Doped Contacts Research Articles

Role of electrostatic doping on the resistance of metal and two-dimensional materials edge contacts

In this theoretical study, we compare electrostatically doped metal-transition metal dichalcogenide (TMD) edge-contacts versus substitutionally doped edge-contacts in terms of their contact resistance. Our approach involves the utilization of electrostatic doping achieved by applying back-gate bias to the metal-TMD edge contacts, where carrier injection is primarily governed by the Schottky barrier at the interface. To analyze these contacts, we employ the Wentzel-Kramers-Brillouin (WKB) approximation to calculate the transmission coefficient and use density functional theory (DFT)-derived band structures. We numerically solve the Poisson equation to capture the electrostatic potential. We also account for the impact of the image force using Green's function for the Poisson equation with boundary conditions appropriate to our specific geometry. Our findings reveal that electrostatically doped TMD edge contacts exhibit higher contact resistance compared to impurity-doped edge contacts at equivalent carrier concentrations. At the same time, we find that, among the electrostatically doped edge contacts, a low-κ back-gate oxide in conjunction with low-κ top oxide is preferable in terms of improvement in contact resistance. For instance, in a metal-TMD edge contact scenario involving a monolayer MoS2 as the channel, SiO2 as the infinitely thick top oxide, and a SiO2 back-gate oxide with an equivalent oxide thickness (EOT) of 1nm, we demonstrate that it is possible to achieve an impressively low contact resistance of 50Ωµm when the back-gate bias exceeds or equals 2 V. Published by the American Physical Society 2024

Flexible silicon solar cells with high power-to-weight ratios.

Silicon solar cells are a mainstay of commercialized photovoltaics, and further improving the power conversion efficiency of large-area and flexible cells remains an important research objective1,2. Here we report a combined approach to improving the power conversion efficiency of silicon heterojunction solar cells, while at the same time rendering them flexible. We use low-damage continuous-plasma chemical vapour deposition to prevent epitaxy, self-restoring nanocrystalline sowing and vertical growth to develop doped contacts, and contact-free laser transfer printing to deposit low-shading grid lines. High-performance cells of various thicknesses (55-130 μm) are fabricated, with certified efficiencies of 26.06% (57 μm), 26.19% (74 μm), 26.50% (84 μm), 26.56% (106 μm) and 26.81% (125 μm). The wafer thinning not only lowers the weight andcost, but also facilitatesthecharge migration and separation.It is found thatthe 57-μm flexible and thin solarcell shows the highest power-to-weight ratio (1.9 W g-1) and open-circuit voltage (761 mV)compared to the thick ones. All of the solar cells characterized have an area of 274.4 cm2, and the cell components ensure reliability in potential-induced degradation and light-induced degradation ageingtests. This technological progress provides a practical basis for the commercialization of flexible, lightweight, low-cost and highly efficient solar cells, and the ability to bend or roll up crystalline silicon solar cells for travel is anticipated.

Ferroelectric‐Gated HfZrO2/AlGaN/GaN High‐Electron‐Mobility Transistors with Regrown Contacts for Radio Frequency and Millimeter‐Wave Switch Applications

Emerging millimeter‐wave wireless communication systems require high‐performance radio frequency switches to support advanced functionality such as frequency agility, circuit reconfigurability, and beamforming. Ferroelectric (FE)‐gated AlGaN/GaN high‐electron‐mobility transistors (HEMTs) (FeHEMTs) are developed and demonstrated as a technology option for this important role. AlGaN/GaN HEMT heterostructures grown by metal‐organic chemical vapor deposition are augmented with a FE gate stack comprising atomic layer deposition‐grown HfZrO2. In addition, highly doped regrown source and drain Ohmic contacts are integrated into an improved fabrication process flow. This improved process leads to contact resistances of 0.105 Ω mm and a reduction in switch on‐resistance as compared to prior reports, as well as drain current densities of 1 A mm−1 and measured ft = 54 GHz for gate lengths of 0.1 μm. The improved contact and gate length scaling makes FeHEMTs a promising candidate for mm‐wave switch applications achieving a reported switch figure of merit of fCO = 1.55 THz.

Exchange current density model for the contact-determined current-voltage behavior of solar cells

An analytic expression for the current–voltage [J(V)] behavior of a solar cell as limited by equilibrium exchange current densities of both carriers at both contacts is derived. The partial currents at both contacts to a generic semiconductor absorber are assumed to be linearly proportional to the excess carrier concentration at the interface with the contacts (e.g., as with Schottky-like contacts). The assumption that the quasi-Fermi levels in the absorber are approximately flat leads to an algebraic solution for the applied voltage as a function of current, which is inverted to obtain the analytic J(V) curve. The J(V) curve reveals distinct behavior associated with electrons and holes, separately, and allows for the determination of all critical performance parameters. In particular, it demonstrates how the characteristic features of the J(V) curve depend on the relative rate at which a particular carrier (electron or hole) is collected at one contact vs the other, rather than the relative rate of electron vs hole collection at a single contact. Furthermore, the model provides a unified explanation of how majority carrier extraction limitations cause nonideal J(V) behaviors such as S-shaped curves and dark/light crossover (i.e., failure of superposition). The efficacy and limitations of the model when applied to Schottky-type and doped semiconductor contacts are discussed. The work serves as a theoretical guide to scientists studying solar cells that are thought to be primarily limited by their contacts.

Effect of Lubricant Formulations on the Tribological Performance of Self-Mated Doped DLC Contacts: a review

Diamond-like carbon (DLC) coatings have emerged as one of the most promising surface coatings for applications involving boundary lubrication regime. Some of the characteristics that distinguish DLC coatings from other hard coatings include high hardness, low friction coefficient, wear resistance, and chemical inertness. Because of their low surface energies, these coatings cannot react effectively with different lubricant constituents. Doping of DLC coatings with metals and nonmetals, such as titanium, tungsten, silicon, chromium, fluorine, and nitrogen, can improve their interaction with lubricants to form tribologically beneficial films. Many experimental studies have been conducted on different types of DLC coatings using various lubricant formulations in the last few decades. The results obtained from these experimental studies are very scattered and contradictory, so the data should be consolidated in a more organized and apprehensible manner. By doing so, tribological behavior of various DLC–lubricant combinations can be understood in a better way, and a more logical continuation of research on DLC coatings can be carried out. In this review paper, most widely investigated metal- and nonmetal-doped DLC coatings, such as Ti-DLC, W-DLC, Si-DLC, Cr-DLC, WC-DLC, and multilayered a-C:H/W-DLC coatings, are considered for evaluation. Tribological performance of the aforementioned DLC coatings, in combination with various base oils and lubricant additives, is analyzed by comparing their average friction and wear coefficient values, which have been calculated from published experimental data. Only self-mated doped DLC contacts are considered in this study to eradicate the interference of ferrous and nonferrous counterbodies.

Contact-dependent performance variability of monolayer MoS2 field-effect transistors

Using self-consistent quantum transport simulations, we investigate the performance variability of monolayer molybdenum disulfide (MoS2) field-effect transistors (FETs) with various contact properties. Varying the Schottky barrier in MoS2 FETs affects the output characteristics more significantly than the transfer characteristics. If doped contacts are realized, the performance variation due to non-ideal contacts becomes negligible; otherwise, channel doping can effectively suppress the performance variability in metal-contact devices. Our scaling study also reveals that for sub-10-nm channels, doped-contact devices can be more robust in terms of switching, while metal-contact MoS2 FETs can undergo the smaller penalty in output conductance.

Rear Contact Pattern Optimization based on 3D Simulations for IBC Solar Cells with Point-like Doped Contacts

In this work 3D simulations are used to study the impact of technological parameters on device performance of c-Si interdigitated back-contacted IBC solar cells with point-like doped contacts. In these cells, the highly-doped regions are defined in a point-like structure instead of the more classical arrangement of fully doped fingers. Numerical simulations allow us to optimize rear contact geometry, i.e. optimum pitch between contacts, depending on the substrate resistivity and the passivation quality of base contacts. Results show a trade-off between recombination and base resistive losses demonstrating limit efficiencies over 27% for a perfectly passivated structure on p- and n-type substrates. More realistic devices where state-of-the-art surface passivation is considered reach efficiencies beyond 22% on 2 ± 1Ωcm substrates with optimum pitch values of 200 ± 50μm.

Hydrodynamic study of terahertz three-dimensional plasma resonances in InGaAs diodes

Using a hydrodynamic model self-consistently coupled to a Poisson solver, we investigate the time and frequency response of InGaAs diodes excited at room temperature by an optical photoexcitation presenting a beating in the terahertz frequency domain. The analysis of the main physical quantities, such as the local electric field and the conduction current density, evidences the presence of strong resonances that are interpreted as three-dimensional plasma oscillations excited by the optical beating. By studying the influence of the geometry and doping of the diode, it is shown that, in most cases, the highly doped contacts mainly control the frequency of the plasma mode while the diode length is a crucial parameter to evidence a second resonance related to the diode active region. Moreover, the amplitude of the plasma resonances can be enhanced at high doping levels and by increasing the level of the optical photoexcitation.

Performance comparison of zero-Schottky-barrier and doped contacts carbon nanotube transistors with strain applied

Abstract Atomistic quantum simulation is performed to compare the performance of zero-Schottky-barrier and doped source-drain contacts carbon nanotube field effect transistors (CNTFETs) with strain applied. The doped source-drain contact CNTFETs outperform the Schottky contact devices with and without strain applied. The off-state current in both types of contact is similar with and without strain applied. This is because both types of contact offer very similar potential barrier in off-state. However, the on-state current in doped contact devices is much higher due to better modulation of on-state potential profile, and its variation with strain is sensitive to the device contact type. The on/off current ratio and the inverse subthreshold slope are better with doped source-drain contact, and their variations with strain are relatively less sensitive to the device contact type. The channel transconductance and device switching performance are much better with doped source-drain contact, and their variations with strain are sensitive to device contact type.

Gate dielectric scaling of top gate carbon nanoribbon on insulator transistors

The effects of gate dielectric constant and thickness on the performance of top gate carbon nanoribbon on insulator transistors are studied using a π-orbital quantum simulation model. The focus is on both the zero Schottky barrier (SB) source-drain contacts and the metal-oxide-semiconductor field effect transistor (MOSFET)-like doped source-drain contacts. The gate dielectric constant has little effect on the on/off current ratio, channel transconductance gm, and switching performance in SB contact devices. However, the on/off current ratio, the channel transconductance, and the switching performance significantly improve with high-K gate dielectric in doped contact devices. The physics is related to the modulation of the tunnel barrier. In SB contact devices, the on-state current is limited by the SB, and therefore, the improvement in on/off current ratio and transconductance is insignificant. However, the tunnel barrier modulation in the subthreshold regime is similar in both types of contact and the inverse subthreshold slope has similar improvement with high-K gate dielectrics. The unity current gain frequency (fT=gm/2πCg) degrades with high-K gate dielectric in SB contact devices and improves in doped contact devices. This is because the gate capacitance does not change much with dielectric constant and gm has a significant improvement in doped contact devices. The device performance improves with thinner gate oxide. The on/off current ratio and the inverse subthreshold slope scale as square root of oxide thickness.

Using a four-probe scanning tunneling microscope to characterize phosphorus doped ohmic contacts for atomic scale devices in silicon

We use a four-probe scanning tunneling microscope to electrically characterize a Si(1 0 0) 2 × 1 surface, doped with phosphorus. In order to obtain the 2 × 1 surface reconstruction, the sample is flashed to a temperature of ∼ 1100 ∘ C , which is well above the phosphorus desorption temperature. Nonetheless, we find that we are able to obtain good ohmic contact and perform reproducible measurements of the sheet resistivity at the surface. This suggests that phosphorus doped regions may act as macroscopic ohmic contacts to atomic scale devices for in situ four-terminal measurements using a four-probe, UHV STM system.

In situ Ni–Si nanowire junction based on substrate sourced growth and its electrical transport behavior

The authors report on formation of high current carrying Ni–Si nanowires between Ni/heavily doped polycrstaliine Si contacts using substrate sourced growth without any supply of Si source gas. The conductance (G) of a 18nm diameter nanowire bridge in the temperature range of 13–293K showed a power function of temperature Tα, with a typical exponent α≈0.62±0.06, and the dI∕dV were fitted to the Vα with α≈0.55±0.15. Although the power law dependence of the G and dI∕dV, which provides evidence for the interaction of the electrons in a low-dimensional infinite conductor, corresponds to the featured behavior of the interacting electrons within an error tolerance, the strength of the interaction is very weak due to the highly metallic characteristics with finite length and lack of purity of the channel. As a result, the in situ high current carrying Ni-Si nanowire junction can be utilized not only as a nanointerconnector, but also as a tool to study low-dimensional electrical transport properties.

Photorefractive p-i-n diode quantum well spatial light modulators

We demonstrate the performance of all-semiconductor photorefractive p-i-n diodes operating in the longitudinal quantum-confined Stark geometry. Low-temperature-grown shallow quantum wells provide high-mobility vertical transport, and potential steps incorporated into the semiconductor buffer layers increase the transit time across the buffer and therefore increase the quantum efficiency for trapping of charge before it is swept out to the doped p-type and n-type contacts. The buffer design and the doped contacts both make all-semiconductor photorefractive devices possible, with peak transient output diffraction efficiencies approaching 3%, but without the need for dielectric insulating layers. We also redefine device speed by making a distinction between transient rise times and frequency response, showing that in these p-i-n devices the update rate is an order of magnitude slower than the inverse rise time.

Fabrication and performance of thin film transistors, TFTs, incorporating doped μc-Si source and drain contacts, and boron-compensated μc-Si channel layers

PH3 doped n+ μc-Si, and lightly B2H6 doped intrinsic μc-Si, iμc-Si, thin films have been integrated into bottom-gate TFTs. The use of n+ μc-Si as a source/drain contact material in a-Si:H TFTs reduces the threshold voltage compared to n+ a-Si:H contacts. The use of iμc-Si as the TFT channel material, combined with a post-deposition, back-channel exposure to atomic-H yielded low-temperature processed TFTs with effective channel mobilities of ∼6.5 cm2/V-s.

Electrostatic potential of impurities in quantum wells

The method of images is used to calculate the electrostatic potential of an electric charge inside a double barrier structure. This potential is important for finding impurity energy levels in the quantum structure and has never been used before in exact form. The media were modeled by assigning different dielectric constants to the barriers and to the well. The highly doped contacts were assumed to have infinite dielectric constants. This method is based on calculating the Fourier transform parallel to the interfaces of the potential. This function can be obtained in closed form. The potential is then obtained by inverse Fourier transform. This integral is computationally more efficient to evaluate than the direct sum of Coulomb image terms. >

The use of laser doped Li contacts on semiconductor nuclear radiation detectors

Ultra-thin (<0.1 μm), highly doped Li contacts have been fabricated on n- and p-type Ge and Si nuclear radiation detectors by the Q-switched ruby laser melting technique. Fwhm resolutions for 5.48 MeV α-particles of 17.4 keV on 2 kΩ · cm p-type Si, 21.7 keV on 300 Ω · cm p-type Si and 15.3 keV on p-type Ge ( N A - N D = 1.7 × 10 10 cm −3) were obtained. Best resolutions for 662 keV 137Cs γ-rays were 2.3 keV for n-type Ge ( N D - N A = 1 × 10 10 cm ⇔3) and 4.6 keV (pulser resolution 3.4 keV) for p-type Ge ( N A - N D = 6 × 10 10 cm −3). Used as an ohmic contact on a 90 kΩ · cm, n-type Si X-ray detector at 80 K the contact allows operation at 1300 V (depletion at 235 V). Crystal heating data and deep level transient spectroscopy results show significant laser damage in the contacts fabricated on Ge but far less damage on Si. Owing to the narrow fluence range, careful control of the homogenized laser output energy is necessary for good fabrication yields.

Ultra-thin laser-aided doped Li contacts on high-purity germanium nuclear radiation detectors

A Q-switched ruby laser pulse ( λ = 0.6943 μm, 25 ns fwhm) was used to melt vacuum-evaporated Li layers on high purity [ p = (1−6) × 10 10 cm −3] Ge for nuclear radiation detectors. The resulting higly doped contacts were extremely thin (⩽0.1 μm), and stable at room temperature. Full width half maximum (fwhm) resolutions of 15.3 keV for 5.48 MeV 241Am α-particles and 3.1 keV for 662 keV 137Cs γ-rays were obtained and show these contacts may be suitable for many detector applications.

Scribe‐Plating and ‐Etching

A technique is described which permits the fabrication of miniature and microminiature ohmic and doped contacts in complex configurations on a variety of semiconductor materials. This technique involves the preferential nucleation of chemiplated and electroplated metals on microscopically fine lines scribed on semiconductor surfaces and the preferential etching of similarly scribed lines. Lines of the order of 10−4 cm have been plated with a variety of metals including doping elements. Scribe plating has yielded p‐n junctions and offers promise of the fabrication of p‐n junction devices considerably smaller than those currently available. This technique is also potentially useful in the fabrication of conventionally sized devices, some of which are described. Suggestions are advanced for the use of this technique in both microminiature and other device fabrication.