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Oxidation Mechanism of Si1-xGex Nanowires with Au Catalyst Tip as a Function of Ge Content Jungmin Bae, Kwang-Sik Jeong, Woo-Jung Lee, Min Baik, Jaehun Park, and Mann-Ho Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10764 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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ACS Applied Materials & Interfaces

Oxidation Mechanism of Si1−xGex Nanowires with Au Catalyst Tip as a Function of Ge Content

Jung Min Bae1,4, Kwang-Sik Jeong1, Woo-Jung Lee1,3, Min Baik1, Jaehun Park2, Mann-Ho Cho1,*

1

2

Department of Physics and Applied Physics, Yonsei University, Seoul 03722, Korea

Pohang Accelerator Laboratory, Pohang University of Sciencea and Technology, Pohang, 37673, Korea

3

Electronics and Telecommunications Research Institute, Daejeon 34129, Korea

4

Korea Research Institute of Standards and Science, Daejeon 34113, Korea

ABSTRACT

Si1−xGex nanowires (NWs) (0.22 ≤ x ≤ 0.78) were synthesized using a vapor-liquid-solid procedure with a Au catalyst. We measured the intrinsic physical, chemical, and electrical properties of the oxidized

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Si1−xGex NWs using several techniques, including transmission electron microscopy, X-ray photoemission spectroscopy, and optical pump-THz probe spectroscopy. We suggest two distinct oxidation mechanisms depending on the Ge content in the Si1−xGex NWs: (i) when the Ge content is around 0.22, a Au catalytic effect brings about oxidation in both the axial and lateral directions; and (ii) when the Ge content is greater than 0.22, the Au tip is detached from the NW body and does not act as a catalyst, which is a result of the high degree of Ge-atom participation in the oxidation process. Additionally, we measured the photoconductivity decay time distribution for the Si1−xGex NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si1−xGex NWs (0.22 < x), whereas it is maintained for Sirich Si1−xGex NWs (x ~ 0.22) as compared to the as-grown Si1−xGex NWs. It indicates that the number of defect states is generated with the formation of defective Ge oxide at the oxide-shell-layer/Si1−xGex-coreNW interface.

Keyword: Si1-xGex Nanowires, Oxidation mechanism, Ge content, Au catalyst, Optical pump-THz probe spectroscopy, Interfacial defect states


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I. Introduction The continuous scale-down of complementary-metal-oxide-semiconductor (CMOS) devices has generated a requirement for alternatives to standard planar transistor device architectures, such as fin-field effect transistor (FET) and monolithic-FET stacked structures. In particular, fin-FET architectures are becoming primary vehicles for sub-22-nm technology nodes 1-2. However, in the semiconductor industry, other novel device structures are currently being investigated 3-5. One way to enhance the performance of FETs is to incorporate high-mobility SiGe or ultimately strained-Ge channels by utilizing a nanowire (NW) structure in fin-FETs 6-7. SiGe-alloy systems have attracted particular attention as alternatives to Sibased technologies owing to their superior properties, including a tunable band gap and tunable mobility with Ge and Si being completely miscible 8-9. According to several reported results, there are many advantages to using Si1−xGex-NW-based FETs for realizing practical advanced devices 10-12. From a geometrical perspective in NWs structure, the extremely large surface area of Si1−xGex NWs plays a dominant role in determining the electronic and optical properties of nano-scale devices incorporating these materials 13-15. Thus, understanding the physical and optical/electrical properties of Si1−xGex NWs is necessary for preparing advanced nano-sized devices using SiGe channels. In addition, understanding the process of Si1−xGex NWs oxidation to form a gate oxide is essential in FET-device fabrication. Defects at the interface between the gate oxide and the Si1−xGex NWs can be particularly critical in determining the device performance and reliability when using Si1−xGex nano3 ACS Paragon Plus Environment


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structures 6-7, 16-17. Literature studies on SiGe-film oxidation can be summarized by the following key points: (i) Ge does not participate in the oxidation of Si1−xGex films when the Ge content is below 0.5, (ii) there is no loss of Ge content during Si1−xGex films oxidation more than Ge content of 0.5, and (iii) the oxidation rate is different in wet and dry oxygen environments 18. Based on the finding that Si is the main participant in the oxidation of the Si1−xGex films, we expect that good-quality SiO2 and Ge-rich Si1−xGex NWs can be formed by oxidizing Si1−xGex NWs. In this case, the interfacial defects generated in Ge-rich Si1−xGex NWs can be critical in determining the carrier transportation in nano-sized devices under an applied electric field. Thus, understanding the oxidation mechanism of Si1−xGex NWs would be very useful for designing novel nano devices. However, there have been no investigations on the oxidation of Si1−xGex NWs as a function of the Ge content. Si1−xGex NWs are generally grown via the vapor-liquidsolid (VLS) method using a Au catalyst at the Au-Si eutectic temperature of 363 °C. In a previous study, we observed the Au catalytic effect during Si-NW oxidation as a function of the oxidation time; the oxidation rate in axial direction through the Au tip was faster than the rate in the lateral direction 19. Up to now, there have been no in-depth studies on the Au catalytic effect in Si1−xGex-NW oxidation as a function of the Ge content. In this study, we oxidized Si1−xGex NWs and attempted to interpret the oxidation mechanism, with regards to the effect of the Au catalyst, as a function of the Ge content. We observed two oxidation mechanisms for Si1−xGex NWs in terms of the Au catalyst: when the Ge content (x) is around 0.22, the Au 4 ACS Paragon Plus Environment

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catalytic effect results in a faster oxidation rate in the axial direction through the Au catalyst compared with that in the lateral direction; when x > 0.22, the Au tip separates from the NW body and does not act as a catalyst, with a Ge-rich oxide forming on the surface of the Au tip. We analyzed the quality of the oxide formed on the Si1−xGex NWs as a function of the Ge content by measuring the chemical states of the oxide species using X-ray photoemission spectroscopy (XPS). Additionally, we investigated the optically excited carrier dynamics on an ultrashort timescale before and after oxidation using optical pump-THz probe (OPTP) spectroscopy, the results of which are dependent on the defect states at the NW surface and the oxide/Si1−xGex-NW interface. The OPTP measurements indicated that there were fewer defect states post-oxidation in the (x < 0.22) Si1−xGex NWs compared with the (x > 0.22) Si1−xGex NWs, which arises from the degree of Ge-atom participation in the oxidation process. II. Experimental Si1−xGex NWs were synthesized using a vapor-liquid-solid (VLS) method with an ultrahigh-vacuum chemical-vapor-deposition (UHV-CVD) system. First, a 2-nm-thick Au-film catalyst was deposited by thermal evaporation onto a clean Si (111) substrate. The sample was then transferred in situ to the main chamber of the UHV-CVD system where it was annealed at 450 °C for 5 min under a pressure of ~1 × 10−8 torr, resulting in the formation of Au-Si alloy droplets on the Si substrate. After formation of the droplets, Si1−xGex NWs were synthesized by filling the main chamber with a mixture of SiH4 and GeH4 as precursors, and H2 as a carrier gas, while maintaining a fixed total pressure of 2 torr 20. The Si-Ge alloy 5 ACS Paragon Plus Environment


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composition was varied by adjusting the SiH4/GeH4 ratio (18/2, 16/4, 13.3/6.7, or 10/10 (sccm)) and maintaining a constant 20 sccm SiH4/GeH4 flow rate under a fixed H2 flow rate of 100 sccm. The Si1−xGex NWs were thermally oxidized in a tube-zone furnace at a temperature of 475 °C using a constant oxygen flow rate of 10 sccm. The furnace chamber was filled with Ar gas while it was heating up to the 475 °C oxidation temperature; on reaching 475 °C, the gas was switched from Ar to O2 19. The melting point of Si1−xGex films is given by the equation 1412 − 738x + 263x2; thus, in our case, oxidation temperatures between 989 °C (x ~ 0.22) and 723 °C (x ~ 0.78) should be appropriate. However, the melting point of Si1−xGex NWs is lower than that of bulk Si1−xGex because of nano size effects, which result from the defect states located on the wire surface. Based on our empirical evidence, we found that 475 °C is the highest temperature that can be used for oxidizing Si1−xGex NWs without any melting occurring. Therefore, 475 °C was used as the oxidation temperature in this study. The oxidation process was performed for 12 h for all Si1−xGex NWs. This was found to be the time necessary, under our oxidation conditions, to produce a distinct Si1−xGex oxide layer, with a remaining Si1−xGex-NW core. The morphology and crystalline structure of the Si1−xGex NWs were investigated using field-emission scanning electron microscopy (FE-SEM, JSM 6500F, Jeol) and transmission electron microscopy (TEM) (Tecnai F20, FEI). Energy dispersive X-ray (EDX) spectroscopy was used to determine the chemical composition of the Si1−xGex NWs. For OPTP measurements, the 800 nm pump pulse was collinearly injected into the sample with the THz pulse. After excitation of the sample by the ultrafast optical pulses, 6 ACS Paragon Plus Environment

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the transient change of the sample in the 0.2–2.6 THz frequency range was probed by THz pulses via electro-optic sampling. The time evolution of the pump-probe signal was collected by scanning the time delay of the pump pulses with respect to the THz pulse 21-22.

III. Results and Discussion The morphology and crystal structure of the Si1−xGex NWs was investigated as a function of the Ge content using SEM and TEM measurements. All of the Si1−xGex NWs exhibited a straight shape, with a single crystal structure in the [111] growth direction 20, 23. The length of the Si1−xGex NWs decreased with increasing GeH4 gas ratio, as shown in Figure S1. However, the shape and length of the Si1−xGex NWs does not affect the oxidation process, as they have no effect on the oxidation rate and kinetics in the lateral and axial directions under the oxidation conditions used here. The relative concentration of Si and Ge in the Si1−xGex NWs synthesized at various SiH4/GeH4-gas ratios was examined using EDX measurements; SiH4/GeH4-gas ratios of 18/2, 16/4, 13.3/6.7, and 10/10 resulted in Ge contents (x) of 0.22, 0.35, 0.51, and 0.78, respectively (Fig. 1(b)). We measured the diameters of numerous individual Si1−xGex NWs as a function of the Ge content using TEM. The Si1−xGex NWs exhibited a Gaussian distribution of diameters, with an average of about 40 nm, regardless of the Ge content (Figure S2). The diameters of the Si1−xGex NWs with different Ge contents are summarized in Table 1. We also used high-resolution X-ray diffraction (XRD) measurements to determine the relative concentration of Ge atoms in the Si1−xGex NWs 7 ACS Paragon Plus Environment


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for comparison with the values determined from the EDX measurements. θ − 2θ XRD patterns for the Si1−xGex NWs are shown in Fig. 1(a). Asymmetric peaks associated with the Si substrate appear between 28.4° and 28.6°, rather than at exactly 28.43°, which is attributed to the use of a (111) vicinal Si substrate with a 4° tilt for growing the Si1−xGex NWs . Although we found that the Si (111) substrate peak position can be changed with tilting θ, chi, and phi axis with a little asymmetry shape, this had no effect on the position of the Si1−xGex-NW (111) diffraction peak; i.e. it is reasonable to determine the Ge content from the Si1−xGex NW XRD spectra. The Si1−xGex-NW (111) diffraction peaks appeared between 27° and 28.5°, with a lower angle corresponding to higher Ge content. Since the Si1−xGex NWs are precipitated from a Au–Si alloy and grown with a high aspect ratio via the VLS mechanism, therefore, strain effects along the vertical direction in the Si1−xGex NWs do not need to be considered. Thus, based on the growth kinetics, we can determine the Ge content (x) corresponding to the lattice constants from the XRD data in Fig. 1(a) using Vegard’s law 20. The values of x for the Si1−xGex NWs determined from the XRD and EDX measurements were almost same, as shown in Fig. 1(b). TEM images and EDX line profiles of the Si1−xGex NWs with different Ge contents are shown before oxidation in Fig. 2(a)–(d) and after oxidation in Fig. 2(a’)–(d’). Before oxidation, the morphology of the different Si1−xGex NWs was similar regardless of the Ge content, with a Au tip at the top of the NWs. Moreover, the EDX line profiles of all pre-oxidation samples show a constant distribution of Si and Ge along the axial direction, with an abrupt interface between the Au tip and NW body. After oxidation at 8 ACS Paragon Plus Environment

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475 °C, there are distinguishable changes in the Si1−xGex-NW morphology depending on the Ge content. When x > 0.22 the Au tip is separated from NW body, while the Au tip is in contact with the NW body when x ~ 0.22. This difference arises from the presence or absence of a Au catalytic effect during oxidation; when x ~ 0.22 a Au catalytic effect is present (Fig. 2(a’)) and when x > 0.22 there is no Au catalytic effect (Fig. 2(b’)–(d’)). Fig. 2(a’) shows that oxidation layers are formed on the two surface regions of the (x ~ 0.22) Si1−xGex NWs along the axial and lateral directions. The oxide layer in the axial direction is thicker than that in the lateral direction. From the EDX measurements, we found that SiO2 only forms on the Au tip in the axial direction, indicating that the Si atoms precipitated on the Au tip react easily with oxygen atoms. This enhances the oxidation rate in the axial direction compared with that in the lateral direction (this is referred to as the ‘Au catalytic effect’). We previously observed this phenomenon in oxidized Si NWs with a Au tip 19. Moreover, when x ~ 0.22, we observed that a Ge-rich Si1−xGex neck part forms below the elongated Au tip, as indicated by the red rectangle in the EDX map and the yellow arrow in the TEM image in Fig. 2(a’). We investigated the oxidation process in detail to better understand this phenomenon. First, to avoid any pre-oxidation, we heated up to the oxidation temperature (475 °C) in Ar gas, rather than oxygen gas. In this state, Si and Ge atoms can melt into the Au tip at the eutectic temperature of AuSi(Ge). On introducing oxygen gas at 475 °C, the Si atoms into the Au tip are preferentially oxidized because the heat of formation for SiO2 (−204 kcal/mol) is lower than that for GeO2 (−119 kcal/mol) 24. 9 ACS Paragon Plus Environment


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Consequently, only a SiO2 layer forms in the axial direction, and the Ge concentration in the Au tip increases. After the oxidation process is finished and the temperature is decreased to room temperature, the Si and Ge atoms melted into the Au tip can re-precipitate below the Au tip in contact with the NW body. In particular, since Ge atoms do not participate in the oxidation process, Ge-rich Si1−xGex is formed in the neck region between the Au tip and the NWs. On the other hand, for the (x > 0.22) Si1−xGex NWs, the Au tip became spherical and separated from the Si1−xGex NWs during oxidation, as shown in the TEM images in Fig. 2(b’)–(d’). In this case, we found that a very-thin Ge-containing Si1−xGex oxide layer surrounds the spherical Au tip (indicated by the yellow arrow): the concentration of Si and Ge in the oxide layer was determined from the EDX line profiles shown within the red rectangles. The composition of the Si and Ge in the oxide layer was also investigated in detail using EDX mapping, with the x ~ 0.51 Si1−xGex NWs selected as a representative sample (Fig. 3). A Ge-containing Si1−xGex oxide layer also formed in the inner region of the Si1−xGex NWs, while a SiO2 layer primarily formed in the outer region. This phenomenon can also be explained by the heats of formation for SiO2 (−204 kcal/mol) and GeO2 (−119 kcal/mol) 24. The participation of Ge atoms in the oxidation of a Si1−xGex film when x > 0.22 is unprecedented; in general, Ge atoms in Si1−xGex films only react with oxygen when x > 0.5 18. In contrast to the (x ~ 0.22) Si1−xGex NWs, we observed separation of the Au tip from the (x > 0.22) Si1−xGex NWs after oxidation. This is attributed to the formation of the Ge-containing Si1−xGex oxide layer at interface between the Au tip and Si1−xGex NWs in 10 ACS Paragon Plus Environment

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the (x > 0.22) Si1−xGex NWs. After separation of the Au tip from the NWs, the Au transforms into a spherical shape due to the strong Au–Au metallic bonds. Also, Si atoms melted into the Au tip are preferentially oxidized and the melted Ge atoms are sequentially oxidized, as observed in the EDX maps in Fig. 3. We measured O1s, Si2p, and Ge3d XPS spectra for the oxide layer formed in the lateral direction to determine its chemical composition and states. There are two distinct components in the Si2p (Ge3d) spectra at 99.2 eV (29.5 eV) and 103 eV (33.5 eV), which are related to pure Si (Ge) bonding and SiO2 (GeO2), respectively (Figure S3). In all of Si1−xGex NWs (0.22 ≤ x ≤ 0.78) pre-oxidation, we observed that both Si and Ge oxide are present in their native state. After oxidation of all Si1−xGex NWs, both Si and Ge oxide were present in oxide layers. We quantified the different Si and Ge oxide states present before and after oxidation, as shown in Fig. 4(a), to determine the effect of Ge content on the oxide species formed in the Si1−xGex NWs. Based on quantification of oxide states, we discovered that Si oxide is mainly formed in all cases of as-grown Si1-xGex NWs, while after oxidation process Ge oxide is significantly increased in the case of x ≥ 0.35 in terms of relative concentration of Si and Ge peaks (expressed with dotted red line). Taking into account the penetration depth of XPS, these results indicate that a Ge-oxide layer around 5 nm thick formed at the outer shell of the Si1−xGex NWs is dramatically increased with increase of Ge content. Based on the EDX and XPS results, we concluded that Ge oxide is primarily distributed at both regions of interface between oxide layer/Si1-xGex NWs and surface on oxide layer. 11 ACS Paragon Plus Environment


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The distribution of Ge atoms in the NWs can be estimated from the behavior of the Ge atoms during oxidation of the Si1−xGex NWs. According to the literature, simultaneous oxidation of Si and Ge easily occurs in Si1−xGex films when x > 0.5, while Si is selectively oxidized when x < 0.5; i.e. Ge atoms can be completely excluded from the oxidation process when x < 0.5 18. Thus, the simultaneous formation of Ge oxide and Si oxide observed here when x ~ 0.22 is unexpected. We propose that this simultaneous oxidation of Ge and Si in films with low Ge contents (in our case, x ~ 0.22 in Si1−xGex NWs) is due to a geometrical effect arising from the extremely large surface area of the NWs. In the oxidized Si1−xGex NWs, Ge oxide is present at the oxide layer/Si1−xGex-NW interface and on the oxide-layer surface. In a previous study, Ge3d XPS measurements showed that interstitial Ge (Gei) atoms were distributed in Si1−xGex NWs, and the Gei-atom content increased with increasing Ge content in the Si1−xGex NWs 20, 23. The formation of Ge oxide near the NW surface can be related to the Gei content. During oxidation, Gei atoms can easily diffuse out toward the NW surface and incorporate O atoms, resulting in the Ge oxide layer. Although the heat of formation for Si oxide is lower than that for Ge oxide, it is easy that Gei atoms react with O atoms due to the diffusion limited process, which event occur at initial states of oxidation process. For all as-grown Si1−xGex NWs, we observed the formation of a Ge-oxide layer on the NW surface. These results indicate that the Gei atoms contribute to the formation of the Ge oxide near the NW surface. Moreover, we found that the Ge oxide layer near the NW surface increased with increasing Ge content in the Si1−xGex NWs (Fig. 4(a)). Thus, after the Gei and O atoms have reacted, Si oxide 12 ACS Paragon Plus Environment

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dominantly forms on the Si1−xGex NWs and Ge atoms then pile up at the oxide-layer/Si1−xGex-NW interface, resulting in the Ge-rich Si1−xGex layer on the NWs. Since a Ge oxide layer forms on the Si1−xGex NWs even when the Ge content is low, the presence of the Ge-rich Si1−xGex layer is sufficient for formation of Ge oxide; i.e. the surface structure consists of Gei-oxide/SiO2/Ge-oxide (or Ge) rich-SiGe oxide (or Si1−xGex layer)/Si1−xGex NWs. Additionally, we determined the oxygen content that had reacted with Ge and Si before and after oxidation, as shown in Fig. 4(b). There is abundant oxygen in the form of native Si and Ge oxides for all as-grown Si1−xGex NWs, while the stoichiometric post-oxidation oxygen content is approximately 0.66. Before oxidation, native oxide (Si or Ge oxide) forms at the NW surface, with –OH and –CHO bonds incorporated, leading to the abnormal O content at the NW surface. Moreover, the native-oxide content increases in proportion to the Ge content, which indicates that the Ge oxide is intrinsically defective. After oxidation, a stable and stoichiometric thermal oxide forms. Based on these measurements, we propose two oxidation mechanisms depending on the Ge content, as depicted in Fig. 5(a) and (b). During oxidation of the (x ~ 0.22) Si1−xGex NWs, Si oxide forms dominantly in both the axial and lateral directions due to its low formation energy. Notably, Ge oxide also forms in the lateral direction despite the low Ge content. Since stable Si oxide is the main oxide, the Au tip in contact with the Si1−xGex NWs is stretched after the oxidation process (Fig. 5(a)). In the oxidized (x > 0.22) Si1−xGex NWs, the Au tip is separated from the Si1−xGex NWs because defective Ge-containing 13 ACS Paragon Plus Environment


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Si1−xGex oxide forms at the weaker interface between the Au tip and Si1−xGex NWs. Si oxide is then located in the oxide-layer center and Ge oxide is present in two regions: the surface and interface of the oxide layer with the spherical Au tip, and the lateral direction of the Si1−xGex NWs (Fig. 5(b)). Finally, we used OPTP spectroscopy to investigate the defect states in the Si1−xGex NWs before and after oxidation. OPTP spectroscopy is well known to provide carrier trapping times captured at defect states, occurring within ultrafast time scale of several, or hundreds of, picoseconds 25-28. For OPTP measurements, the Si1-xGex NWs before and after oxidation process were mechanically transferred onto a sapphire substrate to exclude the effects of the Si substrate. Thus, information on carrier dynamics could be directly extracted from the NWs. We assume that numerous defect states exist at the oxide/NW interface due to the extremely large surface area of the NWs. The high surface-charge density and distorted band structure at the NW surface can affect device performance. In a previous study, we found that the chemical structure at a NW surface resulting from defect states induced changes in the electronic structure of the NWs 23. Photo-induced changes in photocarriers excited from the Si1−xGex NWs were investigated as a function of the Ge content, as shown in Fig. 6(a) and (b) for the as-grown and oxidized Si1−xGex NWs, respectively. The data was obtained by comparing the intensity between an injected THz pulse (T) and an absorbed THz pulse [∆T] during passage through the sample, as a function of time. Charge carriers are generated when an intense pump beam is injected into the sample, and ∆T is then determined; i.e. during evolution of ∆T/T spectra over time, the near-edge intensity indicates the 14 ACS Paragon Plus Environment

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photocarrier density and the decay curve reflects photocarrier lifetimes 21. In the as-grown Si1−xGex NWs, the decay curves decreased with decreasing Ge content (x) from 0.22 to 0.51 and then recovered on further increasing x to 0.78. From these results, we propose that the number of defect states in the Si1−xGex NWs is maximized when x is 0.51, while the number of defect states is lower in Si-rich or Gerich Si1−xGex NWs. We previously obtained the same results and explained the change in decay time for the Si1−xGex NWs as a function of the Ge content, which is determined by the number of defect states. 29 The Si1−xGex NWs contain various oxygen defects unstably incorporated with both Si and Ge atoms at the native-oxide/Si1−xGex-NW interface, which can serve as an electron- and hole-trapping site. The decay time for the Si1−xGex NWs is primarily determined by the energy level of the defect states and the electron (or hole) mobility of the Si and Ge atoms based on the relative concentration of Si and Ge. Thus, the minimum decay time observed for the (x ~ 0.51) Si1−xGex NWs is attributed to a number of hole trapping sites that capture holes faster than electrons, resulting in the shortest decay time 29. After oxidation, the decay curves for all Si1−xGex NWs are drastically decreased, with the exception of the Si-rich (x ~ 0.22) Si1−xGex NWs. This clearly shows that once the Ge-containing Si1−xGex oxide forms in the Si1−xGex NWs (when x > 0.22) via the simultaneous oxidation process, the oxide layer contains numerous defect states mainly arising from a Ge oxide. On the other hand, during oxidation of the Si-rich Si1−xGex NWs, Si oxide primarily forms at the wire surface because of the initially large Si content, which results in decay



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curves similar to those for the as-grown NWs. Thus, we consider oxidation an advantageous process in Si-rich Si1−xGex NWs with or without a small Ge oxide content.

IV. Conclusion We synthesized Si1−xGex NWs with varying Ge contents between 0.22 and 0.78 using a VLS method and oxidized the NWs at 475 °C for 12 hrs. We characterized the physical properties, morphology, and chemical states of the Si1−xGex NWs, using TEM, EDX, and XPS measurements, to probe the intrinsic properties of the oxidized Si1−xGex NWs as a function of the Ge content. The results of these measurements indicate that the Ge content affects the Si1−xGex-NW oxidation mechanism. We propose two oxidation mechanisms for the Si1−xGex NWs depending on the Ge content, in contrast to the oxidation of Si1−xGex film. In the oxidized (x ~ 0.22) Si1−xGex NWs, there is a Au catalytic effect that results in Si oxide forming in the axial direction due to the lower formation energy of SiO2 compared with GeO2. While the Si oxide forms in both the axial and lateral directions, Ge oxide only forms in the lateral direction. This is unexpected when compared with Si1−xGex films, for which Ge oxide only forms when the Ge content is greater than 0.5. In the oxidized (x > 0.22) Si1−xGex NWs, the Au tip is separated from the Si1−xGex NWs due to the formation of a defective Ge-containing Si1−xGex oxide at the weaker interface between the Au tip and the Si1−xGex NWs, resulting in the absence of a Au catalytic effect for oxide formation in the axial direction. As the Ge content is increased, the Ge oxide becomes distributed in two 16 ACS Paragon Plus Environment

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regions, at the surface and interface of the oxide layer. Moreover, we used OPTP spectroscopy to investigate the carrier decay time related with defect states and found that the decay time for the oxidized Si1−xGex NWs is very short, with the exception of the Si-rich Si1−xGex NWs (x ~ 0.22), which is a result of the defect states induced by the Ge oxide. We believe that these results related to the defect states will provide useful information for designing and evaluating nano devices.

ASSOCIATED CONTENT

Supporting information

The SEM images for the morphological shape of the Si1-xGexNWs as a function of Ge content x, the average diameters of numerous Si1-xGex NWs as a function of Ge content x and the Si2p, Ge3d XPS spectra to verify the chemical state of as-grown and oxidized Si1-xGex NWs as a function of Ge content x.

AUTHOR INFORMATION

Corresponding Author

*[email protected]

Notes

The authors declare no competing financial interest



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ACKNOWLEDGMENTS

This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant No.2015R1A2A1A01007560) and partially supported by the Korea Research Institute of Standards and Science under the Metrology Research Center project. The authors are grateful for the valuable help in the experiments performed using the fs-THz spectroscopy beamlines at the Pohang Light Source (PLS).


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REFERENCES (1) Hwang, C. H.; Li, Y. In Modeling of Work-Function Fluctuation for 16 Nm Finfet Devices with Tin/Hfsion Gate Stack, Proceedings of 2010 International Symposium on VLSI Technology, System and Application, 26-28 April 2010; 2010; pp 74-75. (2) Cheng, H. W.; Li, Y. In Random Work Function Variation Induced Threshold Voltage Fluctuation in 16-Nm Bulk Finfet Devices with High-K-Metal-Gate Material, 2010 14th International Workshop on Computational Electronics, 26-29 Oct. 2010; 2010; pp 1-4. (3) Dupre, C.; Hubert, A.; Becu, S.; Jublot, M.; Maffini-Alvaro, V.; Vizioz, C.; Aussenac, F.; Arvet, C.; Barnola, S.; Hartmann, J. M.; Garnier, G.; Allain, F.; Colonna, J. P.; Rivoire, M.; Baud, L.; Pauliac, S.; Loup, V.; Chevolleau, T.; Rivallin, P.; Guillaumot, B.; Ghibaudo, G.; Faynot, O.; Ernst, T.; Deleonibus, S. In 15nm-Diameter 3d Stacked Nanowires with Independent Gates Operation, 2008 IEEE International Electron Devices Meeting, 15-17 Dec. 2008; 2008; pp 1-4. (4) Yiming, L.; Hung-Mu, C.; Jam-Wem, L. Investigation of Electrical Characteristics on SurroundingGate and Omega-Shaped-Gate Nanowire Finfets. IEEE Trans. Nanotechnol. 2005, 4 (5), 510-516. (5) Fu-Liang, Y.; Di-Hong, L.; Hou-Yu, C.; Chang-Yun, C.; Sheng-Da, L.; Cheng-Chuan, H.; TangXuan, C.; Hung-Wei, C.; Chien-Chao, H.; Yi-Hsuan, L.; Chung-Cheng, W.; Chi-Chun, C.; Shih-Chang, C.; Ying-Tsung, C.; Ying-Ho, C.; Chih-Jian, C.; Bor-Wen, C.; Peng-Fu, H.; Jyu-Horng, S.; Han-Jan, T.; Yee-Chia, Y.; Yiming, L.; Jam-Wem, L.; Pu, C.; Mong-Song, L.; Chenming, H. In 5nm-Gate Nanowire Finfet, Digest of Technical Papers. 2004 Symposium on VLSI Technology, 2004., 15-17 June 2004; 2004; pp 196-197. (6) Fang, W.-W.; Singh, N.; Bera, L. K.; Nguyen, H. S.; Rustagi, S. C.; Lo, G.; Balasubramanian, N.; Kwong, D.-L. Vertically Stacked Sige Nanowire Array Channel Cmos Transistors. IEEE Electron Device Lett. 2007, 28 (3), 211-213. (7) Villalon, A.; Le Royer, C.; Nguyen, P.; Barraud, S.; Glowacki, F.; Revelant, A.; Selmi, L.; Cristoloveanu, S.; Tosti, L.; Vizioz, C. In First Demonstration of Strained Sige Nanowires Tfets with Ion Beyond 700µa/µm, VLSI Technology (VLSI-Technology): Digest of Technical Papers, 2014 Symposium on, IEEE: 2014; pp 1-2. (8) Pereira, R. N.; Nielsen, B. B.; Dobaczewski, L.; Peaker, A. R.; Abrosimov, N. V. Local Modes of Bond-Centered Hydrogen in Si:Ge and Ge:Si. Phys. Rev. B 2005, 71 (19), 195201. (9) Yang, J.-E.; Jin, C.-B.; Kim, C.-J.; Jo, M.-H. Band-Gap Modulation in Single-Crystalline Si1-Xgex Nanowires. Nano Lett. 2006, 6 (12), 2679-2684. (10) Lew, K. K.; Pan, L.; Dickey, E. C.; Redwing, J. M. Vapor–Liquid–Solid Growth of Silicon– Germanium Nanowires. Adv. Mater. 2003, 15 (24), 2073-2076.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11) Lew, K.-K.; Pan, L.; Dickey, E. C.; Redwing, J. M. Effect of Growth Conditions on the Composition and Structure of Si1−Xgex Nanowires Grown by Vapor–Liquid–Solid Growth. J. Mater. Res. 2006, 21 (11), 2876-2881. (12) Seong, H.-K.; Jeon, E.-K.; Kim, M.-H.; Oh, H.; Lee, J.-O.; Kim, J.-J.; Choi, H.-J. Interface Charge Induced P-Type Characteristics of Aligned Si1−Xgex Nanowires. Nano Lett. 2008, 8 (11), 3656-3661. (13) Huang, Y.; Duan, X.; Lieber, C. M. Nanowires for Integrated Multicolor Nanophotonics. Small 2005, 1 (1), 142-147. (14) Sirbuly, D. J.; Law, M.; Yan, H.; Yang, P. Semiconductor Nanowires for Subwavelength Photonics Integration. J. Phys. Chem. B 2005, 109 (32), 15190-15213. (15) Agarwal, R.; Lieber, C. M. Semiconductor Nanowires: Optics and Optoelectronics. Appl. Phys. A 2006, 85 (3), 209-215. (16) Ma, J.; Lee, W.; Bae, J.; Jeong, K.; Oh, S.; Kim, J.; Kim, S.-H.; Seo, J.-H.; Ahn, J.-P.; Kim, H. Carrier Mobility Enhancement of Tensile Strained Si and Sige Nanowires Via Surface Defect Engineering. Nano Lett. 2015, 15 (11), 7204-7210. (17) Ma, J.; Lee, W.; Bae, J.; Jeong, K.; Kang, Y.; Cho, M.-H.; Seo, J.; Ahn, J.; Chung, K.; Song, J. Effects of Surface Chemical Structure on the Mechanical Properties of Si1–Xgex Nanowires. Nano Lett. 2013, 13 (3), 1118-1125. (18) Liou, H.; Mei, P.; Gennser, U.; Yang, E. Effects of Ge Concentration on Sige Oxidation Behavior. Appl. Phys. Lett. 1991, 59 (10), 1200-1202. (19) Bae, J. M.; Lee, W. J.; Ma, J. W.; Cho, M. H.; Ahn, J. P.; Lee, H. S. The Oxidation Characteristics of Silicon Nanowires Grown with an Au Catalyst. Nano Res. 2012, 5 (3), 152-163. (20) Lee, W.-J.; Ma, J. W.; Bae, J. M.; Park, S. H.; Cho, M.-H.; Ahn, J. P. The Modulation of Si1− Xgex Nanowires by Correlation of Inlet Gas Ratio with H2 Gas Content. CrystEngComm 2011, 13 (16), 52045211. (21) Lee, W.-J.; Cho, D.-H.; Wi, J.-H.; Han, W. S.; Chung, Y.-D.; Park, J.; Bae, J. M.; Cho, M.-H. NaDependent Ultrafast Carrier Dynamics of Cds/Cu(in,Ga)Se2 Measured by Optical Pump-Terahertz Probe Spectroscopy. J. Phys. Chem. C 2015, 119 (35), 20231-20236. (22) Bae, J. m.; Lee, W.-J.; Ma, J. w.; Kim, J. h.; Oh, S. h.; Cho, M.-H.; Chul, K.; Jung, S.; Park, J. Structural Evolution and Carrier Scattering of Si Nanowires as a Function of Oxidation Time. J. Mater. Chem.C 2015, 3 (9), 2123-2131. (23) Ma, J.; Lee, W.; Bae, J.; Jeong, K.; Kang, Y.; Cho, M.-H.; Seo, J.; Ahn, J.; Chung, K.; Song, J. Effects of Surface Chemical Structure on the Mechanical Properties of Si1–X Ge X Nanowires. Nano Lett. 2013, 13 (3), 1118-1125. (24) Samsonov, G. V. The Oxide Handbook, Springer Science & Business Media: 2013.


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(25) Prasankumar, R. P.; Upadhya, P. C.; Taylor, A. J. Ultrafast Carrier Dynamics in Semiconductor Nanowires. Phys. Status Solidi B 2009, 246 (9), 1973-1995. (26) Parkinson, P.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Zhang, X.; Zou, J.; Jagadish, C.; Herz, L. M.; Johnston, M. B. Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core−Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy. Nano Lett. 2009, 9 (9), 3349-3353. (27) Shah, J. Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Springer: 1999; Vol. 115. (28) Othonos, A. Probing Ultrafast Carrier and Phonon Dynamics in Semiconductors. J. Appl. Phys. 1998, 83 (4), 1789-1830. (29) Bae, J. M.; Lee, W.-J.; Jung, S.; Ma, J. W.; Jeong, K.-S.; Oh, S. H.; Kim, S. M.; Suh, D.; Song, W.; Kim, S.; Park, J.; Cho, M.-H. Ultrafast Photocarrier Dynamics Related to Defect States of Si1-Xgex Nanowires Measured by Optical Pump-Thz Probe Spectroscopy. Nanoscale 2017, 9 (23), 8015-8023.



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Tables

Table 1. Summary of the growth conditions for the Si1−xGex NWs and their Ge contents and diameters

SiH4:GeH4 (sccm)

Ge content, x

Average d (nm)

18:2

0.22

38.3 (±11.2)

16:4

0.35

42.3 (±9.1)

13.3:6.7

0.51

39.3 (±8.5)

10:10

0.78

46.2 (±7.8)


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Figure Captions

Figure 1. (a) XRD spectra for the Si1−xGex NWs (0.22 ≤ x ≤ 0.78). (b) Comparison of the Ge content in the Si1−xGex NWs as determined from EDX measurements and using Vegard’s law based on the XRD peak positions in (a).

Figure 2. TEM images and EDX line profiles for Si1−xGex NWs with varying Ge contents before oxidation (a)-(d) and after oxidation (a’)-(d’). For all of the as-grown Si1−xGex NWs, the Au tip is in contact with NW body and exhibits a semi-sphere shape. After oxidation, the Au tip shape changes depending on the Ge content of the Si1−xGex NWs: when x ~ 0.22 (a’) an elongated Au tip is in contact with the NW body and when x > 0.22 (b’-d’) the spherical Au tip is separated from the NW body. The EDX line scans show the relative concentration of Si and Ge from the Au tip to the NW body. For all of the as-grown Si1−xGex NWs, the Au-tip/NW-body interface is very abrupt, a Ge-rich section (indicated by the red box) forms at the Au-tip/NW-body interface after oxidation for all of the Si1−xGex NWs. Each relative concentration of Si and Ge in Si1−xGex NWs as a function of the Ge content is shown in the EDXgraph insets before and after oxidation.



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Figure 3. TEM image (left) and EDX maps of the Au, Si, Ge, and O (right) content for the oxidized Si1−xGex NWs (x ~ 0.51). (The orange dotted line indicates the remaining core Si1−xGex NWs after oxidation). The EDX maps show that Ge oxide and Si oxide form at the inner-shell and outer-shell of the Si1−xGex NWs, respectively; this phenomenon is also seen at the Au tip.

Figure 4. Quantification of Ge-oxide (a) and O (b) content with respect to the total oxide states of the asgrown and oxidized Si1−xGex NWs as a function of the Ge content. Values were calculated from Si2p, Ge3d, and O1s XPS spectra.

Figure 5. A schematic diagram of the two oxidation mechanisms for Si1−xGex NWs when x ~ 0.22 (a) and x > 0.22 (b). The evolution of the Au-tip shape and formation of the dominant oxide layer during oxidation is shown for the different Ge contents.

Figure 6. Photo-induced change in the photocarriers excited from as-grown (a) and oxidized (b) Si1−xGex NWs as a function of the Ge content (pump beam energy of 800 nm).


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TOC


Si1-xGexNWs

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

x = 0.22 x = 0.35 x = 0.51 x = 0.78

27.0

(a)

Ge Concentration (at. %)


27.5

28.0

28.5

80

EDS XRD

70 60 50 40 30 20

29.0

2 theta (degrees)

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0.1

(b)

ACS Paragon Plus Environment

0.2

0.3

0.4

0.5

GeH4 / (SiH4+GeH4)

Figure 1. Bae et al.

80 60

Au Si Ge

Si : 76.3 Ge : 23.7

40 20

(a’)

0 0

10

20

100

60 40 20 0 0

30

10

100

Au Si Ge

100 80 60 40

Si : 52.1% Ge : 47.9%

20

20 nm

0 0

10

20

(b’)

40 20 0 0

10

20

30

40

50

100

Au Si Ge

60 40 20

(c’)

0

Au Si Ge O

Si : 21.0% Ge : 33.8% O : 45.1%

80 60 40 20 0

0

10

20

30

40

0

Position (point) 100 80 60

Au Si Ge

Si : 17.6% Ge : 82.4%

40 20

(d’)

0 0

10

20

30

40

10

20

30

40

50

60

70

80

Position (point)

50

Position (point)

60


Concentration (%)

(d)

Au Si Ge O

Position (point)

Si : 34.8% Ge : 65.2%

80

40

60

30

Concentration (%)

Concentration (%)

100

30

Si : 32.8% Ge : 39.5% O : 27.5%

80

Position (point)

(c)

20

Position (point)

Concentration (%)

Concentration (%)

(b)

Au Si Ge O

Si : 41.8% Ge : 12.9% O : 45.2%

80

Position (point)

Concentration (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


100

Concentration (%)

(a)

Concentration (%)

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Au Si Ge O

Si : 19.6% Ge : 37.7% O : 42.7%

100 80 60 40 20 0 0

10

20

30

40

50

Position (point)



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Au

Si

Ge

O



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(a)

(b) 1.0 0.9

0.85

Thermal oxidation

O / (SiOx+GeOx)

GeOx / (SiOx+GeOx)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


0.8 0.7 0.6 0.5

Native oxide

0.4 0.3 0.2 0.1

0.80 Native oxide

0.75 0.70

0.666

0.65 Thermal oxidation

0.60 0.2

0.3

0.4

0.5

0.6

0.7

0.8

Ge content, x


0.2

0.3

0.4

0.5

0.6

0.7

0.8

Ge content, x



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(a)

(b)

(a) : SiH4 90%, (b) : SiH4 80%, SiH4 67%, SiH4 50% ACS Paragon Plus Environment


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0

Si1-xGexNWs x = 0.22 x = 0.35 x = 0.51 x = 0.78

-1

-2

x = 0.22 x = 0.35 x = 0.51 x = 0.78

-1

-2

-3 0

(a)

Oxidized Si1-xGexNWs

0

ln(T/T)

ln(T/T)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


20

40

60

80

100

Pump delay (ps)

120

140

0

(b)


20

40

60

80

Pump delay (ps)


100

Oxidation Mechanism of Si - ACS Publications - American

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