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Theory and Fabrication of Ohmic and Schottky Contacts to GaN Substrates

Theory and Fabrication of Ohmic and Schottky Contacts to GaN Substrates. Ann Lehman Kirk Price. Senior Design Project - Spring 2002. Today’s Presentation. Basic Theory GaN Properties Processing Characterization Results. Theory of Contacts . Energy Band Diagrams Current Transfer Models

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Theory and Fabrication of Ohmic and Schottky Contacts to GaN Substrates

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  1. Theory and Fabrication of Ohmic and Schottky Contacts to GaN Substrates Ann Lehman Kirk Price Senior Design Project - Spring 2002

  2. Today’s Presentation • Basic Theory • GaN Properties • Processing • Characterization • Results

  3. Theory of Contacts • Energy Band Diagrams • Current Transfer Models • Thermionic Emission Theory • Diffusion Theory • Thermionic-Diffusion Theory • Tunneling Current

  4. Thermionic Emission Theory • Assumption: The transfer rate of electrons across the metal-semiconductor junction is the major limiting factor in the current through the junction.

  5. Diffusion Theory • Assumption: The drift and diffusion rates in the semiconductor are the major limiting factors in the current through the junction.

  6. Thermionic – Diffusion Theory • This is a combination of the two previous theories, which allows for a more realistic albeit more complex view of m-s junctions.

  7. Tunneling Current • Unlike the previous Current Transport Theories, electrons in the tunneling theory have less energy than the barrier height. • As temperature (the energy of electrons) increases, tunneling becomes more possible since the width of the barrier is thinner at higher energies. • Ohmic Contacts

  8. GaN • Wide Band Gap • High Temperature Applications • Direct Band Gap • Optoelectronic devices • High Electron Drift Velocities (1300 V*s/cm2) • High Saturation Velocity • Fast Devices • High Current Devices • High Breakdown Field (4 MV/cm) • High Power Devices

  9. Other High Temperature Applications • HEMTs • Rectifiers • Photo-HBTs • UV Detectors • X-Ray Detectors • Mixers • Gas-Sensing Devices

  10. Schottky Metal Ohmic Metal Contact Layout

  11. Material Growth • PAMBE Growth • Sapphire Substrate • {100-1} plane • 2.5um GaN Material • Defect Doping Density of 5E+16 cm-3

  12. Ohmic Contact Processing • Sample Cleaning • Application of PR • Exposure • Development

  13. Ohmic Contact Processing Con’t • Metal Evaporation • 500Å Ti • 2500Å Al • Lift-Off • Annealing • 2 min at 500 C • 2 min at 550 C

  14. Schottky Contact Processing Variations • Cleaning with Plasma Reactor • Metal Evaporation • Sample 1: 1500Å Au • Sample 2: • 500Å Cr & 1500Å Au • Sample 3: • 500Å Ni & 1500Å Au • No Annealing

  15. Topology of Contacts

  16. Testing • I-V • C-V • I-T

  17. Ohmic I-V Characteristics

  18. Scottky I-V Characteristics

  19. Schottky Characteristics for Sample 2

  20. Schottky C-V Characteristics

  21. Future Recommendations • Ohmic metals – Ti and Al • Schottky metals – Au and Ni • Test other metals – Ag, W, Pt, Pb. • Test High Temperature Integrity of Contacts

  22. Special Thanks • Churoo Park • Professor Kim & Students • Ling Zhou • Matthew Olson

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