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Experimental study of the ultrathin oxides on SiGe alloy formed by low-temperature ozone oxidation
Materials Science in Semiconductor Processing 107 (2020) 104832
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Experimental study of the ultrathin oxides on SiGe alloy formed by low-temperature ozone oxidation Xueli Ma a, b, Xiaolei Wang a, b, Lixing Zhou a, b, Hao Xu a, b, Yuanyuan Zhang a, b, Jiahui Duan a, b, Jinjuan Xiang a, b, *, Hong Yang a, b, Junjie Li a, b, Yongliang Li a, b, Huaxiang Yin a, b, Wenwu Wang a, b a b
Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SiGe Ozone oxidation Ultrathin oxides XPS
The ultrathin oxides (<1.2 nm) on SiGe are formed by directly oxidizing SiGe surface in 10% O3/O2 mixture in Beneq TFS 200 ALD system. The oxide compositions are characterized using X-ray photoelectron spectroscopy technology as a function of processing conditions, including oxidation time, temperature, and pressure. It is found that the oxide composition is very sensitive to ozone partial pressure. That is, under a lower pressure, Ge atoms can’t be oxidized even over a long time and the oxides are pure SiOx. Until the temperature is increased to 300 � C, about 54% of the Ge atoms of the outermost atomic layer of SiGe are oxidized in the initial oxidation stage. Nevertheless, a slight increase in ozone partial pressure cause a rapid changeover to the formation of oxide mixture containing Si oxide and Ge oxide at all temperatures investigated, i.e. 100 � C–300 � C. Moreover, the oxidation rate of Ge atoms maintains almost unchanged from 100 � C to 230 � C, and decreases at 300 � C.
1. Introduction Strained SiGe is one of the potential channel materials for the advanced pMOSFET devices due to its higher hole mobility compared to Si [1–3]. In order to realize a high performance SiGe-based pMOSFET, it is crucial to obtain a superior high-k/SiGe interface. Different strategies for the interface control have been proposed, such as Si-cap-based passivation [4–7], plasma (N2, NH3, or O2) treatment of SiGe surface [8–11], and thermally-grown oxides on SiGe [12–15]. For Si-cap-based passivation method, there exists a trade-off between the interface quality and the scalability of equivalent oxide thickness. Plasma treat ment is inappropriate for the advanced 3-D devices like FinFETs or nanowires, due to its anisotropic property. Early studies of the thermally-grown oxides on SiGe concentrate on the furnace oxidation or rapid thermal oxidation, which are performed in O2 atmosphere at high pressures and temperatures, resulting in thick oxides and Ge-rich layer beneath the oxides. Moreover, high-temperature process causes an un desired stress relaxation of the strained-SiGe channel leading to a degradation of the electrical properties of devices. With the demand for the ultrathin oxide interlayers in nanoscale devices formed at low
temperature, ozone-oxidation passivation method has been explored and superior high-k/SiGe interfaces with low interface state density are realized [3,16,17]. This method has also been employed by Zhang et al. to passivate high-k/Ge interface [18], and the oxidation kinetics of pure Ge surface by ozone at low temperature (�400 � C) has been investigated in our previous work [19]. However, understanding about the growth behavior of ultrathin oxides on SiGe based on low-temperature ozone oxidation has not been reported yet. In this study, the compositions of ultrathin oxides (<1.2 nm) formed by oxidizing SiGe surface are characterized using X-ray photoelectron spectroscopy (XPS) technology as a function of ozone oxidation pro cessing conditions. It is found that the oxide composition is very sensi tive to ozone partial pressure. Under a lower pressure, Ge atoms can’t be oxidized even over a long time and the oxides are pure SiOx. Never theless, a slight increase in the ozone partial pressure leads to a mixture of oxides, in which the atomic ratio of Ge and Si atoms varies with oxidation time and temperature.
* Corresponding author. Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China. E-mail address: [email protected] (J. Xiang). https://doi.org/10.1016/j.mssp.2019.104832 Received 6 September 2019; Received in revised form 23 October 2019; Accepted 7 November 2019 Available online 15 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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2. Experiment The starting substrates were 30 nm thick strained Si0.7Ge0.3 (100) epitaxially grown in a reduced pressure chemical vapor deposition system (ASM E2000 plus). The ozone oxidation was performed in 10% O3/O2 mixture in Atomic-Layer-Deposition (ALD) chambers (Beneq TFS 200 system) with the pressures of ~3.1 Torr (LPO) and ~3.9 Torr (HPO). Thus, the ozone partial pressures were 0.31 Torr and 0.39 Torr based on Dalton’s law, respectively [20]. Firstly, the native oxides of the samples were removed by using 2% HF solution and deionized (DI) water. Then, they were loaded into ALD chambers for the oxidation processing immediately. The Si0.7Ge0.3 surfaces were treated under two pressures (LPO and HPO) at different temperatures, i.e. 100 � C, 165 � C, 230 � C, and 300 � C. And the oxidation time ranges from 1 min to 30 min. The oxidation amounts and states of Si and Ge of the oxides were determined by X-ray photoelectron spectroscopy (XPS). The thicknesses of the ox ides were calculated using their respective Si 2p and Ge 3d XPS data. The XPS measurements were carried out in a Thermo Scientific ESCALAB 250xi system using a monochromatic Al Kα source (1486.7 eV). The photoelectron emission take-off angle was 90� relative to the sample surface and the pass energy was 15 eV. 3. Results and discussion 3.1. XPS characterization of the oxides of the LPO-samples Fig. 1(a)-(b) are the fitted Si 2p and Ge 3d core-level spectra of an ascleaned sample by HF solution and DI water. The backgrounds are removed by the standard Shirley subtraction. A spin-orbit splitting of 0.61 eV and a branching ratio of 2:1 are used for Si 2p3/2, 1/2 doublets. And a spin-orbit splitting of 0.58 eV and a branching ratio of 3:2 are used for Ge 3d5/2, 3/2 doublets. The Si 2p and Ge 3d spectra are both well fitted by their respective doublets of the ground states, and no signals from oxidation states are observed. This suggests that the native oxide has been completely removed after HF clean. Fig. 2(a)-(d) are the Si 2p core-level spectra of the LPO-samples oxidized at 100 � C, 165 � C, 230 � C, and 300 � C, respectively. The ozone partial pressure is 0.31 Torr and oxidation time ranges from 1 min to 30 min. The spectra are fitted by the Si 2p3/2, 1/2 doublets of the elemental Si in bulk SiGe (Si0) and Si oxide components (i.e. Si1þ, Si2þ, Si3þ, and Si4þ). The chemical shifts of Si1þ3/ 2þ 3þ 4þ 0 2, Si 3/2, Si 3/2, and Si 3/2 relative to Si 3/2 are 0.6, 1.8, 2.8 and 3.5 eV, respectively. The full widths at half maximum (FWHM) for Si03/2, 1þ 2þ 3þ 4þ 1/2, Si 3/2, 1/2, Si 3/2, 1/2, Si 3/2, 1/2 and Si 3/2, 1/2 are taken as 0.5 eV, 0.5 eV, 0.68 eV, 1.18 eV, and 1.37 eV, respectively [21–23]. It is
Fig. 2. The fitted Si 2p core-level spectra of LPO-samples oxidized at (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C with oxidation time ranging from 1 min to 30 min. The ozone partial pressure is 0.31 Torr. The black, red, green, or ange, and magenta lines denote the 2p3/2 components of each chemical state, i. e. Si0, Si1þ, Si2þ, Si3þ, and Si4þ, respectively. Ioxide and I0 represent the areal intensities of Si 2p photoelectrons emitted from oxides and bulk SiGe, respectively.
Fig. 1. The fitted (a) Si 2p (b) Ge 3d core-level spectra of an as-cleaned sample by HF and DI-water.
worth noting that only Si 2p3/2 components are shown. The areal in tensity ratios (Ioxide/I0) of Si oxide to Si0 are also given in Fig. 2, from which the relative SiOx thicknesses can be evaluated. Based on the fitting results, Fig. 3(a)-(d) summarize the changes in the distributions of the oxidation states with oxidation time for different temperatures. The same trend is observed for each temperature, that is, a long oxidation-time causes an increase in the percent of the high-state oxides (I3þþI4þ) and decrease in the percent of the low-state oxides (I1þþI2þ). Moreover, the content of the low-state oxides is much smaller than that of the high-state oxides for high temperature oxidations. In other words, increasing oxidation time and temperature is favorable to enhance the oxidation condition of Si oxide. The distribution of Ge atoms is also important for the understanding of SiGe oxidation process. It should be noted that GeO is volatile when the temperature is above ~ 420 � C [24], and the highest temperature in our experiment is 300 � C. Hence, the effect of the volatilization of GeO is negligible in our work. Fig. 4(a)-(d) are the fitted Ge 3d core-level spectra of the LPO-samples. Compared with the control sample (shown in Fig. 1(b)), there is no obvious change in the Ge 3d peaks for the samples oxidized at 100 � C, 165 � C, and 230 � C independent on the oxidation time. However, for the 300 � C oxidation, peak fitting results 2
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Ge0 for 1 min, 5 min, 10 min, 15 min, and 30 min oxidation are 0.068, 0.061, 0.064, 0.059, and 0.061, respectively. We can see that the amounts of the Ge oxides are independent on the oxidation time. Furthermore, from the Ioxide/I0 values of GeOx and SiOx and their cor responding sensitivity factors, the atomic ratios (GeO/SiO) of the oxides are calculated to be 0.233, 0.142, 0.140, 0.121, and 0.120, respectively. The Ge/Si atomic ratio of the bulk SiGe in our experiment is 0.43, thus the ratios of non-oxidized Ge (GeN-O) to oxidized Ge (GeO) for 1 min, 5 min, 10 min, 15 min, and 30 min oxidation is about 0.838, 2.015, 2.058, 2.532, and 2.557, respectively. Obviously, the majority of the Ge atoms maintain non-oxidized condition especially for a long-time oxidation. These results indicate that Ge atoms of SiGe can hardly be oxidized, especially at low temperatures. This can be explained by the fact that SiOx is thermodynamically favored to be formed than GeOx [25]. Furthermore, the oxides thicknesses are determined through the following equation [22,26]: � Ioxide Λoxide βoxide d ¼ βoxide sin θ ln½ Þ þ 1� ð I0 Λ0 β 0
Fig. 3. Changes in areal intensity ratio of Si oxide components to Si oxide with oxidation time for the (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C oxidation.
where βoxide and β0 are the inelastic mean free paths (IMFP) of Si 2p (or Ge 3d) photoelectrons through oxide layer and bulk SiGe. Λoxide and Λ0 represent the atomic density of Si (or Ge) in oxide layer and bulk SiGe. θ is the photoelectron emission take-off angle of 90� relative to the sample surface. It is difficult to determine these parameters precisely, thus we evaluate the value of the factor (Λoxide/Λ0)(βoxide/β0) relying on the high-resolution transmission electron microscope (HRTEM) images of some samples. Fig. 5(a) and (b) are the HRTEM of the samples
Fig. 4. The fitted Ge 3d core-level spectra of LPO-samples oxidized at (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C.
show the presence of small components at 0.6 eV higher binding energy relative to the elemental Ge of bulk SiGe (Ge0). This indicates the for mation of Ge oxide. The areal intensity ratios (Ioxide/I0) of Ge oxide to
Fig. 5. HRTEM of the samples oxidized at (a) 230 � C (b) 300 � C for 30 min. (c) SiOx thickness vs. oxidation time with oxidation temperature ranging from 100 � C to 300 � C. 3
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oxidized at 230 � C and 300 � C for 30 min, from which the oxides thicknesses are evaluated to be 0.62 nm and 0.87 nm, respectively. For 230 � C/30 min-sample, the oxide is a pure SiOx layer, and its thickness can be estimated from the corresponding Ioxide/I0 value of SiOx of 0.2995. The value of βoxide for Si 2p is 3.76 nm [27,28], thus the factor (Λoxide/Λ0)(βoxide/β0) for SiOx layer is calculated to be 1.67. All the SiOx thicknesses are estimated and shown in Fig. 5(c). As can be seen, the oxidation rate of Si atoms is much larger in the initial stage, then it becomes slow as the oxidation time increases, indicating two different oxidation mechanisms. We suppose that in the initial stage the Si oxidation is predominantly determined by the chemical reaction of ox ygen atoms and Si atoms of SiGe at SiGe surface, then it is limited by the diffusion process of oxygen atoms through SiOx. The SiOx thicknesses of the samples oxidized at 100 � C, 165 � C, 230 � C, and 300 � C for 30 min are about 0.22 nm, 0.25 nm, 0.62 nm, and 0.72 nm, respectively. Obvi ously, as the oxidation temperature increasing, the oxidation rate of Si atoms increases. For 300 � C/30 min-sample, the GeOx/SiOx double layers cannot be distinguished from the HRTEM (Fig. 5(b)), which is attributed to low contrast in the HRTEM image between GeOx and SiOx, and the extremely small thicknesses of both layers. But the GeOx thickness can be determined to be 0.15 nm by subtracting the estimated SiOx thickness of 0.72 nm from the total oxide of 0.87 nm. The Ioxide/I0 value of GeOx is 0.061 for 0.15 nm GeOx. The value of βoxide for Ge 3d is 2.48 nm [28]. Hence, the factor (Λoxide/Λ0)(βoxide/β0) for GeOx layer is 0.98, which is consistent with the value of 0.94 determined based on GeOx/Ge structure in our previous work [19]. The GeOx thicknesses of the samples oxidized at 300 � C for 1 min, 5 min, 10 min, and 15 min are calculated to be 0.166 nm, 0.149 nm, 0.156 nm, and 0.145 nm, respec tively. Based on the Ge–O bond length of 0.16 nm [19] and Ge (100) atomic structure, we deduce that only the two upward bonds of the Ge atoms in the outermost atomic layer of SiGe may be terminated by ox ygen atoms. Combined with the GeN-O/GeO value of 300 � C/1 min-sample of 0.838, we work out that about 54% of the Ge atoms of the outermost layer bond with oxygen atoms in the initial oxidation stage.
30 min, and the oxidation temperature ranges from 100 � C to 300 � C. Fig. 6(a)-(b) show the fitted Si 2p core-level spectra of the HPO-samples. Compared with the LPO-samples (Fig. 2), the areal intensities of Si4þ component are higher indicating higher oxidation condition of the SiOx layers, especially for the 230 � C and 300 � C oxidations. Fig. 7(a)-(b) are the Ge 3d core-level spectra of the samples. Different from the LPOsamples, obvious peaks at high binding energy corresponding to GeOx are observed for all the time and temperatures investigated. Oxide mixtures occur. This oxidation behavior can be attributed to the slightly increased ozone partial pressure, under which Si atoms are still prefer entially oxidized, but some Ge atoms bond with the excess oxygen atoms available at SiGe surface. In order to obtain the distributions of the oxidation states, the Ge 3d spectra are decomposed into the Ge 3d5/2, 3/2 doublets of the elemental Ge in bulk SiGe (Ge0) and Ge oxide compo nents (i.e. Ge1þ, Ge2þ, Ge3þ, and Ge4þ). The chemical shifts of Ge1þ5/2, Ge2þ5/2, Ge3þ5/2, and Ge4þ5/2 relative to Ge05/2 are 0.6, 1.8, 2.8, and 3.7 eV, respectively. The full widths at half maximum (FWHM) for Ge05/ 1þ 2þ 3þ 4þ 2, 3/2, Ge 5/2, 3/2, Ge 5/2, 3/2, Ge 5/2, 3/2 and Ge 5/2, 3/2 are 0.6 eV, 0.6 eV, 0.9 eV, 1.12 eV, and 1.2 eV [19,29,30]. It is noticeable that only Ge 3d5/2 components are shown. Fig. 8(a)-(d) summarize the changes in the oxidation-state distributions of SiOx and GeOx with oxidation tem perature for different time. As shown in Fig. 8(a) and (c), Si oxide components vary with oxidation temperature in the same manner for 1 min and 30 min, that is, the amounts of Si suboxide (Si1þ, Si2þ, Si3þ) decrease and SiO2 (Si4þ) increase with oxidation temperature increasing. For Ge oxide, it is found that the oxidized Ge atoms mainly exist in Ge1þ and Ge4þ oxidation states, which means a more abrupt transition from elemental Ge of bulk SiGe (Ge0) to GeO2 (Ge4þ). Besides, higher temperature and longer time are favorable for enhancing the oxidation condition of Ge oxide, which is similar to the Si oxide. Fig. 9(a) shows the SiOx and GeOx thicknesses as a function of oxidation temperature. As can be seen, with temperature increasing from 100 � C to 230 � C, the SiOx thickness monotonically increases and GeOx thickness approximates to a constant value, i.e. ~ 0.3 nm for 1 min and ~ 0.4 nm for 30 min. These results indicate that both Si and Ge atoms participate in the oxidation processes, but there is a difference in the temperature dependence of oxidation rate between them. In other words, the oxidation rate of Si atoms increases with temperature while that of Ge atoms is independent of temperature. Furthermore, consid ering the Ge–O bond length of 0.16 nm, it can be deduced that the two
3.2. XPS characterization of the oxides of the HPO-samples For the purpose of investigating the effect of ozone partial pressure on the oxidation process of SiGe, the oxides formed under 0.38 Torr ozone partial pressure are examined. The oxidation time are 1 min and
Fig. 6. The fitted Si 2p core-level spectra of HPO-samples oxidized under 0.39 Torr ozone partial pressure for (a) 1 min (b) 30 min. Oxidation tempera ture ranges from 100 � C to 300 � C. The black, red, green, orange, and magenta lines denote the Si 2p3/2 components of each chemical state, i.e. Si0, Si1þ, Si2þ, Si3þ, and Si4þ, respectively. The areal intensity ratios (Ioxide/I0) of Si oxide to Si0 are also given.
Fig. 7. The fitted Ge 3d core-level spectra of HPO-samples oxidized under 0.39 Torr ozone partial pressure for (a) 1 min (b) 30 min. Oxidation tempera ture ranges from 100 � C to 300 � C. The black, red, green, orange, and magenta solid lines denote the Ge 3d5/2 components of each chemical state, i.e. Ge0, Ge1þ, Ge2þ, Ge3þ, and Ge4þ, respectively. The areal intensity ratios (Ioxide/I0) of Ge oxide to Ge0 are also given. 4
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GeO/SiO values decrease to 0.361 and 0.257. As temperature increasing to 300 � C, the oxidation rate of Si atoms still increases but that of Ge atoms decreases, causing that the GeO/SiO value further decreases to 0.134. This trend of oxide composition vs. temperature is also observed for 30 min oxidation process. Moreover, the atomic percentage of Ge atoms in non-oxidized (GeN-O) conditions are evaluated and given in Fig. 9(b). As can be seen, about 8.2% and 3.5% Ge atoms of the outer most atomic layer of SiGe can’t be oxidized at 100 � C within 1 min and 30 min, respectively. With temperature increasing to 300 � C, the percent of GeN-O increases to 69% for 1 min and 75% for 30 min. These results indicate that higher temperature is adverse for the oxidation of Ge atoms. 4. Conclusion In summary, the ultrathin oxides on SiGe surfaces, which are formed by ozone oxidation over a range of process parameters (i.e. temperature, time, and oxidant pressure), are examined in detail. It is found that the oxide composition is very sensitive to ozone partial pressure. Under a lower ozone partial pressure (0.31 Torr in our experiment), Ge atoms can’t be oxidized at low temperatures even over a long time and the formed oxides are pure SiOx layers. Until the oxidation temperature is increased to 300 � C, about 54% of the Ge atoms of the outermost atomic layer of SiGe can be oxidized in the initial stage of oxidation. Never theless, as the oxidation time increasing, no more Ge atoms take part in the oxidation process and they may accumulate at the oxide/SiGe interface. When the ozone partial pressure is slightly increased to 0.39 Torr, there is a rapid changeover to the formation of oxide mixture, in which the GeO/SiO atomic ratio varies with oxidation temperature and time. The GeO/SiO value of 100 � C oxidation is 0.393 for 1 min and 0.413 for 30 min, which are close to the Ge/Si atomic ratio in the initial bulk SiGe. These results indicate that the oxidation rates of Ge and Si atoms are nearly equal at 100 � C. With temperature increasing from 100 � C to 230 � C, the oxidation rate of Si atoms increases while that of Ge atoms maintains unchanged, thus the GeO/SiO value decreases. When the oxidation time is increased from 1 min to 30 min, there is a small increase in the GeO/SiO values. For the oxidation at 300 � C, the decrease in the GeO/SiO value arises from two factors, one is the reduced oxida tion rate of Ge atoms and the other is the transfer of O atoms from GeOx to SiOx. This can be attributed to the fact that O atoms are thermody namically favored to bond with Si over Ge atoms. For Si oxide, both higher oxidation temperature and ozone partial pressure result in higher oxidation condition. Moreover, two different oxidation mechanisms are involved in the observed time-evolutional Si oxide thickness, which are similar to the oxidation of Si substrate.
Fig. 8. Areal intensity ratios of (a) Si oxide components to Si oxide for the 1 min-samples (b) Ge oxide components to Ge oxide for the 1 min-samples (c) Si oxide components to Si oxide for the 30 min-samples (d) Ge oxide components to Ge oxide for the 30 min-samples as a function of oxidation temperature.
Fig. 9. Changes in (a) Oxide (SiOx and GeOx) thicknesses (b) Atomic ratios of Ge and Si in the oxides (GeO/SiO) with oxidation temperature. Atomic per centages of Ge atoms in non-oxidized condition (GeN-O) are also estimated and given. Oxidation time are 1 min and 30 min.
upward and two downward bonds of the Ge atoms in the outermost atomic layer of SiGe are terminated by oxygen atoms. The activation energy of this reaction has been experimentally determined to be 0.06 eV in our previous work [19]. Such a small activation energy sug gests a nearly barrier-less reaction, which can occur even when the temperature is as low as 100 � C. As oxidation temperature is increased to 300 � C, the SiOx thickness increases to 0.95 nm for 1 min and 1 nm for 30 min, but there is a decrease in the GeOx thickness indicating a reduced oxidation rate. This can be understood by that more oxygen atoms participate in the Si oxidation process at 300 � C and the amounts of oxygen atoms available to Ge atoms get fewer. In addition, there should be other factors responsible for the further decrease in the GeOx thickness when the oxidation time is increased to 30 min. Considering the fact that O atoms are thermodynamically favored to bond with Si over Ge atoms, this may be caused by the transfer of O atoms from Ge oxide into Si oxide. The atomic ratios (GeO/SiO) of Ge and Si in the oxides are estimated. As shown in Fig. 9(b), the GeO/SiO value of 100 � C oxidation for 1 min is 0.393 close to the value of Ge/Si in the initial bulk SiGe, from which we infer that the oxidation rates of Ge and Si atoms are nearly equal. For oxidation at 165 � C and 230 � C, the oxidation rates of Ge atoms maintain unchanged while that of Si atoms increase, thus the
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is financially supported in part by the National Key Project of Science and Technology of China (Grant no. 2017ZX02315001-002), in part by CAS Pioneer Hundred Talents Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104832.
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[1] Pouya Hashemi, K. Balakrishnan, S.U. Engelmann, J.A. Ott, A. Khakifirooz, A. Baraskar, et al., First demonstration of high-Ge-content strained-Si1-xGex (x¼0.5) on insulator PMOS FinFETs with high hole mobility and aggressively scaled fin dimensions and gate lengths for high-performance applications, IEDM Tech. Dig. (Dec. 2014) 402–405. [2] P. Hashemi, T. Ando, K. Balakrishnan, J. Bruley, S. Engelmann, J.A. Ott, et al., High-mobility high-Ge-content Si1-xGex-OI PMOS FinFETs with fins formed using 3D germanium condensation with Ge fraction up to x~0.7, scaled EOT~8.5Å and ~10nm fin width, in: VLSI Symp. Tech. Dig, Jun. 2015, pp. 16–17. [3] H. Mertens, R. Ritzenthaler, H. Arimura, J. Franco, F. Sebaai, A. Hikavyy, et al., Sicap-free SiGe p-channel FinFETs and gate-all-around transistors in a replacement metal gate process: interface trap density reduction and performance improvement by high-pressure deuterium anneal, in: VLSI Symp. Tech. Dig, Jun. 2015, pp. 142–143. [4] A. Sareen, Y. Wang, U. Sodervall, P. Lundgren, B. S, Effect of Si cap layer on parasitic channel operation in Si/SiGe metal–oxide–semiconductor structures, J. Appl. Phys. 93 (2003) 3545–3552. [5] Zhonghai Shi, Xiangdong Chen, David Onsongo, Eduardo J. Quinones, S. K. Banerjee, Simulation and optimization of strained SixGex buried channel pMOSFETs, Solid State Electron. 44 (2000) 1223–1228. [6] Minjoo L. Lee, Eugene A. Fitzgerald, Mayank T. Bulsara, Matthew T. Currie, A. Lochtefeld, Strained Si, SiGe, and Ge channels for high-mobility metal-oxidesemiconductor field effect transistors, J. Appl. Phys. 97 (2005). [7] J. Sato-Iwanaga, A. Inoue, H. Sorada, T. Takagi, A. Rothschild, R. Loo, et al., Optimized design of Si-cap layer in strained-SiGe channel p-MOSFETs based on computational and experimental approaches, Solid State Electron. 91 (2014) 1–8. [8] T. Yu, C.G. Jin, Y. Yang, L.J. Zhuge, X.M. Wu, Z.F. Wu, Effect of NH3 plasma treatment on the interfacial property between ultrathin HfO2 and strained Si0.65Ge0.35 substrate, J. Appl. Phys. 113 (2013), 044105. [9] J.-H. Han, R. Zhang, T. Osada, M. Hata, M. Takenaka, S. Takagi, Impact of plasma post-nitridation on HfO2/Al2O3/SiGe gate stacks toward EOT scaling, Microelectron. Eng. 109 (2013) 266–269. [10] M. Mukhopadhyay, S.K. Ray, C.K. Maiti, D.K. Nayak, Y. Shiraki, Electrical properties of oxides grown on strained SiGe layer at low temperatures in a microwave oxygen plasma, Appl. Phys. Lett. 65 (1994) 895–897. [11] K. Sardashti, K.-T. Hu, K. Tang, S. Madisetti, P. McIntyre, S. Oktyabrsky, et al., Nitride passivation of the interface between high-k dielectrics and SiGe, Appl. Phys. Lett. 108 (2016), 011604. [12] P.-E. Hellberg, S.-L. Zhang, F.M. d’Heurle, C.S. Petersso, Oxidation of silicon–germanium alloys. I. An experimental study, J. Appl. Phys. 82 (1997) 5773–5778. [13] S.J. Kilpatrick, R.J. Jaccodine, P.E. Thompson, Experimental study of the oxidation of silicon germanium alloys, J. Appl. Phys. 93 (2003) 4896–4901.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Experimental study of the ultrathin oxides on SiGe alloy formed by low-temperature ozone oxidation Xueli Ma a, b, Xiaolei Wang a, b, Lixing Zhou a, b, Hao Xu a, b, Yuanyuan Zhang a, b, Jiahui Duan a, b, Jinjuan Xiang a, b, *, Hong Yang a, b, Junjie Li a, b, Yongliang Li a, b, Huaxiang Yin a, b, Wenwu Wang a, b a b
Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SiGe Ozone oxidation Ultrathin oxides XPS
The ultrathin oxides (<1.2 nm) on SiGe are formed by directly oxidizing SiGe surface in 10% O3/O2 mixture in Beneq TFS 200 ALD system. The oxide compositions are characterized using X-ray photoelectron spectroscopy technology as a function of processing conditions, including oxidation time, temperature, and pressure. It is found that the oxide composition is very sensitive to ozone partial pressure. That is, under a lower pressure, Ge atoms can’t be oxidized even over a long time and the oxides are pure SiOx. Until the temperature is increased to 300 � C, about 54% of the Ge atoms of the outermost atomic layer of SiGe are oxidized in the initial oxidation stage. Nevertheless, a slight increase in ozone partial pressure cause a rapid changeover to the formation of oxide mixture containing Si oxide and Ge oxide at all temperatures investigated, i.e. 100 � C–300 � C. Moreover, the oxidation rate of Ge atoms maintains almost unchanged from 100 � C to 230 � C, and decreases at 300 � C.
1. Introduction Strained SiGe is one of the potential channel materials for the advanced pMOSFET devices due to its higher hole mobility compared to Si [1–3]. In order to realize a high performance SiGe-based pMOSFET, it is crucial to obtain a superior high-k/SiGe interface. Different strategies for the interface control have been proposed, such as Si-cap-based passivation [4–7], plasma (N2, NH3, or O2) treatment of SiGe surface [8–11], and thermally-grown oxides on SiGe [12–15]. For Si-cap-based passivation method, there exists a trade-off between the interface quality and the scalability of equivalent oxide thickness. Plasma treat ment is inappropriate for the advanced 3-D devices like FinFETs or nanowires, due to its anisotropic property. Early studies of the thermally-grown oxides on SiGe concentrate on the furnace oxidation or rapid thermal oxidation, which are performed in O2 atmosphere at high pressures and temperatures, resulting in thick oxides and Ge-rich layer beneath the oxides. Moreover, high-temperature process causes an un desired stress relaxation of the strained-SiGe channel leading to a degradation of the electrical properties of devices. With the demand for the ultrathin oxide interlayers in nanoscale devices formed at low
temperature, ozone-oxidation passivation method has been explored and superior high-k/SiGe interfaces with low interface state density are realized [3,16,17]. This method has also been employed by Zhang et al. to passivate high-k/Ge interface [18], and the oxidation kinetics of pure Ge surface by ozone at low temperature (�400 � C) has been investigated in our previous work [19]. However, understanding about the growth behavior of ultrathin oxides on SiGe based on low-temperature ozone oxidation has not been reported yet. In this study, the compositions of ultrathin oxides (<1.2 nm) formed by oxidizing SiGe surface are characterized using X-ray photoelectron spectroscopy (XPS) technology as a function of ozone oxidation pro cessing conditions. It is found that the oxide composition is very sensi tive to ozone partial pressure. Under a lower pressure, Ge atoms can’t be oxidized even over a long time and the oxides are pure SiOx. Never theless, a slight increase in the ozone partial pressure leads to a mixture of oxides, in which the atomic ratio of Ge and Si atoms varies with oxidation time and temperature.
* Corresponding author. Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China. E-mail address: [email protected] (J. Xiang). https://doi.org/10.1016/j.mssp.2019.104832 Received 6 September 2019; Received in revised form 23 October 2019; Accepted 7 November 2019 Available online 15 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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2. Experiment The starting substrates were 30 nm thick strained Si0.7Ge0.3 (100) epitaxially grown in a reduced pressure chemical vapor deposition system (ASM E2000 plus). The ozone oxidation was performed in 10% O3/O2 mixture in Atomic-Layer-Deposition (ALD) chambers (Beneq TFS 200 system) with the pressures of ~3.1 Torr (LPO) and ~3.9 Torr (HPO). Thus, the ozone partial pressures were 0.31 Torr and 0.39 Torr based on Dalton’s law, respectively [20]. Firstly, the native oxides of the samples were removed by using 2% HF solution and deionized (DI) water. Then, they were loaded into ALD chambers for the oxidation processing immediately. The Si0.7Ge0.3 surfaces were treated under two pressures (LPO and HPO) at different temperatures, i.e. 100 � C, 165 � C, 230 � C, and 300 � C. And the oxidation time ranges from 1 min to 30 min. The oxidation amounts and states of Si and Ge of the oxides were determined by X-ray photoelectron spectroscopy (XPS). The thicknesses of the ox ides were calculated using their respective Si 2p and Ge 3d XPS data. The XPS measurements were carried out in a Thermo Scientific ESCALAB 250xi system using a monochromatic Al Kα source (1486.7 eV). The photoelectron emission take-off angle was 90� relative to the sample surface and the pass energy was 15 eV. 3. Results and discussion 3.1. XPS characterization of the oxides of the LPO-samples Fig. 1(a)-(b) are the fitted Si 2p and Ge 3d core-level spectra of an ascleaned sample by HF solution and DI water. The backgrounds are removed by the standard Shirley subtraction. A spin-orbit splitting of 0.61 eV and a branching ratio of 2:1 are used for Si 2p3/2, 1/2 doublets. And a spin-orbit splitting of 0.58 eV and a branching ratio of 3:2 are used for Ge 3d5/2, 3/2 doublets. The Si 2p and Ge 3d spectra are both well fitted by their respective doublets of the ground states, and no signals from oxidation states are observed. This suggests that the native oxide has been completely removed after HF clean. Fig. 2(a)-(d) are the Si 2p core-level spectra of the LPO-samples oxidized at 100 � C, 165 � C, 230 � C, and 300 � C, respectively. The ozone partial pressure is 0.31 Torr and oxidation time ranges from 1 min to 30 min. The spectra are fitted by the Si 2p3/2, 1/2 doublets of the elemental Si in bulk SiGe (Si0) and Si oxide components (i.e. Si1þ, Si2þ, Si3þ, and Si4þ). The chemical shifts of Si1þ3/ 2þ 3þ 4þ 0 2, Si 3/2, Si 3/2, and Si 3/2 relative to Si 3/2 are 0.6, 1.8, 2.8 and 3.5 eV, respectively. The full widths at half maximum (FWHM) for Si03/2, 1þ 2þ 3þ 4þ 1/2, Si 3/2, 1/2, Si 3/2, 1/2, Si 3/2, 1/2 and Si 3/2, 1/2 are taken as 0.5 eV, 0.5 eV, 0.68 eV, 1.18 eV, and 1.37 eV, respectively [21–23]. It is
Fig. 2. The fitted Si 2p core-level spectra of LPO-samples oxidized at (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C with oxidation time ranging from 1 min to 30 min. The ozone partial pressure is 0.31 Torr. The black, red, green, or ange, and magenta lines denote the 2p3/2 components of each chemical state, i. e. Si0, Si1þ, Si2þ, Si3þ, and Si4þ, respectively. Ioxide and I0 represent the areal intensities of Si 2p photoelectrons emitted from oxides and bulk SiGe, respectively.
Fig. 1. The fitted (a) Si 2p (b) Ge 3d core-level spectra of an as-cleaned sample by HF and DI-water.
worth noting that only Si 2p3/2 components are shown. The areal in tensity ratios (Ioxide/I0) of Si oxide to Si0 are also given in Fig. 2, from which the relative SiOx thicknesses can be evaluated. Based on the fitting results, Fig. 3(a)-(d) summarize the changes in the distributions of the oxidation states with oxidation time for different temperatures. The same trend is observed for each temperature, that is, a long oxidation-time causes an increase in the percent of the high-state oxides (I3þþI4þ) and decrease in the percent of the low-state oxides (I1þþI2þ). Moreover, the content of the low-state oxides is much smaller than that of the high-state oxides for high temperature oxidations. In other words, increasing oxidation time and temperature is favorable to enhance the oxidation condition of Si oxide. The distribution of Ge atoms is also important for the understanding of SiGe oxidation process. It should be noted that GeO is volatile when the temperature is above ~ 420 � C [24], and the highest temperature in our experiment is 300 � C. Hence, the effect of the volatilization of GeO is negligible in our work. Fig. 4(a)-(d) are the fitted Ge 3d core-level spectra of the LPO-samples. Compared with the control sample (shown in Fig. 1(b)), there is no obvious change in the Ge 3d peaks for the samples oxidized at 100 � C, 165 � C, and 230 � C independent on the oxidation time. However, for the 300 � C oxidation, peak fitting results 2
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Ge0 for 1 min, 5 min, 10 min, 15 min, and 30 min oxidation are 0.068, 0.061, 0.064, 0.059, and 0.061, respectively. We can see that the amounts of the Ge oxides are independent on the oxidation time. Furthermore, from the Ioxide/I0 values of GeOx and SiOx and their cor responding sensitivity factors, the atomic ratios (GeO/SiO) of the oxides are calculated to be 0.233, 0.142, 0.140, 0.121, and 0.120, respectively. The Ge/Si atomic ratio of the bulk SiGe in our experiment is 0.43, thus the ratios of non-oxidized Ge (GeN-O) to oxidized Ge (GeO) for 1 min, 5 min, 10 min, 15 min, and 30 min oxidation is about 0.838, 2.015, 2.058, 2.532, and 2.557, respectively. Obviously, the majority of the Ge atoms maintain non-oxidized condition especially for a long-time oxidation. These results indicate that Ge atoms of SiGe can hardly be oxidized, especially at low temperatures. This can be explained by the fact that SiOx is thermodynamically favored to be formed than GeOx [25]. Furthermore, the oxides thicknesses are determined through the following equation [22,26]: � Ioxide Λoxide βoxide d ¼ βoxide sin θ ln½ Þ þ 1� ð I0 Λ0 β 0
Fig. 3. Changes in areal intensity ratio of Si oxide components to Si oxide with oxidation time for the (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C oxidation.
where βoxide and β0 are the inelastic mean free paths (IMFP) of Si 2p (or Ge 3d) photoelectrons through oxide layer and bulk SiGe. Λoxide and Λ0 represent the atomic density of Si (or Ge) in oxide layer and bulk SiGe. θ is the photoelectron emission take-off angle of 90� relative to the sample surface. It is difficult to determine these parameters precisely, thus we evaluate the value of the factor (Λoxide/Λ0)(βoxide/β0) relying on the high-resolution transmission electron microscope (HRTEM) images of some samples. Fig. 5(a) and (b) are the HRTEM of the samples
Fig. 4. The fitted Ge 3d core-level spectra of LPO-samples oxidized at (a) 100 � C (b) 165 � C (c) 230 � C (d) 300 � C.
show the presence of small components at 0.6 eV higher binding energy relative to the elemental Ge of bulk SiGe (Ge0). This indicates the for mation of Ge oxide. The areal intensity ratios (Ioxide/I0) of Ge oxide to
Fig. 5. HRTEM of the samples oxidized at (a) 230 � C (b) 300 � C for 30 min. (c) SiOx thickness vs. oxidation time with oxidation temperature ranging from 100 � C to 300 � C. 3
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oxidized at 230 � C and 300 � C for 30 min, from which the oxides thicknesses are evaluated to be 0.62 nm and 0.87 nm, respectively. For 230 � C/30 min-sample, the oxide is a pure SiOx layer, and its thickness can be estimated from the corresponding Ioxide/I0 value of SiOx of 0.2995. The value of βoxide for Si 2p is 3.76 nm [27,28], thus the factor (Λoxide/Λ0)(βoxide/β0) for SiOx layer is calculated to be 1.67. All the SiOx thicknesses are estimated and shown in Fig. 5(c). As can be seen, the oxidation rate of Si atoms is much larger in the initial stage, then it becomes slow as the oxidation time increases, indicating two different oxidation mechanisms. We suppose that in the initial stage the Si oxidation is predominantly determined by the chemical reaction of ox ygen atoms and Si atoms of SiGe at SiGe surface, then it is limited by the diffusion process of oxygen atoms through SiOx. The SiOx thicknesses of the samples oxidized at 100 � C, 165 � C, 230 � C, and 300 � C for 30 min are about 0.22 nm, 0.25 nm, 0.62 nm, and 0.72 nm, respectively. Obvi ously, as the oxidation temperature increasing, the oxidation rate of Si atoms increases. For 300 � C/30 min-sample, the GeOx/SiOx double layers cannot be distinguished from the HRTEM (Fig. 5(b)), which is attributed to low contrast in the HRTEM image between GeOx and SiOx, and the extremely small thicknesses of both layers. But the GeOx thickness can be determined to be 0.15 nm by subtracting the estimated SiOx thickness of 0.72 nm from the total oxide of 0.87 nm. The Ioxide/I0 value of GeOx is 0.061 for 0.15 nm GeOx. The value of βoxide for Ge 3d is 2.48 nm [28]. Hence, the factor (Λoxide/Λ0)(βoxide/β0) for GeOx layer is 0.98, which is consistent with the value of 0.94 determined based on GeOx/Ge structure in our previous work [19]. The GeOx thicknesses of the samples oxidized at 300 � C for 1 min, 5 min, 10 min, and 15 min are calculated to be 0.166 nm, 0.149 nm, 0.156 nm, and 0.145 nm, respec tively. Based on the Ge–O bond length of 0.16 nm [19] and Ge (100) atomic structure, we deduce that only the two upward bonds of the Ge atoms in the outermost atomic layer of SiGe may be terminated by ox ygen atoms. Combined with the GeN-O/GeO value of 300 � C/1 min-sample of 0.838, we work out that about 54% of the Ge atoms of the outermost layer bond with oxygen atoms in the initial oxidation stage.
30 min, and the oxidation temperature ranges from 100 � C to 300 � C. Fig. 6(a)-(b) show the fitted Si 2p core-level spectra of the HPO-samples. Compared with the LPO-samples (Fig. 2), the areal intensities of Si4þ component are higher indicating higher oxidation condition of the SiOx layers, especially for the 230 � C and 300 � C oxidations. Fig. 7(a)-(b) are the Ge 3d core-level spectra of the samples. Different from the LPOsamples, obvious peaks at high binding energy corresponding to GeOx are observed for all the time and temperatures investigated. Oxide mixtures occur. This oxidation behavior can be attributed to the slightly increased ozone partial pressure, under which Si atoms are still prefer entially oxidized, but some Ge atoms bond with the excess oxygen atoms available at SiGe surface. In order to obtain the distributions of the oxidation states, the Ge 3d spectra are decomposed into the Ge 3d5/2, 3/2 doublets of the elemental Ge in bulk SiGe (Ge0) and Ge oxide compo nents (i.e. Ge1þ, Ge2þ, Ge3þ, and Ge4þ). The chemical shifts of Ge1þ5/2, Ge2þ5/2, Ge3þ5/2, and Ge4þ5/2 relative to Ge05/2 are 0.6, 1.8, 2.8, and 3.7 eV, respectively. The full widths at half maximum (FWHM) for Ge05/ 1þ 2þ 3þ 4þ 2, 3/2, Ge 5/2, 3/2, Ge 5/2, 3/2, Ge 5/2, 3/2 and Ge 5/2, 3/2 are 0.6 eV, 0.6 eV, 0.9 eV, 1.12 eV, and 1.2 eV [19,29,30]. It is noticeable that only Ge 3d5/2 components are shown. Fig. 8(a)-(d) summarize the changes in the oxidation-state distributions of SiOx and GeOx with oxidation tem perature for different time. As shown in Fig. 8(a) and (c), Si oxide components vary with oxidation temperature in the same manner for 1 min and 30 min, that is, the amounts of Si suboxide (Si1þ, Si2þ, Si3þ) decrease and SiO2 (Si4þ) increase with oxidation temperature increasing. For Ge oxide, it is found that the oxidized Ge atoms mainly exist in Ge1þ and Ge4þ oxidation states, which means a more abrupt transition from elemental Ge of bulk SiGe (Ge0) to GeO2 (Ge4þ). Besides, higher temperature and longer time are favorable for enhancing the oxidation condition of Ge oxide, which is similar to the Si oxide. Fig. 9(a) shows the SiOx and GeOx thicknesses as a function of oxidation temperature. As can be seen, with temperature increasing from 100 � C to 230 � C, the SiOx thickness monotonically increases and GeOx thickness approximates to a constant value, i.e. ~ 0.3 nm for 1 min and ~ 0.4 nm for 30 min. These results indicate that both Si and Ge atoms participate in the oxidation processes, but there is a difference in the temperature dependence of oxidation rate between them. In other words, the oxidation rate of Si atoms increases with temperature while that of Ge atoms is independent of temperature. Furthermore, consid ering the Ge–O bond length of 0.16 nm, it can be deduced that the two
3.2. XPS characterization of the oxides of the HPO-samples For the purpose of investigating the effect of ozone partial pressure on the oxidation process of SiGe, the oxides formed under 0.38 Torr ozone partial pressure are examined. The oxidation time are 1 min and
Fig. 6. The fitted Si 2p core-level spectra of HPO-samples oxidized under 0.39 Torr ozone partial pressure for (a) 1 min (b) 30 min. Oxidation tempera ture ranges from 100 � C to 300 � C. The black, red, green, orange, and magenta lines denote the Si 2p3/2 components of each chemical state, i.e. Si0, Si1þ, Si2þ, Si3þ, and Si4þ, respectively. The areal intensity ratios (Ioxide/I0) of Si oxide to Si0 are also given.
Fig. 7. The fitted Ge 3d core-level spectra of HPO-samples oxidized under 0.39 Torr ozone partial pressure for (a) 1 min (b) 30 min. Oxidation tempera ture ranges from 100 � C to 300 � C. The black, red, green, orange, and magenta solid lines denote the Ge 3d5/2 components of each chemical state, i.e. Ge0, Ge1þ, Ge2þ, Ge3þ, and Ge4þ, respectively. The areal intensity ratios (Ioxide/I0) of Ge oxide to Ge0 are also given. 4
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GeO/SiO values decrease to 0.361 and 0.257. As temperature increasing to 300 � C, the oxidation rate of Si atoms still increases but that of Ge atoms decreases, causing that the GeO/SiO value further decreases to 0.134. This trend of oxide composition vs. temperature is also observed for 30 min oxidation process. Moreover, the atomic percentage of Ge atoms in non-oxidized (GeN-O) conditions are evaluated and given in Fig. 9(b). As can be seen, about 8.2% and 3.5% Ge atoms of the outer most atomic layer of SiGe can’t be oxidized at 100 � C within 1 min and 30 min, respectively. With temperature increasing to 300 � C, the percent of GeN-O increases to 69% for 1 min and 75% for 30 min. These results indicate that higher temperature is adverse for the oxidation of Ge atoms. 4. Conclusion In summary, the ultrathin oxides on SiGe surfaces, which are formed by ozone oxidation over a range of process parameters (i.e. temperature, time, and oxidant pressure), are examined in detail. It is found that the oxide composition is very sensitive to ozone partial pressure. Under a lower ozone partial pressure (0.31 Torr in our experiment), Ge atoms can’t be oxidized at low temperatures even over a long time and the formed oxides are pure SiOx layers. Until the oxidation temperature is increased to 300 � C, about 54% of the Ge atoms of the outermost atomic layer of SiGe can be oxidized in the initial stage of oxidation. Never theless, as the oxidation time increasing, no more Ge atoms take part in the oxidation process and they may accumulate at the oxide/SiGe interface. When the ozone partial pressure is slightly increased to 0.39 Torr, there is a rapid changeover to the formation of oxide mixture, in which the GeO/SiO atomic ratio varies with oxidation temperature and time. The GeO/SiO value of 100 � C oxidation is 0.393 for 1 min and 0.413 for 30 min, which are close to the Ge/Si atomic ratio in the initial bulk SiGe. These results indicate that the oxidation rates of Ge and Si atoms are nearly equal at 100 � C. With temperature increasing from 100 � C to 230 � C, the oxidation rate of Si atoms increases while that of Ge atoms maintains unchanged, thus the GeO/SiO value decreases. When the oxidation time is increased from 1 min to 30 min, there is a small increase in the GeO/SiO values. For the oxidation at 300 � C, the decrease in the GeO/SiO value arises from two factors, one is the reduced oxida tion rate of Ge atoms and the other is the transfer of O atoms from GeOx to SiOx. This can be attributed to the fact that O atoms are thermody namically favored to bond with Si over Ge atoms. For Si oxide, both higher oxidation temperature and ozone partial pressure result in higher oxidation condition. Moreover, two different oxidation mechanisms are involved in the observed time-evolutional Si oxide thickness, which are similar to the oxidation of Si substrate.
Fig. 8. Areal intensity ratios of (a) Si oxide components to Si oxide for the 1 min-samples (b) Ge oxide components to Ge oxide for the 1 min-samples (c) Si oxide components to Si oxide for the 30 min-samples (d) Ge oxide components to Ge oxide for the 30 min-samples as a function of oxidation temperature.
Fig. 9. Changes in (a) Oxide (SiOx and GeOx) thicknesses (b) Atomic ratios of Ge and Si in the oxides (GeO/SiO) with oxidation temperature. Atomic per centages of Ge atoms in non-oxidized condition (GeN-O) are also estimated and given. Oxidation time are 1 min and 30 min.
upward and two downward bonds of the Ge atoms in the outermost atomic layer of SiGe are terminated by oxygen atoms. The activation energy of this reaction has been experimentally determined to be 0.06 eV in our previous work [19]. Such a small activation energy sug gests a nearly barrier-less reaction, which can occur even when the temperature is as low as 100 � C. As oxidation temperature is increased to 300 � C, the SiOx thickness increases to 0.95 nm for 1 min and 1 nm for 30 min, but there is a decrease in the GeOx thickness indicating a reduced oxidation rate. This can be understood by that more oxygen atoms participate in the Si oxidation process at 300 � C and the amounts of oxygen atoms available to Ge atoms get fewer. In addition, there should be other factors responsible for the further decrease in the GeOx thickness when the oxidation time is increased to 30 min. Considering the fact that O atoms are thermodynamically favored to bond with Si over Ge atoms, this may be caused by the transfer of O atoms from Ge oxide into Si oxide. The atomic ratios (GeO/SiO) of Ge and Si in the oxides are estimated. As shown in Fig. 9(b), the GeO/SiO value of 100 � C oxidation for 1 min is 0.393 close to the value of Ge/Si in the initial bulk SiGe, from which we infer that the oxidation rates of Ge and Si atoms are nearly equal. For oxidation at 165 � C and 230 � C, the oxidation rates of Ge atoms maintain unchanged while that of Si atoms increase, thus the
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is financially supported in part by the National Key Project of Science and Technology of China (Grant no. 2017ZX02315001-002), in part by CAS Pioneer Hundred Talents Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104832.
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Materials Science in Semiconductor Processing 107 (2020) 104832
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