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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

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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),

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Outline

  • 1. A few “universal” observations
  • 2. Hypotheses for degradation mechanisms
  • 3. Many questions…

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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

GaN HEMT: breakthrough RF power technology

Sumitomo Remote Radio Head for Japanese Base Station

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GaN HEMT: Electrical reliability concerns

High-voltage OFF and semi-ON: – Degradation of IDmax, RD, IGoff – VT shift – Electron trapping – Trap creation High-power: – Not accessible to DC stress experiments – Device blows up instantly ON: – Mostly benign

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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 in DC step-stress experiments

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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SLIDE 6

Critical voltage: a universal phenomenon

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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SLIDE 7

Structural degradation; correlation with electrical degradation

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

  • Pit at edge of gate
  • Pit depth and IDmax degradation correlate

Pit depth Joh, MR 2010 Chowdhury, EDL 2008

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SLIDE 8

Structural damage at gate edge: a universal phenomenon

Barnes, CS-MANTECH 2012 Dammann, IIRW 2011 Marcon, MR 2010 Chang, TDMR 2011 Liu, JVSTB 2011 Christiansen, IRPS 2011 Cullen, TDMR 2013

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2 4 6 8 10 12 50 100 150

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

Structural degradation: planar view

200 nm 200 nm

Unstressed

  • Vstress>Vcrit: pits along gate edge
  • Pit cross-sectional area correlates

with ID degradation

OFF-state stress: VDG=57 V, Tbase=150 °C

Makaram, APL 2010

averaged

  • ver 1 µm

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SLIDE 10

Holzworth, ECST 2014

Structural damage at gate edge: a universal phenomenon

Barnes, CS-MANTECH 2012 Whiting, MR 2012 Brunel, MR 2013 Monte Bajo, APL 2014

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SLIDE 11

Time evolution of degradation for constant Vstress > Vcrit

IGoff and VT degradation:

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

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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The role of temperature in time evolution

Different degradation physics:

  • IG: weak T dependence
  • IDmax: T activated, Ea similar

to life-test data*

Incubation time

28 30 32 34 36

  • 5

5 10 15 1/kT (eV-1) ln(τinc) (s)

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

* Saunier, DRC 2007; Meneghesso, IJMWT 2010

Joh, IRPS 2011

Incubation time

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SLIDE 13

DC semi-ON stress experiments

  • Pits and trenches under gate

edge on drain side

  • Trench/pit depth and width

correlate with IDmax degradation

Stress: ID=100 mA/mm, VDS=40 or 50 V Step-T experiments: 50<T

a<230oC

(Tj~110-330oC)

SEM AFM

Drain

Wu, JAP 2015

5 10 15 20 25 30 10 20 30 40 50 60 70 80 90

Trench/pit width, depth (nm) Permanent IDmax degradation (%) Trench/pit width Trench/pit depth

Average of 5 1 µm x1 µm scans at finger center 13

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SLIDE 14

20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Ea=1.04 eV ln(1/|slope|) 1/kTchannel (eV

  • 1)
  • Pit/trench depth increase towards center of gate finger

 self heating + thermally activated process

  • Permanent IDmax degradation thermally activated with Ea~1.0 eV

Wu, MR 2014

ΔID=21.6%

Thermally activated degradation

Drain Source Source Distance from center of gate finger Gate fingers

20 40 60 80 100 120 140 160 180 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Depth of damage (nm) Distance from center of gate finger (µm)

ID degradation rate

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1. IG degradation

  • Fast
  • Electric-field driven
  • Weak temperature

sensitivity (Ea~0.2 eV)

  • Tends to saturate

Correlates with appearance of shallow groove and small pits

Summary of electrical and structural degradation under OFF and Semi-ON bias

2. IDmax degradation

  • Much slower
  • Electric-field driven
  • Temperature activated (Ea~1 eV)
  • Starts after IG saturated
  • Does not saturate

Correlates with growth of pits and merging into trenches

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defect state

ΔΦbi EC EF G S D AlGaN GaN 2DEG

AlGaN GaN

defect state

ΔΦbi 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

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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 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/IG degradation below critical voltage
  • Sequential nature of IG and ID degradation
  • Presence of oxygen in pit
  • Role of atmosphere during stress

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IG degradation not critical; TDDB*-like

Meneghini, IEDM 2011 Marcon, IEDM 2010

Vcrit=75 V

  • IG starts increasing for Vstress<Vcrit
  • Onset enhanced by Vstress
  • Weibull distribution
  • Preceded by onset of IG noise

* TDDB = Time-Dependent Dielectric Breakdown 18

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SLIDE 19

IG correlates with EL; EL hot spots correlate with pits, pits are conducting

Montes Bajo, APL 2012

Shallow pits responsible for IG degradation

Normal AFM Conducting AFM EL picture AFM topography

Zanoni, EDL 2009

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1E-4 1E-3 0.01 0.1 1 10 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

IDmax/IDmax(0) |IGoff| (mA/mm)

evolution of stress experiment

“Universal” degradation pattern:

  • IG degradation first without ID degradation
  • ID degradation next without further IG degradation
  • “Corner” of IG and ID same for all samples

Wu, MR 2014

Sequential IG and ID degradation

Semi-ON stress: ID=100 mA/mm, VDS=40 or 50 V Step-Temperature: 50<T

a<230oC

START

Wu, ROCS 2014

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SLIDE 21

Oxygen inside pit

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

formation? (Smith, ECST 2009)

Park, MR 2009 Conway, Mantech 2007

EDX LEES

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Stressed in water- saturated gas (Ar)

SEM Top View TEM Cross Section

Stressed in dry gas (Ar)

  • Moisture enhances surface pitting
  • Results reproduced with dry/wet O2, N2, CO2, air and vacuum

Gao, TED 2014

ΔID=0.3% ΔID=28.8% Off-state stress: Vds = 43 V, Vgs = -7 V for 3000 s in dark at RT

Role of atmosphere

  • n structural degradation

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SLIDE 23

New phenomenon: AlGaN corrosion

2AlxGa1-xN + 3H2O ↔ xAl2O3 + (1-x)Ga2O3 + N2 + H2

Electrochemical cell formed at drain edge of gate Electrochemical reaction (requires holes): Source of holes: trap-assisted BTBT Gao, TED 2014 Source of water: diffusion through SiN

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Tentative complete model?

Step 1: formation of shallow pits/continuous groove in cap

  • TDDB-like formation of small conducting

paths: IG↑

Step 2: growth of pits through anodic

  • xidation of AlGaN
  • IDmax↓ as electron concentration under gate

edge reduced

Exponential dependence of tunneling current on electric field  origin of “critical voltage” behavior?

200 nm

VDG=19 V (Vcrit)

Makaram, APL 2010

200 nm

VDG=57 V

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Many questions…

  • Why weak temperature activation of IG degradation?
  • Why does IG degradation tend to saturate?
  • Why does ID degradation start as IG degradation saturates?
  • Does mechanical stress and inverse piezoelectric effect play role?
  • Why large variability in reliability?
  • Is this all relevant under RF power conditions?

Small pit (2 nm x 3 nm) increases mechanical stress in AlGaN by 3X

Ancona, JAP 2012

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