ppt.ppt

21th RD50 Workshop, CERN, 14th – 16th November 2012

Charge carrier detrapping in irradiated siliconsensors after microsecond laser pulses

Markus Gabrysch1, Mara Bruzzi2, Riccardo Mori2,

Michael Moll1, Hannes Neugebauer1,

Marcos Fernandez Garcia3

1 PH-DT-DD, CERN (CH)

2 INFN and University of Florence (IT)

3 IFCA-Universidad de Cantabria (ES)

21th RD50 Workshop, CERN, 14th - 16th November 2012

21th RD50 Workshop, CERN, 14th – 16th November 2012

Outline

1.Motivation/Aim

2.Setup and diodes

3.Photocurrent during illumination

4.TCAD simulations

5.Measurements and method

6.Conclusions

21th RD50 Workshop, CERN, 14th – 16th November 2012

Motivation/Aim

•Knowledge of energy levels and cross-sections of de-trapping centresis crucial for defect characterization

•These parameters can be determined by investigating the temperaturedependence of the time-constant  for de-trapping

•For defects deep in the bandgap the de-trapping happens on aµs-timescale (around RT)

•Previous work: see e.g. Kramberger, Cindro, Mandic, Mikuz, Zavrtanika,“Determination of detrapping times in semiconductor detectors”(2012 JINST 7 P04006)

21th RD50 Workshop, CERN, 14th – 16th November 2012

Setup and Diodes

21th RD50 Workshop, CERN, 14th – 16th November 2012

CERN TCT setup

for µs-pulses

•Red laser illumination with variable pulse width (0.5 - 20µs)

•Variable bias voltage with T-Bias (20kHz-10GHz, HV < 200V)

•Amplifier was not used (to have maximal bandwidth, but: ~1mV signals)

•Temperature controlled (flushed with dry air for T < 10°C)

660 nm

Pulsed laserrandomized trigger (100Hz)

DUT

T-Bias

Oscilloscope

Laser gets absorbedin first 3µm

21th RD50 Workshop, CERN, 14th – 16th November 2012

Investigated diodes

•Material: Micron, 300µm, FZ p- and n-type

•Irradiation: 24GeV protons with  = 51014, 11014, 51013, 11013 p/cm-2

•Annealing: 80min at 60°C

•Illumination: front (back fully metalized)

Pad: 4.5  4.5 mm2

window:

1.5  1.5 mm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121109 FZ_2328-11_A_Set1612 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1612Set=12Pulses0.25to5us.png$

Example of transient current

•TCT signals have been measured (up to 50µs after illumination)

•Temperature range investigated: ca. -10 to 30°C

•Stability of signal confirmed by recording 10 times the (same) waveformwhich itself is an average of 1024 shots

Example:

•1e14p/cm2

•n-in-p (holes transport)

•T = 21C

•Bias: 50, 100, 150V

•Pulse length: 0.25-5µs

•660nm, top illumination

21th RD50 Workshop, CERN, 14th – 16th November 2012

Photocurrent during illumination

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613Set=0Pulses0.5to2us.png$

Photocurrent during illumination

•Current can drop during illumination

•Happens mostly for lower bias voltage and lower temperature

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613Set=7Pulses0.5to2us.png$

T = 21C

Example: 5e14p/cm2, n-in-p (holes transport)

T = 1C

21th RD50 Workshop, CERN, 14th – 16th November 2012

Photocurrent during illumination

•Current can drop during illumination

•Happens mostly for lower bias voltage and lower temperature

This effect is also visible in TCAD simulationsand can be explained by a reduction of the active volume due totrapped charges

21th RD50 Workshop, CERN, 14th – 16th November 2012

TCAD Simulations

(Synopsis)

21th RD50 Workshop, CERN, 14th – 16th November 2012

Synopsis TCAD Simulation of TCT signal of irradiated diode

Device

•silicon: p-type; 5e11cm-3 boron (Vdep(300µm)=35V; 25kcm)

•bulk dimensions: 300101 µm3 (x-y-z)

•diode: n-p-p

•1 defect: D01 – CiOi (donor): E = 0.36eV, e= 2.1e-18 cm2, h = 2.5e-15 cm2 generation rate : g = 0.7 cm-1 (24GeV/c protons) such that Ndefect = g  

Light pulse

•660 nm, 10mW/cm2

•linear rise and linear fall in intensity from 0 to 100% in 1ns

•pulse length (variable, µs-order)

Physics

•Simulation: Synopsys TCAD F-2011.09

•Temperature: 300K

•Leakage current via SRH lifetime “generated” (more details in M.Moll’s talk tomorrow)

21th RD50 Workshop, CERN, 14th – 16th November 2012

Synopsis TCAD Simulation of TCT signal of irradiated diodes

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-pulse-01-50-150V.png$

=1e13 p/cm2

=1e14 p/cm2

=1e15 p/cm2

simple lines = 50V

marked lines = 150V

Note:

•All currents for thesimulated volume of300101 µm3

•In experiment wehave diodes with30050005000 µm3

we need a scalingfactor of 2.5e6 suchthat, e.g., Idark for1e14p/cm2 is 30µA(but in exp.: 100µA)

Idark

10µs pulse

Light OFF

Light ON

Light OFF

21th RD50 Workshop, CERN, 14th – 16th November 2012

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-pulse-01-50-150V-close.png$

Why do we have higher transients for lower bias voltage?

=1e13 p/cm2

=1e14 p/cm2

=1e15 p/cm2

simple lines = 50V

marked lines = 150V

Observations:

•High transientamplitude after lightpulse for lower voltage

We have a better trapfilling for lower voltage

This can be seen in thedefect occupancy ploton next slide

Light OFF

Light ON

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121109 FZ_2328-11_A_Set1612 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1612Set=12Pulses0.25to5us.png$

Example from slide 7

Example:

•1e14p/cm2

•n-in-p (holes transport)

•T = 21C

•Bias: 50, 100, 150V

•Pulse length: 0.25-5µs

•660nm, top illumination

21th RD50 Workshop, CERN, 14th – 16th November 2012

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-pulse-01-50-150V-occupancy.png$

Defect Occupancy at x=150µm

=1e13 p/cm2

=1e14 p/cm2

=1e15 p/cm2

simple lines = 50V

marked lines = 150V

Observations:

•Higher defectoccupancy for lowervoltage

This is a result of higherhole density for lowervoltage

10µs pulse

150V

50V

Light OFF

Light ON

Light OFF

21th RD50 Workshop, CERN, 14th – 16th November 2012

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-pulse-01-50-150V-densities.png$

Electron and Hole Densities at x=150µm for =1e14 p/cm2

Hole density

Electron density

simple lines = 50V

marked lines = 150V

Explanation:

•Higher hole density forlower voltage higher occupancy larger transient

•Note: the transientcurrents for 50 and 100Vin the case of 1e14p/cm2were the same. Since vdriftis lower for 50V we needhigher carrier density toobtain the same current.

10µs pulse

150V

50V

Light OFF

Light ON

Light OFF

21th RD50 Workshop, CERN, 14th – 16th November 2012

Why do we see a current drop for high fluence and low bias?

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-pulse-01-50-150V.png$

=1e13 p/cm2

=1e14 p/cm2

=1e15 p/cm2

simple lines = 50V

marked lines = 150V

Idark

10µs pulse

Observations:

•Current drops duringillumination for 50Vand = 1e15 p/cm2

Light OFF

Light ON

Light OFF

21th RD50 Workshop, CERN, 14th – 16th November 2012

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\inspect-mm014-007-1e15-300K.png$

Observation:

•Decrease in current for50V for highestfluence = 1e15 p/cm2

This is a result of areduction of the activevolume due to theopposing field fromoccupied defects

The leakage currentdrops and gives anegative contribution totransient photocurrent

Current drop for high fluence and low voltage (Voltage scan)

10µs pulse

20V

50V

Light OFF

Light ON

300K, =1e15 p/cm2

time [s]

21th RD50 Workshop, CERN, 14th – 16th November 2012

Time evolution of E-field and space charge density

$O:\Michael\backup\PCRD5004-TCAD-120830\STDB_mmoll\mm014\tecplot-0014-n417-efield.png$

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Conferences & Visits\121114 - CERN - RD50 Workshop\Talk\tecplot-0014-n417-space-charge.png$

40V

For voltages below 60V thedetector goes into underdepletionat the front junction during thepulse which leads to a reduction inphotocurrent

Figure shows space charge signinversion (from neg. to pos. spacecharge)

21th RD50 Workshop, CERN, 14th – 16th November 2012

Measurements and Method

21th RD50 Workshop, CERN, 14th – 16th November 2012

Signal Components

•For irradiated detectors we expect the current AFTER illumination to be ofthe form:

•In the case of two time-constants:with free parameters A1, A2, 1 and 2.

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\Plots for Talk\FZ_2328-11_A_Set1613_FitTau2_Bias = -50V_Set=0Pulses1.5us.png$

Tau extraction for “slow” transient: 2

General model:

I(t)= a*exp(-t/tau)+z

a = 0.92µA

tau = 26µs

Fitting process:

•Averaged data witherrorbars = std of10 “identical”measurements

•Only fitting to “tail” oftransient: 2

•95% confidenceinterval also obtained

=5e14 p/cm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613-tau2vsTemp_Bias-50V_Sets0to21_sep.png$

Tau extraction for “slow” transient: 2

•Each temperature wasmeasured twice (0 to30C and down to 0C)

Data shows correct trendand is reproducible

Tau extraction seems towork for “slow” transient

=5e14 p/cm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\Plots for Talk\FZ_2328-11_A_Set1613_FitTau1_Bias = -50V_Set=0Pulses1.5us.png$

Tau extraction for “fast” transient: 1

General model:

I(t)= a*exp(-t/tau)+z

a = 0.52µA

tau = 1.9µs

Fitting process:

•“Slow” transientsubtracted beforefitting toaveraged data witherrorbars = std of10 “identical”measurements

•Nearly all points afterillumination are taken

=5e14 p/cm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613-tau1vsTemp_Bias-50V_Sets0to21_sep.png$

Tau extraction for “fast” transient: 1

•Each temperature wasmeasured twice (0 to30C and down to 0C)

Data shows big spreadfor low temperature

Tau extraction seems tofail for “fast” transient

=5e14 p/cm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

Parameter extraction: -fitting & Arrhenius plot

•The detrapping time constant is linked to defect parameters by:

•Looking explicitly on T-dependence:

•And we can analyse data in an Arrhenius plot:

absolute value of the energy level calculatedfrom valence band maximum

h… hole detrapping cross-section

vh… thermal hole velocity

NV… effective density of states in valence band maximum

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613_Arrhenius_tau2_Bias-50V_Sets0to21_sep.png$

Arrhenius plot for “slow” transient: 2

from slope:

Eth = 0.32eV

from Intercept &h = 1.8x1025 s-1(mK)-2

h = 1.4x10-16 cm2

=5e14 p/cm2

21th RD50 Workshop, CERN, 14th – 16th November 2012

$D:\_WinXP_Partition\_brennen\_Jobb\CERN\Mara DLTS-TCT\1211\121111 FZ_2328-11_A_Set1613 (Red,-15C_35C,0.25-5us)\FZ_2328-11_A_Set1613_Arrhenius_tau1_Bias-50V_Sets0to21_sep.png$

Arrhenius plot for “fast” transient: 1

from fit:

Eth  0.1eV

h  1x10-19 cm2

=5e14 p/cm2

Not physicallyreasonable

 maybe effect from electronics?

21th RD50 Workshop, CERN, 14th – 16th November 2012

http://www.distinctiveleadership.com.au/wp-content/uploads/2011/04/conclusion.jpg

Conclusions

Varying both pulse length and bias voltage together with TCAD simulations gave better understanding of the current transient formation
But we need to …
improve the extraction of the time constants i from 𝐼 𝑡 or 𝐼 𝑡 𝑑𝑡 (fitting to exp+exp is numerically very ill-conditioned)
increase the detrapping signal by choosing the “optimal” pulse length(for given temperature and bias voltage)
Study behaviour with IR laser (1060nm, instead of 660nm)which has much higher penetration depth
…

21th RD50 Workshop, CERN, 14th – 16th November 2012

Thanks for your attention!