Function Of Electron Concentration Research Articles

Electron transport of perovskite oxide BaSnO3 on (110) DyScO3 substrate with channel-recess for ferroelectric field effect transistors

We report an electron transport study of an La-doped perovskite oxide BaSnO3 thin film grown by molecular beam epitaxy on (110) DyScO3 as a function of electron concentration, by etching the film step-by-step with nanometer precision. Inductively coupled plasma-reactive ion etching with BCl3/Ar plasma is used for etching depth control. The local doping and electron density are experimentally determined after each etching step. The results show that the electron mobility is dominated by threading dislocations if the electron concentration is below 7.8 × 1019 cm−3, while ionized impurities and phonon scattering become more dominant at electron concentrations greater than 1.2 × 1020 cm−3. The charging state of thread dislocations is estimated to be 6.2. Furthermore, using the etch process to control the electron concentration and channel thickness, a gate-recessed ferroelectric field effect transistor is fabricated with 10 nm HfO2 as a gate dielectric. The device exhibits a saturation current of 29.9 mA/mm with a current on/off ratio of Ion/Ioff = 8.3 × 108 and a ferroelectric polarization charge density of 1.9 × 1013 cm−2. Under the forward gate bias sweep, the device operates in the enhancement mode with a threshold voltage of 3 V. Under the reverse gate sweeping bias, the device operates in the depletion mode with a threshold voltage of –1.5 V.

Superconducting dome in LaAlO3/SrTiO3 interfaces as a direct consequence of the extended s -wave symmetry of the gap

The two-dimensional electron gas (2DEG) at the LaAlO$_3$/SrTiO$_3$ interface exhibits gate tunable superconductivity with a domelike shape of $T_{\rm{C}}$ as a function of electron concentration. Here, we propose that the experimentally observed behavior can be explained as a direct effect of the dominant $extended$ $s$-$wave$ symmetry of the superconducting gap. Our results agree very well with the experimental data. As shown, neither the correlation effects nor the spin-orbit coupling influence significantly the physical picture of the paired state steaming out from our analysis.

Effect of antimony doping on the energy of optical transitions in n-Ge layers grown on Si (001) and Ge (001) substrates

Comparative studies of the bandgap narrowing in antimony doped Ge layers grown on Si(001) and Ge(001) substrates are reported. The doping level in Ge:Sb layers was varied in such a way as to obtain structures with both full and partial electrical activation of the impurity atoms. It was shown that the direct bandgap narrowing as a function of electron concentration can be fitted rather well by a root power dependence in Ge layers grown on both types of substrates. Taking into account the doping-induced deformation of Ge lattices and a careful determination of electron concentration made it possible to accurately distinguish the “structural” contribution to the bandgap narrowing caused by the embedding of large Sb atoms into the Ge matrix and the “electrical” one caused by the interaction of charge carriers. The presented results shed light on some optical properties of heavily and ultra-heavily doped n-Ge and, thus, can be useful for the development of Ge-based electronic or photonic devices.

Fermi energy and electron thermopower in quantum films of an asymmetric profile

The behavior of the Fermi energy and the electron thermoelectric power in an asymmetric quantum well of the Morse potential type as a function of electron concentration are studied. It is shown that the Fermi energy is a piecewise-linear function, and the electron thermoelectric power is an oscillating function. The analysis shows that the nature of these oscillations differs significantly from that in films of a parabolic and rectangular profile, namely, the oscillation period is a nonmonotonic function and reaches a maximum at some value of the electron concentration. The possibility of estimating the parameters of the film profile (quantum well) is proposed. A comparison with the experiment shows a qualitative agreement.

Detection of deuterium retention by LIBS at different background pressures

ITER plans foresee the quantitative diagnostics of fuel retention in reactor walls at near-atmospheric pressures. Using laser induced breakdown spectroscopy (LIBS) for this purpose assumes a reliable resolving of Balmer α-lines of hydrogen isotopes in spectra of plasma produced by focused laser radiation onto the target surface. To develop LIBS for quantitative diagnostics of fuel retention during the maintenance breaks of ITER, the effect of background gas pressure on the laser-induced plasma characteristics has been studied. The background pressure limits the expansion rate of plasma and as a result it leads to higher plasma concentrations. At the same time the limiting factor of the resolving of hydrogen isotope lines is the lines broadening by Stark effect, which is the function of electron concentration. The resolving of lines become possible recording spectra at longer delay times after the laser pulse. On the other hand, at longer delays the signal-to-noise ratio decreases. As a compromise, we found that at atmospheric pressure and at delay times >2000 ns, a fitting of Hα and Dα lines by Voigt contours allows a reliable discrimination of these lines.

Transport Properties of Graphene and Suspended Graphene with EMC: The Role of Various Scattering Mechanisms

The electronic transport properties of graphene and suspended (intrinsic) graphene sheets are studied using an ensemble Monte Carlo (EMC) technique. The combined scattering mechanisms that are taken into account for both cases are nonpolar optic and acoustic phonons, ionized impurity, interface roughness, and surface polar phonon scatterings. The effect of screening is also considered in the ionized impurity and surface polar phonon scatterings of electrons. A rejection technique is used in EMC simulations to account for the occupancy of the final states. Velocity-field characteristics of graphene and suspended graphene sheets are obtained using various values of acoustic deformation potential constants. The variation of electron mobility of graphene is studied as a function of electron concentration and its variation as a function of temperature are investigated for the case of suspended graphene. For the former case, the mobility increases with electron concentration first and after a certain value of electron concentration it begins to decrease, while for the latter case the mobility decreases almost linearly with temperature. The mobility results from EMC simulations are compatible with the existing experimental studies for the unsuspended graphene case.

Laser thermal effect on silicon nitride ceramic based on thermo-chemical reaction with temperature-dependent thermo-physical parameters

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

Effects of Band Nonparabolicity and Band Offset on the Electron Gas Properties in <i>InAs/AlSb</i> Quantum Well

One-band effective mass model is used to simulation of electron gas properties in quantum well. We calculate of dispersion curves for first three subbands. Calculation results of Fermi energy, effective mass at Fermi level as function of electron concentration are presented. The obtained results are good agreement with the experimental dates.

Transport properties and seebeck coefficient of the quasi-twodimensional electron gas in ALP quantum wells

We consider the mobility of a quasi-twodimensional electron gas in a GaP/AlP/GaP quantum well with a valley degeneracy g 1   for quantum well width L < Lc = 45.7 Å and a valley degeneracy of g 2   for quantum well width L > Lc = 45.7 Å. We calculate the mobility as a function of electron density for interface-roughness and impurity scattering with using different approximations for the local-field correction. In the case of zero temperature and Hubbard local-field correction our results reduce to those of [16]. We also study the dependence of resistivity on temperature and parallel magnetic field. The Seebeck coefficient as a function of electron concentration and quantum well width are also calculated.

Stress-induced anomalous shift of optical band gap in Ga-doped ZnO thin films: Experimental and first-principles study

In this work, highly c-axis oriented Ga-doped ZnO thin films have been deposited on glass substrates by RF magnetron sputtering under different sputtering times. The optical band gap is observed to shift linearly with the electron concentration and in-plane stress. The failure of fitting the shift of band gap as a function of electron concentration using the available theoretical models suggests the in-plane stress, instead of the electron concentration, be regarded as the dominant cause to this anomalous redshift of the optical band gap. And the mechanism of stress-dependent optical band gap is supported by the first-principles calculation based on density functional theory.

Anion-controlled passivation effect of the atomic layer deposited ZnO films by F substitution to O-related defects on the electronic band structure for transparent contact layer of solar cell applications

Anion-controlled chemistry in ZnO films is essential to obtain stable charge–carrier transport and to prevent degradation in the performance of the transparent contact layer in solar cells through passivation of O-related defects by F substitution. Therefore, in this work, the passivation effect of F doping in ZnO was confirmed by analysis for the chemical state of O in ZnO films using XPS surface analysis in the O 1s region. Furthermore, we investigated the effect of F doping in a ZnO matrix on the electronic structure of the resulting films as a function of electron concentration by optical band gap shift (Burstein–Moss theory) and near-band-edge transition (photoluminescence with Stokes shift theory) measurements in ZnO films with different F doping concentrations. The band gap narrowing effect was not significant in the F-doped ZnO films due to valence band perturbation from anion doping rather than conduction band perturbation. Finally, the band structure could be approximated by the relationship between experimental analysis and the formation of valence and conduction bands in ZnO orbitals.

The electronic transport of top subband and disordered sea in an InAs nanowire in the presence of a mobile gate

We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire (Rwire ∼ 30 kΩ) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasi-periodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demonstrate the influence of the tip-to-sample distance on the ability to locally affect the top subband electrons as well as the electrons in the disordered sea. Furthermore, we introduce a new method of detection of the subband occupation in an InAs wire, which allows us to evaluate the number of electrons in the conductive band of the wire.

Electrical transport, electrothermal transport, and effective electron mass in single-crystalline In2O3films

A comprehensive study of the room-temperature electrical and electrothermal transport of single-crystalline indium oxide (In${}_{2}$O${}_{3}$) and indium tin oxide (ITO) films over a wide range of electron concentrations is reported. We measured the room-temperature Hall mobility ${\ensuremath{\mu}}_{H}$ and Seebeck coefficient $S$ of unintentionally doped and Sn-doped high-quality, plasma-assisted molecular-beam-epitaxy-grown In${}_{2}$O${}_{3}$ for volume Hall electron concentrations ${n}_{H}$ from $7\ifmmode\times\else\texttimes\fi{}{10}^{16}$ cm${}^{\ensuremath{-}3}$ (unintentionally doped) to $1\ifmmode\times\else\texttimes\fi{}{10}^{21}$ cm${}^{\ensuremath{-}3}$ (highly Sn-doped, ITO). The resulting empirical $S({n}_{H})$ relation can be directly used in other In${}_{2}$O${}_{3}$ samples to estimate the volume electron concentration from simple Seebeck coefficient measurements. The mobility and Seebeck coefficient were modeled by a numerical solution of the Boltzmann transport equation. Ionized impurity scattering and polar optical phonon scattering were found to be the dominant scattering mechanisms. Acoustic phonon scattering was found to be negligible. Fitting the temperature-dependent mobility above room temperature of an In${}_{2}$O${}_{3}$ film with high mobility allowed us to find the effective Debye temperature (${\ensuremath{\Theta}}_{D}=700$ K) and number of phonon modes (${N}_{\mathrm{OPML}}=1.33$) that best describe the polar optical phonon scattering. The modeling also yielded the Hall scattering factor ${r}_{H}$ as a function of electron concentration, which is not negligible (${r}_{H}\ensuremath{\approx}1.4$) at nondegenerate electron concentrations. Fitting the Hall-scattering-factor corrected concentration-dependent Seebeck coefficient $S(n)$ for nondegenerate samples to the numerical solution of the Boltzmann transport equation and to widely used, simplified equations allowed us to extract an effective electron mass of ${m}^{*}=(0.30\ifmmode\pm\else\textpm\fi{}0.03){m}_{e}$ (with free electron mass ${m}_{e}$). The modeled mobility and Seebeck coefficient based on polar optical phonon and ionized impurity scattering describes the experimental results very accurately up to electron concentrations of ${10}^{19}$ cm${}^{\ensuremath{-}3}$, and qualitatively explains a mobility plateau or local maximum around ${10}^{20}$ cm${}^{\ensuremath{-}3}$. Ionized impurity scattering with doubly charged donors best describes the mobility in our unintentionally doped films, consistent with oxygen vacancies as unintentional shallow donors, whereas singly charged donors best describe our Sn-doped films. Our modeling yields a (phonon-limited) maximum theoretical drift mobility and Hall mobility of $\ensuremath{\mu}=190$ cm${}^{2}$/Vs and ${\ensuremath{\mu}}_{H}=270$ cm${}^{2}$/V$\phantom{\rule{0.16em}{0ex}}$s, respectively. Simplified equations for the Seebeck coefficient describe the measured values in the nondegenerate regime using a Seebeck scattering parameter of $r=\ensuremath{-}0.55$ (which is consistent with the determined Debye temperature), and provide an estimate of the Seebeck coefficient to lower electron concentrations. The simplified equations fail to describe the Seebeck coefficient around the Mott transition (${n}_{\mathrm{Mott}}=5.5\ifmmode\times\else\texttimes\fi{}{10}^{18}$ cm${}^{\ensuremath{-}3}$) from nondegenerate to degenerate electron concentrations, whereas the numerical modeling accurately describes this region.

Imaging ambipolar diffusion of photocarriers in GaAs thin films

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (1 1 1) by plasma-assisted MBE

The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics of a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements.

Thermoelectric transport properties of then-type impurity Al in PbTe

Because Tl and In are known to be resonant levels in IV--VI semiconductors, here we synthesize and electrically characterize lead telluride doped $n$-type with aluminum. The results show that Al behaves as a normal donor in PbTe, reaching a maximum electron concentration of 4 10${}^{19}$ cm${}^{\ensuremath{-}3}$. At 300 K, the thermopower, when plotted as function of electron concentration (the Pisarenko relation), follows the calculated line for the conduction band of PbTe, and no enhancement is observed that could indicate the presence of a resonant level.

Anisotropic Scattering of Elongated GaSb/GaAs Quantum Dots Embedded Near Two-Dimensional Electron Gas

Geometrically anisotropic GaSb/GaAs quantum dots elongated along [110] direction, with a lateral aspect ratio of 2-3, are embedded in the vicinity of AIGaAs/GaAs two-dimensional electron gas, using the molecular beam epitaxy by carefully controlling the Sb4 beam flux and As4 background pressure. The electron mobilities micro parallel, micro perpendicular parallel and perpendicular to the direction of dot elongation axis are measured as functions of electron concentration n(2D) at 4.2 K, respectively. As the increase of n(2D), the mobility discrepancy (micro parallel-micro perpendicular) is found to be getting larger. This mobility difference is believed to be due to the anisotropic scattering potential of the elongated GaSb quantum dots. Under Born approximation, we propose a model with the constant squared-barrier potential to calculate the n(2D) dependence of mobility and reasonable agreement is achieved between the experimental results and theoretical simulation.

Electron concentration effects on the Shastry-Sutherland phase stability in Ce2−xPd2+yIn1−zsolid solutions

The stability of a Shastry-Sutherland ShSu phase as a function of electron concentration is investigated through the field dependence of thermal and magnetic properties of the solid solution Ce_{2-x}Pd_{2+y}In_{1-z} in the antiferromagnetic branch. In these alloys the electronic (holes) variation is realized by increasing $Pd$ concentration. The AF transition T_M decreases from 3.5K to 2.8K as $Pd$ concentration increases from y=0.2 to y=0.4. By applying magnetic field, the ShSu phase is suppressed once the field induced ferromagnetic polarization takes over at a critical field B_{cr} which increases with $Pd$ content. A detailed analysis around the critical point reveals a structure in the maximum of the dM/dB derivative, which is related with incipient steps in the magnetization M(B) as predicted by the theory for the ShSu lattice. The crossing of M(B) isotherms, observed in ShSu prototype compounds, is also analyzed. The effect of $In$ substitution by $Pd$ is interpreted as an increase of the number of 'holes' in the conduction band and results in a unique parameter able to describe the variation of the magnetic properties along the studied range of concentration.

Differential Gain of Closely Spaced Energy States in Quantum Dashes

Simple quasi-equilibrium model for quantum dash (QDash) active material is derived for closely spaced energy states. The model is used to study the differential gain of Qdash for different key parameters. Expressions for the electron and the hole occupation probabilities as a function of electron concentration are derived. The derived analytical model shows excellent agreement with numerical simulation. The differential gain of Qdash active layer is calculated for different doping concentration and different electron energy separation between adjacent states. We find that when the electron energy states are widely separated, the differential gain can be slightly enhanced at low-energy detuning by doping the dashes by p-type doing. On the other hand, our calculations reveal that when the electron energy states are close to each other, doping the dashes by either n-type or p-type concentration will not enhance the differential gain.

Raman spectroscopic determination of electron concentration in n-type GaInAsSb

Phonon-plasmon coupled mode Raman spectra of n-type GaInAsSb were measured at room temperature as a function of electron concentration. A relatively simple spectral model for the electronic contribution to the dielectric function was evaluated to determine the electron concentration from the bulk coupled mode spectra. The electron concentration was determined from a Raman spectrum by minimizing the sum of the squared residuals between a measured and a simulated spectrum. The only two fitting parameters were the Fermi energy and a plasmon damping parameter. The electron concentrations determined from the fits to the Raman spectra were compared to the electron concentrations determined from single magnetic field Hall effect measurements that were corrected to account for carriers in two conduction band minima. Compared to the results obtained from the Hall effect measurements, the electron concentrations obtained using Raman spectroscopy were as much as ≈19% lower at low doping levels but not more than ≈1% higher at higher doping levels. At lower carrier concentrations, the deviations are attributed to limitations of the spectral model. At higher carrier concentrations, the two methods were in good agreement. However, given the known limitations of this relatively simple spectral model, this agreement may be fortuitous; i.e., elements of the spectral model that tend to increase the apparent carrier concentration may be offset by elements that decrease the apparent carrier concentration.