Matthias Offer

Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.

Probing the band structure of InAs/GaAs quantum dots by capacitance-voltage and photoluminescence spectroscopy

The band structure of self-assembled InAs quantum dots, embedded in a GaAs matrix, is probed with capacitance-voltage spectroscopy and photoluminescence (PL) spectroscopy. The electron energy levels in the quantum dots with respect to the electron ground state of the wetting layer (WL) are determined from the capacitance-voltage measurements with a linear lever arm approximation. In the region where the linear lever arm approximation is not valid anymore (after the charging of the WL), the energetic distance from the electron ground state of the WL to the GaAs conduction band edge can be indirectly inferred from a numerical simulation of the conduction band under different gate voltages. In combination with PL measurements, the complete energy band diagram of the quantum dot sample is extracted.

Temperature-induced crossover between bright and dark exciton emission in silicon nanoparticles

The excitonic fine structure of silicon nanoparticles is investigated by time-resolved and magnetic-field–dependent photoluminescence. The results are analyzed using the common model of an excitonic fine structure consisting of a bright and a dark exciton. We find that the radiative recombination rates of both excitons differ only by a factor of eight. Therefore, we observe a thermal crossover in the character of the emission from bright-exciton-like to dark-exciton-like. The validity of our model is further supported by magnetic-field–dependent measurements, in which effects of state mixing are observed.

Silicon Nanoparticles: Excitonic Fine Structure and Oscillator Strength

In this review, recent results on optical spectroscopy on silicon nanoparticles are summarized. We will demonstrate the quantum size effect observed in the photoluminescence for nanoparticles with diameters below

Spatially resolved photovoltaic performance of axial GaAs nanowire pn-diodes

10 nm

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

. Moreover, the excitonic fine structure splitting caused by the exchange interaction is investigated using time-resolved and magnetic-field-dependent photoluminescence measurements. From these results, it is possible to estimate the rate of non-radiative recombinations in these nanoparticles, which allows to determine the oscillator strength and the quantum yield independently.

Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics.

Nanowires allow for a combination of highly mismatched materials needed for optimized broad spectrum absorption [1], and the use of appropriate substrates. III–V nanowires offer in addition a high absorption coefficient which is indispensable if a high efficiency energy transformation is aimed at the nanoscale. Currently, both radial [2] as well as axial III–V nanowire pn-junctions are under discussion for photovoltaic applications. The axial device enables a high forward current, ultra-low leakage, and a possible staggered integration of multiple cells. Moreover, the axial device gives full access for a high resolution photovoltaic analysis that is especially important at the current status of evaluation since a detailed device understanding is pending and the published yield data of nanowire devices are still low [1–2].