Dopant Concentration Profiles Research Articles

Joule-class sub-nanosecond pulses produced by end-pumped direct bonded YAG/sapphire modular amplifier

A Joule-class room-temperature diode-pumped solid-state laser was developed. The energy scaling of the 100 mJ 1064 nm seed pulse was realized by a series of two diode-pumped amplifiers. The gain medium consists in free combinations of Nd:YAG ceramics bonded to sapphire transparent heat sinks, to relax the thermal load induced by the 34 kW pump power. At low repetition rate, parasitic lasing was the main limitation to energy scaling. By choosing a gain module combination producing a step-like gradual doping concentration profile, mitigation of parasitic oscillations was observed, and the system delivered 2.8 J, 800 ps pulses at 2 Hz.

Simultaneous Control of Spectral And Directional Emissivity with Gradient Epsilon-Near-Zero InAs Photonic Structures.

Controlling both the spectral bandwidth and directionality of emitted thermal radiation is a fundamental challenge in contemporary photonics. Recent work has shown that materials with a spatial gradient in the frequency range of their epsilon-near-zero (ENZ) response can support broad spectrum directionality in their emissivity, enabling high total radiance to specific angles of incidence. However, this capability is limited spectrally and directionally by the availability of materials with phonon-polariton resonances over long-wave infrared wavelengths. Here, an approach is designed and experimentally demonstrated using doped III-V semiconductors that can simultaneously tailor spectral peak, bandwidth, and directionality of infrared emissivity. InAs-based gradient ENZ photonic structures that exhibit broadband directional emission with varying spectral bandwidths and directional ranges as a function of their doping concentration profile and thickness are epitaxially grown and characterized. Due to its easy-to-fabricate geometry, it is believed that this approach provides a versatile photonic platform to dynamically control broadband spectral and directional emissivity for a range of emerging applications in heat transfer and infrared sensing.

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.

Pulsed Laser Annealed Ga Hyperdoped Poly‐Si/SiOx Passivating Contacts for High‐Efficiency Monocrystalline Si Solar Cells

Polycrystalline Si (poly‐Si)‐based passivating contacts are promising candidates for high‐efficiency crystalline Si solar cells. We show that nanosecond‐scale pulsed laser melting (PLM) is an industrially viable technique to fabricate such contacts with precisely controlled dopant concentration profiles that exceed the solid solubility limit. We demonstrate that conventionally doped, hole‐selective poly‐Si/SiOx contacts that provide poor surface passivation of c‐Si can be replaced with Ga‐ or B‐doped contacts based on non‐equilibrium doping. We overcome the solid solubility limit for both dopants in poly‐Si by rapid cooling and recrystallization over a timescale of ∼25 ns. We show an active Ga dopant concentration of ∼3 × 1020 cm−3 in poly‐Si which is six times higher than its solubility limit in c‐Si, and a B dopant concentration as high as ∼1021 cm−3. We measure an implied open‐circuit voltage of 735 mV for Ga‐doped poly‐Si/SiOx contacts on Czochralski Si with a low contact resistivity of 35.5 ± 2.4 mΩ cm2. Scanning spreading resistance microscopy and Kelvin probe force microscopy show large diffusion and drift current in the p‐n junction that contributes to the low contact resistivity. Our results suggest that PLM can be extended for hyperdoping of other semiconductors with low solubility atoms to enable high‐efficiency devices.

Wideband mid infrared absorber using surface doped black silicon

Black silicon (BSi) is a synthetic nanomaterial with high aspect ratio nano protrusions inducing several interesting properties such as a very large absorptivity of incident radiation. We have recently shown that heavily doping the BSi in volume enables to significantly enhance its mid-infrared absorptivity and tune its spectral range of interest up to 20 μm. In the present letter, we explore the effect of surface doping on BSi radiative properties and its absorptance in particular since surface doping enables reaching even larger dopant concentrations than volume doping but at more limited penetration depths. We considered 12 different wafers of BSi, fabricated with cryogenic plasma etching on n- and p-type silicon wafers and doped using ion-implantation with different dopant types, dosages, and ion beam energies, leading to different dopant concentrations and profiles. The different wafers radiative properties, reflectance, transmittance, and absorptance are experimentally measured using Fourier transform infrared spectroscopy. We show that doping an n-type BSi wafer with phosphorous with a dose of 1017 atm/cm2 and an energy of 100 keV increases its absorptivity up to 98% in the spectral range of 1–5 μm. We propose a simple phenomenological explanation of the observed results based on the dopant concentration profiles and the corresponding incident radiation penetration depth. Obtained results provide simple design rules and pave the way for using ion-implanted BSi for various applications, such as solar energy harvesting, thermo-photovoltaics, and infrared radiation sensing, where both high absorptance and variable dopant concentration profiles are required.

Carrier lifetimes in gold–hyperdoped silicon—Influence of dopant incorporation methods and concentration profiles

Incorporating ultrahigh concentrations of deep-level dopants in silicon drastically alters silicon’s optoelectronic properties. Photodiodes built from silicon hyperdoped with gold extend light sensitivity into the shortwave infrared region, far beyond the absorption edge of a pristine silicon sample. Deep-level dopants, however, also enhance carrier recombination; even though hyperdoped silicon has great light absorption properties, short charge carrier lifetime limits its applications. In this work, using terahertz spectroscopy, we investigate the charge carrier lifetime of gold–hyperdoped silicon, where the gold dopants are introduced by either film deposition or ion implantation, followed by pulsed laser melting. Using reactive ion etching, we measure how carrier lifetime changes when dopant concentration profiles are altered. Furthermore, using a 1D diffusion and recombination model, we simulate carrier dynamics when electrons are excited by sub-bandgap light. Our results show that the dopant distribution profile heavily influences excited carrier dynamics. We found that etching improves the half-life by a factor of two. In the short-wave-infrared range, the gold dopants are both light absorption centers and recombination centers. Focusing on optoelectronic properties in the short-wave-infrared region, our results suggest that these samples are over doped—etching much of the gold dopants away has little impact on the number of excited electrons at a later time. Our results suggest that dopant profile engineering is important for building efficient optoelectronic devices using hyperdoped semiconductors.

Model of boron diffusion in silicon used for solar cell fabrication based on boric acid solutions

PurposeThe purpose of this paper is to develop a model that allows determining the boron concentration profile in silicon based on duration and temperature of the diffusion process.Design/methodology/approachThe model was developed on the basis of the Fick’s second law, which is fundamental for describing the diffusion process. The explicit scheme of the finite difference method was used in the conducted simulations. Results of measurements made using the secondary ion mass spectrometry (SIMS) were used as template dopant concentration profiles. Solution of boric acid in ethanol is the dopant source for which this model was developed.FindingsBased on the conducted simulations, it was proposed that besides the influence of electric field of ionized dopants, which is already described in literature, an appropriate factor reflecting the influence of the threshold concentration on the coefficient of diffusion of boron in silicone should also be introduced.Originality/valueThe developed model enables determination of the boron concentration profile in silicon consistent with the results of SIMS measurements. A factor taking into account the influence of threshold concentration on the coefficient of diffusion was introduced. The influence of concentration of boric acid in the dopant solution on the concentration profile was also considered.

A numerical modeling approach to estimate the piezoresistance of diffused resistors with experimental validation

Silicon piezoresistive sensors have been widely used for many applications in recent decades. Monocrystalline silicon resistors are realized using an ion-implantation or a thermal diffusion process with a Gaussian or complementary error function profile. However, over the years, most researchers have neglected the doping concentration profile of the piezoresistor in the modeling stages resulting in erroneous responses that are far removed from experimental results of fabricated resistive sensors. In the present work, we propose a simulation approach to accurately estimate the piezoresistance of thermally diffused resistors with a non-uniform doping profile. We have modeled the diffused resistor as a parallel combination of several small slices, each having a unique piezoresistive coefficient. Three different slicing strategies were investigated to evaluate the impact of the piezoresistive coefficients, the electrical resistivity of the resistor slices, and the stress profile across the thickness of the resistor embedded in an accelerometer device. The cumulative impact of these parameters on the sensor’s overall sensitivity is evaluated. Further, we have also studied the influence of the accelerometer’s flexure thickness on the sensor’s sensitivity. It is observed from the simulation results that one of the slicing strategies with more slices at the surface of the resistor results in less than 1% error compared to the experimental results of an accelerometer device with a 60 µm flexure thickness.

Determination of the doping profile of lanthanum impurities in thin-film barium titanate structures

The paper presents a calculation of doping concentration profile in a two-layer barium titanate film, one of the layers of which contains lanthanum impurity. The theoretically predicted distributions were experimentally verified. Influence of the doping profile on the magnitude of the electrocaloric (EC) response of ferroelectric films is assessed by numerical simulation. It is demonstrated that at the same La concentration, the magnitude of the EC effect can change by 40% due to the distribution of the lanthanum concentration in the structure. The results obtained allow optimizing the technological process to ensure the required characteristics of ferroelectric thin films.

Characterization Of Gallium Oxide With A Novel Non-Contact Electrical Metrology, CnCV, For Wide Bandgap Semiconductors

Cost savings and short feedback time are significant advantages of the corona non-contact capacitance voltage (CnCV) technique, recently introduced for wide bandgap SiC, GaN, and AlGaN/GaN as a replacement for C-V measurements on fabricated electrical test structures. The goal of this work is to verify the basic applicability and effectiveness of CnCV in characterization of Ga2O3. The results demonstrate excellent measurement conditions on Ga2O3 epitaxial and bulk grown wafers, enabling the determination of important electrical properties such as the dopant concentration profile, initial surface charge, electron affinity and depletion current.

Micro-photoluminescence studies of shallow boron diffusions below polysilicon passivating contacts

Micro-photoluminescence is utilised in this work to study the shallow boron diffusions that occur underneath p + polysilicon passivating contacts. Regions with high boron concentrations emit a photoluminescence peak at a higher wavelength compared to the substrate crystalline silicon due to band-gap narrowing. We demonstrate the presence of this peak with samples following optimised fabrication procedures based on previous studies for thermally BBr3 diffused polysilicon passivating contacts. The intensities of the diffusion peaks, when normalised against the substrate crystalline silicon peak, change with the dopant concentration profiles, which in turn vary with the BBr3 diffusion temperatures as well as the thickness of the interfacial oxide layer. Comparisons are made between the photoluminescence spectra from p + polysilicon passivating contacts and n + polysilicon contacts reported previously [1]. We find that the sub-band-gap photoluminescence peaks arising from the n + polysilicon layers themselves are absent in the p + polysilicon contacts. This simplifies the procedure for photoluminescence spectra measurements in the case of p + polysilicon contacts. Further demonstration of micron-scale spectral photoluminescence mapping with localised p + polysilicon structures demonstrates the feasibility of the method in detecting underlying boron diffused regions with a spatial resolution of below 10 μm. Such micron-scale mapping will be useful for characterising localised polysilicon structures, such as polysilicon under the fingers on the front side of solar cells, or in interdigitated back contact devices.

Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction.

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

Defect states and grading induced bandgap variability analysis of CIGS solar cells through device simulations

Copper-indium-gallium-diselenide (CIGS) is one of the promising absorbing material in thin film solar cells. It is a direct band gap material with the band gap (Eg) ranging from 0.98 to 1.68 eV. The band gap of this material is dependent on the composition of the material. The band gap enhancement leads to significant reduction in the extent of exciton recombination but on the other hand it also reduces the absorption in the absorber layer. We simulated and analyzed the effect of band gap variations in CIGS layer, induced by grading of doping concentration profile and defect states over the performance of solar cell. The performance analysis has been done using SCAPS-1D (Solar Cell Capacitance Simulator) simulator. It is intended to develop specially to simulate the AC and DC characteristics of hetero-junction thin film solar cells, especially CIGS and CdTe solar cells. By the introduction of grading in the CIGS material, an enhancement of 2.48% has been observed in the overall efficiency. This enhancement can be attributed to the reduction in recombination and increase in the exciton generation.

Dopant segregation in YAG single crystal fibers grown by the laser heated pedestal growth technique

In this study, we report self-segregation of dopants in a crystal matrix within a single crystal (SC) fiber. Neodymium and holmium-doped yttrium aluminum garnet (YAG) fibers were grown using the Laser Heated Pedestal Growth (LHPG) technique and cross-sectional dopant concentration was measured using electron-probe micro-analysis. It was observed that the degree of auto-segregation of the rare-earth dopant depended on the difference in ionic size of the dopant ion and the Y3+ ion in the YAG matrix. While holmium showed little tendency to self-segregate, the concentration of neodymium ions varied as much as 25% across the cross-section of the fiber. Strong correlation between the dopant concentration profile and fiber draw speed was also demonstrated. Since the local refractive index depends on the concentration of dopants, a refractive index profile can be achieved by a dopant profile across the fiber cross-section. Engineered index profiles can be realized by varying growth conditions, dopants, crystal matrix, etc. Such an approach is promising in applications such as the development of monolithic SC fibers with graded-index profiles.

Design guidelines for edge‐coupled waveguide unitravelling carrier photodiodes with improved bandwidth

This study presents experimental and simulation results for edge-coupled waveguide unitravelling-carrier (UTC) photodiodes based on an InGaAs/InP heterostructure. Experimental results are used to calibrate the numerical device simulator. The authors study how different aspects of the UTC photodiode epistructure and contacts impact on the overall device bandwidth, calculating the photoresponse for different structural parameters and doping concentration profiles. The effect of these parameters on the 3-dB cut-off frequency is studied, and design guidelines for UTC photodiodes with improved performance are presented. The UTC photodiode simulated using authors’ design guidelines has a 3 dB cut-off frequency of 49 GHz, a factor of 2 larger than the 25 GHz cut-off of the fabricated UTC photodiode.

Analytical Model for the Breakdown Voltage in Punch-through IGBTs

Insulated-gate bipolar-transistors (IGBTs) are becoming the main switching devices in medium- and high-power applications, such as switched-mode power supplies, power switches, variable frequency driver circuits, traction motors, and inducting heating. Although the switching speed of IGBTs is lower than in the case of metal-oxide-semiconductor field-effect-transistors (MOSFETs), the forward voltage drop is significantly lower, particularly in higher blocking voltage rated devices, which provides major advantage for using IGBTs in many applications. In addition, IGBTs have higher breakdown voltage and currant handling capacity than traditional bipolar-junction-transistors (BJTs), which makes them attractive for a variety of applications related to electric power switches. In this presentation, we introduce two new analytical models for the breakdown voltage of the punch-through IGBTs. The models are based on the accurate analytical estimation of the ionization integral and the solution of the Poisson equation in the drift region, which lead to a nonlinear equation for the breakdown voltage as a function of the doping concentrations in the drift region, buffer layer, and gate and emitter junctions, as well as of the widths of these regions and junctions. In general, this equation does not have an analytical solution and needs to be solved numerically in order to compute the breakdown voltage. Analytical solutions can be found only in a number of particular cases that will be discussed at the conference. The numerical and analytical solutions of the breakdown voltages are compared to more accurate results obtained using Synopsys’s Sentaurus Device Simulator [1], which is a commercial-grade finite element semiconductor simulator. We observe a very good agreement between our models and the finite element simulations for a wide range doping concentration and geometric dimensions of the IGBTs. In addition, at the conference, we will also compare our analytical results with previous analytical results published in the literature [2]. The proposed analytical model provides an efficient way to design the geometry and doping concentration profiles in punch-through IGBTs and helps the optimization process of the breakdown voltage in these devices [3]. More details about the approximations used in our analytical calculations, the results of the comparison with the more precise finite element simulations, and the existing literature will be presented at the conference. [1] 2016 Sentaurus Device Simulator User Manual, www.synopsys.com[2] J. Baliga, Fundamentals of Power Semiconductor Devices: Springer, 2008. [3] Zhu, C., & Andrei, P. (2017). Sensitivity of Breakdown Voltage of Power Transistors to Dopant Impurities. ECS Transactions, 80(7), 171-179.

Creation of Acceptor Centers in ZnO Single Crystals by Annealing in Sb Vapor

We have investigated the influence of Sb on the carrier concentration of ZnO single crystals. ZnO single crystal substrates were doped in a sealed ampule by annealing in the Sb vapor at temperatures from 700 to 900 °C to create acceptor centers in ZnO. The dopant concentration profiles were determined by secondary ion mass spectrometry. A significant distinction in dopant concentrations as well as depth of penetration was observed for different temperatures of annealing. Electrical properties of the samples were studied by the Hall Effect and resistivity measurements. Valence states of Sb in the samples were investigated by the X-ray photoelectron spectroscopy method to prove the incorporation of the latter in the matrix of ZnO before and after oxygen annealing. The evolution of the carrier concentration of ZnO along with the valence state of Sb is explained.

Predicting damage production in monoatomic and multi-elemental targets using stopping and range of ions in matter code: Challenges and recommendations

The computer code, Stopping and Range of Ions in Matter (SRIM), is widely used to describe energetic processes of ion-solid interactions; its predictive power relies on the accuracy of energy loss/transfer and collision processes being considered. While the SRIM code is commonly applied in radiation effects research to predict damage production and in the semiconductor industry to estimate ion range and dopant concentration profiles, two challenges exist that affect its use: (1) inconsistency in estimations of atomic displacements between full-cascade and quick (modified Kinchin–Pease) options and (2) overestimation of electronic stopping power for slow heavy ions in light targets (e.g., Be and Si) or in compound targets containing light elements (e.g., C, N and O in carbides, nitrides and oxides). Based on a literature review and our experimental investigations, we discuss the underlying reasons for the discrepancies, clarify the physical limitations of the SRIM predictions, and, more importantly, provide recommendations to address the two challenges.

Effect of Misfit Dislocations in Junctions and Base Thickness on Latch-up Characteristics in Two-Terminal Vertical Thyristor-Based Capacitorless Memory

Conventional dynamic random access memory (DRAM) has been facing a severe challenge to scale down to 10 nm size. The cell capacitor should be able to store sufficient charges of 25~30 fF/cell. In order to meet this requirement, high aspect ratio of cell capacitor is inevitable, resulting in capacitors leaning into each other. To overcome this issue coming from capacitor, the vertical thyristor-based two-terminal capacitorless memory was proposed as a promising candidate to replace current DRAM, which consists of p++-anode / n+-base / p+-base / n++-cathode vertical structure using conventional Si technology. The two-terminal vertical thyristor-based capacitorless memory cell having p++-anode / n+-base / p+-base / n++-cathode vertical structure is easy to construct cross-point memory array cells, as shown in Figure 1. Furthermore, Two-terminal vertical thyristor-based capacitorless memory array cells can be operated without selector since sneak current is suppressed by latch-down process. In our study, three types of two-terminal vertical thyristor-based capacitorless memory cells consist of top-p++-anode / n+-base / p+-base / bottom-n++-cathode with 80-nm-thick base region (PNPN80), top-p++-anode / n+-base / p+-base / bottom-n++-cathode with 160-nm-thick base region (PNPN160), and top-n++-cathode / p+-base / n+-base / bottom-p++-anode with 160-nm-thick base region (NPNP160) were fabricated, individually. The dopant concentration profile of each device was analyzed by secondary ion mass spectroscopy (SIMS), as shown in Figure 2. By correlating HR TEM images with SIMS analyses, misfit dislocations were observed at the depth where dopant pile-up was found, as shown in Figure 3. Figure 4 (a), (b) and (c) show I-V measurements of PNPN80, PNPN160 and NPNP160, respectively. The difference of latch-up voltages between Figure 4 (b) and (c) can be explained by carrier life-time difference. NPNP160 had less misfit dislocations compared to PNPN160, resulting in longer excess carrier life-time, since dislocations act as recombination centers. Thus, NPNP160 needed smaller anode voltage to induce sufficient excess carriers in base regions, which had smaller latch-up voltage than PNPN160. In addition, the difference of latch-up voltages between Figure 4 (a) and (b) resulting from base thickness difference can be explained by electric field. The electric field of PNPN160 is weaker than that of PNPN80 in base regions when equivalent anode voltage is applied. Therefore, PNPN160 needed larger anode voltage to lower the injection barrier of excess carriers. Besides, the effect of misfit dislocations and base dopant concentration on memory margin and off-state leakage current will be discussed. Finally, the memory cell pulse operations will be exhibited as well. Acknowledgment * This research was supported by Brain Korea 21 PLUS Program in 2018, the MOTIE (Ministry of Trade, Industry & Energy 10069063) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. Figure 1

A differential Hall effect measurement method with sub-nanometre resolution for active dopant concentration profiling in ultrathin doped Si1-x Ge x and Si layers.

In this paper, we present an enhanced differential Hall effect measurement method (DHE) for ultrathin Si and SiGe layers for the investigation of dopant activation in the surface region with sub-nanometre resolution. In the case of SiGe, which constitutes the most challenging process, we show the reliability of the SC1 chemical solution (NH4OH/H2O2/H2O) with its slow etch rate, stoichiometry conservation and low roughness generation. The reliability of a complete DHE procedure, with an etching step as small as 0.5 nm, is demonstrated on a dedicated 20 nm thick SiGe test structure fabricated by CVD and uniformly doped in situ during growth. The developed method is finally applied to the investigation of dopant activation achieved by advanced annealing methods (including millisecond and nanosecond laser annealing) in two material systems: 6 nm thick SiGeOI and 11 nm thick SOI. In both cases, DHE is shown to be a uniquely sensitive characterisation technique for a detailed investigation of dopant activation in ultrashallow layers, providing sub-nanometre resolution for both dopant concentration and carrier mobility depth profiles.