Buffer Layer Thickness Research Articles

Effect of various layers on improving the photovoltaic efficiency of Al/ZnO/CdS/CdTe/Cu2O/Ni solar cells

The photovoltaic (PV) cell structure containing Al/ZnO/CdS/CdTe/Cu2O/Ni has been simulated using the SCAPS-1D software. The PV device includes a zinc oxide (ZnO) transparent conductive oxide (TCO) window layer, a cadmium sulfide (CdS) buffer layer, and a cadmium telluride (CdTe) absorber layer. Additionally, an electron reflected-hole transport layer (ER-HTL) comprising cuprous oxide (Cu2O) is introduced between the absorber layer and the back metal contact. Aluminum (Al) and nickel (Ni) serve as the upper/top and back contact materials, respectively, interconnecting the layers. The back contact materials, the thickness of the absorber, buffer, and window layers, the acceptor density of the absorber layer, the donor density of the buffer layer, the series and shunt resistance, as well as temperature, were all modified to investigate the PV performance of this structure. The PV performance parameters are evaluated through the open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power conversion efficiency (PCE). To achieve optimal performance, it is recommended to set the acceptor and donor densities for the absorber and buffer layers at 1017 cm−3. These desired densities can be attained by using a window and buffer layer thickness of 100 nm, an absorber layer thickness of 2500 nm, and an ER-HTL of 50 nm. The optimized model demonstrates PV performance characteristics of 1.4811 V for VOC, 28.682434 mA/cm2 for JSC, 74.91 % for FF, and 31.82 % for PCE under the AM 1.5 G spectrum. Furthermore, it exhibits a quantum efficiency of around 100 % at visible wavelengths.

Optimized Doping of Diffusion Blocking Layers and Their Impact on the Performance of Perovskite Photovoltaics.

The roll-to-roll printing production process for hybrid organic-inorganic perovskite solar cells (PSCs) demands thick and high-performance solution-based diffusion blocking layers. Inverted (p-i-n) PSCs usually incorporate solution-processed PC70BM as the electron-transporting layer (ETL), which offers good electron charge extraction and passivation of the perovskite active layer grain boundaries. Thick fullerene diffusion blocking layers could benefit the long-term lifetime performance of inverted PSCs. However, the low conductivity of PC70BM significantly limits the thickness of the PC70BM buffer layer for optimized PSC performance. In this work, we show that by applying just enough N-DMBI doping principle, we can maintain the power conversion efficiency (PCE) of inverted PSCs with a thick (200 nm) PC70BM diffusion blocking layer. To better understand the origin of an optimal doping level, we combined the experimental results with simulations adapted to the PSCs reported here. Importantly, just enough 0.3% wt N-DMBI-doped 200 nm PC70BM diffusion blocking layer-based inverted PCSs retain a high thermal stability at 60 °C of up to 1000 h without sacrificing their PCE photovoltaic parameters.

Artificial tailoring of MI transition at LAO/STO interface with SrSnO3 buffer layer

Buffer layers, capping layers, gate voltage, etc., can all have a major impact on the interface conducting properties of oxide heterostructures; in some cases, the thickness of a layer significantly alters the transport properties. Here, we report the impact of the SrSnO3 buffer layer of different thicknesses at the LAO/STO interface. The metallic behavior is observed in heterostructures with a buffer layer thickness smaller than 4 uc and the interface becomes insulating for thickness ≥ 4 uc. XPS results showed that the oxidation state of the Sn atom had changed in the insulating samples.

The impact of buffer thickness upon the transport-limited buffer trapping effects in carbon-doped power GaN-on-Si devices

In this work, we focused on investigating the transport-limited trapping effects in GaN-on-Si buffer layers as well as impact of the thickness of buffer layers (TBuf) upon such effects. Vertical transport dynamics of charges within the buffer layers and their key energy levels are quantitatively and statistically investigated and analyzed. The results show that an increased TBuf diminishes both impurity conduction of the defect band formed by carbon doping as well as the injection of electrons from the substrate, greatly diminishing the current collapse and improving the stability of the device. Such enhancement is mainly attributed to the reduced vertical electric field within the thickened epitaxy, which provides an additional pathway to address the current collapse and yields more efficient power GaN-on-Si devices.

Influence of buffer layer on optoelectronic properties of B-Mg co-doped ZnO ultrathin film for high-performance transparent conductor application

In order to develop transparent conductors for optoelectronic devices, B-Mg co-doped ZnO (BMZO) thin films and ZnO/BMZO bilayer films were prepared by using a magnetron sputtering system in this work. The properties of the films with varying operating pressure and buffer layer thickness were measured and investigated systematically. It is found that all films have high transparency, which are almost independent of the buffer layer thickness and the working pressure. Working pressure affects the chance of collision between sputtered particles and gas molecules, which impacts on the average free range of sputtered particles and the energy of the particles. Thus, the BMZO films with high crystallinity and low resistivity can be obtained at appropriate working pressure. The introduction of ZnO buffer layer makes the BMZO films more suitable for TCO applications compared to monolayer BMZO. The results showed that the thickness of the buffer layer with the introduction of ZnO buffer layer decreased the surface roughness and internal stress of the films, which increased the electrical and optical properties of the film. In particular, the ZnO/BMZO film prepared at a working pressure of 3 Pa, exhibits the optimized optoelectrical properties when the buffer layer thickness is 12 nm.

Non-Buffer Epi-AlGaN/GaN on SiC for High-Performance Depletion-Mode MIS-HEMTs Fabrication.

A systematic study of epi-AlGaN/GaN on a SiC substrate was conducted through a comprehensive analysis of material properties and device performance. In this novel epitaxial design, an AlGaN/GaN channel layer was grown directly on the AlN nucleation layer, without the conventional doped thick buffer layer. Compared to the conventional epi-structures on the SiC and Si substrates, the non-buffer epi-AlGaN/GaN structure had a better crystalline quality and surface morphology, with reliable control of growth stress. Hall measurements showed that the novel structure exhibited comparable transport properties to the conventional epi-structure on the SiC substrate, regardless of the buffer layer. Furthermore, almost unchanged carrier distribution from room temperature to 150 °C indicated excellent two-dimensional electron gas (2DEG) confinement due to the pulling effect of the conduction band from the nucleation layer as a back-barrier. High-performance depletion-mode MIS-HEMTs were demonstrated with on-resistance of 5.84 Ω·mm and an output current of 1002 mA/mm. The dynamic characteristics showed a much smaller decrease in the saturation current (only ~7%), with a quiescent drain bias of 40 V, which was strong evidence of less electron trapping owing to the high-quality non-buffer AlGaN/GaN epitaxial growth.

Effect of Ag buffer layer thickness on the microstructure and hardness distribution of Cu alloy coating on Al substrate

The alloy coating is commonly used to enhance the hardness and wear resistance of Al substrate. However, the coating always cracks because of the brittle intermetallic compound (IMC) and thermal stress. In this investigation, the Ag buffer layer was employed to fabricate the crack-free Cu alloy coating. The effect of thickness of the Ag buffer on the phase transformation and hardness of Cu alloy coating was systematically investigated. The results showed that a thinner Ag buffer contributed to the dissolution of more Al and less Ag in the Cu alloy coating due to the dissolving of high-Al region of Ag buffer layer. A lower content of Ag in the Cu alloy coating brought more primary Cu-rich phase. The high Al in the Cu alloy coating contributed to the formation of β phase. During cooling stage, the β phase transformed into β′ phase through martensitic transformation. The more fine-grained β′ phase contributed to a higher hardness of 310 HV of the Cu alloy coating on the Ag buffer layer with a thickness of 450 μm. The thicker buffer layer contributed to the coating with less Al and more Ag. The Cu alloy coating exhibited a microstructure with more eutectic phase. The primary Cu-rich phase became Cu solid solution due to the lower Al concentration. The coarse-grained primary Cu solid solution and low-hardness eutectic phase contributed to the low hardness of 275 HV.

On the Impact of Cadmium Sulfide Layer Thickness on Kesterite Photodetector Performance

Kesterites are currently viewed as one of the most promising candidates for earth abundant and benign elements to substitute critical raw materials in photovoltaic technologies and may also be suitable for low‐noise, room‐temperature, self‐powered photodetectors. However, while the impact of buffer layers on kesterite solar cell efficiency has been an active area of investigation, links between photodetector performance and intermediate layers are yet to be addressed. Herein, the impact of cadmium sulfide buffer layers on the performance of kesterite (Cu2ZnSnS4) photodetectors is probed. Specifically, the effect of buffer layer thickness on various photodetector performance metrices is clarified, including noise current, spectral responsivity, noise equivalent power, frequency response, and specific detectivity. Devices with a 100 nm cadmium sulfide layer perform the best, achieving a linear dynamic range of 180 dB and frequency responses in the range of tens of kHz. The key loss mechanisms are identified, and it is found that the photodetector performance to be primarily limited by shunt resistance‐induced thermal noise and defect‐induced nonradiative losses. Furthermore, we estimate the upper radiative limit of specific detectivity to be approximately Jones. Our results highlight the potential of kesterites to be used as an interesting earth abundant candidate for photodetection applications.

Improved electrical characteristics of AlxGa1−xN/GaN High Electron Mobility Transistor by effect of physical and geometrical parameters

This research aims to study the impact of some physical and structural parameters on the I–V characteristics of a high electron mobility transistors (HEMTs) based on AlxGa1-x N/GaN, we investigate the effect of the GaN buffer layer thickness and the impact of other properties of the materials such as aluminum mole fraction and doping concentration, the Al0.2Ga0.8 N/GaN heterostructures with 400 nm of buffer layer and a layer doped with n = 4 x 1018 cm-3 , for this structure we find the maximum saturation current of 420 mA/mm . The proposed model included GaN buffer layer and Al content were derived from our developed I-V characteristics. The proposed model is in excellent agreement with the simulated I-V characteristics and experimental results.

Electrothermal Performance of AlGaN/GaN Lateral Transistors with >10 μm Thick GaN Buffer on 200 mm Diameter‐Engineered Substrates

Herein, the electrical and thermal performance of lateral AlGaN/GaN high electron mobility transistors (HEMTs) and metal‐insulator‐semiconductor field effect transistors (MISFETs) fabricated with 11 μm thick GaN buffer layers on 200 mm diameter Qromis Substrate Technology (QST) substrates are investigated. The QST substrate has a polycrystalline core engineered to be coefficient of thermal expansion (CTE)‐matched to GaN to minimize wafer bow and residual stress in the GaN film as a result of epitaxial growth. Raman spectroscopy is used to determine the biaxial residual stress in the GaN buffer of the as‐fabricated devices. Electrical characterization is demonstrated on the HEMTs including DC and pulsed output characteristics, DC transfer characteristics, Hall mobility, carrier concentration, sheet resistance, median transition frequency, and maximum stable gain. Finally, the thermal performance of the AlGaN/GaN MISFET is assessed via thermoreflectance thermal imaging at DC power densities up to 19 W mm−1. The thermal resistance of the MISFET, calculated using the peak temperature rises on the gate electrode for DC power densities <10 W mm−1, is measured to be 15.4 mm K W−1, which is comparable with state‐of‐the‐art GaN‐on‐Si lateral transistors.

Effect of GaN/AlGaN Buffer Thickness on the Electrothermal Performance of AlGaN/GaN High Electron Mobility Transistors on Engineered Substrates

AlGaN/GaN high electron mobility transistors on QST engineered substrates are grown with different GaN/AlGaN buffer layer thickness. The as‐grown heterostructures are evaluated for their structural quality via atomic force microscopy, high‐resolution X‐ray diffraction, Raman spectroscopy, and steady‐state thermoreflectance. Transistor devices are fabricated and evaluated via DC and pulsed electrical techniques, as well as thermoreflectance imaging. It is reported that buffer layer thickness of at least 10 μm can result in lateral high electron mobility transistors (HEMTs) with simultaneously high GaN quality, low stress, good DC electrical performance, low current collapse, and low thermal resistance.

Effects of Growth Temperature on the Morphological, Structural, and Electrical Properties of CIGS Thin Film for Use in Solar Cell Applications

Cu-In-Ga-Se nanoparticles (NPs) were synthesized using a colloidal route process. The effects of growth temperature (GT) on the properties of CuInGaSe2 (CIGS) thin films made from these nanoparticles were investigated using TEM, PL, XRD, and SEM techniques. The Cu-In-Ga-Se NPs were synthesized at growth temperatures ranging from 90 °C to 105 °C and then annealed at 550 °C for 7 min under a Se ambient. The resulting CIGS thin film, formed from Cu-In-Ga-Se NPs synthesized at a GT of 90 °C (referred to as GT90-CIGS), showed a tetragonal structure, large grain size, and high sunlight absorption. It had a band gap energy (Eg) of approximately 0.94 eV. Non-vacuum GT90-CIGS-based solar cells were investigated and fabricated using varying thicknesses of a CdS buffer layer. The maximum power conversion efficiency achieved was approximately 8.3% with an optimized device structure of Al/ITO/ZnO/CdS/CIGS/Mo.

Numerical Simulation Study on the Blast Loading Performance of Reactive Fragment

In this paper, numerical simulation is used to study the buffer layer material, thickness and structure on the reactive fragment load and its action time under blast impact loading conditions. The results show that the buffer layer can significantly reduce the peak pressure of the detonation wave acting on the reactive fragment and prolong the action time; the buffering capacity of different buffer materials is different; for the same material, the buffering capacity is proportional to the material thickness; compared with a single layer of buffer layer, the buffer layer with the same thickness and composite structure has a stronger capacity.

Achievement of High Perpendicular Anisotropy and Modification of Heat Treatment Peeling in Micron-Thickness Nd-Fe-B Films Used for Magnetic MEMS.

Thick Nd-Fe-B permanent magnetic films with good perpendicular anisotropy have important applications in magnetic microelectromechanical systems (MEMSs). However, when the thickness of the Nd-Fe-B film reaches the micron level, the magnetic anisotropy and texture of NdFeB film will become worse, and it is also prone to peeling during heat treatment, which seriously limits their applications. In this paper, Si(100)/Ta(100 nm)/NdxFe91-xB9(x = 14.5, 16.4, 18.2)/Ta (100 nm) films with thicknesses of 2-10 μm are prepared by magnetron sputtering. It is found that gradient annealing (GN) could help improve the magnetic anisotropy and texture of the micron-thickness film. When the Nd-Fe-B film thickness increases from 2 μm to 9 μm, its magnetic anisotropy and texture do not deteriorate. For the 9 μm Nd-Fe-B film, a high coercivity of 20.26 kOe and high magnetic anisotropy (remanence ratio Mr/Ms = 0.91) are achieved. An in-depth analysis of the elemental composition of the film along the thickness direction is conducted, and the presence of Nd aggregation layers at the interface between the Nd-Fe-B and the Ta layers is confirmed. The influence of thicknesses of the Ta buffer layer on the peeling of Nd-Fe-B micron-thickness films after high-temperature annealing is investigated, and it is found that increasing the thickness of the Ta buffer layer could effectively inhibit the peeling of Nd-Fe-B films. Our finding provides an effective way to modify the heat treatment peeling of Nd-Fe-B films. Our results are important for the development of Nd-Fe-B micron-scale films with high perpendicular anisotropy for applications in magnetic MEMS.

Ti/HfO2-Based RRAM with Superior Thermal Stability Based on Self-Limited TiOx

HfO2-based resistive random-access memory (RRAM) with a Ti buffer layer has been extensively studied as an emerging nonvolatile memory (eNVM) candidate because of its excellent resistive switching (RS) properties and CMOS process compatibility. However, a detailed understanding of the nature of Ti thickness-dependent RS and systematic thermal degradation research about the effect of post-metallization annealing (PMA) time on oxygen vacancy distribution and RS performance still needs to be included. Herein, the impact of Ti buffer layer thickness on the RS performance of the Al/Ti/HfO2/TiN devices is first addressed. Consequently, we have proposed a simple strategy to regulate the leakage current, forming voltage, memory window, and uniformity by varying the thickness of the Ti layer. Moreover, it is found that the device with 15 nm Ti shows the minimum cycle-to-cycle variability (CCV) and device-to-device variability (DDV), good retention (105 s at 85 °C), and superior endurance (104). In addition, thermal degradation of the Al/Ti(15 nm)/HfO2/TiN devices under different PMA times at 400 °C is carried out. It is found that the leakage current increases and the forming voltage and memory window decrease with the increase in PMA time due to the thermally activated oxidation of the Ti. However, when the PMA time increases to 30 min, the Ti can no longer capture oxygen from HfO2 due to the formation of self-limited TiOx. Therefore, the device shows superior thermal stability with a PMA time of 90 min at 400 °C and no degradation of the memory window, uniformity, endurance, or retention. This work demonstrates that the Ti/HfO2-based RRAM shows superior back-end-of-line compatibility with high thermal stability up to 400 °C for over an hour.

Improving crystal quality of β-phase MgGaO thin films by using low-temperature homo-buffer layer

Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer.

Biophotonic sensor for swift detection of malignant brain tissues by using nanocomposite YBa2Cu3O7/dielectric material as a 1D defective photonic crystal

In the present research work we have theoretically examined the biosensing capabilities of proposed one dimensional defective photonic crystal for swift detection of malignant brain tissues. The transfer matrix formulation and MATLAB computational tool have been used to examine the transmission properties of proposed structure. The identical buffer layers of nanocomposite superconducting material have been used either side of cavity region to enhance the interaction between incident light and different brain tissue samples poured into the cavity region. All the investigations have been carried out under normal incidence to suppress the experimental liabilities involved. We have investigated the biosensing performance of the proposed design by changing the values of two internal parameters (1) the cavity layer thickness (d4) and (2) volume fraction (η) of nanocomposite buffer layers one by one to get the optimum biosensing performance from the structure. It has been found that the sensitivity of the proposed design becomes 1.42607 μm/RIU when the cavity region of thickness 15dd is loaded with lymphoma brain tissue. This value of sensitivity can be further increased to 2.66136 μm/RIU with η = 0.8. The findings of this work are very beneficial for designing of various bio-sensing structures composed of nanocomposite materials of diversified biomedical applications.

Large area solid oxide fuel cells with room temperature sputtered barrier layers: Role of the layer thickness and uniformity in the enhancement of the electrochemical performances and durability

We investigate the effects of the layer thickness and uniformity in solid oxide fuel cells with Gd0.1Ce0.9O1.95-δ buffer layers deposited by room temperature sputtering followed by in air annealing. Buffer layers with different thickness (in the range 200–400 nm) have been deposited using two different sputtering configurations resulting in different thickness uniformity and coverage of the Gd0.1Ce0.9O1.95-δ thin layers. By comparing the electrochemical performances at different temperatures of the cells with sputtered and screen printed buffer layers assembled in the same short stack, we have observed improvements, particularly at lower temperatures, of all the analysed properties (current density vs. voltage curves, Area Specific Resistance, Electrochemical Impedance Spectroscopy) in the case of the cells with sputtered buffer layers, with those having better thickness uniformity always showing the higher and more regular values. Percentage voltage changes of about +8% along with polarization resistance percentage reduction around −10% have been observed at 650 °C. Durability test at 750 °C performed over about 3000 h on the cell with 300 nm thick buffer layer having larger thickness uniformity, has shown percentage voltage changes more than halved respect to those observed for cells with screen printed buffer layers. The obtained results address the importance of the thickness uniformity control when fabricating thin buffer layers of few hundreds of nanometers on anode/electrolyte half-cells with surface roughness of the same order of magnitude.

Growth Twins and Premartensite Microstructure in Epitaxial Ni-Mn-Ga Films

Magnetic shape memory alloys have been examined intensively due to their multifunctionality and multitude of physical phenomena. For both areas, epitaxial films are promising since the absence of grain boundaries is beneficial for applications in microsystems and they also allow to understand the influence of a reduced dimension on the physical effects. Despite many efforts on epitaxial films, two particular aspects remain open. First, it is not clear how to keep epitaxial growth up to high film thickness, which is required for most microsystems. Second, it is unknown how the microstructure of premartensite, a precursor state during the martensitic transformation, manifests in films and differs from that in bulk.Here, we focus on micrometer-thick austenitic Ni-Mn-Ga films and explain two distinct microstructural features by combining high-resolution electron microscopy and X-ray diffraction methods. First, we identify pyramid-shaped defects, which originate from {1 1 1} growth twinning and cause the breakdown of epitaxial growth. We show that a sufficiently thick Cr buffer layer prevents this breakdown and allows epitaxial growth up to a thickness of at least 4 µm. Second, premartensite exhibits a hierarchical microstructure in epitaxial films. The reduced dimension of films results in variant selection and regions with distinct premartensite variants, unlike its microstructure in bulk.

Efficient Environmentally Friendly Flexible CZTSSe/ZnO Solar Cells by Optimizing ZnO Buffer Layers.

Flexible CZTSSe solar cells have attracted much attention due to their earth-abundant elements, high stability, and wide application prospects. However, the environmental problems caused by the high toxicity of the Cd in the buffer layers restrict the development of flexible CZTSSe solar cells. Herein, we develop a Cd-free flexible CZTSSe/ZnO solar cell. The influences of the ZnO films on device performances are investigated. The light absorption capacity of flexible CZTSSe solar cells is enhanced due to the removal of the CdS layer. The optimal thickness of the ZnO buffer layers and the appropriate annealing temperature of the CZTSSe/ZnO are 100 nm and 200 °C. Ultimately, the optimum flexible CZTSSe/ZnO device achieves an efficiency of 5.0%, which is the highest efficiency for flexible CZTSSe/ZnO solar cells. The systematic characterizations indicate that the flexible CZTSSe/ZnO solar cells based on the optimal conditions achieved quality heterojunction, low defect density and better charge transfer capability. This work provides a new strategy for the development of the environmentally friendly and low-cost flexible CZTSSe solar cells.