One of the most promising applications, the 1D semiconductor nanowires (NWs) offer, is the development of next generation solar cells at low-cost and higher efficiency. Group III–V semiconductor NWs, especially GaAs and InP, are the ideal semiconductor materials to build such devices on low-cost platforms.
The aim of this work was to fabricate and study the fundamental working mechanisms of single nanowire solar cell and photodetector based on novel radial p-n junction in a core-shell geometry. The Au-assisted GaAs NWs were grown and doped in situ using metalorganic vapour phase epitaxy (MOVPE). In this study, a unique lithography-free technique was employed on an ensemble of NWs to isolate the core (p-type) from the shell (n-type) on the growth substrate. Electron beam lithography (EBL) was used to make metal contacts to the single core-shell NWs. In that respect, specific contact schemes were developed to realize ohmic contacts between the metal electrodes and the NWs. Electrical measurements of single nanowire device revealed perfect I-V behaviour confirming the formation of radial p-n junction diode. For characterization of NWs, scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) and photocurrent spectroscopies were employed.
Furthermore, surface passivation of NWs was identified as one of the key issues in functioning of the photovoltaic device as no photo response was detected without the same. The surface passivation of GaAs NWs with higher band gap AlGaAs layers resulted in dramatic enhancement of the PL intensity, and the photocurrent measurements of the device revealed a broad photo response in the visible spectrum range of the light, indicating a successful working of a single nanowire as a photodetector. However, although a functioning radial p-n junction was demonstrated, no solar response was detected from the single nanowire structure because of yet many unaddressed challenges such as high contact resistance, doping optimisation, absence of intrinsic region and unoptimised geometry. In summary, a single nanowire based photodetector was successfully demonstrated, and a full fabrication process was studied and developed for making progress towards realising the functional single nanowire based radial p-n junction solar cells in the future
