Amphoteric Doping Research Articles

Insights into enhanced thermoelectric performance of the n-type Mg3Sb2-based materials by amphoteric Al doping

Doping has the potential to alter the levels of anharmonicity in compounds by attenuating bonding strength. In this study, we explore the efficacy of amphoteric Al doping for stimulating anharmonicity in n-type Mg3.25AlxSb1.48Bi0.48Te0.04 to attain enhanced phonon scattering and thermoelectric performance. First-principles calculations and experimental data reveal the occupation of both Sb and Mg2 sites by amphoteric Al atoms in the anionic framework of Mg3.25AlxSb1.48Bi0.48Te0.04. A marginal variation in both carrier concentration and mobility sustains the high power factor without affecting the Seebeck coefficient, implying amphoteric doping induced charge compensation. While phonon velocity, Grüneisen parameter, and crystal orbital Hamilton population calculations results indicate that phonon softening and bond weakening are realized via Al doping, leading to an enhanced lattice anharmonicity and a reduced lattice thermal conductivity. A remarkable enhancement ∼16% in the peak figure of merit ZTpeak and the average ZTavg, was attained for the x = 0.015 sample, when compared with the un-doped sample. Hence, the amphoteric doping can serve as an effective means to optimize ZT values by decoupling the intertwined thermoelectric transport properties.

Two-dimensional n-type pyrite with tuned hydrogen interstitials as a highly selective CO2 reduction catalyst

Density functional theory (DFT) calculations were employed to elucidate the impact of hydrogen interstitial (Hi) defects on the optical and efficacy of iron pyrite (FeS2) as a selective carbon dioxide reduction catalyst (CO2RR). Three different charge states were considered, namely Hi1, Hi0 or Hi−1. Upon examining the formation energy of the various possible charge states that the Hi can accommodate, it was found that Hi will be incorporated in the structure as an n-type defect in the (+1) state across almost the entire bandgap. However, under heavy n-type growth conditions, it can act as an amphoteric dopant, and can accommodate the 3 states simultaneously. The performance of FeS2-H in CO2RR is compared to the pristine counterpart. The calculations show that the addition of Hi facilitates the methanol pathway by prioritizing the attachment of the COOH* intermediate. Consequently, Hi promotes the formation of C1 compounds with higher performance. This is achieved by inhibiting the competitor hydrogen evolution reaction (HER), leading to a significant improvement in the efficiency.

Role of silicon on the conductivity GaSb surface: A first-principles study

Gallium antimonide (GaSb) has exceptional semiconductor properties, making it a promising material for various applications, leading to extensive theoretical and experimental studies. The prevalence of GaSb antisite defects in unintentionally doped GaSb leads to intrinsic p-type conductivity, placing a limitation on its utility. Introducing dopants as electron donors to compensate for the vacancies induced by GaSb antisite is viewed as a feasible strategy to ameliorate this issue. This study explores the transformation and enhancement of GaSb conductivity by analyzing the mechanism of Si doping using first-principles calculations. The calculated results show, the GaSb(001) surface with GaSb antisite exhibits unintentional p-type conductivity. When Si as a dopant source, GaSb-3SiGa complex exhibits n-type conductivity. The pure GaSb(001) surface exhibits intrinsic conductivity, and when Si atoms are doped into the system, it only exhibits n-type conductivity. Taken together, the above observations suggest that Si, as an amphoteric dopant, can serve as an n-type dopant for GaSb. Additionally, the upward shift of the Fermi level upon the incorporation of Si atoms exhibits a consistent trend, irrespective of the presence of the GaSb antisite. Furthermore, charge analysis indicates that the conduction band electron count increases, resulting in an improvement of conductivity. This study opens a new avenue for the preparation of potential high-performance GaSb-based semiconductor devices.

On the flash sintering behavior of Li:ZnO—Evidence of electrode asymmetry driven by electrochemical reactions

AbstractThe synthesis, subsequent flash sintering (FS) characteristics and microstructures of pure and Li‐incorporated ZnO powders are reported. At low concentrations, Li is a substitutional occupant in ZnO but becomes an amphoteric dopant (substitutional and interstitial occupant) at higher concentrations inferred from a contraction reversal of the unit cell volume. Increasing Li reduces the average flash temperature of ZnO modestly by 15°C, and a doubling of the linear shrinkage. A discernible color smear (yellow–white–dark) stretching from the anode to cathode imputable to strong electromigration is also observed. Microanalyses of the electrode regions establish clear evidence of electrochemical (EC) lithiation into ZnO and the formation of Li–Zn compounds not observed in conventional sintering (CS). Interestingly, in contrast to CS, the addition of Li enhances coarsening during FS, suggestive of a dissolution–reprecipitation process concurrent with the EC lithiation process. Evidence for considerable (local) yet tangible temperature, chemical and microstructural asymmetry among electrodes driven by EC reactions is presented. Probable mechanisms, leading to these observations, are discussed.

Direct evidence of Be as an amphoteric dopant in GaN

The interest in Be as an impurity in GaN stems from the challenge to understand why GaN can be doped $p$ type with Mg, while this does not work for Be. While theory has actually predicted an acceptor level for Be that is shallower than Mg, it was also argued that Be is not a suitable acceptor because its amphoteric nature, i.e., its tendency to occupy substitutional Ga as well as interstitial sites, would be considerably more pronounced than for Mg and hence lead to self-compensation. Using the emission channeling technique at the ISOLDE/CERN facility, we determined the lattice location of $^{11}\mathrm{Be}$ $({t}_{1/2}=13.8\phantom{\rule{0.16em}{0ex}}\mathrm{s})$ in different doping types of GaN as a function of implantation temperature. We find within an accuracy of 0.08 \AA{} that the location of interstitial Be is the one predicted by theory. The room temperature interstitial fraction of $^{11}\mathrm{Be}$ was correlated with the GaN doping type, being highest (up to \ensuremath{\sim}80%) in $p$ type and lowest in $n$-GaN, thus giving direct evidence for the amphoteric character of Be. We find that interstitial $^{11}\mathrm{Be}$ fractions are generally much higher than for Mg, which confirms that indeed self-compensation should be considerably more pronounced for Be. With rising implantation temperature, an increasing conversion of interstitial to substitutional Be is observed, involving at least two clearly identifiable steps at 50--150 \ifmmode^\circ\else\textdegree\fi{}C and 350--500 \ifmmode^\circ\else\textdegree\fi{}C. This suggests that the migration of interstitial Be may be subject to two different activation energies.

Investigation of p-type doping in β- and κ-Ga2O3

We have systematically investigated the effects of all possible combinations of vacancies and silicon substitutions on the electronic structure of the β and κ phases of Ga2O3 using plane-wave density functional theory (DFT) methods. It was found that VGa defects are associated with a sufficient shift of the Fermi level to lower energy to induce p-type behavior, with formation energies in the range of 9.0 ± 0.2 eV. Calculations with single atom substitutions in the κ phase, including nitrogen, phosphorous, and silicon, did not show p-type character, although NO substitutions may lead to shallow acceptor states. In the pursuit of elucidating how MOCVD growth of Ga2O3 can result in p-type behavior, as indicated by experimental results in the literature, we examined the role of combining hydrogen and silicon substitutions. The results showed that p-type behavior is observable when gallium atoms are substituted for hydrogen within the coordination sphere of SiO substitutions. This shows that silicon can act as an amphoteric dopant for p-type Ga2O3 semiconducting materials when hydrogen is included with formation energies<6.0 eV.

Enhanced thermoelectric performance in Ti(Fe, Co, Ni)Sb pseudo-ternary Half-Heusler alloys

TiFe0.5Ni0.5Sb-based half-Heusler compounds have the intrinsic low lattice thermal conductivity and the adjustable band structure. Inspired by the previously reports to achieve both p- and n-type components by tuning the ratio of Fe and Ni based on the same parent TiFe0.5Ni0.5Sb, we selected Co as the amphoteric dopants to prepare both n-type and p-type pseudo-ternary Ti(Fe, Co, Ni)Sb-based half-Heusler alloys. The carrier concentration, as well as the density of states effective mass was significantly increased by Co doping, contributing to the enhanced power factor of 1.80 mW m−1 K−2 for n-type TiFe0.3Co0.2Ni0.5Sb and 2.21 mW m−1 K−2 for p-type TiFe0.5Co0.15Ni0.35Sb at 973 K. Combined with the further decreased lattice thermal conductivity due to the strain field and mass fluctuation scattering induced by alloying Hf on the Ti site, peak ZTs of 0.65 in n-type Ti0.8Hf0.2Fe0.3Co0.2Ni0.5Sb and 0.85 in p-type Ti0.8Hf0.2Fe0.5Co0.15Ni0.35Sb were achieved at 973 K, which is of great significance for the thermoelectric power generation applications.

Gate-polarity-dependent doping effects of H2O adsorption on graphene/SiO2 field-effect transistors

Due to its huge surface-to-volume ratio, graphene has become one of the most popular materials for sensors. However, H2O molecules in atmospheric environments can cause the instability of graphene devices, which greatly limits the practical applications of graphene devices. The charge transfer between graphene and adsorbed H2O molecules has been proved previously by first-principle studies, but experimental demonstrations are still lacking. Here, gate-polarity-dependent doping behaviors of adsorbed H2O molecules on a graphene/SiO2 field-effect transistor (GFET) are experimentally investigated. The results indicate that the orientation of the adsorbed H2O can be affected by the gate-voltage polarity due to the dipolar interaction, which leads to the amphoteric doping behavior of adsorbed H2O molecules. With reducing the graphene layer number, the amphoteric doping behavior is more sensitive to the gate voltage polarity. Our results are helpful for constructing high performance graphene-based sensors and enhancing the stability of GFETs.

Stability and amphotericity analysis in rhombohedral ABO3 perovskites

Perovskite structures (ABO3) exhibit several desirable physical properties. These properties relate closely to composition and structure. For instance, structural distortions such as off-centering of B-site cation and/or tilting of BO6 polyhedra allow tuning of properties. Notably, the tilting of BO6 polyhedra yields a rhombohedral structure. This structure is increasingly relevant for functional perovskites. However, there exists no predictive approach thus far that allows for stability analysis in rhombohedral perovskites, and the determination of amphotericity domains for dopants. Here, we derive a tolerance factor equation (tR) that works well for the rhombohedral ABO3 perovskites. The predictive capability of the equation is benchmarked using sodium bismuth titanate (NBT), as a case in point. Using tR, we determined the following rare-earth doping trends in NBT: La to Nd occupy A-site; Tm to Lu go to B-site; and intermediate ions (Sm to Er) occupy both the sites (i.e., show amphotericity). The approach is consistent with experimental literature and hence benchmarked duly. The tolerance factor based analysis described here is hence plausibly relevant for the broader category of rhombohedral perovskites. In fact, it would be useful for accelerated prediction and determination of (a) stable rhombohedral structures, and (b) amphoteric dopants.

Amphoteric imidazole doping induced large-grained perovskite with reduced defect density for high performance inverted solar cells

Intrinsic defect density in polycrystalline halide perovskite films are required to be low enough to suppress charge recombination loss for improvement in performance of perovskite solar cells (PeSCs). In this paper, we propose the use of amphoteric imidazole to achieve high crystalline quality of CH3NH3PbI3 perovskite absorption layer. The imidazole additive plays a synergistic role in controlling the perovskite crystal growth for large grain size and passivating the uncoordinated ions (e.g., Pb2+) defects, resulting in improved carrier transport/lifetime and suppressed non-radiative recombination. The champion power conversion efficiency (PCE) of PeSCs with imidazole is improved to 16.88%, from the control device with a PCE value of 14.65%. Besides, the stability of imidazole modified perovskite films is further improved by limiting ion immigration at grain boundaries against moisture and heat stresses. The findings pave an avenue for synergistically modulating crystallization and healing defect in perovskite to achieve efficient and stable solar cells.

Higher electrical activation of ion-implanted Si over S in GaSb epitaxial layers

Ion implantation of the amphoteric dopant Si compared to the natural n-type dopant S in GaSb epitaxial films results in a higher dopant activation with better surface morphology and crystallinity. GaSb films were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs (001) substrates. Ion implantation for both ions was carried out at an energy of 50 keV and a fluence of 1 × 1015 ions/cm2. Raman spectroscopy revealed a better crystallinity as well as a lower Sb out-diffusion for Si implantation as compared to S after annealing. Van der Pauw Hall measurements confirm the donor nature of the amphoteric dopant Si and with a dopant activation that is 3 times higher than the natural donor S. The electrical activation for both implants increases significantly after thermal annealing. Using temperature (20 K–300 K) vs conductivity measurement and displacement per atom (dpa) estimation, implantation-induced defect-related conduction mechanism is discussed. The results suggest the scope of Si as a viable n-type dopant in GaSb.

Resolving the mystery of the concentration-dependence of amphoteric dopant diffusion in III-V semiconductors

Amphoteric dopants, such as silicon, exhibit a concentration-dependent diffusivity in III-V compounds characterized by a low to negligible diffusion at low dopant concentrations, which increases by orders of magnitude at high concentrations. The reason for this anomalous and non-intuitive diffusivity has largely remained a mystery. Leading models from literature, built to explain existing experimental results, make contradictory assumptions on the nature of the diffusing species, based only on thermodynamic considerations. In this work, we perform ab initio calculations to characterize the kinetics of dopant diffusion in III-V materials revealing how the nature of the primary diffusing species changes with concentration. We use these results to derive a new analytical model which fits historical, as well as more recent, experimental data on Si diffusion in GaAs and In0.57Ga0.43As. This work provides a fundamentals-based understanding of Si diffusion in III-V alloys as well as a new predictive analysis tool while showcasing a mutiscale approach of solving diffusion problems. We hope this work will help guide the design of the next generation of III-V based electronic and photonic devices.

Electrical properties of Si and Be doped InSb and InAlSb/InSb superlattice applied to improve the doping efficiency

Abstract 0.8 to 1 μm Si and Be doped InSb were grown by molecular beam epitaxy. X-ray diffraction and atomic force microscope indicate that growth temperature below 340 °C severely results in dislocations and undulating surface. Silicon shows amphoteric doping nature in InSb at higher growth or Si cell temperatures, for the calculation result reveals it needed more energy to take the place of Sb. ECV measurement demonstrates that InSb/In0.9Al0.1Sb superlattice buffer layer can effectively inhibit Be dopant from diffusing to the substrate in the 420 °C InSb, with which both the doping level and crystalline quality are significantly improved. 790–870 °C Be doped InSb transfer from p- to n-type during the Hall measurement temperature increasing from 77 K to 330 K. The transition temperature of each sample is linearly correlated to the Be doping temperature, which is theoretically interpreted by the excitation mechanism and carrier transportation principles inside InSb.

Charge Compensation Modulation of the Thermoelectric Properties in AgSbTe2 via Mn Amphoteric Doping.

In thermoelectric research, the introduction of a dopant can suppress lattice thermal conductivity (κ1) through phonon scattering and optimize the power factor (PF) by changing the behavior of carriers, which are the key prerequisites for high thermoelectric performance. However, the electrical thermal conductivity (κe) can also increase with the increase of electrical conductivity (σ), which may override the optimization in PF and be detrimental to the improvement of final ZT. In this work, we highlight an amphoteric doping method by using Mn atoms to substitute both Ag and Sb atoms in AgSbTe2. The MnSb positive doping in p-type AgSbTe2 can improve the σ through increasing the hole concentration while maintaining a relative high Seebeck coefficient (S), thus substantially improving the PF. On the other hand, the MnAg negative doping can introduce electrons into the matrix, which will recombine with the major hole carriers and lead to a decrease of σ to suppress exorbitant κe induced by the MnSb doping. The combination of the both functions by Mn amphoteric doping can further improve the thermoelectric property through charge compensation modulation. By virtue of amphoteric doping, though σ is decreased, PF is further optimized because of increased S, while the total thermal conductivity (κtotal) is further decreased due to suppressed κe and additional phonon scattering, which are beneficial for the improvement of the final ZT value. As a result, 5 mol % MnAg-MnSb amphoteric doping AgSbTe2 sample achieves a maximum ZT value of ∼0.74 at 550 K, which is higher than that of the pristine sample and other Mn monodoped counterparts. The present work suggests charge compensation modulation via amphoteric doping as an effective avenue to simultaneously achieve low thermal conductivity and high power factor for better thermoelectric performance.

Reducing-atmosphere resistant mechanism on microwave dielectric enhancement of CaMgSi2O6 glass-ceramics

This work highlights a reducing-atmosphere resistance using amphoteric ion doping to reduce defects and to enhance microwave dielectric properties in CaMgSi2O6 glass-ceramics. Amphoteric Al3+ ion (Al2O3 doping) was selected with both donor and acceptor roles to restrict free electrons and oxygen vacancies in specimens sintered in reducing atmosphere. The X-ray absorption and X-ray photoelectron spectra indicate decreased oxygen vacancies and a valence shift from a mixture of Si2+ and Si3+ to Si4+ in the Al3+–doped CaMgSi2O6 glass-ceramics sintered in reducing-atmosphere. The amphoteric Al3+ ion plays an acceptor role as entering Si4+ site to form oxygen vacancies, meanwhile Al3+ plays a donor role as entering Mg2+ site to compensate free electrons. The amphoteric Al–doped diopside glass-ceramic can increase the electric resistivity and quality factors. This work can provide an efficient approach for the base-metal-electrode (BME) in low-temperature co-fired ceramics (LTCC) and multilayer ceramic capacitors (MLCC).

Non-metallic dopant modulation of conductivity in substoichiometric tantalum pentoxide: A first-principles study

We apply density-functional theory calculations to predict dopant modulation of electrical conductivity (σo) for seven dopants (C, Si, Ge, H, F, N, and B) sampled at 18 quantum molecular dynamics configurations of five independent insertion sites into two (high/low) baseline references of σo in amorphous Ta2O5, where each reference contains a single, neutral O vacancy center (VO0). From this statistical population (n = 1260), we analyze defect levels, physical structure, and valence charge distributions to characterize nanoscale modification of the atomistic structure in local dopant neighborhoods. C is the most effective dopant at lowering Ta2Ox σo, while also exhibiting an amphoteric doping behavior by either donating or accepting charge depending on the host oxide matrix. Both B and F robustly increase Ta2Ox σo, although F does so through elimination of Ta high charge outliers, while B insertion conversely creates high charge O outliers through favorable BO3 group formation, especially in the low σo reference. While N applications to dope and passivate oxides are prevalent, we found that N exacerbates the stochasticity of σo we sought to mitigate; sensitivity to the N insertion site and some propensity to form N-O bond chemistries appear responsible. We use direct first-principles predictions of σo to explore feasible Ta2O5 dopants to engineer improved oxides with lower variance and greater repeatability to advance the manufacturability of resistive memory technologies.

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (&lt;1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT. The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb0.9875Sb0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m−1 K−1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.

Codoping and Interstitial Deactivation in the Control of Amphoteric Li Dopant in ZnO for the Realization of p-Type TCOs.

We report on first principle investigations about the electrical character of Li-X codoped ZnO transparent conductive oxides (TCOs). We studied a set of possible X codopants including either unintentional dopants typically present in the system (e.g., H, O) or monovalent acceptor groups, based on nitrogen and halogens (F, Cl, I). The interplay between dopants and structural point defects in the host (such as vacancies) is also taken explicitly into account, demonstrating the crucial effect that zinc and oxygen vacancies have on the final properties of TCOs. Our results show that Li-ZnO has a p-type character, when Li is included as Zn substitutional dopant, but it turns into an n-type when Li is in interstitial sites. The inclusion of X-codopants is considered to deactivate the n-type character of interstitial Li atoms: the total Li-X compensation effect and the corresponding electrical character of the doped compounds selectively depend on the presence of vacancies in the host. We prove that LiF-doped ZnO is the only codoped system that exhibits a p-type character in the presence of Zn vacancies.

(Magic Dopant) Amphoteric Behavior of a Redox‐Active Transition Metal Ion in a Perovskite Lattice: New Insights on the Lattice Site Occupation of Manganese in SrTiO3

It is demonstrated that a transition metal redox‐active ion can exhibit amphoteric dopant substitution in the SrTiO3 perovskite lattice. In stoichiometric SrTiO3, the manganese dopant is preferably accommodated through isovalent substitution as Mn2+ on the strontium site and as Mn4+ on the titanium site. Previous studies have suggested that either type of substitution is possible for compositions with tailored Sr/Ti stoichiometry. Using electron paramagnetic resonance (EPR) spectroscopy, the site occupancy of dilute concentrations of manganese is investigated in SrTiO3 as a function of the Sr/Ti ratio. The tuned Sr/Ti ratio can be used to manipulate the nature of the manganese substitution, and it is shown that Sr‐rich compositions (Sr/Ti &gt; 1.001) processed in air result in B‐site isovalent doping. For B‐site substituted manganese ions, a new EPR signal for aliovalent Mn2+ is observed in compositions annealed under reducing atmosphere. The concentration of oxygen vacancies observed with EPR is also shown to depend on the Sr/Ti stoichiometry. With improved control over the site of substitution and valence state, doping with a transition metal redox‐active ion may facilitate the ability to engineer new electronic functionality into the perovskite lattice.

Amphoteric doping of praseodymium Pr3+ in SrTiO3 grain boundaries

Charge compensation in rare-earth Praseodymium (Pr3+) doped SrTiO3 plays an important role in determining the overall photoluminescence properties of the system. Here, the Pr3+ doping behavior in SrTiO3 grain boundaries (GBs) is analyzed using aberration corrected scanning transmission electron microscopy. The presence of Pr3+ induces structural variations and changes the statistical prevalence of the GB structures. In contrast to the assumption that Pr3+ substitutes on the Sr site in the bulk, Pr3+ is found to substitute on both Sr and Ti sites inside the GBs, with the highest concentration at the Ti sites. This amphoteric doping behavior in the boundary plane is further confirmed by first principles theoretical calculations.