TCAD Simulation of Radiation Damage for LGAD Sensor · 2020. 7. 1. · Φeq = 1e15 normal scale after MIP hits 1ns log scale after MIP hits 1ns electric field distribution Vbias =

TCAD Simulation of Radiation Damage for LGAD Sensor

Tao Yang

2020-6-4

On be half of IHEP HGTD group

2020-6-4Institute of High Energy Physics, CAS [email protected]

Introduction of Low Gain Avalanche Detector

Radiation damage in LGAD

Simulated radiation effects

Acceptor removal

Bulk defects (DLs model)

Effective acceptor density

Gain factor

Leakage current

Conclusion

Motivation


n+p+ gain layerJTE JTE

SiO2 SiO2Aluminum

p++

e h

multiplication

Electric Field

Dept

h VGL

VFD

~3e16 cm-3 acceptor density and >300 kV/cm electric field on gain layer, the electrons acquire

sufficient kinetic energy to generate additional e/h pairs under this condition.

The JTE overlaps the main junction edge to prevent premature breakdown caused by edge effect, then

high bias voltage could be applied to satisfy appropriate gain (~10) and high time resolution (~50ps).

Effective acceptor density on gain layer and bulk are demonstrated on 1/C2 - V. Gain layer depletion

voltage(VGL) and full depletion voltage(VFD)

Schematic cross section Electric Field distribution 1/C2 - V


Lack one reliable method / model to predict characteristics of irradiated LGAD accurately.

Transient current technology measures complex electric field distribution (multi-Junctions) in

irradiated LGAD which is not fully understood.

Performance of present fabricated LGAD devices drops sharply when Φeq up to 4e15[cm-2], acquiring

satisfied σtime, gain and S/N is still a challenge.

Drift velocity profiles obtained through eTCT(left) and TPA-TCT(right) on the LGAD, -20◦C


JTE JTEn++ cathode

p+ gain layer

p-stop p-stopcollector-ring collector-ring

p++ anode

50μm Neumann boundary conditionp-type epi ~1000Ω • cm

Doping distribution Gain factor with various NA(0) VBD and VGL with various NA(0)

linear

Schematic cross section of LGAD with JTE, P-stop and N-well collector-ring

Breakdown voltage (VBD) and gain layer depletion voltage (VGL) are related to initial gain layer peak NA(0) if other

parameters are fixed (n++ cathode process, gain layer width...)

Notice that VGL is linear with NA(0), reduction of NA(0) could be estimated through change of VGL.

High NA(0) accompanies with lower VBD though higher relative gain factor.

NA(0)


The sensitivity of local inhomogeneous doping / electric field makes it difficult to

construct one TCAD structure which closes fabricated device.

• Doping information extracted 1/C2-V is limited by “one-side junction” approximation and complex

electric field in irradiated LGAD, discrepancies are inevitable. (see p18-p19 in backup)

• N+ cathode doping is significant that compensates gain layer, only acquires it from fabricated process

detail.

IHEP-ZASM LGAD

by M.Zhao, K.Wu

p+ gain layer dose : 4.8e12

deliver: 2019.12

radiation campaign：

~ 2020.9

(CIAE 100MeV proton up to 4e15)

Mask layout

Fabricated device


Grounded collector-ring reforms electric field on the LGAD surface and weakens “edge effect”

(premature breakdown). If breakdown caused by active region (high NA(0)), grounded collector-ring can not

enhance VBD. All of simulations are collector-ring grounded unless noted otherwise.

NA(0) = 1.44e16

T = 300K

NA(0) = 2.80e16

T = 300K

Breakdown on edge

Breakdown on active region

Grounded

Grounded

Grounded

Grounded

Grounded

Grounded

Reverse Voltage = -650V

Reverse Voltage = -350V

NA(0) = 1.44e16

NA(0) = 2.80e16

eCurrentDensity

when breakdown


The initial gain layer acceptor density NA(0) of most of LGAD devices are between 1016 ~ 1017 cm-3

(yellow band). One reasonable value c = 4e-16 are chosen in simulation (minimum acceptor reduction

depends on Ferrero‘s theory) .

Measurements Theoretical prediction


• Φeq : up to 7e15 or 7e15~2.2e16

• Temperature : 300K

• Feature : charges collection, depletion voltage,IV

Defect Type Energy Level [eV] σe [cm-2] σh [cm-2] η [cm-1]

V2 Acceptor EC - 0.42 1e-15 1e-14 1.613

V3 Acceptor EC - 0.46 7e-15 or 3e-15 7e-14 or 3e-14 0.9

CiOi Donor EV + 0.36 3.23e-13 3.23e-14 0.9

• Φeq : >1e15

• Temperature : 253K

• Feature : charges collection, IV, CV, depletion

voltage, double junction

Defect Type Energy Level [eV] σe [cm-2] σh [cm-2] η [cm-1]

E30K Donor EC - 0.1 2.300e-14 2.920e-16 0.0497

V3 Acceptor EC - 0.458 2.551e-14 1.511e-13 0.6447

Ip Acceptor EC - 0.545 4.478e-15 6.790e-15 0.4335

H220 Donor EV + 0.48 4.166e-15 1.965e-16 0.5978

CiOi Donor EV + 0.36 3.230e-17 2.036e-14 0.3780

Bulk acceptor change dominated by bulk defects Previous LGAD radiation simulation:• Bulk defects[Silvaco TCAD]


y = 26.485e-6E-16x

0

5

10

15

20

25

30

0 1E+15 2E+15 3E+15 4E+15 5E+15 6E+15

Volta

ge [V

]

Φeq [cm-2]

VGL

25

30

35

40

45

50

55

0 1E+14 2E+14 3E+14 4E+14 5E+14 6E+14

Volta

ge [V

]

Φeq [cm-2]

VBULK = VFD - VGL

effective bulk acceptor:

generation (Φeq> ~2e14)

HPTM + acceptor removal

T = 253K

[Local acceptor removal + global bulk defects] coupling method could reproduce reduction of VGL and

VBULK which relate to change of gain layer acceptor density and bulk acceptor density. (p20 in backup)

acceptor removal constant : 10-16 ~ 10-15 [cm2]

p-type epi : 1000Ω • cm



T = 253K

Vbias = -500V

NA(0) = 2.34e16

New Perugia + acceptor removal

T = 300K

Vbias = -500V

NA(0) = 2.34e16

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Base on bulk defects coupled acceptor removal model, gain factor decrease 80% when Φeq up to 2e15,

with a medium initial gain layer acceptor density.

Low temperature restrains carrier thermal action which acquires higher gain factor.

Unanticipated multiplication occurs in PIN and LGAD when Φeq up to 5e15, T=253K (HPTM+acceptor

removal).

Definition of gain factor:


y = 3E-18x - 0.0006

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

2.5E-02

0.0E+00 2.0E+15 4.0E+15 6.0E+15

I leak

age /

gai

n/ (d

•S)

Φeq [cm-2]

HPTM(200V) HPTM(400V)

y = 3E-16x - 0.0345

0.0E+00

2.0E-01

4.0E-01

6.0E-01

8.0E-01

1.0E+00

1.2E+00

1.4E+00

1.6E+00

0.0E+00 2.0E+15 4.0E+15 6.0E+15

I leak

age /

gai

n /

(d•S

)

Φeq [cm-2]

New Perugia(200V) New Perugia(400V)

T = 253K

T = 300K

Igen - generation current

MI - the current multiplication factor,

α - leakage current damage constant

d - the detector thickness

S - the active surface.

Ileakage /gain/(d•S) is linear with fluence that agrees the generation

current equation in LGAD.

Simulated slopes have one order of magnitude underestimated

with respect to typical experimental findings. [α (80min,60°C) =

(3.99 ± 0.03) × 10−17 A/cm, measurements taken at 300K]

α(T=253K) = 3e-18 A/cm

α(T=300K) = 3e-16 A/cm

Note: leakage current dominated by bulk defects and the influence of

gain layer acceptor reduction is faint. [see p21 in backup]


Simulation shows the reduction of electric field on gain layer region is dominated by bulk

defects and acceptor removal on static condition:

Effect Electric field (Vbias = -500V)

HPTM(front side) ↑(∝～1/NAeff)

acceptor removal ↓dominate

The influence of deep energy levels for front side (traped electron) seemingly consists of

effective acceptor density and has compensation effect between them.



T = 253K

Vbias= -500V

Φeq = 1e15

normal scale after MIP hits 1ns log scale after MIP hits 1ns electric field distribution

Vbias = -50V

Vbias = -100V

Vbias = -200V

Vbias = -500V

Due to holes traped and accumulate on backside, transient electric field shows one

backside peak.

With reverse voltage up to ~200V, backside peak is recovered in bulk uniform electric

field. Note that drift velocity when different reverse voltage and number of generated

carriers are potential interfering factor for transient electric field.


To enhance radiation resistance, higher initial gain layer acceptor density is considered with less gain reduction rate,

however needs to balance premature breakdown and one controlled multiplication(low gain ~10) .

VGL and VBULK predicted by [HPTM+acceptor] agree previous experimental finding.

Ileakage /gain/(d•S) is linear with fluence that could be explained by generation current equation in LGAD and also be

proved on measurements.

Electric field simulated distribution shows qualitative interpretation in irradiated LGAD where bulk defects coupled

acceptor removal but still unexplain and fully understand complex electric field in irradiated LGAD.

One qualitative method to evaluate irradiated LGAD performance by coupling global

bulk defects and local acceptor removal.

It’s usability needs compare measured data for fabricated device (IHEP-ZASM LGAD) after

radiation (CIAE 100MeV proton ~4e15 cm-2 on 2020.9).



Backup


VGL

VFD

calculate doping

“Metallurgical junction line”

Use “one-side junction ” approximation to calculate doping.

Calculated doping is small than practical doping due to “one-side junction” approximation,

but it not affects to estimate acceptor removal.


VGL

acceptor removal

T = 300K acceptor removal

T = 300KTCAD configuration：

NA(0) = 2.34e16

NA(0) =1.92e16



T = 253K


T = 300K


T = 253KNew Perugia + acceptor removal

T = 300K



T = 253K


T = 300K

“Two steps” degenerating from acceptor removal

acceptor removal

T = 300K


2020-6-4

T = 253K

Vbias= -6V

Backside peak is not obvious when fluence up to 5e15 on static condition.

Electric distribution applied small reverse voltage (-6V) shows a longer depletion region in

irradiated LGAD which consists of accptor density.

After full depletion (-500V), electric field on bulk increase rapidly near gain layer side.

T = 253K

Vbias= -500V

Institute of High Energy Physics, CAS [email protected]

Documents

TCAD Simulation of Radiation Damage for LGAD Sensor · 2020. 7. 1. · Φeq = 1e15 normal scale after MIP hits 1ns log scale after MIP hits 1ns electric field distribution Vbias =