High-Resolution Alpha-Particle Spectrometry Using 4H ... - IEEE Xplore

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006

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High-Resolution Alpha-Particle Spectrometry Using 4H Silicon Carbide Semiconductor Detectors Frank H. Ruddy, John G. Seidel, Haoqian Chen, Abdul R. Dulloo, Member, IEEE, and Sei-Hyung Ryu, Member, IEEE

Abstract—SiC detectors with active volume dimensions sufficient to stop alpha particles have been manufactured and tested. A linear energy response and excellent energy resolution have been obtained for various alpha emitters in the 3.18-MeV to 8.38-MeV energy range. Evaluation of the contributing factors to the SiC detector energy resolution indicates that the measured values for the full width at half maximum (FWHM) are limited by energy straggling of the alpha particles as they pass through the metallic contact layers that comprise the entrance window to the detector. Even with this component included in the measured FWHM, the measured values are comparable to those achievable with silicon alpha spectrometers. The possibility that the energy resolution that can be achieved with SiC may surpass that of silicon can not be excluded. SiC alpha spectrometers are expected to be useful in many nuclear applications where the ability to operate in high-temperature and high-radiation environments is required. Such applications include monitoring of alpha particles, neutrons, and low-energy gamma rays and X-rays in actinide waste-tank environments as well as neutron and gamma-ray monitoring of spent nuclear fuel assemblies. Index Terms—Radiation detectors, resolution, silicon carbide, spectroscopy.

I. INTRODUCTION ETECTORS based on silicon carbide (SiC) semiconductor offer many advantages for nuclear applications. SiC is a wide band-gap semiconductor that is able to operate at elevated temperatures. Furthermore, SiC is more resistant to the effects of radiation than conventional semiconductor materials such as silicon or germanium[1]. Excellent alpha-particle energy resolution was reported for SiC detectors based on a Schottky diode design [2]. More recently, SiC detectors with thicker active volumes were tested and improved energy resolution was observed [3]. Previous measurements [2]–[4] were limited by the fact that the thickest active volumes that could be obtained were thinner than the ranges of alpha particles in SiC resulting in only a fraction of the alpha energy being converted to electron–hole pairs in the detector. SiC detectors with active volumes thicker than alpha

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Manuscript received December 8, 2005; revised March 31, 2006. This work was supported by the Office of Science (BER), U.S. Department of Energy, under Grant DE-FG02-04ER63734. F. H. Ruddy, J. G. Seidel, and A. R. Dulloo are with the Westinghouse Electric Company Science and Technology Department, Pittsburgh, PA 15235-5081 USA (e-mail: [email protected]; [email protected]; [email protected]). H. Chen is with Princeton University, Princeton, NJ 08544 USA (e-mail: [email protected]). S.-H. Ryu is with Cree, Inc., Durham, NC 27703 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/TNS.2006.875155

Fig. 1. Schematic drawing of the Schottky diode SiC detector design.

particle ranges have now become available, and an alpha peak full width at half maximum (FWHM) value less than 20 keV has been reported [5] using such a detector. In the present work, SiC Schottky diodes with 100- m thick epitaxial layers have been tested with a variety of alpha particle sources, and total FWHM values have been evaluated as a function of energy. Factors that contribute to the observed total FWHM have been evaluated. II. SILICON CARBIDE DETECTOR DESIGN AND MANUFACTURE A schematic drawing of the SiC Schottky diode detector design is shown in Fig. 1. Lightly doped n-type epilayers, with cm , were a target nitrogen doping concentration of grown onto commercially available n 4H-SiC substrate wafers with 7.62-cm diameters (obtained from Cree, Inc., Durham, NC). The typical thickness and the resistivity of the substrate were 300 m and 20 m -cm, respectively. An epilayer thickness of 100 m was chosen for the detectors. Multiple floating guard ring termination structures were formed by aluminum implantations, which were annealed at approximately 1600 C in a silicon overpressure to suppress surface roughness. Thirteen guard rings were used, and the distance between the guard rings varied to optimize the blocking characteristics. The minimum spacing between the guard rings was 2 m. After the implant activation, the wafers received a thermal oxidation to grow approximately 500- -thick oxide layers, on which 1.5- m-thick silicon oxide layers were deposited by chemical vapor deposition (CVD). Backside ohmic contacts were formed with sintered nickel. After etching away the oxide layers in the active area, thin titanium layers were deposited by electron-beam evaporation to form Schottky contacts. Titanium (800 ), platinum (1000 ), and gold (9000 ) layers were sequentially electron-beam evaporated to form the overlayer, to be used as bonding pads. The same metal stack

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Fig. 2. Doping profile of the epilayer measured using reverse bias CV technique.

Fig. 4. The decay scheme for U. The isotopes obtained from a separated source of Ac.

Fig. 3. Total leakage current as a function of applied negative reverse bias for two 15.9 mm Schottky diodes.

was deposited on the backside for soldering the devices into packages. The doping of the epilayer was verified after the completion of device fabrication. The doping and the thickness were measured by reverse bias capacitance–voltage (CV) measurements. A representative doping profile is given in Fig. 2. A typcm ical nitrogen doping concentration of was measured. For this doping concentration, a reverse bias of (1130–1260) V is required to fully deplete the epilayer. Diodes with 4.5-mm and 6.0-mm diameter equivalents were manufactured. These diode shapes were close to rectangular to make better use of the SiC wafer area, but had areas corresponding to the diameter equivalents. The total leakage current as a function of applied reverse bias is plotted in Fig. 3 for the two 4.5-mm diodes used in the present measurements. III. MEASUREMENTS The SiC detectors described in the previous section were exposed in vacuum to Gd, Pu, Ac, Fr, At, and Po alpha particles. The reverse bias was 200 V in all measurements in order to form a depletion layer that is thicker Po 8376-keV alpha particles while than the range of the limiting the diode leakage current to less than 0.5 nanoamperes. Po alpha particles After penetrating the Au/Pt/Ti layers, the have an energy of 8022 keV and a residual range of 32.6 m

Fr,

At, and

Po were

in the SiC active layer. The SiC depletion depth is 39.3 m at 200 V. Check sources, which were prepared by electroplating carrier free isotopic material onto a stainless steel backing, were used Gd ( 3182.787 keV [100%]) and Pu ( in the 5499.2 keV [70.91%]; 5456.5 keV [28.98%]) cases. A Ac was prepared by recoil-ion implantation [6] source of Th source that had been previously from an electroplated separated from U. The isotopes Fr, At, and Po were present in this source as they are formed in the decay chain of Ac, which is shown in Fig. 4. The alpha particle energies for these recoil- separated isotopes range from 5830 to 8375 keV. The diameter of the Ac source is 6.35 mm and the Gd and Pu source diameters are approximately 1 mm. The sources were positioned 14 mm from the SiC detector face and the alpha particles passed through a 3.2-mm double collimator in order to reach the detector. This geometry restricts the alpha particles to near normal incidence on the detector and reduces broadening of the alpha particle energy distribution in the SiC active volume due to alphas with angles less than normal incidence passing through the Au/Pt/Ti layers. In all cases, the source thickness was negligible compared to the alpha particle range, resulting in minimal source contributions to the observed FWHMs. A reverse bias was applied to the SiC detector, which was connected to a Canberra 2003BT preamplifier. Bias to the SiC diode was provided by an Ortec 428 Bias Supply through the 2003 BT preamplifier. The preamplifier was connected to a Canberra DSA1000 digital multichannel analyzer (MCA). An HP Omnibook 6100 laptop computer was used with the Canberra software program Genie 2000 to obtain MCA pulse height spectra. A Canberra 814FP pulser was connected to the preamplifier through a modified preamplifier power cable. Pu peak centroid position as a A plot of the observed function of reverse bias potential is shown in Fig. 5. The depletion depth increases with applied voltage, and exceeds the

RUDDY et al.: HIGH-RESOLUTION ALPHA-PARTICLE SPECTROMETRY

Fig. 5.

Pu peak centroid as a function of applied reverse bias.

Fig. 6. Alpha counts as a function of pulse height (channel number) for various isotopes.

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Fig. 7. Observed channel number as a function of alpha particle energy for the Gd, Fr, At, and Po peaks shown in Fig. 2. The linear fit to the data has an R value of 0.99988.

Fig. 8. FWHM for a Gaussian fit to the

Gd Peak in SiC.

range in SiC of the Pu alpha particles at a bias of approximately 80 V. The value of the peak centroid increases correspondingly, as a greater fraction of the alpha particle energy creates ionization in the depleted layer until it reaches a thickness equal to the range of the alpha particles. Application of additional voltage to the diode will continue to increase the depletion thickness but does not result in a substantial additional increase in the peak centroid channel. IV. RESULTS A plot of alpha particle counts as a function of pulse height (channel number) for all of the alpha sources is shown in Fig. 6. Sharp alpha-particle peaks are observed for each of the isotopes. The alpha-emitters Gd, At, and Po are mono-energetic Fr provides two energy-resolved peaks. A plot of oband served peak centroid value as a function of alpha particle energy is shown for these isotopes in Fig. 7. Pulse height (channel number) versus energy is linear with an average pulse-height deviation of 0.3% from the straight-line fit to alpha energy. Gd alpha particles is shown The peak for 3182.787-keV in Fig. 8. A Gaussian function with a FWHM of 41.5 keV has been fit to the data. Data are shown for Fr, At, and Po in Figs. 9–11, respectively, with the corresponding Gaussian fits and FWHMs.

Fig. 9. FWHM for Gaussian fits to the

Fr peaks in SiC.

Generally, the high-energy side of the peak closely fits a Gaussian shape whereas a small amount of low- energy tailing is present in all of the peaks. The tailing is most pronounced for the sources produced by recoil-ion implantation and may be a result of energy degradation in the sources. Alternatively, the tailing may be a result of incomplete charge collection due either to charge trapping in SiC or ion-pair recombination along the alpha track. FWHM values could not be extracted Ac data shown in Fig. 12, because the multiple from the alpha peaks could not be completely resolved in energy by the

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Fig. 13. Fig. 10. FWHM for a Gaussian fit to the

Pu peaks in SiC. Gaussian fits to the doublet peak are shown.

At peak in SiC.

Fig. 14. Full widths at half maximum for alpha particle peaks in SiC as a function of alpha particle energy. Fig. 11. FWHM for a Gaussian fit to the Th/ Th impurities in the results from

Po peak in SiC. The Th source.

Po peak

V. DISCUSSION A plot of the FWHM values obtained for the Gaussian fits Gd, Pu, Fr, At, and Po peaks versus corto the responding incident alpha particle energy is shown in Fig. 14. The line width in the SiC detector is seen to increase with alpha particle energy. The FWHM that is measured with a semiconductor alpha spectrometer such as SiC is comprised of several contributions, which combine in quadrature as follows [7]:

where Fig. 12. Multiple alpha peaks from

Ac.

is the observed peak FWHM value; SiC detector. In the case of Pu, shown in Fig. 13, the two major alpha peaks are not resolved, but the doublet peak can be fit by the sum of two Gaussian shapes corresponding to the Pu peaks. A 46.4-keV peak intensities and energies for the FWHM results from this doublet fit.

is related to the noise from the front-end electronics; is related to the detector leakage current and any charge-collection limits within the active volume of the sensor;

RUDDY et al.: HIGH-RESOLUTION ALPHA-PARTICLE SPECTROMETRY

results from statistical fluctuation of the number of charge carriers produced by each alpha particle;

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TABLE I MEASURED AND CALCULATED FWHM VALUES FOR ALPHA PEAKS IN SIC

is inherent to the charge collection properties in SiC semiconductor; results from other sources, such as variations in energy attenuation in the entrance window caused by differences in the angles of incidence of the alpha particles on the entrance window, self-attenuation of the alpha energy in thick sources, etc. The broadening due to the detector electronics chain and leakage current fluctuations can be determined by injecting test pulses into the detector preamplifier. The leakage currents for the diodes tested were very small ( 1 nanoampere), and pulse height variations due to fluctuations in the leakage current will be a negligible contribution to the FWHM measured for the pulser. The energy equivalent of the electronics chain noise was determined from the test-pulse width to correspond to a Gd, the first FWHM component of 7.96 keV. In the case of two FWHM components are expected to contribute only 3.7% Gd. (8.0 keV) to the measured 41.5-keV FWHM for The statistical broadening component can be estimated from Fig. 15. Silicon PIP Gd alpha-response peaks measured before and after apA)/Pt(1000 A)/Ti(800 A) layers to the entrance window. plication of Au (9000

where is the energy necessary to produce an electron–hole pair in SiC, is the Fano factor, and is the alpha particle energy eV [5], [9]. [8]. For SiC, has been reported to be The Fano factor for SiC has been reported to be 0.04 [10]. If a value of 0.04 is used as an approximation of the SiC value, the , is estimated to statistical broadening component, Gd alpha particles. Therefore, this compobe 2.33 keV for nent is expected to contribute only a small fraction ( 0.3%) of Gd peak. the observed 41.5 keV FWHM for the As noted above, some of the components of should be negligible, because extremely thin sources were used in a distant, collimated geometry. However, the Schottky contact and cover metals, which comprise the entrance window to the SiC detector active volume, can produce a significant broadening of the energy distribution entering the active volume of the detector due to range straggling resulting from alpha energy loss processes within the entrance window. Calculations of the range straggling in the entrance window, which consists of gold (9000 ), platinum (1000 ) and titanium (800 ) layers, were carried out with the range-energy code SRIM [11]. The results are summarized in Table I. With Po, the calculated FWHMs resulting from the exception of range straggling caused by the window are greater than the measured total peak widths, indicating that the calculated values are incorrect. As a check on the present calculations, the energy straggling component reported by Schmid et al. [12] for a Ti(1000 )/Pt(2000 )/Au(10 000 ) contact on a diamond detector, 58.8 keV, was re-calculated and found to agree with the reported value.

Because of the unreasonable values calculated in the SiC case, we conclude it is not possible at present to accurately calculate the contribution of energy straggling from the entrance window to the measured SiC FWHM. An alternative is to measure this component. To accomplish this goal, the alpha energy resolution of a silicon PIP detector with a 500- silicon equivalent entrance windows was evaluated. After Au/Pt/Ti layers with dimensions identical to those of the SiC detectors were applied over the silicon detector window, the alpha energy Gd response resolution was re-evaluated. The resulting spectra are shown in Fig. 15. If the broadening in the silicon detector caused by apis large plication of the Au/Pt/Ti window silicon equivcompared to that caused by the 500alent layer alone where is the broadening caused by addition of the Au/Pt/Ti window. The FWHMs measured before and after application of the Au/Pt/Ti layers are 14.9 keV and 38.7 keV, respectively, re. If this FWHM sulting in a value of 35.7 keV for component and the FWHM components due to electronic and statistical broadening are subtracted in quadrature from the Gd alpha particles in SiC, an inherent measured FWHM for of 19.4 keV can be inferred. This value is comparable to the 20-keV value reported by Ivanov et al. [5]. The data reported in [5] were taken with a Schottky diode with a 1000 chromium entrance window, which causes much less broadening than the Au/Pt/Ti window used in the present work. However, the thinner chromium window will still contribute considerable peak broadening,

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indicating that the Ivanov value [5] also overestimates the true energy resolution for SiC. The data of Fig. 14 show that the FWHM increases with alpha-particle energy. This increase cannot be accounted for by the increase in statistical broadening alone and is not fully understood at this time. However, the increase is probably related to a combination of recoil ion implantation in the source backing and in the detector window before subsequent alpha decay. VI. CONCLUSION SiC detectors with active volume dimensions sufficient to stop alpha particles have been manufactured and tested. A linear energy response and excellent energy resolution have been obtained for various alpha emitters in the 3.18-MeV to 8.38-MeV energy range. Evaluation of the contributing factors to the SiC detector energy resolution indicates that the measured values are limited by energy straggling of the alpha particles as they pass through the metallic contact layers that comprise the entrance window to the detector. Based on a recently reported limit for the SiC Fano factor ( 0.04) [10] and the reported value for the energy per electron–hole pair in SiC (7.7 eV) [5], [9], the predicted statistical limit for the SiC energy resolution at 5486 keV is 3.06 keV. The corresponding statistical limit for Si detectors has been calculated to be 3.47 keV [13] using the Si energy per electron–hole value of 3.62 eV and a Fano factor of 0.11. The SiC statistical limit is predicted to be at least 12% less than the limit for Si. Therefore, it may be possible to obtain better energy resolution with SiC detectors than with Si detectors. A similar conclusion was reached by Phlips et al. [10] for the attainable SiC X-ray energy resolution. Bertuccio and Casiraghi [9] have also reported excellent energy resolution results for SiC X-ray detectors. Strokan et al. [14] have calculated the limiting alpha-particle energy resolution for SiC using Monte Carlo methods and obtained a FWHM value that is about half of the measured [5] value. The authors attribute the discrepancy to nonoptimal design of the SiC detector window. Regardless of the energy broadening caused by the entrance window, the SiC detectors used in the present work are useful for alpha spectrometry purposes. Fabrication of SiC diodes with Schottky contact thicknesses of 500 appears to be possible, and with such diodes, the possibility that the energy resolution that can be achieved with SiC may surpass that of silicon cannot be excluded. SiC alpha spectrometers offer a viable alternative to silicon detectors. SiC detectors have been demonstrated to operate at elevated temperatures of 89 C [2], 94 C [15], and even 304 C [16], whereas silicon detectors are limited by thermally generated charge carriers at temperatures as low as 30 C. Furthermore, SiC detectors have been shown to be much more resistant to gamma, neutron, and charged-particle irradiation effects [1], [17] than silicon detectors. SiC alpha spectrometers are expected to be useful in many nuclear environmental remediation applications where the ability to operate in high-temperature and high-radiation environments is required. Such applications include monitoring of alpha particles and neutrons in actinide waste-tank environ-

ments [18] as well as neutron and gamma-ray monitoring of spent nuclear fuel assemblies [19], [20]. REFERENCES [1] S. Seshadri, A. R. Dulloo, F. H. Ruddy, J. G. Seidel, and L. B. Rowland, “Demonstration of an SiC neutron detector for high radiation environments,” IEEE Trans. Electron Devices, vol. 46, no. 3, pp. 567–571, Mar. 1999. [2] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, S. Seshadri, and L. B. Rowland, “Development of a silicon carbide radiation detector,” IEEE Trans. Nucl. Sci., vol. 45, no. 3, pp. 536–541, Jun. 1998. [3] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, J. W. Palmour, and R. Singh, “The charged particle response of silicon carbide semiconductor radiation detectors,” Nucl. Instr. Meth. A, vol. 505, p. 159, 2003. [4] F. Nava, P. Vanni, C. Lanzieri, and C. Canali, “Epitaxial silicon carbide charge particle detectors,” Nucl. Instr. Meth. A, vol. 437, p. 354, 1999. [5] A. Ivanov, E. Kalinina, G. Kholuyanov, N. Strokan, G. Onushkin, A. Konstantinov, A. Hallen, and A. Kuznetsov, “High energy resolution detectors based on 4H-SiC,” in Proc. 5th Eur. Conf. Silicon Carbide and Related Materials 2004, R. Nipoti, A. Poggi, and A. Scorzoni, Eds. Zurich, Switzerland: Trans Tech Publications Inc., 2005, pp. 1029–1032. [6] F. H. Ruddy, A. R. Dulloo, J. G. Seidel, and B. Petrovic´ , “Separation of the alpha-emitting radioisotopes actinium-225 and bismuth-213 from thorium-229 using alpha recoil methods,” Nucl. Instr. Meth. B, vol. 213, pp. 351–356, 2004. [7] G. F. Knoll, Radiation Detection and Measurement, 3rd ed. New York: Wiley, 2000, p. 466. [8] U. Fano, “Ionization yield of radiations II. The fluctuations of the number of ions,” Phys. Rev., vol. 72, pp. 26–29, 1947. [9] G. Bertuccio and R. Casiraghi, “Study of silicon carbide for X-ray Detection and spectroscopy,” IEEE Trans. Nucl. Sci., vol. 50, no. 1, pp. 175–185, Feb. 2003. [10] B. F. Phlips, K. D. Hobart, F. J. Kub, R. E. Stahlbush, M. K. Das, B. A. Hull, G. D. Geronimo, and P. O’Conner, “Silicon carbide PiN diodes as radiation detectors,” in 2005 IEEE Nuclear Sciences Symp., Fajardo, Puerto Rico, Oct. 2005, Paper N34-6. [11] J. F. Ziegler and J. P. Biersack, SRIM-2003.26: The stopping and range of ions in matter. SRIM.com, Annapolis, MD, 2003. [12] G. J. Schmid, J. A. Koch, R. A. Lerche, and M. J. Moran, “A neutron sensor based on a single crystal CVD diamond,” Nucl. Instr. Meth., vol. A527, pp. 554–561, 2004. [13] G. F. Knoll, Radiation Detection and Measurement, 3rd ed. New York: Wiley, 2000, p. 393. [14] N. B. Strokan, A. M. Ivanov, A. A. Lebedev, M. Syväjärvi, and R. Yakimova, “The limiting energy resolution of SiC detectors in ion spectrometry,” Semiconductors, vol. 39, p. 1420, 2005. [15] G. Bertuccio, R. Casiraghi, A. Cetronio, C. Lanzieri, and F. Nava, “A new generation of X-ray detectors based on silicon carbide,” Nucl. Instr. Meth. A, vol. 518, p. 433, 2004. [16] F. H. Ruddy, A. R. Dulloo, B. Petrovic´ , and J. G. Seidel, “Fast neutron spectrometry using silicon carbide detectors,” in Reactor Dosimetry in the 21th Century, J. Wagemans, H. A. Abderrahim, P. D’hondt, and C. D. Raedt, Eds. London, U.K.: World Scientific, 2003, pp. 347–355. [17] F. H. Ruddy and J. J. Seidel, “The effects of intense gamma-ray irradiation on the alpha-particle response of silicon carbide semiconductor radiation detectors,” in 6th Int. Topical Meeting on Industrial Radiation and Radioisotope Measurement Applications (IRRMA-6), Hamilton, Canada, June 2005, Paper #63, (to be published in Nucl. Instr. Meth. B). [18] A. H. Fero and F. H. Ruddy, “Use of solid state track recorders and neutron transport calculations to characterize the actinide contents of a high-level waste tank,” in Reactor Dosimetry in the 21st Century, J. Wagemans, H. A. Abderrahim, P. D hondt, and C. De Raedt, Eds. London, U.K.: World Scientific, 2003, pp. 158–165. [19] A. R. Dulloo, F. H. Ruddy, J. G. Seidel, T. Flinchbaugh, C. Davison, and T. Daubenspeck, “Neutron and gamma ray dosimetry in spent-fuel radiation environments using silicon carbide semiconductor radiation detectors,” in Reactor Dosimetry: Radiation Metrology and Assessment, ASTM STP 1398, J. G. Williams, D. W. Vehar, F. H. Ruddy, and D. M. Gilliam, Eds. West Conshohoken, PA: American Society for Testing and Materials, 2001, pp. 683–690. [20] T. Natsume, H. Doi, F. H. Ruddy, J. G. Seidel, and A. R. Dulloo, “Spent fuel monitoring with silicon carbide semiconductor neutron/gamma detectors,” J. ASTM Int., Mar. 2006.