Home
Search
Collections
Journals
About
Contact us
My IOPscience
Designer germanium quantum dot phototransistor for near infrared optical detection and amplification
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 055203 (http://iopscience.iop.org/0957-4484/26/5/055203) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 05/06/2017 at 15:43 Please note that terms and conditions apply.
You may also be interested in: Matrix and quantum confinement effects on optical and thermal properties of Ge quantum dots J E Chang, P H Liao, C Y Chien et al. Advances in graphene–based optoelectronics, plasmonics and photonics Bich Ha Nguyen and Van Hieu Nguyen Enhanced 400–600 nm photoresponsivity of metal–oxide–semiconductor diodes withmulti-stack germanium quantum dots S S Tzeng and P W Li Formation of Ge quantum dots array in layer-cake technique for advanced photovoltaics C Y Chien, Y J Chang, J E Chang et al. Photonics and optoelectronics of two-dimensional materials beyond graphene Joice Sophia Ponraj, Zai-Quan Xu, Sathish Chander Dhanabalan et al. Nanocrystals for silicon-based light-emitting and memory devices S K Ray, S Maikap, W Banerjee et al. Semiconductor ultraviolet photodetectors based on ZnO and MgxZn1xO Yaonan Hou, Zengxia Mei and Xiaolong Du Comparison of photoresponse of transistors based on graphene-quantum dot hybrids with layered and bulk heterojunctions Yating Zhang, Xiaoxian Song, Ran Wang et al. Organic phototransistors using poly(3-hexylthiophene) nanofibres Wouter Dierckx, Wibren D Oosterbaan, Jean-Christophe Bolsée et al.
Nanotechnology Nanotechnology 26 (2015) 055203 (9pp)
doi:10.1088/0957-4484/26/5/055203
Designer germanium quantum dot phototransistor for near infrared optical detection and amplification M H Kuo1, W T Lai1, T M Hsu1, Y C Chen2, C W Chang2, W H Chang2 and P W Li1 1
Department of Electrical Engineering and Center for Nano Science and Technology, National Central University, ChungLi, Taiwan, 32001, People’s Republic of China 2 Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, 300, People’s Republic of China E-mail: [email protected] Received 27 August 2014, revised 14 December 2014 Accepted for publication 16 December 2014 Published 15 January 2015 Abstract
We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (Ioff ∼ 0.27 pA μm−2), a high on-off current ratio (Ion/Ioff ∼ 106), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on- and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (Iphoto/Idark) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 × 106 and 1.7 A W−1, respectively, under an incident power of 0.9 mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects. Keywords: germanium quantum dots, MOS phototransistor, optical interconnect (Some figures may appear in colour only in the online journal) 1. Introduction
replacing the long metallic interconnects with an optical interconnect. Such hybrid optical/electronic interconnects have great promise in facilitating better performance even with larger designs [5–8]. In order to effectively utilize optical links in chip-to-chip [7] or on-chip interconnections [8], many material, fabrication and packaging challenges must be solved, in particular, when optical/electrical signal conversions and amplifications are considered. Recently 1–2 μm scale thick Ge-on-Si photodetectors have demonstrated superior efficiencies over their hybrid counterparts and are fast becoming an effective replacement for the conventional building blocks of the transmitter end in an optical link operating at communication standard wavelengths [9–13]. However, the growth of high-
The relentless miniaturization of electronic components has been a widely accepted approach for boosting integratedcircuit (IC) performance [1]. Recently, feature sizes of metaloxide-semiconductor (MOS) transistors, the core building block of ICs, have been aggressively driven into nanometer scales in order to achieve the desired improvement in drive current and operating speed [2, 3]. However, gains from individual transistors have not yet contributed satisfactory improvement in the overall speed of ICs because tight packing of miniaturized metal wires leads to a tremendously enlarged signal propagation delay [4]. It is highly envisaged that this looming interconnect crisis can be overcome by 0957-4484/15/055203+09$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 1. Schematic diagram, key fabrication process flow and CTEM micrograph of the experimental Ge-QD MOS phototransistor.
quality μm-scale-thick Ge epitaxial films on Si is very challenging because of the large lattice mismatch of 4.2% between these two materials. Several approaches, such as buffer layers, two-step growth, annealing and selective growth [14–17], have been proposed to reduce threading dislocation densities and to improve surface roughness in order to achieve high photodetection efficiency. However, the μm-scale Ge/Si heterostructures are too thick to be directly integrated with prevailing submicron or even nanometerscale-thick Si electronic devices. Phototransistors are a unique optical transducer in which light detection/photoelectric conversion (photodiodes) and electrical signal amplification (transistors) are combined within a single device and thus have none of the associated concerns of noise increments, high voltages and high cost [18–20]. Among the possible material and device structure choices for phototransistors, Ge MOS transistors offer promising solutions for low-cost, short-reach optical interconnects, thanks to Ge’s high optical absorption coefficient and enhanced carrier mobility. Most importantly, Ge MOS transistor has emerged as a leading contender to replace its well-established counterpart, Si, for post complementary MOS transistors [21, 22]. Our previous report demonstrated Ge-nanocrystallite phototransistors for blue to near ultraviolent (400–550 nm) photodetection based on a double-gated poly-Si thin-film transistor (TFT) structure [23, 24]. In these Ge-nanocrystallite photo-TFTs, self-assembled clusters of 7.62 ± 0.94 nm, irregular-shaped Ge nanocrystallites were embedded within a 45 nm thick gate oxide. External quantum efficiency as high as >200% is achieved from these doubled-gated Ge-nanocrystallite photo-TFTs under 400–550 nm illumination, yet the on-off current ratio (Ion/Ioff) of 104 and the transient speed, which is of the sub-millisecond [24], need to be further improved. Recently, we have demonstrated a unique capability to precisely place spherical Ge quantum dots (QDs) of desired sizes at targeted spatial locations over the Si substrate using selective oxidation of lithographically-patterned SiGe nano-pillars over buffer Si3N4 layers on the Si substrate [25].
In this approach, the presence of a thin, 3–4 nm thick amorphous interfacial oxide between the Ge QD and the Si substrate is indeed beneficial to de-couple the lattice mismatch constraints between Ge and Si, allow the Ge QD morphology to achieve a spherical shape and improve the crystallinity of the Ge QDs. The excellent hetero-interfacial quality of our designer Ge QDs on Si has been confirmed by an extremely low dark current, together with the significantly improved photocurrent and quantum efficiency of Ge QD/Si MOS photodiodes. We further demonstrated size-tunable photoluminescence of 370–1350 nm in the wavelength from 6.5–50 nm Ge QDs [26], as well as size-dependent tensile or compressive strains generated from local embedded Ge QDs within either SiO2 or Si3N4 [27]. These exquisite size-tunable optical and structural properties indicate the feasibility of wide-ranging band gap engineering in our designer Ge QDs, enabling the production of high-performance Ge phototransistors for broad-band photo detection and photoelectric amplification. In this paper, we have exploited this ‘designer heterostructure’ to construct high performance Ge-QD MOS phototransistors. The Ge-QD MOS phototransistors feature extremely low dark current densities (Ioff ∼ 0.27 pA μm−2), high Ion/Ioff (>106), steep subthreshold swing (∼175 mV/dec at 300 K), superior external quantum efficiency (>100%) and fast response time in nanoseconds under illumination of 850 nm, providing a core building block for Ge optical transducers for on-chip optical interconnect applications.
2. Experiments The experimental procedure, cross-sectional transmission electron microscopy (CTEM) micrographs and schematic diagram of the Ge-QD MOS phototransistors are illustrated in figure 1. The fabrication of the Ge-QD MOS phototransistor started from a (100) p-Si substrate with a resistivity of 0.2–1 Ω cm. After local oxidation isolation processes, arsenic (5 × 1015 cm−2, 25 keV) and BF2 (5 × 1015 cm−2, 10 keV) 2
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 2. (a) STEM micrograph, (b) EDX and (c) Raman spectra show good crystallinity and high purity of the 50 nm Ge QD embedded within buffer Si3N4. A considerable blueshift of 12.7 cm−1 for the Ge-Ge phonon mode was measured on the 50 nm Ge QDs compared to the bulk Raman line.
operating frequency response of the Ge-QD MOS phototransistors was characterized using an ultrafast optical pulse laser (850 nm) driven by a pulse generator in conjunction with an Agilent 86100C digital communication analyzer for recording the pulse response from Ge-QD MOS phototransistors. The PL measurements were conducted using the 488 nm line of an argon ion laser as the excitation source. The PL signal was analyzed using a 0.5 m monochromator and was detected by an InGaAs photomultiplier tube. For the time-resolved PL (TRPL) measurements, the excitation source was replaced by a 50 ps pulsed laser diode (635 nm/ 80 MHz). The PL decay traces were recorded using the timecorrelated single photon counting technique with an overall time resolution of ∼200 ps.
dopants were implanted to form source/drain and substrate electrodes, respectively. The source-drain length and junction depth are 20 μm and 150 nm, respectively. Next, a tri-layer deposition of 30 nm thick Si3N4/60 nm thick polySi0.85Ge0.15/5 nm thick SiO2 was conducted sequentially over the Si substrate. The topmost SiO2 layer is deposited as a hard mask for subsequent plasma etching for producing SiGe nanopillars. The SiO2 cap also prevents the evaporation of Ge during the next high-temperature oxidation process for the generation of Ge QDs from SiGe nanopillars. The buffer Si3N4 layer between the SiGe nanopillars and the Si substrate serves as the initial, local source of Si interstitials for facilitating the migration of Ge QDs. This thin Si3N4 layer also acts as an oxidation mask to protect the Si substrate from oxidation during the thermal oxidation of SiGe nano-pillars. Using a combination of electron-beam lithography and SF6/ C4F8 plasma patterning processes, SiGe nanopillar structures 100 nm in diameter were fabricated. The SiGe nanopillars were then subjected to thermal oxidation at 900 °C for 60 min within an H2O ambient for generating spherical Ge QDs 50 nm in diameter. Lastly, a 150 nm thick indium tin oxide (ITO) was deposited and then patterned as a transparent gate electrode with gate length (Lg) of 20 μm and gate width (W) of 20 μm, followed by source/drain metallization and sintering processes to complete the device fabrication. CTEM, electron dispersive x-ray (EDX), Raman and photoluminescence (PL) spectroscopies were utilized to examine the structural and optical properties of the Ge QDs. The current-voltage (I–V) characteristics of Ge-QD MOS phototransistors were measured using a KEITHLEY 4200 semiconductor parameter analyzer in the darkness or under illumination through a transparent ITO electrode. The
3. Results and discussions The good crystallinity and high purity of spherical Ge QDs were verified from extensive TEM and EDX examinations (figure 2). The EDX analysis showed no significant amount of Si concentration ( 0) under illumination. The photovoltaic effect induced by photo-holes generated in the Ge QDs leads to significant enhancement in Ion at VG > 0.
coefficient of αSi ≅ 5.35 × 102 cm−1 for Si at 850 nm, we estimate that approximately 0.08% of the incident light is effectively absorbed therein, contributing slight photocurrent enhancement, whereas for Ge-QD MOSFETs, a positive gate voltage is conducive to inject photoholes generated within the Ge-QD layer of 50 nm into the Si channel (figures 6(d)–(f)). The injected holes are immediately swept toward the source by the lateral drain E-field and are blocked by a built-in potential barrier at the source/channel junction. This accumulation of holes effectively lowers the potential barrier for electron injection from the source, leading to a significant enhancement in Ion. This is the so-called photovoltaic effect, which commonly happens in hydrogenated amorphous-Si [33, 34].
hole in the vicinity of the gate oxide/Si interface (approximately 150 nm in thickness for the substrate concentration at 1016 cm−3) and in turn forms an inversion layer (approximately 10 nm in thickness) containing mobile electrons adjacent to the Si surface. An inverting gate bias (VG > 0) therefore induces a conducting channel connecting the source and drain. For the control MOSFET containing no QDs, only photoelectrons generated near the surface channel and depletion region (the total depth of 150–200 nm) are apt to contribute photoconductive currents (figures 6(a)–(c)). The photoelectron-hole pairs generated at the place below the source/drain and depletion regions have a negligible contribution for the drain current. Considering the absorption
7
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 7. (a) Spectral response, (b) temporal response and frequency response of the Ge-QD MOS phototransistors under 0.6 mW 850 nm
illumination. The time-resolved photoluminescence spectrum of 50 nm Ge QDs is included for deducing the photocarrier lifetime.
beneficial for the optimal response speed for the Ge-QD MOS phototransistors.
Both spectral responses of the Ge-QD MOS phototransistors biased at the on-state with VG = +5 V and off-state with VG = −5 V show a decline in the photo current-to-dark current ratio when the wavelength of illumination goes beyond 950 nm (figure 7(a)). Notably, there appears to be considerable enhancement in the photocurrent by a factor of more than 30 in magnitude as the incident light wavelength approaches 1310 nm under 0.6 mW illumination, indicating that the Ge QDs are optically active. To assess the temporal response speed of the Ge-QD MOS phototransistor, an impulse light modulation with a pulse width of approximately 68 ps was conducted on the MOS transistors containing no QDs and the Ge-QD MOS phototransistors, respectively. The invisible impulse response peak for the MOS phototransistors containing no QDs (not shown here) excludes the artefact coming from the electrical set-up. A very fast temporal response time of 1.4 ns, together with a 3 dB bandwidth of 410 MHz, were measured on our Ge-QD MOS phototransistor biased at VG = +6 V and VD = +3 V (figure 7(b)). The response speed is essentially limited by the lifetime of photogenerated carriers in the absorption region of the Ge QD and by the gate RC delay, which is mainly determined by the thickness of the gate dielectrics of MOS diodes [35]. The measured PL lifetime of the Ge QDs at 20 K is approximately 1.7 ns (figure 7(b)). According to the temperature dependency of PL intensities in the range of 20–300 K, we estimate that the recombination lifetime at 300 K is shorter than 0.2 ns (which is beyond the temporal resolution limit of our TRPL system). The deduced photocarrier lifetime is approximately 1/10 of the temporal response time measured on the Ge-QD phototransistors. The measured photocarrier lifetime of 1.7 ns at 20 K for Ge QDs is three orders longer than the lifetime (∼1 ps) for oxygenimplanted Ge thin film [36], while it is significantly shorter than the microseconds-to-milliseconds lifetime (2–100 μs) of Si nanocrystals [37, 38]. The photo-carrier lifetime, which was as short as a sub-nanosecond, measured on our Ge QDs suggests the great promise of fast operation of Ge-QD photoconductive switches for on-chip optical interconnect applications. A large QD and a thin gate dielectric layer are
4. Conclusions In summary, high-performance Ge-QD MOS phototransistors have been demonstrated in a CMOS compatible approach. The Ge-QD phototransistor exhibits a significant enhancement in photocurrents and in high photoresponsivity thanks to a strong absorption of the Ge QDs and to the effective hole confinement within the Ge QDs based on both photoconductive and photovolatic effects. A fast temporal response of 1.4 ns of the Ge-QD MOS phototransistor offers great promise for future Si-based optical interconnection applications.
Acknowledgments This work was supported by the Ministry of Science and Technology of Taiwan, Republic of China (NSC-102-2221E-008-111-MY3) and by the Asian Office of Aerospace Research and Development under contract no. FA 2386-141-4008.
References [1] Moore G E 1965 Electronics 38 114 http://web.eng.fiu.edu/ npala/eee6397ex/gordon_moore_1995_article.pdf [2] Bohr M 2011 IEEE Int. Elect. Dev. Meet. 1.1 1 [3] Colinge J P 2007 Microelectron. Eng. 84 2071 [4] Srivastava N and Banerjee K 2004 TMS J. Mater. 56 30 [5] Miller D A B 2000 Proc. IEEE 88 728 [6] Heck M J R, Chen H W, Fang A W, Koch B R, Liang D, Park H, Sysak M N and Bowers J E 2011 IEEE J. Sel. Top. Quantum Electron. 17 333 [7] Forbes M, Gourlay J and Desmulliez M 2001 Electron. Commun. Eng. J. 13 221 [8] Haurylau M, Chen G, Chen H, Zhang J, Nelson N A, Albonesi D H, Friedman E G and Fauchet P M 2006 IEEE J. Sel. Top. Quantum Electron. 12 1699 8
M H Kuo et al
Nanotechnology 26 (2015) 055203
[9] Wang K L, Cha D, Liu J and Chen C 2007 Proc. IEEE 95 1866 [10] Liu J and Michel J 2008 ECS Trans. 16 575 [11] Michel J, Liu J and Kimerling L C 2010 Nat. Photonics 4 527 [12] Samavedam S B, Currie M T, Langdo T A and Fitzgerald E A 1998 Appl. Phys. Lett. 73 2125 [13] Colace L, Ferrara P, Assanto G, Fulgoni F and Nash L 2007 IEEE Photon. Technol. Lett. 19 1813 [14] Giussani A, Rodenbach P, Zaumeil P, Dabrowski J, Kurps R, Weidner G, Mussig H J, Storck P, Wollschlager J and Schroedar T 2009 J. Appl. Phys. 105 033512 [15] Koster S J, Schaub J D, Delinger G and Chu J O 2006 IEEE. J. Sel. Top. Quant. Electron. 12 1489 [16] Choi D, Ge Y, Harris J S, Cagnon J and Stemmer S 2008 J. Cryst. Growth 310 4273 [17] Langdo T A, Leitz C W, Currie M T, Fitzgerald E A, Lochtefeld A and Antoniadis D A 2000 Appl. Phys. Lett. 76 3700 [18] Chandrasekhar S, Campbell J C, Dentai A G, Joyner C H, Qua G J, Gnauck A H and Feuer M D 1988 Electron. Lett. 24 1443 [19] Schaub J D, Koester S J, Dehlinger G, Ouyang Q C, Guckenberger D, Yang M, Rogers D L, Chu J and Grill A 2004 Proc. SPIE 5353 1 [20] Shieh J M, Lai Y F, Ni W X, Kuo H C, Fang C Y, Huang J Y and Pan C L 2007 Appl. Phys. Lett. 90 051105 [21] Yu H Y, Ishibashi M, Park J-H, Kobayashi M and Saraswat K C 2009 IEEE Electron Device Lett. 30 675 [22] Zhang R, Iwasaki T, Taoka N, Takenaka M and Takagi S 2012 IEEE Trans. Electron Devices 59 335
[23] Chen I H, Tseng S S and Li P W 2009 IEEE Photonics Technol. Lett. 21 1674 [24] Tzeng S S, Chen I H and Li P W 2008 Appl. Phys. Lett. 93 191112 [25] Kuo M H, Wang C C, Lai W T, George T and Li P W 2012 Appl. Phys. Lett. 101 223107 [26] Chien C Y, Lai W T, Chang J R, Wang C C, Kuo M H and Li P W 2014 Nanoscale 6 5303 [27] Liao P H, Hsu T C, Chen K H, Cheng T H, Hsu T M, Wang C C, George T and Li P W 2014 Appl. Phys. Lett. 105 172106 [28] Ishikawa Y, Wada K, Cannon D D, Liu J, Luan H C and Kimerling L C 2003 Appl. Phys. Lett. 82 2044 [29] Reparaz J S, Bernardi A, Goñi A R, Lacharmoise P D, Alonso M I, Garriga M, Novák J and Vávra I 2007 Appl. Phys. Lett. 91 081914 [30] Schmidt T, Lischka K and Zulehner W 1992 Phys. Rev. B 45 8989 [31] Jin S, Zheng Y and Li A 1997 J. Appl. Phys. 82 3870 [32] Chien C Y, Chang Y R, Chang R N, Lee M S, Chen W Y, Hsu T M and Li P W 2010 Nanotechnology 21 505201 [33] Kaneko Y, Koike N, Tsutsui K and Tsukada T 1990 Appl. Phys. Lett. 56 650 [34] GadelRab S M and Chamberlain S G 1997 IEEE Trans. Electron Devices 44 1789 [35] Liu P et al 2012 J. Appl. Phys. 112 083103 [36] Sekine N, Hirakawa K, Sogawa F, Arakawa Y, Usami N, Shiraki Y and Katoda T 1996 Appl. Phys. Lett. 68 3419 [37] Dao L V, Wen X, Do M T T, Hannaford P, Cho E C, Cho Y H and Huang Y 2005 J. Appl. Phys. 97 013501 [38] Hartel A M, Gutsch S, Hiller D and Zacharias M 2013 Phys. Rev. B 87 035428
9
Search
Collections
Journals
About
Contact us
My IOPscience
Designer germanium quantum dot phototransistor for near infrared optical detection and amplification
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 055203 (http://iopscience.iop.org/0957-4484/26/5/055203) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 05/06/2017 at 15:43 Please note that terms and conditions apply.
You may also be interested in: Matrix and quantum confinement effects on optical and thermal properties of Ge quantum dots J E Chang, P H Liao, C Y Chien et al. Advances in graphene–based optoelectronics, plasmonics and photonics Bich Ha Nguyen and Van Hieu Nguyen Enhanced 400–600 nm photoresponsivity of metal–oxide–semiconductor diodes withmulti-stack germanium quantum dots S S Tzeng and P W Li Formation of Ge quantum dots array in layer-cake technique for advanced photovoltaics C Y Chien, Y J Chang, J E Chang et al. Photonics and optoelectronics of two-dimensional materials beyond graphene Joice Sophia Ponraj, Zai-Quan Xu, Sathish Chander Dhanabalan et al. Nanocrystals for silicon-based light-emitting and memory devices S K Ray, S Maikap, W Banerjee et al. Semiconductor ultraviolet photodetectors based on ZnO and MgxZn1xO Yaonan Hou, Zengxia Mei and Xiaolong Du Comparison of photoresponse of transistors based on graphene-quantum dot hybrids with layered and bulk heterojunctions Yating Zhang, Xiaoxian Song, Ran Wang et al. Organic phototransistors using poly(3-hexylthiophene) nanofibres Wouter Dierckx, Wibren D Oosterbaan, Jean-Christophe Bolsée et al.
Nanotechnology Nanotechnology 26 (2015) 055203 (9pp)
doi:10.1088/0957-4484/26/5/055203
Designer germanium quantum dot phototransistor for near infrared optical detection and amplification M H Kuo1, W T Lai1, T M Hsu1, Y C Chen2, C W Chang2, W H Chang2 and P W Li1 1
Department of Electrical Engineering and Center for Nano Science and Technology, National Central University, ChungLi, Taiwan, 32001, People’s Republic of China 2 Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, 300, People’s Republic of China E-mail: [email protected] Received 27 August 2014, revised 14 December 2014 Accepted for publication 16 December 2014 Published 15 January 2015 Abstract
We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (Ioff ∼ 0.27 pA μm−2), a high on-off current ratio (Ion/Ioff ∼ 106), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on- and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (Iphoto/Idark) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 × 106 and 1.7 A W−1, respectively, under an incident power of 0.9 mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects. Keywords: germanium quantum dots, MOS phototransistor, optical interconnect (Some figures may appear in colour only in the online journal) 1. Introduction
replacing the long metallic interconnects with an optical interconnect. Such hybrid optical/electronic interconnects have great promise in facilitating better performance even with larger designs [5–8]. In order to effectively utilize optical links in chip-to-chip [7] or on-chip interconnections [8], many material, fabrication and packaging challenges must be solved, in particular, when optical/electrical signal conversions and amplifications are considered. Recently 1–2 μm scale thick Ge-on-Si photodetectors have demonstrated superior efficiencies over their hybrid counterparts and are fast becoming an effective replacement for the conventional building blocks of the transmitter end in an optical link operating at communication standard wavelengths [9–13]. However, the growth of high-
The relentless miniaturization of electronic components has been a widely accepted approach for boosting integratedcircuit (IC) performance [1]. Recently, feature sizes of metaloxide-semiconductor (MOS) transistors, the core building block of ICs, have been aggressively driven into nanometer scales in order to achieve the desired improvement in drive current and operating speed [2, 3]. However, gains from individual transistors have not yet contributed satisfactory improvement in the overall speed of ICs because tight packing of miniaturized metal wires leads to a tremendously enlarged signal propagation delay [4]. It is highly envisaged that this looming interconnect crisis can be overcome by 0957-4484/15/055203+09$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 1. Schematic diagram, key fabrication process flow and CTEM micrograph of the experimental Ge-QD MOS phototransistor.
quality μm-scale-thick Ge epitaxial films on Si is very challenging because of the large lattice mismatch of 4.2% between these two materials. Several approaches, such as buffer layers, two-step growth, annealing and selective growth [14–17], have been proposed to reduce threading dislocation densities and to improve surface roughness in order to achieve high photodetection efficiency. However, the μm-scale Ge/Si heterostructures are too thick to be directly integrated with prevailing submicron or even nanometerscale-thick Si electronic devices. Phototransistors are a unique optical transducer in which light detection/photoelectric conversion (photodiodes) and electrical signal amplification (transistors) are combined within a single device and thus have none of the associated concerns of noise increments, high voltages and high cost [18–20]. Among the possible material and device structure choices for phototransistors, Ge MOS transistors offer promising solutions for low-cost, short-reach optical interconnects, thanks to Ge’s high optical absorption coefficient and enhanced carrier mobility. Most importantly, Ge MOS transistor has emerged as a leading contender to replace its well-established counterpart, Si, for post complementary MOS transistors [21, 22]. Our previous report demonstrated Ge-nanocrystallite phototransistors for blue to near ultraviolent (400–550 nm) photodetection based on a double-gated poly-Si thin-film transistor (TFT) structure [23, 24]. In these Ge-nanocrystallite photo-TFTs, self-assembled clusters of 7.62 ± 0.94 nm, irregular-shaped Ge nanocrystallites were embedded within a 45 nm thick gate oxide. External quantum efficiency as high as >200% is achieved from these doubled-gated Ge-nanocrystallite photo-TFTs under 400–550 nm illumination, yet the on-off current ratio (Ion/Ioff) of 104 and the transient speed, which is of the sub-millisecond [24], need to be further improved. Recently, we have demonstrated a unique capability to precisely place spherical Ge quantum dots (QDs) of desired sizes at targeted spatial locations over the Si substrate using selective oxidation of lithographically-patterned SiGe nano-pillars over buffer Si3N4 layers on the Si substrate [25].
In this approach, the presence of a thin, 3–4 nm thick amorphous interfacial oxide between the Ge QD and the Si substrate is indeed beneficial to de-couple the lattice mismatch constraints between Ge and Si, allow the Ge QD morphology to achieve a spherical shape and improve the crystallinity of the Ge QDs. The excellent hetero-interfacial quality of our designer Ge QDs on Si has been confirmed by an extremely low dark current, together with the significantly improved photocurrent and quantum efficiency of Ge QD/Si MOS photodiodes. We further demonstrated size-tunable photoluminescence of 370–1350 nm in the wavelength from 6.5–50 nm Ge QDs [26], as well as size-dependent tensile or compressive strains generated from local embedded Ge QDs within either SiO2 or Si3N4 [27]. These exquisite size-tunable optical and structural properties indicate the feasibility of wide-ranging band gap engineering in our designer Ge QDs, enabling the production of high-performance Ge phototransistors for broad-band photo detection and photoelectric amplification. In this paper, we have exploited this ‘designer heterostructure’ to construct high performance Ge-QD MOS phototransistors. The Ge-QD MOS phototransistors feature extremely low dark current densities (Ioff ∼ 0.27 pA μm−2), high Ion/Ioff (>106), steep subthreshold swing (∼175 mV/dec at 300 K), superior external quantum efficiency (>100%) and fast response time in nanoseconds under illumination of 850 nm, providing a core building block for Ge optical transducers for on-chip optical interconnect applications.
2. Experiments The experimental procedure, cross-sectional transmission electron microscopy (CTEM) micrographs and schematic diagram of the Ge-QD MOS phototransistors are illustrated in figure 1. The fabrication of the Ge-QD MOS phototransistor started from a (100) p-Si substrate with a resistivity of 0.2–1 Ω cm. After local oxidation isolation processes, arsenic (5 × 1015 cm−2, 25 keV) and BF2 (5 × 1015 cm−2, 10 keV) 2
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 2. (a) STEM micrograph, (b) EDX and (c) Raman spectra show good crystallinity and high purity of the 50 nm Ge QD embedded within buffer Si3N4. A considerable blueshift of 12.7 cm−1 for the Ge-Ge phonon mode was measured on the 50 nm Ge QDs compared to the bulk Raman line.
operating frequency response of the Ge-QD MOS phototransistors was characterized using an ultrafast optical pulse laser (850 nm) driven by a pulse generator in conjunction with an Agilent 86100C digital communication analyzer for recording the pulse response from Ge-QD MOS phototransistors. The PL measurements were conducted using the 488 nm line of an argon ion laser as the excitation source. The PL signal was analyzed using a 0.5 m monochromator and was detected by an InGaAs photomultiplier tube. For the time-resolved PL (TRPL) measurements, the excitation source was replaced by a 50 ps pulsed laser diode (635 nm/ 80 MHz). The PL decay traces were recorded using the timecorrelated single photon counting technique with an overall time resolution of ∼200 ps.
dopants were implanted to form source/drain and substrate electrodes, respectively. The source-drain length and junction depth are 20 μm and 150 nm, respectively. Next, a tri-layer deposition of 30 nm thick Si3N4/60 nm thick polySi0.85Ge0.15/5 nm thick SiO2 was conducted sequentially over the Si substrate. The topmost SiO2 layer is deposited as a hard mask for subsequent plasma etching for producing SiGe nanopillars. The SiO2 cap also prevents the evaporation of Ge during the next high-temperature oxidation process for the generation of Ge QDs from SiGe nanopillars. The buffer Si3N4 layer between the SiGe nanopillars and the Si substrate serves as the initial, local source of Si interstitials for facilitating the migration of Ge QDs. This thin Si3N4 layer also acts as an oxidation mask to protect the Si substrate from oxidation during the thermal oxidation of SiGe nano-pillars. Using a combination of electron-beam lithography and SF6/ C4F8 plasma patterning processes, SiGe nanopillar structures 100 nm in diameter were fabricated. The SiGe nanopillars were then subjected to thermal oxidation at 900 °C for 60 min within an H2O ambient for generating spherical Ge QDs 50 nm in diameter. Lastly, a 150 nm thick indium tin oxide (ITO) was deposited and then patterned as a transparent gate electrode with gate length (Lg) of 20 μm and gate width (W) of 20 μm, followed by source/drain metallization and sintering processes to complete the device fabrication. CTEM, electron dispersive x-ray (EDX), Raman and photoluminescence (PL) spectroscopies were utilized to examine the structural and optical properties of the Ge QDs. The current-voltage (I–V) characteristics of Ge-QD MOS phototransistors were measured using a KEITHLEY 4200 semiconductor parameter analyzer in the darkness or under illumination through a transparent ITO electrode. The
3. Results and discussions The good crystallinity and high purity of spherical Ge QDs were verified from extensive TEM and EDX examinations (figure 2). The EDX analysis showed no significant amount of Si concentration ( 0) under illumination. The photovoltaic effect induced by photo-holes generated in the Ge QDs leads to significant enhancement in Ion at VG > 0.
coefficient of αSi ≅ 5.35 × 102 cm−1 for Si at 850 nm, we estimate that approximately 0.08% of the incident light is effectively absorbed therein, contributing slight photocurrent enhancement, whereas for Ge-QD MOSFETs, a positive gate voltage is conducive to inject photoholes generated within the Ge-QD layer of 50 nm into the Si channel (figures 6(d)–(f)). The injected holes are immediately swept toward the source by the lateral drain E-field and are blocked by a built-in potential barrier at the source/channel junction. This accumulation of holes effectively lowers the potential barrier for electron injection from the source, leading to a significant enhancement in Ion. This is the so-called photovoltaic effect, which commonly happens in hydrogenated amorphous-Si [33, 34].
hole in the vicinity of the gate oxide/Si interface (approximately 150 nm in thickness for the substrate concentration at 1016 cm−3) and in turn forms an inversion layer (approximately 10 nm in thickness) containing mobile electrons adjacent to the Si surface. An inverting gate bias (VG > 0) therefore induces a conducting channel connecting the source and drain. For the control MOSFET containing no QDs, only photoelectrons generated near the surface channel and depletion region (the total depth of 150–200 nm) are apt to contribute photoconductive currents (figures 6(a)–(c)). The photoelectron-hole pairs generated at the place below the source/drain and depletion regions have a negligible contribution for the drain current. Considering the absorption
7
M H Kuo et al
Nanotechnology 26 (2015) 055203
Figure 7. (a) Spectral response, (b) temporal response and frequency response of the Ge-QD MOS phototransistors under 0.6 mW 850 nm
illumination. The time-resolved photoluminescence spectrum of 50 nm Ge QDs is included for deducing the photocarrier lifetime.
beneficial for the optimal response speed for the Ge-QD MOS phototransistors.
Both spectral responses of the Ge-QD MOS phototransistors biased at the on-state with VG = +5 V and off-state with VG = −5 V show a decline in the photo current-to-dark current ratio when the wavelength of illumination goes beyond 950 nm (figure 7(a)). Notably, there appears to be considerable enhancement in the photocurrent by a factor of more than 30 in magnitude as the incident light wavelength approaches 1310 nm under 0.6 mW illumination, indicating that the Ge QDs are optically active. To assess the temporal response speed of the Ge-QD MOS phototransistor, an impulse light modulation with a pulse width of approximately 68 ps was conducted on the MOS transistors containing no QDs and the Ge-QD MOS phototransistors, respectively. The invisible impulse response peak for the MOS phototransistors containing no QDs (not shown here) excludes the artefact coming from the electrical set-up. A very fast temporal response time of 1.4 ns, together with a 3 dB bandwidth of 410 MHz, were measured on our Ge-QD MOS phototransistor biased at VG = +6 V and VD = +3 V (figure 7(b)). The response speed is essentially limited by the lifetime of photogenerated carriers in the absorption region of the Ge QD and by the gate RC delay, which is mainly determined by the thickness of the gate dielectrics of MOS diodes [35]. The measured PL lifetime of the Ge QDs at 20 K is approximately 1.7 ns (figure 7(b)). According to the temperature dependency of PL intensities in the range of 20–300 K, we estimate that the recombination lifetime at 300 K is shorter than 0.2 ns (which is beyond the temporal resolution limit of our TRPL system). The deduced photocarrier lifetime is approximately 1/10 of the temporal response time measured on the Ge-QD phototransistors. The measured photocarrier lifetime of 1.7 ns at 20 K for Ge QDs is three orders longer than the lifetime (∼1 ps) for oxygenimplanted Ge thin film [36], while it is significantly shorter than the microseconds-to-milliseconds lifetime (2–100 μs) of Si nanocrystals [37, 38]. The photo-carrier lifetime, which was as short as a sub-nanosecond, measured on our Ge QDs suggests the great promise of fast operation of Ge-QD photoconductive switches for on-chip optical interconnect applications. A large QD and a thin gate dielectric layer are
4. Conclusions In summary, high-performance Ge-QD MOS phototransistors have been demonstrated in a CMOS compatible approach. The Ge-QD phototransistor exhibits a significant enhancement in photocurrents and in high photoresponsivity thanks to a strong absorption of the Ge QDs and to the effective hole confinement within the Ge QDs based on both photoconductive and photovolatic effects. A fast temporal response of 1.4 ns of the Ge-QD MOS phototransistor offers great promise for future Si-based optical interconnection applications.
Acknowledgments This work was supported by the Ministry of Science and Technology of Taiwan, Republic of China (NSC-102-2221E-008-111-MY3) and by the Asian Office of Aerospace Research and Development under contract no. FA 2386-141-4008.
References [1] Moore G E 1965 Electronics 38 114 http://web.eng.fiu.edu/ npala/eee6397ex/gordon_moore_1995_article.pdf [2] Bohr M 2011 IEEE Int. Elect. Dev. Meet. 1.1 1 [3] Colinge J P 2007 Microelectron. Eng. 84 2071 [4] Srivastava N and Banerjee K 2004 TMS J. Mater. 56 30 [5] Miller D A B 2000 Proc. IEEE 88 728 [6] Heck M J R, Chen H W, Fang A W, Koch B R, Liang D, Park H, Sysak M N and Bowers J E 2011 IEEE J. Sel. Top. Quantum Electron. 17 333 [7] Forbes M, Gourlay J and Desmulliez M 2001 Electron. Commun. Eng. J. 13 221 [8] Haurylau M, Chen G, Chen H, Zhang J, Nelson N A, Albonesi D H, Friedman E G and Fauchet P M 2006 IEEE J. Sel. Top. Quantum Electron. 12 1699 8
M H Kuo et al
Nanotechnology 26 (2015) 055203
[9] Wang K L, Cha D, Liu J and Chen C 2007 Proc. IEEE 95 1866 [10] Liu J and Michel J 2008 ECS Trans. 16 575 [11] Michel J, Liu J and Kimerling L C 2010 Nat. Photonics 4 527 [12] Samavedam S B, Currie M T, Langdo T A and Fitzgerald E A 1998 Appl. Phys. Lett. 73 2125 [13] Colace L, Ferrara P, Assanto G, Fulgoni F and Nash L 2007 IEEE Photon. Technol. Lett. 19 1813 [14] Giussani A, Rodenbach P, Zaumeil P, Dabrowski J, Kurps R, Weidner G, Mussig H J, Storck P, Wollschlager J and Schroedar T 2009 J. Appl. Phys. 105 033512 [15] Koster S J, Schaub J D, Delinger G and Chu J O 2006 IEEE. J. Sel. Top. Quant. Electron. 12 1489 [16] Choi D, Ge Y, Harris J S, Cagnon J and Stemmer S 2008 J. Cryst. Growth 310 4273 [17] Langdo T A, Leitz C W, Currie M T, Fitzgerald E A, Lochtefeld A and Antoniadis D A 2000 Appl. Phys. Lett. 76 3700 [18] Chandrasekhar S, Campbell J C, Dentai A G, Joyner C H, Qua G J, Gnauck A H and Feuer M D 1988 Electron. Lett. 24 1443 [19] Schaub J D, Koester S J, Dehlinger G, Ouyang Q C, Guckenberger D, Yang M, Rogers D L, Chu J and Grill A 2004 Proc. SPIE 5353 1 [20] Shieh J M, Lai Y F, Ni W X, Kuo H C, Fang C Y, Huang J Y and Pan C L 2007 Appl. Phys. Lett. 90 051105 [21] Yu H Y, Ishibashi M, Park J-H, Kobayashi M and Saraswat K C 2009 IEEE Electron Device Lett. 30 675 [22] Zhang R, Iwasaki T, Taoka N, Takenaka M and Takagi S 2012 IEEE Trans. Electron Devices 59 335
[23] Chen I H, Tseng S S and Li P W 2009 IEEE Photonics Technol. Lett. 21 1674 [24] Tzeng S S, Chen I H and Li P W 2008 Appl. Phys. Lett. 93 191112 [25] Kuo M H, Wang C C, Lai W T, George T and Li P W 2012 Appl. Phys. Lett. 101 223107 [26] Chien C Y, Lai W T, Chang J R, Wang C C, Kuo M H and Li P W 2014 Nanoscale 6 5303 [27] Liao P H, Hsu T C, Chen K H, Cheng T H, Hsu T M, Wang C C, George T and Li P W 2014 Appl. Phys. Lett. 105 172106 [28] Ishikawa Y, Wada K, Cannon D D, Liu J, Luan H C and Kimerling L C 2003 Appl. Phys. Lett. 82 2044 [29] Reparaz J S, Bernardi A, Goñi A R, Lacharmoise P D, Alonso M I, Garriga M, Novák J and Vávra I 2007 Appl. Phys. Lett. 91 081914 [30] Schmidt T, Lischka K and Zulehner W 1992 Phys. Rev. B 45 8989 [31] Jin S, Zheng Y and Li A 1997 J. Appl. Phys. 82 3870 [32] Chien C Y, Chang Y R, Chang R N, Lee M S, Chen W Y, Hsu T M and Li P W 2010 Nanotechnology 21 505201 [33] Kaneko Y, Koike N, Tsutsui K and Tsukada T 1990 Appl. Phys. Lett. 56 650 [34] GadelRab S M and Chamberlain S G 1997 IEEE Trans. Electron Devices 44 1789 [35] Liu P et al 2012 J. Appl. Phys. 112 083103 [36] Sekine N, Hirakawa K, Sogawa F, Arakawa Y, Usami N, Shiraki Y and Katoda T 1996 Appl. Phys. Lett. 68 3419 [37] Dao L V, Wen X, Do M T T, Hannaford P, Cho E C, Cho Y H and Huang Y 2005 J. Appl. Phys. 97 013501 [38] Hartel A M, Gutsch S, Hiller D and Zacharias M 2013 Phys. Rev. B 87 035428
9
