Optical properties and band structure of short-period GaAs/AlAs superlattices

We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 Å and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.     

Appearance of resonant Γ-X mixing in gradient GaAs/AlAs short-period superlattices

We have experimentally investigated the low-temperature (10 K) luminescence and reflection spectra of a gradient GaAs/AlAs superlattice. We have examined the behavior of phonon satellites in the vicinity of the X-Γ resonance. Smooth passage through the resonance was achieved by scanning an exciting light beam along the surface of a gradient sample. Based on our experimental results, we have determined the functional dependence of the Γ-X mixing potential on the resonance detuning. Fiz. Tverd. Tela (St. Petersburg) 40, 822–823 (May 1998)     

Refractive indices of (AlAs)(GaAs) short-period superlattices

The refractive indices of short-period binary (AlAs)_m(GaAs)_n superlattices, wherem = n, were investigated by measuring the beam divergences of optical waveguides using these superlattices as the core material. It is demonstrated that the refractive index depends on the period of the superlattice and not simply on the average composition. The refractive index is shown to depend in a systematic way on the direct bandgap of the superlattice, although the relationship may not be quite the same as that for a random alloy.     

Wannier–Stark resonances in Zener tunneling diodes with GaAs/AlAs superlattices

We present DC transport measurements of the valence to conduction band (Zener) tunneling current in ap–i–ndiode with an ultrathin intrinsic layer containing a (GaAs)₅/(AlAs)₂multi-quantum well structure. According to recent theoretical predictions, the DC current should show maxima as a function of the reverse bias voltage that reflect the formation of Wannier–Stark resonances. So far, Wannier–Stark resonances have only been observed optically and never in a regime of strong Zener tunneling. Experimentally, we find the second derivative of the current-voltage characteristics to show a weak oscillatory structure indeed, indicating the existence of Wannier–Stark resonances in Zener tunneling.     

Effect of a terahertz cavity on the conductivity of short-period GaAs/AlAs superlattices

The miniband conductivity in short-period GaAs/AlAs superlattices with a terahertz cavity has been studied. A stepwise decrease in the current caused by the formation of the electrical domains has been observed in current–voltage characteristics at a certain threshold voltage. It has been found that this threshold voltage changes considerably under the variation of the cavity parameters. The shift of the threshold has been explained by the excitation of high-amplitude oscillations in the cavity.     

Determination of the roughness of heteroboundaries from photocurrent spectra of short-period AlAs/GaAs superlattices

Photocurrent spectroscopy is used to study the nature of the roughness of heteroboundaries in (AlAs)_m/(GaAs)_n short-period superlattices (m=3−5, n=10−13) grown by molecularbeam epitaxy. The formation of minibands broadens the optical spectra of superlattices in comparison with isolated quantum wells; therefore to analyze the degree of perfection of the boundaries we used the decay of the minibands into a series of discrete Wannier-Stark levels in an electric field parallel to the superlattice axis. Exciton lines were observed in the photocurrent spectra in an electric field corresponding to direct and indirect (in space) transitions between the Wannier-Stark levels. Comparison of experimental data with calculation indicates that even in the better structures, in addition to monotonic variation of the thickness of the layers over area, roughnesses in the heteroboundaries one monolayer in height are present with characteristic lateral dimension not exceeding 10 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2085–2089 (November 1997)     

Angular dependence of Raman scattering selection rules for long-wavelength optical phonons in short-period GaAs/AlAs superlattices

The angular dependence of Raman scattering selection rules for optical phonons in short-period (001) GaAs/AlAs superlattices is calculated and experimentally studied. Experiments are performed using a micro-Raman setup, in the scattering geometry with the wavevectors of the incident and scattered light lying in the plane of superlattices (so-called in-plane geometry). Phonon frequencies are calculated using the Born model taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra are calculated in the framework of the deformation potential and electro-optical mechanisms. Calculations show an angular dependence of the selection rules for optical phonons with different directions of the wavevectors. Drastic differences in the selection rules are found for experimental and calculated spectra. Presumably, these differences are due to the Fröhlich mechanism in Raman scattering for short-period superlattices.     

Identification of zone boundary and interface phonon recombinations in photoluminescence from type-II GaAs/AlAs short-period superlattices

We present the first identification of the direct participation of the GaAs interface (IF) mode in the phonon-assisted recombinations from type-II GaAs/AlAs short-period superlattices (SPSL). This is achieved by utilizing a novel first-order resonant Raman process associated with the type-II X_z–Γ band gap to enable a direct comparison, in the same sample, of zone boundary phonons with the photoluminescence (PL) phonon satellite energies. We present PL measurements on two complementary SPSLs to identify the reversal in the parity of the GaAs IF branch, where coupling is only allowed to the IF(+) branch. We also identify weak satellites, not previously reported, between the IF(+) and LA(X) satellites.     

Donor-acceptor recombination in type-II GaAs/AlAs superlattices

A study is reported of steady-and nonsteady-state photoluminescence of intentionally undoped and uniformly silicon-doped type-II (GaAs)₇(AlAs)₉ superlattices grown by MBE simultaneously on (311)A-and (100)-oriented GaAs substrates. It has been established that at elevated temperatures (160>T>30 K) the superlattice spectra are dominated by the line due to the donor-acceptor recombination between donors in the AlAs layers and acceptors located in the GaAs layers. The total carrier binding energy to the donor and acceptor in a pair has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1734–1739 (September 1998)     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.