Development of current-based microscopic defect analysis methods and associated optical filling techniques for the investigation on highly irradiated high resistivity silicon detectors

Nuclear Instruments and Methods in Physics Research A 364 (1995) 108-117

ELSEVIER

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Sect,onA

Development of current-based microscopic defect analysis methods and associated optical filling techniques for the investigation on highly irradiated high resistivity silicon detectors * C . J . L i 1 Z. L i * Brookhat,en National Laboratory Upton, NY 11973, USA

Received 30 January 1995; revised form received 30 March 1995 Abstract Current based microscopic defect analysis methods such as current deep level transient spectroscopy (I-DLTS) and thermally stimulated current (TSC) have been further developed in accordance with the need for the defect analysis of highly irradiated (qbn > 1013 n / c m 2) high resistivity silicon detectors. The new I - D L T S / T S C system has a temperature range of 8 K _< T < 450 K and a high sensitivity that can detect a defect concentration of less than 1 0 m / c m 3 (background noise as low as 10 fA). A new filling method using different wavelength laser illumination has been applied, which is more efficient and suitable than the traditional voltage pulse filling. It has been found that the filling of a defect level depends on such factors as the total concentration of free carriers generated or injected, the penetration length of the laser (laser wavelength), the temperature at which the filling is taking place, as well as the decay time after the filling (but before the measurement). The mechanism of the defect filling can be explained by the competition between trapping and detrapping of defect levels, possible capture cross section temperature dependence, and interaction among various defect levels in terms of charge transferring. Optimum defect filling conditions have been suggested for highly irradiated high resistivity silicon detectors.

1. Introduction

The major factor that may limit the application of high resistivity silicon detectors in the newly approved Large Hadron Collider (LHC) at CERN is now believed to be the degradation of the effective concentration of the space charge density (Neff) [1-8]. This N~n. degradation exhibits a two-stage behavior: (11 an increase of N~n- due to the creation of ionized acceptors in the depletion region (or space charge region, SCR) during the neutron irradiation; and (2) further increases of N~ff at room temperature even after the irradiation in the period of room temperature (RT) annealing. The first effect is a short term effect ( 7 < 10 days) and is about 30% or less of the Neft degradation, and the second effect, termed reverse anneal, is a long term effect (7-> 1 yr) and accounts for 70% or higher of the

":' This research was supportcd by lhc US Department of Energy, Contract No. DE-AC02-76CH00016. " Corresponding author. Tel. 516 282 7fi04, fax 516 282 5773, e-mail: zhengl@bnlcl l.bnl.gov. ~Permanent address: Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China.

total Nef~ degradation [6]. For simplicity, we will term all defects that cause Neff degradation as "Neff defects". The estimated neutron fluence of a 10 yr LHC run is about 1 × 1014 n / c m 2, which would generate about 1 X 1013/cm 3 net deep acceptors in the SCR (or Non= l(J13/cm 3) for a high resistivity ( p > 4 kl'~ cm) silicon detector after reverse anneal [6]. This means that for a 300 I.tm thick high resistivity silicon detector, one needs more than 600 V to fully deplete it after the reverse anneal. This is definitely not desirable for real operation considerations because high bias would cause problems such as high leakage current and breakdown, and high power consumption. This problem has led intensive efforts to find favorable conditions for high resistivity detector operations and to make silicon detector radiation hardened in terms of N~n degradation. Some early successes include: 1) 0°C operation [3-4], which can freeze the reverse anneal in a 10 yr period [1]; 2) use of low resistivity ( p = 500 f~ cm) and moderate resistivity ( p = 1500 to 2000 f~ cm) silicon as starting material, which utilizes phosphorus donors recovered from the annealing of P - V center (E-center) to compensate Neff defects [1,7]. For low starting resistivity silicon detectors, a preirradiation of 5 X 1013/cm 2 is needed and for moderate starting resistivity, no preirradia-

0168-9002/95/$09.5t) © 1995 Elsevier Science B.V. All rights reserved SSDI 0168-90/12(951(1(/437-8

C.J. Li, Z. Li / Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117

tion may be needed to retain an operation bias of 200 V (280 Ixm thickness) [7]. Recently, we have started a program to address this problem by impurity engineering, by purposefully introducing impurities into silicon during the detector processing. The two principles for this impurity engineering are: 1) gettering - use of impurities to compete with Net t defects in the process of defect formation; and 2) compensation - use of impurities a n d / o r impurity-vacancy (V) or impurity-interstitial (I) complexes to compensate the Neff defects. Although our first attempt has shown no direct dependence of Neff reverse anneal on the concentrations of carbon and oxygen (in the range of 1016 to 1018 cm -3 for carbon and 1015 to 1017 cm 3 for oxygen) [8], other impurities may be more promising in the search of rad-hard high resistivity silicon detectors. The guiding and absolutely necessary tool for such impurity engineering is the microscopic analysis methods including DLTS and TSC to directly observe the defects generated. Data of DLTS and TSC can correlate the microscopic information associated with various defects to the macroscopic information such as detector leakage current and full depletion voltage. This will make it possible to identify the defect(s) a n d / o r defect clusters responsible to the Neff reverse anneal, as well as impurity(s) and or impurity-V (I) complexes that may hinder such anneal. The traditional capacitance DLTS, or C-DLTS developed by Lang [9], has been mainly applied to low resistivity ( p < 10 ~ cm) materials [10] and recently to high resistivity materials irradiated to low neutron fluences (qbn < 1012 n / c m 2) [11-16]. The application of C-DLTS to highly irradiated high resistivity silicon is limited by the requirement of concentration of defect (N t) being much less than that of initial doping (No), or N t / N d << 1. In addition, at a fluence higher than 4 × 1012 n / c m 2, detector junction capacitance becomes frequency dependent and is flat (no bias dependence) at high frequencies ( f > 100

kHz) [17-18], which makes C-DLTS impossible. On the other hand, I-DLTS [19] and TSC [20-22] have been applied successfully to high resistivity silicon detector irradiated to high neutron fluences. One of the problems discovered in the I-DLTS and TSC measurements on highly irradiated high resistivity silicon detectors was the dependence of measured defect concentration on the filling voltage Vfi. (usually a positive or forward voltage pulse) [21,23]. This dependence is caused by the degradation of detector forward I - V characteristic due to neutron irradiation induced high, close to intrinsic resistivity neutral bulk and it varies with fluence [24]. This filling problem makes the conventional I-DLTS and TSC more difficult: one may need hundreds of volts of filling bias for qOn --~ 1014 n / c m 2 fluence and it is difficult to achieve a "standard" filling voltage. The advantage of using optical means, such as laser illumination, is that one can control the amount of injected free carriers and work in such a condition that defect levels of interest are all filled close to capacity, as demonstrated in our early paper on I-DLTS [10]. In this paper, we present the development of a low-noise, combined ID L T S / T S C defect analysis system and the associated optical filling techniques using lasers with different wavelengths. The dependence of defect filling on the total free carriers generated or injected, laser penetration depth, filling temperature, and decay time will be studied in detail.

2. I - D L T S / T S C system with laser filling 2.1. System setup

The semiconductor microscopic defect analysis system (SMDAS) (including I-DLTS/TSC, as shown in Fig. 1) at Brookhaven National Laboratory (BNL) consists of two parts: the cryostat with sample chamber and the measure-

TSC (I/Laser filling)

I-DLTS (Laser filling) Switch

_•Current l flllir~ (Keithley 487~

Elet~met~r (Kcithley 817 OR 8517)

488 PC

+

Generator (HP S110A)

T,8°o,[ ~ g cont~oUc~ lip LaserJet 4 Printer

109

1

OsciUoseope

(Tek-7?O4A)

Bi~ & switch Box

SignalProeessor (EC4kO 44O2)

Current ~ Amplifier (Keithiey 428)

Boxcar AverBllel

T

(zGke ~ZO)

+

Fig. l. Block diagram of BNL's I-DLTS/TSC defect analysis system with current and optical filling.

110

C.J. Li, Z. Li /Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117

Sample Chamber

Eleetr~)de • , -~ Z~/ i " ~ '/ ~

i

Sample i Chamber]

A1

AI: 2 0 0 0 - 4 0 0 0 S[02 : 4 5 0 0

r-Si02

p~ n+ : Si02

1000 1000 step: 1000~,

nt A

Fig. 3. Schematics of the sample structure used in this study. u~

~

Vaeu'm

=¢

, Pump

L

tC°mp

Fig. 2. Schematics of the 6.5 K cryostat system with sample chamber•

ment apparatuses. As shown in Fig. 2, the sample chamber includes the sample holder and laser diode with focus optics that focuses the laser light into a 1 mm 2 spot on the sample. The sample holder has an area that can hold a sample with an area of 2.2 × 2.2 cm 2 or smaller. Detectors used in this work, as shown in Fig. 3, are our standard p + - n - n ÷ implanted planar junction diodes (0.25 cm 2, 300 Ixm thick) fabricated on n-type, (111), Wacker high resistivity ( 4 - 6 k f l cm) material in BNL's Silicon Detector Development and Processing Lab. However, samples with different structures, such as MOS capacitors, surface barrier diodes, and resistors made on high resistivity materials (GaAs, highly irradiated St, diamond, etc.), can also be measured by this system provided the overall physical size is less than 2.2 x 2.2 cm 2 X 0.5 cm (height). Laser diodes with different wavelengths are powered by a HP 8110A Pulse Generator. The measurement apparatuses are divided into two halves: TSC and I-DLTS. Deep levels can be filled by current (voltage pulse) or laser pulse or the combination of both. The main apparatuses for I-DLTS are Keithley 428 Current Amplifier and an E G & G 4400 Boxcar Averager and Signal processor. The key instrument for TSC is a Keithley 6517 Electrometer. The temperature of the eryostat is controlled by a Scientific Instrument 9650 Digital Temperature Controller. The temperature controller and the apparatuses for both TSC and I-DLTS are integrated with a 486 PC in a LabWindow CVI environment.

8 K in about 1 h. The cooling rate is nearly a constant from 300 to 50 K and increases below 50 K. Heating of the sample with a constant heating rate is accomplished by the balance between cooling of the cryostat and heating using a heater feedback controlled by the temperature controller. Ia this way, constant temperature ramp-up rate (/3) is achieved. For rates higher than 20 K / m i n , it starts to become nonlinear at high temperatures (T > 200 K) due to the limited heating power of the heater. However, for practical purposes, TSC (the one requiring a constant /~) measurements may just need to go as high as 200 K. Fig. 4 shows the system noise at an open circuit condition. By careful shielding and wire arrangement, the noise of the TSC at BNL has been lowered to below 10 fA, which implies a sensitivity to as low as 1 X 109/cm 3 defect concentration in a 5 / 1 signal to noise ratio measurement. Fig. 5 shows the leakage current and TSC signals for an undamaged silicon detector. The base current with a detector is of the order of 10 fA. As expected, the defect concentration in an undamaged detector is minimum (less than 1 × 10m/cm3). This closed-cycle system has been improved compared to the drop-in TSC system (open-cycle) described in Ref. [25] in terms of more controllable heating rate (which is essential for TSC) and much lower noise (about 10 fA as compared to 1-5 pA in Refs. [21-23,25]) and it does not have the induced current problem from the heater (which is not coiled) as seen in Ref. [25]. In addition, since the sample is held in a vacuum

~) 10-,4

o%o oO~--

o o

o o

%~o~%~oO~e ~o ~ ~o o

°

o

~

~S

o

o°

2.2. System qualification 10_,s

The cryostat used in this work is an APD 6.5K-Displex System (Fig. 2) designed to go down to 6.5 K without load. With the load of a sample and associated wire connections, the cryostat can reach a temperature as low as

~'~:7

o

~

0

. . . .

l

50

,

,

,

i

.

~

o

o

ooZi

o oO

o o

100 150 200 Temperature (K)

,

250

,

,

300

Fig. 4. System noise of the TSC setup without samples. Solid line is the average noise which is about 5 fA.

C.J. Li, Z. Li / Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117 4.

I-T (No fill)

A

f°'[ .....................

T S C ( 3 6 m A fill)

traps), if it is below the Fermi level, the filling rate is proportional to no:

10.° !

dn t

d---t = Cnn°( NT -- nt) - ennt + ep( NT -- nt) - ( cpp°nt)

~'10"

"

i

where Nv is the total concentration of level Et, and c,,,e and e,,,p are, respectively, capture rate and emission rate for electrons and holes: .

is

0

(1)

7

10 ~4 lo

111

.

50

.

.

100

.

.

150 T (K)

~

200

.

.

.

.

.

.

.

250

.

300

Fig. 5. Leakage current (l-T) and TSC spectrum of an unirradiated silicon detector: the base current is 10-20 fA and defect concentration in the order of less than l0 w cm 3.

instead of in a He ambient, it may minimize possible surface effect. The shortcoming of this closed-cycle system is its long cooling time of about 1 h as compared to that of 10-15 min for open-cycle systems. Note here that, in a TSC run, the leakage current can be measured during the cooling a n d / o r during the heating-up without prior defect filling at the lowest temperature. The TSC signal can only be obtained during the heating cycle and, as we will show later, with prior defect filling at the lowest temperature by current or laser pulses. The contribution of defects to the TSC is the subtraction of the TSC signal and leakage current. On the other hand, I-DLTS can be carried out in both directions (T down and up), and it is not sensitive to the temperature changing rate in the temperature range used here (/3 < 1 K / s ) .

Cn, p = O'n,pOth, e n = Cngce-(Ec-Et)/kT,

ep = CpNv e-(e'-ev)/kr

If during each filling, we can fill the defect level to be detected to its capacity, ArT, then we may be able to measure the total defect concentration by TSC a n d / o r I-DLTS. Since there is no more filling during carrier emission, Eq. (1) is now reduced to: dn t

d---t = - e n n t + ep(NT -- n t ) ' t / t ( / = 0) = N T.

(3)

The solution of Eq. (3) is:

e,,[ep

nt =N T -

e n + ep

--+e

en

]

- (e+ep)t " .

(4)

For electron traps, which are normally in the upper half of the band gap, e,, >> ep usually stands, we have: ep

+ e-~ne-e,t ]

nt = N T - - [ 1

e n + ep [

ep

N.r

3. Defect filling Both TSC and I-DLTS are current based microscopic defect analysis methods, which measure current or current transient of free carriers emitted from deep levels, respectively. TSC can be regarded as the integration version of the method which measures the total current of the carrier emission in a time scale of tens of seconds. The filling of defects is carried out at lowest temperature before heating. In contrast, I-DLTS looks into the current transient by carrier emission in a time scale of milliseconds. The filling of defects is done by pulsed current or laser before each transient at a given temperature. TSC and I-DLTS can be comparable and complementary to each other in the defect identifications. The filling of each defect level depends on the position of the level relative to the Fermi level, free electron concentration n o and hole concentration P0, and carrier capture cross section o-. For an electron trap level (for simplicity, we only limit our self to electron traps in the discussion. Similar arguments can be derived for hole

(2)

=

] (t=0)

ep N T e n + ep

-- 0

(t--,

~).

(5)

Therefore the concentration of trapped electrons goes through a transient from N T to near 0 with a time constant i/e,,. Note here that for a level near the middle of the band gap, e,, and ep may be in the same order of magnitude. In this case, the transient will not go to 0. In this case, TSC signal may only correspond to a fraction of the total defect concentration since not all the trapped carriers have emitted to contribute to the TSC current. For most cases, however, activation energy for deep levels in irradiated silicon are smaller than 0.5 eV and Eq. (5) is valid. As we have discussed in Section 1, defect filling has become an important issue in the microscopic defect analysis measurements. In particular, for highly irradiated (q~n > 1013 n / c m 2) high resistivity silicon detectors, filling may be the dominant issue in determining defect parameters due to the problem with radiation induced high resistivity

112

('.J. Lt, Z. Li/Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117 15

-

. ~

.

.

.

.

.

.

.

.

.

.

.

.

,

.

.

.

.

.

.

.

J

J

Filling Current + I-T (nofilling)

.

RT ! linj= 500 uA

....

10 -8

10 9

~10 •

10.10 ~O

1

I. ~

Filling Temperature 10K Filling Time 30see.

J

Decay Time 60 ....

A

1- O

lOOpA 1 nA

~

,j O'10 11

O9

1--10.12 ~ o P ~ % 10 ~3 i 0

.

0

20

.

.

.

.

.

.

.

.

.

.

.

.

.

40 60 80 Neutron Fluence (E12 n/cm2)

10 ~s Fig. 6. Neutron induced degradation o f detector f o r w a r d current

= 0

,, . . . . . . . . 50

injection at RT.

3.1. Forward bias filling current dependence

As stated previously, traditional filling using forward bias becomes increasingly difficult as radiation fluence increase beyond 1013 n / c m 2 - higher forward bias is needed for detectors damaged by larger fluence of neutron irradiation [21,23]. This filling problem is caused by the forward I - V degradation due to neutron induced high resistivity bulk [24,26]. At a given injection current, the forward bias increases with neutron fluence, as shown in Fig. 6 for RT I - V characteristics. In addition, as the bulk resistivity increases with decreasing temperature, one can expect even worse forward I - V degradation and even higher biases (hundreds of volts) may be needed for filling at low temperatures. Another problem for current filling is that at low temperatures (T < 100 K) we have observed instabilities of forward I - V characteristics of highly irradiated silicon detectors: at a given low temperature the forward bias to reach the same injection current is not unique - it varies from run to run and depends on the bias history.

& luA []

10uA

200-°-

1 mA

J

120

bulk. In this section, we will present data on defect filling dependence on forward bias filling current, laser filling intensity and wavelength, filling temperature, and decay time. Neutron irradiation was carried out at the University of Lowell's Van de Graaff which generates neutrons with 1 MeV average energy from 7Li(p, n) reaction using 4 MeV protons.

./

/

.

100

o~

100 T (K)

. . . . . . . 150

Fig. 7. TSC spectra of an irradiated (1.85 X 1014 n/cm z) detector at various forward current filling conditions. Detector leakage current was not subtracted from the TSC spectra.

Fig. 7 shows TSC spectra for a detector irradiated to 1.85 × 1014 n / c m 2 with filling current Ifitl as a parameter. As the forward I - V characteristic is not stable at low temperatures, the filling current is controlled by a series resistor R during forward biasing. The values of R, total voltage used and forward voltage on the detector, and corresponding filling current are listed in Table 1. It is clear that in general, all TSC peaks, especially main TSC peaks at temperatures between 130 to 200 K (Ci-Oi, V V - , P - V (or E-center), and other V - V related centers) increases with filling current. 3.2. Laser filling dependence

Laser illumination on silicon detectors generates nonequilibrium free carriers (electrons and holes) which can fill the deep levels to be detected. The depth of free carrier generation ( X ) at RT depends on the laser wavelength A [27]: X = (84.732/A - 76.417) -2 (cm), A in Ixm

and this free carrier generation is not sensitive to neutron induced high resistivity bulk. To illustrate that laser filling is a more reliable technique in defect filling for highly irradiated high resistivity

Table 1 Parameters for TSC current filling using a current limiting series resistor Filling current Resistor [M 1) ] Total bias [V] Forward bias on detector IV]

100 pA 78 000 15 7

1 nA 78 000 86 8

100 nA 90 20 11

(6)

1 IxA

90 107 17

10 p,A 9.6 188 92

100 ~ A 9.6 201 101

1 mA 0.1 208 108

113

CJ. Li, Z. Li /Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117 10

+

![

1.85E14 n/cm2

Laser and/or Forward Bias

VR - 20 V 10 ~

Filling Method

b-2OKlmin ÷"

Laser (0

t

~

,oov

2

0

kaser+100 V

V)

U W'1 03urn Tfil~ = 70 K

0 10 ,o p-

!"~

4/

~ 10 ~

i

1 10.~2

50

strong filling. The saturation of defect filling by laser illumination is demonstrated in Fig. 9b, where the main TSC peak height as a function of filling energy (or filling time at a constant laser power of 1.2 mW) is plotted for detectors irradiated to various neutron fluences. It can be observed in Fig. 10 that the saturation filling energy increases with neutron fluence and starts to reach a constant at about 8 x 1013 n / c m 2 at a value of about 16 mJ. For laser diodes used in this study, the laser power is in the range of 1-5 mW. Therefore a filling time of more than 16

. . . .

100

150

2O0

Temperature(K) T

Fig. 8. TSC spectra of an irradiated (1.85 × 1014 n/cm 2) detector obtained from laser and current filling and their combination. Detector leakage current was not subtracted from the TSC spectra.

silicon detector, current filling is compared with laser filling. Fig. 8 shows the TSC spectra for the same detector described above at various filling conditions at 70 K: current filling only, laser filling only (at V = 0 V), and laser plus current filling (at V 4= 0). It is obvious that laser filling is more efficient (by a factor of 2 - 3 ) than current filling, especially for main peaks above 130 K. At strong filling condition (laser filling), the peak at T = 117 K, the V-V center, decreases, while the main peaks at higher temperatures increase. There may be competition between the filling of the V - V - - center and the main centers at higher temperatures. This filing problem of V - V can be solved by laser filling at lower temperatures as we will see in the next section. We note here that Eq. (6) is valid for RT. At lower temperatures, as silicon energy bandgap becomes wider (1.12 eV at RT and 1.2 eV at 0 K), laser penetration length will be longer than that at RT. The concentration of the defect levels within the main peak in Fig. 8 can be estimated by [28]:

(a)_

1

I

I

I

I

# ~ l e ( 1 . 1 x l 0 1 4 n / e m 2)

]

I t [

O

?;

~,~

,-

[_.

,

O2

F

f -.

i

I

100 120 140 (b)

12

24

36

I

I

160

180

200

48

60

T (K)

F i l l i n g E n e r g y (m J)

4OO

N

4 - - - ~ - / ' r ~ * I ( T ) dT qWA ~ JL

(7)

which gives a value for all defects of Nt = 2 - 5 x 10 z4 cm -3 >> Nd0 = 1 x 1012 cm -3 for 1.85 X 1014 n / c m 2 irradiation. Here in Eq. (7) W is the depletion depth and A is the area of the detector. It is clear that this is well out of the validity of the traditional C-DLTS. The key for a successful laser filling is to chose the filling conditions at the saturation for defect level(s) to be detected. Fig. 9a shows the TSC spectra of the main peak between 130 and 160 K for a detector irradiated to 1.1 × 1014 n / c m 2 at various filling energies (laser power X filling time) or total free carriers generated. The filling temperature was 80 K and the TSC curves were shifted along the y-axis for comparison purposes. It is clear that this main TSC peak increases with the filling energy Efill and starts to saturate at Era1 = 10 mJ. Again, we have seen that the peak at 117 K decreases with filling energy at

L.W

~'350 O.

0.93 um

_--A -.&1-&-~J + 4/Af 4- 4~ o ~ j , O - - 0 . 0. . . . 0. .

300 ~e. 250 200

•

. . . . .

1

•--

5E12/cm2

4-

.

~

7.2E12/cm2

0

I

0

3,6E13/cm2

+

8.3E12/cm2

&_

1,1E141crn2

13,.

I

._c 150 ¢1

~E

1 O0

I

+o - - e - -

o

so 0

10

.o

20

i

30

40

50

60

Filling T i m e ( s e c . ) Fig. 9. Dependence of TSC on laser filling energy: (a) TSC spectra of an irradiated (1.1X 10 TM n/cm 2) detector at various laser filling energy. Detector leakage current was not subtracted from the TSC spectra; and (b) TSC peak height as function of laser filling time (or energy) for detectors irradiated to various neutron fluences.

114

C.J. Li, Z. Li /Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117

2O

J jf J U.I

~10 it

5

/

/

CO

0

20

/

40 60 80 100 Neutron Fluence (E 12 n/cm2)

120

Fig. 10. Saturation laser filling energy as a function of neutron fluence.

s should be enough to fill defect levels to saturation. The standard filling time used in this study for TSC is set at tf = 30 s. The filling saturation study on I-DLTS with laser filling has been reported in our early paper and an effective saturation laser intensity of 500 m W / c m 2 has been found [19], which corresponds well to the 5 mW (per mm 2) laser diode used in this study. After the intensity or power of the laser is fixed to best fill defect levels close to saturation, the laser wavelength may come to play an important role. In particular, for highly irradiated high resistivity silicon detectors, the active junction of the detector may shift to the back side (n ÷ side) due to neutron induced high acceptor-like space charge density which are much higher than initial donor concentration [24], an effective filling may require a long wavelength laser diode that can penetrate the entire thickness of the detector. Although laser filling by illuminating the n ÷ side may also be an alternative, the preliminary try of such filling has proved much more difficult in terms of more mechanical complication if illuminating through a hole in the substrate, or more electronic noise introduced through the guard ring if just simply turning the sample over (p + as the substrate, illuminating the n ÷ contact from front). Also it has been observed by us that at extremely high fluences (@, > 1014 n/cm2), a rectifying junction may exist on both sides when the detector is partially depleted. In this case, filling through the entire detector thickness by using a long wavelength laser diode may be the best method. Fig. l l a shows the I-DLTS spectra of detector 433-23 (qbn = 1.85 × 1014 n/cm2). Three laser wavelengths have been used with penetration length of 18.5 p,m (X = 0.85 p,m), 83.6 I,tm (A = 0.97 txm), and 293 p~m (A = 1.03 ~m) in silicon at RT. Therefore for detectors used in this study with standard 300 p,m thickness, the laser with 1.03 p,m wavelength should be good enough for filling at

temperatures lower than RT. The dominant effect of laser wavelength is however on the filling of the main I-DLTS peak from 160 to 260 K ( C i - O i , V - V - , P - V , and other V - V related centers). The main peak increases with wavelength while the shallow peaks (T < 150 K) are affected little by the laser penetration length. We note here that I-DLTS is more sensitive to the shallow defect levels than TSC, which means that the filling of I-DLTS is more effective than TSC for shallow levels, especially the Acenter ( O - V ) at 90 K. One possible explanation is that shallow levels may not be easily filled due to their fast carrier emission time and that I-DLTS is instant filling and measuring (minimum emission before measurement) while TSC has a relative long delay time after filling (decay time td + r a m p time A T / f l ) before reaching the maximum temperature of the charge emission from a particular level. This effect, however, is a proof of the fact that TSC and I-DLTS do complement each other in the defect analysis for highly irradiated high resistivity silicon detectors. It is also easier for I-DLTS to obtain the values of defect energy levels from the Arrhenius plot of emission rate using the data of various rate windows [19]. The concentration of a defect level measured by I-DLTS with laser filling (or optical filling) is proportional to the correspond-

(a) 0.30

o-~°cc~?(°tsev)

c i - o i , ~ d P-v

025

Filling Wavelength 010eV 1

<=~ 0.20

~

~'~

1.03urn

0.15

~--- 0.97urn

03 0.10 03

0 85urn

o.o5

-0.00

tl=2ms

~

VR=SV

-0,05 0 (b)

50

100 150 200 Temperature(K)

300

250

Filling Wavelength

10-a

-- + -- 1.03um

Filling Ternperture 10K Filling Energy

10-9

t 00-200 mJ

td - 80sec.

~,~

B-2OK/sec.

A

jl~

0.97urn

J~ j=

/

-- O -- 0.85urn []

0,78urn (36mJ)

-- O -- 0.67urn

10.~

/+ .J ==~tl'Bwlm'--

10-~2

0

-- @--

I-T

+

50 100 150 TEMPERATURE (K)

200

Fig. 11. Laser wavelength dependence of (a) I-DLTS; and (b) TSC, detector leakage current was not subtracted from the TSC spectra.

115

C.J. LL Z. Li / NucL Instr. and Meth. in Phys. Res. A 364 (1995) 108-117

ing peak height IgLTS at peak temperature Tm and it can be modeled to obtain the following equation [29]:

5.5/gLTStl ( NT

t~ ) tj

I-DLTS, for ~ = 4 ,

qWA

(8)

where t I and t 2 (usually in ms) are the first and second sampling points, respectively. Note here that the rate window, en(Tm), for I-DLTS is almost equal to 1 / t v Alternatively, it is straightforward to get the total concentration of defect levels since it is proportional to the TSC integral (Eq. (7)). By comparing Eqs. (7) and (8) we notice that the peak signal of TSC, lmTsc, is much smaller than that of I-DLTS, lmoLTs for the same defect concentration. In fact, Eq. (7) may be rewritten as the following: AT 21TSC. _ _ Nz

qWA

(TSC, AT is the peak full width at half maximum).

AT/~3 >_ lOs >> t I --- ms,

10 ~

------~ L W = 1 0 3 urn, Tf ~ 30 s. Id - 60 s

~

1

Filling at

~

~

4

1OK

A-

3oK

e- 5OK

d"

V

70K

10-~2 0

50

~ 10-~o

lOsec. --8--

10-12 ~,~pe~tL

Filling at 70K VR - 20 V

1 mln

o- 80rain

10 13 70

90

110 130 150 170 Temperature (K)

190 210

Fig. 13. TSC spectra of an irradiated (1.8 × 1014 n/cm 2) detector

after various delay times. Detector leakage current was not subtracted from the TSC spectra. This maybe explained by free carrier diffusion during the TSC filling period that is normally greater than a few seconds.

is found to be minimum, as shown in Fig. l l b . Especially at high temperatures (T > 100 K, main peaks), there is virtually no wavelength effect.

(10)

This point can be easily confirmed by comparing Figs. l l a and 11b. For the main peak, the I-DLTS peak height (at 225 K) is about 3 × 10 -7 A, while the TSC peak height (at 150 K) is about 2 × 1 0 - ~0 A. The large value of AT~~3 is also the reason why TSC peaks shift towards lower temperature than the corresponding I-DLTS peaks: TSC is equivalent to the I-DLTS with sampling times (t~) in the tens of seconds and I-DLTS peaks shift towards lower temperature with larger tl. The dependence of TSC signal to the filling wavelength

}--

1

3.3. Filling temperature dependence

/m°LTs = 103 to 10 4 X/mTsc.

VR - 20 V

10"9 S

' ~DecayTime .~1 J.

(9)

It is clear that A T / ~ in TSC is equivalent to t 1 in I-DLTS. Since AT is in the order of 10 K and /3 is normally smaller than 1, we have that:

"10-1°

10-8 [ ' ' ' L i L.W.- 1.03urn,If- 30s

100 150 Temperature (K)

200

Fig. 12, TSC spectra of an irradiated (1.85 x 1014 n/cm 2) detector filled at various temperatures. Detector leakage current was not

subtracted from the TSC spectra.

As we have seen before, strong filling may not be suited to fill all the levels to saturation in a TSC run. This effect may be caused by the competition among various defect levels in filling and carrier emission during long decay at a given temperature and possible dependence of capture cross section on temperature. One way to fix this problem is that one can fill defect levels at different temperatures. It is clear from Fig. 12 that the main TSC peak between 130 and 160 K (Ci-Oi, V - V - and P - V centers) and the peak at 90 K (we denote it as peak B) increases with filling temperature (/'fill). On the other hand, the peak at 117 K ( V - - ) decreases with Tfia,. Again we have observed a general trend: the main peak is in competition a n d / o r interacting with the V peak. The peak between 160 and 200 K (we denote it as peak C), however, increases with the Tfin initially and decreases at the end when /'fill = 120 K, possibly also in competition or interaction with the main peak. One probable interaction between the deep levels is the charge transferring between the levels. One practical filling procedure is to divide the filling into three parts: one TSC run with filling at 10 K to effectively fill the V - - center, one TSC run with filling at 70 K to effectively fill the B peak and the C peak, and finally a TSC run with filling at 120 K to effectively fill the main peak. A typical TSC run normally starts with a filling followed by a decay t a. During the period of the decay, some carriers trapped in all levels, especially those near and below the filling temperature, will emit to the conduction

116

C.J. Li. Z. Li / Nucl. Instr. and Meth. in Phvs. Res. A 364 (1995) 108-117

or valence band. A long decay time will assure a low base current for defect level resolution, as shown in Fig. 13 where TSC spectra has been plotted for detector #433-23 with various decay time at 70 K. On the other hand, long decay time will also affect the effective filling of the deeper levels at higher temperatures. It seems that the optimum decay time is in the range of 1 0 - 3 0 s. In an actual measurement however, the optimum t d may change with filling temperature.

[3] [4] [5] [6]

[7]

4. Summary It has been demonstrated that current-based microscopic analysis methods such as TSC and I-DLTS with optical filling techniques can be effective and complementary tools in the investigation of highly neutron irradiated high resistivity silicon detectors and related radiation hardness study. Optical filling using laser diodes instead of the traditional current filling has been shown more effective and controllable. An effective filling scheme using a long wavelength (1.03 I~m) laser diode is suited for I-DLTS and a practical and effective filling scheme using a long wavelength (1.03 ~ m ) laser diode with three selected filling temperatures has been developed for TSC. For highly irradiated silicon detectors, defect filling has become the major issue in defect identification, for which roles of competition between filling and emission and competition and interaction among various defect levels may have to be examined and taken into account. It is obvious that more work needs to be done to better understand the physics in defect filling as well as quantitative analysis of the I-DLTS and TSC data. Other methods such as infrared spectroscopy (IRS) may be a useful alternative, especially for shallow impurity levels.

[8]

[9] [10]

[11]

[12] [13]

[14]

[15] [16]

[17] [18]

Acknowledgements The authors would like to thank Dr. V. Eremin of PTI for his help during the period of setting up of the T S C / I DLTS system, M. Li of BNL and H. Feick of Univ. of Hamburg for their work in the interfacing of the T S C / I DLTS system with a computer.

References [1] Z. Li, BNL-49013, Int. Symp. on Development and Application of Semiconductor Tracking Detectors, Hiroshima, Japan, May 22-24, 1993, Nucl. Instr. and Meth. A 342 (1994) 105. [2] E. Fretwurst, H. Feick, M. Glaser, C. Gobling, E.H.M Heijine, A. Hess, F. Lemeilleur, G. Lindstroem, K.H. Mahlmann, A. Rolf, T. Schulz and C. Soave, Int. Symp. on Development and Application of Semiconductor Tracking

[19]

[20] [21]

[22]

[23] [24]

Detectors, Hiroshima, Japan, May 22-24, 1993, Nucl. Instr. and Meth. A 342 (1994) 119. H.J. Ziock et al., IEEE Trans. Nucl. Sci. 40 (1993) 344. H.J. Ziock et al., IEEE Nucl. Sci. Symp., San Francisco, CA, Nov. 2-5, 1993. T. Schulz et al., IEEE Nucl. Sci. Symp., San Francisco, CA, Nov. 2-5, 1993, IEEE Trans. Nucl. Sci. 41 (1994) 791. Z. Li, BNL-60274, IEEE Nucl. Sci. Symp., Norfolk, VA, Oct. 30-Nov. 4, 1994, IEEE Trans. Nucl. Sci. 42 (4) (1995) in press. Z. Li, BNL-60155, 6th Pisa Meeting on Advanced Detectors, Elba, Italy, May 22-28, 1994, Nucl. Instr. and Meth. A360 (1995) 445. Z. Li, W. Chen, L. Dou, V. Eremin, H.W. Kraner, C.J. Li, G. Lindstroem and E. Spiriti, BNL-60294, IEEE Nucl. Sci. Symp. Norfolk, VA, Oct. 30-Nov. 4, 1994, IEEE Trans. Nucl. Sci. 42 (4) (1995) in press. D.V. Lang, J. Appl. Phys. 45 (1974) 3023. L.C. Kimerling, in: Radiation Effects in Semiconductors (The Institute of Physics, Bristol, UK, 1976), Institute of Physics Conf. Series 31 (1977) 221. L. Vismara, SICAPO Collaboration, Int. Conf. on Advanced Technology and Particle Physics, Como, Italy, June 13-16, 1988. Z. Li, W. Chen and H.W. Kraner, Nucl. Instr. and Meth. A308, 585 (1991). E. Borchi, C. Bertrand, C. Leroy, M. Bruzzi, C. Furetta, R. Paludetto, P.G. Rancoita, L. Vismara, and P. Giubellino, Nucl. Instr. and Meth. A 279 (1989) 277. Z. Li, H.W. Kraner, E. Verbitskaya, V. Eremin, A. Ivanov, M. Rattaggi, P.G. Rancoita, F.A. Rubinelli, S.J. Fonash, C. Dalc and P. Marshall, BNL-47550, IEEE Nucl. Radiat. Effects Conf., New Orleans, LA, July 13-17, 1992, IEEE Trans. Nucl. Sci. 39 (1992) 1730. V. Eremin, A. Ivanov, E. Verbitskaya and Z. Li, BNL-60143. V. Eremin, A. Ivanov, E. Verbitskaya and Z. Li, BNL-60273, IEEE Nucl. Sci. Symp., Norfolk, VA, Oct. 30-Nov. 4, 1994, IEEE Trans. Nucl. Sci. 42 (4) (1995) in press. Z. Li, BNL-44648, IEEE Nucl. Sci. Symp., Oct. 23-27, 1990, Arlington, VA, IEEE Trans. Nucl. Sci. 38 (1991) 244. Z. Li, BNL-49058, IEEE Nucl. Sci. Symp., San Francisco, CA, (1993), IEEE Trans. Nucl. Sci. 41 (1994) 948. C.J. Li and Z. Li, BNL-49045, Symp. on Development and Application of Semiconductor Tracking Detectors, Hiroshima, Japan, May 22-24, 1993, Nucl. Instr. and Meth. A 342 (1994) 137. H.M. Heijine, J.C. Muller and P. Siffert, Radiat. Eft. 29 (1976) 25. U. Biggeri, E. Borchi, M. Bruzzi, S. Lazanu, Z. Li and S. Lazanu; BNL-49017, IEEE Nucl. Sci. Symp., San Francisco, CA, Nov. 2-5, 1993, IEEE Trans. Nucl. Sci. 41 (1994) 964. Z. Li, H.W. Kraner, S. Lazanu, U. Biggeri, E. Borchi and M. Bruzzi, BNL-60905, 1st Int. Conf. on Large Scale Applications and Radiation Hardness of Semiconductor Detectors, Florence, Italy, July 7-9, 1993, Italian Physical Soc. Conf. Proc., vol. 46, Large Scale Applications and Radiation Hardness of Semiconductor Detectors, A. Baldini and E. Focardi, eds., SIF, Bologna (1994) p. 123. M. Bruzzi, E. Borchi and A. Baldini, J. Appl. Phys. 72 (1992) 4007. Z. Li. BNL-49822, 1st Int. Conf. on Large Scale Applica-

C.J. Li, Z. Li / Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 108-117

tions and Radiation Hardness of Semiconductor Detectors, Florence Italy, July 7-9, 1993, Italian Physical Soc. Conf. Proc., vol. 46, Large Scale Applications and Radiation Hardness of Semiconductor Detectors, A. Baldini and E. Focardi, Eds., SIF, Bologna (1994) p. 101. [25] A. Baldini and M. Bruzzi, Rev. Sci, Instr. 64 (1993) 932. [26] V. Eremin and Z. Li, BNL-60072, IEEE Nucl. and Space

117

Radiat. Effects Conf., Tucson, AZ, July 18-22, 1994, IEEE Trans. Nucl. Sci. 41 (1994) 1907. [27] E.S. Nartowitz and A.M. Goodman, J. Electrochem. Soc. 132 (1985) 2992. [28] L. Forbes and C.T. Sah, Solid State Electronics 14 (1971) 182. [29] Z. Li, to be published.