Comparison of Full Depletion Voltage from C-V, I-V, and Q-V Characteristics for Irradiated Epi-Detector

1. Comparison of full depletion voltage extracted from C-V, I-V and Q-V characteristics for a highly irradiated Epi-detector • E. Fretwurst, N. Hoffmann*, F. Hönniger, G. Lindström • Institute for Experimental Physics, Univ. of Hamburg • *DESY summer student 2005 • Motivation • C-V measurements, frequency dependence (10kHz – 800 kHz), I-V, Q-V measurements and annealing behavior,comparison of extracted Vfd values • Conclusion 1 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

2. Motivation • What is the meaning of „full depletion voltage Vfd“ in case of heavily damaged epitaxial silicon detectors? • Comparison of Vfd values extracted from different measurements:C-V characteristics, frequency dependence: space charge concentration, shallow and deep defect levelsI-V characteristics: concentration of generation centersQ-V characteristics (charge collection measurements): electric field distribution and trapping • Which Vfd values are relevant for the evaluation of the detector properties under practical operation in experiments at S-LHC? 2 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

3. Experimental Conditions • Epitaxial Si on Cz substrate:epi-layer: 50 µm, n-type 50 cmConstant P doping profile[O]: inhomogeneous depth profile, O out-diffusion from Cz[C]: < 1016 cm-3, near to detection limitCz substrate: n-type 0.01 cm, Sb doped • Irradiation: 24 GeV/c protons, p = 4·1015 p/cm² • Measurements:C-V for frequencies between 10 kHz and 800 kHzall C-values correspond to series mode values (Cs, Rs)I-V measurements: (pad current and guard ring current)TCT measurements: generation of charge carriers by a pulsed 1060 nm laser  simulating mipsQ-V characteristics derived by integrating current transients with a time window of 30 nsAnnealing measurements: 80 °C, ta = 0min – 240 min 3 E . Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

4. C-V frequency dependence Frequency dependence measured at room temperature: • strong decrease of C-values with increasing frequency f • shift of C-V transition to constant value Cg (geometrical value) to lower bias voltages with increasing f  decrease of Vfd • specific C-V shape at low f (10 kHz, 50 kHz) indicates non-homogeneous distribution of electrically active defects  possibly correlated withnon-homogeneous [O] distribution 4 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

5. Annealing Effect Annealing at 80 °C: • After 240 minutes the C-V curves are “shifted” to lower bias voltages • The overall shape in the voltage range 1V-10 V is not influenced but the strong decay is shifted to lower voltages  shift in Vfd 5 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

6. Vfd frequency dependence • Vfd decreases with increasing frequency but saturates • Saturation at about 300 kHz for 0 min and 8 min • Saturation between 100 kHz and 200 kHz for 120 min • Relative change Vfd,sat/Vfd,10kHz 50 % 6 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

7. Simulation one deep acceptor level model Parameter: ND = 6·1013 cm-3 NA = 0.98·ND EFn Et = 0.1 eV R = 4·105 s-1 ND - NA X=W-λ W.G. Oldham, S.S. Naik; Solid State Electronics 15 (1972) 1085 Deep acceptor level  transition region λ, defined by crossing of the quasi-Fermi level EFn and trap level Et Capacitance: C = dQ/dV  2 contributions: dQ = dQx + dQw dQx = dQx() depends on frequency due to the emission rate en of the trap Low frequency limit: CL  q0·NA·dx/dV+q0·(ND-NA)dw/dV,High frequency limit: CH q0·(ND-NA)·dw/dV  C() = CH + (CL  CH)/(1 + (/R)2) with R  2·en·(1+K), K counts for the coupling of dQx and dQw 7 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

8. Comparison I-V and C-V • Comparison I-V and C-V curves for same annealing time: • (I-V curves are shifted by an arbitrary value, C-V curves as measured) • Vfd from I-V (crossing point of fit lines) much smaller compared to values from C-V • Saturation of I-V not as clean as expected for a 10 µm gap between central pad and guard ring (possible surface damage effect?) • C-V shape after 60 min annealing shows a double shoulder vanishing after 120 min 8 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

9. Vfd Annealing Curves • Nearly identical time dependence for all frequencies • Shift due to frequency dependent charging and discharging of deep defects • Slightly different annealing behavior of Vfd from I-V, values are comparable with values from C-V at high frequencies 9 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

10. TCT-Measurements Pulsed laser 1060 nm Bias Epi-layer: 50 µmCz-substrate: 300 µm • Signal shape dominated by laser pulse and R-C time constant(diode capacitance, 50  input resistance of the amplifier, charge collection time in the order of 500 ps at 150 V) • Collected charge: integration of the current pulse with a time window of 30 ns 10 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

11. Q-V characteristics and annealing Q-V curves shiftedby an arbitrary value Measurements performed at 20 °C • Extraction of Vfd indicated for the Q-V curves taken at 0 min and 240 min annealing • In the log-log presentation the slope of the increasing part of Q-V increases from about 0.8 at 0 min to 1.1 at 240 min • Charge trapping is clearly seen above “full depletion” and annealing time  120 min 11 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

12. Comparison of Vfd annealing from Q-V and C-V • Vfd values from Q-V are in between the values from C-V taken at 10 kHz and 50 kHzat 0 min and for ta > 30 min the Q-V values coincide almost with those at 10 kHz • The time dependence is nearly identical • This indicates that “full charge collection” is achieved when the transition region λof the space charge region (low field region) becomes zero as approximately deduced from C-V measurements at low frequencies 12 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005

13. Conclusion • The meaning of “full depletion voltage” Vfd has to be taken with care:Vfd depends strongly on type of measurement (C-V, I-V, Q-V) and method of extraction • Systematic studies on a highly irradiated Epi-device show:C-V: Vfd decreases with increasing frequency, but saturatesI-V: Vfd comparable with those from C-V at high frequencies (> 500 kHz)Q-V: Vfd comparable with those from C-V at low frequencies( 10 kHz) • Which Vfd value is relevant for detector operation?The values extracted from Q-V which coincide with values from C-V at low frequencies, but keep in mind:Vfd from Q-V depends on integration time and trapping: For 50 µm thick layers the integration time is less important than for 300 µm (matter of collection time) • Proposed interpretation of relevant Vfd for non-inverted Epi-detectors:The “voltage for full depletion” is achieved when the low field region of the transition region λ vanishes or the crossing point x of the quasi-Fermi level with the deep acceptor level approaches the rear contact (x=d, d=detector thickness) 13 E. Fretwurst, Univ. Hamburg, RD50 workshop, CERN, November 2005