Electrical Characterization of Silicon Nanocrystal Films
Neema Rastgar, David Rowe, Lance Wheeler, Eray Aydil, Uwe Kortshagen
Thin films of semiconductor nanocrystals continue to receive attention as potential materials for making light-emitting diodes, photodiodes and solar cells. This approach to making optoelectronic devices may be promising because semiconductor nanocrystals are inexpensive to synthesize and their optoelectronic properties can be tuned by changing their size. However, devices based on thin films of nanocrystals typically show high electrical resistivity, and establishing control over electronic properties is difficult. The understanding of electronic transport in these nanocrystal films is in its infancy compared to bulk semiconductors. To improve this understanding and to learn how to manipulate charge carrier transport in semiconductor nanocrystal films, we study electronic transport in thin films of intrinsic and doped silicon nanocrystals.
Silicon nanocrystals with diameters ranging from 5-20 nm were synthesized through decomposition of silane in a radio-frequency plasma reactor. Thin films of these nanocrystals were deposited either through ballistic aerosol impaction onto substrates or through spin coating from colloidal dispersions of the nanocrystals. The former approach is in situ and the nanocrystals are deposited onto the substrate immediately after they leave the plasma. In the latter approach, the nanocrystals emerging from the plasma are collected, dispersed in a solvent and cast onto the substrate. In both cases, the nanocrystals are deposited between two 100 nm-thick thermally evaporated aluminum contacts to form thin films of randomly packed nanoparticles. Current-voltage characteristics of the nanocrystal films were measured as a function of doping and temperature between 100 and 300 K. Preliminary results show that the films exhibit space charge limited current above applied electric fields of 1000 V/cm, and Ohmic behavior at lower electric fields. The conductivity of annealed undoped films exhibits Arrhenius dependence on temperature, with an activation energy of 0.60 eV between room temperature and 225 K, indicative of conduction mediated by intrinsic carriers. Annealed boron-doped films, on the other hand, show moderate Arrhenius temperature dependence near room temperature, and weak temperature dependence below 225 K, characteristic of either dopant ionization or tunneling conduction.
Many films also demonstrate hysteresis in the current-voltage characteristics, ranging from insignificant to severe. The hysteresis is thought to arise from a parasitic capacitance in the film due to charging and appears to be most significant in films made of nanocrystals with ligands such as hexene. Resistance-capacitance (RC) time constants on the order of seconds describe the hysteresis. Conduction is sensitive to the surface conditions of the nanocrystals.
For example, the conductivity of nanocrystal films in vacuum increase by an order of magnitude when nitrogen, argon, and oxygen gases flow over the films and the chamber is purged continuously to reduce water partial pressure in the chamber. This change is reversible over a time scale of two hours and suggests that desorption of water from the film is the likely reason for improved conductivity.
This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885.