A. Saher Helmy

Quasi phase matching in GaAs--AlAs superlattice waveguides through bandgap tuning by use of quantum-well intermixing.

We report the observation of second-harmonic generation by type I quasi phase matching in a GaAs-AlAs superlattice waveguide. Quasi phase matching was achieved through modulation of the nonlinear coefficient chi((2))(zxy), which we realized by periodically tuning the superlattice bandgap. Second-harmonic generation was demonstrated for fundamental wavelengths from 1480 to 1520 nm, from the third-order gratings with periods from 10.5 to 12.4microm . The second-harmonic signal spectra demonstrated narrowing owing to the finite bandwidth of the quasi-phase-matching grating. An average power of ~110 nW was obtained for the second harmonic by use of an average launched pump power of ?2.3mW .

Selective control of self-organized In0.5Ga0.5As/GaAs quantum dot properties: Quantum dot intermixing

Selective postgrowth control of the photoluminescence (PL) wavelength has been demonstrated for a single layer self-organized In0.5Ga0.5As/GaAs quantum dot (QD) structure. This was achieved by rapid thermal processing of dots using different dielectric caps. Selective band gap shifts of over 100 meV were obtained between samples capped with sputtered and plasma enhanced silica deposition, with the band gap shift under regions covered with plasma enhanced chemical vapor deposition SiO2 less than 70 meV. The effects of different caps on the PL linewidth were also observed. The differential band gap shift will enable the integration of passive and active devices in QD systems.

Suppression of quantum well intermixing in GaAs/AlGaAs laser structures using phosphorus-doped SiO2 encapsulant layer

A phosphorus-doped silica (SiO2:P) cap containing 5 wt% P has been demonstrated to inhibit the bandgap shifts of p-i-n and n-i-p GaAs/AlGaAs quantum well laser structures after rapid thermal processing. The intermixing suppression has been attributed to the fact that SiO2:P is more dense and void free compared with standard SiO2 together with a strain relaxation effect of the cap layer during annealing. Band gap shift differences as large as 100 meV have been observed from samples capped with SiO2 and with SiO2:P. The n-i-p structure showed a higher degree of intermixing compared to p-i-n structure. This behaviour has been attributed to the rise of Fermi level in the n doped structure, through which the formation energy of Ga vacancies is reduced compared to the p doped structure.

Control of silica cap properties by oxygen plasma treatment for single-cap selective impurity free vacancy disordering

By exposing the SiO2 films used as annealing caps in the process of impurity free vacancy disordering (IFVD) to an oxygen plasma, which is produced in a reactive ion etching machine, the effect of the exposed caps on quantum well intermixing can be substantially controlled. The effect of the oxygen treatment is manifested in inhibiting the Ga outdiffusion from GaAs/AlGaAs heterostructures. A selective IFVD process using identical silica caps has been obtained by selective exposure of the caps to oxygen plasma. Differential band gap shifts in excess of 100 meV were achieved with control samples exhibiting band gap shifts less than 10 meV.

SILICA CAPPING FOR AL0.3GA0.7AS/GAAS AND IN0.2GA0.8AS/GAAS QUANTUM WELL INTERMIXING

Spin-on silica capping has been demonstrated to be an effective dielectric encapsulant layer for quantum well (QW) intermixing at temperatures significantly lower than for conventionally deposited silica. A blueshift of up to 125 meV was observed in the photoluminescence (PL) peak energy of both GaAs and InGaAs QWs after annealing for less than 60 s at 850 °C, without noticeable degradation in the PL emission intensity. A threshold temperature was identified below which no significant QW disordering took place. The activation energy for Al diffusion in Al0.3Ga0.7As/GaAs QWs was about 2.55 eV. Broadly similar effects were seen for In0.2Ga0.8As/GaAs QWs but, in addition, strain effects appear to enhance disordering during the early stages of the anneal.

Engineering quantum-dot lasers

We discuss recent progress in the engineering of quantum-dot (QD) lasers, focusing on the spectral output, dynamics and techniques for integration. Two approaches to such engineering are discussed. Firstly, it is suggested that control of lasing spectra in QD lasers is possible by making use of waveguiding-related phenomena (substrate leakage and reflection) which, in unoptimised laser structures, result in the mode grouping effect (quasiperiodic spectral modulation). Secondly, first experimental studies of quantum dot intermixing are reported, suggesting that this technique is capable of both improving the performance of active QD media and integrating active QD sections with passive waveguides.

High-spatial-resolution quantum-well intermixing process in GaInAs/GaInAsP laser structure using pulsed-photoabsorption-induced disordering

Raman spectroscopy was used to study the spatial resolution of pulsed-photoabsorption-induced quantum-well intermixing in a GaInAs/GaInAsP laser structure. A differential band gap shift of up to 60 meV has been obtained from a sample masked with SixNy/Au and exposed to the laser irradiation. Intermixing was detected in the irradiated regions through the shift of GaAs-like modes to lower frequencies. In addition, the intermixing induced GaInP longitudinal optical modes in the irradiated regions, which is evidence of the intermixing between the upper GaInAs cap and the GaInAsP layer. The spatial resolution of this process, which was obtained from micro-Raman spectra when scanned across the interface of the intermixing mask, was found to be better than 2.5 μm.

The kinetics of intermixing of GaAs/AlGaAs quantum confined heterostructures

An atomic-scale model for the kinetics of intermixing of GaAs/AlGaAs, quantum confined heterostructures is presented. It quantifies the effects of the statistical nature of defect diffusion through heterostructures on the Ga/Al interdiffusion across such an interface. The model has been validated by successfully predicting the observed amounts of quantum well intermixing induced by a hydrogen plasma induced defect layer intermixing process. Agreement within 30% of the measurements was obtained for values of the surface release velocity>1 μm s1.

Raman spectroscopy for characterizing compositional intermixing in GaAs/AlGaAs heterostructures

Compositional intermixing induced by the process of impurity-free vacancy (dielectric cap annealing induced) disordering in GaAs/AlGaAs is studied using Raman spectroscopy. The degree of intermixing in multiple-quantum-well structures was detected through the energy shift of certain Raman modes of the lattices. In addition, localized intermixing, with band-gap shifts as low as 6 nm realized in 1:1 band-gap grating patterns with different periods (⩾4 μm), was also detected through the energy shift and the full width at half maximum of the structures’s Raman modes.

Optical characterization of GaAs/AlGaAs quantum well wires fabricated using arsenic implantation induced intermixing

We report the fabrication of GaAs/AlGaAs quantum well wires using implantation of As at 45 keV to induce quantum well intermixing. The intermixing process was first characterized giving optimized annealing parameters of 875 °C for 30 s and an implantation dose of 1×1013 cm−2. Wire widths from 35 to 1000 nm were defined using e-beam lithography followed by lift-off. Photoluminescence spectra from the lateral wells and barriers were observed from samples with wires as narrow as 50 nm. The energies of the lateral wells were found to remain constant for wire widths between 1000 and 150 nm, and start to shift significantly towards high energy for 80 nm wires, the signal from the lateral well eventually merging with that from the lateral barrier for 35 nm wires. An intermixing radius of about 17 nm was estimated for the process. Photoreflectance measurements were also carried out on these wire samples, showing that the wires appear to have a parabolic lateral potential and clear interwire coupling was observed ...