Reduced Carrier Mobility Research Articles

Revisiting Band Tails and Localized States in Amorphous Semiconductors

Amorphous semiconductors, such as hydrogenated amorphous silicon (a-Si:H) and amorphous indium gallium zinc oxide (a-IGZO) have been central to the development of thin film transistors (TFTs) for large-area electronic applications such as displays. This is a result of the exceptional material uniformity that can be achieved when depositing over very large areas (~10 m2). This means that all TFTs are essentially identical over an entire large display panel. However, there is a technological price to pay – the carrier mobility is generally less in amorphous semiconductors compared to their crystalline equivalent.The origin of this reduced carrier mobility is a result of a key difference in the density of states between crystalline semiconductors and their amorphous counterparts. The long-range order of a crystalline lattice means that all carrier states in the conduction and valence bands exist over long distances. These ‘extended states’ are associated with generally high mobility carriers. However, there is no long-range order in amorphous semiconductors, but only short-range order over one to two bond lengths associated with local atomic coordination requirements (and to some degree a medium-range order over a few bond lengths). The result is that the bottom of the conduction band and top of the valence band gain ‘tail’ states which are localized in nature.Not only do these localized states frequently dominate (or at least significantly affect) conduction in amorphous semiconductors and hence the switching characteristics of TFTs, but they also have a role to play in other important processes such as hysteresis and bias stress instability. But what are these localized band tail states, how should we think about them in the context of conduction and how do they relate to extended states?These questions are revisited from the starting point that there is a perturbation in the delocalized charge concentration with position in an amorphous semiconductor compared to its crystalline counterpart, and this can be quantified as a spatially varying excess charge concentration (where a deficit is just a negative excess). The rate of spatial variation is long relative to the rate of change in the electron wavefunction, and reflects the short- and medium-range order distances. Charge conservation means that that this excess charge concentration will have a mean of zero and a Gaussian probability distribution is assumed. A model using this as a starting point has been created which reproduces band tails and which provides new insights into their physics which are discussed.

Improved thermoelectric properties of the β-Cu2+xSe/CuInSe2 multilayer films by layer interface scattering

The β-Cu2+xSe/CuInSe2 multilayer films with different modulation period were prepared and studied. The results showed that the deposited films possessed obvious layered structure. The room temperature carrier concentration, mobility, electrical conductivity and thermal conductivity decreased, but the Seebeck coefficient and power factor and relative thermoelectric figure of merit increased with reducing modulation period of deposited β-Cu2-xSe/CuInSe2 multilayer films. The linear reduction of carrier concentration and mobility and the decrease in thermal conductivity with modulation period was attributed to the scattering of carriers and phonons by layer interface and grain boundary, respectively. The sample with the smallest modulation period (160 nm) possessed the highest power factor of ∼0.74 at room temperature and ∼1.56 mW m−1 K−2 at 405 °C. The insertion of heterogeneous layer into films is an effective method to increase Seebeck coefficient and decrease thermal conductivity, thus increasing thermoelectric figure of merit of films.

Enhanced Activation in Phosphorous-Doped Silicon via Dual-Beam Laser Annealing.

In this study, we conduct a comparative analysis of single-beam laser annealing (SBLA) and dual-beam laser annealing (DBLA) techniques for semiconductor manufacturing. In the DBLA approach, two laser beams were precisely aligned to simultaneously heat a phosphorus-doped silicon (Si) wafer. The main objective was to investigate the impact of the two annealing techniques on the electrical properties, crystalline structure, and diffusion profile of the treated phosphorus-doped Si at equivalent laser powers. Both SBLA and DBLA improved the electrical properties of the phosphorus-doped Si, evidenced by increased carrier concentration and reduced carrier mobility. Additionally, the crystalline structure of the phosphorus-doped Si showed favorable modifications, with no defects and improved crystallinity. While both SBLA and DBLA produced similar phosphorus profiles with no significant redistribution of dopants compared to the as-implanted sample, DBLA achieved a higher activation ratio than SBLA. Although the results suggest improved dopant activation with minimal diffusion, further studies are needed to clearly confirm the effect of DBLA on dopant activation and diffusion.

Highly Stable Flexible Organic Electrochemical Transistors with Natural Rubber Latex Additives.

Organic electrochemical transistors (OECTs) have attracted considerable interest in the context of wearable and implantable biosensors due to their remarkable signal amplification combined with seamless integration into biological systems. These properties underlie OECTs' potential utility across a range of bioelectronic applications. One of the main challenges to their practical applications is the mechanical limitation of PEDOT:PSS, the most typical conductive polymer used as a channel layer, when the OECTs are applied to implantable and stretchable bioelectronics. In this work, we address this critical issue by employing natural rubber latex (NRL) as an additive in PEDOT:PSS to improve flexibility and stretchability of the OECT channels. Although the inclusion of NRL leads to a decrease in transconductance, mainly due to a reduced carrier mobility from 0.3 to 0.1 cm2/V·s, the OECTs maintain satisfactory transconductance, exceeding 5 mS. Furthermore, it is demonstrated that the OECTs exhibit excellent mechanical stability while maintaining their performance even after 100 repetitive bending cycles. This work, therefore, suggests that the NRL/PEDOT:PSS composite film can be deployed for wearable/implantable applications, where high mechanical stability is needed. This finding opens up new avenues for practical use of OECTs in more robust and versatile wearable and implantable biosensors.

Lattice Mismatch at the Heterojunction of Perovskite Solar Cells.

Lattice mismatch significantly influences microscopic transport in semiconducting devices, affecting interfacial charge behavior and device efficacy. This atomic-level disordering, often overlooked in previous research, is crucial for device efficiency and lifetime. Recent studies have highlighted emerging challenges related to lattice mismatch in perovskite solar cells, especially at heterojunctions, revealing issues like severe tensile stress, increased ion migration, and reduced carrier mobility. This review systematically discusses the effects of lattice mismatch on strain, material stability, and carrier dynamics. It also includes detailed characterizations of these phenomena and summarizes current strategies including epitaxial growth and buffer layer, as well as explores future solutions to mitigate mismatch-induced issues. We also provide the challenges and prospects for lattice mismatch, aiming to enhance the efficiency and stability of perovskite solar cells, and contribute to renewable energy technology advancements.

Lattice Mismatch at the Heterojunction of Perovskite Solar Cells

AbstractLattice mismatch significantly influences microscopic transport in semiconducting devices, affecting interfacial charge behavior and device efficacy. This atomic‐level disordering, often overlooked in previous research, is crucial for device efficiency and lifetime. Recent studies have highlighted emerging challenges related to lattice mismatch in perovskite solar cells, especially at heterojunctions, revealing issues like severe tensile stress, increased ion migration, and reduced carrier mobility. This review systematically discusses the effects of lattice mismatch on strain, material stability, and carrier dynamics. It also includes detailed characterizations of these phenomena and summarizes current strategies including epitaxial growth and buffer layer, as well as explores future solutions to mitigate mismatch‐induced issues. We also provide the challenges and prospects for lattice mismatch, aiming to enhance the efficiency and stability of perovskite solar cells, and contribute to renewable energy technology advancements.

Optimizating TiO2 electron transport layer for MAPbBr3 perovskite solar cells by way of Ga doping

The exceptional electron transport performance and high stability of titanium dioxide (TiO2) make it widely used in the electron transport layer (ETL). However, the presence of numerous surface defects in TiO2 layer can lead to unnecessary non-radiative recombination, which reduces carrier mobility and affects the photoelectric conversion efficiency (PCE) of perovskite solar cells (PSCs). Therefore, tremendous researches have been focused on reducing surface defects and improving carrier mobility in TiO2 layer. In this study, we adopted metal gallium (Ga) with high conductivity at room temperature as an optimization strategy to enhance the performance of TiO2 layer. By incorporating metal Ga into the TiO2 precursor solution, we successfully fabricated a TiO2-Ga layer that exhibits reduced surface defects and optimized energy level structure. This provides an efficient pathway for electron transport in perovskite layer. Experimental results demonstrate that (methyl ammonium lead tribromide) MAPbBr3 perovskite solar cell devices based on TiO2-Ga layer achieve nearly 24% higher PCE compared to those based on pure TiO2 layer. This successful optimization strategy utilizing metal Ga offers novel insights and approaches for enhancing the performance of PSCs.

Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.

Tailoring ohmic contact of CdZnTe based device using a ZnTe:Cu film interlayer

ZnTe materials exhibit distinctive characteristics that render them exceptionally suitable for the utilization in circumventing the electrode contact issue for the CdZnTe based devices for photoelectric applications. This work employed the co-sputtering technique to fabricate ZnTe:Cu films, probing the influences of Copper (Cu) sputtering power on the film's structural, morphological, optical, and electrical properties. It is disclosed that the sputtering power of the Cu target leads to a proportional increase in CuxTe compounds within the ZnTe:Cu films. Notably, an increase in the sputtering power of the Cu target is found to enhance the carrier concentration of the film, but not decrease the resistivity. This unnormal arose is due to a substantial reduction in carrier mobility at higher Cu target sputtering powers. The ZnTe:Cu film exhibits relatively lower resistivity when the sputtering power of the Cu target is approximately 3 W. The optimized ZnTe:Cu film as a buffer layer for the electrode is favor of remarkable improving the ohmic contact properties between the electrode and the CdZnTe, exhibiting a low contact resistivity of 0.62 Ω cm2. This work offers a promising approach to ameliorate the contact performance for the CdZnTe devices with high performance and high reliability.

Significant reduction of lattice thermal conductivity observed in CuInTe2-CuAlTe2 solid-solution alloys.

CuInTe2 and CuAlTe2, which are ternary chalcogenide compounds with the same tetragonal structure, are considered as thermoelectric materials owing to high Seebeck coefficients with large bandgaps of ∼1.08 and 1.96 eV, respectively. In this study, the electrical, thermal, and thermoelectric properties of a CuInTe2-CuAlTe2 solid solution alloy system were systematically investigated by synthesizing a series of CuIn1-xAlxTe2 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) compositions. CuInTe2 and CuAlTe2 form the complete solid solutions as reported, and the electrical conductivity and Seebeck coefficient decrease simultaneously to x = 0.8 due to a significant reduction in carrier mobility, thereby reducing the power factor. For CuAlTe2, the power factor suddenly increased owing to its very high electrical conductivity. On the other hand, the total and lattice thermal conductivity is greatly reduced by additional phonon scattering originating from solid-solution alloying. For instance, the largely reduced lattice thermal conductivity was measured to be 1.8 and 1.9 W m-1 K-1 for the sample with x = 0.4 and x = 0.6 at 300 K, whereas those for CuInTe2 and CuAlTe2 were 4.8 and 5.6 W m-1 K-1, respectively. Nevertheless, the thermoelectric figure of merit zT was significantly reduced by the solid solution alloying due to a significant reduction of power factors despite the reduction in thermal conductivity.

Accounting for Carrier Mobility Reduction due to the Normal Field in the Saturation Current Modeling of Extrinsic MOSFETs

Accounting for Carrier Mobility Reduction due to the Normal Field in the Saturation Current Modeling of Extrinsic MOSFETs

Study of High-Energy Proton Irradiation Effects in Top-Gate Graphene Field-Effect Transistors

In this article, the effects of high-energy proton irradiation on top-gate graphene field-effect transistors (GFETs) were investigated by using 20 MeV protons. The basic electrical parameters of the top-gate GFETs were measured before and after proton irradiation with a fluence of 1 × 1011 p/cm2 and 5 × 1011 p/cm2, respectively. Decreased saturation current, increased Dirac sheet resistance, and negative drift in the Dirac voltage in response to proton irradiation were observed. According to the transfer characteristic curves, it was found that the carrier mobility was reduced after proton irradiation. The analysis suggests that proton irradiation generates a large net positive charge in the gate oxide layer, which induces a negative drift in the Dirac voltage. Introducing defects and increased impurities at the gate oxide/graphene interface after proton irradiation resulted in enhanced Coulomb scattering and reduced mobility of the carriers, which in turn affects the Dirac sheet resistance and saturation current. After annealing at room temperature, the electrical characteristics of the devices were partially restored. The results of the technical computer-aided design (TCAD) simulation indicate that the reduction in carrier mobility is the main reason for the degradation of the electrical performance of the device. Monte Carlo simulations were conducted to determine the ionization and nonionization energy losses induced by proton incidence in top-gate GFET devices. The simulation data show that the ionization energy loss is the primary cause of the degradation of the electrical performance.

Constructing cell-membrane-mimic grain boundaries for high-performance n-type Ag2Se using high-dielectric-constant TiO2

The coupling of charge carrier and phonon transport limits the application of Ag2Se as a low-toxic near-room-temperature thermoelectric material. Strategies that reduce the thermal conductivity via enhancing the phonon scattering usually lead to reduced carrier mobility due to high grain boundary potential barrier. In this study, we developed a cell-membrane-mimic grain boundary engineering strategy for decoupling the charge carrier and phonon scattering through decorating high-dielectric-constant rutile TiO2 at Ag2Se grain boundaries to enable the charge carrier/phonon selective permeability. The nano-sized TiO2 with high dielectric permittivity can secure the charge carrier transport by shielding the interfacial Coulomb potential to lower the energy barrier of grain boundaries, rendering an enhanced power factor. Additionally, benefited from the enhanced phonon scattering by TiO2 nanoparticles, a significantly decreased lattice thermal conductivity of ∼0.20 W m−1 K−1 and a high zT of ∼0.97 at 390 K are obtained in the Ag2Se-based nanocomposites. This work demonstrates that such cell-membrane-mimic grain boundary engineering strategy may shed light on developing high-performance thermoelectric materials.

Electrical properties enhancement of natural ester insulating oil by interfacial interaction between KH550-TiO2 and oil molecules

Bio-based sustainable natural ester has a great application prospect in the field of electrical equipment insulation due to its characteristics such as environmental friendliness. In recent years, the use of nanotechnology to improve the electrical and physical and chemical properties of liquid dielectrics has come under the spotlight. Therefore, two bio-based sustainable natural ester insulating oils were prepared in this paper, modified by nano-TiO2 and KH550-TiO2, respectively. The basic physicochemical and electrical properties of two kinds of nano-TiO2 modified insulating oils were tested and compared. And the interfacial interaction between KH550-TiO2 and oil molecules was explored by molecular dynamics. The results showed that the improving effect of KH550 surface modified nano-TiO2 on the electrical properties of insulating oil was superior to that of nano-TiO2, and the optimal concentration was 0.03 g/L. Compared with pure insulating oil, when the concentration of KH550-TiO2 is 0.03 g/L, the breakdown voltage of nano-modified insulating oil was 75.7 kV, increased by 37.9 %, and the dielectric loss factor was 1.03 %, decreased by 59.8 %. DFT calculations show that there are a large number of electron traps at the interface of nano-TiO2. Electron traps can capture free electrons, reduce carrier mobility, and thus improve the breakdown voltage of insulating oil. The band gap width of KH550-TiO2 is 4.15 eV, which is much larger than that of TiO2. The wider the band gap, the harder it is for electrons to transition from the valence band to the conduction band, which explains why the breakdown voltage of KH550-TiO2 modified oil is higher than that of TiO2 modified oil. More importantly, the amino and silicon hydroxyl groups on the surface of KH550-TiO2 carry strong positive electrostatic potential, and can adsorb free electrons and polar molecules in insulating oil. Moreover, the interface adsorption energy between KH550-TiO2 and oil molecular is -806.03 kJ/mol, which greatly improves the dispersion of KH550-TiO2 in natural esters.

15 MeV proton damage in NiO/β-Ga2O3 vertical rectifiers

15 MeV proton irradiation of vertical geometry NiO/β-Ga2O3 heterojunction rectifiers produced reductions in reverse breakdown voltage from 4.3 kV to 3.7 kV for a fluence of 1013ions·cm−2 and 1.93 kV for 1014 ions·cm−2. The forward current density was also decreased by 1–2 orders of magnitude under these conditions, with associated increase in on-state resistance R ON. These changes are due to a reduction in carrier density and mobility in the drift region. The reverse leakage current increased by a factor of ∼2 for the higher fluence. Subsequent annealing up to 400 °C further increased reverse leakage due to deterioration of the contacts, but the initial carrier density of 2.2 × 1016 cm−3 was almost fully restored by this annealing in the lower fluence samples and by more than 50% in the 1014 cm−2 irradiated devices. Carrier removal rates in the Ga2O3 were in the range 190–1200 for the fluence range employed, similar to Schottky rectifiers without the NiO.

Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS2-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS2) is prepared via a sample solution process. Our results show that 4NTP-MoS2 exhibits higher response (increased by 200 %) to ppb-level NO2 with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS2. Notably, the limit of detection (LOD) toward NO2 of 4NTP-MoS2 is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS2 and the corresponding increment of surface absorption energy to NO2. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe2, WS2, and WSe2.

Remarkable modification of transferring characteristics for both positive and negative charges in XLPE-PS composite

Insulating materials with excellent performance have been one of the key factors that keep the safe operation of high-voltage direct-current (HVDC) cable systems, and investigating their charge trap distribution characteristics is of great significance to improve electrical properties, such as reducing conductivity, suppressing space charge accumulation, and elevating breakdown strength. In this paper, surface potential decay processes of low-density polyethylene (LDPE), low-density polyethylene/polystyrene (LDPE/PS), crosslinked polyethylene-polystyrene (XLPE-PS) and crosslinked polyethylene (XLPE) were investigated, and migration properties of positive charges and negative charges as well as related charge trap characteristics were analyzed. The results demonstrate that for positive charges and negative charges, the formation of featured structure in XLPE-PS composites reduces carrier mobility and deepens charge trap depth but does not change the width of charge trap energy distribution, which remains 0.27eV. Compared with XLPE, XLPE-PS shows a more unified trap distribution under positive and negative polarity, which may be attributed to the featured structure. The findings are helpful to clarify the relationship between multi-phase structure and electrical insulation performance of XLPE, and to provide a reference for the design of insulating materials for extruded HVDC cables.

Single-Crystalline BaZr0.2 Ti0.8 O3 Membranes Enabled High Energy Density in PEI-Based Composites for High-Temperature Electrostatic Capacitors.

Dielectric capacitors are promising for high power energy storage, but their breakdown strength (Eb ) and energy density (Ue ) usually degrade rapidly at high temperatures. Adding boron nitride (BN) nanosheets can improve the Eb and high-temperature endurance but with a limited Ue due to its low dielectric constant. Here, freestanding single-crystalline BaZr0.2 Ti0.8 O3 (BZT) membranes with high dielectric constant are fabricated, and introduced into BN doped polyetherimide (PEI) to obtain laminated PEI-BN/BZT/PEI-BN composites. At room temperature, the composite shows a maximum Ue of 17.94Jcm-3 at 730MVm-1 , which is more than two times the pure PEI. Particularly, the composites exhibit excellent dielectric-temperature stability between 25and 150°C. An outstanding Ue =7.90Jcm-3 is obtained at a relatively large electric field of 650MVm-1 under 150°C, which is superior to the most high-temperature dielectric capacitors reported so far. Phase-field simulation reveals that the depolarization electric field generated at the BZT/PEI-BN interfaces can effectively reduce carrier mobility, leading to the remarkable enhancement of the Eb and Ue over a wide temperature range. This work provides a promising and scalable route to develop sandwich-structured composites with prominent energy storage performances for high-temperature capacitive applications.

Observation of photoinduced polarons in semimetal 1T-TiSe2

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron–phonon coupling in 1T-TiSe2. The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron–phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe2, and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.

Realize high thermoelectric properties in n-type Bi2Te2.7Se0.3 materials via ZrO2 ceramic nanoparticles mediated heterogeneous interface

Bismuth telluride (Bi2Te3)-based thermoelectric materials have been commercially utilized for refrigeration and energy conversion at room/near room temperature. However, n-type Bi2Te3 compounds always exhibit inferior performance compared with their p-type counterparts, which is pronouncedly impeding Bi2Te3-based energy converters from further optimizations. Herein, a promising ZT value (ZT = 1.33) was obtained in n-type Bi2Te2.7Se0.3 (BTS) materials by using an interface engineering strategy to synergistically optimize the electron and phonon transport properties. In detail, ZrO2 ceramic nanoparticles (NPs) were entered into BTS matrix to construct high-density heterogeneous interfaces. The heterogeneous interfaces strongly scatter phonons and lower energy carriers, leading to decreased thermal conductivity and increased Seebeck coefficient, while the electrical conductivity is not sacrificed due to the compromise of the slightly reduced carrier mobility by interfacial scattering and the increased carrier concentration by ZrO2 NPs doping. A planar module consisting of 127 pairs of p-n single legs were assembled. At the temperature difference of 200 K, the output power and energy conversion efficiency reached 9.6 W and 6.2%, respectively. This strategy provides a feasible way to rationally design advanced n-type Bi2Te2.7Se0.3 materials with excellent thermoelectric and mechanical properties induced by high-density heterogeneous interfaces.