Optical strain measurement in ultrathin sSOI wafer

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 253 (2006) 18–21 www.elsevier.com/locate/nimb

Optical strain measurement in ultrathin sSOI wafer J. Munguı´a a

a,*

, H. Chouaib a, J. de la Torre a, G. Bremond a, C. Bru-Chevallier a, A. Sibai a, B. Champagnon b, M. Moreau c, J.-M. Bluet a

Laboratoire de Physique de la Matie`re (LPM), CNRS UMR-5511, INSA de Lyon, 7, av. Jean Capelle, Baˆt. Blaise Pascal, 69621 Villeurbanne, France b Laboratoire de Physico-Chimie des Mate´riaux Luminescents (LPCML), CNRS UNR-5620, Universite´ Claude Bernard Lyon 1, 12 rue Ampe`re, Baˆt. Lippmann, 69622 Villeurbanne, France c Horiba Jobin Yvon SAS division Raman, 231, rue de Lille, 59650 Villeneuve d’ascq, France Available online 9 November 2006

Abstract Three optical characterizations, Raman spectroscopy (RS), low temperature photoluminescence (LTPL) and photoreflectance (PR) have been used for strain characterization in thin (15 nm) strained silicon on insulator (sSOI) layer. The RS using visible and UV excitation allows to determine the strain in the thin overlayer and a value of 1% has been deduced from the LO phonon energy shift. The LTPL spectra analysis shows a bandgap shrinkage at D point of 140 meV. This is due to the D6 conduction band splitting and leads to a strain value of 1%. Finally from PR measurements a bandgap shrinkage at C point of 0.19 eV and 0.08 eV for light holes and heavy holes respectively has been measured, thus confirming the 1% strain value obtained by Raman spectroscopy and LTPL measurements. Ó 2006 Elsevier B.V. All rights reserved. PACS: 07.10.Pz; 78.55. m; 71.20.Mg; 71.20.Nr; 78.30.Am; 78.30.Fr Keywords: Strained silicon; sSOI; Raman Spectroscopy; Photoluminescence; Photoreflectance

1. Introduction Strained silicon on insulator (sSOI) is a new material system that combines the carrier transport advantages of strained silicon layers [1] and SOI technology which allows an increase in performance and a reduction in power consumption due to the active layer insulation [2]. Improved transistor performance occurs as a direct result of increased electron and hole mobility due to energy band changes caused by the tensile strain. For electrons, the sixfold degeneracy of the conduction band minima at the D point is split, in such a way that the two out-of-plane D2 valleys are at a lower energy than the four in-plane D4 valleys. This results in a reduced inter-valley carrier scattering and in a preferential occupation of the twofold valleys where the effective mass is smaller. For holes, the degeneracy of the

valence band minimum at the C point is lifted and the light hole band is preferentially populated, resulting again in a reduced inter-valley carrier scattering and a reduced effective mass [3]. Biaxial tensile strain also impacts interband at highest energy gaps. The fully depleted MOSFET architecture requires SOI substrates with ultra-thin silicon overlayer (<15 nm). The challenge for such structures (sSOI) is to develop a rapid and reliable strain measurement in order to evaluate the strain uniformity across the wafer. In this work we present optical characterization carried out using Raman spectroscopy (RS), low temperature photoluminescence (LTPL) and photoreflectance (PR) techniques in order to analyze the strain effect induced in the silicon overlayer.

2. Experimental details *

Corresponding author. Tel.: +33 4 72 43 87 32; fax: +33 4 72 43 85 31. E-mail address: [email protected] (J. Munguı´a). 0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.10.007

The 15 nm thick silicon layer was epitaxially grown on a Si1 xGex virtual substrate. The so-called virtual substrate

J. Munguı´a et al. / Nucl. Instr. and Meth. in Phys. Res. B 253 (2006) 18–21

Intensity (a.u.)

Bulk Si

Strained Si

505

3.1. Raman spectroscopy In the back scattering geometry used for the experiment, only the LO phonon, with a Raman shift of 520 cm 1, can be observed. This mode, related to the silicon handling substrate, is clearly observed (Fig. 1(a)) using the 514.5 nm excitation. Nevertheless, we can observe an additional peak at the low energy side of the main peak. This low energy component may arise from the strained Si overlayer, as indicated on the Fig. 1(a) with a frequency shift of Dx = 5.6 cm 1. In order to verify this assumption, an additional Raman measurement has been performed using a 325 nm laser excitation. In this case, because of the shorter wavelength, the laser line is fully absorbed by the strained Si overlayer. Consequently the Raman spectrum contains only the LO phonon peak corresponding to the strained layer which presents a significant shift (6.5 cm 1) from the bulk position (Fig. 1(b)). The higher frequency shift (6.5 cm 1) obtained in the case of UV excitation may be due to sample heating by the laser beam. The average in plane stress considering r[0 0 1] = r[0 1 0] as a function of the optical phonon energy shift from the stress free bulk value (Dx = xs x0) can be calculated using the relation

510

515

520

525

530

535

Wavenumber (cm-1)

325 nm

Δω = 6.5 cm-1

Bulk Si

Strained Si

500

3. Results and discussion

514 nm

Δω = 5.6 cm-1

Intensity (a.u.)

is obtained by the growth of a relaxed Si0.8Ge0.2 layer on top of a Si1 wGew graded Ge content buffer layer (1 < w < 20) on a Si substrate [4]. The lattice constant mismatch between Si and SiGe leads to a biaxial tensile strain in the Si layer. The germanium content (x) in the SiGe layer sets the amount of strain induced in the subsequently grown Si layer. In this work a composition with 20% Ge was used in order to achieve 1% of strain. The strained silicon layer is then transferred on a Si handle wafer using the SmartcutTM process [5]. Finally, a selective etching of the SiGe layer leads to a strained silicon on insulator wafer called sSOI. The Raman spectroscopy measurements were carried out using a LabRam HR system under a microscope in back scattering configuration along the (0 0 1) direction. A visible excitation, using the 514.5 nm line of an Ar+ laser, was used to probe the whole structure while a UV excitation, using the 325 nm line of an HeCd laser, was used to probe only the strained Si over layer. LTPL was excited with an Ar+ laser line at 363.8 nm. The photoluminescence signal was dispersed using a Jobin-Yvon HRS2 spectrometer and then detected by a Ge photodiode. All spectra were corrected from the spectral response of the system. The PR measurements were performed using an halogen lamp dispersed by a Jobin-Yvon type HR 640 as a probe beam. The optical modulation of the electric field was realized using the 244 nm line of a doubled Ar+ laser. Finally, the reflectance signal was detected by a GaAs (Hamamatsu H5701-50) photomultiplier. This technique allows getting intrinsic information related to the density of states.

19

505

510

515

520

525

530

535

Wavenumber (cm-1) Fig. 1. Raman spectra corresponding to the sSOI sample using visible (514 nm) (a) and UV (325 nm) excitation (b).

r(Mpa) = 250 Dx (cm 1), deduced from the phonon deformation potentials [6,7]. Therefore, using the shift (Dx = 5.6 cm 1) extracted form Raman measurements and considering the [1 0 0] Young modulus value, a strain value of 1.06 % is extracted. Thus, considering a pure elastic deformation this strain would correspond to a SiGe virtual substrate layer with content between 23% and 25% using the Vegard law or a corrected quadratic law [8] for the SiGe lattice parameter, respectively, this value is in agreement with the 20% nominal value and confirms the previously observed [9] strain conservation during the SmartcutTM process. 3.2. Low temperature photoluminescence The LTPL spectra obtained from a sSOI sample and from a reference sample (bulk silicon after strained silicon overlayer and buried oxide layer chemical etching) are compared in Fig. 2. In both cases we observe a broad band at low energy. This PL band can be interpreted as the

J. Munguı´a et al. / Nucl. Instr. and Meth. in Phys. Res. B 253 (2006) 18–21

20

0.7

sSOI

Pexc=30 mW

Si substrate

Tmeas=8 K

1.20

Richard et al. Schäffler. Rieger et al. Yang et al.

1.15 1.10

0.6

Strained Si Eg (eV)

Photoluminescence intensity (a.u.)

0.8

0.5

(X 5) 0.4 0.3 0.2

1.05 1.00 0.95 0.90 0.85

0.1

0.80 0.00

0.0 0.8

0.9

1.0

1.1

1.2

0.05

0.10

0.15

Energy (eV)

convolution of two emission bands related to dislocations defects in bulk silicon and located at 0.807 eV (D1) and 0.870 eV (D2) [10]. Moreover, the difference between the two samples is observed from the near band edge emission. For bulk material, the classical TO replica of excitonic band gap is observed at 1.098 eV whereas in the case of sSOI two different bands at 0.96 eV and 1.05 eV are clearly observed in addition to the 1.098 eV emission band. In order to differentiate the luminescence mechanisms for each band, one sSOI sample was chemically etched to remove the stressed layer and the PL spectrum was analyzed. The results showed a disappearance of the 0.96 eV peak while the 1.05 eV PL band was still present, thus confirming that the PL band at 0.96 eV is coming only from the silicon stressed layer. Therefore, it can be assigned the 0.96 eV emission band to a recombination via the TO replica of transition between the two out-of-plane D2 valleys band in D-point and the light hole band in C-point in the strained Si overlayer. Taking into account the TO phonon energy and the exciton binding energy, the 0.96 eV would correspond to a 1.03 eV band gap of the strained Si. According to different theoretical calculation [11–14] this strained Si band gap value corresponds to an equivalent Ge content in the SiGe layer between 18% and 21% (Fig. 3) which agrees very well with the nominal value of 20% reported for our samples. 3.3. Photoreflectance The PR spectrum obtained at room temperature for the strained Si layer is shown in Fig. 4. While only one transition is observed in the case of strain free bulk Si, we can observe for the sSOI layer a complex PR spectrum. The data were fitted according to the Aspnes model for low field intensity [15]. Three transitions energies are deduced from this modelling (i) 3.19 eV, (ii) 3.30 eV and (iii) 3.45 eV. From theoretical band diagram calculation in strained Si [11] we attribute these transitions to (i) the lower conduc-

0.25

0.30

0.35

0.40

Fig. 3. Calculated change in direct band gap energy as a function of the Ge content in an SiGe layer with lattice constant equivalent to the strained Si. -5

6.0x10

Lineshape fit Experiment

-5

4.0x10

sSOI T=300K Γ

Eg0 (hh) -5

2.0x10

ΔR/R

Fig. 2. LTPL spectra for an sSOI sample and the corresponding handling Si substrate.

0.20

Ge content (x)

Γ

Eg 0 (lh) Γ

Eg 1 (lh)

0.0 -5

-2.0x10

-5

-4.0x10

3.0

3.2

3.4

3.6

3.8

Energy (eV) Fig. 4. PR spectrum of strained silicon layer at room temperature (dotted curve). The solid line is corresponds to the theoretical fit.

tion band to lh band transition ECg0 (lh) = 3.19 eV, (ii) the lower conduction band to hh band transition ECg0 (hh) = 3.30 eV and (iii) a higher conduction band to lh band ECg1 (lh) = 3.45 eV. Using the deformation potential values reported by Pollak and Cardona [16] and the band gap shrinkage measured in our experiment (DEg(e-lh) = 0.19 eV and DEg(e-hh) = 0.08 eV) we extracted a tensile strain of 1% in the strained Si overlayer. This value is again in good agreement with the nominal tensile strain and with the values obtained from RS and LTPL. 4. Conclusion Three different optical techniques (Raman spectroscopy, low temperature photoluminescence and photoreflectance) have been used for strain characterization in ultra thin sSOI samples (strained silicon overlayer thickness of 15 nm). A good agreement with the nominal strain value is obtained by using these three techniques. We reported

J. Munguı´a et al. / Nucl. Instr. and Meth. in Phys. Res. B 253 (2006) 18–21

first observation of the strained silicon band gap TO replica by PL in a very thin layer (0.96 eV). The strained silicon band gap in C-point is observed using PR characterization, ECg0 (lh): 3.19 eV and ECg0 (hh): 3.30 eV. Acknowledgements This work was supported by CONACYT. The authors thank SOITEC and CEA-LETI for sample preparation. References [1] M.V. Fischetti, E. Ga´miz, W. Ha¨nsh, J. Appl. Phys. 92 (2002) 7320. [2] S. Crsitoloveanu, VLSI Handbook, 2001. Available from: . [3] K. Rim, R. Anderson, D. Boyd, F. Cardone, K. Chan a, H. Chen b, S. Christansen, J. Chu, K. Jenkins, T. Kanarsky, S. Koester, B.H. Lee, K. Lee, V. Mazzeo, A. Mocuta, D. Mocuta, P.M. Mooney, P. Oldiges, J. Ott, P. Ronsheim, R. Roy, A. Steegen, M. Yang, H. Zhu, M. Ieong, H.-S.P. Wong, Solid State Electron. 47 (2003) 1133. [4] J.M. Hartmann, Y. Bogumilowicz, P. Holliger, F. Laugier, R. Truche, G. Roland, M.N. Se´me´ria, V. Renard, E. Olshanetsky, O. Estibal, D. Kvon, J.C. Portal, L. Claverie, Semicond. Sci. Technol. 19 (3) (2004) 311.

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