Recent Progress in Understanding the Electrical Reliability of GaN - PowerPoint PPT Presentation

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 100 mm GaN-on-SiC (CREW) volume manufacturing Palmour, MTT-S 2010 200 W GaN HEMT for cellular base station Kawano, APMC 2005 Sumitomo Remote Radio Head for Japanese Base Station 3

GaN HEMT: Electrical reliability concerns High-power: – Not accessible to DC ON: stress experiments – Mostly benign – 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 4

Critical voltage for degradation in DC step-stress experiments 1.2 1.E+01 OFF-state, V GS =-10 V V DS 1.15 1.E+00 V GS =-10 V I Dmax /I Dmax (0), R/R(0) 1.1 1.E-01 R D |I Goff | (A/mm) G S D R S 1.05 1.E-02 AlGaN 1 1.E-03 2DEG 0.95 1.E-04 I Dmax I Goff 0.9 1.E-05 GaN V crit 0.85 1.E-06 Joh, EDL 2008 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 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 Meneghini, IEDM 2011 Ivo, MR 2011 Liu, JVSTB 2011 GaN HEMT on Si GaN HEMT on Si GaN HEMT on sapphire Demirtas, ROCS 2009 Ma, Chin Phys B 2011 Marcon, IEDM 2010 6

Structural degradation; correlation with electrical degradation Pit depth 50 Permanent I Dmax Degradation (%) 40 30 20 10 Joh, MR 2010 0 0 2 4 6 8 Pit depth (nm) • Pit at edge of gate • Pit depth and I Dmax degradation correlate Chowdhury, EDL 2008 7

Structural damage at gate edge: a universal phenomenon Marcon, MR 2010 Barnes, CS-MANTECH 2012 Dammann, IIRW 2011 Cullen, TDMR 2013 Liu, JVSTB 2011 Christiansen, IRPS 2011 Chang, TDMR 2011 8

Structural degradation: planar view Unstressed 12 Permanent I Dmax Degradation (%) 200 nm averaged over 1 µm 10 8 6 4 2 OFF-state stress: 0 V DG =57 V, T base =150 °C 0 50 100 150 200 nm 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 Monte Bajo, APL 2014 Barnes, CS-MANTECH 2012 Holzworth, ECST 2014 Whiting, MR 2012 Brunel, MR 2013 10

-4 -4 Time evolution of 0.25 0.25 10 10 Stress: V GS =-7 V and V DS =40 V Stress: V GS =-7 V and V DS =40 V 125 °C 125 °C 0.2 0.2 degradation for -5 -5 10 10 I Goff  I Goff  0.15 0.15 |I Goff | (A) |I Goff | (A) | ∆ V T | (V) | ∆ V T | (V) constant V stress > V crit -6 -6 10 10 0.1 0.1  | Δ V T |  | Δ V T | -7 -7 10 10 0.05 0.05 I Goff and V T degradation: -8 -8 0 0 10 10 -4 -4 -2 -2 0 0 2 2 4 4 6 6 • fast (<10 ms) 10 10 Initial Initial 10 10 10 10 10 10 10 10 10 10 Stress time (s) Stress time (s) 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 15 Joh, IRPS 2011 Permanent I Dmax degradation E a =1.12 eV 10 Different degradation physics: ln( τ inc ) (s) Current collapse 5 • I G : weak T dependence E a =0.59 eV • I Dmax : T activated, E a similar 0 to life-test data* I Goff , E a =0.17 eV -5 28 30 32 34 36 1/kT (eV -1 ) 12 * Saunier, DRC 2007; Meneghesso, IJMWT 2010

DC semi-ON stress experiments Stress: I D =100 mA/mm, V DS =40 or 50 V Average of 5 1 µm x1 µm scans a <230 o C Step-T experiments: 50<T 90 at finger center (T j ~110-330 o C) 80 Trench/pit width, depth (nm) 70 SEM Trench/pit width 60 50 40 Drain 30 20 Trench/pit depth 10 AFM 0 0 5 10 15 20 25 30 Permanent I Dmax degradation (%) • Pits and trenches under gate edge on drain side • Trench/pit depth and width correlate with I Dmax degradation Wu, JAP 2015 13

Thermally activated degradation Source I D degradation rate Drain Gate fingers 4.0 7.5 Source 3.5 7.0 Distance from center of gate finger Depth of damage (nm) 3.0 6.5 2.5 ln(1/|slope|) 6.0 2.0 E a =1.04 eV 5.5 1.5 5.0 1.0 4.5 0.5 Δ I D =21.6% Wu, MR 2014 4.0 0.0 0 20 40 60 80 100 120 140 160 180 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 -1 ) Distance from center of gate finger ( µ m) 1/kT channel (eV • 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

Summary of electrical and structural degradation under OFF and Semi-ON bias 1. I G degradation 2. I Dmax degradation • Fast • Much slower • Electric-field driven • Electric-field driven • Weak temperature • Temperature activated (E a ~1 eV) sensitivity (E a ~0.2 eV) • Starts after I G saturated • Tends to saturate • Does not saturate Correlates with appearance of Correlates with growth of pits and shallow groove and small pits merging into trenches 15

Initial hypothesis: Inverse Piezoelectric Effect Mechanism Strong piezoelectricity in AlGaN  |V DG | ↑  tensile stress ↑  crystallographic defects beyond critical elastic energy G G Defects: S S D D Trap electrons AlGaN AlGaN ΔΦ bi ΔΦ bi  n s ↓ → R D ↑, I D ↓ 2DEG 2DEG Strain relaxation GaN GaN  I D ↓ E C E C Provide paths for I G defect defect Joh, IEDM 2006 state state Joh, IEDM 2007  I G ↑ E F E F Joh, MR 2010b AlGaN GaN AlGaN GaN 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 Normal AFM Zanoni, EDL 2009 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 START 1.1 Semi-ON stress: I D =100 mA/mm, 1.0 V DS =40 or 50 V 0.9 a <230 o C Step-Temperature: 50<T 0.8 I Dmax /I Dmax (0) evolution of 0.7 stress experiment 0.6 0.5 0.4 Wu, ROCS 2014 0.3 Wu, MR 2014 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 • O, Si, C found inside pit • Anodization mechanism for pit formation? (Smith, ECST 2009) Park, MR 2009 21

Role of atmosphere on structural degradation Off-state stress: TEM Cross Section SEM Top View V ds = 43 V, V gs = -7 V for 3000 s in dark at RT Stressed in water- saturated gas (Ar) Δ I D =28.8% Stressed in dry gas (Ar) Δ I 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 Source of holes: trap-assisted BTBT Electrochemical cell formed at drain edge of gate Source of water: diffusion through SiN Electrochemical reaction (requires holes): 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? Makaram, APL 2010 Step 1 : formation of shallow V DG =19 V 200 nm pits/continuous groove in cap (V crit ) • TDDB-like formation of small conducting paths: I G ↑ Step 2 : growth of pits through anodic V DG =57 V 200 nm oxidation of AlGaN I Dmax ↓ as electron concentration under gate • edge reduced Exponential dependence of tunneling current on electric field  origin of “critical voltage” behavior? 24