Electrical Characterization of Semiconductor Surfaces and Thin Film Structures using Near- Surface Photoconductance Decay Technique

Open Access
Arora, Aakash
Graduate Program:
Electrical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 03, 2014
Committee Members:
  • Jerzy Ruzyllo, Thesis Advisor
  • Effective Carrier Lifetime
  • Photoconductance Decay
  • Silicon-on-Insulator
  • Electrical Characterization
Electrical characterization of semiconductor surfaces has been a subject of interest for decades. The realization of devices using ultra-thin film semiconductor specimens entails a thorough understanding of surface properties, it’s response to various physical and chemical treatments, behavior of electrical defects present at the surface, near-surface carrier profiles, mobilities and potential distributions. These parameters have been effectively measured for bulk semiconductors using well-established commercial tools, and fairly reproducible results have been produced and published. However, the final device performance will depend on and be reflected by surface dominated phenomena because only the near-surface region is contributing towards the ‘active’ layer in the device operation, and most of the tools fail to characterize this region. Therefore, a fast, reliable and non-destructive in-situ characterization technique is required to monitor and quantify the quality of the surface and sub-surface regions in thin-film structures. Near-surface photo-conductance decay tool utilized in this work makes an attempt in this regard by measuring the near surface minority carrier decay lifetime after the cessation of an external excitation provided in the form of a laser pulse. Previous works done using this tool demonstrate its sensitivity to surface treatments and the ability of the tool to capture changes in the semiconductor surfaces, interfaces, and three-dimensional structures. This work particularly focusses on exploring the capability of the tool to respond to different excitation wavelengths and intensities. Bulk silicon and germanium samples have been used to demonstrate this experiment by measuring the ‘effective’ minority carrier lifetime. An attempt is made to distinguish the surface lifetime from its bulk counterpart, depending on the difference in the nature of defects, thus leading to ‘defect specific photo-conductance’. Theoretical background of this concept has been summarized and verified by experimental data on electronic grade bulk n-type and p-type silicon under different surface conditions. Silicon-on-Insulator (SOI) technology is a keystone in the development of thin-film nano-scale devices. This work presents the characterization of SOI samples with top layer thicknesses in the same range, but manufactured using different fabrication techniques. Because of the inherent physical separation of the ‘active’ layer from the bulk through a dielectric, the lifetimes measured essentially reflect the properties of the active film. A comparative study of the carrier decay curves with surface changes is done for these thin films, and calculations for the intensities in terms of concentration of photo-carriers generated are also included.