Hope College Department of Physics and Engineering
Research Experiences for Undergraduates
Summer 2004
Project Summary

 

Project Title: Performing the Haynes-Shockley Experiment
Student Name: Kathleen McCreary
Student home institution: Wooster College
Research Advisor(s): Dr. Mark Little
Source of Support: NSF-REU

The goal of this summer was to measure transit time and mobility of carriers injected into semiconductor samples using a modified version of the Haynes-Shockley method 1 . Amorphous and crystalline metal-nitride semiconductors samples were grown on p-silicon, n-silicon or sapphire substrates. Films were grown in a vacuum chamber with an atmosphere composed of Nitrogen and/or Argon. Using RF or AC, the desired metal was transformed into plasma and directed toward the substrate. Using conductive epoxy to attach wires to gold films sputtered on the sample was found to be most effective for creating electrical contacts. A large voltage pulse was triggered 1m sec before the laser fired, so the laser pulse would be incident on the sample during the voltage pulse. Ideally, the electric field produced by the voltage pulse would sweep the injected holes or electrons across the sample. During their journey, a gold collector pin in contact with the sample, would pick up the injected carriers. Modifications were made in attempt to detect the injected carriers. Circuits were created to amplify the input voltage pulse and the current produced by the injected carriers. A differential amplifier was used at one point. The current method of experimentation utilizes a square pulse produced by the pulse generator for the input voltage pulse. The frequency, width, amplitude, and phase can be altered as necessary. The current measured from the collector pin is connected to an amplifier before being analyzed on the oscilloscope. Currently, the input pulse is detectable through the collector pin with little noise. The current of laser injected carriers should be visible as a small, broad peak, but are yet to be detected

1J.R. Haynes and W. Shockley, “The mobility and life of injected holes and electrons in germanium,” Phys. Rev. 81 835-843 (1951)