- Email: [email protected]
On the use of pulsed microbeam in IBIC
Nuclear Instruments and Methods in Physics Research B 210 (2003) 176–180 www.elsevier.com/locate/nimb
On the use of pulsed microbeam in IBIC M. Jaksic *, Z. Medunic, N. Skukan Department of Experimental Physics, Rud-er Boskovi c Institute, P.O. Box 180, 10002 Zagreb, Croatia
Abstract In order to expand the application possibilities of ion beam induced charge (IBIC) technique to a wider range of semiconductor materials and devices, three novel modes of measurements have been developed. The fast beam deflector was used to produce ns pulses of single ions and the trigger signal provides a time scale for time resolved IBIC measurements. Longer pulses (>100 ns) containing tens or hundreds of ions were used to enhance IBIC signals in materials with poor charge transport properties. Continuous beam current (10 pA to 1 nA) of 3 MeV protons was used to measure ion beam induced voltage (IBIV) in thin amorphous silicon films. In these samples it was possible to correlate the charge collection properties with elemental distribution measured simultaneously with PIXE. Radiation damage was shown to be negligible for doses below 107 protons lm2 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Charge transport processes in semiconductor materials and devices became, in recent decade, an important topic of nuclear microprobe application in material science and electronics [1]. However, in practice, materials with high concentration of traps and in general with poor charge transport properties, or sample regions where only diffusion contribute to the ion beam induced charge (IBIC) signal, could not be studied with current capability of IBIC setup. In this work, an attempt towards the better utilization of IBIC is demonstrated by the use of recently constructed fast electrostatic deflector that provides ion pulses over a wide range of durations.
*
Corresponding author. Tel.: +385-1-4680-942; fax: +385-14680-239. E-mail address: [email protected] (M. Jaksic).
Two modes of pulsed beam operation were developed. The first one is a short pulse mode with a single ion arriving at a sample within the ns time. This mode could be important in time resolved IBIC experiments with the required time resolution close to 1 ns. The trigger signal is needed to provide the exact time scale between the moment of ion hit and time evolution of detected charge signal. The second mode is a long pulse (100 ns to 10 ls) mode with tens, hundreds, or thousands of ions arriving in bunch to the sample. Such an ion bunch could be particularly useful for the IBIC analysis of high defect density materials where single ion does not produce measurable signal. In samples such as amorphous silicon (a-Si) thin film solar cells, where the typical thickness of PIN layer is of several hundred nm, the energy loss for 3 MeV protons is less than 10 keV. Under such condition measurable signal can be obtained from sample irradiation with proton beam with constant
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01051-6
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
current of about 10 pA. As it will be explained, one can measure ion beam induced voltage (IBIV) at the electrodes.
V
2. Experimental
V
The Zagreb nuclear microprobe setup has been recently upgraded with a new scattering chamber and a fast beam deflector positioned immediately after the object slits. Scattering chamber of a spherical shape is equipped with large number of ports, where most of them are in backward angles, to provide simultaneous use of different ion beam analysis (IBA) techniques. With a load lock system, samples are introduced into the sample holder having controllable temperature. The beam deflector uses fast high voltage push– pull MOSFET switch (Behlke Electronics Gmbh) having 40 ns rise time for 1 kV pulses. For the short pulse mode operation, a beam passes through the collimator slits during the rise time of the switch between positive and negative voltage at one of the vertically positioned deflectors. First measurement of the time distribution, using STIM Si particle detector and conventional NIM modules, showed time resolution of 0.9 ns. In the long pulse mode operation, the 0 V pulses with duration between 200 ns and 40 ls are send to the deflector plate that is connected to HV. In both cases the second deflector plate is grounded. Frequency of pulses can be adjusted to maximum 8 kHz. Alternatively, external trigger pulse can be used as well. The deflector system is schematically presented in Fig. 1. In such conditions, object and collimator slit openings remain unchanged which makes operation of low current techniques such as STIM and IBIC very convenient to use. For the 10 nA beam current measured in the microprobe chamber with pulser turned off, a short pulse mode of 800 Hz repetition rate will typically provide about 200 cps of single ion pulses. The ion rate can be adjusted by pulser frequency, and beam current, to any desired value. On the other side, long pulse mode allows ions to pass through the microprobe system in bunches of tens, hundreds or thousands of ions within a microsecond. The beam current and the
+1kV 0 -1kV
0 -1kV
177
short pulses < 1ns
long pulses > 200ns
V
deflector
Fig. 1. Schematic presentation of high voltage pulses that drive deflector and ion beam path between the object and collimator slits of the microprobe system.
pulse length will determine the number of ions and therefore the total particle energy of the bunch. Such bunches of the broad beam were already used by Taccetti [2]. They could be applied to test linearity of detector and/or detector electronics providing total energies of up to GeV.
3. Measurements 3.1. Short pulse mode Short pulses of single ion produced in the present setup provide a trigger signal with exact known time of ion hitting a sample. This ability is significant in IBIC studies of charge transport behavior in regions where only diffusion contribute to the charge pulse. Depending on the distance between the ion hit position and region where charge carriers start to drift in the electric field, one can simultaneously measure diffusion length and diffusion constant of the material under study [3]. As an example we have analyzed a simple EFG (edge-defined film-fed grown) silicon Schottky diode in the frontal geometry, close to the edge of the contact and including the vertical twin boundary [4]. Fig. 2 shows images of a conventional pulse height IBIC image as well as two
178
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
well as for the samples that have much larger diffusion length as compared to the one presented here. Due to the difficult processing of the small and slow rise time signals from diffusion regions using conventional timing electronics, errors are possible. These can be avoided if a digitizing oscilloscope of the TRIBIC setup is used in such circumstances. 3.2. Long pulse mode The main motivation behind the use of long pulse mode is to study by IBIC samples that have too weak response to single ions. With a very stable beam current, larger ion bunches following the Poisson distribution have well defined number. This number can be increased according to the need of the sample under investigation. In Fig. 3, a spectrum of a series of 3 MeV proton bunches, has been measured by silicon STIM detector. The mean of distribution is at 210 MeV that corresponds to 70 particles in the bunch. The width of distribution is in this case dominated by beam current fluctuations. Application example is given in Fig. 4, where a high defect density EFG Si sample has been imaged by 3 MeV single proton IBIC, as well as with multiple ion IBIC. It can be seen that long pulse mode can provide higher data statistics and significant improvement in image contrast.
400
Intensity
300
Fig. 2. Optical photograph of the EFG silicon Schottky diode (up), IBIC image of this sample with low and high pulse height windows (middle) and images of time distribution in fast and slow time windows (down). The scan size is 500 1000 lm2 .
200
100
images that correspond to period of pulse arrival, namely t ¼ 0 and t ¼ 100 ns (50 ns). One can see, as expected, delayed arrival of pulses further away from contact region where only diffusion contribute to the carrier transport. It is expected however that much better ÔtimeÕ resolution should be obtained in lateral geometry of IBIC experiment, as
0
0
200
400
600
800
1000
1200
Channel number
Fig. 3. Energy spectrum of the mode with long pulses as measured by STIM detector. Discrete peaks correspond to the number of 3 MeV protons in the bunch.
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
179
Fig. 4. IBIC images of high defect density EFG silicon Schotky diode in frontal geometry. Left image correspond to a single proton IBIC measurement, while the right spectrum corresponds to the bunches containing in average 10 protons of 3 MeV. The scan size is 500 1000 lm2 .
Although similar effect can be obtained using heavy ions, flexibility in determination of ion energy to be deposited in pulse may be advantageous in practice. 3.3. Continuous beam – measurements of voltage Studies of charge collection properties by IBIC in layers that have thickness less than a micrometer, are difficult to perform since the energy loss of ions typically used in nuclear microprobes is in-
sufficient (e.g. 3 MeV proton in Si loses 20 keV in 1 lm). However, since radiation damage is in such conditions lower as well, it could be possible to study such samples with much higher currents, as those that are typically used in high current experiments used for elemental imaging by PIXE and other IBA techniques. In our studies of amorphous silicon solar cells (a-Si) in which PIN layer is 350 nm thick over the areas of some cm2 , conventional IBIC setup can not be used due to high sample capacitance.
Fig. 5. PIXE and IBIV image of a-Si solar cell. The scan size is 800 800 lm2 .
180
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
Therefore, we connected sample electrodes to a sensitive digital multimeter (DMM) for the measurement of ion beam induced voltage (IBIV). In order to test influence of radiation induced damage for a-Si samples, proton microbeam was scanned over different areas of sample. With a maximum 1000 1000 lm2 scanning area and rather high current of 1200 pA, measured voltage dropped to the half of the initial value after 400 s. From these measurements we showed that the dose limit when radiation damage becomes noticeable (voltage decreases to 95% of initial value) is at 107 3 MeV protons per lm2 . Following this result, the beam current was decreased to 10 pA value for the 800 800 lm2 scanning area. The DMM was adjusted to measure the IBIV in a slow sampling mode. In such condition one measurement point had to be at least 2 s which gives 800 s measurement time for the 20 20 pixel image. Although this was not needed for the samples we studied, simultaneous PIXE imaging is also possible. The example of simultaneous PIXE and IBIV analysis is given in Fig. 5, where charge collection properties can be correlated with elemental distribution, which is in this case Al in contact layer.
4. Conclusions During the development of IBIC as technique, several improvements in technology of measure-
ments were made. Time resolved measurements, temperature dependence of charge transients, use of different ions having wide range of sampling depths, etc., are some of these that opened new areas of IBIC applications. The use of pulsed beam controlled by deflector presented here is justified in simpler operation needed to switch from the high current to the low current mode in nuclear microprobe, as well as in wider application possibilities. Triggered signal of ns resolution provided by deflector is shown to be useful in timing measurements where a charge collection process is slow. Larger ion bunches were showed to be useful to increase the imaging capabilities for materials with poor charge collection properties. Finally the use of constant ion beam current of pA range has been demonstrated on measurements of IBIV on a-Si solar cell.
References [1] M.B.H. Breese, D.N. Jamieson, P.J.C. King, Materials Analysis Using a Nuclear Microprobe, Wiley, New York, 1996. [2] N. Taccetti, L. Giuntini, G. Casini, A.A. Stefanini, M. Chiari, M.E. Fedi, P.A. Mand o, Nucl. Instr. and Meth. B 188 (2002) 255. [3] E. Vittone, F. Fizzoti, E. Gargioni, R. Lu, P. Polesello, A. Lo Guidice, C. Manfredotti, S. Galassini, M. Jaksic, Nucl. Instr. and Meth. B 158 (1999) 476. . Pastuovic, I. Bogdanovic [4] M. Jaksic, V. Borjanovic, Z Radovic, N. Skukan, B. Pivac, Nucl. Instr. and Meth. B 181 (2001) 298.
On the use of pulsed microbeam in IBIC M. Jaksic *, Z. Medunic, N. Skukan Department of Experimental Physics, Rud-er Boskovi c Institute, P.O. Box 180, 10002 Zagreb, Croatia
Abstract In order to expand the application possibilities of ion beam induced charge (IBIC) technique to a wider range of semiconductor materials and devices, three novel modes of measurements have been developed. The fast beam deflector was used to produce ns pulses of single ions and the trigger signal provides a time scale for time resolved IBIC measurements. Longer pulses (>100 ns) containing tens or hundreds of ions were used to enhance IBIC signals in materials with poor charge transport properties. Continuous beam current (10 pA to 1 nA) of 3 MeV protons was used to measure ion beam induced voltage (IBIV) in thin amorphous silicon films. In these samples it was possible to correlate the charge collection properties with elemental distribution measured simultaneously with PIXE. Radiation damage was shown to be negligible for doses below 107 protons lm2 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Charge transport processes in semiconductor materials and devices became, in recent decade, an important topic of nuclear microprobe application in material science and electronics [1]. However, in practice, materials with high concentration of traps and in general with poor charge transport properties, or sample regions where only diffusion contribute to the ion beam induced charge (IBIC) signal, could not be studied with current capability of IBIC setup. In this work, an attempt towards the better utilization of IBIC is demonstrated by the use of recently constructed fast electrostatic deflector that provides ion pulses over a wide range of durations.
*
Corresponding author. Tel.: +385-1-4680-942; fax: +385-14680-239. E-mail address: [email protected] (M. Jaksic).
Two modes of pulsed beam operation were developed. The first one is a short pulse mode with a single ion arriving at a sample within the ns time. This mode could be important in time resolved IBIC experiments with the required time resolution close to 1 ns. The trigger signal is needed to provide the exact time scale between the moment of ion hit and time evolution of detected charge signal. The second mode is a long pulse (100 ns to 10 ls) mode with tens, hundreds, or thousands of ions arriving in bunch to the sample. Such an ion bunch could be particularly useful for the IBIC analysis of high defect density materials where single ion does not produce measurable signal. In samples such as amorphous silicon (a-Si) thin film solar cells, where the typical thickness of PIN layer is of several hundred nm, the energy loss for 3 MeV protons is less than 10 keV. Under such condition measurable signal can be obtained from sample irradiation with proton beam with constant
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01051-6
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
current of about 10 pA. As it will be explained, one can measure ion beam induced voltage (IBIV) at the electrodes.
V
2. Experimental
V
The Zagreb nuclear microprobe setup has been recently upgraded with a new scattering chamber and a fast beam deflector positioned immediately after the object slits. Scattering chamber of a spherical shape is equipped with large number of ports, where most of them are in backward angles, to provide simultaneous use of different ion beam analysis (IBA) techniques. With a load lock system, samples are introduced into the sample holder having controllable temperature. The beam deflector uses fast high voltage push– pull MOSFET switch (Behlke Electronics Gmbh) having 40 ns rise time for 1 kV pulses. For the short pulse mode operation, a beam passes through the collimator slits during the rise time of the switch between positive and negative voltage at one of the vertically positioned deflectors. First measurement of the time distribution, using STIM Si particle detector and conventional NIM modules, showed time resolution of 0.9 ns. In the long pulse mode operation, the 0 V pulses with duration between 200 ns and 40 ls are send to the deflector plate that is connected to HV. In both cases the second deflector plate is grounded. Frequency of pulses can be adjusted to maximum 8 kHz. Alternatively, external trigger pulse can be used as well. The deflector system is schematically presented in Fig. 1. In such conditions, object and collimator slit openings remain unchanged which makes operation of low current techniques such as STIM and IBIC very convenient to use. For the 10 nA beam current measured in the microprobe chamber with pulser turned off, a short pulse mode of 800 Hz repetition rate will typically provide about 200 cps of single ion pulses. The ion rate can be adjusted by pulser frequency, and beam current, to any desired value. On the other side, long pulse mode allows ions to pass through the microprobe system in bunches of tens, hundreds or thousands of ions within a microsecond. The beam current and the
+1kV 0 -1kV
0 -1kV
177
short pulses < 1ns
long pulses > 200ns
V
deflector
Fig. 1. Schematic presentation of high voltage pulses that drive deflector and ion beam path between the object and collimator slits of the microprobe system.
pulse length will determine the number of ions and therefore the total particle energy of the bunch. Such bunches of the broad beam were already used by Taccetti [2]. They could be applied to test linearity of detector and/or detector electronics providing total energies of up to GeV.
3. Measurements 3.1. Short pulse mode Short pulses of single ion produced in the present setup provide a trigger signal with exact known time of ion hitting a sample. This ability is significant in IBIC studies of charge transport behavior in regions where only diffusion contribute to the charge pulse. Depending on the distance between the ion hit position and region where charge carriers start to drift in the electric field, one can simultaneously measure diffusion length and diffusion constant of the material under study [3]. As an example we have analyzed a simple EFG (edge-defined film-fed grown) silicon Schottky diode in the frontal geometry, close to the edge of the contact and including the vertical twin boundary [4]. Fig. 2 shows images of a conventional pulse height IBIC image as well as two
178
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
well as for the samples that have much larger diffusion length as compared to the one presented here. Due to the difficult processing of the small and slow rise time signals from diffusion regions using conventional timing electronics, errors are possible. These can be avoided if a digitizing oscilloscope of the TRIBIC setup is used in such circumstances. 3.2. Long pulse mode The main motivation behind the use of long pulse mode is to study by IBIC samples that have too weak response to single ions. With a very stable beam current, larger ion bunches following the Poisson distribution have well defined number. This number can be increased according to the need of the sample under investigation. In Fig. 3, a spectrum of a series of 3 MeV proton bunches, has been measured by silicon STIM detector. The mean of distribution is at 210 MeV that corresponds to 70 particles in the bunch. The width of distribution is in this case dominated by beam current fluctuations. Application example is given in Fig. 4, where a high defect density EFG Si sample has been imaged by 3 MeV single proton IBIC, as well as with multiple ion IBIC. It can be seen that long pulse mode can provide higher data statistics and significant improvement in image contrast.
400
Intensity
300
Fig. 2. Optical photograph of the EFG silicon Schottky diode (up), IBIC image of this sample with low and high pulse height windows (middle) and images of time distribution in fast and slow time windows (down). The scan size is 500 1000 lm2 .
200
100
images that correspond to period of pulse arrival, namely t ¼ 0 and t ¼ 100 ns (50 ns). One can see, as expected, delayed arrival of pulses further away from contact region where only diffusion contribute to the carrier transport. It is expected however that much better ÔtimeÕ resolution should be obtained in lateral geometry of IBIC experiment, as
0
0
200
400
600
800
1000
1200
Channel number
Fig. 3. Energy spectrum of the mode with long pulses as measured by STIM detector. Discrete peaks correspond to the number of 3 MeV protons in the bunch.
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
179
Fig. 4. IBIC images of high defect density EFG silicon Schotky diode in frontal geometry. Left image correspond to a single proton IBIC measurement, while the right spectrum corresponds to the bunches containing in average 10 protons of 3 MeV. The scan size is 500 1000 lm2 .
Although similar effect can be obtained using heavy ions, flexibility in determination of ion energy to be deposited in pulse may be advantageous in practice. 3.3. Continuous beam – measurements of voltage Studies of charge collection properties by IBIC in layers that have thickness less than a micrometer, are difficult to perform since the energy loss of ions typically used in nuclear microprobes is in-
sufficient (e.g. 3 MeV proton in Si loses 20 keV in 1 lm). However, since radiation damage is in such conditions lower as well, it could be possible to study such samples with much higher currents, as those that are typically used in high current experiments used for elemental imaging by PIXE and other IBA techniques. In our studies of amorphous silicon solar cells (a-Si) in which PIN layer is 350 nm thick over the areas of some cm2 , conventional IBIC setup can not be used due to high sample capacitance.
Fig. 5. PIXE and IBIV image of a-Si solar cell. The scan size is 800 800 lm2 .
180
M. Jaksic et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 176–180
Therefore, we connected sample electrodes to a sensitive digital multimeter (DMM) for the measurement of ion beam induced voltage (IBIV). In order to test influence of radiation induced damage for a-Si samples, proton microbeam was scanned over different areas of sample. With a maximum 1000 1000 lm2 scanning area and rather high current of 1200 pA, measured voltage dropped to the half of the initial value after 400 s. From these measurements we showed that the dose limit when radiation damage becomes noticeable (voltage decreases to 95% of initial value) is at 107 3 MeV protons per lm2 . Following this result, the beam current was decreased to 10 pA value for the 800 800 lm2 scanning area. The DMM was adjusted to measure the IBIV in a slow sampling mode. In such condition one measurement point had to be at least 2 s which gives 800 s measurement time for the 20 20 pixel image. Although this was not needed for the samples we studied, simultaneous PIXE imaging is also possible. The example of simultaneous PIXE and IBIV analysis is given in Fig. 5, where charge collection properties can be correlated with elemental distribution, which is in this case Al in contact layer.
4. Conclusions During the development of IBIC as technique, several improvements in technology of measure-
ments were made. Time resolved measurements, temperature dependence of charge transients, use of different ions having wide range of sampling depths, etc., are some of these that opened new areas of IBIC applications. The use of pulsed beam controlled by deflector presented here is justified in simpler operation needed to switch from the high current to the low current mode in nuclear microprobe, as well as in wider application possibilities. Triggered signal of ns resolution provided by deflector is shown to be useful in timing measurements where a charge collection process is slow. Larger ion bunches were showed to be useful to increase the imaging capabilities for materials with poor charge collection properties. Finally the use of constant ion beam current of pA range has been demonstrated on measurements of IBIV on a-Si solar cell.
References [1] M.B.H. Breese, D.N. Jamieson, P.J.C. King, Materials Analysis Using a Nuclear Microprobe, Wiley, New York, 1996. [2] N. Taccetti, L. Giuntini, G. Casini, A.A. Stefanini, M. Chiari, M.E. Fedi, P.A. Mand o, Nucl. Instr. and Meth. B 188 (2002) 255. [3] E. Vittone, F. Fizzoti, E. Gargioni, R. Lu, P. Polesello, A. Lo Guidice, C. Manfredotti, S. Galassini, M. Jaksic, Nucl. Instr. and Meth. B 158 (1999) 476. . Pastuovic, I. Bogdanovic [4] M. Jaksic, V. Borjanovic, Z Radovic, N. Skukan, B. Pivac, Nucl. Instr. and Meth. B 181 (2001) 298.