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Inter strip resistance in silicon position-sensitive detectors

Inter strip resistance in silicon position-sensitive detectors. E. Verbitskaya , V. Eremin, N. Safonova* Ioffe Physical-Technical Institute of Russian Academy of Sciences St. Petersburg, Russia *also Saint-Petersburg Electrotechnical University “LETI”, Russia N. Egorov, S. Golubkov

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Inter strip resistance in silicon position-sensitive detectors

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  1. Interstrip resistance in silicon position-sensitive detectors E. Verbitskaya, V. Eremin, N. Safonova* Ioffe Physical-Technical Institute of Russian Academy of Sciences St. Petersburg, Russia *also Saint-Petersburg Electrotechnical University “LETI”, Russia N. Egorov, S. Golubkov Research Institute of Material Science and Technology (RIMST) Zelenograd, Russia 15 RD50 Workshop CERN, Geneva, November 16-18, 2009

  2. Outline • Motivation • Physical model of interstrip resistance • Experimental results on interstrip resistance in as-processed Si detectors • Influence of nonequilibrium carrier generation • RISin n-type FZ and CZ Si samples • Conclusions E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  3. Motivation Current subjects:  Development of operational model for voltage terminating structure (VTS) and current terminating structure (CTS, edgeless detectors)  Strip detector performance at SLHC: very high fluences and enhanced bulk generated current  Noise performance of spectroscopic strip detectors (GSI, Darmstadt) E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  4. 1 2 Special design of test structures p+-n-n+ structure - area 1x1 mm2 Strips: two interpenetrating “combs”: - pitch 25 mm  increased length of interstrip gap  equivalent to 4 cm strips FZ n-Si, r>5 kW d = 300 mm Vfd  20 V E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  5. Physical model j1 = j2 - potentials at the strips • Components that control • interstrip resistance RIS: • surface leakage • interface current Distortion of symmetric distributions of potential and electric field may stimulate excess current flow between strips E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  6. Measurements of interstrip gap characteristics U1 – bias voltage applied to p-n junction U2– bias voltage between the strips E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  7. I-V characteristics of interstrip gap FZ n-Si, # WP 3-6-2 I = Id + Iinst Id – strip dark current Iinst – interstrip current E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  8. DU2cr hole drift Current flow in interstrip gap DU2cr - range of bias voltage in which Iinst is small  DU2cr  with U1 E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  9. Interstrip current Iinst Iinst(U2): dark current is subtracted Interstrip resistance: RIS = dU2/dIinst Regions with different slopes: A and A*: RW = (dIinst/dU2)-1 - ohmic isolation resistance, independent on U1, related mainly with surface leakage B: current step dIinst dIinstand dIinst/dU2 as U1 E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  10. Origin of interstrip current step dIinst j1≠ j2 • Current switching – • redistribution of strip hole • currents • In detector: • Switching acts as • negative feedback • recovery of potential balance Rsw = (dIinst/dU2)-1 in dIinst region E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  11. Interstrip resistance vs bias voltage RW 200 GW irrespective to U1 and at ±U2 Rsw = dU2/dIis at U2 0 Rsw  with U1 RW > Rsw E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  12. Influence of nonequilibrium carrier generation Carrier generation: - by LED illuminating p+ side - white light on n+ side I = Iph + Iinst E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  13. Influence of nonequilibrium carrier generation on RIS • RW and Rsw with carrier generation RW: p+ illumination – high n and p under SiO2 • Rsw at carrier generation: no dependence on U1 – switching iscontrolled by photocurrent rather than dark current Ratio of Rswis about one half of current ratio since ½ of a total structure current is switched E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  14. Influence of Si type FZ n-Si CZ n-Si Similar behavior of Iinst vs. U1 and U2 E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  15. Comparison of Iinst for different Si types DU2cris smaller in CZ Si dU2corresponding to dIinst is smaller in CZ Si RWis similar E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  16. Comparison of Rsw for different Si types Rsw is smaller in CZ Si Parameterization: Rsw = A - B(U1)0.5 FZ: Rsw= 7.21010 – 5.9 109(U1)0.5 CZ: Rsw= 2.51010 – 2.2109(U1)0.5 Rsw depends on bulk generation current E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  17. Future studies •  Different wafer orientation  Detectors with different configuration •  Study of irradiated Si detectors E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  18. Conclusions The factors that define interstrip isolation resistance are: - surface leakage, - interface current, - new mechanism - distortion of symmetric potential distribution at the strips and switching of strip currents.  Switching of strip currents is a negative effect since it decreases interstrip isolation. This effect may control interstrip resistance rather than ohmic conductance between the strips. RWis about 200 GWirrespective to the bias voltage while Rsw is bias dependent and decreases with bias voltage rise down to few GW. Results are partially published in: V. Eremin et al., Semiconductors43(2009) 796. E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

  19. Acknowledgments • This work was made in the framework of RD50 collaboration • and supported in part by: • RF President Grant # 2951.2008.2 • Fundamental Program of Russian Academy of Sciences • on collaboration with CERN Thank you for attention! E. Verbitskaya et al., 15 RD50 Workshop, CERN, Geneva , Nov 16-18, 2009

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