Advances in the characterization of nanowire photovoltaic devices

III-V nanowires (NWs) have a great potential for solar energy applications due to their diameter-dependent optical properties, which may enhance absorption of light. In addition, core-shell radial p-i-n structures, in which the direction of light absorption is orthogonal to the carrier collection, can provide efficient carrier collection. The main goal of this thesis is the experimental study of the challenges of NW-based solar cells, related to materials and device fabrication. In the first part of the thesis, we present an analysis of where the electrical losses can be originated. By applying an equivalent circuit analysis approach, we classified them into three main groups: (i) the non-uniformity of NWs which may result in a reduction of the parallel resistance, (i) potential barriers originated at the different materials interfaces in the solar cell structure may result in an increase of the series resistance or addition of a second diode and (iii) surface recombination resulting in the reduction of the open-circuit voltage. In this thesis, we propose separate strategies to characterize and tackle these factors. The electric scheme of a NW-based solar cell consists of an ensemble of p-n junctions connected in parallel. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of these parallel-connected NW devices. We demonstrate topography and current mapping of the NW arrays, combined with current-voltage (IV) measurements of the individual NW junctions from the ensemble. Our results provide discussion elements on some of the factors limiting the performance of a NW-based solar cell, such as uniformity and photosensitivity of the individual NW p-n junctions within the array, and thereby a path for their improvement. Besides parallel losses due to uniformity issues, barriers in the carrier collection through the various heterointerfaces composing the device is discussed. To analyze it, we illuminate GaAs NW-based solar cells at different levels of light intensity and extract IV characteristics. This analysis helps to separately study the NW p-n junction response and the series resistance. The high series resistance of the NW-ensemble device can be attributed to the following interfaces: 1) GaAs-ITO, forming a photoactive Schottky diode, which suppresses the p-n junction at high concentrations of light, and 2) Si-GaAs heterojunction, disturbing the flow of majority carriers. Finally, the characterization of surface passivation in high-aspect-ratio nano/micro structures is addressed by electrochemical impedance spectroscopy (EIS). The method is applied to Si micropillars, as a proof-of-concept prior to the application to III-V nanowires. We tested structures passivated by a dielectric layer. The effect of different surface treatments on the interface state density were quantified by the analysis of the capacitance-voltage and conductance-voltage characteristics. This method allows the electrical measurements on rough vertical surfaces, which would otherwise suffer from high gate leakage currents if tested using solid-state metal-insulator-semiconductor scheme. The results and characterization methods, demonstrated in this work, contribute to the overall efforts of the scientific community on how to reveal the main engineering challenges in NW-based solar cells. It thus paves the way to approach the fundamental conversion efficiencies predicted by theory

Thick GaN growth via GaN nanodot formation by HVPE

We demonstrate a 400 mu m-thick GaN layer on 4 inch (0001) Al2O3 substrates through GaN nanodot formation as a seed for stress relaxation layers, which were formed by an in situ special surface treatment using HVPE. The size and density of the GaN nanodots determined the thickness of the stress relaxation layers and the structural properties of thick GaN. The 400 mu m-thick GaN layer exhibits a smooth surface and high crystal quality with FWHM of 104 arcsec and 163 arcsec in the (002) and (102) X-ray rocking curves, respectively. The dislocation density estimated via micro-PL measurements was 2 x 10(6) cm(-2). This can provide an efficient and simple way to fabricate thick GaN layers on an Al2O3 substrate without ex situ buffer layer formation or additional complicated processes

The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE

We investigate the stress evolution of 400 mu m-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices

Nearly perfect GaN crystal via pit-assisted growth by HVPE

We demonstrated 5 mm-thick bulk gallium nitride (GaN) with nearly perfect crystal quality. To achieve this, intentional etch pit formation by in situ dry HCl etching and ex situ wet etching in H3PO4 solution was employed on freestanding GaN templates followed by regrowth by hydride vapor phase epitaxy (HVPE). This consecutive etching gave rise to a number of large and deep etch pits on the whole surface of the freestanding GaN. We believe that these can reveal the etch pits originating from the dislocations with the edge, and mixed components. Intentionally formed etch pits were partially covered with the fresh regrown bulk GaN layer, being transformed into voids. Dislocations cannot propagate into new GaN layers through the voids, thus resulting in the reduction of dislocation density. 5 mm-thick bulk GaN exhibits a smooth, transparent surface and extremely high crystal quality with full width at half maximum (FWHM) of 21 arcsec in (0002) X-ray rocking curve and etch pit density (EPD) of 3 x 10(2) per cm(2). This method can provide a promising way to fabricate bulk GaN with extremely low dislocation density, suitable for the fabrication of high-performance devices

Impact of the Ga Droplet Wetting, Morphology, and Pinholes on the Orientation of GaAs Nanowires

Ga-catalyzed growth of GaAs nanowires on Si is a candidate process for achieving seamless III/V integration on IV. In this framework, the nature of silicon’s surface oxide is known to have a strong influence on nanowire growth and orientation and therefore important for GaAs nanowire technologies. We show that the chemistry and morphology of the silicon oxide film controls liquid Ga nucleation position and shape; these determine GaAs nanowire growth morphology. We calculate the energies of formation of Ga droplets as a function of their volume and the oxide composition in several nucleation configurations. The lowest energy Ga droplet shapes are then correlated to the orientation of nanowires with respect to the substrate. This work provides the understanding and the tools to control nanowire morphology in self-assembly and pattern growth