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Sputtered Ge-on-Si heteroepitaxial pn junctions: Nanostructure, interface morphology and photoelectrical properties
Microelectronic Engineering 88 (2011) 518–521
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Sputtered Ge-on-Si heteroepitaxial pn junctions: Nanostructure, interface morphology and photoelectrical properties S.M. Pietralunga a,⇑, M. Feré a, M. Lanata a, G. Radnóczi b, F. Misják b, A. Lamperti c, M. Martinelli d, P.M. Ossi e a
CoreCom, via G. Colombo, 81, 20133 Milano, Italy Research Institute for Technical Physics and Materials Science, Konkoly-Thege M. út 29-33, H-1121 Budapest, Hungary c Laboratorio MDM, IMM-CNR, via C. Olivetti 2, 20041 Agrate Brianza (MB), Italy d Politecnico di Milano, Dept. of Electronics and Information, via Ponzio 34-5, 20133 Milano, Italy e Politecnico di Milano, Dept. of Energy, via Ponzio 34-3, 20133 Milano, Italy b
a r t i c l e
i n f o
Article history: Available online 19 October 2010 Keywords: Germanium thin films DC Magnetron Sputtering Ge-on-Si heteroepitaxy TEM Ge optical properties Ge photodetectors
a b s t r a c t Ge thin films are epitaxially grown onto (1 0 0) Si substrates by DC-Pulsed Magnetron Sputtering. Relaxed single crystalline layers, with slightly misoriented domains are identified by XRD, TEM and HREM. Planar defects and threading dislocations are the relevant lattice imperfections. As-deposited Ge films are p-type without the need for intentional doping, even in the absence of grain boundaries. A pronounced flatness in the near IR absorption spectra is evident, in the absence of strong interfacial strain. This could be traced to a bandgap narrowing effect due to intragap states related to defects in the interfacial region. Photoconductive response around k = 1.5 lm is flat and an equivalent responsivity Reff|Vbias = 1V = 1.0088 A/W at k = 1.5 lm has been estimated. DC-Pulsed Magnetron Sputtering is therefore an attractive solution, deserving further development, to build near-infrared C-MOS compatible photodetectors, particularly suitable for low-speed applications. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Germanium films grown onto silicon platforms are an attractive solution to monolithically integrate near-infrared photodetectors in C-MOS compatible photonic integrated circuits [1–8]. Heteroepitaxial pn-doped Ge-on-Si junctions, realized either by CVD or by MBE, are the solutions for high-speed signal photodiodes (PDs) [2–7]. To realize low-speed detectors, less critical technological approaches can be pursued, based on poly-crystalline p-Ge thin films [8], the p-type conductivity being thermodynamically justified as proceeding from grain boundaries and related defects [9–12]. Thus pn heterojunctions can directly be obtained at the interfaces between as-grown poly-Ge films and n-Si substrates. In case of evaporated Ge-on-Si films, this is claimed to be the case [8], despite recent findings showed p-type conductivity in presence of epitaxial growth [13]. By sputtering deposition techniques, GexSi1x (0 < x < 1) thin films can be provided in a wide range of crystallinity [14–16]. By DC-Pulsed Magnetron Sputtering (DC-PMS), epitaxial Ge films have been obtained onto (0 0 1) Si substrates by some of the authors and details on the growth process can be found in [17,18]. Sputtered ⇑ Corresponding author. Present address: PoliCom Fondazione Politecnico di Milano, via G. Colombo, 81, 20133 Milano, Italy. Tel.: +39 02 2399 8924; fax: +39 02 2399 8922. E-mail address: [email protected] (S.M. Pietralunga). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.10.014
Si/Ge junctions are qualified by a perfect epitaxy, by the absence of interfacial silicon oxide and by sub-nanometer roughness. A strongly defective interface region exists, which extends in the bulk of the film for about 50 nm, in agreement with the critical thickness for Ge-on-Si epitaxy [22,23]. The presence of dislocations slightly misorients the domains, without the evidence of grain boundaries [17,18]. Undoped DC-PMS Ge films are p-type, like poly-Ge films, without the occurrence of a poly-crystalline structure and in the absence of significant contamination [18]. Experimental data for the concentration and mobility of majority carriers approach typical values for undoped epitaxial Ge films [5]. In this work we focus on the linear optical properties of Ge-onSi sputtered epitaxial thin films and on the responsivity of pn PDs at k = 1.5 m; the results are correlated with the nanostructure and interface morphology.
2. Experimental Epitaxial Ge thin films were grown onto n-type 6” Si (1 0 0) wafers, at substrate temperatures Tsubstrate > 640 K by a DC-PMS process [17]. Film crystallinity and interface morphology were investigated by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) with a CM 20 TEM at 200 kV and a JEOL 3010 TEM at 300 kV accelerating voltage. X-ray diffraction (XRD) data were collected on a PANalytical X’Pert
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
PRO MRD diffractometer equipped with standard X-ray beam, monochromated at CuKa (0.154 nm) by a four-bounce hybrid monochromator and sized down to 1 mm 1 mm by beam slits. The diffracted intensity is collected by PIXcel detector. Measurements were collected in Bragg–Brentano geometry. Reciprocal space maps around Ge (0 0 4) reflection were also collected. The linear optical properties of sputtered film samples between k = 300 and 1700 nm were measured by variable angle spectroscopic ellipsometry (VASE) [19]. The dispersion of the real and imaginary parts of the refractive index n = n + ik for the Ge films have been obtained by fitting experimental data to specifically developed oscillator models for the Si–Ge bilayer [20,21]. Heterojunction Ge/Si test mesa PDs have been realized, by patterning 120 nm-thick crystalline Ge films and by evaporating Ti– Au contacts. Multiple series of 12 squared and 12 circular mesas, with areas from 0.03 to 8.04 mm2, were replicated at different positions on the 6” wafers. PDs were backside-illuminated from the Si substrate by beams of FWHM = 110 lm each, from a pigtailed tunable semiconductor laser, spanning the third window of optical communications at k = 1.5 lm and effectively illuminating only the center of the mesas. The optical power impinging on the Si substrate is Popt = 48.5 mW. 3. Results and discussion Fig. 1 is a HREM image of the epitaxial Si/Ge interface, with the (0 0 1) planes parallel to the interface. In the corresponding electron diffraction pattern, in the inset, the (2 2 0) Si and Ge reflections are evident. This allows estimating any residual strain in the Si/Ge system by measuring the distance between them. The position of the peaks marks a difference 0.008 ± 0.0010 nm between (2 2 0) lattice plane distances of Si and Ge. The corresponding difference in the bulks is 0.008 nm, meaning that, within the measurement error, the Ge layer is relaxed and its lattice parameter corresponds to the bulk value. Further confirmation of the Si/Ge
Fig. 1. HREM image of the Si/Ge system. Epitaxial growth is evident. At the interface a defective structure also appears. In the inset the diffraction pattern from the imaged area allows for the revealing of Si and Ge (2 2 0) reflections.
519
relaxation comes from the local measurement of the spacing of the lattice fringes across the Si/Ge interface from HREM image. In bulk Si and Ge, 23 times of a Ge spacing equal 24 times a Si spacing, for the same family of lattice planes. Fig. 2(a) shows an image of the Si/Ge interface where such a measurement has been carried out. Fig. 2(b) shows the intensity distribution of (1 1 1) planes across the interface. The total distance measured by considering 24 fringes in Si, equals 23 fringe spacings in Ge, thus claiming that the relaxation of the Ge crystal is practically complete. Equivalent results are obtained by performing the same type of measurement at different interfacial locations. Misfit dislocations and twinned areas are also evident in the HREM image. XRD confirmed TEM findings on Ge film structure. The diffraction pattern in Fig. 3 shows the Si (0 0 4) and Ge (0 0 4) reflections, indicating film heteroepitaxy. In the inset, the reciprocal space map around the above mentioned reflections shows a sharp circular spot related to the Si (0 0 4), while the Ge (0 0 4) has broader and elongated shape. Further, no asymmetries along x direction can be detected, indicating that the film is fully relaxed. Ge films show different linear optical properties in dependence on the growth process parameters. The spectral dispersion of the real and imaginary part of the refractive index for sputtered epitaxial Ge films are shown in Fig. 4 and compared to corresponding data for an amorphous film (sputtered at Tsubstrate = 503 K [18]) and to reference data for bulk Ge [22]. Epitaxial films (1 and 2 in the figure) underwent nominally identical growth processes; film 2 was also post-growth thermally treated, as reported in [18]. A common feature of DC-PMS grown films, representing a distinguished mark when compared to films obtained by other epitaxial growth techniques [2,5], is the absence of any sharp decrease in absorption over the spectral range 15001600 nm (a|@1500 nm = 5308 cm1, and a|@1560 nm = 4823 cm1 being a = 4p/kk). A corresponding extended spectral flatness towards the IR was experimentally proven also for the responsivity of the test PDs, as described hereafter. The photodiode responsivity is defined as RðkÞjV bias ¼V ¼ Iph =P opt ðkÞ ¼ ðITOT Idark Þ=Popt ðkÞ; where Iph = ITOT Idark is the photogenerated current, being the difference between the mea-
Fig. 2. (a) HREM image of the Ge–Si interface and (b) the intensity distribution in it along line AB. 24 Si/23 Ge lattice spacing along (1 1 1) results.
520
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
Fig. 3. XRD Bragg–Brentano measurement of Si (0 0 4) and Ge (0 0 4) peaks. The reciprocal space map around those peaks, in the inset, evidences an elongated and symmetric shape for Ge (0 0 4) reflection, typical of relaxed thin films.
Fig. 4. Imaginary (a) and real (b) components of complex refractive index for DCPMS Ge-on-Si films. Traces 1 and 2 refer to epi-layers, 2 having undergone a postgrowth rapid thermal annealing process [18].
sured total current ITOT and the dark current Idark, and Popt(k) is the k-tuned incident optical power. Fig. 5 reports the spectral dependence of R(k) between 1525 and 1570 nm, at Vbias = 1 V, as measured onto several test photodiodes realized on the same wafer. In all cases a definite spectral flatness is evident, which agrees with that reported in [18] and represents a distinguishing feature of Ge PDs made of DC-PMS. The low absolute values in R(k) are due to the small film thickness, and to the reduced photogeneration volume, as compared to the whole PD volume, responsible for Idark. By estimating the effective illuminated area to be Aeff = 5600 lm2 and by introducing a normalized effective dark current as Volume eff Idark;eff ¼ Idark Illuminated ¼ Idark PDAArea , the effective responsivity PD Volume I Idark;eff can be defined as: Reff ðkÞ ¼ ph Popt . Reff(k) is an indicator of the ultimate response featured by a PD of optimized geometry. By accounting for measured values of Idark and Iph, Reff(k) up to Reff|Vbias = 1V = 1.0088 A/W at k = 1.5 lm has been calculated. The PDs are then characterized by the absence of the sharp decrease in R(k) around k = 1550 nm, which is a common feature of both single crystalline epitaxial films [1–5] and evaporated polycrystalline Ge [8] and corresponds to the fall in optical absorption below the Ge direct bandgap. The spectral flatness in R(k) could in turn be a direct consequence of the flatness in the absorption spectrum, as proved by ellipsometry. Both phenomena are compatible with a bandgap shrinking due to thermally induced local tensile strains at the Ge/Si interface [25]. Strain management has been proposed to widen the spectral sensitivity of Ge/Si photodetectors [23–25]. However, in the present case TEM and XRD show the films to be fully relaxed, so a different explanation must be provided for the IR-extended response. In principle, an increased IR absorption, corresponding to an effective bandgap narrowing, could arise from a strong concentration of defects and the related localized intragap energy levels in the Ge layer. This in turn should be consistent with the origin of p-type conductivity in thin films [12], where the defective region is a significant portion of the whole volume. Further investigations are required to properly identify the defect nature, so as to directly prove this interpretation.
Fig. 5. Spectral behavior of R(k) for backside-illuminated test PDs. Different curves correspond to various selected PD on the 6” wafer. In the inset, the cross-section of the mesa PD with the illumination geometry is shown.
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
4. Conclusions DC-PMS results to be a promising technique to realize Ge-on-Si pn junctions to be used as PDs in the near IR, showing an extended spectral response. Ge structure, as seen at nanoscale by TEM and XRD, evidenced relaxed epitaxial growth. Further, the presence of defective region extending in the film from the interface may account for the p-type conductivity, mediated by intragap states. These, in turn, may ultimately induce a band gap narrowing leading to the experimentally proved extended absorption and photoconductive response in the near IR. The growth technique, presented here, can also be considered for waveguide photodiode geometries to be monolithically integrated in silicon-based photonic circuits. Acknowledgment Authors thank Dr. R. Birjega, NILPRP (Magurele-Bucharest, Romania) for support in XRD measurements. References
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20]
[1] P.R. Bandaru, S. Sahni, E. Yablonovitch, J. Liu, H.-Jun Kim, Y.-H. Xie, Mater. Sci. Eng., B 113 (2004) 79. [2] O.I. Dosunmu, D.D. Cannon, M.K. Emsley, B. Ghyselen, J. Liu, L.C. Kimerling, M.S. Unlü, IEEE J. Sel. Top. Quant. 10 (2004) 694. [3] L. Vivien, J. Osmond, J.M. Fédéli, D. Marris-Morini, P. Crozat, J.F. Damlencourt, E. Cassan, Y. Lecunff, S. Laval’, Opt. Expr. 17 (2009) 6252. [4] L. Ter-Hoe, W. Jian, N. Hoai-Son, M. Ramana, M.B. Yu, W.L. Loh, G.Q. Lo, N. Balasubramanian, D.L. Kwong, in: Proceedings of 4th IEEE International
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Conference on group IV Photonics, Tokyo, Japan, 19–21 September 2007, Paper FD3. G. Isella, J. Osmond, M. Kummer, R. Kaufmann, H. von Känel, Semicond. Sci. Technol. 22 (2007) S26. J. Liu et al., in: Proceedings of IEEE LEOS 2004 Annual meeting, Paper MP3. J. Yu, E. Kasper, M. Oehme, Thin Solid Films 508 (2006) 396. L. Colace, G. Masini, G. Assanto, Appl. Phys. Lett. 87 (2005) 203507. V. Dutta, P. Nath, K.L. Chopra, Phys. Status Solidi A 48 (1978) 257. H. Kobayashi, N. Inoue, T. Uchida, Y. Yasuoka, Thin Solid Films 300 (1997) 138. G. Wallis, E.E. Pittelli, J. Pantano, Thin Solid Films 4 (1969) 309. H.F. Matarè, J. Appl. Phys. 59 (1986) 97. V. Sorianello, L. Colace, G. Assanto, A. Notargiacomo, N. Armani, F. Rossi, C. Ferrari, Microelectr. Eng., this issue, doi:10.1016/j.mee.2010.09.024. K.E. Haq, J. Electrochem. Soc. 112 (1965) 500. W.K. Choi, L.K. Teh, L.K. Bera, W.K. Chim, A.T.S. Wee, Y.X. Jie, J. Appl. Phys. 91 (2002) 444. G. Bajor, K.C. Cadien, M.A. Ray, J.E. Greene, P.S. Vijayakumar, Appl. Phys. Lett. 40 (1982) 696. S.M. Pietralunga, M. Feré, M. Lanata, D. Piccinin, G. Radnóczi, F. Misják, A. Lamperti, M. Martinelli, P.M. Ossi, Europhys. Lett. 88 (2008) 28005. M. Feré, M. Lanata, D. Piccinin, S.M. Pietralunga, A. Zappettini, P.M. Ossi, M. Martinelli, in: Proceedings of the IEEE 5th International Conference on Group IV Photonics, 17–19 September 2008 Sorrento, Italy, pp. 270–272, doi:10.1109/GROUP4.2008.4638169. R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, Elsevier, Amsterdam, 1992. C.M. Herzinger, B. Johs, W.A. McGahan, J.A. Woollam, W. Paulson, J. Appl. Phys. 83 (1998) 3323. G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 (1996) 371 (and Erratum, Appl. Phys. Lett., 69, (1996) 2137). E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1985. R. People, IEEE J. Quant. Electron. 22 (1986) 1696. D.J. Paul, Adv. Mater. 11 (1999) 191. Y. Ishikawa, K. Wada, J. Liu, D.D. Cannon, H.-C. Luan, J. Michel, L. Kimerling, J. Appl. Phys. 98 (2005) 013501.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Sputtered Ge-on-Si heteroepitaxial pn junctions: Nanostructure, interface morphology and photoelectrical properties S.M. Pietralunga a,⇑, M. Feré a, M. Lanata a, G. Radnóczi b, F. Misják b, A. Lamperti c, M. Martinelli d, P.M. Ossi e a
CoreCom, via G. Colombo, 81, 20133 Milano, Italy Research Institute for Technical Physics and Materials Science, Konkoly-Thege M. út 29-33, H-1121 Budapest, Hungary c Laboratorio MDM, IMM-CNR, via C. Olivetti 2, 20041 Agrate Brianza (MB), Italy d Politecnico di Milano, Dept. of Electronics and Information, via Ponzio 34-5, 20133 Milano, Italy e Politecnico di Milano, Dept. of Energy, via Ponzio 34-3, 20133 Milano, Italy b
a r t i c l e
i n f o
Article history: Available online 19 October 2010 Keywords: Germanium thin films DC Magnetron Sputtering Ge-on-Si heteroepitaxy TEM Ge optical properties Ge photodetectors
a b s t r a c t Ge thin films are epitaxially grown onto (1 0 0) Si substrates by DC-Pulsed Magnetron Sputtering. Relaxed single crystalline layers, with slightly misoriented domains are identified by XRD, TEM and HREM. Planar defects and threading dislocations are the relevant lattice imperfections. As-deposited Ge films are p-type without the need for intentional doping, even in the absence of grain boundaries. A pronounced flatness in the near IR absorption spectra is evident, in the absence of strong interfacial strain. This could be traced to a bandgap narrowing effect due to intragap states related to defects in the interfacial region. Photoconductive response around k = 1.5 lm is flat and an equivalent responsivity Reff|Vbias = 1V = 1.0088 A/W at k = 1.5 lm has been estimated. DC-Pulsed Magnetron Sputtering is therefore an attractive solution, deserving further development, to build near-infrared C-MOS compatible photodetectors, particularly suitable for low-speed applications. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Germanium films grown onto silicon platforms are an attractive solution to monolithically integrate near-infrared photodetectors in C-MOS compatible photonic integrated circuits [1–8]. Heteroepitaxial pn-doped Ge-on-Si junctions, realized either by CVD or by MBE, are the solutions for high-speed signal photodiodes (PDs) [2–7]. To realize low-speed detectors, less critical technological approaches can be pursued, based on poly-crystalline p-Ge thin films [8], the p-type conductivity being thermodynamically justified as proceeding from grain boundaries and related defects [9–12]. Thus pn heterojunctions can directly be obtained at the interfaces between as-grown poly-Ge films and n-Si substrates. In case of evaporated Ge-on-Si films, this is claimed to be the case [8], despite recent findings showed p-type conductivity in presence of epitaxial growth [13]. By sputtering deposition techniques, GexSi1x (0 < x < 1) thin films can be provided in a wide range of crystallinity [14–16]. By DC-Pulsed Magnetron Sputtering (DC-PMS), epitaxial Ge films have been obtained onto (0 0 1) Si substrates by some of the authors and details on the growth process can be found in [17,18]. Sputtered ⇑ Corresponding author. Present address: PoliCom Fondazione Politecnico di Milano, via G. Colombo, 81, 20133 Milano, Italy. Tel.: +39 02 2399 8924; fax: +39 02 2399 8922. E-mail address: [email protected] (S.M. Pietralunga). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.10.014
Si/Ge junctions are qualified by a perfect epitaxy, by the absence of interfacial silicon oxide and by sub-nanometer roughness. A strongly defective interface region exists, which extends in the bulk of the film for about 50 nm, in agreement with the critical thickness for Ge-on-Si epitaxy [22,23]. The presence of dislocations slightly misorients the domains, without the evidence of grain boundaries [17,18]. Undoped DC-PMS Ge films are p-type, like poly-Ge films, without the occurrence of a poly-crystalline structure and in the absence of significant contamination [18]. Experimental data for the concentration and mobility of majority carriers approach typical values for undoped epitaxial Ge films [5]. In this work we focus on the linear optical properties of Ge-onSi sputtered epitaxial thin films and on the responsivity of pn PDs at k = 1.5 m; the results are correlated with the nanostructure and interface morphology.
2. Experimental Epitaxial Ge thin films were grown onto n-type 6” Si (1 0 0) wafers, at substrate temperatures Tsubstrate > 640 K by a DC-PMS process [17]. Film crystallinity and interface morphology were investigated by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) with a CM 20 TEM at 200 kV and a JEOL 3010 TEM at 300 kV accelerating voltage. X-ray diffraction (XRD) data were collected on a PANalytical X’Pert
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
PRO MRD diffractometer equipped with standard X-ray beam, monochromated at CuKa (0.154 nm) by a four-bounce hybrid monochromator and sized down to 1 mm 1 mm by beam slits. The diffracted intensity is collected by PIXcel detector. Measurements were collected in Bragg–Brentano geometry. Reciprocal space maps around Ge (0 0 4) reflection were also collected. The linear optical properties of sputtered film samples between k = 300 and 1700 nm were measured by variable angle spectroscopic ellipsometry (VASE) [19]. The dispersion of the real and imaginary parts of the refractive index n = n + ik for the Ge films have been obtained by fitting experimental data to specifically developed oscillator models for the Si–Ge bilayer [20,21]. Heterojunction Ge/Si test mesa PDs have been realized, by patterning 120 nm-thick crystalline Ge films and by evaporating Ti– Au contacts. Multiple series of 12 squared and 12 circular mesas, with areas from 0.03 to 8.04 mm2, were replicated at different positions on the 6” wafers. PDs were backside-illuminated from the Si substrate by beams of FWHM = 110 lm each, from a pigtailed tunable semiconductor laser, spanning the third window of optical communications at k = 1.5 lm and effectively illuminating only the center of the mesas. The optical power impinging on the Si substrate is Popt = 48.5 mW. 3. Results and discussion Fig. 1 is a HREM image of the epitaxial Si/Ge interface, with the (0 0 1) planes parallel to the interface. In the corresponding electron diffraction pattern, in the inset, the (2 2 0) Si and Ge reflections are evident. This allows estimating any residual strain in the Si/Ge system by measuring the distance between them. The position of the peaks marks a difference 0.008 ± 0.0010 nm between (2 2 0) lattice plane distances of Si and Ge. The corresponding difference in the bulks is 0.008 nm, meaning that, within the measurement error, the Ge layer is relaxed and its lattice parameter corresponds to the bulk value. Further confirmation of the Si/Ge
Fig. 1. HREM image of the Si/Ge system. Epitaxial growth is evident. At the interface a defective structure also appears. In the inset the diffraction pattern from the imaged area allows for the revealing of Si and Ge (2 2 0) reflections.
519
relaxation comes from the local measurement of the spacing of the lattice fringes across the Si/Ge interface from HREM image. In bulk Si and Ge, 23 times of a Ge spacing equal 24 times a Si spacing, for the same family of lattice planes. Fig. 2(a) shows an image of the Si/Ge interface where such a measurement has been carried out. Fig. 2(b) shows the intensity distribution of (1 1 1) planes across the interface. The total distance measured by considering 24 fringes in Si, equals 23 fringe spacings in Ge, thus claiming that the relaxation of the Ge crystal is practically complete. Equivalent results are obtained by performing the same type of measurement at different interfacial locations. Misfit dislocations and twinned areas are also evident in the HREM image. XRD confirmed TEM findings on Ge film structure. The diffraction pattern in Fig. 3 shows the Si (0 0 4) and Ge (0 0 4) reflections, indicating film heteroepitaxy. In the inset, the reciprocal space map around the above mentioned reflections shows a sharp circular spot related to the Si (0 0 4), while the Ge (0 0 4) has broader and elongated shape. Further, no asymmetries along x direction can be detected, indicating that the film is fully relaxed. Ge films show different linear optical properties in dependence on the growth process parameters. The spectral dispersion of the real and imaginary part of the refractive index for sputtered epitaxial Ge films are shown in Fig. 4 and compared to corresponding data for an amorphous film (sputtered at Tsubstrate = 503 K [18]) and to reference data for bulk Ge [22]. Epitaxial films (1 and 2 in the figure) underwent nominally identical growth processes; film 2 was also post-growth thermally treated, as reported in [18]. A common feature of DC-PMS grown films, representing a distinguished mark when compared to films obtained by other epitaxial growth techniques [2,5], is the absence of any sharp decrease in absorption over the spectral range 15001600 nm (a|@1500 nm = 5308 cm1, and a|@1560 nm = 4823 cm1 being a = 4p/kk). A corresponding extended spectral flatness towards the IR was experimentally proven also for the responsivity of the test PDs, as described hereafter. The photodiode responsivity is defined as RðkÞjV bias ¼V ¼ Iph =P opt ðkÞ ¼ ðITOT Idark Þ=Popt ðkÞ; where Iph = ITOT Idark is the photogenerated current, being the difference between the mea-
Fig. 2. (a) HREM image of the Ge–Si interface and (b) the intensity distribution in it along line AB. 24 Si/23 Ge lattice spacing along (1 1 1) results.
520
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
Fig. 3. XRD Bragg–Brentano measurement of Si (0 0 4) and Ge (0 0 4) peaks. The reciprocal space map around those peaks, in the inset, evidences an elongated and symmetric shape for Ge (0 0 4) reflection, typical of relaxed thin films.
Fig. 4. Imaginary (a) and real (b) components of complex refractive index for DCPMS Ge-on-Si films. Traces 1 and 2 refer to epi-layers, 2 having undergone a postgrowth rapid thermal annealing process [18].
sured total current ITOT and the dark current Idark, and Popt(k) is the k-tuned incident optical power. Fig. 5 reports the spectral dependence of R(k) between 1525 and 1570 nm, at Vbias = 1 V, as measured onto several test photodiodes realized on the same wafer. In all cases a definite spectral flatness is evident, which agrees with that reported in [18] and represents a distinguishing feature of Ge PDs made of DC-PMS. The low absolute values in R(k) are due to the small film thickness, and to the reduced photogeneration volume, as compared to the whole PD volume, responsible for Idark. By estimating the effective illuminated area to be Aeff = 5600 lm2 and by introducing a normalized effective dark current as Volume eff Idark;eff ¼ Idark Illuminated ¼ Idark PDAArea , the effective responsivity PD Volume I Idark;eff can be defined as: Reff ðkÞ ¼ ph Popt . Reff(k) is an indicator of the ultimate response featured by a PD of optimized geometry. By accounting for measured values of Idark and Iph, Reff(k) up to Reff|Vbias = 1V = 1.0088 A/W at k = 1.5 lm has been calculated. The PDs are then characterized by the absence of the sharp decrease in R(k) around k = 1550 nm, which is a common feature of both single crystalline epitaxial films [1–5] and evaporated polycrystalline Ge [8] and corresponds to the fall in optical absorption below the Ge direct bandgap. The spectral flatness in R(k) could in turn be a direct consequence of the flatness in the absorption spectrum, as proved by ellipsometry. Both phenomena are compatible with a bandgap shrinking due to thermally induced local tensile strains at the Ge/Si interface [25]. Strain management has been proposed to widen the spectral sensitivity of Ge/Si photodetectors [23–25]. However, in the present case TEM and XRD show the films to be fully relaxed, so a different explanation must be provided for the IR-extended response. In principle, an increased IR absorption, corresponding to an effective bandgap narrowing, could arise from a strong concentration of defects and the related localized intragap energy levels in the Ge layer. This in turn should be consistent with the origin of p-type conductivity in thin films [12], where the defective region is a significant portion of the whole volume. Further investigations are required to properly identify the defect nature, so as to directly prove this interpretation.
Fig. 5. Spectral behavior of R(k) for backside-illuminated test PDs. Different curves correspond to various selected PD on the 6” wafer. In the inset, the cross-section of the mesa PD with the illumination geometry is shown.
S.M. Pietralunga et al. / Microelectronic Engineering 88 (2011) 518–521
4. Conclusions DC-PMS results to be a promising technique to realize Ge-on-Si pn junctions to be used as PDs in the near IR, showing an extended spectral response. Ge structure, as seen at nanoscale by TEM and XRD, evidenced relaxed epitaxial growth. Further, the presence of defective region extending in the film from the interface may account for the p-type conductivity, mediated by intragap states. These, in turn, may ultimately induce a band gap narrowing leading to the experimentally proved extended absorption and photoconductive response in the near IR. The growth technique, presented here, can also be considered for waveguide photodiode geometries to be monolithically integrated in silicon-based photonic circuits. Acknowledgment Authors thank Dr. R. Birjega, NILPRP (Magurele-Bucharest, Romania) for support in XRD measurements. References
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