Electron-hole Recombination Lifetime Research Articles

Control of Polaronic Behavior and Carrier Lifetimes via Metal and Anion Alloying in Chalcogenide Perovskites

Transition-metal perovskite chalcogenides (TMPCs) have emerged as lead-free alternatives to lead-halide perovskites and have been currently of increasing interest for optoelectronic applications because of their suitable band gaps, high carrier mobility, strong light absorption, and high stability. Here, we systematically report a study on the effects of Ti- and Se-alloying strategies on polaron behavior and carrier lifetimes in nonradiative recombination. Although such alloying can effectively tune the band gap of BaZrS3, we observe localized small polaron formation upon Ti alloying and large polarons generating in Se alloying. Ti-alloying strengthens the electron-phonon coupling, leading to a reduced carrier lifetime. Remarkably, Se-alloying weakens the electron-phonon coupling and prolongs the nonradiative electron-hole recombination lifetime by up to 60% compared to that in pristine BaZrS3 material. The simulations rationalize the difference in carrier lifetimes in TMPC alloys and provide guidelines for further improvements in TMPC-based photoelectronic devices.

Transistor Laser-Integrated Photonics for Optical Logic: Unlocking Unique Electro-Optical Integration Potential to Open Up New Possibilities for Logic Processors

The transistor laser benefits from a fast electron-hole recombination lifetime in the base due to its heterojunction bipolar transistor (HB T)-like operation and can be modulated using the base current or the collector voltage through intracavity photon-assisted tunneling (ICPAT). Both modulation processes are inherently high speed and put the transistor laser at a direct advantage over the diode laser as an optical transmitter. However, the true potential of the technology is the natural coexistence of transistor and laser on the same epitaxial structure, enabling monolithic integration of electrical and optical functions on the same wafer without the need for hybrid methods, such as wafer bonding and regrowth. As such, the transistor laser device structure is the ideal platform for integrated photonics. A transistor laser circuit combined with an optical receiver can process signals without the need for discrete electronic processing and electro-optical conversion components. This unique trait opens the possibility for a transistor laser-based all-optical logic processor, as demonstrated using a transistor laser all-optical NOR gate.

Robust type-II band alignment in Janus-MoSSe bilayer with extremely long carrier lifetime induced by the intrinsic electric field

van der Waals (vdW) bilayers have many attractive novel properties that offer an ideal platform for various electronic and optoelectronic applications. However, due to the strong interlayer overlapping, the electron-hole recombination lifetime in vdW bilayers is usually very short, which limits their applications. Here, based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that the Janus-MoSSe bilayer can have a staggered band gap, in which the maximum valence and minimum conduction bands are spatially separated on different layers. Such type-II band alignment is surprisingly robust against external perturbations, such as twisting or stacking modes, which can be ascribed to the intrinsic out-of-plane electric field. Further analysis indicates that due to the existence of such a built-in electric field, the overlapping within the interlayer is suppressed, leading to the completely spacial detachment of the electrons and holes. We then evaluate the photogenerated electron and hole recombination dynamics. The predicted recombination time is extremely long, with a value up to 16.5 ns, which is even longer than that of the ${\mathrm{MoS}}_{2}/{\mathrm{WS}}_{2}$ vdW heterostructure. Our findings propose that introducing an intrinsic electric field is an efficient way to tune the electronic structures of bilayers, which can significantly extend the recombination time of a photoexcited electron and hole, and thus facilitate the design of advanced two-dimensional semiconducting devices.

Quasi-Steady-State Free Carrier Absorption Measurements of Effective Carrier Lifetime in Silicon

The paper presents a semiconductor carrier lifetime measurement technique, quasi-steady-state free carrier absorption (QSS-FCA), that is an all-optical analog of the quasi-steady-state photoconductivity technique. The effective electron–hole recombination lifetime of silicon is measured under quasi-steady-state conditions using an above-bandgap pump and a below-bandgap probe of the free carrier absorption (FCA). A mathematical model is developed to relate the experimentally measured probe signal to the lifetime. QSS-FCA is calibrated self-consistently by measuring the quasi-steady-state signal simultaneously with the true recombination lifetime in the same apparatus. This directly establishes the relationship between the quasi-steady-state signal and lifetime, ensuring the accuracy of QSS-FCA measurements. The calibration constant is the FCA cross section of silicon, which is a material constant. QSS-FCA is a simplification of traditional FCA lifetime measurement approaches where the lifetime is extracted from a time- or frequency-resolved carrier decay. Both of these approaches require a bandwidth commensurate with the inverse carrier lifetime of the wafer and require fitting multiple data points in order to extract the lifetime. In contrast, QSS-FCA is a quasi-steady-state measurement, so it is inherently low frequency, and as a single point measurement technique, fitting is not required. Finally, since QSS-FCA is a purely optical technique, it opens up the possibility of in situ lifetime characterization that would be inaccessible to the traditional RF/microwave-based lifetime measurement techniques.

Unraveling the Mechanism of Photoinduced Charge-Transfer Process in Bilayer Heterojunction.

Charge transfer is a fundamental process that determines the performance of solar cell devices. Although great efforts have been made, the detailed mechanism of charge-transfer process across the two-dimensional van der Waals (vdW) heterostructure remains elusive. Here, on the basis of the ab initio nonadiabatic molecular dynamics simulation, we model the photoinduced charge-transfer dynamics at the InSe/InTe vdW heterostructures. Our results show that carriers can follow either the R-scheme or Z-scheme transfer path, depending on the coupling between the interlayer states at the band-edge positions. In addition, the charge-transfer dynamics can be effectively controlled by the external parameters, such as strains and interlayer stacking configurations. The predicated electron-hole recombination lifetime in the R-scheme transfer path is up to 1.4 ns, whereas it is shortened to 1.2 ps in the Z-scheme transfer path. The proposed R-scheme and Z-scheme are further verified by the quantum transport simulations on the basis of the density functional theory (DFT) method combined with nonequilibrium Green's functions (NEGF-DFT). The analysis reveals that the system dominated by the Z-scheme shows better performance, which can be attributed to the built-in electric field that facilitates the charge transfer. Our work may pave the way for the designing of next-generation devices for light detecting and harvesting.

Resonance-free optical response of a vertical cavity transistor laser

Optical resonance in a semiconductor laser is a major limitation in high speed data communications, resulting in bit error rate degradation and requiring additional power consuming error-correction circuits to counter these effects. In this work, we report the microwave bandwidth measurement of a vertical cavity transistor laser with an oxide-confined aperture of 4.7 × 5.4 μm2 and demonstrate a 3 dB bandwidth of 11 GHz resonance-free optical response via base-current or collector-voltage modulation. The emission spectra exhibit single-mode operation around 970 nm with a narrow linewidth of Δλ ∼ 0.23 Å (cavity Q of 42 216). The resonance-free optical response is explained by the absence of carrier “accumulating” due to the fast base electron-hole recombination lifetimes and a gradient in the minority carrier charge in the transistor active mode.

Long Electron-Hole Separation of ZnO-CdS Core-Shell Quantum Dots.

The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core-shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron-hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core-shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.

Redshift of edge emission from AlGaInP light-emitting diodes and correlation with electron-hole recombination lifetime

The edge emission from AlGaInP light-emitting diodes showed a red-shifted peak in addition to the peak of surface emission. This shift resulted from the quantum-well absorption of the guided wave. Although the shift degrades the color quality and the extraction efficiency of the device, it helps elucidate many important optical properties of the material and the dynamics of carrier recombination, including the electron-hole recombination lifetime, the optical joint density of state, the spontaneous emission spectrum and the absorption spectrum. A simple concept of the bimolecular recombination is established. The corresponding coefficient can be expressed by a simple formula and was therefore determined.

Temperature independence of the spin-injection efficiency of a MgO-based tunnel spin injector

The spin polarization of electrons injected into GaAs from a CoFe∕MgO(100) tunnel spin injector is inferred from the circular polarization of light emitted from a GaAs-based quantum well (QW) detector. The circular polarization strongly depends on the spin and electron hole recombination lifetimes in the QW. Using time-resolved optical techniques, we show that these lifetimes are highly temperature dependent. A peak in the charge lifetime versus temperature is likely responsible for the previously observed dip in the electroluminescence polarization. Evidence for a temperature-independent spin injection efficiency of ∼70% from 10 K to room temperature is found.

Room-Temperature Spin Memory in Two-Dimensional Electron Gases

Time-resolved Kerr reflectivity of two-dimensional electron gases in II-VI semiconductors provides a direct measure of electron spin precession and relaxation over a temperature range from 4 to 300 kelvin. The introduction of n -type dopants increases the electronic spin lifetimes several orders of magnitude relative to insulating counterparts, a trend that is also observed in doped bulk semiconductors. Because the electronic spin polarization in these systems survives for nanoseconds, far longer than the electron-hole recombination lifetime, this technique reveals thousands of spin precession cycles of 15 gigahertz per tesla within an electron gas. Remarkably, these spin beats are only weakly temperature dependent and persist to room temperature.

The role of point defects and arsenic precipitates in carrier trapping and recombination in low-temperature grown GaAs

GaAs epilayers were grown with a wide range of excess arsenic concentrations and subjected to various anneals to study the role of the point defects and arsenic precipitates in carrier trapping and recombination. Prior to anneal, the point defects rapidly trap photogenerated electrons and holes—usually on subpicosecond time scales. However, full electron-hole recombination occurs on a significantly longer time scale. After anneal, the full electron-hole recombination lifetime appears to be greatly reduced, indicating that the arsenic precipitates play a significant role.

A Novel Contactless Approach for Accurate Measurements of Electron-Hole Recombination Lifetime

A Novel Contactless Approach for Accurate Measurements of Electron-Hole Recombination Lifetime

Carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum-well photoluminescence

A quantum-mechanical calculation of the carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum wells is performed, under steady-state optical excitation conditions and in the high-temperature regime. The variables are the continuous-wave (cw) laser intensity, well widths, and acceptor distribution in the well. Radiative recombination of electrons with free holes and holes bound at neutral acceptors are considered. Our calculations for the dependence of the electron density on laser intensity are in quantitative agreement with recent experimental results for multiple asymmetric coupled quantum wells at T=300 K and for intermediate excitation. Also, results for the carrier-density-dependent e-h recombination decay time at T=155 K are in good agreement with recent experimental data in semiconductor quantum wells.

Microwave photoconductive mixing in n- and p-type compensated silicon

Low-temperature photoconductive mixing measurements have been performed on n- and p-type compensated Si at a microwave beat frequency of 253 MHz. The dependence of the transport parameters (electron-hole recombination lifetime and drift velocity) on temperature and electric field obtained from this experimental technique, are discussed. At high temperatures (T≥35 K for n-type and T≥60 K for p-type), the drift velocity is determined by neutral impurity and ionized impurity, and charge carrier-phonon scattering. In the carrier freeze-out regime (T≤35 K for n-type, T≤60 K for p-type) hopping charge transport is the dominant mechanism determining the drift velocity.

Electron-hole recombination lifetimes in a quasi-zero-dimensional electron system in CdSxSe1−x

The recombination lifetimes for the radial and angular quantum number conserved 1S–1S and 1P–1P transitions from three-dimensionally confined electrons in CdSxSe1−x were measured by time-resolved photoluminescence (PL). The assignment of the observed transitions was supported by calculations of eigen energy levels and squared matrix element ratio for these transitions as well as well-resolved PL peaks arising from 1S–1S and 1P–1P transitions.

Field-induced lifetime enhancement and ionization of excitons in GaAs/AlGaAs quantum wells

Picosecond and nanosecond luminescence studies of GaAs/AlGaAs quantum wells exposed to electric fields perpendicular to the layers are reported. The drastic red-shift of the photoluminescence with the electric field strength is accompanied by a strong increase of the electron-hole recombination lifetime. These features are most dominant for the wider wells due to the stronger polarization of the confined electron-hole pairs. Our observations are consistent with the model of the Quantum-Confined Stark Effect. In contrast, for high fields in the regime of ∼100 kV/cm field ionization limits the confinement of electrons and holes and leads to a strong decrease of the photoluminescence yield and lifetime with increasing field strength.

Time Dependence of Current Injection Phenomenon in Nickel-Doped MgO Crystals at High Temperatures

ABSTRACTThe electrical conduction mechanisms in MgO:Ni with Pt electrodes at 1473 K are studied over electric field ranges up to ∼10 kV/cm. The current transients induced by pulsed changes of the applied voltage as well as transient shortcircuit currents have been measured and analyzed to yield two electron hole recombination lifetimes of 10 and 250 s. The main features of the results are consistent with the double injection model of the electrical breakdown of MgO at high temperatures.