Doping Profile Research Articles

Two-well injector direct-phonon terahertz quantum cascade lasers

We present an experimental study on a terahertz quantum cascade laser (THz QCL) design that combines both two-well injector and direct-phonon scattering schemes, i.e., a so-called two-well injector direct-phonon design. As a result of the two-well injector direct-phonon scheme presented here, the lasers benefit from both a direct phonon scattering scheme for the lower laser level depopulation and a setback for the doping profile that reduces the overlap of the doped region with active laser states. Additionally, our design also has efficient isolation of the active laser levels from excited and continuum states as indicated by negative differential resistance behavior all the way up to room temperature. This scheme serves as a good platform for improving the temperature performance of THz QCLs as indicated by the encouraging temperature performance results of the device with a relatively high doping level of 7.56 × 1010 cm−2 and Tmax ∼ 167 K. With the right optimization of the molecular beam epitaxy growth and interface quality, the injection coupling strength, and the doping density and its profile, the device could potentially reach higher temperatures than the latest records reached for the maximum operating temperature (Tmax) of THz QCLs.

Efficient Integrated Multimode Amplifiers for Scalable Long-Haul SDM Transmission

Scaling up cable capacity in a power- and cost-efficient manner is a goal of spatial-division multiplexing (SDM) in long-haul optical communication systems. Integration is a key to scaling SDM beyond the parallel single-mode fiber paradigm. Multicore or multimode fiber systems promise higher integration, but native multicore or multimode amplifiers pose challenges in efficient coupling of pump light into the amplifier, and in control of mode-dependent gain (MDG). We propose an integrated multimode amplifier design for signals in six spatial modes (12 spatial and polarization modes) that relies on excitation of only four pump spatial modes at 980 nm. Simulations of an exemplary amplifier predict a relatively flat gain above 10 dB, noise figure below 5 dB, and MDG standard deviation below 0.1 dB over the C-band (1530–1565 nm), with a power-conversion efficiency close to 30%. Key design features include a graded-index multimode fiber amplifier with optimized ring erbium doping profile, length and pump mode powers; and a cascade of wavelength- and mode-selective couplers that efficiently couples four pump diodes to four pump modes, while passing signal modes with minimal loss. Our design approach, which uses fewer pump diodes per signal mode than parallel single-mode systems, may provide an option for efficient, economical scaling of SDM long-haul systems.

A new line tunneling SiGe/Si iTFET with control gate for leakage suppression and subthreshold swing improvement

This article presents a new line tunneling dominating metal–semiconductor contact-induced SiGe–Si tunnel field-effect transistor with control gate (CG-Line SiGe/Si iTFET). With a structure where two symmetrical control gates at the drain region are given a sufficient negative bias, the overlap of the energy bands at the drain in the OFF-state is effectively suppressed, thus reducing the tunneling probability and significantly decreasing leakage current. Additionally, the large overlap area between the source and gate improves the gate’s ability to control the tunneling interface effectively, improving the ON-state current and subthreshold swing characteristics. By using the Schottky contact characteristics of a metal–semiconductor contact with different work functions to form a PN junction, the need to control doping profiles or random doping fluctuations is avoided. Furthermore, as ion implantation is not required, issues related to subsequent annealing are also eliminated, greatly reducing thermal budget. Due to the different material bandgap characteristics selected for the source and drain regions, the probability of overlap of the energy bands in the source region in the ON-state is increased and that in the drain region in the OFF-state is reduced. Based on the feasibility of the actual fabrication process and through rigorous 2D simulation studies, improvements in subthreshold swing and high on/off current ratio can be achieved simultaneously based on the proposed device structure. Additionally, the presence of the control gate structure effectively suppresses leakage current, further enhancing its potential for low-power-consumption applications.

Endurance Behavior of Z-Shaped Charge Plasma Tunnel FET for Biosensing Application

The tunneling field effect transistor (TFET) is a viable candidate for designing a highly sensitive biosensor. In this work, a doping-less Z-shaped dielectrically modulated charge plasma TFET (CPTFET) structure with misaligned cavity region on channel and source has been proposed for label-free biosensing applications. To design the device, charge plasma technique has been employed, where appropriate metal workfunction is used over intrinsic silicon to create n[Formula: see text] drain and p[Formula: see text]source regions. The charge plasma approach reduces thermal budget, random dopant fluctuation (RDF) and steps required for fabrication. The Z-shaped CPTFET includes the advantage of abrupt profile of doping at source-channel (tunneling) junction. Because cavities are created in source and gate oxide region, the abrupt doping profile suppresses ambipolar behavior and improves sensitivity. The performance of the proposed device for both charged and neutral biomolecules in terms of electric field, band energy, transfer characteristics, [Formula: see text] ratio and subthreshold swing (SS) has been examined. The response of other parameters like cavity thickness and cavity length on ON current has been analyzed using the Silvaco ATLAS device simulator.

Enhanced Yb:YAG Active Mirrors for High Power Laser Amplifiers

The work is aimed at the investigation of the influence of nonlinear active ions concentration profiles in Yb:YAG laser elements on temperature distribution and wavefront distortions during amplification using sub-kilowatt level diode pumping. A mathematical model is presented for the theoretical study of the amplification process in crystals with cubic crystal system. A detailed comparison of Yb:YAG active elements with the same thickness and absorbed pumping power, but with various concentration profiles of Yb3+, ions is carried out. It is shown that the use of active elements with an increasing dopant concentration in the pump beam direction allows one to optimize the temperature profile inside the active element and, thus, reduce the thermal-induced wavefront distortions of the amplified radiation. Modeling is carried out for the experimentally grown crystal with linear concentration gradient profile. It is shown that the linear doping profile with a gradient of 0.65 at.%/mm allows increasing the small-signal gain up to 10% and decreasing the thermal-induced wavefront distortions by ~15%.

Omni-junction silicon modulators of unequal proportions for high-speed datacom

Doping profiles are always a key factor in the design of silicon modulators. As the doping in the waveguide cross-section is limited by the compact footprint, three-dimensional complex optimization presents itself as a viable option for improved performance. This work proposes two different types of unequal-proportioned omni-junction modulators obtained by the linear-decline weight particle swarm optimization algorithm with appropriate figure of merits (FOM). With FOM targeting at overall performance, the obtained modulator composed of unequal-proportioned vertical and lateral junctions has high bandwidth of 40 GHz with the VπL of 1.07 V cm. With FOM for higher modulation efficiency, the modulator composed of unequal-proportioned U-shaped and L-shaped junctions demonstrates low VπL of 0.72 V cm and bandwidth maintaining over 36 GHz. The algorithm together with the 3D design method provides greater flexibility in improving the device performance.

X-Intersected Silicon Modulator of Well-Rounded Performance.

In silicon modulator design, implantation is always a key factor, significantly influencing the doping profile and carrier distribution. As waveguide doping is limited by the compact footprint of the modulator rib, three-dimensional complex optimization is a viable option to improve performance. This work proposes an X-intersected modulator based on two inversely slanted junctions using the effective 3D Monte Carlo method for junction generation. The optimized results show that the modulation efficiency of the design is 1.09 V·cm, while the loss is 18 dB/cm, and the 3 dB bandwidth reaches over 35 GHz owing to the decreased resistance and capacitance of the 3D junction. This work demonstrates the benefits of 3D doping design in silicon modulators, contributing to higher efficiency and avoiding additional PN overlap to introduce lower capacitance. The design of 3D doping profiles well balances the DC and AC performance, and provides novel modulator solutions for high-speed datacom.

Laser Activation for Highly Boron-Doped Passivated Contacts

Passivated, selective contacts in silicon solar cells consist of a double layer of highly doped polycrystalline silicon (poly Si) and thin interfacial silicon dioxide (SiO2). This design concept allows for the highest efficiencies. Here, we report on a selective laser activation process, resulting in highly doped p++-type poly Si on top of the SiO2. In this double-layer structure, the p++-poly Si layer serves as a layer for transporting the generated holes from the bulk to a metal contact and, therefore, needs to be highly conductive for holes. High boron-doping of the poly Si layers is one approach to establish the desired high conductivity. In a laser activation step, a laser pulse melts the poly Si layer, and subsequent rapid cooling of the Si melt enables electrically active boron concentrations exceeding the solid solubility limit. In addition to the high conductivity, the high active boron concentration in the poly Si layer allows maskless patterning of p++-poly Si/SiO2 layers by providing an etch stop layer in the Si etchant solution, which results in a locally structured p++-poly Si/SiO2 after the etching process. The challenge in the laser activation technique is not to destroy the thin SiO2, which necessitates fine tuning of the laser process. In order to find the optimal processing window, we test laser pulse energy densities (Hp) in a broad range of 0.7 J/cm2 ≤ Hp ≤ 5 J/cm2 on poly Si layers with two different thicknesses dpoly Si,1 = 155 nm and dpoly Si,2 = 264 nm. Finally, the processing window 2.8 J/cm2≤ Hp ≤ 4 J/cm2 leads to the highest sheet conductance (Gsh) without destroying the SiO2 for both poly Si layer thicknesses. For both tested poly Si layers, the majority of the symmetric lifetime samples processed using these Hp achieve a good passivation quality with a high implied open circuit voltage (iVOC) and a low saturation current density (J0). The best sample achieves iVOC = 722 mV and J0 = 6.7 fA/cm2 per side. This low surface recombination current density, together with the accompanying measurements of the doping profiles, suggests that the SiO2 is not damaged during the laser process. We also observe that the passivation quality is independent of the tested poly Si layer thicknesses. The findings of this study show that laser-activated p++-poly Si/SiO2 are not only suitable for integration into advanced passivated contact solar cells, but also offer the possibility of maskless patterning of these stacks, substantially simplifying such solar cell production.

A Review of Cr2+ or Fe2+ Ion-Doped Zinc Sulfide and Zinc Selenide Ceramics as IR Laser Active Media

Zinc chalcogenides doped with Cr2+ or Fe2+ ions are of considerable interest as active media for IR lasers operating in the 2–5 µm wavelength range. Such lasers are in demand in various fields of medicine, remote sensing and atmospheric monitoring, ranging, optical communication systems, and military applications. In recent years, however, the rate of improvement in the characteristics of zinc chalcogenide laser sources has slowed considerably. Unwanted thermally induced effects, parasitic oscillations, and laser-induced damage of the active element have hindered the scaling of output power and efficiency. However, the physical and chemical properties of the materials leave ample room for further improvements. In particular, the control of the dopant concentration profile in the active element is of great importance. Zero concentration of Cr2+ or Fe2+ ions on the radiation input/output surfaces can significantly increase the laser-induced damage threshold; the designed concentration distribution in the element volume allows regulation of heat dissipation and reduction of parasitic oscillations. The zinc chalcogenide ceramic technology seems to be the most suitable to solve this challenge. This review presents and discusses the state of the art in ZnS and ZnSe optical and laser ceramics and the directions for further development of their technology.

The Effect of Collector Region Design on Large-Signal Performance of Horizontal Current Bipolar Transistor (HCBT)

The impact of collector region design on large-signal performance of horizontal current bipolar transistor (HCBT) technology is investigated by calibrated time-domain load-pull measurements. Three representative high-performance devices of the same geometry but with different collector doping profiles, namely, uniform, n-well, and low-doped, are subjected to large-signal measurements wherein the radio frequency (RF) time-domain waveforms of current and voltage are measured to facilitate the determination of the maximum output power for each device. The extensive measurements outline the boundaries of the maximum collector current and voltage swings at RF determining linear operating area, which are a result of the physical phenomena in the high-current and high-voltage regimes, namely, the Kirk effect and impact ionization, respectively. While the highest-doped n-well HCBT provides the highest output power of 25.3 dBm at 1-dB gain compression, and the lowest-doped HCBT enables a wideband match at the collector, the HCBT with a uniform doping profile provides a tradeoff between the output power and bandwidth of the output match. The results show the versatility of HCBT technology as an alternative to CMOS and/or sillicon–germanium (SiGe) large-signal RF circuits for the front-ends (FEs) in the sub-6 GHz wireless communications range.

Impact of Dimensions and Doping on the Breakdown Voltage of a Trench 4H-SiC Vertical JFET

In this paper we study the feasibility of the design/fabrication of a vertical trench 4H-SiC Junction Field Effect Transistor (JFET), assuming realistic constraints of the depth of the P+ implantation. The P+ doping profile is obtained using a Monte Carlo implantation simulation. The calculation used a drift-diffusion approach. The JFET aims to achieve a threshold voltage of-3V. We found that this constraint in concomitance with the proposed structure limits the breakdown voltage to approximately 200V. This is the result of a premature breakdown induced by short channel effects, namely Drain Induced Barrier Lowering (DIBL). However, a negative increase in the gate bias represses this short channel effect and improves the breakdown voltage to roughly 1800V. At this gate bias, the breakdown is induced by reaching the critical field strength of 4H-SiC at the gate P+/N junction, which causes avalanche generation of carriers. In addition, we have calculated the dependence of the threshold voltage on the drift doping and pillar width. This work also shows the vulnerability of the design to random fluctuation in the doping profile.

Inverse Gaussian Doped Nanoscale Junctionless Gate All Around-based Biosensor

This article discusses the electrical properties of an Inverse Gaussian Junctionless Gate All Around (IG-JLT-GAA) field effect transistor for identifying neutral and charged bioanalytes. In this work, the inverse Gaussian doping profile enables the potent command of gate over charge in channel in the GAA structure, resulting in improved performance attributes such as OFF current, ON current, current ratio, subthreshold slope, drain-induced barrier lowering, threshold voltage, and transconductance for the proposed biosensor. The leakage current (OFF) is 10−17 A, and the drain current (ON) is 10−6 A, with their ratio of 10−11 fetched at K = 10. This article investigates the effect of varying dielectric constants of biomolecules immobilized in cavity reflects the enhancement in biosensor sensitivity due to the immobilization of high dielectric constant protein bioanalytes. The drain-induced barrier lowering (DIBL) and subthreshold swing (SS) are 4.4 mV/V and 60.6688 mV/dec, respectively, for K = 10. The estimated threshold voltage sensitivity (SVT ) for various dielectric constant biomolecules was found to be 9.7 and 110.5 mV for K = 1 and K = 10, respectively. Further, the performance of the biosensor using well-known biotin biomolecules with K = 2.63 is studied, and the result shows that the biosensor performs outstandingly in all performance attributes required for the detection of charged and neutral biomolecules.

Gaussian doping profile in the channel region: A technology booster for junctionless transistors

In this work, we have developed analytical models for the channel potential, threshold voltage and drain current for a cylindrical Gate-All-Around Junctionless Metal-Oxide-Semiconductor Field Effect Transistors (GAA-JLTs) with Gaussian doped channel region along the radial direction with uniformly doped source and drain regions. It has been found that the variation in doping parameters, such as, straggle, projected range and peak doping concentration, significantly affects the device characteristics such as threshold voltage, Ion/Ioff and so on and therefore, tuning them may bring about optimized device design, avoiding excessive leakage current yet maintaining high on-state current. The results obtained through our proposed analytical model are validated by ATLAS 3-D device simulator. Further, the variation of channel potential with doping parameters and bias potentials have been investigated. We have shown that by tuning the doping profile parameters together with the aspect ratio (AR) of a non-circular cross-sectional JLT, the threshold voltage and Ion/Ioff can be optimized along with reduction in DIBL. The effects of Gaussian doping on few figures-of-merit (FOMs), as applicable to analog domain, have also been analyzed. Computations with necessary discussions have been presented to explore the impact of key doping parameters on the linearity of GAA-JL devices.

A study of activated phosphorus distribution within silicon substrate for polysilicon passivating contacts based on an in-line PVD system

An in-line PVD system consisting of plasma oxidation chamber and phosphorus-doped amorphous silicon deposition chamber was used to fabricate highly-doped polysilicon passivating contact structure for commercial TOPCon solar cells, which can help to achieve simple fabrication process and excellent electrical properties. In this paper, we studied how the doping profiles of ionized phosphorus atoms in the passivating contact formed by this method varies with the deposition conditions, especially the in-diffusion profile in the silicon substrate. Increasing the RF power of plasma oxidation can prevent phosphorus from penetrating through SiOx to form a shallow profile, and reducing the power is conducive to the formation of more pinholes on SiOx, resulting in a wide profile following the shallow profile. The shallow profile effectively inhibits the surface recombination rate and thus achieve good passivation quality, while the emergence of the following wide profile helps to reduce the contact resistivity by providing more majority carrier transport channels. The 100 W condition constructed a shallow and gradually widening doping profile, exhibiting a low recombination current density of 16.5 fA/cm2 and a low contact resistivity of 1.87 mΩ·cm2. However, the SiOx thickness and density obtained at 80 W are insufficient for achieving a shallow-first-then-widen doping profile, even with adjustments to thermal activation conditions. Additionally, we found the engineering of surface microstructure on silicon wafers also has an impact on the doping profile.

Photovoltaic performance of lattice-matched gallium indium arsenide/germanium stannide dual-junction cell

Based on the photovoltaic properties and tandem solar cells theory, Gallium Indium Arsenide/Germanium Stannide (GaInAs/GeSn)-based double-junction (DJ) solar cells have been numerically simulated for the first time. In this study, we explore the band gap combination under lattice matching and obtain the content of In/Sn at optimal efficiency, which is expressed as Ga0.84In0.16As/Ge0.93Sn0.07 DJ solar cell (1.20/0.58 eV). Afterward, it is optimized in terms of variation in the doping contents and active layer thickness. To take full advantage of the electron mobility of the material, the optimal ‘inverted doping profile’ concentration N a(d) is 1.5(5)/5(20) × 1018 cm−3. In addition, the reasonable p(n) layer thickness could be comprised of 0.2–0.8(0.2–1)/0.5–3(1–4) μm of the DJ solar cells with less material consumption. When the p(n) layer thickness is 0.30(0.25)/0.9(1.35) μm, the tandem device can achieve an optimal efficiency of 31.00% with 28.98 mA cm−2 (J sc), 1.25 V (V oc) and 85% (FF). This study highlights that GeSn materials have the potential to combine with III–V materials to form low-cost and high-efficiency tandem devices.

A Near-Infrared Enhanced Field-Line Crowding Based CMOS-Integrated Avalanche Photodiode

This paper presents a CMOS-integrated linear-mode avalanche photodiode based on electric field-line crowding (EFLC-APD) to form an effective multiplication zone and a wide absorption zone. The EFLC-APD possesses a hemispherical avalanching electric field at the n-well/p- epi junction formed due to the curvature of the half-sphere cathode. A lower electric field extends radially across the entire volume of the EFLC-APD towards the substrate and towards the surface anode. Because of such a distribution of the electric field, electrons photogenerated within the whole volume drift towards the cathode. Therefore, the EFLC-APD provides a large sensitive-area to total-area ratio while offering high responsivity and bandwidth for red and near-infrared light due to its thick absorption zone and drift-based carrier transport. It is shown that the electric field distribution can be modified by the design parameters such as cathode radius and diode size in addition to doping profiles. The EFLC-APD achieves a responsivity-bandwidth (R-BW) product of 49.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\frac{\mathrm{A}}{\mathrm{W}}\cdot$</tex-math></inline-formula> GHz, corresponding to the responsivity and bandwidth of 33 A/W and 1.5 GHz, respectively, at the wavelength of 850 nm. In addition, a maximum responsivity of 3.05 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times\, 10^{\mathrm{3}}$</tex-math></inline-formula> A/W at 2 nW optical power is achieved for the red and near-infrared spectral range. Noise characterization resulted in an excess noise factor F = 6 measured at an avalanche gain of 56.7. Due to the high sensitive-area to total-area ratio, high responsivity, large bandwidth, and CMOS compatibility, this APD is a promising optical detector for many applications.

Study of Impact Ionization Coefficients in Silicon With Low Gain Avalanche Diodes

Impact ionization in silicon devices has been extensively studied and several models for a quantitative description of the impact ionization coefficients have been proposed. We evaluate those models against gain measurements on Low Gain Avalanche diodes (LGADs) and derive new parameterizations for the impact ionization coefficients optimized to describe a large set of experimental data. We present pulsed IR-laser based gain measurements on 5 different types of $50\mu m$-thick LGADs from two different producers (CNM and HPK) performed in a temperature range from $-15^oC$ to $40^oC$. Detailed TCAD device models are conceived based on SIMS doping profiles measurements and tuning of the device models to measured C-V characteristics. Electric field profiles are extracted from the TCAD simulations and used as input to an optimization procedure (least squares fit) of the impact ionization model parameters to the experimental data. It is demonstrated that the new parameterizations give a good agreement between all measured data and TCAD simulations which is not achieved with the existing models. Finally, we provide an error analysis and compare the obtained values for the electron and hole impact ionization coefficients against existing models.

Optimized dopant imaging for GaN by a scanning electron microscopy.

Two-dimensional dopant profiling is vital for the modelling, design, diagnosis and performance improvement of semiconductor devices and related research and development. Scanning electron microscopy (SEM) has shown great potential for dopant profiling. In this study, the effects of secondary electron (SE) detectors and imaging parameters on the contrast imaging of multilayered p-n and p-i junction GaN specimens via SEM were studied to enable dopant profiling. The doping contrast of the image captured by the in-lens detector was superior to that of the image captured by the side-attached Everhart-Thornley detector at lower acceleration voltages (Vacc ) and small working distances (WD). Furthermore, the doping contrast levels of the in-lens detector-obtained image under different combinations of Vacc and WD were studied, and the underlying mechanism was explored according to local external fields and the refraction effect. The difference in the angular distributions of SEs emitted from different regions, the response of the three types of SEs to detectors, and the solid angles of detectors toward the specimen surface considerably influenced the results. This systematic study will enable the full exploitation of SEM for accurate dopant profiling, improve the analysis of the doping contrast mechanism, and further improve doping contrast for semiconductors.

Towards Sic-Based VUV Pin-Photodiodes - Investigations on 4H-SiC Photodiodes with Shallow Implanted Al Emitters

4H silicon carbide (SiC) based pin photodiodes with a sensitivity in the vacuum ultraviolet spectrum (VUV) demand newly developed emitter doping profiles. This work features the first ever reported 4H-SiC pin photodiodes with an implanted p-emitter and a noticeable sensitivity at a wavelength of 200 nm. As a first step, Aluminum doping profiles produced by low energy ion implantation in 4H-SiC were characterized by secondary-ion mass spectrometry (SIMS). Photodiodes using these shallow emitters are compared to one with a deep p-emitter doping profile employing IV characteristics and the spectral response. SIMS results demonstrate the possibility of shallow Alimplantation profiles using low implantation energies with all emitter profiles featuring characteristic I-V results. For some shallow doping profiles, a meassurable signal at the upper limit of the VUV spectrum could be demonstrated, paving the way towards 4H-SiC pin photodiodes with sensitivities for wavelengths below 200 nm.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.