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Materials Science in Semiconductor Processing 9 (2006) 288–291

Isothermal deep-level transient spectroscopy study of metastabledefects in hydrogen-implanted n-type silicon

Yutaka Tokuda�, Wakana Nakamura, Hiroshi Terashima

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

Available online 10 February 2006

Abstract

Isothermal deep-level transient spectroscopy (DLTS) with a single pulse has been used to study the transformation

behavior of a metastable defect labeled EM1 (Ec�0.29 eV) which is observed in n-type silicon implanted with hydrogen

ions at 88K and subsequently heated to room temperature. It is found that EM1 shows a decrease in isothermal DLTS

peak height with filling pulse duration time in the range from 1 ms to 1000 s. Isothermal DLTS allows us to detect another

metastable defect labeled EM2 (Ec�0.41 eV) as well as EM1 at the same temperature and reveals a corresponding increase

in EM2 peak height with filling time. This indicates that EM1 filled with electrons is transformed into EM2 during

application of filling pulse.

r 2006 Elsevier Ltd. All rights reserved.

PACS: Dlts/71.55.cn; 72.20.jv

Keywords: Silicon; Hydrogen; Metastable defects; Isothermal

1. Introduction

It has been reported that hydrogen implantationat low temperature induces hydrogen-related defectsinto n-type silicon which show the metastability[1–7]. Two hydrogen-related metastable defectslabeled E30 [1] and E300 [2,3], observed by deep-leveltransient spectroscopy (DLTS), have been identifiedas isolated hydrogen at a bond-center site and bondcentered hydrogen perturbed by a nearby oxygenatom, respectively which are unstable at roomtemperature. On the other hand, room-temperaturestable ones labeled EM1 (Ec�0.29 eV), EM2

e front matter r 2006 Elsevier Ltd. All rights reserved

ssp.2006.01.053

ing author. Tel.:+81565 48 8121;

0020

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

(Ec�0.41 eV) and EM3 (Ec�0.55 eV) have beenobserved in oxygen-rich n-type silicon implantedwith hydrogen ions at 88K and subsequently heatedto room temperature [4–7], in which temperature-scan DLTS with repeated bias pulses has been usedto study their transformation behavior. It has beensuggested that there is the transformation of EM1which occurs during DLTS measurements [7].However, it seems difficult to extract the transfor-mation kinetics for such a complex system bytemperature-scan DLTS with repeated pulses [8].

In this work, we apply the isothermal DLTS witha single pulse to obtain the transformation behaviorof EM1 in hydrogen-implanted n-type silicon. Theisothermal DLTS allows us to measure the EM1and EM2 signals at the same temperature. It will beshown that there is the transformation between

.

ARTICLE IN PRESSY. Tokuda et al. / Materials Science in Semiconductor Processing 9 (2006) 288–291 289

EM1 and EM2 which are the stable states underreverse bias and zero bias, respectively.

2. Experimental

The samples used were prepared from phos-phorus-doped, n-type (1 0 0) Czochralski-grownwafers with a resistivity of 1–2O cm. Samples wereimplanted at 88K with 100 keV hydrogen ions at adose of 2� 1010 cm�2 and subsequently heated toroom temperature. Schottky contacts were fabri-cated by resistive evaporation of gold in vacuum onthe implanted sides of samples. Ohmic contacts wereformed by rubbing eutectic Ga–In on the back side.

Isothermal DLTS with a single pulse was carriedout with a reverse bias of 3V and filling pulseamplitude of 3V. This bias condition enables us toprobe the implanted region for the present samples.The filling pulse duration times were varied in therange from 1ms to 1000 s. Isothermal DLTSspectrum was constructed from the capacitancetransient by means of the rate scan method with abipolar rectangular weighting function [9,10].

3. Experimental results and discussion

Fig. 1 shows the typical temperature-scan DLTSspectra obtained after reverse-bias cooling forhydrogen-implanted samples with the filling times

140 150 160 170 1800

2

4

6

8

10

12

14

16

18

20

1 s

0.1s

1 ms

VOHEM1

DL

TS

sign

al (

fF)

Temperature (K)

Fig. 1. Typical DLTS spectra with the filling times of 1 ms, 0.1

and 1 s obtained after reverse-bias cooling for n-type silicon

implanted with 100 keV hydrogen ions at 88K and subsequently

heated to room temperature.

of 1ms, 0.1and 1 s, where EM1 and the vacancy-oxygen-hydrogen (VOH) center are observed [4–7].It is found that EM1 shows a decrease in DLTSsignals with filling time. We have already suggestedthat this anomalous response of EM1 on filling timeis due to the occurrence of the transformation ofEM1 during DLTS measurements [7].

Fig. 2 shows the schematic diagram of thetransformation behavior of EM1 during DLTSmeasurements which we have presented [7]. EM1is the stable state under reverse bias and isdesignated as state A. State A� (filled state) istransformed to the stable state under zero bias, stateC � (filled state), during the application of fillingpulses at temperatures where EM1 is observed inDLTS spectra. This causes a decrease in EM1DLTS peak height with filling time as shown in Fig.1. The transformation C 0 (empty state)-A0 (emptystate) occurs with fast rates during the applicationof reverse bias in DLTS measurements, whichconfirms the reproducibility for the filling timedependence of EM1 peak heights. In Fig. 2, we usedA� and C � for the filled states, and A0 and C 0 forthe empty states, but it does not mean that thesestates are acceptor-like traps.

In this way, the transformation between states A

and C cycles with repeated pulses during tempera-ture-scan DLTS measurements. For such a case, thenumber of cycles is thought to affect the transfor-mation behavior of metastable defects [8]. There-fore, we used a single cycle in Fig. 2 to obtain thetransformation behavior for A-C. The cycle startsby applying a reverse bias of 3V in the durationwhich is long enough to establish state A0. Next, thefilling pulse at zero bias is applied with the durationtime of tp to fill state A with electrons andsubsequently to cause the partial transformation

emissionemission

EM10.29 eV

transformation(fast)

transformation

A0

capture

transformationabove 200 K

A- C-

C0

B-

Fig. 2. Schematic diagram of the transformation behavior of

EM1 during DLTS measurements.

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0.1 1 10 100 1000

0

EM2

EM1

EM1+EM2

Isot

herm

al D

LT

S pe

ak h

eigh

t (fF

)

Filling time (s)

10

20

30

Fig. 4. Variation of isothermal DLTS peak heights for EM1 and

EM2 with filling time at 151K.

Y. Tokuda et al. / Materials Science in Semiconductor Processing 9 (2006) 288–291290

A�-C � depending on tp in the range from 1 ms to1000 s. This means that the concentration of stateA� decreases with filling time, while that of state C �

increases. By applying reverse bias, states A� andC � turn into states A0 and C 0, respectively, withelectron emission. The resultant capacitance tran-sient is converted into the isothermal DLTSspectrum [10]. The cycle ends by the transformationfrom state C 0 into state A0 with fast rates.

Fig. 3 shows the isothermal DLTS spectrameasured at 151K with the filling times of 10, 50and 1000 s. Each spectrum is obtained by subtract-ing the spectrum after zero-bias cooling from thatafter reverse-bias cooling. Therefore, the signalsfrom the VOH center disappear by this subtraction.We measured the capacitance transients in the timerange up to 60 s, which made it possible to observeEM2 as well as EM1. The EM1 isothermal DLTSpeak height decreases with filling time, which isconsistent with the result obtained from thetemperature-scan DLTS as shown in Fig. 1. EM2is found to show an increase in peak height withfilling time.

Fig. 4 shows the variation of isothermal DLTSpeak heights for EM1 and EM2 with filling time at151K. It is noted that EM1 and EM2 peak heightsvary in the same range in filling time. Furthermore,the sum of both heights remains constant irrespec-tive of filling time. This means that the decrement ofEM1 coincides with the increment of EM2. Theseresults strongly suggest that EM1 is transformed toEM2 during filling time. We have already pointedout that EM1 (state A), stable under reverse bias, is

10-3 10-2 10-1 100 101 102-5

0

5

10

15

20

25

30

1000 s

50 s

10 s

EM2

EM1

Isot

herm

al D

LT

S si

gnal

(fF

)

Time (s)

Fig. 3. Isothermal DLTS spectra measured at 151K with the

filling times of 10, 50 and 1000 s.

transformed to state C, stable under zero bias,during the application of filling pulses as shown inFig. 2 although state C has not been identified [7].The present result obtained from isothermal DLTSindicates that EM2 is state C. Moreover, it is foundthat the filling time dependence of EM1 and EM2can be fitted to the decreasing and increasingexponential functions with the same time constant.Solid lines are calculated ones with the timeconstant of 35 s, which corresponds to the transfor-mation time constant at 151K from EM1 to EM2during the application of zero bias.

It is found that there is transformation betweenEM1 and EM2. EM1 is the stable state underreverse bias, while EM2 is the stable state underzero bias. The EM2 DLTS signals are observedupon reverse bias cooling together with the EM1signals since the formation of EM2 occurs via thetransformation from EM1 during filling time.However, no DLTS signals for EM2 appear uponzero bias cooling from room temperature. This isdue to the transformation from EM1 to anotherstate stable under zero bias, called state B [7], whichis included in Fig. 2. State B has not been observedwith DLTS in the temperature range up to 290K.Thus, EM1 (state A) stable under reverse bias hastwo stable states under zero bias, EM2 (state C )and state B.

4. Summary

Isothermal DLTS has been used to study thetransformation behavior of a metastable defectEM1 occurring during the application of filling

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pulse in hydrogen-implanted n-type silicon. Thistechnique has allowed us to detect metastabledefects EM1 and EM2 at the same temperature. Itis found that there is the transformation betweenEM1 and EM2 which are stable states under reversebias and zero bias, respectively. These are differentconfigurations of the same hydrogen-related defect.

Acknowledgements

This work was supported in part by a Grant-in-Aid from the Nitto Foundation for the Promotionof Science.

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