Capping Layer Thickness Research Articles

Optimization of (In)GaN Heterostructures for Sensing Applications

Herein, the optimization of (In)GaN heterostructures for chemical sensing is presented. The metalorganic vapor phase epitaxy (MOVPE)‐grown sensor consists of an InxGa1−xN quantum well (QW) placed close to the surface of a GaN substrate with a thin GaN cap layer on top. The photoluminescence (PL) wavelength of this QW is sensitive to surface potential changes and thus its optical signal is used as sensor response. Simulations are performed with nextnano to improve its sensitivity. Sensor parameters such as the cap layer thickness d, QW thickness Lz, background buffer layer doping concentration N, and indium concentration x of the QW are varied. It is found that a thin cap layer, together with high background doping and medium QW thickness, is ideal. The indium content does not show a strong influence on sensitivity. The trends found in the simulations are mostly confirmed in real‐world experiments performed in a chemical sensing setup, yet quantitative deviations exist.

Design of Phase‐Change Memory Using Apertureless Scanning Near‐Field Optical Microscopy in the Near‐Infrared Region

A novel optical secondary storage memory operated in the near‐infrared (NIR) region is developed through the integration of an apertureless scanning near‐field optical microscopy probe with the conventional Ge2Sb2Te5 storage stack. The probe composite and the optical coefficients of the storage media stack, particularly for the indium tin oxide capping layer, are optimized according to a previously developed electromagnetic wave model, and a newly established density functional theory model, respectively. The dielectric core and the metal coating medium of the probe are devised to be glass and barium, respectively, to provide high electric field in the NIR region, while the thickness of the ITO capping layer is chosen to be 3 nm here to provide high optical transmittance simultaneously with exemption of complex fabrication process. The recording operation of the aforementioned optimized device is subsequently simulated by a newly established thermo‐optics model, and its feasibility of achieving ≥Tbit in−2, 108 bits per second, and picojoule energy consumption per bit is theoretically demonstrated.

Electrical Performance and Stability Improvements of High-Mobility Indium–Gallium–Tin Oxide Thin-Film Transistors Using an Oxidized Aluminum Capping Layer of Optimal Thickness

We examined the effects of aluminum (Al) capping layer thickness on the electrical performance and stability of high-mobility indium–gallium–tin oxide (IGTO) thin-film transistors (TFTs). The Al capping layers with thicknesses (tAls) of 3, 5, and 8 nm were deposited, respectively, on top of the IGTO thin film by electron beam evaporation, and the IGTO TFTs without and with Al capping layers were subjected to thermal annealing at 200 °C for 1 h in ambient air. Among the IGTO TFTs without and with Al capping layers, the TFT with a 3 nm thick Al capping layer exhibited excellent electrical performance (field-effect mobility: 26.4 cm2/V s, subthreshold swing: 0.20 V/dec, and threshold voltage: −1.7 V) and higher electrical stability under positive and negative bias illumination stresses than other TFTs. To elucidate the physical mechanism responsible for the observed phenomenon, we compared the O1s spectra of the IGTO thin films without and with Al capping layers using X-ray photoelectron spectroscopy analyses. From the characterization results, it was observed that the weakly bonded oxygen-related components decreased from 25.0 to 10.0%, whereas the oxygen-deficient portion was maintained at 24.4% after the formation of the 3 nm thick Al capping layer. In contrast, a significant increase in the oxygen-deficient portion was observed after the formation of the Al capping layers having tAl values greater than 3 nm. These results imply that the thicker Al capping layer has a stronger gathering power for the oxygen species, and that 3 nm is the optimum thickness of the Al capping layer, which can selectively remove the weakly bonded oxygen species acting as subgap tail states within the IGTO. The results of this study thus demonstrate that the formation of an Al capping layer with the optimal thickness is a practical and useful method to enhance the electrical performance and stability of high-mobility IGTO TFTs.

Civil and Environmental Engineering, West Virginia University, Morgantown, WV 26506

Abstract. A coal refuse pile located in Greenbrier County, West Virginia was studied to restrict generation of acid mine drainage through the use of a cap and cover system. This paper presents results of a finite element method seepage analysis on a proposed reclamation design. The proposed reclamation incorporates a cap and cover system with a 0.3-m thick surface vegetation cap layer over a 0.6-m thick low permeability layer. The low permeability layer is directly above the coal refuse. Unsaturated soil mechanics was utilized, adopting the Fredlund and Xing equation for soil-water characteristic curve (SWCC) estimation. SWCC fitting parameters were calculated using the Zapata and the Hernandez estimation techniques. Different precipitation events were used to evaluate seepage throughout the reclamation area and assess the effectiveness of the cap and cover system. A steep area (>4H:1V) and a flat area were considered. The water balance analysis showed a 50% to 88% reduction in water volume at the coal refuse layer and a reduction in the time for the refuse to return to initial water content due to the cap and cover system implementation. Moisture detainment was observed in the growth layer and is important for supporting vegetation persistence.

Non Destructive Evaluation of AlGaN/GaN HEMT structure by cathodoluminescence spectroscopy

We have investigated radiative defects in various component layers in an AlGaN/GaN HEMT structure by measuring Cathodoluminescence (CL) spectrum as a function of accelerating voltage. Low electron beam accelerating voltages (below 1 kV) CL spectra are used to get information about defects in the cap layer and the barrier layer. Samples with good quality GaN cap layer showed surface quantum well (SQW) transition in the CL spectrum. The cap layer thickness and its quality can be determined from the SQW emission energy and its integrated intensity. We show that low intensity electron beam provides information about the defects in the cap layer as well as barrier layer through the intensity and energy of the yellow luminescence peak. The observed changes in the CL intensity of the barrier layer towards the surface have been correlated to the formation of aluminium oxide, giving rise to higher concentration of deep acceptor defects probably related to the adsorbed oxygen on the uncapped AlGaN barrier layer samples. The results are supported by X-ray photoelectron spectroscopy (XPS), High resolution X-ray diffraction (HRXRD) and Secondary ion mass spectrometry (SIMS) techniques.

716 mV Open‐Circuit Voltage with Fully Screen‐Printed p‐Type Back Junction Solar Cells Featuring an Aluminum Front Grid and a Passivating Polysilicon on Oxide Contact at the Rear Side

This article reports the recent progress of p‐type back junction solar cells featuring an aluminum front grid and an n+‐type passivating polysilicon on oxide (POLO) contact at the cell rear side. The best cell has an efficiency of 22.6% and an open‐circuit voltage of 716 mV, independently confirmed by Institute for Solar Energy Research Hamelin (ISFH) CalTeC. The cell area is 244.5 cm2. The increase in the SiNx capping layer thickness at the cell rear side reduces the deterioration of passivation quality of the POLO contact by screen‐printed silver. This increases the open‐circuit voltage by 22 mV compared with cells with a thinner nitride layer thickness. The investigation with scanning electron microscopy shows that the damage of the POLO contacts underneath the screen‐printed metal contacts is avoided by increasing the SiNx capping layer thickness. A contact resistivity of 2 mΩ cm2 is measured using the transfer length method.

The role of SiN/GaN cap interface charge and GaN cap layer to achieve enhancement mode GaN MIS-HEMT operation

The thickness increase of gallium nitride (GaN) cap layer from 2 nm to 35 nm to achieve an enhancement mode GaN MIS-HEMT (Metal-Insulator-Semiconductor High-Electron-Mobility Transistor) with a threshold voltage (Vth) of +0.5 V is studied using TCAD simulations. The simulations are calibrated to measured I-V characteristics of the 1 μm gate length GaN MIS-HEMT with the 2 nm thick GaN cap. A good agreement at low and high drain voltages (VDS=1 V and 5 V) between simulations and measurements is achieved by using a quantum-corrected drift-diffusion transport model. The enhancement mode GaN MIS-HEMT with a GaN cap thickness of 35 nm achieves Vth = + 0.5 V thanks to positive interface traps occurring between the SiN passivation layer and the GaN cap as reported experimentally. The simulations indicate that a parasitic channel is created at the interface between the SiN layer and the 35 nm GaN cap. Our study also shows an increase in the breakdown voltage from 100 V to 870 V when a thickness of the GaN cap layer increases from 15 nm to 35 nm.

Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO3 Ultrathin Film with a SrTiO3 Capping Layer

AbstractOxide heterostructures have great potential for spintronics applications due to their well‐defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO3 ultrathin heterostructures is achieved by oxygen vacancy (VO) engineering. Pulsed laser deposition of an oxygen‐deficient SrTiO3 capping layer can trigger VO migration into the SrRuO3 layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO3 capping layer, the value of the coercive field (HC) in the ferromagnetic SrRuO3 layer can be continuously tuned. The maximum enhancement of HC at 5 K is 3.2 T. Such a wide‐range tunability of HC may originate from a VO‐induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling physical properties of oxide heterostructures via VO engineering, which may facilitate the development of oxide‐based functional devices.

Effect of Device Dimensions, Layout and Pre-Gate Carbon Implant on Hot Carrier Induced Degradation in HKMG nMOS Transistors

The hot carrier (HC) induced degradation has become a major concern in advanced CMOS technologies because of non-scalable $\text{V}_{\mathrm{ DD}}$ . In this work, we have shown that the HC induced degradation in gate-first HKMG nMOS transistors can be modulated by optimizing the device width, lanthanum capping layer thickness, and pre-gate carbon (C) implant. The physics responsible for these observations are investigated and attributed to the reduction in the number of defects (traps) in hafnium oxide (HfO2) and reduction in carrier injection into these defects. It is also shown that the HC performance of these transistors could be further improved by increasing the active-to-active spacing.

Enhanced electric control of magnetic anisotropy via high thermal resistance capping layers in magnetic tunnel junctions

We studied nonlinear magnetic anisotropy changes to the DC bias voltage of magnetic tunnel junctions (MTJs) with capping layers of different thermal resistances. We found that increasing the thickness of MgO capping layers (in the range 0.3–0.5 nm) in MTJs enhances the Joule heating-induced magnetic anisotropy change, which indicates an enhancement of the interfacial thermal resistance at the FeB|MgO capping layer interface. This enhanced interfacial thermal resistance may be attributed to roughness at the FeB|MgO interface. Moreover, we observed a larger power-driven magnetic anisotropy change of 3.21 µJ W−1m−1 in the MTJ with a composite MgO (0.3 nm)|W (2 nm)|MgO (0.4 nm) capping layer. This research supports methods of efficient spin manipulation of spintronic devices such as microwave devices and magnetic memories.

Glutathione-magnetite nanoparticles: synthesis and physical characterization for application as MRI contrast agent

In this work a one-pot synthesis of water soluble glutathione capped magnetite nanoparticles is reported. The magnetic characterization of the samples shows the expected superparamagnetic behavior, but a wide range of blocking temperatures is found, since the size and interparticle interactions are very sensitive to preparation conditions. These properties are correlated with the glutathione-iron ratio and oxidant dose, in order to optimize the aqueous colloidal stability and magnetic properties of the superparamagnetic iron oxide nanoparticles for Magnetic Resonance Imaging applications. The efficiency of the glutathione coated nanoparticles as contrast agent is then evaluated by means of the determination of the relaxation times T1 and T2 in 1H Nuclear Magnetic Resonance experiments. Moreover, the influence of the thickness of the glutathione capping layer on the colloidal stability and, thus, on relaxation times has been studied. Finally, the relaxitivity of the sample that shows the best performance has been determined. A novel one-pot synthesis for “in situ” functionalized hydrophilic magnetite nanoparticles at atmospheric pressure and a temperature lower than 200 °C for Magnetic Resonance Imaging applications is reported. Their properties are analyzed in terms of the synthesis process: glutathione-iron ratio and oxidant dose.

Insights Into the Effect of TiN Thickness Scaling on DC and AC NBTI Characteristics in Replacement Metal Gate pMOSFETs

Fast characterization methods are utilized to investigate DC and AC negative bias temperature instability (NBTI) characteristics in pMOSFETs with different TiN capping layer thicknesses ( ${t} _{\mathrm{ TiN}}$ ). The impacts of ${t} _{\mathrm{ TiN}}$ scaling on the threshold voltage shift ( ${\Delta }\text{V}_{\mathrm{ T}}$ ), pre-existing hole traps ( ${\Delta }\text{V}_{\mathrm{ HT}}$ ), generated traps (GTs), and their relative contributions are studied. The time exponents of ${\Delta }\text{V}_{\mathrm{ T}}$ and GTs, and the impacts of stress voltage, temperature, frequency, and duty cycle on the NBTI degradation are analyzed. DC and AC NBTI degradations increase with ${t} _{T}{}_{iN}$ . When ${t} _{\mathrm{ TiN}}$ is scaled down from 3 nm to 1 nm, the maximum operation field is improved by 60%, which originates from the reduction in both ${\Delta }\text{V}_{\mathrm{ HT}}$ and GTs. Moreover, we experimentally demonstrate that bulk trap generation is an ${f}$ -dependent process and that its relative contribution to ${\Delta }\text{V}_{\mathrm{ T}}$ increases with ${t} _{\mathrm{ TiN}}$ , which is an important factor for the improvement of NBTI through ${t} _{\mathrm{ TiN}}$ scaling.

Epitaxial growth of a silicon capping layer to mitigate roughness after the selective chemical etching of Si1-xGex

The gate-all-around structure is a promising candidate for future generations of advanced metal-oxide semiconductor field-effect transistor technologies, as it provides higher drive current and low-power operation. The commonly-suggested nanowire structure uses a selective etching technique in the silicon/silicon-germanium multilayer to reveal the channel area. The revealed Si surface is rough, however, due to the missing atoms and remnants of Si1-xGex. This leads to increased interfacial state density, which degrades device performance, such as subthreshold voltage swing. Regarding the usage of high-k metal gates on the channel area, the issue of this rough surface, and the subsequent increase of the dangling bonds after etching, should be properly addressed.In this study, we report the enhanced roughness and density of interface state by applying a Si capping layer. The epitaxial Si0.87Ge0.13 layer, with a thickness of 30 nm on the Si substrate, was removed by an etchant consisting of nitric acid, hydrofluoric acid, and acetic acid, after which a 1-nm thick Si capping layer was epitaxially grown onto the surface. Atomic Force Microscopy was used to measure the roughness, and X-ray Photoelectron Spectroscopy was conducted to characterize the atomic bonds on the surface. A 10-nm thick SiO2 film was deposited using the Atomic Layer Deposition process to characterize the capacitance-voltage curve and interface trap density. The Si-capped sample exhibited half of the surface roughness of the un-capped sample, and exhibited a 30% lower interface trap density, by mitigating the impact of wet etching.

Tracing the two- to three-dimensional transition in InAs/GaAs sub-monolayer quantum dot growth

A two- to three-dimensional (2D to 3D) transition in InAs/GaAs sub-monolayer quantum dot (SML-QD) growth was investigated, and its evolution was traced. From the AFM results, the SML-QDs disappeared gradually and plateaus around each island formed with increasing GaAs capping layer thickness (tCL) from 1.5 ML to 4.5 ML. The SML-QDs become fully covered with a GaAs capping layer, leading to a planarization process caused by strain relaxation. In addition, when the individual InAs coverage was is insufficient to cause a 3D transition, similar plateaus formed when stacking with an increasing number of stacking layers (S) from 2 to 6. On the other hand, as the InAs coverage was increased from 0.3 to 0.5 ML, 3D dots with a comparable size to typical SK-QDs were observed beyond S = 4. This means that as the total InAs deposition thickness increases above a critical thickness (~2.0 ML), 2D-small islands transform spontaneously into 3D-pyramid QDs. Such structures then show similar optical characteristics to SK-QDs. Overall, the transition from 2D to 3D in InAs/GaAs SML-QD growth could be controlled by careful choice of the parameters tcl, θ, and S.

X-ray atomic mapping of quantum dots

We report a model-independent atomic-mapping technique for quantum dots (QDs) by combining Bragg reflection x-ray standing wave (XSW) and grazing incidence diffraction (GID) measurements. In this study, we choose GaAs capped InGaAs QDs/GaAs(001) as a model system to show the locations and arrangements of indium atoms within the QDs along various $[hkl]$ directions. This technique directly reveals the actual amount of positional anisotropy and ordering fraction of indium atoms within the QDs by probing the ($\overline{1}11$), (111), (311), ($\overline{1}31$), (113), and ($\overline{1}13$) crystallographic planes. We find that indium atoms are outwardly shifted along the [001] direction by small fractions of the lattice constant, $0.04{a}_{\mathrm{GaAs}}$ and $0.06{a}_{\mathrm{GaAs}}$ from Ga sites for 50- and 150-\AA{} GaAs capped InGaAs QDs, respectively. We observe that an improved coherency factor of the indium atoms within the QDs by 45--60% along the [001] and [011] directions reduces the photoluminescence linewidth by 22%, thus making the QDs efficient for QD-laser and optoelectronic device applications. We also find that the position and ordering of In atoms along the (113) and ($\overline{1}13$) planes are most sensitive to the thickness of the GaAs cap layer. Our XSW-based results are supported by numerical calculations using a QD-macroscopic structural model based on our GID study. We thus show that this atomic-mapping technique will be useful for studying various quantum structures and tuning their properties.

Quantum Dots-Sensitized High Electron Mobility Transistor (HEMT) for Room-Temperature Gas Detection

Introduction Nowadays there is an increasing demand for low power and low-cost gas sensors for a wide range of applications. With the advantages of miniaturization, low power consumption and good compatibility with the standard CMOS circuit, gas sensing field effect transistors (FETs) are the most attractive approaches.Since the first Pd gate field effect transistor gas sensor came into our insight [1], researchers have done a lot of work to materialize the device potential, starting with the choice of the sensing materials and sensor structure [2]. Due to the proximity of two-dimensional electron gas (2DEG) to the sensing surface and high sheet carrier concentration in the order of 1012~1013 cm−2, high electron mobility transistors (HEMTs) are extremely sensitive to the surface charge and variation in the environmental conditions, thereby enabling the sensing of the gases at low concentrations.In this paper, we demonstrate ppm level NO2 detection using AlGaAs/GaAs HEMT-based gas sensor with controlling gate and a PbS quantum dots sensitization layer. Experiment The HEMT structures in this paper were grown by molecular beam epitaxy (MBE) on GaAs substrate [3]. It was composed of an undoped 300 nm thick GaAs buffer layer, 15 nm thick In0.2Ga0.8As channel layer, 4 nm thick Al0.3Ga0.7As spacer layer, Si δ-doping layer, 25 nm thick Al0.3Ga0.7As barrier layer, and 30 nm thick GaAs cap layer. The sheet electron concentration of the HEMT was 2.3 × 1012 cm−2 with a mobility of 5.0 × 103 cm2V−1s−1 at 300 K.The sensing materials, PbS CQDs, were prepared by wet chemical synthesis technique according to previously paper [4], and the PbS quantum dots sensitization layer was formed as a sensing receptor on the exposed Au gate electrode using an electrohydrodynamic jet printer (HWK Co. Ltd., China).The CQDs capped with abundant oleic acid and oleylamine have stable dispersibility, a certain viscosity in the solvent and excellent solution processability, which makes it readily capable of being integrated with the electrohydrodynamic jet printing process at room temperature. The principle is to apply a high voltage power supply between the conductive nozzle (first electrode) and the conductive substrate (second electrode) and use a strong electric field force between nozzle and the substrate to pull the liquid out of the nozzle to form a droplet.Gas sensing characteristics were measured in a vacuum chamber probe station. The gas mixed in the mixing chamber flowed through the inlet of the chamber and out through the outlet. The flow rates of the gases were controlled by mass flow controllers (MFCs). Gas pressure and humidity in the probe station chamber remained constant for stable gas measurement. A series of electrical measurements of the fabricated sensors were carried out using the probe station and a semiconductor parameter analyzer (B1500A, Agilent Technologies). And the sensor response was defined as R=|Ig-I0|/I0, the I0 and Ig is the source to drain current in air and target gas atmosphere, respectively. Results and Conclusions The HEMT-type sensor showed high responsibility and excellent repeatability towards ppm level NO2. In this paper, extremely low concentration of the NO2 (0~25 ppm) was introduced, which shows a high response of 36.1% towards 25 ppm NO2 at room temperature, much higher than reported results [5-7], which even operated at a high temperature over 300 oC.In summary, through gate sensitization, the HEMT shows high sensitivity for ppm level NO2 detection at room temperature, the sensing performance can be both attributed to the receptor function of PbS quantum dots and the transducer function of HEMT. And the results of this paper will pave our future way for multiple target gas detection with in one chip.

Tuning the carrier density in SrTiO3/LaTiO3/SrTiO3 quantum wells

We discuss methods of built-in carrier density control in SrTiO3/LaTiO3/SrTiO3 heterostructures that exhibit quasi-two-dimensional carrier confinement in an interfacial quantum well. Unlike the electronically similar LaAlO3/SrTiO3 heterostructures, where the polar discontinuity at the interface defines the accumulated carrier density, the LaTiO3 heterostructures offer two means of carrier density control—changing the La doping level and utilizing the effect of surface depletion through the change in the SrTiO3 capping layer thickness. Dynamic carrier tuning over a limited range is possible by the application of a back-gate bias, which primarily affects the depth distribution of carriers. We find that small changes in the pre-annealing conditions of a SrTiO3 substrate can have a dramatic effect on the low-temperature sheet resistance of the heterostructures.

Conformal doping strategy for fin structures: tailoring of dopant profile through multiple monolayer doping and capping layer control

Monolayer doping (MLD) has attracted attention as a next-generation doping method because of its great potential for conformal doping of non-planar nanostructures. However, fundamental limitations are identified in conformal doping of fin structures using conventional MLD process, and fin structures doped with conventional MLD presents a poor dose conformality (surface concentration ratio between the side-wall and the top of the fin) of 0.32. The poor dose conformality is mainly caused by the hydrogen-terminated silicon surface formed differently depending on the orientation of each face of the fin structure and the capping layer deposited with low step coverage on the fin structure. As a strategy to overcome the limitations without increasing process complexity, multiple-MLD for dose compensation and capping layer thickness control for adjusting the dopant diffusivity are proposed. These methods are found to be effective in tailoring doping profiles on each face of the fin structure with a fine dose controllability from about 70% to 200%. Finally, it is demonstrated that the proposed approach allows conformal doping of the fin structure with a high surface concentration of 5 × 1020 atoms cm−3 and an excellent dose conformality of 0.98.

Effect of low-temperature GaN cap layer thickness on the optoelectronic performance of InGaN green LEDs with V-shape pits

The low-temperature GaN cap layer was designed after quantum wells for the V-pits contained InGaN-based green light-emitting diodes (LEDs). It is found that the LEDs’ performances are greatly improved with a thicker cap layer. The indium distribution in InGaN/GaN multiple quantum wells (MQWs) becomes more homogeneous. The quality of interface between the quantum well and quantum barrier is promoted, and the carrier confinement effect is enhanced. Thereby the external quantum efficiency (EQE) increases at high current density. However, the sample with a relative thicker cap layer exhibits the lower EQE at low current density, which is attributed to there being more defects at the upper interface of QWs, and the increased probability of carriers meeting at these non-radiative recombination centers. Meanwhile, only the major emission peak emitted from c-plane MQWs can be observed in the electroluminescence spectrum of 100 K, and other emission peaks are hardly visible, which indicates that the sample with the thicker cap has better carrier transport performance.

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.