Recent Progress in Understanding the Electrical Reliability of GaN High-Electron Mobility Transistors J. A. del Alamo Microsystems Technology Laboratories Massachusetts Institute of Technology 2015 MRS Spring Meeting Symposium AA: Materials for Beyond the Roadmap Devices in Logic, Power and Memory San Francisco, CA, April 6-10, 2015 Acknowledgements: C. Y. Chen, F. Gao, J. Jimenez, D. Jin, J. Joh, T. Palacios, C. V. Thompson, Y. Wu ARL (DARPA-WBGS program), NRO, ONR (DRIFT-MURI program),
Outline 1. A few universal observations 2. Hypotheses for degradation mechanisms 3. Many questions 2
GaN HEMT: breakthrough RF power technology Counter-IED Systems (CREW) 200 W GaN HEMT for cellular base station Kawano, APMC 2005 100 mm GaN-on-SiC volume manufacturing Palmour, MTT-S 2010 Sumitomo Remote Radio Head for Japanese Base Station 3
4 GaN HEMT: Electrical reliability concerns ON: Mostly benign High-power: Not accessible to DC stress experiments Device blows up instantly High-voltage OFF and semi-on: Degradation of I Dmax, R D, I Goff V T shift Electron trapping Trap creation
Critical voltage for degradation in DC step-stress experiments S AlGaN GaN V GS =-10 V V DS G D 2DEG Joh, EDL 2008 I Dmax /I Dmax (0), R/R(0) 1.2 1.15 1.1 1.05 1 0.95 0.9 0.85 OFF-state, V GS =-10 V R D R S I Dmax I Goff V crit 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 10 20 30 40 50 V DGstress (V) I Dmax : V DS =5 V, V GS =2 V I Goff : V DS =0.1 V, V GS =-5 V I Goff (A/mm) I D, R D, and I G start to degrade beyond critical voltage (V crit ) + increased trapping behavior current collapse 5
Critical voltage: a universal phenomenon GaN HEMT on SiC GaN HEMT on SiC GaN HEMT on SiC Liu, JVSTB 2011 Meneghini, IEDM 2011 Ivo, MR 2011 GaN HEMT on Si GaN HEMT on Si GaN HEMT on sapphire Marcon, IEDM 2010 Demirtas, ROCS 2009 Ma, Chin Phys B 2011 6
Structural degradation; correlation with electrical degradation Permanent I Dmax Degradation (%) 50 40 30 20 10 0 Pit depth 0 2 4 6 8 Pit depth (nm) Joh, MR 2010 Chowdhury, EDL 2008 Pit at edge of gate Pit depth and I Dmax degradation correlate 7
Structural damage at gate edge: a universal phenomenon Barnes, CS-MANTECH 2012 Dammann, IIRW 2011 Marcon, MR 2010 Cullen, TDMR 2013 Liu, JVSTB 2011 Chang, TDMR 2011 Christiansen, IRPS 2011 8
Structural degradation: planar view Unstressed OFF-state stress: V DG =57 V, T base =150 C 200 nm 200 nm Permanent I Dmax Degradation (%) 12 10 8 6 4 2 0 averaged over 1 µm 0 50 100 150 Average Defect Area (nm 2 ) V stress >V crit : pits along gate edge Pit cross-sectional area correlates with I D degradation Makaram, APL 2010 9
Structural damage at gate edge: a universal phenomenon Barnes, CS-MANTECH 2012 Monte Bajo, APL 2014 Holzworth, ECST 2014 Whiting, MR 2012 Brunel, MR 2013 10
V T (V) 0.25 0.2 0.15 0.1 0.05 Stress: V GS =-7 V and V DS =40 V 125 C I Goff ΔV T 0 10 Initial -4 10-2 10 0 10 2 10 4 10 6 Stress time (s) 10-4 10-5 10-6 10-7 10-8 I Goff (A) Time evolution of degradation for constant V stress > V crit I Goff and V T degradation: fast (<10 ms) saturate after 10 4 s Permanent I Dmax degradation: much slower does not saturate with time Joh, IRPS 2011 11
The role of temperature in time evolution Incubation time Incubation time ln(τ inc ) (s) 15 10 5 0 Permanent I Dmax degradation E a =1.12 ev Current collapse E a =0.59 ev I Goff, E a =0.17 ev -5 28 30 32 34 36 1/kT (ev -1 ) Different degradation physics: I G : weak T dependence Joh, IRPS 2011 I Dmax : T activated, E a similar to life-test data* * Saunier, DRC 2007; Meneghesso, IJMWT 2010 12
DC semi-on stress experiments Stress: I D =100 ma/mm, V DS =40 or 50 V Step-T experiments: 50<T a <230 o C (T j ~110-330 o C) SEM AFM Drain Trench/pit width, depth (nm) 90 80 70 60 50 40 30 20 10 0 Average of 5 1 µm x1 µm scans at finger center Trench/pit width Trench/pit depth 0 5 10 15 20 25 30 Permanent I Dmax degradation (%) Wu, JAP 2015 Pits and trenches under gate edge on drain side Trench/pit depth and width correlate with I Dmax degradation 13
Thermally activated degradation Depth of damage (nm) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 ΔI D =21.6% Source Drain Gate fingers Source Distance from center of gate finger 0 20 40 60 80 100 120 140 160 180 Distance from center of gate finger (µm) ln(1/ slope ) 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 I D degradation rate E a =1.04 ev 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 1/kT channel (ev -1 ) Wu, MR 2014 Pit/trench depth increase towards center of gate finger self heating + thermally activated process Permanent I Dmax degradation thermally activated with E a ~1.0 ev 14
15 Summary of electrical and structural degradation under OFF and Semi-ON bias 1. I G degradation Fast Electric-field driven Weak temperature sensitivity (E a ~0.2 ev) Tends to saturate Correlates with appearance of shallow groove and small pits 2. I Dmax degradation Much slower Electric-field driven Temperature activated (E a ~1 ev) Starts after I G saturated Does not saturate Correlates with growth of pits and merging into trenches
Initial hypothesis: Inverse Piezoelectric Effect Mechanism Strong piezoelectricity in AlGaN V DG tensile stress crystallographic defects beyond critical elastic energy Defects: Trap electrons n s R D, I D Strain relaxation I D Provide paths for I G I G ΔΦ bi defect state AlGaN S AlGaN GaN GaN E C E F G D 2DEG Joh, IEDM 2006 Joh, IEDM 2007 Joh, MR 2010b 16
Predictions of Inverse Piezoelectric Effect model borne out by experiments To enhance GaN HEMT reliability: Reduce AlN composition of AlGaN barrier (Jimenez, ESREF 2011) Thin down AlGaN barrier (Lee, EL 2005) Use thicker GaN cap (Ivo, IRPS 2009; Jimenez, ESREF 2011) Use InAlN barrier (Jimenez, ESREF 2011) Use AlGaN buffer (Joh, IEDM 2006; Ivo, MR 2011) Electric field management at drain end of gate (many) Can t explain: Groove formation/i G degradation below critical voltage Sequential nature of I G and I D degradation Presence of oxygen in pit Role of atmosphere during stress 17
I G degradation not critical; TDDB*-like V crit =75 V Marcon, IEDM 2010 Meneghini, IEDM 2011 I G starts increasing for V stress <V crit Onset enhanced by V stress Weibull distribution Preceded by onset of I G noise * TDDB = Time-Dependent Dielectric Breakdown 18
I G correlates with EL; EL hot spots correlate with pits, pits are conducting Zanoni, EDL 2009 Normal AFM EL picture Montes Bajo, APL 2012 Conducting AFM AFM topography Shallow pits responsible for I G degradation 19
Sequential I G and I D degradation Semi-ON stress: I D =100 ma/mm, V DS =40 or 50 V Step-Temperature: 50<T a <230 o C Wu, ROCS 2014 Wu, MR 2014 I Dmax /I Dmax (0) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 START evolution of stress experiment 1E-4 1E-3 0.01 0.1 1 10 I Goff (ma/mm) Universal degradation pattern: I G degradation first without I D degradation I D degradation next without further I G degradation Corner of I G and I D same for all samples 20
Oxygen inside pit EDX LEES Conway, Mantech 2007 Park, MR 2009 O, Si, C found inside pit Anodization mechanism for pit formation? (Smith, ECST 2009) 21
Off-state stress: V ds = 43 V, V gs = -7 V for 3000 s in dark at RT Role of atmosphere on structural degradation SEM Top View TEM Cross Section Stressed in watersaturated gas (Ar) ΔI D =28.8% Stressed in dry gas (Ar) ΔI D =0.3% Gao, TED 2014 Moisture enhances surface pitting Results reproduced with dry/wet O 2, N 2, CO 2, air and vacuum 22
New phenomenon: AlGaN corrosion Electrochemical cell formed at drain edge of gate Source of holes: trap-assisted BTBT Electrochemical reaction (requires holes): Source of water: diffusion through SiN 2Al x Ga 1-x N + 3H 2 O xal 2 O 3 + (1-x)Ga 2 O 3 + N 2 + H 2 Gao, TED 2014 23
Tentative complete model? Step 1: formation of shallow pits/continuous groove in cap TDDB-like formation of small conducting paths: I G Makaram, APL 2010 V DG =19 V (V crit ) 200 nm Step 2: growth of pits through anodic oxidation of AlGaN I Dmax as electron concentration under gate edge reduced V DG =57 V 200 nm Exponential dependence of tunneling current on electric field origin of critical voltage behavior? 24
Many questions Why weak temperature activation of I G degradation? Why does I G degradation tend to saturate? Why does I D degradation start as I G degradation saturates? Does mechanical stress and inverse piezoelectric effect play role? Ancona, JAP 2012 Small pit (2 nm x 3 nm) increases mechanical stress in AlGaN by 3X Why large variability in reliability? Is this all relevant under RF power conditions? 25