Fabrication and Characterization of Thermomechanically Processed Sulfur and Boron Doped Amorphous Carbon Films

Authors

Lonnie Carlson, University of Nebraska-LincolnFollow

Date of this Version

8-2012

Document Type

Article

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Materials Engineering), Under the Supervision of Professor Jennifer I. Brand. Lincoln, Nebraska: August, 2012

Copyright (c) 2012 Lonnie Carlson

Abstract

Small scale, high power density, reliable, and long-life power supplies would be useful or even critical for space missions or the growing number of microdetectors, microsensors, and miniature vehicles. Alpha or beta particle voltaic devices could satisfy these requirements but have been shown to degrade quickly due to radiation damage. Amorphous carbon (a-C) PN junctions or PIN devices could provide radiation hardness and sufficiently high efficiency. As the range of alpha and beta particles in a-C is ~20-120μm, much thicker films than are typical are needed to maximize collection of the particle energy.

In this work, the fabrication of thermomechanically processed p- and n-type doped a-C films were investigated as a first step in the future development of radiation hard voltaic devices. Boron carbide (B₄C) powder was mixed with a-C nanopowders as a possible p-type dopant with sulfur powder utilized as a possible n-type dopant. Doping levels of 2.5at%, 5.0at%, and 10.0at% were investigated for both dopants with films pressed at 109°C over a pressure range of 0.3-5.0GPa. Initial attempts to fabricate rectifying PN junctions and PIN devices was unsuccessful.

Bonding properties were characterized using Raman spectroscopy with electronic properties primarily assessed using the van der Pauw method. Undoped a-C and boron-doped films were found to be slightly p-type with sulfur-doped films converting to n-type. All films were found to consist almost entirely of nano-graphitic sp² rings with only slight changes in disorder at different pressures. Sulfur doped films were less brittle which is indicative of crosslinking.

Boron doping did not significantly change the film electronic properties and is not an effective dopant at these temperatures and pressures. Sulfur doping had a greater effect and could likely be utilized as basis for an n-type material in a device.

Initial irradiation studies using alpha particles showed that boron and undoped films became more p-type with sulfur films converting to p-type. The sulfur doped films returned to n-type after isothermal annealing.