Study of Semiconductor Near-Surface Region using Photoconductive Decay (PCD) Technique

Open Access
Author:
Bhatia, Divij
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 17, 2011
Committee Members:
  • Jerzy Ruzyllo, Thesis Advisor
Keywords:
  • near surface
  • photoconductive decay
  • lifetime
Abstract:
The photoconductive decay (PCD) technique is a commonly used to study bulk characteristics of semiconductor materials. This is work takes forward the idea of using solid state laser technology for studying the near-surface region of a semiconductor or the interface region of a semiconductor - dielectric interface. The choice of laser (wavelength) is material dependent and the tool, built around a probe station, has the ability to measure pre-patterned small geometry samples. A detailed description of the experimental procedure is provided. The tool has the ability to form reliable ohmic contact with dielectric of thickness less than 50nm without the need for any additional etching steps. The tool is then used to monitor silicon surface change after an HF clean following which it is left to stand in the ambient over time. The results are compared with a surface sensitive contact angle tool. The near-surface PCD (ns-PCD) technique is compared to conventional microwave PCD commonly used to measure bulk lifetime of a semiconductor. The near-surface tool shows a shorter lifetime value for the back surface than the polished front surface of the sample, while microwave PCD gives the same lifetime value for both surfaces. Thus the surface sensitivity of the tool is established. Next thin dielectrics grown on semiconductors are explored using the ns-PCD technique. The time limitation for the diffusion/oxidation furnace to form high quality silicon oxide on silicon is determined to be 8 minutes. This is shown by observing the PCD lifetime that reflects the interface quality of the oxide - silicon. Two different dielectrics in alumina and tantalum oxide are grown on silicon and the PCD plots are compared. The importance of laser choice is demonstrated by taking measurements using 658nm and 980nm lasers. The former shows a more defective interface in the case of tantalum oxide. A different semiconductor material in gallium antimonide is considered in another experiment. One sample is degreased using acetone and IPA clean while the other sample is cleaned using HCl. Alumina is grown on both samples using atomic layer deposition. ns-PCD shows that HCl degrades the surface of GaSb. However forming gas anneal shows an improvement in near surface lifetime implying that HCl clean is needed to remove the uncontrolled GaO and SbO formed under ambient. Lastly it is shown that ns-PCD can be used to explore three dimensional fin structures. An ungated InGaAs quantum well fin structure is grown for this purpose. Nine devices with decreasing fin width are measured using ns-PCD. The quality of the grown fins is reflected in the plots obtained.