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The features of contact resistivity behavior at helium temperatures for InP- and GaAs-based ohmic contacts  [PDF]
A. V. Sachenko,A. E. Belyaev,N. S. Boltovets,S. A. Vitusevich,R. V. Konakova,S. V. Novitskii,V. N. Sheremet
Physics , 2014,
Abstract: Contact resistivity rc of InP and GaAs based ohmic contacts was measured in the 4.2/300 K temperature range. Nonmonotonic dependences rc(T), with a minimum at temperature 50 K (150 K) for InP (GaAs) based contacts were obtained. The results can be explained within the framework of the mechanism of current flow through metal shunts (associated with dislocations) penetrating into the semiconductor bulk, with allowance being made for electron freeze-out at helium temperatures. Contact ohmicity in the 4.2/30K temperature range is due to accumulation band bending near shunt ends at the metal/semiconductor interface.
The annealing mechanism of AuGe/Ni/Au ohmic contacts to a two-dimensional electron gas in GaAs/AlGaAs heterostructures  [PDF]
E. J. Koop,M. J. Iqbal,F. Limbach,M. Boute,B. J. van Wees,D. Reuter,A. D. Wieck,B. J. Kooi,C. H. van der Wal
Physics , 2008, DOI: 10.1088/0268-1242/28/2/025006
Abstract: Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive X-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism and proposing a model that can predict optimal annealing parameters for a certain heterostructure.
Effect of annealing temperature on the morphology of ohmic contact Ti/Al/Ni/Au to n-AlGaN/GaN heterostructures  [PDF]
Wojciech Macherzynski,Andrzej Stafiniak,Adam Szyszka,Jacek Gryglewicz
Optica Applicata , 2009,
Abstract: Ohmic contacts to AlGaN/GaN heterostructures which have low contact resistance and good surface morphology are required for the development of high temperature, high power and high frequency electronic devices. One of the keys to the advancement of such devices is the understanding of ohmic contacts formation to epitaxial aluminium gallium nitride layers. The paper presents the investigation of Ti/Al/Ni/Au based ohmic contact to n-AlGaN/GaN heterostructures grown by LP-MOVPE technique. Multilayer metallization of Ti/Al/Ni/Au with thicknesses of 10/100/40/150 nm, respectively, was evaporated by an electron gun (Ti, Ni) and resistance heater (Al, Au). The contacts were annealed at RTA (rapid thermal annealing) system in nitrogen ambient atmosphere over the temperature range from 775 °C to 850 °C. The time of annealing process was 60 seconds. The morphology of Ti/Al/Ni/Au ohmic contacts to n-AlGaN/GaN heterostructures was studied as a function of the annealing process conditions by an optical microscope and AFM (atomic force microscope). Simultaneously, the electrical parameters of Ti/Al/Ni/Au ohmic contacts were studied as a function of the annealing process conditions by the current-voltage (I-V ) method on dedicated test structures. The characteristic resistances of the Ti/Al/Ni/Au/n-AlGaN/GaN ohmic contacts were evaluated from the circular transmission line method (CTLM). The formation and deterioration mechanisms of the ohmic contacts to n-AlGaN/GaN hesterostructures were studied. One of the mechanisms of agglomerates enlargement during the thermal annealing of Ti/Al/Ni/Au metallization has been proposed.
Ti/Al Ohmic Contacts to n-Type GaN Nanowires  [PDF]
Gangfeng Ye,Kelvin Shi,Robert Burke,Joan M. Redwing,Suzanne E. Mohney
Journal of Nanomaterials , 2011, DOI: 10.1155/2011/876287
Abstract: Titanium/aluminum ohmic contacts to tapered n-type GaN nanowires with triangular cross-sections were studied. To extract the specific contact resistance, the commonly used transmission line model was adapted to the particular nanowire geometry. The most Al-rich composition of the contact provided a low specific contact resistance (mid ? ) upon annealing at 600? for 15?s, but it exhibited poor thermal stability due to oxidation of excess elemental Al remaining after annealing, as revealed by transmission electron microscopy. On the other hand, less Al-rich contacts required higher annealing temperatures (850 or 900? ) to reach a minimum specific contact resistance but exhibited better thermal stability. A spread in the specific contact resistance from contact to contact was tentatively attributed to the different facets that were contacted on the GaN nanowires with a triangular cross-section. 1. Introduction The wide bandgap semiconductor GaN has attracted great attention for its use in electronic and optoelectronic devices, and its growth and properties have recently been explored in the form of a semiconductor nanowire (NW) because of its potential for transistors [1] and light emitting devices [2–4]. Most devices fabricated from GaN NWs require low-resistivity and reliable electrical contacts. Researchers have reported the formation of both Schottky and ohmic contacts to GaN NWs. Sometimes a contact such as Ti/Al [5] has been observed to form either Schottky or ohmic contacts as a result of nonuniformities in the NW surface. The metallizations Ti/Al/Pt/Au [6], Ti/Au [7, 8], Ni/Au [9], as well as Pt deposited by a focused ion beam [10, 11], have all been successfully used to form Ohmic type contacts to n-type GaN nanowires, sometimes with the assistance of annealing or premetallization ultraviolet (UV) ozone surface treatments. On the other hand, Ti, Cr, and Au have been used to form Schottky diodes [7, 12]. These reports also demonstrate that the condition of the NW surface [5, 6] and the size of the GaN nanowire [10] can affect the characteristics of the contact. In this study, we examine Ti/Al contacts to n-type GaN nanowires by electrically probing the contacts and by examining them in cross-section using transmission electron microscopy (TEM). The contact resistance and specific contact resistance were extracted taking into account the geometry of these nanowires, which are tapered and have triangular cross-sections. The effect of changing the ratio of Ti?:?Al on the specific contact resistance was also studied as a function of annealing
Study of Ni/Pt Ohmic Contacts to P-type GaN
p-GaN与Ni/Pt欧姆接触的研究

CHENG Cai-jing,ZHANG Xiang-feng,DING Jia-xin,
成彩晶
,张向锋,丁嘉欣

红外 , 2008,
Abstract: In this paper,the formation and current transmission mechanisms of Ni/Pt Ohmic contacts to p-type GaN are investigated.The linear I-V curves indicate that the formation of the Ohmic contact between the Ni/Pt and the GaN is attributed to the decrease of the effective barrier height with the increase of the hole concentration at the interface of p-type GaN contacts with the Ni/Pt when annealing in air.In the range from 148K to 323K,the specific contact resistance R_c is decreased exponentially with the test temperature T.This indicates that the current transmission mechanism of the Ni/Pt Ohmic contacts to the p-type GaN abides by the thermionic emmission theory.
Structural characterization of V/Al/V/Au Ohmic contacts to n-type Al0.4Ga0.6N

Li Tao,Qin Zhi-Xin,Xu Zheng-Yu,Shen Bo,Zhang Guo-Yi,

中国物理 B , 2011,
Abstract: This paper investigates the temperature dependence of the specific resistance in annealed V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts on n-Al0.4Ga0.6N. Contacts annealed at 700 du and higher temperatures show Ohmic behaviour. Annealing at 800 du produces the lowest contact resistance. Samples annealed at 800 du have been analysed by using cross-sectional transmission electron microscopy and an energy dispersive x-ray spectrum. Limited reaction depths are observed between V-based contacts and n-AlGaN. The VN grains are found to form in the contact layer of the annealed samples, which can be considered as the key to the successful formation of Ohmic contact. The contact layer adjacent to AlGaN material consists of V-Al-Au-N, AlN and AlAu alloys.
Current-Voltage Characteristics of ITO/p-Si and ITO/n-Si Contact Interfaces  [PDF]
Gopal G. Pethuraja, Roger E. Welser, Ashok K. Sood, Changwoo Lee, Nicholas J. Alexander, Harry Efstathiadis, Pradeep Haldar, Jennifer L. Harvey
Advances in Materials Physics and Chemistry (AMPC) , 2012, DOI: 10.4236/ampc.2012.22010
Abstract: We investigated the electrical contact characteristics of indium tin oxide (ITO)/doped hydrogenated amorphous silicon (a-Si:H) junctions. For efficient collection of photo-generated carriers, photovoltaic and photodetector devices require good ohmic contacts with transparent electrodes. The amorphous-Si thin films were sputter deposited on ITO coated glass substrates. As-deposited p-type a-Si:H on ITO formed nearly ohmic type contacts and further annealing did not improve the contact characteristics. On the other hand, as-deposited n-type a-Si:H on ITO formed an ohmic contact, while further annealing resulted in a Schottky type contact. The ITO contact with p-type silicon semiconductor is a ro-bust ohmic contact for Si based optoelectronic devices.
Influence of Carbon Layer on the Properties of Ni-Based Ohmic Contact to n-Type 4H-SiC  [PDF]
A. Kuchuk,V. Kladko,Z. Adamus,M. Wzorek,M. Borysiewicz,P. Borowicz,A. Barcz,K. Golaszewska,A. Piotrowska
ISRN Electronics , 2013, DOI: 10.1155/2013/271658
Abstract: Nickel-based contacts with additional interfacial layer of carbon, deposited on n-type 4H-SiC, were annealed at temperatures ranging from 600 to 1000°C and the evolution of the electrical and structural properties were analyzed by I-V measurements, SIMS, TEM, and Raman spectroscopy. Ohmic contact is formed after annealing at 800°C and minimal specific contact resistance of about ???cm2 has been achieved after annealing at 1000°C. The interfacial carbon is amorphous in as-deposited state and rapidly diffuses and dissolves in nickel forming graphitized carbon. This process activates interfacial reaction between Ni and SiC at lower temperature than usual and causes the formation of ohmic contact at relatively low temperature. However, our results show that the specific contact resistance as well as interface quality of contacts was not improved, if additional layer of carbon is placed between Ni and SiC. 1. Introduction Owing to its excellent intrinsic properties such as high thermal conductivity, high electric field breakdown strength, and high saturation, electron silicon carbide (SiC) is well recognized as an attractive material for application in high-power devices operating in high-temperature environment [1]. Much effort has been undertaken to master the SiC growth, both in form of ingots and of the epitaxial thin films, and important progress has been made in these fields during last two decades. However, in order to fully exploit this potential it is still necessary to overcome several technical issues related to the semiconductor processing and fabrication of power electronic devices; development of reliable ohmic contacts is one of the key problems in this respect [2]. The fabrication of ohmic contacts to SiC may be achieved by using various metallization schemes; as for the n-type SiC, Ni, and Ni-based contacts are the most commonly used ones. They are formed by high temperature annealing at temperatures in the range 950–1050°C for time of 2–15 minutes [3–6]. Although many experimental works have been performed in order to understand the mechanism of ohmic contact formation and different models were proposed to explain Schottky to Ohmic transition, the final picture is still far from completeness. There is no doubt that Ni very easily react with SiC forming the whole spectrum of nickel silicides, depending on details of ohmic contact fabrication. On the other hand there is a strong evidence that fabrication of silicides, via contact reaction of Ni with SiC or via deposition of the specific silicide does not provide solely an ohmic contact with
Nanoscale electro-structural characterisation of ohmic contacts formed on p-type implanted 4H-SiC  [cached]
Frazzetto Alessia,Giannazzo Filippo,Lo Nigro Raffaella,Di Franco Salvatore
Nanoscale Research Letters , 2011,
Abstract: This work reports a nanoscale electro-structural characterisation of Ti/Al ohmic contacts formed on p-type Al-implanted silicon carbide (4H-SiC). The morphological and the electrical properties of the Al-implanted layer, annealed at 1700°C with or without a protective capping layer, and of the ohmic contacts were studied using atomic force microscopy [AFM], transmission line model measurements and local current measurements performed with conductive AFM. The characteristics of the contacts were significantly affected by the roughness of the underlying SiC. In particular, the surface roughness of the Al-implanted SiC regions annealed at 1700°C could be strongly reduced using a protective carbon capping layer during annealing. This latter resulted in an improved surface morphology and specific contact resistance of the Ti/Al ohmic contacts formed on these regions. The microstructure of the contacts was monitored by X-ray diffraction analysis and a cross-sectional transmission electron microscopy, and correlated with the electrical results.
Robust recipe for low-resistance ohmic contacts to a two-dimensional electron gas in a GaAs/AlGaAs heterostructure  [PDF]
M. J. Iqbal,D. Reuter,A. D. Wieck,C. H. van der Wal
Physics , 2014,
Abstract: The study of electron transport in low-dimensional systems is of importance, not only from a fundamental point of view, but also for future electronic and spintronic devices. In this context heterostructures containing a two-dimensional electron gas (2DEG) are a key technology. In particular GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the surface, are widely studied. In order to explore electron transport in such systems, low-resistance ohmic contacts are required that connect the 2DEG to macroscopic measurement leads at the surface. Here we report on designing and measuring a dedicated device for unraveling the various resistance contributions in such contacts, which include pristine 2DEG series resistance, the 2DEG resistance under a contact, the contact resistance itself, and the influence of pressing a bonding wire onto a contact. We also report here a robust recipe for contacts with very low resistance, with values that do not change significantly for annealing times between 20 and 350 sec, hence providing the flexibility to use this method for materials with different 2DEG depths. The type of heating used for annealing is found to strongly influence the annealing process and hence the quality of the resulting contacts.
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