Nonradiative Electron-hole Recombination Research Articles

Development of nanoimprinted InP QDs decorated polyaniline solar cell with conversion efficiency 3%

Organic–inorganic hybrid materials received considerable attention due to promising industrial applications. The originality of novel chemical recipes, allowing incorporation of well-defined nanoparticle structures into complex hybrid architectures, opens new possibilities for multidisciplinary fields and in particular in optoelectronic devices. The rate of non-radiative recombination and energy transfer through a hybrid inorganic/organic nanocomposite is mainly governing the ability of charge transfer from semiconductor quantum dots to conjugated polymers. Herein, we report that the electron–hole non-radiative recombination in polymer can be constricted by funneling the diffusion of exciton by engineering a proper morphology of a hybrid nanostructure. InP quantum dots have been selected due to their efficient exciton generation and polyaniline as a conjugated polymer for its potency to suppress non-radiative recombination by restraining exciton diffusion. The hole transfer was monitored via bi-exponential kinetic model and time of flight method. The conversion efficiency of the prepared films increased from 0.23% to 3.1% when the thickness is increased from 14nm to 157nm.

Ultrafast Spectral Migration of Photoluminescence in Graphene Oxide

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.

Ultrafast Charge Transfer Dynamics in Polycrystalline CdSe/TiO2 Nanorods Prepared by Oblique Angle Codeposition

Ultrafast exciton dynamics of aligned polycrystalline nanorod arrays composed of CdSe or CdSe/TiO2 grown on conductive glass substrates using oblique angle deposition/codeposition have been studied using femtosecond transient absorption (TA) spectroscopy. Scanning electron microscopy images show that the morphology of the two samples are comparable in height, width, and tilt angle. X-ray diffraction and Raman spectroscopy indicate that the as-deposited CdSe nanorod arrays are in the hexagonal phase, while the TiO2 is amorphous. In the TA studies, a pump wavelength of 580 nm was used to determine the exciton lifetimes of CdSe in the two samples. Transient bleach dynamics probed at 695 nm can be fit with triple exponential functions with lifetimes of 7 ps, 84 ps, and ∼1.0 ns for CdSe nanorods versus 0.5 ps, 3 ps, and 24 ps for the CdSe/TiO2 composite-nanorods. These lifetimes are independent of the pump power, indicating that nonlinear processes are not involved. For CdSe nanorods, the two fast decays are mainly due to nonradiative electron–hole recombination or exciton relaxation mediated by trap states. The overall much faster decay in CdSe/TiO2 nanorods is due to electron transfer from the conduction band of CdSe to the conduction band of TiO2. The electron injection rate from CdSe into TiO2 was calculated to be 1.7 × 1011 s–1 based on the average lifetime measured for CdSe with and without TiO2. This very high rate of electron injection is attributed to the large interfacial area and strong coupling between the two materials in CdSe/TiO2 composite-nanorods. Such strongly coupled semiconductor–metal oxide heterostructures are desired for applications in solar energy conversion.

Impurity Diffusion in InGaAs Esaki Tunnel Diodes of Varied Defect Densities

We have fabricated and investigated InGaAs Esaki tunnel diodes, grown on GaAs or InP substrates, of varied defect densities. The tunnel diodes exhibit the same I-V characteristics in spite of the variation of defect density. Under the simple thermal annealing and forward current stress tests, the change in the valley current was not observed, indicating that defects were not increased. On the other hand, the reduction in the peak current due to the carbon diffusion was observed under both tests. The diffusion was enhanced by the stress current owing to the energy dissipation associated with the nonradiative electron-hole recombination. From the reduction rates of the peak current, we obtained the thermal and current-enhanced carbon diffusion constants in InGaAs, which are independent of defect density. Although thermal diffusion of carbon in InGaAs is comparable with that in GaAs, the current-induced enhancement of diffusion in InGaAs is extremely weaker than that in GaAs. The difference between activation energy of thermal and current-enhanced diffusion is 0.8 eV, which is independent of stress current density and close to InGaAs bandgap energy. This indicates that the current-enhanced diffusion is dominated by the energy dissipation associated with nonradiative band-to-band recombination. This enhancement mechanism well explains that the current-induced enhancement is independent of defect density and extremely weak. We also have found that the current-enhanced diffusion constant is approximately proportional to the square of current density, suggesting that the recombination in the depletion layer dominates the current-enhanced diffusion.

Nonradiative electron–hole recombination in ZnSSe epitaxial layers examined by piezoelectric photothermal spectroscopy

Piezoelectric photothermal (PPT) and photoluminescence (PL) spectra could be successfully observed at low temperatures for nondoped ZnSe and ZnS0.08Se0.92 epitaxial layers grown by molecular beam epitaxy. The donor bound exciton (I2) and free-to-acceptor (FA) emission bands were dominant in the PL spectra of both samples. A dominant acceptor-type defect on the FA emission might not be due to S-related defects because the PL emission from this defect was observed in both the ZnSe and ZnSSe samples. Furthermore, this acceptor-type impurity behaved as both electron–hole nonradiative and radiative recombination centers because the distinct signals appeared in the PPT and PL spectra of the ZnSSe sample.

Investigation of the characteristic changes on SrTiO3:Pr,Al,Ga phosphors during low voltage electron irradiation

We have investigated the mechanism of degradation of the low voltage cathodoluminescence (CL) efficiency of SrTiO3:Pr,Al,Ga phosphors. Based on Auger electron spectroscopy and x-ray photoelectron spectroscopy study, it is understood that prolonged irradiation of the phosphor with an electron beam of low voltage and high current density causes characteristic changes [(1) accumulation of overlying carbon and (2) reduction of oxygen] to occur on the phosphor surface. These changes are responsible for the rapid degradation of low voltage CL of SrTiO3:Pr,Al,Ga phosphors. The two aforementioned changes are shown to impact CL output in an important way. An accurate accounting of the total impact of the two changes warrants assessment of the importance of other degradation mechanisms. These other degradation mechanisms include both carbon- and noncarbon-related enhanced nonradiative electron-hole recombination at surfaces.

Modeling gallium arsenide heterojunction bipolar transistor ledge variations for insight into device reliability

It is widely known that under normal bias conditions, GaAs heterojunction bipolar transistor (HBT) device degradation proceeds by a gradual buildup of defects in the base and base–emitter junction depletion regions. The buildup of these defects is associated with a solid-state phenomenon known as recombination enhanced defect reaction, which is the formation and migration of defects associated with nonradiative electron–hole recombination events. These defects are often associated with midgap traps, which serve as additional recombination centers for electron–hole pairs. The resulting increased recombination current is an additional base leakage current, which reduces current gain. By extension, a high electron–hole recombination density in a region with an initially high defect density––such as an unpassivated or poorly passivated base surface––will lead to quick device degradation. This paper reports the modeling of the effects of various different extrinsic base passivation ledge parameters––material composition, thickness, width, and spacing from ledge to base contact––to determine the microscopic effects these parameters have on electron–hole recombination density. Through this we can qualitatively predict the effects these parameters will have on HBT reliability.

Nonradiative electron-hole recombination by a low-barrier pathway in hydrogenated silicon semiconductors

A microscopic pathway for nonradiative electron-hole recombination by large structural reconfiguration in hydrogenated Si is found with first-principles calculations. Trapped-biexciton formation leads to a low-barrier reconfiguration of the H atom, accompanied by crossing of doubly occupied electron and hole levels in the band gap. This crossing represents the nonradiative recombination of the carriers, without multiphonon emission. The proposal provides a mechanism for carrier-induced H emission during metastable degradation of hydrogenated amorphous silicon.

Femtosecond study of photo-induced electron dynamics in AgI and core/shell structured AgI/Ag2S and AgBr/Ag2S colloidal nanoparticles

Direct measurements of the dynamics of photoinduced electrons in AgI, AgI/Ag2S, and AgBr/Ag2S semiconductor colloidal nanoparticles have been performed using femtosecond pump–probe laser spectroscopy. In AgI the transient absorption signal features a laser pulsewidth limited rise followed by a double exponential decay with time constants of 2.5 ps and >0.5 ns. The decay dynamics were found to be independent of pump power, indicating that the decays are not second order kinetic processes. The dynamics were also independent of the probe wavelengths, suggesting that the absorption spectrum of the photogenerated electrons is fully developed within ∼150 fs. The fast 2.5 ps decay is attributed to trapping and nonradiative electron–hole recombination mediated by a high density of trap states while the slow decay is attributed to reaction of deep trapped electrons with silver ions to form Ag atoms. For core/shell structured cocolloids of AgI/Ag2S, similar decay dynamics were observed. The only interesting difference is that the fast decay becomes faster with increasing concentration of Ag2S. This is due to direct excitation of Ag2S, which has a faster initial decay (800 fs) than in AgI. This is supported by the results of AgBr/Ag2S for which the same 800 fs decay was observed while no signal was observed for AgBr colloids alone, indicating clearly that the signal is from Ag2S. These findings on the early time dynamics of photo-induced excitons in AgI, AgI/Ag2S, and AgBr/Ag2S are important in better understanding the photographic process involving silver halides.

Cathodoluminescence, reflectivity changes, and accumulation of graphitic carbon during electron beam aging of phosphors

We demonstrate that extended e-beam exposure produces a contaminating overlayer on phosphors whose opacity increases roughly linearly with time. Raman scattering data and optical analysis indicate that this layer is graphitic in nature, arising from the electron-beam-stimulated conversion of hydrocarbons adsorbed from the vacuum ambient. The presence of this contamination optically attenuates emitted cathodoluminescence, prevents many low energy electrons from ever reaching the phosphor grains, and exacerbates surface charging which reduces the arrival energy of electrons above 1.5–2 keV. All of these effects are shown to impact cathodoluminescent output in an important way, but an accurate accounting of their total impact will be required to assess the importance of other degradation mechanisms like enhanced nonradiative electron-hole recombination at surfaces, both carbon and noncarbon related.

Non-radiative electron-hole pair recombination in degraded GaAs/GaAlAs double heterostructure

The rapid degradation of the luminescence emitted by GaAs/AlGaAs double heterostructures has been studied by spatially resolved photoluminescence at 4 k. A detailed analysis of the recombinations shows that a non-radiative process is efficient at low temperature and high injection in the degraded areas. We propose that the deformation potential induced by the strain field around dislocations is responsible for this new non-radiative process.

Reaction of silicon with chlorine and ultraviolet laser induced chemical etching mechanisms

The reaction of 〈100〉Si with Cl2 and the excimer laser induced chemical etching process using 308 and 248 nm radiation have been investigated. Our results show that even at high Cl2 pressures, only a thin passivating chlorinated surface layer is built up which impedes further reaction. Pulsed excimer laser etching at high energy fluences is dominated by thermal evaporation. At lower energy fluences the melt depth decreases and good etch profiles with excellent spatial resolution are observed. At the lowest laser fluences nonthermal, wavelength dependent etching occurs. The laser etching at 308 nm is based on locally enhanced surface chlorination due to effective photodissociation of Cl2 in the gas phase. Photoinduced desorption, most likely due to nonradiative electron-hole recombination, leads to etching. Conversely, at 248 nm, negligible gas phase photodissociation occurs and the surface reaction of Cl2 molecules limits the etch rate. However, the higher photon energy (5.0 eV for 248 nm) leads to efficient photodesorption of surface atoms and molecules by direct bond breaking which can give rise to higher etch rates than at 308 nm. Depending on the actual conditions, etching at 308 or 248 nm is more efficient. High etch rates can be achieved by combining the effective photodesorption process at 248 nm with a high degree of surface chlorination due to generation of chlorine gas phase radicals in a microwave discharge. In addition, the spatial resolution capability for direct pattern transfer by excimer laser induced chemical etching has been analyzed as a function of energy fluence, gas pressure, and gas phase radical concentration.

Effect of macroscopic stress on accelerated aging of GaInAsP channeled substrate buried heterostructure lasers

We report the results of the measurement of radius of curvature of 1.3 and 1.5 μm wavelength GaInAsP-InP channeled substrate buried heterostructure lasers. The objective of this investigation is to quantify the macroscopic stress present in the device and correlate it with device reliability. The change in dc threshold current (ΔIth) after an accelerated aging test was used as a measure to access device reliability, with high ΔIth indicating decreased reliability. Changes were made in the p-side metallization to bring about a change in either ΔIth or radius of curvature and they included two different contact widths and different thicknesses of the Au bonding pad. It is observed that no correlation between device curvature and ΔIth exists even though the modifications in the p metallization caused significant changes in both quantities. It is suggested that it is not the macroscopic device stress that is measured by the radius of curvature but localized stresses that may exist in the vicinity of the lasing active layer which would affect device reliability. It is surmised that the most important role of stress is its effect on the direction of defect migration with the principal driving force coming from the nonradiative electron-hole recombination occurring in the vicinity of the active layer.

Degradation of GaAs lasers and light-emitting diodes on silicon substrates

The degradation of single-quantum-well graded-refractive-index GaAs lasers and light-emitting diodes grown on silicon substrates by metallo-organic vapor deposition is studied. The mechanism resulting in degradation appears to be identical with that observed in GaAs devices on GaAs substrates. However, the devices degrade very rapidly, even under pulsed operation, owing to the presence of a very high density of non-radiative regions which results from the dislocations caused by the 4.1% lattice mismatch between the GaAs and the silicon substrate. Continuous operation has been obtained up to 160 K. At higher temperatures the dark regions quench the emission 3–5 μs after the start of a current pulse. This prevents the continuous operation at room temperature of these lasers. During operation the dark regions grow rapidly in size by non-radiative electron-hole recombination in the presence of the large tensile stress in the epilayer. We estimate that it will be necessary to reduce the defect density to less than about 10 4 cm −2 and to prevent the growth of defects in the active layer in order to achieve reliable operation of light-emitting devices.

Nonradiative electron-hole recombination through deep centers in semiconductors

Dynamical processes associated with nonradiative multiphonon capture of injected minority carriers by a deep-level defect are described, including the capture itself, the recombination enhanced defect reaction and the coherent electron-hole capture. A simple formula is given their rates with a standard basis put on the adiabatic limit for electron transition much faster than lattice motion.

Defect Reactions in GaP: (Zn,O)

We have observed photoinduced reactions between pairs of zinc and oxygen impurities in gallium phosphide. From photoluminescence studies we find that the nearest-neighbor (Zn, O) pairs are dissociating, after which they re-form as further separated pairs. The activation energy for the dissociation is found to be 0.60\ifmmode\pm\else\textpm\fi{}0.07 eV for the photoinduced reaction, and 2.6\ifmmode\pm\else\textpm\fi{}0.6 eV for the purely thermal reaction. We tentatively identify the photoinduced reaction as being due to excitation of local phonon modes by nonradiative electron-hole recombination.