- Nano Express
- Open Access
Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping
© Dalapati et al; licensee Springer. 2012
- Received: 24 November 2011
- Accepted: 2 February 2012
- Published: 2 February 2012
Electrical and physical properties of a metal-oxide-semiconductor [MOS] structure using atomic layer-deposited high-k dielectrics (TiO2/Al2O3) and epitaxial GaAs [epi-GaAs] grown on Ge(100) substrates have been investigated. The epi-GaAs, either undoped or Zn-doped, was grown using metal-organic chemical vapor deposition method at 620°C to 650°C. The diffusion of Ge atoms into epi-GaAs resulted in auto-doping, and therefore, an n-MOS behavior was observed for undoped and Zn-doped epi-GaAs with the doping concentration up to approximately 1017 cm-3. This is attributed to the diffusion of a significant amount of Ge atoms from the Ge substrate as confirmed by the simulation using SILVACO software and also from the secondary ion mass spectrometry analyses. The Zn-doped epi-GaAs with a doping concentration of approximately 1018 cm-3 converts the epi-GaAs layer into p-type since the Zn doping is relatively higher than the out-diffused Ge concentration. The capacitance-voltage characteristics show similar frequency dispersion and leakage current for n-type and p-type epi-GaAs layers with very low hysteresis voltage (approximately 10 mV).
- Ge out-diffusion and auto-doping
- high-k dielectrics.
In recent years, there had been increasing interest in the introduction of III-V semiconductors as high-mobility channel materials in nanoscale silicon-based [Si-based] complementary-metal-oxide-semiconductor [CMOS] devices [1–7]. This migration from the present strained Si channels is due to two reasons: First, with the replacement of silicon oxide [SiO2] and silicon oxynitride (SiOxNy) by high-permittivity [high-k] dielectrics as the gate insulator [1–8], the choice of a channel material is no longer restricted to Si. Second, the incorporation of appropriate stressors, such as silicon nitride, can enhance both electron and hole mobilities in sub-90-nm devices; there could be scaling limits to such approaches. An inversion n-channel GaAs field effect transistor [FET] with a metal gate high-k dielectric was fabricated on GaAs wafers by de Souza et al. . Ye et al.  characterized the Al2O3/GaAs metal-oxide-semiconductor field effect transistor [MOSFET] and found a very high drain current and a relatively high transconductance. Also, the studies on the effect of the atomic layer-deposited [ALD] Al2O3 blocking layer indicates that it can suppress the growth of an interfacial layer and that the ALD Al2O3 could reduce the formation of native arsenic oxides to below the detection level of X-ray photoelectron spectroscopy .
For high-volume manufacturing, it is of great interest to develop epitaxial III-V high-mobility channel materials on a silicon platform to realize CMOS devices with increased carrier mobility and device flexibility [9–11]. Convergence of the Si and compound semiconductor industries promises the best of both worlds for device manufacturers due to the high performance, flexibility, and enhanced functionality of III-V compounds coupled with the low manufacturing cost and sheer scale of the Si process. In particular, GaAs has received much attention due to its lower effective mass and, hence, an intrinsic superior transport property than Si. Moreover, it is possible to grow epitaxial GaAs [epi-GaAs] on a Si-based CMOS technology-compatible Ge substrate since the lattice parameter of GaAs (0.5653 nm) is almost identical to that of Ge (0.5658 nm), and both have similar thermal conductivity . In addition, Ge has the added advantage of having a high hole mobility of 1,900 cm2 V-1 s1 at 300 K which is about four times higher than that of Si . This suggests the possibility of a heterogeneous integration of GaAs n-channel FETs with Ge p-channel FETs on a common Si platform.
One of the key considerations in fabricating a surface channel MOSFET using epi-GaAs is to achieve a good interface quality between the epi-GaAs substrate and gate oxide, which is vital for the device performance . Fortunately, atomic layer deposition provides a unique opportunity to integrate high-quality gate dielectrics on bulk and epi-GaAs [2, 14, 15]. It was observed that ALD Al2O3 provides a better interface with GaAs interface compared with other ALD high-k dielectrics [6, 8]. Although, by continuing effort on surface passivation, it is possible to grow a high-quality interface with low defect density, the hysteresis voltage for ALD high-k/GaAs gate stack is still high [2, 7, 15]. There are some attempts to achieve low hysteresis voltage using ALD SiO2, directly deposited titanium oxide [TiO2], and Si passivation on GaAs substrates [16–19]. On the other hand, GaAs grown at its optimum temperature on Ge will result in high Ge contamination, such as auto-doping and formation of Ge-based complexes, as significant Ge atoms will diffuse into the GaAs epilayer during growth. Chia et al.  suggested that a thin 10-nm AlAs interfacial layer is sufficient to effectively block the out-diffusion of Ge atoms at a high growth temperature of 650°C, eliminating Ge-based complexes and auto-doping effects in the GaAs layer. It is highly desirable to grow p-type epi-GaAs with good structural and electronic qualities for n-MOSFET device applications. However, to the best of our knowledge, there is no report of a metal-oxide-semiconductor [MOS] capacitor using p-type epi-GaAs grown on Ge substrates.
TiO2 [17, 18] gate dielectric provides low hysteresis voltage, and thin ALD Al2O3 is a promising gate dielectric for surface passivation [6, 7] as well as improved interface quality. In this paper, we demonstrate ALD TiO2/Al2O3 gate stack on undoped (which is n-type) and Zn-doped (p-type) epi-GaAs grown by metallorganic chemical vapor deposition [MOCVD] technique. The epi-GaAs device characteristics are compared with that of undoped and Zn-doped epi-GaAs for different concentrations. Further, we have identified the minimum Zn dopant concentration required for p-type epi-GaAs substrates. Electrical and physical analyses and simulation using SILVACO software (SILVACO, Inc., Santa Clara, CA, USA) have also been performed to understand the impact of the material and processing conditions for a high-quality gate stack on epi-GaAs substrates and the impact of Ge diffusion on the performance of MOS characteristics. The surface topography of epi-GaAs and high-k/epi-GaAs surfaces was examined via atomic force microscopy [AFM]. Interfacial reaction of high-k/epi-GaAs and Ge out-diffusion was studied by time of flight secondary ion mass spectrometry [ToF-SIMS] for all the structures. Capacitance-voltage [C-V] and current-voltage [I-V] characteristics were measured using an Agilent 4284A LCR (Agilent Technologies Inc., Santa Clara, CA, USA) and a Hewlett-Packard 4140B semiconductor parameter analyzer (Hewlett-Packard Company, Palto Alto, CA, USA), respectively
MOS capacitors were fabricated on epi-GaAs substrates. The epi-GaAs substrates were grown at 620°C to 650°C by MOCVD technique. Vicinal Ge (100) substrates with 6° offcut toward the (111) plane were used to ensure that the epitaxial GaAs grown on Ge is free from APD defects. Prior to the growth of GaAs layers, the Ge substrate was heated up to and kept at 650°C for 5 min under H2 environment to remove the native oxide layer. Tertiarybutylarsine and trimethylgallium were introduced into the reactor for the growth of the Zn-doped 300-nm-thick GaAs layer at 620°C. For undoped epi-GaAs, the GaAs substrate was grown on Ge(100) samples with an AlAs interfacial layer at 650°C by MOCVD technique. The details of the film growth and their properties are reported elsewhere . The as-grown wafers were then degreased using isopropanol, cleaned in HF solution (1%) for 3 min to remove the native oxide, and then dipped in NH4OH solution for 10 min. A thin layer of Al2O3 was deposited on epi-GaAs using trimethylaluminium (SAFC Hitech, Haverhill, MA, USA; 99.9%) and H2O as the precursors in a viscous flow-type (0.6 Torr working pressure) atomic layer deposition equipment (f·XALD ALD equipment, Azimuth Technologies Pte Ltd., Singapore) with a N2 flow rate of 50 sccm at 170°C. After that, TiO2 films were deposited under similar conditions. Vapors of TiCl4 (Merck & Co., Inc., Whitehouse Station, NJ, USA; 99%) and H2O precursors were sequentially introduced into the chamber with an exposure time of 0.1 s and purged by 50-sccm N2 flow for 10 s between the two exposures. Post-deposition annealing was carried out in a N2 ambient at 500°C for 1 min by rapid thermal annealing technique. The Au metal, deposited by sputtering, was used as the gate electrode (area, 7.8 × 10-3 cm2). Finally, a low-resistance ohmic back contact was formed by depositing Ti/Pt/Au alloy on the p-GaAs substrate, AuGeNi alloy on the n-GaAs substrate, and Au on the Ge substrate.
It was observed from the SIMS analysis of the epi-GaAs layer grown at 620°C that a significant amount of Ge was diffused into the epi-GaAs thereby converting it to n-type due to auto-doping of Ge to GaAs. By introducing a high density of Zn, it is possible to convert the n-type epi-GaAs to p-type. It is worth noting that although, from SIMS analysis, there was Ge atom diffusion in the epi-GaAs layer up to 20 nm with an AlAs interlayer and up to 100 nm without the AlAs interlayer, however, for both the cases, the epi-GaAs layer shows an n-type behavior. This suggests that although AlAs effectively reduced the Ge atom diffusion into GaAs, there is still some Ge which could possibly be below the SIMS detection limit. From the simulation of Ge diffusion and C-V characteristics, it was also observed that Ge atoms were present in the epi-GaAs even for the AlAs barrier layer, but the concentration is very low (approximately 1015 cm-3). The hysteresis voltage for ALD TiO2/Al2O3 gate stack was very low (approximately 10 mV) as shown in the inset of Figure 4.
In summary, epi-GaAs MOS capacitors were fabricated and characterized using electrical and physical analysis. Atomic layer-deposited TiO2/Al2O3 gate stack is used to fabricate epi-GaAs MOS capacitors on a Ge substrate for III-V CMOS applications. The epi-GaAs MOS capacitor shows an nMOS behavior for undoped and even for Zn-doped epi-GaAs with low concentration due to Ge auto-doping, which is confirmed by the SIMS analysis and simulation. Zn-doped epi-GaAs with a high concentration > 1018 cm-3 converts epi-GaAs into p-type. Interfacial reaction mechanisms between epi-GaAs and ALD TiO2/Al2O3 have been discussed through SIMS analysis and capacitance-voltage characteristics. Although the simulated and experimentally obtained C-V result showed frequency dispersion due to the presence of the interfacial lossy dielectric layer between Al2O3 and epi-GaAs, the hysteresis voltage for epi-MOS device is very small for the ALD Al2O3/TiO2 gate stack. Therefore, using suitable surface passivation with ALD TiO2/Al2O3 on epi-GaAs can pave the way for the next generation of Si-based CMOS technology for ultrahigh-speed devices or multifunctional devices on a Si platform.
One of the authors, A. Das, would like to acknowledge the Department of Science and Technology (DST), India, for providing the Inspire Fellowship scholarship to pursue her research work.
- Bentley SJ, Holland M, Li X, G Paterson GW, Zhou H, Ignatova O, Macintyre D, Thoms S, Asenov A, Shin B, Ahn J, McIntyre PC, Thayne IG: Electron mobility in surface- and buried-channel flatband In0.53Ga0.47As MOSFETs with ALD Al2O3gate dielectric. IEEE Elect Dev Lett 2011, 32: 494.View ArticleGoogle Scholar
- Dalapati GK, Tong Y, Loh WY, Mun HK, Cho BJ: Electrical and interfacial characterization of atomic layer deposited high-κ gate dielectrics on GaAs for advanced CMOS devices. IEEE Trans Elect Dev 2007, 54: 1831.View ArticleGoogle Scholar
- Benedicto M, Galiana B, Molina-Aldareguia JM, Monaghan S, Hurley PK, Cherkaoui K, Vazquez L, Tejedor P: Fabrication of HfO2 patterns by laser interference nanolithography and selectivity dry etching for III-V CMOS application. Nanoscale Res Letts 2011, 6: 400. 10.1186/1556-276X-6-400View ArticleGoogle Scholar
- De Souza JP, Kiewra E, Sun Y, Callegari A, Sadana DK, Shahidi G, Webb DJ, Fompeyrine J, Germann R, Rossel C, Marchion C: Inversion mode n-channel GaAs field effect transistor with high-k/metal gate. Appl Phys Lett 2008, 92: 153508. 10.1063/1.2912027View ArticleGoogle Scholar
- Ye PD, Wilk GD, Yang B, Kwo J, Chu SNG, Nakahara S, Gossmann HJL, Mannaerts JP, Hong M, Ng KK, Bude J: GaAs metal-oxide-semiconductor field-effect transistor with nanometer-thin dielectric grown by atomic layer deposition. Appl Phys Lett 2003, 83: 180. 10.1063/1.1590743View ArticleGoogle Scholar
- Shahrjerdi D, Garcia-Gutierrez DI, Tutuc E, Banerjee SK: Chemical and physical interface studies of the atomic-layer-deposited Al2O3on GaAs substrates. Appl Phys Lett 2008, 92: 223501. 10.1063/1.2937404View ArticleGoogle Scholar
- Lee HD, Feng T, Yu L, Mastrogiovanni D, Wan A, Gustafsson T, Garfunkel E: Reduction of native oxides on GaAs during atomic layer growth of Al2O3. Appl Phys Lett 2009, 94: 222108. 10.1063/1.3148723View ArticleGoogle Scholar
- Shi L, Liu Z: Characterization upon electrical hysteresis and thermal diffusion of TiAl3Oxdielectric film. Nanoscale Res Lett 2011, 6: 557. 10.1186/1556-276X-6-557View ArticleGoogle Scholar
- Suthram S, Sun Y, Majhi P, Ok I, Kim H, Harris HR, Goel N, Parthasarathy S, Koehler T, Acosta T, Nishida T, Tseng H-H, Tsai W, Lee J, Jammy R, Thompson SE: Strain additivity in III-V channels for CMOSFETs beyond 22 nm technology node. Dig Tech Pap - Symp VLSI Technol 2008: 182.Google Scholar
- Hill RJW, Moran DAJ, Li X, Zhou H, Macintyre D, Thoms S, Asenov A, Zurcher P, Rajagopalan K, Abrokwah J, Droopad R, Passlack M, Thayne IG: Enhancement-mode GaAs MOSFETs with an In0.3Ga0.7As channel, a mobility of over 5000 cm2/V·s, and transconductance of over 475 μS/μm. IEEE Electron Device Lett 2007, 28: 1080.View ArticleGoogle Scholar
- Passlack M, Zurcher P, Rajagopalan K, Droopad R, Abrokwah J, Tutt T, Park YB, Johnson E, Hartin O, Zlotnicka A, Fejes P, Hill RJW, Moran DAJ, Li X, Zhou H, Macintyre D, Thorns S, Asenov A, Kalna K, Thayne IG: High mobility III-V MOSFETs for RF and digital applications. Tech Dig - Int Electron Devices Meet 2007, 621.Google Scholar
- Brammertz G, Mols Y, Degroote S, Leys M, Sttenbergen JV, Borghs G, Cyamax M: Selective epitaxial growth of GaAs on Ge by MOCVD. J Cryst Growth 2006, 297: 204. 10.1016/j.jcrysgro.2006.09.015View ArticleGoogle Scholar
- Dalapati GK, Chattopadhyay S, Kwa KSK, Olsen SH, Tsang YL, Agaiby R, Dobrosz P, Bull SJ, O'Neill AG: Impact of strained-Si thickness and Ge out-diffusion on gate oxide quality for strained-Si surface channel n-MOSFETs. IEEE Trans Elect Dev 2006, 53: 1142.View ArticleGoogle Scholar
- Xuan Y, Wu YQ, Lin HC, Shen T, Ye PD: Submicrometer inversion-type enhancement-mode InGaAs MOSFET with atomic-layer-deposited Al2O3as gate dielectric. IEEE Elect Dev Lett 2007, 28: 935.View ArticleGoogle Scholar
- Dalapati GK, Kumar MK, Chia CK, Gao H, Wang BZ, Wong ASW, Kumar A, Chiam SY, Pan JS, Chi DZ: Interfacial and electrical characterization of atomic-layer-deposited HfO2gate dielectric on high mobility epitaxial GaAs/Ge channel substrates. J Electrochem Soc 2010, 157: H825. 10.1149/1.3453935View ArticleGoogle Scholar
- Dalapati GK, Chia CK, Mahata C, Das T, Maiti CK, Kumar MK, Gao H, Chiam SY, Tan CC, Chua CT, Cheng YB, Chi DZ: Surface passivation of GaAs substrates with SiO2deposited using ALD. Electrochem Solid-State Lett 2011, 14: G52. 10.1149/1.3615963View ArticleGoogle Scholar
- Dalapati GK, Sridhara A, Wong ASW, Chia CK, Lee SJ, Chi DZ: Characterization of sputtered TiO2gate dielectric on aluminum oxynitride, passivated p-GaAs. J Appl Phys 2008, 103: 034508. 10.1063/1.2840132View ArticleGoogle Scholar
- Lee MK, Yen CF, Huang JJ: Electrical characteristics of liquid-phase-deposited TiO2films on GaAs substrate with (NH4)2Sxtreatment. J Electrochem Soc 2006, 153: F77. 10.1149/1.2181438View ArticleGoogle Scholar
- Ok I, Kim H, Zhang M, Kang CY, Rhee SJ, Choi C, Krishnan SA, Lee T, Zhu F, Thareja G, Lee JC: Metal gate-HfO2MOS structures on GaAs substrate with and without Si interlayer. IEEE Electron Device Lett 2006, 27: 145.View ArticleGoogle Scholar
- Chia CK, Dong JR, Chi DZ, Sridhara A, Wong ASW, Suryana M, Dalapati GK, Chua SJ, Lee SJ: Effects of AlAs interfacial layer on material and optical properties of GaAs/Ge (100)epitaxy. Appl Phys Lett 2008, 92: 141905. 10.1063/1.2908042View ArticleGoogle Scholar
- Hudait MK, Krupanidhi SB: Self-annihilation of antiphase boundaries in GaAs epilayers on Ge substrates grown by metal-organic vapor-phase epitaxy. J Appl Phys 2001, 89: 5972. 10.1063/1.1368870View ArticleGoogle Scholar
- Kamata Y: High-k/Ge MOSFETs for future nanoelectronics. Mater Today 2008, 11: 30.View ArticleGoogle Scholar
- Kita K, Nomura H, Nishimura T, Toriumi A: Impact of dielectric material selection on electrical characteristics of high-k/Ge devices. ECS Trans 2006, 3: 71.View ArticleGoogle Scholar
- Bai W, Lu N, Ritenour AP, Lee ML, Antoniadis DA, Kwong DL: The electrical properties of HfO2dielectric on germanium and the substrate doping effect. IEEE Trans Electron Devices 2006, 53: 2551.View ArticleGoogle Scholar
- Sze SM: Physics of Semiconductor Devices. 3rd edition. New York: Wiley; 2007.Google Scholar
- Tang H, Prasad K, Sanjines R, Schmid PE, Levy F: Electrical and optical properties of TiO2anatase thin films. J Appl Phys 1994, 75: 2042. 10.1063/1.356306View ArticleGoogle Scholar
- Mikhelashvili V, Eisenstein G: Effects of annealing conditions on optical and electrical characteristics of titanium dioxide films deposited by electron beam evaporation. J Appl Phys 2001, 89: 3256. 10.1063/1.1349860View ArticleGoogle Scholar
- Seo Y, Lee S, An I, Song C, Jeong H: Conduction mechanism of leakage current due to the traps in ZrO2thin film. Semicond Sci Technol 2009, 24: 115016. 10.1088/0268-1242/24/11/115016View ArticleGoogle Scholar
- Zhu WJ, Ma TP, Tamagawa T, Kim J, Di Y: Current transport in metal/hafnium oxide/silicon structure. IEEE Electron Device Lett 2002, 23: 97. 10.1109/55.981318View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.