Photoelectron Kinetics Research Articles

Nanoscale engineering of photoelectron processes in quantum well and dot structures for sensing and energy conversion

Advanced selective doping provides effective tool for nanoscale engineering of potential barriers and photoelectron processes in quantum well (QW) and quantum dot (QD) optoelectronic nanomaterials for IR sensing and wide band photovoltaic conversion. Photoelectron kinetics and device characteristics are investigated theoretically and experimentally. Asymmetrical doping of QWs is employed in a double QW structure for tuning electron transitions in QWs by voltage bias. These QW devices demonstrate bias-tunable multicolor detection and capability of remote temperature sensing. The QD structures with bipolar doping are proposed to independently control photocarrier lifetime (photocurrent) and dark current. The bipolar doping allows us to increase the height of nanoscale potential barriers around QDs without changing the electron population in QDs, which determines dark current. The QD devices with bipolar doping demonstrate significant enhancement of photocurrent, while dark current is close to that in corresponding reference devices with unipolar doping.

Improvement of QDIP performance due to quantum dots with built-in charge

The charging of quantum dots provides two strong effects which improve Quantum Dot Infrared Photodetector (QDIP) performance. First, electrons placed in the quantum dots enhance IR-induced transitions and increase electron coupling to IR radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers strongly suppress the photoelectron capture and exponentially increase the photoelectron lifetime. Both effects enhance the IR photoresponse. Long photoelectron lifetime decreases the generation–recombination noise and increases the device sensitivity. To investigate the potential profiles around charged dots, we used the nextnano3 software which allows for simulation of multilayer structures combined with realistic geometries in one, two, and three spatial dimensions. In weak electric fields the photoelectron kinetics and transport in the potential created by charged dots have been studied analytically. In strong fields the results were based on Monte-Carlo modeling. The effects of dot charging have been investigated in QD structures which were fabricated using molecular beam epitaxy. InAs quantum dots were grown on AlGaAs surfaces by deposition of approximately 2.1 monolayers of InAs. In the obtained structures the dot charging is realized via intra-dot and inter-dot doping. The increase in photoresponse due to dot charging is in good agreement with the model which takes into account anisotropy of potential barriers around QDs in QD layers.

Asymmetrically Doped GaAs/AlGaAs Double-Quantum-Well Structure for Voltage-Tunable Infrared Detection

We fabricate, characterize, and analyze tunable mid-infrared photodetectors based on asymmetrically doped coupled quantum well GaAs/AlGaAs structures. The peak of photoresponse detection varies from 7.5 to 11.1 µm when switching bias from -5 to +5 V. The spectral tunability is defined by the interplay of several effects. First, the electron energy levels are shifted due to the Stark effect. Second, the applied electric field causes the charge redistribution in the coupled wells and shift of electron energy levels due to modification of self-consistent potential. Here we show that effect of electric field on tunneling processes (the Poole–Frenkel effect) and the field-induced decrease of thermo-emission barrier (the Fowler–Nordheim effect) also play a critical role in photoelectron kinetics, strongly enhancing the carrier extraction from quantum wells. The model which takes into account Poole–Frenkel and Fowler–Nordheim effects provides a quantitative description of the data obtained.

Effective harvesting, detection, and conversion of IR radiation due to quantum dots with built-in charge

We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.

ELECTRON HEATING IN QUANTUM-DOT STRUCTURES WITH COLLECTIVE POTENTIAL BARRIERS

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, dot occupation, and electric field. Results of our simulations demonstrate that the capture processes are substantially suppressed by the potential barriers and enhanced in strong electric fields. Detailed analysis shows that the effects of the electric field can be explained by electron heating, i.e. field effects become significant, when the shift of the electron temperature due to electron heating reaches the barrier height. Optimized photoelectron kinetics in quantum-dot structures with collective barriers allows for significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors based on these structures.

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers.

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano3 software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.

Nanostructures with Quantum Dot Clusters: Long Photocarrier Lifetime

Major restrictions of semiconductor optoelectronic devices operating at room temperatures are caused by short photoelectron lifetime, which strongly reduces the photoresponse and puts strong limitations on manipulations with photoelectrons. Here we present results of modeling novel optoelectronic materials, based on structures with correlated dot clusters. The main distinctive characteristic of these quantum-dot structures are collective potential barriers around dot clusters. The barriers provide an effective control of photoelectron capture due to separation of highly mobile electron states transferring the photocurrent from the localized electron states in quantum dots. The novel nanostructured materials combine manageable photoelectron lifetime, high mobility, and quantum tuning of localized and conducting states. Thus, these structures have strong potential to overcome the limitations of traditional quantum dot and quantum-well structures. Besides manageable photoelectron kinetics, the advanced quantum-dot structures will also provide high coupling to radiation, low generation-recombination noise, and high scalability.

Structural and electronic transformations at the Cs/GaAs [formula omitted] interface

Combined study of the structure and electronic properties of the Cs/GaAs(1 0 0) interface is performed by means of low-energy electron diffraction, electron energy loss spectra (EELS), X-ray photoelectron spectroscopy and photoreflectance (PR) techniques. Under Cs adsorption on As-rich (2×4)/c(2×8) surface at room temperature, the order-to-disorder transition was observed at small coverages θ∼0.1 ML, while on Ga-rich surface cesium adsorbs in an ordered way, so that sharp (4×1) structure is observed up to θ∼0.5 ML. At Cs coverages θ∼0.5 ML, a phase transition takes place at which the isolated Cs adatoms condense into closely packed two-dimensional metallic islands with local plasmon excitations detected by EELS. At the same coverages, by means of PR spectroscopy, we observed the decrease of surface photovoltage and acceleration of photoelectron kinetics due to the increase of lateral conductance and, therefore, of the surface recombination velocity.

Drift mobility, electron trapping, and diffusion-limited kinetics in sulfur-sensitized AgBr microcrystals

The room-temperature photoelectron kinetics in 1.3- and 0.4-μm AgBr octahedral emulsion microcrystals have been investigated for time scales ranging from several nanoseconds to several microseconds after exposure. Two first-order decay processes were observed in 1.3-μm octahedra. The fast process is attributable to relatively shallow electron trapping that equilibrates with thermal detrapping after ∼60 nsec; the longer decay may involve ionic processes. Sulfur sensitization of the emulsion enhanced the fast electron decay rate (decay time=14 nsec). At higher levels of sulfur sensitization, the decay rate did not change with increasing sulfur levels; such behavior can indicate diffusion-limited surface trapping. A bulk-to-surface electron drift mobility of 0.8 cm2/V sec on nanosecond time scales has been deduced from these kinetics. Implications of the data to current theories of photographic image formation and sulfur sensitization are discussed.

Optical spin orientation. application to the study of photoelectron kinetics

Recent results obtained with the use of the optical spin orientation method on spin relaxation, recombination and transport of photoelectrons in III–V compounds are presented.

Effect of pre-exposure on the photoconductive response of lead-doped silver bromide

Pre-exposure of lead-doped AgBr binderless powder precipitates to several seconds of light greatly enhanced the microwave photoconductivity at 100K. Photon energies both greater and less than the bandgap of pure AgBr could produce such an effect. The dependence of pre-exposure enhancement on wavelength and intensity and the effect of pre-exposure on photoelectron kinetics were observed. The results suggest that the increase in photoconductivity is caused by two different mechanisms, depending on whether the photon energy is greater or less than the pure AgBr energy gap. It appears that pre-exposure can affect the processes involved in photoelectron generation as well as electron decay mechanisms.

Photoconductive response of lead-doped silver bromide to sub-bandgap light

To increase understanding of the effects of doping silver bromide with divalent lead ions, both the photoconductive response and the photoelectron kinetics of doped binderless powder precipitates were studied as a function of exciting wavelength. Excitation wavelengths both above and below the pure silver bromide bandgap energy were included. Lead doping enhanced the microwave photoconductivity signal and extended the photoresponse to photon energies well below the AgBr bandgap energy. Analysis of the photoelectron kinetics suggests that a sequential two-photon absorption process involving a lead(II) impurity level deep in the AgBr gap provides the mechanism for the extended wavelength response.