Doped Semiconductors Research Articles

Colloidal Synthesis of Semiconductor Films for Efficient Photoelectrochemical Hydrogen Generation

The semiconductor-based photoanodes have shown great potential on photoelectrochemical (PEC) hydrogen generation. Compared to the pristine semiconductor, photoanodes fabricated with doped semiconductors exhibit modulated bandgap structure and enhanced charge separation efficiency, demonstrating improved optoelectronic properties. In this work, we develop a colloidal cation exchange (CE) strategy on versatile synthesis of heterovalent doped chalcogenide semiconductor thin films with high surface roughness. Using Ag-doped CdSe (CdSe:Ag) thin films as an example, the organized centimeter-scale CdSe:Ag films with nanometer-scale thickness (thickness around 80 nm, length × width around 1.5 cm × 1.2 cm) exhibit enhanced optical absorbance ability and charge carrier density by tuning the energy levels of conduction and valence bands as well as improved electrical conductivity by Ag dopants compared to the pristine CdSe film obtained by the vapor-phase vacuum deposition strategy. In the meantime, the surface roughness of the as-prepared semiconductor thin films is also increased with abundantly exposed active sites to facilitate accessibility to water for hydrogen generation and suppress photogenerated carrier recombination. The CdSe:Ag film photoanodes exhibit superb PEC hydrogen generation performance with a photocurrent density of 0.56 mA/cm 2 at 1.23 V versus reversible hydrogen electrode, which is nearly 3 times higher than the pristine CdSe film. This work provides a new strategy on colloidal synthesis of photoelectrodes with modulated heterovalent doping and surface roughness for PEC applications.

Synergistic effects of rare earth doping and carbon quantum dots on BiOF/Bi2MoO6 heterojunctions for enhanced visible-near-infrared photocatalysis.

Herein, we designed and synthesized a series of rare earth doped BiOF/Bi2MoO6 heterojunctions. The doping locations of rare earth ions were altered to determine the influence on the visible and near-infrared photocatalytic activity of heterojunctions. It is experimentally and theoretically confirmed that doping with Tm3+/Yb3+ in one semiconductor of the heterojunction produces superior photocatalytic efficiency than doping in both semiconductors. In addition, the near infrared photocatalytic efficiency strongly relied on upconversion luminescence from the Re3+ doped semiconductor in the heterojunction. By further modifying with CQDs, the CQDs/BiOF:Tm3+,Yb3+/Bi2MoO6 sample shows excellent visible and near-infrared photocatalytic performance, with 90% degradation of RhB occurring in the first 20 min under visible irradiation. This can be attributed to the large BET area, efficient photoinduced carrier separation and the upconversion process of the composite. This research will provide a systematic solution for realizing full-spectrum responsive and highly efficient photocatalysis by combination of rare earth ion doping, quantum dot modification and Z-scheme heterojunctions.

Surface doping of rubrene single crystals by molecular electron donors and acceptors.

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

Efficient and air-stable n-type doping in organic semiconductors.

Chemical doping of organic semiconductors (OSCs) enables feasible tuning of carrier concentration, charge mobility, and energy levels, which is critical for the applications of OSCs in organic electronic devices. However, in comparison with p-type doping, n-type doping has lagged far behind. The achievement of efficient and air-stable n-type doping in OSCs would help to significantly improve electron transport and device performance, and endow new functionalities, which are, therefore, gaining increasing attention currently. In this review, the issue of doping efficiency and doping air stability in n-type doped OSCs was carefully addressed. We first clarified the main factors that influenced chemical doping efficiency in n-type OSCs and then explain the origin of instability in n-type doped films under ambient conditions. Doping microstructure, charge transfer, and dissociation efficiency were found to determine the overall doping efficiency, which could be precisely tuned by molecular design and post treatments. To further enhance the air stability of n-doped OSCs, design strategies such as tuning the lowest unoccupied molecular orbital (LUMO) energy level, charge delocalization, intermolecular stacking, in situ n-doping, and self-encapsulations are discussed. Moreover, the applications of n-type doping in advanced organic electronics, such as solar cells, light-emitting diodes, field-effect transistors, and thermoelectrics are being introduced. Finally, an outlook is provided on novel doping ways and material systems that are aimed at stable and efficient n-type doped OSCs.

Advancements in lanthanide-based perovskite oxide semiconductors for gas sensing applications: a focus on doping effects and development.

Lanthanide-based perovskite oxide semiconductors have garnered significant attention due to their exceptional electrical and sensing properties, making them promising candidates for gas sensing applications. This review paper focuses on developments and the impact of doping in lanthanide-based perovskite oxide semiconductors for gas sensing purposes. The review explores the factors influencing gas sensing performance, such as operating temperature, dopant selection, and target gas species. The role of dopants in enhancing gas sensing selectivity, sensitivity, response/recovery times, and stability is discussed in detail. Comparisons are drawn between doped perovskite oxide semiconductors, undoped counterparts, and other gas-sensing materials. Practical applications of lanthanide-based perovskite oxide semiconductor gas sensors are outlined, including environmental monitoring, industrial process control, and healthcare. The review also identifies current challenges and future perspectives in the field, such as the exploration of novel doping strategies and integration with emerging technologies like the Internet of Things (IoT). The findings emphasize the potential of these materials in advancing gas sensing technology and the importance of continued research in this field.

Post-synthetic modification of semiconductor nanoparticles can generate lanthanide luminophores and modulate the electronic properties of preformed nanoparticles

Post-synthetic modification of inorganic nanoparticles (NPs) provides a unique lesser synthetically demanding opportunity to access nanomaterials those are oftentimes not directly realizable by conventional synthetic routes. Trivalent lanthanide (Ln3+) incorporated (doped) semiconductor NPs can benefit from individual properties of the NPs and Ln3+ moieties. This work summarizes key outcomes from experiments when (a) ZnS /CdS /CdSe NPs are post-synthetically treated with Ln3+ to generate ZnS/Ln or CdSe/Ln [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] and CdS/Ln [Eu, Tb] NPs, (b) synthetically Tb3+ doped Zn(Tb)S NPs are post-synthetically modified with varying concentration of heavy metals like Pb2+/Cd2+ to generate Zn(Tb)S/M [M = Pb, Cd] NPs, and (c) the pH of Zn(Tb)S NPs aqueous dispersion is varied post-synthetically. Key observations from these experiments include (a) incorporation of Ln in all the post-synthetically prepared CA/Ln NPs, with presence of host sensitized dopant emission in select cases that can be rationalized by a charge trapping mediated dopant emission sensitization processes, (b) existence of rich photophysics in the sub-stoichiometric reactant concentration ratio, and (c) identifying the alteration of surface capping ligand structure as an important variable to control the Ln3+ emission. In summary, these experimental observations provide an easy control of reaction conditions either to generate Ln3+ inorganic NP luminophores or to control their electronic properties by modulating either the NP’s core or surface properties, and are of potential usefulness in various luminescence based applications.

XXVII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 13-16 марта 2023 г. Формирование заряженных вакансий в анионной подрешетке AlAs

The vacancy formation dynamics in doped semicon- ductor heterostructures with quantum dots (QDs) formed in the AlAs anionic sublattice has been studied. A theoretical model that describes the effect of doping on the vacancy generation dynamics is constructed. It is shown that the generation of positively charged arsenic vacancies is more probable than the generation of neutral ones at high hole concentrations. On the other hand, at high electron concentrations, the formation of neutral arsenic vacancies is more efficient than that positively charged ones. It has been experimentally revealed that the vacancy-stimulated high- temperature diffusion of antimony is enhanced (suppressed) in p-(n-)-doped heterostructures with Al(Sb,As)/AlAs QDs.

Significant increase of the photoresponse range and conductivity for a chalcogenide semiconductor by viologen coating through charge transfer.

Widening the photoresponse range while enhancing the electrical properties of semiconductors could reduce the complexity and cost of photodetectors or increase the power conversion efficiency of solar cells. Surface doping through charge transfer with organic species is one of the most effective and widely used approaches to achieve this aim. It usually features easier preparation over other doping methods but is still limited by the low physicochemical stability and high cost of the used organic species or low improvement of electrical properties. This work shows unprecedented surface doping of semiconductors with highly stable, easily obtained, and strong electron-accepting viologen components, realizing the significant improvement of both the photoresponse range and conductivity. Coating the chalcogenide semiconductor KGaS2 with dimethyl viologen dichloride (MV) yields a charge-transfer complex (CTC) on the surface, which broadens the photoresponse range by nearly 300 nm and improves the conductivity by 5 orders of magnitude. The latter value surpasses all records obtained by surface doping through charge transfer with organic species.

Investigation of effect of doping in perovskite solar cells: A numerical simulation approach

In n -i-p structure perovskite solar cells, TiO 2 is a widely used thermally stable and low-cost electron transport layer. But in CsSnI 3 based perovskite solar cells, the undoped-TiO 2 electron transport layer does not provide proper band alignment for efficient charge transportation. The use of doped semiconductors in heterojunction solar cells has improved device performance significantly by shifting the energy levels of the electron and hole transport layers. We have investigated the effect of doping density in the absorber layer, electron transport layer, and hole transport layer on the performance of a device with an FTO/TiO 2 /CsSnI 3 /CuSCN/Au configuration. It is found that the doping density of the electron transport layer (TiO 2 ) and absorber layer (CsSnI 3 ) has a significant effect on the band alignment and thus on the device performance; however, an increase in the acceptor doping density of the hole transport layer (CuSCN) does not make any significant changes in the device performance. In addition, we have optimized the variable parameters of different layers. The optimized device shows an efficiency >25%. These results could help to optimize the performance of CsSnI 3 based n -i-p structure perovskite solar cells with TiO 2 electron transport layer.

Machine learning-aided band gap prediction of semiconductors with low concentration doping.

The important physical quantities of materials, such as band gap, have been predicted efficiently with the help of machine learning (ML), in addition to the widely used experimental or computational methods. Under this scheme, by combining density functional theory (DFT) calculations and ML predictive models, the band gaps of doped semiconductors with normal doping concentrations are predicted successfully. Our present work provides a solution to the problem of how to obtain the band gaps of semiconductors doped with extremely low concentrations, which are important cases for some device designs. The structures were constructed by configuration screening with a symmetric criterion, and three-dimensional spatial structural variation was mapped to one-dimensional features, which are the key steps for the ML predictive model. The biggest error in the predicted band gaps of dilute nitride-doped GaAs by ML models is not more than 10%, compared with values obtained from DFT. Considering the limitation of material data, a few-shot learning method was further adopted to check the performance of the predictive models. The performance of the ML models was validated using data out of training and testing datasets. Our method will efficiently accelerate the prediction of physical properties of semiconductors with extremely low-concentration doping.

High-Performance Digital Lock-In Amplifier Module Based on an Open-Source Red Pitaya Platform: Implementation and Applications

A lock-in amplifier (LIA) is an instrument that can extract weak signals from extremely noisy environments, and it is widely used in various precision measurement fields. However, there are still challenges in lock-in techniques in experiments that require high-frequency or multiple lock-in channels, such as high-frequency vibration detection and experiments with quantum sensors. The direct use of commercial LIAs will result in high experimental costs. Herein, a high-performance digital LIA module based on an open-source Red Pitaya (RP) platform is constructed. The quantitative description of the demodulation results, efficient data transfer method, and the development of the automated measurement functions in experiments are carried out in this module. As demonstrations, the phase difference and thermal noise measurements are conducted using the module. The research on temperature sensing with the frequency shift of quartz crystals and semiconductor doping concentration measurements has also been implemented. These demonstrations show that the digital lock-in module has good application prospects.

High p doped and robust band structure in Mg-doped hexagonal boron nitride.

In two dimensional materials, substitutional doping during growth can be used to alter the electronic properties. Here, we report on the stable growth of p-type hexagonal boron nitride (h-BN) using Mg-atoms as substitutional impurities in the h-BN honeycomb lattice. We use micro-Raman spectroscopy, angle-resolved photoemission measurements (nano-ARPES) and Kelvin probe force microscopy (KPFM) to study the electronic properties of Mg-doped h-BN grown by solidification from a ternary Mg-B-N system. Besides the observation of a new Raman line at ∼1347 cm-1 in Mg-doped h-BN, nano-ARPES reveals p-type carrier concentration. Our nano-ARPES experiments demonstrate that the Mg dopants can significantly alter the electronic properties of h-BN by shifting the valence band maximum about 150 meV toward higher binding energies with respect to pristine h-BN. We further show that, Mg doped h-BN exhibits a robust, almost unaltered, band structure compared to pristine h-BN, with no significant deformation. Kelvin probe force microscopy (KPFM) confirms the p-type doping, with a reduced Fermi level difference between pristine and Mg-doped h-BN crystals. Our findings demonstrate that conventional semiconductor doping by Mg as substitutional impurities is a promising route to high-quality p-type doped h-BN films. Such stable p-type doping of large band h-BN is a key feature for 2D materials applications in deep ultra-violet light emitting diodes or wide bandgap optoelectronic devices.

Efficient doping to synthesize high-performance Co/Fe-BiOCl photocatalyst assisted by the ion release from novel CoFe2O4 nanofiber reservoir

Metal ion doping is an efficient method to achieve high photocatalytic performance of semiconductors. However, the current synthetic methods by using inorganic salts as doped sources in the precursor reaction system might suffer from ion waste and second pollution. Herein, we developed a novel method by employing electrospun nanofibers as ion reservoir to release metal ions for the synthesis of ion-doped semiconductors with high photocatalytic activity. As demonstrated, Co/Fe co-doped BiOCl photocatalyst (Co/Fe-BOC) was facilely prepared in the presence of CoFe2O4 nanofibers as Co and Fe ion reservoir via a simple solvothermal method. Compared with BiOCl, the resulted Co/Fe-BOC shows increased specific surface area, visible light response as well as separation and transportation efficiencies of photogenerated carriers. Thus, the Co/Fe-BOC exhibits significantly enhanced visible light photocatalytic activity to degrade 99.4% rhodamine B (RhB) within 4 min. This new doping approach not only avoids the ion waste and pollution during the reaction process but also facilitates the separation of the photocatalyst. Moreover, the CoFe2O4 nanofibers could be repeatedly used as ion reservoir to prepare high-performance Co/Fe-BOC. This nanofiber-based ion doping approach provides a novel, universal and promising insight to obtain doped semiconductors with high photocatalytic activity.

Magnetic, Phonon and Optical Properties of Transition Metal and Rare Earth Ion Doped ZnS Nanoparticles.

The surface, size and ion doping effects on the magnetic, phonon and optical properties of ZnS nanoparticles are studied based on the s-d model including spin-phonon and Coulomb interaction, and using a Green's function theory. The changes of the properties are explained on a microscopic level, due to the different radii between the doping and host ions, which cause different strains-compressive or tensile, and change the exchange interaction constants in our model. The magnetization increases with increasing small transition metal (TM) and rare earth (RE) doping concentration. For larger TM dopants the magnetization decreases. The phonon energies increase with increasing TM, whereas they decrease by RE ions. The phonon damping increases for all doping ions. The changes of the band gap energy with different ion doping concentration is also studied. Band gap changes in doped semiconductors could be due as a result of exchange, s-d, Coulomb and electron-phonon interactions. We have tried to clarify the discrepancies which are reported in the literature in the magnetization and the band gap energy.

Anion‐Modulated Chemical Doping of Organic Hole Conductor Boosts Efficiency and Stability of Perovskite Solar Cells

AbstractChemical doping of organic semiconductors enables significant progress in improving their optoelectronic performance. However, the correlation between doping counter ions and charge‐transport mechanism has not been yet well‐understood. In this study, it is discovered that the anion‐dependent degree of delocalization (DOD) of lithium‐based dopants significantly determines the doping kinetics as well as the conductivity of organic hole transport layer (HTL), leading to large variation in solar cell efficiency and device stability. Specifically, the incorporation of bis(pentafluoroethanesulfonyl) imide (PFSI−) as the anion with a high DOD results in one order of magnitude higher film conductivity and thus an elevated power conversion efficiency (PCE) exceeding 22.1%, much higher than the state‐of‐the‐art lithium bis(trifluoromethane)sulfonimide (LiTFSI) (21.1%) and lithium hexafluorophosphate (LiPF6) (20.0%). Moreover, the dopant LiPF6 with a smaller DOD produces higher doping yield of HTL accompanied by stronger light‐induced PCE fluctuation. Structural analysis reveals anion‐modulated ion exchange kinetics determine the hole‐transport mechanism and device photostability. To mitigate these detrimental effects, a versatile strategy of Li+ solvation is developed to modulate the anion dissociation, enabling simultaneous improvement of device efficiency and stability. This study elucidates an intriguing and generally applicable doping mechanism, and envisages a bright future to further developing efficient and stable organic electronics.

Surface-Doping-Induced Mobility Modulation Effect for Transport Enhancement in Organic Single-Crystal Transistors.

Molecular doping has conventionally been an effective way to improve the electrical-transport performances in organic field-effect transistors (OFETs), while corresponding mechanisms associated with specific doping techniques have been less investigated and discussed in detail. Here, based on ultrathin dinaphtho[2,3-b:2',3'-f]-thieno[3,2-b]thiophene (DNTT) single crystals, robust transconductance enhancements are realized in OFETs upon surface molecular doping realized via van der Waals epitaxially growing crystalline 1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) onto the single crystal's surface. It is proposed that it is the mobility modulation effect (MME) from the interactions between charge-transfer interface and gate electric field, that contributes to more weighted bulk carriers, and finally improves charge-transport performances. The evaluations are further supported by scanning Kelvin probe microscopy(SKPM) surface potential characterizations, which manifest the gate-induced more delocalized holes near the charge-transfer interfaces. Space-charge-limited current (SCLC) investigations, numerical calculations, and theoretical mobility modeling are also performed to corroborate the analysis. This study can deepen the understanding of charge transport in doped semiconductors and provide effective ways for optimizing the electrical performance of organic devices.

Generation–recombination noise in uniformly doped semiconductors with contacts of finite surface recombination velocities

The formula for the voltage noise spectrum due to generation–recombination (G–R) rate fluctuations is derived for uniformly doped majority-carrier n-type semiconductor samples with contacts of finite surface recombination velocities. In the derivation of the formula, a new boundary condition with finite surface recombination velocities considered is used instead of the ohmic boundary condition. The G–R voltage noise spectra are calculated to become saturated and equal in the high electric field region irrespective of surface recombination velocities. However, the behaviors of slow surface recombination velocities in the low electric field region differ from those of fast ones. The profiles of the electron density fluctuations of slow ones differ significantly from those of fast ones in the low-field region. In the high field region, a steep rise in the magnitude of the electron density fluctuation occurs near the voltage contact toward which electrons drift. The various location dependencies of the contribution of noise sources to the voltage power spectral density are also derived for low and high field regions as well as low and high surface recombination velocities, and analytical expressions are developed for the voltage power spectral densities and electron density fluctuations.

Impact of electron-phonon interactions on phonon transport in diamond and c-BN

Diamond and cubic polymorph of boron nitride (c-BN) are two promising next-generation ultrawide bandgap semiconductor materials owning superior thermal properties. Although lattice vibration is the dominant mechanism of heat conduction in semiconductors, electron-phonon interactions exist and may affect phonon transport in doped semiconductors; yet such effects in wide bandgap materials have not received much attention. In this study, we explore the effects of electron-phonon interactions on the lattice thermal conductivity and phonon transport in electron-doped Si, diamond, and c-BN under various electron concentrations and in a wide temperature range from 300 K to 900 K based on the first-principles calculation. It is found that the electron-phonon interactions will bring down the thermal conductivity of doped materials, and the depletion impact increases as the electron concentration increases but decreases as the temperature increases. This depression effect in ultrawide bandgap diamond and c-BN is apparent, though it is not as severe as in Si. At room temperature with a high electron concentration of 10 21 cm −3 , the reduction of thermal conductivity reaches 36 % in Si, and 17.4 % and 16.1 % in diamond and c-BN, respectively. • Thermal effect of electron-phonon interactions in doped c-BN and diamond is revealed. • Temperature and electron concentration dependent lattice thermal conductivity is quantified. • Thermal depletions exist in doped c-BN and diamond, but not as severe as in doped Si. • Depletion lessens as temperature rises, but increases with increasing electron concentration. • Ultrawide bandgap materials have great potentials in high-power high-temperature electronics.

Structural and electronic properties with respect to Si doping in oxygen rich ZnSnO amorphous oxide semiconductor

The effect of Si doping in the presence of excess oxygen on the zinc-tin-oxide (ZTO) thin-film transistor is investigated. The threshold voltage increases from 3.86 to 10.8 V, and the ON current reduces from 4 × 10 -4 to 1.1 × 10 -4 A for the undoped and 1 wt% Si-doped ZTO transistor, respectively. The changes in the threshold voltage and the ON current are attributed to the suppression of free charge carriers due to excess oxygen in the film. The annihilation of oxygen vacancy-related defects states is substantiated by the density of states extracted from the photonic capacitance-voltage characteristic of the undoped and Si-doped ZTO thin-film transistors. Moreover, due to the high binding energy of Si and oxygen, excess oxygen is introduced into the film. The excessive oxygen forms the O-O ex dimer that acts as the electron trap and degrades the device's stability. From the bias stress measurement and the density of states extraction, it is concluded that 0.5 wt% doping of Si in ZTO is optimal for moderate device performance, less defects density, and high stability.

Modulation Doping of Single-Layer Semiconductors for Improved Contact at Metal Interfaces.

Single layers of two-dimensional (2D) materials hold the promise for further miniaturization of semiconductor electronic devices. However, the metal-semiconductor contact resistance limits device performance. To mitigate this problem, we propose modulation doping, specifically a doping layer placed on the opposite side of a metal-semiconductor interface. Using first-principles calculations to obtain the band alignment, we show that the Schottky barrier height and, consequently, the contact resistance at the metal-semiconductor interface can be reduced by modulation doping. We demonstrate the feasibility of this approach for a single-layer tungsten diselenide (WSe2) channel and 2D MXene modulation doping layers, interfaced with several different metal contacts. Our results indicate that the Fermi level of the metal can be shifted across the entire band gap. This approach can be straight-forwardly generalized for other 2D semiconductors and a wide variety of doping layers.