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Steady-State and Transient Electron Transport in AlN

NC STATE UNIVERSITY. Samples were provided by A. Roskowski and R.F. Davis of NCSU. Steady-State and Transient Electron Transport in AlN. R. Collazo , R. Schlesser, and Z. Sitar. February 12,2002. Goals & Experimental Approach. Electron transport

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Steady-State and Transient Electron Transport in AlN

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  1. NC STATE UNIVERSITY Samples were provided by A. Roskowski and R.F. Davis of NCSU Steady-State and Transient Electron Transport in AlN R. Collazo, R. Schlesser, and Z. Sitar February 12,2002

  2. Goals & Experimental Approach • Electron transport • Electrons accelerate in an applied electric field. • Electrons decelerate by the emission of polar optical phonons. • Goals • Derive origin of electrons (relative to CBM) from EED spectra. • Determine carrier temperature as a function of applied field. • Establish conditions for transient transport and steady-state transport. • Estimate mean free path, drift velocities under different transport conditions. • Experimental approach • Extract conduction electrons from WB material into vacuum • Directly measure energy distribution (EED) of extracted electrons with an electron spectrometer

  3. Direct measurement of the electron energy. Measurement of intrinsic material. Electron Energy Distributions (EED)

  4. Test Structure Contact requirements: back contact: - no potential drop across Au/Ti/SiC top contact: - semitransparent for electrons - well defined potential at the surface Au 800Å Ti 560Å SiC 300 µm AlN ~1000Å Au 200Å Vacuum Spectrometer injection transport extraction – V V - VBIAS GND z

  5. Band Diagram • Band bending of AlN/Au interface determined by core-level XPS.

  6. EED Field Dependence/ Steady-State

  7. Carrier Energy Balance/Steady State Electric Field • Electron Temperature Approximation • Maxwellian distribution with small drift component • Solution to Boltzmann Transport Equation E gain rate Energy in the carrier gas Steady State transport condition loss rate JZ: current density EZ:electric field Energy in the crystal lattice

  8. Mobility/Energy Relaxation Rate Ratio/ Energy Balance Approach Specifically;

  9. Drift Velocity Characteristic Curves • Assumptions: • Two different effective masses (0.48 m and 0.31 m) • Constant relaxation times ratio (7 - 10) Using the Mobility/Energy relaxation rate ratio:

  10. Mean Free Path LO Phonon 99.2 meV Average Mean Free Path 5 nm ± 13.5 %

  11. Transient Transport Average Carrier Energy Thermal Component Drift Component not negligible Drifted Electron Distribution

  12. EED Field Dependence/ Transient Transport Length 80 nm

  13. Drift Velocity Characteristic Curves/ Transient Velocity Overshoot • Transient effect length At 630 kV/cm

  14. Material Degradation

  15. Summary of Results • EED results for intrinsic AlN: • Spectra show presence of hot electron transport • Electron temperature increases with the applied field • Observed secondary EED peak at fields > 400kV/cm • scattering into L-M satellite valley • Peak position compatible with band calculations • Estimated allowable ratio between electron mobility and energy relaxation time • Drift Velocity • Mean Free Path • Observed transient transport at fields > 520 kV/cm at a transport length of 80 nm • Velocity Overshoot • Sample quality is crucial (breakdown, leakage)

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