Enno Malguth

Lithium related deep and shallow acceptors in Li-doped ZnO nanocrystals

We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with an acceptor energy of 800 meV could be identified in both EPR and PL measurements and is responsible for the yellow luminescence at 2.2 eV. Additionally, a shallow acceptor state at 150 meV above the valence band maximum is made responsible for the observed donor-acceptor pair and free electron-acceptor transitions at 3.235 and 3.301 eV, possibly stemming from the formation of Li-related defect complexes acting as acceptors.

Doping-level-dependent optical properties of GaN:Mn

The optical properties of molecular-beam-epitaxy-grown GaN with different Mn-doping levels (5–23×1019 cm−3) were studied by cathodoluminescence (CL) and optical transmission spectroscopy. Transmission measurements at 2 K revealed an absorption peak at 1.414±0.002 eV, which was attributed to an internal 5T2→5E transition of the neutral Mn3+ state. The intensity of this Mn-related transmission peak was found to scale with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm−3 reduced the near band edge emission intensity by around one order of magnitude. A complete quenching of the donor–acceptor-pair band at 3.27 eV and strong decrease of the yellow luminescence centered at 2.2 eV were attributed to a reduced concentration of VGa. In the infrared spectral range of 0.8–1.4 eV three broad, Mn-doping related CL emission bands centered at 1.01±0.02, 1.09±0.02, and 1.25±0.03 eV were observed. Their origin is attributed to deep donor complexes, which are generated as a ...

Structural and optical inhomogeneities of Fe doped GaN grown by hydride vapor phase epitaxy

We present the results of cathodoluminescence experiments on a set of Fe doped GaN samples with Fe concentrations of 5×1017, 1×1018, 1×1019, and 2×1020 cm−3. These specimens were grown by hydride vapor phase epitaxy with different concentrations of Fe. The introduction of Fe is found to promote the formation of structurally inhomogeneous regions of increased donor concentration. We detect a tendency of these regions to form hexagonal pits at the surface. The locally increased carrier concentration leads to enhanced emission from the band edge and the internal T41(G)–A61(S) transition of Fe3+. In these areas, the luminescence forms a finely structured highly symmetric pattern, which is attributed to defect migration along strain-field lines. Fe doping is found to quench the yellow defect luminescence band and to enhance the blue luminescence band due to the lowering of the Fermi level and the formation of point defects, respectively.

Mn charge states in GaMnN as a function of Mn concentration and co-doping

In the context of the pursuit of a dilute magnetic semiconductor for spintronic applications, a set of GaMnN samples with varying Mn concentration and Si or Mg co-doping was investigated by optical and electron spin resonance spectroscopy. The results clearly demonstrate how the charge state of Mn is changed between 2+, 3+ and 4+ by Mg and Si co-doping. For p-type GaMnN we show that the introduction of the Mn 3+/4+ donor can be compensated by Mg co-doping lowering the Fermi energy below the Mn 3+/4+ level. While our results are in agreement with the hypothesis that the infrared photoluminescence appearing in GaMnN upon Mg doping originates from Mn 4+ , an unambiguous proof is still to be presented. Under this assumption, our measurements show that the Mn 4+ center must be excited via an extra-center process at 2.54 eV.

Fe-Centers in GaN as Candidates for Spintronics Applications

The potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 400 „m thick freestanding HVPE grown GaN:Fe crystals with difierent Fe-concentration levels ranging from 5£10 17 cm i3 to 2£10 20 cm i3 was studied by means of photoluminescence, photoluminescence excitation (PLE) and Fourier transform infrared (FTIR) transmission experiments. The Fe 3+=2+ charge transfer (CT) level was determined to be at 2.86 § 0.01 eV above the valence band maximum considerably lower than the previously reported value of 3.17 § 0.10 eV. A bound state of the form (Fe 2+ , hV B) with a binding energy of 50 § 10 meV has been established as an excited state of Fe 3+ . FTIR transmission measurements revealed an internal ( 5 E| 5 T2) transition of Fe 2+ around 400 eV which, until now, was believed to be degenerate with the conduction band. Consequently, a second CT band was detected in PLE spectra.

Mn- and Fe- doped GaN for spintronic applications

In the context of ferromagnetic spin-coupling in dilute magnetic semiconductors, we present optical investigations on Mg co-doped GaMnN and Fe doped GaN. A complex luminescence feature occurring in Mg co-doped GaMnN around 1 eV was previously attributed to the internal 4 T 2 (F)— 4 T 1 (F) transition of Mn 4+ involved in different complexes. Selective excitation studies indicate the presence of at least three different complexes. Photoluminescence excitation spectra suggest that the internal Mn 3+ transition may represent an excitation mechanism. Magneto photoluminescence spectra indicate equal g values for the ground and excited state. Low temperature infrared absorption spectra of Fe doped GaN allow to unambiguously establish the electronic structure of the Fe 2+ center in GaN. Our results suggest that the Fe 2+ ( 5 T 2 ) state is stabilized against Jahn-Teller coupling by the reduced site-symmetry of the hexagonal lattice.

Optical Properties of Mn-doped GaN

Molecular beam epitaxy-grown GaN with different Mn concentrations (5-23 x 10 19 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 5 T2 5 E transition of the deep neutral Mn 3+ state since its intensity scaled with the Mn 3+ concentration. The CL measurements showed that Mn-doping concentrations around 10 20 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.