W. Jantsch

Paramagnetic GaN : Fe and ferromagnetic ( Ga , Fe ) N : The relationship between structural, electronic, and magnetic properties

We report on the metalorganic chemical vapor deposition (MOCVD) of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), spatially-resolved energy dispersive X-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably FexN with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. In layers with iron content x<0.4% the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in- and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.

Initial stage of the two-dimensional to three-dimensional transition of a strained SiGe layer on a pit-patterned Si(001) template

We investigate the initial stage of the 2D-3D transition of strained Ge layers deposited on pit-patterned Si(001) templates. Within the pits, which assume the shape of inverted, truncated pyramids after optimized growth of a Si buffer layer, the Ge wetting layer develops a complex morphology consisting exclusively of {105} and (001) facets. These results are attributed to a strain-driven step-meandering instability on the facetted side-walls of the pits, and a step-bunching instability at the sharp concave intersections of these facets. Although both instabilities are strain-driven, their coexistence becomes mainly possible by the geometrical restrictions in the pits. It is shown that the morphological transformation of the pit surface into low-energy facets has strong influence on the preferential nucleation of Ge islands at the flat bottom of the pits.

On the local structure of optically active Er centers in Si

We report high resolution (<0.05 cm−1) photoluminescence (PL) spectra of erbium implanted float‐zone (FZ) and Czochralski grown (CZ) silicon. We show that the PL spectrum of cubic Er centers observed in CZ‐Si annealed at 900°C is the dominant emission in FZ‐Si for the same annealing conditions. We assign it to isolated, interstitial erbium. We observe also two other kinds of optically active Er centers with lower than cubic site symmetry: (i) O‐related (found only in CZ Si) and (ii) those related to radiation defects. We conclude that coimplantation with light elements does not lead to the formation of Er‐codopant complexes, but rather to Er forming complexes with implantation induced lattice defects.

Characterization of deep levels in CdTe by photo-EPR and related techniques

Abstract In photo-EPR, the EPR signature of microscopically identifiable defects is used to monitor light-induced electronic transitions. In this paper we summarize photo-EPR investigations of group IV donors, 3d transition metal impurities and complexes in CdTe. Energy levels and Huang-Rhys factors are derived for the 3d impurities by fitting a model calculation to the photo-ionization cross-section which is derived from constant photo-EPR. Comparison with results for other II–VI compounds confirm host-independent relative zero-phonon level positions for different 3d metal impurities and large lattice relaxation energies.

Deep donor levels due to isolated Fe in CdTe

We report investigations of the photoionization energy of substitutional Fe in CdTe by means of photo-EPR and photoluminescence. Non-illuminated p-type samples exhibit an EPR signal due to the ionized donor, Fe3+ (3d5). Under illumination the EPR signal of high resistivity samples increases strongly for photon energies exceeding 1.45 eV. This increase is attributed to the excitation of the donor electron to the conduction band. Photoluminescence spectra of Fe doped CdTe samples show a pronounced, sharp line peaked at 1.475 eV and a broad luminescence band at about 1.1 eV. We show that both emission lines are related to iron impurities. The experimentally determined level energy of Ec − 1.45 eV for Fe2+3+ is in good agreement with recent theoretical predictions.

Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells

Exploiting the spin resonance of a two-dimensional (2D) electron in SiGe/Si quantum wells, we determine the carrier density dependence of the magnetic susceptibility. Assuming weak interaction, we evaluate the density of states at the Fermi level, D(E{sub F}) , and the screening wave vector, q{sub TF} . Instead of the constant values of an ideal 2D system, we observe a gradual decrease towards the band edge. Calculating the mobility from q{sub TF} yields good agreement with experimental values justifying the approach. The decrease in D(E{sub F}) is explained by potential fluctuations which lead to tail states that make screening less efficient and, in a positive feedback, cause an increase of the potential fluctuations.

Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x . 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+.

Pure spin currents induced by spin-dependent scattering processes in SiGe quantum well structures

We show that spin-dependent electron-phonon interaction in the energy relaxation of a two-dimensional electron gas results in equal and oppositely directed currents in the spin-up and spin-down subbands yielding a pure spin current. In our experiments on SiGe heterostructures the pure spin current is converted into an electric current applying a magnetic field that lifts the cancellation of the two partial charge flows. A microscopic theory of this effect, taking account of the asymmetry of the relaxation process, is developed and is in good agreement with the experimental data.

Direct excitation spectroscopy of Er centers in porous silicon

We report direct excitation of optically active Er centers in porous Si. Excitation spectroscopy performed close to the intracenter 4I15/2→4I11/2 and 4I15/2→4I9/2 transitions of Er3+ (4f11) ions allows us to identify two kinds of Er centers in porous Si: (i) Er diffused into porous nanograins with lower than cubic symmetry and (ii) Er centers incorporated in an amorphous silicalike matrix. The latter show much weaker thermal quenching of the Er3+ emission which decreases only by a factor of eight when the temperature is increased from 4.2 K up to 360 K.

On the environment of optically active Er in Si-electroluminescence devices

We report sharp, atomlike electroluminescence spectra close to 1.54 μm from a low-dose (3.5×1018 cm−3) erbium-implanted silicon light-emitting diode operating under forward bias. The well-resolved Stark splitting identifies the isolated interstitial Er with cubic site symmetry as the source. The full width at half maximum of the most intense line is 0.5 nm. A comparison with a highly Er (5×1019 cm−3) and O (1×1020 cm−3) doped diode with a high doping gradient grown by molecular beam epitaxy and with Er-implanted silica is given with respect to fine structure and thermal quenching. The room-temperature emission of the highly Er and O doped diode is ascribed to Er containing silica precipitates within the c-Si matrix.