Injected Carrier Concentration Research Articles

Modeling and simulation of optical gain of GaP0.3Sb0.7/InP0.7Sb0.3 type-II heterostructure for IR optoelectronics under variable well width, temperature and external electric field

Abstract The type-II GaP0.3Sb0.7/InP0.7Sb0.3 nanoscale heterostructure is modelled and simulated for its optical properties in near-infrared lasing applications. The optical gain of GaP0.3Sb0.7/InP0.7Sb0.3 type-II heterostructure is analyzed under different external values of electric fields, temperatures and well widths at room temperature 300 K. The complete structure is grown on InP substrate. The effects of a varied temperature (290 K–320 K), quantum well width (2 nm, 3 nm, and 4 nm) and applied electric field (20 kV cm−1–8 0 kV cm−1) are explored regarding the band alignment, wavefunction, band dispersion, matrix elements, gain and wavelength. The Luttinger-Kohn model is utilized to compute the band structure. The gain computation involves the evaluation of the 6 × 6 k·p Hamiltonian matrix. The proposed heterostructure at 2 nm quantum well width exhibits a high optical gain of 14998 cm−1 in x-polarization and 16572 cm−1 in y-polarization for injected carrier concentration of 4 × 1012 cm2. Under variable temperature and electric field, a significant optical gain is achieved in x, y and z input polarizations. This heterostructure is regarded as new because of its very high optical gain in NIR regime, that makes it beneficial for optoelectronics.

Performance simulation of an InGaSb/GaSb based quantum well structure for laser diode applications

The present study reports the impact of structural parameters on optoelectronic properties of InGasb/GaSb based quantum well structures (Qws). The laser diodes are designed to operate at 2.3 μm at 300 K. Numerical calculations of the emission wavelength, optical and modal gain of TE mode in InGaSb/GaSb laser diode structure have been carried out for various well material compositions, well thickness, number of quantum wells and temperature. The optical confinement factor and threshold current density are also simulated and reported. The calculations were performed using the 8-bands k.p model. For an injected carrier concentration of 1.56 × 1018 cm−3 at 300 K, peak gain value of the order of 1400 cm−1 is reached and a modal gain of 94 cm−1 can be obtained. A threshold current density around 3 kA/cm2, is expected to be obtained through optical losses of about 50 cm−1. The results show that InGaSb/GaSb quantum wells are appropriate for mid-infrared lasers operating at 300 K.

Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure

Strain and band gap engineered epitaxial germanium (ε-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of an InGaAs layer to simultaneously provide tensile strain in Ge (0.7–1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth), and loss mechanisms that dominate in the ε-Ge QW laser structure were calculated using first-principles-based 30-band k·p electronic structure theory, at injected carrier concentrations from 3 × 1018 to 9 × 1019 cm–3. The higher strain in the ε-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a Jth of 300 A/cm2 can be reduced to <10 A/cm2 by increasing strain from 0.2% to 1.96% in ε-Ge lasing media. The measured room-temperature photoluminescence spectroscopy demonstrated direct band gap optical emission, from the conduction band at the Γ-valley to heavy-hole (0.6609 eV) from 1.6% tensile-strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k·p theory. The detailed plan-view transmission electron microscopic (TEM) analysis of 0.7% and 1.2% tensile-strained ε-Ge/InGaAs structures exhibited well-controlled dislocations within each ε-Ge layer. The measured dislocation density is below 4 × 106 cm–2 for the 1.2% ε-Ge layer, which is an upper bound, suggesting the superior ε-Ge material quality. Structural analysis of the experimentally realistic 1.95% biaxially strained In0.28Ga0.72As/13 nm ε-Ge/In0.28Ga0.72As QW structure demonstrated a strained Ge/In0.28Ga0.72As heterointerface with minimal relaxation using X-ray and cross-sectional TEM analysis. Therefore, our monolithic integration of a strained Ge QW laser structure on GaAs and ultimately the transfer of the process to the Si substrate via an InGa(Al)As/III–V buffer architecture would provide a significant step toward photonic technology based on strained Ge on a Si platform.

Memristive and magnetoresistance effects of SnSe&lt;sub&gt;2&lt;/sub&gt;

Memristor and magnetoresistance (MR) are widely used in the field of information storage. In recent years, SnSe&lt;sub&gt;2&lt;/sub&gt;, as an information storage material with both memristor and MR effects, has received a lot of attention of the researchers. It is of great significance to further explore its electrical transport mechanism. In this paper, the high-purity bulk SnSe&lt;sub&gt;2&lt;/sub&gt; samples are prepared by melting method together with spark plasma sintering. The &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curves are measured under different temperatures and magnetic fields. The memristive and MR effect of SnSe&lt;sub&gt;2&lt;/sub&gt; are systematically investigated. After the memristive characteristics are excluded from interfacial junction effect, phase transition and conductive wire channels, the memristive effect at different temperatures is attributed to the space charge limiting current effect under defect control. Under low electric field conditions, the internal carrier concentration of material is much higher than the injected carrier concentration and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve obeys ohmic conduction. When the voltage increases to the switching voltage &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;on&lt;/sub&gt;, the internal defects of the material are filled with the injected carriers as the transport time of the injected carrier is less than the dielectric relaxation time, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curves deviate from ohmic conductivity. When the voltage reaches the transition voltage &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;TFL&lt;/sub&gt;, the injected carrier increases exponentially, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve presents negative differential phenomenon. Finally, the space charge inside the material will limit the further injection of external carriers, and the &lt;i&gt;V&lt;/i&gt;-&lt;i&gt;I&lt;/i&gt; curve follows the Child law. As the temperature decreases to 10 K, the memristive phenomenon weakens because a large number of defects for accepting the injected carriers are reduced due to the decrease of impurity ionization at low temperatures. At the same time, the sample exhibits a large negative MR at 10 K and 100 K. When impurity scattering predominates, the electrons will be subjected to multiple scattering by the impurities, resulting in localization of carriers. The negative MR effect is related to the inhibition of the carrier localization by the magnetic field. In our work, a large negative MR of about –37% at 0.6 T and 10 K are obtained, which is likely to originate from the disordered distribution of Se vacancy in the material. With the increase of temperature, the scattering mechanism gradually evolves from the impurity scattering into the lattice scattering, and the negative MR effect gradually develops into the positive MR effect.

In-situ characterization of electron-assisted regeneration of Cz-Si solar cells

We examine the regeneration kinetics of passivated emitter and rear solar cells (PERCs) fabricated on boron-doped p-type Czochralski-grown silicon wafers in darkness by electron injection via application of a forward bias voltage at elevated temperature (140 °C) in order to discriminate between electronic and photonic effects. Based on these dark regeneration experiments, we address the existing inconsistency regarding the measured linear dependence of the regeneration rate constant on the excess carrier density. Using the method of dark regeneration by current injection into the solar cell, we are able to measure the total recombination current of the solar cell at the actual regeneration temperature under applied voltage, i.e., at the physically relevant regeneration conditions. The direct comparison of the regeneration rate constant as a function of electronically injected carrier concentration in the dark and the regeneration rate constant during illumination clearly shows that the regeneration is a purely electronically stimulated effect and that photons are not directly involved.

Injected carrier concentration dependence of the expansion of single Shockley-type stacking faults in 4H-SiC PiN diodes

We investigated the relationship between the dislocation velocity and the injected carrier concentration on the expansion of single Shockley-type stacking faults by monitoring the electroluminescence from 4H-SiC PiN diodes with various anode Al concentrations. The injected carrier concentration was calculated using a device simulation that took into account the measured accumulated charge in the drift layer during diode turn-off. The dislocation velocity was strongly dependent on the injected hole concentration, which represents the excess carrier concentration. The activation energy of the dislocation velocity was quite small (below 0.001 eV between 310 and 386 K) over a fixed range of hole concentrations. The average threshold hole concentration required for the expansion of bar-shaped single Shockley-type stacking faults at the interface between the buffer layer and the substrate was determined to be 1.6–2.5 × 1016 cm−3 for diodes with a p-type epitaxial anode with various Al concentrations.

Current-Induced Phase Segregation in Mixed Halide Hybrid Perovskites and its Impact on Two-Terminal Tandem Solar Cell Design

Mixed halide hybrid perovskites are of significant interest because their bandgap can be tuned as a current-matched top-cell in tandem photovoltaics. However, several mixed halide perovskites phase segregate under illumination, exhibit large voltage deficits, and produce unstable photocurrents. We investigate the origin of phase segregation and implication for tandems with mixed halide large-bandgap (∼1.75 eV) perovskites. We show explicitly that MAPb(I0.6Br0.4)3 and (MA0.9,Cs0.1)Pb(I0.6,Br0.4)3, termed “MA” and “MACs”, respectively, rapidly phase segregate in the dark upon 1 sun equivalent current injection. This is direct experimental evidence that conduction band electrons or valence band holes are the culprit behind phase segregation. In contrast, (FA0.83,Cs0.17)Pb(I0.66,Br0.34)3, or “FACs,” prepared at only 75 °C resists phase segregation below 4 sun injection. FACs prepared at 165 °C yields larger grains and withstands higher injected carrier concentrations before phase segregation. The FACs and MAC...

Investigation of charge trapping and detrapping dynamics in LDPE, HDPE and XLPE

In this paper, charge trapping and detrapping dynamics of PE-based materials, i.e., LDPE, HDPE, XLPE were investigated by isothermal surface potential decay method (ISPD). The whole charge transport process including ions generation process in the air, charge trapping in the top surface, the surface potential establishing process, the charge migration process in the bulk was considered in these PE-based materials. The results showed that the trapping process was closely related to the capture cross section, the injected carrier concentration and the trap level. The established surface potential was mainly determined by the captured carriers in the top surface of the PE-based materials. In addition, it was found that the mobility of LDPE was much larger than that of XLPE and HDPE. And it is shown that deep traps may play a more important role in the charge migration process than shallow traps in these PE-based materials. It is indicated that the discrepancy of charge trapping and detrapping dynamics between these PE-based materials may be attributed to the different microscopic morphology.

Short minority carrier lifetimes in highly nitrogen-doped 4H-SiC epilayers for suppression of the stacking fault formation in PiN diodes

We investigated the dependency of minority carrier lifetimes on the nitrogen concentration, temperature, and the injected carrier concentration for highly nitrogen-doped 4H-SiC epilayers. The minority carrier lifetimes greatly shortened when the nitrogen concentration exceeded 1018 cm−3 through enhancing direct band-to-band and Auger recombination and showed a slight variation in the temperature range from room temperature (RT) to 250 °C. The epilayer with a nitrogen concentration of 9.3 × 1018 cm−3 exhibited a very short minority carrier lifetime of 38 ns at RT and 43 ns at 250 °C. The short minority carrier lifetimes of the highly nitrogen-doped epilayer were confirmed to maintain the values even after the subsequent annealing of 1700 °C. 4H-SiC PiN diodes were fabricated by depositing a highly nitrogen-doped epilayer as a “recombination enhancing layer” between an n− drift layer free from basal plane dislocations and the substrate. The PiN diodes showed no formation of stacking faults and no increase in forward voltage during current conduction of 600 A/cm2 (DC), demonstrating that a highly nitrogen-doped buffer layer with a short minority carrier lifetime successfully suppresses the “bipolar degradation” phenomenon.

Optimization of high optical gain in type-II In0.70Ga0.30As/GaAs0.40Sb0.60 lasing nano-heterostructure for SWIR applications

Most of the nano-heterostructures exhibiting lasing action in NIR (near infra-red) region that have been modeled and simulated are based on type-I category. The nano-scaled lasing heterostructures, however, of type-II category operating in SWIR (short wave infra-red) region have not been well studied. In this paper, for SWIR generation, an M-shaped type-II In0.70Ga0.30As/GaAs0.40Sb0.60 symmetric lasing nano-heterostructure has been designed. In order to simulate the optical gain, firstly the wave functions associated with conduction and valence sub-bands, carrier densities within the bands, energy band dispersion relations for the quantum well structure, optical matrix elements and finally optical gain have been studied by utilizing the six band k.p method. For the injected carrier concentration of 5 × 1012/cm2, the optimized optical gain within TE mode is as high as ∼9000/cm at the wavelength of ∼1.95 μm, thus providing a very important alternative material system for the generation of SWIR wavelength region.

Direct monitoring of minority carrier density during light induced degradation in Czochralski silicon by photoluminescence imaging

In this paper, we present a new method for studying the light induced degradation process, in which the minority carrier density is monitored directly during light soaking by photoluminescence imaging. We show experimentally that above a certain minority carrier concentration limit, Δnlim, the boron oxygen (B-O) defect generation rate is fully independent of the injected carrier concentration. By simulation, we determine Δnlim for a range of p-type Czochralski silicon samples with different boron concentrations. The normalized defect concentrations, Nt*, are determined for the same samples by time-resolved Quasi Steady State Photoconductance measurements. After 10 min of light degradation, no correlation between Δnlim, and Nt* is observed. These results indicate that the role of the excess carriers during the rapid decay is to first change the charge state of the defects by shifting the electron quasi-Fermi level across the energy level of the defect centre in its passive state (Elat = EV + (635 ± 18) meV) and that, subsequently, another rate-determining step proceeds before the defect centre becomes recombination active.

Quantitative Analysis of Initial Short-Term Aging Behavior and Its Implication for the Efficiency Droop in Blue Light-Emitting Diodes

We have quantitatively analyzed the short-term aging behavior of blue light-emitting diodes using the current-component analysis method. The internal quantum efficiency and luminescence output decrease monotonically with the aging time and are stabilized at low operation current, owing to the increase and subsequent stabilization in the tunneling and nonradiative currents, while they show complicated behavior at high operation current due to the interplay of different changing rates in the radiative and nonradiative recombination current components. The current-component analysis enables both the quantitative understanding of the aging behavior and the identification of the aging mechanisms. Also shown is that the loss current, which is responsible for the efficiency droop, is approximately proportional to the cube of the injected carrier concentration. This suggests that the origin of the efficiency droop is a three-carrier process.

InAsN/GaSb/InAsN ‘W’ quantum well laser for mid-infrared emission: from electronic structure to threshold current density calculations

The electronic band-structure and optical gain properties of dilute-nitride InAsN/GaSb/InAsN type-II ‘W’ quantum wells based mid-infrared laser diodes on InAs substrate are numerically investigated with an accurate 10-bands kp model. The laser diodes are designed to operate at 3.3 µm at room temperature. The dispersion relations, the optical gain and the threshold current density including computation of non-radiative Auger recombination rate are calculated. For typical injected carrier concentration of 1.5 × 1018 cm−3 at 300 K, peak gain values of the order of 1000 cm−1 are reached and a modal gain value equal to 70 cm−1 can be achieved. By evaluating reasonable optical losses, a threshold current density around 500 A cm−2 is expected, showing that dilute-nitride InAsN/GaSb/InAsN ‘W’ quantum wells are suitable for mid-infrared laser operation at room temperature.

Coulomb interaction of electron gas in MQWs Si/Si 1 − xGe x/Si

We present a theoretical analysis of the conduction and valence-band diagrams of SiGe/Si Multiple Quantum Wells (MQWs), having a specific “W” geometry, and designed for emission or photodetection around the 1.55 μm wavelength. Peculiar features have been extrapolated by solving self-consistent Schrödinger and Poisson equations, taking into account the electrostatic attraction induced by carrier injection. As a result, Coulomb interaction strongly modifies the band profiles and increases the electron probability density at the quantum well interfaces; the injected carrier concentration enhances electron–hole wave functions overlap and the in-plane oscillator strength. These MQWs structures, strain-compensated on relaxed Si 0.75Ge 0.25 pseudo-substrates, are potentially interesting for telecom applications.

Nitrogen effect on optical gain and radiative current density for mid-infrared InAs(N)/GaSb/InAs(N) quantum-well laser

Dilute-nitride InAsN/GaSb/InAsN “W” laser structure is theoretically investigated and compared with similar nitride-free InAs/GaSb/InAs “W” structure. The two laser diodes, to be grown on (0 0 1) InAs substrate, are designed to operate at 3.3 μm at room temperature. Their performances are evaluated in terms of modal gain and radiative efficiency characteristics deduced from optical gain calculation. We find that the inclusion of nitrogen in the laser active region improves optical gain performances leading to peak gain values of the order of 1000 cm −1 for typical injected carrier concentration of 1.5×10 18 cm −3. Modal gain value equal to 70 cm −1 can be achieved and radiative current density inferior to 100 A/cm 2 is predicted. These results demonstrate that the dilute-nitride InAsN/GaSb/InAsN laser structure is very attractive for room temperature operation in the midwave infrared domain.

Induced electrostatic confinement of electron gas in W-designed strain-compensated Si/Si 1–xGe x/Si type-II quantum wells

We present a numerical modeling of the conduction- and the valence-band diagrams of W designed Si/Si 0.4Ge 0.6/Si type II quantum wells. These W structures, strain-compensated on relaxed Si 0.75Ge 0.25 pseudo-substrates, are potentially interesting for emission and photo-detection around a 1.55μm wavelength. Two main features have been extrapolated by solving self-consistently Schrödinger and Poisson equations, taking into account the electrostatic attraction induced by carrier injection: (i) Coulomb attraction strongly modifies the band profiles and increases the electron probability density at the quantum well interfaces. (ii) The injected carrier concentration enhances the in-plane oscillator strength and the electron–hole wave-function overlap.

Prediction of SOI single-event effects using a simple physics-based SPICE model

Expensive and time-consuming three-dimensional (3-D) simulations have been previously employed to characterize single-event upsets of integrated circuits. This paper outlines a more practical engineering approach to SOI single-event effects modeling that involves simple closed-form one-dimensional (1-D) solutions to the carrier transport equations. Using Fourier analysis, the transport equations have direct 1-D closed form solutions for conditions. One-dimensional numerical solutions are also obtained for conditions (i.e., when the excess injected carrier concentration exceeds the background doping), including other nonlinearities such as the dependence of mobility on both the background concentrations and excess carrier concentrations. The 1-D equations for low injection are reasonable approximations for high injection in n-type material, but not for high injection in p-type material. However, a simple 1-D equation can still be used for high injection in p-type material. The 1-D equations are used with existing simple SPICE models, including models for the parasitic bipolar transistors. The 1-D model agrees well with predictions from 2-D and 3-D simulations. A circuit cell layout can be broken into various regions with differing SEU sensitivities, each with its own characterizing 1-D equation. This simple method leads to a theoretical prediction of cross-sectional area versus upset linear energy transfer (LET) that is in good agreement with experimental heavy ion SEU data for an SRAM. The method also accounts for strike angles in three dimensions.

Carrier tunneling in nanocrystalline silicon–silicon dioxide superlattices: A weak coupling model

Differential conductivity measurements in partially disordered nanocrystalline Si--amorphous ${\text{SiO}}_{2}$ superlattices reveal a double-peak structure associated with tunneling via energy levels of light and heavy holes. The theoretical model and numerical simulations presented here show good agreement with experiment and predict that this system does not have stable solutions for an injected carrier concentration greater than ${10}^{17}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}3}$. Similar to a weakly coupled superlattice, a larger carrier concentration results in current instabilities. These instabilities have been observed and can be partially suppressed by using pulsed carrier photoinjection.

First Principles Calculations of Band Discontinuity at Ultrathin SiO2/Si Interfaces

The energy barrier and penetration depth of a hole are studied for ultrathin SiO2/Si interfaces. Layer thickness dependence and injected carrier concentration dependence are calculated for the superlattice structure, by using a first-principles pseudo-potential approach based on the density-functional theory. A new method to evaluate the energy barrier is proposed, which is derived from calculation of both the energy band structure and behavior of the Bloch function. The energy barrier for a hole is reduced with a decrease in Si layer thickness by confinement effect and is reduced also with the monolayer thickness of SiO2. The energy barrier increases by electron injection and decreases by hole injection.

An Analytical Method of Analyzing Insulated-Gate Bipolar Transistor Characteristics in Terms of Applied Terminal Voltages

An analytical method of analyzing insulated-gate bipolar transistor (IGBT) current–voltage characteristics is established in this paper. Important internal device parameters such as the injected carrier concentrations, electron and hole current densities and depletion length of the drift region are calculated as functions of the terminal voltages. The parasitic transistor current gains αnpn and αpnp are also extracted as functions of applied voltages. The temperature effect on both latch-up criteria can also be predicted from this method.