[Paper Review] Hopping conduction in assemblies of hydrosilylated silicon nanocrystals

Ting Chen, Brian Skinner+3 more|arXiv (Cornell University)|Jan 27, 2014

Silicon Nanostructures and Photoluminescence1 citations

TL;DR

This study investigates electron transport in alkyl-ligand-terminated silicon nanocrystal (Si NC) films, revealing that conductivity is governed by thermally activated ionization of rare donor-impurity-containing NCs. At low bias, ionization follows a Poole-Frenkel mechanism with an ionization energy twice the NC charging energy; at high bias and low temperature, cold ionization via tunneling (1 nm characteristic length) dominates, providing a comprehensive model for electron transport in lightly doped nanocrystal films.

ABSTRACT

Silicon nanocrystals (Si NCs) have shown great promise for electroluminescent and photoluminescent applications. In order to optimize the properties of Si NC devices, however, electronic transport in Si NCs films needs to be thoroughly understood. Here we present a systematic study of the temperature and electric field dependence of conductivity in films of alkyl-ligand-terminated Si NCs, which to date have shown the highest potential for device applications. Our measurements suggest that the conductivity is limited by the ionization of rare NCs containing donor impurities. At low bias, this ionization is thermally activated, with an ionization energy equal to twice the NC charging energy. As the bias is increased, the ionization energy is reduced by the electric field, as determined by the Poole-Frenkel effect. At large bias and sufficiently low temperature, we observe cold ionization of electrons from donor-containing NCs, with a characteristic tunneling length of about 1 nm. The temperature- and electric-field-dependent conductance measurements presented here provide a systematic and comprehensive picture for electron transport in lightly doped nanocrystal films.

Motivation & Objective

• To understand the electronic transport mechanisms in films of alkyl-ligand-terminated silicon nanocrystals, which are promising for optoelectronic devices.
• To identify the dominant conduction mechanism under varying temperature and electric field conditions.
• To determine the role of donor impurities in individual nanocrystals in limiting conductivity.
• To characterize the transition from thermally activated ionization to field-assisted and cold ionization processes.
• To provide a quantitative framework for electron transport in lightly doped nanocrystal films for device optimization.

Proposed method

• Conducting temperature- and electric field-dependent conductivity measurements on films of alkyl-ligand-terminated silicon nanocrystals.
• Analyzing the data using the Poole-Frenkel model to extract field-dependent ionization energy.
• Modeling the ionization energy as a function of electric field and temperature to identify the transition to cold ionization.
• Using the observed scaling of conductivity with temperature and field to infer the effective tunneling length for electron emission.
• Relating the ionization energy to the nanocrystal charging energy, assuming it is twice the charging energy for donor-impurity states.
• Fitting the experimental data to distinguish between thermally activated and field-assisted ionization regimes.

Experimental results

Research questions

• RQ1 What is the dominant conduction mechanism in alkyl-ligand-terminated silicon nanocrystal films under varying temperature and electric field?
• RQ2 How does the ionization energy of donor-impurity states in individual nanocrystals depend on applied electric field and temperature?
• RQ3 What is the role of the nanocrystal charging energy in determining the ionization barrier for electrons?
• RQ4 At what field and temperature conditions does cold ionization via tunneling become significant?
• RQ5 Can the observed conductivity behavior be explained by a unified model combining Poole-Frenkel and tunneling effects?

Key findings

• Conductivity in the films is limited by the ionization of rare nanocrystals containing donor impurities.
• At low bias, ionization is thermally activated with an ionization energy equal to twice the nanocrystal charging energy.
• At elevated electric fields, the ionization energy decreases due to field enhancement, consistent with the Poole-Frenkel effect.
• At large bias and low temperature, cold ionization of electrons from donor-containing nanocrystals is observed, with a characteristic tunneling length of approximately 1 nm.
• The combined temperature- and field-dependent conductance measurements reveal a comprehensive picture of electron transport across multiple regimes.
• The results provide a quantitative framework for understanding and optimizing electron transport in lightly doped nanocrystal films for device applications.