Interfacial reactions of Ni/Si0.76Ge0.24 and Ni/Si1-x-yGexCy by vacuum annealing and pulsed KrF laser annealing

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Interfacial reactions of Ni/Si

0:76

Ge

0:24

and Ni/Si

1ÿxÿy

Ge

x

C

y

by

vacuum annealing and pulsed KrF laser annealing

Jian-Shing Luo

a

, Wen-Tai Lin

a,*

, C.Y. Chang

b

, P.S. Shih

b

a_{Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC} b_{Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC}

Abstract

Interfacial reactions of Ni on Si0:76Ge0:24 and Si1ÿxÿyGexCy ®lms by vacuum annealing and pulsed KrF laser

an-nealing were studied by transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS) and X-ray diraction (XRD). For the Ni/Si0:76Ge0:24 and Ni/Si1ÿxÿyGexCy®lms annealed at a temperature of

200±600°C Ge segregation and agglomeration occurred at an extent becoming more severe at higher temperatures. Upon pulsed KrF laser annealing the agglomeration structure was improved. The retardation of phase transformation in the Ni/Si1ÿxÿyGexCy system was observed upon either vacuum annealing or pulsed laser annealing. Multiple-pulse

annealing is an eective method to fabricate smooth Ni(Si0:76Ge0:24)2and Ni(Si1ÿxÿyGexCy)2®lms without inducing Ge

1. Introduction

Recently, Si1ÿxGex on Si has been extensively

studied for applications in the ®eld of optoelec-tronics and high speed heterojunction bipolar transistors [1,2]. Since the lattice spacing of Ge is 4.2% larger than that of Si, compressive strains developed in the Si1ÿxGexoverlayer create stability

problems that limit the thickness of the pseudo-morphic Si1ÿxGex overlayer. Carbon introduced

substitutionally into Si1ÿxGex reduces the lattice

mismatch between Si1ÿxGex and Si and then

thicker pseudomorphic Si1ÿxGex ®lms with a high

Ge content can be fabricated.

For device applications the formation of metal/ Si1ÿxGex and metal/Si1ÿxÿyGexCy ohmic or

recti-fying contacts is required. Thus, the interfacial reactions of some metals such as Ni [3,4], Pt [5,6], Pd [6±8], Ti [9±12], Co [13±15], W [16], Cu [17,18], and Zr [19] on Si1ÿxGex, and Co [20], Zr [21], and

Ti [11] on Si1ÿxÿyGexCy by conventional furnace

annealing and pulsed laser annealing have been studied, respectively. For conventional furnace annealing the formation of a ternary phase was generally accompanied with Ge segregation. Ad-ditionally, an agglomeration structure also ap-peared at higher annealing temperatures. These

*_{Corresponding author. Tel.: 275-7575; fax:} +886-6-2745-985.

E-mail address: [email protected] (W.-T. Lin).

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phenomena could be attributed to the higher heat formation for metal-Si than for metal-Ge [22]. Rapid thermal annealing and pulsed laser an-nealing can shorten the anan-nealing time, resulting in a reduction of Ge segregation. Furthermore, multiple-pulse laser annealing can produce a smooth and continuous germanosilicide ®lm without inducing strain relaxation in the unreacted Si1ÿxGex ®lm [8].

In the present study, the interfacial reactions such as phase transformation, Ge segregation, the formation of agglomeration, and strain relaxation in the Ni/Si0:76Ge0:24 and Ni/Si1ÿxÿyGexCy systems

upon vacuum annealing and pulsed KrF laser annealing were studied by using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS) and X-ray diraction (XRD).

2. Experimental

Strained and partially relaxed Si0:76Ge0:24 ®lms

about 1000 and 1500 A thick were grown on n-type Si(1 0 0) at 550°C by ultra-high-vacuum chemical-vapor deposition (CVD), respectively. Si1ÿxÿyGexCy ®lms were prepared by C ions

im-planted into the partially relaxed Si0:76Ge0:24 ®lms

and subsequent pulsed KrF laser annealing at an energy density of 0.8±1.0 J/cm2_{. Details of the}

preparation of Si1ÿxÿyGexCy ®lms and their

char-acterization were described elsewhere [23]. Prior to deposition the substrates were cleaned by the RCA method and then immediately loaded into the chamber. Ni about 250 A thick was deposited onto the Si0:76Ge0:24 and Si1ÿxÿyGexCy ®lms at room

temperature by electron gun evaporation at a rate of 1 A/s. The base pressure was around 1±2 10ÿ6

Torr. Furnace annealing was carried out at a temperature of 200±700°C in a vacuum of 1±2 10ÿ6 _{Torr. Pulsed KrF laser annealing was}

performed at an energy density of 0.1±0.3 J/cm2_in

a vacuum around 2 10ÿ2 _{Torr. The pulse length}

is 14 ns. The laser beam was focused onto an area of 4 4 mm2_{. Phase formation, the}

microstruc-tures, and chemical compositions of the reacted layer were analyzed by EDS/TEM which was equipped with a ®eld emission gun with an

elec-tron probe 12 A in size. The strain relaxation of the unreacted Si0:76Ge0:24®lms after annealing was

analyzed by XRD.

3. Results and discussion 3.1. Vacuum annealing

For the Ni/Si0:76Ge0:24 ®lms annealed at a

tem-perature of 200±500°C Ni(Si1ÿxGex) was formed.

From EDS/cross-section TEM (XTEM) analysis Ge segregation from the Ni(Si1ÿxGex) layer to the

underlying Si0:76Ge0:24 substrate apparently

ap-peared at temperatures above 300°C at an extent becoming more severe at higher temperatures. Agglomeration at 400°C occurred as shown in Fig. 1, in which the Si1ÿxGex ®lm exposed to the

®lm surface is Ge-rich. The formation of heat for NiSi and NiGe has been determined to be about )45 and )32 kJ/mol, respectively [22]. These val-ues suggest that Ni tends to react preferably with Si. Above 550°C Ni(Si1ÿxGex)2 was formed, in

which only a trace amount of Ge was present. For the Ni/Si1ÿxÿyGexCy ®lms annealed at

200°C Ni(Si1ÿxÿyGexCy) was formed concurrently

with Ni2(Si1ÿxÿyGexCy). After annealing at a

tem-perature of 250±550°C only Ni(Si1ÿxÿyGexCy) was

present. As seen in Fig. 2 the extent of agglomer-ation was signi®cantly alleviated when compared with the Ni/Si0:76Ge0:24system. From EDS/XTEM

analysis the Ni(Si1ÿxÿyGexCy) layer was also

de®-cient in Ge with the extent being more severe at higher temperatures. One example is shown in

Fig. 1. Plan-view TEM image of the Ni/Si0:76Ge0:24sample an-nealed at 400°C showing the agglomeration structures.

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Fig. 3. At 600°C, Ni(Si1ÿxÿyGexCy)2 was formed

concurrently with the apparent appearance of ag-glomeration. Evidently, the phase transformation and appearance of agglomeration for the Ni/ Si1ÿxÿyGexCy system are sluggish in comparison

with those for the Ni/Si0:76Ge0:24 system.

The retardation of phase formation has also been observed in the Ti/Si1ÿxÿyGexCy and Co/

Si1ÿxÿyGexCysystems [11,20]. It was suggested that

C accumulation at the germanosilicide/epilayer interface blocked the interfacial reactions.

3.2. Pulsed KrF laser annealing

In our previous studies for the Pd/Si0:76Ge0:24

system [13] multiple-pulse annealing could ho-mogenize the Pd concentration in the germanosil-icide layer without inducing Ge segregation to the underlying Si0:76Ge0:24®lms. For the Ni/Si0:76Ge0:24

®lms annealed at 0.2 J/cm2 _{for 10 pulses}

Ni(Si1ÿxGex) was formed concurrently with

Ni(Si1ÿxGex)2 as shown in Fig. 4. After annealing

for 20 pulses a smooth Ni(Si1ÿxGex)2 layer was

formed as shown in Fig. 5, in which the upper layer is polycrystalline, while the lower layer is epitaxial. Meanwhile the EDS/XTEM data show that although some Ge segregate down to the lower part of the germanosilicide layer, they do not segregate out of the germanosilicide to the underlying Si0:76Ge0:24 ®lm. Correspondingly, in

Fig. 6 the XRD patterns of the sample annealed at

Fig. 3. (a) XTEM image and (b) the depth pro®les of the chemical elements for the Ni/Si1ÿxÿyGexCysample annealed at 500°C showing that the Ni(Si1ÿxÿyGexCy) ®lm is de®cient in Ge.

Fig. 4. (a) Plan-view image and (b) electron diraction pattern (DP) of the Ni/Si0:76Ge0:24sample annealed at 0.2 J/cm2for 10 pulses showing that Ni(Si1ÿxGex) coexists with Ni(Si1ÿxGex)2, where Ni(Si1ÿxGex) and Ni(Si1ÿxGex)2are denoted as ``X'' and ``O'', respectively.

Fig. 2. Plan-view image of the Ni/Si1ÿxÿyGexCy sample an-nealed at 400°C showing no agglomeration structures.

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0.2 J/cm2 _{for 20 pulses show that the lattice}

con-stant of the unreacted Si0:76Ge0:24 ®lm in the

di-rection perpendicular to the ®lm surface remains nearly unchanged in comparison with that of the as-grown Si0:76Ge0:24 ®lm, revealing that no strain

relaxation appears in the unreacted Si0:76Ge0:24

®lm. For the samples annealed at 0.3 J/cm2 _{for 5}

pulses a single Ni(Si1ÿxGex)2 ®lm was formed,

however, Ge segregation to the underlying Si0:76Ge0:24 ®lm occurred.

For the Ni/Si1ÿxÿyGexCy samples annealed at

0.2 J/cm2_{for 10 pulses only Ni(Si}

1ÿxÿyGexCy) was

formed as shown in Fig. 7. It is worth to note that for the Ni/Si1ÿxGex samples annealed at 0.2 J/cm2

for 10 pulses Ni(Si1ÿxGex)2 has been formed

al-ready as shown in Fig. 4, indicating that upon pulsed laser annealing the sluggish phase trans-formation appears in the Ni/Si1ÿxÿyGexCy system

as well. Ni(Si1ÿxÿyGexCy)2 was formed upon

an-nealing at 0.2 J/cm2 _{for 30 pulses. The}

Ni(Si1ÿxÿyGexCy)2 layer was very smooth and no

apparent Ge segregation into the underlying Si1ÿxÿyGexCy ®lm occurred.

It can be concluded that upon multiple-pulse laser annealing smooth Ni(Si0:76Ge0:24)2 and

Ni(Si1ÿxÿyGexCy)2 ®lms can be grown without

showing Ge segregation to the unreacted

Fig. 7. (a) Plan-view image and (b) DP of the Ni/Si1ÿxÿyGexCy sample annealed at 0.2 J/cm2_{for 10 pulses showing that only} Ni(Si1ÿxÿyGexCy) is present.

Fig. 5. (a) XTEM image and (b) the depth pro®les of the chemical elements for the Ni/Si0:76Ge0:24sample annealed at 0.2 J/cm2 _{for 20 pulses showing that a continuous Ni(Si}

1ÿxGex)2 ®lm was formed without inducing Ge segregation to the un-derlying Si0:76Ge0:24®lm.

Fig. 6. XRD patterns of (a) the as-grown Si0:76Ge0:24®lm and (b) the unreacted Si0:76Ge0:24 ®lm after annealing at 0.2 J/cm2 for 20 pulses.

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Si0:76Ge0:24 ®lms and inducing strain relaxation.

However, the addition of C to the Si1ÿxGex ®lms

signi®cantly retards the phase transformation. 4. Summary and conclusions

1. For the Ni/Si0:76Ge0:24 and Ni/Si1ÿxÿyGexCy samples subjected to vacuum annealing Ge seg-regation to the underlying substrates and ag-glomeration occurred at an extent becoming more severe at higher annealing temperatures. The extent of agglomeration was signi®cantly alleviated for the Ni/Si1ÿxÿyGexCy system. When subjected to multiple-pulse laser annealing smooth Ni(Si0:76Ge0:24)2 and Ni(Si1ÿxÿyGexCy)2 ®lms could be grown with-out inducing Ge segregation to the unreacted substrates and strain relaxation.

2. C plays an important role in delaying phase transformation upon either vacuum annealing or pulsed KrF laser annealing.

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