GaN紫外探测器的Ti/Al接触的电学特性

在非故意掺杂的和掺Si的GaN薄膜上蒸镀Ti(24nm)/Al(nm)薄膜,氮气环境下400～800℃范围内进行退火。实验结果表明,在非故意掺杂的样品上,随退火温度的升高,肖特基势垒高度下降,理想因子升高,表面状况逐渐变差,600℃退火形成较低接触电阻的欧姆接触,比接触电阻率为3.03×10-4Ωcm2,而载流子浓度为5.88×1018cm-3的掺Si的样品未退火就形成欧姆接触,比接触电阻可达到4.03×10-4Ωcm2。     

快速热退火对Ti/Al-GaN接触的影响

利用伏安特性和俄歇能谱深度分布研究了快速热退火对Ti/Al-GaN接触的影响,氮气中600℃退火60s可以获得欧姆接触.实验结果表明,退火温度的升高导致N元素向表面扩散和界面反应的发生.N空位的产生形成了重掺杂的界面,有利于电流的隧穿,界面处Al,Ti,Ga,N三元系或四元系反应产物起到降低Ti/GaN接触的势垒作用.较高温度退火形成的欧姆接触是势垒高度降低和隧穿电流机制共同作用的结果     

GaN上的欧姆接触

主要描述了在ｎ 型和ｐ 型ＧａＮ上制备欧姆接触的方法,分析了欧姆接触的特性及其形成机理, 并讨论了该领域未来的研究趋势     

The Cu/n-GaAs schottky barrier diodes prepared by anodization process

The metal-insulating semiconductor (MIS) Cu/n-GaAs diodes with thin anodic-insulating layer, which is formed by anodic oxidazation on the n-GaAs substrate in aqueous 4C₂H₆O₂+2H₂O+0.1H₃PO₄ electrolyte with pH=2.02; anodically untreated control Cu/n-GaAs diodes; and anodically treated Cu/n-GaAs diodes (several steps of anodization in the same electrolyte followed by a dip in diluted aqueous HCl solution and a subsequent rinse in deionized water) have been prepared. The anodization has increased the barrier heights as well as the ideality factors. We have obtained barrier heights of approximately 0.68 eV, 0.90 eV, and 0.92 eV for the control sample, anodically treated sample, and MIS sample, respectively, adding the contribution caused by image-force effect only. Thus, the barrier height has been increased by at least 140 meV. Furthermore, we have calculated a mean tunneling-barrier height of x=0.025 eV for the MIS Cu/n-GaAs Schottky barrier diode (SBD).     

Characterization of Epitaxial Indium Nitride Interlayers for Ohmic Contacts to Silicon Carbide

Ohmic contacts to n-type 4H- and 6H-SiC without postdeposition annealing were achieved using an interlayer of epitaxial InN beneath a layer of Ti. The InN films were grown by reactive dc magnetron sputtering at 450°C, whereas the Ti films were deposited by electron-beam evaporation at room temperature. The InN films were characterized by x-ray diffraction (XRD), secondary electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM), and Hall-effect measurements. Both XRD and TEM observations revealed that the Ti and InN films have epitaxial relationships with the 6H-SiC substrate as follows: (0001)[]_Ti∥(0001)[]_InN∥(0001)[]_6H-SiC. The Ti/InN/SiC contacts displayed ohmic behavior, whereas Ti/SiC contacts (without an InN interlayer) were nonohmic. These results suggest that InN reduces the Schottky barrier height at the SiC surface via a small conduction-band offset and support previous reports of an electron accumulation layer at the surface of InN.     

Formation of Thick Porous Anodic Alumina Films and Nanowire Arrays on Silicon Wafers and Glass

A method for the fabrication of thick films of porous anodic alumina on rigid substrates is described. The anodic alumina film was generated by the anodization of an aluminum film evaporated on the substrate. The morphology of the barrier layer between the porous film and the substrate was different from that of anodic films grown on aluminum substrates. The removal of the barrier layer and the electrochemical growth of nanowires within the ordered pores were accomplished without the need to remove the anodic film from the substrate. We fabricated porous anodic alumina samples over large areas (up to 70 cm²), and deposited in them nanowire arrays of various materials. Long nanowires were obtained with lengths of at least 9 μm and aspect ratios as high as 300. Due to their mechanical robustness and the built‐in contact between the conducting substrate and the nanowires, the structures were useful for electrical transport measurements on the arrays. The method was also demonstrated on patterned and non‐planar substrates, further expanding the range of applications of these porous alumina and nanowire assemblies.     

Electrical characteristics of thermally stable Ru and Ru/Au ohmic contacts to surface-treated p-type GaN

The electrical and thermal properties of Ru and Ru/Au ohmic contacts on two-step-surface-treated p-GaN have been investigated using current-voltage (I–V) measurements and Auger electron spectroscopy. It is shown that annealing at 700°C for 2 min in a flowing N₂ atmosphere improves the I–V characteristics of the contacts. For example, the annealed Ru and Ru/Au schemes produce a specific contact resistance of 3.4 (±0.9)×10⁻³ and 1.2 (±1.1)×10⁻³ Ωcm², respectively. It is also shown that annealing results in a large reduction (by ∼100 meV) in the Schottky barrier heights of the Ru and Ru/Au contacts, compared to the as-deposited ones. The electrical properties of the two-step-surface-treated Ru/Au contacts are compared with those of the conventionally treated contacts.     

p-GaN与Ni/Pt欧姆接触的研究

本文对p-GaN与Ni/Pt形成欧姆接触及其电流传输机制进行了研究。I-V变温测试曲线是线形的,表明Ni/Pt与p-GaN之所以能形成欧姆接触,是因为Ni/Pt与p-GaN接触在空气中退火时界面处的p-GaN空穴浓度增加了,从而降低了有效势垒高度。在148K～323K范围内,单位接触电阻R_c随测试温度T的升高趋于呈指数下降,表明Ni/Pt与p-GaN欧姆接触的电流传输机制遵循热电子发射。     

Metal contacts to n-type GaN

Contacts consisting of various single layer metals to n-type GaN have been formed and characterized. The current-voltage characteristics were measured for 17 different metals (Sc, Hf, Zr, Ag, Al, V, Nb, Ti, Cr, W, Mo, Cu, Co, Au, Pd, Ni, and Pt) deposited on the same epitaxial growth layer. The barrier height, ideality factor, breakdown voltage, and effective Richardson coefficients were measured from those metals which exhibited strong rectifying behavior. The barrier heights for these metal contacts were measured using current-voltage-temperature and capacitance-voltage techniques. It was found that an increase in metal work function correlated with an increase in the barrier height. The surface state density of GaN was approximated to be very similar to CdS and almost a factor of ten less than GaAs.     

无凹槽AlGaN/GaN肖特基势垒二极管正向电流输运机制

研究了无凹槽AlGaN/GaN肖特基势垒二极管(SBD)的正向电流输运机制。分别采用Ni/Au和TiN作为阳极金属材料制备了无凹槽AlGaN/GaN SBD,对比了两种SBD的直流特性。并通过测量器件的变温I-V特性,研究了器件的正向电流输运机制。结果表明,TiN-SBD(0.95 V@1 mA·mm^-1)与Ni/Au-SBD(1.15 V@1 mA·mm^-1)相比实现了更低的开启电压,从而改善了正向导通特性。研究发现两种SBD的势垒高度和理想因子都强烈依赖于环境温度,通过引入势垒高度的高斯分布模型解释了这种温度依赖性,验证了正向电流输运机制为与势垒高度不均匀分布相关的热电子发射机制。     

Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN

We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 × 10¹⁷ cm⁻³). It is shown that annealing at 500°C for 2 min in a N₂ ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(±0.2) × 10⁻⁴ and 2.4(±0.2) × 10⁻⁵ Ωcm² for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltage-temperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.     

SiC肖特基接触的直接隧穿效应

通过精确求解一维定态薛定谔方程得到电子通过三角形势垒的隧穿几率,模拟了SiC肖特基接触的直接隧穿效应.结果显示该方法比WKB近似更精确,同时也更适合工作在高场条件下的SiC材料,并且能够连续地计算热电子发射电流和隧穿电流.     

Highly Stable Nickel Hexacyanoferrate Nanotubes for Electrically Switched Ion Exchange

Nickel hexacyanoferrate (NiHCF) nanotubes are fabricated by an electrokinetic method based on the distinct surface properties of porous anodic alumina. By this method, nanotubes can be formed rapidly with the morphologies faithfully replicating the nanopores in the template. The prepared nanotubes were carefully characterized using SEM and TEM. Results from IR, UV, EDX, and electrochemical measurements show that the NiHCF nanotubes exist only in the form of K₂Ni[Fe(CN)₆]. Because of this single composition and unique nanostructure, NiHCF nanotubes show excellently stable cesium‐selective ion‐exchange ability. The capacity for electrodes modified with NiHCF nanotubes after 500 potential cycles retains 95.3 % of its initial value. Even after 1500 and 3000 cycles, the NiHCF nanotubes still retain 92.2 % and 82.9 %, respectively, of their ion‐exchange capacity.     

Barrier height determination for n-type 4H-SiC schottky contacts made using various metals

We have studied Schottky barrier contacts to n-type 4H-SiC with Cr, Mo, Ta, W, Au, and Ni. We have focused on effects of the metal work function, measurement technique and interface behavior on the Schottky barrier heights (SBHs). The contacts were prepared by metal deposition via high frequency cathodic sputtering on chemical vapor epitaxially grown epitaxial layers with low residual doping (2 × 10¹⁵ cm⁻³). Prior to deposition on Si-terminated surfaces, they were in-situ cleaned by Ar-ion sputtering for 5 and 10 min, respectively. The contacts have been characterized by means of current-voltage, capacitance-voltage (C-V), and internal photoemission (IPE) methods at room temperature. With deep level transient spectroscopy, interface deep electron traps have been detected with thermal ionization energies of 0.68, 0.77, and 1.04 eV, respectively. These traps have been attributed to structural defects formed during epitaxial growth termination. The diodes have relatively low reverse leakage current, but the ideality factor is larger than one. The SBHs have been determined from C-V and IPE measurements. It has been shown that the C-V data may contain errors resulting in a low SBH if electron deep traps are present in the interface region. In general, the SBH of various metals are influenced by the metal work function and also by the semiconductor surface preparation. The interface homogeneity is an important characteristic of the Schottky contacts, which may be improved by an optimized ion sputtering prior to metal deposition. We have considered the SBHs determined by IPE measurements as most reliable.     

Cu contact on NiSi substrate with a Ta/TaN barrier stack

The thermal and electrical stabilities of Cu contact on NiSi substrate with and without a Ta/TaN barrier stack in between were investigated. Four-point probe (FPP), X-ray diffraction (XRD), scanning electron microscopy (SEM), depth-profiling X-ray photoelectron spectroscopy (XPS), and Schottky barrier height (SBH) measurement were carried out to characterize the diffusion barrier properties. The SBH measurement provides a very sensitive method to characterize the diffusion barrier properties for the copper contact on NiSi/Si. The results show that the Ta/TaN stack can be both thermally and electrically stable after annealing at 450 °C for 30 min and it will have a potential application as a diffusion barrier for Cu contact on NiSi.     

The effects of the temperature and annealing on current-voltage characteristics of Ni/n-type 6H-SiC Schottky diode

The temperature dependence of current-voltage (I-V) characteristics of as-fabricated and annealed Ni/n-type 6H-SiC Schottky diode has been investigated in the temperature range of 100-500 K. The forward I-V characteristics have been analysed on the basis of standard thermionic emission theory. It has been shown that the ideality factor (n) decreases while the barrier height (Φ_b) increases with increasing temperature. The values of Φ_b and n are obtained between 0.65-1.25 eV and 1.70-1.16 for as-fabricated and 0.74-1.70 eV and 1.84-1.19 for annealed diode in the temperature range of 100-500 K, respectively. The I-V characteristics of the diode showed an increase in the Schottky barrier height, along with a reduction of the device leakage current by annealing the diode at 973 K for 2 min.     

Achieving Ultrahigh Electron Mobility in PdSe2 Field-Effect Transistors via Semimetal Antimony as Contacts

Even though atomically thin 2D semiconductors have shown great potential for next-generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high-mobility field-effect transistors (FETs) by introducing few-layer PdSe₂ flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect-free van der Waals (vdW) stackings at the Sb–PdSe₂ heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on-current density reaching 96 µA µm⁻¹, and high electron mobility of 383 cm² V⁻¹ s⁻¹. Furthermore, metal–insulator transition (MIT) is observed in the PdSe₂ FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe₂ vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T₀, and ultrahigh electron mobility of 2,184 cm² V⁻¹ s⁻¹ at 10 K. These findings are expected to be significant for developing high mobility 2D-based quantum devices.     

Characterization of Non-Alloyed Ohmic Contacts to Si-Implanted AlGaN/GaN Heterostructures for High-Electron Mobility Transistors

This paper reports results of a study of non-alloyed ohmic contacts on Si-implanted AlGaN/GaN heterostructures, obtained from current–voltage characteristics of transfer-length method (TLM) test structures. It is shown that the measured contact resistance from the Ti/Au/Ni metal contacts, deposited on Si-implanted regions, to the two-dimensional electron gas channel at the AlGaN/GaN heterointerface of the non-implanted region, is formed by three different components: (i) contact resistance between the metal␣and the semiconductor (0.60 ± 0.16 Ω mm), (ii) resistance of the implanted region (0.62 ± 0.03 Ω mm) and (iii) an additional resistance (0.72 ± 0.24 Ω mm) giving a total value of 1.9 ± 0.3 Ω mm. The specific ohmic contact resistance was determined to be (2.4 ± 0.5) × 10⁻⁵ Ω cm².     

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