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Atomic layer deposition of single-crystalline BeO epitaxially grown on GaN substrates Seung Min Lee, Jung Hwan Yum, Seonno Yoon, Eric S. Larsen, Woo Chul Lee, Seong Keun Kim, Shahab Shervin, Weijie Wang, Jae-Hyun Ryou, Christopher W. Bielawski, and Jungwoo Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13487 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Atomic layer deposition of single-crystalline BeO epitaxially grown on GaN substrates Seung Min Lee1,2, Jung Hwan Yum3,4, Seonno Yoon1,2, Eric S. Larsen3,4, Woo Chul Lee5, Seong Keun Kim5, Shahab Shervin6, Weijie Wang6, Jae-Hyun Ryou6,7, Christopher W. Bielawski3,4,8 and Jungwoo Oh1,2* 1

School of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea 2

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Yonsei Institute of Convergence Technology, Incheon 21983, Republic of Korea

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

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Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea 5

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul, 20792, Republic of Korea

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Department of Mechanical Engineering, University of Houston, Houston, Texas 77204-4006, USA 7

Materials Science and Engineering Program and Texas Center for Superconductivity at UH (TcSUH), University of Houston, Houston, Texas 77204, USA 8

Department of Energy Engineering, UNIST, Ulsan 44919, Republic of Korea

KEYWORDS: Beryllium oxide, Gallium nitride, Atomic layer deposition, Domain-matching epitaxy, Power devices

ACS Paragon Plus Environment

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ABSTRACT: We have grown a single-crystal beryllium oxide (BeO) thin film on a gallium nitride (GaN) substrate by atomic layer deposition (ALD) for the first time. BeO has a higher thermal conductivity, bandgap energy, and dielectric constant than SiO2. As an electrical insulator, diamond is the only material on Earth whose thermal conductivity exceeds that of BeO. Despite these advantages, there is no chemical vapor deposition technique for BeO thin film deposition, and thus, it is not used in nanoscale semiconductor device processing. In this study, the BeO thin films grown on a GaN substrate with a single crystal showed excellent interface and thermal stability. Transmission electron microscopies showed clear diffraction patterns, and the Raman shifts associated with soft phonon modes verified the high thermal conductivity. The x-ray scan confirmed the out-of-plane single-crystal growth direction and the in-plane 6-fold symmetrical wurtzite structure. Single-crystalline BeO was grown on GaN despite the large lattice mismatch, which suggested a model that accommodated the strain of hexagonal-on-hexagonal epitaxy with 5/6 and 6/7 domain matching. BeO has a good dielectric constant and good thermal conductivity, bandgap energy, and single-crystal characteristics, so it is suitable for the gate dielectric of power semiconductor devices. The capacitance–voltage (C– V) results of BeO on a GaN metal-oxide-semiconductor exhibited low frequency dispersion, hysteresis, and interface defect density.


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1. INTRODUCTION Gallium nitride (GaN)-based high-electron mobility transistors (HEMTs) are of great interest for high-frequency, high-power, high-voltage switching, and low-noise applications, because they can provide a low gate leakage current and a large gate swing.1-2 Si-based power devices are physically unable to meet these demands due to the narrow bandgap and low breakdown field. GaN-based heterojunctions are expected to replace Si devices in future power applications owing to their wide bandgap and high breakdown field.3-4 However, conventional GaN/AlGaN quantum-well HEMTs have high gate leakage currents mainly due to the limited Schottky barrier height, resulting in a low forward gate bias swing and poor on/off current ratio.5-6 Metal-oxidesemiconductor (MOS) HEMTs with high-k dielectrics between the gate metals and GaN/AlGaN channels were proposed.7-8 High-k dielectrics, such as Al2O3, HfO2, ZrO2, and Sc2O3, have been used for GaN MOS-HEMTs.9-12 These exotic oxides often form electrical traps due to the mismatch with the GaN interface, which can cause problems, such as decomposition, diffusion, and amorphous crystallization due to poor thermal stability. Native oxides, such as Ga2O3, also tend to form between high-k gate dielectrics and GaN layers, which can shift the threshold voltages of GaN MOS-HEMTs.13-14 For III-V and III-N MOS devices, it was reported that high-k dielectrics by atomic layer deposition (ALD) were usually amorphous or polycrystalline; thus, the high density of dangling bonds at the oxide/semiconductor interface cannot be avoided.15-16 Recently, single-crystal lanthanum oxide (La1.8Y0.2O3) and magnesium calcium oxide (MgxCa1-xO) were epitaxially grown via ALD on gallium arsenide (GaAs) (111) and GaN (001) substrates, respectively.17-18 In both cases, the interface defect densities were reduced by the decreased number of dangling bonds. Epitaxial growth of gate dielectrics on semiconductors naturally provides an effective


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passivation of dangling bonds or grain boundaries, so the desired features of low interface trap density, low gate leakage current, and high mobility can be achieved.19-20 MgxCa1-xO has a rocksalt structure, and its coordination geometry is octahedral, while GaN has a wurtzite structure, and its coordination geometry is tetrahedral. Because of the lattice mismatch of MgO and CaO with GaN, precise composition control of MgxCa1-xO is required to match their lattices. Stoichiometric tuning in ALD is usually complicated and not thermally stable.21 Beryllium oxide (BeO) has the second-highest thermal conductivity (330 W/K·m)22 among insulating materials after diamond. BeO is known as the most thermally stable dielectric, based on its Gibbs free energy of formation. Its high thermal conductivity suggests a smaller number of soft phonon modes compared with other high-k dielectrics, primarily owing to the similarity in size between the Be and O atoms.23-24 Soft phonon modes in gate dielectrics interfere with the movement of channel carriers, degrading the carrier mobility in Si and III-V metal-oxidesemiconductor field-effect transistors (MOSFETs). Thus, a small number of soft phonons is preferred in high-k gate dielectrics.25 The dielectric constant of BeO (6.8) is relatively high among the high-k dielectrics,26 and its bandgap energy (10.6 eV) is the largest among all known high-k dielectrics.27-28 Thus, a large dielectric constant and bandgap energy can potentially reduce the gate leakage current, improving the gate controllability of field-effect transistors.29 BeO and GaN crystallize in the hexagonal wurtzite structure, and their coordination geometry is tetrahedral about cations and anions. Binary BeO does not require complicated stoichiometric tuning to be grown on GaN using ALD. Despite these advantages, BeO is not used in nanoscale semiconductor front-end processes, because BeO is mostly produced as an amorphous powder and sintered into larger shapes.


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In this work, we demonstrate the epitaxial growth of BeO on GaN (001) via ALD for the first time. A high-quality Be(CH3)2 precursor was used for ALD BeO processing. The GaN/AlGaN heterostructures for power transistors were used as substrates for the epitaxial growth of BeO. Transmission electron microscopy (TEM), selective area diffraction (SAD), and X-ray diffraction (XRD) revealed that BeO/GaN hexagonal-on-hexagonal heteroepitaxy was achieved in the [001] growth direction. An atomically sharp interface between the BeO and GaN surfaces was observed. The thermal stability was measured by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy confirmed the identical structural characteristics of thin film BeO and bulk BeO. In addition, the capacitance–voltage (C–V) curves of BeO-gated MOS capacitors suggested the low interface defect density of the BeO/GaN interface. Therefore, the single crystallinity of ALD BeO on GaN and its superior intrinsic properties, such as its high thermal conductivity, large energy bandgap, and high dielectric constant, make the ALD BeO an excellent candidate for power device and other nanoscale device applications.

2. EXPERIMENTAL SECTION Atomic layer deposition of BeO. After cleaning a GaN substrate surface with an aqueous solution of HCl (37%) : DI water = 1 : 1 for 1 min, BeO was deposited on GaN (001) at 250 °C employing ALD using dimethylberyllium as the precursor (see Supporting Information Figures S-1 and S-2). BeO was deposited using an Atomic Classic ALD instrument (load-lock Module, CN1) with a reaction chamber temperature of 250 °C, precursor sublimation temperature of 110 °C, Be pulse of 3 s, H2O pulse of 1 s, Be purge of 20 s, H2O purge of 30 s, and chamber pressure of 0.8 Torr by performing 200 cycles at 0.8 Å/cycle.


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Characterization. TEM measurements: TEM images were obtained using a JEOL JEM-ARM 200F system. A Schottky field-emission electron beam was generated at 80–200 kV (magnification: 50 to 2,000,000×, resolution: 0.1 nm). A focused ion beam (JIB-4601F) was used on electronically transparent samples that were obtained using a Disco 321 DAT dicing saw. XPS measurements: XPS was performed using a Thermo Fisher Scientific K-ALPHA system. The X-ray source was generated using monochromatic Al kα 1,486.6 eV irradiation (spot size: 30–400 µm, energy resolution: 0.85 eV). XRD measurements: XRD analyses were performed using Rigaku SmartLab and Bruker D8 Discover systems. The X-ray tube of the diffractometer was operated at 20–60 kV and 60 mA, and data were obtained using CuKα X-rays (scan axis: 2θ, range: 20–80°, step width: 0.02°, scan speed/duration: 1.00°/min). Raman spectroscopy: All spectra were collected using a Horiba-Lab Ram ARAMIS Raman microscope equipped with a 325 nm He-Cd laser, (spectral range: 350–1,000 cm-1, grating: 2,400 gr/mm) and motorized stage. The instrument was operated using the Thermo Scientific OMNIC 8 software suite. OMNIC™ Atlµs™ mapping software was used to collect and analyze the data.

3. RESULTS AND DISCCUSION Figure 1 shows cross-sectional and plan-view TEM images of epitaxial BeO thin films grown on GaN/AlGaN substrates. The direction of the incident electron beam was parallel to the underlying [1-10] directions for both GaN and BeO. Two layers of BeO and GaN/AlGaN are clearly identified in Figures 1-a1 and a2. The single crystallinity of 16 nm BeO was maintained over a wide range, and the interface between BeO and GaN was sharp. The 200 ALD cycle corresponded to a deposition rate of 0.8 Å/cycle. The inter-planar spacing (d-spacing) of the BeO (002) in the growth direction was 2.196 Å in the lattice image (Figure 1-a3), which corresponds


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to the d-spacing of 2.189 Å of the (002) plane of BeO on the Joint Committee on Powder Diffraction Standards (JCPDS) card. Thus, BeO [001] was confirmed to be the growth direction of as-grown ALD BeO on the GaN (001) substrate. The SAD patterns clearly show that the ALD BeO thin film was grown on a GaN substrate with a wurtzite crystal structure (Figure 1-a4). To study the crystal matching in the basal planes between BeO and GaN, a plan-view TEM analysis was performed. In Figure 1-b1, the left area is only BeO (GaN is fully removed during sample preparation) and the right area is BeO/GaN. The insets in Figures 1-b2 and b3 show SAD patterns of BeO and BeO/GaN, respectively. These images show that ALD BeO was grown on GaN as a hexagon-on-hexagon without any tilt, which is further evidenced by XRD measurements. The thermal stability of the BeO/GaN interface was studied using XPS. The interfaces between high-k gate dielectrics and GaN substrates tend to leave native oxides, such as Ga2O3, during ALD processes.30 However, in the case of the 2 nm BeO/GaN interface, the Ga 3d data for ascleaned GaN and as-grown BeO on the GaN exhibited negligible intensities of Ga-O bonds (20.9 eV), as shown in Figures 2a and 2b. The suppression of native oxide is associated with the selfcleaning effect of the Be(CH3)2 precursor that absorbs oxygen from native oxides, such as Ga-O, As-O, and In-O, because of its high reactivity.31-32 After thermal treatment at 500°C followed by 800°C (1 min) in a nitrogen atmosphere, there was still no increase of the native Ga2O3 signal. This result indicates the high thermal stability of the ALD BeO and is in agreement with a previous report.33 The crystallinity and thermal conductivity of the epitaxial BeO were indirectly assessed by Raman spectroscopy (Figure 3), with a capability of mapping with micrometer resolution.34 The method utilizes a focused laser beam to thermally excite a sample, which then undergoes Raman


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scattering at the excitation spot. The temperature dependence of the phonon energies is used as a local thermometer. It has been reported that small Raman shifts are associated with soft phonon modes, and soft phonon modes cause poor thermal conductivity.35 Spectra recorded for the molecular-beam epitaxial (MBE) BeO (001) film were used as a reference.36 The Raman shift of 753 cm-1 of the ALD BeO was similar to both experimentally and theoretically obtained ones of 762.6 cm-1 and 725 cm-1 , respectively,28 indicating that the ALD BeO film’s material properties are similar with those of bulk BeO. The Raman shift is result of the phonon frequencies measured at the zone center. Thermal conductivity is determined by a heat capacity and a mean free path of phonon (inversely proportional to heat capacity and proportional to mean free path of phonon). Furthermore, the heat capacity is inversely proportional to Debye frequency (the average of summed phonon frequencies). If the main phonon frequencies (maximum intensity value of the Raman shifts) are high, the Debye frequency increases and heat capacity decreases. The mean free path of phonon is inversely proportional to the radius of the atom. In case of BeO, the mean free path of phonon is long due to small atomic radius of Be and O.37-38 The Raman shift for the epitaxial BeO was greater than those reported for GaN (567.28 cm-1), and for other high-k dielectrics, such as Al2O3 (644 cm-1), HfO2 (674 cm-1), and ZrO2 (640 cm-1), indicating that BeO has a higher thermal conductivity than other oxides.39 The heat energy generated by the movement of the channel carrier can be quickly dispersed owing to the high thermal conductivity of the epitaxial ALD BeO. Thus, ALD BeO can provide a way to solve the self-heating problem, which is one of the major issues that arise in power devices. High-resolution XRD was used to accurately assess the crystal orientation and lattice spacing. A 2-Theta-Omega scan was performed on the BeO/GaN sample to characterize the out-of-plane orientation of BeO, as shown in Figure 4a. The main peaks with significant intensity are the


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diffractions of the GaN (002), GaN (004), and Si (111) substrates. The other peaks are the diffractions of the superlattice composed of several layers to improve the structural and electrical properties of the GaN/AlGaN heterostructure on Si substrates for power transistors. BeO (002) exhibited a low diffraction intensity due to its relatively low thickness, but it was clearly detected at the 2-Theta angle of 41.22°. This is almost identical to that of 41.206° in the JCPDS reference. No other peaks having this diffraction intensity were detected. This reflects the fact that BeO grew as a single crystal in the (002) plane. The XRD phi scans for the asymmetric plane of (102) for the BeO film and GaN substrate are shown in Figure 4b. The six separate peaks of BeO repeated every 60° confirm that the BeO grains were well aligned along the a-axis of the wurtzite structure. The lower intensity of the BeO peak is related to the BeO layer being thinner (16-nm) than that of the GaN substrate. Note that the perfect overlap in the peak positions of BeO and GaN in the phi scan of the (102) planes shows that the (100) m-planes of BeO and GaN were aligned, not rotated to reduce the strain despite the large lattice mismatch, which also agrees with the SAD analysis in TEM (see Figure 1-a4). The XRD analysis indicates that the BeO is an epitaxially grown single-crystalline material on GaN. The BeO (002) reflection was slightly shifted from the BeO reference peak only by 0.014°, and the lattice constant (c = 4.39 Å) calculated from the measured d-spacing was nearly identical to the theoretical one (c = 4.38 Å). The results indicate that the epitaxial BeO film has weak strain, which is consistent with the TEM observations. The rocking curve (Omega scan) and asymmetric rotational scan (phi scan) were used to estimate the out-of-plane tilt and in-plane alignment of the BeO crystal, respectively. The full-width-at-half-maximum (FWHM) of the rocking curve for the BeO (002) plane was ~1.9 degrees, which showed minimal spread in the out-of-plane tilt of such a thin film.


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The atomic configurations of the BeO (001) and GaN (001) planes are shown in Figures 5a and 5b, respectively. BeO (001) exhibits a quasi-two-dimensional hexagonal structure.40 To validate our model, the BeO (001) plane was superimposed on the GaN (001) plane by preparing 2D overlays of the respective atomic positions in Figures 5c and 5d. BeO was structurally matched with GaN, as the two had the same wurtzite crystal structure. Epitaxy is the result of a match between the atoms of the deposited film and the metal ion sites of the seed plane. This matching is not necessarily limited to a one-to-one correspondence between the sites in the two lattices.4142

It has been reported that films on substrates that are epitaxially grown in the form of single

crystals by domain-matching epitaxy (DME) exhibit large lattice mismatches (> 7–8%), where the integer multiples of the lattice constants match at the film/substrate interface.43 The fact that the lattice mismatch between BeO (aBeO = 2.69 Å) and GaN (aGaN = 3.19 Å) was as high as 18.2% but BeO on GaN still grew as a single crystal implies that BeO grew in the form of DME on GaN. When the 6 lattice planes of BeO and the 5 lattice planes of GaN were repeated in 6/5 domains, the mismatch between the two domains was greatly reduced to 1.5%. The lengths of the 6/5 domains were 16.19 Å / 15.95 Å and 28.04 Å / 27.63 Å in the vertical and horizontal directions, respectively. Therefore, the compressive strain remained in the in-plane BeO of the 6/5 domain matching. When BeO and GaN were repeated in 7/6 domains, the mismatch between the two domains was further reduced to 1.3%. The length of the two domains was 18. 89 Å / 19.13 Å and 32.71 Å / 33.14 Å in the vertical and horizontal directions, respectively. Therefore, the tensile strain remained in the in-plane BeO of the 7/6 domain matching (see Supporting Information Figures S-3). MOS capacitors with BeO gate dielectrics were fabricated on GaN/AlGaN HEMT substrates. The heterostructures consisted of a 2-nm GaN cap layer, a 20-nm AlxGa1-x N (x = 0.25) barrier, a


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300-nm GaN, and Si (111) substrates. The schematic in Figure 6 (a) represents the capacitor structure used to measure the C–V characteristics in the frequency range of 30 kHz–1 MHz. The C–V curves in Figure 6 (b) show that small frequency dispersion and hysteresis were observed, which indicates that the interface trap density may have been reasonably low, resulting from the efficient passivation of GaN dangling bonds with crystalline BeO. This result is well supported by the theoretical prediction that BeO is an efficient gate dielectric because of its high binding energy and large bandgap energy.44 A 8.5-nm effective oxide thickness (EOT) was decoupled from the 16-nm BeO and GaN/AlGaN heterostructure, which corresponded to the BeO dielectric constant of 7.3.

4. CONCLUSION In summary, we demonstrated the growth of high-quality epitaxial BeO (001) on GaN (001) by ALD for the first time. The crystallographic relationship of BeO-on-GaN was examined using TEM, SAD, and XRD, which supported 5/6 and 6/7 DME in the atomic configuration. The thermal and structural stabilities of the BeO were found to be high through XPS and Raman analysis. Moreover, the BeO dielectric quality at the interface was evaluated using C–V measurements, which revealed that the interface trap density was low. This was attributed to the small number of dangling bonds, the self-cleaning effect, and the superior thermal and structural stabilities of the ALD BeO. Thus, single-crystalline ALD BeO is a promising material that can be applied to the isolation and passivation of nanoscale power transistors that require high electron mobility and fast heat dissipation in high electric fields.

ASSOCIATED CONTENT


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Supporting Information. Synthesis process of ALD precursor and domain matching epitaxy (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Seung Min Lee, Jung Hwan Yum, Eric Larsen, Woo Chul Lee, Seong Keun Kim carried out the experiments. All the authors contributed for designing the experiments, analyzing the data, and writing the manuscript.

ACKNOWLEDGMENT This research was supported by the Ministry of Science and ICT (MSIT), Korea, under the ICT Consilience Creative program (IITP-2017-2017-0-01015) supervised by the IITP (Institute for Information & communications Technology Promotion). It was also supported by the Future Semiconductor Device Technology Development Program (10044735, 10048536), which is funded by the Ministry of Trade, Industry & Energy and the Korea Semiconductor Research Consortium. We are indebted to Gong Gu and Lifen Wang for their insightful discussions and suggestions. 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. The work at the University of Houston was


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supported by the IT R&D program of MOTIE/KEIT (Grant No. 10048933, Development of epitaxial structure design and epitaxial growth system for high-voltage power semiconductors). J.H.R. also acknowledges partial support from the Texas Center for Superconductivity at the University of Houston (TcSUH).

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Fig. 1. TEM images of (a) cross-sectional and (b) plan views of epitaxial BeO thin films on GaN/AlGaN substrates. High-quality crystalline BeO (001) was grown on GaN (001) via vaporphase ALD processes. SAD patterns confirm crystallization of BeO and GaN in hexagonal wurtzite structure.


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Fig. 2. Ga 3d core-level spectra of (a) as-cleaned GaN, (b) as-grown ALD BeO, (c) ALD BeO after 1 min of annealing at 500 °C, and (d) after 1 min of annealing at 800 °C. Interfacial native oxides, oxides (Ga2O3) that typically form after ALD high-k dielectric deposition, were hardly detected even after subsequent heat treatment.


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Fig. 3. Raman shifts of epitaxial BeO thin films on GaN/AlGaN substrates via vapor-phase ALD processes. The Raman shift of ALD BeO was greater than those of GaN (567.28 cm-1), and other high-k dielectrics, indicating a higher thermal conductivity of BeO.


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Fig. 4. High-resolution XRD of BeO-on-GaN (a) 2-Theta-Omega scan showing diffractions of GaN (002), GaN (004), and superlattice. BeO (002) is clearly detected at 41.22°. (b) Rocking curve (Omega scan) and asymmetric rotational scan (phi scan) for asymmetric plane of (102) of BeO-on-GaN.


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Fig. 5. Atomic configurations of (a) BeO (001) and (b) GaN (001) planes. (c) Overlays of atomic positions in two dimensions and (d) three dimensions, showing DME of Be-on-GaN with 6/5 and 7/6 repetition.


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Fig. 6. (a) Schematic of MOS capacitors with BeO gate dielectrics fabricated on AlGaN/GaN HEMT substrates. (b) C–V curves in frequency range of 30 kHz–1 MHz. A 8.5-nm EOT was decoupled from the 16-nm BeO and GaN/AlGaN heterostructure, which corresponded to a high dielectric constant of 7.3.


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