Ga-implantation in Ge: Electrical activation and

JOURNAL OF APPLIED PHYSICS 106, 013518 共2009兲

Ga-implantation in Ge: Electrical activation and clustering G. Impellizzeri,1,a兲 S. Mirabella,1 A. Irrera,1 M. G. Grimaldi,1 and E. Napolitani2 1

MATIS CNR-INFM and Dipartimento di Fisica e Astronomia, Università di Catania, Via S. Sofia 64, 95123 Catania, Italy 2 MATIS CNR-INFM and Dipartimento di Fisica, Università di Padova, Via Marzolo 8, 35131 Padova, Italy

共Received 9 April 2009; accepted 31 May 2009; published online 9 July 2009兲 The electrical activation and clustering of Ga implanted in crystalline Ge was investigated in the 共0.3– 1.2兲 ⫻ 1021 Ga/ cm3 concentration range. To this aim, Ge samples implanted with 50 keV gallium, and annealed at several temperatures up to 650 ° C, have been subjected to a detailed structural and electrical characterization. The substrate was maintained at 77 K during implantation to avoid the formation of the honeycomb structure that occurs during implantation at room temperature of heavy ions at high fluence. Secondary ion mass spectrometry analyses indicated a negligible Ga diffusion and dopant loss during the thermal annealing. The carrier concentration in the recrystallized samples measured by Hall effect showed a maximum concentration of active Ga of ⬃6.6⫻ 1020 Ga/ cm3. A remarkable Ga deactivation occurred with increasing the annealing temperature from 450 to 650 ° C although the sheet resistance did not change considerably in this temperature range. It turned out that the carrier concentration reduction is balanced by the enhancement of the hole mobility that exhibits a steep variation with the concentration of the ionized scattering centers in this range. A simple model is proposed to explain the experimental results taking into account the thermally activated Ga clustering. These studies, besides clarifying the mechanism of Ga deactivation in Ge, can be helpful for the realization of future generation devices based on Ge. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3159031兴 I. INTRODUCTION

Despite silicon is still the prince semiconductor in microelectronics, today there is a growing scientific interest in other semiconducting materials. Germanium is one of these materials1 because of its higher carrier mobility2 together with its high compatibility with the already existing Si-based technology. Even if the first bipolar transistor, developed in 1947 by Bardeen, Brattain, and Shockley, was made of elemental Ge, it was totally replaced by Si in the 1960s and since then almost abandoned. Thus, the knowledge on this semiconductor has remained almost frozen for about 50 years and much basic information are lacking. Among the p-type dopants of Ge, boron has been widely studied in terms of diffusion3–5 and electrical activation.6–8 We have recently shown that a concentration of ⬃5.8 ⫻ 1020 B / cm3 of active B can be achieved.9,10 On the contrary, a detailed scientific knowledge about Ge material implanted with Ga is still lacking. It is known that heavy ion implantation in Ge at room temperature forms a unique damaged structure, which consists of voids separated by interconnected amorphous Ge walls, usually called honeycomb structure.11–17 This damage alters drastically and permanently the near-surface morphology since it is stable against thermal treatments,11,12,14,15 and compromises the Ge-based microelectronic device performances. The honeycomb structure observed in selfimplanted Ge wafers11,12,14,17 is very likely to occur even during Ga-implantation 共having a similar atomic weight to Ge兲 although it has not been observed.15 a兲

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Gallium is a slow diffuser in Ge3,18–20 共the equilibrium diffusion coefficient is about 0.007 nm2 / s at 650 ° C18兲 and its maximum solid solubility is 4.9⫻ 1020 cm−3 at T = 670 ° C.21 These two latter observations suggest Ga as a good choice of p-type dopant in Ge 共see also Ref. 22兲. However, the electrical activation of Ga in Ge has not been investigated in detail and the only information available, by Satta et al., reported a sheet resistance 共RS兲 of 85 ⍀ / sq in Ge implanted with 3 ⫻ 1015 Ga/ cm2 at 40 keV and annealed at 600 ° C for 60 s.23 The aim of this work is to study in detail the electrical activation and stability of Ga implanted at high concentration in Ge. With this purpose, we performed an extensive structural and electrical characterizations of Ge samples implanted with Ga ions in a large range of concentrations and thermal budgets. We evidenced the formation of the honeycomb structure during implantation at room temperature. We obtained a very-high concentration of active Ga. However, we revealed a strong deactivation of Ga as the annealing temperatures increases above 550 ° C. We developed a simple model to explain the experimental results in terms of Ga clustering. II. EXPERIMENTAL

Ge Czochralski wafers, 共100兲-oriented, n-type 共Sb doped, with a resistivity higher than 40 ⍀ cm兲 were implanted with 50 keV Ga+ at different fluences: 1, 2, or 5 ⫻ 1015 Ga/ cm2. The used energy led to a Ga projected range 共R P兲 of ⬃25 nm.24 The average current density was ⬃0.1 ␮A / cm2 and the substrates were held either at room temperature 共RT兲 or at ⬃77 K 共LN2T兲 by means of a liquid-

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nitrogen-cooled sample holder. The ion beam was misoriented by 7° to the normal to the wafer surface to avoid channeling effects. After implantation some samples were annealed, using a conventional furnace, at 450, 550, 600, and 650 ° C for 1 h, in N2 atmosphere, with a proximity cap of Si in physical contact with them to avoid any degradation of the Ge surface. In detail, the thermal treatments were done sequentially, i.e., annealing at 650 ° C for 1 h were done on samples previously annealed at 450 ° C for 1 h plus 550 ° C for 1 h plus 600 ° C for 1 h. Scanning electron microscopy 共SEM兲 was managed by a field emission scanning electron microscope Zeiss Supra 25, using an acceleration voltage of 6 kV, so to investigate the morphology of the implanted samples. Rutherford backscattering spectrometry 共RBS兲 analyses were carried out with a 3.5 MeV HVEE Singletron accelerator, using a 2 MeV He+ beam in channeling configuration, along the 具100典 axis, in order to investigate the lattice damage of the as-implanted and annealed samples. The scattering angle was 110° to improve the depth resolution. Secondary ion mass spectrometry 共SIMS兲 was used to obtain the chemical concentration depth profiles of Ga. The SIMS analyses were performed with a CAMECA IMS-4f instrument, using a 3 keV O2+ analyzing beam. The electrical characterization was carried out by means of four-point probe and Hall measurements, using a BioRad HL5560 equipment. The applied magnetic field was 0.322 T. The samples, patterned according to Van der Pauw geometry, were square-shaped 共1 ⫻ 1 cm2兲. The sheet resistance and the Hall carrier fluence are affected by a maximum error of about 5%. III. RESULTS AND DISCUSSION

In this section we will show our experimental results, first presenting the structural characterization of the asimplanted and annealed samples 共Sec. III A兲 followed by the investigation of the electrical properties of the annealed samples 共Sec. III B兲. In Sec. IV we will discuss the results and we will propose a simple model to describe the Ga deactivation at high temperatures. A. Structural characterization

Despite the growing scientific interest in Ge, the knowledge of the damage induced by the ion implantation process in Ge is lacking compared to Si. One main difference is the modification of the Ge morphology during implantation with heavy ions. Figure 1共a兲 shows a plan-view SEM analysis of the Ge sample implanted at RT with 50 keV Ga at a fluence of 5 ⫻ 1015 Ga/ cm2. It is evident the formation of the honeycomb structure. The Ge walls formed a network that surrounds the voids, with a size in the 10–30 nm range. This porous layer was stable against annealing and no change of the surface morphology was detected by SEM analyses up to 650 ° C, in good agreement with previous results.11,12,14,15 The formation of the honeycomb structure caused by Ga implants in Ge has never been reported and it was not observed in Ge implanted at RT with 30 keV Ga at a fluence of 3 ⫻ 1015 Ga/ cm2.15 The reason for this discrepancy could be the different ion energy 共since in shallow implants the sur-

J. Appl. Phys. 106, 013518 共2009兲

FIG. 1. Plan-view SEM images of Ge samples implanted with 5 ⫻ 1015 Ga/ cm2 at 50 keV at 共a兲 RT or 共b兲 LN2T.

face could play a key role in the point-defect recombination兲 or a different current density 共since a high beam current could increase the target temperature and enhance the dynamic annealing of the defects兲. In fact, although the exact mechanism responsible for this unique damaged structure is still uncertain, it is very likely related to the supersaturation of the vacancies produced by the ion irradiation.11,12,14 A detailed study developed by Strizker et al.14 showed three distinct temperature regimes: low 共⫺180 to −80 ° C兲, intermediate 共⫺80 to 200 ° C兲 and high 共⬎200 ° C兲. Only in the intermediate temperature regime the honeycomb structure was observed. At low temperature the vacancies are frozen and no clustering occurs. At intermediate temperature vacancies are relatively mobile; this leads to clustering of the vacancies and the growth of voids. At high temperature both vacancies and interstitials are highly mobile and there is a high probability of point defect recombination.14 In order to avoid the formation of the honeycomb structure, we lowered the substrate temperature to 77 K during implantation. The SEM image of the surface of a sample implanted at LN2T with 50 keV Ga at a fluence of 5 ⫻ 1015 Ga/ cm2 is shown in Fig. 1共b兲. The surface is featureless with a roughness comparable to that of the unimplanted Ge, confirming the suppression of the honeycomb by low temperature implantation. In the following we will always refer to the samples implanted at LN2T. Figure 2 shows the Ga concentration profiles, measured by SIMS, in samples implanted with different fluences. The Ga maximum concentration spans from 0.3 to 1.2 ⫻ 1021 Ga/ cm3. We investigated, by RBS-channeling analyses, the effect of the Ga implants on the Ge crystalline order. The analyses


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FIG. 2. SIMS profiles of Ga implanted at 50 keV, LN2T, with three different fluences: 1 ⫻ 1015 共dotted line兲, 2 ⫻ 1015 共dashed line兲, or 5 ⫻ 1015 Ga/ cm2 共continuous line兲.

FIG. 4. SIMS profiles of 5 ⫻ 1015 Ga/ cm2 共50 keV, LN2T兲 as-implanted 共continuous line兲, and after thermal annealing at 450 ° C 共dashed-dotted line兲 and at 650 ° C 共open squares兲.

were performed in channeling configuration, along the 具100典 axis. In Fig. 3 the channeling spectra of the samples implanted with 1 ⫻ 1015 Ga/ cm2 共dotted line兲 and 5 ⫻ 1015 Ga/ cm2 共closed circles兲 are reported. In the surface region the yield reaches the random level, indicating the formation of a continuous amorphous layer. The thickness of the surface damaged region increases with the ion fluence, and the amorphous layer extends from the surface to ⬃70 nm for 1 ⫻ 1015 Ga/ cm2, and to ⬃80 nm for 5 ⫻ 1015 Ga/ cm2. Annealing at 450 ° C for 1 h induced a complete recrystallization of the amorphous layers by the solid phase epitaxy process. The RBS-channeling spectrum 共open circles in Fig. 3兲 is typical of a free-of-defect crystal not distinguishable from that of an unimplanted Ge. RBSchanneling spectra of the samples annealed at higher temperatures up to 650 ° C showed no further changes with respect to 450 ° C annealed sample. Figure 4 shows the concentration profiles of Ga in the sample implanted with 5 ⫻ 1015 Ga/ cm2 共continuous line兲 and after annealing up to 650 ° C. The Ga concentration pro-

file of the 450 ° C annealed sample coincides with that of the as-implanted one. After annealing up to 650 ° C there is an ⬃10% reduction of the Ga fluence, probably due to outdiffusion. SIMS analyses allow to exclude any remarkable dopant loss25 and concentration profile broadening during thermal annealing. These latter results are in good agreement with the reported diffusion studies of Ga in Ge.3,18–20

FIG. 3. Channeling RBS spectra for Ge samples implanted at 50 keV, LN2T, with 1 ⫻ 1015 共dotted line兲 or 5 ⫻ 1015 Ga/ cm2 共closed circles兲. The spectrum of the 5 ⫻ 1015 Ga/ cm2 sample after thermal annealing at 450 ° C for 1 h is also reported 共open circles兲.

B. Electrical characterization

We performed electrical investigation with the aim of studying the electrical activation of Ga in Ge. In Figs. 5共a兲–5共c兲 we reported the RS, the Hall carrier fluence and the Hall mobility, respectively, as a function of the Ga fluence for samples annealed at different temperatures. The samples were annealed at 450 ° C for 1 h in order to regrow the amorphous layers realized by the ion implantation at low temperature 共see Fig. 3兲; annealing at higher temperatures 共up to 650 ° C兲 were done to test their thermal stability. Figure 5共a兲 shows that the RS decreases with increasing the Ga fluence, and at a given fluence it varies very slightly with annealing temperature, unlike the Hall carrier fluence and mobility. In fact, the Hall carrier fluence 关Fig. 5共b兲兴 decreases by about one order of magnitude with increasing the annealing temperature. This is a clear indication of a remarkable Ga deactivation. The Hall mobility 关Fig. 5共c兲兴 increases by about one order of magnitude with the annealing temperature. It turns out that the reduction of the carrier concentration is balanced by the increase of the carrier mobility and the sheet resistance remains almost constant. It is well known that the recrystallization by SPE leaves structural defects beyond the original amorphous-crystalline interface, which are commonly referred to as the end-ofrange 共EOR兲 defects. The EOR defects represent concerns for junction leakage. In our samples the EOR stays beyond ⬃70 nm. Since the reliability of the electrical measurements depends on the good insulation between the measured layer 共p+-type兲 and the substrate 共n-type兲, a leakage, if any, could invalidate our electrical data. Considering that the diode leakage decreases with the EOR depth26 we repeated this experiment using preamorphized Ge samples. The preamor-


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FIG. 6. Maximum active Ga concentration vs annealing temperature for the three different fluences: 1 ⫻ 1015 共open squares兲, 2 ⫻ 1015 共closed circles兲, and 5 ⫻ 1015 Ga/ cm2 共open triangles兲. The dashed line is the value of the maximum solid solubility calculated by Fistul.21

FIG. 5. 共a兲 Sheet resistance 共RS兲, 共b兲 Hall carrier fluence, and 共c兲 Hall mobility versus Ga fluence for Ga implanted at 50 keV, LN2T, and annealed in the 450– 650 ° C temperature range.

phization implant was made at RT using a 300 keV, 3 ⫻ 1015 Ge/ cm2 ion beam, in such a way to place the EOR defects deeper than 280 nm. In this case the 50 keV implanted Ga is entirely contained inside the amorphous region, with the EOR defects well apart from the Ga dopant. The four-point probe and Hall measurements showed electrical properties 共not shown兲 almost equal to the ones of the crystalline Ge samples directly implanted with Ga. This allowed to exclude any error on the electrical data arising from the junction leakage. In Fig. 6 we plotted the concentration of active Gamax versus the annealing temperatures for the different implanted fluences: 1 共open squares兲, 2 共closed circles兲, or 5 ⫻ 1015 共open triangles兲 Ga/ cm2. The maximum active Ga concentrations were determined using the following method. Starting from the SIMS profile and the measured active fluence 共assuming the Hall scattering factor equal to 1兲 we simulated

the active Ga fluence with different Gamax values, considering all the dopant distribution below Gamax as fully ionized. The dashed line in Fig. 6 is the value of the maximum solid solubility calculated by Fistul21 共i.e., 4.9⫻ 1020 cm−3 at T = 670 ° C兲. The active Gamax concentration is almost constant from 450 to 550 ° C and at higher temperatures a noticeable deactivation occurs. For temperatures below 650 ° C a metastable regime allows a remarkable Ga activation, while the equilibrium regime seems to be reached at 650 ° C. In detail, the sample implanted with 5 ⫻ 1015 Ga/ cm2 and recrystallized at 450 ° C showed an active Gamax concentration of ⬃6.6⫻ 1020 Ga/ cm3, that is higher than the one obtained with B 关⬃5.8⫻ 1020 B / cm3 共Refs. 9 and 10兲兴. In Fig. 7 the Hall mobility is plotted as a function of the active dopant concentration for the different fluences 共1, 2, or 5 ⫻ 1015 Ga/ cm2兲 and annealing temperatures 共closed squares 450 ° C, open circles 550 ° C, closed triangles 600 ° C, open diamonds 650 ° C兲. The dotted line is only a guide for eyes. It is worth to underline that the hole mobility in Si is almost constant 共within the 30– 60 cm2 / V s range27兲

FIG. 7. Hall mobility vs active dopant concentration for Ga-doped Ge after thermal annealings up to 650 ° C 共symbols兲. Dotted line is only a guide for eyes.


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in this concentration regime unlike Ge in which, besides being higher than in Si, it varies of about one order of magnitude 共from ⬃40 to ⬃700 cm2 / V s, see Fig. 7兲. This remarkable variation of carrier mobility with the concentration of the ionized impurities is in good agreement with the literature.9,28 IV. DISCUSSION

After the structural and electrical characterization presented in Secs. III A and III B, we develop here a simple model to explain the deactivation of the Ga dopant on the basis of our experimental results. The Hall measurements showed that the Hall carrier fluence decreases with the annealing temperature of about one order of magnitude from 450 to 650 ° C 关Fig. 5共b兲兴 indicating a remarkable Ga deactivation, but the SIMS analyses were not able to evidence any Ga clustering because the Ga diffusion length is below the SIMS sensitivity. To test and validate the hypothesis of thermally activated clustering we compared the dopant diffusion length 共for the different thermal treatments兲 to the average distance between two Ga atoms 共that depends on the Ga concentration兲. We can suppose that the Ga atoms can meet each other and eventually form clusters if the diffusion length 共␭兲 is greater than the Ga–Ga distance 共dGa–Ga兲. The probability for such event is expected to increase with increasing the diffusion length. The diffusion length was calculated by the formula: ␭ = 冑2Dt, with D diffusion coefficient18 and t annealing duration. Even if ␭ at stake is very small 共from 0 to 7.2 nm兲, at temperatures higher than 450 ° C it becomes greater than dGa–Ga 共from 0.9 to 1.5 nm兲 and the probability of clustering is not null, in good agreement with the experimental results. It is reasonable to write the fraction of clusterized Ga 共f兲 as the fraction of mobile Ga 共1 − f兲 times the probability 共p兲 of clustering: df = 共1 − f兲dp.

共1兲

If the clustering probability is diffusion dependent, as above proposed, we can write dp =

a dGa–Ga

d␭,

共2兲

where a is an adimensionate coefficient of proportionality and dGa–Ga is dependent only on the Ga concentration. Inserting relation 共2兲 in Eq. 共1兲 and integrating both members, we obtained the following relation that correlates the fraction of clusterized Ga 共f兲 to the diffusion length 共␭兲: f = 1 − e−a␭/dGa–Ga .

共3兲

The fraction of clusterized Ga 共f兲 can be easily calculated by the following relation: f = 1 − 共active Ga fluence兲/共Ga fluence兲, being the active Ga fluence determined by the Hall carrier measurements 关see Fig. 5共b兲兴, assuming an Hall scattering factor equal to 1. We reported in Fig. 8 the fraction of Ga clusterized 共f兲 versus the ratio ␭ / dGa–Ga for the Ga-doped samples 共we excluded the 450 ° C data because the diffusion length is negligible18兲. The data revealed that f exponentially grows with ␭ / dGa–Ga up to 1 共i.e., all Ga atoms clusterized兲 strongly supporting the hypothesis that the clustering is dif-

FIG. 8. Fraction of clusterized Ga 共f兲 vs the ratio Ga diffusion length to Ga–Ga distance 共␭ / dGa–Ga兲 for the different implanted and annealed samples 共open circles兲. With continuous line is reported the fit of the data with the relation: f = 1 − e−a ␭/dGa–Ga.

fusion limited. The continuous line in Fig. 8 is the fit of the experimental data using relation 共3兲 with a = 0.7⫾ 0.1. This simple model can be profitably employed in simulation of electrical behavior of highly Ga-doped germanium, so to evaluate and minimize the inactive Ga fraction.

V. CONCLUSIONS

In conclusion, we presented a detailed structural and electrical investigations of crystalline Ge samples implanted with gallium in the high concentration regime 共from 0.3 to 1.2⫻ 1021 Ga/ cm3兲. We evidenced the formation of the honeycomb structure during implantation at room temperature, which can compromise the Ge-based device performances. We suppressed the honeycomb structure formation by lowering the substrate temperature during implantation. We excluded a remarkable dopant loss during thermal annealing by SIMS analyses. No diffusion of Ga was observed up to 650 ° C. Electrical characterization by Hall technique showed a maximum concentration of active Ga of ⬃6.6 ⫻ 1020 Ga/ cm3. The Hall carrier fluence decreases with the annealing temperature of about one order of magnitude indicating a considerable Ga deactivation. We developed a simple model to explain the experimental results taking into account the thermally activated Ga clustering. An empirical law, able to correlate the fraction of Ga clusterized with the ratio of dopant diffusion length to the Ga–Ga distance, was found, indicating that Ga clustering is limited by the dopant diffusion itself. These experimental data and modeling improve the current understanding of the Ga performance as dopant for Ge, allowing also the optimization of the Ga use for the realization of advanced microelectronic devices. Indeed, from these results we can deduce that Ga, as dopant for Ge, is an optimum choice, since its low diffusivity and its high value of active concentration.


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ACKNOWLEDGMENTS

The authors wish to thank C. Percolla 共MATIS CNRINFM兲, S. Tatì 共MATIS CNR-INFM兲, and R. Storti 共University of Padova兲 for their expert technical assistance. 1

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