- Email: [email protected]
Optically activated planar GaAs switches for DC applications
Nuclear Instruments and Methods in Physics Research A 418 (1998) 434—439
Optically activated planar GaAs switches for DC applications A. Cola *, F. Quaranta , L. Vasanelli , E. Bertolucci, M. Conti, P. Russo, G. Bisogni, U. Bottigli, M. Fantacci, A. Stefanini, R. Fucci, G. Melone, R. Rossi, G. Stefanini Istituto per lo Studio di Nuovi Materiali per l+Elettronica (I.M.E.), C.N.R, Via Arnesano, I-73100 Lecce, Italy Dipartimento Scienze Fisiche, Universita% di Napoli e Sezione INFN Napoli, via Cintia, I-80126 Napoli, Italy Dipartimento di Fisica, Universita% di Pisa e Sezione INFN Pisa, Via Vecchia Livornese 582a, I-56010 S. Piero a Grado (Pi), Italy Optel InP, Cittadella della Ricerca, SS7 per Mesagne Km 7#300, I-72100 Brindisi, Italy Cern, Geneva, Switzerland Received 27 February 1998; received in revised form 14 May 1998
Abstract We report the successful demonstration of an optically activated GaAs switches for DC applications. The results obtained suggest that such devices can be used to control a static voltage drop of several hundreds of volts across gas microstrip detectors for high-energy experiments. The response of passivated/non-passivated devices and also for proton irradiated devices has been evaluated as a function of the incident light intensity and of the applied voltage. 1998 Elsevier Science B.V. All rights reserved. PACS: 85.60.G Keywords: GaAs; PCSS; High-voltage switches
1. Introduction Photoconductive semiconductor switches (PCSS) have unique advantages such as high breakdown field, high speed, negligible time jitter and long lifetime. Optically activated photoconductive switches made from semi-insulating (SI) GaAs were proposed in the late 1970s for use as both closing and opening high power switches [1]. Up to now, the application field regards pulsed-
* Corresponding author. Tel.: #39 0 832 320 243; fax: #39 0 832 325 299; e-mail: [email protected].
power systems with typical applied voltage in the range of 10 kV, flowing current of hundreds of Ampere, and modulation on the timescale of nanoseconds. However, since the static conductivity can be switched over many orders of magnitude at relatively low optical excitation density and low applied voltage, a simple application of these devices can be controlling the DC voltage drop across a load. In high-energy physics experiments one problem to face is the control of the voltage bias of microstrip gas chambers in large hadron collider (LHC) experiments. This is due to the fact that these
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 7 6 - 1
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
microstrips can become leaky and not efficient; when this happens, the applied bias must be reduced to recover the correct gas conditions in the microstrip detectors; afterwards, it is possible to rise the applied voltage again to the working value. For this application, there are many strict requirements to be satisfied: the control system should be cheap, simple and able to switch efficiently the voltage drop across the detector between a low value and the operative one, which is in this case about 600 V. Moreover, the switches should be installed locally, close to the detector which are in turn adjacent to the point of the particles generation; therefore it should be trusty, radiation resistant, compact, and with an efficient remote control. The simplicity and the low-cost of the semiconductor optical switches, the possibility of integrating many devices on typical microelectronic scale, and of controlling the state of the switch by coupling the devices with optical fibres, lead us to believe that semiconductor switches are good candidates for the microstrip voltage control and other application with similar constraints. In this work, we have studied the behaviour of GaAs optical switches in planar configuration based on a conventional metal/Si-GaAs/metal structure. In order to obtain low dark currents we have used blocking contacts and, therefore, our structures could be better classified as photodetectors rather than as photoconductors (which make use of ohmic contacts). We have designed, built and characterized many switches. We performed a test of the optical switches in series with a resistive load supposed to simulate a microstrip detector load. The study has been carried out as a function of the external voltage bias and of the intensity of the optical laser beam used to turn the switch into the on state. The effect of surface passivation of the device has been studied. Finally, the devices have been irradiated with protons in order to evaluate the degradation of the device performance.
435
10 cm V\ s\. The etch pit density is lower than 5000 cm\ and the thickness of the wafers is 200 lm. The geometry of the switches has been defined by standard photolitography using lift-off technique. Before contact depositions the patterned surface was etched in a solution HCl : H O"1 : 1 at 0°C for 10 s. After rinsing in deionized water and drying with nitrogen flow, a 500 A> Ti/750 A> Pd/2000 A> Au multilayer was evaporated in a fully automated electron beam system at a base pressure of 10\ mbar. The surface of some samples were passivated by depositing a silicon nitride (Si N ) film. The 2000 A> thick films were deposited at a temperature of 300°C in a PECVD diode reactor from a silane, ammonia and nitrogen mixture. After the deposition, the Si N film has been etched from the contact regions in a standard RIE reactor. In Fig. 1 shape and dimensions of the contacts are shown. The common contact (anode) is comb shaped and it is constituted by eight teeth; it has pitch of 600 lm, a tooth width of 100 lm and it works as ground; the eight individual contacts (cathodes), each of them placed in front of the corresponding tooth, have the dimension 100; 200 lm. We have built devices with cathode—anode distances of 300, 450, and 600 lm. A three terminal box has been assembled to bias three of the eight switches at the same time; the box
2. Experimental The switches were prepared starting from VGF (100) SI-GaAs wafers. Resistivity is 2;10— 10;10 )cm and mobility is 5.8;10—7.1;
Fig. 1. Geometry of planar contacts used. The distance a between the electrodes was 300, 450, and 600 lm. The individual switch contacts on the right were negatively biased, and the other contact was grounded.
436
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
is able to monitor the voltage drop across a load of 5 G) which is in series with each of the three switches. The value of the resistive load has been chosen equal to the expected load of a group of 64 microstrips which should be driven by each switch. The external voltage has been provided by a 237 Keithley source-measure unit. The voltage drop across the load has been measured across a 1 M) precision resistance (in series with the 5 G) resistance) using a HP 34401A multimeter. This measurement scheme allows us to measure either the total current flowing in the three devices and the individual ones. The bias has been applied using probe points pressed against the contacts and varied from 0 to !700 V in steps of 50 V. An average of one hundred voltage readings has been made to reduce the error. Some current measurements have been performed without the resistive load; in this case, under optical excitation, the current measurements have been taken during voltage pulses of 20 ms, to avoid heating. We have used a focused laser beam to individually activate the switches. The laser emits a continuous power of 3 mW, the spot at the focus is about 50 lm and the wavelength is 630 nm. Neutral filters have been used to reduce the incident intensity to a factor 10\ of the original one. The laser has been mounted on a multi-axis stage in order to control the xyz positioning with a precision of the order of a few microns. The sample has been placed in a cryostat thus allowing us to perform measurements at a controlled temperature. The proton irradiation has been performed at CERN. The total proton fluence was 1.1; 10 cm\ and the energy of the proton beam was 24 GeV. After the irradiation the samples have been kept at about 260 K for about two weeks before the measurements.
3. Results As a preliminary check, the current—voltage curve of a single switch has been measured under dark and under laser illumination in order to qualitatively verify that an adequate variation was reached for the current. The results are reported in
Fig. 2. Typical current—voltage curve of a single device under dark (empty circles) and illuminated (full circles) conditions. For comparison, the curve corresponding to a resistive load of 5 G) is reported (full line).
Fig. 2 and, for comparison, the curve corresponding to 5 G) resistance has been also reported. We note that the dark curve does not show breakdown effect up to the highest voltage and the current values are quite lower than those corresponding to 5 G) resistance. In order to avoid a device degradation, the measurement in the on condition has been performed only up to 250 V; even so, the photocurrent is more than five orders of magnitude greater than the dark current and much higher than the current flowing through a 5 G) resistance. Indeed, the photocurrent values easily reach 10 mA at 200 V. When the load is inserted in series the current is limited to much lower values. Such characteristics indicate that these devices, in principle, can be efficient switches. Thus, we have characterised all the devices using the measurement scheme previously described. Since the optical switch is in series with the load (5 G)) it is clear that when the light is incident on the switch, the applied voltage should drop mainly across the load. Therefore, this case corresponds to the case of the microstrip correctly working. On the contrary, when the light is off, most of the applied voltage should drop at the electrodes of the switch. We measured the ratio » /» which repre sents the voltage dropping on the load » over the total applied voltage » applied to switch and load. Fig. 3a reports » /» versus » when the switch is off (dark), for the devices with different
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
437
Fig. 4. » /» ratio as a function of » ; I is the fraction of the maximum beam intensity (3 mW over a 50 lm spot). The data refer to the passivated sample having a distance between contacts a"450 lm. Lines are guides for the eyes.
Fig. 3. » /» ratio as a function of the total applied voltage » , for the devices in the switch-off state (dark). The squares, triangles and circles refer to switches with different distances between the contacts: 300, 450, and 600 lm, respectively; Fig. 3a refers to non-passivated samples and Fig. 3b to passivated samples. Lines are guides for the eyes.
distances between the contacts (300, 450, and 600 lm). This quantity is lower than 10% around 600 V. The slight decrease of » /» with » is due to the corresponding increase of the static resistance of the switch. Fig. 3b concerns the sample with the GaAs surface passivated. In this case the values are lower, of the order of 2.6% at the maximum voltage (when 700 V are applied, 18 V drop on the load). In both cases, the sample with the closest contacts (300 lm) shows the highest value of » /» . This could be due to the begin ning of a soft reversible break-down. From these results we can draw the first indication that the switch is able to sustain almost the totality of the applied voltage when it is in the off-state.
Fig. 5. » /» ratio as a function of the voltage, for the proton irradiated passivated devices in the switch-off state (dark). The squares, triangles and circles refer to the different distances between the contacts: 300, 450, and 600 lm. Lines are guides for the eyes.
In Fig. 4 we report the behaviour of a passivated switch for various values of the intensity of the laser beam; in particular » /» is reported as a func tion of » for the beam not attenuated, for beam intensity attenuated up to six orders of magnitude, and for dark conditions. After the proton irradiation there is an acceptable degradation of the device performance. Fig. 5 shows the behaviour of the passivated sample in the switch-off state; as can be seen, at high voltages, » /» increases up to 10—12%. The nonpas sivated sample has the same behaviour, and » /» increases up to 12—14%.
438
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
Fig. 6. » /» ratio at 700 V, as a function of the intensity of the laser beam, for the proton irradiated (circles) and the nonirradiated (squares) device. The data refer to the passivated sample having distance between the contacts a"450 lm. Lines are guides for the eyes.
In Fig. 6 we report » /» at 700 V as a func tion of the light intensity for the irradiated and non-irradiated devices (intensity equal to 1 means that no optical filters have been used). From the figure it can be seen that the effect of the irradiation can be described as a shift to the right of the curve corresponding to the non-irradiated device by almost three orders of magnitude and a raising of the dark baseline from 2.5% to 10%. Therefore, the switch-on state has been affected by the irradiation, but » /» stays over 95%. 4. Discussion The presented results are very encouraging for DC applications of planar optically activated SIGaAs switches. The current flowing through each device does not scale with the distance between contacts as a simple resistive model could suggest. This is consistent with the fact that the contacts are not ohmic but blocking ones and therefore the current in our devices is electrode-limited. The active region is localised around the negatively biased electrodes, which are the individual switches. Moreover, the main geometrical dimension of the contacts affecting the current is the width (which is constant in our switches and equal to 100 lm). This should be taken into account especially when
planning a given dark current flowing in planar devices. From Fig. 2 we notice that the photo-current over dark current ratios greater than 10 are achieved. Such ratio can be further increased by rising the light power or by decreasing the contact dimensions, responsible of the dark current. The high photocurrents are indicative of gain mechanism, i.e. the current flowing per photo-generated electron/hole pair is greater than the current due to the single pair itself. It is worthwhile to observe that gain usually occurs in standard photoconductive devices, which make use of ohmic contacts. However, even in metal/semiconductor/metal structures like ours, gain mechanisms have been recently observed and interpreted [2]. The devices can sustain average electric fields up to about 20 kV/cm which is greater than the saturation field observed [3] in SI-GaAs X-ray detectors (10—15 kV/cm), but in agreement with the value reported for planar optically activated GaAs switches [4]. As can be inferred from Fig. 1 the geometry of the contact gives rise to a remarkably non-uniform electric field distribution in the device. This reduces the effective depth of the active region (the high field region between the anode and the cathode) but nevertheless it does not reduce the photo-current to low useless value. Even if in our devices, the values of dark current and of the ratio between light on and light off currents are satisfactory for the envisaged application, improvements can be obtained by reducing the contacts width and/or their relative distance. The used intensity of the laser is high enough to efficiently put the switch in a state much more conductive than the load. For the irradiated samples, the curve reported in Fig. 6 shows a shift of almost three orders of magnitude on the horizontal axis and the maximum intensity of the laser should be increased to possibly reach the 100% of » /» . The curves in Fig. 6 are not very steep, and for each curve, a variation of the laser intensity over three order of magnitude can easily control the quantity » /» over the 10%—90% range. However, even though the switch performance worsens after the proton irradiation, it is still possible to recover the 10%—90% range of » /» by vary ing the light intensity over five orders of magnitude.
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
The planar geometry has many advantages such as the simplicity of the coupling with light (semitransparent contacts are not necessary), and the possibility to process only one surface; furthermore, the lines of force of the electric field are concentrated beneath the surface in a limited layer where the light is absorbed; this assures an optimum collection of the photo-generated carriers, as we have indeed detected. Another advantage of planar switches is the independence of contact distance with respect to the wafer thickness. On the other hand, surface effects are more important. Our study on the devices with the passivated surface confirms a better performance of these treated devices. Acknowledgements Thanks are due to Dr. F. Lemeilleur and Dr. G. Casse from CERN for their support in carrying out
439
the proton irradiation. This work has been partially supported by INFN and by the European Community.
References [1] G. Mourou, W. Knox, Appl. Phys. Lett. 35 (1979) 492. [2] M. Klingenstein, J. Kuhl, J. Rosenzweig, C. Moglestue, A. Hulsmann, Jo. Schneider, K. Kohler, Solid State Electron. 37 (1994) 333. [3] A. Castaldini, A. Cavallini, L. Polenta, C. Canali, C. del Papa, F. Nava, Phys. Rev. B 56 (1997) 9201; Y.F. Hu, C.C. Ling, C.D. Beling, S. Fung, J. Appl. Phys. 82 (1997) 3891. [4] G.M. Loubriel et al., IEEE Trans. Electron. Dev. 38 (1991) 692.
Optically activated planar GaAs switches for DC applications A. Cola *, F. Quaranta , L. Vasanelli , E. Bertolucci, M. Conti, P. Russo, G. Bisogni, U. Bottigli, M. Fantacci, A. Stefanini, R. Fucci, G. Melone, R. Rossi, G. Stefanini Istituto per lo Studio di Nuovi Materiali per l+Elettronica (I.M.E.), C.N.R, Via Arnesano, I-73100 Lecce, Italy Dipartimento Scienze Fisiche, Universita% di Napoli e Sezione INFN Napoli, via Cintia, I-80126 Napoli, Italy Dipartimento di Fisica, Universita% di Pisa e Sezione INFN Pisa, Via Vecchia Livornese 582a, I-56010 S. Piero a Grado (Pi), Italy Optel InP, Cittadella della Ricerca, SS7 per Mesagne Km 7#300, I-72100 Brindisi, Italy Cern, Geneva, Switzerland Received 27 February 1998; received in revised form 14 May 1998
Abstract We report the successful demonstration of an optically activated GaAs switches for DC applications. The results obtained suggest that such devices can be used to control a static voltage drop of several hundreds of volts across gas microstrip detectors for high-energy experiments. The response of passivated/non-passivated devices and also for proton irradiated devices has been evaluated as a function of the incident light intensity and of the applied voltage. 1998 Elsevier Science B.V. All rights reserved. PACS: 85.60.G Keywords: GaAs; PCSS; High-voltage switches
1. Introduction Photoconductive semiconductor switches (PCSS) have unique advantages such as high breakdown field, high speed, negligible time jitter and long lifetime. Optically activated photoconductive switches made from semi-insulating (SI) GaAs were proposed in the late 1970s for use as both closing and opening high power switches [1]. Up to now, the application field regards pulsed-
* Corresponding author. Tel.: #39 0 832 320 243; fax: #39 0 832 325 299; e-mail: [email protected].
power systems with typical applied voltage in the range of 10 kV, flowing current of hundreds of Ampere, and modulation on the timescale of nanoseconds. However, since the static conductivity can be switched over many orders of magnitude at relatively low optical excitation density and low applied voltage, a simple application of these devices can be controlling the DC voltage drop across a load. In high-energy physics experiments one problem to face is the control of the voltage bias of microstrip gas chambers in large hadron collider (LHC) experiments. This is due to the fact that these
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 7 6 - 1
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
microstrips can become leaky and not efficient; when this happens, the applied bias must be reduced to recover the correct gas conditions in the microstrip detectors; afterwards, it is possible to rise the applied voltage again to the working value. For this application, there are many strict requirements to be satisfied: the control system should be cheap, simple and able to switch efficiently the voltage drop across the detector between a low value and the operative one, which is in this case about 600 V. Moreover, the switches should be installed locally, close to the detector which are in turn adjacent to the point of the particles generation; therefore it should be trusty, radiation resistant, compact, and with an efficient remote control. The simplicity and the low-cost of the semiconductor optical switches, the possibility of integrating many devices on typical microelectronic scale, and of controlling the state of the switch by coupling the devices with optical fibres, lead us to believe that semiconductor switches are good candidates for the microstrip voltage control and other application with similar constraints. In this work, we have studied the behaviour of GaAs optical switches in planar configuration based on a conventional metal/Si-GaAs/metal structure. In order to obtain low dark currents we have used blocking contacts and, therefore, our structures could be better classified as photodetectors rather than as photoconductors (which make use of ohmic contacts). We have designed, built and characterized many switches. We performed a test of the optical switches in series with a resistive load supposed to simulate a microstrip detector load. The study has been carried out as a function of the external voltage bias and of the intensity of the optical laser beam used to turn the switch into the on state. The effect of surface passivation of the device has been studied. Finally, the devices have been irradiated with protons in order to evaluate the degradation of the device performance.
435
10 cm V\ s\. The etch pit density is lower than 5000 cm\ and the thickness of the wafers is 200 lm. The geometry of the switches has been defined by standard photolitography using lift-off technique. Before contact depositions the patterned surface was etched in a solution HCl : H O"1 : 1 at 0°C for 10 s. After rinsing in deionized water and drying with nitrogen flow, a 500 A> Ti/750 A> Pd/2000 A> Au multilayer was evaporated in a fully automated electron beam system at a base pressure of 10\ mbar. The surface of some samples were passivated by depositing a silicon nitride (Si N ) film. The 2000 A> thick films were deposited at a temperature of 300°C in a PECVD diode reactor from a silane, ammonia and nitrogen mixture. After the deposition, the Si N film has been etched from the contact regions in a standard RIE reactor. In Fig. 1 shape and dimensions of the contacts are shown. The common contact (anode) is comb shaped and it is constituted by eight teeth; it has pitch of 600 lm, a tooth width of 100 lm and it works as ground; the eight individual contacts (cathodes), each of them placed in front of the corresponding tooth, have the dimension 100; 200 lm. We have built devices with cathode—anode distances of 300, 450, and 600 lm. A three terminal box has been assembled to bias three of the eight switches at the same time; the box
2. Experimental The switches were prepared starting from VGF (100) SI-GaAs wafers. Resistivity is 2;10— 10;10 )cm and mobility is 5.8;10—7.1;
Fig. 1. Geometry of planar contacts used. The distance a between the electrodes was 300, 450, and 600 lm. The individual switch contacts on the right were negatively biased, and the other contact was grounded.
436
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
is able to monitor the voltage drop across a load of 5 G) which is in series with each of the three switches. The value of the resistive load has been chosen equal to the expected load of a group of 64 microstrips which should be driven by each switch. The external voltage has been provided by a 237 Keithley source-measure unit. The voltage drop across the load has been measured across a 1 M) precision resistance (in series with the 5 G) resistance) using a HP 34401A multimeter. This measurement scheme allows us to measure either the total current flowing in the three devices and the individual ones. The bias has been applied using probe points pressed against the contacts and varied from 0 to !700 V in steps of 50 V. An average of one hundred voltage readings has been made to reduce the error. Some current measurements have been performed without the resistive load; in this case, under optical excitation, the current measurements have been taken during voltage pulses of 20 ms, to avoid heating. We have used a focused laser beam to individually activate the switches. The laser emits a continuous power of 3 mW, the spot at the focus is about 50 lm and the wavelength is 630 nm. Neutral filters have been used to reduce the incident intensity to a factor 10\ of the original one. The laser has been mounted on a multi-axis stage in order to control the xyz positioning with a precision of the order of a few microns. The sample has been placed in a cryostat thus allowing us to perform measurements at a controlled temperature. The proton irradiation has been performed at CERN. The total proton fluence was 1.1; 10 cm\ and the energy of the proton beam was 24 GeV. After the irradiation the samples have been kept at about 260 K for about two weeks before the measurements.
3. Results As a preliminary check, the current—voltage curve of a single switch has been measured under dark and under laser illumination in order to qualitatively verify that an adequate variation was reached for the current. The results are reported in
Fig. 2. Typical current—voltage curve of a single device under dark (empty circles) and illuminated (full circles) conditions. For comparison, the curve corresponding to a resistive load of 5 G) is reported (full line).
Fig. 2 and, for comparison, the curve corresponding to 5 G) resistance has been also reported. We note that the dark curve does not show breakdown effect up to the highest voltage and the current values are quite lower than those corresponding to 5 G) resistance. In order to avoid a device degradation, the measurement in the on condition has been performed only up to 250 V; even so, the photocurrent is more than five orders of magnitude greater than the dark current and much higher than the current flowing through a 5 G) resistance. Indeed, the photocurrent values easily reach 10 mA at 200 V. When the load is inserted in series the current is limited to much lower values. Such characteristics indicate that these devices, in principle, can be efficient switches. Thus, we have characterised all the devices using the measurement scheme previously described. Since the optical switch is in series with the load (5 G)) it is clear that when the light is incident on the switch, the applied voltage should drop mainly across the load. Therefore, this case corresponds to the case of the microstrip correctly working. On the contrary, when the light is off, most of the applied voltage should drop at the electrodes of the switch. We measured the ratio » /» which repre sents the voltage dropping on the load » over the total applied voltage » applied to switch and load. Fig. 3a reports » /» versus » when the switch is off (dark), for the devices with different
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
437
Fig. 4. » /» ratio as a function of » ; I is the fraction of the maximum beam intensity (3 mW over a 50 lm spot). The data refer to the passivated sample having a distance between contacts a"450 lm. Lines are guides for the eyes.
Fig. 3. » /» ratio as a function of the total applied voltage » , for the devices in the switch-off state (dark). The squares, triangles and circles refer to switches with different distances between the contacts: 300, 450, and 600 lm, respectively; Fig. 3a refers to non-passivated samples and Fig. 3b to passivated samples. Lines are guides for the eyes.
distances between the contacts (300, 450, and 600 lm). This quantity is lower than 10% around 600 V. The slight decrease of » /» with » is due to the corresponding increase of the static resistance of the switch. Fig. 3b concerns the sample with the GaAs surface passivated. In this case the values are lower, of the order of 2.6% at the maximum voltage (when 700 V are applied, 18 V drop on the load). In both cases, the sample with the closest contacts (300 lm) shows the highest value of » /» . This could be due to the begin ning of a soft reversible break-down. From these results we can draw the first indication that the switch is able to sustain almost the totality of the applied voltage when it is in the off-state.
Fig. 5. » /» ratio as a function of the voltage, for the proton irradiated passivated devices in the switch-off state (dark). The squares, triangles and circles refer to the different distances between the contacts: 300, 450, and 600 lm. Lines are guides for the eyes.
In Fig. 4 we report the behaviour of a passivated switch for various values of the intensity of the laser beam; in particular » /» is reported as a func tion of » for the beam not attenuated, for beam intensity attenuated up to six orders of magnitude, and for dark conditions. After the proton irradiation there is an acceptable degradation of the device performance. Fig. 5 shows the behaviour of the passivated sample in the switch-off state; as can be seen, at high voltages, » /» increases up to 10—12%. The nonpas sivated sample has the same behaviour, and » /» increases up to 12—14%.
438
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
Fig. 6. » /» ratio at 700 V, as a function of the intensity of the laser beam, for the proton irradiated (circles) and the nonirradiated (squares) device. The data refer to the passivated sample having distance between the contacts a"450 lm. Lines are guides for the eyes.
In Fig. 6 we report » /» at 700 V as a func tion of the light intensity for the irradiated and non-irradiated devices (intensity equal to 1 means that no optical filters have been used). From the figure it can be seen that the effect of the irradiation can be described as a shift to the right of the curve corresponding to the non-irradiated device by almost three orders of magnitude and a raising of the dark baseline from 2.5% to 10%. Therefore, the switch-on state has been affected by the irradiation, but » /» stays over 95%. 4. Discussion The presented results are very encouraging for DC applications of planar optically activated SIGaAs switches. The current flowing through each device does not scale with the distance between contacts as a simple resistive model could suggest. This is consistent with the fact that the contacts are not ohmic but blocking ones and therefore the current in our devices is electrode-limited. The active region is localised around the negatively biased electrodes, which are the individual switches. Moreover, the main geometrical dimension of the contacts affecting the current is the width (which is constant in our switches and equal to 100 lm). This should be taken into account especially when
planning a given dark current flowing in planar devices. From Fig. 2 we notice that the photo-current over dark current ratios greater than 10 are achieved. Such ratio can be further increased by rising the light power or by decreasing the contact dimensions, responsible of the dark current. The high photocurrents are indicative of gain mechanism, i.e. the current flowing per photo-generated electron/hole pair is greater than the current due to the single pair itself. It is worthwhile to observe that gain usually occurs in standard photoconductive devices, which make use of ohmic contacts. However, even in metal/semiconductor/metal structures like ours, gain mechanisms have been recently observed and interpreted [2]. The devices can sustain average electric fields up to about 20 kV/cm which is greater than the saturation field observed [3] in SI-GaAs X-ray detectors (10—15 kV/cm), but in agreement with the value reported for planar optically activated GaAs switches [4]. As can be inferred from Fig. 1 the geometry of the contact gives rise to a remarkably non-uniform electric field distribution in the device. This reduces the effective depth of the active region (the high field region between the anode and the cathode) but nevertheless it does not reduce the photo-current to low useless value. Even if in our devices, the values of dark current and of the ratio between light on and light off currents are satisfactory for the envisaged application, improvements can be obtained by reducing the contacts width and/or their relative distance. The used intensity of the laser is high enough to efficiently put the switch in a state much more conductive than the load. For the irradiated samples, the curve reported in Fig. 6 shows a shift of almost three orders of magnitude on the horizontal axis and the maximum intensity of the laser should be increased to possibly reach the 100% of » /» . The curves in Fig. 6 are not very steep, and for each curve, a variation of the laser intensity over three order of magnitude can easily control the quantity » /» over the 10%—90% range. However, even though the switch performance worsens after the proton irradiation, it is still possible to recover the 10%—90% range of » /» by vary ing the light intensity over five orders of magnitude.
A. Cola et al./Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 434—439
The planar geometry has many advantages such as the simplicity of the coupling with light (semitransparent contacts are not necessary), and the possibility to process only one surface; furthermore, the lines of force of the electric field are concentrated beneath the surface in a limited layer where the light is absorbed; this assures an optimum collection of the photo-generated carriers, as we have indeed detected. Another advantage of planar switches is the independence of contact distance with respect to the wafer thickness. On the other hand, surface effects are more important. Our study on the devices with the passivated surface confirms a better performance of these treated devices. Acknowledgements Thanks are due to Dr. F. Lemeilleur and Dr. G. Casse from CERN for their support in carrying out
439
the proton irradiation. This work has been partially supported by INFN and by the European Community.
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