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

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

3. Introduction n Unique material properties of GaN u Wide bandgap, 3.4 eV at RT u High breakdown field, 3 MV/cm u High electron saturation velocity, 1.3x10 7 cm/s u Excellent thermal stability u Strong polarization effect

5. Introduction n GaN-based devices u Great achievement in blue LEDs and laser diodes u Potential microwave high power devices u Next generation wireless communication system, especially in the base station power amplifiers, high Vbk is required n Next generation wireless communication u Access multi-media information using cell phones or PDAs at any time anywhere u High-efficiency base station PAs u 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. R on of GaN HEMT Switches

10. 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 GaN-based devices for various applications

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

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

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

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

15. Ni mask fabrication n Suitable etching mask for ICP etching of GaN u PR, Ni and SiO2 n Ni mask fabrication u Wet chemical etching by HNO 3 : H 2 O (1:1) u Lift-off

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

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 Bias power: 5 w Bias power: 30 w Bias power: 10 w Bias power: 20 w Ni: 2000 Å GaN: 2 um

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 -Cl 2 /Ar flow ratio decrease

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

21. Dry etching parameters: Cl 2 flow rate Higher Cl flow rate -Generate more reactive Cl radicals to participate in the surface chemical reaction

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

23. Summary n Good Ni mask fabrication by lift-off n Dry etching parameters u Bias power u Ar flow rate u Cl 2 flow rate n Smooth etched surface and vertical sidewall profile

24. Low resistance Ohmic contacts to n-GaN n GaN-based materials u Wide bandgap u Not easy to obtain low resistance Ohmic contacts n Approaches to improve the contact resistance u Select proper contact metal: Ti, Al, TiAl, TiAlTiAu,… u Surface treatment: HCl, HF, HNO 3 : HCl (1:3),… u Plasma treatment: Cl 2 /Ar, Cl 2, Ar, …

25. Our work n Plasma treatment u n-GaN with Nd=8.7x10 16, 3.3x10 18 cm -3 u Cl 2 /Ar and Ar plasma n Thermal stability issue n Forming gas ambient treatment

26. Plasma treatment n-GaN sapphire n-GaN sapphire n + -GaN Plasma treatment -> create N vacancies (native donors) -> increase surface electron concentration Cl 2 /Ar or Ar plasma

27. Plasma treatment: Cl 2 /Ar, Ar N D =8.7x10 16 cm -3 No.123456789 ICP power (w) Bias power (w) Pressure (mtorr) Cl 2 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) ρ s (Ω/ □ ) ρ c (  Ω ‧ cm 2 ) 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

28. Plasma treatment: Ar N D =3.3x10 18 cm -3 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

29. Plasma treatment: Ar flow rate Before annealing

30. Plasma treatment: Ar flow rate After annealing

31. Plasma treatment: time

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

34. Thermal aging tests: N 2 ambient

35. Thermal aging tests: Air ambient

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

37. Forming gas ambient treatment n Thermal annealing in N 2 ambient for nitride processing u To avoid hydrogen passivation of dopants u Especially for p-GaN n Forming gas annealing ambient u Better reduction capability due to the H 2 u Reduce the oxidation reaction of metal at high T u Cause carrier reduction of n-GaN due to the H passivation ??

38. Forming gas ambient treatment

39. Summary n Proper plasma treatment by Cl 2 /Ar or Ar u Very effective in the improvement of contact resistance n Thermal stability issue u Plasma-treated Ohmic contacts to n-GaN exhibited excellent thermal stability n Forming gas ambient treatment u No electrical degradation observed u Even lower contact resistance obtained

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

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

42. E-beam system n JEOL 6500 SEM + nano pattern generation system (NPGS) u Max. acceleration voltage: 35 kV u Beam current: tens of pA ~ 1 nA u Thermal field emission (TFE) gun n Thermal field emission gun u Large beam current u 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 n As the spacing between the central stripe and the side stripe<< stripe width u Sub 100 nm T-gate can be easily obtained u Forward scattering effect was dramatically improved u Thus side exposure influences significantly the final e-beam energy density profile

48. Comparison: dose, dose ratio 50 kV15 kV PMMA (uC/cm 2 )600140-200 PMMA-MAA (uC/cm 2 )20040 Dose ratio Copolymer/PMMA 3-43.5-5 Lower dose, higher sensitivity

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

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

51. High performance GaN HFETs n In addition to develop device processing technologies n Design rules u High sheet charge density u High carrier mobility u Maintain high breakdown voltage -> high Al composition AlGaN/GaN heterostructures

52. n Higher band discontinuity u Better carrier confinement u Al=0.3,  Ec=0.5 eV n Higher spontaneous polarization and piezoelectric effect u Higher 2DEG sheet charge density n Higher bandgap of AlGaN u higher breakdown field High Al composition AlGaN/GaN heterostructures

53. Crystal structure and polarity n JAP, 1999 n Wurtzite crystal structure u Hexagonal Bravais lattice (a, c, u) u Both spontaneous and piezoelectric polarization n Polarity Ga-face: MOCVD or PIMBE N-face: PIMBE only

54. Polarization, polarization-induced sheet charge and formation of 2DEG Ga-faceN-face

55. Comparison of calculated and measured 2DEG ns AlGaN: 200Å, ■ / □ : undoped/doped

56. Difficulties in the growth of AlGaN n Atomically smooth surface is not easy to obtain, especially in high Al content n Local variation in the alloy composition n Strain in the AlGaN layer due to the lattice mismatch bet. AlGaN and GaN u Formation of structural defects u Island growth mode u Electrical property of heterostructure, piezoelectric effect -> decrease in electron mobility with high Al composition

57. Our work n Design AlGaN/GaN heterostructures with different Al compositions, different AlGaN thickness and modulation-doping n Surface morphology n Electron transport properties n Device characteristics

58. Structure: Al=0.17 i-AlGaN 18 nm (Al=0.17) i-GaN 3 µ m Buffer layer Sapphire Undoped i-AlGaN 50 nm (Al=0.17) i-GaN 3 µ m Buffer layer Sapphire

59. Structure: Al=0.3 i-AlGaN 28 nm (Al=0.3) i-GaN 3 µ m Buffer layer Sapphire i-AlGaN 5 nm n-AlGaN: 5E18 20 nm i-AlGaN 3 nm i-GaN 3 µ m Buffer layer Sapphire UndopedModulation-doped

60. Surface morphology: Al=0.17 Top AlGaN: 18 nm Undoped AlGaN/GaN structure Step flow structure RMS: 0.176 nm Other location 0.108 nm 0.161 nm Top AlGaN: 50 nm Undoped AlGaN/GaN structure Step flow structure RMS: 0.176 nm

61. undoped AlGaN/GaN structure Step flow structure RMS: 0.096 nm Contact mode Surface morphology: Al=0.3 Modulation-doped AlGaN/GaN structure Step flow structure RMS: 0.131 nm Contact mode

62. Discussion n Surface morphology u Step-like structure u Surface roughness ~ 0.15 nm u Very smooth surface, indicating good crystal quality u Comparable to previous reports Step like

63. Hall data: Al composition

64. Hall data: AlGaN thickness Strain relaxation ??

65. Hall data: Al=0.3, structure Thermal activation of Si donors

66. Discussion n Higher Al composition u Higher ns, lower mobility n Larger AlGaN thickness u Higher ns, lower mobility n Ns: 2DEG formation mechanism u Spontaneous polarization and piezoelectric effect u Strain relaxation u Thermal activation (modulation-doped structure) n Mobility: scattering mechanism u Phonon scattering dominates at high T u Interface roughness scattering dominates at low T

67. Carrier profile: Al=0.3 UndopedModulation-doped

68. 0.15 um AlGaN/GaN HFETs

69. DC characteristics 0.15x75 Al=0.3 undoped -Good dc performance -Vt ~ -7 -wide g m profile over Vg, good linearity

70. Schottky I-V 0.15x75 Al=0.3 undoped

71. Small-signal characteristics 0.15x75 Al=0.3 Undoped V gs : -3.5 V ds : 6

72. DC characteristics 0.15x75 Al=0.3 Modulation-doped -Good dc performance -Vt ~ -9 -narrow g m profile over Vg

73. Schottky I-V 0.15x75 Al=0.3 Modulation-doped

74. Small-signal characteristics 0.15x75 Al=0.3 M-doped V gs : -6 V ds : 6

75. Summary n Surface morphology u Step-like structure u Surface roughness ~ 0.15 nm, indicating that very smooth surface and good crystal quality n Electron transport properties u For undoped structure, due to the strong spontaneous and piezoelectric polarization, high 2DEG density obtained, ~1e13 cm -2 u Additional doping, modulation doping or channel doping, is not necessary n Device characteristics of AlGaN/GaN HFETs u Very large output drain current available, the undoped (~700 mA/mm) and modulation-doped structures (~1000 mA/mm) u High breakdown voltage u High operation frequency

76. n For microwave high-power devices, the stability of device over temperature is extremely important n The thermal conductivity of substrate u Sapphire (0.5 W/cm·K), SiC (4.5 W/cm·K) u Self-heating effect n Device structure u Undoped structure u Modulation-doped structure u Channel-doped structure u Exhibit different electrical behavior at high temperature due to their different transport properties Thermal effect of AlGaN/GaN HFETs

77. Our work n Compare undoped and modulation-doped AlGaN/GaN HFETs, Al=0.3 n Temperature-dependent electron transport properties n Device high temperature performance

78. Electron transport properties v.s. T Thermal activation of Si donors

79. Charge neutrality condition: give more accurate E d n: electron concentration(exp. data, eq (2)) N A : acceptor concentration (N A << N D ) N D : donor concentration (N D =5e18) Nc: effective density of state in conduction band (~T 3/2 ) g d : donor spin-degeneracy factor (g d =2) E d : activation energy (fit parameter) d AlGaN : effective AlGaN thickness(fit parameter) Calculation of E d --- (2) --- (1) (charge neutrality condition)

80. Thermal activation energy

81. Si donor in GaN, AlGaN n Si level in GaN: u Ed~20 meV (for n=1e17 cm -3 ) n Si level in AlGaN: u Al composition and Si doping concentration dependent n Si level in Al 0.3 Ga 0.7 N Ed (meV)Growth 1997, MSE-B110MBE 1998, SSE40MOCVD 2000, PRB100MBE 2002, MSE-B40MBE 2002, APL50Calculation

82. DC characteristics v.s. T 0.15x75 Al=0.3 undoped -Good dc performance from RT to 200°C -Id reduction due to 2DEG mobility degradation -Vt ~ -7, const over temperature, stable gate -wide g m profile over Vg, good linearity

83. Schottky I-V 0.15x75 Al=0.3 undoped Slight increase in Ig, stable Schottky gate High Rg

84. 0.15x75 Al=0.3 Modulation-doped DC characteristics V.s. T -Good dc performance from RT to 200°C -Id reduction due to 2DEG mobility degradation -Vt ~ -9, const over temperature, stable gate -narrower g m profile over Vg

85. Schottky I-V 0.15x75 Modulation-doped Slight increase in Ig, stable Schottky gate Lower Rg than undoped

86. Comparison: dc Larger Id in M-doped structure -additional modulation doping Larger gm in M-doped structure -smaller parasitic Rs -Rs at RT(undoped/M-doped): 3.4/2.6 Ωmm

87. Comparison: small-signal No obvious degradation observed as T< 100ºC -weak temperature dependence of the electron transport property higher f T for M-doped - smaller parasitic Rs

88. Comparison undoped M-doped Nsconstantincrease with T Mobilitycomparable at high T for both I d (T)lowerhigher g m (T)lowerhigher g m profilewidernarrower R s (T)higherlower R g (T)higherlower f T (T)lowerhigher Modulation-doped structure: better performance over temperatures

89. Conclusion n ICP etching of GaN u Smooth etched surface and vertical sidewall profile obtained n Low resistance Ohmic contacts to n-GaN u Plasma-treated Ohmic contacts exhibit low Rc and excellent thermal stability u Even lower Rc obtained using forming gas ambient n Narrow T-gate fabrication u 40 nm narrow T-gate was successfully fabricated using a lower accelerating voltage, 15 kV u A specially designed writing pattern

90. Conclusion n Polarization effect u Design different structures u Electron transport properties: high 2DEG concentration u Device characteristics: high output current u Polarization effect plays a crucial role n Thermal effect u In addition to the substrate, device structure plays a significant role u Compared undoped and modulation-doped structure u Electron transport properties: thermal activation of Si donors u Device high temperature performance: modulation-doped devices exhibit better performance

91. Comparison: GaN HFETs on sapphire 2DEG Ns 2DEG µ Lg (  m) Id, max (mA/mm) Gm,ext (mS/mm) f T (GHz) f max (GHz) 2002 EDL 1.3E13 1330 0.18920212101140 2002 EL 1.2E13 1200 0.25140040185151 2001 IEDM 1.2E13 1200 0.15 recess 1310402107148 2001 EL 1.5E13 1170 0.25139021667136 Our best result 1.23E13 953 0.1510602007590

92. Comparison: GaN HFETs on SiC 2DEG Ns 2DEG µ Lg (  m) Id, max (mA/mm) Gm,ext (mS/mm) f T (GHz) f max (GHz) 2003 EL 1.61E13 993 0.131250250103170 2002 EDL 1.1E13 1300 0.121230314121162 2001 ED 1.2E13 1200 0.121190217101155 2000 EL 1.1E13 1100 0.051200110140 Our best result 1.23E13 953 0.1510602007590