Majority Carrier Concentration Research Articles

Controlled Chemical Doping of Semiconductor Nanocrystals Using Redox Buffers

Semiconductor nanocrystal solids are attractive materials for active layers in next-generation optoelectronic devices; however, their efficient implementation has been impeded by the lack of precise control over dopant concentrations. Herein we demonstrate a chemical strategy for the controlled doping of nanocrystal solids under equilibrium conditions. Exposing lead selenide nanocrystal thin films to solutions containing varying proportions of decamethylferrocene and decamethylferrocenium incrementally and reversibly increased the carrier concentration in the solid by 2 orders of magnitude from their native values. This application of redox buffers for controlled doping provides a new method for the precise control of the majority carrier concentration in porous semiconductor thin films.

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10at.% to 40at.% and 0 to 50at.%, respectively. X-ray diffraction analysis showed the formation of SnO2 cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS2 and Sn2S3 in SnO2:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO2:Al (40at.%) film with 20at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 1017–1018cm−3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO2:Al (40at.%) thin films. The maximum Seebeck coefficient of +774μV/K (at T=370K) was obtained for S-doped SnO2:Al (10at.%) film with 50at.% S-doping. The band gap values were obtained in the range of 3.8–4.2eV. The S-doped SnO2:Al (40at.%) films have shown considerably photoconductivity more than S-doped SnO2:Al (10at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO2 thin film with 40at.% Al and 5at.% S concentration in solution.

Dimers as Fast Diffusing Species for the Aggregation of Oxygen in Boron-Doped Czochralski Silicon: Formation of New Thermal Donors

The thermal behaviors of oxygen-related complexes in boron doped Czochralski Silicon (Cz-Si) wafers at 450°C and 800°C were investigated using Fourier transform infrared spectroscopy (FTIR) and Hall mobility measurements. Activation of thermal donors (TDs) at 450°C leads to a decrease of both mobility and majority carrier concentration using the four point probes configuration of Van Der Pauw. It was found that annealing at 450°C would possibly affect the electronic properties of the Si wafers via the formation of interstitial dioxygen defects (IO2i), which exhibit an IR absorption band positioned at 545 cm–1. A strengthening of the IR bands peaking at around 1595 cm–1, 1667 cm–1, 1720 cm–1 and 1765 cm–1 occurs at 450°C, while they disappear at 800°C. At high temperatures, the precipitation of interstitial oxygen becomes predominant over all other oxygen-related reactions. The dynamic of oxygen-thermal donor generation-annihilation in Cz-Si involving the formation of small oxygen clusters is discussed.

Drive current of accumulation-mode p-channel SOI-based wrap-gated Fin-FETs

Comparisons are performed to study the drive current of accumulation-mode (AM) p-channel wrap-gated Fin-FETs. The drive current of the AM p-channel FET is 15%–26% larger than that of the inversion-mode (IM) p-channel FET with the same wrap-gated fin channel, because of the body current component in the AM FET, which becomes less dominative as the gate overdrive becomes larger. The drive currents of the AM p-channel wrap-gated Fin-FETs are 50% larger than those of the AM p-channel planar FETs, which arises from effective conducting surface broadening and volume accumulation in the AM wrap-gated Fin-FETs. The effective conducting surface broadening is due to wrap-gate-induced multi-surface conduction, while the volume accumulation, namely the majority carrier concentration anywhere in the fin cross section exceeding the fin doping density, is due to the coupling of electric fields from different parts of the wrap gate. Moreover, for AM p-channel wrap-gated Fin-FETs, the current in channel along (110) is larger than that in channel along (100), which arises from the surface mobility difference due to different transport directions and surface orientations. That is more obvious as the gate overdrive becomes larger, when the surface current component plays a more dominative role in the total current.

Relationship between the evolutions of the microstructure and semiconductor properties of yttrium-doped barium titanate ceramics

Intricate connections among the microstructural effect, semiconducting tendency and charge compensation behaviour of yttrium (Y3+) dopants in near-stoichiometric barium titanate (BaTiO3; Ba/Ti atomic ratio = 0.999) ceramics sintered at 1460 °C in air are examined. It is found that with increasing Y3+ doping up to 2.0 mol%, the microstructure of BaTiO3 evolves from a liquid-phase-assisted dense-sintered microstructure to a highly porous microstructure characterized by connected pores and loose lattices of fused submicrometre grains. During such evolution, a transitional microstructure characterized by large distinctive pores and grains with abnormal morphology is also identified. When Y3+ doping is increased progressively from 0.02 to 0.2 mol%, the (negative) majority carrier concentration and conductivity are increased substantially by 8 orders of magnitude. This increase in n-type semiconductor characteristics is contributed not only by the increasing substitution of Y3+ for Ba2+ in host BaTiO3, but also by the formation of yttrium-rich and/or oxygen-deficient precipitates at the grain boundaries. The grain boundary phases would therefore stabilize the mechanism of free electron compensation and enable the transportation of electrons through the grain boundaries. The measured Hall effect data indicate the shift from the n-type to p-type semiconductor properties with increasing Y3+ doping. The carrier mobilities of 1.0 and 2.0 mol% Y-doped BaTiO3 are high; this is attributed to their highly porous microstructures which provide easy diffusion paths for the charge carriers. Through a combined interpretation of the diffractometry, microscopy, mass spectrometry and Hall effect data, Y3+ doping at 1.0 mol% is found to be the critical doping amount separating different site-occupying behaviours of Y3+ in the BaTiO3 cation sites, which eventually lead to different charge compensation mechanisms and semiconductor properties.

Reduction in Majority-Carrier Concentration in Lightly-Doped 4H-SiC Epilayers by Electron Irradiation

The mechanisms for the reduction in the hole concentration in lightly Al-doped p-type 4H-SiC epilayers by electron irradiation as well as in the electron concentration in lightly N-doped n-type 4H-SiC epilayers by electron irradiation are investigated. In the p-type 4H-SiC epilayers, the temperature dependence of the hole concentration, , is not changed by 100 keV electron irradiation, while the is decreased by 150 keV electron irradiation. The density of Al acceptors with energy level eV decreases with increasing fluence of 150 keV electrons, whereas the density of deep acceptors with energy level eV increases. In the n-type 4H-SiC epilayers, the temperature dependence of the electron concentration, , is decreased by 200 keV electron irradiation. The density of N donors located at hexagonal C-sublattice sites decreases significantly with increasing fluence of 200 keV electrons, whereas the density of N donors located at cubic C-sublattice site decreases slightly.

Isochronal Studies of the Structural and Electrical Properties of CdTe Films

The paper reports the influence of annealing temperature under vacuum for one hour on the some structural and electrical properties of p-type CdTe thin films were grown at room temperature under high vacuum by using thermal evaporation technique with a mean thickness about 600nm. X-ray diffraction analysis confirms the formation of CdTe cubic phase at all annealing temperature. From investigated the electrical properties of CdTe thin films, the electrical conductivity, the majority carrier concentration, and the Hall mobility were found increase with increasing annealing temperatures.

Isochronal Studies of the Structural and Electrical Properties of CdTe Films

The paper reports the influence of annealing temperature under vacuum for one hour on the some structural and electrical properties of p-type CdTe thin films were grown at room temperature under high vacuum by using thermal evaporation technique with a mean thickness about 600nm. X-ray diffraction analysis confirms the formation of CdTe cubic phase at all annealing temperature. From investigated the electrical properties of CdTe thin films, the electrical conductivity, the majority carrier concentration, and the Hall mobility were found increase with increasing annealing temperatures.

Neutron irradiation-induced enhancement of electronic carrier transport in ZnO

Irradiation of ZnO single crystals by thermal neutrons with a dose up to 7×1017 cm−2 and subsequent annealing at 400 °C for 1 h leads to a significant increase in majority carrier mobility and concentration in this material, with the corresponding decrease of its sheet resistance. Additionally, cathodoluminescence spectra taken before and after neutron irradiation are consistent with the growing carrier lifetime. The observed effects are attributed to irradiation-induced formation of electrically active species of interstitial Zn and improvement of lattice crystallinity due to annealing.

Electro-analytical characterization of photovoltaic cells by combining voltammetry and impedance spectroscopy: voltage dependent parameters of a silicon solar cell under controlled illumination and temperature

This work reports a detailed electro-analytical framework for direct determination of a broad range of performance-indicator parameters of silicon solar cells. A mono-crystalline Si cell, equipped with the efficiency-boosting back surface field (BSF) feature, is used as a model system for this study. Independently controlled illumination (0–1 Sun, from a solar simulator) and temperature of the cell prevent the interference of photothermal and photovoltaic processes during the measurements. The technique of D.C. voltammetry is employed to obtain current–voltage plots, fill-factors, efficiencies and effective cell resistances. The same experimental platform also supports A.C. impedance spectroscopic probing of the solar cell, which, in combination with complex nonlinear least square analysis of the experimental data, provides detailed information about both the emitter–base and the BSF components of the photovoltaic device. These impedance measurements lead to straightforward determination of the diffusion and depletion layer capacitances, diode resistance, series resistance, concentration of majority carriers as well as lifetime of minority carriers in the base, resistance and capacitance of the BSF junction, and relaxation time of holes in the BSF. The results presented here demonstrate how relatively simple electro-analytical experiments can be strategically utilized for quantitative characterization of photovoltaic systems.

Simulation of Schottky and Ohmic contacts on CdTe

This study focuses on finite element method simulations of various contacts on high resistivity, “semi-insulating,” semiconductors in general, and detector grade CdTe, in particular. The results of this study confirm that the simplified approach assuming depletion approximation is not suitable for analyses of semi-insulating semiconductors. Furthermore, the space charge in such devices is usually dominated by the free charge carriers. The forward current in Schottky diodes may be much higher than expected from the bulk majority carrier concentration. It is shown that the minority current is often much higher than the majority current. These results indicate that a full computer simulation is vital in order to describe the microscopic conditions inside the device, as well as the macroscopic behavior of the device.

Interface-Trap Analysis by an Optically Assisted Charge-Pumping Technique in a Floating-Body Device

An optically assisted charge-pumping (CP) technique is proposed for the characterization of interface traps in floating-body (FB) devices. Even without a body contact, majority carriers can be supplied into the FB by light illumination, which contributes to enabling the CP process. Under a strong inversion enabled by a back gate, the front gate triggers the CP process with a designed pulse waveform. Consequently, modulation of the majority-carrier concentration at the front interface is monitored by the change of the drain current. Thus, the interface-trap density is extracted from the monitored drain current and the developed analytical model.

Thermoelectric properties of n-type CoSb 3 fabricated with high pressure sintering

Highly densified single-phase n-type polycrystalline CoSb 3 samples were prepared with high pressure sintering method followed by annealing under argon environment. The microstructure and thermoelectric properties of the samples were characterized. The Hall coefficient measurements indicate that the majority carrier concentration is about 10 19 cm −3 at 300 K for the annealed samples. The dimensionless figure of merit, ZT, maintains a relatively high value over a wide range of measured temperature. The maximum ZT of 0.15 is achieved at 523 K for the sample annealed at 773 K.

A First-Principles Study of the Role of Na Vacancies in the Thermoelectricity of Na x CoO2

First principles calculations using the generalized gradient approximation to density functional theory have been carried out to evaluate formation energies of defects and the resultant changes in electronic structure of NaCoO2 and Na0.5CoO2. The calculated formation energies confirm that Na vacancies form readily in this material, particularly through volatilization at elevated temperatures, consistent with experimentally observed behavior. Numerical analysis of the change in charge distribution upon Na vacancy formation shows that the vacancy plays a crucial role in modifying the electronic properties of the material. In these p-type thermoelectric materials, Na vacancies act as a reservoir for the minority carrier (electrons), removing them from the CoO2 layer while simultaneously increasing the concentration of majority carriers (holes) available for conduction in the CoO2 layer.

Charge trapping in detector grade thallium bromide and cadmium zinc telluride: Measurement and theory

Carrier trapping times were measured in detector grade thallium bromide (TlBr) and cadmium zinc telluride (CZT) from 300 to 110 K and the experimental data were analyzed using a trapping model. In CZT, because the majority carrier concentration is close to the intrinsic carrier concentration, the trapping time increases exponentially as the temperature decreases below about 160 K. In TlBr, the majority carrier concentration is many orders of magnitude greater than the intrinsic carrier concentration and the trapping time followed a (1/ T) 1/2 temperature dependence over the range of temperatures studied. The results of the model suggest that a moderately deep compensation center could be used to significantly increase the room temperature trapping time in TlBr.

Thermoelectric power measurements of wide band gap semiconducting nanowires

We investigated the temperature-dependent thermoelectric power (TEP) of individual wide band gap ZnO and GaN semiconducting nanowires by fabricating the devices with good Ohmic contacts. In the temperature range of 10–300 K, the measured TEP of both nanowires was linearly dependent on temperature, indicating the degenerate doping nature of these nanowires. The room temperature TEP value of ZnO nanowires was as high as −400 μV/K while an order of magnitude smaller TEP value was observed in GaN. The negative sign of TEP values shows that electrons are the majority carriers in these wide band gap nanowires. More importantly, in comparison with gate-dependent transport measurements of the nanowire field effect transistors, analysis of temperature-dependent TEP measurements provides a reliable way of estimating the majority carrier concentration of nanowires, where conventional Hall effect measurements cannot be used.

Minoritary Transport in Heavily Doped p-type InGaAs

AbstractMinoritary carrier’s transport properties are of fundamental importance in the HBT’s physics. The base transit time is a key parameter to improve microwave figure of merit. Some recent minoritary electron mobilities measurements versus acceptor doping level using magneto transport method exhibit a dramatic increase at very high majority carrier concentration. This effect has been attributed to the coupling of polar optical phonons with hole plasmons (LOPC) which controls the balance between enegy gain by electric field acceleration and energy loss by polar optical phonon emission. We present minoritary mobilities as a function of majority carrier doping calculated in the frame of electrons and holes Monte Carlo modelling including LOPC.

Dependence of the mechanism of current flow in the in-n-GaN alloyed ohmic contact on the majority carrier concentration

Based on the study of the temperature dependence of resistance of the In-n-GaN alloyed ohmic contacts, it is found that the mechanism of current flow in them substantially depends on the concentration N of uncompensated donors in GaN. At N = 5 × 1016 − 1 × 1018 cm−3, current mainly flows along the metallic shunts, and at N ⩾ 8 × 1018 cm−3 it flows by tunneling.

Investigation of electrical properties of nanostructured carbon films derived from block copolymers

Electrical properties of nanostructured carbon (ns-C) films fabricated by pyrolysis of PAN–b–PBA copolymers were investigated. Films having cylindrical morphology and pyrolyzed at 400, 500 and 600 °C were investigated. Both carbide forming (Zr, Ti) and non-carbide forming (Cu, Pt) metals spanning a wide range of electron work functions (4.1–5.5 eV) formed ohmic contacts to the ns-C films in the as-deposited state. The conductivity of the ns-C films increased roughly three orders of magnitude for every 100 °C increase in the pyrolysis temperature. Hall-effect measurements showed that the films pyrolyzed at 600 °C were n-type with a majority carrier concentration and mobility of 5.8 × 10 18 cm −3 and 0.97 cm 2/V s, respectively. Current–voltage measurements as a function of temperature ( I– V– T) were performed on films pyrolyzed at 600 °C, whereas films pyrolyzed at 400 and 500 °C were too resistive for reliable resistivity–temperature and Hall-effect measurements. The resistivity as a function of temperature was analyzed by using the reduced activation energy method and was determined to follow variable-range hopping (VRH) mechanisms at and below room temperature. The data indicates a crossover from Efros–Shklovskii VRH [J. Phys. C 8, (1975) L49] to Mott VRH [J. Non-Cryst. Solids 1, (1968) 1] at temperatures above 100 K.

Characterization of Intrinsic Defects in High-Purity High-Resistivity p-Type 6H-SiC

The densities, cross sections, and energy levels of intrinsic defects in high-purity high-resistivity (approximately 106 Ω cm) p-type 6H-SiC are determined using isothermal capacitance transient spectroscopy (ICTS). Five intrinsic defects are detected ranging from 0.76 to 1.35 eV above the valence band. Since the sum of the densities of intrinsic defects detected is the same order of magnitude as the acceptor density in the p-type 6H-SiC, the intrinsic defects are found to decrease the majority-carrier concentration making its resistivity as high as approximately 106 Ω cm.