Polysilicon Thin Film Research Articles

Poly‐Si active matrix organic light‐emitting diode pixel circuit with compensation for threshold voltage and mobility variations

A new pixel circuit and driving method for large-area, high-luminance active matrix organic light-emitting diode (AMOLED) displays were studied to improve the image quality. The AMOLED display was designed with low-temperature poly-silicon thin film transistors (TFTs), which have good stability but poor uniformity. To overcome the uniformity problem, the proposed pixel circuit compensates for variations in the threshold voltage and the mobility of the driving TFT. The proposed pixel circuit can operate in two compensation modes for high-luminance operation; and the black data insertion was introduced to improve the characteristics of moving images. The pixel circuit was designed for an 11.6″WXGA top-emission AMOLED panel, and its non-uniformity was estimated to be < 4% with a mobility compensation time of 1 μs.

Synthesis and Characterization of LPCVD Polysilicon and Silicon Nitride Thin Films for MEMS Applications

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated MEMS devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not very well controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition conditions; experiments were carried out to study the polysilicon and silicon nitride deposited by Low Pressure Chemical Vapor Deposition (LPCVD) method at wide range of process conditions. High temperature annealing effects on stress in case polysilicon are also reported. The reduced residual stress levels can significantly improve device performance, reliability, and yield as MEMS devices become smaller.

Hydrogenation Effect on Electrical Behavior of Polysilicon Thin Films

Despite their wide availability and relatively low prices, the conventional energy sources have harmful consequences on the environment and are exhaustible. In order to circumvent these negative effects, the renewable energies in general and the photovoltaic energy in particular are becoming more and more attractive. In this type of conversion, the low cost polycrystalline silicon solar cells are promising. However, the photovoltaic parameters of these cells are affected by the existence of the grain boundaries (GBs) and the segregation of impurities at these GBs. In this work, we are interested in studying the effect of hydrogenation on the electrical behavior in polysilicon thin films. The results have shown that the introduction of hydrogen in a polysilicon films reduces the density of trap states at the GBs, and hence their electronic activity. This is translated in the reduction of the resistivity, the improvement the mobility of carriers and an increase in the concentration of free carriers. In addition, it has been shown that the effect of hydrogen on the electrical characteristics is more pronounced, on the one hand, for low doping concentrations, and on the other hands, in boron doped films than in arsenic doped films. This is mainly due to the strong segregation of arsenic atoms at the GBs, which reduces the effect of hydrogen on the density of trap states by reducing its penetration in the GBs.

The role of H-plasma on aluminum-induced crystallization of amorphous silicon

We propose a technique to improve and accelerate aluminum-induced crystallization (AIC) by hydrogen plasma. Raman spectroscopy and secondary ion mass spectrometry of crystallized poly-silicon thin films show that hydrogen plasma radicals reduce the crystallization time of AIC. This technique shortens the annealing time from 10 to 4 h and increases the Hall mobility from 22 to 42 cm2/V s. The possible mechanism of AIC assisted by hydrogen radicals is also discussed.

Flexible Thermoelectric Devices Based on Indium Phosphide Nanowire Networks on Copper

ABSTRACTMore than 50% of total input energy is wasted as heat in various industrial processes. If we could harness a small fraction of the waste heat while satisfying the economic demands of cost versus performance, then thermoelectric (TE) power generation could bring substantial positive impacts. To meet these demands single-crystal semiconductor nanowire networks have been investigated as a method to achieve advanced TE devices because of their predicted large reduction in thermal conductivity and increase in power factor.To further our goal of developing practical and economical TE devices, we designed and developed a material platform that combined a semiconductor nanowire network and a semiconductor thin film integrated directly on a mechanically flexible metallic substrate. We assessed the potential of this platform by using indium phosphide (InP) nanowire networks and a doped poly-silicon (poly-Si) thin film combined on copper sheets. InP nanowires were grown by metal organic chemical vapor deposition (MOCVD). In the nanowire network, InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. Subsequently, plasma-assisted CVD was utilized to form a poly-Si thin film to prevent electrical shorting when an ohmic copper top contact was made. An additional facet to this design is the utilization of multiple materials to address the various temperature ranges at which each material is most efficient at heat-to-energy conversion. The utilization of multiple materials could enable the enhancement of total power generation for a given temperature gradient. We investigated the use of poly-Si thin films combined with InP nanowires to enhance TE properties. TE power production and challenges of a large area nanowire device on a flexible metallic substrate were presented.

An ambient-light sensor system with startup correction for LTPS-TFT LCD

An ambient-light sensor system using low-temperature poly-silicon thin film transistors was studied, to reduce the power consumption of mobile applications. The proposed system with startup correction can correct the panel-to-panel variation of the ambient-light sensors, without additional equipment. The digital output of the readout circuit has 8 levels per decade for the input luminance ranges from 100 to 10000lux, and the readout rate is 40Hz. The proposed method provides maximum differential non-uniformity below 0.3 LSB.

Improving the Microstructure and Electrical Properties of Aluminum Induced Polysilicon Thin Films Using Silicon Nitride Capping Layer

We investigated the capping layer effect of SiNx (silicon nitride) on the microstructure, electrical, and optical properties of poly‐Si (polycrystalline silicon) prepared by aluminum induced crystallization (AIC). The primary multilayer structure comprised Al (30 nm)/SiNx (20 nm)/a‐Si (amorphous silicon) layer (100 nm)/ITO coated glass and was then annealed in a low annealing temperature of 350°C with different annealing times, 15, 30, 45, and 60 min. The crystallization properties were analyzed and verified by X‐ray diffraction (XRD) and Raman spectra. The grain growth was analyzed via optical microscope (OM) and scanning electron microscopy (SEM). The improved electrical properties such as Hall mobility, resistivity, and dark conductivity were investigated by using Hall and current‐voltage (I‐V) measurements. The results show that the amorphous silicon film has been effectively induced even at a low temperature of 350°C and a short annealing time of 15 min and indicate that the SiNx capping layer can improve the grain growth and reduce the metal content in the induced poly‐Si film. It is found that the large grain size is over 20 μm and the carrier mobility values are over 80 cm2/V‐s.

In-situ determination of the coefficient of thermal expansion of polysilicon thin films using micro-rotating structures

A simple method for in situ determination of the coefficient of thermal expansion (CTE) of polysilicon thin films by using micro-rotating structures is presented. The structures are heated electrically and deflect due to the thermal expansion. An analytical expression is derived to relate the CTE of the polysilicon thin film with the lateral displacement, geometrical parameters and the average temperature of the test structure. The lateral displacement is designed to be a constant value (2μm), while the average temperature of the test structure can be obtained from the measurement of resistance of the test structure. Instead of an optical or visual readout, electrical input and electrical readout are utilized. In the experiments, a current–voltage measurement system only is required and all measurements can be carried out in atmosphere. Finite element analysis and experimental results with surface micromachined polysilicon thin films are used to demonstrate the effectiveness of the proposed method. The average value of the obtained CTE is (2.76±0.09)×10−6K−1 with temperature ranging from 450K to 500K.

Polysilicon thin films fabricated by solid phase crystallization using reformed crystallization annealing technique

In this work, a reformed crystallization annealing technique is presented for the solid phase crystallization (SPC) of amorphous silicon (a-Si) on SiNx-coated quartz substrate. This technique includes a two-step annealing process which consists of a low-temperature (475°C) classical furnace annealing for nucleation of Si and a high-temperature (900°C) grain growth process of polycrystalline silicon (poly-Si) during thermal annealing in classical tube furnace. The aim of this reformed two-step annealing technique is reducing the long (up to 48h) crystallization annealing duration of single step annealing at low temperatures (~600°C) while maintaining the film quality, as low-temperature single step annealing, by using reformed technique. Continuous p-type poly-Si film was formed on quartz substrate thanks to exodiffusion of boron, which was deposited prior to a-Si, through Si film by thermal annealing. The stress and degree of crystallinity of the p-type poly-Si were studied by the micro-Raman Spectroscopy. The crystallization fraction value of 95% was deduced for annealed samples at 900°C, independent from crystallization technique. On the other hand, the Raman analysis points out that compressive stress was induced by increasing the annealing duration at 900°C. X-ray diffraction (XRD) analysis reveals that the preferred crystallite orientation of the films, independent from crystallization temperature and substrates, is <111>. Additionally, the average crystallite size calculated from XRD patterns increases from 69Å to 165Å by using reformed two-step annealing instead of single step annealing at 900°C for 90min. The exodiffusion of boron into the silicon film was deduced from secondary ion mass spectrometry (SIMS) analysis and the p+/p graded boron profile was obtained, which may result higher carrier diffusion length and longer carrier life time. Finally, the annealing duration dramatically decrease to 9h by using reformed two-step annealing technique instead of conventional single step annealing at 600°C. The results show that reformed SPC annealing technique has major advantages by combining the lower annealing duration and high crystal quality.

Structure disorder degree of polysilicon thin films grown by different processing: Constant C from Raman spectroscopy

Flat, low-stress, boron-doped polysilicon thin films were prepared on single crystalline silicon substrates by low pressure chemical vapor deposition. It was found that the polysilicon films with different deposition processing have different microstructure properties. The confinement effect, tensile stresses, defects, and the Fano effect all have a great influence on the line shape of Raman scattering peak. But the effect results are different. The microstructure and the surface layer are two important mechanisms dominating the internal stress in three types of polysilicon thin films. For low-stress polysilicon thin film, the tensile stresses are mainly due to the change of microstructure after thermal annealing. But the tensile stresses in flat polysilicon thin film are induced by the silicon carbide layer at surface. After the thin film doped with boron atoms, the phenomenon of the tensile stresses increasing can be explained by the change of microstructure and the increase in the content of silicon carbide. We also investigated the disorder degree states for three polysilicon thin films by analyzing a constant C. It was found that the disorder degree of low-stress polysilicon thin film larger than that of flat and boron-doped polysilicon thin films due to the phase transformation after annealing. After the flat polysilicon thin film doped with boron atoms, there is no obvious change in the disorder degree and the disorder degree in some regions even decreases.

Electrical Activation of Ion Mass Doped Phosphorous

In this study, three different crystalline states of silicon were prepared to be doped with phosphorous by IMD, amorphous, poly crystalline and single crystalline silicon. The dose was controlled by IMD duration time and heat treatment for electrical activation was done in RTA and Furnace. In case of RTA, annealing temperature was controlled by the duration time of power application. In case of a single crystal substrate, the resistance was measured to be 20-50 omega/square depending on the dose and annealing temperature. In case of poly crystal, we could observe segregation of the dopant at grain boundaries, which caused increase of the resistance with increase of annealing temperature. In case of amorphous silicon thin film, this phenomenon could not be observed due to lack of the grain boundaries and the minimum resistance of this work was about 300 omega/square, which was about the same to that in a poly silicon thin film.

Direct Laser Doping of Poly-Silicon Thin Films Via Laser Chemical Processing

Laser chemical processing (LCP) is an attractive doping technique for thin films due to its process simplicity, high achievable doping concentrations, and relatively shallow doping depths. During LCP processing, an infinite supply of dopants is available from the pressurized doping medium. In this paper, LCP is employed for n-type doping of poly-silicon thin films on glass. We achieved a peak doping concentration of 6 × 1018 to 1 × 1019 cm-3 and a junction depth up to 350 nm, as determined by electrochemical capacitance-voltage profiling and secondary ion mass spectrometry. We evaluate the structural quality of the LCP-doped layers with cross-sectional transmission electron microscopy (XTEM), as well as ultraviolet reflectance measurements. The LCP-doped regions are of suitable material quality for device fabrication. The resulting sheet resistance and doping levels are promising for a back surface field for poly-silicon thin-film solar cells on glass (e.g., an n+/ n-/ p+/glass layer structure in superstrate configuration).

Infrared Spectroscopy Study of a Poly-Silicon Film for Optimizing the Boron-Implanting Dose

An experiment was carried out for recording the infrared transmission between 3 and 16 μm for a free-standing poly-silicon (Si) thin film as a function of the boron-ion implanting. The optical constants were extracted in order to optimize the postdeposition implanting of the membranes in a surface micromachined Fabry-Perot interferometer for the thermal infrared. The free-carrier concentration must not degrade the refractive index n significantly since a high and constant value over the application wavelength range is desired. Moreover, an increase in the extinction coefficient k is detrimental for the interferometer performance. On the other hand, poly-Si electrical conductivity must be doped high enough to avoid static charging and to prevent any layout-dependent distribution of the applied control voltage. We applied the variable-angle transmission spectrometry for recording the data of suspended poly-Si membranes, doped with different levels of implanting dose. The Drude model dispersion formula was exploited for extracting the optical constants n(λ) and k(λ). The optical constants are presented as a function of the dopant concentration and the electrical resistivity.

Investigation of the instability of low-temperature poly-silicon thin film transistors under a negative bias temperature stress

In this work, we analyzed and correlated the hysteresis characteristics and instability under negative bias temperature instability (NBTI) stress in p-channel low-temperature poly-silicon (LTPS) thin-film transistors (TFTs). Positive VTH shifts were observed under the NBTI stress. The hysteresis does not appear to be affected by the NBTI stress; however, when the VG stress voltage is −40 V at 100°C, the hysteresis increases as the stress time increases and VTH shifts with sub-threshold slope (SS) degradation. The hysteresis may increase under the extreme stress condition due to the generation of trap-states.

High voltage characteristics of junctionless poly-silicon thin film transistors

The breakdown voltage (VBD) and breakdown mechanism of junctionless (JL) poly-Si thin film transistor (TFT) were compared to the conventional inversion-mode (IM) TFT using fabricated devices and 3D quantum-corrected hydrodynamic transport device simulation. The simulated results are correspondent with experimental ones. The analyses of electric field distributions in on-state show that the channel of JL devices can equally share the voltage like a resistor, because there are no junctions formed between channel and source/drain. The JL TFT shows excellent breakdown characteristics; the off-state VBD of 53.4 V is several times larger than VBD of 9.5 V in IM TFT with same device size. JL devices have large potential for high voltage power metal-oxide-semiconductor devices and circuit applications.

Estimation of Thermal Conductivity of Amorphous Silicon Thin Films from the Optical Reflectivity Measurement

Amorphous silicon (a-Si) thin film material is widely used in liquid crystal display and solar cell applications. Knowledge of its properties is important in enhancing device performance. The properties of a-Si thin film have not been well understood due to the lack of periodicity of the structure. Furthermore, thermal conductivity of a-Si thin film is a key parameter to understand the complex phase transformation mechanism from a-Si thin film to polysilicon thin film by analyzing the transient temperature during the laser recrystallization process. In this work, thermal conductivity of a-Si thin film was determined by measuring optical reflectivity. A-Si thin film was irradiated with a KrF excimer laser beam to raise its temperature. The raised film temperature affects temperature-dependent optical properties such as refractive indices and extinction coefficients. The temperature-dependent optical properties of refractive indices and extinction coefficients of a-Si thin film were measured by ellipsometry. In-situ transient reflectivity at the wavelength of 633 nm was obtained during the excimer laser irradiation. The numerical simulation of one-dimensional conduction equation was solved so that transient reflectivities were calculated with temperature-dependent optical properties combined with thin film optics. Therefore, a well-fitted thermal conductivity was determined by comparing the numerically obtained transient reflectivity with the experimentally measured reflectivity data. The determined thermal conductivity of a-Si thin films was 1.5 W/mK.

N-channel polysilicon thin film transistors as gamma-ray detectors

N-type thin film transistors (TFTs) fabricated with fluorinated and hydrogenated polysilicon were exposed to gamma radiation to a cumulative dose up to 1200 Gy(Si). During irradiation, the gate electrode was biased with a positive voltage up to 4.8 V. The effect of irradiation on the electrical characteristics of the devices was monitored after the irradiation steps and in a prolonged period after irradiation. The main monitoring parameter was the threshold voltage which was found to have linear dependence on irradiation dose. This, together with obtained low fading, suggests that thin transistors have potential as radiation dosimeters.

Studies on High Strain Point Aluminosilicate Glasses

A family of alkaline earth aluminosilicate glasses has been introduced in this paper, that exhibit a high strain point temperature in excess of 740°C, the coefficient of thermal expansion of (27~38) ×10-7/°C. The glasses consist of SiO2 59-80 mol%, Al2O3 10-25 mol%, CaO 5-10 mol%, SrO 1-4 mol%, ZnO 4-6 mol%, SnO2 0.08 mol%, ZrO2 0-4 mol%, Y2O3 0-2 mol%. Their properties show that these glasses can be used as substrates with polysilicon (p-Si) thin film coating on their surface, in particular, for flat panel displays.

System-Level Simulation and Fabrication of On-Chip Fatigue Bending Test Structure for Micro-Scale Polysilicon Films

Two kind of on-chip integrated fatigue bending test structures are designed through system-level simulation method based on macromodels to measure the fracture strength and fatigue mechanical properties of polysilicon thin films. The first on-chip fatigue test structure is actuated by V-beam thermal actuator, and the other test structure actuated by electrostatic comb. The static and dynamic analysis was performed by Coventorware Architect module using self-bulid reduced order model described with the MAST hardware language and some other commercial parts from Coventorware parts library. The structural dimension parameters are determined and optimized according to system-level simulation and the computing result has shown that the self-build macromodels and the on-chip integrated test system are efficient and reliable. Two kinds of polysilicon on-chip fatigue bending test structure were fabricated with two-layer polysilicon surface micromachining process in Institute of Microelectronics, Peking University.

Explicit calculation for grain boundary barrier height in polysilicon TFTs based on quasi-two-dimensional approach

A physical-based explicit calculation to the height of grain boundary barrier has been derived based on the quasi-two-dimensional approach at discrete grain boundaries. The analytical solution is obtained by using the Lambert W function, combining both the uniform distributed deep states and the exponential tail states. The proposed scheme is demonstrated as an accurate and computationally efficient solution in a closed form, which can serve as a basis for the discrete-grain-based models of mobility and drain current in polysilicon thin film transistors. It is verified successfully by comparisons with both numerical simulation and experimental data.