Pulsed KrF Excimer Laser Research Articles

High-density nanodot structures on silicon solar cell surfaces irradiated by ultraviolet laser pulses below the melting threshold fluence

The surface morphology of silicon solar cells irradiated with KrF excimer laser pulses (λ = 248 nm, τ = 20 ns) was investigated below the experimentally observed melting threshold fluence (F th) of 0.47 J cm−2 (±20%). At laser fluences of 0.23–0.48 J cm−2 (equivalent to 0.49F th to 1.0F th), nanodot structures with a height and width of approximately 60–120 nm were periodically formed with an interdot spacing similar to the laser wavelength. The observed nanodot density (29 dots μm2) was higher than that previously obtained at longer wavelengths. Furthermore, crystallinity analysis by micro-Raman spectroscopy revealed a Raman shift of 519.56 cm−1 after irradiation (N= 1500 pulses), compared with 518.27 cm−1 prior to irradiation. A laser fluence of 0.41 J cm−2 ( = 0.87F th) was found to induce compressive stress on the silicon solar cell surface.

Photochemical modification of a diamond surface using a pulsed UV laser as the energy source

The use of an excimer benchtop pulsed UV laser as the energy source for the photochemical modification of the surface of diamond films is reported. Boron-doped nanocrystalline diamond (NCD) films were deposited on Si wafers via plasma-enhanced chemical vapour deposition (PECVD). The as-deposited films were characterised by scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). Following characterisation, the surfaces of the films were subjected to a photochemical reaction with 2,2,2-trifluoroethyl undec-10-enoate using two different sources of radiation: a benchtop UV lamp (254 nm) and a KrF excimer pulsed UV laser (248 nm). The photochemically treated films were characterised by contact angle measurements and X-ray photoelectric spectroscopy (XPS) using non-treated films as a control. The use of a pulsed UV laser source was shown to reduce the photochemical reaction time from several hours to a few minutes.

High conductivity characteristics of phosphorus-doped nanocrystalline silicon thin films by KrF pulsed excimer laser irradiation method.

The microstructure and high conductivity properties of phosphorus-doped nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique and the KrF pulsed excimer laser irradiation method. The results of Fourier transform infrared spectroscopy and Raman spectroscopy show that Si nanocrystallites with an average diameter of 2 nm to 3 nm are formed in the film. The degree of crystallinity increases with the increase of laser radiation intensity, while the content of hydrogen decreases gradually. More phosphorus atoms are substitutionally incorporated into the nc-Si dots under higher laser irradiation fluence, which is responsible for the high dark conductivity. By controlling the laser fluence at 1.0 J cm-2, the dark conductivity as high as 25.7 S cm-1 can be obtained. Based on the measurements of temperature-dependent conductivity, the carrier transport processes are discussed. The phosphorus doping and the increase of electron concentration are considered to be the reason for high dark conductivity and extremely low conductivity activation energy.

Fabrication of highly-oriented β-Ga2O3 thin film on ZnO/AlO x -buffered cyclo-olefin polymer substrate by excimer laser annealing at room temperature

The fabrication of highly-oriented β-Ga2O3 thin films on cyclo-olefin polymer (COP) substrates with oxide-buffer layers of amorphous AlO x and c-axis-oriented crystalline ZnO by excimer laser annealing at RT was investigated. An amorphous GaO x thin film deposited on a COP substrate by pulsed laser deposition at RT was irradiated with a pulsed KrF excimer laser from the film surface at RT in air. X-ray diffraction, reflection-high-energy-electron diffraction, and high-resolution transmission-electron microscopy results revealed solid-state crystallization of the uniaxial-oriented β-Ga2O3 (01) thin film on the ZnO/AlO x -buffered COP substrate. The obtained β-Ga2O3 thin film exhibited green-, blue-, and UV-band cathode-luminescence.

Surface evolution of crystalline SrTiO3, LaAlO3 and Y3Al5O12 targets during pulsed laser ablation

Pulsed laser ablation and deposition find applications in various technological and scientific fields, where precise control of the laser target interaction is crucial for achieving reproducible results. In this work, we investigated pulsed laser ablation of single crystalline (001), (011) and (111) oriented SrTiO3 (STO), (102) LaAlO3 (LAO) and (001) Y3Al5O12 (YAG) targets. The morphology, oxygen loss, and crack formation on the target surfaces after irradiation with a series of KrF excimer laser pulses will be discussed. The target surface morphology was imaged by atomic force, scanning electron microscopy and confocal laser scanning microscopy. Electron backscatter diffraction analysis and energy-dispersive X-ray spectroscopy were used to analyze the crystallographic changes of the surface and the elemental composition. The target material STO shows a significant crack formation and layer separation increasing with surface crystal orientation from (001) through (011) to (111) and laser pulse fluence. In contrast, the laser-ablated surfaces of LAO show only thin hairline cracks and YAG stays free of cracks in the whole laser fluence range investigated but presents large chipped areas.

Solid-phase epitaxial crystallization of β-Ga2O3 thin film by KrF excimer laser irradiation from backside of NiO (111)-buffered α-Al2O3 (0001) substrate at room temperature

We have investigated the solid-phase epitaxial crystallization of β-Ga2O3 thin films on NiO (111) (3 nm thick)-buffered α-Al2O3 (0001) substrates prepared by pulsed KrF excimer laser irradiation onto amorphous Ga2O3 thin films from the backside of the substrate at room temperature. The results of x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy indicated that the solid-phase crystallization of the amorphous Ga2O3 thin film to epitaxial β-Ga2O3 film started from the film/substrate interface toward the film surface with an average growth rate of about 0.1 nm thickness per one pulse irradiation. The optical bandgap of the epitaxial β-Ga2O3 thin films (∼80 nm thick) was estimated to be 4.9 eV from the UV/Vis transmittance measurement.

Effect of Target Morphology on Morphological, Optical and Electrical Properties of FTO Thin Film Deposited by Pulsed Laser Deposition for MAPbBr3 Perovskite Solar Cell

Three targets for pulsed laser deposition (PLD), having different morphology were used to deposit FTO thin film on three glass substrates to fabricate three FTO/SnO2/perovskite/spiro-OMeTAD/Silver perovskite solar cells. 5000 pulses of KrF excimer laser (348 nm, 50 Hz, 10 ns), having 1.2J energy were used to ablate FTO targets. Absorption, transmittance, resistivity, charge mobility and bandgap of deposited FTO thin film were observed to be dependent on target morphology. SEM micrographs of FTO layer reviled significant changes in smoothness and compactness of the thin films due to change in target morphology. Random three dimensional structures of FTO are observed on FTO layer by AFM. These random structures played very important role in improvement of solar cell performance. Maximum efficiency of 12.5% was achieved for proposed FTO/SnO2/perovskite/spiro-OMeTAD/Silver solar cell.

Pulsed Laser Irradiation of GaAs-Based Light-Emitting Structures

The effects of KrF excimer laser pulses on the crystalline and optical properties of structures with four InxGa1 – xAs/GaAs quantum wells (x ranging from 0.08 to 0.25) are studied. The results obtained by Raman spectroscopy and reflection spectroscopy show that the high crystalline quality of the GaAs cap layer is retained after exposure to laser radiation with an energy density from 200 to 360 mJ/cm2. It is experimentally shown by photoluminescence spectroscopy and laser-annealing simulation, and by solving the problem of heat propagation in a one-dimensional GaAs-based system, that the thermal effects occurring in heterostructures under pulsed laser irradiation below the GaAs melting threshold lead to mechanical-stress relaxation. At the initial stages of this process, point defects appear in InxGa1 – xAs/GaAs quantum wells which lead to a “red” shift of the photoluminescence emission peaks of the quantum wells and serve as nonradiative-recombination centers, which causes photoluminescence quenching.

SiO2-SnO2:Er3+ planar waveguides: Highly photorefractive glass-ceramics

For different applications in laser photonics and integrated optics, photorefractive materials are surely of great interest and play an important role in miniaturization of devices and fabrication of functional photonic structures such as gratings and waveguides. In this work, high photorefractivity of sol-gel derived SiO2-SnO2:Er3+ glass-ceramic planar waveguides, with negative effective refractive index change, is demonstrated. Through the photorefractivity investigation employing the UV pulsed KrF excimer laser (λ = 248 nm), the glass-ceramic planar waveguides show fast photorefractive response after a cumulative dose of only 0.3 kJ/cm2. A higher SnO2 content in the glass-ceramics leads to a greater change of saturated effective refractive index: Δneff = - (2.60 ± 0.20) x 10-3 at 1550 nm on the TE0 mode, for the 30 mol% SnO2 planar waveguides and Δneff = - (2.00 ± 0.20) x 10-3 on the 1550 nm TE0 mode, for the 20 mol% SnO2 ones. To investigate the structural modifications involved in the photorefractivity of the SiO2-SnO2:Er3+ glass-ceramics, XRD and Raman characterizations are employed. The grating fabrication by direct UV writing on the SiO2-SnO2:Er3+ glass-ceramic planar waveguide as well as an important reduction in the employed energy is demonstrated.

Effects of substrates on the crystalline growth and UV photosensitivity of glancing angle deposited porous ZnO nanostructures

We prepared ZnO nanostructures on three different substrates such as quartz, sapphire (Al2O3) (001) and MgO (100) by glancing angle deposition (GLAD) technique using KrF excimer pulsed laser and investigated the effects of various substrates on the growth and optical properties. The GLAD technique promotes a vertical growth of the nanostructures and hence creates necessary porosity for penetration of light through the top of the surface. Although these nanostructures are porous, they are highly crystalline and oriented along c-axis of the wurtzite structure. Notably, despite the amorphous nature of the quartz substrate, nanostructures on quartz are preferentially oriented towards the c-axis. All three samples show intense ultraviolet (UV) emission with a highly suppressed visible emission, where the suppression depends on the growth and porous morphology of nanostructures. The variation in morphology and the porosity strongly influenced the electrical properties and UV photo-detection of these nanostructures when exposed under UV (254 nm) light. Although we used the same technique of the growth, the lowest dark current (0.14 nA) with the highest UV photosensitivity (182 %) is achieved for the highly porous and yet crystalline nanostructures grown on the sapphire substrate. Generally, it is extremely difficult to maintain crystalline structure with porosity; however, here we demonstrate a promising approach in designing visible-blind photodetectors by combining these two opposites, porosity and crystallinity, in ZnO nanostructures using the GLAD-assisted pulsed laser deposition method.

Импульсное лазерное облучение светоизлучающих структур на основе GaAs

The effects of KrF excimer laser pulses on the crystalline and optical properties of structures with four InxGa1−xAs/GaAs quantum wells (x ranged from 0.08 to 0.25) were studied. The results obtained by Raman spectroscopy and reflection spectroscopy showed that the high crystalline quality of the GaAs cap layer is retained after exposure to laser radiation with an energy density of 200 to 360 mJ/cm2. It was established experimentally by photoluminescence spectroscopy and by modeling the laser annealing process, which is a solution to the problem of heat propagation in a one-dimensional GaAs-based system, that the thermal effects that occur in heterostructures under pulsed laser irradiation below the GaAs melting threshold lead to relaxation of mechanical stresses. At the initial stages of this process, the point defects appear in InxGa1−xAs/GaAs quantum wells. The latter lead to a “red” shift of the photoluminescence emission peaks of quantum wells and serve as centers of nonradiative recombination, which causes the quenching of the photoluminescence.

Laser Surface Hardening of Gun Metal Alloys.

The effect of laser irradiation with different numbers of laser shots on the microstructure, the surface, and the hardness of gun metal alloy was studied by a KrF pulsed excimer laser system, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Vickers hardness test. The influence of 100–500 laser shots was irradiated on the surface hardness profile and on the microstructure of gunmetal alloy. XRD results showed the maximum 2θ shift, the maximum full width of half maximum FWHM, the maximum dislocation density, and the minimum crystallite size for the sample irradiated with 300 laser shots. The hardness was measured in three different regions at the laser irradiated spot, and it was found that maximum hardness was present at the heat affected zone for all samples. The hardness value of the un-irradiated sample of gun metal was 180, and the value increased up to 237 by raising the number of laser shots up to 300. The peak value of surface hardness of the laser treated sample was 32% higher than the un-irradiated sample. The Raman shift of the un-exposed sample was 605 cm−1 and shifted to a higher value of wave number at 635 cm−1 at 300 laser shots. The hardness value was decreased by further increasing the number of laser shots up to 500. The samples irradiated with 400 and 500 laser shots exhibited smaller hardness and dislocation defect density, which was assigned to possible annealing caused by irradiation.

Improved Structural and Electrical Properties of ZnO-Based Thin Film Transistors by Using Pulsed KrF Excimer Laser Irradiation

We report on the characteristics of ZnO-based thin film transistors (TFT) fabricated by radio frequency magnetron sputtering. The surface of the ZnO channel layers were treated by pulsed KrF excimer laser irradiation (ELI) to improve the structural and electrical properties. The ZnO TFT device is of bottom gate type, which consists of SiO2 as a gate insulator and indium tin oxide as a gate deposited onto Corning glass substrates. The root-mean-square surface roughness and structural property of ZnO channel layer was significantly improved by the ELI treatment. The laser-treated ZnO TFT exhibited a saturation mobility of 19.27 cm2/V s, an on/off ratio greater than 105, the off current of less than 10−7 A, and a threshold voltage of 1.1 V. These results revealed the significant improvement of device characteristics and demonstrated that the pulsed KrF ELI treatment is an effective way to improve the ZnO TFT device performance.

Growth of two-dimensional WS2 thin films by pulsed laser deposition technique

Tungsten disulfide (WS2) is a promising material for potential applications in next-generation 2D-electronics. However, for such applications large-area films will be required. Fabrication techniques involving exfoliation give films of high crystal quality but suffer from the problem of extremely small size of the films which makes it very challenging to realize real devices for industrial applications. In this study, we are reporting the growth of large area two-dimensional (2D) films of WS2 using pulsed laser deposition technique. A polycrystalline WS2 target was ablated using a pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns). The ablated material was deposited over a c-axis aligned single crystal sapphire substrate maintained at 500°C in an ambient vacuum of 10−5 Torr. Thickness of the resulting films was controlled by controlling the number of incident laser pulses. Detailed characterization of the films was carried out using several state-of-the-art techniques including atomic force microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy. Relationship between the number of monolayers in the WS2 films and the spacing between the A1g(Γ) and E2g1 (Γ) Raman peaks (Δf) was determined. Effect of post annealing on the optical properties of the films was also investigated.

Instantaneous imaging of ozone in a gliding arc discharge using photofragmentation laser-induced fluorescence

Ozone vapor, O3, is here visualized in a gliding arc discharge using photofragmentation laser-induced fluorescence. Ozone is imaged by first photodissociating the O3 molecule into an O radical and a vibrationally hot O2 fragment by a pump photon. Thereafter, the vibrationally excited O2 molecule absorbs a second (probe) photon that further transits the O2-molecule to an excited electronic state, and hence, fluorescence from the deexcitation process in the molecule can be detected. Both the photodissociation and excitation processes are achieved within one 248 nm KrF excimer laser pulse that is formed into a laser sheet and the fluorescence is imaged using an intensified CCD camera. The laser-induced signal in the vicinity of the plasma column formed by the gliding arc is confirmed to stem from O3 rather than plasma produced vibrationally hot O2. While both these products can be produced in plasmas a second laser pulse at 266 nm was utilized to separate the pump- from the probe-processes. Such arrangement allowed lifetime studies of vibrationally hot O2, which under these conditions were several orders of magnitude shorter than the lifetime of plasma-produced ozone.

The Improvement of Electrical Characteristics of Pt/Ti Ohmic Contacts to Ga-Doped ZnO by Homogenized KrF Pulsed Excimer Laser Treatment

We investigated the effect of KrF excimer laser surface treatment on Pt/Ti ohmic contacts to Ga-doped n-ZnO (Nd = 4.3 × 1017 cm−3). The treatment of the n-ZnO surfaces by laser irradiation greatly improved the electrical characteristics of the metal contacts. The Pt/Ti ohmic layer on the laser-irradiated n-ZnO showed specific contact resistances of 2.5 × 10−4 ∼ 4.8 × 10−4 Ω cm2 depending on the laser energy density and gas ambient, which were about two orders of magnitude lower than that of the as-grown sample, 8.4 × 10−2 Ω cm2. X-ray photoelectron spectroscopy and photoluminescence measurements showed that the KrF excimer laser treatments increased the electron concentration near the surface region of the Ga-doped n-ZnO due to the preferential evaporation of oxygen atoms from the ZnO surface by the laser-induced dissociation of Zn-O bonds.

Increased doping depth of Al in wet-chemical laser doping of 4H-SiC by expanding laser pulse

Al is doped into 4H-SiC by irradiating pulsed KrF excimer laser to 4H-SiC immersed in AlCl3 aqueous solution. Impact on doping depth of the use of expanded laser-pulse width is investigated. Expanded laser pulse is produced by splitting and recombining the laser beam with mirrors. The laser pulse width was expanded from its original width of 55–100ns, while the peak power of the expanded pulse is as half as that of the original pulse under the same laser fluence. Multiple shots of the expanded laser pulses increased the doping depth at the Al concentration of 1×1016 /cm3 to 100nm from 30nm of the single shot of the original short, high-peak power laser. The increased doping depth could be due to enhanced diffusion by extra vacancies generated by the repeated laser irradiations. Due to the smaller laser peak power, the expanded pulse laser can suppress damage generation under multiple laser shots and, as a result, leakage current of the pn junction diode is kept low.

Structural, electrical, and mechanical characterization of Al 5086 alloy irradiated with 248 nm–20 ns KrF excimer laser

Williamson-Hall analysis of XRD patterns of Al 5086 alloy specimens (10 mm × 10 mm × 6 mm) irradiated with 100–500 KrF excimer laser pulses in air (1 bar) as well as in vacuum (10−3 mbar) was done to evaluate structural changes on laser irradiation. Both crystallite size (30 nm–99 nm) and lattice strain (0.00043–0.00241) were found, in general, to increase with the number of laser shots first rapidly up to 200 and later on rather slowly. Also, for a given number of laser shots, the crystallite size of the specimen laser-irradiated in vacuum was higher than that of the specimens laser-irradiated in air. Harris analysis of XRD patterns revealed that the most preferentially oriented plane was (200) with texture coefficient in the range 2.359–2.982. Electrical resistivity of the specimens was measured by four-point probe technique. It was found to increase with the number of laser shots up to 200, and later on decreases monotonically. However, for a given number of laser shots, its value was higher for the specimen laser-irradiated in air than that for the specimen laser-irradiated in vacuum. On plotting combined surface hardness data for un-irradiated and laser-irradiated specimens in air as well as in vacuum as a function of inverse square-root of crystallite size, a cross over from classical Hall-Petch relation (99 nm–55 nm) to inverse Hall-Petch relation (55 nm–30 nm) occurred at about 55 nm. This is true not only for the surface hardness but also for the hardness measured at 0.5 and 1.0 mm depth below the specimen surface. The intensity of laser-hardening effect gradually diminishes as one goes down from the uniformly laser-irradiated specimen surface to a depth of 3.0 mm below it. The relationship between electrical resistivity and surface hardness was linear.

Surface-pattern geometry, topography, and chemical modifications during KrF excimer laser micro-drilling of p-type Si (111) wafers in ambient environment of HCl fumes in air

Eight mirror-like polished p-type Si (111) wafers were irradiated with 100, 200, 300, 400, 800, 1200, 1600, and 2000 KrF excimer laser pulses in ambient environment of HCl fumes in air. The laser parameters were: wavelength = 248 nm, pulse width = 20 ns, pulse energy = 20 mJ, and repetition rate = 20 Hz. For each set of laser pulses, characterization of the rectangular etched patterns formed on target surface was done by optical/scanning electron microscopy, XRD, and EDX techniques. The average etched depth increased with the increase in number of laser pulses from 100 to 2000 in accord with Sigmoidal (Boltzmann) function, whereas the average etch rate followed an exponential decay with the increase in number of laser pulses. However, the etched area, maximum etched depth, and maximum etch rate were found to increase linearly with the number of laser pulses, but the rate of increase was faster for 100–400 laser pulses (region I) than that for 800–2000 laser pulses (region II). The elemental composition for each etched-pattern determined by EDX shows that both O and Cl contents increase progressively with the increase in the number of laser shots in region I. However, in region II both O and Cl contents attain saturation values of about 39.33 wt.% and 0.14 wt.%, respectively. Perforation of Si wafers was achieved on irradiation with 1200–2000 laser pulses. XRD analysis confirmed the formation of SiO2, SiCl2 and SiCl4 phases in Si (111) wafers due to chemical reaction of silicon with both HCl fumes and oxygen in air.

Effect of KrF Pulsed Excimer Laser Treatment on Surface Microstructure of Al-Si Alloy

In the present research, the Al-Si alloy surface is treated by KrF excimer pulse laser for different number of laser pulses in ambient condition at energy 4.75 J/cm2. The surface microstructural characterization was done by the optical microscope, in situ video recording during laser pulsing, SEM and TEM. The fretting wear test was undertaken to assess the tribological behavior. In situ video recording showed changes in the surface reflectivity with the number of pulses which is related to progressive changes in the surface compositional homogeneity. After ten pulses, signs of rippled structure were observed. The 15 pulse samples showed star-like morphological feature at the central region. The TEM observations showed high density of stacking faults/twins in Si after first pulse treatment. After 15 pulses, nano-crystalline Si precipitates in the size range <5 nm are seen to be homogeneously distributed. A cellular structure with the cell size <100 nm formed in the matrix. These cell boundaries were decorated with the Si nanocrystals. A positive effect in wear resistance property is observed after the 15 pulses treatment.