Characterization of Si1-x-yGexCy films grown by C+ implantation and subsequent pulsed laser annealing

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Characterization of Si

1ÿxÿy

Ge

x

C

y

®lms grown by C

implantation

and subsequent pulsed laser annealing

Jian-Shing Luo

a

_{, Wen-Tai Lin}

a,*

_{, C.Y. Chang}

b

_{, P.S. Shih}

b

_{, F.M. Pan}

c

_{, T.C. Chang}

c a_{Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan}

b_{Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan} c_{National Nano Device Laboratory, Hsinchu, Taiwan}

Received 15 October 1998; received in revised form 23 February 1999; accepted 11 March 1999

Abstract

Epitaxial Si1ÿxÿyGexCy®lms have been grown by Cimplantation into Si0.76Ge0.24®lms with a dose of 1.0 1016/cm2and subsequent

pulsed KrF laser annealing at an energy density of 0.3±1.6 J/cm2_{. Upon laser annealing Ge segregation to the ®lm surface and diffusion to}

the underlying Si appeared at energy densities above 0.8 J/cm2_{and 1.4 J/cm}2_{, respectively, while the depth pro®les of C remained nearly}

unchanged as in the as-implanted Si1ÿxÿyGexCy®lm. Concurrently, no SiC and twin were observed. The amount of C incorporated into

substitutional sites initially increased with the energy density in the range of 0.3±1.0 J/cm2_{, and then saturated at an energy density of 1.0±}

1.6 J/cm2_{. For the Si}

1ÿxÿyGexCy®lms grown at 1.0 J/cm2for 5 and 20 pulses SiC was formed with its amount increasing with the pulse

number because of C segregation to the ®lm surface and the original amorphous/crystal interface where the EOR defects were present. For the Si1ÿxÿyGexCy ®lms grown at energy densities below 1.0 J/cm2 the reduction of tensile stress mainly resulted from the effect of

Keywords: Si1ÿxÿyGexCy®lms; Cimplantation; Pulsed laser annealing

1. Introduction

Si1ÿxGex ®lms grown on Si have great potential for

fabricating high-speed electronic and optoelectronic devices [1,2]. The band gap of Si1ÿxGex®lms decreases

monotoni-cally with the Ge concentration. Carbon substitutionally introduced into Si1ÿxGex ®lms may change the band gap

[3±5], providing an additional design parameter in band structure engineering on Si. In addition, the addition of C can also reduce the lattice mismatch between Si1 ÿ xGexand

Si, opening up the opportunities for fabricating thicker pseudomorphic Si1ÿxGex ®lms with a high Ge content.

Recently, pseudomorphic Si1ÿxÿyGexCy ®lms with carbon

concentrations in the range of 1±2 at.% have been grown by many methods such as molecular beam epitaxy [6,7], che-mical vapor deposition [8,9], and solid phase epitaxy [10± 12]. Since the maximum solubility of substitutional C in Si is much lower (10ÿ5_{) than that required for strain}

compensa-tion, SiC phase may form in the Si1ÿxÿyGexCy®lms during

growth, especially at temperatures above 6008C [13]. Pulsed laser annealing is a promising technique for growing

epi-taxial thin ®lms under nonthermal equilibrium conditions. By this method, above 1.0 at.% of substitutional C could be introduced into Si due to the fast melting and resolidi®cation process [14±16]. Ion implantation is a technique highly compatible with the standard silicon process. As we know, few papers concerning the Si1ÿxÿyGexCy®lms grown by ion

implantation with subsequent pulsed laser annealing have been reported [16±19]. In the present work, we explore the effects of energy density and pulse number on the char-acterization of epitaxial Si1ÿxÿyGexCy®lms grown by C

implantation into Si0.76Ge0.24®lms followed by pulsed KrF

laser annealing. 2. Experimental

Epitaxial Si0.76Ge0.24 ®lms about 0.15 mm thick were

grown on n-type (100)Si at 5508C by ultra-high vacuum chemical vapor deposition (CVD). The as-grown Si0.76Ge0.24 ®lms were partially relaxed. C ions were

implanted at an acceleration voltage of 80 keV with a dose of 1.0 1016_/cm2_{. During implantation the temperature of}

the samples remained below 2008C. In order to con®ne most

*Corresponding author. E-mail: [email protected]

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of the implanted ions in the Si0.76Ge0.24 ®lms, a SiO2

overlayer about 1500 AÊ thick was grown on the as-grown Si0.76Ge0.24®lms. The maximum of the implanted pro®le in

the Si0.76Ge0.24®lms was estimated to be 900 AÊ by TRIM

simulation [20]. Before pulsed laser annealing the SiO2

layer was chemically removed by 5% HF solution. Pulsed KrF laser annealing was performed at an energy density of 0.1±1.6 J/cm2_{in a vacuum around 2 10}ÿ6_Torr.

The laser beam was focused onto an area of 4 4 mm2_{. The}

duration time was 14 ns. The repetition rate was 1 Hz. For each annealing the sample was illuminated by one pulse unless otherwise speci®ed. The microstructure and chemical compositions of Si1ÿxÿyGexCy ®lms were analyzed by

energy dispersive spectrometry (EDS)/transmission elec-tron microscope (TEM) which was equipped with a ®eld emission gun with an electron probe 12 AÊ in size. The variation of the lattice constant of Si1ÿxÿyGexCy ®lms

was analyzed by X-ray diffraction (XRD) with Cu K radiation. The depth pro®le of C in the Si1ÿxÿyGexCy®lms

was examined by secondary ion mass spectrometry (SIMS). Absorption measurements were performed on a Fourier transform infrared spectrometer (FTIR). Samples with lar-ger irradiated areas (10 10 mm2_{) made of nine adjacent}

4 4 mm2_{areas irradiated under identical conditions were}

prepared for XRD, SIMS, and FTIR analyses. 3. Results and discussion

After C_{implantation an amorphous layer about 900 AÊ}

thick was formed on the Si0.76Ge0.24®lm as shown in Fig. 1.

Upon subsequent laser annealing polycrystal Si1ÿxÿyGexCy

®lms were formed at 0.2 J/cm2_{, while epitaxial}

Si1ÿxÿyGexCy®lms were formed at 0.3±1.6 J/cm2as shown

in Fig. 2, in which the end-of-range (EOR) defects are present in the original amorphous/crystal interface. For laser annealing of Si the melting depth is a function of the laser wavelength, pulse length, energy density, and the thickness

of the amorphous Si layer [16,21,22]. In the present study the melting depth at 0.3 J/cm2_{was about 900 AÊ since the}

amorphous Si1ÿxÿyGexCylayer about 900 AÊ thick started to

transform to an epitaxial layer at this ¯uence. No SiC and twin were observed from electron diffraction analysis. At energy densities above 0.8 J/cm2_{the Ge concentration in the}

upper surface of the Si1ÿxÿyGexCy®lms was enriched with

the extent becoming more severe at higher energy densities from EDS/cross-sectional TEM (XTEM) analysis. One example is shown in Fig. 3, in which the strain contrast associated with some defects is present in the upper surface of the Si1ÿxÿyGexCy®lm. It has been reported that surface

segregation of Ge appears in the epitaxial growth of Si

1ÿx-Gex, presumably driven by the surface energy reduction

[23]. In the present study, laser annealing at higher energy densities further enhanced this phenomenon. In addition, at 1.4 J/cm2_{Ge started to diffuse into the underlying Si}

sub-strate, revealing that the melting depth at 1.4 J/cm2 _was

about 1500 AÊ, which was comparable to the thickness, 1500 AÊ, of the as-grown Si0.76Ge0.24 ®lm. In contrast to

Ge the depth pro®le of C in the ®lms annealed at an energy density of 0.3±1.6 J/cm remained nearly unchanged as in the as-implanted ®lm from SIMS analysis. One example is shown in Fig. 4. The slight decrease of the ion yield for the annealed Si1ÿxÿyGexCysample can be attributed to the

change of the chemical states of ions relatively to their loosely bound states in the amorphous Si1ÿxÿyGexCy®lm

after implantation [14].

At energy densities above 0.4 J/cm2_{signi®cant amounts}

of the implanted carbon were incorporated into substitu-tional sites as evidenced by the peak of the substitusubstitu-tional C (Cs) local vibration mode (LVM) at 607 cmÿ1 _{shown in}

Fig. 5. The concentration of substitutional C initially increased with the energy density in the range of 0.4± 1.0 J/cm2 _{and then saturated approximately at an energy}

density of 1.0±1.6 J/cm2_{. No SiC was formed. It has been}

reported that the maximum concentration of C which can be incorporated into substitutional site of Si or Si1ÿxGexupon

Fig. 1. XTEM image of the as-implanted Si1ÿxGex film showing the

formation of an amorphous Si1ÿxÿyGexCylayer.

Fig. 2. XTEM image of an epitaxial Si1ÿxÿyGexCyfilm grown at 0.4 J/

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pulsed laser annealing is about 1.5% from XRD measure-ment, over which SiC is formed [14±17,19]. In the present study, the peak concentration of implanted C is about 2.0% calculated from the equation, n Nd/RP, where n is the

average dopant concentration in the region around Rp, Rpis

the projected range, and Ndis the number of implanted C

atoms per unit area [20]. Upon annealing the maximum concentration of C incorporated into the substitutional site may be lower than its peak concentration [11].

Upon annealing at 1.0 J/cm2 _{for multiple pulses the}

concentration of substitutional C in the Si1ÿxÿyGexCy®lms

decreased with the pulse number, and the SiC peak at around 800 cmÿ1_{apparently appeared after irradiation of 20 pulses}

as shown in Fig. 6. This result is consistent with the plan-view TEM observation. The SIMS depth pro®les in Fig. 7 for the sample annealed at 1.0 J/cm2_{for 20 pulses show that}

carbon segregated to the original amorphous/crystal inter-face and the surinter-face of the ®lm, where the C concentration could be high enough to form SiC. The presence of EOR defects in the original amorphous/crystal interface after laser annealing was con®rmed by XTEM observation. This result implys that upon multiple pulse annealing the BOR defects play an important role in the gathering of C. Similar results have been reported in the formation of Si1ÿxCxby C

implantation and subsequent 7008C annealing [12]. The presence of EOR defects in conjunction with Ge segregation to the ®lm surface could induce severe strain in the ®lms. The driving force for reduction of the strain energy may be responsible for the enrichment of C in the ®lm surface and the original amorphous/crystal interface upon pulsed laser annealing at 1.0 J/cm2_{for larger pulse numbers.}

Kantor et al. [14] have reported that for the Si samples implanted with a carbon dose of 1 1016_/cm2_and

subse-Fig. 3. (a) XTEM image and (b) Ge depth profile of a Si1ÿxÿyGexCyfilm

grown at 1.0 J/cm2_{showing the strong strain contrast and Ge segregation}

to the film surface. Ge/Si is the atomic concentration ratio, x/(1 ÿ x), of Si1ÿxGex. The areas probed by the electron beam for EDS analysis are

denoted as `0' and assigned as 1, 2, and 3, respectively.

Fig. 4. SIMS depth profiles of Si, Ge, and C for the Si1ÿxÿyGexCyfilm

grown at 1.4 J/cm2_{and the as-implanted Si}

1ÿxÿyGexCyfilm, respectively.

Fig. 5. FTIR absorbance spectra of the as-implanted Si1ÿxÿyGexCyfilm

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quently annealed at 1.0 J/cm2 _{for one pulse no SiC was}

observed. This result is consistent with ours. However, in the case of 5 1016_/cm2_{SiC appeared in the samples annealed}

at 1.0 J/cm2 _{for 1±50 pulses. With increasing the pulse}

number the amount of SiC reduced, while that of the substitutional C in Si increased. Correspondingly, their SIMS data revealed that upon annealing for 3 pulses C started to diffuse deep into Si and the extent became more severe for higher pulse numbers. Comparing with the pre-sent study, it is evident that in addition to the annealing parameters the C concentration also plays an important role

in the formation of SiC, Si1ÿxCx, and Si1ÿxÿyGexCy by

pulsed laser annealing.

From XRD analysis the implanted Si1 ÿ x ÿ yGexCy®lms

were well crystallized after annealing at energy densities above 0.6 J/cm2_{as shown in Fig. 8. The satellite (004) peaks}

from the Si1ÿxÿyGexCy ®lms grown at various energy

densities are closer to the Si (004) peak than that from the as-grown Si0.76Ge0.24 ®lm, indicating that the tensile

stress in the Si1ÿxÿyGexCy ®lms after laser annealing was

smaller than that of the as-grown Si0.76Ge0.24 ®lm. The

reduction of the tensile stress in the Si1ÿxÿyGexCy®lms may

result from combining the effects of substitutional C incor-poration and Ge redistribution induced by pulsed laser annealing. In order to explore what extent Ge redistribution exerts on the reduction of the tensile stress in the Si1ÿxÿyGexCy ®lms after laser annealing some as-grown

Si0.76Ge0.24®lms were annealed at energy densities ranging

from 0.4 to 1.6 J/cm2_{and then followed by XRD analysis.}

The XRD patterns in Fig. 9 show that after annealing at 1.0 J/cm2 _{the position of (004) peak remains nearly}

unchanged except that it becomes slightly broadening as compared with that of the as-grown Si0.76Ge0.24®lm.

How-ever, at energy densities above 1.4 J/cm2_{it shifts to higher}

angles because of Ge diffusion to the underlying Si sub-strate. Therefore, it can be concluded that for the Si1ÿxÿyGexCy ®lms grown at energy densities below

Fig. 6. FTIR absorbance spectra of the epitaxial Si1ÿxÿyGexCy films

grown at 1.0 J/cm2_{for various pulse number.}

Fig. 7. SIMS depth profiles of C for the Si1ÿxÿyGexCyfilms grown at

1.0 J/cm2_{for 1 and 20 pulses, respectively.}

Fig. 8. XRD patterns of (a) the as-implanted Si1ÿxÿyGexCyfilm, (b) the

as-grown Si0.76Ge0.24film, and the Si1ÿxÿyGexCyfilms grown at (c) 0.6,

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1.0 J/cm2_{the reduction of tensile stress mainly results from}

the effect of substitutional carbon incorporation. 4. Summary and conclusions

For the Si0.76Ge0.24®lms after Cimplantation at a dose

of 1 1016_/cm2 _{epitaxial Si}

1ÿxÿyGexCy ®lms could be

grown by subsequent pulsed KrF laser annealing at energy densities above 0.3 J/cm2_{. At 0.8 J/cm}2_{Ge segregation to}

the ®lm surface was enhanced and Ge started to diffuse into the underlying Si at an energy density of 1.4±1.6 J/cm2_,

while the depth pro®les of C remained nearly unchanged as in the as-implanted Si1ÿxÿyGexCy ®lm. No SiC and twin

defects were observed. Below 1.0 J/cm2 _{the amount of}

substitutional carbon increased with the energy density, while above 1.0 J/cm2 _{it nearly saturated. For the}

Si1ÿxÿyGexCy®lms grown at 1.0 J/cm2for 5 and 20 pulses,

respectively, SiC appeared and its amount increased with the pulse number because of C segregation to the ®lm surface and the original amorphous/crystal interface where the EOR defects were present. For the Si1ÿxÿyGexCy®lms grown at

energy densities below 1.0 J/cm2 _{the reduction of tensile}

stress mainly resulted from the effect of substitutional carbon incorporation.

Acknowledgements

The authors gratefully appreciate Prof. S.C. Lee, from National Taiwan University, for expert assistance in the FTIR measurements. This work was sponsored by the Republic of China National Science Council under Contract No.NSC 87-2215-E-006-013.

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Fig. 9. XRDpatterns of (a) theas-grown Si0.76Ge0.24film, and the Si0.76Ge0.24

Characterization of Si1-x-yGexCy films grown by C+ implantation and subsequent pulsed laser annealing