Paul Plate

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 Å per cycle and a thickness inhomogeneity of ≤1.2% across 4-in. substrates in the temperature range of 100–150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off-stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates.

Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.

The ever-increasing energy demand and environmental concerns associated with fossil-based fuels have driven the urgent need for sustainable methods to produce chemical fuels. One approach is to perform solar water splitting to produce hydrogen and oxygen. For this purpose, metal oxide-based semiconductor are promising photoelectrode materials due to their relatively high abundance and stability in aqueous solutions. However, metal oxides typically have only slow surface reaction kinetics for water oxidation [1]. To overcome this, water oxidation co-catalysts, such as CoP_i, FeOOH, NiFeO_x, and MnO_x, have been deposited on the surface of numerous metal oxide semiconductors resulting in an improved performance [2-6]. However, the processes at the semiconductor/co-catalyst interface are not yet well understood. Here, we investigate the role of MnO_x and Ni:MnO_x co-catalysts (thickness < 10 nm) deposited by atomic layer deposition (ALD) on Ta-O-N thin films as a model system. By systematically controlling the partial pressure of NH₃, H₂ and H₂O during post-annealing of Ta thin films, different phases (e.g. Ta₂O₅, TaO_xN_y, Ta₃N₅) with varying valence band maximum positions were successfully prepared. The valence band position of the co-catalyst was also varied by modifying the Ni content. Photoelectrochemical studies with and without hole scavenger reveal that MnO_x and Ni:MnO_x enhances the photocurrent of Ta-O-N films by a factor of ~ 4, but they do not affect the charge injection efficiency. Instead, the photocurrent increase is likely a result of higher charge separation efficiency. We attribute this to the formation of a band bending at the Ta-O-N/MnO_x and Ta-O-N/Ni:MnO_x interface, as suggested by the open circuit potential (OCP) and in-line X-ray photoelectron spectroscopy (XPS) data. This is further supported by analyzing Ta-O-N films with varying thickness of the co-catalysts; thicker co-catalysts result in increasing band bending, which correlates with the photocurrent trend as well as the effective carrier diffusion length measured by time-resolved microwave conductivity (TRMC). Finally, the influence of the band offsets between the different photoanode in the Ta-O-N systems and the co-catalysts is discussed. References [1] M. De Respinis, Chem. Mater. 2015, 27, 7091. [2] F. F. Abdi, ChemCatChem 2012, 5, 490. [3] W. D. Chemelewski, J. Am. Chem. Soc. 2014, 136, 2843. [4] J.-W. Jang, Nat. Commun. 2015, 6, 7447. [5] D. Xu, Electrochim. Acta. 265, 2018, 10. [6] A. Ramirez, J. Phys. Chem. C. 2014, 118, 14073.