A switch element has been developed so that a kilovolt step voltage should be supplied to a 50Ω transmission line with a subnanosecond rise time. The element is of silicon substrate with high purity and a pair of electrodes is attached by the evaporation process. The switch action is performed by the photoconductivity produced by the laser light pulse. This paper deals with a preliminary analysis, manufacturing processes and experimental results of the
optoelectronic switch. A performance of 320V output with less than 4 ns risetime was obtained with sufficient persistence for more than 2×10(5) pulse shots. This switch was successfully applied to an optical waveform monitor for laser light pulses giving a resolution less than 2 ns

Ebara, Masami

Koga, Ryuji

Kosaka, Megumi

Sano, Hiroya

Okayama University Digital Information Repository

A High-powered Optoelectronic Switch with Picosecond Risetime

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering, Ok~yama University Vol. 14-2. February 1980A High~powered Optoelectronic Switch with PicosecondRisetimeRyuji KOGA*, Masami EBARA*, Megumi KOSAKA*and Hiroya SANO*(Received February 4,1980)SynopsisA switch element has been developed so that a kilo-volt step voltage should be supplied to a SOQ transmis-sion line with a subnanosecond rise time. The elementis of silicon substrate with high purity and a pair ofelectrodes is attached by the evaporation process.The switch action is performed by the photoconductivityproduced by the laser light pulse.This paper deals with a preliminary analysis, manu-facturing processes and experimental results of theoptoelectronic switch. A performance of 320V outputwith less than 4 ns risetime was obtained with suffici-ent persistence for more than 2xlO s pulse shots. Thisswitch was successfully applied to an optical waveformmonitor for laser light pulses giving a resolution lessthan 2 ns.1. IntroductionA step electric signal with picosecond rise-time and a kilo-voltage on a SOQ line is often required for many applications in high-speed technologies such as Pockels-cell-drive or a streak-camera-trigger. Auston et aZ. proposed a silicon optoelectronic switch anddemonstrated its subnanosecond response and 1.5 kV capabilities [1].* Department of Electronics.1314 Ryuji KOGA. Masami EBARA, Megumi KOSAKA and Hiroya SANOSuccessive efforts have been done for higher voltage up to 10 kV byMourou etal. [2], and for better versatilities with alternate galliumarsenide for Pockels-cell drive by Lee [3]. Mourou and Knox [4] em-phasized a picosecond precision for in-line mounted gallium arsenideswitch for a Pockels-cell drive.The authors are developing a picosecond optical pulse waveformmonitor using optical-fiber delaylines incorporated with a in-linePockels cell for which a clearly step~shaped and high-voltage electricsignal is necessitated. The optolelctronic switch is the indispens-able element to this purpose.This paper reports detailed fabrication processes of optoelectro-nin switches made of silicon and describes experimental results.2. Requirements on the switch elementThe performance characteristics required for the switch elementare specified by the optical gate for a pulsed light signal withpicoseconds duration, whose schematic diagram of the combined electro-optical gate is shown in Fig.l. The lithium tantalate (LiTa0 3 ) crys-tal steers the polarization direction of the transmitted light.This phenomenon is governed by the electric field intensity appliedin the crystal. The conductors form a 50Q transmission line sinceone picosecond corresponds to about a propagation length of 0.2 mmand a fully micros trip transmission line configuration is requiredfor such high-speed operation. The values prepared for the siliconswitch and the requirements from the optical gate are listed inTable 1. The source power is applied in a pulsed waveform for theTRIGGERLIGHTOPTOELECTRONICSWITCHFig.l A configuration of an optical gate for a subnanosecond opticalpulse utilizing the optoelectronic switch.A Silicon Optoelectronic Switchsake of the protection against a thermal breakdown. Provided thatthe rise time is enough short compared to a propagation time in thefeeder line, the surge effect offers an advantage that a half supplyvoltage is enough for the same output voltage compared to a dc powersupply. The LiTa03 crystal requires 2 kV to attain 90 0 swingin polarization direction along the 1 mm optical pathlength with I mmdistance between electrodes. The high voltage and the rather low 50Qcharacteristic impedance result in 20A peak current in the line con-ductors, which require good electrical contacts between differentelements. The configuration of the switch is drawn in Fig.2.Table I Requirements of an optical gate.Source voltage 2 kVLine characteristic impedance 50· QPeak current 20 AApplied source pulse duration 1 )JSExciting optical pulseWavelength 460 nmduration 4 ns FWHMOutput pulse risetime « 4 nsSURFACEELECTRODES,.<E-+/- \ ~I/ \ 1Fig.2 The configuratlon of the opto-electronic switch. Aluminium electrodesform a 50Q microstrip line. Dimensionsare d=lmrn, W=L=lO mrn, r=0.6-1.0mrn.GROUND ELECTRODE3. Principles and analysisThe optoelectronic silicon switch utilizes the photoconductivitybetween two electrodes opposing on one surface excited by incidentlaser light pulse. Since the excitement take place very quicklyafter the injection of the laser light pulse, this switch action canoperate very fast so far as the light pulse has enough short duration.A time dependent transfer characteristics of this switch is ana-lyzed as follows. For the sake of simplicity within an effectiveaccuracy, the population of the electron-hole pairs are dependent1516 Ryuji KOGA, Masami EBARA, Megumi KOSAI<A and Hiroya SANOon the x axis vertical to the surface of the silicon substrate intoit. Let one absorbed photon generate a pair, then its density n ata depth x is expressed asn(x) n .exp(-ax),s( 1)where n s and a are density at the surface and a constant, respectively.Denoting the mobilities of electrons and holes as ~n and ~P' respec-tively, an equivalent conductance G between two electrodes as definedby Fig.3 becomesG(t) e 1 rt t-TT ~ .., (~ +~ )-~ i p(T)exp(- )dTs TLw n p r 10 TO (2)where r stands for both width and distance of the electrodes, p(,) isa temporal laser power, ~ the relaxation time of the pairs, and T s 'the light transmittance across the silicon surface, being given byTs(3)with nr a reflectance of silicon. Substancial values of silicon arelisted in Table 2.Table 2 Parameters ofsilicon.rlr 3.6%+~ 2000 cm2/v·shw/e 2.7 VTO 100 nsOPTOELECTRONIC SWITCH50\1 TRANS}lISSION~Fig.3 An equivalent circuit of theswitch element connected to zO=50n trans-mission lines on both sides.Resulting transmittance T, the temporal ratio of the outputvoltage to the input, is given byT (t) (4)where Zo stands for the characteristic impedance of the transmissionlines. Though G is linear to the incident laser light energy as seenin Eq.-(2), the transmittance shows saturation for G-1«zO correspond-A Silicon Optoelectronic Switching to a large exciting laser power. This results in a shorter rise-time in the output voltage than that of the incident light pulse powerwhich is a replica of an object light pulse.In Fig.4, the peak transmittance is shown for varying effectiveenergy nE of exciting light pulse. It is found that an easily avail-able 2 ~J gives rise to a practical 90% transmittance.1. a r----.----.....---="""17Fig.4 A calculated transmittance T foreffective energy nE of exciting laserpulse, where n is the quantum efficiency.4. Test apparatusE-irLIU~E-tH~UJ~ aE-t 0.01 0.1 1EFFECTIVE ENERGY nE10[].1J]Performances of the experimentally fabricated optoelectronicswitch elements were tested with the system shown in Fig.5. In orderto avoid reflectances which disturb the input and output waveforms,lengths of the coaxial lines are taken enough long, ten meters. Thepulse generator is constructed of a L-C simulated transmission lineand a miniature cold-cathode thyratron, KRYTRON (EG&G). The availablerepetition rate is one pulse per second because of the small capacityof a power supply and a thermal dissipation limit of the KRYTRON.A TEKTRONICS 7623A syncroscope with 4 ns risetime is used to monitorthe output waveforms.~DYELASERIPULSE GEGENERATORATTENU-ATORFig.5 A test system for the fabricated silicon switch elements.18 Ryuji KOGA, Masami EBARA, Megumi KOSAKA and Hiroya SANO5. ExperimentsThree different fabrication processes were tried and performancesof the switch elements were tested. Fundamental process is common,that is, the electrodes are formed by evaporation with a metal maskshown by Fig.6 on a silicon substrate polished to a mirror surface.Two silicon substrates of different purities, aluminium and goldfor evaporated electrodes and diferent heat treatments were combined.Fig.6 A silicon wafer with evaporatedelectrodes (left) , which is scribed andsplit into a switch element after heattreatment (right) .~1 cmFirst sample was made of not so much pure, 10 Q'cm, silicon sub-strates. This was for an establishments of processing only and theresult for the switching performance was quite insufficient mainlydue to the low specific resistance of the silicon wafer.Table 3 Fabrication processes of the optoelectronic switches.Sample II IIIMaterial Silicon single crystal, (111) surface10 4 ~'cm, p-type, 1 rom thickSurface treatment before the evaporationCarborundum #1500 Alumina 0.5 fl toa mirror surfaceChemical etching HN03 :HF - 5:1, 20°C, 10 min.ElectrodeMaterial,Thicknessgold<1 flmaluminium'" 2 flmEvaporation,Pressure 10- 5 TorrSubstrate temPerature200°CHeat treatment after the evaporationno< 10-5 Torr250 °c550°C, 10 min.in flowing N2 gas.A Silicon Optoelectronic SwitchSecond and third versions were fabricated with very pure siliconwafer with the specific resistance of more than 10 4 g·cm. Processingconditions are listed in Table 3.Results of the test done with the system in Fig.4 for a sampleprepared by the process IT are shown by Fig.7. In Fig.7(a), the rise-time of the output is found as 6 ns. In Fig.7(b) , an exponentialdecay due to the extinction of electron-hole pairs with time constantof 100 ns, and reflectance from the 10m coaxial line ends are ob-served. The peak transmittance T is 0.5. This sample was, however,broken down after some tens shots under 800V supply voltage, the goldelectrodes being blown off. This was attributed to that the contactbetween the electrode and the silicon substrate was not enough tightand the gold electrodes were too thin.19(a) (b)Fig.7 Experimental responses of a silicon optoelectronic switch II.Waveforms of output voltages are shown with different timescales. Supplied power is lOOV de and 25 V output is obtainedwith 6 ns rise-time.The third version was designed for better persistence. Elec-trode material was changed from gold to aluminium which allows morethickness in evaporation with a lower ohmic power dissipation andprevent a formation of junctions at the metal and silicon boundary.More careful polishing was made for the surface for reducing a trapdensity,and an accurate heat-treatment process was added to form agood ohmic contacts. A result is given in Fig.8 which shows a 4 nsrisetime, close to the lowest limit of the syncroscope resolution,3.5 ns, and 320 volts output. A more voltage is expected but was nottried because of a fear of electric discharge along the silicon sur-face. The persistence was remarkably improved and the element bythis process is still alive after 2xl0 5 shots with 300 volts output.20 Ryuji KOGA, Masami EBARA, Megumi KOSAKA and Hiroya SANOFig. 8 Response of the switch element Ill. A negative 800V step voltageis supplied through the coaxial cable. Output pulse hight is320V and the rise time 4 ns,which is limited by the bandwidth ofthe syncroscope.6. Application for an optical gateThe last version of the switch element was applied to an opticalgate having been stated in section 2. The assembled optical gate ofthe LiTa0 3 Pockels cell combined with the silicon optoelectronicswitch element is shown in Fig.9. A light pulse from a dye laserwith wavelength of 460 nm was monitored by this gate for its shape.The result is shown in Fig.10. Principle of this method is to takea convolution of a light pulse as a signal and the step waveformgenerated by the optoelectronic switch with varying delay time. Theoriginal waveform is reproduced by calculation. Obtained waveformgives a 2 ns pulse width (full width at half maximum) which is con-sistent with a value expected from the dye laser and the N2 laserdesign.7. Concluding remarksThe fabrication process for a silicon optoelectronic switchwas established. A combination of aluminium electrodes and a siliconsubstrate with high-purity of 10 4 Q·cm specific resistance resultedin the best performance. Careful polishing before the evaporation ofelectrodes and precise heat treatment were necessary for satisfactorypersistence.Output voltage of 320V was obtained and the risetime is conjec-tured to be less than 2 ns from an experiment of optical gating.A Silicon Optoelectronic SwitchFig.9 An optical gate consisting of the switch element anda LiTa03 crystal assembled on an aluminium bulk headwhich is temperature controlled within 0.1 °C accuracy.The assembly is accomodated in an casing of acryl resinfor thermal insulation.1.0...........•~ \w~00..g: 0.5HE-<•...;H \[:! •0 40 2 6TIME [nslFig.10 A result of optical-pulse monitoring for its waveformby the system utilizing the optical gate shown inFig.9. A two nanosecond full width at half maximumis resolved.2122 Ryuji KOGA, Masami EBARA, Megumi KOSAKA and Hiroya SANOAcknowledgemeQtsThe authors were favored to have the collaboration of K. Senoowho contributed his devotional effort as a graduate student to theearlier stage of this study. Thanks are expressed to Dr. S. Nonakaof Nippon Electric Co., Ltd., and to K. Shinohara of Fujitsu Labo-ratory, Ltd., for .their valuable suggestions and considerable helpin providing optical and semiconductor materials.This work was partially supported by the Fund of the Ministryof Education in Aid of Scientific Researches.References[1] D. H. Auston, App1. Phys., VoL 26 (1975) 101.[2] A. Antonetti, M. M. Malley, G. Mourou, and A. Orszag, Opt.Cornrnun., Vol.23(1977)435.[3] C. Lee, Appl. Phys. Lett., Vol.30(1977)84.[4] G. Mourou and W. Knox, Appl. Phys. Lett., Vol.35(1979)492.

A High-powered Optoelectronic Switch with Picosecond Risetime

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