Tayeb Mohammed-Brahim

Nd:YVO 4 Laser Crystallization for Thin Film Transistors with a High Mobility

We crystallize amorphous silicon films with a frequency doubled Nd:YVO 4 laser operating at a repetition frequency of up to 50 kHz. A sequential lateral solidification process yields polycrystalline silicon with grains longer than 100 μm and a width between 0.27 and 1.7 μm depending on film thickness and laser repetition frequency. The average grain size is constant over the whole crystallized area of 25 cm 2 . Thin film transistors with n- type and p-type channels fabricated from the polycrystalline films have average field effect mobilities of μ n = 467 cm 2 /Vs and μ p = 217 cm 2 /Vs respectively. As a result of the homogeneous grain size distribution, the standard deviation of the mobility is only 5%.

Analysis of the activation energy of the subthreshold current in laser- and solid-phase-crystallized polycrystalline silicon thin-film transistors

Analysis of the thermal and gate-voltage dependences of the current in the subthreshold region is performed on both low-temperature laser-crystallized and solid-phase-crystallized polycrystalline silicon (polysilicon) thin-film transistors (TFTs). Temperature measurements are made at first in order to extract the variations of the activation energy EA of the drain current with the gate voltage. The plot of the subthreshold current versus the measured activation energy leads to an apparent activation energy EA/n, where the n factor is extracted from the slope of this plot. The n factor is close to 1 for laser-crystallized polysilicon TFTs while it is rather close to 2 for solid-phase-crystallized ones. These two values can be attributed to a different defect distribution in the two differently crystallized TFTs polysilicon active layers.

Epoxy Based Ink as Versatile Material for Inkjet-Printed Devices.

Drop on Demand inkjet printing is an attractive method for device fabrication. However, the reliability of the key printing steps is still challenging. This explains why versatile functional inks are needed. Epoxy based ink described in this study could solve this critical issue because it can be printed with low drawbacks (satellites droplets, long-lived filaments, etc.). Moreover, a wide concentration range of solute allows the fabrication of films from thin to high aspect ratio. Optimizing experimental parameters (temperature, overlap) and ink composition (single or cosolvent) is useful to tune the film profile. As a result, many shapes can be obtained such as donuts or hemispherical caps for a droplet and smooth or wavy shape for a thin film. This study demonstrates that epoxy based versatile ink can be used in numerous fields of applications (organic electronics, optics, sensors, MEMS, etc.). To prove this assertion, organic field effect transistors and light emitting films have been fabricated.

Study of the solid phase crystallization behavior of amorphous sputtered silicon by X-ray diffraction and electrical measurements

In situ X-ray diffraction (XRD) measurements have been used to study the amorphous-to-crystalline transformation in hydrogenated amorphous silicon (a-Si:H) thin films deposited by DC-Magnetron Sputtering at 300°C. The a-Si:H layers of 2.85 μm thickness were solid phase crystallized (SPC) and the crystallization kinetic was studied from in situ XRD measurements and also by in situ electrical conductance measurements during isothermal annealing at 630°C. The apparition and the evolution of the (1 1 1) peak in the XRD spectra during the annealing of the layer permit to follow the SPC kinetic which is the same as the electrical conductance kinetic (G=f(t)) performed in the same annealing conditions as in the XRD experiment. Several isothermal annealings at different temperatures permit to extract the characteristic parameters of the crystallization from the G=f(t) evolutions. These parameters are the thermally activated crystallization characteristic time and its activation energy.

Transport mechanisms in hydrogenated microcrystalline silicon

Transport properties of microcrystalline (μc-Si/H) and polycrystalline (p-Si) silicon films are analyzed by time resolved microwave conductivity (TRMC), diffusion-induced TRMC (DTRMC), and Hall measurements. The comparison of carrier mobilities in microcrystalline silicon determined by TRMC as well as DTRMC shows that trapping in the disordered part of these films is not the main limiting parameter for transport in microcrystalline silicon. Besides, it is demonstrated that TRMC measurements are not sensitive to barriers between the crystallites. Our measurements reveal that, contrary to the case of p-Si, the influence of barriers in μc-Si/H can be neglected. Transport in μc-Si/H is consequently mainly limited by defects inside the crystallites.

Low temperature (≦600°C) unhydrogenated in-situ doped polysilicon thin film transistors: Towards a technology for flat panel displays

Abstract Low temperature unhydrogenated in-situ doped polysilicon thin film transistors with a SiO 2 deposited gate insulator were fabricated using a four-mask aluminium gate process. Several processes were varied—in particular the deposition pressure of the polysilicon layers, thermal annealing, and cleaning process of the surface of the active layer. The two polysilicon layers, which make up the active layer and the in-situ doped source and drain regions, were deposited at an optimized pressure ( P = 90 Pa) in the amorphous state and crystallized by a thermal annealing. This procedure was performed before plasma etching of the source/drain polysilicon layer. An oxygen plasma + RCA-type wet cleaning were used to ensure the obtainment of a good APCVD SiO 2 gate insulator/active layer interface quality. Therefore, these thin film transistors exhibit good electrical properties: a low threshold voltage (≈ 2 V), a high field effect mobility (> 60 cm 2 Vs −1 ), and a high On/Off state current ratio (≧ 10 7 ) for a drain voltage V ds = 1 V. It is worth noting that these results are similar to those of hydrogenated TFTs made using this type of process. Consequently the TFTs described here could be good candidates for flat panel display applications.

From amorphous to polycrystalline thin films : dependence on annealing time of structural and electronic properties

Some new results about the amorphous to polycrystalline transition of silicon thin films obtained by low pressure chemical vapor deposition (LPCVD) at 550°C are presented. From in situ (monitored during the crystallization annealing) conductance, electron spin resonance, photoluminescence and modulated photocurrent experiments, the density of states is shown to increase in the so-called nucleation regime and reach a maximum just before crystal grain growth starts. These results indicate that crystallization needs the creation of a defected material with large dangling bond densities and wide band-tails. It appears that the electronic quality of the final polycrystalline material is linked to the maximum density of states reached just at the end of the nucleation phase. This level may be associated to a viscous structure.

Polycrystalline silicon thin films for MEMS applications

Abstract Thanks to its interesting mechanical and electrical properties, silicon represents the first candidate as a structural material in the Micro Electro Mechanical Systems field. Doped polycrystalline silicon films are generally used, particularly when electrostatically movable mechanical structures are needed. Here, we investigate the effects of the doping type as well as the post-deposition thermal treatments on the mechanical behaviour of in situ doped polycrystalline silicon films deposited by low pressure chemical vapour deposition from a mixture of silane SiH4 and phosphine or diborane. Stress measurements, performed using micro-Raman spectroscopy, are related to the behaviour of micro fixed–fixed beams as determined from optical and scanning electron microscopy observations. The films, regardless of their doping type, are found tensely stressed when the amorphous deposited films are solid phase crystallised at 600 °C. The tensile stress is reduced becoming compressive when the crystallization temperature is increased. An optimum tensile stress value, corresponding to the maximum of the beam free length, is determined. Finally, air-gap thin film transistors (TFTs) using these doped fixed–fixed beams are realised. Electrical parameters of these TFTs (field effect mobility, threshold voltage, and subthreshold slope) may be considered as good. Particularly the low value of the threshold voltage, 2.5 V, is very interesting for handling devices where the power consumption saving is crucial.

In situ doping of silicon deposited by LPCVD: pressure influence on dopant incorporation mechanisms

The influence of the silane pressure on the dopant incorporation during LPCVD silicon deposition at 550C using silane and phosphine or diborane (H) is examined, for a range of pressure from 1 to 100 Pa. We conclude that different deposition and dopant incorporation mechanisms occur according to the deposition pressure. It is shown that, under low-pressure conditions, silane remains the preponderant host species, while it is silylene at high pressure. At low pressure, and or H are separately but not independently adsorbed. At high pressure, the presence of silylene promotes the formation of monosilylphosphine and monosilylborane which are found to be the adsorbed dopant species. The usual change of the growth rate caused by the addition of the dopant, i.e. a reduction with phosphine and an increase with diborane, is a function of the silane pressure; the dopant content of the solid films causes a significant variation of the growth rate only when it is superior to a threshold of about in both cases.

Effect of the starting amorphous structure on the solid-phase crystallization of silicon

The effect of the deposition rate of amorphous silicon films grown by low-pressure chemical vapour deposition on the quality of furnace-crystallized films is studied. Numerous physical, optical and electrical characterization techniques have been used. The use of the X-ray diffraction technique in the study of polycrystalline silicon microstrains is particularly presented. Results show that, the higher the deposition rate, the higher is the quality of the polycrystalline silicon obtained. This quality is obtained near the limiting rate above which powder formation begins. This quality may be explained by the microstructure of the as-deposited amorphous films and in particular by the role of the low hydrogen content which delays the nucleation and increases the subsequent crystallization rate.