Temperature behavior and modeling of ohmic contacts to Si implanted n-type GaN

The behavior of an ohmic contact to an implanted Si GaN n-well in the temperature range of 25-300 °C has been investigated. This is the sort of contact one would expect in many GaN based devices such as (source/drain) in a metal-oxide-semiconductor transistor. A low resistivity ohmic contact was achieved using the metal combination of Ti (350 Å)/Al (1150 Å) on a protected (SiO₂ cap) and unprotected samples during the post implantation annealing. Sheet resistance of the implanted layer and metal-semiconductor contact resistance to N⁺ GaN have been extracted at different temperatures. Both, the experimental sheet resistance and the contact resistance decrease with the temperature and their characteristics are fitted by means of physical based models.     

Analytical modeling and parameter extraction of top and bottom contact structures of organic thin film transistors

This paper proposes a structure based model of an organic thin film transistor (OTFT) and analyzes its device physics. The analytical model is developed for the top contact structure by mapping the overlap region to the resistance (in the vertical direction) that includes the contact and the bulk sheet resistances. Total device resistance includes the vertical resistance per unit area of the contact region and the sheet resistance of the channel. In addition, the drain and the gate voltages take into account the potential drop across the respective contacts. The gate bias dependent mobility is considered in place of constant mobility, since; it is more realistic and relevant to the organic TFTs. The proposed analytical model is also applied to the bottom contact structure and the current–voltage (I–V) characteristics are obtained. Furthermore, a differential method is employed to extract the parameters, such as, mobility enhancement factor γ, threshold voltage V_T, mobility µ_B, characteristic length L_C, vertical resistance R_V and contact resistance R_C. Finally, the model is validated in terms of electrical characteristics and performance parameters for both top and bottom contact structures. The analytical model results are in close agreement with the experimental results.     

An exact derivation of contact resistance to planar devices

An imperfect, rectangular contact to a semiconducting sheet is formulated as a mixed boundary-value problem. This problem is solved by conformal mapping to yield the complex potential function as an eigenfunction expansion. The result can be used to calculate exact contact resistance if specific contact resistivity is known. With voltage probing data, it can also be used to confirm estimates of specific contact resistivity. The expression for contact resistance resembles that for a perfect (lossless) contact^1–2 but includes an extra term involving the lowest-order coefficient a₀ from the expansion. Contact resistances are calculated and compared with the values obtained from three approximate models; the lossless contact, transmission line,^3–4 and extended transmission line models.^5–6 Up to a common normalization constant, the contact resistances predicted by all models are completely determined by two dimensionless parameters. These define the validity ranges for the approximate models. Compared to the present model, the extended transmission line model appears to be a very satisfactory approximation if the ratio of electrode length to sheet thickness is not less than 0.5.     

Finite metal-sheet-resistance in contact resistivity measurements: Application to Si/TiN contacts

The standard transmission line model cannot be applied to evaluate the contact resistivity of thin TiN layers on highly doped p⁺ and n⁺ substrates because the finite sheet resistance of the TiN must be accounted for. We present two ways to include this effect using existing analytical models. The results are shown to agree with measurements where the effect of the finite sheet resistance of TiN is eliminated with a metallic overlayer. With the help of these evaluation techniques, it is shown that the contact resistivity of TiN changes in opposite ways for p⁺ and n⁺Si after vacuum annealing at 600°C for 15 min. This result is consistent with an increase of the barrier height φ_Bn of the contact by ?0.1 V to near midgap value.     

Highly conductive silver nanowire networks by organic matrix assisted low-temperature fusing

In silver nanowire (AgNW) percolation network electrodes, the junction resistance at the wire-to-wire contact is crucial for the electrode performance. We introduce an organic sublayer between the substrate and the AgNW electrode to improve the sheet resistance and avoid the frequently applied high-temperature post-annealing process. Upon introduction of such a sublayer, a strong reduction of the sheet resistance (R_S) in the same order as it would be expected from a post-annealing process (e.g. 90 min@210 °C) is observed. This effect is investigated in-depth by using different sublayer material classes such as polyelectrolytes, tensides or differently charged polymers. Independent of charge and material class, especially those materials with hydrophobic and hydrophilic parts, dramatically reduce the sheet resistance. Consequently, hydrophobic interaction between substrate and nanowires and capillary forces during drying can be attributed as driving force. Using polyvinylpyrrolidone (PVP), also used as stabilizer for the AgNWs, leads to the largest reduction in sheet resistance of the investigated materials resulting in AgNW electrodes exhibiting R_S = 10.8 Ω/□ at 80.4% transparency (including substrate) without additional post-annealing at high temperatures.     

Electrical properties of fine line printed and light‐induced plated contacts on silicon solar cells

The properties of fine‐line printed contacts on silicon solar cells, in combination with light‐induced plating (LIP), are presented. The seed layers are printed using an aerosol system and a new metallization ink called SISC developed at Fraunhofer ISE. The influence of multiple layer printing on the contact geometry is studied as well as the influence of the contact height on the line resistivity and on the contact resistance. The dependence between contact resistance and contact height is measured using the transfer length model (TLM). Further on, it is explained by taking SEM images of the metal–semiconductor interface, that a contact height of less than 1 µm or a minimum ink amount of only 4–6 mg is sufficient to contact a large area (15·6 cm × 15·6 cm) silicon solar cell on the front side and results in a contact resistance R_c × W < 0·5 Ω cm. As the line resistivity of fine‐line printed fingers needs to be reduced by LIP, three different plating solutions are tested on solar cells. The observed differences in line resistivity between ρ_f = 5 × 10⁻⁸ and 2 × 10⁻⁸ Ω m are explained by taking SEM pictures of the grown LIP‐silver. Finally, the optimum LIP height for different line resistivities is calculated and experimentally confirmed by processing solar cells with an increasing amount of LIP silver. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrical and thermoelectric properties of <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te in the β phase

The conductivity σ, Hall coefficient R, and thermoelectric power α₀ of p-Ag₂Te were studied in the temperature range of 300–550 K. Inconsistency between the signs of R and α₀ was observed at 420–550 K. These results are interpreted within the two-phase model with spherical constant energy surfaces. It is established that the inconsistency between the R and α₀ signs is due to the emergence of the scattering mechanisms with the parameters r_0ac, r₀₀, and r_0d an increase of about 50% in the ratio of the effective electron and hole masses as a result of the transition α → β.     

n-Doped-layer free amorphous silicon thin film solar cells fabricated with the CuMg alloy as back contact metal

The feasibility of using CuMg alloy as back contact metal for n⁺-doped-layer free a-Si:H thin film solar cell (TFSC) has been investigated in this work. The ohmic-contact characteristic has been achieved by using the CuMg alloy as back contact metal. The proposed structure showed the typical solar cell current-voltage (I-V) characteristic. An initial efficiency of 4.3% has been obtained with a open-circuit voltage V_oc = 0.79 V, short-circuit current J_sc = 13.4 mA/cm² and fill factor F.F. = 0.40. Furthermore, the experimental results also showed the CuMg alloy was suitable for the replacement of n⁺-doped-layer with the production cost reduction of a-Si:H TSFC.     

Effect of a thin W, Pt, Mo, and Zr interlayer on the thermal stability and electrical characteristics of NiSi

It is reported that the thermal stability of NiSi is improved by employing respectively the addition of a thin interlayer metal (W, Pt, Mo, Zr) within the nickel film. The results show that after rapid thermal annealing (RTA) at temperatures ranging from 650 °C to 800 °C, the sheet resistance of formed ternary silicide Ni(M)Si was less than 3 Ω/□, and its value is also lower than that of pure nickel monosilicide. X-ray diffraction (XRD) and raman spectra results both reveal that only the Ni(M)Si phase exists in these samples, but the high resistance NiSi₂ phase does not. Fabricated Ni(M)Si/Si Schottky barrier devices displayed good I-V electrical characteristics, with the barrier height being located generally between 0.65 eV and 0.71 eV, and the reverse breakdown voltage exceeding to 40 V. It shows that four kinds of Ni(M)Si film can be considered as the satisfactory local connection and contact material.     

Characterization of ohmic contacts in polymer organic field-effect transistors

It is well known that contact resistance R_c limits the performance of organic field-effect transistors (OFETs) that have high field-effect mobilities (μ_FET ≳ 0.3 cm² V⁻¹ s⁻¹) and short channel lengths (L_ch ≲ 30 μm). The usual transfer-line method (TLM) to analyze R_c calls for extrapolation of total resistance to zero L_ch at constant drain and gate voltages. This requires an unrealistic assumption that R_c does not vary with source−drain current I_sd (nor with channel carrier density σ). Here we describe a self-consistent TLM analysis that instead imposes the condition of constant I_sd and σ. The results explicitly reveal the dependence of R_c on I_sd and σ. We further describe how this R_c(I_sd, σ) surface can be modelled to yield the specific contact resistivity ρ_c of the metal/organic semiconductor (OSC) interface, a key parameter that has so far been neglected in OFETs. We illustrate the application of these analyses to high-performance staggered top-gate bottom-contact poly(2,5-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole-3,6-diyl-terthiophene-2,5″-diyl) (DPPT2-T) OFETs fabricated on bottom Au source–drain electrode arrays, with high contact-corrected μ_FET of 0.5 cm² V⁻¹ s⁻¹. We show that when these electrodes are modified to impose weak, and then strong hole-doping of the DPPT2-T interface, R_c diminishes and its dispersion, i.e. dependence on I_sd and σ, weakens. The ultimate ρ_c attained for the strongly hole-doped contact is ca. 1 Ω cm², broadly independent of I_sd and σ, which we propose is a hallmark of a true metal/OSC ohmic contact. For comparison, the bare Au/DPPT2-T contact gives ρ_c of the order of 10 Ω cm² with a marked σ dependence. The lowest ρ_c reached here shortens the current transfer length down to ca. 5 μm, enabling short electrode lengths to be advantageously employed in technology.     

Interphase interactions and the mechanism of current flow in Au-TiB x -AuGe-n-GaP ohmic contacts

Au-TiB _x -AuGe-n-GaP ohmic contacts have been investigated before and after rapid thermal annealing at T = 723, 773, and 873 K for 60 s in a hydrogen atmosphere. It is shown that the contact resistivity decreases with an increase in temperature in the range 77–232 K due to the thermionic nature of current flow in inhomogeneous ohmic contacts, while in the range 232–386 K the contact resistivity increases, which can be related to the conduction through metal shunts.     

Temperature dependence of Al/Ti-based Ohmic contact to GaN devices: HEMT and MOSFET

In this paper two types of Al/Ti-based Ohmic contacts to Gallium Nitride (GaN) based devices are presented; ImplantedN⁺GaN (like the ones found in the Source/Drain of GaN Metal Oxide Semiconductor Field Effect Transistors-MOSFET) and heterojunction (HJ) AlGaN/GaN contacts (Source/Drain of High Electron Mobility Transistors-HEMT). Sheet resistance (R_sh) and contact resistance (R_c) have been investigated in the temperature (T) range of 25-250 °C. It was found that the R_sh (850/700 Ω□) (25/250 °C) and R_c (2.2/0.7 Ωmm) decrease with T for ImplantedN⁺GaN contact and R_sh (400/850 Ω□) and R_c (0.2/0.4 Ωmm) (weakly for R_c) increase with T for HJAlGaN/GaN contact. Numerical computation based models are used to determine the theoretical R_sh and R_c behavior with T and to fit the experimental values.     

Influence of Sn content on properties of ZnO:SnO2 thin films deposited by ultrasonic spray pyrolysis

The present work is devoted to the preparation of zinc oxide (ZnO): tin oxide (SnO₂) thin films by ultrasonic spray technique. A set of films are deposited using a solution formed with zinc acetate and tin chloride salts mixture with varied weight ratio R=[Sn/(Zn+Sn)]. The ratio R is varied from 0 to 100% in order to investigate the influence of Sn concentration on the physical properties of ZnO:SnO₂ films. The X rays diffraction (XRD) analysis indicated that films are composed of ZnO and SnO₂ distinct phases without any alloys or spinnel phase formations. The average grain size of crystallites varies with the ratio R from 17 to 20 nm for SnO₂ and from 24 to 40 nm for ZnO. The obtained films are highly transparent with a transmission coefficient equal to 80%. An increase in Sn concentration increases both the effective band gap energy from 3.2 to 4.01 eV and the photoluminescence intensity peak assigned defects to SnO₂. The films electrical characterization indicated that films are resistive. Their resistivities vary between 1.2×10² and 3.3×10⁴ (Ω cm). The higher resistivity is measured in film deposited with a ratio R equal to 50%.     

Effect of the tin content on the composition and optical and electrical properties of ITO films deposited onto silicon and glass by ultrasonic spray pyrolysis

With the aim of optimizing the properties of tin-doped indium oxide (ITO) films as applied to silicon solar cells, ??100-nm-thick ITO films were deposited onto (nn ⁺)-Cz-Si and glass substrates by ultrasonic spray pyrolysis in argon at a temperature of 380°C. The relative Sn and In content in the film-forming solution was varied in the range of [Sn]/[In] = 0?C12 at %. Optimal parameters are exhibited by the films produced at [Sn]/[In] = 2?C3 at % in the solution ([Sn]/([In] + [Sn]) = 5.2?C5.3 at % in the film). For such films deposited onto glass substrates, the effective absorptance weighted over the solar spectrum in the wavelength range from 300 to 1100 nm is 1.6?C2.1%. The sheet resistance R _s of the films deposited onto silicon and glass is, correspondingly, 45?C55 and 165?C175 ???^?1. After eight months of storage in air, the resistance R _s of the optimal films remained unchanged; for the other films, the resistance R _s increased: for the films on silicon and glass, the resistance R _s became up to 2 and 14 times higher, respectively.     

LaNiO3 and Cu3Ge contacts to YBa2Cu3O7-x films

Metallization of high-T_c superconductors using low resistivity metal oxides and Cu-Ge alloys has been investigated on high quality pulsed laser deposited epitaxial YBa₂Cu₃O_7-x (YBCO) films. Epitaxial LaNiO₃ (LNO) thin films have been grown on YBCO films at 700°C using pulsed laser deposition. The specific resistivity of LNO was measured to be 50 μΩ-cm at 300K which decreases to 19 μΩ-cm at 100K indicating good metallicity of the LNO films. The contact resistance of LNO-YBCO thin film interface was found to be reasonably low (of the order of 10^-4Ω-cm² at 77K) which suggests that the interface formed between the two films is quite clean and LNO can emerge as a promising metal electrode-material to YBCO films. A preliminary investigation related to the compatibility of Cu₃Ge alloy as a contact metallization material to YBCO films is discussed. The usage of other oxide based low resistivity materials such as SrRuO₃ (SRO) and SrVO₃ (SVO) for metallization of high-T_c YBCO superconductor films is also discussed.     

Impact of electroplated copper thickness on copper CMP and Cu/Coral™ BEOL integration

A study was carried out to establish the impact of electrochemical plated (ECP) Cu thickness on the effect of dishing during Cu chemical mechanical planarization and the electrical and reliability performance of 0.13 μm Cu/Coral devices. The roughness of Cu films at the wafer edge was found to increase with increasing film thickness while it remained constant at the wafer centre. This resulted in different Cu grain morphology across the wafers. The reduction in sheet resistance (R_s) for the Cu film after annealing, as well as the as-deposited and post annealed film stresses were also found to be dependent on the ECP Cu thickness. As the thickness increased, the R_s reduction increased while the as-deposited and post annealed film stresses decreased. The different ECP Cu thickness did not show any significant difference in the amount of Cu dishing at the centre of the wafers. However, at the wafer edge, the Cu dishing amount was found to be significantly affected by the Cu thickness in which the amount of dishing increased as the thickness increased. The via chain, Kelvin via, M1 line and M2 line resistances also showed a strong dependence on the ECP Cu thickness. The thinnest Cu film of 0.7 μm gave the lowest results with the tightest spread for the four resistances tested. For the via chain and M1 line resistance, it was followed by the 1.0 μm Cu film and the 1.3 μm film yielded the worst data. In the case of Kelvin via and M2 line resistance, the thicker plated Cu films gave similar worse results. All the electrical results showed good coincidence with the Cu dishing data. The voltage ramp (v-ramp) data showed no significant difference in the electrical field leading to dielectric breakdown at both M1 and M2 lines for all the three types of ECP Cu thickness split.     

Solid phase epitaxial growth of Dy-germanide films on Ge(0 0 1) substrates

Dy thin films are grown on Ge(0 0 1) substrates by molecular beam deposition at room temperature. Subsequently, the Dy film is annealed at different temperatures for the growth of a Dy-germanide film. Structural, morphological and electrical properties of the Dy-germanide film are investigated by in situ reflection high-energy electron diffraction, and ex situ X-ray diffraction, atomic force microscopy and resistivity measurements. Reflection high-energy electron diffraction patterns and X-ray diffraction spectra show that the room temperature growth of the Dy film is disordered and there is a transition at a temperature of 300-330 °C from a disordered to an epitaxial growth of a Dy-germanide film by solid phase epitaxy. The high quality Dy₃Ge₅ film crystalline structure is formed and identified as an orthorhombic phase with smooth surface in the annealing temperature range of 330-550 °C. But at a temperature of 600 °C, the smooth surface of the Dy₃Ge₅ film changes to a rough surface with a lot of pits due to the reactions further.     

Permittivity of modified polyimide layers on LTCC

In this work, the permittivity of a tailored compound material was investigated consisting of a polyimide matrix in which hollow glass microspheres with a mean diameter of 30 μm are implemented as filler material. Choosing this approach the dielectric constant compared to that of the pure polyimide material is further decreased due to the enclosed air targeted to improve the high-frequency performance of patch antennas operated in the GHz range. Furthermore, the thickness of one single layer can be increased substantially from a maximum of about 10 μm for pure polyimide films to values above 80 μm by simply adding this type of filler material to the liquid polyimide precursor so that cavities in LTCC (low temperature co-fired ceramics) substrates can be filled more reliable. Two different variations of this compound material with filler to polymer ratios of 1:7.5 and 1:10 are realized. Basically, the film thickness depends on the spin coating speed and the microsphere content, respectively. The high initial surface roughness can be decreased to an average value of about 3 μm by applying additional layers of pure polyimide on top enabling thin film technology. The dielectric constant of the complete substrate comprising the LTCC and the compound material is measured using a ring resonator in microstrip configuration. From the resonances occurring in the transmission S-parameter |S₂₁| spectrum between 1 and 10 GHz, the relative dielectric constant can be determined. Using 820 μm thick LTCC substrates the permittivity can be reduced from originally ε_r = 7.8-6.6. By applying numerical calculations, a reduced permittivity of the pure polymer film from ε_r = 3.3 to about 2.9 can be determined when adding the glass microspheres.