Elias Towe

Self-assembled (In,Ga)As/GaAs quantum-dot nanostructures: strain distribution and electronic structure

Abstract This paper presents a simple analytical method for calculating the strain distribution in and around self-assembled (In,Ga)As/GaAs quantum-dot nanostructures. The dots are assumed to be buried in an infinite medium so that the effects of free surfaces can be neglected. This assumption is based on the relative size of the dot, compared to that of the overlayer. The model—based on classical continuum elasticity—is capable of handling dots of arbitrary shapes; here, however, only dots with pyramidal and truncated-pyramidal shapes are considered. The approximate shape of the dots is extracted from high-resolution transmission electron microscope observations. The electronic energy levels in the dots are calculated by solving the three-dimensional effective mass Schrodinger equation. The carrier confinement potential in this equation is modified by the strain distribution. Because the dots are in a strong confinement regime, the effects of Coulomb interactions are neglected. The calculated confined eigen-energies agree with our experimental photoluminescence data. The calculations also support previous results reported by others.

Nanowire lasers with distributed-Bragg-reflector mirrors

A self-consistent, coupled optoelectronic simulation model is used to study microcavity GaN nanowire lasers. This model is applied to show that it is possible to design a nanowire laser with distributed-Bragg-reflector mirrors. Several advantages are expected to be enjoyed from the use of such mirrors. These include reduction of lasing threshold current density (or pumping density rate), dramatic reduction of cavity losses, and the achievement of stable, single-mode operation.A self-consistent, coupled optoelectronic simulation model is used to study microcavity GaN nanowire lasers. This model is applied to show that it is possible to design a nanowire laser with distributed-Bragg-reflector mirrors. Several advantages are expected to be enjoyed from the use of such mirrors. These include reduction of lasing threshold current density (or pumping density rate), dramatic reduction of cavity losses, and the achievement of stable, single-mode operation.

Application-oriented nitride substrates: The key to long-wavelength nitride lasers beyond 500 nm

We present results based on quantum mechanical estimates of the longest emission wavelength for nitride laser diodes grown on c-plane GaN/sapphire substrates. The results indicate that the absence of polarization-induced electric fields in nonpolar/semipolar GaN substrates does not necessarily guarantee that nitride lasers will operate at the longest possible wavelength for a given set of parameters. Our calculations suggest that the limit on the longest possible wavelength of nitride lasers is constrained by the lattice mismatch rather than by the strength of the polarization-induced electric field. Although it may be possible to develop lasers that approach the green portion of the electromagnetic spectrum (∼520 nm) by growing the structures on nonpolar/semipolar GaN substrates, the development of red and near-infrared nitride lasers appears extremely difficult by merely growing the structures on any crystallographic orientation of the GaN substrate. We suggest that efficient lasers emitting at the gree...

Optical properties of nanometer-sized gold spheres and rods embedded in anodic alumina matrices

This letter discusses theoretical studies of optical properties of gold nanospheres and nanorods with various aspect ratios; these are embedded in a porous anodic alumina matrix. The nanoparticles have potential applications in optical filters and sensors.

Oscillator strength for intraband transitions in (In,Ga)As/GaAs quantum dots

This letter reports on theoretical calculations of the oscillator strength associated with electron intraband transitions in (In,Ga)As/GaAs quantum dots. We study the effect of dot size and lateral separation between adjacent dots on the oscillator strength. The calculations indicate that transitions induced by p-polarized light from the electronic ground state to the first excited state are stronger than those induced by s-polarized light for large size dots with wide lateral interdot spacing. This situation changes, however, for small size dots in close proximity with one another. We discuss the relevance and implication of these results for applications in quantum-dot structures designed for mid-infrared detection.

Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopy

InAs/GaAs quantum dot (QD) heterostructures grown by molecular beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. The images reveal individual InAs QDs having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Analysis of strain relaxation of the QDs reveals an indium composition varying from 65% at the base of the QD, to 95% at its center, and back to 65% at its apex. Room-temperature tunneling spectra acquired 3–4 nm from the center of a dot show a peak located in the upper part of the GaAs band gap originating from the lowest electron confined state of the QD, along with a tail in the conductance extending out from the valence band and originating from QD hole states. A computational method is developed for simulating the tunneling spectra using effective-mass bands treated in an envelope function approximation. By comparison of the computations to low-current spectra, the energy of the lowest electron, and highest hole QD states are det...

Design of high-Q microcavities for proposed two-dimensional electrically pumped photonic crystal lasers

Electrically pumped photonic crystal lasers are of practical importance for future integrated photonic circuit systems. This paper proposes a methodology for achieving high quality (Q) factor photonic crystal defect cavities that allow current injection into their active regions. It is shown that by combining certain high Q-factor photonic crystal cavity designs with the technique of wet oxidation of (Al,Ga)As layers, Q factors of up to /spl sim/10/sup 4/ can be obtained within the scope of existing semiconductor planar process technology. The proposed device structures can be optimized through use of finite-difference time-domain methods to obtain optimal separation of the high refractive index substrate from the active core; furthermore, the effects of the top ohmic contact layer, the top and bottom cladding layers of the structure, and the current injection opening can be taken into account to achieve an optimal Q factor in electrically pumped lasers.

Characteristics of high-operating-temperature InAs∕GaAs quantum-dot infrared detectors

Characteristics of high-operating-temperature InAs∕GaAs quantum-dot infrared detectors with a wide band gap current-blocking layer are reported. Clean photoresponse spectra were observed up to 220K. A high peak responsivity of about 4.3A∕W was measured at 100K; this responsivity decreased to 34mA∕W at 220K. The typical corresponding detectivity for the devices at 100K was ∼2×109cmHz1∕2∕W. This detectivity can be improved by increasing the areal quantum-dot density and by carefully optimizing the dopant impurity concentration in the device.

Intersublevel photoresponse of (In,Ga)As/GaAs quantum-dot photodetectors: Polarization and temperature dependence

We report on the polarization dependence of intraband photoresponse of (In,Ga)As/GaAs quantum-dot device structures for light polarized parallel and perpendicular to the layers. Strong photoresponse due to intersublevel transitions induced by both s- and p-polarized infrared light was observed. Within the plane of the layers, it is found that the photoresponse for s-polarized light aligned along the [110] crystallographic direction is virtually identical to that in the [110] direction, suggesting that, at least in the x-y plane, the dots are symmetric. The devices studied were found to operate up to a temperature of around 100–105 K.

On ternary nitride substrates for visible semiconductor light-emitters

No nitride or other substrate material exists for growing lattice-matched nitride device structures. Use of bulk GaN or sapphire substrates is complicated by lattice and thermal mismatches that lead to defect and dislocation generation. To alleviate this problem, we recently proposed ternary nitride substrates on which lattice-matched structures could be grown for lasers within specified spectral bands. These proposed application-oriented nitride substrates have one drawback: several would be required to cover the visible spectrum. By taking advantage of the complex (but feature-rich) valence band structure of nitrides, we have determined that a single substrate (In0.15Ga0.85N) could be used for the development of efficient blue, green, and red laser diodes.