Recent Progress in Understanding the DC and RF Reliability of GaN - - PowerPoint PPT Presentation

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

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ID, RD, and IG start to degrade beyond critical voltage (Vcrit) (+ increased trapping behavior – current collapse)

IDmax: VDS=5 V, VGS=2 V IGoff: VDS=0.1 V, VGS=-5 V

Critical voltage for degradation of GaN HEMTs

Joh, EDL 2008

G S D AlGaN GaN 2DEG

VGS VDS

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 10 20 30 40 50 |IGoff| (A/mm) IDmax/IDmax(0), R/R(0) VDGstress (V)

IDmax RS RD IGoff Vcrit

OFF-state, VGS=-10 V

=-10 V

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Critical voltage: a universal phenomenon?

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Meneghini, IEDM 2011 Ivo, MR 2011

GaN HEMT on SiC GaN HEMT on SiC

Demirtas, ROCS 2009

GaN HEMT on Si

Liu, JVSTB 2011

GaN HEMT on SiC

Ma, Chin Phys B 2011

GaN HEMT on sapphire GaN HEMT on Si

Marcon, IEDM 2010

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defect state

∆Φbi EC EF G S D AlGaN GaN 2DEG

AlGaN GaN

defect state

∆Φbi EC EF G S D AlGaN GaN 2DEG

AlGaN GaN

Initial hypothesis: Inverse Piezoelectric Effect Mechanism

Strong piezoelectricity in AlGaN  |VDG|↑  tensile stress ↑  crystallographic defects beyond critical elastic energy Defects: Trap electrons  ns↓ → RD↑, ID ↓ Strain relaxation  ID ↓ Provide paths for IG  IG↑

Joh, IEDM 2006 Joh, IEDM 2007 Joh, MR 2010b

G S D AlGaN GaN 2DEG

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 0.4 0.6 0.8 1 1.2 10 20 30 |IGoff| (mA/mm) IDmax/IDmax(0), R/R(0) VDGstress (V)

10 20 30 40 Device #1 #2 #3 #4 #5 IDmax IGoff

Joh, MR 2010

#6: unstressed

AlGaN GaN Gate

‐ Small dimple in early stages of IG degradation; ‐ ID degradation delayed

VDS=0 stress

Structural degradation: cross section

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Correlation between pit geometry and IDmax degradation

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10 20 30 40 50 2 4 6 8 Permanent IDmax Degradation (%) Pit depth (nm) 5 10 15 20 2 4 6 8 Current collapse (%) Pit depth (nm)

Pit depth and IDmax degradation correlate:  both permanent degradation and current collapse (CC)

Pit depth Joh, MR 2010

Structural degradation: planar view

200 nm 200 nm 200 nm 200 nm 200 nm

Unstressed VDG=15 V VDG=19 V (Vcrit) VDG=42 V VDG=57 V

• Continuous groove appears for Vstress<Vcrit
• Deep pits formed along groove for Vstress>Vcrit

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OFF-state step-stress, VGS=-7 V, Tbase=150 °C Makaram, APL 2010

2 4 6 8 10 12 50 100 150

Post‐Stress Current Collapse (%) Average Defect Area (nm2)

2 4 6 8 10 12 50 100 150

Permanent IDmax Degradation (%) Average Defect Area (nm2)

IDmax degradation and pit cross-sectional area correlate

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Makaram, APL 2010

Correlation between pit geometry and IDmax degradation

Cross-sectional area averaged over 1 µm

Planar degradation: the role of time

VDS=0, VGS=-40 V, Tbase=150 °C

• Very fast groove formation (within 10 s)
• Delayed pit formation
• Pit density/size increase with time
• Good correlation between IDmax degradation and pit area

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Joh, IWN 2010

Time evolution of degradation for constant Vstress > Vcrit

IGoff and VT degradation:

• fast (<10 ms)
• saturate after 104 s

CC degradation:

• slower
• hint of saturation for long time

Permanent IDmax degradation:

• much slower
• does not saturate with time

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0.05 0.1 0.15 0.2 0.25 Stress time (s) |VT| (V) 10

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|IGoff| (A)

IGoff |ΔVT| Initial Stress: VGS=‐7 V and VDS=40 V 125 °C

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0.05 0.1 0.15 0.2 0.25 Stress time (s) |VT| (V) 10

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|IGoff| (A)

IGoff |ΔVT| Initial Stress: VGS=‐7 V and VDS=40 V 125 °C

Joh, IRPS 2011

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Joh, IRPS 2011

The role of temperature in time evolution

• IG: weak T dependence
• CC, IDmax:

strongly T activated

Incubation time

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Temperature acceleration

• f incubation time

28 30 32 34 36

• 5

5 10 15 1/kT (eV-1) ln(inc) (s)

Permanent IDmax degradation Ea=1.12 eV Current collapse Ea=0.59 eV IGoff, Ea=0.17 eV

• Different Ea for IGoff, CC, IDmax reveal different degradation physics
• Ea for permanent IDmax degradation similar to life test data*

* Saunier, DRC 2007; Meneghesso, IJMWT 2010

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• 1. IG degradation
• Fast
• Voltage enhanced
• Little temperature sensitivity
• Tends to saturate

Correlates with shallow groove

• Groove continuous; on S and D side
• Groove appears for Vstress< Vcrit

Mechanisms:

• Groove: reduction of interfacial oxide at drain end of gate 

field‐induced oxidation?

• IG rise: hopping through defects? Lower B due to gate

interface reconstruction?

Summary of degradation after OFF‐state stress for Vstress > Vcrit

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Electroluminescence correlates with IG degradation

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Zanoni, EDL 2009 Meneghini, IEDM 2011

• Gate current electrons produce EL in GaN substrate
• EL spots tend to merge into a continuous line

VDS=0

• 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 Vstress, time and temperature
• Pits eventually merge

Mechanism:

• Pits: relieve mechanical stress arising from inverse

piezoelectric effect (but detailed process?); electrochemical

• xidation?
• CC: trapping associated with facets of pits?

Summary of degradation after OFF‐state stress for Vstress > Vcrit

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Time evolution of pit growth and CC

100 1000 10000 1 10 100 1000 10000 Average Pit Area (nm2) Stress Time (s)

Pit area~t1/4

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Stress Time (s) Current collapse (%)

150 °C 100 °C 75 °C 125 °C Slope=0.22 Stress: VGS=‐7 V and VDS=40 V

Similar time dependence in current collapse and pit formation

Joh, IWN 2010 Joh, IRPS 2011

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• Dominant trap created by stress already present in virgin

sample, Ea=0.56 eV

• CC associated with surface (?) (pit creates new surfaces right

next to gate edge!)

Current collapse time‐constant spectrum

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0x 10

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Detrapping time constant (sec) Amplitude (A.U.)

Stress time <1s 10s 100s 1000s >10ks DP1 VDS=0 pulse 1s, VGS=‐10 V Ta=30 °C

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0x 10

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Detrapping time constant (sec) Amplitude (A.U.)

Stress time <1s 10s 100s 1000s >10ks DP1 VDS=0 pulse 1s, VGS=‐10 V Ta=30 °C

Using current-transient methodology of Joh TED 2010

Joh, IRPS 2011

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• 3. IDmax, RD 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?

Summary of degradation after OFF‐state stress for Vstress > Vcrit

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Can degradation occur for Vstress < Vcrit?

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Meneghini, IEDM 2011 Marcon, IEDM 2010

Vcrit=75 V

• Sudden irreversible increase in IG,

enhanced by Vstress

• No reported ID degradation
• Appearance of IG noise, EL hot‐spots
• Ea=0.12 eV
• Consistent with groove formation?

Oxygen inside pit

• O, Si, C found inside pit
• Anodization mechanism for pit

formation? (Smith, ECST 2009)

• Electrical stress experiments

under N2 inconclusive

Park, MR 2009 Conway, Mantech 2007

EDX LEES

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Groove: essential for pit formation?

Appearance of groove increases mechanical stress due to inverse piezoelectric effect at drain end of gate

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Ancona, SISPAD 2011

• 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
• f current

2D distribution of structural degradation under high‐power stress

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• Prominent pits under gate edge on drain side
• Pits more prominent towards center of finger
• Typical pit: 50 nm wide, 8 nm deep

Stress conditions: ID=250 mA/mm, VDS=40 V @120C, t=523 h SEM: finger end AFM

Lin, APL 2012

SEM: finger center

4x100 µm devices

• Pits evaluated along 10 µm

segments

• Pit cross sectional area and

density increase towards center

• f device
• Consistent with local T activating

pit formation

2D distribution of pits

characterized area

• uter finger

inner finger

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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

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Tentative new IPE model for GaN HEMT electrical degradation

Step 1: electrochemical (?) formation of continuous groove in cap

• IG↑ 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 (?)
• IDmax↓ as electron concentraon under gate edge

reduced

Model very sensitive to cap design, interface treatments, residual O, and passivation

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