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Growth and Characterization of BeO Thin Films Grown by Atomic Layer Deposition Using HO and O as Oxygen Sources 2

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Woo Chul Lee, Cheol Jin Cho, Sangtae Kim, Eric S. Larsen, Jung Hwan Yum, Christopher W. Bielawski, Cheol Seong Hwang, and Seong Keun Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05240 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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The Journal of Physical Chemistry

Growth and Characterization of BeO Thin Films Grown by Atomic Layer Deposition Using H2O and O3 as Oxygen Sources Woo Chul Lee,†,‡ Cheol Jin Cho,†,‡ Sangtae Kim,† Eric S. Larsen,§,⊥ Jung Hwan Yum,§,⊥ Christopher W. Bielawski,§,⊥ Cheol Seong Hwang, ‡ and Seong Keun Kim†,* †

Center for Electronic Materials, Korea Institute of Science and Technology, Seoul, 02792, South Korea ‡

Department of Materials Science and Engineering, and Inter-University Semiconductor

Research Center, College of Engineering, Seoul National University, Seoul, 08826, South Korea §

Department of Chemistry and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea ⊥

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan, 44919, South Korea

ACS Paragon Plus Environment

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Abstract

Growth characteristics and properties of BeO films grown by atomic layer deposition (ALD) are investigated. ALD chemistries between dimethylberyllium and two different oxygen sources, H2O and O3, are governed by different reaction mechanisms, resulting in different film properties. At the growth temperatures ranging from 150 to 300 °C, the properties of the BeO films grown using H2O are temperature-independent. In contrast, the BeO films grown using O3 at low temperatures (< 200 °C) show high concentrations of carbon and hydrogen, possibly owing to the incomplete removal of the ligands of the precursor, leading to a low film density. This correlates with the evolution of the rough surface and the microstructure composed of few nanometer-sized grains. The low-quality BeO films grown using O3 at low temperatures (< 200 °C) show a decreased band gap (Eg: 7.7–7.9 eV) and dielectric constant (εr: 5.6–6.7). Above 250 °C, these properties recovered to the levels (Eg: ~9.4 eV and εr: ~8.1) of the BeO films grown using H2O, which show high values of Eg: ~9.1-9.4 eV and εr: ~8. Collectively, these findings demonstrate that the O3-ALD process requires relatively more thermal energy than H2O-ALD does, to produce high-quality BeO films.

KEYWORDS BeO, atomic layer deposition, H2O, O3


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Introduction

A capacitor, which stores electric charges in an electric field, requires very high resistivity to suppress the leakage of the stored electric charges. The insulating properties under the application of an electric field are intimately linked to the band gap of the insulating material. Use of a large band gap material as a capacitor dielectric is crucial to enhance the resistivity. BeO has a very large band gap of 10.6 eV,1 which is significantly larger than that of SiO2 (~9 eV) and Al2O3 (~8 eV), two commonly used large band gap materials. The dielectric constant (~6.9 along the a-axis and ~7.7 along the c-axis)2 of wurtzite-structured BeO is higher than that of SiO2 (3.9) and comparable to that of Al2O3 (~8). Furthermore, the structural transformation of BeO into rocksalt has been recently predicted to achieve a very high dielectric constant (~275) as well as a very large band gap of 10.1 eV by ab initio calculations.3 The unexpected combination of the properties of rocksalt BeO suggests that the rocksalt BeO film could be a prospective contender to break the scaling limit found in the dynamic random access memory (DRAM) capacitor technology. Atomic layer deposition (ALD) of BeO thin films has been proposed for the application of a high-k layer on Si and III-V semiconductors.4-5 Formation of a low-k interfacial layer, which significantly decreases the capacitance density of gate stack, could be effectively suppressed in the BeO/Si stacks because BeO is thermodynamically stable when in contact with Si substrates.6 Using dimethylberyllium (DMB, Be(CH3)2) as a Be precursor in ALD efficiently reduces the interfacial layers in BeO/III-V stack structures.5 Although the electrical properties of the ALDgrown BeO films as the gate oxide have been investigated, the growth behavior of ALD BeO has


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not been studied extensively. Likewise, the influence of the oxygen source in the BeO ALD process also remains to be examined. Water (H2O) is a common oxygen source for ALD of oxide films. In particular, ALD with metal alkyl precursors such as trimethylaluminum (TMA) and H2O is known to be close to an ideal ALD chemical reaction.7 The ALD reaction between a metal alkyl precursor and H2O produces low impurity concentrations in the oxide films and excellent step coverage over the three-dimensional structures, including those with high aspect ratios.8-9 Ozone (O3) is also a widely used oxygen source in ALD of oxide films. O3 has higher activity compared to H2O, and renders ALD reaction with stable metal precursors such as β-diketonates possible.10-11 Most of the studies on ALD with O3 for oxide films indicate that the films grown with O3 show lower concentrations of impurities such as carbon, nitrogen, or chlorine12-13 when compared to the films grown with H2O. However, contradictory results have been reported in the case of ALD Al2O3 films grown from TMA, which has ligands identical to those of DMB used in this work. Kim et al. reported that the use of O3 in the Al2O3 ALD led to fewer defects in the films.14 In contrast, Elliott et al. reported that void formation was observed in the Al2O3 films fabricated by O3-ALD, resulting in low density.10 Therefore, it is necessary to carefully examine the properties of BeO films grown by ALD, particularly with respect to the types of oxygen sources. In this work, the growth behaviors of BeO films grown by ALD, using DMB and H2O and O3 as the oxygen source are systematically studied. After confirming the ALD process window, the film properties, such as the band gap and the dielectric constant of the BeO films, are examined. The film properties depend on both the oxygen source and growth temperature, due to the differences in the impurity contents and microstructures of the respective films.


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Experimental Section

BeO thin films were grown in a traveling-wave type reactor by ALD (Atomic-Classic, CN-1. Co.) on p-type Si (100) substrates. The ALD of the BeO thin films was performed at the substrate temperatures ranging from 150 to 300 °C. DMB was used as the Be precursor. A bottle containing DMB was maintained at 115 °C to achieve a sufficient vapor pressure and the delivery line for the DMB precursor was heated to 120 °C to prevent condensation of the DMB vapors. The DMB vapors were delivered to the reactor along with an N2 carrier gas (346 standard cubic centimeter per min, sccm) for supplying a sufficient dose of the precursor. H2O and O3 were used as oxygen sources. A bottle containing H2O was cooled to 5 °C. The O3 gas was produced by an inductive O3 generator (CN-1, OZONE TECH) in which a mixture gas of O2 (970 sccm)/N2 (30 sccm) was used. The O3 concentration was fixed at 180 Ng/m3. No film growth occurred when pure O2 gas was used as the oxygen source. The injection time for each precursor was varied to verify the corresponding self-limiting behavior. The purging times for the precursors were optimized to prevent the reactants from intermixing. Caution: BeO is carcinogenic and may cause chronic beryllium disease. The BeO film thickness was evaluated by spectroscopic ellipsometry (MG-1000, NANO VIEW). The measured spectra was fitted in a wavelength range of 250 – 820 nm based on a Cauchy model. X-ray reflectivity (XRR, ATX-G, Rigaku) was utilized to confirm the film thickness and estimate the film density. The impurity contents in the BeO films were estimated by time-of-flight secondary ion mass spectrometry (ToF-SIMS, TOF-SIMS5, ION-TOF). Atomic force microscopy (AFM, XE-100, Park Systems) was used to observe the surface


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morphologies of the films. The chemical analysis of the films was performed by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC PHI). The microstructures of the BeO films were observed by high-resolution transmission electron microscopy (HRTEM, Titan TEM, FEI). A sputtered Pt layer was grown on top of the BeO films to evaluate the dielectric constant of the BeO films. The capacitance of the metal-insulator-semiconductor (MIS) capacitors was estimated by an Agilent 4294A impedance analyzer at 1 MHz. The dielectric constant of the BeO films was calculated from the accumulation capacitance of the MIS capacitors.

Results and Discussion

Figures 1 (a) and (b) show the variation in the thickness of the BeO films grown at 250 °C over 100 cycles, as a function of feeding times of DMB and the oxygen source, respectively. The feeding times of the oxygen sources were fixed at 1 s for H2O and 2 s for O3 (Fig. 1 (a)) and the feeding time of DMB was fixed at 3 s (Fig. 1 (b)). The film thickness increasingly approaches ~8 nm for H2O and 15 nm for O3 with respect to the DMB feeding time (Fig. 1 (a)). The film thickness also saturates with the oxygen source feeding times for H2O and O3 above 1 s and 2 s, respectively. This result suggests that both the chemical reactions between DMB and the two oxygen sources, H2O and O3, are governed by ALD chemistry that is based on a self-limiting behavior. Based on the saturation behavior, the feeding times of DMB, H2O, and O3 were fixed at 3 s, 1 s, and 2 s in the subsequent experiments, respectively. From the saturated film thickness for the same number of cycles, shown in Figs. 1 (a) and (b), it can be understood that the growth per cycle (GPC) of O3-ALD for BeO is higher than that of


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H2O-ALD for BeO at the growth temperature of 250 °C. GPC in ALD is generally sensitive to the growth temperature, because the number of functional groups on the reaction surface may vary with temperature.7 Degree of removal of bulky ligands, which affects the steric hindrance effect, is also dependent on the growth temperature and types of reactants. Therefore, it is necessary to examine the variation in the GPC of BeO by ALD with respect to the growth temperature and oxygen source. Figure 1 (c) shows the variation in the GPC of BeO ALD as a function of the growth temperature. The GPC values in Fig. 1 (c) were calculated from the slope of the graphs of film thickness vs. the number of cycles, to exclude the influences of the incubation cycles. In the H2O-ALD of BeO films, GPC decreases upon increasing the growth temperature, from 0.14 nm/cycle at 150 °C to 0.06 nm/cycle at 300 °C. Consideration of the ALD processes with a different metal precursor with alkyl ligands similar to that in DMB and H2O could provide an insight into the temperature dependence of the GPC of H2O-ALD for BeO. GPC of the Al2O3 ALD from TMA and H2O was reported to decrease with an increase in the growth temperature,7, 15-16 consistent with the temperature dependence of the GPC of BeO grown by H2O-ALD shown in Fig. 1 (c). Puurunen claimed that a decrease in the surface hydroxyl group concentration with increasing temperature caused the decrease in the GPC of Al2O3 ALD from TMA and H2O precursors.7, 17 In addition, ALD of ZnO films from H2O and diethylzinc which has alkyl ligands similar to that of DMB exhibited a similar temperature dependence.18 In light of the results obtained from ALD processing using metal alkyl precursors and H2O, the decrease in the GPC of H2O-ALD for BeO at higher temperatures in Fig. 1 (c) is likely due to the decrease in the density of the surface hydroxyl groups. Interestingly, the GPC of the BeO O3-ALD does not seem to show significant temperature dependence in the growth temperature range in contrast to that of BeO H2O-ALD. The GPC of


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the BeO O3-ALD is 0.01 nm/cycle at 150 °C, which is lower than the GPC of the BeO H2O-ALD at the same growth temperature. The BeO O3-ALD process shows a relatively high GPC of 0.11 nm/cycle at 300 °C. This value is almost two times larger than that of the BeO H2O-ALD at 300 °C. The ALD growth from a given metal alkyl precursor and O3 is reported to be governed by a very different reaction mechanism compared to that from the same metal precursor and H2O.10, 19 The difference in reaction mechanism may lead to a different temperature dependence of the GPC of O3-ALD processes. For example, the ALD of Al2O3 films from TMA and O3 has been reported to show a drastic decrease in the GPC with increasing temperature, compared to that using TMA and H2O.10 In addition, Kim et al. reported that the GPC of the O3-ALD for ZnO decreased with increasing growth temperature, although the GPC of ALD of ZnO films from diethylzinc and O3 was higher than that of ALD-ZnO films from diethylzinc and H2O.18 Considering the temperature dependence of GPC in O3-ALD for other materials, the temperature independence in the GPC of the BeO O3-ALD process is a rather unexpected phenomenon. A careful investigation of the reaction mechanism of DMB and O3 would be required to further understand the observed temperature dependence. Different reaction pathways for film growth could lead to differences in the GPC of the ALD process as well as variations in the properties of the grown films. The density of the BeO films grown with the two oxygen sources was examined by XRR. The film density is linearly proportional to the square of the critical angle (θc), where the intensity of the reflected X-ray is at half maximum in the XRR spectra. The θc values for the BeO films grown using H2O (Fig. S1 (a), Supporting Information (SI)) appear almost constant, irrespective of the growth temperature, while the θc values for the BeO films grown using O3 increase with growth temperature. (Fig. S1 (b), SI) The variation in the film density was calculated by fitting the measured XRR spectra.


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Figure 2 (a) shows the variation in the film density calculated from the XRR spectra as a function of the growth temperature. The density of the BeO films grown using H2O is in the narrow range of 2.65 to 2.80 g/cm3 and does not show significant temperature dependence. However, the density of the BeO films grown using O3 at 150 °C is as low as 2.16 g/cm3 and increases with increasing growth temperature; above 250 °C, the film density saturated to approximately 2.8 g/cm3, a value close to that of the BeO films grown using H2O. The highest value corresponds to approximately 93% of the bulk density. (3.0 g/cm3) The similar trend is also found in the variation in the refractive indices for those films. Figure 2 (b) shows the variation in the refractive index of the grown films at a wavelength of 588 nm as a function of the growth temperature. The refractive index of the films grown using H2O does not show significant temperature dependence while the refractive index of the films grown using O3 increases from 1.72 at 150 oC to 1.77 at 300 oC with increasing growth temperature. In general, film density is proportional to the refractive index. This tendency in the refractive index is consistent with the change in the film density measured by XRR in Fig. 2 (a). The low density of the BeO films grown using O3 at low temperatures (< 200 °C) indicates that defective films are formed under the growth conditions. In general, such low-density ALDgrown films could be induced by the incorporation of light impurities, including carbon and hydrogen as a result of incomplete removal of the precursor ligands.10 Based on the aforementioned results, the contents of the impurities in the BeO films were examined. Figures 3 (a) and (b) show ToF-SIMS depth profiles of carbon in the BeO films grown between 150 to 300 °C using H2O and O3, respectively. The BeO films grown using H2O exhibit low carbon concentrations and show no notable growth-temperature-dependence on the carbon contents. In contrast, the intensity of the carbon signal is nearly one order of magnitude


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larger in the BeO film grown using O3 at 150 and 200 °C than that in the BeO film grown using H2O. Although the carbon contents are reduced upon increasing the growth temperature, their intensity in the BeO film grown using O3 above 250 °C is still slightly higher than that in the BeO film grown using H2O. For other alkaline earth metals, carbonate formation is a commonly met problem when using ozone as the oxygen source in ALD. However, this should not be an issue in this experiment because beryllium carbonate is not stable. The hydrogen concentration in the BeO films was also compared. Figures 3 (c) and (d) show ToF-SIMS depth profiles of hydrogen in the BeO films grown between 150 to 300 °C using H2O and O3, respectively. In the case of BeO films grown using H2O, the intensity of the hydrogen signal is slightly higher for its growth at 150 °C than at 200 °C. The intensities of the hydrogen signals for the films grown at above 200 °C appear very similar to each other. However, in the films grown using O3, the hydrogen concentration decreases with increasing growth temperature. The intensity of the hydrogen signal is approximately one order of magnitude larger in the BeO film grown using O3 than that of the film grown using H2O, at 150 °C. The intensity of the hydrogen signal in the BeO film grown using O3 above 250 °C is comparable to that of the BeO films grown using H2O at the same temperature. The use of O3 in ALD is generally considered to have a higher potential to decrease the impurity concentrations in ALD-grown oxide films compared to the use of H2O.14, 20 However, this is not the case for O3-ALD BeO. The BeO films grown using O3 below 200 °C contain higher concentrations of impurities such as carbon and hydrogen compared to those in the BeO films grown under other conditions. Therefore, the high concentrations of the impurities in the BeO films grown using O3 below 200 °C can be correlated to the low density of the films in Fig. 2 (a).


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Since the Be ions in the films are lighter compared to carbon, nevertheless, the incorporation of carbon in the films per se is not expected to result in the low density of the BeO films. Therefore, the microstructures of the films were examined by HRTEM to further investigate the origin of the low density of the BeO films grown using O3 at low temperatures. Figures 4 (a) and (b) show cross-sectional HRTEM images of the BeO films grown at 150 °C using H2O and O3, respectively. Although both as-grown BeO films are crystallized into wurtzite structures at 150 °C, (Fig. S2, SI) their microstructures are significantly different from each other. The BeO film grown using H2O contains an approximately 5-nm-thick amorphous layer immediately on top of the Si wafer and a 17-nm-thick crystalline BeO upper layer on the amorphous layer. The crystalline BeO upper layer exhibits dense and columnar grains with a lateral size of 10–20 nm. In contrast, as observed in Fig. 4 (b), randomly oriented grains with few nm diameters are uniformly distributed in the BeO film grown at 150 °C using O3. Therefore, this microstructure could have several low-density grains and large area of grain boundaries with open structure which were caused by incomplete removal of the ligands of DMB. This microstructure character accounts for the low densities of the BeO films grown using O3 at low temperatures of < 200 °C (see Fig. 2 (a)). Above the growth temperature of 250 °C, the density of the BeO films grown using O3 increases and is close to that of the BeO films grown using H2O (Fig. 2 (a)). It is necessary to observe the microstructure of the high-density BeO films grown at higher temperatures to understand the correlation between the microstructure and the density of the BeO films. Figures 4 (c) and (d) show the cross-sectional HRTEM images of the BeO films grown at 250 °C using H2O and O3, respectively. The BeO films grown at 250 °C maintain high density and exhibit a columnar structure with dense grains, irrespective of the oxygen source. This result corroborates


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that the low density of the BeO films grown using O3 at low temperatures correlates with the microstructure with the fine grains. In addition, the amorphous interfacial layer is found to be much thinner in the BeO films grown at 250 °C compared with the film shown in Fig. 4 (a). If the amorphous layer observed in the BeO film grown at 150 °C using H2O in Fig. 4 (a) is a mixture of SiO2 and BeO or SiO2 alone, the interfacial layer should be thicker in the BeO films grown at higher temperatures because the formation reaction of the interfacial layer should be more active at higher temperature. However, this is not the case for BeO ALD. Therefore, it is likely that the interfacial layer in Fig. 4 (a) corresponds to amorphous BeO. It is supposed that, in the H2OALD process at a relatively low temperature of 150 °C, the BeO film begins to grow into an amorphous phase and then a crystalline BeO film is formed above a thickness of ~5 nm. The surface morphology of the BeO films was also examined. Figures 5 (a) and (b) show AFM images of the BeO films grown using H2O and O3, respectively, at growth temperatures of 150, 200, 250, and 300 °C. The corresponding film thickness was controlled in the range of 18–22 nm. The surface of the BeO films grown in H2O-ALD is smooth although the root-mean-squared (RMS) roughness slightly increases from 0.19 to 0.66 nm with increasing growth temperature. In contrast, the BeO films grown in the O3-ALD display different surface morphologies. The BeO films grown at low temperatures (< 200 °C), which exhibit low film density, show rough surface morphologies (RMS roughness: > 2 nm). This surface roughness is consistent with the microstructure composed of few nm-sized grains (Fig. 4 (b)). Both results suggest that incorporation of carbon or hydrogen impurities change the grain morphology during film growth. The surface of the BeO films grown at 200 °C is much rougher than that of the BeO film grown at 150 °C, although the impurity contents in the film are relatively low when grown at 200 °C,


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particularly when compared to that at 150 °C. This result may be attributed to the increased size of grains with random orientation of the film grown under thin condition. On the other hand, the surface of the BeO films grown using O3 above 250 °C (RMS roughness: ~0.3 nm) is very smooth, consistent with a microstructure comprised of dense and columnar grains. These findings demonstrate that the type of the oxygen source used in the BeO ALD influences both the surface morphology and the chemical properties of the corresponding film. The differences in the impurity contents and the microstructure result in different properties of the BeO films. Before investigating the properties of the BeO films, the chemical bonding states of the BeO films with respect to the growth temperature and the oxygen source were investigated. All of the BeO films show a single peak in the XPS spectra of Be 1s and O 1s core levels, which correspond to Be–O bonding of BeO (Fig. S3, SI), indicating that these films are mostly composed of BeO. However, further examination of the BeO film properties reveals differences in the band gap and dielectric constant, when the growth temperature and oxygen source vary. Figure 6 (a) shows the variation in the band gap of the BeO films grown using H2O and O3 on the growth temperature. The band gap of the BeO films was estimated from the XPS O 1s energy loss spectra (Fig. S4, SI) using a method described elsewhere.21 The BeO films grown using H2O show large band gaps ranging from 9.1 to 9.4 eV, irrespective of the growth temperature. In contrast, the BeO films grown using O3 at low temperatures (< 200 °C) show much smaller band gaps of 7.7–7.9 eV. The band gap of the BeO films grown above 250 °C is approximately 9.3–9.4 eV, close to that of the BeO films grown using H2O. Figure 6 (b) shows the variation in the dielectric constant of the BeO films grown using H2O and O3 as a function of the growth temperature. The dielectric constant of the BeO films grown using H2O is 7.3 at 150 °C and approaches 8.1 above 200 °C, which is consistent with the reported values (~6.9 along


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the a-axis and ~7.7 along the c-axis)2 of wurtzite BeO. The slightly low dielectric constant of the BeO films grown at 150 °C is attributed to the presence of an amorphous interfacial layer. Although the dielectric constants of the BeO films grown using O3 show a similar temperature dependence, their values are as low as 5.6 at 150 °C and 6.7 at 200 °C, which is consistent with the low density of the films (as shown in Fig. 2 (a)). The BeO films grown using O3 above 250 °C, which show high density and low impurity concentrations, exhibit a higher dielectric constant of ~8.3. This value is consistent with those of the BeO films grown using H2O. The H2O-ALD process for the growth of BeO is nearly independent of the growth temperature below 200 °C, whereas the O3-ALD process for the growth of BeO produces impurities such as carbon and hydrogen and results in a low film density and, consequently leads to low band gaps and dielectric constants of the respective films. At higher temperatures (> 250 °C), the impurity concentration and the density of the BeO films recover to the levels of the BeO films grown using H2O. This indicates that the O3-ALD for the growth of BeO is different from that of H2OALD and the O3-ALD process requires more thermal energy to generate high-quality BeO films.

Conclusion

The growth characteristics of BeO thin films grown by ALD with DMB and different oxygen sources such as H2O and O3 were investigated in the temperature range of 150 to 300 °C. Although ALD reactions using H2O or O3 show a self-saturation behavior, the growth behavior and film properties are strongly dependent on the oxygen source. With increasing growth temperatures, the GPC of H2O-ALD of BeO decreases, while that of O3-ALD of BeO is almost constant. The properties of the BeO films grown in H2O-ALD are nearly temperature-


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independent, whereas the BeO films grown by O3-ALD at low temperatures (< 200 °C) reveal high impurity concentrations and a low film density. This may be attributed to the incomplete removal of the ligands of the precursor during the O3-ALD at low temperatures, which is supported by the evolution of the BeO microstructure composed of few nanometer-sized grains. The high impurity concentration and low film density of the BeO films in O3-ALD cause lowering of the band gap and dielectric constant of the films. The properties of the BeO films grown by O3-ALD are drastically improved above 250 °C, and are comparable to those of the BeO films grown by H2O-ALD. Therefore, the H2O-ALD process for BeO would be compatible with processes performed at low temperatures. The high GPC and smooth film surface obtained by O3-ALD of BeO at high temperatures should render the O3 ALD process favorable at high temperatures (> 250 °C).

ASSOCIATED CONTENT Supporting Information. Additional information on XRR spectra, TEM data, XPS spectra, and XPS O 1s loss spectra of the BeO films grown by ALD. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the Future Semiconductor Device Technology Development Program (10047231) funded by MOTIE (Ministry of Trade, Industry & Energy) and KSRC (Korea Semiconductor Research Consortium) and by the Korea Institute of Science and Technology (KIST through 2E27160). CSH acknowledges the support from the Global Research Laboratory Program (2012K1A1A2040157) of the National Research Foundation of Korea. JHY, ESL, and CWB are grateful to the Institute for Basic Science (IBS-R019- D1) as well as the BK21 Plus Program funded by the Ministry of Education and the National Research Foundation of Korea for their support.


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FIGURES

Figure 1. Variations in the thickness of BeO films grown at 250 °C over 100 cycles, as a function of (a) DMB and (b) oxygen source feeding times. (c) The variation in the GPC of the BeO ALD as a function of the growth temperature.


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Figure 2. (a) The variation in the density of the BeO films obtained from XRR spectra as a function of the growth temperature. (b) The variation in the refractive index of the films at a wavelength of 588 nm as a function of the growth temperature.


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Figure 3. ToF-SIMS depth profiles of carbon in the BeO films grown in the temperature range of 150 to 300 °C, using (a) H2O or (b) O3. ToF-SIMS depth profiles of hydrogen in the BeO films grown between 150 to 300 °C, using (c) H2O or (d) O3. The hump in the graphs indicates the interface between BeO and Si substrate.


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Figure 4. Cross-sectional HRTEM images of BeO films grown on Si at 150 °C using (a) H2O or (b) O3. Cross-sectional HRTEM images of the BeO films grown on Si at 250 °C using (a) H2O or (b) O3. Scale bar: 10 nm.


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Figure 5. AFM images of BeO films grown using (a) H2O or (b) O3 at growth temperatures of 150, 200, 250, and 300 °C (indicated).


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Figure 6. Variations in the (a) band gap and (b) dielectric constant of BeO films grown using H2O or O3 as a function of the growth temperature.


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REFERENCES

1.

Sashin, V. A.; Bolorizadeh, M. A.; Kheifets, A. S.; Ford, M. J., Electronic Band Structure

of Beryllium Oxide. J. Phys.: Condens. Matter 2003, 15, 3567. 2.

Subramanian, M. A.; Shannon, R. D.; Chai, B. H. T.; Abraham, M. M.; Wintersgill, M.

C., Dielectric Constants of BeO, MgO, and CaO Using the Two-Terminal Method. Phys. Chem. Minerals 1989, 16, 741-746. 3.

Yim, K.; Yong, Y.; Lee, J.; Lee, K.; Nahm, H.-H.; Yoo, J.; Lee, C.; Hwang, C. S.; Han,

S., Novel High-κ Dielectrics for Next-Generation Electronic Devices Screened by Automated Ab Initio Calculations. NPG Asia Mater. 2015, 7, e190. 4.

Yum, J. H.; Akyol, T.; Lei, M.; Ferrer, D. A.; Hudnall, T. W.; Downer, M.; Bielawski, C.

W.; Bersuker, G.; Lee, J. C.; Banerjee, S. K., Electrical and Physical Characteristics for Crystalline Atomic Layer Deposited Beryllium Oxide Thin Film on Si and GaAs Substrates. Thin Solid Films 2012, 520, 3091-3095. 5.

Yum, J. H.; Akyol, T.; Ferrer, D. A.; Lee, J. C.; Banerjee, S. K.; Lei, M.; Downer, M.;

Hudnall, T. W.; Bielawski, C. W.; Bersuker, G., Comparison of the Self-Cleaning Effects and Electrical Characteristics of BeO and Al2O3 Deposited as an Interface Passivation Layer on GaAs MOS Devices. J. Vac. Sci. Technol. A 2011, 29, 061501. 6.

Hubbard, K. J.; Schlom, D. G., Thermodynamic Stability of Binary Oxides in Contact

with Silicon. J. Mater. Res. 2011, 11, 2757-2776.


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7.

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Puurunen, R. L., Surface Chemistry of Atomic Layer Deposition: A Case Study for the

Trimethylaluminum/Water Process. J. Appl. Phys. 2005, 97, 121301. 8.

Matero, R.; Rahtu, A.; Ritala, M.; Leskelä, M.; Sajavaara, T., Effect of Water Dose on

the Atomic Layer Deposition Rate of Oxide Thin Films. Thin Solid Films 2000, 368, 1-7. 9.

Ritala, M.; Leskelä, M.; Dekker, J.-P.; Mutsaers, C.; Soininen, P. J.; Skarp, J., Perfectly

Conformal TiN and Al2O3 Films Deposited by Atomic Layer Deposition. Chem. Vap. Deposition 1999, 5, 7-9. 10. Elliott, S. D.; Scarel, G.; Wiemer, C.; Fanciulli, M.; Pavia, G., Ozone-Based Atomic Layer Deposition of Alumina from TMA: Growth, Morphology, and Reaction Mechanism. Chem. Mater. 2006, 18, 3764-3773. 11. Putkonen, M.; Sajavaara, T.; Johansson, L. S.; Niinistö, L., Low-Temperature ALE Deposition of Y2O3 Thin Films from β-diketonate Precursors. Chem. Vap. Deposition 2001, 7, 44-50. 12. Cho, M.; Jeong, D. S.; Park, J.; Park, H. B.; Lee, S. W.; Park, T. J.; Hwang, C. S.; Jang, G. H.; Jeong, J., Comparison between Atomic-Layer-Deposited HfO2 Films Using O3 or H2O Oxidant and Hf[N(CH3)2]4 Precursor. Appl. Phys. Lett. 2004, 85, 5953-5955. 13. Park, H. B., et al., Comparison of HfO2 Films Grown by Atomic Layer Deposition Using HfCl4 and H2O or O3 as the Oxidant. J. Appl. Phys. 2003, 94, 3641-3647. 14. Kim, J. B.; Kwon, D. R.; Chakrabarti, K.; Lee, C.; Oh, K. Y.; Lee, J. H., Improvement in Al2O3 Dielectric Behavior by Using Ozone as an Oxidant for the Atomic Layer Deposition Technique. J. Appl. Phys. 2002, 92, 6739-6742.


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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


15. George, S. M., Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111-131. 16. Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M., Low-Temperature Al2O3 Atomic Layer Deposition. Chem. Mater. 2004, 16, 639-645. 17. Puurunen, R. L., Correlation between the Growth-Per-Cycle and the Surface Hydroxyl Group Concentration in the Atomic Layer Deposition of Aluminum Oxide from Trimethylaluminum and Water. Appl. Surf. Sci. 2005, 245, 6-10. 18. Kim, S. K.; Hwang, C. S.; Park, S.-H. K.; Yun, S. J., Comparison between ZnO Films Grown by Atomic Layer Deposition Using H2O or O3 as Oxidant. Thin Solid Films 2005, 478, 103-108. 19. Goldstein, D. N.; McCormick, J. A.; George, S. M., Al2O3 Atomic Layer Deposition with Trimethylaluminum and Ozone Studied by in Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry. J. Phys. Chem. C 2008, 112, 19530-19539. 20. Kim, S. K.; Lee, S. W.; Hwang, C. S.; Min, Y. S.; Won, J. Y.; Jeong, J., Low Temperature (< 100 Degrees C) Deposition of Aluminum Oxide Thin Films by ALD with O3 as Oxidant. J. Electrochem. Soc. 2006, 153, F69-F76. 21. Miyazaki, S., Photoemission Study of Energy-Band Alignments and Gap-State Density Distributions for High-k Gate Dielectrics. J. Vac. Sci. Technol. B 2001, 19, 2212-2216.


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TOC GRAPHIC


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Figure 1. Variations in the thickness of BeO films grown at 250 ○C over 100 cycles, as a function of (a) DMB and (b) oxygen source feeding times. (c) The variation in the GPC of the BeO ALD as a function of the growth temperature. 317x82mm (300 x 300 DPI)



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Figure 2. (a) The variation in the density of the BeO films obtained from XRR spectra as a function of the growth temperature. (b) The variation in the refractive index of the films at a wavelength of 588 nm as a function of the growth temperature. 311x117mm (300 x 300 DPI)


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○

Figure 3. ToF-SIMS depth profiles of carbon in the BeO films grown in the temperature range of 150 to 300 C, using (a) H2O or (b) O3. ToF-SIMS depth profiles of hydrogen in the BeO films grown between 150 to 300 ○ C, using (c) H2O or (d) O3. The hump in the graphs indicates the interface between BeO and Si substrate. 239x156mm (300 x 300 DPI)



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Figure 4. Cross-sectional HRTEM images of BeO films grown on Si at 150 °C using (a) H2O or (b) O3. Crosssectional HRTEM images of the BeO films grown on Si at 250 °C using (c) H2O or (d) O3. Scale bar: 10 nm. 169x149mm (300 x 300 DPI)


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Figure 5. AFM images of BeO films grown using (a) H2O or (b) O3 at growth temperatures of 150, 200, 250, and 300 ○C (indicated). 276x146mm (300 x 300 DPI)



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Figure 6. Variations in the (a) band gap and (b) dielectric constant of BeO films grown using H2O or O3 as a function of the growth temperature. 316x118mm (300 x 300 DPI)


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Growth and Characterization of BeO Thin Films ... - ACS Publications

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