1. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Steady-State and Transient Electron Transport in AlN R. Collazo, R. Schlesser, and Z. Sitar NC STATE UNIVERSITY Samples were provided by A. Roskowski and R.F. Davis of NCSU February 12,2002

2. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Electron Energy Distributions (EED) Direct measurement of the electron energy. Measurement of intrinsic material.

4. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Test Structure Au 800Å Ti 560Å SiC 300 µm AlN ~1000Å Au 200Å – V z - V BIAS SpectrometerVacuum GND Contact requirements: back contact: - no potential drop across Au/Ti/SiC top contact:- semitransparent for electrons - well defined potential at the surface VV injectionextractiontransport

5. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Band Diagram Band bending of AlN/Au interface determined by core-level XPS.

6. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB EED Field Dependence/ Steady-State

7. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Carrier Energy Balance/Steady State Steady State transport condition Electron Temperature Approximation Maxwellian distribution with small drift component Solution to Boltzmann Transport Equation Energy in the crystal lattice Energy in the carrier gas loss rate E gain rate Electric Field J Z : current density E Z : electric field

8. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Mobility/Energy Relaxation Rate Ratio/ Energy Balance Approach Specifically;

9. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Drift Velocity Characteristic Curves Using the Mobility/Energy relaxation rate ratio: Assumptions: Two different effective masses (0.48 m and 0.31 m) Constant relaxation times ratio (7 - 10)

10. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Mean Free Path LO Phonon 99.2 meV Average Mean Free Path 5 nm ± 13.5 %

11. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Transient Transport Drifted Electron Distribution Average Carrier Energy Drift Component not negligible Thermal Component

12. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB EED Field Dependence/ Transient Transport Length 80 nm

13. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Drift Velocity Characteristic Curves/ Transient Velocity Overshoot Transient effect length At 630 kV/cm

14. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Material Degradation

15. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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)