnetic anisotropy specific to Mn3+ ions in GaN. The corresponding changes in the magnetization can be quantitatively described by a theory developed here.

Quantitative secondary ion mass spectrometric analysis of secondary ion polarity in GaN films implanted with oxygen

Quantitative analyses of N and O ions in GaN thin films implanted with oxygen ions (16O+) were conducted by secondary ion mass spectrometry (SIMS). Positive (CsM+) and negative secondary ions extracted by Cs+ primary ion bombardment were analyzed for oxygen quantitative analysis. The oxygen depth profiles were obtained using two types of primary ion beams: a Gaussian-type beam and a broad spot beam. The oxygen peak concentrations in GaN samples were from 3.2 × 1019 to 7.0 × 1021 atoms/cm3. The depth profiles show equivalent depth resolutions in the two analyses. The intensity of negative oxygen ions was approximately two orders of magnitude higher than that of positive ions. In contrast, the O/N intensity ratio measured using CsM+ molecular ions was close to the calculated atomic density ratio, indicating that the SIMS depth profiling using CsM+ ions is much more effective for the measurements of O and N ions in heavy O-implanted GaN than that using negative ions.

Electronic structure of transition metal ions in GaN and AlN: Comparing GGA+U with experiment

Electronic structure of Cr, Mn, Fe, and Co transition metal (TM) ions in GaN and AlN is studied within the Generalized Gradient Approximation augmented by the +U corrections, which are treated as free parameters. The main impact of U(N) is the opening of the band gap by shifting both valence and conduction band energies. The intracenter transitions between the gap states of TM impurities are weakly affected by U(N), but they are sensitive to U(TM) which directly act on d(TM) orbitals. The impact of U(TM) depends on both the level symmetry and its occupancy (i.e., on the charge state of the TM ions). Degeneracies of partially occupied multiplet levels are lifted, the effect being mainly driven by the +U correction and not by the Jahn-Teller mechanism. The role of the U-induced hybridization between TM and host states is elucidated.A comparison with experimental intracenter optical transition energies demonstrate that the best agreement is obtained for U ≈ 0 for Fe, and even negative U ≈ −1 eV for Mn. The linear response calculations give U(TM)≈3.5–4.0 eV, which leads to severe discrepancies of about 1 eV with experiment. Importantly, the results obtained with U(Fe) = U(Mn) = U(N) = 5 eV are very close to those obtained with hybrid functionals, but the discrepancies with experiment are even larger. Comparison of the results for GaN and AlN shows that relative energies of gap levels of a given TM ion in the two hosts, as well as their dependencies on U(TM), are the same to within ∼0.2 eV. The estimated valence band offset between GaN and AlN is about 0.5 eV, in agreement with experiment.

Zeeman spectroscopy of the internal transition 4T1 to 6A1 of Fe3+ ions in wurtzite GaN

Internal transitions of Fe3+ ions in wurtzite gallium nitride were analyzed by means of photoluminescence, Zeeman, and transmission spectroscopy in order to investigate the fine structure. Magnetic fields up to 14 T were applied perpendicular or parallel to the crystal c-axis, causing a characteristic splitting pattern of the luminescence related to the transition from the 4T1 excited state to the 6A1 ground state of Fe3+. The complete Hamiltonian matrix is constructed taking into account the crystal field in cubic and trigonal symmetry, spin-orbit interaction, and the influence of external magnetic fields. Numerical solution yields the exact energy level scheme of the excited state 4G of Fe3+ ions in GaN, which partly revises assumptions based on a qualitative treatment considering group theory only and invoking the influence of a Jahn-Teller effect. The coincidence of the calculated energy levels with the experimental data verifies the derived fine structure of the 3d metal ion.

Lattice sites, charge states and spin–lattice relaxation of Fe ions in 57Mn+ implanted GaN and AlN

The lattice sites, valence states, resulting magnetic behaviour and spin–lattice relaxation of Fe ions in GaN and AlN were investigated by emission Mössbauer spectroscopy following the implantation of radioactive 57Mn+ ions at ISOLDE/CERN. Angle dependent measurements performed at room temperature on the 14.4keV γ-rays from the 57Fe Mössbauer state (populated from the 57Mn β− decay) reveal that the majority of the Fe ions are in the 2+ valence state nearly substituting the Ga and Al cations, and/or associated with vacancy type defects. Emission Mössbauer spectroscopy experiments conducted over a temperature range of 100–800K show the presence of magnetically split sextets in the “wings” of the spectra for both materials. The temperature dependence of the sextets relates these spectral features to paramagnetic Fe3+ with rather slow spin–lattice relaxation rates which follow a T2 temperature dependence characteristic of a two-phonon Raman process.

Transition Metal Ions in Semiconductors: LDA, LDA+U, and Experiment

and particularly drastic in transition metal (TM) oxides. A considerable improvement is obtained by adding the +U correction for particular atomic orbitals. While the impact of +U terms was extensively discussed for ideal crystals, its impact on the electronic structure of defects is less understood. We analysed the impact of the +U term for Cr, Mn, Fe, and Co ions in GaN and AlN. The +U term was treated as a free parameter, and it was applied to p(N) and d(TM) orbitals. The results of LDA+U calculations were compared to available experimental data. The band gap of GaN is correct with U(N)=4 eV. The +U terms strongly aect the electronic structure of TM impurities. Surprisingly, for U(TM)=0, the energies of the gap levels induced by these centers, and of the intra-center optical transitions, agree well with experiment. In contrast, for U(N)=U(TM)=4 eV, these energies are in substantial disagreement with experimental values by about 1-2 eV.

Europium‐doped GaN(Mg): beyond the limits of the light‐emitting diode

AbstractRare‐earth doped III‐N semiconductors have been studied for decades on account of their possible application in visible light‐emitting diodes (LED) with built‐in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O'Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p‐type GaN thin films that were co‐doped with Mg and Eu (O'Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290–i03–06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE‐doped III‐N beyond the limited goal of improving LED efficiency into the realm of novel magneto‐optic and quantum‐optical devices. In particular we will describe in this presentation the spectroscopy of ion‐implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Threshold voltage control by gate oxide thickness in fluorinated GaN metal-oxide-semiconductor high-electron-mobility transistors

This paper demonstrates the compensation of the intrinsic positive charges in Al2O3 gate dielectric by fluorine ions in GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs). Negatively-charged fluorine ions diffused into the oxide from the AlGaN barrier during the 250 °C atomic layer deposition compensate the intrinsic positive charge present in the Al2O3. This compensation is key to control the threshold voltage (Vth) of enhancement-mode (E-mode) transistors. A comprehensive analytical model for the Vth of fluorinated MOS-HEMTs was established and verified by experimental data. This model allows the calculation of the different charge components in order to optimize the transistor structure for E-mode operation. Using the proposed charge compensation, the Vth increases with gate dielectric thickness, exceeding 3.5 V for gate dielectrics 25 nm thick.

Optical excitation cross section of erbium in GaN

Epilayers of erbium-doped GaN (GaN:Er) were synthesized by metal-organic chemical vapor deposition, and the optical excitation cross section (σ(exc)) of Er ions in this host material were determined. Photoluminescence (PL) measurements were made using laser diodes at excitation wavelengths of 375 and 405 nm, and the integrated emission intensity at 1.54 μm was measured as a function of excitation photon flux. Together with time-resolved PL measurements, values of σ(exc) of Er ions in GaN:Er were obtained. For excitation at 375 nm, the observed excitation cross section was found to be 4.6×10(-17) cm(-2), which is approximately three orders of magnitude larger than that using resonant excitation. Based on the present and previous works, the optical excitation cross section σ(exc) of Er ions in GaN:Er as a function the excitation wavelength has been obtained. The large values of σ(exc) with near-band-edge excitation makes GaN:Er attractive for realization of chip-scale photonic devices for optical communications.

Local Structures around Eu Ions in GaN Studied by X-ray Absorption Fine Structure (VII)

Local Structures around Eu Ions in GaN Studied by X-ray Absorption Fine Structure (VII)

Induced magnetic moment of Eu3+ ions in GaN

Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0→7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.

Structural and optical study of irradiation effect in GaN epilayers induced by 308 MeV Xe ions

Wurtzite GaN epilayers irradiated at room temperature with 308 MeV 129Xe 35+ ions to fluences of 1 × 10 13 and 3 × 10 13 cm −2 have been studied by contact mode atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), micro-Raman scattering and photoluminescence (PL) spectroscopy. The AFM images showed that the surface of GaN films was etched efficiently due to the Xe ion irradiation. The initial step-terrace structure on GaN surface was eliminated completely at a fluence of 3 × 10 13 cm −2. HRXRD and Raman results indicated that the Xe ion irradiation led to a homogenous lattice expansion throughout the entire ∼3 μm-thick GaN films. The lattice expansion as well as the biaxial compressive stress of the films was increasing with the increase of ion fluence. PL measurements showed that a dominant yellow luminescence band in the as-grown GaN films disappeared, but a blue and a green luminescence bands were produced after irradiation. Based on these results, the strong electronic excitation effect of 308 MeV Xe ions in GaN is discussed.

Local atomic structure around Mn ions in GaN:Mn thin films: Quantitative XANES analysis

GaN:Mn dilute magnetic semiconductors with zinc-blende type of lattice and room temperature ferromagnetism were investigated by the X-ray absorption near edge structure (XANES) with a high accuracy approach of the multidimensional interpolation, which makes it possible to determine the nanoscale local atomic structure around Mn impurities. It is found that Mn atoms are substantially incorporated into the GaN lattice and Jahn–Teller distortion around Mn atom is observed. Our results show that symmetry changes around Mn atom influence on XANES spectrum significantly. Furthermore, the possible impact of local distortions on the magnetic properties is discussed.

Ferromagnetic Fe-Doped GaN Nanowires Grown by Chemical Vapor Deposition

We report the first synthesis and characterization of Fe-doped GaN nanowires. The Fe-doped nanowires were grown by chemical vapor deposition process, using pretreated iron oxide as dopant precursors. The morphological and structural analysis showed that the obtained nanowires were triangular cross section and single crystalline wurtzite GaN structure. It was proved that the pretreatment could induce doping effectively and higher substrate temperature could produce higher dopant concentration. The highest concentration of Fe ions in GaN was 0.12% in our condition. Fe 2p core-level X-ray photoelectron spectroscopy (XPS) spectra showed that the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration. Moreover, the change in the binding energy of Ga 3p in the XPS spectra could be attributed to position change of Fermi level governed by Fe concentration. In the photoluminescence (PL) spectra, the relative intensity of exciton luminescence increased with Fe concentration. The magnetic measurement revealed the Fe-doped GaN nanowires were ferromagnetic at room temperature. The XPS results provided the evidence for the charge state of Fe dopant, indicating the intrinsic nature of room-temperature ferromagnetism of Fe-doped GaN nanowires.