Fingerprinting Electronic Structure in Nanomaterials: A Methodology Illustrated by ZnSe Nanowires.

Characterizing point defects that produce deep states in nanostructures is imperative when designing next-generation electronic and optoelectronic devices. Light emission and carrier transport properties are strongly influenced by the energy position and concentration of such states. The primary objective of this work is to fingerprint the electronic structure by characterizing the deep levels using a combined optical and electronic characterization, considering ZnSe nanowires as an example. Specifically, we use low temperature photoluminescence spectroscopy to identify the dominant recombination mechanisms and determine the total defect concentration. The carrier concentration and mobility are then calculated from electron transport measurements using single nanowire field effect transistors, and the measured experimental data were used to construct a model describing the types, energies, and ionized fraction of defects and calculate the deviation from stoichiometry. This metrology is hence demonstrated to provide an unambiguous means to determine a material's electronic structure.

Enhancement of transport properties in single ZnSe nanowire field-effect transistors.

Wide-gap semiconductors are excellent candidates for next-generation optoelectronic devices, including tunable emitters and detectors. ZnSe nanowire-based devices show great promise in blue emission applications, since they can be easily and reproducibly fabricated. However, their utility is limited by deep level defect states that inhibit optoelectronic device performance. The primary objective of this work is to show how the performance of ZnSe nanowire devices improves when nanowires are subjected to a post-growth anneal treatment in a zinc-rich atmosphere. We use low temperature photoluminescence spectroscopy to determine the primary recombination mechanisms and associated defect states. We then characterize the electronic properties of ZnSe nanowire field effect transistors fabricated from both as-grown and Zn-annealed nanowires, and measure an order-of-magnitude improvement to the electrical conductivity and mobility after the annealing treatment. We show that annealing reduces the concentration of zinc vacancies, which are responsible for strong compensation and high amounts of scattering in the as-grown nanowires.

Electrical characteristics and photocurrent spectral response of Si nanowires p-i-n junctions.

P-i-n junctions were fabricated along Si nanowires (SiNWs) via the conventional top-down approach using optical lithography. Each device comprises 500 identical SiNWs connected in parallel, and each SiNW has triangular cross-section with dimensions of ~6 nm (base) by ~8 nm (height). The photodiodes exhibit very good rectifying electrical characteristics with a low reverse bias current of ~0.2 fA per SiNW. The photocurrent spectral response exhibits three peaks between 400 nm to 700 nm, which arise due to local optical field enhancement associated with diffraction by the periodic SiNW array and interference in an air/SiO2/Si cavity.