swift heavy ion-induced recrysallization of silicon-on-insulator (soi) structures

Swift heavy ion-induced recrysallization of silicon-on-insulator(SOI) structures

G.S. Virdi a,*, B.C. Pathak a, D.K. Avasthi b, D. Kanjilal b

a Central Electronics Engineering Research Institute, Pilani, Rajasthan 333 031, Indiab Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India

Received 3 April 2001; received in revised form 4 September 2001

Abstract

Buried Si3N4–Si interfaces and overlayer in silicon-on-insulator (SOI) structures were improved by irradiation with

100 MeV 107Ag after the synthesis of buried silicon nitride layers by high dose nitrogen ion-implantation. Auger electron

spectroscopy (AES) depth profile analysis illustrates that the MeV heavy ions irradiation in the SOI structure, modifies

the distribution of nitrogen that results in better stoichiometry of the buried silicon nitride layers and abrupt Si3N4–Si

interfaces. Electron spin resonance (ESR) technique shows the improvement in the crystalline structure of the Si over

layer. Current–voltage and high frequency capacitance–voltage (C–V) characteristics were studied, and electrical

breakdown measurements were performed on metal nitride silicon (MNS) structures fabricated after removing the Si

over layer in the SOI structure. In the ion-beam irradiated SOI specimens, buried silicon nitride layer show a high

breakdown field strength of 4.5–6.5 MV=cm as compared to that of 3.0–3.9 MV=cm in the unirradiated one. The C–V

analysis of the MNS capacitors reveals that the buried Si3N4–Si substrate interface exhibits a better quality with re-

duced fixed insulator charge and interface state densities after the ion-beam irradiation. Mid-gap interface state density

at the buried Si3N4–Si substrate interface was as low as 1:0� 1011 cm�2 V�2 after the ion-beam irradiation, which is

comparable to that of silicon nitride films deposited on silicon (Si) by the conventional low pressure chemical vapor

deposition technique. The role of MeV ion-beam irradiation in improving the properties of SOI structures has been

discussed on the basis of various models. � 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Traditionally, the silicon-on-insulator (SOI)substrates are obtained by high dose (�1017–1018ions=cm2). Implantation of nitrogen or oxygen

ions in to silicon (Si) at an energy in the range 100–200 keV. This process leads to amorphization ofthe surface silicon layer together with rough in-terfaces due to defect generated by inelastic nu-clear collisions of incident ions with silicon atoms.Recrystallization of top layer is necessary to de-velop SOI structure for device applications. Nor-mally, thermal annealing of SOI-substrate at atemperature of �1400 �C for P4 h is for de-fect removal after implantation. However, some

Nuclear Instruments and Methods in Physics Research B 187 (2002) 189–200

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residual defects remain even after high tempera-ture annealing. Considerable efforts are being madeto synthesize better quality SOI substrate by highdose nitrogen implantation. An SOI substrate,suitable for the fabrication of higher performancedevices should possess: a device-worthy siliconlayer at the top, a good dielectric buried siliconnitride layer, and two high quality abrupt inter-faces of the buried silicon nitride layer with the topsilicon layer and the substrate silicon. Earlier, wehave reported [1] that the fluorine implantation inSOI structure before or after the synthesis im-proves the crystalline structure of the silicon over-layer as well as the quality of the buried siliconnitride layer. In this study infrared transmissionspectroscopy and glancing angle X-ray diffractionanalysis has shown that the fluorine implanta-tion in SOI structure removes the undesired poly-silicon grains from the transition layer.Swift heavy ion (SHI) irradiation through an

amorphous/crystalline (a/c) interface causes solidphase epitaxy or layer by layer amorphization de-pending on temperature and ion-beam parameters[2,3]. From the application point of view ion-beam-induced epitaxial crystallization (IBIEC) is of in-terest because it occurs at temperature much lowerthan that necessary for the thermally-inducedcrystallization. Further unique feature of this pro-cess is the possibility to control precisely the lateraland vertical extent of recrystallization. The phe-nomenon of IBIEC has been actively pursued in Si[4–6]. In the IBIEC it is assumed that nuclear en-ergy deposition is a dominant parameter. However,it has recently been reported that electronic energydeposition also plays a significant role on thecrystallization of amorphous Si (a-Si) layer [7,8].In the present work SHI-induced recrystalliza-

tion of top silicon layer and improvement of theroughness of interfaces of SOI structures havebeen studied. Auger electron spectroscopy (AES)depth profiling was used to study the distributionof nitrogen in silicon. Recrystallization of the SOIstructure was analyzed by electron spin resonance(ESR) technique. Metal nitride silicon (MNS)capacitors fabricated on the buried silicon nitridelayers after removing the top layers of silicon,were used as an analytical tool to study the fieldstrength and current–voltage (I–V) characteristics

of buried Si3N4 layers. High frequency capaci-tance–voltage (C–V) measurements on the MNScapacitors were carried out to examine the buriedSi3N4–Si substrate interface properties.

2. Experimental

The SOI structure was synthesized by highdose nitrogen ion-implantation with device grade,60–80 X cm resistivity, h100i orientation, p-typesingle crystal having 75 mm diameter. Prior to ion-implantation, wafers were cleaned with standardRCA procedure and a 300 �AA thick screen oxidewas grown by thermal oxidation to reduce any pos-sible channeling during ion-implantation. Waferswere implanted to the dose of 9:0� 1017 cm�2

with atomic nitrogen ions (14Nþ) at 150 keVenergy. The substrate temperature was kept at400� 10 �C and the ion-beam current was main-tained at �60 lA. The implantation of nitrogenwas done using NV-3204 implanter. After nitrogenimplantation thermal annealing was carried outin a conventional furnace at 1200 �C for 2 h innitrogen ambient.After SOI synthesized wafers were irradiated by

100 MeV 107Ag ions to a dose of 1� 1014 ions=cm2. During MeV irradiation sample temperaturewas kept at �400 �C throughout the irradiation.The high energy ion irradiation was carried outusing the beam from 16 MV tandem Pelletronaccelerator [9] at a vacuum of 2:3� 10�7 mbar. Abeam flux of about 5� 1010 ions=cm2=s was used.The total fluence of 1� 1014 was estimated by in-tegrating the charges by current integrator fallingon the sample kept in a cylindrical electron sup-pressed geometry and then counting the pulsesgenerated by each 10�10 C by a counter. The ir-radiation was carried out at an angle of 7� to avoidchanneling during recrystallization.Following specimens were prepared to study the

effect of swift heavy ion (SHI) irradiation of SOIstructures:

• PA: As implanted with 14Nþ (dose 9�1017 cm�2, at 150 keV).

• PB:14Nþ implanted (dose 9� 1017 cm�2, at 150

keV) and furnace annealed (1200 �C, 2 h).

190 G.S. Virdi et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 189–200

• PC:14Nþ implanted (dose 9� 1017 cm�2, at 150

keV), furnace annealed (1200 �C, 2 h), irradiatedwith 107Ag (dose 1� 1014 cm�2, at 100 MeV).

• PD:14Nþ implanted (dose 9� 1017 cm�2, at 150

keV), furnace annealed (1200 �C, 2 h). Irradi-ated with 100 MeV 107Ag (dose 1� 1014 cm�2).Samples were post-annealed (900 �C, 0.5 h).

The AES studies were performed using a Perkin–Elmer model 570 AM scanning Auger microp-robe. The AES depth profiles were evaluated bymultiplexing silicon LMM and nitrogen KLLpeaks at different depths inside the substrate. Thethickness of the buried silicon nitride layer in thesynthesized SOI structure was evaluated by AESdepth profile analysis and also by ellipsometryafter removing the Si over layer.The amorphization and recrystallization recov-

ery after SHI irradiation in the synthesized in SOIstructure was investigated by ESR technique.Pure aluminum was evaporated on the exposed

buried silicon nitride layer and MNS capacitorswere fabricated by defining square dots of 2:5�10�3 cm2 area using conventional photolithogra-phy techniques. For good ohmic contacts, alumi-num metallization was carried out on the backsideof the specimens after removing the oxide.Dielectric breakdown field strength of the bur-

ied nitride layers was monitored on a Tektronix(Model 575) curve tracer and also by I–V mea-surements using Keithley model 598 quasistaticC–V meter. High frequency C–V characteristicsof MNS capacitors were monitored at 1 MHz, onC–V meter from MSI, USA.

3. Results and discussions

AES depth profiles of nitrogen ion-implantedsilicon specimens were recorded after removing thescreen oxide. As seen in Fig. 1 the distribution ofnitrogen in as implanted silicon specimen lookssimilar to a Gaussian profile with a peak laying atdepth of �3400 �AA corresponding to sputter etchtime of 16 min. In Fig. 2 the AES depth profileof annealed specimen clearly illustrates that theGaussian profile changes to a trapezoidal profileafter thermal annealing in a conventional furnace

at 1200 �C for 2 h. The nitrogen peak in theGaussian profile and the center of the flat portionof the profiles of the annealed samples falls veryclose to the projected range (�3600 �AA) of the im-planted nitrogen ions in the silicon simulated usingstopping and range of ions in matter SRIM-2000[10] calculations. For all the annealed specimens,the atomic percent of nitrogen and silicon variedfrom 54% to 57% and from 46% to 43%, respec-tively, in the flat region of the AES depth pro-file. These atomic concentrations of silicon andnitrogen may be considered well within the stoi-chiometric level (Si¼ 43%, N¼ 57%) required tosynthesize the buried silicon nitride layers. Thus,the flat regions in the AES depth profiles of thenitrogen implanted and subsequently annealed sili-con specimens correspond to the nearly stoichio-metric buried silicon nitride layers. This completesthe synthesis of the SOI structure by high dosenitrogen ion-implantation and subsequently ther-mal annealing.In Fig. 2(a), a small bump is observed in the

center portion of the AES profile for the specimenannealed at 1200 �C for 2 h. It is believed that thisbump corresponds to a porous region containingnitrogen bubbles within the buried silicon nitridelayer [11]. These bubbles are formed from excessnitrogen exceeding the amount required to syn-thesize of stochiometric silicon nitride layers. Itis worth noting that AES depth profiles of 14Nþ

implanted specimens annealed at 1200 �C, 2 h

Fig. 1. AES depth profile of as implanted specimen with 14Nþ

(dose 9:0� 1017 cm�2, at 150 keV). The profile peaks at depth

of �3400 �AA corresponding to sputter etch time of 16 min.

G.S. Virdi et al. / Nucl. Instr. and Meth. in Phys. Res. B 187 (2002) 189–200 191

followed by 100 MeV 107Agþ ion-irradiation donot show the bumps (Figs. 2(b) and (c)). Hereconcentration of nitrogen reaches very close to thestoichometric level in a very smooth fashion over a

significant part in the flat region of the AES pro-files. It appears to remove excess nitrogen due toSHI beam irradiation. As a result the formation ofamorphous zone consisting bubbles disappear dueto change of amorphous zone into crystallineSi3N4 with MeV irradiation.It is also observed from Fig. 2 that the AES

depth profiles of ion-beam irradiated specimensshow a major difference near the transition region.The nitrogen distribution of unirradiated specimenis somewhat broad (Fig. 2(a)) and is graded at boththe Si–Si3N4 interfaces. But the nitrogen profiles inthe irradiated specimens near the Si–Si3N4 inter-faces are steeper than in the specimen without theSHI irradiation in SOI structure. This indicatesthat the SHI irradiation modifies the distributionof nitrogen. In the SHI irradiated samples nitrogenpiles up from the wings towards the Si–Si3N4 in-terfaces. Therefore, more abrupt interfaces of theburied silicon nitride layer with the substrate andthe over layer silicon are observed. In the flatportion of the AES profiles, the atomic percent ofnitrogen in the SHI irradiated SOI specimen isvery close to the stoichometric level required insilicon nitride, whereas it is slightly lower in thenon-irradiated SOI specimen. Thus, the buriedsilicon nitride layer in SHI irradiated SOI speci-men are nearly stoichiometric. However, the beststoichiometry of the buried silicon nitride layeris achieved only when synthesis SHI irradiatedSOI specimens is further annealed to 900 �C for0.5 h (Fig. 2(c)). Here the atomic nitrogen con-centration almost equals to the required stoicho-metric level in most parts of the flat region. Itis also evident from the AES depth profiles thatboth the interfaces of the buried silicon nitridelayer with the surface and the substrate siliconare more abrupt for this specimen (Fig. 2(c)).The estimated thickness of the Si over layer and

the buried silicon nitride layer in each of the syn-thesized SOI structure is listed in Table 1. Thethickness of the Si over layer was found to be2450� 25 �AA from AES depth profile analysis. Thewidth of the flat portion in the AES depth profilesdetermines the thickness of stochiometric buriedsilicon nitride layer. The thickness of the buriedsilicon nitride estimated from the AES depthprofile is nearly the same in all SOI specimens and

Fig. 2. AES depth profile of the different specimens: (a) PB:

Implanted with 14Nþ (dose 9:0� 1017 cm�2 at 150 keV and

furnace annealed 1200 �C, 2 h): (b) PC: Irradiated 107Ag dose

1� 1014 ions=cm2 at 100 MeV after 14N implant and furnance

annealed (dose 9� 1017 ions=cm2 at 150 keV annealed 1200

�C, 2 h): (c) PD: Irradiated107Ag dose 1� 1014 ions=cm2 at

100 MeV after 14N implant and furnance annealed (dose

9� 1017 ions=cm2 at 150 keV annealed 1200 �C, 2 h) and post-annealed (900 �C, 0.5 h).


was within the range of 1720� 20 �AA. The thick-ness of these layers were also computed from elli-psometric data after removing the Si over layer.Here the thickness of the buried silicon nitridelayer was also evaluated theoretically. The samecomes out to be 1800 �AA which is very close to theexperimental values. It is also interesting to notethat the thickness of the buried nitride layer irra-diated with SHI ions is slightly higher (�35–36 �AA)than that in the specimen synthesized without SHIirradiation.Refractive index (g) of the synthesized buried

silicon nitride layer was also determined from el-lipsometric data. The value of g determined as 2.20in unirradiated samples and 2.10 and 2.07 in thespecimens irradiated with SHI beam and irradi-ated as well as annealed. The silicon-rich Si3N4

film exhibits a refractive index relatively higherthan that of stoichometric films (g ¼ 2:06) grownby conventional low pressure chemical deposition(LPCVD) method. The higher values of the re-fractive index suggest that these synthesized buriedsilicon nitride layers are silicon rich. Moreover, thelower refractive indices of buried silicon nitridelayers in the SHI irradiated SOI specimens indicatethat the amount of embedded polycrystalline sili-con grains in these specimens is lower as compared

to that in the one which was synthesized withoutSHI irradiation. Thus it may be concluded thatthe SHI irradiation has reduced the presence ofthe embedded polysilicon grains near the buriedsilicon nitride–silicon interfaces. This supplementsour results of the AES analysis, where steeperslopes are observed near the Si3N4–Si interfaces inthe profiles of the MeV irradiated SOI specimensthan in that of the unirradiated ones.The ESR spectrum of unimplanted silicon

substrate shown in Fig. 3(a) gives negligible ESRsignal in the sample indicating that the unpairedelectrons, which constitute the spin electrons areabsent. The nitrogen implanted silicon before an-nealing and SHI irradiation shows ESR signalcharacteristics of a center exhibiting an isotropicg-value of 2.0028 and the signal line width of 3.75G (Fig. 3(b)). The spin density was found to be6� 1017 cm�2. The shape of the signal is closelyLorentzian which is determined by using the der-ivation of curve method [12]. The ESR signal de-cays after high temperature annealing and SHIirradiated using 100 MeV 107Ag ions and post-annealed at 900 �C for 0.5 h as shown in Fig. 3(c).This is attributed to the reduction of Si danglingbonds and the strain in the buried nitride SOIsynthesized structure. The ESR study reveals that

Table 1

Summary of results

S. no. Results Samples specifications

PBa PC

b PDc

1 Thickness of top silicon layer by AES (�AA) 2400 2452 2475

2 Theoretical thickness (�AA) of buried Si3 N4 layer 1800 1800 1800

3 Thickness of buried Si3N4 layer by AES (�AA) 1704 1721 1739

4 Thickness of buried Si3N4 layer by Ellipsometry (�AA) 1725 1750 1753

5 Refractive index (g) of Si3N4 layer 2.20 2.10 2.07

6 Breakdown voltage of buried Si3N4 layer (V) 55–65 75–82 85–95

7 Breakdown field strength of buried Si3N4 (MV cm�1) 3.0–3.9 4.5–6.0 4.5–6.5

8 Fixed insulator charged density at the buried Si3N4–Si substrate

ð1 cm�2Þ interface– 3:68� 1011 1:2� 1011

9 Mid-gap interface state density at buried Si3N4–Si substrate interface

ðeV�1 cm�2Þ– 3:2� 1011 1� 1011

a PB:14Nþ implanted (dose 9� 1017 cm�2, at 150 keV) and furnace annealed (1200 �C, 2 h).

b PC:14Nþ implanted (dose 9� 1017 cm�2, at 150 keV), furnace annealed (1200 �C, 2 h), irradiated with 107Ag (dose 1� 1014 cm�2, at

100 MeV).c PD:

14Nþ implanted (dose 9� 1017 cm�2, at 150 keV), furnace annealed (1200 �C, 2 h), irradiated with 100 MeV 107Ag (dose

1� 1014 cm�2). Samples were post-annealed (900 �C, 0.5 h).


after SHI irradiation the amorpirization layer isconverted into crystalline layer.The I–V and dielectric breakdown characteris-

tics of the MNS capacitors were carried out toevaluate the dielectric nature of the buried siliconnitride layers. From the results shown in Table 1, itis clear that there is a considerable improvement ofthe breakdown voltage of buried Si3N4 layer afterSHI irradiation. Fig. 4 represents the breakdowncharacteristics of the typical MNS capacitor fab-ricated on each of the SOI specimens, taken byramping method on the curve tracer.The threshold limit for leakage current was 1

lA and the ramp rate was 0.2 V/s. The electricalbreakdown for SHI irradiated samples is in therange 75–95 V which is higher than that (inthe range 55–65 V) of the sample which was fur-nace annealed. The corresponding breakdown fieldstrength of these nitride layers comes out to be4.5–6.5 and 3.0–3.9 MV=cm, respectively, for theSOI specimens synthesized with and without SHIirradiation.The histograms in Fig. 5 show the percentage

failure of the MNS capacitors fabricated on dif-ferent specimens for the positive applied bias. Alarge number of capacitors (1 0 0) were chosenrandomly on each specimen. As seen from the

histogram in Fig. 5(a), the maximum number ofbreakdown failure of the MNS capacitors in theunirradiated samples occurred in the range 55–65V. The maximum failure of the MNS capacitorson the SHI irradiated specimen occurred at ahigher range 75–90 V (Fig. 5(b) and (c)). Only 46%of MNS capacitors could sustain a voltage morethan 55 V in specimen without SHI irradiation.50% of the specimen that was only irradiated withSHI beam could bear voltage more than 75 V.However, a large number (more than 75%) ofMNS capacitor fabricated on SHI irradiated andpost-annealed SOI specimen shows a breakdownvoltage above 85 V. This reflects that a reasonablygood dielectric uniformity of buried silicon nitridelayer exists across this specimen. Thus the syn-thesized buried silicon nitride layer of the specimenirradiated with SHI ions followed by annealing

Fig. 3. (a) ESR spectrums of different specimens. Unimplanted

silicon sample. (b) As implanted with 14Nþ (dose 9� 1017

ions=cm2, at 150 keV). (c) Irradiated 107Ag dose 1� 1014

ions=cm2 at 100 MeV after 14N implant and furnance annealed

dose (9� 1017 ions=cm2 at 150 keV annealed 1200 �C, 2 h) andpost-annealed (900 �C, 0.5 h).

Fig. 4. Breakdown characteristics of MNS capacitors fabri-

cated on different specimen: (a) PB: Implanted with14Nþ (dose

9:0� 1017 cm�2 at 150 keV and furnace annealed 1200 �C, 2 h).(b) Pc: Irradiated

107Ag dose 1� 1014 ions=cm2 at 100 MeV

after 14N implant and furnance annealed dose (9� 1017

ions=cm2 at 150 keV annealed 1200 �C, 2 h). (c) PD: Irradiated107Ag dose 1� 1014 ions=cm2 at 100 MeV after 14N implant and

furnance annealed dose (9� 1017 ions=cm�2 at 150 keV an-

nealed 1200 �C, 2 h) and post-annealed (900 �C, 0.5 h).


exhibits a superior quality, e.g. higher electricalbreakdown voltage and better dielectric uniformityas compared to those specimens which are eitherirradiated with SHI ions after SOI synthesis orsynthesized without SHI irradiation.Breakdown and possible current conduction

mechanism in the buried silicon nitride layers werealso studied by I–V characteristics of the MNScapacitors, obtained after applying an electric fieldin steps and recording the corresponding value ofthe leakage current. In all the specimens, it was

observed that the initial instant current was 2–3times higher than the final steady-state current,when the applied bias was changed from one valueto another. This current decay process with timewas very steep in the initial state and became muchslower after 3–4 min, the current reaching to finalsteady-state value only after 25–30 min. The cur-rent decay was more pronounced at lower electricfields of 0.1–2.0 MV=cm or an applied bias of 0.1–34 V. The decay of current with time is attributedto de-trapping of charge or filling up of trap levelspresent in the buried silicon nitride layers of thesynthesized SOI structure [13].To obtain quick and reproducible data, each

capacitor was first charged by biasing it to a volt-age slightly less than the final breakdown voltage.The I–V data were acquired after reducing theapplied voltage in steps. The I–V characteristics ofthe MNS capacitors fabricated on the furnanceannealed specimens are shown in Fig. 6 for anapplied positive bias at the gate with respect to thesubstrate. The leakage current observed from this

Fig. 5. Histograms showing percentage failure of MNS ca-

pacitors with positive applied voltage for different specimens:

(a) PB: Implanted with14Nþ (dose 9:0� 1017 cm�2 at 150 keV

and furnace annealed 1200 �C, 2 h). (b) Pc: Irradiated107Ag

dose 1� 1014 ions=cm2 at 100 MeV after 14N implant and

furnance annealed dose (9� 1017 ions=cm2 at 150 keV annealed

1200 �C, 2 h). (c) PD: Irradiated 107Ag dose 1� 1014 ions=cm2

at 100 MeV after 14N implant and furnance annealed dose

(9� 1017 ions=cm2 at 150 keV annealed 1200 �C, 2 h) and post-annealed (900 �C, 0.5 h).

Fig. 6. I–V characteristics for MNS capacitors (area ¼ 2:5�10�3 cm�2): (a) PB: Implanted with

14Nþ (dose 9:0� 1017 cm�2

at 150 keV and furnace annealed 1200 �C, 2 h). (b) Pc: Irradi-ated 107Ag dose 1� 1014 ions=cm2 at 100 MeV after 14N im-

plant and furnance annealed dose (9� 1017 ions=cm2 at 150 keV

annealed 1200 �C, 2 h). (c) PD: Irradiated107Ag dose 1�

1014 ions=cm2 at 100 MeV after 14N implant and furnance

annealed dose (9� 1017 ions=cm2 at 150 keV annealed 1200 �C,2 h) and post-annealed (900 �C, 0.5 h).


figure was below 1 ngA up to 50 V for MNScapacitor on the unirradiated specimen, whereasthe MNS capacitors fabricated on the irradiatedsamples responded to the same current level evenup to an applied voltage of 80 V. The final break-down voltage was found polarity dependent andwas lower in case of positive bias on the gate elec-trode. The electrical breakdown voltage with nega-tive biasing was at least 6–8 V higher than that withpositive bias on the gate electrode.The I–V characteristics of Fig. 6 are reproduced

in Fig. 7 with Schottky coordinates (pE versus

ln J). It is very difficult to interpret these I–Vcurves at very low electric fields (up to 0.25MV=cm) or an applied bias below 5 V since theyare complicated due to extremely low current

values and long transients. The buried silicon nit-ride layer in the SHI irradiated samples shows anOhmic behavior up to an electric field of 4.0MV=cm (up to an applied bias of 42 V), whereas asimilar behavior was seen only up to 2.0 MV=cm(up to an applied bias of 34 V) in the unirradiatedsamples. At relatively higher electrical fields (2.5–5.2 MV=cm) or an applied bias of 42–89 V, a linearrelationship exists between ln (J) and

pE in the

irradiated samples. The estimated values of con-duction coefficient (b) from the linear portion of I–V curves of the MNS was found to be 2:78� 10�3

and 2:89� 10�3 eV V�1=2 cm1=2, respectively, forthe SHI irradiated and SHI irradiated followed bypost-anneal specimens. The value of b was 2:81�10�3 eV V�1=2 cm1=2 for the non-irradiated speci-men. These experimental values do not match withthe theoretical value of b for a Schottky conduc-tion mechanism, but are very close to the theo-retical value ðb � 3:1� 10�3 eV V�1=2 cm1=2Þ in aPoole–Frenkel mechanism for the silicon nitridefilms.The high frequency C–V characteristics of the

MNS capacitors were measured at 1 MHz tocharacterize the buried Si3 N4–Si substrate inter-

Fig. 7. I–V Characteristics (in Schottkey coordinates) for MNS

capacitors fabricated on (area 2:5� 10�3 cm�2): (a) PB: Im-

planted with 14Nþ (dose 9:0� 1017 cm�2 at 150 keV and fur-

nace annealed 1200 �C, 2 h). (b) Pc: Irradiated107Ag dose

1� 1014 ions=cm2 at 100 MeV after 14N implant and furnance

annealed dose (9� 1017 ions=cm2 at 150 keV annealed 1200

�C, 2 h). (c) PD: Irradiated107Ag dose 1� 1014 ions=cm2 at

100 MeV after 14N implant and furnance annealed dose (9�1017 ions=cm2 at 150 keV annealed 1200 �C, 2 h) and post-an-nealed (900 �C, 0.5 h).

Fig. 8. C–V characteristics of MNS capacitors fabricated on:

(a) PB: Implanted with14Nþ (dose 9:0� 1017 cm�2 at 150 keV

and furnace annealed 1200 �C, 2 h). (b) Pc: Irradiated107Ag

dose 1� 1014 ions=cm2 at 100 MeV after 14N implant and

furnance annealed dose (9� 1017 ions=cm2 at 150 keV annealed

1200 �C, 2 h). (c) PD: Irradiated 107Ag dose 1� 1014 ions=cm2

at 100 MeV after 14N implant and furnance annealed dose (9�1017 ions=cm2 at 150 keV annealed 1200 �C, 2 h) and post-

annealed (900 �C, 0.5 h).


face and are presented in Fig. 8. Fig. 8(a) corre-sponds to the specimen annealed without SHI ir-radiation. This does not show any modulation inthe capacitance value for an applied voltage in therange from �60 to þ60 V. A similar behavior inthe range from �50 to þ50V was reported earlier[14]. One of the most likely reasons for such a C–Vbehavior seems to be the presence of very highcharge density at the lower Si3N4–Si interfaces(between the buried Si3N4 layer and the Si sub-strate) due to which bulk electrode behaves likea metallic contact. The embedded polycrystallinesilicon grains in the buried silicon nitride layer atthe Si3N4–Si substrate interface causes roughnessin the interface. Consequently, a high charge den-sity is expected at such an interface. It may also bedue to high doping caused by nitrogen impuritytail in the substrate silicon layer just under theburied silicon nitride layer. However, no quanti-tative information could be inferred on the inter-face state density at the lower Si3N4–Si interfaceexcept that it must be very high.The MNS capacitors fabricated on the buried

Si3N4 layers formed after SHI irradiation showcapacitance modulation with the variation in theapplied voltage (Fig. 8(b) and (c)). These C–Vcurves look similar to that of a metal–insulator–silicon (MIS) capacitors fabricated on a p-typesilicon substrate. It is evident from these charac-teristics of MNS capacitors that the SHI irradia-tion in SOI substrate helps to achieve betterinterface between the buried silicon nitride layerand the substrate silicon. These observations alsoimply that the silicon layer just beneath the buriedsilicon nitride layer is p-type. The effective dopingconcentrations, calculated from the C–V char-acteristics, are 1:3� 1015 and 1:9� 1015 cm�3, re-spectively, for the layer in the specimens irradiatedwith SHI beam and post-annealed after irradiationof SOI specimens. The calculated doping concen-tration in specimen irradiated and post-annealedis very close to the initial concentration of 2:3�1015 cm�3 for the substrate. This illustrates thatthe post-annealing of SHI irradiated SOI is moreeffective to achieve better quality of Si over layer,buried silicon nitride layer, and Si3N4 interface.The fixed insulator charge densities calculated

from the flatband shift in experimental C–V curves

were 3:68� 1011 and 1:2� 1011 cm�2, respectively,for irradiated and post-annealed after irradiationspecimens.The interface state density at the mid-band-gap

for the lower Si–Si3N4 interface was also calcu-lated from C–V characteristics by the techniquesdescribed by Nicollian and Brews [15]. This wasfound to be 3:2� 1011 and 1� 1011 cm�2 eV�1,respectively, for irradiated and post-annealed afterirradiation specimens.From the analysis of the C–V characteristics of

the MNS capacitors, we may conclude that SHIirradiation in SOI structures reduced the mid-band-gap interface state density as well as the fixedinsulator charge density in the buried interface ofsilicon nitride–silicon substrate. Thus the interfacestate density which refers to the energy stateswithin the silicon energy band gap is reduced. SHI-induced recrystallization and consequent reduc-tion of the interface trap density can be attributedto the passivation of interfacial dangling bonds atthe buried Si3N4–Si substrate interface. The bondsformation with Si near the interface results inbreaking of the Si–N bond and inducement ofrelaxation of the interfacial strain. The discussionpresented above explains the role of ion-beam-induced recrysallisation in improving the basicelectrical properties of the buried Si3N4–Si sub-strate interface in the SOI structure by the SHIheavy ion-beam irradiation.Our present investigations clearly indicate that

the SHI irradiation into the SOI structures syn-thesized by high dose nitrogen implantation im-proves the dielectric behavior of the buried siliconnitride layers and the roughness as well as the in-terface properties of the buried Si3N4–Si inter-faces. The effect of SHI irradiation in the buriedsilicon nitride SOI structure can be understoodwith the help of the models proposed by variousauthors [7,8,16,17].Some authors have developed the phenomeno-

logical models to explain the mechanism of IBIEC[16,17]. In most of these models point defects ther-mally produced in displacement collisions are as-sumed to be responsible for the low temperaturerecrystallization under ion-beam irradiation. In thesemodels it is assumed that simple point defectsor defect clusters produced in collision cascades


should mediate the recrystallization process at thea/c interface. But it is not yet clear whether theyare consumed by the recrystallization or they actas ‘catalyst’ only.When an energetic ion passes through a mate-

rial, it loses energy mainly via two nearly inde-pendent processes: (i) elastic collisions with thenuclei known as nuclear energy loss ðdE=dxÞn,which dominates at an energy of about 1 keV/amu;and (ii) inelastic collisions of the highly chargedprojectile ion with the atomic electron of thematter known as electronic energy loss ðdE=dxÞewhich dominates at an energy of about 1 MeV/amu or more. In the inelastic collision (cross-sec-tion �10�16 cm2) the energy is transferred fromthe projectile to the atoms of the matter throughexcitation and ionization of the surrounding elec-trons. The amount of electronic loss in each col-lision varies from tens of eV to a few keV perAngstrom (�AA). For a SHI moving at a velocitycomparable to the Bohr velocity of the electron thelater is the dominant mechanism for transfer ofenergy to the material for modifying it. The pas-sage of SHI in materials mainly produces elec-tronic excitation of the atoms in the materials. SHIcauses exotic effects in different classes of materialswhich otherwise cannot be generated by any othermeans. Quantitatively, it is capable of depositingelectronic excitation of hundreds of eV/�AA to afew keV/�AA in the materials. Such a large densityof electronic excitation brings out changes in theproperty of materials. There have been attempts tounderstand it by various models. The commonlyreferred models are known as Coulomb explosion[18] model and thermal spike [19] model. Duringthe passage of SHI through materials neighboringpositive target ions are produced by electronicexcitation-induced ionization. These positive ionsare mutually repulsive. The time to cover atomicsites is short in comparison to the response time ofthe conduction electrons. So during the passage ofthe ion a long cylinder containing charged ionsis produced. This cylinder containing the chargedions explodes radially due to conversion of elec-trostatic energy to coherent radial atomic move-ments under Coulomb forces until the ions arescreened by conduction electrons. Due to the re-sulting cylindrical shock wave, modification takes

place along the trajectory of the ion due to radialCoulomb explosion. The thermal spike model is theother competing process which is also responsiblefor the modification of materials.According to this model during the passage of

SHI the kinetic energy of the electrons ejected dueto inelastic collision-induced electronic excitationis transmitted to the lattice by electron–phononinteraction in a way efficient enough to increasethe local lattice temperature above the meltingpoint of the material. The temperature increaseis then followed by a rapid quenching ð1013–1014K=sÞ that may result in a modification of thestructure when the melt solidifies.In Fig. 9, the variation of ðdE=dxÞe and ðdE=

dxÞn for 100 MeV 107Ag beam with depth as itpasses through the Si–Si3N4–Si substrate is plottedbased on Monte Carlo simulation [10] of stoppingand range of ions in matter (SRIM-2000). Theenergy of the beam at different depths are alsomentioned on the top axis. From this figure onecan see that ðdE=dxÞe dominates up to a depth ofabout 10 lm. Near the end of range (�14.2 lm) of100 MeV silver beam ðdE=dxÞn dominates beforeimplantation of the beam at the end of its range.So point defects and defect clusters may be formedmainly near the end of the range due to elasticcollisions. For a typical thickness of about 0.5 lm

Fig. 9. Variation of ðdE=dxÞe and ðdE=dxÞn for 100 MeV 107Ag

beam with depth as it passes through the Si–Si3N4–Si substrate.

The energy of the beam at different depths is mentioned on the

top axis.


near the surface where SOI is synthesized, thevalue of ðdE=dxÞe remains constant and the effectdue to ðdE=dxÞn is comparatively insignificant. Infact the effect of elastic collision is comparativelynegligible up to a depth of 10 lm as evident fromFig. 9.In the present case the values of energy losses

near the a/c interface, as calculated by SRIM-2000,are: Se ¼ 1119 eV=�AA, Sn ¼ 5:935 eV=�AA in top Si-layer, Se ¼ 843 eV=�AA, Sn ¼ 4:196 eV=�AA in Si3N4-layer and Se ¼ 1103 eV=�AA, Sn ¼ 6:144 eV=�AA in theSi-substrate at the Si3N4–Si interface. Electronicenergy loss (Se) is approximately three orders ofmagnitude higher than the nuclear loss (Sn).The energy release mechanism explain the

enhancement of IBIEC by inelastic electronicscattering. Electron–hole pairs produced by theinelastic electronic scattering of mega-electron-voltheavy-ion-beam irradiation recombine and releaseenergy in three ways: radiative recombination,Auger capture, and phonon capture or emission.Radiative recombination of carriers can be band toband or band to localized defect state, and the ex-cess energy is converted to a form of photon. Thisreleased energy cannot aid diffusion of a defect oran atom at the recombination site. Auger captureof carriers can occur as either band-to-band re-combination or band-to-defect capture. Releasedenergy is given to another electron (Auger electron)and is not available to the enhanced diffusion of adefect or an atom at the recombination site.The phonon-aided carrier capture process, how-

ever, can enhance diffusion. There are two types ofsuch process: cascade capture and multiphononemission. The energy liberated in such processes byelectron–hole recombination can be transferredinto the vibrational energy of Si atoms in anamorphous layer near the interface. Although thisliberated energy alone is not enough to move Siatoms from the amorphous phase to the crystallinephase, it probably contributes to the rearrange-ment of Si atoms near vacant spaces from randomsites to substitutional sites. Thus, the inelasticelectronic scattering of mega electron-volt heavy-ion-beam irradiation can lead to the enhancementof IBIEC. This phonon-aided energy release mech-anism is equivalent to subthreshold collisions byelastic nuclear scattering. That is, in this elastic

collision, energy under the threshold for displace-ment of a Si atom from its substitutional site istransferred to the target nucleus but does not pro-duce a vacancy. The energy transferred by suchcollisions is converted to the vibrational energy ofSi atoms. Thus, it is possible that both phonon-aided carried capture by inelastic electronic scat-tering and subthreshold collision by elastic nuclearscattering contribute to the enhancement of IBIECby mega-electron-volt heavy-ion-beam irradiation.Therefore it is certain that the IBIEC rate per unitvacancy produced at the a/c interface by the elasticnuclear scattering of mega-electron-volt heavy-ion-beam irradiation increases with the inelasticelectronic scattering for ionization and excitation.

4. Conclusions

The effect of SHI irradiation on recrystalliza-tion of SOI structure was thoroughly investigated.The nitrogen profile is modified after SHI irradi-ation at the interface resulting in better stochio-metry of the Si3N4 layers and creation of abruptSi3N4–Si substrate interfaces in the SOI structure.The ESR analysis confirms recrystallization ofthe amorphized layers produced by high dose ofnitrogen implantation. The improvement in theelectrical breakdown strength of the synthesizedburied silicon nitride layers in SOI structure dueto irradiation of SHIs are attributed to the re-moval of polycrystalline inclusions from the Si3N4

layer near the buried Si3N4 substrate interface.The present studies also illustrate that the post-annealing (900 �C, 0.5 h) after SHI irradiation ofthe synthesized SOI structure by high dose nitro-gen implantation is a better choice as far as theimprovement the quality of these SOI structure isconcerned. The improvement in the stochiometry,dielectric breakdown field strength, dielectric in-tegrity of the buried silicon nitride layer and thereduction in the fixed insulator charge density aswell as the interface state density at the buriedSi3N4–Si substrate interface in these SOI structureis explain on the mechanism of the enhancementby inelastic electronic scattering. The results of thepresent studies are very useful for achieving im-proved SOI structure.


Acknowledgements

The authors are thankful to Council of Scien-tific and Industrial Research (CSIR) for financialassistance in carrying out this research. They aregrateful to Pelletron accelerator group for pro-viding high quality SHI beam for recrystallization.

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swift heavy ion-induced recrysallization of silicon-on-insulator (soi) structures

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