Phonon-assisted Auger Recombination Research Articles

Electrochemical Charging Effect on the Optical Properties of InP/ZnSe/ZnS Quantum Dots.

Semiconductor quantum dots (QDs) are spotlighted as a key type of emissive material for the next generation of light-emitting diodes (LEDs). This work presents the investigation of the electrochemical charging effect on the absorption and emission of the InP/ZnSe/ZnS QDs with different mid-shell thicknesses. The excitonic peak is gradually bleached during electrochemical charging, which is caused by 1Se (or 1Sh ) state filling when the electron (or hole) is injected into the InP core. Additional charges also lead to photoluminescence (PL) intensity reduction, however, it is greatly mitigated as the mid-shell thickness increases. Various PL measurements reveal that the PL reduction under electrochemical charging is attributed to the acoustic phonon-assisted Auger recombination. Here, the Auger recombination in QDs with a thick mid-shell is reduced under the electrochemically charged condition, indicating that QDs with larger volume are more stable emitters in charge-injecting devices such as LEDs. Furthermore, the negative and positive trion Auger recombination rate constants are estimated, respectively, via electrochemical charging. The negative trion Auger rate constants decrease with an increase in the mid-shell thickness increases, whereas the positive trion Auger rate constants are not heavily reliant on the mid-shell thickness.

Anomalous Auger Recombination in PbSe.

Using first-principles approaches we find that the Auger recombination in PbSe is anomalous in three distinct ways. First, the direct Auger coefficient is 4 orders of magnitude lower than that of other semiconductors with similar band gaps, a result that can be attributed to the lack of involvement of a heavy-hole band. Second, phonon-assisted indirect Auger recombination prevails, contrary to the common belief that direct Auger is dominant in narrow-gap semiconductors. Third, an unexpectedly weak temperature dependence of the Auger coefficient is observed, which we can now attribute to the indirect nature of the Auger process. The widely accepted explanation of this behavior in terms of an unusual temperature dependence of the band gap is only a secondary effect. Our results elucidate the mechanisms underlying the anomalous Auger recombination in IV-VI semiconductors in general, which is critical for understanding and engineering carrier transport.

Inter-valley phonon-assisted Auger recombination in InGaAs/InP quantum well

Nonradiative processes are studied in InGaAs/InP quantum wells (QWs) as a function of the pump power and the temperature, using time-resolved photoluminescence. Intravalley disorder induced direct and intervalley phonon-assisted indirect Auger processes are found to be responsible for nonradiative recombination in low mobility and high mobility quantum wells, respectively. Both Auger processes are spatially separated: the phonon-assisted and direct Auger recombinations take place in the central part of the QW and near the interfaces, respectively. The recombination rate corresponding to the phonon-assisted Auger process is shown to increase with the temperature, while no influence of the temperature was detected for the rate of the disorder induced direct Auger process. The presented data point to the X and/or L valleys as final states for the Auger electron in the intervalley Auger process. Moreover, Auger recombination associated with different confined levels is studied. We show that the conditions for the intervalley phonon-assisted Auger processes are fulfilled for the ground states of the confined electrons and holes, while the nonradiative recombination due to the excited states is dominated by the direct Auger process.

Numerical study of the intrinsic recombination carriers lifetime in extended short-wavelength infrared detector materials: A comparison between InGaAs and HgCdTe

Intrinsic carrier lifetime due to radiative and Auger recombination in HgCdTe and strained InGaAs has been computed in the extended short-wavelength infrared (ESWIR) spectrum from 1.7 μm to 2.7 μm. Using the Green's function theory, both direct and phonon-assisted indirect Auger recombination rates as well as the radiative recombination rates are calculated for different cutoff wavelengths at 300 K with full band structures of the materials. In order to properly model the full band structures of strained InGaAs, an empirical pseudo-potential model for the alloy is fitted using the virtual crystal approximation with spin-orbit coupling included. The results showed that for InxGa1−xAs grown on InP substrate, the compressive strain, which presents in the film when the cutoff wavelength is longer than 1.7 μm, leads to decrease of Auger recombination rate and increase of radiative recombination rate. Since the dominant intrinsic recombination mechanism in this spectral range is radiative recombination, the overall intrinsic carrier lifetime in the strained InGaAs alloys is shorter than that in the relaxed material. When compared to the relaxed HgCdTe, both relaxed and compressively strained InGaAs alloys show shorter intrinsic carrier lifetime at the same cutoff wavelength in room temperature which confirms the potential advantage of HgCdTe as wide-band infrared detector material. While HgCdTe offers superior performance, ultimately the material of choice for ESWIR application will also depend on material quality and cost.

Direct and phonon-assisted indirect Auger and radiative recombination lifetime in HgCdTe, InAsSb, and InGaAs computed using Green's function formalism

Direct and phonon-assisted (PA) indirect Auger and radiative recombination lifetime in HgCdTe, InAsSb, and InGaAs is calculated and compared under different lattice temperatures and doping concentrations. Using the Green's function theory, the electron self energy computed from the electron-phonon interaction is incorporated into the quantum-mechanical expressions of Auger and radiative recombination, which renders the corresponding minority carrier lifetime in the materials due to both direct and PA indirect processes. Specifically, the results of two pairs of materials, namely, InAs0.91Sb0.09, Hg0.67Cd0.33Te and In0.53Ga0.47As, Hg0.38Cd0.62Te with cutoff wavelengths of 4 μm and 1.7 μm at 200 K and 300 K, respectively, are presented. It is shown that for InAs0.91Sb0.09 and Hg0.67Cd0.33Te, when the lattice temperature falls below 250 K the radiative process becomes the limiting factor of carrier lifetime in both materials at an n-type doping of 1015 cm−3, while at a constant temperature of 200 K, a high n...

Auger recombination in sodium-iodide scintillators from first principles

Scintillator radiation detectors suffer from low energy resolution that has been attributed to non-linear light yield response to the energy of the incident gamma rays. Auger recombination is a key non-radiative recombination channel that scales with the third power of the excitation density and may play a role in the non-proportionality problem of scintillators. In this work, we study direct and phonon-assisted Auger recombination in NaI using first-principles calculations. Our results show that phonon-assisted Auger recombination, mediated primarily by short-range phonon scattering, dominates at room temperature. We discuss our findings in light of the much larger values obtained by numerical fits to z-scan experiments.

Auger Recombination in GaAs from First Principles

Auger recombination is a significant loss mechanism in many optoelectronic devices. We use first-principles methods based on density functional theory to study the relative importance of direct and indirect phonon-assisted Auger recombination in GaAs and related alloys. Energy and momentum of the recombining electron–hole pair can be transferred to an Auger electron (eeh process) or an Auger hole (hhe process). For eeh processes, the direct process is negligibly small compared to the phonon-assisted indirect process in GaAs, while in hhe processes the direct and phonon-assisted processes contribute almost equally. The hole processes are about 5 times stronger than the electron processes. In alloys with lower band gaps, the eeh processes become stronger, and below a band gap of 0.8 eV they are as relevant as the hole processes. Our results highlight the importance of indirect processes, even at low band gaps.

Evidence of Phonon-Assisted Auger Recombination and Multiple Exciton Generation in Semiconductor Quantum Dots Revealed by Temperature-Dependent Phonon Dynamics

Auger processes, multiple exciton generation, and Auger recombination, provide and disturb a potential route to increase solar cell efficiencies by creating multiple charge carriers, respectively. Physical mechanisms of the Auger processes can be deduced from the temperature dependence. Our real-time ab initio simulation found logarithmic temperature dependence of the Auger rates in semiconductor quantum dots (QDs), which agrees well with the recent experimental observations. This anomalous temperature dependence is not only determined by static electronic structures of the QDs depending on temperature, but also attributed to dynamical electron-phonon couplings, directly demonstrating that the Auger processes are actually induced by the electron-phonon couplings and can be controlled by phonon modes. Our findings suggest that high-frequency and broad phonon modes of a QD including the surface ligands dictate efficient Auger dynamics in a QD.

Temperature- and excitation-dependent carrier diffusivity and recombination rate in 4H-SiC

Time-resolved optical ‘pump-probe’ techniques were applied to monitor carrier dynamics in CVD-grown 4H-SiC for investigation of carrier diffusivity and recombination rate over a wide temperature (80–800 K) and excitation (1016–1019 cm−3) range. The light-induced transient grating technique provided the dependence of ambipolar diffusion coefficient on nonequilibrium carrier density at various temperatures, attributable to bandgap renormalization at low excitation levels and to plasma degeneracy at high ones. The free carrier absorption technique allowed us to monitor carrier lifetimes at various excitation levels and revealed its essential decrease from microseconds to a few nanoseconds. The latter effect was attributed to phonon-assisted Auger recombination with a temperature-independent, unscreened coefficient, C0 = (5 ± 1) × 10−31 cm6 s−1, which was Coulomb-enhanced at low excitations (up to C300 K ∼ 10−30 cm6 s−1 at 1018 cm3) but gradually decreased with increasing excitation due to screening of the carrier–polar optical phonon interaction. The measurements provided an ambipolar diffusion length of ∼30 µm which was almost temperature independent at low excitations (up to ΔN = 1017 cm−3) but decreased to 1 µm at high excitations, mainly due to the impact of nonlinear recombination.

Numerical analysis of indirect Auger transitions in InGaN

Indirect phonon-assisted Auger recombination mechanisms in bulk InGaN are investigated in the framework of perturbation theory, using first-principles phonon spectral density functions and electronic structures obtained by nonlocal empirical pseudopotential calculations. Nonpolar carrier-phonon interactions are treated within the rigid pseudoion framework, thus avoiding the introduction of empirical deformation potentials. The calculated indirect Auger coefficients exhibit a weak temperature dependence and dominate over direct processes for alloy compositions corresponding to the entire visible spectrum. The present results suggest that indirect Auger processes may be relevant in the operation of InGaN-based light-emitting diodes and lasers, at least in the yellow-green spectral region.

Radiative phonon-assisted and Auger recombination in Si nanocrystals

Recent analysis of the literature shows that the photoluminescence (PL) of Si nanocrystals and porous silicon is caused by phonon-assisted exciton radiative recombination, as well as by direct radiative electron transfer from the second to the first conduction sub-band, which is related to the Auger recombination. The PL decay curve for porous silicon after excitation with ultraviolet laser pulse has been established experimentally. We have constructed continuity equations for the first and the second conduction sub-bands, including radiative phonon-assisted exciton recombination, Auger recombination and direct radiative transition from the second to the first conduction sub-band. The solution of these equations yields the theoretical PL decay curve. These equations contain adjustable parameters. We have estimated their values by using the least-squares fit of the theoretical curve to the experimental data. We have shown that in our case both radiative components are approximately equal at the beginning, but the relative part of the phonon-assisted recombination increases with time and finally tends to 100%. Using the obtained values of the fit parameters, we have constructed the decay curves at different excitation intensities. We conclude that, at a small excitation power, the decay curve is exponential, the phonon-assisted radiative exciton recombination is dominating, and quantum efficiency is close to one. At a high excitation power, the decay curve is approximately stretched-exponential, the direct radiative transitions from the second to the first conduction sub-band are dominating, and quantum efficiency is about 1/3 for porous silicon sample.

The analysis of the variation of the threshold current with pressure in semiconductor quantum well lasers

Since the loss mechanisms in a semiconductor laser are often strongly wavelength dependent and hydrostatic pressure can vary the bandgap, the pressure dependence of the threshold current can then provide a clear picture of the dominant loss mechanisms in a given laser structure. The pressure dependence of the optical confinement factor, band structure, transparency and threshold carrier density and the combined effect of these on both radiative and non-radiative current contributions have been evaluated for lasers operating at a range of wavelengths using basic equations which makes our analysis particularly attractive owing to its simplicity and ability to predict the main effects involved. We investigate in particular the predicted pressure dependence of several Auger processes in long-wavelength (1.55 µm) lasers. It is found that the rate of decrease of phonon-assisted Auger recombination with pressure is close to that observed experimentally, implying that phonon-assisted Auger is the dominant loss mechanism.

Auger recombination in semiconductor quantum wells

The principal mechanisms of Auger recombination of nonequilibrium carriers in semiconductor heterostructures with quantum wells (QW's) are investigated. It is shown that there exist three fundamentally different Auger recombination mechanisms of (i) thresholdless, (ii) quasithreshold, and (iii) threshold types. The rate of the thresholdless Auger process depends on temperature only slightly. The threshold energy of the quasithreshold process essentially varies with QW width and is close to zero for narrow QW's. It is shown that the thresholdless and the quasithreshold Auger processes dominate in narrow QW's, while the threshold and the quasithreshold processes prevail in wide QW's. The limiting case of a three-dimensional (3D) Auger process is reached for infinitely wide QW's. The critical QW width is found at which the quasithreshold and threshold Auger processes merge into a single 3D Auger process. Also studied is phonon-assisted Auger recombination in QW's. It is shown that for narrow QW's the act of phonon emission becomes resonant, which in turn increases substantially the coefficient of phonon-assisted Auger recombination.

Band-structure engineering in strained semiconductor lasers

The influence of both compressive and tensile strain on semiconductor lasers and optical amplifiers is reevaluated in the light of recent experimental and theoretical work. Strain reduces the three-dimensional symmetry of the lattice and helps match the wave functions of the holes to the one-dimensional symmetry of the laser beam. It can also decrease the density of states at the valence band maximum and so reduce the carrier density required to reach threshold. These two effects appear to adequately explain the TE and TM gain in compressive and tensile structures, including polarization-independent amplifiers, the behavior of visible lasers and the improved frequency characteristics of InGaAs/GaAs lasers. In 1.5 /spl mu/m InGaAsP/InP lasers phonon-assisted Auger recombination appears to remain the dominant current path and can explain why the temperature sensitivity parameter to remains >

Temperature sensitivity and high temperature operation of long wavelength semiconductor lasers

The temperature dependence of long wavelength (1.5 μm) quantum-well lasers has been studied theoretically assuming that the dominant contribution to the threshold current is from phonon-assisted Auger recombination. It is found that the best possible value of T0 at room temperature is ≊100 K. Gain calculations based on the InGaAs/InGaAsP/InP system operating at 1.5 μm indicate that the main cause of the reduction from this ideal value is due to the temperature dependence of the threshold carrier density. We also comment on the implication of this for the high temperature operation of tensile and compressive lasers.

Phonon-assisted auger recombination in quantum well semiconductors

Phonon-assisted Auger recombination (AR) is shown to be an important loss mechanism in a quantum well semiconductor in addition to the direct AR. Theoretical investigations demonstrate that it is of the same order of magnitude and has the same temperature dependence as in bulk material, just as direct AR, provided that the material parameters and the carrier concentrations are the same as in the bulk.

Auger recombination in quantum well InGaAs

The application of a realistic band structure shows that phonon-assisted Auger recombination with its weak temperature dependence dominates in quantum well semiconductors, a result in contrast to bulk materials. The experimental result where the Auger coefficient of quantum well InGaAs is nearly temperature independent is explained.

Phonon-assisted Auger recombination in a quasi-two-dimensional structure semiconductor

A theory is presented of the phonon-assisted Auger recombination of the excess carriers which are confined in a quasi-two-dimensional structure laser. The theory, which is based on the Green's-function formalism, is applied to a quantum well of ${\mathrm{In}}_{0.72}$${\mathrm{Ga}}_{0.28}$${\mathrm{As}}_{0.6}$${\mathrm{P}}_{0.4}$. It is shown that the Auger recombination is strongly enhanced by phonon assistance. The Auger recombination in the quasi-two-dimensional structure is also compared with that in the three-dimensional structure.

Carrier recombination and lifetime in highly doped silicon

The predominance of phonon-assisted band-band Auger recombination in highly doped silicon is demonstrated by showing that no recombination mechanism involving common (unavoidable) defects in silicon can yield carrier lifetimes that are consistent with the measured lifetimes, which exhibit an inverse-quadratic doping-density dependence, and/or with their temperature dependence. Both trap-assisted-Auger and Shockley-Read-Hall recombination mechanisms are considered, and dependences of the defect density on the doping density, which are implied by theory and experiment, are accounted for.

Auger recombination in InGaAsP

Auger recombination in InGaAsP is calculated taking into account a realistic band structure instead of the usual parabolic approximation. It follows that the direct conduction-band process is negligible (at T=300 K), contrary to the parabolic approximation. Actually the valence-band process prevails by far as long as only direct Auger processes are considered. However, if phonon-assisted Auger recombination is included the phonon-assisted conduction-band process dominates. Altogether, a decrease of Auger recombination results, which is in agreement with experiments.