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.     

Carrier diffusion in InGaAs/GaAs quantum wells grown on vicinal GaAs(0 0 1) substrates

We have investigated the influence of vicinal GaAs substrates on the optical and electronic properties of InGaAs/GaAs quantum wells (QWs). A single In_0.10Ga_0.90As QW was grown by molecular-beam epitaxy on a vicinal GaAs(0 0 1) substrate with a miscut angle of 0° (nominal), 2°, 4° and 6° towards [1 1 0]. The carrier diffusion was obtained by a micro-photoluminescence scan technique that permits to observe the effective diffusion length characterized by the lateral spread of carriers in the QW followed by radiative recombination. The carrier diffusion length was obtained parallel (L_||) and perpendicular (L) to the atomic steps. The diffusion length decreases as the temperature increases up to 100 K. Above this temperature we found different behaviours that depend on the sample miscut angle.     

Carriers escape mechanisms in shallow InGaAs/GaAs quantum wells grown on vicinal (001) GaAs substrates

We used photoluminescence spectroscopy in order to investigate the carriers escape mechanisms in In_0.15Ga_0.85As/GaAs quantum wells grown on top of nominal (001) and 2°-, 4°- and 6°-off (001) towards (111)A GaAs substrates. We described the escape processes using two models that fit the Arrhenius plot of the integrated PL intensity as a function of the inverse of the sample temperature. In the first model, we considered equal escape probability for electrons and holes. In the second one, we assumed that a single type of carrier can escape from the well. At high temperature, the first model fits the experimental data well, whereas, between 50 K and 100 K, the second model has to be taken into account to describe the data. We observed that the escape activation energy depends on the misorientation angle. An unusual behavior was noted when the full width at half maximum of the photoluminescence main emission was plotted as a function of the sample temperature. We showed that the escape process of the less-confined carriers drives this behavior.     

X-ray diffraction pattern of quasiperiodic (Fibonacci) Nb-Cu superlattices

1-D quasiperiodic Nb-Cu metallic superlattices have been prepared by magnetron sputtering. The X-ray diffraction pattern may be indexed by the projection method from the high-dimensional periodic structure.     

Partial intermixing of strained InGaAs/GaAs quantum wells

Shallow ion implantation and rapid thermal annealing (RTA) was used to modify the optical properties of strained InGaAs/GaAs quantum wells (QWs). After RTA, QW exciton energies, determined from peak positions of the photoluminescence spectra, shifted significantly to higher energies in the implanted areas, whereas they remained basically unaffected in the unimplanted regions. The magnitudes of the energy shifts depend on the well width, RTA temperature and ion implantation fluence. The shifts were interpreted as arising from modification of the shapes of the as-grown QWs due to diffusion of In out of the well material. This process is enhanced by diffusion of vacancies generated near the sample surface by ion implantation. QWs with compositions near the critical thickness exhibit different behaviour from that of fully pseudomorphic layers, due to the presence of dislocations in these layers.     

X-ray photoelectron diffraction study of discommensurate Cu/Ge(111)

We have studied the atomic structure in the interior of discommensurate domains of the Cu/Ge(111) surface by using scanned-angle X-ray photoelectron diffraction (XPED). XPED patterns of Cu 2p_3/2 intensity provided direct information on the local structure in the vicinity of photoelectron emitters. It has been found that a certain number of Cu atoms are embedded within the surface layer, so that the surface has some structural similarity with the discommensurate Cu/Si(111)-‘5×5’.     

Characterization of (In,Ga)As/GaAs strained-layer multiple quantum wells with high-resolution X-ray diffraction and computer simulations

Pseudomorphic, highly strained (In,Ga)As/GaAs multiple quantum well structures were grown by molecular beam epitaxy and characterized by high-resolution X-ray diffraction. Thickness, lattice mismatch and chemical composition of the quantum wells were determined from measurements of satellite Bragg reflections and comparison with calculated rocking curves. In periodic structures, quantum wells with a width of less than 10 nm can be characterized by this technique. The results are compared with transmission electron microscopy, optical absorption and optical emission spectroscopy.     

Self-formation of quantum wire networks in porous InGaAs/GaAs superlattices

The structures containing networks of one-dimensional quantum wires were obtained through self-formation in the course of electrochemical etching of multilayer heteroepitaxial structures with a two-dimensional charge-carrier gas in InGaAs layers. The fact that the dimensionality of the electron-hole subsystem is reduced and the charge-carrier energy spectrum transforms from two-dimensional to quasi-one-dimensional was demonstrated by the shift and narrowing of resonance lines in the photoluminescence spectrum.     

Lineshape analysis of photoreflectance spectra from InGaAs/GaAs quantum wells

The fundamental optical transitions in In_0.15Ga_0.85As/GaAs single symmetric quantum wells (QWs) are studied through photoreflectance (PR) measurements and their dependence on the well distance from the surface. A phase rotation of the lineshape of the PR signal is observed as was predicted in our previous works. PR spectra of several samples, measured at 77 K, are compared with results of PR lineshape calculations, and a fairly good agreement is found. The quantum-confined Stark effect is shown to be the dominant modulation mechanism in the QW. Pronounced interference effects make PR spectra from QWs sensitive to the cap layer thickness.     

Magneto-optic determination of the light-hole effective masses in InGaAs/GaAs strained-layer superlattices

Low-temperature magneto-luminescence studies on strained layer superlattices are used to determine in-plane light-hole band effective masses in both n- and p-type InGaAs/GaAs structures.     

Raman study of confined TO phonons in GaAs/AlAs superlattices grown on GaAs (311)A and B surfaces

The use of Raman scattering in different polarization geometries makes it possible to observe the splitting of transverse optical (TO) phonon modes confined in GaAs/AlAs superlattices grown on faceted GaAs (311)A surfaces. The frequencies of TO modes with atomic displacements in the direction along the facets were observed to be higher than in the transverse one. Increased splitting, up to 3.5 cm ^− 1, was observed for (311)A superlattices when the average thickness of the GaAs layers was 6 monolayers or less. The splitting was absent in superlattices grown on (311)B surfaces under the same conditions. The effect of splitting is reputed to be caused by corrugation of GaAs/AlAs (311)A interfaces and formation of lateral superlattices or arrays of quantum wires, depending on the GaAs layer thickness.     

Light-emitting structures based on nanocrystalline (Si/CaF2) multiquantum wells

Periodic nanocrystalline (Si/CaF₂) multilayers, deposited on (111) silicon by Molecular Beam Epitaxy (MBE) at room temperature, were used to fabricate simple light emitting structures and to study their electrical and optoelectronic properties. Photoluminescence (PL) and electroluminescence (EL) spectra from the same area of the devices are approximately the same, indicative of the same emission mechanism. Current–voltage characteristics reveal important phenomena in vertical carrier transport. Regions of negative differential resistance and current oscillations were observed and were tentatively attributed to resonant tunneling at high electric fields under field domain formation.     

Localized optical phonons in GaAs/AlAs superlattices grown on (311)A and (311)B surfaces

A study is reported of optical vibrational modes in [311]-grown GaAs/AlAs superlattices. An analysis of the TO and LO localized modes observed in IR reflectance spectra showed that the difference between the TO and LO mode frequencies in superlattices grown on (311)A and (311)B surfaces is due to the different localization lengths of these modes. The dispersion of transverse optical phonons in GaAs derived from IR reflectance spectra is in a good agreement with Raman scattering data. Fiz. Tverd. Tela (St. Petersburg) 40, 550–552 (March 1998)