Carrier Recombination Mechanisms Research Articles

Recombination and radiation damage in crystalline silicon solar cell material

This article reviews the carrier recombination mechanisms which degrade crystalline solar cells as a result of exposure to light (photo-degradation) and in the case of satellite cells after exposure to energetic electrons in space. The role of impurities is considered and in particular contamination with iron and the formation of metastable boron-oxygen complexes. The application of high resolution (Laplace) deep level transient spectroscopy in the structural identification of the induced defects is discussed with a view to controlling their formation by defect engineering. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dominant carrier recombination mechanisms in GaInNAs∕GaAs quantum well light-emitting diodes

The electroluminescence of GaInNAs∕GaAs quantum well light-emitting diodes is analyzed as a function of temperature and injection current. The relative influence of nonradiative carrier recombination, recombination from localized states, and conduction-band to valence-band recombination is discussed. The localized states are found to dominate the emission and the external quantum efficiency only at low temperatures and currents. When temperature and∕or injection level are increased, band-to-band transitions become the main recombination mechanism. Nonradiative recombination is strongly thermally activated, and becomes the dominant process above 75K. As a result of postgrowth rapid thermal annealing, the device luminescence efficiency increases by over one order of magnitude due to a decrease in the density of nonradiative recombination centers.

Interpretation of anomalous temperature dependence of anti-Stokes photoluminescence at GaInP2/GaAs interface

In this letter, we report on temperature-dependent anti-Stokes photoluminescence (ASPL) at an interface between partially ordered GaInP2 epilayer and GaAs substrate. It is found that the intensity of the ASPL depends strongly on temperature accompanying with a clear blueshift in energy. A localized-state luminescence model was employed to quantitatively interpret temperature dependence of the ASPL. Excellent agreement between the theory and experiment was obtained. Radiative recombination mechanism of the up-converted carriers was discussed.

Formation mechanism and low-temperature instability of exciton rings.

The macroscopic rings observed in the photoluminescence patterns of excitons in coupled quantum wells are explained by a mechanism of carrier imbalance, transport, and recombination. The rings originate from the spatial separation of p and n carriers, and occur at the interface of the p and n domains, where excitons are generated. We explore the states of excitons in the ring over a range of temperatures down to 380 mK and report a transition of the ring into a periodic array of aggregates, a new low-temperature ordered exciton state.

Optical investigation of GaAs1−xNx/GaAs heterostructure properties

AbstractWe have investigated, by photoluminescence (PL), the optical properties of GaAs1−xNx/GaAs epilayers and GaAs1−xNx/GaAs quantum well (QW) structures grown by metal organic vapor phase epitaxy (MOVPE) on (001)‐oriented GaAs substrates. Different behaviors have been observed for the bulk epilayer and for the QW structures, respectively: (i) a blue shift of the PL bands in both kinds of structures when increasing the excitation density, (ii) an S‐shaped PL peak energy versus temperature dependence has been observed for the GaAsN epilayer but a usual behavior is obtained for the QWs. Based in these experimental results, we have suggested that the carrier recombination mechanisms in the epilayer and in the quantum well structure are different. An enhanced exciton‐localization‐like mode for the epilayer is observed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical turn-on characteristics of vertical-cavity surface-emitting lasers

We present a detailed comparison of the electrical turn-on characteristics of 980 nm vertical-cavity surface-emitting lasers (VCSELs) with simulations using a recently extended laser simulator. It is shown that the three recombination mechanisms, spontaneous emission, Shockley–Read–Hall recombination, and Auger recombination, result in distinctly different exponential current–voltage dependencies below threshold. Therefore, information can be extracted about the relative strength of the recombination rates due to those processes, and their relative contributions to the threshold current can be assessed. We show that for the VCSELs studied in this work, spontaneous emission is the dominant contribution to the threshold current.

Minority carriers recombination in n-InP single crystals

We present a comprehensive study of minority carrier lifetime and recombination mechanisms in n-InP single crystals. The study is based on steady state and time-resolved photoluminescence measurements in a wide temperature range (15–300 K). Various recombination and trapping mechanisms are analyzed and compared in order to assess their influence on the effective hole lifetime. It was found that two main processes govern the hole lifetime: radiative band-to-band recombination at temperatures >100 K and nonradiative trapping in unintentional induced acceptor centers at low temperatures (<100 K). A fit of the measured effective bulk lifetime to a recombination model based on these two processes enabled us to extract the rate constant for the radiative band-to-band recombination, and the density of the acceptor-like trapping centers.

Electron irradiation of a-Si:H films prepared from hydrogen-diluted silane

The effect of 18 MeV electron irradiation on the optoelectronic and structural properties of a-Si:H films has been studied. It has been established that the irradiation leads to a strong decrease of photo- and dark conductivities, causes a change in the state distribution of the valence band-tail, as well as in the recombination mechanism of the photoexcited carriers. The structural properties of the films have been studied by means of Raman spectroscopy. The observed change in the short and medium-range order of the amorphous silicon network suggests that the high-energy electron irradiation induces structural defects, as well.

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.

Recombination mechanism of excess carriers in hydrogenated amorphous germanium

In this work we report a study of the recombination mechanism of electrons trapped in the conduction band-tail states with holes trapped in the valence band-tail states as a function of the light intensity, G, in hydrogenated amorphous germanium (a-Ge:H). The density of trapped electrons increases with light excitation, but the saturation spin density depends only weakly on the light intensity and increases as G 0.17. On the other hand, the characteristic time constant for growth of the light-induced electron spin resonance (LESR) signal, which is related to the recombination lifetimes, follows a power law function, G −0.65. The growth and decay curves can be well fitted by a dispersive model, with a dispersive parameter of about 0.5, assuming bimolecular recombination.

Low-temperature recombination kinetics of photoexcited persistent charge carriers in conjugated polymer/fullerene composite films

The recombination kinetics of long-lived photoexcited charge carriers in a composite of $\mathrm{poly}[2\ensuremath{-}\mathrm{methoxy}{\ensuremath{-}5\ensuremath{-}(3}^{\ensuremath{'}}{,7}^{\ensuremath{'}}\ensuremath{-}\mathrm{dimethyloctyloxy})\ensuremath{-}1,4\ensuremath{-}\mathrm{phenylene}$ vinylene] (MDMO-PPV) and $1\ensuremath{-}(3\ensuremath{-}\mathrm{methoxycarbonyl})\ensuremath{-}\mathrm{propyl}\ensuremath{-}1\ensuremath{-}\mathrm{phenyl}\ensuremath{-}(6,6){\mathrm{C}}_{61}$ (PCBM) at low temperatures $(T=40\mathrm{K})$ are investigated by light induced electron-spin resonance (LESR). These long-lived (persistent) photoinduced charge carriers exhibit recombination times that extend over several hours after cessation of the photoexcitation. These long relaxation times can be explained by nongeminate recombination of randomly distributed carriers assuming charge neutrality. The decay curves fit well to a model in which the recombination mechanism of photoexcited carriers consists of tunneling processes and in which the recombination rate only depends on the intrapair distance between the photoexcited carriers. It is shown that the residual photoexcited carrier concentration after long times tends to be independent of the generation rate. The presented model has already been successful in describing the recombination kinetics of photoexcited carriers in inorganic, amorphous semiconductors, which indicates that the presented recombination mechanism is common to disordered organic and inorganic materials.

Effect of temperature on photoconductivity and its decay in microcrystalline silicon

The effect of temperature on steady-state photoconductivity and its relaxation after illumination cut-off in lightly boron-doped microcrystalline hydrogenated silicon films has been studied. The measurements were performed in the temperature range 150–430 K with an incident photon energy of 1.4 eV. The temperature dependences of the photoresponse time and drift mobility in microcrystalline silicon have been obtained from the data on the steady-state photoconductivity and its decay. Mechanisms of carrier transport and recombination that governing these temperature dependences are discussed.

Luminescence of plasma-treated polymer surfaces at ambient temperature

The aim of the present work is to detect the light emitted by a polypropylene film at ambient temperature once its surface has been subjected to a cold plasma, in other words plasma-induced luminescence (PIL). The general topics is to develop a tool for investigating surface polymer transformation. Kinetic and spectral features of PIL emission were recorded after discharge switch off, thanks to a photomultiplier and a liquid-nitrogen-cooled CCD camera. Three spectral components were clearly identified with their own excitation processes. The two fastest luminescence decay processes are due to radiative de-excitation of energy levels populated during plasma contact, termed as ‘direct excitation mechanisms’. They are related to the short wavelength range of the PIL spectrum which mainly characterizes chromophores initially present in the material. The third one, called a ‘carrier-recombination mechanism’, corresponds to a slow luminescence decay which becomes more and more dominant with the increase of the treatment duration.

Physical Insight into Thermal Behaviour of Power DMOSFET and IGBT: A Two-Dimensional Computer Simulation Study

To acquire a physical understanding of the influence of temperature on the electrical characteristics of power DMOSFET (double-diffused metal-oxide-semiconductor field effect transistor) and IGBT (insulated gate bipolar transistor), and to understand their failure mechanisms, a comprehensive thermal characterization of DMOSFET and IGBT unit cells of various structural parameters and impurity diffusion profiles has been carried out as a function of lattice temperature, encompassing the broad temperature range from 200 to 600 K. Device electrical characteristics have been generated in the steady-state forward conduction, blocking and transfer modes of operation using the computer program ISE-TCAD tools and incorporating the relevant temperature-dependent physical models of carrier transport properties and recombination mechanisms during the analysis. This computer-aided analysis has revealed that at a given temperature, within their respective operating ranges, for a particular forward voltage, the IGBT cell always yielded a much higher current than its DMOSFET cell counterpart. DMOSFET behaviour has been found to be maintained from 200 to 450 K with changes in terminal electrical parameters like ON-resistance, transconductance, saturated drain current, threshold voltage, blocking voltage and leakage current. On the contrary, IGBT cells have shown to be capable of working up to 600 K, much higher than and in striking dissimilarity to the DMOSFET cells. Unlike the DMOSFET cell characteristic, the IGBT cell forward characteristics did not fail completely with temperature. Their partial failure has been observed as a more rapid increase in current at a given voltage so that the curve for higher temperature intersected the one at lower temperature. The current density of failure has been found to decrease with increasing temperature. This was ascribed to latching of the parasitic bipolar thyristor. In addition, a moderate variation of IGBT cell forward characteristics with temperature has been observed, distinguishing them from DMOSFET cells. Further, the transfer characteristic of the IGBT cell has not exhibited collector current saturation at high gate voltages. This saturation behaviour was a notable feature of the DMOSFET cell with the current saturation level depending on temperature. The simulated results have been interpreted in terms of the physics of device operation by invoking the power DMOSFET ON-resistance model, and combinational bipolar transistor-DMOSFET model of the IGBT. Physical explanation of the higher operating temperature capability of IGBT involves the simultaneous action of oppositely-directed thermally varying forward voltage drops in the p–i–n rectifier and the DMOSFET. The simulation study has also established that a distinct correlation of the maximum safe operating temperature with device structural and profile design parameters was possible for the IGBT but was not so apparent for the DMOSFET, making it an inherent device property. Controllability of latching current density and hence peak temperature-withstanding capability of IGBT cells by design parameter modification has been demonstrated by enhancing the current density of safe operation of IGBT cell at a given temperature with the adoption of suitable design techniques.

Near-field photoluminescence study of GaNAs alloy epilayer at room and cryogenic temperature

We have measured the spatial distribution of the optical properties of a GaNAs (N∼0.8%) epilayer to investigate the carrier recombination mechanism at both room temperature and cryogenic temperature using a near-field scanning optical microscope. A difference between the macro and near-field photoluminescence (PL) spectra at room temperature was not observed. At low temperature, we found spatial inhomogeneity of the optical properties and sharp features in the near-field PL spectrum. These findings indicate that the dominant emission mechanism changes from recombination of delocalized carriers at room temperature to recombination of localized carriers (excitons) trapped in the local potential minimum due to compositional fluctuation at low temperature.

Design and analysis of vertical-cavity semiconductor optical amplifiers

The authors present detailed, yet largely analytical, models for gain, optical bandwidth, and saturation power of vertical-cavity semiconductor optical amplifiers (VCSOAs) in reflection and transmission mode. Simple formulas for the gain-bandwidth product are derived. The saturation model considers a sublinear material gain, gain enhancement by the standing-wave effect, and all relevant carrier recombination mechanisms. Excellent agreement with measurements on novel 1.3-/spl mu/m VCSOAs is obtained. The models are used to analyze device performance and to investigate optimization options. Parameter plots are given which allow for an easy exploration of the VCSOA design space, matching desired performance data with the required mirror reflectivity and pump current.

Mechanism of carrier generation and recombination in conjugated polymers

Transient excited-state absorption measurements in the spectral region spanning the infrared active vibrational active (IRAV) modes in prototypical luminescent polymers, poly(phenylene vinylene) (PPV), and poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV), reveal charge carrier generation within 100 fs after photoexcitation. The photocarrier quantum efficiency in MEH-PPV is φ 0≈0.1 in zero applied electric field. There is no correlation between the temporal behavior of the photoinduced IRAV signals and the exciton lifetime. Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation. Our data indicate that the carrier recombination rate is sensitive to the strength of the interchain interaction.

Molecular Organic Light-Emitting Diodes Based on a Guest-Host Active Layer: Approaches for Enhancing Device Performance

We present a comprehensive study of enhancement mechanisms in electroluminescence devices where the active layer is doped with fluorescent dye. We demonstrate two typical systems where the emission mechanism is based on either energy transfer or direct carrier recombination. For the energy transfer mechanism, we find that the maximum quantum efficiency of multilayered devices depends on the nature of the host in addition to the luminescent properties of the guest, by comparing electroluminescence and absolute photoluminescence quantum yields in various guest-host systems. For the direct carrier recombination mechanism using rubrene as the dopant and TPD as the host, we find that the dual electronic nature of rubrene acting as a hole trap and an electron hopping site is responsible for maximizing the carrier recombination on the dopant molecule.

A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se 2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se) 2 solar cells result from alloying CuInSe 2 base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.

Applications of optically detected magnetic resonance in semiconductor layered structures

A short introduction is given on the physics, method, capabilities and limitations of the optically detected magnetic resonance (ODMR) technique. The advantages of the optical detection method in terms of sensitivity and of its direct probe in recombination processes, as compared with the traditional spin resonance technique, will be demonstrated. The importance of these advantages for the ODMR applications in semiconductor layered and quantum structures will be emphasized. The ability of the ODMR technique to provide important information on physical properties of semiconductor layered structures will be highlighted. These include chemical identification, electronic and geometric structure of both radiative and non-radiative defects, carrier recombination mechanism, electronic excitation, etc. Representative cases from CVD-SiC and MBE-Si/SiGe based layered structures will be discussed as examples. The most recent progress, on-going efforts and prospects in achieving unprecedentedly high spectral, time and spatial resolution of the ODMR technique will also be outlined.