微量Hf对大角度晶界含Re双晶合金高温持久性能的影响

采用籽晶法制备含有大角度晶界(约20°)的双晶试板,通过分析不同Hf含量(质量分数:0%,0.4%)的含Re合金晶界处析出相、γ/γ′组织、晶界成分及1100℃/100MPa横向持久性能,研究Hf对晶界组织及高温力学性能的影响。结果表明:Hf显著提高了铸态合金大角度晶界处共晶和碳化物体积分数;热处理后,Hf显著抑制了晶界胞状再结晶组织的形成,含Hf合金的1100℃/100MPa横向持久寿命均显著提高。晶界持久性能与晶界析出相种类、形貌、含量和成分密切相关,而Hf元素在晶界未发现显著的偏聚。本研究对先进镍基单晶合金中晶界缺陷的评价及Hf元素晶界强化作用机制的认识具有一定的指导意义。     

Low temperature short channel polycrystalline silicon thin film transistors with high reliability for flat panel display

We have investigated a short channel (L ≤ 1 μm) effect on the electrical reliability of the low temperature poly-Si thin film transistors (TFT) on a glass substrate. The threshold voltage of the p-type poly-Si TFT was observed to be decreased due to the drain induced barrier lowering as the channel length decreased. In the n-type poly-Si TFT with a lightly-doped-drain (LDD), the threshold voltage was slightly decreased when a high drain voltage was applied, while the field effect mobility decreased due to the series resistance of the LDD region in the short channel poly-Si TFT. As the temperature increased, the field effect mobility increased about 80% due to the increase of the thermal activated carrier concentration. We have also investigated the degradation of a short channel poly-Si TFT under hot carrier and self-heating stress. After hot carrier stress (V_GS = 2V, V_DS = 15V), the field effect mobility was considerably decreased up to 20% due to the trap state generation induced by the hot carrier. The subthreshold slope and threshold voltage were scarcely degraded. After the self-heating stress (V_GS = V_DS = 15V), the subthreshold slope, mobility, and threshold voltage were degraded. Transfer characteristics measured at the high drain voltage (V_DS = 10V) were shifted to a negative direction because of hole trapping at the backside interface between the polysilicon film and buffer oxide on the glass substrate.     

Application of fluorine doped oxide (SiOF) spacers for improving reliability in low temperature polycrystalline thin film transistors

The novel process of self-aligned fluorine doped oxide (SiOF) spacers on low temperature poly-Si (LTPS) lightly doped drain (LDD) thin film transistors (TFTs) is proposed. A fluorine doped oxide spacers were provided to generate the lower dissociation Si-F bonds adjusted to the interface of the drain which is the largest lateral electric field region for lightly doped drain structure. The stronger Si-F bonds can reduce the bonds broken by impact ionization. It is found that the output characteristics of SiOF spacers TFTs show the superior immunity to kink effect. The degradations in Vth shifting, subthreshold slope, drain current and transconductance of SiOF spacers after DC stress are improved.     

Energy equilibrium of cleavage front transmission across a high-angle grain boundary in a free-standing silicon thin film

In this article, the crack growth driving force and the resistance to cleavage cracking associated with crack front transmission across a high-angle grain boundary in a silicon thin film are analyzed, and a closed-form solution of grain boundary toughness is obtained. It is noticed that the fracture resistance of the grain boundary is a function of the film thickness. This size effect is attributed to the nonuniform nature of cleavage front advance.     

Influence of process steps on the performance of microcrystalline silicon thin film transistors

Bottom gate microcrystalline silicon thin film transistors (μc-Si TFT) have been realized with two types of films: μc-Si(1) and μc-Si(2) with crystalline fraction of 80% and close to 100% respectively. On these TFTs we applied two types of passivation (SiN_x and resist). μc-Si TFTs with resist as a passivation layer present a low leakage current of about 2.10^− 12 A for V_G = − 10 and V_D = 0.1V an ON to OFF current ratio of 10⁶, a threshold voltage of 7 V, a linear mobility of 0.1 cm²/V s, and a sub-threshold voltage of 0.9 V/dec. Microcrystalline silicon TFTs with SiN_x as a passivation present a new phenomenon: a parasitic current for negative gate voltage (− 15 V) causes a bump and changes the shape of the sub-threshold region. This excess current can be explained by and oxygen contamination at the back interface.     

The influence of a threshold stress for grain boundary sliding on constitutive response of polycrystalline Al during high temperature deformation

A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s⁻¹, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by , where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249].     

Effects of neutral particle beam on nano-crystalline silicon thin films, with application to thin film transistor backplane for flexible active matrix organic light emitting diodes

A novel deposition process for nano-crystalline silicon (nc-Si) thin films was developed using neutral beam assisted chemical vapor deposition (NBaCVD) technology for the application of the thin film transistor (TFT) backplane of flexible active matrix organic light emitting diode (AMOLED). During the formation of a nc-Si thin film, the energetic particles enhance nano-sized crystalline rather microcrystalline Si in thin films. Neutral Particle Beam (NPB) affects the crystallinity in two ways: (1) NPB energy enhances nano-crystallinity through kinetic energy transfer & chemical annealing, and (2) heavier NPB (such as Ar) induces damage & amorphization through energetic particle impinging. Nc-Si thin film properties effectively can be changed by the reflector bias. As increase of NPB energy limits growing the crystalline, the performance of TFT supports this NPB behavior. The results of nc-Si TFT by NBaCVD demonstrate the technical potentials of neutral beam based processes for achieving high stability and reduced leakage in TFT backplanes for AMOLEDs.     

Production of very fine grained AISI 304 steel with high special grain boundary density by grain boundary engineering

Abstract

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries.     

Comprehensive studies on the stabilities of a-In-Ga-Zn-O based thin film transistor by constant current stress

Stability under constant current stress, along with hysteresis characteristics, was studied for a-In-Ga-Zn-O thin-film transistors (TFTs) in several atmospheres and at several temperatures. Unannealed TFTs showed rather large instability; i.e., large hysteresis in transfer curves (ΔV_G > 0.8 V) and large positive threshold voltage shift (ΔV_th > 10 V for 50 h tests at 5 µA) with deterioration of subthreshold voltage swing was observed. The instability for the unannealed TFT had a strong dependence on the stress atmosphere and the stress temperature, which suggests that trap states generated by the stress test is related to oxygen vacancy formed by breaking weak chemical bonds. Wet annealing improved stability; the hysteresis disappeared and the ΔV_th was reduced to < 2 V. The improvement is considered to be related to the reduction of weak chemical bonds by wet annealing with the strong oxidation power of water molecules.     

Device degradation behavior and polysilicon film morphology of thin film transistors fabricated using advanced excimer laser lateral solidification techniques

The degradation of polysilicon thin film transistors fabricated in films obtained using variations of advanced through-mask excimer laser anneal sequential lateral solidification (SLS) schemes was investigated. The morphology and grain structure of these 50 nm thick polysilicon films was studied using SEM and AFM. Very elongated or square-like polycrystalline silicon grains were observed, as shaped by each crystallization technique. Hot carrier stressing measurements, under gate and drain DC biases, were performed and the TFT device parameters and characteristics were extracted for various stressing times. The threshold voltage V_th, subthreshold slope S and transconductance G_m were observed to exhibit shifts with stressing time, indicating some active layer and interface degradation ascribed to hot carrier injection and trap generation. These shifts depended both on stress conditions and on the fabrication technique used. The hot carrier stressing results thus indicate that the material structure affects the degradation rates of the TFT parameters and trap densities. Furthermore, the device structure and the crystallization conditions, with the resulting film morphology, affect not only the TFT degradation behavior but also other aspects of device performance; the susceptibility to drain current avalanche effects was found to be lower for TFTs in 2^N-shot polysilicon compared to ones in very elongated grain (directional) material.     

Study on electrical degradation of p-type low-temperature polycrystalline silicon thin film transistors with C-V measurement analysis

Laser recrystallized low-temperature poly-silicon (LTPS) films have attracted attention for their application in thin-film transistors (TFTs), which are widely used in active matrix display. However, the degradation behavior of p-type LTPS TFTs is not quite clarified yet. In this paper, the instability mechanisms of p-channel LTPS TFTs under DC bias stress have been investigated. From the IV transfer curves, it was observed that LTPS TFT's mobility increases after stress at some bias conditions. This degradation is most likely caused by interface traps between the poly-Si thin film and the gate insulator, as well as the damaged junction of the drain from stress. In this work, the assumption is examined via C-V measurement. It is found that the C_GD curves of the stressed TFT slightly increase for the gate voltage smaller than the flat band voltage V_FB. However, the C_GS curves of the stressed device are almost the same as those before stress. By employing simulation, it is found that the degradation of p-type TFTs under this stress condition is mainly caused by the trapped charges at the interface between the gate and the drain region, which is generated by the high voltage difference applied during DC bias stress.     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.     

Slip behaviour near a grain boundary in high-cycle fatigue of poly-crystal copper

In order to investigate the characteristic slip behaviour near a grain boundary in high‐cycle fatigue, a high‐cycle fatigue test is carried out using a copper poly‐crystal specimen, which consists of several tens of grains. Seventeen persistent slip bands (PSBs) are observed along the grain boundaries. Their location and the activated slip system are different from those expected by the Schmid factor. After the fatigue test, the crystalline orientation and the three‐dimensional shape of each grain are specified by the repetition of polishing and observation by means of an orientation‐imaging microscope (OIM). A finite‐element method (FEM) analysis is also conducted for the specimen with the same orientation and shape of grains taking into account the anisotropy. This analysis reveals that the shear stress concentrates near the grain boundaries where the PSBs are formed. The activated slip systems predicted by the maximum resolved shear stress agree well with those observed in the fatigue experiment. Thus, the characteristic slip near the grain boundary in the poly‐crystal is governed by the concentrated resolved shear stress on the specific slip system due to the deformation constraint by neighbouring crystals.     

Potential energy of two structures of Σ = 110 1 1 tilt grain boundary in silicon and germanium with empirical potentials and tight-binding methods

Two atomic structures A and B of the Σ = 110 1 1 grain boundary were observed in silicon and germanium. We have performed a complete study of the stability of these two grain boundaries using some empirical potentials and also the semiempirical, tight-binding (TB) method. The TB method has confirmed the experimental observations at low temperatures. The A structure is more stable in silicon whereas for germanium the B structure is obtained. The empirical potentials, such as those of Keating (1966), Baraff et al. (1980) and of Stillinger and Weber (1985), give the A structure as the most stable for both germanium and silicon. The non-ability of these empirical potentials to make a difference between germanium and silicon and the advantage of TB method are discussed.     

应力对多层薄膜窄带滤光片透射特性的影响

利用薄膜应力公式和弹性力学的小挠度弯曲理论,分析了多层薄膜应变与薄膜厚度变化的关系,建立起基片曲率与薄膜厚度变化关系的理论模型,提出了多层薄膜各层厚度不均匀变化的关系式。膜厚均匀变化只影响中心波长的漂移,而这种膜厚不均匀变化不仅引起中心波长也引起光谱的退化。在基片由10mm厚减薄到1．0mm时,对100GHz的窄带滤光片的模拟结果为中心波长减小0．977nm;0．5dB带宽减小0．19nm。引入膜厚随机误差的模拟结果为中心波长减小1．066nm;纹波增加0．36dB;峰值插损增加0．32dB,光谱进一步退化了。说明了基片曲率变化引起的光学薄膜厚度变化的不均匀性是引起这种窄带干涉滤光片光谱退化的主要原因之一。     

Effect of the simultaneous application of a high static magnetic field and a low alternating current on grain structure and grain boundary of pure aluminum

Effect of the simultaneous application of a high static magnetic field and a low alternating electric current on the solidification structure of pure aluminum has been investigated. Results show that the refinement of the solidification structure is enhanced by the electric current under a certain magnetic field. However, when the magnetic field intensity exceeds a certain value, the refinement is impaired under a certain electric current. The observation by electron backscattered diffraction (EBSD) shows the complex fields have led to the increase of the low angle boundaries with the refinement. Moreover, the application of the static gradient magnetic field is capable of modifying the distribution of the refined grains. The above results may be attributed to the formation of the cavities during the electromagnetic vibration process and the high magnetic field.     

偏置应力对InAlN/GaN HEMT直流特性的影响

采用直流偏置应力法对蓝宝石衬底上的InAlN/GaN HEMT器件的电流崩塌效应进行了研究。实验结果表明,在关态和开态应力后,器件直流特性明显退化,退化程度随偏置应力电压和应力时间的累积而增大。理论分析和器件仿真结果表明,关态应力引起的性能退化主要是由栅泄漏电流填充表面态形成的虚栅造成的;而开态应力引起的退化是沟道热电子被势垒层陷阱及表面态俘获产生的。因此,只有消除表面态和势垒层陷阱或者隔绝表面态形成的虚栅才能有效抑制电流崩塌。偏置应力引起的性能退化是可逆过程,在无外界激励时,经过10d左右的静置,器件基本恢复初始性能。     

Fabrication of high frequency ZnO thin film SAW devices on silicon substrate with a diamond-like carbon buffer layer using RF magnetron sputtering

Diamond-like carbon (DLC) film is a promising candidate for surface acoustic wave (SAW) device applications because of its higher acoustic velocity. A zinc oxide (ZnO) thin film has been deposited on DLC film/Si substrate by RF magnetron sputtering; the optimized parameters for the ZnO sputtering are RF power density of 0.55 W/cm², substrate temperature of 380 °C, gas flow ratio (Ar/O₂) of 5/1 and total sputter pressure of 1.33 Pa. The results showed that when the thickness of the ZnO thin films was decreased, the phase velocity of the SAW devices increased significantly.     

The effect of thermal annealing sequence on amorphous InGaZnO thin film transistor with a plasma-treated source–drain structure

In this paper, the effects of thermal annealing and the plasma treatment sequence on the performance of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) without conventional source/drain (S/D) layer deposition were investigated. We fabricated TFTs using two different processes, one where S/Ds were plasma-treated after thermal annealing, the second where the S/Ds were plasma-treated before annealing. The performance of the former exhibited a linear mobility of 4.97 cm²/V s, an on/off ratio of 4.6 × 10⁶, a V_th of 2.56 V, and a subthreshold slope of 0.65 V/decade. However, the TFT parameters of the latter sample were reduced to a linear mobility of 0.07 cm²/V s, an on/off ratio of 1.5 × 10⁵, a V_th of 2.33 V, and a subthreshold slope of 3.54 V/decade. It was shown that the sheet resistance of plasma-treated S/D areas increased after thermal annealing by about three orders of magnitude. As a result, the increase of the sheet resistance caused a decrease of TFT performance.     

Effects of l‐carnitine supplement on serum inflammatory cytokines,C‐reactive protein,lipoprotein (a), and oxidative stress in hemodialysis patients with Lp (a) hyperlipoproteinemia

Inflammation, oxidative stress, and high concentration of serum lipoprotein (a) [Lp (a)] are common complications in hemodialysis patients. The present study was designed to investigate the effects of l ‐carnitine supplement on serum inflammatory cytokines, C‐reactive protein (CRP), Lp (a), and oxidative stress in hemodialysis patients with Lp (a) hyperlipoproteinemia [hyper Lp (a)]. This was an unblinded, randomized clinical trial. Thirty‐six hyper Lp (a) hemodialysis patients (23 men and 13 women) were randomly assigned to either a carnitine or control group. Patients in the carnitine group received 1000 mg/d oral l ‐carnitine for 12 weeks, whereas patients in the control group did not receive any l ‐carnitine supplement. At baseline and the end of week 12, 5 mL of blood were collected after a 12‐ to 14‐hours fast and serum free carnitine, CRP, interleukin‐1β, interleukin‐6 (IL‐6), tumor necrosis factor‐α, Lp (a), and oxidized low‐density lipoprotein were measured. Serum free carnitine concentration increased significantly by 86% in the carnitine group at the end of week 12 compared with baseline (P<0.001), while serum CRP and IL‐6 showed a significant decrease of 29% (P<0.05) and 61% (P<0.001), respectively. No significant changes were observed in serum free carnitine, CRP, and IL‐6 in the control group. There were no significant differences between the two groups in mean changes of serum interleukin‐1β, tumor necrosis factor‐α, Lp (a), and oxidized low‐density lipoprotein concentrations. l ‐carnitine supplement reduces inflammation in hemodialysis patients, but has no effect on hyper Lp (a) and oxidative stress.