High-low Junction Research Articles

Tunnel oxide passivating contact enabled by polysilicon on ultra-thin SiO2 for advanced silicon radiation detectors

Conventional silicon junction detectors encounter significant carrier recombination within the heavily doped p+ and n+ layers, as well as beneath the metal contact regions, creating the so-called “dead layers”, especially on the detector side. In this study, we present the tunnel oxide passivating contact with doped polysilicon on oxide, which demonstrates exceptional surface passivation and carrier selectivity. The key innovation lies in an ultra-thin (~ 1.5 nm) interfacial oxide layer that facilitates efficient majority carrier transportation via tunneling while effectively block minority carriers. Remarkably low saturation current densities, ranging from 5 to 10 fA/cm2 even with the metal contact, underscore the superiority of both n-type and p-type tunnel oxide passivating contacts. In contrast, conventional p–n junction or high-low junction exhibit saturation current densities ranging from 10 to 90 fA/cm2 in the studied p+ and n+ layers with surface passivation schemes due to Auger recombination and surface recombination, and 1000–6000 fA/cm2 with metal contacts due to intense metal-induced recombination at the interface. These findings indicate the potential and superiority of implementing n-type tunnel oxide passivating contact on the detector side and p-type contact on the back side for advanced silicon radiation detectors. This approach would enable thorough collection of generated charge carriers along the track of incident ionizing radiation particles, leading to improved energy resolution and reduced noise levels.

Interfacial chemical bond and oxygen vacancies modulated Mo2S3/BiOBr high-low junctions for enhanced photocatalysis gatifloxacin degradation

Insufficient light absorption and rapid recombination of photogenerated charges are considered to be the main obstacles limiting the efficiency of photocatalytic degradation of organic pollutants. In this study, a novel Mo2S3/BiOBr high-low junction was constructed by coupling narrow bandgap Mo2S3 to improve the light absorption capacity of BiOBr and enhance the utilization of photogenerated charges. Under simulated visible light irradiation, the optimised Mo2S3/BiOBr-6% high-low junction (90.1%) exhibited higher degradation efficiency of gatifloxacin compared to BiOBr (74.9%) and Mo2S3 (72.4%). The improved photocatalytic activity is mainly attributed to the interfacial chemical bonding and the formation of high-low junctions facilitating the rapid transfer of photogenerated charges between semiconductors, resulting in effective separation of photogenerated charges. In addition, the abundant oxygen vacancies in Mo2S3/BiOBr can act as electron traps to trap electrons and effectively reduce the photogenerated charge complexation. This study provides new perspectives for the construction of high-low junctions with interfacial chemical bonds and abundant oxygen vacancies and their use in the photocatalytic degradation of organic pollutants.

Novel MIL-88B(Fe)/ZnTi-LDH high-low junctions for adsorption and photodegradation of tetracycline: Characteristics, performance, and mechanisms

In this paper, a high-efficient MIL-88B(Fe)/ZnTi-LDH high-low junction photocatalyst was successfully prepared by a simple method for the photodegradation of tetracycline (TC). The combination of MIL-88B(Fe) and ZnTi-LDH created an internal electric field and high-low junction, facilitated the separation of carriers, and thus enhanced photocatalytic activity. The optimized MZ-30% sample showed the highest photocatalytic degradation of TC about 93.51%. The photocatalytic kinetic removal rate of MZ-30% reached 0.0941 min−1, 31.36 times of MIL-88B(Fe) (0.003 min−1) and 1.7 times of ZnTi-LDH (0.0550 min−1). Furthermore, MZ-30% possessed the highest adsorption capacity for TC (543.89 mg/L). The effects of co-existing ions, initial pH, water source, and humic acid (HA) on TC degradation were examined. The main active substances of MIL-88B(Fe)/ZnTi-LDH in photocatalytic reaction were 1O2, h+, and ·O2- confirming by reactive species elimination experiment and electron spin resonance (ESR) tests. The molecular structure and toxicity of intermediates were investigated by a high-performance liquid chromatography-mass spectrometer (HPLC-MS) and Toxicity Estimation Software Tool (T.E.S.T), and the possible degradation pathway of TC was revealed. The results highlight that MIL-88B(Fe)/ZnTi-LDH high-low junction composite is a desirable photocatalyst for removing antibiotics and other emerging contaminants in water.

BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production

Through a two-step solvothermal method, different molar ratios of BiPO4 were grown in situ on the surface of oxygen-vacancy-rich BiOBr (Ov-BiOBr), successfully constructing a BiPO4/Ov-BiOBr heterojunction composite material. By constructing a novel type I high-low junction between the semiconductor BiPO4 and Ov-BiOBr, stronger oxidative holes or reductive electrons were retained, thereby improving the redox performance of the photocatalyst. The composite catalyst with a 10% molar content of BiPO4 demonstrated the highest degradation rate of tetracycline (TC), degrading over 95% within 90 min, with a rate constant of 0.02534 min−1, which is 2.3 times that of Ov-BiOBr and 22 times that of BiPO4. The 10% BiPO4/Ov-BiOBr sample displayed the best photocatalytic activity, producing 139 μmol·L−1 H2O2 in 120 min, which is 3.6 times the efficiency of Ov-BiOBr and 19 times that of BiPO4. This was due to the appropriate bandgap matching between BiPO4 and Ov-BiOBr, the photo-generated electron transfer channel via Bi-bridge, and efficient charge separation. It was inferred that the free radical species ·OH and ·O2− played the dominant role in the photocatalytic process. Based on experimental and theoretical results, a possible photocatalytic mechanism was proposed.

Novel BiOBr/Bi2S3 high-low junction prepared by molten salt method for boosting photocatalytic degradation and H2O2 production

The molten salt method focuses on improving the crystallinity of synthetic materials and avoiding the high energy consumption of traditional synthesis processes. In this work, a novel BiOBr/Bi2S3 high-low junction with large contact area was constructed by the molten salt method combined with the ion exchange strategy. Its unique energy band structure and new charge transfer mechanism realize the rapid migration of photogenerated charges between different components. Specifically, Bi2S3 was grown on BiOBr in situ by a high-temperature molten salt reaction. Due to the deep valence band position of BiOBr and the narrow band gap of Bi2S3, an intrinsic internal electric field and band bending are produced at the interface, forming a high-low junction photocatalyst with an intimate interface. In addition, the BiOBr/Bi2S3 composite maintains a high oxidation potential and produces high and robust photocatalytic oxidation activity. In the molten state, the close binding of BiOBr and Bi2S3 can be promoted through the ion-exchange strategy, resulting in excellent photocatalytic degradation rates of bisphenol A and tetracycline and in-situ generation of H2O2. Finally, the mechanism of carriers separation and transfer in BiOBr/Bi2S3 high-low junction is also discussed. Density functional theory (DFT) results found that the improvement of O2 adsorption ability would promote the occurrence of oxygen reduction reaction (ORR), and make positive contributions to the enhanced H2O2 production activity. This study will provide a new perspective for broadening the spectral response range of Bi-based photocatalytic materials and preparing high-low junction photocatalysts with dense interface by the molten salt method.

Ultraviolet photodetectors based on Si-Zn-SnO thin film transistors with a stacked channel structure and a patterned NiO capping layer

Ultraviolet photodetectors (UVPDs) based on Si-Zn-SnO (SZTO) thin-film transistors (TFTs) with a stacked dual-channel layer (DCL) structure with different carrier concentration and NiO capping layer (CL) to alleviate the trade-off between dark current (I dark) and photocurrent (I ph) are reported. Experimental results show that under 275 nm irradiation, the proposed SZTO TFT UVPD with a 30 nm thick upper layer stacked on a 50 nm thick channel layer and a patterned NiO CL exhibit excellent photoresponsivity and photosensitivity up to 1672 A W−1 and 1.03 × 107 A A−1, which is about 272 and 137 times higher than conventional 30 nm thick single-channel layer SZTO TFT. These improvements are due to the use of a DCL which forms a high-low junction to reduce the effective channel thickness and increasing the space for UV illumination and the use of NiO CL lowers the I dark and causes a considerable negative threshold voltage shift under UV irradiation to significantly boost the I ph.

Paper based self-powered UV photodiode: Enhancing photo-response with AZO back-field layer

The development of self-powered UV photodiode on green substrate is still a challenge. In this study, highly conductive Al doped ZnO (AZO) film was sputter-deposited on paper substrate at room temperature; subsequently, paper-based Schottky photodiodes were successfully fabricated by following Au-ZnO-Al and Au-ZnO-AZO-Al structure designs, respectively. These UV light-based devices operate successfully without external power. Au-ZnO-AZO-Al photodiode shows improved response and faster speed compared to Au-ZnO-Al photodiode. The one-dimensional Poisson simulation results demonstrate that the improvement is attributed to the strong electric field in the ZnO-AZO n-n+ high-low junction, thus, enhancing the collection rate and drifting speed of the photogenerated carriers. In addition, AZO can form favorable ohmic contact with the Al electrodes, further improving the device performance. The photo-response of the paper-based photodiode device shows high reproducibility and stability. Based on these outstanding results, we conclude that these paper-based electronic devices have great potential to replace conventional petroleum-based flexible electronic devices, thus, contributing to the green/sustainable development.

Novel ZnFe2O4/Bi2S3 high-low junctions for boosting tetracycline degradation and Cr(VI) reduction

The fabrication of photocatalysts with high reaction activity for decomposition of pharmaceutical contaminants and detoxication of heavy metal ions in wastewater is challenging. In this study, novel ZnFe2O4/Bi2S3 high-low junctions with different work functions and tight interface were constructed by the growth of Bi2S3 over ZnFe2O4 under hydrothermal conditions, and the content of ZnFe2O4 was optimized. The optimal 12 % ZnFe2O4/Bi2S3 sample exhibited the best visible-light photocatalytic performance of 91.6 % tetracycline removal at solution pH of 4.72 and 96.7 % reduction efficiency of Cr(VI) at solution pH of 5.51 within 2 h. Holes (h+), hydroxyl radicals (OH) and superoxide radicals (O2−) jointly attacked tetracycline, leading to efficacious decomposition of tetracycline and sharp toxicity reduction. The toxicity prediction of degradation intermediates by ECOSAR software and E. coli growth further confirmed the significant role of ZnFe2O4/Bi2S3 high-low junction in toxicity reduction of tetracycline. The photogenerated electrons were involved in Cr(VI) reduction. The relationship between structure and photocatalytic activity was set forth from the view of light absorption, band structure, internal electric field at interface, charge separation and migration behaviors. Importantly, the photocatalytic mechanism of ZnFe2O4/Bi2S3 high-low junctions with different work functions and intimate interface was first provided based on edge energy offset. This work will provide new insights for the preparation and application of high-low junction photocatalytic materials based on CB/VB edge energy shift and unique photogenerated charge transfer mechanism.

Double-defect-induced polarization enhanced OV-BiOBr/Cu2−xS high-low junction for boosted photoelectrochemical hydrogen evolution

In order to improve the carrier transfer driving force and separation efficiency, OV-BiOBr/Cu2−xS high-low junction with double defects (Oxygen vacancy and Copper vacancy) was successfully prepared by a facile two-step solvothermal in situ growth technique. OV-BiOBr/Cu2−xS-0.2 showed the highest hydrogen evolution rate (509.75 μmol·cm−2·h−1), which was 12.45, 6.94 and 3.44 times that of pure BiOBr, OV-BiOBr and BiOBr/Cu2−xS composite, respectively. Obvious free radicals (·OH and ·O2-) were observed in the dark state for the first time. Theoretical and experimental results demonstrate that the charge imbalance within OV-BiOBr/Cu2−xS intensifies after the introduction of the double defects, resulting in enhanced interfacial polarization phenomena. Boosted photoelectrochemical hydrogen evolution activities were achieved with the synergistic effect of the internal electric field and polarization electric field. This study will lay a scientific and experimental foundation for the preparation of high-performance photoelectrochemical anode materials with double defect heterojunction.

Novel Znfe2o4/Bi2s3 High-Low Junctions for Boosting Tetracycline Degradation and Cr(Vi) Reduction

Novel Znfe2o4/Bi2s3 High-Low Junctions for Boosting Tetracycline Degradation and Cr(Vi) Reduction

Fabrication of silicon PIN diode with SiGe junction for soft x-ray detector using low-temperature technology

In this paper, a low-temperature technology was introduced for the fabrication of PIN photodiode. A double dielectric oxide layers were prepared by nitric acid oxidation and PECVD, respectively. The selective epitaxial growth of SiGe thin layers, which were in-situ doped in a reactive thermal chemical vapor deposition system, were applied for the preparation of PN junction and high-low junction. A SiNx passivation layer combined with a post-annealing was adopted to lower the reverse current. The temperature was controlled below 450 °C for the whole fabrication process. Finally, the reverse current of 0.7 nA was achieved at room temperature for our 5 mm-diameter PIN detector and the energy resolution of 240 eV was obtained at 5.9 keV, −60 °C. The results indicated that the low-temperature technology is suitable for the fabrication of high quality PIN detector, and the cost can be reduced effectively.

Mathematical modelling of a novel heterojunction SIS front surface and interdigitated back-contact solar cell

In this paper, we propose the design and fabrication of a novel heterojunction semiconductor–insulator–semiconductor (SIS) front surface and interdigitated back-contact (IBC) solar cell. We approximate the performance parameters and loss analysis of the proposed solar cell using MATLAB software programming. Many studies have reported the experimental analysis of amorphous silicon (a-Si) IBC solar cells. A number of silicon heterojunction solar cell designs with promising efficiency have been reported in the past few decades. In this study, a long-lifetime (~ 2 ms) n-Si substrate was considered so that a sufficient number of photogenerated carriers could reach the interdigitated layer and be absorbed. The availability of carriers at the interdigitated back surface was further enhanced by considering a high-low junction created by a ZnO n+ layer at the front surface. A very thin layer of thermally deposited insulator SiO2 was considered between the ZnO and n-Si. This layer reduces the detrimental effects of interface defects. This is the first study in which we have theoretically investigated an IBC solar cell using metal oxide semiconductor layer deposition, thereby avoiding the expensive and complicated doping and diffusion process. In general, a high-concentration n+ layer is doped to create a high-low junction at the front to accelerate the transport of carriers to the back junctions. We propose a cost-effective method using thermal deposition of a SiO2 layer followed by sol–gel ZnO layer deposition, which serves the same purpose as an n+ layer by introducing an SIS junction potential at the front. The interdigitated back surface was designed with sequential n+ a-Si and p+ a-Si vertical junctions.

Analysis of the generation mechanism of the S-shaped J–V curves of MoS2/Si-based solar cells

Amorphous–microcrystalline MoS2 thin films are fabricated using the sol-gel method to produce MoS2/Si-based solar cells. The generation mechanisms of the S-shaped current density–voltage (J–V) curves of the solar cells are analyzed. To improve the performance of the solar cells and address the problem of the S-shaped J–V curve, a MoS2 film and a p+ layer are introduced into the front and back interfaces of the solar cell, respectively, which leads to the formation of a p–n junction between the p-Si and the MoS2 film as well as ohmic contacts between the MoS2 film and the ITO, improving the S-shaped J–V curve. As a result of the high doping characteristics and the high work function of the p+ layer, a high–low junction is formed between the p+ and p layers along with ohmic contacts between the p+ layer and the Ag electrode. Consequently, the S-shaped J–V curve is eliminated, and a significantly higher current density is achieved at a high voltage. The device exhibits ideal p–n junction rectification characteristics and achieves a high power-conversion efficiency (CE) of 7.55%. The findings of this study may improve the application of MoS2 thin films in silicon-based solar cells, which are expected to be widely used in various silicon-based electronic and optical devices.

Analytical study of electrical performance of SiGe-based n+-p-p+ solar cells with BaSi2 BSF structure

This manuscript reports on an analytical study required to determine and enhance the effect of the high–low junction based back surface field (BSF) of thin n-CdS/p-SiGe/p + -BaSi2 heterojunction solar cell. A 0.3 µm thick BSF layer made of extensively available and low-cost barium silicide (BaSi2) is introduced into the basic SiGe solar cell for the first time. SCAPS-1D simulator is employed to analyze different parameters such as open circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), quantum efficiency (QE) and conversion efficiency (η) of the device. The SiGe absorber layer thickness is varied from 2 to 50 µm to analyze the device performance, however optimum results are observed between 10 and 15 µm. The BSF BaSi2 layer thickness changing from 0.1 to 0.8 µm to check the viability of device for optimal efficiency. The present structure anticipates an efficiency of 17.05% for BSF layer thickness of 0.8 µm. In addition, the use of SiGe earth abundant material instead of CIGS having rare earth metals indium and gallium, reduces the overall fabrication cost of the solar cell.

Full-spectrum-responsive Bi2S3@CdS S-scheme heterostructure with intimated ultrathin RGO toward photocatalytic Cr(VI) reduction and H2O2 production: Experimental and DFT studies

Heterostructure-based photocatalysis offers significant potential for developing ultraviolet–visible (UV–Vis) to near-infrared (NIR) light-responsive catalysts with abundant beneficial physicochemical properties to boost environmental remediation upon solar-light irradiation. In this study, for the first time a novel ternary heterostructure photocatalysts containing Bi2S3@CdS@RGO (BCG) were rationally constructed using the hydrothermal approach. The resultant ternary composite with 5 wt% of CdS and 20 wt% of reduced graphene oxide (RGO) contents (BCG-5) was adopted as an optimal sample for photo-reduction of Cr(VI) under simulated solar-light irradiation. The apparent rate constant of the photo-reduction process over BCG-5 was ~ 13, 4, and 3 times higher than those of Bi2S3, CdS, and BC-5, respectively, within 150 min. The enhanced photocatalytic activity of ternary composite could be linked predominantly to the formation of heterostructural, synergistic behavior between the components, hierarchical morphology, the formation of n-n type high-low junctions, efficient interfacial charge-transfer capability, large specific surface area, full-spectrum light-absorption, and outstanding photo-stability. Electron spin resonance and reactive radical-scavenging results demonstrated that the hydroxyl and superoxide active species were primarily responsible for Cr(VI) removal. Furthermore, the photocatalytic activity of BCG-5, as an optimal sample, was further assessed regarding the photocatalytic production of H2O2, with 1.37 and 15.14 times higher efficiency than binary and bare samples, respectively. Assisted by the density functional theory calculations, ultraviolet photoelectron spectroscopy analyses, the charge-carriers pathway and possible photocatalytic mechanism were systematically discussed in S-scheme heterojunction. We expect that our findings will open new horizons for significant applications of bismuth-rich-based heterostructures under both visible- and NIR-light irradiation to address environmental and energy issues.

CN/rGO@BPQDs high-low junctions with stretching spatial charge separation ability for photocatalytic degradation and H2O2 production

Reduced graphene oxide modified black phosphorus quantum dots (rGO@BPQDs) were obtained through surface modification of BPQDs with rGO via an ultrasound-assisted liquid phase method. The rGO@BPQDs could effectively enhance the chemical and structural stability of BPQDs. Zero-dimension rGO@BPQDs were firmly anchored on mesoporous g-C3N4 (CN) via a self-trapping pore confinement effect and π-π interactions to form CN/rGO@BPQDs, which remarkably improved the photoelectric properties of CN. The responsive wavelength of CN/rGO@BPQDs could be extended to 800 nm. The kinetic constant of Rhodamine B and tetracycline degradation reached 0.183 and 0.0194 min−1, respectively. The H2O2 production rate of CN/rGO@BPQDs was 2.6 times that of porous CN. The improved photocatalytic performance and remarkable increase in free radicals are attributed to the formation of n-n type high-low junctions as well as the internal electric field based on different Fermi levels between CN and BPQDs. These collectively promote spatial separation of photogenerated carriers.

Three-dimensional a-Si/a-Ge radial heterojunction near-infrared photovoltaic detector

In this work, three-dimensional (3D) radial heterojunction photodetectors (PD) were constructed over vertical crystalline Si nanowires (SiNWs), with stacked hydrogenated amorphous germanium (a-Ge:H)/a-Si:H thin film layer as absorbers. The hetero absorber layer is designed to benefit from the type-II band alignment at the a-Ge/a-Si hetero-interface, which could help to enable an automated photo-carrier separation without exterior power supply. By inserting a carefully controlled a-Si passivation layer between the a-Ge:H layer and the p-type SiNWs, we demonstrate first a convenient fabrication of a new hetero a-Ge/a-Si structure operating as self-powered photodetectors (PD) in the near-infrared (NIR) range up to 900 nm, indicating a potential to serve as low cost, flexible and high performance radial junction sensing units for NIR imaging and PD applications.

Improved electrical performance of MOCVD-grown GaN p-i-n diodes with high-low junction p-layers

We report that using a high/low p-type junction anode structure results in improved hole injection over a uniformly-doped p-anode layer in GaN pin junction diodes. Quasi-vertical diodes with a 20 nm thick, magnesium concentration of 2 × 1020 cm−3 layer on top of a 480 nm thick layer with a magnesium concentration of 1018 cm−3 show greatly increased forward current density – >100× higher – than those with a 500 nm thick uniformly 3 × 1019 cm−3 Mg-doped p-layer. Forward knee voltage and ideality factor are reduced by a factor of more than two, and reverse leakage current density is also reduced. Additionally, the specific differential series resistance is reduced significantly. With photoluminescence measurements, we found that these improvements are largely due to improved p-GaN material quality of the high/low junction sample with lower average Mg concentration.

A high di/dt 4H-SiC thyristor with ‘-shaped’ n-base

A high di/dt 4H-SiC thyristor with ‘-shaped’ n-base is proposed and investigated by simulation. Unlike the conventional SiC thyristors, the proposed SiC thyristor features a lightly-highly-lightly doped (‘-shaped’) n-base. By modulating the concentration difference of the top p+-n emitter junction its injection efficiency increases. An extra electric field is induced by the existence of an abrupt high-low junction in n-base to force the transit of holes and therefore reduce the recombination. As a result, the turn-on time and turn-on di/dt of the new SiC thyristor are 145 ns and 948 A/cm2/μs, respectively, which are reduced by approximately 72% and increased by approximately 400% compared to conventional SiC thyristors (turn-on time is 514 ns and di/dt is 188 A/cm2/μs).

Lithium-ion battery fiber constructed by diverse-dimensional carbon nanomaterials

Graphene fibers are supposed to be ideal electrodes for fiber-shaped lithium-ion batteries. However, a big challenge remains in how to bring effectively the remarkable properties of graphene onto macroscopic graphene fibers. Here, the assembly of 2D reduced graphene oxides is coordinated by 1D carbon nanotubes resulting in a novel composite fiber. A brick–bridge network with high alignment, optimal porosity and low junction contact resistance is formed, in which reduced graphene oxides serve as the brick and carbon nanotubes as the bridge. Consequently, the composite fiber provides a fast, robust and massive transport network both for electrons and Li-ions. A large specific capacity of 522 mAh g−1 at 50 mA g−1 is achieved. Moreover, high retentions of 143% and 72% are obtained after 800 cycles at 500 mA g−1 and when current density is increased from 50 to 500 mA g−1, respectively.