The mode control of asymmetric Fabry-Perot optical modulators with multiple quantum wells

The asymmetric Fabry-Perot (ASFP) mode position with the thickness of different index coating layer is calculated. The reason for the blue shift of the ASFP mode with the increasing thickness of low index coating layer is analyzed and this phenomenon is observed in experiments. With the wet-etching method, the ASFP mode can be tuned to the desired wavelength and thus the deviation of growth can be compensated. This method is used to improve the contrast ratio of modulators. With the ASFP mode located at different positions relative to the unbiased e-hh peak, different modulation characteristics are demonstrated.

Study of GSMBE growth and characteristics of high-quality strained In0.63Ga0.37As/InP quantum wells

High-quality compressively strained In0.63Ga0.37As/InP quantum wells with different well widths (1-11 nm) have been grown coherently on InP substrates using a home-made gas source molecular beam epitaxy (GSMBE) system. The indium composition in the wells of the sample was determined by means of high-resolution X-ray diffraction and its computer simulation. it is found that the exciton transition energies determined by photoluminescence (PL) at 10 K are in good agreement with those calculated using a deformation potential model. Sharp and intense peaks for each well can be well resolved in the 10 K PL spectra. For wells narrower than 4 nm, the line width of the PL peaks are smaller than the theoretical values of the line-width broadening due to 1 hit interface fluctuation, showing that the interface fluctuation of our sample is within 1 ML. For wells of 7 and 9 nm, the PL peak widths are as low as 4.5 meV.

Intraband optical absorption in semiconductor coupled quantum dots

In the framework of effective mass envelope function theory, absorption coefficients are calculated for intraband (intersubband in the conduction band) optical transition in InAs/GaAs coupled quantum dots. In our calculation the microscpic distributon of the strain is taken into account. The absorption in coupled quantum dots is quite different from that of superlattices. In superlattices, the absorption does not exist when the electric vector of light is parallel to the superlattice plane (perpendicular incident). This introduces somewhat of a difficulty in fabricating the infrared detector. In quantum dots, the absorption exists when light incident along any direction, which may be good for fabricating infrared detectors.

Photoluminescence studies on the interaction of near-surface GaAs/AlxGa1-xAs quantum wells with chemical adsorbates

The chemical adsorption of sodium sulphide, ferrocene, hydroquinone and p-methyl-nitrobenzene onto the surface of a GaAs/AlxGa1-xAs multiquantum well semiconductor was characterized by steady state and time-resolved photoluminescence (PL) spectroscopy. The changes in the PL response, including the red shift of the emission peak of the exciton in the quantum well and the enhancement of the PL intensity, are discussed in terms of the interactions of the adsorbed molecules with surface states.

Subband separation energy dependence of intersubband relaxation time in wide quantum wells

Subband separation energy dependence of intersubband relaxation time in a wide quantum well (250 Angstrom) was studied by steady-state and time-resolved photoluminescence. By applying a perpendicular electrical field, the subband separation energy in the quantum well is continuously tuned from 21 to 40 meV. As a result, it is found that the intersubband relaxation time undergoes a drastic change from several hundred picoseconds to subpicoseconds. It is also found that the intersubband relaxation has already become very fast before the energy separation really reaches one optical phonon energy. (C) 1997 American Institute of Physics.

Calculation of the band structure of semiconductor quantum wells using scattering matrices

The transfer-matrix method widely used in the calculation of the band structure of semiconductor quantum wells is found to have limitations due to its intrinsic numerical instability. It is pointed out that the numerical instability arises from free-propagating transfer matrices. A new scattering-matrix method is developed for the multiple-band Kane model within the envelope-function approximation. Compared with the transfer-matrix method, the proposed algorithm is found to be more efficient and stable. A four-band Kane model is used to check the validity of the method and the results are found to be in good agreement with earlier calculations.

High-pressure study of optical transitions in strained In0.2Ga0.8As/GaAs multiple quantum wells

We have measured low-temperature photoluminescence (PL) and absorption spectra of In0.2Ga0.8As/GaAs multiple quantum wells (MQW's) under hydrostatic pressures up to 8 GPa. In PL, only a single peak is observed below 4.9 GPa corresponding to the n = 1 heavy-hole (HH) exciton in the InxGa1-xAs wells. Above 4.9 GPa, new PL lines related to X-like conduction band states appear. They are assigned to the type-II transition from the X(Z) states in GaAs to the HH subband of the InxGa1-xAs wells and to the zero-phonon line and LO-phonon replica of the type-I transition involving the X(XY) valleys of the wells. In addition to absorption peaks corresponding to direct exciton transitions in the wells, a new strong absorption feature is apparent in spectra for pressures between 4.5 and 5.5 GPa. This absorption is attributed to the pseudodirect transition between the HH subband and the X, state of the wells. This gives clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance in MQW structures. From experimental level splittings we determine the valence band offset and the shear deformation potential for X states in the In0.2Ga0.8As layer.

Effective-mass theory for InAs/GaAs strained coupled quantum dots

In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Valence hole subbands and optical gain spectra of GaN/Ga1-xAlxN strained quantum wells

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc-blende GaN/Ga0.85Al0.15N strained quantum well (100 Angstrom well width) have been investigated using a 6 X 6 Hamiltonian model including the heavy hole, Light hole, and spin-orbit split-off bands. At the k = 0 point, it is found that the light hole strongly couples with the spin-orbit split-off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin-orbit split-off hole. Optical transitions between the valence subbands and the conduction subbands obey the Delta n=0 selection rule. At the k not equal 0 points, there is strong band mixing among the heavy hole, light hole, and spin-orbit split-off hole. The optical transitions do not obey the Delta n=0 selection rule. The compressive strain in the GaN well region increases the energy separation between the so1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode optical gain, and strongly depresses the TM mode optical gain. Even when the carrier density is as large as 10(19) cm(-3), there is no positive TM mode optical gain. The TE mode optical gain spectrum has a peak at around 3.26 eV. The transparency carrier density is 6.5 X 10(18) cm(-3), which is larger than that of GaAs quantum well. The compressive strain overall reduces the transparency carrier density. The J(rad) is 0.53 kA/cm(2) for the zero optical gain. The results obtained in this work will be useful in designing quantum well GaN laser diodes and detectors. (C) 1996 American Institute of Physics.

GSMBE growth and PL investigation of lattice-matched InGaAs/InP quantum wells

Using a home-made gas-source molecular beam epitaxy system, high quality InGaAs quantum wells with different well widths lattice-matched to a (001) InP substrate have been obtained. Sharp and intense peaks for each well can be well resolved in the PL spectra for the sample. For well widths larger than similar to 60 Angstrom, the exciton energies are in good agreement with those of calculation. For wells narrower than 40 Angstrom, our line widths are below the theoretical values of line width broadening due to one monolayer interface fluctuation, showing that the interface fluctuation of our sample is within one monolayer.