Plasma-enhanced Atomic Layer Deposition Research Articles

ALD-Deposited NiO Approaches the Performance of Platinum as a Hydrogen Evolution Cocatalyst on Carbon Nitride

The photocatalytic hydrogen evolution reaction is one of the most promising approaches to utilize solar energy and produce hydrogen as an alternative energy source to traditional fossil fuels. In this work, we focused on further exploring the cocatalyst potential of the transition metal nickel and prepared nickel oxide/carbon nitride (NiO/CNx) heterojunctions through thermal polymerization as well as plasma-enhanced atomic layer deposition (PEALD). The optimal NiO loading thickness on the surface of the CNx was found to be 6.5 nm, prepared from 65 cycles of ALD. This NiO(65)/CNx had a photocatalytic hydrogen evolution rate of 3.9 μmol h–1 in a 2 mg mL–1 aqueous photocatalyst solution (195 μmol h–1 g–1), corresponding to an AQY of 3.1% and exhibiting 54% of the activity of 3 wt % Pt/CNx under 405 nm light illumination with a sacrificial reagent. A 3 wt % Ni/CNx prepared through a photodeposition process from a dispersion of CNx and simple NiCl2 had a photocatalytic hydrogen evolution rate of 2.8 μmol h–1 (140 μmol h g–1) and an AQY of 2.2% with 2 mg mL–1 photocatalyst in the reactant solution, corresponding to 39% of the activity of 3 wt % Pt/CNx. An induction period at the beginning of the hydrogen evolution process was found in all Ni-based photocatalysts, which we related to a Ni in situ photoreduction from NiII to Ni0 and the reverse reaction. In addition, the reproducibility of the induction period even in an oxygen-free environment suggests that electrons flow back from reduced Ni species to CNx under dark conditions. Furthermore, a large population of trap states was detected in the bulk CNx materials via photoinduced absorption spectroscopy, and the trapping severely hinders cocatalysts (even the benchmark Pt cocatalyst) from extracting charges from CNx for subsequent interfacial redox reactions. We conclude that not only does the surface/interface engineering of the photocatalyst/catalyst heterojunctions need to be considered but also trap states in CNx that severely limit the transfer of photogenerated charges to cocatalysts.

Growth of GaN Thin Films Using Plasma Enhanced Atomic Layer Deposition: Effect of Ammonia-Containing Plasma Power on Residual Oxygen Capture.

In recent years, the application of (In, Al, Ga)N materials in photovoltaic devices has attracted much attention. Like InGaN, it is a direct band gap material with high absorption at the band edge, suitable for high efficiency photovoltaic devices. Nonetheless, it is important to deposit high-quality GaN material as a foundation. Plasma-enhanced atomic layer deposition (PEALD) combines the advantages of the ALD process with the use of plasma and is often used to deposit thin films with different needs. However, residual oxygen during growth has always been an unavoidable issue affecting the quality of the resulting film, especially in growing gallium nitride (GaN) films. In this study, the NH3-containing plasma was used to capture the oxygen absorbed on the growing surface to improve the quality of GaN films. By diagnosing the plasma, NH2, NH, and H radicals controlled by the plasma power has a strong influence not only on the oxygen content in growing GaN films but also on the growth rate, crystallinity, and surface roughness. The NH and NH2 radicals contribute to the growth of GaN films while the H radicals selectively dissociate Ga-OH bonds on the film surface and etch the grown films. At high plasma power, the GaN film with the lowest Ga-O bond ratio has a saturated growth rate, a better crystallinity, a rougher surface, and a lower bandgap. In addition, the deposition mechanism of GaN thin films prepared with a trimethylgallium metal source and NH3/Ar plasma PEALD involving oxygen participation or not is also discussed in the study.

Optimization of the structural and optical properties of ALD grown ZnO thin films for photocatalytic applications: thickness dependence

Thin films of ZnO with different thicknesses (ranging from 8 to 40 nm) have been prepared by plasma-enhanced atomic layer deposition. Grazing incidence x-ray diffraction shows the nano-crystalline structure of the films with high degree of disorder. The films have also lattice oxygen and non-lattice oxygen where the film with 20 nm thickness has the highest percentage of the non-lattice oxygen. These films have indirect optical transitions. The energy gap increases slightly with decreasing the film thickness (2.96, 3.03 and 3.16 eV for the thicknesses 40, 20 and 8 nm, respectively). These films have strong photocatalytic activity to treat the water from the organic dyes such as Levafix Brilliant Red. The film with thickness 20 nm has the optimum photocatalytic activity and the lowest contact angle with water. The photoinduced super-hydrophilic nature of ZnO film (20 nm) renders this film suitable for antifogging application. The high photocatalytic activity and super-hydrophilicity are due to the low recombination rate of charge carriers accompanied to the excess of oxygen vacancies and the high degree of structural disorder.

Hydrogen-Doping-Enabled Boosting of the Carrier Mobility and Stability in Amorphous IGZTO Transistors.

This study investigated the effect of hydrogen (H) on the performance of amorphous In-Ga-Zn-Sn oxide (a-In0.29Ga0.35Zn0.11Sn0.25O) thin-film transistors (TFTs). Ample H in plasma-enhanced atomic layer deposition (PEALD)-derived SiO2 can diffuse into the underlying a-IGZTO film during the postdeposition annealing (PDA) process, which affects the electrical properties of the resulting TFTs due to its donor behavior in the a-IGZTO. The a-In0.29Ga0.35Zn0.11Sn0.25O TFTs at the PDA temperature of 400 °C exhibited a remarkably higher field-effect mobility (μFE) of 85.9 cm2/Vs, a subthreshold gate swing (SS) of 0.33 V/decade, a threshold voltage (VTH) of -0.49 V, and an ION/OFF ratio of ∼108; these values are superior compared to those of unpassivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs (μFE = 23.3 cm2/Vs, SS = 0.36 V/decade, and VTH = -3.33 V). In addition, the passivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs had good stability against the external gate bias duration. This performance change can be attributed to the substitutional H doping into oxygen sites (HO) leading to a boost in ne and μFE. In contrast, the beneficial HO effect was barely observed for amorphous indium gallium zinc oxide (a-IGZO) TFTs, suggesting that the hydrogen-doping-enabled boosting of a-IGZTO TFTs is strongly related to the existence of Sn cations. Electronic calculations of VO and HO using density functional theory (DFT) were performed to explain this disparity. The introduction of SnO2 in a-IGZO is predicted to cause a conversion from shallow VO to deep VO due to the lower formation energy of deep VO, which is effectively created around Sn cations. The formation of HO by H doping in the IGZTO facilitates the efficient connection of atomic states forming the conduction band more smoothly. This reduces the effective mass and enhances the carrier mobility.

Characteristics of High‐Crystallinity ZrO2Thin Films Deposited Using Remote Plasma Atomic Layer Deposition

ZrO2deposition using the plasma‐enhanced atomic layer deposition (PEALD) process with a cyclopentadienyl tris(dimethylamino) zirconium (CpZr(NMe2)3) precursor is investigated. The effect of deposition temperature and plasma power on ZrO2thin films’ bonding and electrical properties is studied. CpZr(NMe2)3is used for the deposition at 100 W and 200–400 °C and at 50–1000 W at 300 °C. When generating O2plasma, Ar gas and O2gas are used together to generate more radicals. It is found that CpZr(NMe2)3's PEALD process window is 200–300 °C and the film's crystallinity is strongly influenced by deposition temperature and plasma power. In both process conditions, only the tetragonal phase changes. As a result, the ZrO2film deposited at 50 W and 300 °C has the highest dielectric constant. This is due to the higher O radical ratio that corresponds to lower plasma power. A high O radical ratio causes a chemical reaction rather than an ion bombardment.

Atomic-Level Sn Doping Effect in Ga2O3 Films Using Plasma-Enhanced Atomic Layer Deposition

In this work, the atomic level doping of Sn into Ga2O3 films was successfully deposited by using a plasma-enhanced atomic layer deposition method. Here, we systematically studied the changes in the chemical state, microstructure evolution, optical properties, energy band alignment, and electrical properties for various configurations of the Sn-doped Ga2O3 films. The results indicated that all the films have high transparency with an average transmittance of above 90% over ultraviolet and visible light wavelengths. X-ray reflectivity and spectroscopic ellipsometry measurement indicated that the Sn doping level affects the density, refractive index, and extinction coefficient. In particular, the chemical microstructure and energy band structure for the Sn-doped Ga2O3 films were analyzed and discussed in detail. With an increase in the Sn content, the ratio of Sn-O bonding increases, but by contrast, the proportion of the oxygen vacancies decreases. The reduction in the oxygen vacancy content leads to an increase in the valence band maximum, but the energy bandgap decreases from 4.73 to 4.31 eV. Moreover, with the increase in Sn content, the breakdown mode transformed the hard breakdown into the soft breakdown. The C-V characteristics proved that the Sn-doped Ga2O3 films have large permittivity. These studies offer a foundation and a systematical analysis for assisting the design and application of Ga2O3 film-based transparent devices.

Investigation of trapping/de-trapping dynamics of surface states in AlGaN/GaN high-electron mobility transistors based on dual-gate structures

The mechanism of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) passivated with low-pressure chemical vapor deposited (LPCVD)-SiNx layers, is revealed by a double-sweep current-voltage measurement of a dual-gate HEMT structure. It provides an in-situ observation of the trapping/de-trapping process of surface states in HEMTs under high-voltage switching stress, by examining the gate-to-gate surface-hopping leakage IS along with several critical features, such as sweeping-rate and voltage-dependent onset voltage, turn-on slope, onset voltage hysteresis, and current tailing, etc. It is also verified that the plasma-enhanced atomic layer deposited (PEALD) 3-nm -SiNx interfacial layer inserted between the LPCVD-SiNx passivation and the AlGaN barrier, is beneficial for suppressing the surface-trap-induced current collapse in AlGaN/GaN HEMTs.

High-performance proton ceramic fuel cells using a perovskite oxide cathode surface decorated with CoOx nanoparticles

This study reports on the performance improvement of a protonic ceramic fuel cell (PCFC) after a CoOx nanoparticle treatment has been applied to a PrBa0.5Sr0.5Co2-xFexO5+δ(PBSCF) cathode with a perovskite structure. CoOx nanoparticles are deposited on the sintered PBSCF surface using a plasma-enhanced (PE) atomic layer deposition (ALD) process, thereby avoiding any unwanted reactions or phase changes. The CoOx nanoparticles are successfully deposited uniformly onto the entire surface of the porous and complex cathode structure. A constant deposition rate is observed because of the self-limiting characteristics of the ALD process by a thickness difference as a function of a change in the cycle count. In our experiment, the performance of the fuel cells increases by approximately 36 % compared with the untreated cells at an operating temperature of 650 °C. In addition, all cells feature long-term stability. Impedance analysis reveals that the CoOx nanoparticle treatment results in a significant polarization and some ohmic loss improvement within all temperature regions. This is due to the synergistic effect with PBSCF and self-catalytic effects. The results imply that the proposed method enables high-performance PCFC fabrication; additionally it helps lowering the operating temperature.

Deposition Mechanism and Properties of Plasma-Enhanced Atomic Layer Deposited Gallium Nitride Films with Different Substrate Temperatures

Gallium nitride (GaN) is a wide bandgap semiconductor with remarkable chemical and thermal stability, making it a competitive candidate for a variety of optoelectronic applications. In this study, GaN films are grown using a plasma-enhanced atomic layer deposition (PEALD) with trimethylgallium (TMG) and NH3 plasma. The effect of substrate temperature on growth mechanism and properties of the PEALD GaN films is systematically studied. The experimental results show that the self-limiting surface chemical reactions occur in the substrate temperature range of 250-350 °C. The substrate temperature strongly affects the crystalline structure, which is nearly amorphous at below 250 °C, with (100) as the major phase at below 400 °C, and (002) dominated at higher temperatures. The X-ray photoelectron spectroscopy spectra reveals the unintentional oxygen incorporation into the films in the forms of Ga2O3 and Ga-OH. The amount of Ga-O component decreases, whereas the Ga-Ga component rapidly increases at 400 and 450 °C, due to the decomposition of TMG. The substrate temperature of 350 °C with the highest amount of Ga-N bonds is, therefore, considered the optimum substrate temperature. This study is helpful for improving the quality of PEALD GaN films.

Ultrathin epitaxial NbNx film deposited by PEALD method on C-plane sapphire: Growth, structure and superconducting properties

Here we report on the results of obtaining and study of epitaxial ultrathin superconductive films of niobium nitride (NbNx) grown on a C-plane sapphire substrate. The films were deposited from the TBTDEN metal-organic precursor using the plasma-enhanced atomic layer deposition (PEALD) technique at 350 °C and NH3/Ar and H2/Ar gas mixtures as reactants. We employed X-ray diffraction (XRD), X-ray reflectometry (XRR), and high-resolution transmission electron microscopy (HRTEM) techniques to study the structural properties of the films. The investigation demonstrated that NbN layers grow in the plane (111) and consist of twins which axes 11¯0 are parallel to the substrate axes 11¯00 in the basal plane. The cubic lattice of layers is subject to rhombohedral deformation involving distortion of right angles up to 89° due to the four NbNx lattices coincide with the three sapphire lattices with misfit of 0.31%. We have also determined the quasiparticle diffusion constant and the coherence length of the NbNx films. The values of superconducting transition temperature and critical current density were determined as 12 K and 6.2 МА/cm2, respectively.

Passivating Graphene and Suppressing Interfacial Phonon Scattering with Mechanically Transferred Large-Area Ga2O3.

We demonstrate a large-area passivation layer for graphene by mechanical transfer of ultrathin amorphous Ga2O3 synthesized on liquid Ga metal. A comparison of temperature-dependent electrical measurements of millimeter-scale passivated and bare graphene on SiO2/Si indicates that the passivated graphene maintains its high field effect mobility desirable for applications. Surprisingly, the temperature-dependent resistivity is reduced in passivated graphene over a range of temperatures below 220 K, due to the interplay of screening of the surface optical phonon modes of the SiO2 by high-dielectric-constant Ga2O3 and the relatively high characteristic phonon frequencies of Ga2O3. Raman spectroscopy and electrical measurements indicate that Ga2O3 passivation also protects graphene from further processing such as plasma-enhanced atomic layer deposition of Al2O3.

High performance solar-blind photodetectors based on plasma-enhanced atomic layer deposition of thin Ga2O3 films annealed under different atmosphere

Solar-blind photodetectors (SBPDs) have attracted great interests for potential applications such as missile guidance, biological detection, and ultra-violet imaging. Although a large number of Ga2O3-SBPDs have been reported, most of them with thick light absorption layers still cannot effectively alleviate the trade-off between high responsivity and fast response time simultaneously. In this work, the high-performance Ga2O3 SBPDs with 20 nm ultra-thin film were fabricated. After annealing in nitrogen ambient at 900 ℃, the Ga2O3 SBPD demonstrated with a low dark current of 0.76 pA, a great photo-dark current ratio of 106, a high responsivity of 6.18 A/W, and an ultra-fast decay time of ∼45 ms. Moreover, the Ga2O3 films annealed in nitrogen exhibited a more pronounced amorphous-to-polycrystalline transition, while the samples showed a lower recrystallization and less oxygen vacancy concentration after oxygen annealing. This work provides a desirable strategy for developing high-performance SBPDs

Developing Subthreshold-Swing Limit of PEALD In–Sn–Ga–O Transistor via Atomic-Scaled Sn Control

The In–Sn–Ga–O (ITGO) thin-film transistor (TFT) is promising in that it possesses enhanced electrical characteristics and stability because the tin (Sn) has large spherical s orbitals and a high binding energy with oxygen (O). Recently, there have been several reports of ITGO material fabricated via sputtering. Therefore, studies that control Sn composition to achieve unique characteristics and obtain conformal films have been limited. For these reasons, we evaluated plasma-enhanced atomic layer deposition (PEALD)-derived ITGO materials with varied Sn composition. The electrical characteristics are enhanced with the Sn subcycle ratio from 1 to 5 in the ITGO films (carrier concentration: 6.4 × 1020 to 2.2 × 1021 cm–3, resistivity: 8.6 × 10–4 to 6.1 × 10–4 Ω cm). These results are due to carrier generation by the Sn, a decrease in oxygen-related defects (22.9% to 19.5%), and suppressed indium-oxide (In2O3) crystallinity. To demonstrate their applicability to practical devices, the ITGO films were applied to TFTs as active channel layers. The ITGO TFTs with increasing Sn content show a decrease then increase of subthreshold swing (S.S.) characteristics, while the field-effect mobility (μFE) slightly increases. In addition, the ITGO TFTs show improved negative bias illumination stress (NBIS) stability with increasing Sn content, while the turnaround point is observed in the Sn 5 subcycle. As a result, the ITGO TFT with the Sn 3 subcycle shows an extremely low S.S. of 64.8 ± 1.9 mV/decade and an NBIS result of −0.5 V. The outstanding performance of the ITGO TFT with Sn 3 subcycle is attributed to its low number of oxygen-related defects, amorphous structure, and low valence band maximum (VBM) level. Therefore, the ITGO TFTs with optimized Sn content have the advantages of low power consumption and resistance to an environment with illumination.

Moisture barrier coating of AlN and Al2O3 multilayer film prepared by low-temperature atomic layer deposition

AlN and Al2O3 multilayer films intended as moisture barriers were deposited on polyethylene naphthalate films by remote-type plasma-enhanced atomic layer deposition. The deposition temperatures for AlN and Al2O3 were 160 and 20 °C, respectively. It was assumed that the AlN and Al2O3 interface would suppress the formation of dislocations and pinholes that lead to moisture diffusion. The AlN top layer was expected to act as a water-resistant layer. The surface morphology and the crystallinity of the deposited film were investigated by atomic force microscopy (AFM) and x-ray diffraction, respectively. The gas barrier property of the multilayer film was determined by the water vapor transmission rate, which was measured as 1.3 × 10−3 g/m2/day at a temperature of 40 °C and a relative humidity (RH) of 90%. The AFM image showed that the AlN top layer remained unchanged during water vapor contact for 120 h at 40 °C and 90% RH. The applicability of the multilayer film as a moisture barrier coating for compound semiconductor devices is discussed.

Temperature study of atmospheric-pressure plasma-enhanced spatial ALD of Al2O3 using infrared and optical emission spectroscopy

Atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) is considered a promising technique for high-throughput and low-temperature deposition of ultrathin films for applications where volume and costs are particularly relevant. The number of atmospheric-pressure PE-s-ALD processes developed so far is rather limited, and the fundamental aspects of their growth mechanisms are largely unexplored. This work presents a study of the atmospheric-pressure PE-s-ALD process of Al2O3 using trimethylaluminum [TMA, Al(CH3)3] and Ar–O2 plasma within the temperature range of 80–200 °C. Thin-film analysis revealed low impurity contents and a decreasing growth-per-cycle (GPC) with increasing temperature. The underlying chemistry of the process was studied with a combination of gas-phase infrared spectroscopy on the exhaust plasma gas and optical emission spectroscopy (OES) on the plasma zone. Among the chemical species formed in the plasma half-cycle, CO2, H2O, CH4, and CH2O were identified. The formation of these products confirms that the removal of CH3 ligands during the plasma half-cycle occurs through two reaction pathways that have a different temperature dependences: (i) combustion reactions initiated by O2 plasma species and leading to CO2 and H2O formation and (ii) thermal ALD-like reactions initiated by the H2O molecules formed in pathway (i) and resulting in CH4 production. With increasing temperature, the dehydroxylation of OH groups cause less TMA adsorption which leads to less CO2 and H2O from the combustion reactions in the plasma step. At the same time, the higher reactivity of H2O at higher temperatures initiates more thermal ALD-like reactions, thus producing relatively more CH4. The CH4 can also undergo further gas-phase reactions leading to the formation of CH2O as was theoretically predicted. Another observation is that O3, which is naturally produced in the atmospheric-pressure O2 plasma, decomposes at higher temperatures mainly due to an increase of gas-phase collisions. In addition to the new insights into the growth mechanism of atmospheric-pressure PE-s-ALD of Al2O3, this work presents a method to study both the surface chemistry during spatial ALD to further extend our fundamental understanding of the method.

Crystallinity Effect on Electrical Properties of PEALD-HfO2 Thin Films Prepared by Different Substrate Temperatures.

Hafnium oxide (HfO2) thin film has remarkable physical and chemical properties, which makes it useful for a variety of applications. In this work, HfO2 films were prepared on silicon through plasma enhanced atomic layer deposition (PEALD) at various substrate temperatures. The growth per cycle, structural, morphology and crystalline properties of HfO2 films were measured by spectroscopic ellipsometer, grazing-incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), field-emission scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy. The substrate temperature dependent electrical properties of PEALD-HfO2 films were obtained by capacitance-voltage and current-voltage measurements. GIXRD patterns and XRR investigations show that increasing the substrate temperature improved the crystallinity and density of HfO2 films. The crystallinity of HfO2 films has a major effect on electrical properties of the films. HfO2 thin film deposited at 300 °C possesses the highest dielectric constant and breakdown electric field.

Plasma-enhanced atomic layer deposition of amorphous Ga2O3 for solar-blind photodetection

Wide-bandgap gallium oxide (Ga2O3) is one of the most promising semiconductor materials for solar-blind (200 ​nm–280 ​nm) photodetection. In its amorphous form, a-Ga2O3 maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature, thus it is compatible with Si integrated circuits (ICs) technology. Herein, the a-Ga2O3 film is directly deposited on pre-fabricated Au interdigital electrodes by plasma enhanced atomic layer deposition (PE-ALD) at a growth temperature of 250 ​°C. The stoichiometric a-Ga2O3 thin film with a low defect density is achieved owing to the mild PE-ALD condition. As a result, the fabricated Au/a-Ga2O3/Au photodetector shows a fast time response, high responsivity, and excellent wavelength selectivity for solar-blind photodetection. Furthermore, an ultra-thin MgO layer is deposited by PE-ALD to passivate the Au/a-Ga2O3/Au interface, resulting in the responsivity of 788 A/W (under 254 ​nm at 10 ​V), a 250-nm-to-400-nm rejection ratio of 9.2 ​× ​103, and the rise time and the decay time of 32 ​ms and 6 ​ms, respectively. These results demonstrate that the a-Ga2O3 film grown by PE-ALD is a promising candidate for high-performance solar-blind photodetection and potentially can be integrated with Si ICs for commercial production.

Low-Temperature Self-Limiting Growth of Cubic Boron Nitride Via Hollow-Cathode Plasma-Enhanced Atomic Layer Deposition

The two main polymorphs of boron nitride are hexagonal (h-BN) and cubic (c-BN) phases and both show attractive material properties to be employed under extreme environments. h-BN is also known as “white graphene” due to the matching hexagonal crystal symmetry and shows a layered structure which provides advantages as a lubricant material and as gate dielectrics for two-dimensional (2D) transistor channels. On the other hand, c-BN is analogous to another important elemental carbon material, diamond. While the mechanical strength of c-BN is lower than diamond, the thermal and chemical stability is even superior. Both h-BN and c-BN films are therefore strong candidates for applications under harsh ambient, including high-temperature, high-pressure, and corroding factors.BN films and coatings are conventionally grown using high-temperature CVD methods with typical substrate temperatures over 1000 oC. To reduce the deposition temperatures of this material while preserving the attractive crystal properties, alternative lower temperature physical and chemical vapor deposition strategies have been offered in the literature. Plasma-enhanced atomic layer deposition (PEALD) provides significant advantages over other methods such as precision thickness control, ultimate conformality, while reducing the growth temperature substantially. In our previous work, we had demonstrated the crystalline synthesis of h-BN films at substrate temperatures as low as 350 oC, while no growth was observed at lower substrate temperatures.In this work, we aim to demonstrate the feasibility of growing c-BN at substrate temperatures reaching 200 oC via a customized PEALD reactor featuring a large-diameter capacitively-coupled hollow-cathode plasma source with a relatively short source-to-substrate distance. We used Si(100), sapphire, and glass as the substrates to deposit the films. The growth temperature is kept between 150 and 250 °C, 10 sccm of N2 flow is used as a carrier gas and Ar-purging gas flow was kept at 50 sccm. The triethylboron (TEB) precursor and various nitrogen plasma chemistries including different compositions of N2, H2, and Ar gases are utilized. The optimized growth condition is determined from in-situ monitored saturation studies, where the growth-per-cycle (GPC) parameter is noted for different precursor pulse time (15 and 30 ms), rf-plasma power (100 W to 250 W) and plasma-gas chemistries (N2-only, N2/H2, Ar/N2, Ar/N2/H2 at varying ratios). While the 200 °C as-grown BN films exhibited nanocrystalline c-BN with relatively weak crystal peaks as verified by XRD measurements, vacuum annealing at 5 mTorr N2 flow and 700 °C transformed the film structure into single-phase polycrystalline c-BN. The spectroscopic ellipsometer, x-ray reflectivity (XRR), and x-ray photoelectron spectroscopy (XPS) studies will be presented as well as the influence of post-deposition vacuum annealing to further improve the crystalline structure of c-BN films. Basic electrical properties of the grown BN films will be characterized using metal-insulator-semiconductor (MIS) diode temperature-dependent current-voltage (I-V) measurements.A specific effort will be devoted to enhance the as-grown film crystal properties via implementing novel atomic layer plasma annealing processes, which showed its potential for the ultrawide bandgap Ga2O3 alloy. Another research question to be investigated is whether there is a possibility of tuning the material phase from cubic to hexagonal by carefully adjusting growth and in-situ Ar-plasma annealing conditions.

Electrical and Optical Properties of P-Type NiO Films Grown Via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor

Electrical and Optical Properties of P-type NiO Films Grown via Nickelocene Precursor in a Hollow-Cathode Plasma-ALD Reactor In contrast to the relative abundance of as-grown unintentionally doped n-type semiconductor materials, there are only a few alternative alloys showing p-type conduction without needing extra high-temperature doping processes. Among them, CuO, Bi2O3, SnO, and NiO represent the most widely studied metal oxides which exhibit native p-type properties. NiO is of particular interest mainly due to its stability and promising performance as hole-transport layers in emerging solar cell device structures. However, to broaden the NiO application domain towards potential back-end-of-the-line (BEOL) transistor devices as potential p-type channel layers, the transport properties of NiO needs to be improved significantly. While low-temperature thermal, plasma, and ozone-assisted ALD efforts have resulted in p-type NiO films with atomic-level precision and large-area uniformity, the hole mobility is far from being sufficient, typically lower than 1 cm2/Vs.In an attempt to enhance the hole transport properties in low-temperature as-grown NiO layers, in this study we have carried out a systematic study on plasma-enhanced ALD (PEALD) of NiO films on Si and glass substrates using nickelocene (NiCp2) and O2/Ar plasma mixture in two separate reactors: (i) 8” plasma-ALD reactor with a quartz-based inductively-coupled plasma (ICP) source; (ii) 4” plasma-ALD reactor with a stainless steel-based hollow-cathode plasma (HCP) source, equipped with an in-situ ellipsometer. 300 cycle ICP-ALD growth experiments with 300 W O2/Ar plasma at 150 °C substrate temperature were carried out to identify the optimal NiCp2 precursor temperature (50 - 110 °C) and observe the influence of precursor pulse time (0.1 - 1 s). Detailed saturation studies using in-situ ellipsometer monitoring were carried out in the HCP-ALD reactor, scanning for NiCp2 pulse time, O2 plasma exposure time, purge time, and plasma power. Optimal growth conditions were identified as 90 ms NiCp2 pulse / 5 s purge / 10 s O2-plasma at 100 W plasma exposure / 5 s purge. To check the growth linearity and obtain thicker films for materials characterization, 800 cycle runs were conducted to evaluate the substrate temperature impact (100 - 250 °C) on growth-per-cycle (GPC) and film properties. The last growth parameter studied was the plasma gas composition, where several O2/Ar mixtures (20/0, 20/20, 50/0 sccm) were tested at the optimal substrate temperature of 200 °C.The resulting NiO films are characterized for their structural, optical, and electrical properties. Films grown at optimal conditions (200 °C) exhibit refractive index values reaching 2.3, which is in good agreement with reported values for the best polycrystalline NiO films in the literature. NiO films deposited on both Si and glass substrates exhibit polycrystalline single-phase cubic structure (c-NiO). Hall measurements of the as-grown NiO layers on glass substrates, revealed film resistivity values about 40 and 150 ohm-cm for HCP and ICP samples, respectively. Post-deposition annealing within 200 – 400 °C temperature range resulted in a significant reduction of resistivity for more than two orders of magnitude. However, the relatively volatile and unstable Hall voltage values led to unreliable carrier concentration and Hall mobility values, therefore we were not able to confirm the type of transport via Hall measurements.In order to determine the carrier type and extract field-effect mobility values, we have fabricated vertical NiO/n-Si p-n junction diodes as well as NiO/Al2O3/Si thin-film-transistor (TFT) device prototypes to characterize diode I-V behavior and confirm p-type layer structure for NiO, and extract the field-effect carrier mobility, respectively. Our I-V measurement results confirmed p-n junction diode characteristics with decent ON/OFF ratios for forward and reverse diode current values. TFT characterization results and findings will be presented as well.

Influence of plasma species on the early-stage growth kinetics of epitaxial InN grown by plasma-enhanced atomic layer deposition

Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation.