Band Tail Research Articles

Tin Halide Perovskite Solar Cells with Open-Circuit Voltages Approaching the Shockley-Queisser Limit.

The power conversion efficiency of tin-based halide perovskite solar cells is limited by large photovoltage losses arising from the significant energy-level offset between the perovskite and the conventional electron transport material, fullerene C60. The fullerene derivative indene-C60 bisadduct (ICBA) is a promising alternative to mitigate this drawback, owing to its superior energy level matching with most tin-based perovskites. However, the less finely controlled energy disorder of the ICBA films leads to the extension of its band tails that limits the photovoltage of the resultant devices and reduces the power conversion efficiency. Herein, we fabricate ICBA films with improved morphology and electrical properties by optimizing the choice of solvent and the annealing temperature. Energy disorder in the ICBA films is substantially reduced, as evidenced by the 22 meV smaller width of the electronic density of states. The resulting solar cells show open-circuit voltages of up to 1.01 V, one of the highest values reported so far for tin-based devices. Combined with surface passivation, this strategy enabled solar cells with efficiencies of up to 11.57%. Our work highlights the importance of controlling the properties of the electron transport material toward the development of efficient lead-free perovskite solar cells and demonstrates the potential of solvent engineering for efficient device processing.

Growth and Characterization of Lanthanum Germanide Thin Films by the Thermal Evaporation Technique

AbstractLanthanum germanide (La6Ge) thin films are successfully fabricated using the thermal evaporation technique under a vacuum pressure of 10−5 mbar. The resulting films display an orthorhombic structure, characterized by lattice parameters of a = 8.725 Å, b = 8.063 Å, and c = 5.569 Å. Optical analysis of the La6Ge thin films reveal their high transparency, with an energy bandgap of 3.75±0.04 eV. The bandgap exhibits indirect allowed transitions and featured energy band tails with widths measuring (1.64±0.14) eV. In addition, dielectric dispersion analyses indicate the presence of two dominant dielectric resonance peaks centered at 3.15 and 2.08 eV. Moreover, the La6Ge films demonstrate a terahertz (THz) cutoff frequency of 1.0 THz when illuminated with infrared and visible light. This cutoff frequency increases to 45.6 THz in the ultraviolet range. Furthermore, by utilizing the Drude–Lorentz method, the investigation of optical conductivity parameters reveals that the lanthanum germanide optical filters can achieve free hole density and drift mobility values of 14.44 cm2 V−1 s−1 and 2.8×1018 cm−3, respectively, under infrared light irradiation. The outstanding optical and dielectric properties exhibited by the La6Ge thin films make them excellent candidates for highly transparent optical filters suitable for terahertz technology.

Crystalline and optical properties of AlN films with varying thicknesses (0.4-10 µm) grown on sapphire by metalorganic chemical vapor deposition

Structural and optical properties of AlN films grown on c-sapphire by metalorganic chemical vapor deposition (MOCVD) with varied thicknesses (0.4-10 µm) are investigated. AlN with good crystal quality, large grain size, low dislocation density and micro-strain were grown. The band gap Eg values obtained from Optical transmission (OT) measurement (5.9-6.0 eV) are lower than from spectroscopic ellipsometry (SE) experiments (6.1-6.2 eV). This is because SE measures bandgap primarily due to the lattice structure while OT also includes effects of impurities- and defects-related transitions in the bandgap measurement. The absorption tails below the band gap, i.e., the Urbach band tails, from OT are relatively wider than from SE. OT measurements showed the Urbach band tails from thick AlN films larger than from thinner films, indicating that more unintended doped impurities and defects were incorporated in thicker samples. As per multi-wavelengths (532-325-266nm) excitation Raman spectroscopy, AlN films have compressive stress as whole and tensile stress towards the surface. E2(high) phonon lifetime reduces with an increase of temperature, due to lattice thermal expansion and increased phonon-phonon interaction. This work provides insight into the structural and optical properties of AlN and could help in understanding AlN for optoelectronic applications.

Morphometric Variation in Lepidogrammus cumingi and Dasylophus superciliosus from Greater Luzon, Philippines based on Museum Specimens

The monotypic Scale-feathered Malkoha Lepidogrammus cumingi (Fraser 1839) and polytypic Red-crested Malkoha Dasylophus superciliosus (Dumont 1823) are two of the endemic cuckoos in the Philippines. The taxonomy and distribution of these endemic cuckoos are poorly known. Recent distribution data of these cuckoos showed broader distribution patterns in the islands of Luzon, Catanduanes, and Marinduque; however, morphometric variations among these island populations are still unknown. While both species are sexually monomorphic, the differences between sexes were mainly based on plumage coloration. Dasylophus superciliosus is composed of two subspecies: ssp. superciliosus (Southern Luzon) and ssp. cagayanensis (Northern Luzon). The separation of the two subspecies of D. superciliosus was determined by the differences in superciliary feather, the coloration of breast plumage, and the tail band. In this study, we investigated the morphometric variations (1) between subspecies of D. superciliosus, (2) among island populations, and (3) between sexes (sexual dimorphism) of the two malkoha species. Four linear measurements (bill length, wing length, tarsus length, and tail length) were measured from 184 museum specimens. Results showed that the Catanduanes group (n=10) of L. cumingi exhibited a smaller body size than the Marinduque group (n=9). Red-crested Malkoha D. superciliosus showed significant differences between its two subspecies where the ssp. cagayanensis (n=28) exhibited smaller body size (bill length, wing length, and tail length) compared to ssp. superciliosus (n=81). Lastly, this study confirms that the two malkoha species are sexually monomorphic based on the analyzed morphometric data.

Revealing the interaction mechanism of pulsed laser processing with the application of acoustic emission

The mechanisms of interaction between pulsed laser and materials are complex and indistinct, severely influencing the stability and quality of laser processing. This paper proposes an intelligent method based on the acoustic emission (AE) technique to monitor laser processing and explore the interaction mechanisms. The validation experiment is designed to perform nanosecond laser dotting on float glass. Processing parameters are set differently to generate various outcomes: ablated pits and irregular-shaped cracks. In the signal processing stage, we divide the AE signals into two bands, main and tail bands, according to the laser processing duration, to study the laser ablation and crack behavior, respectively. Characteristic parameters extracted by a method that combines framework and frame energy calculation of AE signals can effectively reveal the mechanisms of pulsed laser processing. The main band features evaluate the degree of laser ablation from the time and intensity scales, and the tail band characteristics demonstrate that the cracks occur after laser dotting. In addition, from the analysis of the parameters of the tail band very large cracks can be efficiently distinguished. The intelligent AE monitoring method was successfully applied in exploring the interaction mechanism of nanosecond laser dotting float glass and can be used in other pulsed laser processing fields.Graphical

Fabrication of electrical semi-conductive SiCN ceramics by vat photopolymerization

The emergence of additive manufacturing (AM) enables ceramics to be fabricated with customized geometry, and polymer-derived ceramics (PDCs) has attracted growing attention owing to their irreplaceable advantages. The combination of 3D printing and PDCs endows the resultant ceramics with both precision and performance. However, AM of ceramics from preceramic polymers is still challenging, and insufficient investigation of functionality also limits the versatility of precursor and its derived ceramics. Herein, we propose a novel paradigm for 3D printing dense silicon carbonitride ceramic and study its electrical semi-conducting properties. The formulated photosensitive precursor inks could achieve self-polymerization and cross-linking under the radiation of UV light (405 nm). The green body with intricate structures is fabricated by digital light processing (DLP). Lightweight (1.79–2.08 g cm–3) and low porosity (< 5%) amorphous ceramics were obtained after thermal treatments. Processes of cross-linking, decomposition, and ceramization are monitored and analyzed. Furthermore, the semi-conducting behaviors of resultant ceramics are identified where the conductivity (10–5–10–1 S m–1) has a monotonic correspondence with the testing temperatures (25–1000 °C). The numerical relationship is fitted by exponential functions, and its conducting mechanism could be interpreted by the band tail hopping (BTH) model. This work could provide alternative solutions for the fabrication of PDCs and potentials for sensing applications.

Influence of 120 MeV Au Ion Irradiation on Phase Transition, Surface, and Optical Properties of Lead‐Free (K,Na)NbO3 Films

High energy ion beam irradiation is an important tool to engineer the properties of multifunctional materials for diverse applications. The present study reports the impact of 120 MeV Au ion beam irradiation on structural and luminescence properties of (K,Na)NbO3 (KNN) films. The increase in ion fluence has resulted in crystalline‐to‐crystalline and crystalline‐to‐amorphous phase transitions at room temperature. The dense grain‐like morphology of crystalline films gets transformed into clusters of nano‐sized grains after irradiation, and the grain boundaries are diffused at higher fluence. X‐ray photoelectron spectroscopy results indicate the evaporation of K‐species from the surface after irradiation. The relative elemental ratio and film's thickness were estimated using Rutherford backscattering spectrometry, and subsequently, KNN/Si interface analysis was also carried out. The increase in optical band gap after irradiation is correlated to the suppression of extended band tails. An intense photoluminescence emission in the UV region is observed, and the intensity increases abruptly after the crystalline‐to‐amorphous phase transition, where the grain boundaries merge with nearby ones. The impact of high electronic energy loss in KNN enhanced the probability of radiative recombination. The obtained results demonstrate the possible use of KNN for optoelectronic applications.

Accurate determination of band tail properties in amorphous semiconductor thin film with Kelvin probe force microscopy

The disordered microscopic structure of amorphous semiconductors causes the formation of band tails in the density of states (DOS) that strongly affect charge transport properties. Such band tail properties are crucial for understanding and optimizing thin-film device performance with immense relevance for large area electronics. Among the available techniques to measure the DOS, Kelvin Probe Force Microscopy (KPFM) is exceptional as it enables precise local electronic investigations combined with microscopic imaging. However, a model to interpret KPFM spectroscopy data on amorphous semiconductors of finite thickness is lacking. To address this issue, we provide an analytical solution to the Poisson equation for a metal–insulator–semiconductor junction interacting with the atomic force microscope tip. The solution enables us to fit experimental data for semiconductors with finite thickness and to obtain DOS parameters, such as band tail width, doping density, and flat band potential. To demonstrate our method, we perform KPFM experiments on Indium–Gallium–Zinc Oxide (IGZO) thin-film transistors (IGZO-TFTs). DOS parameters compare well with values obtained with photocurrent spectroscopy. We demonstrate the relevance of the developed method by investigating the impact of ionizing radiation on DOS parameters and TFT performance. Our results provide clear evidence that the observed shift in threshold voltage is caused by static charge in the gate dielectric, leading to a shift in flat band potential. Band-tails and doping density are not affected by the radiation. The developed methodology can be easily translated to different semiconductor materials and paves the way for quantitative microscopic mapping of local DOS parameters in thin-film devices.

Toward Low‐Power Cryogenic Metal‐Oxide Semiconductor Field‐Effect Transistors

Herein, cryogenic field‐effect transistors (FETs) are discussed. In particular, the saturation of the subthreshold swing due to band tailing is studied. It is shown with simulations and experiments that engineering of the oxide‐channel interfaces and a strong increase of the gate oxide capacitance are effective in improving the switching behavior of the device. The implication of scaling the oxide capacitance on the power consumption of cryogenic devices is investigated, too. Furthermore, an alternative for conventional doping in cryogenic transistors is discussed. Based on synchrotron X‐Ray absorption spectroscopy at total fluorescence (XAS‐TFY) and ultraviolet photoemission spectroscopy (UPS) measurements, it is shown experimentally that in true nanoscale devices, a simple coating yields a shift of the conduction band that is equivalent to a very high dopant concentration. As a result, nanoscale cryogenic steep slope FETs with strongly improved electrical characteristics become feasible.

Influence of Ancillary Ligand on the Photosensitization and Photophysical Properties of Red Luminescent Tri-fluorinated Β-diketonate Eu(III) Complexes.

Highly emissive ternary Eu(III) complexes were synthesized with a tri-fluorinated β-diketone as principal ligand and heterocyclic aromatic compounds as ancillary ligands to assess their utility as an illuminating material for display devices and other optoelectronics.The general characterizations, regarding the coordinating facets of complexes were accomplished via various spectroscopic techniques. Thermal stability was investigated via TGA/DTA. Photophysical analysis was accomplished by PL studies, Band gap value, color parameters and J-O analysis. DFT calculations were performed adopting geometrically optimized structure of complexes.Superb thermal stability has been achieved in complexes, which decides their concrete candidature for display devices. The bright red luminescence of complexes is ascribed to 5D0 → 7F2 transition of Eu(III) ion. Colorimetric parameters unlocked the applicability of complexes as warm light source and J-O parameters adequately summarized the coordinating surrounding around the metal ion. Various radiative properties were also evaluated which suggested the prospective use of complexes in lasers and other optoelectronic devices. The band gap and Urbach band tail, procured from absorption spectra, revealed the semiconducting behavior of synthesized complexes. DFT studies rendered the energies of FMO and various other molecular parameters.It can be summarized from the photophysical and optical analysis of synthesized complexes that these complexes are virtuous luminescent materials and possess potentiality to be used in diverse domain of display devices.

Characterization of PbWO4 thin films formed by the pulsed laser welding technique

Herein thin films of PbWO4 were formed from bilayers of Pb/WO3 by the pulse laser welding technique (PLW) within couple of seconds in an argon atmosphere. Thin films of Pb (100 nm)/WO3 (500 nm) were deposited by the thermal evaporation technique under a vacuum pressure of 10−5 mbar. The films were then exposed to a pulse laser beam of fixed amplitude and variable pulse width (0.1–10 ms). The produced films were characterized by means of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical spectrophotometry techniques. An almost pure phase (∼90%) of tetragonal PbWO4 was achieved after the pulse width exceeds 5.0 ms. PbWO4 thin films produced by the PLW technique exhibited a direct and indirect energy band gaps of 2.27 eV and 3.52 eV, respectively. Energy band tails of widths of 1.35 eV were found dominant in these films. In addition fitting of the dielectric resonance spectra for Pb/WO3 and PbWO4 thin films indicated that the formation of PbWO4 was accompanied with a decrease in the high frequency dielectric constant, a decrease in the free hole concentration and an increase in the electronic friction in the films. The fast method of obtaining PbWO4 thin films and the formation of the direct band gap being 2.27 eV together with the nonlinear dielectric and optical properties make the PbWO4 films obtained by the PLW technique suitable for optoelectronic technology.

Analytical Modeling of Source-to-Drain Tunneling Current Down to Cryogenic Temperatures

The subthreshold swing (SS) of MOSFETs decreases with temperature and then saturates below a critical temperature. Hopping conduction via the band tail has been proposed as the possible cause for the SS saturation. On the other hand, numerical simulations have shown the source-to-drain tunneling (SDT) current limits the SS at low temperatures. It has been argued which transport mechanism dominates the cryogenic subthreshold current. Hence, for the first time, this paper presents an analytical model of the SDT current and the corresponding SS, which is validated by cryogenic measurement on devices from an advanced 16nm FinFET technology.

Effectiveness of Sn-addition on optical properties and physicochemical parameters of SnxSb2-xSe3 thin films

This research work is allocated to prepare the novel thin SnxSb2-xSe3 films, TSSe by thermal evaporation procedure at various Sn-content, where 0≤x≤2 at.%. The field-emission scanning electron microscope has been used to investigate the surface nature of these SnxSb2-xSe3 films. X-ray diffraction revealed that all TSSe films disclosed that the films have the polycrystalline nature with the orthorhombic structure without regard to the Sn-content. The optical characteristics of these layers were studied via transmission and reflection spectrophotometric measurements. The Archimedes' principle has been used to measure the density of samples, which was found that its values increase from 5.766 gm/cm3 to 6.136 gm/cm3. Along with, the molar volume decreases from 83.316 cm3/mol to 77.291 cm3/mol. The optical energy gap investigation of thin SnxSb2-xSe3 layers indicated that these layers exposed a direct allowed electronic transition. The optical band-gap energy reduces from 1.191 eV to 1.050 eV, while the band tail width increases from 0.093 eV to 0.155 eV by boosting the Sn-content. Impact of Sn-content on the linear optical parameters also refers to increasing the absorption coefficient, refractive index (from 3.2086 to 3.3317), molar refraction (from 0.756 to 0.771), the interaction strength between the electrons and photons (from 2.439 to 4.065). On the contrary, the steepness parameter, reflection factor, and electronic polarizability decrease as Sn-percentages increase from 0.0 to 2.0 at%. In addition, many other parameters have been discussed, like the optical conductivity, the real and imaginary parts of the dielectric indices, some nonlinear optical parameters, and the semiconductor-type of these films, which detect that they are p-type semiconductors. The investigated parameters and properties showed that these films can be used in many optical applications, like solar cells, photodetectors, batteries, memory devices, and others.

Structural-disorder-driven critical quantum fluctuation and localization in two-dimensional semiconductors

Quantum fluctuations of wavefunctions in disorder-driven quantum phase transitions (QPT) exhibit criticality, as evidenced by their multifractality and power law behavior. However, understanding the metal-insulator transition (MIT) as a continuous QPT in a disordered system has been challenging due to fundamental issues such as the lack of an apparent order parameter and its dynamical nature. Here, we elucidate the universal mechanism underlying the structural-disorder-driven MIT in 2D semiconductors through autocorrelation and multifractality of quantum fluctuations. The structural disorder causes curvature-induced band gap fluctuations, leading to charge localization and formation of band tails near band edges. As doping level increases, the localization-delocalization transition occurs when states above a critical energy become uniform due to unusual band bending by localized charge. Furthermore, curvature induces local variations in spin-orbit interactions, resulting in non-uniform ferromagnetic domains. Our findings demonstrate that the structural disorder in 2D materials is essential to understanding the intricate phenomena associated with localization-delocalization transition, charge percolation, and spin glass with both topological and magnetic disorders.

Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se2 Solar Cells Fabricated using Different Methods

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute &gt;75% of the total compared to &lt;25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.

Electrical Stability Modeling Based on Surface Potential for a-InGaZnO TFTs under Positive-Bias Stress and Light Illumination.

In this work, an electrical stability model based on surface potential is presented for amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) under positive-gate-bias stress (PBS) and light stress. In this model, the sub-gap density of states (DOSs) are depicted by exponential band tails and Gaussian deep states within the band gap of a-IGZO. Meanwhile, the surface potential solution is developed with the stretched exponential distribution relationship between the created defects and PBS time, and the Boltzmann distribution relationship between the generated traps and incident photon energy, respectively. The proposed model is verified using both the calculation results and experimental data of a-IGZO TFTs with various distribution of DOSs, and a consistent and accurate expression of the evolution of transfer curves is achieved under PBS and light illumination.

Insights into the Efficiency Improvement for CZTSSe Solar Cells with over 12% Efficiency via Ga Incorporation

AbstractSuppressing the band tailing and nonradiative recombination caused by massive defects and defect clusters is crucial for mitigating open‐circuit voltage (Voc) deficit and improving the device performance of CZTSSe thin film solar cells. Cation substitution is one of the most commonly used strategies to address the above issues. The latest world record efficiency of 13.0% is obtained through this strategy (Ag substitution for Cu). Nevertheless, the importance of the approach to implementing metallic ion doping is easily overlooked by researchers. Here, different approaches are adopted to realize Ga doping and the differences in the efficacy and mechanism are thoroughly investigated. It is found that the secondary phase easily emerged when the physical‐based method is employed, and thus challenging to regulate the doping effect. In the case of the chemical‐based method, Ga doping can enlarge the depletion region widthand lower the defect activation energyand Urbach energy. Furthermore, GaSn defects located at grain boundaries can expand the energy band bending between GBs and grain interiors (GIs), thereby suppressing the deep defect states and nonradiative recombination. Consequently, power conversion efficiency as high as 12.12% with a Voc of 522 mV is achieved at 2% Ga doping.

Characterization of N Rich-Silicon Nitride Thin Films Deposited by PECVD

Silicon nitride thin films are very important for their possible use in semiconductor industry and electronic applications. Changing the deposition parameters, silicon nitrides which have many varying optical properties can be produced. In this work, silicon nitride (SiNx) thin films were deposited on silicon substrates using Plasma enhanced chemical vapor deposition (PECVD) technique. The silane (SiH4) and ammonia (NH3) were used as reactant gases. Using these reactant gases, nitrogen (N) rich SiNx films were obtained. In order to get information about absorption and bond types in the films, films were analyzed by the help of Fourier transform infrared spectroscopy (FTIR) was performed. The refractive index, extinction coefficient and band gap energy of the films were changed from 1.86, 0 and 5.38 eV to 2.05, 0.0048 and 4.26 eV, respectively. Using the refractive index, composition of the films were estimated that is [N]/[Si] ratio of the films varied from 1.38 to 1.62. For possible applications of the SiNx films, learning the origin of the light-emission of the films is very important so the photoluminescence (PL) measurements were also used to see the luminescent of the SiNx films which is related to the electronic transitions between the K-center level and the conduction band tail states.

Understanding the Excess 1/f Noise in MOSFETs at Cryogenic Temperatures

Characterization, modeling, and development of cryo-temperature CMOS technologies (cryo-CMOS) have significantly progressed to help overcome the interconnection bottleneck between qubits and the readout interface in quantum computers. Nevertheless, available compact models still fail to predict the deviation of 1/f noise from the expected linear scaling with temperature ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}$ </tex-math></inline-formula> ), referred to as “excess 1/f noise,” observed at cryogenic temperatures. In addition, 1/f noise represents one of the main limiting factors for the decoherence time of qubits. In this article, we extensively characterize low-frequency noise on commercial 28-nm CMOS and on research-grade Ge-channel MOSFETs at temperatures ranging from 370 K down to 4 K. Our investigations exclude electron heating and bulk dielectric defects as possible causes of the excess 1/f noise at low temperatures. We show further evidence for a strong correlation between the excess 1/f noise and the saturation of the subthreshold swing (SS) observed at low temperatures. The most plausible cause of the excess noise is found in band tail states in the channel acting as additional capture/emission centers at cryogenic temperatures.

Study of Substrate Temperatures Effects on the Properties of Ultrasonically Sprayed SnS&lt;sub&gt;2&lt;/sub&gt; Thin Films

Tin disulfide (SnS2) thin films have drawn worldwide attention because of their outstanding performance and earth-abundant constituents. However, problems such as coexistence of complex secondary phases (SnS, Sn2S3), the band tailing issue, and bulk defects need to be addressed for further efficiency improvement. In this regard, the present work is intended for the treatment of one of these problems. Herein, a single phase SnS2 has been obtained using an ultrasonic spray pyrolysis method. which is confirmed by X-ray diffraction (XRD) and energy dispersive X-rays (EDXs) characterization techniques. The substrate temperatures (Ts) were increased from 250 °C to 450 °C, and this significantly improved the film's characteristics, which varied from an amorphous phase and a mixture of crystalline phases, SnS2 and SnS (for the films obtained at Ts = 250 and 300 °C) to a SnS2 pure phase with a hexagonal structure (for Ts ≥ 350 °C). The morphological, optical, and electrical properties of SnS2 films are greatly improved by temperature increases too, especially for the film obtained at 450 °C. This suggests that there are opportunities for further efficiency by using the as-deposited SnS2 thin film at 450 °C.