Ting -Chi Lee OES, ITRI 11/07/2005

1. Ting-Chi Lee OES, ITRI 11/07/2005 GaN-based Heterostructure Field-Effect Transistors

2. Outline • Introduction to GaN • ICP etching of GaN • Low resistance Ohmic contacts to n-GaN • Narrow T-gate fabrication on GaN • Polarization effect in AlGaN/GaN HFETs • Thermal effect of AlGaN/GaN HFETs • Conclusion

3. Introduction • Unique material properties of GaN • Wide bandgap, 3.4 eV at RT • High breakdown field, 3 MV/cm • High electron saturation velocity, 1.3x107 cm/s • Excellent thermal stability • Strong polarization effect

5. Introduction • GaN-based devices • Great achievement in blue LEDs and laser diodes • Potential microwave high power devices • Next generation wireless communication system, especially in the base station power amplifiers, high Vbk is required • Next generation wireless communication • Access multi-media information using cell phones or PDAs at any time anywhere • High-efficiency base station PAs • Present base station PAs: Si LDMOS, low efficiency

6. Device Power Performance vs. Frequency

7. The Wide Band Gap Device Advantages GaN HEMT and Process

8. Suitable Specifications for GaN-based Power Devices

9. Ron of GaN HEMT Switches

10. GaN-based devices for various applications • For high-power switching applications •  GaN schottky diodes •  GaN p-i-n diodes •  GaN HEMT-based switching devices •  GaN MOSHFET-based switching devices • For microwave power amplifications •  GaN Schottky diode •  AlGaN/GaN HEMTs •  AlGaN/GaN MOSHFETs •  GaN-based microwave circuits • For pressure sensor application •  AlGaN/GaN HEMTs

11. R & D activity in GaN HFET • Company • RF Micro Devices, Cree Inc., Sensor Electronic Technology, ATMI • Epi wafers for GaN FET • Lab. • USA: US Naval Research Lab., Hughes Research Lab., Lucent Technologies Bell Lab., TRW, nitronex • USA: Cornell U., UCSB, RPI, U. Texas, USC, NCSU • Germany: Water-Schottky Institute, DaimlerChrysler lab. • Sweden: Chalmers U., Linkopings U., • Japan: Meijo U., NEC and Sumitomo • Military contracting lab. • Raytheon, GE, Boeing, Rockwell, TRW, Northrop Grumman, BAE Systems North America

12. ICP Etching of GaN • GaN-based materials • Inert chemical nature • Strong bonding energy • Not easy to perform etching by conventional wet etching or RIE • New technology • High-density plasma etching (HDP) • Chemically assisted ion beam etching (CAIBE) • Reactive ion beam etching (RIBE) • Low electron energy enhanced etching (LE4) • Photoassisted dry etching

13. ICP Etching of GaN • High density plasma etching (HDP) • Higher plasma density • The capability to effectively decouple the ion energy and ion density • Inductively coupled plasma (ICP) • Electron cyclotron resonance (ECR) • Magnetron RIE (MRIE)

14. Our work • ICP etching • Ni mask fabrication • Dry etching parameters

15. Ni mask fabrication • Suitable etching mask for ICP etching of GaN • PR, Ni and SiO2 • Ni mask fabrication • Wet chemical etching by HNO3: H2O (1:1) • Lift-off

16. Ni 20 um Lift-off Smooth edge Good dimension control Ni mask fabrication PR Ni 20 um Wet etching Rough edge Poor dimension control

17. Dry etching parameters: bias power Larger bias power -Increase the kinetic energy of incident ions -Enhance physical ion bombardment -More efficient bond breaking and desorption of etched products

18. Dry etching parameters: bias power Ni: 2000 Å GaN: 2 um Bias power: 5 w Bias power: 10 w Bias power: 20 w Bias power: 30 w

19. Dry etching parameters: Ar flow rate Higher Ar flow rate -Increase the density of incident Ar ions -Enhance physical ion bombardment Ar flow rate> 15 sccm -Cl2/Ar flow ratio decrease

20. Dry etching parameters: Ar flow rate Ni: 2000 Å GaN: 2 um Ar flow rate: 5 sccm Ar flow rate: 15 sccm Ar flow rate: 20 sccm Ar flow rate: 25 sccm

21. Dry etching parameters: Cl2 flow rate Higher Cl flow rate -Generate more reactive Cl radicals to participate in the surface chemical reaction

22. Dry etching parameters: Cl2 flow rate Ni: 2000 Å GaN: 2 um Cl flow rate: 10 sccm Cl flow rate: 20 sccm Cl flow rate: 30 sccm Cl flow rate: 50 sccm

23. Summary • Good Ni mask fabrication by lift-off • Dry etching parameters • Bias power • Ar flow rate • Cl2 flow rate • Smooth etched surface and vertical sidewall profile

24. Low resistance Ohmic contacts to n-GaN • GaN-based materials • Wide bandgap • Not easy to obtain low resistance Ohmic contacts • Approaches to improve the contact resistance • Select proper contact metal: Ti, Al, TiAl, TiAlTiAu,… • Surface treatment: HCl, HF, HNO3: HCl (1:3),… • Plasma treatment: Cl2/Ar, Cl2, Ar, …

25. Our work • Plasma treatment • n-GaN with Nd=8.7x1016, 3.3x1018 cm-3 • Cl2/Ar and Ar plasma • Thermal stability issue • Forming gas ambient treatment

26. Cl2/Ar or Ar plasma n+-GaN n-GaN sapphire Plasma treatment n-GaN sapphire Plasma treatment -> create N vacancies (native donors) -> increase surface electron concentration

27. No. 1 2 3 4 5 6 7 8 9 ICP power (w) Bias power (w)Pressure (mtorr) Cl2 flow (sccm) Ar flow (sccm) Time (min.) - - - - - - 300 5 15 50 30 1 300 5 15 50 10 2 300 5 15 50 15 2 300 5 15 50 20 2 300 5 15 50 30 2 300 5 15 - 10 1 300 5 15 - 30 1 300 5 15 - 50 1 Rc (Ω‧mm) ρs (Ω/□) ρc (mΩ‧cm2) 0.638 621.0 6.6 0.614 656.3 5.7 0.48 692.2 3.4 0.45 696.3 2.8 0.21 668.3 0.68 0.28 671.5 1.2 0.87 673 11 0.57 649.3 5.0 0.3 803 0.87 Plasma treatment: Cl2/Ar, ArND=8.7x1016 cm-3

28. ICP power (W) Bias power (W) Pressure (mTorr) Ar flow (sccm) Time (min.) - - - - - 300 5 15 10 1 300 5 15 30 1 300 5 15 50 1 300 5 15 50 2 300 5 15 50 3 Plasma treatment: ArND=3.3x1018 cm-3

29. Plasma treatment: Ar flow rate Before annealing

30. Plasma treatment: Ar flow rate After annealing

31. Plasma treatment: time

32. Plasma treatment: time

33. Thermal stability issue • Important for devices • Several studies on the thermal stability of Ohmic contacts to n-GaN have been performed • Thermal stability of plasma-treated Ohmic contacts to n-GaN • If the damages created or defects generated by plasma treatment have any effect on the device reliability ?? • Thermal aging tests at different temperatures for 2h were performed to observe it

34. Thermal aging tests: N2 ambient

35. Thermal aging tests: Air ambient

36. Discussion • After thermal annealing • TiN form at M/GaN interface, thermodynamically stable over a wide temperature • N vacancies and other defects form at interface • High-temperature thermal aging • Improve the crystal quality • Reduce the defect density • No obvious electrical degradation observed • Plasma-treated Ohmic contacts exhibited excellent thermal stability

37. Forming gas ambient treatment • Thermal annealing in N2 ambient for nitride processing • To avoid hydrogen passivation of dopants • Especially for p-GaN • Forming gas annealing ambient • Better reduction capability due to the H2 • Reduce the oxidation reaction of metal at high T • Cause carrier reduction of n-GaN due to the H passivation ??

38. Forming gas ambient treatment

39. Summary • Proper plasma treatment by Cl2/Ar or Ar • Very effective in the improvement of contact resistance • Thermal stability issue • Plasma-treated Ohmic contacts to n-GaN exhibited excellent thermal stability • Forming gas ambient treatment • No electrical degradation observed • Even lower contact resistance obtained

40. Narrow T-gate fabrication on GaN • To realize high performance devices especially for high-frequency application • Conventional approach • A high accelerating voltage of around 40-50 kV • Much reduced forward scattering effect • A lower accelerating voltage for e-beam lithography • Less backscattering from the substrate • Lower doses needed • Much reduced radiation damage • But larger forward scattering effect

41. Our work • E-beam system • E-beam resist processing • PMMA (120 nm)/Copolymer (680 nm) • Narrow T-gate fabrication using a lower accelerating voltage, 15 kV • Writing pattern design • Especially for the reduction of forward scattering with a lower accelerating voltage

42. E-beam system • JEOL 6500 SEM + nano pattern generation system (NPGS) • Max. acceleration voltage: 35 kV • Beam current: tens of pA ~ 1 nA • Thermal field emission (TFE) gun • Thermal field emission gun • Large beam current • Good beam current stability

43. Bi-layer PMMA/Copolymer process

44. Write strategy Central stripe (50 nm): foot exposure Side stripe (75 nm): head exposure Spacing between the central stripe and the side stripe: key point

45. Foot width v.s. central dose

46. 40 nm Narrow T-gate

47. Discussion • As the spacing between the central stripe and the side stripe<< stripe width • Sub 100 nm T-gate can be easily obtained • Forward scattering effect was dramatically improved • Thus side exposure influences significantly the final e-beam energy density profile

48. Comparison: dose, dose ratio Lower dose, higher sensitivity

49. Summary • Narrow T-gate fabrication using a lower accelerating voltage of 15 kV is practical • Specially designed writing pattern • Can significantly improve the forward scattering problem with a lower accelerating voltage • Lower doses are needed for a lower accelerating voltage

50. Polarization effect in AlGaN/GaN HFETs • Design rules for realizing high performance GaN HFETs • High Al content AlGaN/GaN heterostructure • Crystal structure • Polarization-induced sheet charge, 2DEG • Difficulties in the growth of AlGaN