Semiconductor Materials for Power Electronics (SEMPEL) GaN power electronics materials Kjeld Pedersen Department of Physics and Nanotechnology, AAU SEMPEL Semiconductor Materials for Power Electronics AALBORG UNIVERSITY Physics and Nanotechnology 1
Two main applications of GaN Power Electronics and LEDs Power electronics Key enabling technology Today 40% of global energy consumption is electrical 70% expected in 2040: Energy Roadmap 2050, European Commission: http://ec.europa.eu/energy/energy2020/roadmap/index_en.htm GaN technology can reduce losses significantly IGBT-module LED technology 19% of electricity for lighting (14% in EU) > 200 TWh in EU, > 70 Mt CO₂ Light Emitting (GaN) Diodes will reduce losses significantly Huge potential for energy savings Several % just by change of semiconductor!
GaN and SiC potentials Wide bandgap: 3.4 ev for GaN vs 1.1 for Si High switching frequencies, 100 MHz at kw powers Efficiency enhancements of 2-10 % ( e.g. from 96 to 99 %) Smaller footprint and weight down to 10% 3
2D-electron gas polarization effects Spontaneous P SP Piezoelectric P PE Engineering of electronic properties: Polarizations instead of doping! Journal of Applied Physics 85, 3222 (1999) Polarization charge: ρ P = P σ = P SP top + P PE (top) P SP bottom + P PE (bottom) 2D electron gas at interface Journal of Applied Physics 87, 334 (2000) 4
WBG power devices Vertical SiC MOSFET GaN HEMT GaN vs SiC Devices SiC avalable as 8 wafers GaN films on substrate High electron mobility GaN 2DEG Heterojunction GaN/AlₓGa₁ ₓN No doping! Polarization charges Low ON-resistance of HEMT HEMT one-sided structure more space required + higher mobility HEMT Normally ON (Depletion Mode) negative gate voltage to close More complicated structure for normally OFF (Enhancement Mode) Higher thermal conductivity of SiC SiC technology more mature
The main challenges Substrate- epitaxial growth GaN on Si Packaging high power density New type of component HEMT vs IGBT New challenges and opportunities Depletion (normally-on) vs Enhancement (normally-off) mode Reliability long-term stability Ohmic contacts self-heating in drain area Gate dielectrics oxide layers Buffer stack- defects (dislocations) S 2DEG Size of connects vs GaN chip! G Dielectric AlGaN GaN Buffer Substrate D
Epitaxial GaN growth Substrates: SiC - expensive Al₂O₃ -poor thermal conductivity Silicon: Cheap high-quality wafers Lattice mismatch 16% to GaN Alloys with Ga buffer layer needed Mismatch in thermal expansion (54%) Wafer bow with high temp (1100⁰C) growth High density of dislocations
Silicon vs Sapphire substrates GaN-on-Silicon versus GaN-on-Sapphire: Powdec s 1200V devices are GaN-on-Sapphire devices. Not using the Silicon and using Sapphire implies intrinsic differences in the device structure and operation. GaN-on-Si power devices: Thickness of nitride layers for 600V devices is 5μm or more. Field Plates are indivisibly needed for the conductive Si substrate to mitigate the current collapses. Growth on Si substrate needs chamber-cleaning prior to the deposition to avoid the unwanted chemical reactions between GaN and Si interface Advantages of GaN-on-Sapphire for Power devices: Common platform for GaN-LED production. The growth runs successively without chamber-cleaning. Throughput for GaN PSJ-FET/sapphire growth is roughly 10 times larger than that of the GaN FP-HEMT/Si growth. ITME The thermal conductivity of sapphire, 40 [W/m K], is lower than that of Si, 150 [W/m K].
Limitations for high voltage applications V B = ε sε 0 E2 cr 2qN D E cr : Break-down field E cr [V /μm] N D [cm 3 ] ε s V B [V] Buffer break-down a) Silicon-on-insulator wafers b) Remove Si below transistor c) Semi-insulating GaN Si 30 5 10 14 11.8 600 SiC 300 5 10 14 9.7 ~50.000 GaN 340 5 10 14 9.5 ~50.000 Limits HV devices on Si substrates!
Efficiency Static losses resistance of drift region Higher voltage longer drift region higher loss Dynamic losses o Main advantage of HEMT: inherently low C out o + on drain 2DEG depleted no contact to gate No capacitance to gate Only static losses! o SiC: C out = 500 pf (doped layers), charged to 1000 V, switching at 100 khz. P dyn = fc out V c 2 = o 50 W 5% on a 1-kW power supply!
The normally-on problem 1. Cascode configuration Connection with low-voltage Si MOSFET High blocking of HEMT low R ON of Si FET 2. True Enhancement mode Introduction of ions that capture positive charges at interface
Growth facilities MOCVD in Aalborg running since autumn 2015 modified Veco- Showerhead 2 wafers, doping with Si PE-MBE in Aarhus- modified to GaN 2015 4 wafers, both n- and p-doping MOCVD at AAU MBE at AU
SiC on Si an alternative substrate Kukushkin, Semiconductors and Dielectrics 50, 1188 Si(100) Si(111) Advantages: SiC protects against Ga-Si reactions Better lattice match Strain relaxation due to voids Disadvantage: High growth temperature ~1300⁰C Low-temperature process (900 1000⁰C) developed at AU
GaN growth on SiC 4H-SiC on Si(111) as determined from lattice constant AlN buffer layer (100 nm) lattice matching GaN layer (750 nm) Strong correlation between roughness of SiC and pit hoels in GN film Mariia Rozhavskaya et al. Semiconductor Science and Technology (Submitted)
Contacts to GaN semiconductors Ohmic Au Ni Al Annealing A few min 800 C Au/Ni/Al Ni Al Ti Ti Ta TaN/TiN AlGaN AlGaN GaN GaN Annealing leads to diffusion and formation of new elements Formation of TaN/TiN consumes N from the AlGaN layer Forms N vacancies n-type doping Thins down the AlGaN layer easier tunneling to 2D electron gass TiN and TaN conducting elements Schottky: Ni/Au layers
Layout for diodes and contacts Circular and linear structures tested Ohmic contact and diode structures n-gan/sic,~10 16 cm -3 2.5 µm thick Ohmic contacts:ti/al/ti/au 30,60,30, 90nm, annealed at 800 0 C for 5 minutes in N 2 resistivity 10-5 Ω-cm 2 Schotkey contact: Ni/Au (25/50 nm)
I-V curves for Schottkey diodes High quality I-V curves Ideality factor1.1 I = I 0 exp qv nkt Breakdown voltage below -100 V Passivation by Si₃N₄ will be tested Shivakumar D. Thammaiah
Accelerated aging of commercial GaN High Electron Mobility Transistors Reduced thermal conductivity from the GaN transistor to the cooling plate Cracks in the soldering between the elements Electrical conductivity is found to decrease as a consequence of the high operation temperature Sungyoung Song, Stig Munk-Nielsen
Summary Very large potentials in Wide Band Gap semiconductors Higher voltages Higher frequencies Compact systems New applications GaN far from fully developed Still lots of materials issues Still room for device developments New types of devices
References Power electronics with wide bandgap materials: toward greener more efficient technologies, MRS Bulletin vol. 40 p 390 (2015) DOI: http://dx.doi.org/10.1557/mrs.2015.71 Power-switching applications beyond silicon: Status and future prospects of SiC and GaN devices, MRS Bulletin vol. 40 p 399 (2015) DOI: http://dx.doi.org/10.1557/mrs.2015.89 High-voltage normally OFF GaN power transistors on SiC and Si substrates, MRS Bulletin vol. 40 p 418 (2015) DOI: http://dx.doi.org/10.1557/mrs.2015.88 Power GaN Devices, Materials, Applications and Reliability, Springer (2017) (can be downloaded at AAU).