Electrical Transport in Thin Films of Doped Silicon Nanocrystals

Abstract

Colloidal semiconductor nanocrystals (NCs) have shown great potential for thin-film optoelectronics, such as solar cells and light emitting diodes (LEDs), due to their size-tunable electronic properties and solution processability. Significant progress has been made in developing synthetic methods to prepare high quality NCs, achieving controllable doping, and integrating NCs into high performance electronic devices. Most electronic applications rely on the electrical conduction through NC films, therefore, fundamental understanding of the carrier transport in NC films is required to further improve device performance and provide guide for future device design. My research is inspired by the successful achievement of a highly efficient LED with hydrosilylated Si NCs as the emissive layer. To better understand the electrical conduction in the Si NC system, a systematic study of the temperature and electric-field dependence of the film conductivity is performed. It shows that the conductivity of the Si NC film is limited by the ionization of rare NCs containing donor impurities and the carrier transport follows nearest neighbor hopping. The Si NCs are inherently doped with a very small concentration of impurities, about 1 donor per 1000 NCs. This is also the first study of carrier transport in a lightly doped NC system, and results obtained in this work can apply to other NC materials as well. The organic ligands used to passivate NC surface are necessary to achieve strong photoluminescence, however, they inhibit the carrier transport due to the resulting large tunneling barrier between neighboring NCs. The localization length estimated from the temperature data in the high electric field regime is about 1 nm. In addition, the activation energy required for conduction also depends on the surrounding medium of NCs, the electrical conduction can be improved by reducing the activation energy through engineering of the matrix of NC arrays. Doping is critical to enable electrical transport in semiconductor NC films which are otherwise insulating materials. Significant efforts have been made to intentionally introduce substitutional impurities into the NCs, however, only a few attempts have succeeded. One is controllable doping with phosphorus (P) in Si NCs synthesized from a nonthermal plasma gas-phase method. This NC system provides a platform for studying the doping effects on the electronic properties of NCs. In contrast to the Si NCs lightly doped with inherent impurities, the intentional doping with P can easily achieve heavily doped NCs so that each Si NC is metallic. Efros-Shklovskii variable range hopping (ES-VRH) is observed in dense films of P-doped, ligand-free spherical Si NCs over a wide range of doping concentration. The localization length increases with increasing doping concentration and exceeds the diameter of a NC, indicating the approach to the metal-insulator transition (MIT) in the NC film. A theoretical criterion is developed to predict the critical doping concentration for the MIT in a NC film. It reasonably explains a doping-dependent localization length as observed in experiments. Additionally, by varying the separation between NCs through controlled oxidation, the localization length decreases with increasing the interparticle separation, in agreement with the cotunneling theory. Boron (B) doping has also been achieved in the plasma synthesis method, and the electrical properties have shown strong dependence on the surface treatment since most B atoms are sitting on the NC surface. In dense films of B-doped Si NCs, the carrier transport still exhibits ES-VRH conduction but the localization length is doping-independent. The highest doping concentration achieved in this system is actually close to the theoretical critical doping concentration for the MIT in NC films, however, the expected divergence of the localization length does not occur. It is proposed that the degeneracy of conduction band minima or valence band maxima plays a key role in the carrier transport of NC films. Therefore, the hole transport can be completely different from the electron transport in Si NC films. The critical doping concentration derived for P-doped Si NCs cannot be applied to B-doped Si NCs. Moreover, the air stability of Si NCs is significantly affected by the doping. A modified atomic layer deposition (ALD) method is developed to infill B-doped Si NC films and excellent air stability is obtained with a few nanometers coating with alumina. In summary, this thesis focuses on the electrical transport in thin films of doped Si NCs, which are synthesized from a nonthermal plasma gas phase method. Both of inherent and intentional doping have been investigated, and the pictures for carrier transport physics from light doping to heavy doping have been illustrated. This work explores the doping effects on the electrical properties of Si NCs, and also provides a roadmap for the electrical conduction in NC films over a wide range of doping concentration.