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Si quantum well heterostructure
Applied Surface Science 182 (2001) 361±365
Characteristics of UHVCVD grown Si=Si1 quantum well heterostructure
x y Gex Cy =Si
S.K. Raya,*, G.S. Kara, S.K. Banerjeeb a b
Department of Physics and Meteorology, IIT Kharagpur, Kharagpur 721302, India Microelectronics Research Center, The University of Texas, Austin, TX 78712, USA
Abstract Single well strained-layer Si1 x y Gex Cy heterostructures have been grown by ultra-high vacuum chemical vapor deposition. The effect of addition of C on strain and vibrational characteristics of Si1 x Gex y 0 layer has been studied. The results of the carrier con®nement characteristics, device transconductance and optical transitions in a Si/SiGeC/Si quantum well are presented. # 2001 Published by Elsevier Science B.V. Keywords: Heterostructure; Chemical vapor deposition; Transconductance; Quantum well
1. Introduction Lattice strain in silicon alloy heterostructure ®lms is found to have a signi®cant effect on the band structure and carrier transport. Type-I band alignment in a Si/ SiGe/Si heterostructure results in the con®nement of holes in the valence band quantum well that is useful for the fabrication of two-dimensional hole gas (2DHG) devices [1±3]. This leads to dominant carrier transport in the high mobility strained channel layer with reduced scattering from surface roughness and oxide charges. However, the thermal stability of pseudomorphic Si1 x Gex layer has imposed several restrictions on the device structures limiting the applications only for low Ge fractions, thinner active layers and relatively lower process temperature windows. Incorporation of smaller-sized carbon atoms substitutionally into the SiGe lattice enables one to compensate the strain leading to increased thermal stability, which * Corresponding author. Tel.: 91-3222-55221, ext: 4924; fax: 91-3222-55303. E-mail address: [email protected] (S.K. Ray).
offers a new degree of freedom in the band gap, band offset and strain engineering of Si-based materials. In this paper, we report the growth and characteristics of single quantum well Si1 x y Gex Cy heterostructures grown by ultra-high vacuum chemical vapor deposition. The effect of addition of C on lattice strain, vibrational modes, optical transitions and transconductance of the Si1 x Gex heterostructures are reported. 2. Experiment Si1 x y Gex Cy ®lms were grown in an UHVCVD (base pressure 5 10 10 Torr) reactor [4] using ultrahigh purity gases of Si2H6, GeH4 (10% diluted in He), CH3SiH3 (20% diluted in He) and B2H6 as sources of Si, Ge, C and B, respectively. Epitaxial layers of SiGe/ SiGeC alloys of thickness 38±40 nm were grown at a temperature of 5008C on epitaxial Si buffer of 50 nm thick at reactant pressures of 5 10 10 3 Torr. A silicon cap layer of thickness 7 1 nm was grown on top of the active SiGe/SiGeC layers to form the valence band well ¯anked by Si barrier layer on both
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 4 9 - 4
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S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
the sides. SIMS analysis was employed to ®nd out the doping concentration and the compositional pro®le of UHVCVD grown Si1 x Gex =Si1 x y Gex Cy samples. Boron doping concentration was found to vary from 1.5 to 2:0 1017 cm 3 in all the samples. The grown heterostructure was characterized using high-resolution X-ray diffraction (HRXRD), photoluminescence spectra and Raman spectroscopy. HRXRD o±2y scans were taken around the symmetric (0 0 4) and antisymmetric (2 2 4) reciprocal lattice points using a Philips diffractometer to estimate the degree of strain in the sample. Raman spectroscopy using Ar laser of wavelength 488 nm was carried out at room temperature to study the vibrational modes of different bonds in strain-compensated lattices. The spectra were recorded using a SPEX Raman spectrometer equipped with a double monochromator and a multichannel detector. The transconductance of SiGe and SiGeC channel PMOSFET devices were measured to study the hole transport characteristics in the SiGe/SiGeC well away from the Si±SiO2 interface. 3. Results and discussion The schematic cross-section of the grown heterostructure is shown in Fig. 1. In such a structure, the Si1 x Gex =Si1 x y Gex Cy constitutes the sub-surface quantum well channel for holes buried below the Si cap. The energy band diagram (not to the scale) of the
Fig. 1. Schematic diagram of a Si=Si1
x y Gex Cy =Si
heterostructure shown along with the schematic depicts that most of the carriers will be con®ned within the well region at a low gate bias. The valence band discontinuities (DEv) at the heterojunction play a vital role for carrier transport. It has been shown by our group [5] that, for a given mismatch to silicon, the band offset values for the SiGeC samples are larger than those for strained-SiGe, making SiGeC more attractive for CMOS applications. Typical DCXRD (0 0 4) rocking curves of UHVCVD grown Si1 x Gex and Si1 x Gex Cy heterostructures are shown in Fig. 2. The increased perpendicular lattice constant in a compressively strained Si0.8Ge0.2 well (curve a) causes a shift of the X-ray peak by Dy 2119:6 arcsec with respect to the substrate (0 0 4) peak. Adding carbon into substitutional sites within the Si1 x Gex lattice compensates the strain. The X-ray data in Fig. 2(b) and (c) show a shift of the diffraction peak towards the substrate Si peak on incorporation of C, indicating partial strain compensation depending on the amount of carbon in the alloy layers. Carbon in the Si0:8 y Ge0:2 Cy alloy reduces the compressive strain from E 0:96% for y 0 (fully strained SiGe) to E 0:7% for y 0:005 and to E 0:5% for y 0:01. The absence of a Si-cap layer in Si0.79Ge0.2C0.01 sample (Fig. 2(d)) results in a slight reduction of strain to E 0:49%. Fig. 3 shows the typical photoluminescence (PL) spectrum for a sample with 20% Ge and 1% C. The Si related no-phonon (NP) and transverse optical (TO)
SQW structure along with the energy band diagram.
S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
363
Fig. 2. High resolution X-ray diffraction spectra of UHVCVD grown Si1 x y Gex Cy layers: (a) Si0.8Ge0.2; (b) Si0.795Ge0.2C0.005; (c) Si0.79Ge0.2C0.01 and (d) uncapped Si0.79Ge0.2C0.01.
Fig. 3. PL spectra from a Si/Si0.79Ge0.2C0.01/Si SQW at T 12 K.
lines, the PL duplet from the SiGeC well, occur at 1.055 and 1.025 eV, respectively. Two peaks of single quantum well (SQW) of SiGeC emit NP and TO PL lines of comparable intensity. They are attributed to spatially direct transitions of excitons con®ned to the alloy layers. Raman spectroscopy has been performed at room temperature in the backscattering con®guration. Information on the local atomic arrangement can be obtained from a study of the individual Raman peak [6,7]. Fig. 4 shows representative Raman spectra for Si0.8Ge0.2 and Si0.685Ge0.3C0.015 samples. The spectra show the familiar Ge±Ge (301.3 cm 1), Si±Ge (408.5 and 434.25 cm 1) and Si±Si (520.4 cm 1) modes observed in Si1 x Gex alloys. An additional Raman feature near 610 cm 1 in Fig. 4(b), assigned to the localized vibrational mode of C in Si, is observed in the Si0.685Ge0.3C0.015 sample. The absence of Si±C vibrations at 780 cm 1 indicates the incorporation of C in substitutional sites. The spectra in inset of Fig. 4 shows the recorded and the deconvoluted Si±Si Raman peak of Si0.8Ge0.2 and Si0.685Ge0.3C0.015 ®lms. The
Si±Si mode has been deconvoluted by Gaussian ®tting to two constituent peaks attributed to the substrate and strained alloy quantum well. As shown in the ®gure, the summation of individual curves ®ts very well with the original spectrum. The deconvoluted Si substrate peak has been used as a reference to determine the frequency shifts in the binary and ternary alloys. The Si±Si Raman peaks from wells are shifted from the bulk silicon position by 8.0 and 13.86 cm 1 for Si0.8Ge0.2 and Si0.685Ge0.3C0.015 samples, respectively. Though a close agreement with the theoretical prediction [8] is found for fully strained (SiGe) quantum well, the discrepancy is observed for C incorporated partially strain compensated sample. Numerical simulation of Poisson's equation of SiGe heterostructure PMOSFET has been performed using MEDICI [9]. Fig. 5 presents the typical hole density distribution in the Si-cap and SiGe well of the heterostructure shown in Fig. 1. At a low gate bias, the dominant carrier transport is observed in the strained alloy layer due to hole con®nement in the valence band quantum well. At a higher gate bias, the carrier is
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S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
Fig. 5. Variation of hole concentration in Si-cap layer and active SiGe well as a function of gate bias at room temperature.
Fig. 4. Raman spectrum of: (a) fully strained Si0.8Ge0.2 and (b) partially strain compensated Si0.685Ge0.3C0.015 ®lms. Si±Si Raman peaks along with the deconvoluted spectra are shown in the inset.
distributed in both the well and Si-cap (barrier) layer. The relative carrier density in the well determines the device transconductance and hence the drive current of the heterostructure MOSFET. PMOSFET device transconductance has been measured to compare the hole transport characteristics in Si0.8Ge0.2 and Si0.793Ge0.2C0.007 wells. The results for PMOSFETs in the linear region with channel length of 10 mm and width of 10 mm are presented in Fig. 6. For the same Ge concentration, the peak transconductance is found to be higher in partially strain compensated ternary alloy as compared to that in fully strained binary alloy. This is attributed to improved thermal stability of the ternary alloy resulting in reduced process induced strain relaxation. The transconductance for the SiGeC PMOSFET is found to decrease
Fig. 6. Linear transconductance characteristics for 10 10 mm2 heterostructure PMOSFETs. Results for bulk-Si (control-Si) device is presented for comparison.
S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
more rapidly at higher transverse ®eld as compared to control-Si and SiGe devices. This suggests that at strong inversion when the silicon surface channel turns on, the holes in the ternary alloy device undergo higher interface and/or surface roughness scattering. 4. Conclusion Strained Si1 x Gex and Si1 x y Gex Cy SQW heterostructures have been grown by UHVCVD method. Strained wells have been characterized by X-ray diffraction, PL and Raman spectroscopy. Hole con®nement in the valence band well is observed by Poisson simulation of the heterostructure FET. Transconductance of the ternary well device is found to be enhanced as compared to SiGe device due to C induced strain compensation.
365
References [1] S. Verdonckt-Vandebroek, E.F. CrabbeÇ, B.S. Meyerson, D.L. Harame, P.J. Restle, J.M.C. Stork, J.B. Johnson, IEEE Trans. Electron. Dev. 41 (1994) 90. [2] M.Y.A. Yousif, O. Nur, O. Chretien, Y. Fu, M. Willander, Solid-State Electron. 42 (1998) 951. [3] T. Manku, A. Nathan, Can. J. Phys. 70 (1992) 959. [4] S. John, E.J. Quinones, B. Ferguson, S.K. Ray, B. Anantharam, S. Middlebrooks, C.B. Mullins, J. Ekerdt, J. Rawlings, S.K. Banerjee, J. Electrochem. Soc. 146 (1999) 4611. [5] S. Maikap, S.K. Ray, S. John, S.K. Banerjee, C.K. Maiti, Semicond. Sci. Technol. 15 (2000) 761. [6] S.C. Jain, H.J. Osten, B. Dietrich, H. Rucker, Semicond. Sci. Technol. 10 (1995) 1289. [7] L.V. Kulik, C. Guedj, M.W. Dashiell, J. Kolodzey, A. Hairie, Phys. Rev. B 59 (1999) 15753. [8] J. Menendez, P. Gopalan, G.S. Spencer, N. Cave, J. Strane, Appl. Phys. Lett. 66 (1995) 1160. [9] E.J. Quinones, S. John, S.K. Ray, S.K. Banerjee, IEEE Trans. Electron. Dev. 47 (2000) 1715.
Characteristics of UHVCVD grown Si=Si1 quantum well heterostructure
x y Gex Cy =Si
S.K. Raya,*, G.S. Kara, S.K. Banerjeeb a b
Department of Physics and Meteorology, IIT Kharagpur, Kharagpur 721302, India Microelectronics Research Center, The University of Texas, Austin, TX 78712, USA
Abstract Single well strained-layer Si1 x y Gex Cy heterostructures have been grown by ultra-high vacuum chemical vapor deposition. The effect of addition of C on strain and vibrational characteristics of Si1 x Gex y 0 layer has been studied. The results of the carrier con®nement characteristics, device transconductance and optical transitions in a Si/SiGeC/Si quantum well are presented. # 2001 Published by Elsevier Science B.V. Keywords: Heterostructure; Chemical vapor deposition; Transconductance; Quantum well
1. Introduction Lattice strain in silicon alloy heterostructure ®lms is found to have a signi®cant effect on the band structure and carrier transport. Type-I band alignment in a Si/ SiGe/Si heterostructure results in the con®nement of holes in the valence band quantum well that is useful for the fabrication of two-dimensional hole gas (2DHG) devices [1±3]. This leads to dominant carrier transport in the high mobility strained channel layer with reduced scattering from surface roughness and oxide charges. However, the thermal stability of pseudomorphic Si1 x Gex layer has imposed several restrictions on the device structures limiting the applications only for low Ge fractions, thinner active layers and relatively lower process temperature windows. Incorporation of smaller-sized carbon atoms substitutionally into the SiGe lattice enables one to compensate the strain leading to increased thermal stability, which * Corresponding author. Tel.: 91-3222-55221, ext: 4924; fax: 91-3222-55303. E-mail address: [email protected] (S.K. Ray).
offers a new degree of freedom in the band gap, band offset and strain engineering of Si-based materials. In this paper, we report the growth and characteristics of single quantum well Si1 x y Gex Cy heterostructures grown by ultra-high vacuum chemical vapor deposition. The effect of addition of C on lattice strain, vibrational modes, optical transitions and transconductance of the Si1 x Gex heterostructures are reported. 2. Experiment Si1 x y Gex Cy ®lms were grown in an UHVCVD (base pressure 5 10 10 Torr) reactor [4] using ultrahigh purity gases of Si2H6, GeH4 (10% diluted in He), CH3SiH3 (20% diluted in He) and B2H6 as sources of Si, Ge, C and B, respectively. Epitaxial layers of SiGe/ SiGeC alloys of thickness 38±40 nm were grown at a temperature of 5008C on epitaxial Si buffer of 50 nm thick at reactant pressures of 5 10 10 3 Torr. A silicon cap layer of thickness 7 1 nm was grown on top of the active SiGe/SiGeC layers to form the valence band well ¯anked by Si barrier layer on both
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 4 9 - 4
362
S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
the sides. SIMS analysis was employed to ®nd out the doping concentration and the compositional pro®le of UHVCVD grown Si1 x Gex =Si1 x y Gex Cy samples. Boron doping concentration was found to vary from 1.5 to 2:0 1017 cm 3 in all the samples. The grown heterostructure was characterized using high-resolution X-ray diffraction (HRXRD), photoluminescence spectra and Raman spectroscopy. HRXRD o±2y scans were taken around the symmetric (0 0 4) and antisymmetric (2 2 4) reciprocal lattice points using a Philips diffractometer to estimate the degree of strain in the sample. Raman spectroscopy using Ar laser of wavelength 488 nm was carried out at room temperature to study the vibrational modes of different bonds in strain-compensated lattices. The spectra were recorded using a SPEX Raman spectrometer equipped with a double monochromator and a multichannel detector. The transconductance of SiGe and SiGeC channel PMOSFET devices were measured to study the hole transport characteristics in the SiGe/SiGeC well away from the Si±SiO2 interface. 3. Results and discussion The schematic cross-section of the grown heterostructure is shown in Fig. 1. In such a structure, the Si1 x Gex =Si1 x y Gex Cy constitutes the sub-surface quantum well channel for holes buried below the Si cap. The energy band diagram (not to the scale) of the
Fig. 1. Schematic diagram of a Si=Si1
x y Gex Cy =Si
heterostructure shown along with the schematic depicts that most of the carriers will be con®ned within the well region at a low gate bias. The valence band discontinuities (DEv) at the heterojunction play a vital role for carrier transport. It has been shown by our group [5] that, for a given mismatch to silicon, the band offset values for the SiGeC samples are larger than those for strained-SiGe, making SiGeC more attractive for CMOS applications. Typical DCXRD (0 0 4) rocking curves of UHVCVD grown Si1 x Gex and Si1 x Gex Cy heterostructures are shown in Fig. 2. The increased perpendicular lattice constant in a compressively strained Si0.8Ge0.2 well (curve a) causes a shift of the X-ray peak by Dy 2119:6 arcsec with respect to the substrate (0 0 4) peak. Adding carbon into substitutional sites within the Si1 x Gex lattice compensates the strain. The X-ray data in Fig. 2(b) and (c) show a shift of the diffraction peak towards the substrate Si peak on incorporation of C, indicating partial strain compensation depending on the amount of carbon in the alloy layers. Carbon in the Si0:8 y Ge0:2 Cy alloy reduces the compressive strain from E 0:96% for y 0 (fully strained SiGe) to E 0:7% for y 0:005 and to E 0:5% for y 0:01. The absence of a Si-cap layer in Si0.79Ge0.2C0.01 sample (Fig. 2(d)) results in a slight reduction of strain to E 0:49%. Fig. 3 shows the typical photoluminescence (PL) spectrum for a sample with 20% Ge and 1% C. The Si related no-phonon (NP) and transverse optical (TO)
SQW structure along with the energy band diagram.
S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
363
Fig. 2. High resolution X-ray diffraction spectra of UHVCVD grown Si1 x y Gex Cy layers: (a) Si0.8Ge0.2; (b) Si0.795Ge0.2C0.005; (c) Si0.79Ge0.2C0.01 and (d) uncapped Si0.79Ge0.2C0.01.
Fig. 3. PL spectra from a Si/Si0.79Ge0.2C0.01/Si SQW at T 12 K.
lines, the PL duplet from the SiGeC well, occur at 1.055 and 1.025 eV, respectively. Two peaks of single quantum well (SQW) of SiGeC emit NP and TO PL lines of comparable intensity. They are attributed to spatially direct transitions of excitons con®ned to the alloy layers. Raman spectroscopy has been performed at room temperature in the backscattering con®guration. Information on the local atomic arrangement can be obtained from a study of the individual Raman peak [6,7]. Fig. 4 shows representative Raman spectra for Si0.8Ge0.2 and Si0.685Ge0.3C0.015 samples. The spectra show the familiar Ge±Ge (301.3 cm 1), Si±Ge (408.5 and 434.25 cm 1) and Si±Si (520.4 cm 1) modes observed in Si1 x Gex alloys. An additional Raman feature near 610 cm 1 in Fig. 4(b), assigned to the localized vibrational mode of C in Si, is observed in the Si0.685Ge0.3C0.015 sample. The absence of Si±C vibrations at 780 cm 1 indicates the incorporation of C in substitutional sites. The spectra in inset of Fig. 4 shows the recorded and the deconvoluted Si±Si Raman peak of Si0.8Ge0.2 and Si0.685Ge0.3C0.015 ®lms. The
Si±Si mode has been deconvoluted by Gaussian ®tting to two constituent peaks attributed to the substrate and strained alloy quantum well. As shown in the ®gure, the summation of individual curves ®ts very well with the original spectrum. The deconvoluted Si substrate peak has been used as a reference to determine the frequency shifts in the binary and ternary alloys. The Si±Si Raman peaks from wells are shifted from the bulk silicon position by 8.0 and 13.86 cm 1 for Si0.8Ge0.2 and Si0.685Ge0.3C0.015 samples, respectively. Though a close agreement with the theoretical prediction [8] is found for fully strained (SiGe) quantum well, the discrepancy is observed for C incorporated partially strain compensated sample. Numerical simulation of Poisson's equation of SiGe heterostructure PMOSFET has been performed using MEDICI [9]. Fig. 5 presents the typical hole density distribution in the Si-cap and SiGe well of the heterostructure shown in Fig. 1. At a low gate bias, the dominant carrier transport is observed in the strained alloy layer due to hole con®nement in the valence band quantum well. At a higher gate bias, the carrier is
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S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
Fig. 5. Variation of hole concentration in Si-cap layer and active SiGe well as a function of gate bias at room temperature.
Fig. 4. Raman spectrum of: (a) fully strained Si0.8Ge0.2 and (b) partially strain compensated Si0.685Ge0.3C0.015 ®lms. Si±Si Raman peaks along with the deconvoluted spectra are shown in the inset.
distributed in both the well and Si-cap (barrier) layer. The relative carrier density in the well determines the device transconductance and hence the drive current of the heterostructure MOSFET. PMOSFET device transconductance has been measured to compare the hole transport characteristics in Si0.8Ge0.2 and Si0.793Ge0.2C0.007 wells. The results for PMOSFETs in the linear region with channel length of 10 mm and width of 10 mm are presented in Fig. 6. For the same Ge concentration, the peak transconductance is found to be higher in partially strain compensated ternary alloy as compared to that in fully strained binary alloy. This is attributed to improved thermal stability of the ternary alloy resulting in reduced process induced strain relaxation. The transconductance for the SiGeC PMOSFET is found to decrease
Fig. 6. Linear transconductance characteristics for 10 10 mm2 heterostructure PMOSFETs. Results for bulk-Si (control-Si) device is presented for comparison.
S.K. Ray et al. / Applied Surface Science 182 (2001) 361±365
more rapidly at higher transverse ®eld as compared to control-Si and SiGe devices. This suggests that at strong inversion when the silicon surface channel turns on, the holes in the ternary alloy device undergo higher interface and/or surface roughness scattering. 4. Conclusion Strained Si1 x Gex and Si1 x y Gex Cy SQW heterostructures have been grown by UHVCVD method. Strained wells have been characterized by X-ray diffraction, PL and Raman spectroscopy. Hole con®nement in the valence band well is observed by Poisson simulation of the heterostructure FET. Transconductance of the ternary well device is found to be enhanced as compared to SiGe device due to C induced strain compensation.
365
References [1] S. Verdonckt-Vandebroek, E.F. CrabbeÇ, B.S. Meyerson, D.L. Harame, P.J. Restle, J.M.C. Stork, J.B. Johnson, IEEE Trans. Electron. Dev. 41 (1994) 90. [2] M.Y.A. Yousif, O. Nur, O. Chretien, Y. Fu, M. Willander, Solid-State Electron. 42 (1998) 951. [3] T. Manku, A. Nathan, Can. J. Phys. 70 (1992) 959. [4] S. John, E.J. Quinones, B. Ferguson, S.K. Ray, B. Anantharam, S. Middlebrooks, C.B. Mullins, J. Ekerdt, J. Rawlings, S.K. Banerjee, J. Electrochem. Soc. 146 (1999) 4611. [5] S. Maikap, S.K. Ray, S. John, S.K. Banerjee, C.K. Maiti, Semicond. Sci. Technol. 15 (2000) 761. [6] S.C. Jain, H.J. Osten, B. Dietrich, H. Rucker, Semicond. Sci. Technol. 10 (1995) 1289. [7] L.V. Kulik, C. Guedj, M.W. Dashiell, J. Kolodzey, A. Hairie, Phys. Rev. B 59 (1999) 15753. [8] J. Menendez, P. Gopalan, G.S. Spencer, N. Cave, J. Strane, Appl. Phys. Lett. 66 (1995) 1160. [9] E.J. Quinones, S. John, S.K. Ray, S.K. Banerjee, IEEE Trans. Electron. Dev. 47 (2000) 1715.