The dark current characteristics of gallium arsenide doped with silicon and compensated with diffused copper were found to have a pronounced region of current controlled negative differential conductivity (ndc) similar to the characteristics of a thyristor. The resistivity of the semi‐insulating semiconductor was measured to be 105 Ω cm for applied voltages up to 2.2 kV, which corresponds to an average electric field of 38 kV/cm. At higher voltages, a transition to a stable high current state was observed with a current rate of rise exceeding 1011 A/s. There is evidence of the formation of at least one current filament during this transition. A theoretical model based on drift diffusion and boundary conditions that allows double carrier injection at the contacts has been used to show that the observed negative differential resistance is due to the filling of deep copper acceptors. The model also shows that the ndc curves may be tailored by adjusting the copper concentration. Doping of GaAs with various concentrations of copper was shown to change the dark current characteristics in a way predicted by the model

Brinkmann, R. P.

Roush, R. A.

Schoenbach, K. H.

English

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Old Dominion University

Old Dominion UniversityODU Digital CommonsBioelectrics Publications Frank Reidy Research Center for Bioelectrics1992Bistable Behavior of the Dark Current in Copper-Doped Semi-Insulating Gallium ArsenideR. A. RoushOld Dominion UniversityK. H. SchoenbachOld Dominion UniversityR. P. BrinkmannOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/bioelectrics_pubsPart of the Electrical and Electronics CommonsThis Article is brought to you for free and open access by the Frank Reidy Research Center for Bioelectrics at ODU Digital Commons. It has beenaccepted for inclusion in Bioelectrics Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationRoush, R. A.; Schoenbach, K. H.; and Brinkmann, R. P., "Bistable Behavior of the Dark Current in Copper-Doped Semi-InsulatingGallium Arsenide" (1992). Bioelectrics Publications. 264.https://digitalcommons.odu.edu/bioelectrics_pubs/264Original Publication CitationRoush, R. A., Schoenbach, K. H., & Brinkmann, R. P. (1992). Bistable behavior of the dark current in copper‐doped semi‐insulatinggallium arsenide. Journal of Applied Physics, 71(9), 4354-4357. doi:10.1063/1.351365Bistable behavior of the dark current in copper-doped semi-insulating gallium arsenide R. A. Roush,al K. H. Schoenbach, and R. P. Brinkmannbl Physical Electronics Research Institute, Old Dominion University, Norfolk, Virginia (Received 25 November 1991; accepted for publication 29 January 1992) The dark current characteristics of gallium arsenide doped with silicon and compensated with diffused copper were found to have a pronounced region of current controlled negative differential conductivity (ndc) similar to the characteristics of a thyristor. The resistivity of the semi-insulating semiconductor was measured to be 105 ,0, cm for applied voltages up to 2.2 kV, which corresponds to an average electric field of 38 kV /cm. At higher voltages, a transition to a stable high current state was observed with a current rate of rise exceeding 1011 A/s. There is evidence of the formation of at least one current filament during this transition. A theoretical model based on drift diffusion and boundary conditions that allows double carrier injection at the contacts has been used to show that the observed negative differential resistance is due to the filling of deep copper acceptors. The model also shows that the ndc curves may be tailored by adjusting the copper concentration. Doping of GaAs with various concentrations of copper was shown to change the dark current characteristics in a way predicted by the model. INTRODUCTION The high dark resistivity of semi-insulating GaAs, which is often used as a figure of merit for semi-insulating semiconductors, is only defined for relatively low voltages. The ohmic region of the dark current in semi-insulating GaAs reaches up to voltages which correspond to average electric fields in the kV/ cm to several tens of kV/ cm range, dependent on the fabrication technique, doping, and the contact spacing. Above the so-called trap-filled limit volt-age, V TFL• the dark current rises dramatically with voltage. 1'2 This well known behavior of semi-insulating GaAs can be explained as a trap filling effect due to single carrier injection,2 generally electron injection, in samples with only one type of contact (n or p). With increasing current density, the probability for injection of the second type of carriers, generally holes, rises due to the buildup of space charges in the vicinity of the non-injecting contact.3 Eventually, double injection can be expected, leading to filament formation. In semi-insulating semiconductors that contain large concentrations of deep acceptors, and with hole capture cross sections much larger than electron capture cross sec-tions, it has been predicted that double injection leads to negative differential resistivity regions in the dark current characteristics. 2 A material which satisfies these capture cross-section conditions is silicon doped, copper compen-sated gallium arsenide ( GaAs:Si:Cu). The base material is silicon doped GaAs with silicon concentrations in the range from 1016 to 1017 cm - 3• GaAs:Si is an n-type semi-conductor, and it can be compensated with diffused cop-per, which forms several deep acceptors.4 Thermal anneal-ing at temperatures of 560 °C for IO h with a thin layer of •>present address: Dahlgren Division, Naval Surface Warfare Center, Dahlgren, VA 22448-5000. b)Present address: Siemens A. G., Munich, Germany. copper ( 1 µm) deposited on the surface of the GaAs:Si wafer, has resulted in a semi-insulating material with re-sistivities of up to 107 ,0, cm. The compensated semicon-ductor is used as switch material in bulk optically con-trolled switch (BOSS) devices, which are a new type of photoconductive switch that can be turned on and off using light of different wavelengths. 5'6 For best switching results, the semiconductor needs to be slightly overcompensated with copper (slightly p type).7 This is obtained by increas-ing the annealing temperature above the value required for complete compensation. We have investigated experimentally the temporal de-velopment of the dark current in GaAs:Si:Cu over a wide range of voltages which correspond to average electric fields of up to 103 kV /cm. A numerical code which in-cludes the effect of the dominant defects and traps in semi-insulating GaAs8 was used to model the dark current char-acteristics, and to predict its dependence on type and concentration of deep centers. This information is of im-portance for the design of electronic devices, such as pho-toconductive switches or integrated circuits, where the in-sulating properties of GaAs are important. EXPERIMENT Two silicon doped, copper compensated gallium ars-enide samples were studied with respect to their dark cur-rent characteristics. The low voltage dark resistivity was 1 X 105 ,0, cm for sample No. 1, and 2X 106 11 cm for sam-ple No. 2. Using the results of the fabrication procedure, it is assumed that No. 1 contained a higher concentration of deep acceptors than No. 2. The samples were rectangular in size with an area of 5 mm X 5 mm, and thicknesses of 0.058 and 0.061 cm, respectively. The circular contacts with a diameter of 3 mm, were placed on the top and bottom face of the sample. They were formed using a gold-4354 J. Appl. Phys. 71 (9), 1 May 1992 0021-8979/92/094354-04$04.00 © 1992 American Institute of Physics 4354 RG-8 5 Mn 100ft/200ft }---,,------,----<>DIGITIZER #1 520 FIG. I. Circuit used to measure the dark current-voltage characteristics of GaAs:Si:Cu samples. germanium alloy,9 deposited by thermal evaporation and annealed under N2 at 450 •c for 10 min. The dark current measurements were carried out using a voltage pulse generator, shown in Fig. 1, which produces voltage pulses of 300 or 600 ns depending on the length of the RG-8 cable, which was used as the pulse forming net-work. The current through the circuit was obtained by measuring the voltage across the 10 .n current viewing resistor (CVR), which is in series with the GaAs:Si:Cu. The voltage across the GaAs:Si:Cu and CVR was mea-sured in order to obtain the sample voltage. The current and voltage measurements were made using two Tektronix 7912 digitizers. The sample was mounted in a 50 .n mi-crostrip line in order to maintain the necessary circuit bandwidth. To prevent surface flash-over, the 50 .n micro-strip line was mounted in a quartz pressure chamber filled to about 25 psig SF6• The dark current increased monotonically with voltage for applied voltages up to 600 V (9 kV /cm) for No. 2, and 2.2 kV (38 kV /cm) for No. 1. Above these voltage values, a dramatic change in dark current was observed. Figure 2 shows the temporal development of both sample current and voltage (sample No. 1) for an applied voltage of 3.3 kV. For about 120 ns, the dark current is on the order of 100 mA (not within the resolution of current scale in Fig. 2), and then transits into a high current mode with current levels three orders of magnitude above the initial value at 80 4 70 - , ... , 60 I '' •. / ... ., .... , I 3 I I I I I I 50 I I ~ I I > I I I I ~ !z 40 I 2 w I w I \ (!l a: I ' ' ~ a: 30 I ' ::, I -, ...I CJ I '----- .... 0 \ > 20 I I I I I I I I I 10 I I \ I \ I \ 0 I '~ 0 ~ -~ 0 100 200 300 400 500 TIME [ns] FIG. 2. The temporal development of the dark current (solid line), and voltage (dashed line) for sample No. I. 4355 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 ~ !z w a: a: :::, 0 103 102 -•· 101 10° 10·1 10·2 ••• 10-3 10-4 V V ..:pV~ • ..... 10•5 .___...__,____._,__...._~_._ __ _._ _ __._J 0.5 2 VOLTAGE [kV} FIG. 3. The dark current-voltage curves for GaAs:Si:Cu sample, No. I (solid dots) and No. 2 (open triangles). only slightly lower voltages. In Fig. 3, the maximum cur-rent values are plotted versus the corresponding voltage data. Sample No. 1, which is more p type (or more over-compensated) than No. 2, transitions into the high current state at a much higher voltage. The difference in hold-off voltage (maximum voltage with subampere dark current) is even more striking when compared to liquid-encapsu-lated crystal (LEC) grown semi-insulating GaAs of the same thickness, 1 which only holds about 200 V. It should be noted that the highest measured value of the voltage at low currents is not necessarily the maximum hold-off volt-age, but is determined by the load line of the electrical circuit. With higher impedance circuits, even larger values of hold-off voltage might be measured. The delay time for the onset of the dark current devel-opment is strongly dependent on the applied voltage. This is shown in Fig. 4. In this figure, the delay time is defined as the time difference between the onset of the high current 6 0 [ • w 5"" (!l ~ • 0 > • C • w :::i 4 -0. 0. < • • • 3 ' ' ' 0 20 40 60 80 100 120 DELAY TIME [nsl FIG. 4. The dependence of the delay time for the onset of high dark current flow on the applied voltage. Roush, Schoenbach, and Brinkmann 4355 pulse and the time at which the peak voltage is reached (see Fig. 2). Zero onset time implies that the current de-velopment occurs immediately after the peak voltage is reached ( on the time scale of the experiment). The rise-time of the dark current transition is also dependent on the applied voltage. For applied voltages of 6 kV (103 kV/ cm), the initial current rise to 30 A occurred in less than 500 ps, which corresponds to a current rise of greater than 1011 A/s. MODEL The steady-state dark current characteristics of silicon doped copper compensated gallium arsenide, was modeled using the drift-diffusion equations. 8 The GaAs crystal was assumed to contain EL2 centers (activation energy= 0.8 eV) with a concentration of 5 X 1015 cm - 3• Copper is known to form several deep acceptors in GaAs:Si, two of which are called CuA and Cua. The activation energies for CuA, and Cua are 0.14, and 0.44 eV, respectively. 10 The electron and hole capture cross sections of CuA, Cua, and EL2 are as follows: CuA: 2.7X 10- 17 cm2, 4X 10- 15 cm2; Cua: 8 X 10 - 21 cm2, 3 X 10- 14 cm2; EL2: 8 X 10- 13 cm2, 3 X 10- 16 cm2, respectively. 10,11 Note that for the copper centers, the hole capture cross sections are much larger than the electron capture cross sections. This con-dition is used in an analytical analysis by Lampert2 which shows negative differential resistance in the dark current-voltage characteristics of doped GaAs using double injec-tion conditions. Double injection is a condition which allows the free flow of electrons through the cathode contact, and holes through the anode contact. In other words, the electric field is zero at the contacts. Experimentally, this condition is not satisfied at low current densities when Au:Ge con-tacts are used on GaAs due to then-type Ge dopant, which creates a p-n barrier at the anode. At high current densi-ties, however, the boundary conditions may change from single injection to double injection as the anode barrier is overcome. 3 This holds particularly when filamentation oc-curs, which in turn is related to the experimentally ob-served negative differential resistivity. The results of the numerical calculations are shown in Fig. 5. There is clearly a qualitative agreement between this curve and the experimentally obtained current-voltage curves. By proper choice of the defect and impurity con-centrations and their capture cross sections which are only vaguely known, it is certainly possible to also get a quan-titative agreement of model and experiment. However, this is not the purpose of this calculation. The purpose is to demonstrate the strong effect of deep centers on the dark current of semi-insulating semiconductors, which allows us to tailor devices with very peculiar dark current character-istics, such as these with negative differential resistance. The model also accurately predicts the variation in the negative differential resistivity with the concentration of copper in GaAs. By reducing the copper concentration, the ndr becomes less pronounced and disappears completely if all copper centers are removed. 4356 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 104 103 ~ "' E 102 ~ ~ ~ 101 en 100 z w C I- 10·1 z w a: 10·2 a: ::) (.) 10-3 10-4 100 101 102 103 104 VOLTAGE [V] FIG. 5. The computed dark current-voltage curve. SUMMARY The temporal development of the dark current in sili-con doped, copper compensated, GaAs was measured. At voltages that correspond to average electric fields in the tens of kV /cm range, the dark current changes from a low current mode into a high current mode with a current ratio of three to four orders of magnitude. At applied voltages of more than 2 kV across the 0.6 mm sample, these transi-tions from the mA range to the tens of A range occurred on a subnanosecond timescale. The corresponding current-voltage curve shows a pro-nounced region of negative differential resistance. The ex-tent of this region depends on the doping density of copper. Specifically, for a given silicon concentration, an increase in copper density causes a more pronounced S-shaped re-gion of ndr. The highest hold-off voltage before the transi-tion into the high current range was measured to be 2.3 kV across a 0.6 mm sample, which corresponds to an average electric field of 40 kV /cm. These are values which are generally only obtained with semi-insulating GaAs which is heavily damaged by neutron irradiation. The material which we have studied has, in spite of this high hold-off voltage, electron mobilities of about 4000 cm2 /V s. 11 The measured negative differential resistance in GaAs:Si:Cu ex-plains the occurrence of current filamentation in this material. 12•13 Samples destroyed during the experiments were found to have been damaged by current filaments with cross sections of 10- 3 cm2. Using the measured cur-rent values, current densities as high as 105 A/cm2 may have developed through such a filament. The closely com-pensated material was damaged by only a few of the high current pulses; however, the over-compensated material was not damaged after many high current pulses. The shape of the de current-voltage curve can be ex-plained by considering trap filling processes. 1•2 Due to the large hole capture cross section of the copper centers, the hole lifetime at low voltages will be less than the hole transit time from positive to negative contact. With in-Roush, Schoenbach, and Brinkmann 4356 creasing voltage, however, the hole lifetime will increase due to the increased number of filled traps. Eventually the hole lifetime will reach and exceed the transit time, and because of the establishment of an electron-hole plasma in the entire bulk, a large increase in current at reduced volt-ages will result. This mechanism is also assumed to cause lock-on in GaAs:Si:Cu.6•14 After illumination of semi-insulating GaAs with lasers or electron beams, the current continues to flow even after termination of the ionization source, if the voltage across the sample exceeds a certain level, which is in the range of several kV/ cm to several tens of kV/ cm. The radiation only speeds up the process of hole-trap fill-ing, and therefore reduces the delay for the transition into the high dark current phase essentially to zero. The numerical model describes the qualitative behav-ior of the dark current very well. The accuracy of the model is limited by our knowledge of the deep center pa-rameters in semi-insulating material. The model does, how-ever, allow us to predict changes in the electrical charac-teristics of semi-insulating GaAs which can be expected when the type and concentration of deep centers is varied. This allows us to tailor semi-insulating semiconductors with respect to their electrical characteristics. This is of importance for devices where the performance is strongly dependent on such parameters as hold-off voltage, and dark resistance. The high hold-off voltage and the S-shaped 4357 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 dark current curve of GaAs:Si:Cu are of particular impor-tance to high power photoconductive switches. ACKNOWLEDGMENT This work was supported by SDIO/IST and managed by ONR. The program monitor for this project is G. Roy. 1 D. C. Stoudt, K. H. Schoenbach, R. P. Brinkmann, V. K. Lakdawala, and G. A. Gerdin, IEEE Trans. Electron Dev. 37, 2478 (1990). 2 M. A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970), pp. 15-40, 140-154, 207-215, 277-324. 3 M. S. Mazzola, R. A. Roush, D. C. Stoudt, and S. F. Griffiths, Appl. Phys. Lett. 59, 1182 (1991 ). 4J. Blanc, R.H. Bube, and H. E. MacDonald, J. Appl. Phys. 32, (1961). 5 K. H. Schoenbach, V. K. Lakdawala, R. Germer, and S. T. Ko, J. Appl. Phys. 63, 2460 (1988). 6 M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, and S. T. Ko, Appl. Phys. Lett. 55, 2102 ( 1989). 7S. T. Ko, V. K. Lakdawala, K. H. Schoenbach, and M. S. Mazzola, J. Appl. Phys. 67, 1124 (1990). 8 R. P. Brinkmann, J. Appl. Phys. 68, 318 (1990). 9 N. Braslau, J. Vac. Sci. Technol. 19, 803 (1981). 10 D. V. Lang and R. A. Logan, J. Electron. Mater. 4, 1053 (1975). 11 S. T. Ko, PhD dissertation, Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia, 1989. 12 A. M. Barnett, "Current Filament Formation," in Semiconductors and Semimetals-Injection Phenomena, edited by R. K. Willardson and A. C. Beer (Academic, New York, 1970), pp. 141-200. 13 A. M. Barnett and H. A. Jensen, Appl. Phys. Lett. 12, 341 (1968). 14 M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, and R. A. Roush, Trans. Electron Dev. 37, 2499 ( 1990). Roush, Schoenbach, and Brinkmann 4357 

Bistable Behavior of the Dark Current in Copper-Doped Semi-Insulating Gallium Arsenide

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Bistable Behavior of the Dark Current in Copper-Doped Semi-Insulating Gallium Arsenide

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