Quantum confined silicon nanoparticles in disordered matrix

Abstract

Because of the rapid development of nanoscience and nanotechnology, nanoscale materials have drawn much attention. With decreasing the size of crystals to the nanoscale, many quantum-mechanical effects become more observable and can even dominate the electronic and optical properties of the materials. Also due to the promising, tunable, optoelectronic properties and potential for efficiency improvements, quantum confined (QC) semiconductor systems have drawn much attention for device applications. One of the attractive properties arising from quantum-mechanical effects is a size tunable band-gap since the band-gap increases as crystal size shrinks to smaller than the Bohr exciton radius. Quantum confined silicon nanoparticles (SiNPs) have received considerable attention, in part because Si is abundant and non-toxic, and in part because of the historical significance of Si in the microelectronics industry. Hydrogenated amorphous silicon (a-Si:H) has an even longer history of investigation and use in applications than SiNPs. The detailed properties, however, of the hybrid of those two materials when the SiNPs are quantum confined, is still missing. The hybrid material is a nanocomposite material, often called hydrogenated nanocrystalline silicon (nc-Si:H), consisting of SiNPs embedded in an a-Si:H matrix. Conventional nc-Si:H already exists and it has previously been shown that carriers transfer from a-Si:H to SiNPs rapidly. In addition, because of this carriers transfer, conventional nc-Si:H has less photodegradation than a-Si:H [1]. However, in the growth processes of conventional nc-Si:H, the SiNPs form through spontaneous nucleation, so control of the SiNP size, surface termination, and distribution is weak and the Si nanocrystallites in conventional nc-Si:H normally are too large to exhibit quantum confinement effects. The band-gap of conventional nc-Si:H is essentially the same as crystalline silicon (c-Si) [2]. This dissertation examines the optoelectronic and defect properties of quantum confined SiNPs, conventional a-Si:H and in particular of the same SiNPs in a-Si:H (QC-nc-Si:H). With a novel deposition process, we are able to reduce the SiNP size into the quantum confined regime and embed the quantum confined SiNPs into an a-Si:H matrix. QC-nc-Si:H is found to have a band-gap larger than c-Si while smaller than a-Si:H consistent with published model results. Using this QC-nc-Si:H we have seen the photoexcited carriers generated in the aSi:H matrix transfer into SiNPs much as they did with conventional nc-Si:H. In addition, this carrier transfer process dramatically reduces photodegradation (the Staebler-Wronski effect (SWE)) in the material. The QC-nc-Si:H with properties we have explored would open up new device possibilities such as an all nanocrystalline silicon-based multi-junction thin film solar cell or an alternative emitter or collector layer for a c-Si heterojunction (HIT) cell.