Effect of localized traps on the anomalous behavior of the transconductance in nanocrystalline TFTs

It has been reported that nanocrystalline and microcrystalline devices show an anomalous behavior in the transconductance where several rates of increase of the transconductance with applied gate voltage, not present in amorphous TFTs are observed. In this paper we show that the anomalous effect of the transconductance is observed for an acceptor tail states activation energy similar to the normal values for hydrogenated silicon amorphous devices, (a-Si:H), provided that some conditions are met regarding the density of trapped charge in tail and deep states and the density of free charge in the material, which does not necessarily suggest a behavior in between amorphous and polycrystalline. The effect appears if the density of deep tail states, is smaller (higher) than the typical values in a-Si:H. The localized state distribution present in a nanocrystalline TFT prepared by hot wire deposition technique is estimated by comparison of experimental and simulated transconductance curves. In our case a lower density of deep states is obtained, which corresponds with their better light and bias stability.     

简化的非晶硅薄膜晶体管模型

本文基于两点基本假设,提出了一个简化的非晶硅薄膜场效应晶体管模型,在整个电压范围内,得到了电流的解析表达式.与现有的实验结果进行拟合,理论值与实验值符合良好.本文还分析了非晶硅薄膜晶体管输出特性中源漏串联电阻效应,结果表明,源漏串联电阻受栅压调制,近似与栅压的平方成反比.本文还解释了开启电压随温度线性变化规律.模型可用于优化非晶硅薄膜晶体管的设计.     

Narrow width effects of bottom-gate polysilicon thin filmtransistors

The effects of channel width on the characteristics of both hydrogenated and unhydrogenated bottom-gate polysilicon thin-film transistors (TFTs) were investigated in detailed. For unhydrogenated and silane gas formed TFTs, a drastic decrease in threshold voltage is observed due to the grain-boundary traps are reduced when the channel width is reduced to less than grain size, but the minimum drain current sensitive to intragranular tail states are nearly unchanged. After hydrogenation, almost grain boundary traps and intragranular tail states were passivated, the effect of traps along poly channel edges caused by the definition of poly channel pattern will dominate, i.e., threshold voltage and minimum drain current increase with decreasing channel width. Also disilane gas formed TFTs are studied for comparison     

基于陷落电荷效应的非晶硅TFT的开态电流模型

对非晶硅薄膜晶体管,提出基于陷落电荷和自由电荷分析的新方法。考虑到带隙中指数分布的深能态和带尾态,给出了基于阈值电压的开启区电流模型。定义阈值电压为栅氧/半导体界面处陷落于深能级陷阱态的电荷与陷落于带尾态的电荷相等时所对应的栅压。电流模型中,引入一陷落电荷参数β,此参数建立了电子的带迁移率与有效迁移率之间的关系。最后,将电流模型同时与Pao-Sah模型和实验数据进行比较和验证,结果表现出很好的一致性。     

Change in transfer and low-frequency noise characteristics of n-channel polysilicon TFTs due to hot-carrier degradation

Results on the impact of hot-carrier effects on the transfer and low-frequency noise characteristics of n-channel poly-crystalline silicon thin-film transistors (polysilicon TFTs) are presented. After stressing at the condition of maximum substrate current, the experimental data show that TFTs suffer from substantial on-current reduction. Through numerical simulation, it is shown that the stress-induced degradation increases the density of the band tail traps in a region extending 200 nm from the drain and the series resistance on the drain side. It is found that the origin of the noise is reverted from carrier number fluctuation for the unstressed to the mobility fluctuation for the stressed device.     

Modeling of SILC based on electron and hole tunneling. I. Transienteffects

A detailed investigation of the steady-state and transient leakage currents in thin oxides is proposed. The experimental data are compared with numerical results obtained from a model based on an inelastic trap-assisted tunneling process, which includes both electron and hole contributions. In order to accurately reproduce the transient discharge currents, a continuous distribution of oxide traps was adopted. The energies of these levels can be either in correspondence of the conduction or valence band edges of the adjacent silicon/polysilicon layers. Both electrons and holes contribute to the transient stress-induced leakage current (SILC), but the extracted trap densities cannot account for the steady-state SILC. A different mechanism, involving trap levels with energy aligned to the energy gap of the silicon layers is proposed and is developed in the following paper. The model can be applied to any type of device and bias conditions and may be used to correctly recognize the role of electron and hole SILC and the spatial and energy distribution of defect states     

The effects of H2-O2-plasma treatment on thecharacteristics of polysilicon thin-film transistors

The effects of H₂-plasma followed by O₂-plasma treatment on n-channel polysilicon thin-film transistors (TFTs) were investigated. It was found that the H₂-O₂-plasma treatment is more effective in passivating the trap states of polysilicon films than the H₂-plasma or O₂-plasma treatment only. Hence, it is more effective in improving the device performance with regard to subthreshold swing, carrier mobility, and the current ON/OFF ratio. It is also found that thermal annealing of plasma-treated devices increases the deep states but has no effect on the tail states of the devices     

Measurement of density of the gap states ina-Si:H by the normalization of photoconductivity

The spectra of the optical absorption coefficient in low absorption region are obtained by using a normalization procedure for the photoconductivity spectra. The results are explained in terms of the optical transition of electrons from localized states in the exponential valence band tail and in dangling bond states 1.0 eV below the conduction band edge to extended conduction band states. Then the density of the gap states below the Fermi levelE _F is obtained. From the investigation of recombination kinetics, the average density of the gap states over the range of ) and the density of the gap states above the Fermi levelE _F are obtained. These indicate that the width of the conduction band tail is smaller than that of the valence band tail.     

Analytical drain current model for a-Si:H TFTs by simultaneously considering localised deep and tail states

An analytical drain current model for a-Si:H TFTs obtained by considering deep and tail states simultaneously is presented. Using an effective temperature approach, the localised deep and tail states have been considered in the DC model such that no approximations are needed. As verified by the published data, this analytical DC model provides an accurate prediction of the drain current characteristics of an a-Si:H thin film transistor.<>     

Physical models for degradation effects in polysilicon thin-filmtransistors

Experimental data showing the degradation in performance of polysilicon thin-film transistors (TFTs) under a variety of bias stress conditions are presented. A model is proposed to explain these effects whereby device performance degrades due to changes in the effective density of defect states in the material. Unlike single-crystal devices which degrade from hot-carrier effects, poly-Si TFTs are believed to degrade primarily due to the presence of high carrier densities in the channel. Good agreement between computer simulations of the device characteristics and experimental data ia demonstrated. It is shown that stressing under transient conditions leads to a more severe performance degradation than stressing under comparable steady-state conditions     

Illuminating Trap Density Trends in Amorphous Oxide Semiconductors with Ultrabroadband Photoconduction

Under varying growth and device processing conditions, ultrabroadband photoconduction (UBPC) reveals strongly evolving trends in the defect density of states (DoS) for amorphous oxide semiconductor thin-film transistors (TFTs). Spanning the wide bandgap of amorphous InGaZnO_x (a-IGZO), UBPC identifies seven oxygen deep donor vacancy peaks that are independently confirmed by energetically matching to photoluminescence emission peaks. The subgap DoS from 15 different types of a-IGZO TFTs all yield similar DoS, except only back-channel etch TFTs can have a deep acceptor peak seen at 2.2 eV below the conduction band mobility edge. This deep acceptor is likely a zinc vacancy, evidenced by trap density which becomes 5-6× larger when TFT wet-etch methods are employed. Certain DoS peaks are strongly enhanced for TFTs with active channel processing damage caused from plasma exposure. While Ar implantation and He plasma processing damage are similar, Ar plasma yields more disorder showing a ≈2 × larger valence-band Urbach energy, and two orders of magnitude increase in the deep oxygen vacancy trap density. Changing the growth conditions of a-IGZO also impacts the DoS, with zinc-rich TFTs showing much poorer electrical performance compared to 1:1:1 molar ratio a-IGZO TFTs owing to the former having a ∼10 × larger oxygen vacancy trap density. Finally, hydrogen is found to behave as a donor in amorphous indium tin gallium zinc oxide TFTs.     

Effects of hydrogenation on the performance and stability of p-channel polycrystalline silicon thin-film transistors

The effects of hydrogenation on the performance and stability under electrical stress of p-channel polycrystalline silicon thin-film transistors (polysilicon TFTs) are investigated. The hydrogenation is performed in pure H₂ plasma or in plasma of 4% H₂ diluted in He gas. Devices hydrogenated in plasma of H₂/He exhibit lower subthreshold swing with better uniformity and lower leakage current, which indicate passivation of mid-gap trap states arising from dangling bonds at the grain boundaries. Hot-carrier experiments demonstrate that the stability of p-channel TFTs is improved as the hydrogenation becomes more efficient due to the effective removal of donor-type trap states at the grain boundaries.     

Distribution of the density of electronic states in the energy gap of microcrystalline hydrogenated silicon

Photomodulation spectroscopy was applied to a study of the distribution of the density of electronic states in the energy gap of microcrystalline hydrogenated silicon (μc-Si:H) with a varied level of boron doping. The information on the density-of-states distribution was extracted by analyzing temperature dependences of the constant and modulated components of photoconductivity in a sample exposed to modulated light. The distributions of the density of electronic states in the upper and lower halves of the energy gap of μc-Si:H were determined. The study demonstrated that the tail of the density-of-states distribution near the valence band is less steep than that near the conduction band.     

The effects of fluorine passivation on polysilicon thin-filmtransistors

The fluorine implantation on polysilicon was found to improve the characteristics of polysilicon thin-film transistors (TFT's). The fluorine passivates the trap states within the polysilicon channel, as compared with the H₂-plasma passivation. The fluorine implantation passivates more uniformly both the band tail-states and midgap deep-state, while the H₂-plasma treatment is more effective to passivate deep states than tail states. A fluorine-implanted device can be further improved its performance if an H₂-plasma treatment is applied. In contrast to the H₂ -plasma passivation, the fluorine passivation improves the device hot-carrier immunity. Combining the fluorine passivation and H₂ -plasma passivation, a high performance TFT with a high hot-carrier immunity can be obtained     

Modeling of laser-annealed polysilicon TFT characteristics

A model based on two-dimensional (2-D) simulation, for a polysilicon thin-film transistor (poly-Si TFT) with large grains, fabricated in laser recrystallized material, is presented. The importance of differentiating between the density of states of traps within grains and traps localized at grain boundaries is demonstrated. It is shown that the observed lack of saturation in the TFT output characteristics arises due to the effect of high interface trap density within the grain boundaries, whereas the subthreshold slope has a strong dependence on the trap density within the grains. Only by differentiating in this way between grain and grain boundary parameters can both output and subthreshold characteristics of an n-channel poly-Si TFT be accurately modeled using the same set of parameters. Appropriate values for the density of states in both grains and grain boundaries are suggested for laser-annealed TFTs     

a—Si：H带尾态的光致变化

为阐明a-Si∶H的光致变化效应的物理机构,作者用光电流相移法研究了光照前后.a-Si∶H导带尾态的变化情况;同时用等光电导法测量了光吸收Urbach尾,从而确定了价带尾的光致变化情况.实验结果表明:光照以后,导带尾态和价带尾态都增加了.作者认为:在集团氢存在的地方,与光照引起的Si-H键断裂相伴随的Si-Si弱键的增加是带尾态增加的原因.     

Analysis of electrical characteristics of gate overlapped lightly doped drain (GOLDD) polysilicon thin-film transistors with different LDD doping concentration

Different drain field architectures have been recently investigated to reduce field-enhanced effects in conventional self-aligned polysilicon thin-film transistor (TFT) architecture, induced by the intense electric fields at the drain junction. Among these, gate overlapped lightly doped drain (GOLDD) architecture has been shown to be effective in reducing the drain field in both on and off states of the TFT, without introducing appreciable series resistance effects. In this paper, we investigate the electrical characteristics, both in the on- and in the off-regime, of GOLDD polysilicon TFTs, made with different LDD doses, by combining experimental data with two-dimensional (2-D) numerical analysis. We also demonstrate that both the on-state and off-state features of the GOLDD structure can be readily understood in terms of a simple, new model, based upon two TFTs series. This is consistent both with the experimental data and the results of full 2-D simulations. This paper not only clarifies the dependence of kink effect, leakage current, and series resistance upon the LDD-doping, but also provides the guidelines to optimize physical parameters of GOLDD TFTs.     

Electrostatic Tuning of Spray‐Deposited ZnO for Controlled Mobility Enhancement

Spray‐deposited nanocrystalline ZnO films are produced in order to establish empirical relationships between synthetic conditions and the density of states as a means of achieving electrostatic control. By varying the spray‐pyrolysis deposition conditions, i.e., substrate temperature, precursor concentration, and flow rate, a wide range of exponentially distributed density of localized states profiles and field‐effect mobility values ranging over three orders of magnitude (0.02–30 cm² V^?1 s^?1) are obtained for analysis. It is found that mobility can be controlled by appropriately tuning the shape of the density of states profile, increasing the band tail slope and reducing the band edge concentration of shallow states. Most significantly, it is shown that the shape of the density of states can be modified by adjusting the spray‐pyrolysis deposition conditions for electrostatic control. It is found that higher Zn precursor concentration in solution increases the slope of the band tails, leading to higher mobility. Additionally, the band edge concentration is reduced with increased substrate temperature also leading to higher mobility. These results quantify the relationship between defect electrostatics and electron transport while demonstrating electrostatic control via synthetic modification of localized states.     

Localized states in Hg3In2Te6: Cr compounds

The effect of chromium doping on the energy spectrum of the density of states N(E) in the band gap of the Hg₃In₂Te₆ semiconductor compound was studied. Although the chromium impurity has no significant effect on the electrical properties and the Fermi level position, it increases the band tail extent and the density of localized states within the band gap E _g . Comparison of the data on the field effect and on light absorption for photon energies hν lower than E _g made it possible to ascertain that the density of states is distributed continuously and rapidly increases near the edges of the valence and conduction bands. The obtained spectra are discussed on the assumption that the chromium content growth increases the structure disorder and the random potential, which increases N(E). It is shown that localized states control the screening length and pin the Fermi level near the midgap and are also responsible for the compensation of impurity ion charges.     

On-current modeling of large-grain polycrystalline siliconthin-film transistors

Large-grain excimer laser-annealed polysilicon TFTs are studied. Due to the large grain size of the polysilicon film (about 2.5 μm), we propose a model for the on-current (above threshold voltage) taking into account the number of grain boundaries within the channel. This linear-region model considers grain and grain boundaries as two noncorrelated regions within the channel of a polysilicon TFT. The trap density at the grain boundaries and the device parameters involved in this model are determined by fitting the experimental transfer characteristic in the linear regime. Moreover, we show that the proposed model provides reliable results within a temperature range from 150 K to 300 K. Finally, it serves to optimize the energy density of laser annealing and to make predictions about polysilicon TFT technology, since TFTs performances versus grain size plots can be obtained