M. P. Pires

Atomic force nanolithography of InP for site control growth of InAs nanostructures

A combination of atomic force nanolithography and metal organic vapor phase epitaxy has been used to control the nucleation of InAs nanostructures on InP substrates. Pits with controlled width and depth were produced on InP with the use of atomic force nanolithography. The number of nucleated nanostructures depends on the applied force and is independent of the geometry of the pits. Study shows that the density of crystalline defects introduced by nanoindentation is responsible for the number of nucleated nanostructures.

Quantum dot structures grown on Al containing quaternary material for infrared photodetection beyond 10μm

Different InAs quantum dot structures grown on InGaAlAs lattice matched to InP were investigated for quantum dot infrared photodetectors. Extremely narrow photocurrent peaks were observed, demonstrating great potential for fine wavelength selection. Structures which can detect radiation beyond 10μm were developed. Polarization dependence measurements showed that the structures have a zero-dimensional character and are suitable for detection of normal incident light. On the other hand, structures containing coupled quantum wells showed a hybrid two-dimensional/zero-dimensional behavior.

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.

Improved optical properties of InAs quantum dots for intermediate band solar cells by suppression of misfit strain relaxation

The properties of InAs quantum dots (QDs) have been studied for application in intermediate band solar cells. It is found that suppression of plastic relaxation in the QDs has a significant effect on the optoelectronic properties. Partial capping plus annealing is shown to be effective in controlling the height of the QDs and in suppressing plastic relaxation. A force balancing model is used to explain the relationship between plastic relaxation and QD height. A strong luminescence has been observed from strained QDs, indicating the presence of localized states in the desired energy range. No luminescence has been observed from plastically relaxed QDs.

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.

Growth of linearly ordered arrays of InAs nanocrystals on scratched InP

Linear arrays of InAs nanocrystals have been produced by metalorganic vapor phase epitaxy on scratches performed with an atomic force microscope tip along specific crystallographic directions of an (100) InP wafer. Scratches along ⟨110⟩ generate highly mobile defects that extend far from the scratch region along easy-glide directions. On the other hand, ⟨100⟩ scratches result in highly-localized plastic deformation, hardening, and possibly frictional heating. In both cases, growth of nanocrystals was observed only on the scratched areas. Random nucleation of nanocrystals is observed along ⟨110⟩ scratches, while linearly ordered growth occur along ⟨100⟩ scratches. We attribute these observations to the delocalized nature of the dislocations in the ⟨110⟩ case, giving the appearance of random nucleation, while highly localized crystal defects along the ⟨100⟩ scratch lines act as nucleation sites for the growth of linear arrays of nanocrystals.

Detecting Infrared Radiation Beyond the Bandoffset With Intersubband Transitions

InGaAs/InAlAs quantum well (QW) infrared photodetector structures have been investigated to reach operation energies larger than the heterostructure bandoffset. The structures are composed of superlattices having a central QW with a thickness different from the others, behaving as a defect within the periodic structure. The central QW gives rise to localized states in the continuum, allowing absorption and current generation for the radiation of energies beyond the bandoffset. The detection of radiation as large as 587 meV (2.11 μm) has been achieved with intrinsic intersubband transition from the ground state to the second localized excited state in the continuum, as manifested by absorption and photocurrent measurements. Theoretical calculations fully agree with the experimental data, strongly supporting the analysis of the results.

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.