Analysis of contact effects in fully printed p-channel organic thin film transistors

Contact effects have been analyzed in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source–drain contacts. In these devices, contact effects lead to an apparent decrease of the field effect mobility with decreasing L and to a failure of the gradual channel approximation (GCA) in reproducing the output characteristics. Experimental data have been reproduced by two-dimensional numerical simulations that included a Schottky barrier (Φ_b = 0.46 eV) at both source and drain contacts and the effects of field-induced barrier lowering. The barrier lowering was found to be controlled by the Schottky effect for an electric field E < 10⁵ V/cm, while for higher electric fields we found a stronger barrier lowering presumably due to other field-enhanced mechanisms. The analysis of numerical simulation results showed that three different operating regimes of the device can be identified: (1) low |V_ds|, where the channel and the Schottky diodes at both source and drain behave as gate voltage dependent resistors and the partition between channel resistance and contact resistance depends upon the gate bias; (2) intermediate V_ds, where the device characteristics are dominated by the reverse biased diode at the source contact, and (3) high |V_ds|, where pinch-off of the channel occurs at the drain end and the transistor takes control of the current. We show that these three regimes are a general feature of the device characteristics when Schottky source and drain contacts are present, and therefore the same analysis could be extended to TFTs with different semiconductor active layers.     

A unified simulation of Schottky and ohmic contacts

The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated     

Electrically conductive anti‐reflecting nanostructure for chalcogenide thin‐film solar cells

Electrically conducting aluminum (Al)‐doped ZnO nanorods (NRs) film has been introduced as an anti‐reflective (AR) layer for effective light trapping in chalcogenide thin‐film solar cells. Results indicate that the Al‐doping significantly reduced the electrical contact resistance between the Ag top electrode and the AR layer. The Al‐doped ZnO NRs exhibited low average reflectance (4.5%) over the entire visible and near‐infrared range, and changed the nature of electrical contact between the Ag electrode and the AR layer from Schottky to Ohmic. Finally, the CuInS₂ solar cell coated with the Al‐doped ZnO NRs exhibited huge enhancement in photovoltaic efficiency from 9.57% to 11.70% due to the lowering series resistance and the increase in the short‐circuit current density, when compared with that of a solar cell without the AR layer. Copyright © 2014 John Wiley & Sons, Ltd.     

降低肖特基势垒高度的途径探讨

通过深入地分析影响金属－半导体肖特基势垒的各种因素,探讨了几种降低肖特基势垒高度的途径,其中,特别是通过汽相激光诱导化学掺杂技术在金属－半导体界面上制作一足够薄的高掺杂层,可以使肖特基势垒高度得到显著降低。这种技术的应用,对在ＰｔＳｉ／Ｓｉ界面上制作超浅结和半导体器件中制作良好的欧姆接触提供了广阔的应用前景。     

Improved performance in n-channel organic thin film transistors by nanoscale interface modification

We demonstrate that the electrical properties of n-channel thin film transistors can be enhanced by inserting a nanoscale interfacial layer, namely, cesium carbonate (Cs₂CO₃), between organic semiconductor and source/drain electrodes. Devices with the Cs₂CO₃/Al electrode showed a reduction of contact resistance, not only with respect to Al, but also compared to Ca. The improvement is attributed to the reduction in the energy barrier of electron injection and the prevention of unfavorable chemical interaction between the organic layer and the metal electrode. High field-effect mobility of 0.045 cm²/V s and on/off current ratios of 10⁶ were obtained in the [6,6]-phenyl C60 butyric acid methyl ester-based organic thin film transistors using the Cs₂CO₃/Al electrodes at a gate bias of 40 V.     

Experimental and numerical analysis of channel-length-dependent electrical properties in bottom-gate,bottom-contact organic thin-film transistors with Schottky contact

Channel length dependence of field-effect mobility and source/drain parasitic resistance in pentacene thin-film transistors with a bottom-gate, bottom-contact configuration was investigated. Schottky barrier effect such as nonlinear behaviors in transistor output characteristics appeared and became more prominent for shorter channel length less than 10 μm, raising some concerns for a simple utilization of conventional parameter extraction methods. Therefore the gate-voltage-dependent hole mobility and the source/drain parasitic resistance in the pentacene transistors were evaluated with the aid of device simulation accounting for Schottky contact with a thermionic field emission model. The hole mobility in the channel region shows smaller values with shorter channel length even after removing the influence of Schottky barrier, suggesting that some disordered semiconductor layers with low carrier mobility exist near the contact electrode. This experimental data analysis with the simulation enables us to discuss and understand in detail the operation mechanism of bottom-gate, bottom-contact transistors by considering properly each process of charge carrier injection, carrier flow near the contact region, and actual channel transport.     

Bilayer tellurene-metal interfaces

Tellurene, an emerging two-dimensional chain-like semiconductor, stands out for its high switch ratio, carrier mobility and excellent stability in air. Directly contacting the 2D semiconductor materials with metal electrodes is a feasible doping means to inject carriers. However, Schottky barrier often arises at the metal–semiconductors interface, impeding the transport of carriers. Herein, we investigate the interfacial properties of BL tellurene by contacting with various metals including graphene by using ab initio calculations and quantum transport simulations. Vertical Schottky barriers take place in Ag, Al, Au and Cu electrodes according to the maintenance of the noncontact tellurene layer band structure. Besides, a p-type vertical Schottky contact is formed due to the van der Waals interaction for graphene electrode. As for the lateral direction, p-type Schottky contacts take shape for bulk metal electrodes(hole Schottky barrier heights(SBHs) ranging from 0.19 to 0.35 eV). Strong Fermi level pinning takes place with a pinning factor of 0.02. Notably, a desirable p-type quasi-Ohmic contact is developed for graphene electrode with a hole SBH of 0.08 eV. Our work sheds light on the interfacial properties of BL tellurene based transistors and could guide the experimental selections on electrodes.     

Current spreading effects in fully printed p-channel organic thin film transistors with Schottky source–drain contacts

Contact effects have been analyzed, by using numerical simulations, in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source/drain contacts. Considering source–drain Schottky contacts, with a barrier height of 0.46 eV, device characteristics can be perfectly reproduced. From the detailed analysis of the current density we have shown that current spreading occurs at the source contact, thus influencing the effective contact resistance. At low V_ds and for a given V_gs, the current is mainly injected from an extended source contact region and current spreading remains basically constant for increasing V_ds. However, by increasing V_ds the depletion layer of the Schottky contact expands and reaches the insulator–semiconductor interface, causing the pinch-off of the channel at the source end (V_dsat1). For V_ds > V_dsat1 the current injected from the edge of the source contact rapidly increases while the current injected from the remaining part of the source contact basically saturates. Current spreading shows a V_gs-dependence, since the contact injection area depends on the channel resistance and also barrier lowering of the Schottky source contact depends upon V_gs. The injected current from the edge of the source contact can be reproduced using the conventional diode current expression, assuming a constant value for the zero barrier lowering saturation current and considering a V_gs-dependent barrier lowering. The presented analysis clarifies the V_gs-dependence of the contact current–voltage characteristics and points out that the I–V contact characteristics cannot directly be related to a single diode characteristics. Indeed, the contact characteristics result from the combination of two rather different regimes: at low V_ds the current is injected from an extended source contact region with a current spreading related to V_gs, while for V_ds above the pinch-off of the channel at source end, the current is injected primarily from the edge of the source contact and is strongly enhanced by the barrier lowering.     

Influence of pinning trap in Ti/4H–SiC Schottky barrier diode

Ti/4H–SiC Schottky barrier diode without any intentional edge termination is fabricated. The obtained properties, low on-resistance of 3 mΩ cm² and low leakage current of 10⁻⁴ A/cm² at 1000 V, are evaluated by device simulation considering pinning at metal/semiconductor interface. The breakdown voltage is explained by minimization of electric field enhancement at the Schottky electrode edge due to pinning. The leakage current corresponds to Schottky barrier tunneling current depending on drift layer doping and Schottky barrier height.     

Selenium Segregation for Lowering the Contact Resistance in Ultrathin-Body MOSFETs With Fully Metallized Source/Drain

We report the first integration of selenium (Se) segregation contact technology in ultrathin-body (UTB) n-MOSFET featuring Ni fully silicided source and drain. During the Ni silicidation process, the implanted Se segregated at the NiSi-n-Si interface, leading to significant reduction of Schottky barrier height and contact resistance. The UTB n-MOSFETs integrated with Se segregation (SeS) contact technology show significant external series resistance reduction and drive current performance enhancement. Drain-induced barrier lowering and gate leakage current density are not adversely affected by the SeS process.     

用弹道电子发射显微术研究超薄金属硅化物/硅肖特基接触

采用弹道电子发射显微术 ( BEEM)技术对超薄 Pt Si/Si、Co Si2 /Si肖特基接触特性进行了研究 ,并与电流 -电压 ( I- V)及电容 -电压 ( C- V)测试结果进行了对比 .研究了 Ar离子轰击对超薄Pt Si/n- Si肖特基接触特性的影响 .BEEM、I- V/C- V技术对多种样品的研究结果表明 ,I- V/C- V测试会由于超薄硅化物层串联电阻的影响而使测试结果产生严重误差 ;BEEM测试则不受影响 .随着离子轰击能量增大 ,肖特基势垒高度降低 ,且其不均匀性也越大 .用 BEEM和变温 I- V对超薄 Co Si2 /n- Si肖特基二极管的研究结果表明 ,变温 I- V测试可在一定程度上获得肖特基势垒     

Schottky‐Barrier‐Controllable Graphene Electrode to Boost Rectification in Organic Vertical P–N Junction Photodiodes

Monolayer graphene is used as an electrode to develop novel electronic device architectures that exploit the unique, atomically thin structure of the material with a low density of states at its charge neutrality point. For example, a single semiconductor layer stacked onto graphene can provide a semiconductor–electrode junction with a tunable injection barrier, which is the basis for a primitive transistor architecture known as the Schottky barrier field‐effect transistor. This work demonstrates the next level of complexity in a vertical graphene–semiconductor architecture. Specifically, an organic vertical p‐n junction (p‐type pentacene/n‐type N,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C₈)) on top of a graphene electrode constituting a novel gate‐tunable photodiode device structure is fabricated. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device, both of which are critical to improve the performance of a photodiode.     

蒙特卡罗方法模拟金属-半导体接触的直接隧穿效应

运用自洽的蒙特卡罗方法模拟了肖特基接触的隧穿效应 .模拟的内容包括具有不同的势垒高度的金属 -半导体接触在正向和反向偏置下的工作状态 .分析模拟结果可知 ,隧穿电流在反向偏置下起主要的作用 .同时还模拟了引入肖特基效应后 ,SBD的工作特性 ,验证了模拟使用的物理模型 .得到了与理论计算值符合的模拟结果 .分析模拟结果表明 ,由于肖特基效应形成的金属 -半导体接触势垒的降低 ,会在很大程度上影响金属 -半导体接触的输运特性     

High‐Resolution Characterization of Pentacene/Polyaniline Interfaces in Thin‐Film Transistors

We present the first detailed report that directly correlates the reduced contact resistance in organic thin‐film transistors (TFTs) with fundamental structural and morphological characterization at the organic semiconductor/conducting polymer interface. Specifically, the pentacene grains are similar in size and continuous across the channel/electrode interface in bottom‐contact TFTs with polyaniline (PANI) electrodes. On a molecular level, the fused rings of pentacene are oriented perpendicular to the surface both in the channel and on PANI. Accordingly, the contact resistance is small in such devices. In TFTs with gold electrodes, however, the pentacene grains are different in size and are discontinuous across the channel/electrode interface. Further, the fused rings of pentacene are oriented perpendicular to the channel surface and parallel to the gold surface. Such differences across the channel/electrode interface lead to structural and electronic disorder, which manifests itself as a significantly larger contact resistance in devices with gold electrodes.     

Contact-resistance effects in PDI8-CN2 n-type thin-film transistors investigated by Kelvin-probe potentiometry

In this paper, the metal-semiconductor contacts in n-type bottom-contact bottom-gate Organic Field-Effect Transistors (OFET), based on evaporated films of a perylene diimide derivative (PDI8-CN₂), have been investigated by Scanning Kelvin Probe Microscopy (SKPM). OFET were characterized for different thicknesses of the PDI8-CN₂ film and in the light of the gold electrode functionalization with an aromatic thiol self-assembled monolayer. SKPM experiments reveal that aromatic thiol functionalization produces a 30% reduction in the contact resistance (R_c), while lowering the organic film thickness, the contact resistance (R_c) phenomenon tends to be magnified. The experimental observation that the voltage drops occurring close the drain electrode are even larger than those taking place at the source contact suggests that it is not possible, for these devices, to explain the overall R_c effects by simply referring to the presence of a reverse-biased Schottky junction limiting the electron injection process into the active channel.     

Reverse characteristics of Mo-Si epitaxial Schottky diodes

The characteristics of a metal-semiconductor Schottky barrier diode, in which the semiconductor part consists of a heavily doped substrate (sim 10^{-3}Ω.cm) with a thin (∼ 1 µ) epitaxial layer which is much less heavily doped (-∼ 1 Ω.cm) on top of it, were studied. The electric field distribution calculations showed that, for all practical cases, the penetration of the depletion layer into the substrate could be neglected. Expressions for the capacitance-voltage characteristics, the image force barrier lowering effects, and the avalanche breakdown voltages were derived. The expressions can be verified experimentally on Mo-Si Schottky diodes.     

基于过渡金属氧化物薄膜为连接层的叠层有机电制发光器件的数值模型

By utilizing a two-step process to express the charge generation and separation mechanism of the transition metal oxides （TMOs） interconnector layer, a numerical model was proposed for tandem organic light emitting diodes （OLEDs） with a TMOs thin film as the interconnector layer. This model is valid not only for an n-type TMOs interconnector layer, but also for a p-type TMOs interconnector layer. Based on this model, the influences of different carrier injection barriers at the interface of the electrode/organic layer on the charge generation ability of interconnector layers were studied. In addition, the distribution characteristics of carrier concentration, electric field intensity and potential in the device under different carrier injection barriers were studied. The results show that when keeping one carrier injection barrier as a constant while increasing another carrier injection barrier, carri- ers injected into the device were gradually decreased, the carrier generation ability of the interconnector layer was gradually reduced, the electric field intensity at the interface of the organic/electrode was gradually enhanced, and the electric field distribution became nearly linear： the voltage drops in two light units gradually became the same. Meanwhile, the carrier injection ability decreased as another carrier injection barrier increased. The simulation re- sults agree with the experimental data. The obtained results can provide us with a deep understanding of the work mechanism of TMOs-based tandem OLEDs.     

Polypyrrole top-contact electrodes patterned by inkjet printing assisted vapor deposition polymerization in flexible organic thin-film transistors

Highly conductive polymer, polypyrrole (PPy) was successfully patterned as source and drain (S/D) electrodes for flexible pentacene thin film transistors in top-contact structure by combining inkjet printing and vapor deposition polymerization. Facile inkjet printing of initiator and subsequent exposure of pyrrole monomers resulted in selective absorption and polymerization of pyrrole monomers on the patterned initiator region. Pentacene transistors based on printed PPy electrodes exhibited higher electrical characteristics than that of the devices with thermally evaporated Au electrodes. Improved performance of the devices based on PPy electrodes could be attributed to the reduction of contact resistance at the interface between polymer and organic semiconductor. For the replacement of metal electrodes, vapor deposition polymerization assisted inkjet printing technique can provide a versatile method to utilize highly conductive polymer as a functional electrode of flexible organic electronic devices.     

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

A graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (10² S cm^?1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.