N-type Conductivity Research Articles

Reduction of carrier density and enhancement of the bulk Rashba spin-orbit coupling strength in Bi2Te3/GeTe superlattices

Featured with the large Rashba constant (αR ∼ 4.2 eVÅ) coupled with the ferroelectricity, GeTe has attracted much interest, proposing novel energy-efficient spintronic devices. However, those approaches are hampered by the high hole density of GeTe around 1020-1021 cm−3, which deteriorates both the ferroelectricity and the bulk Rashba effect of GeTe. To solve this problem, we have investigated the superlattices composed of Bi2Te3 and GeTe ([BT|GT] SL), the former of which is known as a topological insulator (TI) with typically n-type conduction and strong spin-orbit coupling. We have investigated the magnetotransport properties of 25 [BTx|GTy]z SLs with varying thicknesses (x, y: the thickness in the multiples of the unit cell of the respective layer) of each layer and the repetition number (z) systematically. We have observed a minimum carrier density of 5.7 × 1019 cm−3 in [BT6|GeTe6]6 superlattice sample smaller than that (∼4.4 ×1020 cm−3) of GeTe single film by an order. In addition, we have found that some [BT|GT] SLs with reduced carrier density have a larger Rashba constant compared to that of a single GeTe film. This is explained by the change of the spin polarization which can be modulated by the Fermi level in the Rashba band. These results provide a viable way to realize robust ferroelectricity and strong SOC in GeTe-related material systems simultaneously.

Thermodynamics and robust n-type charge carrier density in Co-doped FeTe2: strain strategy

Herein, we investigated the combined effect of Co-doping and strain (biaxial [110] and hydrostatic [111]) on the thermodynamics and electronic structure of the FeTe2 motif using ab-initio calculations by considering the strong correlation effects. The pristine one has a non-magnetic semiconducting nature with an indirect band gap (E g ) of 0.384 eV. Interestingly, our results revealed that the Co-doping at the Fe site induced an n-type conductivity (i.e. few states are crossing the Fermi level from the valence to conduction band) in the system having a substantial charge carrier density magnitude of 0.14 × 1021 cm−3. The metallicity mainly comprises the Co-3d orbitals along with a significant contribution from Fe-3d states. Thermodynamic, mechanical, and dynamical stability of the Co-doped FeTe2 structure is confirmed by computing the formation energetic, elastic constants, and phonon band structure, respectively. Generally, an increasing and decreasing trend in E g value is evident against the applied compressive and tensile strains having ranged from −5% to +5% for the case of the undoped system, respectively. On the other hand, the Co-doped structure maintained its n-type conduction against considered both types of strains. Moreover, it is demonstrated that compressive strains strengthen the charge carrier density amplitude, while tensile strains show a negative impact. Hence, the present work displays that robust n-type conductivity and stable structure of Co-doped FeTe2 system, makes it a desirable candidate for device applications.

Realizing Excellent Structural and Thermoelectric Performance in Mg3Sb2-Based Alloys by Manipulating Mg Intrinsic Migration Kinetics with Interstitial Ni Doping

N-type Mg3Sb2 is attracting increasing focus for its outstanding room-temperature (RT) thermoelectric (TE) performance; however, achieving reliable n-type conduction remains challenging due to negatively charged Mg vacancies. Doping with compensation charges is generally used but does not fundamentally resolve the high intrinsic activity and easy formation of Mg vacancies. Herein, a robust structural and thermoelectric performance is obtained by manipulating Mg intrinsic migration activity by precisely incorporating Ni at the interstitial site. Density functional theory (DFT) indicates that a strong performance originates from a significant thermodynamic preference for Ni occupying the interstitial site across the complete Mg-poor to -rich window, which dramatically promotes the Mg migration barrier and kinetically immobilizes Mg. As a result, the detrimental vacancy-associated ionized scattering is eliminated with a leading room-temperature ZT up to 0.85. This work reveals that interstitial occupation in Mg3Sb2-based materials is a novel approach promoting both structural and thermoelectric performance.

Optical, electrical, and EPR studies of polycrystalline Al:Cr:ZnSe gain elements

Transition metal-doped II-VI (TM:II-VI) chalcogenides are well-known laser materials for optically pumped middle-infrared lasers. Cr:ZnSe is a key representative of this class of transition metal doped II-VI gain media and is arguably considered the material of choice for optically pumped middle-infrared lasers. In addition to effective mid-IR lasing under optical excitation, these crystals, being wide-band semiconductors, hold the potential for direct electrical excitation. One way to form n-type conductivity in ZnSe crystals is by annealing them in a melt of Zn-Al alloy. However, this annealing of Cr:ZnSe crystals results in their purification and transfer of chromium to the melt of Zn-Al alloy. In this article, we report on optimizing the doping technique for providing n-type conductivity in Al:Cr:ZnSe crystals while preserving the chromium concentration. Al:Cr:ZnSe samples with resistivities ranging from 10.8 to 992 Ω-cm were fabricated. While the 2 + valence state of Cr is typically dominant in Cr:ZnSe, both Cr2+ and Cr+ were detected in Al:Cr:ZnSe samples. The maximum level of Cr+ concentration was measured to be 4 × 1018 cm-3.

Ternary B–C–N compounds layered materials with regulated electronic properties and ultrawide bandgaps

The exploration of novel ultrawide bandgap (UWBG) semiconductors is becoming a challenging and compelling research focus on semiconductor physics, materials, and device applications. Ternary B–C–N compounds have attracted much attention because their electronic structure and semiconductor properties are quite different depending on the chemical composition and atomic arrangement of boron, carbon, and nitrogen elements in the lattice. However, the lack of well-controlled high-quality B–C–N crystals has limited their potential as UWBG devices. In this study, B–C–N compounds are synthesized in bulks from graphite and hexagonal boron nitride (h-BN) using ball milling and high-pressure high temperature technique. The synthesized B–C–N compounds produced are highly crystallized layered-materials with intercalated graphene layers in C-doped h-BN layers. The doped carbon atoms occupy boron sites and nitrogen sites of the h-BN layers unbalanced, giving rise to the n-type conductivity of the B-C-N layered compounds. The measured optical bandgaps range from 3.4 to 6.0 eV, which can be regulated by the carbon content. Their electronic properties are also tunable. Our work is expected to initiate potential applications of the B–C–N material as UWBG semiconductors.

Single Crystalline Transparent Conducting F, Al, and Ga Co-Doped ZnO Thin Films with High Photoelectrical Performance.

Transparent conductive film (TCF) is a material that integrates electrical conductivity and optical transparency. It is widely used as an electrode material in thin-film solar cells. However, considerable progress is needed to facilitate its high performance and low-cost preparation. In this study, a preparation scheme for AlF3 and GaF3 co-doped ZnO (FAGZO) thin films was designed and implemented by magnetron sputtering (MS). The mutual restraint between the electrical properties and the wide-spectrum transmission performance of ZnO films was resolved. First-principles calculations showed that the doped ZnO system had n-type conductivity and that the most stable structure was the FO-AlZn-GaZn system. The experimental results verified the theoretical predictions. Single crystalline ZnO transparent conducting films (ZnO-TCFs) of high quality were achieved by MS. After rapid thermal annealing (RTA) treatment, the mobility reached 49.6 cm2/V s, and the resistivity decreased to 3.82 × 10-4 Ω cm. The AT was 90% between 380 and 1200 nm. Furthermore, the application of the prepared FAGZO film in perovskite solar cells (PSCs) has been verified. Compared to the reference indium tin oxide film, the PSCs using the FAGZO film showed higher JSC and power conversion efficiency. These results demonstrate that MS combined with anion and cation co-doping provides an effective means of exploring high-quality and high-performance ZnO-TCFs.

Highly Efficient Mixed Conduction in a Fused Oligomer n-Type Organic Semiconductor Enabled by 3D Transport Pathways.

Tailoring organic semiconductors to facilitate mixed conduction of ionic and electronic charges when interfaced with an aqueous media has spurred many recent advances in organic bioelectronics. The field is still restricted, however, by very few n-type (electron-transporting) organic semiconductors with adequate performance metrics. Here, a new electron-deficient, fused polycyclic aromatic system, TNR, is reported with excellent n-type mixed conduction properties including a µC* figure-of-merit value exceeding 30 F cm-1 V-1 s-1 for the best performing derivative. Comprising three naphthalene bis-isatin moieties, this new molecular design builds on successful small-molecule mixed conductors; by extending the molecular scaffold into the oligomer domain, good film-forming properties, strong π-π interactions, and consequently excellent charge-transport properties are obtained. Through judicious optimization of the side chains, the linear oligoether and branched alkyl chain derivative bgTNR is obtained which shows superior mixed conduction in an organic electrochemical transistor configuration including an electron mobility around 0.3 cm2 V-1 s-1 . By optimizing the side chains, the dominant molecular packing can be changed from a preferential edge-on orientation (with high charge-transport anisotropy) to an oblique orientation that can support 3D transport pathways which in turn ensure highly efficient mixed conduction properties across the bulk semiconductor film.

Excellent Seebeck coefficient observed in exfoliated N-type Tungsten Disulphide (WS2)

Transition Metal Dichalcogenides (TMDCs) are considered necessary two-dimensional materials in the field of thermoelectricity due to tunable band structure, large effective mass, valley degeneracy and so on. So, in this work, among the TMDCs, the thermoelectric response of mechanically exfoliated Tungsten Disulphide (WS2) has been explored to achieve environment-friendly refrigeration and energy harvesting. Herein, we fabricated the WS2 thermoelectric device through the mechanical exfoliation technique. A few layers of exfoliated WS2 flake were characterized with Raman Spectroscopy, and a thickness of 4.2 nm (∼6 layers) was confirmed through Atomic Force Microscopy. Thermoelectric measurements were carried out using the 2ω method, and the Seebeck coefficient of about ∼ -389 μV/K was recorded at room temperature. The electric and thermoelectric characteristics reveal the n-type conduction in the device and show good mobility of 16.5 cm2V-1s-1. This study experimentally validates the potential of mechanically exfoliated n-type WS2 in thermoelectricity generation.

Deep UV transparent conductive Si-doped Ga2O3 thin films grown on Al2O3 substrates

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices.

Conversion of p-type SnO to n-type SnO2 via oxidation and the band offset and rectification of an all-Tin oxide p-n junction structure

p-Type Tin monoxide (SnO) is a metastable phase and can be oxidized into n-type SnO2 in an O2 environment during growth and post-growth annealing. Here we first grow SnO thin films by RF magnetron sputtering with different oxygen partial pressure in the sputtering gas followed by rapid thermal annealing (RTA) in O2 to obtain n-type conducting SnO2 films. We found that while after RTA in N2 stoichiometric SnO film is p-type conducting, films sputtered with excess O and RTA in O2 crystallize in rutile SnO2 structure and exhibit n-type conductivity. Exploiting the transformation of p-SnO to n-SnO2 through oxidation, we fabricate an all-Tin Oxide transparent p-n quasi-homojunction (p-SnO/n-SnO2) and compare it to a p-SnO/n-ZnO heterojunction structure in term of their rectification behavior. XPS measurements reveal that both SnO2 and ZnO at the Γ point exhibit a type II band offset with SnO with their respective valence band (conduction band) offset of 2.8 eV (1.9 eV) and 2.4 eV (1.33 eV). On the other hand, the indirect conduction band minimum of SnO shows a type I band offset in both junctions with a small conduction band offset of ∼0.1–0.17 eV. The SnO/SnO2 p-n structure shows a reasonable rectification with an ideality factor of ∼12.3, which can be potentially useful in many transparent p-n junction based devices.

Structural and photoluminescence properties of Co-Sputtered p-type Zn-doped β-Ga2O3 thin films on sapphire substrates

This work discusses the growth characteristics, composition, and photoluminescence properties of Zn-doped Ga2O3 (ZnGaO) films. The idea of doping of Zn divalent cation in β-Ga2O3 is to modulate the n-type conductivity of β-Ga2O3 to p-type. Therefore, a series of ZnGaO films with varying Zn contents have been deposited on sapphire substrates using co-sputtering of Ga2O3 and Zn targets at the substrate temperature of 400 °C. The X-ray diffraction analysis revealed that divalent Zn dopant is stable up to 8.62% in ZnGaO films. The X-ray photoelectron spectroscopy defined the increasing amount of Zn content in ZnGaO films. The lowest defect formation energy per atom by first-principles calculations indicates that the favourable site of Zn atoms is substitutional Ga tetrahedral site (T-site) in ZnGaO. The photoluminescence (PL) spectra exhibited that the peak emission wavelength of β-Ga2O3 can be shifted with the inclusion of divalent Zn dopant in Ga2O3 films, which is in accordance with the energy diagram and charge density distribution, indicating the Zn substituted T-site Ga are leading to more defect states, and inducing green luminescence in PL spectra. The ZnGaO films exhibited positive Hall coefficient, which verifies the p-type nature of films. ZnGaO films demonstrate a unique ability to realize p-type characteristics among emerging wide bandgap semiconductors, extending its applications at the forefront of contemporary optoelectronics technologies.

High conductivity in Ge-doped AlN achieved by a non-equilibrium process

Highly conductive Ge-doped AlN with conductivity of 0.3 (Ω cm)−1 and electron concentration of 2 × 1018 cm−3 was realized via a non-equilibrium process comprising ion implantation and annealing at a moderate thermal budget. Similar to a previously demonstrated shallow donor state in Si-implanted AlN, Ge implantation also showed a shallow donor behavior in AlN with an ionization energy ∼80 meV. Ge showed a 3× higher conductivity than its Si counterpart for a similar doping level. Photoluminescence spectroscopy indicated that higher conductivity for Ge-doped AlN was achieved primarily due to lower compensation. This is the highest n-type conductivity reported for AlN doped with Ge to date and demonstration of technologically useful conductivity in Ge-doped AlN.

(002) oriented ZnO and ZnO:S thin films by direct ultrasonic spray pyrolysis: A comparative analysis of structure, morphology and physical properties

In this study, we deposit sulfur incorporated ZnO thin films (ZnO:S) by ultrasonic deposition and analyze changes in the structure, morphology, optical and electrical properties of the films in comparison with the pristine ZnO thin films. Further, electronic band structures of ZnO and (ZnO:S) were theoretically calculated using DFT-VASP method. ZnO and (ZnO:S) thin films formed at different conditions showed hexagonal crystal structure with crystallites of 30–40 nm in size fully oriented along [002] direction perpendicular to substrate. The optical band gap of ZnO was 3.2 eV which was reduced to 2.8 eV due to sulfur incorporation. Vacuum annealed ZnO showed a very low sheet resistance of 67 Ω/□ and the value was increased to 470 Ω/□ in ZnO:S films, nearly metallic behavior. Hall effect measurements confirmed the n-type conductivity with a maximum carrier concentration of 5.6 × 1020 cm−3. These films can find potential applications as window layers with improved charge extraction contacts for solar cell applications.

Sources of n-Type Conductivity in Single Crystal β-Ga2O3 and Monoclinic WO3 Nanoparticles

The source of the commonly observed unintentional n-type conductivity in wide-band-gap oxides is controversial. For high-quality β-Ga2O3 single crystals, the physical characteristics of H-containing defects are studied by solid-state nuclear magnetic resonance (ssNMR) and Fourier transform infrared (FT-IR) spectroscopy. NMR observation demonstrated that the hydrogen atoms occur exclusively in the positive charge state, and three trapped protons are housed in two “half vacancies” left by the Ga vacancy to form a VGa–H2+1 complex. Protons trapped in the vacancies are hardly ever able to diffuse out due to the strong hydrogen bonds they establish, even at relatively high temperatures. The hydrogen atoms in one of the two “half vacancies” are the bridged protons in Ga–O(H2)–Ga bonds, the hydrogen atoms can readily donate electrons to the conduction band, and these hydrogen atoms become the shallow donors. The VGa–H2+1 complex provides high-quality β-Ga2O3 single crystals with the extra-stability and the n-type conductivity. For monoclinic WO3 nanoparticles, NMR, FT-IR, and EPR results show that the bridged proton in W–O(H2)–W groups was assumed to be responsible for the so-called “hidden” hydrogen, which can be converted into a shallow donor in the course of sample processing. Our results provide a guide to more refined experimental studies of point defects and impurities in wide-band-gap oxides and their influence on the control of n-type conductivity.

Density of States Engineering of n-Doped Conjugated Polymers for High Charge Transport Performances.

Charge transport of conjugated polymers in functional devices closely relates to their density of states (DOS) distributions. However, systemic DOS engineering for conjugated polymers is challenging due to the lack ofmodulated methods and the unclear relationship between DOS and electrical properties. Here, the DOS distribution of conjugated polymers is engineered to enhance their electrical performances. The DOS distributions of polymer films aretailored using three processing solvents with different Hansen solubility parameters. The highest n-type electrical conductivity (39 ± 3 S cm-1 ), the highest power factor (63 ± 11 µW m-1 K-2 ), and the highest Hall mobility (0.14 ± 0.02 cm2 V-1 s-1 ) of the polymer (FBDPPV-OEG) areobtained in three films with three various DOS distributions, respectively. Through theoretical and experimental exploration, it is revealed that the carrier concentration and transport property of conjugated polymers can be efficiently controlled by DOS engineering, paving the way for rationally fabricating organic semiconductors.

First-principles study of the native defects with charge states in ZrSe[formula omitted

Study of the native defects in transition metal dichalcogenides (TMDCs) is of fundamental interest and potential technological importance. The atomic and electronic structures of native defects with charge states in ZrSe2 are systematically investigated for the first time based on first-principles calculations with the optB86R-vdW and HSE06 functionals. We identify the configurations of relatively stable defects including Zr interstitial (Zrint), Se vacancy (VSe) and Zr antisite (ZrSe). The positive charged Zr interstitial defect (Zrint2+) has the lowest formation energy and thus is most commonly seen in ZrSe2, which is in good agreement with experimental observation. The ZrSe defect is found to form a DX-like configuration. Moreover, the non-interacting Zr Frenkel pairs are the most favorable form of intrinsic defects. The electronic structure analyses of these defects reveal that the extra Zr atoms adjacent to the defects act as donors, which are responsible for the native n-type conductivity of as-grown ZrSe2. Our study can provide an insight into understanding the properties of native defects in ZrSe2.

Insights into adsorption and high photocatalytic oxidation of ciprofloxacin under visible light by intra-molecular Donor-Acceptor like p-n isotype heterojunction: Performance and mechanism

Antibiotics, e.g. ciprofloxacin (CIP) are frequently detected in natural waters and sewage, however, the conventional sewage treatment techniques presents a great challenge for efficient removal them due to the inhibition of bacterial proliferation. Herein, the intra-molecular Donor-Acceptor like p-n isotype heterojunction of phosphorus and boron co-doped hollow tubular g-C3N4 (xB-PCN) was synthesized for high-efficiency visible light photocatalytic degradation of CIP. The Mott-Schottky curves showed that the xB-PCN exhibited both p and n-type conductivities, which probably because the doped boron has electron acceptor effect and the NH/NH2 groups could act as electron donors. This unique property enables xB-PCN to have excellent charge generation, separation and transfer ability for the highly efficient photodegradation of CIP, reaching 87.56% of decomposition efficiency within 1 h with the apparent rate constant (0.01151 min−1) of 6.85 times that of PCN. Furthermore, the 3B-PCN maintained high degradation percentage in tap water (84.63%) and aquaculture wastewater (78.41%). The results of LC-MS and TOC showed that CIP was gradually mineralized and the degradation pathways were proposed. Based on the radical trapping experiments and ESR characterization, the reactive oxygen species involved in the degradation of CIP were checked. This work offers a promising approach for treating antibiotics polluted water based on a metal-free conjugated polymeric g-C3N4.

Acceptor and compensating donor doping of single crystalline SnO (001) films grown by molecular beam epitaxy and its perspectives for optoelectronics and gas-sensing

(La and Ga)-doped tin monoxide [stannous oxide, tin (II) oxide, SnO] thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from ≈ 5 × 1018 to 2 × 1021 cm−3. In this concentration range, the incorporation of Ga into SnO was limited by the formation of secondary phases observed at 1.2 × 1021 cm−3 Ga, while the incorporation of La showed a lower solubility limit. Transport measurements on the doped samples reveal that Ga acts as an acceptor and La as a compensating donor. While Ga doping led to an increase in the hole concentration from 1 × 1018−1 × 1019 cm−3 for unintentionally doped (UID) SnO up to 5 × 1019 cm−3, La-concentrations well in excess of the UID acceptor concentration resulted in semi-insulating films without detectable n-type conductivity. Ab initio calculations qualitatively agree with our dopant assignment of Ga and La and further predict InSn to act as an acceptor as well as AlSn and BSn as donors. These results show the possibilities of controlling the hole concentration in p-type SnO, which can be useful for a range of optoelectronic and gas-sensing applications.

The stability, mechanical, electronic and thermoelectric properties of unexplored monolayer Ca4Sb2O and Ca4Bi2O

Thermoelectric materials can directly convert heat gradient into electricity, thus have attracted much attention in the field of clean energy and ecological protection. In this work, we proposed two novel two−dimensional materials, Ca4Sb2O and Ca4Bi2O, and investigated their stability, mechanical, electronic and thermoelectric properties. We revealed that these two monolayers are direct semiconductors with bandgaps of 1.57 and 1.08 eV, respectively. Due to the lower electron effective mass, the electron mobilities (252.90–799.97 cm2/Vs) are significantly higher than those of hole (58.16–59.21 cm2/Vs), leading to high n-type conductivities (106–107 S/m), as well as high thermoelectric power factors of 28.80 and 10.83 mW K−2 m−1 at 300 K. Also, owing to the low phonon group velocities and high phonon-phonon scattering rates, the monolayers also possess low lattice thermal conductivities of 1.86 and 1.01 W/mK at room temperature, which are dominated by acoustic modes. As a result, Ca4Sb2O and Ca4Bi2O deliver high n-type thermoelectric figure of merit of 0.27 and 0.29 at 300 K, and increase even further to 1.50 and 1.67 at 700 K, suggesting that they are candidates for low - medium temperature thermoelectric appliances.

Low donor ionization energy in Si-implanted heteroepitaxial AlN

We obtained high n-type conductivity and low donor ionization energy for an AlN film grown on SiC by Si ion implantation. The room temperature conductivity reaches 0.26 Ω−1 cm−1, and the donor ionization energy is only 112 meV, which is the best result of Si-implanted heteroepitaxial AlN. The lattice damage caused by ion implantation can be almost completely repaired by annealing at 1330 °C for 2 h. X-ray photoelectron spectroscopy (XPS) results show few O impurity in the sample. The low donor ionization energy is attributed to avoiding the introduction of O impurities and the mitigation of self-compensating effect. These results show the great potential of Si-implanted heteroepitaxial AlN for device applications.