Tunnel Injection Structure Research Articles

Carrier dynamics in quantum-dot tunnel-injection structures: Microscopic theory and experiment

Tunneling-injection structures are incorporated in semiconductor lasers in order to overcome the fundamental dynamical limitation due to hot carrier injection by providing a carrier transport path from a cold carrier reservoir. The tunneling process itself depends on band alignment between quantum-dot levels and the injector quantum well, especially as in these devices LO-phonon scattering is dominant. Quantum dots with their first excited state near the quantum well bottom profit the most from tunnel coupling. As inhomogeneous broadening is omnipresent in quantum dot structures, this implies that individual members of the ensemble couple differently to the injector quantum well. Quantum dots with higher energy profit less, as the phonon couples to higher, less occupied states. Likewise, if the energy difference between ground state and quantum well exceeds the LO-phonon energy, scattering becomes increasingly inefficient. Therefore, within 20–30 meV, we find quantum dots that benefit substantially different from the tunnel coupling. Furthermore, in quantum dots with increasing confinement depth, excited states become successively confined. Here, scattering gets more efficient again, as subsequent excited states reach the phonon resonance with the quantum well bottom. Our results provide guidelines for the optimization of tunnel-injection lasers. Theoretical results for electronic state calculations in connection with carrier–phonon and carrier–carrier scattering are compared to the experimental results of the temporal gain recovery after a short pulse perturbation.

Comparison between InP-based quantum dot lasers with and without tunnel injection quantum well and the impact of rapid thermal annealing

An InP based tunnel injection quantum dot (QD) laser and a reference quantum dot laser designed to emit at 1.55 μm were grown by molecular beam epitaxy. The lattice matched tunnel injection (TI) structures consist of an InGaAs quantum well (QW), an InAlGaAs barrier and multiple InAs quantum dot layers. Quantum well and quantum dot test structures as well as the actual laser structures were treated with rapid thermal annealing (RTA) at varying temperatures (680–780 °C). Photoluminescence measurements were performed at 10 K on all samples and a strong effect of this annealing process on the emission properties is observed. The laser structures were processed into broad area lasers. Power-current characteristics were measured and evaluated. A strong improvement in device performance is observed after annealing for QD lasers while for TI-QD lasers no significant improvement was obtained, which might be caused by detuning of band energy alignment and different impact of the RTA process on the material quality of QW and QD layers.

Growth and optical characteristics of InAs quantum dot structures with tunnel injection quantum wells for 1.55 [formula omitted]m high-speed lasers

InP based lattice matched tunnel injection structures consisting of a InGaAs quantum well, InAlGaAs barrier and InAs quantum dots designed to emit at 1.55 μm were grown by molecular beam epitaxy and investigated by photoluminescence spectroscopy and atomic force microscopy. The strong influence of quantum well and barrier thicknesses on the samples emission properties at low and room temperatures was investigated. The phenomenon of a decreased photoluminescence linewidth of tunnel injection structures compared to a reference InAs quantum dots sample could be explained by the selection of the emitting dots through the tunneling process. Morphological investigations have not revealed any effect of the injector well on the dot formation and their size distribution. The optimum TI structure design could be defined.

Tunnel injection transistor laser for optical interconnects

Transistor laser (TL) is already an established potential candidate for high speed optical interconnects and present day optical communication networks. This paper investigates theoretically the possibility of having lower base threshold current density and enhanced modulation bandwidth by inserting a tunnel injection structure in a TL having multiple quantum wells (MQW) in the base of the heterojunction bipolar transistor. Transfer of injected charge carriers from bulk to low dimensional nano-structure is assumed to occur via virtual energy states, which contributes to the terminal current. Small signal modulation response is obtained by solving the Statz–De Mars laser rate equations. The optimum threshold base current, confinement of carrier, light power outputs etc. are estimated for three QWs positioned at distances of 39, 59, and 79 nm from the emitter-base junction across the base. Incorporation of tunneling structure substantially lowers the base threshold current and increases the modulation bandwidth as compared to usual MQW transistor laser structure. The changes are more prominent with increasing tunneling probability.

Carrier diffusion as a measure of carrier/exciton transfer rate in InAs/InGaAsP/InP hybrid quantum dot–quantum well structures emitting at telecom spectral range

We investigate the diffusion of photo-generated carriers (excitons) in hybrid two dimensional–zero dimensional tunnel injection structures, based on strongly elongated InAs quantum dots (called quantum dashes, QDashes) of various heights, designed for emission at around 1.5 μm, separated by a 3.5 nm wide barrier from an 8 nm wide In0.64Ga0.36As0.78P0.22 quantum well (QW). By measuring the spectrally filtered real space images of the photoluminescence patterns with high resolution, we probe the spatial extent of the emission from QDashes. Deconvolution with the exciting light spot shape allows us to extract the carrier/exciton diffusion lengths. For the non-resonant excitation case, the diffusion length depends strongly on excitation power, pointing at carrier interactions and phonons as its main driving mechanisms. For the case of excitation resonant with absorption in the adjacent QW, the diffusion length does not depend on excitation power for low excitation levels since the generated carriers do not have sufficient excess kinetic energy. It is also found that the diffusion length depends on the quantum-mechanical coupling strength between QW and QDashes, controlled by changing the dash size. It influences the energy difference between the QDash ground state of the system and the quantum well levels, which affects the tunneling rates. When that QW–QDash level separation decreases, the probability of capturing excitons generated in the QW by QDashes increases, which is reflected by the decreased diffusion length from approx. 5 down to 3 μm.

Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells

We propose a tunnel-injection structure, in which WS2 quantum dots (QDs) act as the injector and InGaN/GaN quantum wells (QWs) act as the light emitters. Such a structure with different barrier thicknesses has been characterized using steady-state and time-resolved photoluminescence (PL). A simultaneous enhancement of the PL intensity and PL decay time for the InGaN QW were observed after transfer of charge carriers from the WS2-QD injector to the InGaN-QW emitter. The tunneling time has been extracted from the time-resolved PL, which increases as the barrier thickness is increased. The dependence of the tunneling time on the barrier thickness is in good agreement with the prediction of the semiclassical Wentzel–Kramers–Brillouin model, confirming the mechanism of the tunnel injection between WS2 QDs and InGaN QWs.

Control of carrier reserving and injection for high-speed semiconductor optical amplifier by using a tunnel injection structure

In this study, we explore a new way of controlling the carrier dynamics of semiconductor optical amplifiers (SOAs) by using a tunneling injection structure, which allows for high-speed operation. The proposed SOA uses carrier reserving to compensate for the decreased carriers caused by signal amplification. We can control the SOA operation properties for high-speed signals by adjusting its characteristics of carrier injection from the reservoir layer to the active layer. Numerical simulations showed that this structure has the potential to suppress signal distortion at a high operation speed of 100 Gbps.

Electronic structure and optical properties of 1.55 µm emitting InAs/InGaAsP quantum dash tunnel injection structures

Optical spectroscopy studies have been reported on an application-relevant system emitting at 1.55 µm and consisting of an InGaAsP injector quantum well (QW), separated by a thin InP barrier (of various thicknesses) from InAs elongated quantum dots called quantum dashes (QDash). The investigated systems constitute tunnel injection structures, whose operation principle is the transfer of carriers captured by the injector QW to the QDash layer by means of tunnelling. Numerical calculations in the eight-band kp model with a realistic dash geometry are used in order to determine the energies of confined levels and their wave functions, results of which are then verified by comparison with measured photoreflectance spectra and used to interpret spectroscopic findings. The tunnelling of carriers from QW to QDash layer is evidenced by photoluminescence excitation measurements and further supported by unusual temperature behaviour of PL peaks broadenings. Careful numerical analysis of confined states and their wave functions explains optical properties of investigated samples. The suggestions for the proper design of structures based on the injection of carriers for applications in telecom lasers are given; most importantly it is shown that care must be taken so that the QW does not significantly influence the dash emitter.

Tunnel injection emitter structures with barriers comprising nanobridges

AbstractNanobridges which bypass the tunnel barrier in tunnel injection structures are investigated. The conditions leading to the formation of confined states within them are determined. It is shown that for tunnel barrier thicknesses in the 4–5 nm range confined hole states are likely to exist within the nanobridges. A new absorption feature, which has been observed only in structures comprising nanobridges, is assigned to transitions involving these hole states. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier wavefunction control in a dilute nitride-based quantum well—a quantum dot tunnel injection system for 1.3 µm emission

There are reported results of numerical calculations of energy structure and carrier wavefunctions in a quantum mechanically coupled complex system consisting of an InGaAsN injector quantum well (QW) (with various N contents), separated by a thin GaAs barrier from an InAs/GaAs quantum dot (QD) layer covered with an InGaAs strain reducing layer. The investigated system consists in a tunnel injection structure, whose operation principle is to transfer the carriers captured by the injector QW to QDs by means of tunnelling. The QDs emit light at the 1.3 µm telecom window at room temperature, so in order to match the energy levels of the confined states in the injector QW to those in the QDs, nitrogen has been added to the former. The obtained calculation results are compared with measured photoreflectance spectra and used to explain their features. A detailed discussion on the spatial distribution of carrier wavefunctions over both parts of the tunnelling system versus nitrogen concentration in the injector QW is presented. The importance of the control of wavefunctions for the performance of tunnel-injection-based devices is discussed.

Excited state coherent resonant electronic tunneling in quantum well-quantum dot hybrid structures

A strong effect of a quantum well (QW) incorporated into a quantum dot (QD) structure on the density of states of the system and the efficiency of carrier transfer from the barrier material to QDs is revealed in InAs/GaAs–InGaAs/GaAs dot-well, tunnel-injection structures. When tuning the QW states in resonance with excited QD states, the carrier flux can be effectively controlled by varying the spacer thickness or barrier height. Enhanced carrier tunneling between QW and QD states is observed by means of photoluminescence excitation spectroscopy for reduced spacer thicknesses. Our results demonstrate that resonant coherent electron tunneling is substantially faster for the second than for the first QW subband and results in the formation of hybrid electronic states delocalized across the QW/QD interface.

Contactless electroreflectance of optical transitions in tunnel-injection structures composed of an In0.53Ga0.47As/In0.53Ga0.23Al0.24As quantum well and InAs quantum dashes

Tunnel-injection structures composed of an In0.53Ga0.47As/In0.53Ga0.23Al0.24As quantum well (QW) and a layer of InAs quantum dashes (QDashes) separated by In0.53Ga0.23Al0.24As barriers of various thicknesses have been investigated by contactless electroreflectance. The observed spectral features have been explained taking into account the optical transitions in a combined system of In0.53Ga0.47As QW and InAs QDash wetting layer. It has been shown that there exist electron and hole states which are localized on both sides of such an asymmetric confinement potential. The latter has allowed concluding that the QDash region in tunnel-injection structures can be easily penetrated by the carriers due to the presence of the wetting layer in the self-assembled structure.

Tunnel injection structures based on InGaAs/GaAs quantum dots: optical properties and energy structure

The limited speed of direct modulation due to the significant population of hot carriers in quantum dots (QD) is a major drawback of the application of QD based lasers in fibre optics telecommunication. One of the most promising methods of alleviating this problem is the design of tunnel injection (TI) structures, where already cold carries are injected by means of tunnelling through a thin barrier from an adjacent quantum well (QW) directly to the QD ground state. We have investigated the properties of TI structures consisting of an InxGa1−xAs quantum well and a layer of self-assembled In0.6Ga0.4As/GaAs quantum dots versus the properties a reference QD sample. Photoreflectance spectroscopy is applied to determine the energies of optical transitions in this complex system. The obtained energies are then used to verify the reliability of the calculations in the 8 band kp model, which take into account the realistic geometry of the dots, influence of the strain and the coupling between the dot layer and the injector quantum well. Finally, time resolved photoluminescence (PL) experiment is performed at low temperature and its results are related to the acquired structure of confined levels. The influence of the tunnelling process on PL rise and decay times is explained.

InGaAs tunnel-injection structures with nanobridges: Excitation transfer and luminescence kinetics

Methods of optical spectroscopy and electron microscopy have been used to study tunnel-injection nanostructures the active region of which consisted of an upper In0.15Ga0.85 As quantum-well layer and a lower layer of In0.6Ga0.4As quantum dots as a light emitter; both layers were separated by a GaAs barrier layer. Deviations from the semiclassical Wentzel-Kramers-Brillouin model are observed in the dependence of the tunneling time on barrier’s thickness. Reduction of the transfer time to several picoseconds at a barrier thickness smaller than 6 nm is accounted for by formation of InGaAs nanobridges between tops of quantum dots and the quantum-well layer; the nanobridges include those with their own hole state. The effect of an electric field induced by tunneling on the carriers’ transfer time in a tunnel-injection nanostructure is taken into account.

Temperature-dependent carrier tunneling for self-assembled InAs/GaAs quantum dots with a GaAsN quantum well injector

We have investigated the carrier tunneling process in a quantum-dot (QD) tunnel injection structure, which employs a GaAs1−xNx quantum well (QW) as a carrier injector. The influence of the barrier thickness between the GaAs1−xNx well and InAs dot layer has been studied by temperature-dependent photoluminescence. Although the 2.5 nm barrier sample exhibits the best tunneling efficiency, a 3.0 nm thickness for the barrier is optimum to retain good optical properties. The carrier capture time from the GaAs1−xNx QW to QD ground states has been evaluated by time-resolved photoluminescence. The result indicates that efficient carrier tunneling occurs at temperatures above 150 K due to the temperature dependent nature of phonon-assisted processes.

Time-resolved photoluminescence spectroscopy of an InGaAs/GaAs quantum well-quantum dots tunnel injection structure

Low temperature carrier dynamics in the InGaAs/GaAs quantum dot-based tunnel injection structure is studied by the time resolved photoluminescence experiment. We observed strongly modified photoluminescence kinetics between tunnel injection and reference quantum dot structures. Slowing down of the photoluminescence rise time in the tunnel injection system under weak and moderate excitation powers, we attributed to a fingerprint of a feeding process of quantum dot states with nonresonant carriers tunneling from the quantum well reservoir. We propose a simple three-level rate equation model to explain qualitatively the observed photoluminescence temporal behavior. Its result shows a good agreement with our experimental data.

Optical properties and energy transfer in InGaAsN quantum well – InAs quantum dots tunnel injection structures for 1.3 μm emission

AbstractTunnel injection structure consisting of InAs self assembled quantum dots emitting light at the second telecom window at 1.3 μm and an InGaAsN injector quantum well is investigated by means of optical spectroscopy and the results compared to the injector‐free reference sample. The photoreflectance measurements, supported by calculations, reveal a complex structure of optical transitions in the system of coupled injector, wetting layer and strain reducing layer. The determined energy structure shows that the tunnelling is possible only through the excited states of quantum dots. Photoluminescence excitation proves the carrier transfer (tunnelling) from the injector to the dots at room temperature. The comparison of photoluminescence spectra confirms the tunnelling efficiency, especially at low temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room temperature free carrier tunneling in dilute nitride based quantum well - quantum dot tunnel injection system for 1.3 μm

We present optical studies of quantum dot tunnel injection structures for 1.3 μm emission with an InGaAsN quantum well injector. Photoreflectance spectroscopy supported by effective mass calculations within the band anticrossing model has been used to identify the optical transitions. Based on that, an evidence of the tunneling from the injector well to the dots could be detected by photoluminescence excitation up to the free carrier regime at room temperature. The latter finds confirmation in shortened photoluminescence rise times, when compared to the injector-free quantum dot reference structure.

Transient carrier transfer in tunnel injection structures

InGaAs tunnel injection nanostructures consisting of a single quantum well as injector and a quantum dot layer as emitter are studied by time-resolved photoluminescence spectroscopy. The quantum dot photoluminescence undergoes substantial changes when proceeding from direct quantum dot excitation to quantum well excitation, which causes an indirect population of the dot ground states. This results in a lowered effective carrier temperature within the dots. Results on the carrier transfer versus barrier thickness are discussed within the Wentzel–Kramers–Brillouin approximation. Deviations for barrier thicknesses <5nm are assigned to the formation of nanobridges that are actually detected by transmission electron microscopy.

Theoretical design of carrier injection rate and recombination rate in tunnel injection quantum well lasers

AbstractCarrier injection rate in tunnel injection quantum well structures used in semiconductor lasers was investigated through theoretical analysis. Carrier capture time from 3D‐state to 2D‐state in a conventional quantum well was several ten picoseconds and it is almost uncontrollable characteristics. The tunnel injection structure can be designed to control the carrier injection rate. The carrier transition time from carrier reservoir‐region of the tunnel injection structure to the active well was a few picoseconds when the structure was designed for high speed transition. As results, carrier injection to ground state of the active well takes less than 10 picoseconds. The change of the radiative recombination rate in the active well also defines the optimal design of the tunnel injection structure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)