лекция 3 дефекты в полупроводниках ga n alsb
TRANSCRIPT
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Дефекты в полупроводниках
GaN и AlSb
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1
4
32
2’
1. Thermally activated to AlGaN Ec
2. Tunneling to Gate3. Tunneling to Channel4. Thermally activated tunneling to
Channel
AlGaN
GaN
Possible trap locations
Egap=3.4 eV
Egap=4.2 eV
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Hydrogenated Antisite
Egain = 2.35 eVExothermic process
NH3
growth
NGaH3 : NEGATIVE FORMATION ENERGY
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CBM(GaN)
, (1)i
f q q bulk
x tot x tot i F vi
occ shiftE H E D H E n q E E V q ED
Defect Complex VGa-ON
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~ 0.5 eV
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“-3”“0”
C.-H. Lin et. al., Appl. Phys. Lett. (2009)
Y. S. Puzyrev, et al., Appl. Phys. Lett. 96, 053505 (2010).
Results of calculations
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Coulomb ScattererTransconductance degradationNeutral defect
Remove H
Yellow Luminescence
“-3”“0”
Hydrogenated Ga vacancy
Candidate defect: hydrogenated Ga vacancy
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MBE-grown devices (passivated)
Positive shift in Ga-rich, N-rich – acceptors created, or donors removed
Negative shift in NH3-rich – donors created or acceptors removed
Electrical stress-induced degradation(Process Splits; Critical Experiments)
Shift in Vpinch-off is permanent.
T. Roy, et al., Appl. Phys. Lett. 96, 133503 (2010).
Electrical stress :
VG = −4 V
VD = 20 V
T = 300 K
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Source of degradation: hydrogenation of Ga-vacancies
“0”
• Hot electrons sequentially remove hydrogens from Ga-vacancies
• Different charge states
EF during stress
Al0.3Ga0.7N
“0”
“-1”
“-2”
“-3”
“-3”
T. Roy, et al., Appl. Phys. Lett. 96, 133503 (2010).
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1
4
32
2’
1. Thermally activated to AlGaN Ec
2. Tunneling to Gate3. Tunneling to Channel4. Thermally activated tunneling to
Channel
AlGaN
GaN
Possible trap locations
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Oxygen complexes
• VGa-ON
• VGa-ON-O
DFT calculation of Defect Candidates
Low formation energies Vacancy complexes with impurities,- O and H
Hydrogen Complexes
• VGa-VN-H
• VGa-VN-H2
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Defect Candidates
Oxygen-Hydrogen Complexes
• VGa-ON-H
• VGa-ON-H2
For example: extended electron state for level ~0.7 eV below CBM of [VGa-ON-H]-2
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Defect Complex VGa-VN-H
CBM(GaN)
~1.eV below AlGaN CBMLocalized state
[VGa-VN-H]-1
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ON
CBM(GaN)
LDA – (0/-1) trap level in conduction band?
Thermodynamic Levels
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CBM(GaN)
LDA – (-1/-2) charge transition level in conduction band?
Defect Complex VGa-ON
LDA state for [VGa-ON-H]-2 is delocalized
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Defect Complex VGa-ON-H
Hybrid Functional calculation Egap = 4.7 V
Localized state for [VGa-ON-H]-2 .
CBM(GaN)
LDA
Level Ec - 0.7 eV
LDA state for [VGa-ON-H]-2 is delocalized
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• H+ diffusion barrier ~2eV
• [VGa]-3 diffusion barrier ~1 eV
Defect Complex VGa-ON-H
Pre-existing either [ON-H ]+1 or [VGa-ON]-2
Both have low formation energies
Formation of the defect?
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Devices from Rockwell
Degradation in AlSb/InAs HEMTs
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EF 0.6 eV
0.1 eV
Ev
Ec
1.7 eV
1.1 eV
AlSb InAs
Ec
Structure Charge upon hole capture
S. Dasgupta, et al., IEEE Trans. Electron Dev. 58, 1499 (2011).
Substitutional oxygen OSb
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Bias Dependence ofElectron Concentration and Energy
(Michigan MC)
Large peak in G-D region
Gate
Electric field
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Electron concentration with energy over 2 eV is significant and exhibits a peak ~ 1.5 eV
Electron Concentration and Energy
Two positions below the channel
Y. Puzyrev et. al “Gate bias dependence of hot-carrier degradation of GaN HEMTs”, submitted to IEEE Electron Device LettersMichigan Monte Carlo
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Defect density from Vpinch-off shifts
• Estimate defect density that contributes to pinch-off voltage shifts
– Charge control model of HEMT
2)()( AlGAN
doffpinch d
tNetV
Experimentally observed shifts in pinch-off voltage:
( ) ( ) ( ) ( ) ( )d dN t N t E n E Et
aE>E
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DFT: activation energy of defect
Electrons having energy greater than activation energy of defect
N(E
)
Eactivation ≈ 1.8 eV
Activation energy of dehydrogenated N-anti-site
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DFT: activation energy of defect
Activation energy of dehydrogenated Ga-vacancies
Electrons having energy greater than activation energy of defect
N(E
)
Eactivation ≈ 0.5 eV
Accelerated testing performed at bias that gives maximum degradation rateSimulations/Calculations allow extrapolation to device operating conditions
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Scattering from bulk and defects
Bulk Polar Optical Phonon scattering
Defect Optical Phonon scattering
Single phonon emission
Hydrogen release or defect reconfigurationMultiphonon emission ΔE
Coulomb scattering
e
e
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For a phonon scattering test calculation we can provide
• total scattering rate as shown on the Figure above• scattering rate dependence on point• energy loss as a function of
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Modeling hierarchy
VGa-Hn
NGa-Hn
( ) ( ) ( ) ( ) ( )a
d dE E
N t N t E n EEt
Monte-Carlo DFT
DFT
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Multiphonon Defect Reconfiguration by Hot Electrons
Ec
E
Release of Hydrogen
E
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V(R,r)= V(R0,r)+q ∂∙ R V(R,r)Linear coupling to phonons
Mutliphonon capture
Henry and Lang, 1977:
Ridley, 1978: Linear coupling is negligible for multiphonon processes Must use non-adiabatic coupling, Kubo 1952
791
94
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Multiphonon capture
2
2NA jj j j j
XH X X
q q q
Non-adiabatic term
Born-Oppenheimer Approximation
Wave function derivative
Wave function 2nd derivative
DFT implementationis time-consuming
( , ) ( ) ( , ),i iX r R R r R Drop ( , )i R r R
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2
22
2
n i i f
n i i f
n f NA i iE E
j f i n i n i f ij j j j
E E
P X H X
X X X Xq q q
Transition probability
Multi-phonon electron scattering
( ) ( ) ( ) ( ) ( )a
d dE E
N t N t E n EEt
( ) ( ) ( ) ( )a
d dE E
N t N t n E Pt
E
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Operating conditions
Overview & Approach Materials and
growth conditions
DFT
• Defect identification
activation process multi-phonon
scattering rate
Simulation
• Electron distribution
in space in energy
Degradation rate
Accelerated Reliability Test
We are here
Process SplitsCharacterization