Fabrication of High Quality SiGe Virtual Substrates by Combining Misfit Strain and Point Defect Techniques

TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll08/21llpp62-67 Volume 14, Number 1, February 2009

Fabrication of High Quality SiGe Virtual Substrates by Combining Misfit Strain and Point Defect Techniques* LIANG Renrong (ॣఅఔ), WANG Jing (ฆ ࡃ)**, XU Jun (༘ ࢋ) Tsinghua National Laboratory for Information Science and Technology (TNList), Institute of Microelectronics, Tsinghua University, Beijing 100084, China Abstract: High quality strain-relaxed thin SiGe virtual substrates have been achieved by combining the misfit strain technique and the point defect technique. The point defects were first injected into the coherently strained SiGe layer through the “inserted Si layer” by argon ion implantation. After thermal annealing, an intermediate SiGe layer was grown with a strained Si cap layer. The inserted Si layer in the SiGe film serves as the source of the misfit strain and prevents the threading dislocations from propagating into the next epitaxial layer. A strained-Si/SiGe/inserted-Si/SiGe heterostructure was achieved with a threading dislocation density of 1×104 cm2 and a root mean square surface roughness of 0.87 nm. This combined method can effectively fabricate device-quality SiGe virtual substrates with a low threading dislocation density and a smooth surface. Key words: strain relaxation; point defects; misfit strain; SiGe virtual substrate; strained Si; inserted Si layer

Introduction High-mobility field effect transistors (FETs) with strained Si, SiGe or Ge channels have strain-relaxed SiGe layers which are usually used as “virtual substrates”[1]. Therefore, thin, highly relaxed, devicequality SiGe buffer layers need to be fabricated with a very smooth surface and a low threading dislocation density. However, due to a 4.2% lattice mismatch between Si and Ge, the growth of defect-free SiGe layers on Si (001) substrates is very difficult. Various approaches have been developed to prepare SiGe virtual substrates. Conventional compositionally graded buffer layers are usually several micrometers thick with the SiGe layers relaxed by a modified Frank-Reed mechanism[2,3]. Another method is to intentionally introduce Received: 2008-03-26; revised: 2008-09-28

* Supported by the National Natural Science Foundation of China (Nos. 60476017 and 60636010) and the Basic Research Foundation of Tsinghua National Laboratory for Information Science and Technology (TNList)

** To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62789152

point defects into the SiGe epitaxial layers[4]. These point defects can be injected into the layers in different ways, such as by growth at low temperatures[5,6], ion bombardment[7], and surfactant mediated growth[8]. The point defects, especially non-equilibrium point defects introduced into the heterostructures from external sources are expected to provide powerful strain relief and to reduce the dislocation formation energy[9]. Concretely, the point defects interact with the dislocations by the following mechanisms: (1) Point defects cause dislocations to climb which helps to annihilate threading dislocation segments with opposite Burgers vectors; (2) Condensation of point defects leads to prismatic dislocations inside the SiGe layer which avoids nucleation from surface sites[4]. These mechanisms can substantially reduce threading dislocation density. A single strained interlayer can also effectively lower the threading dislocation density[10,11]. The strained interlayer acts as a defect filtering layer due to the formation of regularly arranged misfit dislocation networks induced by the misfit strain at the strained interlayer interface. However, there have been few studies of the combination of these two strategies

LIANG Renrong (ॣఅఔ) et alġFabrication of High Quality SiGe Virtual Substrates by Combining ...

(point defects and misfit strain) to improve the quality of SiGe virtual substrates. This work analyzes the effects of an inserted Si layer that provides the misfit strain in the SiGe layer. Then, the feasibility of combining this strategy with the argon ion implantation technique to fabricate high quality SiGe virtual substrates is investigated. The ion implantation technique is used to introduce point defects because of its excellent controllability of defect concentration and position.

1

Experiment

All the structures investigated in this work were grown on 5 inch p-type Si (001) substrates using an Applied Materials Epi-Centura 200 reduced pressure chemical vapor deposition (RP-CVD) system. The Si and Ge sources were pure dichlorosilane (SiH2Cl2) and germane (GeH4). The substrates were prepared by first etching in Piranha solution (Ȟ(H2SO4):Ȟ(H2O2) = 3:1) for 10 min, followed by a dump-rinsing in deionized water, and etching for 1.5 min in a dilute HF solution (Ȟ(HF):Ȟ(H2O) = 1:50). After this treatment, all wafers were hydrophobic. Before the heteroepitaxy, a 10-nm-thick Si buffer layer was grown at 900ć. First, the effects of the misfit strain were studied using a structure consisting of a 10-nm-thick Si cap layer/500-nm-thick Si0.8Ge0.2 film without (Sample A) and with (Sample B) a 50-nm-thick inserted Si layer grown on the Si buffer layer. The inserted Si layer was sandwiched between a 200-nm-thick Si0.8Ge0.2 underlayer and a 300-nm-thick Si0.8Ge0.2 overlayer. The growth temperature of the Si0.8Ge0.2 layer was 900ć, while that of the inserted Si and Si cap layers was 625ć. Second, the feasibility of the combined misfit strain and point defect strategy was investigated using a Sample C fabricated by first growing a 60-nm-thick coherently strained Si0.8Ge0.2 film with a 17-nm-thick inserted Si layer on the Si buffer layer. Then Ar ion implantation was performed with an energy of 70 keV and a dose of 5×1014 cm2. Subsequently, the sample was thermally annealed at 900ć for 2 h in an N2 atmosphere. Finally, an intermediate 120-nm-thick Si0.8Ge0.2 layer was grown, followed by a 17-nm-thick strained Si cap layer at the top of the stack. All of the epitaxial layers were grown at 625ć for Sample C. The dislocation morphology was analyzed using a cross-sectional transmission electron microscope

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(TEM) with JEOL 200CX and JEOL 2010 operating at 200 kV. The surface roughness was examined using a nanoscope IIIa atomic force microscope (AFM) operating in tapping mode. The residual strain and the Ge content were evaluated using a Renishaw RM1000 Raman spectrometer with a 514-nm laser line as the excitation light source. The density of the threading dislocations was estimated by the Schimmel etch method using a diluted CrO3:HF:H2O solution.

2

Results and Discussion

2.1

Misfit strain

Figure 1 shows the cross-sectional TEM images of the Si0.8Ge0.2 films without (Fig. 1a, Sample A) and with (Fig. 1b, Sample B) the 50-nm-thick inserted Si layer. The total Si0.8Ge0.2 thickness was 500 nm and the inserted Si layer was 200 nm above the SiGe/Si buffer layer interface. The mismatch between the SiGe epilayer and the Si substrate can generally be accommodated by the strain and the introduction of misfit dislocations[4]. Once the SiGe layer thickness exceeds a critical thickness during epitaxy, the epilayer relaxes plastically, since the introduction of interfacial misfit dislocations becomes energetically more favorable. As shown in Fig. 1a, the dislocations in the SiGe without the inserted Si layer propagate into the film until they reach the strain-free region and distribute uniformly from the bottom interface up to close to the surface. In contrast, most of the threading dislocations were prevented from propagating into the epitaxial SiGe layer and were trapped at the inserted Si/SiGe interface, as shown in Fig. 1b. This phenomenon is in accordance with previous observations[10-12].

Fig. 1 Cross-sectional TEM images of the samples (a) without and (b) with a 50-nm-thick inserted Si layer

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The reduction of the threading dislocations in Sample B can be attributed to the large misfit strain field created by the inserted Si layer. Figure 1b shows that the dislocations were already present in the lower SiGe layer of Sample B (i.e. this layer is partially relaxed due to the high growth temperature and large thickness) prior to growing the inserted Si layer. On the other hand, Chen et al.[12] claimed that the Si0.8Ge0.2 underlayer thickness does not influence the threading dislocation density and surface roughness of the top SiGe layer. Note that in their work, the 70-nm-thick Si0.8Ge0.2 underlayer was pseudomorphic, which is very different from the case of this work. Therefore, Fig. 1 shows that the inserted Si layer in the SiGe film can act as a source of misfit strain, providing effective nucleation sites for misfit dislocations to relax the mismatch strain and suppressing the propagation of threading dislocations. The thickness of the inserted Si layer may also affect the relaxation of the strains. Here, the inserted Si layer thickness was chosen to be the same as in Lee et al.[11] The Si0.8Ge0.2 had a 0.83% lattice mismatch with Si, so the critical thickness of the inserted Si layer was estimated to be about 13 nm according to Matthews and Blakeslee[13]. Although the inserted Si layer may generate extra misfit dislocations because its thickness is more than the critical thickness, Fig. 1 shows that it effectively reduces dislocations, which suggests that the density of the generated dislocations is much less than that of the dislocations blocked by this layer. More in-depth study of both the optimum thickness and the misfit strain at the heterointerface of the inserted Si layer is expected to further reduce the dislocations and improve the surface roughness. A Schimmel etching solution was used to further

Tsinghua Science and Technology, February 2009, 14(1): 62-67

analyze the threading dislocations penetrating into the SiGe layers. The etch pits were counted with an optical microscope with the statistically obtained threading dislocation density for Sample B being about 2×106 cm2, while for Sample A the density is larger than 1h108 cm2. The root-mean-square (RMS) surface roughnesses of the surfaces were measured to be 10.57 nm (30 ȝm×30 ȝm) for Sample A and 16.64 nm for Sample B. The higher RMS roughness for Sample B than for Sample A could be due to the strain relaxation by modulation of the surface profile[14]. 2.2

Combination of point defects with misfit strains

Figure 2 shows a cross-sectional TEM image of Sample C. The defects were mainly distributed in the ion-implanted SiGe buffer layer and decayed abruptly at the upper interface of the inserted Si layer. According to transport and range of ions in matter (TRIM) simulations, the argon peak position is about 75 nm, which is near the SiGe/Si buffer interface. The high energy, high dose ion implantation into the SiGe layer intentionally introduces point defects into the inserted Si/coherently strained SiGe/Si buffer system. These defects then serve as sources for the formation of dislocation loops during the subsequent high temperature treatment because of their condensation. Hence, they can initiate the strain relaxation and hamper the propagation of threading dislocations in the SiGe buffer layer. If the point defect concentration inside of the sample substantially differs from the equilibrium value, a higher point defect concentration implies a much higher probability of annihilation of threading dislocations with opposite Burgers vectors[9].

Fig. 2 Cross-sectional TEM image of Sample C. Dislocations are seen to be confined near the heterointerface and to decay abruptly at the upper interface of the inserted Si layer/SiGe. No threading dislocations extend to the surface.

LIANG Renrong (ॣఅఔ) et alġFabrication of High Quality SiGe Virtual Substrates by Combining ...

During the regrowth of the intermediate SiGe layer, the inserted Si is biaxial tensile since it is between two SiGe layers. As the total strain energy of the regrowth-SiGe/inserted-Si/SiGe/Si buffer system accumulates to a critical value, the inserted Si layer acts as a new effective nucleation site for the dislocations to relieve the mismatch strain[11]. Figure 3 shows an HRTEM image of a region near the inserted Si layer in Sample C. Most of the misfit dislocations located at the interfaces of the inserted Si layer were extended dislocations consisting of two Shockley partial dislocations and a stacking fault strip[15,16], denoted by D1, D2, and D3 in Fig. 3. No threading dislocation was observed in the TEM experiments, which indicates that the misfit strain generated by the inserted Si layer serves as a defect filtering tool. Thus, the intermediate SiGe layer has a low defect density. The Schimmel etch method was again used to depict the threading dislocations. The etch pits originating from threading

Fig. 3 High resolution TEM micrograph of a region near the inserted Si layer in Sample C. Here, D1, D2, and D3 are extended dislocations at the interfaces of the inserted Si layer and the SiGe.

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dislocations in the intermediate SiGe epilayer were counted with the statistically obtained threading dislocation density being about 1×104 cm2. If only the ion implantation strategy is used, the threading dislocation density in the ion-implanted and annealed SiGe buffer layers is on the order of 106-107 cm2[17,18]. Detailed discussions of the ion implantation method are beyond the scope of this study and can be found elsewhere[19,20]. Figure 4 shows an AFM scan image (10 ȝm×10 ȝm) and the associated line profile of Sample C. A cross-hatch pattern is clearly observed along the orthogonal <110> axes due to misfit dislocation networks that relieve the strain[21]. The RMS surface roughness was measured to be 0.87 nm with a maximum height of 2 nm. This RMS roughness is much smaller than that of either Sample A or B which did not employ the ion implantation strategy. Furthermore, the RMS surface roughness of Sample C is also slightly lower than that measured by Lee et al.[11] (i.e. 3 nm). These results further indicate that the point defects injected from external sources can promote relaxation of the misfit strain. So, these defects can contribute to the reduction of the threading dislocation density and improve the surface roughness. The RMS surface roughness of Sample C before ion implantation was measured to be 0.43 nm. Thus, the RMS roughness increased slightly after the final processing, including the thermal annealing and subsequent regrowth. Therefore, the inserted Si/pseudomorphic SiGe layers should be as flat as possible before ion implantation.

Fig. 4 Tapping-mode atomic force microscopy image (10 ȝm×10 ȝm) and the associated line scan profile showing the surface morphology of Sample C

2.3

Strain relaxation

A detailed analysis of the composition and the residual strain of the SiGe layers was performed using Raman

spectroscopy. The Raman scattering was generated using 514 nm Ar laser light with the optical penetration depth into the SiGe layers estimated to be about

Tsinghua Science and Technology, February 2009, 14(1): 62-67

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300 nm[22]. Figure 5 shows the Raman measurements for Sample C and a reference Si bulk (unstrained) sample. As shown in the figure, three distinct lines whose energies depend on the composition and strain can be observed. They are attributed to Si-Si ( ZSi-Si = 509.60 cm1), Si-Ge ( ZSi-Ge = 405.06 cm1), and Ge-Ge ( ZSi-Ge = 286.64 cm1) atomic vibrations[23]. The

frequency shifts of the Si-Si and Si-Ge lines allow simultaneous analysis of the composition and the residual strain in the SiGe layer using empirical relations for the composition- and strain-induced frequency shift[24,25]: xGe (37ZSi-Ge  24ZSi-Si  2324.10) / 2157.4 (1)

H

(68ZSi-Ge  14.2ZSi-Si  34 626.52) / 2157.4

(2)

where xGe is the Ge content, H is the normalized residual strain, ZSi-Si is the Si-Si phonon frequency, and ZSi-Ge is the Si-Ge phonon frequency. Equations (1) and (2) were solved simultaneously using the data for Sample C in Fig. 5 to obtain a Ge content of 20.1% and a normalized residual strain of 7.1%. The same method was used to show that the Si0.8Ge0.2 layers in both Samples A and B were almost fully relaxed, due to the high growth temperatures for these two samples. The influence of the inserted Si layer thickness on the strain relaxation will be investigated in further studies.

TEM showed that the inserted Si layer in the SiGe film generates a misfit strain that obstructs threading dislocation propagation into the epitaxial layer. The point defects introduced into the inserted Si/pseudomorphic SiGe films by argon ion implantation and high temperature heat treatment significantly reduce the threading dislocation density and improve the surface roughness. A strained-Si/SiGe/inserted-Si/SiGe/Si buffer heterostructure was produced with an RMS surface roughness of 0.87 nm and a threading dislocation density of about 1×104 cm2. This method can be used to fabricate high quality relaxed SiGe buffer layers with large lattice mismatch and may be further developed for heteroepitaxy such as in III-V layers grown on Si based substrates. Acknowledgments The authors thank Dr. Romain Ritzenthaler for helpful discussions.

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Fig. 5 Raman spectra of Sample C and of the Si reference, measured using the Ȝ=514 nm line of an argon laser in air at room temperature

3

Conclusions

The misfit strain and point defect techniques in SiGe epilayers were combined to produce device-quality highly relaxed SiGe films with low threading dislocation densities and smooth surfaces. The cross sectional

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