Developing of the R.D.S.D. software for the research and analysis of radiation defects in semiconductor detectors in static and dynamic cases

V. Mikhaylov, Dr. V. Kushpil

Nuclear Institute of the Academy of Sciences of the Czech Republic PRI, 250 68 Řež near Prague, Czech Republic, National Research Tomsk Polytechnic University, 634050 Tomsk, Russia

A B S T R A CT

Radiation damage is the general reason of deterioration of the operational and detection properties of a detector, due to high doses of irradiation. Long-term ionization to the detector causes damage in form of defects increasing that is more significant in comparison to short-term ionization with the same dose. Depending on the trend of accumulation of defects in a variety of detectors we can determine their effective application time. R.D.S.D. software for the research and analysis of radiation defects in semiconductor detectors in static and dynamic cases was developed. R.D.S.D. Statics provides a possibility to measure and analyze capacitance-voltage, current- voltage and capacitance- frequency characteristics as well as calculate donor concentration in bulk of Silicon, its resistivity and thickness of SiO2, estimate of the flatband voltage for MOS capacitor and plotting the dependency of donor concentration on depletion region. R.D.S.D. Dynamic software provides possibility to measure and analyze curve of transient process of voltage change during switching in diode and integrate it, calculate the charge of switching, internal (inside measurement box) and ambient temperatures as well as estimate level of light inside the measuring box. We can measure transient process for light pulse and investigate behavior of minority carriers into bulk of Silicon. Analyze of capacitance-voltage characteristic proves theoretical influence of defects in form of change from symmetrical characteristic form to non-symmetrical and shift of flatband voltage. Investigation of capacitance-frequency characteristic discovered new effect – waveform fluctuations for high frequencies of C-F characteristic, presumably associated with increasing number of short-lived defects.

1. Introduction Most wide used silicon particle detectors can be created by doping narrow (usually around 1-100 A semiconductor detector is a device that uses a micrometers wide, 1-7 cm length) strips of silicon to semiconductor (usually silicon or germanium) to detect make them into diodes, which are then powered in traversing charged particles or the absorption of reverse-biased. As charged particles pass through photons. detector wafer and then collected by strips, they cause Semiconductor coordinate sensitive detectors small ionization currents which can be detected and have been used in high-energy physics applications in measured. Arranging thousands of these detectors the form of pixel detectors, silicon-drift detectors and around a collision point in a particle accelerator can give microstrip detectors; they are popular due to their an accurate picture of what paths particles take. Silicon unmatched energy and spatial resolution, and have detectors have a much higher resolution in tracking excellent response time. These detectors are charge particles than older technologies such as cloud manufactured mainly of silicon, traditionally on high- chambers or wire chambers. The disadvantage is that resistivity single crystal float-zone material. GaAs is silicon detectors are much more expensive than these perhaps a future alternative to silicon; presently, it older technologies and require sophisticated cooling, seems to be an expensive and not fully mastered control and readout to reduce leakage currents (noise technology of potentially better radiation hardness. source) and for effective application as well as suffer degradation over time from radiation. Silicon now it is the cheapest material for the detectors, and permanent development of the industry In this paper, we describe the program for makes it even cheaper. In addition to it, unlike measurement and analyze radiation defects in Germanium, does not require such a strong cooling. semiconductor detectors in static and dynamic cases. Section 2.1 introduces the radiation damage problem and reviewing the ways of its investigation. Section 2.2 Depending on the trend of accumulation of describes capacitance-voltage characteristic analysis of defects in a variety of detectors we can determine their MOS- and PIN-structures. Capacitance-frequency effective application time until the effect of defects on characteristic analysis of PIN-diode is described in the measurements will not be significant. Therefore, it is Section 2.3 and dynamic characteristics analysis is important to investigate evolution of defect tackled in Section 2.4. In Section 3.1 we speak about accumulation in detectors in long-term exposure (over main points of program development and software used several months). for it. Sections 3.2 and 3.3 are devoted to the consideration of the two sides of software developed – Working lifetime of the detector is an important R.D.S.D. Statics software and R.D.S.D. Dynamics parameter and it is important to consider when planning software. Finally, Section 6 contains the summary and any long-term experiment. For example, it is important conclusions, and some directions are given for future for experiment ALICE at CERN, where silicon detectors work. are now widely used. In addition, the study of radiation defects of the existing detectors will help further the 2. Radiation damage problem development of new devices.

2.1. Overview of the problem 2.2. Capacitance-voltage characteristic analysis

The high particle fluency in the interaction Capacitance-voltage C (V) characteristic is a regions of colliders is a critical parameter for the fundamental characteristic, which is usually carried out operational parameters and detector performance. by the study of properties of semiconductor detectors Radiation damage is the general deterioration of the [2]. We can carry the investigation of ionization defects operational and detection properties of a detector, due to in two ways. high doses of irradiation [1]. First way is to measure CV-characteristic of It is important to note that long-term ionization to irradiated MOS-capacitors. Capacitance-voltage the detector causes damage in form of defects characteristic of MOS before and after irradiation are increasing. And this increasing is more significant in shown on figures 2 and 3 respectively. These two charts comparison to short-term ionization with the same dose. allow us to see the change of symmetrical characteristic In the short exposure in the resulting defects remains a form to non-symmetrical and the shift of flatband chance to recover, whereas in long-term damage to the voltage – the voltage at which maximum capacity is defects become stable and the chances of recovery are reached for irradiated structure in comparison to non- sharply reduced. irradiated one. It is important to note that the non- irradiated characteristic is symmetric of zero-level due to the fact that our investigated detector consists of two oppositely plugged MOS silicon capacitors.

Fig. 1. ALICE SDD detector and hybrid readout chip PASA-SCA used for STAR experiment

The topic is fairly new to the study, because usually while studying for radiation defects detectors are tested for the ability to withstand a high dose of short- term exposure and do not incorporate the effects of long-term. 2 The second way of carrying the investigation of ionization defects is to measure CV-characteristic of irradiated PIN-diode. This characteristic is shown on the figure 5 (yellow dots).In addition to it we can calculate and plot dependency of donor concentration with respect to depletion region that is shown on figure 5 (green dots) [4].

Fig. 2. Capacitance-voltage characteristic of MOS before irradiation

Fig. 5. Capacitance-voltage characteristic of PIN before irradiation and dependency of donor concentration on depletion region

2.3. Capacitance-frequency characteristic analysis

Fig. 3. Capacitance-voltage characteristic of MOS It is also important to obtain capacitance- after irradiation frequency C (f) characteristic. This characteristic is shown on the figure 6 for the range 10kHz-1000kHz If we know the values of relative permittivity and with step of 5kHz. From this chart we can see area for the investigated MOS structure we can calculate dependence of the capacity (which is directly some additional parameters. With help of capacitance- proportional to the number of defects) on the frequency. voltage characteristic of MOS we can calculate donor Since frequency is inversely related to the time we can concentration in capacitor, its resistivity and thickness determine the capacity (which characterizes the number of SiO2 layer [3]. These values calculated for the MOS- of states) for trapping states with small lifetimes. structure before irradiation are shown on the figure 4.

Fig. 4. Additional parameters for MOS before irradiation 3 Fig. 6. Capacitance-frequency characteristic of MOS Fig. 7. Capacitance-frequency and temperature- after irradiation frequency characteristics of MOS after irradiation

We got a plot with waveform fluctuations on the 2. 4 Dynamic characteristics analysis right side. Fluctuations at the left side (low frequency) are probably noise, but at the right side (high frequency) In previous paragraphs we discussed investigation – it looks like a direct proof of a new effect, presumably of static characteristics of detectors – capacitance- associated with appearance and then damage due to voltage and capacitance-frequency characteristics. short-lived defects. But it is also important to investigate dynamic We re-measured the capacitance-frequency characteristic to have a conception of behavior of characteristic for the range 100kHz-400kHz with step of minority carrier’s for fast transient processes during 1kHz and obviously can see the same waveform recharging. When we send to the diode voltage-pulse of fluctuation effect. In addition, on figure 7 we can see a rectangular shape which takes it out of forward state to graph of red dots and the corresponding right vertical the depletion state we should theoretically get a reaction axis. This graph shows the dependency of detector of rectangular form. But in reality we get an immediate temperature with respect to the time of measurement. As higher jump of output voltage in the switch time and the temperature is approximately constant (it changes then output voltage stabilizes. Three different transient only for a 0.1 of degrees) we can conclude that the process curves of change of voltage in diode plotted waveform change in capacitance that we have detected using light-echo function are shown on the figure 8. is not related to the change of the Fermi level due to changes in temperature.

Fig. 8. The transient process curves of change of voltage in diode 4 Depending on the magnitude of the jump we can Both programs consist of many special functions that calculate the value of switching charge and estimate the are necessary for interacting with different devices as life of minority carriers that characterizes silicon quality oscilloscope, electrometer, waveform generator and so and radiation damage. As the result of radiation damage on. There are also developed special functions for to silicon bulk the traps concentration increases and mathematical calculations as differentiation and lifetime of minority carriers decreases. Decreasing of integration. It is possible to see how user interfaces of amplitude of switch-impulse characterizes silicon both utilities do look like in appendix 1. quality near the PN-junction. Studying of dynamic properties of PIN-junction is very important for 3.2. R.D.S.D. Statics software development of higher-quality detectors especially for The R.D.S.D. Statics software is a software with high-energy physics experiments [5]. visual user interface designed for the investigation of 3. R.D.S.D. software interface MOS- and PIN-structures in statistic case. The software consists of 4 tabs. The first three tabs are designed to 3.1. Main points of program development take measurements, the fourth tab – for conducting the analysis using the measured data. Program package National Instruments LabWindows/CVI was used for the development of the For measurement of C(V)- and I(V)- R.D.S.D. software. LabWindows/CVI is a proven ANSI characteristics, we can select range and step of C integrated development environment that provides measurement in volts, and the frequency and amplitude engineers and scientists with a comprehensive set of of input signal. For measurement of C(f)-characteristic, programming tools for creating test and control we can select range and step of measurements in kHz, applications. LabWindows/CVI combines the longevity shifting voltage in volts and amplitude of input signal in and reusability of ANSI C with engineering-specific millivolts. There is also a possibility to hold functionality designed for instrument control, data measurement of C(V)-characteristic with hysteresis. acquisition, analysis, and user interface development. The purpose of such visual programming is automating In the analysis tab we can calculate donor the usage of processing and measuring equipment in any concentration in Silicon near MOS capacitor, its laboratory setup. resistivity, hysteresis charge and thickness of SiO2 layer with help of capacitance-voltage characteristic of MOS- Actually the R.D.S.D. software consists of two structure. These values calculated for the MOS-structure independent utilities. The R.D.S.D. Statics software before irradiation was already shown on the figure 4. It provides a possibility to investigate MOS- and PIN- is also possible to estimate the flatband voltage in semi- structures in statistic case. There are features such as automatic mode. For the PIN-structure we can calculate measurement and plotting of capacitance-voltage and donor concentration, hysteresis charge and resistivity, capacitance-frequency characteristics, calculation of and also plot the dependency of donor concentration on additional static parameters, estimation of flatband depletion region. voltage and so on. The ability to save measured data to file and to The R.D.S.D. Dynamics software provides a download existing file with the data of measurement is possibility to investigate PIN-diodes in dynamic case. also provided in all the tabs. There is a status bar for There are features such as measurement and plotting of diagnostic and run-time messages and temperature bar a the dependency of switching charge on time, as well as that shows the temperature of investigated structure. All the transient process curve of change of voltage, and the GPIB-ports are presented as adjustable values. manual control of measurement box is also provided. Software is also checking if all the devices needed for measurement are connected before the start. Due to LabWindows/CVI is a based on C- language it is the language used for the programming. 3.3. R.D.S.D. Dynamics software Communication with devices is held by GPIB-protocol and TCP-protocol, so html-language is used as well. The R.D.S.D. Dynamics software is software with visual user interface designed for the investigation of 5 PN-diodes in dynamic case. The software is providing a also checking if all the devices needed for measurement possibility to investigate fast transient changes of are connected before the start. voltage in diode and to calculate the charge of recharging as well as lifetime of minor carriers. 4. Conclusions and future work

The R.D.S.D. Dynamics software is software with We developed the R.D.S.D. software for the visual user interface designed for the investigation of research and analysis of radiation defects in PN-diodes in dynamic case. The software is providing a semiconductor detectors in static and dynamic cases. possibility to measure and plot fast transient processes The software is dedicated for making measurements of of changes of voltage during switching in diode and to MOS- and PIN-structures and is built with help of calculate the charge of switching. The R.D.S.D. LabWindows/CVI software. Dynamics can handle measurement in two modes. In R.D.S.D. Statics software provides possibility to single measurement mode one measurement cycle is measure static characteristics as capacitance-voltage, carried. It is possible to investigate transient process current-voltage characteristic and capacitance-frequency curve at the main graph with use of light-echo function characteristic. There is also analysis of measured data for this mode. Monitoring mode allows us to make tool for calculation of donor concentration in capacitor, number of measurement cycles that can be also provided its resistivity and thickness of SiO2 layer estimation of with irradiation. It is possible to set time of one cycle the flat band voltage and plotting the dependency of and number of cycles for this mode. donor concentration on depletion region

Graphical user interface of software consists of R.D.S.D. Dynamic software provides possibility three graphs. The big one shows the dependency of to measure dynamic characteristics as curve of transient switching charge on time during the monitoring mode. process of voltage change during switching in diode, Second graph shows the transient process curve of curve of integrated voltage of transient processes of change of voltage in diode (within nanoseconds). Third voltage change and to calculate the charge of switching, graph shows the curve of integrated voltage of transient lifetime of minor carriers, internal and external process that is important because it is used for temperatures. It is also possible to over watch the level calculation of switching charge. of light inside the measuring box.

R.D.S.D. Dynamics is interacting with the device The results we achieved are in good agreement developed in the lab for measurement, control and with theory. It is obvious that during irradiation irradiation of diode. Due to this fact software has a semiconductor structure changes its properties. Kind of possibility to manually control position of driver that this change is quite predictable for the capacitance- holds irradiating source, connection of the waveform voltage characteristic that changes from symmetrical generator to diode. The temperature control is also characteristic form to non-symmetrical and its flat band possible using internal fan that can be controlled voltage shifts. manually or just turn on if temperature exceeds the limit of 35 degrees Celsius. Some of results are even seem to be innovative. In Section 2.3 we probably found some new effect We got For the oscilloscope it is possible to set the scale a capacitance-frequency characteristic with waveform and position for sweep, amplifier sensitivity and trigger fluctuations on the right side that look like a direct proof position. The ability to save measured data to file and to of a new effect, presumably associated with damage of download existing file with the data of measurement is Silicon and appearance of short-lived defects. It is provided as well. There is a status bar for diagnostic and important to note that these fluctuations are not run-time messages and temperature bar that shows both connected with change of the Fermi level because the internal and external temperatures. It is also possible to measured temperature is approximately constant. over watch the level of light inside the measuring box using photodiode voltage. All the GPIB-ports and IP- In future we will add further investigation and addresses are presented as adjustable values. Software is analysis option to receive the better understanding of internal processes in detectors. We are also very 6 enthusiastic to make new measurements using the And last but not least I want to thank the IAESTE developed software to achieve the better understanding organization for making possible achieving of such an of the new effects we found during this work. international internship experience. Finally I am very thankful for everyone I worked and communicated with Acknowledgements during this work in Nuclear Physics Institute of ASCR.

I would like to thank my mentors References Dr. Vasily Kushpil for granting the job and teaching me so many things all along my work. I would also like to [1] Andrea Candelori, “Radiation-hard detectors for very high thank Dr. Vladimir Wagner for getting me into the luminosity colliders”, NIMA, A 560 (2006) 103–107 [2] J. Hilibrand and R.D. Gold, "Determination of the Impuri- Nuclear Physics Institute for a two month internship and ty Distribution in Junction Diodes From Capacitance-Voltage organization of extremely interesting excursion to Measurements", RCA Review, vol. 21, p. 245, June 1960 Temelin nuclear power plant. I express the great [3] V. Eremin el all., “Development of transient current and gratitude to Dr. Michal Sumbera for the granting of charge techniques for the measurement of effective net con- centration of ionized charges (A&) in the space charge region main financing source for our work. I am very grateful of p-n junction detectors.”, NIMA, A 372 103–p.397-378, to Dr. Svetlana Kushpil for the very nice welcome and (1996) provision of information of different facilities in Czech [4] M.L.Lucia atal.,“Capacitance measurements of p-n junc- Republic. I am saying great thank to head of my tions: depletion layer and diffusion capacitance contributions” , 1993 Eur. J. Phys. 14 86, 1996 department Prof Sergey N. Liventsov for additional [5] Kuno h.j., “Analysis and characterization of P-N junction financial support and making this internship possible. diode switching”, Electron Devices, IEEE Transactions. Jan. 1964

7 Appendix A R.D.S.S. Statics Software – outlook of the program interface

Fig. 1. C(V)-measurement tab interface

8 Appendix A (extension) R.D.S.S. Statics Software – outlook of the program interface

Fig. 2. C(f)-measurement tab interface

9 Appendix A (extension) R.D.S.S. Statics Software – outlook of the program interface

Fig. 3. Analysis tab interface

10 Appendix B R.D.S.S. Dynamics Software – outlook of the program interface

Fig. 1. Interface view

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