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.

Hollow-cathode plasma-generated hydrogen radicals induce crystal phase transformation from h-InN to c-In2O3 during plasma-enhanced atomic layer deposition using trimethyl-indium and Ar/N2 plasma.

In this study, the authors have carried out real-time process monitoring via in situ ellipsometry to understand the impact of rf-plasma power and plasma exposure time on self-limiting aluminum nitride (AlN) growth character and the corresponding film properties. AlN thin films were grown on Si(100) substrates with plasma-enhanced atomic layer deposition using trimethyl-aluminum (TMA) as a metal precursor and Ar/N2/H2 plasma as a coreactant. Saturation experiments have been employed in the range of 25–200 W plasma power and 30–120 s plasma exposure time. In situ multiwavelength ellipsometry identified single chemical adsorption (chemisorption) and plasma-assisted ligand removal events, as well as changes in growth per cycle (GPC) with respect to plasma power. The real-time dynamic in situ monitoring study revealed that GPC and TMA chemisorption thickness gain exhibited plasma power dependent saturation behavior. The amount of chemisorption saturated at ∼2.3 Å for higher rf-power levels, while for 25 and 50 W it went below 1.0 Å, which is mainly attributed to incomplete ligand removal. Besides in situ characterization, ex situ measurements to identify optical, structural, and chemical properties were also carried out on 500-cycle AlN films as a function of plasma power. AlN samples displayed a single-phase hexagonal wurtzite crystal structure with (002) preferred orientation for 150 and 200 W, while the dominant orientation shifted toward (100) at 100 W. 50 W and lower rf-power levels resulted in amorphous material with no apparent crystal signature. Furthermore, it was found that when the plasma exposure time was increased from 30 to 120 s for 25 and 50 W, the amount of chemisorption exceeded the thickness gain values recorded for 150–200 W (∼2.4 Å). However, such a recovery in the chemisorption thickness gain did not restore the crystallinity as the AlN films grown at sub-50 W showed amorphous character independent of plasma exposure time.

Plasmonic nanostructures with electrical connections have potential applications as new electro-optic devices due to their strong light–matter interactions. Plasmonic dimers with nanogaps between adjacent nanostructures are especially good at enhancing local electromagnetic (EM) fields at resonance for improved performance. In this study, we use optical extinction measurements and high-resolution electron microscopy imaging to investigate the thermal stability of electrically interconnected plasmonic dimers and their optical and morphological properties. Experimental measurements and finite difference time domain (FDTD) simulations are combined to characterize temperature effects on the plasmonic properties of large arrays of Au nanostructures on glass substrates. Experiments show continuous blue shifts of extinction peaks for heating up to 210°C. Microscopy measurements reveal these peak shifts are due to morphological changes that shrink nanorods and increase nanogap distances. Simulations of the nanostructures before and after heating find good agreement with experiments. Results show that plasmonic properties are maintained after thermal processing, but peak shifts need to be considered for device design.