Nanoscale characterisation of dielectrics for advanced materials and electronic devices

Abstract

Strained silicon (Si) and silicon-germanium (SiGe) devices have long been recognised for their enhanced mobility and higher on-state current compared with bulk-Si transistors. However, the performance and reliability of dielectrics on strained Si/strained SiGe is usually not same as for bulk-Si. Epitaxial growth of strained Si/SiGe can induce surface roughness. The typical scale of surface roughness is generally higher than bulk-Si and can exceed the device size. Surface roughness has previously been shown to impact the electrical properties of the gate dielectric. Conventional macroscopic characterisation techniques are not capable of studying localised electrical behaviour, and thus prevent an understanding of the influence of large scale surface roughness. However scanning probe microscopy (SPM) techniques are capable of simultaneously imaging material and electrical properties. This thesis focuses on understanding the relationship between substrate induced surface roughness and the electrical performance of the overlying dielectric in high mobility strained Si/SiGe devices. SPM techniques including conductive atomic force microscopy (C-AFM) and scanning capacitance microscopy (SCM) have been applied to tensile strained Si and compressively strained SiGe materials and devices, suitable for enhancing electron and hole mobility, respectively. Gate leakage current, interface trap density, breakdown behaviour and dielectric thickness uniformity have been studied at the nanoscale. Data obtained by SPM has been compared with macroscopic electrical data from the same devices and found to be in good agreement. For strained Si devices exhibiting the typical crosshatch morphology, the electrical performance and reliability of the dielectric is strongly influenced by the roughness. Troughs and slopes of the crosshatch morphology lead to degraded gate leakage and trapped charge at the interface compared with peaks on the crosshatch undulations. Tensile strained Si material which does not exhibit the crosshatch undulation exhibits improved uniformity in dielectric properties. Quantitative agreement has been found for leakage at a device-level and nanoscale, when accounting for the tip area. The techniques developed can be used to study individual defects or regions on dielectrics whether grown or deposited (including high-κ) and on different substrates including strained Si on insulator (SSOI), strained Ge on insulator (SGOI), strained Ge, silicon carbide (SiC) and graphene. Strained SiGe samples with Ge content varying from 0 to 65% have also been studied. The increase in leakage and trapped charge density with increasing Ge extracted from SPM data is in good agreement with theory and macroscopic data. The techniques appear to be very sensitive, with SCM analysis detecting other dielectric related defects on a 20% Ge sample and the effects of the 65% Ge later exceeding the critical thickness (increased defects and variability in characteristics). Further applications and work to advance the use of electrical SPM techniques are also discussed. These include anti-reflective coatings, synthetic chrysotile nanotubes and sensitivity studies.