Low-Temperature As-Grown Crystalline β-Ga₂O₃ Films via Plasma-Enhanced Atomic Layer Deposition

Abstract

We report on the low-temperature growth of crystalline Ga2O3 films on Si, sapphire, and glass substrates using plasma-enhanced atomic layer deposition (PEALD) featuring a hollow-cathode plasma source. Films were deposited by using triethylgallium (TEG) and Ar/O2 plasma as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed within 150–240 °C substrate temperature and 30–300 W radio-frequency (rf) plasma power ranges. Additionally, each unit AB-type ALD cycle was followed by an in situ Ar plasma annealing treatment, which consisted of an extra (50–300 W) Ar plasma exposure for 20 s ending just before the next TEG pulse. The growth per cycle (GPC) of the films without Ar plasma annealing step ranged between 0.69 and 1.31 Å/cycle, and as-grown refractive indices were between 1.67 and 1.75 within the scanned plasma power range. X-ray diffraction (XRD) measurements showed that Ga2O3 films grown without in situ Ar plasma annealing exhibited amorphous character irrespective of substrate temperature and rf power values. With the incorporation of the in situ Ar plasma annealing process, the GPC of Ga2O3 films ranged between 0.76 and 1.03 Å/cycle along with higher refractive index values of 1.75–1.79. The increased refractive index (1.79) and slightly reduced GPC (1.03 Å/cycle) at 250 W plasma annealing indicated possible densification and crystallization of the films. Indeed, X-ray measurements confirmed that in situ plasma annealed films grow in a monoclinic β-Ga2O3 crystal phase. The film crystallinity and density further enhance (from 5.11 to 5.60 g/cm3) by increasing the rf power value used during in situ Ar plasma annealing process. X-ray photoelectron spectroscopy (XPS) measurement of the β-Ga2O3 sample grown under optimal in situ plasma annealing power (250 W) revealed near-ideal film stoichiometry (O/Ga of ∼1.44) with relatively low carbon content (∼5 at. %), whereas 50 W rf power treated film was highly non-stoichiometric (O/Ga of ∼2.31) with considerably elevated carbon content. Our results demonstrate the effectiveness of in situ Ar plasma annealing process to transform amorphous wide bandgap oxide semiconductors into crystalline films without needing high-temperature post-deposition annealing treatment.

Large-Sized Nanocrystalline Ultrathin β-Ga2O3 Membranes Fabricated by Surface Charge Lithography

“…Therefore, the GaN phase can be excluded and only the formation of α and γ-Ga The crystalline quality of the obtained Ga 2 O 3 nanomembranes is comparable to that inherent to ultrathin layers of Ga 2 O 3 obtained by using ALD techniques with subsequent annealing. A recent study reported on ICPE-ALD synthesis of 2D layers of β-Ga 2 O 3 on Si, sapphire, and glass [45]. The authors demonstrated evolution of XRD patterns from an amorphous layer to a fairly-crystallized one after ex situ treatment at 800 • C. Applying various growth and crystallization methods, it remains difficult to combine large-area uniformity and high crystal quality of β-Ga 2 O 3 structures.…”

Ga2O3 and Related Ultra-Wide Bandgap Power Semiconductor Oxides: New Energy Electronics Solutions for CO2 Emission Mitigation

“…Additionally, the temperature during growth depends mainly on the gallium precursor chosen [ 79 , 80 ]. Besides the conventional ALD, the plasma-enhanced atomic layer deposition (PEALD) further permits a lower deposition temperature and better Ga 2 O 3 film properties with very smooth surface roughness (<1 nm) [ 81 , 82 , 83 ].…”

Recent Advances in Hollow Cathode Technology for Plasma-Enhanced ALD—Plasma Surface Modifications for Aluminum and Stainless-Steel Cathodes

“…[28] SiO 2 (PE-ALD) [7,24,25] β-Ga 2 O 3 (PE-ALD) [38] Silicon nitride (PE-ALD) [22,23,27,31,37,40] Plasma treatment (PE-ALD) [26] Electron treatment (CVD based) [36,39] GaN (CVD based) [42,44,45,55] InN (CVD based) [41][42][43]45,47,48,53,57,58] InGaN (CVD based) [49,51,54,56] InN nanopillars (CVD based) [46,50,52,55] The oxygen contamination overview provided in this introduction is followed by an experimental examination of some effects related to the surface modification of cathode materials by the generated plasma. In particular, we examine changes that have been observed for aluminum and stainless-steel cathodes.…”