Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

Increased Photoconductivity Lifetimes in GaAs Nanowires via n-Type and p-Type Shell Doping

Francesca Amaduzzi, Anna Fontcuberta i Morral, Federico Matteini

Reliable doping in GaAs nanowires is essential for the development of novel optoelectronic devices. Previously, GaAs nanowires have been shown to exhibit extremely short photoconductivity lifetimes of a few picoseconds due to their high surface recombination velocity, which is detrimental for nanowire devices, such as solar cells and nanowire lasers. Here, we show that, by exploiting engineered band-bending via selective doping, this parasitic surface recombination can be reduced. We utilise non-contact time-resolved terahertz spectroscopy to characterise the doping efficiency in n-type and p-type doped GaAs nanowire(8) and show high carrier concentrations of the order of 10(18) cm(-3). The carrier lifetimes were increased by an order of magnitude from 0.13ns for undoped to 3.8ns and 2.5ns for n-doped and p-doped GaAs nanowires respectively; showing that surface recombination is greatly suppressed as a result of shell doping. We also present a novel effect of p-doping in GaAs nanowires: a rapid decay in photoconductivity within 25ps after photoexcitation. This fast decay is attributed to rapid electron trapping at the nanowire surface due to doping related band bending. Thus, we demonstrate the advantages of selective doping for enhancement of desirable transport properties in GaAs nanowires, as well as highlighting terahertz spectroscopy as a reliable technique for characterising doped GaAs nanowires(1).

Ieee2016

Controlled growth of CH3NH3PbI3 nanowires in arrays of open nanofluidic channels

Eric Joseph Bonvin, László Forró, Endre Horvath, Bálint Náfrádi, Andrzej Sienkiewicz, Massimo Spina

Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels. The guided growth, visualized in real-time with a simple optical microscope, undergoes through a metastable solvatomorph formation in polar aprotic solvents. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.

Nature Publishing Group2016

Defect chemistry of methylammonium lead iodide

Alessandro Senocrate

The present work deals with the defect chemistry and charge transport properties in halide perovskites, and in particular in the archetypal methylammonium lead iodide. These materials are extensively researched due to their very promising application as light-harvesters in solar cells and in other optoelectronic devices. Notwithstanding the numerous studies dealing with these materials (especially with their optical and electronic properties, and with device application), a significant portion of the underlying physics and chemistry is still poorly understood. Indeed, the physico-chemical features behind their exceptional photo-electrochemical properties are still largely unknown. Moreover, these materials suffer from severe degradation processes presently impeding their practical application. In addition, the charge transport in these materials is not purely electronic, but rather shows a significant ionic portion due to mobile ionic defects. The nature of such ion conduction, alongside its effect on the photo-electrochemical properties and on the materials stability, has never been systematically investigated. The study of these aspects is the aim of this thesis, where: At first, we study the charge transport properties of methylammonium lead iodide, with particular attention to the ionic contribution, and we perform a defect chemical study of the compound. We show how the ionic conductivity, in equilibrium conditions, can be unambiguously assigned to mobile iodine vacancies, with an electronic contribution due to electron holes or conduction electron depending on the iodine activity. Secondly, we investigate the light effect on the charge transport previously characterized. Alongside an expected increase of the electronic contribution, we observe a striking enhancement of ionic conductivity in methylammonium lead iodide upon illumination. This remarkable observation is of fundamental relevance for both photovoltaics and solid state ionics fields. Here we also discuss a mechanism for such photo-enhanced ion conduction that relies on electron-ion interaction. Subsequently, we analyze methylammonium lead iodide and other hybrid halide perovskites under oxygen exposure, both in the dark and under illumination. We show that light strongly affects the kinetics of oxygen interaction, so much so that under illumination methylammonium lead iodide completely degrades, while it is metastable in the dark. The oxygen, when incorporated in a sufficient amount in the lattice, also acts as acceptor dopant, greatly varying the ionic and electronic conductivities. We then investigate stability of several halide perovskites with respect to temperature, oxygen, and illumination through thermodynamic considerations. Here we show how many of these processes are expected to be extremely severe for some of the compounds, underlining an important -and intrinsic- bottleneck for the application of these materials. At last, we investigate the short-range ion dynamics in methylammonium lead iodide, since these are linked to stability and electronic transport properties. Here, we show that methylammonium dynamics conforms well to a fast bi-axial rotation becoming isotropic in the cubic phase. In the inorganic lattice, a strong nuclear coupling between Pb and I is present, alongside highly active iodine dynamics.

EPFL2018

Defect chemistry of methylammonium lead iodide

Alessandro Senocrate

EPFL2018