Nonradiative Recombination Mechanisms Research Articles

Carrier dynamics and photoluminescence quenching mechanism of strained InGaSb/AlGaSb quantum wells

GaSb based quantum wells (QWs) show promising optical properties in near-infrared spectral range. In this paper, we present photoluminescence (PL) spectroscopies of InxGa1−xSb/AlyGa1−ySb QWs and discuss the possible thermal quenching and non-radiative carrier recombination mechanisms of the QW structures. The In and Al concentrations as well as the QW thicknesses were precisely determined with the X-ray diffraction measurements. Temperature dependent time-integrated and time-resolved PL spectroscopies resulted in the thermal activation energies of ∼45 meV, and the overall self-consistent calculation of the band parameters based on the measured physical values confirmed that the activation energies are due to the hole escape from the QW to the barriers. The relation of the present single carrier escape mechanism with the other escape mechanisms reported with other material systems was discussed based on the estimated band offset. The relation of the present thermal hole escape to the Auger recombination was also discussed.

The influence of growth conditions on carrier recombination mechanisms in 1.3 μm GaAsSb/GaAs quantum well lasers

We investigate the temperature and pressure dependence of the threshold current density of edge-emitting GaAsSb/GaAs quantum well (QW) lasers with different device characteristics. Thermally activated carrier leakage via defects is found to be very sensitive to the growth conditions of GaAsSb QWs. An optimization of the growth conditions reduces the nonradiative recombination mechanisms from 93% to 76% at room temperature. This improvement in carrier recombination mechanisms leads to a large improvement in the threshold current density from 533 Acm−2/QW to 138 Acm−2/QW and the characteristic temperature, T0 (T1), from 51 ± 5 K (104 ± 16 K) to 62 ± 2 K (138 ± 7 K) near room temperature.

Upconversion luminescence with tunable lifetime in NaYF4:Yb,Er nanocrystals: role of nanocrystal size

Despite recent achievements to reduce surface quenching in NaYF(4):Yb,Er nanocrystals, a complete understanding of how the nanocrystal size affects the brightness of upconversion luminescence is still incomplete. Here we investigated upconversion luminescence of Yb,Er-doped nanocrystals in a broad range of sizes from 6 nm to 45 nm (cubic or hexagonal phases), displaying an increasing red-to-green luminescence intensity ratio and reduced luminescence lifetimes with decreasing size. By analyzing the upconversion process with a set of rate equations, we found that their asymptotic analytic solutions explain lower decay rates of red compared to green upconversion luminescence. Furthermore, we quantified the effect of the surface on luminescence lifetime in a model where nanocrystal emitters are divided between the near-surface and inside regions of each nanocrystal. We clarify the influence of the four nonradiative recombination mechanisms (intrinsic phonon modes, vibration energy of surface ligands, solvent-mediated quenching, and surface defects) on the decay rates for different-size nanocrystals, and find that the defect density dominates decay rates for small (below 15 nm) nanocrystals. Our results indicate that a defect-reduction strategy is a key step in producing small upconversion nanocrystals with increased brightness for a variety of bioimaging and biosensing applications.

Design and Simulation of Multiquantum-Well AlGaAs/GaAs Single Junction Solar Cell with Back Surface Reflector

This paper reports on the design and simulation of a multiquantum Well (MQW) AlGaAs/GaAs single p-i-n junction cell with a Distributed Bragg Reflector (DBR) placed at back surface. The DBR structure reflects the part of the spectrum that benefits from absorption in the AlGaAs MQW structure, while being transparent to the IR spectrum. The addition of a DBR allows for a possible second bottom cell junction to be placed beneath the DBR in series with the MQW single cell for an additional efficiency enhancement. The efficiencies and short-circuit currents for the AlGaAs MQW cell with and without DBR are simulated. An energy balance equations model coupled with drift-diffusion equations is solved in heterojunction p-i-n with embedded quantum wells to model hot electron effects. The current-voltage characteristics of an MQW single junction AlGaAs/GaAs p-i-n solar cell structure were compared with measured data. The efficiencies and short-circuit currents of an optimized cell with and without DBR are simulated. Results obtained show improvement in short circuit current and efficiency when both MQW and DBR were used. Simulation results predict an efficiency of about 28.4% for the MQW AlGaAs-DBR single p-i-n junction photovoltaic cell under the presence of both radiative and nonradiative recombination mechanisms.

Carrier dynamics in bulk GaN

Carrier dynamics in hydride vapor phase epitaxy grown bulk GaN with very low density of dislocations, 5–8 × 105 cm−2, have been investigated by time-resolved photoluminescence (PL), free carrier absorption, and light-induced transient grating techniques in the carrier density range of 1015 to ∼1019 cm−3 under single and two photon excitation. For two-photon carrier injection to the bulk (527 nm excitation), diffusivity dependence on the excess carrier density revealed a transfer from minority to ambipolar carrier transport with the ambipolar diffusion coefficient Da saturating at 1.6 cm2/s at room temperature. An extremely long lifetime value of 40 ns, corresponding to an ambipolar diffusion length of 2.5 μm, was measured at 300 K. A nearly linear increase of carrier lifetime with temperature in the 80–800 K range and gradual decrease of D pointed out a prevailing mechanism of diffusion-governed nonradiative recombination due to carrier diffusive flow to plausibly the grain boundaries. Under single photon excitation (266 and 351 nm), subnanosecond transients of PL decay and their numerical modeling revealed fast processes of vertical carrier diffusion, surface recombination, and reabsorption of emission, which mask access to pure radiative decay.

Strong charge carrier localization interacting with extensive nonradiative recombination in heteroepitaxially grown m-plane GaInN quantum wells

The development of GaInN quantum well structures with nonpolar crystal orientation for light-emitting diodes and semiconductor lasers is currently one of the main foci of III-nitride-based optoelectronics research. One of the advantages of nonpolar orientations is the absence of polarization fields perpendicular to the quantum well plane. As a consequence, radiative recombination rates are higher compared to quantum wells on polar surfaces. However, due to high densities of threading dislocations and basal plane stacking faults in the case of heteroepitaxially grown nonpolar layers, and due to band gap inhomogeneities in the GaInN quantum wells, characterization of radiative and nonradiative recombination mechanisms is a complex challenge. So far, most published data about band gap fluctuations, charge carrier localization and internal quantum efficiency in nonpolar quantum wells are ambiguous. Here, we present temperature and excitation power density-dependent photoluminescence data featuring multiple characteristics related to strong charge carrier localization in m-plane (1–100) GaInN quantum wells. Thermally activated redistribution of charge carriers between localization sites in these quantum wells is weaker than in polar c-plane ones. The localization strength increases with higher indium concentration in the quantum wells. In the heteroepitaxially grown quantum well structures, the internal quantum efficiency is reduced even at low temperatures (T = 10 K) and especially for m-plane quantum wells with high indium mole fractions.

Photo-catalytic activity of Zn1-xMnxS nanocrystals synthesized by wet chemical technique.

Polyvinyl pyrrolidone capped Zn1-xMnxS (0 ≤ x ≤ 0.1) nanocrystals have been synthesized using wet chemical co-precipitation method. Crystallographic and morphological characterization of the synthesized materials have been done using X-ray diffraction and transmission electron microscope. Crystallographic studies show the zinc blende crystals having average crystallite size approx. 3 nm, which is almost similar to the average particle size calculated from electron micrographs. Atomic absorption spectrometer has been used for qualitative and quantitative analysis of synthesized nanomaterials. Photo-catalytic activity has been studied using methylene blue dye as a test contaminant. Energy resolved luminescence spectra have been recorded for the detailed description of radiative and non-radiative recombination mechanisms. Photo-catalytic activity dependence on dopant concentration and luminescence quantum yield has been studied in detail.

Biexciton quantum yield of single semiconductor nanocrystals from photon statistics.

Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temperature due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, we observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g((2)) photon correlation measurement and show that the normalized amplitude of the cascade feature is equal to the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, we show that the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.

Temperature quenching of photoluminescence of ordered GaInP2 alloy under different excitation densities

AbstractThe photoluminescence (PL) emission from an epi‐structure containing an atomically ordered GaInP2 layer and a GaAs layer was studied under excitation power densities of 0.03 – 3 W/cm2at temperatures of 10 to 300 K. The quenching of the integrated PL intensity from both: the GaInP2and the GaAs layers is stronger under low excitation, than under high excitation density. The temperature dependence, however, have different shapes being the PL decay observed for the GaInP2 layer stronger than that for the GaAs layer. Comparing the temperature dependence of the PL intensity from the ordered GaInP2 and the GaAs layers under different excitation densities and analyzing them together, we conclude that the inhomogeneity of the ordered layer is responsible for the different temperature behavior of the GaInP2 alloy PL emission. To explain the experimentally observed PL intensity temperature dependence an additional nonradiative recombination mechanism due to a thermally activated escape of the carriers from its confinement within regions of lower bandgap has to be taken into account. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of the impurity–host interactions on the nonradiative processes in ZnS:Cr

There is a great deal of controversy about whether the behavior of an intermediate band in the gap of semiconductors is similar or not to the deep-gap levels. It can have significant consequences, for example, on the nonradiative recombination. In order to analyze the behavior of an intermediate band, we have considered the effect of the inward and outward displacements corresponding to breathing and longitudinal modes of Cr-doped ZnS and on the charge density for different processes involved in the nonradiative recombination using first-principles. This metal-doped zinc chalcogenide has a partially filled band within the host semiconductor gap. In contrast to the properties exhibited by deep-gap levels in other systems, we find small variations in the equilibrium configurations, forces, and electronic density around the Cr when the nonradiative recombination mechanisms modify the intermediate band charge. The charge density around the impurity is equilibrated in response to the perturbations in the equilibrium nuclear configuration and the charge of the intermediate band. The equilibration follows a Le Chatelier principle through the modification of the contribution from the impurity to the intermediate band and to the valence band. The intermediate band introduced by Cr in ZnS for the concentrations analyzed makes the electronic capture difficult and later multiphonon emission in the charge-transfer processes, in accordance with experimental results.

Mechanisms of manganese-assisted non-radiative recombination in Cd(Mn)Se/Zn(Mn)Sequantum dots

Mechanisms of non-radiative recombination of electron–hole complexes inCd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations ofMn2 + ions are analyzed within the framework of the single-electron model of deep 3d levels insemiconductors. In addition to the mechanisms caused by Coulomb and exchangeinteractions, which are related because of the Pauli principle, another mechanism due tosp–d mixing is considered. It is shown that the Coulomb mechanism reducesto long-range dipole–dipole energy transfer from photoexcited quantum dots toMn2 + ions. The recombination due to the Coulomb mechanism is allowed for any states ofMn2 + ions and e–h complexes. In contrast, short-range exchange and sp–d recombinationsare subject to spin selection rules, which are the result of strong lh–hh splittingof hole states in quantum dots. Estimates show that efficiency of the sp–dmechanism can considerably exceed that of the Coulomb mechanism. Thephonon-assisted recombination and processes involving the upper excited states ofMn2 + ions are studied. The increase in PL intensity of an ensemble of quantum dotsin a magnetic field perpendicular to the sample growth plane observed earlieris analyzed as a possible manifestation of the spin-dependent recombination.

Effects of the impurity–impurity and impurity–host interactions on the charge density and the related processes

Transitions via impurities, in addition to transitions from the valence band to the conduction band in semiconductors increase the generation of electron–hole pairs. These transitions lead to alterations in the electronic density around the impurities, and as a consequence, to modifications in the forces on the impurities. In many cases it leads to an increase in the non-radiative recombination. The understanding of these processes at an atomic scale is very important for the physics of doped semiconductors, and technologically, for optoelectronic and spintronic devices. Using simple approximate models based on first-principles, the charge density and the related processes are analyzed. The results show that an increase in the impurity–host interaction or an increase in the concentration of the impurity leads to small variations in the density around the impurities for different charge states and a decrease in the effective Coulomb repulsion. These facts reduce the mechanism of non-radiative recombination via multi-phonon emission.

X-ray diffraction, Raman, and photoacoustic studies of ZnTe nanocrystals

Nanocrystalline ZnTe was prepared by mechanical alloying. X-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy, and photoacoustic absorption spectroscopy techniques were used to study the structural, chemical, optical, and thermal properties of the as-milled powder. An annealing of the mechanical alloyed sample at 590 °C for 6 h was done to investigate the optical properties in a defect-free sample (close to bulk form). The main crystalline phase formed was the zinc-blende ZnTe, but residual trigonal tellurium and hexagonal ZnO phases were also observed for both as-milled and annealed samples. The structural parameters, phase fractions, average crystallite sizes, and microstrains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns. Raman results corroborate the XRD results, showing the longitudinal optical phonons of ZnTe (even at third order) and those modes of trigonal Te. Nonradiative surface recombination and thermal bending heat transfer mechanisms were proposed from photoacoustic analysis. An increase in effective thermal diffusivity coefficient was observed after annealing and the carrier diffusion coefficient, the surface recombination velocity, and the recombination time parameters remained the same.

Lifetime recovery in ultrahighly titanium-doped silicon for the implementation of an intermediate band material

The doping of conventional semiconductors with deep level (DL) centers has been proposed to synthesize intermediate band materials. A recent fundamental study of the nonradiative recombination (NRR) mechanisms predicts the suppression of the NRR for ultrahigh DL dilutions as a result of the delocalization of the impurity electron wave functions. Carrier lifetime measurements on Si wafers doped with Ti in the 1020–1021 cm−3 concentration range show an increase in the lifetime, in agreement with the NRR suppression predicted and contrary to the classic understanding of DL action.

Effects of rapid thermal annealing on optical properties of p-doped and undoped InAs/InGaAs dots-in-a-well structures

Postgrowth rapid thermal annealing was used to investigate the intermixing and structural changes in p-doped and undoped InAs/In0.1Ga0.9As dots-in-a-well (DWELL) structures grown by molecular beam epitaxy. Interdiffusion of In and Ga atoms caused by thermal annealing was proven from photoluminescence (PL) measurements, where blueshifts of the energy peaks were observed. The results show that p-doped quantum dot (QD) structures are more resistant to intermixing with higher thermal energy onset, and the reason is explained as the suppressed Ga diffusion resulted from the Be dopant. Rapid quenching of the integrated PL intensity at high temperature was observed in both undoped and p-doped DWELL QDs. Good agreement was obtained by fitting the integrated PL profile using two nonradiative recombination mechanisms, resulting in two activation energies that correspond to loss of carriers to nonradiative centers.

Temperature and injection current dependent electroluminescence study of GaInP/AlGaInP quantum well laser diode grown using tertiarybutylarsine and tertiarybutylphosphine

GaInP/AlGaInP triple quantum well (TQW) lasers, grown by metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP), were fabricated with a pulsed anodic oxidation (PAO) process. The devices worked at room temperature (RT) with the lowest threshold current density (Jth) of 1.5 kA/cm2 ever reported for GaInP/AlGaInP lasers grown using TBAs and TBP. Temperature dependent (35–250 K) electroluminescence (EL) study of the GaInP/AlGaInP laser diode showed almost the same luminescence quenching behavior at a high temperature region (120–250 K), independent of the injection current (100–150 mA). A model involving a nonradiative recombination mechanism was presented to interpret the EL quenching behavior over the experimental temperature range. The nonradiative recombination centers in the Al-containing barrier or cladding layer are believed to contribute to the loss of carriers via nonradiative recombination.

Low temperature characterization of the photocurrent produced by two-photon transitions in a quantum dot intermediate band solar cell

Conceived to exceed the conversion efficiency of conventional photovoltaic devices, the intermediate band solar cell bases its operation on exploiting, besides the usual band-to-band optical transitions, the absorption of two sub-bandgap photons. For the present, the only technology used to implement an intermediate band in real devices has been the growth of an InAs/GaAs quantum dot superlattice. In practice, the obtained material shows two limitations: the narrow energy gap between conduction and intermediate band and the appearance of growth defects due to the lattice stress. The consequences are the presence of non-radiative recombination mechanisms and the thermal escape of electrons from the intermediate to the conduction band, hindering the splitting of the quasi-Fermi levels associated with the intermediate and conduction bands and the observation of photocurrent associated with the two-photon absorption. By reducing the temperature at which the devices are characterised we have suppressed the parasitic thermal mechanisms and have succeeded in measuring the photocurrent caused by the absorption of two below bandgap photons. In this work, the characterization of this photocurrent at low temperature is presented and discussed.

Third order nonlinear susceptibility of InN at near band-gap wavelengths

The authors report room-temperature measurements of the third order nonlinear susceptibility modulus ∣χ(3)∣ of thick (∼600nm) InN layers. Transmission measurements provide a room-temperature value for the optical band gap of the samples slightly above 1500nm. Third order nonlinear optical susceptibility has been measured using degenerate four wave mixing experiments at wavelengths near and above band gap. ∣χ(3)∣ values of (4.2–10)×10−10esu were measured at this wavelength range. The associated relaxation time of the generated population grating at 1500nm was measured. The obtained value of 4.8ps is consistent with a nonradiative recombination mechanism.

Recombination-mechanism dependence of transport and light emission of ambipolar long-channel carbon-nanotube field-effect transistors

The transport properties of ambipolar long-channel carbon-nanotube field-effect transistors are calculated in the framework of a diffusive-transport model. The effects associated with radiative and nonradiative recombinations of injected electrons and holes in the channel are considered, and the spatial dependence of the recombination profile on the gate and drain voltages is explored. Nonradiative recombination is shown to play a decisive role in the transport characteristics. The emitted light spot size is predicted to exhibit sensitive dependence on the nonradiative recombination mechanism. Moreover, the local electric field reaches a maximum but remains relatively small inside the recombination region.

Analysis of Deviation of Threshold Voltage from Hole Accumulation Model at High Excitation

The energy state, concentration, and potential energy for both electrons and holes in the channel of InAlAs/InGaAs high electron mobility transistors (HEMTs) were studied using the theory based on the local density functional method. The numerical result shows that the potential profile changes from the triangle to the square well as the sheet concentration of holes accumulated in the source region ( ps) increases above the sheet concentration of the two-dimensional electron gas (2DEG) because the same amount of electrons as holes was injected from the source to maintain the charge neutrality in the channel. As a result, the quasi-Fermi energy increased and the potential energy of electrons in the channel approached that of the square-well potential, the former led to an increase in the threshold voltage (VTH). The overlap integral between the wavefunctions of an electron and a hole was estimated as a function of the channel thickness (Lz) and was shown to decrease with increasing Lz. A detailed theory concerning the relation between the VTH shift and ps was developed and compared with the experimental results. In this theory, the recombination of holes with 2DEG was taken into account, on the assumption that the dominant process was due to the non-radiative Auger recombination mechanism.