Metal-insulator-semiconductor Contacts Research Articles

Reducing contact resistance of MoS2-based field effect transistors through uniform interlayer insertion via atomic layer deposition.

Molybdenum disulfide (MoS2), a semiconducting two-dimensional layered transition metal dichalcogenide (2D TMDC), with attractive properties enables the opening of a new electronics era beyond Si. However, the notoriously high contact resistance (RC) regardless of the electrode metal has been a major challenge in the practical applications of MoS2-based electronics. Moreover, it is difficult to lower RC because the conventional doping technique is unsuitable for MoS2 due to its ultrathin nature. Therefore, the metal-insulator-semiconductor (MIS) architecture has been proposed as a method to fabricate a reliable and stable contact with low RC. Herein, we introduce a strategy to fabricate MIS contact based on atomic layer deposition (ALD) to dramatically reduce the RC of single-layer MoS2 field effect transistors (FETs). We utilize ALD Al2O3 as an interlayer for the MIS contact of bottom-gated MoS2 FETs. Based on the Langmuir isotherm, the uniformity of ALD Al2O3 films on MoS2 can be increased by modulating the precursor injection pressures even at low temperatures of 150 °C. We discovered, for the first time, that film uniformity critically affects RC without altering the film thickness. Additionally, we can add functionality to the uniform interlayer by adopting isopropyl alcohol (IPA) as an oxidant. Tunneling resistance across the MIS contact is lowered by n-type doping of MoS2 induced by IPA as the oxidant in the ALD process. Through a highly uniform interlayer combined with strong doping, the contact resistance is improved by more than two orders of magnitude compared to that of other MoS2 FETs fabricated in this study.

Enhanced Performance of WS2 Field-Effect Transistor through Mono and Bilayer h-BN Tunneling Contacts.

Transition metal dichalcogenides (TMDs) are of great interest owing to their unique properties. However, TMD materials face two major challenges that limit their practical applications: contact resistance and surface contamination. Herein, a strategy to overcome these problems by inserting a monolayer of hexagonal boron nitride (h-BN) at the chromium (Cr) and tungsten disulfide (WS2 ) interface is introduced. Electrical behaviors of direct metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) contacts with mono- and bilayer h-BN in a four-layer WS2 field-effect transistor (FET) are evaluated under vacuum from 77 to 300 K. The performance of the MIS contacts differs based on the metal work function when using Cr and indium (In). The contact resistance is significantly reduced by approximately ten times with MIS contacts compared with that for MS contacts. An electron mobility up to ≈115 cm2 V-1 s-1 at 300 K is achieved with the insertion of monolayer h-BN, which is approximately ten times higher than that with MS contacts. The mobility and contact resistance enhancement are attributed to Schottky barrier reduction when h-BN is introduced between Cr and WS2 . The dependence of the tunneling mechanisms on the h-BN thickness is investigated by extracting the tunneling barrier parameters.

Stable reverse bias or integrated bypass diode in HIP‑MWT+ solar cells

The Metal Wrap Through+ (HIP-MWT+) solar cell is based on the PERC concept but features two additional electrical contacts, namely the Schottky contact between p-type Si bulk and Ag n-contact and the metal-insulator-semiconductor (MIS) contact on the rear side of the cell below the n-contact pads. To prevent thermal hotspots under reverse bias, both contacts shall either restrict current flow or allow a homogenous current flow at low voltage. In this work we present both options. First the stable reverse bias characteristics up to −15 V with a MIS contact using industrially manufactured SiON passivation and second, an integrated by-pass diode using AlOX as insulator in the passivation stack allowing current flows at approximately Vrev = –3.5 V depending on the chosen screen-print paste. The examined Schottky contacts break down at around Vrev = –2.5 V. Reverse bias testing of the cells reveals a solid performance of the cells under reverse bias and an average conversion efficiency of η = 21.2% (AlOX) and η = 20.7% (SiON), respectively.

Analytical study of Metal-Insulator-Semiconductor contacts for both p- and n-InGaN

This paper has studied the metal–insulator-semiconductor (MIS) contact for both p- and n-InGaN. We found that the insulator layer thickness has a remarkable effect on the Fermi level pinning and barrier height reduction in MIS contacts. Schottky's formation with doped InGaN is explained by extending the MIS contacts' using the metal-induced gap states (MIGS) model. The J-V characteristics clarify the current transport mechanism through the MIS contact with InGaN semiconductor for different temperatures. We observed less control on the barrier height reduction in the case of n-InGaN through changing the thickness of the interfacial layer. The calculated contact resistivities of MIS contact with p- and n-InGaN show better results than the metal–semiconductor contact counterparts. These findings are highly significant to design and fabricate the InGaN based switching devices for converter applications.

Diode-Like Selective Enhancement of Carrier Transport through Metal-Semiconductor Interface Decorated by Monolayer Boron Nitride.

2D semiconductors such as monolayer molybdenum disulfide (MoS2 ) are promising material candidates for next-generation nanoelectronics. However, there are fundamental challenges related to their metal-semiconductor (MS) contacts, which limit the performance potential for practical device applications. In this work, 2D monolayer hexagonal boron nitride (h-BN) is exploited as an ultrathin decorating layer to form a metal-insulator-semiconductor (MIS) contact, and an innovative device architecture is designed as a platform to reveal a novel diode-like selective enhancement of the carrier transport through the MIS contact. The contact resistance is significantly reduced when the electrons are transported from the semiconductor to the metal, but is barely affected when the electrons are transported oppositely. A concept of carrier collection barrier is proposed to interpret this intriguing phenomenon as well as a negative Schottky barrier height obtained from temperature-dependent measurements, and the critical role of the collection barrier at the drain end is shown for the overall transistor performance.

Insulator as Efficient Hole Injection Layer in Perovskite Light‐Emitting Device via MIS Contact

AbstractPolymer insulator is employed as efficient hole injection layer (HIL) in lead halide perovskites light‐emitting diodes (PeLEDs) for the first time, by revealing that metal–insulator–semiconductor (MIS) contact is responsible for the hole‐injection mechanism in PeLEDs with indium tin oxide (ITO)/insulator/perovskite structure. For MIS contact of ITO/insulator/perovskite, the electric field is concentrated in the insulator layer, thereby reducing the tunneling distance from ITO to perovskite. By studying the transient current response, it is confirmed that the accumulation of both electrons and ions at the insulator/perovskite interface is responsible for the screening of electric field. With polystyrene (PS) as hole injection layer, the device exhibits a much better device performance than that of PeLEDs with regular polyvinylcarbazole (PVK) HIL, even though the resistivity of PS is 5 orders of magnitude higher than that of PVK, which confirms the universal applicability of MIS contact. By optimizing the thickness of PS layer, the performance of FAPbBr3‐based device achieves a turn‐on voltage of 2.8 V, a peak luminous efficiency of 44.7 cd A−1, and an external quantum efficiency of 10.6%.

Contact resistivity reduction on lowly-doped n-type Si using a low work function metal and a thin TiOX interfacial layer for doping-free Si solar cells

Eliminating a doping process could be an effective way to reduce the production cost of c-Si cells. However, in absence of highly doped Si, the formation of a high quality contact is not straightforward. The lack of field-effect passivation from a lowly doped region can lead to a high recombination current density at the contacts (J0,metal) and moreover, contact resistivity (ρC) typically increases when doping level is decreasing. In this work we focus on reducing the contact resistivity of an electron-selective contact for doping-free cells. Although the effect of low work function metals (LWMs) in combination with an i-a-Si:H layer has already been reported, the synergy effect of a LWM and a MIS (Metal-Insulator-Semiconductor) contact structure on top of the i-a-Si:H has not been reported yet. Here, we demonstrate a new ATOM (i-a-Si:H / TiOX / low workfunction metal) contact structure as an electron-selective contact using an i-a-Si:H layer, a TiOX interfacial layer and Ca (Φ = 2.9eV) without requiring an additional n+ doping process. The addition of TiOX in between the i-a-Si:H layer and the Ca decreases the ρC by about 2 orders of magnitude. Despite of increased J0,metal due to e-beam processing of TiOX, the Ca based ATOM contact increases the potential max efficiency up to 25 %. To the best of our knowledge, this is the first demonstration of an electron-selective contact comprising a low work function metal, an interfacial TiOX and an i-a-Si:H passivation layer. This type of contact could be a promising route for the optimization of doping-free cells

Undoped and Doped Junctionless FETs: Source/Drain Contacts and Immunity to Random Dopant Fluctuation

By using numerical simulations, we demonstrate that a proper source/drain contacting strategy not only boosts the on-currents, but also eliminates random dopant fluctuation (RDF) of a junctionless FET (JLFET). Two contacting strategies for JLFETs with a wide range of doping concentration ( $10^{10} \sim ~10^{20}$ cm $^{-3})$ are compared and discussed: metal–insulator–semiconductor (MIS) and metal–semiconductor (MS) contacts. The Schottky barrier at the MS contact shows more negative impacts on the on-current when the JLFET is not doped heavily. With dopantless and unintentionally doped Si (doping concentration $= 10^{10} \sim ~10^{17}$ cm $^{-3}$ for both n-type dopants and p-type dopants), the JLFETs with MIS contacts may exhibit higher on-currents than those with MS contacts and show almost the same transfer characteristics. The later result actually implies that the JLFET with MIS contacts possesses the immunity against RDF.

Titanium Dioxide Hole-Blocking Layer in Ultra-Thin-Film Crystalline Silicon Solar Cells

One of the remaining obstacles to approaching the theoretical efficiency limit of crystalline silicon (c-Si) solar cells is the exceedingly high interface recombination loss for minority carriers at the Ohmic contacts. In ultra-thin-film c-Si solar cells, this contact recombination loss is far more severe than for traditional thick cells due to the smaller volume and higher minority carrier concentration of the former. This paper presents a novel design of an electron passing (Ohmic) contact to n-type Si that is hole-blocking with significantly reduced hole recombination. This contact is formed by depositing a thin titanium dioxide (TiO2) layer to form a silicon metal-insulator-semiconductor (MIS) contact. A 2 {\mu}m thick Si cell with this TiO2 MIS contact achieved an open circuit voltage (Voc) of 645 mV, which is 10 mV higher than that of an ultra-thin cell with a metal contact. This MIS contact demonstrates a new path for ultra-thin-film c-Si solar cells to achieve high efficiencies as high as traditional thick cells, and enables the fabrication of high-efficiency c-Si solar cells at a lower cost.

A Dopingless FET With Metal–Insulator–Semiconductor Contacts

By adopting the charge-plasma concept, dopingless FETs with metal-semiconductor and metal–insulator–semiconductor (MIS) contacts in parallel at the source/drain (SD) have been studied in this letter. Currents are found to flow mainly through the MIS contacts for a given SD metal workfunction when the insulator thickness is thin enough. In order to avoid the possible penalty caused by Fermi level pinning, the dopingless FET with only SD MIS contacts has been proposed as well. The impacts of insulator material parameters, such as bandgap, electron affinity, dielectric constant, and physical thickness, on the electrical characteristics of the dopingless FET have been investigated systematically. Based on numerical simulations, this letter provides a general guideline with physical insights for designing dopingless FETs with high-permittivity insulator at the SD MIS contacts.

Electroluminescence from MIS silicon-based light emitters with arrays of self-assembled Ge(Si) nanoislands

We report on the electroluminescence from silicon-based metal–insulator–semiconductor (MIS) diodes with arrays of self-assembled Ge(Si) nanoislands. Aluminum oxide (Al2O3) is used as an insulator material in the MIS contact. Variations in the electroluminescence spectra caused by changing the metal work function are examined. The intense electroluminescence from Ge(Si) nanoislands localized at a distance of 50 nm from the insulator–semiconductor interface is observed at room temperature. The emission spectrum is found to be controlled by choosing the design of the semiconductor structure and the barrier height for injected carriers.

Thermal Stability Concern of Metal-Insulator-Semiconductor Contact: A Case Study of Ti/TiO2/n-Si Contact

This work discusses the thermal stability of metal-insulator-semiconductor (MIS) contacts. A case study is performed on a typical low-Schottky barrier height ( $q\varphi _{b})$ MIS contact: Ti/TiO2/n-Si. By incorporating different levels of donor concentration in n-Si, we perform a systematic Ti/TiO2/n-Si thermal stability study under different electron conduction mechanisms. We find that both $q\varphi _{b}$ and contact resistivity ( $\rho _{c})$ of the Ti/TiO2/n-Si MIS contacts vary dramatically after mere 300 °C–500 °C 1-min rapid thermal treatments. The variations in $q\varphi _{b}$ and $\rho _{c}$ are related to the thermally driven TiO2 decomposition. This thermal stability study of Ti/TiO2/n-Si reveals a general concern for the MIS contact application: since the MIS contacts on n-type semiconductor generally utilize a reactive low-work function metal and an ultrathin insulator, it is difficult to maintain their interface quality considering the thermal budget in standard manufacturing of integrated circuits. Possible solutions to this MIS thermal stability issue are discussed.

Contact resistivities of metal-insulator-semiconductor contacts and metal-semiconductor contacts

Applying simulations and experiments, this paper systematically compares contact resistivities (ρc) of metal-insulator-semiconductor (MIS) contacts and metal-semiconductor (MS) contacts with various semiconductor doping concentrations (Nd). Compared with the MS contacts, the MIS contacts with the low Schottky barrier height are more beneficial for ρc on semiconductors with low Nd, but this benefit diminishes gradually when Nd increases. With high Nd, we find that even an “ideal” MIS contact with optimized parameters cannot outperform the MS contact. As a result, the MIS contacts mainly apply to devices that use relatively low doped semiconductors, while we need to focus on the MS contacts to meet the sub-1 × 10−8 Ω cm2 ρc requirement for future Complementary Metal-Oxide-Semiconductor (CMOS) technology.

Amorphous silicon enhanced metal-insulator-semiconductor contacts for silicon solar cells

Carrier recombination at the metal-semiconductor contacts has become a significant obstacle to the further advancement of high-efficiency diffused-junction silicon solar cells. This paper provides the proof-of-concept of a procedure to reduce contact recombination by means of enhanced metal-insulator-semiconductor (MIS) structures. Lightly diffused n+ and p+ surfaces are passivated with SiO2/a-Si:H and Al2O3/a-Si:H stacks, respectively, before the MIS contacts are formed by a thermally activated alloying process between the a-Si:H layer and an overlying aluminum film. Transmission/scanning transmission electron microscopy (TEM/STEM) and energy dispersive x-ray spectroscopy are used to ascertain the nature of the alloy. Idealized solar cell simulations reveal that MIS(n+) contacts, with SiO2 thicknesses of ∼1.55 nm, achieve the best carrier-selectivity producing a contact resistivity ρc of ∼3 mΩ cm2 and a recombination current density J0c of ∼40 fA/cm2. These characteristics are shown to be stable at temperatures up to 350 °C. The MIS(p+) contacts fail to achieve equivalent results both in terms of thermal stability and contact characteristics but may still offer advantages over directly metallized contacts in terms of manufacturing simplicity.

Reduction in Specific Contact Resistivity to $ \hbox{n}^{+}$ Ge Using $\hbox{TiO}_{2}$ Interfacial Layer

We report a metal-insulator-semiconductor (MIS) contact using a TiO2 interfacial layer on highly doped n+ Ge to overcome the problem of metal-Fermi-level pinning on Ge, which results in a large electron barrier height. A specific contact resistivity of 1.3 × 10-6 Ω·cm2 was achieved, which represents a 70× reduction from conventional contacts. For the first time, interfacial layer conductivity is experimentally identified as an important consideration for high-performance MIS contacts. New insights on the mechanism responsible for contact resistance reduction are presented.

Impact of fixed charge on metal-insulator-semiconductor barrier height reduction

Recently, the insertion of ultrathin insulators to form metal-insulator-semiconductor (MIS) contacts has been used extensively to reduce the Schottky barrier height and to shift the Fermi level pinning. In this paper, we investigate the physical non-idealities of the ultrathin insulator in Al/Al2O3/n-GaAs MIS through stoichiometry, density, and bandgap measurements. These structural non-idealities electrically manifest as bulk and interface fixed charges that are found to contribute to the observed barrier height reduction. The effect of fixed charge has not been considered before, and when combined with the previously reported interface dipoles, it provides a more thorough understanding of the MIS contacts.

Novel contact structures for high mobility channel materials

Abstract

T-gate geometric (solution for submicrometer gate length) HEMT: Physical analysis, modeling and implementation as parasitic elements and its usage as dual gate for variable gain amplifiers

In order to achieve superior RF performance, short gate length is required for the compound semiconductor field effect transistors, but the limitation in lithography for submicrometer gate lengths leads to the formation of various metal–insulator geometries like T-gate [Sandeep R. Bahl, Jesus A. del Alamo, Physics of breakdown in InAlAs/ n +-InGaAs heterostructure field-effect transistors, IEEE Trans. Electron Devices 41 (12) (1994) 2268–2275]. These geometries are the combination of various Metal-Semiconductor (MS)/Metal–Air-Semiconductor (MAS) contacts. Moreover, field plates [S. Karmalkar, M.S. Shur, G. Simin, M. Asif Khan, Field-plate engineering for HFETs, IEEE Trans. Electron Devices 52 (2005) 2534–2540] are also being fabricated these days, mainly at the drain end ( Γ -gate) having Metal–Insulator-Semiconductor (MIS) instead of MAS contact with the intention of increasing the breakdown voltage of the device. To realize the effect of upper gate electrode in the T-gate structure and field plates, an analytical model has been proposed in the present article by dividing the whole structure into MS/MIS contact regions, applying current continuity among them and solving iteratively. The model proposed for Metal–Insulator Semiconductor High Electron Mobility Transistor (MISHEMT) [R. Gupta, S.K. Aggarwal, M. Gupta, R.S. Gupta, Analytical model for metal insulator semiconductor high electron mobility transistor (MISHEMT) for its high frequency and high power applications, J. Semicond. Technol. Sci. 6 (3) (2006) 189–198], is equally applicable to High Electron Mobility Transistors (HEMT) and has been used to formulate this model. In this paper, various structures and geometries have been compared to anticipate the need of T-gate modeling. The effect of MIS contacts has been implemented as parasitic resistance and capacitance and has also been studied to control the middle conventional gate as in dual gate technology by applying separate voltages across it. The results obtained using the proposed analytical scheme has been compared with simulated and experimental results, to prove the validity of our model.

Analysis and circuit model of a multilayer semiconductor slow-wave microstrip line

An analytical single-layer reduction quasi-static formulation to accurately compute all the line parameters of metal insulator semiconductor (MIS) and Schottky contact multilayer slow-wave microstrip lines is presented. It is valid for a wide range of parameters and its validity is compared with the full-wave spectral domain analysis technique. We also obtain a circuit model, which is able to accurately explain the experimental results, including dispersion at the lower end of the frequency range, for both the MIS and Schottky contact microstrip lines. Useful data to design passive components based on these lines are also presented.

High-efficiency OECO Czochralski-silicon solar cells for mass production

Abstract To improve the economy of photovoltaics, efficiencies of solar cells have to be drastically increased without using complex technologies. This work demonstrates that with the obliquely evaporated contact metal-insulator-semiconductor (MIS)-n+p solar cell structure recently developed at ISFH efficiencies exceeding 21% can be obtained using only four simple fabrication steps: (i) mechanical surface grooving, (ii) P-diffusion, (iii) oblique vacuum evaporation of Al, and (iv) plasma silicon nitride deposition. Cell design and processing sequences are outlined together with the importance of MIS contacts as both low-cost and high efficiency features. The custom-made pilot line equipment for mass production of 20% efficient 10×10 cm2 Cz silicon solar cells including Ga doped wafers is described.