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DC and low-frequency-noise characterization of epitaxially grown raised-emitter SiGe HBTs
Thin Solid Films 517 (2008) 6–9
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
DC and low-frequency-noise characterization of epitaxially grown raised-emitter SiGe HBTs Katsuyoshi Washio ⁎ Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan
a r t i c l e
i n f o
Available online 13 August 2008 Keywords: Epitaxial growth Phosphorous Heterojunction Bipolar transistor SiGe Low-frequency noise Polysilicon High-frequency
a b s t r a c t DC and low-frequency-noise characteristics of SiGe HBTs with a raised-emitter structure, fabricated by epitaxial growth of phosphorous-doped Si layers, were investigated. Experimental results indicate unexpected emitter-size dependencies of both base current and low-frequency noise, because mono–poly interfacial native oxides close to the intrinsic emitter-base junction are localized at the emitter periphery. The raised mono-Si emitter SiGe HBT with a scaled emitter exhibits low-frequency noise that is about ten times smaller than a conventional poly-Si emitter SiGe HBT. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to their high-frequency, high-gain, and low-noise performance, silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) are widely used in many high-speed wired communications and microwave wireless applications [1]. To improve their highfrequency capability further, in response to the needs of recent communication systems with high transmission rate, their device geometry has been aggressively scaled down. Poly-Si emitter (PE) bipolar transistors provide high current gain because of a mono–poly interfacial oxide between the poly-Si emitter electrode and mono-Si intrinsic emitter. However, as the emitter of a PE SiGe HBT is scaled down, emitter resistance is increased and transistor characteristics fluctuate largely. Furthermore, low-frequency-noise (1/f noise) characteristics are significantly influenced by the interfacial oxide [2]. To overcome these problems, a solution, replacing conventional poly-Si layers for emitter electrodes with epitaxially grown mono-Si layers, has been proposed, and its effectiveness has been verified [3]. The 1/f noise in microwave front-end analog circuits, such as voltage-controlled oscillators and mixers, is very important because it generates undesired phase noise. There have been therefore numerous studies on measurements and models concerning 1/f noise in the case of PE bipolar transistors. However, few data on an epitaxially grown raised-emitter (ERE) SiGe HBT have been reported. Accordingly, in this work, DC and 1/f-noise characteristics of scaled ERE SiGe HBTs were investigated. This found unexpected size dependencies of
⁎ Corresponding author. Tel.: +81 42 323 1111; fax: +81 42 327 7764. E-mail address: [email protected]. 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.08.020
these characteristics and 1/f noise about ten times smaller than a conventional PE SiGe HBT. 2. Fabrication process of ERE SiGe HBTs A cross-sectional view of the intrinsic region of an ERE SiGe HBT is shown in Fig. 1. After conventional process sequences for fabricating the self-aligned SiGe HBT with maximum cutoff frequency of about 90 GHz, including the selective epitaxial growth of i-Si/p+-SiGe/i-SiGe and the formation of n+ poly-Si layers on the sidewall of the emitterbase isolation oxide, were done, a blanket phosphorous-doped (about 2 × 1020 cm− 3) Si layer for the ERE was formed using low-pressure chemical-vapor deposition (LPCVD). Here, the thickness of the ERE layer was set to be thicker than the hole-diffusion length. The trapeziform ERE layers were grown only on the surface of intrinsic regions and amorphous layers were deposited on other layers. The conditions for epitaxial growth of a phosphorous-doped Si layer were investigated by changing growth temperature, total pressure, and PH3 partial pressure to achieve good crystallinity and low resistivity. The dependence of resistivity on growth temperature is shown in Fig. 2. The resistivity monotonically decreases with lowering growth temperature and is saturated at about 550 °C. In the case of growth temperature over about 600 °C, poor surface morphology due to island-like growth caused by the stacking faults is observed. It is considered that this phenomenon induces deterioration of phosphorous activation. To attain the lowest resistivity and sufficient growth rate, the growth temperature for the phosphorous-doped Si layer was set to 555 °C. The native oxides on the surface of the intrinsic regions were completely removed by H2 cleaning at 800 °C before epitaxial growth
K. Washio / Thin Solid Films 517 (2008) 6–9
7
Fig. 1. Cross-sectional view of intrinsic region of ERE SiGe HBT.
Fig. 4. Typical Gummel plots of ERE and PE SiGe HBTs with emitter area of 0.2 × 1 µm.
Following the formation of ERE, to obtain sufficient thickness for the following silicide formation, a conventional n+ poly-Si layer for the emitter electrode was deposited. An interfacial oxide between the n+ poly-Si and ERE layers therefore existed. Finally, to achieve nearly the same current gain in the ERE SiGe HBTs as that in the PE SiGe HBTs, the annealing time for the emitter drive-in was adjusted. The annealing time for the ERE SiGe HBT is a little longer than that for the PE SiGe HBT. This is because phosphorous concentration in the ERE layer is about two times lower than that in the PE layer, and the effect of enhanced diffusion of phosphorous is smaller for the ERE SiGe HBTs than that for the PE SiGe HBTs. 3. Measurement results 3.1. DC characteristics Fig. 2. Dependence of resistivity on growth temperature for epitaxial growth of phosphorous-doped Si layer.
and this was confirmed by a SIMS profile, as shown in Fig. 3. Compared with the interfacial oxide layer between the ERE (P-doped Si) and i-Si layers (indicated by the high-oxygen-concentration peak of about 1021 cm− 3) in the case of the PE SiGe HBT, no interfacial oxide layer is observed in the case of the ERE SiGe HBT.
Fig. 3. Oxygen depth profile near interface between phosphorous-doped Si and i-Si layers.
Typical Gummel plots of the ERE and PE SiGe HBTs are shown in Fig. 4. The maximum current gain is 400–500 for both HBTs. There is no non-ideal base current in the case of the ERE SiGe HBT like the observed base current with ideal factor of about 1.15 in the PE HBT case. This good DC characteristic of the ERE SiGe HBT is attributed to the removal of the interfacial oxides on the surface of the intrinsic region. In compensation for this, base current at VBE over 0.8 V in the ERE SiGe HBT is, however, two to three times larger than that in the PE HBT, because the interfacial oxide plays the role of a barrier to holecurrent flow. The dependence of normalized values of IB/AE (IB: base current, AE: emitter area) on PE/AE (PE: emitter periphery) is shown in
Fig. 5. Dependence of normalized values of IB/AE on PE/AE for ERE and PE SiGe HBTs.
8
K. Washio / Thin Solid Films 517 (2008) 6–9
Fig. 6. Typical curves of base-current-noise spectral density SIB at IB of 100 nA for ERE and PE SiGe HBTs with emitter area of 0.15 × 0.7 µm.
Fig. 5. In the case of the PE SiGe HBT, IB/AE generally increases with PE/ AE because the ratio of recombination current at the emitter periphery becomes large; on the other hand, it unexpectedly decreases with PE/ AE in the case of the ERE SiGe HBT. It can be considered that only the interfacial oxide between the n+ poly-Si and ERE layers at the emitter periphery acts as a hole-current barrier. This is understandable with respect to the structure of trapeziform ERE layers because the inter-
Fig. 7. Dependence of SIB at 1 Hz on base current for ERE and PE SiGe HBTs; emitter areas of (a) 0.15 × 0.7 µm and (b) 0.2 × 5 µm.
Fig. 8. Dependence of SIB at 1 Hz on AE for ERE and PE SiGe HBTs.
face between the n+ poly-Si and ERE layers is close to the intrinsic emitter-base junction at the facet region of the ERE. 3.2. 1/f Noise Typical curves of the base-current-noise spectral density SIB at an IB of 100 nA for the ERE and PE SiGe HBTs with an emitter area of 0.15 × 0.7 µm are shown in Fig. 6. The ERE SiGe HBT exhibits SIB about ten times smaller than that of the PE HBT. The dependence of SIB at 1 Hz on the base current for the ERE and PE SiGe HBTs, with emitter areas of 0.15 × 0.7 and 0.2 × 5 µm, is shown in Fig. 7(a) and (b), respectively. For the two emitter areas and two emitter structures, the quadratic dependence of base-current-noise spectral density on base current, SIB ~ I2B, was observed. However, there is a large difference in SIB of the SiGe HBTs with an emitter area of 0.15 × 0.7 µm despite having nearly the same SIB as that of the SiGe HBTs with an emitter area of 0.2 × 5 µm. The dependence of SIB at 1 Hz on emitter area AE is shown in Fig. 8. SIB is approximately inversely proportional to AE within the scatter of data obtained for the PE SiGe HBTs. This means that the 1/f-noise sources follow the tunnel diode model, which assumes that the Nyquist noise of the oxide layer modulates the barrier height and hence tunneling probability of carriers (transparency fluctuation) [4]. Here, the 1/f-noise sources are homogeneously distributed over the emitter area and are spatially uncorrelated; that is, excess noise sources are located at the intrinsic emitter-base junction. On the other hand, an anomalous dependence, namely, there is small increase in SIB with AE,
Fig. 9. Dependence of SIB at 1 Hz on PE for ERE SiGe HBTs.
K. Washio / Thin Solid Films 517 (2008) 6–9
9
is observed for the ERE SiGe HBTs. This dependence indicates that the noise sources are inhomogeneously distributed over the emitter area. The dependence of SIB at 1 Hz on PE obtained for the ERE SiGe HBTs is presented in Fig. 9. Within the scatter of data, SIB increases with PE (SIB ∝ PE). This dependence indicates that the excess noise sources are located at the periphery of the emitter-base junction. This result is understandable with respect to the structure of the ERE SiGe HBTs because their interfacial oxide between the n+ poly-Si and ERE layers located at the emitter periphery is close to the intrinsic emitter-base junction. The mono–poly interface on the top of the ERE layer is sufficiently far from the emitter-base junction; therefore, dependency of SIB ∝ A−E 1 was not observed. This result is consistent with the unexpected decrease in IB/AE with increasing PE/AE in the case of the ERE SiGe HBTs, which arises from that the interfacial oxide on top of the ERE layer does not act as a hole-current barrier.
mono–poly interfacial oxides close to the intrinsic emitter-base junction are localized at the emitter periphery. The very small measured SIB of the scaled ERE SiGe HBTs demonstrates their better lowfrequency-noise performance than that of conventional PE SiGe HBTs.
4. Summary
[1] K. Washio, IEEE Trans. Electron Devices 50 (2003) 656. [2] M.J. Deen, F. Pascal, IEE Proc.-Circuits Devices Syst. 151 (2004) 121. [3] T.F. Meister, H. Schäfer, K. Aufinger, R. Stengl, S. Boguth, R. Schreiter, M. Rest, H. Knapp, M. Wurzer, A. Mitchell, T. Böttner, J. Böck, IEEE BCTM 2003, Proceeding of the 2003 Bipolar/BiCMOS Circuits and Technology Meeting, France, Toulouse, Sept. 28–30, 2003, p. 103. [4] H.A.W. Markus, T.G.M. Kleinpenning, IEEE Trans. Electron Devices 42 (1995) 720.
DC and 1/f-noise characteristics of scaled ERE SiGe HBTs fabricated by epitaxial growth of phosphorous-doped Si layer were investigated. Unexpected size dependencies of both base current and 1/f noise were found, and they can be considered to arise from that the fact the
Acknowledgements The author would like to express his sincere thanks to Mr. I. Suzumura of the Central Research Laboratory, Hitachi, Ltd. for his contribution to the development of the phosphorous-doped epitaxialgrowth process. This work was partially supported by “The research and development project for expansion of radio spectrum resources” of the Ministry of Internal Affairs and Communications, Japan. References
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
DC and low-frequency-noise characterization of epitaxially grown raised-emitter SiGe HBTs Katsuyoshi Washio ⁎ Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan
a r t i c l e
i n f o
Available online 13 August 2008 Keywords: Epitaxial growth Phosphorous Heterojunction Bipolar transistor SiGe Low-frequency noise Polysilicon High-frequency
a b s t r a c t DC and low-frequency-noise characteristics of SiGe HBTs with a raised-emitter structure, fabricated by epitaxial growth of phosphorous-doped Si layers, were investigated. Experimental results indicate unexpected emitter-size dependencies of both base current and low-frequency noise, because mono–poly interfacial native oxides close to the intrinsic emitter-base junction are localized at the emitter periphery. The raised mono-Si emitter SiGe HBT with a scaled emitter exhibits low-frequency noise that is about ten times smaller than a conventional poly-Si emitter SiGe HBT. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to their high-frequency, high-gain, and low-noise performance, silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) are widely used in many high-speed wired communications and microwave wireless applications [1]. To improve their highfrequency capability further, in response to the needs of recent communication systems with high transmission rate, their device geometry has been aggressively scaled down. Poly-Si emitter (PE) bipolar transistors provide high current gain because of a mono–poly interfacial oxide between the poly-Si emitter electrode and mono-Si intrinsic emitter. However, as the emitter of a PE SiGe HBT is scaled down, emitter resistance is increased and transistor characteristics fluctuate largely. Furthermore, low-frequency-noise (1/f noise) characteristics are significantly influenced by the interfacial oxide [2]. To overcome these problems, a solution, replacing conventional poly-Si layers for emitter electrodes with epitaxially grown mono-Si layers, has been proposed, and its effectiveness has been verified [3]. The 1/f noise in microwave front-end analog circuits, such as voltage-controlled oscillators and mixers, is very important because it generates undesired phase noise. There have been therefore numerous studies on measurements and models concerning 1/f noise in the case of PE bipolar transistors. However, few data on an epitaxially grown raised-emitter (ERE) SiGe HBT have been reported. Accordingly, in this work, DC and 1/f-noise characteristics of scaled ERE SiGe HBTs were investigated. This found unexpected size dependencies of
⁎ Corresponding author. Tel.: +81 42 323 1111; fax: +81 42 327 7764. E-mail address: [email protected]. 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.08.020
these characteristics and 1/f noise about ten times smaller than a conventional PE SiGe HBT. 2. Fabrication process of ERE SiGe HBTs A cross-sectional view of the intrinsic region of an ERE SiGe HBT is shown in Fig. 1. After conventional process sequences for fabricating the self-aligned SiGe HBT with maximum cutoff frequency of about 90 GHz, including the selective epitaxial growth of i-Si/p+-SiGe/i-SiGe and the formation of n+ poly-Si layers on the sidewall of the emitterbase isolation oxide, were done, a blanket phosphorous-doped (about 2 × 1020 cm− 3) Si layer for the ERE was formed using low-pressure chemical-vapor deposition (LPCVD). Here, the thickness of the ERE layer was set to be thicker than the hole-diffusion length. The trapeziform ERE layers were grown only on the surface of intrinsic regions and amorphous layers were deposited on other layers. The conditions for epitaxial growth of a phosphorous-doped Si layer were investigated by changing growth temperature, total pressure, and PH3 partial pressure to achieve good crystallinity and low resistivity. The dependence of resistivity on growth temperature is shown in Fig. 2. The resistivity monotonically decreases with lowering growth temperature and is saturated at about 550 °C. In the case of growth temperature over about 600 °C, poor surface morphology due to island-like growth caused by the stacking faults is observed. It is considered that this phenomenon induces deterioration of phosphorous activation. To attain the lowest resistivity and sufficient growth rate, the growth temperature for the phosphorous-doped Si layer was set to 555 °C. The native oxides on the surface of the intrinsic regions were completely removed by H2 cleaning at 800 °C before epitaxial growth
K. Washio / Thin Solid Films 517 (2008) 6–9
7
Fig. 1. Cross-sectional view of intrinsic region of ERE SiGe HBT.
Fig. 4. Typical Gummel plots of ERE and PE SiGe HBTs with emitter area of 0.2 × 1 µm.
Following the formation of ERE, to obtain sufficient thickness for the following silicide formation, a conventional n+ poly-Si layer for the emitter electrode was deposited. An interfacial oxide between the n+ poly-Si and ERE layers therefore existed. Finally, to achieve nearly the same current gain in the ERE SiGe HBTs as that in the PE SiGe HBTs, the annealing time for the emitter drive-in was adjusted. The annealing time for the ERE SiGe HBT is a little longer than that for the PE SiGe HBT. This is because phosphorous concentration in the ERE layer is about two times lower than that in the PE layer, and the effect of enhanced diffusion of phosphorous is smaller for the ERE SiGe HBTs than that for the PE SiGe HBTs. 3. Measurement results 3.1. DC characteristics Fig. 2. Dependence of resistivity on growth temperature for epitaxial growth of phosphorous-doped Si layer.
and this was confirmed by a SIMS profile, as shown in Fig. 3. Compared with the interfacial oxide layer between the ERE (P-doped Si) and i-Si layers (indicated by the high-oxygen-concentration peak of about 1021 cm− 3) in the case of the PE SiGe HBT, no interfacial oxide layer is observed in the case of the ERE SiGe HBT.
Fig. 3. Oxygen depth profile near interface between phosphorous-doped Si and i-Si layers.
Typical Gummel plots of the ERE and PE SiGe HBTs are shown in Fig. 4. The maximum current gain is 400–500 for both HBTs. There is no non-ideal base current in the case of the ERE SiGe HBT like the observed base current with ideal factor of about 1.15 in the PE HBT case. This good DC characteristic of the ERE SiGe HBT is attributed to the removal of the interfacial oxides on the surface of the intrinsic region. In compensation for this, base current at VBE over 0.8 V in the ERE SiGe HBT is, however, two to three times larger than that in the PE HBT, because the interfacial oxide plays the role of a barrier to holecurrent flow. The dependence of normalized values of IB/AE (IB: base current, AE: emitter area) on PE/AE (PE: emitter periphery) is shown in
Fig. 5. Dependence of normalized values of IB/AE on PE/AE for ERE and PE SiGe HBTs.
8
K. Washio / Thin Solid Films 517 (2008) 6–9
Fig. 6. Typical curves of base-current-noise spectral density SIB at IB of 100 nA for ERE and PE SiGe HBTs with emitter area of 0.15 × 0.7 µm.
Fig. 5. In the case of the PE SiGe HBT, IB/AE generally increases with PE/ AE because the ratio of recombination current at the emitter periphery becomes large; on the other hand, it unexpectedly decreases with PE/ AE in the case of the ERE SiGe HBT. It can be considered that only the interfacial oxide between the n+ poly-Si and ERE layers at the emitter periphery acts as a hole-current barrier. This is understandable with respect to the structure of trapeziform ERE layers because the inter-
Fig. 7. Dependence of SIB at 1 Hz on base current for ERE and PE SiGe HBTs; emitter areas of (a) 0.15 × 0.7 µm and (b) 0.2 × 5 µm.
Fig. 8. Dependence of SIB at 1 Hz on AE for ERE and PE SiGe HBTs.
face between the n+ poly-Si and ERE layers is close to the intrinsic emitter-base junction at the facet region of the ERE. 3.2. 1/f Noise Typical curves of the base-current-noise spectral density SIB at an IB of 100 nA for the ERE and PE SiGe HBTs with an emitter area of 0.15 × 0.7 µm are shown in Fig. 6. The ERE SiGe HBT exhibits SIB about ten times smaller than that of the PE HBT. The dependence of SIB at 1 Hz on the base current for the ERE and PE SiGe HBTs, with emitter areas of 0.15 × 0.7 and 0.2 × 5 µm, is shown in Fig. 7(a) and (b), respectively. For the two emitter areas and two emitter structures, the quadratic dependence of base-current-noise spectral density on base current, SIB ~ I2B, was observed. However, there is a large difference in SIB of the SiGe HBTs with an emitter area of 0.15 × 0.7 µm despite having nearly the same SIB as that of the SiGe HBTs with an emitter area of 0.2 × 5 µm. The dependence of SIB at 1 Hz on emitter area AE is shown in Fig. 8. SIB is approximately inversely proportional to AE within the scatter of data obtained for the PE SiGe HBTs. This means that the 1/f-noise sources follow the tunnel diode model, which assumes that the Nyquist noise of the oxide layer modulates the barrier height and hence tunneling probability of carriers (transparency fluctuation) [4]. Here, the 1/f-noise sources are homogeneously distributed over the emitter area and are spatially uncorrelated; that is, excess noise sources are located at the intrinsic emitter-base junction. On the other hand, an anomalous dependence, namely, there is small increase in SIB with AE,
Fig. 9. Dependence of SIB at 1 Hz on PE for ERE SiGe HBTs.
K. Washio / Thin Solid Films 517 (2008) 6–9
9
is observed for the ERE SiGe HBTs. This dependence indicates that the noise sources are inhomogeneously distributed over the emitter area. The dependence of SIB at 1 Hz on PE obtained for the ERE SiGe HBTs is presented in Fig. 9. Within the scatter of data, SIB increases with PE (SIB ∝ PE). This dependence indicates that the excess noise sources are located at the periphery of the emitter-base junction. This result is understandable with respect to the structure of the ERE SiGe HBTs because their interfacial oxide between the n+ poly-Si and ERE layers located at the emitter periphery is close to the intrinsic emitter-base junction. The mono–poly interface on the top of the ERE layer is sufficiently far from the emitter-base junction; therefore, dependency of SIB ∝ A−E 1 was not observed. This result is consistent with the unexpected decrease in IB/AE with increasing PE/AE in the case of the ERE SiGe HBTs, which arises from that the interfacial oxide on top of the ERE layer does not act as a hole-current barrier.
mono–poly interfacial oxides close to the intrinsic emitter-base junction are localized at the emitter periphery. The very small measured SIB of the scaled ERE SiGe HBTs demonstrates their better lowfrequency-noise performance than that of conventional PE SiGe HBTs.
4. Summary
[1] K. Washio, IEEE Trans. Electron Devices 50 (2003) 656. [2] M.J. Deen, F. Pascal, IEE Proc.-Circuits Devices Syst. 151 (2004) 121. [3] T.F. Meister, H. Schäfer, K. Aufinger, R. Stengl, S. Boguth, R. Schreiter, M. Rest, H. Knapp, M. Wurzer, A. Mitchell, T. Böttner, J. Böck, IEEE BCTM 2003, Proceeding of the 2003 Bipolar/BiCMOS Circuits and Technology Meeting, France, Toulouse, Sept. 28–30, 2003, p. 103. [4] H.A.W. Markus, T.G.M. Kleinpenning, IEEE Trans. Electron Devices 42 (1995) 720.
DC and 1/f-noise characteristics of scaled ERE SiGe HBTs fabricated by epitaxial growth of phosphorous-doped Si layer were investigated. Unexpected size dependencies of both base current and 1/f noise were found, and they can be considered to arise from that the fact the
Acknowledgements The author would like to express his sincere thanks to Mr. I. Suzumura of the Central Research Laboratory, Hitachi, Ltd. for his contribution to the development of the phosphorous-doped epitaxialgrowth process. This work was partially supported by “The research and development project for expansion of radio spectrum resources” of the Ministry of Internal Affairs and Communications, Japan. References