Platelet defects in hydrogen implanted silicon

Nuclear Instruments North-Holland

PLATELET

and Methods

DEFECTS

in Physics

Research

IN HYDROGEN

B44 (1990) 313-317

IMPLANTED

313

SILICON

S. ROMAN1 ‘)* and J.H. EVANS 2, I) Ion Implantation Section, AEA Technology, HanveIl Laboratory, Oxfordshire, OXI I ORA, UK ‘) Materials Development Division, AEA Technology, Hawell Laboratory Oxfordshire, OX11 ORA, UK Received

27 July 1989

TEM and XTEM analysis of silicon implanted with 40 keV H+ near room temperature, to doses of 2.5 and 3 x 10” Hcmm2, reveal platelet structures. These platelets, lying in (001) and (111) habit planes, exhibit relatively low strain contrast. Conditions required for formation and observation are discussed along with results from annealing experiments. Comparison with platelet observations in the literature indicates that the structure in hydrogen-implanted silicon may differ from that suggested for other gas/solid combinations,

1. Introduction

lack of strong strain fields around platelets in silicon suggests a different structure or mode of formation.

The behaviour of hydrogen in solid materials continues to be of interest in several fields of study. Recently, particular attention has been drawn to dopant passivation in silicon after H/D plasma exposure [l-4] or low energy, low dose, hydrogen ion implantation [5-lo]. The interest in the present work is in the microstructural artefacts produced by hydrogen-implantation. To this end the effects of implanting hydrogen or helium in the dose range 1 X lOI to 2 X lOI cme2 have been studied. This communication reports TEM and XTEM observations of platelets formed by 40 keV H+ implantation into silicon to doses of 2.5 and 3 X 1016 Hem-2. Several observations of platelet-like precipitation of hydrogen in silicon have previously been reported [ll-171. Recent work suggested that impurities may have been involved, e.g. carbon [16,17], but our results with cross-sectional TEM confirm that both sets of platelet images (with (001) and { 111) habit planes) are associated directly with the implanted hydrogen profile and are unlikely to involve interaction with other impurities. Recent studies of damage release in hydrogen-implanted silicon [18] describe platelets formed under particular conditions. The parameters mentioned, 2x lOI6 Hcme2 at about 15 keV, are in accordance with the requirements found in the present work, as discussed shortly. In the discussion we include the Si-H results in a brief survey of other platelet observations in the literature following inert gas or hydrogen implantation. The

* On attachment from the School of Electrical, Electronic and Systems Engineering, University of Wales College of Cardiff, Cardiff, CFl 3YH, Wales, UK. 0168-583X/90/$03.50 (North-Holland)

0 Elsevier Science Publishers B.V.

2. Experimental In this study, samples were Wacker, (OOl), n-type CZ silicon (5-10 Dcm P diffusion doped) cut from 2 in. wafers and cleaned using a standard RCA process. Hydrogen and helium implantations, at 40 and 45 keV respectively, were carried out in a small diffusionpumped target enclosure on the Harwell MkIV ion implanter. The dose was monitored via a charge integrator connected to the target plate. A Faraday cup assembly, with an external magnet, was employed to prevent erroneous current readings due to electron emission. The ion current was maintained at about 1.3 pAcmm2, resulting in maximum temperatures of below 100°C, measured by a Chromal thermocouple mounted on the target plate. The pressure, measured close to the target enclosure, was generally about 2 X 10m5 mbar. For plan-view TEM, foils were prepared by dimpling of 3 mm discs drilled ultrasonically from each sample. Final thinning was achieved using a Gatan Dual Ion Beam Mill operated at 6 kV with a 30 o incident Ar ion beam. A sputter cleaning step (3 kV at 15 ” ) was found to be necessary to remove defects occasionally introduced by the argon ion beam (e.g. small bubbles). The same sputter cleaning process was used to controllably remove silicon from the implanted surface in order to allow imaging of regions deeper than the top 100 nm. The cross-sectional specimen preparation involved glueing two small pieces from a single sample face-to-face and polishing to around 100 pm. The sandwich was then mounted between 3 mm copper microscope grids before ion beam thinning from both sides. TEM analy-

S. Romam, J. H. Evans / Platelet defects in hydrogen implanted sificon

314 sis was carried

out on a Philips EM400 operated at 100 or 120 kV. A Gatan video enhancement system was also used.

3. Results Platelets were observed in samples implanted with doses of 2.5 and 3 x 1016 HcmW2. At higher doses, equal to or above 5 x lOI Hem-‘, bubbles are observed to form in preference to platelets, although infrequent platelet observations have still been made away from the damage peak. At lower doses, e.g. less than about 1 X 1016 Hcme2, no clear platelet or bubble images were located. Examples of platelet images observed after hydrogen-implantation at an intermediate dose level are given in fig. 1. This shows a defocus-contrast pair exhibiting the expected light and dark fringe in under and over-focus respectively. Tilting the foil by the order of 10” or more from the edge-on condition caused the 1 nm-wide images to broaden and disappear - confirming their planar nature. Although fig. I shows only an (001) platelet orientation, all samples examined also contained {lllf platelets in similar concentrations. Two sets of edge-on (111) plateiets and one set of {OOl} platelets can be seen in the plan-view micrograph of fig. 2, in this case imaged along a (011) beam direction. Taking into account the non edge-on habit planes, the platelet concentrations were estimated to be 3 + 1 x lOI6 cmW3. Platelet diameters ranged between about 10 and 25 nm with an RMS average of 20 nm. Fig. 3 shows the damage band produced beneath the surface of a hydrogen-implanted sample, viewed in cross-section. The damage and implant peaks, situated between about 420 to 540 nm, are near’ly coincident, but appear to be about 20% deeper than predicted by Monte Carlo simulations using the TRIM code 119,201. This

Fig.

1. TEM hydrogen platelet images seen in under-focus (left-hand figure) and over-focus (right-hand figure).

Fig.

2. Three

different orientations of platelets plan-view TEM specimen.

seen

in a

discrepancy can be attributed to channeling, since the samples were implanted within a few degrees to the normal. For an implant dose of 3 X lOI6 Hcmm2, TRIM gives an average hydrogen level across the implant zone of about 0.05 II per silicon. The same simulation suggests an induced damage level of about 1 displacement per silicon atom. Details of platelet structures within the damage band of fig. 3 are shown in fig. 4. The association of platelets with the damage/implant profile clearly indicates that platelet formation must be a direct consequence of the hydrogen implantation.

Fig. 3. TEM cross-section of the damage band hydrogen implantation at 40 keV.

produced

by

S. Romanr, J. H. Evans / Platelet defects in hydrogen implanted silicon

2Snm

315

this limit suggested some platelet break up. Since hydrogen is expected to out-diffuse quickly at temperatures in excess of 500 OC (this is particularly clear from desorption studies [28,29]), our results are an indication of the higher stability of hydrogen bound in platelet structures. The removal of most of the hydrogen below 500 o C would imply that only a relatively small fraction of the hydrogen is trapped within platelets. A single experiment using hydrogen implantation into p-type silicon showed identical platelet morphologies and similar concentrations. This result is in contrast to the observations of Johnson et al who reported platelet concentrations lower by a factor of 100 in p-type material [13]. In the present work considerable care was needed to clearly image the platelets. Unless foils were adequately thin and clean, the low platelet contrast made such artefacts difficult to observe.

4. Discussion

Fig. 4. Micrographs illustrating the confinement of platelets within the damage/implant band, imaged along [OIO] (lop) and [Oil] (bottom).

Although both {OOl} and {ill} platelet families are observed to form within the damage band, there was some evidence that formation is slightly more favourable on the (001) planes parallel to the surface. This could be explained by lower formation energy requirements in surface-oriented planes, where strain relief can be more easily accommodated. Similar implantations using helium instead of hydrogen have not exhibited any platelet structures. Instead, helium always appears to precipitate as bubbles, as seen in previous work [21]. Cross-sectional TEM confirms that these bubbles are confined to the depth band defining the helium range. With regard to the hydrogen bubble observations, we reiterate that at doses above 5 X lOI Hcmd2, bubbles form in preference to platelets. This probably suggests an interaction of the higher peak concentration with more extensive damage. A subsequent increase in dose, results in bubble growth and above 5 X lOI Hem-2, eventually, at doses in excess of 10” Hem-*, blister and crater formation occur [22-261, similar to the inert gas induced blistering of metals [27]. The behaviour of platelets on annealing is of some interest. Jeng et al. [15] have indicated that {ill} defects in silicon hydrogenated during reactive ion etching (RIE) are removed between 400 and 800” C. In the present work, anneals using a TEM hot stage gave results consistent with the upper end of this range. Most of the platelets were stable up to 700” C, although occasional observations of small bubble clusters below

From cross-sectional analysis it is clear that platelets only form close to the depth of maximum hydrogen concentration. For the implantation energies used here, the implanted atom and induced damage profiles both peak at similar depths. Although for helium the peaks are less coincident, it is important to note that similar implantations failed to provide any evidence of platelet formation. In fact, under all conditions studied, only bubbles were produced. Clearly the hydrogen is an essential ingredient in the observed platelets. It is interesting to note that over a decade ago, Chu et al. [22,23] carried out implantations under similar conditions (50 keV, 4 x 1016 H cm-*) to those reported here. His reference to “microblisters” beneath the surface is believed to be one of the earliest platelet observations in hydrogen-implanted silicon. It seems useful to compare the Si-H results with other observations of platelet-like structures in the literature - restricting ourselves to those reported after inert gas or hydrogen exposure. These are given in table 1. Surprisingly, no particular pattern emerges with respect to implantation parameters. The amount of displacement damage is variable (and even zero in the MO [30] and Ni [31] cases). Also irradiations near to room temperature are not always sufficient; in the GaAs case, for example, annealing was required for the platelets to form [32-341. However, a significant feature emerges from an examination of the micrographs from the quoted papers; in all cases except for silicon, the platelet strain fields appear very large. These more conventional strain fields arise from high gas pressures within disc-shaped cavities or vacancy loops. The weak strain fields in the Si-H case referred to in the experimental results must reflect differences in formation and/or structure. Certainly it seems impossible that all of the hydrogen could

S. Romani, J. H. Evans / Platelet defects in hydrogen implanted silicon

316

Table 1 ExampIes of reported inert gas and hydrogen platelets Material Gas BC BC MO Ni Ni &As Si Si

Si Si Si MO/W KBr

Conditions

Homogeneous cyclotron implantation Neutron I~a~ati~n, 500-800 *C 100-150 eV He impl~ta~on He 50 eV He implantation He 3 keV He implantation He 300 keV H implantation + anneal H H/D plasma exposure H H/D plasma exposure H CHF, reactive ion etching H CF, reactive ion etching H CF,/H, RIE and N z plasma H He/H 5 keV He/H implantation 75 keV electron irradiation Br He He

Ref. 1351 I361 I301 I31] [371 [32-341 (141 [12,131

I16J71 P] [I51 I38f I391

in platelets. if it were? a combination of hydrogen concentration levels and platelet dimensions would give an average value of 8 x lo4 hydrogens per platelet, This would lead to a packing density of more than 2 X 10z3 Hcmm3, several times that of solid hydrogen. The implied pressure would be massive and therefore incompatible with observations of small strain fields. Although no detailed model of hydrogen platelet structure or formation in s&on has been completely accepted, the lack of a central vacancy loop structure, known to exist in the other examples (see table 1) is generally accepted in the silicon case [12-171. The model favoured by the authors initially fo~Iows that of Ponce et al. fl4& tt is suggested that platelets arise from hydrogen passivation of Si-Si bonds broken during implantation damage processes. Calculations have previously established that the bond-centred site is the most stable configuration for atomic hydrogen in a retaxed silicon lattice [40,41]. Since, however, one broken Si-Si bond requires two hydrogen atoms to passivate it, we can expect a pairing effect, perhaps with the two interstitial hydrogen atoms displaced to opposite sides of the original Si-Si bond. The resulting structure of Si-H bonds, could produce “planes” of hydrogen atoms. These could cause a relatively small amount of bowing in the neighbouring matrix planes, resulting in the Iow strain contrast observed. Any de&&d model needs to account for the roughly equal probability of platelet formation on both (001) and {Ill) orientations. It is worth noting that, for the (111) platelets, the simple model would allow about lo4 hydrogen atoms to be accommodated in our average sized platelet (20 nm diameter). When combined with our eariier caicuiations this implies that only about one-eighth of the implanted hydrogen would be trapped in platelets. Clearly many other trapping sites must be av~lable within the lattice be trapped

for the remaining hydrogen. These traps presumably must include displacement damage defects and small vacancy-hydrogeu clusters, precursors to the bubbles seen at higher doses. Such structures could involve hydrogen-p~sivated mono, di- and t&vacancy centres (see for example Corbett [42]). Raman spectroscopy of a sample area known to contain plateiets certainly did not rule out the existence of multiply bound hydrogen modes, Unhke Johnson et al.% observation of a single, narrow Raman peak in plasma exposed material [13], we located a broad peak between about 1700 and 2100 cm-‘. The expected positions of the silicon mono- and tetra-hydride bond resonances are 1305 and 2100 cm-’ respectively, Without knowing the actual bonding structures and accurate values far scattering cross-sections, direct inference from Raman spectra is difficult. We certainly cannot equate any single mode to hydrogen bound in platelets. All that can sensibly be said is that the assortment of actual bonding modes may involve structures intermediate between single substitutional hydrogen atoms and small clusters containing 2 or 3 hydrogen atoms per silicon. A mixture of the singly, doubly and triply-passivated vacancy-type centres could still apply, perhaps in addition to the simple passivated broken bond structure mentioned above. It is worth noting that infrared studies have suggested several possible Si-H modes 143-471. Some workers have even suggested that small vacancy clusters may be involved f&47], in agreement with the above model. The fact that bubbles instead of platelets have been observed after he~um-~pl~ta~on could suggest that the platelet formation mechanism requires the interaction of implanted hydrogen atoms with non-excessive implant damage. The diffusivity of the implanted species is also expected to be important, since hydrogen is known to diffuse quickly in its atomic form, particularly in damaged material [48-501.

5. Summary The results in this paper show that within a certain dose range, hydrogen impl~ted into silicon at temperatures below 100 o C can precipitate in platelets on two sets of habit planes, namely {OfB) and (111). At higher doses bubbles generally form instead of platdets. The lack of strain field around these platelets suggests that they may arise from the passivation of broken Si-Si bonds and essentially contain little or no gas pressure component, in contrast to other cases of platelets in materials. A comparison of the calculated hydrogen content within typical platelets indicates that only a relatively smaii amount of the implanted hydrogen remains trapped within them. The stability of platelets up to about 700 * C suggests binding energies si~ficantly

S. Romani, J. H. Evans / Platelet defects in hydrogen implanted silicon

above that of hydrogen structures.

bound

in silicon in other defect

The help of Dr. U. Bussmann (University of Surrey) with the initial glueing and grinding stages of XTEM specimen preparation is much appreciated. The assistance of Dr. P. Graves (Harwell), both in obtaining Raman spectra and for related discussions, is gladly acknowledged. Thanks are also due to Dr. G. Dearnaley and members of the Ion Implantation Section at Harwell for continued technical assistance and encouragement. This work was supported by the UKAEA Underlying Research Programme.

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