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Selective epitaxy of InP on Si(1 0 0) substrates prepared by liquid-phase epitaxy
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 411–414 www.elsevier.com/locate/jpcs
Selective epitaxy of InP on Si(1 0 0) substrates prepared by liquid-phase epitaxy Maki Sugaia,, Toshio Kochiyaa, Yutaka Oyamaa, Jun-Ichi Nishizawab a
Department of Materials Science and Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-11-1021, Sendai 980-8579, Japan b Semiconductor Research Institute, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan
Abstract Selective liquid-phase (LPE) epitaxial growth of InP was performed on patterned Si(1 0 0) substrates. The growth temperature was rapidly cooled, then the crystal growth proceeded at constant temperature. As a result, area-selective epitaxy (ASE) and slight epitaxial lateral overgrowth (ELO) layers were achieved in narrow openings and small nuclei were observed in the wide openings. X-ray diffraction results show that the lattice strain in the InP nuclei on the Si substrate was fully relaxed. Huber etching revealed the dislocation-related etch pits in ASE layers, and it is shown that the etch pit density (EPD) is dependent on the length of open seed area. r 2007 Published by Elsevier Ltd. Keywords: A. Semiconductors; A. Thin films; B. Epitaxial growth; C. X-ray diffraction; D. Lattice dynamics
1. Introduction Silicon is widely used for integrated circuits, and it has the advantage of high thermal conductivity availability of large wafers and low cost, while InP is one of the materials for high-speed electronic devices and optical devices in III–V compound semiconductors. In order to utilize these advantages, application of InP on Si heterostructures is very attractive for optoelectronic devices. However, there are serious problems such as a large lattice mismatch (about 8%) and a difference in the thermal expansion coefficient (about 50%). In order to overcome these difficulties, InP growth on Si has been achieved by incorporating buffer layers. However, the buffer layers induced a large number of dislocations and defects into the growth layer, and the diffusion of the buffer layer elements caused impurities. As a countermeasure to these problems, epitaxial lateral overgrowth (ELO) technique shows great promise for growing InP on Si [1]. This remarkable technique can not only reduce the dislocation density [2], but also the strain in the grown layers. For these reasons, and in view of the reduction of defects and lattice strain, it Corresponding author. Tel./fax: +81 22 795 7330.
E-mail addresses: [email protected] (M. Sugai), [email protected] (Y. Oyama). 0022-3697/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2007.07.005
is appropriate that InP layers are directly formed on the Sisubstrate with the ELO technique. In addition, it is expected that the liquid-phase epitaxy (LPE) will reduce defects, because LPE process proceeds under near-thermal equilibrium when compared with other growth methods. In this paper, InP growth on Si(1 0 0) substrate was performed by LPE. The Si substrates with open windows were prepared for the area selective epitaxy (ASE) and ELO. The possibility of InP ASE and ELO was investigated. 2. Experimental details Phosphorous-doped n-type Si(1 0 0) substrates were used, and L-shaped and wide opening patterned SiN masking layers were fabricated. 100–200 nm thick SiN masking layers were deposited using remote-plasma chemical vapor deposition. Then photolithography and wet etching were performed for SiN layers to form the open windows. The lengths of the L-shaped patterns were 300, 500 and 1000 mm, and the width was 30 mm. The wide openings were square in shape with a length of 1.5 mm. These patterns were formed on identical substrates. Before the epitaxy, the Si substrates were degreased, using a boiling H2SO4-based solution and then etched using an HF solution for a few seconds.
ARTICLE IN PRESS M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
412
finish
temperature
start
time Fig. 1. The crystal growth temperature sequence in the present LPE process.
Fig. 4. The surface morphologies of InP ELO layer on an L-shaped patterned Si substrate observed by SEM.
Si substrate
Intensity (a.u)
InP nuclei
31 Fig. 2. The surface morphologies of InP ASE layer on an L-shaped patterned Si substrate observed by SEM.
33 32 -2 (degrees)
34
experimental data simulation (relaxtion = 100%v) Fig. 5. XRD pattern of InP nuclei on a Si substrate.
Fig. 3. SEM micrograph of InP growth nuclei in wide opening patterned Si substrate.
InP was grown in a Pd-diffused H2 atmosphere by LPE. 6N-In was used as a solution, and non-doped InP polycrystal was used as a source. In addition, phosphorous pressure was applied on the In-P solution to control the deviation from the stoichiometric composition. The crystal growth temperature was rapidly lowered by about 2 1C/min during the initial stages of epitaxy to obtain high supersaturation, and then growth temperature was kept constant. The crystal growth started at 650 1C and completed at 610 1C as shown in Fig. 1. The total crystal growth time was 2 h.
ARTICLE IN PRESS M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
The as-grown surface morphologies were observed by scanning electron microscopy (SEM). The structural properties of wide openings were investigated by fivecrystal X-ray diffraction (XRD). The etch pit density (EPD) of InP ASE layers in the L-shaped openings were investigated by defect-etching using an HBr:H3PO4 ¼ 1:2 solution (Huber etching). 3. Results and discussions
Intensity (a.u)
Figs. 2 and 3 show the as-grown surface morphologies. The results represent that ASE layers were achieved in narrow openings with L-shaped patterns (Fig. 2) and only small nuclei were observed in the wide openings (Fig. 3). These differences may be induced by the lattice strain between InP and Si. That is, this can be prospected that the
34.2
34.3
34.4
-2 (degrees) opening under masking layer Si substrate Fig. 6. XRD patterns from: (a) the wide openings, (b) the SiN masking layers, and (c) Si substrate before epitaxy.
413
lattice strain increases as growth area increases in the case of heteroepitaxy. Thus it is explained that quasi-twodimensional ASE layers were formed due to the reduction of lattice strain in narrow openings without the formation of isolated small nuclei which is quasi-three-dimensional growth mode. As shown in Fig. 2, the top surface of the ASE layer was not flat; in other words, the grown surface was surrounded by some inclined facets. It is considered that the vertical growth rate in the [1 0 0] direction is higher than that of sidewall orientations. And it was also shown that slight ELO layers with about 3.2 mm width were observed on the SiN masking layer (Fig. 4). XRD was measured for the InP growth nuclei in the wide opening area. Diffraction peaks due to the Si substrate and the InP nuclei can be observed (Fig. 5). From the simulation results, the lattice strain in the InP nuclei on the Si substrate was fully relaxed. In addition, XRD of the Si substrate in the wide openings after epitaxy was measured in comparison with the area under the SiN masking layers after epitaxy, and the Si substrate before epitaxy (Fig. 6). It is shown that the symmetric Si XRD peak was observed at the higher diffraction angle in wide openings and the SiN masking layers. Here, it is noted that the covalent radius of phosphorous is smaller than that of Si. Thus, it is considered that phosphorus in InP solution diffused into the Si substrate during LPE, and therefore a highly phosphorous-doped n+-Si region was formed at the surface of the Si substrate. Huber etching results of InP ASE layers in the L-shaped pattern were investigated, and clear dislocationrelated deep pits were observed (Fig. 7). It is shown that the EPD is dependent on the length of the open seed region as the width of the open seed area is fixed at 30 mm. As shown in Fig. 7, EPDs of the ASE layers for 500 mm length (a) and 1000 mm length (b) open seed region were estimated to be 9.5 104 and 2.2 105 cm2, respectively. It is shown that the EPD of 1000 mm length ASE is higher when compared with that of the 500 mm length ASE layer. It is considered that the EPD in the longer open seed region is higher because the lattice strain is larger, when the open seed area becomes longer in InP/Si heteroepitaxial system.
Fig. 7. SEM micrographs of InP ASE layers on Si(1 0 0) after Huber etching. (a) 500 mm open seed length region. EPD is 9.5 104 cm2. (b) 1000 mm open seed length region. EPD is 2.2 105 cm2.
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M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
4. Conclusion InP was grown on a Si substrate by the LPE method. Whereas only the small growth nuclei of InP were observed in wide openings, the ASE layers and slight ELO layers around 3.2 mm of InP were formed in the narrow openings with an L-shaped pattern. From the XRD results in the wide openings, the lattice strain of InP nuclei on the Si substrates were fully relaxed and a highly-phosphorousdoped region was formed at the surface of the Si substrate by the diffusion of P atoms during epitaxy. Huber etching results of ASE InP layers revealed that dislocation-related
etch pits were observed, and EPDs increased as the ASE layer is longer, because the longer open seed region made the lattice strain larger. It is concluded that there is a possibility for growing InP ELO layers on Si substrates with narrow openings by an LPE technique. References [1] O. Parillaued, E. Guil-Lafon, B. Gerard, P. Etienne, P. Pribat, Appl. Phys. Lett. 68 (19) (1996) 2654. [2] T. Kochiya, Y. Oyama, T. Kimura, K. Suto, J. Nishizawa, J. Cryst. Growth 281 (2005) 263.
Journal of Physics and Chemistry of Solids 69 (2008) 411–414 www.elsevier.com/locate/jpcs
Selective epitaxy of InP on Si(1 0 0) substrates prepared by liquid-phase epitaxy Maki Sugaia,, Toshio Kochiyaa, Yutaka Oyamaa, Jun-Ichi Nishizawab a
Department of Materials Science and Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-11-1021, Sendai 980-8579, Japan b Semiconductor Research Institute, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan
Abstract Selective liquid-phase (LPE) epitaxial growth of InP was performed on patterned Si(1 0 0) substrates. The growth temperature was rapidly cooled, then the crystal growth proceeded at constant temperature. As a result, area-selective epitaxy (ASE) and slight epitaxial lateral overgrowth (ELO) layers were achieved in narrow openings and small nuclei were observed in the wide openings. X-ray diffraction results show that the lattice strain in the InP nuclei on the Si substrate was fully relaxed. Huber etching revealed the dislocation-related etch pits in ASE layers, and it is shown that the etch pit density (EPD) is dependent on the length of open seed area. r 2007 Published by Elsevier Ltd. Keywords: A. Semiconductors; A. Thin films; B. Epitaxial growth; C. X-ray diffraction; D. Lattice dynamics
1. Introduction Silicon is widely used for integrated circuits, and it has the advantage of high thermal conductivity availability of large wafers and low cost, while InP is one of the materials for high-speed electronic devices and optical devices in III–V compound semiconductors. In order to utilize these advantages, application of InP on Si heterostructures is very attractive for optoelectronic devices. However, there are serious problems such as a large lattice mismatch (about 8%) and a difference in the thermal expansion coefficient (about 50%). In order to overcome these difficulties, InP growth on Si has been achieved by incorporating buffer layers. However, the buffer layers induced a large number of dislocations and defects into the growth layer, and the diffusion of the buffer layer elements caused impurities. As a countermeasure to these problems, epitaxial lateral overgrowth (ELO) technique shows great promise for growing InP on Si [1]. This remarkable technique can not only reduce the dislocation density [2], but also the strain in the grown layers. For these reasons, and in view of the reduction of defects and lattice strain, it Corresponding author. Tel./fax: +81 22 795 7330.
E-mail addresses: [email protected] (M. Sugai), [email protected] (Y. Oyama). 0022-3697/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2007.07.005
is appropriate that InP layers are directly formed on the Sisubstrate with the ELO technique. In addition, it is expected that the liquid-phase epitaxy (LPE) will reduce defects, because LPE process proceeds under near-thermal equilibrium when compared with other growth methods. In this paper, InP growth on Si(1 0 0) substrate was performed by LPE. The Si substrates with open windows were prepared for the area selective epitaxy (ASE) and ELO. The possibility of InP ASE and ELO was investigated. 2. Experimental details Phosphorous-doped n-type Si(1 0 0) substrates were used, and L-shaped and wide opening patterned SiN masking layers were fabricated. 100–200 nm thick SiN masking layers were deposited using remote-plasma chemical vapor deposition. Then photolithography and wet etching were performed for SiN layers to form the open windows. The lengths of the L-shaped patterns were 300, 500 and 1000 mm, and the width was 30 mm. The wide openings were square in shape with a length of 1.5 mm. These patterns were formed on identical substrates. Before the epitaxy, the Si substrates were degreased, using a boiling H2SO4-based solution and then etched using an HF solution for a few seconds.
ARTICLE IN PRESS M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
412
finish
temperature
start
time Fig. 1. The crystal growth temperature sequence in the present LPE process.
Fig. 4. The surface morphologies of InP ELO layer on an L-shaped patterned Si substrate observed by SEM.
Si substrate
Intensity (a.u)
InP nuclei
31 Fig. 2. The surface morphologies of InP ASE layer on an L-shaped patterned Si substrate observed by SEM.
33 32 -2 (degrees)
34
experimental data simulation (relaxtion = 100%v) Fig. 5. XRD pattern of InP nuclei on a Si substrate.
Fig. 3. SEM micrograph of InP growth nuclei in wide opening patterned Si substrate.
InP was grown in a Pd-diffused H2 atmosphere by LPE. 6N-In was used as a solution, and non-doped InP polycrystal was used as a source. In addition, phosphorous pressure was applied on the In-P solution to control the deviation from the stoichiometric composition. The crystal growth temperature was rapidly lowered by about 2 1C/min during the initial stages of epitaxy to obtain high supersaturation, and then growth temperature was kept constant. The crystal growth started at 650 1C and completed at 610 1C as shown in Fig. 1. The total crystal growth time was 2 h.
ARTICLE IN PRESS M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
The as-grown surface morphologies were observed by scanning electron microscopy (SEM). The structural properties of wide openings were investigated by fivecrystal X-ray diffraction (XRD). The etch pit density (EPD) of InP ASE layers in the L-shaped openings were investigated by defect-etching using an HBr:H3PO4 ¼ 1:2 solution (Huber etching). 3. Results and discussions
Intensity (a.u)
Figs. 2 and 3 show the as-grown surface morphologies. The results represent that ASE layers were achieved in narrow openings with L-shaped patterns (Fig. 2) and only small nuclei were observed in the wide openings (Fig. 3). These differences may be induced by the lattice strain between InP and Si. That is, this can be prospected that the
34.2
34.3
34.4
-2 (degrees) opening under masking layer Si substrate Fig. 6. XRD patterns from: (a) the wide openings, (b) the SiN masking layers, and (c) Si substrate before epitaxy.
413
lattice strain increases as growth area increases in the case of heteroepitaxy. Thus it is explained that quasi-twodimensional ASE layers were formed due to the reduction of lattice strain in narrow openings without the formation of isolated small nuclei which is quasi-three-dimensional growth mode. As shown in Fig. 2, the top surface of the ASE layer was not flat; in other words, the grown surface was surrounded by some inclined facets. It is considered that the vertical growth rate in the [1 0 0] direction is higher than that of sidewall orientations. And it was also shown that slight ELO layers with about 3.2 mm width were observed on the SiN masking layer (Fig. 4). XRD was measured for the InP growth nuclei in the wide opening area. Diffraction peaks due to the Si substrate and the InP nuclei can be observed (Fig. 5). From the simulation results, the lattice strain in the InP nuclei on the Si substrate was fully relaxed. In addition, XRD of the Si substrate in the wide openings after epitaxy was measured in comparison with the area under the SiN masking layers after epitaxy, and the Si substrate before epitaxy (Fig. 6). It is shown that the symmetric Si XRD peak was observed at the higher diffraction angle in wide openings and the SiN masking layers. Here, it is noted that the covalent radius of phosphorous is smaller than that of Si. Thus, it is considered that phosphorus in InP solution diffused into the Si substrate during LPE, and therefore a highly phosphorous-doped n+-Si region was formed at the surface of the Si substrate. Huber etching results of InP ASE layers in the L-shaped pattern were investigated, and clear dislocationrelated deep pits were observed (Fig. 7). It is shown that the EPD is dependent on the length of the open seed region as the width of the open seed area is fixed at 30 mm. As shown in Fig. 7, EPDs of the ASE layers for 500 mm length (a) and 1000 mm length (b) open seed region were estimated to be 9.5 104 and 2.2 105 cm2, respectively. It is shown that the EPD of 1000 mm length ASE is higher when compared with that of the 500 mm length ASE layer. It is considered that the EPD in the longer open seed region is higher because the lattice strain is larger, when the open seed area becomes longer in InP/Si heteroepitaxial system.
Fig. 7. SEM micrographs of InP ASE layers on Si(1 0 0) after Huber etching. (a) 500 mm open seed length region. EPD is 9.5 104 cm2. (b) 1000 mm open seed length region. EPD is 2.2 105 cm2.
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M. Sugai et al. / Journal of Physics and Chemistry of Solids 69 (2008) 411–414
4. Conclusion InP was grown on a Si substrate by the LPE method. Whereas only the small growth nuclei of InP were observed in wide openings, the ASE layers and slight ELO layers around 3.2 mm of InP were formed in the narrow openings with an L-shaped pattern. From the XRD results in the wide openings, the lattice strain of InP nuclei on the Si substrates were fully relaxed and a highly-phosphorousdoped region was formed at the surface of the Si substrate by the diffusion of P atoms during epitaxy. Huber etching results of ASE InP layers revealed that dislocation-related
etch pits were observed, and EPDs increased as the ASE layer is longer, because the longer open seed region made the lattice strain larger. It is concluded that there is a possibility for growing InP ELO layers on Si substrates with narrow openings by an LPE technique. References [1] O. Parillaued, E. Guil-Lafon, B. Gerard, P. Etienne, P. Pribat, Appl. Phys. Lett. 68 (19) (1996) 2654. [2] T. Kochiya, Y. Oyama, T. Kimura, K. Suto, J. Nishizawa, J. Cryst. Growth 281 (2005) 263.