Crystalline BeO grown on 4H-SiC via atomic layer deposition: Band

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Crystalline BeO grown on 4H-SiC via atomic layer deposition: Band alignment and interface defects Seung Min Lee, Yoonseo Jang, Jongho Jung, Jung Hwan Yum, Eric S. Larsen, Sangyeon Lee, Hyungtak Seo, Christopher W. Bielawski, Hi-Deok Lee, and Jungwoo Oh ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00098 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Electronic Materials

Crystalline BeO grown on 4H-SiC via atomic layer deposition: Band alignment and interface defects Seung Min Lee 1,2, Yoonseo Jang 1,2, Jongho Jung 1,2, Jung Hwan Yum 3, Eric S. Larsen 3,4, Sang Yeon Lee 5, Hyungtak Seo 5, Christopher W. Bielawski 3,4, Hi-Deok Lee 6, and Jungwoo Oh 1,2* 1School

of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea

2Yonsei 3Center

Institute of Convergence Technology, Incheon 21983, Republic of Korea

for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

4Department

of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

5Department

of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea

6Department

of Electronics Engineering, Chungnam National University, Daejeon 34134, Republic of Korea

KEYWORDS: Beryllium oxide (BeO), Silicon carbide (SiC), Band alignment, Internal photoemission spectroscopy (IPE), Reflection electron energy loss spectroscopy (REELS), Ultraviolet photoelectron spectroscopy (UPS).

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ABSTRACT: A crystalline beryllium oxide (BeO) film was grown on 4H silicon carbide (4HSiC) via thermal atomic layer deposition (ALD). Self-produced diethyl beryllium was a precursor, and H2O was an oxidant. The growth rate of BeO corresponded to 0.8 Å/cycle in the growth temperatures of 150 – 200 °C. Transmission electron microscopy and x-ray diffraction of BeO/4H-SiC demonstrated that wurtzite BeO (0002) was grown on 4H-SiC (0001) substrate. The average crystallite sizes of BeO were 15-16 nm and the compressive strain was applied to the BeO film in the out-of-plane direction. The band alignment and interface defects of BeO/4HSiC were determined by using internal photoemission spectroscopy (IPE), ultraviolet photoelectron spectroscopy (UPS), and reflection electron energy loss spectroscopy (REELS). The conduction band offset (CBO), valence band offset (VBO), energy bandgap of 4H-SiC and BeO corresponded to 2.28 ± 0.1 eV, 2.53 ± 0.01 eV, 3.16 ± 0.1 eV, and 8.3 ± 0.05 eV, respectively. The calculated bandgap (7.97 eV) of a thin BeO film obtained from the sum of CBO (2.28 eV), VBO (2.53 eV), and the SiC bandgap (3.16 eV). The difference between the calculated (7.97 eV) and REELS (8.3 eV) bandgap of BeO film is due to the error bars between the analysis methods. Interface defect levels determined via IPE analysis corresponded to 3.53 ± 0.1 eV (graphitic carbon) and 4.46 ± 0.1 eV (π-bonded carbon) and were formed during the ohmic annealing process.


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1. INTRODUCTION 4H-Silicon carbide (SiC) is a wide-bandgap (approximately 3.26 eV) semiconductor that is used for high-performance power electronics due to its high breakdown field (3 MV/cm), high electron drift velocity (2 × 107 cm/s), and excellent thermal conductivity (370 W/mK).1-5 In addition, carbon antisite-vacancy (CAV) pairs in 4H-SiC serves as a single-photon emitter at visible wavelengths.6 Therefore, the 4H-SiC is a key material for next-generation photonics such as single-photon sources reported in the spectral range of 550 to 1450 nm for cavity optomechanical applications.7 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) widely use silicon dioxide (SiO2) as a gate dielectric. The interface trap density (Dit) values of the SiO2/4H-SiC interface are approximately 1–2 times (1013 cm-2·eV-1) those of the SiO2/Si interface.8-12 The permittivity (κ) of SiO2 (κ = 3.9) is lower than that of 4H-SiC (κ = 10). Therefore, the electric field in SiO2 is 2.5 times that in 4H-SiC. Thus, the SiC MOSFETs are operated at a significantly lower electric field than the 4H-SiC breakdown field to avoid premature SiO2 breakdown.13 The use of high-κ epitaxial oxides is a direct solution to improve the performance of the device.14 Beryllium oxide (BeO) received significant attention as an electrical insulator due to its unique physical properties such as its high bandgap energy, dielectric constant, and thermal conductivity.15-18 In addition, BeO has the same wurtzite crystal structure as that of 4H-SiC. This shows the possibility of epitaxial growth of BeO on 4H-SiC substrate. A previous study examined atomic-layer-deposited (ALD) BeO on 4H-SiC (0001). The results of crystallographic analysis indicated that the BeO (0001) single crystal was successfully grown on 4H-SiC. The lattice mismatch of 12.9% between BeO (a = 2.69 Å, c = 4.38 Å) and 4H-SiC (a = 3.07 Å, c = 10.05 Å) was substantially reduced to 0.3 ~ 1.2% according to domain epitaxy principle. Good


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dielectric properties with high κ (6.9) and small Dit were observed based on the capacitance– voltage curves.19 Furthermore, the excellent thermal conductivity of BeO (330 W/mK)18 plays a major role in mitigating heating problems that arise during the operation of 4H-SiC power devices. BeO is also a promising candidate for the structural and electrical passivation layer of 4H-SiC quantum-emitter due to its high-quality epitaxial growth and wide-bandgap characteristics. Because, charge trapping and fabrication damage in non-passivated surface states can degrade the optical properties of the CAV center in 4H-SiC. With respect to the surface passivation of 4H-SiC based quantum emitter, the molecular termination schemes and epitaxial growth of wide-bandgap inorganic materials are used.20-21 Although the unique characteristics of BeO are especially beneficial for power semiconductors and photonics, there is a paucity of studies on the electrical and physical analysis of ALD BeO since precursors for ALD are synthesized in the laboratory and are not yet commercially available. The toxicities of Be and BeO have also been pointed out as a hindrance to complementary-metal-oxide-semiconductor (CMOS) application. Respirable-sized beryllium particles are known to cause chronic lung cancer. However, other health problems were not reported except these shortcomings. BeO ceramic is not a hazardous waste according to materialsafety-data-sheet (MSDS) and federal law in the USA. Despite environmental health and safety concerns, there is a demand for high-performance devices, and research on new materials is required due to limitations of existing scaling technology. BeO can improve the performance of CMOS devices because it has much higher bandgap and thermal stability than other high-κ dielectrics. In addition, hazardous substances in the past have been commercially used due to the development of environmental and safety technologies.


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In the study, we present the ALD process and electronic band structure of BeO films on 4HSiC substrates. First, the reaction mechanism and process condition of ALD BeO are described. Second, the crystal properties of BeO films determined via transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses are introduced. The energy bandgap, VBO, CBO, and defect energy levels of ALD BeO films on 4H-SiC are determined via reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), and internal photoemission spectroscopy (IPE), respectively. Based on the spectral analysis results, we determine the energy band alignment of ALD BeO on 4H-SiC.

2. EXPERIMENTAL SECTION Atomic layer deposition of BeO. A Si-faced epitaxial 4H-SiC (0001) wafer produced by Cree, Inc., is used in the study. A 4H-SiC active layer with thickness of 6.7 μm and nitrogen doping concentration of 7.54 × 1015 cm-3 were epitaxially grown on highly n+ doped 4H-SiC wafer. Prior to the ALD process, an ohmic contact was formed on the back surface of 4H-SiC. Native oxide on the back side of 4H-SiC was removed with a buffered oxide etchant (BOE) for 1 min, and a 50-nm thick Ni layer was subsequently deposited by using an e-beam evaporator. Ohmic annealing was performed in a nitrogen atmosphere at 1000 °C for 3 min via a rapid thermal process (RTP). For the ALD of BeO, the organic contaminants on the 4H-SiC surface were removed with acetone, isopropanol alcohol (IPA), and deionized (DI) water for each 5 min. The native oxide on the front side of 4H-SiC was removed by BOE (7:1) for 1 min and DI water rinsed for 30 seconds. Thin films (2 nm to 50 nm) of BeO thin film were grown on 4H-SiC (0001) in a shower-head type reactor by using NCD LucidaTM M100 ALD.22 The ALD process of BeO films was performed at a reaction chamber temperature ranging from 100 to 300 °C. Self-


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produced diethyl beryllium (DEB), namely Be(C2H5)2, corresponded to the precursor and the DI H2O corresponded to the oxidant. The detailed synthesis process and safety protocols of DEB is shown in Figure S1. The DEB precursor in liquid form has the saturated vapor pressure of 1.6 torr at room temperature.23 The cylinder containing DEB precursor was maintained at 60 °C. Argon was used as a precursor carrier gas at a flow rate of 20 standard cubic centimeters per minute (sccm). Characterization. TEM measurements: High-resolution TEM (HR-TEM) images were obtained by using a JEOL JEM-ARM 200F system. A focused ion beam (JIB-4601) was used on electronically transparent samples. XRD measurements: Furthermore, XRD and x-ray reflectivity (XRR) analyses were performed by using Rigaku SmartLab systems. The x-ray tube of the diffractometer was operated at 40 kV and 30 mA, and data were obtained by using a CuKα x-ray. IPE measurements: The IPE analyses were performed by using a customized setup shown in Figure S2. The system consisted of a 150 W broadband xenon light source and a monochromator to provide an hv ranging from 1.05 eV (200 nm) to 6.2 eV (1100 nm). With respect to the IPE measurements, the semi-transparent top electrodes were formed through photolithography wherein 10 nm of Au was deposited by a thermal evaporator and patterned via a lift-off process. The output spectra were obtained by measuring the dc photocurrent across a metal-oxide semiconductor (MOS) structure with a Keithley 4200-SCS semiconductor parameter analyzer. REELS and UPS measurements: The REELS and UPS spectra were obtained by using a Thermo Fisher Scientific ESCALAB 250Xi system. Incident electrons with kinetic energy of 1 keV and 3 keV scattering angle of 165º were used in REELS. An ultra violet (UV) He lamp emitting He Ⅰ (21.2 eV) or He Ⅱ (40.8 eV) photons was used as the UPS source. The base pressure of the chamber was typically in the low 10-10 to high 10-11 torr range.


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3. RESULTS AND DISCUSSION A simplified first-order approximation of the reaction mechanism of ALD BeO occurs by alternating surface hydroxyl (-OH) and ethyl termination, and this is expressed as follows: 𝑥( ― OH) + Be(C2H5)2 → O𝑥 ― Be(C2H5)2 ― 𝑥 + 𝑥C2H6

(1)

O𝑥 ― Be(C2H5)2 ― 𝑥 + H2O → O ― Be ― (OH) + (2 ― 𝑥)C2H6

(2)

Be(C2H5)2 + H2O → BeO + 2C2H6

(net)

where the variable x denotes the degree of the ethane-releasing reaction that occurs in reaction 1. Reaction 2 involves rearranging oxygen on the newly formed surface to maintain BeO stoichiometry. The net reaction for the ALD BeO is exothermic, and this allows an exergonic reaction at low temperatures. The aforementioned reaction mechanism of ALD BeO film is schematically illustrated in Figure 1. The first state corresponds to the chemisorption of DEB molecules by the active site (-OH) of the surface. Si was reported to exhibit a H-terminated surface after removal of native oxide with hydrofluoric acid (HF), and 4H-SiC was reported to reveal an OH-terminated surface after cleaning with HF.24-25 The next step corresponds to ligand (C2H5) exchange that is attributed to high reactivity of both DEB and OH groups. The subsequent step corresponds to the purge of remaining DEB and reaction products and ethane (C2H6) from the ALD chamber. During the next step, H2O is injected into reaction chamber, and the oxidation process subsequently proceeds. The final step involves purging the remaining oxidant and ethane gas from the reaction chamber. After the oxidation process is completed, a crystalline BeO film is formed on the surface where all the Be atoms are bonded to each other via oxygen atoms.


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Figures 2a and 2b show the growth per cycle (GPC) of the BeO films as a function of DEB feeding time and growth temperature (Tg), respectively. P-type Si (100) wafers are used to determine the ALD process window of BeO. The oxidant feed time is fixed at 0.5 s, and the purge times for DEB and H2O are fixed at 30 s. The growth temperature is calibrated by using the thermocouple wafer used to determine the difference between the set point and process values. The GPC over 1.5 s saturates to the 0.8 Å/cycle in the growth temperature range of 150 – 200 °C. However, the GPC of ALD BeO increases to source condensation at Tg below 150 °C and to DEB decomposition at Tg above 200 °C. Based on the GPC saturation behavior, the DEB feeding time and growth temperature are fixed at 1.8 s and 200 °C, respectively, in subsequent ALD of the BeO. The film thickness and density of as-grown BeO/4H-SiC determined via the XRR analysis correspond to 23. 5 nm and 2.65 g/cm3 for the 300 ALD cycle and 48.2 nm and 2.80 g/cm3 for the 600 ALD cycle (as shown in Figure S3). The thickness of BeO film based on ALD cycle is consistent with the GPC of 0.8 Å/cycle, which is same result of the GPC of BeO/Si. The densities of BeO films are similar to those reported by Lee et al.26 Figure 3 shows cross-sectional HR-TEM images and diffraction patterns of the as-grown ALD BeO on the 4H-SiC substrate. The high-quality crystalline BeO film is maintained over a wide range (Figure 3a). Selected region electron diffractions in Figure 3b indicate that the ALD BeO film is grown with the same crystal structure as 4H-SiC. In Figure 3c, the interplanar spacing (dspacing) of the BeO in the growth direction is 2.176 Å, and this corresponds to a d-spacing of the BeO (0002) plane in an inorganic crystal structure database (ICSD) card. The HR-TEM results confirm that the BeO (0001) film grows to a wurtzite-on-wurtzite structure without a tilt on a 4H-SiC (0001). In order to examine the crystallinity of BeO films for macroscopic region, highresolution XRD analyses are performed. Figure 4a shows theta-2theta scans for 4H-SiC (blue),


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24 nm-BeO/4H-SiC (black), and 48 nm-BeO/4H-SiC (red) samples. The 4H-SiC (0004) and BeO (0002) peaks are located at 35.52° and 41.20°, respectively. The average crystallite size (Dp) of the BeO film is obtained by using the Debye–Scherrer equation, Dp = (K × λ) / (β × cosθ) where K, λ, β, and θ correspond to the Scherrer constant (here 0.9), full-width-half-minimum (FWHM) of the XRD peak, Bragg angle, and x-ray wavelength (CuKα = 1.542 Å).27 The Dp values for 24 nm and 48 nm-BeO/4H-SiC are approximately 15.17 nm and 16.33 nm, respectively. The rocking curve (omega scan) is performed for 48 nm-BeO/4H-SiC to estimate the out-of-plane strain (as shown in Figure 4b). The FWHM and d-spacing of BeO (0002) are approximately 2.82° and 2.117 Å, which is shorter than the d-spacing of bulk BeO (2.189 Å). The results indicate that the compressive strain is applied to the BeO film in the out-of-plane direction. Figure 5 shows the IPE yield and threshold of BeO/4H-SiC measured in a high-vacuum environment to prevent chemical shift and light scattering due to air and moisture. A positive bias is applied to the top electrode. Thus, electron carriers are provided to the n-type 4H-SiC substrate. In Figure 5a, it is expected that the spectral thresholds of Φ1 and Φ3 correspond to the excitation of the electron from the conduction and VB edge of SiC, respectively. Specifically, Φ2 originates from the interface states distributed in the SiC bandgap. Given the image-force effects induced by external bias, photocarriers are emitted starting at a photon energy level lower than the intrinsic oxide-semiconductor barrier height.28 In order to observe the intrinsic band parameters (for a zero electric field), linear extrapolation is conducted to the spectral thresholds in Figure 5b. The IPE thresholds ΔΦ1, ΔΦ2, and ΔΦ3, were obtained as 2.28 ± 0.1 eV, 4.46 ± 0.1 eV, and 5.44 ± 0.1 eV, respectively. Specifically, ΔΦ1 corresponds to the CBO at the BeO/4HSiC interface, and this is lower than the reported CBO of SiO2 (2.7 eV).29 The bandgap of 4H-


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SiC is determined from ΔΦ3 ‒ ΔΦ1 = 3.16 ± 0.1 eV, and this is slightly narrower than the bulk value (3.26 eV) caused by the sp2(π)-bonded carbon cluster.30 Additionally, ΔΦ2 denotes the valence state of the π-bonded carbon cluster, and this is consistent with the prior IPE threshold for SiO2/a-C:H SiC (4.6 ± 0.1 eV).30 A graphite-like (G-L) state of 3.53 ± 0.1 eV is obtained after correcting ΦG-L for a zero electric field.31 The π-bonded and G-L carbon clusters can be attributed to carbon condensation on the 4H-SiC surface during the ohmic annealing process (1000 °C) and/or residual carbon in metal-organic precursors during the ALD process.32-33 The IPE onset feature at approximately 3.16 ± 0.1 eV photon energy represents the photocurrent of 4H-SiC. The UPS VB spectra of 4H-SiC, ALD BeO, ALD Al2O3, and PECVD SiO2 with a thickness of each oxide layer of 30 Å or less are shown in Figure 6. The linear extrapolation method is used to determine the valence band maximum (VBM) of each sample. The localized UPS data on the turn on point was shown in Figure S3. By using the method, the VBM position of as-cleaned 4HSiC is deduced as 2.43 ± 0.01 eV in Figure 6a, and this is similar to the reported values (2.6 eV for n-type 4H-SiC).34 The VBM values of oxide samples are 4.96 ± 0.01 eV for ALD BeO, 4.09 ± 0.01 eV for ALD Al2O3, and 5.03 ± 0.01 eV for PECVD SiO2. The VBO is calculated by comparing the energy difference between the VBM of oxides and SiC substrate. The obtained VBO values of BeO/SiC, Al2O3/SiC, and SiO2/SiC are 2.53 ± 0.01 eV, 1.66 ± 0.01 eV, and 2.60 ± 0.01 eV, respectively. The VBO values of Al2O3 and SiO2 are consistent with the results in extant studies as determined by VB edge spectra where VBO values of 1.63 eV (Al2O3) and 2.65 eV (SiO2) are reported.35-36 Therefore, the VBO of ALD BeO/SiC obtained in the study is also considered as reasonable.


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Figure 7 shows the REELS spectra for electron beam energies of 1 and 3 keV incident on 10 nm ALD-BeO on the 4H-SiC. Surface (EgS = 8.0 ± 0.05 eV) and inter bandgap (EgI = 8.3 ± 0.05 eV) energies are obtained via the linear extrapolation of the onset of inelastic energy loss. The aforementioned values are close to 7.7–7.9 eV and 7.0–8.0 eV as measured by using the O 1s energy loss spectra and REELS for ALD BeO films on Si, respectively.26, 37 A potential reason for the slightly larger bandgap in the study corresponds to the improved crystallinity of BeO on the 4H-SiC as opposed to that on the Si substrate. The relationship between crystallinity and bandgap is already reported in high-κ dielectrics. For example, the bandgap of an amorphous Al2O3 film is lower than that of a well-ordered bulk of Al2O3.38 The surface bandgap (8.0 eV) is collected by using a 1 keV beam (Figure 7a) and exhibits a reduction of approximately 0.3 eV when compared with the inter value (8.3 eV). The narrowing of the surface band gap can be caused by Be or O vacancies.37 Figure 7b shows the maximum loss peak at approximately 24.5 ± 0.05 eV, and this is consistent with previous REELS results for single-crystal BeO that are induced by the excitation of a bulk plasmon.39-41 The scheme of Figure 8a summarizes all the features of the band alignment between ALD BeO and 4H-SiC. The left side depicts the band structure of 4H-SiC, and the VBO, 4H-SiC bandgap, and CBO values corresponding to 2.53 ± 0.01 eV, 3.16 ± 0.1 eV, and 2.28 ± 0.1 eV, respectively, are determined via UPS and IPE. The center side shows that the graphitic and πbonded carbon clusters are located at the interface, which is located at 1.25 eV and 2.18 eV beneath the conduction band minimum of 4H-SiC. The right side indicates the full energy bandgap of ALD BeO wherein REELS is used to measure the bandgap at 8.3 ± 0.05 eV. The calculated bandgap of BeO (left side) is approximately 7.97 eV (as obtained from the summation of VBO (2.53 ± 0.01 eV), CBO (2.28 ± 0.1 eV), and SiC bandgap (3.16 ± 0.1 eV)). The


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difference between the calculated and REELS bandgap of BeO film is due to the error bars that occur between the analysis methods. The energy bandgap of ALD BeO (8.3 eV) clearly exceeds that of ALD Al2O3 (approximately 6.5–7 eV)42-43 although it is comparable to that of thermal SiO2 (approximately 9.0 eV).44-45 The energy distribution of interface defects were shown in Figure 8b. The sp2-bonded carbon clusters act as interface traps when valence electrons are emitted. The G-L cluster has an amphoteric action. For n-type SiC structure, the interface state is charged negatively by electron trapping. Hole trapping of the p-type SiC positively charges the interface.31

4. CONCLUSION In the study, single-crystalline BeO films epitaxially grown on 4H-SiC substrates were achieved via ALD. Specifically, DEB synthesized in-house was used as a precursor, and DI water was used as an oxidant. The growth rate of BeO corresponded to 0.8 Å/cycle in the growth temperature range of 150 – 200 °C. From the 2theta-theta scans of BeO/4H-SiC, only the wurtzite (0002) peak was detected without any other peaks, thereby demonstrating that BeO was epitaxially grown on 4H-SiC substrate. The omega scan indicated that the compressive strain remained in the epitaxially grown BeO films. We determined the energy bandgap, band alignment, and defect levels of BeO on 4H-SiC by using REELS, UPS, and IPE, respectively. The surface and interface bandgaps for ALD BeO were measured as 8.0 ± 0.05 eV and 8.3 ± 0.05 eV, respectively. The CBO of 2.28 ± 0.1 eV and VBO of 2.53 ± 0.01 eV were obtained across the BeO/4H-SiC interface. Interface defect levels of 3.53 ± 0.1 eV (graphitic carbon) and 4.46 ± 0.1 eV (π-bonded carbon) formed during the ohmic annealing process were also determined via IPE measurement. The results advanced the understanding of the electronic band


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structure of ALD BeO on 4H-SiC, and this is critical for the successful implementation of unique ALD BeO dielectrics in 4H-SiC MOSFETs and photonic applications.

ASSOCIATED CONTENT Supporting Information. Synthesis process and safety protocols of DEB, configuration of IPE setup, XRR spectra, and UPS data (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author contributions Seung Min Lee, Jung Hwan Yum, Eric Larsen, Sang Yeon Lee, and Hyungtak Seo performed experiments. All authors contributed to designing the experiments, analyzing the data, and writing the manuscript.

ACKNOWLEDGMENT Funding: The study was supported by the MIST (Ministry of Science and ICT), Korea, under the “ICT Consilience Creative Program” (IITP-2018-2017-0-01015) supervised by the IITP (Institute for Information & Communications Technology Promotion) and by the Korea Electric Power Corporation (under grant number3: R18XA06-03). The study was supported by the Future Semiconductor Device Technology Development Program (10048536) funded by the MOTIE


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(Ministry of Trade, Industry, & Energy) and the KSRC (Korea Semiconductor Research Consortium). Additionally, 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.

REFERENCES 1.

Choyke, W. J.; Matsunami, H.; Pensl, G., Silicon carbide: recent major advances.

Springer Science & Business Media: 2013. 2.

Baliga, B. J., Fundamentals of power semiconductor devices. Springer Science &

Business Media: 2010. 3.

Spitz, J.; Melloch, M.; Cooper, J.; Capano, M., 2.6 kV 4H-SiC lateral DMOSFETs. IEEE

Electron Device Letters 1998, 19 (4), 100-102. 4.

Phan, H.-P.; Dao, D. V.; Tanner, P.; Han, J.; Nguyen, N.-T.; Dimitrijev, S.; Walker, G.;

Wang, L.; Zhu, Y., Thickness dependence of the piezoresistive effect in p-type single crystalline 3C-SiC nanothin films. Journal of Materials Chemistry C 2014, 2 (35), 7176-7179. 5.

Sugita, T.; Hiramatsu, K.; Ikeda, S.; Matsumura, M., Fabrication of pores in a silicon

carbide wafer by electrochemical etching with a glassy-carbon needle electrode. ACS applied materials & interfaces 2013, 5 (7), 2580-2584. 6.

Castelletto, S.; Johnson, B.; Ivády, V.; Stavrias, N.; Umeda, T.; Gali, A.; Ohshima, T., A

silicon carbide room-temperature single-photon source. Nature materials 2014, 13 (2), 151. 7.

Yamada, S.; Song, B.-S.; Asano, T.; Noda, S., Silicon carbide-based photonic crystal

nanocavities for ultra-broadband operation from infrared to visible wavelengths. Applied Physics Letters 2011, 99 (20), 201102.


14

Page 15 of 29 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


8.

Saks, N.; Mani, S.; Agarwal, A., Interface trap profile near the band edges at the 4H-

SiC/SiO 2 interface. Applied Physics Letters 2000, 76 (16), 2250-2252. 9.

Afanas’ ev, V.; Stesmans, A.; Ciobanu, F.; Pensl, G.; Cheong, K.; Dimitrijev, S.,

Mechanisms responsible for improvement of 4 H–SiC/SiO 2 interface properties by nitridation. Applied Physics Letters 2003, 82 (4), 568-570. 10.

Fukuda, K.; Suzuki, S.; Tanaka, T.; Arai, K., Reduction of interface-state density in 4 H–

SiC n-type metal–oxide–semiconductor structures using high-temperature hydrogen annealing. Applied Physics Letters 2000, 76 (12), 1585-1587. 11.

Rozen, J.; Ahyi, A. C.; Zhu, X.; Williams, J. R.; Feldman, L. C., Scaling between channel

mobility and interface state density in SiC MOSFETs. IEEE Transactions on Electron Devices 2011, 58 (11), 3808-3811. 12.

Afanas’ev, V.; Ciobanu, F.; Dimitrijev, S.; Pensl, G.; Stesmans, A., Band alignment and

defect states at SiC/oxide interfaces. Journal of Physics: Condensed Matter 2004, 16 (17), S1839. 13.

Lipkin, L. A.; Palmour, J. W., Insulator investigation on SiC for improved reliability.

IEEE transactions on Electron Devices 1999, 46 (3), 525-532. 14.

Tanner, C. M.; Toney, M. F.; Lu, J.; Blom, H.-O.; Sawkar-Mathur, M.; Tafesse, M. A.;

Chang, J. P., Engineering epitaxial γ-Al 2 O 3 gate dielectric films on 4H-SiC. Journal of Applied Physics 2007, 102 (10), 104112. 15.

Lee, S. M.; Yum, J. H.; Larsen, E. S.; Lee, W. C.; Kim, S. K.; Bielawski, C. W.; Oh, J.,

Advanced Silicon-on-Insulator: Crystalline Silicon on Atomic Layer Deposited Beryllium Oxide. Scientific reports 2017, 7 (1), 13205.


15


16.

Page 16 of 29

Bosak, A.; Schmalzl, K.; Krisch, M.; van Beek, W.; Kolobanov, V., Lattice dynamics of

beryllium oxide: Inelastic x-ray scattering and ab initio calculations. Physical Review B 2008, 77 (22), 224303. 17.

Morell, G.; Pérez, W.; Ching-Prado, E.; Katiyar, R., Anharmonic interactions in

beryllium oxide. Physical Review B 1996, 53 (9), 5388. 18.

Slack, G. A.; Austerman, S., Thermal conductivity of BeO single crystals. Journal of

Applied Physics 1971, 42 (12), 4713-4717. 19.

Lee, S. M.; Jang, Y.; Jung, J.; Yum, J. H.; Larsen, E. S.; Bielawski, C. W.; Wang, W.;

Ryou, J.-H.; Kim, H.-S.; Cha, H.-Y., Atomic-layer deposition of crystalline BeO on SiC. Applied Surface Science 2019, 469, 634-640. 20.

Hauf, M.; Grotz, B.; Naydenov, B.; Dankerl, M.; Pezzagna, S.; Meijer, J.; Jelezko, F.;

Wrachtrup, J.; Stutzmann, M.; Reinhard, F., Chemical control of the charge state of nitrogenvacancy centers in diamond. Physical Review B 2011, 83 (8), 081304. 21.

Polking, M. J.; Dibos, A. M.; de Leon, N. P.; Park, H., Improving Defect‐Based Quantum

Emitters in Silicon Carbide via Inorganic Passivation. Advanced Materials 2018, 30 (4), 1704543. 22.

http://www.ncdtech.co.kr/2018/theme/basic/subpages/sub020503.php

LucidaTM

M

series ALD. 23.

He, Y.-d.; Luo, J.-s.; Li, J.; Meng, L.-b.; Luo, B.-c.; Zhang, J.-q.; Zeng, Y.; Wu, W.-d.,

Composition and microstructure of beryllium carbide films prepared by thermal MOCVD. Fusion Engineering and Design 2016, 103, 118-124.


16

Page 17 of 29 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


24.

King, S. W.; Nemanich, R. J.; Davisa, R. F., Wet Chemical Processing of (0001) Si

6H‐SiC Hydrophobic and Hydrophilic Surfaces. Journal of The Electrochemical Society 1999, 146 (5), 1910-1917. 25.

King, S. W.; Tanaka, S.; Davis, R. F.; Nemanich, R. J., Hydrogen desorption from

hydrogen fluoride and remote hydrogen plasma cleaned silicon carbide (0001) surfaces. Journal of Vacuum Science & Technology A 2015, 33 (5), 05E105. 26.

Lee, W. C.; Cho, C. J.; Kim, S.; Larsen, E. S.; Yum, J. H.; Bielawski, C. W.; Hwang, C.

S.; Kim, S. K., Growth and Characterization of BeO Thin Films Grown by Atomic Layer Deposition Using H2O and O3 as Oxygen Sources. The Journal of Physical Chemistry C 2017, 121 (32), 17498-17504. 27.

Klug, H. P.; Alexander, L. E., X-ray diffraction procedures: for polycrystalline and

amorphous materials. X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials, 2nd Edition, by Harold P. Klug, Leroy E. Alexander, pp. 992. ISBN 0-471-49369-4. Wiley-VCH, May 1974. 1974, 992. 28.

Tugulea, A.; Dascǎlu, D., The image‐force effect at a metal‐semiconductor contact with

an interfacial insulator layer. Journal of applied physics 1984, 56 (10), 2823-2831. 29.

Afanas’ ev, V.; Bassler, M.; Pensl, G.; Schulz, M.; Stein von Kamienski, E., Band offsets

and electronic structure of SiC/SiO2 interfaces. Journal of Applied Physics 1996, 79 (6), 31083114. 30.

Afanas’ev, V.; Stesmans, A.; Andersson, M., Electron states and microstructure of thin a-

C: H layers. Physical Review B 1996, 54 (15), 10820. 31.

Afanasev, V.; Bassler, M.; Pensl, G.; Schulz, M., Intrinsic SiC/SiO2 interface states.

physica status solidi (a) 1997, 162 (1), 321-337.


17


32.

Page 18 of 29

Muehlhoff, L.; Choyke, W.; Bozack, M.; Yates Jr, J. T., Comparative electron

spectroscopic studies of surface segregation on SiC (0001) and SiC (0001). Journal of Applied Physics 1986, 60 (8), 2842-2853. 33.

Gaskins, J. T.; Hopkins, P. E.; Merrill, D. R.; Bauers, S. R.; Hadland, E.; Johnson, D. C.;

Koh, D.; Yum, J. H.; Banerjee, S.; Nordell, B. J., Investigation and Review of the Thermal, Mechanical, Electrical, Optical, and Structural Properties of Atomic Layer Deposited High-k Dielectrics: Beryllium Oxide, Aluminum Oxide, Hafnium Oxide, and Aluminum Nitride. ECS Journal of Solid State Science and Technology 2017, 6 (10), N189-N208. 34.

O’Brien, M.; Koitzsch, C.; Nemanich, R., Photoemission of the SiO 2–SiC

heterointerface. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 2000, 18 (3), 1776-1784. 35.

Kim, D.-K.; Kang, Y.-S.; Jeong, K.-S.; Kang, H.-K.; Cho, S. W.; Chung, K.-B.; Kim, H.;

Cho, M.-H., Effects of spontaneous nitrogen incorporation by a 4 H-SiC (0001) surface caused by plasma nitridation. Journal of Materials Chemistry C 2015, 3 (19), 5078-5088. 36.

Hosoi, T.; Kirino, T.; Mitani, S.; Nakano, Y.; Nakamura, T.; Shimura, T.; Watanabe, H.,

Relationship between interface property and energy band alignment of thermally grown SiO2 on 4H-SiC (0001). Current Applied Physics 2012, 12, S79-S82. 37.

Koh, D.; Yum, J.-H.; Banerjee, S. K.; Hudnall, T. W.; Bielawski, C.; Lanford, W. A.;

French, B. L.; French, M.; Henry, P.; Li, H., Investigation of atomic layer deposited beryllium oxide material properties for high-k dielectric applications. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 2014, 32 (3), 03D117.


18

Page 19 of 29 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


38.

Costina, I.; Franchy, R., Band gap of amorphous and well-ordered Al 2 O 3 on Ni 3 Al

(100). Applied physics letters 2001, 78 (26), 4139-4141. 39.

Roessler, D.; Walker, W.; Loh, E., Electronic spectrum of crystalline beryllium oxide.

Journal of Physics and Chemistry of Solids 1969, 30 (1), 157-167. 40.

Sashin, V.; Bolorizadeh, M.; Kheifets, A.; Ford, M., Electronic band structure of

beryllium oxide. Journal of Physics: Condensed Matter 2003, 15 (21), 3567. 41.

Jenkins, L. H.; Zehner, D. M.; Chung, M., Characteristic energy gain and loss, double

ionization, and ionization loss events in Be and BeO secondary electron spectra. Surface Science 1973, 38 (2), 327-340. 42.

Zhang, F.; Sun, G.; Zheng, L.; Liu, S.; Liu, B.; Dong, L.; Wang, L.; Zhao, W.; Liu, X.;

Yan, G., Interfacial study and energy-band alignment of annealed Al2O3 films prepared by atomic layer deposition on 4H-SiC. Journal of Applied Physics 2013, 113 (4), 044112. 43.

Wang, Q.; Cheng, X.; Zheng, L.; Shen, L.; Zhang, D.; Gu, Z.; Qian, R.; Cao, D.; Yu, Y.,

Influence of LaSiOx passivation interlayer on band alignment between PEALD-Al2O3 and 4HSiC determined by X-ray photoelectron spectroscopy. Applied Surface Science 2018, 428, 1-6. 44.

Watanabe, H.; Kirino, T.; Kagei, Y.; Harries, J.; Yoshigoe, A.; Teraoka, Y.; Mitani, S.;

Nakano, Y.; Nakamura, T.; Hosoi, T. In Energy band structure of SiO2/4H-SiC interfaces and its modulation induced by intrinsic and extrinsic interface charge transfer, Materials Science Forum, Trans Tech Publ: 2011; pp 386-389. 45.

Li, W.; Zhao, J.; Wang, D., An amorphous SiO2/4H-SiC (0001) interface: Band offsets

and accurate charge transition levels of typical defects. Solid State Communications 2015, 205, 28-32.


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TABLE OF CONTENTS


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FIGURES

Figure 1. Schematic illustration of a growth cycle of epitaxial BeO film on 4H-SiC by ALD.


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Figure 2. Growth per cycle (GPC) of the ALD BeO as a function of (a) DEB feeding time and (b) growth temperature. The GPC over 1.5 s of DEB feeding time saturates to the 0.8 Å/cycle. ALD window is in the range growth temperature of 150 to 200 °C.


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Figure 3. (a) Cross-sectional HR-TEM images of ALD BeO on 4H-SiC at the 10-nm scale. (b) Electron diffraction of the selected region; BeO, interface, and 4H-SiC. (c) d-spacing of BeO in out-of-plane [0002] and in-plane [1100] directions.


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Figure 4. (a) theta-2theta scan spectra as a function of film thickness and (b) rocking curve for the BeO on 4H-SiC.


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Figure 5. (a) IPE yield as a function of photon energy and (b) IPE spectral threshold as a function of the electric field strength in oxide. To excite electrons in the Au(10nm)/BeO(30nm)/4H-SiC structure, the photon energy in the range of 2 to 6 eV was applied to the sample at electrical bias of 1.0 V (black), 1.4 V (red), and 1.7 V (blue). The inset shows a schematic band diagram of the BeO/4H-SiC interface.


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Figure 6. Full range UPS VB spectra of (a) as-cleaned 4H-SiC, (b) ALD BeO/4H-SiC, (c) ALD Al2O3/4H-SiC, and (d) PECVD SiO2/4H-SiC. The red lines denote a linear extrapolation of the leading UPS edge to the baseline.


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Figure 7. REELS spectra obtained for ALD BeO on 4H-SiC at primary electron energies (Ep) of (a) surface: 1.0 keV and (b) inter: 3.0 keV.


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Figure 8. (a) energy band diagram and (b) energy distribution of interface defect at the ALDBeO/4H-SiC interface. The numerical values of Eg for BeO, CBO, and VBO obtained by using REELS, IPE, and UPS are indicated.


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Crystalline BeO grown on 4H-SiC via atomic layer deposition: Band

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