Polarization-dependent Photoluminescence Research Articles

Spin relaxation of excitons in zero-dimensional InGaAs quantum disks

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.

Interband optical transition spectra in GaAs quantum wires with rectangular cross sections

Interband optical transition spectra of rectangular GaAs quantum wires (QWR's) of various cross-sectional sizes are experimentally and theoretically studied. High-quality GaAs QWR's with lateral sizes below 20 nm are formed in AlAs trench structures with (110) vertical sidewalls by using metal-organic chemical vapor deposition. Polarization-dependent photoluminescence excitation (PLE) spectra in the QWR's clearly exhibit absorption peaks corresponding to optical transitions between quantized one-dimensional conduction and valence subbands. It is found that transition strengths and polarization anisotropies in the lowest- and higher-energy PLE peaks significantly vary, depending on the cross-sectional shape of the rectangular wires. The polarization-dependent interband transition matrix elements and the detailed absorption spectra are calculated by a multiband effective-mass theory considering heavy-hole and light-hole subband mixing. The theoretical results clarify the physical origin of observed PLE peaks and explain the strong dependence of interband transition properties on the cross-sectional ratio of QWR's.

Influence of the growth temperature and substrate orientation on the layer properties of MOVPE-growth

The luminescence properties of In 1− x Ga x As y P 1− y layers and heterostructures grown lattice matched to GaAs by metalorganic vapour-phase epitaxy (MOVPE) were studied and correlated to the crystalline properties. For laser structures emitting around 800 nm a red-shift of the emission from the active layer ( y = 0.72) grown at 680°C together with an anomalous temperature behaviour and excitation dependence of the bandgap is observed. Although some degree of ordering is observed for thick layers of this composition, polarization dependent photoluminescence does not indicate ordering of the quantum well to be the main reason for this excitation dependence. Instead, interfacial In-rich layers are found to be responsible. The thickness of these interfacial layers strongly depends on substrate misorientation and growth conditions.

Three-Dimensional Ordering of Molecules: p-Nitroaniline in Aligned ZSM-5 Single Crystals

Single-crystal zeolites can be aligned in silicon grooves with a surface coverage of more than 50%. The crystals are fixed to the substrate with tetramethoxysilane. Access to the microporous system is checked with N2 adsorption measurements. Two different types of ZSM-5 zeolites are used to realize a three-dimensional ordering of p-nitroaniline (PNA). Coupled thermogravimetry/FTIR reveals a loading of PNA in ZSM-5 of 8 wt %, which amounts to almost four molecules/unit cell. FTIR spectra of PNA adsorbed into ZSM-5 zeolites best resemble single-crystal PNA. Polarization dependent photoluminescence experiments at 4.5 K reveal a high degree of orientation of the molecules. Slight differences in the synthesis condition of the two types are responsible for the different peaks at 522 and 560 nm in the emission spectrum.

Effect of sulfur doping on optical anisotropy of CdSiAs2

Polarization dependent photoluminescence and photocurrent measurements were carried out on undoped and sulfur doped single crystals of CdSiAs2. For undoped crystal the interband transition A shows a 40 meV blueshift under perpendicular light polarization with respect to the parallel one. This shift is about twice as large for sulfur doped crystal. A pronounced photoabsorption band at 1.53 eV, related to the sulfur donor, is observed in the low temperature photocurrent spectrum. A significant enhancement of the optical anisotropy of donor-acceptor transition is seen in the low temperature photoluminescence spectra of CdSiAs2 doped with sulfur.

Electromagnetic properties of a dielectric grating. II. Quantum wells excited by surface waves.

A quantum well excited by a rectangular grating is studied as a prototype of optical systems showing periodicity and spatial-dispersion effects. The optical nonlocal response of this system is computed and compared with the optical response of a quantum well excited by a prism in attenuated total reflection and with the ``equivalent-slab model'' of the grating. Additional effects due to the interplay between exciton-polaritons and confined electromagnetic modes are pointed out in such kinds of systems. The measured polarization dependence of photoluminescence from quantum-well excitons excited through a grating is explained. \textcopyright{} 1996 The American Physical Society.

Dimensional crossover and confinement-induced optical anisotropy in GaAs T-shaped quantum wires.

On a series of GaAs T-shaped quantum wires (T-QWR's) fabricated by the cleaved-edge-overgrowth method, the spatially resolved polarization-dependent photoluminescence is studied systematically. The dimensional crossover between one and two dimensions has been demonstrated in terms of the characteristic energy of the lateral confinement and the confinement-induced optical anisotropy in T-QWR's. The Bloch functions of holes in T-QWR's and the confinement potential for holes affected by the Coulomb interaction with electrons have been clarified.

Ordering in GaInP2 studied by optical spectroscopy

AbstractThe optical properties of spontaneously ordered GaInP2 grown by MOVPE are studied by means of photoluminescence, time‐resolved photoluminescence, and polarization dependent photoluminescence excitation spectroscopy. Ordered samples grown on a substrate which is misoriented 6° toward [111]B show clear excitonic features both in luminescence and absorption. In excitation spectra both valence band edges are clearly resolved and are shown to be polarization dependent as predicted by band structure calculations. In these samples an additional non‐excitonic low‐energy photoluminescence emission is observed the peak energy of which depends strongly on excitation density and temperature and which shows extremely long luminescence lifetimes. A density of states model for ordered GaInP2 is proposed which allows to explain the results obtained in the last few years. The density of states model is discussed with respect to the microstructure of the ordered alloy as known from TEM investigations.

Reduction of etching damage in the fabrication of quantum wires

Quantum wire structures were fabricated by patterning quantum well samples with electron beam lithography and various wet chemical etching procedures. Wire structures with 800 Å wire width were achieved by wet etching in NH 4OHH 2O 2H 2O (20:7:973). These samples were characterized by scanning electron microscopy, photoluminescence (PL), and polarization-dependent photoluminescence excitation (PLE) measurements. The PL spectra show significantly stronger peaks than that taken from an unetched quantum well sample. A wire width of 400 Å was estimated from the blue shift of PL peaks. A 22% anisotropy was observed from polarization-dependent PLE spectra, further indicating the existence of two-dimensional quantum confinement.

Optical properties of anatase (TiO 2)

We present a study of optical properties of anatase (TiO 2) using temperature and polarization dependent photoluminescence, optical absorption, and reflectance measurements. Anatase crystals, rutile crystals and thin TiO 2 films have been studied in parallel for comparison purposes. Visible luminescence, due to the emission of self-trapped excitons, is observed in anatase. The optical absorption edge of anatase is 0.2eV larger than that of rutile, and shows a dichroism of about 40 meV with respect to E∥ c and E⊥ c polarizations at 10 K. The absorption edge of anatase displays an exponential energy dependence and is significantly less steep than the absorption edge of rutile. This may reflect the exciton self-trapping and/or the stronger influence of imperfections in anatose. For both anatase and rutile, the strongest interband transitions, governing the long-wavelength dispersion of the refractive index, appear as strong reflectivity maxima around 4 eV.

Direct epitaxial growth of (AlGa)As/GaAs quantum wires by orientation-dependent metalorganic vapour phase epitaxy

The direct growth of (AlGa)As/GaAs quantum wire structures using metalorganic vapour phase epitaxy (MOVPE) is investigated. The present concept takes advantage of the different growth rates of GaAs and (AlGa)As on (100) and {111} planes to transfer a trapezoidal GaAs starting structure with dimensions in the μm-range into a quantum wire structure in the nm-range. For stripe dimensions below 500 nm significant changes in growth behaviour (enhanced growth rates and for special cases also the formation of other crystallographic facets) are observed by using scanning electron microscopy (SEM) studies. Based on these findings (AlGa)As/GaAs quantum wire structures are realized with lateral structure sizes of 30 nm as proved by SEM. The luminescence properties of these structures, which are determined by standard and polarization-dependent photoluminescence excitation spectroscopy, are presented and discussed in comparison to the properties of quantum well structures.

Fractional superlattices and their device applications

Abstract ( AlAs ) 1 2 ( GaAs ) 1 2 fractional-layer superlattices (FLSs) are grown on a (001) vicinal substrate by metalorganic chemical vapor deposition (MOCVD). FLSs are new types of lateral superlattice with a period of 4–20 nm. The uniform superlattice period is observed over a large surface area by bright field transmission electron microscopy (TEM). The results suggest the ideal growth from a kink site occurs during MOCVD growth, and the distances between kink sites are equal. Polarization-dependent photoluminescence and optical absorption spectra of FLSs are also observed. Electron wave interference devices with a lateral periodic potential have been fabricated.

Growth and characterization of serpentine superlattices in the GaSb-AlSb system

We report the growth of GaSb/AlSb serpentine superlattices (SSLs) on vicinal GaAs and GaSb substrates. Cross-sectional transmission electron microscopy confirms the SSL structure and shows excellent lateral uniformity, better than previous arsenide-SSLs. Photoluminescence (PL) measurements indicate a good-quality lateral superlattice with a spectral linewidth between 13 and 15 meV. Polarization-dependent PL measurements give a normalized linear polarization around 60%, the strongest that has been seen for SSL structures. Preliminary estimates suggest much better segregation between the Ga-rich and Al-rich regions than arsenide-SSLs, with the change in aluminum concentration Δx≊0.35.

Electron and hole spin lifetimes in Zn1−xMnxSe and Zn1−yFeySe based heterostructures

We have studied carrier spin relaxation in diluted magnetic semiconductor (DMS) epilayer and spin superlattice samples using polarization and magnetic field dependent photoluminescence. The giant Zeeman splitting of the DMS materials permits unambiguous identification of electron and hole excitonic spin relaxation times. We find that the heavy hole spin lifetime τhs is comparable to the radiative recombination time τr, and substantially longer than the electron spin lifetime τes, with τhs/τr≊4 and τes/ τr≊0.06. We attribute these long spin lifetimes to the strain induced splitting of the bulk degenerate light and heavy hole bands, which inhibits mixing of the hole spin states and subsequent heavy hole spin flip relaxation processes. In the regime of weak confinement, the spin relaxation times are independent of the degree of quantum confinement, the number of interfaces, the magnetic ion concentration, and the magnetic species. We conclude that spin flip via exchange scattering between the carriers and magnetic ions is not the dominant excitonic spin relaxation mechanism.

MOCVD methods for fabricating GaAs quantum wires and quantum dots

Semiconductor low-dimensional structures such as quantum wires and quantum dots fabricated by metalorganic chemical vapor deposition (MOCVD) are reported. GaAs/AlAs quantum wire arrays including fractional-layer superlattices were grown on (001) GaAs vicinal surfaces. Uniform lateral superlattices were observed by transmission electron microscopy. However, polarization-dependent photoluminescence and the optical absorption spectra suggest that significant alloying occurs in AlAs/GaAs vertical interfaces. GaAs tetrahedral quantum dots buried in AlGaAs were also fabricated on a SiO 2 masked (111)B GaAs substrate partially etched to a triangular shape. First, AlGaAs truncated tetrahedral structures with three {110} facets were grown on GaAs triangular areas. Next, GaAs dot structures were sequentially grown on top of the AlGaAs truncated tetrahedron. Finally, AlGaAs was over-grown on {110} sidewall facets with a different growth condition. We observed large quantum size effects for about 100 nm TQD in low temperature photoluminescence, which is in good agreement with calculations.

InGaAs quantum well wires grown on patterned GaAs substrates

Strained InGaAs/GaAs quantum well wires have been grown on 2300 Å period gratings etched into GaAs substrates. Both conventional molecular beam epitaxy and migration-enhanced epitaxy have been used as growth techniques. The low-temperature photoluminescence from the quantum well wires was compared to the photoluminescence from a planar quantum well sample grown simultaneously. The planar samples showed a sharp single peak, whereas the patterned samples displayed two peaks. The samples grown by conventional molecular-beam epitaxy had two fairly broad, overlapping peaks of comparable intensity. The samples grown by migration-enhanced epitaxy had two distinct peaks, with one peak an order of magnitude larger than the other. Polarization-dependent photoluminescence was performed on the migration-enhanced epitaxy grown samples. The larger of the two peaks on the patterned sample showed a 21% linear polarization dependence, while the other peak showed unpolarized emission. In addition, the quantum well grown on the unpatterned substrate exhibited unpolarized emission.

Fractional superlattices grown by MOCVD and their device applications

(AlAs) 1 2 (GaAs) 1 2 fractional-layer-superlattices (FLS's) are grown on (001) vicinal substrates by metalorganic chemical vapor deposition. Various kinds of GaAs substrates were used. When the substrate is misoriented to [ 1 10] by 1.92° and [110] by 0.10°, uniform superlattice periods over a large surface area are observed by bright field transmission electron microscopy (TEM). The results suggest the ideal growth from a kink site occurs during MOCVD growth, and the distances between kink sites are equal. On a substrate misoriented to [110] by 1.90°, the superlattice periods exhibit an undulation. This shows that kink flow mode growth is not dominant in the [110] direction. On a substrate misoriented to [010] by 2.0°, no superlattice periods were observed. From the above results, we discuss the growth mechanisms. Polarization dependent photoluminescence and optical absorption spectra of FLS's were also observed. Electron wave interference devices with lateral periodic potential were fabricated.

Center-of-mass quantization of excitons in GaAs quantum-well wires.

We have studied wide quantum-well-wire structures by polarization-dependent photoluminescence and photoluminescence excitation spectroscopy. We observe an up-to-threefold splitting of the lowest heavy-hole-excition resonance, which arises from a quantization of the excitonic center-of-mass motion perpendicular to the direction of the wires. The energy separation between these transitions depends on the active wire width ${\mathit{L}}_{\mathit{x}}$, which is significantly smaller than the geometrical wire width. Identifying the exciton wave vector ${\mathit{K}}_{\mathrm{ex}}^{\mathit{n}}$ with n\ensuremath{\pi}/${\mathit{L}}_{\mathit{x}}$, we can reconstruct the exciton dispersion and find excellent agreement with theory if we use the k=0 heavy-hole mass.

Anisotropy in photoluminescence and absorption spectra of fractional layer superlattices

Polarization-dependent photoluminescence (PL) and optical absorption spectra of (AlAs)1/2(GaAs)1/2 and (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional layer superlattices (FLSs) were measured at room temperature. For the first time, it was observed that the spectra are dependent on their polarizations. The anisotropies in the PL and absorption spectra show that quantum confinement unambiguously occurs in the direction parallel to the surface, i.e., lateral quantum carrier confinement effect appears. This finding proves that FLSs are promising for novel optical devices.