Doping Concentration Gradient Research Articles

3D Printing of Gradient-Doped Yb:YAG Laser Ceramics by Leveraging Active Mixing

Gradient-doped laser ceramics fabricated via 3D printing have great potential for high-power laser applications. However, it is challenging to produce such types of gain media because it is inconvenient to fabricate different component slurries prepared offline using traditional methods precisely. To address these issues, a facile approach for the additive manufacturing of high-power laser ceramics is proposed for gradient structure fabrication. First, a material-extrusion-based 3D printing device with a homemade active mixing module is developed to fabricate gradient-doped Yb:YAG(Y3Al5O12) laser ceramics using two slurries with different doping concentrations. Subsequently, the active mixing module is systematically investigated to obtain different Yb-doped concentrations of printing slurries with a uniform element distribution. By measuring the switching delay volume between different components, a proper volume adjustment enables the Yb concentration distribution of the green bodies to be consistent with the designed profile in the 3D process. Finally, multi-component (0–5–10–5–0 at% Yb:YAG) green bodies are printed, and experimental tests are conducted to evaluate the performance of these gradient-doped laser ceramics. The results show that the gradient-doped ceramic obtained 82.1% in-line transmittance at 1100 nm (4.6 mm thickness, along with the doping concentration gradient) and the average Yb ions diffusion distance across the interface fitted in the 20–30 μm range. Furthermore, a 1030 nm laser output with an output power of 4.5 W with a slope efficiency of 41.3% is achieved when pumped with a 940 nm laser diode. This study provides useful insights for developing various gradient rare-earth-doped laser ceramics for high-power laser applications.

Performance Evaluation of Junctionless Cylindrical Gate-All-Around FET for Low Power Applications

The advent of device miniaturization techniques and the evolution of very deep submicron technology have led to the increased prominence of short channel effects (SCEs) in conventional transistors (CTs). Now, in the era of nanoengineering and nano-wires, current research is centered around a novel device known as the Junctionless Field Effect Transistor (JLFET), which incorporates gate-all-around engineering applications. Given the challenges associated with scaling transistor sizes, such as creating high-quality junctions and changing doping concentrations (~1019 cm–3) over a 10 nm distance, JLFET emerges as a promising alternative to CTs. Notably, JLFET lacks junctions and doping concentration gradients. In this study, the authors have conducted a comprehensive analysis and performance evaluation of JLFET and CTs, specifically in the context of low-power applications. Various performance parameters of JLFET, including SS, DIBL, transconductance, output conductance, and Ion/Ioff, have been assessed. The findings indicate that JLFET exhibits reduced susceptibility to SCEs compared to CTs and demonstrates exceptional current driving capability.

The effects of source doping concentration and doping gradient on the ON-state current of Si nanowire TFETs

The tunnel field-effect transistor (TFET) is considered a promising next-generation transistor due to its potentially limit-breaking low subthreshold swing and better immunity against short-channel effects. However, the low ON-state current (ION) of TFETs has been a critical problem. In this work, we investigated the effects of the source doping concentration and the source doping gradient (SDG) on the ION of n-type Si gate-all-around (GAA) nanowire (NW) TFETs using an ATLAS device simulator. Unexpectedly, we found that increasing the source doping concentration does not necessarily improve ION, especially for TFETs with a large SDG. Moreover, although reducing the SDG indeed increases ION, for TFETs with low source doping concentration (e.g., 1 × 1019 cm−3), the improvement in ION by reducing the SDG becomes insignificant.

An Alternate Approach to Backside Thinning: A Doping Selective Silicon Wet Etch

Backside illuminated technologies have been used in complementary metal–oxide–semiconductor image sensors for years and are finding a renewed interest and are being investigated for imagers and detectors in light detection and ranging applications. Coupled with the improved performance facilitated through 3D integration, the thinning of bonded wafers has become a crucial processing step. In this study, an alternate and potentially more cost-effective method has been explored using an HNA (hydrofluoric, nitric, acetic)wet etch for backside thinning of bonded epitaxial P+/P- silicon wafers. By utilizing a dopant concentration gradient, P+ silicon is selectively etched and P- silicon can be effectively used as an etch stop. Several HNA concentrations were explored to determine ratios that yield high P+ silicon etch rates while maintaining high P+/P- selectivity. Furthermore, bath life, spiking scheme, and repeatability were examined using a large wafer sample size with the goal of developing a full process flow.

A Novel Step-Doped Channel AlGaN/GaN HEMTs with Improved Breakdown Performance.

The AlGaN/GaN high electron mobility transistor with a step–doped channel (SDC–HEMT) is first proposed in this paper. The potential distribution and the electric field (E–field) distribution of the device are explored by the numerical approach and analytical approach simultaneously. By introducing extra dopants to the channel layer, the E–field distribution along the AlGaN/GaN heterojunction interface is reshaped, resulting in an improved breakdown characteristic. An optimized doping concentration gradient of channel layer of 2 × 1016 cm−3/step is proposed and verified by simulations. The breakdown voltage (BV) of the optimized SDC–HEMT reaches 1486 V with a 59.8% improvement compared with conventional AlGaN/GaN HEMT. In addition, the average E–field in the region between gate and drain improves from 1.5 to 2.5 MV/cm. Based on the equivalent potential method (EPM), an analytical model of the E–field and potential distribution is presented. The veracity and effectiveness of the proposed methodology is verified by the good agreement between the simulated and modeled results.

A Top‐Down Platform Enabling Ge Based Reconfigurable Transistors

AbstractConventional field‐effect transistor (FET) concepts are limited to static electrical functions and demand extraordinarily steep and reproducible doping concentration gradients. Reaching the physical limits of scaling, doping‐free reconfigurable field‐effect transistors (RFETs) capable of dynamically altering the device operation between p‐ or n‐type, even during runtime, are emerging device concepts. In this respect, Ge has been identified as a promising channel material to enable reduction of power consumption and switching delay of RFETs. Nevertheless, its use has been limited to simulations and bottom‐up demonstrators not compatible with complex circuit technology. In this work, a deterministic top‐down fabrication scheme is demonstrated to realize a Ge‐based RFET architecture and exploring realizations with three independent gates. Polarity control and leakage current suppression are enabled by the specific injection of charge carriers through gated Al‐Ge heterojunctions and the introduction of a blocking electrostatic energy barrier. Further, the choice of monolithic Al/Ge contacts alleviates process variability compared to Ni‐germanide contacts presenting a top‐down technology platform for Ge‐based RFETs. Our device concept is a first step toward future integrated high‐performance and low‐power reconfigurable circuits, providing a platform for future energy‐efficient systems as well as hardware security integrated circuits.

Influences of direction and magnitude of Mg2+ doping concentration gradient on the performance of full concentration gradient cathode material

Influences of direction and magnitude of Mg2+ doping concentration gradient on the performance of full concentration gradient cathode material

Analysis of chemical stress and the propensity for cracking during the vertical Bridgman growth of BaBrCl:Eu

Computational models are employed to analyze residual chemical stresses arising from compositional variations in europium-doped BaBrCl crystals grown by the vertical Bridgman method. We find that significant chemical stress is produced by radial segregation of Eu in this system. In particular, the distribution of normal stresses is set by the radial concentration gradient, whose changing sign produces surface states in tension or compression. Crack opening from surface flaws will be promoted or suppressed by tensile or compressive surface stresses, respectively. Thus, crystal growth processing strategies that change the radial dopant concentration gradients are posited to affect the propensity for cracking. For this system, surface stresses are changed from states of tension to compression when the growth rate is increased, thus improving the chances to avoid cracking—a strategy that defies classical wisdom that dictates slower growth to improve outcomes. Similar strategies affecting segregation may prove beneficial to tailor chemical stress fields to reduce cracking in other crystal growth systems.

Analysis of analog and RF behaviors in junctionless double gate vertical MOSFET

The prime obstacle in continuing the transistor’s scaling is to maintain ultra-shallow source/drain (S/D) junctions with high doping concentration gradient, which definitely demands an advanced and complicated S/D and channel engineering. Junctionless transistor configuration has been found to be an alternative device structure in which the junction and doping gradients could be totally eliminated, thus simplifying the fabrication process. In this paper, a process simulation has been performed to study the impact of junctionless configuration on the analog and RF behaviors of double-gate vertical MOSFET. The result proves that the performance of n-channel junctionless double-gate vertical MOSFET (n-JLDGVM) is slightly better than the junction double-gate vertical MOSFET (n-JDGVM). Junctionless device exhibits better analog behaviors as the transconductance (gm) is increased by approximately 4%. In term of RF behaviors, the junctionless device exhibits 3.4% and 7% higher cut-off frequency (fT) and gain band-width product (GBW) respectively over the junction device.

Performance analysis of ultrathin junctionless double gate vertical MOSFETs

The main challenge in MOSFET minituarization is to form an ultra-shallow source/drain (S/D) junction with high doping concentration gradient, which requires an intricate S/D and channel engineering. Junctionless MOSFET configuration is an alternative solution for this issue as the junction and doping gradients is totally eliminated. A process simulation has been developed to investigate the impact of junctionless configuration on the double-gate vertical MOSFET. The result proves that the performance of junctionless double-gate vertical MOSFETs (JLDGVM) are superior to the conventional junctioned double-gate vertical MOSFETs (JDGVM). The results reveal that the drain current (ID) of the n-JLVDGM and p-JLVDGM could be tremendously enhanced by 57% and 60% respectively as the junctionless configuration was applied to the double-gate vertical MOSFET. In addition, junctionless devices also exhibit larger ION/IOFF ratio and smaller subthreshold slope compared to the junction devices, implying that the junctionless devices have better power consumption and faster switching capability.

Atomic layer deposition of Ta-doped SnO2 films with enhanced dopant distribution for thermally stable capacitor electrode applications

Atomic layer deposition (ALD) on Ta-doped SnO2 thin films is proposed as a methodology for the fabrication of capacitor electrodes, for application in dynamic random-access memories (DRAMs). In ALD of doped materials, dopant concentration gradients inevitably occur due to the cyclic process-based characteristics of ALD. In this study, the dopant distribution improves drastically by the decrease in the growth per cycle of TaOx ALD. The modified ALD recipe exploits the low reactivity between Ta and Sn precursors to facilitate a reduction in the growth per cycle. The Ta-doped SnO2 films grown based on this modified ALD recipe exhibit improved crystallinity and conductivity. The ALD process yields excellent conformality of the Ta-doped SnO2 film over a hole structure with a high aspect ratio of ~10, both in terms of physical thickness and composition. Additionally, the Ta-doped SnO2 films serve as a template for the overgrowing dielectric TiO2 film, which induces the formation of a high-temperature phase with a high dielectric constant, rutile TiO2, and exhibit excellent thermal stability even after annealing at 400 °C in forming gas atmosphere. These findings demonstrate that the proposed methodology for the growth of Ta-doped SnO2 can facilitate the fabrication of capacitor electrodes for application in DRAMs.

Gradients of Be-dopant concentration in self-catalyzed GaAs nanowires

Effective and controllable doping is instrumental for enabling the use of III–V semiconductor nanowires (NWs) in practical electronics and optoelectronics applications. To this end, dopants are incorporated during self-catalyzed growth via vapor–liquid–solid mechanism through the catalyst droplet or by vapor–solid mechanism of the sidewall growth. The interplay of these mechanisms together with the competition between axial elongation and radial growth of NWs can result in dopant concentration gradients along the NW axis. Here, we report an investigation of Be-doped p-type GaAs NWs grown by the self-catalyzed method on lithography-free Si/SiOx templates. The influence of dopant incorporation on the structural properties of the NWs is analyzed by scanning and transmission electron microscopy. By combining spatially resolved Raman spectroscopy and transport characterization, we are able to estimate the carrier concentration, mobility and resistivity on single-NW level. We show that Be dopants are incorporated predominantly by vapor–solid mechanism for low Be flux, while the relative contribution of vapor–liquid–solid incorporation is increased for higher Be flux, resulting in axial dopant gradients that depend on the nominal doping level.

Photoexcited Electrons Driven by Doping Concentration Gradient: Flux-Prepared NaTaO3 Photocatalysts Doped with Strontium Cations

Electron–hole recombination always competes with desired reactions on semiconductor photocatalysts. Reducing recombination probability is essential for increasing the quantum efficiency of the reactions. Previous studies demonstrated that doping with lanthanoid or alkaline-earth metal cations reduced recombination probability in NaTaO3 photocatalysts for artificial photosynthesis. The motivation behind this study was to reveal how the guest metal cations reduced recombination probability. NaTaO3 photocatalysts were doped with Sr cations through crystallization in molten NaCl flux to produce 50–100 nm sized particles of NaTaO3–Sr(Sr1/3Ta2/3)O3 solid solution. Intraparticle distribution of Sr cations was sensitive to immersion time in the hot flux with a fixed Sr concentration of 2 mol % relative to Ta. Extended immersion for 60 h resulted in a homogeneous Sr distribution. Curtailed immersion for 1 h yielded particles capped with a 3 nm thick Sr-accumulated layer. The population of electrons bandgap-excited under Hg–Xe lamp irradiation was enhanced in the 1-h immersed photocatalyst by 160 times relative to that in a Sr-free NaTaO3 photocatalyst. In the 60-h immersed photocatalyst, population enhancement was not more than 9 times. We interpreted the large population enhancement in the 1-h immersed photocatalyst with a concentration gradient of Sr cations from the surface to bulk. The concentration gradient induced an energy gradient of conduction-band minimum. Photoexcited electrons were driven on the energy gradient to be separated from holes. The overall water splitting reaction rate was evaluated on the photocatalysts to show a 4-times enhancement on the 1-h immersed photocatalyst relative to the rate on the Sr-free photocatalyst. The reaction-rate enhancement less than the electron population enhancement was ascribed to a limited fraction of electrons overriding the energy gradient and returning back to the surface.

Characteristics of GaAs/GaSb tunnel field-effect transistors without doping junctions: numerical studies

Tunneling field-effect transistor (TFET) suffers from ultra-sharp doping concentration gradients in both the source/channel junction and drain/channel junction. Recently, the junctionless (JL) TFET device has been proposed to avoid the issue of ultra-sharp doping concentration gradients. Employing III---V semiconductor as a drain/channel material and a group IV semiconductor as a source material has been proposed to improve the heterojunctionless (HJL) TFET device performance. GaAs:Ge HJL-TFET has proved more efficient than other HJL-TFET structures in providing more ON-state current, less OFF-state current, and less subthreshold slope (SS). For the first time in this paper, GaSb as the source material and GaAs as the drain/channel material have been proposed. This is the so-called GaAs:GaSb HJL-TFET structure. Simulation results show that the GaAs:GaSb HJL-TFET provides improvement in both $$I_{\mathrm{ON}}/I_{\mathrm{OFF}}$$ION/IOFF ratio and SS as compared to GaAs:Ge HJL-TFETs. We demonstrate that for a 20 nm channel length, the GaAs:GaSb HJL-TFET average SS is improved by 19% and the point slope by 52%, as compared to those of the GaAs:Ge HJL-TFET. Numerical simulations show that the average SS and $$I_{\mathrm{ON}}/I_{\mathrm{OFF}}$$ION/IOFF ratio of GaAs:GaSb HJL-TFET are nearly 9 mV/dec and 3E12, respectively, for a 10 nm channel length. Thus, the GaAs:GaSb HJL-TFET holds promise for future logic transistor applications.

Textures and shapes in nematic elastomers under the action of dopant concentration gradients.

We explore a novel strategy of patterning nematic elastomers that does not require inscribing the texture directly. It is based on varying the dopant concentration that, beside shifting the phase transition point, affects the nematic director field via coupling between the gradients of concentration and nematic order parameter. Rotation of the director around a point dopant source causes topological modification manifesting itself in a change of the number of defects. A variety of shapes, dependent on the dopant distribution, are obtained by anisotropic deformation following the nematic-isotropic transition.

Theoretical analysis of an end-pumped Yb:YAG active mirror thin-disk amplifier with a longitudinal doping concentration gradient.

In this paper, we present a theoretical derivation of the amplification in a longitudinally pumped amplifier with a doping gradient concentration optimized for obtaining a constant longitudinal gain. This technique makes it possible to reduce both the effect of the amplification of the spontaneous emission (ASE) and the temperature gradient. We will express the longitudinal concentration profile as a function of the amplifier parameters and the launched pump intensity. Our computations show the limits of such a technique for controlling the ASE. We will also make a proposal for a technique to achieve such a controlled concentration profile.

Pinch-off voltage modeling for CMOS image pixels with a pinned photodiode structure

A novel analytical model of pinch-off voltage for CMOS image pixels with a pinned photodiode structure is proposed. The derived model takes account of the gradient doping distributions in the N buried layer due to the impurity compensation formed by manufacturing processes; the impurity distribution characteristics of two boundary PN junctions located in the region for particular spectrum response of a pinned photodiode are quantitative analyzed. By solving Poisson's equation in vertical barrier regions, the relationships between the pinch-off voltage and the corresponding process parameters such as peak doping concentration, N type width and doping concentration gradient of the N buried layer are established. Test results have shown that the derived model features the variations of the pinch-off voltage versus the process implant conditions more accurately than the traditional model. The research conclusions in this paper provide theoretical evidence for evaluating the pinch-off voltage design.

A luminescence study of doping effects in InP-based radial nanowire structures

We have used micro-photo- and cathodo-luminescence at low temperatures to study the effects of sulphur doping in InP and radial InP/InAs/InP structured nanowires. Samples with pure wurtzite crystal structure, with modulated wurtzite/zincblende crystal structure and with different radial InAs growth times were investigated. We observed a doping concentration gradient along the nanowires, the location of segments of different crystal structure and thickness fluctuations on the monolayer scale of the InAs layer.

Generation of powerful microwave voltage oscillations in a diffused silicon diode

The mechanism of the generation of powerful microwave voltage oscillations in a diffused silicon diode is studied. A reverse current 2 kA in amplitude is passed through a 0.5-cm2 diode with a structure thickness of 320 μm, a p-n junction depth of 220 μm. At an average diode voltage of ∼300 V and a microwave pulse duration of ∼200 ns, the maximum voltage swing reaches 480 V. The oscillation frequency lies in the range 5 to 7 GHz; the power of the microwave pulse component is ∼300 kW. A theoretical consideration shows that voltage oscillations are caused by periodically repeating processes of breakdown and structure filling with plasma followed by its removal by the reverse current. The frequency and voltage swing are controlled by the current density and dopant-concentration gradient in the vicinity of the p-n junction.

Manipulation of Dopant Distribution in Yttrium-Doped Ceria Particles

Yttrium is one of the most widely used dopants (e.g., in yttrium-doped ceria (YDC) and yttria-stabilized zirconia) for improving performance of solid electrolytes utilized in solid oxide fuel cells. Oxygen ion conductivity is known to be a function of yttrium concentration in the electrolytes and thus it is important to further understand the factors that influence such concentration distribution. In this study, we examine the change in the distribution of yttrium concentration after annealing in YDC particles. The geometry and morphology of YDC particles, synthesized by spray pyrolysis, were characterized using transmission electron microscopy, and the distribution of yttrium was investigated by X-ray energy-dispersive spectroscopy (XEDS) considering electron interaction volume and geometry of the particle in the XEDS profile analysis. In addition, relative concentrations of Ce(lll) ions, which are thought to be proportional to the concentrations of the oxygen vacancies in the YDC particles, were examined using X-ray photoelectron spectroscopy to correlate with yttrium concentration distributions. It was found that in the as-prepared hollow spherical particles the concentration of yttrium increases linearly from the inner surface towards the outer surface. After annealing the distribution of yttrium becomes non-linear and the dopant is found to migrate to the inner and outer surfaces of the particle. The decrease of the concentration of Ce(lll) ions upon annealing followed by the formation of the dopant concentration gradient is likely due to the increase of the size of the crystallites in the particles.