Doped Layer Research Articles

Realization of Conductive n‐Type Doped α‐Ga2O3 on m‐Plane Sapphire Grown by a Two‐Step Pulsed Laser Deposition Process

Structural and electrical properties of undoped and doped α‐Ga2O3 thin films grown by pulsed laser deposition on m‐plane sapphire in a two‐step process are presented. A buffer layer of undoped α‐Ga2O3 is introduced below the electrically active thin film to improve the crystal quality and enable the stabilization of the α‐phase at lower substrate temperatures for sufficient dopant incorporation. Donor doping of the active layers with tin, germanium, and silicon, respectively, is realized below a critical substrate temperature of 600 °C. Depth‐resolved X‐ray photoelectron spectroscopy measurements on tin‐doped samples reveal a lower amount of tin in the bulk thin film compared to the surface and a lower tin incorporation for higher substrate temperatures, indicating desorption or float‐up processes that determine the dopant incorporation. Electron mobilities as high as 17 cm2 V−1 s−1 (at ) and 37 cm2 V−1 s−1 (at ) are achieved for tin‐ and germanium doping, respectively. Further, a narrow window of suitable annealing temperature from 680 to 700 K for obtaining ohmic Ti/Al/Au layer stacks is identified. For higher annealing temperatures, a deterioration of the electrical properties of the thin films is observed suggesting the need for developing low temperature contacting procedures for α‐Ga2O3‐based devices.

Liquid metal gallium-based printing of Cu-doped p-type Ga2O3 semiconductor and Ga2O3 homojunction diodes

As a promising third-generation semiconductor, gallium oxide (Ga2O3) is currently facing bottleneck for its p-type doping. The doping process of conventional semiconductors usually introduces trace impurities, which is a major technical problem in the electronics industry. In this article, we conceived that the process complexity could be significantly alleviated, and a high degree of control over the results could be attained using the selective enrichment of liquid metal interfaces and harvesting the doped metal oxide semiconductor layers. An appropriate mechanism is thus proposed to prepare the doped semiconducting based on multicomponent liquid metal alloys. Liquid metal alloys with the certain Cu weight ratios in bulk are utilized to harvest Cu-doped Ga2O3 films, which result in p-type conductivity. Then, field-effect transistors were integrated using the printed p and n-type Ga2O3 films and demonstrated to own excellent electrical properties and stability. Au electrodes fabricated on the printed Ga2O3 and Cu-doped Ga2O3 layers showed good Ohmic behavior. Furthermore, high-power diodes are realized using printed p and n-type Ga2O3 homojunction through combining van der Waals stacking with transfer printing. The fabricated Ga2O3 homojunction diode exhibited good efficiency at room temperature, involving a rectification ratio of 103 and forward current density at 10 V (J@10 V) of 1.3 mA. This opens the opportunity for the cost-effective creation of semiconductor films with controlled metal dopants. The process disclosed here suggests important strategies for further synthesis and manufacturing routes in electronics industries.

Performance improvement of a sol–gel ZTO-based TFT due to an interfacial SnOxdopant layer

Here we combine the features of SnOxand ZTO semiconductors to produce a high-performance ZTO/SnOx/ZTO TFT.

A Novel Doping Layer Strategy to Realize High Efficiency Laye-by-Layer Organic Solar Cells

Molecular doping is an effective mean to achieve high performance organic solar cells (OSCs). However, the introduction of dopants aggravates the problem of morphological complexity in bulk heterojunction (BHJ) OSCs, and the choice of solvent after mixing with the donor and acceptor is greatly limited. Here, we innovatively propose a solution to solve the above problems by inserting a dopant layer between the donor layer and the acceptor layer to construct OSCs with a stacked structure through layer-by-layer (LbL) spin-coating solution method. Compared with the control devices (16.95%), the performance of PM6/PABA/BO-4Cl devices with the addition of the dopant layer 4-Aminobenzoic acid (PABA) was significantly enhanced, achieving an efficiency of 17.46%. Morphological characterization and charge analysis showed that the performance improvement was attribute to the film morphology optimization by the dopant located at the D/A interface, while effectively increasing the exciton dissociation rate and charge mobility of OSCs. Thus, our work demonstrates that the doping layer strategy coupled with sequential solution deposition is an effective way to construct efficient devices and is a promising alternative to BHJ OSCs.

Design and optimization of a blue fluorescent microcavity-organic light-emitting diode (MOLED) on AZO anode for an algae excitation light source application

In this work, we present simultaneous organic light-emitting diode (OLED) stack optimization and optical modelling for a blue microcavity-OLED (MOLED) based on AZO anode to be used as algae excitation light in optical biosensor. Fluorescent materials (MADN and DPAVBi) were chosen as host:guest for the doped emissive layer due their known stability compared to phosphorescent and thermally activated delayed fluorescence (TADF) materials. The MOLED modelling was performed by targeting 470 nm as the maximal excitation wavelength and suppressing the emission in the algae fluorescence bandwidth (600–800 nm) in order to fulfil the sensor requirements. By using a bilayer hole transport layer/electron blocking layer (HTL/EBL) instead of a single HTL, the total thickness was adjusted to meet the resonance wavelength condition without loss of efficiency, while at the same time preserving a maximum electric field intensity in the emissive layer (antinode position). MOLED devices were fabricated by organic semiconductor evaporation on three dielectric distributed Bragg reflectors (DBRs) with 3 different numbers of TiO2/SiO2 (high-index/low-index) pairs and aluminum-doped zinc oxide (AZO) transparent electrode. Devices with 1.5 pairs as DBR showed not only an improved external quantum efficiency (+33%) compared to a standard OLED but also an increase of about 3 times the intensity of the peak at 470 nm combined to a lower emission in the 600–800 nm bandwidth, as aimed. This increase in the peak intensity should lead to a longer device lifetime, as the current density necessary to excite algae at 470 nm is significantly lower owing to the microcavity effect.

Diffusion of Brønsted acidic dopants in conjugated polymers

An examination of the mechanism of electrical doping of films of poly(3-hexylthiophene) by a strong Brønsted acids reveals a heavily doped surface layer that limits diffusion of the acid into the bulk.

Preparation and characterization of RF sputtered ZnO layers for application in thin films solar cells

Transparent conductive electrodes (TCO) serve as window and contact layers and they should combine high optical transmittance with low electrical resistivity. Recently ZnO has been considered to be the most promising TCO material in view of cost and properties. Chalcopyrite based solar cells commonly consist of Cu(In,Ga)Se2 (CIGS) absorber layer and a sputter sequence of undoped and doped ZnO layers in a structure: glass/Mo/CIGS/CdS/ZnO/ZnO:Al. In some cases, ZnO plays the role of the buffer layer itself as an alternative of the mostly used toxic CdS. An advanced application of this material is considered to be involving in Organic Solar Cells (OSCs) as Electron Transporting Material (ETM). The work presents fabrication of uniform resistive thin (50-100 nm) undoped ZnO films serving as buffer layers and conductive Al-doped ZnO films (Al:ZnO) with thickness in the range of 100-500 nm. In the latter case the film thickness was optimized to combine high transmittance and low resistivity. Both types of films are with good adhesion and possess a transmittance over 90% in the visible region. Highly transparent bi-layer films (ZnO/ZnO:Al) were finally embedded in the solar cell structure: glass/Mo/CIGS/ZnO/ZnO:Al/Al.

Numerical simulation of PbS quantum dot solar cell and optimisation of absorber layer

A Quantum dot Solar Cell (QDSC) can be defined as a photovoltaic cell which utilises quantum dot as the captivating photovoltaic device. It is used as a substitute to the bulk material for example silicon or copper indium gallium selenide. Researchers have proven that nanotechnology may considerably increase the amount of electricity generated by solar cells. On altering the size of the quantum dots, a wide variety of energy levels can be observed with different bandgaps. Further, in CQD devices, the electron transport layer-ETL and hole transport layer-HTL plays a vital role in enhancing the performance. Maximum power conversion efficiency is achieved in this work by treating the HTL (PbS-EDT) with the absorber layer (PbS-TBAI) and ETL(TiO2). In the initial simulation, efficiency obtained is 9.88%. After altering the thickness of the absorber layer from 200 to 600 nm, the optimum result is obtained at 600 nm. Acceptor doping (NA) of HTL layer and Donor doping (ND) of ETL layer is optimised at 1x1019 cm−3 where Interface Defect Density (IDD) is kept constant at 1x1013 cm -2. The maximum power conversion efficiency (PCE) achieved is 22.35% using 1D SCAPS Simulator.

DFT and SCAPS-1D calculations of FASnI3-based perovskite solar cell using ZnO as an electron transport layer

In this work, we used both DFT and TDDFT to investigate the structural, electronic and optical properties of the Hybrid Organic-Inorganic FASnI3 perovskite. Indeed, we apply the DFT approach implemented under Quantum Espresso code to investigate and discuss the solar perovskite FASnI3 applying the calculation approximations: GGA-PBE and GGA-PBESol. When applying the PBE approximation, the band structure demonstrates that this perovskite has a direct band gap of (1.36 eV), which agrees well with the results of the existing experiments. The DOS and PDOS have been illustrated and discussed for the two cases. We have also examined including the spin-orbit coupling effect on the band gap of this material, in addition, the optical properties of FASnI3 have been computed and discussed. The band gap and optical properties allowed us to assume that this material could potentially be the best match for photovoltaic use. Further, applying SCAPS software, an n-i-p planar FASnI3 solar perovskite device was modeled and simulated. The device performances have simulated with (ZnO, ZnS, ZnSe, TiO2 and CdS) materials as an Electron Transport Layers. It is found that various factors influencing the device performance such as the thickness of the FASnI3, different ETLs. Moreover, the impact of temperature, the impact of active layer defect level and doping level were also investigated and discussed. The simulation results demonstrates that high efficiency of 28.13% can be produced with a thickness of FASnI3 absorber around 600 nm, and a total defect density of 1014cm−3 with (ZnO) as an ETL at a temperature of 300k. Finally, these theoretical simulation results could pave the path to design and fabricate efficient, lead-free PSCs.

Influence of highly doped layers on hydrogen in-diffusion into crystalline silicon

In this work, the influence of highly doped layers on the hydrogen in-diffusion into silicon from hydrogen-rich silicon nitride is investigated. Hydrogen in the crystalline silicon bulk of wafers with and without highly doped layers present was examined in solar cell-like structures by resistivity measurements for quantification of BH-pair dynamics. Hydrogen diffusion through phosphorus- and boron-doped Float-Zone samples without highly doped layers and with p + - respectively n + -layers was determined via secondary ion mass spectrometry. Therefore, deuterium concentration in an amorphous silicon layer on the backside was measured. This experiment shows an up to threefold increase of deuterium concentration in the amorphous layer for samples with highly doped layers present. A possible explanation of this observation is given by simulations. • Influence of highly doped layers on H in-diffusion. • BH-pair dynamics in samples with and without highly doped layers. • SIMS data of D diffusing through highly doped layers and bulk c-Si. • Simulations of H-species in samples with n + -p-n + structure after firing.

Study of the internal quantum efficiency of FBK sensors with optimized entrance windows

Single-photon detection of X-rays in the energy range of 250 eV to 1 keV is difficult for hybrid detectors because of the low quantum efficiency and low signal-to-noise ratio. The low quantum efficiency is caused by the absorption of soft X-rays in the entrance window of the silicon sensors. The entrance window consists of an insensitive layer on the surface and a highly doped layer, which is typically from a few hundred nanometers to a couple of micrometers thick and is comparable to the absorption depth of soft X-ray photons (e.g. the attenuation length of 250 eV X-ray photons is ∼100 nm in silicon). The low signal-to-noise ratio is mainly caused by the small signal amplitude (e.g. ca. 70 electrons for 250 eV X-ray photons in silicon) with respect to the electronic noise. To improve the quantum efficiency, the entrance window must be optimized by minimizing the absorption of soft X-rays in the insensitive layer, and reducing charge recombination at the Si-SiO2 interface and in the highly doped region. Low gain avalanche diodes (LGADs) with a multiplication factor between 5 and 10 increase the signal amplitude and therefore improve the signal-to-noise ratio for soft X-rays, enabling single-photon detection down to 250 eV. Combining LGAD technology with an optimized entrance window technology can thus allow hybrid detectors to become a useful tool also for soft X-ray detection. In this work we present the optimization of the entrance window by studying the internal quantum efficiency of eight different process technology variations. The sensors are characterized using light emitting diodes with a wavelength of 405 nm. At this wavelength, the light has an absorption depth of 125 nm, equivalent to that of 276 eV X-rays. The best variation achieves an internal quantum efficiency of 0.992 for 405 nm UV light. Based on this study, further optimization of the quantum efficiency for soft X-rays detection is planned.

Thin-Film Carbon Nitride (C2N)-Based Solar Cell Optimization Considering Zn1−xMgxO as a Buffer Layer

Carbon nitride (C2N), a two-dimensional material, is rapidly gaining popularity in the photovoltaic (PV) research community owing to its excellent properties, such as high thermal and chemical stability, non-toxic composition, and low fabrication cost over other thin-film solar cells. This study uses a detailed numerical investigation to explore the influence of C2N-based solar cells with zinc magnesium oxide (Zn1−xMgxO) as a buffer layer. The SCAPS-1D simulator is utilized to examine the performance of four Mg-doped buffer layers (x = 0.0625, 0.125, 0.1875, and 0.25) coupled with the C2N-based absorber layer. The influence of the absorber and buffer layers’ band alignment, quantum efficiency, thickness, doping density, defect density, and operating temperature are analyzed to improve the cell performance. Based on the simulations, increasing the buffer layer Mg concentration above x = 0.1875 reduces the device performance. Furthermore, it is found that increasing the absorber layer thickness is desirable for good device efficiency, whereas a doping density above 1015 cm−3 can degrade the cell performance. After optimization of the buffer layer thickness and doping density at 40 nm and 1018 cm−3, the cell displayed its maximum performance. Among the four structures, C2N/Zn0.8125Mg0.1875O demonstrated the highest PCE of 19.01% with a significant improvement in open circuit voltage (Voc), short circuit density (Jsc), and fill factor (FF). The recorded results are in good agreement with the standard theoretical studies.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO3/SrTiO3 interfaces through elaborately controllable epitaxial growth of SrRuO3. After inserting a SrRuO3 buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru4+ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

Creation of nanopores and nitrogen doping in the surface layers of reduced graphene oxide electrode via ions implantation resulting in enhanced electrochemical performance for supercapacitor

Nitrogen doping is an effective way to enhance the electrochemical performance of carbon materials. In this study, the surface layers of graphene oxide (GO) film was firstly implanted by nitrogen ions, and then annealed at nitrogen atmosphere to obtain the nitrogen-doped reduced GO (NrGO) electrode for supercapacitor. Uniformly distributed nanopores are created in the surface layers of NrGO electrode, and also as high as 9.9 % nitrogen content is doped into its surface layers. The synergistic effects of these surface modifications significantly improve its electrochemical performance. The specific capacitance is increased from 148 F g−1 (original reduced GO (rGO)) to 270 F g−1 (NrGO), tested at current density of 0.5 A g−1 in symmetrical cells with 6 M KOH electrolyte. The volumetric capacitance of NrGO reaches 416 F cm−3, and its volumetric energy density is 14.3 Wh L−1 with power density of 191.7 W L−1. This study provides an exploration via nitrogen ions implantation to fabricate graphene-based electrodes with high electrochemical performance.

Design of photonic crystal fiber with large mode area for flat-top mode generation

We report a large mode area photonic crystal fiber with a flat-top mode field. The optical fiber has a multi-layer square structure with air holes and doped layer. A flat and top-hat-like mode intensity profile is provided through the reasonable design of the doped layer. The modal characteristics including effective mode area, dispersion, bending loss, confinement loss and field profile are investigated. The proposed fiber with a top-hat mode generation has potential applications in laser drilling, laser cleaning, imaging, spectroscopy, and so on.

Impact of Ge doping on MOVPE grown InGaN layers

The impact of Ge doping on InGaN layers grown with the Metal Organic Vapor Phase Epitaxy technique is investigated, with the main focus on the influence of GeH4 flow and Ga/III ratio on the luminescence, electrical and structural properties of InGaN:Ge layers. It is shown that at doping levels above 1019 cm-3 an increase in GeH4 flow results in a decrease in the In content and lower concentration of free carrier density in InGaN layers. On the contrary, the change of Ga/III ratio has no influence on the luminescence and structural properties. An unintentional Ge doping of InGaN layers due to the Ge memory effect or back diffusion is discussed.

Silicon–Hydrogen Bonding Configuration Modified by Layer Stacking Sequence in Silicon Heterojunction Solar Cells

Recent improvements in highly efficient crystalline silicon (c-Si) solar cells have relied on surface passivation. Hydrogen plays a crucial role in the surface passivation of silicon heterojunction (SHJ) solar cells because Si–H bonds passivate dangling bonds in amorphous silicon and on the c-Si surface. In this work, we demonstrate that the Si–H bonding configuration is modified by layer stacking sequence for SHJs. The quality of surface passivation strongly correlates with low-temperature hydrogen effusion from the SHJ structure. Our results show that the deposition of doped layers on intrinsic amorphous silicon supplies additional hydrogen to the amorphous/crystalline heterostructure. Moreover, the deposition of a p-layer modifies the microstructure of the intrinsic layer underneath, whereas depositing an n-layer does not induce structural changes. We suggest that the low-temperature hydrogen effusion characteristics can be used as a sensitive indicator for examining the passivation quality of SHJ solar cells.

Optimization of high-temperature superconducting bilayer structures using a vortex dynamics simulation

We argue that the current carrying properties of high-temperature superconducting thin films can be further improved, in particular under the mid-field range (B ≈ 0.1–2 T), via introduction of multilayer structures that compromise between good zero field critical current and vortex pinning performance. In this work we focus on a simple bilayer structure consisting of two adjacent layers of pure YBa2Cu3O (YBCO) and BaZrO3 (BZO) doped YBCO under magnetic field within the mid-field range oriented parallel to the c-axis of the YBCO unit cell. We have utilized a computational model to simulate the vortex dynamics limited critical current separately from the associated zero field current, which is addressed analytically. The obtained results have allowed us to estimate the optimal layer thicknesses as a function of magnetic field. Our idealized model suggests that the thickness of the doped layer should be substantially smaller than the undoped one, that is around 30% of the total thickness of the film. We have estimated that the current carrying capability of the optimized bilayer structure can be up to 50% higher when compared with corresponding single layer films. Possible deviations from the obtained results associated with the idealized model, most prominently the effect of natural defects, are comprehensively discussed. Our results provide the foundation for the future experimental realization of the proposed bilayer structures. The comparison between the presented results and experimental realization would enable further study of the underlying primitive vortex interactions.

Influence of the Position of Local Ta Doping in SiNx Resistive Random Access Memory

This article presents an analysis of the effect of the Ta‐doping position on the performance of SiNx‐based resistive random access memories by comparing the electrical properties and conduction mechanisms of the devices. The electrical test results show that when the doping position moves down in the resistive switching (RS) layer, the resistance window of the device increases during the resistive switch process. However, the consistency of the parameter distribution decreases. The conduction mechanism of all Ta‐doped devices behaves as space charge limited current. According to the analysis of the RS model of the device, Ta in the RS layer connects and disconnects with the localized conductive filaments. The adjustment ability of the doping layer on the conductive channel and the reduction of the device stability when the doping position moves up are proposed. Proper Ta doping in the middle of the RS layer of the device allows the device to obtain a large resistance window with relatively stable characteristics.

Design and analysis of novel high-performance III-nitride MQW-based nanowire white-LED using HfO2/SiO2 encapsulation

A novel, white light-emitting diode structure with improved thermal characteristics is designed for providing efficient light which may be used especially in the underground mining environment. It describes a new nano hafnium oxide-silica doped silicone layer as LED encapsulation material that promises enhanced efficiency by 30.1% and reduced efficiency droop of 0.491%. The enhanced power and efficiency of the LED with HfO2/SiO2 doped bi-layer are attributed to the significant minimization of overflow of electrons which is fundamentally responsible for efficiency degradation through p-GaN region. In this article encapsulant material based on nano HfO2/SiO2 not only enhances light extraction but opens a broad new range of encapsulant engineering capabilities composites. Our designed LED is generated from a monolithic composition of blue and yellow spectrum which eventually creates the white light. This minimizes the problems related to multiple numbers of LEDs, green gap and phosphor color rendering.