ALD/ALE 2026 Session EM1-TuA: Next Generation ALD Semiconductors

Ultrawide bandgap metal oxide semiconductor materials have attracted significant interest for high-power, high-frequency, and extreme-environment applications due to their large breakdown fields and thermal stability under harsh operating conditions. In this work, we investigate the growth dynamics of gallium oxide (Ga₂O₃) and aluminum gallium oxide (Al_xGa_1-x)₂O₃ thin films deposited by plasma-enhanced atomic layer deposition (PE-ALD). The deposition process was monitored in real-time using in-situ spectroscopic ellipsometry (SE).

Ga₂O₃ ultrathin films were grown using four trimethylgallium pulses with a total exposure time of 20 msec during the metal-precursor half-cycle, followed by a remote oxygen plasma (75 sec, 300 W). Ternary (Al_xGa_1-x)₂O₃ films were deposited using a hybrid ALD process that alternate trimethylaluminum (60 msec)/H₂O (60 msec) cycles for Al₂O₃ and trimethylgallium/oxygen-plasma cycles for Ga₂O₃. While the substrate temperature is maintained at 250 ^o C, argon was used Argon as the carrier gas during the introduction of precursor material (60 sccm) and plasma process (200 sccm) with process pressure of 0.25 Torr.

To interpret the time-resolved SE data, a dynamic dual-box model approach was employed, providing insight into surface processes such as precursor adsorption and ligand removal [1,2]. Periodic oscillations in surface roughness and subsurface layer thickness were observed, reflecting the cyclic nature of molecule attachment and ligand desorption. To maintain accurate analysis of the in-situ SE data,the complex dielectric functions of Ga₂O₃ and (Al_xGa_1-x)₂O₃ thin films were determined over the spectral range of 0.74–5.04 eV using selected in-situ SE data at different film thickness. The growth rates of Ga₂O₃ and Al₂O₃ were 0.918 Å/cycle and 0.575 Å/cycle, respectively. Additionally, by performing ex-situ SE measurements in a spectral range from 0.64 eV to 9 eV at select angles of incidence from 45^o to 75^o by 10^o steps, we extracted the broad spectral range complex dielectric function at room temperature and identify the higher order band-to-band transitions based on critical point model dielectric function analysis.

Complementary characterization techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy, were used to assess film crystallinity, composition, and surface morphology.

Overall, this study demonstrates the in-situ SE technique as a powerful tool for recipe optimization and for revealing in-cycle growth kinetics during PE-ALD of Ga₂O₃ and (Al_xGa_1-x)₂O₃ thin films.

[1] Kilic, Ufuk, et al. Sci. Rep. 10.1 (2020): 10392.

Monoclinic β-Ga₂O₃ has a rare combination of ultrawide bandgap (~4.8 eV) and extrinsically controllable electron density, n, between ~10¹⁶ and ~10²⁰ cm^-3, making it a strong candidate for high-power, high-frequency, and optoelectronic applications. There have been numerous reports of β-Ga₂O₃ thin film growth via metalorganic chemical vapor deposition (MOCVD), thermal atomic layer deposition (ALD), and plasma-enhanced atomic layer deposition (PEALD), including Ga₂O₃:Ge with n > 10²⁰ cm^-3 via MOCVD¹ and Ga₂O₃:Si with n > 10¹⁸ cm^-3 via PEALD followed by thermal annealing.² In the case of thermal ALD, conformal growth on substrates with high aspect ratio (AR) has been demonstrated, including Si trenches with AR = 11,³ although doping of the conformal Ga₂O₃ was not investigated. For the development of high-performance, high-AR Ga₂O₃ devices, there remains a need for thermal ALD processes for doped Ga₂O₃ that can simultaneously achieve high n and high conformality.

In this work, we demonstrate thermal ALD of Ga₂O₃:Si using trimethylgallium (TMG), bis(t-butylamino)silane (BTBAS), and ozone as precursors, employing supercycles consisting of x TMG/O₃ cycles and 1 BTBAS/O₃ cycle. Amorphous Ga₂O₃:Si is deposited on Si(100), α-Al₂O₃(0001), and insulating β-Ga₂O₃:Fe(010) substrates at 220 °C, and some films are then crystallized via thermal annealing under N₂. Comparing as-grown Ga₂O₃:Si with x = 15 to undoped Ga₂O₃, Si incorporation is confirmed by an increase in the growth-per-cycle (GPC) from 0.69 to 0.73 Å/cyc and a decrease in the refractive index from 1.92 to 1.84. The as-grown films are electrically insulating, but thermal annealing experiments reveal that low electrical resistivity is possible after crystallization. Ga₂O₃:Si (x = 400, thickness = 28 nm) grown on β-Ga₂O₃:Fe and annealed at 900 °C for 10 min exhibits n = 8.4 × 10¹⁸ cm^-3 (dopant activation = 8.8%) and an electron mobility, μ, of 49 cm²V^-1s^-1 for a resistivity, ρ, of 1.4 × 10^-2 Ω-cm. Interestingly, ρ becomes immeasurably high (ρ > 1 Ω-cm) when the annealing temperature is increased to 1000 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicate that annealing at 900 °C produces an epitaxial β-Ga₂O₃:Si layer on top of the β-Ga₂O₃:Fe that spans approximately half of the film thickness (with lower crystalline quality in the top half of the film), whereas annealing at 1000 °C produces fully epitaxial β-Ga₂O₃:Si. The high ρ of the fully epitaxial β-Ga₂O₃:Si—and potential strategies for attaining conformal, conductive β-Ga₂O₃:Si on various substrates—will be discussed in light of known challenges presented by Ga₂O₃ crystallization, including the possible formation of Ga vacancies⁴ and γ-Ga₂O₃ en route to β-Ga₂O₃.⁵

1. Alema et al., APL Mater. 9, 091102 (2021)
2. Zhang et al., Surf. Coat. Technol. 435, 128252 (2022)
3. Comstock & Elam, Chem. Mater. 24, 4011 (2012)
4. Gann et al., J. Appl. Phys. 138, 115302 (2025)
5. Wouters et al., APL Mater. 12, 011110 (2024)

Wurtzite aluminum–scandium nitride (Al_1-xSc_xN) thin films are promising for next-generation electronic and sensing technologies. However, achieving precise composition and uniform coverage on complex three–dimensional architectures remains challenging. Here, we demonstrate the growth of Al_1-xSc_xN by plasma–enhanced atomic layer deposition (PEALD) under ultrahigh purity conditions (UHP-C) using a supercycle sequence composed of alternating AlN and ScN constituent processes. The PEALD process utilized trimethylaluminum (TMA), bis(ethylcyclopentadienyl) scandium chloride [ClSc(EtCp)₂] and N₂−H₂ plasma as co-precursors at substrate temperatures ranging from 215−300 °C.

A 60.3 nm-thick PEALD Al_0.83Sc_0.17N film grown at 300 °C on a {111}-oriented platinum bottom electrode on Si (100) exhibited clear ferroelectric switching. The film showed switched polarization (2P_r) of 163 μC/cm² and 139 μC/cm² for negative and positive pulsing, coercive fields of 5.5 MV/cm and -4.8 MV/cm, and a dielectric constant of 12.8−13.8 at 100 kHz under ±10 V. The effective longitudinal piezoelectric coefficient (d_33,f) of the Al_0.83Sc_0.17N film was measured to be -23.6 pm/V and 22.1 pm/V for the N-and metal-polarities, respectively.

AlN is a wide-bandgap semiconductor with exceptional thermal conductivity, temperature stability, and piezoelectric properties, making it a promising material for high power and high frequency electronics, deep ultraviolet optoelectronics, and microelectromechanical systems. While AlN PEALD has been the subject of extensive research, it typically results in amorphous or polycrystalline films, which have inferior electrical properties compared to epitaxial AlN. Most reports of epitaxial AlN growth by PEALD involve the use of atomic layer annealing (ALA), in which an Ar plasma exposure is incorporated into each cycle to induce crystallization of the surface.[1] While the effectiveness of ALA is proven, its use also increases cycle duration, which significantly prolongs total process time and can promote impurity incorporation. For these reasons, the growth of epitaxial AlN without ALA is desirable, though this is challenged by the complexity of controlling the plasma properties to achieve suitable growth conditions.[2]

In this work, we demonstrate the growth of 30-50 nm thick epitaxial AlN films on Al₂O₃, GaN, and Ga₂O₃ at 300 °C using PEALD without ALA or thermal annealing. The PEALD process uses the commonly-employed combination of trimethylaluminum (TMA) and N₂/H₂/Ar plasma, and is performed in a standard commercial reactor with remote inductively coupled plasma (ICP) source.Plasma diagnostics were used to identify favorable plasma regimes which produce ion energy and flux characteristics comparable to those of plasmas used in ALA. The films were characterized using x-ray reflectivity (XRR), high resolution x-ray diffraction (HRXRD), in-plane grazing incidence diffraction (IP-GID), atomic force microscopy (AFM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and optical measurements. The growth per cycle (GPC) and density ranged from 0.90–1.03 Å and 2.998–3.173 g/cm³, respectively, depending on substrate, with deposition on GaN resulting in the highest GPC and density.The topography of the films closely replicated that of the underlying material with roughness from 0.25–1 nm by AFM and 0.83–2 nm by XRR, indicating that the deposition was highly conformal. The epitaxial nature of the AlN (i.e., exhibiting a highly ordered crystalline structure with well-defined orientational relationship to the substrate) is confirmed by HRXRD and IP-GID, which show the films to be single phase and orientation with 6-fold azimuthal rotational symmetry. While the AlN films on Al₂O₃ and Ga₂O₃ are wurtzite phase with significant mosaicity, the AlN on GaN is metastable zincblende phase with low tilt disorder, as evidenced by narrow out-of-plane rocking curves with FWHM= 0.149 degrees (537 arcseconds). Results of AlN PEALD on nanocrystalline diamond (NCD) and single crystal diamond (SCD) will also be presented.

[1] H. Y. Shih et al., Sci. Rep. 7, 39717 (2017)
[2] D. R. Boris et al., J. Vac. Sci. Technol. A 42, 033008 (2024)

Aluminum nitride (AlN) is an important material for optoelectronics,¹ energy harvesting, and surface-acoustic-wave transducers and resonators.^2–5 It also enables devices operating at high frequencies and in thermally and chemically harsh environments.⁶ This interest stems from its combination of a wide bandgap, high thermal conductivity, favorable lattice constant, and piezoelectric properties. In most applications, (002)-textured wurtzite films are required to achieve the desired device performance.⁷ Furthermore, when fabricating 3D nanostructures such as microelectromechanical systems (MEMS),⁸ memory devices,⁹ and through silicon via (TSV) technology,¹⁰ conformal growth and atomic-scale control are essential. Atomic layer deposition (ALD) has been proven to be an enabling deposition method under these conditions.

However, obtaining AlN films with crystal quality comparable to that of other deposition techniques remains challenging for ALD on Si substrates,^11–13 as oxygen and carbon impurities can significantly degrade crystallinity. Surface blistering is also a critical reliability issue in ALD and PE-ALD AlN,¹⁴ yet systematic wafer-scale studies are scarce in literature. Moreover, the use of electron cyclotron resonance (ECR) plasma sources for AlN growth has received limited attention to date.¹⁵

In this work, AlN films are deposited on 200 mm Si(111) wafers using plasma-enhanced atomic layer deposition with trimethylaluminum (TMA) and NH₃ plasma as reactants. A novel ALD module (Evatec PEALD) is used for film deposition, employing a microwave ECR source for plasma generation. The effects of substrate temperature (200-400 °C) and plasma power (50-200 W) on film properties and blister formation are investigated. It is found that a combination of high power and substrate temperature leads to the formation of blisters on the edge region of the wafer.Applying a combination of characterization techniques, the number of blisters as a function of process parameters could be quantified (0-10 % of wafer area), and the defect formation mechanism was identified as likely caused by stress-induced effects. Fine-tuning of the substrate temperature and plasma power enables the suppression of these defects, resulting in damage-free, crystalline, and chemically pure films on 200 mm Si(111) substrates.

In a nutshell, this work presents effective mechanisms for producing PEALD AlN thin films with preferential c-axis orientation on 200 mm wafers, highlighting the importance of plasma source and parameter selection and giving insights into the suppression of film blistering effects.