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Normal incidence p–i–n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition
Optics Communications 283 (2010) 3404–3407
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Normal incidence p–i–n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition Zhiwen Zhou a,⁎, Jingkai He a, Ruichun Wang a, Cheng Li b, Jinzhong Yu c a b c
Department of Electronic Communication Technology, Shenzhen Institute of Information and Technology, Shenzhen, Guangdong 518029, People's Republic of China Department of Physics, Semiconductor Photonics Research Center, Xiamen University, Xiamen, Fujian 361005, People's Republic of China State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, People's Republic of China
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 24 April 2010 Accepted 24 April 2010 Keywords: Germanium Hererojunction Photodiode Tensile strain
a b s t r a c t We report on normal incidence p–i–n heterojunction photodiodes operating in the near-infrared region and realized in pure germanium on planar silicon substrate. The diodes were fabricated by ultrahigh vacuum chemical vapor deposition at 600 °C without thermal annealing and allowing the integration with standard silicon processes. Due to the 0.14% residual tensile strain generated by the thermal expansion mismatch between Ge and Si, an efficiency enhancement of nearly 3-fold at 1.55 μm and the absorption edge shifting to longer wavelength of about 40 nm are achieved in the epitaxial Ge films. The diode with a responsivity of 0.23 A/W at 1.55 μm wavelength and a bulk dark current density of 10 mA/cm2 is demonstrated. These diodes with high performances and full compatibility with the CMOS processes enable monolithically integrating microphotonics and microelectronics on the same chip. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Si-based devices for optical applications have been substantially researched in recent years and there has been remarkable progress on several of the required components, including modulators [1], light emitters [2], and photo detectors [3]. Ge, due to its large absorption coefficient at near-infrared region (NIR), lower cost and compatibility with Si complementary metal oxide semiconductor (CMOS) processing compared to III–V semiconductors, is perceived as the best candidate for on-chip photodiodes. However, the drawback in using pure Ge is the introduction of surface undulations and threading dislocations which relieve the stress caused by the 4.2% lattice mismatch between Ge and Si. Rough surface creates difficulties in lithograph patterning and process integration. Dense defects degrade the performance of the devices for detrimental generation–recombination centers. Thus, a lot of research has been done in that area with different technological approaches. For example, employing SiGe compositionally graded buffer (CGB) layers to grow Ge layers effectively reduced the threading dislocations, yielding optical detectors with a quantum efficiency of 12.6% at 1.3 μm and a recoded low dark current density of 0.15 mA/cm2 [4]. Unfortunately, these CGB layers often suffer from a thickness of approximately 10 μm, making the integration of devices on the Si-based circuits difficult. Another method is direct deposition of Ge at low temperature as buffer layer combined with cyclic thermal annealing, which reported a responsivity of 0.25 A/W at 1.55 μm wavelength and a dark current
⁎ Corresponding author. E-mail address: [email protected] (Z. Zhou). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.04.098
density of 30 mA/cm2 on 1 μm Ge films grown on Si [5]. The side effects of such high-temperature annealing are incompatibility with standard CMOS process and the fast Si–Ge interdiffusion, resulting in a reduced responsivity for longer wavelength detection [6]. An approach of selective-area heteroepitaxy is also proposed. A vertically illuminated Ge diode demonstrated a responsivity of 0.64 A/W at 1.55 μm with a dark current density of 16 mA/cm2 at 1 V reverse bias [7]. The fabrication of silicon dioxide (SiO2) pattered substrates and/or chemical mechanical polishing makes the whole device processing flows complex. In this paper, a normal incidence p–i–n Ge heterojunction photodiode grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) on a planar Si substrate without thermal annealing is presented. The 0.14% residual tensile strain induced by the thermal expansion mismatch between Ge and Si lowers the direct energy gap by amount of about 20 meV, thus improve the efficiency in the NIR and expand the Ge absorption edge to longer wavelength of approximately 40 nm. A responsivity of 0.23 A/W at wavelength of 1.55 μm and a dark current of about 1 µA at 1 V reverse bias, dominated by the peripheral leakage, was obtained for a 1-μm thick Ge photodiode with an area of 50× 50 µm2. 2. Experiment A schematic cross section view of normal incidence p–i–n Ge heterojunction photodiode on Si substrate is shown in Fig. 1(a). The top p-type contact and the intrinsic layer were made of epitaxial Ge films, while the bottom n-type contact was the Si substrate (resistivity of 0.018–0.1 Ω cm). The intrinsic and boron-doped Ge films for the
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∼1 μm, i.e., down to n-Si substrate (bottom contact). Diameters of round-mesa ranged between 24 and 300 µm and side lengths of square-mesa varied from 20 to 200 µm. A 300-nm-thick lowtemperature SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD) for insulation, passivation and antireflection (not optimized). Openings for the contacts were then patterned in the SiO2 layer and etched in diluted HF. Top and bottom metal contacts were formed by sputter-deposition 500-nm Aluminum (Al) and wet chemical etching. Fig. 1(b) shows the optical microscope top view of the resulting p–i–n Ge diode. The used materials and technological processes are fully compatible with CMOS technology. 3. Results and discussion
Fig. 1. (a) Schematic cross section and (b) optical microscope top view of normal incident p–i–n Ge photodiodes on Si substrate. The thicknesses are 200 nm and 800 nm for i-Ge and p-Ge layers, respectively, and the round-mesa diameter shown is 64 µm. The inset in figure (b) shows the typical optical image of etched Ge surface in diluted I2 solution. The threading dislocation density was approximately 6 × 107 cm− 2.
diodes were epitaxially grown on 4 in. Si(100) substrates by UHVCVD system with a base pressure of 2 × 10− 7 Pa. The gas sources were pure GeH4 and B2H6 diluted to 0.5% in H2, more details can be found in reference [8]. Before being loaded into the chamber, the wafers were cleaned by Radio Corporation of American (RCA) method and dried by N2. After baking the wafer at 850 °C (baking temperature could be lowered, i.e. to 600 °C, if the ultra-thin native oxide formed during RCA cleaning was removed in aqueous HF solution finally with a minor modification for the wafer cleaning procedures) for 30 min to obtain a clean epi-ready surface, the chamber temperature was ramped down to 330 °C and 90 nm Ge buffer layer was initially deposited to relax the misfit strain due to lattice parameter difference between Ge and Si and derive a relative flat growing surface. And then, the temperature was increased to 600 °C and 710 nm undoped Ge (i-Ge) and 200 nm boron-doped Ge (p-Ge) films were grown successively. In-situ boron doping was achieved with doping levels of about 5 × 1017 cm− 3, as evaluated by secondary ion mass spectroscopy. No thermal annealing was performed to reduce defects. The growth rates for Ge at 330 and 600 °C were 0.5 and 1.5 nm/min, respectively. Boron dopant had little effect on the Ge growth rate. Atomic force microscopy images of the epitaxial Ge surface showed no cross-hatch patterns with a root-mean-square roughness of 2.7 nm in a 10 × 10 µm2 area. The threading dislocation densities of Ge films on Si were characterized by counting pits utilizing selectively chemical etching with the help of Nomarski optical microscope. About 200 nm top Ge layer was etched in diluted I2 solution [9] (HF:HNO3:CH3COOH:I2 = 10 ml:20 ml:100 ml:30 mg) at the rate of nearly 20 nm/s. The typical optical image of the etched Ge surface was shown as inset of Fig. 1(b), and the etch pit densities were averaged to be 6 × 107 cm− 2. The p–i–n Ge photodiodes were realized as round and squaremesa structures. Mesas were defined by standard photolithography and etched by CF4/O2 reactive ion etching of the Ge layer to a depth of
In order to investigate the crystalline homogeneity and the microscopic strain state of the Ge epilayer, high-resolution X-ray diffraction (XRD) Ω–2θ scan around (004) order was performed and the rocking curve was showed in Fig. 2. Two peaks respectively originating from Si substrate and Ge epilayer are clearly observed. The Ge diffraction peak located at − 5496″ is symmetric and sharp with no diffraction tails inclining to higher incidence angle generated by Si–Ge interdiffusion and the full width at half maximum is only 90″, suggesting the good quality of epitaxial Ge film. On the other hand, based on the peak position of the Ge epilayer, which is much nearer to Si substrate in comparison with the bulk Ge one (straight dotted line showed in Fig. 2), the in-plane lattice parameter of the Ge layer is calculated as 0.5666 nm, larger than the bulk Ge 0.5658 nm, implying the tensile strain in Ge film with amount of 0.14%. The tensile strain arises from the thermal expansion coefficient mismatch between Ge and Si [10]. During the cooling process after Ge deposition, the decrease in the parallel lattice constant of Ge is suppressed by that of the Si substrate, generating residual tensile strain in Ge layer. We could see in the following section that this tensile strain is beneficial to expand the response wavelength and improve the absorption efficiency of Ge photodiodes. The dark current of photodiode not only is an indication of material quality but also determines optical receiver sensitivity. Fig. 3(a) shows the dark current–voltage characteristics from photodiodes of squaremesa with various side lengths. All the dark current curves exhibit a good rectification. At 1 V reverse bias, the dark currents are 0.34, 0.99, 3.3 and 7.6 μA for lengths of 20, 50, 100 and 200 μm, respectively. Accordingly, the dark current densities are 84, 40, 33 and 19 mA/cm2. Fig. 3(b) shows the dark currents at 1 V reverse bias as a function of photodiode mesa lengths. The dark current level of our p–i–n Ge diodes is comparable to those Ge diodes even grown selectively and/or
Fig. 2. XRD Ω–2θ scan of (004) order rocking curve of 1-µm-thick Ge epitaxially grown on Si substrate by UHVCVD. Straight dotted line indicated the theoretical peak position of bulk Ge.
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photodiode. The responsivity is retrieved by dividing the photocurrent (with dark current subtracted) by the input optical power. The diodes are vertically illuminated using a fiber-pigtailed continuous halogen source or laser diode at 1546 nm. Fig. 4 shows the photocurrent versus reverse bias for the Ge p–i–n photodiode operating at 1546 nm with 9.22 mW incident power. The active absorption area of the device is 50× 50 μm2. There is a gentle increase in the photocurrent with bias increasing. The responsivities of 0.13 and 0.23 A/W were measured at reverse bias of 1 and 2 V, respectively, corresponding to 10.4 and 18.4% external quantum efficiency. This responsivity data is the typical value of normal incidence p–i–n Ge diode with an intrinsic thickness of about 1 μm [5,12–14]. From the responsivity (R), the absorption coefficient (α Ge) of epitaxial Ge film at certain wavelength (λ) can be obtained according to the formula [15]:
RðA = W Þ =
λðμmÞ −α d −α d ð1−rref Þe Ge p ‐Ge 1−e Ge i ‐Ge 1:24
ð2Þ
ð1Þ
where rref is the surface reflectivity, dp-Ge and di-Ge are the thickness of p-Ge and i-Ge films respectively. The extracted absorption coefficient of Ge epilayer at 1.55 µm is about 3000 cm− 1, which is at least three times larger than the bulk value (b1000 cm− 1) [16]. Epitaxial Ge film presents an enhanced absorption coefficient at infrared wavelength, which greatly improves the responsivity of the epitaxial Ge photodiode.The enhancement of absorption is attributed to the residual tensile strain, which is caused by thermal expansion mismatch with a mount of 0.14%, as determined by XRD measurement. This 0.14% tensile strain reduces the direct bandgap of Ge from 0.802 to 0.783 eV [15], accordingly, the effective detection wavelength is expanded from 1.55 to 1.58 µm. Spectral response measurement verified the expansion of the absorption edge because of the 0.14% tensile strain. Fig. 5 shows the normalized spectral responsivity from a 50 × 50 µm2 diode measured under zero bias. For comparison, the simulated result using the bulk Ge absorption data [16] is also included. We note that for our epitaxial Ge diode, a fairly flat response at wavelength range of 1.3–1.55 µm is obtained, and the responsivity at 1.6 µm is still 20% of that measured at 1.5 µm. In contrast, the responsivity decreases rapidly beyond 1.52 µm in bulk Ge diodes. At 1.6 µm, the responsivity is less than 10% of its 1.5 µm value. The measured response edge from our diode is shifted to longer wavelength by nearly 40 nm, in agreement with the theoretical value of 38 nm. These results indicate our epitaxial Ge diodes can be used in optical communication of almost all the relevant bands (with a partial coverage of the L band).
where JBulk is the bulk leakage current density, JSurf is the peripheral leakage current density, and L is the side length of the squaremesa. The data fits this second order polynomial model very well (see Fig. 3(b)), which allows us to correctly extract the peripheral and bulk leakage components of the diodes. JSurf and JBulk are found to be 40 μA/ cm and 10 mA/cm2, respectively. Fig. 3(b) also shows the peripheral and bulk leakage components of the diode leakage as a function of diode mesa lengths. We can see for larger diodes the majority leakage is the bulk, whereas for small diodes peripheral leakage dominates. The cross-over point is at a side length of approximately 160 μm. Since many applications require small diodes (to obtain a high bandwidth), the key to further improving diode leakage is to lower the peripheral leakage current, nevertheless Ge surface cleaning and passivation are both challenging. The properties of the epitaxial Ge film are no longer the limiting factors, indicating the epitaxial Ge films prepared by lowtemperature Ge buffer on the planar Si substrate in a UHVCVD system is suitable for practical applications. External optical responsivity was assessed by calibrating the power at the output of an optical fiber and measuring the photocurrent of the
Fig. 4. Dark current and photocurrent versus voltage for the 50 × 50 µm2 square-mesa diode at a wavelength of 1546 nm.
Fig. 3. (a) Dark current as a function of the applied bias voltage of square-mesa photodiodes with different side lengths: 20, 50, 100, and 200 µm. (b) Dark current versus side length of square-mesa diodes at 1 V reverse bias.
subjected to thermal annealing [5,7], although a certain worse than the best reported one [11]. Furthermore, for our devices, the dark current density decreases as the mesa length increases. This indicates that the source of dark current is not only the bulk leakage, but also the peripheral leakage. To separate the bulk and perimeter components, we fit the current data to the following model: I = JSurf ⋅4L + JBulk ⋅L
2
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Acknowledgements This work was partly supported by the National Basic Research Program of China (973 Program) under grant No. 2007CB613404 and Program for New Century Excellent Talents in University.
References
Fig. 5. Normalized spectral responsivity of p–i–n Ge diode measured at zero bias compared to the simulated result utilizing the bulk Ge absorption coefficient.
4. Conclusion In conclusion, a normal incidence p–i–n Ge heterojunction photodiode grown by UHVCVD on planar Si substrate without hightemperature thermal annealing has been demonstrated. The low bulk dark current density of 10 mA/cm2 confirms the good Ge quality. A substantial response in the NIR was achieved due to the 0.14% residual tensile strain and the responsivity is 0.23 A/W at 1.55 μm wavelength under 2 V reverse bias. These results suggest that a low thermal budget CMOS-comparable process to fabricate a high performance Ge/Si photodiode is possible.
[1] A. Liu, R. Jones, L. Liao, et al., Nature 427 (2004) 615. [2] A. Fang, H. Park, O. Cohen, R. Jones, M. Paniccia, J. Bower, Opt. Express 14 (2006) 9203. [3] Y. Kang, et al., Nat. Photonics 3 (2008) 59. [4] S.B. Samavedam, M.T. Currie, T.A. Langdo, E.A. Fitzgerald, Appl. Phys. Lett. 73 (1998) 2125. [5] L. Colace, G. Masini, G. Assanto, H.C. Luan, K. Wada, L.C. Kimerling, Appl. Phys. Lett. 76 (2000) 1231. [6] G. Dehlinger, S.J. Koester, J.D. Schaub, J.O. Chu, Q.C. Ouyang, A. Grill, IEEE Photonics Technol. Lett. 16 (2004) 2547. [7] H.Y. Yu, S. Ren, W.S. Jung, A.K. Okyay, D.A.B. Miller, K.C. Saraswat, IEEE Electron Device Lett. 30 (2009) 1161. [8] Z. Zhou, C. Li, H. Lai, S. Chen, J. Yu, J. Cryst, Growth 310 (2008) 2508. [9] H.C. Luan, D.R. Lim, K.K. Lee, K.M. Chen, J.G. Sandland, K. Wada, L.C. Kimerling, Appl. Phys. Lett. 75 (1999) 2909. [10] D.D. Cannon, J. Liu, Y. Ishikawa, K. Wada, D.T. Danielson, S. Jongthammanurak, J. Michel, L.C. Kimerling, Appl. Phys. Lett. 84 (2004) 906. [11] J. Osmond, G. Isella, D. Chrastina, R. Kaufmann, M. Acciarri, H. von Käne, Appl. Phys. Lett. 94 (2009) 201106. [12] L. Colace, M. Balbi, G. Masini, G. Assanto, H.C. Luan, L.C. Kimerling, Appl. Phys. Lett. 88 (2006) 101111. [13] G. Isella, J. Osmond, M. Kummer, R. Kaufmann, H. von Känel, Semicond. Sci. Technol. 22 (2007) S26. [14] H.Y. Xue, C.L. Xue, B.W. Cheng, Y.D. Yu, Q.M. Wang, Chin. Phys. B 18 (2009) 2542. [15] Y. Ishikawa, K. Wada, J. Liu, D.D. Cannon, H.C. Luan, J. Michel, L.C. Kimerling, J. Appl. Phys. 98 (2005) 013501. [16] R.F. Potter, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Orlando FL: Academic, 1985, p. 465.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Normal incidence p–i–n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition Zhiwen Zhou a,⁎, Jingkai He a, Ruichun Wang a, Cheng Li b, Jinzhong Yu c a b c
Department of Electronic Communication Technology, Shenzhen Institute of Information and Technology, Shenzhen, Guangdong 518029, People's Republic of China Department of Physics, Semiconductor Photonics Research Center, Xiamen University, Xiamen, Fujian 361005, People's Republic of China State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, People's Republic of China
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 24 April 2010 Accepted 24 April 2010 Keywords: Germanium Hererojunction Photodiode Tensile strain
a b s t r a c t We report on normal incidence p–i–n heterojunction photodiodes operating in the near-infrared region and realized in pure germanium on planar silicon substrate. The diodes were fabricated by ultrahigh vacuum chemical vapor deposition at 600 °C without thermal annealing and allowing the integration with standard silicon processes. Due to the 0.14% residual tensile strain generated by the thermal expansion mismatch between Ge and Si, an efficiency enhancement of nearly 3-fold at 1.55 μm and the absorption edge shifting to longer wavelength of about 40 nm are achieved in the epitaxial Ge films. The diode with a responsivity of 0.23 A/W at 1.55 μm wavelength and a bulk dark current density of 10 mA/cm2 is demonstrated. These diodes with high performances and full compatibility with the CMOS processes enable monolithically integrating microphotonics and microelectronics on the same chip. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Si-based devices for optical applications have been substantially researched in recent years and there has been remarkable progress on several of the required components, including modulators [1], light emitters [2], and photo detectors [3]. Ge, due to its large absorption coefficient at near-infrared region (NIR), lower cost and compatibility with Si complementary metal oxide semiconductor (CMOS) processing compared to III–V semiconductors, is perceived as the best candidate for on-chip photodiodes. However, the drawback in using pure Ge is the introduction of surface undulations and threading dislocations which relieve the stress caused by the 4.2% lattice mismatch between Ge and Si. Rough surface creates difficulties in lithograph patterning and process integration. Dense defects degrade the performance of the devices for detrimental generation–recombination centers. Thus, a lot of research has been done in that area with different technological approaches. For example, employing SiGe compositionally graded buffer (CGB) layers to grow Ge layers effectively reduced the threading dislocations, yielding optical detectors with a quantum efficiency of 12.6% at 1.3 μm and a recoded low dark current density of 0.15 mA/cm2 [4]. Unfortunately, these CGB layers often suffer from a thickness of approximately 10 μm, making the integration of devices on the Si-based circuits difficult. Another method is direct deposition of Ge at low temperature as buffer layer combined with cyclic thermal annealing, which reported a responsivity of 0.25 A/W at 1.55 μm wavelength and a dark current
⁎ Corresponding author. E-mail address: [email protected] (Z. Zhou). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.04.098
density of 30 mA/cm2 on 1 μm Ge films grown on Si [5]. The side effects of such high-temperature annealing are incompatibility with standard CMOS process and the fast Si–Ge interdiffusion, resulting in a reduced responsivity for longer wavelength detection [6]. An approach of selective-area heteroepitaxy is also proposed. A vertically illuminated Ge diode demonstrated a responsivity of 0.64 A/W at 1.55 μm with a dark current density of 16 mA/cm2 at 1 V reverse bias [7]. The fabrication of silicon dioxide (SiO2) pattered substrates and/or chemical mechanical polishing makes the whole device processing flows complex. In this paper, a normal incidence p–i–n Ge heterojunction photodiode grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) on a planar Si substrate without thermal annealing is presented. The 0.14% residual tensile strain induced by the thermal expansion mismatch between Ge and Si lowers the direct energy gap by amount of about 20 meV, thus improve the efficiency in the NIR and expand the Ge absorption edge to longer wavelength of approximately 40 nm. A responsivity of 0.23 A/W at wavelength of 1.55 μm and a dark current of about 1 µA at 1 V reverse bias, dominated by the peripheral leakage, was obtained for a 1-μm thick Ge photodiode with an area of 50× 50 µm2. 2. Experiment A schematic cross section view of normal incidence p–i–n Ge heterojunction photodiode on Si substrate is shown in Fig. 1(a). The top p-type contact and the intrinsic layer were made of epitaxial Ge films, while the bottom n-type contact was the Si substrate (resistivity of 0.018–0.1 Ω cm). The intrinsic and boron-doped Ge films for the
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∼1 μm, i.e., down to n-Si substrate (bottom contact). Diameters of round-mesa ranged between 24 and 300 µm and side lengths of square-mesa varied from 20 to 200 µm. A 300-nm-thick lowtemperature SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD) for insulation, passivation and antireflection (not optimized). Openings for the contacts were then patterned in the SiO2 layer and etched in diluted HF. Top and bottom metal contacts were formed by sputter-deposition 500-nm Aluminum (Al) and wet chemical etching. Fig. 1(b) shows the optical microscope top view of the resulting p–i–n Ge diode. The used materials and technological processes are fully compatible with CMOS technology. 3. Results and discussion
Fig. 1. (a) Schematic cross section and (b) optical microscope top view of normal incident p–i–n Ge photodiodes on Si substrate. The thicknesses are 200 nm and 800 nm for i-Ge and p-Ge layers, respectively, and the round-mesa diameter shown is 64 µm. The inset in figure (b) shows the typical optical image of etched Ge surface in diluted I2 solution. The threading dislocation density was approximately 6 × 107 cm− 2.
diodes were epitaxially grown on 4 in. Si(100) substrates by UHVCVD system with a base pressure of 2 × 10− 7 Pa. The gas sources were pure GeH4 and B2H6 diluted to 0.5% in H2, more details can be found in reference [8]. Before being loaded into the chamber, the wafers were cleaned by Radio Corporation of American (RCA) method and dried by N2. After baking the wafer at 850 °C (baking temperature could be lowered, i.e. to 600 °C, if the ultra-thin native oxide formed during RCA cleaning was removed in aqueous HF solution finally with a minor modification for the wafer cleaning procedures) for 30 min to obtain a clean epi-ready surface, the chamber temperature was ramped down to 330 °C and 90 nm Ge buffer layer was initially deposited to relax the misfit strain due to lattice parameter difference between Ge and Si and derive a relative flat growing surface. And then, the temperature was increased to 600 °C and 710 nm undoped Ge (i-Ge) and 200 nm boron-doped Ge (p-Ge) films were grown successively. In-situ boron doping was achieved with doping levels of about 5 × 1017 cm− 3, as evaluated by secondary ion mass spectroscopy. No thermal annealing was performed to reduce defects. The growth rates for Ge at 330 and 600 °C were 0.5 and 1.5 nm/min, respectively. Boron dopant had little effect on the Ge growth rate. Atomic force microscopy images of the epitaxial Ge surface showed no cross-hatch patterns with a root-mean-square roughness of 2.7 nm in a 10 × 10 µm2 area. The threading dislocation densities of Ge films on Si were characterized by counting pits utilizing selectively chemical etching with the help of Nomarski optical microscope. About 200 nm top Ge layer was etched in diluted I2 solution [9] (HF:HNO3:CH3COOH:I2 = 10 ml:20 ml:100 ml:30 mg) at the rate of nearly 20 nm/s. The typical optical image of the etched Ge surface was shown as inset of Fig. 1(b), and the etch pit densities were averaged to be 6 × 107 cm− 2. The p–i–n Ge photodiodes were realized as round and squaremesa structures. Mesas were defined by standard photolithography and etched by CF4/O2 reactive ion etching of the Ge layer to a depth of
In order to investigate the crystalline homogeneity and the microscopic strain state of the Ge epilayer, high-resolution X-ray diffraction (XRD) Ω–2θ scan around (004) order was performed and the rocking curve was showed in Fig. 2. Two peaks respectively originating from Si substrate and Ge epilayer are clearly observed. The Ge diffraction peak located at − 5496″ is symmetric and sharp with no diffraction tails inclining to higher incidence angle generated by Si–Ge interdiffusion and the full width at half maximum is only 90″, suggesting the good quality of epitaxial Ge film. On the other hand, based on the peak position of the Ge epilayer, which is much nearer to Si substrate in comparison with the bulk Ge one (straight dotted line showed in Fig. 2), the in-plane lattice parameter of the Ge layer is calculated as 0.5666 nm, larger than the bulk Ge 0.5658 nm, implying the tensile strain in Ge film with amount of 0.14%. The tensile strain arises from the thermal expansion coefficient mismatch between Ge and Si [10]. During the cooling process after Ge deposition, the decrease in the parallel lattice constant of Ge is suppressed by that of the Si substrate, generating residual tensile strain in Ge layer. We could see in the following section that this tensile strain is beneficial to expand the response wavelength and improve the absorption efficiency of Ge photodiodes. The dark current of photodiode not only is an indication of material quality but also determines optical receiver sensitivity. Fig. 3(a) shows the dark current–voltage characteristics from photodiodes of squaremesa with various side lengths. All the dark current curves exhibit a good rectification. At 1 V reverse bias, the dark currents are 0.34, 0.99, 3.3 and 7.6 μA for lengths of 20, 50, 100 and 200 μm, respectively. Accordingly, the dark current densities are 84, 40, 33 and 19 mA/cm2. Fig. 3(b) shows the dark currents at 1 V reverse bias as a function of photodiode mesa lengths. The dark current level of our p–i–n Ge diodes is comparable to those Ge diodes even grown selectively and/or
Fig. 2. XRD Ω–2θ scan of (004) order rocking curve of 1-µm-thick Ge epitaxially grown on Si substrate by UHVCVD. Straight dotted line indicated the theoretical peak position of bulk Ge.
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photodiode. The responsivity is retrieved by dividing the photocurrent (with dark current subtracted) by the input optical power. The diodes are vertically illuminated using a fiber-pigtailed continuous halogen source or laser diode at 1546 nm. Fig. 4 shows the photocurrent versus reverse bias for the Ge p–i–n photodiode operating at 1546 nm with 9.22 mW incident power. The active absorption area of the device is 50× 50 μm2. There is a gentle increase in the photocurrent with bias increasing. The responsivities of 0.13 and 0.23 A/W were measured at reverse bias of 1 and 2 V, respectively, corresponding to 10.4 and 18.4% external quantum efficiency. This responsivity data is the typical value of normal incidence p–i–n Ge diode with an intrinsic thickness of about 1 μm [5,12–14]. From the responsivity (R), the absorption coefficient (α Ge) of epitaxial Ge film at certain wavelength (λ) can be obtained according to the formula [15]:
RðA = W Þ =
λðμmÞ −α d −α d ð1−rref Þe Ge p ‐Ge 1−e Ge i ‐Ge 1:24
ð2Þ
ð1Þ
where rref is the surface reflectivity, dp-Ge and di-Ge are the thickness of p-Ge and i-Ge films respectively. The extracted absorption coefficient of Ge epilayer at 1.55 µm is about 3000 cm− 1, which is at least three times larger than the bulk value (b1000 cm− 1) [16]. Epitaxial Ge film presents an enhanced absorption coefficient at infrared wavelength, which greatly improves the responsivity of the epitaxial Ge photodiode.The enhancement of absorption is attributed to the residual tensile strain, which is caused by thermal expansion mismatch with a mount of 0.14%, as determined by XRD measurement. This 0.14% tensile strain reduces the direct bandgap of Ge from 0.802 to 0.783 eV [15], accordingly, the effective detection wavelength is expanded from 1.55 to 1.58 µm. Spectral response measurement verified the expansion of the absorption edge because of the 0.14% tensile strain. Fig. 5 shows the normalized spectral responsivity from a 50 × 50 µm2 diode measured under zero bias. For comparison, the simulated result using the bulk Ge absorption data [16] is also included. We note that for our epitaxial Ge diode, a fairly flat response at wavelength range of 1.3–1.55 µm is obtained, and the responsivity at 1.6 µm is still 20% of that measured at 1.5 µm. In contrast, the responsivity decreases rapidly beyond 1.52 µm in bulk Ge diodes. At 1.6 µm, the responsivity is less than 10% of its 1.5 µm value. The measured response edge from our diode is shifted to longer wavelength by nearly 40 nm, in agreement with the theoretical value of 38 nm. These results indicate our epitaxial Ge diodes can be used in optical communication of almost all the relevant bands (with a partial coverage of the L band).
where JBulk is the bulk leakage current density, JSurf is the peripheral leakage current density, and L is the side length of the squaremesa. The data fits this second order polynomial model very well (see Fig. 3(b)), which allows us to correctly extract the peripheral and bulk leakage components of the diodes. JSurf and JBulk are found to be 40 μA/ cm and 10 mA/cm2, respectively. Fig. 3(b) also shows the peripheral and bulk leakage components of the diode leakage as a function of diode mesa lengths. We can see for larger diodes the majority leakage is the bulk, whereas for small diodes peripheral leakage dominates. The cross-over point is at a side length of approximately 160 μm. Since many applications require small diodes (to obtain a high bandwidth), the key to further improving diode leakage is to lower the peripheral leakage current, nevertheless Ge surface cleaning and passivation are both challenging. The properties of the epitaxial Ge film are no longer the limiting factors, indicating the epitaxial Ge films prepared by lowtemperature Ge buffer on the planar Si substrate in a UHVCVD system is suitable for practical applications. External optical responsivity was assessed by calibrating the power at the output of an optical fiber and measuring the photocurrent of the
Fig. 4. Dark current and photocurrent versus voltage for the 50 × 50 µm2 square-mesa diode at a wavelength of 1546 nm.
Fig. 3. (a) Dark current as a function of the applied bias voltage of square-mesa photodiodes with different side lengths: 20, 50, 100, and 200 µm. (b) Dark current versus side length of square-mesa diodes at 1 V reverse bias.
subjected to thermal annealing [5,7], although a certain worse than the best reported one [11]. Furthermore, for our devices, the dark current density decreases as the mesa length increases. This indicates that the source of dark current is not only the bulk leakage, but also the peripheral leakage. To separate the bulk and perimeter components, we fit the current data to the following model: I = JSurf ⋅4L + JBulk ⋅L
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Acknowledgements This work was partly supported by the National Basic Research Program of China (973 Program) under grant No. 2007CB613404 and Program for New Century Excellent Talents in University.
References
Fig. 5. Normalized spectral responsivity of p–i–n Ge diode measured at zero bias compared to the simulated result utilizing the bulk Ge absorption coefficient.
4. Conclusion In conclusion, a normal incidence p–i–n Ge heterojunction photodiode grown by UHVCVD on planar Si substrate without hightemperature thermal annealing has been demonstrated. The low bulk dark current density of 10 mA/cm2 confirms the good Ge quality. A substantial response in the NIR was achieved due to the 0.14% residual tensile strain and the responsivity is 0.23 A/W at 1.55 μm wavelength under 2 V reverse bias. These results suggest that a low thermal budget CMOS-comparable process to fabricate a high performance Ge/Si photodiode is possible.
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