Energy Band Model Research Articles

Dominant n-type conduction and fast photoresponse in BP/MoS2 heterostructures

In recent years, van der Waals heterojunctions between two-dimensional (2D) materials have garnered significant attention for their unique electronic and optoelectronic properties and have opened avenues for innovative device architectures and applications. Among them, the heterojunction formed by black phosphorus (BP) and molybdenum disulfide (MoS2) stands out as a promising candidate for advanced optoelectronic devices. This study unravels the interplay between BP, MoS2, and Cr contacts to explain the electrical behavior of a BP/MoS2 heterojunction showing rectifying behavior with dominant n-type conduction, and a high ON/OFF current ratio around 104 at ± 20 V. The higher unexpected current observed when applying a negative bias to either MoS2 or BP side is elucidated by an energy band model incorporating a type II heterojunction at the BP/MoS2 interface with Cr forming a Schottky contact with MoS2 and an ohmic contact with BP. The BP/MoS2 heterojunction shows pronounced photoresponse, linearly dependent on the incident laser power, with a responsivity of 100 µA/W under white light at 50 µW incident power. Time-resolved photocurrent measurements reveal a relatively fast response with characteristic rise times less than 200 ms. This work demonstrates that BP/MoS2 van der Waals heterojunctions have unique electrical and photoresponse characteristics that are promising for advanced optoelectronic applications.

The blue color mechanism on sapphires from different gem deposits before and after heating under oxidizing atmosphere

The blue color of sapphire is commonly related to the amount of Fe and Ti impurities replacing Al3+ in the Al2O3 structure. Generally, the color intensity on sapphires is related to the gem deposits including the basaltic-related and metamorphic-related ones. The color of sapphires has been changed after heating under oxidizing atmosphere. However, the explanation about the color mechanism from some previous research contradicted each other and it was still wondered. For this reason, this research is focused on the role of Fe and Ti oxidation states as well as the blue color mechanism on sapphires before and after heating under oxidizing atmosphere. In this study, the sapphire samples were collected from different gem deposits including basaltic-related sapphires from Kanchanaburi province, Thailand and metamorphic-related ones from Sri Lanka before and after heating at 1100 °C under oxidizing atmosphere. As a result, the blue color on sapphires before heating can be described as a hole color center assigned to Fe3+-Ti4+ mixed acceptor states inside an energy band gap that could receive an electron from the valence band for charge-balancing after excitation. After heating, the basaltic-related sapphires turned from dark blue to light blue and the metamorphic-related ones turned from light blue to colorless. The Fe3+-Ti4+ mixed acceptor states were decreased because a hole color center was filled by an electron from oxygen during the heating process instead of an electron from the valence band. Therefore, it can be concluded that the blue color mechanism on sapphires before and after heating under an oxidizing atmosphere can be explained by an energy band model involving the presence or absence of Fe3+-Ti4+ mixed acceptor states as well as a hole color center inside an energy band gap.

THERMALLY-OPTICALLY-THERMALLY STIMULATED LUMINESCENCE

A three-stage energy band model was studied. The model consists of electrons thermally stimulated from the ground state to the first excited state, after which they were optically stimulated into the second excited state and they were finally stimulated thermally into the conduction band. A set of simultaneous differential equations was generated from the models and three assumed conditions were applied to this model, which they were solved analytically and analytical expressions were obtained. The same set of simultaneous equations were solved numerically using ode 15s MATLAB solver. When considering first-order peaks, the kinetic parameters obtained were found to be in good agreement with the analytical expressions. But when considering non first-order peaks, the kinetic parameters obtained numerically were not in good agreement with the analytical expressions and explanations had been given. Second-order peaks could not be obtained despite careful selection of the kinetic parameters because the traps were quickly saturated and the quasi- equilibrium conditions assumed could no longer be satisfied. The stability of the excited TA-OSL signals produced by the model was also studied. The real stability of the excited TA-OSL signals produced by this model was found to be about 46 million years.

Exploring the molecular spectroscopic and electronic characterization of nanocrystalline Metal-free phthalocyanine: a DFT investigation

The molecular structure optimization and harmonic vibrational frequencies of nanocrystalline metal-free phthalocyanine were investigated using the density functional theory (DFT) implemented in the Gaussian 09 program through B3LYP/6-311G basis set. Thermodynamic and non-linear optical (NLO) characteristics, total dipole moment, HOMO–LUMO energies, natural population analysis (NPA), and global chemical reactivity descriptors were examined. TD-DFT evaluations were used to interpret the electronic absorption. The experimental results and computed values match up adequately. An essential component of photovoltaic devices and the optimized HOMO–LUMO energy gap (2.16 eV) were estimated and compared with those obtained experimentally. The energy band model was used to check the probability of the transition type and estimate the values of energy gaps, which were found to be indirectly allowed. To provide a comprehensive picture for the characterization and determine the activation energy of the charge carriers, the DC electrical conductivity was also examined. It was revealed to have a value between 0.767 and 1.288 eV depending on the temperature range.

On the temperature and humidity effects of contact electrification in semiconductor-semiconductor case: An energy band model for electron transfer in triboelectrification

Contact electrification (CE) (or triboelectrication) is a well-known phenomenon, but the identity of the charge carriers and their charge transfer mechanism have been under controversy. In recent years, some progress has been made in the study of CE mechanism in metal/dielectric-dielectric cases (in which cases the charge transfer is mainly dominated by the electron transfer mechanism), but the mechanism of CE in semiconductor-semiconductor (S-S) cases is still not well known. In this study, we chose ZnO-Si as friction pairs and conducted a series of studies on the effects of the conductivity type and doping concentration of Si wafer and further investigated the ambient temperature and humidity effects on CE in S-S cases. The results show that the CE in S-S cases is a charge transfer process dominated by the electron transfer mechanism. We describe the electron transfer in the contact-separation (CS) process with an energy band model, and explain the following principle more intuitively: the larger the Fermi energy level difference between the semiconductor friction layers is, the larger the number of transferred electrons are in CS process and the higher the output of CS-TENG is; the main influence mechanism of temperature on CE in S-S cases is thermal electron emission; and the influence of humidity on CE in S-S cases is mainly caused by the change of the surface state density of the friction layers by the adsorption of water molecules. This work not only clarifies that the charge transfer in S-S CE is dominated by the electron transfer mechanism originated from the Fermi energy level difference, but also proposes a more complete energy band model to explain the semiconductor-semiconductor CE mechanism, including the mechanism of the effect of temperature and humidity on the output performance of S-S CS-TENG, which is of great significance to improve the application potential of S-S TENG.

Controlling the Carrier Injection Efficiency in 3D Nanocrystalline Silicon Floating Gate Memory by Novel Design of Control Layer.

Three-dimensional NAND flash memory with high carrier injection efficiency has been of great interest to computing in memory for its stronger capability to deal with big data than that of conventional von Neumann architecture. Here, we first report the carrier injection efficiency of 3D NAND flash memory based on a nanocrystalline silicon floating gate, which can be controlled by a novel design of the control layer. The carrier injection efficiency in nanocrystalline Si can be monitored by the capacitance-voltage (C-V) hysteresis direction of an nc-Si floating-gate MOS structure. When the control layer thickness of the nanocrystalline silicon floating gate is 25 nm, the C-V hysteresis always maintains the counterclockwise direction under different step sizes of scanning bias. In contrast, the direction of the C-V hysteresis can be changed from counterclockwise to clockwise when the thickness of the control barrier is reduced to 22 nm. The clockwise direction of the C-V curve is due to the carrier injection from the top electrode into the defect state of the SiNx control layer. Our discovery illustrates that the thicker SiNx control layer can block the transfer of carriers from the top electrode to the SiNx, thereby improving the carrier injection efficiency from the Si substrate to the nc-Si layer. The relationship between the carrier injection and the C-V hysteresis direction is further revealed by using the energy band model, thus explaining the transition mechanism of the C-V hysteresis direction. Our report is conducive to optimizing the performance of 3D NAND flash memory based on an nc-Si floating gate, which will be better used in the field of in-memory computing.

Analysis of a novel method of alkalis treatment: Effect on energy band optimization and carrier recombination at the grain boundary

Alkali metal post-deposition treatment (PDT) is a key factor to approach high efficiency in Cu(In, Ga)Se 2 (CIGS) solar cells. In addition to single alkali element treatment, co-doping of light and heavy alkali elements is also applied to the preparation of CIGS absorber for higher efficiency. In this work, the details of different methods of alkali-treatments are revealed by combining several microstructural and compositional analysis methods, especially for the NaF-PDT/CsF-PDT process which introduces extra Na before Cs into the absorber. The ion exchange of Na and Cs in the grain boundaries (GB) of the absorber is analyzed in detail, which reveals alkali-treatment restrains the formation of large bandgap compounds at GB. Extensive material analysis shows that NaF/CsF-PDT can not only tailor the surface nanoscale morphology of CIGS films but also optimize the band structure at GB to reduce carrier recombination. Such modification gains the improvement of the highest power conversion efficiency (PCE) from 15.2% to 17.4% and average PCE from 14.3% to 16.5% (without antireflective coating) after NaF/CsF-PDT. According to the analysis of data, we propose a model of energy band at GB, which explains the reason why NaF/CsF-PDT shows a better effect on CIGS devices than CsF-PDT. These results could serve as a basis for the further understanding of the effects of alkali-PDT on CIGS film and help to achieve even higher efficiency. • Effects of alkali elements Na and Cs on CIGS solar cells are investigated by a series of characterization tools. • NaF/CsF-PDT can tailor the surface nanoscale morphology and optimize the band structure at CIGS films. • An energy band model of GBs is proposed to explain the effect of alkali metal co-doping. • The PCE improved from 15.2% to 17.4% after NaF/CsF-PDT.

Fine modulation of the energy band strategy to control the carrier confinement capability of digital alloys

In this paper, a strategy to finely modulate the energy band structure to control the carrier confinement capability of digital alloys (DA) is proposed. Strain analysis shows that As and Sb atoms are exchanged within the AlAsSb DA. The bottom of the corrected potential well is low on the left and high on the right in the growth direction, resulting in a higher band offset of the AlSb potential barrier layer on the left side of the potential well than on the right side. The modulation of the band leads to a higher probability of electron tunneling in DA under the action of an electric field opposite to the growth direction. Conversely, it is difficult for the electrons to tunnel into the lower energy level potential wells. The I–V curve of DA shows that the current value under positive bias is significantly smaller than the value under negative bias when the voltage is higher. The measured results correspond perfectly with the modified energy band model, which verifies the feasibility of energy band modulation. This is important for the structural design of DA and the reduction of dark current in optoelectronic devices.

MAPbI3 perovskite hybrid metasurface for agile, multidimensional THz wave modulation with 3D Dirac semimetal-like Fermi model optically induced by three wavelength bands

Organic–inorganic hybrid three-dimensional (3D) perovskite materials have emerged as potential candidates for terahertz (THz) modulators. In this study, the metasurface of electromagnetically induced transparency resonance was integrated with 3D perovskite to achieve a high-efficiency dynamic THz modulator. The integration was achieved by modulating the intrinsic optoelectronic properties of perovskite with three wavelengths of optical pump and external voltage excitation. The maximum modulation depth was 466.8 %, with a pump flux of 11.5 mW/cm2 and excitation at 532 nm. The Dirac-like semimetal-Fermi energy band model was proposed to explain the dynamic transitions of free carriers in perovskites and reveal the internal mechanism of the dynamic modulation. In addition, we verified our proposed theoretical model by simulation and theoretical fitting, and calculated the conductivity and number of free carriers of perovskite under different excitations. The system thoroughly explains the observed differences under three wavelengths of optical pump excitation. The active control of THz waves based on Dirac semimetal materials with optical and electrical inputs could have many potential applications that have been proposed by few studies.

Charge transport behaviors in a multi-gated WSe2/MoS2 heterojunction

Heterojunctions and multi-gated structures facilitate the fabrication of high-performance and multifunctional transistors. Here, a WSe2/MoS2 heterojunction structure transistor with a back gate and two top gates is proposed. The back gate controls the carrier transport of the entire heterojunction channel, and the top gates independently control the carrier transports of MoS2 or WSe2 channels. The rectification direction of the heterojunction device could be reversed, and the rectification ratio could be modulated from 10−4 to 104 by changing the back-gate voltage. In addition, an evident negative-differential transconductance phenomenon with a current peak and a current valley are observed in the back-gate transfer characteristic curve, which results from the different control ability of the same gate voltage to the Fermi levels in MoS2 and WSe2. The current peak can be obviously modulated and eliminated by the MoS2 top gate, while the WSe2 top gate can control the position of the current valley from −8 to +12 V, which clearly supports the heterostructure energy band model. Moreover, the diversity of output states under multi-gate modulation makes applications in logic circuits possible. These results demonstrate the potential of this approach for the development of next-generation electronic functional devices.

Structural, optical and DFT studies of disodium phthalocyanine thin films for optoelectronic devices applications

Theoretical and experimental investigations have been carried out on the structural and optical properties of disodium phthalocyanine (Na2Pc) thin films prepared using the thermal evaporation method. The optimized geometry structure, vibrational bands and HOMO-LUMO gap were predicted by density functional theory (DFT) computations. FTIR and XRD techniques were utilized to analyze the structural characteristics of Na2Pc thin films. XRD exhibited that the Na2Pc powder has α-polycrystalline nature with a triclinic structure, whereas the Na2Pc thin films have an amorphous structure. The optical properties of different thicknesses films were studied in the wavelength range of 200–2500 nm using the UV–Vis-NIR spectrophotometer. The absorption coefficient of the Na2Pc molecules showed four absorption bands in the UV–Vis region. The types of optical transitions were determined by applying the energy band model as an indirect allowed transition. The onset gap, optical gap and Urbach energy were estimated to be 1.31, 2.25 and 0.41 eV, respectively. The results of absorption coefficient and energy gap showed that the Na2Pc thin films can be utilized in optoelectronic device applications. The absorption index, k, and refractive index, n, were determined and found to be independent of the film thickness. The dispersion parameters have been evaluated according to a single oscillator model in the non-absorbing region of the spectrum.

(Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge2Sb2Te5

We calculate critical electronic conduction parameters of the amorphous phase of Ge2Sb2Te5 (GST), a common material used in phase change memory. We estimate the room temperature bandgap of metastable amorphous GST to be Eg (300K) = 1.84 eV based on a temperature dependent energy band model. We estimate the free carrier concentration at the melting temperature utilizing the latent heat of fusion to be 1.47 x 1022 cm-3. Using the thin film melt resistivity, we calculate the carrier mobility at melting point as 0.187 cm2/V-s. Assuming that metastable amorphous GST is a supercooled liquid with bipolar conduction, we compute the total carrier concentration as a function of temperature and estimate the room temperature free carrier concentration as p(300K) ≈ n(300K) = 1.69×1017 cm-3. Free electrons and holes are expected to recombine over time and the stable (drifted) amorphous GST is estimated to have p-type conduction with p(300K) ≈ 6×1016 cm-3.

Highly Sensitive H2 Sensors Based on Co3O4/PEI-CNTs at Room Temperature

The highly dispersed Co3O4 on the surface of CNTs modified with polyethylenimine (PEI) was synthesized using the hydrothermal method. In the CNT-Co3O4 composite materials, CNTs not only provide the substrate for the Co3O4 nanoparticles but also prevent their aggregation. Furthermore, the interaction between Co3O4 and CNTs modified with polyethylenimine (PEI) helps to improve the gas sensing performance. In particular, the CNT-Co3O4 composite synthesized at 190°C shows the outstanding sensitive characteristics to H2 with a lower detection limit of 30 ppm at room temperature. The obtained CNT-Co3O4 sensor displays excellent selectivity and stability to H2. The energy band model of the conductive mechanism has been built to explain the resistance change when the gas sensor is exposed to the H2. Hence, the CNT-Co3O4 composite material presents highly promising applications in H2 gas sensing.

Influence of sub-bandgap illumination on electric field distribution at grain boundary in CdZnTe crystals

<sec>Grain boundary is one of the main defects, limiting the large-area application of CdZnTe nuclear radiation imaging detectors. In order to explore the ways to improve the electric field distribution properties near grain boundary, the effect of sub-bandgap illumination on the electric field distribution in CdZnTe detector with grain boundary is studied by Silvaco TCAD simulation technique. The grain boundary potential barrier and electric field dead zone are found in simulation results that significantly affect the carrier transport process in CdZnTe detector. The electric field dead zone caused by the grain boundary disappears under the bias of sub-bandgap illumination. Thus the electric field distribution tends to be linear. Meanwhile, the effects of different wavelengths and intensities of sub-bandgap illumination on the electric field distribution at the grain boundary are also investigated. The results show that the electric field of CdZnTe is distorted by sub-bandgap illumination at an intensity lower than 1×10<sup>–9</sup> W/cm<sup>2</sup>. In contrast, a flatter electric field distribution is achieved at a wavelength of 850 nm and an intensity of 1×10<sup>–7</sup> W/cm<sup>2</sup>. The carriers can be transported by drifting, reducing the probability of being captured or recombined by defects during transport, thus improving the charge collection efficiency of the detector.</sec><sec>In addition, the microscopic mechanism of the modulation of the electric field distribution by sub-bandgap illumination and the energy band model of CdZnTe crystal containing grain boundary are proposed. Owing to the existence of the grain boundary, two space charge regions are formed near the grain boundary. The energy band at the grain boundary is bent upward. Meanwhile, the metal-semiconductor contact forms a Schottky barrier, and the energy band near the electrode is bent upward. When the bias voltage is applied, the energy band structure of the CdZnTe tends to tilt from the cathode to the anode. The sub-bandgap illumination can lower the energy band barrier at the grain boundary and regulate the energy band on both sides of the grain boundary. It is believed that this discussion will also make some contributions to understanding of the effects of illumination and grain boundary in other types of optoelectronic devices, especially the applications of thin films in solar cells and detectors.</sec>

Effects of Temperature on the Tribovoltaic Effect at Liquid‐Solid Interfaces

AbstractEnergy harvesters based on tribovoltaic effect are interesting due to their direct current output characteristic that matches most commercial electronic devices. However, their mechanism remains to be further investigated, especially for liquid‐solid cases. Here, the effect of temperature on the tribovoltaic effect at liquid‐solid interfaces is studied. The results show that the increasing of temperature will lead to the increase of the tribo‐voltage and tribo‐current at the water/Si and water/metal interfaces during sliding. Moreover, the coupling effect of the pH value of the liquid and the temperature on the tribovoltaic effect is demonstrated, and an energy band model is proposed for understanding the received data, in which the energy released (“bindington”) by contacting the liquid and the solid with the formation of chemical bonds is considered to be responsible for the generation of tribovoltaic effect.

Enhanced Band Edge Luminescence in GaAs with Sulfur Passivation

In this work, we perform a sulfur chemical passivation on GaAs substrate, which leads to a change in the photoluminescence (PL) owing to a reduction of surface states. By varying the reaction time of passivation, we can modify the chemical bonds of the GaAs surface and reduce the surface depletion region thickness, which strongly affects the emission efficiency of the GaAs band edge. Based on systematic PL measurements and energy band model with surface states, the sulfur passivation also enhances the number of free and bound excitons. Therefore, sulfur passivation is important to improve the performance of GaAs-based optoelectronics devices.

Features in low-temperature electrical resistivity of amorphous Cd3As2 films due to hopping conductivity with changing activation energy

Amorphous Cd3As2 films were deposited on single-crystalline SrTiO3 substrate by high-frequency non-reactive magnetron sputtering. Electrical resistivity of the film, measured within 3-75 K range, increases at cooling. Below ∼70 K, negative transverse magnetoresistance observed. Features in electrical resistivity and magnertoresistance can be attributed to variable-range hopping conductivity of the Mott type with local activation energy, which is temperature-dependent. Two temperature ranges in the hopping conductivity observed. Appearing two ranges is in qualitative accordance with the Mott-Davis energy-band model developed for amorphous semiconductors. High temperature range of the hopping conductivity is attributed to electron hops between localized states, positioned inside band tails, whereas electron hops between localized states, positioned inside Fermi-peak, can be responsible for low-temperature range. At taking into account temperature dependences of local activation energy, universal expression for various mechanisms of the hopping conductivity could be successfully applied to analyze the experimental data.

Optical processes and apparent increase of optical band gap in Y2O3:Eu2+ nanoparticles synthesised by a gradually-heated solution combustion method

We report optical absorption and emission features of Eu doped Y2O3 nanoparticles synthesised by a gradually-heated solution combustion method. An X-ray diffraction analysis reveals that the samples exhibit body centred cubic structure. The crystallite size of an undoped Y2O3 sample was estimated to be ~19.87 nm but it is found to increase up to 46.38 nm in a sample doped with 4% Eu. The samples have spherical structure of diameter ~23–57 nm. Analysis of optical absorption spectra indicates an apparent increase of optical band gap from 5.00 eV to 5.24 eV after the doping. This apparent widening of band gap is explained with reference to the Burstein-Moss effect. The Eu doped samples produce a primary emission band at 447 ± 1 nm and a secondary band between 480 and 630 nm. Analysis reveals that the primary band is composed of two overlapping components with peak maxima at 446 ± 1 nm and 455 ± 3 nm. These emission bands are attributed to the transition from degenerate 5d → 4f level of Eu2+ ion. The absorption and emission features have been discussed with reference to an energy band model.

Mechanism of bifunctional p-amino benzenesulfonic acid modified interface in perovskite solar cells

• ABSA is firstly used to synergistically modify the interface between SnO 2 and perovskite layer. • The acidic sulfonic acid groups in ABSA connect to the under-coordinated Sn ions. • The alkaline amino groups passivate the defects on surface of perovskite. • The energy band model for SnO 2 nanoparticle and interface is established. • The mechanism of significantly improved PCE is analyzed. The n-type metal oxide SnO 2 exists surface energy band bending due to massive vacancy oxygen. The bending will cause unnecessary electron accumulation and recombination at the interface between electron transport layer (ETL) and perovskite layer. Besides, there exist massive deep-level defects (Pb I , Pb i , I Pb ) for solution-processed polycrystalline perovskite films, which will aggravate the energy band uncoordinated and non-radiative recombination, limiting device efficiency and stability. Herein, amphoteric linking molecule p-amino benzenesulfonic acid (ABSA) is firstly used to synergistically modify the interface between SnO 2 ETL and perovskite layer. The acidic sulfonic acid groups in ABSA modify SnO 2 ETL by inactivating the under-coordinated Sn ions, which reduces the energy band barrier on the surface of SnO 2 film, increasing the conduction and reduces charge recombination. Besides, the alkaline amino groups also passivate the deep-level defects on surface of perovskite to lessen intermediate energy level and non-radiative recombination. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) based on MAPbI 3 is obviously increased from 18.02% to 20.32% after 0.10 mg ml −1 ABSA modification. Simultaneously, the modified devices show better stability due to improved perovskite crystallization and the reduced defects. The energy band model for SnO 2 nanoparticle and interface between perovskite layer and SnO 2 ETL is established to explain the interfacial engineering from a new physical perspective.

Compact Modeling of Schottky Gate-all-around Silicon Nanowire Transistors with Halo Doping

A physics-based compact model for halo doped Silicon Nanowire Transistor with Schottky-barrier (SB) contact at the source/drain (S/D) junctions is developedconsidering the quasi-2D surface-potential solution. The halo implant is considered only at the source side to avoid hot electron effects in the high field drain region. The surface potential and electric field expressions are derived using a systematic approach. The impact of a single halo at the source is analyzed using the energy band model. The analysis shows that the halo increases the electron energy at the source side and suppresses the short channel effects. Moreover, high doping of halo decreases the tunneling barrier width, thereby increasing the ON-state current. Physics-based modeling of the surface potential at the halo boundary has ensured that the model preserves accuracy and continuity.