Excimer Laser Annealing Research Articles

Precision excimer laser annealed Ga-doped ZnO electron transport layers for perovskite solar cells.

Organic–inorganic hybrid perovskite solar cells (PSCs) continue to attract considerable attention due to their excellent photovoltaic performance and low cost. In order to realize the fabrication of PSCs on temperature-sensitive substrates, low-temperature processing of all the components in the device is required, however, the majority of the high-performance PSCs rely on the electron transport layers (ETLs) processed at high temperatures. Herein, we apply excimer laser annealing (ELA) to treat ETLs (Ga-doped ZnO, GZO) at room temperature. A synergetic improvement in optical transparency and electrical conductivity is achieved after ELA treatment, which in turn improves light absorption, enhances electron injection, and depresses charge recombination. Devices fabricated with ELA treated GZO ETL acheived a power conversion efficiency (PCE) of 13.68%, higher than that of the PSCs utilizing GZO with conventional high-temperature annealing (12.96%). Thus, ELA is a promising technique for annealing ETLs at room temperature to produce efficient PSCs on both rigid and flexible substrates.

Microscopic observation of lateral and vertical charge transportation in Si nanocrystals sandwiched by amorphous SiC layers

Charge injection and transportation process is a fundamental problem to Si nanocrystals (Si-ncs) based electric and photonic devices. In the manuscript, a single layer of Si-ncs sandwiched by amorphous Si carbide (a-SiC) was prepared by excimer laser annealing of a-SiC/a-Si/a-SiC multilayers, and the charging effect was then characterized by Kelvin probe force microscopy (KPFM) on the microscopic scale. Opposite charges were injected into Si-ncs through the biased tip and formed a core-ring or up-down shaped distribution. The decay characteristics showed that these opposite charges would not only vertically tunnel through the bottom a-SiC layer to substrate but also laterally transport and recombine with each other driven by the attractive Coulomb force. Besides, the charge retention time was also found dependent on the injection biases, which is tentatively ascribed to the charge trapping by the Si-ncs/a-SiC interface states under high bias scanning. The analysis was further supported by conductive atomic force microscopy (CAFM) measurement, in which the current-voltage curves gradually shifted during the repetition test, probably because of bias screening by the trapped charges at these interface states.

Improved performance of Ge n+/p diode by combining laser annealing and epitaxial Si passivation**Project supported by the High Level Talent Project of Xiamen University of Technology, China (Grant No. YKJ16012R).

A method to improve Ge n+/p junction diode performance by excimer laser annealing (ELA) and epitaxial Si passivation under a low ion implantation dose is demonstrated. The epitaxial Si passivation layer can unpin the Fermi level of the contact of Al/n-Ge to some extent and reduce the contact resistance. In addition, the fabricated Ge n+/p junction diode by ELA plus epitaxial Si passivation exhibits a decreased reverse current density and an increased forward current density, resulting in a rectification ratio of about 6.5×106 beyond two orders magnitude larger than that by ELA alone. The reduced specific contact resistivity of metal on n-doped germanium and well-behaved germanium n+/p diode are beneficial for the performance improvement of Ge n-MOSFETs and other opto-electronic devices.

Excimer laser annealing: An alternative route and its optimisation to effectively activate Si dopants in AlN films grown by plasma assisted molecular beam epitaxy

Si doped AlN films were grown on GaN/Sapphire templates by plasma assisted molecular beam epitaxy (PAMBE) technique. We show that employing an excimer laser annealing with optimized power and frequency rather than the conventional thermal annealing could be a potential alternative route towards improving the structural and electrical properties of AlN layers. Upon optimized laser annealing of the Si-doped AlN layer, the electron concentration was achieved to be as high as ∼4.9×1018cm−3 which was measured by Raman spectroscopy measurement and was further cross checked by standard Hall measurement which estimated the same as ∼7.4×1018cm−3 with a mobility of 109cm2/V-sec. The improvement of free carrier concentration was leveraged by improvement of structural properties. The r.m.s surface roughness of the Si-doped AlN layers measured by atomic force microscopy was reduced to 0.76nm and corresponding residual stress estimated by high resolution XRD and Raman measurement was found to be less than half compared to the in-situ Si-doped sample. Thus laser annealing is proposed to be a suitable method to achieve high electron concentration in Si doped AlN films without compromising the structural quality.

Characteristics of Thin-Film Transistors Fabricated by the Excimer Laser-Annealed Amorphous Silicon in Ultralow Oxygen Concentrations

To integrate circuits into the organic light emitting diode displays, it is necessary to fabricate polycrystalline silicon (poly-Si) based thin-film transistors (TFTs) on the glass substrates. In this work we investigated the correlation between the electrical characteristics and the poly-Si morphology of the excimer laser annealed (ELA) TFTs in ultralow oxygen concentrations (~ ppm). The main feature of ELA poly-Si films is the protrusion at grain boundaries that makes the film surface rough. The surface roughness increases with an increasing oxygen concentration during the laser annealing and degrades the TFT characteristics in the on-state as well as the breakdown voltage of the gate insulator, while the off current is independent of process conditions. This result is attributed to the increased oxygen incorporation in the film in the case of an ELA process. Since oxygen increased the defect density in the polysilicon bandgap, controlling the oxygen concentrations in the process chamber helped to improve the performance of the ELA poly-Si TFTs. Based on these results, we discuss the relationship between performance of active matrix organic light emitting display panels and oxygen concentrations during ELA.

Analysis of polydihydrosilane crystallization by excimer laser annealing

Printing of electronics has been gaining a lot of attention over the past decade as a low cost alternative to conventional electronic fabrication methods. A significant development in this area was the possibility to print a silicon precursor, polydihydrosilane, which can directly be transformed into polycrystalline silicon by an excimer laser treatment. Due to the limited laser heat diffusion, low-cost flexible substrates such as plastics and even paper could be used that typically have low thermal budgets. Since the silicon precursor is sensitive to ultraviolet light and may transform in a photochemical reaction, the question arises whether the excimer laser crystallization is predominantly photochemical or rather a thermal reaction. In this work, a model is developed and reflected to experimental data, to understand the physics behind the process. Through finite-element analysis and experimental characterizations it was observed that the physics behind the process was predominantly thermal, and that instead of an intermediate transition to a-Si, a direct transformation to poly-Si exists. By understanding this process, the treatment can be optimized or more efficient tools can be used that would enable a low cost production of high performing silicon devices.

Electrical and Structural Analysis on the Formation of n-type Junction in Germanium

Germanium (Ge) has re-emerged as a potential candidate to replace silicon (Si) as a substrate, due to its higher carrier mobility properties that are the key point for the realization of devices high drive current. However, the fabrication process of Ge is confronted with many problems such as low dopant electrical activation and the utilization of heavy n-type dopant atoms during ion implantation. These problems result in more damage and defects that can affect dopant activation. This paper reports the electrical and structural analysis on the formation of n-type junction in Ge substrate by ion implantation, followed by excimer laser annealing (ELA) using KrF laser. ELA parameters such as laser fluences were varied from 100 – 2000 mJ/cm2 and shot number between 1 – 1000 to obtain the optimized parameter of ELA with a high degree of damage and defect removal. Low resistance with a high degree of crystallinity is obtained for the samples annealed with less than five shot number. Higher shot number with high laser fluence, shows a high degree of ablation damage.

P‐77: Novel and Highly Reliable XeF and KrF Excimer Laser Annealing for Reducing the Cost of Flat Panel Display Equipment

One of the most important goals of flat panel display (FPD) manufacturing is to reduce the cost of excimer laser annealing (ELA) processes of low‐temperature poly‐Si (LTPS) technology. We developed novel XeF (351 nm) and KrF (248 nm) excimer lasers specially designed for ELA. The semiconductor‐field‐proven excimer laser technologies are applied to stabilize the operation and to reduce the cost.

P‐23: Effect of Buffer Layers on Performance and Reliability of Polycrystalline Silicon TFTs Fabricated on Polyimide

Thickness of buffer layers affects performance of flexible polycrystalline silicon thin film transistors (TFT). A thicker buffer SiO2 film demonstrates better endurance to thermal expansion stress from the polyimide substrate. However, the thickness of buffer SiO2 film would causes the variation of optimal excimer laser annealing (ELA) crystallization condition. In addition, the simulation results shows that the non‐uniform stress in the gate insulator is more pronounced near the poly‐silicon/gate insulator edge and at the two sides of the poly‐silicon protrusion. It would affect the stability results and device performance. By optimization the process condition, the device would still have good device performance after operating under high temperature (60 □) and long period of time (500hrs).

Investigation on Crack Suppression by Thermal-Plasma-Jet Crystallization of Amorphous Silicon Films on Flexible Glass Substrate

Introduction.Ultra-thin glass substrate is attracting much attention as flexible substrates for curved display applications. In fabrication of thin-film transistors (TFTs) on flexible glass, crystallization of amorphous Si (a-Si) is one of the key process technologies [1-3]. Rapid-thermal-annealing (RTA) in microsecond regime is necessary to form high crystallinity Si without thermal damage to glass. We have proposed the application of atmospheric pressure DC ark discharge thermal-plasma-jet (TPJ) to the crystallization of a-Si films [4]. Because of its simple structure and atmospheric pressure process, TPJ enables low cost crystallization compared with excimer laser annealing (ELA) technique. High speed lateral crystallization (HSLC) is induced by sweeping a molten region at a speed as high as 4000 mm/s. However, residual tensile stress is generated because of the densification of glass surface after RTA process. As a result, significant number of cracks are generated on the surface of glass substrate in cases of sever annealing conditions. In this study, we applied µ-TPJ crystallization of a-Si films on ultra-thin glass substrate and investigated the effect of glass thickness on residual stress and crack generation. Experimental.After RCA cleaning and HF treatment of 100-µm-thick flexible glass and 500-µm-thick conventional glass substrates, 100-nm-thick a-Si films were deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 250 °C. Dehydrogenation was carried at 450 °C in N2 ambient for 1 h. TPJ irradiation set up is shown in Fig. 1. µ-TPJ was generated by DC arc discharge under atmospheric pressure with supplying power (P) of 1.7 kW between electrodes. The Ar gas flow rate (f) was 1.0 L/min. µ-TPJ was generated by blowing out the thermal plasma through an orifice with its diameter (ϕ) of 600 µm. The distances between the plasma source and glass substrates (d) were fixed at 1.0 mm and the substrates were linearly moved by a motion stage in front of the µ-TPJ with scanning speed (v) of 2200 mm/s. For the measurement of temperature profile in the glass substrate, transient reflectivity during TPJ annealing was measured by irradiating bare conventional glass substrate with a He-Ne laser (632.8nm) from the back surface. Details of the non-contact temperature measurement are described in Ref [5]. Results and discussion.The microscope images of crystallized Si films on each substrate are shown in Fig. 2. Many cracks were observed in conventional glass substrate. On the other hand, no crack was observed in flexible glass substrate, although both glass substrates were annealed at the same time. Crystallization was conducted under various µ-TPJ irradiation conditions, and almost no cracks were found in flexible glass and it showed significant resistance against RTA. This result suggests that the tensile stress at glass surface is significantly reduced in the case of ultra-thin flexible glass. Previous study calculated residual stress profile in glass substrate on the basis of multilayered beam theory [6]. To obtain stress profile, we measured transient temperature profile during TPJ irradiation by optical probe (Fig .3). We assumed surface densification of glass when the temperature exceeded softening point (1258 K). We conducted residual stress analysis of glass substrate as the residual stress is shown in Fig. 4 [6]. Strong tensile stress is introduced in glass surface due to the shrinkage by RTA. At the same time, compressive stress is introduced inside of glass to compensate the tensile stress at the surface. It should be noted that the depth of stress compensation is in the range of ~400 µm as shown in Fig. 4, which is larger than the thickness of flexible glass. It is expected that the back side of thin glass substrate is free to relax the stress and plays a roll in suppression of residual stress in the surface of glass. Conclusions.We have crystallized a-Si films on flexible glass substrate by µ-TPJ without crack generation. Flexible glass substrate is useful to relieve surface tensile stress and as the result, crack generation is significantly suppressed. Acknowledgments. A part of this work was supported by Research Institute for Nanodevice and Bio Systems (RNBS) Hiroshima University, and JSPS KAKENHI Grant Number (B)(16H0433400).

H and Au diffusion in high mobility a-InGaZnO thin-film transistors via low temperature KrF excimer laser annealing

We report the fabrication of high mobility amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) irradiated by a single shot of a 248 nm KrF excimer laser. Very high mobilities (μ) of up to 43.5 cm2/V s were obtained after the low temperature excimer laser annealing (ELA) process. ELA induces high temperatures primarily in the upper layers and maintains very low temperatures of less than 50 °C in the substrate region. Scanning Transmission Electron micrographs show no laser induced damage and clear interfaces after the laser irradiation. In addition, several characterization studies were performed to determine the μ improvement mechanism. The analysis of Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy suggests incorporation of H mainly from the hybrid passivation layer into the channel. Moreover, Energy-dispersive X-ray Spectroscopy results show that Au diffused into the channel after ELA. Both KrF ELA-induced H and Au diffusion contributed to the higher μ. These results demonstrate that ELA can greatly enhance the electrical properties of a-IGZO TFTs for promising applications in large area, transparent, and flexible electronics.

Surface Engineering of Polycrystalline Silicon for Long-Term Mechanical Stress Endurance Enhancement in Flexible Low-Temperature Poly-Si Thin-Film Transistors.

The surface morphology in polycrystalline silicon (poly-Si) film is an issue regardless of whether conventional excimer laser annealing (ELA) or the newer metal-induced lateral crystallization (MILC) process is used. This paper investigates the stress distribution while undergoing long-term mechanical stress and the influence of stress on electrical characteristics. Our simulated results show that the nonuniform stress in the gate insulator is more pronounced near the polysilicon/gate insulator edge and at the two sides of the polysilicon protrusion. This stress results in defects in the gate insulator and leads to a nonuniform degradation phenomenon, which affects both the performance and the reliability in thin-film transistors (TFTs). The degree of degradation is similar regardless of bending axis (channel-length axis, channel-width axis) or bending type (compression, tension), which means that the degradation is dominated by the protrusion effects. Furthermore, by utilizing long-term electrical bias stresses after undergoing long-tern bending stress, it is apparent that the carrier injection is severe in the subchannel region, which confirms that the influence of protrusions is crucial. To eliminate the influence of surface morphology in poly-Si, three kinds of laser energy density were used during crystallization to control the protrusion height. The device with the lowest protrusions demonstrates the smallest degradation after undergoing long-term bending.

Impacts of excimer laser annealing on Ge epilayer on Si

The impacts of excimer laser annealing on the crystallinity of Ge epilayers on Si substrate grown by low- and high-temperature two-step approach in an ultra-high vacuum chemical vapor deposition system were investigated. The samples were treated by excimer laser annealing (ELA) at various laser power densities with the temperature above the melting point of Ge, while below that of Si, resulting in effective reduction of point defects and dislocations in the Ge layer with smooth surface. The full-width at half-maximum (FWHM) of X-ray diffraction patterns of the low-temperature Ge epilayer decreases with the increase in laser power density, indicating the crystalline improvement and negligible effect of Ge–Si intermixing during ELA processes. The short laser pulse time and large cooling rate cause quick melting and recrystallization of Ge epilayer on Si in the non-thermal equilibrium process, rendering tensile strain in Ge epilayer as calculated quantitatively with thermal mismatch between Si and Ge. The FWHM of X-ray diffraction patterns is significantly reduced for the two-step grown samples after treated by a combination of ELA and conventional furnace thermal annealing, indicating that the crystalline of Ge epilayer is improved more effectively with pre- annealing by excimer laser.

Film forming properties of silicon nanoparticles on SixNy coated substrates during excimer laser annealing

In this article we investigate the film forming properties of excimer laser annealed silicon nanoparticles on non-silicon substrates. In contrast to their film forming properties on oxide free silicon substrates, the nanoparticle thin film tends to dewet and form a porous µ-structure on the silicon nitrite covered glass model substrates considered for our investigation. This is quantified using a SEM study in conjunction with image processing software, in order to evaluate the µ-structure size and inter µ-structure distance in dependence of the laser energy density. To generalize our results, the film forming process is described using a COMSOL Multiphysics ® fluid dynamics model, which solves the Navier Stokes equation for incompressible Newtonian fluids. To account for the porous nanoparticle thin film structure in the simulation, an effective medium approach is used by applying a conservative level set one phase method to our mesh. This effort allows us to predict the Si melt film formation ranging from a porous Si µ-structure to a compact 100% density Si thin film in dependence of the substrate / thin film interaction, as well as the laser energy used for the nanoparticle processing.

Electrical compensation via vacancy–donor complexes in arsenic-implanted and laser-annealed germanium

Highly n-type Ge attained by shallow As implantation and excimer laser annealing was studied with positron annihilation spectroscopy and theoretical calculations. We conclude that a high concentration of vacancy–arsenic complexes was introduced by the doping method, while no sign of vacancies was seen in the un-implanted laser-annealed samples. The arsenic bound to the complexes contributes substantially to the passivation of the dopants.

Post-annealing effect on the optical property of indium tin oxide sputtered films

By using the spectroscopic ellipsometric technique, we investigated the excimer laser annealing (ELA) effect on the electronic properties of indium tin oxides (ITO) films fabricated by the DC-sputtering method, which is the one of the most commonly known methods for the commercial ITO films. We found that while the ELA process was helpful for enhancing the electronic property of the sputtered films, the degree of the enhancement was not so sizable as the case of the sol-gel films. This result appeared to originate from the difference in the physical properties of the sol-gel and sputtered amorphous films. We also examined the thermal annealing (TA) effect on the sputtered amorphous films in various ambient conditions, and compared their physical properties with those of the TA sol-gel films.

Excimer laser annealing for low-voltage power MOSFET

Excimer laser annealing of lumped beam was performed to form the P-base junction for high-performance low-voltage-power MOSFET. An equivalent shallow-junction structure for the P-base junction with a uniform impurity distribution is realized by adopting excimer laser annealing (ELA). The impurity distribution in the P-base junction can be controlled precisely by the irradiated pulse energy density and the number of shots of excimer laser. High impurity activation for the shallow junction has been confirmed in the melted phase. The application of the laser annealing technology in the fabrication process of a practical low-voltage trench gate MOSFET was also examined.

Role of SiOxNy surface passivation layer on stability improvement and kink effect reduction of ELA poly silicon thin film transistors

The role of SiOxNy thin layer growth on excimer laser annealed (ELA) polysilicon, by N2O plasma treatment, on the stability improvement under prolonged operation and bias stress of TFT was investigated. After NH3 plasma treatment, field effect mobility in p-channel TFT devices increased from 50 to 76cm2V−1s−1, ON/OFF current ratio improved to 4.6×107, threshold voltage shrank from −9.8V to −7.7V and kink effect in the ID-VD characteristics was diminished. Using NH3 plasma treatment combined with SiOxNy layer indicated a lesser effectiveness in decreasing interface states, but better in reducing of the influence of mobile charges on TFT performance, than the previous approaches. An improvement in field effect mobility of 60.5cm2V−1s−1, excellent stability under bias stress of ±1MV·cm−1 for 2h with ΔVth~0.1V were achieved. We found that under NH3 plasma treatment combined with SiNxOy thin layer; the effect of hot carrier injection were reduced significantly due to creating the SiN, and SiO bonding at the interface of gate insulator and active layer. This result indicated that thin film transistor using NH3 plasma treatment with SiOxNy passivated layer promises to improve TFT performances with high field effect mobility, high ON/OFF current ratio, low leakage current and excellent stability.

Erratum to: Progress of p-channel bottom-gate poly-Si thin-film transistor by nickel silicide seed-induced lateral crystallization

Excimer laser annealing (ELA) is known to be the most common crystallization technology for the fabrication of low-temperature polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) in the mass production industry. This technology, however, cannot be applied to bottom-gate (BG) TFTs, which are well developed for the liquid–crystal display (LCD) back-planes, because strong laser energy of ELA can seriously damage the other layers. Here, we propose a novel high-performance BG poly-Si TFT using Ni silicide seed-induced lateral crystallization (SILC). The SILC technology renders it possible to ensure low damage in the layers, smooth surface, and longitudinal large grains in the channel. It was observed that the electrical properties exhibited a steep subthreshold slope of 110 mV/dec, high field-effect mobility of 304 cm2/Vsec, high Ion/Ioff ratio of 5.9 × 107, and a low threshold voltage of −3.9 V.

Nano-crystallization in ZnO-doped In2O3 thin films via excimer laser annealing for thin-film transistors

In a previous work, we reported the high field effect mobility of ZnO-doped In2O3 (IZO) thin film transistors (TFTs) irradiated by excimer laser annealing (ELA) [M. Fujii et al., Appl. Phys. Lett. 102, 122107 (2013)]. However, a deeper understanding of the effect of ELA on the IZO film characteristics based on crystallinity, carrier concentrations, and optical properties is needed to control localized carrier concentrations for fabricating self-aligned structures in the same oxide film and to adequately explain the physical characteristics. In the case of as-deposited IZO film used as the channel, a high carrier concentration due to a high density of oxygen vacancies was observed; such a film does not show the required TFT characteristics but can act as a conductive film. We achieved a decrease in the carrier concentration of IZO films by crystallization using ELA. This means that ELA can form localized conductive or semi-conductive areas on the IZO film. We confirmed that the reason for the carrier concentration decrease was the decrease of oxygen-deficient regions and film crystallization. The annealed IZO films showed nano-crystalline phase, and the temperature at the substrate was substantially less than the temperature limit for flexible films such as plastic, which is 50°C. This paves the way for the formation of self-aligned structures and separately formed conductive and semi-conductive regions in the same oxide film.