The influence of the copper concentration in silicon-doped gallium arsenide on the photoionization and photoquenching of charge carriers was studied both experimentally and theoretically. The studies indicate that the compensation ratio (NCu/NSi) is an important parameter for the GaAs:Si:Cu switch systems with regard to the turn-on and turn-off performance. The optimum copper concentration for the use of GaAs:Si:Cu as an optically controlled closing and opening switch is determined

Ko, St. T.

Lakdawala, V. K.

Mazzola, M. S.

Schoenbach, K. H.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsElectrical & Computer Engineering FacultyPublications Electrical & Computer Engineering1990Influence of Copper Doping on the Performance ofOptically Controlled GaAs SwitchesSt. T. KoOld Dominion UniversityV. K. LakdawalaOld Dominion University, vlakdawa@odu.eduK. H. SchoenbachOld Dominion UniversityM. S. MazzolaOld 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 CitationKo, St. T.; Lakdawala, V. K.; Schoenbach, K. H.; and Mazzola, M. S., "Influence of Copper Doping on the Performance of OpticallyControlled GaAs Switches" (1990). Electrical & Computer Engineering Faculty Publications. 90.https://digitalcommons.odu.edu/ece_fac_pubs/90Original Publication CitationKo, S. T., Lakdawala, V. K., Schoenbach, K. H., & Mazzola, M. S. (1990). Influence of copper doping on the performance of opticallycontrolled GaAs switches. Journal of Applied Physics, 67(2), 1124-1126. doi:10.1063/1.345780Influence of copper doping on the performance of optically controlled GaAs switchesS. T. Ko, V. K. Lakdawala, K. H. Schoenbach, and M. S. MazzolaCitation: Journal of Applied Physics 67, 1124 (1990); doi: 10.1063/1.345780View online: http://dx.doi.org/10.1063/1.345780View Table of Contents: http://aip.scitation.org/toc/jap/67/2Published by the American Institute of PhysicsInfluence of copper doping on the performance of optically controlled GaAs switches s. T. Ko, v. K. Lakdawala, K. H. Schoenbach, and M. S. Mazzola Department of Electrical and Computer Engineering. Old Dominion Uniuersity, Norfolk. Virginia 23529 (Received 5 July 1989; accepted for publication 3 October 1989) The influence of the copper concentration in silicon-doped gallium arsenide on the photoionization and photoquenching of charge carriers was studied both experimentally and theoretically. The studies indicate that the compensation ratio (N CU IN Si ) is an important parameter for the GaAs:Si:Cu switch systems with regard to the turn-on and turn-off performance. The optimum copper concentration for the use of GaAs:Si:Cu as an optically controlled closing and opening switch is determined. Recently Sehoenbach et al. introduced a novel concept for a bulk optically controlled semiconductor switch (BOSS), which can be turned on and turned off by two lasers with different wavelengths. I The feasibility of the BOSS concept has been demonstrated with CdS:Cu (Ref. 2) and with GaAs:Si:Cu (Refs. 3 and 4). In this communica-tion we investigated silicon-doped gallium arsenide with dif-fused copper (GaAs:Si:Cu). Si introduces a shallow donor level [Es; = Ee - 0.0058 eV (Ref. 5)] in GaAs and Cu in-troduces two primary deep acceptor levels (Ecu = Ev + 0.14 eV and ECll = Ev + 0.44 eV).6 At thet-A B mal equilibrium, most of the electrons ionized from the shal-low donor are trapped at the deep acceptors. Hence the ini-tial electron occupation of the deep acceptor levels is determined by the density of the shallow donor. The switch concept is based on photoionization of the deep acceptor (tum-on) and then the subsequent photoquenching of the previously generated free carriers (turnoff). Photoquench-ing of the BOSS is obtained by creating free holes through the photoexcitation of electrons from the valence band to the CUB level. The free electrons at the conduction band can then recombine with the free holes at the valence band by direct band-to-band recombination and by recombination through recombination centers. Since GaAs is a direct semi-conductor material, relatively fast turn-off can be achieved through direct band-to-band recombination. The turn-off transient characteristics can be further improved by intro-ducing fast recombination centers such as Cr into the GaAs material, which can lead to subnanosecond turn-off times. 7 Two GaAs samples were prepared.4 One was an n-type sample where the shallow donor, Si, was unciercompensated by the deep acceptor Cu (Nsi = 5X 1O t6 cm- 3, Ncu "",4.5 X 1016 cm-3 ). The other was p type, where the shallow donor Si was overcompensated by the deep acceptor Cu (NSi = 5X 1016 cm- 3,N Cu = 1017 cm -3). The base ma-terial, GaAs material doped with Si at a concentration of 5 X 1016 em -3, has been characterized by deep-level tran-sient spectroscopy (DLTS).8 Two defect levels located at 0.83 and 0.41 eV below the conduction band were detected and identified as theEL 2 level (NEL2 = 7 X lotS cm- 3 ) and the EL 5 (NELS =2X lOIS cm-- 3 ) level, respectively (Fig. 1 ).9 These defects are known as native electron traps in GaAs and affect the on-state performance of the switch by capturing free electrons at the traps. A possible fast recombi-nation center introduced during Cu diffusion is chromium (Cr) located at about the middle of the band gap. The pa-rameters for the fast recombination center were estimated from the photoconductivity decay of the test samples. The estimated value of the density of the recombination center which is denoted as RC in Fig. 1 is NRC = 3 X lO J4 cm- 3• The electron-capture cross section ( Un ) and the hole-cap-ture cross section (ap ) are estimated as Un = 10- 13 cm2 and ap = 1O~ 13 cm2, respectively.7 The energy-level diagram of GaAs:Si:Cu material is shown in Fig. 1. The concentration ofCuA i.s about 20% of CUB where Neu = Neun.n The temporal variation of the photoconductivity of the two switch samples is shown in Fig. 2, ANd: Y AG laser was used to turn the switches on [wavelength A = 1.06,um, peak photon flux >1>1= 1024 cm- 2 S-I, fun width half maximum (FWHM) = 26 ns]. A tunable infrared laser system was used to turn the switches off (wavelength A = 1.8 ,urn, peak photon flux $2 = 1025 cm -2 S-I and FWHM = 7 ns). The conductances and conductivity of the two switch samples were calculated from photocurrent measurements and nor-malized to eliminate the geometrical differences between the two samples. Higher on-state conductivity was obtained in the n-type sample than the p-type sample. The experimental results clearly show the existence of long-lived "tail" conductivity (on-state), which is attribut-ed to the electron contribution from the deep acceptor (eu B ). It can be seen from the figure that the p-type sample (overcompensated) provided better turn-off performance Ec ~~~~~~~~~----~~~----~ Si - TO.cOSS aV 0,41 eV 0.83 eV f El5-~ 0.71 eV RC _______ EL2-i 0.44 eY CUA_ ...L 1 Ev 0,14 eY f 1.42 eV 1 FIG. L Energy-level diagram of the BOSS (GaAs:Si:Cu) system used for the simulation. 1124 J. Appl. Phys. 67 (2).15 January 1990 0021-8979/90/021124-03$03.00 © 1990 American Institute of Physics 1124 1.5 -- g N-rYPE -- ~ P-TYPE >-l->-~ Ul I u :::> a z 0 u 0 w N 3 0.5 .;( ::f: r.r i \ 0 :z I '-...... J -----0.0 0 100 200 300 400 TIME ens] FIG. 2. Experimental resillts (lfthe photo-condllctivity measurement ill the two samples. compared with the n-type (undercompensated) sample and that the turn-off transient time of the switches are on the order of nanoseconds. This fast decay cannot be explained by direct radiative recombination; it indicates the existence of fast recombination centers in the samples. Based on the experimental data and deep-level param-eters from various papers, to the switch systems have been numericaHy simulated. The model uses rate equations for the densities offree electrons, free holes, and bound electrons at aU trap levels. l • 1O The rate equations include the Auger recombination process and two-photon ionization process. In the previous model,l recombination through fast recom-bination centers was included in the direct recombination coefficient kd • Here, recombination through fast recombina-tion centers is considered separately from direct band-to-band recombination. Assumptions made in the model are that the temporal profile of the laser pulses are Gaussian, and that electron transitions between defect levels are negli-gible. Figure 3 shows the result obtained from the numerical simulation for the two switch samples. Two important aspects of the BOSS are clearly visible: the existence of the long tail conductivity (on-state) and fast photoquenching (turn-off). It is also seen from the figure that n-typematerial provides better on-state conductivity while p-type material gives stronger photoquenching during the turn-off phase of the switch. The decay time of the photoquenching is on the order of nanoseconds. Th.e simulated curve has a slower de-cay rate during the on-state (after the turn-on pulse) than the experimental curve. This discrepancy is believed to be due to surface recombination, which was not considered in our model. Two important parameters determining the on-state conductivity and the photoquenching are the photoioniza-tion rate of electrons kef and the photoionization rate of holes kill at the CUB level, which are kef = ~«plnCuB' khf = a;!«PZ(NCUB - neUE)' 1125 J. AppL Phys., Vol. 67, No.2, 15 January 1990 (1) (2) 1.5 >-I-:> i= 1.0 u :::> 0 z 0 u 0 UJ N :::; « 0.5 Ji: a:: 0 z 0.0 0 iA \ --, N-T'fPE ...... - .. I F'-TVPE \., ,., ....... _ ..... _ ... , ............... , J \ 100 200 300 TIME [1'15] y,- -1L TURN-ON LASER 1-\;1 " 1.1 eV TURN-OFF LASER 1"1)) " 0.6S eV 400 FIG. 3. Simulated photoconductivity of the two sampiesduring the switch-ing operation. where <P 1 is the turn-on photon flux, 4>2 the turn-off photon fiux, d~ the electron photo-ionization cross section, O'~ the hole photoionization cross section, ncus the density of CUB centers occupied by electrons, and NCUB the total density of CUB centers. In the BOSS, the electron ionization from the CUB level to the conduction band is the process that deter-mines the on-state conductivity of the switch, because elec-tron ionization at the laser wavelength of 1.06 pm is possible only through the CUB level. The n-type material has a higher electron concentration ncun , and hence it has a higher elec-tron ionization rate kef than the p-type material. Conse-quently higher on-state conductivity is obtained in the n-type sample compared to the p-type sample. In the p-type sample, on the other hand, the hole ionization rate is higher than for the n-type material because of a higher hole concen-tration (Neue - llcuB). Hence the p-type sample provides better photoquenching. These facts are clearly demonstrated in the experimental as wen as in the calculated results. Further numerical simulations were performed to pro-vide a basic guideline for optimizing the Cu concentration with respect to the photoquenching efficiency. The calculat-ed on-state (before tum-off laser irradiation) and turn-off (after turn-off laser irradiation) conductivity as a function of the compensation ratio, N eu IN Si are plotted in Fig. 4. The results show that the higher the eu density the lower is the on-state conductivity. On the other hand, the higher the Cu density the higher is the quenching ratio (the ratio ofturn-on to turn-off conductivity). A small change in the eu con-centration affects the turn-off characteristics of the switch drastically. The choice of the eu concentration in the GaAs switch material depends on the application of the switch itself. The undercompensated or funy compensated material (n type, N CU IN Si < 1) is suitable for closing switches with a high on-Ko etaf. i125 'I ....., E " E J: c3 >-I-> ;::: U :::,) 0 z 0 U 10 2 10' 100 10-1 10-2 10 -3 10-4 10-5 0.5 0--8 ON A- I OFF 1.0 1.5 2.0 FrG. 4. On-state and turn-off conductivity as a function of the compensa-tion ratio (N cu/Nsi)' Parameters used in thc calculation are turn-on laser photon flux (<P,) of2X 1024 cm- 2 8·', turn-olflaser photon flux (<P 2 ) of 1025 cm- 2 s .', and a shallow donor density (N.,;) of 5 X 10'6 cm 3. state conductivity at a given turn-on photon flux. The over-compensated material (p type, ]V eu IN Si > 1), on the other hand, is better suited for opening switches because of more efficient quenching. For the use ofthe system as both a clos-ing and opening switch, there exists an optimum Cu concen-tration because the quenching ratio is proportional to Cu concentration while the on-state conductivity is inversely proportional to it. For example, if strong photoquenching (conductivity change greater than 4 orders of magnitude) and relatively high on-state conductivity is desired, the opti-mum eu density would be 8X 1016 cm- l (N culNsi = 1.6). Both experimentally observed and calculated photocon-ductivities have been presented, showing the dependence of the BOSS performance on the Cu concentration. This depen-dence allows one to generate GaAs:Si:Cu switches for a wide variety of applications by changing the concentration of the impurity in the GaAs material over relatively small ranges. These variations can easily be controlled by changing the Cu diffusion process in GaAs:Si. This work was supported by the Strategic Defense Ini-tiative Organization-Innovative Science and Technology and managed by the Office of Naval Research under Con-tract No. NOOO-14-86-k-S60. The program monitor for this project is G. Roy. 'K. H. Schoenbach, V. K. LakdawaIa, R. Germer, and S. T. Ko, J. AprL Phys. 63, 2460 (1988). "R. K. F. Germer, K. H. Schoenbach, and S. G. E. Pronko, I. App!. Phys. 64,913 (1988). 3M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, R. Germer, G. M. Loubriel, and F. 1. Zutavern, App!. Phys. Lett. 54,742 (1989). 4M. 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. 5S. M. Sze and J. C. Irvin, Solid-State Electron. 11, 599 (1968). "D. V. Lang and R. A. Logan, J. Electron. Mater. 4,1053 (1975). 7J. S. Weiner and P. Y. Yu, J. App\. Phys. 55, 3889 (1984). "D. V. Lang, J. App\. Phys. 45,3023 (1974). 9S. T. Ko, Ph. D. thesis, Old Dominion University, Norfolk, VA (1989). 105. T. Ko. V. K. Lakdawala, K. H. Schoenbach, and M. S. Mazzola, in Technical Digest of the Seventh IEEE Pulsed Power Conference, Monte-rey, California, June 1989. paper P3-8. The theoretical basis of application of fractal to fracture physics Xinhua Wu, Ziyong Zhu, and Wei Ke Institute 0/ Corrosion and Protection of Metals, Academia Sinica. 110015, Shenyang, People's Republic of China (Received 4 May 1989; accepted for publication 18 September 1989) A theoretical basis to support the application of the fractal approach in fracture physics is put forward. According to the theory, if Griffith's criterion is approved, the fractal approach can be used. Fractal and fractal geometry was first proposed by Mandelbrot in 1975. 1 He used this geometry to describe ir-regular or fragmentai objects. In recent years, fractal geome-try has been used in more and more fields of science; for example, fracture physics, materials science, etc. Through the analysis of specimen surface2 or failure surface,3,4 the fractal dimension Df and the relationship between Df and residual life, fracture toughness Kic were obtained. But there is no theoretical basis to support the application of fractal approach in fracture physics. This paper is going to solve this problem. The problem has been studied by Dong.5 The con-clusion is that the fractal approach is in conflict with Grif-fith's criterion. 1126 J. Appl. Phys. 61 (2), 15 January 1990 His calculation is as follows: The criterion of crack growth is 0) where r is the unit surface energy, and rp is unit surface plastic energy. For sman range yield, we can suppose: lp >2r. (2) Thus, G 1C = rp ' 0021-8979/90/021126-02$03.00 © ; 990 American Institute of Physics 1126 

Influence of Copper Doping on the Performance of Optically Controlled GaAs Switches

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Influence of Copper Doping on the Performance of Optically Controlled GaAs Switches

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