Doped Layer Research Articles

Effect of drift layer doping and NiO parameters in achieving 8.9 kV breakdown in 100 μm diameter and 4 kV/4 A in 1 mm diameter NiO/β-Ga2O3 rectifiers

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga2O3 rectifiers is reported. Decreasing the drift layer doping from 8 × 1015 to 6.7 × 1015 cm−3 produced an increase in reverse breakdown voltage (VB) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in VB compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (RON) was increased from 7.1 m Ω cm2 in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm2 in heterojunctions. The maximum power figure of merit (VB)2/RON was 10.2 GW cm−2 for the 100 μm NiO/Ga2O3 devices. We also fabricated large area (1 mm2) devices on the same wafer, achieving VB of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW cm−2 for these devices. These parameters are the highest reported for large area Ga2O3 rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN.

Few-mode erbium-doped fiber with trench and weak coupling for mode gain equalization based on layered-doping technology.

A few-mode erbium-doped fiber (FM-EDF) with a step refractive index and trench structure is designed and proposed to realize the modal gain equalization of a few-mode erbium-doped fiber amplifier (FM-EDFA). The layered-doping technology is used to reduce the mode gain difference (DMG). The doping radius and doping concentration are adjusted to obtain the optimum FM-EDF structure. When the designed FM-EDF is applied to the FM-EDFA, the DMG of the whole C-band is less than 0.15dB and the DMG is less than 0.12dB at 1550nm. The minimum refractive index difference (Δ n eff) between modes can be calculated according to the refractive index and radius of the fiber core; i.e.,1.35×10-3, which will greatly reduce the coupling between modes in a practical application. Tolerances in the fiber manufacturing process are also considered for reliable FM-EDFA performance. When the doping radius and concentration of each doping layer fluctuate by ±15% based on the precise value, the maximum DMG increases to 1.8dB. In general, DMG can maintain a small value, which is beneficial for application in optical communications systems.

Effect of nitrogen introduced at the SiC/SiO2 interface and SiC side on the electronic states by first-principles calculation

In this study, using first-principles calculations, we investigate the behavior of electrons at the SiC/SiO2 interface when nitrogen is introduced as a dopant within a few nm of the SiC surface. When a highly doped nitrogen layer (5 × 1019 cm−3) is introduced within a few nm of the SiC(112̅0) surface, the electronic state is not significantly affected if the doping region is less than 4 nm. However, if the doping region exceeds 4 nm, the effect of quantum confinement decreases, which increases the electron density induced in the inversion layer. As for the wavefunction, even when an electric field is applied, the peak shifts toward the direction in which the electrons are pulled away from the interface. This reduces the effect of electron scattering at the interface and improves electron mobility.

Advances in In Situ Boron and Phosphorous Doping of SiGeSn

Dopant concentrations higher than 1 × 1019 cm−3 are required to improve the performances of various GeSn based devices such as photodetectors, electrically pumped lasers and so on. In this study, the in situ Boron and Phosphorous doping of SiGeSn was investigated, building upon recent studies on in situ B or P doped GeSn. The surfaces of intrinsic and lowly doped pseudomorphic SiGeSn layers were rough. By contrast, a 〈110〉 cross hatch was recovered and surfaces as smooth as the Ge Strain-Relaxed Buffers underneath were obtained for the highest B2H6 or PH3 mass-flows. The surface Root Mean Square roughness and Zrange values were then as low as 0.36 nm and 2.86 nm for SiGeSn:B, and 0.47 nm and 4.60 nm for SiGeSn:P. In addition, Si contents as high as 25% were obtained, notably in SiGeSn:B layers. Dopants were almost fully electrically active in those SiGeSn:B and SiGeSn:P layers, with carrier concentrations as high as 2.0 × 1020 cm−3 and 2.7 × 1020 cm−3, respectively. For SiGeSn:P, the shortcoming of in situ doped GeSn:P was overcome, that is the formation of electrically inactive SnmPnV clusters for high PH3 mass-flows. Such electrically active carrier concentrations will be beneficial for (Si)GeSn based devices, but also for all Group-IV based devices with extremely low thermal budget constraints.

The effect of silicon films impurity compensation on the performance of silicon drift detector

Silicon drift detector (SDD) fabricated using in-situ doping technology has been developed in recent years since its advantages of low process temperature and ultra-shallow junction. However, the N+ and P+ regions on the same substrate surface cannot be formed simultaneously during the process of fabricating SDD by using in-situ doping. To overcome these challenges, we proposed a simple method to form stacked silicon films with phosphorus doped layer and boron doped layer on the same surface of SDD by impurity compensation effect in this work. The stacked silicon films with thicker boron doped layer (stack-1, partially compensated) and thinner boron doped layer (stack-2, totally compensated) were prepared by impurity compensation. Then we further investigated the films properties and electrical properties of stack-1 and stack-2. The experimental results showed that the leakage current of stack-1 was larger than that of stack-2 in PN junction devices. The simulation results indicated that a tunnel junction was formed in the stack-1 which would cause a very large leakage current when a reverse bias was applied to the PN junction. Finally, stack-2 with completely compensated boron doped layer was used to fabricated SDD and the best energy resolution achieved at the temperature of 213 K was 180 eV @ the Mn-Kα line (5.9 keV). The good energy resolution of this SDD indicated that the stacked silicon films with fully compensated boron doped layer are suitable applied to fabricating the silicon drift detectors and can also obtain excellent device performance.

Bandgap engineered, quantum-confined ultrathin Te layer for highly effective back surface field in Si/organic hybrid solar cells

The suppression of back surface carrier recombination is primary importance for efficient Si-based solar cells. It is usually achieved with back surface field (BSF) by introducing a highly doped layer on the rear surface, which require a high temperature doping processes and involves toxic gasses. Here, we report that a high power conversion efficiency of 13.1% with device area of 1cm2 has been achieved for planar Si-PEDOT:PSS hybrid solar cells by depositing a ultrathin tellurium (Te) layer between silicon and rear contact, without any high temperature process or further treatment. Current-voltage, electrochemical impedance spectroscopy, quasi-steady-state-photoconductance measurements are used to investigate the effect of Te BSF layer on the device performance. The insertion of Te BSF layer have successfully reduced charge carrier recombination at rear interface. In addition, it could reduce the contact resistance thanks to the favorable band alignment originated from quantum confinement effect, resulting in improved current density and fill factor. This is the very first demonstration that a quantum-confined material can be used as an effective passivating back surface field layer in Si-organic hybrid solar cells.

Simulation aided design of a high efficient GaSb based single-junction solar cell

AbstractOwing to their elevated absorption coefficient, superior resistance to radiation and reduced effective electron mass, gallium antimonide (GaSb) semiconductors were documented to be suitable for photovoltaic systems applications. However, they were found to suffer from low efficiency. This work has been dedicated to enhance the design of GaSb based single-junction solar cells having window and back surface field made from AlGaAsSb material. The purpose is to maximize their electrical performance by considering doping and thickness of layers as design variables. A methodology of optimization was proposed. It is based on computer aided design through simulations performed under SILVACO-ATLAS software and a numerical procedure that was developed to achieve optimal design of this particular solar cell. The approach encompasses analysis of variance and derivation of response surface models to get explicit expression of the objective function corresponding to efficiency. The obtained results have shown an efficiency which is higher than all the previous known values established in literature.

Broadband Response and a Transformation between Dual‐ and Single‐Wavelength Detection in Coupled Doped‐Well Quantum Cascade Detector

AbstractAs a third‐generation infrared detector, the quantum cascade detector (QCD) exhibits an accurately adjustable wavelength, low noise, and ultrafast response characteristics. By introducing an additional doping layer, QCD also shows excellent application prospects in the broadband response. Herein, a coupled doped‐well QCD with an array structure located at very long‐wave infrared (VLWIR, ≈15 µm) is prepared. Based on the energy levels interaction and carrier distribution, the regulatory mechanism of the applied bias on the response characteristics is explored. At zero bias, the detector exhibits dual‐wavelength detection owing to the splitting of the energy levels, then transforms into single‐wavelength detection with the bias increasing. Simultaneously, the QCD device exhibits a broadband response (≈13–16 µm) from 15 to 300 K and an excellent detectivity of 1.52 × 1012 cm Hz1/2 W−1 at 15 K. A high R0A (>106 Ω cm2) and robust detectivity (>109 cm Hz1/2 W−1) are obtained at room temperature. The results of the response characteristics presented in this work provide a strategy for the flexible application of QCD in infrared detection.

Enhancing Triplet-Triplet Upconversion Efficiency and Operational Lifetime in Blue Organic Light-Emitting Diodes by Utilizing Thermally Activated Delayed Fluorescence Materials.

In the process of triplet-triplet upconversion (TTU), a bright excited singlet can be generated because of the collision of two dark excited triplets. In particular, the efficiency of TTU is crucial for achieving a high exciton production yield in blue fluorescence organic light-emitting diodes (OLEDs) beyond the theoretical limit. While the theoretical upper limit of TTU contribution yield is expected to be 60%, blue OLEDs with the maximum TTU contribution are still scarce. Herein, we present a proof of concept for realizing the maximum TTU contribution yield in blue OLEDs, achieved through the doping of thermally activated delayed fluorescence (TADF) molecules in the carrier recombination zone. The bipolar carrier transport ability of TADF materials enables direct carrier recombination on the molecules, resulting in the expansion of the recombination zone. Although the external electroluminescence quantum efficiency of OLEDs is slightly lower than that of conventional TTU-OLEDs due to the low photoluminescence quantum yield of the doped layer, the TTU efficiency approaches the upper limit. Furthermore, the operational device lifetime of OLEDs employing TADF molecules increased by five times compared to the conventional ones, highlighting the expansion of the recombination zone as a crucial factor for enhancing overall OLED performance in TTU-OLEDs.

Characterization of tunnel oxide passivated contact fabricated by sputtering and ion implantation technique

Tunnel oxide passivated contact (TOPCon) structures using highly doped n-type polycrystalline silicon were fabricated using facing target sputtering and ion implantation techniques for a SiH4-free fabrication process of high-efficiency silicon solar cells. We investigated the structural and electrical properties of the highly doped n-type poly-Si layers to optimize the ion implantation process. We also investigated the surface passivation quality of our TOPCon structure. An effective carrier lifetime of 2.01 ms and an implied open circuit voltage of 704 mV were obtained for our sample annealed at 950 °C. The sample also exhibits a low contact resistance of 3.22 × 10−3 Ω cm−2. Our results open the way for SiH4-free fabrication of silicon solar cells with a TOPCon structure.

Plasmonic nanoparticle doped medium for frequency selective waveguide coupling

We present a powerful technique for frequency selective coupling of dielectric waveguides by incorporating plasmonic nanoparticles within the structure. This is achieved by the inclusion of an optimal density of subwavelength plasmonic nanoparticles within the cladding layer between the coupling waveguides. Frequency selectivity of the coupling is possible by careful manipulation of the nanoparticle characteristics that govern the effective refractive index of the doped layer that supports plasmonic excitations within a narrow frequency range. The efficient coupling arises from inducing modes within the nanoparticle doped cladding layer of the waveguide in the spectral and spatial vicinities of the selected waveguide modes. Our observations are modeled with the transfer matrix method and corroborated with full-wave simulations.

2.1 kV (001)- β -Ga2O3 vertical Schottky barrier diode with high-k oxide field plate

We report a vertical β-Ga2O3 Schottky barrier diode (SBD) with BaTiO3 as field plate oxide on a low doped thick epitaxial layer exhibiting 2.1 kV breakdown voltage. A thick drift layer of 11 μm with a low effective doping concentration of 8 × 1015 cm–3 is used to achieve high breakdown voltage. Using the high-k dielectric with a dielectric constant of 248, the breakdown voltage increases from 816 V for the non-field-plated SBD to 2152 V (>2× improvement) for the field-plated SBD without compromising the on-state performance. The diode dimensions are varied to analyze the effect of edge high-field related leakage with reverse bias and also the effect of current spreading during forward operation. Very uniform distribution of breakdown voltages of 2152 ± 20 V are observed for the diode diameters from 50 to 300 μm for the field-plated SBDs. The on and off state power losses are also analyzed and compared with the non-field-plated devices and the switching losses are estimated analytically.

Theoretical investigation of silicon thin film solar cell for improving short and long wavelength response

Theoretical investigation or numerical simulation of performance of a solar cell can provide useful information to maximize power conversion efficiency. In that respect we carried out a set of numerical simulation using AFORS-HET simulation program. Separately the optical absorption of the individual layers of thin film solar cell was analyzed. Current-voltage characteristic curve of a reference cell (Cell-A) was used as the starting device. The power conversion efficiency (PCE) of the reference device was 8.85% with short circuit current density (Jsc) of 15.43 mA/cm2 and fill factor (FF) of 68.3%. After suitable optimization the PCE of this device (Cell-B2) improves to 11.59% (Jsc and FF became 13.0 mA/cm2 and 87% respectively). The results show that the effective optical absorption in the active layer can be improved significantly by optimizing the device structure. The short wavelength response can be improved by reducing the parasitic optical absorption of the doped window layer, while the long wavelength response improves by raising effective absorption length of the active layer. Furthermore, optimum thickness of the active layer, for the highest possible PCE, is found to be dependent upon its defect density.

Terahertz nanospectroscopy of plasmon polaritons for the evaluation of doping in quantum devices

Abstract Terahertz (THz) waves are a highly sensitive probe of free carrier concentrations in semiconducting materials. However, most experiments operate in the far-field, which precludes the observation of nanoscale features that affect the material response. Here, we demonstrate the use of nanoscale THz plasmon polaritons as an indicator of surface quality in prototypical quantum devices properties. Using THz near-field hyperspectral measurements, we observe polaritonic features in doped silicon near a metal-semiconductor interface. The presence of the THz surface plasmon polariton indicates the existence of a thin film doped layer on the device. Using a multilayer extraction procedure utilising vector calibration, we quantitatively probe the doped surface layer and determine its thickness and complex permittivity. The recovered multilayer characteristics match the dielectric conditions necessary to support the THz surface plasmon polariton. Applying these findings to superconducting resonators, we show that etching of this doped layer leads to an increase of the quality factor as determined by cryogenic measurements. This study demonstrates that THz scattering-type scanning near-field optical microscopy (s-SNOM) is a promising diagnostic tool for characterization of surface dielectric properties of quantum devices.

Optimal Aluminum Doping Method in PEALD for Designing Outstandingly Stable InAlZnO TFT

AbstractOxide semiconductors are promising semiconducting materials for next‐generation thin‐film transistors (TFTs). The role of the cations should be considered in the design of oxide semiconductors suitable for various applications. Ga has been widely used as a carrier suppressor in oxide semiconductors; however, research has recently been conducted to replace it with Al, which is cheaper and forms a stronger bond with O. Deposition using plasma‐enhanced atomic layer deposition (PEALD) is very useful for controlling the composition of the cations in an oxide semiconductor. In this study, an InAlZnO (IAZO) semiconductor was deposited using super‐cycle PEALD for the first time. Al2O3 (AO), AlZnO (AZO), and InAlO (IAO) as Al doping layers in IAZO are evaluated. Considering its electrical characteristics and stability, AZO is selected as the optimal Al doping layer. The IAZO thin film and TFT characteristics according to the Al doping method are examined through the role of the cations and H concentration. In addition, the Al concentration is controlled by adjusting the number of cycles in super‐cycle PEALD. As the Al concentration decreased, the mobility increased; however, the stability degraded significantly, which is induced by the increase in oxygen vacancies. Finally, the high‐Al IAZO system using AZO successfully showed overall excellent properties including mobility over 10 cm2 V−1 s, and outstanding stability under 0.1 V, which are greatly improved compared to previously reported IAZO TFTs.

Significant improvement of injection efficiency in deep-UV LD structures by light Mg doping in p-core layer

Improvement of carrier injection efficiency is essential to achieve lower threshold and shorter wavelength in deep-ultraviolet laser diodes. We have confirmed that the introduction of electron blocking layer and Mg doping layer into the core layer significantly improves the injection efficiency. In this study, we show that optimizing the Mg doping level in the core layer improves the external quantum efficiency by a factor of about 10 compared to the non-doped sample. The dependence of the external quantum efficiency on Mg flow rate can be interpreted in terms of a reduction of the dip at the p-side core/cladding interface in the conduction band. The dip is expected to be suppressed by ionized Mg activated by the Poole–Frenkel effect, resulting in improved carrier injection efficiency. A remarkable improvement in efficiency is also observed when the Mg doping region is limited to the vicinity of the core/cladding layer interface.

Al/Si dopants effect on the electronic and optical behaviors of graphene mono-layers useful for infrared detector devices

Two-dimensional (2D) graphene with different forms is a prosperous class of materials beneficial in nano-electronics, and infrared-detector devices. Herein, we analyze the electronic and optical behaviours of electron acceptor (Al)- and isovalent (Si) inserted into graphene sheets, which are computed by utilizing ab-initio simulations. We find that the individual doping impurities of Al or Si atoms onto monolayer graphene result in p-type and semiconducting behaviours, respectively, attributed to the contribution of the valence electrons number of these atoms to the host 2D honeycomb lattice of graphene. Even though the Al atom contributing one less electron to the host lattice, both individual impurities of the Al- or Si-doped materials are found to cause a splitting in the valence states and conduction states at the K-point, leading to the opening of the Dirac cone. In monolayer graphene, doping two Al atoms into the nearest neighbour sites creates a trivial metallic system, while doping two Si atoms into the nearest neighbour sites causes the Dirac cone to re-emerge. Owing to the stark difference in the electronic structure results of mono- and double-atom substitution of Al/Si in graphene single-layers, we find different optical behaviours in these doped systems. Additionally, X-ray absorption spectroscopy simulations are employed to inspect the core-level spectra of pure and substitutional doped graphene single layers. Accordingly, the optical spectral features of graphene single-layers substituted with foreign impurities, such as Al/Si have revealed the tailoring of optical absorption from the infrared to the visible windows. Due to the outstanding characteristics of this 2D dimensional gapless graphene, our simulated results could provide a guidance for future experimental investigations into the fabrication of doped graphene sheets suitable for infrared detectors, photonics, and modern optoelectronic devices integrated into advanced technologies.

P-n Homojunction perovskite solar cells: effects of ionic density and thickness of the doped layers

The p-n homojunction perovskite solar cells are promising in comparison to planar heterojunction perovskite solar cells. It is observed that the p-n homojunction provides an efficient built-in electric field for the photo-generated electron-hole pairs to dissociate into free charge-carriers and orient them to transport to the respective electrodes, thereby reducing the overall losses due to recombination processes. However, the existence of p-n homojunction, while in operation, is currently undergoing experimental tests. In this study, we simulated the current-voltage characteristics of both planar heterojunction and p-n homojunction perovskite solar cells. For simulation, we utilised a one-dimensional drift-diffusion equation. The current-voltage characteristics show a high fill factor for the p-n homojunction devices indicating better power conversion efficiency as compared to that of planar heterojunction devices. In addition, we report the spatial distribution of electron- and hole-density with variations in (i) the mobile ionic density, inherently present in the perovskite material and (ii) the thickness of p- and n-type perovskite layers. It is observed that the role of p-n homojunction ceases when the ionic density is higher than 1017 cm−3 or when one of the doped layers is substantially thicker as compared to the other. We correlate the cease of p-n homojunction to (i) the ionic effect which screens the built-in field, and (ii) the predominance of the effect of one doped layer over the other. The study provides impetus to the development of the p-n homojunction perovskite solar cells.

Modulation of Schottky barrier at metal/Ge contacts by phosphoric acid coating and excimer laser annealing

Germanium is an attractive material for advanced devices such as high speed transistors and high efficiency optical devices. To realize the high performance operation, low resistive contacts to Ge are required. However, the Fermi level in Ge at metal/Ge contacts is often pinned near the valance band edge. This Fermi level pinning generates high Schottky barrier heights at metal/Ge contacts regardless the work function of the metals, which results in low resistive contacts to p-type Ge but high resistive contacts to n-type Ge. Thus, a technique for modulating the Schottky barrier heights at metal/Ge contacts is desired. For this purpose, effects of surface treatments of Ge using phosphoric acid coating and excimer laser annealing (LA) were investigated. After LA without phosphoric acid coating, the reverse current of the Al/n-type Ge contacts is increased by an order of magnitude. On the other hand, the reverse current is significantly increased by three orders of magnitude after LA with phosphoric acid coating. Here, the Schottky barrier heights are decreased from 0.56 to 0.50 eV and from 0.59 to 0.49 eV after LA for samples without and with phosphoric acid coating, respectively. These phenomena are attributed to formation of very thin GeOx surface layers and very shallow doped layers having very high P concentrations, respectively. This technique is useful for modulation of the Fermi level pinning for metal/Ge contacts with the low thermal budget.

Unconventional Substitution for BiVO4 to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping.

Bismuth vanadate (BiVO4) as a fascinating semiconductor for photoelectrocatalytic (PEC) water oxidation with suitable band gap (Eg) has been limited by the issue of poor separation and transportation of charge carriers. Herein, we propose an unconventional substitution of V5+ sites by Ti4+ in BiVO4 (Ti:BiVO4) for the similar ionic radii and accelerated polaron hopping. Ti:BiVO4 increased the photocurrent density 1.90 times up to 2.51 mA cm-2 at 1.23 V vs RHE and increased the charge carrier density 1.81 times to 5.86 × 1018 cm-3. Compared with bare BiVO4, Ti:BiVO4 improves the bulk separation efficiency to 88.3% at 1.23 V vs RHE. The DFT calculations have illustrated that Ti-doping modification could decrease the polaron hopping energy barrier, narrow the Eg, and decrease the overpotential of the oxygen evolution reaction (OER) concurrently. With further spin-coated FeOOH cocatalyst, the photoanode has a photocurrent density of 3.99 mA cm-2 at 1.23 V vs RHE. The excellent PEC performance of FeOOH/Ti:BiVO4 is attributed to the synergistic effect of the FeOOH layer and Ti doping, which could promote charge carrier separation and transfer by expediting polaron migration.