Bandgap Engineering Method Research Articles

Origin of Intergranular Li Metal Propagation in Garnet-Based Solid Electrolyte By Direct Electronic Structure Analysis and Performance Improvement By Bandgap Engineering

Garnet-structured oxide electrolyte (Li7La3Zr2O12, LLZO) has the significant advantage of being chemically and electrochemically stable against Li metal, which allows the implementation of Li metal batteries. However, a short-circuit failure by Li penetration through the LLZO electrolyte has remained a crucial issue for safety and is a major hurdle for Li-based batteries to overcome. Here, we investigate the mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (LLZTO) by examining their energy band structures and defect states using reflection electron energy loss spectroscopy (REELS), scanning photoelectron microscopy (SPEM), and nanoscale charge-based deep level transient spectroscopy (Nano Q-DLTS) techniques. The experimental results reveal that the Schottky barrier height (SBH) was lowered by 0.5 eV due to the defect states localized in grain boundaries and the metallic Li propagation along the grain boundaries is caused by the SBH reduction. Based on the analytical results, the laser annealing of LLZTO was performed as a bandgap engineering method to suppress the Li dendrite formation by forming a mixed surface layer of amorphous LLZTO and Li2O, which has a wide bandgap to block the electron injection into the grain boundaries. The electrochemical measurements for the laser treated LLZTO demonstrate that the stability and cycling performance are significantly improved. This study sheds light on the importance of electronic structure, in particular, defect states to develop high performance oxide solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.

Origin of intergranular Li metal propagation in garnet-based solid electrolyte by direct electronic structure analysis and performance improvement by bandgap engineering

The mechanism of Li dendrite formation for Ta-doped LLZO (LLZTO) was investigated by examining the electronic structure and the laser annealing of LLZTO was performed as a bandgap engineering method to suppress the Li dendrite formation.

Structural, optical and ferroelectric properties of KNixNb1−xO3 single crystals

Ferroelectric perovskite oxides have attracted considerable attention as photovoltaic materials owing to the bulk photovoltaic effect (BPVE). However, single crystals are necessary to investigate the mechanism underlying the BPVE and bandgap engineering methods are furthermore required in order to increase their optical absorption in the visible region of the solar spectrum. In this study, a high-quality, large-sized (the largest diameter and height were 32 and 29mm, respectively) KNixNb1-xO3 (x = 0.006, 0.010, and 0.012) single crystal was grown using a top-seeded solution growth method. The effect of nickel doping on the optical and ferroelectric properties was investigated. It was observed that while the introduction of nickel induced bandgap narrowing and thus an increase in the visible-light absorption, the concomitant introduction of oxygen vacancies led to slight weakening of the ferroelectric properties. On the whole, however, our results indicate the high potential of KNixNb1-xO3 single crystals for investigating the BPVE and as candidate photovoltaic materials.

Design of Band Engineered HgCdTe nBn Detectors for MWIR and LWIR Applications

In this paper, we present a theoretical study of mercury cadmium telluride (HgCdTe)-based unipolar n-type/barrier/n-type (nBn) infrared (IR) detector structures for midwave IR and longwave IR spectral bands. To achieve the ultimate performance of nBn detectors, a bandgap engineering method is proposed to remove the undesirable valence band discontinuity that is currently limiting the performance of conventional HgCdTe nBn detectors. Our proposed band engineering method relies on simultaneous grading of the barrier composition and doping density profiles, leading to efficient elimination of the valence band discontinuity. This allows the detector to operate at |V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bias</sub> | <;50 mV, rendering all tunneling-related dark current components insignificant and allowing the detector to achieve the maximum possible diffusion current limited performance.

Bandgap engineering through nanoporosity

Considering 105 ZnO polymorphs we use many body GW and density functional based calculations to probe how the band gap is affected by nanoporosity. Within a reasonable range of energetic stability, we predict that nanoporosity can induce band gap increases of up to ~1.5 eV relative to wurtzite ZnO. Our results further imply that structural stability and band gap increase are fundamentally linked to pore system dimensionality. We suggest that nanoporosity could be employed as a general band gap engineering method for morphologically and electronically tailored functional materials.

Non-metal doping of transition metal oxides for visible-light photocatalysis

Transition metal oxides and mixed oxides are the largest group of materials for photocatalytic applications. Many highly active compounds are known from literature for environmental remediation, pollutant degradation and solar fuel generation. However, most of these oxides can only absorb UV light to perform photocatalytic reactions at their surface due to their large band gap. In this review, we present the recent progress in non-metal doping of transition metal oxides and mixed oxides, one of the major strategies to reduce the large band gap of semiconductor materials into the visible light range. We outline the advantages of this strategy compared to other band gap engineering methods, and especially stress the effect of efficient homogeneous non-metal doping on the optical, electronic and photocatalytic properties of photocatalysts, compared to surface doping and surface modification, including the effects of an open crystal structure on the efficiency of the doping process. We then present the highlights and breakthroughs of the last ten years in the research field and point out major improvements important for future applications, covering all the available non-metal doped transition metal oxides concerning photocatalytic reactions.

Double-Crystalline Silicon Channel Thin Film Transistors Fabricated Using Continuous-Wave Green Laser for Large Organic Light-Emitting Diode Displays

We developed double-crystalline silicon channel thin film transistors (TFTs) whose active region consisted of a polysilicon layer and a microcrystalline silicon layer. The polysilicon layer was formed by continuous-wave green laser annealing with scalability to large substrates. The microcrystalline silicon layer formed on the polysilicon layer provides a sufficient channel etching margin for the fabrication of TFTs on large substrates without degradation of TFT mobility. The kink-free output characteristics were realized by a band-gap engineering method using a microcrystalline silicon layer. The TFT characteristics desirable for organic light-emitting diode display applications, namely, high mobility, high reliability, and kink-free output characteristics, have been successfully demonstrated.

Tunable bandgap opening in the proposed structure of silicon-doped graphene

A specific structure of doped graphene with substituted silicon impurity is introduced and ab initio density-functional approach is applied for the energy band structure calculation of the proposed structure. Using the band structure calculation for different silicon sites in the host graphene, the effect of silicon concentration and unit cell geometry on the bandgap of the proposed structure is also investigated. Chemically, silicon-doped graphene results in an energy gap as large as 2 eV according to density-functional theory calculations. As the authors will show, in contrast to previous bandgap engineering methods, such structure has significant advantages including wide gap tuning capability and its negligible dependency on lattice geometry.

Development of NIR Emitted CdTe Quantum Dots by Concentration Control Method

Recently, much research has been performed on quantum dots (QDs) due to the promising applications of LED, laser and bio-imaging. They can show the tunable fluorescent property between UV and IR via the control of particle size. Especially, near-infrared emission between 780-880 nm have attracted attention in bio-imaging area due to the deep penetration of emitted light and avoiding the window between the absorption band of hemoglobin and water. Until now, CdTe/CdSe core-shell type-II QDs, InAs/ZnSe, and InAsxP1-x/InP/ZnSe core-shell QDs were developed and applied for sentinel lymph node mapping as an example of a bio-imaging application. In particular, we are interested in CdTe QDs which have a smaller bandgap than the other IIVI QDs and which, therefore, can emit near-infrared light and provide high quantum efficiency. Bulk CdTe is a typical II-VI QDs with a bandgap of 824 nm, which becomes larger than that of bulk semiconductor with decreasing particle size. Several methods have been reported to tune the bandgap of CdTe QDs: 1) growth temperature and time control, 2) additional precursor injection and 3) chemical surface modification using different surfactants. In this paper, we present a new bandgap engineering method of CdTe QDs with controlling precursor concentration. A similar experiment has been conducted by Peng et al., but they studied sizeand shape-controlled QDs with surfactant change. The synthetic procedure used in the present study is similar to the method developed by Bawendi et al. for the production of CdSe QDs, using Cd(acac)2 and TOP-Te as cadmium and tellurium precursors, respectively. The mixture solution of cadmium and tellurium precursors was injected into the surfactant solution of TOP (trioctylphosphine), TOPO (trioctylphosphine oxide) and HDA (hexadecylamine) at 350 C and then cooled to room temperature. The concentration of precursors was controlled serially from one (9.64 mM of Cd) to six (57.84 mM of Cd). The detailed experimental procedures can be found in the Supporting Information. Scheme 1 depicts the bandgap variation versus size control, which were obtained from the emission peaks and TEM images, respectively. CdTe QDs with similar bandgap size to bulk semiconductors could be obtained by our method. Figure 1 shows that the CdTe QDs which were presented by the concentration control method were engineered to emit in the 670 to 800 nm range and that longer wavelength emission could be obtained at a high concentration than at a low concentration. A longer wavelength emission indicates larger particle size due to the Ostwald ripening in the high concentration. In other words, the monomers react very fast with each other and form nuclei when the precursors are injected at high temperature (350 C). The nuclei number in high concentration is larger than that in low concentration because of the high collision frequency in high concentration. Next, monomer depletion occurs very fast and the nuclei concentration decreases due to the Ostwald ripening which includes the decomposition of emission peaks move to a longer wavelength at high nuclei to monomer and re-growth in another nuclei. The monomer depletion and Ostwald ripening occur faster in high nuclei concentration. A few studies have reported on similar research about kinetics in QDs synthesis. Table 1 and Figure 2 show the emission peak positions and quantum yields (QYs) according to the concentration and molar ratio. The emission peaks appear in

Suppression of the floating-body effect in SOI MOSFET's by the bandgap engineering method using a Si/sub 1-x/Ge/sub x/ source structure

The bandgap engineering method using a SiGe source structure is presented as a means to suppress the floating-body effect in SOI MOSFET's. Experiments using Ge implantation are carried out to form a narrow-bandgapped SiGe layer in the source region. It has been confirmed that Ge-implanted SIMOX exhibited a 0.1 eV bandgap narrowing with a relatively low Ge-dosage of 10/sup 16/ cm/sup -2/. The fabricated N-type SOI-MOSFET's exhibited suppressed parasitic bipolar effects, such as improvement of the drain breakdown voltage or latch voltage, and suppression of abnormal subthreshold slope. Advantages over other conventional methods are also discussed, indicating that the bandgap engineering provides a practical method to suppress the floating-body effect.