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Formation of Zr-oxynitride thin films on 4H-SiC substrate
Thin Solid Films 520 (2012) 6822–6829
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Formation of Zr-oxynitride thin films on 4H-SiC substrate Yew Hoong Wong, Kuan Yew Cheong ⁎ Energy Efficient & Sustainable Semiconductor Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 13 July 2012 Accepted 13 July 2012 Available online 20 July 2012 Keywords: Zr-oxynitride Sputtered Zr Nitrous oxide X-ray photoelectron spectroscopy
a b s t r a c t Formation of Zr-oxynitride by simultaneous oxidation and nitridation in nitrous oxide of sputtered Zr on SiC substrate is reported. Sputtered Zr on SiC substrate and followed by oxidation and nitridation in nitrous oxide ambient for 15 min at different temperatures (400–900 °C) have been systematically investigated. By using X-ray photoelectron spectroscopy, chemical compositions, and depth profile analysis have been evaluated, as well as energy band alignment of Zr-oxynitride/interfacial layer/SiC system. Zr-oxynitride film of Zr–O, Zr–N, and/or Zr–O–N and its interfacial layer composed of mixed Zr–O, Zr–N, Zr–O–N, Zr–Si–O, Si–N, and/or C–N phases were verified. A possible model related to the oxidation and nitridation mechanisms has been proposed and explicated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The advent of technology boom has spurred utilization of wide bandgap (WBG) semiconductors as alternative substrates to replace silicon for high-power, high-temperature, and/or high-radiation metaloxide-semiconductor (MOS)-based switching devices [1,2]. Among the WBG semiconductors, silicon carbide (SiC) has developed into one of the leading contenders owing to its commercial availability and ability to grow native oxide (SiO2) [2,3]. SiC appears in different polytypes [4,5]. 4H-SiC is one of the polytypes that provides a wide bandgap of 3.26 eV, high breakdown field strength of ~3 MV/cm, high saturation electron drift velocity of ~2 × 107 cm/s, and high thermal conductivity of ~3.7 W/cm °C [2,6,7]. These properties potentially enable operation of SiC-based devices under harsh conditions. Nonetheless, a high quality gate oxide must be deposited or grown between a metal electrode and a SiC substrate in order to sustain a high transverse electric field and a low gate leakage current in the MOS-based devices. So far, thermally nitrided-SiO2 is regarded as the most promising gate oxide due to its low interface and slow trap densities, high reliability, and low leakage current [8–15]. However, possession of its low dielectric constant (κ = 3.9) as compared to SiC (κ = 10) limits the permissible electric field in SiC-based devices [16]. As a result, the gate oxide may electrically break down before the SiC substrate thus defeating the purpose of using SiC as the primer substrate for high-power, high-temperature, and/or high-radiation applications. In order to overcome the aforementioned problem, an alternative gate oxide, to replace the low-κ nitrided-SiO2, with higher κ value has been proposed. Several single layers and stacking layers of high κ gate oxides ⁎ Corresponding author. Tel.: +60 4 599 5259; fax: +60 4 594 1011. E-mail address: [email protected] (K.Y. Cheong). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.07.036
on SiC have been investigated, which include MgO [17], Gd2O3 [18], Pr2O3 [19], oxidized Ta2Si [20], Al2O3 [21–23], AlN [24], La2O3 [25], ZrO2 [26,27], HfO2 [28–30], CeO2 [2,31,32], Y2O3 [33], Al2O3/SiO2 [34,35], HfO2/SiO2 [36–39], Ta2O5/SiO2 [40], La2O3/SiO2 [41], oxidized Ta2Si/SiO2 [40], SiNx/SiO2 [42], Al2O3/SiN/SiO2 [43], and Al2O3/AlN/Al2O3/AlN/SiO2 [44]. Of these oxides, ZrO2 is a potential candidate to be integrated, as the gate oxide, in SiC-based MOS devices; as it exhibits excellent properties such as high κ value (22–25), large energy bandgap (5.8–7.8 eV), and easily stabilized in the form of cubic or tetragonal polymorphs, which may further enhance its effective κ value [45–51]. Based on numerous reports, this material has demonstrated encouraging and wellbehaved characteristics when it is applied as gate oxide on Si-based substrate [46–49,52–56]. However, reports on its characteristics on SiC substrate are scarce. Karlsson et al. [26] reported on the growth and thermal stability of ZrO2 thin films on SiC(0001) by chemical vapor deposition technique. A heterogeneous layer was formed after annealing at 1000 °C due to the decomposition of ZrO2, thus it leads to the formation of t-ZrO2 remnants, metallic Zr silicide, and Si aggregates. Kurniawan et al. [27] reported on the formation of ZrO2 by sputtering of Zr on SiC substrate followed by thermal oxidation in ambient oxygen. The study pointed that post-annealing of the oxide has produced a thicker interfacial layer with an increment in capacitance and in dielectric constant as well as reduction of leakage current density but with lower oxide breakdown field. To improve the MOS characteristics and the oxide–SiC interface quality, it has been found that oxidation and/or post oxidation annealing in a nitrogen-containing ambient has two beneficial effects, i.e., enhanced removal of carbon and passivation of silicon dangling bonds [3]. The utilization of nitrous oxide (N2O) gas has contributed to the growth of thermally nitrided-SiO2 as gate oxide
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on SiC with low leakage current and low interface-trap density [11,12]. At high temperature, N2O gas is decomposed exclusively into N and O compounds, which are indispensable in performing oxidation and nitridation processes on the gate oxide [11,47,57]. In general, the beneficial effects of utilizing N2O gas in growing thermally nitrided SiO2 gate on SiC-based MOS structure have been highlighted but there is no pertinent investigation on the effects of N2O gas on oxidizing and nitriding of sputtered Zr based on SiC substrate. Hence, it is worthwhile to investigate the effects of simultaneous thermal oxidation and nitridation of a sputtered Zr/SiC system in ambient N2O. Previously, some works have been carried out on the effects of thermal oxidation and nitridation in ambient N2O of a sputtered Zr/Si system on their chemical, structural, electronic, and electrical properties [47–49]. Based on the reports, a model on oxidation and nitridation mechanism has been proposed. It has been examined that bulk ZrO2 and interfacial layer (IL) of Zr-silicate oxynitride (ZrSiON) have been formed on Si substrate based on X-ray photoelectron spectrometer results. However, using the same technique on a sputtered Zr/SiC system, it is postulated that bulk Zr-oxynitride is formed rather than ZrO2 if the processing parameters are well optimized. In this work, it is aimed to report the formation of Zr-oxynitride from Zr thin films on 4H-SiC substrate by simultaneous thermal oxidation and nitridation in N2O gas at various temperatures.
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available in the investigated films was calculated based on the following equation [11,60,61]: Ix
Cx
. Sx
∑i Ii
100
ð1Þ
Si
where, Ix and Ii were peak intensity of the evaluated element and all other detected elements, respectively. Sx and Si were sensitivity factor of the respective evaluated element and all other detectable elements. The sensitivity factor is dependent on XPS system. In this work, the sensitivity factors for Zr 3d, Si 2p, O 1s, N 1s, and C 1s were 2.576, 0.328, 0.78, 0.47, and 0.278, respectively. The XPS system was also used to determine the band alignment of the dielectric/semiconductor structure. The spectra of wide scan with binding energy range of − 5 to 25 eV, were collected at a take-off angle of 0° with respect to normal surface, with low pass energy of 20 eV and small step size of 0.1 eV. Owing to onset of single particle excitation and band-to-band transition, the energy loss spectrum of O 1s photoelectron provides further insight on the bandgaps of Zr-oxynitride and IL [62]. Subsequently, a detail scan of O 1s was carried out using the same pass energy and step size of 1.0 eV. Band alignment was extracted based on the Kraut method [63,64]. 3. Results and discussion
2. Experimental procedures 3.1. Compositional and depth profile analysis n-type Si-faced 4H-SiC(0001) substrates of 0.8 cm×0.8 cm, with 4.09°-off axis, 0.020 Ω cm of resistivity, and 1-μm thick of n-type epitaxial layer doped with nitrogen at concentration of (1–4)×1016 cm−3, were commercially purchased from CREE Inc. (USA). They were first cleaned by an ultrasonic and standard RCA cleaning, followed by dipping into a diluted HF solution (1 HF:50 H2O) aiming to remove native oxide on the surface. A 5-nm thick Zr film was deposited on the cleaned SiC substrates by a RF sputtering system (Edwards Auto 500). During the sputtering, working pressure, RF power, inert Ar gas flow, and deposition rate were regulated at 1.6 × 10−5 Pa, 170 W, 20 cm3/min, and 0.2 nm/s, respectively. Samples were placed inside a horizontal tube furnace and heated up from room temperature to a set of temperatures (400, 500, 700, and 900 °C) in an ambient Ar flow with a heating rate of 10 °C/min. Once the set temperature was reached, Ar gas was turned off and N2O gas was then purged in with a flow rate of 150 mL/min for 15 min. The samples were eventually taken out at room temperature after the furnace was cooled down to room temperature in ambient Ar. Compositional and depth profile of the produced films were measured by Kratos Axis UltraDLD X-ray photoelectron spectrometer (XPS) with a monochromatic Al-Kα X-ray source (hν=1486.69 eV) performed at Research Center for Surface and Materials Science, The Auckland University, New Zealand. The spectrometer was operated at 150 W and a take-off angle of 0o with respect to normal surface. In order to perform a depth profiling analysis, a 5-kV Ar ion etching was used. The pressure of Ar in the analysis chamber was 4×10−5 Pa, while the area of analysis was 220×220 μm2. The chemical compositions of the films were obtained from a combination of wide and narrow scans. A wide scan was first performed with a passing energy of 160 eV for 9 min to determine the elemental chemical states. The core-level spectra that had been detected were Zr 3d, Si 2p, O 1s, N 1s, and C 1s Subsequently, for a narrow scan, a passing energy of 20 eV for 5 min was used to scan through the binding-energy range of interest. The recorded C 1s peak due to the adventitious carbon-based contaminant on the surface, with respect to the literature value of 284.6 eV [58,59], was used as a reference to compensate for the charging effect of the XPS spectra. Surface charge and linear background were corrected with the help of CasaXPS software (version 2.3.15) before deconvolution of the XPS spectra was performed. The total concentration of an element (Cx)
Fig. 1(a)–(d) depicts the atomic composition depth profiles for Zr, Si, O, N, and C as a function of etching time in all investigated samples. Based on the depth profiles, it is observed that at earlier etching time, Zr and O elements are in dominance with atomic ratio of about 1:2. This indicates that a stoichiometric ZrO2 has been formed at the outmost layer in the bulk region of each sample. Atomic percentages of both Zr and O elements decrease as the etching time is extended and their ratio deviates from 1:2, until they totally vanish. Apart from Zr and O elements, N element can be detected in the bulk region of all investigated films as well. This may infer that nitrogen has been incorporated into the Zr–O network of the film, forming Zr-oxynitride. Zr-oxynitride in this experiment is in amorphous structure [48]. The atomic concentration of N element decreases as the etching time is prolonged, until it totally fades away. This defines the boundary of the oxynitride and SiC, as shown in the figure. Beyond this boundary, atomic percentages of Si and C increase significantly; confirming the appearance of SiC substrate. In between SiC and Zr-oxynitride boundaries, an interfacial layer (IL) with a mixture of Zr, O, Si, N, and C have been detected. A comprehensive analysis of the chemical compounds of the IL was performed by narrow scan of XPS and it will be discussed in the subsequent paragraphs. The thickness of Zr-oxynitride and IL are dependent on the oxidation/nitridation temperature. Based on the figure, thickness of Zr-oxynitride is marginally reduced as the oxidation/ nitridation temperature increases. The distribution profile of nitrogen in all investigated samples has been presented in Fig. 1(e). It demonstrated that the nitrogen in the 500 °C sample is distributed more uniformly throughout the entire film, including IL, as compared to other samples. Narrow scans of each core-level spectrum as a function of etching time for different oxidation/nitridation temperatures (400–900 °C) are shown in Figs. 2[(a)–(d)]–5[(a)–(d)]. A non-linear Gaussian–Lorentzian function (solid lines) was used to deconvolute the measured peaks (dotted lines) using CasaXPS software (version 2.3.15). The Zr 3d spectra [Fig. 2(a)–(d)] are well fitted by Zr–O, Zr–N, Zr–O–N, and/or Zr–Si–O components at their respective binding energies. Zr 3d doublet, which corresponds to 3d5/2 and 3d3/2 spin–orbit split components at 184.4 and 182.0 eV, respectively, has been detected at the top-most surface (etching time of 0 s) of the bulk region for all investigated samples
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Fig. 2. Evolution of Zr 3d core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
Fig. 1. Atomic percentage of Zr, Si, O, N, and C as a function of etching time for sample oxidized/nitrided samples at (a) 400 °C, (b) 500 °C, (c) 700 °C, 900 °C, and (e) nitrogen depth profile of all investigated samples.
(400–900 °C). This indicates that stoichiometric Zr–O (ZrO2) is formed [47,65]. As the etching time increases, it is observed that progressive chemical shift of Zr–O peaks towards lower or higher binding energies, depending on the oxidation/nitridation temperature. This is owing to
the evolution from stoichiometric to non- or sub-stoichiometric Zr–O towards a deeper region of all investigated films [47,66,67]. Besides Zr–O peaks, Zr–N (178.7–179.5 eV [47,68–70]) peaks can be detected in the bulk region towards the IL for all investigated samples (400– 900 °C), whilst Zr–O–N (180.0–182.5 eV [70–74]) can be detected only in the bulk region towards the IL for sample oxidized/nitrided at 500 °C. In addition, Zr–Si–O (177.2 eV [47,75]) compound has been detected at the IL region of the investigated samples, except for the sample oxidized/nitrided at 500 °C. This suggests that the bulk of Zr-oxnitride, which comprises Zr–O, Zr–N, and/or Zr–O–N, with an IL of Zr–O, Zr–N, Zr–O–N, and/or Zr–Si–O, were formed in the investigated samples. As the etching time extends to SiC substrate, the intensity of Zr 3d spectra decreases and disappears. To identify the presence of other components in the samples, Si 2p spectra were investigated [Fig. 3(a)–(d)]. Initially, there is no detectable peak at the earlier etching time for all investigated samples. As the etching
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Fig. 3. Evolution of Si 2p core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
time increases, Si–N, Si–C, and/or Zr–Si–O were detected at binding energies of 101.4–102.4 eV [11,47,61], 99.8 eV [61], and 98.0–99.4 eV [47,61], respectively, in the investigated samples, with an exception that Zr–Si–O peak was absent in the sample oxidized/nitrided at 500 °C. It is noted that the detected Si–C peak originated from the SiC substrate and the detection of Zr–Si–O peak, with respect to the range of etching, is consistent with the results demonstrated in Zr 3d spectra. Based on the O 1s spectra analysis [Fig. 4(a)–(d)], at the top-most surface (etching time of 0 s) of all investigated samples, two peaks can be fitted in the O 1s core-level spectra, with their locations at 530.0 eV [47,76,77] and 531.9–532.0 eV [47,78]. These two peaks are assigned to stoichiometric Zr–O. Similar to the observation made in Zr 3d spectra (Fig. 2), the O 1s spectra have progressive chemical shift of Zr–O peaks towards lower or higher binding energies, depending on the oxidation/nitridation temperature. This is supported by the earlier claim (Fig. 2) that sub- or non-stoichiometric Zr–O is detectable as it
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Fig. 4. Evolution of O 1s core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
moves deeper inside. Apart from Zr–O peaks, Zr–O–N (528.8–529.9 eV [73]) peaks were detected in the bulk region towards the IL for 500 °C sample and Zr–Si–O (529.7–530.7 eV [47,77]) peaks were detected in the IL, in all investigated samples, except in the sample oxidized/nitrided at 500 °C. The intensity of O 1s spectra decreases and vanishes as the etching time prolongs to SiC substrate. These results are in agreement with the results shown in Zr 3d and Si 2p spectra. Four components, i.e. Zr–N, Si–N, C–N, and/or Zr–O–N, can be fitted in N 1s spectra [Fig. 5(a)–(d)]. At earlier etching time, there is no peak being detected, except for 500 °C sample. A Zr–O–N peak is detected with a reference binding energy of 399.2 eV [71,72,74]. With increment of etching time, three other peaks associated with Si–N, C–N, and Zr–N at binding energies of 397.0–397.1 eV [11,47,79], 396.5 eV [80], and 395.9 eV [47,59], respectively were detected. The intensity of these peaks reduces as the etching time continues to extend until it totally
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Y.H. Wong, K.Y. Cheong / Thin Solid Films 520 (2012) 6822–6829 Table 1 Possible compounds in the substrate, interfacial layer, and bulk of the investigated samples. Compounds
Substrate IL
Bulk
SiC Zr–O Zr–N Zr–O–N Zr–Si–O Si–N C–N Zr–O Zr–N Zr–O–N
Oxidation/nitridation temperature (°C) 400
500
700
900
√ √ √
√ √ √ √
√ √ √
√ √ √
√ √ √ √ √
√ √ √ √ √
√ √ √ √ √
√ √ √ √
3.2. Band alignment By using the same extraction method, as described elsewhere [49], energy band alignment of Zr-oxynitride/IL/SiC system can be evaluated. Typical valence band spectra of Zr-oxynitride and IL for all investigated samples have been studied (not shown). The valence band edges (Ev) of Zr-oxynitride and IL were approximated by an intercept of linear extrapolation of a maximum negative slope near the edge to the minimum horizontal baseline [49,82]. As a result, valence-band offsets (ΔEv) of Zr-oxynitride and IL with respect to SiC substrate were 1.00 ± 0.05 eV and 2.50 ± 0.05 eV, respectively, for all investigated samples. To evaluate conduction-band offset (ΔEc) of Zr-oxynitride/IL/SiC system, the bandgaps (Eg) of Zr-oxynitride and IL were first deduced from O 1s plasmon loss spectra [63,64] of Zr-oxynitride and IL (not shown). The Eg values of Zr-oxynitride and IL extracted from their respective O 1s plasmon loss spectra are presented in Fig. 6. As explained earlier, values of Eg were also estimated by an intercept of linear extrapolation. The extracted Eg values of Zr-oxynitride and IL were 5.00–5.20 eV and 7.80–8.50 eV, respectively, with a tolerance of 0.05 eV, dependent on the oxidation/ nitridation temperature (Fig. 6). By using the following equation (Eq. (2)), conduction-band offset of Zr-oxynitride to IL, ΔEc(ZrON/IL) for Zr-oxynitride/IL/SiC system can be determined [83]: ΔEcðZrON=ILÞ ¼ EgðILÞ –ΔEvðIL=SiCÞ þ ΔEvðZrON=SiCÞ –EgðZrONÞ
Fig. 5. Evolution of N 1s core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
vanishes. The obtained peaks in the N 1s spectra also match the analyses obtained from Zr 3d, Si 2p, and O 1s spectra. A typical narrow scan of C 1s core-level spectra has been studied (not shown). There is no peak observed at the earlier etching time. With the increments of etching time until the IL region, three peaks which are associated with Si–C, C–C, and C–N, are detected at binding energies of 282.5 eV [11,81], 283.6 eV [11,81], and 285.0 eV,[10] respectively. The intensity of Si–C, which originated from the SiC substrate, increases as the etching time increases, matching the results shown in Si 2p spectra. On the other hand, C–C and C–N bonds, which appear in the IL decreases gradually as the etching time is extended. Based on the analyses of Zr 3d, Si 2p, O 1s, N 1s, and C 1s spectra, possible compounds being detected in the bulk, interfacial layer, and substrate are presented in Table 1.
ð2Þ
where, Eg(ZrON) and Eg(IL) are the bandgaps of Zr-oxynitride and IL, respectively. ΔEv(ZrON/SiC) and ΔEv(IL/SiC) are the valence band offsets of Zr-oxynitride and IL, respectively, with respect to SiC substrate. The calculated values of Eg(ZrON), Eg(IL), ΔEc(ZrON/SiC), ΔEc(IL/SiC), and ΔEc(ZrON/IL), are presented in Fig. 6. The highest value of ΔEc(ZrON/IL), i.e., 2.00 eV, was attained by sample oxidized/nitrided at 500 °C (Fig. 6) as compared to other investigated samples. A schematic of band alignment of Zr-oxynitride/IL/SiC system is illustrated in Fig. 7. The Eg value of SiC substrate is obtained from literature [1,6]. 3.3. Oxidation and nitridation mechanisms On the basis of chemical analysis that has been carried out by XPS, a possible model of oxidation and nitridation mechanisms of sputtered Zr on SiC substrate using N2O gas at temperatures of 400 to 900 °C can be deduced. Owing to fast initial oxidation of Zr at room temperature, a very thin monolayer (~ 1 nm) of ZrO2 is formed on the top-most surface according to the following reaction [47,84]: Zr þ O2 ➔ZrO2 :
ð3Þ
Based on the experiments conducted by Gupta et al. [85] and Enta et al. [86], decomposition of N2O was at elevated temperatures above
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is supplied [89]. As the thickness of increases, less oxygen may diffuse in and cause an incomplete reaction between Zr and O. As a result, a sub- stoichiometric Zr–O compound is formed towards the interfacial layer between the stoichiometric Zr–O (ZrO2) compound and the SiC substrate. Simultaneously, nitridation process happens. Since N atoms are smaller than O atoms, N atoms may diffuse faster than O atoms at the same processing temperature and duration. Therefore, N atoms may diffuse further inside and react with sub-stoichiometric Zr–O to form Zr–N compound by releasing oxygen (Eq. (5)) [47,90] or react with unreacted Zr to form Zr–N compound (Eq. (6)): →
Zr−Oþ N →Zr−N þ O Fig. 6. The calculated values of Eg(ZrON), Eg(IL), ΔEc(ZrON/SiC), ΔEc(IL/SiC), and ΔEc(ZrON/IL) in the band alignment of Zr-oxynitride/IL/SiC system.
1000 °C. Nonetheless, Miller and Grassian [87] have determined that ZrO2 is an effective catalyst for the decomposition of N2O above 350 °C, whereby N2O may decompose exclusively to N and O compounds. The study also discerned that Zr cations were involved in the decomposition reaction of N2O. In addition, it is reported by Petrucci et al. [88] that there is no free energy change as a function of temperature in the decomposition of N2O into N and O compounds. Therefore, the decomposition is a spontaneous reaction at those investigated temperatures. At 400 °C, Zr cations as catalytic ions are able to decompose N2O into N and O compounds. Since oxygen atoms are easily being sensed and absorbed by Zr metal, the absorbed O atoms may react with Zr to form Zr–O compound. This process may proceed further inward, depending on the availability of oxygen produced at this temperature. The reaction is described by: →
Zrþ O →Zr−O
→
Zrþ N →Zr−N:
ð5Þ ð6Þ
The reported Gibbs free energies of formation of ZrN and stoichiometric ZrO2 at 500 °C are −395.48 kJ/mol [88,90,91] and −1133.08 kJ/mol [88,90,92], respectively. This indicates the formation of ZrN is less favorable than formation of stoichiometric ZrO2 at this temperature. However, in this case, the Zr–O involved is sub- or non-stoichiometric, hence, the formation of ZrN (Eqs (5) and (6)) is more favorable. The Zr–N compound formed may be embedded in the Zr–O layer towards the interfacial layer. Hence, Zr-oxynitride layer which consists of Zr–O and Zr–N has been formed, with Zr–O rich compound located at the topmost surface and Zr–N rich compound located near the interface region. The released O atoms from the formation of Zr–N compound (Eq. (5)) and an inward diffusion of O atoms from the ambient may react with the unreacted Zr and bridge to the topmost Si layer (dangling Si bonds) of the bulk-terminated SiC surface, thus, forming Zr–Si–O compound: →
Zrþ O þSi→Zr−Si−O:
ð7Þ
ð4Þ
where, the arrow pointing to the right above a chemical symbol indicates an inward diffusion process of the particular chemical. A stoichiometric Zr–O (ZrO2) is formed when a sufficiently high concentration of oxygen
Alternatively, O atoms in the sub-stoichiometric Zr–O may react and bridge to the Si dangling bonds of the bulk-terminated SiC surface, forming Zr–Si–O compound: Zr−O þ Si→Zr−Si−O:
ð8Þ
In other words, the O atom in Zr–Si–O compound has passivated the dangling Si bonds and it functions as a connector to connect Zr-oxynitride network with SiC substrate. The excessive N atoms which diffuse further inside towards the interfacial layer also help to passivate the dangling Si bonds by forming Si–N compound (Eq. (7)). In addition, minute amount of Si atoms from the SiC substrate may diffuse out to form Si–N compound as well (Eq. (8)). →
Siþ N →Si−N
ð9Þ
← → Sιþ N →Si−N
ð10Þ
where, the arrow pointing to the left above a chemical symbol indicates an outward diffusion process of the particular chemical. Due to the catalytic effect offered by Zr and/or ZrO2 [50,93,94], SiC may be slightly decomposed at low temperature of 400 °C. As a result of the minute out-diffusion of Si atoms from SiC substrate, some minute C atoms diffuse out as well. These out-diffusing C atoms will react with N atoms in the interfacial layer, forming C\N bonds [10,13]: Fig. 7. Band alignment of Zr-oxynitride/IL/SiC system. Eg(ZrON) = bandgap of Zr-oxynitride, Eg(IL) = bandgap of IL, Eg(SiC) = bandgap of SiC, ΔEv(ZrON/SiC) = valence band offsets of Zr-oxynitride to SiC, ΔEv(IL/SiC) = valence band offsets of IL to SiC, ΔEc(ZrON/SiC) = conduction band offset of Zr-oxynitride to SiC, ΔEc(IL/SiC) = conduction band offset of IL to SiC, ΔEc(ZrON/IL) = conduction band offset of Zr-oxynitride to IL.
← → Cþ N →C−N
ð11Þ
The formation of C\N bonds essentially removes the carbon as well as carbon clusters that have been formed.
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As the oxidation/nitridation temperature is elevated to 500–900 °C, stoichiometric Zr–O (ZrO2) can also be formed when sufficiently high concentration of oxygen is supplied [89]. Moreover, more O atoms are produced and able to diffuse further inside; hence, more Zr\O bonds (sub-stoichiometric Zr–O) are formed towards the interfacial layer (Eq. (4)). Concurrently, more N atoms are able to diffuse in and react with sub-stoichiometric Zr–O to form Zr–N compound by releasing oxygen (Eq. (5)) [47,89] or react with unreacted Zr to form Zr–N compound (Eq. (6)) towards the interfacial layer. Excess N atoms may also diffuse further inside towards the interfacial layer to passivate the dangling Si bonds by forming Si–N compound (Eq. (9)). Minute out-diffusing Si and C atoms from the SiC substrate may react with inward diffusing N atoms to form Si\N bonds (Eq. 10) and C\N bonds (Eq. (11)), respectively. On the other hand, by obeying Eqs. (7) and (8), bridging O atoms in Zr–Si–O compound is formed at elevated temperatures of 700–900 °C. At a specific temperature of 500 °C, it is proposed that mixture of sub-stoichiometric Zr–O and Zr–N compounds are in the midst of exchanging and balancing the O and N atoms among themselves in order to achieve thermodynamic equilibrium condition. As a result, Zr–O–N compound is formed. During the process of exchanging O and N atoms among Zr–O and Zr–N compounds, there are also N atoms from the ambient N2O diffuse in and incorporate substitutionally and/or interstitially into stoichiometric Zr–O (ZrO2) region, forming Zr–O–N compound. Hence, Zr-oxynitride layer formed at this temperature consists of Zr–O and Zr–O–N, with Zr–O rich compound located at the topmost surface and Zr–O–N rich compound located near interface region. As reported by Ngaruiya et al. [95], The nitrogen atoms occupy oxygen sites within the crystalline phase of Zr–O, resulting in an almost unchanged ZrO2 structure with a sevenfold metal–nonmetal coordination and interatomic distances around 2.04–2.27 Ǻ. The reported interatomic distances have assured Zr tetravalency when nitrogen has been incorporated into stoichiometric Zr–O layer. Therefore, Zr–O compound remains stoichiometric at the topmost surface. 4. Conclusions In summary, chemical compositions and depth profile analysis of sputtered Zr oxidized/nitrided in ambient N2O at various temperatures (400–900 °C) for 15 min were presented. Based on the XPS results, Zr-oxynitride film of Zr–O, Zr–N, and/or Zr–O–N and its interfacial layer consisted of mixed Zr–O, Zr–N, Zr–O–N, Zr–Si–O, Si–N, and/or C–N compounds were identified. A possible model related to the oxidation and nitridation mechanisms has been proposed and explained. The reported chemical compositions in the film in this study may serve a vital role in understanding the functional properties of the film to work as advanced high-κ dielectric in SiC-based MOS devices. Acknowledgments The authors would like to acknowledge the support provided by USM fellowship, USM-RU-PRGS (8032051), and The Academy Sciences for the Developing World (TWAS) through TWAS-COMSTECH Research Grant (09–105 RG/ENG/AS_C) during the study. References [1] Z. Luo, T. Chen, D.C. Sheridan, J.D. Cressler, In: in: Z.C. Feng (Ed.), Power Materials SiC, Springer-Verlag, Berlin, New York, 2004, p. 375. [2] W.F. Lim, K.Y. Cheong, Z. Lockman, J. Alloys Compd. 497 (2010) 195. [3] S. Dimitrijev, H.B. Harisson, P. Tanner, K.Y. Cheong, J. Han, In: in: W.J. Choyke, H. Matsunami, G. Pensl (Eds.), Silicon Carbide — Recent Major Advances, Springer-Verlag, Berlin, Germany, 2004, p. 373. [4] J.B. Casady, R.W. Johnson, Solid-State Electron. 39 (1996) 1409. [5] M.T. Soo, K.Y. Cheong, A.F. Mohd Noor, Sens. Actuators B Chem. 151 (2010) 39.
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Formation of Zr-oxynitride thin films on 4H-SiC substrate Yew Hoong Wong, Kuan Yew Cheong ⁎ Energy Efficient & Sustainable Semiconductor Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 13 July 2012 Accepted 13 July 2012 Available online 20 July 2012 Keywords: Zr-oxynitride Sputtered Zr Nitrous oxide X-ray photoelectron spectroscopy
a b s t r a c t Formation of Zr-oxynitride by simultaneous oxidation and nitridation in nitrous oxide of sputtered Zr on SiC substrate is reported. Sputtered Zr on SiC substrate and followed by oxidation and nitridation in nitrous oxide ambient for 15 min at different temperatures (400–900 °C) have been systematically investigated. By using X-ray photoelectron spectroscopy, chemical compositions, and depth profile analysis have been evaluated, as well as energy band alignment of Zr-oxynitride/interfacial layer/SiC system. Zr-oxynitride film of Zr–O, Zr–N, and/or Zr–O–N and its interfacial layer composed of mixed Zr–O, Zr–N, Zr–O–N, Zr–Si–O, Si–N, and/or C–N phases were verified. A possible model related to the oxidation and nitridation mechanisms has been proposed and explicated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The advent of technology boom has spurred utilization of wide bandgap (WBG) semiconductors as alternative substrates to replace silicon for high-power, high-temperature, and/or high-radiation metaloxide-semiconductor (MOS)-based switching devices [1,2]. Among the WBG semiconductors, silicon carbide (SiC) has developed into one of the leading contenders owing to its commercial availability and ability to grow native oxide (SiO2) [2,3]. SiC appears in different polytypes [4,5]. 4H-SiC is one of the polytypes that provides a wide bandgap of 3.26 eV, high breakdown field strength of ~3 MV/cm, high saturation electron drift velocity of ~2 × 107 cm/s, and high thermal conductivity of ~3.7 W/cm °C [2,6,7]. These properties potentially enable operation of SiC-based devices under harsh conditions. Nonetheless, a high quality gate oxide must be deposited or grown between a metal electrode and a SiC substrate in order to sustain a high transverse electric field and a low gate leakage current in the MOS-based devices. So far, thermally nitrided-SiO2 is regarded as the most promising gate oxide due to its low interface and slow trap densities, high reliability, and low leakage current [8–15]. However, possession of its low dielectric constant (κ = 3.9) as compared to SiC (κ = 10) limits the permissible electric field in SiC-based devices [16]. As a result, the gate oxide may electrically break down before the SiC substrate thus defeating the purpose of using SiC as the primer substrate for high-power, high-temperature, and/or high-radiation applications. In order to overcome the aforementioned problem, an alternative gate oxide, to replace the low-κ nitrided-SiO2, with higher κ value has been proposed. Several single layers and stacking layers of high κ gate oxides ⁎ Corresponding author. Tel.: +60 4 599 5259; fax: +60 4 594 1011. E-mail address: [email protected] (K.Y. Cheong). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.07.036
on SiC have been investigated, which include MgO [17], Gd2O3 [18], Pr2O3 [19], oxidized Ta2Si [20], Al2O3 [21–23], AlN [24], La2O3 [25], ZrO2 [26,27], HfO2 [28–30], CeO2 [2,31,32], Y2O3 [33], Al2O3/SiO2 [34,35], HfO2/SiO2 [36–39], Ta2O5/SiO2 [40], La2O3/SiO2 [41], oxidized Ta2Si/SiO2 [40], SiNx/SiO2 [42], Al2O3/SiN/SiO2 [43], and Al2O3/AlN/Al2O3/AlN/SiO2 [44]. Of these oxides, ZrO2 is a potential candidate to be integrated, as the gate oxide, in SiC-based MOS devices; as it exhibits excellent properties such as high κ value (22–25), large energy bandgap (5.8–7.8 eV), and easily stabilized in the form of cubic or tetragonal polymorphs, which may further enhance its effective κ value [45–51]. Based on numerous reports, this material has demonstrated encouraging and wellbehaved characteristics when it is applied as gate oxide on Si-based substrate [46–49,52–56]. However, reports on its characteristics on SiC substrate are scarce. Karlsson et al. [26] reported on the growth and thermal stability of ZrO2 thin films on SiC(0001) by chemical vapor deposition technique. A heterogeneous layer was formed after annealing at 1000 °C due to the decomposition of ZrO2, thus it leads to the formation of t-ZrO2 remnants, metallic Zr silicide, and Si aggregates. Kurniawan et al. [27] reported on the formation of ZrO2 by sputtering of Zr on SiC substrate followed by thermal oxidation in ambient oxygen. The study pointed that post-annealing of the oxide has produced a thicker interfacial layer with an increment in capacitance and in dielectric constant as well as reduction of leakage current density but with lower oxide breakdown field. To improve the MOS characteristics and the oxide–SiC interface quality, it has been found that oxidation and/or post oxidation annealing in a nitrogen-containing ambient has two beneficial effects, i.e., enhanced removal of carbon and passivation of silicon dangling bonds [3]. The utilization of nitrous oxide (N2O) gas has contributed to the growth of thermally nitrided-SiO2 as gate oxide
Y.H. Wong, K.Y. Cheong / Thin Solid Films 520 (2012) 6822–6829
on SiC with low leakage current and low interface-trap density [11,12]. At high temperature, N2O gas is decomposed exclusively into N and O compounds, which are indispensable in performing oxidation and nitridation processes on the gate oxide [11,47,57]. In general, the beneficial effects of utilizing N2O gas in growing thermally nitrided SiO2 gate on SiC-based MOS structure have been highlighted but there is no pertinent investigation on the effects of N2O gas on oxidizing and nitriding of sputtered Zr based on SiC substrate. Hence, it is worthwhile to investigate the effects of simultaneous thermal oxidation and nitridation of a sputtered Zr/SiC system in ambient N2O. Previously, some works have been carried out on the effects of thermal oxidation and nitridation in ambient N2O of a sputtered Zr/Si system on their chemical, structural, electronic, and electrical properties [47–49]. Based on the reports, a model on oxidation and nitridation mechanism has been proposed. It has been examined that bulk ZrO2 and interfacial layer (IL) of Zr-silicate oxynitride (ZrSiON) have been formed on Si substrate based on X-ray photoelectron spectrometer results. However, using the same technique on a sputtered Zr/SiC system, it is postulated that bulk Zr-oxynitride is formed rather than ZrO2 if the processing parameters are well optimized. In this work, it is aimed to report the formation of Zr-oxynitride from Zr thin films on 4H-SiC substrate by simultaneous thermal oxidation and nitridation in N2O gas at various temperatures.
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available in the investigated films was calculated based on the following equation [11,60,61]: Ix
Cx
. Sx
∑i Ii
100
ð1Þ
Si
where, Ix and Ii were peak intensity of the evaluated element and all other detected elements, respectively. Sx and Si were sensitivity factor of the respective evaluated element and all other detectable elements. The sensitivity factor is dependent on XPS system. In this work, the sensitivity factors for Zr 3d, Si 2p, O 1s, N 1s, and C 1s were 2.576, 0.328, 0.78, 0.47, and 0.278, respectively. The XPS system was also used to determine the band alignment of the dielectric/semiconductor structure. The spectra of wide scan with binding energy range of − 5 to 25 eV, were collected at a take-off angle of 0° with respect to normal surface, with low pass energy of 20 eV and small step size of 0.1 eV. Owing to onset of single particle excitation and band-to-band transition, the energy loss spectrum of O 1s photoelectron provides further insight on the bandgaps of Zr-oxynitride and IL [62]. Subsequently, a detail scan of O 1s was carried out using the same pass energy and step size of 1.0 eV. Band alignment was extracted based on the Kraut method [63,64]. 3. Results and discussion
2. Experimental procedures 3.1. Compositional and depth profile analysis n-type Si-faced 4H-SiC(0001) substrates of 0.8 cm×0.8 cm, with 4.09°-off axis, 0.020 Ω cm of resistivity, and 1-μm thick of n-type epitaxial layer doped with nitrogen at concentration of (1–4)×1016 cm−3, were commercially purchased from CREE Inc. (USA). They were first cleaned by an ultrasonic and standard RCA cleaning, followed by dipping into a diluted HF solution (1 HF:50 H2O) aiming to remove native oxide on the surface. A 5-nm thick Zr film was deposited on the cleaned SiC substrates by a RF sputtering system (Edwards Auto 500). During the sputtering, working pressure, RF power, inert Ar gas flow, and deposition rate were regulated at 1.6 × 10−5 Pa, 170 W, 20 cm3/min, and 0.2 nm/s, respectively. Samples were placed inside a horizontal tube furnace and heated up from room temperature to a set of temperatures (400, 500, 700, and 900 °C) in an ambient Ar flow with a heating rate of 10 °C/min. Once the set temperature was reached, Ar gas was turned off and N2O gas was then purged in with a flow rate of 150 mL/min for 15 min. The samples were eventually taken out at room temperature after the furnace was cooled down to room temperature in ambient Ar. Compositional and depth profile of the produced films were measured by Kratos Axis UltraDLD X-ray photoelectron spectrometer (XPS) with a monochromatic Al-Kα X-ray source (hν=1486.69 eV) performed at Research Center for Surface and Materials Science, The Auckland University, New Zealand. The spectrometer was operated at 150 W and a take-off angle of 0o with respect to normal surface. In order to perform a depth profiling analysis, a 5-kV Ar ion etching was used. The pressure of Ar in the analysis chamber was 4×10−5 Pa, while the area of analysis was 220×220 μm2. The chemical compositions of the films were obtained from a combination of wide and narrow scans. A wide scan was first performed with a passing energy of 160 eV for 9 min to determine the elemental chemical states. The core-level spectra that had been detected were Zr 3d, Si 2p, O 1s, N 1s, and C 1s Subsequently, for a narrow scan, a passing energy of 20 eV for 5 min was used to scan through the binding-energy range of interest. The recorded C 1s peak due to the adventitious carbon-based contaminant on the surface, with respect to the literature value of 284.6 eV [58,59], was used as a reference to compensate for the charging effect of the XPS spectra. Surface charge and linear background were corrected with the help of CasaXPS software (version 2.3.15) before deconvolution of the XPS spectra was performed. The total concentration of an element (Cx)
Fig. 1(a)–(d) depicts the atomic composition depth profiles for Zr, Si, O, N, and C as a function of etching time in all investigated samples. Based on the depth profiles, it is observed that at earlier etching time, Zr and O elements are in dominance with atomic ratio of about 1:2. This indicates that a stoichiometric ZrO2 has been formed at the outmost layer in the bulk region of each sample. Atomic percentages of both Zr and O elements decrease as the etching time is extended and their ratio deviates from 1:2, until they totally vanish. Apart from Zr and O elements, N element can be detected in the bulk region of all investigated films as well. This may infer that nitrogen has been incorporated into the Zr–O network of the film, forming Zr-oxynitride. Zr-oxynitride in this experiment is in amorphous structure [48]. The atomic concentration of N element decreases as the etching time is prolonged, until it totally fades away. This defines the boundary of the oxynitride and SiC, as shown in the figure. Beyond this boundary, atomic percentages of Si and C increase significantly; confirming the appearance of SiC substrate. In between SiC and Zr-oxynitride boundaries, an interfacial layer (IL) with a mixture of Zr, O, Si, N, and C have been detected. A comprehensive analysis of the chemical compounds of the IL was performed by narrow scan of XPS and it will be discussed in the subsequent paragraphs. The thickness of Zr-oxynitride and IL are dependent on the oxidation/nitridation temperature. Based on the figure, thickness of Zr-oxynitride is marginally reduced as the oxidation/ nitridation temperature increases. The distribution profile of nitrogen in all investigated samples has been presented in Fig. 1(e). It demonstrated that the nitrogen in the 500 °C sample is distributed more uniformly throughout the entire film, including IL, as compared to other samples. Narrow scans of each core-level spectrum as a function of etching time for different oxidation/nitridation temperatures (400–900 °C) are shown in Figs. 2[(a)–(d)]–5[(a)–(d)]. A non-linear Gaussian–Lorentzian function (solid lines) was used to deconvolute the measured peaks (dotted lines) using CasaXPS software (version 2.3.15). The Zr 3d spectra [Fig. 2(a)–(d)] are well fitted by Zr–O, Zr–N, Zr–O–N, and/or Zr–Si–O components at their respective binding energies. Zr 3d doublet, which corresponds to 3d5/2 and 3d3/2 spin–orbit split components at 184.4 and 182.0 eV, respectively, has been detected at the top-most surface (etching time of 0 s) of the bulk region for all investigated samples
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Fig. 2. Evolution of Zr 3d core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
Fig. 1. Atomic percentage of Zr, Si, O, N, and C as a function of etching time for sample oxidized/nitrided samples at (a) 400 °C, (b) 500 °C, (c) 700 °C, 900 °C, and (e) nitrogen depth profile of all investigated samples.
(400–900 °C). This indicates that stoichiometric Zr–O (ZrO2) is formed [47,65]. As the etching time increases, it is observed that progressive chemical shift of Zr–O peaks towards lower or higher binding energies, depending on the oxidation/nitridation temperature. This is owing to
the evolution from stoichiometric to non- or sub-stoichiometric Zr–O towards a deeper region of all investigated films [47,66,67]. Besides Zr–O peaks, Zr–N (178.7–179.5 eV [47,68–70]) peaks can be detected in the bulk region towards the IL for all investigated samples (400– 900 °C), whilst Zr–O–N (180.0–182.5 eV [70–74]) can be detected only in the bulk region towards the IL for sample oxidized/nitrided at 500 °C. In addition, Zr–Si–O (177.2 eV [47,75]) compound has been detected at the IL region of the investigated samples, except for the sample oxidized/nitrided at 500 °C. This suggests that the bulk of Zr-oxnitride, which comprises Zr–O, Zr–N, and/or Zr–O–N, with an IL of Zr–O, Zr–N, Zr–O–N, and/or Zr–Si–O, were formed in the investigated samples. As the etching time extends to SiC substrate, the intensity of Zr 3d spectra decreases and disappears. To identify the presence of other components in the samples, Si 2p spectra were investigated [Fig. 3(a)–(d)]. Initially, there is no detectable peak at the earlier etching time for all investigated samples. As the etching
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Fig. 3. Evolution of Si 2p core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
time increases, Si–N, Si–C, and/or Zr–Si–O were detected at binding energies of 101.4–102.4 eV [11,47,61], 99.8 eV [61], and 98.0–99.4 eV [47,61], respectively, in the investigated samples, with an exception that Zr–Si–O peak was absent in the sample oxidized/nitrided at 500 °C. It is noted that the detected Si–C peak originated from the SiC substrate and the detection of Zr–Si–O peak, with respect to the range of etching, is consistent with the results demonstrated in Zr 3d spectra. Based on the O 1s spectra analysis [Fig. 4(a)–(d)], at the top-most surface (etching time of 0 s) of all investigated samples, two peaks can be fitted in the O 1s core-level spectra, with their locations at 530.0 eV [47,76,77] and 531.9–532.0 eV [47,78]. These two peaks are assigned to stoichiometric Zr–O. Similar to the observation made in Zr 3d spectra (Fig. 2), the O 1s spectra have progressive chemical shift of Zr–O peaks towards lower or higher binding energies, depending on the oxidation/nitridation temperature. This is supported by the earlier claim (Fig. 2) that sub- or non-stoichiometric Zr–O is detectable as it
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Fig. 4. Evolution of O 1s core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
moves deeper inside. Apart from Zr–O peaks, Zr–O–N (528.8–529.9 eV [73]) peaks were detected in the bulk region towards the IL for 500 °C sample and Zr–Si–O (529.7–530.7 eV [47,77]) peaks were detected in the IL, in all investigated samples, except in the sample oxidized/nitrided at 500 °C. The intensity of O 1s spectra decreases and vanishes as the etching time prolongs to SiC substrate. These results are in agreement with the results shown in Zr 3d and Si 2p spectra. Four components, i.e. Zr–N, Si–N, C–N, and/or Zr–O–N, can be fitted in N 1s spectra [Fig. 5(a)–(d)]. At earlier etching time, there is no peak being detected, except for 500 °C sample. A Zr–O–N peak is detected with a reference binding energy of 399.2 eV [71,72,74]. With increment of etching time, three other peaks associated with Si–N, C–N, and Zr–N at binding energies of 397.0–397.1 eV [11,47,79], 396.5 eV [80], and 395.9 eV [47,59], respectively were detected. The intensity of these peaks reduces as the etching time continues to extend until it totally
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Y.H. Wong, K.Y. Cheong / Thin Solid Films 520 (2012) 6822–6829 Table 1 Possible compounds in the substrate, interfacial layer, and bulk of the investigated samples. Compounds
Substrate IL
Bulk
SiC Zr–O Zr–N Zr–O–N Zr–Si–O Si–N C–N Zr–O Zr–N Zr–O–N
Oxidation/nitridation temperature (°C) 400
500
700
900
√ √ √
√ √ √ √
√ √ √
√ √ √
√ √ √ √ √
√ √ √ √ √
√ √ √ √ √
√ √ √ √
3.2. Band alignment By using the same extraction method, as described elsewhere [49], energy band alignment of Zr-oxynitride/IL/SiC system can be evaluated. Typical valence band spectra of Zr-oxynitride and IL for all investigated samples have been studied (not shown). The valence band edges (Ev) of Zr-oxynitride and IL were approximated by an intercept of linear extrapolation of a maximum negative slope near the edge to the minimum horizontal baseline [49,82]. As a result, valence-band offsets (ΔEv) of Zr-oxynitride and IL with respect to SiC substrate were 1.00 ± 0.05 eV and 2.50 ± 0.05 eV, respectively, for all investigated samples. To evaluate conduction-band offset (ΔEc) of Zr-oxynitride/IL/SiC system, the bandgaps (Eg) of Zr-oxynitride and IL were first deduced from O 1s plasmon loss spectra [63,64] of Zr-oxynitride and IL (not shown). The Eg values of Zr-oxynitride and IL extracted from their respective O 1s plasmon loss spectra are presented in Fig. 6. As explained earlier, values of Eg were also estimated by an intercept of linear extrapolation. The extracted Eg values of Zr-oxynitride and IL were 5.00–5.20 eV and 7.80–8.50 eV, respectively, with a tolerance of 0.05 eV, dependent on the oxidation/ nitridation temperature (Fig. 6). By using the following equation (Eq. (2)), conduction-band offset of Zr-oxynitride to IL, ΔEc(ZrON/IL) for Zr-oxynitride/IL/SiC system can be determined [83]: ΔEcðZrON=ILÞ ¼ EgðILÞ –ΔEvðIL=SiCÞ þ ΔEvðZrON=SiCÞ –EgðZrONÞ
Fig. 5. Evolution of N 1s core level XPS spectra as a function of etching time for different oxidation/nitridation temperature samples: (a) 400 °C, (b) 500 °C, (c) 700 °C, and 900 °C.
vanishes. The obtained peaks in the N 1s spectra also match the analyses obtained from Zr 3d, Si 2p, and O 1s spectra. A typical narrow scan of C 1s core-level spectra has been studied (not shown). There is no peak observed at the earlier etching time. With the increments of etching time until the IL region, three peaks which are associated with Si–C, C–C, and C–N, are detected at binding energies of 282.5 eV [11,81], 283.6 eV [11,81], and 285.0 eV,[10] respectively. The intensity of Si–C, which originated from the SiC substrate, increases as the etching time increases, matching the results shown in Si 2p spectra. On the other hand, C–C and C–N bonds, which appear in the IL decreases gradually as the etching time is extended. Based on the analyses of Zr 3d, Si 2p, O 1s, N 1s, and C 1s spectra, possible compounds being detected in the bulk, interfacial layer, and substrate are presented in Table 1.
ð2Þ
where, Eg(ZrON) and Eg(IL) are the bandgaps of Zr-oxynitride and IL, respectively. ΔEv(ZrON/SiC) and ΔEv(IL/SiC) are the valence band offsets of Zr-oxynitride and IL, respectively, with respect to SiC substrate. The calculated values of Eg(ZrON), Eg(IL), ΔEc(ZrON/SiC), ΔEc(IL/SiC), and ΔEc(ZrON/IL), are presented in Fig. 6. The highest value of ΔEc(ZrON/IL), i.e., 2.00 eV, was attained by sample oxidized/nitrided at 500 °C (Fig. 6) as compared to other investigated samples. A schematic of band alignment of Zr-oxynitride/IL/SiC system is illustrated in Fig. 7. The Eg value of SiC substrate is obtained from literature [1,6]. 3.3. Oxidation and nitridation mechanisms On the basis of chemical analysis that has been carried out by XPS, a possible model of oxidation and nitridation mechanisms of sputtered Zr on SiC substrate using N2O gas at temperatures of 400 to 900 °C can be deduced. Owing to fast initial oxidation of Zr at room temperature, a very thin monolayer (~ 1 nm) of ZrO2 is formed on the top-most surface according to the following reaction [47,84]: Zr þ O2 ➔ZrO2 :
ð3Þ
Based on the experiments conducted by Gupta et al. [85] and Enta et al. [86], decomposition of N2O was at elevated temperatures above
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is supplied [89]. As the thickness of increases, less oxygen may diffuse in and cause an incomplete reaction between Zr and O. As a result, a sub- stoichiometric Zr–O compound is formed towards the interfacial layer between the stoichiometric Zr–O (ZrO2) compound and the SiC substrate. Simultaneously, nitridation process happens. Since N atoms are smaller than O atoms, N atoms may diffuse faster than O atoms at the same processing temperature and duration. Therefore, N atoms may diffuse further inside and react with sub-stoichiometric Zr–O to form Zr–N compound by releasing oxygen (Eq. (5)) [47,90] or react with unreacted Zr to form Zr–N compound (Eq. (6)): →
Zr−Oþ N →Zr−N þ O Fig. 6. The calculated values of Eg(ZrON), Eg(IL), ΔEc(ZrON/SiC), ΔEc(IL/SiC), and ΔEc(ZrON/IL) in the band alignment of Zr-oxynitride/IL/SiC system.
1000 °C. Nonetheless, Miller and Grassian [87] have determined that ZrO2 is an effective catalyst for the decomposition of N2O above 350 °C, whereby N2O may decompose exclusively to N and O compounds. The study also discerned that Zr cations were involved in the decomposition reaction of N2O. In addition, it is reported by Petrucci et al. [88] that there is no free energy change as a function of temperature in the decomposition of N2O into N and O compounds. Therefore, the decomposition is a spontaneous reaction at those investigated temperatures. At 400 °C, Zr cations as catalytic ions are able to decompose N2O into N and O compounds. Since oxygen atoms are easily being sensed and absorbed by Zr metal, the absorbed O atoms may react with Zr to form Zr–O compound. This process may proceed further inward, depending on the availability of oxygen produced at this temperature. The reaction is described by: →
Zrþ O →Zr−O
→
Zrþ N →Zr−N:
ð5Þ ð6Þ
The reported Gibbs free energies of formation of ZrN and stoichiometric ZrO2 at 500 °C are −395.48 kJ/mol [88,90,91] and −1133.08 kJ/mol [88,90,92], respectively. This indicates the formation of ZrN is less favorable than formation of stoichiometric ZrO2 at this temperature. However, in this case, the Zr–O involved is sub- or non-stoichiometric, hence, the formation of ZrN (Eqs (5) and (6)) is more favorable. The Zr–N compound formed may be embedded in the Zr–O layer towards the interfacial layer. Hence, Zr-oxynitride layer which consists of Zr–O and Zr–N has been formed, with Zr–O rich compound located at the topmost surface and Zr–N rich compound located near the interface region. The released O atoms from the formation of Zr–N compound (Eq. (5)) and an inward diffusion of O atoms from the ambient may react with the unreacted Zr and bridge to the topmost Si layer (dangling Si bonds) of the bulk-terminated SiC surface, thus, forming Zr–Si–O compound: →
Zrþ O þSi→Zr−Si−O:
ð7Þ
ð4Þ
where, the arrow pointing to the right above a chemical symbol indicates an inward diffusion process of the particular chemical. A stoichiometric Zr–O (ZrO2) is formed when a sufficiently high concentration of oxygen
Alternatively, O atoms in the sub-stoichiometric Zr–O may react and bridge to the Si dangling bonds of the bulk-terminated SiC surface, forming Zr–Si–O compound: Zr−O þ Si→Zr−Si−O:
ð8Þ
In other words, the O atom in Zr–Si–O compound has passivated the dangling Si bonds and it functions as a connector to connect Zr-oxynitride network with SiC substrate. The excessive N atoms which diffuse further inside towards the interfacial layer also help to passivate the dangling Si bonds by forming Si–N compound (Eq. (7)). In addition, minute amount of Si atoms from the SiC substrate may diffuse out to form Si–N compound as well (Eq. (8)). →
Siþ N →Si−N
ð9Þ
← → Sιþ N →Si−N
ð10Þ
where, the arrow pointing to the left above a chemical symbol indicates an outward diffusion process of the particular chemical. Due to the catalytic effect offered by Zr and/or ZrO2 [50,93,94], SiC may be slightly decomposed at low temperature of 400 °C. As a result of the minute out-diffusion of Si atoms from SiC substrate, some minute C atoms diffuse out as well. These out-diffusing C atoms will react with N atoms in the interfacial layer, forming C\N bonds [10,13]: Fig. 7. Band alignment of Zr-oxynitride/IL/SiC system. Eg(ZrON) = bandgap of Zr-oxynitride, Eg(IL) = bandgap of IL, Eg(SiC) = bandgap of SiC, ΔEv(ZrON/SiC) = valence band offsets of Zr-oxynitride to SiC, ΔEv(IL/SiC) = valence band offsets of IL to SiC, ΔEc(ZrON/SiC) = conduction band offset of Zr-oxynitride to SiC, ΔEc(IL/SiC) = conduction band offset of IL to SiC, ΔEc(ZrON/IL) = conduction band offset of Zr-oxynitride to IL.
← → Cþ N →C−N
ð11Þ
The formation of C\N bonds essentially removes the carbon as well as carbon clusters that have been formed.
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As the oxidation/nitridation temperature is elevated to 500–900 °C, stoichiometric Zr–O (ZrO2) can also be formed when sufficiently high concentration of oxygen is supplied [89]. Moreover, more O atoms are produced and able to diffuse further inside; hence, more Zr\O bonds (sub-stoichiometric Zr–O) are formed towards the interfacial layer (Eq. (4)). Concurrently, more N atoms are able to diffuse in and react with sub-stoichiometric Zr–O to form Zr–N compound by releasing oxygen (Eq. (5)) [47,89] or react with unreacted Zr to form Zr–N compound (Eq. (6)) towards the interfacial layer. Excess N atoms may also diffuse further inside towards the interfacial layer to passivate the dangling Si bonds by forming Si–N compound (Eq. (9)). Minute out-diffusing Si and C atoms from the SiC substrate may react with inward diffusing N atoms to form Si\N bonds (Eq. 10) and C\N bonds (Eq. (11)), respectively. On the other hand, by obeying Eqs. (7) and (8), bridging O atoms in Zr–Si–O compound is formed at elevated temperatures of 700–900 °C. At a specific temperature of 500 °C, it is proposed that mixture of sub-stoichiometric Zr–O and Zr–N compounds are in the midst of exchanging and balancing the O and N atoms among themselves in order to achieve thermodynamic equilibrium condition. As a result, Zr–O–N compound is formed. During the process of exchanging O and N atoms among Zr–O and Zr–N compounds, there are also N atoms from the ambient N2O diffuse in and incorporate substitutionally and/or interstitially into stoichiometric Zr–O (ZrO2) region, forming Zr–O–N compound. Hence, Zr-oxynitride layer formed at this temperature consists of Zr–O and Zr–O–N, with Zr–O rich compound located at the topmost surface and Zr–O–N rich compound located near interface region. As reported by Ngaruiya et al. [95], The nitrogen atoms occupy oxygen sites within the crystalline phase of Zr–O, resulting in an almost unchanged ZrO2 structure with a sevenfold metal–nonmetal coordination and interatomic distances around 2.04–2.27 Ǻ. The reported interatomic distances have assured Zr tetravalency when nitrogen has been incorporated into stoichiometric Zr–O layer. Therefore, Zr–O compound remains stoichiometric at the topmost surface. 4. Conclusions In summary, chemical compositions and depth profile analysis of sputtered Zr oxidized/nitrided in ambient N2O at various temperatures (400–900 °C) for 15 min were presented. Based on the XPS results, Zr-oxynitride film of Zr–O, Zr–N, and/or Zr–O–N and its interfacial layer consisted of mixed Zr–O, Zr–N, Zr–O–N, Zr–Si–O, Si–N, and/or C–N compounds were identified. A possible model related to the oxidation and nitridation mechanisms has been proposed and explained. The reported chemical compositions in the film in this study may serve a vital role in understanding the functional properties of the film to work as advanced high-κ dielectric in SiC-based MOS devices. Acknowledgments The authors would like to acknowledge the support provided by USM fellowship, USM-RU-PRGS (8032051), and The Academy Sciences for the Developing World (TWAS) through TWAS-COMSTECH Research Grant (09–105 RG/ENG/AS_C) during the study. References [1] Z. Luo, T. Chen, D.C. Sheridan, J.D. Cressler, In: in: Z.C. Feng (Ed.), Power Materials SiC, Springer-Verlag, Berlin, New York, 2004, p. 375. [2] W.F. Lim, K.Y. Cheong, Z. Lockman, J. Alloys Compd. 497 (2010) 195. [3] S. Dimitrijev, H.B. Harisson, P. Tanner, K.Y. Cheong, J. Han, In: in: W.J. Choyke, H. Matsunami, G. Pensl (Eds.), Silicon Carbide — Recent Major Advances, Springer-Verlag, Berlin, Germany, 2004, p. 373. [4] J.B. Casady, R.W. Johnson, Solid-State Electron. 39 (1996) 1409. [5] M.T. Soo, K.Y. Cheong, A.F. Mohd Noor, Sens. Actuators B Chem. 151 (2010) 39.
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