N-type Carrier Concentration Research Articles

Oxygen nonstoichiometry and defect chemistry of perovskite-type Ca0.25Sr0.75Fe0.75Mo0.25O3−δ

The oxygen content in mixed-conducting perovskite-type Ca0.25Sr0.75Fe0.75Mo0.25O3−δ was measured as a function of oxygen partial pressure varying in the range 10−21–0.5 atm at temperature 750–950 °C by coulometric titration technique. The experimental data were satisfactorily simulated with a defect equilibrium model involving reactions of iron oxidation, charge disproportionation on Fe sites, and electron exchange between iron and molybdenum. A comparison of the obtained results with similar data for SrFe0.75Mo0.25O3−δ has shown that calcium substitution provides no observable effect on standard enthalpy and entropy of disproportionation and electron exchange reactions, but decrease standard enthalpy and increase standard entropy of the oxidation reaction, resulting in an almost tenfold decrease in the respective equilibrium constant. A decrease in the concentration of p-type carriers and an increase in the concentration of n-type carriers were revealed in response to calcium substitution for strontium in SrFe0.75Mo0.25O3−δ.

Effect of Ag incorporation on structural and opto-electric properties of pyrolized CdO thin films

We have investigated the structure, morphology and opto-electric properties of CdO and Ag-incorporated CdO thin films prepared by chemical spray pyrolysis (CSP) method. The X-ray diffraction (GIXRD) study confirms that CdO samples are highly polycrystalline with cubic structure. The AFM study shows smooth surfaces both for CdO and Ag-CdO thin films. The direct band gap energy varies from 2.25 to 2.50 eV depending on Ag content in the films. Urbach energy reversal in optical band gap indicates that several localized states are present above Fermi level or near at the conduction band. The optical properties, such as refractive index, optical conductivity and nonlinear optical susceptibility, are found dependent on Ag content. A combined effect on the transport properties has been observed with the incorporation of Ag in CdO. The metal-like electric conduction of CdO film has been removed up to a temperature Tc for Ag doping. The n-type carrier concentration of CdO is reduced with increasing Ag content and the concentration is of the order of ~ 1020 cm−3.

Analysis of Carrier Transport in n-Type Hg1−xCdxTe with Ultra-Low Doping Concentration

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for obtaining high performance infrared detectors. Characterizing carrier transport in materials with ultra low doping (ND = 1014 cm−3 and lower), and multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional methods. In this work, Hall effect measurements with a swept B-field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis to analyze multi-layered MCT samples. Low temperature measurements (77 K) were able to identify multiple carrier species, including an epitaxial layer (x = 0.2195) with n-type carrier concentration of n = 1 × 1014 cm−3 and electron mobility of μ = 280000 cm2/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).

Carrier concentration dependent photoluminescence properties of Si-doped InAs nanowires

We report the effects of intentional n-type doping on the photoluminescence (PL) properties of InAs nanowires (NWs). Employing silicon (Si) as a dopant in molecular beam epitaxy grown NWs, the n-type carrier concentration is tuned between 1 × 1017 cm−3 and 3 × 1018 cm−3 as evaluated from Fermi-tail fits of the high-energy spectral region. With the increasing carrier concentration, the PL spectra exhibit a distinct blueshift (up to ∼50 meV), ∼2–3-fold peak broadening, and a redshift of the low-energy tail, indicating both the Burstein-Moss shift and bandgap narrowing. The low-temperature bandgap energy (EG) decreases from ∼0.44 eV (n ∼ 1017 cm−3) to ∼0.41 eV (n ∼ 1018 cm−3), following a ΔEG ∼ n1/3 dependence. Simultaneously, the PL emission is quenched nearly 10-fold, while the pump-power dependent analysis of the integrated PL intensity evidences a typical 2/3-power-law scaling, indicative of non-radiative Auger recombination at high carrier concentrations. Carrier localization and activation at stacking defects are further observed in undoped InAs NWs by temperature-dependent measurements but are absent in Si-doped InAs NWs due to the increased Fermi energy.

Electronic band structure and carrier concentration of formamidinium–cesium mixed cation lead mixed halide hybrid perovskites

The electronic structure of hybrid perovskite compositions of FA0.83 Cs0.17 PbI3−xBrx (x = 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5) is determined using ultraviolet photoelectron spectroscopy (UPS) and UV–Vis–NIR absorption spectroscopy. With the help of UPS, ionization potential and Fermi energy are determined, and using absorption measurements, bandgap values are obtained. It is observed that for FA0.83 Cs0.17 PbI3−xBrx, as the Br content increases, the bandgap increases. The UPS measurements confirm the n-type nature of all compositions. Additionally, the Hall measurements were carried out for the selected compositions and the n-type carrier concentrations were determined.

Enhancement of Thermoelectric Properties in n-Type Cu0.01Bi2Te2.3+xSe0.7 (0 ≤ x ≤ 0.7) Compounds with Te-Excess

The fine control of antisite defects for Bi2Te3-based materials is necessary to improve their thermoelectric performance using the optimization of a carrier concentration. In this work, we attempted to tune the n-type carrier concentration by forming antisite TeBi defects under a Te-rich condition for Cu0.01Bi2Te2.3+xSe0.7 samples (0 ≤ x ≤ 0.7). The electrical resistivity decreases with increasing the amount of excess Te in the sample of Cu0.01Bi2Te2.3+xSe0.7, which is originated from the increase in the electron carrier concentration for the Te-excess samples. The highest power factor of 2.72 mW/m K2 is obtained at 323 K for Cu0.01Bi2Te2.4Se0.7, which is enhanced by ~ 20% compared to the x = 0 sample. The highest ZT of 0.92 is achieved at 473 K for Cu0.01Bi2Te2.4Se0.7, which is 11% higher than that of x = 0 sample (ZT = 0.83). We demonstrate that the optimization of n-type carrier concentration by forming antisite TeBi defects in n-type Bi2Te3-based materials should be effective for enhancing their thermoelectric performance.

Correlated Chemical and Electrically Active Dopant Analysis in Catalyst-Free Si-Doped InAs Nanowires.

Direct correlations between dopant incorporation, distribution, and their electrical activity in semiconductor nanowires (NW) are difficult to access and require a combination of advanced nanometrology methods. Here, we present a comprehensive investigation of the chemical and electrically active dopant concentrations in n-type Si-doped InAs NW grown by catalyst-free molecular beam epitaxy using various complementary techniques. N-type carrier concentrations are determined by Seebeck effect measurements and four-terminal NW field-effect transistor characterization and compared with the Si dopant distribution analyzed by local electrode atom probe tomography. With increased dopant supply, a distinct saturation of the free carrier concentration is observed in the mid-1018 cm-3 range. This behavior coincides with the incorporated Si dopant concentrations in the bulk part of the NW, suggesting the absence of compensation effects. Importantly, excess Si dopants with very high concentrations (>1020 cm-3) segregate at the NW sidewall surfaces, which confirms recent first-principles calculations and results in modifications of the surface electronic properties that are sensitively probed by field-effect measurements. These findings are expected to be relevant also for doping studies of other noncatalytic III-V NW systems.

Structural features and high-temperature transport in SrFe0.7Mo0.3O3−δ

The complex oxide SrFe0.7Mo0.3O3−δ was obtained by combustion of the organometallic precursor in air followed by annealing in an argon flow at 1350°C, and characterized with the help of X-ray and electron diffraction methods. Oxygen nonstoichiometry and electrical conductivity data were collected in the oxygen partial pressure range from 10–19 to 0.5atm at temperatures 750–950°C. The as-prepared single phase oxide SrFe0.7Mo0.3O3−δ with the cubic double perovskite structure (SG Fm3m) is shown to undergo a structural transition to the tetragonal double perovskite phase (SG I4mmm) in the result of reducing treatment at pO2 = 10−12atm and 950°C. The ordered phases are characterized by a strong anti-site disordering of iron and molybdenum and nearly zero long-range ordering parameter. The maximal concentration of n-type carriers is about four times larger than of p-type carriers in the studied limits of oxygen pressure and temperature. The mobility of p-type carriers is found to vary within ~ 0.02–0.03cm2V−1s−1 with the migration energy of about 0.4eV, while the n-type mobility being approximately twice higher does not practically depend on temperature. Such features as good electrical conductivity, which can rise up to 40Scm−1 in reducing conditions and a considerable amount of oxygen vacancies favorable for fast oxygen ion transport are beneficial for application of SrFe0.7Mo0.3O3−δ as anode material in SOFCs and oxygen membrane for hydrogen generation by a water splitting.

Oxygen nonstoichiometry and defect equilibria in La0.49Sr0.5–xBaxFeO3–δ

High level of mixed-conductivity and good stability of oxide La0.5Sr0.5FeO3–δ make it promising for the development of functional materials for such energy-related applications as solid oxide fuel cells and membrane reactors for partial oxidation of methane. The oxygen content in La0.49Sr0.5−xBaxFeO3–δ (x = 0, 0.25, 0.5) was measured by means of coulometric titration in the interval of partial oxygen pressure between 10−20 and 0.5 atm at temperatures 750–950 °C. The obtained results were employed for defect structure analysis of oxides. An ideal solution model enabled satisfactory simulation of experimental data for barium-free oxide. A reasonable data description for barium-containing oxides was achieved in the model, assuming that some of the oxygen vacancies are unavailable for oxygen ions. Partial substitution of barium for strontium was found to decrease the oxidation reaction constant of iron ions, resulting in a raised concentration of n-type electronic charge carriers and a reduced concentration of p-type carriers.

Thermoelectric properties of n and p-type cubic and tetragonal [formula omitted] (X = Ba,Pb): A density functional theory study

Thermoelectric properties of cubic (C) and tetragonal (T) BaTiO3 (BTO) and PbTiO3 (PTO) are investigated using density functional theory together with semiclassical Boltzmann's transport theory. Both electron and hole doped BTO and PTO are considered in 300–500K temperature range. We observed that C-BTO has larger power factor(PF) when doped with holes, whereas n-type carrier concentration in C-PTO has larger PF. Comparing both BTO and PTO, C-PTO has larger figure of merit ZT. Tetragonal distortion reduces the Seebeck coefficient S in n-doped PTO, and the electronic structures revealed that such reduction in S is mainly caused by the increase in the optical band gaps (Γ−Γ and Γ-X).

Temperature-dependent conduction mechanism of vertically aligned graphene nanoflakes incorporated with nitrogen in situ

Carbon nanostructured materials have been widely investigated and explored all over the world. However, the frontiers of applications and basic examinations related to these materials have yet to be opened in terms of their stability and robustness in the required environment. I report the temperature-dependent transport properties of vertically aligned graphene nanoflakes (GNFs) at room temperature to 800 K. Investigation of GNFs incorporated with nitrogen (N2) of varying concentration in situ and their possible conduction mechanism has been carried out over the entire range of temperatures mentioned above. N-type conductivity, carrier concentration, mobility and modulation at various temperatures are observed by means of Hall-effect measurements. The film of GNFs incorporated with nitrogen in situ persists in a linear trend, satisfying conduction behaviour expectations for all measured transport parameters with only a small discrete-point anomaly. Supporting evidence of N2 incorporation and structural modifications is studied by scanning electron microscopy, Raman spectroscopy and x-ray photoemission spectroscopy. The observation findings also provide the thermal stability of N2 incorporated into two-dimensional vertically standing GNFs.

Carrier Lifetimes of Iodine-Doped CdMgTe/CdSeTe Double Heterostructures Grown by Molecular Beam Epitaxy

Iodine-doped CdMgTe/CdSeTe double heterostructures (DHs) have been grown by molecular beam epitaxy and studied using time-resolved photoluminescence (PL), focusing on absorber layer thickness of 2 μm. The n-type free carrier concentration was varied to ∼7 × 1015 cm−3, 8.4 × 1016 cm−3, and 8.4 × 1017 cm−3 using iodine as dopant in DHs. Optical injection at 1 × 1010 photons/pulse/cm2 to 3 × 1011 photons/pulse/cm2, corresponding to initial injection of photocarriers up to ∼8 × 1015 cm−3, was applied to examine the effects of excess carrier concentration on the PL lifetimes. Iodine-doped DHs exhibited an initial rapid decay followed by a slower decay at free carrier concentration of 7 × 1015 cm−3 and 8.4 × 1016 cm−3. The optical injection dependence of the carrier lifetimes for DHs was interpreted based on the Shockley–Read–Hall model. The observed decrease in lifetime with increasing n is consistent with growing importance of radiative recombination.

Compensation of native donor doping in ScN: Carrier concentration control and p-type ScN

Scandium nitride (ScN) is an emerging indirect bandgap rocksalt semiconductor that has attracted significant attention in recent years for its potential applications in thermoelectric energy conversion devices, as a semiconducting component in epitaxial metal/semiconductor superlattices and as a substrate material for high quality GaN growth. Due to the presence of oxygen impurities and native defects such as nitrogen vacancies, sputter-deposited ScN thin-films are highly degenerate n-type semiconductors with carrier concentrations in the (1–6) × 1020 cm−3 range. In this letter, we show that magnesium nitride (MgxNy) acts as an efficient hole dopant in ScN and reduces the n-type carrier concentration, turning ScN into a p-type semiconductor at high doping levels. Employing a combination of high-resolution X-ray diffraction, transmission electron microscopy, and room temperature optical and temperature dependent electrical measurements, we demonstrate that p-type Sc1-xMgxN thin-film alloys (a) are substitutional solid solutions without MgxNy precipitation, phase segregation, or secondary phase formation within the studied compositional region, (b) exhibit a maximum hole-concentration of 2.2 × 1020 cm−3 and a hole mobility of 21 cm2/Vs, (c) do not show any defect states inside the direct gap of ScN, thus retaining their basic electronic structure, and (d) exhibit alloy scattering dominating hole conduction at high temperatures. These results demonstrate MgxNy doped p-type ScN and compare well with our previous reports on p-type ScN with manganese nitride (MnxNy) doping.

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd1−x Mg x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 1018 cm−3 for CdTe and 3 × 1017 cm−3 for Cd0.65Mg0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd0.65Mg0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 1018 cm−3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 1016 cm−3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.

Electrical properties of zinc oxide – Tetracene heterostructures with different n-type ZnO films

Electrical properties of organic-inorganic p–n heterojunction structures with tetracene (Tc) and zinc oxide (ZnO) films were investigated. The ZnO films had different n-type carrier concentrations that varied from ∼1015 cm−3 to 1019 cm−3. Lower n-type ZnO layers resulted in decreased reverse currents in the ZnO:Al/ZnO/Tc/Au structures and in an improvement of their asymmetric properties. Experimentally determined energy level alignments at the ZnO/Tc interfaces were related to the electrical behavior of the structures. An improved rectification was associated with decreased generation-recombination currents at the ZnO/Tc interface due to an increased organic-inorganic interface energy gap. Current-voltage characteristics were analyzed by a differential approach. Electrical conduction mechanisms including bimolecular recombination as well as trap-filled limited conduction were identified in the investigated structures.

Codeposition of amorphous zinc tin oxide using high power impulse magnetron sputtering: characterisation and doping

Thin film zinc tin oxide (ZTO) has been energetically deposited at 100 °C using high power impulse magnetron sputtering (HiPIMS). Reactive co-deposition from Zn (HiPIMS mode) and Sn (DC magnetron sputtering mode) targets yielded a gradient in the Zn:Sn ratio across a 4-inch diameter sapphire substrate. The electrical and optical properties of the film were studied as a function of composition. As-deposited, the films were amorphous, transparent and semi-insulating. Hydrogen was introduced by post-deposition annealing (1 h, 500 °C, 100 mTorr H2) and resulted in significantly increased conductivity with no measurable structural alterations. After annealing, Hall effect measurements revealed n-type carrier concentrations of ∼1 × 1017 cm−3 and mobilities of up to 13 cm2 V−1 s–1. These characteristics are suitable for device applications and proved stable. X-ray photoelectron spectroscopy was used to explore the valence band structure and to show that downward surface band-bending resulted from OH attachment. The results suggest that HiPIMS can produce dense, high quality amorphous ZTO suitable for applications including transparent thin film transistors.

Few-Layer Nanosheets of n-Type SnSe2.

Layered p-block metal chalcogenides are renowned for thermoelectric energy conversion due to their low thermal conductivity caused by bonding asymmetry and anharmonicity. Recently, single crystalline layered SnSe has created sensation in thermoelectrics due to its ultralow thermal conductivity and high thermoelectric figure of merit. Tin diselenide (SnSe2 ), an additional layered compound belonging to the Sn-Se phase diagram, possesses a CdI2 -type structure. However, synthesis of pure-phase bulk SnSe2 by a conventional solid-state route is still remains challenging. A simple solution-based low-temperature synthesis is presented of ultrathin (3-5 nm) few layers (4-6 layers) nanosheets of Cl-doped SnSe2 , which possess n-type carrier concentration of 2×1018 cm-3 with carrier mobility of about 30 cm2 V-1 s-1 at room temperature. SnSe2 has a band gap of about 1.6 eV and semiconducting electronic transport in the 300-630 K range. An ultralow thermal conductivity of about 0.67 Wm-1 K-1 was achieved at room temperature in a hot-pressed dense pellet of Cl-doped SnSe2 nanosheets due to the anisotropic layered structure, which gives rise to effective phonon scattering.

First-principle study of hydrogenation on monolayer MoS2

The structural and electronic properties of hydrogenation on 1H-MoS2 and 1T-MoS2 have been systematically explored by using density functional theory (DFT) calculations. Our calculated results indicate an energetically favorable chemical interaction between H and MoS2 monolayer for H adsorption when increasing concentration of H atoms. For 1H-MoS2, single H atom adsorption creates midgap approaching the fermi level which increases the n-type carrier concentration effectively. As a consequence, its electrical conductivity is expected to increase significantly. For 1T-MoS2, H atoms adsorption can lead to the opening of a direct gap of 0.13eV compared to the metallic pristine 1T-MoS2.

Relationship between microstructure and electronic properties of energetically deposited zinc tin oxide

Thin films of amorphous n-type zinc tin oxide have been energetically deposited from a filtered cathodic vacuum arc at moderate temperatures. The characteristics of these films span a range suitable for semiconductor devices and transparent conducting oxide interconnects with carrier concentration and mobility dependent on local bonding. X-ray photoelectron spectroscopy (XPS) and electron diffraction have revealed that acceptor-like Sn(II) bonding in the films decreased with increasing growth temperature, resulting in higher n-type carrier concentrations. XPS and in situ Ar plasma treatment showed that downward surface band bending resulted from OH attachment. Persistent photoconductivity was attributed to the photoionization of oxygen vacancies.

Effects of MgO buffer annealing on optical and electrical quality of P-MBE grown ZnO films on c-sapphire

Zinc oxide (ZnO) has been attracting much attention because of its potential applications in photonic and optoelectronic devices. In this present study, we investigated the effect of MgO buffer annealing on the optical and electrical quality of P-MBE grown ZnO films on c-sapphire with MgO buffer layer. The optical quality was observed by low-temperature PL (photoluminescence) measurement in the near band edge emission region measured at 10K and at 77K. The emission line located at 3.368eV dominates the spectrum in both samples (ZnO with and without MgO buffer annealing) at 10K and 77K. This emission can be divided into two peaks, 3.367eV and 3.363eV and assigned as I2 (ionized donor bound excitons emission) and I4 (Hydrogen donor related emission), respectively. The relative intensity of these donor bound exactions to free exaction emission of the sample without MgO buffer annealing is greater than that of the sample with MgO buffer annealing. Comparison of the PL spectra of ZnO with and without annealing revealed that the intensity of free exciton emission from the sample with MgO buffer annealing is twice of that from the sample without annealing. We also found that the intensity of deep-level broad emission is reduced by about 1/3 by MgO-buffer annealing. Hence, the decrease of deep level emission intensity and the increase of free exciton emission intensity by annealing of MgO buffer corresponds to the reduction of defects of the ZnO film. The PL properties also suggest that there are fewer nonradiative recombination centers in ZnO layers with MgO buffer annealing than those in ZnO layers grown without MgO buffer annealing. The electrical quality was measured by room temperature Hall measurements. We found that the samples have a background n-type carrier concentration. The ZnO samples with MgO buffer annealing has a carrier concentration of 1.17×1017 cm-3 and Hall mobility of 120 cm2/V.s, while the ZnO sample without MgO buffer annealing has a carrier concentration of 2.63 × 1016 cm-3 and Hall mobility of 105 cm2/V.s. The improvement of electron mobility of ZnO films by MgO buffer annealing is due to a decrease in dislocation density. We conclude that annealing of MgO buffer layer increases the optical and electrical quality of the ZnO films. These results agree with the structural quality as observed by HRXRD.