Elsevier

Applied Surface Science

Volume 469, 1 March 2019, Pages 634-640
Applied Surface Science

Full Length Article
Atomic-layer deposition of crystalline BeO on SiC

https://doi.org/10.1016/j.apsusc.2018.09.239Get rights and content

Highlights:

  • Single-crystalline BeO films are epitaxially grown on 4H-SiC substrates by ALD.

  • Domain structures of BeO/4H-SiC (8/7 and 9/8) have residual mismatches of 0.023 nm.

  • Bandgap (10.6 eV), permittivity (6.9) and thermal conductivity (330 W/m-K) of BeO are suitable for power transistor.

  • Interface trap density of ALD-BeO film is about 6 × 1010 cm−2 eV−1 at Ec-Et = 0.6 eV.

Abstract

For the first time, an epitaxial beryllium oxide (BeO) film was grown on 4H silicon carbide (4H-SiC) by atomic layer deposition (ALD) at a low temperature of 250 °C. The BeO film had a large lattice mismatch with the substrate (>7–8%), but it was successfully grown to a single crystal by domain-matching epitaxy (DME). The bandgap energy, dielectric constant, and thermal conductivity properties of crystalline BeO are suitable for power transistors that require low leakage currents and fast heat dissipation in high electric fields. Physical characterization confirmed the single-crystalline BeO (0 0 2). Raman analysis showed that the E1 and A1 phonon modes of ALD BeO were intermixed with the E2 and A1 phonon modes of SiC, resulting in a significant increase in phonon intensity. After heat treatment at a high temperature, a small amount of SiO2 interfacial oxide was formed but the stoichiometry of BeO was maintained. From the capacitance-voltage (C-V) curves, we obtained a dielectric constant of 6.9 and calculated a low interface trap density of 6 × 1010 cm−2·eV−1 using the Terman method at Ec-Et = 0.6 eV. The high bandgap, thermal conductivity, and excellent crystallinity reduced the dangling bonds at the interface of BeO-on-SiC.

Introduction

Beryllium oxide (BeO) has a wurtzite (P63mc) crystal structure and exhibits excellent physicochemical properties, such as high bandgap energy (10.6 eV, direct) [1], melting point (2532 ± 10 °C) [2], electrical resistivity (>1014 Ω cm) [3], and dielectric constant (6.9) [4]. The most important feature of BeO is its high thermal conductivity (330 W/m-K). Among the well-known oxide materials (SiO2, Al2O3, HfO2, ZrO2), BeO has the highest thermal conductivity [5]. Thus, BeO has been used in refractory materials, the dielectric discharge channels of resonators, the substrate-crystal holders of radio frequency (RF) devices, heat-transfer elements in cryogenic technology, and heat-release elements in nuclear reactors [6], [7]. Furthermore, BeO is considered the best candidate for electrical insulators and/or heat-dissipation layers for high-power electronics.

The molecular beam epitaxy (MBE) of BexZn1-xO alloy films has been reported by several research groups. The MBE growth temperature of BeO (x = 1) has been reported to be about 850 °C (Be source) and 750 °C (substrate) [8], [9], [10]. This conventional growth technique has been challenged in terms of temperature, ultimate thickness control at the atomic level, and in-situ growth in a more complete device structure (e.g., integration of the gate dielectric on the transistor) [11]. Recently, a polycrystalline BeO thin film was grown on a silicon substrate by atomic layer deposition (ALD) at 250 °C. The ALD BeO film exhibited good electrical properties, such as dielectric constant (κ) (6.8), bandgap (7.91 eV), and conduction band offset (2.3 eV) for silicon. It also suppressed the formation of native oxide by effectively preventing oxygen diffusion into silicon [12]. In addition, the self-limiting growth mechanism of ALD provides attractive properties, such as accurate and simple film thickness control, sharp interfaces, wide range uniformity, excellent conformability, good reproducibility, multilayer processing capability, and high film stoichiometry at relatively low temperatures [13]. Therefore, by utilizing ALD as the low-temperature epitaxy of BeO, the excellent thermal-electrical properties of BeO and the attractive features of the ALD process can be applied to the front-end process of nano- and microelectronics.

In this work, we demonstrate the low-temperature epitaxial growth of BeO on 4H silicon carbide (4H-SiC) (0 0 1) via ALD for the first time. Several intrinsic properties of BeO make it the best practical candidate as a gate dielectric material in SiC-based metal-oxidesemiconductor-field-effect-transistors (MOSFETs). First, the high dielectric constant of BeO (κ = 6.9), compared to SiO2 (κ = 3.9), drives the device at lower oxide fields and allows the full utilization of the high breakdown field of SiC [14]. Second, the large bandgap of the BeO gate dielectric (10.6 eV), compared to other gate oxides such as Al2O3 (8.7 eV) and SiO2 (8.9 eV), potentially enables an appropriate barrier height at the interface with 4H-SiC [12]. Third, BeO exhibits extremely high thermal conductivity (330 W/m·K). BeO has relatively few soft phonon modes, as compared to all other high-k dielectrics, due to the similar sizes of the Be and O atoms [4], [15]. Soft phonon modes tend to interact with channel carriers, reducing carrier mobility in MOSFETs [16]. Fourth, BeO (lattice constant: a = 2.69 Å, c = 4.38 Å) has the same wurtzite crystal structure as 4H-SiC (lattice constant: a = 3.07 Å, c = 10.05 Å). Large lattice mismatches can be accommodated by domain-matching epitaxy (DME). The wurtzite-structured BeO can grow into higher-quality crystals on 4H-SiC substrates than other oxides with cubic structures. Since the phonon frequency is high in a single-crystal solid, the high thermal conductivity of BeO grown as a crystalline material on a SiC substrate can be maintained. Thus, using the BeO film as a gate insulator, high channel mobility can be obtained, and at the same time, self-heating problems can be alleviated. Extensive physical characterization using transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and crystal simulation revealed high-quality crystalline BeO thin films epitaxially grown on SiC substrates. For practical use in SiC power transistors, the gate insulator characteristics of BeO were evaluated with MOS capacitors. The frequency dispersion and dielectric constants were measured from capacitance-voltage (C-V) curves. We calculated the Dit as a function of the energy states using the Terman method. Without using the conventional passivation method, the SiC substrate employing the BeO gate insulator exhibited excellent capacitor characteristics. This suggests that high-quality crystalline BeO, which inherently possesses unique physical properties, is very compatible with SiC substrates.

Section snippets

Synthesis of ALD precursor

A precursor dimethylberyllium (DMBe), Be(CH3)2, was synthesized from BeCl2(Et2O)2 by Grignard metathesis. First, HgCl2 was added to a dispersion of Be powder in Et2O to synthesize BeCl2(Et2O)2. Second, methyl magnesium bromide (CH3MgBr) was added to BeCl2(Et2O)2 and stirred for 12 h. Finally, the residual solvent (Et2O) was removed under reduced pressure, and sublimation at 55–70 °C was carried out over a period of 16–24 h to obtain Be(CH3)2 powder (Supporting Information Figs. S-1 and S-2).

Fabrication of MOS capacitor

Crystal properties

Fig. 1(a) and (b) show cross-sectional high-resolution (HR) TEM images of the epitaxial films of 30-nm ALD BeO grown on SiC (0 0 1) substrates at scale magnifications of 20 nm and 5 nm, where two layers of the SiC and BeO can be clearly identified. The crystallinity of BeO is maintained over a wide range. In a Fourier-filtered image taken across the interface region (Fig. 1(c)), quasi-periodic misfit dislocation can be seen at the interface with eight or nine planes of BeO matching with seven

Conclusion

In summary, we demonstrated for the first time the growth of high-quality epitaxial BeO on 4H-SiC at low ambient temperature (250 °C). The crystallographic relationships of the ALD BeO on the SiC were studied using TEM, SAD, crystal simulation, and XRD, which enabled the prediction of the hexagonal-on-hexagonal atomic configuration. The thermal and structural stabilities of the BeO were found to be high via XPS and Raman analyses. Moreover, the BeO dielectric quality at the interface was

Supporting Information

Synthesis of DMBe precursor, optical phonon motion in BeO, and correlation between Dit, bandgap and thermal conductivity (PDF).

Note

The authors declare no competing financial interest.

Acknowledgment

This research was supported by the MSIT (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), by Korea Electric Power Corporation. (Grant number3): R18XA06-03, and by the Basic Science Research Program through the NRF (2015R1A6A1A03031833). CWB, JHY, and ESL are grateful to the Institute for Basic Science (IBS-R019-D1) as well as the BK21 Plus Program

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