Mastering the doping of semiconductor nanowires : the case of zinc oxide – MAD-FIZ

Nanowires were grown by metalorganic chemical vapour deposition (MOCVD), and the incorporation of doping impurities was investigated following in situ processes (during growth), and ex situ diffusion from a gas phase or a «spin on dopant (SOD)« glass. Atom probe tomography has yielded the spatial distribution of dopants. The incorporation and a first insight of the dopant properties was assessed through their optical signatures. Scanning capacitance microscopy (SCM) was developed and used to locally measure the concentration of carriers. Transport experiments in a single doped nanowire were carried out. At the same time, ab initio calculations has brought information on the surface contribution and effects, and help to understand the experimental data.

The project took benefit of the most advanced nano-characterization tools as well as the «state of the art« growth techniques and processes developed for nanostructures. In particular, the consortium opened the question whether size reduction (turning from a thin film to a nanowire) could make it easier to dope ZnO p-type.
We have evidenced the incorporation of nitrogen as a dopant impurity in ZnO NWs, by using atom probe tomography, which reveals a homogeneous distribution of nitrogen atoms, without clusters or aggregates. Optical characterizations suggest the introduction of acceptors, which constitute a first step towards p-type conduction (by holes). Nevetheless, electrical measurements carried out on a single doped NW show that electrons keep on being the majority carriers in the material (n-type). We emphasize that the conduction is ensured by the volume of the NW, and not the surface.

The developed methodology and the results concerning atom probe tomography open new fields of investigation. The measurement of impurity concentrations is now possible for semiconductor nanowires like it is case for 2D layers by using SIMS (secondary ion mass spectrometry).
Electrical characterizations show how the surface of ZnO is reactive, and how adsorbed species influence the transport properties of nanowires. These astonishing surface properties should be exploited to design high sensitive gas sensors.
The development of SCM and SSRM techniques (which use an atom force microscope in an electrical mode) is promising to assess the electrical activity of the core of a semiconductor nanowire, or the separate the contributions in the radial structures (so-called «core-shell«)

Most part of this work has been published in international journals dedicated to fundamental or applied physics (Physical Review Applied, Applied Physic Letters…). Oral presentations (some invited) have been given during international conferences gathering the scientific community working on nanotechnologies, multifunctional materials or developing novel characterization techniques.

Semiconductor nanowires (SC-NWs) open new fields of investigation in fundamental physics, and offer unique opportunities for the future generation of electronics, photonics, sensors, actuators, energy, and medical applications. During the last decade, improvements in crystal growth and characterization at the nanometer scale made it possible to confront experimental observations and theoretical models. Today, an ensemble of advanced nano-characterization tools available at the national level allows us to address the issues related to SC-NWs doping. Our ambition is to yield a clear understanding of the complex relations between doping, size reduction, surface effects and transport properties, in the light of structural, optical and electrical investigations at the nanoscale. Since p-type doping is still a debated issue for ZnO, nanowire of this material will be taken as a case study for the MAD-FIZ project in order to address some of the main issues that concern SC-NWs transport properties. In particular, the consortium will open the question whether size reduction (turning from a thin film to a nanowire) could make it easier to dope ZnO p-type. Six partners with complementary expertises will be present in the consortium : GEMAC (Groupe d'Etude de la Matière Condensée (coordinator), GPM (Groupe de Physique des Matériaux), INL (Institut des Nanotechnologies de Lyon), Institut Néel, CEA-LETI, and CEA-DEN. The project will take benefit of the most advanced nano-characterization tools as well as the "state of the art" growth techniques and processes developed for nanostructures.
Nanowires will be grown by metalorganic chemical vapour deposition (MOCVD), and incorporation of doping impurities will be investigated following in situ processes, and ex situ diffusion from a gas phase or a "spin on dopant (SOD)" glass. Atom probe tomography will yield the spatial distribution of dopants. The incorporation and a first insight of the dopant properties will be assessed through their optical signatures. Scanning capacitance microscopy will be developed and used to locally measure the type and the concentration of carriers. Transport experiments in a single doped nanowire will be carried out, including I(V), Deep Level Transient Spectroscopy, capacitive measurements, and cathodoluminescence coupled with e-beam induced current (EBIC). At the same time, ab initio calculations will bring information on the surface contribution and effects, and help to understand the experimental data.
Besides the fundamental question concerning the relationship between doping and low dimensionality, one ambitious objective of the project is the realisation of electroluminescent radial and/or axial ZnO NW p-n junctions and the observation of near UV electroluminescence.