Interfacial reaction and electrical property of Ge/Ni/ZnSe for blue laser diode

Interfacial reaction and electrical property of the Ge/Ni/ZnSe have been investigated as a fundamental study to develop ohmic contacts to p-ZnSe, using x-ray diffraction, Auger electron spectroscopy, cross-sectional transmission electron microscopy, and capacitance-voltage measurement. Ni/ZnSe, Ni/Ge systems have been also studied for comparison with the Ge/Ni/p-ZnSe system and the electrical property could be related to the interfacial reaction. After annealing at 170°C, Ni₃Se₂ and NiGe were formed at Ni/ZnSe and Ge/Ni interface, respectively. The increase of annealing temperature resulted in the decomposition of Ni₃Se₂ through the reaction with Ge. The change of the Schottky barrier height strongly depended on the annealing temperature. According to the result of Ni/ZnSe interface reaction, Ni₃Se₂ in the Ni/ZnSe interface lowered the Schottky barrier height. At higher annealing temperature, the Schottky barrier height increased and it was mainly due to the thermal decomposition of ZnSe.     

Formation mechanism of InxGa1−xAs ohmic contacts to n-type GaAs prepared by radio frequency sputtering

The formation mechanisms of InAs/Ni/W ohmic contacts to n-type GaAs prepared by radio-frequency (rf) sputtering were studied by measuring contact resistances (R_c) using a transmission line method and by analyzing the interfacial structure mainly by x-ray diffraction and transmission electron microscopy. Current-voltage characteristics of the InAs/Ni/W contacts after annealing at temperatures above 600°C showed “ohmic-like behavior.” In order to obtain the “ohmic” behavior in the contacts, pre-heating at 300°C prior to high temperature annealing was found to be essential. The contacts showed ohmic behavior after annealing at temperatures in the range of 500∼850°C and contact resistance values of as low as ∼0.3Ω-mm were obtained. By analyzing the interfacial structures of these contacts, In_xGa_1−xAs layers with low density of misfit dislocations at the In_xGa_1−xAs and GaAs interface were observed to grow epitaxially on the GaAs substrate upon heating at high temperatures. This intermediate In_xGa_1−xAs layer is believed to divide the high energy barrier at the contact metal and GaAs interface into two low barriers, resulting in reduction of the contact resistance. In addition, Ni was found to play a key role to relax a strain in the In_xGa_1−xAs layer (introduced due to lattice mismatch between the In_xGa_1−xAs and GaAs) by forming an intermediate Ni_xGaAs layer on the GaAs surface prior to formation of the In_xGa_1−xxAs layer.     

Development of InxGa1−xAs-based ohmic contacts for p-type GaAs by radio-frequency sputtering

In_xGa_1−xAs-based ohmic contacts which showed excellent contact properties for n-GaAs were demonstrated to be applicable to p-GaAs ohmic contacts. These contacts, prepared by radio-frequency sputtering, provided low contact resistance (0.2 Ω-mm), excellent thermal stability, smooth surface, and good reproducibility. The contact resistances had a weak dependence on the annealing temperatures, which was desirable in a manufacturing view point. This weak temperature dependence was explained to be due to a unique Schottky barrier height at the metal/p-In_xGa_1−xAs interface which does not depend on the In concentration in the In_xGa_1−xAs layer. The present experiment showed the possibility of simultaneous preparation of ohmic contacts for both n and p-GaAs using the same contact materials.     

Interface observation in Au/Ni/p-GaN studied by HREM and energy-filtering TEM

The contact resistance of Au/Ni/p-GaN ohmic contacts for different annealing conditions was measured. This was then correlated with microstructure, including phase distribution, observed by high-resolution electron microscopy combined with energy-filtering imaging. A contact resistance of 2.22 x 10(-4) ohms cm2 for Au/Ni contacts to p-GaN after annealing at 500 degrees C for 5 min in air ambient was obtained. NiO layers were identified at the interface and upper area of annealed Ni/Au/p-GaN for air ambient. In addition, an Au layer was found at the interface of p-GaN due to a reversal reaction during annealing. Identification of the observed phases is discussed, along with possible formation mechanisms for the ohmic contacts in the Au/Ni/p-GaN system.     

Effects of intermediate semiconductor layers on carrier transport mechanisms through p-ZnSe/metals interfaces

The effects of formation of intermediate semiconductor layers at p-ZnSe/metals interfaces on carrier transport mechanisms were studied by comparing contacts prepared by the deposition and annealing (DA) technique or the molecular beam epitaxy (MBE) technique. Current density vs voltage (J-V) curves of the MBE contact with a p-ZnSe/p-ZnTe superlattice intermediate layer showed ohmic behavior. However, J-V curves of the DA contact with a ZnTe intermediate layer showed rectifying behavior. The difference of the electrical properties between these two contacts was due to existence of a highly resistive intermediate layer with highly dense defects in the DA contact and a low resistance p-type conductive intermediate layer with relatively small densities of crystalline defects in the MBE contacts. From the present results, it was concluded that formation of the highly resistive semiconductor layer with dense crystalline defects prevented the DA contact to transit from non-ohmic J-V behavior to ohmic.     

Lateral diffusion effects in AuGe based source-drain contacts to AllnAs/InGaAs/InP doped channel MODFETs

We have investigated the formation of source-drain AuGe/Au and Ni/AuGe/Ni/Au alloyed ohmic contacts to AlInAs/InGaAs/InP doped channel MODFETs, and observed lateral diffusion of the contact system after the standard annealing procedure at the temperature range of 185 to 400°C. Auger depth profiling of contacts annealed at 250°C, revealed that Au(Ge) diffused through the top InGaAs and AlInAs layers into the active InGaAs layer, but had reduced penetration into the AlInAs buffer layer. This reduction in diffusion along the depth axis at the AlInAs buffer layer boundary is believed to result in enhanced lateral diffusion and the observed lateral encroachment of the contacts. Both Au and Ni containing contact systems showed similar behavior in terms of lateral diffusion with encroachment extending between 0.25 and 0.5 μm at the periphery of the contacts for annealing temperatures between 300 and 400°C. A controlled ramp-to-peak temperature annealing procedure is developed to suppress such lateral diffusion effects. Low temperature annealing (250°C) using this procedure resulted in equally low contact resistance values (∼0.1Θ-mm) and no lateral diffusion. It is concluded that in thin multilayered structures the modified annealing procedure presented here, is necessary for optimal ohmic contact formation.     

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.     

Ohmic contact formation in palladium-based metallizations to n-Type InP

Two Pd-based metallizations have been systematically studied, i.e., Au/Ge/Pd and Pd/Ge contacts to n-type InP, in an attempt to better understand the role of the metallization constituents in forming ohmic contacts. Ohmic contacts were obtained with minimum specific resistances of 2.5 × 10⁻⁶ Ω-cm² and 4.2 × 10⁻⁶ Ω-cm² for the Au/Ge/Pd and the Pd/Ge contacts, respectively. The annealing regime for ohmic contact formation is 300-375°C for the Au/Ge/Pd/InP system and 350-450°C for the Pd/GelnP system. Palladium, in both cases, reacts with InP to form an amorphous layer and then an epitaxial layer at low temperatures, providing good metallization adhesion to InP substrates and improved contact morphology. Ohmic contact formation in both contacts is attributed to Ge doping, based on the solid state reaction-driven decomposition of an epitaxial layer at the metallization/InP interface, producing a very thin, heavily doped InP layer. Gold appears to be responsible for the difference in contact resistance in the two systems. It is postulated that Au reacts strongly with In to form Au-In compounds, creating additional In site vacancies in the InP surface region (relative to the Au-free metallization), thereby enhancing Ge doping of the InP surface and lowering the contact resistance. Both contacts degrade and ultimately become Schottky barriers again if over annealed, due to consumption of additional InP, which destroys the heavily doped InP layer.     

Ni基n型SiC材料的欧姆接触机理及模型研究

研究了Ni基n型SiC材料的欧姆接触的形成机理,认为合金化退火过程中形成的C空位(Vc)而导致的高载流子浓度层对欧姆接触的形成起了关键作用。给出了欧姆接触的能带结构图,提出比接触电阻ρC由ρC1和ρC2两部分构成。ρC1是Ni硅化物与其下在合金化退火过程中形成的高载流子浓度层间的比接触电阻,ρC2则由高载流子浓度层与原来SiC有源层之间载流子浓度差形成的势垒引入。该模型较好地解释了n型SiC欧姆接触的实验结果,并从衬底的掺杂水平、接触金属的选择、合金化退火的温度、时间、氛围等方面给出了工艺条件的改进建议。     

Mechanisms of current flow in metal-semiconductor ohmic contacts

Published data on the properties of metal-semiconductor ohmic contacts and mechanisms of current flow in these contacts (thermionic emission, field emission, thermal-field emission, and also current flow through metal shunts) are reviewed. Theoretical dependences of the resistance of an ohmic contact on temperature and the charge-carrier concentration in a semiconductor were compared with experimental data on ohmic contacts to II–VI semiconductors (ZnSe, ZnO), III–V semiconductors (GaN, AlN, InN, GaAs, GaP, InP), Group IV semiconductors (SiC, diamond), and alloys of these semiconductors. In ohmic contacts based on lightly doped semiconductors, the main mechanism of current flow is thermionic emission with the metal-semiconductor potential barrier height equal to 0.1–0.2 eV. In ohmic contacts based on heavily doped semiconductors, the current flow is effected owing to the field emission, while the metal-semiconductor potential barrier height is equal to 0.3–0.5 eV. In alloyed In contacts to GaP and GaN, a mechanism of current flow that is not characteristic of Schottky diodes (current flow through metal shunts formed by deposition of metal atoms onto dislocations or other imperfections in semiconductors) is observed.     

多层金属-n型4H-SiC的欧姆接触

研究了热退火条件下Au/Ti/Ni-4H-SiC欧姆接触形成机制.通过950 ℃下的快速热退火形成的最低欧姆接触电阻为2.765×10-6 Ω·cm2.SIMS分析表明退火过程中NiSi化合物的形成会带来SiC内部多余C原子的溢出,并在接触面上与Ti形成间隙化合物TiC.这一过程造成接触表面存在由大量C空位形成的缺陷层从而增强了表面间接隧穿.通过界面能带结构图直观地解释了欧姆接触在热退火条件下的形成机制.     

Microstructural analysis of NiInGe ohmic contacts for n-type GaAs

NiInGe ohmic contact materials, which are attractive to use in future GaAs devices, were previously developed in our laboratories. Although the NiInGe contacts provided low contact resistances of about 0.3 Ω-mm and excellent thermal stability, further reduction of the contact resistance (R_C) of the NiInGe contacts was mandatory to use these contacts in submicron devices. In this paper, the microstructural parameters, which influence the R_C values, were investigated by correlating the R_C values with the microstructure at the interface between the contact materials and the GaAs substrate. The R_C values of the NiInGe contacts were found to depend strongly on the volume fraction and the In concentration (x) of the In_xGa_1−xAs compound semiconductor layers, which were formed at the metal/GaAs interface. Both the volume fraction and the In concentration of the In_xGa_1−xAs layers were found to depend on the thickness of the In layer used in the NiInGe contact and the annealing temperature to form the ohmic contact. A R_C value of 0.18 Ω-mm was obtained for the Ni (18 nm)/In (13 nm)/Ge (30 nm) contact (where a slash “/” indicates the deposition sequence) after annealing at temperature of 650°C for 5 sec.     

Au/Zn/Au/p-In P欧姆接触的界面研究

研究了离子束溅射制备的Au/Zn/Au/p-InP欧姆接触的界面特性.在480℃退火15s比接触电阻达到最小,为1.4×1 0-5 Ω·cm 2.利用俄歇电子能谱(AES)和X射线光电子能谱(XPS)研究了接触界面的冶金性质.实验结果表明,在室温下InP中的In就可以扩散到接触的表面,退火后可与Au形成合金.退火后,Zn的扩散可以在p-InP表面形成重掺杂层,从而降低接触势垒高度,减小势垒宽度,有助于欧姆接触的形成;在接触与p-InP的界面产生一个P聚集区,同时Au与InP反应生成Au2P3,其P的2p3/2电子的结合能约为129.2 eV.     

Features of Creating Ohmic Contacts for GaAs/AlGaAs Heterostructures with a Two-Dimensional Electron Gas

The literature concerning the features of creating ohmic contacts for GaAs/AlGaAs heterostructures with a two-dimensional (2D) electron gas with a high level of electron mobility is analyzed. The process of annealing the contacts based on the Ni/Au/Ge system is considered. The recommended published parameters of the layers to be sprayed and the regimes of their annealing are presented, which make it possible to obtain ohmic contacts with low resistance up to temperatures lower than 4 K. Several mechanisms are considered, which can lead to the experimentally observed dependence of the characteristics of the contact on its crystallographic orientation. A method for creating contacts using Au/Ge/Pd metallization, in which the contact is formed due to the mutual diffusion and interaction of metals and a semiconductor in the solid phase at temperatures lower than 200°C, is described. This ensures a higher degree of homogeneity of the contact in the composition and a smooth metal–semiconductor interface, and it can lead to decrease of the influence of orientation effects on the electric characteristics of the contact.     

Cd and Te-based ohmic contact materials to p-Type ZnSe

In order to explore a possibility of forming an intermediate semiconductor layer with low Schottky barrier by the conventional deposition and annealing technique, the electrical properties of Cd and Te-based contacts on the nitrogendoped ZnSe substrates have been investigated. Cd in the Cd/W contact reacted with the ZnSe substrate after annealing at temperatures above 250°C and formed epitaxial Cc_x}Zn_1−xSe layers, leading to reduction of the “turn-on” voltage (V_T) from about 11 to 6 V (here, a slash “/” between Cd and W means the deposition sequence). The reduction of the V_n} value by annealing at elevated temperatures was also observed for the Bi-Cd/W and In-Cd/W contacts. The average Cd composition (x) in the Cd_n}Zn_1−xSe layers was measured to be larger than 0.9, which agreed with the values estimated from the calculated Cd-Zn-Se phase diagrams. The ohmic behavior was strongly influenced by the thickness of the Cd_xZn_1−xSe layer, the density of misfit dislocations formed at the interface between the Cd_x Zn_1−x Se and the ZnSe, and/or the total area of the Cd Zn. Se layers covering the ZnSe surface. The present result suggests that formation of the large-areal Cc_xZn_1−xSe layers with thin thickness is crucial to achieve further reduction of the V_T value by the conventional deposition and annealing technique. Also, the V_T reduction was not obtained for the Te/W contact even after annealing at temperatures close to 300°C, which was explained to be due to absence of ternary ZnSe_1−xTe_n intermediate layers.     

Alloying behaviour of Au-Ge/Pt ohmic contacts to GaAs by pulsed electron beam and furnace heating

Au-Ge/Pt contacts on n-type GaAs have been irradiated by a pulsed electron beam to yield excellent ohmic behaviour. Auger electron spectroscopy (a.e.s.) measurements of the depth profiles of the constituent elements in these irradiated contacts indicate that considerable intermixing of the metals with Ga and/or As is not necessary for good ohmic behaviour. Subsequent isochronal (2 min) furnace heating in H2 of these contacts at different temperatures shows that the Au-Ge/Pt metallisation system is highly reactive, with the inter mixing and the ohmic behaviour being a strong function of the furnace temperature. From a.e.s. measurements the important role of Ge as a dopant in generating a heavily doped GaAs layer at the metal-GaAs interface responsible for the ohmic character of the contacts is confirmed. Low specific contact resistance is closely related to the ability in incorporating Ge in a shallow GaAs layer during the alloy cycle.     

The thermal stability of ohmic contact to n-type InGaAs layer

The thermal stability of ohmic contact to n-type InGaAs layer is investigated. When Ni/Ge/Au is used as the contact metal, the characteristics of the ohmic contact are degraded after thermal treatment. The specific contact resistance of (Ni/Ge/Au)-InGaAs ohmic contact after annealing at 450°C is about 15 times larger than that of as-deposited sample. This is due to the decomposition of InGaAs and the interdiffusion of Ga and Au. A new phase of Au₄ln appears after annealing at 300°C. While in the case of Ti/Pt/Au, Au does not penetrate into the InGaAs layer as revealed by secondary ion mass spectroscopy. The specific contact resistance of (Ti/Pt/Au)-InGaAs ohmic contact after annealing at 450°C is eight times larger than that of as-deposited sample. Therefore, the thermal stability of (Ti/Pt/Au)-InGaAs ohmic contact is better than that of (Ni/Ge/Au)InGaAs ohmic contact.     

非故意掺杂GaN上Ti/Al/Ni/Au欧姆接触研究

基于圆形传输线模型,研究了背景载流子浓度为71016cm3的非故意掺杂GaN与Ti/Al/Ni/Au多层金属之间欧姆接触的形成。样品在N2气氛中,分别经过温度450,550,700,800,900℃的1 min快速热退火处理后发现,当退火温度高于700℃欧姆接触开始形成,随着温度升高欧姆接触电阻持续下降,在900℃时获得了最低比接触电阻6.6106O·cm2。研究表明,要获得低的欧姆接触电阻,需要Al与Ti发生充分固相反应,并穿透Ti层到达GaN表面;同时,GaN中N外扩散到金属中,在GaN表面产生N空位起施主作用,可提高界面掺杂浓度,从而有助于电子隧穿界面而形成良好欧姆接触。     

Laser annealed Ta/Ge and Ni/Ge ohmic contacts to GaAs

Refractory metal ohmic contacts to n-type GaAs have been developed using epitaxial Ge films and pulsed laser annealing. Laser annealing was carried out with a 22 ns pulse from a Q-switched ruby laser operating in the TEM00mode. The specific contact resistivity of the contacts Ta/Ge and Ni/Ge on 2 × 10¹⁷cm^-3dopes GaAs exhibited sharp minima as a function of laser energy density at 1 × 10^-6Ω-cm²and 2 × 10^-6Ω-cm², respectively, which occurred near the melting point of the layered contacts. Auger electron sputter profiles revealed Ge migration into the GaAs surface after laser annealing at sufficient energy density to form ohmic contact. The contacts have applications to high temperature devices and to devices which experience high channel or contact temperatures, such as power FETs and TEDs.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.