HIGH SPEED INTERDIGITAL MSM PHOTODIODES. RICHARD JOHN SEYMOUR, B.Sc., M.Eng. A Thesis. Submitted to the School of Graduate Studies

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1 HIGH SPEED INTERDIGITAL MSM PHOTODIODES '.S) By RICHARD JOHN SEYMOUR, B.Sc., M.Eng. A Thesis Submitted to the School of Graduate Studies in Partial Fulfillment of the Requirements for the degree Doctor of Philosophy McMaster University September 1989

2 HIGH SPEED INTERDIGITAL MSM PHOTODIODES

3 DOCTOR OF PHILOSOPHY (1989) (Physics) McMASTER UNIVERSITY Hamilton, Ontario TITLE: HIGH SPEED INTERDIGITAL MSM PHOTO DIODES AUTHOR: Richard John Seymour, B.Sc. (University of Waterloo) M.Eng. (McMaster University) SUPERVISOR: Dr. B.K. Garside NUMBER OF PAGES: X, 143 ii

4 ABSTRACT A fast, simple photodetector which is compatible with optical integration techniques can be produced Ur.iUb ::. d~~ign consisting of an interdigital metalsemiconductor - metal (MSM) photodiode. The general operation of these devices is discussed with emphasis placed on basic device characteristics such as IV, CV, and steady state and pulsed light response. Ideas and models are presented to allow predictions of generic device performance as well as design and optimization of specific devices. These theoretical aspects are validated through comparison with experimental results. iii

5 ACKNOWLEDGEMENTS The author would like to thank Dr. B.K. Garside for his supervision of this work and for the assistance he provided throughout. Thanks also to Dr. 'l'imusk. Dr.,jessop, and Dr. Thompson for their assistance in completing this thesis. Special thanks to Ann Levy for her invaluable contributions and to my parents for their continuing support. Finally, thanks to all the people whose contributions wec(~ perhaps less tangible but not necessarily less valuable. iv

6 TABLE OF CONTENTS Page INTRODUCTION 1 CHAPTER 1 DEVICE MODELING 5 CHAPTER 2 STEADY STATE CHARACTERISTICS 15 CHAPTER 3 CAPACITANCE 47 CHAPTER 4 PULSE RESPONSE CHARACTERISTICS 75 CHAPTER 5 SAMPLE APPLICATIONS 92 SUMMARY 104 APPENDIX 1 NUMERICAL MODEL 107 APPENDIX 2 DEVICE FABRICATION 137 REFERENCES 140 v

7 LIST OF FIGURES Page Figure 1.1 Typical layout. of an int.erdigital diode. 9 Figure 1.2 Comparison of equipotent.ial plots obtained 12 usin~ the potential routine (top) and t.he stea y state routine (bottom). Figure 1.3 Comparison of the average electron densit.y 13 calculated using Boltzmann's equation and t.he steady state routine. Figure 2.1 Temperature dependence of average electron 18 density in 50S films measured using t.he Hall effect. Figure 2.2 Predicted spatial dependence of photocurrcnt 20 for a carrier lifetime of (a) 4x10- JO sec (b) 10-6 sec, (spot size 1.0 microns). Figure 2.3 Pulse response of the (a) depleted region, 22 and (b) undepleted region, of an int.erdigital detector on SOS. Figure 2.4 Experimental setup used to measure photo- 23 current as a function of position. Figure 2.5 Sample scan between two digits on SOS. 25 Figure 2.6 Sample scan between two digits on bulk single 26 crystal silicon. Figure 2.7 Variation of t.he average depletion width as a 28 function of bias voltage. Figure 2.8 Normalized responsivity for interdigital 29 diodes on (a) semi - insulating GaAs, (b) epi -layer GaAs. Figure 2.9 Normalized responsivity for int.erdigital 30 diodes on SOS. Figure 2.10 Absorption coefficients of GaAs and silicon. 30 Figure 2.11 Responsivityas a function of bias voltage for 34 an epi - layer interdigital detector. vi

8 Figure 2.12 Typical I-V characteristic of an interdigital 36 detector (vertical scale 10 p.a/div, horizontal scale 30 V/ div). Figure 2.13 Operation phases of a one dimensional MSM 37 device, (atv < Vrt, depletion regions not in contact, f ~ V = V rt, depletion rp.~on5 just in contact, c V = Vfb, the electric eld at the forward bias digit equals zero. (d) V > Vfb. Figure 2.14 Notation used to describe a symmetric one 38 dimensional MSM structure, ( Jnt = Jn2 = <Pn, Vdl=Vd2=Vd). Figure 2.15 Comparison of the current calculated by the 41 one dimensional analytical model and the one dimensional numerical model for a silicon MSM device ( L = 5 J1.ID, Nd = 5xl0 15 cm -3, In = 0.8 V ). Figure 2.16 Current calculated USi~ the two dimensional 43 model for an interdigit MSM device on 50S ( t = 1 J1.Il1, x = 5 J.Lffi, G = 5 J1.ffi, Nd = 5xl0 15 cm- 3, <Pn = 0.8 V ). Figure 2.17 Comparison of V fb calculated for a one 44 dimensional silicon MSM device and a two dimensional SOS interdigital device for different contact separations. The limitation of avalanche breakdown is shown for the one dimensional case. Figure 3.1 Equivalent circuit for the capacitance of an 48 interdigital diode. Figure 3.2 Capacitance due to digits as a function of 51 voltage for an interdigital diode on 50S. Figure 3.3 Sample of the measured capacitance of a 6 53 digit diode on SOS (digit width and separation of 10 J1.Il1). Figure 3.4 Geometry used by Smith for calculating the 55 even and odd mode fringing capacitances of coupled lines. Figure 3.5 Definition of capacitances C I and C 2 60 calculated in equations 3.12 and vii

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