Laser-assisted Oxidation Research Articles

Improvement of β-Ga2O3 Junction Field-Effect Transistors By Employing WSe2/β-Ga2O3 p-n Heterojunction Via Laser-Assisted Oxidation

Beta gallium oxide (β-Ga2O3) is a semiconductor material that has been receiving a lot of attention during the past years owing to its suitability for next-generation high-power devices. The attributes of β-Ga2O3 that make it attractive for use in high-power devices include ultra-wide bandgap energy of 4.9 eV, large breakdown field of ~8 MV/cm (estimated), and high Baliga’s and Johnson’s figures of merit of approximately 3214 and 2844, respectively. From the device fabrication point of view, β-Ga2O3 crystal having a monoclinic structure with an anisotropic unit cell facilitates the effortless fabrication of low-scale β-Ga2O3 devices through simple mechanical exfoliation. However, the nature of β-Ga2O3 that it is n-doped by its oxygen vacancies and its difficulties in p-doping limit the application of β-Ga2O3 in integrated structures such as the CMOS (complementary metal-oxide-semiconductor), where the semiconductor substrate is required to have both n- and p-doped areas. Various studies, with the aim of finding alternative solutions to this problem, are being conducted on the fabrication of p-n heterojunctions composed of n-type β-Ga2O3 and p-type or ambipolar semiconductor materials. Among them, transition metal dichalcogenides (TMDs) are studied for the p-n heterojunctions involving the n-type β-Ga2O3. TMDs have a crystal structure composed of layers, consisting of in-plane repeating units with two to three atoms, attached to one another by interplanar van der Waals interaction which gives rise to favorable electrical and structural characteristics, including high in-plane carrier mobility, atomic-scale thickness, and atomically clean surface allowing TMDs to form van der Waals heterostructures without strain from heteroepitaxial growth. In this work, tungsten diselenide (WSe2), which has a moderate bandgap of ~1.2 to 1.7 eV (bulk to monolayer, respectively), is utilized to form the p-n heterojunction with β-Ga2O3. WSe2 exhibits ambipolar transport characteristics in nature and shows excellent doping controllability through processes such as Ar plasma treatment, ultraviolet/ozone treatment, and laser-assisted oxidation. In this work, laser-assisted oxidation was utilized for the selective p-doping of WSe2 in the WSe2/β-Ga2O3 heterojunction area.In this work, β-Ga2O3 and WSe2 flakes were mechanically exfoliated from single crystal bulks with an adhesive tape, respectively. β-Ga2O3 flakes were transferred via dry transfer method onto Si/SiO2 substrates, followed by electron-beam lithography to define the β-Ga2O3 channels. Ti/Au (50/100 nm) source/drain electrodes were deposited by electron-beam evaporator. Rapid thermal annealing was performed in low vacuum (~10-2 Torr) at 500℃ for 60 s to form Ohmic contacts. Selected WSe2 flakes were transferred onto a specific position of the β-Ga2O3 channels by PDMS-film-assisted dry transfer method. Source/drain electrodes of the WSe2 channel were defined using electron-beam lithography, followed by Pt/Au (20/80 nm) electrode deposition with an electron-beam evaporator. Fabricated devices were thermally annealed in a high vacuum condition (~6×10-6 Torr) at 200℃ for 120 min to improve the contact. 325 nm laser with an intensity of 0.135 mW was utilized for the selective oxidation of WSe2 in the WSe2/β-Ga2O3 heterojunction.The fabricated device exhibited distinct rectifying behavior with an ideality factor of 1.16 and an on/off ratio of ~106, where the on-resistance decreased by approximately five orders of magnitude after WSe2 oxidation. β-Ga2O3 JFET with a WSe2 top gate showed outstanding characteristics with a subthreshold swing of 76.84 mV/dec and an on/off ratio of ~107. This work demonstrates low-scale, high-quality β-Ga2O3 JFETs with WSe2, paving the way to the potential applications of high power β-Ga2O3 device with high switching speed and small device footprint.

Chromatic annuli formation and sample oxidation on copper thin films by femtosecond laser.

We report an experimental investigation on the irradiation of copper thin films with high repetition rate femtosecond laser pulses (1040 nm, 50 MHz), in ambient air and liquid water. We observe a novel, striking phenomenon of chromatic copper oxides (CuO and Cu2O) annuli generation. The characteristic features of the chromatic copper oxide annuli are studied by exploiting micro-Raman spectroscopy, optical and scanning electron microscopies. In the case of irradiation in water, the seldom investigated effects of the immersion time, tw, after irradiation with a fixed number of pulses are analyzed, and an intriguing dependence of the color of the chromatic annuli on tw is observed. This remarkable behavior is explained by proposing an interpretation scenario addressing the various processes involved in the process. Our experimental findings show that Cu2O nanoparticles (size of ≈20 nm) and Cu2O nanocubes (nanocube edges of ≈30, ≈60 nm) can be effectively generated by exploiting high repetition rate laser-assisted oxidation.

Laser-assisted oxidation of multi-layer tungsten diselenide nanosheets

We report the structural and electrical characterization of tungsten oxides formed by illuminating multi-layer tungsten diselenide (WSe2) nanosheets with an intense laser beam in the ambient environment. A noninvasive microwave impedance microscope (MIM) was used to perform electrical imaging of the samples. The local conductivity ∼102 S/m of the oxidized product, measured by the MIM and conventional transport experiments, is much higher than that of the pristine WSe2, suggesting the formation of sub-stoichiometric WO3-x polycrystals with n-type carriers. With further efforts to improve the conductivity of the oxides, the laser-assisted oxidation process may be useful for patterning conductive features on WSe2 or forming electrical contacts to various transition metal dichalcogenides.

Selective tuning of high-Q silicon photonic crystal nanocavities via laser-assisted local oxidation

We examine the cavity resonance tuning of high-Q silicon photonic crystal heterostructures by localized laser-assisted thermal oxidation using a 532 nm continuous wave laser focused to a 2.5 μm radius spot-size. The total shift is consistent with the parabolic rate law. A tuning range of up to 8.7 nm is achieved with ∼ 30 mW laser powers. Over this tuning range, the cavity Qs decreases from 3.2×10(5) to 1.2×10(5). Numerical simulations model the temperature distributions in the silicon photonic crystal membrane and the cavity resonance shift from oxidation.

Tuning optical modes in slab photonic crystal by atomic layer deposition and laser-assisted oxidation

The authors experimentally investigate the effects of atomic layer deposition (ALD) and laser-assisted oxidation on the optical modes in GaAs L3 photonic crystal air-bridge cavities, using layers of InAs quantum dots as internal light source. Four distinct optical mode peaks are observed in the photonic bandgap and they show different wavelength-redshifts (0–6.5 nm) as the photonic crystal surface is coated with an Al2O3 layer (0–5.4 nm thick). Numerical finite-difference time-domain (FDTD) simulations can well-reproduce the experimental result and give insight into the origin of the shifts of modes with different spatial profiles. By combining the ALD coating with in situ laser-assisted oxidation, we are able to both redshift and blueshift the optical modes and we attribute the blueshift to the formation of a GaAs-oxide at the expense of GaAs at the interface between GaAs and the Al2O3 layer. This result can be quantitatively reproduced by including a GaAs-oxide layer into the FDTD model. Selective etching experiments, confirm that this GaAs-oxide layer is mainly at the interface between GaAs and Al2O3 layers.

Local tuning of photonic crystal nanocavity modes by laser-assisted oxidation

The authors demonstrate a simple method to achieve local tuning of optical modes in GaAs photonic crystal nanocavities by continuous wave laser-assisted oxidation in air atmosphere. By irradiation with a focused laser beam at power levels of a few tens of milliwatts, photonic crystal nanocavity modes shift to shorter wavelengths by up to 2.5 nm. The mode shifts can be controlled either by varying the laser power or by iterating laser-assisted oxidation steps and are well explained by finite-element-method and finite-difference time-domain simulations. This method provides a simple route to achieve fine spectral tuning of individual nanocavities for photonic devices.

Laser-assisted Zr/ZrO 2 coating on Ti for load-bearing implants

Oxidized Zr alloys have been shown to exhibit lower friction and superior wear properties, suggesting that they could be used in hip and knee implants. However, conventional oxidation of Zr alloys above 500 °C, in dry air, for several hours has been shown to have detrimental effects on the substrate’s properties. In this work, we deposited pure Zr on Ti, then oxidized the coating using a continuous-wave Nd:YAG laser, which facilitated localized heating to elevated temperatures without affecting the substrate. Laser-assisted oxidation resulted in a 7 μm thick fully dense ZrO 2 layer on Zr in which an increase in oxidation kinetics was evident due to an increase in the laser power and/or the oxygen partial pressure. Due to its high surface energy and wettability, the wear rate of laser-oxidized Zr was two orders of magnitude less compared to that of as-deposited Zr. The oxidized coatings showed comparable in vitro biocompatibility to that of pure Ti and excellent in vitro cell–material interactions. This article reports the processing of Zr/ZrO 2 coatings on Ti using lasers, and the influence of laser parameters and oxygen partial pressure on the coating’s mechanical, microstructural, wear and in vitro biological properties using human osteoblast cells.

Fabrication of TiO 2/Ti electrode by laser-assisted anodic oxidation and its application on photoelectrocatalytic degradation of methylene blue

A TiO 2/Ti mesh electrode by laser calcination was prepared in this article. The resulting TiO 2 film was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), and it illuminated that the prepared electrode mainly consisted of anatase TiO 2 nanoparticles on its surface and exhibited a superior photocatalytic activity. The photodegradation of methylene blue (MB) using the proposed electrode under different experimental conditions was investigated in terms of both UV absorbance at 664 nm and chemical oxygen demand (COD) removal. The electrical bias applied in photoelectrocatalytic (PEC) oxidation was also studied. The experimental results showed that under the optimal potential of +0.50 V (versus SCE), UV absorbance and COD removal during the photodegradation of MB by the proposed TiO 2/Ti mesh electrode were 97.3% and 87.0%, respectively. Through the comparison between photocatalytic (PC) oxidation and photoelectrocatalytic (PEC) oxidation, it was found that PEC oxidation was a convenient and effective way to mineralize the organic matters and that laser-treated photoelectrode exceeded the oven-treated one.

Separation of Solubilized Single-walled Carbon Nanotubes by Chirality using Laser-assisted Selective Oxidation

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Self-assembled germanium nano-structures by laser-assisted oxidation

The investigation on the oxidation behaviour of Si1−xGex alloys (x=0.05, 0.15 and 0.25) is carried out. It is found for the first time that on the oxide film a germanium nano-cap with a thickness of 1.8–2.8nm and a few Ge nanoparticles with diameters ranging from 5.5 nm to 10 nm are formed by the low-temperature laser-assisted dry oxidation of Si1−xGex substrate. A new scanning method on the decline cross-section of the multiple-layer sample is adopted to measure the layer thickness and the composition. Some new peaks in photoluminescence (PL) spectra are discovered, which could be related to the nano-cap and the nano-particles of germanium. A suitable model and several new calculating formulae with the unrestricted Hartree–Fock–Roothaan (UHFR) method and quantum confinement analysis are proposed to interpret the PL spectra and the nano-structure mechanism in the oxide.

Reflectance spectra and refractive index of a Nd:YAG laser-oxidized Si surface

The reflectance spectra and refractive index of Nd:YAG laser-oxidized SiO 2 layers with thicknesses from 15 to 75 nm have been investigated with respect to the laser beam energy density and substrate temperature. Thickness and refractive index of films have been determined from reflectance measurements at normal light incidence in the spectral range 300–800 nm. It was found that the oxide-growth conditions at higher substrate temperatures and laser powers greater than 3.36 J cm −2 provides a better film quality in terms of both optical thickness and refractive index. However, the refractive indices of the films are smaller in the whole spectral range studied as compared to that of conventional thermally grown SiO 2. This might be due to the porous structure formed during the laser-assisted oxidation. The results suggest the need of post-oxidation annealing to improve the refractive indices of the films, suitable for Si-device applications.

Observation of exothermic reaction during laser-assisted iron oxide coating on aluminum alloy

Abstract Aluminum and Fe 3 O 4 reacts readily in what is known as thermite reaction to produce large amount of heat. Attempts were made to coat Fe-oxide on A319 cast aluminum alloy employing a high power laser to exploit this reaction. High-speed, high-resolution infrared thermography was employed to study the thermal conditions during the laser treatment. Parallel experiment using a less exothermic oxide (FeO) and the same substrate further emphasized development of higher temperature during highly exothermic reaction. The cooling rate calculated via both steady state and non-steady state relations were one order of magnitude different, which was supported by microstructural observations. Transmission electron microscopy revealed formation of aluminides as a result of reaction between iron oxide and aluminum alloys.

Investigation of surface oxidation of low carbon sheet steel during its treatment with Nd:Glass pulsed laser

The surfaces of hot-rolled and cold-rolled samples of low carbon mild steel have been treated by Nd:Glass laser. The temperature conditions at the surface have been characterised via a numerical simulation. The thermal conductivity equation has been solved for the one-dimensional case. In the model developed for numerical simulation the change in absorption coefficient during the laser treatment due to the formation of surface oxide film has been considered. The temperature gradients during the laser pulse and following the cooling stage have been estimated. It has been found that the maximum temperature reached at the end of the laser pulse on the samples’ surface is 1814 °C, and this relatively high temperature causes oxidation of both samples and surface melting of the cold-rolled specimen. The thickness of the formed oxide film has been evaluated via the developed model for numerical simulation to be 77 nm. Some important parameters of the grown oxide film namely: phase composition; lattice imperfections; and residual macro-stresses have been estimated via X-ray asymmetric grazing incidence beam methods. It has been revealed that the oxide film has a layered macrostructure built up by two mono-phase layers namely Fe 3O 4 and FeO, from which the former is the top and the latter the bottom layer. The comparison between the broadening of the corresponding diffraction lines shows that the FeO layer formed on the cold-rolled steel has a higher concentration of lattice imperfections than the one grown on the hot-rolled specimen. For the top Fe 3O 4 layers formed on both steels the relation between the peaks broadening is the reverse. It has also been revealed that the grown laser-assisted oxide films contain tensile residual macro-stresses.

The laser-assisted iron oxide coating of cast Al auto engines

Cast A319 Al alloy, which is a popular choice for automobile engine cylinder block material, has been limited in that use by its poor tribological properties. Various techniques have been explored to address this problem, including the one addressed in this study, laser surface engineering (LSE.) Engine cylinders made of A319 Al alloy were LSE coated with FeO. The region of coating produced a refined microstructure compared to the cast substrate structure. Also, the FeO coating displayed superior wear performance compared to uncoated A319 Al under dry sliding wear conditions.

Laser-induced oxidation of metals: state of the art

The general features of laser-induced metal oxidation and laser-assisted oxide deposition are discussed. The mechanisms of isothermal and laser-induced oxidation processes are compared. The peculiarities of the laser-assisted processes, e.g. non-linearities and non-equilibriums feedback effects, are emphasized. Instabilities connected with the appearance of positive feedback between the laser and thermal parameters, as well as the optical properties of the system, are considered. Absorptivity changes and variations of the metal oxidation constant, such as due to changes in temperature and wavelength, are presented by using vanadium as an example. Experimental data of laser-assisted metal oxidation of different metals are presented in tabulated form. Some morphological irregularities as a result of laser irradiation are shown. Trends of theoretical and experimental investigations are indicated. © 1997 Elsevier Science S.A.

Laser-assisted formation of metallic oxide microtubes

The fabrication of metallic oxide microtubes is possible directly, without any support structure, by continuous wave infrared laser-assisted oxidation of the metal in air. The particular case presented is the growth of tube-like vanadium pentoxide microcrystals grown in our laboratories.

Metal oxide layer growth under laser irradiation

A review of laser-assisted metal oxidation experiments is given. Results are obtained for V targets, and the manner in which temperature affects the Wagner oxidation constants is studied. The effect of an external electric field on the oxidation rate is considered. Morphological peculiarities of grown oxide layers are presented.

Holograms obtained by laser-assisted oxidation of thin tellurium films

Holograms are recorded by laser-assisted oxidation of thin tellurium films in air. The process of recording is characterized in situ by measuring the evolution of a self-diffracted beam of a cw Ar+ laser. Stationary diffraction efficiency is shown to be independent both on the recording beam power and on the grating period. Amplitude holograms are obtained with an efficiency of 3.2% at a spatial period of 3 μm.

Negative thermal feedback in laser-assisted oxidation of thin Zn films

The processes leading to the control of the lateral dimension of laser-assisted oxidation of Zn films on glass are examined. It is shown that it is determined by negative feedback between temperature and optical absorption.

Laser-assisted oxidation of metallic films on insulators

Abstract When metallic films are irradiated in air by means of powerful laser beams, oxidation takes place. Features specific to the mechanisms of laser-matter interaction are observed, which are not found under furnace-annealing conditions. Two such features are described in this paper. Laser-induced oxidation of thin cadmium films on glass substrates indicates that oxidation accelerates when the metallic film (below a solid oxide film) rapidly melts. In the case of copper films, feedback effects between laser energy absorption and film temperature give rise to oscillating oxidation rates.