Developing Quantum Tunneling Contacts with Ultrathin AlN or ZrN on GaN

  • Author / Creator
    Voon, Kevin J
  • Gallium nitride (GaN) high-electron mobility transistors (HEMTs) have great potential for high-power and high-frequency applications, but current leakage issues compromise their reliability. This research focuses on developing GaN metal-oxide semiconductor field-effect transistors (MOSFETs), which produces higher current densities and breakdown voltages due to their natural off-state. Forming low-resistance ohmic contacts for the device can be accomplished by developing quantum tunneling contacts through an ultrathin AlN layer capable of generating high two-dimensional electron gas (2DEG) concentrations with GaN. This thesis will investigate the change in 2DEG and conductance with AlN thickness and GaN doping concentration, with AlN deposited using plasma-enhanced atomic layer deposition (PEALD) at 250ºC. In addition, ZrNx is being considered as a possible alternative material for high electric field regions of the GaN MOSFET due to its greater breakdown capacity compared to AlN. A peak 2DEG of 2.2e13 cm^-2 was achieved with 4.5 nm of AlN on highly doped N+ GaN (~10^18 cm-3 concentration), measured through capacitance-voltage (C-V) profiling, with strain relaxation occurring at 6 nm. Although the 2DEG was theorized to not significantly change with doping concentration, the peak 2DEG was obtained at 6 nm at 1.9e13 cm^-2 on unintentionally doped N- GaN (10^14 cm^-3 concentration) while significantly lower values were reported for thinner layers. This suggests that higher doping leads to a greater degree of crystallinity with low-temperature ALD on GaN, as the lower 2DEG in N- GaN was attributed to acceptor-like interface traps. The conductance, derived from current-voltage (I-V) plots, was found to vary primarily due to 2DEG for N+ GaN, and a contact resistance of <0.5 Ω∙mm (contact resistivity of 2e-4 Ω∙cm^2) was achieved with 3 nm of AlN deposited. The N- GaN conductance varied primarily by thickness, specifically, the transmission coefficient through AlN. An ohmic I-V curve was reported for 3 nm while higher thicknesses produced a non-ohmic relation. This characteristic was also observed after post-deposition annealing. Initial measurements of semiconducting ZrNx (with x=1.15) indicated a closely matched work function and electron affinity with GaN and Al metal, since high 2DEG and ohmic contacts were only achieved with Al contacts on N+ GaN compared to Cr contacts or N- GaN. In addition, the 2DEG showed minimal variation with ZrNx thickness, with any differences attributed to interface traps. Although as-deposited ZrNx reported high 2e-2 Ω∙cm^2 contact resistivity, post-deposition annealing at 900ºC for 1 minute led to a tenfold decrease to 2e-3 Ω∙cm^2 for 4 nm of ZrNx on N+ GaN. With 1 nm of ZrNx, the minimum contact resistivity was 5e-3 Ω∙cm^2, achieved at 800ºC annealing. Future work will investigate the ZrNx stoichiometry spectrum in order to lower the contact resistivity further.

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    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.