Doped Semiconductors Research Articles

Method for estimating elastic modulus of doped semiconductors by using ab initio calculations—Doping effect on Young’s modulus of silicon crystal

A novel method for estimating the elastic modulus of doped semiconductors using ab initio calculation is demonstrated for a Si crystal. The elastic modulus of various materials basically depends on lattice constants on which dependence is shown by ab initio calculation and actually depends on the temperature, species, and concentration of doped atoms. This paper focuses on the dependences of species and concentrations of doped atoms at room temperature and shows the simplest procedure within Vegard’s law for estimating the elastic modulus of doped semiconductors.

(Sm/Eu/Tm)3+ doped tantalum semiconductor system for photovoltaic and electrochemical functionality amplification

This work reports the synthesis, characterization, and energy focused applications of the novel lanthanides co-doped tantalum pentoxide hetero-system (Sm3+-Eu3+-Tm3+:Ta2O5). Ln3+-doped Ta2O5 express excellent opto-electronic features reflected by the narrow band gap energy of 3.87 eV. Different vibrations confirm the presence of Ta–O–Ta and Ta–O bonds. The synthesized system possesses orthorhombic geometry with 59.46 nm particle size. With the smoother and compact morphology, the synthesized material succeeds in augmenting the performance of different systems aimed at energy applications. Fully ambient perovskite solar cell device fabricated with the Ln3+-doped Ta2O5 as an electron transport layer excels in achieving an efficiency and fill factor of 14.17% and 76% under artificial sun. This device was marked by the negligible hysteresis behavior showing profound photovoltaic performance. The electrochemical activity of the Ln3+-doped Ta2O5 decorated electrode was evaluated for electrical charge storage potential with pseudocapacitive behavior. With the highest specific capacitance of 355.39 F/g and quicker ionic diffusion rate, the designed electrode excels conventionally used materials. Electro-catalysis of water with Ln3+-doped Ta2O5 material indicates its capacity for H2 production with the lowest overpotential and Tafel slope values of 148 and 121.2 mV/dec, while the O2 generation is comparatively lower. With the stable electrochemical output, this rare earth modified material has the potential to replace conventionally used environmentally perilous and costly materials.

Effect of n- and p-Doping on Vacancy Formation in Cationic and Anionic Sublattices of (In,Al)As/AlAs and Al(Sb,As)/AlAs Heterostructures.

The vacancy generation dynamics in doped semiconductor heterostructures with quantum dots (QD) formed in the cationic and anionic sublattices of AlAs is studied. We demonstrate experimentally that the vacancy-mediated high temperature diffusion is enhanced (suppressed) in n- and p-doped heterostructures with QDs formed in the cationic sublattice, while the opposite behavior occurs in the heterostructures with QDs formed in the anionic sublattice. A model describing the doping effect on the vacancy generation dynamics is developed. The effect of nonuniform charge carrier spatial distribution arisen in heterostructures at high temperatures on the vacancy generation and diffusion is revealed.

Dependence relationship between doping modes and surface plasmon resonance properties of doped ZnO nanocrystals

The use of doped semiconductors to achieve plasmonic frequencies in the middle infrared (mid-IR) has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. Plasmonic materials have great static or dynamic tunability. In some cases, choosing a dopant over another is an important trade-off to optimize the plasmonic performance. In this work, trivalent metal ion (M)-doped ZnO nanoparticles (NPs, M=Al, In) were successfully synthesized with highly consistent synthesis conditions. Interestingly, surface plasmon resonance (SPR) can only be observed in Al-doped ZnO (AZO) NPs, but not in In-doped ZnO (IZO) NPs. We used X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and a first principle calculation to clarify the proper dopant sites (replaceable or interstitial doping) of Al3+ and In3+ in the host nanocrystals of ZnO. In addition, these results assisted by density of states (DOS) analysis indicate that different doping modes influenced the electronic structures in different degrees, and may lead to the opposite SPR phenomenon. Our work deepens the understanding of the relationship between doping sites in the host nanocrystals and SPR properties and give important guidance to the develop the doped semiconductors with plasmonic performance.

Chemical Doping of Organic and Coordination Polymers for Thermoelectric and Spintronic Applications: A Theoretical Understanding.

ConspectusThe controlled doping of organic semiconductors (OSCs) is crucial not only for improving the performance of electronic and optoelectronic devices but also for enabling efficient thermoelectric conversion and spintronic applications. The mechanism of doping for OSCs is fundamentally different from that of their inorganic counterparts. In particular, the interplay between dopants and host materials is complicated considering the low dielectric constant, strong lattice-charge interaction, and flexible nature of materials. Recent experimental breakthroughs in the molecular design of dopants and the precise doping with high spatial resolution call for more profound understandings as to how the dopant interacts with the charge introduced to OSCs and how the admixture of dopants alters the electronic properties of host materials before one can exploit controllable doping to realize desired functionalities.By employing state-of-the-art computational tools, we revealed the effects of doping in representative and emerging organic and coordination polymers aiming toward thermoelectric and spintronic applications. We showed that dopants and hosts should be taken as an integrated system, and the type of charge-transfer interaction between them is the key for spin polarization. First, we found doping-induced modifications to the electronic band in a potassium-doped coordination polymer, an n-type thermoelectric material. The charge localization due to the Coulomb interaction between the completely ionized dopant and the injected charge on the polymer backbone and also the polaron band formation at low doping levels are responsible for the nonmonotonic temperature dependence of the conductivity and Seebeck coefficient observed in recent experiments. The mechanistic insights gained from these results have provided important guidelines on how to control the doping level and working temperature to achieve a high thermoelectric conversion efficiency. Next, we demonstrated that the ionized dopants scatter charge carriers via screened Coulomb interactions, and it may become a dominant scattering mechanism in doped polymers. After incorporating the ionized dopant scattering mechanism in PEDOT:Tos, a p-type thermoelectric polymer, we were able to reproduce the measured Seebeck coefficient-electrical conductivity relationship spanning a wide range of doping levels, highlighting the importance of ionized dopant scattering in charge transport.In the two cases described above, charge injection is enabled by integral charge transfer between the dopant and host polymers. In a third example, we showed that a novel type of stacked two-dimensional polymer, conjugated covalent organic frameworks (COFs) with closed-shell electronic structures, can be spin polarized by iodine doping via fractional charge transfer even at high doping levels. We then manifested that magnetization can be attained in nonmagnetic materials lacking metal d electrons and further designed two new COFs with tunable spintronic structure and magnetic interactions after the iodine doping. These findings have suggested a practical route to enable spin polarization in nonradical materials by chemical doping via orbital hybridization, which holds great promise for flexible spintronic applications.

Insights on CaTiS3 films grown by pulsed laser deposition

We attempt to grow perovskite CaTiS3 by pulsed laser deposition (PLD) on various substrates using a CaS:TiS2 target. Using Al2O3 (0001) substrates at 600 °C deposition temperature in vacuum, Ca1.0Ti1.05S2.48Oy stoichiometry is measured by Energy-dispersive X-ray spectroscopy (EDS) and Ca1.0Ti1.0S2.0O0.7 by Rutherford Backscattering Spectrometry (RBS). This indicates that when the films are grown in vacuum at moderate temperature, a high amount of S can be transferred from the target to the film. While no phase segregation of CaS and TiS2 could be observed, no perovskite CaTiS3 phase could be obtained, but rather a phase governed by van der Waals interactions. The films show a highly doped n-type semiconductor behavior with absorption coefficient in the 105 cm−1 range at 350–2500 nm but no surface photovoltage signal. This work should stimulate further experimental efforts in PLD growth of chalcogenides and chalcogenide perovskites.

Investigation of LSPR Coupling Effects toward the Rational Design of CsxWO3–δ Based Solar NIR Filtering Coatings

AbstractThe optical range of localized surface plasmon resonance (LSPR) is extended into the infrared region, thanks to the development of highly doped semiconductor nanocrystals. Particularly, the near‐infrared (NIR) range holds a significant interest in managing solar radiation. However, practical applications necessitate the arrangement of particles, which is known to possibly impact their optical properties through LSPR coupling effects. How such coupling modifies the LSPR response in semiconductor hosts remains largely unexplored. In this study, a protocol for producing composite coatings composed of cesium‐doped tungsten bronze nanocrystals embedded in a silica matrix is presented. Achieving individual dispersion of nanocrystals is made possible through careful selection of a surface polyglycerol ligand exchange. This allows to tune the interparticle distance by adjusting the nanocrystal volume fraction in the composite. The findings demonstrate that LSPR coupling effects significantly influence the LSPR intensity of nanocrystals in the composite when the nanocrystal‐to‐nanocrystal distance matches their size. Beyond elucidating the LSPR coupling effect, this study provides insights into the potential use of Cs‐HTB nanocrystals for solar control applications. Through the optimization of morphology and film structure, remarkable selectivity is obtained in terms of maintaining good transparency in the visible range while achieving high absorption in the NIR.

Construction of photocatalyst by modification of argentum noble metal and hydrogen doping into WO3 nanoparticles: Enhancing the solar energy utilization

Solar energy is considered sustainable green energy to prevent environmental pollution. Tungsten trioxide (WO3) is one of the attractive solar energy-driven photocatalysts and photocatalytic supports. Besides, Hydrogen (H) has a small size and can move into many inorganic compounds, and occupy the sites in microstructure as well, with little space structure expansion. H doping in semiconductors attracts more and more attention due to its effect of providing free carriers, resulting in localized surface plasmon resonances which can improve the utilization efficiency of solar energy. In our research, we developed a new method to prepare the H-doped WO3 whose color turned green, and to prepare Ag/HxWO3, which could extend the solar light response range to near-infrared. Furthermore, the photocatalytic activity to degrade RhB solution under UV light and visible light was both improved. The density of free electrons is based on the Drude model to discuss the mechanism of photocatalysis upon the visible. The contact of noble metal Ag and HxWO3 can play a vital effect in the separation of photogenerated electron-hole pairs. The localized surface plasmon resonances and green coloration bring the tungsten trioxide more remarkable performance together. Results showed that constructed photocatalysis of this work greatly enhanced the utilization rate of solar energy. Photocatalytic for the Degradation of liquid organic pollutants with HxWO3 is believed to have a bright feature.

Direct-current output of silicon–organic monolayer–platinum Schottky TENGs: Elusive friction-output relationship

Triboelectric nanogenerators (TENGs) are an emerging energy harvesting technology able to convert ubiquitous mechanical energy into electricity. Friction, static charging and flexoelectricity are all involved in the mechanism underpinning TENG operation, but their relative contribution has remained elusive. Here we used dynamic and static conductive atomic force microscopy (C-AFM) measurements on monolayer-modified silicon crystals to detect evidence of a relationship between friction and zero-bias current, and between pressure and the direction of the putative flexovoltage. We demonstrate that a static electricity-related tribovoltage is probably responsible for a friction excess, and that surprisingly this friction excess is found to be dependent on the doping level and type of the silicon substrate. Such friction excess is however no longer measurable once current is allowed to flow across the junction. This observation points to an electrostatic origin of friction in silicon-based Schottky TENGs, and suggests that the zero external bias DC current is at least in part an electronic flow to neutralize static charges. Further, the sign of the zero-bias current, but not its magnitude, is independent of the semiconductor doping type, which is again suggestive of surface statics being a main contributor to the zero-bias output rather than exclusively a space-charge effect. We also reveal the presence of a junction flexovoltage under pressures common in AFM experiments (GPa), even for negligible lateral friction. In a static Pt–monolayer–n-type Si junction the flexovoltage carries the same sign as the tribovoltage, and can reach such magnitude to overwrite external voltages as high as 2 V. The immediate implication is that the flexovoltage is likely to have i) a strong contribution to the zero-bias output of a n-Si Schottky TENG, ii) a negative effect on the output of a p-Si TENG, and iii) its detection can be straightforward, as we discovered that flexoelectricity manifests as an “inverted diode”: a n-type Si–platinum diode with negligible current even when the n-type material is negatively biased as long as the “static” diode remains under a large normal pressure.

Ultrathin metal Ni layer on ZnO/TiO2 photoelectrodes with excellent photoeletrochemical performance in multiple electrolyte solutions

It is well known that the oxygen vacancy (Ovac) as the electron-donor dopant in semiconductor can increase the electron-holes separation in photoeletrochemical (PEC) water splitting. Furthermore, the metal Ni can promote the hydrogen evolution reaction (HER) on the surface of semiconductor. In this paper, the ZnO/TiO2 photoelectrodes with rich Ovac was synthesized by electrostatic adsorption through using ZIF-8 as the precursor. Then the ultrathin Ni layer with about 7 nm was deposited on the surface of ZnO/TiO2 (Ni/ZnO/TiO2) by vacuum thermal evaporation method. The Ni/ZnO/TiO2 photoelectrodes showed the highest photocurrent than ZnO/TiO2, Ni/ TiO2 and pure TiO2 photoelectrodes. The durability of Ni/ZnO/TiO2 photoelectrodes was keeping for 10 h in multiple electrolyte solutions under AM 1.5 G illumination and the photocurrent decline can be ignored. The UV–vis absorption spectra demonstrated that the ultrathin Ni layer showed plasma with ZnO/TiO2 for enhancing the water splitting performance. Furthermore, the ultrathin Ni layer enhanced the photogenerated charges transfer for improving the PEC performance. This work provides a new method for ultrathin metal Ni layer with Ovac semiconductor photoelectrode to improve the PEC performance in multiple electrolyte solutions.

Coulomb-induced synchronization of intersubband coherences in highly doped quantum wells and the formation of giant collective resonances

Many-body Coulomb interactions drastically modify the optical response of highly doped semiconductor quantum wells leading to a merger of all intersubband transition resonances into one sharp peak at the frequency substantially higher than all single-particle transition frequencies. Starting from standard density matrix equations for the gas of pairwise interacting fermions within Hartree-Fock approximation, we show that this effect is due to Coulomb-induced synchronization of the oscillations of coherences of all $N$ intersubband transitions and sharp collective increase in their coupling with an external optical field. In the high doping limit, the dynamics of light-matter interaction is described by the analytic theory of $N$ coupled oscillators which determines new collective normal modes of the system and predicts the frequency and strength of the blueshifted collective resonance.

Intrinsic optical absorption in Dirac metals

A Dirac metal is a doped (gated) Dirac material with the Fermi energy (EF) lying either in the conduction or valence bands. In the non-interacting picture, optical absorption in gapless Dirac metals occurs only if the frequency of incident photons (Ω) exceeds the direct (Pauli) frequency threshold, equal to 2EF. In this work, we study, both analytically and numerically, the role of electron–electron (ee) and electron–hole (eh) interactions in optical absorption of two-dimensional (2D) and three-dimensional (3D) Dirac metals in the entire interval of frequencies below 2EF. We show that, for Ω≪EF, the optical conductivity, ℜσ(Ω), arising from the combination of ee and certain eh scattering processes, scales as Ω2lnΩ in 2D and as Ω2 in 3D, respectively, both for short-range (Hubbard) and long-range (screened Coulomb) interactions. Another type of eh processes, similar to Auger–Meitner (AM) processes in atomic physics, starts to contribute for Ω above the direct threshold, equal to EF. Similar to the case of doped semiconductors with parabolic bands studied in prior literature, the AM contribution to ℜσ(Ω) in Dirac metals is manifested by a threshold singularity, ℜσ(Ω)∝(Ω−EF)d+2, where d is the spatial dimensionality and 0<Ω−EF≪EF. In contrast to doped semiconductors, however, the AM contribution in Dirac metals is completely overshadowed by the ee and other eh contributions. Numerically, ℜσ(Ω) happens to be small in almost the entire range of Ω<2EF. This finding may have important consequences for collective modes in Dirac metals lying below 2EF.

Anderson localization in doped semiconductors

We theoretically consider the problem of doping induced insulator to metal transition in bulk semiconductors by obtaining the transition density as a function of compensation, assuming that the transition is an Anderson localization transition controlled by the Ioffe-Regel-Mott (IRM) criterion. We calculate the mean free path, on the highly doped metallic side, arising from carrier scattering by the ionized dopants, which we model as quenched random charged impurities. The Coulomb disorder of the charged dopants is screened by the carriers themselves, leading to an integral equation for localization, defined by the density-dependent mean free path being equal to the inverse of the Fermi wave number, as dictated by the IRM criterion. Solving this integral equation approximately analytically and exactly numerically, we provide detailed results for the localization critical density for the doping induced metal-insulator transition.

Anomalous lattice thermal conductivity driven by all-scale electron-phonon scattering in bulk semiconductors

Electron-phonon coupling (EPC) has been broadly considered to govern the charge transport in solids, whereas its effect on thermal conductivity, especially for semiconductors at ambient temperatures, is a widely debated issue in condensed-matter physics. Employing state-of-the-art first-principles calculations to quantify all the possible scattering factors, we show the dominant role of EPC in phonon transport of $n$-doped GaSb, which yields an unprecedented reduction of lattice thermal conductivity and triggers anomalous temperature-independent behavior. The significant EPC impact hinges on the joint effect of multiple conduction pockets, large EPC strength, and bunched heat-carrying acoustic branches, provoking strong electron-phonon scattering to surpass the intrinsic phonon-phonon scattering for all-scale phonons, in contrast to the case in silicon that only long-wave phonons can be scattered by carriers even at high doping level. This work advances the understating of phonon transport in doped semiconductors and stimulates potential heat management application of anomalous thermal conductivity.

Effect of carrier doping on the electronic states of earth-abundant Fe–Al–Si thermoelectric materials

Fe–Al–Si-based thermoelectric (FAST) materials are non-toxic and low-cost materials that can be used for autonomous power supplies to drive internet-of-things wireless sensor devices. The conduction type can be controlled by changing the Al/Si ratio, which is suitable for fabricating reliable thermoelectric power-generation modules consisting of materials with similar thermal expansion coefficients. In this work, we evaluated the electronic structures of p- and n-type FAST materials with relatively large absolute values of the Seebeck coefficient by photoemission spectroscopy to obtain deeper insight into controlling the p-n characteristics of FAST materials. The core-level spectra suggested that the FAST materials have a covalent bonding nature. The chemical-potential shift should be the dominant factor of the core-level shift, which is consistent with the expected behavior of carrier doping of thermoelectric semiconductors, that is, rigid-band-like behavior. The size of the core-level shift of ∼0.15 eV is close to the band gap of ∼0.18 eV obtained from transport measurements. The observed electronic structure can qualitatively explain the experimental results.

Structural and electronic properties of substitutionally doped SiAs monolayer

The substitutional single-atom doping is proven to be a significant method to alter the electronic properties of low dimensional materials. Based on the first-principles density functional theory, we present a systematic study on the structural and electronic properties of single-atom substitutionally doped SiAs monolayers. SiAs monolayer is a semiconductor with an indirect band gap of 1.6 eV. Through substitutional doping, the band gap (calculated at the PBE level) of the monolayer SiAs ranges from 0.032 eV to 1.625 eV. The Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) systems present magnetic ground states with a magnetic moment of 1.00 μB. Moreover, monolayer Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) have long range and local short range magnetic moments owing to the degree of hybridization between the dopant and its adjacent atoms. Metallic behavior can be found in Si31XAs32 (X = N, P, Sb), Si32As31Y (Y = O, S, Se) systems due to the contribution of doped atoms intersecting the Fermi level. The calculated results could provide a new approach for low dimensional doped SiAs-based photoelectronic and magnetic semiconductor devices.

Effect of Oxygen Exchange between Two Oxide Layers of a Memristive Bilayer Valence-Change Memory Cell on the Switching Polarity

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.

GaSb band-structure models for electron density determinations from Raman measurements.

We investigate the use of Raman spectroscopy to measure carrier concentrations in GaSb epilayers to aid in the development of this technique for the nondestructive characterization of transport properties in doped semiconductors. The carrier concentration is quantified by modeling the measured coupled optical phonon-free carrier plasmon mode spectra. We employ the Lindhard-Mermin optical susceptibility model with contributions from carriers in the two lowest GaSb conduction-band minima, the and minima. Furthermore, we evaluate three conduction-band models: (1) both minima parabolic and isotropic, (2) the minimum non-parabolic and isotropic and the minima parabolic and isotropic, and (3) the minimum non-parabolic and isotropic and the minima parabolic and ellipsoidal. For a given epilayer, the carrier concentration determined from the spectral simulations was consistently higher for the ellipsoidal minima model than the other two models. To evaluate the conduction-band models, we calculated the to electron mobility ratio necessary for the electron concentrations from the Raman spectral measurements to reproduce those from the Hall effect measurements. We found that the model with the ellipsoidal minima agreed best with reported carrier-dependent mobility-ratio values. Hence, employing isotropic minima in GaSb conduction-band models, a common assumption when describing the GaSb conduction band, likely results in an underestimation of carrier concentration at room temperature and higher doping levels. This observation could have implications for Raman spectral modeling and any investigation involving the GaSb conduction band, e.g., modeling electrical measurements or calculating electron mobility.

Towards a comprehensive theory of metal–insulator transitions in doped semiconductors

A review is given on the theory of metal–insulator transitions (MIT) in doped semiconductors. We focus in particular on reviewing theories of their anomalous magnetic properties, which emerge from the interplay of spin and charge correlations and disorder. Building on the review of these existing theories and experiments, we suggest that the finite temperature phase diagram can be structured into 1. a quantum spin liquid phase at subcritical doping and low temperature, as described by the Bhatt–Lee theory of random spin clusters, mostly random singlets. 2. a critical non-Fermi-liquid fan, originating at the MIT, which is dominated by random Kondo singlets with a universal tail of the distribution of their binding energies. This is caused by multifractality and results in an anomalous power law divergence of the magnetic susceptibility with a universal power and 3. a supercritical, low temperature phase. Rare events caused by the random placement of dopants do not allow to define strict phase boundaries. Remaining open problems are reviewed and outlined. Finally, the possibility of finite temperature delocalization transitions is reviewed, which are caused by the correlation induced temperature dependence of the spin scattering rate from magnetic moments. This review article is devoted to the memory of Konstantin B. Efetov.

Negative Magnetoresistance in Hopping Regime of Lightly Doped Thermoelectric SnSe

Semiconducting SnSe, an analog of black phosphorus, recently attracted great scientific interest due to a disputed report of a large thermoelectric figure of merit, which has not been reproduced subsequently. Here we concentrate on the low-temperature ground state. To gain a better understanding of the system, we present magneto-transport properties in high-quality single crystals of as-grown, lightly doped SnSe down to liquid helium temperatures. We show that SnSe behaves as a p-type doped semiconductor in the vicinity of a metal-insulator transition. Electronic transport at the lowest temperatures is dominated by the hopping mechanism. Negative magnetoresistance at low fields is well described by antilocalization, while positive magnetoresistance at higher fields is consistent with the shrinkage of localized impurity wavefunctions. At higher temperatures, a dilute metallic regime is realized where elusive T2 and B2 resistivity dependence is observed, posing a challenge to theoretical comprehension of the underlying physical mechanism.