Energy Band Engineering Research Articles

Progress of long wavelength infrared focal plane arrays based on antimonide compounds superlattice

<p indent=0mm>The research of antimonide compounds began with 1950s. With the appearing of superlattice concept and energy band engineering in the 1970s, the potentials in infrared detection of antimonide compounds gradually emerged. Due to the reality demand, epitaxy and preparation technologies of antimonide-based materials have achieved rapid progress. The rapid advance of T2SL also benefits from the previously five decades ago of development of III-V group materials and devices research. The development of InAs/GaSb type-II superlattices (T2SLs) results from two primary motivations: the InAs/GaSb superlattice has more apparent advantages than HgCdTe in low cost, reproducibility, operability and high uniformity, especially for long wavelength detector. Secondly, the Auger recombination probability of type II superlattices is lower than that of HgCdTe, which means that the infrared detector of type II superlattice has a lower dark current or a higher working temperature than that of HgCdTe detector. And improving the working temperature of the long-wave focal plane is crucial for reducing the power consumption, size and weight of the imaging system. In addition, the atmosphere window with highest transmittance is located at the range of <sc>8−14 μm.</sc> And the emitted infrared radiation by an object at room temperature <sc>(300 K)</sc> has a wavelength of about <sc>10 μm.</sc> Therefore, long wavelength infrared detection is extremely valuable for InAs/GaSb type-II superlattices. Compared with HgCdTe device, T2SL detectors can provide equivalent or better performance under the same cutoff wavelength. But the short minority carrier lifetime of InAs/GaSb superlattice cause a high level of generation-recombination current (<italic>J</italic>_gr). In order to suppress <italic>J</italic>_gr and other dark current, the different device structures like W-structure, PπMN structure, CBIRD and unipolar barrier were put forward and employed in the detectors. As a result, the dark current and impedance was greatly improved. Furthermore, with the putting forward of InAs/InAsSb superlattice, the defects level in band gap introduced by Ga were avoided, and the minority carriers lifetime were effectively improved. The reproducibility, operability and uniformity of T2SL bring more advantages than HgCdTe detector with the sustainable developement of T2SL technologies and theories.

The Optical Blueshift Saturation Behavior of Mg-=SUB=-x-=/SUB=-Zn-=SUB=-1-x-=/SUB=-O Films

A phenomenon of blueshift saturation was observed from cathodoluminescence (CL) spectra of MgxZn1-xO films (0.069≤x≤0.8) which were grown by pulsed laser deposition technology. By analyzing the results of composition-dependent X-ray diffraction spectra, scanning electron microscopy images, absorption spectra and CL spectra, the crystalline structural disorder was determined via Urbach energy value. Furthermore, a competition mechanism between band-filling effect and band tail states was proposed in the composition-dependent CL spectra. This competition is believed to be responsible for the composition-induced blueshift saturation often observed in the disordered alloy semiconductors. The results of this research can provide important reference in energy-band engineering.

Low Interfacial Energy Barrier and Improved Thermoelectric Performance in Te-Incorporated Polypyrrole

Controlled carrier dynamics and energy band engineering in polymer systems can provide a wider scope for studying the thermoelectric (TE) performance of such systems. With such an objective, polypy...

Enhanced photocatalytic performance of ammonia self-etched holely g-C3N4 decorated with anatase nanoflakes by a facile synthesis process

Graphitic carbon nitride (g-C3N4) nanosheets have attracted extensive attention in the field of photocatalysis due to their superior properties. However, the photocatalytic performance of g-C3N4 is still restricted by the limited active sites and high combination rate of photo-generated carriers. To confront these drawbacks, a facile NH3 self-etching and TiO2 decorating method is employed in this work. The self-released NH3 with high concentration in thermal condensation process etches and exfoliates g-C3N4 into less aggregated nanosheets with abundant nano holes (~2.7 nm). The specific surface area is up to 224 m2·g−1, which is higher than the previous reported samples prepared by template-free methods. The formed holes endow g-C3N4 nanosheets with more exposed active edges that greatly accelerate mass exchange and photo-generated carriers transfer. After decorating anatase TiO2 nanoflakes, the composite photocatalyst exhibits a Z-scheme heterojunction behavior and enhances spatial separation efficiency of photo-generated carriers greatly. As a result, the photocatalytic performance is significantly enhanced: the photodegradation rate on rodamine B reaches 99.2% within 8 min and the hydrogen evolution rate reaches as high as 1764 μmol·g−1·h−1. This work provides a facile and low-cost processing method to prepare high active and stable photocatalysts, and show the importance in rational designing the nano-materials by well-directed preparation process and energy band engineering.

Recent progress and challenges on two-dimensional material photodetectors from the perspective of advanced characterization technologies

Atomically thin two-dimensional (2D) materials exhibit enormous potential in photodetectors because of novel and extraordinary properties, such as passivated surfaces, tunable bandgaps, and high mobility. High-performance photodetectors based on 2D materials have been fabricated for broadband, position, polarization-sensitive detection, and large-area array imaging. However, the current performance of 2D material photodetectors is not outstanding enough, including response speed, detectivity, and so forth. The way to further promote the development of 2D material photodetectors and their corresponding practical applications is still a tremendous challenge. In this article, these issues of 2D material photodetectors are analyzed and expected to be solved by combining micro-nano characterization technologies. The inherent physical properties of 2D materials and photodetectors can be accurately characterized by Raman spectroscopy, transmission electron microscopy (TEM), and scattering scanning near-field optical microscope (s-SNOM). In particular, the precise probe of lattice defects, doping concentration, and near-field light absorption characteristics can promote the researches of low-noise and high-responsivity photodetectors. Scanning photocurrent microscope (SPCM) can show the overall spatial distribution of photocurrent and analyze the mechanism of photocurrent. Photoluminescence (PL) spectroscopy and Kelvin probe force microscope (KPFM) can characterize the material bandgap, work function distribution and interlayer coupling characteristics, making it possible to design high-performance photodetectors through energy band engineering. These advanced characterization techniques cover the entire process from material growth, to device preparation, and to performance analysis, and systematically reveal the development status of 2D material photodetectors. Finally, the prospects and challenges are discussed to promote the application of 2D material photodetectors.

Modulating Surface/Interface Structure of Emerging InGaN Nanowires for Efficient Photoelectrochemical Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting provides a promising approach to convert solar energy into hydrogen. Developing active, stable, and cost‐effective semiconductors photoelectrodes is of great significance for achieving high‐efficiency and large‐scale hydrogen production. InGaN nanowires as an important candidate have gained a great upsurge in solar water splitting due to its tunable gap, high electron mobility, large active area, and excellent chemical stability. To obtain state‐of‐the‐art InGaN nanowires‐based photoelectrodes, tremendous efforts have been devoted to enhance light absorption capacity, charge carrier dynamics, and redox activity. In this review, recent advances in InGaN nanowires as photoelectrodes for PEC water splitting are comprehensively presented, with a focus on photoelectrode optimization strategies from the aspects of surface and interface structure modulation including doping engineering, energy band engineering, heterostructure engineering, and micro‐nano engineering. The representative applications of InGaN nanowires‐based PEC tandem cell are also discussed. Finally, perspectives on remaining challenges and future development are outlined.

(Invited) Heteroepitaxy and Strain Engineering of Germanium-Silicon-Tin Ternary Alloy Semiconductor Thin Films for Energy Band Design

Strain engineering of group-IV semiconductor thin film is a key for energy band design and engineering for electronic and optoelectronic applications. In this paper, we report our recent achievements for strain relaxation method of Ge1 −x−y Si x Sn y ternary alloy semiconductor epitaxial layer on various substrates. Virtual Ge substrate effectively enhances the strain relaxation of Ge1 −x−y Si x Sn y layer and also improves the surface morphology and uniformity after the strain relaxation. Ion implantation into Ge substrate is also effective to enhance the strain relaxation and achieves a large in-plane lattice constant as a buffer layer for the growth of strain-relaxed Ge1 −x Sn x epitaxial layer with a Sn content as high as 10%.

Self-integrated effects of 2D ZnIn2S4 and amorphous Mo2C nanoparticles composite for promoting solar hydrogen generation

Abstract To maximize the performance of photocatalytic hydrogen production, photocatalysts need to be modified by various means such as energy band engineering and cocatalyst. Here, we propose a strategy of self-integrated effects for promoting photocatalysts performance. We firstly design and fabricate a 2D ZnIn2S4 and amorphous Mo2C nanoparticles composite photocatalyst by integrating heterojunction effect, cocatalyst effect and photothermal effect in one, and then demonstrate that self-integrated effects of Mo2C/ZnIn2S4 composite can greatly enhance photocatalytic hydrogen evolution. Based on in situ characterization techniques and theoretical calculations, we also establish that the photocatalytic mechanism of self-integrated effects consisting heterojunction effect, cocatalyst effect and photothermal effect is attributed to the increased absorption capacity, the enhanced carrier separation, the reduced ΔGH*, the more active sites, the increased electron density and the enhanced carrier's mobility. Especially, the contribution of photothermal effect can elevate temperature to accelerate the photocatalytic reaction and the photothermal contribution exceeds 100% under irradiation. Consequently, 2D ZnIn2S4/amorphous Mo2C nanoparticles has a remarkable photocatalytic hydrogen evolution rate respectively up to 22.11 and 40.93 mmol/g/h upon visible and AM1.5 illumination, promoting 164% and 156% for the available values reported of modified ZnIn2S4 photocatalysts so far. These findings suggest that the proposed self-integrated effects can greatly promote photocatalytic hydrogen production.

Strong Band Bowing Effects and Distinctive Optoelectronic Properties of 2H and 1T′ Phase‐Tunable MoxRe1–xS2 Alloys

AbstractStructure and energy band engineering of 2D materials via selective doping or phase modulation provide a significant opportunity to design them for optoelectronic devices. Here, the synthesis of high‐quality MoxRe1–xS2 alloys with tunable composition and phase structure via chemical vapor deposition growth is reported, and their novel energy band structures and optoelectronic properties are explored. The phase separation and structure reconstruction, which are found to be two serious problems in the synthesis of these alloys, are successfully suppressed through tuning their growth thermodynamics. As a result, the obtained MoxRe1–xS2 alloys have uniform composition, phase structure, and crystal orientation. Together with X‐ray photoelectron spectroscopy analysis and first‐principle calculation, the Re/Mo doping‐induced Fermi level up‐shift/down‐shift, new electronic states, and “sub‐gap” formation in MoxRe1–xS2 alloys are revealed. Especially, a strong band bowing effect is discovered in the MoxRe1–xS2 alloys with structure transition between 1T′ and 2H phases. Furthermore, these alloys reveal tunable conduction behavior from n‐type to bipolar and p‐type in 1T′ phase, as well as novel “bipolar‐like” electron conduction behavior in 2H alloys. The results highlight the unique alloying effects, which do not exist in the single‐phase 2D alloys, and provide the feasibility for potential applications in building novel electronic and optoelectronic devices.

Interface Engineering and Epitaxial Growth of Single‐Crystalline Aluminum Films on Semiconductors

AbstractSingle crystalline aluminum (Al) films are grown on GaAs substrates by molecular beam epitaxy. The high crystalline quality and sharp interfaces of the epitaxial Al films are confirmed by high‐resolution X‐ray diffraction and transmission electron microscopy. Semi‐metallic ErAs is inserted to the Al/GaAs interface to modify the band structure and reduce the optical loss of Al without degrading the crystalline quality. The optical dielectric properties of the Al films are measured by spectroscopic ellipsometry and the optical loss (ε2) is reduced in a broad spectral region from ultraviolet (UV) to near‐infrared compared to the widely quoted Palik's values. In particular, the loss in the visible region is reduced by a factor of three, indicating the effective energy band engineering by ErAs.

Weak thermal quenching and tunable luminescence in Ce:Y3(Al,Sc)5O12 transparent ceramics for high power white LEDs/LDs

Realizing the high brightness solid-state lighting with the emission spectra similar to that of sun light is significantly challenged by the inherent deficiency of the luminescence spectrum and thermal robustness in the single sturctured Ce3+:Y3Al5O12 (Ce:YAG) transparent ceramic (TC). In this study, based on the idea of energy band engineering, the energy splitting intensity of 5d level and the energy gap between the ground band and the 5d1 energy level of Ce3+ ion in Ce:YAG TC were effectively modified via Sc doping. A series of Ce:Y3Al5−xScxO12 (Ce:YASG) TCs were fabricated with impressive properties, showing a decreased stokes shift and a distinct blue-shift (541 → 520 nm) with broadened full width in emission spectra. Particularly, the prepared TCs exhibited a desired thermal stability (only ~6% luminescent intensity loss at 150 °C) and high internal quantum efficiency (IQE) of 84.2%. The optimal composition between Ce3+ and Sc3+ ions was investigated by constructing the Ce:YASG and (Y1−yCey)3Al4ScO12 (YCASG) TC-based white LEDs/LDs in a remote excitation mode. Both the luminous efficiency of radiation (LER) as high as 218 lm/W and tunable color from pale-green (pale-white) to yellowish white (pale-green) region were obtained. Therefore, the Ce:YASG TC is a highly promising color conversion material for high power lighting and displaying applications in the future.

Large‐scale quantification of aluminum in AlxGa1‐xN alloys by ToF‐SIMS: The benefit of secondary cluster ions

Precise determination of composition is requisite for AlxGa1‐xN‐based energy band engineering. Secondary cluster ions in a time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) full spectrum are introduced for accurate quantification of Al in AlxGa1‐xN for AlGaN‐based devices. It was found that statistical analysis with huge number of large secondary cluster ions (without small ions) show much better linear dependence with apparent compensation of matrix effect, particularly in negative polarity. For AlxGa1‐xN with x = 0 to 0.7, x values calculated based on large negative cluster ions (NCIs) show excellent linear dependence with real values. Besides, atomic count ratio of C (Al)/C (Ga) and C (Ga)/C (Al) also shows great linear relationships with corresponding mole fraction ratios in a large range. Overlap of these linear ranges offers a reliable and convenient quantification protocol of AlxGa1‐xN covering a full range of x = 0 to 1. This study verifies that the secondary cluster ions are beneficial for quantification of AlxGa1‐xN‐based materials.

Fabrication of folded bilayer-bilayer graphene/hexagonal boron nitride superlattices

Carbon-based superlattices represent a novel class of quantum metamaterials having promising prospects. Here, we show the first fabrication of folded bilayer-bilayer graphene (fBBLG)/hexagonal boron nitride (hBN) superlattices. In the transport properties at low temperatures, satellite peaks are observed due to the crystallographic angle alignment of fBBLG and hBN. The peculiar resistance peaks appear away from the charge neutrality point and the hBN-induced satellites, which may be signatures of possible competing orders. Our results suggest the emergence of a unique electronic band-structure in the fBBLG, providing a way for investigating the correlated electron phenomena by performing energy-band engineering with superlattice structures.

Optically Modulated Threshold Switching in Core–Shell Quantum Dot Based Memristive Device

AbstractThe threshold switching (TS) phenomenon in memristors has drawn great attention for its versatile applications in selectors, artificial neurons, true random number generators, and electronic integrations. The transition between nonvolatile resistive switching and volatile TS modes can be realized by doping, varying annealing and voltage sweeping conditions, or imposing different compliance current. Here, a strategy is reported to achieve such transition by the noninvasive UV light stimulus based on InP/ZnS quantum dot (QD) memristor. The core–shell InP/ZnS QDs with quasi‐type II band alignment ensures photoexcited electrons localized in InP core, photoexcited hole state distributed in the outer shell, and subsequent lifetime controlling of conductive filament under light irradiation. Systematic mechanism investigations indicate that UV photogenerated holes are accumulated on the surface of the QD film, which is consistent with rapid transfer of photogenerated holes in the core–shell InP/ZnS structure. Based on the light‐modulated effect, a reconfigurable 9 × 9 visual data storage array with a key pattern and a simple leaky integrate‐and‐fire circuit are constructed. These results suggest the potential of direct optical modulation of memory mode through energy band engineering, leading to future optoelectronic and electronic device for the implementation of neuromorphic visual system and artificial neural networks.

Realizing Stable Artificial Photon Energy Harvesting Based on Perovskite Solar Cells for Diverse Applications.

As the fastest developing photovoltaic device, perovskite solar cells have achieved an extraordinary power conversion efficiency (PCE) of 25.3% under AM 1.5 illumination. However, few studies have been devoted to perovskite solar cells harvesting artificial light, owing to the great challenge in the simultaneous manipulation of bandgap-adjustable perovskite materials, corresponding matched energy band structure of carrier transport materials, and interfacial defects. Herein, through systematic morphology, composition, and energy band engineering, high-quality Cs0.05 MA0.95 PbBrx I3- x perovskite as the light absorber and Nby Ti1- y O2 (Nb:TiO2 ) as the electron transport material with an ideal energy band alignment are obtained simultaneously. The theoretical-limit-approaching record PCEs of 36.3% (average: 34.0 ± 1.2%) under light-emitting diode (LED, warm white) and 33.2% under fluorescent lamp (cold white) are achieved simultaneously, as well as a PCE of 19.5% (average: 18.9 ± 0.3%) under solar illumination. An integrated energy conversion and storage system based on an artificial light response solar cell and sodium-ion battery is established for diverse practical applications, including a portable calculator, quartz clock, and even environmental monitoring equipment. Over a week of stable operation shows its great practical potential and provides a new avenue to promote the commercialization of perovskite photovoltaic devices via integration with ingenious electronic devices.

Novel and dual-mode strain-detecting performance based on a layered NiO/ZnO p–n junction for flexible electronics

A dual-mode piezoelectric transistor based on a NiO/ZnO p–n junction can detect the dynamic strain based on the piezoelectric effect, and further monitor the static strain with the regulation of energy band engineering in the p–n junction.

Energy Band Engineering of Polymeric Carbon Nitride with Indium Doping for High Enhancement in Charge Separation and Photocatalytic Performance

Metal doping is a valid strategy to increase simultaneously photoabsorption, the charge separation efficiency, and thus the photocatalytic activity of polymeric carbon nitride (CN); however, metal ...

Visible-Light-Driven Cleavage of C-O Linkage for Lignin Valorization to Functionalized Aromatics.

Lignin is the most abundant source of renewable aromatics. Catalytic valorization of lignin into functionalized aromatics is attractive but challenging. Photocatalysis is a promising sustainable approach. The strategies for designing well-performing photocatalysts are desired but remain limited. Herein, a facile energy band engineering strategy for promoting the photocatalytic activity of zinc-indium-sulfide (Znm In2 Sm+3 ) for cleavage of the lignol β-O-4 bond under mild conditions was developed. The energy band structure of Znm In2 Sm+3 could be tuned by controlling the atomic ratio of Zn/In. It was found that Zn4 In2 S7 performed best for cleavage of the β-O-4 bond under visible-light irradiation, owing to its appropriate energy band structure for offering adequate visible-light absorption and suitable redox capability. Functionalized aromatic monomers with near 18.4 wt % yield could be obtained from organosolv birch lignin. Mechanistic studies revealed that the β-O-4 bond was efficiently cleaved mainly through a one-step redox-neutral pathway via a Cα radical intermediate. The thiol groups on the surface of Zn4 In2 S7 played a key role in cleavage of the β-O-4 bond.

Interlayer Transition in a vdW Heterostructure toward Ultrahigh Detectivity Shortwave Infrared Photodetectors

AbstractVan der Waals (vdW) heterostructures of 2D atomically thin layered materials (2DLMs) provide a unique platform for constructing optoelectronic devices by staking 2D atomic sheets with unprecedented functionality and performance. A particular advantage of these vdW heterostructures is the energy band engineering of 2DLMs to achieve interlayer excitons through type‐II band alignment, enabling spectral range exceeding the cutoff wavelengths of the individual atomic sheets in the 2DLM. Herein, the high performance of GaTe/InSe vdW heterostructures device is reported. Unexpectedly, this GaTe/InSe vdWs p–n junction exhibits extraordinary detectivity in a new shortwave infrared (SWIR) spectrum, which is forbidden by the respective bandgap limits for the constituent GaTe (bandgap of ≈1.70 eV in both the bulk and monolayer) and InSe (bandgap of ≈1.20–1.80 eV depending on thickness reduction from bulk to monolayer). Specifically, the uncooled SWIR detectivity is up to ≈1014 Jones at 1064 nm and ≈1012 Jones at 1550 nm, respectively. This result indicates that the 2DLM vdW heterostructures with type‐II band alignment produce an interlayer exciton transition, and this advantage can offer a viable strategy for devising high‐performance optoelectronics in SWIR or even longer wavelengths beyond the individual limitations of the bandgaps and heteroepitaxy of the constituent atomic layers.

Dramatically improved electron transport performance by a deep triangular potential well in organic field-effect transistors

Overcoming the intrinsic properties of organic molecules to achieve high device performance, particularly for electron transport, is a challenge in the field of organic semiconductor device physics. Energy-band engineering in modern inorganic electronics has been introduced in organic electronics. Here, we utilized Al2O3 and PbO metal-oxide layers as dielectric modification layers and fabricated high-quality organic heterojunction films with large band bending and sharp interfaces. The large band bending indicated a deeper triangular potential well, which can confine more electrons and form a high carrier density organic two-dimensional electron gas. A sharp heterojunction interface can reduce interface carrier transport scattering and crystal lattice defects and can retain uniform interfacial electronic structures. These two features of organic heterojunctions are the key factors to realize energy-band engineering in organic electronics. By using these heterojunction films as active layers in organic field-effect transistors, the transistors exhibited a high electron mobility of approximately 8 cm2 V−1 s−1. Therefore, we demonstrated the feasibility and solutions of energy-band engineering in organic electronics, and provided a novel strategy to improve device performance.