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

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

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

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

Device Power Performance vs. Frequency

The Wide Band Gap Device Advantages GaN HEMT and Process

Suitable Specifications for GaN-based Power Devices

R on of GaN HEMT Switches

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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, …

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

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

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

Plasma treatment: Ar N D =3.3x10 18 cm -3 ICP power (W) Bias power (W) Pressure (mTorr) Ar flow (sccm) Time (min.)

Plasma treatment: Ar flow rate Before annealing

Plasma treatment: Ar flow rate After annealing

Plasma treatment: time

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

Thermal aging tests: N 2 ambient

Thermal aging tests: Air ambient

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

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 ??

Forming gas ambient treatment

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

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 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

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

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

Bi-layer PMMA/Copolymer process

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

Foot width v.s. central dose

40 nm Narrow T-gate

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

Comparison: dose, dose ratio 50 kV15 kV PMMA (uC/cm 2 ) PMMA-MAA (uC/cm 2 )20040 Dose ratio Copolymer/PMMA Lower dose, higher sensitivity

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Hall data: Al composition

Hall data: AlGaN thickness Strain relaxation ??

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

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

Carrier profile: Al=0.3 UndopedModulation-doped

0.15 um AlGaN/GaN HFETs

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

Schottky I-V 0.15x75 Al=0.3 undoped

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

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

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

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

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

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

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

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

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)

Thermal activation energy

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

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

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

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

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

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

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

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

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

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

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 1.3E EL 1.2E IEDM 1.2E recess EL 1.5E Our best result 1.23E

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 1.61E EDL 1.1E ED 1.2E EL 1.1E Our best result 1.23E