Free Hole Concentration Research Articles

Threshold characteristics of semiconductor lasers under conditions of violation of electroneutrality in quantum wells

The threshold characteristics of semiconductor lasers are studied theoretically when the electroneutrality in quantum wells is violated. It is shown that even with the infinitely large threshold concentration of the charge carriers of one sign in the wells, the minimum threshold concentration of the carriers of the opposite sign is nonzero. It is found that in InGaAs/GaAs/AlGaAs heterostructures emitting near the wavelength , in a wide range of values of the electron concentration in the wells the threshold concentrations of free electrons and holes in the waveguide region are small, the contribution of the recombination current in the waveguide region to the total threshold current is negligible and in the case of a single quantum well, the threshold current density is virtually constant, i.e., the violation of electroneutrality in the InGaAs/GaAs/AlGaAs structures with a single quantum well has almost no effect on the threshold current. In the structures with two or three wells the violation of electroneutrality manifests itself much stronger and can lead to either a decrease or an increase in the threshold current.

Study of the Electrical Properties of Monolayer MoS<sub>2</sub> Semiconductor

We present the study of the electrical properties of monolayer MoS2 in terms of semiconductor theory. The free electron and hole concentrations formulas in two-dimensional (2D) semiconductors have been developed based on three-dimensional (3D) semiconductors theory, and derived the intrinsic carrier concentration equation of 2D system. Using these equations, we simulated the intrinsic carrier concentration in monolayer MoS2 with temperature. The intrinsic carrier density in monolayer MoS2 increases exponentially with temperature, but it lows a few orders of magnitude than that of 3D semiconductor. It means that monolayer MoS2 based devices can operated at very high temperatures. Accordingly, the conductivity and resistivity were simulated for 2D MoS2, the former increases exponentially while the latter decreases with temperature or carrier concentration.

Exciton diffusion length and concentration of holes in MEH-PPV polymer using the surface voltage and surface photovoltage methods

Novelized method of the surface photovoltage (SPV) measurement convenient for evaluation of exciton diffusion length and thickness of the space charge region (SCR) in organic semiconductors is applied to poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) polymer. Exciton diffusion length and thickness of the SCR was found. The experiment is complemented by measurements of surface potential by the Kelvin probe force microscopy yielding the work function and concentration of free holes. The latter value is much lower than the concentration of ionized states determined from the thickness of the space charge region, which can be ascribed to the presence of traps.

Compensation of Native Defect Acceptors in Microcrystalline Ge and Si1-xGex Thin Films by Oxygen Incorporation: Electrical Properties and Solar Cell Performance

Undoped hydrogenated microcrystalline Ge (µc-Ge:H) films grown by plasma-enhanced chemical vapor deposition reveal high concentration of free holes (>1018 cm-3) when the films exhibit a high crystalline volume fraction. ESR and Hall-effect experiments suggest that the acceptor states arise from the native dangling bond defects at Ge crystalline grain boundaries. It is demonstrated that an intentional oxygen incorporation during the µc-Ge:H deposition reduces the hole concentration by two orders of magnitude. Furthermore, µc-Si1-xGex:H (x=0.1–0.3) alloy p–i–n solar cells show marked improvements in photocarrier collection properties upon oxygen incorporation into the i-layer in the order of 5×1018–1020 cm-3. These findings are explained by the effect of the compensation of the negatively charged Ge dangling bonds by oxygen donors.

Vanadium deep impurity level in diluted magnetic semiconductors Pb1 − x − y Sn x V y Te

The crystal structure, Sn and V distribution over the length of single-crystal ingots, and galvanomagnetic effects in low magnetic fields (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) in Pb1−x−y Sn x V y Te alloys (x = 0.05−0.21, y ≤ 0.015) are studied. It is shown that all the samples are single-phase, while the Sn and V concentrations exponentially increase from the beginning to the end of the ingots. Upon doping with V, a decrease in the concentration of free holes and a metal-insulator transition are found. They are related to the appearance of a deep impurity level of V in the band gap, electron redistribution between the level and the valence band, and pinning of the Fermi-level to the impurity level. The shift rate of the V level relative to the conduction band bottom is determined and a diagram of the reconstruction of the electronic structure of the Pb1 − x − y Sn x V y Te alloy upon varying the host composition is suggested.

Taming transport in InN

AbstractThe large electron affinity of InN, close to 6 eV and the largest of any III–V semiconductor, creates a strong driving force for native donor formation, both in the bulk and at surfaces and interfaces. Moreover, all InN surfaces, regardless of crystal orientation or doping, have been observed to have a surface accumulation layer of electrons, which interferes with standard electrical measurements. For these reasons, until recently, it was uncertain whether or not compensation by donor defects would prevent “real” p‐type activity (i.e., existence of sufficiently shallow acceptors and mobile holes). A coordinated experimental approach using a combination of electrical (Hall effect) and electrothermal (Seebeck coefficient) measurements will be described that allows definitive evaluation of carrier transport in InN. In Mg‐doped InN films, the sensitivity of thermopower to bulk hole conduction, combined with modeling of the parallel conducting layers (surface/bulk/interface), enables quantitative measurement of the free hole concentration and mobility. In undoped (n‐type) material, combined Hall and thermopower measurements, along with a considering of the scattering mechanisms, leads to a quantitative understanding of the crucial role of charged line defects in limiting electron transport.

Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films

In this work, we investigated structural, morphological, electrical, and optical properties from a set of ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ thin films grown by sulfurization of metallic precursors deposited on soda lime glass substrates coated with or without molybdenum. X-ray diffraction and Raman spectroscopy measurements revealed the formation of single-phase ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ thin films. A good crystallinity and grain compactness of the film was found by scanning electron microscopy. The grown films are poor in copper and rich in zinc, which is a composition close to that of the ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ solar cells with best reported efficiency. Electrical conductivity and Hall effect measurements showed a high doping level and a strong compensation. The temperature dependence of the free hole concentration showed that the films are nondegenerate. Photoluminescence spectroscopy showed an asymmetric broadband emission. The experimental behavior with increasing excitation power or temperature cannot be explained by donor-acceptor pair transitions. A model of radiative recombination of an electron with a hole bound to an acceptor level, broadened by potential fluctuations of the valence-band edge, was proposed. An ionization energy for the acceptor level in the range 29--40 meV was estimated, and a value of $172\ifmmode\pm\else\textpm\fi{}2$ meV was obtained for the potential fluctuation in the valence-band edge.

Slow Positron Studies in Polymers

The diffusion trapping model has been applied to slow positron annihilation in He+ irradiated polystyrene and polystyrene – polystyrene bilayers. The S-parameter and the positron lifetime have been calculated as a function of the incident positron energy. The effect of the fluence upon the nature of the S-parameter curve has been discussed. It has been found that a change in fluence affects positronium formation. The transition rate for surface to positronium formation has been found to be dependent upon the fluence and the atomic number of the irradiated ion. The lifetime results show that, at low energy, the o-Ps annihilates mainly at the polymeric surface. The free volume hole concentration is found to decrease at low energy, and becomes constant at higher energies.

High-Current-Gain Direct-Growth GaN/InGaN Double Heterojunction Bipolar Transistors

We report high-current-gain n-p-n GaN/InGaN double-heterojunction bipolar transistors (DHBTs) using a direct-growth fabrication processing approach. The impact of the indium composition in the base layer was studied, and a burn-in effect using a constant-base-current stressing method was observed. We found that DHBTs with higher indium composition in the InGaN base layer may help reduce the base resistance and lower the surface recombination current but may result in higher bulk recombination current. A device burn-in effect was also studied. The postprocessing current stressing step helps increase free-hole concentration in the base, reduce the bulk recombination current, and enhance the current gain. As a result, a direct-growth GaN/In0.03Ga0.97N DHBT with a peak current gain of 105 and a collector current density > 6.5 kA/cm2 was demonstrated on a sapphire substrate.

Pulsed laser deposition of BiCuOSe thin films

Undoped and 10% Ca-doped BiCuOSe thin films are prepared by pulsed laser deposition without ex-situ processing. The influence of the preparation conditions on structure and properties of Bi0.9Ca0.1CuOSe thin films on amorphous silica substrates is studied. The highest achieved concentration and mobility of free holes (3.9×1020 cm−3 and 3.5 cm2/Vs) was close to that measured in strongly c-axis oriented samples on SrTiO3 substrates (4.0×1020 cm−3 and 7.5 cm2/Vs). The Bi0.9Ca0.1CuOSe films on SrTiO3 show almost temperature-independent Seebeck coefficient and their resistivity increases with increasing temperature. The Seebeck coefficient of undoped BiCuOSe films on SrTiO3 increases below 150°K, and the resistivity shows a flat plateau centered at this temperature. Optical measurements suggest that BiCuOSe has an indirect bandgap of 0.8 eV and a strong absorption edge at 1.45 eV. Ab-initio calculations of the electronic band structure, effective masses and optical properties of BiCuOSe are also presented.

Factors limiting lifetime of charge carriers in semi-insulated CdTe under high radiation flux

With the growing interest in CdTe and (CdZn)Te X-ray detectors operating at high fluxes of X-ray photons (∼10 10 cm −2 s −1) the questions arises, whether the lifetime of charge carriers is limited by trapping and recombination at deep levels or by band-to-band recombination due to a strongly elevated concentrations of free electrons and holes compared to detectors working at low fluxes. A set of numerical simulations was done to resolve this question. The approach is based on the self-consistent steady state solution of electron and hole drift-diffusion equations using an iterative method. The effect of space charge on the electric field distribution and carrier transport through the sample is evaluated by solving the Poisson equation. The material simulation parameters were chosen to describe typical situations in state-of-the art highly compensated semi-insulated CdTe and CdZnTe (∼10 10 Ωcm) with deep near-midgap levels with concentrations in the range of 10 11–10 12 cm −3.The band-to-band recombination parameter was calculated using the Van Roosbroeck–Shockley relation and was estimated ∼3×10 −10 cm 3/s. The results show, that under all studied conditions the lifetime of carriers and thus the detector performance is limited by carrier trapping and recombination at deep levels.

Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells

Simultaneous and locally resolved determination of the mechanical stress variation and the free hole concentration using Raman spectroscopy is demonstrated in laser crystallized amorphous silicon layers. Such layers are often used for the fabrication of thin film solar cells, e.g., on borosilicate glass substrates. The combined effects of stress and doping on the Raman signal can be separated based on the use of three wavelengths in the visible. The results show that the free hole concentration in the samples investigated varies between 1×1018 and 1.3×1019 cm−3. Stress as well as the free hole concentration vary substantially within the sample. The stress level varies between 575 and 850 MPa (±12 MPa). Cross-sectional transmission electron microscopy images show the presence of extended lattice defects such as dislocations and grain boundaries in the crystallized Si layer which could account for the lateral stress variations detected by Raman spectroscopy. The impact of film inhomogeneity in terms of stress and doping on the performance of a solar cell will be discussed.

Defects in Bi2Te3−x Se x single crystals

Single crystals of a ternary system based on Bi2Te3−x Se x (nominally x=0.0–0.2) were prepared using the Bridgman technique. Samples with varying content of Se were characterized by the measurement of lattice parameters, electrical conductivity σ⊥c and Hall coefficient R H(B‖ c). The actual concentration of selenium c Se in the samples was determined using atomic emission spectroscopy. While a small selenium concentration enhances the free hole concentration P after passing a maximum, the hole concentration decreases at higher selenium concentrations. The extreme-like dependence P=f(c Se) is explained in terms of a change of the native defect concentration due to the substitution of selenium atoms by tellurium ones.

Electron spin resonance of light holes in porous silicon

Abstract A new ESR spectrum labeled Si-AA20 is revealed in porous silicon after annealing at 1000 °C. The AA20 line has a Dyson type that allows to attribute the AA20 to free carrier absorption. The AA20 spectrum is isotropic with g=2.0710. The high value of the g-factor indicates that the absorption can be hole-like. Intensity of the AA20 spectrum considerably changed upon rotation of the magnetic field H in the ( 0 1 ¯ 1 ) plane, at the same time the shape and half-width of the spectrum line did not change. The intensity was maximal with H along 〈1 1 1〉 directions and decreased in 2–3 times with H along 〈0 1 1〉 and 〈0 0 1〉 directions when the intensity reached minimum. The AA20 spectrum can be explained by formation of layers in porous silicon with high concentration of free holes for compensation of negative charged traps.

On the theoretical basis for the duplicitous thermoluminescence peak

The simultaneous release of electrons and holes by what seems to be a single trap has been observed experimentally. We previously performed numerical simulations on a phenomenological model which showed similar behaviour. Here, we provide an analytical solution to this model. This model explains trends in radioluminescence, thermoluminescence and thermally stimulated conductivity of a material with one electron trap, one hole trap and one radiative recombination centre, in which thermal excitation of the electron trap occurs before that of the hole trap. It is shown that TL emission due to electron recombination at centres can be controlled by a hole trap and the electron recombination will have a peak shape associated with the hole trap's parameters. When this happens, the peaks in free electron concentration, free hole concentration and TL all occur nearly simultaneously. The analytical model allows this to be explained along with scaling laws and initial rise behaviour. Under the conditions illustrated by this model, the usual methods used to distinguish between electron traps and hole traps will give incorrect results.

Temperature and doping dependencies of electrical properties in Al-doped 4H-SiC epitaxial layers

The free hole concentration and the low-field transport properties of Al-doped 4H-SiC epilayers with several acceptor concentrations grown on semi-insulating substrates have been investigated in the temperature range from 100to500K by Hall-effect measurements. Samples have been grown by cold-wall chemical vapor deposition (CVD) in the Al acceptor concentration range from 3×1015to5.5×1019cm−3. The dependencies of the acceptor ionization ratio at 300K and the ionization energy on the acceptor concentration were estimated. Numerical calculations of the hole Hall mobility and the Hall scattering factor have been performed based on the low-field transport model using relaxation-time approximation. At the low acceptor concentrations, the acoustic phonon scattering dominates the hole mobility at 300K. At the high acceptor concentrations, on the other hand, the neutral impurity scattering dominates the mobility. A Caughey–Thomas mobility model with temperature dependent parameters is used to describe the dependence of the hole mobilities on the acceptor concentration, and the physical meanings of the parameters are discussed.

Highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures grown by metalorganic vapor phase epitaxy

In this study, we present theoretical and experimental results regarding highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures. Based on simulation results, several multiheterostructures were grown by metalorganic vapor phase epitaxy. Using high resolution x-ray diffraction and x-ray reflectometry, the abruptness of the AlGaN/AlN/GaN interfaces could be determined. Using electron holography, the energetic profile of the valence band could be measured, yielding important information about the vertical carrier transport in such multiheterostructures. The electrical properties of the samples were investigated by measuring the lateral (σL) and vertical (σV) conductivity, respectively. The free hole concentration of a sample optimized in terms of lateral conductivity was measured to be 1.2×1019 cm−3 (295 K) with a mobility of 7 cm2/V s, yielding a record σL of 13.7 (Ω cm)−1. Low temperature Hall measurements (77 K) proved the existence of a two-dimensional hole gas at the AlN/GaN interface, as the lateral conductivity could be increased to 30 (Ω cm)−1 and no carrier freeze out was observable. By substituting the p-GaN layer in a light emitting diode (LED) with an AlGaN/GaN multiheterostructure, the overall voltage drop could be reduced by more than 100 mV (j=65 A/cm2). Furthermore improved current spreading on the p-side of LEDs with integrated AlGaN/AlN/GaN multiheterostructures could be proved by μ-electroluminescence, respectively.

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

AbstractHere, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 1022 m−3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.

Observation of stacking faults formed during homoepitaxial growth of p-type 4H-SiC

Threading dislocations and their transformation into stacking faults (SFs) are observed in p-type 4H-SiC epitaxial layers by high voltage transmission electron microscope. Homoepitaxial growth and in situ aluminum doping of 4H-SiC epitaxial layers are carried out using the organosilicon precursor bistrimethylsilylmethane (C7H20Si2 and the metal-organic precursor trimethylaluminum (C3H9Al), and the free hole concentration of the most heavily aluminum-doped epitaxial layers is >1021 cm−3. Threading dislocations are formed at the interface between the epitaxial layer and the substrate. However, the density of these threading dislocations decreases toward the epitaxial layer surface with their transformations to SFs.

Impact of CBr 4, V/III ratio, temperature and AsH 3 concentration on MOVPE growth of GaAsSb:C

We investigate the impact of the carbon tetrabromine (CBr 4) and arsine (AsH 3) concentrations, the V/III ratio at the gas inlet ( R inlet) and the temperature ( T g) on the growth rate, the solid composition, the incorporated carbon (C) and free hole concentration in GaAsSb:C grown by metal-organic vapor-phase epitaxy (MOVPE). The introduction of CBr 4 deeply perturbs the growth when compared to when no C-doping is used. The growth of GaAsSb:C using CBr 4 behaves as if an effective V/III ratio ( R eff) were defined as the R inlet leading to the same growth rate in the absence of CBr 4 under the same group V inlet flux. This consideration allows (and/or accounts) for the growth of high-quality layers with R inlet≪1, and explains alloy composition and growth rate variations as well as non-monotonic variations of the free hole concentration with increasing CBr 4 flow. We show how R eff∼1 can be experimentally determined to optimize the growth conditions in MOVPE reactors. An increase of the CBr 4 flux enhances the solid arsenic (As) concentration and reduces the growth rate. The As concentration dependence on the CBr 4 flux becomes stronger at higher R inlet. Although the incorporated C density monotonically increases with increasing CBr 4 flow, the hole concentration increases and then drops. This drop is attributed to the amphoteric character of C and/or to the formation of C–C bonds. The peak hole density is enhanced from 2.5×10 19 to over 1.6×10 20 cm −3 by reducing R inlet from 0.72 to 0.25. Furthermore, the hole density decreases from 1.6×10 20 to 5.5×10 19 cm −3 if the AsH 3 concentration increases from 0.31 to 0.51. Thus, alloys with As-rich composition obtained by increasing the AsH 3 composition in the gas phase cannot achieve hole concentrations as high as 1×10 20 cm –3 at T g=550 °C. However, we demonstrate that GaAs 0.7Sb 0.3 with doping levels of 1×10 20 cm –3 could only be achieved by decreasing T g to 510 °C.