Specific Contact Resistance Measurements Research Articles

Defining specifications for accurate Metal/TCO specific contact resistivity measurements by TLM in silicon heterojunction devices

Measuring specific contact resistivity (ρc) in test vehicles representative of the final solar cell is critical for interface engineering in high efficiency devices such as silicon heterojunction (SHJ) solar cell. The Transfer Length Method (TLM) is commonly used for layer sheet resistance (Rsheet) and ρc evaluation and is valid for samples with charge transport confined in one conductive layer only. For metal/Transparent Conductive Oxide (TCO) interface ρc evaluation from SHJ cells precursors, there is a need to prevent any contribution from the conductive c-Si. In this work, we investigated by simulation and experimental approach the parameters restricting the current confinement inside the TCO. The first one is the mid-gap trap density (Dit) at the a-Si:H/c-Si interface which has no strong impact on current confinement on simulated TLM test structures. The second one is the variation of the activation energy (Ea=Ec−EF) of the a-Si:H contact layers. Both simulated and experimental results have demonstrated that three current distribution regimes are possible in the TLM test structure depending on the Ea value with only one regime, for Ea between 0.6 and 1.4 eV, enabling to isolate the metal/TCO interface. This condition is mandatory to maintain a sufficiently high potential barrier for electrons at the interfaces a-Si:H(p)/a-Si:H(i)/c-Si(n) and thus lateral current in the TCO layer to apply TLM on SHJ devices.

Metallization of ZnSb and contact resistance

We present results on electrical resistance of metal contacts to ZnSb. We synthesized the thermoelectric semiconductor ZnSb with specific doping concentrations by adding Cu as an acceptor to the melt, followed by solidification, crushing, ball-milling, hot-pressing, sawing, and polishing yielding wafers suitable for substrates for further processing. Many batches were made yielding different doping concentrations. We defined transmission line geometries in deposited metal films for specific contact resistance measurements. We prepared sets of Cu, Ti, and Ni films, respectively. We measured the contact resistance vs annealing temperatures. For Cu/ZnSb samples, we observed a specific contact resistance from 5 × 10−7 to 4 × 10−5 Ω cm2. We also measured the carrier concentration of ZnSb. The measurement data of the specific contact resistance had systematic dependence on doping concentration and annealing temperature and were analyzed by a model incorporating different transport mechanisms across the energy barrier at the metal–semiconductor interface. The data were discussed in terms of systematic variation in barrier height and density of states effective mass. We proposed these arising as a consequence of interactions at the interface and a nonparabolic valence band. We have also monitored the interface of the ZnSb substrate and metal films with transmission electron microscopy.

Electronic surface, optical and electrical properties of p – GaN activated via in-situ MOCVD and ex-situ thermal annealing in InGaN/GaN LED

Electronic surface properties, optical and electrical characteristics of p-type Mg-doped Gallium Nitride (p-GaN) activated under different thermal annealing condition were investigated. In this work, p-GaN samples were subjected to in-situ and ex-situ thermal annealing process at 650 °C in Nitrogen (N2) rich condition. In-situ annealing process took place in Metal Oxide Chemical Vapor Deposition (MOCVD) chamber while ex-situ annealing process was carried out in the conventional oven. X-Ray and Ultraviolet Photoemission Spectroscopy (XPS/UPS) were used to observe the energy alignment of the p-GaN surface. From PES spectra, the sample subjected to in-situ thermal annealing shown to exhibit lower surface bend bending of 0.28 eV as compared to ex-situ thermal annealing activation with 0.45 eV band bending. This result is in agreement with specific contact resistance measurement that shows in-situ sample exhibits lower resistance resulted in better carrier injection from metal to p-GaN. Photoluminescence (PL) spectra elucidates that in-situ sample has a good surface quality with less nitrogen related vacancies (VN) formed on the p-GaN surface. All these results are further proved with the higher output power from LED with in-situ annealing. At 20 mA, in-situ sample shown an increment of ~14% light output power compared to ex-situ sample. Results obtained in this work suggest that the thermal activation condition for p-GaN activation process plays an active role on the surface quality as well as the energy alignment of the film surface and shows the potential of in-situ p-GaN activation for tunnel junction structure.

Determination of the thermoelectric properties of a skutterudite-based device at practical operating temperatures by impedance spectroscopy

Skutterudite-based thermoelectric materials are promising candidates for waste heat recovery applications at intermediate temperatures (300–500 °C) owing to their high dimensionless figure of merit and power factor. Recently, several researchers have reported the high performance of skutterudite-based thermoelectric devices obtained by optimizing the crystal structure and microstructure of skutterudite materials and developing metallization layers for device fabrication. Despite extensive research efforts toward maximizing the power density and thermoelectric conversion efficiency of skutterudite-based devices, the thermoelectric properties of such devices after fabrication remain largely unknown. Here, we systematically investigated the factors that affect the thermoelectric properties of skutterudite-based devices within the range of practical operating temperatures (23–450 °C). We successfully prepared a two-couple skutterudite-based device with titanium metallization layers on both sides of the thermoelectric legs and characterized it using scanning and transmission electron microscopy and specific contact resistance measurements. Impedance spectroscopy measurements of the two-couple skutterudite-based device revealed the figure of merit of the device and enabled the extraction of three key thermoelectric parameters (Seebeck coefficient, thermal conductivity, and electrical conductivity). The impedance spectra and extracted parameters depended strongly on the measurement temperature and were mainly attributable to the thermoelectric properties of skutterudite materials. These observations demonstrate the interplay between the properties of thermoelectric materials and devices and can aid in directing future research on thermoelectric device fabrication.

Ni/W/Ni ohmic contacts for both n- and p-type 4H-SiC

In this study, we used a Ni/W/Ni-layered structure to provide low-resistive ohmic contacts with good thermal stability for both n-type and p-type 4H-SiC. As reference, we used Ni and Ni/Ti/Ni as control groups with specific contact resistivities, and we verified the thermal stability of the structures by specific contact resistivity measurements and thermal duration tests. We found that for both n-type and p-type semiconductors, Ni/W/Ni is superior in terms of thermal stability and specific contact resistivity. Using XRD, we also analyzed the components involved in ohmic contact and thermal stability tests.

Study on the measurement accuracy of circular transmission line model for low-resistance Ohmic contacts on III-V wide band-gap semiconductors

The accuracy and error propagation for determining the low specific contact resistance of Ohmic contacts on III-V wide band-gap semiconductors based on the circular transmission line model have been analyzed and the validity of this method is discussed in detail. The accuracy is more susceptible to the factors including data fitting method, electrical measurement technique and contact area correction. By using the equations of the original circular transmission line model to extract the fitting parameters, the calculation accuracy is much improved and the inapplicability of the linear least-square fitting is prevented. To further improve the accuracy, a four-probe current-voltage measurement technique was adopted to reduce the parasitic series resistances and the uncertainty bound, especially for the Ohmic contact with low sheet resistance of the semiconductor. Moreover, we have studied the size effect of contact pads of patterns and demonstrated that contact area correction is necessary for the semiconductor with high sheet resistance. A comprehensive error analysis is also performed to fully understand all the impact factors on this advanced method of specific contact resistance measurement, which is benefit for device performance evaluation and failure analysis.

Improvement of the electrical properties of the frontal contact in CdS/CdTe solar cells

The use of a high resistivity transparent oxides (HRT) onto low resistivity transparent oxides (TCO) has been found to improve the performance of CdS/CdTe solar cells. In this work is presented and discussed a study about the use of a commercial conductive glass with SnO2:F as TCO and SnO2 as resistive transparent oxide as front contact in CdS/CdTe solar cells. The SnO2 was deposited by pneumatic spray pyrolysis and its electrical and optical properties were controlled and optimized by varying the substrate temperature and deposition time with the application of a post thermal treatment in O2 atmosphere. The transfer length method (TLM) was implemented to characterize electrically the SnO2:F/SnO2 as well the SnO2/CdS and SnO2:F/SnO2/CdS interfaces through the resistivity and specific contact resistivity measurements and their dependence on O2 treatment were analyzed. The use of the TCO/buffer layer on the CdS/CdTe solar cell in combination with the thermal treatment proposed in this work for TCO and HRT reduced the front contact/CdS contact resistance in a 10 factor and the solar cell efficiency was improved from 6.4% to 12.2%.

Low Energy Ion Implantation and Annealing of Au/Ni/Ti Contacts to n-SiC

ABSTRACTThe electrical characteristics of Au/Ni/Ti/ n-SiC contacts have been examined as a function of implant dose (1013-1014 ions/cm2) at 5 KeV and temperature of annealing (750-1000 °C). Measurements of specific contact resistance, ρc, were approximately constant at lower implant doses until increasing at 1 x 1015 ions/cm2 for both C and P ions. Annealing at a temperature of 1000 °C has reduced the value of ρc by an order of magnitude to ∼1 x 10-6 Ω.cm2 at implant doses of 1013-1014 ions/cm2. Auger Electron Spectroscopy (AES) has shown that annealing at 1000 °C resulted in a strong indiffusion of the metallization layers at the interface.

Ti/Ni/Au contacts to n-SiC after low energy implantation

The effect of low energy implantation of P or C ions in 3 C-SiC on the properties of Ti/Ni/Au contacts has been examined for doses in the range 1013–1015ions/cm2. Measurements of specific contact resistance, ρc, were performed using the two-contact circular test structure. The magnitude of ρc for the Ti/Ni/Au contacts on unimplanted SiC was 1.29×10−6Ωcm2. The value of ρc increased significantly at an implant dose of 1×1015ions/cm2. The dependence of Rsh and ρc on ion dose has been measured using both C and P implant species.

Current Transport Through Lead–Borosilicate Interfacial Glass Layers at the Screen–Printed Silver-Silicon Front Contact

During the firing of screen-printed silver contacts in industrial crystalline silicon solar cells, an interfacial glass layer is formed between the silver bulk contact and the silicon substrate. By means of numerical modeling, we investigate conduction mechanisms across the interfacial glass layer, which consists of lead–borosilicate glass enclosing silver colloids. As there is a considerable lack of parameters for the interfacial glass layer, we revisit properties of similar materials such as as-grown crystalline and polycrystalline lead monoxide. We revise characteristics of applications that use similar glasses, such as photodetectors, thick-film transistors, and surface passivation. We propose that current transport via the interfacial glass is similar to the one in thick-film resistors based on ruthenium-doped lead–borosilicate glass. Device simulations are performed using direct tunneling, trap-assisted tunneling, and a conducting insulator model for current transport across the interfacial glass layer. The tunneling mechanisms are only effective for thin interfacial glass layers up to 2 nm. A combination of direct tunneling and the conducting insulator modeling is well suited to model interfacial glass layers between 0.5 and 1000 nm. Applications to specific contact resistance measurements show that the resistivity of the interfacial glass is in the range of thick-film resistors of similar lead monoxide content.

Effect of Low Energy Implantation on the Properties of Ti/Ni/Au Contacts to n-SiC

ABSTRACTThe effect of low energy implantation of P or C ions in 3C-SiC on the properties of Ti/Ni/Au contacts has been examined for doses in the range 1013-1015 ions/cm2. Measurements of specific contact resistance, ρc, were performed using the two-contact circular test structure. The magnitude of ρc for the Ti/Ni/Au contacts on unimplanted SiC was 1.29 x 10−6 Ω.cm2. The value of ρc increased significantly at an implant dose of 1 x 1015 ions/cm2. The dependence of ρc on ion dose has been measured using both C and P implant species.

Specific Contact Resistance Measurement of Screen-Printed Ag Metal Contacts Formed on Heavily Doped Emitter Region in Multicrystalline Silicon Solar Cells

Multicrystalline silicon (mc-Si) wafers are widely used to develop low-cost high-efficiency screen-printed solar cells. In this study, the electrical properties of screen-printed Ag metal contacts formed on heavily doped emitter region in mc-Si solar cells have been investigated. Sintering of the screen-printed metal contacts was performed by a co-firing step at 725°C in air ambient followed by low-temperature annealing at 450°C for 15 min. Measurement of the specific contact resistance (ρc) of the Ag contacts was performed by the three-point probe method, showing a best value of ρc = 1.02 × 10−4 Ω cm2 obtained for the Ag contacts. This value is considered as a good figure of merit for screen-printed Ag electrodes formed on a doped mc-Si surface. The plot of ρc versus the inverse of the square root of the surface doping level (Ns−½) follows a linear relationship for impurity doping levels Ns ≥ 1019 atoms/cm3. The power losses due to current traveling through various resistive components of finished solar cells were calculated by using standard expressions. Cross-sectional scanning electron microscopy (SEM) views of the Ag metal and doped mc-Si region show that the Ag metal is firmly coalesced with the doped mc-Si surface upon sintering at an optimum firing temperature of 725°C.

Very low specific contact resistance measurements made on a highly p-type doped 4H-SiC layer selectively grown by vapor-liquid-solid transport

This work reports on the performances of ohmic contacts fabricated on highly p-type doped 4H-SiC epitaxial layer selectively grown by vapor-liquid-solid transport. Due to the very high doping level obtained, the contacts have an ohmic behavior even without any annealing process. Upon variation of annealing temperatures, it was shown that both 500 and 800 °C annealing temperature lead to a minimum value of the Specific Contact Resistance (SCR) down to 1.3×10−6 Ω⋅cm2. However, a large variation of the minimum SCR values has been observed (up to 4×10−4 Ω⋅cm2). Possible sources of this fluctuation have been also discussed in this paper.

Measurement of specific contact resistivity using scanning voltage probes

Specific contact resistivity measurements have conventionally been heavy in both fabrication and simulation/calculation in order to account for complicated geometries and other effects such as parasitic resistance. We propose a simpler geometry to deliver current and the use of a scanning voltage probe to sense the potential variation along the sample surface, from which the specific contact resistivity can be straightforwardly deduced. We demonstrate an analytical example in the case where both materials are thin films. Experimental data with a scanning Kelvin probe measurement on graphene from the literature corroborate our model calculation.

Accurate Method of Measuring Specific Contact Resistivity of Interface between Silicide and Silicon and Its Application

Reduction of specific contact resistivity is one of the key items for downscaling of device feature size. This paper discusses an accurate method of measuring specific contact resistivity by using our simple four-point probe test structure having a small area, the interface resistance of which is to be measured, fabricated on a substrate that has low sheet resistance. We also show its application for direct specific contact resistivity measurements of several systems that have dopant dependence, i.e., the NiPtSi/Si, NiPtSi/SiGe, and Pt silicide systems, as well as the metal-metal system between silicide and contact plug metal.

Si implanted reactivation in GaN grown on sapphire using AlN and oxide cap layers

Gallium nitride (GaN) is a promising material for power electronic devices. Due to GaN sensitivity to high temperature treatments, dopant activation, after ion implant, is one of the major critical steps to be overcome. An annealing cap layer is then mandatory during high temperature treatment to avoid degradations. In this work, cap layers, such as AlN and SiO x , were deposited on Si-implanted N-type GaN. Samples were annealed using both classical (FA) and rapid thermal (RTA) annealing for times ranging from 30 s to 8 h and temperatures from 1000 to 1150 °C. Transmission Electron Microscopy has been done to observe the implanted layer structure. After cap layer removal, samples surface has been investigated through Atomic Force Microscopy measurements. Dopant activity was indirectly evaluated by Specific Contact Resistance (SCR) measurements. This work demonstrates that low SCR value (8.2 × 10 −5 Ω cm 2) with low surface roughness (∼1 nm) can be reached using RTA and an oxide cap layer. However, presence of hexagonal pits in GaN layer is difficult to avoid. Compromise between low SCR with low roughness value and low hexagonal pits density on the GaN surface must be found.

Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ c = 2.15 × 10−6 Ω-cm2 indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ c = 8.34 × 10−5 Ω-cm2. These results indicate that the ρ c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.

SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si3N4) deposited over the n+-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si3N4 layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n+-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n+-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρc (in Ω-cm2) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρc = 2.53 × 10−5 Ω-cm2 is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρc is two orders of magnitude lower than the typical value of ρc = 3 × 10−3 Ω-cm2 reported previously for the Ag contacts of the solar cell. Such low value of ρc for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρc versus impurity doping level (Ns) shows that measured value of the ρc follows a linear relationship with the Ns as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n+-Si surface of the silicon solar cell.

Systematic TLM Measurements of NiSi and PtSi Specific Contact Resistance to n- and p-Type Si in a Broad Doping Range

We present the data on specific silicide-to-silicon contact resistance (rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) obtained using optimized transmission-line model structures, processed for a broad range of various n- and p-type Si doping levels, with NiSi and PtSi as the silicides. These structures, despite being attractive candidates for embedding in the CMOS processes, have not been used for NiSi, which is the material of choice in modern technologies. In addition, no database for NiSi-silicon contact resistance exists, particularly for a broad range of doping levels. This letter provides such a database, using PtSi extensively studied earlier as a reference.

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.