Doping-Type Dependence of Damage in Silicon Diodes ... - IEEE Xplore

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 6, DECEMBER 2007

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Doping-Type Dependence of Damage in Silicon Diodes Exposed to X-Ray, Proton, and He+ Irradiations M. Caussanel, A. Canals, S. K. Dixit, M. J. Beck, A. D. Touboul, R. D. Schrimpf, D. M. Fleetwood, and S. T. Pantelides

Abstract—Different amounts of degradation for n-Si and p-Si are observed after X-ray, H+ , and He+ irradiations. Recombination lifetime and forward I-V measurements made on abrupt-junction diodes are compared to theory. Ionizing damage and displacement damage associated with surface and bulk trapping mechanisms, respectively, compete with each other and lead to different behaviors according to the doping type of the silicon on the lightly doped side of the junction. Surface effects are dominant in the n+ /p diodes compared to the p+ /n diodes; bulk trapping prevails in the n-Si compared to p-Si. Independently of ion type or fluence, the lifetime damage factor due to irradiation is worse in the p-Si than in the n-Si by a factor of 2–3 times. Index Terms—Doping, ionization, ion radiation effects, semiconductor junctions.

I. INTRODUCTION OR more than 40 years, differences between the radiation responses of p-doped and n-doped silicon have been reported [1]–[4]. Initially, experiments showed disparities after electron irradiations from very low energy (a few hundred keV [1]) to high energy [2]. In [1] different relative damage rates of n /p and p /n Si junctions are observed as a function of electron energy (from 150 keV to 800 keV). In [2], fission neutron, 3.7 MeV proton and 53 MeV electron irradiations led to dissimilar displacement damage factors versus collector current for 2N2222A and 2N2907A transistors. Comparison of material damage coefficients with particle NIEL below a few tenths of keV cm /g revealed a linear dependence for n-type Si and a quadratic dependence for p-type Si [3].

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Manuscript received July 20, 2007; revised September 3, 2007. This work was supported in part by the AFOSR MURI Program. M. Caussanel is with the Université de Perpignan Via Domitia, Perpignan, 66860, France (e-mail: [email protected]). A. Canals is with TRAD, Labege 31674, France (e-mail: anna.canals@trad. fr). S. K. Dixit is with the Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, TN 37235 USA (e-mail: sriram.k.dixit@ vanderbilt.edu). M. J. Beck and S. T. Pantelides are with the Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 USA (e-mail: [email protected]; [email protected]). A. D. Touboul is with IES, Université Montpellier II, F-34095 Montpellier cedex 5, France (e-mail: [email protected]). R. D. Schrimpf and D. M. Fleetwood are with the Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235 USA (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2007.909021

The magnitude of the difference between p-Si and n-Si has been found to correlate well with the fraction of damage due to low-energy PKA (Primary Knock-on Atom) recoils [2], [4]. This motivates the study of radiation damage due to low-energy displacement-damage events: the creation of vacancy-interstitial pairs (Frenkel pairs), their evolution with time (annealing, dissociation, lifetime), their properties (charge state, vacancy-interstitial distance, crystallographic sites) and their impacts on the electrical/mechanical/optical properties of the material. The purpose of this work is to build upon recent efforts [4], [5] to enhance the understanding of p-Si/n-Si radiation-response dissimilarities. X-rays, H and He irradiations have been carried out on identical abrupt n /p and p /n junctions manufactured by Sandia National Laboratories [6]; two types of characterizations were performed: basic forward I–V measurements, and carrier recombination lifetime measurements based on a modified version of the open-circuit voltage decay method [7]. As mentioned in [8], lifetime is one of the few parameters that permit characterization of a modern device with a low defect density. As a result, surface effects can come into play in a significant manner in lifetime measurements. In this paper we evaluate the impact of radiation-silicon interaction on the recombination lifetime, which describes the time required by minority carriers to recombine (e.g., in quasi-neutral regions) and do not consider generation lifetime, which applies where there is a deficiency of carriers (e.g., in space-charge regions). II. EXPERIMENTAL DESCRIPTION A. Sample, Irradiation and Characterization Description The samples studied here are abrupt p /n and n /p junctions manufactured by Sandia National Laboratories (lot G1916A, wafer I.D. 33 [6]). Their cross-section is presented in Fig. 1, and Fig. 2 illustrates the top-view. The packaged chip has been processed completely, including extensively patterned metal lines connected to active regions through contact openings. The fabrication process involves a 1 m passivation P-glass layer covering everything except the contacts. Doping levels on the lowdoped sides of the junctions are 2.7 10 cm and 4 10 cm for the p /n and the n /p diodes, respectively. The highdoped side is about 10 cm for both types of junction. The calculated width of the depletion region at equilibrium is 0.17 m on the p-side and 0.67 nm on the n -side for the n p junctions. Values for the p n junctions are 0.17 nm on

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TABLE I EXPERIMENTAL DESCRIPTION SUMMARY

Fig. 1. Cross Section of the n /p and p /n junctions manufactured by Sandia National Laboratories (lot G1916A, wafer I.D. 33) [6].

Fig. 2. Top view of the n /p and p /n junctions manufactured by Sandia National Laboratories (lot G1916A, wafer I.D. 33) [6].

the p -side and 0.63 m on the n-side. This emphasizes the relative importance of the lightly doped side of the junction in the lifetime measurements described below. According to [9] and [10], given the moderate doping value of the samples it is reasonable to assume that, for both types of carrier, the minority carrier mobility does not depend on the substrate doping type. This allows estimation of the minority carrier diffusion 320 m length on the low-doped side of each junction: 140 m. By comparing these values with sample and geometry (Figs. 1 and 2), one notices that the diodes are almost short-base diodes, which means that part of the injected minority carriers will recombine at the ohmic contacts. This point helps in understanding first, the low pre-irradiation hole lifetime value (Section II-B), and second, the sample sensitivity to X-ray radiation (Section III-A). Table I presents an overview of the sample type, the kind of radiation and the characterization methods used. The various irradiations and testing were carried out at Vanderbilt University. For all irradiations, all sample pins were grounded. B. Lifetime Measurement Set-Up The lifetime measurement method used here is a slightly modified version of the open-circuit voltage decay method [7]. , is applied across the junction by a voltage First, a bias, source. When the steady state is attained, the circuit is quickly opened and the voltage decay across the junction is measured

Fig. 3. Example of measurement obtained using the Open-Circuit Voltage Decay method on abrupt junctions. Minority carrier lifetime values are extracted from the linear part of the curves. For clarity, only 1 marker for every 4 data values is displayed. The inset presents the experimental set-up, in which a MOSFET has been used as a fast switch.

and recorded with a storage oscilloscope. For accurate measurements, the time required to open the circuit must be much smaller than the time taken by the minority carriers to recombine. In order to do so, an NMOS transistor was placed in series with the junction (inset of Fig. 3) and the circuit was opened by removing the bias, , on its gate. The voltage decay across the junction was measured as a function of time. Assuming that recombination is dominated by quasi-neutral region recombination with the simple exponential voltage de, the lifetime can then be extracted pendence from the linear part of the voltage vs. time curve (Fig. 3) using (1)

(1) The analysis is simple if the useful linear portion extends throughout the full measuring time. However, this is not the case here, and this typical non-linearity is often attributed to the depletion capacitance [11] (sometimes called the transition capacitance). The first steep part of the curve is a transition regime corresponding to the loss of the Ohmic behavior. The average electron and hole lifetimes measured before irradiation are 43.4 3.2 s and 16.9 1.9 s. The electron mean value

CAUSSANEL et al.: DOPING-TYPE DEPENDENCE OF DAMAGE IN SILICON DIODES

Fig. 4. Lifetime measurement curve evolution under fractional X-ray irradiation for n /p and p /n junctions.

agrees very well with data available in the literature [12], confirming the reliability of the method. However, the hole mean value is about 5 times lower than typical values [13]. This can be attributed to surface damage in the n region associated with oxygen diffusion from the SiO into the n epi, which is activated by the high temperature post-oxidation anneal undergone by the wafers. As a result, there is likely to be an enhanced impurity concentration near the surface. References [14]–[16] illustrate the O out-diffusion effect. This mechanism especially has an impact on the measured lifetime because the samples are almost short-base diodes. As mentioned in Section II-A, sample geometry and doping imply some injected minority carrier recombination close to the contacts. The effect of oxygen contamination has also been noticed on hole diffusion length in GaInNAs solar cells [17]. In that work the authors present evidence that certain near-midgap traps are associated with oxygen defects and conclude that the oxygen recombination center is a lifetime-limiting defect that controls the hole diffusion length. For abrupt junctions, reliable values of can only be obtained and measurements are made at times . As if can be seen in Figs. 1 and 2, the samples fulfill the first condition for both types of junction. Fig. 3 presents a lifetime measurement before and after ion irradiation. This figure verifies that is also satisfied by the experithe second condition mental procedure. III. RESULTS AND DISCUSSION A. X-ray Irradiation Impact on Minority Carrier Lifetime X-ray irradiations were performed to check whether or not the measured lifetime is dominated by bulk recombination. A 500 krad(SiO ) dose was delivered in 4 steps, and the samples were measured immediately after each dose. Results show that the voltage vs. time curves are mainly modified in two ways (Fig. 4). The first striking difference is that the p /n junction is much less sensitive to ionization than the n /p junction. The p /n junction exhibits only a very small change after 500 krad(SiO ).

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On the n /p junction a noticeable difference appears above 50 krad(SiO ). At this dose and above, the voltage versus time curve exhibits more curvature and the linear part of the curve is reduced. That the lifetime measurement on the p /n junction is less impacted by ionization than the n /p junction measurement is consistent with the fact that the electron diffusion length L in the n /p junction ( 320 m) is higher than the hole diffusion length in the p n junction ( 140 m) by 2.3 times. A higher percentage of injected carriers recombine further away from the interface in the p n junction compared to the n p junction. In other words, minority carrier recombination is less affected by surface effects in the p n junction than in the n p junction. A second post/pre-irradiation difference concerns the voltage drop across the sample before the circuit is opened. With an applied voltage, , equal to 0.6 V across the sample in series with the MOSFET switch, about 0.55 V drops across the sample before irradiation and 0.05 V across the transistor. This value remains the same 3 mV for all the different samples measured before irradiation. After the X-ray irradiation this value is reduced: the higher the dose, the higher the reduction (Fig. 4; data emphasized by the ellipses). The reduction has a higher magnitude in the n /p case compared to the p /n case. This voltage drop is due to charge trapped at the SiO /Si interface of the sample. This effect is discussed in the analysis of the I-V data (Sections III-B and III-D) and is in addition to the changes in carrier lifetime described in Section III-C. B. I-V Characteristic Changes Under X-Ray Irradiation In an ideal case, the current-voltage characteristics of a silicon diode under forward bias can be divided into four parts. First there is the ideal diode region, in which a 60 mV increase in bias causes an order of magnitude increase in current (corresponding to an ideality factor of 1). At low and high biases, the characteristics are dominated by depletion region recombination and high injection, respectively, each having an ideality factor of 2. At biases higher than the high injection zone, current increases linearly because of the series resistance of the diode. Figs. 5–8 present various irradiation data versus bias, with the portions of the bias range that correspond to either the ideal diode region or the series resistance region indicated. These regions have been determined accurately by an appropriate fit of the pre-irradiation I-V curves. Below the ideal diode region, the samples exhibit no region where current is dominated by trap-assisted recombination in the depletion region. Also, between the ideal diode region and the series resistance region, the high injection zone is not clearly discerned. Figs. 5 and 6 detail the effect of fractional X-ray irradiations on the n p and p /n junctions, respectively. Instead of plotting on the same graph pre- and post-irradiation I-V curves, the ratio of the post-irradiation I-V characteristics to the pre-irradiation I-V characteristics is plotted on a log scale versus bias. This illustrates how the different regions of the I-V curve are differently affected by irradiation. In order to permit an easy comparison with ion irradiation data, the Y-axis range matches that of the proton data in Figs. 7 and 8. We first observe that the p /n I-V characteristics are less affected by X-ray irradiation than the n /p I-V characteristics. A

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Fig. 5. Logarithmic ratio of X-ray irradiated n /p junction I-V to pre-irradiation I-V versus bias.

Fig. 7. Logarithmic ratio of H irradiated n /p junction I-V to pre-irradiation I-V versus bias. For each ion fluence, Table II (Section III-D) presents the total ionizing dose (TID) contribution for ion damage calculated with SRIM.

Fig. 6. Logarithmic ratio of X-ray irradiated p /n junction I-V to pre-irradiation I-V versus bias.

similar trend has been found from the lifetime measurement. Also, most of the damage occurs in the ideal diode region of bias, with highest damage for lowest biases. The p /n junction exhibits no change at all in the series resistance region. These results suggest that carrier trapping occurs at certain surfaces within the sample. Here more trapping is obtained in the oxide over the p-Si compared to that over the n-Si. Moreover, the sensitivity of the surface regions of these diodes to defect buildup is consistent with recent results obtained on VDMOSFETs [18]. C. Ion Irradiation Impact on Minority Carrier Lifetime Ion irradiation experiments were used to evaluate the effects of various H and He fluences ranging from 5.5 10 ions/cm up to 10 ions/cm . A different sample was used for each ion fluence. Due to the limited energy range allowed by the accelerator, particle energies of 1.8 MeV for protons and 1.4 MeV for Helium ions were used. These two particular values were chosen after having assessed the range of both particle types in the sample structure simulated with SRIM (Stopping and Range of Ions in Matter) [19]: 1.8 MeV protons

Fig. 8. Logarithmic ratio of H irradiated p /n junction I-V to pre-irradiation I-V versus bias. For each ion fluence, Table II (Section III-D) presents the total ionizing dose (TID) contribution for ion damage calculated with SRIM.

stop deeply in the substrate (40.9 m depth), and 1.4 MeV He ions stop on average at the metallurgical junctions (4.8 m depth). The data presented in Fig. 3 were obtained after irradiating the p /n junctions with 1.8 MeV protons to fluences of 10 ions/cm and 10 ions/cm . Like the X-ray irradiation, a lower initial voltage drop across the sample is observed after ion irradiation. Fig. 9 shows how much this initial voltage drop across the sample is reduced on the n /p and p /n junctions by various H fluences; the higher the fluence, the higher the reduction. This effect is attributed to interface charge trapping in the case of the X-ray irradiation. For the proton irradiation, the creation of a significant amount of displacement defects in the bulk leads to higher sample resistance, which causes a higher voltage drop across the sample after the irradiation. Since it is not a higher but a lower voltage that is observed after irradiation, this change can uniquely be attributed to the ionization effect of the ions. On the other hand, a much larger voltage drop reduction in the p /n junction case than in the

CAUSSANEL et al.: DOPING-TYPE DEPENDENCE OF DAMAGE IN SILICON DIODES

Fig. 9. Closed circuit voltage drop reduction for the n /p and p /n junction as a function of H fluence.

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Fig. 10. Electron and hole lifetime reduction percentage as a function of fluence for H and He irradiations.

TABLE II TID DAMAGE FRACTION OF H IRRADIATIONS

n /p junction is observed here. The opposite trend was found for X-ray irradiations (Fig. 4). Since this phenomenon cannot be totally explained by surface trapping, another mechanism must be involved. This mechanism is likely bulk trapping by defects related to displacement damage, and it is found to be more efficient in n-type Si than in p-type Si. This trend is consistent with recent theoretical work [4], which demonstrated that small Frenkel pairs (vacancy-interstitial distance about 8 ) with identical crystallographic configuration in both type of material are generally more stable in p-Si than in n-Si. They are more likely to act as electron traps in n-Si than in p-Si and when they do so the release of the trapped charge occurs with the recombination of the Frenkel pair. In Fig. 3, another important difference compared to the X-ray data is that, after having opened the circuit, the slope of the curve gets much steeper. This effect is due to lifetime reduction resulting from new recombination centers created by the ion exposure. Like the X-ray irradiation, the ionizing contribution of the ions makes the lifetime plot more curved, reducing its useful linear part (Fig. 4). Because the linear part of the curve gets not only steeper but is also compressed in time, it is more difficult to extract a reliable lifetime value after a high fluence ion irradiation. A multiple step data treatment procedure had to be used in order to extract consistent lifetime values in a similar way for all data sets. This procedure involves a binomial (Gaussian) smoothing of the data followed by a differentiation aimed at revealing more easily the limit between the end of the initial steep decay and the beginning of the signal corresponding to the carrier recombination. Fig. 10 presents the lifetime reduction percentage for electrons and holes as a function of H and He fluence. The lifetime decreases with increasing fluences up to 10 ions/cm . The most remarkable feature of Fig. 10 is that, for both H and He irradiations, the electron lifetime systematically decreases more than the hole lifetime. Considering for example bipolar gain degradation, more change in response is usually expected for the more lightly doped material [20]. It is then worth noticing that more degradation is observed for the n /p junction than for the p /n junction despite the p-doping (4 10 cm ) being 13 times higher than the n-doping (2.7 10 cm ).

D. I-V Characteristic Changes Under Ion Irradiation To permit a meaningful comparison between ion data and X-ray data (Figs. 5 and 6), the same kind of plotting is used: ratio of the post-irradiation I-V to the pre-irradiation I-V on a log scale vs. bias. Figs. 7 and 8 show the proton results for the n /p and p /n junctions, respectively. Moreover, to emphasize the displacement damage, with respect to ionization, Table II (end of this section) presents for each ion fluence the total ionizing dose (TID) contribution of proton irradiation damage calculated with SRIM. First, in the ideal diode region the current at a given bias is higher after irradiation for both types of junction. The highest fluence results in the highest current increase. Second, in the series resistance region, this trend is reversed. The current at a given bias is lowered after irradiation and the highest fluence results in the lowest current. The series resistance accounts for several contributions: the series resistance of the connecting wires, the contact resistance between the metal and the semiconductor, and finally the resistivity of the semiconductor. The creation by ion irradiation of a significant amount of displacement damage within the material increases its resistivity. Thus, the trend observed in the series resistance region of the I-V curves (for biases from about 0.8 V and higher) is clear. However, a change in bulk resistivity cannot account for the results in the ideal diode region (between 0.2 and 0.6 V). Part of the observed modifications of the ideal diode region is due to surface charge trapping, whose existence has been demonstrated by the X-ray

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irradiations (Figs. 4–6), and another part to the bulk trapping suggested by the closed-circuit voltage drop data (Fig. 9) and confirmed here. Moreover, for either the radiation-induced modifications in the ideal diode region or the series resistance region, the p /n junction is always more damaged than the n /p junction. Also, comparing X-ray I-V data (Figs. 5 and 6) with proton I-V data (Figs. 7 and 8) gives an interesting picture of the experiments. With only X-ray ionization, the p-Si is more damaged than the n-Si. Nevertheless there is clear competition between surface trapping and bulk trapping that affects differently the p-Si and n-Si regions. When displacement damage occurs, the previous trend is reversed and n-Si gets more damaged than p-Si. IV. CONCLUSION X-rays, H and He ion irradiations have been carried out on n /p and p /n junctions. The open-circuit voltage decay method has been employed to assess the degradation due to irradiation of the recombination lifetime. Forward I-V curves have also been recorded as a complementary characterization. Several points come out of the comparison of both sets of data. X-ray irradiations revealed some surface effects due to the buildup of interface traps. The device surface region sensitivity to defect buildup is in agreement with recent results obtained on VDMOSFETs [18]. These surface effects also have been noticed after ion irradiation, and they complicate lifetime value extraction. In addition, we find that the bulk trapping mechanism associated with the displacement damage reversed the damage trend observed between p-Si and n-Si after X-ray irradiation of the material. This mechanism, more efficient in n-Si than in p-Si, is in agreement with recent theoretical work [4]. Consistent with previously published damage coefficients [5], [3], [2], [1], a higher degradation by a factor of 2–3 times is observed in p-Si compared to n-Si despite the doping being lower in the n-Si. This factor is independent of fluence or ion type. Analyzing jointly the I-V curves confirms the occurrence of surface charge trapping, which can partially explain the changes in the ideal diode portion of the I-V characteristics. The trend observed in the series resistance part of the characteristics results from the high bulk resistivity of the material. Considering only the X-ray ionization, the p-Si is more damaged than the n-Si; however when displacement damage intervenes this trend gets reversed and n-Si becomes more damaged than p-Si. ACKNOWLEDGMENT The authors would like to thank Dr. A. Perona for fruitful discussions and a critical review of this manuscript.

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