Threading dislocations in silicon layer produced by separation by implanted oxygen process

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Threading dislocations in silicon layer produced by separationby implanted oxygen process

E. Prieura)European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France

C. GuilhalmencLaboratoire d’Electronique de Technologie et d’Instrumentation, Commissariat a` l’Energie Atomique deGrenoble, 17 Rue des Martyrs, F-38054 Grenoble Cedex 9, France

J. HartwigEuropean Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France

M. OhlerEuropean Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France, and Max PlanckArbeitsgruppe, Ro¨ntgenbeugung an Schichtsystemen, Hausvogteiplatz 5-7, D-10117 Berlin, Germany

A. Garcia and B. AsparLaboratoire d’Electronique et de Technologie de l’Informatique, Commissariat a` l’Energie Atomique,Centre d’Etudes Nucle´aires de Grenoble, 17 Avenue de Martyrs, F-38054 Grenoble Cedex, France

~Received 29 February 1996; accepted for publication 8 May 1996!

Threading dislocations in the silicon layer in three different types of the silicon on insulator samplesproduced by standard and improved separation by implanted oxygen~SIMOX! processes wereinvestigated by synchrotron x-ray topography, scanning electron microscopy~SEM!, and opticalmicroscopy. The densities and Burgers vectors of the dislocations were first determinednondestructively by synchrotron x-ray topography. Then the line directions of the same dislocationswere determined by SEM after chemical Secco etching. Some of these results were compared withresults obtained from optical microscopy of Secco etched samples. The threading dislocations in theSi layer were found to occur mainly in pairs with densities of the order of 105 cm22 in standardSIMOX samples and of the order of 104 cm22 in improved SIMOX samples. These dislocationshave an edge character. Other features of these dislocations, such as the distances between twodislocations forming a pair, orientations of these pairs, and dislocations that change their linedirection, are also discussed. ©1996 American Institute of Physics.@S0021-8979~96!01316-3#

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I.INTRODUCTION

In microelectronics, silicon on insulator~SOI! technolo-gies have mainly been used in specific areas, such as miland space applications, due to their low power consumpand good resistance to radiation damage.1 However, the im-portance of these technologies has increased considerbecause of their low power and low voltage applications.2 Inaddition, fully depleted devices that are not possible in bsilicon offer promising possibilities for SOI technologies3

For all these applications, high quality SOI wafers aneeded.

Separation by implanted oxygen~SIMOX! is one of themost widely used techniques to produce SOI material. Inwafers produced by the standard SIMOX process,4 thethreading dislocation density in the Si layer~Fig. 1! is of theorder of 105 cm22.5 Recent improvements in the SIMOXprocess~labeled here as ‘‘improved SIMOX’’ process! ofcombining Si epitaxial growth and annealing steps reduthe threading dislocation density down to the order of 14

cm22.6 A further reduction is necessary for bipolar transistapplications, thus, motivating the studies of dislocationsSIMOX.

Threading dislocations in SIMOX are found to occmostly in pairs. The origin of these pairs is presumably lar

a!Electronic mail:[email protected]

J. Appl. Phys. 80 (4), 15 August 1996 0021-8979/96/80(4)/21

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dislocation half-loops that form due to stresses duringimplantation process.7–9During annealing, the half-loops expand and create dislocations pairs. The characteristicsthese dislocations in standard and improved SIMOX wafare difficult to determine by transmission electron microcopy, because their density is only 104–53105 cm22. Experi-ments using synchrotron white beam x-ray diffraction toporaphy in transmission~Laue! geometry have shown that withthis method one can obtain the Burgers vector of the threing dislocations in the Si layer in SIMOX samples.10 Thistechnique also allows one to determine the dislocation dsity nondestructively. The density and line direction of thdislocations can be studied after etching the samples withpreferential chemical Secco etch,11 i.e., destructively. In thiscase optical microscopy gives information about the dislotion density and scanning electron microscopy~SEM! aboutthe line direction.

In the present study threading dislocations in three dferent types of the SOI samples from the standard andproved SIMOX processes are investigated by combinsynchrotron x-ray topography,12,13SEM, and optical micros-copy. The work is aimed at determining the character ofdislocations, their density, the distance between the dislotions forming a pair, and their mutual orientation. The dferences between the samples produced by the standardimproved processes are also discussed.

211313/8/$10.00 © 1996 American Institute of Physics

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II. METHODS

A. Samples

The standard SIMOX wafers are produced by implant1.831018 O1 cm22 at 190 keV at approximately 600 °C int~100! Czochralski-grown Si wafers and then annealing thfor 6 h at 1320 °C ~Ar11%O2!. This results in a uniform0.2-mm-thick Si layer on top of a 0.4-mm-thick amorphousSiO2 layer. The Si layer contains threading dislocations ging from the upper Si/SiO2 interface to the sample surfacThe samples produced by the above mentioned procedurlabeled as ST1 in the present study.

The 0.2-mm-thick Si layer was found to be too thin tgive a reasonable contrast on x-ray topographs. For thatson a 10-mm-thick Si layer was grown on the standaSIMOX wafers by chemical vapor deposition. The standSIMOX samples with a 10mm epitaxy are labeled as ST2 ithe present study. It was assumed that the epitaxial grodid not change the density, Burgers vector, and line direcof the threading dislocations in the Si layer. Thus, the resfrom the ST2 samples should also represent the characttics of the ST1 samples.

The improved SIMOX wafers are produced by the saimplantation procedure as the standard SIMOX wafers buaddition an epitaxial Si layer is grown to facilitate the glidinof the dislocations. In the present study, the investigatedproved SIMOX samples~labeled as IM! have a Si epitaxy of10mm. After the epitaxy, a second high temperature anning is made to reduce the threading dislocation density.

A schematic of the studied sample structures is showFig. 1. Before the measurements, small scratches were mwith a diamond edge on the sample surfaces. These scraenabled the localization of images of the same dislocation pictures obtained by different characterization methoand, thus, allowed the comparison of these dislocationages. The scratches did not introduce additional dislocati

B. Synchrotron x-ray topography

Topographs were taken at the BM5 Optics beamlinethe European Synchrotron Radiation Facility~ESRF!. Thephoton spectrum used in the measurements ranged fromto 1.0 Å. The measurements were made in transmission

FIG. 1. Schematic of the measured samples after additional epitaxy

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ometry taking advantage of the very high energies availabat the ESRF. The beam size was 30 mm38 mm allowing theimaging of a large area of the sample in a single exposurBoth polychromatic~white! and monochromatic beams wereused. White beam topography is a fast technique that allowone to record topographs with several diffraction vectors insingle exposure. In monochromatic beam topography onone topograph is recorded for each exposure, but this tecnique has a better resolution and a higher sensitivity to defects than white beam topography. In the monochromatbeam measurements of the present study, often only a smpart of the sample was imaged because of the curvaturethe SIMOX samples~curvature radius about 50–100 m!. Amonochromatic beam was obtained from a~211! Si crystalusing the symmetrical 111 reflection in transmission geometry. Both dispersive and nondispersive double crystal arangements were employed, and both symmetrical and asymetrical reflections from the sample were usedMonochromatic beam topographs were taken at differenworking points on the rocking curve. The sample to filmdistance was 10–20 cm. Kodak SO-343 high resolution filmwere used. The spatial resolution of the measurements wabout 1mm.

III. RESULTS

A. Standard SIMOX process

Several standard SIMOX samples without an additionaepitaxy ~samples ST1! were studied by optical microscopyafter a five step chemical Secco etch developed for the chaacterization of dislocations in thin SOI structures.14 Thethreading dislocation density in the Si layer was found tovary in the range 1–53105 cm22 in different samples. Figure2 shows the typical dislocation configuration found in ST1samples. The threading dislocations are seen as white poinIn most cases, the dislocation etch pits form pairs that havpreferential orientations along@010# and @001# directions.The dislocation etch pits in a pair are separated by a distanvarying from a value less than 1mm up to 3mm.

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FIG. 2. Optical micrograph of a ST1 sample after a five step Secco etch

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FIG. 3. Images from the same area of a ST2 sample.~a! Optical micrograph after a modified Secco etch.~b! and ~c! Monochromatic beam topographs, thwavelength of the diffracted beaml50.33 Å. Diffraction vectorg5@022# ~b! andg5@022# ~c!. Some dislocation pair images are labeled as A and B, andscratch mark as S.

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Several SIMOX samples produced by the standaSIMOX process but with a 10-mm-thick additional epitaxy~samples ST2! were studied by optical microscopy afterone step modified preferential Secco etch of 2 min~etchingrate about 0.5mm/min!. The threading dislocation density in

J. Appl. Phys., Vol. 80, No. 4, 15 August 1996

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ST2 samples was found to be the same as in ST1 sami.e., the epitaxial growth had no effect on the dislocatidensity. Figure 3~a! shows an optical micrograph illustratinthe typical dislocation configuration for ST2 samples. In adition to the scratch mark S, only the etch pits of th

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threading dislocations are visible. Their density is ab23105 cm22. The etch pits appear in pairs and are separaby a distance of 10–13mm. Most of these pairs have preerential orientations close to@011# and @011# directions, i.e.,different from ST1 samples. Several@011# and@011# orientedpairs are labeled in Fig. 3~a! as A and B, respectively.

Figures 3~b!–3~c! show two double crystal x-ray topographs with different diffraction vectorsg taken from thesame area of the ST2 samples as in Fig. 3~a!. In all thetopographs of the present study, the higher optical dencorresponds to the higher diffracted intensity, i.e., the tographs are negatives. The dislocations are seen as spotssisting of black and white regions. In Fig. 3~b! the back-ground is white and, thus, mainly the black region ofdislocation contrast is visible. In Fig. 3~c! the situation is theopposite. This change of background is related to the Mfringes that result from the interference of the wavesfracted by the layer and the substrate.15

All the dislocation images seen in Fig. 3~a! are visible inFigs. 3~b!–3~c! but with different contrasts. The labels A, Band S help in identifying the same dislocations in Figs. 3~a!–3~c!. Dislocations A~@011# oriented pairs! have a strong contrast in Fig. 3~b! but a weak contrast in Fig. 3~c!. For dislo-cations B the situation is the opposite. The weakening ofdislocation contrast is related to the fact that normallyimage of the dislocation in the bulk crystal has a very wecontrast or is not visible on a topograph when the condig–b50 is fulfilled. Because here the dislocations are in10.2-mm-thick layer, the surface relaxation effect of the dlocation is relatively large giving rise to a weak contrast evthoughg–b50. The contrasts of the dislocation images wanalyzed on several white and monochromatic beam tographs with different diffraction vectorsg. Dislocations Bhave weak contrast wheng is along @022# @Fig. 3~b!# and@133# directions. Thus, their Burgers vectorsb are parallel tothe @011# direction. The same analysis for the dislocationsshows, that their contrast weakens in@022# @Fig. 3~c!# and@133# reflections. Thus, the dislocations A have thbi@011#.

Figure 4 shows two SEM images of dislocation pairsa ST2 sample after the Secco etch. Figure 4~a! shows a dis-location pair A that has itsbi@011#, as determined from thtopographs. The shape of the etch pit in Fig. 4~a! indicatesthat the projection of the dislocation line directionl is paral-lel to @011#, i.e., l5@h11# whereh remains to be determinedIn Fig. 4~b! ~dislocations B!, the directions are changebi@011# and the projection ofli@011#. Thus,b'l in all casesand the dislocations are of edge character. The shapes oetch pits in Fig. 4 indicate that the dislocations in the papproach each other when penetrating into the sample.

The line directionl was investigated in more detail bstudying the etch pits of a dislocation pair as a functionthe remaining Si layer thickness after the Secco etch. Fig5~a!–5~c! show three SEM images of etch pits of the sadislocation pair after different etching times. The distanbetween the two etch pits decreases when the layer thickis reduced. In addition, the sizes of the etch pits becolarger but their shapes do not change. When the compoof the edge pit separation distance parallel to the@011# di-


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rection is drawn as a function of the remaining Si layer thicness @Fig. 5~d!# and the dislocations are assumed tostraight, two parameters are obtained: the angle betweendislocation line direction and the surface normal, anddistance between the dislocation ends in the Si/SiO2 inter-face. The former is found to be about 34° and the lattesmall value, less than 1mm. The @211# direction forms andangle of 35.3° with the surface normal. Thus, the dislocatline directions are very likely@211#, @ 211#, @211#, and@211#.These are the edge dislocations typically found in Si.16

B. Improved SIMOX process

Figure 6~a! shows an optical micrograph of the improveSIMOX sample with 10.2mm Si layer thickness~sample IM!after 4 min of the modified Secco etch. Images of dislocatetch pits are visible and their density is about 33104 cm22.They appear in pairs separated by a distance that is typic15mm but varies between 10 and 30mm. The orientation ofthe pairs is not well defined.

Figures 6~b!–6~c! show two topographs of the same paof the IM sample shown in Fig. 6~a!. The contrast of a dis-

FIG. 4. SEM images of dislocation pairs in a ST2 sample after a modifiSecco etch. The pairs are labeled in Fig. 3 as A~a! and B ~b!.

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FIG. 5. SEM images of a dislocation pair in a ST2 sample after a modified Secco etch of 1 min~a!, 4 min ~b!, and 7 min~c!. ~d! shows the distance parallelto the @011# direction between the two dislocation etch pits in the pair as a function of the Si layer thickness.

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location pair on these topographs consists of black and wregions. The inhomogeneous background in Figs. 6~b!–6~c!is due to the Moire´ fringes. The dense interference linesthe lower part of Fig. 6~b! and in the left-hand part of Fig6~c! are parts of the contrasts of the scratch marks madethe sample surface just outside the region shown in FigThe contrasts of the dislocations in IM samples@Figs. 6~b!–6~c!# show finer details than the ones in ST2 [email protected]~b!–3~c!#. The images of the two dislocations formingpair are resolved. This improvement of the visibility of dtails in the dislocation contrast is most likely related to tmore uniform Si/SiO2 interface producing a lower background, i.e., less additional contrast, on the topographs othan those of ST2 samples.

In Figs. 6~b!–6~c!, exactly the same dislocations are viible as in Fig. 6~a!. The positions of the sharp black spotsthe topographs correspond well to those of the etchshown in Fig. 6~a!. As in the case of the ST2 samples~Fig.3!, half of the dislocation images have strong contrast athe other half have weak contrast on the topographs as itrated in Figs. 6~b!–6~c!. The dislocations labeled as A havstrong contrast in Fig. 6~b! ~g5@022#! and weak contrast inFig. 6~c! ~g5@022#!. For the dislocations B the situationreversed. As with ST2 samples, the weak contrast is vlikely related to the surface relaxation and theg–b50 crite-rion can be applied to the dislocation images with the wecontrast. The analysis of the dislocation contrast on sevtopographs with differentg vectors from IM samples givethe same result for the dislocation Burgers vectors astained in ST2 samples, i.e., dislocations A and B have thbi@011# and @011#, respectively. Figure 7 shows a SEM image of the dislocations labeled as A1, B1, and A2 in Fig. 6.Again, the result is identical with ST2 samples, i.e., the dlocations A and B have their line directionsl5@h11# and@h11#, respectively.


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Also for IM samples the etch pits of a dislocation pawere studied under SEM as a function of the Si layer thicness~Fig. 8!. The distance between the two etch pits firdecreases and then remains constant as a function ofetching time. This behavior can be explained assuming tthe dislocations consist of two parts and are so-called brokdislocations. Close to the sample surface their line directioare similar to those observed in ST2 samples. Deeper inSi layer the dislocations change their direction. Their Bugers vectors do not change. The shapes of the SEM imaof the dislocation etch pits after 7 min of the Secco [email protected]~c!# look different from those with shorter etching time@Figs. 8~a!–8~b!# and those of ST2 samples~Fig. 5!. Thisalso indicates that their line direction changes inside thelayer. About 90% of the dislocations in IM samples werfound to consist of broken lines.

IV. DISCUSSION

Table I summarizes the results presented in Sec. III. Tmost important difference between the samples is the threing dislocation density that is an order of magnitude smalin the improved SIMOX than in the standard SIMOX. In thimproved process, during the second annealing, the dislotions move and eliminate each other, thus, reducing thdensity.

In the standard SIMOX, the dislocation etch pits on thsample surface are oriented either along the@010# and@001#directions~ST1! or along@011# and @011# directions~ST2!.This can be explained in the following way: When two dislocations have opposite Burgers vectors a stable pair oritation should be close to the@010# or @001# directions.17

These orientations were found in ST1 samples becausehigh temperature annealing was the last process stepenabled the dislocations to move to the stable positio

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FIG. 6. Images from the same area of a IM sample.~a! Optical micrograph after a modified Secco etch.~b! and ~c! Monochromatic beam topographs, thwavelength of the diffracted beaml50.33 Å. Diffraction vectorg5@022# ~b! andg5@022# ~c!. Several dislocation pair images are labeled as A and B.

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When the Si layer was grown epitaxially thicker~ST2samples!, the dislocations grew with the layer without changing their line directions. Because the line directions of thtwo dislocations in a pair are either@211# and@211# or @211#and@211#, the pair orientation~i.e., the orientation of the twodislocation etch pits on the sample surface! becomes close to@011# and @011# when epitaxy becomes thicker, as observein ST2 samples. When the sample with an epitaxy is anealed for a second time~IM !, the dislocations should moveinto stable positions. Despite this, pair orientations in IM

FIG. 7. SEM images of dislocation pair in a IM sample after a modifieSecco etch. The pairs are labeled in Fig. 6 as A1, B1, and A2.


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samples were not found to be well defined. This is moprobably due to the broken dislocation lines.

The distance between the dislocation etch pits in a pvaries because of several reasons. The origin of dislocapairs, i.e., dislocation half-loops in the Si layer, have diffeent forms resulting in different distances between the discations. In the samples where the layer is grown thicker,distance becomes larger, because dislocation line directare not parallel. In the case of the broken dislocations,distance depends also on the depth at which the line is bken. This may be the dominating parameter in IM sampwhere the distance varied by the largest amount.

When the threading dislocation density becomes so hthat the images of closely situated dislocations can superp~as in the standard SIMOX samples!, then their analysis withx-ray topography may be difficult. This problem can bminimized by using different experimental conditions. In thpresent study, monochromatic beam x-ray topography wused and the working point on the rocking curve was varto obtain an improved image contrast compared to the p

TABLE I. Characteristics of the threading dislocations in the Si layer of tstudied SIMOX samples. In the case of a broken dislocation line, its dirtion concerns only the part closest to the surface. The magnitude ofBurgers vector is taken to be equal toa/&, wherea is the lattice parameterof Si.

Sample type ST1 ST2 IM

Si layer thickness 0.2mm 10.2mm 10.2mmDensity 1–53105 cm22 1–53105 cm22 1–43104 cm22

Pair orientation @010# and @001# @011# and @011# not well definedDistance on surface ,3 mm 10–13mm 10–30mmBroken lines 0 0 '90%Burgers vector 6a/23@011#;@011#Line direction @211# and @211#; @211# and @ 211#Character edge

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FIG. 8. SEM images of a dislocation pair in a IM sample after a modified Secco etch of 1 min~a!, 4 min ~b!, and 7 min~c!. ~d! shows the distance parallelto the @011# direction between the two dislocation etch pits in the pair as a function of the Si layer thickness.

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vious study10 that was made using a white x-ray beam.this white beam study a part~about 1/3! of the dislocationimages could not be completely characterized, becausecontrast did not change strongly in different reflectionBased on the present work, it can be concluded that thimages had their origin in two closely situated dislocatipairs with perpendicular Burgers vectors.

In this work the observed dislocation density, pair orietation, and separation distance in the standard SIMsamples indicate that the density and the character ofdislocations did not change during the epitaxial growth. Tonly changes in the dislocation configurations occurredthe improved process where the dislocations often chantheir line directions~broken dislocations!. This is most likelynot to be related to the epitaxial growth but to the secoheat treatment. Thus, the well-established method for anaing dislocations in bulk crystals, x-ray topography, can abe used for the analysis of the threading dislocationsSIMOX layers when the Si layer is grown epitaxially thickeTo achieve sufficient x-ray contrast of the threading dislotions, an adequate layer thickness is required. In the precase, more than about 3mm. Because there is no lower limifor the dislocation density when studied by x-ray topogphy, it can also be used to characterize the low implantatdose SIMOX samples18 where the threading dislocation density is of the order of 102 cm22.

V. SUMMARY

Threading dislocations in the Si layer in three differetypes of standard and improved SIMOX samples were invtigated by x-ray topography, SEM, and optical microscoafter the Secco etch. The threading dislocation density instandard SIMOX samples was found to be of the order of5

cm22 and in the improved SIMOX samples, an order of manitude lower. Additional 10-mm-thick epitaxy did not changethese densities. The dislocations appear in pairs and tintersections with the sample surface are separated by atance varying from 2 to 30mm depending on the Si layethickness and dislocation configuration. In the standSIMOX samples the dislocation Burgers vectors were demined to be parallel to the6@011# and6@011# directions andtheir line directions to be@211#, @211#, and @211#, @211#,


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respectively. Thus, the dislocations have an edge charactIn the improved SIMOX samples, the dislocation lines wereobserved to consist often of two parts. Their Burgers vectorwere determined to be the same as in the standard SIMOsamples. The comparison of chemical etching micrographand x-ray topographs shows that they give a one to one corespondence for the threading dislocation images.

ACKNOWLEDGMENTS

The authors are very grateful to the SOITEC compan~Grenoble, France! for providing the SIMOX wafers. Dr.Hubert Moriceau from LETI is acknowledged for fruitfuldiscussions.

1K. Izumi, in Proceedings of the Fourth International Symposium on SOITechnology and Devices, edited by D. N. Schmidt~Electrochemical Soci-ety, Pennington, NJ, 1990!, Vol. 90-6, p. 3.

2A. J. Auberton-Herve and B. Aspar, inProceedings of the SemiconductorEast Conference~SEMI, Tokyo, 1994!.

3G. W. Cullen, M. T. Duffy, and A. C. Ipri, inProceedings of the SixthInternational Symposium on SOI Technology and Devices, edited by S.Cristoloveanu~Electrochemical Society, Pennington, NJ, 1994!, Vol. 94-11, p. 5.

4C. Jaussaud, J. Margail, J. M. Lamure, and M. Bruel, Radiat. Eff. DefectSolids127, 319 ~1994!.

5J. Margail, Nucl. Instrum. Methods B74, 41 ~1993!.6B. Aspar, Electron. Lett.~to be published!.7D. Venables and K. S. Jones, Nucl. Instrum. Methods B74, 65 ~1993!.8J. Stoemenos, A. Garcia, B. Aspar, and J. Margail, J. Electrochem. So142, 1248~1995!.

9A. Garcia, Ph.D. thesis, l’Institute National Polytechnique de GrenobleGrenoble, France, 1995.

10E. Prieur, J. Ha¨rtwig, A. Garcia, M. Ohler, J. Baruchel, B. Aspar, and G.Rolland, J. Cryst. Growth~to be published!.

11F. Secco d’Aragona, J. Electrochem. Soc.119, 948 ~1972!.12Characterization of Crystal Growth Defects by X-Ray Methods, edited byB. K. Tanner and D. K. Bowen~Plenum, New York, 1980!.

13R. Barrett, J. Baruchel, J. Ha¨rtwig, and F. Zontone, J. Phys. D28, A250~1995!.

14A. Garcia, B. Aspar, J. Margail, and C. Pudda, inProceedings of theIEEESOI Conference~IEEE, Palm Springs, California, 1993!.

15M. Ohler, E. Prieur, and J. Ha¨rtwig, J. Appl. Crystallogr.~to be pub-lished!.

16F. Shimura,Semiconductor Silicon Crystal Technology~Academic, SanDiego, 1989!, pp. 69-71.

17W. T. Read, Jr.,Dislocations in Crystals~McGraw-Hill, New York,1953!, p. 128.

18B. Aspar, C. Guilhalmenc, C. Pudda, A. Garcia, A. M. Papon, A. J.Auberton-Herve, and J. M. Lamure, Microelectron. Eng.28, 411 ~1995!.

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Threading dislocations in silicon layer produced by separation by implanted oxygen process