Transmission electron microscopy characterization of secondary defects created byMeV Si, Ge, and Sn implantation in siliconJ. Wong-Leung, S. Fatima, C. Jagadish, J. D. Fitz Gerald, C. T. Chou, J. Zou, and D. J. H. Cockayne Citation: Journal of Applied Physics 88, 1312 (2000); doi: 10.1063/1.373819 View online: http://dx.doi.org/10.1063/1.373819 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/88/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Defects in Ge and Si caused by 1MeVSi+ implantationa) J. Vac. Sci. Technol. B 26, 425 (2008); 10.1116/1.2834557 Study of extended-defect formation in Ge and Si after H ion implantation Appl. Phys. Lett. 86, 181910 (2005); 10.1063/1.1906319 Triple crystal diffractometry, x-ray standing wave, and transmission electron microscopy investigation of shallowBF 2 implantation in Si J. Vac. Sci. Technol. B 20, 1436 (2002); 10.1116/1.1491548 The effect of implantation, energy, and dose on extended defect formation for MeV phosphorus implanted silicon Appl. Phys. Lett. 75, 2629 (1999); 10.1063/1.125100 Effect of ion mass on the evolution of extended defects during annealing of MeV ion-implanted p-type Si Appl. Phys. Lett. 74, 1141 (1999); 10.1063/1.123468

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Transmission electron microscopy characterization of secondary defectscreated by MeV Si, Ge, and Sn implantation in silicon

J. Wong-Leung,a) S. Fatima, and C. JagadishDepartment of Electronic Materials Engineering, Research School of Physical Sciences and Engineering,The Australian National University, Canberra ACT 0200, Australia

J. D. Fitz GeraldPetro Physics Group, Research School of Earth Sciences, The Australian National University,Canberra ACT 0200, Australia

C. T. ChouPacific Solar, 82-86 Bay Street, Botany, NSW 2019, Australia

J. ZouAustralian Key Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia

D. J. H. CockayneDepartment of Materials, University of Oxford, Parks Road, Oxford 0X1 3PH, England

~Received 21 October 1999; accepted for publication 24 April 2000!

Extended defects created in Si by ion implantation to doses below the amorphization threshold havebeen studied after annealing at 800 °C for 15 min. The implant species were the group IV elementsSi, Ge, and Sn, and structural defects created by similar damage distribution were compared. Themass of the implanted ion influences the type of defect observed. For all three implant species,rod-like $311% planar defects were observed. Additionally, in Ge and Sn implanted samples, small$111% interstitial faulted dislocation loops were observed. ©2000 American Institute of Physics.@S0021-8979~00!01615-7#

I. INTRODUCTION

Ion implantation is now an important processing step inthe manufacturing of silicon devices. The potential use ofdefect engineering by ion implantation in future silicon de-vices is a driving force for experimental research into param-eters controlling extended defect formation in silicon. Differ-ent types of defects have been observed depending on theimplant dose, the annealing temperature, and the implantspecies. There exists a critical dose for defect formation afterannealing which is the minimum dose for extended defectformation and below which only point defects areobserved.1,2 Above the critical dose, various types of ex-tended defects are observed in annealed Si:3 Namely cat-egory I defects which are formed from implant doses belowthe amorphization threshold and category II defects or end-of-range defects which are formed below the amorphous–crystalline interface for doses above the amorphizationthreshold. Category III defects generally result from an im-perfect amorphous–crystalline interface.

Implantation induced category I damage has been shownto be primarily interstitial in nature. This is attributed to thenonconservative nature of the implantation process and the‘‘ 11 model’’ postulates that the number of interstitialstrapped in the residual defects after annealing is equal to theimplant dose.3 Indeed, heavier mass species will result indenser collision cascades during implantation. Recently,Herneret al.4 have shown that Pb implants resulted in tran-

sient enhanced diffusion~TED! of boron superlattices andPROPHET IV simulations can only model the extent of TEDwith a ‘‘14.5 model.’’ Alternatively, the atomic displace-ment model5 accounts for the different damage created bythe implantation of different mass ions. Even so, other fac-tors such as dynamic annealing cannot be accounted for byusing scaling factors determined fromTRIM6 or FASTRIM7

simulations.In recent studies, we have concentrated on the critical

dose regime.2,8 Both electrical and structural characterizationof Si, Ge, and Sn implanted samples consistently reveal thatthe formation of extended defects occurs at lower doses forheavier mass species.8 In this study, we report details of thetransmission electron microscopy~TEM! characterization ofthe extended defects formed in the various samples studied.

II. EXPERIMENT

p-type Czochralski~Cz! Si wafers were implanted atroom temperature with group IV elements namely Si, Ge,and Sn to energies of 4, 6, and 8.5 MeV, respectively, suchthat the implant depth is similar in all three cases and around2.8mm. The Si substrate was tilted 7° off axis with respect tothe incoming ions to minimize channeling. Various dosesranging from 731012 to 131014cm22 were studied. All thesamples were subject to a 15 min anneal at 800 °C. Cross-section samples with a foil normal within 15° of@011# wereprepared from the various samples by cleaving, mechanicalpolishing, and ion beam thinning. TEM was carried out usinga Philips EM 430T instrument operated at 300 keV. One ofa!Electronic mail: jw1109@rsphysse.anu.edu.au

JOURNAL OF APPLIED PHYSICS VOLUME 88, NUMBER 3 1 AUGUST 2000

13120021-8979/2000/88(3)/1312/7/$17.00 © 2000 American Institute of Physics

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the difficulties of weak beam imaging using a 300 keV beamis the larger Ewald sphere thus increasing the chances ofhaving multiple beams operative. To overcome this, someTEM analysis was also carried out on a Philips CM12 instru-ment operated at 120 keV. Weak beam micrographs wererecorded using ag/ng diffraction arrangement9 with n.3,except forg5400 wheren.2 was used. The conventionused for the coordinate system is the same as that in Ref. 10.Burgers vector analysis was carried out using the standardg•b andg•b∧u criteria.11 For loops, inside/outside analysiswas carried out using6(g•b)s pairs of images.

III. RESULTS

A. Si implantation

Figure 1 shows typical cross-section TEM images of thesample implanted with 4 MeV Si to a dose of 131014cm22 imaged withg5202̄ near the@111# zone axis.All the defects observed in this sample are rod-like defects~RLDs!. Twelve variants exist, elongated along the six^011&line directions with two possible habit planes for each linedirection. Figure 2 is a schematic showing the^011& linedirections in the convention used. The cross-sectional TEM

micrograph in Fig. 1 has the implant surface at the top of thefigure and shows the RLDs at the implant range~'2.8 mmdepth!. The long defects which are parallel to the implantsurface and perpendicular to the foil normal (@01̄1# directionin Fig. 2! lie in a horizontal direction in Fig. 1 and conse-quently show no depth contrast. The other variant with itslength parallel to the implant surface~@011# direction in Fig.2! corresponds to the defects which are approximately paral-lel to the sides of the micrograph in Fig. 1~an example ismarked A in Fig. 1!. This group of defects shows a dottedcontrast consistent with the defect lines running through thefoil thickness. The dotted contrast is essentially a depth con-trast and its periodicity indicates the inclination of the defectin the foil. Finally, any defects oriented along@101̄# are notvisible. We note that the operativeg5202̄ is parallel to theline direction ~u! and the invisibility is consistent with aBurgers vectorb perpendicular tou, an issue discussed inmore detail in the next paragraph. There remain six othervariants of the defects inclined to the surface—two habitplanes for each of the three line directions~see Fig. 2!. Thesethree line directions can be differentiated by~a! the anglebetween the projections of the defect and the normal to theimplant surface and~b! the dotted contrast showing the in-clination of the defect. The two habit planes associated witheach line direction were differentiated by noting changes tothe width of each image when tilting around the Bragg re-flection.

Previous literature on RLDs reports two types of RLDslying along ^011&. First, there are the$311% rod-like defectswith a $311% habit plane:12 their dissolution was shown tocause TED of B in Si.13 These defects have been shown to beinterstitial,14 with a displacement vectorR of typea^611&/25giving a dilation approximately perpendicular to the plane ofthe defect. More recently, some RLDs have been analyzed tohave a$111% habit plane with displacement vectorR of typea^111&/10. These were named$111% RLDs.10 The RLDs inthis study were imaged with 022 type reflections close to the@111# and @ 1̄11# zone axes. By using conventional invisibil-ity criteria wheng•R50 is satisfied, the images of the RLDsobserved in our Si implanted sample were more consistentwith the displacement vector associated with$311% RLDsthan with $111% RLDs.

One feature of the micrographs is the marked change indefect visibility occurring with the reversal of the diffractionvector~see Fig. 1!. This observation can be explained by thefact that the RLDs are extended defects. The two parallel

FIG. 1. Weak beam micrographs of Si wafers implanted with 4 MeV Si toa dose of 131014 cm22 and annealed at 800 °C for 15 min.

FIG. 2. Schematic showing convention used and the positions of^011& withrespect to the foil.

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edges defining the width of the defect can be considered astwo partial dislocations with oppositeb so that their imagesmove to opposite sides of the defect upon reversingg. Whenboth images are ‘‘inside’’ the defect, a different image inten-sity can result compared with when they are ‘‘outside.’’ Thisis a similar explanation to that given for similar observationsof defects in fcc metals.15 This change in visibility makes itessential to use both6g images to establish thatg•R50when determiningR. We also note that the RLDs do notshow fault contrast in most line orientations except thoselying along the@011# direction~close to foil normal! ~variantmarked A in Fig. 1! and this is discussed in Sec. IV.

An identical set of analyses was carried out for the Siimplant doses above the critical dose. In each case, the dis-placement vector and fault plane were identical to those forthe 131014Si cm22 dose. Samples from different Si implantdoses, above the critical dose, were studied by TEM. It wasfound that the density of RLDs in the samples increased withincreasing dose~see Ref. 2!. The average lengths of theRLDs were measured for samples implanted with Si to dosesof 431013, and 131014cm22 and determined to be'0.22mm in both cases.

B. Ge implantation

The defects in Ge implanted samples were different fromthose observed in Si implanted samples even when the doseand energy chosen had been adjusted to create similar peakvacancy distribution in the two samples. Figure 3 showstypical micrographs of a sample implanted with 431013Ge cm22. In addition to RLDs, dislocation loops~e.g.,marked by arrowheads! were observed. Analysis of theRLDs, following the same procedures as for the RLDs in Si,showed that the defects were consistent with$311% RLDs asin Si. TEM analyses of the loops were carried out. Traceanalysis showed the loops to have a$111% habit plane, and,using theg•b invisibility criterion, the Burgers vector of theloops was determined to be perpendicular to the habit plane.Table I summarizes the variousg•b products for the variouspossible$111% habit planes and some of the diffracting vec-tors g used to record the images. Figure 4 shows a typicaldislocation loop observed in the Ge implanted sample im-aged using different reflections. Using the analyses as sum-marized in Sec. II, some of the loops were determined to befaulted and interstitial in nature with a~111! habit plane. Asan example, the loop in Fig. 4 shows faulted contrast whenimaged usingg511̄1 @see Fig. 4~b!#. It shows residual con-trast ~from g•bLu) when imaged using the three 022-typediffraction vectors close to the@111# zone axis@e.g., Fig.4~c!#, namelyg5022̄, 202̄, 22̄0 for whichg•b50 ~see TableI!. These observations are consistent with$111% Frank loops.A feature of the images of the loops is the double imageobserved underg54̄00 andg51̄11̄, which is not present ing5400 and 11̄1 @see Figs. 4~a! and 4~b!#. This feature isobserved for both 300 and 120 keV electron beam energiesand is discussed in Sec. IV.

TEM examination was carried out on samples implantedwith Ge to doses above the critical dose~between 731012

and 131013cm22!. Figures 5~a! and 5~b! show typical mi-

crographs of samples implanted with Ge to doses of 231013 and 431013cm22, respectively, where loop has beenmarked by arrowhead. It is interesting to note that the loopswere observed in the sample implanted with Ge to a dose of431013cm22 while only RLDs were observed in the sampleimplanted with Ge to a dose of 231013cm22. The averagelengths of the RLDs in the Ge implanted samples, 231013

and 431013cm22 were'0.21 and 0.16mm, respectively.

C. Sn implantation

The defects in Sn implanted samples were similar tothose in Ge implanted samples in that both loops and RLDswere observed. Typical micrographs of the sample implantedwith Sn to a dose of 331013cm22 are shown in Fig. 6. ThisSn implanted sample showed a slightly higher density ofloops than the Ge implanted sample (431013Ge cm22) eventhough the damage distribution is identical in these two cases~discussed later!. As in the case of Ge implantation, a fullanalysis of the loops showed them to be$111% Frank loops.As an example of the analysis, Fig. 6 shows three variants~marked a, b, and c! of the four possible habit planes of suchloops. The inside/outside contrast6~g•b!s, observed whengis reversed, differentiates loops a and b from short RLDs~example shown in Fig. 6!. In this orientation, loops a and bare on the two edge-on$111% planes. According to our con-vention, loops a and b have an inclination consistent with

FIG. 3. Weak beam micrographs of Si wafers implanted with 6 MeV Ge toa dose of 431013 cm22 and annealed at 800 °C for 15 min.~The insetshows the highlighted region on a higher magnification.!

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habit planes of (1̄11̄) and (1̄1̄1), respectively. Loop c hastwo possible habit planes. By trace analysis, loop c was de-termined to have a habit plane (11̄1) and withg502̄2 im-aging, satisfies theg•b50 criterion~see Table I!. Again, theloop shows invisibility only where bothg•b50 andg"b`u50 are satisfied, i.e., where the line is locally parallelto the diffracting vector in Fig. 6. All three loops werefaulted and again were determined to be interstitial in nature.

As reported in Ref. 8, the critical dose for Sn implantedsamples is between 131013 and 231013cm22. Figures 7~a!and 7~b! show typical micrographs of samples implantedwith Sn to doses of 231013 and 331013cm22, respectively.The density of defects appears to decrease with decreasingimplant dose. Loops could be observed in both samples butwith significantly fewer loops visible in the sample im-

FIG. 4. Weak beam micrographs of loop in Ge implanted sample~a! g/2.1g, close to@011# ~b! g/5.1 g, close to@011# ~c! g/3.1 g, close to@111#.

FIG. 5. Weak beam micrographs of Si wafers implanted with 6 MeV Ge todoses of~a! 231013 cm22 and~b! 431013 cm22 and annealed at 800 °C for15 min. ~Loop is marked by an arrowhead.!

TABLE I. Showingg.b product for differentg andb.

g 022̄ 400 11̄1 1̄1̄1 202̄ 2̄20 220 202

b g•b

1/3@ 1̄11̄# 4/3 24/3 21 21/3 0 4/3 0 24/3

1/3@ 1̄1̄1# 24/3 24/3 1/3 1 24/3 0 24/3 01/3@111# 0 4/3 1/3 21/3 0 0 4/3 4/31/3@11̄1̄# 0 4/3 1/3 21/3 4/3 24/3 0 0

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planted with the lower dose of 231013cm22. The averagelengths of the RLDs measured in these two samples, im-planted with 231013 and 331013Sn cm22, were 0.09 and0.20mm, respectively. We note that, for low dose Ge and Snimplants, there are very few defects, and in these samples,fewer than 40 defects were measured in contrast to the othercases described above.

IV. DISCUSSION

Si implantation induced defects have been extensivelycharacterized for most dose regimes. The defects created de-pend on the dose regimes and the annealing temperature. Atvery low doses, the damage created by implantation can becompletely removed by a high temperature anneal and thisdose regime is known as subcritical. Above the critical dosebut below doses which cause amorphization, annealing tem-peratures<800 °C have been observed to nucleate RLDs.However, for high Si implant doses which result in an amor-phous layer being formed,16–18 the nucleation of loops hasbeen observed.

FASTRIM simulations show that similar damage distribu-tions are formed by the following three implants, 131014Si cm22, 431013Ge cm22, and 331013Sn cm22.However, the TEM characterization of these three differentsamples in this study clearly show that for different specieswith similar damage distribution, the defects formed are dif-ferent and dislocation loops are preferentially nucleated insamples implanted with higher mass species.8 The observa-tions related to preferential nucleation of loops for heaviermass species appear to be a direct result of the densercascades created by implantation of the heavier species con-sistent with both experimental evidence19 and MD simu-lations.20 Even though the dose in our experiments has beenadjusted to minimize differences in average damage distribu-

tion, the dense cascades created by the heavier mass speciesmay have introduced some local amorphous regions at thepeak of the damage distribution. The crystallization of suchlocal amorphous regions may explain the nucleation of loopsin the case of heavier implant species.FASTRIM simulationscan only predict the average distribution of vacancies andinterstitials. As a consequence, the simulations do not differ-entiate between regions of uniform disorder and sparsehighly disordered regions.

Fatimaet al.2,8 have determined the threshold doses cor-responding to the transformation from point defects to ex-tended defects in annealed samples. The threshold dose wasshown to depend more strongly on the damage productionthan on the dose of the implant. For the heavier mass species,they reported that doses just above the threshold dose forextended defect formation resulted primarily in the formationof RLDs, while increasing the dose further resulted in both

FIG. 6. Weak beam micrographs of Si wafers implanted with 8.5 MeV Sn toa dose of 331013 cm22 and annealed at 800 °C for 15 min.

FIG. 7. Weak beam micrographs of Si wafers implanted with 8.5 MeV Sn todoses of~a! 231013 cm22 and~b! 331013 cm22 and annealed at 800 °C for15 min. ~Loops were marked by arrowheads.!

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RLDs and loops. This trend is consistent with different doseregimes observed for silicon implants defined by the differ-ence in extended defect formation, i.e., RLDs, RLDs, plusloops, and finally only loops with increasing implant dose.21

It is interesting to note that for heavier species, there is asimilar trend for defect formation with implant dose. How-ever, the threshold dose for each defect regime occurs atlower implant doses compared to the Si implanted samples.

For the Si implanted samples, we note that the averagelength of the RLDs was the same for doses different by morethan a factor of 2. This indicates that the RLDs vary in den-sity rather than size as a function of dose. The averagelengths of defects for both Ge and Sn were close to theaverage length determined for Si, indicating little variationwith implant species. The loops were of the order of 1000 Åin diameter in both the Ge and Sn cases. It is interesting tonote that the RLDs observed in our samples are more con-sistent with the$311% RLDs than the$111% RLDs, in contrastto the study of Chouet al.10 We believe this difference to bea result of the different annealing conditions. Indeed, implan-tation induced defects are well known to be dependent on theannealing temperatures and times.

The absence of strong fault contrast from the RLDs, re-ferred to in Sec. III A, can be understood as follows. Thefault contrast is determined byg•R. In this case,R is close tothe normal of the habit plane and assumed to be of typea^611&/25. For all theg used,g•R is small but not an inte-ger. If the normal to the habit plane is nearly parallel to theincident beam direction@e.g., Fig. 8~a!#, g•R is relativelysmall~because the angle betweeng andR is large!, and any

fault contrast will be weak.g•R is largest when the habitplane ~and henceR! is viewed edge on, but the projectedwidth of the fault is narrow and the fault is difficult to see.By comparison, those defects~variant marked A in Fig. 1!elongated along the@011# direction and therefore steeply in-clined with respect to the foil, do show significant stackingfault contrast. The productg•R is significant in this case.

Turning now to the loops, the transformation fromdouble image to single image observed under the reversal ofthe sign ofg ~see Sec. III B! cannot be easily explained byconsidering a$111% Frank loop. This anomaly is also presentin loops identified as Frank loops.16 Seshan and Washburn22

reported a change in contrast for$111% Frank loops in Si onreversingg, which they attributed to the adsorption of dopanton the stacking fault. Furthermore, the loops observed in ourstudy have edges parallel to^110&, which is not characteristicof faulted loops but is typical of prismatic loops.16,18Further-more, the size of the loops observed in our Ge sample islarger than those in Refs. 16 and 18 and faceting of loops hasbeen observed to be size dependent.23 An explanation for theanomaly observed, namely the transformation from double tosingle image under the reversal ofg, is being pursued, in-cluding the possibility of the loops having edge facets oninclined planes.

In conclusion, we have shown that defect nucleation canbe controlled by implant species and dose. In most of thecases studied above the critical doses, RLDs were observedwith an average length of'0.2 mm. In addition, loops formin samples implanted with heavier mass species. The loopswere determined to be interstitial faulted loops with a$111%habit plane. All the defects observed in the implantedsamples were interstitial defects in agreement with previousreports of defect formation for preamorphization implantdoses.

ACKNOWLEDGMENTS

The authors J. W. L. and S. F. acknowledge financialsupport from the Australian Research Council and AUSAID,respectively.

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