Unintentional Doping Research Articles

Disorder-induced decoupled surface transport channels in thin films of doped topological insulators

Nonideal topological insulator (TI) films in which the bulk states are not insulating due to unintentional doping exhibit strong surface--bulk coupling. Such surface--bulk coupling can further induce intersurface coupling that affects the electrical conductivity of the TI films through a quantum interference effect known as weak antilocalization. Increased understanding and control of intersurface coupling is therefore crucial for the use of TI-based quantum devices. In this report on the transport properties of doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films under perpendicular and parallel magnetic fields, we observe a crossover between coupled and decoupled surface channels that is mediated by intentional disorder controlled by a post-annealing process. The intentional disorder causes the surface state carriers to rapidly lose their quantum phase and coherence, and as a result, more disordered ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films exhibit a shorter penetration depth of the surface state into the bulk states and weaker intersurface coupling, even though stronger surface--bulk coupling is expected. In previous studies, the role of disorder has generally been considered by determining its effect on surface--bulk scattering, but our results indicate that the role of disorder must be considered as a source of decoherence.

Multi-carrier transport in ZrTe5 film**Project supported by Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06D348) and Shenzhen Peacock Program (Grant No. KQTD2016022619565991).

Single-layered zirconium pentatelluride (ZrTe5) has been predicted to be a large-gap two-dimensional (2D) topological insulator, which has attracted particular attention in topological phase transitions and potential device applications. Herein, we investigated the transport properties in ZrTe5 films as a function of thickness, ranging from a few nm to several hundred nm. We determined that the temperature of the resistivity anomaly peak (Tp) tends to increase as the thickness decreases. Moreover, at a critical thickness of ∼40 nm, the dominating carriers in the films change from n-type to p-type. A comprehensive investigation of Shubnikov–de Hass (SdH) oscillations and Hall resistance at variable temperatures revealed a multi-carrier transport tendency in the thin films. We determined the carrier densities and mobilities of two majority carriers using the simplified two-carrier model. The electron carriers can be attributed to the Dirac band with a non-trivial Berry phase π, while the hole carriers may originate from surface chemical reaction or unintentional doping during the microfabrication process. It is necessary to encapsulate the ZrTe5 film in an inert or vacuum environment to potentially achieve a substantial improvement in device quality.

Electrical properties of extended defects in strain relaxed GeSn

We report the electrical properties of 60° dislocations originating from the +1.2% lattice mismatch between an unintentionally doped, 315 nm thick Ge0.922Sn0.078 layer (58% relaxed) and the underlying Ge substrate, using deep level transient spectroscopy. The 60° dislocations are found to be split into Shockley partials, binding a stacking fault. The dislocations exhibit a band-like distribution of electronic states in the bandgap, with the highest occupied defect state at ∼EV + 0.15 eV, indicating no interaction with point defects in the dislocation's strain field. A small capture cross-section of 1.5 × 10−19 cm2 with a capture barrier of 60 meV is observed, indicating a donor-like nature of the defect-states. Thus, these dislocation-states are not the source of unintentional p-type doping in the Ge0.922Sn0.078 layer. Importantly, we show that the resolved 60° dislocation-states act as a source of leakage current by thermally generating minority electrons via the Shockley-Read-Hall mechanism.

Effect of alkaline-doping on photoelectrochemical activity of electrodeposited cuprous oxide films

P-type Cu2O films with alkaline ions (Li+, Na+ and K+) of unintentional dopants on indium tin oxide coated glass substrate are successfully fabricated via a simple electrodeposition method for photoelectrochemical (PEC) hydrogen generation. The SEM and XRD analysis show the as-grown films with the pyramid-like morphology and cubic structure, and the composition of alkaline-doped Cu2O films are examined using XPS spectroscopy to demonstrate the substitution of alkaline ions in the Cu2O lattice. The optical analyses, including the absorbance and low-temperature photoluminescence spectra, confirm a bandgap of 2.3 eV and the presence of structural defects in alkaline-doped Cu2O films. The Mott-Schottky plot shows the flat band potentials of the alkaline-doped Cu2O films to be approximately −0.1 V and the hole concentrations in the order of 1017 cm−3. Significantly, the Cu2O:Li film photocathode exhibits a higher photocurrent of −2.2 mA cm−2 at a potential of −0.6 V vs Ag/AgCl which are greater than those of Cu2O:K and Cu2O:Na films due to greater preferred orientation degrees along (111) and less structural defects, because the ionic radii of Cu and Li is similar. These results demonstrate the great potential of alkaline doped Cu2O films in solar-related applications.

Effects of excess carriers on charged defect concentrations in wide bandgap semiconductors

Unintentional doping and doping limits in semiconductors are typically caused by compensating defects with low formation energies. Since the formation enthalpy of a charged defect depends linearly on the Fermi level, doping limits can be especially pronounced in wide bandgap semiconductors where the Fermi level can vary substantially. Introduction of non-equilibrium carrier concentrations during growth or processing alters the chemical potentials of band carriers and allows populations of charged defects to be modified in ways impossible at thermal equilibrium. We demonstrate that in the presence of excess carriers, the rates of carrier capture and emission involving a defect charge transition level determine the admixture of electron and hole quasi-Fermi levels involved in the formation enthalpy of non-zero charge defect states. To understand the range of possible responses, we investigate the behavior of a single donor-like defect as functions of extrinsic doping and charge transition level energy. We find that that excess carriers will increase the formation enthalpy of compensating defects for most values of the charge transition level in the bandgap. Thus, it may be possible to use non-equilibrium carrier concentrations to overcome limitations on doping imposed by native defects. Cases also exist in which the concentration of defects with the same charge polarity as the majority dopant is either left unchanged or actually increases. This surprising effect arises when emission rates are suppressed relative to the capture rates and is most pronounced in wide bandgap semiconductors. We provide guidelines for carrying out experimental tests of this model.

Defect induced structural inhomogeneity, ultraviolet light emission and near-band-edge photoluminescence broadening in degenerate In2O3 nanowires

We demonstrate here defect induced changes on the morphology and surface properties of indium oxide (In2O3) nanowires and further study their effects on the near-band-edge (NBE) emission, thereby showing the significant influence of surface states on In2O3 nanostructure based device characteristics for potential optoelectronic applications. In2O3 nanowires with cubic crystal structure (c-In2O3) were synthesized via carbothermal reduction technique using a gold-catalyst-assisted vapor–liquid–solid method. Onset of strong optical absorption could be observed at energies greater than 3.5 eV consistent with highly n-type characteristics due to unintentional doping from oxygen vacancy defects as confirmed using Raman spectroscopy. A combination of high resolution transmission electron microscopy, x-ray photoelectron spectroscopy and valence band analysis on the nanowire morphology and stoichiometry reveals presence of high-density of defects on the surface of the nanowires. As a result, chemisorbed oxygen species can be observed leading to upward band bending at the surface which corresponds to a smaller valence band offset of 2.15 eV. Temperature dependent photoluminescence (PL) spectroscopy was used to study the nature of the defect states and the influence of the surface states on the electronic band structure and NBE emission has been discussed. Our data reveals significant broadening of the NBE PL peak consistent with impurity band broadening leading to band-tailing effect from heavy doping.

Importance of Paramagnetic Background Subtraction for Determining the Magnetic Moment in Epitaxially Grown Ultrathin van der Waals Magnets

Due to the atomically thin nature of monolayer and few-layer van der Waals magnets, the undesired background signal from the substrate can have significant contribution when characterizing their magnetic properties. This brings challenges in accurately determining the magnitude of the magnetic moment of the epitaxially grown van der Waals magnets on bulk substrates. We discuss the impact of the background subtraction method for accurately determining the magnetic moments in such systems. Using the recently reported intrinsic two-dimensional van der Waals ferromagnet MnSe2 as an example, we show that a normal diamagnetic background subtraction method in analyzing the bulk magnetometry measurement will result in an unexpectedly large magnetic moment (greater than ~10 μB per formula unit). Through our systematic growth study, we identify an additional paramagnetic signal due to unintentional Mn doping of the substrate. To extract the correct magnetic moment, a paramagnetic background should also be considered. This yields a total magnetic moment of ~4 μB per formula unit in monolayer MnSe2, which is in close agreement to the theoretically predicted value.

Design and fabrication of BiV(O, N)4 p-n homojunction solid solutions for enhanced methylene blue degradation via LED light irradiation

Quantum efficiency of photoelectrons and photoholes depends on both their populations and ability to stay separated before reaching the targeted reaction sites. Hence, we present a design and practice of a new strategy to form BiVO4 p-n homojunctions by simply controlling the nitrogen doping levels. This is the first time a BiVO4 p-n homojunction is (1) designed from first principles quantum mechanics, (2) fabricated subsequently using unintentional doping in a single pot, and (3) able to degrade 88% methylene blue dye (MB) in just 20 mins under LED light irradiation. To the best of our knowledge, this study reports the fastest degradation rate of methylene blue dyes using BiVO4. We also compared the results of our p-n homojunction (un-intentionally doped or U-N-doped) with that of a heterojunction (intentionally-doped or I-N-doped Bi2O3/BiVO4) with regards to the photocatalytic performance in MB dye degradation. As expected the p-n homojunction out-performs that of the p-n heterojunction, although the heterojunction enables larger photoelectron/photohole populations than that of homojunction. Further, we propose that the formation of BiVO4 p-n homojunctions was the origin of the recent breakthrough in water splitting research recently reported by Kim et al. (2014). This design strategy and the simple one-pot synthesis technique have immediate implications to solar energy applications beyond dye degradation and water-splitting.

Anisotropic thermal conductivity of 4H and 6H silicon carbide measured using time-domain thermoreflectance

Silicon carbide (SiC) is a wide bandgap (WBG) semiconductor with promising applications in high-power and high-frequency electronics. Among its many useful properties, the high thermal conductivity is crucial. In this letter, the anisotropic thermal conductivity of three SiC samples, n-type 4H-SiC (N-doped 1 × 1019 cm−3), unintentionally doped (UID) semi-insulating (SI) 4H-SiC, and SI 6H-SiC (V-doped 1 × 1017 cm−3), is measured using femtosecond laser based time-domain thermoreflectance (TDTR) over a temperature range from 250 K to 450 K. We simultaneously measure the thermal conductivity parallel to (kr) and across the hexagonal plane (kz) for SiC by choosing the appropriate laser spot radius and the modulation frequency for the TDTR measurements. For both kr and kz, the following decreasing order of thermal conductivity value is observed: SI 4H-SiC > n-type 4H-SiC > SI 6H-SiC. This work serves as an important benchmark for understanding thermal transport in WBG semiconductors.

Performance improvement induced by asymmetric Y2O3-coated device structure to carbon-nanotube-film based photodetectors

Film-based semiconducting carbon nanotube (CNT) photodetectors are promising candidates for industrial applications. However, unintentional doping from the environment such as water/oxygen (H2O/O2) redox, polymers, etc. changes the doping level of the CNT film. Here, we evaluate the performance of film-based barrier-free bipolar diodes (BFBDs), which are basically semiconducting CNT films asymmetrically contacted by perfect n-type ohmic contact (scandium, Sc) and p-type ohmic contact (palladium, Pd) at the two ends of the diode. We show that normal BFBD devices have large variances of forward current, reverse current, and photocurrent for different doping levels of the channel. We propose an asymmetric Y2O3-coated BFBD device in which the channel is covered by a layer of an Y2O3 film and an overlap between the Sc electrode and the Y2O3 film is designed. The Y2O3 film provides p-type doping to the channel. The overlap section increases the length of the base of the pn junction, and the diffusion current of holes is suppressed. In this way, the rectifier factors (current ratio when voltages are at +0.5 V and −0.5 V) of the asymmetric Y2O3-coated BFBD devices are around two orders of magnitude larger and the photocurrent generation is more stable compared to that of normal devices. Our results provide a way to conquer the influence of unintentional doping from the environment and suppress reverse current in pn diodes. This is beneficial to applications of CNT-based photodetectors and of importance for inspiring methods to improve the performances of devices based on other low dimensional materials.

Gate-Tunable Electronic Structure of Black Phosphorus/HfS2 P–N van der Waals Heterostructure with Uniformly Anisotropic Band Dispersion

Black phosphorus (BP)-based heterostructure with tunable band offset has been proven to be promising for rectifier diode and photoelectronic devices. However, it is usually not easy to find a suitable material to construct the heterojunction because the necessary type-II band structure and the strong unintentional p-type doping of BP should be both considered. Therefore, most of studies mainly focused on certain 2D materials, like MoS2 and WSe2. However, the low mobility of these materials greatly hinders the further promotion of device performance. For the first time, we demonstrate that HfS2, which has been proven to possess a much higher mobility of electrons and has been experimentally synthesized recently, fully satisfies conditions of heterostructure with BP. The heterojunction could be used as a tunable optoelectronic device and rectifier diode. With external normal electric field, the efficiency of photon-generated charge separation and rectification ratio could be manipulated. In addition, what is interesting is that the nanostructure presents an unexpected highly anisotropic band dispersion along orthogonal directions, which suggests a superior transport performance with both high mobility and carrier density.

Incomplete Ionization of a 110\u2009meV Unintentional Donor in \u03b2-Ga2O3 and its Effect on Power Devices

Understanding the origin of unintentional doping in Ga2O3 is key to increasing breakdown voltages of Ga2O3 based power devices. Therefore, transport and capacitance spectroscopy studies have been performed to better understand the origin of unintentional doping in Ga2O3. Previously unobserved unintentional donors in commercially available (bar{2}01) Ga2O3 substrates have been electrically characterized via temperature dependent Hall effect measurements up to 1000 K and found to have a donor energy of 110 meV. The existence of the unintentional donor is confirmed by temperature dependent admittance spectroscopy, with an activation energy of 131 meV determined via that technique, in agreement with Hall effect measurements. With the concentration of this donor determined to be in the mid to high 1016 cm−3 range, elimination of this donor from the drift layer of Ga2O3 power electronics devices will be key to pushing the limits of device performance. Indeed, analytical assessment of the specific on-resistance (Ronsp) and breakdown voltage of Schottky diodes containing the 110 meV donor indicates that incomplete ionization increases Ronsp and decreases breakdown voltage as compared to Ga2O3 Schottky diodes containing only the shallow donor. The reduced performance due to incomplete ionization occurs in addition to the usual tradeoff between Ronsp and breakdown voltage.

Enhanced magnetic ordering in V, C codoped hierarchical porous ZnO nanograins

Vanadium doped ZnO macroporous structures are synthesized using solution combustion method. The as-burnt Zn1−xVxO (x = 0, 3mol%) foams are crushed and ground in agate mortar and pestle and calcined at 450°C/1h for further characterization. The structural study using XRD confirms the wurtzite phase of ZnO having hexagonal structure. The crystallite size is found to decrease from 28nm to 11nm on account of addition of vanadium. Grain size and grain boundary modification along with hierarchical porous structures are depicted using SEM micrographs. Further elucidation on the porous nature is made through BET. Quantitative and qualitative determination of intentional and unintentional dopants is estimated. Vibrational spectral studies elucidate the marked role of vanadium's infusion into the lattice and subsequent modification of grain boundary. An order of magnetic enhancement is achieved as illustrated by ambient and low temperature magnetization measurements. Role of vanadium doping on the grain boundary modification are brought to lime-light with structural, morphological and vibrational studies and the same is accounted for the observed magnetic property.

Encoding Highly Nonequilibrium Boron Concentrations and Abrupt Morphology in p-Type/n-Type Silicon Nanowire Superlattices.

Although silicon (Si) nanowires (NWs) grown by a vapor-liquid-solid (VLS) mechanism have been demonstrated for a range of photonic, electronic, and solar-energy applications, continued progress with these NW-based technologies requires increasingly precise compositional and morphological control of the growth process. However, VLS growth typically encounters problems such as nonselective deposition on sidewalls, inadvertent kinking, unintentional or inhomogeneous doping, and catalyst-induced compositional gradients. Here, we overcome several of these difficulties and report the synthesis of uniform, linear, and degenerately doped Si NW superlattices with abrupt transitions between p-type, intrinsic, and n-type segments. The synthesis of these structures is enabled by in situ chlorination of the NW surface with hydrochloric acid (HCl) at temperatures ranging from 500 to 700 °C, yielding uniform NWs with minimal nonselective growth. Surprisingly, we find the boron (B) doping level in p-type segments to be at least 1 order of magnitude above the solid solubility limit, an effect that we attribute to a high incorporation of B in the liquid catalyst and kinetic trapping of B during crystallization at the liquid-solid interface to yield a highly nonequilibrium concentration. For growth at 510 °C, four-point-probe measurements yield active doping levels of at least 4.5 × 1019 cm-3, which is comparable to the phosphorus (P) doping level of n-type segments. Because the B and P dopants are in sufficiently high concentrations for the Si to be degenerately doped, both segments inhibit the etching of Si in aqueous potassium hydroxide (KOH) solution. Moreover, we find that the dopant transitions are abrupt, facilitating nanoscale morphological control in both B- and P-doped segments through selective KOH etching of the NW with a spatial resolution of ∼10 nm. The results presented herein enable the growth of complex, degenerately doped p-n junction nanostructures that can be explored for a variety of advanced applications, such as Esaki diodes, multijunction solar cells, and tunneling field-effect transistors.

Intended or Unintended Doping? A Review of the Presence of Doping Substances in Dietary Supplements Used in Sports.

Introduction: The use of dietary supplements is increasing among athletes, year after year. Related to the high rates of use, unintentional doping occurs. Unintentional doping refers to positive anti-doping tests due to the use of any supplement containing unlisted substances banned by anti-doping regulations and organizations, such as the World Anti-Doping Agency (WADA). The objective of this review is to summarize the presence of unlabeled doping substances in dietary supplements that are used in sports. Methodology: A review of substances/metabolites/markers banned by WADA in ergonutritional supplements was completed using PubMed. The inclusion criteria were studies published up until September 2017, which analyzed the content of substances, metabolites and markers banned by WADA. Results: 446 studies were identified, 23 of which fulfilled all the inclusion criteria. In most of the studies, the purpose was to identify doping substances in dietary supplements. Discussion: Substances prohibited by WADA were found in most of the supplements analyzed in this review. Some of them were prohormones and/or stimulants. With rates of contamination between 12 and 58%, non-intentional doping is a point to take into account before establishing a supplementation program. Athletes and coaches must be aware of the problems related to the use of any contaminated supplement and should pay special attention before choosing a supplement, informing themselves fully and confirming the guarantees offered by the supplement.

Elucidating the unintentional p-type nature of spinel Co3O4: A defect study using ab-initio calculation

Co3O4 is one of the most widely used materials in energy and environmental field due to its unintentional p-type nature, which depends on the preparation conditions. In this study, we investigated the origin of the unintentional p-type conductivity of Co3O4 by calculating all possible intrinsic point defects. We found that the octahedral cobalt vacancy and tetrahedral cobalt vacancy are the sources of unintentional p-type doping. Using charge balance theory, we analyzed the effect of preparation condition on intrinsic defect-induced doping. In most of preparation condition, the formation of these cobalt vacancies plays a dominant role and the spontaneous formation of p-type doping is unavoidable. However, if there is ample oxygen and the temperature is low during the preparation, the unintentional p-type doping can be avoided. This theoretical work on defects provides a crucial clue to optimize Co3O4 for various electrochemical applications.

Determination of higenamine and coclaurine levels in human urine after the administration of a throat lozenge containing Nandina domestica fruit.

Higenamine is a key component of traditional Chinese herbal medicine. The fruit of Nandina domestica (which contains this component) is available as an ingredient in the so-called Nanten-nodo-ame throat lozenge found on the Japanese market, which is an over-the-counter pharmaceutical and is easy to purchase for Japanese athletes. However, higenamine is a non-selective β2-agonist, which is exemplified in the prohibited list of the World Anti-Doping Agency (WADA). Therefore, some have raised a concern regarding the potential cause of increased unintentional higenamine doping cases in the Asian region. This study aimed to investigate components of throat lozenges and develop a mass-spectrometry method for the quantification of higenamine and coclaurine in human urine. Moreover, a population study of Japanese subjects (n=246) and an excretion study (n=4) of the corresponding throat-lozenge recipients were performed to test the applicability of the current reporting threshold (i.e., 10ng/mL) of higenamine set by WADA. The estimates of higenamine and coclaurine were 2.2±0.1μg/drop (mean of n=12) and 0.5±0.01μg/drop (mean of n=12), respectively. The maximum concentrations of higenamine and coclaurine were 0.2-0.4 and 0.3-1.0ng/mL, respectively, at 10-12h after administration of higenamine (nine drops); however, the concentrations in all four volunteers did not reach the positivity criterion of 10ng/mL. No higenamine and coclaurine could be detected in the Japanese subjects. Therefore, there is no risk of detecting unintentional higenamine doping when the WADA reporting threshold is used.

(Invited) Material Challenges in Low Offcut Substrates for High Power SiC Devices

Silicon carbide is a material of interest for high-voltage and high-power switching device applications. Basal plane dislocations (BPDs) are a major concern for SiC bipolar devices as they source Shockley-type stacking faults in the presence of an electron-hole plasma and reduce minority carrier lifetimes [1, 2]. Many researchers have investigated methods to reduce the BPD density by experimenting with pre-growth treatments [3-5], substrate orientation [6], growth parameters [6, 7] and growth interrupts [8]. This work investigates extended defects, morphology and lifetime in 4H-SiC epilayers grown on substrates offcut 2° toward the [11-20].Epilayers were synthesized on 2° offcut substrates in a horizontal hot-wall reactor using the standard chemistry of silane (2% in H2) and propane. Epilayers were grown at various growth rates, C/Si ratios, and growth temperatures. The pressure was maintained at 100 mbar for all growths. Some samples were grown with a 5 µm highly doped n+ buffer layer using ultra high purity nitrogen prior to the low-doped epilayers. Ultraviolet photoluminescence (UVPL) imaging was used to identify BPDs in the low doped epilayers. Time resolved photoluminescence measurements were performed to determine the minority carrier lifetime of the layers and analysis of Raman spectroscopic maps revealed the location of polytype inclusions. Electron trap concentrations were determined using deep level transient spectroscopy (DLTS). Surface roughness was measured by atomic force microscopy and the morphology was also characterized using Nomarski microscopy and white light interferometry. When a 15 µm epilayer was grown without a buffer layer, step bunching was observed and the surface roughness was 6.0 nm RMS. For comparison, a standard 4° offcut sample typically has 3.0 nm RMS for a 20 µm epilayer. Using UVPL, it was found that after 4 µm of epi, 90% of the BPDs had converted in the low doped layer as compared to 70% in a 4° offcut sample, indicating the conversion is faster in the lower offcut material. The conversion results were from an older substrate and vendor A. For newer substrates, vendor B, the density of BPDs at the epilayer/substrate interface was ≤ 0.2 cm-2. 3C-SiC inclusions were present in the epilayers as verified using Raman spectroscopy for both unintentionally doped (UID) and N+ epilayers. These inclusions were eliminated by increasing the growth temperature and lowering the C/Si ratio for N+ epilayers, but by increasing C/Si ratios for UID films. Changing these growth parameters resulted in specular film morphology and resulted in minority carrier lifetimes of approximately 1 µs. [1] J.P. Bergman, et al. Mater. Sci. Forum Vol. 353-356, 299 (2001). [2] R.E. Stahlbush, et al., J. Electron. Mater. 31, 370 (2002). [3] Z. Zhang, et al., Appl. Phys. Lett. 89, 081910 (2006). [4] J.J. Sumakeris, et al., Mater. Sci. Forum 527-529, 529 (2006). [5] H. Tsuchida, et al., Mater. Sci. Forum 483-485, 97 (2005). [6] W. Chen and M.A. Capano J. Appl. Phys. 98, 114907 (2005). [7] T. Ohno, et al., J. Cryst. Growth 271, 1 (2004). [8] R. E. Stahlbush, et al., Jr., Appl. Phys. WeLett. 94, 041916 (2009).

GaN Lateral High Voltage Photoconductive Semiconductor Switches

Wide bandgap GaN photoconductive semiconductor switches (PCSSs) have gained recent attention due to high critical electric field strength and high electron saturation velocity to provide high power ultrafast devices. To reduce leakage currents, PCSSs have previously been demonstrated on semi-insulating GaN achieved by Fe compensation doping. However, Fe is known to have strong memory effects, which can redistribute Fe into subsequent films, as well as a narrow window of acceptable doping levels. PCSSs with fast response time are required for applications requiring high repetition rates. C-doped GaN is used to reduce leakage current and improve the photocurrent of lateral PCSSs. The C-doping is performed by simply tuning the growth parameters, such as temperature, pressure and III/V ratio and creates shallow accepter levels, compensating any unintentional n-type doping. In this work, we demonstrate high voltage GaN:C PCSSs with fast response times measured using a picosecond pulsed laser and high breakdown voltages, over 4000 V, measured in a vacuum probe station.

Effect of Alumina Addition on Li6.75La2.75Ca.25Zr1.5Nb.5O12 Lithium Garnet

Lithium-conducting garnets are a promising solid state electrolyte in the cubic phase for lithium (Li) batteries because of their stability with lithium, wide electrochemical stability window, and high conductivity (~1 mS cm-1). Aluminum (Al) is a widely studied dopant due to its ability to stabilize the cubic phase because of the occurrence of unintentional doping from alumina crucibles used during synthesis. The quantity of aluminum in the material is critically important to sintering conditions and performance in both the bulk and at the grain boundaries. Al can diffuse into the structure of garnets and dope the Li site while removing three total Li for charge balance. Another possibility is the Al reacts with excess lithium or lithium that has left the structure during high temperature processing. This allows for a liquid phase to form, enabling better densification during sintering. In this work, the aluminum content in Li6.75La2.75Ca.25Zr1.5Nb.5O12 was varied and calcined in magnesia crucibles to limit Al contamination. One weight percent of alumina (0.1 moles) was added before the solid state reaction, producing a conductivity of 2.96 x 10-4 S cm-1 after sintering at 1100°C. Specifically in the sample with 0.1 Al added when sintered at 1100°C for 12 hours, the Al contributed to an increase in grain boundary conductivity to 1.96x10-3 S cm-1 and a decrease in total activation energy to 0.35 eV versus samples without any added aluminum. The varying Al content showed little variance in bulk conductivity over several sintering conditions, while showing a wide divergence of 2.72x10-3 S cm-1, in grain boundary conductivity. Possible explanations are discussed with structural, chemical, and EIS measurements detailed.