Hole Escape Times Research Articles

Effect of Thermal Annealing on Absorption and Hole Escape Processes in Type II GaSb/GaAs Quantum Dots: Implications for Solar Cell Design

We present a theoretical study of the rapid thermal annealing process and strain on the electronic, optical, and charge dynamical properties of type II GaSb/GaAs quantum dots (QDs). Our theoretical results show a blueshift and the enhancement of interband absorption spectrum because of inter-diffusion processes and the annealed samples. We have identified that increased annealing temperature makes the hole escape times from such QD shorter while the effect of strain on the hole confinement potential becomes weaker. At the same time, in both structures, unstrained and strained QDs, the intra-band absorption in the valence band (IVBA) is redshifted, and its magnitude is decreased after annealing when compared with the as-grown sample. To explain those effects we discuss how the electron–hole wave function overlap varies with thermal treatment and strain and discussed those effects on radiative and thermal processes. Our results explain and are in agreement with the recent experimental observations on similar structures. We have identified that holes could thermally escape out of the type II GaSb/GaAs QD before they are recombined and will contribute to the enhancement in absorption of QD solar cells. The emission and tunneling times of holes were estimated to be $ 10^{-12}$ to $10^{-14}$ s about one order of magnitude faster than the escape times because of radiative recombination processes, $ 10^{-11}$ to $10^{-13}$ s.

InP-Based Waveguide-Integrated Photodiodes With InGaAs/GaAsSb Type-II Quantum Wells and 10-GHz Bandwidth at 2 $ \mathbf {\mu }$m Wavelength

Waveguide-integrated photodiodes with InGaAs/GaAsSb type-II quantum well absorption regions designed to absorb light at 2 $ \mu$ m are presented. A novel dual-integrated waveguide-depletion layer was used to maximize quantum efficiency in photodiodes designed with thin absorbers for high-speed optical response. Low dark currents (1 nA at $-$ 1 V) and an internal responsivity of 0.84 A/W along with a bandwidth above 10 GHz and an open eye diagram at 10 Gb/s have been demonstrated at 2 $ \mu$ m. The high-speed carrier dynamics within InGaAs/GaAsSb type-II quantum wells are explored for the first time and suggest that the transit time of the photodiode is limited by light hole escape times in the quantum wells.

Picosecond carrier relaxation in type-II ZnSe/BeTe heterostructures

Relaxation of photoexcited carriers in the course of the formation of spatially separated layers of electrons and holes in type-II ZnSe/BeTe heterostructures has been studied based on a high time resolution investigation of fast luminescence kinetics. The hole escape times τ from the ZnSe layer have been measured in structures with different ZnSe layer thicknesses (τ = 2.5, 7.5, and 23 ps for thicknesses d = 10, 15, and 20 nm, respectively). It is shown that the increase in the time τ can be explained by the fact that the escape rate of photoexcited holes from the lowest above-barrier level in the ZnSe layer into the BeTe layer decreases as the thickness of the ZnSe layer increases.

Multibandk∙ptheory of carrier escape from quantum wells

A general theory of multichannel coherent particle escape introducing the escape matrix based on the scattering formalism has been developed and applied to the problem of hole escape from biased quantum wells. This approach incorporates band-mixing effects, such as valence-band nonparabolicity and heavy-to-light (light-to-heavy) hole transformations, in the theory of carrier escape from quantxum wells consistently with the principles of multiband effective-mass theory. The numerical calculations, made in the Luttinger approximation, show significant influence of the band-mixing effects on the hole escape time and are in satisfactory agreement with available experimental data. The effective decrease of the heavy-hole tunneling mass (``lightening'') as a result of the light-hole admixture is found to be the major reason for the escape-time decrease, as compared to the simple parabolic-band effective-mass model. The concept of the escape matrix can be used for investigating multichannel coherent escape processes in different systems such as quantum wires, quantum dots, or photonic structures.

Electron and hole escape times in single quantum wells

The calculation of the carrier escape time is important for quantum well devices. In this article a model for calculating the escape time of both electrons and holes is presented. The escape time is found by solving the Schrödinger equation by the method of the logarithmic derivative of the wave function which yields: the continuum density of states within a biased quantum well, the proper group velocity, and the partitioning between the thermionic emission and tunneling currents. Excellent agreement between the theoretical and previously reported experimental results for electron and hole escape times is achieved.

A frequency response model in multiquantum well lasers with unequilibrium carrier transport

A carrier transport model to explain the high-frequency response in high-speed MQW lasers is described. The ambipolar approximation, which is unsuitable for dealing with the high-speed carrier dynamics in MQW structures, was not adopted for small-signal analysis. The carrier transport effect can be characterized by four time constants: the electron transport time, τbmn; the hole transport time, τbmp; the electron escape time, τwbn; and the hole escape time, τwbp. The frequency response was interpreted as the sum of the constant response term due to the fast electron current and the roll-off term due to the slow hole transport time. The ratio of the electron contribution to the total response was proportional to the ratio of electron contribution to the total differential gain, ξ, and reciprocally proportional to χn0 = 1 + τbmn/τwbn. The value of ξ was calculated to be about 0.5 for typical MQW lasers. The roll-off frequency is mainly determined by . The ratio χp0 = 1 + τbmp/τwbp affects the resonant frequency and the damping rate in the high-bias condition.

Engineering of barrier band structure for electroabsorptionMQW modulators

The introduction of tensile strain to the barriers of InGaAsP multiquantum well, λ = 1.55 µm, electroabsorption modulators is proposed. It decreases the valence band barrier height, and heavy hole escape time, which greatly increases the optical saturation intensity leading to smaller, lower capacitance modulators with greater power handling capabilities.

Phonon-assisted sequential tunneling in biased quantum well structures

We present a theoretical analysis of electron and hole escape mechanisms from a quantum well (QW) in an external electric field. The influence of carriers and dopant ion charges on the band structure is simulated with a self-consistent Poisson-Schrödinger solver. A new escape mechanism called the phonon-assisted sequential tunneling is proposed as an alternative to direct tunneling and thermionic emission from the QW ground state. Our calculation shows that at high forward biases, POP phonon-assisted sequential tunneling and thermionic emission dominate the electron and hole escape times. At high reverse biases, direct electron tunneling from the QW ground state to the reservoir determines the turn-off time constant, while heavy holes take a longer time to escape than electrons.

Charge-induced self-feedback optical bistable device: switching time and spatial resolution

We report extensive experimental and theoretical studies on the properties of an optical bistable device called the charge-induced self-feedback device (CSFD) which is a variation of a self electro-optic effect device (SEED). In the mechanism, a novelty: of the CSFD consists of the feedback due to field-screening, resulting in independent switchings of optical beams focused at different spots on the CSFD without the help of pixellated structures. We experimentally demonstrate independent switchings without pixellation. In addition, a spatial resolution of the CSFD on such a device operation is found to be limited by an in-plane spreading of photoexcited carriers enhanced by a lateral electric-field induced by photoexcitation. Also, it is experimentally demonstrated that shortening carrier escape times leads to an improvement on the spatial resolution in terms of the suppression of the in-plane carrier spreading. Furthermore, an investigation on dynamics of the photoexcited carriers in the device reveals that the switching time of the CSPD is on the order of nanosecond, limited mainly by the hole escape time. >

Simultaneous measurement of electron and hole escape times from biased single quantum wells

Picosecond pump probe measurements on waveguides containing a single GaAs/GaAlAs quantum well have enabled us to determine for the first time both electron and hole escape times from biased quantum wells. Contributions from excitonic saturation and field screening by photogenerated carriers can be clearly identified in the nonlinear transition signal and are quantitatively modeled. Preliminary analysis suggests that thermally assisted escape is dominant at room temperature, but discrepancies are found with conventional thermionic emission models.

Simultaneous measurements of electron and hole sweep-out from quantum wells and modeling of photoinduced field screening dynamics

Electron and hole escape times from a GaAs-AlGaAs quantum well in an electric field at room temperature are measured by picosecond optical pump-probe techniques on samples containing a single quantum well in a waveguide. The use of a single well avoids multiple well transport and resonant tunneling effects. Carriers excited in the quantum well by the pump beam result in a transient bleaching signal from excitonic saturation, and, as they leave the well, a transient electroabsorption signal because the movement of charge partially screens the electric field. Both processes are modeled, including important electrical equilibration processes of the sample as a whole. This modeling and the use of two samples with asymmetric barrier heights allows the measurement of the electron and hole emission as a function of applied electric field. Preliminary analysis suggests that the emission mechanism is thermionic rather than by tunneling, but the results are not well explained by conventional thermionic emission models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>