MEGAWATT SOLID-STATE ELECTRONICS

Transcription

1 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

2 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)

3 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

4 Free-Standing GaN Single Crystal Substrates MOVPE Capping Layer µm MOVPE GaN Thick GaN Layer µm µm HVPE GaN MOVPE GaN Barrier Layer Separation Plane Schematic of the growth sequence to produce free-standing GaN film 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.

5 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.

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

7 Free-Standing HVPE Grown GaN Substrate

8 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 Annealing Temperature ( o C) Ti/Au 3 µm GaN 0.5 µm n + -GaN Al 2 O 3 substrate Ti/Au V B (V) control Depth Removed (Å)

9 Dry Etch Damage in n-gan Current (ma) control sample 40 W rf 100 W rf 150 W rf 250 W rf Voltage (V) H 2 Plasma 500 W ICP V B (V) 12 8 H 2 N rf Power (W) 350 Current (ma) Voltage (V) N 2 Plasma 500 W ICP Self dc Bias (V) H 2 N rf Power (W)

10 Dry Etch Damage in p-gan Breakdown Voltage (V) Ar H 2 Ar H Self-dc Bias (V) -V F (V) n-p junction control diode ICP Power (W) ICP Power (W)

11 Dry Etch Damage in p-gan Breakdown Voltage (V) Breakdown Voltage (V) as-exposed control Depth Removed (Å) Annealing Temperature ( o C)

12 Metal/GaAs(C)/GaN(Mg) Contacts Metal p-gaas (C) p-gan (Mg) 3.4 ev ÄE C ~ 0.2eV 1.43 ev Interface states E F CONCENTRATION (atoms/cc) As (counts) C GaAs/GaN O GaN (counts) SECONDARY ION INTENSITY (cts/sec) ÄE V ~ 1.8eV DEPTH (Angstroms) 10 0

13 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) 9-14x x10-6 p-gan(mg) 8x10-2 4x10-3 p-gaas( C)/p-GaN(Mg) 7x10-3 2x10-3

14 Implant Isolation of n-gan 1.2x10 18 O + 3E12 30 ke Ion Concentration (cm -3 ) 1.0x x x x x E12 60 kev 9E kev 9E kev 2E kev Sum Depth(Å) 1.8x10 18 Ti GaN Ion Concentration (cm -3 ) 1.6x x x x x x x10 17 O + 3E12 30 ke 5E12 60 kev 9E kev 9E kev 2E kev Sum 2.0x Depth(Å)

15 Thermal Stability of Implant Isolation Sheet Resistance ( Ω/square) Pre-implantation Sheet Resistance Sheet Resistance ( Ω/square) Pre-implantation Sheet Resistance Annealing Temperature (ºC) Annealing Temperature (ºC)

16 D (cm -2 s -1 ) CONCENTRATION (atoms/cc) MEGAWATT SOLID-STATE ELECTRONICS 17 O 16 O 18 O Si (counts) SiO 2 /GaN GaN (counts) DEPTH (Angstroms) 17 O in GaN /T (K -1 ) SECONDARY ION INTENSITY (cts/sec) CONCENTRATION (atoms/cc) Oxygen Indiffusion into GaN 500 o C as deposited 700 o C 17 O in GaN GaN (counts) 900 o C DEPTH (Angstroms) SECONDARY ION INTENSITY (cts/sec)

17 PLASMA CHEMISTRY Etch Rate (Å/min) W rf, 2 mtorr, 15 sccm Etch rate (Å/min) W ICP, 2 mtorr, 15 sccm PF 5 BF 3 SF 6 NF ICP-Power (W ) rf-power (W) Selectivity SiC/Al SiC/Ni SiC/ITO Selectivity NF3/5O2 250W rf 2mTorr SiC/Al SiC/Ni SiC/ITO % NF 3 (Total flow NF 3 /O 2 is 15 sccm) ICP Power (W )

18 Etch Rate (µm/min) 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 rf Power (W)

19 EFFECT OF UV ILLUMINATION ON SiC ETCH RATES Etch Rate (Å/min) UV w/o UV dc self-bias 200 W rf 2 mtorr 10Cl 2 /5Ar dc self-bias (-V) Etch Rate (Å/min) W rf 2 m Torr 10SF 6 /5A r U V w /o U V dc self-bias dc self-bias (-V) ICP power (W) ICP pow er (W ) Etch Rate (Å/min) W ICP 2 mtorr 10Cl 2 /5Ar UV w/o UV dc self-bias dc self-bias (-V) Etch Rate (Å/min) W ICP 2 mtorr 10SF 6 /5Ar UV w/o UV dc self-bias dc self-bias (-V) rf power (W ) rf power (W )

20 SEM MICROGRAPHS OF ETCHED SiC FEATURES WITHOUT UV WITH UV

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

22 Material Characterization GaN diode N Concentration (atoms/cc) Si (+ 14 N) H O C Secondary Ion Intensity (cts/sec) Mg 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 cm -3

23 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 Voltage (V) ( ) 2 VRB 2 = 4.2 MW cm R ON

24 V RB Temperature Dependence 330 V RB (V) 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) Temperature ( o C)

25 GaN Diode Reverse Recovery Current Transient Waveform V RB = 356V T = 300K Current Density (A/cm 2 ) Time (µs)

26 GaN Schottky Rectifiers Current (A) Currnet (A) Voltage (V) 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

27 High Breakdown GaN Grown by Modified MOCVD

28 GaN Breakdown Voltage As A Function of Doping and Thickness Breakdown Voltage (V) 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) 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 ( 2K e N ) = B M = 8eN k B 0.125

29 Specific Resistance and Figure-of-Merit Specific Resistance (ohm-cm 2 ) 10 0 GaN-UF Diode Rectifiers GaN-UF SiC-ABB GaN-Caltech SiC-Purdue GaN-UF Si SiC-NCSU 6H-SiC 4H-SiC SiC-RPI GaN Breakdown Voltage (V) UF Diodes R ON = mω cm -2 V RB = 356-2,000 FOM (V RB ) 2 /R ON = MW cm -2 R ON = 4( V ) RB µ E C 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

30 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).

31 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).

32 Contributions of Surface & Bulk Leakage Current 10-6 GaN Schottky Diodes 10-3 GaN Schottky Diodes I R (A) 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 ) V R = 150V V R = 15V Contact Diameter (µm) Perimeter/Area Ratio (cm -1 )

33 SUMMARY OF RECTIFIER RESULTS FROM DIFFERENT GROWTH TECHNIQUES METHOD THICKNESS DOPING V RB (V) COMMENTS (µm) (cm -3 ) HVPE x O, Si Backgrounds MBE 3-5 5x Thickness MOCVD (Normal) x MW cm -2 MOCVD (Modified) ,000 Improve V F, R ON

34 View of High Breakdown Rectifier Design

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

36 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

37 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

38 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

39 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