基于两步退火法提升Al/n~+Ge欧姆接触及Ge n~+/p结二极管性能

锗(Ge)中高激活浓度、低扩散深度的n型掺杂是实现高性能Ge n-MOSFET的重要前提条件.本文采用低温预退火与脉冲激光退火相结合的两步退火法,结合磷离子注入,制备Al/n~+Ge的欧姆接触以及Ge n~+/p结二极管.通过电流-电压特性测试来研究Al/n~+Ge的欧姆接触以及Ge n~+/p结二极管的性能,测试结果表明:低温预退火可初步修复注入损伤,并降低激光退火时杂质的扩散深度;结合离子注入工艺和两步退火工艺,Al/n~+Ge欧姆接触的比接触电阻率降至2.61×10~(-6)Ω·cm~2,Ge n~+/p结二极管在±1V的整流比提高到8.35×10~6,欧姆接触及二极管性能均得到了显著提升.     

金属与半导体Ge欧姆接触制备、性质及其机理分析

金属与Ge材料接触时界面处存在着强烈的费米钉扎效应, 尤其与n型Ge形成的欧姆接触的比接触电阻率高, 是制约Si基Ge器件性能的关键因素之一. 本文对比了分别采用金属Al和Ni 与Si衬底上外延生长的p型Ge和n型Ge材料的接触特性. 发现在相同的较高掺杂条件下, NiGe与n型Ge可形成良好的欧姆接触, 其比接触电阻率 较 Al接触降低了一个数量级, 掺P浓度为2×10¹⁹ cm^-3时达到1.43×10^-5 Ω·cm². NiGe与p型Ge接触和Al接触的比接触电阻率相当, 掺B浓度为4.2×10¹⁸ cm^-3时达到1.68×10^-5 Ω·cm². NiGe与n型Ge接触和Al电极相比较, 在形成NiGe过程中, P杂质在界面处的偏析是其接触电阻率降低的主要原因. 采用NiGe作为Ge的接触电极在目前是合适的选择. 关键词： 金属与Ge接触性质 NiGe 比接触电阻率     

GaN HEMT欧姆接触模式对电学特性的影响

本文制备了AlGaN/GaN HEMT器件中常规结构与带有纵向接触孔结构的两种接触电极,研究了该两种源欧姆接触模式对器件电学特性的影响.在相同条件下进行快速退火,发现在750?C下退火30 s后,常规结构还没有形成欧姆接触,而带有纵向欧姆接触孔的接触电极与外延片已经形成了良好的欧姆接触.同时,比较了Ti/Al/Ti/Au和Ti/Al/Ni/Au电极退火后表面形态,Ti/Al/Ni/Au具有更好的表面形貌.通过测试两种结构的HEMT器件后,发现采用纵向欧姆接触孔结构器件具有更高的跨导和饱和电流,但是也会在栅极电压为0.5—2 V之间产生严重的电流崩塌现象.     

Ni/Au与N掺杂p型ZnO的欧姆接触

研究了Au,In,Ni/Au三种不同金属膜与N掺杂p型ZnO的接触特性,发现Ni/Au双层膜更适合作为其欧姆电极材料,并比较了不同气氛和不同温度退火对Ni/Au电极的影响.发现在O₂中退火电极性能发生蜕变,而在N₂中退火性能得到改善.指出即使在N₂中退火,退火温度的选择也是至关重要的,本实验在400℃,氮气气氛下退火150s,得到了较好的欧姆接触特性.     

TiAl3 和Ti/TiAl3 非合金化电极n型GaN欧姆接触的实现

在不进行合金化的情况下,首次直接采用TiAl3合金材料作为金属接触电极.在蓝宝石衬底上生长的n型载流子浓度为2×1018 cm-3的GaN上,成功地得到低接触电阻的欧姆接触,并由环形传输线模型方法测得比接触电阻率为3×10-5 Ω*cm2.与通常n型欧姆接触采用的Ti/Al双层结构比较,TiAl3合金结构更容易形成非合金化的n型欧姆接触.在此实验基础上,进一步分析了N空位和界面层处的TiAl3在形成非合金化或低温退火欧姆接触中发挥的作用,由此设计的Ti/TiAl3/Ni/Au接触结构,在TiAl3合金结构基础上明显地降低了接触电阻率.     

退火温度和退火气氛对Ni/Au与p-GaN之间欧姆接触性能的影响

研究了不同退火温度和气氛对Ni/Au与p-GaN之间欧姆接触性能的影响. 采用圆形传输线模型方法得到不同退火温度和不同退火气氛下的比接触电阻率. 结果表明, 较适宜的退火温度为500 ℃左右, 退火温度太高或太低都会导致比接触电阻率的增大; 较适宜的退火气氛为适量含氧的氮气气氛, 且氧气含量对比接触电阻率大小的影响并不显著. 经过对退火条件的优化, 得到的比接触电阻率可达7.65×10^-4 Ω·cm². 关键词： p-GaN 欧姆接触 圆形传输线模型 快速热退火     

低接触电阻率Ni/Ag/Ti/Au反射镜电极的研究

研究了Ag的厚度、退火时间、沉积温度对于Ni/Ag/Ti/Au电极的反射率及与p-GaN欧姆接触性能的影响. 利用分光光度计测量反射率, 采用圆形传输线模型计算比接触电阻率. 结果表明: 随着Ag厚度的增加, Ni/Ag/Ti/Au电极的反射率逐渐增大; 在氧气氛围中, 随着退火时间从1 min增至10 min, 300 ℃退火时, 比接触电阻率持续下降, 而对于400-600 ℃退火, 比接触电阻率先减小后增大; 在300和400 ℃氧气中进行1-10 min 的退火后, Ni/Ag/Ti/Au的反射率变化较小, 退火温度高于400 ℃时, 随着退火时间的增加, 反射率急剧下降; 在400 ℃氧气中3 min退火后, 比接触电阻率可以达到3.6×10^-3 Ω·cm². 此外, 适当提高沉积温度可以增加Ni/Ag/Ti/Au的反射率并降低比接触电阻率, 沉积温度为120 ℃条件下的Ni/Ag/Ti/Au电极在450 nm处反射率达到90.1%, 比接触电阻率为6.4×10^-3 Ω·cm². 综合考虑电学和光学性能, 在沉积温度为120 ℃下蒸镀Ni/Ag/Ti/Au (1/200/100/100 nm)并在400 ℃氧气中进行3 min退火可以得到较优化的电极. 利用此电极制作的垂直结构发光二极管在350 mA电流下的工作电压为2.95 V, 输出光功率为387.1 mW, 电光转换效率达到37.5%.     

硅衬底GaN基LED N极性n型欧姆接触研究

在Si衬底GaN基垂直结构LED的N极性n型面上,利用电子束蒸发的方法制作了Ti/Al电极,通过了I-V曲线研究了有无AlN缓冲层对这种芯片欧姆接触的影响.结果显示,去除AlN缓冲层后的N极性n型面与Ti/Al电极在500到600 ℃范围内退火才能形成欧姆接触.而保留AlN缓冲层的N极性n型面与Ti/Al电极未退火时就表现为较好的欧姆接触,比接触电阻率为2×10^-5 Ω·cm²,即使退火温度升高至600 ℃,也始终保持着欧姆接触特性.因此,AlN缓冲层的存在是Si衬底GaN基垂直结构LED获得高热稳定性n型欧姆接触的关键. 关键词： 硅衬底 N极性 AlN缓冲层 欧姆接触     

Ni/Ag/Ti/Au与p-GaN的欧姆接触性能及光反射率

研究了不同Ni厚度的Ni/Ag/Ti/Au电极在不同退火温度和退火气氛下与p-GaN之间的欧姆接触性能以及电极的光反射率的变化.采用矩形传输线模型对各电极的比接触电阻率进行测算,利用分光光度计对电极在不同波长下的反射率进行测量.结果表明,Ni金属层的厚度越小,电极的光反射率越高,而Ni层厚度对比接触电阻率的影响较小;当退火温度高于400℃后,电极的光反射率降低,在氧气氛围中退火后光反射率比在氮气中退火后下降更加明显.但在氧气氛围中退火有利于减小比接触电阻率.综合考虑接触电阻和光反射率,电极Ni(1 nm)/Ag/Ti/Au在400℃氧气中快速退火后得到了较好的结果,其比接触电阻率为5.5×10-3Ω·cm2,在450 nm处反射率为85%.利用此电极制作了垂直结构发光二极管(LED)器件,LED在350 mA注入电流下,工作电压为3.2 V,发光功率为270 mW,电光转换效率达到24%.     

双空位掺杂氟化石墨烯的电子性质和磁性

基于密度泛函理论,计算了外来原子X(Al,P,Ga,As,Si)双空位替代掺杂氟化石墨烯的电子特性和磁性.通过对计算结果分析发现,与石墨烯的双空位掺杂类似,氟化石墨烯的双空位掺杂也是一种较为理想的掺杂方式.通过不同原子掺杂,氟化石墨烯的电子性质与磁性均发生很大变化:Al和Ga掺杂使氟化石墨烯由半导体变为金属,并且具有磁性;P和A8掺杂使氟化石墨烯变为自旋半导体;Si掺杂氟化石墨烯仍是半导体,只改变带隙且没有磁性.进一步讨论磁性产生机制获得了掺杂原子浓度与磁性的关系,并且发现不同掺杂情况的磁性是由不同原子的不同轨道电子引起的.双空位掺杂不仅丰富了氟化石墨烯的掺杂方式,其不同电磁特性也使此类掺杂结构在未来的电子器件中具有潜在应用.     

Schottky barrier at the Al/Si(111) doped and double-doped interfaces: a local-density cluster study

Using the local-density approximation method we have investigated a cluster model of the Al/Si(111) interface. P atoms were placed in the fifth silicon double-atomic layer (DAL) counting from the interface. The second DAL was doped with either Ga or As. The Schottky barrier height was found to be 0.75 eV at the Al/Si(intrinsic) system and 0.73 eV at the Al/Si(P-doped) case. The additional doping of the near-interface layer by Ga increased the barrier to 0.90 eV while the As doping decreased it to 0.5 eV.     

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au（15 nm/220 nm/40 nm/50 nm） multiplayer contacts to n-type GaN （Nd = 3.7 × 10^17 cm^-3, Nd = 3.0 × 10^18 cm^-3）. The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the AI layer even the Ti layer.     

表面化学处理和退火对p-GaN/ZnO:Ga接触特性的影响

ZnO∶Ga(GZO)透明电极沉积在p-GaN表面,用作透明电流扩展层。直接沉积在p-GaN上的p-GaN/GZO存在较大的势垒,容易形成肖特基接触,而良好的欧姆接触对功率LED器件至关重要。为了降低接触势垒,采用盐酸和氢氧化钠溶液对GaN表面进行去氧化层处理,并对p-GaN/GZO进行退火处理,研究表面处理和退火对p-GaN/GZO接触特性的影响。研究表明:碱性溶液处理有利于降低接触势垒;退火处理后,接触势垒略有增加。     

Improved device performance in nonpolar a ‐plane GaN LEDs using a Ni/Al/Ni/Au n‐type ohmic contact

The authors report upon the increased light‐output power (P_out) via a reduction in the forward voltage (V_f) for nonpolar a ‐plane GaN LEDs using Ni/Al/Ni/Au n‐type ohmic contacts. The specific contact resistivity of the Ni/Al/Ni/Au contact is found to be as low as 5.6 × 10^–5 whereas that of a typical Ti/Al/Ni/Au contact is 6.8 × 10^–4 Ω cm², after annealing at 700 °C. The X‐ray photoelectron spectroscopy results show that the upward surface band bending is less pronounced for the Ni/Al contact compared to the Ti/Al contact, leading to a decrease in the effective Schottky barrier height (SBH). The V_f of the nonpolar LEDs decreases by 10% and P_out increases by 15% when the Ni/Al/Ni/Au scheme is used instead of the typical Ti/Al/Ni/Au metal scheme. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors (HEMTs) have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current--voltage (I--V) measurements. A non-linear fitting method was used to extract the contact parameters from the I--V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450°C in N₂ ambience.     

Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300 K is obtained at a fixed forward current after switching from N2 to 10%H2＋N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2＋N2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air azcelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity Of the sensor are calculated.     

Study of Ti/Au,Ti/Al/Au,and Ti/Al/Ni/Au ohmic contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The contacts of Ti/Au, Ti/Al/Au, and Ti/Al/Ni/Au films deposited on n-GaN were studied by current–voltage (I–V) and transmission-line-method measurements. The effect of annealing temperature on specific contact resistivity has been investigated by changing the annealing temperature from 400 to 900 °C. Ti/Al/Au and Ti/Al/Ni/Au films were superior to the bilayer (Ti/Au) in ohmic contact characteristics and thermal stability. The Ti/Al/Ni/Au composite showed the best thermal stability due to the fact that Ni plays a more important role than the alloy of Ti/Al in preventing the interdiffusion of Ti, Al, and Au. The lowest contact resistivity (10^-7cm²) to n-GaN was obtained for the Ti/Al/Ni/Au sample by short-time/high-temperature annealing. The formation mechanism of ohmic contacts to n-GaN is also discussed. PACS 73.40.Cg; 73.61.Ey     

Mechanism of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures via laser annealing

The physical mechanisms of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures by laser annealing and rapid thermal annealing are systematically investigated. The microstructures indicate that a better surface morphology and an intact contact interface are formed after laser annealing. None of the TiN alloy spikes are formed at the interface of the laser annealing sample. The experimental results show that the current transport mechanism through the ohmic contact after laser annealing is different from the conventional spike mechanism, and it is dominated by thermionic field emission.     

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10¹⁸ cm⁻³). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10⁻⁴ (Al), 8.0×10⁻⁵ (Al/Au) and 1.2×10⁻⁵ Ω cm² (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.     

高反射率p-GaN欧姆接触电极

根据光学薄膜原理计算了GaN/Ti/Ag、GaN/Al和GaN/Ni/Au/Ti/Ag、GaN/Ni/Au/Al多层电极结构的反射率,得出Ag基和Al基反射电极均能在全角范围内提供较高的反射率。实验测量结果表明,反射率能高于80%的Ag基反射电极,具有低欧姆接触的电学特性。并将GaN/Ni/Au/Ti/Ag多层反射电极应用在上下电极结构的GaN基LED中。实验上采用两步合金法获得了低接触电阻、高反射率的电极结构,并引入Ni/Au覆盖层克服了Ag高温时的团聚和氧化现象。解决了Ag电极的稳定性问题,显著地提高了LED的出光效率,成功制备了具有上下电极结构的GaN基LED管芯。