Transmission Line Model Test Research Articles

Hybrid TLM‐CTLM Test Structure for Determining Specific Contact Resistivity of Ohmic Contacts

ABSTRACTVarious test structures can be used to determine the specific contact resistivity of ohmic contacts. The transmission line model test structure and circular transmission line model test structure are the most commonly used. The analytical expressions of the former are straightforward and effectively describe the electrical behaviour of a contact, while the concentric geometry of the latter eliminates complications during fabrication. In this article, we present a hybrid test structure that combines the advantages of the transmission line and the circular transmission line models. The analytical expressions of the new structure are presented, and its finite‐element modelling is undertaken. The effect of contact geometry on this test structure is also discussed. Using the presented test structure, determining contact parameters does not require any error corrections.

Temperature dependent characteristics of Ti/Al/Ni/Au Ohmic contact on lattice-matched In0.17Al0.83N/GaN heterostructures

In this work, we fabricated the transmission line model test structures of Ti/Al/Ni/Au Ohmic contact on the lattice-matched In0.17Al0.83 N/GaN heterostructures, grown on the silicon and sapphire substrates respectively, and studied the temperature-dependent characteristics of the sheet resistance (Rsh) and the specific contact resistivity (ρSC). The ln(Rsh)-ln(T) plot has two distinct linearly dependent regions: 1) At T < 200 K, the slope is close to zero, indicating the comparable impurity and phonon scattering components; 2) At T > 200 K, it becomes much smaller than zero, as the phonon scattering effect becomes significant. Increasing the temperature from 100 K to 523 K, ρSC exhibits a typical roll-over behavior: 1) At T < 350 K, a “metal-like” property is important, featuring a power-law coefficient of α ~ 1.95, which is ascribed to the formation of TiN protrusions across the InAlN barrier after the rapid thermal annealing; 2) At T > 350 K, a thermionic field emission current model agrees well with the experimental data points, and the effective barrier heights are extracted to be about 1.21 eV and 0.81 eV for the samples on the silicon and sapphire substrates, respectively.

Reducing metal/graphene contact resistance via N, N-dimethylacetamide-assisted clean fabrication process

Contact resistance (R C ) is of great importance for radio frequency (RF) applications of graphene, especially graphene field effect transistors (FETs) with short channel. FETs and transmission line model test structures based on chemical vapor deposition grown graphene are fabricated. The effects of employing traditional lithography solvent (Acetone) and strong solvents for photo resist, such as N, N-Dimethylacetamide (ZDMAC) and N-Methyl pyrrolidone (NMP), are systematically investigated. It was found that ZDMAC and NMP have more proficiency than acetone to remove the photo-resist residues and contaminations attached on graphene surface, enabling clean surface of graphene. However, strong solvents are found to destroy the lattice structure of graphene channel and induce defects in graphene lattice. Clean surface contributes to a significant reduction in the R C between graphene channel and metal electrode, and the defects introduced on graphene surface underneath metal electrodes also contribute the reduction of R C . But defects and deformation of lattice will increase the resistance in graphene channel and lead to the compromise of device performance. To address this problem, a mix wet-chemical approach employing both acetone and ZDMAC was developed in our study to realize a 19.07% reduction of R C , without an unacceptable mass production of defects.

Multi-wire metallization for solar cells: Contact resistivity of the interface between the wires and In2O3:Sn, In2O3:F, and ZnO:Al layers

Replacing expensive silver with inexpensive copper for the metallization of silicon wafer solar cells can lead to substantial reductions in material costs associated with cell production. A promising approach is the use of multi-wire design. This technology uses many wires in the place of busbars, and the copper wires are “soldered” during the low-temperature lamination process to the fingers (printed or plated) or to the transparent conductive oxide (TCO) layer, e.g. in the case of the α-Si/c-Si heterojunction cells. We have studied the effects of Si surface morphology (textured or planar) and TCO resistivity (ρ) on the contact resistivity (ρC) between wires and TCO (In2O3:Sn (ITO), In2O3:F (IFO), and ZnO:Al (AZO)) layers grown on silicon substrates by ultrasonic spray pyrolysis. To determine ρC by transmission line model (TLM) measurements, we have developed a specialized TLM test structure which takes into account specifics of laminated contacts. It has been shown that, if the longitudinal resistance of the wires is left out of account, the error in ρC determined by TLM measurements may reach tens or hundreds of percent or even more. To eliminate such errors, we have adjusted the TLM measurement procedure. The present results demonstrate the following: (i) In all the groups of our samples, ρC increases with ρ and the ρC(ρ) data can be represented by a power-law trend line. (ii) For ITO and IFO, ρC is lower in the case of a textured surface: 0.3–6mΩcm2 at ρ=0.2–3mΩcm. (iii) In contrast, for the AZO films ρC is lower in the case of a planar Si surface: 5–140mΩcm2 at ρ=9–80mΩcm. These findings have been used to analyze the series resistance (Rs) of AZO/n-Si heterojunction solar cells. The contribution of ρC to Rs has been shown to reach 30–40%.

Modified Linear Transmission Line Model Test Structure for Determining Specific Contact Resistance

ABSTRACTA modified design of the transmission line model test structure uses the simple calculation of specific contact resistance, ρc, based on a two contact linear pattern but without the requirement of a mesa etch. This modified structure uses a linear TLM with semicircular terminations at each end. The function of the semicircular terminations is to confine the fringing fields at the ends of the linear TLM contacts. Simple analytical equations for determining ρc have been developed on the basis of the modified linear TLM pattern. These calculations have shown good agreement with a finite element model (FEM) of the modified TLM test structure using typical parameters for metal/ SiC contacts.

GaInN‐based tunnel junctions with high InN mole fractions grown by MOVPE

We demonstrated low‐resistive GaInN‐based tunnel junctions with high In mole fractions (over 0.35) grown by metalorganic vapor phase epitaxy (MOVPE). A 2 nm heavily Mg‐doped Ga0.6In0.4N/15 nm heavily Si‐doped GaN tunnel junction was grown on a standard‐sized (310 × 310 µm2) LED. The tunnel junction showed 0.06 V drop at 20 mA (26 A/cm2), corresponding to 4 × 10−3 Ωcm2 as the resistivity. This result indicates that MOVPE‐grown tunnel junctions showed an almost comparable resistivity as conventional p‐contact. We also found that the tunnel junction resistivity is decreased to 4 × 10−4 Ω cm2 with an increase of current density up to 5 kA/cm2. A tunneling through midgap states was suggested from forward‐biased I–V characteristics of transmission line model test structures with the tunnel junctions.

A Simplified Method for (Circular) Transmission Line Model Simulation and Ultralow Contact Resistivity Extraction

The metal resistance in the transmission line model (TLM) structures creates a serious obstacle to determine precisely the intrinsic contact resistivity. To tackle this problem, we propose a new model, the lump model, to evaluate the metal resistance influence in both TLM and circular TLM (CTLM) test structures. In this letter, we demonstrate the high simplicity, great robustness, and flexibility of the lump model. The previous reported contact resistivity values extracted with CTLM are usually above 1 χ 10 -7 Ω · cm 2 because the metal resistance impact is commonly neglected. This is the first time that the role of the metal in CTLM is appropriately analyzed. Low contact resistivity, 3.6χ10 -8 Ω · cm 2 , of standard NiSi/n-Si contact has been extracted and this shows the high sensitivity of this method.

Analytical and Finite-Element Modeling of a Two-Contact Circular Test Structure for Specific Contact Resistivity

The specific contact resistivity of a metal-semiconductor ohmic contact can be determined using a number of different test structures, and several of these use the transmission line model approach. In the circular transmission line model test structure, the concentric circular contacts have circular equipotentials in the semiconductor layer, and transmission line model equations can be used to describe their current-voltage behavior. Using test structures with two circular contacts of three different sizes, we present a new technique for determining specific contact resistivity. The analytical expressions are developed and presented, and finite-element modeling results are undertaken to demonstrate the accuracy of the technique. The scaling behavior of this test structure is also discussed. There are no error corrections required for determining contact parameters using the presented test structure.

Correcting separation errors related to contact resistance measurement

A two-layer transmission line model has been proposed to model the sheet resistance of a semiconductor layer of finite thickness outside the ohmic contact region of a transmission line model test pattern. It is shown that the effective sheet resistance is a function of the separation between the contacts and that there is a pivot point in the curve of the total resistance between the contacts versus their separation. The work reveals an error in the measurement of R c using the standard tlm for small contact spacing. The results are in agreement with simulation results obtained from the boundary element method.

Excess noise in AlGaAs/GaAs heterojunction bipolar transistors and associated TLM test structures

Noise measurements both on transmission line model (TLM) test structures and on associated HBT's are presented. Contact noise is proved to be negligible in the TLM's related to the base structure of transistors. A Hooge parameter for p/sup ++/ doped GaAs is extracted. Activation energies are calculated from results versus temperature. Considering the TLM related to the structure of the emitter, it is shown that the g-r levels observed originate from the AlGaAs layer. Noise measurements on HBT's also exhibit excess noise. A value of the cutoff frequency between the equivalent input current white noise and the 1/f component is given. The base current dependencies associated with different measurement configurations suggest the 1/f noise to come from the base or the emitter-base junction. The g-r components are studied as a function of temperature. Activation energies are deduced. Finally a comparison of the TLM and HBT noise results is presented. The presence of the complex DX center and of g-r levels in the base region are proposed as possible origins for the g-r noise in HBT's.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical modeling of transmission line model test structures. I. The effect of inhomogeneities on the extracted contact parameters

Extraction of electrical contact parameters by transmission line model (TLM) test structures is based on the assumption that the electrical and geometrical parameters of the contacts within a structure are identical. Statistical modeling is presented to show that the usual extraction procedure can lead to large errors in the extracted contact parameters in the case of nonideal TLM structures, even if there is no error in the measured electrical and geometrical parameters. Errors in the extracted parameters as functions of the nominal contact parameters are presented graphically. These graphs can be used to assess the accuracy of the TLM measurements and to optimize TLM test structure design. In the parameter intervals examined, the TLM method works best (yields the fewest errors in all extracted parameters) when the geometrical length of a contact is equal to about two times the electrical transfer length.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical modeling of transmission line model test structures. II. TLM test structure with four or more terminals: a novel method to characterize nonideal planar contacts in presence of inhomogeneities

For pt.I see ibid., vol.37, pp.2350-2360, Nov. 1990. A modified transmission line model (redundant TLM or RTLM) test structure (with four or more terminals) and a novel data extraction method are suggested to improve the accuracy of the TLM method. Statistical modeling by both statistical simulation and the method of error propagation shows that with four terminals and independently known sheet resistances between the contacts, the errors of the extracted parameters can be reduced considerably. With the use of five or more terminals, the accuracy of the parameter extraction can also be determined by a single structure, allowing the separation of inhomogeneities within a test structure from the inhomogeneities over the wafer. Experimental data gathered by Kelvin cross-bridge resistors and by traditional and modified transmission line model structures on Al-Si-Cu/TiW/p-n Si contacts clearly show the advantage of the RTLM method over the traditional TLM method. On a wafer with small inhomogeneities, both methods gave consistent results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Obtaining the specific contact resistance from transmission line model measurements

In characterizing ohmic contacts using the transmission line model, it is necessary to make a measurement referred to as the contact end resistance, as a result of modification to the sheet resistance under the contact. In this article we show that this contact end resistance and the consequent specific contact resistance can be deduced from simple resistance measurements carried out between contacts on a standard, transmission line model test pattern.