Peder Bergman

Nanoscale Ln(III)-Carboxylate Coordination Polymers (Ln = Gd, Eu, Yb): Temperature-Controlled Guest Encapsulation and Light Harvesting

We report the self-assembly of stable nanoscale coordination polymers (NCPs), which exhibit temperature-controlled guest encapsulation and release, as well as an efficient light-harvesting property. NCPs are obtained by coordination-directed organization of pi-conjugated dicarboxylate (L1) and lanthanide metal ions Gd(III), Eu(III), and Yb(III) in a DMF system. Guest molecules trans-4-styryl-1-methylpyridiniumiodide (D1) and methylene blue (D2) can be encapsulated into NCPs, and the loading amounts can be controlled by changing reaction temperatures. Small angle X-ray diffraction (SAXRD) results reveal that the self-assembled discus-like NCPs exhibit long-range ordered structures, which remain unchanged after guest encapsulations. Experimental results reveal that the negatively charged local environment around the metal connector is the driving force for the encapsulation of cationic guests. The D1 molecules encapsulated in NCPs at 140 degrees C can be released gradually at room temperature in DMF. Guest-loaded NCPs exhibit efficient light harvesting with energy transfer from the framework to the guest D1 molecule, which is studied by photoluminescence and fluorescence lifetime decays. This coordination-directed encapsulation approach is general and should be extended to the fabrication of a wide range of multifunctional nanomaterials.

Time‐resolved spectroscopy of Zn‐ and Cd‐doped GaN

Photoluminescence transients and time‐resolved luminescence spectra are reported for the violet‐blue emissions from epitaxial layers of Zn‐ and Cd‐doped GaN. A typical decay time of 300 ns is reported for the blue GaN:Zn emission, peaking at about 2.89 eV. For GaN:Cd a somewhat longer decay time of τ≊1 μs dominates for the broad peak centered at ≊2.72 eV. In both cases it is concluded that the simple process involving a free‐to‐bound transition of an electron to a hole bound at the ZnGa, respectively, the CdGa acceptor is the dominating recombination mechanism corresponding to these characteristic decay rates.

Controlling inter-chain and intra-chain excitations of a poly(thiophene) derivative in thin films

The decay of photoexcitations in polythiophene chains has been studied in solid solutions of the polymer from room temperature to 4 K. A strong blue shift of the emission spectrum is observed in th ...

Structural and Optical Properties of Homoepitaxial GaN Layers

The review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.

High‐resolution x‐ray analysis of InGaN/GaN superlattices grown on sapphire substrates with GaN layers

InGaN/GaN superlattice structures grown on (0001) sapphire substrates with GaN layers were investigated by two‐dimensional reciprocal space mapping of high‐resolution x‐ray diffraction. The results show that InGaN/GaN multi‐quantum wells with fairly good crystallinity can be grown coherently on partially relaxed GaN layer irrespective of the large lattice mismatch and thermal incompatibility with the underlying substrate. Narrow and bright band edge related emissions were observed by photoluminescence measurements, indicating high quality of these InGaN/GaN superlattice structures. Our results suggest a larger band‐gap bowing coefficient than the value reported in the literature.

Mn2+ intra-shell recombination in bulk and quantum dots of II-VI compounds

Origin of a fast component of the photoluminescence (PL) decay of Mn 2+ intra-shell 4T1 ? 6A 1 transition in bulk and quantum dot structures is discussed based on the results of PL, PL kinetics and ...

Homoepitaxial On-Axis Growth of 4H- and 6H-SiC by CVD

More than 30 μm thick high quality homoepitaxial layers have been grown on both 4HSiC and 6H-SiC on-axis [00.1] substrates. The epitaxial layers were grown both in a horizontal hotwall CVD reactor at 1580 o C, and in a chimney-type vertical hot-wall CVD at 1800-1850 o C. Photoluminescence (PL) measurements performed at low temperature (2 K) show strong free exciton (FE) related luminescence. The typical residual doping concentration, obtained by capacitance versus voltage measurements, was in the low 10 cm range for 4H and 6H (00.1) face. At 1580 o C and a growth rate of 3 μm/h the homoepitaxial yield is almost 100% on the 4H (00.1 ) and 6H (00.1) faces, while more than 50% has been achieved on the 4H (00.1) face.

Investigations of Possible Nitrogen Participation in the Z1/Z2 Defect in 4H-SiC

We have investigated the possibility of nitrogen involvement in the formation of the Z1/Z2 defect in 4H-SiC. A recent model suggests that nitrogen together with carbon interstitiallike defects is responsible for this deep center. This possibility was tested by irradiation of phosphorus doped epilayers with low residual nitrogen concentration. Nitrogen doped epilayers with similar doping concentration were used for comparison. Irradiation yields identical DLTS spectra for both phosphorus and nitrogen doped samples; the Z1/Z2 peak occurs at the same temperature and its concentration is independent of the dopant. The concentration of the Z1/Z2 defect in irradiated phosphorus doped samples exceeds the atomic nitrogen concentration by nearly one order of magnitude. No saturation of the concentration of the Z1/Z2 defect or other irradiation induced defects was observed. We conclude that nitrogen atoms do not participate in the formation of the Z1/Z2 defect.

Optical and Transport Properties of CdSe/ZnSe Self-Organized Nanostructures: 1-Dimensional versus 3-Dimensional Quantum Confinement

We present comprehensive studies of optical, structural and transport properties of molecular beam epitaxy grown CdSe/ZnSe self-organizing nanostructures, aimed at elucidation of the intrinsic morphology of CdSe layers as a function of their nominal thickness (w). Particular attention has been paid to the layer thickness below the critical value (w < 3.0 monolayer (ML)). It is found that the layer morphology is transformed from pure two-dimensional objects at w<0.7 ML to extended (15–40 nm) flat islands formed in the thicker layers. The density of the islands increases with w, which is accompanied by enhancement of the luminescence quantum efficiency. An optically pumped green laser is fabricated, taking advantage of the superior emission characteristics of the single CdSe layer with w=2.8 ML, used as an active region. The threshold power density of 3.9 kW/cm2 observed at 300 K is fivefold less than that of the reference sample using a conventional ZnCdSe quantum-well active region.

SiC – a semiconductor for high-power, high-temperature and high-frequency devices

SiC has in comparison with Si superior basic properties for applications in high-power, high-frequency and high-temperature electronics. The potential applications of SiC were known decades ago, but the poor quality of the material produced at that time has delayed the device development. However, during the last years the crystal growth process of SiC has been improved considerably. We will present some important properties of SiC, describe the two most common growth processes and discuss fundamental materials problems that remain to be solved. A further aspect, which we will discuss, is the polytypism of SiC, which may allow us to obtain generic knowledge of, for instance, defects in semiconductors.