A model of ohmic contacts to semiconductors

A unified and detailed model of both thermionic and tunneling ohmic contacts to semiconductors is described. The model takes into account the actual profile of the energy barrier as it is determined by the difference between the metal work function and the semiconductor electron affinity, by the ionized impurities in the semiconductor, by the interfacial quantum electric dipole and by the classical and quantum penetration of the charge carriers into the depletion layer. The current, above and below the energy barrier peak, is computed by using the Kemble generalized transmission coefficient, by taking into account the anisotropy of the effective masses as well as their dependence on impurity-concentration, and by employing the Fermi-Dirac statistics. Good agreement between theoretical and experimental results is observed. The model enables the design of ohmic contacts to semiconductors, i.e. the determination of the semiconductor impurity concentration required to achieve pre-assigned features.     

Current crowding on metal contacts to planar devices

A simple transmission line model is applied to the contact region between metal and diffusion layer in planar devices. Taking into account the sheet resistance of the diffusion layer and an ohmic specific contact resistance between metal and semiconductor the theoretical current distribution across the contact is calculated and compared with the results obtained with a model suggested by Kennedy and Murley. Experimental data are given that confirm the validity of the transmission line model.     

Specific contact resistivity of InGaAs/InP p-isotype heterojunctions

The specific contact resistivity of lattice matched InGaAs/InP p-isotype heterojunction has been measured through the use of an interface transmission line model structure. The measured resistance values are comparable to or greater than those of the metal/semiconductor interface and depend heavily on the doping and the abrupt or graded nature of the interface.<>     

A DC and small signal AC model for organic thin film transistors including contact effects and non quasi static regime

We present a compact model for the DC and small signal AC analysis of Organic Thin Film Transistors (OTFTs). The DC part of the model assumes that the electrical current injected in the OTFT is limited by the presence of a metal/organic semiconductor junction that, at source, acts as a reverse biased Schottky junction. By including this junction, modeled as a reverse biased gated diode at source, the DC model is able to reproduce the scaling of the electrical characteristics even for short channel devices.The small signal AC part of the model uses a transmission line approach in order to compute the impedances of the channel and parasitic regions of the device. The overlap capacitances and the presence of non-ideal metal/organic semiconductor junctions are taken in account as well and the model can be easily adapted to different device geometries. The model is particularly well suited for printed devices, often realized with large process tolerances, since it takes into consideration the presence of parasitic regions and their effect on the AC operation.The model has been validated on printed OTFTs using a pentace-derivative as organic semiconductor with a quite peculiar device layout. It has been fully implemented in Verilog-A programming language.     

Static, dynamic, and noise analysis of multisection DFB lasersusing frequency-domain transmission line model

A novel frequency-domain transmission line model for multisection distributed feedback (DFB) lasers is developed. The characteristic impedances of active periodic structures are derived. A multisection DFB laser is described as a transmission line network, with each section represented by a transmission lint segment with a corresponding characteristic impedance. Static, dynamic, and noise analysis of multisection DFB lasers is demonstrated. The reflections at the junctions between sections are evaluated more accurately. The resonant condition of the equivalent transmission line network, instead of cumbersome Wronskian, is used to reformulate the rate equations. The diffusion coefficient of Langevin noise terms of any two different positions is solved for the first time, resulting in a more accurate noise analysis of multisection semiconductor lasers. Analytical expressions of the dynamic responses and noise properties of multisection DFB semiconductor lasers are derived     

RF Transmission Line Method for Wide-Bandgap Heterostructures

We propose a high-frequency extension of the transmission line model method that allows for independent extraction of open-surface sheet resistance of a semiconductor layer and that under metallization. The method is applied for the characterization of an AlGaN/GaN heterostructure with a high ( ~40%) Al content with and without a top dielectric layer. In both cases, the sheet resistance under the metal is approximately 30% smaller than that under the open surface. This reduction is attributed to the surface charge compensation by the top metal.     

判断金属/薄层半导体欧姆接触质量的一个新方法

提出了一个判断金属／薄层半导体欧姆接触质量的新方法－－环内圆形传输线模型外的支,样品制备简单,无需台面绝缘,测试局域在很小的圆环内无需考虑圆环外的任何边界影响,由于结构的对称性,消除了侧向电流聚集效应,通过测试值的直线拟合消除了部分偶然误差,提高了精度。     

Beyond mean field and plane wave theories of bistable semiconductor lasers

The steady-state output of semiconductor lasers containing saturable absorbers is calculated using a model that includes transverse mode variation, high gain and absorption coefficient, and high mirror transmission. The results are compared to the predictions of the mean field theory, and they show that the mean field formulas underestimate the amount of hysteresis and overestimate the required amount of absorption compared to the high gain formulas.     

Impact of Thickness on Contact Issues for Pinning Effect in Black Phosphorus Field‐Effect Transistors

Metal/semiconductor contact is a significant constraint in short‐channel field effect transistors (FETs) comprising black phosphorus (BP) and other 2D semiconductors. Due to the pinning effect at metal/2D semiconductor interface, the Schottky barrier usually does not follow the Schottky–Mott rule, resulting in thickness‐dependent FET performance. In this work, the Schottky barrier in BP FETs is investigated via theory calculation and electrical measurement. A simple metal/BP contact model is presented based upon thickness‐dependent electrical characteristics of BP FETs. The model considers the Schottky barrier as a combined effect of the Schottky–Mott rule and the pinning effect and provides a feasibility to track the conducting behavior of other 2D semiconductor FETs.     

Atomic resolution Z-contrast imaging of semiconductors

The direct interpretability of atomic resolution Z-contrast images obtained from a scanning transmission electron microscope (STEM) makes this imaging technique particularly powerful for the analysis of interfaces and defects in semiconductor materials and devices. In this paper, the principles of the technique are outlined and representative examples of its use are presented. In particular, we show the use of Z-contrast imaging to determine the polarity of a CdTe film grown on a Si substrate, the atomic structures of stacking faults and threading dislocation cores in GaN, and the atomistic structure of an ohmic metal/semiconductor contact of Au/GaAs.     

Asymmetric electric field enhancement in nanocrystal memories

The electrostatic model for nanocrystal memories is used to illustrate the fundamental difference of the metal nanocrystal memory in low-voltage program/erase (P/E) operations in comparison with semiconductor nanocrystal and trap-based memories. Due to repulsion of potential contours inside conductors, the metal nanocrystals will significantly enhance the electric field between the nanocrystal and the sensing channel set up by the control gate bias and, hence, can achieve much higher efficiency in low-voltage P/E. On the other hand, the electric field originated from the stored charge will only be slightly different for metal and semiconductor nanocrystal cases. We presented the electrostatic models by both approximate analytical formulation and three-dimensional numerical simulation in a nanocrystal array. Operations of P/E and read disturbance were analyzed for the cases of homogeneous charge distribution, silicon, and metal nanocrystals. In the P/E condition of +5/-5 V, the metal nanocrystal memory offers around 1.6 times higher peak fields than Si counterparts and almost three times higher than that from the one-dimensional model for homogeneous charge distribution. The field enhancement factor suggests the design criteria of oxide thickness, nanocrystal size, and spacing. The advantage of asymmetric field enhancement of metal nanocrystals will be even more prominent when high-K gate dielectrics are employed.     

Thermoelectric Transport in a ZrN/ScN Superlattice

Metal/semiconductor superlattices have the potential for a high thermoelectric figure of merit. The thermopower of these structures can be enhanced by controlling the barrier height using high-energy electron filtering. In addition, phonon scattering at interfaces can reduce the lattice contribution to the thermal conductivity. In this paper, we present theoretical and experimental studies of the thermoelectric transport in ZrN/ScN metal/semiconductor superlattices. Preliminary measurement results show an exponential increase in the cross-plane electrical conductivity with increasing temperature, which indicates the presence of the barrier. Fit of the Boltzmann transport-based model with the data indicates a barrier height of 280 meV. The cross-plane Seebeck coefficient of the sample is also measured by combining Seebeck voltage transient measurements with the thermal imaging technique. A Seebeck coefficient of 820 μV/K at room temperature is extracted, which is in good agreement with the simulation result of 800 μV/K. Theoretical calculations predict that the ZrN/ScN structure can exhibit a ZT of 1.5 at 1300 K assuming lateral momentum is conserved and that a ZT of 3 is achievable if the lateral momentum is not conserved.     

Mechanisms of temperature dependence of threshold voltage in high-k/metal gate transistors with different TiN thicknesses

The change in temperature coefficient of the threshold voltage (=dV_th/dT) for poly-Si/TiN/high-k gate insulator metal–oxide–semiconductor field-effect transistors (MOSFETs) was systematically investigated with respect to various TiN thicknesses for both n- and p-channel MOSFETs. With increasing TiN thickness, dV_th/dT shifts towards negative values for both n- and p-MOSFETs. A mechanism that changes dV_th/dT, depending on TiN thickness is proposed. The main origins are the work function of TiN (Φ_TiN) and its temperature coefficient (dΦ_TiN/dT). These are revealed to change when decreasing the thickness of the TiN layer, because the crystallinity of the TiN layer is degraded for thinner films, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).     

量子环终端布局对自旋输运的影响

研究了量子环透射终端与入射终端相对位置改变时的变化规律。研究结果表明:透射概率和自旋极化率随半导体环尺寸的增大作周期性振荡,都与透射终端位置的改变相关;铁磁电极、Rashba自旋轨道耦合、外加磁场对透射概率和自旋极化率具有不同影响;透射概率随δ势垒强度增加单调减小,自旋极化率随δ势垒强度增加单调增大。     

Transmission of hot electrons through a metal-semiconductor interface

The expression for the current density of the chemical electron emission from a metal film into a semiconductor is obtained. The dependences of the probability for transmission of hot electrons through the metal-semiconductor interface on the average hot-electron energy, temperature, and electric-field strength are obtained. The conditions at which the probability for transmission of hot electrons through the interface is almost equal to unity are established.     

非零金属电阻值和合金化对确定比接触电阻值的影响

水文提出用圆环测试结构和双传输线模型确定金属-半导体欧姆接触的比接触电阻ρ_C值。考虑了金属电阻值不为零和合金化后金属接触下方半导体电阻率的改变对确定比接触电阻值的影响,导出了计算比接触电阻值的公式。用提出的方法对实验样品进行测量和计算,并将结果与文献中报道的方法所得结果作了比较。     

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     

纳米硅薄膜光学性质的测定与研究

通过测定纳米硅薄膜的透射谱,建立计算模型计算得出薄膜样品的折射率、厚度、吸收系数和光能隙.计算结果表明这种半导体材料在620 nm波长附近的折射率约为3.4,计算得到的厚度与用台阶仪测量的结果吻合很好.在620 nm波长附近的吸收系数介于吸收系数较小的晶体硅与吸收系数较大的非晶硅之间,光能隙约为1.6 eV,两者都随晶态含量增大而呈减小趋势.     

飞秒激光作用下薄膜的热力学研究

为了描述飞秒激光与金属薄膜相互作用过程中的非平衡传热现象,采用有限差分的方法对金属薄膜内的温度场进行了1维数值模拟。对双温模型中电子-晶格耦合系数G、激光脉宽和电子的弹道运动等因素对金属薄膜表层电子和晶格温度的影响进行了理论分析。结果表明,G影响材料表面电子的温升,电子和晶格温度平衡时的延迟时间随着G的增大而减小,二者呈指数变化关系。这一结果对改善半导体元件中薄膜的温升是有帮助的。     

Reversible Degradation of Ohmic Contacts on p-GaN for Application in High-Brightness LEDs

This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current-voltage (I-V) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250degC was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I-V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened.