One-Step Ge/Si Epitaxial Growth

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Published: June 08, 2011

r 2011 American Chemical Society 2398 dx.doi.org/10.1021/am200310c|ACS Appl. Mater. Interfaces 2011, 3, 2398–2401

RESEARCH ARTICLE

www.acsami.org

One-Step Ge/Si Epitaxial Growth

Hung-Chi Wu,

†

Bi-Hsuan Lin,

‡

Huang-Chin Chen,

§,^

Po-Chin Chen,

†

Hwo-Shuenn Sheu,

‡

I-Nan Lin,

§

Hsin-Tien Chiu,

#

and Chi-Young Lee*

,^

†_{Department of Materials Science and Engineering and}^_{Center for Nanotechnology, Materials Science, and Microsystems National}

Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China

‡_{Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan 30076, Republic of China} §_{Department of Physics, Tamkang University, Tamsui, Taiwan 25137, Republic of China}

#

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China

b

S Supporting Information

1. INTRODUCTION

Germanium (Ge) has attracted much attention because of its several superior intrinsic properties towards silicon (Si), such as large excitonic Bohr radius (Ge: 24.3 nm; Si: 4.9 nm), high carrier mobility (Ge has 2.75 times increase in bulk electron and 4 times increase in hole mobility than Si), and small energy bandgap (direct and indirect bandgap for Ge are 0.8 and 0.66 eV, and for Si are 3.2 and 1.12 eV); furthermore, Ge has a strong absorption band at near-infrared wavelength and high refractive index.1 3 These advantageous properties of Ge lead to an alternative semiconductor material for its potential use in electronic and optoelectronic applications.4 10 Along with the progressive miniaturization of Si-based electronics, current small-sized elec-tronic platforms meet a signiﬁcant problem while attempting to match up with the constantly increasing requirements for higher speed and throughput.11,12Consequently, the integration of a high-quality Ge layers on Si enhances device performances and helps to continue traditional silicon scaling which motivates numerous researches in the recent years. High-quality Ge on Si continues to remain technologically important for several nota-ble applications.4 10

Although Ge has better advantageous properties than Si and may be included into the Si-based transistor scaling path, the practical use of bulk Ge for semiconductor industry still meets several problems, especially the lattice mismatch (4.17%) be-tween Ge and Si.13 17The large lattice mismatch would lead to the relaxed epitaxial Geﬁlm growth on Si substrate dominating by islanding and misﬁt dislocations, and therefore a thick GexSi1-x

(0 <x <1) buﬀer layer is formed.14 17

For further applications to

be studied, deterministic control of orientation and crystal structure is necessary. As a result, it is important to overcome the mismatch and to grow high quality crystalline Ge epilayers on Si substrate that are much suitable for Si-based transistor integration.

Various approaches have been used to grow epitaxial Gefilms, including chemical vapor deposition (CVD),18,19ion beam sputter deposition,20 a solution method coupled with annealing,21 and several epitaxial growth methods, such as molecular beam epitaxy (MBE),16,22chemical beam epitaxy,23solid-phase (or surfactant-mediated) epitaxy.24,25So far, CVD and MBE have been mainly employed to fabricate Ge films on Si for producing Ge/Si heterodevices. However, both methods are operating under an ultra-high vacuum (UHV) atmosphere, and the growth rate of MBE is too slow for mass production. On the contrary, CVD offers many advantages, such as high throughput, in situ doping and selective deposition, yet hazardous germane (GeH4) and its

derivatives have been widely utilized as Ge precursors in CVD to prepare Geﬁlms.18,19Therefore,ﬁnding a hazardousless and environment-friendly Ge precursor is necessary.

In this work, a wafer-scaled Ge layer with GexSi1 xbuﬀer layer

was epitaxially grown on a Si substrate in one step process via a facile CVD method without UHV condition by thermal decom-position of a safe precursor, germanium(IV) oxide (GeO2),

under a hydrogen (H2) atmosphere.

Received: March 12, 2011 Accepted: May 31, 2011

ABSTRACT:Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Geﬁlms on silicon wafer with a GexSi1 x(0 < x < 1) graded buﬀer layer was demonstrated

through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO2powder under hydrogen

atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diﬀraction anal-ysis shows that the Geﬁlm with an epitaxial relationship is along

the in-plane direction of Si. The successful growth of epitaxial Geﬁlms on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.

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2399 dx.doi.org/10.1021/am200310c |ACS Appl. Mater. Interfaces 2011, 3, 2398–2401

ACS Applied Materials & Interfaces _{RESEARCH ARTICLE}

2. EXPERIMENTAL SECTION

A. Synthesis.The Ge film was obtained by CVD method26in a horizontal hot-wall quartz tube reactor comprised of a pumping system and a reaction chamber heated by a tubular furnace. After RCA cleaning steps, cleaned Si wafers with [100] orientation were used as the substrates. The highly pure GeO2powder was used as the source material

placed in an alumina boat in the high-temperature zone and the cleaned substrates were placed in the low temperature zone to downstream of the reaction chamber. Before heating, the reaction chamber was pumped to a vacuum pressure of 10 3 Torr. Then, the carrier gas, H2, was

introduced into the reaction chamber at a flow rate of 90 sccm (standard cubic centimeters per minute). The temperature was set as 1100C in the high-temperature zone with chamber pressure maintained about 15 20 Torr for different durations (1.5, 3, and 6 h).

B. Characterization.The as-synthesized samples were studied by X-ray diffraction (Bruker D8) using Cu KR radiation, and ϕ scan

measurements were performed with a four-circle diffractometer (Huber, D-8219 Rimsting). The micro-Raman system (Lab Raman HR800, Jobin Yvon) was using a He Ne laser with a wavelength of 632.8 nm. The surface morphology and crystallinity of the as-synthe-sized samples were observed under a scanning electron microscope (SEM, JEOL 6500F) and a transmission electron microscope (TEM, JEOL 2100F), respectively. The chemical compositions of the pro-ducts were analyzed by energy-dispersive spectroscopy (EDS) that is equipped on SEM and TEM. The Auger depth profile was obtained on a VG Scientific Microlab 350.

3. RESULTS AND DISCUSSION

A silver-grayﬁlm was obtained on the Si (100) substrate at the downstream region of the thermal CVD in one step process by using GeO2powder as a precursor under H2atmosphere in a

tubular furnace. The coverage of this silver-grayﬁlm was about

16 cm, corresponding to a temperature range of 1000 920C, and the SEM/EDS analysis indicates the composition of this silver-gray film was only Ge. From the cross-sectional SEM investigation, the thickness of thisfilm gradually decreased from 10μm to 0.2 μm from upstream to downstream side. In addition to that, the cross-sectional SEM image shown in Figure 1a shows an undulated interface between Gefilm and Si substrate. Further, the Gefilm grown on Si(100) substrate was identified by a micro-Raman system shown in Figure 1b. A sharp peak appeared at around 300 cm-1 is consistent with a Ge Ge bond stretching vibration; in addition, a very weak peak appeared at 390 cm-1 could be assigned to Ge Si bond stretching.27Further, Auger depth analysis of the Gefilm grown on Si substrate indicated that thefilm grown on Si substrate contained several micrometer high purity Ge layer on a gradient GexSi1 x(0 < x <1) layer shown in

Figure 1c.

Further, the structural properties of the Gefilm were char-acterized by XRD. Figure 2a shows the typicalθ 2θ scan XRD profile of the Ge film deposited on Si(100) substrate. Two sharp peaks appeared at around 2θ = 69 and 66 were assigned to the (400) planes of the Si(100) substrate (JCPDS card no. 75-0589) and (400) planes of cubic Ge (JCPDS card no. 89-2768), respectively. Usually, the singular sharp XRD signal crystal resulted from the superiorly preferred orientation growth. There-fore, the normal of the Gefilm has a [100] orientation parallels to the [100] of the underneath Si(100) substrate. In addition, the full width at half maximum (FWHM) value of the (400) rocking curve of Ge is 0.15 (Figure 2b, a FWHM value of the (400) rocking curve for Si substrate is 0.03), which strongly supports the good crystalline quality of the Ge layer. The growth direction of the germaniumfilm was further studied by ϕ scan profile, as shown in Figure 2c. Theϕ scan profile for the off-normal (111) Figure 1. (a) Cross-sectional SEM image of the silver-grayfilm on silicon substrate. (b) Raman spectrum for the as-grown film on silicon substrate; (c) Auger depth analysis for the as-grownfilm with the thickness of about 5 μm grown on Si substrate, and the as-grown film has 4 μm high purity Ge layer on a 1μm gradient GexSi1 xlayer.

Figure 2. (a) XRDθ 2θ scan profile of the as-grown Ge film on Si(100) substrate, the range of 65 70 was magnified and shown in the inset; (b) rocking curve of the Ge(400) reflection; (c) XRD ϕ scan profiles for the off-normal (111) plane of the Ge film and Si(111).

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planes of the Ge film shows a four-fold azimuthal symmetry superposed that of Si(100) substrate indicates the in-plane orientation of the Gefilm and parallels to silicon substrate. Based on these XRD results indicate that the silver-gray Ge film epitaxially grew on Si(100) substrate through the simple CVD method in one step. Further experiments were carried out by using Si(110) and Si(111) as substrates, and the resulting Ge films also show an epitaxial behavior as shown in Figure S1 in the Supporting Information.

In addition, Figure 3a represents the cross-sectional TEM investigation from the interface between the grown thinfilm and Si substrate. The dislocation induced contrast is visible at the interface. Further, Figure 3b shows a high-resolution TEM image from the circled region in Figure 3a, the dislocations were clearly indicated as arrows. TEM/EDS elemental line profiles (Figure S2 in the Supporting Information) of the as-grownfilm on silicon substrate shows a gradient change from the buffer GexSi1 xlayer.

Moreover, the Ge epilayers grown on Si substrate for different deposition time were observed by SEM (Figure S3 in the Supporting Information). When the growth time was 1.5 h, many holes formed on the silicon surface, whereas when the growth time increased to 3 h, some of the holes werefilled up by germanium, and many islands formed on surface instead. As the growth time was prolonged to 6 h, the Gefilm get thicker and the islands become larger. Based on the above investigations, the fundamental growth route was suggested as follows: GeO2

(melting point: 1086C) was initially reduced by H2in the high

temperature regions (1100C) to form Ge, GeOy(0 < y <2) and

H2O, and then subsequently ﬂowed to the lower temperature

region to react with the silicon substrate. H2O and GeOycould be

the etching reagent resulting for holes formation, whereas Ge causes the deposition. Moreover, while surface silicons were oxidized by GeOy, Ge (in GeOy) would take the place of Si,

resulting in the formation of Ge layers on silicon. GexSi1 x

graded layer formed for the accommodation of the large lattice mismatch between Si and Ge,15 17 Following the Ge layer formed above the buﬀer layer by precipitating Ge which was formed by reaction of GeO2and H2. The dislocations appeared

at the interface between GexSi1 x layer and Si substrate may

account for the mismatch induced stress relieving by dislocation networks (Figure 3a).

4. CONCLUSION

In summary, a virtual Ge template composed of Ge epilayer with a GexSi1 xbuﬀer layer on Si substrate can be obtained in

one step process via CVD method without UHV condition by a thermal decomposition of a hazardousless GeO2powder under a

H2atmosphere. Structural analyses show high-crystallinityﬁlms

with preferential orientation and thickness up to several micro-meters could be produced. The successful growth of epitaxial Ge ﬁlms on Si demonstrates the feasibility of integrating various functional devices on the Ge/Si templates.

’ ASSOCIATED CONTENT

b

S Supporting Information. Figure S1, XRD θ 2θ scan profiles of Ge films grew on Si(110), and Si(111) substrates; Figure S2, TEM and EDS elemental line profiles of the as-grown film on silicon substrate; Figure S3, Ge epilayers grown on Si substrate for different deposition time. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

Corresponding Author *E-mail: cylee@mx.nthu.edu.tw.

’ ACKNOWLEDGMENT

The authors thank Miss Shalini Jayakumar for grammatical editing, Miss Ting-Hsun Chang for the experimental help, and the National Science Council of the Republic of China, Taiwan, forﬁnancially supporting this research under Contract NSC-96-2113-M-007-021-MY3, NSC-97-2113-M-009-015-MY3, and NSC-99-2113-M-007-011.

’ REFERENCES

(1) Streetman, B.; Banerjee, S. Solid State Electronic Devices, 6th ed.; Prentice Hall: Upper Saddle River, NJ, 2005.

(2) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; Wiley: New York, 2007.

(3) Colace, L.; Masini, G.; Assanto, G. IEEE J. Quantum Electron. 1999, 35, 1843–1852.

(4) Nayfeh, A.; Chui, C. O.; Yonehara, T.; Saraswat, K. C. IEEE Electron. Device Lett. 2005, 26, 311–313.

(5) Dey, S.; Joshi, S.; Garcia-Gutierrez, D.; Chaumont, M.; Campion, A.; Jose-Yacaman, M.; Banerjee, S. K. J. Electron Mater. 2006, 35, 1607–1612.

(6) Lubyshev, D.; Fastenau, J. M.; Wu, Y.; Liu, W. K.; Bulsara, M. T.; Fitzgerald, E. A.; Hoke, W. E. J. Vac. Sci. Technol., B 2008, 26, 1115–1119. (7) Feng, D. Z.; Liao, S. R.; Dong, P.; Feng, N. N.; Liang, H.; Zheng, D. W.; Kung, C. C.; Fong, J.; Shaﬁiha, R.; Cunningham, J.; Krishnamoorthy, A. V.; Asghari, M. Appl. Phys. Lett. 2009, 95, 261105.

(8) Luan, H. C.; Wada, K.; Kimerling, L. C.; Masini, G.; Colace, L.; Assanto, G. Opt. Mater. 2001, 17, 71–73.

(9) Leonhardt, D.; Sheng, J.; Cederberg, J. G.; Li, Q. M.; Carroll, M. S.; Han, S. M. Thin Solid Films 2010, 518, 5920–5927.

(10) Beeler, R.; Mathews, J.; Weng, C.; Tolle, J.; Roucka, R.; Chizmeshya, A. V. G.; Juday, R.; Bagchi, S.; Menendez, J.; Kouvetakis, J. Sol. Energ. Mat. Sol. C 2010, 94, 2362–2370.

(11) Theis, T. N.; Solomon, P. M. Science 2010, 327, 1600–1601. (12) Lundstrom, M. Science 2003, 299, 210–211.

(13) Silvestri, H. H.; Bracht, H.; Hansen, J. L.; Larsen, A. N.; Haller, E. E. Semicond. Sci. Technol. 2006, 21, 758–762.

(14) Bean, J. C. Science 1985, 230, 127–131.

(15) Samavedam, S. B.; Fitzgerald, E. A. J. Appl. Phys. 1997, 81, 3108–3116.

(16) Herman, M. A. Cryst. Res. Technol. 1999, 34, 583–595. (17) Kahng, S. J.; Ha, Y. H.; Moon, D. W.; Kuk, Y. J. Vac. Sci. Technol., A 2000, 18, 1937–1940.

(18) Mukherjee, C.; Seitz, H.; Schroder, B. Appl. Phys. Lett. 2001, 78, 3457–3459.

(19) Kobayashi, S.; Cheng, M. L.; Kohlhase, A.; Sato, T.; Murota, J.; Mikoshoba, N. J. Cryst. Grwoth 1990, 99, 259–262.

(20) Tomasch, G. A.; Kim, Y. W.; Markert, L. C.; Lee, N. E.; Greene, J. E. Thin Solid Films 1993, 223, 212–217.

(21) Zou, G. F.; Luo, H. M.; Ronning, F.; Sun, B. Q.; McCleskey, T. M.; Burrell, A. K.; Bauer, E.; Jia, Q. X. Angew. Chem. Int. Ed. 2010, 49, 1782–1785.

Figure 3. Interface between the as-grown ﬁlm and silicon substrate: (a) typical TEM image; (b) HR-TEM image form the circled region in (a).

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(22) Liu, J.; Kim, H. J.; Hul’ko, O.; Xie, Y. H.; Sahni, S.; Bandaru, P.; Yablonovitch, E. J. Appl. Phys. 2004, 96, 916–918.

(23) Eres, D.; Lowndes, D. H.; Tischler, J. Z. Appl. Phys. Lett. 1989, 55, 1008–1010.

(24) Lu, G. Q.; Nygren, E.; Aziz, M. J.; Turnbull, D.; White, C. W. Appl. Phys. Lett. 1990, 56, 137–139.

(25) Wietler, T. F.; Bugiel, E.; Hofmann, K. R. Thin Solid Films 2006, 508, 6–9.

(26) Wu, H. C.; Hou, T. C.; Chueh, Y. L.; Chen, L. J.; Chiu, H. T.; Lee, C. Y. Nanotechnology 2010, 21, 455601.

(27) Baranov, A. V.; Fedorov, A. V.; Perova, T. S.; Moore, R. A.; Solosin, S.; Yam, V.; Bouchier, D.; Thanh, V. L. J. Appl. Phys. 2004, 96, 2857–2863.