n-GaN上Ti/Al电极的表面处理与退火

实验研究了不同表面处理方法和不同退火条件对GaN上的Ti/Al电极的影响,用CH3CSNH2/NH4OH处理后的GaN材料的荧光光谱强度最高,在该材料上制作的Ti/Al电极的欧姆接触电阻率最小.通过欧姆接触电阻率,I-V曲线,X射线衍射等手段,分析了GaN与Ti/Al 电极接触表面在退火过程中的固相反应,提出了二次退火的方法.     

金属/n型AlGaN欧姆接触

用传输线模型对n型AlGaN(n-AlGaN)上Au/Pt/Al/Ti多金属层欧姆接触进行了接触电阻率的测量.在850℃退火5min后,测得欧姆接触电阻率达1.6×10-4Ω·cm2.经X射线衍射分析,Au/Pt/Al/Ti/n-AlGaN界面固相反应得出在500℃以上退火过程中,AlGaN层中N原子向外扩散,在AlGaN表面附近形成n型重掺杂层,导致欧姆接触电阻率下降;随退火温度的升高,N原子外扩散加剧,到800℃以上退火在Au/Pt/Al/Ti/n-AlGaN界面形成Ti2N相,导致欧姆接触电阻率进一步下降.     

金属/n型AlGaN欧姆接触

用传输线模型对n型AlGaN(n-AlGaN)上Au/Pt/Al/Ti多金属层欧姆接触进行了接触电阻率的测量.在850℃退火5min后,测得欧姆接触电阻率达1.6×10-4Ω·cm2.经X射线衍射分析,Au/Pt/Al/Ti/n-AlGaN界面固相反应得出在500℃以上退火过程中,AlGaN层中N原子向外扩散,在AlGaN表面附近形成n型重掺杂层,导致欧姆接触电阻率下降;随退火温度的升高,N原子外扩散加剧,到800℃以上退火在Au/Pt/Al/Ti/n-AlGaN界面形成Ti2N相,导致欧姆接触电阻率进一步下降.     

Ti/Al/Ni/Au在GaN N面上的欧姆接触

以Ti/Al/Ni/Au作为欧姆接触金属体系,通过电感耦合等离子体(ICP)刻蚀的预处理,在氢化物气相外延法生长的单晶氮化镓(GaN)材料的N面实现了良好的欧姆接触,其比接触电阻率为3.7×10-4 Ω·cm2.通过扫描电子显微镜、原子力显微镜、阴极荧光和光致发光谱对GaN N面的表面、光学特性进行了对比表征.结果表明:未刻蚀GaN衬底的N面表面存在一定的损伤层,导致近表面处含有大量缺陷,不利于欧姆接触的形成;而ICP刻蚀处理有效地去除了损伤层.X射线光电子能谱(XPS)分析显示刻蚀后样品的Ga 3d结合能比未刻蚀样品向高能方向移动了约0.3 eV,其肖特基势垒则相应降低,有利于欧姆接触的形成.同时对Fe掺杂半绝缘GaN的N面也进行了刻蚀处理,同样实现了良好的Ti/Al/Ni/Au欧姆接触,其比接触电阻率为0.12 Ω·cm2.     

表面处理对p-GaN欧姆接触的影响

分别用稀盐酸、王水以及(NH4)2S溶液处理p-GaN表面,通过测试样品表面Ols的X射线光电子能谱(XPS),比较了这些溶液去除p-GaN表面氧化层的能力;在经不同溶液处理后的样品表面,以相同的条件制作Ni/Au电极,并测试其与p-GaN的比接触电阻,结果表明经稀盐酸处理后的样品表面,由于其氧含量较高,不能与Ni/Au形成良好的欧姆接触,而经王水和(NH4)2S溶液处理后的p-GaN表面,能与Ni/Au形成良好的欧姆接触;最后,通过比较样品表面的Ga/N原子浓度比,探讨了王水处理p-GaN表面能够形成良好欧姆接触的原因.     

表面处理对p-GaN欧姆接触的影响

分别用稀盐酸、王水以及(NH4)2S溶液处理p-GaN表面,通过测试样品表面Ols的X射线光电子能谱(XPS),比较了这些溶液去除p-GaN表面氧化层的能力;在经不同溶液处理后的样品表面,以相同的条件制作Ni/Au电极,并测试其与p-GaN的比接触电阻,结果表明经稀盐酸处理后的样品表面,由于其氧含量较高,不能与Ni/Au形成良好的欧姆接触,而经王水和(NH4)2S溶液处理后的p-GaN表面,能与Ni/Au形成良好的欧姆接触;最后,通过比较样品表面的Ga/N原子浓度比,探讨了王水处理p-GaN表面能够形成良好欧姆接触的原因.     

AlGaN/AlN/GaN肖特基二极管的电学性能

利用金属有机化学气相淀积(MOCVD)方法生长的AlGaN/AlN/GaN/蓝宝石材料制备了AlGaN肖特基二极管.器件的肖特基接触和欧姆接触分别为Ti/Pt和Ti/Al/Ti/Au,均采用电子束蒸发的方法沉积.AlGaN表面欧姆接触的比接触电阻率为7.48×10-4Ω/cm2,器件的I-V测试表明该AlGaN肖特基二极管具有较好的整流特性.根据器件的正向,I-V特性计算得到器件的势垒高度和理想因子分别为0.57eV和4.83.将器件在300℃中温退火,器件的电学性能有所改善.     

AlGaN/AlN/GaN肖特基二极管的电学性能

利用金属有机化学气相淀积(MOCVD)方法生长的AlGaN/AlN/GaN/蓝宝石材料制备了AlGaN肖特基二极管.器件的肖特基接触和欧姆接触分别为Ti/Pt和Ti/Al/Ti/Au,均采用电子束蒸发的方法沉积.AlGaN表面欧姆接触的比接触电阻率为7.48×10-4Ω/cm2,器件的I-V测试表明该AlGaN肖特基二极管具有较好的整流特性.根据器件的正向,I-V特性计算得到器件的势垒高度和理想因子分别为0.57eV和4.83.将器件在300℃中温退火,器件的电学性能有所改善.     

Ti/Al/Ti/Au与AlGaN欧姆接触特性

研究了溅射Ti/Al/Ti/Au四层复合金属与AlGaN/GaN的欧姆接触特性,并就环境温度对欧姆接触特性的影响进行了分析研究.试验证实:溅射的Ti/Al/Ti/Au与载流子浓度为2.24×1018cm-3的AlGaN之间在室温下无需退火即可形成欧姆接触.随快速退火温度的升高接触电阻降低.快速退火时间30s已可实现该温度下最佳欧姆接触.当工作温度不高于300℃时接触电阻几乎不受温度的影响.     

Ti/Al/Ti/Au与AlGaN欧姆接触特性

研究了溅射 Ti/ Al/ Ti/ Au四层复合金属与 Al Ga N / Ga N的欧姆接触特性 ,并就环境温度对欧姆接触特性的影响进行了分析研究 .试验证实 :溅射的 Ti/ Al/ Ti/ Au与载流子浓度为 2 .2 4× 10 1 8cm- 3的 Al Ga N之间在室温下无需退火即可形成欧姆接触 .随快速退火温度的升高接触电阻降低 .快速退火时间 30 s已可实现该温度下最佳欧姆接触 .当工作温度不高于 30 0℃时接触电阻几乎不受温度的影响     

Structural properties of alloyed Ti/Al/Ti/Au and Ti/Al/Mo/Au ohmic contacts to AlGaN/GaN

Two alloyed ohmic contact structures for AlGaN/GaN–Ti/Al/Ti/Au and Ti/Al/Mo/Au were studied. Both structures were optimized for minimum ohmic contact resistance. Structures grown on sapphire and SiC substrates were used to investigate structural properties of ohmic contacts to AlGaN/GaN. Ohmic contacts to AlGaN/GaN on SiC showed higher contact resistance values compared to contacts to AlGaN/GaN on sapphire. Ohmic contact metals were etched on samples after annealing. The alloyed interface was studied with backside illumination under an optical microscope. Alloyed inclusions associated with threading dislocations were observed on the surface. For the AlGaN/GaN on SiC sample the inclusion density was an order of magnitude lower than for the sample on sapphire. Conductive atomic force microscopy with carbon nanotube tip was used to investigate topography and conductivity profile of the surface after ohmic contact metal removal by etching.     

SAW Filters Composed of Interdigital Schottky and Ohmic Contacts on AlGaN/GaN Heterostructures

We proposed surface acoustic wave (SAW) filters composed of interdigital Schottky and ohmic contacts on AlGaN/GaN heterostructures. The contribution of the SAWs appeared in the radio frequency characteristics of the filters when the Schottky contacts were reverse biased. Onsets of the SAW signals and the threshold voltage of simultaneously fabricated high-electron mobility transistors were found to almost agree with one another. We also obtained an isolation of >40 dB. These results suggest that SAW-based functional devices are likely to be realized using AlGaN/GaN heterostructures with interdigital Schottky and ohmic contacts     

AlGaN/GaN HEMT欧姆接触的研究进展

从欧姆接触形成的机理出发,介绍了在AlGaN/GaN HEMT中实现源和漏区欧姆接触的各种方法,如表面处理技术、金属化系统和重掺杂技术等.回顾了近年来这些方法的研究进展.     

Effect of ohmic contacts on buffer leakage of GaN transistors

The effect of ohmic contacts on the buffer leakage of GaN transistors is presented. The buffer leakage for AlGaN/GaN high-electron mobility transistors and GaN MESFETs grown on the same underlying buffer was observed to be different. Controlled experiments show that the increased buffer leakage is due to the nature of the alloyed ohmic contacts and can be minimized if they are screened by the Si doping or by the two-dimensional electron gas.     

Alloyed Si/Al-based ohmic contacts to AlGaN/GaN nitride heterostructures

For the first time in Russia, the Si/Al/Ti/Au alloyed contact composition is investigated for the formation of ohmic contacts to AlGaN/GaN heterostructures using thermal annealing. The obtained results are compared with those for conventional Ti/Al/Ni/Au ohmic contacts. Use of the composition under investigation makes it possible to decrease the annealing temperature to 675–700°C, which results in improvement in the morphology of alloyed ohmic contacts in comparison with conventional contacts. The value of the contact resistance using the Si/Al-based composition to the AlGaN/GaN heterostructure is obtained in relation to the temperature and annealing duration. It is shown that no qualitative change in the resistance occurs at an annealing duration of several minutes in the temperature range of 700–750°C. In the temperature range of 675–700°C, there is an asymptotic decrease in the contact resistance with increasing annealing duration. The smallest value of the contact resistance amounts to 0.41 Ω mm.     

氧等离子体表面处理对AlGaN/GaN HEMT欧姆接触的影响

采用电感耦合等离子体(ICP)刻蚀系统,研究了氧等离子体表面处理对AlGaN/GaN HEMT欧姆接触电阻的影响。利用能量色散X射线光谱仪、光致发光谱和原子力显微镜以及电学测试设备对处理前后样品进行表征分析。结果表明,在最佳的氧等离子体处理条件(ICP功率250 W,射频功率60 W,压强0.8 Pa,氧气流量30 cm³/min,时间5 min)下,欧姆接触电阻为0.41Ω·mm,比参照样品接触电阻降低了约69%。分析认为经过氧等离子体处理后,在近表面处产生了一定数量的N空位缺陷,这些N空位表现为浅能级施主掺杂,有利于欧姆接触的形成。通过采用氧等离子体表面处理工艺制备的AlGaN/GaN HEMT,在+2 V的栅极偏压下获得了0.77 A/mm的最大漏极饱和电流。     

On a two-layer Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> dielectric mask for low-resistance ohmic contacts to AlGaN/GaN HEMTs

The fabrication of a two-layer Si₃N₄/SiO₂ dielectric mask and features of its application in the technology of non-fired epitaxially grown ohmic contacts for high-power HEMTs on AlGaN/GaN heterostructures are described. The proposed Si₃N₄/SiO₂ mask allows the selective epitaxial growth of heavily doped ohmic contacts by nitride molecular-beam epitaxy and the fabrication of non-fired ohmic contacts with a resistance of 0.15–0.2 Ω mm and a smooth surface and edge morphology.     

Nanocrack formation in AlGaN/GaN high electron mobility transistors utilizing Ti/Al/Ni/Au ohmic contacts

AlGaN/GaN HEMTs are poised to become the technology of choice in RF and power electronics applications where high operating frequencies and high breakdown voltages are required. The alloyed contacting scheme utilized in the formation of the source and drain contacts of these devices affects the conduction of electrons through the 2DEG from the moment of ohmic contact formation onward to operation in the field. Analysis of the ohmic contacts of as-fabricated and electrically stressed AlGaN/GaN HEMTs, via chemical deprocessing and Scanning Electron Microscopy, indicates the presence of cracks oriented along the [11-20] directions, which nucleate at metal inclusions present under the alloyed ohmic source/drain contact metal. Cracks which form at the edges of these contact regions can extend into the channel region. It appears that electrical biasing induces additional growth in the longest cracks present within the channel regions of these devices.     

High resolution physical analysis of ohmic contact formation at GaN-HEMT devices

A low ohmic contact resistance is a key request on AlGaN/GaN HEMT devices especially when dealing with higher frequency applications. In such a GaN material system very high annealing temperatures of up to 900 °C are frequently necessary to achieve low ohmic resistance. It is known that high annealing temperatures could reduce electrical performance and reliability of the devices. Therefore, low contact resistances at reduced annealing temperatures are beneficial. In the present work a special SiCl₄ plasma treatment was applied prior to the ohm metallisation resulting in sufficiently low ohmic resistances at a significantly reduced anneal temperature of 750 °C. High resolution physical analysis by transmission electron microscopy (TEM) and time of flight secondary mass spectroscopy (TOFSIMS) have been performed to understand the structural and chemical differences between the treated ohmic contacts compared to contacts processed under standard conditions. An advanced preparation technique was developed to get access to the reaction interfaces for TOFSIMS analysis from backside. It could be shown that the SiCl₄ treated samples reveal less impurities/contaminants on the ohmic metal–GaN/AlGaN interface and a different metal semiconductor alloying. The diffusion of metals like Al and Au into the semiconductor substrate was detected at the front of the TiN forming reaction.     

Study of the leakage current suppression for hybrid-Schottky/ohmic drain AlGaN/GaN HEMT

The leakage current suppression mechanism in AlGaN/GaN High Electron Mobility Transistors (HEMTs) is investigated. It is known that leakage current can cause severe reliability problems for HEMT devices and conventional AlGaN/GaN HEMT devices suffer from detrimental off-state drain leakage current issues, especially under high off-state drain bias. Therefore, a leakage current suppression technique featuring hybrid-Schottky/ohmic-drain contact is discussed. Through the 2-zones leakage current suppression mechanism by the hybrid-Schottky/drain metal including the shielding effect of the rough ohmic-drain metal morphology and the drain side electric field modulation, AlGaN/GaN HEMT featuring this novel technique can significantly enhance the leakage current suppression capability and improve the breakdown voltage. An analytical method using loop-voltage-scanning is proposed to illustrate the optimization procedure of the hybrid-Schottky/ohmic drain metallization on leakage current suppression. Through the comparison of the loop leakage current hysteresis of conventional ohmic drain HEMT and hybrid-Schottky/ohmic drain, the leakage current suppression mechanism is verified through the leakage current considering surface acceptor-like trap charging/discharging model. Device featuring the hybrid-Schottky/ohmic drain technique shows an improvement in breakdown voltage from 450 V (with no Schottky drain metal) to 855 V with a total drift region length of 9 μm, indicating enhanced off-state reliability characteristics for the AlGaN/GaN HEMT devices.