T. Schupp

Dielectric function of zinc-blende AlN from 1 to 20 eV: Band gap and van Hove singularities

The dielectric function (DF) of phase-pure cubic AlN films is determined by ellipsometry. The sharp onset of the imaginary part of the DF defines the direct absorption edge corresponding to a conduction-to-valence band spacing at the center of the Brillouin zone (BZ) of 5.93 eV. Phonon-assisted transitions lead to the pronounced absorption tail below this edge from which the indirect gap of zinc-blende AlN is estimated with 5.3 eV. Transitions due to four additional critical points of the BZ are resolved at higher photon energies. The high-frequency and static dielectric constants are determined with 4.25 and 8.07, respectively.

Anti-phase domains in cubic GaN

The existence of anti-phase domains in cubic GaN grown on 3C-SiC/Si (001) substrates by plasma-assisted molecular beam epitaxy is reported. The influence of the 3C-SiC/Si (001) substrate morphology is studied with emphasis on the anti-phase domains (APDs). The GaN nucleation is governed by the APDs of the substrate, resulting in equal plane orientation and the same anti-phase boundaries. The presence of the APDs is independent of the GaN layer thickness. Atomic force microscopy surface analysis indicates lateral growth anisotropy of GaN facets in dependence of the APD orientation. This anisotropy can be linked to Ga and N face types of the {111} planes, similar to observations of anisotropic growth in 3C-SiC. In contrast to 3C-SiC, however, a difference in GaN phase composition for the two types of APDs can be measured by electron backscatter diffraction, μ-Raman and cathodoluminescence spectroscopy.

Long room-temperature electron spin lifetimes in highly doped cubic GaN

We report on very long electron spin relaxation times in highly n-doped bulk zincblende GaN exceeding 500 ps up to room-temperature. Time-resolved Kerr-rotation measurements show an almost temperature independent spin relaxation time between 80 and 295 K confirming an early prediction of Dyakonov and Perel for a degenerate electron gas.

High temperature electron spin dynamics in bulk cubic GaN: Nanosecond spin lifetimes far above room-temperature

The electron spin dynamics in n-doped bulk cubic GaN is investigated for very high temperatures from 293 K up to 500 K by time-resolved Kerr-rotation spectroscopy. We find extraordinarily long spin lifetimes exceeding 1 ns at 500 K. The temperature dependence of the spin relaxation time is in qualitative agreement with predictions of Dyakonov-Perel theory, while the absolute experimental times are an order of magnitude shorter than predicted. Possible reasons for this discrepancy are discussed, including the role of phase mixtures of hexagonal and cubic GaN as well as the impact of localized carriers.

Growth of cubic GaN quantum dots

Zinc‐blende GaN quantum dots were grown on 3C‐AlN(001) by two different methods in a molecular beam epitaxy system. The quantum dots in method A were fabricated by the Stranski‐Krastanov growth process. The quantum dots in method B were fabricated by droplet epitaxy, a vapor‐liquid‐solid process. The density of the quantum dots was controllable in a range of 108 cm−2 to 1012 cm−2. Reflection high energy electron diffraction analysis confirmed the zinc‐blende crystal structure of the QDs. Photoluminescence spectroscopy revealed the optical activity of the QDs, the emission energy was in agreement with the exciton ground state transition energy of theoretical calculations.

Excitonic complexes in single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy

We study by microphotoluminescence the optical properties of single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy. We show evidences of both excitonic and multiexcitonic recombinations in individual quantum dots with radiative lifetimes shorter than 287 ± 8 ps. Owing to large band offsets and a large exciton binding energy, the excitonic recombinations of single zinc-blende GaN/AlN quantum dots can be observed up to 300 K.

Mechanical Properties of Cubic SiC, GaN and AlN Thin Films

Cubic polytypes of SiC, GaN and AlN were grown on silicon by molecular beam epitaxy. The mechanical properties of the epitaxial layers were investigated by nanoindentation. For 3C-SiC grown on Si(111) and Si(100) a dependence of the mechanical properties on the surface preparation with germanium prior to the carbonization was obtained.

Temperature dependence of the electron Landé g-factor in cubic GaN

The temperature dependence of the electron Lande g-factor in bulk cubic GaN is investigated over an extremely broad temperature range from 15 K up to 500 K by time-resolved Kerr-rotation spectroscopy. The g-factor is found to be approximately constant over the full investigated temperature range. Calculations by k·p-theory predict a negligible temperature dependence g(T) in complete agreement with the experiment as a consequence of the large band-gap and small spin orbit splitting in cubic GaN.

Strain dependent electron spin dynamics in bulk cubic GaN

The electron spin dynamics under variable uniaxial strain is investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. Spin relaxation is found to be approximately independent of the applied strain, in complete agreement with estimates for Dyakonov-Perel spin relaxation. Our findings clearly exclude strain-induced relaxation as an effective mechanism for spin relaxation in cubic GaN.

Long room-temperature electron spin lifetimes in bulk cubic GaN

We report on very long electron spin lifetimes in cubic GaN measured by time-resolved Kerr-rotation-spectroscopy. The spin coherence times with and without external magnetic field exceed 500 ps at room temperature, despite a high n-type doping level of more than 1019 cm-3 in the bulk sample under investigation. Our findings are therefore highly relevant for spin optoelectronics in the blue wavelength regime. The spin lifetimes are found to be almost temperature independent in accord with a prediction for degenerate electron gases of Dyakonov and Perel from 1972. These results are discussed also in comparison to wurtzite GaN, which shows much shorter spin lifetimes and a dependence of spin lifetimes on the spin orientation.