Structural and electrical properties of Au and Ti/Au contacts to n-type GaN

Au and Ti/Au layers were deposited on n-GaN. The samples were annealed at 400, 700 and 900 °C for 10 min in vacuum. The contacts were rectifying up to 700 °C and the highest Schottky barrier height of 1.07 eV was obtained for an Au single layer by current-voltage measurements. A binary phase of Au₂Ga was identified at the interface of the n-GaN/Ti/Au contact after annealing at 900 °C. The formation of Ti₂N and TiN (twin) phases epitaxially grown on GaN was also observed in the same contact as well as some gold diffusion into the topmost region of the GaN epilayer.     

Al and Ti/Al contacts on n-GaN

Al(60 nm) and Ti(40 nm)/Al(160 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) on a c-plane sapphire substrate. The samples have been annealed at 300, 400, 700 or 900 °C for 10 min in vacuum. The microstructural and electrical properties of the contacts have been investigated by electron microscopy, X-ray diffraction and by current-voltage measurements. As-deposited Al and Ti/Al contacts were rectifying with Schottky barrier heights below 0.35 eV and 0.38 eV, respectively. After heat treatment at 300 °C and 400 °C both contacts exhibited linear current-voltage characteristics. After annealing at 700 °C Al contacts became rectifying with a barrier height of 0.42 eV, while Ti/Al contacts remained nearly linear at the same temperature. The electrical characteristics and XRD analysis indicated that the upper metal in Ti/Al contact diffused in the Ti layer already during deposition. Cross-sectional transmission electron microscopy revealed that in the case of Ti/Al contacts, the continuity of the Ti layers ceased when annealing above 700 °C. X-ray diffractions showed, that a Ti₂N interface phase formed in Ti/Al contacts at 700 and 900 °C, and an AlN interface phase developed in the same contact at 900 °C.     

The effect of fast annealing treatment on the interface structure and electrical properties of Au/Hg3In2Te6 contact

The effect of fast annealing treatment on the electrical properties and interface structures of Au/Hg₃In₂Te₆ contact has been studied by means of current–voltage test and high-resolution transmission electron microscopy. The current–voltage characteristics of Au/Hg₃In₂Te₆ indicate an improvement of the Schottky barrier height (SBH) from 0.557 to 0.601 eV after proper annealing treatment. Meanwhile, the orthorhombic AuTe₂ particles with irregular morphology formed near the interface region after 200 °C annealing treatment. These particles were generated by chemical reaction between Au and Te atoms rather than the phase transformation process. Two types of crystallographic orientation relationship were confirmed between AuTe₂ particles and the Hg₃In₂Te₆ matrix owing to the orientation attachment mechanism. Based on the results, it is believed that the formation of the AuTe₂ phase is likely to introduce additional energy level in Hg₃In₂Te₆, leading to the upward of band bending and increment of SBH.     

Metallic contacts to nitrogen and boron doped diamond-like carbon films

Hydrogenated diamond-like carbon (DLC) was deposited using a radio-frequency plasma-enhanced chemical vapor deposition method. Electrical properties of Al, Au, Ti, and Zr contacts to nitrogen and boron doped DLC films have been studied, and mechanisms of the observed current-voltage (I-V) characteristics are investigated. Linear I-V characteristics were observed for Au, Ti, and Zr contacts to both nitrogen and boron doped DLC films. A band structure model for metal-DLC contact is proposed to explain the observed ohmic contacts. Fermi level shifting at the surface of DLC films produces an ohmic resistive layer instead of a Schottky barrier for metal-DLC contacts. Al contacts to both nitrogen and boron doped DLC films show nonlinear I-V characteristics, which are attributed to a dielectric layer of carbide (Al₄C₃) instead of a Schottky barrier suggested by other groups. Inert elements such as Au and Pt, and transition metals such as Ti, Zr and W, which form conductive carbides, are considered good contacting metals for electrical studies of DLC films.     

Junction like behavior in polycrystalline diamond films

We have successfully fabricated polycrystalline diamond rectifying junction devices on n-type (1 0 0) silicon substrates by Hot Filament Chemical Vapor Deposition (HFCVD) using methane/hydrogen process gas and trimethyl borate and trimethyl phosphite dissolved in acetone as p- and n-type dopants, respectively. Impedance spectroscopy and current-voltage analysis indicates that the conduction is vertical down the grains and facets and not due to surface effects. Electrical characteristics were analyzed with In and Ti/Au top metal contacts with Al as the substrate contact. Current-voltage characteristics as a function of temperature showed barrier potentials of 1.1 eV and 0.77 eV for the In and Ti/Au contacts, respectively. Barrier heights of 4.8 eV (In) and 4.4 eV (Ti/Au) were obtained from capacitance-voltage measurements.     

Thermal annealing behaviour on Schottky barrier parameters and structural properties of Au contacts to n-type GaN

We have investigated the thermal annealing effects on electrical and structural properties of Au Schottky contacts on n-type GaN using current–voltage (I–V), capacitance–voltage (C–V), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements. The calculated Schottky barrier height of the as-deposited Au/n-GaN diode was 0.85 eV (I–V) and 1.4 eV (C–V), respectively. However, after annealing at 300 °C it was found that the Schottky barrier height (SBH) slightly decreased to 0.77 eV (I–V) and 1.24 eV (C–V), and then slightly increased to 0.83 eV (I–V) and 1.30 eV (C–V) when the contact was annealed at 400 °C. With further increase in annealing temperature to 500 °C the barrier height was decreased and the respective values are 0.73 eV (I–V) and 1.02 eV (C–V). Based on the X-ray diffraction and RBS results, the formation of gallide phases at the Au/n-GaN interface could be the reason for variation in the Schottky barrier heights upon annealing temperatures.     

ZnS stacking order influence on the formation of Zn-poor and Zn-rich Cu2ZnSnS4 phase

This paper reports the synthesis and characterization of Cu₂ZnSnS₄ (CZTS) absorber films, prepared by a two-step electrodeposition of a ZnS (zinc sulfide) binary and a CZT (copper, zinc and tin) ternary precursors on Mo/Ti/Si substrates. The as-electrodeposited ZnS/CZT and CZT/ZnS stacks were thermally treated in a tubular furnace in sulfur environment at 550 °C. The role of the ZnS buffer layer is to provide a zinc and sulfur reservoir, needed to complete the formation of kesterite phase. X-ray diffraction and Raman analyses revealed the formation of the CZTS phase. The surface morphology and chemical composition of the films were studied using a scanning electron microscope. The bandgap values inferred from diffuse reflectance data, are discussed with respect to the stoichiometry which is considerably affected by the order of the stacks. Room-temperature photoluminescence of the CZT/ZnS sample showed a board PL band of 1.51 eV. It was found that the film with a ZnS layer on top is preferred for the formation of a Zn-rich single CZTS phase.

     

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO(2) and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO(2) as well as observe and analyze the morphology of the film's underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I-V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.     

The influence of rapid thermal annealing on electrical and structural properties of Pt/Au Schottky contacts to n-type InP

The electrical and structural properties of Pt/Au Schottky contacts to n-InP have been investigated in the annealing temperature range of 200–500 °C by current–voltage (I–V), capacitance–voltage (C–V), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) measurements. The barrier height of as-deposited Pt/Au Schottky contact is found to be 0.46 eV (I–V) and 0.68 eV (C–V). For the contacts annealed at 300 °C, the barrier height is increased to 0.51 eV (I–V) 0.89 eV (C–V). Further increase in annealing temperature up to 500 °C, the barrier height has been found to decrease to 0.49 eV (I–V) 0.82 eV (C–V) from those values obtained at 300 °C. It has been found that the electrical characteristics are significantly improved for Pt/Au Schottky contacts upon annealing at 300 °C. Based on the Auger electron spectroscopy and X-ray diffraction results, the formation of Pt–In and Au–In intermetallic compounds at the interface may be the reason for the increase of barrier height after annealing at 300 °C for Pt/Au Schottky contacts. From the atomic force microscopy (AFM) results, it is evident that the surface becomes smooth with RMS roughness of 16.91 nm for the Pt/Au Schottky contacts after annealing at 500 °C compared to the 300 °C annealed sample (RMS roughness of 17.33 nm).     

Au与p-CZT晶体的接触特性及其CZT表面处理状态的影响

对不同腐蚀、钝化表面处理的CZT晶片与Au接触的I-V特性进行了研究.用XPS分析了钝化前后CZT晶体表面成分,发现钝化后CZT晶片表面形成厚度为3.1nm的TeO2氧化层.用Agilent 4339B高阻仪进行未腐蚀、腐蚀与腐蚀钝化的CZT晶片I-V特性测试,结果表明腐蚀和腐蚀钝化均能不同程度提高Au/p-CZT接触的势垒高度,相应地减小了漏电流.     

Metal contacts to n-type AlGaAs grown by molecular beam epitaxy

We report for the first time the barrier heights of Cu, Ni, Ag, Ti on etched n-type Al_0.33Ga_0.67As and their dependence on annealing temperature with I–V and C–V techniques. The barrier heights of Al and Au, measured for comparison, are 0.96 and 1.06 eV, respectively, in excellent agreement with the results reported previously. The barrier heights of the Cu, Ni, Ag and Ti/n---Al_0.33Ga_0.67As diodes are found to be 1.08, 0.90, 0.87 and 0.87 eV, respectively. It is observed that the barrier heights for Al, Au, Cu and Ti contacts monotonically decrease with annealing temperature. For the Ag and Ni contacts, however, they become higher after being annealed at 473 K for 10 min and become lower thereafter, accompanied by a change of their ideality factors in opposite direction. The barrier heights extrapolated from C–V measurements for all metals studied are higher than that deduced from I–V data, and become higher after annealing at high temperatures, indicating the existence of a thin oxide layer at interface and broadening of the oxide after annealing. Our results can be qualitatively explained by the quality of contact and defects created at the semiconductor surface due to interdiffusion.     

The interaction of Al,Ag, Au,and Ti to Pr2O3 thin film dielectrics

We studied the thermal stability of various electrode materials on 2 nm Pr₂O₃ dielectric films. Thin Al, Au, Ag, and Ti layers are deposited, by thermal evaporation or by sublimation (case of Ti). The Pr₂O₃ layers were deposited on Si substrates by electron beam evaporation from a Pr₆O₁₁ powder and by wet chemical deposition. XPS and SR-PES were used to study the effect of annealing on the metal to Pr₂O₃ interaction. XPS results show a strong diffusivity of Al, Ag, and Au in Pr₂O₃ upon annealing to 300 °C with formation of alloying with Si substrate. In contrast, Ti remained stable even upon annealing to 900 °C. SR-PES results show a stable Titanium oxides formation at room temperature. All results demonstrate that Ti is a good diffusion barrier between metal contacts and Pr₂O₃ or can be used as a metal contact.     

Electrical contacts on photoconductive Sb2S3 films

Evaporated metal contacts to layers of deposited Sb₂S₃ 1 μm thick have been investigated. It has been found that Ni, Au and Pt make blocking contacts while contacts of In, Ga, Al and Bi are ohmic. Cu, Ag and Pd contacts show poor stability and have a very low breakdown voltage; they are not considered to be suitable contact materials. If Al is used care must be taken to avoid the formation of a layer of Al₂O₃ between the Al and the Sb₂S₃ which will result in a blocking contact. Au makes a very reproducible and reliable contact capable of withstanding fields up to about 6 x 10⁵ V/ cm.The type of contact made by any of these metals depends on the work function of the metal. The low work function metals In and Ga make the best ohmic contacts. The high work function metals Pt and Au make good blocking contacts. The results can be explained in terms of the conventional theory of contacts if the Sb₂S₃ layer is assumed to be n-type with a low density of surface states. On this basis it is expected that the work function of Sb₂S₃ lies in the range 4.7–4.9 eV.     

Aluminum/nickel silicide contacts on silicon

The results of a study of the interaction occurring at elevated temperatures between nickel silicide contacts on n-type 〈111〉 silicon and a thin aluminum overlayer are presented. The electrical and structural characteristics of the Al-nickel silicide interaction were investigated using Schottky barrier diodes, Auger electron spectroscopy, X-ray diffraction and scanning electron microscopy. As-grown diodes were found to consist mainly of NiSi and the NiSi-Si interface exhibited a Schottky barrier energy φ_Bn of 0.62 eV. Upon heat treatment the NiSi layer was transformed to the intermetallic NiAl₃, and the barrier energy for the resulting NiAl₃-Si interface was found to be 0.76 eV. The electrical characteristics of the NiAl₃ layer were stable up to 500°C and no evidence of aluminum penetration into the silicon substrate was found.     

Au-CZT与Au/Cr-CZT接触性能比较

本文通过附着力测试,应力模拟和IV特性及多道能谱分析对Au-CZT与Au/Cr-CZT接触进行研究。结果表明采用Au/Cr复合电极可提高电极附着强度和热稳定性。在老化实验中,Au-CZT界面附着力下降了61.98%,而Au/Cr-CZT仅降低28%。Au/Cr电极器件的漏电流较低,241Am射线下能谱响应更佳。分析其原因可能是Au与CZT间的Cr层降低了接触层内的热应力,合金化过程促进了金-半界面的互扩散,使Au和CZT更易形成欧姆接触,综合考虑Au/Cr复合电极能获得比Au电极更理想的接触性能。     

Interfacial electronic structures between fullerene and calcium for high performance n-type organic semiconducting devices

The electronic structure of fullerene (C₆₀) deposited on calcium (Ca) was investigated using in-situ ultraviolet photoelectron spectroscopy. The energy level alignment at the C₆₀/Ca interface was estimated by combining both shifts of the highest occupied molecular orbital (HOMO) level and of the vacuum level during the step-by-step deposition of C₆₀ on Ca. The HOMO level of C₆₀ shows shifts relating to band-bending, resulting in an electron injection barrier of 0.2 eV with accumulation contact with the Ca substrate. The vacuum level reveals an interface dipole of 1.11 eV with negative poles on the C₆₀ side. Noticeably, gap states are formed at the interface region, which might pin the Fermi level and be responsible for the formation of the interface dipole. The complete interfacial energy level diagram of C₆₀/Ca is presented.     

Effects of temperature,thickness and atmosphere on mixing in Au-Ti bilayer thin films

The interdiffusion and intermetallic compound formation of Au-Ti bilayer thin films annealed at 125 to 350 °C have been investigated. The bilayer thin films were prepared through electron beam deposition at comparatively low temperature. The interdiffusion of annealed specimens was examined by measuring electrical resistance and the depth-composition profile, and by observation using a transmission electron microscope. Interdiffusion between the thin films was detected at temperature above 175°C in a vacuum of 10^–4 Pa. The starting temperature at which interdiffusion occurred decreases with lowering annealing vacuum. The intermetallic compounds AuTi, Au₄Ti, Au₂Ti and Ti₃Au form during annealing at over 250 °C. The activation energies of Au in Ti and Ti in Au obtained by the penetration depth are approximately 0.45 and 0.41 eV, respectively. These measurements indicate that the diffusion is controlled by a short-circuit mechanism. The diffusion of Ti species in Au depends on the annealing vacuum and Au thickness.     

The effects of Ge content on the microstructure and specific contact resistance of solid-state NiAuGe/ZrB2/Au ohmic contacts to n-InGaAs

The Ge thickness, x, of NiAuGe(5 nm/45 nm/xnm)/ZrB₂(50 nm)/Au(20 nm) ohmic contacts to n-lnGaAs was varied between 0 and 20 nm. The microstructures of these contacts, after annealing at 270°C, were investigated using transmission electron microscopy (TEM) and correlated with the respective specific contact resistances. In the absence of Ge, a Ni-Ga-As phase was formed at the metal-semiconductor interface and the specific contact resistance was high (0.63 mm). When thicknesses of x = 10 nm or x = 15 nm of Ge were added, Ni-Ge-As phases were observed, but they were replaced by AuGeAs and NiGe when x = 20 nm. The specific contact resistance was a minimum (0.11 mm) for this composition. Ge was clearly beneficial for ohmic-contact formation. The low-temperature I–V characteristics of the contact containing the largest amount of Ge (that is, x = 20 nm) indicated that electron tunnelling through degenerately-Ge-doped regions was not the dominant ohmic mechanism in these contacts.     

Microstructural analysis of Au/Pt/Ti contacts to p-type InGaAs

A detailed study on the microstructural changes that occur on annealing of Au/Pt/Ti ohmic contacts to n-type InGaAs has been carried out. The metal layers were deposited sequentially by electron beam evaporation onto InGaAs, doped with Zn to a level of 7 × 10¹⁸ cm^–3, that was epitaxially grown on < 100 > InP substrates. The deposition sequence and metal layer thicknesses were: Ti (25 or 30 nm), Pt (25 or 30 nm) and Au (250 or 300 nm). Samples were annealed at temperatures ranging from 250–425 C in a nitrogen atmosphere. As-deposited contacts were Schottky barriers, while a minimum contact resistance of 2 × 10^–5 cm² was obtained by annealing in the 375–425 C range. Annealing resulted in the inward diffusion of Ti and outward diffusion of In and As, leading to the formation of TiAs, metallic In and Ga-rich InGaAs at the Ti/InGaAs interface. The Pt diffusion barrier was effective in preventing In diffusion into the outer Au layer and minimizing Au diffusion to the semiconductor.     

The effect of thermodynamic properties of Me–Ti (Me = In,Sn, Ga,Au, and Ge) melts on the wetting of the CaF<Subscript>2</Subscript> substrate

The effect of Ti additions on the wetting behavior of CaF₂ by non-reactive liquid metals (In, Sn, Ga, Au, Ge) was investigated. Pure metals do not wet CaF₂ while minor additions of Ti improve wetting. Small changes of the contact angle were observed in the CaF₂/Au–Ti and CaF₂/Ge–Ti systems, which are characterized by strong Me–Ti interaction in the melt, while considerable decrease of contact angle was obtained in the CaF₂/In–Ti, CaF₂/Sn–Ti and CaF₂/Ga–Ti systems, which display a relatively weak Me–Ti interaction. According to a thermodynamic analysis and experiential observations, Ti does not react with the substrate to form condensed phases at the metal/CaF₂ interface. Therefore, it was assumed that the mechanism of the wetting improvement is attributed to the Ti segregation at the interface. The results of the XPS analysis confirm a Ti enrichment of the region close to the interface, moreover, according to the high resolution XPS spectrum, obtained from this region, the position of the In4d peak has a chemical shift, which is typical for In–Ti intermetallic compounds. The XPS analysis does not provide sufficient evidence for the formation of the intermetallic interfacial layer at elevated temperature. Thus, further investigations have to be designed and conducted in order to clarify this issue.