E.H. Li

Theoretical analysis of modulation response and second-order harmonic distortion in vertical-cavity surface-emitting lasers

A rate-equation model is developed, with the consideration of size effects, to analyze the steady state and dynamic behavior of index-guided vertical-cavity surface-emitting lasers. The size dependence of spatial hole burning, cavity loss, as well as thermal resistance of device cavity are taken into account. Using this model, the influence of size effects on the amplitude modulation response and second-order harmonic distortion are studied. It is found that a laser with a small core radius exhibits better modulation response and less harmonic distortion than that of a large waveguide device, however, there is a tradeoff between the output power and modulation efficiency of the lasers.

Eigenstates and absorption spectra of interdiffused AlGaAs-GaAs multiple-quantum-well structures

We present a comprehensive analysis of the inter-well coupled eigenstates in interdiffused AlGaAs-GaAs multiple quantum-well structures. A full numerical calculation is considered without any approximation or presumption of the eigenstates. During the initial interdiffusion in the quantum-well structure with well and barrier thickness equaling 100 /spl Aring/, the wavefunctions behave as in the case of a single quantum well with almost no well-to-well coupling of the states. However, as interdiffusion proceeds, the eigenstate in each subband forms minibands, as in the case of superlattice. Distortion of the coupled wavefunctions can also be observed as a consequence of the nonuniformity of the confined wells at the far sides of the multiquantum-well core. The polarized absorption coefficients, including valence band-mixing, are also calculated. Results show that the blue shift of the absorption edge is greater in the range of 10 /spl Aring//spl les/L/sub d//spl les/30 /spl Aring/. The two-dimensional quantum-well properties are at its strongest in the beginning of interdiffusion. An estimation of the modulator performance also shows an improvement of the contrast ratio and lower absorption loss during the initial stage of interdiffusion. This predicts wavelength tuning range of almost 60 nm.

Anodic-oxide-induced interdiffusion in GaAs/AlGaAs quantum wells

Enhancement of interdiffusion in GaAs/AlGaAs quantum wells due to anodic oxides was studied. Photoluminescence, transmission electron microscopy, and quantum well modeling were used to understand the effects of intermixing on the quantum well shape. Residual water in the oxide was found to increase the intermixing, though it was not the prime cause for intermixing. Injection of defects such as group III vacancies or interstitials was considered to be a driving force for the intermixing. Different current densities used in the experimental range to create anodic oxides had little effect on the intermixing.

Anodic-oxide-induced intermixing in GaAs-AlGaAs quantum-well and quantum-wire structures

Anodic oxides of GaAs were shown to enhance the intermixing in GaAs-AlGaAs quantum wells (QW) during rapid thermal processing. Proximity of the anodic oxide to the QW has been shown to influence the photoluminescence (PL) energy shift due to intermixing. Anodic oxide induced intermixing has been used to enhance quantum-wire PL in the structures grown on V-groove patterned GaAs substrates. This has been attributed to enhanced lateral confinement in these structures. Injection of defects such as group-III vacancies or interstitials was considered to be driving force for the intermixing.

The applications of an interdiffused quantum well in a normally on electroabsorptive Fabry-Perot reflection modulator

A Fabry-Perot reflection-type modulator which uses interdiffused AlGaAs-GaAs quantum wells as the active cavity material has been studied and optimized theoretically. An asymmetric Bragg reflector structure (modeled by transfer matrices), with a doped depletion layer in the heterostructure, has been considered. This is the first study to model such a material system in this type of modulator, and the results show improvement in modulation property over its as-grown rectangular quantum-well modulator. In particular, the change of reflectance in the diffused quantum-well modulator is almost 0.6 to 0.7, which is higher than that of the typically available values (/spl sim/0.5 to 0.6), while the OFF-state on-resonance reflectance is almost close to zero. The operation voltage is also reduced by more than half as the interdiffusion becomes extensive. The finesse of the more extensively diffused quantum well also increases. Both of these features contribute to an improvement of the change of reflectance in the modulator. The operation wavelengths can be adjusted over a range of 100 nm. However, the absorption coefficient change of the diffused quantum well increases only when there is a small amount of interdiffusion.

Semiconductor lasers using diffused quantum-well structures

We assess the relative merits and prospects of using diffused quantum-well (QW) structures in semiconductor lasers. First, different techniques to achieve interdiffusion are analyzed and compared. Second, recent development of semiconductor lasers using interdiffusion technique is also discussed. Third, the optical properties of diffused QWs are studied. In addition, novel design of diffused QWs structures to maintain stable single-mode operation in semiconductor lasers is proposed. Finally, brief discussion and conclusion are given.

The effect of carrier-induced change on the optical properties of AlGaAs-GaAs intermixed quantum wells

The carrier-induced effects in the change of absorption and refractive index on the AlGaAs-GaAs intermixing modified quantum wells (QWs) have been investigated theoretically. Band-filling, bandgap shrinkage, and free-carrier absorption have been included for various carrier concentrations. The Schrodinger and the Poisson equations have been considered self-consistently. The polarized absorption coefficients are calculated using the Kane k/spl middot/p method for a four band model and followed by the Kramers-Kranig transformation to obtain the refractive index change. The results obtained show a more enhanced bandgap renormalization and change of absorption, but a reduced change in refractive index for the larger intermixing extents. It is important to know the carrier-induced optical parameter changes the intermixed QWs because of their recent interests in photonics.

The effect of interdiffusion on the intersubband optical properties in a modulation-doped quantum-well structure

The linear and nonlinear (based on optical field intensity) intersubband absorptions in conduction band, and its change in refractive index in AlGaAs-GaAs interdiffused quantum wells (QWs) are presented. The calculation of the electron energy levels and the envelope wave functions in a modulation doped interdiffused QWs with screening effects are considered. QW interdiffusion shows a wavelength tunability of the intersubband absorption peaks and refractive index dispersions. This shifting of the transition energies is also demonstrated here to be a useful technique for broad-band and multicolor photodetector application. In addition, it can serve to remove noise, such as minor peaks and dispersions, in the optical spectra.

A comparative study of uniaxial pressure effects in intraband AlGaAs/GaAs and interband InAs/AlSb/GaSb resonant tunneling diodes

We report on the effects of uniaxial pressure on (001)-oriented AlGaAs/GaAs and InAs/AlSb/GaSb double barrier resonant tunneling diodes (RTDs). The current–voltage characteristics of the AlGaAs/GaAs RTDs shift asymmetrically due to stress-induced piezoelectric fields in the barriers and well structures. Although all the materials involved are piezoelectric, the interband InAs/AlSb/GaSb resonant tunneling device surprisingly shows, in contrast to the AlGaAs/GaAs one, a symmetrical behavior for the same orientation [110] of the applied pressure. We explain the observed differences considering the different tunneling paths involved in the conduction mechanism of the two heterostructure device types as well as their pressure dependencies.

High-power single-mode operation in DFB and FP lasers using diffused quantum-well structure

Distributed feedback (DFB) and Fabry-Perot (FP) semiconductor lasers with step and periodic interdiffusion quantum-well structures are proposed for high-power single-longitudinal-mode operation. It is shown that the phase-adjustment region formed by the diffusion step (i.e., step change in optical gain and refractive index) counteracts the influence of spatial hole burning, especially for DFB lasers with large coupling-length products biased at high injection current. Furthermore, it is found that with careful design of the diffusion grating (i.e., grating period and amount of diffusion extent) of FP lasers, side-mode suppression ratio can be enhanced and threshold current density can be minimized to a satisfied level.