Electrical Doping Research Articles

Efficient p‐Type Doping of Tin Halide Perovskite via Sequential Diffusion for Thermoelectrics

Metal halide perovskites (MHPs) hold great potential in thermoelectric (TE) applications, thanks to their regular and soft lattice in nature. However, the poor electrical conductivity caused by low charge carrier density (<1014 cm−3 for lead‐based MHPs) strongly impedes its TE development. In this scenario, tin halide perovskites (THPs) emerge as promising TE candidates owing to their high background hole densities (>1019 cm−3). However, further electrical doping remains challenging, originating from the limited capability of accommodating heterogeneous dopants and the heavy compensation in THPs. Herein, a novel diffusion‐mediated doping approach is demonstrated to prominently increase the p‐type doping level of THPs by a sequence of air exposure and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) surface treatments. In paradigm photovoltaic THP materials—CH(NH2)2SnI3 (namely FASnI3), the electrical conductivity is dramatically increased by 300× from 0.06 to 18 S cm−1 in thin films, leading to a remarkable enhancement of power factor by 25× up to 53 μW m−1 K−2. In contrast, only a slight variation of thermal conductivity is observed after F4TCNQ deposition, which is in accordance with the increase in electrical conductivity, indicating that the lattice structures of FASnI3 remain intact after doping. This study paves an illuminating way to ameliorate TE properties in halide perovskites.

Catalyzing n-doping

Electrical doping, i.e., inducing p- or n-type conductivity by adding a dopant, is of fundamental importance for device physics of semiconductors and is used in about every commercially produced device. For organic semiconductors, the conductivity is intrinsically low (typically 10−4–10−8 S/cm) due to the mostly amorphous nature, the weak coupling between molecules, and the low number of free charge carriers in the presence of large energy gaps.

CMOS Compatible Al‐Doped ZnO Sol–Gel Thin‐Film Properties

Zinc oxide (ZnO) is a low‐cost class of n‐type inorganic semiconductors with a large exciton binding energy (≈60 meV), wide direct bandgap (3.37 eV), and the most important material for various fields of industrial and deep‐tech applications. Herein, a complementary metal–oxide–semiconductor (CMOS) temperature compatible (400 °C)‐ integrated circuit (IC) process based on the sol–gel method is described. The properties of aluminum (Al)‐doped ZnO (AZO) thin films were investigated. The Al content, Al/(Al+Zn) ratio, varies from 0 to 10% and exhibits compressive stresses from −4 to −1.8 GPa. At low dopant concentrations, the Al content acts as an electrical dopant, while at higher dopant concentrations, it acts as an impurity. The electrical resistivity, which was only 3 × 10−3 Ω cm, is inversely related to the orientation of the thin film, which was preferably along the (0 0 2) direction. The optical bandgap energy of AZO thin films was determined to be in the range of 3.34–3.87 eV. Herein, a novel method to change the Al content of doped AZO thin films to improve their properties is described, which is suitable for next‐generation flexible, microsystem, and optoelectronic devices.

Spin transport properties in DNA & electrically doped iron QD organo-metallic junction

The DNA sensor has emerged as strong candidates for next generation ultra low power application due to its self assemble technique. Non-Equilibrium Green’s Function (NEGF) along with Density Functional Theory (DFT) based First Principle approach is used in investigation of spin transport properties along with quantum scattering transmission characteristics of DNA sensor via Iron (Fe) quantum dots (QD) electrodes at room temperature. Electrically doped Fe QD plays an important role in spin transport mechanism. This electrical doping concentration and weak coupling strength between DNA and Fe QD organo-metallic junction effect into the tunneling contact resistance (TCR) along with quantum-ballistic transmission and junction conductivity of parallel and anti-parallel configuration of this analytical model representation. It has been observed that higher current has been achieved for parallel configuration when compare with anti-parallel configuration at same bias voltage. This voltage-current characteristic is significantly modulated due to the electrical doping effect. This spin transport property shows that this system can well perform for anti parallel configuration. High tunnel organo-metallic resistance approximately 99.9% is observed even at 0 V bias voltage. Tunneling Organo Metallic Contact Resistance (TOMCR) remains large at upper bias voltage.

Thin films of electron donor–acceptor complexes: characterisation of mixed-crystalline phases and implications for electrical doping

For electron donor–acceptor complexes a link will be established between optical, structural and vibrational properties of EDA complexes as well as the electrical doping by them.

Impact of Bi doping on nonradiative carrier recombination in CsPbI3

Doping with Bi in CsPbI3 promotes the formation of Ii, which is detrimental to carrier collection. Codoping with Na balances electrical doping of Bi and thus inhibits the undesirable Ii formation.

Electrical and Optical Doping of Silicon by Pulsed-Laser Melting

Over four decades ago, pulsed-laser melting, or pulsed-laser annealing as it was termed at that time, was the subject of intense study as a potential advance in silicon device processing. In particular, it was found that nanosecond laser melting of the near-surface of silicon and subsequent liquid phase epitaxy could not only very effectively remove lattice disorder following ion implantation, but could achieve dopant electrical activities exceeding equilibrium solubility limits. However, when it was realised that solid phase annealing at longer time scales could achieve similar results, interest in pulsed-laser melting waned for over two decades as a processing method for silicon devices. With the emergence of flat panel displays in the 1990s, pulsed-laser melting was found to offer an attractive solution for large area crystallisation of amorphous silicon and dopant activation. This method gave improved thin film transistors used in the panel backplane to define the pixelation of displays. For this application, ultra-rapid pulsed laser melting remains the crystallisation method of choice since the heating is confined to the silicon thin film and the underlying glass or plastic substrates are protected from thermal degradation. This article will be organised chronologically, but treatment naturally divides into the two main topics: (1) an electrical doping research focus up until around 2000, and (2) optical doping as the research focus after that time. In the first part of this article, the early pulsed-laser annealing studies for electrical doping of silicon are reviewed, followed by the more recent use of pulsed-lasers for flat panel display fabrication. In terms of the second topic of this review, optical doping of silicon for efficient infrared light detection, this process requires deep level impurities to be introduced into the silicon lattice at high concentrations to form an intermediate band within the silicon bandgap. The chalcogen elements and then transition metals were investigated from the early 2000s since they can provide the required deep levels in silicon. However, their low solid solubilities necessitated ultra-rapid pulsed-laser melting to achieve supersaturation in silicon many orders of magnitude beyond the equilibrium solid solubility. Although infrared light absorption has been demonstrated using this approach, significant challenges were encountered in attempting to achieve efficient optical doping in such cases, or hyperdoping as it has been termed. Issues that limit this approach include: lateral and surface impurity segregation during solidification from the melt, leading to defective filaments throughout the doped layer; and poor efficiency of collection of photo-induced carriers necessary for the fabrication of photodetectors. The history and current status of optical hyperdoping of silicon with deep level impurities is reviewed in the second part of this article.

Two-dimensional bipolar magnetic semiconductors with high Curie-temperature and electrically controllable spin polarization realized in exfoliated Cr(pyrazine)2 monolayers

Exploring two-dimensional (2D) magnetic semiconductors with room temperature magnetic ordering and electrically controllable spin polarization is a highly desirable but challenging task for nanospintronics. Here, through first principles calculations, we propose to realize such a material by exfoliating the recently synthesized organometallic layered crystal Li$_{0.7}$[Cr(pyz)$_2$]Cl$_{0.7}$0.25$\cdot$(THF) (pyz = pyrazine, THF = tetrahydrofuran) [Science 370, 587 (2020)]. The feasibility of exfoliation is confirmed by the rather low exfoliation energy of 0.27 J/m$^2$, even smaller than that of graphite. In exfoliated Cr(pyz)$_2$ monolayer, each pyrazine ring grabs one electron from the Cr atom to become a radical anion, then a strong $d$-$p$ direct exchange magnetic interaction emerges between Cr cations and pyrazine radicals, resulting in room temperature ferrimagnetism with a Curie temperature of 342 K. Moreover, Cr(pyz)$_2$ monolayer is revealed to be an intrinsic bipolar magnetic semiconductor where electrical doping can induce half-metallic conduction with controllable spin-polarization direction.

Potassium versus Sodium in Cu(In,Ga)Se2—Similarities and Differences in the Electrical Characteristics of Solar Cells and Thin Films after NaF or KF Postdeposition Treatment

The improvement of CIGSe solar cells efficiency due to potassium fluoride postdeposition treatment (KF‐PDT) is often interpreted as being caused by changes in the interface region. The impact of KF‐PDT on bulk properties of the CIGSe absorbers is investigated by measuring hole densities in the solar cells and conductivities of corresponding thin films and is then analyzed together with similar data for NaF‐treated absorbers and thin films. It is found that both alkali metals change the electrical properties of the whole CIGSe layer and not only the absorber surface. By measuring electrical conductivity and doping level in thermal equilibrium (relaxed) and metastable state (after light soaking) as well as their evolution with the amount of Na and K after postdeposition treatment (PDT) at various temperatures, one can observe effects that can only be explained by the influence of alkali metals on grain boundary properties. In contrast to Na, low concentration of potassium leads to a decrease in conductivity. Persistent photoconductivity exhibits a linear relationship with the relaxed conductivity in both NaF‐ and KF‐treated samples. These effects directly show that electrical transport in CIGSe is limited by potential barriers at the grain boundaries, which are impacted by the alkali treatment.

Graphene‐Based Microwave Metasurfaces and Radio‐Frequency Devices

Graphene, as a 2D material, has attracted significant attention due to its particular electronic band structure and unique physical properties. Metasurfaces integrated with graphene hold great promise for dynamic manipulation of the electromagnetic waves, where metasurfaces with strong resonances enhance the interaction with incident waves and in turn facilitate a deep modulation through electrical doping of graphene. However, the tunable surface conductivity of graphene is very frequency dependent that makes graphene behave significantly different in THz and microwave band. Due to the novel tuning method of graphene–electrolyte‐based sandwich structures, experimental studies of tunable microwave metadevices have aroused wide interests in the past few years. Herein, the recent progress of graphene‐based microwave metasurfaces and related devices, involved but not limited to tunable absorbers, dynamic beam steering, reconfigurable antennas, and attenuators, are reviewed, enriching the experimental developments of graphene in the microwave region.

Halide Perovskite Precursors Dope PEDOT:PSS (Adv. Electron. Mater. 9/2021)

Halide Perovskite Semiconductors In article number 2100394, Guillaume Wantz & co. demonstrate that coating halide perovskite semiconductors on top of the well-known PEDOT:PSS hole transporting layer is affecting the polymer. Halide precursors are penetrating into the PEDOT:PSS inducing a strong electrical doping. Its conductivity increases by two orders of magnitude having implications on cross-talk currents in LEDs and on short-circuit currents overestimation in solar cells.

Control of Water Adsorption via Electrically Doped Graphene: Effect of Fermi Level on Uptake and H2O Orientation

AbstractThe interaction of graphene with water molecules under an applied electric field is not thoroughly understood, yet this interaction is important to many thermal, fluidic, and electrical applications of graphene. In this work, the effect of electrical doping of graphene on water adsorption is studied through adsorption isotherms and current–voltage (IV) characterizations as a function of the Fermi level. The water adsorption onto graphene increases by ≈15% and the doping levels increase by a factor of three with a gate‐to‐graphene voltage of +20 or −20 V compared to 0 V for sub‐monolayer adsorption. This change in uptake is attributed to the increase in density of state of graphene upon electrical‐doping, which changes the Coulombic and van der Waals interactions. The water adsorption onto graphene is either n‐ or p‐doping depending on the applied gate‐to‐graphene voltage. The ambi‐doping nature of water onto graphene is due to the polar nature of water molecules, so the doping depends on the orientation of the water molecules.

Graphene Photo Memtransistor Based on CMOS Flash Memory Technology with Neuromorphic Applications

Graphene holds a great promise for a number of diverse future applications, in particular related to its easily tunable doping and Fermi level by electrostatic gating. However, as of today, most implementations rely on electrical doping via the application of continuous large voltages to maintain the desired doping. We show here how graphene can be implemented with conventional semiconductor flash memory technology in order to make programmable doping possible, simply by the application of short gate pulses. We also demonstrate how this approach can be used for a memory device, and also show potential neuromorphic capabilities of the device. Finally, we show that the overall performance can be significantly enhanced by illuminating the device with UV radiation. Our approach may pave the way for integrating graphene in CMOS technology memory applications, and our device design could also be suitable for large scale neuromorphic computing structures.

Growth and characterization of ZnO thin films at low temperatures: from room temperature to − 120 °C

ZnO thin films have been grown by e-beam evaporation in the range from room temperature to − 120 °C on two types of substrates, Al2O3 (0001) and Si (100). Although the ZnO/Al2O3 system has been thoroughly characterized, including optical and electrical techniques, the morphological, structural and chemical properties show no significant differences between both substrates. Thus, the general features of the ZnO growth mode at low temperature can be generalized. The relatively low growth temperatures reduce the diffusion of atoms at the surface, which leads to morphological and chemical changes. As the temperature decreases, the growth mode changes from a van der Drift model to a gradual bilayer system composed of an interfacial layer in contact with the substrate and a second columnar-based layer. This second well-ordered film disappears for the lowest temperatures while a Zn-rich interface in contact with the substrate emerges. Precisely from this interface, Zn-rich whiskers develop under the ZnO film and cause the loss of adhesion at temperatures below − 100 °C. These extreme temperatures also affect the crystal size, lattice strain, and total amount of oxygen vacancies. The behavior of the optical and electrical properties in terms of band gap, transparency, electrical resistivity, and Seebeck coefficient is discussed in the light of structural and chemical characterization. Samples grown at 0 °C exhibit an enhanced transmittance compared to those grown at room temperature while preserving similar electrical resistivity values and natural n-type doping. These results open a promising route to enhance ZnO films properties below the typical high temperature window.

The effects of secondary doping on ink-jet printed PEDOT:PSS gas sensors for VOCs and NO2 detection

In the study of conductive conjugated polymers, electrical doping has long played an important role. A new polymeric gas sensor has been successfully fabricated by means of an ink-jet printer using a conductive aqueous formulation of poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS). A simple yet robust treatment method for the irreversible secondary doping was performed (by H2SO4 and MeOH post-treatments) to enhance conductivity and improve gas sensing performance. Real time gas sensing measurements were carried out by exposing the devices with eight different analytes in a low concentrations range of VOCs vapors up to 5 % of the saturated vapor pressure, 10 ppm of NO2 and up to 10 % of relative humidity (RH) at 21 °C, exploiting dry air as carrier and diluting gas. The gas response, obtained as the ratio between the steady-state resistance variation and the baseline resistance of the device, was evaluated for different PEDOT:PSS post-treated sensors. An unexpected behavior of PEDOT:PSS post-treated with concentrated H2SO4 was observed, while MeOH and diluted H2SO4 post-treated sensors exhibited improved response towards all investigated analytes. The best performances were obtained towards 5 % of ammonia and NO2 with a gas response of 6 % and 28 % respectively with the device post-treated with pure methanol and 16 % with the sensor post-treated with diluted sulfuric acid. Furthermore, long-term stability and the influence of temperature were evaluated on the fabricated sensors. Altogether, these promising results allow a better understanding of the secondary doping effects on the electrical and sensing properties, paving the way for electronic nose development.

Effective plasma frequency in tunable THz reflectors based on graphene and phosphorene

In this work, we show that a THz reflector made of a phosphorene (graphene)–dielectric multilayer can be analyzed through a simple effective plasma frequency model. We obtain the cutoff frequency, marking the beginning of the transmission region in terms of the effective plasma frequency, which depends on structural parameters of the multilayer and, more importantly, on the carrier density of phosphorene (graphene) layers. This implies that the cutoff frequency can be varied when considering carrier density tuning. We also show that the frequency regime, where cutoff frequency is obtained by carrier density tuning, goes through a wide range in the THz domain (3–25 THz), making the design of tunable reflectors in these frequencies possible through electrical doping. We numerically analyze the cutoff frequency as a function of carrier density and optical thickness of the multilayer, finding that the cutoff frequency and carrier density of phosphorene (graphene) have a very simple quadratic (quartic) relation. Our analysis allows us to obtain ranges where the effective analytical model fits better with the cutoff frequency computed from photonic bands for the multilayer periodic structure.

Increasing Volume in Conjugated Polymers to Facilitate Electrical Doping with Phosphomolybdic Acid.

Molecular p-type electrical dopants have been proven useful to fine-tune the optoelectronic properties of bulk organic semiconductors and their interfaces. Here, the volume in polymer films and its role in solution-based electrical p-type doping using phosphomolybdic acid (PMA) are studied. The polymer film volume was controlled using two approaches. One is based on heating both the PMA solution and the film prior to immersion. The second is based on coating the polymer film with a liquid blend that contains the PMA solution and a swelling solvent. 31P NMR and FTIR experiments indicate that the Keggin structure appears to be preserved throughout the doping process. Results show that increasing the polymer volume facilitates the infiltration of the PMA Keggin structure, which results in an increased electrical p-type doping level.

Electron transport properties of electrically doped guanine nano-sheet based bio-Zener diode: a first principle paradigm

The quantum ballistic transmission properties of an electrically-doped guanine-nanosheet-based bio-Zener diode are investigated using density functional theory and nonequilibrium Green’s function-based first-principles calculations. The bio-Zener diode is gate-bias modulated, and its various quantum-electronic properties, for example, the V–I characteristic, the transmission spectra, and the device density of states, depend on both the electrical doping concentration and on the applied gate bias voltage. The junctionless highly doped bio-Zener diode shows high levels of reverse-bias current which is dominated by majority charge carriers. It is also found that, due to the presence of a wide bandgap and the backscattering effect, the forward-bias current is highly suppressed. The quantum simulation results confirm a strong reverse gate-bias-modulated biased current–voltage response as well as a charge transport phenomenon through the effective device region. The bio-Zener diode exhibits a specific reverse breakdown that can be varied from −0.78 to −3.2 V without affecting the forward current–voltage characteristic. The current findings are obtained by including the coherent tunneling and incoherent hopping processes with a minimal Hamiltonian model approach.

Recent Advances in Electrical Doping of 2D Semiconductor Materials: Methods, Analyses, and Applications.

Two-dimensional materials have garnered interest from the perspectives of physics, materials, and applied electronics owing to their outstanding physical and chemical properties. Advances in exfoliation and synthesis technologies have enabled preparation and electrical characterization of various atomically thin films of semiconductor transition metal dichalcogenides (TMDs). Their two-dimensional structures and electromagnetic spectra coupled to bandgaps in the visible region indicate their suitability for digital electronics and optoelectronics. To further expand the potential applications of these two-dimensional semiconductor materials, technologies capable of precisely controlling the electrical properties of the material are essential. Doping has been traditionally used to effectively change the electrical and electronic properties of materials through relatively simple processes. To change the electrical properties, substances that can donate or remove electrons are added. Doping of atomically thin two-dimensional semiconductor materials is similar to that used for silicon but has a slightly different mechanism. Three main methods with different characteristics and slightly different principles are generally used. This review presents an overview of various advanced doping techniques based on the substitutional, chemical, and charge transfer molecular doping strategies of graphene and TMDs, which are the representative 2D semiconductor materials.

Electrical doping in halide perovskites

Electrical doping (that is, intentional engineering of carrier density) underlies most energy-related and optoelectronic semiconductor technologies. However, for the intensely studied halide perovskite family of semiconductors, reliable doping remains challenging, owing to, for example, compensation from and facile migration of intrinsic defects. In this Review, we first discuss the underlying fundamentals of semiconductor doping and then investigate different doping strategies in halide perovskites, including intrinsic defect, extrinsic defect and charge transfer doping, from an experimental as well as a theoretical perspective. We outline the advantages and pitfalls of different characterization techniques to assess doping and examine the impact of doping on optoelectronic properties. Finally, we highlight challenges that need to be overcome to gain control over the electronic properties of this important material class. Halide perovskites exhibit outstanding semiconductor properties and are a key component of a variety of devices, including solar cells and light-emitting diodes. This Review discusses electrical doping strategies for halide perovskites and takes a critical look at the challenges that need to be overcome to control the electronic properties of these semiconducting materials.