Reduction Of Carrier Concentration Research Articles

Fully Atomic Layer Deposition Induced InAlO Thin Film Transistors

The atomic layer deposition (ALD) technique has been extensively utilized for depositing metal oxide (MO) thin films, particularly as gate insulators in thin film transistors (TFTs). However, research on MO semiconductors as channel layer materials in TFTs using ALD is limited, especially for fully ALD-fabricated MO TFTs. In this study, fully ALD-fabricated InAlO TFTs were developed, with optimized aluminum doping content and annealing temperature to enhance the electrical properties and stability. The InAlO TFTs, fabricated with a cycle ratio of 15:1 and annealed at 350 °C, exhibit superior performance, including a field-effect mobility of 7.2 cm2/V·s, a subthreshold swing of 165 mV/decade, and an on/off current ratio of 2.3×106, with a threshold voltage of only 0.1 V. Additionally, they demonstrated good stability, with a threshold voltage shift of 0.11 V under positive bias stress. The improved electrical properties and stability are attributed to reduced carrier concentration and the inhibition of oxygen defect generation, achieved through appropriate annealing temperatures and aluminum doping. Thus, the development of fully ALD-fabricated high-performance TFTs holds promise for next-generation display technologies as pixel-driving circuits.

Entropy-assisted low-electrical-conductivity pyrochlore for capacitive energy storage

Capacitors with high energy storage performances are highly desired for the miniaturization, lightweight, and integration of high-end pulse systems. However, the trade-off between dielectric constant and breakdown strength restricts further performance optimization. To improve energy storage properties, a new tactic with rising attention, the high-entropy concept, has been demonstrated to suppress leakage current and enlarge the breakdown strength. However, the mechanisms connecting the insulation properties and entropy are still unclear. Herein, we successfully fabricated a series of entropy-modulated Cd2Nb2O7-based pyrochlore ceramics via the doping of multiple elements, and high-entropy pure pyrochlore was obtained due to the entropy-stabilization effect. Our results revealed that the insulation properties are distinctly enhanced via the enlarged carrier transport barriers and reduced carrier concentration, which should be attributed to entropy-induced large lattice distortion, grain-refining, and fewer defects. The optimized insulation properties significantly enhance breakdown strength from 148 kV cm−1 to 457 kV cm−1. A high energy density of 2.29 J cm−3 with a high energy efficiency of 88% is thus achieved in the high-entropy ceramic, which is 150% higher than the pristine material. This work indicates the effectiveness of high-entropy design in the improvement of energy storage performance, which could be applied to other insulation-related functionalities.

Boosting the thermoelectric properties of layered SnSb2Te4 compound by microstructure regulation combined with heterovalent halogen substitution

The layered IVV2Te4-based compounds have recently received significant attention due to their intrinsically low lattice thermal conductivities. However, the influence of microstructure regulation and halogen doping on their thermoelectric properties has been scarcely studied. In this work, we systematically investigated the anisotropic thermoelectric transport properties of pristine SnSb2Te4 compounds with varied microstructures manipulated by ball milling. With the help of thermoelectric transport models, the optimal ball milling time is estimated based on the calculation of quality factor. Furthermore, the remarkably improved Seebeck coefficient is achieved by the introduction of halogen elements into Te sites, which is attributed to reduced carrier concentration and converged carrier pockets. As a result, a maximal zT ∼0.4 at 725 K is achieved in SnSb2(Te0·94I0.06)4 sample aligned with the SPS pressure direction with a ball-milling time of 1 h, which demonstrates a 35 % enhancement over the pristine sample. This work provides an insightful guide to elevate the thermoelectric properties of layered IVVI2Te4-based compounds.

High Carrier Mobility and Promising Thermoelectric Module Performance of N-Type PbSe Crystals.

The scarcity of Te hampers the widespread use of Bi2Te3-based thermoelectric modules. Here, the thermoelectric module potential of PbSe is investigated by improving its carrier mobility. Initially, large PbSe crystals are grown with the temperature gradient method to mitigate grain boundary effects on carrier transport. Subsequently, light doping with <1mole‰ halogens (Cl/Br/I) increases room-temperature carrier mobility to ~1600cm2V-1s-1, achieved by reducing carrier concentration compared to traditional heavy doping. Crystal growth design and light doping enhance carrier mobility without affecting effective mass, resulting in a high power factor ~40µWcm-1K-2 in PbSe-Cl/Br/I crystals at 300K. Additionally, Cl/Br/I doping reduces thermal conductivity and bipolar diffusion, leading to significantly lower thermal conductivity at high temperature. Enhanced carrier mobility and suppressed bipolar effect boost ZT values across the entire temperature range in n-type PbSe-Cl/Br/I crystals. Specifically, ZT values of PbSe-Br crystal reach ~0.6 at 300K, ~1.2 at 773K, and the average ZT (ZTave) reaches ~1.0 at 300-773K. Ultimately, ~5.8% power generation efficiency in a PbSe single leg with a maximum temperature cooling difference of 40K with 7-pair modules is achieved. These results indicate the potential for cost-effective and high-performance thermoelectric cooling modules based on PbSe.

Mechanism of electrical performance deterioration in a-Si:H TFTs caused by source/drain Decap treatment

During the fabrication process of the display industry, the Decap treatment is usually employed as a means to reuse semiconductor materials of thin film transistors (TFTs), but the influence and mechanism of Decap process on the properties of the decapped samples is unknown. To elucidate this occurrence, the electrical characteristics of the channel layer, as well as the microstructure and components at the interface between the Source/Drain (S/D) and the channel layer were examined. It reveals that the infiltration of impurity ion (O and Mo) has significant impact on the deterioration of device performance for a-Si:H TFTs with a Mo/Al/Mo stacked S/D electrode structure. More specifically, oxygen's intrusion leads to the formation of SiOX and the alterations of the surface energy levels and charge distribution of Si, thus introduces an interface trap states between channel layer and S/D electrodes to scatter and capture of interface carriers. And Mo's intrusion may induce the come into being of MoSi or MoP chemical states, causing the decrease of the doping efficiency of phosphorus and the reduction of carrier concentration (from 5.52 × 1016 cm−3 to 4.44 × 1016 cm−3) of a-Si:H channel layer. The combined effect of these two factors results in the decline of electrical performance of S/D decapped a-Si:H TFTs.

Fabrication of thermoelectric Bi2Te2·5Se0.5 with adjustable porosity

Porous structures have attracted considerable interest for their impact on the transport and mechanical characteristics of materials. The fabrication of porous materials, however, often involves intricate preprocessing steps and typically lacks the ability to tailor porosity with ease. In the present study, we present a straightforward method to prepare porous Bi2Te2·5Se0.5 samples by employing low-pressure spark plasma sintering, with the porosity regulated by preset mass density using a modified graphite mold. This approach resulted in a substantial decrease in thermal conductivity, attributed to the introduction of pores that reduce mass density and disrupt phonon transmission. An accompanying decrease in electrical conductivity was also noted, arising from reduced carrier concentration and mobility. Despite these variations, all porous samples maintained similar levels of thermoelectric performance, with a peak zT of ca. 0.9 at 373 K. The average zT within the temperature range of 298–500 K remained slightly above 0.8 for all samples. Furthermore, the porous samples exhibited greatly enhanced mechanical properties. This work demonstrates a versatile and adaptable method to produce porous materials with controllable porosity, potentially applicable to other porous thermoelectric systems.

Synergistic effect of electrical bias and proton irradiation on the electrical performance of β-Ga2O3 p–n diode

The synergistic impact of reverse bias stress and 3 MeV proton irradiation on the β-Ga2O3 p–n diode has been studied from the perspective of the defect in this work. The forward current density (JF) is significantly decreased with the increase in proton irradiation fluence. According to the deep-level transient spectroscopy results, the increase in the acceptor-like trap with an energy level of EC-0.75 eV within the β-Ga2O3 drift layer, which is most likely to be Ga vacancy-related defects, can be the key origin of the device degradation. The increase in these acceptor-like traps results in the carrier concentration reduction, which in turn leads to a decrease in JF. Furthermore, compared with the case of proton irradiation with no bias, the introduction of −100 V electrical stress induced a nearly double decrease in JF. Based on the capacitance–voltage (C–V) measurement, with the support of the electric field, the carrier removal rate increased from 335 to 600 cm−1. Similar to the above-mentioned phenomenon, the trap concentration also increased significantly. We propose a hypothesis elucidating the synergistic effect of electrical stress and proton irradiation through the behavior of recoil nuclei under the electric field.

Interface‐Enhanced High‐Temperature Thermoelectricity in Cu1.99Se/B4C Composites with Synergistically Improved Mechanical Strength

AbstractDegenerate semiconductors usually demonstrate a metallic‐like conduction behavior, showing limited carrier mobility at high temperatures. Herein, by incorporating B4C nanoparticles into the Cu1.99Se system, an unusual switch from the “degenerate” to “non‐degenerate” semiconducting behavior is revealed as a result of the engineered interfaces. Heterogeneous interfaces and disordered Cu2Se amorphous phases are introduced in the Cu1.99Se/B4C composites, which generate a trapping effect against the mobile Cu ions, leading to a distinctive “hump” signature for electrical conductivity. Benefiting from this rare high‐temperature thermoelectric response, a high power factor is obtained due to reduced carrier concentration and enhanced mobility, and low lattice thermal conductivity is retained because of relatively stronger anharmonic lattice vibrations. Consequently, the Cu1.99Se + 0.9 vol.% B4C sample at least achieves a maximum thermoelectric figure of merit (ZT) of 2.6 at 1025 K with synergistically enhanced mechanical robustness. The present interface engineering strategy may be applicable to other thermoelectric materials with ionic migration characteristics.

Reliability of transparent conductive oxide in ambient acid and implications for silicon solar cells

Transparent conductive oxide (TCO) films, known for their role as carrier transport layers in solar cells, can be adversely affected by hydrolysis products from encapsulants. In this study, we explored the morphology, optical-electrical properties, and deterioration mechanisms of In2O3-based TCO films under acetic acid stress. A reduction in film thickness and carrier concentration due to acid-induced corrosion was observed. X-ray photoelectron spectroscopy and inductively coupled plasma emission spectrometry analyses revealed that TCOs doped with less-reactive metals exhibited enhanced corrosion resistance. The efficiency of silicon heterojunction (SHJ) solar cells with tin-doped indium oxide, titanium-doped indium oxide, and zinc-doped indium oxide films decreased by 10%, 26%, and 100%, respectively, after 200 ​h of corrosion. We also found that tungsten-doped indium oxide could effectively safeguard SHJ solar cells against acetic acid corrosion, which offers a potential option for achieving long-term stability and lower levelized cost of solar cell systems. This research provides essential insights into selecting TCO films for solar cells and highlights the implications of ethylene-vinyl acetate hydrolysis for photovoltaic modules.

High mobility and productivity of flexible In2O3 thin-film transistors on polyimide substrates via atmospheric pressure spatial atomic layer deposition

We fabricate In2O3 films using atmospheric pressure spatial atomic layer deposition (AP S-ALD) and investigate their properties at various temperatures (150 °C–225 °C). As the temperature increased, the growth per cycle (GPC) increases, with the GPC and refractive index reaching values of 1.33 and 2.02, respectively, at 225 °C. The In2O3 thin film indicates a reduction in carrier concentration from 1.53 ± 1.37 × 1021 to 3.09 ± 0.53 × 1020 cm−3 as the temperature increased, and a decrease in the resistance from 3.55 ± 0.07 × 10−3 to 3.70 ± 0.01 × 10−4 Ω∙cm, which is attributed to a decrease in the impurity concentration (150 °C: 1.5 at.%; 175 °C: 1.0 at.%; 200 °C: N/A; 225 °C: N/A) and an increase in crystallinity. We use the In2O3 film as a channel layer in a top-gate bottom-contact thin-film transistor (TG-BC TFT). The device shows excellent performance characteristics, including a field-effect mobility of 69.8 cm2/V·s, a threshold voltage of −0.06 ± 0.22 V, and a subthreshold swing of 0.16 ± 0.01 V/decade. We successfully fabricated high-mobility TFTs and demonstrated their reliability via bias and 50,000 bending tests. The high-performance channel layer of the TFTs fabricated using AP S-ALD are promising for flexible applications.

Elucidating the role of oxygen vacancies on the electrical conductivity of β-Ga2O3 single-crystals

The contribution of oxygen vacancies (VO) to the electrical conductivity of unintentionally doped β-Ga2O3 has been a topic of recent debate. Here, we use a combination of Hall measurements and Raman spectroscopy on as-grown and O2-annealed β-Ga2O3 crystals to investigate the role of VO on electrical conductivity. The annealed samples show a significant decrease in carrier concentration. By comparing the relative Raman shift of individual modes with theoretically calculated contributions of oxygen sites to these modes, we verify the marked reduction of VO in annealed β-Ga2O3 crystals. Furthermore, the IR modes in β-Ga2O3, usually hidden by free carrier absorption, are clearly seen in the annealed sample. The reduction of band tail states as well as free carrier absorption in the annealed samples provides additional evidence for reduced carrier concentration related to VO, making them a key determinant of electrical conductivity in β-Ga2O3.

Enhancing Thermoelectric Performance of CuInTe2 via Trace Ag Doping at Indium Sites.

Thermoelectric technology can be utilized to directly convert waste heat into electricity, aiming at energy harvesting in an environmentally friendly manner. As a promising p-type thermoelectric material, CuInTe2 possesses a high inherent lattice thermal conductivity, which limits the practical implementation in the field of thermoelectricity. Herein, through the combination of vacuum melting and annealing along with hot-pressure sintering techniques, we demonstrated that CuIn0.95Ag0.05Te2 thermoelectric materials with trace Ag doping can exhibit a notably high Seebeck coefficient of 614 μV/K, arising from the high density-of-states effective mass and reduced carrier concentration. Owing to the diminished lattice thermal conductivity derived from Umklapp scattering induced by point defects and dislocation, stemming from the trace Ag doping at In sites rather than Cu sites, CuIn0.95Ag0.05Te2 exhibited a maximum figure of merit (ZT) of 1.38 at 823 K, an 18% enhancement over pristine CuInTe2, leading to a maximum average ZT of 0.67 across temperatures ranging from 303 to 823 K. In essence, our work underscores the efficacy of doping engineering and point defects in tailoring the thermoelectric performance of CuInTe2-based materials. This study not only contributes to advancing the fundamental understanding of thermoelectric enhancement but also lays out a practical pathway toward the realization of high-performance CuInTe2-based thermoelectric materials.

Decreasing the Carrier Concentration of ZrNiSn: An Opposite Way to the Best N-Type Half-Heusler Thermoelectrics.

N-type ZrNiSn-based alloys reach a record thermoelectric figure of merit zT ≈1.2 by increasing the carrier concentration to 4-5 × 1020 cm-3 . In this work, It is reported that a comparable zT can also be realized in trace Ru-doped ZrNiSn-based alloy at even lower temperature by decreasing the carrier concentration. Compared to the previously reported Co doping, the doping of Ru results in a more effective reduction in carrier concentration, and thus higher Seebeck coefficient, lower electronic thermal conductivity, and enhanced thermoelectric performance. The electronic specific heat coefficient of the ZrNi1- x Rux Sn sample remains constant with increasing Ru content, indicating no obvious change in the density of states effective mass. Theoretical calculations show that the doping of Ru has negligible effect on the bottom of conduction band. The lattice thermal conductivity is further reduced by alloying Ti and Hf at the Zr site, and the bipolar diffusion is suppressed by doping of 0.5 at.% Sb. As a result, Ti0.25 Zr0.5 Hf0.25 Ni0.99 Ru0.01 Sn0.995 Sb0.005 reaches not only a zT value of 1.1 at 773 K but also a record average zT value of 0.8 in 300 to 873 K, demonstrating the effectiveness of trace Ru doping on boosting the thermoelectric performance of ZrNiSn-based alloys.

Reversible Total Ionizing Dose Effects in NiO/Ga2O3 Heterojunction Rectifiers

NiO/Ga2O3 heterojunction rectifiers were exposed to 1Mrad fluences of Co-60 g-rays either with or without reverse biases. While there is small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma-photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000x reduction in forward current and 100x increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on-off ratio is also reduced by almost 5 orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of < 4 cm-1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current-voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes. Figure 1

Reversible Total Ionizing Dose Effects in NiO/Ga2O3 Heterojunction Rectifiers

NiO/Ga2O3 heterojunction rectifiers were exposed to 1Mrad fluences of Co-60 g-rays either with or without reverse biases. While there is small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma-photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000x reduction in forward current and 100x increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on-off ratio is also reduced by almost 5 orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of < 4 cm-1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current-voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes.

Ion energy dependence of dry etch damage depth in Ga2O3 Schottky rectifiers

The β-polytype of Ga2O3 has a high critical electric field strength (∼8 MV.cm−1) and can be grown as large diameter (6 in.) single crystal boules by inexpensive melt-growth techniques. This makes it promising for next generation power electronics and solar-blind UV photodetectors. Since wet etching is not feasible for patterning such devices due to the chemical inertness of Ga2O3, attention is focused on dry etching, with the attendant issue of surface modification due to chemical or ion-induced changes. In this work, we demonstrate that dry etch damage in β-Ga2O3 is manifested by a reduction in near-surface carrier concentration, possibly due to the introduction of Gav acceptor states. The depth to which the carrier density is affected depends on the interplay between ion energy and etch rate, with faster etch rates reducing the depth of the remaining damaged region. The maximum damage depth observed experimentally from capacitance-voltage profiling on Schottky rectifiers was ∼ 110 nm, well beyond the range of the ions in the Inductively Coupled Plasmas and emphasizing that rapid diffusion of point defects occurs into the sample during the etch step. Using Schottky rectifier structures as a platform to understand the effect of dry etch damage on device parameters, we found that on-state resistance was more affected than ideality factor or breakdown voltage, with a large increase (5x) for ion energies ≥ 325 eV. Reverse current density characteristics showed the presence of trap-assisted space-charge-limited conduction in the damaged layers. The implications for high power rectifier fabrication are discussed.

Anionic regulation and valence band convergence boosting the thermoelectric performance of Se-alloyed GeSb2Te4 single crystal

GeSb2Te4, a layer-structured pseudo-binary chalcogenide, is a promising thermoelectric material with low lattice thermal conductivity ensured by the van de Waals gap. However, its excessive hole concentration leads to low Seebeck coefficient and limited thermoelectric performance. Herein, GeSb2(Te1-xSex)4 (x = 0, 0.05, 0.07, 0.1, and 0.15) single crystals were grown via Bridgman method for thermoelectric performance investigation, with single crystals to utilize the anisotropic low lattice thermal conductivity, and Se-alloying as the strategy to optimize the thermoelectric performance. It was found that Se alloying increases the Seebeck coefficients, deriving from the reduced carrier concentrations due to the enlarged formation energy of intrinsic GeSb(A1), GeSb(A2), and VGe(A2) defects, as well as the simultaneous enhancement of density-of-states effective mass from a facilitated valence band convergence. Besides, Se-alloying also contributes to the reduction of the lattice thermal conductivity. With the forementioned benefits, a high figure of merit of 1.02 is obtained at 723 K in the out-of-plane direction of the crystal sample GeSb2(Te0.9Se0.1)4, and a 74% improvement on average zT is gained. The study of anionic regulation and valence band convergence in GeSb2Te4-based single crystal provides an effective pathway for performance optimization in related layer-structured thermoelectric materials.

Enhancing the Thermoelectric Performance of GeSb4Te7 Compounds via Alloying Se.

Ge-Sb-Te compounds (GST), the well-known phase-change materials, are considered to be promising thermoelectric (TE) materials due to their decent thermoelectric performance. While Ge2Sb2Te5 and GeSb2Te4 have been extensively studied, the TE performance of GeSb4Te7 has not been well explored. Reducing the excessive carrier concentration is crucial to improving TE performance for GeSb4Te7. In this work, we synthesize a series of Se-alloyed GeSb4Te7 compounds and systematically investigate their structures and transport properties. Raman analysis reveals that Se alloying introduces a new vibrational mode of GeSe2, enhancing the interatomic interaction forces within the layers and leading to the reduction of carrier concentration. Additionally, Se alloying also increases the effective mass and thus improves the Seebeck coefficient of GeSb4Te7. The decrease in carrier concentration reduces the carrier thermal conductivity, depressing the total thermal conductivity. Finally, a maximum zT value of 0.77 and an average zT value of 0.48 (300-750 K) have been obtained in GeSb4Te5.5Se1.5. This work investigates the Raman vibration modes and the TE performance in Se-alloyed GeSb4Te7 sheddinglight on the performance optimization of other GST materials.

Enhanced thermoelectric properties for p-type BiSbTe by incorporating multiferroic BiFeO3 nanoparticles

Thermoelectric materials can directly convert electrical energy and thermal energy by regulating carrier and phonon transport under electrical and thermal fields. However, further enhancement of thermoelectric properties is hindered by limited available methods that can synergistically tune transport properties. Here, multiferroic BiFeO3 nanoparticles with spontaneous electrical and magnetic polarization are synthesized and incorporated into p-type Bi0.5Sb1.5Te3, to probe the impact of simultaneous local electrical field and magnetic field on the transport properties of the BiFeO3/Bi0.5Sb1.5Te3 nanocomposites. The BiFeO3/Bi0.5Sb1.5Te3 atomic-resolution interfacial structure is revealed using aberration-corrected scanning transmission electron microscopy. The selective reaction of Sb cations leads to reduced carrier concentration and decreased electrical conductivity. The Seebeck coefficient is enhanced by additional scattering from local electrical and magnetic field. Phonon scattering is boosted by the nanostructures from interfacial reaction and local lattice distortion caused by the polarization. As a result, the ZT values and the cooling performance are improved by 9% and 31% for the nanocomposites, respectively.

Enhanced thermoelectric performance of MXene/GeTe through a facile freeze-drying method

As a great promising lead-free mid-temperature thermoelectric material, Germanium telluride (GeTe) has attracted recently a great deal of attention. However, the high carrier concentration caused by Ge vacancies and thermal conductivity have been a bottleneck of its widespread use in thermoelectrics. The incorporation of nano-second phase has been considered a promising method to address the above issues. However, how to choose a suitable nano-second phase and achieve uniform dispersion in the matrix is still difficult. In this work, uniform dispersion of 2D material MXene in the GeTe was realized by a simple freeze-drying method, successfully tuning the electron and thermal transport properties of GeTe-based alloys synchronously. The results revealed that the carrier concentration of GeTe-based composite gradually decreased from ∼1021 to ∼1020 cm−3 with the participation of MXene. The lowest thermal conductivity can reach 2.19 W m−1 K−1 for GeTe composites with 1.5 vol% MXene at 700 K, which is ∼25.8% lower than that of pristine GeTe. Benefiting from the reduced carrier concentration and thermal conductivity, a maximum ZT value of 1.13 at 700 K is achieved for the composites with 1.5 vol% MXene, showing a 20.2% improvement compared with that of pristine GeTe (∼0.94). These results point out the potential value of MXene in enhancing the performance of GeTe-based thermoelectric materials.