Persistent photoconductivity in copper-compensated, silicon-doped semi-insulating gallium arsenide with a time constant as large as 30 µs has been excited by sub-band-gap laser radiation of photon energy greater than 1 eV. This photoconductivity has been quenched on a nanosecond time scale by laser radiation of photon energy less than 1 eV. The proven ability to turn the switch conductance on and off on command, and to scale the switch to high power could make this semiconductor material the basis of an optically controlled pulsed-power closing and opening switch

Ko, S. T.

Lakdawala, V. K.

Mazzola, M. S.

Schoenbach, K. H.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsElectrical & Computer Engineering FacultyPublications Electrical & Computer Engineering1989Nanosecond Optical Quenching ofPhotoconductivity in a Bulk GaAs SwitchM. S. MazzolaOld Dominion UniversityK. H. SchoenbachOld Dominion UniversityV. K. LakdawalaOld Dominion University, vlakdawa@odu.eduS. T. KoOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/ece_fac_pubsPart of the Electronic Devices and Semiconductor Manufacturing Commons, Physics Commons,and the Semiconductor and Optical Materials CommonsThis Article is brought to you for free and open access by the Electrical & Computer Engineering at ODU Digital Commons. It has been accepted forinclusion in Electrical & Computer Engineering Faculty Publications by an authorized administrator of ODU Digital Commons. For more information,please contact digitalcommons@odu.edu.Repository CitationMazzola, M. S.; Schoenbach, K. H.; Lakdawala, V. K.; and Ko, S. T., "Nanosecond Optical Quenching of Photoconductivity in a BulkGaAs Switch" (1989). Electrical & Computer Engineering Faculty Publications. 88.https://digitalcommons.odu.edu/ece_fac_pubs/88Original Publication CitationMazzola, M. S., Schoenbach, K. H., Lakdawala, V. K., & Ko, S. T. (1989). Nanosecond optical quenching of photoconductivity in abulk GaAs switch. Applied Physics Letters, 55(20), 2102-2104. doi:10.1063/1.102076Nanosecond optical quenching of photoconductivity in a bulk GaAs switchM. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, and S. T. KoCitation: Appl. Phys. Lett. 55, 2102 (1989); doi: 10.1063/1.102076View online: http://dx.doi.org/10.1063/1.102076View Table of Contents: http://aip.scitation.org/toc/apl/55/20Published by the American Institute of PhysicsNanosecond optical quenching of photoconductivity in a bulk: GaAs sWItch M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawaia, and S. T. Ko Department of Electrical and Computer Engineering, Old Dominion Uni1 Jersity. Norfolk, Virginia 23529 (Received 15 May 1989; accepted for publication 11 September 1989) Persistent photoconductivity in copper-compensated, silicon-doped semi-insulating gallium arsenide with a time constant as large as 30 ItS has been excited by sub-band-gap laser radiation of photon energy greater than 1 eV. This photoconductivity has been quenched on a nanosecond time scale by laser radiation of photon energy less t.han 1 eV. The proven ability to turn the switch conductance on and off on command, and to scale the switch to high power could make this semiconductor material the basis of an optically controlled pulsed-power closing and opening switch. High-power, bulk, laser-controlled semiconductor switches are of interest for use in lightweight, medium to high power pulse generators. They have so far been mainly used as closing switches for applications requiring fast rise times and low jitter.! Closing of the switch is generaHy achieved by illuminating the switch with band-gap radi-ation, thus generating electron-hole pairs. The photocon-ductivity remains high for the duration of the laser pulse, and then decays in the absence of the laser pulse with a time constant determined by bulk recombination of free carriers or by carrier sweep out. In order to achieve short opening times, short electron-hole pair lifetimes are required. How-ever, a short electron-hole pair lifetime represents a large conductivity loss mechanism, which for many applications dictates an unacceptable expenditure of expensive laser pho-tons. An alternative has been proposed2 which we have called a bulk optically controlled semiconductor switch (BOSS). The BOSS utilizes the excitation of electrons from deep centers, such as provided by copper in copper-compensated silicon-doped semi-insulating GaAs (Cu:Si:GaAs), to in-duce photoconductivity. The small cross section for electron trapping into a Cu center allows for long conduction times without continuous laser radiation. Just as important, how-ever, BOSS may be interrupted on command by the applica-tion of a second laser pulse of longer wavelength. This laser pulse floods the valence band with free holes, thus inducing a rapid quenching of the photoconductivity over a time scale given by the electron-hole pair lifetime of the material, which can be subnanosecond. The excitation oflong-lived photoconductivity (known as "tail conductivity") at relatively high power has already been demonstrated. 3 The purpose of this letter is to present recent results that demonstrate strong switch opening by rapid laser-stimulated quenching of the tail conductivity. Low-resistivity, n-type GaAs can be made semi-insulat-ing by the introduction of suitable acceptors into the materi-al through diffusion. One defect is a deep acceptor known as CUB' which is located 0.44 eV above the valence band. The addition of the proper density of CUB levels into GaAs grown with a shallow donor will result in a highly resistive, compensated crystaL 4 The crystals used in this investigation wcre taken from a GaAs wafer grown using the horizontal Bridgman tech-nique. 5 The material was originally doped with silicon to a density of5 X 10 16 cm -:I. The crystals were then covered on one side with copper using a vacuum evaporator. The copper layers were approximately 1 pm thick. The samples were placed in a diffusion fumace and annealed in a low-pressure arsenic environment for 2 h. One sample was annealed at 550°C (:±: 1°C), while another was annealed at 575°C ( ± 1°C). Both samples were then polished and Au-Ge con-tacts were formed on one side in a planar geometry. Both samples are rectangular in shape with a length and width of about 5 mm. They are both 0.25 mm thick. The contacts are 3 mm wide and form a 2.5 mm gap across the surface of the crystal. The contacts were annealed at 450°C for 3 min, after which the dark current-voltage (1- V) characteristics were measured up to a voltage of 100 V. The contacts were ob-served to be injecting (ohmic). From the J- V characteristics, the resistivity of the crystal annealed at 550°C was found to be 103 n cm at 300 K. The resistivity of the crystal annealed at 575°C was found to be 9X 104 n cm, also at 300 K. Based on estimates of the Fermi level determined from these mea-surements, coupled with known solid solubility curves for copper in GaAs,6 we have conduded that the former crystal is undercompensated (n type), while the latter crystal is overcompensated (p type). Further references to the two samples will use these labels. Photoconductivity measure-ments for the p- and n-type samples are compared in Ref. 7, where the n-type sample is shown to be the better closing switch. This letter concentrates on the opening performance of the p-type sample. Photoconductivity measurements were performed to test the BOSS concept. Figure 1 illustrates the experimental apparatus. Two different lasers were used to anow two con-secutive laser pulses to illuminate the crystal, one for closing the switch, and the other to open it. A Nd:Y AG laser (A = 1,06 pm, hv = 1.17 e V) was used to induce the initial conductivity in the crystal (i.e., "turn on" the switch). The laser is Q switched, which results in a Gaussian-shaped tem-poral response with a full width at half maximum (FWHM) of 26 ns and a peak photon flux of 5 X 1023 em· 2 s - 1 (90 k W / cm2 ). The other laser pulse (the "tum-ofl''' pulse) was generated by a tunable laser system formed with a DCR-3 Nd:YAG laser, a PDL pulsed dye laser, and an IR-WEX LiNbO, mixing crystal, aU manufactured by Spectra Phys-ics. The laser has a center wavelength of 1.62 11m, which can betuned ± O.18.um (hv = 0.77 ± O.08eV). This laser has a 2102 Appl. Phys. Lett. 55 (20). 13 November 1989 0003-6951/89/462102-03$01.00 @ 1989 American Institute of Physics 2102 500 DIGITIZER (500 INPUT) NEUTRAL DENSITY FILTER FIG. j. Diagram iIlustrati~lg the experimental setup. Note that the crystal was illuminated en the face containing the contacts. . FWHM of 7 ns, and a peak photon flux of 5 X 1025 em 2 s - 1 (6 MW /cm 2 ) ifthe beam is focused onto the active area between the contacts (3mm X 3mm). The bias circuit consists of a capacitor charged to a dc voltage, and connected to the crystal by a long 50 n cable (two~way transit time = 275 ns). The initial bias voltage was varied from 40 to ~ SOV without affecting the photocon-ductivity of the crystal. In fact, the p-type crystal has been demonstrated to exhibit tail conductivity up to a voltage of 3 kV (12 kVlcm).3 The load is a 50 n oscilloscope termi~ nation, which acts as a current viewing resistor. These impe-dances impose a load line of 100 n on the switching charac-teristics of the crystal. The current waveform is digitized by a Tektronix 7912 digitizer. The complete response of the p-type sample to excita~ tion by both laser pulses is illustrated by a typical current waveform shown in Fig. 2. The crystal has first been excited by the 1.06 pm laser pulse, which causes a current overshoot to a peak of more than 0.6 A. The current then decays to a tail of about 0.4 A. After 200 ns, the 1.62 f-lm laser pulse is incident (photon fiux = 5 X 1025 cm~· 2 s 1), which initial-ly creates an increase in current that rapidly gives way to a o 6 0.5 0.4 -0.3 i\ I \ , \,--'--laO 200 300 TIME [ns] 4CO 500 FIG. 2. Measured current flowing through the p-type crystal. Sample con-ductivity is induced initially by the 1.06 j.lITl laser pulse. The sample conduc-tivity is quenched 200 [IS later by the 1,62 j.lm laser pulse. The initial hias voltage is 67 V. 2103 Appl. Phys. Lett., Vol. 55, No. 20, 13 November 1989 strong quench. The current decreases below the measure-ment resolution in under 10 ns. The response of the current through the p-type crystal when the second laser pulse wavelength was increased to 1.8 ,urn (hv = 0.69 eV) is shown in Fig, 3. The initial current increase in Fig. 2 associated with the second laser ,mise has completely disappeared in Fig. 3, with no appare~t reduc-tion in quenching. The intensity of the second laser pulse was varied from 5 v 1025 t 5 X 1022 ~, I . tId . /, 0 cm - S WItii neutra enslty filters. Figme 4 shows the intensity dependence of the quenching factor Q, which is defined as Q=!.;,o/uT , where !::.a is the difference between the initial tail conductiv-ity and the final quenched conductivity, and (Fr is the initial tail conductivity. The maximum value of Q that could be resolved was about 98%. Figure 4 indicates that Q decreases below 98% at a photon fiux of 3 X 1024 cm ~ 2 S ~ 1 . As the laser intensity decreases, the initial increase in conductivity just prior to the fast quench rapidly diminishes. The initial peak virtually disappears at a photon flux of 5 X 1024 em 2 s t. The sample required approximately 1 mJ of 1.06 pm laser energy to induce a tail conductivity of at least 0.5 (.n cm) ;; however, because of the long absorption length for light at wavelengths exceeding the band-gap cutoff, only a fraction of the incident radiation was absorbed (estimated from known cross sections2 to be slightly more than 1 % ). At the break point on Fig. 4 (photon flux = 3 X 1024 cm 2 s ~ 1 ), ~O.3 m1 oflascr energy at 1.62 f-lm was expended to quench the conductivity to a final value of about 0.01 en cm) ]. The absorption at this wavelength is expected to be stronger, but probably no more than 10%. Therefore, the energy required to turn off the switch is with~ in the same order of magnitude as the energy required to turn it on. The current response of the Cu:Si:GaAs switch to the two incident laser pulses can be understood by considering the electron gain and loss processes in the semiconductor. 0.5 0.5 (\, ~~ .... 0.4 z w I ' '~~-c: '" 0.3 -L_~_ OJ I u WJ -' 0.2 I a.. :>: '" I '" 0.1 L-.. --L.....---L_-"-_-'-----'_-..L_ . ..l-----'_-'---l laO 200 300 500 FIG. 3. Measured current flowing through the p-type crystal. Sample con-dUciivity is induced initially by the 1.06 pm laser pulse. The sample conduc-tivity is quenched 200 IlS later by the 1.8 j.lnl laser pulse. The initial bias voltage is 55 V. Mazzola et af. 2103 100 80 ~ '" 0 >- 60 >-(J ... "-'" z r u 40 z Ul ::> <> 20 0 10 22 :0 25 PHOTON FLUX [cm- 2s- 1] FIG. 4. Quem:hing factor as a function of the 1.62!llTI laser pulse peak inten-sity. The maximum quenching factor that could be resolved was 98%. The initial overshoot of the photocurrent (Figs. 2 and 3) is due to electron-hole generation by a two-step process through deep levels. After the first laser pulse is gone a por-tion of the electrons recombine rapidly with excess holes, either by direct recombination or through fast recombina-tion centers. In addition, electrons may be captured in fast traps, such as EL2. However, the bulk of the conduction electrons decay slowly, because further decay is controlled by the slow capture processes associated with CUB' The second laser pulse applied after about 200 ns quenches the photocurrent. However, for laser wavelengths below 1.75 pm, the photoconductance shows a spike just before the onset of quenching (see Fig. 2). This initial in-crease in photocurrent is thought to be a two-photon process. Based on published values of the two-photon ab-sorption coefficientS we estimate that for our experiment two-photon excitation must be considered when the photon flux exceeds 1025 cm - 2 S - 1 • A two-photon process will not occur for photon energies less than half the band gap. The fact that the current spike became insignificant when the wavelength was increased to a value above 1.75 pm or the intensity was decreased to a value below 1025 cm- 2 s 1 2104 Appl. Phys. Lett., Vol. 55, No. 20, 13 November 1989 supports the two-photon interpretation of the switch re-sponse. As the second pulse intensity was reduced below the break point on Fig. 4, the fall time of the conductivity did not increase beyond 10 ns. Thus, the electron-hole pair lifetime must be substantially less than the laser pulse width. This fact points, in our opinion, to the presence of recombination centers in the Cu:Si:GaAs crystal, which could have been introduced either during the Cu diffusion or during crystal growth. It is known that recombination center concentra-tions as small as 1015 cm 3 in GaAs can result in a suhnano-second electron-hole pair lifetime, 1 which would explain the observed rapid quenching of the photocurrent. The feasibility of the BOSS concept has been successful-ly demonstrated with p-type Cu:Si:GaAs. Optically con-trolled closing and opening can be achieved on a nanosecond time scale. According to a rate equation model of the switch, currents on the order of kiloamperes should be controllable with millijoule laser pulses, thus making the material suit-able for pulsed power applications. Furthermore, since the concept is not restricted to one particular material, as has been demonstrated with CdS,9 the shift of the controlling radiation into the visible range should be possible with wider band-gap semiconductors. The authors acknowledge J. Wigginton and S. Ains-worth for their valuable help in sample preparation. This work was supported by the Strategic Defense Initiative Or-ganization/lrmovative Science and Technology and man-aged by the Office of Naval Research under contract No. NOOO-14-86-k-560. The program monitor for this project is G.Roy. 1 C. H. Lee, ed. Picosecond Optoelectronic Devices (Academic, Orlando, FL, 1985), pp. 75-373. 'K. H. Schoenbach, V. K. Lakdawala, R. Germer, and S. T. Ko, J. App!. Phys. 63, 2460 (1988). 3 M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, R. Germer, G. M. Loubriei, and F. J. Zutavern, App!. Phys. Lett. 54, 742 (1989). 4 J. Blanc, R. H. Bube, and H. E. MacDonald. J. App!. Phys. 32, 1666 (1961 ). 'Grown by Morgan Semiconductor Div., Ethyl Corp., Garland, TX 75047-2376. 6 R. N. Hall and J. H. Racette, J. App!. Phys. 35, 379 ( 1964). 7 M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, and S. T. Ko. in Technical Digest of the Seventh IEEE Pulsed Power Conference, Monte-rey. California, June 1989, paper 16-2. "A. F. Stewart and M. Bass, Appl.l'hyso Lett. 37,1040 (1980). "R. K. F. Germer, K. H. Schoenbach, and S. G. E. Prollko. J. App!. Phys. 64,913 (1988). Mazzola et al. 2104 

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Nanosecond Optical Quenching of Photoconductivity in a Bulk GaAs Switch

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