Maximum Electron Concentration Research Articles

Carrier density distribution in AlGaAs/GaAs superlattices with different numbers of quantum wells determined by capacitance-voltage profiling

The charge carrier density distribution in uniformly doped AlGaAs/GaAs superlattices with layer thicknesses of 1.5/10 nm and different numbers of quantum wells was studied. Both energy band theory and capacitance–voltage profiling were used to determine the carrier concentration profiles of the structures. During the analysis of the experimental and simulated capacitance–voltage characteristics, it was found that the maximum electron concentrations increase with an increase in the number of quantum wells from ∼ 7.1⋅1016cm−3 for three wells to ∼ 9.3⋅1016cm−3 for 25 wells with an overall superlattice doping level of ∼ 1017cm−3.

Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Based on the medians of the F2-layer maximum electron concentration NmF2 for two ionosphericstations, Boulder and Hobart, in 1963–2013, an analysis of the dependence of the annual asymmetry localindex R at noon on solar activity has been performed, where the R index is the January/July ratio of the summaryNmF2 concentration for this pair of stations. Solar activity indices averaged over 81 days are used: Fobs isthe solar radio emission flux at a wavelength of 10.7 cm measured by ground-based radio telescopes and Fadjis the value of Fobs reduced to a fixed distance from the Sun of one astronomical unit. It is found that theregression equations that manifest the NmF2 medians dependence on Fobs make it possible to obtain theannual asymmetry index R for a fixed value of Fobs with allowance for a substitution of Fobs by cFobs in theseregression equations, the c coefficient is equal to 1.03 and 0.97 for January and July, respectively. The c = 1case corresponds to neglecting the annual asymmetry in the Fobs index due to the ellipticity of the Earth’sorbit. For the c = 1 version, the R index increases with a solar activity increase from 1.2 under low activity upto almost 1.4 under high activity. An additional allowance for the annual asymmetry in Fobs leads to anincrease in the R index by approximately 0.1 almost independently of the solar activity level. Apparently, thisconclusion is being drawn for the first time. The Fadj index also makes it possible to obtain a correct estimateof the R index, because the annual asymmetry in the solar radiation flux is indirectly taken into account viathe experimental values of NmF2.

Positive impurity size effect in degenerate Sn-doped GaN prepared by pulsed sputtering

This study reports on the epitaxial growth of heavily Sn-doped GaN films by pulsed sputtering deposition (PSD) and their basic characteristics, which include electrical, optical, and structural properties. Heavily Sn-doped GaN yielded a maximum electron concentration of 2.0 × 1020 cm−3 while keeping an atomically flat surface. The high electron concentration was confirmed by Raman spectroscopy measurements. X-ray diffraction analysis revealed that the Sn dopants exhibited a positive-size effect coefficient, which is opposite to conventional n-type dopants, such as Si and Ge. Furthermore, the shifts toward higher energy of optical bandgap energies and near-band edge emission peaks clearly indicated the highly degenerated nature of the PSD-grown Sn-doped GaN. These results indicate that the introduction of Sn atoms is quite promising for stress control in n-type GaN.

Determination of the electron distribution in thin barrier AlGaAs/GaAs superlattices by capacitance-voltage profiling

Electron density distribution in uniformly doped AlGaAs/GaAs superlattices with respective layer thicknesses 1.5/10 nm and a different number of quantum wells was investigated. Experimental samples containing 3, 5 and 25 periods with the same layer parameters were grown by molecular beam epitaxy. Capacitance-voltage profiling was used to determine the carrier concentration profiles in the structures both numerically and experimentally. During the analysis of experimental capacitance-voltage characteristics it was found that the maximum electron concentration increases with an increase in the number of quantum wells starting from 7,1∙1016 сm–3 for 3 wells up to 9,2∙1016 сm–3 for 25 wells with overall superlattice doping level of 1017 сm–3. In some samples saturation areas are observed on the concentration profiles, that are associated with the region of superlattice. Concentration values, obtained from computer modeling, correspond to the experimental data with an error of less than 10 %. Capacitance-voltage profiling is a suitable technique for determining the carrier concentration profiles in thin barrier superlattices. Despite the fact that the method provides distribution of the “apparent” carrier concentration profile, it can be used to estimate the dopant atoms distribution in the strongly coupled quantum well heterostructures.

Impact of Al-doping on structural, electrical, and optical properties of sol-gel dip coated ZnO:Al thin films

Aluminum-doped ZnO (AZO) thin films were coated on glass substrates using the sol-gel dip-coating technique. We investigated the effect of Al-doping level on the surface morphology, crystal structure, atomic bonding, and optical and electrical properties of the AZO films. The Al-doping levels in the sol-gel solution were 0, 0.25, 0.50, 1.0, 1.5, and 3.0% with a Zn precursor concentration of 0.50 M. The results show that the grain boundary increased with the doping level, while the crystallite size decreased from 27.2 to 14.2 nm. The AZO films were subjected to tensile stress. The Al-doping levels ≤ of 1.5% encouraged the intensity of the (002) x-ray diffraction peak. The center of the (002) peak shifted from 34.46° to 34.51°, and that of the Raman mode shifted from 435.0 to 432.4 cm−1. Doping with 1.5% Al resulted in a maximum electron concentration of 4.7 × 1018 cm−3 with a minimum resistivity of 2.6 × 10−1 Ω cm and a mobility of 5.14 cm2/V.s. The Urbach energy increased from 88 to 120 meV with increasing doping level. For the AZO films doped with Al (≤1.5%), the analysis of the UV–vis spectra reveals that the position of the conduction band (CB) minimum of the films shifted from −0.16 to −0.21 eV and shifted outward from the valence band (VB). Further Al doping to 3.0% resulted in a VB shift toward CB. The optical band gap reached a maximum of 3.33 eV at 1.5% Al. The combination of tensile stress and electron density due to Al doping influences the shift in the optical band gap of the AZO films. The crystal structure, atomic bonding, and electronic band structure of ZnO films can be modified by Al doping.

The optimization of n-type and p-type m-plane GaN grown on m-plane sapphire substrate by metal organic chemical vapor deposition

The properties of non-polar m-plane GaN layers grown on m-plane sapphire substrate using metal organic chemical vapor deposition at different disilane (Si2H6) and bis(cyclopentadienyl)magnesium (Cp2Mg) flow rates were investigated. The surface morphologies for Si- and Mg-doped of m-plane GaN exhibit root mean square of 1.22 and 2.95 nm, respectively. Using x-ray diffraction method, the epi-layer properties of doped m-GaN layers were studied emphasizing on the disilane and Cp2Mg flow rates towards the effect on threading dislocations and basal stacking faults. Room temperature Hall measurement indicate the maximum respective electron and hole concentration of ~1.88 × 1018 cm−3 and 6.50 × 1017 cm−3 were achieved at higher flow rate as well as the mobility of the electron. However, the mobility of hole was recorded to be low of ~5.2 cm2/V at higher Cp2Mg flow rates. The implementation of optimized n- and p-type contact layer demonstrate a 483 nm non-polar LED with light output power ~0.2 mW and forward voltage of 4.8 V at 20 mA current injection.

Experimental studies of the electrical and radiative characteristics of a channel surface discharge

Channel surface discharge (CSD) is a type of surface (sliding) discharges or dielectric barrier discharges in the incomplete stage. Experimental results of both incomplete (low-current) and high-current complete stages of the CSD are presented. The waveform of current pulse in the discharge incomplete stage as well as the influence of the voltage growth rate dU/dt on the discharge electrodes on waveform are studied. Experimental dependence of the CSD current pulse amplitude on the dU/dt rate is obtained in the incomplete stage over a wide range of variation of the dU/dt. For the CSD complete stage, the relation between the intensity of the discharge radiation in the UV region and the amplitude of the current pulse has been received. The dependence of the maximum electron concentration on the UV radiation intensity of the high-current stage CSD was obtained for the case when UV radiation from the CSD was used for ionization of atmospheric pressure nitrogen.

Complexes and compensation in degenerately donor doped GaN

Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions.

First Report of an Eclipse From Chilean Ionosonde Observations: Comparison With Total Electron Content Estimations and the Modeled Maximum Electron Concentration and Its Height

AbstractThe ionospheric responses to the total solar eclipse on 2 July 2019 over low latitudes in southern South America are presented. Ionosonde observations were used within the totality path at La Serena (LS: 29.9°S, 71.3°W) and at Tucumán (TU: 26.9°S, 65.4°W) and Jicamarca (JI: 12.0°S, 76.8°W), with 85% and 52% obscuration, respectively. Total electron content (TEC) estimations over the South American continent were analyzed. The ionospheric impact of the eclipse was simulated using the Sheffield University Plasmasphere‐Ionosphere Model (SUPIM) at the Instituto Nacional de Pesquisas Espaciais (INPE). The significant variability of the diurnal variations of the various ionospheric characteristics over equatorial and low latitudes on geomagnetically quiet days makes it difficult to unambiguously determine the ionospheric responses to the eclipse. Nonetheless, some specific issues can be derived, mainly using simulation results. The E and F1 layer critical frequencies and densities below 200 km are found to consistently depend on decreasing solar radiation. However, the F1 layer stratification observed at both TU and LS cannot be related to the eclipse or other processes. The F2 layer does not follow the changes in direct solar radiation during the eclipse. The SUPIM‐INPE‐modeled F region critical frequency and TEC are overestimated before the eclipse at LS and particularly at TU. However, these overestimations are within the observed large day‐to‐day variability. When an artificial prereversal enhancement is added, the simulations during the eclipse better reproduce the observations at JI, are qualitatively better for LS, and are out of phase for TU. The simulations are consistent with conjugate location effects.

Saturation of electrically activated Sb concentration in heavily Sb-doped n+-Ge1−xSnx epitaxial layers

We investigated the key factors dominating the maximum doping concentration of Sb in heavily Sb-doped Ge1−xSnx epitaxial layers prepared by non-thermal equilibrium molecular beam epitaxy. First, we found that the Sb-doped Ge layer showed a maximum electron concentration of 1020 cm−3 after low-temperature growth without surface degradation, whereas further Sb-doping caused Sb segregation and/or precipitation with a decrease in electron concentration. Next, we analyzed Sb chemical bonding state and showed that the concentration of electrically activated Sb atom, Sb1+, was saturated to approximately 1020 cm−3 and further doping increased Sb0 concentration. To analyze Sb1+ saturation, we simulated the theoretical relationship between ionized-donor and total-donor concentrations and showed that the saturation tendency of Sb1+ is due to the inevitable generation of an unionized substitutional donor, which occurs before Sb segregation and/or precipitation. The results of this study are significant for the design of heavily doped semiconductors.

Thermoelectric properties of La- and Sc-doped Mg3Sb2 synthesized via pulsed electric current sintering

High-efficiency thermoelectric materials that convert heat into electricity are considered as a countermeasure against greenhouse gas emissions and global warming. In this regard, Mg3Sb2-based Zintl compounds, such as Mg3(Sb, Bi)2 and Mg3Sb2–Mg3Bi2, are inexpensive yet suitable candidates in the medium-temperature range due to low-toxicity and relative abundance of their constituent elements. However, n-type Mg3Sb2 without Bi possesses superior oxidation resistance. This study aimed to investigate the suitability of Sc and La as candidates for n-type Mg3Sb2 dopants. La-doped and Sc-doped Mg3Sb2 (Mg3Sb2Mx [M = La, Sc]; x = 0.000 to 0.05) were synthesized by a one-step, rapid pulsed electric current sintering using Mg, Sb, and a small amount of hydrate (La(OH)3) or oxide (Sc2O3). Electron backscatter diffraction revealed that the average grain sizes for the La-doped (x = 0.03) and Sc-doped (x = 0.05) Mg3Sb2 were 17 and 38 μm, respectively. The room temperature maximum electron concentration for the La-doped and Sc-doped Mg3Sb2 increased up to 2.3 × 1019 and 3.7 × 1019 cm−3, respectively, which is comparable to or higher than the maximum values obtained for the reported n-type Te-doped Mg3Sb2 (approximately 2 × 1019 cm−3). The maximum dimensionless thermoelectric figure of merit values for the La-doped and Sc-doped Mg3Sb2 at 770 K were 0.93 and 0.80, respectively, indicating that Sc and La are promising doping candidates for achieving higher ZT values in n-type Mg3Sb2 without Bi.

Electron Concentration Limit in Ge Doped by Ion Implantation and Flash Lamp Annealing.

Controlled doping with an effective carrier concentration higher than 1020 cm−3 is a key challenge for the full integration of Ge into silicon-based technology. Such a highly doped layer of both p- and n type is needed to provide ohmic contacts with low specific resistance. We have studied the effect of ion implantation parameters i.e., ion energy, fluence, ion type, and protective layer on the effective concentration of electrons. We have shown that the maximum electron concentration increases as the thickness of the doping layer decreases. The degradation of the implanted Ge surface can be minimized by performing ion implantation at temperatures that are below −100 °C with ion flux less than 60 nAcm−2 and maximum ion energy less than 120 keV. The implanted layers are flash-lamp annealed for 20 ms in order to inhibit the diffusion of the implanted ions during the recrystallization process.

Enhanced Photoluminescence of Heavily Doped n-Ge/Si(001) Layers

The photoluminescence spectra of epitaxial n+-Ge:P/Si(001) structures are studied. The structures are grown by hot-wire chemical vapor deposition and doped with phosphorus to the maximum electron concentration 1 × 1020 cm–3 from a source based on thermally decomposed GaP. The effects of the doping level and rapid thermal annealing of n+-Ge:P layers on the photoluminescence spectra are studied. It is demonstrated that the epitaxial n+-Ge:P/Si(001) layers grown by hot-wire chemical vapor deposition are promising for application as active regions of light-emitting optoelectronic devices operating in the near-infrared spectral region.

Ionospheric Response to the Space Weather Events of 4-10 September 2017: First Chilean Observations

Objective: The diurnal variations of several ionospheric characteristics during the Space Weather Events of 4-10 September 2017, for Chilean latitudes, will be reported. Materials and Methods: Observations were made using a recently installed ionosonde at the Universidad de La Serena field station (29°52'S; 71°15’W). Also, reported is the total electron content determined using the upgraded Chilean network of dual-frequency Global Navigation Satellite Systems (GNSS) receivers. Results: Sudden ionospheric disturbances are described in terms of the minimum reflection frequency determined from ionosonde records. An attempt to derive the extent of the effect on high frequency propagation paths in the region is made using simultaneous ionosonde observations at other locations. The geomagnetic storm ionospheric effects are discussed in detail using the observed diurnal variation of maximum electron concentration (NmF2), virtual height of the F-region (h’F/F2) and Total Electron Content (TEC). These are complemented with the time-latitude variation of TEC for the 70°W meridian. Conclusion: It is found that large increases of NmF2, h’F/F2 and TEC observed during 8 September 2017 storm are well described in terms of the evolution of the Equatorial Ionospheric Anomaly (EIA) over the same time interval. Known physical mechanisms are suggested to explain most of the observations.

Усиленная фотолюминесценция сильно легированных слоев Ge/Si(001) n-типа проводимости

AbstractThe photoluminescence spectra of epitaxial n ^+-Ge:P/Si(001) structures are studied. The structures are grown by hot-wire chemical vapor deposition and doped with phosphorus to the maximum electron concentration 1 × 10^20 cm^–3 from a source based on thermally decomposed GaP. The effects of the doping level and rapid thermal annealing of n ^+-Ge:P layers on the photoluminescence spectra are studied. It is demonstrated that the epitaxial n ^+-Ge:P/Si(001) layers grown by hot-wire chemical vapor deposition are promising for application as active regions of light-emitting optoelectronic devices operating in the near-infrared spectral region.

Effects of nanoscale vacuum gap on photon-enhanced thermionic emission devices

A new model of the photon-enhanced thermionic emission (PETE) device with a nanoscale vacuum gap is established by introducing the quantum tunneling effect and the image force correction. Analytic expressions for both the thermionic emission and tunneling currents are derived. The electron concentration and the temperature of the cathode are determined by the particle conservation and energy balance equations. The effects of the operating voltage on the maximum potential barrier, cathode temperature, electron concentration and equilibrium electron concentration of the conduction band, and efficiency of the PETE device are discussed in detail for different given values of the vacuum gap length. The influence of the band gap of the cathode and flux concentration on the efficiency is further analyzed. The maximum efficiency of the PETE and the corresponding optimum values of the band gap and the operating voltage are determined. The results obtained here show that the efficiency of the PETE device can be significantly improved by employing a nanoscale vacuum gap.

Effect of Grid Boundary Expansion to Include One Additional Data Source on Ionospheric Imaging Accuracy

Four-dimensional ray tomography of ionospheric electron concentration using the Global Navigation Satellite System data is now a well-established technique. Since its advent, there have been a few studies of practical principles for optimizing crucial yet basic aspects of the problem for real experiments. For instance, optimal grid boundaries, voxel numbers, and voxel sizes must be determined case by case. This experiment examines the consequences of a small (<; 11%) increase in the number of voxels in a grid which has its locally horizontal boundary expanded to include one extra ground receiver station. Three different internal division definitions are used for each boundary, making six imaging runs in total. It is found that, when the maximum electron concentration of the ionospheric F-2 layer (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F2) parameter is compared with independent measurements from an Incoherent Scatter Radar (ISR) over a 12-month period, the expansion of the grid to include the extra data improves the accuracy of the imaging algorithm, regardless of which internal division definition is used. A convolution technique is used to obtain quantitative information about the differences between algorithm runs with different internal divisions and boundaries and the observed ISR values. Examination of the distribution of observed and reconstructed N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> F2 values shows that the imaging algorithm does not produce as many low-valued results as the observed data.

Behavior of dual ion beam sputtered MgZnO thin films for different oxygen partial pressure

Mg-doped ZnO (MgZnO) films were grown on p-Si (001) substrates by dual ion beam sputtering deposition system at a constant growth temperature of 600 °C for different oxygen partial pressure. The impact of oxygen partial pressure on the structural, electrical, elemental and morphological properties was thoroughly investigated. X-ray diffraction (XRD) spectra revealed that the deposited MgZnO films were polycrystalline in nature with preferred (002) crystal orientation. The peak of MgZnO (101) plane was reduced significantly as oxygen partial pressure was increased and disappeared completely at 80 and 100 % O2. The maximum electron concentration was evaluated to be 5.79 × 1018 cm−3 with resistivity of 0.116 Ω cm and electron mobility of 9.306 cm2/V s at room temperature, for MgZnO film grown with 20 % O2. Raman spectra shows a broad peak at 434 cm−1 corresponded to E 2 high phonons mode of MgZnO wurtzite structure. The peak at 560 cm−1 corresponded to the E1 (LO) mode and was associated with oxygen deficiency in MgZnO films. Raman intensity at 560 cm−1 reduced, on increasing oxygen partial pressure. A correlation between structural, electrical, elemental and morphological properties with oxygen partial pressure was also established.

Defects and stresses in MBE-grown GaN and Al0.3Ga0.7N layers doped by silicon using silane

The electric and structural characteristics of silicon-doped GaN and Al0.3Ga0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 1020 cm−3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al0.3Ga0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 1019 cm−3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al0.3Ga0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.

Maximum electron concentration and total electron content of the ionosphere over Concepción, Chile, prior to the 27 February 2010 earthquake

Ionospheric variability observed prior to mayor earthquakes has been studied for decades. In particular, in many such studies the identification of ionospheric precursors of large earthquakes has been regarded as a specific goal. This paper analyses the observations of the maximum electron concentration (NmF2) over Concepción (36.8°S;73.0°W) and of the total electron content (TEC) for an area covering the rupture zone corresponding to the very large Chile earthquake of 27 February 2010. The analyses used here are similar to those published before for many earthquakes in Taiwan, Japan and Russia. Possible NmF2 and TEC precursors are compared with other precursors proposed for the same earthquake using different TEC determinations and satellite observations of electron/ion concentration, energetic particle bursts and electromagnetic emissions. Some possible precursors derived from the various observations are consistent with each other. However, none can be unambiguously associated to the Chilean earthquake.