P. L. Souza

InAs quantum dot growth on AlxGa1-xAs by metalorganic vapor phase epitaxy for intermediate band solar cells

InAs quantum dot multilayers have been grown using AlxGa1−xAs spacers with dimensions and compositions near the theoretical values for optimized efficiencies in intermediate band photovoltaic cells. Using an aluminium composition of x = 0.3 and InAs dot vertical dimensions of 5 nm, transitions to an intermediate band with energy close to the ideal theoretical value have been obtained. Optimum size uniformity and density have been achieved by capping the quantum dots with GaAs following the indium-flush method. This approach has also resulted in minimization of crystalline defects in the epilayer structure.

Photocurrent Calculation of Intersubband Transitions to Continuum-Localized States in GaAs/AlGaAs Multiquantum Wells for Mid-Infrared Photodetector

In this paper, we propose a system based on GaAs heterostructure where it is possible to generate photocurrent with mid-infrared radiation. This system is based on a central quantum well (CQW) embedded in a superlattice. Because of the CQW, which acts as a defect, there are localized states between the mini-bands in the continuum of the conduction band. Unlike the usual systems where the final states are delocalized, the oscillator strength due to the transitions between electrons occupying the ground-state to these continuum-localized states is enhanced. An applied electrical bias mixes the mini-band states with the localized state in the continuum, and due to the combined effects of strong oscillator strength and high transmission coefficients, narrow and sharp peaks are observed in the photocurrent when exciting these final states. We calculate and present results of the absorption and photocurrent for a system built to operate at 4.1 μm and discuss their dependence with the bias applied to the system and with the intensity of the incident radiation.

Intraband Auger effect in InAs∕InGaAlAs∕InP quantum dot structures

InAs quantum dot structures grown on InGaAlAs have been investigated for midinfrared photodetection. Intraband photocurrent and absorption measurements, together with a full three-dimensional theoretical modeling revealed that a bound-to-bound optical transition, where the final state is about 200meV deep below the conduction band continuum, is responsible for the photogenerated current. The reported results strongly suggest that an Auger process plays a fundamental role in generating the observed intraband photocurrent. Photoluminescence and interband photocurrent spectra of the same structures further support the reached conclusions.

Multiple-photon peak generation near the ˜10 μm range in quantum dot infrared photodetectors

We present results from simulations of the photocurrent observed in recently fabricated InAs quantum dot infrared photodetectors that respond with strong resonance peaks in the ∼10μm wavelength range. The results are in good agreement with experimental data generated earlier. Multiphoton scattering of electrons localized in the quantum dots are not only in accordance with the observed patterns, but are also necessary to explain the photocurrent spectrum obtained in the calculations.

Exploring Parity Anomaly for Dual Peak Infrared Photodetection

In this paper, we show a superlattice quantum well infrared photodetector (S-QWIP) grown by metal-organic vapor phase epitaxy with two narrow photocurrent peaks in the mid infrared range due to transitions between the ground state from a quantum well and two excited states localized in the continuum. The structure composed of InGaAs/InAlAs quantum-well lattice matched to InP with a central quantum well acting as an artificial defect. The potential profile is carefully chosen to explore the parity anomaly of the continuum localized states and also to reduce the thermoexcited electrons decreasing the dark current. The photocurrent spectrum shows two peaks with transition energies of 300 and 460 meV (Δλ/λ of 0.13 and 0.12) at 12 K. The peak detectivity is 1.23×1010 Jones at 30 K and +5 V. When compared with a regular multiquantum well sample designed to generate photocurrent at the same wavelength, the S-QWIP shows an increase of 15 K on its background-limited performance temperature and a lower dark current for temperatures above 200 K.

Simultaneous positive and negative photocurrent response in asymmetric quantum dot infrared photodetectors

We study the influence on the photocurrent of the final state in bound-to-quasibound transitions in self-assembled quantum dot infrared photodetectors. We investigate two structures designed to explore different mechanisms of carrier extraction and therefore achieve a better insight on these processes. We observe photocurrent in opposite directions, with positive and negative sign, for different incident frequencies at the same applied external electric field. This phenomenon is attributed to the asymmetry of the potential barriers surrounding the quantum dots.

Delta doping superlattice structures for amplitude modulators: Observation of the Stark effect and improvement of the chirp

A Stark shift for a GaAs/AlGaAs multiple quantum well structure containing a nipi delta-doping superlattice is observed. Photocurrent measurements revealed a Stark shift that varied with increasing applied electric field, from being equivalent to that of an undoped structure to 30% below. Even though these values are still below those predicted theoretically, they demonstrate the possibility of fabricating amplitude modulators using these novel structures. Furthermore, the obtained chirp parameter is within the desired [−1,0] range for long-distance optical communication, contrary to the high positive values for the undoped structure, showing that such device may have a better overall performance.

Effect of growth temperature on C-doped InAlAs layers grown by LP-MOVPE

Abstract Carbon-doped InAlAs layers grown by low-pressure metalorganic vapor phase epitaxy are investigated as a function of growth temperature. Photoluminescence spectra show a significant drop in peak intensity as the growth temperature is reduced, due to the incorporation of non-radiative defects. It is shown that the C-doping is not related to the deterioration of the optical properties of the layers. On the other hand, both the atomic and the net hole concentration increase as the growth temperature is reduced due to a more efficient C-incorporation. C-related deep donors are also incorporated in the layers. Annealing the samples improve their electrical activity but does not affect the optical properties.

Enhancement of the electroabsorption in multiple quantum well structures containing a nipi delta-doping superlattice

Transmission measurements as a function of applied reverse voltage, together with a fit procedure, demonstrate that the introduction of a nipi delta-doping superlattice in a GaAs∕AlGaAs multiple-quantum-well structure, for application in amplitude modulation, enhances the change in absorption with applied electric field, leading to an improvement of the device performance.

Carbon delta-doped AlGaAs grown by metalorganic vapor phase epitaxy

Good-optical-quality C delta-doped AlGaAs layers grown by metalorganic vapor phase epitaxy using CBr4 with the impurities confined over not more than 5 A and with doping levels above 1×1012 cm−2 are obtained. Such layers are found to be adequate for use in the fabrication of nipi superlattices for amplitude modulation. Yet, little flexibility is found in the growth conditions, in particular for the V to III fluxes ratio, for obtaining such layers.