Andre Krtschil

This report concerns investigations of the activation process of nerve cells cultured on large planar interdigitated electrodes. Cultured neurons from embryonic rat cerebral cortex form electrophysiologically active networks after one week in vitro. The treatment with antagonists to ionotropic glutamate and GABA_A receptors blocks synaptic neurotransmission without affecting intrinsic excitability of neurons. Standard stimulation parameters, which were previously determined to trigger a generalized network response in control networks, were ineffective for activation of networks under a synaptic activity blockade. Here we show that in these conditions a pulse sequence of ten biphasic pulses with amplitude of ±2.2 V is adequate for global network stimulation, without cell damage. In specially designed compartment cultures we analyzed further the stimulation process in control and synaptically blocked networks with cell bodies spatially separated from the electrodes. Our results suggest that the direct stimulation of few neurons trigger network-wide responses by the activation of synaptic connections. Further, the stimulation of axonal neuritic extensions is sufficient to active single neurons.

P- and n-type conductivity domains in dual-doped ZnO:As+N layers grown by metal organic vapor phase epitaxy on GaN–sapphire templates were electrically microcharacterized by scanning capacitance microscopy (SCM) and scanning surface potential microscopy (SSPM) techniques with respect to their defect states. The p-type domains were found to be dominated by two acceptors with thermal activation energies of about 80 and 270 meV, as observed by transient SCM scans at different temperatures. Optically excited SSPM scans revealed defect-to-band transitions at 400, 459, and 505 nm omnipresent in both domain types as well as a shallower transition at 377 nm exclusively in the p-type regions. According to the similar energy levels, the optical transitions at 377 and 400 nm are assigned to acceptor states, whereby the 80-meV acceptor is probably responsible for the conversion from n- to p-type regions in the domains.

Quenching of photocurrent spectroscopy and thermally stimulated currents has been investigated in undoped AlGaN layers and GaN/AlGaN heterostructures grown by metalorganic vapor phase epitaxy on sapphire substrates. The quenching was induced by an additional illumination as well as by an excitation with light below bandgap energy, respectively. In GaN layers, mainly the thermal defect emission band between 210 and 300 K is involved in the quenching processes and there are strong indications of a reduction of the band by incorporation of aluminum. In AlGaN layers, a decrease of quenching with increasing Al content is observed in accordance to a reduction of the height of the thermal emission between 210 and 300 K in these layers. These results are supported by quenching experiments in AlGaN/GaN heterostructures.

Quenching effects induced by additional below-bandgap illumination in undoped semi-insulating GaN were investigated using optical admittance spectroscopy (OAS) and photocurrent (PC) spectroscopy as well as optically excited, thermally stimulated currents (TSC). In OAS and PC, a decrease of defect-related signals due to the quenching light was observed. The thermal quenching of the defect band between 2.7 and 3.3 eV shows a good agreement with thermal emissions as measured by TSC, indicating the same defects cause the optical transitions in OAS/PC and the thermal transitions in TSC. The thermal emission in the temperature region between 250 and 300 K, which is responsible for the thermal quenching of the blue band (BB) in OAS, also shows an optical quenching under below-bandgap excitation.