M. Dion

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at ∼707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of ∼8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.

Extremely low threshold current strained InGaAs/AlGaAs lasers by molecular beam epitaxy

Using solid source molecular beam epitaxy we have grown strained layer InGaAs/AlGaAs graded index separate confinement heterostructure lasers operating at 1.01 μm. For broad‐area, uncoated Fabry–Perot devices with cavity lengths in excess of 3000 μm, the threshold current density is 56 A/cm2, a value which we believe to be the lowest ever reported for laser diodes in any materials system. The internal quantum efficiency for these lasers is 88%, while the materials losses are 1.8 cm−1.

QUANTUM-WELL INTERMIXING FOR OPTOELECTRONIC INTEGRATION USING HIGH ENERGY ION IMPLANTATION

The technique of ion‐induced quantum‐well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded‐index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion‐induced QW intermixing, such as ion doses, fluxes, and energies, post‐implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.

COMPOSITION OF ALGAAS

Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.

Defect diffusion in ion implanted AlGaAs and InP: Consequences for quantum well intermixing

InGaAs/GaAs/AlGaAs and InGaAs/InGaAsP/InP laser structures, with InGaAs quantum wells approximately 1.85 μm beneath the surface, were implanted with ions having energies up to 8.6 MeV. Intermixing of the quantum wells, after rapid thermal annealing, was monitored through changes in the energy, linewidth, and intensity of the photoluminescence peak from the quantum wells. Where the defects had to diffuse primarily through Al0.71Ga0.29As, these quantities correlate strongly, for short anneal times, with calculated vacancy generation and ion deposition at the depth of the quantum well prior to annealing. This suggests that the defect diffusion length in the AlGaAs and/or GaAs is quite low. For diffusion primarily through InP, the photoluminescence data correlated well with the calculated total number of vacancies created in the sample, suggesting that defect diffusion is very efficient in InP.

Band‐gap tuning of InGaAs/InGaAsP/InP laser using high energy ion implantation

The technique of ion‐induced quantum well intermixing using broad area, high energy (1 MeV P+) ion implantation has been used to tune the emission wavelength of an InGaAs/InGaAsP/InP multiple quantum well (MQW) laser operating at 1.5 μm. The optical quality of the band‐gap shifted material is assessed using low‐temperature photoluminescence (PL). The band‐gap tuned lasers are characterized in terms of threshold current density and external quantum efficiency and exhibit blue shifts in the lasing spectra of up to 63 nm. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating active as well as passive optoelectronic devices.

Design criteria for structurally stable, highly strained multiple quantum well devices

Strain compensation allows the synthesis of infinitely thick heterostructures with many highly strained quantum wells. Design criteria are given for optimized strain and thickness parameters in several device geometries. Strain compensation, using alternating layers of opposite strain, is quantitatively treated using an energy balance analysis. The upper bound to stability for strained multiple quantum wells with and without strain compensation is defined for geometries typically used in optoelectronic devices. Highly metastable structures (composed of many layers of high strain and/or thickness) require low epitaxy temperatures to avoid strain relaxation during growth of individual strained layers, prior to their stabilization in a strain compensated structure.

Quasi-phase-matched second-harmonic generation in reflection from AlxGa1-xAs heterostructures

Quasi-phase-matched second-harmonic (SH) generation in reflection geometry is described and demonstrated. The SH intensity can be strongly enhanced by spatially modulating the optical properties of the nonlinear medium. This type of quasi-phase-matching is demonstrated by using an Al0.8Ga0.2As/GaAs heterostructure designed for λ = 1.06 μm incident light. The SH light intensity generated in reflection from the heterostructure is enhanced 70 times relative to the SH response of a homogeneous GaAs wafer. A resonant cavity design that employs this structure to make thin films with extremely high SH generation efficiencies is proposed.

Demonstration of an ion-implanted, wavelength-shifted quantum-well laser

A technique for fabricating many different wavelength lasers on the same wafer has been developed. High energy ion implantation was used to selectively blue shift the emission wavelength of an InP-based quantum well laser structure. This structure was then processed into fully functional broad-area lasers whose current threshold was unaffected by the implantation process, indicating extremely high material quality after bandgap-shifting. This process has the potential for the integration of not only different wavelength lasers, but also other devices, such as waveguides, detectors, modulators, etc., on a single wafer.

The enhancement of quantum well intermixing through repeated ion implantation

Quantum well (QW) intermixing has been performed using low-energy broad-area ion implantation to increase the bandgap energy in a spatially selective manner. There is a maximum single dose beyond which further intermixing of the QWS is impeded by damage to the semiconductor surface. We demonstrate that this problem can be overcome by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have observed shifts in optical bandgap for multiple implants greater than 2.5 times that observed for a single implant.