Semiconductor Contacts Research Articles

Design of Semiconductor Contact Grating Terahertz Source with Enhanced Diffraction Efficiency

We report a semiconductor contact grating terahertz source design based on a rectangular profile for phase-matched terahertz generation in the long infrared pump wavelength range. The calculations show that the best diffraction efficiency can be achieved by a filling factor significantly smaller than 50%. Furthermore, the possibility of diffraction efficiency enhancement was investigated by applying three different antireflective coating structures. Numerical simulations have indicated that at 2.06 μm and 3.0 μm pump wavelength, diffraction efficiencies greater than 91% and 89% can be achieved by adding an appropriate antireflective coating to the GaP and GaAs contact grating structure, respectively. In addition, numerical simulations were performed to investigate the influence of wall angles on diffraction efficiency. The results reveal that the wall angle does not significantly affect the diffraction efficiency: while keeping the wall angle deviation from the vertical below 25 degrees, the efficiency drop remains below 5% for otherwise optimal grating parameters.

Physical mechanisms of the influence of γ-ray surface treatment on the characteristics of close AuNi/n–n+-GaN Schottky contacts

The results obtained here suggest that low-dose 60Co γ-irradiation (Dγ ∼ 140 Gy) has a complex effect on close AuNi/n–n+-GaN{0001} Schottky contacts. This manifests in the disappearance of current steps in the initial section of the forward current–voltage curve, improvement in the average values of the ideality factor n, a decrease in the average values of the true Schottky barrier height ϕ bn in the middle section and an increase in series resistance R S and enhancement of the inhomogeneous metal–semiconductor contact series resistance effect in the final section. In all cases, the observed changes are sustainable. A combination of the Zur–McGill–Smith close Schottky contact defect model, the inhomogeneous contact model and the radiation-induced defect formation model provides an explanation for the physical mechanisms of changes observed in electrophysical and instrumental characteristics after γ-irradiation. Such mechanisms are associated with changes in the electrophysical nature of GaN structural defects (dislocations and interface states) and degradation of the homogeneity of contact conductivity. This paper shows that the low-temperature anomaly also manifests itself in close AuNi/n–n+-GaN Schottky contacts subjected to γ-irradiation.

Multiscale modeling of semimetal contact to two-dimensional transition metal dichalcogenide semiconductor

A multiscale simulation approach is developed to simulate the contact transport properties between semimetal and a monolayer two-dimensional transition metal dichalcogenide (TMDC) semiconductor. The results elucidate the mechanisms for low contact resistance between semimetal and TMDC semiconductor contacts from a quantum transport perspective. The simulation results compare favorably with recent experiments. Furthermore, the results show that the contact resistance of a bismuth-MoS2 contact can be further reduced by engineering the dielectric environment and doping the TMDC material to <100 Ω·μm. The quantum transport simulation indicates the possibility to achieve an ultrashort contact transfer length of ∼1 nm, which can allow aggressive scaling of the contact size.

Negative Schottky barrier height and surface inhomogeneity in n-silicon M–I–S structures

The alleviation effect on the Schottky barrier height (SBH) (ΦB) using ultrathin titanium dioxide and hafnium dioxide dielectrics in a single layer and a bilayer stack was demonstrated. ΦB in the Pt/n-Si contact was reduced from 0.53 to −0.058, 0.3, and −0.12 eV using 3 nm TiO2, 1 nm HfO2, and high-k/high-k bilayer insertion, respectively. A maximum of 122% reduction in ΦB was obtained using bilayer dielectric insertion, which is the highest ever reduction reported so far in a Schottky diode. This was achieved by effectively passivating the semiconductor surface states by HF cleaning followed by inserting an ultrathin film produced from the novel Atomic Layer Deposition (ALD) technique. The Gaussian distribution (GD) of barrier heights all over the interface has been investigated for both Metal–Semiconductor (M–S) and Metal–Insulator–Semiconductor (M–I–S) contacts. The nonlinear behavior in a conventional Richardson plot was observed with lower values of the Richardson constant (A*). We have reported the surface inhomogeneity in both M–S and M–I–S contacts through temperature dependency of diode characteristics. The standard deviation (σ) as evidence for the Gaussian distribution of barrier heights was determined using the ln(Is/T2) vs q/2kT plot. The results were validated by a modified Richardson plot where the values of A* obtained were found to be in close agreement with the known values. As the ALD technique is known for conformity and uniformity of thin films, the dielectric insertion has proved effective in mitigating the SBH. However, the inhomogeneity in both M–S and M–I–S points to the role of dipole formation at the interface.

The light detection performance of the congo red dye in a Schottky type photodiode

The Congo red (CR) (3,3′-[(1,1′,-biphenyl)-4,4′-diyl)bis(4-amino-1 amino naphthalene sulphonic)]) is usually used for staining of amyloidosis diseases in the diagnostic application. It shows dichroic behavior, and thus can be employed in optoelectronic applications. In this study, commercially purchased congo red dye was used as an interfacial organic layer for Schottky type photodiode to understand its sensitivity to light. The surface morphology of the CR interlayer was investigated by scanning electron microscopy (SEM), and almost uniform surface was obtained. While cobalt (Co) element was employed as metallic contact, the n-type Si and aluminum (Al) were used as a semiconductor and ohmic contact, respectively. Thus, Co/CR/n-Si device was fabricated by a spin coating and thermal evaporation technique, and characterized by I-V measurements under dark and various light power intensities. The results revealed that the congo red dye can be improved and employed for optoelectronic applications.

GaAs thermophotovoltaic patterned dielectric back contact devices with improved sub-bandgap reflectance

We demonstrate GaAs thermophotovoltaic (TPV) devices with a patterned dielectric back contact (PDBC) architecture, featuring a dielectric spacer between the semiconductor and back metal contact over most of the back surface for high reflectance, and metal point contacts over a smaller area for electrical conduction. In the TPV application, high sub-bandgap reflectance is needed to reflect unused sub-bandgap photons to the thermal emitter to minimize energy losses in this portion of the thermal spectrum. We explore different PDBC fabrication processes with SU-8 and SiO 2 dielectric spacer layers to maximize sub-bandgap reflectance while minimizing series resistance to increase TPV conversion efficiency. We successfully demonstrate GaAs SU-8 PDBC TPV devices with 2200 °C blackbody-weighted sub-bandgap reflectance of 94.9% and 96.5% with and without a front metal grid, respectively. This is 0.7% and 2.3% (absolute) higher than the mean sub-bandgap reflectance of 94.2% for GaAs baseline TPV devices with 100% Au back contact with front metal grid. Lower sub-bandgap reflectance in TPV devices with front grids indicates the front grid induces light scattering leading to additional parasitic absorption in the TPV device. We also show that for higher contact coverage fractions, the PDBC reflectance cannot in general be treated by a linear interpolation using simple 1D transfer matrix method modeling and should be treated instead as a diffraction grating by solving Maxwell's equations in 3D. • We explore different patterned dielectric back contact (PDBC) fabrication processes with SU-8 and SiO 2 for TPV cells. • Sub-bandgap reflectance over 96% demonstrated using dielectric-metal point contact rear mirror structure, a gain of >1%. • PDBC structures should be modeled as a 3-dimensional diffraction grating by solving Maxwell's equations. • Front grids increase diffuse reflectance thus increasing absorption in the device.

All‐van‐der‐Waals Barrier‐Free Contacts for High‐Mobility Transistors

Ultrathin two-dimensional (2D) semiconductor devices are considered with beyond-silicon potential but are severely troubled by high Schottky barriers of metal-semiconductor contacts, especially for p-type semiconductors. Since the severe Fermi-level pinning effect and the lack of conventional semimetals with high work functions, their Schottky hole barriers are hardly removed. Here, we firstly report an all-van-der-Waals barrier-free hole contact between p-type tellurene semiconductor and layered 1T'-WS2 semimetal which achieve a zero Schottky barrier height of 3±9meV and the high field-effect mobility of ∼1304 cm2 V-1 s-1 . The formation of such contacts can be attributed to the higher work function of ∼4.95eV of the 1T'-WS2 semimetal, which is in sharp contrast with low work function (4.1-4.7eV) of conventional semimetals. Our study defines an available strategy for eliminating the Schottky barrier of metal-semiconductor contacts, facilitating 2D semiconductor-based electronics and optoelectronics to extend Moore's law. This article is protected by copyright. All rights reserved.

Dynamics of the transition resistance of Al–(Ti, Ni, Mo)–Si type contacts under conditions of non-stationary electrical loads

This paper analyses the behavioral features of ohmic contacts under the conditions of traditional isothermal annealing. The purpose of this paper is to study the value change of the semiconductor contact resistance when applying electric current pulses of different powers. For the experiments, structures were formed of metal-sublayer-semiconductor wafer Al–(Ti, Ni, Mo)–Si. The quality control of the deposited films was carried out microscopically by the four-probe method. The presented results indicate the solid-phase interaction of components during diffusion annealing Si–Me–Al thin-film systems. This rearrangement of atoms, which occurs with grain-boundary diffusion, significantly affects the electrical and thermal characteristics of the formed contacts. The novelty of the paper is that the optimal parameters of a rectangular current pulse were selected, which made it possible to reduce the value of the transition resistance by 1.6 times.

Benchmarking contact quality in N-type organic thin film transistors through an improved virtual-source emission-diffusion model

Due to nonideal behavior, current organic thin film transistor technologies lack the proper models for essential characterization and thus suffer from a poorly estimated parameter extraction critical for circuit design and integration. Organic thin film transistors are often plagued by contact resistance, which is often less problematic in inorganic transistors; consequently, common models used for describing inorganic devices do not properly work with organic thin film transistors. In this work, we fabricate poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} based organic thin film transistors with reduced contact resistance through the introduction of metallic interlayers between the semiconductor and gold contacts. The addition of 10 nm thick manganese interlayer provides optimal organic thin film transistor device performance with the lowest level of contact resistance. Improved organic thin film transistors were characterized using an improved organic virtual-source emission diffusion model, which provides a simple and effective method to extract the critical device parameters. The organic virtual-source emission diffusion model led to nearly perfect prediction using effective gate voltages and a gate dependant contact resistance, providing a significant improvement over common metal–oxide–semiconductor field-effect transistor models such as the Shichman–Hodges model.

Controlling Catalyst–Semiconductor Contacts: Interfacial Charge Separation in p-InP Photocathodes

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry, but controlling their properties is challenging. Using thin films and nanoparticle arrays of Pt hydrogen-evolution catalysts on p-InP (a high-performance photocathode material), along with macroscopic and nanoscopic electrical and chemical analysis, we show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. The new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

Electron Irradiation of Titanium-Doped Chromium Nanostructured Thin Films for Higher Conductive Electrodes

The low-energy electron irradiation improves the electrical properties of the Ti-doped chromium thin films obtained by Laser-assisted Thermionic Vacuum Arc (LTVA). The irradiation doses, between 0.6 C/m2 and 326.4 C/m2, operate as a nanozonal melting effect, decreasing the metallic alloy's roughness, and increasing the electrical conductivity at a depth up to 8 nm. Therefore, the grain sizes of the surface nanostructures are increased after irradiation, but the surface roughness is substantially improved. By tailoring the surface parameters like crystallinity and roughness of the metallic thin films used as electrodes in the OLED technologies, a reduction of the Schottky contacts between the metal and semiconductor contact is expected. This fact minimizes the contact resistance between the metallic and semiconductor thin films and increases the charge injection across the OLED sandwich structures

Effects of Carbon Incorporation on Electrical Characteristics and Thermal Stability of Ti/TiO2/n-Ge MIS Contact

The effects of carbon incorporation on the thermal stability of the interfacial TiO₂ layer and the electrical characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were investigated. The improved thermal stability and contact characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were characterized in terms of Schottky barrier height (SBH) and specific contact resistivity (&#x03C1;_<i>c</i>) using the Schottky diode and circular transmission line model (CTLM). The values of SBH and &#x03C1;_<i>c</i> increased after the rapid thermal annealing (RTA) above 550 &#x00B0;C. The current density&#x2013;bias voltage (<i>J&#x2013;V</i>) curves of the Schottky diode showed a change of contact characteristics from Ohmic-like behavior to rectifying. This thermal instability was mainly caused by the decomposition of the interfacial TiO₂ layer after high-temperature annealing. The structural degradation was confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. When carbon ions were incorporated into the interfacial TiO₂ layer, the SBH and &#x03C1;_<i>c</i> values showed relatively stable characteristics as the RTA temperature increased up to 600 &#x00B0;C. The EELS mapping showed that the diffusion of oxygen from the interfacial TiO₂ layer was effectively suppressed thanks to the incorporation of carbon. Thus, the carbon incorporation can improve the thermal stability of the interfacial TiO₂ layer and the metal&#x2013;insulator&#x2013;semiconductor contact characteristics for Ge-based device applications.

Modulating the Schottky barrier of MXenes/2D SiC contacts via functional groups and biaxial strain: a first-principles study.

Two-dimensional (2D) graphene-like SiC has attracted intense interest recently due to its unique electrical and physical properties. In implementing 2D semiconductors in device applications, one of the main challenges so far has been the formation of a high-quality Schottky barrier owing to the strong Fermi level pinning (FLP) at the interface of traditional metal-2D semiconductor contacts. In this paper, the 2D MXenes Ti3C2T2 (T = F, O, OH) are proposed to serve as electrodes for 2D SiC. The structural and barrier properties of the Ti3C2T2/SiC contacts were systematically investigated based on first-principles calculations combined with the GGA-PBE and HSE06 functionals. It is found that Ti3C2T2 can be bonded with 2D SiC by van der Waals (vdW) interactions. Weak FLP is exhibited at Ti3C2T2/SiC vdW contacts. The type of contact can be tuned by changing the functional T group of Ti3C2T2. Ti3C2F2/SiC and Ti3C2O2/SiC contacts exhibit a p-type Schottky contact and p-type Ohmic contact, respectively, whereas an n-type Ohmic contact occurs in the Ti3C2(OH)2/SiC contact. In addition, the calculated tunneling possibility (TB) is ∼20% between Ti3C2T2 and SiC, indicating weak bonding at the Ti3C2T2/SiC vdW junctions. Furthermore, the Schottky barrier height and TB of the Ti3C2(OH)2/SiC contacts can be modulated via the biaxial strain. The controllable contact type and barrier in Ti3C2T2/SiC contacts provide guidelines for developing high-performance 2D SiC optoelectronic and electronic devices.

Sliding silicon-based Schottky diodes: Maximizing triboelectricity with surface chemistry

Triboelectric nanogenerators are an emerging energy technology which harvests electricity from mechanical energy. Within this technology there are sliding metal–semiconductor contacts, which can be miniaturized, and having a direct current (DC) output are suitable as autonomous power sources for electronic devices. Herein we explore the scope of engineering the surface chemistry of silicon towards maximizing the output of a Pt–Si Schottky diode-based triboelectric nanogenerator. Through the attachment of covalent Si–C-bound organic monolayers we have engineered silicon surface chemistry to systematically tune friction, wettability and surface work function, with the overall aim of clarifying the interplay between mechanical and electronic properties defining the DC output of a zero-bias sliding Schottky diode. Current outputs increase two-fold in amine- and alcohol-terminated monolayers compared to shorter and carbon-terminated films. This trend parallels the change in friction measured in response to surface functionalization. A pronounced effect of silicon doping on friction and current was revealed by atomic force microscopy, indicating a link between doping and friction, even at zero applied bias. This work reveals an electrical component of friction by demonstrating a friction excess in response to the flow of current, and it opens up novel avenues into the use of silicon, and its surface chemistry, as platform for triboelectric nanogenerators.

Reliability Evaluation of Photovoltaic Modules Fabricated from Treated Solar Cells by Laser‐Enhanced Contact Optimization Process

According to the International Technology Roadmap for Photovoltaics, passivated emitter and rear solar cells dominate the market in 2021 of up to 80% and are forecast to remain state of the art at least for the next 5 years. Within the production process of solar cells, it is typical to have cells with lower efficiency grades due to variations in manufacturing processes or material defects. Reprocessing such solar cells could save cost due to increased production yield and simultaneously reduced cost for recycling of unusable/unsellable low‐efficiency cells. Herein, the impact of the laser‐enhanced contact optimization (LECO) process on the power output and reliability of solar modules using commercial off‐spec cells of different manufacturers is analyzed. LECO is a downstream process for optimizing metal−semiconductor contacts on finished solar cells. The treatment leads to a significant economic gain due to enhanced cell efficiency (Wp ↑, therefore manufacturing cost per Wp ↓) even of already good solar cells. Herein, the first evaluation of the impact of the LECO process on the cell output power on an industrial scale (&gt;1000 cells) and on module reliability is presented. The results for common short‐term effects like light‐induced degradation and light‐ and elevated temperature‐induced degradation are within expected limits and the durability against, for example, potential‐induced degradation is not changed due to the LECO process. The results further show that cell sorting is crucial for a reliable module and to avoid outliers in terms of unexpected degradation and recovery phenomena of individual cells.

Photo-e.m.f. at a metal/layered n-InSe semiconductor contact under heating conditions of current carriers by an electric field

The features of the photo-e.m.f. are experimentally studied on the metal/n-InSe contact under conditions of heating the current carriers by an electric field in the temperature range of T0 =77÷350 K. The dependences of the photo-e.m.f. (Uph ) value have been measured in the absence of and under the condition of heating the current carriers , as well as the value of from the wavelength (λ) and light intensity (I), heating electric field strength (Ê), time (τ), temperature (T0 ), the initial value of the dark resistivity (ρD0 at 77K) n-InSe. It has been established that the heating of current carriers by an electric field significantly affects the magnitude and behavior of the photo-e.m.f. characteristics on the metal/n-InSe contact. The nature of this effect depends on T0,ρD0, I. The value of significantly exceeds the value of . With an increase in Ê, the value of AUph increases linearly (ΔUph~Ê) at relatively small Ê, and the dependence of ΔUph (Ê) reaches saturation at higher Ê. The value of ΔUph decreases with increasing ρD0 , at relatively small Ê. With an increase in ρD0 , the relaxation time of the photo-e.m.f. also increases when turned off the pulse of the electric field. The obtained experimental results are explained on the basis of the dependence of the photo-e.m.f. on the metal/n-InSe contact on the effective temperature of heated current carriers (Te ), considering the effect of the spatial inhomogeneity of n-InSe crystals on the photo-e.m.f. and on the process of heating the current carriers.

Improving Performance of Bifacial‐Grid III–V Solar Cells Bonded on Glass by Selective Contact Annealing

Selective contact annealing is developed to improve the performance of bifacial‐grid III–V top cells bonded on glass by epoxy resin for Si‐based tandem solar cells. The annealing method can minimize the damage to the bonding layer and is compatible with the industrial evaporation technique for contact fabrication. The improvement of the front contact quality by selective annealing treatments is demonstrated through optical and electrical characterization which shows a significantly enhanced III–V device performance. Furthermore, a fitting model is established to reveal the mechanism underlying the performance improvement through the reduction of contact resistances and the bad contact area. Herein, an effective annealing approach to fabricate high‐quality metal–semiconductor contacts for devices with poor thermal resistivity is demonstrated, which can be used in polymer‐bonded top cells in Si‐based tandem solar cells and other flexible optoelectronic devices on polymer substrates.

Performance Limits and Potential of Multilayer Graphene–Tungsten Diselenide Heterostructures

AbstractAtomically thin tungsten diselenide (WSe2) transistors with multilayer graphene (w/ G) as contact electrodes are successfully fabricated by a precise area‐controllable chemical vapor deposition method. The performance and electrical properties of the devices are explored. Compared to those of WSe2 transistors without graphene contacts (w/o G), the maximum current densities of the w/ G–WSe2 transistors increase by one to two orders of magnitude. In addition, the device performance is markedly improved for the w/ G–WSe2 transistors, including an on/off current ratio of ≈107, subthreshold swing of ≈150 mV/decade, and threshold voltage of ≈1.75 V. The improved performance of the w/ G–WSe2 transistors is ascribed to the alleviation of electrical contributions from metal–semiconductor contact resistance, which is consistent with the analysis of low‐frequency noise measurements. In addition, self‐trapping behavior in the WSe2 channel is found, which possibly paves a way to further optimize the performance of layered devices. Finally, an inverter using the w/ G–WSe2 transistors is demonstrated. These transistors represent a step forward in the development of layered graphene‐based electronics suitable for energy‐efficient and high‐performance industrial‐scale products.

Charge Carrier Mobility and Series Resistance Extraction in 2D Field‐Effect Transistors: Toward the Universal Technique

Abstract2D semiconductor field‐effect transistors (2D FETs) have emerged as a promising candidate for beyond‐silicon electronics applications. However, its device performance has often been limited by the metal‐2D semiconductor contact, and the non‐negligible contact resistance (RSD) not only deteriorates the on‐state current but also hinders the direct characterization of the intrinsic properties of 2D semiconductors (e.g., intrinsic charge carrier mobility, μint). Therefore, a proper extraction technique that can independently characterize the metal‐2D semiconductor contact behavior and the intrinsic properties of a 2D semiconducting layer is highly desired. In this study, a universal yet simple method is developed to accurately extract the critical parameters in 2D FETs, including characteristic temperature (To), threshold voltage (VT), RSD, and μint. The practicability of this method is extensively explored by characterizing the temperature‐dependent carrier transport behavior and the strain‐induced band structure modification in 2D semiconductors. Technology computer aided design simulation is subsequently employed to verify the precision of RSD extraction. Furthermore, the universality of the proposed method is validated by successfully implementing the extraction to various 2D semiconductors, including black phosphorus, indium selenide, molybdenum disulfide, rhenium disulfide, and tungsten disulfide with top‐ and bottom‐gated configurations.

Molecular dynamics simulations of ultrafast radiation induced melting at metal–semiconductor interfaces

Metal–semiconductor contacts in silicon carbide (SiC) diodes endure damages at the interface when exposed to harsh radiation environments. Due to the rapid rise in temperature and ultrafast cooling that follows the radiation impact, the structural properties of the materials can be altered through melting, recrystallization, and amorphization. A detailed understanding of the material failure modes at the interface is lacking, specifically at the nanoscale. We use molecular simulations to investigate the ultrafast melting at tungsten (W)–SiC interfaces following radiation damage and apply deep learning techniques to track the transient evolution of the local molecular structures. We show that W near the radiation track undergoes melting and, eventually, most of it recrystallizes with a noticeable degree of undercooling, while SiC is rendered permanently amorphous. The observation of local undercooling in W films is important as it can affect the device performance even before the bulk melting temperature of the material is reached. We also show that at high temperatures, the interface undergoes a fracture-like failure. The results presented here are significant in understating the different failure modes of SiC diode materials.