Formation and dissociation of hydrogen-related defect centers in Mg-doped GaN

Moderately and heavily Mg-doped GaN were studied by a combination of post-growth annealing processes and electron beam irradiation techniques during cathodoluminescence (CL) to elucidate the chemical origin of the recombination centers responsible for the main optical emission lines. The shallow donor at 20-30 meV below the conduction band, which is involved in the donor-acceptor-pair (DAP) emission at 3.27 eV, was attributed to a hydrogen-related center, presumably a (VN-H) complex. Due to the small dissociation energy (<2 eV) of the (VNH) complex, this emission line was strongly reduced by low-energy electron irradiation. CL investigations of the DAP at a similar energetic position in Si-doped (n-type) GaN indicated that this emission line is of different chemical origin than the 3.27 eV DAP in Mg-doped GaN. A slightly deeper DAP emission centered at 3.14 eV was observed following low-energy electron irradiation, indicating the appearance of an additional donor level with a binding energy of 100-200 meV, which was tentatively attributed to a VN-related center. The blue band (2.8-3.0 eV) in heavily Mg-doped GaN was found to consist of at least two different deep donor levels at 350±30 meV and 440±40 meV. The donor level at 350±30 meV was strongly affected by electron irradiation and attributed to a H-related defect

Optical properties of Mn-doped GaN

Molecular beam epitaxy-grown GaN with different Mn concentrations (5-23×1019 cm-3) and codoped with Si were investigated by cathodoluminescence (CL) spectroscopy and optical transmission measurements. In the GaN:Mn, an intense absorption peak at 1.414 +/- 0.002 eV was observed. This peak was attributed to an internal 5T 2→ 5E transition of the deep neutral Mn3+ state since its intensity scaled with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm -3 had three effects on the emission spectrum: (i) the donor bound exciton at 3.460 eV was reduced by more than one order of magnitude, (ii) the donor-acceptor-pair band at 3.27 eV was completely quenched and (iii) the yellow luminescence centered at 2.2 eV was the strongly decreased. The latter two effects were attributed to a reduced concentration of VGa. In the infrared spectral range, three broad, Mn-doping related CL emission bands centered at 1.01 ± 0.02 eV, 1.09 ± 0.02 eV and 1.25 ± 0.03 eV were observed. These bands might be related to deep donor complexes, which are generated as a result of the heavy Mn-doping, rather than internal transitions at the Mn atom

Precise measurements of the properties of the B-1(5721)(0,+) and B-2*(5747)(0,+) states and observation of B-+,B-0 pi(-,+) mass structures

Invariant mass distributions of B+pi- and B0pi+ combinations are investigated in order to study excited B mesons. The analysis is based on a data sample corresponding to 3.0 fb-1 of pp collision data, recorded by the LHCb detector at centre-of-mass energies of 7 and 8 TeV. Precise measurements of the masses and widths of the B_1(5721)^(0,+) and B_2*(5747)^(0,+) states are reported. Clear enhancements, particularly prominent at high pion transverse momentum, are seen over background in the mass range 5850--6000 MeV in both B+pi- and B0pi+ combinations. The structures are consistent with the presence of four excited B mesons, labelled B_J(5840)^(0,+) and B_J(5960)^(0,+), whose masses and widths are obtained under different hypotheses for their quantum numbers.Comment: 29 pages, 5 Figures, 8 Table

Cathodoluminescence efficiency dependence on excitation density in n-type gallium nitride

Cathodoluminescence (CL) spectra from silicon doped and undoped wurtzite n-type GaN have been measured in a SEM under a wide range of electron beam excitation conditions, which include accelerating voltage, beam current, magnification, beam diameter, and specimen temperature. The CL intensity dependence on excitation density was analyzed using a power-law model (ICL ∝ Jm) for each of the observed CL bands in this material. The yellow luminescence band present in both silicon and undoped GaN exhibits a close to cube root (m = 0.33) dependence on electron beam excitation at both 77 K and 300 K. However, the blue (at 300 K) and donor-acceptor pair (at 77 K) emission peaks observed in undoped GaN follow power laws with exponents of m = 1 and m = 0.5, respectively. As expected from its excitonic character, the near band edge emission intensity depends linearly (m = 1) in silicon doped GaN and superlinearly (m = 1.2) in undoped GaN on the electron beam current. Results show that the intensities of the CL bands are highly dependent not only on the defect concentration but also on the electron-hole pair density and injection rate. Furthermore, the size of the focussed electron beam was found to have a considerable effect on the relative intensities of the CL emission peaks. Hence SEM parameters such as the objective lens aperture size, astigmatism, and the condenser lens setting must also be considered when assessing CL data based on intensity measurements from this material

Depth-resolved cathodoluminescence microanalysis of near-edge emission in III-nitride thin films

We describe an approach to assess the quality of III-nitride thin films using depth-resolved cathodoluminescence (CL) microanalysis. In this procedure, the depth-resolved peak shift due to self-absorption of the near-edge CL emission is calculated using Monte Carlo simulation techniques and compared with measured peak shift values. A discrepancy between the experimental and modeled data indicates the presence of an exciton peak shift due to strain, near-edge defects, and alloy fluctuation. Depth-resolved peak shift analysis of the near-edge CL from an undoped 700 nm thick Al0.057Ga0.943N film grown on a (0001) Al2O3 substrate is presented to demonstrate the utility of the method. © 2001 American Institute of Physics

Cathodoluminescence properties of zinc oxide nanoparticiles

Zinc oxide nano-particles (25 nm) have been investigated by cathodoluminescence spectroscopy (300 nm-1700 nm) at 80 K and 300 K following thermal annealing in high purity H2/N2, N2, O2 and Ar gaseous atmospheres. The intensity of the ZnO near band edge peak was significantly increased after heat treatment in hydrogen. Conversely, thermal annealing in the other gas types decreased this emission. This effect is attributed to hydrogen passivation of competitive non-radiative defect centers, most likely bulk zinc vacancy centers. The appearance of a strong green emission centered at 2.4 eV following thermal annealing in all gas atmospheres is ascribed to the formation of bulk oxygen vacancy defects. A strong red shift of the near band edge emission with increasing beam current at 300 K is accredited to electron beam heating rather than to an increase in the carrier density. Electron beam heating is evidenced by the occurrence of a strong black body emission in the near infrared spectral region. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of low-energy electron beam irradiation on defects in activated Mg-doped GaN

The influence of low-energy electron beam irradiation (LEEBI) on residual hydrogen impurities and defects in activated magnesium doped gallium nitride layers was studied. In-plane and depth-resolved cathodoluminescence imaging revealed the correlation between the spatial distribution of the injected carriers and the depth and lateral distribution of activated magnesium acceptors. It was found that the hydrogen dissociation results from electron-hole recombination at hydrogen defect complexes and is associated with the generation of additional defect centers

Electron irradiationinduced electromigration and diffusion of defects in Mgdoped GaN

In-plane and depth-resolved cathodoluminescence (Cl.) microanalysis and spectroscopy was carried out to study the impact of electron injection on electro-migration and diffusion of native defects and residual impurities in rapidly thermally annealed (RTA) Mg-doped p-type GaN. During intense electron beam irradiation (e.g. Eh = 10 keV, h= 80 nA), an electric field is generated within the primary beam interaction volume. We observed the following two electric field-related effects: (i) an increased electron recombination length and a subsequent field-assisted charge spreading, which causes a dissociation of Mg- H complexes beyond the interaction volume of the primary electron beam, and ii) thermally assisted electromigration of positively charged, mobile defects leading to the formation of deeper complexes, which are highly stable and act as nonradiative recombination pathways. Furthermore, the diffusion of electron beam-dissociated hydrogen was found to result in the passivation of Mg acceptors beyond the charge recombination volume

Low-energy electron-beam irradiation and yellow luminescence in activated Mg-doped GaN

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