A. Mazuelas

Atomic layer molecular beam epitaxy growth of InAs on GaAs substrates

InAs layers with thickness ranging from 0.1 to 2.5 μm have been grown directly on highly mismatched (7.4%) (001) GaAs substrates by atomic layer molecular beam epitaxy (ALMBE). This growth method, based on the modulated deposition of one or both component species, provides InAs layers with excellent flat morphology, independently of the total thickness. A detailed study of the evolution of the electron diffraction (RHEED) pattern indicates that a complete decoupling between the InAs epitaxial layer and the GaAs substrate is reached in less that 10 monolayers. Evidence is obtained that layer-by-layer nucleation takes place from the beginning of the growth.

Critical thickness determination of InAs, InP and GaP on GaAs by X-ray interference effect and transmission electron microscopy

Abstract X-ray interference effect, reflection high-energy electron diffraction, and transmission electron microscopy were used to determine the critical thickness of InAs, InP and GaP on GaAs {001} grown by atomic layer molecular beam epitaxy. Three different series of samples consisting in N monolayers of InAs (N=1, 2, 3, 4), M monolayers of InP (M=3, 4, 5, 6, 7) and L monolayers of GaP (L=2, 3, 4, 5, 6, 9) covered by a 200 nm GaAs cap layer were grown at 350°C. The thicknesses of the strained layers were chosen to cover the range from strained layers (below the critical thickness for generation of misfit dislocations) to relaxed layers (where all lattice mismatch is relieved by the generation of misfit dislocations). Sample growth was monitored by reflection high-energy electron diffraction in order to in-situ study the relaxation process. X-ray interference effect was used to determine thickness and strain status of the strained layers comparing experimental diffraction patterns with simulated ones using dynamical theory of X-ray diffraction. Transmission electron microscopy was used to assess thickness, relaxation status and dislocation nucleation in the strained layer.

Atomic layer MBE growth and characterization of AlAsInAs strained layer superlattices on GaAs

A recent development of Molecular Beam Epitaxy (MBE) — the Atomic Layer Molecular Beam Epitaxy — has been used to grow AlAsInAs strained layer superlattices (SLS) on (001)GaAs at low growth temperatures (Ts < 400°C). The growth process basically consists on alternating group V and/or group III beams following an optimum timing established by RHEED oscillations observation during the monolayer formation sequence. This method allows to grow at low substrate temperature with excellent morphology, even for those systems which have extremely different optimum MBE growth conditions and a severe lattice mismatch of 7% like AlAsInAs. X-ray diffraction and optical characterization results for superlattices of different periodicities are presented. In particular, Raman spectra of these samples showing folded acoustic phonons demonstrate their quality.

Strain distribution and structural characterization of short period GaAsGaP strained superlattices by raman and X-Ray scattering

Abstract Raman scattering is used to study structural properties of strained-layer GaAsGaP short period superlattices grown on GaAs substrates. Different thicknesses of the constituent layers and also the effect of different types of buffer layers are studied. From the energy and width of the confined optical phonons observed, information about strain accomodation in the layers, strain relaxation and, in general, about structural quality is achieved.

Folded acoustic phonons in InAs-AlAs strained-layer superlattices

Raman spectroscopy is used to characterize highly mismatched (7%) InAs‐AlAs superlattices grown by atomic layer molecular beam epitaxy. In particular, folded acoustic modes are presented and compared with two different theoretical models (Rytov and linear chain). We find good agreement between theory and experiments. We estimate, with a simple model, the magnitude of the effect of the strain on the phonon frequency shifts.

Growth and characterization of ultrathin GaP layer in a GaAs matrix by X-ray interference effect

An ultrathin two monolayers thick layer of GaP sandwiched within a GaAs matrix was grown by atomic layer molecular beam epitaxy (ALMBE). The X-ray interference effect (Pendellösung) was used to determine the structural parameters such as thickness, lattice parameter, chemical composition, and strain. Excellent agreement between the experimental rocking curve and the simulation using the dynamical theory of X-ray diffraction was found indicating the high quality of the sample. Analysis of the scans in symmetrical (004) and asymmetrical (224) reflections, sensitive to both perpendicular and parallel strain, shows that the GaP layer is coherent with the substrate, i.e., it is below the critical thickness in agreement with critical thickness theories. Despite the competition for incorporation between arsenic and phosphorus the experimental GaP thickness is found to be identical to the nominal growth value, demonstrating full incorporation of phosphorus when growing by ALMBE. No significant out-diffusion or segregation of P is observed.

Determination of in-depth thermal strain distribution in Molecular Beam Epitaxy GaAs on Si

In-depth stress distribution GaAs layers grown by Molecular Beam Epitaxy (MBE) on Si (001) has been studied by X-ray diffraction, photoluminescence and Raman spectroscopy. In order to determine the stress state at different distances to the interface GaAs/Si, layers of different thickness were prepared by chemical etching of the grown samples. We observe a non-uniform residual strain distribution through the GaAs on Si epilayer. Residual strain of thermal origin is larger in the highly defective region (∼ 0.4 μm) near the GaAs/Si interface where we have found a non-elastic relation between measured in-plane (a‖) and in growth direction (a⊥) lattice parameters. However, thermal strain is partially relaxed by formation of 107 cm−2 dislocations in the region of better crystalline quality near the external surface.

Quantum well lasers with InAs monolayers in the active region grown at low temperature by atomic layer molecular beam epitaxy

Strained-layer quantum well lasers have been grown at low substrate temperature (350°C) by atomic layer molecular beam epitaxy (ALMBE). A series of five separate confinement laser structures has been grown. Active regions consist of 80 A of GaAs in which n InAs monolayers are inserted separated by 3 GaAs monolayers, with n = 1, 3, 5 and 7. A sample with 100 A Ga0.8In0.2As active region was also grown at low temperature for comparison. Structural quality is studied by X-ray diffraction. Optical characterization has been performed, and the results are compared with calculations by a four-band envelope-wavefunction method. Broad area lasers have been fabricated, and their performances are studied as a function of increasing InAs content. Lasers with GaAs active regions including InAs monolayers show threshold currents comparable to that of the GaInAs alloy laser, for n≤5 monolayers. These results prove the feasibility of ALMBE for the low temperature growth of strained layer lasers.

Structural characterization of gaas/gap superlattices

Powder and double-crystal X-ray diffraction were used to study the structural properties of highly strained (GaAs)N/(GaP)M short-period superlattices grown on GaAs (001) substrates. In spite of the large lattice mismatch (f=3.6%) between GaAs and GaP and the competition for incorporation between As and P, high-quality short-period superlattices of GaAs/GaP have been grown by a development of conventional molecular beam epitaxy named atomic layer molecular beam epitaxy. The in-plane lattice parameter (a/sub ///) of the different superlattices was measured and studied as a function of the GaP content. The authors found that, for a given total superlattice thickness of 4000 AA, the critical lattice mismatch is fc approximately=0.5% (corresponding to an average GaP content of 13.6% in the superlattice). This means that for an average misfit or lattice mismatch above fc the superlattice starts to relax. This experimental result is compared with predictions of critical thickness theories based on energy criteria. A clear relation of the degree of relaxation with peak width of the superlattice zeroth-order diffraction peak is found. High-resolution transmission electron microscopy has been performed to analyse the type of dislocations that relax the mismatched layers.

High-resolution electron microscopy and X-ray diffraction characterization of alternately strained (GaAs)n(GaP)m(GaAs)n(InP)m superlattices grown by Atomic Layer Molecular Beam Epitaxy.

Abstract Alternately-strained-layer of (GaAs)n(GaP)m(GaAs)n(InP)m, superlattices with n = 10, 90 and m = 2 monolayers, have been characterized by X-ray diffraction and high-resolution transmission electron microscopy. The heterostructures were grown by atomic layer molecular beam epitaxy on GaAs semi-insulating substrates at a substrate temperature of 710K. All the structures were coherent with the substrate and there is an effective compensation of the strains due to GaP and InP layers. X-ray diffraction and high-resolution electron microscopy analysis indicate a high crystalline quality with layers thickness deviations of ± 1 monolayer from the designed value.