In the present work, the potential of group III-nitride nanowire (NW) ensembles for application as bio-chemical sensors with optical readout was evaluated. The sample analysis was divided into two main parts. At first, an electrochemical analysis with impedance spectroscopy and cyclic voltammetry was conducted. The second part comprises a photoelectrochemical examination by means of photoluminescence spectroscopy at room temperature.In the electrochemical characterization by impedance spectroscopy the recorded impedance spectra were evaluated by comparison to an electrical equivalent circuit. The influence of Silicon- and Magnesium-doping on the extracted parameters defining the NW/electrolyte interface, the surface capacitance (CS) and the surface resistance (RS), was investigated. Comparing the n.i.d. GaN NW ensembles to Si-doped ensembles reveals an increase of CS with a simultaneous reduction of RS. This is assigned to the formation of a conductive channel inside the NWs. For Mg-doped NWs only a slight increase of RS in combination with a small decrease of CS was observed and assigned to compensation of the residual n-type doping.According to the extracted electrochemical parameters the NWs were classified into two groups. On the one hand, the conductive NWs, defined by a highCS in combination with a low RS. On the other hand, the resistive NWs which were characterized by a lower CS and higher RS. The differences between these two groups were explained in terms of different bandprofiles within the NWs. Assuming a constant diameter along the NW growth axis the width of the space charge region depends on the doping concentration. Due to Si-diffusion the doping concentration is known to be higher in the region close to the interface to the Si substrate. The interplay between the NW diameter and the surface band bending (SBB) determines the potential barrier Ufb for electrons to reach the surface. For conductive NW a high doping concentration and/or a large diameter allows the formation of a conductive channel in the NW so that the complete NW to contributes to CS and RS. On the contrary, a resistive NW with a thin diameter is fully depleted of charge carriers as the space charge region is extended throughout the whole NW. This model was approved by cyclic voltammetry measurements which indicate the interfacial charge transfer into a redox couple ccurring only for conductive NWs. Bias application allows for the modification of the SBB inside the NWs which was investigated by bias-dependent IS analysis. Cathodic bias application decreases the space charge region for resistive NWs, resulting in an increase of CS accompanied by a significant decrease of RS. Anodic bias application showed the opposite effect of a sharply increased RS and a slightly decreased CS for the conductive NW sample with the highest Si-concentration, indicating that the electrical characteristics of NWs can be controlled by externally applied bias.The photoluminescence of GaN NW ensembles in electrolyte solution was characterized as a function of the externally applied bias (UC) and the pH value. It was shown that the intensity of the PL intensity strongly depends on the bias and the pH, while its energetic position and full width at half maximum (FWHM) remain unchanged. These results were discussed on the basis of the extended "dead layer" model for semiconductors, assuming the absence of radiative recombination within the space charge region. Photo-generated charge carriers can either recombine non-radiatively at the surface or radiatively in the field-free NW center. The application of cathodic bias at constant pH directly impacts the SBB with the result that photo-generated holes are no longer attracted towards the surface. As a consequence the radiative recombination rate increases while non-radiative losses at the surface decrease. At constant bias the decrease of the pH value below 7 leads to a similar increase of the intensity. This was explained in terms of the site-binding model which states that in acidic solutions the surface becomes more positively charged. This impacts the SBB in a similar manner as cathodic bias. The "extended dead layer model" was confirmed by analysis of Si-doped NWs that exhibit a higher SBB due to the smaller width of the SCR. First results on the influence of Mg-doping of GaN NWs were presented.Finally, a conceptional study of NW ensembles as an optical transducer for pH detection was briefly presented. The data indicated that with the setup used here a pH resolution of 0.05 pH can be achieved with high reproducibility. Additionally, two proof of principle measurements were presented where the enzymatically catalyzed production of penicilloic acid accompanied by a subsequent pH change was optically detected with GaN and InGaN/GaN NWs.
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