Electrical, Spectral, and Chemical Properties of 1.8 MeV Proton Irradiated AlGaN

1. Electrical, Spectral, and Chemical Properties of 1.8 MeV Proton Irradiated AlGaN/GaN HEMT Structures as a Function of Proton Fluence Radiation Issues for AlGaN/GaN Structures LEEN & SIMS Micro-Spectroscopy AlGaN/GaN HEMT Electrical, Spectral, and Chemical Changes with Fluence Analysis: Three-Fold Physical Degradation Mechanism Conclusions / Next Steps This work supported by AFOSR/NE under MURI grant F49620-99-1-0289

2. Engineering Advanced Electronic Devices with Wide Band Gap Semiconductors High Breakdown Fields ? High Power, High Speed High Saturated-Electron Drift Velocities ? High Speed, Wide Bandwidth High Melting Points ? High Temperature Operation Low Intrinsic Carrier Densities ? High Temperature Operation Strong Piezoelectric Effect and High ?EC (Nitrides) ? High Sheet Carrier Concentration ? High Current / High Power

3. Basic Mechanism Challenges AlGaN/GaN High Power, High Frequency Transistors Outlooked for Aerospace Bulk GaN Exhibits 100x Improved Radiation Hardness versus GaAs . . . But: Relatively Little Known about Nature of III-N Radiation Damage – Defects, Field Distributions Radiation Effects on Piezoelectric Field-Induced Channel Structure Not Yet Understood Ultrathin Film Structure Difficult to Characterize Electrically, Optically, or Chemically

4. This figure illustrates the experimental setup schematically. The UHV chamber shown in cross section contains a glancing incidence electron gun operating under the range of conditions shown. Electrons incident on the specimen excite electronic transitions at various depths, depending on the incident beam energy Ebeam. Spectra are obtained for energies as low as 100 eV, i.e., in the surface science regime. (The UHV environment enables the surface to be maintained atomically clean or well-controlled during the experiment. Conventional cathodoluminescence spectroscopy (CLS) is usually performed in an electron microscope at much higher energies and under poor vacuum conditions such that the surface is rapidly contaminated with carbon. Such deposits promote non-radiative recombination near the surface.) The resultant optical emission is collected by IR-UV optics including a quartz or CaF2 lens, a prism or grating monochromator and one of a variety of photodetectors. Because of the extremely low (tens of nanometers or less) minority carrier diffusion lengths near surface and interfaces, the depth resolution is maintained close to that calculated by a number of theoretical treatments. Such nanometer depth resolution permits one to probe the electronic structure of the structure, including localized states, heterojunction electronic band gaps, the presence of new compounds at a “buried” interface or at ultra-thin layers (as low as 1-2 nm). This capability is valuable for probing semiconductor interfaces such as Schottky barriers, heterojunctions, quantum wells, high electron mobility transistor structures, reacted or interdiffused layers below a free surface. Such information can be used to modify and optimize the growth or processing of electronic materials.This figure illustrates the experimental setup schematically. The UHV chamber shown in cross section contains a glancing incidence electron gun operating under the range of conditions shown. Electrons incident on the specimen excite electronic transitions at various depths, depending on the incident beam energy Ebeam. Spectra are obtained for energies as low as 100 eV, i.e., in the surface science regime. (The UHV environment enables the surface to be maintained atomically clean or well-controlled during the experiment. Conventional cathodoluminescence spectroscopy (CLS) is usually performed in an electron microscope at much higher energies and under poor vacuum conditions such that the surface is rapidly contaminated with carbon. Such deposits promote non-radiative recombination near the surface.) The resultant optical emission is collected by IR-UV optics including a quartz or CaF2 lens, a prism or grating monochromator and one of a variety of photodetectors. Because of the extremely low (tens of nanometers or less) minority carrier diffusion lengths near surface and interfaces, the depth resolution is maintained close to that calculated by a number of theoretical treatments. Such nanometer depth resolution permits one to probe the electronic structure of the structure, including localized states, heterojunction electronic band gaps, the presence of new compounds at a “buried” interface or at ultra-thin layers (as low as 1-2 nm). This capability is valuable for probing semiconductor interfaces such as Schottky barriers, heterojunctions, quantum wells, high electron mobility transistor structures, reacted or interdiffused layers below a free surface. Such information can be used to modify and optimize the growth or processing of electronic materials.

5. Energy Dependence of LEEN

6. Depth-Resolved CLS: Si-SiO2

8. Cornell 2DEG HEMT Structure

9. Time-of-Flight Secondary Ion Mass Spectrometer

10. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Transistor Properties: I sat

11. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Transistor Properties: Vth, gm

12. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Transistor Properties: ?SB

13. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Transistor Properties: ?c, ?, and n2DEG

14. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Micro-Cathodoluminescence Spectra

15. 1.8 MeV Proton-Induced Changes in AlGaN/GaN Micro-Cathodoluminescence Spectra

16. Effect of AlGaN Strain Relaxation

17. AlGaN/GaN SIMS Interface Profiles vs. p+ Fluence

18. Analysis: < 1014 p+/cm2 Regime Vth, ?c, ?SB Exhibit Sub-Linear Changes with Increasing Fluence Channel Properties (ns and ?) Unaffected

19. Analysis: > 1014 p+/cm2 Regime Further ? and Vth Degradation Extracted ND ~Decreases by Order of Magnitude Significant ? Degradation Above 3x1014 p+/cm2 ? AlGaN/GaN Interface Roughening Indicated by Luminescence and SIMS Significant n2DEG Degradation > 2x1015 p+/cm2 ? Strain Relaxation Suggested by Luminescence Spectra ? Degradation Accounts for gm Changes ? Degradation and Other Effects (e.g., ?c) Account for ID, sat Changes

20. Conclusions

21. Proton Irradiation Issues(from Last Year) Does Polarization Charge Decrease? Does Lattice Relaxation Occur? (Yes, Yes) What Defect Complexes Are Produced? How Many versus Dose? (Acceptors, ~ 1018 cm-3) Do Proton-Induced Defects Compensate or Complex with (Remove) Existing Donors? (Yes) Do Carrier Density or Mobility Changes Dominate Electrical Changes? (Mobility for 1014 cm-3 Range, Carrier Density for 1015 cm-3 Range)

22. Collaborators At Ohio State University B.D.White, M. Bataiev, S.H. Goss, and L.J. Brillson At Vanderbilt University X. Hu, A. Kamarkar, D.M. Fleetwood, and R.D. Schrimpf At Cornell University W.J. Schaff