K. Thonke

Temperature‐dependent lifetime distribution of the photoluminescence S‐band in porous silicon

We study the recombination mechanism of the visible photoluminescence (PL) S‐band in p‐doped porous Si layers by time‐resolved photoluminescence. From the observed ‘‘stretched‐exponential’’ PL decays we present a simple yet accurate evaluation method for lifetime distributions G(τ) and average recombination lifetimes 〈τ〉. The average lifetimes feature a strong temperature dependence and a characteristic thermal activation energy of 10–20 meV for low temperatures. Our results are discussed within the models of quantum‐confined exciton recombination and surface state recombination.

Electroluminescence, photoluminescence, and photocurrent studies of Si/SiGe p‐i‐n heterostructures

Comparative electroluminescence and photoluminescence measurements were performed on Si/Si0.7Ge0.3 p‐i‐n single quantum well structures, and on one p‐i‐n and one undoped multiple quantum well structure in a wide temperature range. The samples were grown pseudomorphically by molecular beam epitaxy, and mesa diodes for electroluminescence and photocurrent measurements were fabricated. In electroluminescence, optical emission comes primarily from the SiGe quantum wells whereas no emission from Si is observed except for high temperatures of ≊200 K and up. All p‐i‐n structures exhibit maximum emission intensities in a temperature range between 80 K and 220 K, depending on the quantum well width. This temperature characteristic is very different from undoped quantum well samples. A model is discussed that accounts satisfactorily for all observed temperature dependent data. As an essential feature, the model includes Auger recombination in addition to radiative recombination in the n+ and p+ sides of the junctio...

Cathodoluminescence investigation of isotope effects in diamond

Abstract We report high-resolution cathodoluminescence (CL) measurements near the indirect gap of isotopically modified diamonds. From the energies of intrinsic phonon-assisted peaks, as well as the no-phonon recombination of boron-bound excitons, we determine the contribution of isotope disorder to the frequencies of various phonons with nonzero wave vector. The results confirm calculations of the phonon self-energy based on ab-initio lattice dynamics. For some modes we predict an asymmetric variation of the disorder-induced shift with the 13 C concentration which is observed in second-order Raman spectra. The no-phonon emission from boron-bound excitons (D 0 and D 0 ′ lines) exhibits a fine structure which arises from a splitting of the neutral boron acceptor ground state (2 meV) and the initial bound excitons (3.3 meV). When varying the 13 C content all components shift with a slope of (14.6 ± 0.5) meV/amu.

Uniaxial stress and Zeeman splitting of the 1.681 eV optical center in a homoepitaxial CVD diamond film

Abstract Observation of the 1.681 eV optical center in a homoepitaxial CVD diamond film at the very narrow photoluminescence (PL) linewidth of 0.2 meV allows us to study the defect under uniaxial stress parallel to 〈100〉 and in magnetic fields along the three main crystal directions. With no external field, the center exhibits a characteristic fourfold no-phonon PL fine structure plus weak additional components. The four main components are further split apart by uniaxial stress p∥〈 100〉 at linear rates. The energy level scheme deduced from these data and from thermalization measurements indicate that the center is under internal uniaxial overpressure at zero external stress. This is ascribed to its large size in the narrow diamond lattice. Complex splitting patterns are observed for applied magnetic fields which cannot fully be analyzed at present. The observed anisotropies indicate a non-cubic symmetry of the defect close to tetragonal, possibly slightly distorted towards lower symmetry. This is consistent with simple models of the defect involving silicon, nitrogen and a vacancy which are the established defect constituents.

Photoluminescence of high-quality SiGe quantum wells grown by molecular beam epitaxy

A variety of SiGe quantum well (QW) samples were grown by solid‐source molecular beam epitaxy (MBE) to study the influence of growth temperature TG and QW width Lz on the photoluminescence (PL) properties. For all growth temperatures investigated (350 °C≤TG≤750 °C) we found intense, well‐resolved PL signals from the SiGe QWs. The PL intensity increases with TG, and the stability against measurement temperature becomes better. A formerly reported PL band below the SiGe band edge is either completely absent, or very weak in the 4.2 K spectra of our samples. Thus, the defects or complexes responsible for this signal are obviously not inherent properties of MBE growth.

Quantum confinement effects in absorption and emission of freestanding porous silicon

Abstract Free-standing heavily p-doped porous silicon films with varying nanocrystallite sizes are analysed in absorption and photoluminescence experiments. Blueshift of the optical absorption and of the PL-IR and S-band occurs with increasing porosity, i.e. reduced nanoparticle size. Absorption and emission data demonstrate a continuous transition from bulk to quantum confinement-controlled Si bandstructure.

Dynamics of the degradation by photo-oxidation of porous silicon: FTPL and FTIR absorption study

Fourier transform photoluminescence (FTPL) and Fourier transform infrared (FTIR) spectroscopy have been performed in situ during etching and on freshly anodized porous silicon after etching. During etching the authors find a continuous increase of Si-H bonds and no additional oxygen species at the inner surface of porous silicon. They demonstrate that photoirradiation in oxygen ambient causes photo-oxidation at the inner surface and thus accelerates aging significantly. Furthermore, they investigate the influence of different ambients and varying laser power on the dynamic process of PL degradation of, and increasing oxygen incorporation into, freshly etched porous silicon.

Characterization of doped and undoped CVD-diamond films by cathodoluminescence

Nominally undoped and boron- and phosphorus-doped CVD-grown diamond films on silicon substrates were studied by cathodoluminescence in the range from 2 eV to 5.5 eV with and without high spatial resolution. Single crystallites in polycrystalline films exhibit strong free exciton luminescence, which is mostly absent in completely closed films, with a few exceptions. The presence of nitrogen in the layers quenches the free exciton radiation. We observe new, fairly narrow luminescence lines at 5.15 eV and 4.97 eV close to the emission of the boron bound exciton. A broad band at h v = 4.4–4.6 eV recently associated with boron shows an unresolved triplet substructure. All these near-band edge peaks seem to originate from optical centers associated with boron doping.

Luminescence properties and surface topography of porous silicon

Abstract We report optical and structural properties of porous silicon by steady-state and time-resolved photoluminescence (PL) and scanning tunneling microscopy (STM). The PL decay is non-exponential and the lifetimes are strongly dependent on temperature. Our experimental data of the recombination lifetime under variation of temperature, emission energy, and excitation density are discussed within the models of tail-state recombination, direct interband, and quantum-confined exciton recombination. Our optical and structural results support the model of quantum-confined carriers in crystalline Si structures.

Observation of boron bound excitons in boron-implanted and annealed natural IIa diamonds

Cathodoluminescence at 77 K was used to study the optical properties of ion-implanted and annealed natural type IIa diamonds. The substrates were implanted at room temperature with 12C+, 11B+, 31P+, and 75As+ -ions with energies of up to 350 keV and doses of up to 3×1013 cm−2. After annealing at 1200 °C, the cathodoluminescence spectra show a number of transitions which are induced by the radiation damage independent of the implanted ion species. Only in the B+implanted samples are there two transitions related to the implanted ion species: the 4.5 eV band and the boron bound exciton. The appearance of the bound exciton spectrum demonstrates the presence of isolated boron on substitutional lattice sites implying electrical acceptor activity. Our annealing studies indicate a minimum annealing temperature of 1000 °C for the activation of the implanted boron atoms onto the acceptor states.