Planar Ohmic Contacts Research Articles

A Novel Analytical Model for Ohmic Contacts to Planar Devices: Theoretical Design and Experimental Verification

Designing a reasonable electrode size and obtaining precise contact resistance parameters are always crucial in the field of chip manufacturing, especially in the cases of tiny chips with very low contact resistance. In this work, an improved analytical model featuring an electrode-pair (EP) layout design is proposed to accurately extract the electrical parameters of the planar Ohmic contact. The key feature of the proposed model layout is the adoption of a series of EPs with a fixed interval distance but varied electrode widths which is just opposite to the conventional transmission line model (TLM). Then, the effective electrode width depending on the conduction current requirement and practical Ohmic contact technology level in production can hence be derived to assist the engineers for a better design in chip layout. Furthermore, the model is analyzed and demonstrated by performing both technology computer-aided design (TCAD) simulations and experimental measurements. The results verify the ability to achieve more precise contact resistance parameters and distinguish the variance of the sheet resistance underneath the contact after annealing treatment by the proposed EP model (EPM). This work provides a distinct perspective to understand and quantify the electrical characteristics in a greater variety of Ohmic contacts.

Analytical model for accurate extraction of metal-semiconductor ohmic contact parameters using a novel electrode-pair layout scheme

In this paper, an electrode-pair model is proposed and demonstrated for the first time to accurately extract the electrical resistance parameters of the planar metal-semiconductor ohmic contacts. Different from the conventional transmission line model, the proposed model layout features a series of separated electrode pairs with the same electrode distance but various widths. Meanwhile, an equivalent circuit for contact resistance composition is set up to clearly specify the contribution of each resistance component to the overall contact performance. The semiconductor sheet resistances underneath the contact and outside the contact area are treated as completely independent variables. The proposed scheme is modeled theoretically and analyzed by the TCAD simulations, and the validity of the model is verified by the experimental data. Finally, the variance of the sheet resistance underneath the contact after annealing treatment can be distinguished by the model and hence more actual and precise specific contact resistance is achieved. This work provides a distinct perspective to understand and quantify the electrical characteristics of the semiconductor ohmic contacts, and it can also assist engineers for a better electrode layout design.

Experimental Observation of Delayed Impact-Ionization Avalanche Breakdown in Semiconductor Structures without p–n Junctions

We have experimentally studied the dynamics of impact-ionization switching in semiconductor structures without p–n junctions when subnanosecond high-voltage pulses are applied. Silicon n+–n–n+ type structures and volume ZnSe samples with planar ohmic contacts exhibit reversible avalanche switching to the conducting state within about 200 ps, which resembles the well-known phenomenon of delayed avalanche breakdown in reverse-biased p+–n–n+ diode structures. Experimental data are compared to the results of numerical simulations.

Экспериментальное наблюдение задержанного ударно-ионизационного пробоя полупроводниковых структур без p-n-переходов

AbstractWe have experimentally studied the dynamics of impact-ionization switching in semiconductor structures without p – n junctions when subnanosecond high-voltage pulses are applied. Silicon n ^+– n – n ^+ type structures and volume ZnSe samples with planar ohmic contacts exhibit reversible avalanche switching to the conducting state within about 200 ps, which resembles the well-known phenomenon of delayed avalanche breakdown in reverse-biased p ^+– n – n ^+ diode structures. Experimental data are compared to the results of numerical simulations.

Planar Ohmic Contacts to Al0.45Ga0.55N/Al0.3Ga0.7N High Electron Mobility Transistors

We present a low resistance, straightforward planar ohmic contact for Al0.45Ga0.55N/Al0.3Ga0.7N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950°C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900°C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900°C anneal) and a Schottky (850°C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρc = 2.5 × 10−5 Ω cm², sheet resistances of RSH = 3.9 kΩ/■, and maximum current densities of 75 mA/mm (at VGATE of 2 V). Electrical measurements from −50 to 200°C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. These contacts enabled state-of-the-art performance of Al0.45Ga0.55N/Al0.3Ga0.7N HEMTs.

The Research and Design of a Room-Temperature Nuclear Radiation CdZnTe Detector

Cadmium zinc telluride (CZT) material is one of the preferred materials for the fabrication of X-ray and gamma-ray detector. In this paper, it is presented a room-temperature nuclear radiation detector system based on CZT material with planar electrode and ohmic-contact. The experimental validation and analysis of the designed system is carried out. At the same time, some research and design on the surface signal-readout method、the charge sensitive preamplifier circuit、pulse signal processing circuit and soft processing program is made. The signal from the CZT material exposed to the radiation through the experiment is successfully required. The signal from the experiment is correspondingly processed and validated. The potential factors that influence the detector performance are discussed. The conductive adhesive is the joint of the ohmic-contact electrode and the preamplifier circuitry. The preamplifier circuitry has high gain, rise time in the nanosecond level and little equivalent charge-noise. The research and experimental design base for the development of subsequent gamma spectrometer.

Power loss calculation as a reliable methodology to assess the ohmic losses of the planar ohmic contacts formed on the photovoltaic devices

The front grid contact is particularly important and requires a low contact resistance which represents the resistance associated with the barrier at the interface of the metal and semiconductor contact structure. Often applied metal contacts are fired at a higher temperature (typically above 700 °C) in air ambient, which produces ohmic contact on both surface of the photovoltaic device. The specific contact resistance is one of the important device parameter on studying the interfacial properties of the metalization system. Therefore, a reliable methodology to assess the ohmic losses of the applied metal contact structure is required. It shows that it is rather simple and reliable to assess the electrical quality of the applied metal contacts by quantifying the total ohmic losses of the solar cell associated with the various resistive components of the solar cell normalized to unit cell area. It has been recently demonstrated that with a new experimental procedure, namely iteration method based on the calculation of power loss (ICPL) associated with the contact resistance of the front Ag thick-film metal contacts, a much reliable value of the specific contact resistance of the order of ≅10−5 Ω cm2 can be extracted for the planar ohmic contacts. In this work, the specific contact resistance of the planar ohmic contacts formed on the heavily doped n+ region of the solar cells were studied on large number of finished cells by two independent methods: (i) standard three-point probe (TPP) and (ii) iteration technique based on the calculation of the power loss (ICPL) associated with the contact resistance of the front Ag contacts of the solar cell normalized to unit cell area. It shows that the value of specific contact resistance measured by both methods are desirably much lower than the expected value of 10−3 Ω cm2 for the screen-printed Ag metal contacts of the photovoltaic cells used for the A.M. 1.5 applications. Using the iteration, each resistive components of the solar cell normalized to unit cell area were directly evaluated. It is shown that by combining the measurements of specific contact resistance of the planar ohmic contacts and ohmic losses of the cell, it gives a direct and non-destructive diagnostic tool to qualitatively check the electrical quality of the applied Ag metal contacts.

Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρc) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρc of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10−5 Ω−cm2 is measured for the Ag metal contacts formed on the n+ silicon surface. This value is much lower than the ρc data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρc of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

Two-level model of longitudinal magnetic field-induced current instability and chaos inn-GaAs

We extend our previous application of the two-impurity-level model to interpret the cross-over current instability for a $n$-GaAs semiconductor with planar ohmic contacts under an external magnetic field $B$ longitudinal to the applied dc bias. The previous assumption of spatial homogeneity in current flow direction as well as previous considerations of the Landau level shifts for the conduction band electrons and the electron mobility with the field and the carrier electron temperature dependence are all retained. In addition, we further add in a phenomenological effective magnetoresistance $h$ factor from the generation-annihilation dynamic of the filamentary current and the carrier temperature dependence of the impact ionization coefficient from the ground level ${X}_{1}$ and that of the capture coefficient ${T}_{1}^{s}$ in the generation-recombination processes. Increasing the magnetic field above a critical magnitude in the post-breakdown region of the S-shaped current density-field characteristic, we are able to find the system bifurcating to chaos via several period-doubling routes at various bias voltages. Our numerical results are consistent with those observed experimentally by Aoki, Kawase, Yamamoto, and Mugibayahi [J. Phys. Soc. Jpn. 59, 20 (1990)].

GaN resistive hydrogen gas sensors

GaN epilayers grown by organometallic vapor phase epitaxy have been used to fabricate resistive gas sensors with a pair of planar ohmic contacts. Detectible sensitivity to H2 gas for a wide range of gas mixtures in an Ar ambient has been realized; the lowest concentration tested is ∼0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. Real-time response to H2 shows a clear and sharp response with no memory effects during the ramping cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. This appears to be consistent with a surface-adsorption-induced change of conductivity; a detailed picture of the gas sensing mechanism requires further systematic studies.

Wide-range (0.33%–100%) 3C–SiC resistive hydrogen gas sensor development

Silicon carbide (SiC) resistive hydrogen gas sensors have been fabricated and tested. NiCr planar ohmic contacts were deposited on both a 4μm 3C–SiC epitaxial film grown on n-type Si(001) and directly on Si to form the resistive sensor structures. Detection at concentrations as low as 0.33% and as high as 100% (H2 in Ar) was observed with the 3C–SiC sensor while the Si sensor saturated at 40%. The 3C–SiC sensors show a remarkable range of sensitivity without any saturation effects typically seen in other solid-state hydrogen gas sensors. Under a constant 2V bias, these sensors demonstrated an increase in current up to 17mA upon exposure to pure H2. Preliminary experiments aimed at determining the gas sensing mechanism of these devices have been conducted and are also reported.

Quasi-two-dimensional transmission line model (QTD-TLM) for planar ohmic contact studies

An analytical quasi-two-dimensional transmission tine model (QTD-TLM) has been formulated to more accurately extract the specific contact resistance (/spl rho//sub c/) of ohmic contacts than the conventional one-dimensional TLM (1D-TLM). Similar to 1D-TLM, the extraction of /spl rho//sub c/ using QTD-TLM is straightforward. By means of the conformal mapping technique, the two-dimensional (2-D) (or lateral) current flow and current crowding, owing to the presence of a gap between the TLM mesa and contacts, are jointly incorporated into our model using a single shunt resistor. QTD-TLM is generalized as it is applicable to a variety of contact dimensions and gap widths, and to both alloyed and nonalloyed contacts. The validity of QTD-TLM has been verified experimentally using two alloyed and two nonalloyed ohmic contacts, and by comparison with results from a 2-D numerical model.

Using TLM principles to determine MOSFET contact and parasitic resistance

Transmission Line Model (TLM) networks are commonly used to model planar metal-semiconductor ohmic contacts. Multiple layer contacts such as non-alloyed n + n , heterojunction and metal-silicide-silicon contacts can also be analyzed using a Tri-Layer TLM (TLTLM) network. In this article, two and three layer contact structures are combined using the appropriate TLM models in order to electrically model the gate-drain/source extension and drain/source contact region of a MOSFET. Important device properties such as contact and parasitic resistance can thus be derived for various device structures in terms of the geometrical and material parameters used in the TLM model. The developed model is used to give an example calculation of the parasitic resistance in the gate-drain extension and the resistance of the ohmic contact.

Numerical investigation of non-ohmic hopping conduction in macroscopically non-uniform thin layers: weak electron-phonon interaction

The current-field and the conductivity-field characteristics of random hopping non-uniform systems in the limit of a weak electron-phonon coupling (band-like hopping transport in disordered solids) have been calculated numerically within the Bottger-Bryksin model. In particular, we consider thin layers placed between two planar Ohmic contacts, with exponential spatial variations in the total centre concentration over the layer thickness. We find that firstly for r-hopping transport in spatially uniform layers the current decreases with increasing field, reaching a constant saturation value, and on increasing the degree of non-uniformity the saturation current becomes lower and secondly both uniform and non-uniform r- epsilon -hopping systems are almost exactly Ohmic in the investigated field range; conductivity decreases on increasing the degree of non-uniformity of the centre distribution. Degree of the layer non-uniformity in the limit of weak electron-phonon coupling are quite different from the dependences in the case of strong electron-phonon coupling. The dependences on the degree of the layer non-uniformity of the centre distribution. The dependences on the degree of the layer non-uniformity in the limit of weak electron-phonon coupling are quite different from the dependences in the case of strong electron-phonon coupling.

An analytical model for alloyed ohmic contacts using a trilayer transmission line model

This paper describes a Transmission Line Model approach to the modeling and analysis of alloyed planar ohmic contacts. It briefly reviews the standard Transmission Line Model (TLM) commonly used to characterize a planar ohmic contact. It is shown that in the case of a typical Au-Ge-Ni alloyed ohmic contact, a more realistic model based on the TLM should take into account the presence of the alloyed layer at the metal-semiconductor interface. In this paper, such a model is described. It is based on three layers and the two interfaces between them, thus forming a Tri-Layer Transmission Line Model (TLTLM). Analytical expressions are derived for the contact resistance Re and the contact end resistance Rc of this structure, together with a current division factor, S. Values for the contact parameters of this TLTLM model are inferred from experimentally reported values of Re and Re for two types of contact. Using the analytical outcomes of the TLTLM, it is shown that the experimental results obtained using a standard TLM can have considerable discrepancies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A New Electrical Model for Calculating the Sheet Resistance Parameter in Alloyed Ohmic Contacts

ABSTRACTThis paper briefly reviews the standard Transmission Line Model (TLM) commonly used to measure the specific contact resistance ρc and the sheet resistance Rsk beneath a planar ohmic contact. In the case of an alloyed ohmic contact, a more realistic three layer (Tri-Layer Transmission Line Model (TLTLM)) can be used for the analysis. This model is based on three layers (metal layer, alloyed semiconductor layer and the unalloyed semiconductor layer) and the two interfaces between them. By using appropriate TLTLM parameters, it is possible to calculate the sheet resistance Rsk that has been experimentally derived from the standard TLM. The new TLTLM model predicts that values of Rsk greater and less than Rsh (the unmodified epitaxial layer sheet resistance) are possible in agreement with experimentally reported observations.

Electrical Modelling and Characterisation of Alloyed Ohmic Contacts

ABSTRACTThis paper briefly reviews the standard Transmission Line Model (TLM) commonly used to measure the specific contact resistance of a planar ohmic contact. It is proposed that in the case of a typical Au-Ge-Ni alloyed ohmic contact, a more realistic model would need to take into account the presence of the alloyed layer at the metal-semiconductor interface. An alternative is described which is based on three contact layers and the two interfaces between them, thus forming a Tri-Layer Transmission Line Model (TLTLM). Expressions are given for the contact resistance Rc and the contact end resistance Re of this structure, together with a current division factor, f. Values for the parameters of this model are inferred from experimentally reported values of Rc and Re for two types of contact.

The Development of Solid Phase Regrowth on GaAs and its applications

AbstractA solid phase regrowth process on GaAs has been observed in Pd- and Ni- based bi-layer structures, e.g. the Si/Ni, the Ge/Pd, the In/Pd, and the Sb/Pd structures. Due to the regrowth, uniform epitaxial layers of Ge, GaAs, InxGa1-xAs, and GaSbl-xAsx on GaAs substrates by solid phase reactions can achieved. The model of this regrowth process will be presented. Based on this regrowth mechanism, a series of non-spiking planar ohmic contacts on n and p type GaAs have been developed. Low contact resistivity in the range of mid 10−7 Ω-cm2 was obtained. The ohmic contact formation mechanism of these contacts will also be discussed. All the studies suggest that the ohmic behavior is a result of the formation of an n+ or p+ surface layer via solid phase reactions. The regrowth process has also been utilized to achieve compositional disordering of GaAs/AlGaAs superlattices, and low loss AlGaAs/GaAs waveguide has been obtained.

Chemical reactions at metal/compound semiconductor interfaces: Au and GaAs

The reaction between Au thin films and GaAs substrates is discussed. The reaction proceeds by dissolution of the GaAs to form a AuGa solid solution, but regrowth of GaAs at the Au GaAs interface is occuring simultaneous with dissolution. The rate of this reaction depends upon the condition of the GaAs surface at the interface. Contrary to most systems, thin native oxides increase the rate of the reaction while oxide-free interfaces react more slowly. Sputter damaged, oxide-free interfaces react at an intermediate rate. Dopant from the substrate may segregate to the decomposed GaAs areas leading to the formation of an ohmic contact after reaction. Finally, heating sandwich structures of AuGe, As and Ga on GaAs to approximately 400°C results in isothermal regrowth of doped, smooth, planar ohmic contacts with low specific contact resistance. This procedure should be suitable for forming ohmic contacts on shallow junction devices.

Modeling of the source resistance in modulation-doped FETs

A distributed multiple layer transmission line (MTL) model has been developed for the planar source ohmic contacts as encountered in modulation-doped FETs. Detailed current and voltage distributions under the MTL system have been obtained by numerical solutions of the coupled differential equations. The results clearly demonstrate that source resistance R S cannot be estimated simply from knowledge of the behavior of series resistance R NG in non-gated TLM test structures. The effects of the heavily-doped cap layer and the variation of the gate-source spacing on R NG and R S have been studied. Differences between our results based on the MTL model and that obtained from the lumped element contact end-resistance (LECR) model are explained satisfactorily. Our results show that the LECR method could be used to calculate the series resistance of the MODFET structures provided appropriate end elements are first determined by the distributed MTL model. Our results clearly indicate that attempts to reduce R S by increasing the cap layer thickness or decreasing the gate-source spacing would have only limited effectiveness. However, reduction of the interfacial contact resistivities is more effective in the reduction of R S , as demonstrated in the case of pseudomorphic structures. Graphical results are presented that clearly show the dependence of R S on the structural parameters of representative conventional and pseudomorphic test MODFETs.