MEGAWATT SOLID-STATE ELECTRONICS

MATERIALS, PROCESS AND DEVICE DEVELOPMENT FOR GaN (and SiC) POWER DEVICES University of Florida: SRI: MCNC: Device Design/Simulation (In Collaboration with Sandia) Process Development Device Fabrication (In Collaboration with Sandia) High Rate GaN Epitaxy Novel Gate Dielectrics Characterization High Field Transport Device Design Packaging Commercialization

OBJECTIVES Develop the Technology Base for GaN-Based Electrical Switches at Power Levels Well Above 1MW - 25kV Stand-Off Voltage - 2 ka Conducting Current - Forward Voltage Drop <2% of Rated Voltage - 50 khz Operating Frequency APPLICATIONS : More Efficient Transmission and Distribution of Electric Power, as part of EPRI s Flexible AC Transmission Systems (FACTS) Concept : Single-Pulse Switching In the Sub-Systems of Hybrid-Electric Combat Vehicles (DARPA/DOD)

CONCEPT MEGAWATT SOLID-STATE ELECTRONICS Low Power MOSFET + Thyristor GTO Thyristor GTO + Power Diodes + Packaging Inverter Module Approach is to Make Devices in Parallel with Materials Development, Modelling and package Development ACHIEVEMENTS Previous: GaN D-Mode MOSFET AlGaN/GaN Bipolar Transistor (Precursor to Thyristor) Process Modules (W contacts, Dry Etching, Implant Doping) This Period: GaN Schottky Rectifiers with V RB 2000 V, FOM 2 ( V ) RB 2 R = 4 48 MW cm ON Process Modules (Etch Damage, Isolation) Free Standing GaN Substrates Initial Packaging Results High Field Transport Theory

Free-Standing GaN Single Crystal Substrates MOVPE Capping Layer 0.1-0.2 µm MOVPE GaN Thick GaN Layer 100-300 µm 0.2-0.3 µm HVPE GaN MOVPE GaN Barrier Layer Separation Plane Schematic of the growth sequence to produce free-standing GaN film. 50-100 Å LiGaO 2 Substrate Nitrided Layer Substrate H 2 + HCl + GaN Hot wall reactor NH 3 GaCl NH 3 /N 2 GaCl + CH 4 Ga(CH 3 ) 3 /N 2 HCl Schematic growth mechanism in the combined MOCVD/ hydride VPE reactor.

Free-Standing GaN Single Crystal Substrates Free standing bulk GaN substrate grown with the MOCVD/hydride VPE process (10 x 10 x 0.3 mm 3 ). XRD scan from 50µm thick, free-standing GaN.

Free-Standing GaN Substrate (Separated From LiGaO 2 Substrate)

Free-Standing HVPE Grown GaN Substrate

Dry Etch Damage on n-gan Ti/Au Pt/Au Ti/Au 5 4 control 3 µm undoped (n~10 17 cm -3 ) GaN 0.5 µm n + (10 18 cm -3 ) GaN Al 2 O 3 substrate V B (V) 3 2 Pt/Au 1 300 400 500 600 700 800 900 Annealing Temperature ( o C) Ti/Au 3 µm GaN 0.5 µm n + -GaN Al 2 O 3 substrate Ti/Au V B (V) 5 4 3 2 control 1 0 50 100 150 200 250 300 Depth Removed (Å)

Dry Etch Damage in n-gan Current (ma) 2 0-2 -4 control sample 40 W rf 100 W rf 150 W rf 250 W rf -6-15 -10-5 0 5 2 Voltage (V) H 2 Plasma 500 W ICP V B (V) 12 8 H 2 N 2 4 0 0 50 100 150 200 250 300 rf Power (W) 350 Current (ma) 0-2 -4-6 -15-10 -5 0 5 Voltage (V) N 2 Plasma 500 W ICP Self dc Bias (V) 250 150 H 2 N 2 50 0 50 100 150 200 250 300 rf Power (W)

Dry Etch Damage in p-gan 50 280 5 Breakdown Voltage (V) 40 30 20 10 Ar H 2 Ar H 2 240 200 160 Self-dc Bias (V) -V F (V) 4 3 2 1 0 n-p junction control diode 0 120 0 500 1000 1500 ICP Power (W) 200 600 1000 1400 1800 ICP Power (W)

Dry Etch Damage in p-gan 20 50 Breakdown Voltage (V) 16 12 8 4 Breakdown Voltage (V) 40 30 20 10 as-exposed control 0 0 100 200 300 400 500 600 700 Depth Removed (Å) 0 300 400 500 600 700 800 900 1000 Annealing Temperature ( o C)

Metal/GaAs(C)/GaN(Mg) Contacts 10 22 10 7 Metal p-gaas (C) p-gan (Mg) 3.4 ev ÄE C ~ 0.2eV 1.43 ev Interface states E F CONCENTRATION (atoms/cc) 10 21 10 20 10 19 10 18 As (counts) C GaAs/GaN O GaN (counts) 10 6 10 5 10 4 10 3 10 2 10 1 SECONDARY ION INTENSITY (cts/sec) ÄE V ~ 1.8eV 10 17 0 1000 2000 3000 4000 DEPTH (Angstroms) 10 0

Structural and Electrical Results Specific Contact Resistance of Ni/Au p-ohmic Contacts (Measured at 250 o C) Material ρ c (ohm cm 2 ) ρ c (ohm cm 2 ) unannealed 800 o C p-gaas(c) 9-14x10-6 3-6x10-6 p-gan(mg) 8x10-2 4x10-3 p-gaas( C)/p-GaN(Mg) 7x10-3 2x10-3

Implant Isolation of n-gan 1.2x10 18 O + 3E12 30 ke Ion Concentration (cm -3 ) 1.0x10 18 8.0x10 17 6.0x10 17 4.0x10 17 2.0x10 17 5E12 60 kev 9E12 125 kev 9E12 200 kev 2E13 325 kev Sum 0.0 0 2000 4000 6000 Depth(Å) 1.8x10 18 Ti GaN Ion Concentration (cm -3 ) 1.6x10 18 1.4x10 18 1.2x10 18 1.0x10 18 8.0x10 17 6.0x10 17 4.0x10 17 O + 3E12 30 ke 5E12 60 kev 9E12 125 kev 9E12 200 kev 2E13 325 kev Sum 2.0x10 17 0.0 0 490 2000 4000 6000 Depth(Å)

Thermal Stability of Implant Isolation 10 13 10 13 Sheet Resistance ( Ω/square) 10 12 10 11 10 10 10 9 10 8 10 7 10 6 10 5 10 4 Pre-implantation Sheet Resistance Sheet Resistance ( Ω/square) 10 12 10 11 10 10 10 9 10 8 10 7 10 6 10 5 10 4 Pre-implantation Sheet Resistance 10 3 0 200 400 600 800 Annealing Temperature (ºC) 10 3 0 200 400 600 800 Annealing Temperature (ºC)

D (cm -2 s -1 ) CONCENTRATION (atoms/cc) 10 23 10 22 10 21 10 20 10 19 10 18 10-12 10-13 10-14 MEGAWATT SOLID-STATE ELECTRONICS 17 O 16 O 18 O Si (counts) SiO 2 /GaN GaN (counts) 10 0 0 1000 2000 3000 DEPTH (Angstroms) 17 O in GaN 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1000/T (K -1 ) 10 7 10 6 10 5 10 4 10 3 10 2 10 1 SECONDARY ION INTENSITY (cts/sec) CONCENTRATION (atoms/cc) 10 20 10 19 10 18 10 17 10 16 Oxygen Indiffusion into GaN 500 o C as deposited 700 o C 17 O in GaN GaN (counts) 900 o C 0 400 800 1200 1600 2000 DEPTH (Angstroms) 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 SECONDARY ION INTENSITY (cts/sec)

PLASMA CHEMISTRY 10000 7000 5 3 6000 Etch Rate (Å/min) 6000 1000 6 3 250W rf, 2 mtorr, 15 sccm Etch rate (Å/min) 5000 4000 3000 2000 750W ICP, 2 mtorr, 15 sccm PF 5 BF 3 SF 6 NF 3 1000 750 1000 1250 1500 ICP-Power (W ) 0 150 250 350 450 rf-power (W) Selectivity 21 18 15 12 9 6 SiC/Al SiC/Ni SiC/ITO Selectivity 6 4 2 10NF3/5O2 250W rf 2mTorr SiC/Al SiC/Ni SiC/ITO 3 0 20 40 60 80 100 % NF 3 (Total flow NF 3 /O 2 is 15 sccm) 0 800 1000 1200 1400 1600 ICP Power (W )

Etch Rate (µm/min) 2 1 0 Etching SiC Al Ni ITO 10NF 3 /5O 2 750W ICP 2mTorr DEEP FEATURE ETCHING Deposition -1 8 Infinite 6 Selectivity 4 2 SiC/Al SiC/Ni SiC/ITO 0 100 200 300 400 500 rf Power (W)

EFFECT OF UV ILLUMINATION ON SiC ETCH RATES Etch Rate (Å/min) 1600 1200 800 400 UV w/o UV dc self-bias 200 W rf 2 mtorr 10Cl 2 /5Ar 450 400 350 300 250 dc self-bias (-V) Etch Rate (Å/min) 5000 4000 3000 2000 200 W rf 2 m Torr 10SF 6 /5A r U V w /o U V dc self-bias 500 450 400 350 300 250 dc self-bias (-V) 0 200 1000 200 0 200 400 600 800 1000 ICP power (W) 150 0 150 0 200 400 600 800 1000 1200 ICP pow er (W ) Etch Rate (Å/min) 1000 800 600 400 200 500 W ICP 2 mtorr 10Cl 2 /5Ar UV w/o UV dc self-bias 350 300 250 200 150 dc self-bias (-V) Etch Rate (Å/min) 4000 3000 2000 1000 500 W ICP 2 mtorr 10SF 6 /5Ar UV w/o UV dc self-bias 350 300 250 200 dc self-bias (-V) 0 100 50 100 150 200 250 300 rf power (W ) 0 150 50 100 150 200 250 300 rf power (W )

SEM MICROGRAPHS OF ETCHED SiC FEATURES WITHOUT UV WITH UV

GaN Schottky Diode Rectifiers Ti/Au Ti/Au 4 µm GaN undoped 1 µm n + GaN(Si) 3 10 18 cm -3 LT GaN buffer Al 2 O 3 substrate Planar (Top) and Mesa (Right) Structures

Material Characterization 10 21 GaN diode 10 5 10 20 14 N Concentration (atoms/cc) 10 19 10 18 10 17 Si (+ 14 N) H O C 10 4 10 3 10 2 Secondary Ion Intensity (cts/sec) 10 16 Mg 10 15 10 1 0 2 4 6 Depth (µm) RMS Roughness 0.2nm (1x1µm 2 ) 1.6nm (10x10µm 2 ) Surface Defect Density 10 8 cm -2 Major Impurities O - 7x10 17 cm -3 Si - 10 17 cm -3

Reverse Breakdown Voltage At Room Temperature 12 8 GaN diode + MOCVD 4µ m UNDOPED / 1µ m n V RB = 356V Current (ma) 4 2 = 3.5V (100A cm ) V F 2 R ON = 28mΩ cm 0-350 -250-150 -50 50 Voltage (V) ( ) 2 VRB 2 = 4.2 MW cm R ON

V RB Temperature Dependence 330 V RB (V) 320 310 300 GaN diode In GaN mesa diodes, we observe a negative temperature coefficient for V RB - surface or defect-assisted breakdown Some reports of positive temperature coefficients in GaN p-i-n diodes and HFETs 290 V F decreases with temperature (3.5V at 25 O C, 2V at 200 o C) 280 0 40 80 120 160 Temperature ( o C)

GaN Diode Reverse Recovery Current Transient Waveform V RB = 356V T = 300K 800 700 Current Density (A/cm 2 ) 600 500 400 300 200 100 0-100 -200-0.2-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time (µs)

GaN Schottky Rectifiers 0.008 0.05 0.04 0.004 0.03 0.02 Current (A) 0.000-0.004 Currnet (A) 0.01 0.00-0.01-0.02-0.03-0.04-0.008-800 -600-400 -200 0 200 Voltage (V) -0.05-2500 -2000-1500 -1000-500 0 500 1000 1500 2000 2500 Voltage (V) 11µm Normal MOCVD V RB = 550V, V F =3.5V, R ON = 6 mω cm 2 (V RB ) 2 /R ON = 48MW cm -2 3µm Modified MOCVD V RB >2,000V, best V F = 15V (more typically 50V due to inconsistent implant activation),r ON = 0.8 Ωcm 2 (V RB ) 2 /R ON > 5MW cm -2

High Breakdown GaN Grown by Modified MOCVD

GaN Breakdown Voltage As A Function of Doping and Thickness Breakdown Voltage (V) 10 6 10 5 10 4 10 3 10 2 8 Thickness 100 µ 50 µ 20 µ 10 µ 5 µ 1 µ Punch-through GaN Diode Avalanche BD this work (3µm) this work (4µm) Caltech (9µm) this work (11µm) Caltech (11µm) 10 1 10 13 10 14 10 15 10 16 10 17 Drift Region Doping Concentration (cm -3 ) Depletion Width at Breakdown W = Reverse Breakdown Voltage V R B Maximum Electric Field at Breakdown E where = permittivity K 8 7 Ke 1 E G 7 7 N B 0.125 3 3 3 ( 2K e N ) 0. 25 = B M = 8eN k B 0.125

Specific Resistance and Figure-of-Merit Specific Resistance (ohm-cm 2 ) 10 0 GaN-UF Diode Rectifiers 10-1 10-2 10-3 10-4 GaN-UF SiC-ABB GaN-Caltech SiC-Purdue GaN-UF Si SiC-NCSU 6H-SiC 4H-SiC SiC-RPI GaN 10 1 10 2 10 3 10 4 Breakdown Voltage (V) UF Diodes R ON = 6-800 mω cm -2 V RB = 356-2,000 FOM (V RB ) 2 /R ON = 4.2-48MW cm -2 R ON = 4( V ) RB µ E C 2 2 + where V RB = breakdown voltage µ = carrier mobility = permittivity E C = critical field for breakdown P sub W sub = resistivity/thickness of substrate P sub W sub + R C

Reverse Leakage of GaN Rectifier J R (A/cm 2 ) SiC AND GaN DIODE RECTIFIERS SiC (NCSU, '95) SiC (Siemens, '97) SiC (Purdue, '97) SiC (Philips, '97) GaN (UF, '99) SiC (Cree, '97) GaN (Caltech, '99) SiC (RPI, '98) SiC (Cincinnati, '97) SiC (D-Benz, '97) SiC (ABB, '97) V R (Volts) GaN (UF, '99) Theory with Barrier Lowering J R = A**T 2 exp where J R = reverse current density A** = Richardson s constant T = φ B = e ( φ φ ) B B kt absolute temperature Schottky barrier height φ B = image-force induced barrier lowering (after Khemka et.al., SSE in press Rhoderick & Williams).

Forward Drop of Rectifier V F (Volts) SiC AND GaN DIODE RECTIFIERS J F = 100A/cm 2 25 o C GaN (UF, '99) GaN (Caltech, '99) SiC (RPI, '98) SiC (Cincinnati, '97) SiC (Philips, '97) Φ B = 1.75 ev 1.50 ev 1.25 ev 1.00 ev SiC (Purdue, '98) V F = nkt e where n = T = ideality factor absolute temperature A** = Richardson s constant φ B = In J F A** T 2 + n φ Schottky barrier height B + R ON J F NCSU, '95 V RB (Volts) Kyoto Univ., '95 R ON = specific on-resistance J F = current density at UF (after Baliga (1994), Khemka et.al. SSE (in press).

Contributions of Surface & Bulk Leakage Current 10-6 GaN Schottky Diodes 10-3 GaN Schottky Diodes 10-7 10-4 I R (A) 10-8 10-9 I R D 2 (b u l k) I R D (p e r i me t e r) V R = 150V V R = 15V J R (A cm -2 ) 10-5 10-6 V R = 150V V R = 15V 10-10 0 200 400 600 800 1000 1200 Contact Diameter (µm) 10-7 0 200 400 600 800 Perimeter/Area Ratio (cm -1 )

SUMMARY OF RECTIFIER RESULTS FROM DIFFERENT GROWTH TECHNIQUES METHOD THICKNESS DOPING V RB (V) COMMENTS (µm) (cm -3 ) HVPE 60-100 2x10 17 60-200 O, Si Backgrounds MBE 3-5 5x10 16 60-80 Thickness MOCVD (Normal) 5-12 2x10 16 350-550 4-48MW cm -2 MOCVD (Modified) 3 10 15 2,000 Improve V F, R ON

View of High Breakdown Rectifier Design

View of Rectifier Fingers Schottky Contact Schottky Contact Ohmic contact SiO 2 p-implant GaN layer n + -implant

View of Junction Barrier Controlled Schottky (JBS) Rectifiers KT m+ s J ln( ) 2 q 2d AT ( x + t)( m+ s) ( m+ s 2d) m+ s 2d FC i V FS = φ B + + ρ ln( ) J FC Where: V FS is the forward voltage drop of the JBS rectifier J L = 2d ( ) AT m + s 2 qφ B q exp[ ( )]exp( kt kt qe 4πε S Where: J L is the reverse leakage current of the JBS rectifiers SiO 2 Schottky Contact Ohmic contact p-implant GaN layer n + -implant

View of Rectifiers with Floating Field Ring Termination Extend the depletion boundary along the surface of dielectric layer and reduce the electric field crowing at the edge V ffr = 2 qn A 2 W S S V qn a + ε 2ε S A W S SiO 2 Schottky Contact Ohmic contact Where: V ffr is the potential of the floating field ring N A is the doping concentration ε S is permittivity W S is the floating field ring spacing Va is the applied Schottky bias p-implant GaN layer n + -implant

View of Rectifiers with Edge Termination Proper Design of the edge termination is critical for both obtaining a high breakdown voltage and reducing the onstate voltage drop and the switching times SiO 2 Schottky Metal Ohmic contact P + Guard Ring: Place a P-type diffused guard ring at the edge of the Schottky barrier metal Metal Overlap: Extend the Schottky barrier metal over an oxide layer at the edge p-implant GaN layer n-implant

FUTURE WORK THYRISTORS - Grow P-N Junctions, Bipolars And Then Thyristors On Free-Standing GaN Substrates (Strong Correlation Of Distance From Heteroepitaxial Interface With High Junction Leakage And Poor Modulation Characteristics) - Lateral, All-implanted Devices RECTIFIERS - Edge Termination/Surface Passivation - P-I-N vs Schottky Tradeoff Comparison Temperature Dependence Of V RB, V F, Switching Characteristics - Stability At 300 o C Under Bias