High resolution n-type Si surface barrier detectors for measurement of conversion electrons below 20 keV

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Nuclear Instruments and Methods m Physics Research 221 (1984) 582- 585 Nort h*Hc~ltand.Amsterdam

H I G H R E S O L U T I O N n-TYPE Si S U R F A C E BARRIER D E T E C T O R S FOR M E A S U R E M E N T O F C O N V E R S I O N E L E C T R O N S BELOW. 20 keV J.L.W. P E T E R S E N Laboratormrn voor Algemene Natuurkunde, Unwerstty Gromngen, Gromngen, The Netherlands

Recewed 29 September 1983

The performance of n-type S1 surface barner detectors manufactured m our laboratory is described Resoluuons of 600 eV and 660 eV fwhm were obtmned for the 13.6 and 7.3 keV conversion electron hnes of S7Co, respectwely These results were obtained using a 50 mm2 detector with an Au electrode of 20 #g/cm 2, dc-coupled to a cooled FET preamplifier with resistwe feedback The window thickness of tlus detector was 370 _+30 ,A Si equivalent at LN2 temperature

1. Introduction For the spectroscopy of electrons with energies > 50 keV cooled Si(Li) detectors coupled to cooled FET preamplifiers are commonly used with a resolution of 1 keV fwhm at 50 keV. However, for measurements of low energy electrons ( < 50 keV) the window thickness has a significant effect on the resolution and line shape Due to straggling of the electrons m the window which is composed of a thin Au electrode (200 A) and a relatively thick interracial dead layer, the resolution worsens with decreasing energy. For this reason Si(L0 detectors are not used for the detection of electrons with energies < 20 keV [1]. The window thicknesses published for Si(Li) detectors are in the range of 2000-5600 Si equivalent [1,21. It is the aim of tlus work to obtain a sdicon detector for the spectroscopy of electrons with energies < 20 keV. A thin window detector with high resolution would be very useful, for instance for multichannel depthselective conversion electron Mbssbauer spectroscopy (DSCEMS) [3] and for nuclear spectroscopy of isomeric transitions from yrast traps and other high spin states. In an accompanying paper [4] the considerable advantages of the use of such detectors, especially with regard to efficiency, are explained for DSCEMS measurements. We started from a normal n-type Si surface-barrier (s.b.) detector. For this type of detector low values of window thicknesses are reported [5-8]. In general, mainly due to the relatively large detector capacitance, the noise of an n-type Si s.b. detector-preamplifier system is higher than that of a Si(Li)-detector-preamplifier system. For this reason, up to now, the use of n-type Si s.b. detectors for the detection of electrons 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physxcs Publishing Division)

with energies ~ 20 keV has not been reported to our knowledge.

2. Description of the detectors and the pgeamplifier modification The detectors were made from n-type Si * with a specific resistivity of 10 kl2cm, using standard techniques. The lapped wafers with a diameter of 20 mm and a thickness of 2 or 3 mm were CP4-etched, rinsed with distilled water and mounted in a pyrex ring with n-type epoxy. Next the Au electrode at the front and the AI electrode at the back were evaporated and thin silver wires were attached to both electrodes using a one component silver glue. Electrode contacts attached with silver epoxy to the electrodes came apart upon cooling. In order to avoid detector breakdown during frequent cooling to LN 2 temperature we did not use epoxy rings around the electrodes. The metal layers at the epoxy interfaces may break upon cooling and for this reason standard commercial s.b. detectors with epoxy rings are only guaranteed to - 3 0 ° C . The detectors were mounted in a modified Si(L0 detector assembly head and de-coupled to a cooled FET preamplifier with resistive feedback. The modification consisted of three points: a) To obtain higher gain and signal-to-noise ratio the original 2N4416 FET was replaced by a selected 2N5397 FET and the feedback circuit was replaced by a 10 GI~ resistor. The stray capacitance of this resistor served as feedback capacitance. b) A positive bias voltage was applied to the A1 elec* Obtained from Wacker Chermtromc

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trode at the back and the signal was taken from the Au front electrode With this unusual set-up the electron peaks do not shift to lower energies since the Au electrode ~s at earth potentml. c) The A1 m o u n t i n g ring was not connected to the detector electrodes or to earth. Tins resulted m a reduced detector-stray capacxtance and thus m lower p r e a m p h f i e r noise. The assembly head is shown m fig 1. The Au electrode is connected to a sliver contact o n the pyrex m o u n t i n g n n g which extends to the back of this ring. To insulate the contact from the AI m o u n t i n g ring, a part of the latter was removed. The F E T with its teflon housing is placed eccentrically on an A1 pdlar to obtain a F E T g a t e - d e t e c t o r contact connection as short as possible To further reduce the stray capacitance a teflon d m p h r a g m instead of a metal one was used to c o l h m a t e the electrons and to protect the bare Sz surface The diameter of the d m p h r a g m was equal to the Au electrode dmmeter

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Ftg 2 The y and conversion electron lines of the 122 and 136 5 keV transition m 57Co measured wtth an LN 2 cooled 50 mm 2 n-type S l s b detector with 30 # g / c m 2 Au electrode and a depleuon layer thickness of 1 2 mm The energy scale is 70 eV per channel

30 # g / c m 2 Au electrode the line resoluuons improved to 500 and 750 eV fwhm, respectwely, using an Enertec Schlumberger type PSC 761 R preamplifier a n d an Ortec type 450 main amplifier with a 5 #s s h a p m g time. The electronic pulser resolution was 430 eV fwhm The results for the higher energy part of the spectrum are shown in fig. 2. The 750 eV fwhm resolution obtained for the 115 keV conversion electron hne may be comp a r e d w~th some published data. For S t ( L ) detectors the best measured value k n o w n to us IS 880 _+ 20 eV fwhm [1]. The result of the 50 m m 2 detector without guard ring compares also favourably with a resolution of 1000 eV fwhm for this line obtained by Avdelchikov [9] using a 30 m m 2 p-type s.b detector of 2.5 m m thickness provided with a guard ring to reduce the leakage current. F o r a 25 m m 2 n-type S I s b. detector of 1 m m thickness the reported [10] resolution of 2.6 keV fwhm of this h n e was mainly governed by the prea m p h f l e r norse F o r a 78 5 m m 2 n-type SI s.h. detector of 2 m m thickness p r o w d e d with a guard rang the reported [11] resoluuon of 1.6 keV fwhm at T--- 200 K

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3. Results To test the p e r f o r m a n c e of the detector a n d prea m p h f i e r system we used a 57Co source t m p l a n t e d at 15 keV in AI to first measure the resolution for the 14.4 keV y-hne a n d the 115 keV K-converston electron line of the 122 keV transition. F o r a 90 m m 2 detector with a 40 f f g / c m 2 Au electrode a n d a bias voltage of 600 V resolutmns of 850 a n d 1000 eV fwhm were obtained for these hnes, respecuvely. A Kevex type 2000 p r e a m p h f i e r a n d an Ortec type 452 main a m p h f i e r with a 2 # s shaping time were used. For a 50 m m 2 detector with a

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for the 320 keV conversion electron line of UJBa is equivalent with a resolution of I keV fwhm at 115 keV~ a result equal to that of our 90 m m 2 s.b.-detector without guard ring. As a next step the resolution for the 13.6 keV L-conversion electron line a n d the 7.3 keV K-conversion electron fine of the 14.4 keV transition was m e a s u r e d T o separate the y lines a n d electron hnes, the 3' spect r u m was m e a s u r e d separately using a n A1 a b s o r b e r foil between source a n d detector. This foil can be inserted w i t h o u t b r e a k i n g the v a c u u m in the cryostat by m e a n s of a magnet. The electron spectrum is then o b t a i n e d by s u b t r a c t i n g the 3' s p e c t r u m from the e + 3' spectrum T h e e + 3' a n d the e spectra o b t a i n e d with the 50 m m 2 detector with 3 0 / t g / c m 2 A u electrode are shown in figs. 3 a n d 4. In b o t h spectra the 13.6 keV conversion elect r o n line is clearly resolved with a resolution of 680 eV f w h m a n d a peak-to-tail ratio of a b o u t 8 was measured in the e spectrum. T h e 7.3 keV line is only resolved in the e spectrum a n d has a resolution of 780 eV. In figs 5

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Fig. 6 The 13.6 and 7 3 conversion electron and Auger electron lines of 57Co measured with an LN 2 cooled 50 mm 2 n-type Si s b. detector with 20/~g/cm 2 Au electrode The energy scale is 83 eV per channel

a n d 6 slrralar spectra are shown, o b t a i n e d with a 50 m m 2 detector, tbas time w~th a 20 # g / c m 2 A u electrode. D u e to reduced straggling in the window the resolution for the 13.6 keV a n d 7.3 keV conversion electron lines i m p r o v e d to 600 eV a n d 660 eV fwhm, respectavely a n d the peak-to-tad ratio i m p r o v e d to a b o u t 15 for the 13.6 keV line in the e spectrum. These resolutions include the c o n t r i b u t i o n of the finite source tluckness as given in table 1. To our knowledge the Auger electron lines have not been published before using a Si detector. The 7.3 a n d 13.6 keV conversion electron hnes have been seen with poor resolution with Si(Li) detectors. For the 14.4 keV 3' line a resolution of 530 eV was o b t a i n e d recently b y Geretschlager [8] using a 25 m m 2 Ortec n-type S1 s.b detector of 0.5 m m thickness, coupled to a special preamplifier with d r a i n feedback.

4. T h i c k n e s s o t d e t e c t o r w i n d o w

T h e energy loss of the 7.3 a n d 13.6 keV conversion electrons in t h e window of the detector provided with a 2 0 / x g / c m 2 A u electrode was extracted from the measured shifts of these lines with respect to the 14.4 keV y line. The results are gdven in table 1. The shifts were f o u n d to be nearly equal to the calculated [12] energy losses in the 2 0 / a g / c m 2 A u layer plus the 57Co source. F r o m these results a n d using k n o w n stopping data it

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follows that the window has a thickness of the eqmvalent of 370 _+ 30 ,~ Sl at L N 2 temperature This implies that the mterfacial dead layer has a thickness of less than 60 ,~ S1 equivalent at LN 2 temperature. This findm g is in agreement with the result of Geretschlager [8] who found 54 5 + 8.5 ,~ S1 equivalent, while Hsieh [6] measured < 200 ,~ $1 equivalent. In a stmilar way we also found that the window thickness of the detector with 30 ~ g / c m 2 Au electrode was almost totally determined by the sum of thicknesses of the Au layer and the source For b o t h detectors no change m the wmdow thickness ~s observed when the bias voltage is reduced from 600 V to 100 V This means that the window thickness is field i n d e p e n d e n t m the range of a b o u t 500 V / r a m to 200 V / m m . The thin window thickness of the n-type S I s b detectors measured at LN 2 temperature is not m accord a n c e with G o u l d m g ' s hypothesis [13] that all $1 detectors possibly have a window thickness of a b o u t 3000 ,~ as a result of back diffusion m t o the surface of charge carriers at T---77 K SI(L1) detectors do have such a window thickness It would be interesting to know ff the thicker mterfacml dead layer of SI(L0 detectors is correlated w~th the L~ c o m p e n s a t i o n itself or w~th the very h~gh (100 kf2cm) specific resIstwlty of the c o m p e n s a t e d material Very high resistivity material rnlght result m low effective fields at the surface leading to thicker windows as ts proposed m ref. [14]. To check this hypothesis it is worthwhile to measure the window thickness of an n-type S l s b. detector m a d e from very high resistivity material. If such detectors would also have a thin window, then such material could be used for large area s.b. detectors with a reduced capacitance a n d a resulting improved resolution. A second t m p r o v e m e n t of the present results may be achieved by a further reduction in the thickness of the front electrode. We experienced that a detector with a 9 / ~ g / c m 2 Au electrode had a poor barrier A detector wtth a 15 ~ g / c m 2 Au electrode survived only a short time m vacuum Also m refs. [5,7,15] it was found that 16 # g / c m 2 Au electrode thickness is a m i n i m u m value below which problems like a poor barrier or increased resistance due to structural effects in the Au layer were encountered. Nevertheless further research on the influences of surface treatment a n d evaporation conditions on the q u a h t y of very thin ( < 16 ~ g / c m 2) Au electrodes a n d their surface barriers might be useful. Also the application of Pd electrodes should be considered G o o d S1 s.b. detectors have been m a d e with Pd electrodes down to 6 1 / z g / c m 2 [16]. For such detectors w~th a very thin window, the height of the low-energy tad may be determined primarily by incomplete charge collection due to backscattering of electrons from the depletion layer T h e fraction of backscattered electrons mcreases with decreasing electron energy and rises for perpendicular incidence from 13% at 100 keV to 20% at

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50 keV [17,18]. We f o u n d n o data in the hterature for backscattering fracttons of electrons m SI detectors w~th energies < 50 keV

5. Conclusions It has been d e m o n s t r a t e d that it ~s possible that low-noise n-type silicon surface barrier detectors dccoupled to a cooled F E T p r e a m p h f i e r with reslstwe feedback can be used w~th high resolution for the energy selection of electrons with energies < 20 keV. These detectors are very useful for high effictency depth-selective conversion electron Mossbauer spectroscopy and we m e n t i o n t h o r use for nuclear spectroscopy of isomeric transitions from yrast and other high-spin states I gratefully t h a n k Dr. J. van Kllnken * for introducing and stlmulatmg this investigation a n d Dr D.O. Boerma for helpful discussions and reading of this paper. This investigation was financed by the Stlchtlng voor F u n d a m e n t e e l Onderzoek der M a t e n e ( F O M ) subsldlzed through the N e t h e r l a n d s Organzzatxon for the A d v a n c e m e n t of Pure Research (ZWO)

References [1] I Ahmad and F Wagner, Nucl Instr and Meth 116 (1974) 465 [2] R G Musket and W Bauer, Nucl Instr and Meth 109 (1973) 593 [3] U Baverstam, Mossbauer effect methodology, vol 9 (Plenum, New York, 1974) pp 259-276 [4] S C Panchoh, H de Waard, A v.d WlJk, J L W Petersen and J van Khnken, Nucl Instr and Meth this issue, p 577 [5] E Elad, C N lnskeep, R A Sareen and P Nestor, IEEE Trans Nucl So 20 (1973)534 [6] K C Hsleh, Nucl Instr and Meth 138 (1976)677 [7] F M lpavlch, R A, Lundgren, B A Lamblrd and G Gloeckler, Nucl Instr and Meth 154 (1978) 291 [8] M Geretschlager, Nucl Instr. and Meth 192 (1982) 117 [9] V V Avdochlkov, E A Ganza and V P Pnkhodtseva, Nucl Instr and Meth 133 (1976) 579 [10] T Tamura and K Kuroyanagl, Nucl Instr and Meth 67 (1969) 38 [11] R D Ryan, IEEETrans Nucl Scl 20(1)(1973)473. [12] E Segr& Nuclei and particles (W A Benjamin, New York, 1964) p 32 [13] F S Gouldlng, Nucl Instr and Meth 142 (1977) 213 [14] J Llacer, E E Hailer and R C Cord1, IEEE Trans Nucl So 24 (1977) 53 [15] J M Jaklevlc and F S Gouldmg, IEEE Trans Nucl Sct 18 (2) (1971) 187 [16] C Inskeep, E Elad and R A Sareen, IEEE Trans Nucl Scl 21 (1974) 379. [17] B Planskoy, Nucl Instr and Meth 61 (1968)285 [18] A Damkjaer, Nucl Instr and Meth 200 (1982) 377 * Kernfysxsch Versneller lnst~tuut, Umvers~ty of Gronlngen