B. Yavich

Chirp dependence in InGaAs/InAlAs multiple quantum well electro-absorptive modulators near polarization-independent conditions

We study the dependence of the chirp parameter on heavy- and light-hole energy splitting by analyzing the change in the absorption curve of different InGaAs/InAlAs p-i-n multiple quantum well structures designed for use in amplitude modulators. We observe, for the transverse electric mode, a high chirp parameter for the sample whose fundamental transition involves the light hole, whereas for samples whose fundamental transition involves the heavy hole, the more polarization sensitive the samples are, the smaller the chirp parameter is. This indicates that it is not possible to have tensile strained InGaAs/InAlAs multiple quantum well structures for electro-absorptive modulators which are simultaneously chirp-free and polarization independent.

On the optimization of InGaAs-InAlAs quantum-well structures for electroabsorption modulators

Several InGaAs-InAlAs multiple quantum-well structures grown by metalorganic vapor phase epitaxy (MOVPE), with various Ga content and quantum-well width, have been investigated for electroabsorption modulators (EAMs). The light-hole heavy-hole splitting, the chirp parameter, the insertion loss and the figure of merit /spl Gamma//spl Delta//spl alpha//F of the different InGaAs-InAlAs structures have been evaluated with photocurrent, photoluminescence, absorption and X-ray measurements. It was then possible to experimentally study the influence of different parameters of the multiple quantum-well structures on the device performance. The use of tensile strained barriers are believed to be responsible for the improvement in the figure of merit. Structures with unresolved light-hole and heavy-hole transitions, with negligible chirp, with adequate insertion loss and with extremely high values for /spl Gamma//spl Delta//spl alpha//F have been obtained, however, not simultaneously.

Anisotropy of the cyclotron mass in superlattices containing two populated minibands

The cyclotron mass in periodically delta-doped InP was studied, using the temperature dependence of the Shubnikov-de Haas (SdH) effect in tilted magnetic fields. The samples had two populated minibands, E1 and E2, both of which contributed with oscillatory components to the SdH spectrum. When the magnetic field is tilted from the direction parallel to the axis of the superlattice, the cyclotron mass associated with E1 electrons increases, as expected for a quasi-two-dimensional system. In contrast, the cyclotron mass of E2 electrons decreases. This decrease is due to Bragg reflections of electrons by the superlattice, which lead to shorter orbits when the magnetic field is tilted. It is estimated that in periodically delta-doped semiconductors the cyclotron mass can decrease at most by a factor of three when the magnetic field is rotated by π/2.

Single and periodically Si -doped InP grown by LP-MOVPE

Single and periodically Si -doped InP layers were grown by LP-MOVPE at . A full width at half maximum of 32 ? was obtained for the net charge concentration profile for a sample with a peak net charge concentration of . Numerical simulations showed that the impurities are localized over less than three InP monolayers. No dopant diffusion or segregation was observed. The periodic structures, grown with barriers varying from 100 to 300 ?, all had nearly the same carrier sheet concentration per dopant layer and impurity localization characteristics.

Characterization of -doped superlattices by Shubnikov - de Haas measurements

The Shubnikov - de Haas oscillations of InP with a periodical planar doping with Si were studied at 4.2 K in fields of 0 - 14 T. By confronting the oscillation frequencies detected experimentally with the ones predicted on the basis of the effective-mass approximation the carrier population of the superlattice minibands and the characteristic width of the doped layer were obtained. The width of the doped layer obtained in this way is in good agreement with the value obtained from C - V profiling measurements on the same structures.

Reliably designing InGaAs-InAlAs strained multiple-quantum-well structures for amplitude modulation

A simulation of the performance parameters of amplitude modulators was carried out for InGaAs-InAlAs multiple-quantum-well structures for operation at 1.55 /spl mu/m. The device parameters were estimated from the calculated absorption spectra with applied reverse bias and from the photoluminescence spectra. The theoretically determined results are compared with the experimental data obtained from the measured photocurrent spectra with light incident in the direction perpendicular to the layers. Good agreement was achieved between experimental and theoretical data, providing a reliable way for designing efficient amplitude modulators. The effect of residual doping level and pre-bias on device parameters is discussed.

Interfacial layers and impurity segregation in InP/In0.53Ga0.47As superlattices

The interfaces in InP=In0.53 Ga0.47As superlattices modulation doped with Si were investigated using magneto-transport, capacitance-voltage, and high resolution X-ray diffraction measurements. Results indicate that a thick interfacial layer is formed when InP is grown on top of InxGa1-xAs, and that Si atoms that fall in the interfacial layer have a high probability of not forming a shallow donor center. Using a simple theoretical model the width of the interfacial layer which was estimated to be 7±1 monolayers.

High magnetic field transport and photoluminescence in doped InGaAs/InP superlattices

Lattice-matched InP/InxGa1- x As short period superlattices (x = 0.53) d-doped with Si in the middle of the InP barriers were studied. The samples had a high carrier concentration which filled two minibands. In addition to a peak associated with the electrons from the second miniband, E2, the Shubnikov-de Haas spectra showed a well resolved doublet structure that is assigned to E1 electrons of superlattice wave vectors kz = 0 and kz = p /d. From the lineshape of the Shubnikov-de Haas oscillations, an E1 quantum mobility of 970 cm2/Vs was deduced, which represents an increase of about 40% over the value for periodically delta-doped semiconductors. The photoluminescence ex- hibits a band at photon energies higher than the InGaAs bandgap and whose FWHM approximates the Fermi energy of the confined carriers. Thus the photoluminescence observed is consistent with the recombination of electrons confined by the superlattice potential and photoexcited holes.

Luminescence from miniband states in heavily doped superlattices

We have studied doped superlattices of GaAs/AlGaAs composition. When the doping atoms are introduced into the barriers surrounding the superlattice, as well as to the inner ones, but with half of the concentration, the photoluminescence due to interband transitions from extended superlattice states is detected. This is demonstrated by a study of the sample´s photoluminescence in a magnetic field, whose intensity oscillates in concomitance with the SdH spectrum of electrons confined in the miniband.

Some key issues on the optimization of multiple quantum well structures for amplitude modulation

Main parameters of InGaAs/InAlAs multiple quantum well amplitude modulators, such as contrast ratio, insertion loss and chirp parameter, were calculated in order to find a quantum well structure which optimizes them. The parameters were estimated from the theoretical absorption curves, which were calculated for different values of applied reverse bias and were compared with experimental data. A study of the device parameters as a function of the Ga content and operation voltage was performed. The study showed that optimum values for the modulator parameters cannot be obtained simultaneously. The influence of the residual doping level and the applied pre-bias are emphasized.