Recent Progress in Understanding the DC and RF Reliability of GaN High Electron Mobility Transistors J. A. del Alamo and J. Joh* Microsystems Technology Laboratories, MIT, Cambridge, MA *Presently with Texas Instruments, Dallas, TX Spring MRS 2012 San Francisco, April 9-13, 2012 Acknowledgements: ARL (DARPA-WBGS program), ONR (DRIFT-MURI program), J. Jimenez, C. V. Thompson, T. Palacios
Outline 1. Critical voltage for GaN degradation 2. Structural degradation of GaN HEMTs 3. Time evolution of degradation 4. Discussion 5. Tentative new model for electrical degradation 2
Critical voltage for degradation of GaN HEMTs 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) 3
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 4
Initial hypothesis: Inverse Piezoelectric Effect Mechanism S AlGaN GaN G D 2DEG 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 Φ bi defect state AlGaN GaN S AlGaN GaN E C E F G D 2DEG Joh, IEDM 2006 Joh, IEDM 2007 Joh, MR 2010b 5
Structural degradation: cross section Small dimple in early stages of I G degradation; I D degradation delayed Gate AlGaN I Dmax /I Dmax (0), R/R(0) 1.2 1 0.8 0.6 0.4 V DS =0 stress #3 #4 #5 #6: unstressed #2 Device #1 I Goff I Dmax 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 I Goff (ma/mm) 0 10 20 30 V DGstress (V) Joh, MR 2010 10 20 30 40 GaN 6
Correlation between pit geometry and I Dmax degradation Joh, MR 2010 50 20 Permanent I Dmax Degradation (%) 40 30 20 10 Pit depth Current collapse (%) 15 10 5 0 0 0 2 4 6 8 0 2 4 6 8 Pit depth (nm) Pit depth (nm) Pit depth and I Dmax degradation correlate: both permanent degradation and current collapse (CC) 7
Structural degradation: planar view OFF-state step-stress, V GS =-7 V, T base =150 C Makaram, APL 2010 200 nm 200 nm Unstressed V DG =15 V V DG =19 V (V crit ) 200 nm V DG =42 V 200 nm V DG =57 V 200 nm Continuous groove appears for V stress <V crit Deep pits formed along groove for V stress >V crit 8
Correlation between pit geometry and I Dmax degradation Permanent I Dmax Degradation (%) 12 10 8 6 4 2 0 Makaram, APL 2010 0 50 100 150 Average Defect Area (nm 2 ) Post Stress Current Collapse (%) 12 10 8 6 4 2 0 Cross-sectional area averaged over 1 µm 0 50 100 150 Average Defect Area (nm 2 ) I Dmax degradation and pit cross-sectional area correlate 9
Planar degradation: the role of time V DS =0, V GS =-40 V, T base =150 C Joh, IWN 2010 Very fast groove formation (within 10 s) Delayed pit formation Pit density/size increase with time Good correlation between I Dmax degradation and pit area 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 CC degradation: slower hint of saturation for long time Permanent I Dmax degradation: much slower does not saturate with time Joh, IRPS 2011 11
The role of temperature in time evolution Incubation time I G : weak T dependence CC, I Dmax : strongly T activated Joh, IRPS 2011 12
Temperature acceleration 15 of incubation time 10 Permanent I Dmax degradation E a =1.12 ev ln( inc ) (s) 5 0 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 E a for I Goff, CC, I Dmax reveal different degradation physics E a for permanent I Dmax degradation similar to life test data * * Saunier, DRC 2007; Meneghesso, IJMWT 2010 13
Summary of degradation after OFF state 1. I G degradation Fast Voltage enhanced Little temperature sensitivity Tends to saturate stress for V stress > V crit Correlates with shallow groove Groove continuous; on S and D side Groove appears for V stress < V crit Mechanisms: Groove: reduction of interfacial oxide at drain end of gate field induced oxidation? I G rise: hopping through defects? Lower B due to gate interface reconstruction? 14
Electroluminescence correlates with I G degradation V DS =0 Zanoni, EDL 2009 Meneghini, IEDM 2011 Gate current electrons produce EL in GaN substrate EL spots tend to merge into a continuous line 15
Summary of degradation after OFF state stress for V stress > V crit 2. Current collapse degradation (trapping) Slower Enhanced by temperature, voltage Tends to saturate for very long times Correlates with pits: Pits randomly located on drain side Pits grow with V stress, time and temperature Pits eventually merge Mechanism: Pits: relieve mechanical stress arising from inverse piezoelectric effect (but detailed process?); electrochemical oxidation? CC: trapping associated with facets of pits? 16
Time evolution of pit growth and CC Average Pit Area (nm 2 ) 10000 1000 100 1 10 100 1000 10000 Stress Time (s) Pit area~t 1/4 Current collapse (%) 10 2 10 1 Stress: V GS = 7 V and V DS =40 V Slope=0.22 150 C 125 C 100 C 75 C 10 0 100-4 10-2 10 0 10 2 10 4 10 6 Stress Time (s) Joh, IWN 2010 Joh, IRPS 2011 Similar time dependence in current collapse and pit formation 17
Current collapse time constant spectrum Amplitude (A.U.) Joh, IRPS 2011-0.2-0.4 0 x 10-4 Stress time <1s 10s 100s 1000s -0.6 V DS =0 pulse -0.8 >10ks 1s, V GS = 10 V T DP1 a =30 C -1 10-3 10-2 10-1 10 0 10 1 10 2 10 3 Detrapping time constant (sec) Using current-transient methodology of Joh TED 2010 Dominant trap created by stress already present in virgin sample, E a =0.56 ev CC associated with surface (?) (pit creates new surfaces right next to gate edge!) 18
Summary of degradation after OFF state stress for V stress > V crit 3. I Dmax, R D degradation Much slower Temperature activated Voltage enhanced Does not saturate Correlate with pits Mechanism: Pit depletes electron concentration in channel below Current loss associated with pit depth? 19
Can degradation occur for V stress < V crit? V crit =75 V Meneghini, IEDM 2011 Marcon, IEDM 2010 Sudden irreversible increase in I G, enhanced by V stress No reported I D degradation Appearance of I G noise, EL hot spots E a =0.12 ev Consistent with groove formation? 20
Oxygen inside pit EDX LEES Park, MR 2009 Conway, Mantech 2007 O, Si, C found inside pit Anodization mechanism for pit formation? (Smith, ECST 2009) Electrical stress experiments under N 2 inconclusive 21
Groove: essential for pit formation? Appearance of groove increases mechanical stress due to inverse piezoelectric effect at drain end of gate 2 nm x 3 nm groove increases mechanical stress in AlGaN from 4.6 GPa to 13 GPa Groove has little effect in current underneath Pit formation brings major loss of current Ancona, SISPAD 2011 22
2D distribution of structural degradation under high power stress 4x100 µm devices SEM: finger end Stress conditions: I D =250 ma/mm, V DS =40 V @120C, t=523 h SEM: finger center AFM Prominent pits under gate edge on drain side Pits more prominent towards center of finger Typical pit: 50 nm wide, 8 nm deep Lin, APL 2012 23
2D distribution of pits inner finger characterized area outer finger Pits evaluated along 10 µm segments Pit cross sectional area and density increase towards center of device Consistent with local T activating pit formation 24
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 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) Surprises: Role of oxygen Role of surface chemistry Groove formation below critical voltage 25
Tentative new IPE model for GaN HEMT electrical degradation Step 1: electrochemical (?) formation of continuous groove in cap I G through leakage path created around groove (?) Step 2: random seeding of pits as AlGaN barrier exposed Step 3: growth of pits through AlGaN and along gate edge to relieve mechanical stress CC due to new surfaces (?) I Dmax as electron concentra on under gate edge reduced Model very sensitive to cap design, interface treatments, residual O, and passivation 26