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Long-time behaviour of Si(Li) detectors
NUCLEAR
AND METHODS 118 (1974) 237-246 ;
NORTH-HOLLAND -HOLLAND PUBLISHING Ct)
LONG-TIME BEHAVIOUR OF Si(IJ) DETECTORS M. SLAPA Institute of Nuclear Research, .gawierk, Poland Received 10 Oktober 1973 The long-time behaviour of silicon lithium-drifted detectors was studied in the course of storage at 60 °C with and without a bias voltage. Some observations on the behaviour of these detectors stored at room temperature for periods between one half and
two years were also made. It has been found that the observed changes of detector parameters are due primarily to migration of lithium ions in silicon.
1. Introduction
experimental re:.=.+les have been compared with theoretical estimations .
Lithium-driftedsilicon detectors exhibit anumber of advantages which promote their growing use in spectroscopy and detection of ionizing radiation . Some
limitations of their appiicability result, however, from an appreciable mobility of lithium ions in silicone), even at room temperature. Consequently, the lithium
distribution varies dueto migration, nucleonization and precipitation of lithium ions. Therefore, in the course of long-term operation, changes of the detector parameters may be afticipmed . These changes should
be particularly pronounced in the case of long-term 2.3) . operation at higher temperatures (40-60°C) Thus far, only a few results have been published on the long-term behaviour of drifted detectors .
Mayer et al .) observed rapid changes of detector capacity during seven days of storage at room temperature without bias . On the other hand . Tuzzolino et al .'-") and I . Hayashi et al') failed to detect any
measurable changes in the basic parameters of a Si(Li) detector stored at room temperature for a period of 100d to 1 y. Studies at elevated temperature were also reported"). The drifted detectors were stored with a bias voltage at 50°C for two weekss), at 85°C for one
week') and at 100°C for several hours') respectively. The detector parameter changes after such short times of observation at elevated temperatures were
negligible. The aim of this work was to study the behaviour of drifted detectors stored at 60°C with and without bias voltage. The measurements cover a period of about 45 weeks in the case of silicon lithium-drifted (SLD) detectors and about seven weeks in the case of silicon lithium-drifted surface barrier(SLDSB;detectors. Some observations on the behadour of SLD and SLDSB detectors stored at room temperature for a period between one half and two years were also made. The
2. Tinte-variation of lithium distribution in drifted detectors A time-dependence of the lith'-um distribution in a drifted detector can be expressed analytically as a solution of the continuity equation with the boundary
conditions determined by the diffusion, drift and clean-up processes'-") (fig. I). Fig. l a presents the case of an ideal compensation in a drifted detector and fig . lb the most frequently encountered case of lithium distribution in drifted detectors . The one-dimensional continuity equation can be written in the form): L átL - DLa_ a
+PL
ax
(NLE)
-ù
,
(1)
where: DL- diffusion constant of lithium ions in Si, NL - mobility of lithium ions in Si, NL- lithium concentration, E - electric field intensity, v - mean rate of the decay of electrical activity of lithium ions in Si.
The first . .two terms on the right-hand side of eq . (1) describe the processes of lithium migration as a superposition of the processes of diffusion and drift . An approximate solution of eq . (1) with thethird tern omitted has been given by Carslawee ) . The predicted changes on the boundaries of the compensated region may be expressed in units (DLt)t or PLEt for the detector stored without and with a bias voltage, respectively. The third term in eq. (1) describes possible changes of the electrical activity of lithium ions in Si, resulting from the processes of lithium
237
238
M. SLAPA
EDGE REGION
Nq
rr
IEPCEWGION
as
1.0
DISTANCE `hllY
a
Fig. I . Distribution of lithium ions in a Si(Li) detector . a) The case of ideal compensation ; b) thecase of a constant concentration gradient of lithium ions in the"compensated" region.
nucleonization and precipitation. These processes may be induced by the interaction of lithium ions with vacancies, substitutional oxygen atoms'), dislocations 2), oxygen complexes' °) and vacancy clusters'2). The probability of such interaction and, consequently, the rate of nucleonization and precipitation decreases with decreasing concentration of lithium atoms in Si. In the case which is of most interest to us, i.e ., when lithium-ion concentrationis of the orderof 10" cm-' (compensated layer) and the silicon parameters correspond to the material commonly used in the detector technology, the above processes are- highly improbable'-") and the third term in eq. (1) may be disregarded. However, these processes are virtually possible and in specific cases may lead to instability of the lithium distribution in the detectór4-t2).
10° cm -2. The oxygen concentration in silicon, as specified by the manufacturer, was :51016 oxygen atoms/cm3. To avoid changes in oxygen concentration from one detector to another, the study of SLD detectors was performed usinga set prepared from one silicon single crystal . 4. Drifted detectors with surface barrier - SLDSB 4.1 .
THE CHARACTERISTICS OF THE SLDSB DETECTORSSTORED AT 600C STUDY OF
The SLDSB detectors studied in this work were provided with guard-rings described previously") . In comparison with typical SLDSB detectors, the structuré of these detectors (fig . 2) enables additional information to be obtained by a measurement of the resistivity of the gap between thecentral electrode and 3. Detectors the guard-ring . The detectors used for measurements were prepared This resistivity may be presented as a surface resistifrom p-type vacuum-grown floating-zone silicon . The vity Si02 layer and the resistivity of an incompletely preparation technology has been described else- compensated surfacelayer, connected in parallel (fig. 2) . where'4" ' 5). Silicon resistivity ranged from 800 to The parameters measured were: I- V characteristics ., 1 SBD D cm, the minority carrier lifetime was about resistivity of the gap between thecentral electrode and 1000 its and the dislocation density was less than the guard-ring, energy resolution for 21 °Po a-particles
LONG-TIME BEHAVIOUR OF MWIMETE Low"' S4TpN LAYER.
Fig. 2. An SLDSBdetector with evaporated guard-rings .
and z°'Bi ß-particles, noise figure of the detector and dead layer thickness . The measurement sequence was as follows: a) After forming the surface barrier, the SLDSB detectors were stored at room temperature without bias for a few days. b) Next, all initial characteristics were measured. k c) The detectors were placed in hermetically sealed containers filled with dry air and stored at 60°C with and without a reserve bias voltage. The bias voltage value was 30 V. d) After a given time, the detectors were removed from the thermostat and the I-V characteristics; and gap resistivities were measured . e) After such measurements, the detectors were again placed in the thermostat and stored at 60°C . f) After a given time, the detectors were removed from thethermostat, thecontainers were opened andthe remaining characteristics were measured . g) It was possible to seal the detectors in the containers again and to repeat the storage and measurement cycle. Fig. 3A presents the I- V characteristics of the detector stored without bias for five weeks and next with bias for about two weeks. Similar characteristics for the detector stored all the time with bias are shown in fig. 3B. Also shown is the block-scheme of the measurement system for detector current measurements. The I-Vcharacteristics of the detectors stored without bias change insignificantly after a five week period while those of the detectors stored with bias vary markedly even after one week of storage. The gap resistivity of the detector stored for five weeks with no bias and next for about two weeks with bias is shown in fig. 4. The time-dependence of this resistivity can easily be explained by lithium distribution changes in the nearsurface layer .
Si(Li) DETECTORS
239
During the period of storing the SLDSB detectors with nc bias (left), a broadening of the concentration profile in the near-surface region occurs as a result of diffusion, and theresistivity decreases. In theright part of the curve, comlx,nsation in thA layer recovers is a result of lithium drift produced by the external bias voltage and the resistivity increases. Fig. 5 presents the spectra of 2tePo a-particle detected by the studied SLDSB detectors: a) following directly on the preparation process, b) after five weeksof storage without bias, c) after an additional twoweeks of storagewith bias . The dead layer thicknesses in this detector, afterfive weeks of storage without bias (fig. 5b), were 2.5 ,um, 0.6 pin and 0.1 pre for bias voltages of 100 V, 200V and 300 V, respectively. After additional storage of the detector for about two weeks with bias, these thicknesses were 0.85 pre and 0.1 pre for the bias voltages 100 V and 200 V, respectively . Thebehaviour of deeper x
, _. 7t
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Al
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Fig. 3. The I-Vcharacteristics of the All detector ; (AI) before annealing, (A2) after 5.3 weeks of annealing without bias, (A3) after the next 2 weeks of storage with a bias . Also shown are the I- V characteristics of the B-71 detector stored with a bias; (Bl) before annealing, (B2) after 1 week of annealing, (B3) after weeks of annealing.
M . SLAPA layers of the detector were studied using deeply penetrating 2o7Bi ß-particles (range 1 .6 mm) . Fig . 6 presents the spectra produced by the detector studied and measured after the same time periods as before. It may be seen from fig . 6 that the observed degradation of energy resolution results from a change of noise properties of the central electrode fwhm of the generator peak.
Fig . 7 presents a dependence of the resistivity of the gap between the central electrode and the guard-ring on the annealing time for the detector stored at a constant bias voltage Up = 30 V . As a result of a continuous process of lithium drift to the insulating SiO2 layer, this resistivity decreases and, consequently, the junction may disappear. After seven weeks of storage, the central electrode current had a break-down
100
10
TlHE
r NEEKS
Fig. 4. The resrstivity of the gap between the central electrode and the guard-ring of an SLDSS detector as a function of annealing at +ó0`C.
LONG-TIME BEHAVIOUR OF
DETECTORS
Si(Li)
2al
character . (In spite of a low value of 1, about 1 pA, it was impossible to measure the signal produced by L MeV particles.)
surface barrier were also studied. After this storage time, these detectors exhibited an energy resolution of 14-18 keV for the 207 Bi ß-particles and a collection coefficient of 99.8%. The dead layer thickness estimated on the basis of the Zt°Po a-particle measurement 4.2 . BEHAVIOUR OF SLDSB DETECTORS STORED ranged between 0.1 and 0.3 ftm at a bias voltage of AT ROOM TEMPERATURE Several SLDSB detectors stored at room temperature 200-300 V.. Fig. 8 presents the I- V characteristics of the central without bias during a period of 2-3 y after forming the .3nev S AFTECTOR All
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fabrication process. (b) after storing for about 5 5. The spectrum of the =t0 Po a-particles detected by the detector ; (a) following the weeks without bias, (c) after storing for the next 2 weeks with a bias .
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The spectra of the ze7Hi ß-particles detected by the detector at the same times as in fig. 5.
rar`
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LON
TIME BEHAVIOUR OF
Si(Li)
DETECTORS
243
5. Drifted detectors- SLD ,Iectrode, measured before and after two years storage with no bias. The form of the characteris Behaviour of SLD detectors stored as 60`C the existence of a obtained after this period indicates The SLD detectors, after the preparation process low-resistivityuncompensatedlayer betweenthecentral described before (section 3), were placed in hermetielectrode and the guard-ring. As the bias voltage cally sealed metal containers in a dry air atmosphere increases, the low resistivity layer saturateswith space- and the initial characteristics were measured. Next,the charge and the proper action . of the guard-ring is detectors were stored at 60 °C without bias voltage, observed . with a bias voltage Up =30 V and with a "minimum" bias (see below) equalto 4.5 V. After a give%, time,the detectors were removed from thethermostat, cooled to room temperature and the measurements were performed. The parameters measured were as follows: I-V characteristics, efficiency of the detection of Yradiation from the t3 'Cs source at fixed conditions, dependence of theabove efficiency on the bias voltage, capacity characteristics anddead layer thickness of the n'and p' sides as a function of bias voltage. The I-V characteristics of the SLD detectors (ex.ept one, see below) did not deteriorate during the observation period . The efficiency of 7-radiation detection is a function of the real active volume of the detecoor and of the charge coefficient. At a sufficiently high electric field intensity in the detector (see fig. 10), this efficiency depends exclusively on the real active volume of thedetector"') . Consequently, theefficiency measurement permitsdeterminationofpossible changes of the detector's active volume during the storage period . A dependence of the efficiency on the detector thickness has been determined on the ground of literature data 16,l7) . Fig. 9 presents theeffi,,.iency of the '"Cs 7-radiation NfF CNFFFS] detection in thecourse c:f annealing at 60°C for about 45 weeks and at 23'C for about 30 weeks. The effiFig. 7. The resistivity of the gap between the central electrode ciency was measured using a fixed source-detector and theguard-ring in an SLDSB detector stared with a bias at geometry, a fixed discrimination level E,, = 100 keV, a 60'C as a function of annealing time . fixed bias voltage Up=80 V and a fixed shaping time Tt = Ty =0.5 its . The measurement error results 2 mainly from instability of the discrimination level avid was estimated to be ±4%. It may be seen from fig. 9 á that the efficiency of the detectors stored at 60`C without bias deteriorates and dec-,eases by 25°! after 46 weeks (fig. 9a). The SLD detectors stored at the W same temperature with a bias voltage Up = 30 V 0.5 DETECTOR R-A exhibit a constant efficiency within the experimental Sr d " ?3mm o - 1N-At ,rrsraGFG.ia.0,3ma; error. K-- AFTER 2 YEAR STORAGE AT ABOUT 23C The minimum bias voltage securing almost constant efficiency was determined from the preliminary measurements as Up = 4.5 V. It may be seen from fig. 9b 100 200 300 that this bias results in about 81 decrease of the APPLIED BIAS [V] detection efficiency for ,-radiation after 50 weeks of Fig. g. The !-V characteristics of the central electrode of an storage at 60'C. SLDSBdetector, before and after storage of2yearswith no bias .
244
M. SLAPA
The efficiency of all detectors studied, which were next stored at 23'C for about 30 weeks, was constant within the experimental error. In the case of the P-8 detector (fig. 9c) stored at 60°C with a bias Up =10Y, the reverse current increased from I uA to about 25 uA after 23 d. At that time, the bias was switched off and the detector was further stored at 60°C. After ten days of storage in these conditions, the reverse current &-creased from 25 NA to about 2 uA . Next, the detector was annealed with a bias Up == 30 V. The initial window thickness of the P-8 detector was abo t 1011m and the observed deterioration of the I-V characteristics was related to the removal of the p + i junction, resulting from lithium drift into the junction region . After a successive annealing without bias, the junction recovers as a result of lithium out-diffusion from the junction region . Fig. 10 presents a dependence of the detection efficiency of the MCS 7-radiation on the bias voltage, measured before and after 45 weeks of storage at 60°C . It is seen from fig . l0a that the efficiency in the detectors stored without bias varies with bias voltage (plateau region missing) and at a detector bias voltage Up = 80 V is lower by 25% after 46 weeks of storage. In the detectors stored with a bias voltage, the change
of active volume after 45 weeks is negligible and'' amounts to about +3% for Up = 30 V (fig . 10c) and about - 9% for Up = 4.5 V (fig . 10b) . Fig. lI presents the capacity characteristics of the SLD detectors measured before and after 45 weeks of storage at 60°C. As a result of a continuous lowtemperature drift process in the detectors stored with a bias voltage, the ideal compensation is attained over the whole compensation region (fig. 1) and the capacity characteristic reveals a plateau starting from a value as low as 30 V. In this case, the capacity has decreased by about 8% as compared with the initial value . In the case of storing the SLD detectors without bias, the diffusion processes [eq. (1)] result in a broadening ofthe compensation region (fig. 1). Consequently, the active layer th ; .kness decreases and hence' the detector capacity increases. The capacity increment for Up = 80 V amounts to 10%, after the -+eriod of storage. The relative contributions to the capacity change, resulting from the thickness changes of the detector dead layers in the course of annealing at 60'C may be obtained from fig. 12 . This figi ;re presents the dead layer thickness of the p + (fig. 12a) and n' (fig. 12b) contacts, measured after the annealing process . The dead layer thickness
----
DETECTOR P-12 DETECTOR P-13
DETECTOR P-15
STORA6E4T.23C----
-- --I
- DETECTOR P-8 " - DETECTOff P"6
!0
30
40 Tl1VE (WEEKS]
SO
60
70
80
Fig. , 9. detection efficiency of the 137CS 7-radiation as a function of storage time for the detectors stored: without bias and with bias voltages of 4 .5 V and 30 V.
245
LONG-TIME BEHAVIOUR OF Si(Li) DETECTORS
was determined by measurement of the line shift of dead layer thickness on thep+ contact side wasslightly monoenergetic 2°'Bí electrons with respect to the reduced. For instance, thechange forthe P-12 detector position obtained, using the SLDSB detector with full amounted to about 1501em. The C-V characteristics and dead layer thickness charge collection . The initial dead layer thickness of the n' contact ranged between 801ím and 1201ím measured after 30 weeks of storage at about 23'C for all detectors studied and was independent of bias exhibited no detectable changes. voltage above 70-80V. The final dead layer thickness at Up=300 V, measured for the detectors stored 6. Conclusions without bias, was equal to the respective initial thickThe results obtained lead to the following concluness within the experimental, error . This observation is sions: confirmed by the C-V characteristics (fig. i la). The 1) Main changesof theparameters of theSLDSBand thickness of the uncompensated-layer (space-charge SLD detectors stored at room temperature and at region) for the P-12 detector stored without bias was 60°C with and withoutbias result from lithium concenestimated on the ground of the data of fig. 12 as tration changes on the boundaries of the compensated 801ím and 1201em on the n*' and p+ side, respectively. region . The thickness on the n' contact side for the detectors 2) The observed lithium concentration changes may annealed with bias voltage UP = 30 V and Up= 4.5 V, be described by migration of lithium ions in silicon were practically equal to the initial values whereas the [eq. (1)j with the third term omitted. The range of concentration change in the case of storing is the detector without bias may !:e ~stir .ac, d,~,5(L'tt)} ").Thisquantity, determined foi ,- `'-12 detector from fig. 12, amounts to 1201ím . Thedifu,sov
DETECTOR P- I? UP . D
á
K- IN:IfAI
DETECTOR P-15 UP-4,5V ä " mn
V
-- AFTER 46 WEEKSTORAGE ATE 'C
-
" -AFTER 50 AEEK SWAGE AT60T . -WITIAL
DETECTOR P-6 up -3u
01-f
- INITIAL
" - AFTER 44 WEEK SWAGE AT60 "6
APRIEP
lm
I%TC6E
150
[VI
30mm'
to
DETECTOR P-6 Y 30V
- arma
*
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Fig. 10 . Detection efficiency of theta7Cs y-radiation for various bias voltages of the detectors stored under the same conditions as those in fig. 9.
. - AFTER 44 WEEK STORAGE Á .'60'C
too
200 APPE1E0 VOITAGE (V]
901)
Fig . 11 . The C- V characteristics of SLD detectors before and after annealingforabout 45 weeks at 60'C.
24 6
M. SLAPA'
constant of lithium ions at 60°C, estimated on thebasis o¬ this value is DL = 1 .7 x 10- 13 cm2/s. This value is in goad agreement with the diffusion constant of lithium
storage . For the detectors studied (d.. 1.4 mm), this voltage was estimated as Up =4.5 V. The value of this voltage can be derived from the condition for : the compensation of the 'flux of diffusing and drifting lithium ions. -
room temperature is equivalent to the behaviour otsermd at 60°C with the diffusion constant of lithium ions reduced by about a factor of 80 2). 4) A 20% reduction in the detectionefficiency of y-
The experimental results indicate that in` agreement with Pell's suggestions, the most suitable material for detectors operating at elevated temperatures is silicon
5) There exists a minimum bias voltage giving negligible changes of the detector parameters during
detectors.
ions at an oxygen concentration of 1016atoms/cm31 6). 3) Thelong-term behaviour of the detector stored at
radiation observed in the detectors annealed' without bias voltage for about 45d combined with only 10% change of the detector capacity, can be explained by a radial contraction of the active layer estimated to be about 0.2 mm .
with an oxygen concentration exceeding' 1011 atoms/ cm'. It also seems that the effect of the third term in eq. (1) should be considered in each specific case, since both the existing literature data and our observations indicate that in some cases this term may significantly affect the lithium distribution stability in Si(Li) T should ' like to express my gratitude to Dr J. Chwaszczewska, Mr E Belca :z'and Mr W . Nowicki for valuable discussions and theirhelp during performance of theexperiment .
References
nb aanga emlr7 Fig. 12. Dead layer thickness of the n+ and p+ contacts measured for selected SLD detectors after annealing at +60°C for about 45 weeks .
1 ) E. M. Pell, J. Appl. Phys. 31 (1960) 291. 2) E. M. Pell, Phys. Rev. 119 (1960) 1222. 3) R. A. Swalin and R. D. Weltrin, Progress in solid state chemistry, vol . 2 (1965) . 4) J. W. Mayer, N. A. Baily and H. L. Dunlap, Conf. on Nuclear Electronics (IAEA, Belgrade, 1961) . s) A. J. Tuzzolino, J. Kristoff and M. A. Perkins, Nucl. Instr. and Meth. 36 (1965) 73. 6) A. J. Tuzzolino, M. A. Perkins and I. Kristofl; Nucl. Instr. and Meth. 48 (1967) 33. 7) I. Hayashi, H. E. Kern, I . W. Rodgers and G. H. Wheatley, IEEE Trans. Nucl . Sci. NS-13 (1966) 214. s) P. E. Gibbons, Nucl. Instr. and Meth. 16 (1962) 284. s) A. Lauber, Report AE-356. lo) G. Bertolini and A. Coche, Semiconductor detectors (NorthHolland Publ. Co., Amsterdam, 1968). 11) H. S. Carslaw and 1.C. Jaeger, Conduction of heat in solids (2nd ed. ; Oxford University Press, 1959) . 1-) H. J. Guislain, W. K. Schoenmaekers and L. H. De Laet, Nucl. Instr. and Meth . 101 (1972) 1. 13) H. R. Kegel, IEEE Trans. Nucl. Sci . NS-15 (1968) 332. 14),J. Chwaszczewska, M. Dakowski, W. Przyborski and M. 'Sowihski, Nukleonika 10 (1965) 251. 15) E. Belcam, J. Chwaszczewska, M. Slapa, M. Szymczak and J. Tys, Nucl . Instr. and Meth. 77 (1970) 21. 16) N. A. Baily and G. Kramer, Radiation Res. 22 (1964) 53 . 17) A. R. Jones, AECL Report No. 3455 (1%9). 1s) E. M. Peil, J. Appl. Phys. 32 (1961) 1048 .
AND METHODS 118 (1974) 237-246 ;
NORTH-HOLLAND -HOLLAND PUBLISHING Ct)
LONG-TIME BEHAVIOUR OF Si(IJ) DETECTORS M. SLAPA Institute of Nuclear Research, .gawierk, Poland Received 10 Oktober 1973 The long-time behaviour of silicon lithium-drifted detectors was studied in the course of storage at 60 °C with and without a bias voltage. Some observations on the behaviour of these detectors stored at room temperature for periods between one half and
two years were also made. It has been found that the observed changes of detector parameters are due primarily to migration of lithium ions in silicon.
1. Introduction
experimental re:.=.+les have been compared with theoretical estimations .
Lithium-driftedsilicon detectors exhibit anumber of advantages which promote their growing use in spectroscopy and detection of ionizing radiation . Some
limitations of their appiicability result, however, from an appreciable mobility of lithium ions in silicone), even at room temperature. Consequently, the lithium
distribution varies dueto migration, nucleonization and precipitation of lithium ions. Therefore, in the course of long-term operation, changes of the detector parameters may be afticipmed . These changes should
be particularly pronounced in the case of long-term 2.3) . operation at higher temperatures (40-60°C) Thus far, only a few results have been published on the long-term behaviour of drifted detectors .
Mayer et al .) observed rapid changes of detector capacity during seven days of storage at room temperature without bias . On the other hand . Tuzzolino et al .'-") and I . Hayashi et al') failed to detect any
measurable changes in the basic parameters of a Si(Li) detector stored at room temperature for a period of 100d to 1 y. Studies at elevated temperature were also reported"). The drifted detectors were stored with a bias voltage at 50°C for two weekss), at 85°C for one
week') and at 100°C for several hours') respectively. The detector parameter changes after such short times of observation at elevated temperatures were
negligible. The aim of this work was to study the behaviour of drifted detectors stored at 60°C with and without bias voltage. The measurements cover a period of about 45 weeks in the case of silicon lithium-drifted (SLD) detectors and about seven weeks in the case of silicon lithium-drifted surface barrier(SLDSB;detectors. Some observations on the behadour of SLD and SLDSB detectors stored at room temperature for a period between one half and two years were also made. The
2. Tinte-variation of lithium distribution in drifted detectors A time-dependence of the lith'-um distribution in a drifted detector can be expressed analytically as a solution of the continuity equation with the boundary
conditions determined by the diffusion, drift and clean-up processes'-") (fig. I). Fig. l a presents the case of an ideal compensation in a drifted detector and fig . lb the most frequently encountered case of lithium distribution in drifted detectors . The one-dimensional continuity equation can be written in the form): L átL - DLa_ a
+PL
ax
(NLE)
-ù
,
(1)
where: DL- diffusion constant of lithium ions in Si, NL - mobility of lithium ions in Si, NL- lithium concentration, E - electric field intensity, v - mean rate of the decay of electrical activity of lithium ions in Si.
The first . .two terms on the right-hand side of eq . (1) describe the processes of lithium migration as a superposition of the processes of diffusion and drift . An approximate solution of eq . (1) with thethird tern omitted has been given by Carslawee ) . The predicted changes on the boundaries of the compensated region may be expressed in units (DLt)t or PLEt for the detector stored without and with a bias voltage, respectively. The third term in eq. (1) describes possible changes of the electrical activity of lithium ions in Si, resulting from the processes of lithium
237
238
M. SLAPA
EDGE REGION
Nq
rr
IEPCEWGION
as
1.0
DISTANCE `hllY
a
Fig. I . Distribution of lithium ions in a Si(Li) detector . a) The case of ideal compensation ; b) thecase of a constant concentration gradient of lithium ions in the"compensated" region.
nucleonization and precipitation. These processes may be induced by the interaction of lithium ions with vacancies, substitutional oxygen atoms'), dislocations 2), oxygen complexes' °) and vacancy clusters'2). The probability of such interaction and, consequently, the rate of nucleonization and precipitation decreases with decreasing concentration of lithium atoms in Si. In the case which is of most interest to us, i.e ., when lithium-ion concentrationis of the orderof 10" cm-' (compensated layer) and the silicon parameters correspond to the material commonly used in the detector technology, the above processes are- highly improbable'-") and the third term in eq. (1) may be disregarded. However, these processes are virtually possible and in specific cases may lead to instability of the lithium distribution in the detectór4-t2).
10° cm -2. The oxygen concentration in silicon, as specified by the manufacturer, was :51016 oxygen atoms/cm3. To avoid changes in oxygen concentration from one detector to another, the study of SLD detectors was performed usinga set prepared from one silicon single crystal . 4. Drifted detectors with surface barrier - SLDSB 4.1 .
THE CHARACTERISTICS OF THE SLDSB DETECTORSSTORED AT 600C STUDY OF
The SLDSB detectors studied in this work were provided with guard-rings described previously") . In comparison with typical SLDSB detectors, the structuré of these detectors (fig . 2) enables additional information to be obtained by a measurement of the resistivity of the gap between thecentral electrode and 3. Detectors the guard-ring . The detectors used for measurements were prepared This resistivity may be presented as a surface resistifrom p-type vacuum-grown floating-zone silicon . The vity Si02 layer and the resistivity of an incompletely preparation technology has been described else- compensated surfacelayer, connected in parallel (fig. 2) . where'4" ' 5). Silicon resistivity ranged from 800 to The parameters measured were: I- V characteristics ., 1 SBD D cm, the minority carrier lifetime was about resistivity of the gap between thecentral electrode and 1000 its and the dislocation density was less than the guard-ring, energy resolution for 21 °Po a-particles
LONG-TIME BEHAVIOUR OF MWIMETE Low"' S4TpN LAYER.
Fig. 2. An SLDSBdetector with evaporated guard-rings .
and z°'Bi ß-particles, noise figure of the detector and dead layer thickness . The measurement sequence was as follows: a) After forming the surface barrier, the SLDSB detectors were stored at room temperature without bias for a few days. b) Next, all initial characteristics were measured. k c) The detectors were placed in hermetically sealed containers filled with dry air and stored at 60°C with and without a reserve bias voltage. The bias voltage value was 30 V. d) After a given time, the detectors were removed from the thermostat and the I-V characteristics; and gap resistivities were measured . e) After such measurements, the detectors were again placed in the thermostat and stored at 60°C . f) After a given time, the detectors were removed from thethermostat, thecontainers were opened andthe remaining characteristics were measured . g) It was possible to seal the detectors in the containers again and to repeat the storage and measurement cycle. Fig. 3A presents the I- V characteristics of the detector stored without bias for five weeks and next with bias for about two weeks. Similar characteristics for the detector stored all the time with bias are shown in fig. 3B. Also shown is the block-scheme of the measurement system for detector current measurements. The I-Vcharacteristics of the detectors stored without bias change insignificantly after a five week period while those of the detectors stored with bias vary markedly even after one week of storage. The gap resistivity of the detector stored for five weeks with no bias and next for about two weeks with bias is shown in fig. 4. The time-dependence of this resistivity can easily be explained by lithium distribution changes in the nearsurface layer .
Si(Li) DETECTORS
239
During the period of storing the SLDSB detectors with nc bias (left), a broadening of the concentration profile in the near-surface region occurs as a result of diffusion, and theresistivity decreases. In theright part of the curve, comlx,nsation in thA layer recovers is a result of lithium drift produced by the external bias voltage and the resistivity increases. Fig. 5 presents the spectra of 2tePo a-particle detected by the studied SLDSB detectors: a) following directly on the preparation process, b) after five weeksof storage without bias, c) after an additional twoweeks of storagewith bias . The dead layer thicknesses in this detector, afterfive weeks of storage without bias (fig. 5b), were 2.5 ,um, 0.6 pin and 0.1 pre for bias voltages of 100 V, 200V and 300 V, respectively. After additional storage of the detector for about two weeks with bias, these thicknesses were 0.85 pre and 0.1 pre for the bias voltages 100 V and 200 V, respectively . Thebehaviour of deeper x
, _. 7t
-
OETLtiRAP stll
i
Al
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Fig. 3. The I-Vcharacteristics of the All detector ; (AI) before annealing, (A2) after 5.3 weeks of annealing without bias, (A3) after the next 2 weeks of storage with a bias . Also shown are the I- V characteristics of the B-71 detector stored with a bias; (Bl) before annealing, (B2) after 1 week of annealing, (B3) after weeks of annealing.
M . SLAPA layers of the detector were studied using deeply penetrating 2o7Bi ß-particles (range 1 .6 mm) . Fig . 6 presents the spectra produced by the detector studied and measured after the same time periods as before. It may be seen from fig . 6 that the observed degradation of energy resolution results from a change of noise properties of the central electrode fwhm of the generator peak.
Fig . 7 presents a dependence of the resistivity of the gap between the central electrode and the guard-ring on the annealing time for the detector stored at a constant bias voltage Up = 30 V . As a result of a continuous process of lithium drift to the insulating SiO2 layer, this resistivity decreases and, consequently, the junction may disappear. After seven weeks of storage, the central electrode current had a break-down
100
10
TlHE
r NEEKS
Fig. 4. The resrstivity of the gap between the central electrode and the guard-ring of an SLDSS detector as a function of annealing at +ó0`C.
LONG-TIME BEHAVIOUR OF
DETECTORS
Si(Li)
2al
character . (In spite of a low value of 1, about 1 pA, it was impossible to measure the signal produced by L MeV particles.)
surface barrier were also studied. After this storage time, these detectors exhibited an energy resolution of 14-18 keV for the 207 Bi ß-particles and a collection coefficient of 99.8%. The dead layer thickness estimated on the basis of the Zt°Po a-particle measurement 4.2 . BEHAVIOUR OF SLDSB DETECTORS STORED ranged between 0.1 and 0.3 ftm at a bias voltage of AT ROOM TEMPERATURE Several SLDSB detectors stored at room temperature 200-300 V.. Fig. 8 presents the I- V characteristics of the central without bias during a period of 2-3 y after forming the .3nev S AFTECTOR All
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fabrication process. (b) after storing for about 5 5. The spectrum of the =t0 Po a-particles detected by the detector ; (a) following the weeks without bias, (c) after storing for the next 2 weeks with a bias .
~7-
DETECTOR A 11 Y0~&
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The spectra of the ze7Hi ß-particles detected by the detector at the same times as in fig. 5.
rar`
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LON
TIME BEHAVIOUR OF
Si(Li)
DETECTORS
243
5. Drifted detectors- SLD ,Iectrode, measured before and after two years storage with no bias. The form of the characteris Behaviour of SLD detectors stored as 60`C the existence of a obtained after this period indicates The SLD detectors, after the preparation process low-resistivityuncompensatedlayer betweenthecentral described before (section 3), were placed in hermetielectrode and the guard-ring. As the bias voltage cally sealed metal containers in a dry air atmosphere increases, the low resistivity layer saturateswith space- and the initial characteristics were measured. Next,the charge and the proper action . of the guard-ring is detectors were stored at 60 °C without bias voltage, observed . with a bias voltage Up =30 V and with a "minimum" bias (see below) equalto 4.5 V. After a give%, time,the detectors were removed from thethermostat, cooled to room temperature and the measurements were performed. The parameters measured were as follows: I-V characteristics, efficiency of the detection of Yradiation from the t3 'Cs source at fixed conditions, dependence of theabove efficiency on the bias voltage, capacity characteristics anddead layer thickness of the n'and p' sides as a function of bias voltage. The I-V characteristics of the SLD detectors (ex.ept one, see below) did not deteriorate during the observation period . The efficiency of 7-radiation detection is a function of the real active volume of the detecoor and of the charge coefficient. At a sufficiently high electric field intensity in the detector (see fig. 10), this efficiency depends exclusively on the real active volume of thedetector"') . Consequently, theefficiency measurement permitsdeterminationofpossible changes of the detector's active volume during the storage period . A dependence of the efficiency on the detector thickness has been determined on the ground of literature data 16,l7) . Fig. 9 presents theeffi,,.iency of the '"Cs 7-radiation NfF CNFFFS] detection in thecourse c:f annealing at 60°C for about 45 weeks and at 23'C for about 30 weeks. The effiFig. 7. The resistivity of the gap between the central electrode ciency was measured using a fixed source-detector and theguard-ring in an SLDSB detector stared with a bias at geometry, a fixed discrimination level E,, = 100 keV, a 60'C as a function of annealing time . fixed bias voltage Up=80 V and a fixed shaping time Tt = Ty =0.5 its . The measurement error results 2 mainly from instability of the discrimination level avid was estimated to be ±4%. It may be seen from fig. 9 á that the efficiency of the detectors stored at 60`C without bias deteriorates and dec-,eases by 25°! after 46 weeks (fig. 9a). The SLD detectors stored at the W same temperature with a bias voltage Up = 30 V 0.5 DETECTOR R-A exhibit a constant efficiency within the experimental Sr d " ?3mm o - 1N-At ,rrsraGFG.ia.0,3ma; error. K-- AFTER 2 YEAR STORAGE AT ABOUT 23C The minimum bias voltage securing almost constant efficiency was determined from the preliminary measurements as Up = 4.5 V. It may be seen from fig. 9b 100 200 300 that this bias results in about 81 decrease of the APPLIED BIAS [V] detection efficiency for ,-radiation after 50 weeks of Fig. g. The !-V characteristics of the central electrode of an storage at 60'C. SLDSBdetector, before and after storage of2yearswith no bias .
244
M. SLAPA
The efficiency of all detectors studied, which were next stored at 23'C for about 30 weeks, was constant within the experimental error. In the case of the P-8 detector (fig. 9c) stored at 60°C with a bias Up =10Y, the reverse current increased from I uA to about 25 uA after 23 d. At that time, the bias was switched off and the detector was further stored at 60°C. After ten days of storage in these conditions, the reverse current &-creased from 25 NA to about 2 uA . Next, the detector was annealed with a bias Up == 30 V. The initial window thickness of the P-8 detector was abo t 1011m and the observed deterioration of the I-V characteristics was related to the removal of the p + i junction, resulting from lithium drift into the junction region . After a successive annealing without bias, the junction recovers as a result of lithium out-diffusion from the junction region . Fig. 10 presents a dependence of the detection efficiency of the MCS 7-radiation on the bias voltage, measured before and after 45 weeks of storage at 60°C . It is seen from fig . l0a that the efficiency in the detectors stored without bias varies with bias voltage (plateau region missing) and at a detector bias voltage Up = 80 V is lower by 25% after 46 weeks of storage. In the detectors stored with a bias voltage, the change
of active volume after 45 weeks is negligible and'' amounts to about +3% for Up = 30 V (fig . 10c) and about - 9% for Up = 4.5 V (fig . 10b) . Fig. lI presents the capacity characteristics of the SLD detectors measured before and after 45 weeks of storage at 60°C. As a result of a continuous lowtemperature drift process in the detectors stored with a bias voltage, the ideal compensation is attained over the whole compensation region (fig. 1) and the capacity characteristic reveals a plateau starting from a value as low as 30 V. In this case, the capacity has decreased by about 8% as compared with the initial value . In the case of storing the SLD detectors without bias, the diffusion processes [eq. (1)] result in a broadening ofthe compensation region (fig. 1). Consequently, the active layer th ; .kness decreases and hence' the detector capacity increases. The capacity increment for Up = 80 V amounts to 10%, after the -+eriod of storage. The relative contributions to the capacity change, resulting from the thickness changes of the detector dead layers in the course of annealing at 60'C may be obtained from fig. 12 . This figi ;re presents the dead layer thickness of the p + (fig. 12a) and n' (fig. 12b) contacts, measured after the annealing process . The dead layer thickness
----
DETECTOR P-12 DETECTOR P-13
DETECTOR P-15
STORA6E4T.23C----
-- --I
- DETECTOR P-8 " - DETECTOff P"6
!0
30
40 Tl1VE (WEEKS]
SO
60
70
80
Fig. , 9. detection efficiency of the 137CS 7-radiation as a function of storage time for the detectors stored: without bias and with bias voltages of 4 .5 V and 30 V.
245
LONG-TIME BEHAVIOUR OF Si(Li) DETECTORS
was determined by measurement of the line shift of dead layer thickness on thep+ contact side wasslightly monoenergetic 2°'Bí electrons with respect to the reduced. For instance, thechange forthe P-12 detector position obtained, using the SLDSB detector with full amounted to about 1501em. The C-V characteristics and dead layer thickness charge collection . The initial dead layer thickness of the n' contact ranged between 801ím and 1201ím measured after 30 weeks of storage at about 23'C for all detectors studied and was independent of bias exhibited no detectable changes. voltage above 70-80V. The final dead layer thickness at Up=300 V, measured for the detectors stored 6. Conclusions without bias, was equal to the respective initial thickThe results obtained lead to the following concluness within the experimental, error . This observation is sions: confirmed by the C-V characteristics (fig. i la). The 1) Main changesof theparameters of theSLDSBand thickness of the uncompensated-layer (space-charge SLD detectors stored at room temperature and at region) for the P-12 detector stored without bias was 60°C with and withoutbias result from lithium concenestimated on the ground of the data of fig. 12 as tration changes on the boundaries of the compensated 801ím and 1201em on the n*' and p+ side, respectively. region . The thickness on the n' contact side for the detectors 2) The observed lithium concentration changes may annealed with bias voltage UP = 30 V and Up= 4.5 V, be described by migration of lithium ions in silicon were practically equal to the initial values whereas the [eq. (1)j with the third term omitted. The range of concentration change in the case of storing is the detector without bias may !:e ~stir .ac, d,~,5(L'tt)} ").Thisquantity, determined foi ,- `'-12 detector from fig. 12, amounts to 1201ím . Thedifu,sov
DETECTOR P- I? UP . D
á
K- IN:IfAI
DETECTOR P-15 UP-4,5V ä " mn
V
-- AFTER 46 WEEKSTORAGE ATE 'C
-
" -AFTER 50 AEEK SWAGE AT60T . -WITIAL
DETECTOR P-6 up -3u
01-f
- INITIAL
" - AFTER 44 WEEK SWAGE AT60 "6
APRIEP
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I%TC6E
150
[VI
30mm'
to
DETECTOR P-6 Y 30V
- arma
*
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Fig. 10 . Detection efficiency of theta7Cs y-radiation for various bias voltages of the detectors stored under the same conditions as those in fig. 9.
. - AFTER 44 WEEK STORAGE Á .'60'C
too
200 APPE1E0 VOITAGE (V]
901)
Fig . 11 . The C- V characteristics of SLD detectors before and after annealingforabout 45 weeks at 60'C.
24 6
M. SLAPA'
constant of lithium ions at 60°C, estimated on thebasis o¬ this value is DL = 1 .7 x 10- 13 cm2/s. This value is in goad agreement with the diffusion constant of lithium
storage . For the detectors studied (d.. 1.4 mm), this voltage was estimated as Up =4.5 V. The value of this voltage can be derived from the condition for : the compensation of the 'flux of diffusing and drifting lithium ions. -
room temperature is equivalent to the behaviour otsermd at 60°C with the diffusion constant of lithium ions reduced by about a factor of 80 2). 4) A 20% reduction in the detectionefficiency of y-
The experimental results indicate that in` agreement with Pell's suggestions, the most suitable material for detectors operating at elevated temperatures is silicon
5) There exists a minimum bias voltage giving negligible changes of the detector parameters during
detectors.
ions at an oxygen concentration of 1016atoms/cm31 6). 3) Thelong-term behaviour of the detector stored at
radiation observed in the detectors annealed' without bias voltage for about 45d combined with only 10% change of the detector capacity, can be explained by a radial contraction of the active layer estimated to be about 0.2 mm .
with an oxygen concentration exceeding' 1011 atoms/ cm'. It also seems that the effect of the third term in eq. (1) should be considered in each specific case, since both the existing literature data and our observations indicate that in some cases this term may significantly affect the lithium distribution stability in Si(Li) T should ' like to express my gratitude to Dr J. Chwaszczewska, Mr E Belca :z'and Mr W . Nowicki for valuable discussions and theirhelp during performance of theexperiment .
References
nb aanga emlr7 Fig. 12. Dead layer thickness of the n+ and p+ contacts measured for selected SLD detectors after annealing at +60°C for about 45 weeks .
1 ) E. M. Pell, J. Appl. Phys. 31 (1960) 291. 2) E. M. Pell, Phys. Rev. 119 (1960) 1222. 3) R. A. Swalin and R. D. Weltrin, Progress in solid state chemistry, vol . 2 (1965) . 4) J. W. Mayer, N. A. Baily and H. L. Dunlap, Conf. on Nuclear Electronics (IAEA, Belgrade, 1961) . s) A. J. Tuzzolino, J. Kristoff and M. A. Perkins, Nucl. Instr. and Meth. 36 (1965) 73. 6) A. J. Tuzzolino, M. A. Perkins and I. Kristofl; Nucl. Instr. and Meth. 48 (1967) 33. 7) I. Hayashi, H. E. Kern, I . W. Rodgers and G. H. Wheatley, IEEE Trans. Nucl . Sci. NS-13 (1966) 214. s) P. E. Gibbons, Nucl. Instr. and Meth. 16 (1962) 284. s) A. Lauber, Report AE-356. lo) G. Bertolini and A. Coche, Semiconductor detectors (NorthHolland Publ. Co., Amsterdam, 1968). 11) H. S. Carslaw and 1.C. Jaeger, Conduction of heat in solids (2nd ed. ; Oxford University Press, 1959) . 1-) H. J. Guislain, W. K. Schoenmaekers and L. H. De Laet, Nucl. Instr. and Meth . 101 (1972) 1. 13) H. R. Kegel, IEEE Trans. Nucl. Sci . NS-15 (1968) 332. 14),J. Chwaszczewska, M. Dakowski, W. Przyborski and M. 'Sowihski, Nukleonika 10 (1965) 251. 15) E. Belcam, J. Chwaszczewska, M. Slapa, M. Szymczak and J. Tys, Nucl . Instr. and Meth. 77 (1970) 21. 16) N. A. Baily and G. Kramer, Radiation Res. 22 (1964) 53 . 17) A. R. Jones, AECL Report No. 3455 (1%9). 1s) E. M. Peil, J. Appl. Phys. 32 (1961) 1048 .