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doi:10.1016/j.m

Materials Science in Semiconductor Processing 6 (2003) 277–279

Isochronal annealing study of hydrogen interaction withimplantation-induced point defects in p-type silicon

Yutaka Tokuda*, Hisanori Sato

Department of Electronics, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan

Abstract

Isochronal annealing with zero and reverse bias applied to Schottky diodes was used to monitor the evolution of

hydrogen interaction with point defects observed in hydrogen-implanted p-type silicon, i.e., divacancy (VV), carbon–

oxygen interstitial pair (CiOi) and two levels at Ev þ 0:28 and Ev+0.50 eV. The VV and CiOi are passivated by

hydrogen liberated from hydrogen-containing defects during annealing in the temperature range 90–150�C and

reappear upon annealing above 180�C under reverse bias due to hydrogen liberation and its field drift. Two levels at

Ev þ 0:50 and Ev+0.28 eV are ascribed to irradiation-induced and hydrogen-related defects, respectively.

r 2003 Elsevier Ltd. All rights reserved.

PACS: 71.55.Cn

Keywords: Silicon; Hydrogen; Point defects; Annealing; DLTS

1. Introduction

Hydrogen in silicon is known to interact with dopants

and various defects [1]. Many studies have been done by

deep level transient spectroscopy (DLTS) on the

hydrogen interaction with irradiation-induced point

defects in silicon such as divacancy (VV), vacancy–

oxygen (VO) pair and carbon–oxygen interstitial pair

(CiOi) [1–6].

The VV is well recognized to be passivated by

hydrogen [2–4,6]. Recent DLTS measurements reveal

that hydrogen interaction with VO results in the

formation of the hydrogen-related complex (VOH) with

the energy level of Ec�0.32 eV [3–5]. Further addition of

hydrogen to VOH leads to the complete passivation of

electrical activity of VO [3,4]. The formation of the

complex of CiOi and hydrogen (Ec�0.36 eV) has beenshown to occur in electron-irradiated n-type silicon by

hydrogen incorporation together with the concentration

decrease of CiOi in p-type silicon [3]. Isochronal

annealing experiments have indicated that VV and CiOi

ing author. Fax: +81-565-48-0020.

ess: tokuda@aitech.ac.jp (Y. Tokuda).

e front matter r 2003 Elsevier Ltd. All rights reserve

ssp.2003.05.006

annihilate upon annealing around 160�C in hydroge-

nated p-type silicon followed by electron irradiation [2],

which again indicates hydrogen interaction with these

defects. However, the comparison of hydrogen- and

silicon-implanted p-type silicon has revealed that hydro-

gen is not involved in annihilation of CiOi by thermal

annealing [6]. There seems some discrepancy on the

effect of hydrogen on CiOi.

In order to clear the effect of hydrogen on CiOi, we

tried to study the isochronal annealing behavior of

defects in hydrogen-implanted p-type silicon with

DLTS. It will be shown that isochronal annealing

experiments with zero bias and reverse bias applied to

Schottky diodes are very useful to study interaction of

defects with hydrogen.

2. Experimental procedure

Wafers used were prepared from boron-doped, p-type

(1 0 0) Czochralski-grown silicon crystals with a resistiv-

ity of between 6 and 8Ocm: Hydrogen implantation

was performed at an energy of 150 keV to a dose of

2� 1010 cm�2 at room temperature.

d.

ARTICLE IN PRESSY. Tokuda, H. Sato / Materials Science in Semiconductor Processing 6 (2003) 277–279278

Schottky contacts were fabricated by evaporation of

Ti on the implanted side of samples. Isochronal

annealing for 20min was carried out in 30�C steps from

60�C to 240�C with zero bias and reverse bias applied to

Schottky diodes. When the zero bias was applied during

annealing, the damaged region was in the neutral region

(zero bias annealing). The reverse bias was changed to

include the damaged region in the depletion region at

each annealing temperature (reverse bias annealing).

DLTS measurements were performed with the reverse

bias of 5V and the filling pulse amplitude of 5V in the

temperature range 80–290K. This bias condition enables

us to probe the damaged region by the present

implantation with DLTS. Concentration depth profiles

of defects were evaluated with DLTS by changing the

bias conditions.

3. Experimental results and discussion

Fig. 1 shows the DLTS spectrum with the time

constant t of 19.1ms from hydrogen-implanted p-type

silicon. Five defects labeled P1–P5 are observed in the

as-implanted spectrum. The energy levels are estimated

to be Ev þ 0:21; Ev þ 0:28; Ev þ 0:35; Ev þ 0:50 and

Ev þ 0:55 eV for P1, P2, P3, P4 and P5, respectively,

with the corresponding hole capture cross-sections of

9.0� 10�15, 3.2� 10�14, 1.9� 10�15, 2.1� 10�13 and

6.5� 10�15 cm2. These values are calculated from the

Arrhenius plots of tT2 determined from DLTS measure-

ments. P1 and P3 are irradiation-induced defects and

correspond to the singly positive charge state of

divacancy (VV) and the CiOi, respectively. According

to the recent assignment [6], P2 and P4 are hydrogen-

related defects. No definite assignment has been avail-

able for P5. Here, we focus on the isochronal annealing

behavior of P1–P4.

Measurements of defect depth profiles for P1–P4 at

each annealing temperature have revealed that the defect

concentration varied upon annealing with the shape of

50 100 150 200 250 3000

P5

P4

P3(CiOi)

P2

P1(VV)

Temperature (K)

DL

TS

sign

al (

a.u.

)

Fig. 1. DLTS spectrum with the time constant t of 19.1ms

from hydrogen-implanted p-type silicon.

depth profiles unchanged. Therefore, peak values of

depth profiles are plotted against annealing temperature

in Fig. 2 for the sample annealed under reverse bias to

show the isochronal annealing behavior of P1–P4. A

significant feature in isochronal annealing behavior is

that P1 (VV), P3 (CiOi) and P4 drastically decrease in

concentration upon annealing in the temperature range

90–150�C, while the concentration of P2 increases. It is

well known that VV and CiOi are thermally stable in this

annealing temperature range [7]. The similar annealing

behavior is observed for P1, P3 and P4 upon zero bias

annealing, which rules out the effect of the charge state

on the thermal stability of these defects. These results

indicate that hydrogen interaction with VV and CiOi

occurs in hydrogen-implanted samples, resulting in the

passivation of electrical activity of these defects as hole

traps. Hereafter, we use the word ‘‘passivation’’ for the

disappearance of energy levels in the lower half of the

band gap by hydrogen interaction. P4 seems also

passivated by hydrogen. The partial annealing of the

A center in the same temperature range has already been

reported in hydrogen-implanted n-type silicon, which is

ascribed to the transformation of the A center into the

complex of the A center and hydrogen [5]. It is thought

that there exist hydrogen-containing defects created by

hydrogen implantation to liberate hydrogen during

annealing in the temperature range 90–150�C which

causes hydrogen passivation of P1, P3 and P4. The

liberated hydrogen also leads to the growth of P2. This

indicates that P2 is an electrically active defect as

containing hydrogen.

As seen from Fig. 2, P1, P3 and P4 reappear upon

annealing above 180�C. This annealing behavior is

explained by the liberation of hydrogen from these

defects, which strongly supports that VV, CiOi and P4

are indeed passivated by hydrogen. On the contrary,

hydrogen passivation of P1, P3 and P4 is found to be

stable upon zero bias annealing above 180�C. This is

shown in Fig. 3 for P3 (CiOi). This means that the

0 50 100 150 200 2500

1

2

3

4

5

6

7

8

9

10

Annealing temperature ( ̊C)

P1(VV)P2P3(CiOi)P4

Tra

p co

ncen

trat

ion

(1013

cm

-3 )

Fig. 2. Isochronal annealing behavior of P1 (VV), P2, P3 (CiOi)

and P4 for the sample annealed under reverse bias.

ARTICLE IN PRESS

0 50 100 150 200 2500

1

2

3

4

5

Tra

p co

ncen

trat

ion

(1013

cm

-3 ) P3(CiOi)

revers bias annealingzero bias annealing

Annealing temperature (˚C)

Fig. 3. Isochronal annealing behavior of P3 (CiOi) under zero

and reverse bias.

Y. Tokuda, H. Sato / Materials Science in Semiconductor Processing 6 (2003) 277–279 279

reappearance of P1, P3 and P4 upon reverse bias

annealing is due to the field drift of hydrogen dissociated

from these defects as positively charged species. It is

thought that the dissociated hydrogen from irradiation-

induced defects again associates with these defects under

no electric field.

The occurrence of hydrogen passivation of VV is

consistent with the previous reports [2–4,6]. However, as

pointed out earlier, there is some discrepancy in the

literature on the effect of hydrogen on CiOi [2,3,6].

Comparison of our isochronal annealing behavior under

zero and reverse bias supports that the CiOi pair is

indeed passivated by hydrogen. On the other hand, the

isochronal annealing behavior of P2 confirms the

previous assignment that this level is the hydrogen-

related defect [6]. It is noted from Fig. 2 that the

annealing behavior of P4 is similar to that for VV and

CiOi. This seems to suggest that P4 is the irradiation-

induced defect rather than the hydrogen-related defect,

which is inconsistent with the previous assignment by

Fatima et al. [6]. In fact, a defect corresponding to P4

has been observed in boron-implanted samples [6,8].

However, no production of this defect has been revealed

in carbon- and helium-implanted samples [6]. Further

study is necessary to identify P4.

4. Summary

Comparison of zero and reverse bias annealing in

hydrogen-implanted p-type silicon indicates that hydro-

gen interaction with CiOi pair occurs together with VV,

resulting in their passivation. These defects remain

passivated at least around 240�C under no electric field.

The hydrogen interaction with levels at Ev þ 0:50 and

Ev þ 0:28 eV suggests that these are irradiation-induced

and hydrogen-related defects, respectively.

References

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[3] Feklisova OV, Yarykin NA. Transformation of deep-level

spectrum of irradiated silicon due to hydrogenation under

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[4] Tokuda Y. Deep-level transient spectroscopy study of

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[5] Tokuda Y, Ito A. Annihilation and formation of electron

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