Spontaneous Lateral Composition Modulation Research Articles

Spontaneous lateral modulation in short-period superlattices investigated by grazing-incidence x-ray diffraction

The process of spontaneous lateral composition modulation in short-period InAs-AlAs superlattices has been investigated by grazing-incidence x-ray diffraction. We have developed a theoretical description of x-ray scattering from laterally modulated structures that makes it possible to determine the lateral composition modulation directly without assuming any structure model. From experimental intensity distributions in reciprocal space we have determined the amplitudes of the modulation and its degree of periodicity and their dependence on the number of superlattice periods. From the data it follows that the modulation process cannot be explained by bunching of monolayer steps and most likely, it is caused by stress-driven morphological instabilities of the growing surface.

Optical properties of spontaneous lateral composition modulation in AlAs/InAs short-period superlattices

The effect of lateral composition modulation, spontaneously generated during the epitaxial growth of an AlAs/InAs short-period superlattice, on the electronic band structure is investigated using phototransmission and photoluminescence spectroscopy. Compared with uniform layers of identical average composition, the presence of the composition modulation considerably reduces the band-gap energy and produces strongly polarized emission and absorption spectra. We demonstrate that the dominant polarization direction can selectively be aligned along the [1̄10] or [010] crystallographic directions. In compressively strained samples, the use of (001) InP substrates slightly miscut toward (111)A or (101) resulted in modulation directions along [110] or [100], respectively, and dominant polarization directions along a direction orthogonal to the respective composition modulation. Band-gap reductions as high as 350 and 310 meV are obtained for samples with composition modulation along [110] and [100], respectively. Ratios of polarized intensities up to 26 are observed in transmission spectra.

Excitons at a single localized center induced by a natural composition modulation in bulkGa0.5In0.5P

Optical properties of ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ grown by gas-source molecular beam epitaxy are reported. A strong optical anisotropy due to the spontaneous lateral composition modulation of Ga and In was observed in macro- and micro-photoluminescence (PL) spectra. The micro-PL study revealed that the PL band is composed of sharp lines that arise from the radiative decay of excitons trapped at local potential minima caused by the composition modulation. Anisotropy in PL rise and decay times was also observed. The detection energy dependence of the PL decay time is well explained by considering the relaxation process of excitons from shallow traps to deep traps.

Long wavelength pseudomorphic InGaPAsSb type-I and type-II active layers grown on GaAs

GaPAsSb type-I quantum wells and symmetric InGaAs/GaPAsSb/InGaAs type-II quantum wells are proposed as long wavelength active layers for GaAs based optoelectronic devices. Room temperature photoluminescence from 1.2 to 1.5 μm is observed from the type-II quantum wells, while low temperature photoluminescence at 1.25 μm is observed from type-I quantum wells. A reflection high-energy electron diffraction peak that is extended parallel to the surface, while its width is perpendicular to the surface is narrow, suggests that spontaneous lateral composition modulation occurs in the GaPAsSb layers.

Photoluminescence studies of lateral composition modulated short-period AlAs/InAs superlattices

We present low temperature photoluminescence data for a series of layers exhibiting spontaneous lateral composition modulation in (AlAs) m (InAs) n short period superlattices grown on InP with differing average lattice constants, i.e. varying global strain. The low temperature photoluminescence peak energies were found to be much lower than the corresponding energy expected for the equivalent In x Al 1− x As alloys. The bandgap energy reductions are found to approach 500 meV and this reduction is found to correlated with the ‘strength’ of the composition modulation wave amplitude.

Strain-dependent morphology of spontaneous lateral composition modulations in (AlAs)m(InAs)n short-period superlattices grown by molecular beam epitaxy

The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and (001)InP substrate. For samples grown under tension, transmission electron and atomic force microscopy reveal composition modulations along directions close to 〈310〉 coupled to a surface cusping. For samples grown under compression, we observe composition modulations roughly along the elastically soft 〈100〉 directions coupled to a surface rippling. For high strains (⩾0.7%), with individual InAs layer thicknesses ⩽1.6 monolayers, we observe weak or no composition modulations.

Spontaneous lateral composition modulation in InAlAs and InGaAs short-period superlattices

The microstructure of spontaneous lateral composition modulation along the [1 1 0] direction has been studied in (InAs) n /(AlAs) m short-period superlattices grown by molecular beam epitaxy on (0 0 1) InP. X-ray diffraction and transmission electron microscopy show that global strain ( ε) in the superlattice reduces the degree of composition modulation, which disappears for | ε|>0.7%. For tensile strains of ε≈+0.4%, we find that In-rich columns become regularly spaced and correlated with cusps in the growth surface. A similar correlation is seen in (InAs) n /(GaAs) m short-period superlattices between the enriched columns and the peaks and valleys of {1 1 4} A facets on the surface. The enriched columns in the (InAs) n /(GaAs) m layer (and the facets) extend for much longer distances (∼0.2–0.4 μm) in the [1 1 ̄ 0] direction than do the columns in the (InAs) n /(AlAs) m layer (∼56 nm).

Detection of lateral composition modulation by magnetoexciton spectroscopy

An experimental signature for detecting spontaneous lateral composition modulation in a (InAs) n /(GaAs) m short period superlattice on a InP substrate based on magnetoexciton spectroscopy is described. We find by aligning the magnetic field in three crystallographic directions, one parallel to and the other two perpendicular to the composition modulation direction, that the magnetoexciton shifts are anisotropic and are a good indicator for the presence of composition modulation.

Spontaneous lateral composition modulation in AlAs/InAs short period superlattices via the growth front

The spontaneous formation of lateral composition modulation in AlAs/lnAs short period superlattices on InP (001) substrates has been investigated. Transmission electron microscopy and x-ray diffraction reciprocal space mapping show that the lateral modulation is very regular, with a periodicity along the [110] direction on the order of 180A. A surprising: result is that this material system also exhibits a lateral modulation along the [1–10] direction, with a periodicity of 330A. Reflection high energy electron diffraction performed during the deposition revealed that the reconstruction changed from (2 × 1) during the InAs deposition cycle to (1 × 2) during the AlAs cycle, which may be related to the presence of the modulation in both directions. High magnification transmission electron micrographs show that the surface is undulated and that these undulations correlate spatially with composition modulation. Detailed analysis of the images shows that the contrast observed is indeed due to composition modulation. Photoluminescence from the modulated layer is strongly polarized and red-shifted by 220 meV.

Spontaneous Lateral Composition Modulation in III-V Semiconductor Alloys

The application of III-V semiconductor alloys in device structures is of importance for high-speed microelectronics and optoelectronics. These alloys have allowed the device engineer to tailor material parameters such as the bandgap and carrier mobility to the need of the device by altering the alloy composition. When using ternary or quaternary materials, the device designer presumes that the alloy is completely disordered, without any correlation between the atoms on the cation (anion) sublattice. However the thermodynamics of the alloy system often produce material that has some degree of macroscopic or microscopic ordering. Short-range ordering occurs when atoms adopt correlated neighboring positions over distances of the order of a few lattice spacings. This can be manifested as the preferential association of like atoms, as in clustering, or of unlike atoms, as in chemical ordering (e.g., CuPt ordering). Long-range ordering occurs over many tens of lattice spacings, as in the case of phase separation. In either short-range or long-range ordering, the band structure and the crystal symmetry are greatly altered. Therefore it is absolutely critical that the mechanisms be fully understood to prevent ordering when necessary or to exploit it when possible.

Lateral composition modulation in AlAs/InAs short period superlattices grown on InP(001)

Spontaneous lateral composition modulation as a consequence of the deposition of a (AlAs)n/(InAs)m short period superlattice on an InP(001) substrate is examined. Transmission electron microscopy images show distinct composition modulation appearing as vertical regions of In- and Al-rich materials alternating in the [1̄10] projection. The periodicity of the modulation is 130 Å, and is asymmetric. The transmission electron and x-ray diffraction patterns from the structure exhibit distinct satellite spots which correspond to the lateral periodicity. Transmission electron microscopy images show that the individual superlattice layers possess cusplike undulations, which directly correlate with the composition modulation. Composition modulation in this sample appears to be coupled to morphological and compositional instabilities at the surface due to strain.

Spontaneously generated effective-mass lateral superlattices.

An alternative approach to synthesizing effective-mass lateral superlattices is proposed. We show theoretically that the coherency strain associated with spontaneous lateral composition modulation provides a negative feedback which results in a band-gap locking phenomenon but that the resulting longitudinal strain wave breaks the symmetry of the valence bands, thereby generating an effective-mass lateral superlattice.