Reactive Radio Frequency Power Research Articles

Power effect on the properties of copper nitride films as solar absorber deposited in pure nitrogen atmosphere

AbstractNowadays, copper nitride (Cu3N) is of great interest as a new solar absorber material, flexible and lightweight thin film solar cells. This material is a metastable semiconductor, nontoxic, composed of earth‐abundant elements, and its band gap energy can be easily tunable in the range 1.4–1.8 eV. For this reason, it has been proposed for many applications in the solar energy conversion field. The main aim of this work is to evaluate the properties of the Cu3N thin films fabricated by reactive radio‐frequency (RF) magnetron sputtering at different RF power values to determine its potential as light absorber. The Cu3N films were fabricated at room temperature from a Cu metallic target at the RF power ranged from 25 to 200 W onto different substrates (silicon and glass). The pure nitrogen flux was set to 20 sccm, and the working pressures were set to 3.5 Pa and 5 Pa. The X‐ray diffraction results showed a transition from (100) to (111) preferred orientations when RF power increased; the atomic force microscopy images revealed a granular morphology, while Fourier transform infrared spectroscopy and Raman spectra exhibited the characteristics peaks related to Cu–N bonds, which became narrower when the RF power increased. Finally, to stablish the suitability of these films as absorber, the band gap energy was calculated from transmission spectra.

Impact of the RF Power on the Copper Nitride Films Deposited in a Pure Nitrogen Environment for Applications as Eco-Friendly Solar Absorber

This material can be considered to be an interesting eco-friendly choice to be used in the photovoltaic field. In this work, we present the fabrication of Cu3N thin films by reactive radio-frequency (RF) magnetron sputtering at room temperature, using nitrogen as the process gas. Different RF power values ranged from 25 to 200 W and gas pressures of 3.5 and 5 Pa were tested to determine their impact on the film properties. The morphology and structure were exhaustively examined by Atomic Force Microscopy (AFM), Fourier Transform Infrared (FTIR) and Raman Spectroscopies and X-ray Diffraction (XRD), respectively. The AFM micrographs revealed different morphologies depending on the total pressure used, and rougher surfaces when the films were deposited at the lowest pressure; whereas FTIR and Raman spectra exhibited the characteristics bands related to the Cu-N bonds of Cu3N. Such bands became narrower as the RF power increased. XRD patterns showed the (100) plane as the preferred orientation, that changed to (111) with the RF power, revealing a worsening in structural quality. Finally, the band gap energy was estimated from transmission spectra carried out with a Perkin Elmer 1050 spectrophotometer to evaluate the suitability of Cu3N as a light absorber. The values obtained demonstrated the capability of Cu3N for solar energy conversion applications, indicating a better film performance under the sputtering conditions 5.0 Pa and RF power values ranged from 50 to 100 W.

Effect of Argon on the Properties of Copper Nitride Fabricated by Magnetron Sputtering for the Next Generation of Solar Absorbers.

Copper nitride, a metastable semiconductor material with high stability at room temperature, is attracting considerable attention as a potential next-generation earth-abundant thin-film solar absorber. Moreover, its non-toxicity makes it an interesting eco-friendly material. In this work, copper nitride films were fabricated using reactive radio frequency (RF) magnetron sputtering at room temperature, 50 W of RF power, and partial nitrogen pressures of 0.8 and 1.0 on glass and silicon substrates. The role of argon in both the microstructure and the optoelectronic properties of the films was investigated with the aim of achieving a low-cost absorber material with suitable properties to replace the conventional silicon in solar cells. The results showed a change in the preferential orientation from (100) to (111) planes when argon was introduced in the sputtering process. Additionally, no structural changes were observed in the films deposited in a pure nitrogen environment. Fourier transform infrared (FTIR) spectroscopy measurements confirmed the presence of Cu-N bonds, regardless of the gas environment used, and XPS indicated that the material was mainly N-rich. Finally, optical properties such as band gap energy and refractive index were assessed to establish the capability of this material as a solar absorber. The direct and indirect band gap energies were evaluated and found to be in the range of 1.70-1.90 eV and 1.05-1.65 eV, respectively, highlighting a slight blue shift when the films were deposited in the mixed gaseous environment as the total pressure increased.

Experimental investigations of plasma dynamics in the hysteresis regime of reactive RF sputter processes

Reactive radio frequency (RF) sputter processes are highly relevant for thin film deposition, but there is no complete understanding of the fundamentals of their operation. While the Berg model describes the hysteresis regime considering the oxygen coverage of the boundary surfaces, a complete fundamental understanding of the plasma–surface interactions and their effects on the discharge is still missing. In this work, we provide such fundamental insights based on an extensive experimental analysis of the physics in the hysteresis regime of magnetized reactive sputter processes, where the same reactive gas admixture can lead to different discharge characteristics depending on the previous state of the plasma. A variety of plasma and surface diagnostics is used to reveal these insights. A low pressure capacitively coupled RF discharge (CCP, 13.56 MHz) with a magnetron-like magnetic field topology adjacent to the target is operated in argon gas with a variable admixture of O2. The applied RF power, the gas flows/pumping speed, as well as the neutral gas pressure are changed systematically to understand the effects of these external control parameters on the hysteresis regime. The magnetic asymmetry effect is found to play an important role, since an axially non-uniform magnetic field is used to realize a local electron confinement at the target. Similar to process control in applications, the DC self-bias is measured to stabilize the surface composition using a feedback controller with the oxygen gas flow as the manipulated variable.

In-rich AlxIn1−xN grown by RF-sputtering on sapphire: from closely-packed columnar to high-surface quality compact layers

The structural, morphological, electrical and optical properties of In-rich AlxIn1−xN (0 < x < 0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire are investigated as a function of the deposition parameters. The RF power applied to the aluminum target (0 W–150 W) and substrate temperature (300 °C–550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented with the c-axis along the growth direction. The aluminum composition is tuned by changing the power applied to the aluminum target while keeping the power applied to the indium target fixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature optical band gap is observed to blue-shift from 1.76 eV to 2.0 eV, strongly influenced by the Burstein–Moss effect. Increasing the substrate temperature, results in an evolution of the morphology from closely-packed columnar to compact. For a substrate temperature of 500 °C and RF power for Al of 150 W, compact Al0.39In0.61N films with a smooth surface (root-mean-square surface roughness below 1 nm) are produced.

Effect of growth parameters on the properties of RF-sputtered highly conductive and transparent p-type NiOx films

Highly conductive and transparent NiOx films can be very useful as buffer layers for the optimization of the p-type contacts of optoelectronic devices. Thin NiOx films were fabricated by reactive radio frequency (RF) sputtering at room temperature starting from a Ni target. A systematic study of the influence of oxygen partial pressure, RF power and sputtering gas pressure on the films’ properties was carried out. The structural, microstructural, optical and electrical properties were affected differently by the sputtering parameters. Resistivity decreased by increasing the oxygen partial pressure and the sputtering total pressure and by decreasing the RF power, while transmittance increased by decreasing the oxygen partial pressure and by increasing the RF power and sputtering pressure. Minimum resistivity of 1.6 × 10−2 Ωcm and a visible transmittance of 40% were achieved for a film grown in a pure oxygen atmosphere, while a higher transmittance of 54% and a resistivity of ρ = 1.1 × 10−1 Ωcm were obtained for a film grown at 30% oxygen partial pressure. The trends of transmittance and resistivity as a function of the oxygen pressure during the sputtering process can be explained in terms of the amount of Ni3+ defects deduced by x-ray photoelectron spectroscopy (XPS) measurements. The full interpretation of the other results is less straightforward and highlights the relevance of the samples’ structural properties.

Gas permeation properties of silicon oxynitride thin films deposited on polyether sulfone by radio frequency magnetron reactive sputtering in various N2 contents in atmosphere

A silicon oxynitride (SiOxNy) thin film was deposited on polyether sulfone (PES) as a barrier layer against water vapor permeation using reactive radio frequency (RF) magnetron sputtering with a pure Si target in an Ar/N2 atmosphere. The coating parameters studied included RF power, N2 content in atmosphere and substrate bias. The water vapor transmission rate, thickness, chemical bonds, microstructure and light transmittance of the films were measured. Taguchi analysis shows that the N2 content has the most significant influence on the permeability of the gas barrier films. Experimental results show that using a fixed working chamber pressure of 1.6Pa and deposition time of 30min results in the lowest water vapor transmission rate which is two orders of magnitude smaller than that of uncoated PES; this result was obtained with an RF power of 250W, N2 content of 100% and without applying substrate bias. By precisely adjusting the N2/Ar flow ratio, the largest light transmittance was obtained, at 95% N2. It was also found that the gas barrier properties of the SiOxNy film are heavily influenced by its microstructure.

Studies on characteristics of resistive power calculated with discrete Fourier transform in a pulse-modulated radio frequency discharge.

In a pulse-modulated (PM) radio-frequency (RF) capacitively coupled plasma, the DFT (Discrete Fourier Transform)-calculated RF power and the corresponding phase shift between voltage and current measured with calibrated voltage and current probes present oscillations in the pulse rising and falling edges. The oscillating phase shift between voltage and current obtained in the falling edge is outside the expected value for a resistive-capacitive RF discharge. Numerical simulation and analytical analysis are made to interpret these abnormal characteristics and seek an approach to obtaining the reliable resistive (active) RF power. The oscillation is proved to be originated from the oscillating non-zero reactive RF power of the capacitor(s) in the load. At the time instant when the reactive RF power within an integer RF period is not zero, the reactive RF power is mistakenly regarded as the active RF power in the DFT analysis, as a result, the corresponding phase is thus incorrect and even outside the expected value for a resistive-capacitive load. The resistive RF power and the phase can be only correctly calculated at the time instant when the reactive RF power is zero. For a series (or parallel) RC (resistor-capacitor) load and a combined RC load with the dominated series (or parallel) RC impedance, the time instant of the zero reactive RF power is calculated with one of the two proposed empirical formulae. In practice, the DFT-calculated resistive RF power is obtained according to the following procedures: (1) applying DFT to the measured RF voltage and current signals to obtain the power and time instants for minimal phase shifts between voltage and current; (2) selecting the empirical formula to calculate time instants of the zero reactive RF power; (3) getting resistive powers at time instants of the zero reactive RF power. In real PM RF capacitively coupled plasmas, the empirical formula for the series RC load is selected to calculate the resistive RF power. The accuracy of DFT-calculated resistive RF power is proved to be related to two kinds of errors. The first is the error of the time instant of the zero reactive RF power calculated using the empirical formula. This error is relatively lower when the requirement that the dominated parallel or series RC impedance is met and is almost independent of the impedance phase angle of a combined RC load. The second is the error of the DFT-calculated resistive RF power compared with the corresponding time integral RF power at the real zero reactive RF power. This error is independent of the load type or the load impedance but varies with the slope of PM RF voltage amplitude vs. time. The two kinds of errors both increase in the pulse rising and falling edges.

Reactive Sputtering Growth and Characterizations of InN Thin Films on Si Substrates

We have studied the effects of deposition conditions on the crystal structure of InN films deposited on Si substrate. InN thin films have been deposited on Si(100) substrates by reactive radio frequency (RF) magnetron sputtering method with pure In target at room temperature. The nitrogen gas pressure, applied RF power and the distance between target and substrate were 2×10-2 Torr, 60 W and 8 cm, respectively. The effects of the Ar–N2 sputtering gas mixture on the structural properties of the films were investigated by using scanning electron microscope, energy-dispersive X-ray spectroscopy, atomic force microscopy and X-ray diffraction techniques.

Residual stress of AlN films RF sputter deposited on Si(111) substrate

AlN films with (002) crystal orientation are widely used in various resonator-based applications. Low intrinsic stress is one of the important requirements for the thin film stacks employed in electroacoustic devices. In this paper, AlN films were deposited by reactive radio frequency (RF) sputtering with various conditions. The effects of N2 concentrations, sputtering pressure, substrate temperature, and RF power on the microstructure and stress of AlN films were investigated. The X-ray diffraction, atomic force microscopy, and field emission scanning electron microscopy were utilized to investigate the orientation and surface morphology of AlN films. A systematic study of the stress of AlN was done as a function of the deposition parameters. As the nitrogen concentration decreased, the sputtering pressure increased. The residual stress has a change tendency from compressive to tensile. The substrate temperature and RF power affect stress slightly. The N2 concentrations is the major fact that influences on stress of sputtering AlN films.

Study on the photocatalytic activities of TiO 2 films prepared by reactive RF sputtering

Titanium oxide (TiO 2) films were deposited on non-alkali glass by reactive radio frequency (RF) magnetron sputtering using a Ti metal target in this study. The deposition parameters employed to realize the photocatalytic activities of TiO 2 films include RF power, deposition time, argon–oxygen ratio (O 2/(Ar + O 2)) and substrate temperature. The orthogonal array and analysis of variance (ANOVA) were adopted to determine the effect of the deposition variables on characteristic properties and the optimal conditions. The results indicated that a higher photocatalytic activity of TiO 2 films could be achieved under RF power of 150 W, deposition time of 3 h, argon–oxygen ratio of 40% and substrate temperature of 80 °C. RF power and argon–oxygen ratio had a higher effect on the methylene blue (MB) absorbance. The validation experiments show an improved photocatalytic activities of 5% when the Taguchi method is used.

High‐surface‐quality nanocrystalline InN layers deposited on GaN templates by RF sputtering

AbstractWe report a detailed study of the effect of deposition parameters on optical, structural, and morphological properties of InN films grown by reactive radio‐frequency (RF) sputtering on GaN‐on‐sapphire templates in a pure nitrogen atmosphere. Deposition parameters under study are substrate temperature, RF power, and sputtering pressure. Wurtzite crystallographic structure with c‐axis preferred growth orientation is confirmed by X‐ray diffraction measurements. For the optimized deposition conditions, namely at a substrate temperature of 450 °C and RF power of 30 W, InN films present a root‐mean‐square surface roughness as low as ∼0.4 nm, comparable to the underlying substrate. The apparent optical bandgap is estimated at 720 nm (1.7 eV) in all cases. However, the InN absorption band tail is strongly influenced by the sputtering pressure due to a change in the species of the plasma.

Influence of deposition conditions on nanocrystalline InN layers synthesized on Si(1 1 1) and GaN templates by RF sputtering

We present a detailed investigation on the influence of deposition conditions on morphological, structural and optical properties of InN films deposited on Si(1 1 1) and GaN-on-sapphire templates by reactive radio-frequency (RF) sputtering. The deposition parameters under study are nitrogen content in the sputtering gas, substrate–target distance, substrate temperature and RF power. X-ray diffraction measurements confirm the (0 0 0 1) preferred growth orientation and the wurtzite crystallographic structure of the material. For optimized deposition conditions, InN on Si(1 1 1) substrates presents smooth surface with root-mean-square roughness ∼1 nm. Surface quality of the InN films can be further improved by deposition on GaN-on-sapphire templates, achieving root-mean-square roughness as low as ∼0.4 nm, comparable to that of the underlying substrate. The room-temperature absorption edge is located at 1.70 eV. Intense low-temperature photoluminescence peaking at 1.60 eV is observed.

Structural and optical properties of amorphous oxygenated iron boron nitride thin films produced by reactive co-sputtering

Amorphous oxygenated iron boron nitride (a-FeBN:O) thin films were prepared by reactive radio-frequency (RF) sputtering, from hexagonal boron nitride chips placed on iron target, under a total pressure of a gas mixture of argon and oxygen maintained at 1 Pa. The films were deposited onto silicon and glass substrates, at room temperature. The power of the generator RF was varied from 150 to 350 W. The chemical and structural analyses were investigated using X-ray photoelectron spectroscopy (XPS), energy dispersive of X-ray and X-ray reflectometry (XRR). The optical properties of the films were obtained from the optical transmittance and reflectance measurements in the ultraviolet–visible–near infrared wavelengths range. XPS reveals the presence of boron, nitrogen, iron and oxygen atoms and also the formation of different chemical bonds such as Fe–O, B–N, B–O and the ternary BNO phase. This latter phase is predominant in the deposited films as observed in the B 1s and N 1s core level spectra. As the RF power increases, the contribution of N–B bonds in the as-deposited films decreases. The XRR results show that the mass density of a-FeBN:O thin films increases from 2.6 to 4.12 g/cm 3 with increasing the RF power from 150 to 350 W. This behavior is more important for films deposited at RF power higher than 150 W, and has been associated with the enhancement of iron atoms in the film structure. The optical band gap decreases from 3.74 to 3.12 eV with increasing the RF power from 150 to 350 W.

RF magnetron-sputtering deposition of pyroelectric lithium tantalate thin films on ruthenium dioxide

Lithium tantalate (LiTaO 3) thin films were deposited on ruthenium dioxide (RuO 2) electrode by reactive radio frequency (RF) magnetron sputtering with a lithium dioxide–tantalum pentoxide (Li 2O 2/Ta 2O 5) (50–50% mole ratio) target. This article presents morphological, structural, dielectric and pyroelectric studies of LiTaO 3 thin films as function of growth conditions (RF power, gas pressure and temperature). The final aim is to improve the pyroelectric coefficient for thermal detectors applications. The best pyroelectric coefficient of LiTaO 3 thin films (400 nm) equal to 60 μC/m 2 K was obtained for a growth temperature of 400 °C, and a pressure of 10 mTorr.

Reactive sputter deposition of AlInN thin films

AlInN films were grown on (0 0 0 1) sapphire and glass substrates by reactive radio-frequency (RF) magnetron sputtering using aluminium and indium targets in an ambience of argon and nitrogen. It was revealed that the Al composition in the AlInN films can be controlled by varying the ratio of the applied RF power of the indium target to that of the aluminium target. The lattice constant for c-axis obtained from the (0 0 0 2) diffraction peak of the AlInN films decreased with the increase of Al composition.

Structural and chemical analysis of amorphous B–N–C thin films deposited by RF sputtering

Ternary Boron–Nitrogen–Carbon (B–N–C) thin films were deposited, onto silicon substrates, by reactive radio frequency (RF) sputtering from a boron carbide (B 4C) target in a gas mixture of nitrogen and argon. The influence of the RF power ( P RF) on the structure and the chemical composition of these films are studied by Fourier transform Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements. The two techniques reveal the presence of B, C and N atoms in the deposited films. The presence of nitrogen in the atmosphere of the deposition chamber produces ternary B–N–C films composed mainly with a mixture of B–N and C N bonds as revealed by these techniques. The boron content increases while carbon and nitrogen contents decrease with P RF. The higher proportion of boron atoms produced a strong contribution of the boron nitride in the final compound B–N–C films.

Photoluminescence and characteristics of terbium-doped AlN film prepared by magnetron sputtering

Tb-doped AlN films are prepared by reactive radio frequency (RF) magnetron sputtering under different work conditions. As the RF power increases from 50 to 250 W, the prepared film changes from amorphous to c-axis oriented crystalline. Lower work pressure and higher RF power enhance the deposition rate and crystallinity. The emission from 5D 4 to 7F J ( J = 6–0) of Tb 3+ are observed on all the films. The intensity of PL spectra of crystalline films is obviously stronger than that of the amorphous films. The PL intensity of 5D 4 to 7F 6 is the strongest for crystalline film, but for amorphous film that of 5D 4 to 7F 5 is the strongest. Time-resolved spectra show that there exist two decay time for 5D 4 to 7F J ( J = 6, 5): one is shorter ranging from 41 to 60 μs, the other is longer ranging from 202 to 287 μs. The decay time of amorphous film is slightly longer than that of crystalline films.