Capping Layer Thickness Research Articles

Effect of InGaN/GaN Superlattice on Lattice Curvature of GaN Layers Grown on Sapphire Substrates

The lattice curvature due to the difference in the thermal expansion coefficients between sapphire and GaN decreases the device performance and production yield. There is room to improve the lattice curvature and radius of curvature of GaN layers grown on sapphire substrates. Herein, an InGaN/GaN superlattice (SL) is grown on a low‐temperature GaN buffer layer through metalorganic vapor‐phase epitaxy to improve the lattice curvature. The lattice curvature of the GaN layer is investigated in terms of In content of the InGaN layer, the number of SL layers, cap layer, and GaN thickness. The radius of curvature increases with the increase in the number of SL layers and In content. The radius of curvature is 45 m when a 10‐period SL structure is used and the In content is 23.4%. When using a 20‐period SL structure, the GaN layer partially separates despite its very low thickness (2 μm). The improvement in the lattice curvature can be attributed to the thermal decomposition of InGaN during the annealing process from the growth temperature of InGaN to that of GaN.

Emission characteristics of transparent organic light-emitting diodes with molybdenum oxide capping layers

In this work, we have studied a molybdenum oxide (MoO3) layer as an effective capping layer (CL) for TOLEDs. We varied the thickness of the capping layer in order to investigate its effects on the device performance. Both the top and bottom emission of the devices were found to be easily controlled by changing the thickness of MoO3 layer. Especially, at the CL thickness of 40 nm, the current efficiency shows 2.2 times enhancement compared to reference device which has no capping layer and exhibited the highest transmittance value. The results also revealed that the transmittance and light outcoupling efficiency are significantly dependent on the capping layer thickness and exhibit identical trend in change. In addition, the changes in the emission characteristics were optically simulated which agreed well with the experimental data.

Dependence of stability and electronic and optical properties of perovskite quantum dots on capping ligand chain length.

Methylammonium lead bromide (MAPbBr3) perovskite quantum dots (PQDs) passivated with capping ligands with different chain length, including butylamine-valeric acid (BUTY-VA), octylamine-caprylic acid (OCTY-CA), and dodecylamine-lauric acid (DODE-LA), are investigated to determine an optimized capping layer thickness for maximizing both electronic and antimoisture properties of perovskite materials in optoelectronic devices. The photoluminescence quantum yield (PLQY) is observed to be chain length dependent, where the PLQY of BUTY-VA, OCTY-CA, and DODE-LA MAPbBr3 PQDs is 82% ± 4%, 68% ± 7%, and 18% ± 2%, respectively. Electrochemical impedance spectroscopy (EIS) measurements of each PQD film reveal that there is a slight increase in conductivity from reducing the capping ligand chain length from 8 carbon atoms (OCTY-CA) to 4 carbon atoms (BUTY-VA). Using the Butler-Volmer equation, the charge transfer factor β for BUTY-VA and OCTY-CA MAPbBr3 PQD films in a tetrabutylammonium hexafluorophosphate-dichloromethane electrolyte solution was calculated to be 0.36 and 0.31, respectively. From an Arrhenius analysis, the activation energy (Ea) for charge transport between the PQD film and the electrolyte was calculated to be 77 and 90 meV for BUTY-VA and OCTY-CA MAPbBr3 PQD films, respectively. Moreover, passivating PQDs with capping ligands with 12 carbon atoms (DODE-LA) almost completely insulates the PQDs and diminishes charge transport. This is also observed in transient photocurrent density measurements. The results suggest that the inter-PQD distance in this solid film is too long for effective tunneling to occur. However, using BUTY-VA capping ligands to improve electronic properties of PQD solid film comes with a cost of stability.

Plasmonic effects of two-dimensional indium-nanoparticles embedded within SiO2 anti-reflective coating on the performance of silicon solar cells

This study experimentally examined the plasmonic near-field and far-field effects of two-dimensional (2-D) indium-nanoparticles (In-NPs) embedded within an anti-reflective SiO2 layer on the optical and electrical characteristics of silicon solar cells. The dimensions and profiles of the 2-D In-NPs were derived from scanning electron microscope images (top-view and side-view) using Image-J software. The surface plasmon resonance of the 2-D In-NPs was estimated from Raman scattering and absorbance measurements. Measurements of optical reflectance and external quantum efficiency revealed that near-field and far-field plasmonic effects depended on the thickness of the spacer and capping layers. The influence of these phenomena on photovoltaic performance was confirmed in terms of photovoltaic current density-voltage under AM 1.5 G illumination. The application of a 90-nm thick SiO2 anti-reflective layer was shown to enhance conversion efficiency by 23.85% (compared to the bare reference cell), and the inclusion of In-NPs between an 18-nm thick spacer layer and 72-nm thick capping layer extended this improvement to 34.16%.

Impact of Digital Alloy Capping Layers on Bilayer InAs Quantum Dot Heterostructures

The influence of digital alloy capping technique on the strain-coupled bilayer InAs quantum dots (QDs) has been presented. Multiple capping layers of different composition have been utilized instead of a single thick capping layer to reduce the overall strain inside the heterostructure. In bilayer QDs, the propagation of residual strain from the lower QD layer to the top QD layer introduces defects and dislocations. By using the digital alloy technique over the conventional or analog alloy capping technique, the surface defects could be minimized and the crystallinity could be improved. In this study, four different capping materials have been considered, viz. two ternary materials (InxGa1-xAs and GaAs1-xSbx) and two quaternary materials (InxAlyGa1-x-yAs and InxGa1-xAs1-ySby) for analysis of both analog alloy and digital alloy techniques. The strain and energy band profile has been computed for all the heterostructures using Nextnano software. The biaxial and hydrostatic strain have been computed and compared to understand the strain distribution within the heterostructures. The ground state emission wavelength was found red shifted for the InAs bilayer QD heterostructure with digital alloy capping layers as compared to that of the analog alloy capping. Moreover, the choice of capping materials exhibits both type-I and type-II energy band alignment which is suitable for different optoelectronic applications. This comparative study of analog and digital alloy capping layer for the bilayer QDs would definitely help in optimization of the device heterostructures with minimal strain and defects.

Temperature dependence of spin pumping and inverse spin Hall effect in permalloy/Pt bilayers

The temperature dependence of the spin pumping process in Fe80Ni20 (Py)/Pt bilayers is analyzed for different Pt capping layer thickness. Samples have been grown by means of dc magnetron sputtering, at room temperature, on top of Si substrates. In order to get an optimized interface between the Py and the Pt layers, the Pt capping layers were deposited In situ. The values of the damping constant, α, obtained for the Py alone samples, around 0.007 at RT, make evident the good quality of the samples. The increase of α due to spin pumping into the Pt layer is clearly appreciated; α values in Py/Pt bilayers are twice the value in Py alone. Spin pumping is confirmed by direct measurements of the inverse spin Hall effect (ISHE) voltage signal (VISHE) measured across the samples. To separate ISHE and spin rectification contributions to VISHE the line shape analysis method has been used and the conditions for its applicability thoroughly discussed. VISHE increases almost linearly on lowering temperature for all the frequencies investigated and smoothly increases as frequency decreases showing roughly a linear dependence with the inverse of the frequency. The Pt layer thickness dependence of α and VISHE suggest a low interfacial spin mixing conductance and high spin memory loss.

In-situ observation of structural and chemical transitions in B4C based layered systems

In the context of multilayer based X-ray optics developments, thin layered systems of C/[Pt/B4C] multilayers and C/Pt/B4C/Cr stacks with variable cap layer thicknesses were deposited on Si wafers using DC magnetron sputtering. The samples were studied with in-situ X-ray reflectivity techniques during annealing in air up to temperatures of 300 °C. Simulated X-ray spectra of B4C/Cr reveal a considerable thickness loss of the B4C layer at elevated temperatures. The effect is amplified and accelerated when a thin Pt top layer is added but attenuated and slowed down by an additional C cap layer. Energy dispersive X-ray spectroscopy data indicate the overall depletion of B in samples after the annealing process. In-depth studies using X-ray photoelectron spectroscopy techniques show clear evidence of chemical modifications in the original B4C layer and confirm the structural modifications derived from the X-ray reflectivity data. This study demonstrates the catalyzing role of Pt in the degradation of B4C based layered structures in air and the potential protective function of C cap layers.

A novel AlGaN/GaN Schottky barrier diode with partial p-AlGaN cap layer and recessed dual-metal anode for high breakdown and low turn-on voltage

A novel AlGaN/GaN Schottky barrier diode with partial p-AlGaN cap layer and recessed dual-metal anode is proposed and investigated by TCAD simulation in this work. By utilizing this structure, the device can achieve both enhanced reverse blocking capacity and superior forward performance. The operating mechanism of the partial p-AlGaN cap layer is analyzed. Carriers underneath the partial cap layer are depleted, and the electric field is redistributed, leading to a lower reverse leakage current and a higher breakdown voltage. Additionally, the recessed dual-metal anode allows the device to turn on at a low forward bias. The effect of the length and thickness of partial p-AlGaN cap layer on the reverse and forward performance has been further investigated, respectively. The optimized device shows excellent performance with a breakdown voltage of 2461 V, a turn-on voltage of 0.43 V, and the highest Baliga’s figure of Merit of 1738 MW cm−2. The fabrication issues of the proposed device have also been presented.

The role of vapor venting and liquid feeding on the dryout limit of two-layer evaporator wicks

Vapor chambers developed for high-heat-flux operation require advanced evaporator wick designs that can sustain capillary flow when boiling occurs over the heater region. A two-layer evaporator wick integrates a thin base wick layer that is supplied with liquid from a thick cap layer through an array of vertical feeding posts distributed over the heated area. This design allows boiling to occur within the thin base layer, while separating the incoming liquid feeding and outgoing vapor venting pathways. In our prior work, boiling in two-layer wicks was experimentally demonstrated to provide high-heat-flux dissipation over larger heater areas and at low thermal resistance. The current study experimentally explores the effect of two-layer wick design parameters, specifically the dimensions that alter the area available for liquid feeding and vapor venting, on the thermal performance and dryout limit of the wick, using water as the working fluid. Four different two-layer wick designs are fabricated over a 1 cm2 evaporator area by sintering 180–212 μm copper particles. Increasing the vapor-venting area from 7% to 16% of the total evaporator area yielded a significant increase in the dryout limit, from 315 W/cm2 to 405 W/cm2. Increasing the liquid-feeding area using wider posts increased the dryout limit further. Finally, a parametrically optimized design with fewer but larger posts and vents resulted in better performance compared to a design with denser features. With this two-layer wick design, we demonstrate an extremely high dryout limit of 512 W/cm2 over the large 1 cm2 heated area at a thermal resistance of 0.08 K/W.

InAlGaAs encapsulation of MOVPE-grown InAs quantum dots on InP(0 0 1) substrate

Stranski-Krastanow (S-K) grown self-assembled quantum dots (QDs) are considered as an efficient active layer for many optoelectronic applications, such as semiconductor laser, photodetector, due to the possibility to confine carriers in three dimensions. Towards the device application, the subsequent growth of an encapsulation layer is a key step to obtain the desired QDs-based heterostructures. In this work, we have studied step-by-step the encapsulation process of S-K grown InAs QDs/InP(0 0 1) by InAlGaAs (lattice matched to InP) in MOVPE. A detailed investigation with Electron Channeling Contrast Imaging (ECCI) and cross-sectional Transmission Electron Microscopy (TEM), indicated that after the InAlGaAs capping the partially relaxed, dome shaped InAs QDs are transformed into fully biaxially strained, truncated pyramids however without introducing any defects in the heterostructure. The study also presents the important role of the capping layer thickness on the QDs photoluminescence efficiency. The gradual change of the surface morphology during the encapsulation process, has been investigated step-by-step in-depth by atomic force microscopy. The morphological changes can be explained by a capping model based on the localized growth rates. We show that two growth steps of Non-uniform capping, where growth rates are different from each other, result in the successful planarization of the InAlGaAs capping without forming any defect.

Schottky barrier diodes fabricated with metal oxides AgOx/IGZO

In this work, Schottky barrier diodes were fabricated using silver oxide (AgOx) as the Schottky contact and amorphous indium gallium zinc oxide (a-IGZO) as the n-type semiconductor. The devices were fabricated with four photolithography steps on glass substrates. The materials used for the fabrication were: 150 nm thick Indium tin oxide (ITO) as the cathode, a 45 nm thick IGZO layer deposited by sputtering, a 500 nm thick SU-8 layer as passivation layer, a 40 nm thick AgOx layer deposited by reactive sputtering and a 70 nm thick Au capping layer deposited by e-beam evaporation as the anode. The fabricated Schottky barrier diodes area was 80um x 160um. From the current-voltage characteristics, based on thermionic emission theory, the following electrical parameters were obtained: an ideality factor of 1.71 ± 0.14, a rectification ratio of 1.38 × 109, a Schottky barrier height of 1.14 ± 0.01 eV, and a saturation current density of 4.5 × 10−13 A/cm2.

Surface reverse etching analysis of (Al)GaN-capped AlGaNGaN high electron mobility transistors

AlGaN/GaN-based high electron mobility transistor heterojunction with different thickness (Al)GaN cap layers were grown by metalorganic vapor phase epitaxy (MOVPE). The thickness of the cap layers was examined by X-ray photoelectron spectroscopy. It could be seen that the actual thickness of the GaN cap layer was about 1 nm thinner than the theoretical thickness, and however the results of the Al0.04Ga0.96N cap layer was basically the same. The surface quality was tested by atomic force microscopy and the strain properties of the cap layer were characterized by photoluminescence.

Internal screening and dielectric engineering in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MA-tBLG) has appeared as a tunable testing ground to investigate the conspiracy of electronic interactions, band structure, and lattice degrees of freedom to yield exotic quantum many-body ground states in a two-dimensional Dirac material framework. While the impact of external parameters such as doping or magnetic field can be conveniently modified and analyzed, the all-surface nature of the quasi-2D electron gas combined with its intricate internal properties pose a challenging task to characterize the quintessential nature of the different insulating and superconducting states found in experiments. We analyze the interplay of internal screening and dielectric environment on the intrinsic electronic interaction profile of MA-tBLG. We find that interlayer coupling generically enhances the internal screening. The influence of the dielectric environment on the effective interaction strength depends decisively on the electronic state of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric environment, e.g. by varying the capping layer composition and thickness, as a promising pursuit to provide further evidence for resolving the hidden nature of the quantum many-body states in MA-tBLG.

Direct observation of terahertz emission from ultrafast spin dynamics in thick ferromagnetic films

We have experimentally investigated the field- and fluence-dependent terahertz (THz) emission behaviors by a femtosecond optical pump in Co ferromagnetic films, together with the time-resolved magneto-optical Kerr effect measurement. The thickness of Co and Ta capping layers has been systematically varied. For thick films, THz emission behavior is found to be directly matched with the photoinduced ultrafast demagnetization and remagnetization dynamics. The angle-dependent THz emission measurement also confirms a direct relation between the THz emission and the photoinduced demagnetization/remagnetization dynamics in the case of thick films, while THz emission from the inverse spin Hall effect also plays a role in the case of thin Co films.

Revealing Temperature-Dependent Absorption and Emission Enhancement Factors in Plasmon Coupled Semiconductor Heterostructures

Localized surface plasmon resonances can increase the quantum efficiency of photon emitters through both absorption and spontaneous emission enhancement effects. Despite extensive studies, experimental results that clearly distinguish the two plasmonic enhancement effects are rarely available. Here, we present clear spectral signatures of the plasmonic enhancement effects on the absorption (excitation) and spontaneous emission (Purcell factor) by analyzing the temperature-dependent photoluminescence (PL) properties of an InGaAs/GaAs single quantum well (QW) coupled to colloidal gold nanorods (AuNRs) at different GaAs capping layer thicknesses (d). We find that when the emitting InGaAs layer is close to the AuNRs (d = 5 nm), the plasmonic enhancement effect on the QW PL is dominated by the Purcell factor, which significantly increases the external quantum efficiency of the QW that otherwise barely emits. When d is increased to 10 nm, the temperature dependence of the PL enhancement factor (F) reflects absorption enhancement in the capping layer followed by carrier diffusion and capture by the well. First, F increases with temperature and then decreases following the temperature dependence of the carrier diffusion coefficient in GaAs. By factoring out the contribution of the captured carriers to F, it is shown that carrier transfer to the well reaches saturation with increasing incident laser power. In addition to providing insight into the plasmonic enhancement mechanism, the results presented in this work suggest that colloidal plasmonic nanoparticles can be used as simple probes for understanding carrier transport phenomena in arbitrary semiconductor heterostructures.

Theoretical study of stress and strain distribution in coupled pyramidal InAs quantum dots embedded in GaAs by finite element method

Stress and strain distributions in and around a single or two-coupled pyramidal InAs quantum dots (QDs) embedded in GaAs are calculated by finite element methods according to the continuum elasticity theory. By changing the quantum dot spacing and thickness of cap layer, the results about strain and stress distributions show compressive strain and stress distribution in the QDs and relaxation undergoes two stages with different speeds for different quantum dot height, quantum width and thickness of cap layer. The stress and strain distributions of pyramidal QDs would not vary monotonously with geometric dimensions. The height of quantum dot and cap layer thickness can effectively adjust the vertical correlation of self-assembly QDs according to the calculation. The shape of stress distribution at surface of cap layer can be tuned from a quadrangle into a circle by increasing the thickness of cap layer or decreasing the height of quantum dot. Also, a new approach to grow quantum ring is found in this paper. The calculations of two-coupled QDs show that the self-assembly technology might fail if the horizontal distance between two QDs is not large enough. The stress induced by upper QDs will be relaxed to zero with a longer distance downwards is found in this paper.

Magnetic exchange interactions and phase transition in Fe/V(100), V/Fe/V(100) and Fe/V/Fe/V(100) films

We present the magnetic properties of bcc Fe films on and embedded in bcc-V(100) substrates in their paramagnetic phase above the Curie temperature. We have used a first-principles electronic structure method based on the spin-density functional theory and the spin fluctuations in the paramagnetic phase are described using the disordered local moments model. We have calculated the local magnetic moments, magnetic correlations and the Curie temperature of several Fe/V(100) and V/Fe/V(100) systems. In general, the Fe layers in close proximity to V have their magnetic moments suppressed, while in the interior of the film, the value of the magnetic moment oscillates from one layer to the other. When the films are capped by a V layer, the magnitudes of the magnetic moments decreases. The Curie temperature also shows an interesting oscullatory behaviour as the V capping layer thickness is increased. In the Fe/V/Fe/V(100) films, we observed that the exchange interactions between the two Fe layers across the spacer exhibits an oscillating behaviour.

Effects of Cu(In,Ga)3Se5 defect phase layer in Cu(In,Ga)Se2 thin film solar cells

Cu(In,Ga)3Se5 (135-CIGS) layers with various thicknesses were deposited on the surface of Cu(In,Ga)Se2 (112-CIGS) photon absorber in the fabrication of CIGS thin film solar cells. This significantly affects the shift of the optical band gap energy from 1.15 eV (112-CIGS) to 1.19 eV, with only 10 nm thick of 135-CIGS capping layer, leading to the increase in open-circuit voltage (Voc) of the solar cells. The optical transmission spectra show no sign of single 135-CIGS layer. The maximum Voc of 670 mV was obtained from 5 to 10 nm thick 135-CIGS capping layer on 112-CIGS compared to 646 mV of only 112-CIGS. The power conversion efficiencies of the devices covered with 135-CIGS with thickness of less than 80 nm are slightly lower than that of the uncovered 112-CIGS solar cells due to lower generated photocurrents. The solar cell parameters become dramatically deteriorate with thicker 135-CIGS capping layer. The XRD also shows the shift of the diffraction peak toward larger 2θ without peak broadening or splitting when the thickness of 135-CIGS is increased. The external quantum efficiency (EQE) measurements indicate the shift of absorption threshold towards shorter wavelength when the thickness of 135-CIGS is increased, which is consistent with the optical transmission measurements.

A systematic study of device structure on DC and small‐signal characteristics of millimeter‐wave AlGaN/GaN HEMT

AbstractIn this paper, the relationship between the gate structure, passivation structure, short‐channel effect, source‐drain distance, DC, and the small‐signal characteristic of a device is studied through simulations. By studying the gate structure and passivation layer, it is concluded that the operating frequency of a device increases as the gate length decreases. However, as the gate length decreases, the gate resistance becomes the main factor restricting the small‐signal performance. In order to solve the contradiction between the gate resistance and gate length, a T‐shaped gate structure is studied, in which the gate height, gate cap, and passivation layer thickness are optimized. It is found that when the gate height and gate cap are 120 and 500 nm, respectively, the parasitic capacitance introduced by the gate cap can be minimized. Meanwhile, the influence of gate resistance on the small‐signal gain can also be reduced. Besides, the short‐channel effect is analyzed, and the scheme for improving the short‐channel effect is proposed. By analyzing the source‐drain distance, and considering the influence of parasitic capacitance and resistance on the small‐signal characteristic, the gate‐source spacing is determined to be 0.9 μm. The design of a gate‐drain spacing is mainly based on the influence of a breakdown voltage; therefore, the gate‐drain spacing should be 1.5 to 2 μm.

Equisensitive adjustment of planar Hall effect sensor’s operating field range by material and thickness variation of active layers

A novel technique of operating field range adjustment using varying thicknesses of spacer and capping layers is proposed for planar Hall effect (PHE) magnetic sensors. In terms of this technique, the spacer and capping layer thicknesses are varied interdependently, in a way that the variation of the operating magnetic field range is performed without a change of sensitivity. The relationship between the thicknesses of spacer and capping layers required for this ‘equisensitive’ variation of field range are calculated and experimentally approved. The active layer material substitution effect on the performance of PHE sensor is studied. The sensitivity, output voltage, operating field range, and shunt current are compared for PHE sensors, based on NiFe and NiFeMo active ferromagnetic layers. The NiFe/IrMn and NiFeMo/IrMn interface coupling energies are compared and the effect of IrMn crystallinity on their difference is discussed. The range of active layer thicknesses, at which the operating field range can be varied while maintaining the sensitivity at the same value, has been determined.