2009 Undergraduate Research and Scholarship Conference

Title

Experimental Evidence for Polaron Hopping Conduction in HfO₂ – A Cryogenic Study

Presenter(s)

Justin Reed, Departments of Electrical & Computer Engineering and Materials Science & Engineering, Boise State University
Richard G. Southwick, Departments of Electrical & Computer Engineering and Materials Science & Engineering, Boise State University
Ross Butler, Departments of Electrical & Computer Engineering and Materials Science & Engineering, Boise State University
Chris Buu, Departments of Electrical & Computer Engineering and Materials Science & Engineering, Boise State University

Faculty Sponsor

Dr. Bill Knowlton

Information

Metal-oxide-semiconductor field effect transistors (MOSFETs) are used in nearly all electronic devices including computers and cell phones. The pursuit to create faster lighter electronics manifests subsequent decreases in MOSFET size and thus oxide thickness. Silicon dioxide (SiO₂) has traditionally been used as the dielectric material due to its ease of formation and adequate dielectric properties. SiO₂ in a current MOSFET is now 1.2 nm thick which is only several monolayers of Si-O. At this thickness, SiO₂ is losing its insulative properties allowing electrons to transport (i.e., leakage current) through which results in high power consumption and significant heat dissipation in electronic devices. New insulative materials, called high K dielectrics, are being investigated to replace SiO₂. The high K dielectric material halfnium oxide (HfO₂) is a primary candidate to replace SiO₂ as it can be grown thicker to minimize leakage current but maintain the necessary capacitance for high performance MOSFETs. However, leakage current analysis techniques fail to fully explain electron transport mechanisms in HfO₂ at low electric fields over an extended temperature range. Sir Neville Mott suggested electron transport in highly disordered materials (e.g., SiO₂ and HfO₂) can be attributed to three transport mechanisms: 1) localized to extended state excitation (tunneling), 2) polaron nearest College of Engineering Poster Presentations 53 neighbor hopping conduction (NNH), and 3) variable range hopping conduction (VRH)^[1,2] To investigate these transport mechanisms, the leakage current through HfO₂ was measured over a variety of voltages and a temperature range from 5-300K. We present data analysis and a subsequent model that incorporates Mott’s mechanisms to describe electron transport in HfO₂.

[1] N. F. Mott, “Conduction in noncrystalline materials II. Localized states in a pseudogap and near extremities of conduction and valence bands,” Philosophical Magazine, vol. 19, pp. 835-852, 1969.

[2] N. Mott, “Electrons in glass,” Physics Nobel Prize Lecture, pp. 403-413, 1977.