Majority Carrier Type Research Articles

Controlling Film Formation and Host-Guest Interactions to Enhance the Thermoelectric Properties of Nickel-Nitrogen-Based 2D Conjugated Coordination Polymers.

2D conjugated coordination polymers (cCPs) based on square-planar transition metal-complexes (such as MO4, M(NH)4, and MS4, M = metal) are an emerging class of (semi)conducting materials that are of great interest for applications in supercapacitors, catalysis, and thermoelectrics. Finding synthetic approaches to high-performance nickel-nitrogen (Ni-N) based cCP films is a long-standing challenge. Here, a general, dynamically controlled on-surface synthesis that produces highly conductive Ni-N-based cCP films is developed and the thermoelectric properties as a function of the molecular structure and their dependence on interactions with ambient atmosphere are studied. Among the four studied cCPs with different ligand sizes hexaminobenzene- and hexaaminotriphenylene-based films exhibit record electrical conductivity (100-200 S cm-1) in this Ni-N based cCP family, which is one order of magnitude higher than previous reports, and the highest thermoelectric power factors up to 10 µW m-1 K-2 among reported 2D cCPs. The transport physics of these films is studied and it is shown that depending on the host-guest interaction with oxygen/water the majority carrier type and the value of the Seebeck coefficient can be largely regulated. The high conductivity is likely reflecting good interconnectivity between (small) ordered domains and grain boundaries supporting disordered metallic transport.

IrGe4: A Predicted Weyl-Metal with a Chiral Crystal Structure.

Polycrystalline IrGe4 was synthesized by annealing elements at 800 °C for 240 h, and the composition was confirmed by energy-dispersive X-ray spectroscopy. IrGe4 adopts a chiral crystal structure (space group P3121) instead of a polar crystal structure (P31), which was corroborated by the convergent-beam electron diffraction and Rietveld refinements using synchrotron powder X-ray diffraction data. The crystal structure features layers of IrGe8 polyhedra along the b axis, and the layers are connected by edge- and corner-sharing. Each layer consists of corner-shared [Ir3Ge20] trimers, which are formed by three IrGe8 polyhedra connected by edge-sharing. Temperature-dependent resistivity indicates metallic behavior. The magnetoresistance increases with increasing applied magnetic field, and the nonsaturating magnetoresistance reaches 11.5% at 9 T and 10 K. The Hall resistivity suggests that holes are the majority carrier type, with a carrier concentration of 4.02 × 1021 cm-3 at 300 K. Electronic band structures calculated by density functional theory reveal a Weyl point with a chiral charge of +3 above the Fermi level.

Controlled p-Type Doping of Pyrite FeS2.

Pyrite FeS2 has extraordinary potential as a low-cost, nontoxic, sustainable photovoltaic but has underperformed dramatically in prior solar cells. The latter devices focus on heterojunction designs, which are now understood to suffer from problems associated with FeS2 surfaces. Simpler homojunction cells thus become appealing but have not been fabricated due to the historical inability to understand and control doping in pyrite. While recent advances have put S-vacancy and Co-based n-doping of FeS2 on a firm footing, unequivocal evidence for bulk p-doping remains elusive. Here, we demonstrate the first unambiguous and controlled p-type transport in FeS2 single crystals doped with phosphorus (P) during chemical vapor transport growth. P doping is found to be possible up to at least ∼100 ppm, inducing ∼1018 holes/cm3 at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is increased in crystals natively n-doped with S vacancies, the majority carrier type inverts from n to p near ∼25 and ∼55 ppm P, as detected by Seebeck and Hall effects, respectively. Detailed temperature- and P-doping-dependent transport measurements establish that the P acceptor level is 175 ± 10 meV above the valence band maximum, explain details of the carrier inversion, elucidate the relative mobility of electrons and holes, reveal mid-gap defect levels, and unambiguously establish that the inversion to p-type occurs in the bulk and is not an artifact of hopping conduction. Such controlled bulk p-doping opens the door to pyrite p-n homojunctions, unveiling new opportunities for solar cells based on this extraordinary semiconductor.

Photo Seebeck Effect of Air Stable MAPbI3

With increasing interest in energy harvesting using heat as a next-generation eco-friendly energy source, organic-inorganic perovskite materials have emerged as promising materials for thermoelectric devices. In particular, the photo-Seebeck effect of halide perovskite materials has attracted attention due to their wide optical absorption spectrum and large diffusion length, depending on their composition. MAPbI3, a representative organic perovskite component, has been reported to have a Seebeck coefficient of only hundreds of µV/K. In this manuscript, we report a photo-Seebeck effect for bulk MAPbI 3 perovskite in which the magnitude of the Seebeck coefficient significantly increased by 700 µV/K under illumination with a green laser diode. An air-stable perovskite pellet was synthesized using the alcohol substitution synthesis method, and both the Seebeck coefficient and the photocurrent increased in air, proving that enhanced Seebeck coefficient is associated with the formation of excitons in MAPbI3. X-ray diffraction analysis found that the remnant PbI2 led to n/i>-type electronic transport characterized by a negative Seebeck coefficient. Photo-induced electron transfer from MAPbI3 to the PbI2-rich phase under illumination led to dedoping of electrons, to form an MAPbI3 pellet. The significant enhancement in the Seebeck coefficient was found to depend on the composition of the remnant PbI2, which alters the majority carrier type in the bulk MAPbI3.

The Effect of doping by Fe and Sn on the physical properties of Nickel. oxide thin films prepared by Spin coating

We have prepared pure nickel oxide films and those codoped Which are With iron and tin on glass substrate using spin coating . (XRD) measurement shows that the films prepared with Molars concentration (0.3M) and annealing temperature (573 k) were found to be Amorphous structure and annealing at (723k) temperature, change into polycrystalline. the optical properties show ,they transport records value as between (72-85)% for all films. The energy gap decreases with the increase of doping while Urbach energy increases with doping . The electrical properties were studied from variation of conductivity and increase with temperature range (308-373) 0k, while the activation energy decrease with temperature. The type of majority carriers of films were found using Hall effect measurements. It has also been found that the Nickel oxide is P-type while the Tin oxide is N-type. The carries concentration decreases with the increase of doping while Mobility, coefficient Hall and resistance increases with doping

P-type conductivity in Sn-doped Sb2Se3

Antimony selenide (Sb2Se3) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices (Hobson et al 2020 Chem. Mater. 32 2621–30). Herein, Sn-doped Sb2Se3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 1014 cm−3) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, x-ray and hard x-ray photoemission spectroscopies are employed to obtain a non-destructive depth profile of the valence band maximum, confirming p-type conductivity and indicating a majority carrier type inversion layer at the surface. Finally, these results are supported by density functional theory calculations of the defect formation energies in Sn-doped Sb2Se3, showing a possible limit on the carrier concentration achievable with Sn as a dopant. This study sheds light on the effectiveness of Sn as a p-type dopant in Sb2Se3 and highlights avenues for further optimisation of doped Sb2Se3 for solar energy devices.

Investigation on copper doped CdSe thin films for photodetector applications

Cu doped CdSe thin films of thickness 300 ± 20 nm were grown on glass substrates by thermal co-evaporation technique with high purity CdSe and Cu2Se powders as source materials. Different spectroscopic analysis technique was adopted to study the impact of Cu incorporation on the properties of the CdSe films. As Cu concentrations increased in the system, XRD peak position and optical absorption edge have shifted systematically. The formation of Cu acceptor levels and occurrence of radiative transitions from defect levels to valence band were confirmed by photoluminescence. The chemical state and crystal phase of deposited films were investigated by Raman analysis. XPS spectra showed a minor quantity of oxygen present in the doped films. Hall measurement revealed that Cu dopant altered the majority carrier type from n to p. The prepared films are photosensitive materials, and sample with 15 at% Cu showed good photoresponse and is suitable for photodetector applications.

Structural and electronic transitions in thulium-substituted manganese selenide

The structural, transport, optical, and acoustic properties of a new TmXMn1‒XSe (0 ≤ Х ≤ 0.2) chalcogenide system have been studied in the temperature range of 80–500 K. The morphology and microstructure of the polycrystalline samples have been studied by scanning electron microscopy. Temperatures of maxima of the thermal expansion and sound attenuation coefficients related to the lattice strain and electronic transitions have been determined. The variation in the energy of activation of carriers near the percolation concentration caused by a change in the thulium valence has been revealed. The type of majority carriers has been established from the thermopower data. The temperature range of the anomalous compressibility associated with delocalization of electrons has been found from the temperature dependence of the thermal expansion coefficient. Jahn‒Teller polarons have been found above the percolation concentration using the infrared spectroscopy data.

Complementary Tunneling Behaviors in van der Waals Vertical Heterostructures

Vertical tunneling transistors are of interest in the development of next-generation electronics. However, due to nonideal metal-semiconductor interfaces, the majority tunneling carrier type and device polarity are largely fixed, posing a key limitation for practical application. The authors demonstrate complementary tunneling behaviors in vertical heterostructures using ultrathin BN as a tunneling barrier and damage-free van der Waals metal as the top contact, in which the majority carrier type can be switched from electrons to holes by a bias voltage. This work provides proof of principle and comprehensive understanding of the tunneling mechanism in these high-performance devices.

Spin Hall effect driven by the spin magnetic moment current in Dirac materials

The spin Hall effect of a Dirac Hamiltonian system is studied using semiclassical analyses and the Kubo formula. In this system, the spin Hall conductivity is dependent on the definition of spin current. All components of the spin Hall conductivity vanish when spin current is defined as the flow of spin angular momentum. In contrast, the off-diagonal components of the spin Hall conductivity are non-zero and scale with the carrier velocity (and the effective $g$-factor) when spin current consists of the flow of spin magnetic moment. We derive analytical formula of the conductivity, carrier mobility and the spin Hall conductivity to compare with experiments. In experiments, we use Bi as a model system that can be characterized by the Dirac Hamiltonian. Te and Sn are doped into Bi to vary the electron and hole concentration, respectively. We find the spin Hall conductivity ($\sigma_\mathrm{SH}$) takes a maximum near the Dirac point and decreases with increasing carrier density ($n$). The sign of $\sigma_\mathrm{SH}$ is the same regardless of the majority carrier type. The spin Hall mobility, proportional to $\sigma_\mathrm{SH}/n$, increases with increasing carrier mobility with a scaling coefficient of $\sim$1.4. These features can be accounted for quantitatively using the derived analytical formula. The results demonstrate that the giant spin magnetic moment, with an effective $g$-factor that approaches 100, is responsible for the spin Hall effect in Bi.

Study of charged object sensing properties via an organic nanobelt

A flexible wearable electronic skin (e-skin) that is able to sense non-touch objects is strongly required for the improvement and advancement of next-generation high-performance sensing systems. Here, a facile two-terminal organic device is employed for studying the sensing properties of charged objects including proximity and contact stimuli. By adjusting the amount of charges on the stimuli, the sensing response of the organic device was confirmed to rely on the charge quantity of stimuli rather than the applied pressure. In addition, the carrier concentration in the organic semiconductor is effectively accumulated or depleted, depending on the carrier type in the semiconductor and the polarity of voltage at the atomic force microscope (AFM) tip. These results show a potential application in ultrahigh-precision automatic operation, for example, humanoid robots to sensitively detect and safely grab a charged-object without damage. • The flexible organic sensor applies the two-terminal device configuration. • The sensing characteristics of charged objects are studied by organic nanobelt sensor. • The organic semiconductor sensor can sense both proximity and contact stimuli. • The amount of charges on the stimuli is used to confirm the sensing mechanism. • The flexible sensor can identify the majority carrier type in semiconductor materials.

Elucidating the electronic properties of single-wall carbon nanohorns

Thermopower measurements directly illustrate that as-grown SWCNHs are p-type semiconductors, while chemical doping can also be used to tune the majority carrier density and type.

Asymmetric transport in boron intratube p-i-n junction via gas storage for diode applications

Boron nanotubes (BNTs) possess higher conductivities than carbon nanotubes (CNTs), making them potential candidates for nanoscale p-i-n diode applications. Herein, we designed a novel boron intratube p-i-n junction via gas storage, by using ab initio quantum transport simulations. NO and NH3 are the optimal oxide and hydride to inject holes and electrons into (5,5) buckled triangular BNT (BT-BNT), respectively. The device performance in (5,5) BT-BNT FET indicates that the gas storage can rationally change the majority carrier type and concentration, by which (5,5) BT-BNT-based p-i-n junction with large negative rectification can be constructed. We further investigated the dependence of asymmetric transport on the intrinsic region length in the formed p-i-n junction. It is found that the insertion of intrinsic region can act as a buffer layer to reduce the carrier concentration difference on p-type and n-type regions, thus weakening the transport asymmetry. A short intrinsic region with length of 6.22 Å, i.e., n = 2, contributes to improve the rectifying ratio. Our results demonstrate the significance of gas storage combined with the intrinsic region in the formation of p-i-n junction for diode applications based on one-dimensional nanotubes.

Ultrafast spatiotemporal photocarrier dynamics near GaN surfaces studied by terahertz emission spectroscopy

Gallium nitride (GaN) is a promising wide-bandgap semiconductor, and new characterization tools are needed to study its local crystallinity, carrier dynamics, and doping effects. Terahertz (THz) emission spectroscopy (TES) is an emerging experimental technique that can probe the ultrafast carrier dynamics in optically excited semiconductors. In this work, the carrier dynamics and THz emission mechanisms of GaN were examined in unintentionally doped n-type, Si-doped n-type, and Mg-doped p-type GaN films. The photocarriers excited near the surface travel from the excited-area in an ultrafast manner and generate THz radiation in accordance with the time derivative of the surge drift current. The polarity of the THz amplitude can be used to determine the majority carrier type in GaN films through a non-contact and non-destructive method. Unique THz emission excited by photon energies less than the bandgap was also observed in the p-type GaN film.

Doping-free complementary WSe2 circuit via van der Waals metal integration

Two-dimensional (2D) semiconductors have attracted considerable attention for the development of ultra-thin body transistors. However, the polarity control of 2D transistors and the achievement of complementary logic functions remain critical challenges. Here, we report a doping-free strategy to modulate the polarity of WSe2 transistors using same contact metal but different integration methods. By applying low-energy van der Waals integration of Au electrodes, we observed robust and optimized p-type transistor behavior, which is in great contrast to the transistors fabricated on the same WSe2 flake using conventional deposited Au contacts with pronounced n-type characteristics. With the ability to switch majority carrier type and to achieve optimized contact for both electrons and holes, a doping-free logic inverter is demonstrated with higher voltage gain of 340, at the bias voltage of 5.5 V. Furthermore, the simple polarity control strategy is extended for realizing more complex logic functions such as NAND and NOR.

Doping of MoTe2 via Surface Charge Transfer in Air.

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

Strain-induced majority carrier inversion in ferromagnetic epitaxial LaCoO3−δ thin films

Tensile-strained $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films are ferromagnetic, in sharp contrast to the zero-spin bulk, although no clear consensus has emerged as to the origin of this phenomenon. While magnetism has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, structure, magnetism, and transport are studied in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films (10--22-nm thick) on various substrates (from 1.4% compressive to 2.5% tensile strain), using synchrotron x-ray diffraction, scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect. High quality, smooth films are obtained, exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains. Consistent with prior work, ferromagnetism with an approximately 80--85 K Curie temperature is observed under tension; polarized neutron reflectometry confirms a relatively uniform magnetization depth profile, albeit with interfacial dead layer formation. Electrical transport is found to have similar semiconducting nature to bulk, but with reduced resistivity and activation energy. Hall effect measurements, however, reveal a striking inversion of the majority carrier type, from $p$-type in the bulk and under compression to n-type under tension. While thus far overlooked, ferromagnetism in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films is thus directly correlated with $n$-type behavior, providing important insight into the ferromagnetic state in this system. Aided by density functional theory calculations, these results are interpreted in terms of tensile-strain-induced orbital occupation and band structure changes, including a rapid decrease in effective mass at the ${e}_{g}$-derived conduction band minimum, and corresponding increase at the valence band maximum.

Interfacial Built-In Electric Field-Driven Direct Current Generator Based on Dynamic Silicon Homojunction.

Searching for light and miniaturized functional device structures for sustainable energy gathering from the environment is the focus of energy society with the development of the internet of things. The proposal of a dynamic heterojunction-based direct current generator builds up new platforms for developing in situ energy. However, the requirement of different semiconductors in dynamic heterojunction is too complex to wide applications, generating energy loss for crystal structure mismatch. Herein, dynamic homojunction generators are explored, with the same semiconductor and majority carrier type. Systematic experiments reveal that the majority of carrier directional separation originates from the breaking symmetry between carrier distribution, leading to the rebounding effect of carriers by the interfacial electric field. Strikingly, NN Si homojunction with different Fermi levels can also output the electricity with higher current density than PP/PN homojunction, attributing to higher carrier mobility. The current density is as high as 214.0 A/m2, and internal impedance is as low as 3.6 kΩ, matching well with the impedance of electron components. Furthermore, the N-i-N structure is explored, whose output voltage can be further improved to 1.3 V in the case of the N-Si/Al2O3/N-Si structure, attributing to the enhanced interfacial barrier. This approach provides a simple and feasible way of converting low-frequency disordered mechanical motion into electricity.

Applied Trace Alkali Metal Elements for Semiconductor Property Modulation of Perovskite Thin Films.

With the rapid consumption of energy, clean solar energy has become a key study and development subject, especially the when new renewable energy perovskite solar cells (PSCs) are involved. The doping method is a common means to modulate the properties of perovskite film. The main work of this paper is to incorporate trace amounts of alkali metal elements into the perovskite layer and observe the effects on the properties of the perovskite device and the majority carrier type of the perovskite film. Comparative analysis was performed by doping with Na+, K+, and Rb+ or using undoped devices in the perovskite layer. The results show that the incorporation of alkali metal ions into the perovskite layer has an important effect on the majority carrier type of the perovskite film. The majority carrier type of the undoped perovskite layer is N-type, and the majority carrier type of the perovskite layer doped with the alkali metal element is P-type. The carrier concentration of perovskite films is increased by at least two orders of magnitude after doping. That is to say, we can control the majority of the carrier type of the perovskite layer by controlling the doping subjectively. This will provide strong support for the development of future homojunction perovskite solar cells. This is of great help to improve the performance of PSC devices.

Impact of K+ Doping on Modulating Majority Charge Carrier Type and Quality of Perovskite Thin Films by Two-step Solution Method for Solar Cells

Traditional hetero-junction perovskite solar cells are composed of light-absorbing layers, charge carrier-transporting layers, and electrodes. Recently, a few papers on homo-junction perovskite solar cells have been studied. Here, we studied the effect of K+ doping on TiO2/PbI2 interface quality, perovskite film morphology, photo-physical properties, and majority carrier type. In particular, the K+ extrinsic doping can modulate the majority carrier type of the perovskite thin film. The study indicated that the interface between the perovskite layer and the TiO2 layer deteriorates with the increase of K+ doping concentration, affecting the electron transport ability from the perovskite film to the TiO2 layer and the photo-physical properties of the perovskite layer by K+ doping. In addition, the majority charge carrier type of perovskite thin films can be changed from n-type to p-type after K+ extrinsic doping, and the corresponding hole concentration increased to 1012 cm−3. This approach of modulating the majority charge carrier type of perovskite thin film will pave the way for the investigation of perovskite homo-junction by extrinsic doping for solar cells.